Volume 105. number 4 CHEMICAL PHYSICS LETTERS 23 March 1984 COOPERATIVITY OF HYDROPHILIC AND HYDROPHOBIC EFFECTS IN THE AQUEOUS HYDRATIOK OF POLYFUNCTIONAL SOLUTES Francrs T. MARCHESE and David L. BEVERIDGE Depnrfment of Chemistry. Hunter College of rhe cify Uniuersiry of New York. 695 Park Avenue. New York. New York 100~1. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDC USA Received 9 August 1983; in final form 9 December 1983 Liquid state computer simulation ti reported on a dllutc aqueous solution of formamide at 25°C. Analysis of the results reveals essential characteristics of hydrophilic and hydrophobic hydration in the fust shell of the hydration complex. A novel cooperativlty between the hydrophilic hydration of the carbonylgoup and the hydrophobic hydration of the methyne poup is observed. The structural chemistry of aqueous hydration can be conveniently considered in terms of ionic, hydro- phobic, and hydrophilic effects. Liquid state computer srrnulation has been recently applied to dilute aqueous solutions of prototypes of ionic, hydrophobic, and hydrophilic solutes, and the essential characteristrcs of each mode of aqueous hydration have been delin- eated [l] _ We report herein computer simulation studies of the aqueous hydration of a simple polyfunc- tional solute containmg both hydrophilic and hydro- phobic functional groups in close proximity and inves- tigate the articulation of the various modes of aqueous hydration at the molecular level. The analyses of Monte Carlo computer simulations on aqueous solutions of monofunctional solutes at ambient temperatures foml the background for this study_ Results on alkali metal cations and h&de amens [2,3] showed that the essential characteristrc of ionic hydration is the organization of water into closed polygonal forms about the solute, which can be iden- tified wrth simple geometric structures familiar from coordination chemistry. Simulation studies of a dilute aqueous solution of methane [4] produced evidence for a clathrate-lrke model of hydrophobic hydration, which in the case of [CH4] aq was found to be derived from pentagonal dodecahedra. Water pentagons, ves- tiges of this charactenstic structure, have been found in the local solution envrronment of CH,, CH,, and CH groups studred by computer simulation [5-71. The essential characteristrc of hydrophilic hydration is a simple two-center sequentral hydrogen bond or, in proprtrous cases, a multi-center structure with wa- ter bridges. We have recently extended these studies to dilute aqueous solutions of small organic polyfunctional mol- ecules, and the results obtained from simulation on W,NCHOl aq at 25°C are particularly interesting. A Monte Carlo computer simulation was performed on a unit cell consistmg of one formamide molecule and 215 water molecules at a density of I g/cc under periodic boundary conditions. The intermolecular po- tential functions used were the MCY function [S] for the water-water interactions and the QPEN [9] po- tential for the formamide-water interaction. The total simulation spanned 4800k configurations chosen by the Metropolis method augmented with the force bias and preferential sampling for convergence acceleration [ 10,l l] _ Analysis encompassed the final 1 OOOk con- figurations. A detailed statistical picture of the structure and energetics of [H2NCHOlaq based on the proximity criterion [5] is forthcoming. For present purposes, we note the first solvation shell (hydration complex) of the formamide molecule was found to contain an average of 11.9 water molecules_ The N, H (trans to the carbonyl oxygen atom), and H (cis) atoms of the 431