Rotational spectra and conformational structures of 1-phenyl-2-propanol, methamphetamine, and 1-phenyl-2-propanone.

Microwave spectra have been recorded for 1-phenyl-2-propanol, methamphetamine, and 1-phenyl-2-propanone from 11 to 24 GHz using a Fourier-transform microwave spectrometer. Only one spectrum from a single conformational isomer was observed for each species. The rotational transitions in the spectrum of 1-phenyl-2-propanone were split into separate transitions arising from the A- and E-torsional levels of the methyl rotor. The fit of the E-state transitions to a "high-barrier" internal rotation Hamiltonian determines V3 = 238(1) cm-1 and rotor-axis angles of thetaa = 87.7(5) degrees, thetab = 50.0(5) degrees, and thetac = 40.0(5) degrees. Ab initio optimizations (MP2/6-31G**) and single-point calculations (MP2/6-311++G**) were used to model the structures of 1-phenyl-2-propanol, methamphetamine, and 1-phenyl-2-propanone. The lowest energy conformations of these species were found to be stabilized by weak OH-pi, NH-pi, and CH-pi hydrogen-bonding interactions. Moments of inertia, derived from the model structures, were used to assign the spectra to the lowest energy conformation of each species. A series of MP2/6-31G* partial optimizations along the internal rotation pathway were used to estimate the barrier to methyl rotation to be 355 cm-1 for 1-phenyl-2-propanone.