Through-space polarization

In aromatic compounds carbon-13 shifts are largely determined by mesomeric (resonance) and inductive effects. Field effects arising from through-space polarization of the n system by the electric field of a substituent, and the influences of steric (y) effects on the ortho carbon nuclei should also be considered. Substituted carbon (C-l) shifts are further influenced by the anisotropy effect of triple bonds (alkynyl and cyano groups) and by heavy atom shielding. 

The remarkably low reactivity of tetrafluoroparacyclophane (29) to Friedel-Crafts acylation in comparison with para-xylene indicates the deactivation of the nonfluorinated aromatic ring of 29 because of electron-deficiency due to a through-space polarization (Scheme 1.33) [26]. The tt-tt stacking interaction between arenes and perfluoroarenes has been employed for modification of the liquid crystals [27]. 

Chen, C. T., and Siegel, J. S. "Through Space Polar-ir Effects on the Acidity and Hydrogen Bonding Capacity of Carboxylic Acids." /. Am. Chem. Soc., 116,5959-5960 (1994). predictable pK shifts... 

The second component is called afield effect and is attributed to through-space interactions of the electric dipoles resulting from polar bonds. 

As suggested by Roberts and Moreland many years ago (1953), the acidity constants of 4-substituted bicyclooctane-l-carboxylic acids provide a very suitable system for defining a field/induction parameter. In this rigid system the substituent X is held firmly in place and there is little possibility for mesomeric delocalization or polarization interactions between X and COOH (or COO-). Therefore, it can be assumed that X influences the deprotonation of COOH only through space (the field effect) and through intervening o-bonds. On this basis Taft (1956, p. 595) and Swain and Lupton (1968) were able to calculate values for o and crR. 

In addition to sample rotation, a particular solid state NMR experiment is further characterized by the pulse sequence used. As in solution NMR, a multitude of such sequences exist for solids many exploit through-space dipolar couplings for either signal enhancement, spectral assignment, interauclear distance determination or full correlation of the spectra of different nuclei. The most commonly applied solid state NMR experiments are concerned with the measurement of spectra in which intensities relate to the numbers of spins in different environments and the resonance frequencies are dominated by isotropic chemical shifts, much like NMR spectra of solutions. Even so, there is considerable room for useful elaboration the observed signal may be obtained by direct excitation, cross polarization from other nuclei or other means, and irradiation may be applied during observation or in echo periods prior to... 

Where the positive charge on the atom adjacent to the nucleus is real rather than formal, i.e. NR3 rather than N02, there is evidence that its effect on o complex stability is exerted through a field effect (cf. p. 22) operating through space, in addition to any polar (inductive) effect operating through the bonds. The deactivating effect of Y on the nucleus declines, i.e. the overall rate of substitution increases, in the approximate order ... 

Many authors have tried to identify other contributions to y-gauche substituent effects instead of or in addition to sterically induced bond polarizations. For electronegative substitutents, for example F, OR, NR2, and Cl, the most obvious interpretation is to assume a through-space electric-field transmission of yg-SCS... 

---