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201.
Janet E. Del Bene 《Chemical physics letters》1973,23(2):287-291
Ab initio SCF CI calculations with a minimal STO-3G basis set have been performed on the hydrogen bonded dimers in which H2O is the proton donor to H2CO in its relaxed singlet and triplet n→π* states. Two dimers which are easily interconverted are found in the singet n→π* state with hydrogen bond energies of 1.82 and 1.71 kcal/mole. The equilibrium dimer in the triplet state has a hydrogen bond energy of 2.97 kcal/mole. In both states, hydrogen bond formation occurs at the carbon atom. The structures of the dimers and the nature of the intermolecular surfaces in the regions of hydrogen bond formation are examined. Electron densities and distributions are also discussed. 相似文献
202.
Park DL Whitaker TB Simonson J Morris HF Durr B Njapau H 《Journal of AOAC International》2007,90(4):1036-1041
The number of elevator facilities with laboratories to test shelled corn for aflatoxin on site is increasing. The inherent difficulty in accurately determining the true aflatoxin concentration of a lot of corn may have serious implications. Deviations from the true value are of even greater significance at busy locations where a high throughput is desired. This study was instituted to measure (1) the differences in aflatoxin test results between elevator laboratories and the Louisiana Agricultural Chemistry (LAC) laboratory and (2) the variability in aflatoxin test results associated with sampling, sample preparation, and analysis of shelled corn at such locations. One hundred lots of shelled corn from 10 elevators in Louisiana were analyzed for aflatoxin using the Aflatest method (at elevators and at the LAC laboratory) and high-performance column liquid chromatography (HPLC; LAC laboratory only). Mean aflatoxin levels determined at elevator laboratories were significantly (P < 0.05) lower from those obtained in the LAC laboratory using the Aflatest method. Overall, Aflatest method results were lower than those obtained by HPLC. This difference may be attributed to analyst technical dexterity, difficulty in providing careful attention to detail in a high throughput environment, and/or substandard facilities found at elevators. The total variance was partitioned into the combined sampling plus subsampling variance and analytical variance. The sampling and sample preparation steps accounted for about 91.5% of the total variability. When using the HPLC analytical method, the analytical step contributed only 8.5% to the total variance. 相似文献
203.
Carbon fibre microelectrodes were made by sealing the fibres into glass and by using heat-shrinkable tubing. The electrodes can be coated with mercury by deposition at ?0.9 V vs. SCE from 0.1 M thiocyanate containing 0.05 mM mercury(II) at pH 2.5. Coulometric measurements and square-wave voltammetry were used to establish the properties of the deposit. Conditions for the deposition and stripping of cadmium are outlined. 相似文献
204.
Silver is deposited electrolytically on carbon fibers from acetate buffer at pH 4.6 in the potential range from ?0.3 to ?0.8 V vs. SCE. Chronoamperometric and cyclic voltammetric measurements confirm that the mecanism of deposition is nucleation; the rate is higher than that at large circular glassy carbon electrodes. The silver deposits are more stable, especially with respect to oxidation by air, than are similar deposits of mercury. Preliminary results on codeposition of silver and mercury are reported. 相似文献
205.
206.
High-level ab initio quantum chemical calculations (G4(MP2)//MP2/6-311+G(2df,p)) have been used to examine homodimers of second-row
bases, and to compare the results with those obtained previously for the first-row analogs. The relationship between the binding
energies of the dimers and the proton affinities (PAs) of the bases follows the same pattern as that for the first-row systems,
with the binding energies initially increasing with increasing proton affinity but subsequently decreasing. This may be attributed
to the opposing effects of increased PA on the hydrogen-bond donor and hydrogen-bond acceptor. The binding energies are generally
smaller for the second-row dimers than for the corresponding first-row dimers. There is an increased tendency for asymmetrical
hydrogen bonds in homodimers of the second-row compared with first-row dimers. This may be attributed to the lower electronegativities
of second-row atoms relative to their first-row counterparts, and to the longer internuclear separation between the hydrogen-bonded
second-row atoms. 相似文献
207.
Xiang Zheng Wan Ruixuan Zou Bingjie Qi Xiemin Huang Qing Kumar Shalen Pitman Janet L. Zhou Guohua Song Qinxin 《Analytical and bioanalytical chemistry》2018,410(26):6751-6759
Analytical and Bioanalytical Chemistry - Detection of EGFR mutations in circulating cell-free DNA (cfDNA) is beneficial to monitor the therapeutic effect, tumor progression, and drug resistance in... 相似文献
208.
209.
Dr. Qian Gou Lorenzo Spada Prof. Juan Carlos Lòpez Prof. Jens‐Uwe Grabow Prof. Walther Caminati 《化学:亚洲杂志》2015,10(5):1198-1203
The rotational spectra of four isotopologues of the 1:1 complex between chloromethane and water revealed the presence of only one rotamer in a pulsed jet expansion. The two subunits are linked through two weak hydrogen bonds, O? H???Cl (RH???Cl=2.638(2) Å) and C? H???O (RH???O=2.501(2) Å), forming a five‐membered ring. All transitions display the hyperfine structure due to the 35Cl (or 37Cl) nuclear quadrupole effects. Dynamical features in the spectrum are caused by two large‐amplitude motions. Each component line appears as an asymmetric doublet with a relative intensity ratio of 1:3. The splittings led to the determination of barrier to internal rotation of water around its symmetry axis, V2=320(10) cm?1. Finally, an unexpected small value of the inertial defect (?0.96 uÅ2 rather than ?3.22 uÅ2) allowed the estimation of the barrier to the internal rotation of the CH3 group, V3≈8 cm?1. 相似文献
210.
William D. Fiers Greg J. Dodge Yang Li Janet L. Smith Robert A. Fecik Courtney C. Aldrich 《Chemical science》2015,6(8):5027-5033
Polyketide synthase (PKS) β-processing domains are responsible for much of the stereochemical complexity of polyketide natural products. Although the importance of β-processing domains has been well noted and significantly explored, key stereochemical details pertaining to cryptic stereochemistry and the impact of remote stereogenic centers have yet to be fully discerned. To uncover the inner workings of ketoreductases (KR) and dehydratases (DH) from the tylosin pathway a didomain composed of TylDH3-KR3 was recombinantly expressed and interrogated with full-length tetraketide substrates to probe the impact of vicinal and distal stereochemistry. In vitro product isolation analysis revealed the products of the cryptic KR as d-alcohols and of the DH as trans-olefins. Steady-state kinetic analysis of the dehydration reaction demonstrated a strict stereochemical tolerance at the β-position as d-configured substrates were processed more than 100 times more efficiently than l-alcohols. Unexpectedly, the k
cat/K
M values were diminished 14- to 45-fold upon inversion of remote ε- and ζ-stereocenters. This stereochemical discrimination is predicted to be driven by a combination of allylic A1,3 strain that likely disfavors binding of the ε-epimer and a loss of electrostatic interactions with the ζ-epimer. Our results strongly suggest that dehydratases may play a role in refining the stereochemical outcomes of preceding modules through their substrate stereospecificity, honing the configurational purity of the final PKS product. 相似文献