Theoretical dipole derivatives for HCN and DCN

Hartree–Fock–Roothaan self-consistent-field (SCF) and multireference double-excitation configuration interaction (MRD–CI) methods have been used to compute the potential and dipole moment surfaces for the HCN molecule, from which a variety of molecular properties, such as equilibrium bond distances, force constants, equilibrium dipole moment and derivatives with respect to the normal coordinates, and integrated intensities for HCN and DCN, have been derived and compared with available experimental data. Satisfactory agreement is generally found between the MRD–CI and the experimental results, e.g., the calculated dipole derivatives with respect to the stretching normal coordinates are, with experimental values given in parentheses, −8.6 (±4.5) and −88.9 (±88) esu for HCN and −22.6 (±20) and −61.1 (±66) esu for DCN. Nevertheless, the calculated dipole derivatives with respect to the bending normal coordinate (80.1 and 46.4 esu, respectively, for HCN and DCN) are found to be much larger than the corresponding experimental values of 61 and 32 esu, discrepancies which would appear to be far too large to be attributed entirely to deficiencies in the present theoretical treatment.