Microwave Spectra, Hyperfine Structure, and Electric Dipole Moments for Conformers I and II of Glycine

A number of low-J rotational transitions have been measured for two conformations of the simplest amino acid, glycine, providing precise hyperfine transition frequencies and electric dipole moments for radioastronomy searches for glycine in interstellar clouds. Measurements were made between 12 and 25 GHz with a pulsed-nozzle, Fabry-Perot-cavity, Fourier transform microwave spectrometer equipped with either a heated nozzle or laser ablation-nozzle for vaporization of solid phase glycine. Nuclear quadrupole coupling constants were determined for both conformers from measurements of the hyperfine-split rotational transitions. The quadrupole coupling constants for conformer I are eQq_aa = -1.208(9) MHz and eQq_bb = -0.343(8) MHz and for conformer II are eQq_aa = 1.773(2) MHz and eQq_bb = -3.194(4) MHz. The dipole moment components of both conformers were determined to higher precision. We obtain μ_a = 3.039(10) × 10^-30 C m [0.911(3)D] and μ_b = 2.025(17) × 10^-30 C m [0.607(5) D] for conformer I, and μ_a = 17.92(11) × 10^-30 C m [5.372(34)D] and μ_b = 3.10(3) × 10^-30 C m [0.93(1)D] for conformer II, where the uncertainties shown are 1 standard deviation.