Transcript RI10_Presentationx

Infrared Spectroscopy
of Alanine in Solid
Parahydrogen
Shin Yi Toh, Ying-Tung Angel Wong, Pavle Djuricanin,
and Takamasa Momose
Department of Chemistry
University of British Columbia
Vancouver, BC, Canada.
Introduction
Motive – To study amino acids in relation to interstellar
chemistry
Start with simple amino acid  β-alanine
β-alanine in Astrochemistry:
• Found in various classes of carbonaceous meteorites.
• Most abundant type of amino acid in CI chondrites (class of
carbonaceous meteorites).
• Formed preferentially over α-alanine in ion-molecule reactions with
smaller molecules found in interstellar medium.
[1] J.G. Lawless, Geochim. Cosmochim. Ac., 37, 2207 (1978).
[2] O. Botta, Z. Martins, and P. Ehrenfreund, Meteorit. Planet. Sci., 42, 81 (2007).
[3] V. Blagojevic, S. Petrie, and D.K. Bohme, Mon. Not. R. Astron. Soc., 339, L7 (2003).
Previous studies on β-alanine
Computation:
• First began with Ramek (1990)
M. Ramek, J. Mol. Struct. (Theochem), 208, 301 (1990)
• Derived 20 stable conformers of β-alanine at HF/4-31G level of theory.
Gas Phase Spectroscopy:
• McGlone and Godfrey (1995)
S.J. McGlone and P.D. Godfrey, J. Am. Chem. Soc., 117, 1043 (1995)
• Free-expansion jet spectrometry; observed conformers I, V.
•
Sanz et al. (2006)
M.E. Sanz, A. Lesarri, M.I. Peña, and V. Vaquero, J. Am. Chem. Soc., 128, 3812 (2006)
• Fourier transform microwave spectroscopy; found conformers II and III, in addition to I, V.
Matrix-isolation spectroscopy:
• Dobrowolski et al. (2008)
J.C. Dobrowolski, M.H. Jamróz, R. Kolos, J.E. Rode, and J. Sadlej, Chem. Phs. Chem., 9, 2042 (2008)
• Matrix-isolation IR spectroscopy (argon matrices); detected at least 3 conformers: I, II, IV.
•
Stepanian et al. (2012)
S. G. Stepanian, A. Y. Ivanov, D. A. Smyrnova, and L. Adamowiez, J. Mol. Struct., 1025, 6 (2012)
• Matrix-isolation FT-IR spectroscopy (argon matrices) + irradiation + matrix annealing +
deuteration; detected the presence of at least 5 conformers: I, II, IV, V, VII.
Objective
Matrix isolation spectroscopy of amino acids:
• Identify vibrational frequency of different conformers
• Identify stable conformer under various condition (low temperature, UV
irradiation, etc)
Parahydrogen as matrix host
• The soft and more inert property of pH2 allow for the trapping of highly
energetic states  possibility of more conformers detection.
In on this study
Using solid pH2 matrix-isolation FT-IR spectroscopy, we aim to:
• determine the gas-phase conformational composition of β-alanine, and
compare to that in argon matrix.
• investigate the outcomes of UV photochemistry on gas-phase β-alanine.
Method
Experimental
Ortho-Para Converter:
• Operates at 14K – triple
point of hydrogen
• Magnetic catalyst:
(FeOH)O
• Yields parahydrogen gas of
99.95% purity
Cartridge
Heater
Knudsen Cell:
• Β-alanine sublimation
temperatures - 390±1K or
380±1K
18mm
1.6mm
NTC Thermistor
Method
• FT-IR spectrometer: KBr beamsplitter, MIR glowbar light source, liquid cooled
MCT detector. 0.2 cm-1 resolution, 1000 scans, approx. 5000-700 cm-1 range
• UV-irradiation with D2 lamp (λ = 180 – 270 cm-1)
Method
Computational
• Theoretical frequencies and intensities calculated for the 11 lowest energy
β-alanine conformers at the B3LYP/aug-cc-pVTZ level of theory.
Results and Discussions
Conformational Composition
Absorbance, relative unit
Absorbance, relative unit
ω(NH2) region
II
ν (C=O) region
0.25
0.04
0.08
I
III
0.06
I
0.05
0.10
I
0.03
0.04
VII
0.02
IV
0.02
0.01
0.00
0.00
780
0.20
790
800
810
820
-1
Wavenumber, cm
830
Para-H2
II
IV
III not
observed in
Ar
IV
0.10
0.05
Ar
1720
III
0.15
0.00
1740
1760
-1 1780
Wavenumber, cm
1800
Results and Discussions
UV Photochemistry
Conformer
Population before UV
Population after UV
Change in population
I
1.00
0.73
- 0.27
II
0.63
0.46
-0.17
III
0.19
0.25
+0.06
IV
0.36
0.49
+0.13
VII
0.11
0.03
-0.08
 All population numbers are normalized to the population of conformer I at deposition.
II & I
ν (C-O) region
I
Deposition Spectra
Absorbance, relative unit
II
IV
VII
IV
VII
III
Para-H2
Ar
UV-Irradiation Difference Spectra
Para-H2
Ar
1090
1100
1110
1120
1130
-1
Wavenumber, cm
1140
1150
1160
Results and Discussions
Experimental population ratio, %
Temperature dependence of conformational population
40
30
Conformer I
Conformer II
Conformer III
Conformer IV
Conformer VII
20
10
380
382
384
386
388
390
Sublimation temperature, K
Conformer
ΔEZPE, kJ/mol
Boltzmann distribution at 390 K, %
Boltzmann distribution at 380K, %
I
0.0
46.4
47.5
II
2.0
25.4
25.4
III
7.4
4.7
4.5
IV
3.9
14.1
13.8
VII
5.4
9.0
8.7
Conclusions
Conformational Studies:
• Five conformers of β-alanine were identified: I, II, IV, III, and VII.
• Conformer III was found for the first time under matrix isolation
technique.
• Conformer V: possibility of inconclusive assignment by previous
research.
UV Photochemistry:
• Conformational change:
• I & II  IV
• VII  III
Sublimation Temperature:
• Trend follows Boltzmann distribution. With increasing temperature:
• Lowest energy conformer (conformer I) decrease.
• Higher energy conformers (conformer II, IV, III, and VII) increase.
• Useful technique for conformational studies of other similar molecules:
• Aid in identifying the most stable conformer.
Summary of Parahydrogen as matrix host for studies of amino acids
Advantages:
• Conserves the room temperature conformations of the gas-phase sample.
• Lighter collision of the host molecule to the gaseous sample preserve the
highly energized states of the target molecule.
• More profound isomerization of amino acids under UV irradiation in pH2
matrix compare to in Ar matrix.
• Comparable spectra line-width of the sample with that performed in Ar
matrix.
• Expectation: narrower line-width.
Disadvantage:
• No matrix annealing experiment.
• The range of temperature change without distorting the pH2 crystal is very
narrow (4-7K) as compared to solid Ar (4-40K).
Alternative technique – sublimation temperature experiment
• Preservation of amino acids’ conformational population at sublimation
temperature due to the softer collision effect on the sample.
Current Work
Absorbance, relative intensity
UV Photolysis on β-alanine
New product peak
from UV photolysis
1840
1842
1844
1846
1848
1850
CO2
After 4hrs UV Irradiation
At Deposition
2200
2250
2300
-1
Wavenumber, cm
2350
2400
Aside from conformational changes, β-alanine also seems to photodissociate
into CO2 molecule and other fragments. We are now performing computation
calculations on some possible candidates in hope to assign these β-alanine
UV photolysis products.
Future Work
Using similar experimental settings and concepts as the ones employed for
gas phase β-alanine, we aim to further expand the present spectroscopic
knowledge on amino acids by conducting solid parahydogen matrixisolation FTIR spectroscopy on other simple amino acids. We also aim to
study the zwitterion form of the molecules in study. Currently under
investigation:
• α-alanine
• β-alanine zwitterion
Acknowledgement
Supervisor: Takamasa Momose
Collaborator: Ying-Tung Angel Wong
Technician: Pavle Djuricanin