Dimers collinear

The vinyliodonium salts 52,55 and 56 exist as head-to-tail dimers in the solid state. The dimeric unit of 52 exhibits nearly equidistant intra- and intermodular P—0--T contacts and has been described as a pseudo-octahedral 12-1-4 system95. The dimeric structures of 55 and 56 are held together by P=0--T contacts collinear with the vinyl ligands, while the fluoroborate and perchlorate ions are nearly collinear with the phenyl ligands140. 

The most satisfactory treatments of hydrogen bonding use either ab initio or semi-empirical molecular orbital theory. The most stable form of the dimer has the configuration shown in Fig. 1 (Shipman et al., 1974 Stillinger and Lemberg, 1975), It is generally assumed, in the absence of information to the contrary, that in water, ice and hydrates one H-atom is collinear with the two oxygen atoms. 

HX molecules contain a single proton but three lone electron pairs on the halogen atom. Since none of these lone pairs is collinear with the HX bond, and the HX dimer does not contain all four atoms on a single axis, it is unlikely that a chain of such molecules will be linear. 

It is of some interest to compare the different structures resulting from linear monomers (HF)2 (bent) and (HCN)2 (collinear). In both dimers, the observed intermolecular separations11 are such that rule 7 is ineffective, even for the collinear structure of (HCN)2, where steric effects might be expected to be important. As already seen in Section 5.2.1 and... 

This proliferation of reaction paths may be subdivided into those which involve the transfer of only one electron (paths 1 and 3) and those in which a second electron jump also occurs (paths 2, 4, 5 and 6). Molecular orbital symmetry favours45 the first electron jump in collisions where the halogen molecule has a roughly collinear orientation with respect to the K2 dimer, but places no requirement on the orientation of the K2 dimer itself. Examination54 of the electronic structure of the incipient KJ XY- complex, suggests that a second electron jump is inhibited when the orientation of the K2 dimer is broadside to the halogen molecule axis and enhanced when the K2 axis is collinear. In particular, chemi-ionization seems most favoured when the KKX Y complex is most nearly in a linear configuration. Thus, the reaction dynamics may be summarized as illustrated in Fig. 10. 

Fig. 9.3. The energy level diagram and schematic representation of the dipoles in the collinear and parallel arrangement of a pair of molecules in a dimer.

Experimental work of various sorts indicates that the dimer, trimer, and probably tetramer of HCN are all fully linear, consistent with what might be expected in terms of the collinearity of each HCN molecule and the N lone pair [210-216]. (There is... 

A parabolic dependence on hydrophobicity and molecular weight was noticed for the enzyme inhibitory activity in QSAR 10. K[ values used for deriving QSAR 10 were measured by inhibiting recombinant single-chain dimeric HIV protease. The same authors reported another statistically better QSAR model based on ClogP and the CMR term, however, C log P and CMR were highly collinear (r = 0.70). QSAR 11 reported for antiviral activity has less predic-... 

Interaction energy components for the water dimer, benzene dimer, and the CI2--OH2 complex. The water dimer is in its equilibrium geometry, the benzene dimer is in the parallel stacked geometry with center-of-mass separation 3.7A, and the Cl2"-OH2 complex Is In a symmetrical complex with all atoms in a plane and Cl-Cl 0 collinear with r(CI-"0)= 2.755 A. All energies are reported InkJmor ... 

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