FTIR spectroscopy of Si2C and C2H in solid argon

Abstract

The techniques of matrix isolation and Fourier transform infrared (FTIR) spectroscopy have been applied to isotopic studies of the mid-infrared spectra of SiC(,2) and C(,2)H. The results reveal important, new information about the molecular structures of these two free radicals. The FTIR spectra of the products from vaporizing silicon carbide at 2900 K and quenching in argon at 7 K show two SiC(,2) absorptions at 1741.1 and 824.4 cm('-1). Under the assumption of various possible geometries, calculations based on these two vibrations were made to predict frequencies for SiC(,2) singly and doubly substituted with carbon-13. Comparison with the spectra observed on isotopic substitution shows conclusively that SiC(,2) possesses C(,2v) symmetry and that the observed vibrations are the (nu)(,1) and (nu)(,2) fundamentals, respectively. This geometry is in agreement with recent results from a rotational analysis of the band system at 498 nm. Evidence is presented in the present study which supports a structure which is T-shaped rather than cyclic and suggests that the molecule might best be understood as Si('+) ionically bonded to a C(,2)('-) group. An extensive FTIR isotopic matrix study using deuterium and carbon-13 for C(,2)H produced by vacuum UV photolysis of acetylene has been conducted in search of information on the presently unobserved (nu)(,2) bending vibration. Based on isotopic data, an absorption at 2104 cm('-1) is assigned to the (nu)(,2) + (nu)(,3) combination band of C(,2)H, which implies an unexpectedly low value of (TURN)260 cm('-1) for the (nu)(,2) bending vibration. Identification of the combination band gives the first significant indication of the value of the (nu)(,2) fundamental. The effects of carbon-13 substitution on a band at 3610 cm('-1) are inconsistent with its previous assignment as the (nu)(,1) vibration of C(,2)H; however, carbon-13 shifts for the C(,2)D band at 2798 cm('-1) do agree with its (nu)(,1) assignment. Absorptions corresponding to several vibronic bands of the A('2)(PI) (<---) X('2)(SIGMA)('+) transition, which have recently been reported in studies using color center laser spectroscopy have been observed in the matrices, thus confirming that they originate from the lowest vibrational level of the ground site. An additional C(,2)H band at 4618 cm('-1) which has never been revealed in any previous study, has also been observed in the argon matrices. The effects of carbon-13 substitution on these bands are reported.