Exploring nuclear motion through conicalintersections in the UV photodissociation ofphenols and thiophenolMichael N. R. Ashfold†, Adam L. Devine, Richard N. Dixon, Graeme A. King, Michael G .D. Nix, and Thomas A. A. Oliver

School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom

Edited by F. Fleming Crim, University of Wisconsin, Madison, WI, and accepted May 12, 2008 (received for review January 18, 2008)

High-resolution time-of-flight measurements of H atom productsfrom photolysis of phenol, 4-methylphenol, 4-fluorophenol, andthiophenol, at many UV wavelengths (�phot), have allowed sys-tematic study of the influence of ring substituents and the het-eroatom on the fragmentation dynamics. All dissociate by XOH(X � O, S) bond fission after excitation at their respectiveS1(1��*)–S0 origins and at all shorter wavelengths. The achievedkinetic energy resolution reveals population of selected vibrationallevels of the various phenoxyl and thiophenoxyl coproducts, pro-viding uniquely detailed insights into the fragmentation dynamics.Dissociation in all cases is deduced to involve nuclear motion on the1��* potential energy surface (PES). The route to accessing thisPES, and the subsequent dynamics, is seen to be very sensitive to�phot and substitution of the heteroatom. In the case of thephenols, dissociation after excitation at long �phot is rationalized interms of radiationless transfer from S1 to S0 levels carrying suffi-cient OOH stretch vibrational energy to allow coupling via theconical intersection between the S0 and 1��* PESs at longer OOHbond lengths. In contrast, H � C6H5O(X2B1) products formed afterexcitation at short �phot exhibit anisotropic recoil-velocity distri-butions, consistent with prompt dissociation induced by couplingbetween the photoprepared 1��* excited state and the 1��* PES.The fragmentation dynamics of thiophenol at all �phot matches thelatter behavior more closely, reflecting the different relative dis-positions of the 1��* and 1��* PESs. Additional insights areprovided by the observed branching into the ground (X2B1) andfirst excited (2B2) states of the resulting C6H5S radicals.

photofragment translational spectroscopy � nonadiabatic �dissociation dynamics

Heteroaromatic molecules such as pyrroles, imidazoles, andphenols are key components of the long-wavelength chro-

mophores in nucleobases and aromatic amino acids (e.g., histi-dine, tryptophan, and tyrosine), which dominate the UV-absorption spectra of many biological molecules. �*4�transitions are responsible for the strong UV absorptions, butthese heteroaromatics also possess excited states formed by�*4� electron promotions. Absorption to the 1��* states is verymuch weaker, but these states can still be populated by directphotoexcitation and/or radiationless transfer from 1��* (or1n�*) states. Recent theoretical studies by Sobolewski et al. (1)alerted photochemists to the likely importance of 1��* states inpromoting XOH (X � N, O) bond fission in such molecules. Inthe case of phenol, the ground state correlates diabatically withan excited (2B2) electronic state of the phenoxyl radical afterOOH bond extension, the 1��* state is bound with respect toRO-H, and the 1��* state‡ correlates diabatically with phenoxylproducts in their ground (X2B1) state. Thus, a cut through thepotential energy surface (PES) for the 1��* state along RO-Hintersects both the 1��* and 1�� PESs, as depicted in Fig. 1a.These crossings develop into conical intersections (CIs) whenthe out-of-plane coordinates are considered (2, 3). The �*orbital in phenol is largely 3s Rydberg at short RO-H but evolves

to an antibonding �* valence orbital centered on the OOH bondas RO-H increases. The corresponding orbital in thiophenol hassubstantial 4s Rydberg character at short RS-H, but the potentialsfor the respective diabatic states in this molecule (4, 5) show thesame parent-to-product correlations as phenol (as Fig. 1bshows). More careful scrutiny of the respective PESs reveals anumber of differences. First, the relative energies of the 1��*and 1��* states in thiophenol are such that the CI between them(labeled 1��*/1��* CI in Fig. 1b) lies at much lower energywithin the 1��* manifold than in phenol. Second, the highest(singly) occupied molecular orbital in the radical product is a p�orbital that, particularly in the case of thiophenoxyl, is largelylocalized on the heteroatom [and aligned perpendicular (X2B1state) or parallel (2B2 state) to the molecular plane]. Thestrength of any interaction between orbitals on the ring and onthe heteroatom declines as RC-X increases, so that the energyseparation between these two states of thiophenoxyl is onlyapproximately one-third that between the corresponding statesof the phenoxyl radical. As Fig. 1b shows, a consequence of thesedifferences is that the 1��*/1�� CI in thiophenol occurs atlonger RX-H than in phenol, and the gradients of the respectivepotentials in the CI region are less steep.

Here we report high-resolution time-of-f light (TOF) spectraof the H atom products resulting from photolysis of phenol, anillustrative selection of substituted phenols (4-methylphenol andthree 4-halophenols), and thiophenol, each of which have beenmeasured at a range of UV wavelengths (�phot) by using the H(Rydberg) photofragment translational spectroscopy (PTS)technique (6, 7). Total kinetic energy release (TKER) spectraare derived by rebinning measured TOF spectra by using therelationship

TKER�12

mH�1�mH

mR� �d

t �2

, [1]

where mH and mR are the mass of the H atom (1.0079 a.u.) andthe cofragment, t is the measured TOF, and d is the distanceseparating the interaction region from the detector. Energy,momentum, and signal intensity conservation arguments thenenable determination of the population distribution among theinternal energy states of the respective cofragments (at each�phot) and, thus, an opportunity to explore systematically theinfluence of (i) ring substituents and (ii) the heteroatom on thefragmentation dynamics of these prototypical heteroaromatics.

Author contributions: M.N.R.A. designed research; A.L.D., G.A.K., and M.G.D.N. performedresearch; A.L.D., R.N.D., G.A.K., and M.G.D.N. analyzed data; and M.N.R.A. and T.A.A.O.wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

†To whom correspondence should be addressed. E-mail: mike.ashfold@bris.ac.uk.

‡It proves convenient to label the diabatic states (and PESs) of interest 1��, 1��*, and 1��*and to use the descriptors S0, S1, etc. for the corresponding adiabatic states.

© 2008 by The National Academy of Sciences of the USA

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Results and DiscussionPhenol. Photoexcitation of phenol at all absorbing wavelengthsbelow the S1(1��*)–S0 origin [�phot � 275.113 nm (8)] results inOOH bond fission (7). As Fig. 2a illustrates, the TKER spec-trum derived by measuring the H atom TOFs after excitation atthis origin shows a series of peaks on a broad background signal.The latter (observed here and in all of the other heteroaromaticsystems discussed in this work) is attributed to the ‘‘statistical’’decay of highly internally excited parent molecules formed byboth one-photon and multiphoton excitations and subsequentradiationless transfer, intramolecular vibrational redistribution(IVR), and eventual dissociation on the ground-state PES.Vibrationally excited CO products have been reported recentlyas another product from the unimolecular decay of ‘‘hot’’ground-state phenol molecules (9). The focus of the presentstudy is the sharp structure shown in Fig. 2a, which is associatedwith formation of phenoxyl radicals in specific vibrational levelsof the X2B1 ground state, with modest levels of rotationalexcitation. More product states are populated when exciting atshorter �phot, and the structure gradually coalesces into a broadfeature centered at TKER � 6,500 cm�1 (Fig. 2b). Theseproducts show no recoil anisotropy. Once �phot � 248 nm,additional structure starts to appear at higher TKER values (seeFig. 2c). This structure, which is much more evident in thespectrum recorded at �phot � 232.00 nm (Fig. 2d), is alsoindicative of C6H5O(X2B1) fragment formation in a very limitedsubset of the available vibrational level density. Again, aftershifting to yet-shorter �phot, this structure fuses into a broadunresolved feature, centered at TKER � 12,000 cm�1. Thisfeature appears more strongly in spectra measured with thepolarization vector of the photolysis laser, �phot, aligned perpen-dicular to the TOF axis, implying dissociation on a time scale thatis fast compared with the rotational period of the photoexcitedmolecule. Equivalent photolysis studies of C6D5OD reveal nostructured D atom signal when exciting near the S1–S0 origin buta similar progression of peaks to that seen with C6H5OH whenexciting at �phot � 248 nm (7).

Assignment involves matching the peak separations in suchspectra, recorded across the range 279 � �phot � 206 nm, to thecalculated (anharmonic) wavenumbers for vibrational modes ofthe ground-state phenoxyl radical. The latter were calculated byusing Gaussian 03 (10) at the DFT-B3LYP/6-311��G(d,p)level, the reliability of which for such problems has been testedextensively by now. PTS studies generally offer a direct route todetermining bond-dissociation energies D0(XOH) from the

difference between the photolysis photon energy and the fastestpeak in the TKER spectrum (which will typically be associatedwith formation of v � 0 products). In the case of phenol,however, all attempts to assign the fastest peak in spectrameasured at long and short �phot to the formation of H �phenoxyl (v � 0) products returned mutually inconsistent valuesfor D0(C6H5OOH). Internal consistency can be obtained, how-ever, by assigning the TKERmax peak in Fig. 2a to formation ofphenoxyl fragments carrying one quantum of excitation in mode�16a. The Wilson mode-numbering scheme (11) is used through-out, because it is insensitive to the loss of the three XOH modesand, thus, serves to illustrate the complementarity of the ringvibrations in the parent and radical products. The dominantstructure is then assigned to two short progressions involving(odd quanta of) �16a built on either zero or one quantum of �18b(the COO in-plane wag), as shown in Fig. 3a. The nuclearmotions associated with the former out-of-plane ring mode[which has a�(a2) symmetry in Cs(C2v)] and the latter in-plane

a b

Fig. 1. Cuts along RO-H and RS-H through the PESs for the ground (S0) and first three excited singlet (two 1��* and one 1��*) states of phenol (a) and thiophenol(b), adapted from refs. 1 and 4 to match experimentally determined excited-state parent and product term values and the respective bond-dissociation energies.The different fragmentation mechanisms deduced by analysis of the vibrational energy disposal in the C6H5O and C6H5S products are illustrated also.

a

b

c

d

Fig. 2. TKER spectra derived from H atom TOF measurements after photol-ysis of jet-cooled phenol molecules at �phot � 275.11 nm (a), 257.41 nm (b),244.00 nm (c), and 232.00 nm (d) with �phot aligned at 90° to the detection axis.The fast H atom signal attributable to dissociation via the 1��*/1��* CI iscircled in c.

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[a�(b2)] bending mode are shown in Fig. 4. All other peaks inspectra recorded at long �phot are similarly assigned to popula-tion of overtone or combination levels of the C6H5O(X) radicalwith overall a� vibrational symmetry (i.e., with an odd numberof quanta in �16a or another out-of-plane mode). The progressionof peaks evident in spectra recorded at short �phot (e.g., at 232.00nm; Fig. 2d) is associated with �18b; again, however, to obtain aninternally consistent value for D0(C6H5OOH), it is necessary toassume that this progression is built on one quantum of anotherout-of-plane mode: �16b. The best-fit value of D0(C6H5OOH)returned by this analysis is 30,015 � 40 cm�1 (7), and this valueis assumed in deriving fragment internal energy (Eint) spectrasuch as that shown in Fig. 3a. Similar analyses of the structurein TKER spectra obtained when exciting C6D5OD at �phot � 248nm serve to reinforce this assignment in that a mutually consis-tent value for D0(C6D5OOD), after appropriate correcting fordifferences in zero-point energies, is obtained only if, again, theobserved progression in �18b is assumed to be built on onequantum of �16b.

The S0 and S1 states of phenol both have A� electronicsymmetry. Any excitation from S0(v�0) to S1, therefore, willpopulate levels with A�Va� � A� vibronic symmetry. The esti-mated lifetime of the S1(v�0) level of phenol is �2 ns, with thatfor phenol-d6 at least an order of magnitude longer (see ref. 12and references therein). Pump–probe experiments have sug-gested that internal conversion (IC) to the ground state is thedominant nonradiative decay process (13). The proposed ratio-nale for the observations at long �phot involves initial �*4�excitation and subsequent dissociation via IC to S0 levels carry-ing sufficient OOH stretch vibrational energy to allow couplingto the 1��* PES and dissociation to H � C6H5O(X) products viathe lower adiabatic surface of the CI at larger RO-H (7, 14). Theidentification of OOH stretching overtones as key acceptormodes in the initial (rate-limiting) IC process accords withexpectations based on maximizing the overlap between initialand acceptor vibrational wavefunctions (i.e., Fermi’s goldenrule). It is also consistent with previous explanations for the large

increase in S1 lifetime after deuteration (see ref. 12 and refer-ences therein) and with our nonobservation of an equivalentOOD bond fission channel after photolysis of phenol-d6 at long�phot. Theory (3) identifies OOH torsion (an a� vibration) as thedominant coupling mode at the 1��/1��* CI. Such is eminentlyplausible in that it is precisely the type of nuclear motionrequired to mix the �* and � orbitals in phenol. However, theobservation that the C6H5O(X) products are formed in levelswith an odd number of quanta in �16a (or, very weakly, �11), thetwo lowest frequency a� modes that survive the OOH bondfission, suggests that the torsional motion still must be partiallycoupled to these out-of-plane ring vibrations in the region of theCI. Given that the 1��* state has A� electronic symmetry, thefinding that the populated product levels all have a� vibrationalsymmetry is consistent with conservation of vibronic symmetrythroughout the parent 3 fragment evolution. The observedactivity in product-mode �18b, the COO in-plane wag, reflectsthe impulse exerted by the departing H atom. Henceforth,population of product vibrations such as this, induced by forcesacting during the fragmentation process, will be termed aconsequence of dynamic Franck–Condon (dFC) effects.

The TKER of the H � C6H5O(X) products in Fig. 2a reflectsthe drop in the potential energy as the dissociating moleculeevolves from S1(v�0) to the ground-state dissociation asymptote(recall Fig. 1a). As Fig. 2 b and c shows, reducing �phot initiallycauses little change in the mean TKER of products attributed todissociation via the 1��* PES. This apparent constancy of themean TKER is understandable, because the S1 levels excited atshorter �phot will involve those modes with the best Franck–Condon factors for v 0 transitions, which in the case of a�*4� excitation will tend to be symmetric (a�) ring breathingvibrations. If the nuclear motions associated with such a modeare only weakly coupled to the dissociation coordinate, theassociated vibrational energy can act as a ‘‘spectator’’ in theparent (S1) 3 product evolution and, thus, not be released asTKER. We will use the shorthand description vertical Franck-Condon (vFC) for such product vibrational excitation that can betraced back to vFC parent excitation. Careful studies followingexcitation to many different vibrational levels of phenol (S1)have identified a range of behaviors, including efficient retentionof the photoprepared mode of vibration, redistribution of thevibrational energy into other (a�) modes of the product, and lossof the vibrational energy to translation, with the net effect thatany increase in the mean TKER of the products is very much lessthan the increase in photon energy (7).

a

b

c

Fig. 3. Internal energy spectra (Eint) of radicals formed as a result of H atomloss after excitation on the S1–S0 origin transitions of phenol at �phot � 275.11nm (a), 4-methylphenol at �phot � 284.03 nm (b), and 4-fluorophenol at �phot �284.77 nm (c). The combs assign levels of the respective radical productspopulated in these dissociations, with Eint � 0 defining the energy of (unob-served) v � 0 products.

Fig. 4. Nuclear motions associated with selected product modes of vibrationin the phenoxyl radical, calculated with Gaussian 03 at the DFT-B3LYP/6-311��G(d,p) level.

Ashfold et al. PNAS � September 2, 2008 � vol. 105 � no. 35 � 12703

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�phot � 246 nm marks the onset of a step change in the TKERspectra, with the appearance of an additional, fast, and aniso-tropic component within the distribution of recoiling H atomsfrom phenol (and D atoms from phenol-d6), centered atTKER � 12,000 cm�1. This new feature is also attributable toOOH bond fission on the 1��* PES, yielding C6H5O(X) frag-ments, but the route to populating the 1��* PES is different. Onepossibility is direct �*4� excitation, but this transition ispredicted to be weak (15). An alternative would be �*4�excitation and subsequent nonadiabatic transfer to the 1��*PES. The mechanism of such nonadiabatic transfer is open todebate. Coupling at the 1��*/1��* CI is one obvious possibility,which has been advanced in earlier theoretical and experimentalstudies (2, 3, 5). The (nonlimiting) perpendicular recoil anisot-ropy of these fast H atoms accords with the assumption that Hatom formation involves initial �*4� excitation, prompt disso-ciation, and recoil of the H atom at an angle to the transitionmoment, � (which lies in the plane but at an angle to the OOHbond). Impulsive effects alone cannot account for the extent ofthe observed progression in product-mode �18b. Transfer to the1��* PES results in a build-up of electron density in the �*orbital, much of which, at short RO-H, is localized in the regionbetween the OOH hydrogen and the ortho ring hydrogen. Thisbuild-up of electron density causes repulsion of the OH groupand an increase in the COCOO bond angle (by �10°) anddistorts the heavy atom geometry further from C2 (2, 3). Thus,the dissociating molecules experience a COCOO bending forcein the region of the 1��*/1��* CI, which manifests itself in theform of COO in-plane wagging vibration in the product as RO-Hincreases. OOH torsion is likely to be a dominant coupling modeat the 1��*/1��* CI also; the deduction that the populatedproduct states carry a quantum of excitation in �16b would implya degree of coupling between this mode and the torsion duringextension of RO-H on the 1��* PES. Rather than focusing on thesomewhat restricted region of configuration space associatedwith the 1��*/1��* CI, it is also possible to envisage a vibroniccoupling route to the same result. In this scenario, the oscillatorstrength is carried by an initial �*4� excitation, but thepopulated levels are vibronically mixed with the continuum of1��* levels. Parent-mode �16b has the appropriate a� symmetry(b1 in C2v) to promote this vibronic coupling, and its orthogo-nality to the dissociation coordinate would explain its mappinginto the phenoxyl products.

Substituted Phenols. The introduction of a ring substituent can beexpected to perturb the electronic states of phenol and alter therelative stabilities of the parent molecule and radical, which mayinfluence the photodissociation dynamics. Here we focus on thecase of methyl radical and halogen substituents, at the 4-(para-)position. Methyl substitution will potentially increase the rate ofIVR as a result of the additional vibrational state densityassociated with the methyl internal rotor modes. Enhanced IVRprobabilities could encourage depopulation of the S0 OOHstretch overtone levels, which are seen as crucial in relaying fluxfrom the photoprepared S1 state to the 1��/1��* CI and, thence,to dissociation on the 1��* PES. Halogen substitution causeselectron withdrawal from the � orbitals of the ring, but this iscountered by � back-donation into the aromatic system.

In the case of 4-methylphenol (4-MePhOH), excitation on itsS1–S0 origin transition at �phot � 283.023 nm yields H atom TOF(and TKER) spectra similar to those observed for bare phenol.Analyses of these and other TKER spectra obtained across thewavelength range 284 � �phot � 222 nm reveal many parallelswith behavior identified in the case of phenol (16). Theseparallels include (i) identification of H atom products attribut-able to eventual dissociation on the 1��* PES at all �phot, (ii) theobservation of vFC modes in the products, leading to a nearconstancy of the mean TKER (�5,500 cm�1) of the feature(s)

attributable to H � 4-MePhO product formation at long �phot,(iii) the appearance of additional, faster features in TKERspectra obtained at short �phot (�246 nm in this case also, but incontrast to phenol, these fast H atoms display no discerniblerecoil anisotropy), and (iv) a requirement that the TKERmaxfeatures observed at long and short �phot be identified with4-MePhO radicals carrying one quantum of, respectively, �16aand �16b to arrive at an internally consistent value for thebond-dissociation energy D0(4-MePhOOH) � 29,320 � 50cm�1. The Eint spectrum of the 4-MePhO radical productsformed when exciting at �phot � 283.023 nm shows the charac-teristic progression in (odd quanta of) �16a, along with a broaderfeature attributable to population of the combination modes�9b � �16a and �18b � �16a (Fig. 3b). Modes �9b and �18b involve,respectively, concerted in-plane (a�) trans and cis motion of theCH3 and O species relative to a line linking the C1 and C4 atoms.Compared with phenol, the introduction of a substituent (CH3)with similar mass to O on the opposite side of the benzene ringleads to a significant coupling between the motions of thesependant groups. As in phenol, such product energy disposal canbe rationalized in terms of IC to high OOH stretch overtonelevels of the S0 state and subsequent nonadiabatic transfer at the1��/1��* CI at extended RO-H. The long progression of peakscentered at TKER � 11,500 cm�1 observed after excitation at�phot � 246 nm (which we regard as the threshold at which the1��* state can be populated directly or by radiationless transferfrom the optically ‘‘bright’’ 1��* excited state) is assigned interms of progressions involving �9b and �7a(a�) [or �19a(a�)], againbuilt on one quantum of �16b(a�). Both �7a and �19a involve anin-plane symmetrical ring breathing motion in combination witha COO stretch.

As with the methylphenols, UV photoexcitation of 4-fluorophe-nol (4-FPhOH) yields two �phot-dependent families of structured Hatom TOF (and thus TKER) spectra. The structured featureobserved when exciting at long wavelength (285 � �phot � 270 nm)is also centered at TKER � 5,500 cm�1; this feature persists atshorter �phot, but as in phenol, the associated structure eventuallycoalesces into an unresolved envelope. Once �phot � 238 nm, a newstructured feature appears, centered at TKER � 12,000 cm�1. Asin the methylphenols, neither recoil-velocity distribution shows anydiscernible anisotropy, and an internally consistent interpretationof all of the recorded spectra requires that the TKERmax featuresobserved at long and short �phot be assigned to the formation of4-FPhO radicals carrying one quantum of, respectively, �16a and�16b, thereby yielding the bond-dissociation energyD0(FPhOOH) � 29,370 � 50 cm�1 (17).

Replacing the fluorine atom in 4-FPhOH by chlorine orbromine leads to more obvious differences in photofragmenta-tion behavior. The TKER spectrum obtained from TOF mea-surements of H atoms from excitation of 4-ClPhOH at long �photshows some, albeit weak, structure reminiscent of that seen with4-FPhOH, but excitation of 4-BrPhOH at similar wavelengthsshows no such H signal. Exciting 4-ClPhOH at �phot � 238 nmyields a structured feature at higher TKER, similar to thatobserved from both phenol and 4-FPhOH in the same wave-length region. As in the case of 4-FPhOH, these products showan isotropic recoil-velocity distribution. 4-BrPhOH shows nosuch signal at short �phot either. Analyses of the 4-ClPhOHspectra yields an OOH bond strength, D0(ClPhOOH) �29,520 � 50 cm�1, very similar to that in 4-FPhOH. This modestreduction in OOH bond strength relative to that in phenol likelyreflects a degree of stabilization of the radical product by theelectron-withdrawing halogen substituent. Possible reasons forthe perceived decline in H atom yield attributable to dissociationon the 1��* PES across the series 4-FPhOH � 4-ClPhOH �4-BrPhOH include increased IC probabilities to S0 (on accountof the greater vibrational state density after halogen addition)and an increased probability for intersystem crossing to triplet

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states (as a result of the heavy atom effect). Time-dependentdensity functional theory calculations allude to another possibleexplanation. These calculations show the nature and localizationof the lowest �* orbital to be very sensitive to halogen substi-tution. In 4-FPhOH, as in phenol, this �* orbital is centered onthe OOH group (at geometries near the ground-state mini-mum), whereas in 4-BrPhOH it is localized on the COBr bond;in 4-ClPhOH the calculations suggest some �* amplitude in theregions of both the OOH and COCl bonds. The most straight-forward explanation for the decrease in H atom yield, therefore,is increased competition from the rival COY (Y � F, Cl, Br)bond-fission process, which becomes progressively less endo-thermic across this series (17).

Thiophenol. As noted earlier, the energy separation between theground (X2B1) and excited (2B2) states of the thiophenoxylradical is much smaller than the corresponding splitting inphenoxyl, and the Eint spectrum obtained from H atom TOFmeasurements after excitation of thiophenol at its S1–S0 origin(�phot � 285.80 nm; Fig. 5a) shows short progressions of peaksattributable to formation of both electronic states of the C6H5Sradical. The distribution of populated vibrational levels withinboth states is deduced to spread, and to shift to higher Eint, as�phot is reduced, as exhibited in Fig. 5 b and c. The recoil-velocitydistributions of the partner H atoms are isotropic at �phot � 275nm, whereas the H atoms associated with the resolved absorptionfeatures in spectra recorded at shorter �phot (e.g., Fig. 5 b and c)show a preference for recoiling perpendicular to �phot (4, 5). Incontrast to the phenols, we attribute the TKERmax peak in thesespectra to formation of C6H5S(X)��0 products. The neighboring

peaks in Fig. 5a then indicate population of levels involvingexcitation in �18b (the COS in-plane wag), �6a, and/or �1 � twoa1 (in C2v, a� in Cs) ring breathing modes. The set of peaksstarting at Eint � 2,960 cm�1 is assigned to the H � C6H5S(2B2)product channel. However, the separations between these peaksare too small for them to be assigned to population of differentlevels of the C6H5S(2B2) radical if the peak at Eint � 2,960 cm�1

is assigned to formation of v � 0 products. These features, andthe companion peaks in all spectra measured at long �phot, canbe assigned consistently only if the 2,960 cm�1 feature isattributed to formation of C6H5S(2B2) radicals with one quantumof excitation in the lowest frequency a� mode (�11, a ringpuckering vibration with b1 symmetry). The peak at Eint � 3,215cm�1 is then assigned to C6H5S(2B2) products with �16a � 1 (�16ais another low frequency a� mode with a2 symmetry), and thoseat Eint � 3,420 cm�1 and �4,090 cm�1 are assigned to 2B2 stateradicals carrying a quantum of �11 and, respectively, a quantumof �1 and �6a [i.e., two of the vFC active a� modes identified inthe H � C6H5S(X) product channel also]. Given these assign-ments, we obtain D0(C6H5SOH) � 28,030 � 100 cm�1 and anenergy separation between the electronic origins of the X2B1 and2B2 states of C6H5S, T00 � 2,800 � 100 cm�1, which is in goodaccord with a recent theoretical estimate [2,674 cm�1 (4)]. Theseassignments imply that all populated levels in the C6H5S(X2B1)and C6H5S(2B2) products have, respectively, a� and a� vibrationalsymmetry.

Given the evident similarities between the low Eint part of Fig.5a and the corresponding spectrum from phenol (Fig. 3a), it isworth considering alternative assignments before discussing thesignificance of this interpretation. By analogy with the phenols,the intuitive assignment would involve progressions in (oddquanta of) �16a, built on zero or one quantum of �18b. As the redcombs in Fig. 5a show, such an assignment is not inconceivablebut would require that (i) �16a is 6% smaller than predicted byGaussian 03 at the DFT-B3LYP/6-311��G(d,p) level and (ii)the ‘‘progression’’ involves just members with v16a � 1 and 3.Such a large discrepancy between the observed and calculatedwavenumber for a ground-state vibration of this type would besurprising in our experience, and similar calculations for the 2B2state of the radical succeed in predicting wavenumbers for �16a,�6a, and �11 that reproduce the observed differences in the TKERspectrum (Fig. 5a) to within experimental accuracy. The energyseparation between the electronic origins of the X2B1 and 2B2states of C6H5S has been inferred in the present work, but to ourknowledge, no independent experimental determination hasbeen reported. Thus, we appreciate that one or both of theprogressions in Fig. 5a could, in principle, be offset by one ormore quanta of some other mode and that the value forD0(C6H5SOH) reported above is strictly an upper limit. For now,we persist with the simplest and neatest assignment of thepopulated product modes indicated by the black combs shown inFig. 5.

In contrast to the phenols, the TKER spectra obtained fromUV photolysis of thiophenol appear to evolve smoothly with�phot, with the most notable change being an onset of productrecoil anisotropy once �phot � 275 nm. Analyses of these TKERspectra, the parent absorption spectrum, the excitation spectrumfor forming H atom products (5), and the available ab initio data(4) leads to the following conclusions. Excitation at �phot � 275nm populates quasi-bound levels of the S1 state, which decay bytunneling to the 1��* PES (see Fig. 1b). In contrast to phenol,the barrier to SOH torsion is much lower, with the result that asubstantial fraction of the parent thiophenol molecules in ourmolecular beam are in levels with torsional quantum number � �0. Such torsional motion is proposed as a coupling modefacilitating population transfer at the 1��*/1��* CI; asymptot-ically, the torsional energy will be released mainly as TKER. Fluxevolving on the 1��* PES samples the 1��*/1�� CI at longer

a

b

c

Fig. 5. Internal energy spectra (Eint) of radicals formed by H atom loss afterexcitation of thiophenol at �phot � 285.80 (a), 272.50 (b), and 252.50 nm (c)with �phot aligned at 90° to the detection axis. The combs show the mostprobable assignment of the populated levels of the C6H5S products, whereasthe red comb above the features at low Eint in a shows the degree of ‘‘mis-fit’’associated with an alternative assignment (involving population of oddquanta of �16a) that might be considered by analogy with phenol.

Ashfold et al. PNAS � September 2, 2008 � vol. 105 � no. 35 � 12705

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RS-H, where the branching between ground- and excited-stateC6H5S fragment formation is established. Which vibrationallevels are populated in each product channel is controlled bysymmetry. All populated levels of C6H5S(X2B1) identified in theTKER spectra have a� vibrational symmetry; activity in theseparticular in-plane modes can be understood in terms of vFCand/or dFC effects. The energy disposal within the C6H5S(2B2)products is very different, involving levels with a� vibrationalsymmetry, particularly modes �11 and �16a. As in phenol, wededuce that population of these modes reflects the couplingforces acting at the 1��*/1�� CI.

Once �phot � 275 nm, the fastest C6H5S fragments in bothproduct channels display anisotropic recoil-velocity distribu-tions, with a preference for v being perpendicular to �phot (andthus to �). This recoil anisotropy can be understood in terms ofdirect �*4� excitation and prompt dissociation on the 1��*PES, with subsequent product branching determined by the1��*/1�� CI, as at longer �phot. �*4� absorption occurs at thesesame wavelengths also. Franck–Condon considerations requirethat the resulting 1��* parent molecules will be vibrationallyexcited but predominantly in ‘‘spectator’’ modes that are, at best,weakly coupled to the dissociation coordinate. Such moleculesdissociate by tunneling to the 1��* PES, as at longer �phot, withexcitation in the spectator modes mapping through into thecorresponding product vibrational levels. Thus, not all of theexcess energy is available for release as kinetic energy, and theformation of such products leads to a progressive broadeningand loss of structure in TKER spectra measured at shorter �phot.Once �phot � 260 nm, absorption to a higher 1��* statedominates, but the observed energy disposal and the evidentrecoil anisotropy of the fastest C6H5S(X2B1) and C6H5S(2B2)products indicate that nonadiabatic transfer from this state to the1��* PES must be both fast and efficient. As in the phenols, theincreased relative yield of slow H atoms after excitation at short�phot indicates that other fragmentation channels (most probablyinvolving unimolecular decay of highly vibrationally excitedground-state molecules) also gain in importance.

ConclusionsThere are clear similarities, but also significant differences,between the UV photochemistry of phenol and thiophenol.

These molecules dissociate by XOH bond fission after excitationat their respective S1(1��*)–S0 origins and at all shorter �phot. Inboth phenol and thiophenol, the decay rate from low levels of therespective S1 states is sufficiently slow that the resulting H atomsshow isotropic recoil-velocity distributions. However, the life-time of thiophenol(S1) molecules is much shorter than that ofphenol(S1) at comparable levels of excitation. The reduced S1lifetime in thiophenol can be traced to improved coupling to thedissociative 1��* state, the energy of which is lower in thiophe-nol (relative to that of the 1��* state) than in the phenols. Thus,photoexcited C6H5SH molecules are deduced to dissociate eitherby tunneling to the 1��* PES via the 1��*/1��* CI and/or, once�phot � 275 nm, after direct population of the 1��* state. Ineither scenario, the dissociating flux samples the upper cone ofthe subsequent 1��*/1�� CI at longer RS-H, where the branchinginto specific vibrational levels of the ground (X2B1) and firstexcited (2B2) states of the C6H5S fragment is determined.Phenol(S1) molecules, in contrast, are deduced to dissociate byIC to vibrational levels of the S0 state. Subsequent OOHstretching motion on the S0 PES projects f lux to the lower coneof the 1��/1��* CI and, thence, to specific symmetry-allowedvibrational levels of the ground state of the C6H5O radical. Thisalternative route through the 1��/1��* CI, and/or the muchgreater energy separation between the X2B1 and 2B2 states inC6H5O, apparently precludes formation of excited-state C6H5Oradicals at all �phot studied.

Materials and MethodsThe experimental apparatus and procedures used have been described else-where (7). All precursors were obtained commercially (Aldrich). Solid samplesof phenols were each packed into an in-line filter placed behind a pulsedsolenoid valve, both of which were heated (typically to �50°C) to generatesufficient vapor pressure and seeded in argon at �760 torr. Thiophenol is aliquid at room temperature. Its vapor (pressure � 2 torr) was vacuum distilledinto a glass bulb and then made up to �760 torr with argon.

ACKNOWLEDGMENTS. We are extremely grateful to Drs J. N. Harvey and B.Cronin and to K. N. Rosser for their many and varied contributions to this workand to the Engineering and Physical Sciences Research Council for financialsupport via the pilot portfolio partnership LASER.

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