Detection of cyclic carbon clusters. I. Isotopic study of the ν4(e′) mode of cyclic C6 in solid Ar

The cyclic ${\mathrm{C}}_{\mathrm{6}}$ 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 $\text{∼10\hspace{0.17em}}\mathrm{K}.$ Measurements on spectra produced using both ${}^{12}\mathrm{C}$ - and ${}^{13}\mathrm{C}$ -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, ${\nu }_4{\mathrm{(e}}^{\prime }\text{)=1694.9\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}}$ of the cyclic ${\mathrm{C}}_{\mathrm{6}}$ isomer with $D_{\text{3h}}$ symmetry. This assignment is based on excellent agreement of the frequency, ${}^{13}\mathrm{C}$ isotopic shifts, and relative intensities with the theoretical predictions.