From hydrogen bond to bulk: Solvation analysis of the n-pi* transition of formaldehyde in water

Supermolecular calculations that treat both the solute and the solvent quan tum mechanically are performed to analyze the n-pi* transition of formaldeh yde in water. The liquid structures are generated by canonical (constant vo lume, temperature, and number of particles) (NVT) Metropolis Monte Carlo si mulation. Autocorrelation function is calculated to obtain efficient ensemb le average. Full quantum mechanical intermediate neglect of differential ov erlap/singly excited configuration interaction (INDO/CIS) calculations are then performed in the supermolecular clusters corresponding to the hydrogen bond shell and the first, second, and third solvation shells. The largest cluster, corresponding to the third solvation shell, includes 1 formaldehyd e and 80 water molecules. INDO/CIS calculations are performed on a properly antisymmetric reference ground-state wave function involving all valence e lectrons. The results are then extrapolated to the bulk limit. The estimate d Limit value for the solvatochromic shift of the n-pi* transition of forma ldehyde in water, compared to gas phase, is 2200 cm(-1). (C) 2000 John Wile y & Sons, Inc.