A Spectroscopic Study of the Torsional Modes of
1, 4-bis(phenylethynyl)benzene

Rydberg atoms
Cavity Ring Down Spectroscopy
Reaction Mechanisms
Cold Molecules
Group members

Last updated 24/04/2012

Torsional Modes of 1,4-bis(phenylethynyl)benzene

The CRDS experiment has been used to study the torsional vibrational modes of 1, 4-bis(phenylethynyl)benzene (BPEB).

anti-symmetric twist
Anti-symmetric twisting mode of BPEB

symmetric twist
Symmetric twisting mode of BPEB

Only the symmetric torsional vibration moves the central phenyl ring, thus isotopic substitution of the hydrogens on the central ring can be used to identify spectral peaks belonging to the individual modes.

symmetric twist of the D-BPEB molecule

1,4-bis(phenylethynyl)-2,3,5,6-tetradeuterobenzene (D-BPEB)

UV-Vis Spectrum of BPEB

UV-vis spectrum of the BPEB

The UV-vis spectrum of BPEB in the gas phase was recorded and compared with the spectrum recorded on cyclohexane solution. Due to the 20 nm blue shift in the gas phase the CRDS experiment was targeted under the shoulder of the spectrum at 320 nm.


The CRDS experiment was modified to seed the molecular beam with BPEB. An 'oven' design was adapted for use in our system.

nozzle and oven for seeding the molecular beam

When heated the solid sample sublimes in the vacuum of the chamber and creates a cloud of BPEB vapour in the exit of the oven. When the nozzle is fired the Ar pulse 'picks-up' and cools the BPEB as it travels towards the ring-down cavity axis. The 40 deg cone in the oven aids cooling and spatial focusing of the beam.

This allowed us to record the spectra of BPEB and D-BPEB.


Comparison of UV cavity ring-down spectra of BPEB and D-BPEB
Comparison of the UV cavity ring-down spectra of BPEB and D-BPEB. The vertical dotted lines indicate anti-symmetric torsional transitions which are not shifted upon isotopic substitution of the central ring. The arrows indicate the shifts of some symmetric torsional transitions caused by the slower twisting vibration of the heavier central ring in D-BPEB.

From the spectra, we were able to deduce the tortional potentials of the electronic ground and excited states and to successfully simulate the spectra.

The results of this study are published at:

Cavity Ring-Down Spectroscopy of the Torsional Motions of 1,4-Bis(phenylethynyl)benzene.
Stuart J. Greaves, Emma L. Flynn, Emma L. Futcher, Eckart Wrede, Donocadh P. Lydon, Paul J. Low, Simon R. Rutter, and Andrew Beeby.
J. Phys. Chem. A, 110 (2006), 2114-2121.

The article has been released on the web and can be downloaded by subscribers of ASC journals using the following link: 10.1021/jp054426h

Links to home pages:     University of Durham     Chemistry Department     Eckart Wrede