N/a-interaction

Both species exhibit the expected linear geometry that maximizes the dominant n- - a interaction. However, these isomers are rather perplexing from a dipole-dipole viewpoint. The dipole moment of CO is known to be rather small (calculated Fco = 0.072 D), with relative polarity C- 0+. 40 While the linear equilibrium struc-ture(s) may appear to suggest a dipole-dipole complex, robust H-bonds are formed regardless of which end of the CO dipole moment points toward HF This isomeric indifference to dipole directionality shows clearly that classical dipole-dipole interactions have at most a secondary influence on the formation of a hydrogen bond. 

The H-bonded H HOH product species was previously depicted in Fig. 5.16, while the structure and leading n— a interaction for the corresponding H2 OH-reactant species are shown in Fig. 5.33. Figure 5.34 similarly depicts the structure of the transition-state species and principal n—a interaction for the reactant-like Lewis structure that better describes the resonance hybrid (see below). 

The dominant interaction is the n—o interaction and since it is maximized in an anti arrangement, it is concluded that HN=NH will be expected to exist in the sterically crowded cis geometry. Of course, if the n—a interactions are weak they will not be able to reverse the trans over cis preference dictated by steric effects. 

We can increase the magnitude of the n—a interactions by replacing one or both H s by a more electronegative atom or group. For example, consider the molecule difluorodiazene. The stabilizing interactions which obtain in the cis and trans geometries are specified below ... 

However, ab initio calculations show that the Y conformation is more stable than the W conformation314. As in the case of HOOH, n-a interactions are present but weak and cannot overide the preference for the Y conformation dictated by conventional steric effects. Nonetheless, the increase in n—a conjugative interactions... 

Once more, replacement of the hydroxyl hydrogen by a group or an atom more electronegative than hydrogen, replacement of the hydroxyl oxygen by its third period counterpart, or, a combination thereof, will enhance n—a interaction and should lead to a preference for the crowded W conformation over the Y con-. formation. This expectation is confirmed by ab initio calculations in which the preferred conformation for NH2OF is found to be the more crowded W con-former314. ... 

In Part II we examined in detail various experimental tests of nonbonded interactions. Here, we again focus on specific physical and reactivity probes in order to test the importance of n—a interactions in organic problems. Specifically, we shall examine the following two areas ... 

In this section, results of Photo Electron Spectroscopy (PES) as well as infrared spectroscopy shall be examined in terms of n— a interactions. 

In this section we shall examine in detail the role of n—a interactions in carbanion chemistry. Carbanion stability as well as relative acidities of organic molecules are important probes of n—a interactions. 

A natural bond orbital-based CI/MP through-space/bond interaction analysis of the Sn2 reaction between allyl bromide and ammonia17 showed that allyl bromide reacted faster than propyl bromide because the a - n and n — a interactions stabilize the allyl bromide transition state equally. 

The idea of radicals selectively abstracting hydrogen atoms from C—H bonds weakened by this n-a interaction was also used by Deslongchamps et al. (1972a) in their initial treatment of the ozonolysis of [1] and [2], Although the balance of evidence now points to this reaction being one of hydride abstraction, at the time a hydrogen atom abstraction mechanism was entirely reasonable. 

Intramolecular proton transfer in electronically excited transfer in, say, salicylic acid ester and other aromatic compounds leads to deexcitation of the energized electron [43-45]. In photoreduction processes, electron transfer often precedes proton transfer [46] the stability of the protonic bond is at least partially due to an n-a interaction [47]. The strength of the protonic interaction appears to be proportional to the ionization potential of the donor and is sensitive to solvent polarity [48]. These effects have hardly been touched on in biologically important transitions and represent an important new field of research. 

Results of X-ray diffraction analysis of SP2335 confirm the effect of the electronic state of the N atom on the length ofthe Cspiro-Obond. In SP23, due to the amide conjugation of the N-atom LEP with the it-electron system of the carbonyl group, the activity of the LEP and the efficiency of the n-a interaction must be lower than in SP5. Therefore, the C8piro-N bond (1.47 A) is not shortened while the Cspiro-0 bond (1.44 A) is much less elongated than in SP5. 

It was demonstrated in Section 7.2.1.3 that the efficiency of orbital hyperconjugation in the spiro center depends both on the geometric configuration of the LEPs of the heteroatoms and adjacent bonds and on the electronic properties of the heteroatoms. Since symmetrical spiropyrans contain the same heteroatoms in the spiro center, efficiency of n-a interactions depends mainly on the electronic state of the heteroatoms as determined by the nature and positions of substituents in the benzopyran moiety. 

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