Detection of a Higher Energy Conformer of 2-Phenylethanol by Millimeter-Wave Spectroscopy

We have detected a second conformer of 2-phenylethanol and determined its shape using Stark-modulated free jet microwave spectroscopy with an elevated preexpansion temperature of 300 °C. By comparison of observed and ab initio predicted spectroscopic rotational constants and hydroxyl-deuteration isotopic substitution coordinates the species has been identified with the Cs trans conformer, designated as 5. This is in agreement with the previously published results from pulsed-jet UV−IR laser spectroscopy. These workers identified 5 together with the lower energy conformer 1 that also was detected by us in an earlier microwave study. Stark effect measurements on 5 yielded electric dipole moment values of μa = 0.3282(6) D, μb = 0.0 D (assumed), μc = 1.33(7) D, and hence μtotal = 1.37(7) D. Previously published large-basis MP2 level ab initio calculations predicted that, because of conformational relaxation, just two conformers should be present following argon jet cooling. However, the C1 trans conformer 4 was predicted to be more stable than 5. New ab initio calculations which include radially diffuse basis functions at the MP2/6-311++G(d,p) level now indicate that 5 is more stable than 4.