Simulation of ultrafast photodynamics of pyrrole with a multiconfigurational Ehrenfest method.

We report the first results of ab initio multiconfigurational Ehrenfest simulations of pyrrole photodynamics. We note that, in addition to the two intersections of 1(1)A2 and 1(1)B1 states with the ground state 1(1)A1, which are known to be responsible for N-H bond fission, another intersection between the 1(2)A2 and 1(2)B1 states of the resulting molecular radical becomes important after the departure of the H atom. This intersection, which is effectively between the two lowest electronic states of the pyrrolyl radical, may play a significant role in explaining the branching ratio between the two states observed experimentally. The exchange of population between the two states of pyrrolyl occurs on a longer scale than that of N-H bond fission.

Collision-induced dissociation of halide ion-arginine complexes: evidence for anion-induced zwitterion formation in gas-phase arginine.

We report the first low-energy collisional-induced dissociation studies of the X(-)·arginine (X(-) = F(-), Cl(-), Br(-), I(-), NO(3)(-), ClO(3)(-)) series of clusters to investigate the novel phenomenom of anion-induced zwitterion formation in a gas-phase amino acid. Fragmentation of the small halide ion clusters (F(-)·arginine and Cl(-)·arginine) is dominated by deprotonation of the arginine, whereas the major fragmentation channel for the largest ion clusters (I(-)·arginine and ClO(3)(-)·arginine) corresponds to simple cluster fission into the ion and neutral molecule. However, the fragmentation profiles of Br(-)·arginine and NO(3)(-)·arginine, display distinctive features that are consistent with the presence of the zwitterionic form of the amino acid in these clusters. The various dissociation pathways have been studied as a function of % collision energy and are discussed in comparison to the fragmentation profiles of protonated and deprotonated arginine. Electronic structure calculations are presented for Br(-)·arginine to support the presence of the zwitterionic amino acid in this complex. The results obtained in this work provide important information on the low-energy potential energy surfaces of these anion-amino acid clusters and reveal the presence of several overlapping surfaces in the low-energy region for the Br(-)·arginine and NO(3)(-)·arginine systems.

Evidence for hydrogen bond network formation in microsolvated clusters of Pt(CN)4(2-): collision induced dissociation studies of Pt(CN)4(2-)·(H2O)(n) n = 1-4, and Pt(CN)4(2-)·(MeCN)(m) m = 1, 2 cluster ions.

Low-energy collision induced dissociation has been used to investigate the structure and stability of microsolvated clusters of the prototypical, aprotic multiply charged anion, Pt(CN)(4)(2-), i.e. Pt(CN)(4)(2-)·(H(2)O)(n) n = 1-4, Pt(CN)(4)(2-)·(MeCN)(m) m =1, 2, and Pt(CN)(4)(2-)·(H(2)O)(3)·MeCN. For all of the systems studied, the lowest energy fragmentation pathway was found to correspond to decay of the cluster with loss of the entire solvent ensemble. No sequential solvent evaporation was observed. These observations suggest that the Pt(CN)(4)(2-) solvent clusters studied here form hydrogen-bonded "surface solvated" structures. Electronic structure calculations are presented to support the experimental results. In addition, the detailed fragmentation patterns observed are interpreted with reference to the differential solvation of the ionic fragmentation and electron detachment potential energy surfaces of the core Pt(CN)(4)(2-) dianion. The results described represent some of the first experiments to probe the microsolvation of this important class of multiply charged anions.

Noncovalent interactions in the gas-phase conformers of anionic iduronate (methyl 2-o-sulfo-α-L-iduronate): variation of subconformer versus ring conformer energetics for a prototypical anionic monosaccharide studied using computational methods.

The conformational preferences of the prototypical anionic monosaccharide (methyl 2-O-sulfo-α-L-iduronate) have been studied at various computational levels to investigate the energetic variation of 17 subconformers associated with the (4)C(1), (2)S(0), (5)S(1), and (1)C(4) ring conformers. These calculations include the first fully optimized MP2 calculations that have been performed for an anionic sugar system, and therefore allow an assessment of the performance of a group of DFT functionals (B3LYP, PW91PW91, and M05-2X) for treating the noncovalent dispersion and anomeric effects that are present in this system. We find that the recently developed M05-2X functional of Truhlar and co-workers [Y. Zhao, N. E. Schultz, D. G. Truhlar, J. Chem. Theory Comput., 2006, 2, 364] reproduces the MP2 results most closely, thus indicating that it may well be suitable for computational studies of larger ionic saccharides. Most importantly, the results presented indicate that it is crucial to consider the subconformers (which correspond to rearrangements of the sugar-ring side-chains) of the main ring-conformers in order to obtain a reliable overview of the potential energy surface of such systems. We find that the lowest isolated (gas-phase) conformer corresponds to a (4)C(1) chair conformer, which displays a pair of strong C(3)-OH···SO(3)(-) and OMe···SO(3)(-) electrostatic hydrogen-bonding interactions, coupled with a looser C(4)-OH···SO(3)(-) interaction. Overall, the relative energies of the subconformers appear to be straightforwardly related to the number of hydrogen-bonding interactions that each conformer displays among its pendant functional groups.

Exploring the mechanisms of H atom loss in simple azoles: Ultraviolet photolysis of pyrazole and triazole.

The photophysics of gas phase pyrazole (C(3)N(2)H(4)) and 2H-1,2,3-triazole (C(2)N(3)H(3)) molecules following excitation at wavelengths in the range 230 nm>or=lambda(phot)>or=193.3 nm has been investigated using the experimental technique of H (Rydberg) atom photofragment translational spectroscopy. The findings are compared with previous studies of pyrrole (C(4)N(1)H(5)) and imidazole (C(3)N(2)H(4)), providing a guide to H atom loss dynamics in simple N-containing heterocycles. CASPT2 theoretical methods have been employed to validate these findings. Photoexcitation of pyrazole at the longest wavelengths studied is deduced to involve pi( *)<--pi excitation, but photolysis at lambda(phot)or=193.3 nm. The N-H bond strength of pyrazole is determined as 37 680+/-40 cm(-1), significantly greater than that of the N-H bonds in pyrrole and imidazole. The correlation between the photochemistry of azoles and the number and position of nitrogen atoms within the ring framework is discussed in terms of molecular symmetry and orbital electron density. A photodissociation channel yielding H atoms with low kinetic energies is also clearly evident in both pyrazole and 2H-1,2,3-triazole. Companion studies of pyrazole-d(1) suggest that these slow H atoms arise primarily from the N-H site, following pi( *)<--pi excitation, and subsequent internal conversion and/or unintended multiphoton absorption processes.

Pi sigma* excited states in molecular photochemistry.

The last few years have seen a surge in interest (both theoretical and experimental) in the photochemistry of heteroaromatic molecules (e.g. azoles, phenols), which has served to highlight the importance of dissociative excited states formed by electron promotion to sigma* molecular orbitals. Such excited states--which, for brevity, are termed pi sigma* states in this Perspective article--may be populated by direct photo-excitation (though the transition cross-sections are intrinsically small), or indirectly, by non-adiabatic coupling from an optically 'bright' excited state (e.g. an excited state resulting from pi* <--pi excitation). The analogous pi sigma* excited states in prototypical hydride molecules like H(2)O and NH(3) have long been recognised. They have served as test-beds for developing concepts like Rydbergisation, conical intersections (CIs) between potential energy surfaces, and for investigating the ways in which non-adiabatic couplings at such CIs influence the eventual photofragmentation dynamics. This Perspective article seeks to highlight the continuity of behaviour revealed by the earlier small molecule studies and by the more recent studies of heteroaromatic systems, and to illustrate the photochemical importance of pi sigma* excited states in many broad families of molecules. Furthermore, the dynamical influence of such excited states is not restricted to closed shell species; the Article concludes with a brief consideration of the consequences of populating sigma* orbitals in free radical species, in molecular cations, and in dissociative electron attachment processes.

Ultraviolet photodissociation dynamics of 2-methyl, 3-furanthiol: tuning pi-conjugation in sulfur substituted heterocycles.

H atom loss following ultraviolet photoexcitation of 2-methyl, 3-furanthiol (2M,3FT) at many wavelengths in the range 269 nm > or = lambda(phot) > or = 210 nm and at 193 nm has been investigated by H (Rydberg) atom photofragment translational spectroscopy. The photodissociation dynamics of this SH decorated aromatic ring system are contrasted with that of thiophenol (Devine et al. J. Phys. Chem. A 2008, 112, 9563), the excited electronic states of which show a different energetic ordering. Ab initio theory and experiment find that the first excited state of 2M,3FT is formed by electron promotion from an orbital comprised of an admixture of the S lone pair and the furan pi system (n/pi) to a sigma* orbital centered on the S-H bond. Photoexcitation at long wavelengths results in population of the (1)(n/pi)sigma* excited state, prompt S-H bond fission, H atoms displaying a (nonlimiting) perpendicular recoil velocity distribution, and partner radicals formed in selected low vibrational levels of the ground state. This energy disposal can be rationalized by considering the forces acting as the excited molecules evolve on the (1)(n/pi)sigma* potential energy surface (PES). Energy conservation arguments, together with the product vibrational state analysis, yield a value of 31320 +/- 100 cm(-1) for the S-H bond strength in 2M,3FT. Excitation at shorter wavelengths (lambda(phot) < or = 230 nm) is deduced to populate one or more (diabatically bound) (1)(n/pi)pi* excited states which decay by coupling to the (1)(n/pi)sigma* PES and/or to high vibrational levels of the electronic ground state.

Comparison of the resonance-enhanced multiphoton ionization spectra of pyrrole and 2,5-dimethylpyrrole: Building toward an understanding of the electronic structure and photochemistry of porphyrins.

The photophysical properties of porphyrins have relevance for their use as light-activated drugs in cancer treatment and sensitizers in solid-state solar cells. However, the appearance of their UV-visible spectra is usually explained inadequately by qualitative molecular-orbital theories. We intend to gain a better insight into the intense absorption bands, and excited-state dynamics, that make porphyrins appropriate for both of these applications by gradually building toward an understanding of the macrocyclic structure, starting with studies of smaller pyrrolic subunits. We have recorded the (1+1) and (2+1) resonance-enhanced multiphoton ionization (REMPI) spectra of pyrrole and 2,5-dimethylpyrrole between 25 600 cm(-1) (390 nm) and 48 500 cm(-1) (206 nm). We did not observe a (1+1) REMPI signal through the optically bright (1)B(2) (pipi( *)) and (1)A(1) (pipi( *)) states in pyrrole due to ultrafast deactivation via conical intersections with the dissociative (1)A(2) (pisigma( *)) and (1)B(1) (pisigma( *)) states. However, we did observe (2+1) REMPI through Rydberg states with a dominant feature at 27 432 cm(-1) (two-photon energy, 54 864 cm(-1)) assigned to a 3d<--pi transition. In contrast, 2,5-dimethylpyrrole has a broad and structured (1+1) REMPI spectrum between 36 000 and 42 500 cm(-1) as a result of vibronic transitions to the (1)B(2) (pipi( *)) state, and it does not show the 3d<--pi Rydberg transition via (2+1) REMPI. We have complemented the experimental studies by a theoretical treatment of the excited states of both molecules using time-dependent density functional theory (TD-DFT) and accounted for the contrasting features in the spectra. TD-DFT modeled the photochemical activity of both the optically dark (1)pisigma( *) states (dissociative) and optically bright (1)pipi( *) states well, predicting the barrierless deactivation of the (1)B(2) (pipi( *)) state of pyrrole and the bound minimum of the (1)B(2) (pipi( *)) state in 2,5-dimethylpyrrole. However, the quantitative agreement between vibronic transition energies and the excited-state frequencies calculated by TD-DFT was hampered by inaccurate modeling of Rydberg orbital mixing with the valence states, caused by the lack of an asymptotic correction to the exchange-correlation functionals used.

Intramolecular amide stacking and its competition with hydrogen bonding in a small foldamer.

Attractive interactions between two carboxamide groups in a "stacked" geometry are explored under isolated molecule conditions. Infrared spectra of single conformations of a small gamma-peptide, Ac-gamma(2)-hPhe-NHMe, reveal the presence of a conformation in which the two amide planes are approximately parallel with the amide dipoles in an antialigned orientation. This stacked conformation is energetically comparable to conformations that contain an intramolecular amide-amide H-bond. Amide stacking interactions can compete with H-bonding in circumstances where the amide groups can be brought into a stacking configuration with minimal strain, opening the way for its use in the design of future foldamer structures.

Exploring the time-scales of H-atom detachment from photoexcited phenol-h(6) and phenol-d(5): statistical vs nonstatistical decay.

The prevalence of (1)pi sigma* states in the photochemistry of heteroaromatics is becoming increasingly clear from the recent literature. Photodissociation measurements have shown that following excitation of phenol molecules above the S(1)/S(2) conical intersection, H-atoms are eliminated with two distinct ranges of kinetic energy release. Those with high kinetic energy are attributed to direct dissociation while those with low kinetic energy are traditionally attributed to indirect dissociation or statistical unimolecular decay, both pathways giving electronic ground-state phenoxyl fragments. Using a combination of femtosecond pump/probe spectroscopy and velocity map ion imaging techniques, the time and energy resolved H-atom elimination in phenol-h(6) and phenol-d(5), following excitation at 200 nm has been measured. At the lowest kinetic energies, the H-atom elimination from phenol-d(5) occurs in <150 fs, in sharp contrast to what one expects from a statistical decay process. This implies that these H-atoms are formed through a direct dissociation process yielding electronically excited phenoxyl fragments.

High resolution photofragment translational spectroscopy studies of the ultraviolet photolysis of phenol-d(5).

The dissociation dynamics of gas phase phenol-d(5) molecules (C(6)D(5)OH) following excitation at numerous wavelengths in the range 275 > or = lambda(phot) > or = 193.3 nm have been investigated using the techniques of H (Rydberg) atom photofragment translational spectroscopy and resonance enhanced multiphoton ionization spectroscopy. The results are compared with those from recent studies of the fully hydrogenated and fully deuterated isotopologues (C(6)H(5)OH and C(6)D(5)OD), and various halo- and methyl-substituted phenols. Analysis of the vibrational energy disposal within the phenoxyl-d(5) dissociation products identifies three distinct O-H bond fission pathways, involving nonadiabatic coupling to dissociative states of (1)pisigma* character, following initial pi* <-- pi excitation. Dissociation at lambda(phot) > 248 nm involves internal conversion (IC) to high vibrational levels of the electronic ground ((1)pipi) state and subsequent coupling to the lowest (1)pisigma* potential energy surface (PES) via a conical intersection (CI) between the (1)pipi/(1)pisigma* PESs at extended O-H bond lengths (R(O-H)). Once lambda(phot) < or = 248 nm, dissociation proceeds directly, via a (1)pipi*/(1)pisigma* CI. Both pathways yield phenoxyl-d(5) products in selected vibrational levels of the ground (X(2)B(1)) electronic state. The detailed energy disposal within the phenoxyl-d(5)(X) products shows many parallels with that deduced from companion studies of other phenol isotopologues and various substituted phenols, but a notable isotope effect is identified, thus providing yet greater insights into the factors controlling the vibrational energy disposal in the phenoxyl products. A hitherto unobserved O-H bond fission channel yielding phenoxyl-d(5) fragments in the electronically excited B(2)A(2) state is identified at the shortest excitation wavelength (lambda(phot) = 193.3 nm) and rationalized in terms of nonadiabatic coupling to, and subsequent dissociation on, the second excited (1)pisigma* PES. Selective deuteration as in phenol-d(5) causes little reduction in the intensity of the "slower" H atom products that are observed from all phenol systems, suggesting that C-H/D bond fission makes at most a minor contribution to this feature.

High-resolution and dispersed fluorescence examination of vibronic bands of tryptamine: spectroscopic signatures for L(a)/L(b) mixing near a conical intersection.

The vibronic spectrum of tryptamine has been studied in a molecular beam up to an energy of 930 cm(-1) above the S(0)-S(1) electronic origin. Rotationally resolved electronic spectra reveal a rotation of the transition dipole moment direction from (1)L(b) to (1)L(a) beginning about 400 cm(-1) above the (1)L(b) origin. In this region, vibronic bands which appear as single bands at low resolution contain rotational structure from more than one vibronic transition. The number of these transitions closely tracks the total vibrational state density in the (1)L(b) electronic state as a function of internal energy. Dispersed fluorescence spectra show distinct spectroscopic signatures attributable to the (1)L(b) and (1)L(a) character of the mixed excited-state wave functions. The data set is used to extrapolate to a (1)L(a) origin about 400 cm(-1) above the (1)L(b) origin. DFT-MRCI calculations locate a conical intersection between these two states at about 900 cm(-1) above the L(a) origin, whose structure is located along a tuning coordinate which is close to a linear interpolation between the two excited-state geometries. Along the branching coordinate, there is no barrier from (1)L(a) to (1)L(b). A two-tier model for the vibronic coupling is proposed.

Near-UV photolysis of substituted phenols. Part II. 4-, 3- and 2-methylphenol.

The photodissociation of jet-cooled 4-, 3- and 2-methylphenol molecules has been investigated using the experimental techniques of resonance enhanced multiphoton ionisation and H (Rydberg) atom photofragment translational spectroscopy. O-H bond fission is found to occur, via a repulsive (1)pisigma state, in a manner analogous to that occurring in phenol and 4-fluorophenol. Excitation to the (1)pipi manifold results in H-atom loss either directly (via a (1)pipi/(1)pisigma conical intersection) or indirectly, following internal conversion to the ground state and subsequent coupling to the (1)pisigma state via a second conical intersection at extended O-H bond lengths. The resulting methylphenoxyl radicals are created with specific vibrational excitation, reflecting the nuclear distortions required to access the (1)pisigma potential energy surface and the geometry changes induced by subsequent H atom loss. The position of the methyl group on the benzene ring is observed to influence the product vibrational energy disposal-not least through its influence on the mode(s) that are activated as a result of coupling to the repulsive (1)pisigma state. O-H bond strengths are reported for 4-, 3- and 2-methylphenol. These are in good agreement with values derived from recent combustion calorimetry studies and serve to highlight the relative destabilisation of the radical caused by methyl substitution at the 3-position.

Quantitative (upsilon, N, Ka) product state distributions near the triplet threshold for the reaction H2CO --> H + HCO measured by Rydberg tagging and laser-induced fluorescence.

In this paper, we report quantitative product state distributions for the photolysis of H2CO --> H + HCO in the triplet threshold region, specifically for several rotational states in the 2(2)4(3) and 2(3)4(1) H2CO vibrational states that lie in this region. We have combined the strengths of two complementary techniques, laser-induced fluorescence for fine resolution and H atom Rydberg tagging for the overall distribution, to quantify the upsilon, N, and Ka distributions of the HCO photofragment formed via the singlet and triplet dissociation mechanisms. Both techniques are in quantitative agreement where they overlap and provide calibration or benchmarks that permit extension of the results beyond that possible by each technique on its own. In general agreement with previous studies, broad N and Ka distributions are attributed to reaction on the S0 surface, while narrower distributions are associated with reaction on T1. The broad N and Ka distributions are modeled well by phase space theory. The narrower N and Ka distributions are in good agreement with previous quasi-classical trajectory calculations on the T1 surface. The two techniques are combined to provide quantitative vibrational populations for each initial H2CO vibrational state. For dissociation via the 2(3)4(1) state, the average product vibrational energy (15% of E(avail)) was found to be about half of the rotational energy (30% of E(avail)), independent of the initial H2CO rotational state, irrespective of the singlet or triplet mechanism. For dissociation via the 2(2)4(3) state, the rotational excitation remained about 30% of E(avail), but the vibrational excitation was reduced.

Near-ultraviolet photodissociation of thiophenol.

H(D) Rydberg atom photofragment translational spectroscopy has been used to investigate the dynamics of H(D) atom loss C6H5SH(C6H5SD) following excitation at many wavelengths lambda phot in the range of 225-290 nm. The C6H5S cofragments are formed in both their ground (X(2)B1) and first excited ((2)B2) electronic states, in a distribution of vibrational levels that spreads and shifts to higher internal energies as lambda(phot) is reduced. Excitation at lambda(phot) > 275 nm populates levels of the first (1)pi pi* state, which decay by tunnelling to the dissociative (1)pi sigma* state potential energy surface (PES). S-H torsional motion is identified as a coupling mode facilitating population transfer at the conical intersection (CI) between the diabatic (1)pi pi* and (1)pi sigma* PESs. At shorter lambda(phot), the (1)pi sigma* state is deduced to be populated either directly or by efficient vibronic coupling from higher (1)pipi* states. Flux evolving on the (1)pi sigma* PES samples a second CI, at longer R(S-H), between the diabatic (1)pi sigma* and ground ((1)pi pi) PESs, where the electronic branching between ground and excited state C6H5S fragments is determined. The C6H5S(X(2)B1) and C6H5S((2)B2) products are deduced to be formed in levels with, respectively, a' and a'' vibrational symmetry-behavior that reflects both Franck-Condon effects (both in the initial photoexcitation step and in the subsequent in-plane forces acting during dissociation) and the effects of the out-of-plane coupling mode(s), nu11 and nu16a, at the (1)pi sigma*/(1)pi pi CI. The vibrational state assignments enabled by the high-energy resolution of the present data allow new and improved estimations of the bond dissociation energies, D0(C6H5S-H) < or = 28,030 +/- 100 cm(-1) and D0(C6H5S-D) < or = 28,610 +/- 100 cm(-1), and of the energy separation between the X(2)B1 and (2)B2 states of the C6H5S radical, T(00) = 2800 +/- 40 cm(-1). Similarities, and differences, between the measured energy disposals accompanying UV photoinduced X-H (X = S, O) bond fission in thiophenol and phenol are discussed.

Exploring nuclear motion through conical intersections in the UV photodissociation of phenols and thiophenol.

High-resolution time-of-flight measurements of H atom products from photolysis of phenol, 4-methylphenol, 4-fluorophenol, and thiophenol, at many UV wavelengths (lambda(phot)), have allowed systematic study of the influence of ring substituents and the heteroatom on the fragmentation dynamics. All dissociate by X-H (X = O, S) bond fission after excitation at their respective S(1)((1)pipi*)-S(0) origins and at all shorter wavelengths. The achieved kinetic energy resolution reveals population of selected vibrational levels of the various phenoxyl and thiophenoxyl coproducts, providing uniquely detailed insights into the fragmentation dynamics. Dissociation in all cases is deduced to involve nuclear motion on the (1)pisigma* potential energy surface (PES). The route to accessing this PES, and the subsequent dynamics, is seen to be very sensitive to lambda(phot) and substitution of the heteroatom. In the case of the phenols, dissociation after excitation at long lambda(phot) is rationalized in terms of radiationless transfer from S(1) to S(0) levels carrying sufficient O-H stretch vibrational energy to allow coupling via the conical intersection between the S(0) and (1)pisigma* PESs at longer O-H bond lengths. In contrast, H + C(6)H(5)O(X(2)B(1)) products formed after excitation at short lambda(phot) exhibit anisotropic recoil-velocity distributions, consistent with prompt dissociation induced by coupling between the photoprepared (1)pipi* excited state and the (1)pisigma* PES. The fragmentation dynamics of thiophenol at all lambda(phot) matches the latter behavior more closely, reflecting the different relative dispositions of the (1)pipi* and (1)pisigma* PESs. Additional insights are provided by the observed branching into the ground (X(2)B(1)) and first excited ((2)B(2)) states of the resulting C(6)H(5)S radicals.

State-selective photodissociation dynamics of formaldehyde: near threshold studies of the H+HCO product channel.

The laser-induced photodissociation of formaldehyde in the wavelength range 309

Near-UV photolysis of substituted phenols, I: 4-fluoro-, 4-chloro- and 4-bromophenol.

The experimental techniques of H (Rydberg) atom photofragment translational spectroscopy and resonance-enhanced multiphoton ionisation time-of-flight spectroscopy have been used to investigate the dynamics of H atom loss processes from gas phase 4-fluorophenol (4-FPhOH), 4-chlorophenol (4-ClPhOH) and 4-bromophenol (4-BrPhOH) molecules, following excitation at many wavelengths, lambda(phot), in the range between their respective S(1)-S(0) origins (284.768 nm, 287.265 nm and 287.409 nm) and 216 nm. Many of the Total Kinetic Energy Release (TKER) spectra obtained from photolysis of 4-FPhOH show structure, the analysis of which reveals striking parallels with that reported previously for photolysis of bare phenol (M. G. D. Nix, A. L. Devine, B. Cronin, R. N. Dixon and M. N. R. Ashfold, J. Chem. Phys., 2006, 125, 133318). The data demonstrates the importance of O-H bond fission, and that the resulting 4-FPhO co-fragments are formed in a select fraction of their available vibrational state density. All spectra recorded at lambda(phot)> or = 238 nm show a feature centred at TKER approximately 5500 cm(-1). These H atom fragments show no recoil anisotropy, and are rationalised in terms of initial S(1)<-- S(0) (pi* <--pi) excitation and subsequent dissociation via two successive radiationless transitions: internal conversion to ground (S(0)) state levels carrying sufficient O-H stretch vibrational energy to allow efficient transfer to (and round) the Conical Intersection (CI) between the S(0) and S(2)((1)pi sigma*) Potential Energy Surfaces (PESs) at larger R(O-H), en route to H atoms and ground state 4-FPhO products. The vibrational energy disposal in the 4-FPhO products indicates that parent mode nu(16a) promotes non-adiabatic coupling at the S(0)/S(2) CI. Spectra recorded at lambda(phot)< or = 238 nm reveal a faster (but still isotropic) distribution of recoiling H atoms, centred at TKER approximately 12 000 cm(-1), attributable to H + 4-FPhO products formed when the optically excited (1)pi pi* molecules couple directly with the (1)pi sigma* PES. Parent mode nu(16b) is identified as the dominant coupling mode at the S(1)((1)pi pi*)/S(2)((1)pi sigma*) CI, and the resulting 4-FPhO radical co-fragments display progressions in nu(18b) (the C-O in-plane wagging mode) and nu(7a) (an in-plane ring breathing mode involving significant C-O stretching motion). Analysis of all structured TKER spectra yields a C-F bond dissociation energy: D(0)(H-OC(6)H(4)F) = 29 370 +/- 50 cm(-1). The photodissociation of 4-ClPhOH shows many similarities, though the 4-ClPhO products formed together with faster H atoms at shorter wavelengths (lambda(phot)< or = 238 nm, by coupling through the S(1)/S(2) CI) show activity in an alternative ring breathing mode (nu(19a) rather than nu(7a)). Spectral analysis yields D(0)(H-OC(6)H(4)Cl) = 29 520 +/- 50 cm(-1). H atom formation via O-H bond fission is (at best) a very minor channel in the photolysis of 4-BrPhOH at all wavelengths investigated. Time-dependent density functional theory calculations suggest that this low H atom yield is because of competition from the alternative C-Br bond fission channel, and that the analogous C-Cl bond fission may be responsible for the weakness of the one photon-induced H atom signals observed when photolysing 4-ClPhOH at longer wavelengths.

Ultraviolet photolysis of adenine: dissociation via the 1pi sigma* state.

High resolution total kinetic energy release (TKER) spectra of the H atom fragments resulting from photodissociation of jet-cooled adenine molecules at 17 wavelengths in the range 280>lambda(phot)>214 nm are reported. TKER spectra obtained at lambda(phot)>233 nm display broad, isotropic profiles that peak at low TKER ( approximately 1800 cm(-1)) and are largely insensitive to the choice of excitation wavelength. The bulk of these products is attributed to unintended multiphoton dissociation processes. TKER spectra recorded at lambda(phot)

High resolution photofragment translational spectroscopy studies of the near ultraviolet photolysis of imidazole.

The fragmentation dynamics of imidazole molecules following excitation at 193.3 nm and at many wavelengths in the range of 210< or =lambda(phot)< or =240 nm have been investigated by H Rydberg atom photofragment translational spectroscopy. Long wavelength excitation within this range results in population of the 1 (1)A(")((1)pisigma(*)) excited state, but the 2 (1)A(')<--X (1)A(')(pi(*)<--pi) transition becomes the dominant absorption once lambda(phot)< or =220 nm. The measured energy disposals show parallels with those found in recent studies of the UV photolysis of pyrrole [Cronin et al., Phys Chem. Chem. Phys. 6, 5031 (2004)]. The total kinetic energy release (TKER) spectra display a "fast" feature, centred at TKER approximately 9200 cm(-1). The analysis of the structure evident in the fast feature reveals the selective population of specific in-plane stretching vibrational levels of the imidazolyl cofragment; these fragments are deduced to carry only modest amounts of rotational excitation. Comparison with calculated normal mode vibrational frequencies allows the assignment of the populated levels and a precise determination of the N-H bond strength in imidazole: D(0)=33,240+/-40 cm(-1). The observed energy disposal can be rationalized using Franck-Condon arguments, assuming that the potential energy surface (PES) for the 1 (1)A(")((1)pisigma(*)) state has a topology similar to that of the corresponding (1)pisigma(*) state of pyrrole. As in pyrrole, photoexcitation populates skeletal motions in the S(1) state (in-plane motions in the present case) that are only weakly coupled to the N-H dissociation coordinate and thus map through into the corresponding product vibrations. A second, "slow" feature is increasingly evident in TKER spectra recorded at shorter lambda(phot). This component, which exhibits no recoil anisotropy, is attributed to H atoms formed by the "statistical" decay of highly vibrationally excited ground state molecules. The form of the TKER spectra observed at short lambda(phot) is rationalized by assuming two possible decay routes for imidazole molecules excited to the 2 (1)A(')((1)pipi(*)) state. One involves fast 2 (1)A(')((1)pipi(*)) right arrow-wavy 1 (1)A(")((1)pisigma(*)) radiationless transfer and subsequent fragmentation on the 1 (1)A(')((1)pisigma(*)) PES, yielding fast H atoms (and imidazolyl cofragments)-reminiscent of behavior seen at longer excitation wavelengths where the 1 (1)A(")((1)pisigma(*)) PES is accessed directly. The second is assumed to involve radiationless transfer to the ground state, most probably by successive 2 (1)A(') right arrow-wavy 1 (1)A(") right arrow-wavy X (1)A(') couplings, mediated by conical intersections between the relevant PESs and the subsequent unimolecular decay of the resulting highly vibrationally excited ground state molecules yielding slow H atoms.