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Notes: These experiments investigate the competition between C–C and C–Br bond fission in bromoacetone excited in the 1[n(O),π*(C=O)] absorption, elucidating the role of molecular conformation in influencing the probability of adiabatically traversing the conical intersection along the C–C fission reaction coordinate. In the first part of the paper, measurement of the photofragment velocity and angular distributions with a crossed laser-molecular beam time-of-flight technique identifies the primary photofragmentation channels at 308 nm. The time-of-flight spectra evidence two dissociation channels, C–Br fission and fission of one of the two C–C bonds, BrH2C–COCH3. The distribution of relative kinetic energies imparted to the C–Br fission and C–C fission fragments show dissociation is not occurring via internal conversion to the ground electronic state and allow us to identify these channels in the closely related systems of bromoacetyl- and bromopropionyl chloride.In the second part of the work we focus on the marked conformation dependence to the branching between C–C fission and C–Br fission. Photofragment angular distribution measurements show that C–Br fission occurs primarily from the minor, anti, conformer, giving a β of 0.8, so C–C fission must dominate the competition in the gauche conformer. Noting that the dynamics of these two bond fission pathways are expected to be strongly influenced by nonadiabatic recrossing of the reaction barriers, we investigate the possible mechanisms for the conformation dependence of the nonadiabatic recrossing with low-level ab initio electronic structure calculations on the C–Br reaction coordinate and qualitative consideration of the conical intersection along the C–C reaction coordinate. The resulting model proposes that C–C bond fission cannot compete with C–Br fission in the anti conformer because the dissociation samples regions of the phase space near the conical intersection along the C–C fission reaction coordinate, where nonadiabaticity inhibits C–C fission, while from the gauche conformer C–C fission can proceed more adiabatically and dominate C–Br fission. A final experiment confirms that the branching ratio changes with the relative conformer populations in accord with this model.

Notes: Previous crossed laser-molecular beam experiments on bromoacetyl chloride investigated the influence of electronic nonadiabaticity on the branching between C–Cl and C–Br fission upon 1[n(O),π*(C=O)] excitation at 248 nm. This note identifies an additional contribution to the data from a minor primary C–C fission channel. Using the distribution of kinetic energies imparted to C–C fission products in bromoacetone following 1[n(O),π*(C=O)] excitation as a characteristic one, we reanalyze the Br+ and Cl+ time-of-flight spectra from bromoacetyl chloride at 248 nm to identify any contribution from C–C bond fission. We find that a small portion of the Br+ signal results from CH2Br fragments from C–C fission. Although this in no way alters the major conclusions from this previous work, it does slightly alter the P(ET) derived for C–Br fission and the C–Cl:C–Br branching ratio from bromoacetyl chloride.

Notes: This study uses emission spectroscopy of H2S at excitation energies near 200 nm to probe the dissociation dynamics from a conical intersection in the Franck–Condon region to the H+SH product exit channel. Photoexcitation accesses these coupled surfaces near the transition state region of the lower adiabat, a potential surface for the excited state H+SH→HS+H reaction. Excitation wavelengths from 199–203 nm tune through the first of the resonances in the absorption spectrum assigned to recurrences in the motion along the symmetric stretch orthogonal to the reaction coordinate and also access energies just above and at the conical intersection. We disperse the emission from the dissociating molecules at each of five excitation wavelengths in this region to probe several features of the reaction dynamics on the coupled potential energy surfaces. The resulting emission spectra cover the range of final vibrational eigenstates from 500 to 11 000 cm−1 above the initial ground vibrational state for all five excitation wavelengths, and go out to 16 500 cm−1 for the 199 and 201 nm excitation wavelengths. The resulting spectra, when considered in conjunction with recent scattering calculations by Heumann and Schinke on ab initio potential energy surfaces for this system, evidence a progression of emission features to low vibrational eigenstates in the SH stretch that result from coupling of the nuclear motion from the bound to the dissociative region of the potential energy surfaces.This emission, into local mode eigenstates such as 00+1, 11+0, 11+1, 21+0, 21+1, evidences the antisymmetric dissociative motion and bending induced near the conical intersection, and dominates the spectrum at excitation wavelengths only near 200 nm. We analyze the excitation wavelength dependence of these features and also of the n0+0 progression for n≥4, which reflect the exit channel dynamics. The excitation wavelength dependence shows that while the emission spectra do not reveal any dynamics unique to scattering states that access a symmetric stretch resonance in the Franck–Condon region, they do reveal the energy location of and the dynamics at the conical intersection. A reanalysis of other workers' measurements of the SH product vibrational state distribution shows that v=0 products are strongly favored at excitation wavelengths near the conical intersection.

Notes: These experiments on bromopropionyl chloride investigate a system in which the barrier to C–Br fission on the lowest 1A‘ potential energy surface is formed from a weakly avoided electronic configuration crossing, so that nonadiabatic recrossing of the barrier to C–Br fission dramatically reduces the branching to C–Br fission. The results, when compared with earlier branching ratio measurements on bromoacetyl chloride, show that the additional intervening CH2 spacer in bromopropionyl chloride reduces the splitting between the adiabatic potential energy surfaces at the barrier to C–Br fission, further suppressing C–Br fission by over an order of magnitude. The experiment measures the photofragment velocity and angular distributions from the 248 nm photodissociation of Br(CH2)2COCl, determining the branching ratio between the competing primary C–Br and C–Cl fission pathways and detecting a minor C–C bond fission pathway. While the primary C–Cl:C–Br fission branching ratio is 1:2, the distribution of relative kinetic energies imparted to the C–Br fission fragments show that essentially no C–Br fission results from promoting the molecule to the lowest 1A‘ potential energy surface via the 1[n(O),π*(C=O)] transition; C–Br fission only results from an overlapping electronic transition. The results differ markedly from the predictions of statistical transition state theories which rely on the Born–Oppenheimer approximation.While such models predict that, given comparable preexponential factors, the reaction pathway with the lowest energetic barrier on the 1A‘ surface, C–Br fission, should dominate, the experimental measurements show C–Cl bond fission dominates by a ratio of C–Cl:C–Br=1.0:〈0.05 upon excitation of the 1[n(O),π*(C=O)] transition. We compare this result to earlier work on bromoacetyl chloride, which evidences a less dramatic reduction in the C–Br fission pathway (C–Cl:C–Br=1.0:0.4) upon excitation of the same transition. We discuss a model in which increasing the distance between the C–Br and C=O chromophores decreases the electronic configuration interaction matrix elements which mix and split the 1n(O)π*(C=O) and np(Br)σ*(C–Br) configurations at the barrier to C–Br bond fission in bromopropionyl chloride. The smaller splitting between the adiabats at the barrier to C–Br fission increases the probability of nonadiabatic recrossing of the barrier, nearly completely suppressing C–Br bond fission in bromopropionyl chloride. Preliminary ab initio calculations of the adiabatic barrier heights and the electronic configuration interaction matrix elements which split the adiabats at the barrier to C–Br and C–Cl fission in both bromopropionyl chloride and bromoacetyl chloride support the interpretation of the experimental results. We end by identifying a class of reactions, those allowed by overall electronic symmetry but Woodward–Hoffmann forbidden, in which nonadiabatic recrossing of the reaction barrier should markedly reduce the rate constant, both for ground state and excited state surfaces.

Notes: These experiments use molecular photodissociation of CH3SH to probe the dynamics and the influence of nonadiabatic coupling in the transition state region of the CH3+SH→CH3S+H reaction. Photoexcitation at 222 and 248 nm in the first of two absorption bands accesses the lower of the two coupled potential energy surfaces near the saddle point of the excited state reaction coordinate. Measurement of the resulting photofragments' velocities and angular distributions determine the branching between the CH3+SH and the CH3S+H exit channels. At all wavelengths within the first absorption band, we observe preferential fission of the stronger S–H bond over the weaker C–S bond. Fission of the C–S bond occurs only to a small degree at 222 nm and is not observable at 248 nm. Comparison with our earlier data at 193 nm, corresponding to excitation to the upper bound adiabat which is nonadiabatically coupled to the lower dissociative surface reached at 222 nm, shows that the branching ratio between C–S bond fission and S–H bond fission is a factor of eight larger at 193 nm.To probe the forces in the Franck–Condon region, we also measure the photoemission spectrum from dissociating CH3SH excited at 222 nm and compare it to the previous measurement at 193 nm. The 222 nm spectrum evidences emission into the S–H stretch and methyl stretch vibrations but not into C–S stretching modes, consistent with the dominance of S–H fission on the lower adiabat, while the 193 nm emission spectrum, reassigned here, has only a progression in the C–S stretch. The comparison of the spectra suggests a model in which stretching along the C–S coordinate on the bound upper state occurs as the amplitude couples nonadiabatically to the lower dissociative surface, allowing the molecule to access the region near the saddle point on the lower surface at extended C–S bond lengths. This results in better overlap with the C–S fission exit channel and thus an increased branching to C–S bond fission over that observed upon direct excitation to the lower dissociative surface at 222 nm. To further advance the experimental conclusions, we present collaborative calculations of the potential energy surfaces using the effective valence-shell Hamiltonian method developed by Freed and co-workers.