A STUDY OF CHEMICAL-EXCHANGE IN UNEQUALLY POPULATED SYSTEMS BY NOVEL NMR METHODOLOGIES - APPLICATION TO THE CIS-TRANS ISOMERIZATION IN FURFURAL

The internal rotation of the aldehyde group with respect to the ring in 2-furanaldehyde (furfural) provides an example of chemical exchange in a two-site system with unequal populations. In a polar solvent, the two isomers are present in unequal amounts. In the present study, rates of exchange for the cis-trans isomerization process of furfural in acetone are measured using a combination of the offset-saturation method for determination of fast rates of exchange, time-domain line shape analysis for measurement of intermediate rates of exchange, and selective-inversion measurements of slow exchange rates. This combination of NMR techniques permits the determination of rates of exchange over 5 orders of magnitude, covering a temperature range of 177-318 K. The utility of the offset-saturation method for fast rate measurement is highlighted in the present system, where the exchange rapidly enters the fast exchange regime. The corresponding values of the activation parameters are the energy of-activation E(2) = 41.9 +/- 0.9 kJ mol(-1), the enthalpy of activation Delta H* = 40.0 +/- kJ mol(-1), and the entropy of activation Delta S* = -26.8 +/- 5.2 J K-1 mol(-1).