Hyperconjugation absorption

Delocalization of a free electron pair into an antibonding Hyperconjugation Resonance Absorption Spectroscopy... 

The weakening of a bonds by negative hyperconjugation with lone electron pairs also reveals itself in IR and NMR spectra. Thus, C-H, N-H, or O-H bonds oriented trans or antiperiplanar to an unshared, vicinal electron pair are weakened and have therefore a significantly reduced IR vibrational frequency [17]. The C-H vibrational frequency in aldehydes is, for example, lower than that in alkenes (Scheme 2.7). Polycyclic amines with at least two hydrogen atoms antiperiplanar to the lone pair on nitrogen have characteristic absorption bands at 2800-2700 cm-1 which have been used to infer the relative configuration of such amines [18]. 

Electronic interactions in [2.2]paracyclophane and its derivatives have been extensively studied [27]. In addition to the original absorption bands, the cyclophanes exhibit the additional cyclophane band at longer wavelengths (Amax = 302 nm for the parent [2.2]paracyclophane). The existence of the additional bands can be explained in principle by the exciton/charge resonance interaction between the two chromophores. However, the complete assignment of the individual absorption bands in each cyclophane molecule is still a challenging task. It is known that the UV spectra of cyclophanes can be reasonably explained only if the ct-7I interaction, or a special type of hyperconjugation, is taken into consideration [28]. 

It has been demonstrated that, in accord with theoretical predictions, the presence of resonance attracting substituents in position 5 (either standard or hyperconjugative) of the basic chromophore 1 produces the desired red shift of the second electronic transition. The sulfonate group fulfills this function, in fact in the case of aryl derivatives such as 8, a bit too well for the strong red shift of the second transition in aryl sulfonates results in very little residual absorption in the mid-UV region particularly at 313 nm. 

Several objections have been raised for this interpretation17 Unlike in the 260 m/x band, no definite trend has been found in the position of the 200 m/x bands in alkyl benzenes or p-alkyl substituted benzenes. In many cases, the observed wavelengths are actually found to be in the reverse order (i.e., in the inductive order) to that predicted by C-—H hyperconjugation. At this point it may be stated that the evidence for hyperconjugation from the absorption spectra of alkyl substituted benzene derivatives is not definitive. 

The magnitudes of the barriers to rotation of many small organic molecules have been measured. The experimental techniques used to study rotational processes include microwave spectroscopy, electron diffraction, ultrasonic absorption, and infrared spectroscopy. Some representative barriers are listed in Table 2.1. As with ethane, the barriers in methylamine and methanol appear to be dominated by hyperconjugative stabilization of the anti conformation. The barrier decreases (2.9 2.0 1.1) in proportion to the number of anti H-H arrangements (3 2 1). (See Topic 1.1 for further discussion.) ... 

In the low-absorptivity region, where the saturation effect is insignificant, the electronic and vibrational contributions to the absorptivity become comparable (Ballester et al., 1964b) and consequently acceptable correlations are obtained only in a very few instances, such as in alkylbenzenes. Actually, the square law has allowed the evaluation of the carbon-carbon and carbon-hydrogen bond hyperconjugation (Baker-Nathan effect) (Ballester and Riera, 1964a March, 1985, p. 65). 

The UV spectra of M(CH3)3CH2C6H5 compounds (M = C, Si, Ge, Sn) have been compared [12, 15, 19, 28] and studied for the crystalline compounds at low temperatures [28]. The similarities in the UV spectra of the Si and Ge compounds indicate that their crystal lattices are isomorphous [28]. For UV data and calculated transition energies, see also [46]. The hyperconjugation effect of the CH2 group and the changes of UV absorptions and dipole moments in the M(CH3)3CH2C6H5 series are discussed [15]. 

Kaya et al. measured FUV spectra of six kinds of amides in the gas phase. They assigned the major absorption band to the Tr-ir transition and found a peak shift to lower energy upon CH3 substitution on the N atom. The peak shifts to low energy region were attributed to hyperconjugation with the substitution of methyl groups [26]. 

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