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  • 1
    Electronic Resource
    Electronic Resource
    Temperature dependence of nuclear shielding and quadrupolar coupling of noble gases in liquid crystals (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 8977-8985 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A theoretical model for nuclear magnetic resonance (NMR) observables of noble gas atoms in nematic and smectic A liquid crystal environments is developed. It is used to account for the behavior of the nuclear shielding of 129Xe (σ) and the quadrupolar coupling of 21Ne (B) measured in 1-butyl-c-4-(4'-octylbiphenyl-4-yl)-r-1-cyclo-hexan-carbonitrile (NCB 84). The analysis provides detailed information on the orientational and positional order of the liquid crystal solvent and of the solute atoms. In the nematic phase, the decrease of σ with decreasing temperature is attributed to the increase of the density and orientational order parameter S of the solvent. The former factor determines the variation of the isotropic part of σ, while the latter governs the change in the anisotropic part (the anisotropy of the shielding tensor Δσ is of the order of −10 ppm). The prominent increase of σ in the smectic A phase is ascribed to the tendency of the atoms to occupy the interspaces between the smectic layers rather than their interiors (the deviation from the uniform distribution of the atoms is of the order of 5%). The main result is that the isotropic part of σ increases as the density of the solvent in the immediate neighborhood of the solute decreases. In the case of the quadrupolar coupling, the effect of the redistribution of the atoms is small, as there is no isotropic contribution to B. The behavior of the quadrupolar coupling of 21Ne in the nematic phase indicates that there are two distinct contributions to B. One is due to the distortion of the electron cloud from the spherical symmetry and the other arises directly from the external electric field gradient produced by the neighboring molecules. The temperature dependence of the latter contribution is not determined by the variation of S alone.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463324
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  • 2
    Electronic Resource
    Electronic Resource
    A special method for analyzing anisotropic nuclear magnetic resonance parameters: Acetonitrile in liquid crystals (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 8514-8523 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A reliable analysis of the nuclear magnetic resonance (NMR) spectral parameters of partially oriented molecules requires the calculation of the effects of the correlation between the molecular vibration and rotation. However, in many cases the information content of the spectral data is not sufficient for an unambiguous determination of all the adjustable parameters involved in such an analysis. The present paper describes a special method to simplify the analysis significantly, so as to make seemingly underdetermined problems solvable. The method is applicable to the molecules which contain segments composed of one or more light bonds attached to a heavier bond. It is applied to the anisotropic couplings Dij of acetonitrile (CH3CN) oriented in various liquid crystals. The analysis leads to the following rα geometry: (angle)HCH=109.22°±0.06°, rCH/rCC =0.751±0.002 and rCN/rCC =0.788±0.005. In addition, detailed information on (1) the indirect coupling anisotropies ΔJCC and 2ΔJCN, (2) the 1H and 13C chemical shift anisotropies, (3) the external torques acting on the CH bonds, and (4) the orientational order parameters of the CH3C segment of the acetonitrile molecule is obtained.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459289
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  • 3
    Electronic Resource
    Electronic Resource
    Xe〈sup LOCATION="PRE"〉129 adsorbed in AlPO4-11 molecular sieve: Molecular dynamics simulation of adsorbate dynamics and NMR chemical shift (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 6470-6478 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Molecular dynamics simulation of 129Xe adsorbed in the AlPO4-11 molecular sieve has been carried out in the constant NVT ensemble at T=300 K. The host material was modeled as a fully dynamic framework. Single-particle and pair distribution functions of the adsorbed gas in the unidirectional channels of the material with different adsorbate loadings are presented. Self-diffusion coefficients, DXe, have been calculated for the dynamics of the adsorbates between the adsorption cells in AlPO4-11. The dynamic framework is shown to decrease the effective potential barrier for intercellular jumps. An estimate is given for the jump frequency. The NMR chemical shifts of 129Xe were evaluated using parametrizations based on first principles calculations of Xe–Xe and Xe-OH2 model systems. No adjustable parameters were used to improve agreement with experimental results. The results of a phenomenological three-site dynamic averaging model proposed earlier by others were reproduced with a good accuracy using chemical shift parameters from gradient corrected density-functional theory. Finally, the approximations inherent to the present computational approach are discussed. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475135
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  • 4
    Electronic Resource
    Electronic Resource
    Nuclear magnetic shielding of noble gases in liquid crystals (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 6381-6388 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A theoretical model for nuclear magnetic shielding of noble gas atoms soluted in liquid crystal solvents is developed. It is found that the solvent effect on the shielding can be represented as a linear combination of products of the liquid crystal orientational order parameters of varying rank. In a special case of pairwise additive shielding perturbations, most of the coefficients vanish and the shielding reduces to a sum of two terms, the isotropic and anisotropic parts. Both contributions are directly proportional to the density of the liquid crystal, and the anisotropic part is also directly proportional to the second rank orientational order parameter of the liquid crystal. The developed model is used to account for the behavior of the 129Xe shielding in the nematic liquid crystal 4-ethoxybenzylidene-2,6-dideutero-4′-n-butylaniline (d2-EBBA). The pairwise additivity approximation of the shielding perturbations is found to explain the observed temperature dependence of the 129Xe shielding satisfactorily. In particular, the temperature dependence of the isotropic part is mostly due to the change in the liquid crystal density, whereas the anisotropic part is mainly controlled by the temperature dependence of the Xe–liquid crystal molecule pair correlation function and the second rank orientational order parameter of the liquid crystal. This result differs from the results of the phenomenological theory of Lounila et al. [J. Chem. Phys. 97, 8977 (1992)], where only the density and the orientational order parameter were assumed to be significantly temperature dependent. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.478541
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  • 5
    Electronic Resource
    Electronic Resource
    Isotope and temperature effects on the 13C and 77Se nuclear shielding in carbon diselenide (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 1350-1361 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A comprehensive theoretical and experimental study of the 13C and 77Se nuclear magnetic shieldings and their rovibrational corrections in carbon diselenide (CSe2) has been undertaken. The 13C and 77Se shielding tensors as well as all their first and second derivatives with respect to the internal displacement coordinates of the molecule have been calculated by several first principles gauge-including atomic orbital (GIAO) methods. Hartree-Fock (HF), multiconfiguration Hartree-Fock (MCHF), and density-functional (DFT) theories have been compared, the latter both in the local density approximation (LDA) and by using two gradient corrected exchange-correlation functionals. The shielding derivatives calculated with MCHF and DFT are very much smaller in magnitude than the derivatives obtained by using HF, being in reasonable mutual agreement. By using the theoretical shielding derivatives and the cubic anharmonic force constants calculated within LDA, together with an experimental harmonic force field, all the first and second order terms in the rovibrational contributions to the shielding constants and anisotropies have been worked out. The contributions to the shielding constants have been calculated for the various isotopomers of CSe2 at several temperatures, and the resulting theoretical temperature dependencies of the shielding constants, the isotope shifts and the temperature dependencies of the isotope shifts have been compared with the experimental results. There is excellent agreement between the theoretical and experimental results for 77Se. The agreement is not quite as good for the (anomalously small) shielding constant of 13C and its rovibrational corrections. Contrary to what has been frequently assumed, none of the first and second order terms in the rovibrational contributions to the shielding constants can safely be neglected. In particular, the first order isotope effect due to change in the bond not directly attached to the observed nucleus is very important. Furthermore, the second order terms — including the bending and even cross terms — are essential in order to give a correct description of the isotope shifts. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.474482
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  • 6
    Electronic Resource
    Electronic Resource
    Quantification of metabolites from single-voxelin vivo 1H NMR data of normal human brain by means of time-domain data analysis (1995)
    Springer
    Magnetic resonance materials in physics, biology and medicine 3 (1995), S. 129-136 
    Details
    ISSN: 1352-8661
    Keywords: brain ; metabolites ; 1H NMR spectroscopy ; quantification
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine , Physics
    Notes: Abstract We present here a combination of time-domain signal analysis procedures for quantification of human brainin vivo 1H NMR spectroscopy (MRS) data. The method is based on a separate removal of a residual water resonance followed by a frequency-selective time-domain line-shape fitting analysis of metabolite signals. Calculation of absolute metabolite concentrations was based on the internal water concentration as a reference. The estimated average metabolite concentrations acquired from six regions of normal human brain with a single-voxel spin-echo technique for theN-acetylaspartate, creatine, and choline-containing compounds were 11.4±1.0,6.5±0.5, and 1.7±0.2 mmol kg−1 wet weight, respectively. The time-domain analyses ofin vivo 1H MRS data from different brain regions with their specific characteristics demonstrate a case in which the use of frequency-domain methods pose serious difficulties.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01771697
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  • 7
    Electronic Resource
    Electronic Resource
    Conformational analysis. XIIIFor Parts XI and XII, see Ref. 1. - A 300 MHz 1H NMR study of 2-methyl-, 2-phenyl-, 2-(2-chlorophenyl)-and 2-(4-chlorophenyl)-oxetanes (1975)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 7 (1975), S. 286-290 
    Details
    ISSN: 0030-4921
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The 300 MHz spectra of 2-methyl-, 2-phenyl-, 2-(2-chlorophenyl)- and 2-(4-chlorophenyl)-oxetanes were recorded in CCl4 and C6D6. The chemical shifts and the proton-proton coupling constants were solved through iterative calculations. The results were discussed in the light of structural factors and theoretical calculations. The average angle of pucker for the oxetane ring was estimated at c. 5°, in fair agreement with the value of 3·3° derived from a microwave study.
    Additional Material: 7 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/mrc.1270070607
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  • 8
    Electronic Resource
    Electronic Resource
    13C—13C spin-spin coupling constants and 13C isotope effects on 13C chemical shifts in some 4-membered rings (1978)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 11 (1978), S. 157-159 
    Details
    ISSN: 0030-4921
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The 13C—13C spin-spin coupling constants in natural abundance oxetane, thietane, cyclobutanone, bromo-and chlorocyclobutane have been measured. Furthermore, the 13C isotope-induced changes in the chemical shifts of the different 13C nuclei in the molecules mentioned above are reported. These shifts are normally to higher magnetic field; in cyclobutanone, however, the resonance of the carbonyl carbon has shifted to lower field because of the substitution of 13C-3 for 12C-3.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/mrc.1270110313
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  • 9
    Electronic Resource
    Electronic Resource
    Secondary isotope effect on the 19F shielding in xenon and krypton difluorides in solution (1994)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 32 (1994), S. 242-247 
    Details
    ISSN: 0749-1581
    Keywords: NMR ; Isotope shift ; 19F,129Xe chemical shifts ; Xe,F coupling constants ; Xenon difluoride ; Krypton difluoride ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The effects of the various xenon and krypton isotopes on the nuclear shielding of 19F in xenon and krypton difluoride in solution were studied by 19F NMR spectroscopy. For both molecules, linear relationships were observed between the one-bond secondary isotope shift of the 19F nucleus, 1Δ; 19F(m′/128Xe) and 1Δ 19F(m′/78Kr), and the relative mass factor, (m′ - m)/m′, and also between the shifts and the quadratic mean bond displacement, 〈(Δr)2〉. On the basis of the latter correlation and the linear relationship between 〈Δr〉 and 〈(Δr)2〉 for the linear combination of the first and second derivatives of the 19F shielding, the values -1742 and -5804 ppm Å-2, respectively, were determined. Further, the temperature dependence of the 129Xe and 19F chemical shifts and the 129Xe, 19F indirect spin-spin coupling of XeF2 were determined. The 129Xe chemical shift is extremely sensitive to temperature with Δδ/ΔT = -0.4718 ppm K-1, whereas for 19F Δδ/ΔT = +0.0042 ppm K-1. In contrast, the 129Xe, 19F indirect spin-spin coupling is independent of temperature over the range 243-303 K, the mean value being 5644.2 ± 0.6 Hz.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/mrc.1260320410
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  • 10
    Electronic Resource
    Electronic Resource
    Solute molecular structure determination by NMR: Furan in liquid crystal mixtures (1985)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 23 (1985), S. 725-727 
    Details
    ISSN: 0749-1581
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The rα structure of furan dissolved in various thermotropic nematogens and in their mixtures was studied. The results indicate that the structure is not very sensitive to the solvent, contradicting the observations made earlier on the structure of thiophene. The distance ratios and bond angles are observed to be in fair agreement with the results derived from a microwave spectrum.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/mrc.1260230909
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