TITLE

High resolution spectroscopy of carboxylic acid in the gas phase: Observation of proton transfer in (DCOOH)[sub 2]

AUTHOR(S)
Madeja, F.; Havenith, M.
PUB. DATE
October 2002
SOURCE
Journal of Chemical Physics;10/15/2002, Vol. 117 Issue 15, p7162
SOURCE TYPE
Academic Journal
DOC. TYPE
Article
ABSTRACT
We report the first fully analyzed high resolution spectrum of a carboxylic acid dimer in the gas phase. High resolution spectra in the region of the C-O stretch 1241.7-1250.7 cm[SUP-1] have been recorded for (DCOOH)[SUB2]. The data could be fit within experimental uncertainty to a rigid rotor Watson S reduced Hamiltonian. The vibrational frequency of the C-O stretch in (DCOOH)[SUB2] is determined to be 1244.8461 (2) cm[SUP-1]. Our spectra are the first direct experimental evidence for proton transfer tunneling in formic acid dimer (FAD), with FAD serving as a prototype for double hydrogen bonded organic complexes. Previous theoretical studies predicted proton transfer times covering a range of several orders of magnitude. Our measurements on (DCOOH)[SUB2] established a proton transfer time of 5.8 ns [tunneling splitting of 0.00286(25) cm[SUB-1]]. The proton transfer was found to accelerate upon vibrational excitation of the skeleton motion (the vibrational C-O mode), corresponding to a tunneling splitting of 0.00999(21) cm[SUP-1]. For this state the proton transfer time decreased to 1.7 ns, which corresponds to a vibrationally enhanced proton transfer mechanism. The moments of inertia are in agreement with a C[SUB2h] structure which indicates a high tunneling barrier. The observed spin statistical weights follow a description of formic acid dimer in the molecular symmetry group G[SUB8], which is isomorphic to the point group D[SUB2h]. Our results suggest an out-of-plane contribution to the proton transfer mechanism for the otherwise planar dimer.
ACCESSION #
7431019

 

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