Infrared photodissociation spectra of the C–H stretch vibrations of C6H6+–Ar, C6H6+–N2, and C6H6+–(CH4)1–4

Vibrational infrared photodissociation spectra of mass selected ${\mathrm{C}}_{\mathrm{6}}{\mathrm{H}}_{\mathrm{6}}^{\mathrm{+}}\mathrm{–Ar},$ ${\mathrm{C}}_{\mathrm{6}}{\mathrm{H}}_{\mathrm{6}}^{\mathrm{+}}{\mathrm{–N}}_{\mathrm{2}},$ and ${\mathrm{C}}_{\mathrm{6}}{\mathrm{H}}_{\mathrm{6}}^{\mathrm{+}}{\mathrm{–(CH}}_{\mathrm{4}}{\mathrm{)}}_{\mathrm{1–4}}$ ionic complexes are recorded in the spectral range of the C–H stretching vibrations. Transitions at 3095±15 cm⁻¹ occur in all spectra and are assigned to C–H stretch fundamentals of the benzene cation in its ${}^2{E}_{\text{1g}}$ electronic ground state. In the case of the ${\mathrm{C}}_{\mathrm{6}}{\mathrm{H}}_{\mathrm{6}}^{\mathrm{+}}{\mathrm{–(CH}}_{\mathrm{4}}{\mathrm{)}}_{\mathrm{1–4}}$ complexes, additional transitions at 2904±7 and 3010±24 cm⁻¹ are observed and attributed to the symmetric and antisymmetric C–H stretch vibrations of the ${\mathrm{CH}}_{\mathrm{4}}$ ligands, ${\nu }_1$ and ${\nu }_3.$ The deduced C–H stretching vibrations of ${\mathrm{C}}_{\mathrm{6}}{\mathrm{H}}_{\mathrm{6}}^{\mathrm{+}}$ in the ${}^2{E}_{\text{1g}}$ ground state are roughly 30 cm⁻¹ higher than the corresponding frequencies in the ${}^1{A}_{\text{1g}}$ electronic ground state of the neutral species, indicating that the C–H bonds become stronger upon removal of an electron from the highest occupied $e_{\text{1g}}$ orbital of ${\mathrm{C}}_{\mathrm{6}}{\mathrm{H}}_{\mathrm{6}}.$