THE EFFECT OF CYANO-SUBSTITUTION ON THE HEATS OF FORMATION OF ORGANICIONS AND NEUTRALS

In this paper the effect of the cyano group (CN) on the heats of forma tion of small, gas-phase organic cations was investigated. The CN subs tituent has a predictable and significant influence on cationic heats of formation which does not vary greatly for saturated and unsaturated ions. It was found to increase the DELTA(f)H-degrees-298 of radical c ations by 42-44 kcal mol -1 compared with their H-substituted analogue s. The effect of CN substitution on neutral molecules is ca. 32 kcal m ol-1, and so the ionization energy of a CN-substituted molecule is lar ger than for its unsubstituted analogue by approximately 10-12 kcal mo l-1. This is supported by calculations which show that the CN group do es not conjugate significantly with the positive charge center, i.e. i t behaves as an inductive electron-withdrawing substituent. The ioniza tion energies of the three neutral molecules, cyanoketene, benzoyl cya nide, benzoylacetonitrile (9.93 +/- 0.26, 9.89 +/- 0.09, and 9.54 +/- 0.09 eV, respectively), and their cationic heats of formation (253 +/- 3, 256 +/- 2, 237 +/- 2 kcal mol -1, respectively) were determined fo r the first time, along with the heat of formation of neutral cyanoket ene, 24 +/- 5 kcal mol-1, the cyanoacetyl cation, 198 +/- 3 kcal mol-1 and the cyanoacetyl radical, 32 +/- 1 kcal mol-1.