Quantitative Four‐State Conformational Analysis by Ring Current NMR Anisotropy: A Family of Molecules Capable of Intramolecular π‐Stacking

Abstract

A novel, multi-state, conformational analysis based on the magnetic anisotropy of molecules undergoing fast dynamic exchange is described. Calculated chemical shift tensors combined with experimental data from proton NMR studies were used to quantify conformational distributions as a function of solvent and temperature for a hydrocarbon and two fluorocarbon derivatives of N, ]bis(2-phenylpyridinium) dibromide. Inspection for ad-

Intramolecular π‐Stacking in Isostructural Conformational Probes Depends Strongly on Charge Separation, a Proton NMR Study

“…Solving these equations gave the distributions of the conformational classes C, F and S of 1a-c under various conditions. [13][14][15] Compound 1d was approximated to have no connection between conformational changes and chemical shift. We estimated that 1d corrected the drift in chemical shifts of Ha-Hc due to bulk solvent effects and due to changes in ion pairing.…”

“…Equations 1-4 were used to determine the mol fractions of the conformers present in solution with chemical shift as the independent variable. [15] …”

“…In the mathematical model these go from small to zero to correct for shielding by solvent (coefficient for Sef, Hc in Table 1). [15] Preoccupation about these issues instigated checks of Equations 2-4 for molecules 1a-c by averaging the S state expressions and excluding the hard-wired mass balance of Equation (1); the mass balanced in these systems of equations within ca. 10 %.…”

“…Descriptions of these checks are available in greater detail. [15] The mathematical model that connected experimental chemical shift differences to the conformation of 1a should perform as well for 2a once the ideal chemical shift reference is changed to 2d. Successful application requires near equivalent NMR anisotropy of monosubstituted phenyl rings so that the substituents: N-xylylpyridinium-2-yl of 1a, 2-xylylphen-1-yl of 2a and the F atom substituent in the model exert similar magnetic effects on the chemical shifts of proximal atoms.…”

“…The constants in Equations 2-4 had to be calculated for each of these separately. [15] To probe the applicability of the chemical shift constants used for 1a to the conformational analysis of 2a, chemical shift calculations of a series of monosubstituted benzene rings that spanned the usual linear free energy relationship were run and the results were plotted against sigmapara. [20,21] An H atom in H 2 was put at (x, y, z: 0.49, 0.49, 2.6 Å) with respect to a phenyl ring positioned with C1, and C4 along the horizontal axis; all C atoms were in the xy plane and the phenyl centroid was at 0, 0, 0.…”