Coherent Proton Transfer in Formic Acid Dimer

Coherent Proton Transfer in Formic Acid Dimer 

We will now focus in this subsection on the experimental study providing accurate information on the proton transfer tunneling in isolated carboxylic acid dimers in the gas phase. 

In an early paper by Robertson and Lawrence [63] the tunneling splitting of the ground state was estimated to be about 6 cm i. They assumed that the observed fine structure in the first excited 0-H stretch of 140 cm i is attributed to the transfer tunneling splitting. However, in view of new theoretical studies and the recent experimental results [39], their conclusions were probably based on a misassign-ment. 

In order to determine exact quantitative data it is necessary to obtain high resolution spectra of the dimer. Since a resolution of 0.001 cm i is beyond the scope of FTIR spectrometers, laser spectroscopic techniques were required. This could be aciiieved in an IR study using sensitive absorption measurements by a lead salt diode laser. 

In the paper of Madeja and Havenith the observation of 409 lines in the frequency range from 1241.7 to 1250.7 cm i was reported. All lines could be assigned to the C-O stretch vibrational band. The splitting due to proton transfer tunneling was fully resolved (see Fig. 2.3). The frequency range from 1241.6 to 1242.6 cm is displayed it can cdearly be seen that each rotational vibrational line is split into two fully resolved tunneling components. 

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