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. 2018 Sep 6;3(9):10743–10747. doi: 10.1021/acsomega.8b01840

Quantum Tunneling Tautomer of N,N-Dimethyl-p-toluidine Dehydrogenates Identified by Deep-UV Laser Ionization Mass Spectroscopy

Haiming Wu †,‡, Chengqian Yuan †,‡, Chenghui Zeng †, Zhixun Luo †,‡,*
PMCID: PMC6645372  PMID: 31459191

Abstract

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Utilizing customized deep-ultraviolet laser ionization mass spectroscopy, here we report a finding of remarkable dehydrogenation product of N,N-dimethyl-p-toluidine (DMT). The DMT dehydrogenates find comparable mass abundance with the DMT molecule ions showing decent stability at the loss of one electron and one H atom from the DMT molecule. First-principles calculation reveals that the dehydrogenation most readily occurs at the N-connected methyl group. Furthermore, at the removal of a hydrogen atom, a neighboring hydrogen atom on the other methyl come close and interact with the dehydrogenated methylene group, pertaining to C–H···C weak interactions which give rises to a resonant structure (C···H–C) on a basis of hydrogen atom quantum tunneling effect. The quantum tunneling tautomer of DMT dehydrogenates displays reversible donor–acceptor charge-transfer interactions as demonstrated by natural bonding orbital analysis and vibrational spectroscopy. It is worth noting that the novel dehydrogenation product was also observed for another small organic molecule o-phenylenediamine, which bears two neighboring amino groups and the subsequent dehydrogenation product pertains to resonant structures of N–H···N and N···H–N. The deep ultraviolet laser not only produces fragmentation-free mass spectra for such small organic molecules but also tailors the interesting quantum tunneling tautomer from such specific molecules.

Introduction

Dehydrogenation, as the name implies, is a chemical reaction involving the removal of atomic or molecular hydrogen.1−7 The dehydrogenation from C–H activation is recognized of significance enabling to convert inert and low-valued molecules such as alkanes to reactive and valuable products such as olefins.8,9 However, such processes are often endothermic and thus need high temperature conditions and/or well-designed catalysts.10,11 This is because the C–H bond (1.09 Å, 413 kJ/mol) is a covalent bond, that is, carbon shares its outer valence electrons with hydrogens, leading to both-filled outer shells hence reasonable stability; also, the electronegativity difference between C (2.55) and H (2.2) atoms is 0.35 according to Pauling’s scale, small enough to be regarded as being nonpolar.12−14 Therefore, the C–H bond is relatively very strong and generally unreactive, allowing only participation in radical substitution or utilizing well-designed catalysts. For example, catalytic dehydrogenation of hydrocarbons was found a representative process to produce styrene which is the monomer for synthetic polymers;15−18 oxidative dehydrogenation of hydrocarbons also received reasonable research interest by using various oxides as efficient catalysts.19−25

Although the C–H bond is strong, its bond strength could vary over 30% in magnitude even for fairly stable organic compounds, even in the absence of heteroatoms, allowing selectivity of C–H bond activation.26 The use of light to control selective chemical reactions is highly attractive and has been recognized of importance from early on. Recently, astronomers reported that the very basic chemical ingredients of life, hydrocarbon molecules (CH, or methylidyne radical) and positive ion (CH+) and carbon ion (C+), are largely resulted from interactions of ultraviolet light radiation from stars, rather than other ways as thought earlier, like turbulent events related to supernovae and young stars.27 It is worth noting that photochemical synthesis has also attracted significant attention owing to a global drive toward renewable, clean, and sustainable energy technologies.28,29

N,N-Dimethyl-p-toluidine (DMT, C9NH13) known as a component in dental materials is widely used in medical devices with annual production estimates of 1 to 10 million pounds (U.S. Environmental Protection Program 2011).30 Also, it was used in the preparation of acrylic bone cements 50 years ago,31−33 but in recent years it was recognized a lack of information regarding its toxicity and carcinogenicity available in the literature.30 In view of this, precise and efficient identification of such chemicals and their photo-induced fragments is of significance. Utilizing customized all-solid-state deep ultraviolet (DUV) laser, recently we developed a reflection time-of flight mass spectrometer (Re-TOF-MS),34 which bears advantages for precise low-fragment mass spectrometric analysis. On the basis of this instrument, here we report an interesting finding of remarkable dehydrogenation product of DMT. The ionization potential of this molecule is right at the single-photon energy of the DUV laser pertaining to the high-efficient single-photon ionization (SPI) process. The DMT dehydrogenates find comparable mass abundance with the DMT molecule ions, showing decent stability of both and dehydrogenation-dominated photo-induced dissociation of the DMT molecules. Along with first-principles calculations, we rationalize the stability of dehydrogenation at the N-connected methyl group, where a neighboring hydrogen atom on the other methyl could come close and form H-bond weak interactions with the dehydrogenated methylene group, pertaining to C–H···C and C···H–C resonant structures, hence giving rises to quantum tunneling tautomer.

Results and Discussion

Figure 1A presents six mass spectra of DMT ionized by the 177.3 nm DUV laser, sampled by a bubbling source combined with a pulsed valve and He buffer gas. DMT is light yellow liquid in normal pressure at room temperature, and it has an ionization potential of 6.93 ± 0.02 eV.35 Considering that the single-photon energy of the DUV laser is 7 eV, it is inferred that the DUV-LIMS of DMT aims at high-efficient SPI free of fragmentation. However, there are two brother peaks dominating the mass spectra at the varied laser power intensity, one refers to its molecule ion (C9NH13+•) and the other pertains to a dehydrogenation product (C9NH12+). This is in sharp contrast to the other samples, which also bear ionization potentials close to 7 eV, such as para-phenylenediamine (PPD) and benzophenone (BP), where solely dominant molecule ion peaks are addressed in Figure 1B(a,d). Very clean mass spectra are observed, showing unique advantages of the SPI mass spectrometry for chemical identification. Nevertheless, there is a similar case: ortho-phenylenediamine (OPD) also takes on remarkable dehydrogenation product (C6N2H7+) at the DUV-LIMS measurement (Figure 1B(c)).

Figure 1.

Figure 1

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(A) DUV-LIMS of DMT at varied laser power intensity. (a–f) Refer to 30, 40, 50, 60, 70, and 80% of the full energy at about 10 μJ. (B) Comparison of the DUV-LIMS of PPD (a), DMT (b), OPD (c), and BP (d) at ∼5 μJ energy power, respectively.

To unravel the unusual dehydrogenation fragment (C9NH12+) ionized by DUV laser, simply we have checked the structures of the four molecules. As is shown in the insets of Figure 1B, DMT have two neighboring −CH3 groups and OPD also have two nearby −NH2 groups. In comparison, the other two molecules PPD and BP do not have such groups close to each other. It is worth noting the distinct difference of DUV-LIMS spectra of the two isomers, PPD and OPD, which only bear different position of the two amino groups. Does the positions of −CH3 and −NH2 groups account for selective dehydrogenation products for these small organic molecules?

To verify this, we have first checked the strengths of all the C–H bonds in DMT after ionization and found that the C–H bonds in −CH3 groups are very close to that of the C–H on the skeleton carbon ring. However, when we scan the potential energies for these C–H bonds dissociation (or, equivalently, H elimination) for the ionic DMT by first-principles calculations, as shown in Figure 2, we find that the dehydrogenation with H atom elimination from the methyl group on N atom (red line, i.e., no. 2 H atom) displays the least activation energy (4.15 eV), which is 1.1 eV less than that for the removal of no. 5 H atom. The photo-induced ionization and C–H bond dissociation processes can be summarized as

graphic file with name ao-2018-018407_m001.jpg 1
graphic file with name ao-2018-018407_m002.jpg 2

Figure 2.

Figure 2

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Varying potential energy profiles associated with the different H atom removal off DMT calculated at the b3lyp/6-311++g(d,p) level of theory. Inset is the structure of DMT, where the atoms in blue, dark, and gray refer to N, C, and H, respectively.

Furthermore, dynamics calculation for the most stable dehydrogenization product found that the dehydrogenization of DMT from a −CH3 group gives rise to resonating structures (tunneling tautomer), as shown in Figure 3, pertaining to intramolecular H atom tunneling effect through a 3.11 eV energy barrier of a transition state with a H-location right at the center of the other two NH2 groups. The tunneling tautomer under the DUV laser excitation creates a dynamic condition profiting to the stability of dehydrogenization product. This is well consistent with the previous finding on malonaldehyde, which is known as a typical system for the study of intramolecular H atom transfer.36,37 Also, the intramolecular quantum tunneling effect of DMT dehydrogenization product is applicable to OPD dehydrogenization product.

Figure 3.

Figure 3

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Potential energy profile along the isomerization path of DMT at the b3lyp/6-311++g(d,p) level of theory. Insets are the electrostatic potentials on the surface of cationic tautomer and transition state of dehydrogenation products. Red and blue colors indicate negative and positive regions, respectively, with the maxima and minima corresponding to 0.11 and 0.19 a.u. The atoms in blue, dark, and gray refer to N, C, and H, respectively.

Furthermore, to reveal the role of interactions between transferred H atom and its adjacent atoms in determining the photo-induced quantum tunneling effect of DMT, utilizing natural bonding orbital (NBO) analysis we have depicted the donor–acceptor overlaps of the cationic tunneling tautomer and the transition state for the dehydrogenation products, as shown in Figure 4. Simply considering charge-transfer interactions between BDC–H and BDN–C* first, the first-principles calculations show that the dominant interactions of two tautomers are donor–acceptor overlaps between the bonding orbitals on the C–H bond (BDC–H) and the antibonding orbitals on the N–C bond (BDN–C), read as BDC–H → BDN–C* (3.71 kcal/mol, Figure 4a,d), which is weaker than in the transition state (7.61 kcal/mol, Figure 4c). Also, the transition state displays large donor–acceptor charge-transfer interactions around the dominant BDC–H → BDC–N (67.26 kcal/mol, Figure 4b). Besides, the transferred H atom find the same charge-transfer interactions with C–H bonds in the methyl group, BDC–H → BDC–H* (18.31, 4.89, 3.88 kcal/mol, Figure 4e–g). The large charge-transfer interactions provide noncovalent interactions between the H atom and adjacent C atom on methyl group allowing the formation of the quantum tunneling tautomer, with decent stability and thus comparable mass abundance with the parent ion peak.

Figure 4.

Figure 4

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NBO donor–acceptor overlaps of the cationic (a,d) tunneling tautomer and (b,c,e–g) transition state of tautomers. BD and BD* refer to bonding orbitals and antibonding orbitals. Donor and acceptor orbitals are plotted in purple/yellow and blue/green, respectively. The atoms in blue, dark, and gray refer to N, C, and H, respectively.

Conclusions

We report a finding of remarkable dehydrogenation product of DMT utilizing customized deep-ultraviolet laser ionization mass spectroscopy (DUV-LIMS). The DMT dehydrogenates find comparable mass abundance with the molecule ions because of the structure stability and the hydrogen prefer to be dissociated at the −CH3 groups. At the removal of a hydrogen atom, a neighboring hydrogen atom on the other methyl come closer and interact with the dehydrogenated methylene group, giving rises to resonant structures pertaining to C–H···C weak interactions and hydrogen atom quantum tunneling effect. The quantum tunneling tautomer of DMT dehydrogenates displays reversible donor–acceptor charge-transfer interactions as demonstrated by NBO analysis. In addition to the dehydrogenation at the N-connected methyl group, we also found that this principle is applicable to OPD, where the neighboring −NH2 groups facilitate the subsequent dehydrogenation product pertaining to N–H···N interactions and resonant structures. Thanks to the deep ultraviolet laser which not only produces fragmentation-free mass spectra for such small organic molecules but also tailors the interesting quantum tunneling tautomer from such specific molecules.

Experimental and Computational Methods

Experimental Methods

The experiments were performed on a customized Re-TOF-MS34 (Figure S1, Supporting Information) based on the all-solid-state DUV laser system (177.3 nm wavelength, 15.5 ps pulse duration, ∼15 μJ pulse energy, 10 Hz repeating rate) by frequency-doubling of 355 nm laser through a KBBF–CaF2 prism-coupled device.38 The powder samples of BP (99+ % purity, Acros), PPD, and OPD (99+ % purity, Acros Organics) were put in a customized thermal evaporation source and heated to a certain proper temperature that could produce continuous vapor to form a stable molecular beam. DMT (99% purity, Alfa Aesar) is a liquid at room temperature, and hence, a bubbling source is used along with a pulsed valve and He buffer gas as a representative sampling method to attain supersonic expansion molecular beam.

Calculation Methods

All the optimization, frequency, and energy calculations were performed on a basis of density functional theory (DFT) embedded within the Gaussian 09 program package.39 Geometries of all species were fully optimized at the unrestricted b3lyp/6-311++g(d,p) level of theory.40,41 All the energies were corrected with zero-point-vibrations. All transition states structures were checked and confirmed by intrinsic reaction coordinate42 calculations. To estimate the amount of charge of quantum tunneling tautomer of DMT dehydrogenates, NBO43 analysis of neutral phenylenediamine isomers was also performed. The NBO orbitals were plotted via VMD and Multiwfn software.44

Acknowledgments

We thank Prof. Wensheng Bian for his friendly discussion. This work is financially supported by the “National Project Development of Advanced Scientific Instruments Based on Deep Ultraviolet Laser Source” (no. Y31M0112C1), the Key Research Program of Frontier Sciences (CAS, Grant QYZDBSSW-SLH024), and the National Natural Science Foundation of China (grant no. 21722308). Z.L. acknowledges the National Thousand Youth Talents Program.

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b01840.

  • Additional details of experimental methods, 355 nm-LIMS, DFT-calculated details (PDF)

Author Contributions

The manuscript was written through contributions of all authors.

The authors declare no competing financial interest.

Supplementary Material

ao8b01840_si_001.pdf (1,023.6KB, pdf)

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Supplementary Materials

ao8b01840_si_001.pdf (1,023.6KB, pdf)

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