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

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

3. 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

4. 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.

5. 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)

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

7. 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, (2004); Chocholoušová, PCCP 4, 2119 (2002); Gantenberg, Chem. Eur. J. 6, 1865 (2000)

8. 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, (2008); Kirby, Nature 456, 45 (2008); Stratton, Biochem. 40, (2001)

9. 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

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

11. 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°

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

13. 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).

14. 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

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

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

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

18. Band Assignment (2/2) Is it an acyclic FAD (II)?
Quantum chemistry calculations:
MP2/ G(3df,2p)a cm-1
B3LYP/ G(3df,2p)a cm-1
Exp ±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.

19. Band Assignment (2/2) Is it an acyclic FAD (II)?
Quantum chemistry calculations:
MP2/ G(3df,2p)a cm-1
B3LYP/ G(3df,2p)a cm-1
Exp ±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.

20. 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

21. 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)

22. 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/ G(3df,2p); ΔE is presented in kcal mol-1 , ΔS is presented in cal mol-1 K-1

23. 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/ G(3df,2p); ΔE is presented in kcal mol-1 , ΔS is presented in cal mol-1 K-1

24. 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/ G(3df,2p); ΔE is presented in kcal mol-1 , ΔS is presented in cal mol-1 K-1

25. 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.

26. 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

27. Experimental Setup