Tunneling splitting and level ordering in a CO-stretch fundamental of the formic acid dimer

The reversed order of the levels in which a CO-stretch fundamental in the formic acid dimer is split, as recently observed and ascribed to perturbation by a nearby state, is interpreted as due to accidental degeneracy between the CO-stretch level and a combination level. This interpretation allows a proper assessment of the hindering effect of antisymmetric CO-stretch excitation on proton tunneling in the dimer.