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 e2g 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.