NMR spectroscopy investigation of the cooperative nature of the internal rotational motions in acetophenone

De Luca, Giuseppina, Longeri, Marcello, Pileio, Giuseppe and Lantto, Perttu (2005) NMR spectroscopy investigation of the cooperative nature of the internal rotational motions in acetophenone. ChemPhysChem, 6 (10), 2086-2098. (doi:10.1002/cphc.200500134).

Abstract

The proton NMR spectrum of the doubly enriched acetophenone-carbonyl, methyl-¹³C₂ isotopomer dissolved in a liquid-crystalline solvent (LXNMR) was analyzed to yield a data set of 19 dipolar couplings. The presence of so many couplings, and in particular the dependence of some of them on the acetyl carbons enabled the investigation of the structure of the acetyl moiety and of possible cooperative motions about the aryl-carbonyl and carbonyl-methyl bonds. Methodological aspects, and approximations relating to the application of the vibrational correction procedure in the presence of large-amplitude torsional motions, are discussed. Results show that it is possible to discriminate between a continuous and a discrete conformer distribution about the angle φ₁ but not among a few proposed continuous shapes of U_iso({φ}). In this study, the use of dipolar couplings with a non-negligible contribution from the indirect spin-spin coupling tensor J, (D_C8C9 in our case), for structural determination is extended from rigid to flexible molecules. The 1/2J_C8C9 ^aniso contribution was derived theoretically using the density functional theory linear response (DFT-LR) first-principles calculation of the J_C8C9 spin-spin coupling tensor.

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Contributors

Author: Giuseppe Pileio

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