Quantum-chemical calculations are reported, based on an upgraded version of the QCFF/PI Hamiltonian, of ground ( 1A) and excited ( 1B) state properties of 4a,4b_dihydrophenanthrene (DHP), the second most important product of cis-stilbene deactivation. The computed equilibrium geometries and adiabatic vibrational frequencies are used to estimate the activity of the totally symmetric modes in a recently observed resonance Raman (RR) spectrum, under the assumption that the Franck-Condon (FC) mechanism dominates. The agreement between the observed and calculated spectrum is very satisfactory. In particular it reproduces the frequency of the mode showing the highest Raman activity; this frequency is much lower than the frequencies of corresponding CC stretchings in s-trans-polyenes and was not reproduced satisfactorily in earlier calculations. Diabatic and adiabatic force fields are analyzed and compared to those computed for s-trans-polyenes. Vibronic coupling effects evaluated from the difference between these two force fields are found to be similar to those of s-trans-polyenes. The anomalous frequency is shown to be due to the presence of cis-configurations and, to a lesser extent, to cross links in DHP. Model calculations on a polyene with the same chain structure as DHP suggest that a similar effect should be observed for other conjugated systems having a high number of cis-linkages. Recent estimates indicate that this process may contribute as much as 30% to the transformation of excited cis-stilbene [ 41.
Until recently, little was known about DHP.