Rotation of Aromatic Groups

The elucidation of the intramolecular dynamics of tryptophan residues became possible due to anisotropy studies with nanosecond time resolution. Two approaches have been taken direct observation of the anisotropy kinetics on the nanosecond time scale using time-resolved(28) or frequency-domain fluorometry, and studies of steady-state anisotropy for xFvarying within wide ranges (lifetime-resolved anisotropy). The latter approach involves the application of collisional quenchers, oxygen(29,71) or acrylamide.(30) The shortening of xF by the quencher decreases the mean time available for rotations of aromatic groups prior to emission. 

Rotations of aromatic groups should be associated with a considerable... 

Scheme 65). The conformational state and barrier to rotation of methyl groups have attracted much theoretical and experimental interest. The barriers to rotation of methyl in various aza aromatics have been determined in the solid state from H-spin-lattice relaxation times (85JOC2972). Such barriers for 84a-i are given (in kJ mol-1) in Scheme 66. 

RECIPROCAL RELATIONS IN OPTICAL ROTATION AND THE PROXIMITY OF AROMATIC GROUPS... 

Internal rotation of hydroxy-group against an aromatic frame has been assumed to be restricted by a potential with 2-fold symmetry. The —OH internal rotation barriers in three cresols have been assumed to be the... 

NMR spectroscopy is a powerful method for the detection of steric effects in molecules in which steric interactions are characterized by rotation of functional groups around single bonds to relieve van der Waals interactions or on rigid systems in which steric interactions are partially accommodated by bond angle and bond length distortions. Applications include aromatic and heteroaromatic nitro compounds, aryl ketones, aldehydes, 1,2-diketones, aromatic carboxylic acids, esters and amides. 

On page 132, atropisomerism was possible when ortho substituents on biphenyl derivatives and certain other aromatic compounds prevented rotation about the bond. The presence of ortho-substituents can also influence the conformation of certain groups. In 88, R= alkyl, the carbonyl unit is planar with the trans C=0 - F conformer more stable when X=F. When X=CF3, the cis and trans are planar and the trans predominates. When R = alkyl there is one orthogonal conformation but there are two interconverting nonplanar conformations when R=0-alkyl. In 1,2-diacylbenzenes, the carbonyl units tend to adopt a twisted conformation to minimize steric interactions. " ... 

FIGURE 6.16 ortho-Quinone methide 3 stabilization of the zwitterionic rotamer in a complex with /V-methyImorpholine /V-oxide (17). The zwitterionic, aromatic precursor 3a affords the common quinoid form of the o-QM 3 by in-plane rotation of the exocyclic methylene group. 

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