Symmetry enforced gauge invariance of nuclear magnetic shielding constants

E.S.R. experiments performed at 1·3 K by optical detection are reported for the photo-excited triplet state of palladiumporphin in a single crystal of n-octane, and the observation of a level anticrossing signal is described.

In the crystal the orbital degeneracy of the ³ E _u state of the free molecule is lifted by the crystal field and in n-octane the energy difference between the two orbital components |x> and |y> is found to be 58 ± 2 cm^-1. The spinorbit coupling (SOC) and the orbital Zeeman interaction couple the triplet manifolds of |x> and |y>, and for a proper understanding of the magnetic properties of these states it is necessary to work in the basis of the six spin-orbit functions deriving from the ³ E _u state of the free molecule. It is shown that either of the two triplet states can be described by an effective spin hamiltonian of the common form and expressions for the zero-field parameters D and E and the principal values of the g tensor are given. The experimental values of the parameters in the lowest triplet state are D = -24·38 ± 0·03 GHz, |E| = 320 ± 60 MHz, g _‖ = 1·677 ± 0·001 and g _⊥ = 1·989 ± 0·002. The matrix element of the SOC connecting the |x> and |y> triplet manifolds amounts to qZ = 15 ± 3 cm^-1 and the vibronic orbital angular momentum (in units of ?) in the ³ E _u state of the free molecule to qΛ = 1·5 ± 0·3. A tentative value of 0·63 for the orbital reduction factor q is obtained by comparison with a theoretical estimate of Λ. The value of q is indicative of weak Jahn-Teller coupling.     

The magnetic susceptibilities of NO and OH

A non-isothermal kinetic equation for the distribution function of a sub-system weakly coupled to a bath is derived by modification of the analysis and assumptions of a previous paper [1]. The equation has the form of a generalized non-isothermal Fokker-Planck equation when it is linearized in thermodynamic gradients and only terms through second order in the coupling parameters are retained. Higher order terms in the coupling parameter do not diverge with time. The equation is compared with certain ‘exact’ model results of Ullersma and with the coherence time method. The equation is used to calculate a jump rate for diffusion of a harmonic particle weakly coupled to a lattice and it is found that the jump rate becomes independent of the mass of the particle for a heavy enough particle. The source of the discrepancy of this result with a similar calculation of Prigogine and Bak is indicated. The model of the jump rate is inappropriate for diffusion in a thermal gradient and more appropriate models of the jump are briefly considered. A brief comparison of the derivation of the kinetic equation with the Fano coherence time approximation is made and a difference is noted.     

Calculations of intermolecular interactions and their influences on structure and13C,1H magnetic shielding constants of solute molecules

On the basis of an improved method of molecular mechanics the spatial structures of molecular clusters modelling the solvate shell of methane in acetone and benzene, 2,9,10-trimethyl-1,3-dithia-5,6-benzocycloheptene in carbon disulfide have been calculated. The nuclear magnetic shielding constants for these structures are calculated by quantum chemical methods in the approach of the density functional theory of the B3LYP/6-31(d,p) level with the usage of gauge-invariant atomic orbitals. It is shown that the increase of the number of the solvent molecules results in better agreement of the calculated and experimental values of¹H and¹³C chemical shifts.     

Nonempirical quantum-chemical calculations of the magnetic shielding tensor of nuclei 47, 49Ti in crystalline titanium oxides

Nonempirical computations are carried out for the chemical shift tensor for the ^{47, 49}Ti nuclei in crystalline modifications of titanium dioxide and barium titanate. Computations are made using the cluster approach and the GAUSSIAN-94 program. A comparison with the experiment shows that such an approach can be used for titanium oxides containing TiO₆ complexes. It is noted that the choice of the basic wave functions for the titanium atoms plays a significant role.     

Intrinsic and extrinsic magnetic susceptibilities of some TCNQ complexes

The magnetic susceptibilities (χ) of quinolinium·(TCNQ)₂, N-methyl phenazinium·TCNQ and Li·TCNQ were measured from 2 to 300 K and are discussed in connection with the low-temperature specific heats (C) measured by other authors, χ is decomposed into three parts: χ_d the temperature-independent part, χ_c, Curie-Weiss type paramagnetism, and χ_p, the remainder. Correspondingly, C is composed of three terms, γT, H/T² and αT³. The electronic state of these substances is discussed in terms of each type of susceptibility.The model, on which the above separation of χ and C is based, defines two types of electrons: localized electrons associated with a magnetic moment and band electrons. Though this model is useful phenomenologically, it is shown that the analysis of χ on the basis of this model indicates less band electrons and more localized electrons or stronger magnetic interactions than does that of C.     

Initial magnetic susceptibilities of iron and nickel aerosol powders

The dependence on angle is considered for the hysteresis-free initial susceptibilities _a and _ar together with the initial susceptibility _o for iron and nickel aerosol powders, as well as the effects on these characteristics from changes in powder packing density. Estimates are made of the negative-interaction fields, and a quantitative comparison is made between the observed _ar() and theoretical values by means of the pair model. The correlation between the experimental and theoretical relationships confirms the concepts used on the predominant effects from magnetostatic interaction in the mechanism for magnetization reversal in powders of materials with low anisotropy.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, pp. 40–43, July, 1984.     

Effects of Debye shielding in the electron broadening calculations

The impact broadening by charged electron gas is calculated. A proper account of the Debye screening has been taken as also the curvilinearity of the perturber’s trajectories. The results have been given in the form of graphs for a ready comparison with earlier results.     

Optical and magnetic susceptibilities for semiconductors and alkali halides

Optical susceptibility, electronic polarizability and magnetic susceptibility values have been evaluated from optical electronegativities. The validity of the proposed empirical relations has been tested in the case of II–VI, III–V group semiconductors and alkali halides. The estimated values of these parameters are in good agreement with the available experimental data. There has been no report on the direct estimation of optical susceptibility and magnetic susceptibility using the concept of optical electronegativity.     

Magnetic structures and magnetic susceptibilities of terbium oxysulfide and oxyselenide

For the two isomorphous compounds Tb₂O₂S and Tb₂O₂Se, the magnetic susceptibility measurements on powder samples show an antiferromagnetic ordering with Néel temperatures of about 7.7 and 7K respectively. Differing in this respect from the other rare earth oxyselenides, the magnetic anisotropy of Tb₂O₂Se at low temperature is weaker than that of Tb₂O₂S.We also determine the magnetic structures of these two compounds by neutron diffraction experiments at 1.5K. The magnetic cell is orthohexagonal and doubled along the c-axis; the magnetic moments make an angle, with the c-axis, of 47 ± 10° for Tb₂O₂S and 30 ± 10° for Tb₂O₂Se and the moment values at 1.5K are 8.14 ± 0.2μ_B and 6.5 ± 0.2μ_B, respectively.It is rather exceptional that in a rare earth uniaxial compound the magnetic moment makes an angle with the c-axis. However we interpret this situation by the fact that several levels exist very near to the ground state. The crystal field calculations are in good agreement with the experimental results.     

Anomalous magnetic susceptibilities of the alkaline earth hexammines

The magnetic susceptibility of the metallic compounds Ca(NH₃)₆, Sr(NH₃)₆ and Ba(NH₃)₆ has been measured by the Faraday method in the range 2–200 K. The susceptibility of Ca(NH₃)₆ is strongly dependent on temperature and exhibits a broad minimum near 120 K and a peak near 10 K. In contrast to Ca(NH₃)₆, the susceptibilities of Sr(NH₃)₆ and Ba(NH₃)₆ are diamagnetic and decrease rapidly as the temperature is lowered. At low temperatures Ba(NH₃)₆ has the largest mass susceptibility of any nonsuperconducting metal.     

Proton magnetic shielding anisotropy in acetylene

A high resolution N.M.R. study of acetylene in different liquid crystals is reported. The anisotropy in proton chemical shift has been measured accurately. The present result differs considerably from the values previously reported and is found to be independent of liquid crystal medium, concentration or density and reference of chemical shift measurement. Additional chemical shift data in gaseous acetylene and methane-tetramethylsilane (TMS) relative shifts in nematic and isotropic phases of the liquid crystals are also given.     

AlB2弹性常数的第一性原理计算

利用基于局域密度近似框架下的第一原理平面波赝势方法,结合HGH型相对论分析赝势,对AlB2的晶格参数和弹性常数进行了计算.结果显示:当晶格参数c和a的比率c/a为1.084时,具有最稳定的几何结构,与实验值及其他理论得到的计算值相符合.     

Nuclear shielding and magnetic hyperfine structure of hydrogen cyanide

A millimeter-wave molecular beam maser has been used to resolve the magnetic hyperfine structure of hydrogen cyanide. The spin-rotation interaction constants have been measured for the three nuclei ¹⁴N, ¹³C, and H. The paramagnetic nuclear shielding factors have been calculated for the three nuclear sites. The spin-rotation constants for ¹⁴N in H¹²C¹⁴N, for H in H¹²C¹⁴N and for ¹³C in D¹³C¹⁴N are + 10.4 kHz, −3.7 kHz, and +15.0 kHz, respectively. The respective paramagnetic shielding factors are −408.98 × 10⁻⁶, −73.40 × 10⁻⁶, and −249.52 × 10⁻⁶.     

超导体MgB2弹性常数的第一原理计算

利用全势线性muffin-tin轨道(FP-LMTO)方法, 结合在密度泛函理论框架下的广义梯度近似, 研究了六角密堆积结构超导体MgB2的晶格参数, 弹性常数, 以及体积模量及其对压强的微分. 计算结果显示当晶格参数c和a的比率c/a大约为1.138时, MgB2的结构最稳定.本文所得到的计算结果与实验值及其他作者利用不同方法得到的计算值相符合.     

ZnSe弹性常数和相变的从头计算

利用平面波密度泛函理论研究了ZnSe从闪锌矿结构到盐石结构的相变.结果发现通过H相等得到的相变压力为16.8 GPa,与通过高压弹性常数值判断所得到的结果相符.