A Reconsideration of the Validity of Correlating Substituent-induced Proton Chemical Shifts in Aromatic Derivatives with Reactivity Parameters

Abstract

Comparisons of 'H chemical shifts and charge densities (determined by C N D 0 / 2 MO calculations) for 4-substituted derivatives of styrene, toluene, benzylchloride, and N,N,N-trimethylphenylammonium ion indicate that the chemical shifts primarily reflect intramolecular electronic effects. These effects are reflected by correlations of 'H chemical shifts with the F and R reactivity parameters of Swain and Lupton. It is concluded that it is valid to correlate substituent-induced chemical shifts for aromatic derivatives with divided reactivity parameters provided that ( I ) chemical shifts are measured at infinite dilution in a non-polar medium and'(2) at least ten derivatives of each family are investigated.Calculations for 4-substituted toluenes indicate that there will be a small conformational dependence for substituent-induced benzylic proton chemical shifts.La comparaison des deplacements chimiques ' H et des densites de charge (dktermintes par la mkthode CNDO/2 MO) pour des derives substitues en position 4 du styrene, du toluene, du chlorure de benzyle et de I'ion N,N,N-trimkthylphinyl ammonium indiquent que les deplacements chimiques refletent principalement les effets Clectroniques intramolCculaires. Ces effets apparaissent sous forme de correlation entre les deplacements chimiques 'H avec les parametres de rkactivite F e t R de Swain et Lupton. On en conclut qu'il est valide de faire une correlation entre le substituant et les diplacements chimiques induits dans les derives aromatiques possedant des parametres de reactivite divises pourvu (I) q u e les deplacements chimiques soient mesurks dilution infinie dans un solvant non-polaire et (2) qu'au moins dix derives de la famille soient etudits.Les calculs pour les toluenes substituCs en position 4 indiquent qu'il y a une petite dependance conformationnelle pour les dkplacements chi~niques des protons benzyliques qui ont ett induits p a r les substituants.[ (1) derivatives) should reflect resonance effects since R = 0 for the N(CH,),+ group (3). They also [I]

Investigations of Substituent Effects by Nuclear Magnetic Resonance Spectroscopy and All-valence Electron Molecular Orbital Calculations. IV. 4-Substituted Phenylacetylenes

“…We have already discussed this criticism in some detail (10). The results presented above support our previous conclusions that these correlations are meaningful when chemical shifts are measured for a large number of substituents under inert conditions (1, 2, 10).…”

“…They conclude that 'the statistical superiority of DSP correlations should not be used as a reason for proposing unusual mechanisms of transmission of substituent effects' and suggest that these apparent mechanisms may simply be artifacts of the correlation (7). relate with o,, this approach should not be of general validity since different systems will often have different resonance-field ratios (10). Even for compounds of type 6, where Mitchell and Phillips claim that 19F chemical' shifts adequately correlate with o, (7), recent results on further families of this type show that h varies by more than 2 as G is changed (23).…”

“…Since CCl, is not an inert solvent for 1 H chemical shifts (9), this prevented a meaningful comparison with the styrene data which were determined in C,Hl, (1). In addition, it has been suggested that a minimum of 10 substituents should be included in an investigation of this type (6,10). Consequently, a more detailed investigation seemed necessary.…”

Substituent-induced 1H chemical shifts for 4-substituted (2, 2-dichlorocyclopropyl)benzenes: further evidence for intramolecular electric field effects on 1H chemical shifts

“…While field effects appear to dominate over inductive effects a t long range (28,29), calculations on aliphatic co~npounds suggest that inductive effects may be important at distances up to three bonds (29). The sinalier shifts for CH, protons than CH, protons for ethylbenzenes (8)(9)(10) are consistent with inductive effects, possibly combined with weak hyperco~ljugative interactions of the alkyl and phenyl groups (25). The apparent resonance contribution to 6,(,, -6 ,,(,, may be due to small substituent induced distortions of the side-chain group.…”

Synthesis, Spectral Correlations and Antimicrobial Activities of 2-Pyrimidine Schiff’s Bases

“…Table 1. These chemical shift values (ppm) are correlated with different Hammett substituent constants and F & R parameters using single and multi-linear regression analyses according to the approach of Reynolds [32,33]. In this correlation, the structureparameter equation employed is shown in equation (7).…”