The structure and electric dipole moment of camphor determined by rotational spectroscopyElectronic supplementary information (ESI) available: Measured and fitted frequencies of field-free rotational transitions and of Stark components, and the results of fitting the molecular geometry. See http://www.rsc.org/suppdata/cp/b2/b212029a/

The rotational spectrum of camphor was investigated for the first time. Accurate spectroscopic constants for the parent isotopomer were determined from measurements in supersonic expansion at 6.7–18.5 GHz, and on a room-temperature sample at 173–222 GHz. All three types of allowed dipole moment transitions, μa, μb, μc, were observed. The supersonic expansion spectrum allowed measurement, in natural abundance, of all ten singly substituted 13C isotopomers, as well as of the 18O isotopomer. The rotational constants were used to determine the rs, r0, and the r(1)m gas-phase geometries of the camphor molecule. The electric dipole moment of camphor was determined from Stark effect measurements, and is |μa| = 2.9934(23) D, |μb| = 0.7298(6) D, |μc| = 0.0804(7) D, μtot = 3.0821(22) D. The experimental results are compared with predictions from ab initio calculations and with the geometry of solid camphor.