Detection of cyclic carbon clusters. II. Isotopic study of the ν12(eu) mode of cyclic C8 in solid Ar

The cyclic ${\mathrm{C}}_{\mathrm{8}}$ cluster was detected and identified for the first time in Fourier transform infrared spectra of the products from the laser evaporation of ${}^{13}\mathrm{C}{/}^{12}\mathrm{C}$ rods trapped in Ar at ∼10 K. Measurements on spectra produced using both ${}^{12}\mathrm{C}$ - and ${}^{13}\mathrm{C}$ -enriched rods combined with ab initio predictions have resulted in the assignment of the most intense infrared active mode, ${\nu }_{12}{\mathrm{(e}}_u\text{)=1844.2\hspace{0.5em}}{\mathrm{cm}}^{\mathrm{-1}}$ of the cyclic ${\mathrm{C}}_{\mathrm{8}}$ isomer with $C_{\text{4h}}$ symmetry. A new isotopic perturbation model (IPM) in which normal mode information from ab initio calculations is combined with experimental vibrational frequencies was developed in order to investigate the isotopic vibrational spectrum that is complicated by the fact that the mode is doubly degenerate and mixes with two infrared inactive modes lying nearby. The assignment of ${\nu }_{12}{\mathrm{(e}}_u)$ is based on excellent agreement of the frequency, ${}^{13}\mathrm{C}$ isotopic shifts, and relative intensities with the predictions of earlier density functional theory (DFT) calculations and perturbation treated DFT/B3LYP/cc-pVTZ calculations using the IPM model carried out in the present study.