Three dimensional alignment of molecules using elliptically polarized laser fields

We demonstrate, theoretically and experimentally, that an intense, elliptically polarized, nonresonant laser field can simultaneously force all three axes of a molecule to align along given axes fixed in space, thus inhibiting the free rotation in all three Euler angles. Theoretically, the effect is illustrated through time dependent quantum mechanical calculations. Experimentally, 3, 4-dibromothiophene molecules are aligned with a nanosecond laser pulse. The alignment is probed by 2D ion imaging of the fragments from a 20 fs laser pulse induced Coulomb explosion.     

Fluctuations in flow produced by competition between apparent wall slip and dilatancy

Dense suspensions can exhibit a dramatic stress-induced transition from liquid-like to solid-like behavior. In many materials, the solid-like flow state is characterized by large flow fluctuations and instabilities. Although various experiments have been performed to characterize flow fluctuations, the mechanisms that govern the flow instabilities remain poorly understood. To elucidate these mechanisms, we characterize a system that rapidly fluctuates between two flow states. One of the flow states is dominated by apparent wall slip, and the other is dominated by dilatancy. The dilatant regime occurs at elevated stresses and is associated with reduced wall slip, whereas the wall slip-dominated regime occurs at lower stresses. At stresses that are intermediate between these two regimes, the material fluctuates between the two regimes in a semi-regular fashion. Our analysis of the fluctuations at millisecond timescales shows that fluctuations occur because neither regime is capable of supporting a constant stress in a stable manner. We rationalize our results in terms of the differences in the shear-induced particle pressure between regions that are particle-rich and regions of slip that are particle-depleted.     

A method for C2 acylation of 1,3-indandiones

The 1,3-indandione scaffold is an important structural motif used in the preparation of a large number of industrial chemical and pharmaceutical compounds. However, few approaches allow for the direct C2 acylation on these building blocks. A method was developed using DMAP and EDCI, which is mild in reactivity, covers a diverse range of carboxylic acid acylating agents, is compatible with electron releasing and withdrawing substituents on the 1,3-indandione partner, and performs well in a polar aprotic solvent (for solubility reasons) This method cleanly afforded twenty five different products in yields of 32–96%.     

Squeezing the [Cu-OH...H2O-Cu]3+ bridge by cryptate encapsulation

Treatment of cryptand L(1) with Cu(II) generates a H3O2(-)-bridged dicopper(II) cryptate, 2, where the guest anion has responded to steric constraint by a significant shortening of the O-O distance to 2.325(9) A; computational optimization at the B3LYP/6-31(d) level suggests that the bridging O-H...O H-bond is bent (approximately 157 degrees) but that the barrier to interchange of the bridging H atom is low (<4 kJ mol(-1)). This cryptate, rather than the [Cu2L(1)muCN]3+ species recently claimed to derive from cleavage of the C-C bond of the solvent, is the product of acetonitrile recrystallization of the initially formed reaction product, 1.     

Crystal Engineering of Supramolecular 1,4-Benzene Bisamides by Side-Chain Modification – Towards Tuneable Anisotropic Morphologies and Surfaces

Benzene bisamides are promising building blocks for supramolecular nano-objects. Their functionality depends on morphology and surface properties. However, a direct link between surface properties and molecular structure itself is missing for this material class. Here, we investigate this interplay for two series of 1,4-benzene bisamides with symmetric and asymmetric peripheral substitution. We elucidated the crystal structures, determined the nano-object morphologies and derived the wetting behaviour of the preferentially exposed surfaces. The crystal structures were solved by combining single-crystal and powder X-ray diffraction, solid-state NMR spectroscopy and computational modelling. Bulky side groups, here t-butyl groups, serve as a structure-directing motif into a packing pattern, which favours the formation of thin platelets. The use of slim peripheral groups on both sides, in our case linear perfluorinated, alkyl chains, self-assemble the benzene bisamides into a second packing pattern which leads to ribbon-like nano-objects. For both packing types, the preferentially exposed surfaces consist of the ends of the peripheral groups. Asymmetric substitution with bulky and slim groups leads to an ordered alternating arrangement of the groups exposed to the surface. This allows the hydrophobicity of the surfaces to be gradually altered. We thus identified two leitmotifs for molecular packings of benzene bisamides providing the missing link between the molecular structure, the anisotropic morphologies and adjustable surface properties of the supramolecular nano-objects.     

Local orbitals by minimizing powers of the orbital variance

It is demonstrated that a set of local orthonormal Hartree-Fock (HF) molecular orbitals can be obtained for both the occupied and virtual orbital spaces by minimizing powers of the orbital variance using the trust-region algorithm. For a power exponent equal to one, the Boys localization function is obtained. For increasing power exponents, the penalty for delocalized orbitals is increased and smaller maximum orbital spreads are encountered. Calculations on superbenzene, C(60), and a fragment of the titin protein show that for a power exponent equal to one, delocalized outlier orbitals may be encountered. These disappear when the exponent is larger than one. For a small penalty, the occupied orbitals are more local than the virtual ones. When the penalty is increased, the locality of the occupied and virtual orbitals becomes similar. In fact, when increasing the cardinal number for Dunning's correlation consistent basis sets, it is seen that for larger penalties, the virtual orbitals become more local than the occupied ones. We also show that the local virtual HF orbitals are significantly more local than the redundant projected atomic orbitals, which often have been used to span the virtual orbital space in local correlated wave function calculations. Our local molecular orbitals thus appear to be a good candidate for local correlation methods.     

Magnetic properties of a manganese(III) Chain with monoatomic bridges: catena-MnF(salen)

In the solid state, MnF(salen) forms chains wherein fairly linear fluoride bridges between high-spin Mn(III) centers are observed. We interpret the magnetic properties of these chains by use of the classical Fisher model and by use of the high-temperature expansion approach, as well as by exact matrix diagonalization of the spin Hamiltonian, of model rings. In solution, electron paramagnetic resonance shows the chains to be symmetrically cleaved to monomeric MnF(salen).     

Monitoring the aggregation of single casein micelles using fluorescence microscopy

The aggregation of casein micelles (CMs) induced by milk-clotting enzymes is a process of fundamental importance in the dairy industry for cheese production; however, it is not well characterized on the nanoscale. Here we enabled the monitoring of the kinetics of aggregation between single CMs (30-600 nm in diameter) by immobilizing them on a glass substrate at low densities and subsequently imaging them with fluorescence microscopy. We validated the new method by a quantitative comparison to ensemble measurements of aggregation. Single-particle statistics allowed us to observe for the first time several heterogeneities in CM aggregation. We observed two types of CM growth: a slow increase in the size of CMs and a stepwise increase attributed to interactions between aggregates preformed in solution. Both types of growth exhibit a lag phase that was very heterogeneous between different CMs, suggesting significant differences in their composition or structure. Detailed size histograms of CMs during aggregation also revealed the presence of two distinct subpopulations with different growth amplitudes and kinetics. The dependence of these distinct nanoscale processes/parameters on aggregation conditions is not accessible to bulk measurements that report only ensemble-average values and may prove important to an in-depth understanding of CM aggregation.