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141.
A new Pd-catalyzed reaction for the stereoselective synthesis of cyclopentane-fused benzocyclobutenes is described. These transformations likely proceed via carbamate-directed carbopalladation followed by intramolecular C-H activation of an alkylpalladium intermediate. The mechanistic relationship between these transformations and Pd-catalyzed reactions of gamma-(n-Boc-amino)alkenes with aryl bromides that afford pyrrolidines is discussed. Differences in reactivity between Pd-amino and Pd-amido complexes appear to play a key role in the outcome of these transformations. 相似文献
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143.
Smith J Rey G Dickens P Fletcher N Hollenberg L Wolfe J 《The Journal of the Acoustical Society of America》2007,121(1):547-558
Traditional didjeridus have a broad range of bore geometries with many details not immediately apparent to a player, and are therefore suitable for examining the relationship between perceived quality and physical properties. Seven experienced players assessed the overall playing quality of 38 didjeridus that spanned a wide range of quality, pitch, and geometry, as well as 11 plastic cylindrical pipes. The ranking of these instruments was correlated with detailed measurements of their acoustic input impedance spectra. Most significantly, the ranked quality of a didjeridu was found to be negatively correlated with the magnitude of its acoustic input impedance, particularly in the frequency range from 1 to 2 kHz. This is in accord with the fact that maxima in the impedance of the player's vocal tract can inhibit acoustic flow, and consequently sound production, once the magnitude of these impedance maxima becomes comparable with or greater than those of the instrument. This produces the varying spectral peaks or formants in the sound envelope that characterize this instrument. Thus an instrument with low impedance and relatively weak impedance maxima in this frequency range would allow players greater control of the formants in the output sound and thus lead to a higher perceived playing quality. 相似文献
144.
Development of Enantioselective Palladium‐Catalyzed Alkene Carboalkoxylation Reactions for the Synthesis of Tetrahydrofurans 下载免费PDF全文
Brett A. Hopkins Zachary J. Garlets Prof. Dr. John P. Wolfe 《Angewandte Chemie (International ed. in English)》2015,54(45):13390-13392
The Pd‐catalyzed coupling of γ‐hydroxyalkenes with aryl bromides affords enantiomerically enriched 2‐(arylmethyl)tetrahydrofuran derivatives in good yield and up to 96:4 e.r. This transformation was achieved through the development of a new TADDOL/2‐arylcyclohexanol‐derived chiral phosphite ligand. The transformations are effective with an array of different aryl bromides, and can be used for the preparation of products bearing quaternary stereocenters. 相似文献
145.
Daltrop S Kotlicki A Waltham C Wolfe N Elie B Gautier F 《The Journal of the Acoustical Society of America》2012,131(1):837-843
The "gothic" style of harp was popular across most of Europe from the late medieval period to the Renaissance. These harps have a one-piece, longitudinally oriented hardwood soundboard, as opposed to the transverse-oriented bonded softwood structure on a modern instrument. In addition, the one-piece back is flexible, whereas the back of a modern instrument is a rigid molded shell. To study the gothic harp, one was constructed from plans created by the Boston Museum of Fine Arts from a late German model in their collection. The vibrational behaviors of the soundboard and soundbox were measured at various stages of construction. The completed instrument was subjected to modal analysis and radiativity measurements. The sound radiation is dominated by two breathing modes at 188 and 273 Hz, each with strong motion of the back, and modes around 350 Hz. Taken together, these modes function like the A0/T1 resonance pairs seen in the soundboxes of other instruments, and a comparison is made with the guitar. Also observed is that as the frequency increases, radiation is emitted from higher up the soundboard, and from higher soundholes. This feature has been observed in other harps, and is a consequence of the harp family's unique geometry. 相似文献
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147.
J. Waldman T.S. Chang H.R. Fetterman G.E. Stillman C.M. Wolfe 《Solid State Communications》1974,15(8):1309-1312
Recombination radiation from Landau states in impact ionized high-purity GaAs has been observed. The narrow band (~ 3 cm?1) emission has been magnetically tuned from 80–120 cm?1, and used to observe rotational transitions in water vapor. 相似文献
148.
Virginia Wolfe PhD Joanne Long Heather Conner Youngblood Henry Williford Michelle Scharff Olson 《Journal of voice》2002,16(1):52-60
Aerobic instructors frequently experience vocal fatigue and are at risk for the development of vocal fold pathology. Six female aerobic instructors, three with self-reported voice problems and three without, served as subjects. Measures of vocal function (perturbation and EGG) were obtained before and after a 30-minute exercise session. Results showed that the group with self-reported voice problems had greater amounts of jitter, lower harmonic-to-noise ratios, and less periodicity in sustained vowels overall, but no significant differences in measures of perturbation and EGG were found before and immediately after instruction. Measures of vocal parameters showed that subjects with self-reported voice problems projected with relatively greater vocal intensity and phonated for a greater percentage of time across beginning, middle, and ending periods of aerobic instruction than subjects with no reported voice problems. 相似文献
149.
150.
Luis M. Tel Saul Wolfe Imre G. Csizmadia 《International journal of quantum chemistry》1973,7(3):475-490
Nonempirical molecular orbital calculations of the energies of CH3CH (ethylcarbonium ion) and HOCH (hydroxymethylcarbonium ion) as a function of rotation about the C? C or C? O bonds and deviation from coplanarity at the carbonium ion center are reported. As expected, and in agreement with previous work, both carbonium centers are planar and there is no barrier to rotation in the planar ethylcarbonium ion. However, for the planar configuration at carbon, the conjugative interaction between oxygen and carbon produces a barrier to rotation about the C? O bond of HOCH of 19.6 Kcal/mole. When a pyramidal geometry is imposed upon the carbonium ion center of CH3CH, a typical three-fold barrier results. As the deviation from coplanarity increases there is a regular increase in the barrier height (1.72 Kcal/mole at the tetrahedral geometry), but the energy minimum remains at the same position in each case (60°). For HOCH, imposition of a pyramidal geometry on the carbonium ion center causes a change in both rotational barriers. One decreases slightly (from 19.6 to 15.4 Kcal/mole) and the other increases to 30.5 Kcal/mole. There is an accompanying change in the position of the minimum of the rotational potential, from 90° towards the gauche structure. 相似文献