NMR detection of oriented association via dilution shifts in tetramethylsilane solvent. 1. n-Alkyl halides

Infinite-dilution deshielding values, in one or more specified pure solvents, serve to characterize a known or unidentified molecule, even in mixtures. The usual solvents lend to inlemel strongly wilh solutes, thus concealing weak mlermoiccular associations. By definition, all solvents interact somewhat with solutes. The most favorable solvent in which to detect weak self-association of a solute must be a non-polar one having low and spherically symmetrical polarizability. Tetramethylsilane (TMS) is the most practical choice. The slopes, hsv and csv linear plots (by volume in TMS) of ^H and ^C deshielding values for alkyl halides are reported. These slopes (termed ʹslicking indicesʹ) differ substantially within the molecules, being largest at the halogen position and thus demonstrating oriented self-association. The slopes increase with substituent polarizability, not with electronegativity, Substantial negative (c)S(v) but near-zero HSv for all chain methylene groups is tentatively ascribed to alteration of conformational equilibria upon dilution. The reference TMS line position was located by means of its 29Si satellites (for13C) and its 13C and 29Si satellites (for 1H). The latter show an unusual previously unreported positive isotope shift of +0.7pph. Although perdeuterocyclohexane was used for the NMR lock, perdeutero-TMS, if available, would permit much greater latitude in dilutions. Copyright © 1999 John Wiley & Sons, Ltd.