Solvent effects on molecular spectra. I. Normal pressure and temperature Monte Carlo simulations of the structure of dilute pyrimidine in water

In this series, our aim is to develop a new scheme based upon a perturbation expansion of the weak intermolecular interactions for the solvent (solvatochromatic) shift of the center of an electronic absorption band in a condensed phase. It is tested by calculation of the shift of the 1(n,π*) absorption and fluorescence spectra of pyrimidine in water. Herein, NPT-ensemble Monte Carlo simulations are performed to determine the structure of dilute pyrimidine in water at 25 °C and 1 atm. pressure. Six different intermolecular effective pair potentials are used to produce a wide range of hydrogen-bond structures ranging from 0 to 2 hydrogen bonds per pyrimidine molecule, suitable for subsequent investigations into the correlation of spectral shift with solvent structure. One potential obtained using Kollman’s Lennard-Jones parameters combined with ab initio electrostatic-potential charges produces the most accurate potential function; this correctly reproduces the observed enthalpy of hydration and partial specific volume of pyrimidine, and produces a chemically reasonable description of the hydrogen-bond structure.