Rydberg spectra of bis(η6-benzene)chromium derivatives: Gas-phase photoabsorption study of bis(η6-1,2,4-trimethylbenzene)chromium and bis(η6-1,2,4,5-tetramethylbenzene)chromium

The gas-phase electronic absorption spectra of (η6-1,2,4-C6H3Me3)2Cr (1) and (η6-1,2,4,5-C6H2Me4)2Cr (2) have been measured for the first time and compared with those recorded in n-pentane solution. The gas-phase spectra of both compounds show transitions from the non-bonding chromium 3dz2 orbital to molecular Rydberg s, p and d levels. The first ionisation potentials have been determined from the Rydberg frequencies as 4.994±0.009 and 4.862±0.009 eV for 1 and 2, respectively. Detailed assignments of Rydberg bands have been made on the basis of analysis of the quantum defects and term values. The Rydberg structures agree with the C1, Cs or C2 conformation for 1 and the D2d geometry for 2 in the gas phase. In crystal, however, the molecule of 2 appears to adopt an eclipsed conformation close to D2h as indicated by the X-ray diffraction. The influence of ring methylation on the Rydberg term values has been analysed for the first time.