Isotropic Coupling Constants Research Articles

Isotropic coupling constant for the nitrogen atom from correlated calculations based on spin-unrestricted wave functions

The isotropic hyperfine coupling constant for the ground electronic state of the nitrogen atom is computed from the quadratic configuration interaction correlation procedure based on a single unrestricted Hartree–Fock (UHF) determinant. The splitting is determined from the normalized spin density at the nucleus, derived by finite-field perturbation theory. Results obtained are compared with previous work based on spin-restricted reference spaces. Close accord is found between present estimates and full configuration interaction values for small basis sets. Contributions from shells of higher angular momentum functions in the basis set are similar in both approaches and for the largest unrestricted calculation the correlated results are in good accord with experiment. The UHF wave function is shown to provide a reasonable account of the K-shell contribution to the isotropic coupling and hence the UHF-based correlated calculations show much less sensitivity to the neglect of core–electron correlation than the dramatic effects observed in spin-restricted treatments.

The hyperfine structure in the ethylene radical cation: An accurate CI study

The hyperfine structure in the ethylene radical cation, including isotropic and anisotropic coupling constants for H and 13C, has been computed from accurate CI wavefunctions calculated at the experimental geometry. The isotropic hyperfine coupling constant for the protons is obtained as −4.5 G and for the carbon atoms −3.0 G for a torsion angle 25°. The calculated a H value was found to be very sensitive to the torsion angle around the CC bond. This fact was used to obtain an independent estimate of 28° for the torsion angle from recent ESR measurements.

Configuration interaction calculations on the propane radical cation, C3H 8 +

Proton isotropic hyperfine coupling constants have been calculated for three low-energy nuclear conformations on the ground state potential surface of the propane cation, using a multireference singles and doubles configuration interaction (MR-SDCI) wave function. The lowest point found on the potential surface hadC 2v symmetry and the electronic wave function at this point had2B2 symmetry. At this point, the largest isotropic coupling constant is calculated to be 88.6 G, which is in fair agreement with the experimental value of 98 G obtained in an SF6 matrix at 4 K. No support is found for a “long-bond” ground state of lower symmetry thanC 2v . AnotherC 2v minimum on the ground state potential energy surface was found at which the wave function had2 B 1 symmetry. At this point, two large coupling constants of 198 G and 35 G were calculated. AC 2v stationary point was also found on the ground state potential surface at which the wave function had2 A 1 symmetry. At this point, couplings of 86 G and 25 G were obtained. None of these agree closely with the other experimental result of couplings at both 100–110 G and 50–52.5 G which was obtained in freon matrices. It is suggested that the latter spectra might correspond to a dynamical average of two distorted2 A' states inC s symmetry.

ENDOR study of nitronylnitroxides in frozen solution

ENDOR spectra of differently α-substituted nitronylnitroxides in a frozen solution of n-butylphthalate have been analysed to evaluate the hyperfine coupling tensor A of the radical protons. The EPR spectra in frozen solution showed a good resolution of the spectral features corresponding to the gx, gy, and gz values. Therefore different ENDOR spectra were recorded by saturating the EPR signals at different magnetic fields. The tensor components were obtained by computing the ENDOR spectra, and the relative signs were determined by means of TRIPLE experiments. Also ENDOR spectra of liquid solutions were performed to obtain the isotropic coupling constants. Two different sets of Ai, H components and director cosines were evaluated for the methyl protons at 2,3-positions of the nitronylnitroxide ring. The discrepancies between the two sets diminished on going from the α-CH3 radical to the other α-substituted radicals because of structural effects of the large substituents. The Ai, H components of the α-CH3 protons were also obtained. Only the 2,6-protons of the α-phenyl or α-pyridine contributed to the ENDOR signal and the Ai, H components of the pyridine protons were all greater because of the inductive effect of the nitrogen in the pyridine ring.

A b i n i t i o calculation of the hyperfine coupling constants in B2

Hyperfine coupling constants for the ground electronic state of the B2 molecule, [3∑−g] , are computed using correlation procedures based on spin-unrestricted wave functions. Two distinct methods have been employed. Firstly, a variational configuration mixing approach with all configurations arising from single and double replacements in the reference wave function has been augmented by an approximate size-consistency correction. Secondly, a many-body perturbation theory procedure based on the coupled–cluster expansion with infinite-order doubles has been approximately corrected for single and triple excitations through fourth order. Both isotropic and dipolar coupling constants have been obtained as expectation values over the appropriate wave functions for the unrestricted Hartree–Fock (UHF) and the variational configuration-interaction (CI) methods. The anisotropic coupling is already satisfactorily accounted for at the UHF level, while even with the approximate correction for the effect of higher excitations, the variational CI approach still underestimates the isotropic splitting by a factor of 2. The origin of this deficiency is traced using the other correlation method in which the isotropic coupling is derived from the normalized spin density at the nucleus, obtained by finite-field perturbation theory. The effect of the inclusion of triple excitations from the UHF wave function is seen to contribute about one third of the observed value. Such excitations are not accounted for in the size-consistency corrected variational CI approach. This conclusion is confirmed by a similar finding in a full fourth-order Mo/ller–Plesset (MP) calculation.

Isotropic coupling constants for the atoms boron-fluorine from correlated calculations based on spin-unrestricted wave functions

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTIsotropic coupling constants for the atoms boron-fluorine from correlated calculations based on spin-unrestricted wave functionsIan CarmichaelCite this: J. Phys. Chem. 1989, 93, 1, 190–193Publication Date (Print):January 1, 1989Publication History Published online1 May 2002Published inissue 1 January 1989https://doi.org/10.1021/j100338a040RIGHTS & PERMISSIONSArticle Views88Altmetric-Citations18LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (586 KB) Get e-Alerts

ChemInform Abstract: Applications of ENDOR Spectroscopy to Radical Cations in Freon Matrices. Part 2. The Radical Cation of 2,3‐Diazabicyclo(2.2.2)oct‐2‐ene

The radical cation of 2,3-diazabicyclo[2.2.2]oct-2-ene (1), generated by γ-rays in a CF2ClCFCl2 matrix, has been fully characterized by its hyperfine data with the use of ESR, ENDOR, and TRIPLE resonance spectroscopy. The isotropic coupling constants for are +3.14 mT for the two 14N Nuclei in the azo group, −0.336 mT for the two protons at the bridgehead C atoms, and +151 and +0.135 mT for the sets of four exo- and four endo-protons, respectively. MO calculations indicate that these values are consistent with an electron removal from an orbital represented by the antibonding combination (n_) of the lone-pair atomic orbitals at the two N atoms. The corresponding radical anion is persistent in fluid solution and its hyperfine data, combined with MO calculations, point to an electron uptake into the antibonding π orbital (π*) of the azo group. It is, thus, reasonable to classify as a σ and as a π radical.

The radical cation of naphthalene: First correct analysis of its ESR spectrum

The radical cations of naphthalene ( 1) and its 1,4-dideutero derivative ( 1- d 2), generated from the corresponding neutral compounds by γ-rays in a CFCl 3 matrix at 77 K, have been studied by ESR and ENDOR spectroscopy. At 140 K, the hyperfineanisotropy tensors of the protons in 1 +• appear to be quasi-axial, presumably as a consequence of the fast rotation about the twofold symmetry axis perpendicular to the molecular plane. The isotropic coupling constants are −0.74±0.01 and −0.187 ± 0.014 mT for the two sets of four equivalent protons in the 1,4,5,8- and 2,3,6,7-positions, respectively. The relative increase in the coupling constant of the 1,4,5,8-protons on going from the radical anion 1 −• (−0.495 mT) to the radical cation 1 +• (−0.74 mT) is similar to that found for the 1,4-protons in the corresponding radical ions ( 2 −• and 2 +•) of s-trans-buta-1,3-diene ( 2).

Applications of ENDOR spectroscopy to radical cations in freon matrices. Part 2. The radical cation of 2,3‐diazabicyclo[2.2.2]oct‐2‐ene

AbstractThe radical cation of 2,3‐diazabicyclo[2.2.2]oct‐2‐ene (1), generated by γ‐rays in a CF2ClCFCl2 matrix, has been fully characterized by its hyperfine data with the use of ESR, ENDOR, and TRIPLE resonance spectroscopy. The isotropic coupling constants for \documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\scriptstyle {{\relbar}\kern-4pt {.} }}$\end{document} are +3.14 mT for the two 14N Nuclei in the azo group, −0.336 mT for the two protons at the bridgehead C atoms, and +151 and +0.135 mT for the sets of four exo‐ and four endo‐protons, respectively. MO calculations indicate that these values are consistent with an electron removal from an orbital represented by the antibonding combination (n_) of the lone‐pair atomic orbitals at the two N atoms. The corresponding radical anion \documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\scriptstyle {{\relbar}\kern-4pt {.} }}$\end{document} is persistent in fluid solution and its hyperfine data, combined with MO calculations, point to an electron uptake into the antibonding π orbital (π*) of the azo group. It is, thus, reasonable to classify \documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\scriptstyle {{+}\kern-4pt {.} }}$\end{document} as a σ and \documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\scriptstyle {{\relbar}\kern-4pt {.} }}$\end{document} as a π radical.

Triplet-state E.P.R. studies of exchange coupled copper(II) and silver(II) pairs in zinc(II) bis[N.N.-diisopropyldithiocarbamate

Ambient temperature single crystal triplet-state E.P.R. studies on exchange coupled Cu2+-Cu2+ pairs in zinc(II) bis[N,N-diisopropyldithiocarbamate] matrix and temperature dependent E.P.R. studies on powder samples of Cu2+-Cu2+ and Ag2+-Ag2+ pairs, separated by a distance of 3·545 Å, in the same host lattice are reported. In addition to the doublet spectra, we observed the triplet lines arising due to spin-spin interactions. The directions of g and D-tensors in Cu2+-Cu2+ pairs vary by 42° in the xz-plane. The D-tensor seems to receive its contribution both from the dipolar and pseudo-dipolar interactions. The zero-field splitting (Z.F.S.) tensor, ‘D’, increases linearly with decrease in temperature down to 80K. This effect is seen in both Cu2+-Cu2+ and Ag2+-Ag2+ pairs: it is tentatively attributed to a crystal contraction at low temperatures. The integrated intensities of ΔM s = ±2 E.P.R. transitions of the triplets have been used to estimate the magnitude and sign of isotropic exchange coupling constants (2J). The values of 2J for Cu2+-Cu2+ and Ag2+-Ag2+ pairs are found to be 40·2 cm-1 and 62·5 cm-1, respectively. The interaction in both the systems has been found to be ‘ferromagnetic’.

Nitrogen hyperfine splitting of nitroxide solutions: Differently structured and charged nitroxides as probes of environmental properties

AbstractThe dependence of the nitrogen isotropic coupling constants, aN, on the structure and charge of the radicals, temperature and solvent polarity was analysed for solutions containing the following nitroxides: 4‐hydroxy‐2,2,6,6‐tetramethylpiperidine‐1‐oxyl (Tempol); 4‐trimethylammonium‐2,2,6,6‐tetramethylpiperidine‐1‐oxyl, iodide salt (TempTMA+); 2,2,5,5‐tetramethylpyrrolin‐1‐oxyl‐3‐carboxamide (Tempyo); and 2,2,5,5‐tetramethylpyrrolin‐1‐oxyl‐3‐carboxylate, sodium salt (Tempyca−). Polar hydrogen bond acceptor solvents and hydroxylic solvents were used. In each solvent used, Tempol and Tempyca− showed aN values which were higher, respectively, than those of TempTMA+ and Tempyo due to inductive, structural and charge effects. Such effects were analysed on the basis of the proton and 13C coupling constants. Non‐hydroxylic solvents gave temperature coefficients for aN that were almost independent of solvent polarity. Lower daN/dT values were found for charged radicals. A gradual decrease of temperature coefficients with decreasing hydrogen bond donor (HBD) properties were related to free volume changes of radical molecules. The analysis of the aN dependence on polarity parameters gave the best fitting using a simplified form of the Wertheim reaction field. Tempyca− was greatly affected by changes in polarity. A parameter to account for HBD abilities was also analysed.

Glutaryl phosphatidylcholine effects on pH and composition dependent behavior of liposomes studied by spin-labeling method

The influence of glutaryl phosphatidylcholine on the molecular organization of phosphatidylcholine liposomes was studied by spin-labeling technique. The ESR signals given by the 5-nitroxide stearic acid label showed that the presence of glutaryl lecithin (i) significantly increased the negative charge density of the polar liposome surface with increasing proton concentration depending on the bulk solution pH, and (ii) apparently decreased the packing (order) of the hydrophobic region close to the surface, essentially in the presence of saturated phospholipids. The spectral information— S (order parameter) and a N (isotropic nitrogen coupling constant)—resulted in the location of the probe near or in the polar zone of the membrane or in the hydrophobic region, depending on the protonation/deprotonation of the fatty acid carboxyl group of the probe. The microviscosity of the inner region of the membrane monitored by the 12- and 16-probes was not significantly altered by glutaryl lecithin. On the other hand, glutaryl lecithin has a lesser effect on liposomes containing anionic polar head groups, such as dipalmitoyl phosphatidylglycerol or phosphatidylinositol, the anionic charge of which already had the same effect on protonation of the polar surface. The temperature dependence of dipalmitoyl phosphatidylcholine liposome dynamic behavior indicates that the glutaryl lecithin effect is completely different above and below the gel-to-liquid crystalline phase transition point.

Aspects of high-resolution multinuclear magnetic resonance of solid organometallic compounds

Experimental results for high-resolution NMR of spin- 1 2 metals (particularly 119Sn, 195Pt and 207Pb) in solid compounds are presented. The spectra are generally obtained using the high-power-decoupling, cross-polarisation and magic-angle-rotation suite of techniques. The question of optimizing spectrometer performance by choice of suitable test compounds is addressed. Information on molecular structure in solids and on crystallographic sites, derived from NMR, is discussed. Results for isotropic spin-spin coupling constants to spin 1 2 metals in solids are presented. The use of spinning sideband manifolds to obtain information on the interplay of the shielding, dipolar coupling and indirect (“scalar”) coupling tensors is mentioned.

EPR spectrum features of the tetrameric copper compounds of formula Cu4OX6L4

The recent experimental observation of S=2 EPR signals from tetrameric Cu(II) complexes having the general formula Cu4OX6L4 is interpreted on the basis of a static quasi-Heisenberg model for the complex, where the anisotropic symmetric and skew-symmetric interactions have been taken into account with the approximation of the second order of the perturbation theory. It is found that the observed g value fits the single-ion mean value g=(gCu∥+2gCu⊥)/3, as expected, and the constant for the cubic zero-field splitting may be simply written as B4=(D2a+D2b+D 2c)/120J0, where J0 is the isotropic exchange coupling constant and Da,b,c are the principal values of the symmetric component of the anisotropic interaction. It is found, in contrast, that both the symmetric and skew-symmetric parts may contribute to the not yet observed S=1 signals. When the model is applied to the case of Cu4OCl6(TPPO)4 the B4 value measured by EPR and the J0 value derived by magnetic susceptibility data seem to support a symmetric exchange component very large compared with the dipolar energy.

Slow magic-angle rotation 13C NMR studies of solid phosphonium iodides. The interplay of dipolar, shielding, and indirect coupling tensors

High-resolution 13C NMR spectra were recorded for solid [Me 2Ph 2P] +I − and [Me 3PhP] +I − using high-power decoupling, cross-polarization, and magic-angle rotation. Spectra of the ipso carbon, obtained with the nonquaternary suppression (dipolar dephasing) pulse sequence, show an unequal distribution intensity of spinning sidebands for the two peaks split by the isotropic coupling constant 1 J PC. This effect is traced to the joint influence of (P, C) dipolar coupling and 13C shielding anisotropy. The existence of scalar coupling enables magic-angle rotation to distinguish the two subspectra which are present. Analysis of the sideband patterns yields values of the NMR parameters. Under certain assumptions, including that the axis of greatest shielding is perpendicular to the PC bond, it is shown that 1 J PC is positive. In contrast to traditional methods of determining signs, this conclusion does not rely on knowledge of signs of other coupling constants.

Quenched random-bond Ising ferromagnet: Anisotropic concentrations.

We have recently introduced an effective-field framework which, without mathematical complexities, enables calculation of the phase diagram (and magnetization) associated with a quenched mixed-bond spin-1/2 Ising model in an anisotropic simple cubic lattice. The case corresponding to anisotropic coupling constants, but isotropic concentrations, was discussed in detail in that occasion. Herein we discuss the case corresponding to isotropic coupling constants but anisotropic concentrations. Certain interesting phase diagrams are exhibited; whenever comparison with other available data is possible, the present results provide a satisfactory qualitative (and to a certain extent quantitative) agreement.

2D Ligand ENDOR and TRIPLE resonance studies of Cu(II)-doped DTGS single crystals

The deuteron hyperfine and quadrupole coupling tensors in the bis(glycinato)-Cu(II) ((ND2CH2COO)2Cu) complex in ferroelectric triglycine sulfate (DTGS) single crystals were measured by ENDOR spectroscopy. These data indicate that the molecular symmetry of the Cu complex is C1. From TRIPLE resonance experiments the signs of the isotropic coupling constants were determined. Structural information was obtained from the principal values of the quadrupole coupling tensors of the deuterium atoms and their direction cosines. Using the relationship between the deuterium quadrupole constant and ND bond lengths, the positions of the deuterons in bis(glycinato)-Cu(II) were estimated. The negative signs of the isotropic hyperfine coupling constants were explained by spin polarization of the deuteron 1s orbitals. The values of the anisotropic hyperfine coupling constants are discussed in the framework of conventional molecular-orbital theory and can be understood quite satisfactorily.

Unusual endor spectra and triple resonance of 14N with small hyperfine couplings in Cu(II)/Zn(II) bis-(diethy-dithiocarbamate) Cu/Zn(et 2dtc) 2

The hfs and quadrupole tensors of the nitrogen atoms of Cu(II) bis-(diethyl-dithiocarbamate) diluted in single crystals of the corresponding zinc complex have been determined by electron nuclear double resonance. The isotropic coupling constants are negative indicating, that the unpaired s-spin densities are mainly due to polarization effects.

1 H and 31 P ENDOR study of bis(O,O′-dimethyl-dithiophosphato)copper(II) and bis(diethyl-dithiophosphinato)copper(II) complexes

31 P and 1 H ENDOR spectra of powdered samples of the title compounds, Cu(Me2-dtp)2 and Cu(Et2-dtph)2, magnetically diluted in the corresponding Ni(II) host lattice and in solid toluene-d8 solutions of 63Cu(Me2-dtp)2, were studied at 100 K. The 31 P isotropic coupling constants and anisotropic coupling tensor parameters of Cu(Me2 and Cu(Et2-dtph)2 and the 1 H parameters of Cu(Me2 are estimated. The matrices (Ni(Me2 or toluene-d8) have a small influence on the parameters. The appearance of additional 31 P coupling is found in the case of 63Cu/Ni(Et2-dtph)2, this being attributed to the intermolecular interaction of the unpaired electron with 31 P nuclei from neighbouring Ni(Et2 molecules.

Deuteron chemical shift and EFG tensors in α-glycine

By computer fitting of resonance lines in deuteron spectra of perdeuterated crystals, deuteron chemical shifts in crystals are measured with an accuracy of ±0.3 ppm. Second-order quadrupole shifts must be taken into account. An important advantage of the method is that the measured chemical shifts and the shift tensors are automatically related to EFG tensors which can be assigned easily. The method is applied to α-glycine for which a shift anisotropy Δσ = 21 ± 9 ppm has been reported for the methylene protons. Full EFG and chemical-shift tensors are reported at both methylene deuteron sites, and motionally averaged tensors are reported for the ND 3 + sites. The shift anisotropies Δσ for the methylene deuterons are 4.7 and 9.4 ppm, respectively. Significant differences of the isotropic shifts and quadrupole coupling constants are also observed for these two deuterons. They are interpreted as evidence for the formation of a weak CH … O hydrogen bond in α-glycine.