Molecular electronic properties and vibrational characteristics of PCCN…HX/DX (X = F, Cl, Br) inter-molecular complexes

We have investigated theoretically the existence of linear H-bonding formed between C-cyanophosphaethyne (as proton acceptor) and hydrogen halides (as proton donors) in their ground states. DFT (B3LYP and B3PW91) methods in conjunction with 6-311++g(3df,3pd) basis set under C∞v point group symmetry have been utilized to obtain molecular optimized geometries, dipole moments, rotational constants, 14N nuclear quadrupole coupling constants, binding energies, dissociation energies, zero-point difference energies, and vibrational frequencies of PCCN…HX/DX (X = F, Cl, Br) complexes. The dipole moments compared to the sum of the monomer dipole moments were found to be increased and those of the 14N nuclear quadrupole coupling constants compared to the corresponding value of PCCN were decreased upon the complex formation. The binding and dissociation strengths of these complexes were found to be ordered as follows: PCCN…HF > PCCN…HCl > PCCN…HBr. The D-bonded complexes were shown to be more stable than the H-bonded complexes according to their dissociation energies. Investigation reveals red-shifting of the HX/DX stretching modes and blue-shifting of the CN stretching mode(s) upon the complex formation. For the former case, the corresponding band intensity increases significantly.