International Translation Centre

There is also a database World Translations Index (WTI) which records translations of literature in all fields of science and technology. It corresponds to the publication World Transindex . Translations covered are those from all languages into Western languages and those from other Western languages into French, Spanish and Portuguese. World Translation Index is available on the European Space Agency Information Service. The database is updated monthly and is produced by the International Translation Centre, Delft, The Netherlands, and the Institut d Information Scientifrque et Technique, Centre National de la Recherche Scientifrque, Paris, France. 

Electronic spectra are almost always treated within the framework of the Bom-Oppenlieimer approxunation [8] which states that the total wavefiinction of a molecule can be expressed as a product of electronic, vibrational, and rotational wavefiinctions (plus, of course, the translation of the centre of mass which can always be treated separately from the internal coordinates). The physical reason for the separation is that the nuclei are much heavier than the electrons and move much more slowly, so the electron cloud nonnally follows the instantaneous position of the nuclei quite well. The integral of equation (BE 1.1) is over all internal coordinates, both electronic and nuclear. Integration over the rotational wavefiinctions gives rotational selection rules which detemiine the fine structure and band shapes of electronic transitions in gaseous molecules. Rotational selection rules will be discussed below. For molecules in condensed phases the rotational motion is suppressed and replaced by oscillatory and diflfiisional motions. 

The operators of discrete rotational groups, best described in terms of both proper and improper symmetry axes, have the special property that they leave one point in space unmoved hence the term point group. Proper rotations, like translation, do not affect the internal symmetry of an asymmetric motif on which they operate and are referred to as operators of the first kind. The three-dimensional operators of improper rotation are often subdivided into inversion axes, mirror planes and centres of symmetry. These operators of the second kind have the distinctive property of inverting the handedness of an asymmetric unit. This means that the equivalent units of the resulting composite object, called left and right, cannot be brought into coincidence by symmetry operations of the first kind. This inherent handedness is called chirality. 

There are three translational dispersion curves and three rotational dispersion curves. In the absence of rotation-translation coupling these vibrations are also distinctly separate. However, external modes and internal modes of the same character will become mixed, the closer they are in frequency the greater will be the mixing. Atomic displacements are no longer controlled by exclusively internal or external forces but by both. Modes of a mostly internal (or mostly external) character now involve displacements of the centre of mass (or deformations of the molecular frame). This is a general feature of all molecular crystals but it is usual to ignore such complications when the frequency of the lowest... 

Generally in molecular beam studies, both beams have comparable velocities and intersect one another at 90°, and thus the CM velocity vector points at a wide angle intermediate between the two beams. Measurement of the displacement of the laboratory angular distribution of products from the centre-of-mass vector enables an estimate of the velocity of the products to be derived. Reaction products have been velocity analysed (e.g. see refs. 8 and 231) and the results support the view that the product relative translational energy is usually within ca. 1 kcal mole of the reactant relative translational energy. Most of the alkali metal reactions studied to date are exothermic, thus the products must be internally excited. It is believed [8] that, for most reactions, the internal excitation consists mainly of vibrational excitation however, the partition of the vibrational energy between, for example, KI and CH3 is as yet unknown. There are a few exceptions, e.g. the K + HBr reaction where KBr is rotationally excited rather than vibrationally excited [8], and the... 

Here, the Ms and Fa are the masses and the energy of the particle indicated by the subscripts, and E and Q are the centre-of-mass energy and the energy transformed from internal to translational motions, respectively. The positive and negative signs in eqn. (75) refer to x = 0° and 180°, respectively. Thus it follows that if product ions are observed at 0 = 0° with energies less than... 

In Table 1 are shown some typical values of these entropy terms. For a standard state of 1 mole/1 the translational entropy is normally about 120-150 J/K/mole this value is not very dependent upon the molecular weight. For the majority of medium-sized molecules the entropy of rotation is about 85-115 J/K/mole and not normally very dependent upon the size and structure of the molecule. The exceptionally small value for water, 44 J/K/mole, is because in this molecule nearly all the mass is concentrated at the centre of gravity and hence the moment of inertia, and thus the entropy, is small. The entropy of internal rotation makes a comparatively small contribution to the total entropy of the molecule and the vibrational contribution is even smaller except for the very low vibrational frequencies. 

The Schrodinger equation which describes the internal motion of a diatomic molecule consisting of n electrons of masses > o and two nuclei with masses M and M, respectively, after the translational motion has been split off (20) is as follows (referred to centre of mass co-ordinates) ... 

Fig. 5.2 A schematic energy diagram J2(K) of the internal and the external molecular vibrations in molecular crystals. Q is the frequency, hS2 the energy and K is the magnitude of the wavevector in a particular direction, e.g. in the direction a. (C = 0 is the centre and K = itja the boundary of the Brillouin zone, with the lattice constant a. P is the usual notation for the centre of the Brillouin zone. MSi is a low-frequency internal molecular oscillation with a small or vanishing dispersion const.). MSi is a high-frequency internal molecular oscillation. All together, there are 3N-6 internal modes N is the number of atoms per molecule. OP is an optical phonon in which whole molecules are excited to carry out translational or libration oscillations whose frequencies are...

The main variation of MC methods is how the perturbing step is done. For a system composed of spherical particles (atoms), the only variables are the centre of mass of each particle, and the trial moves are simple translations of particles. For rigid non-spherical particles, the three rotational degrees of freedom must also be sampled, while for flexible molecules, it is usually also of interest to sample the internal degrees of freedom (conformations, vibrations). The latter can be done in Cartesian coordinates, or selectively in for example only the torsional variables. 

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