Authors:
AB Sahakyan, AA Shahkhatuni, AG Shahkhatuni, HA Panosyan
Journal name: 
Magn Reson Chem
Citation info: 
46(1):63-68
Abstract: 
Dielectric permittivity (epsilon) and temperature effects on indirect spin-spin coupling constants were studied using acetonitrile as a probe molecule. Experiments were accompanied by hybrid DFT (density functional theory) studies, where the solvent was modeled using the polarization continuum model. Owing to its numerous types of J-couplings, acetonitrile is a very convenient molecule against which various basis sets can be tested or the best basis set can be selected for a given study. The results show reasonable agreement between calculated and experimental values. According to our data, scalar spin-spin coupling constants undergo substantial shifts at lower values of the dielectric constant. Thus J-coupling values are not transferable between measurements made at differing epsilon-conditions, and the assumption of the epsilon-independence of the J-coupling can lead to crucial mistakes in experiments using low-epsilon media. Dielectric permittivity also causes small geometric fluctuations within the molecule, which themselves can affect J-coupling values. Examinations of the results computed with frozen and relaxed geometries show that geometry mediation mostly affects the spin-dipole term of the J-coupling; hence, for accurate evaluation of the latter, frozen geometries are not acceptable. Another interesting fact revealed is the connection between the solvent dielectric properties and the temperature-dependence slopes of J-couplings in corresponding media.
DOI: 
http://doi.org/10.1002/mrc.2137
E-pub date: 
31 Jan 2008