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Intramolecular motion and reorientational barriers in 9‐fluorenone, 4‐methyl‐9‐fluorenone and 4,5‐dimethyl‐9‐fluorenone
Author(s) -
Wilson Nancy K.
Publication year - 1985
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/mrc.1260230106
Subject(s) - chemistry , fluorenone , methyl group , intramolecular force , arrhenius equation , steric effects , activation energy , photochemistry , crystallography , stereochemistry , organic chemistry , group (periodic table) , polymer , fluorene
Carbon‐13 nuclear magnetic resonance spin‐lattice relaxation times and nuclear Overhauser enhancements were measured as functions of temperature for the hydrogen‐bearing carbons in 9‐fluorenone, 4‐methyl‐9‐fluorenone and 4,5‐dimethyl‐9‐fluorenone. Reorientational diffusion constants were obtained from the dipolar T 1 values. Isotropic overall molecular motion was assumed, and the methyl group rotation was separated from the overall molecular reorientation by the method of Woessner. Arrhenius plots of the dependence of the rotation rates on temperature provided values of the energy barriers for molecular reorientation and methyl group rotation. The energy barriers for molecular reorientation are essentially the same: 9‐fluorenone 11.8 ± 0.5 kJ mol −1 , 4‐methyl‐9‐fluorenone 11.9 ± 0.5 kJ mol −1 and 4,5‐dimethyl‐9‐fluorenone 11.7 ± 0.2 kJ mol −1 . The barriers to methyl group rotation are 4‐methyl‐9‐fluorenone 7.8 ± 0.9 kJ mol −1 and 4,5‐dimethyl‐9‐fluorenone 11.0 ± 0.5 kJ mol −1 . Thus the bay methyl‐methyl steric interaction in 4,5‐dimethyl‐9‐fluorenone increases the methyl rotational barrier by 3.2 kJ mol −1 .