Rotational spectroscopic and theoretical study of the perfluorobutyric acid⋯formic acid complex
Introduction
Perfluorinated carboxylic acids (PFCAs) consist of a terminal carboxylic group and an alkyl chain which is fully fluorinated. PFCAs have been used as surfactants in industrial processes and also as ion-pairing agents in analytical chemistry [1]. PFCAs are known to be toxic and the effects of some longer chain PFCAs, for example perfluorooctanoic acid, on the environment have been extensively investigated by the United States Environmental Protection Agency (EPA) [2]. We note that perfluorooctanoic acid was recently included in the list of the Stockholm Convention on Persistent Organic Pollutants [3]. Perfluorobutyric acid (PFBA), a shorter chain PFCA, was identified as one of several possible candidates [4] to replace perfluorooctanoic acid. However, these shorter chain PFCAs are more water soluble and might pose other serious environmental issues. Some of us have recently reported rotational spectroscopic studies of several short-chain PFCAs, such as perfluoropropionic acid [5], PFBA [6], and perfluoropentanoic acid [7], as well as their mono and/or dihydrates [4], [8]. In this context, it is also of considerable interest to investigate structural properties of the hydrogen (H)-bonded complexes between PFBA and other simple acids, such as formic acid (FA), and with itself.
Carboxylic acids have a strong tendency to dimerize or form larger aggregates even at room temperature [9] because of the favorable formation of strong intermolecular H-bonds. It is well known that the two carboxylic acid groups favor the formation of an eight-membered ring featuring two H-bonds [10]. Detection of rotational transitions requires the molecular system to be studied to have a permanent electric dipole moment and they can therefore not possess a center of symmetry. For this reason, many of the carboxylic acid dimers which have been studied with microwave spectroscopy are heterodimers which consist of two different acids. Since FA is the simplest carboxylic acid and has sufficient vapor pressure at room temperature, it is not surprising that many FA containing heterodimers with other carboxylic acids have been studied by using rotational spectroscopy. Some examples are propiolic acid⋯FA [11], [12], [13], benzoic acid⋯FA [14], and nitric acid⋯FA [15], where tunneling splittings due to a double proton transfer between the two acids were detected experimentally. The potential energy surfaces of these planar complexes have a double-minimum potential, similar to systems that undergo a single proton tunneling motion, such as malonaldehyde [16], [17], [18], [19] and tropolone [20], [21]. A generic double-minimum potential for a double proton transfer in planar molecules is shown in Fig. S1, Supporting Information, with the equivalent minima indicated. Howard and co-workers [22] published a beautiful paper on acetic acid⋯FA, a ‘large’ nonrigid molecule, and showed how the double proton tunneling motion is strongly coupled to the methyl internal rotation. In that work, they also developed a suitable framework to extract meaningful spectroscopic constants, such as the principal moments of inertia and the methyl bond axis orientation from the experimental spectra. In other FA containing acid dimers, such as fluoro/difluoro/trifluoroacetic acid⋯FA [23], [24], [25], cyclopropanecarboxylic acid⋯FA [26], 3,3,3-trifluoro-2-(trifluoromethyl)propanoic acid⋯FA [27], and acrylic acid⋯FA [28], no splittings due to a double proton tunneling process were observed experimentally. In these systems, the double proton transfer would require a simultaneous CF3 internal rotation (or other heavy group motions) to reach an equivalent potential energy minimum and the double proton tunneling is quenched. In the difluoroacetic acid⋯FA case [24], Caminati and co-workers observed some narrow splittings between 10 and 20 kHz, which may be attributed to either tunneling between the gauche +/− forms of difluoroacetic acid or double proton tunneling coupled to a simultaneous CHF2 internal rotation in order to reach an equivalent potential energy minimum. They could attribute the splitting to gauche +/− tunneling of the difluoroacetic acid subunit by studying all three deuterated isotopologues and detecting splittings of similar magnitude in all spectra. Their conclusion is consistent with the proposal that the double proton transfer is quenched if a simultaneous heavy atom motion is required to reach an equivalent potential minimum. On the other hand, Obenchain et al. had recently recorded rotational spectra of the perfluoropropionic acid⋯FA complex [29] where each a-type rotational transition is split into a doublet, with spacings of less than 1 MHz. The exact origin of the observed splittings, however, is still unclear. It is therefore interesting to explore the closely related PFBA⋯FA heterodimer to obtain a more complete picture of double proton tunneling and the nature of the H-bonding interactions in these perfluorinated acid containing hetero acid dimers.
Fourier transform microwave spectroscopy (FTMW) has been used extensively to probe molecular structure and dynamics of small molecules and their non-covalently bound complexes in the gas phase [30], [31]. In recent years, it has been applied to studies of molecules [32] and complexes [33] of important environmental interest, some of which are discussed above [34]. In this paper, we report the first FTMW spectroscopic investigation of the PFBA⋯FA heterodimer, complemented by ab initio calculations. Both broadband chirped pulse FTMW [35], [36] and cavity-based FTMW [37] spectrometers at the University of Alberta were utilized in the current study. The latter has a higher resolution capability than the former one. Our aim was not only to measure rotational transitions to extract structural properties of the acid heterodimer, but also to examine if there are any splittings associated with double proton transfer or other tunneling motions.
Section snippets
Theoretical and experimental methods
A computational search for low energy conformers of the PFBA⋯FA complex was carried out using the GAUSSIAN 09 package [38]. Geometry optimizations were performed using the second order Møller-Plesset perturbation (MP2) and the dispersion corrected density functional theory (DFT), B3LYP-D3BJ [39], methods. The 6-311++G(2d,p) basis set was used. Harmonic vibrational frequencies were also determined after geometry optimization to ensure that the optimized structures are true minima.
Initial
Results and discussion
Two conformers of the PFBA monomer were predicted theoretically, although the second conformer is significantly less stable and had not been detected experimentally [6]. The MP2 conformational search located six low energy conformers for the PFBA⋯FA heterodimer. Similar results were obtained at the DFT level. The calculated relative equilibrium dissociation energies, ΔDe, relative zero-point energy (ZPE) corrected dissociation energies, ΔD0, molecular rotational constants, A, B, C, and electric
Conclusions
The PFBA⋯FA complex was studied using rotational spectroscopy and high-level ab initio calculations. Conformational searches at the B3LYP-D3BJ and MP2 levels of theory identified the same six possible conformers with the lowest energy one being more stable by at least 22 kJ mol−1 than the others. This is consistent with the experimental detection of one single conformer. The experimental rotational constants are in excellent agreement with those from B3LYP-D3BJ calculations, indicating that the
Acknowledgments
This research was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC). Computing resources were provided by Westgrid and Compute/Calcul Canada. MJC, AGIII, and WL gratefully acknowledge the Welch Foundation (#BX-0048) for financial support. MJC and WL also thank the Undergraduate Research Initiative (URI) of the University of Texas Rio Grande Valley (UTRGV) for financial support. YX holds a Tier I Canada Research Chair in Chirality and Chirality Recognition; WJ
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Rotational spectra of 4,4,4-trifluorobutyric acid and the 4,4,4-trifluorobutyric acid-formic acid complex
2018, Journal of Molecular SpectroscopyCitation Excerpt :In case the energy levels of the two conformers are close, both will be observed in the supersonic cooling process, such as acrylic acid-trifluoroacetic acid [23] and acrylic acid-difluoroacetic acid [24]. But many other heterodimers have a higher energy difference between the two conformers so that only one conformer will be observed, such as, for example, cyclopropanecarboxylic acid-formic acid [25] and perfluorobutyric acid-formic acid [26]. It will be interesting to extend the study to more carboxylic acid heterodimers to get more detailed understanding of the concerted double proton tunneling.
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