Matrix isolation study of the interaction of excited neon atoms with allene and propyne: Infrared spectra of H2CCCH2+ and H2CCCH-

When a Ne:allene or a Ne:propyne sample was codeposited at approximately 5 K with a sample of pure neon that had been excited in a microwave discharge to provide a 16.6-16.85 eV energy source, prominent new infrared absorptio ns which can be assigned to the H2CCCH2+ cation appeared. Also present in t he allene experiments were two absorptions which can be assigned to H2CCCH- . In the propyne experiments, an absorption is tentatively attributed to th e strongest infrared fundamental of CH3CCH+. The structures and vibrational fundamentals obtained from density functional and ab initio calculations f or various isotopomers of H2CCCH2+, CH3CCH+, cyc-C3H3+, H2CCCH+, and H2CCCH - are given. The infrared absorption pattern of the cation common to the al lene and propyne experiments matches that of the predominant gas-phase prod uct, cyc-C3H3+, reasonably well, except for the presence of extra peaks in the CH-stretching region. However, comparison of the results of experiments on isotopically substituted samples with the calculated spectra excludes t hat assignment and supports the identification of the neon-matrix product a s H2CCCH2+. It is suggested that collisions with the excess of neon atoms i n the sampling region rapidly remove excess energy from the initially forme d allene and propyne cations, inhibiting the loss of an H atom from those t wo species. (C) 2001 American Institute of Physics.