Valence-bond structures, linear molecular sizes

The separation of chemical species by size exclusion chromatography is more reproducible than any other type of chromatography. Once the SEC columns, the mobile phase (most often a pure solvent like THF or toluene), and the flow rate are selected, the retention volume (or retention time assuming the flow rate does not change) is primarily a function of linear molecular size, which can be obtained from the valence bond structure if the compound is known. Some of the chemical species can interact with the solvent forming complexes with an effective linear size greater than that of the molecule. This causes the expected retention volume, based on "free" molecular structure, to shift to a lower but very reproducible retention volume. Phenols in coal liquids form 1 1 complex with THF (9,10) and carry the effective linear molecular size to increase. As a result phenolic species elute sooner than expected from their... 

The retention volume of several aliphatic, phenolic, heterocyclic, amine and aromatic compounds in THF and toluene have been reported elsewhere ( ). It is clear that aromatic compounds, as expected from their valence bond structures, have smaller linear molecular sizes compared to n-alkanes of similar molecular weight It is expected that most of the con-... 

Cadmium clusters have been treated by Baetzold (47) using EH and CNDO calculations. With atomic valence electron configuration 4dl05s25p, the clusters are calculated to be weakly stable. Linear geometry is more stable than symmetric three-dimensional geometries or even the bulk crystal structure for small Cd clusters. Poor stability is a consequence of the closed atomic 5s shell in Cd. Unstable antisymmetric 5s molecular orbitals are filled in the small clusters, but the amount of bonding by 5p orbitals increases with size. This leads to the trend of increasing stability with size as observed in Table VIII. Compari-... 

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