Investigation of the Amide I Band of N-Methylacetamide in Solid Parahydrogen using
FTIR Spectroscopy
Leif O. Paulson and David T. Anderson
Department of Chemistry
University of Wyoming, Laramie, WY 82071
Monday, June 22, 2009
Overview
• N-Methylacetamide• Experimental setup• Examination and discussion of the Amide I feature• Summary
N-Methylacetamide (NMA)
• H• H
• H
• H
• trans-N-Methylacetamide
wavenumber/cm-1
1700 1705 1710 1715 1720
log 10
(I0/
I)
0.00
0.25
0.50
0.75
1.00
1.25
NMA IR Transition1
FWHM = 4.5 cm-1
1. L. O. Paulson and D. T. Anderson. 61st Ohio State University International Symposium on Molecular Spectroscopy, talk R008 (2006)
Why NMA?
Amide I Vibrational Mode
• Simple model of peptide bond• Well studied specimen• Amide I mode is extremely
sensitive to its environment2
• Large molecule to study
Rationale and Challenges
2. K. E. Amunson and J. Kubelka. J. Phys. Chem. B. 111, 9993 (2007)
Producing Variable Amounts of Orthohydrogen and Parahydrogen3,4
• nH
2
• pH
2
3. S. Tam and M. E. Fajardo. Rev. Sci. Instrum. 70, 1926 (1999)4. Yoshioka, K., Raston, P. L., and D. T. Anderson. Int. Rev. Phys. Chem. 25, 469 (2006)
Cryostat cold tip
Fe(OH)3 catalyst
T=14-80K
Obtain variable amounts of parahydrogen (pH2) and orthohydrogen (oH2)
FTIR Beam
nH2
Bruker IFS 120 HR FTIR
FTIR Beam
MCT Detector
BaF2 substrate
o/p converter
Chemical dopant
Synthesis of NMA-doped pH2 Crystals5
5. M. E. Fajardo and S. Tam. J. Chem. Phys. 108, 4237 (1998)
Experimental Setup
pH2
NM
AFTIR
1695 1700 1705 1710 1715
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
wavenumber (cm-1)
1695 1700 1705 1710 1715
log 1
0(I
0/I
)
0.00
0.20
0.40
0.60
0.80
1764 1766 1768 1770
0.00
0.05
0.10
0.15
0.20
0.25
0.30
1764 1766 1768 1770
0.00
0.10
0.20
0.30
0.40
NMA Compared with Formic Acid6
6. L. O. Paulson and D. T. Anderson. J. Phys. Chem . A. 113, 1770 (2009)
NMA0.005% oH2
NMA51% oH2
Formic Acid0.005% oH2
Formic Acid51% oH2
Matrix Shift as a Function of oH2 Concentration
7. J. Kubelka and T. A. Kiederling. J. Phys. Chem. A. 105, 10922 (2001)
wavenumber (cm-1)
1690 1700 1710 1720 1730 1740
log 10
(I0/
I)
0.0
0.2
0.4
0.6
0.8
1.0
1.2Gas phase
• NMA Amide I gas phase frequency7 is 1731 cm-1
0.005% oH2
51% oH2
Δνmatrix=νpara-νgas (cm-1)
51% ortho 0.005% ortho
-27.6 (1.59%) -23.5 (1.36%)
Intermolecular Interactions in the Matrix
J=0pH2
J=1oH2
coscossinsin21cos3cos2
3,,,
222
2
2
24 HHNMAHNMA
HNMAHNMAquaddip R
RV
022 pHVpHV quaddip 022 oHVoHV quaddip
NMAoHNMApH VV 22
2
ang2
ang
Matrix Shift EffectpH2 oH2
NMA
ν=1
ν=0
Gas phase
In pH2
In pH2 with trace oH2
In pH2/oH2 mixture
1731 cm-1
1710.0 cm-1
1707.5 cm-1
1703.4 cm-1
wavenumber (cm-1)
1695 1700 1705 1710 1715
log 10
(I0/
I)
0.0
0.5
1.0
1.5
Frequency Shift due to Orthohydrogen Amount
0.005% oH2
51% oH2
27.6 cm-1
23.4 cm-1
The Environment of the Matrix
• NMA
NMA
Temp. increases
• Increased diffusion8 allows for oH2 molecules to move about in the matrix at 4.3K
• There is a greater electrostatic interaction between the oH2 quadrupole and NMA dipole moments, causing the oH2 molecules to agglomerate around the NMA dopant9
8. J. van Kranendonk. Solid Hydrogen (Plenum, New York, 1983)9. K. Yoshioka and D. T. Anderson. J. Chem. Phys. 119, 4731 (2003)
1.8K 4.3K
Temperature Effects
51% oH2
wavenumber (cm-1)
1695 1700 1705 1710 1715
log 1
0(I
0/I
)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
0.005% oH2
wavenumber (cm-1)
1695 1700 1705 1710 1715
log 1
0(I
0/I
)
0.0
0.2
0.4
0.6
0.8
1.0
1.99K
4.36K
1.92K
1.83K
4.34K
1.65K
NMA Widths with Variable oH2 Concentrations
wavenumber (cm-1)
1695 1700 1705 1710 1715
log 10
(I0/
I)
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
wavenumber (cm-1)
1695 1700 1705 1710 1715
log 1
0(I
0/I
)0.0
0.2
0.4
0.6
0.8
0.005% oH2
FWHM=4.5 cm-151% oH2
FWHM=1.8 cm-1
NMA Matrix Environment at High Orthohydrogen Concentrations
• oH
2
• pH
2
• NMA
• NMA is surrounded by oH2 molecules • Results in a primarily homogeneous environment
Linewidths due to the Matrix Environment
wavenumber (cm-1)
1695 1700 1705 1710 1715
log 10
(I0/
I)
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
wavenumber (cm-1)
1695 1700 1705 1710 1715
log 1
0(I
0/I
)
0.0
0.2
0.4
0.6
0.8
0.005% oH2
FWHM=4.5 cm-1
51% oH2 FWHM=1.8 cm-1
Inhomogeneous Homogeneous
• Broad• Asymmetric
• Narrow• Symmetric
Summary
• NMA Amide I mode is extremely sensitive to its environment
• NMA Amide I mode remains broad in solid pH2 due to residual oH2
1700 1710 1720 1730
0.0
0.2
0.4
0.6
0.8
wavenumber (cm-1)
1740 1750 1760 1770
log 1
0(I 0/
I)
0.0
0.1
0.2
0.3
0.4
-27.6 cm-1
-23.5 cm-1
-8.2 cm-1
-11.4 cm-1
Δνp-o=νortho-νpara (cm-1)
Formic Acid NMA
-3.2 -4.1
NMA
Formic Acid
Red=0.005% oH2
Blue=51% oH2
Acknowledgments
Dr. David T. Anderson
Ms. Sharon C. Kettwich
Ms. Elsbeth Klotz
NSF for the funding
Thank you for listening!!
See S.C.K.’s talk on Wednesday
Top Related