The effect of cyano substitution on the heats of formation of organic ions and neutrals

Abstract

In this paper the effect of the cyano group (CN) on the heats of formation of small, gas-phase organic cations was investigated. The C N substituent 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 AfHo298 of radical cations by 42-44 kcal mol-' compared with their H-substituted analogues. The effect of C N substitution on neutral molecules is ca. 32 kcal mol-', and so the ionization energy of a CN-substituted molecule is larger than for its unsubstituted analogue by approximately 10-12 kcal mol-'. This is supported by calculations which show that the C N group does not conjugate significantly with the positive charge center, i.e. it behaves as an inductive electron-withdrawing substituent. The ionization energies of the three neutral molecules, cyanoketene, benzoyl cyanide, benzoylacetonitrile (9.93 f 0.26, 9.89 f 0.09, and 9.54 f 0.09 eV, respectively), and their cationic heats of formation (253 f 3,256 f 2,237 f 2 kcal mol-', respectively) were determined for the first time, along with the heat of formation of neutral cyanoketene, 24 f 5 kcal mol-', the cyanoacetyl cation, 198 i 3 kcal mol-', and the cyanoacetyl radical, 32 f 1 kcal mol-'.

Theoretical study of OCCHCN as a potential alternative insulation gas for SF6

“…The IP(a) values of intermediates IM2 and IM6, and the product HCCN cannot be obtained and are labeled by "-" in Table IV as well as SF 6 , which can be dissociated by electron attachment. 31 The results show that the theoretical values of IP(a) and EA(a) are in good agreement with the corresponding experimental values, 30 indicating that our theoretical results are reasonable and reliable. The IPs and EAs of molecules are very important parameters to estimate their abilities of reduction and oxidation, respectively.…”

A Combined Mass Spectrometric and Thermochemical Examination of the C2,H2,N Family of Cations and Radicals

“…It was evident that the cyano functionality had a predictable effect on the thermochemistry of several classes of radical cations, raising their heats of formation by 42-44 ± 3 kcal mol-1 relative to a hydrogen substituent. 1 The resulted indicated that the CN group acted predominantly as an inductive electron-withdrawing group and did not greatly participate in charge delocalization of the cationic center. The appearance energy (AE) technique3•4 was employed to determine new thermochemistry for the even electron cations +CH2CN, CH3+CHCN, and (CH3)2+CCN and to confirm the Af7f029s(*CH2CN) derived from gas phase kinetic studies.…”

“…The degree of -delocalization in the linear cations and radicals was estimated by using the above thermochemical data in isodesmic reactions 1 and 2. +CH2CN(NC) + CH4 + AE -*• CH3CN(NC) ++CH3 (1) *CH2CN(NC) + CH4 + AE -CH3CN(NC) + *CH3 (2) Alone, the values for the exothermicity (-ve AE) or endothermicity (+ve AE) are of limited use. To put them into perspective, the CN and NC substituents were compared to other groups using their respective forms of eqs 1 and 2.…”

Cyanoketene: the microwave spectrum and structure of an unstable molecule

“…All five quartic centrifugal distortion constants, five of the seven sextic, and one octic could be determined with sufficient accuracy and yielded the best fits. Only for the 18 O-species we had to restrict the measurements from 18 to 26 GHz and to 13 a-type transitions with a standard deviation of 54 kHz. Besides the rotational constants only two quartic distortion constants could be fitted; the rest was fixed to the corresponding values of the parent molecule.…”

“…Extensive theoretical calculations about the structure, reactivity, and energetic quantities have been carried out [15][16][17]. Production of the molecule under high vacuum conditions by flash thermolysis of different precursors allowed the measurement of its heat of formation and an estimate of the ionization energy by mass spectroscopic techniques [18], the determination of the IR-bands in an Ar matrix at 12 K [19], and to find the pathways of its formation also by trapping the different species in an Ar matrix at 14 K [16]. The latter paper contradicts strongly a finding of Efremov et al [20] who claim that cyanoketene can be isolated as a liquid, boiling at )34°C, and who published an NMR spectrum.…”