Understanding solvent effects on hyperfine coupling constants of cyclohexadienyl radicals

Quantum chemical calculations have been carried out to understand better solvent effects on the isotropic muon and proton hyperfine coupling constants in the C₆H₆Mu^• radical. Both polarizable continuum solvent models and explicit inclusion of water molecules into supermolecular complexes were used. Changes in the hyperfine couplings of in-plane hydrogen atoms are very small and difficult to discuss, partly due to relatively large experimental error bars. In contrast, the out-of-plane proton and muon hyperfine couplings exhibit more pronounced changes. These are partly due to structural changes of the radical and partly due to direct electronic polarization effects. Polarizable continuum solvent models agree well with experimental changes for benzene but overshoot the enhancement of the hyperfine couplings for water. Explicit inclusion of water molecules reduces this overestimated spin density increase and thereby tends to bring theory and experiment into closer agreement. The enhancement of the spin density on the out-of-plane hydrogen or muon atoms by the solvent environment is mainly due to an increased polarization of the singly occupied MO towards this side. Electronic Supplementary Material: Supplementary material is available in the online version of this article at dx.doi.org/10.1007/s00214-005-0680-x     

Modeling solvent effects on electron-spin-resonance hyperfine couplings by frozen-density embedding

In this study, we investigate the performance of the frozen-density embedding scheme within density-functional theory [J. Phys. Chem. 97, 8050 (1993)] to model the solvent effects on the electron-spin-resonance hyperfine coupling constants (hfcc's) of the H2NO molecule. The hfcc's for this molecule depend critically on the out-of-plane bending angle of the NO bond from the molecular plane. Therefore, solvent effects can have an influence on both the electronic structure for a given configuration of solute and solvent molecules and on the probability for different solute (plus solvent) structures compared to the gas phase. For an accurate modeling of dynamic effects in solution, we employ the Car-Parrinello molecular-dynamics (CPMD) approach. A first-principles-based Monte Carlo scheme is used for the gas-phase simulation, in order to avoid problems in the thermal equilibration for this small molecule. Calculations of small H2NO-water clusters show that microsolvation effects of water molecules due to hydrogen bonding can be reproduced by frozen-density embedding calculations. Even simple sum-of-molecular-densities approaches for the frozen density lead to good results. This allows us to include also bulk solvent effects by performing frozen-density calculations with many explicit water molecules for snapshots from the CPMD simulation. The electronic effect of the solvent at a given structure is reproduced by the frozen-density embedding. Dynamic structural effects in solution are found to be similar to the gas phase. But the small differences in the average structures still induce significant changes in the computed shifts due to the strong dependence of the hyperfine coupling constants on the out-of-plane bending angle.     

Environmental effects on phenoxyl free radical spin densities and hyperfine couplings

The effect of the environment, as modelled by hydrogen bonding, ion-pairing and/or general continuum model effects, is investigated for the phenoxyl free radical. All are shown to lead to a redistribution of spin density from the phenoxyl O atom to the C_ipso position. Isotropic and anisotropic hyperfine couplings are calculated at the B3LYP level of theory confirming this trend. Introduction of the continuum model has a significant effect the Na-O bond length of the ion-pair model significantly altering its calculated EPR properties in comparison with gas-phase values. The trends identified are of immediate significance for biological environmental effects on tyrosyl free radicals.     

Generation and detection of the cyclohexadienyl radical in phosphonium ionic liquids

The formation of the cyclohexadienyl radical, C(6)H(6)Mu, in ionic and molecular solvents has been compared. This is the first time that a muoniated free radical is reported in an ionic liquid. In marked contrast to molecular liquids, free radical generation in ionic liquids is significantly enhanced. Comparison of the hyperfine interactions in the ionic liquid and in molecular solvents and with theoretical calculations, suggests significant and unforeseen solvent interaction with the cyclohexadienyl radical.     

Kinetic solvent effects on peroxyl radical reactions

Kinetic solvent effects on peroxyl radical reactions are easily determined using a new peroxyester-based radical clock method.     

Molecular orbital study of hyperfine splitting constants in ethyl and cyclohexadienyl radicals

Hyperconjugated models are used for a m.o. calculation of proton h.f.s. constants in Ethyl and Cyclohexadienyl radicals. A slightly modified Pariser-Parr-Pople SCF method is employed including extensive configuration interaction. The calculated spin densities for the hyperconjugated protons are in good agreement with experiment.

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Zusammenfassung Zur Berechnung von Protonenhyperfeinstrukturkonstanten von Äthyl- und Cyclohexadienyl-Radikalen werden Modelle mit Hyperkonjugation verwendet. Die Rechnung gründet sich auf eine leicht modifizierte Pariser-Parr-Pople-Methode einschließlich ausgedehnter Konfigurationswechselwirkung. Die berechneten Spin-Dichten für die hyperkonjugierten Protonen stimmen mit experimentell ermittelten Werten gut überein.

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Résumé Des modèles tenant compte à la hyperconjugaison sont usés pour le calcul des constantes de structure hyperfine protonique des radicaux éthyl et Cyclohexadiényl. Le calcul se base sur une méthode Pariser-Parr-Pople un peu modifiée et comprenant une interaction de configurations extensive. Les densités de spin calculées pour les protons hyperconjugués sont en bon accord avec des valeurs expérimentales.

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We thank the Italian National Research Council (Chemistry Committee, Research Group IV) for financial support. One of us (P.L.N.) thanks Sicedison S.p.A. for a grant.     

Esterification in ionic liquids: the influence of solvent basicity

The second-order rate constant (k2) for the esterification of methoxyacetic acid with benzyl alcohol is reported in a range of ionic and molecular solvents. The solvent effects on esterification rate are examined by using a linear solvation energy relationship based on the Kamlet-Taft solvent scales (alpha, beta, and pi*). It is shown that the hydrogen bond basicity of the solvent is the dominant parameter in determining the esterification rate and that the best rates are achieved in low basicity solvents.     

Non-empirical calculations on the conformation and hyperfine structure of the silyl radical influence of vibrational effects

An ab-initio spin-restricted SCF and perturbative configuration interaction study of the silyl radical is presented. The vibrational dependence of isotropic coupling constants is investigated using double-zeta and double-zeta plus polarization basis sets. The calculations predict a nearly tetrahedral geometry for the radical with an inversion barrier of 5.85 kcal/mol. The vibrational treatment leads to coupling constants (α_Si = ?190.20 G;α_H = +5.03 G) in excellent agreement with experiment.     

Spin relaxation effects in photochemically induced dynamic nuclear polarization spectroscopy of nuclei with strongly anisotropic hyperfine couplings

We describe experimental results and theoretical models for nuclear and electron spin relaxation processes occurring during the evolution of 19F-labeled geminate radical pairs on a nanosecond time scale. In magnetic fields of over 10 T, electron-nucleus dipolar cross-relaxation and longitudinal DeltaHFC-Deltag (hyperfine coupling anisotropy--g-tensor anisotropy) cross-correlation are shown to be negligibly slow. The dominant relaxation process is transverse DeltaHFC-Deltag cross-correlation, which is shown to lead to an inversion in the geminate 19F chemically induced dynamic nuclear polarization (CIDNP) phase for sufficiently large rotational correlation times. This inversion has recently been observed experimentally and used as a probe of local mobility in partially denatured proteins (Khan, F.; et al. J. Am. Chem. Soc. 2006, 128, 10729-10737). The essential feature of the spin dynamics model employed here is the use of the complete spin state space and the complete relaxation superoperator. On the basis of the results reported, we recommend this approach for reliable treatment of magnetokinetic systems in which relaxation effects are important.     

Laser-induced fluorescence of cyclohexadienyl (c-C6H7) radical in the gas phase

A laser-induced fluorescence spectrum was observed in the 500-560 nm region when a mixture of 1,4-cyclohexadiene and oxalyl chloride was photolyzed at 193 nm. The observed excitation spectrum was assigned to the A (2)A(2)<--X (2)B(1) transition of the cyclohexadienyl radical c-C6H7, produced by abstraction of a hydrogen atom from 1,4-cyclohexadiene by Cl atoms. The origin of the A<--X transition of c-C(6)H(7) was at 18 207 cm(-1). From measurements of the dispersed fluorescence spectra and ab initio calculations, the frequencies of several vibrational modes in both the ground and excited states of c-C(6)H(7) were determined: nu(5)(C-H in-plane bend)=1571, nu(8)(C-H in-plane bend)=1174, nu(10)(C-C-C in-plane bend)=981, nu(12)(C-C-C in-plane bend)=559, nu(16)(C-C-C out-of-plane bend)=375, and nu(33)(C-C-C in-plane bend)=600 cm(-1) for the ground state and nu(8)=1118, nu(10)=967, nu(12)=502, nu(16)=172, and nu(33)=536 cm(-1) for the excited states.     

13C NMR studies on radical anions. Special solvent effects

The determination of radical ions is crucial in mechanistic studies of reactions in which single electron transfer is suspected. A ¹³C NMR method based on the interaction of radical anions with tetrahydrofuran (THF) molecules is presented; measurements of the broadening of the THF α-carbon signal allow determination of the substrate concentration in the range 0.2–1.2 M . The spectacular effect observed on addition of small amounts of benzene greatly improved the method. In addition, studies of the spin-lattice, T₁, and of the transverse nuclear, T₂, relaxation times and of the effects of added co-solvents allow the proposal of the bonding situation and of the degree of coordination in the radical anion-cation-THF molecules' complex.     

SIMPRE1.2: Considering the hyperfine and quadrupolar couplings and the nuclear spin bath decoherence

SIMPRE is a fortran77 code which uses an effective electrostatic model of point charges to predict the magnetic behavior of rare‐earth‐based mononuclear complexes. In this article, we present SIMPRE1.2, which now takes into account two further phenomena. First, SIMPRE now considers the hyperfine and quadrupolar interactions within the rare‐earth ion, resulting in a more complete and realistic set of energy levels and wave functions. Second, and to widen SIMPRE's predictive capabilities regarding potential molecular spin qubits, it now includes a routine that calculates an upper‐bound estimate of the decoherence time considering only the dipolar coupling between the electron spin and the surrounding nuclear spin bath. Additionally, SIMPRE now allows the user to introduce the crystal field parameters manually. Thus, we are able to demonstrate the new features using as examples (i) a Gd‐based mononuclear complex known for its properties both as a single ion magnet and as a coherent qubit and (ii) an Er‐based mononuclear complex. © 2016 Wiley Periodicals, Inc.     

Structural and solvent effects on the C-S bond cleavage in aryl triphenylmethyl sulfide radical cations

Steady-state and laser flash photolysis (LFP) studies of a series of aryl triphenylmethyl sulfides [1, 3,4-(CH(3)O)(2)-C(6)H(3)SC(C(6)H(5))(3); 2, 4-CH(3)O-C(6)H(4)SC(C(6)H(5))(3); 3, 4-CH(3)-C(6)H(4)SC(C(6)H(5))(3); 4, C(6)H(5)SC(C(6)H(5))(3); and 5, 4-Br-C(6)H(4)SC(C(6)H(5))(3)] has been carried out in the presence of N-methoxyphenanthridinium hexafluorophosphate in CH(3)CN, CH(2)Cl(2), CH(2)Cl(2)/CH(3)CN, and CH(2)Cl(2)/CH(3)OH mixtures. Products deriving from the C-S bond cleavage in the radical cations 1(?+)-5(?+) have been observed in the steady-state photolysis experiments. Time-resolved LFP showed first-order decay of the radical cations accompanied by formation of the triphenylmethyl cation. A significant decrease of the C-S bond cleavage rate constants was observed by increasing the electron-donating power of the arylsulfenyl substituent, that is, by increasing the stability of the radical cations. DFT calculations showed that, in 2(?+) and 3(?+), charge and spin densities are mainly localized in the ArS group. In the TS of the C-S bond cleavage an increase of the positive charge in the trityl moiety and of the spin density on the ArS group is observed. The higher delocalization of the charge in the TS as compared to the initial state is probably at the origin of the observation that the C-S bond cleavage rates decrease by increasing the polarity of the solvent.     

Kinetic solvent effects on hydrogen abstraction reactions from carbon by the cumyloxyl radical. The importance of solvent hydrogen-bond interactions with the substrate and the abstracting radical

A kinetic study of the hydrogen atom abstraction reactions from propanal (PA) and 2,2-dimethylpropanal (DMPA) by the cumyloxyl radical (CumO?) has been carried out in different solvents (benzene, PhCl, MeCN, t-BuOH, MeOH, and TFE). The corresponding reactions of the benzyloxyl radical (BnO?) have been studied in MeCN. The reaction of CumO? with 1,4-cyclohexadiene (CHD) also has been investigated in TFE solution. With CHD a 3-fold increase in rate constant (k(H)) has been observed on going from benzene, PhCl, and MeCN to TFE. This represents the first observation of a sizable kinetic solvent effect for hydrogen atom abstraction reactions from hydrocarbons by alkoxyl radicals and indicates that strong HBD solvents influence the hydrogen abstraction reactivity of CumO?. With PA and DMPA a significant decrease in k(H) has been observed on going from benzene and PhCl to MeOH and TFE, indicative of hydrogen-bond interactions between the carbonyl lone pair and the solvent in the transition state. The similar k(H) values observed for the reactions of the aldehydes in MeOH and TFE point toward differential hydrogen bond interactions of the latter solvent with the substrate and the radical in the transition state. The small reactivity ratios observed for the reactions of CumO? and BnO? with PA and DMPA (k(H)(BnO?)/k(H)(CumO?) = 1.2 and 1.6, respectively) indicate that with these substrates alkoxyl radical sterics play a minor role.     

Comments on the indo calculation of hyperfine coupling in the σ phenyl radical

The relative magnitudes and signs of the hyperfine coupling constants in the phenyl radical, calculated by INDO, are explained by a combination of delocalization of electron spin density into the sigma framwork and a modification of the exchange polarization effects commonly observed for π-type radicals.     

The effects of nonlocal gradient corrections in density functional calculations of hydrocarbon radical hyperfine structures

Ground-state equilibrium geometries and hyperfine structures of a number of organic neutral and charged radical compounds are computed using the linear combination of Gaussian-type orbitals–density functional theory method. In addition to the local spin-density approximation, we also use two different nonlocal (gradient corrected) schemes for the calculations of the exchange and correlation potentials. The different functional forms are found to generate slightly different total and unpaired spin-density distributions in the molecules, and as a result, the computed isotropic hyperfine coupling constants vary markedly. The smallest variations are found for the hydrogens, where the results are generally in satisfactory agreement with experiment. For the carbon atoms, however, large differences in isotropic coupling constants are observed. The anisotropic hyperfine structures are generally very well described at all levels of theory. © John Wiley & Sons, Inc.     

Understanding kinetic solvent effects on hydrogen abstraction reactions from carbon by the cumyloxyl radical

A kinetic study of the hydrogen abstraction reactions from tetrahydrofuran (THF) and cyclohexane (CHX) by the cumyloxyl radical was carried out in different solvents. With THF, a 4.5-fold decrease in rate constant (k(H)) was observed on going from isooctane to 2,2,2-trifluoroethanol. An opposite behavior was observed with CHX, where k(H) increased by a factor 4 on going from isooctane to 2,2,2-trifluoroethanol. The important role of substrate structure and of the solvent hydrogen bond donor ability is discussed.     

Solvent effects on radical copolymerization of acrylonitrile and methyl acrylate: solvent polarity and solvent-monomer interaction

Abstract

New insights for the effects of organic solvent polarities and solvent-monomer interactions on the radical copolymerization for an important copolymer, poly(acrylonitrile-co-methyl acrylate) (PAN-co-MA), were provided in this research. Solvents, dimethylformamide (DMF), dimethylacetamide (DMAc) and dimethyl sulfoxide (DMSO), were used as reaction media. The polarity of these solvents was in the sequence of DMAc?<?DMF?<?DMSO. By studying the reactivity ratios of AN and MA, the triad fractions of the resultant copolymers, the interactions between monomers and solvents, and the compositions of copolymers at various conversions, we concluded that the solvent polarity had minimal influence on the copolymerization of AN and MA, while the solvent-monomer interactions played important roles. The interactions between monomer-monomer, monomer-solvent, and solvent-solvent, were calculated based on quantum chemistry methods. Both theoretical calculations and experimental results suggested that AN and MA in DMSO tended to aggregate locally, while they could be homogeneously dissolved in DMAc and DMF. The interactions between solvent and monomers could cause local monomer concentration variations, or ‘bootstrap’ effect, which is one of the critical factors affecting the copolymerization process of AN and MA and the chemical structures of the resultant polymers.     

Some calculations of solvent effects on one bond NC spin–spin couplings

The solvaton model, incorporating INDO parameters and commonly encountered perturbation procedures, is employed to obtain the variation of ¹J(N≡C) as a function of the dielectric of the medium (?) for some cyanides and isocyanides. In all cases considered ¹J(N≡C) is predicted to become increasingly negative as ? increases. Changes of up to 2 Hz in ¹J(N≡C) are expected, with the isocyanides being more sensitive than the cyanides to a change in ?.