Rebecca A. Rose

Vibrationally quantum-state-specific reaction dynamics of H atom abstraction by CN radical in solution

Molecular vibrations in a solution-phase reaction are detected at a level of detail rivaling that of gas-phase studies. Solvent collisions can often mask initial disposition of energy to the products of solution-phase chemical reactions. Here, we show with transient infrared absorption spectra obtained with picosecond time resolution that the nascent HCN products of reaction of CN radicals with cyclohexane in chlorinated organic solvents exhibit preferential excitation of one quantum of the C-H stretching mode and up to two quanta of the bending mode. On time scales of approximately 100 to 300 picoseconds, the HCN products undergo relaxation to the vibrational ground state by coupling to the solvent bath. Comparison with reactions of CN radicals with alkanes in the gas phase, known to produce HCN with greater C-H stretch and bending mode excitation (up to two and approximately six quanta, respectively), indicates partial damping of the nascent product vibrational motion by the solvent. The transient infrared spectra therefore probe solvent-induced modifications to the reaction free energy surface and chemical dynamics.

Chemical Reaction Dynamics in Liquid Solutions

The dynamics of bimolecular chemical reactions can be examined in liquid solutions using infrared absorption spectroscopy with picosecond time resolution. On such short time scales, the transient absorption spectra reveal vibrational mode and quantum-state-specific energy disposal, followed by vibrational relaxation as the energy is dissipated to the surrounding solvent. Comparison with energy disposal measurements for gas-phase reactions under single-collision conditions offers direct insights into the modification of the energy landscape and the nuclear dynamics in the presence of the solvent. The reactions of CN radicals with organic molecules in chlorinated solvents exemplify the dynamical information that can be obtained. The potential to extend such experiments to a range of reactions and solvents is discussed.

Reaction dynamics of CN radicals with tetrahydrofuran in liquid solutions

Transient, broadband infra-red absorption spectroscopy with picosecond time resolution has been used to study the dynamics of reactions of CN radicals with tetrahydrofuran (THF) and d(8)-THF in liquid solutions ranging from neat THF to 0.5 M THF in chlorinated solvents (CDCl(3) and CD(2)Cl(2)). HCN and DCN products were monitored via their v(1) (C≡N stretching) and v(3) (C-H(D) stretching) vibrational absorption bands. Transient spectral features indicate formation of vibrationally excited HCN and DCN, and the onsets of absorption via the fundamental bands of HCN and DCN show short (5-15 ps) delays consistent with vibrational relaxation within the nascent reaction products. This interpretation is confirmed by non-equilibrium molecular dynamics simulations employing a newly derived analytic potential energy surface for the reaction in explicit THF solvent. The rate coefficient for reactive formation of HCN (as determined from measurements on both the 1(1)(0) and 3(1)(0) fundamental bands) decreases with increasing dilution of the THF in CDCl(3) or CD(2)Cl(2), showing pseudo-first order kinetic behaviour for THF concentrations in the range 0.5-4.5 M, and a bimolecular rate coefficient of (1.57 ± 0.12) × 10(10) M(-1) s(-1) is derived. Simultaneous analysis of time-dependent HCN 1(1)(0) and 3(1)(0) band intensities following reaction of CN with THF (3.0 M) in CD(2)Cl(2) suggests that C-H stretching mode excitation is favoured, and this deduction is supported by the computer simulations. The results extend our recent demonstration of nascent vibrational excitation of the products of bimolecular reactions in liquid solution to a different, and more strongly interacting class of organic solvents. They serve to reinforce the finding that dynamics (and thus the topology of the reactive potential energy surface) play an important role in determining the nascent product state distributions in condensed phase reactions.

Vibrationally quantum-state-specific dynamics of the reactions of CN radicals with organic molecules in solution

The dynamics of reactions of CN radicals with cyclohexane, d(12)-cyclohexane, and tetramethylsilane have been studied in solutions of chloroform, dichloromethane, and the deuterated variants of these solvents using ultraviolet photolysis of ICN to initiate a reaction. The H(D)-atom abstraction reactions produce HCN (DCN) that is probed in absorption with sub-picosecond time resolution using ∼500 cm(-1) bandwidth infrared (IR) pulses in the spectral regions corresponding to C-H (or C-D) and C≡N stretching mode fundamental and hot bands. Equivalent IR spectra were obtained for the reactions of CN radicals with the pure solvents. In all cases, the reaction products are formed at early times with a strong propensity for vibrational excitation of the C-H (or C-D) stretching (v(3)) and H-C-N (D-C-N) bending (v(2)) modes, and for DCN products there is also evidence of vibrational excitation of the v(1) mode, which involves stretching of the C≡N bond. The vibrationally excited products relax to the ground vibrational level of HCN (DCN) with time constants of ∼130-270 ps (depending on molecule and solvent), and the majority of the HCN (DCN) in this ground level is formed by vibrational relaxation, instead of directly from the chemical reaction. The time-dependence of reactive production of HCN (DCN) and vibrational relaxation is analysed using a vibrationally quantum-state specific kinetic model. The experimental outcomes are indicative of dynamics of exothermic reactions over an energy surface with an early transition state. Although the presence of the chlorinated solvent may reduce the extent of vibrational excitation of the nascent products, the early-time chemical reaction dynamics in these liquid solvents are deduced to be very similar to those for isolated collisions in the gas phase. The transient IR spectra show additional spectroscopic absorption features centered at 2037 cm(-1) and 2065 cm(-1) (in CHCl(3)) that are assigned, respectively, to CN-solvent complexes and recombination of I atoms with CN radicals to form INC molecules. These products build up rapidly, with respective time constants of 8-26 and 11-22 ps. A further, slower rise in the INC absorption signal (with time constant >500 ps) is attributed to diffusive recombination after escape from the initial solvent cage and accounts for more than 2/3 of the observed INC.

Quasi-classical trajectory study of the dynamics of the Cl + CH4 → HCl + CH3 reaction

We present an on-the-fly classical trajectory study of the Cl + CH(4)→ HCl + CH(3) reaction using a specific reaction parameter (SRP) AM1 Hamiltonian that was previously optimized for the Cl + ethane reaction [S. J. Greaves et al., J. Phys Chem A, 2008, 112, 9387]. The SRP-AM1 Hamiltonian is shown to be a good model for the potential energy surface of the title reaction. Calculated differential cross sections, obtained from trajectories propagated with the SRP-AM1 Hamiltonian compare favourably with experimental results for this system. Analysis of the vibrational modes of the methyl radical shows different scattering distributions for ground and vibrationally excited products.

Velocity map imaging the dynamics of the reactions of Cl atoms with neopentane and tetramethylsilane

The reactions of ground state Cl((2)P(3/2)) atoms with neopentane and tetramethylsilane have been studied at collision energies of 7.9+/-2.0 and 8.2+/-2.0 kcal mol(-1), respectively. The nascent HCl(v=0,J) products were probed using resonance enhanced multiphoton ionization spectroscopy combined with velocity map imaging (VMI) to determine the rotational level population distributions, differential cross sections (DCSs), and product translational energy distributions. The outcomes from PHOTOLOC and dual beam methods are compared and are discussed in light of previous studies of the reactions of Cl atoms with other saturated hydrocarbons, including a recent crossed molecular beam and VMI investigation of the reaction of Cl atoms with neopentane [Estillore et al., J. Chem. Phys. 132, 164313 (2010)]. Rotational distributions were observed to be cold, consistent with the reactions proceeding via a transition state with a collinear Cl-H-C moiety. The DCSs for both reactions are forward peaked but show scatter across a broad angular range. Interpretation using a model based on linear dependence of scattering angle on impact parameter indicates that the probability of reaction is approximately constant across all allowed impact parameters. Product translational energy distributions from dual beam experiments have mean values, expressed as fractions of the total available energy, of 0.67 (Cl+neopentane) and 0.64 (Cl+tetramethylsilane) that are consistent with a kinematic model for the reaction in which the translational energy of the reactants is conserved into product translational energy.

Adiabatic and nonadiabatic dynamics in the CH3(CD3)+HCl reaction

The scattering dynamics leading to the formation of Cl (2P(3/2)) and Cl* (2P(1/2)) products of the CH(3)+HCl reaction (at a mean collision energy =22.3 kcal mol(-1)) and the Cl (2P(3/2)) products of the CD(3)+HCl reaction (at =19.4 kcal mol(-1)) have been investigated by using photodissociation of CH(3)I and CD(3)I as sources of translationally hot methyl radicals and velocity map imaging of the Cl atom products. Image analysis with a Legendre moment fitting procedure demonstrates that, in all three reactions, the Cl/Cl* products are mostly forward scattered with respect to the HCl in the center-of-mass (c.m.) frame but with a backward scattered component. The distributions of the fraction of the available energy released as translation peak at f(t)=0.31-0.33 for all the reactions, with average values that lie in the range =0.42-0.47. The detailed analysis indicates the importance of collision energy in facilitating the nonadiabatic transitions that lead to Cl* production. The similarities between the c.m.-frame scattering and kinetic energy release distributions for Cl and Cl* channels suggest that the nonadiabatic transitions to a low-lying excited potential energy surface (PES) correlating to Cl* products occur after passage through the transition state region on the ground-state PES. Branching fractions for Cl* are determined to be 0.14+/-0.02 for the CH(3)+HCl reaction and 0.20+/-0.03 for the CD(3)+HCl reaction. The difference cannot be accounted for by changes in collision energy, mass effects, or vibrational excitation of the photolytically generated methyl radical reagents and instead suggests that the low-frequency bending modes of the CD(3)H or CH(4) coproduct are important mediators of the nonadiabatic couplings occurring in this reaction system.

Velocity map imaging of the dynamics of the CH3 + HCl → CH4 + Cl reaction using a dual molecular beam method

The reactions CH3 + HCl → CH4 + Cl(2P3/2) and CD3 + HCl → CD3H + Cl(2P3/2) have been studied by photo-initiation (by CH3I or CD3I photolysis at 266 nm) in a dual molecular beam apparatus. Product Cl(2P3/2) atoms were detected using resonance enhanced multi-photon ionisation and velocity map imaging, revealing product translational energy and angular scattering distributions in the centre-of-mass frame. Image analysis is complicated by the bimodal speed distribution of CH3 (and CD3) radicals formed in coincidence with I(2P3/2) and I(2P1/2) atoms from CH3I (CD3I) photodissociation, giving overlapping Newton diagrams with displaced centre of mass velocities. The relative reactivities to form Cl atoms are greater for the slower CH3 speed group than the faster group by factors of ∼1.5 for the reaction of CH3 and ∼2.5 for the reaction of CD3, consistent with the greater propensity of the faster methyl radicals to undergo electronically adiabatic reactions to form Cl(2P1/2). The average fraction of the available energy becoming product translational energy is ⟨ft ⟩ = 0.48 ± 0.05 and 0.50 ± 0.03 for reaction of the faster and slower sets of CH3 radicals, respectively. The Cl atoms are deduced to be preferentially forward scattered with respect to the HCl reagents, but the angular distributions from the dual beam imaging experiments require correction for under-detection of forward scattered Cl products. A correction function is deduced from separate measurements on the Cl + C2H6 reaction, for which the outcomes can be compared with published differential cross-sections from crossed molecular beam experiments. Monte Carlo simulations of the dual beam experimental method suggest that the source of the depletion is secondary collisions of the slowest moving reaction products (in the laboratory frame) with unreacted reagents or carrier gas in one of the molecular beams.

Imaging the nonadiabatic dynamics of the CH3 + HCl reaction

LAB-frame velocity distributions of Cl-atoms produced in the photoinitiated reaction of CH(3) radicals with HCl have been measured for both the ground Cl ((2)P(3/2)) and excited Cl* ((2)P(1/2)) spin-orbit states using a DC slice velocity-map ion imaging technique. The similarity of these distributions, as well as the average internal excitation of methane co-products for both Cl and Cl* pathways, suggest that all the reactive flux proceeds through the same transition state on the ground potential energy surface (PES) and that the couplings which promote nonadiabatic transitions to the excited PES correlating to Cl* occur later in the exit channel, beyond the TS region. The nature of these couplings is discussed in light of initial vibrational excitation of CH(3) radicals as well as previously reported nonadiabatic reactivity in other polyatomic molecule reactions. Furthermore, the scattering of the reaction products, derived using the photoloc method, suggests that at the high collision energy of our experiment (E(coll) = 22.3 kcal mol(-1)), large impact parameter collisions are favoured with a reduced kinematic constraint on the internal excitation of the methane co-product.