ABSTRACT
Deuterium isotope substitution in one part of a molecule could produce a significant effect on chemical shifts of neighbouring nuclei as well as on nuclei, located far from the site of replacement. To estimate how far this influence could extend the reaction of proton-deuterium exchange of several 2-alkanones in deuterated methanol solutions of 1-methyl 3-ethyl imidazolium acetate ionic liquid (IL) was studied in detail using 13C NMR spectroscopy. Deuteration occurs in alkyl groups of 2-alkanones neighboring the ketonic group via keto-enol tautomerization catalyzed by IL. In the course of the reaction, various isotopomers with various deuteration levels are formed, among which a dynamic equilibrium is established. The number of substituted deuterons affects not only the multiplicity and chemical shifts of directly bonded carbon, but carbons in the groups further along the alkyl chain. Moreover, the latter groups better indicate the level and site of substitution.
ABSTRACT
The reaction of the proton-deuterium exchange of acetone in imidazolium-based ionic liquid (IL)-deuterium oxide mixtures was studied in detail via NMR spectroscopy. Certain ILs exhibit considerable catalytic properties and contribute to the course of reaction up to the complete deuteration. The efficiency of deuterium exchange crucially depends on the features of ILs; the type of anion and chain length of cation. The linear secondary isotope effects on the NMR chemical shifts of the 13C atoms in acetone were observed depending on the deuteration level of the molecule.
ABSTRACT
Electric field (EF) induced changes of one-bond indirect spin-spin coupling constants are investigated on a wide range of molecules including peptide models. EFs were both externally applied and internally calculated without external EF application by the hybrid density functional theory method. Reliable agreement with experimental data has been obtained for calculated one-bond J-couplings. The role of the EF sign and direction, internal and induced components, hydrogen bonding, internuclear distance and hyperconjugative interactions on the one-bond J-coupling vs EF interconnection is analyzed. A linear dependence of 1J on EF projection along the bond is obtained, if the bound atoms possess different enough electron densities and an EF determined by the electronic polarization exists along the bond. Accentuating the 1JNH couplings as possible EF sensitive parameters, a systematic study is done in two sets of molecules with a large variation of the native internal EF value. The most EF affected component of the 1JNH coupling constant is the spin-dipole term of Ramsey's formulation; however, in the total J-coupling formation, the EF influence on the Fermi contact term is the most significant. The induced EF projection along the bond is 6.7 times weaker in magnitude than the simulated external uniform field. The absolute EF dependence of the one-bond J-coupling involves only the internal field, which is the sum of the induced field (if the external field exists) and the internuclear field determined by the native polarization. That linear and universal dependence joins the corresponding couplings in a diverse set of molecules under various electrostatic conditions. Many types of the one-bond J-couplings can be potentially measured in biomolecules, and the study of their relation with the electrostatic properties at the corresponding sites opens a new avenue to the full exploitation of the NMR measurable parameters with novel and exciting applications.
Subject(s)
Electrons , Models, Chemical , Quantum Theory , Acetamides/chemistry , Electricity , Formamides/chemistry , Hydrogen Bonding , Peptides/chemistry , Sensitivity and Specificity , VibrationABSTRACT
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.
ABSTRACT
To estimate the torsion sensitivity of dipolar coupling, biphenylic molecules were chosen as probes due to their relatively simple structure and the surprisingly high ambiguity of the only flexible parameter-the interring torsion angle. Solution structures of 4,4'-dibromobiphenyl and 4,4'-diiodobiphenyl are reported for the first time in two liquid crystals I52 and ZLI 1695. The comparison of NMR structures of various para-substituted biphenyls (BPs), calculated by the additive potential maximum entropy (APME) approach, shows that the small spread of torsion angle values in case of different solvents and para-substituents is in good agreement with theoretical expectations from hybrid density functional theory (DFT) methods. Furthermore, the real structural changes of interring torsion and the prevalence of solvent effects over para-halosubstitution can be correctly revealed from these small fluctuations.
ABSTRACT
Alignment media used for measuring residual dipolar couplings, such as solutions of filamentous phages, phospholipid mixtures, polyacrylamide gels and various lyotropic liquid crystalline systems were investigated with respect to solvent effects on molecular structure. Structural parameters of the small rigid model compound 13C-acetonitrile were calculated from dipolar couplings and variations from expectation values were used for assessment of solvent effects. Only minor solvent effects were observed for most of the media employed and the measured structural data are in good agreement with microwave data and theoretical predictions.
