排序方式: 共有83条查询结果,搜索用时 93 毫秒
51.
M Stone E P Davis A S Douglas M NessAiver R Gullapalli W S Levine A Lundberg 《The Journal of the Acoustical Society of America》2001,109(6):2974-2982
A new technique, tagged Cine-Magnetic Resonance Imaging (tMRI), was used to develop a mechanical model that represented local, homogeneous, internal tongue deformation during speech. The goal was to infer muscle activity within the tongue from tissue deformations seen on tMRI. Measurements were made in three sagittal slices (left, middle, right) during production of the syllable /ka/. Each slice was superimposed with a grid of tag lines, and the approximately 40 tag line intersections were tracked at 7 time-phases during the syllable. A local model, similar to a finite element analysis, represented planar stretch and shear between the consonant and vowel at 110 probed locations within the tongue. Principal strains were calculated at these locations and revealed internal compression and extension patterns from which inferences could be drawn about the activities of the Verticalis, Hyoglossus, and Superior Longitudinal muscles, among others. 相似文献
52.
K C Kumara Swamy S Kumaraswamy P Kommana 《Journal of the American Chemical Society》2001,123(50):12642-12649
Very short C-H...O, N-H...O, and O-H...O hydrogen bonds have been generated utilizing the cyclic phosphate [CH2(6-t-Bu-4-Me-C6H2O)2]P(O)OH (1). X-ray structures of (i) 1 (unsolvated, two polymorphs), 1...EtOH, and 1...MeOH, (ii) [imidazolium](+)[CH2(6-t-Bu-4-Me-C6H2O)2PO2](-)...MeOH [2], (iii) [HNC5H4-N=N-C5H4NH](2+)[(CH2(6-t-Bu-4-Me-C6H2O)2PO2)2](2-)...4CH3CN...H2O [3], (v) [K, 18-crown-6](+)[(CH2(6-t-Bu-4-Me-C6H2O)2P(O)OH)(CH2(6-t-Bu-4-Me-C6H2O)2PO2)](-)...2THF [4], (vi) 1...cytosine...MeOH [5], (vii) 1...adenine...1/2MeOH [6], and (viii) 1...S-(-)-proline [7] have been determined. The phosphate 1 in both its forms is a hydrogen-bonded dimer with a short O-H...O distance of 2.481(2) [triclinic form] or 2.507(3) A [monoclinic form]. Compound 2 has a helical structure with a very short C-H...O hydrogen bond involving an imidazolyl C-H and methanol in addition to N-H...O hydrogen bonds. A helical motif is also seen in 5. In 3, an extremely short N-H...O hydrogen bond [N...O 2.558(4) A] is observed. Compounds 6 and 7 also exhibit short N-H...O hydrogen bonds. In 1...EtOH, a 12-membered hydrogen-bonded ring motif, with one of the shortest known O-H...O hydrogen bonds [O...O 2.368(4) A], is present. 1...MeOH is a similar dimer with a very short O(-H)...O bond [2.429(3) A]. In 4, the deprotonated phosphate (anion) and the parent acid are held together by a hydrogen bond on one side and a coordinate/covalent bond to potassium on the other; the O-H...O bond is symmetrical and very strong [O...O 2.397(3) A]. 相似文献
53.
A reversed-phase high-performance liquid-chromatographic method for monitoring of reactions involved in process development of a key intermediate of antihypertensive drugs, e.g, doxazosin mesylate, prazosin, alfuzosin, terazosin, etc., has been developed and validated. The HPLC profiles of impurities of 4-amino-2-chloro-6,7-dimethoxyquinazoline were used as fingerprints to follow the synthetic procedures in the manufacturing unit. The separation was accomplished on an Inertsil ODS-3 column with isocratic elution using acetonitrile-ammonium acetate (10 mM; pH 4.0; 50:50 v/v) as mobile phase and a photodiode array detector set at 240 nm at ambient temperature. The method was validated with respect to accuracy, precision, linearity, and limits of detection and quantification. The method could detect the impurities at a level of 0.01 to 0.20 microg/mL and it was found to be suitable not only for monitoring of reactions but also for quality assurance of 4-amino-2-chloro-6,7-dimethoxyquinazoline. 相似文献
54.
K. R. Vishnu Mahesh H. N. Narasimha Murthy B. E. Kumaraswamy N. Raghavendra R. Sridhar M. Krishna Niranjan Pattar Ratna Pal B. S. Sherigara 《Frontiers of Chemistry in China》2011,6(2):153-158
The Sodium Montmorillonite (Na-MMT) is not susceptible to polymer due to its organophilic character and low basal spacing.
The primary objective of this study was to improve Na-MMT platelets separation by organically modifying it with cation and
anion exchanges using Cetyl trimethyl ammonium bromide (CTAB) and Sodium dodecyl sulfate (SDS) respectively. Basal spacing,
presence of functional groups, Zetapotential with particle size analysis and thermal stability of the Organomodified Na-MMT
(OMMT) were characterized using XRD, FTIR, zeta-potential analyzer and TGA respectively. The basal spacing of CTAB modified
OMMTincreased to 19.5 ? from 11.0 ? which corresponds to the basal spacing of Na-MMT. The SDS modified OMMT did not show any
increase in the basal spacing. FTIR spectra of CTAB modified Na-MMT illustrated the attachment of CTAB functional groups to
Na-MMT, while the same was absent in the case of SDS modified Na-MMT. The zeta-potential of Na-MMT shifted from −24.88 mV
to 15.66 mV in the case of CTAB modified Na-MMT and 12.49 mV for SDS modified Na-MMT, indicating a greater surface potential
of the modified nanoclay. The TGA showed greater weight loss for CTAB modified Na-MMT than that for Na-MMT, indicating the
effective Na+ ion exchange with alkyl amines. 相似文献
55.
Diallyltin dibromide reacts with β-keto-aldehydes possessing no aromatic side groups and with (hydroxy) aryl imines to afford the expected homoallyl alcohols or amines respectively. With β-ketoaldehydes having aromatic side groups, skipped or conjugated dienes are obtained depending on whether or not an aqueous work up procedure is used. 相似文献
56.
An efficient enantioselective synthesis of (+)-centrolobine and 5-epi-diospongin-A was achieved by the use of asymmetric transfer hydrogenation (ATH)/tandem Grubbs cross-metathesis/oxy-Michael reaction. Furthermore, this strategy allows for diastereodivergent access to every representative member of the family. 相似文献
57.
Gullapalli Kumaraswamy Duggirala Ramakrishna Kondapalli Santhakumar 《Tetrahedron: Asymmetry》2010,21(5):544-548
A concise enantioselective total synthesis of (+)-epi-muricatacin, a potent cytotoxic agent, is described. A key feature of this protocol is a catalytic asymmetric hydrogenation and a chiral auxiliary mediated intramolecular iodoetherification to ensure a high degree of distereo- and enantiocontrol. 相似文献
58.
First stereoselective total synthesis of nonenolide stagonolide G involving a convergent strategy is described. The key reactions include Keck allylation and Grubbs ring closing metathesis reaction. 相似文献
59.
A carbohydrate derived synthesis of C16-C29 fragment of sorangicin A is described utilizing regioselective epoxide opening, segment-coupled Prins cyclization reaction and cross metathesis as the key steps. 相似文献
60.
K. C. Kumaraswamy Sudha Kumaraswamy Praveen Kommana N. Satish Kumar K. Senthil Kumar 《Journal of Chemical Sciences》2002,114(4):367-377
The compounds [CH2(6-t-Bu-4-Me-C6H2O)2]PCl (1), (OCH2CMe2CH2O)-PCl (2), and [ClPN(t-Bu)]2 (3) have been utilized as precursors in the synthesis of (i) new pentacoordinate phosphorus compounds [e.g. CH2(6-t-Bu-4-Me-C6H2O)2 P(NRR′)(O2C6C14), CH2(6-t-Bu-4-Me-C6H2O)2PX[OC(O-i-Pr)N=N(C(O)O-i-Pr)],(ii) cyclic phosphates and their complexes [e.g. imidazolium+CH2(6-t-Bu-4-Me-C6H2O)2PO2
-.MeOH], (iii) new cycloaddition products [e.g. CH2(6-t-Bu-4-Me-C6H2O)2PC(CO2Me)C(CO2Me)C(O)N, (iv) macrocyclic compounds [e.g. [(t-BuN)P]2[-OCH2CMe2CH2O-]h2] and (v) phosphonates [e.g. (OCH2CMe2CH2O)P (O)CH2C(CN)=CHC5H4FeC5H5]. The synthetic and structural aspects of these new products are discussed. 相似文献