Solvent Effects on Internal Rotational Barriers in Furfural. NMR Measurements and ab-Initio Molecular Orbital Methods Using Continuum Models

Abstract

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 (IPC) model, predict an increase in barrier with increasing solvent dielectric constant, ε. 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 ΔH ⧧ and ΔS ⧧, respectively. In acetone and toluene large ΔS ⧧ values of −26 and 20 J/(mol K) were found, along with a ΔS° of 10 J/(mol K) in both solvents. In methanol no appreciable values for ΔS ⧧ and ΔS° were measured. The ΔH ⧧ for toluene, acetone, and methanol are 48.6, 40.2, and 46.4 kJ/mol, respectively, which do not obey a simple relationship with ε. This indicates that the solvent effect is likely more complex than just the effect of a solvent reaction field. The large ΔS ⧧ values support this and also imply that equating ΔG ⧧ and ΔH ⧧ is not always justified, even for aprotic solvents. The behavior of these three barriers and their corresponding ΔS ⧧ are discussed in terms of direct solvent−solute interactions.

Conformational stability of furfural in aqueous solution: the role of hydrogen bonding

“…Here, the average number and the strength of hydrogen bonds formed between the solute and the solvent are essentially constant along the internal rotation, giving again no contribution to the activation entropy. These results are in fair agreement with previous observations from Bain and Hazendonk [28] for the investigation of furfural in methanol.…”

“…Recently, Baldridge et al [12] showed that the conformer with larger dipole moment, OOcis, prevails in solvents with dielectric constants higher than 5. However, the stability between both conformers does not obey a simple relationship with the dielectric constant [28]. This, of course, indicates that the solvent effects are more complex than could be inferred by a simple reaction field.…”

“…The solvent dependence of the activation barrier for the cistrans interconversion of furfural is really an important topic in biophysical chemistry. Bain and Hazendonk [28] have used NMR measurements to determine the thermodynamic parameters for the rotation of the carbonyl group in furfural dissolved in toluene, acetone, and methanol. They have found large entropy effects for the non protics solvents, toluene and ace-tone, and none for the protic solvent methanol.…”

Theoretical model of internal rotation in monosubstituted derivatives of furfural

“…The conformational equilibrium between planar forms of furfural (Fig. 1) has received special attention in both experimental and theoretical articles,1–14 including the effect of solvent on the OO‐cis/OO‐trans population ratio and on rotational barriers. The relative facility for the industrial production of this compound from pentose‐rich biomass and its two well‐defined, self‐exchangeable and stable isomers have sparkled imagination as a prospect for a single molecule bit of information storage.…”

Boundary element methods for dielectric cavity construction and integration

“…37 The cavity surface of a molecular solute is usually defined either as a union of atomic spheres, 38 often with additional constructions to fill in the regions where adjacent spheres intersect, 39,40 or as a solute electronic isodensity contour. 5,19,41,42 In practice, these prescriptions may not differ too much from one another. We prefer the isodensity contour construction, which automatically provides a smooth surface and is completely specified by a single parameter, namely the numerical value chosen for the contour.…”