Extending the range of neutral N-donor ligands available for metal catalysts: N-[1-alkylpyridin-4(1H)-ylidene]amides in palladium-catalyzed cross-coupling reactions

N-[1-Alkylpyridin-4(1H)-ylidene]amides (PYAs) are a new class of easily prepared, neutral N-donor ligands that share some features in common with N-heterocyclic carbenes. They are strongly electron-donating toward metal centers, and a palladium(II) complex of one of these ligands has been shown to successfully catalyze both the Heck-Mizoroki and Suzuki-Miyaura cross-coupling reactions.     

Transfer of lipid and phase reorganisation in self-assembled liquid crystal nanostructured particles based on phytantriol

The internal structure of dispersed liquid crystal nanostructured particles of the V(2) and H(2) phases, termed cubosomes and hexosomes respectively, is integral to their application in the pharmaceutical, agricultural and food industries. However the nanostructure is susceptible to change upon incorporation of other lipids and hence it is important to understand the potential for interparticle lipid transfer for such particles when they encounter a particle of dissimilar lipid content. Using time resolved synchrotron small angle X-ray scattering, we have investigated the transfer of material between cubosomes composed of phytantriol with three different particle types of dissimilar composition, (i) hexosomes and (ii) emulsified microemulsion composed of phytantriol and vitamin E acetate, and (iii) cubosomes prepared from glycerol monooleate. It was found that material was transferred between the different dispersed nanostructured particles, with the transfer being caused by compositional ripening. Somewhat counter-intuitively the transfer was bidirectional with phytantriol being more rapidly transferred than the minor component vitamin E acetate. The greater lipophilicity of vitamin E acetate supports previous studies suggesting greater mobility for the less lipophilic components, regardless of the more efficient transfer route to achieve uniform composition. When particles comprising lipids with similar lipophilicities were mixed, the transfer was limited and did not achieve completion; a phase change between cubic nanostructures required to achieve complete mixing provides an apparent barrier to further compositional ripening. The conclusions from this study provide additional support to lipid transfer mechanisms, and highlight some subtleties in using dissimilar lipid mixtures in e.g. food applications.     

Spin density distributions in radical anions of heterocyclic amine N-oxides

The unrestricted Hartree-Fock (UHF) method of Snyder and Amos is used to calculate, in the -electron approximation, the spin density distributions in radical anions of heterocyclic amine N-oxides. The computed spin densities are observed to be in good agreement with the experimental values. The computed spin density distribution of the radical anion of pyridine N-oxide is consistent with the greater susceptibility of pyridine N-oxide relative to pyridine to electrophilic nitration. Also, the calculations are consistent with the lower basicity of the N-oxides relative to the parent bases.     

AB initio calculations of 2H and 14N quadrupolar coupling constants in hydrogen bonded dimers

SCF MO calculations at the 6-31G^** level of approximation are reported for ²H and ¹⁴N electric field gradients in HCN?HCN, HCN?HF, and CH₃CN?HF dimers, with emphasis on the configurational dependence of these quantities in (HCN)₂. In comparison with available experimental nuclear quadrupolar coupling constants, the calculated values for the monomers and dimers exhibit an accuracy of ≈ 10%, which is comparable to that of other spectroscopic parameters. The implications of hydrogen bonding for quadrupolar spin-lattice relaxation rates are briefly discussed.     

Experimental investigation of the molecular ions of the xylenes

A mass spectrometric method of distinguishing between molecular ions of the three isomeric xylenes (dimethylbenzenes) was sought, in light of recent findigs that photoexcited ions could be distinguished via measurements of kinetic energy release accompanying expulsion of a methyl radical. Provided the molecular ions are formed with low internal energies, reproducible differences were found between relative intensities of collision induced reactions of higher critical energies, than for methyl expulsion. These differences exist both for collision energies in the kilovolt range (double focusing mass spectrometers) and in the range of a few tens of volts (triple quadrupole instrument). Though statistically significant, these differences were small. The mechanism of isomerization and fragmentation was investigated via isotopic labelling studies and measurements of kinetic energy release. Most of the present findings can be rationalized in terms of the most recent version of established mechanisms for reactions of ionized methylbenzenes.     

An SCF-MS-Xα study of the bonding and nuclear quadrupole coupling in diatomic halogens and interhalogens in the ground X Σ and B Π 0 excited states

SCF-MS-Xα calculations of the electronic structure of diatomic halogens and interhalogens XY (X = I, Br, Cl; Y = I, Br, Cl, F) have been used to investigate the bonding and nuclear quadrupole coupling in these molecules. Calculations have been carried out for the ground X ¹ Σ electronic state, and for the excited B ³ Π₀ state in the case of I₂, Br₂, ICl and IBr. Good agreement (to within 10% in most cases) is obtained between the calculated and observed nuclear quadrupole coupling constants for the molecules in the ground state. For the excited state the agreement is not as good, but the calculation does reproduce the observed decrease in the coupling constants to less than one quarter of their ground state values, and analysis of the contributions to the field gradients clearly shows the reasons for this. The electric dipole moments and electric quadrupole moments of the molecules have also been calculated. However, these prove to be much more strongly dependent on the variables used in the calculation (atomic sphere radii, inclusion of d orbitals). The results of the calculations have also been used to test some of the assumptions made in the Townes and Dailey method of analysis of nuclear quadrupole coupling data.     

Molecular Dynamics Simulations of Energetic Solids

A continuing objective in the area of energetic materials is to reduce sensitivity toward impact and shock. One approach is to develop a better understanding of how factors related to the crystal lattice, e.g., defects, influence the initiation and propagation of detonation. Molecular dynamics is a useful tool for this purpose. This paper presents an overview of molecular dynamics treatments of energetic solids. Some of these have simulated initiation and propagation in idealized systems; others have focused on developing a satisfactory procedure for describing molecular crystals of practical significance. Our emphasis in this discussion is on the progress that has been made along the second lines.     

Hyperaromatic stabilization of arenium ions

Benzene-cis- and trans-1,2-dihydrodiols undergo acid-catalyzed dehydration at remarkably different rates: k(cis)/k(trans) = 4500. This is explained by formation of a β-hydroxycarbocation intermediate in different initial conformations, one of which is stabilized by hyperconjugation amplified by an aromatic no-bond resonance structure (HOC(6)H(6)(+) ? HOC(6)H(5) H(+)). MP2 calculations and an unfavorable effect of benzoannelation on benzenium ion stability, implied by pK(R) measurements of -2.3, -8.0, and -11.9 for benzenium, 1-naphthalenium, and 9-phenanthrenium ions, respectively, support the explanation.     

The synthesis of N-substituted ketimines from oxazirdines

N-Unsubstituted diaryl, alkyl aryl and dialkyl ketimines are prepared in good yield at ambient temperature by the reaction of KOBu^t on oxaziridines having an α-hydrogen atom on the N-alkyl substituent; the mechanism and stereochemistry of the reaction are discussed.