MRC - Magnetic Resonance in Chemistry - up to Jan. 2008

MRC - Magnetic Resonance in Chemistry - up to Jan. 2008

Magnetic Resonance in Chemistry
Volume 45, Issue 9 , Pages 797 - 800
Published Online: 30 Jul 2007

Spectral Assignments and Reference Data
Statistical analysis of 13C and 15N NMR chemical shifts from GIAO/B3LYP/6-311 + + G** calculated absolute shieldings
Fernando Blanco, Ibon Alkorta, José Elguero
Instituto de Química Médica, CSIC, Juan de la Cierva 3, E-28006 Madrid, Spain

Keywords - 13C NMR • 15N NMR • GIAO

Abstract
The 13C and 15N absolute shieldings of 28 compounds have been calculated at the GIAO/B3LYP/6-311 + + G** level to complete a collection of data already published. This has allowed us to devise new equations relating and for these nuclei based on 461 points (13C) and 70(72) points (15N).
Received: 22 May 2007; Revised: 11 June 2007; Accepted: 14 June 2007

Digital Object Identifier (DOI) - 10.1002/mrc.2053 About DOI

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Magnetic Resonance in Chemistry
Volume 45, Issue 10 , Pages 814 - 818
Published Online: 9 Aug 2007

Research Article
High-resolution magic-angle spinning NMR for the identification of reaction products directly from thin-layer chromatography spots
Scott A. Bradley *, Rebekah L. McLaughlin
Department of Drug Metabolism, Merck Research Laboratories, Rahway, NJ 07065, USA

Keywords
NMR • 1H • thin-layer chromatography • TLC • magic-angle spinning • HRMAS • Nano probe

Abstract
We have investigated the prospect of identifying organic reaction products directly from separated thin-layer chromatography (TLC) spots with high-resolution magic-angle spinning (HRMAS) NMR. The concept is to use the TLC spots for NMR analysis so that spectra can be obtained before the reaction is worked up, but without having to elute the product from the TLC stationary phase. Thus, the separated spot is scraped from the plate, transferred to an HRMAS sample rotor, and suspended with a deuterated solvent. Herein, we describe the effects of having the stationary phase present during NMR acquisition. Using a Varian 4 mm gHX Nanoprobe and rotenone as a test compound, we found that the presence of the stationary phase during NMR acquisition resulted in (i) a large, broad background signal near 4.6 ppm and (ii) a decrease in the signal-to-noise ratio due to the adsorption of the product molecules to the adsorbent. However, both effects could be adequately and conveniently eliminated. The background signal was removed by using either a CPMG pulse sequence or chemical exchange. The adsorption was avoided by using a more polar solvent system. Finally, we found that spectra with good signal-to-noise ratio and resolution could be acquired in a matter of minutes even for cases of limited product concentration. Therefore, we believe the technique has value and provides the organic chemist with another option to obtain NMR data critical for structural elucidation or verification.

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Magnetic Resonance in Chemistry
Volume 45, Issue 10 , Pages 860 - 864
Published Online: 29 Aug 2007

Research Article
Dependence of 35Cl NQR on hydrogen bonding and temperature in dichlorophenol-aniline charge transfer complexes
D. Ramananda, K. P. Ramesh, J. Uchil

*Correspondence to D. Ramananda, Department of Materials Science, Mangalore University, Mangalore, India.

Keywords
hydrogen bonded • NQR • charge transfer • chemical bond • ionic bond • double bond

Abstract
The hydrogen-bonded charge transfer complexes of aniline with -acceptors (or proton donors) such as 2,5-, 2,6-, 3,4- and 3,5-dichlorophenol were prepared. The 35Cl nuclear quadrupole resonance (NQR) frequencies of these charge transfer complexes in the temperature range 77-300 K were measured to ascertain the existence or otherwise of a phase transition upon complex formation. Further, the NQR frequency and asymmetry parameter of the electric field gradient at the site of quadrupole nucleus were used to estimate the chemical bond parameters, namely ionic bond, double bond character of the carbon-chlorine(CCl) bond and the percentage charge transfer between the donor-acceptor components in charge transfer complexes. The effect of hydrogen bonding and temperature on the charge transfer process is analysed.
Received: 14 February 2007; Revised: 21 June 2007; Accepted: 26 June 2007

Digital Object Identifier (DOI) - 10.1002/mrc.2059

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Magnetic Resonance in Chemistry
Volume 45, Issue 11 , Pages 962 - 970
Published Online: 8 Oct 2007

Research Article
Solid state NMR characterization of individual compounds and solid solutions formed in Sc2O3V2O5Nb2O5Ta2O5 system
Dzhalil Khabibulin, Konstantin Romanenko, Mikhail Zuev, Olga Lapina
*Correspondence to Olga Lapina, Boreskov Institute of Catalysis, SB RAS, pr. Lavrentieva, 5, 630090 Novosibirsk, Russia.

Keywords
solid state NMR • SATRAS • MAS • 51V • 45Sc • 93Nb • solid solutions • bulk oxides • vanadates • niobates

Abstract
In this study, 51V, 45Sc and 93Nb MAS NMR combined with satellite transition spectroscopy analysis were used to characterize the complex solid mixtures: VNb9(1-x)Ta9xO25, ScNb(1-x)TaxO4 and ScNb2(1-x)Ta2xVO9 (x = 0, 0.3, 0.5, 0.7, 1.0). This led us to describe the structures of Sc and V sites. The conclusions were based on accurate values for 51V quadrupole coupling and chemical shift tensors obtained with 51V MAS NMR/SATRAS for VNb9O25, VTa9O25 and ScVO4. The 45Sc NMR parameters have been obtained for Sc2O3, ScVO4, ScNbO4 and ScTaO4. On the basis of 45Sc NMR and data available from literature, the ranges of the 45Sc chemical shift have been established for ScO6 and ScO8. The gradual change of the 45Sc and 51V NMR parameters with x confirms the formation of solid solutions in the process of synthesis of VNb9(1-x)Ta9xO25 and ScNb(1-x)TaxO4, in contrast to ScNb2(1-x)Ta2xVO9. The cation sublattice of ScNb(1-x)TaxO4 is found to be in octahedral coordination. The V sites in VNb9(1-x)Ta9xO25 are present in the form of slightly distorted tetrahedra. The 93Nb NMR parameters have been obtained for VNb9O25. Copyright © 2007 John Wiley & Sons, Ltd.
Received: 1 May 2007; Revised: 22 June 2007; Accepted: 20 August 2007

Digital Object Identifier (DOI) - 10.1002/mrc.2086

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Magnetic Resonance in Chemistry
Volume 46, Issue 1 , Pages 9 - 16
Published Online: 26 Nov 2007

Research Article
Critical behavior of KDCO3 from 2H and 39K single crystal NMR
Christophe Odin *
*Correspondence to Christophe Odin, Groupe Matière Condensée et Matériaux, UMR6626 au CNRS, Université Rennes I, Campus de Beaulieu, 35042 Rennes Cedex, France.

Keywords
NMR • solid-state NMR • phase transition • critical behavior • hydrogenocarbonate • deuterium NMR

Abstract
Potassium hydrogenocarbonate KDCO3 presents an order/disorder phase transition at Tc 353 K. The critical behavior of this phase transition was studied by single crystal 2H and 39K NMR. The evolution of the order parameter as a function of temperature is quantified, and the critical exponent was determined, indicating a transition close to a tricritical point. The 2H Zeeman relaxation rate is strongly increased near the transition temperature. By calculating the noncritical contribution to the Zeeman relaxation rate, we show that the observed relaxation rate clearly presents a pseudo-divergent behavior near Tc, with a logarithmic singularity. The nature of the phase transition is discussed in the light of these results. Copyright © 2007 John Wiley & Sons, Ltd.
Received: 31 May 2007; Revised: 30 August 2007; Accepted: 18 September 2007

Digital Object Identifier (DOI) - 10.1002/mrc.2112

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Magnetic Resonance in Chemistry
Volume 46, Issue 1 , Pages 30 - 35
Published Online: 20 Dec 2007

Research Article
Inverse H-C ex situ HRMAS NMR experiments for solid-phase peptide synthesis
Timothy R. Ramadhar, Fernando Amador, Michael J. T. Ditty, William P. Power
*Correspondence to William P. Power, Department of Chemistry, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada, N2L 3G1.

Keywords
NMR • 1H • 13C • heteronuclear shift correlation • inverse experiments • gel-phase HRMAS • BIRD-HMQC • CT-HMBC-1 • solid-phase peptide synthesis

Abstract
The growing importance of solid-phase peptide synthesis (SPPS) has necessitated the development of spectroscopic experiments that can be used to obtain structural and conformational information on resin-bound peptides. Despite the utility of two-dimensional high-resolution magic angle spinning (HRMAS) NMR experiments that provide homonuclear shift correlations, experiments that provide heteronuclear shift correlations are necessary for complex conformational and structural elucidatory problems. Here we report the optimization and implementation of non-gradient inverse NMR experiments for acquiring the 1H13C shift correlations of resin-bound peptides. The use of non-gradient experiments is advantageous as many magic angle spinning (MAS) probes do not possess gradient coils. An HRMAS BIRD-HMQC experiment with a reduced 1JCH constant has proven very suitable for obtaining one-bond correlations. Long-range correlations can be interpolated by using a non-gradient HRMAS CT-HMBC-1 experiment where the resulting data is processed with forward linear prediction. It has been shown that removing the effects of 1H1H J-modulation is crucial in order to view cross peaks that correspond to long-range correlations. Additionally, both experiments prove extremely useful over routine one-dimensional 13C HRMAS experiments for extracting carbon chemical shift data. The non-gradient HRMAS BIRD-HMQC and CT-HMBC-1 experiments can be used to assist in conformational analysis and to identify and deconvolute situations where accidental equivalence and seemingly correlated isochronous signals arise.
Received: 23 August 2007; Revised: 21 September 2007; Accepted: 25 September 2007