Isotropic Coupling Constants Research Articles

Electron spin resonance spectroscopy, stability and spin-probe properties of dithiazolyl, dithiadiazolyl, benzodithiazolyl and disulphenimidyl free radicals

A comparative investigation has been made of the e.s.r. spectra of 1,3,2-dithiazol-2-yl, 1,3,2,4-dithiadiazol-2-yl, 1,3,2-benzodithiazol-2-yl and dibenzenesulphenimidyl free radicals, both in solution and in rigid matrices. The powder spectra were analysed to give values for the anisotropic g-factors and hyperfine coupling constants, and, in turn, these results were used by means of a simple e.s.r. tumbling theory to calculate radical radii from low-temperature fluid e.s.r. spectra. A simple correction was incorporated into the theory to include a contribution from ‘slip’ rotation. The values of the nitrogen isotropic coupling constants provide a useful guide to the electronic and spatial structures of the radicals. It is shown that the 1,3,2-dithiazol-2-yl radicals have the greatest potential as spin probes.

Detection and identification of free radicals in the gaseous-phase ozonolysis of ethylene: microreactor-matrix ESR spectroscopy

An investigation of the gas-phase ozonolysis of ethylene with the aid of a combination of a microreactor with matrix ESR spectroscopy is reported. Formation of radicals by the ESR signal is unequivocally proven. Concentration and type of radicals are shown to depend both on residence time and initial concentration, in particular on initial O 2 concentration. For millisecond residence times and O 2 initially present predominantly HOO (ROO) is formed: for reaction times ⩾ 40 s the radicals disappear. In absence of O 2 at short reaction time, H atoms and the CH 3(C 2H 5) radical are identified, in addition to HOO (ROO). Besides these radical species NO 2 and NO 3 were found to be present. UV irradiation of the reaction matrices leads to formation of H atoms. CH 3− C 2H 5 and HCO and destruction of peeroxy-type radicals. Kinetic calculations are in qualitative agreement with the observed dependence of radical concentration on reaction time. Reaction schemes explaining the observed behavior are proposed. For the two radicals HOO and CH 3OO extended ab initio SCF MCI calculations were carried out. Besides electronic energies for ground ( 2A″) and first excited ( 2A′) electronic states, predictions for isotropic coupling constants for all nuclei, molecular electric dipole and quadrupole values, spin densities and electric-field gradients at the oxygen nuclei are given. A short discussion is devoted to the discrepancies between observed and calculated coupling constants.

Electron spin resonance investigation of sulphur-33 and nitrogen-15 substituted dithiazol-2-yl and dithiazolidin-2-yl free radicals

Thermal cyclo-addition reactions of tetrasulphur dinitride with certain alkynes generate a new series of stable free radicals possessing the 1,3,2-dithiazolyl ring. The radicals have simple e.s.r. spectra and are potentially useful spin probes or labels. The sharpness of the spectral lines has enabled us to observe natural abundance lines from nitrogen-15, sulphur-33 and carbon-13. The presence of two equivalent sulphur atoms in the heterocyclic ring has been demonstrated by isotopic double labelling with nitrogen-15 and sulphur-33. An isotopic triple labelling experiment with the latter two isotopes and with deuterium was carried out with the related 1,3,2-dithiazolidinyl radicals. Temperature coefficients of coupling constants were measured for both classes of radicals and INDO calculations showed that nitrogen out-of-plane wagging is responsible for all the temperature coefficients.Molecular orbital calculations were performed for the 1,3,2-dithiazolyl and 1,3,2-dithiazolidinyl structures using semi-empirical INDO and ab inito Gaussian 76 computer programs: the optimised structural parameters from both calculations are in good agreement. The INDO calculations gave signs and magnitudes of isotropic coupling constants in overall agreement with those observed experimentally.

α-Hydroxyalkyl radicals by photoreduction in aprotic solvents: ESR spectra, AB initio potential function, inversion—internal rotation dynamics and temperature dependence of coupling constants of 1-hydroxyethyl

A study of the formation of α-hydroxyalkyl radicals by photoreduction of formaldehyde, acetaldehyde, acetone and cyclobutanone in aprotic solvents (methylcyclohexane, isopentane,n-octane) by means of ESR spectra is reported. ESR spectra of 1-hydroxyethyl (CH3CHOH) and 1-hydroxy-1-methylethyl (CH3COHCH3) resolved to second order are observed, whereas from the other two carbonyl compounds only unstructured spectra are observed. Extended ab initio CI calculations were carried out for 1-hydroxyethyl within the framework of a molecular model featuring finite degrees of freedom for inversion at the α-carbon center and for internal rotation of the OH group, providing a two dimensional electronic potential function and values for the isotropic coupling constants for the hydroxylic, the α and the methyl protons. The ESR spectra of CH3CHOH show a characteristic temperature dependence of the coupling constants of each of the three types of protons between 150 and 360 K. For interpretation the inversion—internal rotation problem has been treated in two dimensions. The nuclear motion states were then used for calculation of the T dependence of spin densities and coupling constants within the framework of Boltzmann averaging. Theoretical predictions suggest that inversion is essential for interpretation of the observed T dependence of the coupling constants. For the hydroxylic proton the temperature dependence is predicted considerably too low, whereas for the α and CH3 protons it is predicted approximately correct. Possible effects of hydrogen bonding on ESR spectra are briefly discussed.

Generation of Cd +1 in x-irradiated frozen aqueous solutions: Similar hydration shells for Cd +2 and Cd +1 species

Cd +1 has been generated in frozen aqueous solution by X-irradiation at 4.2 and 77 K. No difference in the g-factors or isotropic hyperfine constant has been detected for the two irradiation temperatures in contrast to Ag° species. Although the isotropic coupling constant indicates that 30% of the unpaired electron spin density of Cd +1 is delocalized onto first solvation shell waters it appears that the equilibrium solvation environments for Cd +2 and Cd +1 are essentially the same.

Automated analysis of high-resolution NMR spectra. I. Principles and computational strategy

A general iterative method for analyzing high-resolution NMR spectra for chemical shifts and isotropic coupling constants is presented that differs from conventional procedures in that it does not depend on the assignment of energy levels or transitions; makes use of the full information content of a spectrum; and is in principle, and in the great majority of cases also in practice, capable of arriving at the correct solution from starting parameters chosen randomly within liberally specified boundaries. The basic equations are derived from a generalized formulation of the least-squares problem. One is led to an ordered sequence of correlation matrices W k in data space, whose off-diagonal elements provide a general mechanism for smoothing out the multitude of false local minima on the error hypersurface of the ordinary least-squares method. The conditions to be satisfied by the W k matrices to establish a unique pathway to the global minimum are stated in a formally rigorous way. It is demonstrated that although specific information about the W k matrices is not accessible in practice, and paradoxically only because of this, one may conclude that they must be symmetric band matrices with diagonal elements w ii = 1 and off-diagonal elements w ii decaying monotonically with absolute distance from the principal diagonal and satisfying the condition 0 ⩽ w ij < 1. The characteristics of an efficient computational strategy based on this formalism are outlined.

One-electron reduction of di-imidosulphur compounds, S(NR)2, and some complexes of Group 6A metal carbonyl derivatives containing S(NR)2 ligands: studies of the radical products by electron spin resonance spectroscopy

Reduction of the di-imidosulphur compounds, S(NR)2(R = But, Ph, 4-MeC6H4, or 4-O2NC6H4), in 1,2-dimethoxyethane solution with a potassium mirror in vacuo gives fairly stable solutions of the corresponding anion radicals [S(NR)2]– which have been studied by e.s.r. spectroscopy. The frozen-solution e.s.r spectrum of K[S(NBut)2] has also been recorded. Isotropic coupling constants A(14N), A(1H) and, for R = But, A(33S) have been deduced and analysed in terms of the electronic structures of the radicals using empirical electron-density relationships. Some CNDO/2 and INDO calculations have been made and the experimental and calculated electron-spin densities compared. Variable-temperature e.s.r. measurements have given no indication of the presence of distinct geometric isomers of the anions in solution. The model calculations suggest that the E/E conformation is most stable and that interconversion to give the E/Z or Z/Z forms is unlikely. The experimental evidence in all cases indicates the two nitrogen atoms to the magnetically equivalent. Alkali-metal isotropic coupling constants A(MI) for the species MI[S(NBut)2](MI=7Li, 23Na, or 39K) have been estimated and indicate that the metal–anion interaction increases in the order K < Na < Li.Potassium-metal reduction of [M(Co)4{S(NBut)2}](M = Cr, Mo, or W) produces the corresponding anions with well defined isotropic e.s.r spectra. Values for A(14N) and A(M)(M =95/97Mo or 183W) have been deduced and the spectra analysed. The results suggest that the unpaired electron is located mainly in the S(NBut)2 ligand. CNDO/2 calculations for the chromium anion are consistent with this observation. Attempts to produce [M(CO)4{S(NPh)2}]– have been unsuccessful except for M = Cr, when an unstable species is formed. A tentative analysis of the e.s.r spectrum of this radical anion is presented.

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.

Temperature and Pressure Dependence Studies of the Motions of the +NH3 Radical Trapped in γ Irradiated Ammonium Perchlorate Single Crystal

Abstract The temperature and pressure dependences of the +NH3 coupling tensors have been investigated by ESR spectroscopy. The linear variation of the reduced correlation time of the “rotational diffusion” motion with reciprocal of the temperature is shown to be unrelated to the dilatation of the matrix. Comparison with +ND3 results gives the contribution to the nitrogen isotropic coupling constant of the out of plane vibration of the nitrogen atom.

Electron spin resonance investigations of the photo-oxidation of decacarbonyldimanganese and related complexes

An e.s.r. study has been made of the photolytic reactions of decacarbonyldimanganese derivatives in the presence of air or dioxygen in solution at low temperatures. E.s.r. parameters for the species [Mn(CO)4L(O2)][L = CO, PPh3 PBun3, or P(OEt)3] are presented and oxygen-17 labelling of the species where L = CO has given isotropic coupling constants for the two atoms of the dioxygen moiety of 2.2 ± 0.1 and 2.05 ± 0.1 mT. Frozen-solution spectra of [Mn(CO)5(O2)] have been analysed and the results are discussed in terms of the bonding in the complex. Related photo-oxidation studies of other binuclear carbonyl complexes are reported and a species which is probably [Fe(η-C5H5)(CO)2(O2)] has been identified by e.s.r. spectroscopy.

Optically detected E.P.R. and low-field ENDOR of triplet benzophenone

The ENDOR spectrum of ^(13)C substituted (carbonyl carbon) triplet excited benzophenone was studied at ca. 100 G applied magnetic field by means of optical detection. The benzophenone was substitutionally dissolved in dibromodiphenylether (DDE). The ENDOR transitions and the E.P.R. transitions were studied as a function of applied field direction in the ab and bc crystal planes. The angle ϕ between the line joining the phenyl group centres and a principal axis of the fine structure tensor was found to be 23·6° ± 0·02° (standard deviation limited to one set of measurements). The principal axes of the 13C hyperfine tensor were within experimental error (±10°) coincident with the fine structure axes. The principal values of the hyperfine tensor were found to be : |A_(xx)/h| = 43·16 ± 1·84, |A_(yy)/th| = 22·66 ± 2·50, |A_(zz)/h| = 10·25 ± 1·30 MHz. The low-field ENDOR does not provide an indication of the signs of these tensor elements, so the value of the isotropic coupling constant cannot be measured. Two values of the anisotropic (dipole) hyperfine tensor elements were deduced on the assumption that the transitions frequencies are mainly determined by interactions between the nuclear spin and electron density on the carbonyl carbon atom. These values were (i) A_(xx)/^d/h = 17·82, A_(yy)/h = -22·68, A_(zz)/h = -15·14 MHz and (ii) A_(xx)/h = 39·73, A_(yy)/h = -26·09, A_(zz)/h = -13·63 MHz. Set (ii) is consistent with recent calculations of the spin density distribution in triplet benzophenone in which the orbital spin densities are 0·64 for the carbonyl carbon π-orbital, 0·17 for the oxygen π-orbital, and 0·88 for the non-bonding orbital on oxygen (all expressed as fractions of one electron). Isotope effects on the fine structure constants of triplet benzophenone were measured and found to be consistent with the changes occurring in the spin-orbit coupling with the ground state. The kinetic parameters for the excitation and de-excitation of the triplet substates were deduced from transient ODMR studies of benzophenone in DDE. The T_z spin state is mainly populated (85–88 per cent) and this is also the most strongly radiative state (k_z^r = 0·88). The steady-state populations of the three triplet levels are similar.

Matrix dependence of the electron spin resonance spectra of the radical trapped in different single crystals

Nitrogen and proton coupling tensors have been measured for the [Formula: see text] radical trapped in five different single crystal matrices. As all isotropic coupling constants are the same within experimental error (aN = 21.2 G, aH = 28.4 G), no measurable crystal field effect is found. In these solids the motions of the radical depend on the dimension of the trapping cage and the motions of the surrounding molecules.

Solvation spectra. Part 52.—The aquation of silver atoms and cations: a radiation and electron spin resonance study

Exposure of a range of aqueous solutions containing AgI cations to 60Co γ-rays at 77 K gave AgII and AgO together with other paramagnetic products. The AgII centres have [graphic omitted] configurations, which means that linear solvation for Ag+ is unlikely, since this would lead to a [graphic omitted] configuration. A range of distinct AgO centres displaying low (∼80 %) atom character was detected. Their yield was enhanced, but their distribution largely unaltered, when certain additives, such as potassium fluoride, were used to give glassy solids. However, their distribution varied with dose and with temperature. The major initial product exhibited an extra 6 G doublet splitting due to hyperfine coupling to a single proton. On further irradiation, this centre was superceded by others exhibiting no resolved coupling to protons but, in some cases, well defined g and A anisotropy. For all the AgO centres, gav was less than the free-spin value. When water enriched in 17O was used, superhyperfine coupling was detected for some of the AgO centres. From the apparent anisotropic and isotropic coupling constants, delocalisation of ∼5% per water molecule was deduced. The intensities indicated the presence of several equivalent water ligands but solvation numbers could not be derived accurately.These results for AgO are significant for two reasons; (1) they show that solvation is not lost in aqueous glasses when the charge on an ion is neutralised. This contrasts with the fact that when small neutral molecules become charged, solvation is acquired, even at 77 K. (2) They indicate the presence of a range of discrete AgI solvates in solution rather than a single preferred unit.

Unstable intermediates. Part 175. Effect of ionizing radiation on N-nitrosodimethylamine: an electron spin resonance study

Exposure of N-nitrosodimethylamine to 60Co γ-rays at 77 K gave two paramagnetic centres detected by e.s.r. spectroscopy. One had hyperfine coupling to two non-equivalent nitrogen atoms whose estimated isotropic coupling constants agreed with those for the anions R2NNO– in fluid solution. These anions, formed in methanol at 77 K, became protonated on annealing. The other centre had a large isotropic coupling (ca. 40 G) indicative of a σ-radical, and we suggest that it is the species Me2NṄO+, which is isoelectronic with the well known iminoxyl radicals, R2CNO. The results indicate an increased spin-density on nitrogen on going from R2CṄO to R2NṄO+.

ENDOR studies of the benzyl radical trapped in an X-irradiated single crystal of potassium-hydrogen bisphenylacetate

Using ENDOR we have measured the total hyperfine coupling tensors for all seven protons of th benzyl radical in an X-irradiated single crystal of potassium-hydrogen bisphenylacetate. Analysis of the anisotropic couplings confirms the planarity and C 2V symmetry of the radical, and shows that the HCH bond angle at the methylene group is 118 ± 2°. The magnitudes of the anisotropic couplings are related to the spin density distributions deduced from isotropic coupling constants and molecular orbital calculations; the discrepancies are discussed.

Tester strains for the identification of specific base pair substitution mutations in bacteriophage T4

Solid-state 13C CP/MAS NMR experiments on 5-chlorouracil and 5-bromouracil have revealed that the line of the carbon atom bonded to the halogen atoms shows an asymmetric splitting in each compound. The patterns of the splittings depend strongly on the strength of the magnetic field. The spectral patterns were reproduced well by theoretical calculations of the direct dipole interaction between 13C and the S=32 nucleus of chlorine in the case of 5-chlorouracil. The sign of the chlorine quadrupole coupling constant was assigned to be negative. In addition to the direct dipole interaction, the indirect spin coupling between 13C and bromine (S=32) contributes to the line splitting in 5-bromouracil. The sign of the bromine quadrupole coupling is positive. The isotropic indirect spin coupling constant was estimated to be −170 Hz for Br. For 5-iodouracil, an NMR signal of the carbon spin coupled directly to the iodine spin (S=52) was not observed, probably due to the effect of the quadrupolar relaxation of the iodine.

Geometrical structure of solvated electrons in γ-irradiated 3-methylpentane glass at 77 K from an analysis of the modulation of the electron spin echo decay envelope

Deuterium modulation of the electron spin echo decay envelope from trapped electrons in γ-irradiated 3-methylpentane-d14 glass at 77 K has been analyzed to give a solvation structure characterized by 18–21 equivalent nearest neighbor deuterons to the electron at a distance of 3.0–3.2 Å and with an isotropic coupling constant of essentially zero. From the structure of 3-methylpentane it is suggested that there are six to seven first solvation shell molecules around the electron.

Structures of some fluorinated pyridines from the analysis of NMR spectra of nematic solutions

Proton and fluorine NMR spectra of 2,6-difluoro- and 2,4,6-trifluoropyridine dissolved in different nematic phases have been recorded and analyzed. Sample rotation was used to determine relative signs of isotropic coupling constants. The analyses of the spectra of EBBA solutions of the trifluoro compound was carried out with the aid of an iterative computer program which does not rely on the assignment of lines in observed and computed spectra. The anisotropic coupling constants obtained from the analysis are consistent with a negligible anisotropic contribution to J 26 FF in 2,6-difluoropyridine, but two, or more, J ij values in 2,4,6-trifluoropyridine contain small but significant anisotropic contributions. These conclusions were reached after allowing for the effect of vibrational motion on the dipolar couplings, which necessitated computation of normal coordinates for these molecules. Vibrational corrections were also computed for dipolar couplings in pentafluoropyridine.

ChemInform Abstract: PROTON HYPERFINE SPLITTINGS IN THE ESR OF SOME ALKYLPHOSPHINE DIMERIC RADICALS

Chemischer InformationsdienstVolume 7, Issue 11 Organoelement Compounds ChemInform Abstract: PROTON HYPERFINE SPLITTINGS IN THE ESR OF SOME ALKYLPHOSPHINE DIMERIC RADICALS MASAMOTO IWAIZUMI, MASAMOTO IWAIZUMISearch for more papers by this authorTAKASHI KISHI, TAKASHI KISHISearch for more papers by this authorFUMIO WATARI, FUMIO WATARISearch for more papers by this authorTARO ISOBE, TARO ISOBESearch for more papers by this author MASAMOTO IWAIZUMI, MASAMOTO IWAIZUMISearch for more papers by this authorTAKASHI KISHI, TAKASHI KISHISearch for more papers by this authorFUMIO WATARI, FUMIO WATARISearch for more papers by this authorTARO ISOBE, TARO ISOBESearch for more papers by this author First published: March 16, 1976 https://doi.org/10.1002/chin.197611282AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume7, Issue11March 16, 1976 RelatedInformation

Electron spin resonance analysis of irrversible changes induced by calcium perturbation of erythrocyte membranes

Introduction of calcium during hemolysis of erythrocytes causes irreversible membrane changes, including protein aggregation. These changes have been investigated by incorporation of one protein and three fatty acid spin label probes into washed membranes from erythrocytes hemplyzed with a range of Ca 2+ concentrations. Electron spin resonance spectra of the lipid probes were analyzed for changes in the order parameters, isotropic coupling constants and mean angular deviations of the lipid hydrocarbon chains. The results generally indicated an increased freedom of mobility of the probes with increased Ca 2+ concentration during hemolysis, but the response of each probe showed a different concentration dependence. The maximal response was obtained with the I(5,10) probe. Variations in the responses were interpreted to reflect different modes of protein-lipid or protein-probe interactions arising from Ca 2+-induced membrane protein alterations. Spectra from membranes treated with the protein spin label showed an increased ratio of immobilized to mobile label with increased Ca 2+ concentrations at hemolysis. This is consistent with the membrane protein aggregation phenomena previously observed. It is suggested that the increased protein-protein interactions formed as a result of calcium treatment permit an increased lipid mobility in the membrane regions monitored by the fatty acid probes.