Gauche attractive

It is quite clear from the data that there is substantial "excess attractive interaction in compound 4 i.e. the slight experimental preference for the axial conformation contrasts with a substantial calculated preference for the equatorial one the difference has been ascribed to the "gauche attractive effect" between oxygens (14,17). In contrast, for compounds 1 and 2 there is a substantial "excess" repulsion, i.e. the equatorial conformation is preferred more than calculated. The effect, which has been called "gauche repulsive effect" (14) is especially marked for gauche interactions between sulfur atoms, as in 2. Only in the case of compound 3 is the difference between calculated and observed preference for the equatorial position sufficiently small to make any conclusion uncertain, especially in view of the fact that, in the absence of energy minimization, the calculated energy difference (1.50 kcal/mol) may be somewhat overestimated. 

The origin of this gauche attractive effect, which overcomes unfavorable steric and/or dipolar interactions, can be rationalized in terms of a a (or n ff ) energy-lowering orbital interactions between the best combinations of donor and acceptors bonds (or lone pairs of electrons) in an antiperiplanar orientation (Scheme 19) (73TL1645, 77]A8379, 79JA1700). 

Examples of attractive nonbonded interactions can be found in certain halogenated hydrocarbons. In 1-chloropropane, for example, the gauche ccMiformation is slightly... 

Similar conclusions were reached for sulfoxides 157. Conformation 158 was preferred for (RS/SR)-157 but with some contribution from conformer 159. The (RR/SS) dias-tereomers preferred the reverse conformer 161 was preferred to 160161. An attractive force between Ph/Ar and Ph/R was thought to be the primary factor in determining the conformational preference of sulfoxides 152 and 157. MM2 calculations were carried out on a series of molecules of general structure PhCHR—X—R with X equal to CHOH, C=0, S and S=0151. The main conformers of these molecules have the Ph (or aryl) and R (alkyl) groups gauche. The calculations supported the existence of CH-tr attractive interactions with minor contributions from other effects. 

R (alkyl) groups gauche. The calculations supported the existence of CH-jt attractive interactions with minor contributions from other effects. 

Fig. 7.5 Illustration of how dispersion forces affect gauche (G) conformations. Compared to structures with gauche forms devoid of dispersion forces (i.e., HF-optimized), structures with gauche forms subject to dispersion forces (MP2 optimized) contract in such a way that the 1. ..5 nonbonded interactions in an attractive part of the van der Waals potential are shortened. Thus, in GG-pentane (shown above), MP2-optimized torsional angles are contracted by several degrees compared to the HF-optimized geometry, causing a reduction in the 1...5 nonbonded distances by several tenths of an A. For additional details and the numerical values see R. F. Frey, M. Cao, S. Q. Newton, and L. Schafer, J. Mol. Struct. 285 (1993) 99.

On the basis of the above discussion, we are led to the conclusion that sigma nonbonded attractive interaction in the form of a hydrogen bond will tend to favor a syn conformation opposing the inherent preference of ethane molecules for a staggered conformation. A compromise is expected to be reached in the gauche conformation. However, severe steric effects may force an anti conformational preference. 

Experimental results shown in Table 10 suggest that a hydrogen bond, Le., a strong nonbonded attractive interaction, is instrumental in dictating gauche preference in CH3CH2CH2X and XCH2CH2OH systems where X carries at least one lone pair. 

In other words, the apparent order of stability of 1,3-butadiene is trans > gauche > cis. We conclude, therefore, that the preferred conformation of 1,3-butadiene is dictated by steric effects which overwhelm attractive pi and sigma nonbonded interactions. 

Since the C—Br bond is a much better donor and a much better acceptor than the C—H bond, the prediction is that the gauche conformation will be stabilized more than the cis conformation by a—it interactions. These interactions may then dominate nonbonded attractive effects present in the cis isomer. 

Microwave spectroscopic studies of l-fluoro-2-propene show that the cis conformation is more stable than the gauche conformation by approximately 306 cal/mol293 However, when fluorine is replaced by chlorine, bromine, or iodine, the gauche conformation becomes more stable294. These results confirm our expectations that the conformational preferences of allyl halides may depend on a balance of nonbonded attractive effects and a—it interaction effects. 

A typical R4R3 A—ARi R2 molecule is hydrazine, a system which has attracted an enormous amount of theoretical interest. The syn, anti and gauche conformers of NH2—NH2 are depicted below along with an enumeration of the dominant interactions which obtain in each case ... 

The tendency of polyphenylethanes to take up anti conformations was mentioned above (Sect. IV-B). Some confusion arose, however, when Ivanov (MMI, ref. 147) and Jacob (BIGSTRN, ref. 148) independently reported that MM calculations gave lower energies for the gauche-1,2-diphenylethane 31a than for the anti conformer 31b, while the reverse was actually observed in crystals. Ivanov even reasoned that the calculated higher stability of 31a in the vapor state arose from attractive nonbonded Q, interactions. MM2... 

By equilibrating (BF3) the cis/trans isomers of 2-Ph-5-OR-l,3-dioxanes, the conformational free energy differences were determined [-AG° = 0.24 kcal mol- (OMe), 0.01 (OTs), -0.34 (OSO2C6H4-PNO2), -0.48 (OSO2 CH3)] and discussed with respect to the steric and the attractive gauche O-C-C-O effect (92T5941). 

In this study, the dipole moments at 298 K and the corresponding temperature coefficient of PDTC are calculated in the RIS approximation. The results are compared to the values determined by experiment. The results indicate that an extra stabilization energy of about 3.8 kj mol-1 must be added to these gauche states relative to trans state. This provides evidence of a large attractive sulfur gauche effect in the polysulfides, about equal in magnitude to that found for the analogous POM. 

The experimental value of the dihedral angle25 in HOOH is 111°. The interactions between adjacent lone pairs are repulsive, but the other gauche effects result from attractive influences (see below). Had we use the rabbit ear structure 22, with equivalent lone pairs, the experimental structure would be hard to explain. 

---