DO PROTONS TUNNEL IN THE FORMIC-ACID DIMER AND IN THE CRYSTAL

Abstract

We investigate whether excited state proton tunnelling through the barrier of a double minimum potential contributes to the fine structure of the νs(OH) infrared absorption band of the gas phase formic acid dimer, as has been suggested by Excoffon and Maréchal. We adopt their estimate of 140 cm−1 for the excited state splitting in (HCOOH)2, and use this to parameterise a realistic proton potential for the dimer. The same potential, adjusted for the shorter oxygen-oxygen separation across the hydrogen bond in the crystal, enables us to assign features in the infrared spectrum of crystalline DCOOH as |b0⪢ → |3⪢ and |1⪢ → |2⪢ transitions. We estimate the splitting between the lowest symmetric a..