The semiempirical derivation of ¹³C nuclear magnetic resonance chemical shifts. Hydrocarbons, alcohols, amines, ketones, and olefins

Abstract

. Can. J. Chem. 55,2813Chem. 55, (1977.Substituent parameters were derived for the semiempirical determination of 13C chemical shifts in saturated hydrocarbons, alcohols, amines, ketones, and olefins. The olefin parameters are valid for six-membered rings and the remaining parameters for six-membered rings in chair conformations. We have shown that P-carbon substituent double bond subslituent effects suggested that effects on 13C resonances are related to the the y-group has not introduced a new shielding number of hydrogen-carbon (HC) and carbonmechanism which fortuitously cancels the HC carbon (CC) gauche interactions (1). Similarly, shift increment, but rather that it has eliminated P-heterosubstituent effects depend on the num-the HC shift contribution (3).' In other words, ber of hydrogen-heterosubstiluent (HX) and the observed gauche y-effect is not associated carbon-heterosubstiluent (CX) gauche inter-with the y-group but with the elimination of the actions (2). If a gauche y-substituent is added to H an HC configuration, one observes a displacement which is equal in size, but opposite in sign, to the HC shift increment. An investigation of For personal use only.

Semiempirical calculation of 13C nuclear magnetic resonance chemical shifts of acyclic hydrocarbons. Application to the stereochemical analysis of steroidal side chains

“…In Table 1 a comparison is shown between the experimental data and the values calculated with the program. The standard deviation is within the expected values (13,14) although the errors may accumulate in some unfavourable cases. However, the power of the method rests in the prediction of relative shieldings or deshieldings, and is not intended to give reliable absolute values.…”

“…Several rules for evaluating chemical shifts of sp3 carbons have been proposed (8)(9)(10)(11)(12), but they cannot deal with molecular asymmetry or conformationally rigid molecules. The semiempirical method developed by Beierbeck et al (13,14) allows us to predict, with a satisfactory degree of accuracy, 13C chemical shifts of saturated aliphatic hydrocarbons and rigid cyclohexanes having different substituents, and has been recently extended to epoxides (1 5).…”

“…14. Individual 13c resonances are predicted using the substituent parameter set derived for rigid cyclic systems (13), and are conformationally averaged using the populations pj . Since the number of conformers grows exponentially with the number of rotated C-C bonds, the calculation of a medium size hydrocarbon requires considerable work.…”

Aliphatic diazo compounds. XIV. The synthesis of 7-substituted 3-diazo-2-norbornanones and the infrared and proton and carbon-13 nuclear magnetic resonance spectra of these diazo ketones and their precursors

“…If the effect is primarily steric in origin, this circumstance would be nicely accommodated by our earlier proposal of preferred conformations of type 50 for the latter groups. This interpretation must be treated with caution, since it is possible that the 6 effect is not primarily steric in nature and devolves from removal of a y hydrogen, in analogy to the proposal of Saunders and ApSimon and their co-workers that the y effect arises from removal of a p hydrogen (30,35).…”

Conformational analysis of 1-oxaspiro[5.5]undecanes. A powerful probe for the endo and the exo anomeric effects in acetals

“…Considering the semiempirical methods for the determination of chemical shift in 13C nmr (7,12) and the analysis of the steric effects observed in each conformer, we can predict that major differences will be observed, in terms of chemical shifts for C5, C7, and C l l . Effectively, at room temperature, C7 and C I l are averaged at 35.2 as well as C8 and Clo at 21.9 ppm.…”