EPR and FTIR spectroscopy of C4, C3H and OCCCH in solid Ne and Ar

Abstract

The electron-paramagnetic-resonance spectrum of C$\sb4$ in a neon matrix at ${\sim}$4 K formed by the vacuum-ultraviolet photolysis of 1,3-butadiene has been investigated to seek evidence for the bending of the molecule, which had previously been indicated by the results of EPR and FTIR measurements made on C$\sb4$ trapped in argon at 10 K. The observed spectra, which include $\sp{13}$C hyperfine splitting and zero-field splitting data, confirm the splitting of the perpendicular $xy\sb1$ and $xy\sb2$ lines of triplet C$\sb4$ into separate x and y components, which is indicative of the unequivalent axes of a bent molecule. A Fourier transform infrared isotopic study has been made of the deuterium and carbon-13 substituted C$\sb3$H radical produced by trapping in solid Ar the products of the vacuum ultraviolet photolysis of CH$\rm\sb3CHCH\sb2$ (propylene) or a mixture of methane and acetylene. The 1824.7 and 1159.8 cm$\sp{-1}$ absorptions, which in earlier work using only deuterium isotosopic data could be assigned to either the cyclic or linear isomer, can now be definitely identified on the basis of extensive $\sp{13}$C isotopic measurements as the $\nu\sb2(\sigma\sp+$) and $\nu\sb3(\sigma\sp+$) fundamentals of linear C$\sb3$H. The frequencies of the fundamentals and D and $\sp{13}$C isotopic shifts are in good agreement with the predictions of preliminary ab initio calculations for the linear isomer. The identification of a fundamental vibration of the HCCCO radical has been made, for the first time, in a FTIR study involving extensive isotopic measurements. An absorption at 2308.6 cm$\sp{-1}$ has been assigned to the $\nu\sb2$, antisymmetric CCCO stretching mode of HCCCO which was produced by trapping in Ar the products of the vacuum ultraviolet (VUV) photolysis of a mixture of acetylene and carbon monoxide. The assignment is confirmed by measurements for a variety of D, $\sp{18}$O, and $\sp{13}$C substituted isotopomers.