Fourier transform infrared measurements on the spectra of the products of the vaporization of silicon/carbon mixtures trapped in solid argon in concert with ab initio calculations using second order many body perturbation theory have resulted in the identification for the first time of two vibrational fundamentals, 3 (b 1u )ϭ982.9 and 4 (b 2u )ϭ382.2 cm Ϫ1 , of the rhombic ground state structure of Si 2 C 2 . The observed frequencies, intensities, and isotopic shifts are in good agreement with the ab initio predictions. Tentative assignments are also made for fundamentals of the linear ͑SiCCSi͒ and distorted trapezoidal isomers. The relative energies of the three isomers have been estimated at various ab initio levels.

The cyclic C6 cluster has been identified for the first time in Fourier transform infrared spectra of the products from the laser evaporation of graphite rods trapped in Ar at ∼10 K. Measurements on spectra produced using both 12C- and 13C-enriched rods combined with the results of new density functional theory calculations performed in the present work as well as previous calculations by Martin and Taylor, have resulted in the assignment of the most intense infrared active mode, ν4(e′)=1694.9 cm−1 of the cyclic C6 isomer with D3h symmetry. This assignment is based on excellent agreement of the frequency, 13C isotopic shifts, and relative intensities with the theoretical predictions.

The Si2C ground state has been reinvestigated using various basis sets at the Hartree–Fock level as well as with the use of many-body perturbation theory to second order. In agreement with previous theoretical investigations, the ground state is found to be a closed-shell C2v symmetry structure, whereas a linear D∞h structure is a transition state. Vibrational frequencies, infrared intensities, and isotopic shifts are presented and found to be in excellent agreement with recent experimental results, supporting a reinterpretation of the infrared spectrum of Si2C.

The products of vaporization of boron/carbon mixtures around 3000 K were trapped in argon matrices at 10 K and their Fourier transform infrared spectra were measured. Analysis of the spectra combined with the predictions of density functional theory (DFT) calculations have resulted in the assignment of a previously observed vibration at 1194.4 cm−1 to the ν2 fundamental of cyclic BC2, which is effectively symmetric. The assignment is supported by 10B, 11B, and 13C isotopic data and is in good agreement with the theoretical predictions.

Fourier transform infrared observation of the ν2 stretching mode of the HCCCO radical in solid Ar J. Chem. Phys. 98, 9251 (1993); 10.1063/1.464405Fourier transform infrared isotopic study of the ν4 and ν5 stretching modes of linear C6 in Ar at 10 K A second stretching fundamental 7 ( u ), of the linear C 9 carbon cluster has been observed for the first time at 1601.0 cm Ϫ1 by means of a Fourier transform infrared investigation of the products of the evaporation of graphite trapped in solid Ar at ϳ10 K. Comparison of the measured isotopic shifts of the single 13 C and single 12 C isotopomers with the predictions of new density functional theory and coupled cluster ab initio calculations carried out as part of this study, conclusively confirms the assignment of the new mode.