Tucker et al. 1971
Tucker, K.D., G.R. Tomasevich, and P. Thaddeus, 1971: Laboratory measurement of the 6-centimeter formaldehyde transitions. Astrophys. J., 169, 429-440, doi:10.1086/151155.
The 110-111 rotational transition of H212C160 in its ground and first two excited vibrational states, and that of H213C16O, 212C180, and HD12C160 in their vibrational ground states, all of which lie at a wavelength of about 6 cm, have been studied with a beam-maser spectrometer. Line widths in the range 1-3 kHz have been attained, allowing the hyperfine structure of the transitions to be rather thoroughly resolved, and transition frequencies to be measured to an accuracy of the order of 100 Hz. All significant features of the observed spectra have been successfully interpreted in terms of the current theory of molecular hyperfine structure, and the appropriate hyperfine coupling constants have been determined.