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. 2024 Nov 6;15(45):11361–11373. doi: 10.1021/acs.jpclett.4c02733

Unique Applications of para-Hydrogen Matrix Isolation to Spectroscopy and Astrochemistry

Isabelle Weber †, Prasad Ramesh Joshi †, David T Anderson ‡,*, Yuan-Pern Lee †,§,*
PMCID: PMC11571221  PMID: 39503286

Abstract

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Cryogenic solid para-hydrogen (p-H2) exhibits pronounced quantum effects, enabling unique experiments that are typically not possible in noble-gas matrices. The diminished cage effect facilitates the production of free radicals via in situ photolysis or photoinduced reactions. Electron bombardment during deposition readily produces protonated and hydrogenated species, such as polycyclic aromatic hydrocarbons, that are important in astrochemistry. In addition, quantum diffusion delocalizes hydrogen atoms in solid p-H2, allowing efficient H atom reactions with astrochemical species and introducing new concepts in astrochemical models. Some H atom reactions display anomalous temperature behaviors, highlighting the rich chemistry in p-H2. The investigation on quantum diffusion of heavier atoms and molecules is also important for our understanding of the chemistry in interstellar ice. Additionally, matrix shifts of electronic transitions of polycyclic aromatic hydrocarbons in p-H2 are less divergent than those in solid Ne such that systematic measurements in p-H2 might help in the assignment of diffuse interstellar bands.

The matrix-isolation technique using noble-gas matrix hosts (Ne, Ar, Kr, Xe, and N2) has been widely employed to preserve unstable species for spectroscopic or chemical studies.1−5 However, because of the cage effect associated with the rigid crystal structure of the matrix host, to produce free radicals via in situ photolysis or photoinduced reactions is challenging. For the past several decades, para-hydrogen (p-H2) has emerged as a novel matrix host due to its unique properties associated with its quantum-mechanical nature.6 Initially, focus was placed on developing experimental techniques to grow transparent, chemically doped p-H2 solids, investigating the unique properties of solid p-H2, performing high-resolution solid-state spectroscopy of molecules and clusters, producing free radicals via in situ photolysis, and studying nuclear-spin relaxation with infrared spectroscopy; several earlier reviews are available.5−10 In more recent years, more and more applications that are unique to p-H2, including using in situ photolysis and photoinduced reactions to produce radicals, using electron bombardment to produce protonated and hydrogenated species, and making use of the H atom mobility in solid p-H2 to carry out low-temperature hydrogen tunneling reactions, have been developed, as summarized in several articles.11−14 This Perspective will hence focus on the more recent advances on photoinduced fragmentation and reactions, production of protonated species, H atom reactions, anomalous temperature effects in chemical reactivity, heavy-atom diffusion, and electronic transitions of trapped guest species that are unique to p-H2 matrix-isolation spectroscopy; the last three topics were not covered in detail in previous reviews. Possible applications to astrochemistry and future perspectives are also discussed. Due to limited space, the nuclear-spin relaxation of species in p-H2 is not included in this article.

Photolysis and Photoinduced Reactions In Situ

The advantage of the diminished cage effect in solid p-H2 for producing free radicals was first demonstrated by Momose and co-workers to produce •CH3 and •C2H5 upon UV irradiation of CH3I and C2H5I trapped in solid p-H2.7,15 Later, Lee and co-workers demonstrated that CH3•S and CH3•O can be readily produced by photolysis of CH3SSCH3 (or CH3SCH3)16 and CH3ONO,17 respectively; whereas similar experiments in solid Ar produced only H2CS + CH3SH and H2CO + HNO because the solid Ar solvent cage constrained the two nascent fragments so that they reacted further with each other to form stable molecules. With the diminished cage effect, producing free radicals from in situ photolysis becomes straightforward in solid p-H2; for example, many alkyl radicals can be readily produced from UV irradiation of alkyl halides.18 More recently, (Z)- and (E)-C2H3•C(CH3)I radicals were produced upon in situ photodissociation of (Z)- and (E)-(CH2I)HC=C(CH3)I, respectively,19 and (Z)- and (E)-•CH2C(CH3)CHI radicals were observed upon photodissociation of (Z)- and (E)-(CH2I)(CH3)C=CHI, respectively;20 these molecules are precursors of Criegee intermediates. The allylic C–I bond, not the vinylic one, was dissociated, and the (Z)- and (E)-conformation was retained upon photolysis. Momose and co-workers employed UV or vacuum UV irradiation (193, 213, or 266 nm) to photolyze 1,3-cyclohexadiene21 and several amino acids including α-alanine,22 glycine, leucine, proline, and serine23 to observe HOCO and imines rather than amine radicals; they proposed that HOCO could be a tracer for the search of amino acids in interstellar space.

The diminished cage effect also makes photoinduced bimolecular reactions feasible in solid p-H2. A series of reactions of halogen atoms with various alkenes to produce the haloalkyl radicals have been demonstrated.14 For example, photolysis of a Cl2/isoprene/p-H2 matrix produced 1-chloromethyl-1-methylallyl and 1-chloromethyl-2-methylallyl radicals, showing site selectivity on the addition of the Cl atom to the carbon skeleton;24 in contrast, only stable dichloroalkanes were produced from analogous experiments in an Ar matrix. Formation and the infrared spectrum of the open-form of the 2-bromoethyl radical (2-•C2H4Br) from UV irradiation of a C2H4/Br2/p-H2 matrix were also reported.25 This feature of a diminished cage effect in p-H2 matrix isolation opens a new paradigm for producing free radicals that were challenging to prepare either in the gaseous phase or in noble-gas matrices.

In addition to the production of unstable species, the properties of weakly bound radicals in solid p-H2 provide insight into the nature of the reaction. Unlike the •C6H6Cl, an open-form η1 σ-complex (6-chlorocyclohexadienyl radical) reported previously in solid p-H2,26 the •C6H6Br radical produced via in situ photolysis of a Br2/C6H6/p-H2 matrix is an open-form η1 π-complex with the benzene ring performing a bevel-gear-type rotation (Figure 1) with respect to Br, according to its IR absorption spectrum that shows the two predicted CH out-of-plane bending modes associated with mainly even- and odd-numbered carbons merged into one broad band; this motion was also supported by quantum-chemical computations showing a small barrier ∼1 kJ mol–1.27 Furthermore, the observation of only trans-ortho- and trans-para-C6H6Br2, but not cis-ortho- and cis-para-C6H6Br2, when the mixing ratio of Br2 was increased suggests that this gear-type motion allows the second Br atom to attack the •C6H6Br radical from only the opposite side of the Br atom in •C6H6Br. Previously, it was generally accepted that this stereo selectivity was associated with the formation of an η2 complex, with Br bound to two carbon atoms, even though no direct detection of the η2 complex has been reported. This finding indicates that an η1 complex with a bevel-gear-type motion, which might also be feasible in solutions, can explain this stereo selectivity.

Figure 1.

Figure 1

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Bevel-gear-type motion of •C6H6Br and its infrared spectrum. The benzene ring undergoes a bevel-gear-type motion with respect to Br in η1-•C6H6Br. η2-•C6H6Br is a transition state with a barrier of ∼1 kJ mol–1. Spectral lines indicated with B (green) are due to η1-•C6H6Br. In the green box, two lines of out-of-plane CH bending modes were predicted for η1-•C6H6Br, but only one broad feature was observed. Partially reproduced from ref (27). Copyright 2023 American Chemical Society.

One of the chief limitations to using solid p-H2 as a matrix host to generate free radicals or ions for spectroscopic characterization is the fact that H2 is not chemically inert, as pointed out previously;11 examples include reactions of radicals •CH2, •C2H3, and •C2H2Cl, produced via photolysis or photoinduced reactions, with the H2 host, so that these radicals could not be isolated in p-H2. However, while H2 molecules are obviously more reactive than noble gas atoms, the large bond energy of H2 (436 kJ mol–1) combined with the high thermal conductivity of p-H2 crystals7 makes solid p-H2 relatively chemically inert. Nonetheless, if the target reactive species can react with the p-H2 host via an exergonic reaction, then even if the barrier to reaction is very high, the possibility of tunneling reactions with the host can prevent stabilization of the target species at enough concentrations to detect it. Sometimes if the tunneling reaction with the surrounding hydrogen molecules is on a time scale of a few hours, the reactive species can still be observed by conventional spectroscopy, and this disadvantage can be turned into an advantage. For example, infrared studies of the in situ photodissociation of NH3 in solid p-H2 at 193 nm showed that the nascent-•NH2 photoproduct is rapidly cooled within the p-H2 matrix to the ground vibrational and rotational state before a tunneling-driven reaction (τ1/2= 0.37 min, EA = 47.5 kJ mol–1) with the p-H2 host to produce ortho-NH3 in a defect site.28

Protonated and Hydrogenated PAH

Electron bombardment during deposition of a p-H2 matrix produces a significant amount of H3+ and H atoms from the reaction H2+ + H2; H3+ can readily transfer a proton to a species that has a proton affinity greater than that (422 kJ mol–1) of H2. Most polycyclic aromatic hydrocarbons (PAH) have proton affinities greater than 800 kJ mol–1, and therefore the production of protonated PAH (H+PAH) via proton transfer from H3+ becomes feasible. The side products are hydrogenated PAH, which can be produced from either the neutralization of a protonated PAH or the reaction of an H atom with the PAH. In general, this method has advantages over Ar-tagging or IR multiphoton dissociation methods for recording the IR spectrum of cold ions;11 among them, the FTIR absorption spectrum has a wide spectral coverage and provides accurate relative IR intensities for better comparison with theoretical predictions. Also, because guest ions or molecules typically have narrow absorption line widths in p-H2, spectra of various isomers can be resolved and identified by grouping observed lines of each isomer according to their distinct photolytic behaviors upon secondary photolysis at varied wavelengths. The protonation of aniline (C6H5NH2) using this technique serves as a good example; para-, amino-, and ortho-H+C6H5NH2 isomers were clearly identified according to their IR spectra.29 Similarly, isomers of protonated fluoranthene, 3-, 9-, and 10-C16H11+, were all identified with IR spectra in a single experiment.30

Protonated PAH were proposed to be possible carriers of the interstellar unidentified infrared (UIR) emission bands.31 Earlier work on planar protonated PAH isolated in p-H2 with increasing size from naphthalene (1-C10H9+),32 pyrene (1-C16H11+),33 coronene (1-C24H13+),34 to ovalene (7-C32H15+)35 indicated that, as the size of PAH increases, the IR absorption bands are trending toward, but not quite reaching, the UIR emission bands.11,14 Furthermore, the IR absorption bands of protonated corannulene (hub-H+C20H10),36 a nonplanar PAH which has a carbon framework as a fragment of C60, also agree satisfactorily with those of the UIR bands. More recent work focused on polycyclic aromatic nitrogen heterocycles (PANH). The N atom in the aromatic ring of protonated PANH (H+PANH), such as protonated quinoline (C9H7NH+)37 and protonated isoquinoline (iso-C9H7NH+),38 induces a blue shift of the CC-stretching modes of PAH near 6.3 μm, so that these bands match better with the UIR band near 6.2 μm. However, other bands of C9H7NH+ and iso-C9H7NH+ do not fit with the UIR bands, suggesting that these two species are unlikely to be carriers of the UIR bands, consistent with the expectation that they are too small to survive the intense UV fields in interstellar media.

The associated coproducts monohydrogenated PANH (HPANH) were produced either from the neutralization of the protonated H+PANH or reactions of H with PANH in the electron-bombardment experiments. These HPANH also can be produced by a more efficient method via UV/IR irradiation of the p-H2 matrix with trace Cl2 (typically <200 ppm) added to produce H atoms via Cl + H2 (v = 1), to be discussed in the next section. Because of the small mixing ratio of H atoms employed in these experiments, mainly only monohydrogenated PAH or PANH were produced. In general, PAH/PANH hydrogenated at various nonfused carbon sites have similar energy, so that many isomers might be produced. These isomers were distinguished according to their photolytic behavior upon irradiation at varied wavelengths and by comparison with quantum-chemically predicted IR spectra. For quinoline (C9H7N), hydrogenation at the 1-, 3-, 4-, 7-, and 8-positions was observed,37 whereas for isoquinoline (iso-C9H7N), hydrogenation at all eight feasible carbon sites except the two carbons on the fused ring was observed.38 According to the CCSD(T)/6-311++G(d,p)//B3LYP/6-311++G(d,p) method, the lowest-energy isomer of iso-•C9H8N is 2-iso-•C9H8N (or iso-•C9H7NH, hydrogenation at the N site), other isomers with hydrogenation on the nonfused carbon sites are higher in energy by only 2–24 kJ mol–1; the barriers for the hydrogenation range from 21 to 27 kJ mol–1, so they are all accessible from H atom tunneling reactions. Some missing isomers in hydrogenated quinoline might be due to interference from other species or their small IR intensities. This technique is unique in producing nearly all possible monohydrogenated PAH in a single experiment.

Hydrogen Atom Reactions

One convenient method used to generate H atoms in solid p-H2 is to take advantage of the diminished cage effect to photolyze trace Cl2 (<1000 ppm) to form isolated Cl atoms; Cl atoms are stable in p-H2 because the reaction Cl + H2 is endothermic by ∼4 kJ mol–1 (∼370 cm–1) with a barrier of ∼21 kJ mol–1 (∼1720 cm–1).13 Next, to generate the H atoms, the matrix is irradiated with near-IR radiation which is absorbed by the p-H2 matrix in the 4200–4700 cm–1 range and generates H2(v = 1) vibrons that travel through the solid and induce the Cl + H2(v = 1) reaction (Figure 2a). Using this UV/IR method, the H atom is initially produced with some fraction of the exothermicity (∼45.3 kJ mol–1) of the Cl + H2(v = 1) reaction; however, this energy is quickly dissipated by the p-H2 matrix, and the H atom is rapidly thermalized.39 Once the H atom is generated in solid p-H2 it becomes delocalized by a chemical diffusion mechanism (H + H2 → H2 + H) whereby the H atom reacts repeatedly with adjacent H2 molecules (Figure 2b) and in this way moves through the solid.40 Typically, researchers that use this UV/IR method to generate H atoms also perform experiments in the dark to test that the reaction still occurs, even in the absence of any near-IR radiation. By studying the reaction both during the near-IR exposure and in the dark, researchers gain further insight into the reaction mechanism. Alternatively, the group in Laramie employed 193 nm irradiation of N2O to produce O(1D), which reacted with the surrounding H2 host to produce H atoms (along with OH); reaction of H atoms with N2O produced both cis- and trans-HNNO.41

Figure 2.

Figure 2

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H atom tunneling reaction in solid p-H2. (a) Potential energy scheme of Cl + H2. The reaction is endothermic by ∼4 kJ mol–1 (370 cm–1) with a barrier of ∼21 kJ mol–1 (1720 cm–1). The reaction proceeds upon IR irradiation. (b) H atom tunneling in the solid p-H2. The pair of blue balls indicates H2, and the orange, green, and brown balls indicate H atom, reactant, and product, respectively. The red box indicates the formation of a new bond. The H atom does not have to diffuse physically through the matrix to approach the reactant.

Reactions of H atoms play key roles in astrochemical models for the formation of complex organic molecules because H atoms are abundant, mobile, and reactive. A detailed model for the formation of H2O, H2CO, and CH3OH involving H-abstraction and H-addition reactions has been reported;42 for example, H atom addition can convert CO sequentially to H*CO, H2CO, CH3*O or *CH2OH, and CH3OH.42−44 However, many astrochemical models typically focus on mainly H atom addition reactions.45,46 For H atom addition reactions, infrared spectra of cis- and trans-HNNO from H + N2O;41 1-•C4H7 from H + 1,3-butadiene (C4H6);47 1,1-dimethylallyl [(CH3)2•CCH=CH2] and 1,2-dimethylallyl [H2C=C(CH3) •CH(CH3)] radicals from H + isoprene [H2C=C(CH3)CH=CH2];48•C5H5NH and 4-C5H6•N from H + pyridine;49 2,3-dihydropyrrol-2-yl (3-H•C4H4NH) and 2,3-dihydropyrrol-3-yl (2-H•C4H4NH) radicals from H + pyrrole (C4H4NH);50•ONH(OH) from H + nitrous acid (HONO);51 H•SO2, •S(OH)2, HS(O)OH from H + SO/SO2;52 2-hydrofuran-3-yl, 3-hydrofuran-2-yl, 2,3,4-trihydrofuran-5-yl, and 2,3,5-trihydrofuran-4-yl from H + furan;53 and H2C•NO from H + fulminic acid (HCNO)54 were recently observed by several groups. These H atom reactions were all studied using this UV/IR technique, demonstrating the generality of this approach, and none of these reactions could be studied at these temperatures in a noble gas matrix simply because the H atom is not mobile enough under such conditions.

However, when H atoms reacted with formamide HC(O)NH2, the H atom abstraction played an important role; the radical intermediate H2N•CO and the product of the second H atom abstraction, HNCO, were observed.55 A dual-cycle mechanism containing two sets of H atom abstraction and H atom addition reactions chemically links HC(O)NH2, H2N•CO, and HNCO together and might explain the nearly constant ratio of [HNCO]/[HC(O)NH2] in interstellar media. H atom abstraction reactions to produce radicals were also observed for methanol (CH3OH),56 ethanol (C2H5OH),57 methyl formate [HC(O)OCH3],58 acetamide [CH3C(O)NH2],59 acetic acid [CH3C(O)OH],60 glycine [NH2CH2C(O)OH],61 methyl amine [CH3NH2],62N-methyl formamide (NMF) [HC(O)NHCH3],63 and formaldoxime (H2CNOH);54 a recent review is available.13 From these experiments, we learned that in addition to the well-known H atom addition reactions, H atom abstraction reactions can also produce radicals and open up new channels for further reactions to form larger or more complex molecules. Furthermore, the coupling of H atom abstraction and H atom addition reactions can enable the uphill (endothermic) isomerization of molecules, as was observed in the conversion of the lower-energy trans-NMF to the higher-energy cis-NMF in darkness.63 The H atom addition can also induce fragmentation to form more stable smaller species, such as HNCO + CH4 and CH2NH + CO from H + trans-·C(O)NH(CH3).63 Similarly, CH3•CHOH was produced from H + C2H5OH.64

Recently, rich chemistry was observed for H + glycolaldehyde [HOCH2C(O)H, GA] in solid p-H2 by two groups.65,66 GA was detected in the interstellar medium (ISM); it is a primitive sugar-like molecule and a potential precursor for complex sugars. Radical intermediates HOCH2CH2•O (1), HOCH2•CHOH (2), HOCH2•CO (3), HO•CHC(O)H (4), and •OCH2C(O)H (5) derived from GA might lead to the formation of glyceraldehyde, dihydroxyacetone, and ethylene glycol in the interstellar medium. The group in Taiwan recently conducted reactions of the H + Cis-cis conformer of GA (Cc-GA) in solid p-H2 at 3.2 K and identified IR spectra of radicals produced from H atom abstraction, Cc-HOCH2•CO (3) and Cc-HO•CHC(O)H (4), as well as the closed-shell species HOCHCO (6), produced via consecutive H atom abstraction reactions with GA. In addition, cc-HOCH2CH2•O (1) and •CH2OH + H2CO were produced through the H atom addition and the H atom induced fragmentation channels, respectively. The reaction of H + GA indicates the multiple roles that GA might play in astronomical chemistry via its rich chemistry with four channels of three distinct types. Furthermore, the formation of cc-HO•CHC(O)H (4) and •CH2OH + H2CO was found to be enhanced during near-IR irradiation of the sample as compared to reactions in darkness. Such a significant difference in the branching among different reaction channels, as was observed depending on whether the p-H2 matrix was being exposed to near-IR radiation or not, illustrates nicely that when studying such low-temperature chemical reactions, all possible energy sources must be characterized, as it appears that the H atoms in the near-IR irradiated p-H2 samples react differently from those in the p-H2 samples with no near-IR exposure.

Coupling the strongly exothermic reaction H + H → H2 (exothermicity ∼480 kJ mol–1) with potential endothermic reactions of guest species means that even endothermic reactions become feasible in the dark under interstellar conditions. This effect is similar to the coupling of ATP → ADP to drive originally endothermic reactions in biological systems; H atoms hence can be regarded as the ATP in the ISM. Because of the abundance of H atoms in astronomical environments, one must consider all possible H atom-assisted reactions, including the previously overlooked ones such as H atom abstraction, uphill isomerization, and H atom induced fragmentation, in astrochemical models.

On the other hand, because of the coupling of H atom abstraction and H atom addition reactions, H atoms might be catalytically converted to H2 by these astrochemical species; that means, mobile H atoms might be converted to molecular H2 without going through the direct H + H recombination reaction. These previously overlooked catalytic reactions might help to explain the observation of a smaller [H]/[H2] ratio in interstellar media as compared with the predicted ratio based on the recombination of H atoms in the gaseous phase and on dust grains.67

Anomalous Temperature Effect

Typically, reaction rate coefficients increase with temperature; this Arrhenius temperature dependence is related to the barrier for the reaction. However, in solid p-H2, several H atom reactions have been found to occur only at temperatures lower than ∼3 K and to cease above that temperature. For example, the group in Laramie reported that the addition reaction H + N2O → cis-HNNO/trans-HNNO41,68 occurs below ∼2.4 K but ceases above 2.4 K. Ab initio calculations69 that take zero-point energy corrections into account predict that this H + N2O reaction has a barrier of 47 kJ mol–1, which means that this reaction can only take place via quantum-mechanical tunneling under these low-temperature conditions. In addition, H atom abstraction reactions conducted in solid p-H2 such as H + HC(O)OH → HOCO + H2,70,71 H + CH3OH → •CH2OH + H2,72 and H + HC(O)OCH3 → •C(O)OCH3/HC(O)O•CH2 (ref (58)) also showed similar anomalous temperature behaviors. All of these H atom abstraction reactions have sizable barriers, suggesting that this might be required to observe this nonintuitive low-temperature reactivity.

The reaction of H atoms with formic acid was one of the first reaction conducted in solid p-H2 that showed this anomalous low-temperature reaction kinetics.70,71 The reactivity of H atoms with formic acid has also been studied in noble-gas matrix-isolation experiments.73 H-addition products dominated the matrix-isolation studies conducted in a Kr matrix at 31 K, where the final product was identified73 as the simplest geminal diol radical trans–cis-HC(OH)2. This shows that the chemical reactivity is qualitatively different under these two contrasting low-temperature conditions. We believe that the differences stem from the different H atom diffusion mechanisms; in a noble-gas matrix, the chemistry is thermally induced such that the H atoms only start to diffuse above some threshold temperature, whereas in solid p-H2 the H atoms are mobile at all temperatures via a quantum-mechanical tunneling mechanism. Thus, this anomalous low-temperature chemistry cannot be studied in a noble-gas matrix and, therefore, is likely related to the details of H atom quantum diffusion in solid p-H2.

We point out that this anomalous low-temperature reaction kinetics is different from what has been observed in several gas-phase reactions in which the reaction accelerates very rapidly at very low temperatures.74 For gas-phase hydrogen abstraction reactions of OH with organic compounds containing alcohol, ether, carbonyl, and ester functional groups, the rate coefficients at very low temperatures can be up to 1000 times larger than those at room temperature, despite the barrier to products.74 This anti-Arrhenius behavior has been explained by the formation of a weakly bound hydrogen-bonded complex of the reactants, such that at low temperatures the lifetime of the complex against redissociation back to reactants becomes much longer, and hence the probability of quantum-mechanical tunneling under the reaction barrier to form products becomes much larger. A comparable mechanism could be operative for H atoms in solid p-H2 where only at the lowest temperatures the H atom gets trapped in a weakly bound complex with a potential reactant, such that there is an increased probability of reaction. However, for all of the reactions conducted in solid p-H2 that have shown this anomalous temperature behavior, we do not measure an upturn in the reaction rate coefficient similar to what has been observed under gas-phase conditions.

Detailed kinetic studies conducted by Mutunga et al. on the H + N2O reaction41 in solid p-H2 showed that the rate coefficient does not increase at lower temperatures, but rather the reaction mechanism switches abruptly such that H atoms do not react with N2O at temperatures above 3 K. Shown in Figure 3 are kinetic plots for the H atom reaction study with formic acid in which the temperature is varied during the reaction. As seen in Figure 3, the reaction is initiated at 4.3 K by partial in situ photodissociation of formic acid at 193 nm to generate H atoms, and the HOCO that is generated during the photolysis step slowly decays after the photolysis laser is stopped while the temperature is maintained at 4.3 K. There are five open photodissociation channels75 for formic acid at 193 nm, and the radical channels produce HCO, OH, HOCO, and H atoms. However, once the temperature is reduced to ∼2.7 K, all of a sudden the HOCO concentration begins to grow due to H atom abstraction reactions with formic acid to produce HOCO and H2. Then at later stages of the experiment, the temperature is again increased and, when the temperature reaches ∼3.6 K, the kinetics again switch to HOCO decay. However, when we measure the kinetics as a function of temperature, we do not observe an increased rate coefficient at lower temperatures; the rate coefficient is essentially temperature-independent over the temperature range 1.5–4.3 K. This lack of the upturn in the rate coefficients with a decrease in temperature led us to speculate that the reaction mechanism responsible for the observed kinetics in solid p-H2 is different from what is observed in some gas-phase reactions in which a long-lived prereactive complex speeds up the kinetics, but this idea might be premature. In general, gas-phase and condensed-phase reaction kinetics are very different, such that participation of a long-lived prereaction complex in the reaction mechanism could have very different effects on the reaction kinetics under different reaction conditions. The group in Laramie is conducting detailed kinetic studies on H atom reactions with O2, CO, and NO in which these anomalous temperature effects have not been observed. Indeed, the reactions that show anomalous low-temperature reaction kinetics involve reactions with appreciable barriers and involve reactant molecules that contain functional groups such as carbonyls, alcohols, and esters. Thus, the anomalous low-temperature reaction kinetics may be related to preferred trapping sites adjacent to the coreactant that are only populated at low temperatures. What is really needed at this point is computational studies that can treat H atom quantum-diffusion in chemically doped p-H2 solids, such as the path integral molecular dynamics simulations conducted by Voth and co-workers in the early 2000s.76

Figure 3.

Figure 3

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Kinetic plots for the 193 nm photolysis (10 min, 18 mW cm–2) induced reaction study conducted on a HC(O)OH/p-H2 sample as a function of temperature. The top graph shows the temperature of sample TB, and the bottom graph shows the measured HOCO (blue circles) and HCO (red circles) concentrations. This experiment shows that for a sample photolyzed at 4.3 K, the reaction that produces HOCO after photolysis only starts the first time the temperature is lowered below ∼2.7 K. The kinetics of this reaction again qualitatively changes at later times when the temperature is raised above 3.6 K. Partially reproduced from ref (70). Copyright 2014 American Chemical Society.

Heavy-Atom Diffusion

As discussed previously, H atom diffusion in solid p-H2 has been known for over 50 years, but the diffusion of heavier species remains largely unexplored.77 However, two studies have provided preliminary information about molecular diffusion in solid p-H2. One group studied HF diffusion in solid p-H2 at ∼4 K by monitoring the temporal decay of the HF monomer absorption due to dimerization using infrared absorption spectroscopy.78 These researchers hypothesize that because the activation energy for the dimerization reaction, HF + HF → (HF)2, must be small or near zero, this reaction is diffusion-controlled in the solid phase, so that the rate coefficient k for dimerization is proportional to the diffusion coefficient D of HF in solid p-H2. They found that the decay in the HF monomer absorption peaks was well fit by second-order rate equations, which they used to extract the rate coefficient for the growth of HF clusters (dimer) as a function of time after deposition. In this study, they showed that the diffusion rates were affected by the sample temperature, the initial HF concentration, and annealing the sample. They also showed that the infrared light from their FTIR spectrometer speeds up the diffusion, presumably by excitation of rovibrational motion of HF or p-H2.

More recently, Momose and co-workers79 studied the diffusion of H2O molecules in solid p-H2 using a similar approach. By monitoring the temporal decay in H2O monomer absorption features and the growth of cluster peaks (dimer, trimer, and tetramer), they could fit their data to models of diffusion-controlled nucleation. Similar to the studies of HF diffusion,78 they found that the diffusion rate is inversely proportional to the concentration of water molecules due to a lowering of the periodicity of the solid. It is known that the efficiency of a quantum-tunneling process is strongly enhanced when the energy levels between the initial and final states of the tunneling process are nearly resonant. Kagan and Leggett80 pointed out that the rate of quantum diffusion therefore substantially decreases with a lowering of the periodicity of the solid due to poor resonance. In the water diffusion experiments,79 it is the H2O molecule itself that lowers the periodicity of the lattice, and thus, quantum diffusion is slower in more concentrated samples. By measuring how the diffusion coefficient explicitly depends on the water concentration, they found evidence that there might be correlated motion of the water molecules, a signature of quantum diffusion.79 These researchers also showed, through detailed modeling of the kinetics, that both the water monomers and dimers migrate through solid p-H2. However, the group in Laramie also point out that both Br atoms81 and Li atoms82 are thought to only diffuse in solid p-H2 at elevated temperatures of 4.3–4.4 K, which is interpreted as thermal diffusion. Adding to the confusion, the authors of the HF diffusion paper78 stated that H2O does not diffuse in solid p-H2 at 3.6 K, nor does CO, but the later experiments of Momose and co-workers79 showed that H2O does quantum-diffuse in solid p-H2. Kumada et al. reported83 that Ne atoms cannot diffuse in solid p-H2 at 4.2 K despite their size and mass being comparable to those of HF. Thus, there is considerable uncertainty in whether other heavy atoms and molecules can diffuse in solid p-H2. Is there something special about HF and H2O that permits facile quantum diffusion? Some conflicting results are likely related to the various experimental conditions used to look for quantum diffusion, where lower dopant concentrations and lower temperatures are advantageous to observe quantum diffusion.

The group in Laramie recently demonstrated that oxygen atoms can also quantum diffuse through solid p-H2 at appreciable rates.84 Experiments on the 193 nm in situ photolysis of O2 trapped in solid p-H2 were performed to measure the diffusion-controlled kinetics of the O(3P) + O2 → O3 reaction via infrared spectroscopy of the O3 reaction product. Short-term exposure of an O2-doped p-H2 solid to 193 nm radiation produces O atoms in their ground electronic state O(3P); the 193 nm photon energy is not capable of producing O(1D).85 Therefore, once the O(3P) atom becomes equilibrated at liquid helium temperatures, it cannot react with the p-H2 host even by quantum-mechanical tunneling because the O(3P) + H2 → OH + H reaction is endothermic (ΔH = +944 cm–1).86 However, during in situ photolysis the nascent O(3P) atoms are generated with a maximum translational energy of 62.6 kJ mol–1, which is greater than the calculated barrier, 55.2 kJ mol–1, for the reaction, so the possibility of reaction seems high. Yet, the light mass of the p-H2 host means that the collision energy in the center-of-mass frame between a fast-moving O atom and a stationary H2 molecule is very low, and thus, short-term 193 nm photolysis can be used to generate O atoms in solid p-H2 with little evidence of O(3P) reactions. After photolysis, continued growth in the O3 concentration was observed for up to 600 min due to the O(3P)-atom quantum diffusion. A representative trace of the kinetics of the O3 is shown in Figure 4 for an annealed O2/p-H2 sample. The O(3P) + O2 reaction is barrierless,87,88 and therefore, we expect that the reaction is diffusion-controlled so that by measuring the reaction kinetics we can estimate the diffusion coefficient for O(3P) atoms in solid p-H2. The reaction rates and O3 yields are strongly affected by the p-H2 crystal morphology, where in as-deposited solids that contain mixed crystal structures and more defects, the rate coefficient is 2× slower than in annealed solids. As expected for quantum diffusion-controlled reaction kinetics for a barrierless reaction, increasing the temperature from 1.7 to 4.0 K did not increase the rate coefficient significantly.

Figure 4.

Figure 4

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Ozone growth kinetics for an annealed O2/p-H2 sample recorded at 1.71(1) K after a 0.5 min 193 nm photolysis exposure. The O3 concentration is plotted as black circles, and the red line is the result of a least-squares fit of the data to a first-order kinetics equation. Partially reproduced from ref (74). Copyright 2023 American Chemical Society.

The surprising result from the O(3P)-atom reaction studies was that the average O(3P)-atom diffusion coefficient is comparable to the best literature values for H atom quantum diffusion in solid p-H2.84 Specifically, the diffusion coefficient extracted from the O(3P) atom reaction at 1.7 K for annealed p-H2 solids is D = 7.6(2) × 10–17 cm2 s–1 and the diffusion coefficient for H atom quantum diffusion is D = 2.7(3) × 10–16 and D = 1.7(2) × 10–16 for n-H2 and p-H2, respectively, at 4.2 K.84 The previous measurements of the diffusion coefficient for H atoms in solid n-H2 and p-H2, respectively, were based on measuring the reaction kinetics of H atom recombination (H + H → H2). Measuring the diffusion coefficient indirectly in this way may introduce systematic errors due to the nature of quantum diffusion. When the distance between the recombining H atoms is small, the major potential inhomogeneity for quantum tunneling is associated with the interaction of the pairing particles.83 This can cause the final stages of recombination to be very slow or not to occur at all because the tunneling bandwidth is much smaller than the energy level mismatch caused by the interacting particles. This possibility is pointed out in the paper that measured the H atom diffusion coefficient where they observed an “absence” of recombination in heavily enriched p-H2 solids.89 However, in more recent measurements90 on H2 thin films at temperatures below 1 K, the H atom recombination kinetics can be distinguished from spatial diffusion driven by a concentration gradient, allowing the researchers to test for this systematic bias. These researchers showed that for both n-H2 and p-H2 thin films, the H atom diffusion coefficient measured for pure spatial diffusion is 1 to 2 orders of magnitude larger than the diffusion coefficient extracted from H atom recombination.90 This is consistent with our picture of quantum diffusion, where the diffusion process is the fastest when the H atom propagates through a periodic matrix potential via resonant tunneling. Therefore, more diffusion studies in solid p-H2 involving other atoms and molecules are needed to better characterize the conditions that lead to fast quantum diffusion in solid p-H2.

Electronic Transitions

Investigations of the electronic transitions in p-H2 are scarce. The best example to show the advantages associated with the unique properties of p-H2, the softness of the matrix, is the Rydberg transitions of NO.91 The electronic transition from the ground X2Π state to the Rydberg state A2Σ+ is known to produce a bubble-like electronic cloud. For noble-gas matrix hosts, the rigidity of the solid limits the expansion of the electronic bubble upon excitation, hence inducing a large blue shift for this transition. In the gaseous phase, the NO (A–X) transition origin was reported to be 44080.5 cm–1,92 whereas those in solid Ne and Ar were reported to be 45536 and 46377 cm–1,93,94 respectively, with blue matrix shifts greater than 1450 cm–1. In contrast, the NO (A–X) transition origin in p-H2 was observed at 44105 ± 20 cm–1, with a blue shift of only ∼25 cm–1.

On the basis of the electronic spectroscopy of NO in solid p-H2, one would expect small and less-divergent matrix shifts for electronic transitions of species of various types in solid p-H2. If this would be true, because producing protonated, cationic, and hydrogenated species in p-H2 is much easier than producing these species in the gaseous phase or in noble-gas matrices, as discussed previously, one would be able to identify possible carriers of diffuse interstellar bands (DIBs) by comparison of electronic spectra of potential molecules trapped in solid p-H2 with observed DIBs, taking the expected matrix shift and uncertainties into consideration as a preliminary test. However, so far, only fluorescence excitation spectra and dispersed fluorescence of 1-hydronaphthyl radical (1-C10H9),95 sumanene (C21H12),96 and peri-hexabenzo-coronene (C42H18)97 have been reported; a lot of data need to be collected before any conclusion can be drawn.

Another advantage of investigating electronic transitions with the matrix-isolation technique is that the transition origin can be unambiguously identified because vibrational relaxation is rapid in matrices, and typically emission comes only from the vibrational ground state and often from the lowest electronic state of the same spin multiplicity. In the case of ovalene, a literature report indicated a transition origin of S1–S0 at 466.22 nm (21449 cm–1) in the gaseous phase,98 but experiments in solid p-H2 indicated that the band was misassigned by one vibrational quantum; the origin should be ∼473.1 nm (21135 cm–1) in the gaseous phase and was observed at 475.1 nm (21050 cm–1) in p-H2 with a lifetime of 1.5 μs.99 Furthermore, according to the spectral pattern of electronic emission (corresponding to dispersed fluorescence) and absorption (corresponding to fluorescence excitation) predicted with the Franck–Condon Herzberg–Teller approach, the reported transition should be the S2(B3u)–S0(Ag) rather than the S1(B2u)–S0(Ag) transition. This example also demonstrates the advantage of using p-H2 for the correct spectral assignments.

The group in Taiwan has recorded the dispersed fluorescence and fluorescence excitation spectra of several PAH and a few hydrogenated and protonated PAH, of which the gaseous transition origins have been reported. Figure 5 shows the matrix shifts (including unpublished results) of several PAH and their derivatives as a function of mass m; solid symbols represent values for solid p-H2 and open symbols represent literature values in the Ne matrix. Among them, circles indicate PAH (black for planar PAH and blue for nonplanar PAH in solid p-H2), squares indicate hydrogenated PAH, and triangles indicate protonated PAH. The average matrix shift, νgas – νmatrix, of planar PAH in solid p-H2 is 93 ± 9 cm–1, whereas that for all species in solid p-H2 is 70 ± 28 cm–1; listed errors represent one standard deviation in fitting. As compared with solid Ne, even though the matrix shifts of a majority of species in p-H2 are greater than those in Ne, they show shifts only to the red and with small variations.

Figure 5.

Figure 5

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Matrix shifts for species in solid p-H2 and Ne as a function of mass m. Solid symbols: species in solid p-H2; open symbols: species in solid Ne. Symbols: circles: PAH (black for planar PAH and blue for nonplanar PAH in solid p-H2); square: hydrogenated PAH; triangles: protonated PAH. The average matrix shift of planar PAH in p-H2 (black solid circles) is 93 ± 9 cm–1 and that of all species in p-H2 (solid symbols) is 70 ± 28 cm–1, as indicated with dashed lines and gray regions representing listed uncertainties as one standard deviation in fitting.

The limited data showing that the matrix shifts for electronic transitions of PAH and their derivatives in p-H2 are always red-shifted and with small variations look promising. If the average value can be reliably applied, then the wavenumbers of absorption bands observed in solid p-H2 can be corrected to yield the estimated gas-phase values with uncertainties <60 cm–1 at the 95% confidence level; for a band near 600 nm, this uncertainty corresponds to ±2.2 nm. The additional advantage is that many protonated and hydrogenated PAH can be readily produced only in solid p-H2, not in noble-gas matrices.

Summary and Future Perspectives

In summary, several advantages of using p-H2 as a host for matrix isolation as compared with traditional noble-gas hosts have been demonstrated. These include the production of free radicals or unstable species via in situ photoirradiation; the production of protonated and hydrogenated species by using electron bombardment; efficient low-temperature H atom reactions in complete darkness via quantum diffusion of the H atoms; the anomalous temperature effects in reactions; the diffusion of heavy atoms or molecules; and the less divergent, consistently red matrix shifts for most electronic transitions.

Photoirradiation of guest species isolated in solid p-H2, either via direct photodissociation or using photoinduced bimolecular reactions, is an efficient and convenient way to produce free radicals for spectral characterization; in most cases, this method is not possible in noble-gas matrices due to the cage effect. The infrared and electronic spectra of many radicals await to be explored using p-H2 matrix-isolation spectroscopy. Furthermore, with clever design, more complicated experimental procedures such as irradiation–reaction–irradiation may be applied to generate radicals that are challenging to produce with irradiation in a single step. The “soft” environment of the p-H2 lattice is also conducive to the study of the relative motion in molecular complexes such as the bevel-gear-type motion in C6H6Br; future investigations on this type of motion and their associated chemistry such as stereoselectivity, chirality, or photochemistry become possible.

The production of protonated and monohydrogenated species using electron bombardment of a p-H2 matrix is straightforward and unique. The band positions in the IR spectra of protonated coronene and protonated corannulene seem to be approaching those in the interstellar UIR bands, but even larger protonated PAH awaits exploration to have improved matches. Unfortunately, the availability of these large amounts of PAH is rather limited. Perhaps a top-down method using graphene or C60 might give some hints on this puzzle. Furthermore, a comparison of the UV-induced IR emission with IR absorption spectra is necessary to ensure that assigning UIR emission bands according to IR absorption is practical. Even though monohydrogenated PAH appear to be unrelated to the UIR emission, they may play some roles in the interstellar catalytic H2 formation. The photochemistry and additional hydrogen reactions of PAH are required for further investigations.

The method for inducing H atom reactions in darkness using Cl2 as the initiator is very efficient and distinct from the H atom bombardment experiments in which the matrix or ice sample is bombarded with large amounts of H atoms. In the latter experiments, the large number of H atoms might induce secondary reactions such that reaction intermediates are typically difficult to identify using this approach. In p-H2 matrix experiments without input of energy (in darkness), the temporal evolution of each species (reactants and products) is followed in real time and hence provides valuable information to deduce the reaction mechanism. Although the p-H2 environment is obviously different from interstellar ices, investigations of H atom reactions in solid p-H2 provide a fundamental understanding of the possible low-temperature reaction mechanisms that might lead to new synthetic routes. As p-H2 matrix-isolation research has demonstrated, H atom abstraction and H-atom-induced fragmentation likely play more important roles in astrochemical environments than previously thought. Furthermore, by coupling H atom abstraction and addition reactions, two interstellar species might be chemically linked and form a quasi-equilibrium; transformation to an isomer with a higher energy is also possible. So far, H atom reactions of only a limited number of amides and small amino acids have been investigated. Investigations on the reactions of H atoms with larger amino acids, nucleobases, and other biologically related species might provide new insights into their reactions. The difference in the branching among various channels of H atom reactions between periods with near-IR irradiation and that in darkness, as was observed in the H + glycolaldehyde system, deserves further experimental and theoretical investigations.

The method of using UV/IR irradiation of a p-H2 matrix containing trace Cl2 to perform H atom reactions has one major disadvantage; the available H atoms for reactions in complete darkness are limited to the remnant H atoms that remain unreacted after near-IR irradiation, which places a constraint on the extent of H atom reactions that can proceed in darkness. Recently, a new method employing UV photolysis of a p-H2 matrix containing trace H2O2 was developed; the OH thus produced upon photolysis can react with H2 via tunneling to produce H2O and H atoms without the need for near-IR irradiation. This method allows for continuous generation of H atoms in darkness after UV photolysis of H2O2. Preliminary experiments on a UV-irradiated H2O2/CH3NH2/p-H2 matrix indicated that the H atom reactions can proceed further in this system, so that a significant amount of CH2NH and HCN was produced, as compared with those from the UV/IR-irradiated Cl2/CH3NH2/p-H2 matrix, indicating that abstraction of more H atoms in CH3NH2 is possible using this method. The disadvantages of this method are that a shorter wavelength is needed to dissociate H2O2, the byproduct H2O might interfere with the measured chemistry, and OH might compete with H atoms for reactions. These two methods are, hence, complementary to each other and can be used to study the same reaction. In view of the limitations and possible interference in these two methods, further new methods with high efficiency and less interference are waiting to be developed.

The anomalous temperature dependence observed for several H atom abstraction reactions conducted in solid p-H2 points to the complexity of the low-temperature solid-state reaction kinetics. Using solid p-H2 matrix isolation to study low-temperature chemistry allows the potential energy landscapes and reaction mechanisms to be investigated at unprecedented levels of detail. It is the mobility of the H atom in solid p-H2 that makes this all possible, and thus, the details of H atom quantum diffusion in solid p-H2 should continue to be studied to support our conviction that solid p-H2 can be used as a model system to probe the reactions occurring in and on interstellar ices. As discussed in this Perspective, the greater reactivity at lower temperatures is believed to be related to the pathway of H atom quantum diffusion through the solid and not that the reaction rate coefficient is increased at lower temperatures. The facile quantum diffusion of H atoms through solid p-H2 means that low-temperature bimolecular reactions can be studied in the laboratory on multiple-hour time scales, and the analogous studies in interstellar ice analogues or noble gas matrices is simply not possible because the H atom diffusion rates are much slower and thermally activated. While mass transport in solid p-H2 is very different from that in any other cryogenic matrix host, the chemical reactivity should be comparable. This is why the studies of the O atom reactions are also exciting because O atom chemistry can also be explored using p-H2 matrix isolation if efficient ways of O atom generation are developed.

Even though the present data support a consistently red and less-divergent matrix shift for electronic transitions in solid p-H2, many more data need to be collected before one can make such a conclusion. The problem that one faces is that limited data are available for PAH and PAH-derivatives in the gaseous phase or trapped in solid Ne for comparison, because investigations on the electronic transitions of these species are challenging. In contrast, producing these PAH-derivatives using p-H2 is less challenging, so their electronic transitions may be investigated without much difficulty. Finding a promising candidate of DIB would provide a big boost for the investigation of electronic transitions in solid p-H2. For proper assignments of the observed bands, quantum-chemically predicted spectra based on Franck–Condon Hertzberg–Teller or even higher-level calculations are indispensable. However, for some “difficult” electronically excited states, accurate predictions using the standard TD-DFT method are challenging. More sophisticated methods are needed to tackle these “difficult” states in order to support the assignments of observed bands.

Acknowledgments

Work in the Lee group was supported by National Science and Technology Council, Taiwan (grants MOST112-2639-M-A49-001-ASP and MOST112-2634-F-009-026) and Center for Emergent Functional Matter Science of National Yang Ming Chiao Tung University from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan. We thank the National Center for High-performance Computing (NCHC) of National Applied Research Laboratories (NARLabs) in Taiwan for providing computational and storage resources. Work in the Anderson group was sponsored, in part, by the Chemistry Division of the U.S. National Science Foundation (grant no. CHE-2101719).

Biographies

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Isabelle Weber received her doctorate in chemistry from the Karlsruhe Institute of Technology (KIT), Germany, in 2017 for her work on the gas-phase kinetics of hydrocarbons under combustion-relevant conditions. After a postdoc in atmospheric chemistry at the University of Lille, France, she first joined the Y.-P. Lee laboratory at National Yang Ming Chiao Tung University (NYCU), Taiwan, as a postdoc in 2020 to study electronic spectra of PAH isolated in solid para-H2 and characterize the influence of solid para-H2 on electronic transitions of isolated guest molecules. She is now an assistant research fellow at NYCU.

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Prasad Ramesh Joshi is an assistant research fellow in Y.-P. Lee’s Laboratory in the National Yang Ming Chiao Tung University (NYCU), Taiwan. He has received a BSc in Chemistry and MSc in Chemistry from the University of Pune, India, and PhD in Analytical and Physical Chemistry from the Sorbonne University (formerly University of Pierre and Marie Curie), Paris, France. His research interests focus on investigating free radicals that are important in astrochemistry using solid para-hydrogen.

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David T. Anderson is a full professor in the Department of Chemistry at the University of Wyoming. He received his BS degree from George Washington University in 1989 and his PhD degree in 1993 from Dartmouth College working with John S. Winn. He then went on to an NRC postdoctoral position at JILA, University of Colorado where he worked with David J. Nesbitt, and then to the University of Pennsylvania in 1996 for a postdoctoral position with Marsha I. Lester’s group. His research at UW focuses on matrix-isolation spectroscopy which is a technique for maintaining molecules in a cryogenic solid at very low temperatures for spectroscopic study. Traditionally, noble gases (Ne, Ar, Kr, and Xe) are used as the matrix host for preserving reactive species in an inert solid environment, but the Anderson group takes a new twist on this venerable technique and uses solid molecular hydrogen (H2) as the matrix host.

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Yuan-Pern Lee received his BS degree in 1973 from National Taiwan University and his PhD degree in 1979 from U. C. Berkeley working with George C. Pimentel. He conducted postdoctoral research at NOAA, Boulder, Colorado, where he worked with Carleton J. Howard. He returned to Taiwan in 1981 to teach at the National Tsing Hua University and became a chair professor in Applied Chemistry, National (Yang Ming) Chiao Tung University in 2004. He is now a contract professor after his retirement in 2022. He became a Fellow of the American Physical Society in 1999 and the Academician of Academia Sinica in 2008. He received the Pimentel Prize for Advances in Matrix Isolation (2018) and the Presidential Science Prize in Taiwan (2019). His research focuses on spectroscopy, kinetics, and dynamics of free radicals or unstable species that are important in astrochemistry, atmospheric chemistry, or combustion chemistry. The techniques employed include step-scan FTIR, matrix isolation (p-H2), quantum-cascade laser, cavity ringdown, IR-VUV photoionization/time-of-flight mass detection, laser-induced fluorescence, and shock tube.

The authors declare no competing financial interest.

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