The torsional The torsional spectrum of disilanespectrum of disilane

N. Moazzen-Ahmadi, University of Calgary

V.-M. Horneman, University of Oulu, Finland

Ethane and disilane are often used as Ethane and disilane are often used as benchmark molecules to investigate benchmark molecules to investigate torsion around C-C and Si-Si bonds. torsion around C-C and Si-Si bonds.

Large amplitude internal rotation Large amplitude internal rotation interacts strongly with small amplitude interacts strongly with small amplitude vibrational modes. Hence, the fine vibrational modes. Hence, the fine structure of the vibrational bands are structure of the vibrational bands are significantly altered by the torsional significantly altered by the torsional motion.motion.

Molecular systemsMolecular systems

The following molecules are under The following molecules are under investigation:investigation:

CHCH33CHCH33 CH CH33CHCH33

CHCH33CDCD3 3 CHCH33SiHSiH33,CH,CH33SiDSiD33

CDCD33CDCD3 3 SiHSiH33SiHSiH33

TorsionTorsion

This motion is basically the internal This motion is basically the internal twisting of the two groups with twisting of the two groups with respect to one another about the respect to one another about the symmetry axis of the molecule.symmetry axis of the molecule.

There is a barrier to internal There is a barrier to internal rotation (several hundred of cmrotation (several hundred of cm-1-1).).

Torsional motionTorsional motion

Near the bottom

of the barrier

Above the top

of the barrier

HamiltonianHamiltonianThe Hamiltonian describing the The Hamiltonian describing the vibration-torsion-rotation for a vibration-torsion-rotation for a symmetric top is much simpler than symmetric top is much simpler than that for an asymmetric top. that for an asymmetric top.

Much fewer interactions are Much fewer interactions are allowed between the vibrations.allowed between the vibrations.

Assignment of the lines is relatively Assignment of the lines is relatively straightforward.straightforward.

Torsional energyTorsional energy

Torsional potentialTorsional potential

0

100

200

300

400

500

600

700

cm-1

0

1

4

3

2

5

gs32

10

01

23

Δ = .015 cm-1

Δ = .51 cm-1

Δ = 6.5 cm-1

Torsional potentialTorsional potential

0

100

200

300

400

500

600

700

cm-1

0

1

4

3

2

5

gs32

10

01

23

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

Resolution 0.0025 cm-1

Path Length 172 m

Pressure 10 Torr

Temperature 296 K

CH3CH3

N. Moazzen et al., Journal of Molecular Spectroscopy (2001).

Resolution 0.007 cm-1

Path Length 172 m

Pressure 22 Torr

Temperature 296 K

SiH3SiH3

115.4 115.8 116.2 116.6 117.0

cm-1

0.88

0.94

1.00

Inte

nsity

128.500 129.875 131.250

cm-1

Inte

nsity

0.6

0.8

1.0

Journal of Chemical Physics (2006).

0

100

200

300

400

500

600

700

cm-1

0

1

4

3

2

5

gs

Molecular parameters for the gs of Si2H6

L. Goodman et al. Int. J. Quantum Chem. 90, 657 (2002).

V. Pophristic et al. J. Phys. Chem. A 105, 7457 (2001).

V. Pophristic and L. Goodman, Nature 411, 565 (2001).

HyperconjugationHyperconjugation in chemistry is in chemistry is the stabilising interaction that results the stabilising interaction that results from the interaction of the electrons from the interaction of the electrons in a sigma bond (usually C-H or C-C) in a sigma bond (usually C-H or C-C) with an adjacent empty (or partially with an adjacent empty (or partially filled) antibonding p-orbital to give an filled) antibonding p-orbital to give an extended molecular orbital that extended molecular orbital that increases the stability of the system. increases the stability of the system.

Hemoglobin

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

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