Proton Magnetic Resonance Shifts and the Electron Density Distribution in Aromatic Systems

Abstract

The quantitative nature of the empirical linear correlation between the proton resonance shifts in aromatic molecules and the local electron density on the carbon atom to which the proton is bonded has been investigated. The application of this relationship t o the determination of the electron density distribution of a variety of aromatic molecules, as well as the main limitations inherent in this application, are discussed. Electron density distributions have been derived for aniline, anisole, azulene, acepleiadylene, pyridine, pyridinium ion, and for the aromatic ions pentalenyl, indenyl, fluorenyl, triphenylmethyl cation, and azulene dianion, and are compared with those calculated by molecular orbital methods. In general there is a fairly good correspondence between the experimental and calculated density values, although for the aromatic ions the excess charge tends t o be somewhat more uniformly distributed over the molecule than would be indicated by simple MO calculations.

Comment on the Relationship between 1H Chemical Shifts and Net Charges

“…The value a = 1 1 . 1 ppm/electron is similar to the one proposed ear lier [4]. 6° = 5.2 ppm agrees approximately with those values given for benzenoid and olefinic protons [1], Using these a and (5° parameters we calculate proton shifts 6 which agree well with the observed ones.…”

“…A value o f a = 10.7 ppm/electron has been obtained [4] by a re gression analysis between observed shifts and net charges in the diatropic [5] annulenes C5H 5_ , C6H 6, C 7 H 7+, and C 8H 82~. Herewith it was assumed that RC and L A terms do not differ considerably in these compounds and that the counterion does not affect the shifts.…”

“…(3° contains all iso tropic contributions to d. <5 q is related to the electronic charge distribution [2]. Experimental and theoretical studies [3,4] provided evidence that <3q = aq (2) where q is the jz net charge (q < 0 for excess electron density) on the adjacent carbon atom. A value o f a = 10.7 ppm/electron has been obtained [4] by a re gression analysis between observed shifts and net charges in the diatropic [5] annulenes C5H 5_ , C6H 6, C 7 H 7+, and C 8H 82~.…”

“…Experimental and theoretical studies [3,4] provided evidence that <3q = aq (2) where q is the jz net charge (q < 0 for excess electron density) on the adjacent carbon atom. A value o f a = 10.7 ppm/electron has been obtained [4] by a re gression analysis between observed shifts and net charges in the diatropic [5] annulenes C5H 5_ , C6H 6, C 7 H 7+, and C 8H 82~. Herewith it was assumed that RC and L A terms do not differ considerably in these compounds and that the counterion does not affect the shifts.…”

The crystal structure of 2,4,6-tribromoaniline

“…Lide, 1962) as well as from the delocalization of the lone-pair electrons of the donor group which, according to both theoretical and experimental investigations (see e.g. Fischer-Hjalmars, 1962;Kwiatkowsky, 1967;and Schaefer & Schneider, 1963), plays a relatively important role in aniline.…”