Correlation, effects structural isomer energies

It should be pointed out that the simulations for the Lig cluster were carried out in the framework of the AIMD method based on the HF as well as on the DF (B-LYP) approximation. Comparison of results allows one to draw conclusions about the influence of electron correlation effects on the dynamics of the Lig cluster. The global features concerning the different temperature behavior of the most stable T4 isomer with respect to the other Isomers remain unchanged, as do their dynamical properties. However, the temperature at which the T4 structure isomerizes is lower at the DF-B-LYP level, since in this particular case the correlation effects lowered the energy differences between individual isomers. For other studied neutral and cationic Li clusters this is not the case (cf. Ref. 18). 

Our conclusion then is In most ground stereochemical problems, monodeterminantal SCF-MO theory provides reasonable quantitative estimates of geometrical preferences. In addition, an analysis of the SCF-MO (or EHMO) wavefunction can reveal the reasons for stereoselection, i.e., the operative electronic mechanisms responsible for the preference for one geometry over another. These same mechanisms are responsible for differential Cl corrections of the energies of isomers so that "Cl effects", or, "correlation effects" are reflections of the electronic structures of molecules as grossly reproduced by monodeterminantal MO theory. To put it crudely, a proper analysis of the electronic structure of molecules at the SD MO level automatically predicts the way in which "correlation effects" will come into play ... 

Gas-phase basicities of several substituted benzaldehydes (62 X = o-/m-/p-Me/F, o-j 77 -Cl) have been measured, relative to benzaldehyde or mesitylene as reference bases, over a range of temperatures.101 The tolualdehydes are more basic than benzaldehyde, the halobenzaldehydes less so, following classical aromatic substituent effects. The data also correlate well with solution-based linear-free-energy substituent constants, as well as with theoretical (MNDO) calculations. Some deviations are noteworthy (i) the o-halobenzaldehydes (especially chloro) have higher basicities than predicted, but calculations tend to rale out the hydrogen-bonded isomer (63), which is also contraindicated by a normal A,S value, inconsistent with the expected restriction of— hOH rotation in such a structure (ii) anomalies in the high-temperature behaviour of m-fluorobenzaldehyde in the presence of mesitylene reference base are consistent with a specific catalysed isomerization to the ortho- or para-isomer. 

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