Can Nitriles Be Stronger Bases Than Proton Sponges in the Gas Phase? A Computational Analysis

Abstract

DFT calculations have been performed for a series of push-pull nitriles [(R2N)n(X═Y)iC≡N, where i = 0, 1, or 2, n = 1, 2, or 3, R2N = H2N, Me2N, or C4H8N, X = CH, N, or P, Y = CH or N]. The possible protonation N-sites (N-cyano, N-imino, and N-amino) have been examined and their proton affinities (PA) estimated. For all compounds in the series, even for those containing the guanidino, phosphazeno, and diphosphazeno pushing groups, the N-cyano atom is the favored site of protonation. The n-π conjugation strongly decreases the PA value of the pushing amino group in favor of the pulling cyano one. Nitriles with the phosphazeno groups [(R2N)3P═N-P(R2N)2═N and (R2N)3P═N] exhibit the strongest basicity in the series. Some of them (with PA > 1000 kJ mol(-1)) are stronger bases than DMAN, the so-called "proton sponge". Nitriles bearing the guanidino group [(R2N)2C═N] are less basic than those with the phosphazeno group [(R2N)3P═N] but more basic than those with the formamidino group (R2N-CH═N) containing the same substituent R. The N-imino atoms, present in the transmitter group (X═N, X = CH, N, or P), display PA values lower than those of the N-cyano site by more than 30 kJ mol(-1). When proceeding from the unsubstituted derivatives (R = H) to the methylated ones (R = Me), the Me groups at the N-amino atom increase the PA value of the N-cyano site for Me2N-X═Y-C≡N (X, Y = CH or N) by ca. 30-60 kJ mol(-1). For the guanidino and phosphazeno derivatives containing two and three amino groups, respectively, this effect is not additive. The four Me groups for (Me2N)2C═N-C≡N and the six Me groups for (Me2N)3P═N-C≡N increase the PA(N-cyano) values by only 30-50 kJ mol(-1). The C≡N bond lengths of the neutral forms are well correlated with the PA(N-cyano) values.

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

“…Using Δ p S° = 6.0 J · mol −1 · K −1 , the resulting experimental proton affinity is PA(NH 2 CN) = 800.9 kJ/mol (Table ), a value in correct agreement with our G4MP2 computation which give PA(NH 2 CN) = 803.9 kJ/mol (Table ). Computation also show that protonation at the cyano nitrogen is strongly favored against protonation at the amino nitrogen; this work). The difference amount to ∼105 kJ/mol at the G4MP2 level (Table ).…”

“…Introducing alkyl groups or extending the conjugation is expected to further increase the basicity of this kind of cyanide. This is confirmed by B3LYP/6‐311+G(d,p) calculations as illustrated in Scheme for selected examples (note that, at this level of theory, proton affinities are overestimated, we thus anchored the original data to PA(P(NH 2 ) 3 =NCN) = 962 kJ/mol, as calculated at the G2 level by the same authors).…”

“…20,69 The results converge toward PA-((CH 3 ) 2 NCN) ∼ 850 kJ/mol and, from these data, a value of 852 kJ/mol has been retained by Hunter and Lias. 6 Calculations at the G2, G3, and G4MP2 levels provide a slightly higher value of 860 kJ/ Moreover the incoming proton is in a plane perpendicular to the Finally, it is also of interest to note that the proton affinity at the amino nitrogen is close to 700 kJ/mol for cyanamide and attains ∼790 kJ/mol for dimethyl cyanamide (G4MP2 calculation, Table 12; see also Raczinska et al 110 ). Thus, the gain in basicity afforded by the two additional methyl groups is larger for the amino nitrogen than for the cyanide nitrogen.…”

“…The results converge toward PA((CH 3 ) 2 NCN) ∼ 850 kJ/mol and, from these data, a value of 852 kJ/mol has been retained by Hunter and Lias . Calculations at the G2, G3, and G4MP2 levels provide a slightly higher value of 860 kJ/mol (Raczinska et al; this work). An additional remark concerns the geometrical change occurring during the protonation of NH 2 CN and (CH 3 ) 2 NCN.…”

“…Moreover the incoming proton is in a plane perpendicular to the planar NH 2 CN or NC 2 CN skeletons: in other words, the protonated forms are allenic‐like species. Finally, it is also of interest to note that the proton affinity at the amino nitrogen is close to 700 kJ/mol for cyanamide and attains ∼790 kJ/mol for dimethyl cyanamide (G4MP2 calculation, Table ; see also Raczinska et al). Thus, the gain in basicity afforded by the two additional methyl groups is larger for the amino nitrogen than for the cyanide nitrogen.…”