NMR chemical shifts as probes for steric effects in mono‐ and disubstituted adamantanes

Abstract

Substituent effects are widely used to investigate the factors affecting carbon‐13 chemical shifts. Adamantane and its derivatives are convenient probes for relative contributions to these factors owing to their symmetry and relative absence of ring strain. We have extended our studies on NMR chemical shifts of 1‐ and 2‐methyladamantanes to hydroxy, bromo, methoxy and acetamide substituents and also certain disubstituted (one of the groups being methyl) analogs. DFT/GIAO calculations at the B3LYP/6–31G(d,p) level show that, except for α‐effects, steric interactions are mainly responsible for substituent effects on chemical shifts. The CHARGE program is particularly well suited for localizing these effects and estimating their approximate shape and range.

1H NMR spectra of alcohols and diols in chloroform: DFT/GIAO calculation of chemical shifts

“…[19] In contrast, stereoelectronic effects on shifts in selected conformers of acyclic and polycyclic alcohols were investigated at the Becke, threeparameter, Lee-Yang-Parr (B3LYP)/6-31G(d,p) level. [20][21][22][23][24][25] Computed OH proton shifts of unsaturated alcohols [26] and alkane diols [27][28][29] in benzene are in fair agreement with experimental data, but the higher the CH proton shift, the more it is overestimated. Because Gaussian 09 [30] provides a better description of OH shifts in alkane diols [27][28][29] than Gaussian 03 [31] used in the earlier work, [26] it seemed opportune to revise these calculations, to reconsider both unsaturated alcohols and diols in chloroform and, at the same time, to examine a broader range of saturated alcohols.…”

“…= 0.99455. Seidl et al computed shifts for adamantan‐1‐ol, 33 , and adamantan‐2‐ol, 34 , by the same approach, but did not compare them with the experimental data, and they are not included in Abraham's study . If they are, the correlation is somewhat poorer: intercept = 0.013 ± 0.034 ppm, gradient = 0.988 ± 0.016, corr.…”

NMR investigation of steric effects in alkyl‐ and haloadamantanes

“…1,[9][10][11][12] Hydrogen chemical shifts also evidence steric interactions, nuclei under compression being deshielded relative to equivalent positions on the unsubstituted system. 2 A combination of these effects which reflects a shortening of the C-H bond, an increase in charge on the carbon nucleus and a decrease in charge on the hydrogen nucleus is referred to as bond polarization. 13 Changes in bond and dihedral angles often accompany steric effects and, in situations where the molecular framework is distorted, also may be associated with steric effects.…”

“…These data could also be used to separate steric contributions from those that are mainly of electronic nature and to evaluate the shape and volume of the groups that are involved in the respective interactions. 2 Changes in geometry are reflected mainly by carbon chemical shifts and hydrogen chemical shifts can be used to verify the direction in which the substituent is pointing and how far it extends. 2 The steric effects that will result from a substituent in a certain position can therefore be well established.…”

“…2 Changes in geometry are reflected mainly by carbon chemical shifts and hydrogen chemical shifts can be used to verify the direction in which the substituent is pointing and how far it extends. 2 The steric effects that will result from a substituent in a certain position can therefore be well established. These aspects were investigated in the series of adamantanes with alkyl (1-9) and halogen substituents (10)(11)(12)(13)(14)(15) that are commonly associated with steric effects 3 and given in Fig.…”