Nikita N. Lukzen

Investigation of the magnetic field dependence of CIDNP in multinuclear radical pairs. 1. Photoreaction of histidine and comparison of model calculation with experimental data

Magnetic field effects on CIDNP formed in the diamagnetic products of geminate recombination of photo-generated radical pairs are calculated at arbitrary field strength. The simulations are based on a model that uses a full quantum mechanical description of the polarized nuclei and a semiclassical account of all other nuclear spins. The results are compared with CIDNP experiments on the photoreaction between N-acetylhistidine and 2,2′-dipyridine in fields between 0 and 7 T employing field cycling between a variable field of polarization and a fixed field of detection. To allow quantitative comparison the simulation takes into account the effects of adiabatic field change and of RF excitation pulse angle on the NMR intensity. Both net and multiplet polarization as observed in two coupled spin pairs (CH2 protons in β position of histidine, and protons in H2,H4 position of its ring) are well reproduced over the whole field range. The wide applicability of the theoretical model and the possibility to extend it to larger spin systems make it particularly useful for the analysis of CIDNP in protein systems.

Consistent Treatment of Spin-Selective Recombination of a Radical Pair Confirms the Haberkorn Approach

In the present work, we have shown that consistent derivation of the kinetic equations describing the electron spin-selective recombination of radical pairs confirms the conventional Haberkorn approach. The derivation has been based on considering the interaction of the reactive system (radical pair and product state) with the thermal bath. The consistency of this approach has also been substantiated by numerical simulations performed for the purely quantum mechanical model of the recombining radical pair. Finally, we have shown that the quantum Zeno effect on radical pair recombination is not an exclusive feature of the approach recently proposed by Kominis, as it should be present at any rate of the singlet-triplet dephasing in the radical pair, which always accompanies the recombination process.

Theory of spin-peierls transitions in chains of exchange clusters.

A theoretical approach to the analysis of magnetostructural phase transitions of chain polymeric heterospin complexes is suggested. The approach is based on a model of the spin-Peierls transition in chains of exchange clusters. The chain elasticity parameter is found to be a main factor determining the order of phase transition.

The influence of the singlet radical-pair decay rate on RYDMR and SNP spectra, and the mean RP lifetime

Abstract Calculations within the slope of a two-site model have revealed that fast decay of a singlet radical pair (RP) could inhibit T→S transition of the RP. In this case, the splittings observed in ESR (RYDMR, SNP) spectra should be equal to half of the corresponding hfi constants even in the absence of exchange interaction. Calculations of SNP spectral for acyl-alkyl flexible biradicals have shown that the halved splitting as compared to the hfi constant, which is observed experimentally, can be explained with no exchange interaction considered. it is shown that in certain cases, the mean RP lifetime can increase with increasing singlet RP decay rate.

Diffusion-influenced reactions of particles with several active sites.

The main concern of the present work is consideration of sterically specific liquid-phase reactions in the case where one of the reactants has several active sites. In kinematic approximation we derived compact formulas for partial rate constants of individual active sites. Reaction rates are expressed via the kinetic rate constants and convolutions of the free Green functions over reaction zones. These results are valid for arbitrary geometry of reactants and active sites. Effects of mutual influence of the active sites were studied, their simple estimate was proposed.

Application of integral encounter theory to account for the spin effects in radical reactions: I. Δg and spin relaxation effects on recombination kinetics of free radicals

Abstract Integral encounter theory is applied to investigate spin effects in radical recombination. Effects of electron spin relaxation and of S/T 0 mixing by an external magnetic field are considered. An analytical expression for the stationary rate constant is obtained from which appropriate conditions for experimental investigations are predicted.

The integral encounter theory of multistage reactions containing association–dissociation reaction stages

The matrix-form integral encounter theory (IET) is generalized to consider bulk reversible association-dissociation reactions. The many-particle derivation of the IET kinetic equations is based on the extension of the formalism of effective particles developed earlier. As a result, an infinite hierarchy for vector correlation patterns is obtained and truncated in the way suggested earlier for elementary reactions. We believe that such an extension of the IET is utterly important for the kinetic description of a great number of diffusion-influenced chemical reactions.

Highly Efficient Polarization of Spin-1/2 Insensitive NMR Nuclei by Adiabatic Passage through Level Anticrossings.

A method is proposed to transfer spin order from para-hydrogen, that is, the H2 molecule in its singlet state, to spin-1/2 heteronuclei of a substrate molecule. The method is based on adiabatic passage through nuclear spin level anticrossings (LACs) in the doubly rotating frame of reference; the LAC conditions are fulfilled by applying resonant RF excitation at the NMR frequencies of protons and the heteronuclei. Efficient conversion of the para-hydrogen-induced polarization into net polarization of the heteronuclei is demonstrated; the achieved signal enhancements are about 6400 for (13)C nuclei at natural abundance. The theory behind the technique is described; advantages of the method are discussed in detail.

Spin crossover in an elastic chain of exchange clusters beyond mean field approximation

Exact analysis of spin crossover in infinite elastic chain of two-spin exchange clusters was performed theoretically beyond mean-field approximation. Statistical mechanics of the system was exactly calculated by means of the transfer matrix technique. A possibility of spin-Peierls-like magneto-structural transition in a one-dimensional chain was shown as a result of interrelation of spin and elastic subsystems of the chain. The importance of the boundary conditions for spin crossover to occur was shown. The smoothness of spin crossover depends on the parameters of chain elasticity and the crossover position on the temperature scale is defined by the dependence of exchange integral on the cluster deformation. The proposed model qualitatively describes the main scenario of spin crossover effect in the chain polymer heterospin complex of copper(ii) hexafluoroacetylacetonate with methyl pyrazol-substituted nitronyl nitroxide containing two-spin exchange clusters.

Analysis of Nutation Patterns in Fourier-Transform NMR of Non-Thermally Polarized Multispin Systems

Abstract The complex spin order of hyperpolarized multispin systems giving rise to anomalous NMR spectral patterns that vary with the RF excitation angle is analyzed by decomposing its nutation behavior in a superposition of Fourier harmonics. The product operator formalism is applied to calculating the spectral contributions of the various mutual alignments of scalar coupled spins. Two cases are treated, namely systems exhibiting only differences in population of their spin states and systems showing in addition zero-quantum coherences between states, a situation often seen at hyperpolarization. After deriving the general solution a number of representative examples are discussed in detail. The theoretical treatment is applied to analyzing the spin order observed in a hyperpolarized two-spin system that is prepared in the singlet state by para-hydrogen induced polarization.