Magnetic Resonance in Chemistry

doi: 10.1002/mrc.1260301001pmid: N/A

Cheng, Jinlong; Xenopoulos, Alexander; Wunderlich, Bernhard

doi: 10.1002/mrc.1260301002pmid: N/A

The 13C chemical shifts of symmetric tetra‐n‐alkylammonium cations with n = 1–18 were identified unambiguously using two‐dimensional homonuclear (1H) chemical shift correlation (COSY) and heteronuclear (1H, 13C) chemical shift correlation spectroscopy (XHCORR). The 13C chemical shifts in small tetra‐n‐alkylammonium cations with less than five carbons in a chain are found to decrease from C‐1 (bonded to the nitrogen) (52‐60 ppm) to the methyl chain end (7‐14 ppm). For the cations with longer chains the values of chemical shifts are not in sequence with the carbon positions in an alkyl chain. The chemical shifts of C‐2 (22.5 ppm) are close to that of the methylene carbon next to the methyl group. The C‐3 carbon resonates at 26.4 ppm and is more shielded than the third carbon from the chain end (31.4 ppm). The chemical shifts of the remaining carbons are very similar to those of carbons in a corresponding linear alkane. The differences of the chemical shifts in alkylammonium salts from the corresponding n‐alkanes are largest for C‐1, C‐2 and C‐3, and this can be ascribed to the effects of the positive charge on C‐1, and the restricted segmental motion for the innermost region of the cations. Empirical additivity rules for the chemical shifts of C‐1, C‐2, and C‐3 are derived from the experimental results on 26 symmetric tetra‐n‐alkylammonium salts, and are tested for some asymmetrical systems with unequal chain lengths in the cation.

Windle, John J.; Scherrer, Rene

doi: 10.1002/mrc.1260301003pmid: N/A

Aqueous suspensions of dextran‐based Sephadex® G microbeads of progressively increasing degrees of covalent crosslinking and concomitantly decreasing water content formed a two‐phase system consisting of the waterswollen microbeads of diameter 100‐300 μm, the hydrogel phase, and of the surrounding water, the aqueous phase. These suspensions were analyzed by ESR spectroscopy using a series of small amphipathic nitroxide spin probes which partitioned between the two phases. Probe partition coefficients increased with decreasing gel water content for the more hydrophobic probes and decreased for the more hydrophilic probes. The ESR spectra of the suspensions consisted of the superposition of the spectra for the probes in the two phases. From the partition coefficients and the spectral parameters of the probes in the suspension and in the aqueous phase, relative rotational correlation times, τc, and isotropic nitrogen nuclear hyperfine splitting constants, aN, for the probes in the aqueous hydrogel interior were determined. For each probe, τc increased and aN decreased with increasing crosslinking and decreasing gel water content, indicating increasing microviscosity and decreasing dielectric constant and hydrophilicity for the gel water. The alterations in the gel water properties resulted from increasing gel matrix–water interactions and were further enhanced for water within the restricted volumes (compartments) in the most highly crosslinked hydrogels (Sephadex G‐15 and G‐10). In addition, the aN values indicated that two hydrogen bonds were formed to the nitroxide group of the probes in water and that, in the hydrogels, the hydrogen bonding contribution to aN progressively increased as the water content decreased.

Suryaprakash, N.; Khetrapal, C. L.; Diehl, P.

doi: 10.1002/mrc.1260301004pmid: N/A

The results of an NMR study of the interaction of quinazoline with iodine in the nematic phase indicate the formation of at least two different types of charge‐transfer complexes. Significant changes in the molecular geometry of the quinazoline moiety were observed as a result of complexation with iodine. Detailed information on the formation of the charge‐transfer complexes was derived from the changes in the molecular structure, order parameters and chemical shifts as functions of iodine concentration. The observed changes in the order parameters are interpreted in terms of bond interaction tensors.

Anselmi, Cecilia; Centini, Marisanna; Scotton, Mirella; Sega, Alessandro

doi: 10.1002/mrc.1260301005pmid: N/A

The dynamics and conformation ofN,N‐dimethyl‐N‐[3‐(4‐methoxy‐trans‐cinnamoylamino)propyl]‐N‐n‐dodecylammonium bromide (1) have been established in two solvents (CDCl3 and DMSO‐d6) by the use of 13C spin–lattice relaxation rates, non‐selective and selective proton spin–lattice relaxation rates and 1H–(1H) NOE experiments. The two solvents affect the conformation of the cinnamide moiety in different ways. This alters the anchor capacity of the moiety towards the alkyl chain and, as a consequence, the dynamics of 1 in CDCl3 and DMSO‐d6 show understandable differences. However, the main mean conformations of 1 in both solvents are ‘linear’. The data do not allow the rationalization of the relationship between conformation and sunscreen efficiency.

Kupč, Ēriks; Wrackmeyer, Bernd

doi: 10.1002/mrc.1260301006pmid: N/A

The magnitude and signs of geminal 29SiZ13C coupling constants were determined for nine derivatives of representative organosilicon compounds [Z = C(sp2, sp3), N, O, (two‐ and three‐coordinate), Si, S] from 1H‐detected two‐dimensional (2D) Ψ‐BIRD‐HSQC and Ψ‐BIRD‐HMQC pseudo‐triple resonance 1H29Si(13C) and 1H13C(29Si) experiments. The high sensitivity of the method allowed the measurements to be carried out in less than 1 h using 2–10 mg of the respective compound. Couplings as small as 0.45 Hz have been resolved. The couplings ranged from −21.4 to +2.2 Hz, showing regular dependence on the nature, hybridization and coordination of the intervening Z atom.

Rossi, Claudio; Donati, Alessandro; Picchi, Maria Pia; Sansoni, Maria Rosaria; Corbini, Gianfranco; Corti, Piero

doi: 10.1002/mrc.1260301007pmid: N/A

The NMR spectral properties of the antibiotic tylosin were investigated. DQ‐COSY and HETCOR two‐dimensional spectra were used for the assignment of both the proton and protonated carbon signals. Selective (H)C‐NOE, were proposed for the controversial quaternary carbon assignments. The analysis of molecular motion in solution based on carbon spin–lattice relaxation rates shows that tylosin exhibits dynamics that are typical of each molecular moiety. A higher degree of free motion is observed with increasing distance from the macrolide nucleus.

Goez, Martin

doi: 10.1002/mrc.1260301008pmid: N/A

Different one‐dimensional methods for the measurement of the effective flip angles of NMR observation pulses are discussed, and their stability against the noise of the experimental input data is compared. A new multipulse scheme is presented which is much better suited for the determination of small flip angles. Effects of dipolar interactions in a solid on the accuracy of the results are estimated. Experimental tests are given, showing the feasibility of the proposed calibration method.

Vedova, C. O. Della; Duddeck, H.; Praas, H.‐W.

doi: 10.1002/mrc.1260301009pmid: N/A

The 17O chemical shifts of nine sulphinylimines and isocyanates are reported and discussed in terms of electronic and mesomeric effects. Unusually large values were found for the sulphinylimines. The differences in acyl fluorides compared with acyl chlorides are rationalized on the basis of a mesomeric form, implying an increase of the carbonyl bond order.

Wrackmeyer, Bernd; Kupče, Ēriks; Kehr, Gerald; Sebald, Angelika

doi: 10.1002/mrc.1260301010pmid: N/A

Absolute coupling signs of 1J(119Sn, 13C) (<O) and 2J(119Sn, 1H) (<O) in (n5‐C5H5)2Sn (1) and 1J(119Sn, 13C) (<O)2J(119Sn, 13C) (<O) and 3J(119Sn, 1H) (>O) in (n5‐C5Me5)2Sn (2) were determined using advanced direct and indirect two‐dimensional methods. These are based on Ψ‐BIRD techniques and z‐filtering, leading to pseudotriple‐resonance experiments of the type 1H{119Sn(13C)} and 13C{1H(119Sn)}, respectively. A mixed n5‐cyclopentadienyltin(II) amide was prepared and characterized by 1H, 13C, 29Si and 119Sn NMR in solution in the presence of its symmetrical redistribution products 1 and [(Me3Si)2N]2Sn. Solid‐state 119Sn CP/MAS NMR spectra of 1 shows the presence of two tin sites (in accord with the x‐ray analysis) with a 62.2 ppm difference in 119Sn chemical shifts, the mean value being in close agreement with the solution‐state δ119Sn value. The symmetries of the shielding tensor for the two tin sites are different and there appears to be a correlation between this property and the ring centroid—Sn—ring centroid angle α. 13C CP/MAS NMR proved insensitive to the structural differences between the two types of molecules of 1 in the unit cell.