Identification of New Dimers of Formic Acid: The Use of a Continuous

LETTER pubs.acs.org/JPCL

Identification of New Dimers of Formic Acid: The Use of a Continuous-Wave Optical Parametric Oscillator in Matrix Isolation Experiments Kseniya Marushkevich, Mikael Siltanen, Markku R€as€anen, Lauri Halonen, and Leonid Khriachtchev* Department of Chemistry, University of Helsinki, P.O. Box 55, FIN-00014 Finland

bS Supporting Information ABSTRACT: We report on the first photochemical use of a continuous-wave optical parametric oscillator (OPO) in a matrix isolation experiment. The narrow-band highpower OPO light promotes efficient conformational changes of matrix-isolated formic acid by selective vibrational excitation, which is combined with thermal annealing of matrices. This approach allows us to find five new dimers of formic acid in an argon matrix (presumably two trans-trans and three trans-cis structures), as shown by infrared spectroscopy. The assignment of the newly observed absorptions is discussed on the basis of the recent ab initio calculations. As a result of this work, all of the trans-trans and trans-cis dimers of formic acid theoretically predicted to date have now been observed in matrix isolation experiments. The structure of dimers can be influenced by the solid matrix surrounding, which may explain differences from the computational spectra obtained for species in vacuum. SECTION: Kinetics, Spectroscopy

F

ormic acid (HCOOH, FA) is the smallest organic acid that has been extensively studied due to its fundamental, biological, and environmental value. The trans and cis conformers of FA differ by the orientation of the hydroxyl group (OH). The trans conformer is lower in energy by ∼1365 cm-1 than the cis form,1 which explains the negligible concentration of cis-FA molecules at normal conditions. The cis-FA molecules can be prepared in large amounts in solid rare gas matrices by vibrational excitation of trans-FA.2-4 The high-energy cis conformer decays back to the trans form via tunneling of the hydrogen atom through the torsional barrier.4-6 Vibrational spectroscopy is ideally suited to study different conformations of organic molecules.4,7-9 Formic acid is a classical model to study hydrogen bonding.10,11 Different calculations of the trans-trans dimers are known and seem to predict up to six stable structures.12-18 Two of these trans-trans structures have been observed experimentally in the gas phase,19-21 rare gas matrices, 17,22-24 and helium nanodroplets.25 In our recent experiments in rare gas matrices,18 four of the trans-trans structures (tt1, tt2, tt3, and tt6 following notation of that work) have been found. Up to five trans-cis dimers have been predicted,15,16,18,26 and two of them (tc1 and tc4 in notation of ref 18) have been prepared and identified in our laboratory.18,23,27 The six trans-trans and five trans-cis dimers computationally studied in ref 18 are shown in the Supporting Information. Two cis-cis dimers have been computed by Roszak et al.,15 but no experimental observation of these structures has been reported. Our previous matrix isolation experiments with the FA conformers have been done using pulsed infrared (IR) light from an r 2011 American Chemical Society

optical parametric oscillator (OPO Sunlite with IR extension, Continuum) tuned to various fundamental, overtone, and combination vibrational transitions.4 This approach was sometimes limited by insufficient photon density in the mid-IR region provided by nonlinear mixing of the signal or idler OPO light with the fundamental Nd:YAG laser light. In the present work, we use high-power light from a continuous-wave (CW) OPO to promote efficient conformational changes by vibrational excitation of FA combined with thermal annealing of argon matrices. This approach allows us to prepare several new FA dimers. The assignment of the newly observed absorptions is discussed on the basis of the recent ab initio calculations [MP2=full/6-311þþG(2d,2p)] performed by Lundell and co-workers.18 The gaseous samples were prepared by mixing formic acid HCOOH with argon (AGA, 99.9999%) in a ∼1:800 proportion. The matrices were deposited onto a CsI substrate at 12 K in a closed-cycle helium cryostat (APD, DE 202A). The IR absorption spectra were measured with a Nicolet SX-60 FTIR spectrometer at the sample temperature of 8.5 K with a resolution of 1 cm-1 coadding up to 500 interferograms. Optical excitation was done with a CW OPO designed and built in our laboratory, and it was similar to our earlier developments.28,29 It has a singly resonant design, which provides an excellent combination of a relatively simple setup, high power, narrow spectrum (27 K leads to efficient dimerization of FA molecules. When the initial matrix contains only trans-FA, the annealing produces deformation bands at 1227 (cyclic dimer tt1), 1180 and 1131 (tt2), and 1114 cm-1 (tt3), and no evidence of higher multimers is observed at these FA concentrations in an argon matrix (Figure 1b, upper trace).18 New bands are seen after annealing when the initial matrix is enriched with cis-FA. The most characteristic spectral features to be discussed below are a doublet at 3406-3411 cm-1 and a band at 3363 cm-1 in the OH stretching region, bands at 1797, 1793, and 1787 cm-1 in the CdO stretching region, and a number of bands at around 1254 and 1158 cm-1 in the deformation region (Figure 1b, lower trace). No definite bands of the new absorbers are observed in the torsional region, despite the relatively high absorption intensities predicted by theory. The new bands observed in the OH stretching region presumably correspond to the hydrogenbonded OH stretching modes and may originate from both transFA and cis-FA units of dimers. On the other hand, the bands in the CdO stretching and deformation regions show that cis-FA participates in these absorbers because the trans-trans dimers cannot absorb at the observed frequencies according to the available calculations.18 The complexes of trans-FA and cis-FA with natural matrix impurities (water and nitrogen) can be ruled out from the consideration because their absorptions are known in an argon matrix.31,32 The cis-cis dimers may also contribute to the observed absorptions, and moreover, the cis-cis dimers should be formed in some amounts upon annealing of a matrix containing cis-FA molecules. However, we suggest that the new bands found after annealing rather originate from the trans-cis dimers. The cis-cis dimers should have at least one free OH bond (not involved in hydrogen bonding).15 It has been shown that substantial stabilization of the cis-FA conformer occurs when

Figure 1. FTIR spectra of FA structures in an argon matrix (from top to bottom). (a) Spectra after deposition and shortly after excitation of trans-FA at 3550.3 cm-1 with the CW OPO. The structures of the FA conformers are shown following ref 1. (b) Spectra after annealing at 34 K of matrices containing mainly trans-FA and after enrichment with cis-FA. The new dimeric bands associated with the presence of cis-FA are marked with asterisks. The spectra were measured at 8.5 K.

the corresponding OH bond is involved in intermolecular interaction with a sufficient interaction energy.31-33 Strong hydrogen bonding can stop tunneling of hydrogen at low temperatures. It follows that the cis-cis dimers should undergo tunneling decay to lower-energy (trans-cis and trans-trans) forms. In contrast, the bands under discussion are quite stable in time (with an exception of the band at 3406 cm-1, which cannot be from a cis-cis dimer for spectroscopic reasons explained below). It is plausible to suggest that the cis-cis dimers (if they are formed) decay at the elevated temperatures to the trans-cis and trans-trans dimers. Such intermediate structures were not detected in our experiments. In order to discriminate the observed absorbers, we use different experimental conditions (see Figure 2). If the asprepared matrix is left in the dark, the band at 3406 cm-1 (absorber II) slowly (in time scale of hours) decreases, and a new band at 3423 cm-1 rises (absorber IV). The 3423 cm-1 band can be efficiently bleached by CW OPO resonant light, which builds up a band at 3387 cm-1 (absorber V). Optical excitation at 3406 cm-1 bleaches the corresponding band and produces mainly the band at 3387 cm-1 (absorber V) and only a minor amount of absorber IV (3423 cm-1). Absorber V (3387 cm-1) has another band at 3544 cm-1, and the excitation at 3544 cm-1 696

dx.doi.org/10.1021/jz2001858 |J. Phys. Chem. Lett. 2011, 2, 695–699

The Journal of Physical Chemistry Letters

LETTER

bleaches the 3387 cm-1 band and produces the band at 3406 cm-1 (absorber II). In contrast, if absorber V is decomposed by excitation at 3387 cm-1, it gives rise to absorber IV (3423 cm-1). Absorber I is quite stable under excitation of the OH stretching mode at 3411 cm-1, but it can be destroyed by the broad-band IR light from the spectrometer source (Globar), which mainly leads to absorber V. The band at 3363 cm-1

(absorber III) can be slowly (in a couple of hours) decomposed by the resonant excitation (∼1 W cm-2), which produces absorber V. Annealing of the matrix at 34 K completely decomposes absorber V. Other structures (e.g., the most stable transtrans dimer tt1) rise in some of these experiments. The described changes allow us to identify other bands of the same absorbers (Table 1). The substantial distinction between the vibrational spectra suggests that these absorbers should be discussed in terms of different dimeric structures rather than different matrix sites of the same dimer. Table 1 presents our assignments of the observed bands to the various FA dimers predicted theoretically18 and shown in Figure 3. Absorbers I and II are, in principle, suitable to both theoretical structures tc2 and tc5; therefore, discrimination between them is not straightforward. On the basis of the experimental spectra, the tc2 structure looks more suitable for absorber II due to the large CdO stretching shift for the trans unit (-41 cm-1), which agrees better with the theory (-31.9 cm-1 for tc2 versus -12.4 cm-1 for tc5). In fact, the observation of this CdO stretching band at 1726 cm-1 shows that this absorber cannot be a cis-cis dimer. However, we assign absorber II to tc5. The tc5 structure has a free OH bond in the cis unit, which allows the tunneling decay, and this is observed for absorber II. In contrast, the tc2 structure should be stable in the dark because the OH group of the cis-FA unit is involved in the hydrogen bonding. The free OH stretching mode of the cis unit of tc5 (calculated shift -4.6 cm-1) is not observed, but this can happen because the OH stretching modes involved in the hydrogen bonding are substantially intensified compared to the free modes, and the latter can overlap with the monomer bands. Some of the characteristic modes of FA dimers are often not seen in experiments.18

Figure 2. FTIR spectra of FA structures in an argon matrix. Difference spectra showing (from top to bottom) the results of ∼10 h in the dark, excitations at 3423, 3406, 3544, and 3387 cm-1, excitation by broadband IR (Globar) light for ∼10 h, excitation at 3363 cm-1, and annealing at 34 K. The solid samples may have a complex prehistory. The trans-cis FA dimers were prepared by irradiation at 3550.3 cm-1 and annealing at 34 K. Labels I-V correspond to the groups of absorptions presented in Table 1. Positions of the monomeric bands of trans-FA and cis-FA monomers are marked with t and c, and the water dimer band is marked with w.34 The spectra were measured at 8.5 K.

Figure 3. Theoretical structures discussed in this work. The values in parentheses are the interaction energies (kJ mol-1). The data are from ref 18. See Figure S1 (Supporting Information) for other possible structures of trans-trans and trans-cis dimers.

Table 1. Experimental Groups of Absorption Bands I-V (in cm-1) and Probable Assignmentsa I νOH νCdO def.

tc2b

II

tc5b

tc3b

III

(-4.6)

3411 (-139)

(-128.7)

1793 (-14)

(-279.1) (-9.3)

3406 (-144) 1797 (-10)

(-197.6) (-14.2)

(-31.9)

1726 (-41)

(-12.4)

1241 (-2)

(þ18.4)

1254 (þ11)

(þ3.2)

1155 (þ52)

(þ63.6)

1158 (þ55)

(þ50.4)

(-9.4)

IV

tt4b

V

tt5b

3423 (-127)

(-107.1)

3544 (-6)

(-4.4)

(-236.0) (-11.0)

1771 (þ4)

(-217.7) (þ3.3)

3387 (-163) 1774 (þ7)

(-149.1) (þ6.3)

(-23.3)

1734 (-33)

(-27.8)

1750 (-17)

(-16.1)

1338 (þ95)

(þ109.3)

1159 (þ56)

(þ56.6)

1154 (þ51)

(þ58.0)

1151 (þ48)c

(þ51.2) (þ36.3)

1110 (þ7)

(þ11.3)

1074 (-29)

(-44.5)

3363 (-253) 1787 (-20)

a

Experimental and calculated complexation-induced shifts are given in parentheses; values corresponding to the cis-FA unit are presented in italics. Calculated complexation-induced shifts from ref 18; the corresponding structures are shown in Figure 3. c The assignment of this band to the cis or trans unit is unclear. b

697

dx.doi.org/10.1021/jz2001858 |J. Phys. Chem. Lett. 2011, 2, 695–699

The Journal of Physical Chemistry Letters Absorber III agrees with the theoretical structure tc3. As fingerprints, it has a strongly red-shifted OH stretching mode and a strongly blue-shifted deformation mode for the cis-FA unit. The OH stretching (free) mode of the trans-FA unit is not observed, but it may overlap with trans-trans dimers or the trans monomer. The CdO stretching band of the trans-FA unit is not observed either, but it may overlap with the corresponding absorption of tt1 at ∼1730 cm-1. The tc3 dimer has the OH bond of the cis unit involved in hydrogen bonding, which agrees with the high stability of absorber III. Two new absorbers appear under conformational changes of the trans-cis dimers. Absorber IV is assigned to the theoretical structure tt4 (Figure 3). The observed OH stretching band is predicted to be the stronger one of the two OH stretching absorptions of tt4. The agreement for the CdO and deformation modes is fairly good within this assignment. Absorber V is assigned to the theoretical structure tt5. All characteristic bands are observed at proper spectral positions. The tt5 dimer is observed after deposition, but it decays at elevated temperatures, as confirmed in Figure 2. The changes shown in Figure 2 are consistent with the proposed structural assignments. The decay of the trans-cis dimers is accompanied by the appearance of trans-trans dimers. As mentioned earlier, the decomposition of absorber II in the dark is an important argument to assign it to the tc5 structure, in contrast to tc2. We put the earlier qualitative arguments on a quantitative basis by calculating the tunneling barriers for tc5 and tc2 in the adiabatic approximation (i.e., without movement of heavy atoms), as described in detail elsewhere.32 For tc2, we found that the final tunneling state is higher in energy by 6240 cm-1 than the initial state, and the barrier is 7920 cm-1, which makes the dark decay impossible at low temperatures.4,31 For tc5, the final tunneling state is lower in energy than the initial state by 940 cm-1, and the barrier is 3485 cm-1, allowing tunneling. Importantly, the tunneling decay of tc5 should presumably lead to the tt4 dimer (Figure 3); thus, the assignment of absorbers II and IV is consistent in this respect. When tc5 is excited at 3406 cm-1, the tt1 (3074 cm-1), tt4, and tt5 structures are formed because more energy is available for the structural reorganization in the solid matrix as compared to that in the tunneling process. Excitation of the hydrogen-bonded OH stretching modes interconverts the tt4 and tt5 dimers, which shows that this photon energy is insufficient for a conformational change due to the hydrogen bonding, but a structural change is possible. In contrast, if the free OH stretching mode of tt5 is excited, the trans-cis dimer tc5 appears, which is reasonable due to the lower barrier. The annealing-induced decay of tt5 mainly leads to tt1, that is, the lowest-energy trans-trans dimer. It should be admitted that the structural assignments are not straightforward in some cases, and we present the most probable assignments. Not all characteristic bands are found for the absorbers under discussion, which decreases the confidence of the assignments. Importantly, the computational structures refer to vacuum, whereas the experiments are for the clusters in an argon matrix. It is probable that the solid matrix substantially influences the structures of the dimers and hence the vibrational spectra. It follows that we may discuss only the matrix-isolated species, which are most suitable to the structures calculated in vacuum but not the exact structures. We believe that the matrixinduced perturbation can be substantial, which can be responsible for the absence of some lines in the experimental spectra, for example, due to the line broadening. Agreement between

LETTER

experiment and theory is the best for the tt5 dimer, probably suggesting that this structure fits well in the matrix. The effect of the matrix on molecular complexes is a complicated and practically unstudied area, which is a challenge for computational science. The quality of the assignment can be improved by using deuteration and Raman spectroscopy; however, this exceeds the scope of the present paper. In summary, vibrational excitation of the ground-state transFA by a CW OPO prepares the higher-energy cis-FA molecules in an argon matrix, and thermal annealing leads to the formation of different dimers. Three new structures containing the cis-FA molecules are experimentally found and tentatively assigned to the trans-cis dimers (tc2, tc3, and tc5) theoretically studied previously.18 Two of these dimers (tc2 and tc3) are practically stable at low temperatures due to hydrogen bonding of the OH group of the cis-FA unit, whereas tc5 slowly decays, presumably via the tunneling mechanism. The conformational changes on these trans-cis dimers allowed us to identify two new transtrans dimers (tt4 and tt5). These five FA dimers discussed in this work have not been identified so far. As a result, all of the theoretically predicted trans-trans and trans-cis structures18 seem to be observed in matrix isolation experiments to date.

’ ASSOCIATED CONTENT Supporting Information. Six trans-trans and five trans-cis dimers computationally studied in ref 18 (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org.

bS

’ AUTHOR INFORMATION Corresponding Author

*E-mail: leonid.khriachtchev@helsinki.fi.

’ ACKNOWLEDGMENT The work was supported by the Finnish Centre of Excellence in Computational Molecular Science and by the Magnus Ehrnrooth foundation. We thank Alexandra Domanskaya for her help in organizing this work and Markku Vainio for his contribution in the design and construction of the optical parametric oscillator. ’ REFERENCES (1) Hocking, W. H. The Other Rotamer of Formic Acid, cisHCOOH. Z. Naturforsch. A 1976, 31, 1113–1121; in the present Letter, we use the notation of the trans and cis forms following ref 1. The opposite labeling also can be found in the literature. (2) Pettersson, M.; Lundell, J.; Khriachtchev, L.; R€as€anen, M. IR Spectrum of the Other Rotamer of Formic Acid, cis-HCOOH. J. Am. Chem. Soc. 1997, 119, 11715–11716. (3) Pettersson, M.; Mac-^ oas, E. M. S.; Khriachtchev, L.; Fausto, R.; R€as€anen, M. Conformational Isomerization of Formic Acid by Vibrational Excitation at Energies below the Torsional Barrier. J. Am. Chem. Soc. 2003, 125, 4058–4059. (4) Khriachtchev, L. Rotational Isomers of Small Molecules in Noble-Gas Solids: From Monomers to Hydrogen-Bonded Complexes. J. Mol. Struct. 2008, 880, 14–22. (5) Pettersson, M.; Mac-^oas, E. M. S.; Khriachtchev, L.; Lundell, J.; Fausto, R.; R€as€anen, M. Cis f trans Conversion of Formic Acid by Dissipative Tunneling in Solid Rare Gases: Influence of Environment on the Tunneling Rate. J. Chem. Phys. 2002, 117, 9095–9098. 698

dx.doi.org/10.1021/jz2001858 |J. Phys. Chem. Lett. 2011, 2, 695–699

The Journal of Physical Chemistry Letters

LETTER

(27) Marushkevich, K.; Khriachtchev, L.; R€as€anen, M. High-Energy Conformer of Formic Acid in Solid Neon: Giant Difference between the Proton Tunneling Rates of cis Monomer and trans-cis Dimer. J. Chem. Phys. 2007, 126, 241102. (28) Vainio, M.; Peltola, J.; Persijn, S.; Harren, F. J. M.; Halonen, L. Thermal Effects in Singly Resonant Continuous-Wave Optical Parametric Oscillators. Appl. Phys. B: Lasers Opt. 2009, 94, 411–427. (29) Vainio, M.; Peltola, J.; Persijn, S.; Harren, F. J. M.; Halonen, L. Singly Resonant cw OPO with Simple Wavelength Tuning. Opt. Express 2008, 16, 11141–11146. (30) Mac-^ oas, E. M. S.; Lundell, J.; Pettersson, M.; Khriachtchev, L.; Fausto, R.; R€as€anen, M. Vibrational Spectroscopy of cis and trans Formic Acid in Solid Argon. J. Mol. Spectrosc. 2003, 219, 70–80. (31) Marushkevich, K.; Khriachtchev, L.; R€as€anen, M. Hydrogen Bonding between Formic Acid and Water: Complete Stabilization of the Intrinsically Unstable Conformer. J. Phys. Chem. A 2007, 111, 2040–2042. (32) Marushkevich, K.; R€as€anen, M.; Khriachtchev, L. Interaction of Formic Acid with Nitrogen: Stabilization of the Higher-Energy Conformer. J. Phys. Chem. A 2010, 114, 10584–10589. (33) Khriachtchev, L.; Domanskaya, A.; Marushkevich, K.; R€as€anen, M.; Grigorenko, B.; Ermilov, A.; Andrijchenko, N.; Nemukhin, A. Conformation-Dependent Chemical Reaction of Formic Acid with an Oxygen Atom. J. Phys. Chem. A 2009, 113, 8143–8146. (34) Engdahl, A.; Nelander, B. Water in Krypton Matrices. J. Mol. Struct. 1989, 193, 101–109.

(6) Domanskaya, A.; Marushkevich, K.; Khriachtchev, L.; R€as€anen, M. Spectroscopic Study of cis-to-trans Tunneling Reaction of HCOOD in Rare Gas Matrices. J. Chem. Phys. 2009, 130, 154509. (7) James, W. H.; Baquero, E. E.; Shubert, V. A.; Choi, S. H.; Gellman, S. H.; Zwier, T. S. Single-Conformation and Diastereomer Specific Ultraviolet and Infrared Spectroscopy of Model Synthetic Foldamers: R/β-Peptides. J. Am. Chem. Soc. 2009, 131, 6574–6590. (8) Balabin, R. M. The First Step in Glycine Solvation: The Glycine-Water Complex. J. Phys. Chem. B 2010, 114, 15075–15078. (9) Balabin, R. M. The Identification of the Two Missing Conformers of Gas-Phase Alanine: A Jet-Cooled Raman Spectroscopy Study. Phys. Chem. Chem. Phys. 2010, 12, 5980–5982. (10) Chang, Y. T.; Yamaguchi, Y.; Miller, W. H.; Schaefer, H. F. An Analysis of the Infrared and Raman Spectra of the Formic Acid Dimer (HCOOH)2. J. Am. Chem. Soc. 1987, 109, 7245–7253. (11) Hobza, P.; Havlas, Z. Counterpoise-Corrected Potential Energy Surfaces of Simple H-Bonded Systems. Theor. Chem. Acc. 1998, 99, 372–377. (12) Turi, L. Ab Initio Molecular Orbital Analysis of Dimers of cisFormic Acid. Implications for Condensed Phases. J. Phys. Chem. 1996, 100, 11285–11291. (13) Qian, W. L.; Krimm, S. Spectroscopically Determined Molecular Mechanics Model for the Intermolecular Interactions in HydrogenBonded Formic Acid Dimer Structures. J. Phys. Chem. A 2001, 105, 5046–5053. (14) Chocholousova, J.; Vacek, J.; Hobza, P. Potential Energy and Free Energy Surfaces of the Formic Acid Dimer: Correlated Ab Initio Calculations and Molecular Dynamics Simulations. Phys. Chem. Chem. Phys. 2002, 4, 2119–2122. (15) Roszak, S.; Gee, R. H.; Balasubramanian, K.; Fried, L. E. New Theoretical Insight into the Interactions and Properties of Formic Acid: Development of a Quantum-Based Pair Potential for Formic Acid. J. Chem. Phys. 2005, 123, 144702. (16) Yavuz, I.; Trindle, C. Structure, Binding Energies, and IRSpectral Fingerprinting of Formic Acid Dimers. J. Chem. Theory Comput. 2008, 4, 533–541. (17) Olbert-Majkut, A.; Ahokas, J.; Lundell, J.; Pettersson, M. Raman Spectroscopy of Formic Acid and Its Dimers Isolated in Low Temperature Argon Matrices. Chem. Phys. Lett. 2009, 468, 176–183. (18) Marushkevich, K.; Khriachtchev, L.; Lundell, J; Domanskaya, A.; R€as€anen, M. Matrix-Isolation and Ab Initio Study of Trans-Trans and Trans-Cis Dimers of Formic Acid. J. Phys. Chem. A 2010, 114, 3495–3502. (19) Zielke, P.; Suhm, M. A. Raman Jet Spectroscopy of Formic Acid Dimers: Low Frequency Vibrational Dynamics and Beyond. Phys. Chem. Chem. Phys. 2007, 9, 4528–4534. (20) Ito, F. Infrared Spectra of (HCOOH)2 and (DCOOH)2 in Rare Gas Matrices: A Comparative Study with Gas Phase Spectra. J. Chem. Phys. 2008, 128, 114310. (21) Balabin, R. M. Polar (Acyclic) Isomer of Formic Acid Dimer: Gas-Phase Raman Spectroscopy Study and Thermodynamic Parameters. J. Phys. Chem. A 2009, 113, 4910–4918. (22) Gantenberg, M.; Halupka, M.; Sander, W. Dimerization of Formic Acid — An Example of a “Noncovalent” Reaction Mechanism. Chem.—Eur. J. 2000, 6, 1865–1869. (23) Marushkevich, K.; Khriachtchev, L.; Lundell, J.; R€as€anen, M. cis-trans Formic Acid Dimer: Experimental Observation and Improved Stability Against Proton Tunneling. J. Am. Chem. Soc. 2006, 128, 12060–12061. (24) Mac-^oas, E. M. S.; Myllyperki€o, P.; Kunttu, H.; Pettersson, M. Vibrational Relaxation of Matrix-Isolated Carboxylic Acid Dimers and Monomers. J. Phys. Chem. A 2009, 113, 7227–7234. (25) Madeja, F.; Havenith, M.; Nauta, K.; Miller, R. E.; Chocholousova, J.; Hobza, P. Polar Isomer of Formic Acid Dimers Formed in Helium Nanodroplets. J. Chem. Phys. 2004, 120, 10554–10560. (26) Zhou, P. P.; Qiu, W. Y. Red- and Blue-Shifted Hydrogen Bonds in the cis-trans Noncyclic Formic Acid Dimer. ChemPhysChem 2009, 10, 1847–1858. 699

dx.doi.org/10.1021/jz2001858 |J. Phys. Chem. Lett. 2011, 2, 695–699