Magnetic Resonance in Chemistry

Hayton, Lynda J.; Mile, Brynmor; Timms, Peter L.

doi: 10.1002/mrc.1870pmid: 16835900

Co atoms were reacted with ethene at 77 K and the paramagnetic products studied by electron spin resonance (ESR) at X‐ and K‐bands. The ESR spectra of the major product at both frequencies showed eight cobalt multiplets (ICo = 7/2) indicating a mono‐cobalt complex. The spectra have orthorhombic g and cobalt hyperfine tensors and were simulated by the parameters; g1 = 2.284, g2 = 2.0027, g3 = 2.1527; A1 < − 25 MHz, A2 = − 109 MHz, A3 = − 198 MHz. Proton and 13C (1% natural abundance) hyperfine couplings were lower than the line widths (<2 MHz) indicating less than 0.5 spin transfer to the ethene ligands. We assigned the spectrum to a Jahn—Teller‐distorted planar trigonal mono‐cobalt tris‐ethene [Co(η‐C2H4)3] complex in C2v symmetry. The SOMO is either a 3dx2−y2 (2a1) orbital in a T‐geometry or a 3dxy (b1) orbital in a Y‐geometry but there is only a spin density, a2, of 0.30 in these d orbitals. The spin deficiency of 0.70 is attributed to two factors; spin transfer from the Co to ethene π/π* orbitals and a 4p orbital contribution, b2, to the SOMO. Calculations of a2 and b2 have been made at three levels of spin transfer, θ. At θ = 0.00a2 is 0.23 and b2 is 0.78, at θ = 0.25a2 is 0.25 and b2 is 0.52 and at θ = 0.50a2 is 0.28 and b2 is 0.23. The other possible assignment to a mono‐cobalt bis‐ethene complex [Co(η‐C2H4)2] cannot be discounted from the ESR data alone but is considered unlikely on other grounds. The complex is stable up to ∼220 K indicating a barrier to decomposition of ∼50 kJ Mol−1 Copyright © 2006 John Wiley & Sons, Ltd.

Stærk, Dan; Chapagain, Bishnu P.; Lindin, Therese; Wiesman, Zeev; Jaroszewski, Jerzy W.

doi: 10.1002/mrc.1879pmid: 16871644

The main saponin (1) present in the mesocarp of Balanites aegyptiaca fruit is a mixture of 22R and 22S epimers of 26‐(O‐β‐D‐glucopyranosyl)‐3‐β‐[4‐O‐(β‐D‐glucopyranosyl)‐2‐O‐(α‐L‐rhamnopyranosyl)‐β‐D‐glucopyranosyloxy]‐22,26‐dihydroxyfurost‐5‐ene. This structure differs from a previously reported saponin isolated from this source by the site of attachment of the rhamnosyl residue, and presumably represents a structural revision of the latter. The main saponin (2) present in the kernel is a xylopyranosyl derivative of 1. The use of high‐field NMR enabled the practically complete assignment of 1H and 13C chemical shifts of these complex saponins, existing as a mixture of C‐22 epimers. Moreover, the work represents a new approach to structural elucidation of saponins: direct preparative‐scale HPLC‐RID of crude extracts followed by high‐field NMR investigations supported by ESI‐MSn. Copyright © 2006 John Wiley & Sons, Ltd.

Bushell, James A.; Claybourn, Mike; Murphy, Damien M.; Williams, Helen E.

doi: 10.1002/mrc.1880pmid: 16847997

Gamma radiation of poly (lactide‐co‐glycolide) raw polymers and processed microspheres under vacuum and at 77 K results in the formation of a series of free radicals. The resulting powder electron paramagnetic resonance (EPR) spectrum contains a distribution of several different radicals, depending on the annealing temperature, and is therefore difficult to interpret. By utilising the selectivity of the electron nuclear DOuble resonance (ENDOR) and associated ENDOR induced EPR (EIE) techniques, a more direct approach for the deconvolution of the EPR spectrum can be achieved. Using this approach, the radiolytically induced CH3•CHC(O)R‐ chain scission radical was identified at 120 K by simulation of the EIE spectrum. At elevated temperatures (250 K), this radical decays considerably and the more stable radicals ‐O•CHC(O)‐, CH3•C(OR)C(O)‐ and CH3•C(OH)C(O)‐ predominate. This work demonstrates the utility of the EIE approach to supplement and aid the interpretation of powder EPR spectra of radicals in a polymer matrix. Copyright © 2006 John Wiley & Sons, Ltd.

Ramesh, V.; Chandrakumar, N.

doi: 10.1002/mrc.1883pmid: 16856210

We propose and demonstrate a 1D pulse sequence to convert double quantum coherence (DQC) of y phase with optimal efficiency, relying on single transition selection. Our sequence has a larger high‐sensitivity bandwidth with respect to the coupling, compared to other reconversion strategies. A modified version of the new pulse sequence provides the missing chemical shift and coupling information, at minor cost in sensitivity. Application to 1D 13C INADEQUATE is demonstrated.

Piotto, Martial; Bourdonneau, Maryse; Elbayed, Karim; Wieruszeski, Jean‐Michel; Lippens, Guy

doi: 10.1002/mrc.1884pmid: 16883554

The acquisition time and quality of 1D 13C{1H} spectra can be improved substantially by using a modified driven equilibrium Fourier transform (DEFT) sequence, which is specifically designed to compensate for the effects of B1 inhomogeneity, pulse miscalibration and frequency offsets. The new sequence, called uniform driven equilibrium Fourier transform (UDEFT), returns the carbon magnetization with a high accuracy along its equilibrium position after each transient is complete. Thus, the sequence allows the use of relaxation delays (RD), which are much shorter than the carbon T1 of the molecule, thereby speeding up the acquisition process of 1D 13C{1H} spectra. To achieve this level of performance, UDEFT employs a refocusing element constituted by a composite adiabatic carbon pulse surrounded by two 90° carbon pulses whose phases are designed to compensate for 90° pulse miscalibrations in an MLEV manner (90° +x − τ(FID)− 180+y(Adia)− τ− 90° +x − 180° +x(Adia)). A version of the UDEFT sequence allows recording 1D 13C{1H} spectra devoid of heteronuclear NOE by using a matched adiabatic 1H decoupling scheme where an even number of 180° adiabatic pulses is applied during the UDEFT module. Spectra of a solution of 300 mM camphor that contains some carbon nuclei with very long T1 relaxation times (90 s and 78 s) were acquired with 128 scans in 10 min using a 5 s relaxation delay. Copyright © 2006 John Wiley & Sons, Ltd.

Boyd, Maruta; Hay, Michael P.; Boyd, Peter D. W.

doi: 10.1002/mrc.1886pmid: 16900565

1H, 13C and 15N NMR measurements (1D and 2D including 1H15N gs‐HMBC) have been carried out on 3‐amino‐1, 2,4‐benzotriazine and a series of N‐oxides and complete assignments established. N‐Oxidation at any position resulted in large upfield shifts of the corresponding N‐1 and N‐2 resonances and downfield shifts for N‐4 with the exception of the 3‐amino‐1,2,4‐benzotriazine 1‐oxide in which a small upfield shift of N‐4 was observed. Density functional GIAO calculations of the 15N and 13C chemical shifts [B3LYP/6‐31G(d)//B3LYP/6‐311 + G(2d,p)] gave good agreement with experimental values confirming the assignments. The combination of 13C and 15N NMR provides an unambiguous method for assigning the 1H and 13C resonances of N‐oxides of 1,2,4‐benzotriazines. Copyright © 2006 John Wiley & Sons, Ltd.

Hiller, Achim; Patt, Jörg T.; Steinbach, Jörg

doi: 10.1002/mrc.1875pmid: 16900572

Two 4‐substituted aromatic iodosyl compounds were investigated with regard to their solubility, stability and chromatographic behaviour. 1‐Iodosyl‐4‐methoxy‐ and 1‐iodosyl‐4‐nitro‐benzene are soluble in methanol and provide acceptable 1H and 13C NMR spectra; however, gradual oxidation of the solvent was observed. LC‐MS analyses suggest that unlike the parent substance, iodosylbenzene, which has a polymeric structure, both compounds rather exist in the monomeric form. Copyright © 2006 John Wiley & Sons, Ltd.

Pérez‐Picaso, Lemuel; Rios, María Yolanda; Hernández, Ana Niurka; Martínez, Jorge

doi: 10.1002/mrc.1864pmid: 16826554

The rhizobacteria Burkholderia cepacia biosynthesized the new tetrapeptide Cyclo[N‐(Lys‐Phe)‐Orn‐Val] (1), a 2,5‐diketopiperazine, and the known siderophore azurechelin (salicylic acid). The structure of 1 was established by means of IR, UV, 1H and 13C NMR, double dimension experiments and MS. Copyright © 2006 John Wiley & Sons, Ltd.