CH4–N2, NH3–N2, H2O–N2 and HF–N2 complexes: Ab initio studies and comparisons—transition to hydrogen bonding

Abstract

Geometry optimizations on a set of structures for the CH4–N2, NH3–N2, H2O–N2 and HF–N2 complexes were performed using coupled cluster CCSD(T) methods with augmented correlation consistent basis sets up to the five-zeta level (AV5Z). Corrections for the basis set superposition error were applied. Most stable for CH4–N2 is a structure with N2 facing three hydrogens of CH4 in T-shape. For NH3–N2, two structures were found to have equal dissociation energies, one with N2 facing N of NH3, corresponding to the structure predicted and confirmed by microwave spectroscopy, the other being hydrogen-bonded. For H2O–N2 and HF–N2, the hydrogen-bonded structure (X–H···N) is most stable. Dissociation energies De increase from 159 cm−1 to 246 cm−1 to 428 cm−1 to 800 cm−1 along this series. For the hydrogen-bonded structures, the X–N and X–H distances decrease along the series. Both X–H–N and H–N–N angles are around 145° (most bent) for NH3–N2, around 170° (near linear) for H2O–N2 and 180° (linear) for HF–N2. Upon complexation, dipole and quadrupole moments generally increase. Harmonic vibrational frequencies and IR intensities were calculated by the Moller–Plesset MP2/AVQZ method. Frequencies of the intermolecular vibrational modes increase from CH4–N2 to HF–N2. Infrared intensities of the highest frequency intermolecular modes increase from 0.004 km/mol for CH4–N2 to 120 km/mol for HF–N2. Intensities of the stretching modes increase well over the monomer values in going across this series of complexes, particularly for H2O–N2 and HF–N2. Calculated redshifts of the stretching modes are 83 cm−1 for HF–N2 (43 cm−1 experimentally). Results are compared with those of corresponding XHn–O2 complexes.