CH4-N2, NH3-N2, H2O-N2and HF-N2complexes: Ab initio studies and comparisons-transition to hydrogen bonding

Geometry optimizations on a set of structures for the CH4-N-2, NH3-N-2, H2O-N(2)and HF-N(2)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-N(2)is a structure with N(2)facing three hydrogens of CH(4)in T-shape. For NH3-N-2, two structures were found to have equal dissociation energies, one with N(2)facing N of NH3, corresponding to the structure predicted and confirmed by microwave spectroscopy, the other being hydrogen-bonded. For H2O-N(2)and HF-N-2, the hydrogen-bonded structure (X-H center dot center dot center dot N) is most stable. Dissociation energiesD(e)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 degrees (most bent) for NH3-N-2, around 170 degrees (near linear) for H2O-N(2)and 180 degrees (linear) for HF-N-2. 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-N(2)to HF-N-2. Infrared intensities of the highest frequency intermolecular modes increase from 0.004 km/mol for CH4-N(2)to 120 km/mol for HF-N-2. Intensities of the stretching modes increase well over the monomer values in going across this series of complexes, particularly for H2O-N(2)and HF-N-2. Calculated redshifts of the stretching modes are 83 cm(-1)for HF-N-2(43 cm(-1)experimentally). Results are compared with those of corresponding XHn-O(2)complexes.