INTERMOLECULAR HYDROGEN-BONDING IN NLO - THEORETICAL-ANALYSIS OF THE NITROANILINE AND HF CASES

Monomer and dimer structures of p-nitroaniline have been theoretically optimized both in vacuo and in the presence of an electric dipole fie ld by means of ab initio molecular orbital procedures at the Hartree-F ock level of theory with the 6-31G and 6-31 + G* basis sets. The elec tron correlation has been estimated at the MP2/6-31 + G* level and by calculations based on density functional theory (B3LYP/6-31 + G*). T he hydrogen-bonded structure of the dimer has been studied in detail a nd H bond energies of 2.5-5.7 kcal mol(-1) have been obtained. The hyp erpolarizabilities have been also estimated at all levels of calculati on and a clear increase in the value of beta has been observed in goin g from the monomer to the dimer structure. A similar study has been pe rformed in H-bonded chains of HF. The theoretical study of the HF chai ns leads to no increase in the value of beta, while the hypothetical l inear structure clearly leads to an increase of this value with the nu mber of molecules in the cluster. The dependence of the geometries of the stationary structures and the nonlinear optical (NLO) properties u pon theoretical methods is discussed. This analysis suggests that the hyperpolarizabilities strongly depend on the number of molecules aggre gated through H bonds. We have now shown that connecting molecules via intermolecular hydrogen bonding, especially when this gives rise to a linear structure, must be considered as a useful second way of linkin g donor and acceptor.