Intensity of d-d symmetry-forbidden electronic transition in Cr(CO)6

Absolute absorption intensities (oscillator strengths) are calculated for the d-d symmetry-forbidden transition in hexacarbonyl chromium. The vibronic coupling mechanism is taken into account in a way that represents an alternative to the traditional perturbative approach of Herzberg and Teller. In the so-called direct method, the electronic transition moment is directly expanded in a power series of the vibrational normal coordinates of suitable symmetry. In the present case, i.e., d-d ligand field transitions, or more specifically (1)A(1g) -> T-1(1g) and (1)A(1g) -> T-1(2g) transitions, the dipole selection rule is broken by vibronic interaction induced by normal modes that transform like T-1u and T-2u representations of the O-h group. An analysis of the relative importance of normal modes in promoting electronic transitions is carried out.