The Formic Acid–Nitric Acid Complex: Microwave Spectrum, Structure, and Proton Transfer

Abstract

Rotational spectra are reported for seven isotopologues of the complex HCOOH-HNO3 in a supersonic jet. The system is planar and bound by a pair of hydrogen bonds, much like the more widely studied carboxylic acid dimers. Double proton exchange interconverts the system between a pair of equivalent structures, as revealed by a splitting of the a-type spectrum that disappears when one of the hydrogen bonding protons is replaced by deuterium. The observation of relative intensities that are consistent with nuclear spin statistics in a symmetric and antisymmetric pair of tunneling states provides additional evidence for such a motion. The observed splittings in the pure rotational spectrum are 1-2 orders of magnitude smaller than those recently reported in the pure rotational spectra of several related carboxylic acid dimers. This is a curious difference, although we note that because the observed spectra do not cross the tunneling doublet, the splittings are a measure of the difference in effective rotational constants for the two states, not the tunneling frequency itself. The observed rotational constants have been used to determine an accurate vibrationally averaged structure for the complex. The two hydrogen bond lengths, 1.686(17) Å and 1.813(10) Å for the hydrogen bonds involving the HNO3 and HCOOH protons, respectively, differ by 0.127(27) Å. Likewise, the associated oxygen-oxygen distances determined for the parent species, 2.631 and 2.794 Å, differ by 0.163 Å. These results suggest that the double proton transfer is necessarily accompanied by substantial motion of the heavy atom frame, and thus this system, in principle, provides an excellent prototype for multidimensional tunneling processes. Ab initio calculations of the binding energy and the barrier height are presented. Excellent agreement between the calculated equilibrium structure and the experimental, vibrationally averaged structure suggests that the vibrational wave function is not highly delocalized in the region between the equivalent potential wells. (14)N nuclear quadrupole hyperfine structure is interpreted in terms of the degree to which the HNO3 releases its proton in either of the equivalent potential energy minima.

How CO2 Interacts with Carboxylic Acids: A Rotational Study of Formic Acid–CO2

“…FA is one of the most abundant acids in the atmosphere, with an important influence on precipitation chemistry and acidity, and many model simulations have been proposed to explain the atmospheric FA acidity levels 22. Regarding the rotational spectroscopy investigations, the characterised FA complexes involve those with: argon,23 water,24 formaldehyde,25 formamide,26 nitric acid,27 and other carboxylic acids 28–34. In all cases the FA carboxyl group acts as a bidentate ligand forming a closed planar cycle with the bound counterpart.…”

Microwave spectra and structure of the cyclopropanecarboxylic acid-formic acid dimer

“…Several heterodimers of carboxylic acids that maintain C 2vM symmetry have shown to exhibit the concerted double proton tunneling dynamics in the gas phase, such as the propiolic acid-formic acid dimer, 1,11,12 benzoic acid-formic acid dimer, 13 acetic acid-formic acid dimer, 14 and the nitric acid-formic acid dimer. 15 One of the interesting features of this work is to determine if tunneling splittings can be observed in doubly H-bonded heterodimers without the C 2vM symmetry that is present for the propiolic acid-formic acid dimer. So far these tunneling splittings have only been seen in dimers with the C 2vM symmetry.…”

A Hybrid Program for Fitting Rotationally Resolved Spectra of Floppy Molecules with One Large-Amplitude Rotatory Motion and One Large-Amplitude Oscillatory Motion

“…A second class of complexes to which the program should be applicable are hydrogen-bonded acid pairs like acetic acid - formic acid (CH 3 COOH - - HOOCH) 25 or the acetic acid - nitric acid (CH 3 COOH - - HOONO) analog of HCOOH - - HOONO, 26 where the back-and-forth motion is the exchange of both protons in the double hydrogen bond of the complex. Here too, the relatively few existing measurements for the ground torsional state have been fit to experimental error (a few tens of kHz) using other models.…”