Intermolecular hydrogen bonding in NLO. Theoretical analysis of the nitroaniline and HF cases

Abstract

Monomer and dimer structures of p-nitroaniline have been theoretically optimized both invacuo and in the presence of an electric dipole field by means of abinitio molecular orbital procedures at the Hartree–Fock level of theory with the 6-31G and 6-31+G** basis sets. The electron correlation has been estimated at the MP2/6-31+G** level and by calculations based on density functional theory (B3LYP/6-31+G**). The hydrogen-bonded structure of the dimer has been studied in detail and H bond energies of 2.5–5.7 kcal mol^-1 have been obtained. The hyperpolarizabilities have been also estimated at all levels of calculation and a clear increase in the value of β has been observed in going from the monomer to the dimer structure. A similar study has been performed in H-bonded chains of HF. The theoretical study of the HF chains leads to no increase in the value of β, while the hypothetical linear structure clearly leads to an increase of this value with the number of molecules in the cluster. The dependence of the geometries of the stationary structures and the nonlinear optical (NLO) properties upon theoretical methods is discussed. This analysis suggests that the hyperpolarizabilities strongly depend on the number of molecules aggregated 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 linking donor and acceptor.