CH₅⁺:  Chemistry's Chameleon Unmasked

Abstract

The nuclear vibrational wave function and zero-point vibrational energy of CH5(+) are calculated using quantum diffusion Monte Carlo techniques on an interpolated potential energy surface constructed from CCSD(T)/aug'-cc-pVTZ ab initio data. From this multidimensional wave function, the vibrationally averaged rotational constants and radial distribution functions for atom-atom distances within the molecule are constructed. It is found that the distributions of all 10 H-H distances are bimodal and identical. The radial distribution functions obtained for the five C-H distances are also identical, but unimodal. The three rotational constants were found to be 3.78, 3.80, and 3.83 cm(-1). These values indicate that the ground state of CH5(+) is significantly more symmetric than its global minimum energy structure. We conclude that the zero-point motion of CH5(+) renders all five protons equivalent in the ground state and precludes the assignment of a unique structure to the molecule.

Evolution of Structure in CH5+ and Its Deuterated Analogues

“…If we compare the values, obtained here with those reported by Thompson et al 10 we find that the present values are larger by 0.06-0.08 cm -1 . This would reflect somewhat smaller a These equilibrium rotational constants differ from those that were obtained directly from a minimization at the CCSD(T)/aug-cc-pVTZ level of electronic structure theory/basis setby less than 0.001 cm -1 .…”

“…A third potential surface was developed independently by Thompson, Crittenden, and Jordan. 10 This surface is based on electronic energies and Hessians, evaluated at the CCSD(T)/aug′-cc-pVTZ level of theory/basis using GAUSSIAN 98. In contrast to the fits of Bowman and co-workers, this potential is constructed using the Shephard interpolation, within the GROW 2.2 package of Collins and coworkers.…”

“…9 While this approach works for these cases, the partially deuterated species are not expected to be spherical tops. Following the work of Jordan and co-workers, 10 we employ an Eckart embedding of the body-fixed axis system, as described by Louck and Galbraith. 29 As this algorithm is based on the separation of rotational and vibrational motions of the system as it moves away from a reference geometry, it is not clear that this should be an appropriate choice for a species as fluxional as CH 5 + .…”

Ionization of Methane Clusters in Helium Nanodroplets

“…In the case of CH 5 ions, the calculations provide clear evidence for a CH 5 + unit. As demonstrated in several theoretical studies, [21][22][23][24][25][26] and also confirmed by the interpretation of infrared spectra, [27][28][29][30][31][32] CH 5 + is best viewed as an H 2 + moiety bound to CH 3 by a threecenter bond, although the structure is thought to be highly fluxional. In line with previous calculations on CH 5 + (CH 4 ) nÀ1 clusters, [33] this description of the CH 5 + unit is maintained when methane molecules are added.…”

Ultra-sensitive high-precision spectroscopy of a fast molecular ion beam

“…The limitations of current techniques are highlighted by the case of CH + 5 , a molecular ion of considerable interest due to its lack of a classical structure 10 and as the prototypical non-classical carbocation. 11 The only published highresolution spectrum of CH + 5 was recorded by Oka and coworkers 12 in a liquid nitrogen cooled discharge of H 2 and CH 4 , and was identified by a process of elimination, by removing the known spectral lines of H + 3 , CH + 3 , C 2 H + 3 , HCO + , HCNH + , the Rydberg spectrum of H 2 , and strong lines of CH 4 that remained due to a slight asymmetry in the AC plasma.…”