Atomic triplets

The reactions of reactive ground state atomic triplets with organic compounds have been studied most extensively with oxygen atoms. Direct photolysis of oxygen or nitrogen dioxide yields O P) atoms, but Cvetanovic s method of photosensitizing the decomposition of N20 with excited mercury triplets9 in the presence of various pressures of 

With perfluoroolefins atomic oxygen does not give normal adducts, but instead causes cleavage of the double bond.11 With terminal olefins triplet difluoromethylene apparently is a major product, and its addition to excess olefin to form perfiuorocyclopropanes is quenched by molecular oxygen. 

Sulfur atoms are produced by photolysis of carbonyl sulfide. At low COS pressure and at high pressures of inert gas, the S(1D) atoms initially formed are collisionally deactivated to S(3P) atoms (A = 26.4 kcalmole-1) so that no insertion into C—H bonds is observed.12 Instead the S(3P) atoms add nonstereospecifically to olefins.13 

Selenium (3P) atoms have been produced by photolysis of CSe2 in the presence of excess nitrogen, and they too add to olefins with a selectivity similar to that of a very reactive but electrophilic radical.14 

Recoil UC atoms have been produced by nuclear transformations and allowed to react with ethylene.15 Both Q1/)) and C(3P) atoms are formed, and both add to the double bond and insert into the vinylic C—H bond. The resulting hot singlet adducts relax primarily to allene and methylacetylene, whereas the hot triplet adducts decompose to acetylene or are stabilized as carbenes, which mainly add to more ethylene to yield various C5 products. 

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The structure of a liquid is conventionally described by the set of distributions of relative separations of atom pairs, atom triplets, etc. The fundamental basis for X-ray and neutron diffraction studies of liquids is the observation that in the absence of multiple scattering the diffraction pattern is completely determined by the pair distribution function. 

In general, the total interatomic potential between any pair of atoms is the sum of the pair-wise interaction and the interactions between three atoms (triplets), four atoms (quartets), etc. The problem is pair potentials are by far the easiest to compute, however, their exclusive use gives results that are only semiquantitative (even with ionic solids), accounting for only up to 90 percent of the total cohesive energy in a solid. The three-body term simply cannot be neglected, although the higher-order terms often can be. 

If in a molecule M an atom triplet simultaneously fulfilling the above-mentioned conditions can be found, then the fingerprint of M will highlight the bit i corresponding to this basis triangle. 

Fig. 6.15 Stereo packing diagrams of the red form (upper) and the light stable orange form (lower) of 6-XXVIII. Both structures are plotted on the same reference plane which is the N-C(Et) = N atomic triplet in a molecule located near the centre of the cell.

A set of realistic intermolecular potentials are arguably the most important part of a successful molecular dynamics simulation. For a simple atomic system, the potential energy can be written in terms of the coordinates of individual atoms, atom pairs, atom triplets, etc. ... 

At the beginning of the simulation, a fixed list can be established which contains all indices of atom pairs, atom triplets, and atom quadruples that participate in the bonded interactions. The bonded interactiOTi potentials can be calculated on the basis of this fixed list at each time step instead of checking all the atoms in the system. 

In the superposition-free category, techniques are typically based on exploiting interatomic distances in a way that is independent of molecular orientation and position. One group of superposition-free methods measuring molecular shape was based on atom triplet distances. Bemis and Kuntz [16] devised a method that considered each molecule as the set of its atom triplets. Molecular shape histograms were calculated with the perimeters of the triangle formed by each atom triplet and... 

Ojik, the angle formed by atoms j, i, and k with atom i at the vertex. The three-body term is a penalty function that increases the potential (and repulsive forces) whenever a three atom triplet deviates from an ideal angle. Oy is given as... 

The formalism of collisional TCFs can be conveniently applied to interactions involving polyatomics and solid surfaces, using a many-atom description of the collision partners [30,31,35]. Interaction potentials are decomposed into atom-pair, atom-triplet, etc., terms, and the collision can be understood as a sequence of atom-atom encounters by analogy to the three-atom approaches to atom-diatom collisions [36,37]. It is then possible to describe scattering involving polyatomics, solid surfaces and molecules adsorbed at solid surfaces. 

Alternatively, V can be written as a sum of atom-pair, atom-triplets, etc., terms. 

The 3D similarity measures described thus far are all based on interatomic distance information. Bath et al. " have described two measures for 3D similarity searching that are based on angular information and have compared the effectiveness of searches using these measures with those of the atom-triplet measures described in the previous section. It should be noted that all of these angular and atom-triplet measures make use only of geometrical information, that is, distances and angles between the constituent atoms of the pairs of molecules that are being compared, and do not involve chemical information such as atomic type, polarity, hydrophobicity, etc. 

The efiectiveness of the BNB and NBN measures was assessed by simulated property-prediction experiments. These experiments involved the QSAR data sets studied previously by Pepperrell and Willetti" for the evaluation of distance-based similarity measures and a large set of 6-deoxyhexopyranose carbohydrates, which had previously been classified into 14 shape classes using numerical clustering methods based on torsional dissimilarity coefficients. The comparison encompassed the Bemis-Kuntz and Lederle measures, including not just the atom-triplet but also the atom-pair and atom-quadruplet versions of the former measure. The results were equivocal, in that it was impossible to... 

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