Pressure effects on the electron spin relaxation of several radicals in solution

The pressure effect on the decay rate of chemically induced dynamic electron spin polarization (CIDEP) was investigated on several free-radical intermediates in photolysis, and the spin-lattice relaxation times for these radicals were estimated from the decay rates of CIDEP signals at various pressures. The spin-lattice relaxation rates were retarded by increasing external pressure. From the pressure dependence of the spin-lattice relaxation rates the activation volume was estimated. The activation volumes of these radicals divide into two groups; ≈30 cm³ mol⁻¹ for negative ions and ≈10 cm³ mol⁻¹ for neutral radicals.     

Transfer of spin density in molecular complexes of paramagnetic metalloporphyrins

Using the¹H and¹³C NMR method, we have observed delocalization of spin density from the paramagnetic metalloporphyrins to aromatic nitro compounds in chloroform solutions. We have shown that spin transfer occurs directly from the metal atoms. The effectiveness of spin transfer varies in the order Co²⁺ > Mn³⁺ > Fe³⁺ > Cu²⁺, correlating with the accessibility of the d_z2 orbital of the metal containing the unpaired electrons.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1009–1015, May, 1990.We thank A. B. Solov'eva and G. V. Ponomarev for providing the metalloporphyrins.     

Proton spin relaxation in some paramagnetic transition-metal acetylacetonate complexes. Comparison between theory and experiment

Proton spin-lattice (T₁) and spin-spin (T₂) relaxation times have been measured for CH₃ protons in a series of paramagnetic transition-metal acetylacetonate complexes and the results interpreted in terms of current relaxation theory, τ_r, the correlation time for molecular reorientation, was estimated from the ¹³C T₁ in the analogous diamagnetic Co(III) and Pd(II) complexes. Using this approach and treating in detail the effects of fast CH₃ group internal motion good agreement is obtained between theory and experiment. In all cases electron-nuclear dipolar coupling dominates T₁ whereas the hyperfine contribution can be important for T₂.     

Theory of electron paramagnetic relaxation in liquid solutions

Theoretical and Experimental Chemistry -     

Self-decelerating relaxation of the light-induced spin states in molecular magnets Cu(hfac)2L(R) studied by electron paramagnetic resonance

Molecular magnets Cu(hfac)(2)L(R) (hfac = hexafluoroacetylacetonate) called "breathing crystals" exhibit thermally and light-induced magnetic anomalies very similar to iron(II) spin-crossover compounds. They are physically different systems, because the spin-state switching occurs in exchange-coupled nitroxide-copper(II)-nitroxide clusters, in contrast to classical spin crossover in d(4)-d(7) transition ions. Despite this difference, numerous similarities in physical behavior of these two types of compounds have been observed, including light-induced excited spin-state trapping (LIESST) phenomenon recently found in the Cu(hfac)(2)L(R) family. Similar to iron(II) spin-crossover compounds, the excited spin state in breathing crystals relaxes to the ground state on the time scale of hours at cryogenic temperatures. In this work, we investigate this slow relaxation in a series of breathing crystals using electron paramagnetic resonance (EPR). Three selected compounds represent the cases of relatively strong or weak cooperativity and different temperature of thermal spin transition. They all were studied in a neat magnetically concentrated form; however, sigmoidal self-accelerating relaxation was not observed. On the contrary, the relaxation shows pronounced self-decelerating character for all studied compounds. Relaxation curves and their temperature dependence could be fitted assuming a tunneling process and broad distribution of effective activation energies in these 1D materials. A number of additional experimental and theoretical arguments support the distribution-based model. Because self-decelerating relaxation behavior was also found in 1D polymeric iron(II) spin-crossover compounds previously, we compared general relaxation trends and mechanisms in these two types of systems. Both similarities and differences of copper-nitroxide-based breathing crystals as compared to iron(II) spin-crossover compounds make future research of light-induced phenomena in these new types of spin-crossover-like systems topical in the field of molecule-based magnetic switches.     

Nuclear spin relaxation in benzyl fluoride : v- 19f intra and intermolecular relaxation of benzyl fluoride in solution

¹⁹F relaxation times of benzyl fluoride in acetone-d₆ and in methanol-d₄ were measured and extrapolated at infinitely dilute solution. The fluorine relaxes through intramolecular dipole-dipole (DDa) and spin-rotation (SR) mechanisms in acetone-d₆ ,and through DDa,SR and inter-molecular dipole-dipole mechanisms in methanol-d₄. The DDa contribution can be recalculated from the overall and internal reorientational motions through ²D measurements on the same solutions.The separation of the different contributions are consistent with those made on the proton relaxation times and correlates more closely with poor solvation of benzylfluoride in acetone-d₆ and with greater solvation in methanol-d₄.     

Nuclear spin relaxation study of aqueous raffinose solution in the presence of a gadolinium contrast agent

Paramagnetic enhancement of nuclear spin-lattice relaxation rates (PREs) was measured in aqueous solution of the trisaccharide raffinose in the presence of a gadolinium(III) complex, GdDTPA-BMA, used as a magnetic resonance imaging contrast agent. The relaxation enhancement of aqueous protons was measured over a broad range of magnetic fields, using field-cycling apparatus in addition to conventional spectrometers. The nuclear magnetic relaxation dispersion profile thus obtained was interpreted with a recently developed model, allowing for both inner- and outer-sphere relaxation. The relaxation enhancement for the carbon-13 nuclei in raffinose was studied under high-resolution conditions at three magnetic fields, whereas the sugar proton PRE was measured at two fields. The PRE of the sugar nuclei could be interpreted in a consistent way, assuming that it was caused by the outer-sphere mechanism. The electron spin relaxation was found to be a less important source of modulation of the electron-nuclear dipole-dipole interaction than the mutual translational diffusion.     

NMR relaxation,spectra, and structure of paramagnetic complexes of 1,3-diaryl-5-quinoxalylformazans in solution

All-Union Scientific-Research Institute of Chemical Reagents and Ultrapure Chemical Substances. Institute of Catalysis, Academy of Sciences of the USSR, Siberian Branch. Translated from Zhurnal Strukturnoi Khimii, Vol. 31, No. 2, pp. 57–64, March–April, 1990.     

Electrostatic and relaxation effects in ionization of molecular dimers and intermolecular complexes

The shifts in ionization energies which occur when a molecule is incorporated as an asymmetric dimer or in an intermolecular complex are analyzed theoretically. MO ? SCF calculations with 4–31G basis sets were performed on closed- and open-shell states of (HF)₂, H₂O·HF, and their valence–hole ions, as well as on the heterodimers incorporating the higher homologues CH₃F, CH₃OH, and (CH₃)₂O. The analysis concerns the influence of electrostatic, polarization, and charge transfer effects associated with complexation on the initial molecular state of each monomer system, as well as monomer–dimer differences in the electronic relaxation mechanism considered as a final state effect in the ionization process. The calculated ionization energy shifts which agree well with the experimental data available for (CH₃)₂O·HF, show that the shifts are dominated by electrostatic effects, but some effects arising from differences in molecular size and electric polarizability of the monomers can be discerned.     

Nuclear spin relaxation of mesogenic fluids in spherical microcavities

We discuss the nuclear spin relaxation resulting from molecular translational diffusion of a liquid crystal in the isotropic phase confined to spherical microcavities. The relaxation is induced by the time modulation of spin interactions as molecules diffuse between the ordered surface layer into the isotropic interior volume and back. The calculated spin-lattice relaxation rate T(1) (-1) shows three distinct dispersion regimes: a plateau at the lowest frequencies, practically independent of the size of the cavity, an intermediate power-law dispersion regime with an exponent between -0.7 and -1, depending on the spatial profile of the order parameter and cavity radius, and at frequencies above 1 MHz a strong dispersion tending toward the quadratic dependence of the relaxation rate on the Larmor frequency in the high-frequency limit. The pretransitional increase in T(1) (-1) depends drastically on the Larmor frequency. The frequency and temperature dependences of T(1) (-1) yield not only information on the magnitude of the surface order parameter, but also on its spatial profile, revealing the type of liquid-crystal-substrate interactions. Apart from thermotropic liquid crystals in the isotropic phase, this analysis can be also applied to other fluids in porous media.     

Nuclear quantum effects induce metallization of dense solid molecular hydrogen

We present an accurate computational study of the electronic structure and lattice dynamics of solid molecular hydrogen at high pressure. The band‐gap energies of the , Pc, and structures at pressures of 250, 300, and 350 GPa are calculated using the diffusion quantum Monte Carlo (DMC) method. The atomic configurations are obtained from ab initio path‐integral molecular dynamics (PIMD) simulations at 300 K and 300 GPa to investigate the impact of zero‐point energy and temperature‐induced motion of the protons including anharmonic effects. We find that finite temperature and nuclear quantum effects reduce the band‐gaps substantially, leading to metallization of the and Pc phases via band overlap; the effect on the band‐gap of the structure is less pronounced. Our combined DMC‐PIMD simulations predict that there are no excitonic or quasiparticle energy gaps for the and Pc phases at 300 GPa and 300 K. Our results also indicate a strong correlation between the band‐gap energy and vibron modes. This strong coupling induces a band‐gap reduction of more than 2.46 eV in high‐pressure solid molecular hydrogen. Comparing our DMC‐PIMD with experimental results available, we conclude that none of the structures proposed is a good candidate for phases III and IV of solid hydrogen. © 2017 Wiley Periodicals, Inc.     

Nuclear quantum effects on the nonadiabatic decay mechanism of an excited hydrated electron

We present a kinetic analysis of the nonadiabatic decay mechanism of an excited state hydrated electron to the ground state. The theoretical treatment is based on a quantized, gap dependent golden rule rate constant formula which describes the nonadiabatic transition rate between two quantum states. The rate formula is expressed in terms of quantum time correlation functions of the energy gap and of the nonadiabatic coupling. These gap dependent quantities are evaluated from three different sets of mixed quantum-classical molecular dynamics simulations of a hydrated electron equilibrated (a) in its ground state, (b) in its first excited state, and (c) on a hypothetical mixed potential energy surface which is the average of the ground and the first excited electronic states. The quantized, gap dependent rate results are applied in a phenomenological kinetic equation which provides the survival probability function of the excited state electron. Although the lifetime of the equilibrated excited state electron is computed to be very short (well under 100 fs), the survival probability function for the nonequilibrium process in pump-probe experiments yields an effective excited state lifetime of around 300 fs, a value that is consistent with the findings of several experimental groups and previous theoretical estimates.     

The electron spin in the quasi-ergodic interpretation of quantum mechanics

According to classical electrodynamics, a charged particle radiates energy and absorbs that arising from all the systems which constitute the universe. Under the effect of the electromagnetic field created by this (the universe field), the particle describes a very complicated trajectory which can be considered as being formed both by a non-relativistic trajectory governed by the electric component of the universe field, and by an ultra-relativistic motion around this trajectory, arising from the magnetic component. The spin appears as being the kinetic momentum associated with this latter motion. The chief characteristics of spin are quantitatively obtained as time-average values.     

The motional model in the electron spin relaxation of large Mn(II) complexes: Rotational or fluctuational process?

The solution ESR spectra of Mn(II) bound to ATP have been analyzed at three microwave frequencies. The liquid-type relaxation process induced by rapid fluctuations of the crystal-field symmetry does not account for the frequency dependence of the linewidth. The ESR lineshapes are better explained in terms of a static distribution of crystalfield sites which induces field dependent inhomogeneous broadening.     

Isotope effects in the relaxation of ion nuclei in paramagnetic solutions

Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2428–2429, October, 1988.     

The Stern-Gerlach experiment and the effects of spin relaxation

The classical Stern-Gerlach experiment is analyzed with an emphasis on the spin dynamics. The central question asked is whether there occurs a relaxation of the spin angular momentum during the time the particle passes through the Stern-Gerlach magnet. We examine in particular the transverse relaxation, involving angular momentum exchange between the spin of the particles and the spins of the magnet. A method is presented describing relaxation effects at an individual particle level. This leads to a stochastic equation of motion for the spins. This is coupled to a classical equation of motion for the particle translation. The experimental situation is then modeled through simulations of individual trajectories using two sets of parameter choices and three different sets of initial conditions. The two main conclusions are: (A) if the coupling between the magnet and the spin is solely described by the Zeeman interaction with the average magnetic field the simulations show a clear disagreement with the experimental observation of Stern and Gerlach. (B) If one, on the other hand, also allows for a T(2) relaxation time shorter than the passage time one can obtain a practically quantitative agreement with the experimental observations. These conclusions are at variance with the standard textbook explanation of the Stern-Gerlach experiment.