A butterfly motion of formic acid and cyclobutanone in the 1: 1 hydrogen bonded molecular cluster

Upon supersonic expansion, formic acid and cyclobutanone (CBU) form a molecular cluster in which the two constituent molecules, linked by OH center dot center dot center dot O and CH center dot center dot center dot O hydrogen bonds, undergo a rapid interconversion between two equivalent forms. The tunneling motion takes place through the rupture and reformation of the C-H center dot center dot center dot O hydrogen bond between the carbonyl oxygen of HCOOH and one of the two hydrogen atoms of the methylenic group adjacent to the cyclobutanone keto group. From the microwave spectra, tunneling energy splittings (Delta E-01) have been determined for the parent (1122.756(3) MHz), DCOOH center dot center dot center dot CBU (1084.538(1) MHz) and HCOOD center dot center dot center dot CBU (1180.282(4) MHz) isotopic species. From these splittings, the potential barrier to interconversion has been calculated to be B-2 = 39.7(5) cm(-1). The tunneling pathway is an asymmetric butterfly-like motion between the two moieties of the adduct, with a barrier at a configuration in which the ring plane of cyclobutanone is coplanar with formic acid.