Microwave Spectrum for a Second Higher Energy Conformer of Cyclopropanecarboxylic Acid and Determination of the Gas Phase Structure of the Ground State

Abstract

Microwave spectra for a higher-energy conformer of cyclopropanecarboxylic acid (CPCA) were measured using a Flygare-Balle-type pulsed-beam Fourier transform microwave spectrometer. The rotational constants (in megahertz) and centrifugal distortion constants (in kilohertz) for this higher-energy conformer are A = 7452.3132(57), B = 2789.8602(43), C = 2415.0725(40), DJ = 0.29(53), and DJK = 2.5(12). Differences between rotational constants for this excited-state conformation and the ground state are primarily due to the acidic OH bond moving from a position cis relative to the cyclopropyl group about the C1-C9 bond to the more stable trans conformation. Calculations indicate that the relative abundance of the higher-energy state should be 15% to 17% at room temperature, but the observed relative abundance for the supersonic expansion conditions is about 1%. The measurements of rotational transitions for the trans form of CPCA were extended to include all of the unique (13)C singly substituted positions. These measurements, along with previously measured transitions of the parent and -OD isotopologues, were used to determine a best-fit gas-phase structure.

Microwave spectra and structure of the cyclopropanecarboxylic acid-formic acid dimer

“…The steric interactions of the FA hydrogen bonding to CPCA may prevent the CPCA molecule from rotating about the C 1 -C 9 bond to the high energy form, which may be why we were unable to observe this high energy conformer of CPCA-FA, even though this conformer of CPCA was observed with the monomer. 20 The Kraitchman determined coordinates of the 13 C atoms and the D are in fair agreement with the best fit coordinates, with the exception of some coordinates varying by ∼0.1 Å and more for the D substituted atom. The errors in some of the substituted coordinates are large, a result of the substituted atoms lying close to the principle axes or center of mass within the dimer.…”

“…To confirm that these splittings were indeed Doppler splittings, a 50/50 argon/neon mixture was used as the carrier gas and, as expected with the presence of argon, these Doppler splittings decreased slightly. In the study of the monomer of CPCA, there were no signs of splittings caused by internal rotation of the cyclopropane group 17,18,20 and the same seemed to hold true for the CPCA-FA dimer. Tunneling splittings were not observed, most likely because the dimer lacks the C 2vM symmetry, which may be necessary to create the symmetric double well potential with similar energies at the minima, allowing for the protons to tunnel through the energy barrier.…”

“…We recently continued to measure isotopologue transitions of the CPCA monomer and were able to assign unknown transitions that corresponded to this high energy conformer of the CPCA monomer. 20 It may be possible to observe the dimer conformer corresponding to this high energy state; however, these transitions may be considerably weaker and more difficult to observe because of the low abundance of this higher energy structure under the present experimental conditions. These transitions have not yet been observed.…”

“…16 The cyclopropanecarboxylic acid (CPCA) monomer has been studied by microwave spectroscopy previously and gas phase structural information was determined. 17,18,20 The microwave spectrum of trifluoroacetic acid-cyclopropanecarboxylic acid dimer was also measured and no tunneling splittings were observed. 19 One criterion for observing the tunneling splittings would be if the energies of the two minima in the double well potential are very close.…”