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71.
Shi Y Zhang J Grazier N Stein PD Atwal KS Traeger SC Callahan SP Malley MF Galella MA Gougoutas JZ 《The Journal of organic chemistry》2004,69(1):188-191
N,N'-Disubstituted ketene aminals are bioisosteres of thioureas and are useful building blocks in many synthetic operations. A convenient one-pot synthesis of N,N'-disubstituted ketene aminals from activated methylene compounds and isothiocyanates is described. Most of these aminals exist in rotameric equilibrium around the central C=C bonds in solution, and the rotamers are stabilized by intramolecular hydrogen bonding both in solution and in solid states. 相似文献
72.
Ronald Callahan Kimberly Marshall Robert Rothchild Kerstin Rosmarion 《The Chemical Educator》2001,6(4):227-234
Undergraduate organic chemistry special laboratory projects based upon the synthesis of phencyclone, 1 (a potent Diels-Alder diene), and the preparation of a series of highly hindered Diels-Alder adducts of the phencyclone, were described earlier. Details of the synthesis of an analogue of 1 are presented here as an extension of these projects. The analogue, 3,6-dibromophencyclone, 2, and adducts from a wide range of dienophiles, can be prepared by undergraduate organic chemistry students. These adducts (from 1 or 2) are eminently suitable for student characterization by NMR to examine aspects of hindered rotation, magnetic anisotropy, and dynamic NMR spectroscopy, using modern one- and two-dimensional multinuclear methods with a medium-field instrument (7 T), to observe 1H, 13C, and 19F. Use of 2 effectively doubles the range of potential target compounds for students. The Diels-Alder adducts (and their precursors) have been studied by molecular modeling methods. This present paper describes the reaction of 2 with 4, the dienophile N-(4-dimethylamino-3,5-dinitrophenyl)maleimide (Tuppys maleimide), to form the adduct 5. Compound 5 has been well-characterized by 1D and 2D 1H and 13C NMR, and is illustrative of the wide range of adducts that can be made from 2 by students. The structure of 5, as determined by geometry optimization at the semiempirical (AM-1) level, is included here. 相似文献
73.
NMR methods, including one- and two-dimensional techniques (at 7.05 T) for 1H, 13C and 19F, have been applied to studies of hindered rotations and magnetic anisotropy in some crowded Diels-Alder adducts of phencyclone (1). Symmetrically substituted N-aryl maleimides (2) bearing numerous halogens on the N-aryl ring, were employed as dienophiles to form the target adducts (3). The maleimides included: N-(4-bromo-2,6-difluorophenyl)maleimide (2a); N-(2,3,5,6-tetrafluorophenyl)maleimide (2b); N-(4-bromo-2,3,5,6-tetrafluorophenyl)maleimide (2c); N-(2,3,4,5,6-pentachlorophenyl)maleimide (2d); and N-(2,4,6-tribromophenyl)maleimide (2e). Maleimides (2a-2c) were prepared from the precursor N-aryl maleamic acids (5a-5c). Ambient temperature fluorine-19 NMR of these maleamic acids in d6-acetone showed substantial unusual peak broadening consistent with intermediate exchange rate processes, which may correspond to the N-aryl rotation process. Maleimides (2d) and (2e) were produced in one step from pentachloroaniline or 2,4,6-tribromoaniline, respectively, and maleic anhydride with anhydrous ZnCl2 at ca. 200 °C. For the adducts (3), we observed slow exchange limit spectra on the 1H, 13C, [and 19F, for (3a-3c)] NMR timescales for the rotation of the unsubstituted bridgehead phenyls about the C(sp3)C(sp2) bonds, and for the rotations of the N-aryl rings about the N(sp2)C(aryl sp2) bonds. Ab initio calculations for geometry optimizations at the Hartree-Fock level with 6-31G* (or LACVP*) basis sets were performed for the adducts. We believe that this is the first report of detailed 1H, 13C, and 19F NMR data for a substantial collection of N-aryl maleamic acids, maleimides and their phencyclone adducts bearing multiple fluorines or other halogens directly on the N-aryl ring, together with complementary quantitative geometric parameters from high-level HF/6-31G* (or LACVP*) calculations. 相似文献
74.
Zehong Wan Hongxing Yan Ralph F. Hall Xichen Lin Stefano Livia Tomasz Respondek Katherine L. Widdowson Chongjie Zhu James F. Callahan 《Tetrahedron letters》2009,50(3):370-3479
Chemoselectivity of chlorine versus methylthio (-SCH3), methylsulfinyl (-SOCH3), or methylsulfonyl [-S(O)2CH3] in either functionalization or substitution of pyridopyrimidinone and pyrimidopyrimidinone was studied. Utilization to prepare final targets for accelerated SAR studies via two-dimensional array syntheses has been demonstrated in both systems. 相似文献
75.
The research described in this paper was supported by research grant DE-FG02-86ER250125 of the Applied Mathematical Science subprogram of the Office of Energy Research, U.S. Department of Energy, and National Science Foundation grants DMS-8503350 and DMS-8611574 相似文献
76.
J.P McCaffrey B.T Sullivan J.W Frase D.L Callahan 《Micron (Oxford, England : 1993)》1996,27(6):407-411
In preparing silicon-based semiconductors for transmission electron microscopy (TEM), it has been widely observed that a backlit silicon sample displays a series of colors in the thinnest regions. These colors results from absorption and optical interference of light within the silicon sample and depend upon the type of light source and the silicon sample thickness. These colors can range from deep red for thickness ≥5 μm, through orange and yellow in the thinner regions, to essentially colorless at the thinnest regions. In this work we present the first direct measurement of silicon color versus thickness and discuss the factors that influence this relationship. 相似文献
77.
Janice D. Callahan Stephen W. Sorensen 《The Journal of the Operational Research Society》1991,42(3):227-234
An expert system was desired for a group decision-making process. A highly variable data set from previous groups' decisions was available to simulate past group decisions. This data set has much missing information and contains many possible errors. Classification and regression trees (CART) was selected for rule induction, and compared with multiple linear regression and discriminant analysis. We conclude that CART's decision rules can be used for rule induction. CART uses all available information and can predict observations with missing data. Errors in results from CART compare well with those from multiple linear regression and discriminant analysis. CART results are easier to understand. 相似文献
78.
J. Mass M. Avella J. Jiménez M. Callahan E. Grant K. Rakes D. Bliss B. Wang 《Applied Physics A: Materials Science & Processing》2007,88(1):95-98
We present a study of the visible luminescence in ZnO hydrothermal crystals. The study is carried out on oxygen polarity basal
plane (000-1) plates that are sliced through several distinct growth sectors of the crystal. The plates reveal three distinct
regions that display differences in the luminescence emission, especially in the visible range. These differences should be
related to the variation in the distribution of impurities and native defects in the various growth sectors of hydrothermal
ZnO. CL images also expose the presence of nonradiative recombination centers that are associated with point defects or complexes
of point defects. CL measurements varying the acceleration voltage of the e-beam reveal that the deep levels responsible for
the green luminescence, and in a minor extension those responsible for the yellow-orange luminescence are depleted near the
surface. A discussion about the main hypothesis accounting for such behavior is presented.
PACS 71.55.Gs; 71.55.-i; 78.55.Et; 78.60.Hk 相似文献
79.
Kline JL Glenzer SH Olson RE Suter LJ Widmann K Callahan DA Dixit SN Thomas CA Hinkel DE Williams EA Moore AS Celeste J Dewald E Hsing WW Warrick A Atherton J Azevedo S Beeler R Berger R Conder A Divol L Haynam CA Kalantar DH Kauffman R Kyrala GA Kilkenny J Liebman J Le Pape S Larson D Meezan NB Michel P Moody J Rosen MD Schneider MB Van Wonterghem B Wallace RJ Young BK Landen OL MacGowan BJ 《Physical review letters》2011,106(8):085003
The first soft x-ray radiation flux measurements from hohlraums using both a 96 and a 192 beam configuration at the National Ignition Facility have shown high x-ray conversion efficiencies of ~85%-90%. These experiments employed gold vacuum hohlraums, 6.4 mm long and 3.55 mm in diameter, heated with laser energies between 150-635 kJ. The hohlraums reached radiation temperatures of up to 340 eV. These hohlraums for the first time reached coronal plasma conditions sufficient for two-electron processes and coronal heat conduction to be important for determining the radiation drive. 相似文献
80.
Gengeliczki Z Callahan MP Kabeláč M Rijs AM de Vries MS 《The journal of physical chemistry. A》2011,115(41):11423-11427
We report the structure of clusters of 2,4-diaminopyrimidine with 3,7-dimethylxanthine (theobromine) in the gas phase determined by IR-UV double resonance spectroscopy in both the near-IR and mid-IR regions in combination with ab initio computations. These clusters represent potential alternate nucleobase pairs, geometrically equivalent to guanine-cytosine. We have found the four lowest energy structures, which include the Watson-Crick base pairing motif. This Watson-Crick structure has not been observed by resonant two-photon ionization (R2PI) in the gas phase for the canonical DNA base pairs. 相似文献