Abstract The rotational spectrum of the planar cis rotamer of formic acid, cis-HCOOH, has been detected for the first time. Twenty transitions belonging to the qRK, qQ1, qQ2 , rP0 , rP1,rP2 branches 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: μa= 2.65(1) D, μb = 2.71(1) D and , μ= 3.79(1) D. The energy difference between the ground vibrational states of cis-and trans-HCOOH has been determined by microwave relative intensity measurements. The cis 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 Other Rotamer of Formic Acid, cis-HCOOH1
