Chemical shifts conjugation effects

Konovalov and colleagues145 studied the chemical shifts of the CH2 protons for a series of derivatives of arylsulfonylacetic acids and /i-disulfones in DMSO or CDClj. The transmission of electronic effects through the S02 bridge was detected and attributed to the possibility of participation of the 3d orbitals of sulfur in the formation of the S=0 bond and to the existence of dn-pn conjugation . The paper contains various references to related (mainly Soviet) work. 

A study39 of substituent effects on the 15N chemical shift (515N) (Table 10) for 4-substituted anilines in DMSO was interpreted in terms of substituent solvation-assisted resonance (SSAR) effects. Solvation of certain conjugated jr-electron-acceplor (+R) substituents has been found to give significant enhancements in the acidities of anilines, phenols and other acids40,41, and the magnitudes of these enhancements increase with... 

For reasons of this sort Taft and Rakshys considered that the chemical shift of a Fluorine atom meta to a particular substituent is a measure of only the inductive effect of that substituent while of course the shift of an ortho or para i F nucleus is a measure of both inductive and conjugative effects. They proposed that the difference between the meta chemical shift and the para chemical shift be taken as a measure of the conjugative effect of a substituent. Their results for some of the systems of interest in the present context are summarised in Table 5. In both cases the second-row substituent appears to have a considerable electron accepting conjugative effect compared to its first-row analogue. This of course can be rationalised by partial double bonding. 

Commonly, the increase in the conjugation chain length leads to the bathochromic shift of the absorption bands. The optical and photoelectrical properties analysis needs to take into account not only formal polyconjugation following from the molecule chemical structure but effective conjugation defined by sterical factors too. 

An empirical increment system permits prediction of charge distribution in a,/ -unsaturated carbonyl compounds, assuming additivity of electronic effects and neglecting the conformational dependence of carbon-13 chemical shifts [290]. Moreover, carbonyl and alkenyl carbon shifts of a, /3-unsaturatcd ketones may be used to differentiate between planar and twisted conjugated systems, as shown in Table 4.29 [291] and outlined for phenones in Section 3.1.3.8. 

The 13C spectrum of crotonaldehyde (CH3 CH=CH CHO Fig. 3.52) provides a good example of the way in which the 13C chemical shift is determined both by the state of hybridisation of the carbon atom and the nature of the substituent. The four carbon atoms have markedly different chemical shifts. The methyl carbon appears at 3 17.1. It is shifted downfield slightly compared to the methyl carbon at the end of a chain of methylene groups as in 3-methylheptane (Fig. 3.41) and hex-l-ene (Fig. 3.51). The two alkenyl carbons appear at <5133.4 and 3 152.9. The effect of conjugation of the carbon-carbon double bond is that the (i-carbon is shifted further downfield. The carbon of the carbonyl group is sp2-hybridised and is directly bonded to an electronegative atom. It is shifted furthest downfield and appears at 3 192.2,... 

The X-confused systems 100 and 101 exhibit borderline macrocyclic aromaticity, which results from the onium-type contributions 100 and 101, characterized by the presence of 18-electron aromatic circuits. For instance, the NH protons in 100 resonate at ca. 5.8 ppm, while those in the corresponding dication are observed at 5.3—4.4 ppm. These chemical shifts are intermediate between values expected of a nonaromatic porphyrinoid (S > 10 ppm) and those typical of aromatic systems (S < 0 ppm). The macrocyclic aromaticity is fully restored in the 3-substituted derivatives containing an sp3 carbon (104-105) or a carbonyl group (102-103), which is an additional reason for the enhanced reactivity of 100 and 101. In comparison, the pyrrolyl-substituted derivative 106 shows only moderate diatropicity, which is noticeably enhanced upon protonation [247], This effect is explained by cross-conjugation of the pyrrolyl substituent in the dication 107, which enables an 18-electron aromatic circuit in the macrocycle. 

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