Abstract
Recent experimental evidence suggests again the existence of the metastable methane anion in plasma swarms. In order to test the reliability of the complete basis set (CBS) extrapolation scheme with augmented correlation-consistent basis sets for anionic molecules, we study the evolution of the electron affinity of methane with benchmark ab initio calculations with aug-cc basis sets up to aug-cc-pV6Z + diffuse. Geometry optimizations and vibrational analysis were done at the MP2 level. The electron affinity (EA) was calculated at the MP2 and CCSD(T) levels with and without frozen core and including the extrapolations to the CBS limit. Using the aug-cc-pVnZ basis sets it is found that two non-decreasing CCSD(T) CBS limits exist for the EA (0.29 and 0.53 eV) obtained with the n = 3, 4, 5 and n = 4, 5, 6 series, respectively. A new scheme is proposed which can be generalized for very accurate quantum chemical description of molecular anions: the standard aug-cc-pVnZ basis sets can be supplemented with extra-diffuse orbitals using a simple even-tempered scheme. This yields a reliable CBS extrapolation method to develop a (discrete approximation of a) continuum anionic state near ionization, viz., one that closely matches the energy of the corresponding neutral state. These results show that CH4 has no stable anions of 2A1 symmetry, implying that plasma swarms with anionic methane consist of metastable rather than stable methane anions.
Highlights
It is known that methane is a very stable molecule
As mentioned in the previous section, in order to calculate the electron affinity (EA) with the CCSD and CCSD(T) methods, we have used the MP2 optimized geometries of methane and its anion corresponding to a given basis set with and without the frozen core orbital
In order to test the reliability of the electron affinity with the widely used complete basis set extrapolation scheme in this complex case, geometry optimizations and vibrational calculations for methane and its anion were performed at the MP2 level with the aug-cc-pVnZ (n = 2, 3, 4, 5, 6 and 6-diffuse) basis sets
Summary
It is known that methane is a very stable molecule. It is characterized by five closed shells and its electronic configuration resembles than of the neon atom. Resembling a noble gas atom, it is natural to expect that the methane anion is unstable with respect to the neutral molecule. Sharp and Dowell [2], who used a beam technique and found that dissociative attachment in CH4 took place over the range of electron energies 8 - 13 eV, with H− and CH. Later, using a pulsed Townsend technique, Hunter et al [3] measured the density-normalized electron-attachment coefficient over the density-normalized electric field strength, E/N, in the range 52.5 - 250 Td (1 Townsend = 10−17 V cm2)
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