Results from Quantum Chemical Calculations

Our discussion concentrates on experimental information providing some insight into the difficulties and limitations of these studies. In places, results from quantum chemical calculations will be invoked for comparison however, a critical analysis of the application of these methods to sulfones and sulfoxides is beyond the scope of this section. As in previous reviews in this series3,6,7, we shall be concerned primarily with the geometrical aspects of molecular structures. 

Third, currently available force fields have not been parameterized to handle non-equilibrium forms, in particular, reaction transition states. Note, however, that there is no fundamental reason why this could not be done (using results from quantum chemical calculations rather than experiment as a basis for parameterization). 

We acknowledge that although thermochemical estimates will occasionally be used in the current study, we forego the results from quantum chemical calculations largely in the name of brevity. We are forced to use benzalaniline (XIII), there being no enthalpy of formation known for its acyclic counterpart CH2=CH-CH=N-CH=CH2, for either of its isomers CH2=CH-CH=CH-CH=NH and CH2=CH-CH=CH-N=CH2, nor an unequivocal measured value for CH2=NH or any of its methylated derivatives (see the summaries of imine thermochemistry [21, 22]). We additionally note that benzalaniline, taken as its trans-isomer like its hydrocarbon counterpart stilbene (XII), is a conventional,... 

The description of intermolecular interactions as it results from quantum chemical calculations of the explicitly interacting species (i.e. a typical supramolecular approach). 

As the signs of the coupling constants in 42 and 254 have been deduced from comparisons with results from quantum-chemical calculations, one cannot be quite sure that the preceding deductions are correct. 

Quantum chemistry has nowadays reached such an advanced level that highly accurate results can be achieved for energies and properties of small- to mediumsized molecules. As stressed in the previous section, the requirements for these high-level calculations are efficient treatment of electron correlation via coupled-cluster theory, basis set extrapolation techniques, incorporation of core correlation, and relativistic as well as vibrational effects together with the use of suitable additivity schemes. Nevertheless, despite all the progress made so far, it is still essential to benchmark the results from quantum chemical calculations, and, as pointed out above, rotational spectroscopy offers such an opportunity. 

---