Does 2-methylacetophenone comply with steric inhibition of resonance? A direct experimental proof of its nonplanar conformation from a joint ab initio/electron diffraction analysis

D. Hnyk, S. Samdal, O. Exner, D.A. Wann, D.W.H. Rankin

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Abstract

(Figure presented) The structure and conformations of 2-methylacetophenone (1) have been investigated by ab initio calculations carried out at the MP2(full)/6-311++G* level and by gas electron diffraction (GED). According to both methods, 1 exists predominantly as a form with the CεO bond synclinal with respect to the Car-C(O) bond (1B), with a torsional angle [C(6)-C(1)-CεO] of 32.7(24)° as determined by GED and 26.0° from MP2(full)/6-311++G. Calculations also predict the presence of a second conformer, the anticlinal structure (1C), with φ = 140.0°, with an abundance of less than 6%, an amount hardly detectable by GED. Different DFT computational protocols both support a nonplanar form of the predominant conformer (B2PLYP) and are in contradiction (B3LYP, M052x, B98, B97-D) with this experimental finding. The GED results, supported by the calculations that involve long-range correlation, are in a good agreement with 13C NMR spectroscopic investigations, UV spectra, and dipole moment studies. However, previous claims that assumed steric inhibition of resonance caused by a significantly nonplanar conformation with φ close to 90° have been disproved. Steric crowding is evident from the geometrical parameters, particularly from the C(1)-C(2) bond length and from the C(1)-C(2)-C(H 3) and C(2)-C(1)-C(O) bond angles. It is concluded that any explanation of reactivity by steric inhibition of resonance and by other steric factors must be supported by experimental and/or theoretical investigation of the actual molecular shape. © 2010 American Chemical Society.
Original languageEnglish
Pages (from-to)4939-4943
JournalJournal of Organic Chemistry
Volume75
Issue number15
DOIs
Publication statusPublished - 6 Aug 2010

Bibliographical note

Cited By (since 1996):2

Export Date: 1 October 2013

Source: Scopus

CODEN: JOCEA

doi: 10.1021/jo100291r

Language of Original Document: English

Correspondence Address: Hnyk, D.; Institute of Inorganic Chemistry of the ASCR, v.v.i., No. 1001, CZ-250 68 Husinec-Řež, Czech Republic; email: hnyk@iic.cas.cz

Chemicals/CAS: carbon, 7440-44-0; oxygen, 7782-44-7

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Keywords

  • Ab initio calculations
  • Anticlinal structure
  • Bond angle
  • Computational protocols
  • Diffraction analysis
  • Gas electron diffraction
  • Geometrical parameters
  • Long range correlations
  • Molecular shapes
  • Nonplanar conformation
  • Spectroscopic investigations
  • Steric crowding
  • Steric factor
  • Theoretical investigations
  • Torsional angle
  • UV spectrum
  • Conformations
  • Electron diffraction
  • Resonance
  • Ultraviolet spectroscopy
  • Bond length
  • 2 methylacetophenone
  • acetophenone derivative
  • carbon
  • methyl group
  • oxygen
  • unclassified drug
  • ab initio calculation
  • article
  • carbon nuclear magnetic resonance
  • chemical bond
  • chemical reaction
  • chemical structure
  • conformation
  • dipole
  • electron diffraction
  • gas
  • reaction analysis
  • stereospecificity
  • structure analysis
  • ultraviolet spectroscopy

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