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An ab initio quantum-chemical study of the compounds HC≡CXCH3 (X = O, S, Se)

“…One lone electron pair is a hybrid orbital (n s ), and the other, virtually pure p x orbital (n p ). In the hybrid orbitals of the lone electron pairs n s , the These values are close to those obtained previously in the series of aromatic [16] and vinyl [17] compounds with the corresponding heteroatoms. According to the results given in Table 1, in terms of the NBO formalism, n s and n p of the X atoms appreciably differ in the energy.…”

“…and Se atoms with the acetylenic moiety exceeds the energy of the resonance interaction of n p of the respective heteroatoms with the aromatic ring in the C 6 H 5 XMe molecules [16] and with the double bond in the H 2 C=CHXMe molecules [17].…”

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.…”

“…According to the results of NBO analysis, the energy of interaction of the unshared electron pair on the oxygen atom with the antibonding orbital of the vinyl C=C bond [electron density transfer n(O σ ) → σ*(C=C)] in the s-cis conformer considerably exceeds the corresponding energy for the s-trans conformer due to more favorable orientation of the interacting orbitals [16]. This contribution to the total energy gives rise to appearance of an energy minimum corresponding to the s-cis structure and hence to s-cis-trans rotational isomerism in the series of vinyl ethers.…”

Long-range 19F-1H spin-spin coupling as an indication of conformational equilibrium of fluoro-substituted aryl vinyl sulfides

“…It was found that alkyl vinyl sulfides give rise to rotational isomerism related to internal rotation about the S-C α bond. According to the results of nonempirical quantum-chemical calculations with account taken of electron correlation in terms of the second-order perturbation theory using Pople's triplesplit basis set, the molecule of methyl vinyl sulfide adopts preferentially planar s-cis conformation [3]. The second, less energetically favorable conformer is represented by two quasi-degenerate nonplanar gauche structures separated by a low rotational barrier corresponding to the planar trans conformer [3].…”