SOLVENT EFFECTS ON MOLECULAR-SPECTRA .1. 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 pertur bation expansion of the weak intermolecular interactions for the solve nt (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,pi) absorption and fluorescence spectra of pyrimidine in wat er. Herein, NPT-ensemble Monte Carlo simulations are performed to dete rmine the structure of dilute pyrimidine in water at 25-degrees-C and 1 atm. pressure. Six different intermolecular effective pair potential s are used to produce a wide range of hydrogen-bond structures ranging from 0 to 2 hydrogen bonds per pyrimidine molecule, suitable for subs equent investigations into the correlation of spectral shift with solv ent structure. One potential obtained using Kollman's Lennard-Jones pa rameters combined with ab initio electrostatic-potential charges produ ces the most accurate potential function; this correctly reproduces th e observed enthalpy of hydration and partial specific volume of pyrimi dine, and produces a chemically reasonable description of the hydrogen -bond structure.