The Other Rotamer of Formic Acid, cis-HCOOH¹

Abstract

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.

Role of water in formic acid decomposition

“…For example, Figure 7 shows that the trans isomer is 4.1 kcal/mol more stable than the cis isomer in the absence of water. This energy difference is within 0.1 kcal/mol of previous experimental determinations (Lide, 1966;Hocking, 1976) and electronic structure calculations (Francisco, 1992;Goddard et al, 1992). Since the trans isomer is more stable, most of the formic acid will be in that form.…”

Formic acid above the Jungfraujoch during 1985–2007: observed variability, seasonality, but no long-term background evolution

“…In particular, an integrated HCOOH band intensity over the 1045-1150 cm −1 interval was found equal to 38.8 × 10 −18 cm −1 /(molec cm −2 ) at 296 K, thus nearly twice the value reported by Goldman and Gillis (1984). Perrin and Vander Auwera (2007) synthesized the original improvements of this research with respect to earlier ones, pointing out that the new band intensity refers to the trans-form of HCOOH, as natural formic acid only contains about 0.1% of the HCOOH cis-form (Hocking, 1976). They further compiled an improved line list for fitting HCOOH in the 9-micron region, containing individual positions and intensities of almost 50 000 lines between 940.20 and 1244.41 cm −1 .…”

Trans-cismolecular photoswitching in interstellar space

“…The energy barrier to internal rotation (the conversion from trans to cis) is approximately 4827 cm −1 (Hocking 1976), approximately 7000 K in temperature units. This is much higher than the thermal energy available in molecular clouds (having typical temperatures from approximatley 10 to 300 K).…”