Trimethyl phosphate-water interaction: A matrix-isolation infrared and ab initio study

Trimethyl phosphate (TMP) and water were co-deposited in nitrogen and argon matrices, and adducts of these species were identified using infrared spectroscopy. Formation of the adducts was evidenced by shifts in the vibrational frequencies of TMP and water. We computed the structures of these adducts and the vibrational frequencies at the HF/6-31G** level. The computed vibrational frequencies in the adducts involving the TMP submolecule compared well with the observed frequencies, while the agreement was rather poor for the modes involving the H2O submolecule. Both experimental and computational studies indicated that two types of TMP-water complexes were formed: one in which the hydrogen in water was bonded to the phosphoryl oxygen of TMP and another in which the bonding was at the alkoxy oxygen of the phosphate. The stabilization energy of these adducts, corrected for zero-point energies and basis set superposition errors, was computed at both the HF/6-31G**//HF/6-31G** and MP2/6-31G**//HF/6-31G** levels. Our computations indicated that both the phosphoryl and alkoxy oxygen bonded TMP-H2O complexes had a cyclic structure determined by a combination of two hydrogen-bonded interactions, one involving a hydrogen in water and another involving a hydrogen in the methyl group of TMP.