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  • 1
    Electronic Resource
    Electronic Resource
    Extended Hueckel calculation of the electron paramagnetic resonance parameters of copper(II) bis(dithiocarbamate) (1972)
    s.l. : American Chemical Society
    Inorganic chemistry 11 (1972), S. 1338-1343 
    Details
    ISSN: 1520-510X
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/ic50112a037
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  • 2
    Electronic Resource
    Electronic Resource
    Vibron band structure in chlorinated benzene crystals: Lattice dynamics calculations and Raman spectra of 1,2,4,5-tetrachlorobenzene (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 4023-4034 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: From a combined theoretical and experimental study of 1,2,4,5-tetrachlorobenzene (TCB) we conclude that this crystal shows many interesting effects. In agreement with earlier optical measurements, which probe the complete vibron band structure of TCB, we calculate that several of the vibron modes have the dispersion of a one-dimensional crystal with stacks of molecules along the a axis. The inclusion of fractional atomic charges in the atom–atom potential used in the calculations is absolutely necessary to obtain the correct vibron bandwidths. Also the sign of the vibrational coupling matrix elements, which is given correctly by the calculations, is determined by these charges. For other properties, such as the crystal stability, the phonon frequencies, the site splitting in the vibron bands, and also the dispersion of the lower frequency vibrations, the interactions between the "one-dimensional'' stacks are essential, however. The calculations predict further, in qualitative agreement with the Raman spectra, that the splitting between the pairs of inequivalent vibron bands in α-TCB (the site splitting) is significantly larger than the factor group splitting in β-TCB. Finally we have found that, for those vibrations where the conditions are shown by the calculations to be favorable, the 35Cl/37Cl isotope effects are clearly visible in the Raman spectra.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.454835
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  • 3
    Electronic Resource
    Electronic Resource
    Magnetic coupling and dynamics in solid α and β-O2. I. An ab initio theoretical approach (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 3583-3596 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This paper describes a new approach to the dynamic and magnetic properties of solid α and β oxygen which is based on two theoretical developments. First, we have constructed the lattice and spin Hamiltonian for solid O2 by including explicitly the interactions between the triplet ground state O2 molecules as obtained mainly from recent ab initio calculations. The spin coupling parameters in this Hamiltonian, especially the Heisenberg exchange parameter J, are strongly dependent on the positions and orientations of the molecules. Secondly, we have developed an integrated scheme for lattice dynamics and spin wave calculations which uses this Hamiltonian. The actual mixing between the lattice modes, phonons and librons, and the magnons appears to be small; their interaction can be largely taken into account by renormalization of the coupling terms. In the lattice dynamics part of the calculation it is essential to include the Heisenberg term, since it is the extremely strong anisotropy of the coupling parameter J that explains the anomalously large libron splitting in α-O2. The spin-wave calculation with the Hamiltonian averaged over the lattice vibrations yields reasonable values for the magnon frequencies with no empirical fit parameters.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.451963
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  • 4
    Electronic Resource
    Electronic Resource
    Magnetic coupling and dynamics in solid α and β-O2. II. Prediction of magnetic field effects (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 3597-3601 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Via simple thermodynamic arguments and via quantitative lattice dynamics and spin-wave calculations, we predict that the phase transition temperature Tαβ in solid oxygen and the optical libron frequencies in the α and β phases will both be lowered by an external magnetic field. The lowering of Tαβ varies from about 0.1 K at 7.5 T to about 1.2 K at 30 T. The lowering of the Bg and Ag libron peaks in the Raman spectrum of α-O2 and of the Eg peak in the β-O2 spectrum varies between 1.5 and 2.9 cm−1 at 30 T. These shifts can be explained by the magnetic field induced changes in the sublattice magnetizations, which affect the Heisenberg exchange contribution to the intermolecular potential. From ab initio calculations it is known that the Heisenberg coupling parameter J is extremely anisotropic.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452737
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  • 5
    Electronic Resource
    Electronic Resource
    Lattice dynamics of α-CO from an ab initio potential (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 8402-8407 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Harmonic lattice dynamics calculations have been performed for the antiferroelectric P213 ordered phase of α-carbon monoxide and for two related structures with a different head–tail ordering of the CO molecules. The potential used is an ab initio CO–CO potential with its anisotropy expanded in spherical harmonics, as well as a site–site model fitted to this ab initio potential. Good agreement with experiment is obtained for the structure, the cohesion energy, and the phonon frequencies without any adjustment of the potential. Head–tail reordering of the molecules is energetically almost neutral if it is accompanied by a translational displacement along the bond axes, which is symmetric with respect to the inversion centers of the average Pa3 structure. This displacement is induced in particular by the strong head–tail anisotropy in the short-range repulsion. Translation–rotation coupling is found to be important; it affects especially the frequency of the lowest optical mode.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460072
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  • 6
    Electronic Resource
    Electronic Resource
    The O2–O2 dimer: Magnetic coupling and spectrum (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 5346-5360 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A theoretical analysis has been made of the van der Waals vibration–rotation–electron spin states of the O2–O2 dimer in its 3Σ−g –3Σ−g electronic ground state. This analysis is based on a Hamiltonian that includes a spin-dependent O2–O2 interaction potential and it involves also the permutation–inversion symmetry of the system. We have constructed some hindered internal rotor models for the vibrational states of the O2–O2 dimer which correspond with different equilibrium geometries, and for each of these models we have numerically calculated the spin-rotation fine structure. This fine structure appears to be determined not only by the Heisenberg exchange interaction between the O2 monomer triplet states, but also by intramolecular spin–orbit and spin–spin coupling and, to a smaller extent, by the intermolecular spin–spin (magnetic dipole) interaction and by the Coriolis terms in the kinetic energy. The resulting fine-structure spectrum is very complex, and very sensitive to the geometry of the O2–O2 dimer, to the nature of its internal motions and to the various magnetic couplings. This implies that detailed measurements of this spectrum, which can be interpreted with the help of the theory presented here, will yield interesting information on the properties of the O2–O2 dimer and, at the same time, verify our assumptions on the magnetic interactions between O2 molecules which have important consequences for the properties of solid oxygen.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453654
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  • 7
    Electronic Resource
    Electronic Resource
    The infrared photodissociation spectra and the internal mobility of SF6, SiF4, and SiH4 dimers (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 2837-2847 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present an analysis of the couplings originating from different intermolecular interactions (electrostatic, exchange, dispersion, induction) which split and shift the frequencies of the vibrational transitions in van der Waals dimers, and determine their intensities. Model potential calculations illustrate the importance of the various contributions in (SF6)2, (SiF4)2, and (SiH4)2 and their dependence on the monomer orientations. The results, in conjunction with calculated equilibrium structures, barriers to internal rotation and (harmonic) van der Waals vibrational frequencies, lead to several observations which are relevant for the interpretation of the infrared photodissociation spectra of these complexes. We confirm that in (SF6)2 and (SiF4)2 (orientation independent) resonant dipole–dipole coupling dominates the appearance of the spectra. For (SiH4)2 we conclude, however, that other than electrostatic terms are not negligible and, moreover, that the electrostatic coupling leads to orientation-dependent vibrational frequencies and intensities. This orientational dependence is related to the large displacements of the hydrogen atoms in the ν4 mode of SiH4. We also find that the internal rotations in (SF6)2 and (SiF4)2 are more strongly locked than those in (SiH4)2. Especially the geared internal rotations in the latter dimer could easily occur at the experimental molecular beam temperatures.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457930
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  • 8
    Electronic Resource
    Electronic Resource
    Ab initio potential energy surfaces of Ar–H2O and Ar–D2O (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 8096-8104 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present an ab initio potential energy surface for the van der Waals system argon–water. The points on the surface are computed as a sum of Heitler–London short range repulsion, damped dispersion, and damped induction. Damping is done in the manner first proposed by Tang and Toennies. We give the surface analytically in the form of a spherical harmonic expansion through l=7. The expansion coefficients are represented by functions depending on the distance between the centers of mass of the monomers. We also present an Ar–D2O potential obtained from the Ar–H2O potential by translating the center of mass of the water molecule. An analytic formula for the translation of polarizabilities is derived and applied to the computation of the long range energy of Ar–D2O. The short range part of the energy is translated numerically. Finally, the potential is checked by comparison of computed interaction virial coefficients with experimental data.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460092
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  • 9
    Electronic Resource
    Electronic Resource
    Ab initio potential energy surfaces of Ar–NH3 for different NH3 umbrella angles (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 491-500 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Four ab initio potential energy surfaces of the van der Waals system argon–ammonia are computed for the following four different HNH ("umbrella'') angles of ammonia: 100°, 106.67°, 113.34°, and 120°. These potentials have been obtained by Heitler–London short-range calculations and from multipole-expanded dispersion and induction long-range contributions. A Tang–Toennies-like damping is applied to the long-range energy. Each surface is given analytically in the form of a spherical harmonic expansion through l=7, where the expansion functions depend on the polar angles of the argon atom with respect to the principal axes of NH3. The expansion coefficients are represented by functions depending on the distance between the monomers. The potential for the equilibrium HNH angle 106.67° is applied to the computation of interaction virial coefficients in which quantum effects through (h-dash-bar)2 are included.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460365
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  • 10
    Electronic Resource
    Electronic Resource
    The van der Waals rovibrational states of the Ar–NH3 dimer (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 501-510 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Starting from an ab initio anisotropic potential surface, we have calculated the bound rovibrational states of the Ar–NH3 dimer for J=0, 1, 2, and 3. There is good agreement with data available from molecular beam spectroscopy in the microwave and far-infrared regions. The nature of the states is discussed and they are correlated with free internal rotor states. In spite of the substantial barriers to internal rotation, Ar–NH3 resembles much more a free internal rotor complex than a nearly rigid dimer. Still, the van der Waals vibrations show characteristic differences from free internal rotations. In particular, there is strong mixing between the fundamental stretch vibration of the dimer and the lowest bending mode. We assign the band observed in the far-infrared region to the lowest, primarily bending, of these modes. Further transitions in the yet unexplored far-infrared regions are predicted.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460366
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