Studies in hydrogen bonding: modeling of water in crystalline hydrates

Abstract

The orientations and symmetries of the water molecules in the crystalline hydrates NaCl {times} 2H{sub 2}O, NaBr {times} 2H{sub 2}O, NaI {times} 2H{sub 2}O and LiI {times} 3H{sub 2}O have been investigated by use of computer modeling of the hydrates. The interatomic pair potential function EPEN/2 (involving exp({minus}6) plus Coulombic terms) was applied using previously determined parameters for the alkali-metal and halide ions and the water molecule. Rotational orientations, O-H bond lengths, and H-O-H bond angles of the water molecules were adjusted subject to symmetry constraints, using the computer program WMIN, to give structures with minimum energy. The hydrogen bonding in the modeled structures is compared with experimental evidence on the nature of the hydrogen bonding, particularly that given by infrared spectroscopy of the partially deuterated hydrates. Satisfactory agreement is found between experimental and calculated lattice energies, which provides justification for transferring the EPEN/2 parameters among the various ionic and molecular structure types. The enthalpy of formation of NaCl {times} 2H{sub 2}O is calculated from this result to be {minus}255.9 kcal/mol ({plus minus}5%); this value has not heretofore been reported.