# Polar (acyclic) Isomer of Formic Acid Dimer: Raman Spectroscopy Study

Roman Balabin

► To cite this version:

Roman Balabin. Polar (acyclic) Isomer of Formic Acid Dimer: Raman Spectroscopy Study. École thématique. 64th International Symposium On Molecular Spectroscopy, Ohio State University, Columbus, OH, United States. 2009, pp.30. hal-04909579

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# POLAR (ACYCLIC) ISOMER OF FORMIC ACID DIMER: RAMAN SPECTROSCOPY STUDY

Roman M. Balabin

D-CHAB, ETH Zurich, Switzerland

<figure><img src="image_3.png" /></figure>
# Content

I. **Introduction and motivation:** formic acid

II. **Experimental:** high-sensitivity Raman setup

III. **Spectrum assignment**

IV. **Thermodynamic parameters:** $K_D$, $\Delta G$, $\Delta H$, and $\Delta S$
# Part I

I. **Introduction and motivation:** formic acid

II. **Experimental:** high-sensitivity Raman setup

III. **Spectrum assignment**

IV. **Thermodynamic parameters:** $K_D$, $\Delta G$, $\Delta H$, and $\Delta S$
Formic Acid (FA)

* The simplest carboxylic acid (RCOOH, R=H)

* Formic acid dimer (FAD) is a prototype of molecular complexes with double hydrogen bond

<figure><img src="image_4.png" /></figure>

* Hydrogen tunneling splitting

* Environmental importance (clouds and fog)

* Role in human metabolism etc.

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>
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

<figure><img src="image_7.png" /></figure>
Other Dimer Structures

<figure><img src="image_8.png" /><figcaption>trans-FAM</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>cis-FAM</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>cyclic FAD (c-FAD, I) [-14.46 kcal mol⁻¹]</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>acyclic FAD (a-FAD, II) [-8.82]</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>cis acyclic FAD (a-FAD, IIcis) [-5.44]</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>III [-6.80]</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>IV [-5.60]</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>V [-3.87]</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>VI [-2.93]</figcaption></figure>
# 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)

<figure><img src="image_17.png" /></figure>
# Project Goals and Motivation

* **Goals:**

* Find acyclic FAD in a gas-phase

* Identify acyclic FAD structure

* Evaluate thermodynamic parameters

<figure><img src="image_18.png" /></figure>

* **Motivation:**

* Molecular dynamics (MD): force field parameterization

* Biochemistry and biophysics (enzymes, DNA/RNA base pairs, etc.)

* H-bonding: benchmark experimental data

<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /></figure>
# Part II

I. Introduction and motivation: formic acid

II. Experimental: high-sensitivity Raman setup

III. Spectrum assignment

IV. Thermodynamic parameters: $K_D$, $\Delta G$, $\Delta H$, and $\Delta S$
High-Sensitivity Raman Setup

<figure><img src="image_21.png" /></figure>
# 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: $P_{a-FAD} < 1$ Pa [$10^{14}$ cm$^{-3}$]

* Formic acid decomposition ($k_r \sim 10^{-4}$ h$^{-1}$):

$$HCOOH \rightarrow CO + H_2O$$

$$HCOOH \rightarrow CO_2 + H_2$$

* Formic acid adsorption onto metal surfaces (cell walls) and optical elements
# Part III

I. Introduction and motivation: formic acid

II. Experimental: high-sensitivity Raman setup

III. Spectrum assignment

IV. Thermodynamic parameters: $K_D$, $\Delta G$, $\Delta H$, and $\Delta S$
Raman Spectrum of Formic Acid

<figure><img src="image_22.png" /></figure>
Band Assignment (1/2)

* Do we observe a dimer?

- Vapor pressure influence

<figure><img src="image_23.png" /></figure>
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

<footnote>a Scaled according to c-FAD v₁₄ vibration.</footnote>
# Band Assignment (2/2)

* Is it an acyclic FAD (II)?

- Quantum chemistry calculations:

MP2/6-311++G(3df,2p)<sup>a</sup> 851 cm<sup>-1</sup>

B3LYP/6-311++G(3df,2p)<sup>a</sup> 843 cm<sup>-1</sup>

Exp. 864.1±2.1 cm<sup>-1</sup>

- Isotope substitution

<figure><img src="image_24.png" /></figure>

No appropriate vibration
in any deuterated FA

- Depolarisation ratio

- Thermodynamic parameters

<footnote><sup>a</sup> Scaled according to c-FAD v<sub>14</sub> vibration.</footnote>
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 ρ(exp.)= 0.743(11); ρ.calc.=0.75

- Thermodynamic parameters

<footnote>a Scaled according to c-FAD v₁₄ vibration.</footnote>
# Part IV

I. Introduction and motivation: formic acid

II. Experimental: high-sensitivity Raman setup

III. Spectrum assignment

IV. Thermodynamic parameters: $K_D$, $\Delta G$, $\Delta H$, and $\Delta S$
Thermodynamic parameters of a-FAD

<figure><img src="image_25.png" /></figure>

$$ \Delta H_{\text{a-FAD}} = -8.6 \pm 0.2 \text{ kcal mol}^{-1} \quad [-35.8 \pm 1.0 \text{ kJ mol}^{-1}] $$

$$ \Delta S_{\text{a-FAD}} = -36 \pm 2 \text{ cal mol}^{-1} \text{ K}^{-1} \quad [-150 \pm 9 \text{ J mol}^{-1} \text{ K}^{-1}] $$
Thermodynamic parameters: exp. vs. calc.

<table><thead><tr><th></th><th>I</th><th>II</th><th>IIc</th><th>III</th><th>IV</th><th>V</th><th>VI</th></tr></thead><tbody><tr><td>ΔE</td><td>-14.46</td><td>-8.82</td><td>-5.44</td><td>-6.8</td><td>-5.6</td><td>-3.87</td><td>-2.93</td></tr><tr><td>ΔS</td><td>-43.7</td><td>-37.5</td><td>-37.8</td><td>-34.1</td><td>-34</td><td>-31.2</td><td>-27.2</td></tr></tbody></table>
# Thermodynamic parameters: exp. vs. calc.

<table><thead><tr><th></th><th>I</th><th>II</th><th>IIc</th><th>III</th><th>IV</th><th>V</th><th>VI</th></tr></thead><tbody><tr><th>ΔE</th><td>-14.46</td><td>-8.82</td><td>-5.44</td><td>-6.8</td><td>-5.6</td><td>-3.87</td><td>-2.93</td></tr><tr><th>ΔS</th><td>-43.7</td><td>-37.5</td><td>-37.8</td><td>-34.1</td><td>-34</td><td>-31.2</td><td>-27.2</td></tr></tbody></table>

$$ \Delta H_{\text{a-FAD}} = -8.6 \pm 0.2 $$

$$ \Delta S_{\text{a-FAD}} = -36 \pm 2 $$
Thermodynamic parameters: exp. vs. calc.

<table><thead><tr><th></th><th>I</th><th>II</th><th>IIc</th><th>III</th><th>IV</th><th>V</th><th>VI</th></tr></thead><tbody><tr><th>ΔE</th><td>-14.46</td><td>-8.82</td><td>-5.44</td><td>-6.8</td><td>-5.6</td><td>-3.87</td><td>-2.93</td></tr><tr><th>ΔS</th><td>-43.7</td><td>-37.5</td><td>-37.8</td><td>-34.1</td><td>-34</td><td>-31.2</td><td>-27.2</td></tr></tbody></table>

$$ \Delta H_{\text{a-FAD}} = -8.6 \pm 0.2 $$

$$ \Delta S_{\text{a-FAD}} = -36 \pm 2 $$

<figure><img src="image_26.png" /></figure>

<figure><img src="image_27.png" /></figure>
Conclusions

* A polar (acyclic) isomer of formic acid dimer (II, a-FAD) is observed in a gas phase. Its $v_{14}$ 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 ($\Delta H_{a-FAD}$) is found to be -8.6(2) kcal mol$^{-1}$; the entropy ($\Delta S_{a-FAD}$) is estimated to be -36(2) cal mol$^{-1}$ K$^{-1}$.
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)

ETH

Eidgenössische Technische Hochschule Zürich
Swiss Federal Institute of Technology Zurich

<figure><img src="image_28.png" /></figure>

<figure><img src="image_29.png" /></figure>

<figure><img src="image_30.png" /></figure>

<figure><img src="image_31.png" /></figure>
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

ETH

Eidgenössische Technische Hochschule Zürich
Swiss Federal Institute of Technology Zurich

<figure><img src="image_32.png" /></figure>

<figure><img src="image_33.png" /></figure>

<figure><img src="image_34.png" /></figure>

<figure><img src="image_35.png" /></figure>
Experimental Setup

<figure><img src="image_36.png" /></figure>
Thermodynamic parameters: exp. vs. calc.

<table><thead><tr><th></th><th>I</th><th>II</th><th>IIc</th><th>III</th><th>IV</th><th>V</th><th>VI</th></tr></thead><tbody><tr><th>ΔE</th><td>-14.46</td><td>-8.82</td><td>-5.44</td><td>-6.8</td><td>-5.6</td><td>-3.87</td><td>-2.93</td></tr><tr><th>ΔS</th><td>-43.7</td><td>-37.5</td><td>-37.8</td><td>-34.1</td><td>-34</td><td>-31.2</td><td>-27.2</td></tr></tbody></table>

$$ \Delta H_{\text{a-FAD}} = -8.6 \pm 0.2 $$

$$ \Delta S_{\text{a-FAD}} = -36 \pm 2 $$

<figure><img src="image_37.png" /></figure>
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