Experimental and calculated geometries

Table 2 Experimental and calculated geometries of N-heterocyclic silylenes using different methods (bond lengths in pm, bond angles in degrees)...

Table I. Comparison of experimental and calculated geometries for CUTiCHa...

Figure 16. Experimental and calculated IR resonance enhanced photodissociation spectra of Fe" (CH4)3 and Fe" (CH4)4. Experimental spectra were obtained by monitoring loss of CH4. Calculated spectra are based on vibrational frequencies and intensities calculated at the B3LYP/ 6-311+G(d,p) level. Calculated frequencies are scaled by 0.96. The calculated spectra have been convoluted with a 10-cm full width at half-maximum (FWHM) Gaussian. The D2d geometries of Fe (CH4)4 are calculated to have very similar energies, and it appears that both isomers are observed in the experiment.

The results, presented in Table 8, show that in most cases the conformer with the lowest steric energy indeed corresponds to the experimentally most favored one. In addition, several molecules containing the N—C—N moiety were retrieved from the Cambridge Structural Database and calculated with the new parameter set. A comparison between MM2 and X-ray geometries (selected structural parameters only) for two conformers of 1,4,5,8-tetraazadecalin (25, 26) is provided in Table 9 and shows good fit between the experimental and calculated data. 

Since steric effects of the F-strain variety are not an inherent property of the acid or base, but a function of the geometry of the adduct, we would not expect our parameters to incorporate this effect. When this data is included in an attempted computer fit, large deviations between experimental and calculated values are observed for these systems. Consequently, in attempting to obtain meaningful parameters, data in... 

A conformational analysis of thiacycloalkanes including 2-thiacyclobu-tane, utilizing molecular mechanics for the calculation of the geometries and energies, was made by Allinger and Hickey. The calculated C—C, C—S, C—S—C, C—C—C, and C—C—S bond parameters for the thietane structure, in comparison to those that were derived from electron diffraction, NMR, and microwave measurement, are reported. The experimental and calculated heat of formation of the thiacyclobutane are 14.49 and 14.58 kcal/mol, respectively. 

Table 7. Experimental and calculated N NMR shieldings in models for P3N5 (6-31G optimized geometries and 6-31G or 6-311(2d,p) GIAO NMR).

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