Communications: Evidence for proton tunneling from the microwave spectrum of the formic acid-propriolic acid dimer

The microwave spectrum of the formic acid-propriolic acid dimer was measured in the 5-13 GHz range using a pulsed-beam, Fourier transform spectrometer. 22 a-dipole rotational transitions and 3 b-dipole rovibrational transitions were measured for the normal isotopomer. All of these observed transitions were split into doublets by the effects of the concerted tunneling of the two acid protons. The smaller splittings of 1-1.5 MHz for the a-dipole transitions are due to the differences in rotational constants for the upper and lower tunneling states. The b-dipole transitions are rovibrational (combination) transitions with a change in rotational state and tunneling state and provide direct information on the tunneling splittings since these observed splittings are the sum of the tunneling level splittings for the two rotational states involved in the transition. The b-dipole splittings are 55.16 (0 ₀₀ - 1₁₁), 58.58 (1₀₁ - 2₁₂), and 71.24 MHz (2₀₂ - 3₁₃). No similar splittings were observed when deuterium was substituted for either or both of the hydrogen bonding protons. For the lower tunneling state (ν₀⁺), A=5988.7 (7), B=927.782 (7), and C=803.720 (7) MHz. For the upper tunneling state (ν₀^-), A=5988 (1), B=927.78 (1), and C=804.06 (1) MHz. Using a simple model with potential function V= ax⁴ - bx² the splittings could be reproduced reasonably well with a barrier height of H_e =3800^cm-1.