The Surface Chemistry of

acetic acid and its α-amino derivative (glycine) on/in ice

by Fourier Transform Infrared Reflection

Absorption Spectroscopy (FTIR-RAS)

By Qiang Gao

CHEM 794, March 27nd, 2003 2

Outline

• Introduction• Proposal• Summary• Acknowledgements

CHEM 794, March 27nd, 2003 3

Ice structure

• The structure of ice films deposited from the vapor phase :

T < 140 K a mixture of amorphous and cubic ices

140 K< T < 180 K a mixture of cubic and hexagonal ices

T > 180 K hexagonal ice

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The perfect hexagonal ice structure

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Noncrystalline ice structure

• The low-density amorphous icea highly porous open network with nano-scale pores and a significant concentration of dangling OH groups

• The high-density amorphous icesimilar to the low-density amorphous ice but with additional water molecules occupying interstitial sites

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The infrared spectrum of ice

• 3800-2800 cm-1

• near 3700 cm-1

• 1680-1620 cm-1

• 900-700 cm-1

• coupled OH stretching modes

• dangling OH bonds

• bending mode

• librational mode

CHEM 794, March 27nd, 2003 7

The surface chemistry of ice

• Three regions in ice :surface

subsurface

bulk

• The weak adsorbates CF4, H2, N2, and CO

• The stronger adsorbates HCN, SO2, H2S, and acetylene

• The strongest adsorbates NH3, HCl and ethylene oxide

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Acetic acid structure

• In gas phase, the monomers and dimers of acetic acid coexist

• In liquid phase, it mainly forms cyclic dimers but open chain dimers have also been reported

• In solid phase it forms a chain-like polymer

• It may exist in ionic form

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Infrared spectrum of acetic acid

ν (C=O) :

1788 and 1730 cm-1 (g)

1715 cm-1 (l)

1648 cm-1 (s)

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The surface chemistry of acetic acid

• On metal surfacestrongly depends on the nature of the substrate and on the coverage

• On iceBy classical molecular dynamics simulation Compoint et al. found :

the formation of two H-bonds

strongly trapped at 250 K

the penetration is more favorable than desorption

CHEM 794, March 27nd, 2003 11

Glycine structure

• At room temperature, glycine is a white polycrystalline solid which exists as zwitterion, NH3

+ CH2COO- .

• Upon annealing glycine vaporizes and converts to its non-ionic form, NH2CH2COOH.

• In solution, it changes from cationic through zwitterionicto anionic with decreasing acidity

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The infrared spectrum of glycine

• The frequencies associated with the NH2 and NH3+

groups at ~1600 cm-1 appear to be too close to be resolved

• The asymmetric (~1600 cm-1) and symmetric (~1400 cm-1) stretches of carboxylate should be clearly distinguishable from the carbonyl stretch (~1720 cm-1) of the acid group –COOH.

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The surface chemistry of glycine

• On metal surfaces

• On Silicon surface

• No study on ice Glycine adsorbed on Cu(110)

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Proposal

• The goal - the surface chemistry of acetic acid and its α-amino derivative (glycine) on/in ice

• The method - Fourier Transform Infrared Reflection Absorption Spectroscopy (FTIR-RAS) and ab initio quantum mechanical and molecular dynamics computation

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Why?

• Ice has attracted a lot of attention over the past decade due to its important role as a common medium at different temperature

• Only a limited amount of information about the adsorption of organic acids on ice is known

• Glycine is an important model biomolecule that can be used to provide insights into more complex systems

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Experimental method - FTIR-RAS

• Identification of functional groups

• Information about the chemical bonding of the adsorbate and substrate

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Schematic layout of

the experimental set up

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How to introduce chemicals?

• Water and acetic acid vapors are introduced after purified by several freeze-pump-thaw cycles.

• A temperature-controlled beam doser with a liquid nitrogen-cooled cooling shroud will be constructed to provide an effusive molecular beam with minimal cracking to glycine. Before evaporation the glycine powder will be outgassedfor several hours.

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How to determine film thickness?

Film thickness will be determined by optical interference using a helium-neon laser (λ = 632.8 nm) :

• The incident laser beam is directed at a growing film at 22° from the surface normal .

• The reflected light is then detected by a photodiode and converted to a digital signal.

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The photodiode signal at any given time (t) is related to film thickness (x) by the following relation :

))(21arccos(2

)( 0

hSSftx t−

−=π

s0 -- the initial photodiode signalst -- the photodiode signal at time t. h -- the signal difference between complete constructive and destructive interference

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f is the thickness at the first constructive interference determined by the equation :

λθθθ

mxnfn=− 211

2

2 tansin2cos2

n1 -- the refractive indices for vacuumn2 -- the refractive indices for adsorbateθ1 and θ2 -- the incident angle and refractive angle satisfying Snell’s law (n1sin θ1= n2sin θ2 ) m -- an integer

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Experimental procedure

Preparation of ice

Adsorption of adsorbate

Desorption of adsorbate

noncrystalline iceT = 120 K

crystalline iceT = 160 K

P = 1.0E-7 torrt = 10 min(~ 20 nm)

P = 5.0E-7 torrt = 10 min(~100nm)

P = 1.0E-6 torrt = 10 min(~200nm)

P = 2.0E-6 torrt = 10 min(~400nm)

P = 1.0E-7 torrt = 10 min(~ 20 nm)

P = 5.0E-7 torrt = 10 min(~100nm)

P = 1.0E-6 torrt = 10 min(~200nm)

steadily heating

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Theoretical calculations

• CHARMM (Chemistry at HARvard Molecular Mechanics)

• Gaussian 98

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CHARMM

• a program based on classical molecular dynamics simulation

• The simplicity of the potential energy function makes possible simulation of mesoscopic systems

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Gaussian 98

• A connected system of programs for performing a variety of semi-empirical and ab initio molecular orbital calculations

• Capability of predicting many properties of molecules

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The equilibrium geometry for an acetic acid monomer on a model hexagonal ice surface

1.92 Å2.15 Å

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The fundamental frequencies of acetic acid

monomer and on the modeled hexagonal ice

3480.02980.52926.72867.01755.21440.71432.91419.21286.41056.61012.5825.6

3687.6 (3583)2971.6 (3051)2927.9 (2996)2867.5 (2944)1820.6 (1788)1432.2 (1439)1425.3 (1434)1395.3 (1380)1316.0 (1259)1049.6 (987)988.0 (847)643.9 (581)

ν(OH)νas(CH3)

νs(CH3)ν(C=O)δas(CH3)

δs(CH3)ν(C–O)ρas(CH3)ρs(CH3)γ (OH)

Acetic acid on a modeled ice(cm-1)

CH3COOH(cm-1)

Assignment

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Optimized geometries and binding energies of glycine-water complexes

bonding with the amino group and carboxyl group

ΔE = -19.4 kJ/mol ΔE = -37.5 kJ/mol

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Summary• The surface chemistry of acetic acid and glycine has

attracted a lot of attention.

• Ice plays an important role in surface chemistry.

• My project will focus on the surface chemistry of acetic acid and glycine on/in ice by FTIR-RAS.

• Computation by CHARMM and Gaussian 98 will be used to assist in the interpretation of the data.

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Acknowledgements

• Dr. K.T. Leung

• Serge Miltlin, Xiaojing Zhou, Xiang Yang

• Committee members