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