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. Due to its numerous types of J couplings, acetonitrile
is a very convenient molecule against which to test various basis sets
or to select the best basis set for a given study. The results show reasonable
agreement between calculations and experiment. 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 assuming
(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.
Aleksandr B. Sahakyan /go to my home page/