Rotation of methyl radicals in a solid krypton matrix

Electron spin resonance (ESR) measurements were carried out to study the rotation of methyl radicals (CH(3)) in a solid krypton matrix at 17-31 K temperature range. The radicals were produced by dissociating methane by plasma bursts generated by a focused 193 nm excimer laser radiation during the krypton gas condensation on the substrate. The ESR spectrum exhibits only isotropic features at the temperature range examined, and the intensity ratio between the symmetric (A) and antisymmetric (E) spin state lines exhibits weaker temperature dependence than in a solid argon matrix. However, the general appearance of the methyl radical spectrum depends strongly on temperature due to the pronounced temperature dependency of the E state linewidths. The rotational energy level populations are analyzed based on the static crystal field model, pseudorotating cage model, and quantum chemical calculations for an axially symmetric, planar rotor. Crystal field strength parameter values of -140 cm(-1) in Ar and -240 cm(-1) in Kr match most closely the experimentally observed rotational energy level shifts from the gas phase value. In the alternative model, considering the lattice atom movement in a pseudorotating cage, the effective lowering of the rotational constants B and C to 80%-90% leads to similar effects.

Rotation of methyl radicals in a solid argon matrix

Electron spin resonance (ESR) measurements were carried out to study the rotation of methyl radicals (CH3) in a solid argon matrix at 14-35 K temperatures. The radicals were produced by dissociating methane by plasma bursts generated either by a focused 193 nm laser radiation or a radio frequency discharge device during the gas condensation on the substrate. The ESR spectrum exhibits axial symmetry at the lowest temperature and is ascribed to ground state molecules with symmetric total nuclear spin function I=3/2. The hyperfine anisotropy (Aparallel)-Aperpendicular) was found to be -0.01 mT, whereas that of the g value was 2.5x10(-5). The anisotropy is observed for the first time in Ar and is manifested by the splitting of the low-field transition. Elevation of temperature leads reversibly to the appearance of excited state contribution having antisymmetric I=1/2. As a function of the sample temperature, the relative intensities of symmetric and antisymmetric spin states corresponding to ground and excited rotor states, respectively, proton hyperfine and electron g-tensor components, and spin-lattice relaxation rates were determined by a numerical fitting procedure. The experimental observations were interpreted in terms of a free rotation about the C3 axis and a thermal activation of the C2-type rotations above 15 K. The ground and excited rotational state energy levels were found to be separated by 11.2 cm-1 and to exhibit significantly different spin-lattice coupling. A crystal field model has been applied to evaluate the energy levels of the hindered rotor in the matrix, and crystal field parameter varepsilon4=-200 cm-1, corresponding to a 60 cm-1 effective potential barrier for rotation of the C3 axis, was obtained.

Theoretical Study of the Complex-Forming CH + H2 → CH2 + H Reaction

The complex-forming CH + H2 --> CH2 + H reaction is studied employing a recently developed global potential energy function. The reaction probability in the total angular momentum J = 0 limit is estimated with a four-atom quantum wave packet method and compared with classical trajectory and statistical theory results. The formation of complexes from different reactant internal states is also determined with wave packet calculations. While there is no barrier to reaction along the minimum energy path, we find that there are angular constraints to complex formation. Trajectory-based estimates of the low-pressure rate constants are made and compared with experimental results. We find that zero-point energy violation in the trajectories is a particularly severe problem for this reaction.

Test of a chemical timing method for measuring absolute vibrational relaxation rate constants for S1 p-difluorobenzene

A chemical timing (CT) method for measuring absolute rate constants for collisional vibrational relaxation has been tested for the 5(1) state of S(1) p-difluorobenzene (pDFB) where an alternative method exists to provide benchmark values. The CT method was originally developed to treat vibrational energy transfer (VET) in large molecules excited to high vibrational levels where the intramolecular vibrational redistribution (IVR) resulting from large vibrational state densities completely eliminates vibrational structure in the emission spectrum. Here we apply the same method to a low-lying state (5(1) with epsilon(vib) = 818 cm(-1)) located in the low-density region of the vibrational manifold where IVR plays no role. For high vibrational levels, the chemical timing method involves addition of high O(2) pressures (kTorr) to a low-pressure pDFB sample, introducing vibrational structure in the fluorescence spectrum. Response of this spectrum to vibrational relaxation by Ar is then examined. For levels such as 5(1), the fully structured fluorescence spectrum allows the rate constant for single-collision VET into the surrounding vibrational field to be measured directly without the presence of O(2). The measurements of 5(1) VET have been repeated with various O(2) pressures (kTorr) for comparison with the O(2)-free benchmark. In the presence of O(2), the rate constant for VET by Ar is (4.0 +/- 0.5) x 10(6) Torr(-1) s(-1) and independent of high O(2) pressure variations. The rate constant as found by the standard O(2)-free method is (3.6 +/- 0.4) x 10(6) Torr(-1) s(-1). This comparison suggests that the chemical timing method is capable of providing a reasonably accurate measure of the VET rate constant for high vibrational levels provided that details of the kinetics are known.