¹³C Nuclear Magnetic Resonance Studies on Molecular Association. I. Self-association of Dipolar Molecules

Abstract

Self-association of dipolar substances is studied by 13C magnetic resonance. The dilution shifts in carbon tetrachloride are analyzed in terms of the monomer–dimer equilibrium. Downfield 13C shifts of the nitrile and carbonyl groups are observed for the formation of the dimer of acetone, acetonitrile, and ethyl acetate. Upfield 13C shifts, on the other hand, are observed in the methyl and methylene groups of acetone, methylethyl ketone, ethyl acetate, acetonitrile, propionitrile, nitromethane, nitroethane, N,N-di-methylformamide, and N,N-dimethylacetamide. The equilibrium constants are 0.1–0.5 in units of mole fraction. The downfield 13C shifts of the nitrile and carbonyl groups are interpreted in terms of electric (reaction) field effects imposed by another molecule through dimer formation. The alternation of polarity by polarization of the X–Y group explains the upfield 13C shifts of methyl and methylene groups as shown in the following scheme: Hs+—C−—X+—Y−, where X—Y stands for C=O, C≡N, and NO2 groups which have been classified as –I− groups by Pople and Gordon.

Solvent effects on 13C and 17O chemical shifts of polar molecules. Comparison between experiment and theory

“…Although our results are in fairly good agreement with those of Saito et a1.,12 the concentrations used by us for extrapolation at infinite dilution' are lower than those in Ref. 12 and are, therefore, likely to be more reliable. Since the data reported by Ando et aL8 diverge widely from ours' and from Saito's,'' they merit re-examination.…”

Effect of Temperature on the Organization of Acetonitrile at the Silica/Liquid Interface

“…Despite the relatively simple structure of an acetonitrile molecule, the organization of liquid acetonitrile exhibits fascinating behavior. Given the substantial dipole moment of this molecule (3.92 D), acetonitrile has some propensity for dipole pairing in the bulk. , However, at a silica surface, acetonitrile takes on a remarkably different organization. Because acetonitrile can accept a hydrogen bond, there is a layer of molecules whose cyano groups point toward the silica surface.…”

Liquid crystalline materials: physical properties and intermolecular interactions

“…Summarizing, we can say that the formation of dimers is a conse quence of the antiferroelectric interactions between dipole moments and probably also induced dipoles, in combination with the liquid structure. The idea of molecular association of nitriles in both the nematic and the isotropic phase is supported by X-ray measurements (Leadbetter 1975) and nuclear magnetic resonance studies (Saito et al 1973). Additional information can sometimes be obtained from the layer spacing of a smectic A phase, when this phase occurs at lower temperatures.…”