Hf Coupling Research Articles

Photoinduced Isomerization of trans-Acetylene Radical Anion to Vinylidene Radical Anion in 2-Methyltetrahydrofuran

Photoinduced reactions of trans-HC⋮CH- in a glassy 2-MTHF matrix were studied by ESR and electron absorption (EA) spectroscopy. ESR and EA spectra were interpreted with the aid of the results of molecular orbital (MO) calculations. The trans-HC⋮CH- was generated and stabilized in the 2-MTHF matrix at 77 K by ionizing irradiation. The ESR parameters observed for the radical anion are A1 = 5.3, A2 = 4.6, A3 = 4.5 mT for two equivalent 1H atoms, A⊥ = 1.5, A∥ = 4.2 mT for two equivalent 13C atoms, and g1 = 2.0009, g2 = 2.0024, g3 = 2.0029 for the g tensor. By illuminating with light of λ ≥ 430 nm, the trans-HC⋮CH- was found to be irreversibly isomerized into the vinylidene radical anion, H2CC-, in the matrix. The experimental ESR parameters of H2CC- [A1 = 5.4, A2 = 6.0, A3 = 5.7 mT for two equivalent protons, g1 = 2.0026, g2 = 2.0008, g3 = 2.0026] were fully consistent with the results obtained by semiempirical MO methods, (1) INDO and Analytical Dipolar Calculation method (ANADIP) for isotropic and anisotropic 1H hyperfine (hf) couplings, respectively, and (2) AM1 for the g tensor, for a geometrical structure optimized by ab initio method (Gaussian90/uhf/6-31++G**). The structure of the anions was in agreement with the results obtained from the EA spectroscopic study. The photoinduced isomerization was initiated by the light absorption at λ ≥ 430 nm of trans-HC⋮CH-. The EA band at λmax = 374 nm observed for H2CC- was attributed to the electron transition from the 2B2 ground state and to the 2A1 excited state.

Intramolecular electric field effect on a 1J(C,H) NMR spin–spin coupling constant. an experimental and theoretical study

An experimental study of the effects of intramolecular electric fields on 1J(C,H) coupling constants involving a formyl proton in a series of 5-X-salicylaldehydes is reported. The particularly large electric field component along the CcHf bond, calculated for X=NO2 , leads to a measured substituent effect of 7.36 Hz. For other X substituents a linear correlation was found between the calculated substituent electric field component along the Cc—Hf bond and the substituent effect on 1J(Cc ,Hf). To determine if 5-X substituents affect the intramolecular CO· · ·HO hydrogen bond, NBO analyses were carried out on 5-NO2 − and 5-H-salicylaldehyde, and they were compared with those corresponding to benzaldehyde and phenol. The effects of interactions other than the electrostatic one on 1J(Cc ,Hf) couplings were ruled out. The experimental results are in agreement with theoretical predictions published previously. Copyright © 1999 John Wiley & Sons, Ltd.

Intramolecular electric field effect on a1J(C,H) NMR spin-spin coupling constant. an experimental and theoretical study

An experimental study of the effects of intramolecular electric fields on 1J(C,H) coupling constants involving a formyl proton in a series of 5-X-salicylaldehydes is reported. The particularly large electric field component along the CcHf bond, calculated for X=NO2 , leads to a measured substituent effect of 7.36 Hz. For other X substituents a linear correlation was found between the calculated substituent electric field component along the Cc—Hf bond and the substituent effect on 1J(Cc ,Hf). To determine if 5-X substituents affect the intramolecular CO· · ·HO hydrogen bond, NBO analyses were carried out on 5-NO2 − and 5-H-salicylaldehyde, and they were compared with those corresponding to benzaldehyde and phenol. The effects of interactions other than the electrostatic one on 1J(Cc ,Hf) couplings were ruled out. The experimental results are in agreement with theoretical predictions published previously. Copyright © 1999 John Wiley & Sons, Ltd.

1H, 13C and 19F NMR Studies of the Diels-Alder Adduct of p-Fluoranil with Phencyclone. II. Hindered Phenyl Rotations and Anisotropic Effects in a Model Compound for Drugs

The Diels-Alder adduct of phencyclone, compound 1, with p-fluoranil, compound 3, has been prepared in refluxing toluene. The adduct, compound 2, has been examined by 1H, 13C and 19F NMR spectroscopy at 300, 75 and 282 MHz, respectively. At ambient temperature, the unsubstituted bridgehead phenyl groups in adduct 2 are found to exhibit hindered rotation, resulting in slow exchange limit (SEL) 1H NMR spectra. Full aryl proton assignments are made based on 1D and 2D (COSY45) NMR. The 19F NMR (proton coupled) reveals one of the two 19F signals to be a triplet. This resonance collapses to a singlet in the proton decoupled 19F spectrum, implying an unexpected long range 1H-19F coupling. For the 13C NMR spectrum, tentative assignments are presented. Data for compound 2 as a model compound for drugs are discussed in terms of the hindered aryl rotation and evidence of magnetic anisotrppic effects.

Structure and dynamics of radicalized hydrocarbon derivatives included in perhydrotriphenylene single crystals

The radical obtained by γ-irradiation of the nonadecan-10-one/perhydrotriphenylene (10-NDO/PHTP) inclusion compound has been studied by EPR spectroscopy. The radical is formed by removal of one of the protons in the α position to the carbonyl group and the unpaired electron spin is coupled to the α proton and to two equivalent β protons. The hyperfine (hf) coupling constants vary slowly on lowering the temperature until an abrupt change occurs at T = 150 K. The radical reorients quickly on the EPR timescale inside the PHTP channels. The reorientational correlation time τr is obtained from the linewidth values and it is found to be shorter than the τr value measured previously for the same radical included in urea. The EPR spectrum changes in the timescale of weeks and after two months displays the presence of an additional hf coupling with a pair of equivalent protons, the other spectral parameters being unchanged with respect to those of the freshly irradiated radical. This modification is probably due to a folding of the radicalized guest molecule inside the host channel.

Radical Cation of Naphthalene on H−ZSM-5 Zeolite and in CFCl3 Matrix. A Theoretical and Experimental EPR, ENDOR, and ESEEM Study

The naphthalene radical cation generated from the corresponding neutral compound by UV- or X-irradiation on H−ZSM-5 zeolite and in frozen CFCl3 solution has been studied by EPR, ENDOR, and ESEEM spectroscopies. The g and ring proton hyperfine (hf) tensors have been determined and are identical in the two media, within experimental error. The isotropic hf couplings of the two sets of four equivalent protons are |a(1)| = 16.4 and |a(2)| = 4.7 MHz (0.587 and 0.167 mT, respectively). Semiempirical calculations of isotropic and dipolar hf couplings are found to be in good agreement with experiment. The hf tensors obtained for the radical cation of naphthalene-d8 were analogous to those of the protonated compound, scaled by the gyromagnetic ratio, γH/γD. The nuclear quadrupole interaction (nqi) tensors of the deuterium nuclei were also determined by ESEEM, being the only resolved method for the present nqi strength.

EPR and ENDOR studies of the water oxidizing complex of Photosystem II

A comparative study of X-band EPR and ENDOR of the S2 state of photosystem II membrane fragments and core complexes in the frozen state is presented. The S2 state was generated either by continuous illumination at T=200 K or by a single turn-over light flash at T=273 K yielding entirely the same S2 state EPR signals at 10 K. In membrane fragments and core complex preparations both the multiline and the g=4.1 signals were detected with comparable relative intensity. The absence of the 17 and 23 kDa proteins in the core complex preparation has no effect on the appearance of the EPR signals. (1)H-ENDOR experiments performed at two different field positions of the S2 state multiline signal of core complexes permitted the resolution of four hyperfine (hf) splittings. The hf coupling constants obtained are 4.0, 2.3, 1.1 and 0.6 MHz, in good agreement with results that were previously reported (Tang et al. (1993) J Am Chem Soc 115: 2382-2389). The intensities of all four line pairs belonging to these hf couplings are diminished in D2O. A novel model is presented and on the basis of the two largest hfc's distances between the manganese ions and the exchangeable protons are deduced. The interpretation of the ENDOR data indicates that these hf couplings might arise from water which is directly ligated to the manganese of the water oxidizing complex in redox state S2.

Automatic computer simulations of ESR spectra

A versatility of automatic ESR simulation procedures has been developed to obtain high quality of fitting and produce additional information. The following examples are treated: derivation of long-range proton hyperfine coupling constants from unresolved lines; determination of13C hf couplings from the naturally abundant isotope satellites; analyzing chemical exchange phenomena with two-sites model; decomposition of superimposed spectra consisting of poorly resolved components. In order to achieve best agreement between calculated and experimental spectra within a reasonable number of iteration cycles, we combined various approaches, like consecutive independent parameter optimization, least-square approach, optimization on “serpentines” and optimization of compound parameters. The spectra can be computed both for liquid and solid state samples, for non-oriented, partially oriented and single crystal samples. Second order perturbation formulas are used for the spin Hamiltonian includingg- and hyperfine tensors that have isotropic, axial and rhombic symmetry. For chemical exchange the modified Bloch equation for two-site model is used. Various lineshapes, including Lorentzian, Gaussian, mixed, modulation and dispersion distorted forms are applied. Third order parabolic interpolations are used for building up spectra from individual lines. In order to correct sweep non-linearity a third order interpolation converts the spectrum to become equidistant. This procedure can occasionally improve square deviation by an order of magnitudes for well resolved spectra. In the analysis of strongly overlapping superimposed spectra, simultaneous adjustment of two independent experimental spectra can give an exact decomposition. The inclusion of long-range couplings offers highly perfect fitting and allows one to resolve contributions of naturally abundant13C isotopes.

Rotational Effects on Radiationless Transition IV: Hyperfine Interaction and Deuterium Isotope Effects

Abstract The theory of the rotational effects on radiationless transitions in typical azines is extended to include the hyperfine (hf) interaction term, while examining their deuterium (isotope) effects on the decay behaviors after excitation into the rotational levels belonging to their S10 levels, i.e. their ground vibrational levels of S1 states. The Fermi-contact hf term is shown to be enough to endorse the coupling between rotational levels that are different either in symmetries or in some angular-momentum quantum numbers. The deuterium (D) effect is found to be not simply described as a conventional isotope effect: Changes by D substitution are brought about in densities of triplet states and hf coupling terms, as well as molecular symmetry groups. While interpreting the experimental data, modifications of the schemes are made from those presented earlier in this series.

The hyperfine structures of small radicals from density functional calculations

The isotropic and anisotropic hyperfine (hf) structures of a set of anionic, neutral and cationic radicals are investigated by means of local and nonlocal gradient-corrected density functional theory (DFT). The molecules under study are formed by H, C, N, O, F, and Cl atoms, and the hf structures are computed at both the experimental (where present) and various DFT and CI optimized geometries. The agreement with experiment and with results from previous CI or MRCI calculations is generally very satisfactory. The anisotropic hf couplings are relatively insensitive to basis set effects and functional form, whereas the isotropic hf couplings are highly dependent on the form of the nonlocal corrections to the exchange functional, particularly for heteroatoms. Using the functional by Perdew and Wang (‘‘PW86’’), an excellent agreement with experiment is obtained for all neutral and cationic radicals, whereas for the halide containing anions somewhat elongated bond lengths, and thus less accurate hf structures, are obtained.

19F-13C, 1H-13C Multi-Bond and Single-Bond 2D Correlations and Determinations of Coupling-Constant Signs

Abstract Fluorine-labeled metabolic products of pesticide or drug models often involve CF3-C- moieties. The first multibond as well as direct fluorine-carbon and proton-carbon correlations are reported for several simple aliphatic fluorocarbon molecules containing CF3-C-. The relative sign and magnitude of the 19F-13C, 1H-13C, and 1H-19F coupling constants are determined with facility by the skewing and separation of multibond and direct cross peaks in the 2D heteronuclear NMR spectra. The ability to obtain the coupling information potentially provides vital clues for the structural identification of CF3-C- metabolites.

The effect of a neon matrix on the hyperfine structure of CH +4. A model study

The influence of up to six Ne atoms on the isotropic hyperfine (hf) coupling constants of the hydrogen atoms in CH + 4 is investigated using density functional theory. It is found that at very short NeH distances ( R = 1.8 Å), the hf structure is strongly affected, whereas at the more realistic van der Waals distance ( R ≈ 2.5 Å), the methane radical experiences a very weak field from the surrounding Ne atoms. At larger distances, the effects of the Ne matrix are negligible. It is furthermore found that at small R, the changes in a iso (H) are strongly dependent on the relative positions of the Ne atoms.

Temperature effects on the 1H1H, 1H19F and 1919F coupling constants of fluorobenzene and 1,2‐difluorobenzene

AbstractThe 1H NMR spectra of fluorobenzene and 1,2‐difluorobenzene were measured in acetonitrile‐d3 and cyclohexane‐d12 at four temperatures to study the temperature dependence of 1H1H, 1H19F and 19F19F coupling constants. In acetonitrile‐d3 the 5J(F,H) coupling constant of fluorobenzene decreases by 5%, the 5J(H,H) coupling increases by 0.5% and all other couplings decrease by less than 1% when the temperature increases from 283 to 333 K. In cyclohexane‐d12 the change in 5J(F,H) for fluorobenzene is much smaller than in acetonitrile‐d6; the other changes are normally slightly smaller than in acetonitrile‐d5. The changes for 1,2‐difluorobenzene are generally of the same order as those for fluorobenzene.

A Novel Approach in the Determination of Magnitudes and Relative Signs of Long-Range Heteronuclear J Couplings Involving Fluorines

Fluorine-labeled metabolic products of pesticide or drug models often involve CF3-C- moieties. The first multibond as well as direct fluorine-carbon and proton-carbon correlations are reported for several simple aliphatic fluorocarbon molecules containing CF3-C-. The relative sign and magnitude of the 19F-13C, 1H-13C, and 1H-19F coupling constants are determined with facility by the skewing and separation of multibond and direct cross peaks in the 2D heteronuclear NMR spectra. The ability to obtain the coupling information potentially provides vital clues for the structural identification of CF3-C- metabolites.

Hindered motions in a ketonea-urea inclusion compound. An ESR and ENDOR study

We have studied by ESR and ENDOR spectroscopy the free radicals produced in γ-irradiated inclusion compound formed between the ketone 10-nonadecanone and urea. Only one type of long lived radical is formed by the removal of an α-proton from the ketone. The hyperfine (hf) coupling constants of the α- and β-protons of the radicals have been measured by ESR at different temperatures in the range 110–292 K and at different orientations of the crystals. The hf coupling of the γ-protons of the radical and of the urea protons have been studied by ENDOR. The temperature and angular dependences of the coupling constants have been analyzed in terms of the internal and overall motions of the radical inside the hexagonal channels formed by the urea molecules. It has been found that the radical cannot perform complete reorientations around the long molecular axis, but it undergoes restricted rotational diffusion. This process is explained by assuming a coupling between the rotational and translational degrees of freedom of the radical inside the urea channels.

Solvent effects on the 1H1H, 1H19F and 19F19F coupling constants of fluorobenzene and 1,2‐difluorobenzene

AbstractThe 1H and 19F NMR spectra of fluorobenzene and 1,2‐difluorobenzene were measured in several pure solvents and in binary solvent mixtures to study the solvent dependence of 1H19F and 19F19F coupling constants. The solvents are distinctly divided into two groups on the basis of their hydrogen bonding properties. Most the coupling constants increase with increasing polarity of the solvent within both groups. The 3J(F,H) and 4J(F,H) coupling constants are typically increased by 4‐7% when the solvent is changed from cyclohexane to acetone. For the 5J(F,H) coupling of fluorobenzene the change is as large as 70%.

A theoretical investigation of the electronic structure, hyperfine properties and binding energies of muonium centres in cuprous chloride

A series of self-consistent local-spin-density molecular-cluster calculations has been performed to investigate the electronic structure, the hyperfine (hf) properties and binding energies of muonium centres in curprous chloride. Spin-polarization was taken into account on a finite, stoichiometric cluster including Cu 7Cl 7 and an interstitial muonium (MU) atom. Different Mu positions along the direction 〈111〉 of the CuCl zinc blende structure between the two inequivalent tetrahedral interstitial sites (hereafter T Cu and T Cl) were investigated. Our results indicate a localization of Mu close to the T Cu site, shifted about 0.5 Å toward the apical Cu atom of the employed cluster. Such a fact nicely agrees with experimental findings and it has been discussed and rationalized on the basis of the different bonding interaction between MU and the nearest neighbours. In the light of our theoretical outcomes, the single-electron model proposed by Cox and Symons for the reduction of the Fermi contact term A of the hf coupling constant in I–VII semiconductors seems to be rather inadequate. The accord between experimental and theoretical hyperfine parameters is satisfactory. In particular, theoretical data would indicate that the two different muonium centres, experimentally detected in CuCl, have the same chemical environment. These results are encouraging for the study of other compound semiconductors.

EPR spectra of DMPO spin adducts of superoxide and hydroxyl radicals in pyridine.

Electron spin resonance spectroscopy and the spin trapping technique were used to study the formation of the superoxide radical in pyridine. 5,5-Dimethyl-1-pyrroline-N-oxide (DMPO) was employed as a trapping agent. Superoxide radical was generated using chemical (potassium superoxide) and photochemical methods with anthralin, benzanthrone, rose bengal, 1,8-dihydroxyanthraquinone and zinc tetraphenylporphyrine as photoactive pigments. Hyperfine coupling (hf) constants for DMPO/O2.- were determined to be aN = 12.36 G, a beta H = 9.85, G, a gamma H = 1.34 G. The aN and a beta H hf constants are in good agreement with values calculated from a previously determined relationship between hf constants and solvent acceptor number (Reszka et al., (1992) Free Radical Res. Commun., in press). When concentrated hydrogen peroxide was added to DMPO in pyridine a similar EPR spectrum was observed. It is suggested that in this case the DMPO/.O2H adduct is formed by nucleophilic addition of H2O2 to DMPO to give a hydroxylamine, followed by oxidation to the respective nitroxide. The EPR spectrum observed when tetrapropylammonium hydroxide and H2O2 were added to DMPO in pyridine had hf couplings aN = 13.53 G, a beta H = 11.38 G, a gamma H = 0.79 G and it was assigned to a DMPO/.OH adduct. This assignment was based on similarity of this spectrum to the one produced by UV photolysis of hydrogen peroxide and DMPO in aqueous solution and subsequent transfer to pyridine.

Phenoxides and thiophenoxides of aluminum and gallium. Evidence of a 1H-19F coupling in (R2MOC6F5)2 in solution. Crystal and molecular structures of (Me2AlOC6F5)2 and (Me2GaSC6F5)2

The reaction of trialkylaluminum and -gallium derivatives with pentafluorophenol and pentafluorothiophenol yields the corresponding phenoxides and thiophenoxides, (R 2 MEC 6 F 5 ) 2 (M=Al, E=O, R=Me (1a), Et (1b), and i-Bu (1c); M=Ga, E=O, R=Me (2a); M=Ga, E=S, R=Me (2b)) quantitatively. Variable-temperature 1 H NMR studies established that the Al 2 S 2 and Ga 2 S 2 ring systems undergo rapid inversion/exchange, which leads to equivalence of the alkyl groups attached to the metal atoms

NMR analysis of fluorinated corticosteroids related to fluocinonide: Detection of long‐range 1H–19F couplings using the heteronuclear equivalent of the COSY experiment

AbstractThe analysis of the 1H and 19F NMR spectra of the corticosteroid fluocinonide and five related steroids is reported. The 500 MHz 1H NMR spectra revealed several previously undetected long‐range 1H–19F coupling constants. These couplings were assigned by the application of the heteronuclear equivalent of the COSY pulse sequence. The fluorine substituent effects and long‐range heteronuclear couplings were examined in relation to structures derived from molecular mechanics (MMX) calculations.