Conformational Equilibria in Bimolecules of Carboxylic Acids: A

Letter pubs.acs.org/JPCL

Conformational Equilibria in Bimolecules of Carboxylic Acids: A Rotational Study of Fluoroacetic Acid−Acrylic Acid Qian Gou, Gang Feng, Luca Evangelisti, and Walther Caminati* Dipartimento di Chimica, “G. Ciamician” dell’Università, Via Selmi 2, I-40126 Bologna, Italy S Supporting Information *

ABSTRACT: The rotational spectra of three conformers of the fluoroacetic acid−acrylic acid bimolecule have been assigned by pulsed jet Fourier Transform microwave spectroscopy. Information on their relative populations in the jet has been obtained by relative intensity measurements. In addition, the Ubbelohde effect has been quantitatively estimated from the changes of the planar moment of inertia Paa upon H→D isotopic substitution of the carboxylic hydrogens. The dissociation energies of the three conformers, as estimated from centrifugal distortion effects, have almost the same values for the three species.

SECTION: Spectroscopy, Photochemistry, and Excited States

T

chemical and spectroscopic properties of this kind of molecular system. Bauder and co-workers reported a rotational analysis for the adducts of trifluoroacetic acid with formic and with acetic acids11 and Antolinez et al. reported the MW spectrum of the trifluoroacetic acid-cyclopropanecarboxylic acid bimolecule.12 In none of these cases was splitting attributed to a double proton transfer tunneling observed. The failure to observe the tunneling splittings is due to the high values of the reduced mass of CF3 which, in order to connect equivalent minima, has to rotate by 60°. As a consequence, splittings are too small to be detected. A few years ago, the tunneling caused by the double proton transfer has also been observed by PJFTMW spectroscopy on formic acid-propriolic acid13 and formic acid-acetic acid.14 Very recently, the double proton transfer dynamics in the homodimer of acrylic acid15 and the heterodimer benzoic acid-formic acid16 has been successfully described by the tunnelling splittings of several isotopologues obtained from the PJFTMW spectra. For the distinctly different case, where the double proton transfer connects to unequal conformers, only one investigation has been reported in the literature, i.e., the heterodimer acrylic acid-formic acid17 which adopts two different conformational shapes, arising from the cis or trans forms of the acrylic acid moiety. Another interesting experimental evidence related to the investigation of bimolecules of carboxylic acids is the Ubbelohde effect,18 that is an increase of the O···O distances

he dimers of carboxylic acids are characterized by an eight-membered ring motif, which includes two relatively strong OH···O hydrogen bonds (HBs). The corresponding dimerization energy is larger than 60 kJ/mol, and for this reason a considerable portion of dimers is found in the gas phase, even at room temperature. It was possible, indeed, to observe the low-resolution microwave spectra in a standing cell of some carboxylic acid bimolecules already by Costain in 19611 and Bellot and Wilson in 1975.2 This kind of adduct is interesting because a double proton transfer takes easily place, connecting either two equivalent, or two nonequivalent molecular conformations. In the first case, tunnelling effects are expected, which can lead to the determination of the potential energy surface for the motion. In the second case, we expect a conformational equilibrium to characterize the molecular system. Various spectroscopic techniques, such as femtosecond degenerate four-wave mixing and Raman spectroscopy, have been applied to determine the tunnelling splittings in dimers of carboxylic acids. Most of them have been focused on the proton transfer rate in homodimers, and mainly on the simplest carboxylic acid dimer of the series, the formic acid dimer.3−9 Interesting results have been obtained on the ground and first electronic excited states of the benzoic acid dimer10 by rotationally resolved laser induced fluorescence. Microwave spectroscopy is particularly suitable to structurally and energically characterize the molecular interaction, and to reveal the internal dynamics. However, limited by the requirement of permanent dipole moment, the microwave investigations of acids dimers are mostly focusing on heterodimers. Pulsed jet Fourier transform spectroscopy (PJFTMW) should allow obtaining precise details on the © 2013 American Chemical Society

Received: July 17, 2013 Accepted: August 9, 2013 Published: August 9, 2013 2838

dx.doi.org/10.1021/jz4015037 | J. Phys. Chem. Lett. 2013, 4, 2838−2842

The Journal of Physical Chemistry Letters

Letter

It was not possible to assign the spectrum of the fourth conformer (Ct). All measured transitions could be fit, independently for each conformer, with Watson’s semirigid Hamiltonian (S-reduction; Ir-representation),26 obtaining the rotational and two first order centrifugal distortion constants reported in Table 2. The

upon H→D isotopic substitution of the hydrogen atoms involved in the HB. This effect was already outlined in the first MW study of this kind of bimolecule.1 It is interesting to note that, in contrast to the rO···O increase observed in carboxylic dimers, a shrinking of the O···O distance is observed (reverse Ubbelohde effect) for complexes where the two subunits are held together by a single OH···O HB.19−21 Here we report the pulsed jet FTMW spectrum of the hetero dimer of acrylic acid (AA) and fluoroacetic acid (FAA), two carboxylic acids that can assume more than one configuration.22,23 Up to four conformers are then expected, for the dimer, to be observable with MW spectroscopy. In addition, structural information leading to a precise measure of the above-mentioned Ubbelohde effect will be provided. Before collecting the rotational spectra, full geometry optimizations of the four more stable configurations of the dimer were carried out at the MP2/6-311++G(d,p) level using the Gaussian03 program package.24 The corresponding drawings, as well as the calculated rotational constants, electric dipole moments and relative energies are listed in Table 1.

Table 2. Experimental Spectroscopic Parameters of the Observed Conformers of FAA−AA Bimolecule (SReduction, Ir Representation) Tc A/MHz B/MHz C/MHz DJ/kHz DJK/kHz σb/kHz Nc

4350.3931(7) 470.1161(2) 425.7949(2) 0.017(1) −0.014(2) 2.1 48

a

Tt

Cc

4046.0566(8) 475.4679(2) 426.9990(2) 0.018(1) −0.091(2) 2.4 48

3997.401(1) 477.7362(2) 428.2972(2) 0.017(1) −0.064(2) 2.0 35

Error in parentheses in units of the last digit. bRMS error of the fit. Number of lines in the fit.

a c

Table 1. MP2/6-311++G(d,p) Calculated Parameters of the Plausible Conformers of FAA−AA Bimolecule

Absolute energy is −594.2018134 Eh . −594.1965657 Eh. a

b

comparison between the experimental and predicted rotational constants (see Tables 1 and 2) gives unequivocal conformational assignments of conformers Tc, Tt, and Cc. The spectroscopic rotational constants of the three detected conformers agree with the calculated values within 2% discrepancies. Therefore, no further structural fitting has been done for the geometric structures, apart for the Ubbelohde effect, as described below. Then we analyzed the rotational spectra of the OD deuterated species, obtained by mixing the acids with D2O. The spectra of these species turned out to be quite weak, probably due to the presence of adducts of the carboxylic acids with water. For this reason, only five OD isotopologues have been observed (3 for the Tc and 2 for the Tt species, respectively). A few transitions have been measured for each deuterated species, and for this reason the centrifugal distortion constants have been fixed to the values of the corresponding parent species in the parameter fits. The determined spectroscopic parameters of all deuterated isotopologues are listed in Table 3. From the rotational constants it is easy to calculate, for each species, the values of the Paa planar moments of inertia (defined as Paa = ∑imia2i ) through the relation Paa = h/(16π2)(−1/A + 1/B + 1/C). These quantities give a measure of the mass extension perpendicular to the b,c-plane. We report in Table 4 the Paa values, along with the calculated and experimental values of their shifts upon OD deuteration. One can note that the experimental ΔPaa values are considerably larger than the calculated ones, according to the Ubbelohde effect previously described.1,17 To reproduce the experimental ΔPaa values expected to be originated by the H→D substitutions, it has been necessary to slightly elongate the rC···C distances of the carboxylic carbon atoms, according to the values shown in Table 5. The intermolecular stretching motion that leads to the dissociation is almost parallel to the a-axis for all conformers of the complex. In this case, a pseudo diatomic approximation can be used to roughly estimate the force constant of the stretching mode leading to the dissociation through the equation:27

Absolute energy is

We label the four species with symbols of the type Xx, where X indicates the configurations of FAA (T for trans, C for cis) while x indicates the configurations of AA (t for trans, c for cis). In order to have a better estimate of the energy differences, all intermolecular binding energy values were counterpoise corrected for basis set superposition error (BSSE).25 These calculations suggest configurations with trans FAA to lie at lower energies. According to the calculated rotational constants, the μa-type R branch bands were expected to appear in narrow frequency regions, separated by a B+C spacing. We searched first for the μa-type transitions of the global minimum Tc, which are expected to be the most intense lines. Transitions with J = 9 to 12 and with K−1 = 0 to 4 could be measured. Then some μb-type R and Q branch lines were also identified. Once conformer Tc was removed from the spectrum, a number of less intense transitions could be easily assigned to conformer Tt. With further signal accumulation, some much weaker lines were found which were assigned to the Cc transitions. It is interesting to note that cis-fluoroacetic acid, which appears in the Cc form of the dimer, was not observed in the MW spectrum of the monomer (a second conformer was observed, but with a cis configuration of the COOH group),23 probably because of the very small values of its dipole moment components. So, acrylic acid acts as an electrophore for cisfluoroacetic acid, leading to its indirect observation. 2839



Letter

Table 3. Experimental Spectroscopic Parameters for the Deuterated Species of FAA−AA (S-Reduction, Ir Representation)a Tc A/MHz B/MHz C/MHz σb/kHz Nc

Tt

FAA(OD)-AA

FAA-AA(OD)

FAA(OD)-AA(OD)

FAA(OD)-AA

FAA-AA(OD)

4295.258(5)a 469.4828(2) 424.7434(2) 1.3 11

4292.946(5) 469.3397(2) 424.6039(2) 1.7 11

4238.737(5) 468.7634(2) 423.5976(2) 3.3 11

3983.443(5) 474.5850(2) 425.5827(2) 3.1 11

4019.761(5) 474.6199(2) 426.0249(2) 2.6 11

Error in parentheses in units of the last digit. bRMS error of the fit. cNumber of lines in the fit. aThe DJ and DJK centrifugal distortion parameters have been fixed to the values of the corresponding parent species.

a

Table 4. Paa Values for the Various Isotopologues of the Tc and Tt Conformers of FAA−AA, and Calculated and Experimental Values of Their Shifts upon OD Deuteration Tc

Tt ΔPaa/uÅ2

Paa/uÅ2 FAA-AA FAA(OD)-AA FAA-AA(OD) FAA(OD)-AA(OD)

exptl.

1072.874 1074.323 1074.650 1075.964

1.449 1.777 3.091

ΔrC···C/Å FAA(OD)−AA FAA−AA(OD) FAA(OD)−AA(OD)

Tt

0.0040(3) 0.0045(3) 0.0082(3)

0.0053(3) 0.0044(3) -

ks = 16π 4(μD R CM)2 [4B4 + 4C 4 − (B − C)2 × (B + C)2 ]/(hDJ)

(1)

where μD is the pseudo diatomic reduced mass, RCM is the distance between the centers of the mass of the two subunits, and B, C, and DJ are the spectroscopic parameters of Table 2. Assuming a Lennard-Jones-type potentia, the zero point dissociation energy of the complex can be derived applying the approximate expression:28 2 E B = ksR CM /72

(2)

The results are listed in Table 6. The dissociation energies of the three conformers have similar values, in accord with ab initio calculations results. Relative intensity measurements of some pairs of nearby μatype lines, allowed the estimation of the relative population of the three conformers in the jet. We obtained NTc/NTt/NCc ≈ 7/

Tc

Tt

Cc

40 134 4.81 77 67 53

39 132 4.81 75 67 54

42 137 4.80 80 67 53

exptl.

calc.

0.009 0.173 0.183

1060.781 1062.757 1062.675 -

1.976 1.894 -

0.073 0.318 -

EXPERIMENTAL SECTION

Molecular clusters were generated in a supersonic expansion, under conditions optimized for the dimer formation. Details of the Fourier transform microwave spectrometer30 (COBRAtype31), which covers the range 6.5−18 GHz, have been described previously.32 Inert gas helium at a stagnation pressure of ∼0.25 MPa was passed over a mixture sample of fluoroacetic acid and acrylic acid in a almost 1:1 ratio (commercial sample bought from Apollo or Aldrich, and used without further purification) and expanded through a solenoid valve (General Valve, Series 9, nozzle diameter 0.5 mm) into the Fabry-Pérot cavity. The spectral line positions were determined after Fourier transformation of the time-domain signal with 8k data points, recorded with 100 ns sample intervals. Each rotational

a c

Paa/uÅ2

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Table 6. Dissociation Energy Data (See Text) kS/Nm−1 v/cm−1 RCM/Å EBa/kJ mol−1 EB0b/kJ mol−1 EBbssec/kJ mol−1

calc.

4/1. As outlined in other papers,29 the jet plume is removed from an equilibrium situation, and the relative concentration of the dimer conformers depend also on the concentrations of the conformers of the monomers, such that it is difficult to derive the relative energies. The relative populations are, however, qualitatively in agreement with the order of the calculated ab initio relative energies. In summary, with Fourier transform microwave technique, we assigned the rotational spectra of FAA−AA dimer including the deuterated species. Three conformers were identified. The successful assignment of the conformer Cc involving cisfluoroacetic acid evinced the existence of cis-fluoroacetic acid in the supersonic jet, in spite of the failure of observing it in isolation. The increase of the hydrogen bond length upon H→ D isotopic substitution (Ubbelohde effect) has been deduced from the elongation of the carboxylic carbons C···C distance. This is the first case in which the rotational spectra of three different conformers are observed for carboxylic acid dimer. The concerted double proton transfer in FAA−AA connects unequal conformers, differently with respect to the cases of AA−AA or benzoic acid-formic acid, where this motion connects structurally equivalent minima, generating tunneling splittings.

Table 5. Increases of the C···C Distances (ΔrC···C) upon H→ D Substitution Tc

ΔPaa/uÅ2

Pseudo diatomic approximation. bMP2/6-311++G(d,p) values. Counterpoise corrected MP2/6-311++G(d,p) values. 2840



Letter

(11) Martinache, L.; Kresa, W.; Wegener, M.; Vonmont, U.; Bauder, A. Microwave Spectra and Partial Substitution Structure of Carboxylic Acid Bimolecules. Chem. Phys. 1990, 148, 129−140. (12) Antolinez, S.; Dreizler, H.; Storm, V.; Sutter, D. H.; Alonso, J. L. The Microwave Spectrum of the Bimolecular Trifluoroacetic Acid··· Cyclopropanecaboxylic Acid. Z. Naturforsch. 1997, 52a, 803−806. (13) Daly, A. M.; Douglass, K. O.; Sarkozy, L. C.; Neill, J. L.; Muckle, M. T.; Zaleski, D. P.; Pate, B. H.; Kukolich, S. G. Microwave Measurements of Proton Tunneling and Structural Parameters for the Propiolic Acid-Formic Acid Dimer. J. Chem. Phys. 2011, 135 (154304), 1−12. (14) Tayler, M. C. D.; Ouyang, B.; Howard, B. J. Unraveling the Spectroscopy of Coupled Intramolecular Tunneling Modes: A Study of Double Proton Transfer in the Formic−Acetic Acid Complex. J. Chem. Phys. 2011, 134 (054316), 1−9. (15) Feng, G.; Favero, L. B.; Maris, A.; Vigorito, A.; Caminati, W.; Meyer, R. Proton Transfer in Homodimer of Carboxylic Acids: The Rotational Spectrum of the Dimer of Acrylic Acid. J. Am. Chem. Soc. 2012, 134, 19281−19286. (16) Evangelisti, L.; Écija, P.; Cocinero, E. J.; Castaño, F.; Lesarri, A.; Caminati, W.; Meyer, R. Proton Tunneling in Heterodimers of Carboxylic Acids: A Rotational Study of the Benzoic Acid-Formic Acid Biomolecule. J. Phys. Chem. Lett. 2012, 3, 3770−3775. (17) Feng, G.; Gou, Q.; Evangelisti, L.; Xia, Z.; Caminati, W. Conformational Equilibria in Carboxylic Acid Bimolecules: A Rotational Study of Acrylic Acid-Formic Acid. Phys. Chem. Chem. Phys. 2013, 15, 2917−2922. (18) Ubbelohde, A. R.; Gallagher, K. J. Acid-base Effects in Hydrogen Bonds in Crystals. Acta Crystallogr. 1955, 8, 71−83. (19) Tang, S.; Majerz, I.; Caminati, W. Sizing the Ubbelohde Effect: The Rotational Spectrum of tert-Butylalcohol Dimer. Phys. Chem. Chem. Phys. 2011, 13, 9137−9139. (20) Snow, M. S.; Howard, B. J.; Evangelisti, L.; Caminati, W. From Transient to Induced Permanent Chirality in 2-Propanol upon Dimerization: A Rotational Study. J. Phys. Chem. A 2011, 115, 47−51. (21) Evangelisti, L.; Feng, G.; Rizzato, R.; Caminati, W. Conformational Equilibria in Adducts of Alcohols with Ethers: The Rotational Spectrum of Ethylalcohol-Dimethylether. ChemPhysChem 2011, 12, 1916−1920. (22) Bolton, K.; Lister, D. G.; Sheridan, J. Rotational Isomerism, Barrier to Internal Rotation and Electric Dipole Moment of Acrylic Acid by Microwave Spectroscopy. J. Chem. Soc., Faraday Trans. 2 1974, 70, 113−123. (23) Van Eijck, B. P.; Brandts, P.; Maas, J. P. M. Microwave Spectra and Molecular Structures of Rotational Isomers of Fluoroacetic Acid and Fluoroacetyl Fluoride. J. Mol. Struct. 1978, 44, 1−13. (24) Frisch, M. J. et al. Gaussian 03, revision B.01; Gaussian, Inc.: Pittsburgh, PA, 2003. (25) Boys, S. F.; Bernardi, F. The Calculations of Small Molecular Interactions by the Differences of Separate Total Energies. Some Procedures with Reduced Errors. Mol. Phys. 1970, 19, 553−566. (26) Watson, J. K. G. In Vibrational Spectra and Structure; Durig, J. R., Ed.; Elsevier: New York/Amsterdam, 1977; Vol. 6, pp 1−89. (27) Millen, D. J. Determination of Stretching Force Constants of Weakly Bound Dimers from Centrifugal Distortion Constants. Can. J. Chem. 1985, 63, 1477−1479. (28) Novick, S. E.; Harris, S. J.; Janda, K. C.; Klemperer, W. Structure and Bonding of KrClF: Intermolecular Force Fields in Van Der Waals Molecules. Can. J. Phys. 1975, 53, 2007. (29) See, for example, Caminati, W.; López, J. C.; Blanco, S.; Mata, S.; Alonso, J. L. How Water Links to Cis and Trans Peptidic Groups: the Rotational Spectrum of N-Methylformamide−Water. Phys. Chem. Chem. Phys. 2010, 12, 10230−10234. (30) Balle, T. J.; Flygare, W. H. Fabry−Perot Cavity Pulsed Fourier Transform Microwave Spectrometer with a Pulsed Nozzle Particle Source. Rev. Sci. Instrum. 1981, 52, 33−45. (31) Grabow, J.-U.; Stahl, W.; Dreizler, H. A Multioctave Coaxially Oriented Beam-Resonator Arrangment Fourier-Transform Microwave Spectrometer. Rev. Sci. Instrum. 1996, 67, 4072−4084.

transition appears as a doublet due to Doppler effect. The line position is calculated as the arithmetic mean of the frequencies of the Doppler components. The estimated accuracy of the frequency measurements is better than 3 kHz. Lines separated by more than 7 kHz are resolvable. The deuterated species were derived from directly mixing with deuterated water.

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ASSOCIATED CONTENT

S Supporting Information *

(1) Completion of ref 24; (2) tables of transition frequencies; (3) ab initio relative energies of the conformers of the monomers of acrylic acid and 2-F-acetic acid. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. All authors contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS G.F. and Q.G. thank the China Scholarships Council (CSC) for financial support. Financial support was also received from Italian MIUR (PRIN08, Project KJX4SN_001) and the University of Bologna (RFO).

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REFERENCES

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(32) Caminati, W.; Millemaggi, A.; Alonso, J. L.; Lesarri, A.; López, J. C.; Mata, S. Molecular Beam Fourier Transform Microwave Spectrum of the Dimethylether−Xenon Complex: Tunnelling Splitting and 131 Xe Quadrupole Coupling Constants. Chem. Phys. Lett. 2004, 392, 1−6.

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Conformational Equilibria in Bimolecules of Carboxylic Acids: A

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