1 he empirical correlation between ?~~eso-substitutio~~ and the phyllo-type spectrum in the porphyrin series is shown to be n1~1ch less general than had previously been supposed. The variation of the spectrum with the electronic nature of the substituent is discussed.I t became apparent a t an early stage in our study of the nitration (I) of octa-substituted porphyrins that deductions froin visible and nuclear magnetic resonance spectra \\ere in conflict. Thus, although the nuclear magnetic resonance spectrum of nitrooctaethylporphyrin clearly corresponded to a meso-substituted structure, the visible spectrum \\.as of an etio type, \\Thereas, on the basis of the correlation proposed by Stern and 'CVenderlein ( 2 ) , a phyllo-type spectrum ( Fig. 1) would have been expected. Indeed, Stern and hlolvig (3) had applied this correlation to reject, incorrectly as it now einerges (1, 4), the nzesonitro structure for nitroetioporphyrin I. I t thus became desirable to reconsider the correlation bet\\reen meso-substitution and the phyllo-type spectrum.The general correlation, which was proposed in 1936, and which has in recent years ( 5 , 6) received a certain arnount of theoretical interpretation, is sun~inarized in Fig. 1. I t is well documented (2, 7, 8)' for both the etio-and rhodo-type assignments, where it operates effectively in the sense that all porphyrins of the stated substitution pattern are expected ETlO PHYLLO RHODO I . 1 Spectral correlations in the porphyrin series (diagrammatic, after Stern and W e n d e r l e i~~ (2)).Rand l a is omitted. R = alkyl, substituted allcyl, and, within certain limits, other groups (e.g. oximinoformyl, vinyl, and hydrogen). R' is not specified (but is generally an allcyl group in the earlier examples).R" = hydrogen, alkyl, aryl, or OR. For personal use only.