Calculations of the indirect nuclear spin–spin coupling constants of PbH4

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     

The optimum contraction of basis sets for calculating spin–spin coupling constants

The previously proposed pcJ-n basis sets, optimized for calculating indirect nuclear spin–spin coupling constants using density functional methods, are re-evaluated for finding the optimum contraction scheme as a compromise between computational efficiency and minimizing contraction errors. An exhaustive search is performed for the H₂, F₂ and P₂ molecules, and candidates for optimum contraction schemes are evaluated for a larger test set of 21 molecules. Using the criterion that the contraction error should not exceed the basis set error relative to the basis set limit, the optimum contraction is defined for each basis set. The results show that it is difficult to contract basis sets for calculating spin–spin coupling constants to any significant degree without losing the inherent accuracy. The work provides guidelines for searching for optimum contraction schemes for other properties and/or at theoretical levels where a systematic search is impractical.     

NMR chemical shielding and spin–spin coupling constants across hydrogen bonds in uracil–α-hydroxy-N-nitrosamine complexes

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 H_U?O_NP and H_NP?O_U 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.     

Massively parallel spin–orbit configuration interaction

We describe a new approach to incorporating quantum effects into chemical reaction rate theory using quantum trajectories. Our development is based on the entangled trajectory molecular dynamics method for simulating quantum processes using trajectory integration and ensemble averaging. By making dynamical approximations similar to those underlying classical transition state theory, quantum corrections are incorporated analytically into the quantum rate expression. We focus on a simple model of quantum decay in a metastable system and consider the deep tunneling limit where the classical rate vanishes and the process is entirely quantum mechanical. We compare our approximate estimate with the well-known WKB tunneling rate and find qualitative agreement.     

Mössbauer study of novel iron(II) complexes with oximes in low spin and high spin states

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.     

A local spin study for magnetic model complex HHeH

Chemists, physicists and material scientists havegiven great attention to the theoretical studies of mag-netic systems owing to their significance in the fun-damental research areas as well as their potential ap-plications in the field of high technologie…     

One-bond CC spin—spin coupling constants in derivatives of benzene. Non-linearity of 1J(CC) vs substituent electronegativity

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.     

Ab initio study of the nuclear spin—spin coupling constants and magnetic shielding constants in silicon derivatives via the equations-of-motion method

Non-empirical equations-of-motion calculations of the nuclear spin—spin coupling constants and magnetic shielding constants in a representative series of molecules featuring siliconsilicon or siliconcarbon 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.     

Distribution profiles of nitroxide spin probes in human skin—a combined study using spatially resolved electron spin resonance spectroscopy and mass spectrometry

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.     

19F and 1H NMR study of five-membered ring triflones and related σ -adducts. Long range fluorine-proton spin spin coupling.

Fluorine and proton chemical shifts for various trifluoromethylsulfonyl thiophene, selenophene, furan and N-methylpyrrole derivatives are reported and compared with those for nitro analogs. The ¹⁹F chemical shift of the SO₂CF₃group is found to be insensitive to its ring position as well as to the nature of the heterocycle. It is also unaffected by the loss of aromaticity and the presence of a negative charge which result from a nucleophilic addition to the ring. In most triflones studied, long range fluorine-proton coupling constants are observed but the proton involved in these couplings depends upon the α or β position of the SO₂CF₃ group as well as the nature of the ring heteroatom.     

Nitrogen nuclear spin flips in nitroxide spin probes of different sizes in glassy o-terphenyl: possible relation with α- and β-relaxations

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).     

Can large magnetic anisotropy and high spin really coexist?

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.     

Spying on the boron–boron triple bond using spin–spin coupling measured from 11B solid-state NMR spectroscopy

There is currently tremendous interest in the previously documented example of a stable species exhibiting a boron–boron triple bond (Science, 2012, 336, 1420). Notably, it has recently been stated using arguments based on force constants that this diboryne may not, in reality, feature a boron–boron triple bond. Here, we use advanced solid-state NMR and computational methodology in order to directly probe the orbitals involved in multiple boron–boron bonds experimentally via analysis of ¹¹B–¹¹B spin–spin (J) coupling constants. Computationally, the mechanism responsible for the boron–boron spin–spin coupling in these species is found to be analogous to that for the case of multiply-bonded carbon atoms. The trend in reduced J coupling constants for diborenes and a diboryne, measured experimentally, is in agreement with that known for alkenes and alkynes. This experimental probe of the electronic structure of the boron–boron multiple bond provides strong evidence supporting the originally proposed nature of the bonds in the diboryne and diborenes, and demonstrates that the orbitals involved in boron–boron bonding are equivalent to those well known to construct the multiple bonds between other second-row elements such as carbon and nitrogen.     

Theory of chemical bonds in metalloenzymes XII: Electronic and spin structures of metallo–oxo and isoelectronic species and spin crossover phenomena in oxygenation reactions

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.     

Reactive spin state dependent enantiospecific photocyclization of axially chiral α-substituted acrylanilides

The enantiomeric ratio (e.r.) in the 3,4-dihydroquinolin-2-one photoproduct during 6π-photocyclization of α-substituted axially chiral ortho-tert-butyl-acrylanilides depends on the nature of the reactive spin state (singlet or triplet), where the singlet-spin state reactivity gives a racemic mixture and the triplet reactivity gives an e.r. value >95?:?5.     

Hysteretic spin crossover between a bisdithiazolyl radical and its hypervalent σ-dimer

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.     

Identification and dosimetric features of γ-irradiated cefadroxil by electron spin resonance

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.