共查询到20条相似文献,搜索用时 31 毫秒
1.
SE Korchak KL Ivanov AN Pravdivtsev AV Yurkovskaya R Kaptein HM Vieth 《The Journal of chemical physics》2012,137(9):094503
Effects of spin-spin interactions on the nuclear magnetic relaxation dispersion (NMRD) of protons were studied in a situation where spin [fraction one-half] hetero-nuclei are present in the molecule. As in earlier works [K. L. Ivanov, A. V. Yurkovskaya, and H.-M. Vieth, J. Chem. Phys. 129, 234513 (2008); S. E. Korchak, K. L. Ivanov, A. V. Yurkovskaya, and H.-M. Vieth, ibid. 133, 194502 (2010)], spin-spin interactions have a pronounced effect on the relaxivity tending to equalize the longitudinal relaxation times once the spins become strongly coupled at a sufficiently low magnetic field. In addition, we have found influence of (19)F nuclei on the proton NMRD, although in the whole field range, studied protons and fluorine spins were only weakly coupled. In particular, pronounced features in the proton NMRD were found; but each feature was predominantly observed only for particular spin states of the hetero-nuclei. The features are explained theoretically; it is shown that hetero-nuclei can affect the proton NMRD even in the limit of weak coupling when (i) protons are coupled strongly and (ii) have spin-spin interactions of different strengths with the hetero-nuclei. We also show that by choosing the proper magnetic field strength, one can selectively transfer proton spin magnetization between spectral components of choice. 相似文献
2.
One- and two bond spin–spin coupling constants, 1 J, 1h J , and 2h J across X–H?O hydrogen bonds and shielding constants of bridging hydrogens have been computed for complexes formed from interaction between the α-hydroxy-N-nitrosamine (NP) and four preferential binding sites of the uracil (U) at B3LYP/6-311++G(2d,2p)//MP2/6-311++G(2d,2p) level of theory. All complexes are stabilized by two HU?ONP and HNP?OU hydrogen bonds. Very good correlations were found between NMR spin–spin coupling constant as well as isotropic shielding constant and the binding energy, H-bond distance, red-shift of vibration frequency, charge transfer energy, and electron density at H-bond critical point. 相似文献
3.
Kizashi Yamaguchi Mitsuo Shoji Hiroshi Isobe Syusuke Yamanaka Jiro Shimada Yasutaka Kitagawa Mitsutaka Okumura 《Polyhedron》2009,28(9-10):2044-2052
The broken-symmetry (BS) and multideterminant approaches to atomic oxygen (O), molecular oxygen (O2) and iron–oxo (Fe(IV)O) core in P450 have elucidated electronic structures of the ground triplet and excited singlet states, which indicate isoelectronic characteristics of the species. The dissociation processes of the O–O and Fe–O double bonds are also examined to clarify the radical character, namely O-atom property responsible for radical mechanism of hydroxylations of alkanes and epoxidation of alkenes. This isolobal analogy has indeed enabled us to propose possible reaction mechanisms of oxygenation reactions by the Fe(IV)O species on the basis of available theoretical and experimental results for O and O2. Similarly, an isolobal analogy of the σ* bond among Fe(IV)O, dioxirane, peracids, etc. indicates the common electrophilic property for the oxygenation reactions. The small energy gaps between the high- and low-spin states of the transition structures and intermediates generated in the oxygenation reactions are found to be origins for spin crossover phenomena along the reaction pathways of these reactions. 相似文献
4.
《Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy》1976,32(11):1671-1673
The chemical shifts and coupling constants of the hydroxyl and ring protons of the following substituted phenols and anisoles have been obtained in basic and nonbasic solvents: 2-adamant-1-yl-4-tert-butylphenol, 6-bromo-2-adamant-1-yl-4-tert-butylphenol, 3,5-di-tert-butylphenol, 3,5-dichlorophenol, 6-bromo-2-adamant-1-yl-4-tert-butyl anisole and 6-bromo-2,4-di-tert-butyl anisole. Hydrogen bonding of the hydroxyl group of the substituted phenols to the solvent does not cause any detectable change in the spin—spin coupling between the meta protons, but causes a very small increase (<0.10 Hz) in the coupling between the ortho and para protons. 相似文献
5.
Ruiz E Cirera J Cano J Alvarez S Loose C Kortus J 《Chemical communications (Cambridge, England)》2008,(1):52-54
This theoretical study discusses the interplay of the magnetic anisotropy and magnetic exchange interaction of two Mn6 complexes and suggests that large magnetic anisotropy is not favoured by a high spin state of the ground state. 相似文献
6.
Dionisia Sanz Rosa M. Claramunt François Mathey Ibon Alkorta Goar Sánchez-Sanz José Elguero 《Comptes Rendus Chimie》2013,16(10):937-944
This paper reports the study by NMR spectroscopy and ab initio methods of the structure of 3,4-dimethyl-1-cyanophosphole and its dimer. The dimer presents a P···P interaction of the pnictogen type due to the presence of σ-holes. NMR of the monomer was recorded in CDCl3 solution while NMR of the dimer corresponds to the solid state (CPMAS) experiments. The 2pJPP spin–spin coupling constant has not been measured, but calculated at the B3LYP level. AIM, NBO and ELF methodologies have been used to describe the electronic structure of the dimer. 相似文献
7.
The spin chirality and spin structure of the Cu3 and V3 nanomagnets with the Dzialoshinsky–Moriya (DM) exchange interaction are analyzed. The correlations between the vector κ and the scalar χ chirality are obtained. The DM interaction forms the spin chirality which is equal to zero in the Heisenberg clusters. The dependences of the spin chirality on magnetic field and deformations are calculated. The cluster distortions reduce the spin chirality. The vector chirality is reduced partially and the scalar chirality vanishes in the transverse magnetic field. In the isosceles clusters, the DM exchange and distortions determine the sign and degree of the spin chirality κ. The correlations between the chirality parameters κn and the intensities of the EPR and INS transitions are obtained. The vector chirality κn describes the spin chirality of the Cu3 and V3 nanomagnets, the scalar chirality describes the pseudoorbital moment of the DM cluster. It is shown that in the consideration of the DM exchange, the spin states DM mixing and tunneling gaps at level crossing fields depend on the coordinate system of the DM model. The calculations in the DM exchange models in the right-handed and left-handed frame show opposite magnetic behavior at the level crossing field and allow to explain the opposite schemes of the tunneling gaps and levels crossing, which have been obtained in different treatments. The results of the DM model in the right-handed frame are consistent with the results of the group-theoretical analysis, whereas the results in the left-handed frame are inconsistent with that. The correlations between the spin chirality of the ground state and tunneling gaps at the level crossing field are obtained for the equilateral and isosceles nanoclusters. 相似文献
8.
E. Kuzmann Z. Homonnay Z. Németh A. Vértes Z. Klencsár Cs. Várhelyi Cs. Várhelyi 《Radiation Physics and Chemistry》2012,81(6):632-634
New iron(II) dioximato complexes [Fe(DioxH)2L2] (DioxH: methyl-ethyl-glyoxime, dimethyl-glyoxime, and benzyl-methyl-glyoxime) without and with axially coordinated ligands L (L: 4-dimethyl-amino-pyridine; 3-OH-aniline; 2-imidazolidone; 4-nitrobenzyl-pyridine; 2-amino-pyridine) have been synthesized by reaction of the components dissolved in ethanol at room temperature in inert atmosphere, and were studied by 57Fe Mössbauer spectroscopy. Characteristic isomer shift and quadrupole splitting values of the individual new compounds were determined. It was suggested that iron is in the iron(II) low spin state in all compounds having axially coordinated ligands; however, the high spin iron(II) state is characteristic when no axial ligands are bound to the iron center. Low spin state complexes could be categorized into two groups on the basis of isomer shifts. The difference in the isomer shift was explained on the basis of the type of ligating nitrogens. 相似文献
9.
《Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy》1993,49(11):1613-1619
A set of one-bond CC coupling constants has been determined for mono- and disubstituted benzenes. Large 1J(CC) values have been found within the benzene rings bearing highly electronegative substituents such as halogens, methoxy and nitro groups and small values for those with electropositive substituents. The total range of 1J(CC) couplings observed in our work is larger than 50 Hz. These large variations of CC spin—spin couplings are interpreted in terms of Fermi-contact contributions and the redistribution of s-electrons within a CC bond under influence of substituents. Contrary to some previous findings the data obtained in the present work indicate that the relationship between 1J(CC) and the substituent electronegativity is not linear. 相似文献
10.
The [Fe(II)(Cys)(4)](2-) site of rubredoxin from Clostridium pasteurianum (Rd(red)) has been studied by M?ssbauer spectroscopy in both purified protein and whole cells of Escherichia coli overproducing it. Excellent fits were obtained to an S = 2 spin Hamiltonian for D = 5.7(3) cm(-1), E/D = 0.25(2), delta = 0.70(3) mm/s, DeltaE(Q) = -3.25(2) mm/s, eta = 0.75(5), A(x) = -20.1(7) MHz, A(y) = -11.3(2) MHz, and A(z) = -33.4(14) MHz. These parameters were analyzed with crystal-field theory for the (5)D manifold of iron(II), revealing a d(z(2)) orbital ground state that is admixed by approximately 0.21 d(x(2) - y(2)). The spin-Hamiltonian parameters are consistent within the (5)D theory, apart from the zero-field splitting parameter, D. This problem was solved by extending the crystal-field treatment with spin-orbit coupling to spin-triplet d-d excited states of the iron. Theoretical estimates are given for the spin-triplet (D(T)) and spin-quintet contributions (D(Q)) to D based on excitation energies derived from time-dependent density functional theory, TD-DFT. The computational results were interpreted in terms of crystal-field theory, yielding the Racah parameters B = 682 cm(-1) and C = 2583 cm(-1). The theoretical analysis gives the relative magnitudes D(Q):D(T):D(ss) = 51%: 42%:7% (D(ss) originates from spin-spin interaction). The DFT analysis corroborates the pivotal role of the torsion angles (omega(i)) of the C-S(i) bonds in shaping the electronic structure of the iron(II) site. Rd(red) in overexpressing whole cells accounts for 60% of the M?ssbauer absorption. The Rd(red) spectra from whole cells are virtually identical to those of the purified protein. By using the theoretical omega dependence of the spin Hamiltonian parameters, the torsions for Rd(red) in whole cells and purified protein samples are estimated to be the same within 2 degrees. These findings establish M?ssbauer spectroscopy as a structural tool for investigating iron sites in whole cells. 相似文献
11.
Lekin K Winter SM Downie LE Bao X Tse JS Desgreniers S Secco RA Dube PA Oakley RT 《Journal of the American Chemical Society》2010,132(45):16212-16224
The bisdithiazolyl radical 1a is dimorphic, existing in two distinct molecular and crystal modifications. The α-phase crystallizes in the tetragonal space group P4?2(1)m and consists of π-stacked radicals, tightly clustered about 4? points and running parallel to c. The β-phase belongs to the monoclinic space group P2(1)/c and, at ambient temperature and pressure, is composed of π-stacked dimers in which the radicals are linked laterally by hypervalent four-center six-electron S···S-S···S σ-bonds. Variable-temperature magnetic susceptibility χ measurements confirm that α-1a behaves as a Curie-Weiss paramagnet; the low-temperature variations in χ can be modeled in terms of a 1D Heisenberg chain of weakly coupled AFM S = (1)/(2) centers. The dimeric phase β-1a is essentially diamagnetic up to 380 K. Above this temperature there is a sharp hysteretic (T↑= 380 K, T↓ = 375 K) increase in χ and χT. Powder X-ray diffraction analysis of β-1a at 393 K has established that the phase transition corresponds to a dimer-to-radical conversion in which the hypervalent S···S-S···S σ-bond is cleaved. Variable-temperature and -pressure conductivity measurements indicate that α-1a behaves as a Mott insulator, but the ambient-temperature conductivity σ(RT) increases from near 10(-7) S cm(-1) at 0.5 GPa to near 10(-4) S cm(-1) at 5 GPa. The value of σ(RT) for β-1a (near 10(-4) S cm(-1) at 0.5 GPa) initially decreases with pressure as the phase change takes place, but beyond 1.5 GPa this trend reverses, and σ(RT) increases in a manner which parallels the behavior of α-1a. These changes in conductivity of β-1a are interpreted in terms of a pressure-induced dimer-to-radical phase change. High-pressure, ambient-temperature powder diffraction analysis of β-1a confirms such a transition between 0.65 and 0.98 GPa and establishes that the structural change involves rupture of the dimer in a manner akin to that observed at high temperature and ambient pressure. The response of the S···S-S···S σ-bond in β-1a to heat and pressure is compared to that of related dimers possessing S···Se-Se···S σ-bonds. 相似文献
12.
13.
《Radiation Physics and Chemistry》2008,77(1):79-86
In the present work, electron spin resonance (ESR) identification of γ-irradiated cefadroxil monohydrate (CM), duricef capsule (DC) and duricef suspension (DS) and their potential use as normal and/or accidental dosimetric materials were investigated in the dose range of 1–25 kGy. Although unirradiated samples did not exhibit any ESR signals, irradiated samples were observed to present ESR spectra with many resonance lines originating from radiation induced radical or radicals. Dose-response curves associated with the resonance peak heights of CM (I1, I2) and DS (I3, I4, I5, I6) were found to follow linear and power functions of applied radiation dose, respectively. Simulation calculations were performed to determine the structure and spectral parameters of the radiation-induced radicalic species involved in the formation of experimental ESR spectrum of CM using, as input, the room temperature signal intensity data obtained for a sample irradiated at dose of 10 kGy. Kinetic behaviors and activation energies of the radicalic species were also calculated using the data obtained from annealing studies performed at five different temperatures. The presence of detectable signal intensities even after a storage period of 100 days was considered as providing an opportunity in the discrimination of irradiated CM and DS from unirradiated ones. Basing on room temperature signal intensity decay and dose–response data, it was concluded that CM and DS present the features of a good dosimetric material. 相似文献
14.
Popkov Alexander Langer Vratislav Manorik Peter A. Weidlich Tomáš 《Transition Metal Chemistry》2003,28(4):475-481
Long-range [
n
J(13C, 13C),
n
J(15N, 15N) and
n
J(15N, 13C)] interactions in the 13C-n.m.r. spectra of CDCl3 solutions of selectively labeled nickel(II) complexes of the Schiff base derived from (S)-N-benzylproline (2-benzoylphenyl)amide and glycine provide experimental evidence for the necessity of the donation of electron density from the -system of the benzyl ring to the nickel orbitals. No such interactions were observed in (2H)DMSO solutions, where the complex exists in a different conformation. 相似文献
15.
《Chemical physics》1986,103(1):29-33
Non-empirical equations-of-motion calculations of the nuclear spin—spin coupling constants and magnetic shielding constants in a representative series of molecules featuring siliconsilicon or siliconcarbon single, double and triple bonds are presented. The EOM results, which include the main portion of the electron correlation effects, are in resonable agreement with the available experimental data. On passing from single to double and triple bonding situation the pattern for the 1J(SiY) parameters resembles that exhibited by 1J(CY) in the structurally related carbocompounds, whereas an inversion in the relative position of the triply bonded atom is predicted in the case of the 29Si resonance relative to the 13C sequence. 相似文献
16.
《Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy》1993,49(3):329-338
The spin—lattice relaxation time of the 31P nucleus was measured for 11 phosphorylated molecules (phosphine oxides, trialkylphosphates and phosphoramides) dissolved in nitromethane at three different frequencies and as a function of the temperature for three compounds. The different contributions to the relaxation rate due to dipolar, chemical shift anisotropy and spin—rotation interactions were determined and the reorientational correlation times of the molecules were deduced when the anisotropy of the chemical shift tensor of the 31P nucleus could be (re)determined. The quadrupolar coupling constant of the 17O nucleus was also determined from the linewidth of the nuclear magnetic resonance signals, for phosphine oxides and triphenylphosphate, giving some information on the electronic distribution into the phosphoryl bond. The spin—rotation coupling constants for trimethylphosphine oxide and triphenylphosphine oxide were deduced and the chemical shift anisotropy Δσ of trialkylphosphates estimated. 相似文献
17.
Hochkirch U Herrmann W Stösser R Linscheid M Borchert HH 《Analytical and bioanalytical chemistry》2011,401(3):901-907
Electron spin resonance spectroscopy and mass spectrometry are two analytical methods that are very rarely used in combination.
In this paper, we will show that the methods complement one another in the example of the distribution of stable nitroxide
radicals in human skin, including the spatial resolution of these distribution processes. There are many ESR investigations
dealing with this subject, but unfortunately, they are all limited to the detection of paramagnetic species. The combination
with MS allows the successful examination of the distribution profile of the main biotransformation product of the nitroxide
radicals, the respective “ESR-silent” hydroxylamines. In order to maintain the biological state of the sample material as
far as possible, atmospheric pressure matrix-assisted laser desorption/ionization with ion trap detection has been used for
the mass spectrometric investigations. The results validate the former findings of the strong reduction of stable free radicals
by biological material; moreover, the diamagnetic species formed during these processes have been identified. 相似文献
18.
The pulsed electron-electron double resonance (ELDOR) technique was employed to study nitroxide spin probes of three different sizes dissolved in glassy o-terphenyl. A microwave pulse applied to the central hyperfine structure (hfs) component of the nitroxide electron paramagnetic resonance spectrum was followed by two echo-detecting pulses of different microwave frequency to probe the magnetization transfer (MT) to the low-field hfs component. The MT between hfs components is readily related to flips in the nitrogen nuclear spin, which in turn are induced by molecular motion. The MT on the time scale of tens of microseconds was observed over a wide temperature range, including temperatures near and well below the glass transition. For a bulky nitroxide, it was found that MT rates approach dielectric α (primary) relaxation frequencies reported for o-terphenyl in the literature. For small nitroxides, MT rates were found to match the frequencies of dielectric β (secondary) Johari-Goldstein relaxation. The most probable motional mechanism inducing the nitrogen nuclear spin flips is large-angle angular jumps, between some orientations of unequal occupation probabilities. The pulsed ELDOR of nitroxide spin probes may provide additional insight into the nature of Johari-Goldstein relaxation in glassy media and may serve as a tool for studying this relaxation in substances consisting of non-rigid molecules (such as branched polymers) and in heterogeneous and non-polar systems (such as a core of biological membranes). 相似文献
19.
20.
We report ab initio calculations of the indirect nuclear spin–spin coupling constants of PbH4 using a basis set which was specially optimized for correlated calculations of spin–spin coupling constants. All nonrelativistic
contributions and the most important part of the spin–orbit correction were evaluated at the level of the random phase approximation.
Electron correlation corrections to the coupling constants were calculated with the multiconfigurational linear-response method
using extended complete and restricted active space wavefunctions as well as with the second-order polarization propagator
approximation and the second-order polarization propagator approximation with coupled-cluster singles and doubles amplitudes.
The effects of nuclear motion were investigated by calculating the coupling constants as a function of the totally symmetric
stretching coordinate. We find that the Fermi contact term dominates the Pb‐H coupling, whereas for the H‐H coupling it is
not more important than the orbital paramagnetic and diamagnetic contributions. Correlation affects mainly the Fermi contact
term. Its contribution to the one-bond coupling constant is reduced by correlation, independent of the method used; however,
the different correlated methods give ambiguous results for the Fermi contact contribution to the H‐H couplings. The dependence
of both coupling constants on the Pb‐H bond length is dominated by the change in the Fermi contact term. The geometry dependence
is, however, overestimated in the random phase approximation.
Received: 16 November 1998 / Accepted: 30 March 1999 / Published online: 14 July 1999 相似文献