DFT-PBE, DFT-D, and MP2 Studies on the H2O•••HNH and HOH•••NH2 Hydrogen Bonds in Water-Aniline Complexes

DFT-GGA method of Perdew-Burke-Ernzerhof (PBE) is used with aug-cc-PVTZ, 6-311++G**, and Def2-TZVP large basis sets to study the hydrogen bond interactions between oxygen lone pair as a donor electron with hydrogen atom connected to the aniline’s nitrogen as an electron acceptor (H2O···HNH-Ph), and nitrogen lone pair with hydrogen of water molecule (Ph-H2N···HOH), both in the gas phase. In some cases, MP2/Def2-TZVP is also carried out to test the results of DFT. To analyze donor-acceptor interactions of two above hydrogen bonds, natural bond orbital (NBO), natural energy decomposition analysis (NEDA), natural population analysis (NPA), and quantum theory of atoms in molecules (QTAIM) methods have been used in a detailed manner. Results show that the hydrogen bond in PhH2N···HOH is partially covalent, while hydrogen bond in PhHNH···OH2 is totally electrostatic. For the PhHNH···OH2 complex, there is a large gap between MP2 and PBE results which can be filled by incorporating dispersion terms in the DFT calculations. In all calculations, nitrogen atom of aniline is an stronger electron donor than the oxygen atom of water in the gas phase. PhH2N···HOH has higher electron density than PhHNH···OH2. NBO data shows that the stabilization energy due to the charge transfer for HOH···NH2Ph complex is more than that in H2O···HNHPh complex. The inversion barrier energy was also calculated at the level of PBE/Def2-TZVP without and with dispersion term, and results show that the barrier energy for PhH2N···HOH and PhHNH···OH2 complexes, are about 104 and 103          kcal mol-1, without, and 8.14 and 7.03 kcal mol-1, with dispersion, respectively.