Journal of Molecular Structure, 299 (1993) 29-41 0022-2860/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved 29 Conformational properties of carbonyl isocyanates - stereoelectronic effects favouring the cisoid conformation Dieter Klapstein *, Werner M. Nau Department of Chemistry, St. Francis Xavier University, Antigonish, N.S., Canada B2G lC0 (First received 27 January 1993; in final form 19 April 1993) Abstract The uC=O regions of the gas phase infrared spectra of six carbonyl isocyanates X-CO-NC0 with X = F, OMe, OEt, SMe, NC0 and NMe2 have been analyzed. For all compounds except X = NMez two absorptions were found in the yC=O region with a separation of at least 30 cm-‘. The clearly defined rotational band contours allowed assignments to the cisoid and transoid conformers of these molecules derived from rotation around the C-N bond. For X = OMe, OEt and SMe the interpretation of the K-N regions suggests the alkyl groups adopt the Z-arrangement. The relative intensity changes in the vC=O regions via variable-temperature measurements were used to determine the relative conformational stabilities. For all compounds the cisoid conformer was found to be more abundant in the gas phase. The results are explained in terms of varying contributions of mesomeric structures as supported by semiempirical MO calculations. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGF Introduction The concept of electronic stabilization via delocalization of bonded and non-bonded elec- tron pairs is well established and to a con- siderable extent chemical reactivity can be under- stood on this basis. In many cases these inter- actions determine the conformations of organic molecules and they are then referred to as stereo- electronic effects. In turn, the study of the spatial structures of molecules allows statements to be made about the stereoelectronic effects that govern their conformational behaviour. The latter information may then be used to predict or explain the chemical reactivity of the molecule on the basis of electronic activation or deactivation, or on the basis of the preferred geometric arrangement, both criteria being * Corresponding author. directly related to the stereoelectronic nature of the interaction [ 11. Two classes of organic compounds, molecules containing conjugated double bonds V=W-X=Y and molecules containing the ester function -CO-OR, have been well-examined with respect to their stereoelectronic effects. For the conjugate double bond systems, three general conformations [2] (cisoid, transoid and gauche) have to be considered (Scheme 1). On the one hand, the conjugate systems present challenges for MO calculations since the reproduction of experimental data requires the proper balance of steric strain (favouring the gauche isomer) and resonance energy (favouring the planar conformers) [3,4]. On the other hand, the isomerism is of importance for con- certed [4 + 21 cycloaddition reactions which require the diene to adopt the cisoid conformation r51.