Generalized Anomeric Effect on N-Alkyl-amino Cation Affinities: A G2(+)_M Investigation

Abstract

The gas-phase N-alkyl-amino-cation affinities (NAACA) of archetypal anionic main-group element hydrides across the Periodic Table have been investigated by means of a modified G2(+) scheme. The reactions studied include R(2)NB → R(2)N(+) + B(-) (R = H, Me; B = XH(n), n = 0-3; X = F, Cl, Br, O, S, Se, N, P, As, C, Si, Ge). Our calculations indicate that the reasonable linear correlations between NAACA and proton affinities (PA) only exist within the Period 2 anions, including H(3)C(-), H(2)N(-), HO(-), and F(-), or the anions within Periods 3-4 in the Periodic Table, which is significantly different from the alkyl cation affinities, where there is a reasonable correlation between the computed alkyl cation affinity and PA values of the set of anionic main-group element hydrides. The interesting differences can be ascribed to the generalized anomeric effect induced by n(N) → σ*(X-H) negative hyperconjugation found in R(2)NXH(n), with central atom X belonging to Groups 14-16 (X = O, S, Se, N, P, As, C, Si, Ge).

G2(+)M Study on N-Alkylamino Cation Affinities of Neutral Main-Group Element Hydrides: Trends Across the Periodic Table

“…Two minima, anti - and syn -conformations, and two TS structures, N-inv -TS, and N-X-rot -TS, are located on the G2(+) M PES of HXNH 2 + (X = F, Cl, Br). The anti -conformer is favored over the syn -conformer by 25.0 kJ/mol for HFNH 2 + , but by less than about 5.0 kJ/mol for HClNH 2 + and HBrNH 2 + , which is similar to our previous results on the anti - versus syn -comformers of H 2 NOH and H 2 NXH (X = S, Se) . It can be mainly ascribed to the stronger n (N) → σ*(X–H) (X = Cl, Br) negative hyperconjugation that leads to a N–X (X = Cl, Br) bond in the syn -conformer that is shorter than that in the anti -conformers.…”

“…This trend, AMCA > NDMAMCA, resembles the previously reported MCA > i -PCA . More recently, we calculated AMCA and NDMAMCA of anionic MGH of groups 14–17 within the periods 2–4 by the G2(+) M method . It is found that there is an important intramolecular orbital interaction, i.e., negative hyperconjugation, in H n –1 XNR 2 (R = H, Me), which describes the donation of electron density from a filled p -orbital to a neighboring antibonding σ*-orbital.…”

“…ΣE (2) [n(N) → σ*(X−H)] and ΣE (2) [n(X) → σ*(N− H)] in H n XNH 2 + and H n−1 XNH 2 (in Parentheses); ΣE (2) [n(N) → σ*(X−H)] and ΣE (2) [n(X) → σ*(N−C)] in H n XNMe 2 + and H n−1 XNMe 2 (in Parentheses) (X = N, P, As, O, S, Se, F, Cl, Br) a E (2) values are under printing threshold. 23 there is another n(X) → σ*(N−H) hyperconjugation in H n XH 2 N + in addition to the main n(N) → σ*(X−H) interaction. Obviously the latter interaction is stronger than the former one.…”

“…The anti-conformer is favored over the syn-conformer by 25.0 kJ/mol for HFNH 2 + , but by less than about 5.0 kJ/mol for HClNH 2 + and HBrNH 2 + , which is similar to our previous results on the anti-versus syn-comformers of H 2 NOH and H 2 NXH (X = S, Se). 23 It can be mainly ascribed to the stronger n(N) → σ*(X−H) (X = Cl, Br) negative hyperconjugation that leads to a N−X (X = Cl, Br) bond in the syn-conformer that is shorter than that in the anti-conformers. The hyperconjugation partially counterbalances the Pauli repulsion in these synconformers.…”

Strong Hyperconjugative Interactions in Isolated and Water Complexes of Desflurane: A Theoretical Investigation

“…This value is not the energy difference between the two conformers but rather represents the major impact on their conformational preference. The local interaction (attraction or repulsion) between the nonbonded atoms can also influence the conformational preference. − In the present systems, the A conformer can be stabilized by an electrostatic interaction between the positively charged H8 atom and the negatively charged F7 atom (H8···F7 = 2.469 Å). Structure B can be stabilized by a electrostatic interaction between the H5 and F9 atoms (H5···F9 = 2.677 Å).…”