Identification of Two Stable Side-Chain Orientations of Valine Methyl Ester by Microwave Spectroscopy

The rotational spectra of two valine methyl ester (ValOMe) conformers have been measured using a cavity-based Fourier-transform microwave spectrometer in the range of 9–18 GHz. Ten conformers of ValOMe were modeled using the ωB97XD/6-311++G(d,p) level of theory, and separate spectra arising from two lowest-energy conformations were observed and assigned. 44 rotational transitions were assigned to conformer I, the lowest-energy configuration, and were fit to Watson’s A-reduced Hamiltonian: A = 2552.0145(5) MHz, B = 1041.8216(3) MHz, and C = 938.54890(22) MHz. ¹⁴N nuclear quadrupole hyperfine splittings were resolved, and 231 hyperfine components were fit to χ_aa = −4.187(7) MHz, and χ_bb–χ_cc = 1.269(5) MHz. The spectrum of conformer I also reveals tunneling splittings arising from the methyl rotor. XIAM was used to fit the barrier to the internal rotation of the methyl rotor, and the best-fit V₃ barrier was found to be 401.64(19) cm^–1. 47 rotational transitions were assigned for conformer II (ΔE = 2.08 kJ mol^–1), and the fitted rotational constants are A = 2544.2837(3) MHz, B = 1092.3654(15) MHz, and C = 896.3131(12) MHz. 264 hyperfine components were fit to χ_aa = −4.187(7) MHz and χ_bb–χ_cc = 1.518(6) MHz, and the best-fit V₃ barrier was found to be 409.74(16) cm^–1.