Intrinsic information content of NMR dipolar couplings: a conformational investigation of 1,3-butadiene in a nematic phase

Celebre, Giorgio, Concistré, Maria, De Luca, Giuseppina, Longeri, Marcello and Pileio, Giuseppe (2006) Intrinsic information content of NMR dipolar couplings: a conformational investigation of 1,3-butadiene in a nematic phase. ChemPhysChem, 7 (9), 1930-1943. (doi:10.1002/cphc.200600220).

Abstract

The conformational equilibrium of 1,3-butadiene in a condensed fluid phase is investigated by liquid-crystal NMR spectroscopy. The full set of D _HH and D_CH dipolar couplings is determined from the analysis of the ¹H spectra of the three 1,3-butadiene most-abundant isotopomers (i.e. the all ¹²C and the two single-labeled ¹³C isotopomers) for a total of 21 independent dipolar couplings. A very good starting set of spectral parameters for the analysis of the ¹H spectrum is determined in a semiautomated way by the analysis of the (N-1) (specifically, N = 6, the number of 1/2 spin nuclei in the spin system) quantum refocused (5QR), and not (5Q), spectra. As an alternative approach, a Monte Carlo (MC) numerical simulation, capable of predicting the solute ordering, is tested to simulate the 5QR spectrum. The set of D _ij couplings is very good, proving that the MC method can represent a novel, valid alternative to the existing spectral simplification procedures. The experimentally determined dipolar-coupling data set is fully compatible with the 1,3-butadiene conformational distribution reported in the literature for isolated molecules, indicating the presence of about 99% of s-trans conformer. With regards to the remaining 1 %, in spite of the direct and very strong dependence of the observables on the molecular structure, it was not possible to discriminate between the planar s-cis and s-gauche forms, both of which produce a very good fit of the dipolar couplings. Vibrational corrections, up to the anharmonic term, were applied; the calculated geometrical parameters are in good - although not exact - agreement with those reported in the literature from experimental and theoretical investigations. This result can be considered as supporting the methodology used for obtaining the structure and conformational distribution of a flexible molecule in a liquid phase.

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Contributors

Author: Giuseppe Pileio

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