1.2- Difluoroethane: conformation

Two wider ranging, more systematic investigations of conformational dependence have since been performed to establish whether the conformational sensitivity noted in the above PECD smdies may generally provide a means for identifying and distinguishing gas-phase structure of suitable chiral species. The B-spline method has been applied to the model system (l/f,2f )-l,2-dibromo-l,2-dichloro-l,2-difluoroethane [60]. Rotation around the C C bond creates three stable conformational possibilities for this molecule to adopt. The results for both core and valence shell ionizations reaffirm an earlier conclusion a and p are almost unaffected by the rotational conformation adopted, whereas the PECD varies significantly. Eor the C Ij ionization to show any sensitivity at aU to the relative disposition of the halogen atoms further reinforces the point made previously in connection with the core level PECD phenomenon. 

Figure 3.63 illustrates the gauche effect for vicinal lone pairs and polar C—F bonds with the examples of (a) hydrazine and (b) 1,2-difluoroethane, respectively. As seen in Fig. 3.63(a), the

lone pairs are anti to one another (thus squandering their powerful donor strength on vicinal moieties with no acceptor capacity) is disfavored by 3.2 kcal mol-1 relative to the preferred = 93.9° conformer in which each nN hyperconjugates effectively with... 

Our approach will be illustrated by using 1,2-difluoroethane as the model system. This molecule can be dissected as shown below. The sigma nonbonded interaction of the fluorine 2px lone pairs and its impact upon conformational preference can be... 

Good quality ab initio calculations have not yet been carried out in order to test whether lone pair nonbonded attraction obtains in 1,2-difluoroethane. Results of INDO calculations shown below indicate the presence of a nonbonded attractive interaction on the basis of the partial bond order p (F2px, F2px). By contrast, the presence of a nonbonded repulsive interaction favoring the anti conformation is indicated on the basis of the bond order p(F2px, F2px). 

Fig. 18. Pi type stabilizing orbital interactions in syn and anti 1,2-difluoroethane. The symmetry labels are assigned with respect to a mirror plane (syn conformer) or a rotational axis (anti con-former)...

The sigma nonbonded interaction between the two substituents fall into pattern d of Scheme 1. Here, unlike the case of 1,2-difluoroethane, we conclude that there will be a preference for the syn conformation due to the sigma nonbonded interaction of the pi systems of the substituents. This will be counteracted by the inherent preference of any ethane molecule for the staggered geometry and a compromise is expected to be reached in the gauche conformation, barring adverse steric effects. 

We find that the OP is the most important term and the absolute values of the four contributions follow the order OP > SD > F C > OD for all the molecules studied here with the exception of the synperiplanar conformations of 1,2-difluoroethane and 1,2-difluoropropane, where the FC term is the most important... 

Fig. 3.11 Molecules with low symmetry (n) plicv pboryl bromide chloride fhiondc. C, (b nitrosyl chloride. C (c) the anu conformer of R.S-1.2-dichloro-l.2-difluoroethane.

In the case of a number of vicinal difluoro systems, such as 2,3-difluoro-2,3-diphenylethane or 2,3-difluorosuccinic acid derivatives, the coupling systems are AA XX, which means that they will produce second-order spectra (see Section 2.3.7). A case in point are the fluorine and proton spectra of 1,2-difluoroethane, which have been analyzed carefully both experimentally and computationally in order to determine details of the conformational distribution of this molecule (Scheme 3.24).7 As is commonly known, the gauche conformation is preferred thermodynamically over the anticonformation. 

Figure 2. Gauche conformation of 1,2-difluoroethane with maximum overlap between the best donor (oC-h) and best acceptor (Oc-f).

The 1,2-dihaloethanes show a marked conformational sensitivity to physical state and medium137-142 (see Table 18). In the vapor phase, 1,2-difluoroethane is more stable in the gauche form but the dichloro, dibromo and diiodo compounds are more stable in the anti form. The gauche form is favored in the liquid state when at least one of the halogen atoms is fluorine, but in other instances the anti form is preferred. 

Figure 1.7 Orbital interaction of 1,2-difluoroethane (14) and conformation of 2-fIuoroetha-nol (15).

Hirano, T., Nonoyama, S., Miyajima, T., el al. (1986) Gas-phase fluorine-19 and proton high-resolution NMR spectroscopy application to the study of unperturbed conformational energies of 1,2-difluoroethane. J. Chem. Soc., Chem. Commun., 606-607. 

Femholt, L. and Kveseth, K. (1980) Conformational analysis. The temperature effect on the structure and composition of the rotational conformers of 1,2-difluoroethane as studied by gas electron diffraction. Acta Chem. Scartd, A., 34, 163-170. 

Friesen, D. and Hedberg, K. (1980) Conformational analysis. 7. 1,2-Difluoroethane. An electron-diffraction investigation of the molecular structure, composition, trans-gauche energy and entropy differences, and potential hindering internal rotation. J. Am. Chem. Soc., 102, 3987-3994. 

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