Far-infrared relaxation in the isolated formic acid dimer molecule: Possibility of fast proton tunneling in a hydrogen bond

The band contours of the far-infrared hydrogen bond stretching fundamental of the low-pressure vapors of the ring dimer molecules of (HCOOH)2 and (HCOOD)2 were measured and Fourier inverted. The resulting dipole correlation functions decay appreciably faster than those of the vibrational fundamentals not directly involved in hydrogen bonding, such as the C–H stretching and deformation modes of the molecules. However, it is shown conclusively that proton tunneling in the hydrogen bonds of the isolated dimer molecule is not the cause of the band broadening and, if proton tunneling occurs at all, it must have an inversion rate considerably slower than 1012 sec−1.