Density Functional Theory Study of the Hydrogen-Bonded Pyridine - H₂O Complex: A Comparison with RHF and MP2 Methods and with Experimental Data

As a test for the applicability of the density functional theory (DFT) to hydrogen-bonded systems, various local and nonlocal exchange correlation functionals have been used to calculate the equilibrium structure and the harmonic vibrational frequencies of the pyridine - water complex. We also performed restricted Hartree -Fock (RHF) and Moller - Plesset (MP2) calculations for comparison. Three different basis sets (6-31G**, 6-31++G**, and 6-311++G(2d, 2p) 6d)) were used in the study. The best agreement between the DFT computed and the experimental rotational constants for monomeric pyridine were obtained with the B3-PW91 and B3-LYP functionals. For the vibrational frequencies of the H-bonded complex, the experimental data from matrix FT-IR spectrometry as well as the results obtained from the ab initio methods were best reproduced with the DFT method with the nonlocal exchange and correlation functionals. The hybrid approaches, which mix HF and Slater exchange, are particularly effective in these types of calculations.