The AA′BB′ system of conformational populations for derivatives of choline, tryptamine and tyramine

Abstract

Equations relating the magnetic resonance spectral parameters N and L of the AA'BB' system to the conformational populations of 1,2-disubstituted ethanes of the type XCH2CH2Y are derived for a variety of substituents. These equations are used to obtain conformational information for choline, tryptamine and tyrarnine derivatives in solution.

Conformational populations for antihistamines and antihypertensives in solution

“…For other choline derivatives, recent opinion has favored an electrostatic interaction (2, 16, 20); but in the case of guanoclor, the NH group of the guanidinium head may provide extra stability through a sixmembered ring involving the 0-C-C-N-N-H atoms (17). Comparison of the chemical shifts of phenocholine and xylocholine showed that for xylocholine in solution the aromatic ring was considerably restricted in rotation to a region orthogonal to the plane of the COC bond (2). The chemical shift of the OCH2 protons for guanoclor indicates that the bulky ortho-dichloro substituents are also restricting rotation.…”

“…more comblications due to overlapping absorptions, long-range couplings, unfavorable linewidths, or uncertainties in some parameters. However, the parameter N. the sum of the vicinal coupling constants JAB and JAB,, is estimated from the separation between the strongest pair of lines in one of the multiplets, either A or B, depending on which is clearer (2 Quantitative interpretation of N values is based on the correlation between the vicinal coupling constants in the individual rotamers and the electronegativities of substituents X and Y derived by Abraham and Gatti (3). Relations for the parameter N as a function of nt, the proportion of trans-rotamer, can be derived for the OCHzCHzN and NCHzCHzN fragments and are given in Eqs.…”

“…Although the spectral parameters can, in principle, provide rotameric populations, this is not always straightforward. The difficulties are partly experimental and numerical and partly interpretative (2). Recent work clarified the effects of the substituents X and Y on the parameters obtained from an analysis of the high-resolution PMR spectrum of the XCH2CH2Y system (3) (an AA'BB' spectrum), and quantitative rotameric populations can now be obtained.…”

Nuclear magnetic resonance spectroscopy

“…Theory of NMR spectra of symmetrical spin systems with 1 = lk (270) and arbitrary 1 (271) has been presented. Analyses of [A *=1 XíJ2 (271a), AA'BB' (272,287,289), A2a+iB2b+c etc. (274), AB2X and AB2MX (275), A3XX'X"X'"A3' (276), ABX (277,281), AA'BB'CX (278), AMX (279), AA'A"A'"XnXn' (282), AA'X (283), AX (284), AAX (284), AA'XX'Z (285), AB2 (288) and An-A"'XX' (290) systems, many applied to specific compounds, have been reported as have calculations for the NMR spectra of a finite chain containing two types of / = V2 nuclei (273), and discussions of the uniqueness of NMR spectral analysis for a general system of nuclei with I = lk (271b), measurement of unresolved couplings in first-order NMR spectra (280) and determination of relative signs of spin-spin coupling constants (286).…”