Monday, February 26, 2007

MRC: Magnetic Resonance in Chemitry - Update to February 2007

Magnetic Resonance in Chemistry
Vol: 45, Issue: 2, February 2007
pp. 118 - 122

Title: Structure of a paramagnetic supermicroporous silica-based material via a multinuclear solid-state NMR monitoring
Authors: Bakhmutov, Vladimir I.a; Shpeizer, Boris G.a; Clearfield, Abrahama
Affiliations: a. Department of Chemistry, Texas A & M University, P.O. Box 30012, College Station, Texas 77842‐3012, USA
Keywords: 29Si; 27Al; 1H; 13C MAS NMR; paramagnetic solids; T1 relaxation; porous materials
Abstract (English):

The paramagnetic Si/Al material doped with 19.8% (weight) of Mn2+ has been characterized by the 1H, 13C, 27Al, 29Si MAS NMR spectra and the 1H, 29Si T1MAS relaxation measurements as a gel dried at 130 °C and as samples calcined at 504, 540 and 650 °C. It has been demonstrated that in spite of strong paramagnetic effects, NMR monitoring in combination with 29Si T1 relaxation experiments can be successful for structural descriptions of porous materials. Copyright © 2007 John Wiley & Sons, Ltd.

Magnetic Resonance in Chemistry
Vol: 45, Issue: 1, January 2007
pp. 5 - 23

Title: Structural elucidation of nitro-substituted five-membered aromatic heterocycles utilizing GIAO DFT calculations
Authors: Katritzky, Alan R.a; Akhmedov, Novruz G.a; Doskocz, Jacekb; Hall, C. Dennisa; Akhmedova, Rena G.a; Majumder, Sumana
Affiliations: a. Center for Heterocyclic Compounds, Department of Chemistry, University of Florida, Gainesville, FL 32611‐7200, USA
b. Faculty of Bioorganic Chemistry, Department of Chemistry, Wroclaw University of Technology, Wybrzeże Wyspiańkiego 27, 50‐370, Wroc3aw, Poland
Keywords: GIAO; long-range coupling constants; five-membered heterocycles; proton-coupled 13C NMR spectra
Abstract (English):

The GIAO (Gauge Including Atomic Orbitals) DFT (Density Functional Theory) method is applied at the B3LYP/6–31+G(d,p)//B3LYP/6–31+G(d), B3LYP/6–311++G(d,p)//B3LYP/6–31+G(d), B3LYP/6–311+G (2d,p)//B3LYP/6–31+G(d) and B3LYP/6–311++G(d,p)//B3LYP/6–311++G(d,p) levels of theory for the calculation of proton and carbon chemicals shifts and coupling constants for 25 nitro-substituted five-membered heterocycles. Difference (1D NOE) spectra in combination with long-range gHMBC experiments were used as tools for the structural elucidation of nitro-substituted five-membered heterocycles. The assigned NMR data (chemical shifts and coupling constants) for all compounds were found to be in good agreement with theoretical calculations using the GIAO DFT method. The magnitudes of one-bond (1JCH) and long-range (nJCH, n > 1) coupling constants were utilized for unambiguous differentiation between regioisomers of nitro-substituted five-membered heterocycles. Copyright © 2006 John Wiley & Sons, Ltd.

Magnetic Resonance in Chemistry
Vol: 44, Issue: 11, November 2006
pp. 985 - 988

Title: 29Si spin-lattice NMR relaxation in microporous silica-based materials with high Mn2+ concentrations
Authors: Bakhmutov, Vladimir I.a; Shpeizer, Boris G.a; Clearfield, Abrahama
Affiliations: a. Department of Chemistry, Texas A&M University, College Station, Texas 77842‐3012, USA
Keywords: 29Si MAS NMR; T1 relaxation; microporous materials
Abstract (English):

The 29Si T1 time measurements have been performed under magic angle spinning for supermicroporous Si/Mn materials 1–6 containing Mn2+ in concentrations of 0 (1), 1.2 (2), 2.5 (3), 3.8 (4), 15 (5) and 19.8 wt% (6). It has been found that the central lines, corresponding to isotropic chemical shifts, and their sidebands relax differently in samples 2–4 with relatively small Mn2+ contents. Because the relaxation curves for 1–6 are nonexponential and identical for 6 at spinning rates of 5, 10 and 12 kHz, dipolar relaxation via the paramagnetic centers is dominant. The relaxation data have been interpreted as a function of Mn2+ concentrations in terms of incorporation of the Mn2+ ions into the silica lattice when their concentration is small (£2.5 wt%). At higher concentrations, most of the manganese is located on the surface of the silica. Copyright © 2006 John Wiley & Sons, Ltd.

Magnetic Resonance in Chemistry
Vol: 44, Issue: 11, November 2006
pp. 989 - 1007

Title: Density functional computations of 99Ru chemical shifts: relativistic effects, influence of the density functional, and study of solvent effects on fac—[Ru(CO)3I3]-
Authors: Autschbach, Jochena; Zheng, Shaohuia
Affiliations: a. Department of Chemistry, 312 NSC, State University of New York at Buffalo, Buffalo, NY 14260‐3000, USA
Keywords: density functional theory; computational chemistry; ruthenium; solvent effects; relativistic effects
Abstract (English):

Solvent effects on the 99Ru NMR chemical shift of the complex fac—[Ru(CO)3I3]- are investigated computationally using density functional theory. Further, benchmark calculations of the 99Ru shift for a set of ten Ru complexes have been performed in order to calibrate the computational model and to determine the importance of relativistic effects on the 99Ru nuclear magnetic shielding and on the chemical shift. A computational model for fac—[Ru(CO)3I3]- that includes both explicit solvent molecules and a continuum model is shown to yield the best agreement with experiment. Relativistic corrections are shown to be of minor importance for determining 99Ru chemical shifts. On the other hand, the nature of the density functional is of importance. In agreement with literature data for ligand trends of 99Ru chemical shifts, the chemical shift range for different solvents is also best reproduced by a hybrid functional. Copyright © 2006 John Wiley & Sons, Ltd.

Magnetic Resonance in Chemistry
Vol: 44, Issue: 10, October 2006
pp. 909 - 916

Title: 7Li and 13C solid-state NMR spectra of lithium cuprates
Authors: Jost, Steffena; Kühnen, Martina; Günther, Haralda
Affiliations: a. University of Siegen, FB 8, OC II, D‐57068 Siegen, Germany
Keywords: NMR; solid-state NMR; 7Li NMR; 13C NMR; quadrupole interaction; ion pairs; SIP; CIP; organolithium compounds
Abstract (English):

7Li and 13C solid-state MAS NMR spectra of three lithium cuprates with known X-ray structures—lithium([12]crown-4)2 dimethyl and diphenyl cuprate (1,2) and lithium(thf)4-[tris(trimethylsilyl) methyl]2 cuprate (3)—have been measured and analysed with respect to the quadrupolar coupling constants of lithium-7, c(7Li), and the asymmetry parameters of the quadrupolar interactions, (7Li), as well as the 6, 7Li and 13C chemical shifts. The c(7Li) values of 23, 30, and 18 kHz for 1, 2 and 3, respectively, are in line with the high symmetry around the lithium nucleus in the solvent-separated structures and may be used as reference data for this structural motif. Calculations based on charges derived from ab initio 6-31 G HF computations using the point charge model (PCM) and the program GAMESS support the experimental findings. Copyright © 2006 John Wiley & Sons, Ltd.

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