Global fit analysis Global fit analysis including including 9 9 + + 44 44 hot hot

band of ethane: band of ethane: Evidence of interaction Evidence of interaction with the with the 12 12 fundamentalfundamental

J.R. Cooper and N. Moazzen-Ahmadi

University of Calgary

Ethane is often used as benchmark molecules to Ethane is often used as benchmark molecules to investigate torsion around C-C bond. Its investigate torsion around C-C bond. Its significance lies in the fact that much more significance lies in the fact that much more complex molecules that occur in biological complex molecules that occur in biological environments undergo similar internal motions and environments undergo similar internal motions and insight obtained in mechanisms responsible for insight obtained in mechanisms responsible for internal rotation in ethane will be useful in internal rotation in ethane will be useful in understanding the corresponding effects occurring understanding the corresponding effects occurring in complex biological systems.in complex biological systems.

N6-methyl adenine: modification of DNA structure by the addition of methyl groups (black) affects protein:DNA interactions and so can control gene expression.

ImportanceImportance

ImportanceImportance

The C-H stretching vibrations are used The C-H stretching vibrations are used to monitor ethane in the Earth's to monitor ethane in the Earth's atmosphere.atmosphere.

The lowest frequency vibrational The lowest frequency vibrational fundamental (12 fundamental (12 µµm) has been used to m) has been used to determine the abundance of ethane in determine the abundance of ethane in the atmospheres of Jupiter, Saturn, the atmospheres of Jupiter, Saturn, Titan, Neptune and comets.Titan, Neptune and comets.

Importance of hindered internal rotationImportance of hindered internal rotation

Hindered internal rotation is a Hindered internal rotation is a very very low-frequency vibrational mode low-frequency vibrational mode and strongly anharmonic.and strongly anharmonic.

Ground vibrationalstate

Excited vibrationalstate

• Bath states State mixing IVR

Torsionalsublevels

260.25 260.50 260.75 261.00

WAVENUMBER /cm-1

TRA

NS

MIT

TAN

CE

0.90

0.92

0.94

0.96

0.98

1.00

0.92

0.94

0.96

0.98

1.00

287.9 288.0 288.1 288.2 288.3 288.4 288.5 288.6 288.7

WAVENUMBER / cm-1

TRA

NS

MIT

TAN

CE

0.70

0.80

0.90

1.00

0.80

0.90

1.00

*

**

** ** s

r rs

r

srs

rs

rsr ssr

Torsion

TORSIONAL ANGLE

0

200

400

600

800

1000

1200

cm

-1

N. Moazzen-Ahmadi et al., Journal of molecular spectroscopy (2001).

0

200

400

600

800

1000

1200

1400

cm

-1

v4 = 1

v4 = 0

V9 = 1

3

2

1

0

Methyl rock

gs

The The 99 fundamental fundamental

0

200

400

600

800

1000

1200

1400

cm

-1

High overtone bandHigh overtone band v4 = 1

v4 = 0

V9 = 1

3

2

1

0

34

gs

0

200

400

600

800

1000

1200

1400

cm

-1

V9 = 1

3

2

1

0

34

gs

Methyl rock

Two-band analysisTwo-band analysis

0

1

N. Moazzen-Ahmadi et al., Journal of chemical physics (1999).

843.5 843.6 843.7 843.8 843.9 844.0

cm-1

TR

AN

SM

ITT

AN

CE

rQ8

0.00

0.20

0.40

0.60

0.80

1.00

0.20

0.40

0.60

0.80

1.00

827.18 827.28 827.38 827.48 827.58 827.68

cm-1

TR

AN

SM

ITT

AN

CE

0.00

0.20

0.40

0.60

0.80

1.00

0.20

0.40

0.60

0.80

1.00

rQ2

** *

801.22 801.32 801.42 801.52 801.62 801.72

cm-1

TR

AN

SM

ITT

AN

CE

pQ8

0.00

0.20

0.40

0.60

0.80

1.00

0.20

0.40

0.60

0.80

1.00

738.50 738.65 738.80 738.95 739.10

cm-1

TR

AN

SM

ITT

AN

CE

0.80

0.90

1.00

0.90

1.00

pP12(39)pP15(34)

pP18(29)

pP19(27)

pP9(44)

P18(28)

*

pP21(24)

0.4 Torr, 2 m, -140 C, 0.0024 cm-1

Ethane on Saturn in emission

F.M. Flasar et al., Science (2005).

Ethane on Titan

F.M. Flasar et al., Science (2005).

0

500

1000

1500

cm

-1

0

1

2

3

4

gs

0

1

V9 = 1

0

1V3 = 1

3

C-C stretch

0

500

1000

1500

cm

-1

0

1

2

3

4

gs

N. Moazzen-Ahmadi Journal of molecular spectroscopy (2001).

0

1

V9= 1

0

1

V3= 1

Three-band analysisThree-band analysis

C-C stretching fundamentalC-C stretching fundamentalCH3CH3CH3CH3

993 994 995

WAVENUMBERS

(a)

(b)

(c)

A. Al-Kahtani et al. JCP 98, 101 (1993).

0

500

1000

1500

cm

-1

0

1

2

3

4

gs

N. Moazzen-Ahmadi Journal of molecular spectroscopy (2001).

0

1

V9= 1

0

1

V3= 1

9 9 + + 4 4 4 4 bandband

High temperature spectrum (High temperature spectrum (TT = 295 K) = 295 K)

gs0

12

3

9 = 1

01

2

Low temperature spectrum (Low temperature spectrum (TT = 128 K) = 128 K)

gs0

12

3

9 = 1

01

2

No data for 12!

12 12 9 9 bandband

Energy pathways in ethaneEnergy pathways in ethane