Acidities in cyclohexanediols enhanced by intramolecular hydrogen bonds

Equilibrium gas-phase acidities of the six isomeric cyclohexanediols were measured in a Fourier transform ion cyclotron resonance mass spectrometer. Although all six cyclohexanediols have the same functional groups and similar structures, the acidities vary over 11 kcal/mol. This large difference is due mostly to the balance between hydrogen bonding and geometric strain. To understand the origins of the acidity differences in more detail, the conformations and energetics of the cyclohexanediols were studied using density functional theory, which gave good agreement with the experimental acidities. Finally, methanol-methoxide and methanol-methanol interactions were used as a model for the hydrogen bonding.     

Ring-closing metathesis-based synthesis of (3R,4R,5S)-4-acetylamino-5-amino-3-hydroxy- cyclohex-1-ene-carboxylic acid ethyl ester: a functionalized cycloalkene skeleton of GS4104

(3R,4R,5S)-4-Acetylamino-5-amino-3-hydroxy-cyclohex-1-ene-carboxylic acid ethyl ester, a functionalized cyclohexene skeleton of GS4104, was diastereoselectively synthesized. A major advantage of this synthesis is the use of readily available L-serine to replace frequently used (-)-shikimic acid or (-)-quinic acid as the starting material. Ring-closing metathesis and diastereoselective Grignard reactions successfully served as the key steps. Absolute configurations of the key intermediates were confirmed by corresponding two-dimensional NMR studies.     

Calculations on the octupolar molecules with enhanced two-photon absorption cross sections based on the Zn (II) and Cu (I) as centers

The equilibrium geometries, electronic structures, as well as one- and two-photon absorption cross sections of a series of octupolar chromophores with Zn(2+) or Cu(+) as coordinate centers and 4,4'-bis(dibutylaminostyryl)-[2,2']-bis(bipyridyl) as ligands have been determined by using B3LYP/6-31G and ZINDO methods. These molecules are designed by controlled combination of two or three bipyridyl ligands with the metal centers. The results show that Zn(2+) is an effective template for the design of octupolar structures which enable it to form tetrahedral and octahedral coordinated complexes; while Cu(+) only exists in a tetrahedral coordinated complex, comparing the tetrahedral complex with Zn(2+) as the center with that of Cu(+) as the center, it is found that the complex with the Cu(+) center is a better two-photon absorption material than the former as far as the transparency/nonlinearity is concerned. Furthermore, for the same metal center of Zn(2+), both one- and two-photon absorptions of the tetrahedral complex are redshifted relative to those of the octahedral complex, is attributed to the spiroconjugation effect in the tetrahedral complex. Our theoretical findings are consistent with recent experimental observations and provide an important foundation for the design of improved transparency-nonlinearity two-photon absorption materials.     

Electric potential induced dissociation of hybridized DNA with hairpin motif immobilized on silicon surface

DNA probes were immobilized on silicon surfaces through ester condensation between the -COOH group on the surface and the terminal -OH group in the oligonucleotide, and the surface density of DNA probes against the concentration of immobilization solution was measured by radioactive labeling. The dissociation of DNA duplex on the surface by an electric potential was studied with the scanning potential hairpin denaturation/dissociation (SPHD). The influence of the stem length in the hairpin probe on the SPHD curve was systematically investigated. It was found that the capability of discrimination on single nucleotide polymorphism (SNP) by a hairpin probe was related to the free energy of formation of the secondary structure in the probe (DeltaG(ss)). In our system, when DeltaG(ss) was around -3 kcal/mol, an optimal recognition of SNP was reached and the SPHD curve was sigmoid. In contrast, the equivalent SPHD curve from a linear probe was exponential-decay alike with a poor discrimination of SNP. The concentration dependent experiments showed good linearity between the melting potential and logarithm of target concentration in the range of 1 x 10(-9) to 5 x 10(-7) M.     

Self-assembled nanowire networks of aryloxy zinc phthalocyanines based on Zn-O coordination

Tetrakis(aryloxy)phthalocyanine (4c) and its Zn congeners (4a and 4b) and Ni congener (4d) were synthesized, and their self-assembling properties in coordinating and non-coordinating solvents were investigated by absorption and fluorescence spectroscopy, matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS), and transmission electron microscopy (TEM). Compounds 4a and 4b in non-coordinating solvents exhibit red-shifted and split Q-bands in absorption spectra even at very low concentrations, suggesting J-aggregate formation. The MALDI-TOF MS for the samples of 4a and 4b prepared from chloroform solutions gives the monomer and aggregate signals. The TEM images of such samples display an indefinite two-dimensional network structure. The aggregates break up into monomers when a coordinating solvent is added to the solution. The driving force for the aggregate formation is proposed to be the complementary coordination of the ether oxygen in the aryloxy groups of one molecule to the core Zn of another molecule of phthalocyanine.     

Chromatographic retention prediction and octanol-water partition coefficient determination of monobasic weak acidic compounds in ion-suppression reversed-phase liquid chromatography using acids as ion-suppressors

Although simple acids, replacing buffers, have been widely applied to suppress the ionization of weakly ionizable acidic analytes in reversed-phase liquid chromatography (RPLC), none of the previously reported works focused on the systematic studies about the retention behavior of the acidic solutes in this ion-suppression RPLC mode. The subject of this paper was therefore to investigate the retention behavior of monobasic weak acidic compounds using acetic, perchloric and phosphoric acids as the ion-suppressors. The apparent octanol-water partition coefficient (K^″ow) was proposed to calibrate the octanol-water partition coefficient (Kow) of these weak acidic compounds, which resulted in a better linear correlation with log kw, the logarithm of the hypothetical retention factor corresponding to neat aqueous fraction of hydroorganic mobile phase. This log K^″ow−log kw linear correlation was successfully validated by the results of monocarboxylic acids and monohydrating phenols, and moreover by the results under diverse experimental conditions for the same solutes. This straightforward relationship not only can be used to effectively predict the retention values of weak acidic solutes combined with Snyder-Soczewinski equation, but also can offer a promising medium for directly measuring Kow data of these compounds via Collander equation. In addition, the influence of the different ion-suppressors on the retention of weak acidic compounds was also compared in this RPLC mode.     

Polyoxometalate Derivatives with Multiple Organic Groups. 3. Synthesis and Structure of Bis(phenyltin) Bis(decatungstosilicate), [(PhSnOH(2))(2)(gamma-SiW(10)O(36))(2)](10)(-)

A new phenyltin tungstosilicate derivative, [(PhSnOH(2))(2)(gamma-SiW(10)O(36))(2)](10)(-) (1), has been prepared by reaction of phenyltin trichloride with K(8)[gamma-SiW(10)O(36)].xH(2)O. The new heteropolyanion was characterized by elemental analysis, infrared spectroscopy, multinuclear NMR, and X-ray crystallography. The crystals of Cs(9)H[(PhSnOH(2))(2)(gamma-SiW(10)O(36))(2)].16H(2)O (Cs salt of 1) are triclinic, space group P&onemacr;, with lattice constants a = 12.401(3) ?, b = 13.832(3) ?, c = 16.313(3) ?, alpha = 96.17(2) degrees, beta = 109.73(2) degrees, gamma = 97.13(2) degrees, V = 2579.9(10) ?, and Z = 1. Anion 1 has a structure of virtual C(2)(h)() symmetry with two phenyltin groups sandwiched between two gamma-SiW(10) groups. Such a structure is different from all previously reported polytungstates derived from [gamma-SiW(10)O(36)](8)(-) lacunary anions.     

Determination of radix ginseng volatile oils at different ages by comprehensive two-dimensional gas chromatography/time-of-flight mass spectrometry

Comprehensive 2-D GC (GC x GC) coupled with TOF MS or flame ionization detector (FID) was employed to characterize and quantify the chemical composition of volatile oil in the radixes of Panax ginseng C. A. Mey. (ginseng) at different ages. Thirty-six terpenoids were tentatively identified based on the MS library search and retention index in a ginseng sample at the age of 3 years. An obvious group-type separation was obtained in the GC x GC-TOF MS chromatogram. The data collected by GC x GC-FID were processed using a principal component analysis (PCA) method to classify the samples at different ages. The compounds responsible for the significant differentiation among samples were defined. It was found that the relative abundances of alpha-cadinol, alpha-bisabolol, thujopsene, and n-hexadecanoic acid significantly rise with the increase in age.     

Systematic investigation on the geometric dependence of the calculated nuclear magnetic shielding constants

We perform a systematic examination on the dependence of the calculated nuclear magnetic shielding constants on the chosen geometry for a selective set of density functional methods of B3LYP, PBE0, and OPBE. We find that the OPBE exchange-correlation functional performs remarkably well when either the optimized geometries or the experimental geometries are used. The popular B3LYP and PBE0 functionals have a clear tendency of deshielding, giving shieldings that are usually too low and shifts that are usually too high, at the experimental geometries. Combined with the Hartree-Fock geometries, however, much improved magnetic constants are obtained for B3LYP and PBE0, due to the compensation effect from the systematic underestimation of bond lengths by the Hartree-Fock method.     

Remote substituent effects on bond dissociation energies of para-substituted aromatic silanes

UB3LYP/6-31G(d) and ROMP2/6-311++G(d,2p) methods were used to calculate the Si-X bond dissociation energies (BDEs) of a number of para-substituted aromatic silanes (4-Y-C(6)H(4)-SiH(2)X, where X = H, F, Cl, or Li). It was found that the substituent effect on the Si-H BDE of 4-Y-C(6)H(4)-SiH(3) was small, as the slope (rho(+)()) of the BDE- regression was only 0.09 kJ/mol. In comparison, the substituent effect on the Si-F BDE of 4-Y-C(6)H(4)-SiH(2)F was much stronger, whose rho(+ )()value was -2.34 kJ/mol. The substituent effect on the Si-Cl BDE of 4-Y-C(6)H(4)-SiH(2)Cl was also found to be strong with a rho(+)() value of -1.70 kJ/mol. However, the substituent effect on the Si-Li BDE of 4-Y-C(6)H(4)-SiH(2)Li was found to have a large and positive slope (+9.12 kJ/mol) against. The origin of the above remarkably different substituent effects on the Si-X BDEs was found to be associated with the ability of the substituent to stabilize or destabilize the starting material (4-Y-C(6)H(4)-SiH(2)X) as well as the product (4-Y-C(6)H(4)-SiH(2)* radical) of the homolysis. Therefore, the direction and magnitude of the effects of Y-substituents on the Z-X BDEs in compounds such as 4-YC(6)H(4)Z-X should have some important dependence on the polarity of the Z-X bond undergoing homolysis. This conclusion was in agreement with that from earlier studies (for example, J. Am. Chem. Soc. 1991, 113, 9363). However, it indicated that the proposal from a recent work (J. Am. Chem. Soc. 2001, 123, 5518) was unfortunately not justified.