1. Infrared Spectroscopy of N-Methylacetamide in Solid Parahydrogen
Leif O. Paulson and David T. Anderson
Department of Chemistry, University of Wyoming
Laramie, WY
61th International Symposium on Molecular Spectroscopy, June 19-23, 2006

2. Outline Why N-methylacetamide (NMA)?
High-resolution vibrational spectroscopy
Methyl rotors and nuclear spin conversion

3. trans-N-methylacetamide
The peptide bond H H H H
trans-N-methylacetamide

4. Vibrational modes of the peptide bond
amide A
amide II
amide I
amide III

5. Infrared spectrum of NMA
amide A
amide I

6. Deposit crystals from the gas-phase
cryostat
cold tip
vacuum shroud
optical substrate
T = 2.4 K
catalytic converter
Fe(OH)3
IR beam
vacuum
radiation shield
room temperature
dopant (e.g., NMA)
atmosphere
S. Tam and M. E. Fajardo, Rev. Sci. Instrumen. 70, 1926 (1999).

7. Infrared spectrum of NMA in solid pH2
4.32 K, 0.05 cm-1, 121 scans
assignment based on S. Ataka, H. Takeuchi, and M. Tasumi, J. Mol. Struct. 113, 147 (1984).

8. High-resolution infrared spectra of amide A
0.75 cm-1
absorption shows fine structure

9. Amide A – intensity changes with time
main feature ( cm-1) sharpens and increases in intensity at 4.32 K
blue shoulder at cm-1 decreases in intensity
lineshape irregular – unresolved fine structure?
- 337 min, 4.32 K
- 70 min, 4.35 K
- 36 min, 4.35 K

10. Amide A – intensity changes with time
at low temperature, 1.77 K, main feature decreases in intensity while new feature at 3499 cm-1 grows
blue shoulder at cm-1 remains constant
could this be evidence of clustering with ortho-H2? K
elapsed = 176 min H

11. Amide I – extremely broad IR transition
4.5 cm-1
annealed, 1.77 K
amide I
as-deposited, 1.91 K
amide I is known to be especially sensitive to structure
linewidths extremely broad – is this lifetime broadening?

12. Amide I – intensity changes with time
- 337 min, 4.32 K
- 70 min, 4.35 K
- 36 min, 4.35 K
similar to amide A, amide I changes with time at 4.3 K
unresolved fine structure?

13. Amide I – intensity changes with timeK
elapsed = 176 min
at 1.73 K, intensity constant but shifts slowly to higher energy
unresolved fine structure?

14. Could fine structure be caused by methyl rotor torsional motion?
NMA contains two methyl tops with low barriers to rotation
nuclear spin statistics could “trap” high energy forms that decay slowly
amide A and amide I vibrations sensitive to methyl rotor orientation EE E4 E3 E EA E1 AE E2 A AA A1
no torsion
top 1
top 2
G18
energy diagram from N. Ohashi, J. T. Hougen, R. D. Suenram, F. J. Lovas, Y. Kawashima, M. Fujitake, and J. Pyka, J. Mol. Spectrosc. 227, 28 (2004).

15. Infrared spectra sensitive to Ramachandran angles
gas-phase N-CH3 torsional barrier y f
torsional barriers measured with microwave spectroscopy
does high-resolution infrared spectrum contain similar information?

16. Summary NMA/pH2 amide A and amide I absorptions show fine structure
amide I absorptions very broad – lifetime broadening?
intensity changes with temperature and time – methyl rotors and intramolecular NSC
Currently studying per-deuterated (d7) NMA and
N-methylformamide (one methyl rotor)