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861.
The infrared absorption spectrum of 3,5-dimethyl-4-phenylpyridine in the 400–2000 cm–1 range was obtained. A valence-force model was used to compute the fundamental frequencies and normal vibrational modes. The experimental and computed data were found to be in good agreement. The addition of two methyl groups in the 3,5-positions of the-phenylpyridine molecule reduces conjugation between the benzene and pyridine rings, rendering the molecule noncoplanar. The free rotation of the methyl groups about the C-C bond is retarded by the free electron pair in the nitrogen atom of the pyridine ring. 相似文献
862.
A dielectric study on polyacrylonitrile (PAN), poly-2-hydroxyethyl methacrylate (P-HEMA) and copolymers of acrylonitrile and HEMA in the temperature range 30–150° is reported. The loss peaks occur in the frequency region 0.1–100 kHz at temperatures between 110° and 140 for PAN. In copolymers of acrylonitrile with HEMA, these loss peaks were present although their positions on temperature scale and their peak heights depended on the HEMA content. For P-HEMA no relaxation peak was observed in this range of frequency and temperature. The results for copolymers compared with those for the homopolymers suggest that the loss peaks in PAN are due to molecular motion in amorphous regions. 相似文献
863.
Vijay M. Rao Donald G. Mitchell Robert M. Steiner Matthew D. Rifkin D.Lawrence Burk Jr. David Levy Samir K. Ballas 《Magnetic resonance imaging》1988,6(6):661-667
Since considerable expansion of hematopoietic marrow occurs in patients with sickle cell anemia (SCA), magnetic resonance images of 20 hips in 10 patients with known homozygous SCA were reviewed to determine a) if low signal hematopoietic marrow extended into the femoral capital epiphysis and b) if the MR characteristics of avascular necrosis (AVN) differed depending on the type of epiphyseal marrow. Our results revealed variable epiphyseal marrow type; mixed (fatty and hematopoietic) marrow (42%), fatty marrow (32%), hematopoietic marrow (16%) and hemosiderotic marrow (10%). AVN occurred irrespective of the underlying marrow. Segmental areas of low signal intensity in variable shapes (ring, band, crescent or large homogeneous area) was the most consistent MR manifestation of AVN in SCA. A low signal intensity peripheral rim surrounding a central zone, isointense with epiphyseal marrow on T1 and T2 weighted images, was most frequently observed similar to that described in patients without hemoglobinopathy. The notable difference, however, was of segmental areas within the same femoral head that demonstrated variable central zone signal on T2 weighted images. Further, while an increase in hip joint fluid is commonly seen with both early and advanced AVN in patients without hemoglobinopathy; it was increased in only one hip in patients with SCA. The observed differences in MR characteristics may be due to different pathophysiology of AVN in patients with SCA. 相似文献
864.
Specific incorporation of didehydroreticuline and reticuline into (±)- (+)-, and (-)-tetrahydropalmatines in Cocculus laurifolius and of (R)- and (S)-reticulines into (R)- and (S)-tetrahydropalmatines respectively has been demonstrated. Feeding of[l-3H, 4'-methoxy-14C]reticuline suggested that reticuline was not converted in the plants into didehydroreticuline and racemisation of optically active forms of tetrahydropalmatine did not take place via dehydrotetrahydropalmatine 相似文献
865.
Laurence D. Richards Shiv K. Gupta 《The Journal of the Operational Research Society》1985,36(9):838-843
An information society is one in which informational (or interactional) as opposed to material entities represent the dominant form of social and economic transaction. This paper argues that in such an information-rich environment, the conventional view of the systems approach may no longer be relevant, and may in fact be a serious barrier to the development and implementation of creative management technologies. We present an alternative perspective on the systems approach that emphasizes the primacy of informational concepts as opposed to the primacy of matter/energy concepts. This alternative focuses on interactions, on the logic of change, on circular causality, and on multiple concepts of time. There are important implications for organizational design, managerial control and information systems. 相似文献
866.
Suchitra Gupta 《BIT Numerical Mathematics》1985,25(1):233-241
A class of finite difference schemes for the solution of a nonlinear system of first order differential equations with two point boundary conditions which shares properties with Runge-Kutta processes and gap schemes is discussed. The order conditions for the coefficients of these processes, techniques for reducing these order conditions in number and the symmetry conditions are given. A symmetricA-stable eight order process which has second, fourth and sixth orderA-stable processes embedded in it is given as an example.Research supported in part by the United States Air Force under contract AFOSR-89-0383. 相似文献
867.
A generalized Reynolds equation is derived in the present paper by taking into account the effect of rotation in lubrication problems. The existence of certain fundamental solutions is shown in this extended framework which is not allowed in the classical Reynolds theory. Results concerning the pressure and the load capacity of the resulting bearing system are obtained and interpreted in the respective cases when the film thickness is a linear or an exponential functions of the coordinate along the bearing length. One of the important results is that while the load capacity decreases with increasing values of α for an exponentially inclined slider in the classical Reynolds theory, it increases with increasing values of α in the present context. 相似文献
868.
869.
870.
An inverse optimal control problem is formulated to develop robust control laws for purely oscillatory systems. The optimal control solution requires output feedback with specified constraints, leading to robustness with respect to unmodeled modes and a large class of parameter variations. The robustness properties are proved directly from known properties of control laws resulting from quadratic performance indices. The control laws are useful for poorly damped flexible structures.This research was supported by the Office of Naval Research, Contract No. N00014-77-C-0247. 相似文献