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

Abstract The gas-phase electronic absorption spectra of (η 6 -1,2,4-C 6 H 3 Me 3 ) 2 Cr ( 1 ) and (η 6 -1,2,4,5-C 6 H 2 Me 4 ) 2 Cr ( 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 3d z 2 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 C 1 , C s or C 2 conformation for 1 and the D 2 d geometry for 2 in the gas phase. In crystal, however, the molecule of 2 appears to adopt an eclipsed conformation close to D 2 h as indicated by the X-ray diffraction. The influence of ring methylation on the Rydberg term values has been analysed for the first time.