The isomers of ionized ethane

The previously reported ²A_g, ²A_1g, and ²B_g states of ionized ethane are characterized at several levels of theory. The diborane-like ²A_g state, which gives rise to the observed ESR spectrum, is predicted by SCF and CCD calculations not to exist in a separate minimum from the ²A_1g state formed by ionization of the C(SINGLE BOND)C bond. However, as reported by Lunell and Huang, second-order Moller-Plesset theory places the ²A_g lowest, provided polarization functions are included on carbon. QCISD theory predicts that both A states correspond to potential energy minima, but places the long-bond ²A_1g state lower, at least with moderately large basis sets. F orbitals on carbon stabilize the diborane structure more than the long-bond one. When a potential energy surface is generated for a series of fixed C(SINGLE BOND)C bond lengths by optimizing all variables except for the C(SINGLE BOND)C bond length with MP2 theory and calculating the energy with QCISD(T), the ²A_g state is predicted to be the lowest energy state with the ²A_1g state 1.83 kJ/mol above it. The two A states are predicted to be separated by a barrier 2.79 kJ/mol above the lower state. This barrier is above the zero-point energy in the C(SINGLE BOND)C stretch for the lower state but below the ZPE for this stretch in the upper state, which is therefore predicted not to exist as a stable species. A single quantum of vibrational excitation in the low frequency C(SINGLE BOND)C stretch is predicted to yield an ion with a poorly defined C(SINGLE BOND)C bond length. The highest levels of theory employed give poor agreement with the experimental hyperfine coupling constants. The discrepancy could either be due to neglect of vibrational effects, to poor inherent accuracy of the calculation, as one author has concluded, or to compression of the ion by the matrix as suggested by another. The ²B_g state is found to be higher in energy than the A states at all theoretical levels and is predicted to have a large (160.2–177.4 G) hyperfine coupling from four hydrogens. The transition state for simultaneous exchange of two hydrogen atoms between the carbons by a diborane structure is predicted to lie above the lowest energy fragmentation threshold, in agreement with experiment. © 1996 by John Wiley & Sons, Inc.     

Factors affecting stability and equilibria of free radicals—IX: Non-equivalence of aryl groups in 1,1-diaryl-2-benzenesulphonyl-hydrazyls and related compounds

1,1-Bis-(3',5'-di-t-butylphenyl)-2-benzenesulphonyl-hydrazine 11 and the corresponding 2-picryl-hydrazine 13 were synthesized. Oxidation of 11 afforded the stable hydrazyl 12 whose ESR spectrum could be simulated in terms of three hyperfine coupling constants. These experimental data leave no alternative but to assign larger proton hyperfine coupling constants to one aryl group in 12 than to the other. This non-equivalence was interpreted in terms of the Linnett theory. The picrylhydrazyl 14 does not give completely resolved ESR spectra. Structures were ascertained by IR, UV-vis, ¹H NMR and ¹³C NMR spectra.     

The Radical Anions of N,N′-Dicyanoquinone Diimines,a New Class of Electron Acceptors

The radical anions of 12 N,N′-dicyanoquinone diimines, a new class of electron acceptors, hace been characterized by their hyperfine data with the use of ESR and ENDOR spectroscopy. The largest coupling constant (0.30–0.45 mT), due to the two ¹⁴N nuclei in the exocyclic positions, gives rise to a conspicuous broadening of the peripheral ESR lines by an incomplete averaging of the hyperfine anisotropy. The most plausible interpretation of the experimental results for the radical anions of N,N′-dicyano- 1,4-benzoquinone diimine ( 1 ) and N,N′-Dicyano-9,10-anthraquinone diimine ( 9 ) is in terms of both ‘syn’- and ‘anti’-configurations contributing to the ESR and ENDOR spectra and having equal proton- and ¹⁴N-coupling constants. The π-spin distribution in the radical anions of N,N′-dicyanoquinone diimines is compared with those in the analogous ions of tetracyanoquinodimethanes and quinones.     

ESR single-crystal study of radical and radical-pair formation in some γ-irradiated vinyl monomers

Single crystals of methyl methacrylate (MMA), methyl acrylate (MA), and acrolein (A) have been prepared by a low-temperature technique. After irradiation with γ-rays at 77°K the paramagnetic species were identified by ESR spectroscopy. MMA gave a seven-line single spectrum from radicals formed by hydrogen addition. The hyperfine coupling constants are slightly anisotropic with a mean value of 22 G. Radical pairs were observed as ΔM_s = 1 and ΔM_s = 2 transitions; the hyperfine coupling was 11 G. From the strongly anisotropic dipolar interaction, upper limits for the distances between the pair components were calculated to be 5.45 Å and 6.3 Å. MA gave a five-line main spectrum with the same hyperfine coupling values and two radical pairs, one with a distance 5.9 Å between the components. In a there was also a strongly anisotropic interaction. The hyperfine coupling of the ΔM_s = 2 transition was 9.8 G. The number of radical pairs compared to the total number of radicals increases only slightly with the radiation dose. This makes it likely that pair formation occurs in the spurs and blobs formed by the γ-radiation. At an increased temperature the radical pairs disappeared; the spectrum of MMA changed to that characteristic of propagating polymer radicals.     

Molecular geometry of benzosemiquinone radicals: Radicals from resorcinol and its derivatives

The observed proton hyperfine coupling constants of radicals from resorcinol and 2-carboxylic, 5-carboxylic, and 5-methyl resorcinols in acidic and alkaline solution were reproduced by means of the INDO (intermediate neglect of differential overlap) with molecular geometry adjusting method. The bond lengths of C? O were longer in acidic solution than in alkaline solution and the total energies were lower in acidic solution than in alkaline solution. The assignments of the coupling constants of the radicals from resorcinols were |A₂| > |A₅| in alkaline oxidation and |A₂| < |A₅| in acidic oxidation.     

The effects of nonlocal gradient corrections in density functional calculations of hydrocarbon radical hyperfine structures

Ground-state equilibrium geometries and hyperfine structures of a number of organic neutral and charged radical compounds are computed using the linear combination of Gaussian-type orbitals–density functional theory method. In addition to the local spin-density approximation, we also use two different nonlocal (gradient corrected) schemes for the calculations of the exchange and correlation potentials. The different functional forms are found to generate slightly different total and unpaired spin-density distributions in the molecules, and as a result, the computed isotropic hyperfine coupling constants vary markedly. The smallest variations are found for the hydrogens, where the results are generally in satisfactory agreement with experiment. For the carbon atoms, however, large differences in isotropic coupling constants are observed. The anisotropic hyperfine structures are generally very well described at all levels of theory. © John Wiley & Sons, Inc.     

An indo investigation of magnetic resonance constants of azaaromatic systems

INDO–MO calculations have been performed on the ESR hyperfine coupling constants of the pyridinyl radical and the radical anions of pyrimidine, quinoline, isoquinoline, acridine and benzcinnoline. The nuclear spin coupling constants and the ¹⁴N nuclear quadrupole coupling constant of pyridine have also been calculated. In general, calculated values are in satisfactory agreement with the experimental data.     

Calculated hyperfine coupling constants for 5,5-dimethyl-1-pyrroline N-oxide radical products in water and benzene

The optimized structures of some radical adducts of 5,5-dimethyl-1-pyrroline N-oxide were computed by different methods on ESR spectra. As trapped radicals, H, N₃, NH₂, CH₃, CCl₃, OOH in water and F, OH, CF₃, CH₂OH, OC₂H₅ in benzene solutions were used. The calculated isotropic hyperfine coupling constants of all the trapped radicals were compared with the corresponding experimental data. The hyperfine coupling constant due to the β proton of the nitroxide radical was seen to be consist with the McConnel’s relation α_β = B ₀ + B ₁cos²θ and, to be effected with the opposite spin density of oxygen nucleus bonded to the nitrogen. It was concluded that in hyperfine calculations the DFT(B3PW91)/LanL2DZ level is superior computational quantum model relative to the used other level. Also, the study has been enriched by the computational of the optimized geometrical parameters, the hyper conjugative interaction energies, the atomic charges and spin densities for all the radical adducts.     

Zero-point vibrational corrections to isotropic hyperfine coupling constants in polyatomic molecules

The present work addresses isotropic hyperfine coupling constants in polyatomic systems with a particular emphasis on a largely neglected, but a posteriori significant, effect, namely zero-point vibrational corrections. Using the density functional restricted-unrestricted approach, the zero-point vibrational corrections are evaluated for the allyl radical and four of its derivatives. In addition for establishing the numerical size of the zero-point vibrational corrections to the isotropic hyperfine coupling constants, we present simple guidelines useful for identifying hydrogens for which such corrections are significant. Based on our findings, we critically re-examine the computational procedures used for the determination of hyperfine coupling constants in general as well as the practice of using experimental hyperfine coupling constants as reference data when benchmarking and optimizing exchange-correlation functionals and basis sets for such calculations.     

High-level coupled-cluster methods for electron spin resonance spectra: on the experimental spectrum of the silacyclobutane radical cation

The silacyclobutane radical cation is a prototype intermediate in chemical reactions involving Si based organic molecules. In the interest of its full characterization, the experimentally determined isotropic hyperfine coupling constants of the hydrogens in silacyclobutane radical cation (c-SiC(3)(+)) have raised some interesting questions, leading to different interpretations of the spectrum. To help resolve this discrepancy, we report very high-level theoretical results with coupled-cluster theory using its analytical, response density matrix procedure, and recently proposed basis sets that are specific to ESR. The detailed studies of geometries, basis set effects, and electron correlation tend to support the B3LYP/6-31G-based reassignment of the ESR spectrum of the c-SiC(3)(+) radical cation by F?ngstr?m et al.     

Hyperfine interactions in aqueous solution of Cr3+: an ab initio molecular dynamics study

We performed an ab initio molecular dynamics simulation of the paramagnetic transition metal ion Cr³⁺ in aqueous solution. Isotropic hyperfine coupling constants between the electron spin of the chromium ion and nuclear spins of all water molecules have been determined for instantaneous snapshots extracted from the trajectory. The coupling constant of first sphere oxygen, A _iso(¹⁷O_I)=1.9±0.3 MHz, is independent on Cr–O_I distance but increases with the tilt angle for the water molecule approaching 180°. First sphere hydrogen spins have A _iso(¹ H_I)=2.1±0.2 MHz which decreases with increasing tilt angle and shows a Cr–H_I distance dependence. The hyperfine coupling constants for second sphere ¹⁷O is negative and an order of magnitude smaller (−0.20±0.02 MHz) compared to first sphere.     

Investigations of the sodium and lithium D-lines in strong magnetic fields

Experimental and theoretical investigations of the splitting of the hyperfine structure of the sodium and lithium-D-lines in magnetic fields between 0 and 1 T were performed. In this magnetic field region the fine structure levelsJ=1/2 andJ=3/2 of the excited term² P begin to influence each other. In case of lithium crossings and anticrossings of hyperfine states stemming from different fine structure energy levelsJ=1/2 andJ=3/2 can be observed. The measurements were performed by laseratomic-beam spectroscopy in dependence on the applied external magnetic field strength. The experimental spectra were compared with computed spectra. Spectra were simulated by calculations using for the hyperfine hamiltonian two hyperfine constantsA andB in case of sodium and four hyperfine constantsa _c ,a _d ,a ₀ andb in case of lithium. Values for this constants could be derived by fitting the theoretical splittings to the experimental ones. For the first time theg _I — factor of sodium could be determined in a purely optical way.     

Estimates of magnetic resonance parameters in semi-empirical quantum chemistry

The possibility to reliably estimate the magnetic resonance parameters (MRP) can significantly increase the information content of experimentally recorded spectra. Here we consider in detail semi-empirical estimates of the ESR spectra parameters: isotropic hyperfine coupling constants, anisotropic hyperfine coupling tensors and g-tensors. The results show that the semi-empirical procedures give estimates of MRP comparable in quality with those of complicated ab initio and DFT schemes. It was underlined that a special attention should be given to geometric parameters of free radicals. The automatic procedure to determine molecular geometries of free radicals on the ground of their spectral characteristics was discussed.     

Electrochemical and ESR study of 5-nitrofuryl-containing thiosemicarbazones antiprotozoal drugs

Cyclic voltammetry and electron spin resonance (ESR) techniques were used in the investigation of several potential antiprotozoal thiosemicarbazones nitrofurane derivatives. A self-protonation process involving the protonation of the nitro group due to the presence of an acidic proton in the thiosemicarbazone moiety was observed in the first step of a CEE(rev) reduction mechanism of these derivatives. ESR spectra of the free radicals obtained by electrolytic reduction were characterized and analyzed. AM1 methodology was used to obtain the optimized geometries and UB3LYP calculations were performed to obtain the theoretical hyperfine coupling constants. The theoretical study exhibited an unusual assignment of the spin densities showing a free radical centered in the thiosemicarbazone moiety rather than the nitro which are in agreement with the experimental hyperfine pattern.     

DFT calculations of hyperfine coupling constants of organic π radicals and comparison with empirical equations and experiment

Density functional theory calculations (UB3LYP/EPR‐III) for a series of radicals and radical ions were performed to check the internal consistency of the method and its implications to the theoretical concepts of electron paramagnetic resonance such as π–σ spin polarization, hyperconjugation and phenyl hyperconjugation. In the second part, experimental data for seven radicals (43 hyperfine coupling constants) are compared with calculations, yielding a correlation of r² = 0.97. Copyright © 2016 John Wiley & Sons, Ltd.     

L-tryptophan radical cation electron spin resonance studies: connecting solution-derived hyperfine coupling constants with protein spectral interpretations

Fast-flow electron spin resonance (ESR) spectroscopy has been used to detect a free radical formed from the reaction of l-tryptophan with Ce (4+) in an acidic aqueous environment. Computer simulations of the ESR spectra from l-tryptophan and several isotopically modified forms strongly support the conclusion that the l-tryptophan radical cation has been detected by ESR for the first time. The hyperfine coupling constants (HFCs) determined from the well-resolved isotropic ESR spectra support experimental and computational efforts to understand l-tryptophan's role in protein catalysis of oxidation-reduction processes. l-Tryptophan HFCs facilitated the simulation of fast-flow ESR spectra of free radicals from two related compounds, tryptamine and 3-methylindole. Analysis of these three compounds' beta-methylene hydrogen HFC data along with equivalent l-tyrosine data has led to a new computational method that can distinguish between these two amino acid free radicals in proteins without dependence on isotope labeling, electron-nuclear double resonance, or high-field ESR. This approach also produces geometric parameters (dihedral angles for the beta-methylene hydrogens) that should facilitate protein site assignment of observed l-tryptophan radicals as has been done for l-tyrosine radicals.     

The Radical Anions of syn-1,6:8,13-Bridged [14] Annulenes. A Compilation of Hyperfine Data

Hyperfine coupling constants and g-factors determined by ESR and ENDOR spectroscopy are given for the radical anions of 21 syn-1,6:8,13-bridged [14]annulenes. The great majority of these values is reported for the first time. The hyperfine data are consistent with the single occupancy of an orbital which resembles a π-perimeter-LUMO of the same nodal properties. The sum of the coupling constants of the protons attached to the π-perimeter can be used as a planarity criterion for this perimeter.     

ESR study of spin-adducts of dialkoxyphosphoryl radicals with di(p-methoxyphenyl)methanofullerene

The addition of phosphoryl radicals to C₆₀C(p-C₆H₄OMe)₂ was shown (ESR) to result in the formation of at least seven isomers differing in their hyperfine coupling constants (HFC) with the phosphorus nuclei and in their kinetic behavior and thermodynamic stability.     

Ab initio calculations of nuclear quadrupole coupling constants of low-lying rovibrational levels in the X1Σ+ and a1Σ+ states of all isotopic species of LiH

The ²H, ⁶Li, and ⁷Li quadrupole coupling constants of the low-lying rovibrational levels in the X¹∑⁺ and A¹∑⁺ electronic states of ⁷Li²H, ⁶Li²H, ⁷Li¹H, and ⁶Li¹H are calculated from molecular wave functions which explicitly describe nuclear motion. Except for the lithium coupling constants in the A¹∑⁺ state, the vibrational dependence of the nuclear quadrupole coupling is found to be significant for all the studied isotopic species. On the other hand, the rotational dependence appears to be important only for the deuteron coupling constant of ⁷Li²H and ⁶Li²H in the A¹∑⁺ state. Special attention is paid to relative magnitudes of the Li and D quadrupole coupling constants in the ⁷Li²H and ⁶Li²H isotopic species. The information about the relative magnitudes and their changes with vibrational excitation is used to outline a theoretical approach to the interpretation of the hyperfine structure of the spectra of these species.     

Rotational isomerism as studied by nuclear quadrupole coupling.: Theoretical and experimental 14NX tensors for hydrazine,methylhydrazine, and 1,2-dimethylhydrazine

The nuclear quadrupole coupling constants of the ¹⁴N nuclei for hydrazine, the inner and outer conformers of methylhydrazine, and the inner-outer and the outer-outer conformers of 1,2-dimethylhydrazine are calculated by an ab initio SCF method, and also by a CI calculation for hydrazine. The results are compared with available experimental values. Characteristic dependence of the X tensors on the conformational structure is demonstrated. An application of theoretical hyperfine structures to a spectral analysis is discussed.