Effect of Pr-Ca substitution on the transport and magnetic behavior of LaMnO3 perovskite

The effect of simultaneous substitution of a fluctuating cation and a divalent cation in LaMnO₃ perovskite modifies the properties of the material to exhibit large valence colossal magnetoresistance (CMR) effect. A good example of these properties is (La_1−2x Pr _x Ca _x )MnO₃ (LPCMO) type CMR material. In this communication it is reported that, with the increase in x (for x=0.1, 0.15, 0.2), the T _c varies between 100 and 120 K with improvisation in metal-insulator transition. Interestingly, resistance increases with x from few hundred ohms to few kilo ohms with corresponding decrease in the unit cell volume. The results of the studies using X-ray diffraction (XRD), electrical resistivity, magnetoresistance and ac susceptibility measurements on LPCMO samples for understanding the structural, transport and magnetic properties are discussed in detail.     

The inelastic neutron scattering spectrum of H3B:NH3 and the reproduction of its solid-state features by periodic DFT

The inelastic neutron scattering (INS) and periodic density functional theory (DFT) vibrational spectra of H3B:NH3 are reported to 1600 cm-1. The H3B:NH3 structural and INS features, specifically the reduced solid-state B:N dative bond length and the altered B:N stretching frequency, are reproduced by the periodic DFT calculations, placing the B:N stretching mode at 800 cm-1, in excellent agreement with experiment relative to previous nonperiodic theoretical treatments of this molecule.     

Vicinal deuterium perturbations on hydrogen NMR chemical shifts in cyclohexanes

The substitution of a deuterium for a hydrogen is known to perturb the NMR chemical shift of a neighboring hydrogen atom. The magnitude of such a perturbation may depend on the specifics of bonding and stereochemical relationships within a molecule. For deuterium-labeled cyclohexanes held in a chair conformation at -80 degrees C or lower, all four possible perturbations of H by D as H-C-C-H is changed to D-C-C-H have been determined experimentally, and the variations seen, ranging from 6.9 to 10.4 ppb, have been calculated from theory and computational methods. The predominant physical origins of the NMR chemical shift perturbations in deuterium-labeled cyclohexanes have been identified and quantified. The trends defined by the Delta delta perturbation values obtained through spectroscopic experiments and by theory agree satisfactorily. They do not match the variations typically observed in vicinal J(H-H) coupling constants as a function of dihedral angles.     

Investigation of (1R,2S)‐(−)‐Ephedrine by Cryogenic Terahertz Spectroscopy and Solid‐State Density Functional Theory

The terahertz (THz) spectrum of the pharmaceutical (1R,2S)‐(?)‐ephedrine from 8.0 to 100.0 cm^?1 is investigated at liquid‐nitrogen (78.4 K) temperature. The spectrum exhibits several distinct features in this range that are characteristic of the crystal form of the compound. A complete structural analysis and vibrational assignment of the experimental spectrum is performed using solid‐state density functional theory (DFT) and cryogenic single‐crystal X‐ray diffraction. Theoretical modeling of the compound includes an array of density functionals and basis sets with the final assignment of the THz spectrum performed at a PW91/6‐311G(d,p) level of theory, which provides excellent solid‐state simulation agreement with experiment. The solid‐state analysis indicates that the seven experimental spectral features observed at low temperature consist of 13 IR‐active vibrational modes. Of these modes, nine are external crystal vibrations and provide approximately 57 % of the predicted spectral intensity. This study demonstrates that the THz spectra of complex pharmaceuticals may be well reproduced by solid‐state DFT calculations and that inclusion of the crystalline environment is necessary for realistic and accurate simulations.     

关于K个复相关矩阵相等的检验

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Theoretical analysis of the terahertz spectrum of the high explosive PETN.

The experimental solid-state terahertz (THz) spectrum (3 to 120 cm(-1)) of the high explosive pentaerythritol tetranitrate (PETN, C(5)H(6)N(4)O(12)) has been modeled using solid-state density functional theory (DFT) calculations. Solid-state DFT, employing the BP density functional, is in best qualitative agreement with the features in the previously reported THz spectrum. The crystal environment of PETN includes numerous intermolecular hydrogen-bonding interactions that contribute to large (up to 80 cm(-1)) calculated shifts in molecular normal-mode positions in the solid state. Comparison of the isolated-molecule and solid-state normal-mode calculations for a series of density functionals reveals the extent to which the inclusion of crystal-packing interactions and the relative motions between molecules are required for correctly reproducing the vibrational structure of solid-state THz spectra. The THz structure below 120 cm(-1) is a combination of both intermolecular (relative rotations and translations) and intramolecular (torsions, large amplitude motions) vibrational motions. Vibrational-mode analyses indicate that the first major feature (67.2 cm(-1)) in the PETN THz spectrum contains all of the optical rotational and translational cell modes and no internal (molecular) vibrational modes.     

The terahertz spectrum and quantum chemical assignment of 2,2′-bithiophene in cyclohexane

The room temperature solution-phase terahertz (THz, 7 to 165 cm^?1) spectrum of 2,2′-bithiophene in cyclohexane is reported. Density functional theory (B3LYP) and ab initio (MP2) methods employing the 6-311++G(2d,2p) and aug-cc-pVDZ basis sets are used to assign the THz vibrational structure and determine the relative populations of the cis and trans conformations, as well as the trans–trans rotational barrier height and the effects of the cyclohexane solvent on the predicted molecular geometries and vibrational frequencies. Significant differences are seen in the performance of the different theoretical methods, with the best performing method dependent upon the molecular property of interest. The best fit model of the experimental THz spectrum is achieved using MP2/aug-cc-pVDZ, which places the relative trans and cis populations at 54% and 46%, respectively.     

Theoretical analysis of the solid-state terahertz spectrum of the high explosive RDX

The solid-state terahertz (THz) spectrum (2–120 cm⁻¹) of α-form cyclotrimethylenetrinitramine (RDX) has been simulated using solid-state density functional calculations at a BP/DNP level of theory. BP/DNP features are in good agreement with both 298 K and a new 7 K polycrystalline RDX THz spectrum. The 7 K RDX spectrum is noteworthy for several mode shifts and spectral detail that greatly aids mode assignments. Previous RDX isolated-molecule calculations (with six calculated modes below 125 cm⁻¹) are incapable of accurately predicting the numerous features in this region, highlighting the importance of solid-state theoretical methods for solid-state terahertz feature assignments.