Experimental reference data conformational energies

As in the case of the MM2 force field, parameterization of MM3 for amines was based mainly on experimental data with occasional references to ab initio calculations, mainly to evaluate relative conformational energies and derive appropriate torsional parameters. As mentioned above, one notable difference between the two force fields is the removal of lp on sp3 nitrogens from MM3. This simplifies the treatment of vibrational spectra and allows for a realistic treatment of nitrogen inversion which could not be handled by MM2. As usual with MM3, parameterization was aimed at reproducing a variety of molecular properties such as structure, steric energy, dipole moments, moments of inertia, heat of formation and vibrational spectra. A complete list of MM3 parameters for amines is provided in Reference 6. 

TABLE 22. A comparison of rotational barriers and conformational energies (kcal mol-1) for several amino compounds and a single nitro compound between several commonly used force fields and experiment (all experimental and calculated data are taken from Reference 60 unless otherwise noted) die last entry provides the Average Absolute Error (kcal mol-1) between theory and experiment60. Reproduced by permission of John Wiley Sons, Inc. from Ref. 60... 

Fig. 5.16 Comparison of the computed conformational energies and experimentally determined crystal energies for 5-XII (see text for discussion of the generation of these data points). Modification YN is the reference point (E = 0) for both comparisons. (From Yu et al. 2000, by permission.)...

This discussion requires the usual caution. Most important, the calculations refer in general to isolated molecules while the experimental data refer to the crystal or solution. A direct comparison between the two, although frequently made, must therefore be carried out with caution. This is particularly true when the different computed conformations differ little in energy, so that the effect of the environmental forces may be rather drastic. The same applies, a fortiori, to attempts at... 

Table 1.17 Fluorescence energies (eV) at planar and twisted conformations for DMABN in vacuo and in two solvents ( acn indicates acetonitrile and eye cyclohexane). Experimental data refer to fluorescence maxima (the parentheses indicate that such value is very uncertain).

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