Anomalous chemical shifts in the nuclear magnetic resonance spectra of some neopentyl compounds

Possibilities and results of "C-DMR line shape analysis are explored with IX-2,2-dimethylpropanes (neopentanes, I), 2X-3,3-dimethylbutanes (2) and 2X-2,3,3-trimethylbutanes (3) with X = H, Hal, OR or CH,. The observed barriers AG· consistently increase in this sequence, e.g. for X = Cl from 6.5 (I) to 7.7 (2) to 10.4 (3) kcal/mol, Molecular mechanics force field calculations for I, although giving barriers too low by 0.5-1.5 kcal/rnol, correctly reproduce the observed trends and explain the puzzling low barriers observed for X = CH, in comparison to X = Hal, as the result of the higher ground state energy of the methyl compounds. Conformational equilibria are investigated with the non-topomeric 2X-2,3-dimethylbutanes. The AGo values obtained from low temperature signal integration agree with force field predictions only for X = Hand F, which dominate, as X = OR does, in the all-gauche conformation; in contrast, the predominance of the trans conformer for X = CI, Br and J is not rationalized. The calculated ground state geometries show no significant geometry distortion upon change of the substituents X, which provides a basis for the I'C shielding comparison to corresponding cyclohexanes of similar substitution pattern. The observed substituent effects on the "C shifts are in agreement with other models and mechanistic proposals for the screening constant variations.

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“…and used without purification. "C_'''F couplings [Hz) (CFCI" 200 K): 168.4 ('J), 22.1 fJ to /3CH), 25 fJ to /3CH,), 5.9 ('J).…”

Section: Methodsmentioning

confidence: 99%

¹³C‐NMR Spectroscopic and Conformational Studies on Rotations in Substituted Ethanes

Freitag

Schneider

1980

Israel Journal of Chemistry

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An empirical correlation of NMR chemical shifts and conformations in ethers and amines

Price

1971

Tetrahedron Letters

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Anomalous chemical shifts in the nuclear magnetic resonance spectra of some neopentyl compounds

Cited by 2 publications

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¹³C‐NMR Spectroscopic and Conformational Studies on Rotations in Substituted Ethanes

¹³C‐NMR Spectroscopic and Conformational Studies on Rotations in Substituted Ethanes

An empirical correlation of NMR chemical shifts and conformations in ethers and amines

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Anomalous chemical shifts in the nuclear magnetic resonance spectra of some neopentyl compounds

Cited by 2 publications

References 0 publications

13C‐NMR Spectroscopic and Conformational Studies on Rotations in Substituted Ethanes

13C‐NMR Spectroscopic and Conformational Studies on Rotations in Substituted Ethanes

An empirical correlation of NMR chemical shifts and conformations in ethers and amines

Contact Info

Product

Resources

About

¹³C‐NMR Spectroscopic and Conformational Studies on Rotations in Substituted Ethanes

¹³C‐NMR Spectroscopic and Conformational Studies on Rotations in Substituted Ethanes