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

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 X[combining tilde](2)T2 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 Td to C3v minimum energy configuration occurs via JT active vibrations of t2 symmetry, whereas CH4(+) rearranges to the C2v structure through a combination of JT active e and t2 bending vibrations.

Size effects on cation heats of formation. III. Methyl and ethyl substitutions in group IV XH₄, X = C, Si, Ge, Sn, Pb

The heats of formation of alkyl-substituted XH4 (X = C, Si, Ge, Sn, Pb) ΔfH(M(+)) = IEad(M) + ΔfH (M) molecular ions have often never been measured or are not known to better than 10 or 20 kJ/mol. The present study on these Main Group IV compounds adopts a graphical approach that relates cation heats of formation to cation size. A study of methyl substitution in XH4 is followed by a similar examination of heat of formation data on ethyl substitution in methane, silane, germane, stannane, and plumbane. The results provide tests of the validity of this graphical method as well as its use as a predictive tool for determining cation and neutral heats of formation. Suggestions are made of the need to investigate or reinvestigate the ionization energies and/or the heats of formation of several of the molecules studied. Results and conclusions are tabulated.