Accuracy of theoretical potential energy profiles along proton-transfer coordinate for XH-NH3 (X = F, Cl, Br) hydrogen-bonded complexes

The Study of ab initio potential energy curves along the proton-transfer coordinate in XH-NH3 (X = F, Cl, Br) hydrogen-bonded complexes is reported. Equilibrium geometries, harmonic vibrational frequencies, one-dimensional energy profiles, and anharmonic proton stretching frequencies have been calculated at B3-LYP/6-311++G(d,p), MP2/6-311++G(d,p), MP2/6-311++G(2df,2pd), and CCD/6-311++G(2df,2pd) levels of theory. The results have been compared to more accurate CCSD(T)/6-311++G(2df,2pd) values. It has been found that the structures and most of the harmonic vibrational frequencies of the complexes agree well, but the proton stretching frequencies differ significantly. These differences are enhanced when anharmonicity along the proton transfer coordinate is taken into account, and even small inaccuracies in the PES cause large errors in the computed frequencies. These results show that the proper treatment of electron correlation is extremely important for the correct description of anharmonic energy profiles in hydrogen-bonded complexes.