Molecular nonrigidity,

Molecular symmetry, and point groups, 53-59 Molecules reactivity of. 203 stereochemically nonrigid, 723-730... 

Figure 2. Forms of the potential wells described in the chapter and the features pertinent to the corresponding molecular models, if and 2% denote, respectively, ensembles of librating and rotating dipoles p denotes a dipole moment VIB refers to the charges 8 of a nonrigid dipole vibrating along the H-bond p(t) denotes a given harmonically changing component of a dipole moment.

In Section VILA a strongly idealized picture was described. The dielectric response of an oscillating nonrigid dipole was found in terms of collective vibrations of two charged particles. Now a more specific picture pertinent to an idealized water structure will be considered. Namely, we shall briefly consider thermal motions of a dipole as (i) pure rotations in Fig. 56b and (ii) pure translations in Fig. 58a. Item (i) presents the major interest for us, since we would like to roughly estimate on the basis of a molecular dynamics form of the absorption band stipulated by rotation of a dipole. Of course, even in terms of a simplified scheme, the internal rotations of a molecule should also be accompanied by its translations, so the Figs. 56a and 56b should somehow interfere. However, in Section IX.B.l we for simplicity will neglect this interference. This assumption approximately holds, since, as will be shown in Section IX.B.2, the mean frequencies of these two types of motion substantially differ. 

The case of 7-methoxycoumarin with no associated nonrigid groups was analyzed for librational motion about the molecular axes using the program THMB... 

The molecular structure is essentially unaffected by the change in crystal structure Structurally nonrigid molecules can be deformed along soft deformational paths by the change in crystal structure... 

It might be more pronounced for nonrigid molecular systems. However, the overall optical activity of the solution might be smaller as consequence of cancellation effects of the chirality transfer of different conformers. 

In the "nonrigid symmetric-top rotors" (such as NH ), the second-order Stark effect is observed under normal circumstances. Indeed, field strengths of the order of 1 600 000 [V/m] are required to bring the interaction into the first-order regime in this case [18]. In contrast, very weak interactions suffice to make the mixed-parity states and appropriate for the description of optically active systems. Parity-violating neutral currents have been proposed as the interaction missing from the molecular Hamiltonian [Eq.(1)] that is responsible for the existence of enantiomers [14,19]. At present, this hypothesis is still awaiting experimental verification. 

Conformational analysis of 1,4,7-trithiacyclononane 10 in the gas phase was done using ab initio molecular orbital calculations at the HF and MP2 levels as well as microwave and photoelectron spectroscopies. The photoelectron spectroscopic data showed evidence for at least two conformations with different ionization energies. Using the calculated photoelectron spectra, the observed sulfur 3p-ionization peaks can be assigned to Cj and U2 conformations. Forty of the observed microwave transitions can be assigned to a Cj symmetry, while additional microwave lines are believed to be due to a nonrigid U2-symmetry conformation <1997PCA9180>. 

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