AB-INITIO CALCULATIONS OF THE GEOMETRY AND VIBRATIONAL FREQUENCIES OFTHE TRIPLET-STATE OF TUNGSTEN PENTACARBONYL AMINE - A MODEL FOR THE UNIFICATION OF THE PRERESONANCE RAMAN AND THE TIME-RESOLVED INFRARED EXPERIMENTS

Ab initio calculations of the vibrational frequencies of W(CO)(5)NH3 i n its ground electronic (1)A(1) (b(2)(2)e(4)) and lowest excited state (3)E (b(2)(2)e(3)a(1)(1)) have been performed at the HF level. The ca lculated frequencies of the nu(CO) bands are in agreement with observe d data on experimentally studied W(CO)(5)(amine) molecules. The optimi zed geometries of the ground and the excited states show that the W-N, W-C-eq, W-C-ax, C-O-eq bonds lengthen and the C-O-ax bond shortens on excitation. Our results resolve an apparent disagreement between the fast time-resolved infrared (TRIR) spectroscopy and the preresonance R aman (PRR) spectroscopy. The unexpected simultaneous lengthening of bo th W-C-eq and C-O-eq is due to C-O-eq antibonding character in the a(1 ) orbital which more than offsets its loss from the e. In addition a n ew band, predicted but as yet unresolved in the TRIR, accounts for the C-O-ax shortening as expected from the PRR W-C-ax lengthening.