Proton Hfs Research Articles

EPR Spectra and Spin Density Distribution of O,S-Dimethyl 4,4-Dithioterephthalate Radical Anions

The preparation of O-methyl S-trideuteromethyl 4,4-dithioterephthalate and S-methyl O-trideuteromethyl 4,4-dithioterephthalate is described. The EPR spectra of the corresponding radical anions are measured. Comparison with the spectrum of O,S-dimethyl 4,4-dithioterephthalate radical anions allows the unequivocal assignment of the proton hyperfine structure (proton “hfs”) coupling constants in the above asymmetric species. Assignment of the arene proton hfs coupling constants is achieved by PM6 and density functional theory MO calculations of the spin density distribution and application of McConnell's relationship aH μ = −2.4·ρπ μ. The spin density distribution in the asymmetric title compound is compared with those in the radical anions of dimethyl terephthalate and the corresponding symmetric sulfur analogs.

ChemInform Abstract: Structure of the Cyclohexa-1,4-diene Radical Cation: An ab initio Molecular Orbital and Electron Paramagnetic Resonance Study.

The structure of the radical cation of cyclohexa-1,4-diene, (1+), was explored using ab initio MO calculations and EPR spectroscopy. Geometry optimizations were carried out on neutral 1 and 1+ at the (U)HF/6-31G(d, p) and MP2/6-31G(d, p) levels of theory. The ground states of both 1 and 1+ were found to possess D2h structures, in agreement with experimental data. The ground electronic state of 1+ was found to be 2B1u and not 2B3g, and this is the result of through-bond orbital interactions. The computed MP2/6-31G(d, p) proton isotropic hfs values for 1+ were found to be in good agreement with the experimental values. The EPR spectra and proton hfs values for 1+ are similar to those obtained earlier by Shida et al.

EPR Studies on Carboxylic Esters. Part 17.1 EPR Spectra and Spin Densities in the Radical Anions of Thio and Dithiocoumarins

The radical anions of 4-methyl-1-thiocoumarins and 4-methyl-1-thio-2-thionocoumarins are generated by internal electroreduction and studied by EPR spectrosopy. The spin density distribution is evaluated from the proton hfs coupling constants and by MO calculations. The radical anions of the corresponding coumarins and 2-thionocoumarins are not persistent enough for EPR spectroscopic measurements.

EPR Studies on Carboxylic Esters. Part 15.1 Spin Density Distribution in the Radical Anions of Naphthalenecarboxylic Esters

The spin density distribution in the radical anions of naphthalene-mono-, di- and -tetra-carboxylic esters is determined from the proton hfs coupling constants as measured by EPR spectroscopy and discussed in terms of semi-empirical MO calculations.

Structure of the cyclohexa-1,4-diene radical cation: an ab initio molecular orbital and electron paramagnetic resonance study

The structure of the radical cation of cyclohexa-1,4-diene, (1+), was explored using ab initio MO calculations and EPR spectroscopy. Geometry optimizations were carried out on neutral 1 and 1+ at the (U)HF/6-31G(d, p) and MP2/6-31G(d, p) levels of theory. The ground states of both 1 and 1+ were found to possess D2h structures, in agreement with experimental data. The ground electronic state of 1+ was found to be 2B1u and not 2B3g, and this is the result of through-bond orbital interactions. The computed MP2/6-31G(d, p) proton isotropic hfs values for 1+ were found to be in good agreement with the experimental values. The EPR spectra and proton hfs values for 1+ are similar to those obtained earlier by Shida et al.

ENDOR spectroscopy of ion pairs derived from tropones and alkali metals: evidence for stereoisomerism

ESR and ENDOR/TRIPLE resonance studies are reported for the ion pairs derived from the non-planar ketyls of tribenzotropone (1), dibenzotropone (2) and dibenzosuberone (3) with different alkali metal cations. The observation of three different alkali hyperfine splitting constants (hfs) in some ENDOR spectra may indicate the presence of three different stereoisomers of these ion pairs. All hfs data and their temperature dependence can be interpreted in terms of three different ion pair structures. The proton hfs are well interpreted in terms of McLachlan's spin-density calculations. Support for the geometries proposed for the three stereoisomers is provided by INDO calculations of the spin densities at the lithium cation.

Experimental and theoretical study on cation radicals of cyclopropane, cyclobutane and cyclopentane

Abstract Cation radicals of cycloalkanes have been produced for the first time in γ-irradiated rigid solutions using fluoromethane as a matrix. Observed ESR spectra are analyzed by ab initio MO calculations; molecular geometries of the cations are optimized for several low-lying doublet states by the energy gradient method. Based on the optimized geometry thus determined the hyperfine splitting constants are calculated by the pseudo-orbital theory. Large Jahn-Teller distortions are calculated for the carbon rings and the H-C-H frames of the cation radicals. The distortions are consistent with the nodal picture of singly occupied orbitals. The calculated hfs constants are sensitive to the change in geometry due to a large contribution of spin-delocalization. The average of the calculated proton hfs constants is compatible with the observed ESR spectra which indicates that all the protons of the individual cations have become equivalent owing to the dynamic Jahn-Teller distortion.

Field Theoretical Treatment of the Proton hfs in Aromatic Ion Radicals

Abstract Field theoretical method to estimate the proton hfs in aromatic ion radicals is presented, and applied to explain why the proton hfs of the positive ion is larger than that of the negative ion of the same molecule. Since the Hartree-Fock orbitals of the neutral molecule are adequate for describing the negative ion state of this molecule, but not for the positive ion state, the correlation effect in the latter should be effectively taken into account, so that the particle propagators in the positive ion are renormalized. The hyperfine energy consists of two terms, the hyperfine self-energy term and the pole strength which is interpreted as the existence probability of the state under investigation. The electron correlation affects these differently between positive and negative ions.

Proton hyperfine data and temparature dependence of the dynamics of ring rotation in bis-benzene-vanadium: a powder endor study

The quasi-free benzene rotation of paramagnetic V(C 6H 6) 2 substituted into Fe(C 5H 5) 2 and Cr(C 6H 6) 2 hosts has been studied by ENDOR spectroscopy between T = 15 K and T = 180 K. In Fe(C 5H 5) 2, two different coalescence temperatures have been found. The full proton hfs tensor of the rigid molecule has been determined from powder samples.

An E.S.R. study of some alkyl substituted benzene anion radicals : an experimental evaluation of the extent of vibronic interaction

E.S.R. studies of the proton hyperfine splitting in the anion radicals of benzene, toluene, ethylbenzene, iso-propylbenzene, t-butylbenzene, and p-xylene show that it is strongly dependent upon temperature. In the case of the monoalkylbenzenides there are two contributions to this observed temperature dependence. First, there is a rapid thermal averaging over two near-degenerate states which have different spin-density distributions. The energy separations between these two near-degenerate states have been measured for the above series of monoalkylbenzenides and are reported here. It is also possible to measure experimentally the spin-density distributions for the two states between which the averaging takes place. With this information one can obtain an experimental estimate of the extent of vibronic interaction which occurs in these systems. Such estimates are presented here and compared with those obtained theoretically. The second source of the strong temperature dependence of the proton hfs lies in t...

ESR-Untersuchungen von Nitroxyl-Radikalen / ESR Studies of Nitroxyl Radicals

Mono-, di- and trinitroxylradicals are investigated in solution by ESR spectroscopy. The ESR spectra of all multispin systems studied can be interpreted under the assumption of an exchange interaction of the radical electrons. The exchange energies J are evaluated by comparison of experimental and theoretical line positions. The temperature dependence of J leads to activation energies around 1.5 Kcal/Mol. The analysis of proton hfs of the biradicals indicates the existence of two radical species which slow and fast exchange rates.

EPR study of NO2 and NO3 produced in urea nitrate by uv irradiation, x-irradiation, and constituent tritium atom decay

NO2 and NO3 radicals have been produced by uv irradiation, x-irradiation, and constituent tritium atom decay in single crystals of urea nitrate. Two types of NO3 and five types of NO2 were produced, and which of these were observed depended on the type of irradiation and the temperature. At 77 °K two types (two different orientations) of NO2 were formed from tritium decay while only one of these was produced by x-irradiation. The various forms of NO2 were found to irreversibly interconvert from one to another as the temperature increased. Different mechanisms for defect formation have been proposed to account for the different types of radicals formed by x-irradiation and tritium decay at room temperature and 77 °K. The NO3 species which resulted from any of the methods of irradiation at room temperature showed no 14N hfs but did exhibit a significant interaction with five neighboring hydrogen bonded protons. Deuterium substitution caused the proton hfs to collapse into one line, thus simplifying the analysis. This NO3 was the same one reported formed by γ-irradiation. A different type of NO3 without proton hfs was formed when either x-irradiation or tritium substitution was used at 77 °K. These two types did not interconvert. From evaluation of the spin-Hamiltonian parameters the electronic ground states for these NO3 radicals have been elucidated. The low temperature form is found to be a π radical and the high temperature form is a σ radical.

ENDOR study of pentaphenylcyclopentadienyl radicals in solution. Lifting of orbital degeneracy by methyl substitution

An electron-nuclear-double-resonance (ENDOR) study has been made of pentaphenylcyclopentadienyl (PPCPD) and a series of para-methylated PPCPD radicals in liquid solution. The hyperfine splitting (hfs) constants in PPCPD and pentamethyl PPCPD (MPPCPD) do not show a significant π-σ spin delocalization effect. This indicates that the phenyl substituents are twisted less than 60° out of the plane of the five-membered ring. A MO calculation based on σ-π separability using the McLachlan approximation gives good agreement between experimental and theoretical hfs constants for a twist angle of 40°. The ring proton hfs constants in PPCPD and MPPCPD are practically identical, indicating a negligible effect of the methyl groups on the spin density distribution. Moreover, the hfs constants in both radicals are not affected noticeably by temperature variations. The ring proton splittings in the partially methylated PPCPD radicals, on the other hand, vary widely and show a marked temperature dependence. The same is tru...

Semi-empirical Unrestricted SCF-MO Treatment for Valence Electron Systems. I. Application to Small Doublet Radicals

Abstract A semi-empirical SCF method for valence electron systems including a differential overlap previously proposed by the present authors is here extended to molecules with an open-shell structure. The unrestricted Hartree-Fock method is applied, and the one-centre and part of two-centre σ-π-type exchange repulsion integrals, playing an essential role in spin-density calculations, are included in the calculations. The calculated spin densities are divided into the mechanistic (spin-polarization and spin-delocalization) contributions. The spin-polarization mechanism is shown to be important even in cases where the spin delocalization mechanism has usually been considered to be dominant (e.g., ethyl and vinyl radicals). The calculated spin densities of π-electron radicals (methyl, ethyl, allyl, and trans-butadienyl anion radicals) and of σ-electron radicals (vinyl, formyl, NO2, CO2−, CN) are discussed. Their hfs constants and mechanistic contributions are calculated; for the methyl and vinyl radicals these are shown to be strongly angular-dependent. The calculated potential curve and the hfs constants of the vinyl radical lead to the CCHa angle, θ\simeq135°; furthermore, the Hα and Cβ hfs constants are shown to be negative. Generally, the calculated proton hfs constants agree satisfactorily with the experimental data and with other calculations except in the case of the formyl radical. The atomic dipoles of some σ-electron radicals are calculated, and some interesting features common to all the σ-electron radicals studied are found.

ESR Spectrum and Structure of HNO2− in KCl

If a KCl melt containing traces of OH− and CN− is subjected to controlled oxygenation, then ultraviolet photolysis of crystals grown from this melt produces the HNO2− radical anion as a substitutional impurity. The radical is also produced by photolysis of KCl:OH−:NO2− crystals. Formation of this radical is attributed to reaction of H atoms, produced by photolytic decomposition of OH−, with NO2− ions, which in the former case are produced by oxidation of CN−. At 77°K the electron spin resonance (ESR) spectrum of this radical is an isotropic nitrogen hyperfine structure (hfs) triplet. At 4°K, however, the spectrum is anisotropic and contains both nitrogen and hydrogen hfs. The magnetic constants of HNO2− are: g1 = 2.0020, g2 = g3 = 2.0074; A1(N) = 39.2, A2(N) = A3(N) = 12.0 Oe; and A1(H) = − 5.0, A2(H) = − 12.5, A3(H) = 13.0 Oe. Axis 2 is perpendicular to the only symmetry plane of the slightly nonplanar molecule. Axes 1 and 3 are, respectively, approximately perpendicular and parallel to the NH bond. Axes 1 and 2 lie in the same {110} crystal plane, and axis 1 makes a 26.6° angle with the (100) crystal axis contained in this plane. At roughly 25°K a spectrum of (111) symmetry is observed which is the result of motional averaging about that (111) crystal axis which makes the smallest angle with molecular axis 2. It can be shown from the nitrogen and proton hfs constants that the deviation of the molecule from planarity is roughly 10°, and that the unpaired electron density on the nitrogen atom is 0.57.

Solvent and Temperature Effects on the Proton hfs in the ESR Spectra of the Organic Radical

Abstract A theoretical investigation is made into the solvent and temperature effects on the proton hfs in the ESR spectra of such organic radicals as: (Remark: Graphics omitted.) where X stands for the CH3 group, in several solvents. Both effects are explained on the assumption that a molecular complex is formed between radical and solvent, so that the charge-transfer occurs from radical to solvent: The solvent effect is caused by the fact that, in various solvent molecules, slight changes take place in their lowest vacant levels, which are occupied by the electron transferred from the radical; on the other hand, the temperature effect is explained by the thermal motion of the molecules, which changes the probabilities of the radical being in the solvated state and in the free state.

Spin Polarization in an Aliphatic Fragment: Comparison between Proton Hyperfine Interaction and Spin–Spin Coupling

Expressions are derived for proton hyperfine interaction constants (hfs) in an aliphatic chain arising from a single unpaired pπ electron on the first carbon atom. The method used is the Dirac vector model. Only spin polarization is considered, while electron delocalization is neglected. The resulting expressions are very similar to those derived by Alexander for proton spin–spin coupling. This makes it possible to use experimental spin–spin data for a semiempirical estimate of the contribution of spin polarization to the aliphatic proton hfs. It is found that spin polarization contributes substantially to the hfs of protons removed up to five bonds from the pπ center. The contribution of spin polarization to α-proton hfs is positive and amounts to about 50% of the observed value. For β and γ protons spin polarization contributes, respectively, negative and positive spin densities of the order found experimentally. Deviations from the calculated values are discussed in terms of spin delocalization and conformational effects.

Anisotropic Hyperfine Interactions in the ESR Spectra of Alkyl Radicals

We have studied the effect of anisotropic hyperfine interactions on the electron spin resonance (ESR) spectra of alkyl radicals trapped in polycrystalline matrices. The anisotropy broadens some or all of the hfs components, thus complicating the spectra. In alkyl radicals one has both isotropic and anisotropic hfs interactions with the α protons, but only an isotropic interaction with the β protons. Computed and experimental line shapes for the ethyl and propyl radicals are in qualitative agreement, provided that the hfs interaction is averaged over the various equilibrium orientations of the —CH2· group. This implies a rapid reorientation of the —CH2· group. It is noteworthy that when the two α protons are antiparallel the hyperfine anisotropy cancels and sharp intense hfs components result, while parallel orientations of the α protons give broad weak hfs lines. In particular, if the broad weak lines associated with the α proton hfs interactions are overlooked, the propyl radical spectrum appears to be a triplet, implying hfs interactions with the β protons only. In radicals where there is an odd number of α hydrogens (e.g. R′ — ĊH — R) there is no possibility of cancellation, and all the lines will be broad and weak. Our attempts to observe radicals of this type have been unsuccessful, most likely for this reason.

Proton Hyperfine Interactions in Semiquinone Ions

The proton hyperfine structure in a series of methylsubstituted semiquinone ions has been shown to arise in a general sense from configuration interaction. A crude calculation of the proton hfs to be expected in a hypothetical solution of C2H4+ ions indicates that the σ—π interaction is responsible for the hfs of protons attached directly to an aromatic ring. The molecular orbital theory of hyperconjugation accounts for the detailed hfs associated with the methyl protons.