Se–N interactions in selenohydroxylamine: a theoretical study

Abstract

The potential energy surfaces of thiohydroxylamine HS-NH 2 , 1, and selenohydroxylamine HSe-NH 2 , 2, have been searched, using ab initio and density functional methods, to study the conformational preferences. There are two minima on the path of rotation around the Se-N bond in 2. High accuracy G2MP2 calculations showed that the Se-N rotational barrier in 2 is 5.41 kcal mol Ϫ1 , which is 1.16 kcal mol Ϫ1 less than the S-N rotational barrier in 1. The inversion around N in 1 and 2 goes through low energy barriers of 1.79 and 2.44 kcal mol Ϫ1 at the same level respectively. Charge analysis using the natural population analysis (NPA) method has been performed to understand the electronic factors responsible for the observed trends in the Se-N interactions. The strength of the negative hyperconjugation in 2 has been estimated using natural bond orbital (NBO) analysis and by studying the substituent effect.

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

“…There have been several previous computational studies on the conformations of NH 2 XH (X = O, S, Se). − It is found that the anti - is favored over the syn -conformer by about 5 kcal mol –1 for H 2 NOH, but only by less than 1 kcal mol –1 for H 2 NSH and H 2 NSeH, which can be mainly ascribable to the stronger n (N) → σ*(X–H) (X = S, Se) hyperconjugation in syn , leading to a shorter N–X (X = S, Se) bond length in syn -conformers than those in anti -conformers.…”

Dynamic1H NMR Spectroscopic Study of the Ring Inversion inN-Sulfonyl Morpholines—Studies on N−S Interactions

“…These conformational preferences of syn- or anti periplanar structures 8a and 8c have been generally observed in most of the sulfonamides, even in the planar sulfonamides 6 and 7 (cf. Scheme ), and have been rationalized in terms of negative hyperconjugation (anomeric effect) between the nitrogen lone pair and the electron-deficient C−S bond (σ* C−S ). ,,− The relevant negative hyperconjugation is suggested theoretically to be strongly enhanced in α-sulfonyl carboanions, which are isoelectronic to pyramidal sulfonamides. , The localization of the nitrogen lone pair in the pyramidal arrangement, which increases the electron-donating ability of the nitrogen nonbonding orbital, was postulated to counterbalance the attenuation of negative hyperconjugation due to the elongation of the N−S bond.…”

Anomeric Answer to Sulfenamide Stability and α-Nucleophilicity

“…Sulfenamides are also known to produce diastereomeric compounds according to NMR, where the S–N bond is the chiral axis because of a rotational barrier about S–N. This barrier has been attributed to steric interactions , between N- and S-substituents, and in some cases, a negative hyperconjugative effect between n N → σ* S–C has been suggested but not quantified . Such interactions between lone-pair electrons and σ* S–C orbitals have been exploited to bias specific conformers in drug molecules .…”

“…21 We demonstrate the existence of an electronic interaction between the −NH 2 lone-pair electrons and the σ* S−C bond via crystallographic and in silico methods. Previously, this proposed interaction was referred to as negative hyperconjugation; 28 however, given that the electronic interaction is ∼4−5 kcal/mol and similar in energy to classical anomeric interactions, this interaction is better described as a true anomeric interaction. We further propose that the poor stability associated with S-unsubstituted sulfenamides (as in 1−3) can, in part, be attributed to a lack of a suitable, low-lying σ* S−C acceptor orbital to stabilize the adjacent S−N bond.…”