Conformational and vibrational studies of isomeric hydrogen cyanide tetramers by quantum chemical methods.

The results of structural studies and detailed harmonic and anharmonic vibrational analysis on two hydrogen cyanide (HCN) tetramers diaminomaleonitrile (DAMN) and diaminofumaronitrile (DAFN), which are important molecules for understanding the chemistry of interstellar space and nitrile rich environments, are being reported on the basis of density functional theory using second-order perturbation theory. Both the molecules are found to have C1 symmetry. While all the heavy atoms of DAMN lie in the same plane (maximum deviation 6°), the two nitrogen atoms in DAFN are out of plane by about 15°. The two amino groups are tetrahedral and do not have significant bond angle anisotropy. Detailed conformational studies are reported on the two molecules and their possible rotational isomers are identified. Complete vibrational analysis based on harmonic and anharmonic frequencies, intensity of infrared and activity of Raman bands and potential energy distribution over the internal coordinates has been provided for the two molecules. Affect of hydrogen bonding on molecular geometry and frequencies of the NH stretch modes has been studied by calculations on the dimers of the two molecules. A close agreement has been observed between the experimental and calculated frequencies. Vibrational-rotational constants such as rotational constants in the ground vibrational state (A0, B0, C0) and the effective rotational constants (Ae, Be and Ce), including terms due to quartic centrifugal distortion constants, rotation-vibration coupling constants, Wilson and Nielsen's centrifugal distortion constants have been calculated using B3LYP and B97-1 functionals and 6-31G**, 6-311+G** and TZVP basis sets.