The challenging conformer assignment of proline methyl ester from rotational spectroscopy†

The conformational structures of proline methyl ester (PrOMe) were modeled using CREST and further optimized using ωB97XD and MP2 methods with the 6-311++G(d,p) and aug-cc-pVDZ basis sets. Among the seven lowest energy conformers, two unique conformers, C^γ-exo/C^δ-endo and C^γ-endo, were found to be very close to the minimum energy. A rotational spectrum consisting of 51 rotational transitions was recorded for PrOMe using a cavity-based Fourier-transform microwave spectrometer in the range 9–20 GHz. The rotational transitions, split into resolved ¹⁴N-nuclear quadrupole hyperfine components for the A- and E-methyl-internal-rotation tunneling states, were fit using XIAM: A = 3678.4360(7) MHz, B = 1037.5616(3) MHz, and C = 944.2045(3) MHz, and the barrier to methyl torsion was found to be 393.54(9) cm⁻¹. Comparison of model and spectroscopic moments of inertia is insufficient to conclusively assign the conformational structure. Analysis of second moments of inertia, dipole moment projections, and nuclear quadrupole hyperfine constants provides sufficient additional evidence to determine that the rotational spectrum is from a structure with an intramolecular hydrogen bond from the imino hydrogen to the carbonyl oxygen and with C^γendo.