Proton Hyperfine Constants Research Articles

Semiquinone radical anions derived from 2,3-dimethylchrysazin, 7-deoxyaklavinone, and aclacinomycin T. Computational studies of the influence of aprotic and protic solvents on the electron spin resonance spectra

The influence of the solvent on the molecular structures and the ESR (EPR) spectra of the three title semiquinones is investigated by theoretical calculations, using density functional theory (DFT) and polarizable continuum models (PCM). The reordering of the relative magnitudes of the isotropic hyperfine constants observed when passing from an aprotic to a protic solvent is well reproduced by calculations using the integral equation formalism PCM (IEFPCM), provided discrete solute-solvent hydrogen-bonding interactions in the protic solvent is considered explicitly. A mixed discrete-continuum procedure with a simplified super-molecular model leads to excellent prediction of all proton hyperfine constants measured in methanol solution, including methine as well as alkyl protons. The dependence of the alkyl proton hyperfine constants on the conformation of the A rings in the semiquinones of 7-deoxyaklavinone and aclacinomycin T is discussed.

Characterization of free radicals in clathrate hydrates of pyrrole, thiophene, and isoxazole by muon spin spectroscopy

Gas hydrates have long been of interest to the petrochemical industry, but there has been growing interest in potential applications for carbon dioxide sequestration and hydrogen storage. This has prompted many fundamental studies of structure and host–guest interactions, but there has been relatively little investigation of chemical reactions of the guest molecules. In previous work, we have shown that it is possible to use muon spin spectroscopy to characterize H atom like muonium and muoniated free radicals formed in clathrate hydrates. Muonium (Mu) forms in clathrate hydrates of cyclopentane and tetrahydrofuran, whereas furan and its dihydro derivatives form radicals. The current work extends studies to clathrates hydrates of other five-membered heterocycles: thiophene, pyrrole, and isoxazole. All form structure II hydrates. In addition to the clathrates, pure liquid samples of the heterocycles were studied to aid in the assignment of radical signals and for comparison with the enclathrated radicals. Similar to furan, two distinct radicals are formed when Mu reacts with thiophene and pyrrole. However, only one muoniated radical was detected from isoxazole. Muon, proton, and nitrogen hyperfine constants were determined and compared with values predicted by DFT calculations to aid the structure assignments. The results show that Mu adds preferentially to the carbon adjacent to the heteroatom in thiophene and pyrrole and to the carbon adjacent to oxygen in isoxazole. The same radicals are formed in clathrates, but the spectra have broader signals, suggesting slower tumbling. Furthermore, additional signals in the avoided level-crossing spectra indicate anisotropy consistent with restricted motion of the radicals in the clathrate cages.

Free Radicals of N-Donor-Stabilized Silicon(II) Compounds Probed by Muon Spin Spectroscopy

Transverse-field muon spin rotation (TF-μSR) spectra have been obtained for muoniated free radicals formed by positive muon irradiation of a series of four-membered cyclic silylenes. Muon-electron hyperfine coupling constants (Aμ) were determined from the spectra; values range from 28.5 to 59.3 MHz. The radicals are formed by muonium (Mu) addition to the silylenes, in a fashion similar to the expected reaction of the H atom. However, it is unclear a priori which is the site of addition. Accordingly, density functional theory (DFT) calculations were carried out for two classes of radicals: silyl radicals formed by H addition to the backbone C atom of the four-membered ring and alkyl radicals formed by H addition to the unsaturated Si(II) center. By comparison of the predicted proton hyperfine constants with the experimentally determined Aμ values it is possible to assign structures to each detected radical. Examples of both addition sites were found.

Density Functional Theory Study of the β-Carotene Radical Cation and Deprotonated Radicals

The beta-carotene radical cation and deprotonated neutral radicals were studied at the density functional theory (DFT) level using different density functionals and basis sets: B3LYP/3-21G, SVWN5/6-31G*, BPW91/DGDZVP2, and B3LYP/6-31G**. The geometries, total energies, spin distributions, and isotropic and anisotropic hyperfine coupling constants of these species were calculated. Deprotonation of the methyl group at the double bond of the cyclohexene ring of the carotenoid radical cation at 5 or 5' produces the most stable neutral radical because of retention of the pi-conjugated system while less stable deprotonation at 9 or 9' and 13 or 13' of the chain methyl groups causes significant distortion of the conjugation. The predicted methyl hyperfine coupling constants of 13-16 MHz of the neutral radicals are in good agreement with the previous electron nuclear double resonance (ENDOR) spectrum of photolyzed beta-carotene on a solid support. DFT calculations on the beta-carotene radical cation in a polar water environment showed that the polar environment does not cause significant changes in the proton hyperfine constants from those in the isolated gas-phase molecule. DFT calculated methyl proton hyperfine coupling constants of less than 7.2 MHz are in agreement with those reported for the radical cation in photosystem II (PS II) and those found in the absence of UV light for the radical cation on a silica alumina matrix.

Free radicals formed by H(Mu) addition to triphenylene and dodecahydrotriphenylene

Muonium has been used as an H atom analogue to investigate the free radicals formed by H addition to the fused polyaromatic hydrocarbon triphenylene. Although there are three inequivalent sites of attack in this molecule, only two radicals were detected. Muon and proton hyperfine constants were determined by transverse field μSR and LCR, respectively. With the help of quantum calculations, all the signals can be assigned to radicals formed by Mu addition to C–H sites, while there is no evidence for addition to the tertiary carbons at ring junctions. To force attack on a tertiary carbon, a complementary study was conducted on the dodecahydrotriphenylene molecule. As expected, only one radical is formed. From LCR measurements the number of non-equivalent protons are interpreted as the results of the conformational effect of the saturated side rings.

Free radicals formed by H(Mu) addition to fluoranthene

Muonium has been used as an H atom analogue to investigate the free radicals formed by H addition to the polyaromatic hydrocarbon fluoranthene. There are nine unique carbons in the molecule, but only five radicals were detected. Muon and proton hyperfine constants were determined by transverse field µSR and µLCR, respectively, and compared with calculated values. All signals were assigned to radicals formed by Mu addition to C-H sites. There is no evidence for addition to the tertiary carbons at ring junctions.Key words: muonium, fluoranthene, free radical, hyperfine constants.

Free radicals formed by H(Mu) addition to pyrene

Muonium addition to pyrene has been investigated by means of transverse field muon spin rotation and muon level-crossing spectroscopy. Two hydropyrenyl radicals are produced and easily detected while a third is formed in low abundance. Muon and proton hyperfine constants were experimentally determined and compared with semiempirical and ab initio calculations. It is concluded that the radicals are of cyclohexadienyl type, i.e., the Mu adds only to carbon atoms that can assume a tetrahedral -CHMu- configuration without distorting the planar carbon skeleton of pyrene.Key words: muonium, pyrene, radical, hyperfine.

Free radicals formed by H(Mu) addition to pyrene

Muonium addition to pyrene has been investigated by means of transverse field muon spin rotation and muon level-crossing spectroscopy. Two hydropyrenyl radicals are produced and easily detected while a third is formed in low abundance. Muon and proton hyperfine constants were experimentally determined and compared with semiempirical and ab initio calculations. It is concluded that the radicals are of cyclohexadienyl type, i.e., the Mu adds only to carbon atoms that can assume a tetrahedral -CHMu- configuration without distorting the planar carbon skeleton of pyrene.

Far-Infrared Rotational Spectra of ZnH and ZnD in theX2Σ+(v= 0) State

Several rotational transitions of zinc hydride and deuteride within thev= 0 level of theX2Σ+state have been measured in both electron spin components over the rangesN″ = 2 to 10 for ZnH andN″ = 9 to 21 for ZnD. A least-squares fit to these data in combination with low-Nmicrowave data measured by other workers has resulted in improved values of the rotational, fine, and hyperfine structure constants. The values of the proton hyperfine constants are discussed in the context of a molecular orbital analysis of zinc hydride.

Temperature dependence of the muon and proton hyperfine constants of an \alpha‐muonium‐substituted methyl radical

Muon hyperfine constants A_μ have been measured by transverse field μSR for (CH3)3Si\mbox\.CHMu in hexane from 167 K to 332 K. In addition, avoided level‐crossing resonance was used to determine \alpha‐proton coupling constants Ap over a similar range of temperatures. The two hyperfine constants can be described by a common temperature dependence, d|Ai|/ dT=1.4\times 10-3 MHz\,K-1, where Ai represents Ap or the reduced muon constant A^\prime_μ=0.3141A_μ. There is a small isotope effect (A^\prime_μ is 2.2 % larger than Ap) consistent with zero‐point motion in the anharmonic C–H bond stretch. The common temperature dependence is tentatively attributed to a coupled deviation of the C–H and C–Mu bonds out of the nodal plane of the p orbital containing the unpaired electron.

NH 3+ EPR in single crystal potassium ammonium sulphate, KNH 4SO 4

An EPR study has been performed on X-irradiated potassium ammonium sulphate, KNH 4SO 4 (PAS), single crystals in the temperature range −120°C to 150°C. The samples were characterized by powder XRD. Three types of NH 3 + have been identified, along with SO 3 −. While the first centre, (NH 3 +) I is characterized with equal and isotropic A N and A H (the nitrogen and the proton hyperfine constants) with an isotropic g = 2.0042, and other two, (NH 3 +) II and (NH 3 +) III are trapped at two physically inequivalent sites and possess isotropic g and A H but rhombically symmetric A N tensors. (NH 3 +) I seems to be undergoing fast rotation/slow tumbling whereas the other two are relatively rigid. DSC scans did not reveal any phase transitions but the EPR hyperfine components broadened on cooling. The results are discussed in detail.

Investigation of electron distribution and hyperfine properties of hemin by first-principles Hartree-Fock self-consistent field procedure

As part of a program for first-principles quantitative study of hyperfine properties of large biological molecules, we have applied the Self-Consistent Field Unrestricted Hartree-Fock procedure to investigate the electronic structure of hemin, the associated charge and unpaired spin population distributions and the hyperfine constants of57Fe,14N,13C,1H and35Cl nuclei. The results of the present work indicate significantly more localized charge and spin distributions than were found earlier by the semi-empirical Self-Consistent Charge Extended Huckel procedure. The theoretical values of the57Fe,14N and methene proton hyperfine constants, the only nuclei for which experimental data are available, are found to be −34.99, 11.89 and −1.50 MHz in satisfactory agreement with the experimental values of −26.47, 7.55 and −1.014 MHz, respectively. The contact and dipolar contributions to the hyperfine constants and their breakdowns into direct and exchange polarization contributions are analyzed. Possible sources that could bridge the remaining gap between experiment and theory will be analyzed.

Hyperfine interactions and relaxivities in divalent and trivalent aquoion systems

The electronic structures of aquoions involving the divalent paramagnetic ion Mn+2 and the two trivalent ions Fe+3 and Cr+3 have been studied using the unrestricted Hartree–Fock procedure, with six H2O molecules surrounding the metal ion. From the results of our investigations, good agreement is found between the metal–oxygen distances obtained from the present calculation and those from hydrated ionic crystals and aqueous solutions. From the analysis of the charges on the various atoms of the cluster, the covlaent bonding between the metal ion and water molecules is found to be negligible in the Mn+2 system and significant for Fe+3 and Cr+3. The spin populations however indicate strong localization on the metal ions in all three cases. Very good agreement is found between the calculated contact proton hyperfine constants for all three systems and measured values and for the 17O hyperfine constant in the Mn+2 aquoion system, the only one of those systems for which it is available. The contributions of the dipolar hyperfine fields at the proton sites to the proton relaxivities are studied and it is shown that in all three cases, the use of the point dipole model in earlier literature to explain the data on these relaxivities is well justified. Physical reasons for the various features of our results are discussed.

Muon level-crossing spectroscopy of organic free radicals

Muon level-crossing spectroscopy has been applied to the study of muonium-substituted radicals formed in liquid benzene, hexadeuterobenzene, furan, 2-methylpropene, 2,3-dimethyl-2-butene, and gaseous ethene. The magnitudes and signs of the proton and deuteron hyperfine constants are reported, and are discussed in terms of isotope effects and intramolecular motion.

ChemInform Abstract: A COMPARISON OF THE CARBON‐13 HYPERFINE CONSTANTS OF CARBOXYLATED CYCLOHEXADIENYL AND HYDROXYCYCLOHEXADIENYL RADICALS

/sup 13/C ESR spectra of the H atom adducts to pyromellitic and benzenepentacarboxylic acids have been observed. Comparison of the /sup 13/C hyperfine constants with those previously obtained for the OH adducts shows essentially identical values at the carboxyl positions and significant differences for the ring atoms only at the C/sub 3/ and C/sub 6/ positions. The spin density distribution in the ..pi.. system is clearly not affected to any significant extent by OH substitution. In spite of this the /sup 13/C hyperfine coupling constants for C/sub 6/, the site of addition, are 13 and 23% lower, respectively, in the OH adducts. This finding is at variance with correlation of /sup 13/C couplings strictly in terms of the local and adjacent ..pi.. spin densities. Since there is no appreciable loss of spin density from the ..pi.. system the effects of OH substitution cannot arise directly from transfer of spin density to the C/sub 6/ position. Rather these effects must result from changes in the interactions between the sigma and ..pi.. systems introduced by the substitution. It is suggested that the differences noted here, and the related drop of approx. 35% in the proton hyperfine constants at C/sub 6/, result mainlymore » from the loss of symmetry in the OH substituted radicals. 12 references, 1 figure, 1 table.« less

Xα multiple scattering calculations on iron(II) porphine

Spin-restricted and spin-unrestricted Xα multiple scattering calculations are reported for iron porphine with D4h symmetry in the absence of axial ligands. A 3A2g ground state is predicted when the radius of the porphyrin ‘‘hole’’ is less than 2.0 Å, whereas a quintet ground state is expected for larger Fe–N distances. A large asymmetry is found in the occupations of the three Fe 4p orbitals. This has important implications for the interpretation of Mössbauer quadrupole coupling constants. Magnetic susceptibility and NMR results can be understood in terms of a model that involves spin-orbit mixing of an excited 3Eg state with the ground 3A2g state; an essential element in explaining the NMR data is that the proton hyperfine constants are not the same in these two states. Tentative assignments are given for observed absorptions in the optical and near-infrared regions. It is shown that spin-unrestricted wave functions are particularly suited for the qualitative interpretation of electronic structure of metallophorphyrins.

The strong influence of the solvent on the spin distribution in 1,2-benzosemiquinones

A recently developed procedure to treat solvent effects of ring proton hyperfine constants of semiquinones is applied to 1,2-benzosemiquinone and its two mono-methyl derivatives. The results imply that the spin distribution is largely determined by the influence of the solvent. This explains the failure of traditional calculations to describe the ESR spectra of these radicals.

Solvent sensitivities of the hyperfine constants of methyl- and t-butyl-substituted 1,4-benzosemiquinones

The effect of solvation on the spin densities in methyl- and t-butyl-1,4-benzosemiquinone anion radicals is studied by means of an iterative version of the Energy Weighted Maximum Overlap (EWMO) method within a simple solvation model. Calculated ring carbon spin densities and their solvent sensitivities are compared with experimental proton hyperfine constants measured in a number of different H 2O-DMF mixtures. The results indicate that the spin distribution in the t-butyl derivatives depends critically on the interaction with the solvent. The effects due to association of solvent molecules are found to be larger than the perturbation caused by the t-butyl substitution itself, implying that the largest ring proton coupling constant in 2-t-butyl-1,4-benzosemiquinone can be unambiguously assigned to the 6-position, in line with a simple additivity prediction but in disagreement with recent theoretical interpretations.

ESR spectra of the carboxylated cyclohexadienyl radicals: 13C hyperfine constants and the effect of carboxylation on the g factors and proton hyperfine constants

The g factors and proton hyperfine constants of 15 multiply carboxylated cyclohexadienyl radicals and all 19 of the carboxylated hydroxycyclohexadienyl radicals have been determined by the in situ radiolysis method. The g factors were measured with sufficient accuracy (±0.00002) to establish that they increase monotonically with total spin density on the positions substituted. The proton hyperfine data are interpreted to indicate that there is a 13% loss in spin density on the ring in the pentacarboxylate radical. Pronounced effects of asymmetric substitution are noted even when this substitution is at a position of low spin density. For the more highly charged hydroxycyclohexadienyl radicals the ESR spectra are sufficiently intense that the 13C containing radicals can be observed at the natural abundance level. Complete sets of 13C hyperfine data are reported for the 1,3,5-tricarboxylate, the three tetracarboxylate and the pentacarboxylate radicals. Definitive assignment of 13C hyperfine constants to the carboxyl groups attached to the conjugated system is possible. Values of 4.8, 1.3, and 6.8 G are attributed to positions 1, 2, and 3, respectively, in the pentacarboxylate radical. These hyperfine constants parallel almost exactly the proton values in the unsubstituted radical so that one can conclude that they can be used as a good indicator of the spin density on the adjacent ring carbon via a relation of the form aC(CO2−) = QC(CO2−) ρα. Correlation of the 13C and proton hyperfine data give a value of 15.8 G for QC(CO2−). Ring 13C hyperfine constants of ∼ 18, 12(2), 11(2), and 8.5 G are observed for each radical with the largest and smallest values being assigned, respectively, to the carbon atoms at positions 3 and 6. Comparisons among the different sets of values indicate variations from predictions based on the Karplus–Fraenkel treatment of the order of 10% so that specific assignment of the remaining four values is difficult. The hyperfine constants of the methylene protons in the substituted cyclohexadienyl radicals reflect very closely the loss in spin density to the carboxyl groups. Substitution in the hydroxycyclohexadienyl radicals, however, produces a far more pronounced and complicated effect that indicates a strong interaction between the OH group and the carboxyl groups substituted on the adjacent positions.

ESR Spectra of the Fluorinated Silyl Radicals

Isotropic ESR spectra of · SiH2F, · SiHF2, and · SiF3 are reported. The radicals were produced by radiolysis of H3SiF, H2SiF2, and HSiF3 in an SF6 matrix. The hyperfine parameters in gauss are: for · SiH2F, aH=34.6, aF=55.2; for · SiHF2, aH=89.9, aF=77.8; and for · SiF3, aF=136.6, aSi=(−) 498.0. The large 29Si hyperfine constant for · SiF3 shows that the radical is strongly pyramidal, probably as a result of the high ionic character of the Si–F bonds. The proton hyperfine constants are believed to be positive and the parallel of · SiHF2 with · HPO2− (aH=82.5 G) is noted. The radical (CH3)2SiF · was also produced in liquid phase photolysis experiments and has the parameters aH=4.53 and aF=65.21 G.