Kalasinsky who gave so much to so many in the short time he was with us Keywords: Ab initio calculations Infrared and Raman spectra r 0 structural parameters F 2 C@NX (X = H, F, Cl, Br) molecules a b s t r a c t Ab initio calculations with full electron correlation by the perturbation method to second order and hybrid density functional theory calculations by the B3LYP method utilizing the 6-31G(d) and 6-311+G(d,p) basis sets have been carried out for the F 2 C@NX (X = H, F, Cl, Br) molecules. From these calculations, force constants, vibrational frequencies, infrared intensities, Raman activities, and structural parameters have been determined and compared to the experimental quantities when available. By combining previously reported rotational constants for all four F 2 C@NX molecules with the ab initio MP2/6-311+G(d,p) predicted structural values, adjusted r 0 parameters have been obtained. The r 0 values for F 2 C@NH and F 2 C@NF are in excellent agreement with the previously determined values except for the difference in the two CF distances of F 2 C@NH which differ by 0.017 ร
instead of the 0.008 ร
estimated distances. For F 2 C@NCl [Br] the values are: distances .308(3)] and angles (ยฐ) ClNC = 113.2(5) [113.5(5)], F t CN = 120.3(5) [120.4(5)], F c CN = 129.7(5) [130.4(5)]. These reported values have much lower uncertainties than those previously reported from the microwave data alone but considering the relatively large uncertainties in the values given initially the two results are in near agreement. The centrifugal distortion constants have been calculated and are compared to the experimentally F 2 C@NX (X = H, F, Cl, Br) determined values. These results are compared to several of the corresponding parameters of related molecules.