FIR

The demodulation algorithm is very simple the DSP multiplies the received signal by each carrier, and then filters the result using a FIR filter. This kind of digital filter is phase linear, (constant group delay important for the EC combinations). Other filters may be programmed, other demodulation algorithms may be used. 

In the dense interstellar medium characteristic of sites of star fonuation, for example, scattering of visible/UV light by sub-micron-sized dust grains makes molecular clouds optically opaque and lowers their internal temperature to only a few tens of Kelvin. The thenual radiation from such objects therefore peaks in the FIR and only becomes optically thin at even longer wavelengths. Rotational motions of small molecules and rovibrational transitions of larger species and clusters thus provide, in many cases, the only or the most powerfiil probes of the dense, cold gas and dust of the interstellar medium. 

THz. Therefore, for laboratory absorption experiments, a typical FIR detector provides an estimated detection limit (NEP/source power) of 10 with a source output of 20 nW. In general, high-sensitivity bolometers... 

Rotational transition frequencies acquired in the THz region expand upon and complement those acquired in the microwave. Two types of molecules undergo rotational transitions that fall in the FIR molecules witli rotation about an axis having a small moment of inertia, and molecules in high-J states. FIR spectra of the first type of molecules are... 

A number of mixing experiments have therefore been used to generate both pulses and CW THz radiation. Among these, diode-based mixers used as upconvertors (that is, heterodyne spectroscopy m reverse ) have been the workliorse FIR instruments. Two such teclmiques have produced the bulk of the spectroscopic results ... 

Molecules like those presented in figure B 1.4.4 fonn another interesting suite of targets from a TFIz perspective. Such chains can be treated as rigid rods, and as they get longer their lowest bending frequencies move rapidly into the FIR. For example, the lowest frequencies of a variety of chains are as follows ... 

FIR) [25], where the recycle delay is made shorter than 5T and the experiment is carried out under the steady-state rather than equilibrium conditions. A still more time-saving variety, the super-fast inversion recovery (SUFIR) has also been proposed [26]. 

In the fir.st iteration proces.s, the class values of the atoms of the structure. show information already known (the degree of the nodes). Hence Morgan takes the neighboring atoms into account. He considers the environment of an atom by summing class values of all directly adjacent atoms. This process results in a new class value called the extended connectivity (EC) value of the atom. The new EC value expresses indirectly the neighborhood of the adjacent atoms in a second sphere (Figure 2-43). 

In the example, we proceed from the right (rotated molecule) to the left (reference molecule). The first transposition is done with ligands 4 and 2, in order to obtain the fir.st part of the reference. scqttencc 1 2". Then, only a pcrmtttation of ligands 3 and 4 has to be done to obtain the reference matrix on the left-hand side. Thus, in total, we have executed two transpositions (4 2) and (3 4),... 

The fir.-fit line of the file (see Figure 2-110) - the HEADER record - hold.s the moleculc. s classification string (columns 11-50), the deposition date (the date when the data were received by the PDB) in columns 51-59, and the PDB (Dcode for the molecule, which is unique within the Protein Data Bank, in columns 63-66. The second line - the TITLE record - contains the title of the experiment or the analysis that is represented in the entry. The subsequent records contain a more detailed description of the macromolecular content of the entiy (COMPND), the biological and/or chemical source ofeach biological molecule in the entiy (SOURCE), a set ofkeywords relevant to the entiy (KEYWDS). information about the experiment (EXPDTA), a list of people responsible for the contents of this entiy (.AUTHOR), a history of modifications made to this entiy since its release (REVDAT), and finally the primaiy literature citation that describes the experiment which resulted in the deposited dataset ()RNL). 

TIte NCC water model. (After Corongiu G 1992. Molecular Dynamics Simulation fir Liquid Water Using risable and Flexible Potential. International Journal of Quantum Chemistry 42 1209-1235.)... 

We want to derive an expression for the rate of change of the energy with time, dE/dl. Firs we differentiate the kinetic energy term with respect to time ... 

It can thus be seen that most of the variation in the data (85.9%) is explained by the fir principal component, with all but a fraction being explained by the first two componeni These two principal components can be plotted as a scatter graph, as shown in Figu 9.33, suggesting that there does indeed seem to be some clustering of the conformatioi of the five-membered ring in this particular data set. 

The most ambitious approaches to the protein folding problem attempt to solve it from firs principles (ab initio). As such, the problem is to explore the coirformational space of th molecule in order to identify the most appropriate structure. The total number of possibl conformations is invariably very large and so it is usual to try to find only the very lowes energy structure(s). Some form of empirical force field is usually used, often augmente with a solvation term (see Section 11.12). The global minimum in the energy function i assumed to correspond to the naturally occurring structure of the molecule. 

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