POLAR (ACYCLIC) ISOMER OF FORMIC ACID DIMER: RAMAN SPECTROSCOPY STUDY Roman M. Balabin D-CHAB, ETH Zurich, Switzerland

Content I. Introduction and motivation: formic acid II. Experimental: high-sensitivity Raman setup III. Spectrum assignment IV. Thermodynamic parameters: KD, ΔG, ΔH, and ΔS

Part I I. Introduction and motivation: formic acid II. Experimental: high-sensitivity Raman setup III. Spectrum assignment IV. Thermodynamic parameters: KD, ΔG, ΔH, and ΔS

Formic Acid (FA) The simplest carboxylic acid (RCOOH, R=H) Formic acid dimer (FAD) is a prototype of molecular complexes with double hydrogen bond Hydrogen tunneling splitting Environmental importance (clouds and fog) Role in human metabolism etc.

Cyclic Dimer of Formic Acid (c-FAD) Vapor pressure measurements Gas-phase electron diffraction Infrared spectroscopy Raman spectroscopy Dielectric spectroscopy NMR spectroscopy Laser temperature jump Shock-tube technique Coolidge, JACS 50, 2166 (1928); Millikan, JACS 80, 3515 (1958); Lazaar, JACS 107, 3769 (1985); Ito, CPL 318, 571 (2000)

Other Dimer Structures Chocholoušová, PCCP 4, 2119 (2002); Balabin, Chem. Phys. 352, 267 (2008); Balabin, JPCA 113, 4910 (2009) [MP2/aug-cc-pVTZ]

Acyclic Dimer of Formic Acid (a-FAD) Molecular dynamics simulations Ab initio studies IR study – Pulse deposition into Ar matrices (7 K) IR study – Ultracold He nanodroplets (0.37 K) Madeja, JCP 120, 10554 (2004); Chocholoušová, PCCP 4, 2119 (2002); Gantenberg, Chem. Eur. J. 6, 1865 (2000)

Project Goals and Motivation Find acyclic FAD in a gas-phase Identify acyclic FAD structure Evaluate thermodynamic parameters Motivation: Molecular dynamics (MD): force field parameterization Biochemistry and biophysics (enzymes, DNA/RNA base pairs, etc.) H-bonding: benchmark experimental data Valdes, PCCP 10, 2747 (2008); Balabin, JCP 129, 164101 (2008); Kirby, Nature 456, 45 (2008); Stratton, Biochem. 40, 10411 (2001)

Part II I. Introduction and motivation: formic acid II. Experimental: high-sensitivity Raman setup III. Spectrum assignment IV. Thermodynamic parameters: KD, ΔG, ΔH, and ΔS

High-Sensitivity Raman Setup Hill, Appl. Opt. 16, 2044 (1977); Zielke, PCCP 9, 4528 (2007); Balabin, JPCA 113, 1012 (2009); Balabin, JPCA 113, 4910 (2009)

Setup Parameters Laser type: DPSS Laser wavelength: 457 nm Laser power: 19.8 W Number of passes 67 Cell gain: 47.5±0.5 Raman scattering angle: 90° Collecting condenser type: four-lens Collecting angle: 2×84°

Experimental Parameters Temperature range: 25–45 °C Temperature step: 1.00±0.04 °C Temperature accuracy: 0.01 °C Vapor pressure range: 3000-9000 Pa Spectral resolution: 2 cm-1 Accumulation time: 2.5 min (15×10 s)

Experimental Challenges Laser instability Retroreflecting cell instability Low concentration of a-FAD: Pa-FAD < 1 Pa [1014 cm-3] Formic acid decomposition (kr  10-4 h-1): HCOOH → CO + H2O HCOOH → CO2 + H2 Formic acid adsorption onto metal surfaces (cell walls) and optical elements Coolidge, JACS 50, 2166 (1928); Gibson, Chem. Rev. 69, 673 (1969); Mathews, J. Chem. Soc. A, 2203 (1969).

Part III I. Introduction and motivation: formic acid II. Experimental: high-sensitivity Raman setup III. Spectrum assignment IV. Thermodynamic parameters: KD, ΔG, ΔH, and ΔS

Raman Spectrum of Formic Acid Balabin, JPCA 113, 4910 (2009)

Band Assignment (1/2) Do we observe a dimer? Vapor pressure influence

Band Assignment (2/2) Is it an acyclic FAD (II)? Quantum chemistry calculations: MP2/6-311++G(3df,2p)a 851 cm-1 B3LYP/6-311++G(3df,2p)a 843 cm-1 Exp. 864.1±2.1 cm-1 Isotope substitution Depolarisation ratio Thermodynamic parameters a Scaled according to c-FAD ν14 vibration.

Band Assignment (2/2) Is it an acyclic FAD (II)? Quantum chemistry calculations: MP2/6-311++G(3df,2p)a 851 cm-1 B3LYP/6-311++G(3df,2p)a 843 cm-1 Exp. 864.1±2.1 cm-1 Isotope substitution Depolarisation ratio Thermodynamic parameters No appropriate vibration in any deuterated FA a Scaled according to c-FAD ν14 vibration.

Band Assignment (2/2) Is it an acyclic FAD (II)? Quantum chemistry calculations: MP2/6-311++G(3df,2p)a 851 cm-1 B3LYP/6-311++G(3df,2p)a 843 cm-1 Exp. 864.1±2.1 cm-1 Isotope substitution Depolarisation ratio Thermodynamic parameters ρ(exp.)= 0.743(11); ρ(calc.)=0.75 a Scaled according to c-FAD ν14 vibration.

Part IV I. Introduction and motivation: formic acid II. Experimental: high-sensitivity Raman setup III. Spectrum assignment IV. Thermodynamic parameters: KD, ΔG, ΔH, and ΔS

Thermodynamic parameters of a-FAD ΔHa-FAD = -8.6±0.2 kcal mol-1 [-35.8±1.0 kJ mol-1] ΔSa-FAD = -36±2 cal mol-1 K-1 [-150±9 J mol-1 K-1] Balabin, JPCA 113, 4910 (2009); Balabin, JPCA 113, 1012 (2009); Balabin, Chemom. Intell. Lab. Syst. 88, 183 (2007)

Thermodynamic parameters: exp. vs. calc. II IIc III IV V VI ΔE -14.46 -8.82 -5.44 -6.8 -5.6 -3.87 -2.93 ΔS -43.7 -37.5 -37.8 -34.1 -34 -31.2 -27.2 MP2/6-311++G(3df,2p); ΔE is presented in kcal mol-1 , ΔS is presented in cal mol-1 K-1

Thermodynamic parameters: exp. vs. calc. II IIc III IV V VI ΔE -14.46 -8.82 -5.44 -6.8 -5.6 -3.87 -2.93 ΔS -43.7 -37.5 -37.8 -34.1 -34 -31.2 -27.2 ΔHa-FAD = -8.6±0.2 ΔSa-FAD = -36±2 MP2/6-311++G(3df,2p); ΔE is presented in kcal mol-1 , ΔS is presented in cal mol-1 K-1

Thermodynamic parameters: exp. vs. calc. II IIc III IV V VI ΔE -14.46 -8.82 -5.44 -6.8 -5.6 -3.87 -2.93 ΔS -43.7 -37.5 -37.8 -34.1 -34 -31.2 -27.2 ΔHa-FAD = -8.6±0.2 ΔSa-FAD = -36±2 MP2/6-311++G(3df,2p); ΔE is presented in kcal mol-1 , ΔS is presented in cal mol-1 K-1

Conclusions A polar (acyclic) isomer of formic acid dimer (II, a-FAD) is observed in a gas phase. Its v14 peak position is found to be at 864(2) cm-1. The presence of acyclic dimer is confirmed by the pressure dependence of Raman band intensity, temperature influence, and a comparison with quantum chemistry data. Thermodynamic parameters of polar formic acid dimer are experimentally evaluated for the first time. The enthalpy difference (ΔHa-FAD) is found to be -8.6(2) kcal mol-1; the entropy (ΔSa-FAD) is estimated to be -36(2) cal mol-1 K-1.

Thank you for your attention! Acknowledgements: Prof. M. Suhm (Göttingen University, Germany) P. Zielke (Göttingen University, Germany) E. Lomakina (Moscow State University, Russia) A. Borisov (Ministry of Defence, Russia) I. Samoilenko (Moscow State University, Russia) The Ministry of Defence of the Russian Federation Swiss Federal Institute of Technology (ETH Zurich) Saint-Petersburg State University of Information Technologies, Mechanics and Optics ITERA International Group of Companies

Experimental Setup