Theoretical study of the structure, stability and vibrational spectrum of the hydrogen-bonded nitric acid trimer

The structure, stability and vibrational spectrum of the hydrogen-bonded nitric acid trimer have been investigated using ab initio MP2/6-31G(d,p) and B3LYP/6-31G(d,p) calculations. Full geometry optimization was performed for the complex studied. It was established that the hydrogen-bonded nitric acid trimer has a planar cyclic structure. As a result of the hydrogen bonding a charge rearrangement occurs. The hydrogen and oxygen atoms, participating in the hydrogen bonding are the most sensitive to the complexation. The oxygen atoms act as acceptors of electric charge. The hydrogen bonding between three nitric acid molecules leads to the substantial changes in the vibrational characteristics of the stretching O-H vibrations. In the complex studied these vibrations are shifted to lower wave numbers. The calculated frequency shifts with the MP2/6-31G(d,p) are -88, -107 and -224 cm(-1) and with the B3LYP/6-31G(d,p) calculations are -157, -198 and -274 cm(-1). The magnitudes of the wavenumber shifts are indicative of O-(HO)-O-... hydrogen-bonded interactions with a medium strength. The intensities of these vibrations increase dramatically upon hydrogen bonding. The calculated increase at the MP2 level is up to 12 times and with the B3LYP/6-31G(d,p) calculations is up to 13 times. (C) 2004 Elsevier B.V. All rights reserved.