Proximity and conformational effects on carbon-13 chemical shifts at the .gamma. position in hydrocarbons

Abstract

Using the modified-INDO finite perturbation theory of l3C chemical shifts, a set of "computer experiments" was performed to explore the geometrical dependence of the shielding of methyl groups in systems where steric effects may be expected. The conformational dependences of the computed methyl-group l3C shielding constants in «-butane and 2-butene systems were found to be substantial and to be qualitatively consistent with trends that have been popularly referred to as the 7 effect. However, inspection of the computed electron density distributions and details of the geometrical dependences lead to the suggestion that the mechanism of the 7 effect may be considerably more complex than the popularly accepted C-H bond polarization by nonbonded H~*H interactions. Calculations on systems in which two methane or ethane molecules are brought close enough together to emphasize intermolecular ~ interactions and calculations on «-pentane conformers similarly lead to a conclusion that the nature and conformational relationship of the bonding connecting the interacting methyl groups may be at least as important as the proximity of nonbonded hydrogens.(A) Steric Effects on 13C Shielding and the 7 Effect. In the early 1960's, as Paul and Grant1 were examining the l3C chemical shifts of linear alkanes with attention focused upon the possible existence of additivity relationships, an interesting pattern was uncovered. As Grant and Paul noted, and as can be seen in Table I in proceeding down the series from methane to propane, the chemical shift of the terminal carbon (Ci) moves increasingly toward lower shielding. The trend is reversed, however, for butane, in which the terminal carbon is more shielded than the terminal carbon of propane. This apparent reversal in trend has been noted in many saturated systems and has become known as "the 7 effect". This effect, as noted by Woolfenden and Grant,2 is also present in the alkenes but not in cumulenes or aromatics. An example of the 7 effect in alkenes is included in Table I, where it can be seen that the methyl carbons on both isomers of 2-butene are more shielded than the methyl carbon on propene. The effect is far greater for the cis isomer. This type of data led Woolfenden and Grant2 to suggest that the 7 effect was due to a through space interaction of "groups which have coiled back upon one another". Grant et al.2•3 went on to propose that this effect arose from electron repulsions between interacting C-H systems, which cause significant increases in the electron density about the carbon nucleus and, hence, it was argued, an increase in shielding. As a shorthand notation, we will refer to this as the "C-«-H-~*H-*-C" interpretation. As a specific example, let us consider «-butane, which is shown in both cis and trans configurations in Figure 1. In a gauche or cis configuration, the hydrogen atoms on the terminal methyl groups are close enough to one another to give rise, in the popular interpretation of the 7 effect, to electronic repulsions, inducing a polarization of electr...

Dibenzo-15-crown-5 ether and its sodium thiocyanate complex. X-ray crystallographic and NMR studies in the solid phase and in solution

“…In terms of the order of predicted shielding at carbons ortho to oxygen, this leads to the prediction that C-6 should be the most shielded, followed by C-11, C-14, and C-3. It is notable that this is the same order of shielding that one predicts based on the simple dihedral angle dependence of the y-steric shift in butane (23).…”

“…5 pprn relative to counterparts in y-trans orientations. Also there is a tendency to maximum shielding for cis coplanar networks (23) with maximum deshielding for a dihedral angle of 120°. 2 Comparison of the torsional networks for C-15 vs. C-18 reveals important differences, which would be expected to lead to increased shielding for the C-18 site.…”

NMR Analysis of Substituent Effect of Arymethylene‐malononitriles and Malonates

“…The solvents also alter the anisotropy ratio. The greater ratios are observed in CDCl 3 than that in dioxane-d 8 and DMSO-d 6 . The high polar DMSO-d 6 might associate with the solute to give more spherical cluster to enhance the symmetry and to low anisotropic ratio.…”

“…This indicates that the van der Waals interaction between analyte and solvent plays an important role. 8 The variation of the chemical shifts resulting from different solvents can reveal structural information of the solution phase. The 13 C chemical shifts of malononitriles 4A in nine solvents are listed in Table 4A.…”

Solid State 13C NMR of 30-Crown-10 Ether and 30-Crown-10.4H2O

“…It may be that other influences, such as long range intraring C-H interactions, provide significant contributions to the observed chemical shifts. Indeed, it has been demonstrated for butane that the orientations of remote C-H interactions do have a substantial influence on the INDO calculated 13 C shieldings [9]. Also, more recent work [10] has shown that lattice effects as well as torsionally dependent shielding tensors can be important in the computation of accurate 13 C shielding tensors in solids.…”