A Monte Carlo-quantum mechanics study of the solvent-induced spectral shift and the specific role of hydrogen bonds in the conformational equilibrium of furfural in water

The solvation shift of the lowest absorption transition of furfural in water is analyzed as a function of the rotation angle for the interconversion between the two conformations, OO-cis and OO-trans, of furfural. In total, 20 Monte Carlo NPT simulations are performed, corresponding to different rotation angles of the carbonyl group. The solvation shift of the n-pi* state is calculated to be 1230 +/- 45 cm(-1) in the most stable OO-cis form. This calculated shift is found to be essentially independent of the rotation angle. The hydrogen bonds between furfural and water are also analyzed along the interconversion path. These hydrogen bonds are found to be equivalent, both in number and in binding energy, for all rotation angles. The results for the solvent-induced spectral shift and the hydrogen bond interactions confirm that in water they make no preference for any rotamer of furfural and lead to small contribution to the entropic activation barrier of furfural in protic solvents.