Journal of Molecular Structure,51 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJI (1979) 17-26 o Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands zyxwvutsrqponm INFRARED SPECTRA OF NITROSYL CYANIDE AND 8 ISOTOPICALLY SUBSTITUTED SPECIES. A GENERAL HARMONIC FORCE FIELD DETERMINED FROM EXPERIMENTAL DATA AND AB INITIO CALCULATIONS BQRGE BAK, FLEMMING M. NICOLAISEN and OLE J. NIELSEN Chemical Laboratory V, University of Copenhagen, The H.C. Brsted Institute, 5 Universitetsparken, DK-2100 Copenhagen 0 (Denmark) STEEN SKAARUP Physical Laboratory III, Technical University of Denmark, Building 309 C, DK-2800 Lyngby (Denmark) (Received 31 May 1978) ABSTRACT Infrared gas phase spectra of ONCN, ONCISN, ONWN, OISNCN, ‘*ONCN 0”NCL5N, OL5N1”CN, ‘80NCLSN and ‘aON’“CN have been recorded. A general harmonic’force field (with six constraints) has been derived based on the assigned fundamental vibration fre- quencies and checked by calculation of experimental values for centrifugal distortion con- stants, a Coriolis coupling constant and the inertial defect. The harmonic force field of a fully optimized structure of ONCN as found by the Pulay force method is reported and compared to the experimental force field. No essential disagreements were found. INTRODUCTION zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Since the appearance of the “force method” of Pulay [l] a steadily increas- ing interest in ab initio calculations of harmonic force constants exists. The reliability of such calculations has been tested by making comparisons with experimentally determined force fields for several molecules [ 21, In almost all cases calculated diagonal force constants are overestimated to a considerable degree dependent on the applied basis set. This is due partly to inherent limitations of the Hartree-Fock wavefunction and partly to the fact that the ab initio force constants and frequencies are harmonic values expected to be larger than the observed effective values because the experimental fre- quencies contain anharmonic contributions. However, the rather good theor- etical prediction of off-diagonal (interaction) force constants, which are less accessible experimentally than the diagonal ones, is useful in fixing a force field or in choosing between alternative force fields. Thus experiment and computations may cooperate. Experimental force fields for ONF [3], ONCl [ 41 and ONBr [ 51 have been determined and are still under investigation. Only ONF has been subjected to