Charlotte E. Hinkle

Characterizing Excited States of CH5+ with Diffusion Monte Carlo

The spectroscopy and dynamics of protonated methane have been of long-standing interest due to the unusual and highly fluxional behavior of CH5+. This reflects the fact that the ground-state wave function for CH5+ has nearly equal amplitude at the 120 equivalent minima and at the saddle points that connect these minima. While low-resolution spectra of CH5+ have been assigned, the nature of the couplings between the CH stretches and the low-frequency modes is not as well characterized. An understanding of this will be important in the interpretation of rotationally resolved spectra. In this work, fixed-node diffusion Monte Carlo techniques are used to calculate energies and probability amplitudes for several excited states. The calculated energies are shown to be in good agreement with previously reported vibrational configuration interactions calculations. Analysis of the 12-dimensional probability amplitudes shows that there are strong couplings between the high-frequency CH stretch and HCH bend motions and the low-frequency modes that lead to isomerization CH5+.

Isotopic effects on the dynamics of the CH3(+) + H2 → CH5(+) → CH3(+) + H2 reaction.

Diffusion Monte Carlo is used to investigate the anharmonic zero-point energy corrected energies for the CH(3)(+) + H(2)→ CH(5)(+) → CH(3)(+) + H(2) process as a function of the center of mass separation of the two fragments. In addition to the title reaction, all possible deuterated and several tritiated (CH(4)T(+) and CH(3)T(2)(+)) analogues of this reaction are investigated. As anticipated, the replacement of one or more of the hydrogen atoms with deuterium or tritium atoms lowers the zero-point energy of the system. Further, in the partially deuterated or tritiated isotopologues, the lowest energy configuration generally has the heavy atoms in the CH(3)(+) fragment. Analysis of the wave functions allows us to study how zero-point energy influences the approach geometries sampled during low-energy collisions between CH(3)(+) and H(2), and to gain insights into how the dynamics is affected by the substitution of heavier isotopes for one or more of the hydrogen atoms. Differences between quantum and classical descriptions of the title reaction are also discussed.

Theoretical Investigations of Mode Mixing in Vibrationally Excited States of CH5

Using diffusion Monte Carlo, five vibrational excited states of CH(5)(+) and CD(5)(+) are evaluated and analyzed. Here, we focus on the fundamentals in the five modes that are generated by requiring that the wave functions change sign at specified values of the five symmetry-adapted linear combinations (SALCs) of the CH or CD bond lengths. Even though the definitions of these modes are based on displacements of the CH or CD bond lengths, the frequencies are found to be low compared to previously calculated CH vibrational frequencies in this molecule. The totally symmetric mode, with A(1)(+) symmetry, has a calculated frequency of 2164 and 1551 cm(-1) for CH(5)(+) and CD(5)(+). The frequencies of the four fundamentals that arise from excitation of the four SALCs that transform under G(1)(+) symmetry have frequencies that range from 1039 to 1383 cm(-1) and from 628 to 893 cm(-1) in CH(5)(+) and CD(5)(+), respectively. The origins of the broken degeneracy are investigated and explained to reflect extensive coupling to the two low-frequency modes that lead to isomerization of CH(5)(+).