Hydrogen bonding of urea-salicylic acid,U·SA

Urea (U) and salicylic acid (SA) crystallize from aqueous solution as a 11 adduct whose structure shows them to be linked via several weak and one strong hydrogen bonds. The ir spectra of the adduct and its deuterated counterpart have been analyzed and the stretching modes of the various hydrogen bonds identified. The¹H and¹³C nmr. spectra are also interpreted to show that discrete adducts of U·SA persist in solution. On heating, U·SA decarboxylates at a much lower temperature than SA itself.     

Determination of carbon nanotube density by gradient sedimentation

Density gradient centrifugation is a high-resolution technique for the separation and characterization of large molecules and stable complexes. We have analyzed various nanotube structures by preparative centrifugation in sodium metatungstate-water solutions. Bundled, isolated and acid-treated single-walled nanotubes (SWNTs) and multiwall nanotubes (MWNTs) formed sharp bands at well-defined densities. The structure of the material in each band was confirmed by transmission electron microscopy and Raman spectroscopy. Our data suggest respective densities of 1.87, 2.13, 1.74, and 2.1 g/cm(3) for bundled, isolated, and acid-treated SWNTs and MWNTs. These measured results compare well with their calculated densities.     

Tetraethyl stannane: structure, conformations, and orientational order when dissolved in a nematic liquid crystalline solvent

Quantum chemical calculations are used to investigate the structure and the distribution of conformers of tetraethyl stannane, and these are compared with those of tetraethyl methane. The calculations predict that the most abundant conformers in tetraethyl stannane are those with symmetry C1, in contrast to tetraethyl methane, in which the D2d and S4 conformers are dominant. The structural and conformational information on tetraethyl stannane are then used, together with the additive potential model, to explain the finite quadrupolar splittings observed in the deuterium NMR spectrum of tetraethyl stannane dissolved in a nematic liquid crystalline solvent. This analysis emphasizes that to understand the orientational order of a flexible molecule in a liquid crystalline phase it is essential to consider the symmetry of individual conformers rather than that of an average structure.     

The Preparation and Properties of the α,ω-bis(4,4′-Cyanobiphenyloxy)Alkanes: Nematogenic Molecules with a Flexible Core

The first twelve members of the homologous series of α,ω-bis(4,4′-cyanobiphenyloxy) alkanes have been synthesised. The compounds are nematogenic, although the mesophases for the first and third members of the series are monotropic. Both the nematic-isotropic transition temperature and the entropy of transition exhibit a pronounced dependence on the length of the flexible core; this is analogous to that found for main chain thermotropic liquid crystal polymers.     

Robust cross-validation of linear regression QSAR models

A quantitative structure-activity relationship (QSAR) model is typically developed to predict the biochemical activity of untested compounds from the compounds' molecular structures. "The gold standard" of model validation is the blindfold prediction when the model's predictive power is assessed from how well the model predicts the activity values of compounds that were not considered in any way during the model development/calibration. However, during the development of a QSAR model, it is necessary to obtain some indication of the model's predictive power. This is often done by some form of cross-validation (CV). In this study, the concepts of the predictive power and fitting ability of a multiple linear regression (MLR) QSAR model were examined in the CV context allowing for the presence of outliers. Commonly used predictive power and fitting ability statistics were assessed via Monte Carlo cross-validation when applied to percent human intestinal absorption, blood-brain partition coefficient, and toxicity values of saxitoxin QSAR data sets, as well as three known benchmark data sets with known outlier contamination. It was found that (1) a robust version of MLR should always be preferred over the ordinary-least-squares MLR, regardless of the degree of outlier contamination and that (2) the model's predictive power should only be assessed via robust statistics. The Matlab and java source code used in this study is freely available from the QSAR-BENCH section of www.dmitrykonovalov.org for academic use. The Web site also contains the java-based QSAR-BENCH program, which could be run online via java's Web Start technology (supporting Windows, Mac OSX, Linux/Unix) to reproduce most of the reported results or apply the reported procedures to other data sets.     

Better characterization of surface organometallic catalysts through resolution enhancement in proton solid state NMR spectra

Delayed-acquisition methods, namely, echo and constant-time-acquisition approaches, allow a significant improvement in resolution in the proton solid state NMR spectra of surface organometallic catalysts such as [syn-(SiO)Mo(=NAr)(=CH(t)Bu)(CH2(t)Bu)] and [(SiO)Re(C(t)Bu)(=CH(t)Bu)(CH2(t)Bu)] (syn/anti ratio = 1:1). This enables the observation of all of the proton resonances, which is not possible with the simple proton single-pulse technique under magic-angle spinning. For example, the methylene protons of the neopentyl ligands, buried in the large peak associated with all of the methyls in the 1H MAS spectrum, can easily be identified by recording a delayed-acquisition spectrum (resolution enhancement of a factor of 3 is obtained). Moreover, combining constant-time acquisition with heteronuclear carbon-proton correlation spectroscopy also improves the resolution of the 2D HETCOR spectra.