Isomer structure assignment, response

Table I. Isomer Structure Assignment, Response Factors, and Concentration in 1 1 1 1 Mixture of Aroclor 1242 1248 1254 and 1260.

The interpretation of molecular orbital calculations on conformational isomers is not as straightforward as for molecular mechanics methods. Because MO calculations treat all of the bonding forces of the molecule, the difference between two conformations represents only a small part of the total energy. Furthermore, unlike the molecular mechanics model in which energies are assigned to specific interatomic interactions, the energy of a specific molecular orbital may encompass contributions from a number of intermolecular interactions. Thus, the identification of the structural features responsible for the energy difference between two conformers may be very difficult. 

The influence of the approximations used in the geometry optimization at the correlated level of theory can be crucial for determination of the energy sequence of isomers very close in energy. The calculation of optically allowed spectroscopic patterns for all isomers with comparable stabilities for a given cluster size and comparison with experimental data, obtained at low temperature, usually allows one to identify one isomer which is responsible for the measured features. However, in order to check the reliability of the assignment of the recorded spectrum to a specific cluster structure, it is useful to establish the relation between different structural forms and increasing temperature using ab initio molecular dynamics (cf. Section 5). 

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