MOLECULAR-STRUCTURE AND CONFORMATIONS OF TETRAHYDROFURFURYL ALCOHOL FROM A JOINT GAS-PHASE ELECTRON-DIFFRACTION AND AB-INITIO MOLECULAR-ORBITAL INVESTIGATION

The molecular structure and conformational composition of tetrahydrofu rfuryl alcohol in the gas-phase has been studied by a joint electron d iffraction/ab initio method. The most abundant (84 +/- 8%) conformer ( A) in the gas-phase mixture was found to be stabilized by hydrogen bon ding and had the O-H group placed over the furanose ring, which had a distorted T-4(3) conformation. The other conformer (B) with the relati ve abundance of 16 +/- 8% had the O-H group located outside the ring a nd directed toward the ring oxygen also participating in the hydrogen bond formation. The barrier heights to pseudorotation in different con formers were estimated from ab initio calculations, Torsion strain was found to contribute mainly to the higher barrier near the E I form of the ring due to considerable ring flattening. The flattening might be a consequence of unfavorable axial position of the substituent at tha t point along the pseudorotation pathway. Differences between paramete rs in the same conformers as well as differences between parameters in the various conformers were assumed in the electron diffraction struc ture analysis from the MP2(FC)/6-311++G* ab initio calculations. The following values (r(g) bond lengths and r(a) angles with total errors) were found for the main parameters in the most stable conformer: r(C- C)(mean) = 1.538 +/- 0.004 Angstrom; r(C-O)(mean) = 1.430 +/- 0.003 An gstrom; r(C-H)(mean) = 1.109 +/- 0.003 Angstrom; r(O-H) = 0.925 +/- 0. 012 Angstrom; angle O-1-C-2-(6) =105.5 +/- 1.4 degrees; angle C-3-C-2- C-6 = 112.3 +/- 0.80; angle C-2-C-6-O-7 = 112.9 +/- 1.0 degrees; angle C-5-O-1-C-2 = 113.1 +/- 1.60; angle O-1-C-2-C-3 = 102.9 +/- 0.6 degre es; pseudorotation puckering amplitude q(0) = 9.8 +/- 0.3 degrees; pse udorotation phase angle f= 85.5 +/- 6.4 degrees. (C) 1998 Elsevier Sci ence B.V. All rights reserved.