N-substituted akyl- and nonalkylpiperidines: Equatorial, axial or intermediate conformations?

Journal of Molecular Structure

Gao

et al. 2017

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a b s t r a c tMolecular structure and conformational behavior of N-phenylpiperidine (NPhP) were investigated by synchronous gas-phase electron diffraction/mass spectrometry (GED/MS) and quantum chemistry. Due to influence of steric repulsion and hyperconjugation, NPhP may exist in two conformers, equatorial and axial chair forms. Both experiment and theoretical calculations suggest a C 1 symmetry of the conformers, with the plane perpendicular to the phenyl group turned by ca. 30e40 (equatorial) and 0e20 (axial) about the plane perpendicular to the piperidine ring symmetry plane. According to the QC calculations, NPhP may exist as two conformers, equatorial and axial, with a ratio of Eq:Ax ¼ 92:8 (B3LYP), 87:13 (B3LYP-GD3), 84:16 (M06-2X), 83:17 (MP2/6-311G**) and 76:24% (MP2/cc-pVTZ). Except for the latter, these values are in good agreement with the experimental GED data of 90(10):10(10)%.A comparative analysis of similar compounds, phenylcyclohexane and 1-phenylheterocyclohexanes, was performed. Conformational properties depend on the C Ph eX bond distance and hyperconjugation between the phenyl ring and the lone pair on the heteroatom. The contribution of the axial form of 1-phenylcyclohexane derivatives increases in the series of the heteroatom X in the cyclohexane ring: C / N / Si / P.

Section: Nitrogen Inversionmentioning

confidence: 84%

“…Recently we started a systematic study of the molecular structures and conformational properties of N-substituted piperidines, including N-cyclohexylpiperidine [6] and alkyl-and nonalkylpiperidines [7], as well as piperazines [8,9].…”

Section: Introductionmentioning

confidence: 99%

Structure and conformational behavior of N-phenylpiperidine studied by gas-phase electron diffraction and quantum chemical calculations

Shlykov

Journal of Molecular Structure

Gao

et al. 2017

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“…Molecular models of compounds 1 and 2 are shown in figure 1. As in cases of formyl-and acetylpiperidine, [5,13] as regards molecule 1, the results of quantum chemical calculation DFT showed that the trifluoroacetyl group was in an intermediate position (B3LYP) or was near to the "axial" structure (B3LYP-D3 and M062X) towards the piperidine ring. The results are almost the same as the result of MP2 calculation.…”

Section: Energies and Barriersmentioning

confidence: 99%

“…However, with regard to formyl-and acetyl-piperidine, substituents are placed in an intermediate position, which means definitions of equatorial and axial structure do not apply. [5] Heterocyclic compounds, containing the N-oxide group, have obvious biological activities. [6] Among representatives of this group, there are compounds having sterilizing, analgesic and anti-convulsant activities as well as active substances having apoptotic, antifungal and antibacterial activities.…”

Section: Introductionmentioning

confidence: 99%

Molecular structure of N‐Trifluoroacetylpiperidine 1 and 4‐Methylpyridine‐N‐Oxide studied by Gas Electron Diffraction and Quantum Chemical Calculations

Long

Vietnam Journal of Chemistry

Trang

et al. 2020

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Molecular structures of N-Trifluoroacetylpiperidine 1 and 4-Methylpyridine-N-oxide 2 are studied by gas-phase electron diffraction/mass spectrometry (GED / MS) and quantum chemical (QC) calculations. The strong conjugation of the electron lone pair on the nitrogen atom and the double bond of the carbonyl cause molecule 1 to exist solely in the form of structure whereby the trifluoroacetyl group is in an intermediate position. As regards compound 2, the results of quantum chemical calculations and GED showed that molecule 2 has molecular symmetry in the form of CS with the flat pyridine ring. There were no hydrogen atoms on the flat surface of the pyridine ring. Structure parameters obtained from the experiments are appropriate for the parameters from quantum chemical calculation DFT for irreplaceable Noxide molecule. The presence of the CH3 substituent in 2 results in an increase of electron density and a decrease of the ipso angle, and an increase of r (N-O) in comparison with irreplaceable pyridine-N-oxide.

“…For a variety of alkyl-, alkenyl-, alkynyl-, aryl-piperidine and R n X-piperidine (R=H, CH 3 ; n=1: X=O, S; n=2: X=N, P) derivatives the conformations and conformational preferences were described recently [9][10][11][12]. The present study continues our systematic investigations of Npiperidine derivatives by exploring the conformational behavior and geometric structure of CNP, which was studied by gas-phase electron diffraction/mass spectrometric (GED/MS) experiments, IR spectroscopy and extended quantum chemical (QC) calculations.…”

Section: Introductionmentioning

confidence: 99%

Intramolecular inversions, structure and conformational behavior of gaseous and liquid N-cyanopiperidine. Comparison with other 1-cyanoheterocyclohexanes

Shlykov

Journal of Molecular Structure

Weber

2017

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The molecular structure and intramolecular inversions of N-cyanopiperidine (CNP) were studied by various quantum chemical methods and using synchronous gas electron diffraction/mass spectrometry (GED/MS) and IR spectroscopy. The contribution of the equatorial conformer was found GED 52(6), IR 60(6), B3LYP 77, B3LYP-GD3 67, M062X 59, MP2 54%. Interpolation using CCSD(T)-F12/CBS predicts E axE eq =0.26 kcal/mol. The NBO analysis shows a strong hyperconjugation of the nitrogen atom lone pair with the triple bond making the r(N-C CN)=1.355(3) Å distance as short as a typical sesquilateral bond. A low energy barrier for the Eq→Ax transition of 1.0-2.3 kcal/mol in CNP leads to difficulties in NMR conformational ratio studies. This leaves the GED method almost as the only experimental method to study both structural and conformational properties. Conformational properties of the related compounds are found to depend on the X-C CN bond distance and hyperconjugation between the CN group and the lone pair on the heteroatom. The contribution of the axial form of 1-cyano-heterocyclohexanes increases in the series of heteroatoms X in the cyclohexane ring: NCSiP.