K. C. Mishra

Theory of orbital contributions to magnetic hyperfine fields in hemoglobin derivatives—application to azidomyoglobin

A variational procedure has been applied in conjunction with the linear combination of atomic orbitals (molecular orbitals) [LCAO(MO)] method for electronic structure investigations to study the unquenching of the orbital angular momentum due to the influence of spin–orbit interaction in large biomolecular systems and its influence on the magnetic hyperfine fields, especially at 57mFe nuclei. Application is made to azidomyoglobin and it is found that incorporation of spin–orbit effects leads to drastic effects on the hyperfine fields as suggested from earlier investigations based on the crystal field model. In contrast to the values of 37, 166, and 13 kG obtained without incorporation of orbital unquenching effects and with the external magnetic field applied along the a, b, and c axes of the crystal, the values we obtained when orbital unquenching effects are included are 252, 133, and 116 kG which agree considerably better with the experimental values of 185±5, 133±5, and 99±5 kG from Mossbauer measurem...

Electronic structure and hyperfine interaction of muonium in semi-conductors

The Unrestricted Hartree-Fock self-consistent field cluster procedure is being utilized for first-principle investigations of the electronic structures and hyperfine interactions in normal and anomalous muonium states in semi-conductors. Our results for the total energy for the normal muonium state for a twenty-seven atom cluster in diamond, including the muonium and its neighboring atoms, show a minimum at the tetrahedral site and a maximum at the hexagonal site indicating that normal muonium is located in the tetrahedral region and avoids the hexagonal region. Using the calculated spin-density as a function of the position of muonium and carrying out averaging over the vibrational motion of the muon governed by the total energy curve obtained from our work, we have derived a muon hyperfine constant which is about 75% of that in free muonium, in good agreement with experiment. The natures of the total energy and spindensity curves permit us to draw conclusions regarding the origin of the observed trend in the hyperfine constants for normal muonium in diamond, silicon and germanium. The UHF cluster procedure is also applied to study a model of a muon in a positively charged environment for the anomalous muonium center in diamond. This model leads to a hyperfine interaction tensor with the observed feature of strong anisotropy but significantly weaker than experiment. The results obtained for this model indicate the importance for the anomalous muonium state with its relatively weak hyperfine interaction, of exchange polarization effects inherent in the UHF procedure.

First principles cluster investigation of electronic structure and hyperfine interaction for nitrogen in diamond

The self-consistent field Unrestricted Hartree-Fock cluster procedure has been used to study the location, electronic structure and hyperfine properties of nitrogen impurity in diamond. From the analysis of the potential energy curve for nitrogen along the <111> axis, it was found that nitrogen is located at a position 0.3A away from the substitutional site towards the plane formed by the three nearest neighbour carbon atoms. The calculated values of the magnetic hyperfine constants and nuclear quadrupole coupling constants for14N agree very well with values obtained from electron paramagnetic resonance and electronnuclear double resonance measurements.

Hartree-Fock cluster investigations of electronic structure and nuclear quadrupole interaction in the superionic conductor Li3N

The nuclear quadrupole interactions for7Li and14N nuclei are studied for lithium nitride crystal using the Hartree-Fock cluster procedure to obtain the electronic structure. The influence of the Madelung potential due to the ions outside the cluster was incorporated in a first-principle manner. Our results for the quadrupole coupling constants will be compared with experimental data and the results of a recent band calculation.

Hartree-Fock cluster investigation of location and hyperfine properties of normal muonium in silicon-trend with respect to diamond

Using the first-principles Hartree-Fock Cluster procedure employed earlier for normal muonium (Mu) in diamond, the total energy and hyperfine field at the muon site in silicon have been studied as a function of muon position along the <111> direction. The muon was found to be localized in the tetrahedral interstitial region, although the potential was significantly shallower as compared to diamond. The vibrationally averaged hyperfine constant for the muon shows a correct trend compared to diamond but is somewhat larger than experiment, possible reasons for which will be discussed. Results for the superhyperfine constants in silicon will be presented and compared with those for diamond.

Evaluation of the magnetic moments of radium isotopes

Using the relativistic linked cluster many-body perturbation procedure we have obtained the hyperfine field at the nucleus of the Ra+ ion in the2S1/2 ground state. There is good agreement between the calculated magnetic moment of213Ra and the results of a recent Zeeman measurement by the collinear laser beam technique. Detailed comparison is carried out between our result and earlier ones.

Influence of coulomb corrections in the self-consistent charge extended Hückel (SCCEH) procedure on hyperfine properties of biological molecules

For the sake of simplicity, the Coulomb interaction between electrons of an atom with the cores and valence electrons belonging to neighbouring atoms is not completely incorporated in the SCCEH procedure often used for the study of electronic structures and hyperfine properties of biological systems. In the present work, the influence of this neglected interaction on the charge distribution in both small molecules and the large biologically important system, hemin, as well as on the hyperfine properties of the latter is analysed.

Self-consistent field cluster study of hyperfine properties of normal and anomalous muonium in elemental semiconductors

Using the Unrestricted Hartree Fock (UHF) Cluster Procedure, it is shown that for the normal muonium (Mu) center, the tetrahedral site is the most favorable in the two systems diamond and silicon investigated, while for the anomalous muonium (Mu*) center, a site displaced in the <111> direction with respect to a vacancy in a double-positively charged environment is the appropriate one for all three elemental semiconductors. Using our calculated electronic wave-functions, one is able to explain all features of the observed hyperfine properties of both centers and, in a number of cases, obtain good quantitative agreement with experiment.

Vacancy associated model for anomalous muonium in diamond, silicon and germanium

Through first-principles investigations on a number of models for anomalous muonium in diamond using the Unrestricted Hartree-Fock Cluster procedure, it is demonstrated that a muonium trapped near a double-positively charged vacancy is the most viable model for this center. This model is shown to successfully explain all the observed features of the hyperfine tensors A⇉ in diamond, silicon and germanium, namely, oblateness, opposite signs of A│ and A┼ in diamond and same signs for silicon and germanium, the trend in the strengths of the hyperfine tensors from diamond to germanium and the negative sign for A┼ in diamond.

Theory for position of muonium in α-quartz and associated hyperfine interaction

The electronic structures and hyperfine interactions of muonium and hydrogen in α-quartz are investigated by the unrestricted Hartree-Fock cluster procedure. The muonium is found to be trapped near the center of the line joining two silicon atoms. On including vibrational effects, the muon hyperfine constant comes out as 1.09 times that for free muonium, this ratio being larger than unity and smaller than for protons in trapped hydrogen, both features being in agreement with experiment.