Stephen J. Heyes

The study of a monocotyledon abscission zone using microscopic, chemical, enzymatic and solid state 13C CP/MAS NMR analyses.

We have investigated distinguishing features in cells of the abscission zone of a monocotyledon fruit, the oil palm Elaeis guineensis. The cell walls of the abscission zone and the subtending mesocarp and pedicel have been analysed by light and transmission electron microscopy, by chemical methods and by solid state 13C CP/MAS NMR spectroscopy. Results show that these abscission zone cells have specific characteristics which include high levels of unmethylated pectin in the walls and an inducible (x35) polygalacturonase enzyme expression. Together these findings help to explain the localised precision of cell separation events.

207Pb NMR chemical shifts of lead tetracarboxylates

207Pb NMR spectroscopy was used to investigate 20 lead(IV) tetracarboxylates both in solution and in the solid state by cross‐polarization magic angle spinning (CP/MAS) NMR. There is generally good correlation between isotropic chemical shifts in the solid state and those in solution. Multiple CP/MAS NMR isotropic resonances are observed for three of the compounds and are interpreted in terms of multiple molecules in the crystallographic asymmetric unit. From an analysis of the range of, and trends in, the NMR chemical shifts, circumstantial evidence is presented for augmentary coordination by Lewis basic ortho‐aromatic substituents of the carboxylate groups. The 207Pb chemical shift anisotropy (CSA) parameters were extracted from analysis of the spinning sideband manifolds of the CP/MAS spectra. The CSA spreads are small in comparison with others reported for 207Pb, and all have positive skew, in contrast to the typical case for lead(II) compounds. Thirteen of the 20 CSA analyses performed show effectively axial CSA tensors. A simple shielding model which rationalizes this typical CSA pattern is presented, based on an analysis of geometric information derived from the two known single‐crystal x‐ray structures of lead(IV) carboxylates. Copyright

Discrimination of chiral guests by chiral channels: variable temperature studies by SXRD and solid state 13C NMR of the deoxycholic acid complexes of camphorquinone and endo-3-bromocamphor.

3alpha,12alpha-dihydroxy-5beta-cholan-24-oic acid (deoxycholic acid DCA) is able to discriminate between the R- and S-enantiomers of camphorquinone and endo-(+)-3-bromocamphor and select only the S-enantiomers from a racemic mixture. DCA forms novel well ordered 1:1 adducts with (1S)-(+)-camphorquinone and (1S)-endo-(-)-3-bromocamphor, both of which have been characterized by single crystal X-ray diffraction (SXRD). When DCA is cocrystallized with (RS)-camphorquinone and (RS)-endo-3-bromocamphor, 1:1 adducts of the S-enantiomers are produced together with crystals of the free racemic guest. In contrast, in the absence of (1S)-(+)-camphorquinone, DCA forms a 2:1 adduct with (1R)-(-)-camphorquinone. In this 2:1 adduct the guest is disordered at ambient temperature and undergoes a phase change in the region 160-130 K similar to that observed for the ferrocene adduct, but with only partial ordering of the guest. The SXRD structure of the low temperature form and the variable temperature (13)C CP/MAS NMR are reported. Cocrystallizing DCA with (1R)-endo-(+)-3-bromocamphor gives the free guest and a glassy solid.

The role of molecular shape and axial symmetry in the solid state molecular dynamics of metal carbonyl compounds included in cyclodextrin cavities

13C CP/MAS NMR spectra of the metal carbonyl complexes Cr(CO)6, Fe(CO)5 and Mn2(CO)10 included in β- and γ-cyclodextrin (CD) cavities have been studied in the temperature range 133 to 423 K. Cr(CO)6 molecules reorient effectively isotropically within γ-cyclodextrin, Fe(CO)5 molecules in β-cyclodextrin reorient effectively isotropically and show rapid axial–equatorial exchange, but Mn2(CO)10 in the super-cage defined by two γ-cyclodextrin molecules shows MAS sidebands at lower temperatures indicative of a more restricted motion such as molecular gyration. The important role of guest and host molecular shape and symmetry match in the reorientational dynamics of the metal carbonyls is discussed.

Modification of MCM-41 via ring opening of a strained[1]ferrocenophane

The walls of the mesoporous silicate MCM-41 can be functionalised with ferrocenyl end groups by reaction with an alkane solution of (1,1′-ferrocenediyl)dimethylsilane; the product is studied by solid-state NMR and EXAFS spectroscopies.

Molecular dynamics in solid bis(η-arene)molybdenum complexes studied by solid-state deuterium nuclear magnetic resonance spectroscopy

Variable-temperature solid-state 2H NMR spectra have been recorded on microcrystalline samples of [Mo(η-C6D6)2], [Mo{η-C6D3(CD3)3-1,3,5}2] and [ZrS2{Mo[η-C6D3(CD3)3-1,3,5]2}0.15]. In the case of [Mo(η-C6D6)2] the spectra are invariant in the range 160–360 K and are consistent with rapid ring rotation at all accessible temperatures. For both [Mo{η-C6D3(CD3)3-1,3,5}2] and [ZrS2{Mo[η-C6D3(CD3)3-1,3,5]2}0.15] the η-C6D3(CD3)3 ligands were found to be static at all temperatures up to 360 K.

Structural investigations of a lead(IV) tetraacetate–pyridine complex

A 1 : 1 crystalline complex of lead(IV) tetraacetate and pyridine (LTA-py) has been prepared. The single-crystal X-ray structure, at 296 and 150 K, establishes the presence of a relatively short Pb-N bond (2.307 A) within an intriguing seven-coordinate lead inner sphere consisting of the pyridine ligand and two bidentate and two monodentate acetate ligands. The pyridine occupies a surprising amount of the available coordination space and has induced a dramatic change in coordination compared to the four chelating acetate ligands found in lead tetraacetate (LTA). Thermal measurements (TGA/DSC) indicate the de-coordination of pyridine and its loss from the solid between 360 and 380 K. (207)Pb CP/MAS NMR spectroscopy also demonstrates the existence of the Pb-N bond through observation of (1)J((207)Pb,(14)N)= 63 Hz and a (207)Pb-(14)N dipolar coupling constant, of 149 Hz. The solid-state (207)Pb NMR parameters are used to give insight into the coordination environment of Pb(iv) in LTA-py. In solution, ligand exchange is rapid on chemical shift and J-coupling time scales. A (207)Pb NMR study of the titration of an LTA solution by pyridine yields a stability constant for LTA-py of K = 1.5 M(-1) and predicts it to have a (207)Pb NMR chemical shift essentially identical to that observed by CP/MAS NMR in the solid state. This correlation between the solid state and solution indicates that the seven-coordinate LTA-py structure found in the crystalline state does persist in solution, and this could further explain why the addition of pyridine has such profound effects on lead(IV) carboxylate-mediated organic reactions. Simulations of exchange-broadened line shapes of (13)C CP/MAS NMR spectra in the temperature regime above 280 K indicate local motion of the pyridine rings in the form of 180 degrees jumps (activation energy 72.5 kJ mol(-1)); these are first such ring flips reported for a coordinated pyridine ligand.

Structure and dynamics of cobaltocene intercalated in tantalum disulphide investigated by 2H n.m.r. spectroscopy

2 H N.m.r. spectroscopy reveals the existence of two orientations of the cobaltocenium ion in the [Co(η-C5H5)2]1/4TaS2 intercalate, with different dynamic properties in each orientation.