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Through space spin-spin coupling 19F-1H as the base for comparative analysis of conformational equilibrium in fluorine-substituted aryl vinyl selenides and sulfides

“…This conclusion was later confi rmed by quantum-chemical calculations on the level MP2/6-31(d) that revealed the presence of the main minimum on the curve of potential energy of internal rotation corresponding to the s-cis-form [4]. The second minimum of the methyl vinyl selenide corresponded to two degenerate gauche-forms divided by a relatively small energy barrier (~0.5 kcal mol -1 ) in the region of s-trans-form [4].The quantum-chemical calculation on the level MP2/6-311(d,р) and the analysis of coupling constants 77 Se-1 H in divinyl selenide revealed that this compound exists as three rotational isomers: non-fl at s-cis/s-trans and two non-fl at s-trans/s-trans distinguished by the values of the torsional angles at the Se-C bonds [5]. Therewith the s-cis/s-trans-form is considerably more populated [5].…”

“…This is governed by the weighted average value of the torsion angles ϑ and Ψ at the bonds Х-C α and Х-C ipso (Х = S, Se) respectively (see structure А). The analysis of the intramolecular interactions in phenyl and vinyl sulfi des and selenides in the framework of the formalism of the Natural Bond Orbitals,NBO [13] showed that the energy of the interaction of the unshared electron pairs of sulfur or selenium atoms with the π*-orbitals of the double bond is signifi cantly higher than the interaction with the π*-orbitals of the aromatic ring [4,14]. This fact results in considerably larger value of the weighted average torsion angle Ψ than ϑ, and in the sterically strained s-cis-conformation of molecules I and II the torsion angle Ψ should be especially large [15,16].…”

“…The decrease in the parameters Δδ and δН Х in the fl uorine-substituted aryl vinyl sulfi de II compared to phenyl vinyl sulfi de (IV) is due to the change in the infl uence of the anisotropy of the aromatic fragment caused by the increase in the torsion angle Ψ [15,16]. The difference between vinyl selenides from vinyl sulfi des consists in weaker energy of the interaction of the unshared electron pairs of the selenium atom with the π*-orbitals of the double bond as compared with the similar interactions of the sulfur atom, and consequently in lower value of the barrier to the internal rotation in the vinyl selenides [4]. Therefore it is a logical expectation that the sterical infl uence of the fl uorine atom in vinyl selenide I would lead to the increase in the torsion angle ϑ at the Se-C α bond.…”

“…It was demonstrated that the methyl vinyl selenide existed as a mixture of rotatonal isomers among which the most stable was the fl at s-cisconformer [3]. This conclusion was later confi rmed by quantum-chemical calculations on the level MP2/6-31(d) that revealed the presence of the main minimum on the curve of potential energy of internal rotation corresponding to the s-cis-form [4]. The second minimum of the methyl vinyl selenide corresponded to two degenerate gauche-forms divided by a relatively small energy barrier (~0.5 kcal mol -1 ) in the region of s-trans-form [4].…”

Effect of aromatic ring anisotropy on the 1H NMR shielding constants and conformational equilibrium of sterically strained aryl vinyl ethers

“…However, analysis of intramolecular interactions in phenyl and vinyl ethers in terms of the natural bond orbital (NBO) approach [13] showed that the energy of interaction between unshared electron pairs on the oxygen atom and π* orbital of the double bond is considerably higher than the energy of interaction with π* orbitals of the aromatic ring [14][15][16][17]. Therefore, the barrier to internal rotation about the O-C i bond is lower than the barrier to rotation about the O-C α bond, and the torsion angle ψ is more labile than φ.…”