Origin of distinct structural symmetry of the neopentane cation in the ground electronic state compared to the methane cation

Abstract

An ab initio quantum dynamics study has been performed to explore the distinct structural symmetry of C(CH₃)₄⁺ in the ground electronic state compared to CH₄⁺. Additionally, the underlying details of the highly diffuse and complex vibronic structure of the first photoelectron band of C(CH₃)₄ have been investigated. Associated potential energy surfaces over the two-dimensional space of nuclear coordinates, subject to the T₂ ⊗ (e + t₂) Jahn–Teller effect, are established from extensive electronic structure calculations and (then) the nuclear dynamics calculations are done on them via wave packet propagation including the nonadiabatic coupling of the three electronic sheets. The theoretical results are in good agreement with experimental observations. The JT stabilization energies due to T₂ ⊗ e, T₂ ⊗ t₂ and T₂ ⊗ (e + t₂) distortions in the ²T₂ electronic manifold of C(CH₃)₄⁺ illustrate that the highest stabilization occurs through the T₂ ⊗ t₂-JT distortion (in the ground state of C(CH₃)₄⁺). However, CH₄⁺ gains such maximum stabilization due to T₂ ⊗ (e + t₂)-JT distortion. From this novel result and applying the epikernel principle, we propose that the structural evolution of C(CH₃)₄⁺ from T_d to C_3v minimum energy configuration occurs via JT active vibrations of t₂ symmetry, whereas CH₄⁺ rearranges to the C_2v structure through a combination of JT active e and t₂ bending vibrations.