Abstract: | Ab initio molecular orbital calculations using a 3-21G basis set have been used to optimize geometries for pyrrole, CH3(X)CCH2, CH3(H)CCHX (both cis and trans), c-C3H5X, and CH2CHCH2X, where X is CN and NC. In all the alkenyl derivatives methyl groups are found to adopt the conformation in which the methyl hydrogen eclipses the double bond. 6-31G*∥3-21G level calculations show the alkenyl cyanides to be of similar energy to pyrrole, but the isocyanides are ~20 kcal mol?1 higher in energy. For both substituents the cyclopropyl derivatives are higher in energy by ~10 kcal mol?1. At the 6-31G* level ring strain is 27.7 kcal mol?1 for the cyanide and 30.6 kcal mol?1 for the isocyanide. Data on the relative energies of RCN and RNC are compared when R is (i) a saturated hydrocarbon, (ii) an unsaturated hydrocarbon, (iii) an α-carbenium ion, (iv) an allyl cation, and (v) an α-carbanion. |