ARTICLE pubs.acs.org/JPCA
Spectra and Integrated Band Intensities of the Low Order OH Stretching Overtones in Peroxyformic Acid: An Atmospheric Molecule with Prototypical Intramolecular Hydrogen Bonding Montu K. Hazra and Amitabha Sinha* Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093-0314, United States
bS Supporting Information ABSTRACT: The gas phase spectra of several vibrational bands of peroxyformic acid (PFA), an atmospheric molecule exhibiting intramolecular hydrogen bonding, are presented. In the fundamental region, Fourier transform infrared (FT-IR) spectroscopy is used to probe the CO, OH and CH stretching vibrations, while in the region of the first and second OH-stretching overtones (2νOH and 3νOH) photoacoustic spectroscopy is used. Integrated absorption cross sections for the PFA vibrational bands are determined by comparing their respective peak areas with that for the OH-stretching bands of n-propanol for which the absorption cross section is known. The measured integrated intensities of the OH stretching bands are then compared with a local mode model using a one-dimensional dipole moment function in conjunction with the OH stretching potential computed at both the MP2/aug-ccpVDZ and CCSD(T)/aug-cc-pVDZ levels. The data allow us to investigate changes in the OH stretch band position and intensity as a function of overtone order arising from the influence of hydrogen bonding. Furthermore, calculations at the MP2/aug-cc-pVDZ level show that there are three stable conformers of PFA with relative energies of 0, 13.54, and 13.76 kJ/mol, respectively. In the room temperature spectra, however, we see evidence for transitions from only the lowest energy conformer. The geometrical parameters and vibrational frequencies of the most stable conformer are presented.
1. INTRODUCTION Hydrogen-bonded (H-bonded) complexes play an important role in atmospheric chemistry, where they can potentially influence climate through the absorption and scattering of solar radiation.18 In addition, intermolecular interactions associated with H-bonding can result in spectral shifts of a molecule’s absorption profile, thus altering the wavelength dependence of its absorption cross section and leading to changes in its atmospheric photochemistry.915 Vibrational overtone spectroscopy can, in principle, provide a direct window to probe the H-bonded stretching coordinate of these complexes. However, the low concentration of these complexes, combined with the inherent weakness and spectral broadening associated with H-bonded vibrational overtones,16 have generally limited the applicability of overtone spectroscopy for the study of gas phase H-bonded complexes. An alternate approach for exploring hydrogen bonded systems is to investigate the overtone spectroscopy of molecules with internal hydrogen bonding.1725 These intramolecular H-bonded systems are convenient as they can be generated in comparatively larger concentration and yet display many of the complexities found in systems with intermolecular hydrogen bonds. One such feature, that has been reported in many H-bonded systems in the liquid phase, has been the strong intensity increase in the stretching fundamental of the H-bonded proton donor OH/CH stretch and excessive weakness (relative r 2011 American Chemical Society
to that found in non-H-bonded systems) of their corresponding first overtone.2630 A similar trend has also been noted in matrix isolation and helium droplet experiments.3134 Gas phase studies of intramolecular H-bonded molecules and H-bonded complexes in the fundamental and overtone regions of their OH stretching vibrations also support this trend, as studies find that overtone transitions of H-bonded hydroxyl groups become more difficult to detect with increasing H-bond strength and that the intensity enhancement due to H-bonding is limited to the OH stretching fundamental.1725,3538 Interestingly, however, even though the number of studies on H-bonded systems is quite extensive, published experimental values of absolute integrated absorption cross sections for both the fundamental and overtone transitions involving gas phase H-bonded molecules are rather limited. In the literature, intensity analysis of gas phase H-bonded systems has been reported primarily in terms of relative oscillator strengths or theoretically predicted absolute oscillator strengths.1725,3542 Thus, measurements that quantify the influence of H-bonding on absolute band intensities in various gas phase H-bonded systems provide a useful complement to the currently available database.35 In this work we present first results Received: December 17, 2010 Revised: April 13, 2011 Published: May 09, 2011 5294
dx.doi.org/10.1021/jp112028c | J. Phys. Chem. A 2011, 115, 5294–5306
The Journal of Physical Chemistry A
Figure 1. Optimized structures of the three PFA (HCOOOH) conformers calculated at the MP2/aug-cc-pVDZ level. Relative energies are given with respect to conformer I, the most stable conformer.
examining the OH stretching states of peroxyformic acid (PFA, HC(O)OOH). Due to its molecular structure, PFA is not only a potentially important source of tropospheric OH radicals through photodissociation, but being the simplest organic peroxyacid to exhibit internal hydrogen bonding, the molecule also provides a model system for investigating the influence of H-bonding on OH stretching overtones. PFA is formed in the atmosphere through the gas phase reaction of O3 with chloroethenes as well by the photo-oxidation of hydrocarbons.4346 To date, only the ground vibrational state and a few bands in the region of the fundamental have been investigated in PFA.4649 In 1952 Giguere and Almos were the first to measure the infrared spectra of gaseous and liquid PFA.47 The first microwave spectrum of this molecule was recorded in 1983 by Oldani et al.48 From the microwave measurements it was concluded that the molecule is planar and exhibits intramolecular hydrogen bonding. These studies also concluded that there are no higher energy conformers within ∼700 cm1 of the most stable conformer corresponding to the H-bonded conformer; labeled conformer I in Figure 1. Later on, in 1990, high resolution Fourier transform infrared (FT-IR) spectroscopy of the ν6 (CO stretch) fundamental band was performed by Bauder et al.49 and this study provided rotational and centrifugal distortion constants as well as an estimate of the band’s integrated absorption cross section. In addition to the experimental work, there have also been several theoretical studies on PFA at both the HartreeFock (HF) theory and MøllerPlesset perturbation (MP2) theory level directed toward examining its potential energy surface for the possible existence of additional conformers apart from the minimum energy H-bonded conformer.5052 An examination of these computational studies, however, reveal some disagreement regarding the geometry of the stable conformers associated with the molecule. While the earlier study by Langley et al.51 and the more recent study by Solimannejad et al.52 both suggest that there are just two stable conformers for PFA, Langley et al. suggest that conformers I and III, shown in Figure 1, are stable, whereas the results of Solimannejad et al. prefer conformers I and II. More recently, ab initio calculations have also been used to examine intermolecular H-bonded complexes involving PFA, most notably with water,53 the hydroperoxy radical,54 and with itself.52 To the best of our knowledge, there is no prior information available on the vibrational overtones of PFA, and in this article we report on the integrated band intensities of several of its fundamental bands as well as its low-order OH stretching overtones. In the fundamental region we use FT-IR spectroscopy to record the CO (CO), OH and CH stretching modes, while in the region of the first and
ARTICLE
second OH-stretching overtones (2νOH and 3νOH), photoacoustic spectroscopy is used. Integrated absorption cross sections are determined by comparing respective peak areas of the PFA bands with the OH bands of n-propanol for which absolute integrated absorption cross sections are available.55 The measured band intensities are then compared with the results of ab initio harmonic intensities, calculated at the MP2/aug-cc-pVDZ and MP2/aug-ccpVTZ levels. In the case of the OH stretching bands, intensities are also calculated using a local mode model utilizing a one-dimensional (1-D) dipole moment function and OH stretching potential calculated at the MP2/aug-cc-pVDZ and CCSD(T)/aug-cc-pVDZ levels. This allows us to investigate the influence of hydrogen bonding on the OH stretching band position and intensity. Finally, in this study we also address the issue of the number of conformational minima in PFA by independently varying the dihedral angles around OO and CO bonds using relaxed scans at MP2/aug-ccpVDZ level and report on the relative energies and vibrational frequencies of the conformers.
2. EXPERIMENTAL METHOD PFA is known to be a potentially dangerous and explosive substance.56 In the laboratory, it can be generated through the reaction of hydrogen peroxide with formic acid. The reaction is described by the following equation: HCðOÞOH þ H2 O2 h HCðOÞOOH þ H2 O
ð1Þ
Typically during synthesis, concentrated sulfuric acid is also added and acts as a catalyst to shift the equilibrium toward the product side by removing the product water molecules.57 We have synthesized small batches of PFA (∼2.5 mL) by following the procedure outlined by Oldani et al.48 Briefly, 0.55 mL of ∼95% hydrogen peroxide (H2O2) is added dropwise to 1.10 mL of 99.5% sulfuric acid (H2SO4) kept in a 100 mL round-bottom flask cooled to 0 C and stirred with a magnetic stirring bar. After the addition is completed, 0.85 mL of 99.5% formic acid (HCOOH) is slowly added to the mixture. The mixture is then stirred at ice temperature for approximately ∼1 h and finally at room temperature for an additional hour thereafter. After synthesis, the sample is purified through several freezepumpthaw cycles using a liquid nitrogen bath in order to remove volatile impurities, which primarily consists of CO2. Liquid PFA samples decompose even at ice temperatures. Thus after synthesis, the purified sample was stored at liquid nitrogen temperatures. Interestingly, in contrast to the liquid samples, vapor phase PFA appears to decompose much more slowly. The infrared spectra in the region of the fundamental (700 4000 cm1) were recorded at a resolution of 0.5 cm1 using a commercial FT-IR spectrometer (Thermo Fisher, Nicolet 6700) equipped with a liquid-nitrogen-cooled mercurycadmium telluride-A (MCT-A) detector and KBr beam splitter. The spectrometer was purged continuously with dry nitrogen to minimize IR absorption by atmospheric gases. The spectra were recorded at room temperature using several different sample pressures covering a range between 0.4 Torr and 2.0 Torr. The IR cell consisted of a 15 cm long cylindrical Pyrex glass tube having a diameter of 2.54 cm. Spectra were recorded using both CaF2 and NaCl windows, which were attached to the ends of the IR cell using Torr Seal epoxy. After removal of the initial CO2, formic acid (FA) was found to be the only other detectable impurity (
