Gas phase basicities of polyfunctional molecules. Part 6: Cyanides and isocyanides

Abstract

This paper gathers structural and thermochemical informations related to the gas-phase basicity of molecules containing cyanides (nitriles) and isocyanides (isonitriles) functional groups. It constitutes the sixth part of a general review devoted to gas-phase basicities of polyfunctional compounds. A large corpus of cyanides and isocyanides molecules is examined under seven major chapters. In the first one, a rapid overview of the definitions and methods leading to gas-phase basicity, GB, proton affinity, PA, and protonation entropy, Δ S°, is given. In the same chapter, several aspects of the gas phase chemistry of protonated cyanides and isocyanides are also presented. Chapters II-VI detail the protonation energetics of aliphatic, unsaturated, and heteroatom substituted (halogens, O, S, N, P) cyanides. A seventh chapter is devoted to isocyanides. Experimental data available in the literature (120 references) were reevaluated according to the presently adopted basicity scale that is the NIST database anchored to PA(NH ) = 853.6 kJ/mol and GB (NH ) = 819 kJ/mol. In this latter source, however, several erroneous values have been identified which were corrected in the present review. Structural and energetic information given by G4MP2 quantum chemistry computations on ca. 60 typical systems are presented. The present review includes the GB, PA, and Δ S° values of ca. 110 cyanides and isocyanides, and, for selected examples, is completed by a set of computed heats of formation (Δ H°) at 0 and 298 K.

Computational studies into urea formation in the interstellar medium

“…The most likely pathway using charged species, involves isocyanic acid protonated on the nitrogen atom. This is in agreement with available literature data, both for the gas phase as for the solid state (Dekock & Jasperse 1983;Hop et al 1989;Hunter & Lias 1997;Hudson et al 2005;Gupta et al 2013;Bouchoux 2018;Marcelino et al 2018). This species then reacts barrierlessly with ammonia to form protonated urea.…”

“…Moreover, there is a clear correlation with the abundances of formamide (Bisschop et al 2007;López-Sepulcre et al 2015). There is ample evidence of HNCO molecules functioning as an imine base, both in the gas phase and in the solid state (Dekock & Jasperse 1983;H o pe ta l .1989;Hunter & Lias 1997;Hudson, Khanna & Moore 2005;Gupta et al 2013;Bouchoux 2018;Marcelino et al 2018). The proton affinity of HNCO is higher than that of water, so that one would expect protonation to occur even in water-based ices (Dekock & Jasperse 1983;Bouchoux 2018).…”

Push–Pull Effect on the Gas-Phase Basicity of Nitriles: Transmission of the Resonance Effects by Methylenecyclopropene and Cyclopropenimine π-Systems Substituted by Two Identical Strong Electron Donors

“…This effect occurs when a strong electrondonating group pushes its electron density toward the electron-accepting cyano group through a conjugated system [1]. Several examples can be found in a review reporting on the gas-phase basicity values of nitriles [2]. For example, 3-(dimethylamino)acrylonitrile (CH 3 ) 2 N−CH=CH−C≡N, which is protonated preferentially on the cyano group, displays a proton affinity (897 kJ mol −1 ) close to that of methylamine (899 kJ mol −1 ) [3,4].…”

“…For example, 3-(dimethylamino)acrylonitrile (CH 3 ) 2 N−CH=CH−C≡N, which is protonated preferentially on the cyano group, displays a proton affinity (897 kJ mol −1 ) close to that of methylamine (899 kJ mol −1 ) [3,4]. The calculated proton affinities of nitriles bearing strong electron-donating groups, such as phosphazeno and diphosphazeno substituents, can exceed 1000 kJ mol −1 [2,5,6]. Inserting a group with at least one double bond (e.g., vinyl group) in the dimethyl cyanamide skeleton (CH 3 ) 2 N−C≡N, leading to (CH 3 ) 2 N−CH=CH−C≡N, significantly enhances the push-pull effect, increasing the proton affinity from 852 to 897 kJ mol −1 [3,4].…”

“…In this study, we extended earlier quantum chemical investigations of strong basic nitriles in the gas phase [5,6] to two series of nitriles (I and II) possessing interesting π-πand n-π-conjugated systems or groups (Figure 1). First, we included in our study, for comparison with previous reports and for investigation of the substituent effects, nitriles containing simple acyclic π-systems such as H 2 C=Z (1) and H 2 C=CH−CH=Z (2) [2], as well as nitriles with different cyclic scaffolds displaying strong electron delocalization (aromaticity), such as cyclopropene (3) [7], 1,3,5-cycloheptatriene (5) [8,9], and "quinoid" fragments (6 and 7) [10]. Additionally, the methylenecyclopropene (3)-conjugated system (with Z: CH) and its heteroatomic analog (with Z: N) was hypothesized to be an efficient resonance-transmitting unit between various pushing substituents and the site of protonation [7,[10][11][12][13][14][15][16][17]; therefore, we also decided to study the gas-phase protonation of the nitrile series shown in Figure 2, for which the two X substituents are electron-donating or pushing groups of medium or strong power, such as Me, NR 2 , N=C(NR 2 ) 2 , and N=P(NR 2 ) 3 (R: H and Me) attached to the cyclopropenylidene ring.…”