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31.
A simple quantum-mechanics/molecular-mechanics (QM/MM) approach for calculating NMR shielding tensors (sigma) is presented. The method involves capping the QM region with quantum capping potentials (QCPs) and representing the MM region with point charges. Test calculations on simple systems without MM charges show that calculated sigma values improve relative to the full QM results with increasing distance between the capped bond and chromophore. Calculations on the histidine amino acid and cytosine monophosphate (CMP) nucleic acid show that the use of QCPs with point charges result in mean errors in the isotropic component of sigma that are less than 1.6 ppm. The results also reveal that, contrary to previous work, the explicit effect of point charges on sigma through coupling with gauge factors, as in the gauge including atomic orbital approach, is minimal for the CMP molecule. The present QM/MM approach for calculating sigma is easy to apply and requires no code modification. 相似文献
32.
Misra SK Andronenko SI Rinaldi G Chand P Earle KA Freed JH 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2003,160(2):131-138
Multifrequency electron paramagnetic resonance studies on the Mn(2+) impurity ion in a mixed single crystal NH(4)Cl(0.9)I(0.1) were carried out at 9.62 (X-band) in the range 120-295 K, at 35.87 (Q-band) at 77 and 295 K, and at 249.9 GHz (far-infrared band) at 253 K. The high-field EPR spectra at 249.9 GHz are well into the high-field limit leading to a considerable simplification of the spectra and their interpretation. Three magnetically inequivalent, but physically equivalent, Mn(2+) ions with their respective magnetic Z-axes oriented along the crystallographic [100], [010], [001] axes were observed. Simultaneous fitting of EPR line positions observed at X-, Q-, and far infra-red bands was performed using a least-squares procedure and matrix diagonalization to estimate accurately the Mn(2+) spin-Hamiltonian parameters. The temperature variation of the linewidth and peak-to-peak intensities of the EPR lines indicate the presence of lambda-transitions in the mixed NH(4)Cl(0.9)I(0.1) crystal at 242 and 228 K consistent with those observed in the pure NH(4)Cl and NH(4)I crystals, respectively. A superposition-model analysis of the spin-Hamiltonian parameters reveals that the local environment of the Mn(2+) ion is considerably reorganized to produce axially symmetric crystal fields about the respective Z-axes of the three magnetically inequivalent ions as a consequence of the vacancy created due to charge-compensation when the divalent Mn(2+) ion substitutes for a monovalent NH(4)(+) ion in the NH(4)Cl(0.9)I(0.1) crystal. This reorganization is almost the same as that observed in NH(4)Cl and NH(4)I single crystals, although the latter two are characterized by different, simple cubic and face-centered cubic, structures. 相似文献
33.
Callegari A Marcaccio M Paolucci D Paolucci F Tagmatarchis N Tasis D Vázquez E Prato M 《Chemical communications (Cambridge, England)》2003,(20):2576-2577
Amidoferrocenyl-functionalised single wall carbon nanotubes (Fc-SWNT) are efficient exoreceptors for the redox recognition of H2PO4-. 相似文献
34.
35.
Gino Fano 《Monatshefte für Mathematik》1898,9(1):17-29
Ohne ZusammenfassungVortrag, gehalten auf dem internationalen Mathematiker-Congress in Zürich den 10. August 1897. 相似文献
36.
37.
38.
The physical behavior of a fluid in contact with solid layers is still not fully understood. The present work focuses on the
study and understanding of thermodynamic and structural properties of gold–water nanolayer mixtures using molecular dynamics
simulations. Two different systems are considered, where approximately 1,700 water molecules are confined between gold nanolayers
with separations of 7.4 and 6.2 nm, respectively. Novelties of the present work are in the use of accurate force fields for
modeling the inter- and intra-molecular interactions of the components, and providing comprehensive thermodynamic properties
of the mixtures. The results are validated by examination of the pure fluid and pure solid properties. Results indicate that
the thermodynamics of the system does not behave as an ideal mixture. The structure of the pure fluid is also analyzed and
compared against the structure of the confined fluid in the mixture. Anisotropicity is observed in the fluid structure close
to the surface of the nanolayer. Higher ordering and higher flux are detected in the fluid molecules close to the fluid–solid
interface. Unusual thermodynamic behavior, anisotropicity, liquid layering, and higher interfacial fluid flux could be just
some of the factors leading to the enhanced energy transport observed in mixtures involving at least one nanoscale component,
such as nanofluids. 相似文献
39.
Riccardo Barbieri Paolo Campli Gino Isidori Filippo Sala David M. Straub 《The European Physical Journal C - Particles and Fields》2011,71(12):1812
We study CP asymmetries in rare B decays within supersymmetry with a U(2)3 flavour symmetry, motivated by the SUSY flavour and CP problems, the hierarchies in the Yukawa couplings and the absence
so far of any direct evidence for SUSY. Even in the absence of flavour-blind phases, we find potentially sizable CP violating
contributions to b→s decay amplitudes. The effects in the mixing-induced CP asymmetries in B→ϕK
S
and B→η′K
S
, angular CP asymmetries in B→K
∗
μ
+
μ
− and the direct CP asymmetry in B→X
s
γ can be in the region to be probed by LHCb and next generation B factories. At the same time, these effects in B decays are compatible with CP violating contributions to meson mixing, including
a non-standard B
s
mixing phase hinted by current tensions in the CKM fit mostly between SyKS, eKS_{\psi K_{S}}, \epsilon_{K} and DMBs/DMBd\Delta M_{B_{s}}/\Delta M_{B_{d}}. 相似文献
40.