ALBERT

Notes: We report the polarized emission spectra from photodissociating nitromethane excited at 200 and 218 nm. At both excitation wavelengths, the emission spectra show a strong progression in the NO2 symmetric stretch; at 200 nm a weak progression in the NO2 symmetric stretch in combination with one quantum in the C–N stretch also contributes to the spectra. We measure the angular distribution of emitted photons in the strong emission features from the relative intensity ratio between photons detected perpendicular to versus along the direction of the electric vector of the excitation laser. We find the anisotropy is substantially reduced from the 2:1 ratio expected for the pure CH3NO2 X(1A1)→1B2(ππ*)→X(1A1) transition with no rotation of the molecular frame. The intensity ratios for the features in the NO2 symmetric stretching progression lie near 1.5 to 1.6 for 200 nm excitation and 1.7 for 218 nm excitation. The analysis of the photon angular distribution measurements and consideration of the absorption spectrum indicate that the timescale of the dissociation is too fast for molecular rotation to contribute significantly to the observed reduction in anisotropy. The detailed analysis of our results in conjunction with electron correlation arguments and previous work on the absorption spectroscopy and final products' velocities results in a model which includes two dissociation pathways for nitromethane, an electronic predissociation pathway and a vibrational predissociation pathway along the 1B2(ππ*) surface. Our analysis suggests a reassignment of the minor dissociation channel, first evidenced in photofragment velocity analysis experiments which detected a pathway producing slow CH3 fragments, to the near threshold dissociation channel CH3 + NO2(2 2B2).

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 work measures the change in branching between the CF3+I(2P3/2) and I(2P1/2) product channels when one photodissociates vibrationally excited rather than cold CF3I at 248.5 nm. The experiment tests a model for the dependence of branching at a conical intersection on the amplitude of the dissociative wave function at bent geometries, a model which we propose here to explain previously observed differences in branching between the I(2P1/2) and I(2P3/2) channels at 248 nm for CH3I versus CD3I. In the CF3I experiment, we observe an increase in the branching from 13% to 17% I(2P3/2) products when the temperature of the CF3I parent is increased from 100 to 400 °C, in agreement with the qualitative prediction of the model. We analyze the angular distributions of the photofragments to eliminate the possibility that the change in branching is due to an increased contribution from direct absorption to the electronic state correlating with I(2P3/2) products.

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: This work measures the change in branching between the CN+I(2P3/2) and the CN+I(2P1/2) product channels when one photodissociates vibrationally excited rather than cold ICN at 248.5 nm. The crossed-laser molecular beam experiment tests a model for the dependence of branching at a conical intersection on the amplitude of the dissociative wave function at bent geometries. The results provide a critical comparison between the long-standing empirical surfaces for ICN photodissociation and more recent ab initio potential energy surfaces for ICN's first absorption band. In the experiment, we observe an increase in branching from 49% to 58% I(2P3/2) products when the temperature of the ICN parent photodissociated is increased in going from a 100 °C vs a 400 °C nozzle expansion. We analyze the angular distributions of the photofragments to eliminate the possibility that the change in branching is due to an increased contribution from direct absorption to the electronic state correlating with I(2P3/2) products. We discuss the relationship between these studies, which mediate the effective off-diagonal potential coupling by forcing the nuclear dynamics to sample more strongly coupled regions of the intersecting potential surfaces, with intramolecular electron transfer studies which examine how the coupling depends on the particular conformer.

Notes: This work investigates how molecular dissociation induced by local 1[n(O),π*(C=O)] electronic excitation at a carbonyl functional group can result in preferential fission of an alpha bond over a weaker bond beta to the functional group and how nonadiabaticity in the dynamics drives the selectivity. The experiment measures the photofragment velocity and angular distributions from the photodissociation of acetyl chloride and bromoacetyl chloride at 248 nm, identifying the branching between bond fission channels and the mechanism for the selectivity. The anisotropic angular distributions measured shows dissociation occurs on a time scale of less than a rotational period, resulting in primary C–X (X=Cl, Br) bond fission, but no significant C–C bond fission. While the selective fission of the C–Cl over the C–C alpha bond can be predicted from the adiabatic correlation diagram for this special class of Norrish type I cleavage, the preferential fission of the C–Cl alpha bond over the C–Br bond beta to the carbonyl group would not be predicted on the adiabatic potential energy surface. In bromoacetyl chloride, fission of the C–Cl and C–Br bonds occurs with a branching of 1.0:1.1 (approximately 1.0:0.5 from the 1nπ* transition) compared with a predicted statistical branching ratio of 1:30. This preferential α-bond fission is attributed to a dissociation mechanism on the coupled [n,π*(C=O)] and [n(X),σ *(C–X)] electronic states, a model consistent with the lack of C–C fission and the measured kinetic energy and angular distributions.The selectivity results from the relative strengths of the electronic coupling between the initially excited [n,π*(C=O)] bound configuration and the two [n(X),σ *(C–X)] states, the weaker coupling inhibiting the adiabatic crossing over the barrier to C–Br bond fission. The results demonstrate the need to go beyond the Born–Oppenheimer approximation to gain predictive ability in any reactive system where the electronic configuration changes along the reaction coordinate, particularly at barriers due to configuration crossings. In addition, the Cl product angular distribution determines the orientation of the 1[n(O),π*(C=O)] transition dipole moment and shows it is governed by the C2v symmetry of the localized carbonyl electronic orbitals and not by the asymmetric substitution at the carbonyl group. Spectra of the Br atoms from direct dissociation at 193 nm help separate the contribution from the overlapping nσ *(C–Br) transition at 248 nm.

Notes: We investigate the origin of the observed fission of the stronger S–H bond over the weaker C–S bond in CH3SH excited at 193 nm using the complementary techniques of mass-resolved photofragment time-of-flight spectroscopy and emission spectroscopy. The velocities and angular distributions of the CH3S and SH photofragments show that both C–S and S–H bond fission occur on a subpicosecond timescale and impart considerable energy to relative product translation. The dispersed emission from photoexcited CH3SH molecules in a flow cell evidences a progression in the CH3 umbrella mode and combination bands with one quantum in the C–S stretch, but no progression with S–H stretch. Examination of the results with reference to previous ab initio calculations of the excited state surfaces reveals the importance of nonadiabatic coupling in the dissociation dynamics. This is a clear example of selective bond fission upon excitation of an electronic state that is not repulsive in the bond that breaks. We discuss the implication of the work with respect to using the Born–Oppenheimer approximation in reactive collisions near a saddle point along the reaction coordinate.