Ab initio calculations of the geometry and vibrational frequencies of the triplet state of tungsten pentacarbonyl amine: A model for the unification of the preresonance Raman and the time-resolved infrared experiments
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Ab initio calculations of the vibrational frequencies of W(CO)5NH3 in its ground electronic 1A1 (b22e4) and lowest excited state 3E (b22e3a11) have been performed at the HF level. The calculated frequencies of the (CO) bands are in agreement with observed data on experimentally studied W(CO)5(amine) molecules. The optimized 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 Raman (PRR) spectroscopy. The unexpected simultaneous lengthening of both W-C(eq) and C-O(eq) is due to C-O(eq) antibonding character in the a1 orbital which more than offsets its loss from the e. In addition a new 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.