Substituent effects on scalar J(13C, 13C) couplings in pyrimidines. An experimental and DFT study

One- two- and three 13C, 13C (n = 1, 2, 3) scalar couplings, (n)J(C,C) in a set of pyrimidine derivatives were studied both experimentally at natural abundance and theoretically by their DFT calculation of all four contributions. Trends of non-contact terms are discussed and substituent effects are rationalized, comparing some of them with the corresponding values in benzene and pyridine. Although substituent effects on non-contact terms are relatively important, the whole trend is dominated by the Fermi contact term. According to the current literature, substituent effects on 1J(C,C) couplings in benzene derivatives are dominated by the inductive effect, which, apparently, is also the case in nitrogen heteroaromatic compounds. However, some differences observed in this work for substituent effects on 1J(C,C) couplings in pyrimidine derivatives suggest that in the latter type of compounds substituent effects can be affected by the orientation of the ring nitrogen lone pairs.     

Direct measurement of dynamic frequency shift induced by cross-correlations in 15N-enriched proteins.

We describe a new NMR experimental scheme that allows the direct determination of the dynamic frequency shift induced by chemical shift anisotropy/dipolar interaction (CSA/DD) cross-correlations in 15N-enriched proteins. Its principle consists of comparing two rates of polarisation transfer between the amide proton and nitrogen. The first rate, which is independent of the dynamic frequency shift, is based on a selective Hartmann-Hahn coherence transfer. The second rate, which depends on the dynamic frequency shift, is based on a free evolution of the transverse magnetisation. We report experimental validation of this approach by measuring the average dynamic frequency shift due to CSA/DD cross-correlations in the calcium-binding protein D9k. The method may also be applicable to the measurement of dynamic frequency shift induced by cross-correlations between the Curie spin and dipolar interactions.     

Simple non-empirical calculations of the zero-field splitting in bis(aquo) bis(malonato) nickel(II)

Summary A simple non-empirical method is applied to calculate the splittings of the ground state triplet, caused by the spin-orbit coupling, in bis(aquo) bis(malonato) nickel(II), allowing for a certain geometry variation. The calculations yield splittings on the order of 10–20 cm^–1. The comparison of exact (within model) and second-order perturbation theory calculations indicate that the spin Hamiltonian formalism is valid. The implications of the results for the theory of nuclear spin relaxation in paramagnetic complexes in solution are discussed.     

Finite perturbation MCSCF and CI calculations of nuclear spin-spin coupling constants for some molecules with multiple bonds

Non-empirical finite perturbation calculations at the Hartree-Fock, multiconfigurational self-consistent field and configuration interaction levels of approximation are presented for the Fermi contact contribution in multiply-bonded molecules ethene, formimide, formaldehyde, ethyne and hydrogen cyanide. The finite perturbation-multiconfigurational SCF (FPMC) method (with few configurations) is free from the UHF triplet instabilities normally present in single configuration coupled Hartree-Fock (CHF) calculations of the Fermi contact (and spin-dipolar) contribution for π-bonded systems. The behaviour of coupling constants, calculated using FPMC for multiply-bonded systems parallels the behaviour of the CHF coupling constants in comparable systems with single bonds only. The effects of dynamic electron correlations are important and are obtained using the Cl method. After accounting for the orbital contribution by means of the single configuration CHF method, agreement with experiment is excellent for systems containing only carbon and hydrogen, when a double-zeta quality basis set is used. For systems containing nitrogen and oxygen agreement is still reasonable, but the use of larger basis sets seems to be necessary if good agreement with experiment is to be obtained.     

Quinuclidine complex with alpha-cyclodextrin: a diffusion and 13C NMR relaxation study

The stability of an inclusion complex of quinuclidine with alpha-cyclodextrin in solution was investigated by NMR measurements of the translational diffusion coefficient. A 1:1 stoichiometry model yielded an association constant of 35 +/- 3 M(-1). The guest molecules exchange rapidly between the host cavity and the bulk solution. The reorientational dynamics of the guest and host molecules was studied using carbon-13 NMR relaxation at two magnetic fields. The relaxation of the host nuclei showed very little dependence on the guest-host concentration ratio, while the 13C spins in quinuclidine were sensitive to the solution composition. Using mole-fraction data, it was possible to extract the relaxation parameters for the bound and free form of quinuclidine. Relaxation rates of the guest molecule, free in solution, were best described by an axially symmetric model, while the data of the complex species were analyzed using the Lipari-Szabo method. Applying the axially symmetric model to the complexed quinuclidine indicated that the anisotropy of its reorientation in the bound form was increased.     

Comparison of different methods for calculating the paramagnetic relaxation enhancement of nuclear spins as a function of the magnetic field

The enhancement of the spin-lattice relaxation rate for nuclear spins in a ligand bound to a paramagnetic metal ion [known as the paramagnetic relaxation enhancement (PRE)] arises primarily through the dipole-dipole (DD) interaction between the nuclear spins and the electron spins. In solution, the DD interaction is modulated mostly by reorientation of the nuclear spin-electron spin axis and by electron spin relaxation. Calculations of the PRE are in general complicated, mainly because the electron spin interacts so strongly with the other degrees of freedom that its relaxation cannot be described by second-order perturbation theory or the Redfield theory. Three approaches to resolve this problem exist in the literature: The so-called slow-motion theory, originating from Swedish groups [Benetis et al., Mol. Phys. 48, 329 (1983); Kowalewski et al., Adv. Inorg. Chem. 57, (2005); Larsson et al., J. Chem. Phys. 101, 1116 (1994); T. Nilsson et al., J. Magn. Reson. 154, 269 (2002)] and two different methods based on simulations of the dynamics of electron spin in time domain, developed in Grenoble [Fries and Belorizky, J. Chem. Phys. 126, 204503 (2007); Rast et al., ibid. 115, 7554 (2001)] and Ann Arbor [Abernathy and Sharp, J. Chem. Phys. 106, 9032 (1997); Schaefle and Sharp, ibid. 121, 5387 (2004); Schaefle and Sharp, J. Magn. Reson. 176, 160 (2005)], respectively. In this paper, we report a numerical comparison of the three methods for a large variety of parameter sets, meant to correspond to large and small complexes of gadolinium(III) and of nickel(II). It is found that the agreement between the Swedish and the Grenoble approaches is very good for practically all parameter sets, while the predictions of the Ann Arbor model are similar in a number of the calculations but deviate significantly in others, reflecting in part differences in the treatment of electron spin relaxation. The origins of the discrepancies are discussed briefly.