Correlation of molecular orbital indices of reactivity and proton chemical shifts: Acenaphtho[1,2‐b]‐ and ‐[1,2‐c]thiophens

Abstract

Abstract-Nuclear magnetic resonance proton chemical shifts may be correlated with free valence, a representative reactivity index, for fluoranthene and two novel sulphur heterocyclic analogues, acenaphtho [l,2-b]-and -[I ,2-c]thiophens. Such correlations arise fortuitously, however, and an attempt to explain them in terms of intrinsic properties of the Huckel-McWeeny theory of proton shielding is proved to be fallacious.APPARENT correlations between nuclear magnetic resonance proton chemical shifts (&), and both Hiickel MO and SCF reactivity indices, to and experimental reactivities5 in alternant polynuclear benzenoid hydrocarbons, have led to a number of applications of this approach to non-alternant hydrocarbons,6 hydrocarbon ions6 and heterocyclic molecules.'~* Thus, Clin and Salier have pointed out a relation between carcinogenicity and the sign of the deviation for K (or Krebs) region protons from a graph of 6, against superdelocalisability.* This idea is attractive from the standpoint of predicting both the reactivity towards electrophilic, nucleophilic and radical reagents, and even the carcinogenicity of novel heterocyclic analogues of fluoranthene (l), such as acenaphtho For example, Fig. 1 shows the relation, for analogous protons of 1 to 3, between 6, and free valence (F,), our chosen reactivity index, which gives predictions similar to many other indices,, and suggests correctly that electrophilic substitution (e.g. detritiation,13 bromination,14 nitrationl5 and sulphonation16) takes place preferentially at the 3 position for 1. The reactions of 2 and 3 have not apparently been studied. We believe, however, for a number of reasons that the basis of such correlations is, if not spurious, at least coincidental. We have already shown3 that if the range of protons of alternant benzenoid hydrocarbons is extended, substantial deviations from plots of 6, against F, occur which are not readily exp1ained.l' Even if these deviations are ignored, the remaining correlation arises as a fortuitous consequence of the geometry of polynuclear aromatic molecules, so that in a causative sense 6, is unrelated to F,.. Values of the latter fall into broad groups, each roughly corresponding to a different position type, e.g. relative proximity to ring centres, the values of 6, for the proton types of 4 are 6, > 6, > 6,, and for the protons of 1, 2 and 3, 6, e 6, > 6z;18*z0 apparent, but noncausatiue, relations between 6, and F, thus arise of the type shown in Fig. 1 , for which the correlation coefficient (0.87) is statisticazly significant at the 0.1 % level.* While contributions to 6, from unequal charge densities (q,, itself a reactivity index) are expected for nonalternants, hydrocarbon ions and heterocycles, 6, is still dominated by effects arising from .rr-electron circulations. A recent attemptz1 to argue a fundamental relation between a, , F, and q, depends on equating a general expression for the change in partial .rr-energy at the r'th atomic centre, (AE"), due to a general perturbation ( A m , = k,q, + k,F, + k...

Aromaticity and Ring Currents

“…The Haigh−Mallion experimental data 188,189 were also used (though not without some reservations about and criticisms of their qualityespecially where chrysene was concerned) in an extensive attempt by Blustin − to account, à la Musher, − for 1 H NMR chemical shifts in the condensed, benzenoid hydrocarbons by means of his ‘localized π-bond' model. − Blustin claimed 200 that his “simple calculations give results which compare well with experiment and with calculations based on the ring current model.” Like Musher 5-7 indeed, Blustin's third paper 200 was enticingly entitled Aromatic Proton Chemical Shifts Without Ring Currents Blustin felt that 200 “the overall results suggest that there is no need to associate a special ring current attribute to π-conjugation in aromatic molecules.” Blustin also proposed 200 a scale of aromaticity based on his localized π-bond model; he was of the opinion that a comparison of the π-electron contributions to anisoptropy should provide “a straightforward method for ordering [the aromaticity of] a group of molecules” but he pointed out that “this approach fails when an increase in anisotropy is due to factors unconnected with π-conjugation.” 200 There have also been suggestions − that correlations can be obtained between 1 H NMR chemical shifts in the planar, condensed, benzenoid hydrocarbons and reactivity indices (such as free valence) in these molecules. The proposition 201-203 that ‘causative' relations could be expected between these quantities has, however, been vigorously contested. − The Haigh−Mallion experimental data 188,189 on 1 H NMR chemical shifts in the planar, condensed, benzenoid hydrocarbons have even been correlated with graph-theoretical, ‘topological' indices that Mekenyan et al call ‘hierarchically extended connectivities'. Furthermore, Westermayer et al have had some success by comparing the Haigh−Mallion data 188,189 with predictions from what they describe as an ‘empirical atomic point-dipole model'.…”

“…The proposition [201][202][203] that 'causative' relations could be expected between these quantities has, however, been vigorously contested. [205][206][207] The Haigh-Mallion experimental data 188,189 on 1 H NMR chemical shifts in the planar, condensed, benzenoid hydrocarbons have even been correlated with graph-theoretical, 'topological' indices that Mekenyan et al 208 call 'hierarchically extended connectivities'. Furthermore, Westermayer et al 209 have had some success by comparing the Haigh-Mallion data 188,189 with predictions from what they describe as an 'empirical atomic point-dipole model'.…”