Eric B. Brouwer

Combination of X-ray photoelectron and solid-state 13C nuclear magnetic resonance spectroscopy in the structural characterisation of humic acids

Leonardite humic acid (LHA) and soil humic acid (SHA), purchased from the IHSS, as well as purified humic acid from Aldrich (HA) are characterised by cross-polarisation magic angle spinning (CP-MAS) and Bloch decay 13C NMR spectroscopy and X-ray photoelectron spectroscopy (XPS). For the 13C CP-MAS NMR experiments, the effects of both the sample spinning sidebands and the influence of the cross-polarisation contact time are investigated. The functional groups distribution obtained by CP-MAS NMR is compared to the one resulting from chemical analyses. It is found that CP-MAS NMR overestimates the concentration of CO2H groups for certain humic compounds, whereas the concentration of phenolic groups is in reasonable agreement with the values determined by chemical methods. XPS was used as a complementary technique to understand the overestimation of carboxylic groups by NMR. The results from XPS, a surface characterisation technique, are in good agreement with those measured by elemental analyses of the bulk structure. High resolution C 1s and N 1s spectra show that the overestimation of the CO2H content by NMR is due to the presence of amide groups which give overlapping signals in the CO2H region of the NMR spectra.

t-Butylcalix[4]arene compounds with long chain guests ☆: structures and host–guest interactions

Abstract This crystal structure study examines systematically the effect of guest size and guest substituents on the guest–host interactions in a number of simple p-tert-butylcalix[4]arene host–guest compounds. With increasing chain length, the guest conformation becomes increasingly complex, going from a simple all-trans (C6) to s-shaped (C8) and helical (C12) conformations. For Cl-substituted alkanes, the Cl atom is excluded from the deep cavity, again by the chain taking on complex conformations. However, the Cl atom can be forced into the deep cavity with rigid guests such as 1,4-dichlorobenzene. The results suggest that it is space filling rather than specific interactions that give these compounds stability, and that they should be properly described as clathrates rather than as stoichiometric host–guest materials.

The complex relationship between guest-free polymorphic products and desolvation of p-tert-butylcalix[4]arene inclusion compounds

Depending on the details of the technique employed, desolvation of tBC inclusion compounds leads to either a dense, guest-free polymorph, or a guest-free low density polymorph, the latter also having distinct high and low temperature forms.

t-Butylcalix[4]arene host-guest compounds: Structure and dynamics

Despite the proliferation of structural studies on calixarene host-guest compounds, some important structural details remain obscure. In particular, the ubiquitous disorder of the calixt-butyl group often hinders a detailed determination of the non-bonding interactions between host and guest. We propose and develop a synoptic approach in which several complementary characterization techniques (X-ray diffraction and NMR) provide a powerful means of assessing the interplay of structure and dynamics in calixarene-guest compounds. The simplet-butylcalix[4]arene-guest compounds (guest = toluene, nitrobenzene, cyclohexane,n-pentane, l-chlorobutane) are examined with this approach, and new structural and dynamic features are uncovered.

A reexamination of the low-temperature crystal structure of the p-tert-butylcalix[4]arene–toluene inclusion compound. Differences in spatial averaging with Cu and Mo Kα radiation

The low-temperature crystal structure of the p-tert-butylcalix[4]arene-toluene inclusion compound, C(44)H(56)O(4) x C(7)H(8), exhibits significant guest-induced distortion of the host calixarene molecule. This distortion persists long enough to establish a correlation in the orientation of guests in adjacent host molecules. Remarkably, the space group and unit cell that best describe the structure appear to depend on the wavelength of the incident radiation. This behaviour appears to arise from spatial averaging over the different scattering volumes required to establish the diffraction peaks.

Self-inclusion and paraffin intercalation of the p-tert-butylcalix[4]arene host: a neutral organic clay mimic

Two newly identified structures arise upon the crystallisation of the p-tert-butylcalix[4]arene host molecule from tetradecane: the guest-free, self-included host structure, and a 1:1 host–guest structure in which the paraffin guest is anchored in the bowl-shaped host cavity and intercalates layers of host molecules in a lamellar structure reminiscent of clays.

Some New Halogen-containing Hydrate-formers for Structure I and II Clathrate Hydrates1

Abstract The clathrate hydrate-forming capabilities of 15 hydrofluorocarbon (HFC), fluorocarbon (FC), hydrochlorofluorocarbon (HCFC) and chlorofluorocarbon (CFC) guests are reported. These molecules form stable clathrate hydrates at T > 0°C and have been characterized in detail by X-ray powder diffraction and the measurement of the decomposition temperature, T decomp. The clathrate hydrate structure type depends on the size of the guest molecule and is consistent with the known trend: guest r VDW = 4.1–5.9 A gives structure I, guest d VDW < 4.1 A or = 5.6–6.9 A gives structure II. In addition, the phase diagram of the 1,1-dichloro-1-fluoroethane (R-141b) clathrate hydrate system is reported with a quadruple point of T = 8.4°C and P = 415 mbar. A number of guests give hydrates with relatively high decomposition temperatures (T ∼ 10°C) at low vapour pressures (P < 1 bar) that may be suitable for industrial applications.

Amine guest size and hydrogen-bonding influence the structures of p-tert-butylcalix[4]arene inclusions

A novel structure involving a 2(p-tert-butylcalix[4]arene)·3.5(1,4-butanediamine) inclusion compound shows amine sites both exo and endo to the cavity, with the amine hydrogen-bonding to itself as well as to the host hydroxyl groups; for bulky amines with large pKa values, steric factors are more important than basicity, and the formation of a 1∶1, high-symmetry host–guest structure is preferred over a low-symmetry hydrogen-bonded structure.

Thermodynamic and Resonance Studies of Structural Changes in Crystals

When disordered crystalline materials are cooled, they often show phase transitions to more ordered phases which are accompanied by rearrangements in structure and/or changes in the dynamics of the constituent molecules or molecular fragments. It then becomes necessary to study all of these aspects (structure, dynamics and thermodynamics) in order to understand the system more completely. We give examples of (a) the naphthalene-tetracyanobenzene system, where a disorder observed in the room-temperature structure led to extensive NMR and heat capacity investigations in order to obtain an understanding of the disordered and ordered phases; (b) the t-butylcalix[4]arene toluene clathrate, where a disordered room-temperature structure posed a number of problems regarding the symmetry and dynamic state of the system, and where subsequent NMR and heat-capacity measurements led to a reinvestigation of the structure and a better structural model; (c) a set of xenon cyclodextrin compounds with different hydration states where no structural information was available, and NMR and thermodynamic measurements were used to construct a partial phase diagram.

π-Methyl interactions and p-tert-butylcalix[4]arene-guest stability: NMR and crystallographic studies of cyclohexane and n-pentane inclusion compounds1

Abstract Despite the absence of π(guest)-methyl(host) interactions, p-tert-butylcalix[4]arene-cyclohexane [C12.5H17O, tetragonal, P 4n, a = 12.9446(4), c = 12.8156(4) A, D c = 1.134 g cm−3, Z = 8, 1697 observed data I > 2.5σ(I), R = 0.054] and p-tert-butylcalix[4]arene-n-pentane [C12.25H17O, tetragonal, P 4n, a = 13.0230(19), c = 12.6190(12) A, D c = 1.119 g cm−3, Z = 8, 1641 observed data I > 2.5σ(I), R = 0.048], characterized by NMR and X-ray diffraction, are stable and isoluble compounds.