The rotational spectrum of the planar eis rotamer of formic acid, cis-HCOOH, has been detected for the first time. Twenty transitions belonging to the IRK , "JQi, QQ2 > r Po» r Pi r P2 brandies of the parent isotopic species in its ground state have been assigned and measured. The rotational constants and quartic centrifugal distortion constants have been determined using Watson's reduced Hamiltonian. Stark effect measurements have yielded the molecular electric dipole moment: /na= 2.65(1) D, ^6 = 2.71(1) D and ,« = 3.79(1) D. The energy difference between the ground vibrational states of eis-and iraras-HCOOH has been determined by microwave relative intensity measurements. The eis rotamer is found to lie at higher energy than the trans rotamer by 1365 + 30 cm -1 . A one dimensional potential energy curve has been calculated for the OH torsional vibration of formic acid.

The microwave and millimetre wave spectra of six isotopic species of isocyanic acid, HNCO, have been measured. The molecule is a very slightly asymmetric prolate rotor. Besides the a type R branches, previously reported for two species, we have assigned for the first time a type Q branches and also some intense b type P and R branch transitions. Accurate values of the rotational constants have been determined. The rotational constant A0 is particularly large, with the result that the molecule undergoes very large centrifugal distortion, and terms up to the 12th power in the angular momentum are required to fit the spectrum. 14N nuclear quadrupole hyperfine splitting has been measured and accurate values for both χaa and (χbb − χcc) have been obtained, the latter for the first time. From the Stark effect of the b type transitions we have found the b component of the dipole moment to be 1.35 ± 0.1 D, an order of magnitude larger than previously supposed, and have used it to reinterpret earlier data indicating a variation of the a component with Ka. An improved molecular structure has been obtained. Astrophysical consequences of the large b component of the dipole moment are discussed. A table of transitions of potential astrophysical interest is presented.

The very weak microwave spectra of six isotopically substituted species of formic acid in the eis rotamer form (i.e. with the two hydrogen atoms eis to each other) have been studied. From the data on HC 18 OOH, HCO^OH, H^COOH, HCOOD, DCOOH and DCOOD, together with the previously published data for HCOOH, the complete substitution structure of the eis rotamer of formic acid has been derived. The structure is: rs(C-H) = 1.105 (4)

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