Knowledge Bank

Sigma bond electronic transitions are readily predicted by N-V theory, yet actual assignments have, in many cases, not been forthcoming. Electronic absorption spectra of methane and ethane, as well as of some higher homologs, have been recorded to $95,000cm-1$. The methane spectrum can be regarded as consisting of four identical spectra, one for each C-H bond. If, for each C-H in ethane, one subtracts from the full ethane spectrum a unit C-H spectrum he finds what is apparently a single electronic band centered around $75,000cm-1$. Moreover, the same procedure applied to higher alkanes gives absorption seemingly composed of several bands with total intensity proportional to the number of C-C bonds---this with surprising exactness. The ethane band consists of some nine vibronic transitions with average spacing ca $11,500cm-1$. This is probably an H-C-H bending mode ($14,500cm-1$ in the ground state) because no other ground state vibrational mode is nearby and of higher frequency. The spectra of the higher homologs have been studied using an independent systems model and parameters derived from the methane and ethane spectra. Interactions between bond excitations are taken as involving the transition moments as extended dipoles. It is suggested that these findings should be quite generally helpful in the interpretation of vacuum UV spectra of compounds containing alkyl groups.