Consequences of the dynamical approach to the nuclear spin relaxation induced by molecular rotations in an external magnetic field

The evolution of two nuclear spins, interacting between themselves and with an external magnetic field, and situated on molecules of a fluid, is treated, under the assumption of the mixing property of molecular rotations, by the method of limit theorems for random differential equations. The limit diffusion behaviour of the system is derived and the results on relaxation times and (non-thermodynamical) equilibrium are obtained and discussed.     

Solvent-Induced Deuterium Isotope Effects in 13C- and 15N-NMR. Spectra of Enaminones

The effect of deuterium on the ¹³C and ¹⁵N chemical shifts of enaminones has been investigated. D/H isotope shifts are reported for neutral and protonated species, i.e., when the isotope is exchanged on the C(2)-, N-, or O-atoms. In cases of slow exchange the isotope shifts were obtained from solutions containing both isotopomers, whereas for fast exchange (acidic solutions) either separate NMR. sample tubes (¹⁵N-NMR.) or coaxial tubes (¹³C-NMR.) were used. In neutral molecules the isotope effects δ_C(D, H) are intrinsic in nature. In acidic solutions, the enaminocarbonyl cations formed exhibit δ_C(D, H)- and δ_N(D, H)-values which are discussed in terms of the proton transfer. The mesomeric character of the cations is reflected by characteristic features in the δ_C(D, H)- and δ_N(D, H)-values, which can be ascribed to isotopic perturbation of resonance. O-Protonation shifts in the ¹⁵N-resonance, observed for the first time, are large and positive (+60 to +76 ppm), in contrast to amides, where the effects are of the same sign but an order of magnitude smaller. Both protonation shifts and solvent-induced isotope effects are discussed in connection with the nucleophilic character of the reactive centers in the enaminone synthon.     

The solution structure of [Cu(aq)]2+ and its implications for rack-induced bonding in blue copper protein active sites

The structure of [Cu(aq)]2+ has been investigated by using full multiple-scattering theoretical (MXAN) analysis of the copper K-edge X-ray absorption (XAS) spectrum and density functional theory (DFT) to test both ideal Td and square-planar four-coordinate, five-coordinate square-pyramidal, and six-coordinate octahedral [Cu(aq)]2+ models. The best fit was an elongated five-coordinate square pyramid with four Cu-O(eq) bonds (2 x 1.98 +/- 0.03 A and 2 x 1.95 +/- 0.03 A) and a long Cu-O(ax) bond (2.35 +/- 0.05 A). The four equatorial ligands were D2d-distorted from the mean equatorial plane by +/-(17 +/- 4) degrees, so that the overall symmetry of [Cu(H2O)5]2+ is C2v. The four-coordinate MXAN fit was nearly as good, but the water ligands (4 x 1.96 +/- 0.02 A) migrated +/-(13 +/- 4) degrees from the mean equatorial plane, making the [Cu(H2O)4]2+ model again D2d-distorted. Spectroscopically calibrated DFT calculations were carried out on the C2v elongate square-pyramidal and D2d-distorted four-coordinate MXAN copper models, providing comparative electronic structures of the experimentally observed geometries. These calculations showed 0.85e spin on Cu(II) and 0.03e electron spin on each of the four equatorial water oxygens. All covalent bonding was restricted to the equatorial plane. In the square-pyramidal model, the electrostatic Cu-O(ax) bond was worth only 96.8 kJ mol(-1), compared to 304.6 kJ mol(-1) for each Cu-O(eq) bond. Both MXAN and DFT showed the potential well of the axial bond to be broad and flat, allowing large low-energy excursions. The irregular geometry and D2d-distorted equatorial ligand set sustained by unconstrained [Cu(H2O)5]2+ warrants caution in drawing conclusions regarding structural preferences from small molecule crystal structures and raises questions about the site-structural basis of the rack-induced bonding hypothesis of blue copper proteins. Further, previously neglected protein folding thermodynamic consequences of the rack-bonding hypothesis indicate an experimental disconfirmation.     

Reductive alkylation of thioureas: a highly practical synthesis of unsymmetrical N,N′-disubstituted thioureas

A highly practical synthesis of unsymmetrical N,N′-disubstituted thioureas by the reductive alkylation of N-monosubstituted thioureas with aldehydes is described. N-Monosubstituted thioureas can in turn be synthesized by the reductive amination of thiourea with an appropriate aldehyde. This reductive alkylation methodology was also extended to carbamates.     

14N and 13C NMR of tautomeric systems of hydroxy-pyridines

¹⁴N NMR is shown to be an efficient method of investigation of the tautomeric equilibria in hydroxy-and related substituted pyridines. Large relative ¹⁴N chemical shifts within tautomeric pairs estimated from N-Me and O-Me derivatives make possible quantitative determinations of the equilibria. Qualitative conclusions may be drawn from ¹⁴N line widths as well as from ¹³C and proton chemical shifts.     

119Sn NMR Spectra of Some Carbohydrate Organotin Derivatives

Abstract

The ¹¹⁹Sn NMR spectra of several sugar-tin derivatives were recorded. The geometric and steric isomers of all of the organotin derivatives studied were easily differentiated by ¹¹⁹Sn NMR. The appropriate ¹¹⁹Sn resonances are: ca - 50 ppm for trans and ?60 ppm for cis vinyltin derivatives (1-3), ca 16 ppm for allyltins 4-6, and ca ?32 ppm for tin-carbinols 9 and 11. When the hydroxyl group in carbinol 9 was converted to an O-acetyl group, the chemical shift of ¹¹⁹Sn was shifted to ?22 ppm.     

Polarity and Conformations of Phosphazacompounds in Solution

Abstract

A series of iminophosphoranes o-X-C₆H₄-N=PPh₃, where X=H(I),Me (II),Et (III),OMe(IV),OEt(V) were investigated by means of dipole moments method. Because of an orientation of aromatic fragment determines intramolecular interactions in the system (benzene ring - double N=P bond), the problem of internal rotation of N-C_ar bond is important. DM (exp.) of molecules (I-V), determined in dioxane are 4.30(I), 4.34(II), 4.53(III), 4.75(IV), 4.79 D(V), respectively.