A theoretical study of nuclear spin coupling constants and hyperfine coupling constants of se-heterocyclics

MO calculations based on the finite perturbation theory in the INDO approximation have been carried out on selenophene, eighteen of its monosubstituted derivatives and benzo (b)selenophene. The calculated nuclear spin coupling constants satisfactorily reproduce signs, magnitudes, internal orders and some trends of the experimental values. Comparison of different ZDO calculations provides information on the relative importance of σ and π pathways for the various coupling constants in selenophene and benzo(b)selenophene. Unrestricted Hartree-Fock calculations at the INDO level have been performed on the radical anions of dibenzoselenophene and 2,1,3-benzoselenadiazole, the phenoselenazine radical cation, the phenoselenazine neutral radical and the phenoselenazine nitroxide. The isotropic hyperfine coupling constants have been found to be generally in satisfactory agreement with experiment.     

The rotational spectrum,nuclear spin—spin coupling,nuclear quadrupole coupling,and molecular structure of KrHF

Rotational spectra have been assigned for the ⁸²Kr, ⁸³Kr, ⁸⁴Kr, and ⁸⁶Kr isotopic species of the KrHF and KrDF van der Waals molecules by using pulsed microwave Fourier transform spectroscopy in a Fabry—Perot cavity with a pulsed supersonic nozzle molecular source. The rotational, centrifugal distortion, nuclear spin—spin, and nuclear quadrupole coupling constants are used to determine the structure and obtain intramolecular potential binding information. The ⁸³Kr nuclear quadrupole coupling constants are 10.28 ± 0.08 MHz and 13.83 ± 0.13 MHz for KrHF and KrDF respectively. The electric field gradient at the krypton nucleus is calculated from the coupling constant and the known nuclear quadrupole moment and explained by Sternheimer shielding and formation of the van der Waals bond. There is a negligible charge transfer in the KrHF bond.     

Analysis of in vivo and in vitro ESR spectra of H-flavin,an intrinsic spin label of flavoproteins

ESR experiments with fully deuterated flavoprotein have made possible the first determination of g-tensors and coupling constants for the radical form of this protein. Rotational diffusion correlation times for several in vitro and in vivo flavoprotein systems have been calculated and the utility of the saturation behavior of ²H-flavin radicals is demonstrated.     

Probing the Electronic Structure of Bacteriochlorophyll Radical Ions—A Theoretical Study of the Effect of Substituents on Hyperfine Parameters

In reaction centers (RCs) of photosynthesis, a light‐induced charge separation takes place creating radical cations and anions of the participating cofactors. In photosynthetic bacteria, different bacteriochlorophylls (BChl) are involved in this process. Information about the electronic structure of the BChl radical cations and anions can be obtained by measuring the electron spin density distribution via the electron–nuclear hyperfine interaction using EPR and ENDOR techniques. In this communication, we report isotropic hyperfine coupling constants (hfcs) of the BChl b and g radical cations and anions, calculated by density functional theory, and compare them with the more common radical ions of BChl a and with available experimental data. The observed differences in the computed hyperfine data are discussed in view of a possible distinction between these species by EPR/ENDOR methods. In addition, ¹⁴N nuclear quadrupole coupling constants (nqcs) computed for BChl a, b, g, and also for Chl a in their charge neutral, radical cation and radical anion states are presented. These nqcs are compared with experimental values obtained by ESEEM spectroscopy on several different radical ions.     

An SCF-MS-Xα study of the bonding and nuclear quadrupole coupling in diatomic halogens and interhalogens in the ground X Σ and B Π 0 excited states

SCF-MS-Xα calculations of the electronic structure of diatomic halogens and interhalogens XY (X = I, Br, Cl; Y = I, Br, Cl, F) have been used to investigate the bonding and nuclear quadrupole coupling in these molecules. Calculations have been carried out for the ground X ¹ Σ electronic state, and for the excited B ³ Π₀ state in the case of I₂, Br₂, ICl and IBr. Good agreement (to within 10% in most cases) is obtained between the calculated and observed nuclear quadrupole coupling constants for the molecules in the ground state. For the excited state the agreement is not as good, but the calculation does reproduce the observed decrease in the coupling constants to less than one quarter of their ground state values, and analysis of the contributions to the field gradients clearly shows the reasons for this. The electric dipole moments and electric quadrupole moments of the molecules have also been calculated. However, these prove to be much more strongly dependent on the variables used in the calculation (atomic sphere radii, inclusion of d orbitals). The results of the calculations have also been used to test some of the assumptions made in the Townes and Dailey method of analysis of nuclear quadrupole coupling data.     

13C Fourier transform NMR and partially relaxed Fourier transform NMR relaxation study on the ethanol–manganese(II) system

The paramagnetic contribution to the ¹³C and ¹H nuclear relaxation rates in the ethanol-Mn(II) system has been calculated. Both T₁ and T₂ experiments have been performed by means of Fourier transform and partially relaxed Fourier transform NMR spectroscopy. The correlation times for the dipolar and scalar parts have been discussed and evaluated. Calculations of distances and of hyperfine coupling constants have been carried out.     

Theoretical nuclear spin–spin coupling constants using atom–atom polarizabilities. 13C?H and H?H′ coupling constants of some [2.2.1] bicyclic compounds using the INDO approximation

Theoretical nuclear spin–spin coupling constants are calculated using mutual and self atom–atom polarizabilities according to a theory where no semi-empirical parameters are used, except Slater exponents which can be obtained from other sources. As an application, the ¹³C? H and H? H′ couplings of some [2.2.1] bicyclic compounds are calculated with the aid of INDO molecular orbitals and compared with the experimentally obtained coupling constants.     

Precise values of the 13C component vicinal coupling constants for calculating side-chain conformations in amino acids

From consideration of ¹H–¹H vicinal coupling constants and ¹³C? ¹H long-range coupling constants in a series of amino acid derivatives, the precise values of ¹³C component vicinal coupling constants have been calculated for the three minimum energy staggered rotamers for the C(α)H? C(β)H₂ side-chains of amino acids.     

An NMR,ESR, ENDOR and TRIPLE resonance investigation of a cation radical formed from 1,2,4,5-tetramethoxybenzene

Several methods have been established for preparing cation radicals from 1,2,4,5-tetramethoxybenzene that allow highly resolved ESR spectra to be recorded. Precise values of the hyperfine coupling constants for the aromatic and methoxy protons have been obtained; the values are 0.2268±0.0004 and 0.0863±0.0002 mT, respectively, with dichloromethane as solvent. No temperature dependence is evident. TRIPLE resonance experiments showed that both coupling constants have the same sign. NMR experiments provided contact shift and line broadening measurements; these proved that both the above constants are positive and led to a value of 3.1 (±0.3)×10⁸M^?1 s^?1 at 23°C for the rate constant for electron exchange between the cation radical and the parent compound.     

NMR studies of verdazyls with aliphatic substituents

Using PMR the isotropic hyperfine H coupling constants of three series of substituted verdazyls have been measured and assigned. The experimental data are compared with series of other radical groups and with theoretical concepts published recently.^2,3 The position of an atom relative to the radical centre is denoted in the usual way:     

An INDO investigation of the electronic structure and hyperfine coupling constants of the radicals HBO−, HCO and HCN−

The equilibrium geometry and hyperfine coupling constants in the isoelectronic radicals HBO^–, HCO and HCN^– have been calculated using the INDO method. The calculated coupling constants are in reasonable agreement with experiment for these -radicals, provided the geometry is optimised in the calculations.     

Theoretical study of N quadrupole coupling constants in some NO-containing complexes: N2O3 and FNO

The nuclear quadrupole coupling constants at nitrogen centers have been computed for N₂O₃ and FNO by employing the complete-active-space self-consistent field, internally contracted multireference configuration interaction and single-configuration coupled-cluster methods with correlation-consistent basis sets at the levels of attainable accuracy. To examine the overall quality of the wave functions used in our calculations, also electric dipole moments and potential energy characteristics were calculated and compared with available experimental and recent theoretical data. The effects of the choice of the basis set and reference configuration space were investigated. The robust changes in the electric field gradients occurring in the course of complex formation from isolated subunits were interpreted in terms of wave function composition. Our calculations confirm the assignment of the ¹⁴N nuclear quadrupole coupling constants to nuclear centers in N₂O₃ provided by the microwave measurements of Cox et al. [A.P. Cox, J. Randell, A.C. Legon, Chem. Phys. Lett. 126 (1986) 481.].     

Application of semi-empirical calculations of 13C chemical shifts and 1J(CH) coupling constants to the analysis of carbonium ion structures

¹³C chemical shifts and ¹³CH coupling constants for classical and non-classical structures of some typical carbonium ions have been calculated by the method of summation over excited states in the INDO approximation using a special parameterization. Comparison of the data obtained with the experimental values shows that the calculation of the chemical shifts and spin-spin coupling constants is superior to energy calculations, and provides a useful means for the study of the structure of carbonium ions.     

Use of the semiempirical MNDO method for the prediction of equilibrium geometries and relative stability of exo- and endo-5-nitro-2-norbornenes

The structural and energetic parameters for exo- and endo-5-nitro-norbornenes have been obtained via the MNDO method. The results have been compared with the geometrical parameters calculated from the vicinal coupling constants deduced from the ¹H NMR spectra.     

13C NMR spectra and carbon-proton coupling constants of various methylangelicins

The ¹³C-nmr spectra of various methyl derivatives of angelicin are reported. The assignment of chemical shifts for all the C atoms has been achieved by using carbon-proton coupling constants, nuclear Overhauser effect consideration and shift effects caused by the introduction of methyl groups on various positions of the angelicin nucleus. Substituent effects on ¹³C chemical shifts and carbon-proton coupling constants are discussed.     

14N quadrupole coupling in COANP

The ¹⁴N electric quadrupole coupling constants and asymmetry parameters have been determined for the three chemically non-equivalent nitrogen sites in 2-cyclo-octyl-amino-5-nitropyridine (COANP) with the help of proton-nitrogen nuclear quadrupole double resonance.     

The structure of phosphino and phosphoranyl radicals

Equilibrium geometries and isotropic nuclear hyperfine coupling constants have been calculated for the radicals PX₂ and PX₄ (X = H, F, Cl) using the INDO approximations; PH₂ and PF₂ have also been investigated by ab initio methods.     

Electron spin resonance of gamma-irradiated (CH3)4NPF6 and (CH3)4NBF4 single crystals

The electron spin resonance spectra of γ-irradiated tetramethylammonium hexafluorophosphate and tetramethylammonium tetrafluoroborate single crystals and powders have been investigated between 120–350 K. The dominant species in both compounds have been attributed to (CH₃)₃N⁺ radical. All hyperfine coupling constants of the methyl protons and the nitrogen nucleus have been found to be temperature independent and determined. It is concluded that the motional behaviour of (CH₃)₃N⁺ does not differ in tetramethylammonium compounds. The outer lines of the main spectrum of (CH₃)₄NPF₆ have been attributed to PF^-₅ and FPO^-₂ radicals and the anisotropic behaviour of the hyperfine coupling constants has been presented.     

One-electron redox reactions of trifluoperazine in aqueous solutions

Absolute rate constants have been measured for the reactions of the primary and specific one-electron oxidant radicals with the protonated form of trifluoperazine (TFP). The primary radicals, e^- _aq and OH·, react with TFP at diffusion controlled rates. The transients thus produced have been characterized. Halogenated aliphatic peroxyl radicals oxidize TFP with rate constants between 10⁷ and 10⁸ dm³ mol^-1 s^-1, depending on the structure of the peroxyl radical. The reactivity of peroxyl radicals has been found to vary with Taft's inductive parameter. Oxidation of TFP at acidic pH has been studied using stopped-flow technique. The reaction between TFP radical cation and ascorbic acid has also been examined using pulse radiolysis technique. The results indicate that TFP radical cation is repaired by ascorbate. One-electron reduction potential of TFP · + /TFP at pH 3.5 has been calculated to be 0.964 V vs. NHE.