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951.
Guido F. Smits Maarten C. Krol Pieter N. Van Kampen Cornelis Altona 《Journal of Molecular Structure》1986,139(3-4):247-253
Contrary to expectation, the gauche conformer of 1,2-difluoroethane is more stable than the trans conformer in the gas phase. In order to understand the underlying causes of the “gauche effect”, a complete geometry relaxation was performed for the gauche and trans conformers of 1,2-difluoroethane with the 4-21G, 4-31G and 4-31G** basis sets. The 4-31G** optimized geometry of the gauche conformer compares well with the experimental values obtained from a number of electron-diffraction studies. A correction for the correlation energy, calculated by means of second-order Møller—Plesset perturbation theory with the 6-31G** basis set, proves to be essential to obtain a correct estimate of the energy difference between the gauche and trans conformers 1,2-difluoroethane. 相似文献
952.
Frequency resolution was evaluated for two normal-hearing and seven hearing-impaired subjects with moderate, flat sensorineural hearing loss by measuring percent correct detection of a 2000-Hz tone as the width of a notch in band-reject noise increased. The level of the tone was fixed for each subject at a criterion performance level in broadband noise. Discrimination of synthetic speech syllables that differed in spectral content in the 2000-Hz region was evaluated as a function of the notch width in the same band-reject noise. Recognition of natural speech consonant/vowel syllables in quiet was also tested; results were analyzed for percent correct performance and relative information transmitted for voicing and place features. In the hearing-impaired subjects, frequency resolution at 2000 Hz was significantly correlated with the discrimination of synthetic speech information in the 2000-Hz region and was not related to the recognition of natural speech nonsense syllables unless (a) the speech stimuli contained the vowel /i/ rather than /a/, and (b) the score reflected information transmitted for place of articulation rather than percent correct. 相似文献
953.
954.
20-Deoxoluteone has been synthesized from sclareol. Sclareol was brominated with phosphorus tribromide to form a mixture of
primary allyl bromides, from which, by the malonic synthesis, a mixture of bicyclogeranylgeranylacetic acids was obtained
which was cyclized with fluorosulfonic acid to form a mixture of two diastereomeric δ-lactones. The predominating lactone
was converted by successive reduction with lithium tetrahydroaluminate, oxidation with oxalyl chloride in dimethyl sulfoxide,
reaction with methylmagnesium iodide, and oxidation by the chromium trioxide/pyridine complex into 20-deoxyluteone.
Institute of Chemistry, Moldavian SSR Academy of Science, Kishinev. Translated from Khimiya Prirodnykh Soedinenii, No. 3,
pp. 346–353, May–June, 1990. 相似文献
955.
G. Langouche B. Hlimi O. El Hajjaji G. Marest I. Berkes M. Van Der Heyden M. Tong 《Hyperfine Interactions》1986,28(1-4):849-852
125Te and129I Mössbauer spectra were studied of AuTe2, AuAgTe4, and AuCuTe4. A distribution in hyperfine parameters is needed to obtain a satisfactory fit of the129I spectra, and is interpreted in favor of lattice displacement waves recently observed in these compounds. 相似文献
956.
957.
Pitch perception by cochlear implant subjects 总被引:9,自引:0,他引:9
B Townshend N Cotter D Van Compernolle R L White 《The Journal of the Acoustical Society of America》1987,82(1):106-115
Direct electrical stimulation of the auditory nerve can be used to restore some degree of hearing to the profoundly deaf. Percepts due to electrical stimulation have characteristics corresponding approximately to the acoustic percepts of loudness, pitch, and timbre. To encode speech as a pattern of electrical stimulation, it is necessary to determine the effects of the stimulus parameters on these percepts. The effects of the three basic stimulus parameters of level, repetition rate, and stimulation location on subjects' percepts were examined. Pitch difference limens arising from changes in rate of stimulation increase as the stimulating rate increases, up to a saturation point of between 200 and 1000 pulses per second. Changes in pitch due to electrode selection depend upon the subject, but generally agree with a tonotopic organization of the human cochlea. Further, the discriminability of such place-pitch percepts seems to be dependent on the degree of current spread in the cochlea. The effect of stimulus level on perceived pitch is significant but is highly dependent on the individual tested. The results of these experiments are discussed in terms of their impact on speech-processing strategies and their relevance to acoustic pitch perception. 相似文献
958.
Proactive heuristic procedures for robust project scheduling: An experimental analysis 总被引:1,自引:0,他引:1
Stijn Van de Vonder Erik DemeulemeesterWilly Herroelen 《European Journal of Operational Research》2008
Solution-robust project scheduling is a growing research field aiming at constructing proactive schedules to cope with multiple disruptions during project execution. When stochastic activity durations are considered, including time buffers between activities is a proven method to improve the stability of a baseline schedule. 相似文献
959.
Samuel P. Molesworth Michael J. Van Stipdonk 《Journal of the American Society for Mass Spectrometry》2010,21(8):1322-1328
There is now strong evidence for the existence of macrocyclic isomers of bn+ ions, the formation and subsequent opening of which can lead to loss of sequence information from protonated peptides in
multiple-stage tandem mass spectrometry experiments. In this study, the fragmentation patterns of protonated YARFLG and permuted
isomers of the model peptide were investigated by collision-induced dissociation. Of interest was the potential influence
of the arginine residue, and its position in the peptide sequence, on formation of the presumed macrocyclic b5 ion isomer and potential loss of sequence information. We find that regardless of the sequence position (either internal
or at the N- or C-terminus), only direct sequence ions or ions directly related to fragmentation of the arginine side chain
are observed. 相似文献
960.