Photoelectron spectroscopic correlation of the molecular orbitals of methane, ethane, propane, isobutane and neopentane

Abstract

Energy level trends across the series CH_4–n(CH₃)_n have been studied by He(I) PE spectroscopy and are discussed in terms of ab-initio SCF calculations and an “equivalent orbital” treatment. The first band in the spectra of isobutane and neopentane is split by Jahn-Teller forces into two and three components respectively, implying that these bands originate from MOs localized mainly in the CC bonds. From this the symmetry of the highest occupied MO in propane (4b₁, CC) and ethane (3a_1g, CC) is deduced, an assignment which in the latter case differs from that of other workers. Formally degenerate pseudo-π CH orbitals of the methyl groups are split by “through space” interactions which amount to 2.7 eV in the case of neopentane. Earlier attempts to interpret the PE spectra of linear and branched alkanes are considered to be inadequate because this latter effect was not taken into full account.