The torsional spectrum of disilane N. Moazzen-Ahmadi, University of Calgary V.-M. Horneman, University of Oulu, Finland

Ethane and disilane are often used as benchmark molecules to investigate torsion around C-C and Si-Si bonds. Large amplitude internal rotation interacts strongly with small amplitude vibrational modes. Hence, the fine structure of the vibrational bands are significantly altered by the torsional motion.

Molecular systems The following molecules are under investigation: CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CD 3 CH 3 SiH 3,CH 3 SiD 3 CH 3 CD 3 CH 3 SiH 3,CH 3 SiD 3 CD 3 CD 3 SiH 3 SiH 3 CD 3 CD 3 SiH 3 SiH 3

Torsion This motion is basically the internal twisting of the two groups with respect to one another about the symmetry axis of the molecule. There is a barrier to internal rotation (several hundred of cm -1 ).

Torsional motion Near the bottom of the barrier Above the top of the barrier

Hamiltonian The Hamiltonian describing the vibration-torsion-rotation for a symmetric top is much simpler than that for an asymmetric top. Much fewer interactions are allowed between the vibrations. Assignment of the lines is relatively straightforward.

Torsional energy

Torsional potential  Δ =.015 cm -1 Δ =.51 cm -1 Δ = 6.5 cm -1

Torsional potential 

Resolution cm -1 Path Length 172 m Pressure 10 Torr Temperature 296 K CH 3 N. Moazzen et al., Journal of Molecular Spectroscopy (2001).

Resolution cm -1 Path Length 172 m Pressure 22 Torr Temperature 296 K SiH 3 Journal of Chemical Physics (2006).

Molecular parameters for the gs of Si 2 H 6

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).

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

Hemoglobin

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

Importance of hindered internal rotation Hindered internal rotation is a very low-frequency vibrational mode and strongly anharmonic. Ground vibrational state Excited vibrational state Bath statesState mixing IVR Torsional sublevels