Solvent effects in emission spectroscopy: A Monte Carlo quantum mechanics study of the n←π* shift of formaldehyde in water

Abstract

Supermolecular calculations that treat both the solute and the solvent quantum-mechanically are performed to analyze the solvatochromism of the first emission transition of formaldehyde in water. The liquid structures are generated by NVT Metropolis Monte Carlo simulation assuming a fully relaxed excited state. The autocorrelation function is calculated to obtain an efficient ensemble average. A detailed analysis of the hydrogen bonds and their contribution to the solvation shift is presented. On average, 0.7 hydrogen bonds are formed in the excited state, about three times less than in the ground state. Quantum-mechanical calculations using the intermediate neglect of differential overlap with singly excited configuration interaction (INDO/CIS) are then performed in the supermolecular clusters corresponding to the hydrogen bond shell and the first, second, and third solvation shells. The third solvation shell extends up to 10 Å from the center of mass of formaldehyde, showing the very long-range effects on the solvation shift of this polar molecule. The largest cluster includes one formaldehyde and 142 water molecules. INDO/CIS calculations are performed on this cluster with a properly antisymmetric reference ground state wave function involving all valence electrons. The estimated limit value for the solvatochromic shift of the n-π* emission transition of fully relaxed formaldehyde in water, compared to the gas phase, is ≈1650 cm−1. The total Stokes shift of formaldehyde in water is calculated as ≈550 cm−1.