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NMR Studies of Internal Rotation about the C‐C Bond in 1, 1, 2‐Trisubstituted Ethanes of the type XCH 2 ‐CHX 2
Author(s) -
Lin WeiChuwan,
Chen JennShing,
Chen LiShu,
Lin Shin
Publication year - 1976
Publication title -
journal of the chinese chemical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.329
H-Index - 45
eISSN - 2192-6549
pISSN - 0009-4536
DOI - 10.1002/jccs.197600019
Subject(s) - chemistry , vicinal , coupling constant , rotation (mathematics) , yield (engineering) , conformational isomerism , atmospheric temperature range , dielectric , solvent , constant (computer programming) , internal rotation , reaction rate constant , computational chemistry , thermodynamics , analytical chemistry (journal) , molecule , organic chemistry , quantum mechanics , mechanical engineering , physics , geometry , mathematics , computer science , engineering , programming language , kinetics
The vicinal H‐H coupling constant in the AB 2 spectrum, J AB , of 1, 1, 2‐trisubstituted ethanes of the type XCH 2 ‐CHX 2 has been measured in the medium of various solvents and at several different temperatures over the range of 238°‐368° K . Based on rotational averaging and on the simple model of internal rotation about the C‐C bond, a refiend Karplus equation for the vicinal coupling constant could yield the analytical expression which describes an explicit functional dependence of the vicinal coupling constant on internal rotation parameters, and permits evaluation of ΔE, the energy difference between rotamers, at any fixed temperature. For neat 1, 1, 2‐trichloroethane ΔE was found to keep constant over the range of temperature variation studied, while in the case of neat 1, 1, 2‐tribromoethane ΔE increases with increasing temperature in the same temperature range. In contrast to a small temperature effect a much pronounced medium effect on ΔE was observed for both 1, 1, 2‐tribromoethane and 1, 1, 2‐trichloroethane. At any fixed temperature ΔE decreases as dielectric constant of the solvent increases.