Vibrational frequencies from anharmonicab initio/empirical potential energy functions IV. Diacetylene: frequencies and infrared intensities of stretching vibrations

An anharmonic potential energy function for the stretching vibrations of diacetylene has been constructed from ab initio SCF calculations and little experimental information. Low-lying vibrational states are calculated by diagonalization of an approximate vibrational hamiltonian which neglects the anharmonic coupling between stretching and bending motions. The potential energy terms of diacetylene show close resemblance with those obtained previously for cyanoacetylene. Equilibrium geometry, rotational and centrifugal distortion constants are predicted. Infrared intensities of several diacetylene isotopes are calculated from the anharmonic vibrational wavefunctions and SCEP dipole moment functions.