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. 2022 Apr 4;144(14):6321–6325. doi: 10.1021/jacs.1c13336

Why the Photochemical Reaction of Phenol Becomes Ultrafast at the Air–Water Interface: The Effect of Surface Hydration

Tatsuya Ishiyama †, Tahei Tahara ‡,, Akihiro Morita §,∥,*
PMCID: PMC9012180  PMID: 35377635

Abstract

graphic file with name ja1c13336_0005.jpg

Photochemical reactions at the air–water interface can show remarkably different rates from those in bulk water. The present study elucidates the reaction mechanism of phenol characteristic at the air–water interface by the combination of molecular dynamics simulation and quantum chemical calculations of the excited states. We found that incomplete hydrogen bonding to phenol at the air–water interface affects excited states associated with the conical intersection and significantly reduces the reaction barrier, resulting in the distinctively facilitated rate in comparison with the bulk phase. The present study indicates that the reaction dynamics can be substantially different at the interfaces in general, reflecting the difference in the stabilization energy of the electronic states in markedly different solvation at the interface.

Introduction

Solute species located at water surfaces are affected by the solvation environment different from those in bulk water, and consequently, they can manifest different reactivity when the solvation plays an important role.1 The different reactivity at water surfaces has drawn particular attention in the field of heterogeneous atmospheric chemistry or organic synthesis to date.2−7 However, it is challenging to extract detailed microscopic information on the reactions at the water surfaces when the reaction takes place at the water surface and in the bulk simultaneously. Recently, new experimental evidence of different reactivity at the air–water interface was reported at ultrafast time resolution. The photochemical reaction of phenol was revealed to be much faster at the air–water interface than that in bulk water by several orders of magnitude.8 This is a remarkable example to demonstrate the distinctive reactivity of molecules at the air–water interface. Stimulated by this finding, the present paper elucidates the microscopic mechanism of the photochemical reaction characteristic at the air–water interface.

Phenol is known to undergo hydrogen detachment after photoexcitation, which has been intensively studied as a typical photochemical reaction.9−17 The reaction is known to be an activation process from the potential energy surfaces of a phenol molecule.11,14,18,19 After phenol absorbs a photon and is excited to the S1 state, it passes a conical intersection (CI) between the S1 and S2 states to go to the exit channel of detachment.10,20,21 The energy barrier that determines the reaction dynamics is directly associated with the location of the CI, which may be sensitive to the solvation environment.22−24 Accordingly, the present work investigates the effect of hydration on the CI at the air–water interface in contrast to bulk water. We think that the present approach offers direct microscopic information on the solvation effect on the reaction mechanism, though the computation requires the statistical sampling of solvating water molecules and the CI search of phenol surrounded by them.

Computation

The ab initio calculations of the excited states and CI were performed by 10-in-10 complete active space self-consistent field (CASSCF). The active space consists of seven π orbitals and three σ orbitals associated with the O–H bond and Rydberg orbital. The calculations were conducted based upon the state-averaged (SA) reference wave function for the three states S0, S1, and S2 with equal weights. We mainly deal with the effect of hydration by SA-CASSCF, which can reasonably describe the perturbation of hydration on these states. Although further quantitative improvement could be expected by incorporating the dynamic correlation, it is prohibitively demanding to investigate the CI in the hydrated cluster systems at that level.

The basis functions of 6-311++G(2d,p) and 6-21G were employed for phenol and water, respectively. We note that the diffuse functions of phenol are essential for an accurate prediction of the excited states, as discussed in the Supporting Information. All the ab initio calculations were carried out with the MOLPRO program (version 2020.1),25 and the details of ab initio calculations are described in the Supporting Information. The energy barrier was evaluated with the difference in energies between the S1 and CI states at their optimized geometries (denoted by S1(OPT) and CI(OPT) hereafter), i.e., ΔE = E(CI(OPT)) – E(S1(OPT)).

Results and Discussion

Isolated Phenol

Before treating the effects of solvation, we discuss the excited states of an isolated phenol molecule. The energies of the S1 state at the S1(OPT) and CI(OPT) structures are E(S1(OPT)) = −305.564 382 69 a.u. and E(CI(OPT)) = −305.554 528 62 a.u., respectively. Consequently, the barrier of ΔE = 6.19 kcal/mol was obtained for the isolated phenol molecule.

The structures of S1(OPT) and CI(OPT) are compared in Table 1. The CI structure is characterized with a shortened 3–2 (C–O) bond and 5–4 and 8–7 (C–C) bonds, which are preferable for the resonance electronic structure of quinone-like diradical C6H5O• + H• (see Figure 1(b)) because the CI structure should stabilize the S2 state of πσ* character. We also notice that ΔR21 = 0.050 Å in Table 1 indicates the slightly elongated O–H bond of phenol at the CI(OPT) compared to that at S1(OPT). Meanwhile, if we plot the potential energy surface (PES) as a function of OH distance (R21) with the other degrees of freedom fixed, the crossing of S1 and S2 states takes place at ΔR21 = 0.2 Å with a barrier height ΔE = 21.2 kcal/mol. This result is consistent with a previous PES scan of phenol along the OH distance, yielding a barrier of about 1 eV (∼23 kcal/mol).13,26 The present calculation indicates that the main reaction path from S1(OPT) to CI(OPT) is along the ring deformation rather than the elongation of the OH bond.

Table 1. Optimized Geometries of S1 and CI of an Isolated Phenol Moleculea.

    isolated phenol
 (surface (II)/bulk (III))
i j Rij(S1) Rij(CI) ΔRij ΔRij
2 1 0.962 1.012 0.050 (0.034/0.020)
3 2 1.349 1.281 –0.068 (−0.073/–0.086)
4 3 1.427 1.435 0.008 (0.025/0.013)
5 4 1.428 1.386 –0.042 (−0.058/–0.089)
6 5 1.430 1.417 –0.013 (−0.014/–0.012)
7 6 1.428 1.417 –0.011 (−0.011/–0.022)
8 7 1.430 1.385 –0.045 (−0.058/–0.076)
9 4 1.073 1.069 –0.004 (0.000/0.000)
10 5 1.072 1.072 0.001 (0.003/0.000)
11 6 1.073 1.073 –0.000 (−0.001/–0.002)
12 7 1.071 1.072 0.001 (0.002/0.001)
13 8 1.071 1.071 0.000 (−0.001/0.001)
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a

Rij(S1) and Rij(CI) denote the i–j bond lengths at S1(OPT) and CI(OPT) structures, respectively. Unit: Å. The numbering of atoms is shown in Figure 1(a), and ΔRij = Rij(CI) – Rij(S1). The last column with parentheses shows the average values of ΔRij in the solvated environments (surface(II)/bulk (III)) for comparison.

Figure 1.

Figure 1

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(a) Serial numbers of atoms and (b) canonical formula of the diradical structure of phenol. (c) Typical snapshots of solvated phenol clusters in the three regions.

The CI(OPT) structure is planar, and the S0, S1, and S2 states are of A′, A′, and A″ symmetry, respectively. The dipole moments of these states at the CI(OPT) structure are 1.63, 1.69, and 11.7 D, respectively. The S2 state has a remarkably large dipole moment due to its πσ* character with intramolecular charge transfer. Figure 2 illustrates the partial atomic charge of the H(1) site at these states. It is worth noting that the H(1) site of phenol OH has a negative charge at S2 (−0.145 at CI(OPT) of isolated phenol), which is indicative of the πσ* character of S2. The full list of atomic partial charges is given in the Supporting Information.

Figure 2.

Figure 2

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Natural bond orbital (NBO) charges of H at S0, S1, and S2 states of isolated and solvated phenol at S1(OPT) and CI(OPT) structures. The results in the solvated environment were obtained by the average of 10 clusters in “bulk (III)”.

Calculation of Hydration Effects

Next, we discuss the effect of hydration on the barrier. The microscopic interface structure of the phenol/water solution was investigated by our previous MD study,27 where a slab of a solution containing 600 water and 25 phenol molecules in an MD cell was calculated under the periodic boundary conditions. Here we use the MD trajectories to sample the solvation structures around phenol molecules. Figure 3 displays the density profiles of water and phenol near the air–solution interface (cf. Figure S5 of ref (27)). The density of phenol exhibits a maximum at the interface, indicating its surface activity, and consequently, the net density of phenol in the bulk is depleted. The density profile of water (black) shows that the 10–90 thickness of water density is 8.78 Å and that of phenol (red) shows that the full width at half-maximum (fwhm) of the phenol density is 8.15 Å. Based on the density profiles, we decompose the interface into three spatial regions, “surface (I)”, “surface (II)”, and “bulk (III)”, as illustrated in Figure 3. “Surface (II)” is the region within the fwhm of the phenol density, and “surface (I)” and “bulk (III)” are defined as the vapor and bulk sides of it, respectively.

Figure 3.

Figure 3

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Number density profiles of the oxygen sites of water (black line) and phenol (red line) as a function of the coordinate normal to the air–water interface z.27 Three regions, “surface (I)”, “surface (II)”, and “bulk (III)”, are shown, where the shaded area indicates surface (II) within the full width at half-maximum of the phenol density.

Then instantaneous configurations of the solvated phenol clusters were sampled from the MD trajectories. A solvated cluster around an arbitrary phenol molecule, illustrated in Figure 1(c), was extracted to include the phenol molecule and surrounding water molecules. The water molecules in the cluster were determined on condition that the O sites of water are located within 4.4 Å distance from one of the phenol C sites or within 4.0 Å from the phenol O site. The threshold distances, 4.4 and 4.0 Å, were determined from the first minimum positions of the corresponding site–site radial distribution functions to define the first solvation shell of phenol. We randomly sampled 10 hydrated phenol clusters from each of the three regions, where the region of a cluster was assigned by the position of its phenol O site. Typical snapshots of the hydrated clusters in the three regions are illustrated in Figure 1(c). The average and standard deviations of the numbers of solvated water molecules in the three regions are (I) 0.8 ± 1.0, (II) 5.5 ± 2.5, and (III) 10.4 ± 2.9, respectively, indicating that the phenol molecules at surface regions are less hydrated than those in the bulk and that those in surface (II) are “half-hydrated”.28 For each cluster, an ab initio search of S1(OPT) and CI(OPT) and the calculation of ΔE were carried out. During the geometry optimizations, the surrounding water molecules were fixed to maintain the instantaneous configurations of hydrated clusters. Table 2 summarizes the calculated results of ΔE. All the calculated ΔE values are substantially larger than the value calculated for isolated phenol. At the topmost surface region [surface (I)], ΔE shows 12.11 kcal mol–1, which is closest to the value calculated for the above-mentioned isolated phenol, ΔE = 6.19 kcal/mol, because the phenol molecules of surface (I) are surrounded with smallest numbers of water molecules. Table 2 corroborates that the hydration augments the barrier height.

Table 2. Summary of Calculated Barriers ΔE of Hydrated Phenol in the Three Regions (I, II, III)a.

surface (I) surface (II) bulk (III) bulk (III′)
12.11 20.46 23.16 20.35
±1.67 ±6.10 ±7.13 ±5.36
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a

The average and standard deviation are shown in the first and second rows. Unit: kcal/mol.

The calculation also reveals the associated change in the location of the CI. In Table 1, the last column with parentheses indicates the average displacements of i–j bond lengths ΔRij between S1(OPT) and CI(OPT) in surface (II) and bulk (III). By comparing the three sets of ΔRij in the isolated and the solvated (surface (II)/bulk (III)) environments, we find that these sets of ΔRij have very similar directions, characterized with shortened 3–2, 5–4, and 8–7 bonds. Furthermore, the displacements of Rij become larger as the phenol molecule is more hydrated. The average displacements of Rij in surface (II) are in between those in the isolated and bulk (III), arguably because the phenol molecules in surface (II) are half-hydrated. Because the barrier crossing dynamics associated with the CI proceeds mainly along the ring deformation to the quinone-like form (see Figure 1(b)) in either environment, the potential energy surface of the excited states along the ring deformation can be sketched as shown in Figure 4 with the location of the CI. This figure clearly illustrates the mechanism of how the solvation shifts the location of the CI and changes the barrier height in a correlated manner. The effect of solvation (or desolvation) on the reaction kinetics will be further discussed below.

Figure 4.

Figure 4

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Perturbation mechanism on the conical intersection and barrier by the hydration, where the reaction coordinate is considered to be the ring deformation (see the text).

Surface Hydration

Next, we compare the solvation environments of surface (II) and bulk (III). We find that most phenol molecules in surface (II) and bulk (III) form the hydrogen bonds of the phenol OH group with the surrounding water. However, the hydration shell around the phenyl group of phenol is rather incomplete in surface (II), which is in contrast to that in bulk (III), where both the phenol OH and phenyl groups are fully hydrated (see Figure 1(c)). To estimate the effect of water molecules around the phenyl group, we tentatively removed the water molecules around the C sites from the 10 snapshot clusters of bulk (III) and thus defined the clusters of “bulk (III′)”. As shown in Table 2, ΔE = 20.35 kcal/mol calculated for bulk (III′) is smaller than that of bulk (III) (23.16 kcal/mol) and comparable to ΔE = 20.46 kcal/mol for surface (II). This implies that the solvation effect on ΔE is mainly attributed to the hydration of the phenol OH group, although the water around the phenyl group has a considerable effect on the barrier height.

Then we analyze the solvation effect of phenol OH, which is related to hydrogen bonding. We pointed out in Figure 2 that the H(1) site of isolated phenol OH has a negative charge in the S2 state, which should be quite unfavorable for the hydrogen bond formation. Therefore, the partial charges of solvated phenol were examined in comparison with those of isolated phenol in Figure 2. We find that the H(1) site of solvated phenol OH has a positive charge at S2 (0.505 at CI(OPT)), indicative of significant polarization of the S2 state to adjust itself to the hydrogen bond. The whole atomic charges for the S0, S1, and S2 states are summarized in the Supporting Information.

The perturbation of the electronic state was further investigated in comparison with the fully optimized 1:1 phenol–water cluster, where a hydrogen bond between phenol and water is formed. We found that one water molecule that accepts a hydrogen bond from phenol OH suffices to cause the remarkable polarization of the S2 state with the positive H site (∼0.86), while its πσ* character is retained. The barrier height of the CI is augmented to be 9.3 kcal/mol for the fully optimized 1:1 cluster. This investigation of the 1:1 cluster supports the mechanism that the hydrogen bond formation of the phenol OH distorts and destabilizes the S2(πσ*) state and consequently augments the barrier of the CI. This perturbation should reduce the rate of the photochemical reaction as hydration around the phenol molecule proceeds.

Conclusion

The present work clarified the mechanism of the hydration effect on the photochemical reaction of phenol, as illustrated in Figure 4. Based on the calculations of isolated phenol, the 1:1 cluster, surface (I) and (II), and bulk (III), we found a clear tendency that a less hydrated environment leads to a lower barrier of the CI crossing. In other words, the reaction of hydrogen detachment proceeds via the CI with the S2 state, and the S2 state of πσ* character is quite unfavorable for hydrogen bonding. Thus, a hydrogen bond of phenol OH and adjacent water deforms the S2 state and augments the barrier height of the CI. Besides this local hydrogen bond effect of the phenol OH, we mention that the hydration of phenol can also tend to augment the CI barrier according to Marcus’ theory of electron transfer.29,30 Because the S2 state involves the substantial intramolecular charge transfer, a large solvation reorganization energy is expected to be associated with the CI crossing in the more complete hydrated environment. Therefore, since the air–water interface offers an incomplete hydration environment to phenol molecules, they can show a distinctively facilitated rate in comparison with those in bulk water.

This work focuses on the CI crossing in the photodetachment reaction of phenol since the dynamics of the ultrafast reaction should be pertinent to the initial activation step. Besides the barrier of the initial step, the overall kinetics and yield of product (PhO• + e– + H3O+) also depend on the subsequent steps of proton and/or electron transfer in water. We can assume that the product of separated ions is eventually stabilized in the hydrated environment, in contrast to the isolated environment, and hence the detailed dynamics of the exit channel needs to be properly considered for a quantitative discussion of the overall change in the reaction. Nevertheless, the results of the present work strongly suggest that the fast photochemical reaction of phenol at the air–water interface mainly proceeds at the topmost layer with the help of the reduced barrier. The present calculations demonstrate that a combination of electronic state calculations and molecular dynamics (MD) sampling of the hydrated environment can elucidate the microscopic reaction mechanism at the water surfaces.

Lastly, we note a broader implication obtained from the present work. In general, the CI is an essential element that governs the mechanism and kinetics of chemical reactions. Because the CI is the point where two electronic states of different characters cross at the potential energy surface, its location shifts as the relative energy of the two electronic states is changed. Because each electronic state usually has a different character, the sensitivity of its energy to the environment, i.e., hydrogen bonding, the polarity and anisotropy of interaction, etc., is different. These quantities are substantially different between the interface and bulk, so the location of the CI, as well as the height of the barrier that the CI generates, should be significantly different between the interface and bulk. Therefore, for a reaction in which the CI determines the barrier of the rate-determining step, the reaction kinetics at the interface naturally becomes different from the bulk, although the reaction may be accelerated or decelerated. The present case of the phenol reaction provides a typical example and, thus, exhibits distinct reaction rates at the air–water interface.

Acknowledgments

The authors thank Drs. R. Kusaka and S. Nihonyanagi for valuable comments. Some of the MD and ab initio calculations were performed using the supercomputers at Research Center for Computational Science, Okazaki, Japan, and MASAMUNE-IMR at CCMS, IMR, Tohoku University, Sendai, Japan. This work was supported by the Grants-in-Aid (Nos. 18H05265, 20H00368, 21H01878) by the Japan Society for the Promotion of Science (JSPS), Fugaku Supercomputer Project (No. JPMXP1020200308) by the Ministry of Education (MEXT), Japan, and the Asahi Glass Foundation.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.1c13336.

  • Vertical excitation energies in comparison with experiment; basis set dependence; characters of active orbitals; characters of S0, S1, and S2 states at the S1(OPT) and CI(OPT) geometries; natural bond orbital charges at the S0, S1, and S2 states of isolated and hydrated phenol at the S1(OPT) and CI(OPT) geometries (PDF)

The authors declare no competing financial interest.

Supplementary Material

ja1c13336_si_001.pdf (253.8KB, pdf)

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Supplementary Materials

ja1c13336_si_001.pdf (253.8KB, pdf)

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