An NH3+⃛phenyl interaction in l‐­phenyl­alanyl‐l‐valine

One of the amino H atoms of l ‐phenyl­alanyl‐l ‐valine, C₁₄H₂₀N₂O₃, participates in a rare secondary interaction in being accepted by the aromatic ring of the phenyl­alanine side chain. The phenyl group is also a donor in a weak hydrogen bond to the peptide carbonyl group.     

Theoretical characterization of cyclic thiolated gold clusters

The density functional theory is used to explore structural, energetic, vibrational, and optical properties of thiolated gold clusters (MeSAu)x with x = 2-12. Clusters up to (MeSAu)4 adopt Au-S ring conformations, and crownlike structures are formed for larger sizes. The clusters are essentially polymeric and show convergence in structural and energetic properties at (MeSAu)4. The nature of the Au-S bond is polar covalent with a degree of cyclic electron delocalization. Vibrational analysis reveals characteristic Au-S stretch vibrations at approximately 300 cm(-1). Effects of ligand substitution are studied in the case of the tetramer by comparing the results for methylthiolate with hexylthiolate, benzenethiolate, and glutathionate (GS). The choice of ligand has clear effects on electronic properties. For example, the optical gap is approximately 1.5 eV lower for (GSAu)4 than for (MeSAu)4.     

Fast noniterative orbital localization for large molecules

We use Cholesky decomposition of the density matrix in atomic orbital basis to define a new set of occupied molecular orbital coefficients. Analysis of the resulting orbitals ("Cholesky molecular orbitals") demonstrates their localized character inherited from the sparsity of the density matrix. Comparison with the results of traditional iterative localization schemes shows minor differences with respect to a number of suitable measures of locality, particularly the scaling with system size of orbital pair domains used in local correlation methods. The Cholesky procedure for generating orthonormal localized orbitals is noniterative and may be made linear scaling. Although our present implementation scales cubically, the algorithm is significantly faster than any of the conventional localization schemes. In addition, since this approach does not require starting orbitals, it will be useful in local correlation treatments on top of diagonalization-free Hartree-Fock optimization algorithms.     

Solid-state (14)N MAS NMR of ammonium ions as a spy to structural insights for ammonium salts

The high resolution offered by magic-angle spinning (MAS), when compared to the static condition in solid-state NMR of powders, has been used to full advantage in a (14)N MAS NMR study of some ammonium salts: CH(3)NH(3)Cl, (NH(4))(2)(COO)(2) x H(2)O, (CH(3))(3)(C(6)H(5)CH(2))NCl, (CH(3))(3)(C(6)H(5))NI, [(n-C(4)H(9))(4)N](2)Mo(2)O(7), (NH(4))(2)HPO(4), and NH(4)H(2)PO(4). It is shown that the high-quality (14)N MAS NMR spectra, which can be obtained for these salts, allow determination of the (14)N quadrupole coupling parameters, i.e. C(Q) (the quadrupole coupling constant) and eta(Q) (the asymmetry parameter), with very high precision. In particular, it is shown that precise C(Q), eta(Q) parameters can be determined for at least two different (14)N sites in case the individual spinning-sideband (ssb) intensities arise from a single manifold of ssbs, i.e. the ssbs for the two sites cannot be resolved. This feature of (14)N MAS NMR, which is the first demonstration for manifolds of ssb in MAS NMR without the potential information from a central transition, becomes especially useful at the slow spinning frequencies (nu(r) = 1000-1500 Hz) applied to some of the ammonium salts studied here. The detection of the number of sites has been confirmed by the corresponding crystal structures determined from single-crystal X-ray diffraction (XRD), either in this work for the unknown structure of benzyl trimethylammonium chloride or from reports in the literature. The magnitudes of the (14)N quadrupole coupling constants for the ammonium salts studied here are in the range from C(Q) approximately 20 kHz to 1 MHz while the asymmetry parameters span the full range 0 < or = eta(Q) < or = 1. Clearly, the (14)N quadrupole coupling parameters (C(Q), eta(Q)) for ammonium ions appear highly sensitive toward crystal structure and therefore appreciably more informative for the characterization of ammonium salts in comparison to the isotropic (14)N (or (15)N) chemical shifts.     

Protolytic properties of polyamine wasp toxin analogues studied by 13C NMR spectroscopy

Acid-base properties of the natural polyamine wasp toxin PhTX-433 (1) and seven synthetic analogues [PhTX-343 (2), PhTX-334 (3), PhTX-443 (4), PhTX-434 (5), PhTX-344 (6), PhTX-444 (7), and PhTX-333 (8)], each having four protolytic sites, were characterized by 13C NMR spectroscopy. Nonlinear, multiparameter, simultaneous fit of all chemical shift data obtained from the NMR titration curves yielded macroscopic pKa values as well as intrinsic chemical shift data of all differently protonated macrospecies. Analyses of the chemical shift data demonstrated strong interactions between all four sites and provided information about complex relationships between chemical shift values and protonation state. Deprotonation of fully protonated forms starts at the central amino group of the polyamine moiety, and the extent of this trend depends on the distance to the flanking, protonated amino groups. The pKa1 values of 1-8 are in the range 8.2-9.4. Hence, some of the toxins are incompletely protonated at the pH and ionic strength conditions used for assessment of their interactions with ionotropic glutamate and nicotinic acetylcholine receptors, and the degree of protonation is expected to have pharmacological importance in the ion-channel binding event.     

Silylenes,Silenes, and Disilenes: Novel Silicon‐Based Reagents for Organic Synthesis?

Silylenes, silenes, and disilenes are silicon analogues of carbenes and alkenes. Since the first detection and isolation of these species a few decades ago, focus has been given to their fundamental structure and reactivity properties. Recent developments show that the time is set to exploit their unique chemistry in applied areas. Emerging applications in catalysis and stereoselective synthesis point to a new field within synthetic organosilicon chemistry.     

AC electroosmotic pump with bubble-free palladium electrodes and rectifying polymer membrane valves

We present the design, test and theoretical analysis of a novel micropump. The purpose is to make a pump with large flow rate (approximately 10 microL min-1) and high pressure capacity (approximately 1 bar) powered by a low voltage DeltaV<30 V. The pump is operated in AC mode with an electroosmotic actuator in connection with a full wave rectifying valve system. Individual valves are based on a flexible membrane with a slit. Bubble-free palladium electrodes are implemented in order to increase the range of applications and reduce maintenance.