Solvent effects on internal rotational barriers in furfural. NMR measurements and ab-initio molecular orbital methods using continuum models

Modern experimental and theoretical methods for determining solvent effects on internal rotational barriers in small molecules are compared. The barrier to rotation of the aldehyde group in furfural dissolved in toluene, acetone, and methanol is used as a test case. Ab-initio molecular orbital methods such as self consistent reaction field (SCRF) calculations, performed with the Onsager and isodensity surface polarized continuum (TPC) model, predict an increase in barrier with increasing solvent dielectric constant, epsilon. A combination of three nuclear magnetic resonance experiments are used to obtain rate data over 6 orders of magnitude representing an approximately 150 K temperature range. Activation parameters were obtained with errors less than 1 kJ/mol and 6 J/(mol K) for Delta H-double dagger and Delta S-double dagger, respectively. In acetone and toluene large Delta S-double dagger values of -26 and 20 J/(mol K) were found, along with a Delta S degrees of 10 J/(mol K) in both solvents, In methanol no appreciable values for Delta S-double dagger and Delta S degrees were measured. The Delta H-double dagger for toluene, acetone, and methanol are 48.6, 40.2, and 46.4, kJ/mol. respectively, which do not obey a simple relationship with epsilon. This indicates that the solvent effect is likely more complex than just the effect of a solvent reaction field. The large Delta S-double dagger values support this and also imply that equating Delta G(double dagger) and Delta H-double dagger is not always justified, even for aprotic solvents, The behavior of these three barriers and their corresponding Delta S-double dagger are discussed in terms of direct solvent-solute interactions.