High accuracy ab initio studies of Li6+, Li6-, and three isomers of Li6

The structures and energetics of Li(6) (+), Li(6) (-) and three isomers of Li(6) are investigated using the coupled-cluster singles, doubles and perturbative triples [CCSD(T)] method with valence and core-valence correlation consistent basis sets of double- to quadruple-zeta quality (cc-pVXZ and cc-pCVXZ, where X=D-Q). These results are compared with qualitatively different predictions by less reliable methods. Our results conclusively show that the D(4h) isomer is the global minimum structure for Li(6). It is energetically favored over the C(5v) and D(3h) structures by about 5.1 and 7.1 kcal mol(-1), respectively, after the inclusion of the zero-point vibrational energy (ZPVE) correction. Our most accurate total atomization energies are 123.2, 117.6, and 115.7 kcal mol(-1) for the D(4h), C(5v), and D(3h) isomers, respectively. Comparison of experimental optical absorption spectra with our computed electronic spectra also indicate that the D(4h) isomer is indeed the most stable structure. The cation, anion, and some higher spin states are investigated using the less expensive cc-pCVDZ basis set. Adiabatic ionization energies and electron affinities are reported and compared with experimental values. Predictions of molecular properties are found to be sensitive to the basis set used and to the treatment of electron correlation.     

The phenomenon of conglomerate crystallization. XX. Spontaneous resolution in coordination compounds. XVIII.

The title compound, [cis-Co(en)₂(NO₂)₂](NO₂) (1), crystallizes in the polar, nonenantiomorphic, monoclinic space group, Cc, with lattice constants:a=9.198(2) Å,b=12.444(2),c=9.963(3), and=96.76(2)°;V=1132.39 ų andd(calc;Z=4) =1.860 g cm^–3. Thus, with NO_2– as the counteranion, [cis-Co(en)₂(NO₂)₂] crystallizes in a heterochiral lattice containing racemic pairs of cations. A total of 2699 data were collected over the range of 4°270°; of these, 1859 (independent and withI3(I)) were used in the structural analysis. Data were corrected for absorption (=15.465 cm^–1) and the relative transmission coefficients ranged from 0.9934 to 0.7112. Refinement was carried out for both lattice polarities and the finalR(F) andR _w (F) residuals were, respectively, 0.0242 and 0.0202 for (–––) and 0.0264 and 0.0243 for (+++). Thus, the former was selected as correct for our specimen.Unlike all previous X-ray diffraction studies of the structural properties of the cation [cis-Co(en)₂(NO₂)₂]⁺, which are found to have a pair of oppositely configured en rings [i.e., () or ()], we find that in1 the cations are in the lowest energy conformation and configuration; i.e., () or (). We attribute this change in configuration to the formation of strong interionic hydrogen bonds between nitrite anion oxygens and the axial—NH₂ hydrogens, which markedly weaken the intermolecular and intramolecular hydrogen bonds between ligand—NO₂ oxygens and the hydrogens of those same amine moieties. Thus, the nitrite anions behave exactly as nitrate anions, except that the hydrogen bonds found here are stronger than those formed by the latter. This is as expected since the negative charge is delocalized over two, instead of three, oxygens.     

Hydrogen Bond Cooperativity and the Three‐Dimensional Structures of Water Nonamers and Decamers

Broadband rotational spectroscopy of water clusters produced in a pulsed molecular jet expansion has been used to determine the oxygen atom geometry in three isomers of the nonamer and two isomers of the decamer. The isomers for each cluster size have the same nominal geometry but differ in the arrangement of their hydrogen bond networks. The nearest neighbor O? O distances show a characteristic pattern for each hydrogen bond network isomer that is caused by three‐body effects that produce cooperative hydrogen bonding. The observed structures are the lowest energy cluster geometries identified by quantum chemistry and the experimental and theoretical O? O distances are in good agreement. The cooperativity effects revealed by the hydrogen bond O? O distance variations are shown to be consistent with a simple model for hydrogen bonding in water that takes into account the cooperative and anticooperative bonding effects of nearby water molecules.     

Mass spectrometric fragmentation patterns of isomeric 5-methyl-3-alkyl-2-alkenylpyrazines

The detailed mass spectral fragmentation pathways of a series of both naturally occurring and synthetic 5-methyl-3-alkyl-2-aIkenylpyrazines have been elucidated with the aid of deuterium labels placed specifically in the alkenyl side-chain. The influence on the fragmentation pattern of the stereochemistry of the alkenyl group as well as the relative placement of the groups on the pyrazine nucleus have also been evaluated. This provides information of some diagnostic value in the assignment of structures by mass spectrometry to new compounds within this class.     

Synthesis of Spirocyclic Ethers by Enantioselective Copper‐Catalyzed Carboetherification of Alkenols

Spirocyclic ethers can be found in bioactive compounds. This copper‐catalyzed enantioselective alkene carboetherification provides 5,5‐, 5,6‐ and 6,6‐spirocyclic products containing fully substituted chiral carbon centers with up to 99 % enantiomeric excess. This reaction features the formation of two rings from acyclic substrates, 1,1‐disubstituted alkenols functionalized with either arenes, alkenes, or alkynes, and clearly constitutes a powerful way to synthesize chiral spirocyclic ethers.     

Sizing large proteins and protein complexes by electrospray ionization mass spectrometry and ion mobility

Mass spectrometry (MS) and ion mobility with electrospray ionization (ESI) have the capability to measure and detect large noncovalent protein-ligand and protein-protein complexes. Using an ion mobility method of gas-phase electrophoretic mobility molecular analysis (GEMMA), protein particles representing a range of sizes can be separated by their electrophoretic mobility in air. Highly charged particles produced from a protein complex solution using electrospray can be manipulated to produce singly charged ions, which can be separated and quantified by their electrophoretic mobility. Results from ESI-GEMMA analysis from our laboratory and others were compared with other experimental and theoretically determined parameters, such as molecular mass and cryoelectron microscopy and X-ray crystal structure dimensions. There is a strong correlation between the electrophoretic mobility diameter determined from GEMMA analysis and the molecular mass for protein complexes up to 12 MDa, including the 93 kDa enolase dimer, the 480 kDa ferritin 24-mer complex, the 4.6 MDa cowpea chlorotic mottle virus (CCMV), and the 9 MDa MVP-vault assembly. ESI-GEMMA is used to differentiate a number of similarly sized vault complexes that are composed of different N-terminal protein tags on the MVP subunit. The average effective density of the proteins and protein complexes studied was 0.6 g/cm(3). Moreover, there is evidence that proteins and protein complexes collapse or become more compact in the gas phase in the absence of water.     

Flow of Newtonian Fluid in Non-Uniform Tubes with Application to Renal Flow: A Numerical Study

In this paper, a numerical method employing a finite difference technique is used for an investigation of viscous, incompressible fluid flow in a tube with absorbing wall and slowly varying cross-section. The effect of fluid absorption through permeable wall is accounted by prescribing flux as a function of axial distance. The method is not restricted by the parameters in the problem such as wave number, permeability parameter, amplitude ratio and Reynolds number. The effects of these parameters on the radial velocity and mean pressure drop are studied and the results are presented graphically. Comparison is also made between the results obtained by perturbation method of solution and present approach.     

Oxidative self-decomposition of the nickel(III) complex of glycylglycyl-L-histidylglycine

Self-decomposition of the nickel(III) doubly deprotonated peptide complex of Gly2HisGly occurs by base-assisted oxidation of the peptide. At < or =p[H+] 7.0, the major pathway is a four-electron oxidation (via 4 Ni(III) complexes) at the alpha carbon of the N-terminal glycyl residue. The product of this oxidation is oxamylglycylhistidylglycine, which hydrolyzes to yield ammonia and oxalylglycylhistidylglycine. Both of these peptide products decompose to give isocyanatoacetylhistidylglycine. A small amount (2%) of oxidative decarboxylation also is observed. In another major pathway above p[H+] 7.0, two Ni(III)-peptide complexes coordinate via an oxo bridge in the axial positions to form a reactive dimer species. This dimer generates two Ni(II)-peptide radical intermediates that cross-link at the alpha carbons of the N-terminal glycyl residues. In 0.13 mM Ni(III)-peptide at p[H+] 10.3, this pathway accounts for 60% of the reaction. The cross-linked peptide is subject to oxidation via atmospheric O2, where the 2,3-diaminobutanedioic acid is converted to a 2,3-diaminobutenedioic acid. The products observed at 相似文献   

Water Triggers Hydrogen‐Bond‐Network Reshaping in the Glycoaldehyde Dimer

Carbohydrates are ubiquitous biomolecules in nature. The vast majority of their biomolecular activity takes place in aqueous environments. Molecular reactivity and functionality are, therefore, often strongly influenced by not only interactions with equivalent counterparts, but also with the surrounding water molecules. Glycoaldehyde (Gly) represents a prototypical system to identify the relevant interactions and the balance that governs them. Here we present a broadband rotational‐spectroscopy study on the stepwise hydration of the Gly dimer with up to three water molecules. We reveal the preferred hydrogen‐bond networks formed when water molecules sequentially bond to the sugar dimer. We observe that the dimer structure and the hydrogen‐bond networks at play remarkably change upon the addition of just a single water molecule to the dimer. Further addition of water molecules does not significantly alter the observed hydrogen‐bond topologies.     

Lipase-mediated regioselective modifications of macrolactonic sophorolipids

Chemoenzymatic synthesis and modification of well-defined macrolactonic sophorolipid (SLML) analogues via a series of successive regioselective de-esterification/transesterification reactions is investigated. Of the lipases screened, Candida antartica lipase- B (Novozyme-435) successfully deacylated the C-6′ acetoxy group of natural and peracylated SLMLs. Subsequent transesterification with acylating agents (esters of fatty acids) was successful only with the C-6′ deacetylated natural SLML providing an avenue to well-defined analogues of varying amphilicity. The macrolactonic motif was essential for enzymatic recognition of the sophorose rings of these complex glycolipids. In the absence of the lactonic motif, the peracylated sophorose rings are not deacylated, rather the carboxyl end of the non-lactonic forms that was preferentially transesterified. All macrolactonic derivatives were characterized by IR, ¹H, ¹³C, ¹H-¹H and ¹H-¹³C NMR spectroscopy, as well as HRMS where applicable.