Distinguishing tunneling pathways for two chiral conformer pairs of 1,3-propanediol from the microwave spectrum.

The microwave spectrum of the sugar alcohol 1,3-propanediol (CH(2)OHCH(2)CH(2)OH) has been measured over the frequency range 6.7 to 25.4 GHz using both cavity and broadband microwave spectrometers. The tunneling splittings from two structurally chiral conformer (enantiomeric) pairs of 1,3-propanediol have been fully resolved and assigned. The tunneling frequency of the lowest-energy inverting pair is 5.4210(28) MHz and found to increase by more than 7-fold to 39.2265(24) MHz for the higher-energy form. From the observed selection rules, three possible inversion pathways along the two OH concerted torsional modes have been identified and theoretically investigated. Quantum chemical calculations (MP2/aug-cc-pVTZ level) have been performed on the eight lowest-energy forms and three transition-state structures. Two of these pathways cross through C(S) transition states associated with each of the enantiomeric pairs and a third common pathway of lowest energy has a transition state of C(1) symmetry. For only the C(1) pathway is good agreement found between predictions from a 1D WKB analysis and the observed tunneling frequencies and 7-fold ratio. The conformer interconversion barrier is calculated to be about 3-fold smaller than that for the inversion suggesting the wave functions of the four inversion levels are partially delocalized over the four surface minima. Accurate dipole moment components have also been obtained from Stark effect measurements for the lowest-energy form.