Tammannstr. 6, Göttingen, Germany 2 Department of Chemistry, Technical University of Denmark

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1 Supplementary material for: The highest frequency hydrogen bond vibration and an experimental value for the dissociation energy of formic acid dimer F. Kollipost 1, R. W. Larsen 2, A. V. Domanskaya 1, M. Nörenberg 1, and M. A. Suhm 1 1 Institut für Physikalische Chemie, Universität Göttingen, Tammannstr. 6, Göttingen, Germany 2 Department of Chemistry, Technical University of Denmark Kemitorvet, Building 206, DK 2800 Kgs. Lyngby, Denmark Table S1. Fundamental vibrational wavenumbers of the cyclic formic acid dimer. Mode Symmetry /cm 1 A g 2900 * A g 2949 a A g 1670 a A g 1415 a A g 1375 a A g 1214 a A g 677 a A g 194 c A g 161 c B g 1060 a B g 911 d B g 242 c A u 1050 e A u 922 b A u b A u 69.2 b B u 3084 b B u b B u 1746 b B u 1454 b B u 1364 b B u 1218 b B u 698 b B u 264 f * Rough estimate only. a John E. Bertie and Kirk H. Michaelian. The Raman spectra of gaseous formic acid h 2 and d 2. J. Chem. Phys., 76: , b R. Georges, M. Freytes, D. Hurtmans, I. Kleiner, J. Vander Auwera, and M. Herman. Jet cooled and room temperature FTIR spectra of the dimer of formic acid in the gas phase. Chem. Phys., 305: , c Z. Xue and M. A. Suhm. Probing the stiffness of the simplest double hydrogen bond. The symmetric d Z. Xue. Raman spectroscopy of carboxylic acid and water aggregates. Doctoral thesis, Göttingen, ISBN e R. C. Millikan and K. S. Pitzer. The infrared spectra of dimeric and crystalline formic acid. J. Am. Chem. Soc., 80:3515, f This work. 1

2 Table S2. Experimentally observed intermolecular vibrational modes of the formic acid dimer and their assignments. Species Mode /cm 1 (HCOOH) 2 P Q R a (DCOOH) (HCOOD) (DCOOD) a Z. Xue and M. A. Suhm. Probing the stiffness of the simplest double hydrogen bond. The symmetric Figure 1S. FTIR jet spectra of in the symmetric isotopologues of formic acid dimer: a) (HCOOH) 2 (250 scans) b) (DCOOH) 2 (20 scans) c) (HCOOD) 2 (20 scans) d) (DCOOD) 2 (15 scans). The absorptions of the deuterated dimers are characteristically shifted to lower wavenumber. The values for (DCOOH) 2, (HCOOD) 2 and (DCOOD) 2 reflect the increasing effective mass and the comparable amplitude of OH and CH motion. Like the deuterium free dimer, the observed bands feature a higher frequency shoulder, which is attributed to larger clusters. 2

3 Table S3. Anharmonicity constants a / cm 1 of the intermolecular vibrational modes of the formic acid dimer. Mode 0 b c b b 1 b b 6 d 0.5 b b 1 b 7 d 1 b d a Numbers in boldface are experimental values, others are estimated by the formula 2 0.5%. b Z. Xue and M. A. Suhm. Probing the stiffness of the simplest double hydrogen bond. The symmetric c R. Georges, M. Freytes, D. Hurtmans, I. Kleiner, J. Vander Auwera, and M. Herman. Jet cooled and room temperature FTIR spectra of the dimer of formic acid in the gas phase. Chem. Phys., 305: , d This work. 3

4 Table S4. Collection of available experimental dissociation enthalpies Δ and K p (296 K) determinations for p 0 =10 5 Pa. Temperature ranges which require substantial extrapolation are marked in italics. refers to methods based on vapor density, including acoustic phenomena. S(X) refers to spectroscopic determination of species X. The K p (296 K) values were obtained from the linear fits of ln as a function of 1/, thus assuming that Δ is not temperature dependent. The error introduced by this assumption is only significant for the italic values, as verified by graphical extrapolation. / /Pa / Year Method a b S(D) c d e f g h 361(45) S(M) i (50) S/ j a H. C. Ramsperger and C. W. Porter. The ultraviolet absorption spectrum of formic acid. J. Am. Chem. Soc., 48:1267, b A. S. Coolidge. The vapor density and some other properties of formic acid. J. Am. Chem. Soc., 50:2166, c R. C. Herman. Vibration Spectra and Molecular Structure IX. Further Studies of the Vapors of the Fatty Acid Series. J. Chem. Phys., 8:252, d M. D. Taylor and J. Bruton. The Vapor Phase Dissociation of Some Carboxylic Acids. Formic and Propionic Acids. J. Am. Chem. Soc., 74:4151, e J. R. Barton and C. C. Hsu. P V T X Properties of Associated Vapors of Formic and Acetic Acids. J. Chem. Eng. Data, 14: f R. Büttner, G. Maurer. Dimerisierung einiger organischer Säuren in der Gasphase. Berichte der Bunsengesellschaft für physikalische Chemie, 87: , g A. Winkler and P. Hess. Study of the Energetics and Dynamics of Hydrogen Bond Formation in Aliphatic Carboxylic Acid Vapors by Resonant Photacoustic Spectroscopy. J. Am. Chem. Soc., 116:9233, h S. Miyamoto, S. Nakamura, Y. Ivai, Y. Arai. Measurement of Vapor Phase Compressibility Factors of Monocarboxylic Acids Using a Flow Type Apparatus and Their Association Constants. J. Chem. Eng. Data, 44:48 51,1999. i J. Vander Auwera, K. Didriche, A. Perrin, and F. Keller. Absolute line intensities for formic acid and dissociation constant of the dimer. J. Chem. Phys., 126:124311, j This work 4

5 Table S5. Anharmonically modeled vibrational partition function of formic acid monomer and dimer, and dissociation enthalpy Δ at different temperatures T. / / (4) (5) 5