Sigma Nonbonded Interactions

Our recent computational work has provided the basis for a more detailed atudysis which provides a more realistic appreciation of sigma nonbonded interactions. The results of this analysis are not as clear cut as the ones reached on the basis of the previous more simplistic approach. However, they are important insofar as they can affect the type of reasoning employed in the unraveling of through bond and through space interactions by means of photoelectron spectroscopy. 

Focusing now on the sigma lone pair interactions, we can simplify the interaction diagram as shown in Fig. 11. The relative stabilization of the cis and trans geometries due to the interaction of the lone pairs with the central C—C bond can be assessed from consideration of all orbital interactions shown in Fig. 11. These interactions and their impact upon geometrical preference are discussed below. 

The analysis provided above, leads us to anticipate trends in sigma nonbonded interactions between lone pairs. Thus, in the case of the model system FH X—YFH , the possibility of lone pair nonbonded attraction is expected to increase as the splitting of the and rrg levels of the X—Y fragment gets smaller. Typical energy 

As can be seen, the splitting remains always substantial and does not fluctuate drastically. However, this does not constitute proof that the ttu orbital will be the one which will control the lone pair nonbonded interaction in the model systems simply because the relative orbital energies of an isolated diatomic X-Y are certainly different in a quantitative sense from those of a diatomic X—Y within a molecule FH X-YFH ,. 

While the sigma nonbonded interaction of lone pairs is ambiguous, the situation improves when one considers nonbonded interactions of sigma bonds. This is illustrated by reference to the molecule 1,3-butadiene. The orbitals of the C—C coupling unit are assumed to be the same as the ones shown in Fig. 11 for the C-C coupling unit of 1,2-difluoroethylene. The sigma MO s of the CH2 group and the 

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MO s of the component system can be easily developed. The final simplified interaction diagram is shown in Scheme 1. Clearly, we now have an additional interaction, as compared to the 1,2-difluoroethylene case, which stabilizes the cis isomer. Hence, the sigma nonbonded interaction of bonds is expected to favor a cis structure when the two bonds are coupled through another sigma bond. 

Comment Sigma nonbonded interactions are predicted to be attractive in most cases. 

Comment Computational test is needed to determine the sign of sigma nonbonded interaction. 

A final useful index of sigma nonbonded interactions between lone pairs is the partial bond order p (Xm, Yn) which is evaluated over the MO s which result from the interaction of the lone pair group MO s with the sigma HOMO and vacant MO s of the coupling unit. This index is intimately connected with the type of analysis employed in this work. In our survey of a variety of problems of molecular structure we shall provide computational results pertinent to the analysis outlined, i e. all or some of the following indices will be provided ... 

We are now prepared to examine how attractive or repulsive nonbonded interactions determine molecular geometry. We shall discuss representative examples where pi and/or sigma nonbonded interactions obtain. In each case, we provide computational data in support of general theoretical arguments as well as pertinent experimental results. It should be mentioned that only crucial indices of nonbonded interactions are provided and the survey of the experimental work is by necessity incomplete, i.e., it would take volumes to consider all available data. Nonetheless, at the end of this chapter, the reader should be able to apply the key ideas to problems of direct interest to him. 

Once we have considered pi nonbonded interaction control of the FCF angles in F2CH2 and CF2=CH2, we turn our attention to sigma nonbonded interaction control in all molecules or fragments of the type FAF. Here, we focus upon the interaction of the px lone pairs of fluorine with the py AO or uy type MO s of A. 

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... 

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. 

Consider the molecule 1-fluoropropane dissected in the manner shown below. First, we consider the sigma nonbonded interaction between methyl and fluorine and we construct the group MO s of the CH3—F fragment as shown by the interaction diagram of Fig. 19. 

A similar analysis can be given for the pi nonbonded interaction between the methylene group and the fluorine 2pz lone pair. However, due to much poorer spatial overlap, this interaction will be of limited significance compared to the sigma nonbonded interaction. 

The approach used in this section can be used to treat any 1,2-disubstituted ethane. In general, sigma nonbonded interactions may favor the syn geometry, the... 

In this case both pi and sigma nonbonded interaction between the fluorine lone pairs are important in dictating the preferred isomer of N2F2. Arguing as before, we predict the following ... 

Arguing as before we predict that cis C1NSO is stabilized more than tram C1NSO by pi lone pair nonbonded attraction. The results of CNDO/2 calculations are shown above and it can be readily seen that pi nonbonded attraction favors the cis isomer. On the other hand, the indices of sigma nonbonded interaction imply that the tram geometry is stabilized more by sigma lone pair nonbonded attraction than the cis. 

Turning our attention to the cis isomer, the Css conformation involves a potential sigma nonbonded interaction between the hybrid in-plane oxygen lone pairs, the Cse involves a hydrogen bond, and the Cee conformation involves a sigma nonbonded interaction between the hydroxyl hydrogen atoms. These interactions are pictorialized below ... 

Now, since sigma nonbonded interactions are weaker than pi nonbonded interactions, there will be an electronic bias in favor of Css. However, steric effects favor the Cee conformation. Hence, steric effects may make it appear that sigma nonbonded interactions dominate pi nonbonded interactions. 

Experimental results indicate that the sigma nonbonded interactions introduced by the substituents X and Y do not play an important role in determining the conformational preference of XCHjCOY molecules. Specifically, the IR and Raman spectra of XCHjCOY for the case X=Y=Br, X=Y=C1 and X=C1, Y=Br show that the most stable conformation is the S conformation143, 144 ... 

Sigma nonbonded interactions in dimethyl ether can be analyzed in the same way as before and the conclusion is that sigma nonbonded attraction is greatest for the Cee conformation. We expect pi nonbonded attraction to dominate sigma nonbonded attraction and the final result is that the C conformation will be the most stable torsional isomer of R—X-R molecules. 

Sigma nonbonded attraction also obtains in the isopropyl cation and, arguing as before, we conclude that sigma nonbonded attractive interactions are maximized in the Cee conformation. The 2-propyl cation constitutes a good example where pi and sigma nonbonded interactions reinforce each other. 

Indices of pi and sigma nonbonded interactions in the Css and Cee conformations of the 2-propyl cation as calculated by the INDO method are shown above. As can be seen, both pi and sigma nonbonded interactions favor the Cee relative to the conformation. 

The appropriate interaction diagrams are similar to that of Fig. 28. Reasoning as before, we conclude that pi nonbonded interactions favor the staggered conformation. Sigma nonbonded interactions, on the other hand, favor the eclipsed conformation. We expect pi nonbonded attractive interactions to dominate sigma interactions and the resulting order of stability is predicted to be > Cee. 

On the basis of a previous discussion, we predicted that pi nonbonded interactions will produce an order of stability of gauche > tram > cis, assuming appreciable Ci—C4 pi overlap in 1,3-butadiene. However, if sigma nonbonded interactions dominate pi interactions the order of stability will be cis > gauche > tram. Finally, in the event that steric effects are the controlling factor the order of stability will become trans > gauche > cis. 

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. 

As was discussed earlier, cis 1,3-butadiene is favored by attractive sigma nonbonded interactions. However, this conformation is destabilized relative to the trans... 

We will now consider an interesting case where sigma nonbonded interactions of the hydrogen bond type help to reinforce the conformational preference of sub-... 

In a molecule of this type, trans conformational preference is not only due to effects inherent in any diene molecule but also to an attractive sigma nonbonded interaction between the Xpx lone pair and the Is AO of the hydrogen attached on C3. Consequent-... 

Sigma lone pair nonbonded interaction in 1,2-difluorocyclopropane presents a similar situation to that observed in 1,2-difluoroethane (see Pattern b Scheme 1). Specifically, sigma nonbonded attraction may favor the trans isomer. This can be seen in the results of CNDO/2 calculations shown above where it appears that the sigma nonbonded interaction is repulsive in the cis isomer. 

In Part II we commented that pi and sigma nonbonded interactions can reinforce or oppose each other. The same situation obtains in the case of pi conjugative interactions. As an example consider the three conformations of diazabutadiene ... 

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