Ortho effect in fluorobenzenes: cross-correlated relaxation and quantum chemical studies

✍ Scribed by Arunima; N. D. Kurur

• Publisher: John Wiley and Sons
• Year: 2005
• Tongue: English
• Weight: 151 KB
• Volume: 43
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.1520

✦ Synopsis

An early solid-state NMR study of the shielding tensors in substituted fluorobenzenes had indicated the presence of the 'ortho effect'. This was confirmed recently in the liquid state from a study of cross-correlated relaxation, which gives a handle on the shielding tensor. We report here a combined experimental and computational study on substituted fluorobenzenes where the ortho substituent is varied systematically. Experimental measurements of the longitudinal relaxation of 19F indicate the cross-correlation between the chemical shift anisotropy (CSA) of fluorine and its dipolar interaction with the ortho proton, and provide a measure of the CSA orientation parameter. This parameter is obtained also from quantum chemical calculations of the 19F CSA tensor. We establish a correlation between the CSA orientation parameter and linear free energy parameters by resorting to a multi-parameter regression analysis. Excellent correlation is obtained for most of these substituents only when a parameter for the ortho effect is included.