Theoretical study of the molecular structure and normal coordinate analysis of hydrogen cyanide addition compound with boron trifluoride, HCN-BF(3).

An extensive HF, MP2, B3LYP and CCSD study of the molecular structure and normal vibrations have been performed for the HCN-BF(3) molecule. Calculations with a wide range of basis sets were classified into two groups based on the optimized N-B bond distance. The results for Group A are compared with the experimental structure of the solid phase molecules. The N-B lengths of Group A are approximately linear related to the N-B-F valence angles and also to the N-B stretching frequencies. HF/DZV calculation was used to represent the solid phase model. The N-B lengths of Group B are close to those of the gas phase molecule and both N-B-F angles and N-B sensitive frequencies have roughly the same values. Differences in the chemical bond between gaseous and solid phase HCN-BF(3) are discussed based on the calculated force constants, vibrational frequencies and potential energy distributions. Vibration mode analysis indicates that the nu(4) mode in the 600-700 cm(-1) region can be assigned to the BF(3) symmetric deformation, which shifts upon (10)B/(11)B isotopic substitution. The nu(5) mode which is insensitive to isotope substitution and changes band position with the N-B distance is assigned to the N-B bond stretching vibration.