the surface chemistry of acetic acid and its α-amino ...atom.uwaterloo.ca/group meetings/2003...
TRANSCRIPT
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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
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CHEM 794, March 27nd, 2003 2
Outline
• Introduction• Proposal• Summary• Acknowledgements
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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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CHEM 794, March 27nd, 2003 4
The perfect hexagonal ice structure
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CHEM 794, March 27nd, 2003 5
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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CHEM 794, March 27nd, 2003 6
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
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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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CHEM 794, March 27nd, 2003 8
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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CHEM 794, March 27nd, 2003 9
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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CHEM 794, March 27nd, 2003 10
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
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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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CHEM 794, March 27nd, 2003 12
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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CHEM 794, March 27nd, 2003 13
The surface chemistry of glycine
• On metal surfaces
• On Silicon surface
• No study on ice Glycine adsorbed on Cu(110)
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CHEM 794, March 27nd, 2003 14
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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CHEM 794, March 27nd, 2003 15
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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CHEM 794, March 27nd, 2003 16
Experimental method - FTIR-RAS
• Identification of functional groups
• Information about the chemical bonding of the adsorbate and substrate
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CHEM 794, March 27nd, 2003 17
Schematic layout of
the experimental set up
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CHEM 794, March 27nd, 2003 18
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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CHEM 794, March 27nd, 2003 19
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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CHEM 794, March 27nd, 2003 20
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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CHEM 794, March 27nd, 2003 21
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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CHEM 794, March 27nd, 2003 22
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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CHEM 794, March 27nd, 2003 23
Theoretical calculations
• CHARMM (Chemistry at HARvard Molecular Mechanics)
• Gaussian 98
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CHEM 794, March 27nd, 2003 24
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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CHEM 794, March 27nd, 2003 25
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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CHEM 794, March 27nd, 2003 26
The equilibrium geometry for an acetic acid monomer on a model hexagonal ice surface
1.92 Å2.15 Å
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CHEM 794, March 27nd, 2003 27
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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CHEM 794, March 27nd, 2003 28
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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CHEM 794, March 27nd, 2003 29
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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CHEM 794, March 27nd, 2003 30
Acknowledgements
• Dr. K.T. Leung
• Serge Miltlin, Xiaojing Zhou, Xiang Yang
• Committee members