On quantum-chemical estimates of constants of isotropic hyperfine coupling with protons in free radicals

A scheme of consitent quantum-chemical calculations of constants of isotropic hyperfine coupling (IHFC) with protons (a _H ^iso ) in free radicals is considered for the case where the spin populations ρ_s ^H are determined in the basic set of symmetrically orthogonalized atomic orbitals taking model σ-and π-electron fragments as an example. The competence of using two coefficients of proportionalityK(H) when estimating the proton IHFC constants by semiempirical quantum-chemical methods was demonstrated. Theoretical substantiation of empirical values of the above coefficients previously used is revealed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 245–249, February, 1999.     

Protonenresonanz-Spektren des CycIohexa-3,5-dien-1-on-Systems (ortho-Dienone)

Proton magnetic resonance spectra of a scries of ortho-dienones have been analyzed using multiple resonance and computer techniques. The sequence of the chemical shifts are in agreement with the calculated π-electron distribution. The coupling constants of the ring protons are of like sign, whereas those of the methyl protons with the ring protons alternate in sign with the number of intervening bonds.     

Nuclear magnetic resonance studies of hydrazyl radicals

A study of the nuclear magnetic resonance spectra of six hydrazyl radicals in solution, including α,α′-diphenyl-β-picryl-hydrazyl (DPPH), has been carried out. From the values of the paramagnetic shifts the constants (α_H) of hyperfine coupling with the protons were found. For DPPH these have the following values: On the basis of the analysis of the hyperfine constants a conclusion is drawn concerning the substantial role of σ-electron spin density delocalization in aromatic rings. The geometric structure of radicals investigated is also discussed in the paper.     

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

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.     

From alcohols to sugars-how does the reactivity of alpha-hydroxyalkyl radicals change with structure? A quantitative examination by pulse radiolysis

Rate constants are reported for the 1-electron reduction of the azo dye Orange II in water (pH 7.0) by 10 different alpha-hydroxy radicals. The radicals were created by pulse radiolysis of aqueous solutions of the corresponding alcohol/sugar. The rate constants varied from 1 x 10(8) to 2.7 x 10(9) mol(-1) dm(3) s(-1) and radicals with beta-hydroxy groups had the lowest rate constant. The reaction was found to be controlled by the reduction potentials of the radicals, with steric influences having little effects. Good fits of the data were obtained using the Marcus equation with lambda =140 kJ/mol.     

Semi-empirical π-electron calculations of proton,nitrogen and fluorine hyperfine-coupling constants

The application of the parameterization scheme of Beveridge and Hinze for π-electron molecular orbital calculations to spin density calculations is examined for a series of radicals including anions derived from pyridine derivatives, substituted styrenes, aromatic carboxylic acids, benzonitriles and fluoronitrobenzene derivatives and azabenzyl and fluorobenzyl radicals. The method correlates proton hyperfine coupling constants satisfactorily and gives a reasonable correlation of nitrogen coupling constants. The application to fluorine couplings is less satisfactory.     

Electron Spin Resonance of α-(Alkoxycarbonyl)alkyl Radicals in Solution

High-resolution ESR. spectra of the radicals CH₂COOR, CH₃CHCOOR and (CH₃)₂CCOOR with R?CH₃, CH₂CH₃, CH(CH₃)₂ and C(CH₃)₃ in liquid solution confirm planar energy-minimum structures with substantial barriers to internal rotation about the ?, CO-bonds (?40 kJ/mol) and partial π-electron delocalization. The assignments of coupling constants to protons in isomeric positions and the conclusions on radical structures are supported by INDO-calculations.     

The 1H NMR spectra of quinoline,quinoline N-oxide,the quinolinium ion and of their monomethyl derivatives

¹H chemical shifts of quinoline, quinoline N-oxide and the quinolinium ion were obtained by complete analysis of their NMR spectra and interpreted critically in an attempt to quantify the possible different effects acting on the shielding constant of protons in these systems. Semi-empirical SCF calculations of electron charge densities and ring current contributions were also performed. It was found that the same effects that act on the shielding of protons directly bonded to the heterocyclic system also act, in an attenuated form, on the proton chemical shift of methyl groups in monomethyl derivatives. Vicinal coupling constants were rationalised in terms of π-electron bond order and electro-negativity of neighbouring atoms.     

Phylloquinone and related radical anions studied by pulse electron nuclear double resonance spectroscopy at 34 GHz and density functional theory

1H hyperfine (hf) coupling constants of semiquinone radical anions of 1,4-naphthoquinone, 2-methyl-1,4-naphthoquinone, and 2-methyl-3-phytyl-1,4-naphthoquinone in frozen alcoholic solutions were measured using pulse Q-band electron nuclear double resonance spectroscopy. The resolved signals of the quinone protons as well as from hydrogen bond and solvent shell protons were analyzed and assigned. Both in-plane and out-of-plane hydrogen bonding with respect to the pi-plane of the radical is observed. Interactions with nonexchangeable protons from the surrounding matrix are detected and assigned to solvent protons above and below the quinone plane. Density functional theory was used to calculate spin Hamiltonian parameters of the radical anions. Solvent molecules of the first solvent shell that provide hydrogen bonds to the quinones were included in the geometry optimization. The conductor-like screening model was employed to introduce additional effects of the solvent cage. From a comparison of the experimental and calculated hf tensors it is concluded that four solvent molecules are coordinated via hydrogen bonds to the quinone oxygens. For all radicals very good agreement between experimental and calculated data is observed. The influence of different substituents on the spin density distribution and hydrogen bond geometries is discussed.     

All electron calculations of open-shell polyatomic molecules. III. Perturbation treatment of the spin polarization in vinyl and methyl radicals

A first order perturbation treatment starting with SCF-MO 'S in canonical or equivalent quasi-localized form is presented for the hyperfine coupling constants of vinyl and methyl radicals. The spin-polarisation contribution to hyperfine splittings is found to be large, negative for the proton of the radical center in both radicals and positive for the β protons of vinyl.     

AB initio MO LCAO UHF calculations of magnetic hyperfine interactions in π-radicals. Isotropic and anisotropic couplings of ·CH3, ·NH3+, ·C2H5 and ·N2H4+

A series of π-electron radicals have been investigated with ab initio methods, using the unrestricted Hartree-Fock approximation. The geometries have been partially optimized, and magnetic hyperfine coupling constants have been evaluated. For the dipolar hyperfine couplings, the results obtained are in fair agreement with experiment, whereas the isotropic coupling constants show greater deviations. These values are also shown to be sensitive to annihilation of the quartet contamination of the wavefunction.     

EPR INVESTIGATIONS CONCERNING THE STRUCTURE OF THE ASCORBIC ACID RADICAL IN WATER, ALCOHOL, AND DIMETHYLSULFOXIDE

Abstract— Electron paramagnetic resonance (EPR) spectra of the ascorbic acid radical in water, methanol, ethanol, n -propanol, and dimethylsulfoxide are reported. The radical was produced by photolyzing ascorbic acid solutions containing acetone. Lines from ¹³C in natural abundance are observed. Four different ¹³C-coupling constants can be distinguished. A comparison of the experimental ¹H-coupling constants with values calculated by the INDO method allows assignment to specific protons in the radical. An effect of solvent on the spectrum is observed and is interpreted in terms of intramolecular hydrogen bonding. Data are presented which indicate that ascorbic acid is able to scavenge alcohol radicals by H-atom transfer.     

Analysis of 1H NMR spectra of fluorene and azafluorenes: Long-range coupling constants

Analyses of the ¹H NMR spectra of fluorene, 2-nitrofluorene, 1-azafluorene, 2-azafluorene, 4-azafluorene, 7-nitro-4-azafluorene and 1,4-diazafuorene have been carried out using computer calculations and homo double resonance techniques. The relative signs and magnitude of the ⁴J, ⁵J and ⁶J long-range coupling constants of the 9-CH₂ group protons with the pyridine and phenylene ring protons have been measured by five- and six-spin calculations. It was established that the presence of substituents and the N heteroatom has only a very weak effect on the values and signs of these coupling constants. The usual alternating signs of long-range coupling constants involving π-electron systems were confirmed, giving negative signs for even and positive signs for odd numbers of bonds separating the coupling protons.     

Magnetic properties of single crystal alpha-benzoin oxime: An EPR study

The electron paramagnetic resonance (EPR) spectra of gamma irradiated single crystals of alpha-benzoinoxime (ABO) have been examined between 120 and 440 K. Considering the dependence on temperature and the orientation of the spectra of single crystals in the magnetic field, we identified two different radicals formed in irradiated ABO single crystals. To theoretically determine the types of radicals, the most stable structure of ABO was obtained by molecular mechanic and B3LYP/6-31G(d,p) calculations. Four possible radicals were modeled and EPR parameters were calculated for the modeled radicals using the B3LYP method and the TZVP basis set. Calculated values of two modeled radicals were in strong agreement with experimental EPR parameters determined from the spectra. Additional simulated spectra of the modeled radicals, where calculated hyperfine coupling constants were used as starting points for simulations, were well matched with experimental spectra.     

ESR study of free radicals produced from the reaction of o-substituted phenyl methacrylates with tert-butoxy radical

ESR spectra of radicals produced from the reactions of phenyl methacrylate (PMA) and four o-substituted PMAs, 2,4,6-trimethyl-, 2,6-diisopropyl-, 2,6-di-tert-butyl-, and 2,6-di-tert-butyl-4-methyl-PMAs, with tert-butyoxy radical were measured in 2-methyltetrahydrofuran over the temperature range of ?53 to ?15°C. The coupling constants of the β-methylene protons observed varied with the bulkiness of the o-substituents, whereas the p-substitution did not affect the pattern of the spectra. 2,6-Diisopropyl- and 2,4,6-timethyl-PMAs, which can form homopolymers, gave 5- and 13-line spectra, respectively. For the radicals from 2,6-di-tert-butyl- and 2,6-di-tert-butyl-4-methyl-PMAs, the same 8-line spectrum was observed, indicating that the coupling constant of one of the β-methylene protons was too small to detect. Conformations of the radicals were deduced from the coupling constants of the β-methylene protons. Variation of the ESR spectrum according to the bulkiness of the o-substituent was interpreted as a consequence of steric interactions between the polymer chain bound to the β carbon and the substituents, and the α-methyl group.     

Structure and ethanol complexation of cyclic tetrasaccharide in aqueous solution studied by NMR and molecular mechanics

The structure and ethanol complexation of a cyclic tetrasaccharide (CTS) in aqueous solution were investigated by proton NMR spectroscopy and molecular mechanics calculations. Two glucose units, A and B, of CTS are alternatively bonded by alpha-1,3 and alpha-1,6 linkages. The overlapped signals of protons A5, A6S, A6R, B3, B6S and B6R were resolved by spectral simulations to determine their chemical shifts and vicinal coupling constants. All vicinal coupling constants except for the A5-A6 spin system are consistent with the dihedral angles in the X-ray crystal structure. Each of protons A5, A6S, and A6R in the two units of A is equivalent with respect to the chemical shift. The vicinal coupling constants of (3)J(5-6S) and (3)J(5-6R) for unit A are close to the average of two rotamers that are present in crystals. The intensities of cross-peaks in the rotating frame nuclear Overhauser effect spectroscopy (ROESY) spectrum were rather well correlated with the effective distances calculated for the X-ray structure and molecular mechanics structures calculated in vacuo and water, although they are slightly better correlated with molecular mechanics structure in vacuo than with the other structures. From the changes of the chemical shifts of several CTS protons with increasing ethanol concentration, it was suggested that adsorption sites of ethanol on the plate structure of CTS are protons B2 and B4 (site B) in the concave face side and protons A1 and A2 (site A) in the convex back side. The binding constants for sites A and B are 0.0061 and 0.0176 M(-1), respectively. These binding constants are much smaller than a value of 4.1 M(-1) for the ethanol-alpha-cyclodextrin complex.     

Radikale aus Polymethin-Farbstoffen: III—Polymethinoxonole

The e.s.r. spectra of neutral radicals in solution obtained by electrolytic reduction of a series of dinuclear polymethine oxonols have been interpreted. The values of the e.s.r. coupling constants, ¹H n.m.r. results from the parent compounds and HMO calculations are used to discuss conformation, π-electron density distribution and C? C bond lengths in these radicals.     

Theoretical prediction and assignment of vicinal 1H-1H coupling constants of diastereomeric 3-alkoxy-6,7-epoxy-2-oxabicyclo[3.3.0]octanes

Spin-spin coupling constants between nuclei in NMR spectroscopy reflect their spatial arrangement. A number of calculation methods, applying different levels of theory, have been developed to support the stereochemical assignment of novel compounds. Nevertheless, revisions of the assignment of structures in the literature are not rare. In the present work, the reliability of the calculation methods amenable for a theoretical prediction of spin-spin coupling constants of vicinal protons to support correct stereochemical assignment of substitution at five-membered rings of 3-alkoxy-6,7-epoxy-2-oxabicyclo[3.3.0]octanes was studied. Experimental (3)J(H,H) coupling constants were compared with the coupling constants calculated for all possible diastereomers. The fully quantum chemical approach provided theoretical (3)J(H,H) coupling constants with an absolute deviation of no more than 1.1 Hz for 91% of the experimentally studied coupled spins, whereas the methods without quantum chemical geometry optimization resulted in completely unreliable predictions. Consequently, for a reliable stereochemical assignment of small and medium size molecules, the protocol for calculating the coupling constants based on the results of the quantum chemical geometry optimization is recommended.