Ju l 2 00 5 Density functional theoretical ( DFT ) study for the prediction of spectroscopic parameters

DFT and RHF level calculations in conjunction with three different of basis sets have been used to investigate the variations in the bond lengths, dipole moment and rotational constants, IR frequencies, IR intensities and rotational invariants of ClCCCN. The nuclear quadrupole constants of chlorine and nitrogen of ClCCCN have been calculated on the experimental rs structure as well as on the B3PW91/6-311++g(d,p) optimized geometry and are found to be within the scale length of the experimental uncertainty. Linear regression analysis between the B3LYP/6-311++g(d,p) level calculated and experimental Bo values have been achieved for ClCCCN and BrCCCN. The slopes and intercepts obtained from the regression analysis were used to calculate the reasonable values of rotational constants of all the rare isotopic species of ClCCCN and BrCCCN as well having standard deviations ±0.048 MHz and ±0.015 MHz respectively. All the spectroscopic parameters obtained from DFT calculation shows satisfactory agreement with the available experimental data.