Torsion change

Table 3.25 shows how the main NBO descriptors vary in the perp —anti transition of NH2CH2F. As expected, this torsional change leads to strong increases in overlap (from iS ,0 = 0.01 to, Vno = 0.29) and hyperconjugative stabilization (from 0 to 21.7 kcal mol-1), as well as significant population shifts ( 0.07e) fromnN to ocxf. 

Figure 13. The conformer of 7 showing the potential for intramolecular hydrogen bonding between the hydroxyl substituents at Cl and C3. The C3-03 bond torsion change induced during 3-2IG geometry optimization of this structure was presumably driven by this interaction.

For accurate comparison of relative energies, one must add to the BO-optimized energy the zero-point vibrational energy (ZPVE), which in the harmonic approximation is half the sum of the fundamental frequencies. This correction is most critical for the calculation of activation energies. The contribution of the ZPVE of the mode corresponding most closely to the reaction coordinate is lost completely. Processes that involve breaking of a bond to H are the most seriously affected torsional changes are the least affected. 

To account for the effects of conformational and torsional changes, for example those caused by rotational isomers, one has to introduce the properly weighted conformationally averaged electron density ... 

The photophysical behavior of GFP described above and summarized in the inset in Fig. 5.3 is further complicated by transitions between bright and dark fluorescent states. At the single molecule level these transitions are responsible for the reversible fast blinking and photobleaching that has been observed in single protein experiments [37-39]. The most commonly accepted models used to explain these observations are based on nonradiative relaxation pathways between the excited and ground state that involve torsional changes of the (p and x dihedrals ofthe chromophore shown in Fig. 5.4. 

Figure 4.12. The torsional changes involved in hydrogen-addition.

Nevertheless, the main features of the correlated torsional changes were exhibited all over the intramolecular motions and the complex system strongly resembled a mechanical clockwork. 

Figures 2-5 present structures of the complexes taken from the Monte Carlo simulations. The illustrated structures are the last ones from the different simulations. Though their choice is effectively random, they are good representaticsis of typical structures sampled during the calculations. Relatively little motion was chtained for the host exc t for torsional changes for the ei t methojQ groips. However, the guests rattled in the cavity with some twisting and translations of 2-3 A.

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