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NMR detection of oriented association via dilution shifts in tetramethylsilane solvent. 1. n ‐Alkyl halides
Author(s) -
Tiers George Van Dyke
Publication year - 1999
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/(sici)1097-458x(199909)37:9<609::aid-mrc516>3.0.co;2-4
Subject(s) - chemistry , tetramethylsilane , alkyl , polarizability , substituent , dilution , solvation , solvent , halide , molecule , chemical shift , intermolecular force , dissociation (chemistry) , solvent effects , computational chemistry , stereochemistry , organic chemistry , thermodynamics , physics
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 tend to interact strongly with solutes, thus concealing weak intermolecular 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, H S V and C S V , of linear plots (by volume in TMS) of 1 H and 13 C deshielding values for n ‐alkyl halides are reported. These slopes (termed ‘sticking indices’) 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 H S V 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 29 Si satellites (for 13 C) and its 13 C and 29 Si satellites (for 1 H). The latter show an unusual previously unreported positive isotope shift of +0.7 ppb. 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.