Trimethyl phosphate-acetylene interaction: a matrix-isolation infrared and ab initio study

Trimethyl phosphate (TMP) and acetylene were codeposited 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 the modes involving the TMP and acetylene submolecules. The structures of these adducts, energies and the vibrational frequencies were computed at the HF/6-31G** level. Both the experimental and computational studies indicated that two types of TMP-acetylene complexes were formed; one in which the hydrogen in acetylene was bonded to the phosphoryl oxygen and another in which the bonding was at the alkoxy oxygen of the phosphate. In addition to the primary hydrogen bonded interaction at the phosphoryl oxygen, this complex, also appeared to be stablilized by a secondary and weaker interaction involving a methyl hydrogen in TMP and the pi cloud in acetylene--a case of a H...pi interaction. The computed vibrational frequencies in the adducts agreed well with the observed frequencies for the modes involving the TMP submolecule, while the agreement was relatively poor for the modes involving the acetylene submolecule. The stabilization energies of these adducts, corrected for both zero-point energies and basis set superposition errors, were approximately 3 kcal/mol for the phosphoryl complex and, approximately 1 kcal/mol for the alkoxy complex.