Conformational schemes: An available tool for the assignment of NMR-measured barriers, demonstrated with the example of crowded piperidines

Anatoly M. Belostotskii, Hugo E. Gottlieb, Pinchas Aped, Alfred Hassner

Research output: Contribution to journalArticlepeer-review

44 Scopus citations

Abstract

MM3-derived full conformational schemes are proposed as a powerful and convenient tool for the currently problematic assignment of NMR-measured barriers for flexible systems that possess more than one type of independent intramolecular motion. Hindered piperidines were chosen as a molecular model with seven possible intramolecular dynamic processes. The free energies of activation for methyl group topomerization in 1,2,2,5,5-pentamethyl-, 1-ethyl-2,2,6,6-tetramethyl-, and 1-butyl-2,2,6,6-tetramethylpiperidines were determined at different temperatures by means of line-shape analysis of 13C NMR spectra. Schemes of conformational transformations for the N-Me and N-Et compounds were created with MM3-based methodology. These schemes permit the assignment of the measured barriers to ring inversion for the N-Me compound and to ring inversion - nitrogen inversion, ring inversion, and C - N rotation for the N-Alkprimary piperidines (for the N-Et and N-Pr derivatives the experimental barriers had previously been attributed to isolated C - N rotation only). A unique dynamic process for tertiary amines, isolated nitrogen inversion, is described for the N-alkylpiperidines with an N-substituent bulkier than Me.

Original languageEnglish
Pages (from-to)449-455
Number of pages7
JournalChemistry - A European Journal
Volume5
Issue number2
DOIs
StatePublished - 1999

Keywords

  • Conformation analysis
  • Molecular dynamics
  • NMR spectroscopy
  • Nitrogen heterocycles

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