Rotational Isomerism Involving an Acetylenic Carbon II : Effect of 1-Halogen Substituents on Rotamer Population and Rotational Barrier around C(sp)-C(sp3) Bonds in Bis(9-triptycyl)ethynes

Autor:	Takao Yamamori, Michinori Oki, Toshiaki Makino, Shinji Toyota
Rok vydání:	2000
Předmět:	Steric effects education.field_of_study Chemistry Population General Chemistry Kinetic energy symbols.namesake Crystallography Computational chemistry Halogen symbols Proton NMR Van der Waals radius education Isomerization Conformational isomerism
Zdroj:	Bulletin of the Chemical Society of Japan. 73:2591-2597
ISSN:	1348-0634 0009-2673
DOI:	10.1246/bcsj.73.2591
Popis:	A series of (1,4-dimethyl-9-triptycyl)(1-halo-9-triptycyl)ethynes were synthesized to examine the effect of halogen substituents on the rotational isomerism about C(sp)-C(sp3) bonds. The kinetic and thermodynamic data of the isomerization between the ap and sc rotamers were determined by the total lineshape analysis and the intensity of the 1H NMR signals, respectively. The rotational barriers are enhanced with the steric size of halogen atoms from 11.6 (F) to 17.3 (I) kcal mol-1, and there is a linear correlation between the barrier height and van der Waals radius of the 1-substituent. Although large halogen atoms decrease the ratio of the sterically congested sc form, its population tends to be larger than that expected from the steric effect. This finding is attributable to the C-H…X(halogen) interactions between the 1-Me and X groups in the sc form. The possibility of the weak interactions is discussed on the basis of molecular structures optimized by the MM2 calculations.
Databáze:	OpenAIRE
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