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31.
32.
Gooseman NE O'Hagan D Slawin AM Teale AM Tozer DJ Young RJ 《Chemical communications (Cambridge, England)》2006,(30):3190-3192
The structures of 3-fluoroazetidinium hydrochloride and 3-fluoro-1,5-diazacyclooctane hydrobromide are explored both by X-ray diffraction analysis and DFT calculations, and the conformations of these molecules are shown to be significantly influenced by the through space C-F...N+ interaction. 相似文献
33.
Samson RS Wheeler-Kingshott CA Symms MR Tozer DJ Tofts PS 《Magnetic resonance imaging》2006,24(3):255-263
B1 errors are a problem in magnetization transfer ratio (MTR) measurements because the MTR value is dependent on the amplitude of the magnetization transfer (MT) pulse. B1 errors can arise from radiofrequency (RF) nonuniformity (caused by the RF coil, or skin effect and dielectric resonance in the subject's head) and also from inaccurate setting of the transmitter output when compensating for varying amounts of loading of the RF coil. B1 errors, and hence MTR errors, may be up to 5-10%, a large source of error in quantitative MR measurements. Radiofrequency nonuniformity may cause MTR histograms to be broadened. The dependence of MTR on B1 was modeled using binary spin bath theory, with a continuous wave (CW) approximation. For B1 reductions of up to 20%, normalized plots for different brain tissue types could be approximated by a single line, indicating that a systematic correction could be applied to MTR measurements with a known B1 error, regardless of tissue type. On a 1.5-T scanner with a birdcage coil, MTR was measured in 18 tissue types in five controls. The MT pulse amplitude was reduced in steps from its nominal value by up to 20%. Averaging data over all controls and tissue types resulted in a line fitting mtr(normalized)=0.812b(1normalized)+0.193, where mtr(normalized) is the normalized value of MTR (relative to its value at the nominal B1) and b(1normalized) is the normalized value of B1 (relative to its nominal value). For a 20% reduction in MT pulse amplitude (i.e., b(1normalized)=0.80), the mean MTR value for the 18 tissue types was 7.0 percent units (pu) below the correct value. After correction using the single equation above for all tissue types, all MTR values were within 1.5 pu of their correct value [root mean square (rms) error=0.7 pu]. Magnetization transfer ratio values tended to be slightly overcorrected because the simple linear correction scheme is only an approximation to the true MTR dependence on B1. A B1 field mapping technique was implemented, based on the double angle method (DAM), with fast spin-echo (FSE) readout, and TR=15 s; this took a total of 6 min of imaging time. This was used to quantify B(1) errors and correct MTR maps and histograms. However, the cerebrospinal fluid (CSF) T1 is very long (approximately 4.2 s); thus, to achieve complete longitudinal relaxation (a requirement of the DAM B1 mapping method), an increase in TR and, hence, acquisition time would be required. In general, however, we are not interested in calculating the B1 in the CSF, although it is important that the B1 is determined in partial volume voxels around the CSF. Using our birdcage head coil, whole-brain B1 histograms were found to have full-width half maximums (FWHMs) ranging from just 6.8% to 11.5% of the nominal B1 value. The FSE DAM B1 field mapping technique was shown to be robust, although a longer TR time may be desirable to ensure complete elimination of CSF partial volume errors. The procedure can be applied on any scanner where the Euro-MT sequence is available, or alternatively, where the amplitude of B1 or of the MT pulse can be manually reduced in order to perform this type of "calibration" experiment for the particular MTR sequence used. The MTR is known to be highly dependent on the parameters of the sequence used, in particular, the MT pulse shape, flip angle, duration, and offset frequency, and the repetition time TR' between successive MT pulses. Therefore, correction schemes will differ for different MTR sequences, and new data sets would be required to calculate these different correction schemes. 相似文献
34.
Peach MJ Helgaker T Sałek P Keal TW Lutnaes OB Tozer DJ Handy NC 《Physical chemistry chemical physics : PCCP》2006,8(5):558-562
The recently proposed CAM-B3LYP exchange-correlation energy functional, based on a partitioning of the r operator in the exchange interaction into long- and short-range components, is assessed for the determination of molecular thermochemistry, structures, and second order response properties. Rydberg and charge transfer excitation energies and static electronic polarisabilities are notably improved over the standard B3LYP functional; classical reaction barriers also improve. Ionisation potentials, bond lengths, NMR shielding constants and indirect spin-spin coupling constants are comparable with the two functionals. CAM-B3LYP atomisation energies and diatomic harmonic vibrational wavenumbers are less accurate than those of B3LYP. Future research directions are outlined. 相似文献
35.
E. D. Jones S. W. Tozer T. Schmiedel 《Physica E: Low-dimensional Systems and Nanostructures》1998,2(1-4)
We report the measurement of the pressure dependence for the band-gap energy
and conduction-band mass
for an 80 Å-wide n-type
strained-single-quantum well at 4.2 K for pressures between 0 and 35 kbar and fields up to 30 T. The band-gap energy
, at each pressure, was determined by extrapolating the magnetoluminescence “fan-diagram” to zero magnetic field. The pressure dependence of the band-gap energy was found to be quadratic with a linear term of about 10.3 meV/kbar and a small,
, quadratic contribution. Analyses of the pressure-dependent 4.2 K magnetoluminescence data yield a conduction-band mass logarithmic pressure derivative
. 相似文献
Full-size image
36.
It is demonstrated that there is still scope for improvement in the quality of conventional, semiempirical hybrid exchange-correlation functionals in density-functional theory. A new functional, denoted B97-3, is determined from a fit to eight chemical properties (316 data points). For a series of 25 chemical assessments (850 data points) including 17 assessments and 10 chemical properties absent from the fitting data, B97-3 provides the lowest or joint-lowest mean absolute error on 15 occasions, compared to 6, 5, and 4 occasions for B3LYP, PBE0, and B97-2, respectively [A. D. Becke, J. Chem. Phys. 98, 5648 (1993); M. Ernzerhof and G. E. Scuseria, J. Chem. Phys. 110, 5029 (1999); C. Adamo and V. Barone, J. Chem. Phys. 110, 6158 (1999); P. J. Wilson, T. J. Bradley, and D. J. Tozer, J. Chem. Phys. 115, 9233 (2001)]. Mean absolute errors from B97-3 are, on average, 21%, 18%, and 12% smaller than from these three functionals. The most notable improvements are obtained for classical reaction barriers, where the error reductions are 60%, 54%, and 27%. 相似文献
37.
Background
Amyotrophic lateral sclerosis (ALS) is an age-dependent neurodegenerative disease that causes motor neuron degeneration, paralysis and death. Mutations in Cu, Zn superoxide dismutase (SOD1) are one cause for the familial form of this disease. Transgenic mice expressing mutant SOD1 develop age-dependent motor neuron degeneration, skeletal muscle weakness, paralysis and death similar to humans. The mechanism whereby mutant SOD1 induces motor neuron degeneration is not understood but widespread mitochondrial vacuolation has been observed during early phases of motor neuron degeneration. How this vacuolation develops is not clear, but could involve autophagic vacuolation, mitochondrial permeability transition (MPT) or uncharacterized mechanisms. To determine which of these possibilities are true, we examined the vacuolar patterns in detail in transgenic mice expressing mutant SOD1G93A. 相似文献38.
van Eis MJ van Der Linde BS de Kanter FJ de Wolf WH Bickelhaupt F 《The Journal of organic chemistry》2000,65(14):4348-4354
In contrast to the terminal phosphinidene complex PhPW(CO)(5) (2), which adds to [5]metacyclophane (1) in a 1,4-fashion, dichlorocarbene preferentially adds in a 1,2-fashion to the formal "anti-Bredt" type double bond of the aromatic ring of 1 to afford the norcaradiene 11b, which immediately rearranges to the bridged cycloheptatriene 12b and further by a [1,5] sigmatropic chlorine migration to the isomeric 13b as the first observable product. More slowly, the latter isomerizes via a dissociative mechanism to give 15b. A computational study supports the notion that the [1,5] chlorine migration in the rearrangement 12b --> 13b, for which an activation barrier of 70.2 kJ mol(-)(1) was calculated, is essentially concerted with minor charge separation. In contrast, the analogous [1,5] chlorine migration in the flat model compound 7,7-dichlorocycloheptatriene (12a) displays features of a dissociative pathway. 相似文献
39.
40.
Gibson SE Jones JO Kalindjian SB Knight JD Steed JW Tozer MJ 《Chemical communications (Cambridge, England)》2002,(17):1938-1939
The syntheses and structures of three cyclophanes containing two (Z)-dehydrophenylalanine residues are reported; the length of the tethers between the two amino acid residues is easily altered and changing this parameter has a significant effect on the solid state structures of the cyclophanes. 相似文献