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  • 1
    Electronic Resource
    Electronic Resource
    Generalized Langevin equations for molecular dynamics in solution (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 4463-4471 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This paper rigorously derives several classes of generalized Langevin equations (GLE) which describe the motion of an interacting many-body system in a heat bath. The fundamental assumption in deriving these GLEs is that the Liouville operator representing the system–heat bath interaction LSR commutes with the global Liouville operator L. An important second dissipation–fluctuation theorem is put forward relating the memory kernel of the friction force to the correlation function between the potential force of the system and the random force from the heat bath and to the auto- and cross-correlation functions of the random forces. Unlike most of the previous treatments in which the system, the heat bath and the system–heat bath interaction are greatly simplified, the properties of the system, the heat bath and the system–heat bath interaction remain general in this study. In particular, the interaction Hamiltonian may be arbitrarily nonlinear, so that the present theory is applicable to any physical system. Finally, a unified treatment for the GLEs, originally proposed by Ciccotti and Ryckaert for the systems in which there is no interaction among particles, is extended to many-body systems with internal interactions and in a heat bath.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460602
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  • 2
    Electronic Resource
    Electronic Resource
    Stochastic dynamics of n-nonane and related molecules in solution compared with nuclear magnetic resonance coupled relaxation times (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 7576-7590 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The motion of chain molecules in solution has been analyzed using both generalized Langevin equation (GLE) and ordinary Langevin equation (OLE) simulations. A numerical algorithm for solving the GLEs is developed in which the integrations over various forces have been performed explicitly. It is shown in the GLE simulations that the motion of chain segments is correlated closely with solvent relaxation giving significantly reduced friction forces. At temperatures higher than 233 K, a hydrodynamic description with a structure relaxation mode in the solvent (diglyme) is sufficient to yield Cartesian correlation times in good agreement with the NMR coupled relaxation results on n-nonane. The relative contributions of both overall tumbling and internal motion to the Cartesian and end-to-end direction relaxation and the possible couplings of these two motions are analyzed by calculating apparent activation energies for various motional modes and by using a harmonic approximation. It is found that the OLE model underestimates the contribution of internal motion to the relaxation of local Cartesian modes. The finite structural relaxation rate in the solvent can substantially alter not only the correlation times, but the dynamic features of the relevant relaxation processes in a full GLE calculation. In particular, it is shown that the short-time decay of the Cartesian correlation functions is underdamped oscillation in contrast with the overdamped behavior found from the OLE simulations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.461384
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  • 3
    Electronic Resource
    Electronic Resource
    The effect of transverse cross relaxation on nuclear magnetic resonance dipolar spectra (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 5542-5546 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The 13C dipolar powder spectra of 13CH3 31PO(OH)2 at room temperature and 13CH3 19F at low temperature (T=25 K) are reported. An intense peak is observed at the center of the Pake doublet. This phenomenon is explained by the transverse cross relaxation mechanism between the 13C and either the 31P or 19F nuclei. This theory shows the importance of the nonsecular terms in the Redfield relaxation theory. The chemical shielding tensors and dipolar coupling constants of these two molecules are obtained by fitting the experimental spectra.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455698
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  • 4
    Electronic Resource
    Electronic Resource
    A new high pressure sapphire nuclear magnetic resonance cell (1996)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 67 (1996), S. 240-243 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A new version of a single-crystal sapphire high pressure nuclear magnetic resonance (NMR) cell is described that is capable of controlling the sample pressure independent of the temperature. A movable piston inside the cell adjusts and controls the sample pressure from ambient conditions to 200 atm within ±0.3 atm. The linewidth at half-height for a 13C spectrum of carbon dioxide at 15 °C and 57.8 atm is found to be 0.5 Hz. The carbon dioxide gas/liquid phase transition is clearly observed by measuring 13C chemical shifts as the sample pressure approaches equilibrium. The time required for this NMR cell to reach equilibrium with its surroundings is relatively short, usually 15–30 min. The cell body has the same outer dimensions of a standard spinning turbine and fits into a standard 10 mm commercial probehead capable of controlling the sample temperature using the spectrometer's variable temperature unit. The flexibility of the device and the increased speed in making the measurement is demonstrated. Such control of important thermodynamic variables facilitates the NMR study of important biochemical and chemical reactions in gas, liquid, and supercritical fluid environments. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1146578
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  • 5
    Electronic Resource
    Electronic Resource
    Pulsed NMR probe-monitoring system (1987)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 58 (1987), S. 1128-1130 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A simple device for measuring the instantaneous reflected power from a NMR probe is described. It is built from commercially available directional power-meter components coupled to a low-speed oscilloscope. The device can be used to monitor the performance of a probe while data are being acquired. It is particularly useful for adjusting probes whose characteristics change during a high-power pulse because of component heating.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1139571
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  • 6
    Electronic Resource
    Electronic Resource
    Methods for analyzing spectroscopic line shapes. NMR solid powder patterns (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 84 (1986), S. 3717-3725 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new method is described for numerically computing theoretical NMR powder patterns which achieves a many-fold increase in speed and accuracy over previous techniques. The method incorporates a simple and efficient technique for selecting the set of crystal orientations over which the spectral frequencies are calculated. The orientation selection technique is then integrated with an interpolation scheme which transfers the intensities at these frequencies to the computed spectrum. The method will be useful whenever an average over a sphere is computed numerically. The new efficiency of the method makes practical least squares fitting of theoretical spectra to experimental NMR data. The fits provide unbiased estimates of the NMR parameters and their errors. The technique is illustrated by extracting chemical shift tensors from a proton decoupled carbon-13 NMR spectrum.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.450211
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  • 7
    Electronic Resource
    Electronic Resource
    The calculation of 13C chemical shielding tensors in ionic compounds utilizing point charge arrays obtained from Ewald lattice sums (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 9236-9243 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An embedded ion method is proposed for accurately calculating the 13C chemical shift tensors in ionic compounds. The method models an ionic crystal by embedding an ion of interest inside an array of point charges. The potential, produced by an infinite ionic lattice, at the location of the ion of interest can be simulated accurately utilizing a point charge array obtained by the Ewald summation method. The Ewald summation method, as implemented in the computer program EWALD, in conjunction with the quantum-mechanics program GAUSSIAN 98 is used to generate a self-consistent point charge array that simulates the Ewald potential in a defined region at the center of the array. Subsequently, the chemical shift tensor calculation is performed using GAUSSIAN 98 on the ion of interest positioned in the region inside the point charge array in which the Ewald potential is established. The embedded ion method was tested on potassium methyl-trithiocarbonate (KS2CSCH3) whose crystal lattice is composed of potassium cations and molecular S2CSCH3− anions. The principal values of the 13C chemical shift tensors in KS2CSCH3 were measured in a stationary cross polarization nuclear magnetic resonance experiment. It is shown that the correlation between experimental and calculated principal values improves significantly when the C–H bond distances are optimized from their x-ray values. It is further demonstrated that a substantial improvement in the correlation is obtained when the chemical shielding tensor calculation is performed on an S2CSCH3− anion embedded inside a point charge array obtained by the Ewald summation method. The embedded ion method was completed applying the B3P86/cc-pVTZ, B3LYP/cc-pVTZ, and MP2/cc-pVDZ quantum-mechanical computations and the various results are compared and analyzed. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1356016
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