Space-group frequency 207

C. Pratt Brock and L. L. Duncan. Anomalous space group frequencies for monoalcohols C HmOH. Chem. Mater. 6, 1307 1312 (1994). 

The CSD has been used to classify the occurrence and connectivity of crystal hydrates43 44 (Section 8.6.3), symmetry and space group frequency (as in the sub-database CSDSymmetry), 45 frequencies of low symmetry packing where there is more then one molecule in the asymmetric unit (Section 8.7)46 and the occurrence of polymorphism (Section 8.5),47 CSD-based tables for bond length distributions for organic and coordination compounds have been derived48 49 and efforts are currently underway to develop an automated library of such parameters called Mogul.50... 

Donohue, J. (1985). Revised space group frequencies for organic compounds. Acta Crystallogr. A, 41, 203. [183]... 

Padmaja, N., Ramakumar, S. and Viswamitra, M. A. (1990). Space-group frequencies of proteins and of organic compounds with more than one formula unit in the asymmetric unit. Acta Crystallogr. A, 46, 725-30. [178]... 

One of the attractions of supramolecular chemistry is the extraordinary potential for synthesis of new materials that can be achieved much more rapidly and more effectively than with conventional covalent means. For supramolecular synthesis to advance, it is obviously important to characterize, classify, and analyze structural patterns, space group frequencies, and symmetry operators [118], However, at the same time we also need to bring together this information with the explicit aim of improving and developing supramolecular synthesis - the deliberate combination of different discrete molecular building blocks within periodic crystalline materials. 

In the analysis of space group frequencies, one should be aware of the biases in the CSD. For example, in crystalline oxohydrocarbons a predominance of P2] and P2]2i2] was detected [4], but this is because many of the compounds are derivatives of sugars or other natural products, studied in their naturally occurring, enan-tiomerically pure form, so that the choice of a chiral space group is forced. 

N. Padmaja, S. Ramakumar, and M. A. Viswamitra, Acta Crystallogr., Sect. A, 46, 725 (1990). Space-Group Frequencies of Proteins and of Organic Compounds with More than One Formula Unit in the Asymmetric Unit. 

An analysis of the Cambridge Struetural Database (CSD) for the space group frequency distribution of crystals is shown in Table 4.1. The 13 most... 

Predicted vibration modes for a graphite crystal with the space group (frequencies are determined from Raman and neutron scattering the vibration peak at 870 cm is predicted to exist but not detected in the experiment). (Adapted from McCreery, R.L., Carbon electrodes Structural effects on electron transfer kinetics. In Electroanalytiml Chemistry A Series of Advances, vol. 1, Bard, A.J. [ed.]. New York Marcel Dekker, 1991.)... 

The two exponential tenns are complex conjugates of one another, so that all structure amplitudes must be real and their phases can therefore be only zero or n. (Nearly 40% of all known structures belong to monoclinic space group Pl c. The systematic absences of (OlcO) reflections when A is odd and of (liOl) reflections when / is odd identify this space group and show tiiat it is centrosyimnetric.) Even in the absence of a definitive set of systematic absences it is still possible to infer the (probable) presence of a centre of synnnetry. A J C Wilson [21] first observed that the probability distribution of the magnitudes of the structure amplitudes would be different if the amplitudes were constrained to be real from that if they could be complex. Wilson and co-workers established a procedure by which the frequencies of suitably scaled values of F could be compared with the tlieoretical distributions for centrosymmetric and noncentrosymmetric structures. (Note that Wilson named the statistical distributions centric and acentric. These were not intended to be synonyms for centrosyimnetric and noncentrosynnnetric, but they have come to be used that way.)... 

The IR and Raman frequencies for the alkali tetrathiomolybdates and -tungstates (156) follow in Table 21. All these compounds crystallize in the jS-K2S04 lattice, space group D2 with Z = 4 having anionic site symmetry C (757). The number... 

The vibrational frequencies for compounds of these anions with various cations are given in Table 24. The splitting of the degenerate vibration is only occasionally observed. It is not possible to differentiate between the dsym(MS3) and i3asym(MS3). The same has been found for the solution spectra. All these compounds crystallize in the space group Dlh—Pnma (106, 167). 

The electrical conductivity of CoOP as a function of temperature is shown in Figure 6. Above room temperature the compound exhibits metallic behaviour but coincidental with the development of the superstructure the conductivity falls rapidly with decreasing temperature. Below 250 K CoOp behaves as a semiconductor with an activation energy of meV.74 The conduction has been shown to be frequency dependent below 250 K.75 Thermopower studies have also clearly demonstrated the changeover from metallic behaviour above 300 K. to semiconductor behaviour below 250 K.72 The behaviour of ZnOP is very similar to that of CoOp, with the phase transition from the Cccm to Pccn space group occurring at 278 K. Superstructure formation is complete by about 260 K.77... 

N-(2-acetamido-4-nitropheryl)pyrrolidene (PAN) is dimorphic. The thermodynamically stable form is non-polar (Fig.2), (14). The crystal space group is P21 with two independent molecules per asymmetric unit which are related by a pseudo center of inversion. This crystal does not display measurable frequency doubling generation. 

From the symmetry analysis we know that in the R3>c space group only five modes are active in RS (Alg + 4 g). The experiment on LaA103 [1] clearly shows four Raman active modes and one that has a very low frequency of 33 cm-1 was also observed [9]. Comparing the RS of rhombohedral LaMn03 one should note the strong distinction to RS of LaA103. The main difference is the appearance of a new... 

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