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The formic acid-nitric acid complex: microwave spectrum, structure, and proton transfer
Rebecca B Mackenzie1, Christopher T Dewberry, Kenneth R Leopold
1Department of Chemistry, University of Minnesota , 207 Pleasant Street, SE, Minneapolis, Minnesota 55455, United States.
The Journal of Physical Chemistry. A
|August 22, 2014
Summary
The formic acid-nitric acid complex exhibits double proton tunneling, evidenced by rotational spectra. This motion involves significant heavy atom rearrangement, making it a model for multidimensional tunneling.
Area of Science:
- Molecular spectroscopy
- Quantum chemistry
- Intermolecular forces
Background:
- Carboxylic acid dimers are well-studied for hydrogen bonding and proton transfer dynamics.
- Understanding proton tunneling in complexes is crucial for chemical reaction mechanisms.
Purpose of the Study:
- Investigate the structure and dynamics of the formic acid-nitric acid (HCOOH-HNO3) complex.
- Characterize the double proton exchange mechanism and its impact on molecular structure.
- Provide experimental data to validate theoretical calculations of tunneling phenomena.
Main Methods:
- High-resolution rotational spectroscopy in a supersonic jet.
- Analysis of rotational spectra for seven isotopologues.
- Ab initio quantum chemical calculations.
Main Results:
- Observed rotational spectra indicate a planar HCOOH-HNO3 complex bound by two hydrogen bonds.
- Spectral splittings confirm double proton exchange, which disappears upon deuteration.
- Experimental rotational constants yield a vibrationally averaged structure with distinct hydrogen bond lengths.
- (14)N nuclear quadrupole hyperfine structure provides insight into proton delocalization.
Conclusions:
- The HCOOH-HNO3 complex serves as an excellent prototype for studying multidimensional tunneling.
- Double proton transfer is coupled with substantial heavy atom motion.
- Experimental and theoretical structures show excellent agreement, suggesting limited delocalization of the vibrational wave function.
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