The magnetic circular dichroism spectra of matrix-isolated benzene

The magnetic circular dichroism (MCD) spectra of the ¹ B _2u ←¹ A _1g transition of benzene in nitrogen and argon matrices at 20 K have been measured in order to obtain a spectrum devoid of hot bands and complicating rotational structure. The spectrum is dominated by three progressions of B terms, two of positive sign and one of negative. One positive progression and the negative one can be definitely assigned to the e_2g modes v ₆ and v ₉ respectively, whilst the other positive progression may be the e _2g mode v ₈. This is in marked contrast with the absorption spectra of the same matrices which reveal only a single progression built upon v ₆. The MCD spectra are nicely accounted for in terms of a magnetic mixing between the ¹ B _1u and ¹ B _2u states, the electric dipole intensity arising from the mixing in of a ¹ E _1u state via e _2g vibrational modes.