The Barrier to Proton Transfer in the Dimer of Formic Acid: A Pure Rotational Study.

The rotational spectra of three C-deuterated isotopologues of the dimer of formic acid have been measured, thanks to the small dipole moment induced by asymmetric H→D substitution(s). For the DCOOH-HCOOH species, the concerted double proton transfer of the two hydroxy hydrogen atoms takes place between two equivalent minima and generates a tunneling splitting of 331.2(6) MHz. This splitting can be reproduced by a 3D model with a barrier of 2559 cm-1 (30.6 kJ mol-1 ) as obtained from theoretical calculations.