Laser Raman Spectroscopy and Modelling of Diamondoids
We have been using our Renishaw Raman spectrometer to take Raman spectra of a number of diamondoid crystals, supplied to us by Molecular Diamond Technologies (a part of Chevron USA). We have also been performing ab initio DFT calculations to simulate these spectra, in order to assign the peaks to specific vibrational modes.
Diamondoids are hydrocarbon molecules that are totally or largely superimposable on the diamond lattice. The first in the series is adamantane, and higher diamondoids are made by fusing together successive adamantane units. A large number of such units makes diamond.
Diamondoids occur naturally in virtually all petroleum (1-10 ppm). Molecular Diamond Technologies extract them by reverse phase HPLC, and then recrystallise them by standard organic chemistry techniques. Samples are available for many of the smaller diamondoids, either as powders or as small crystals around 1 mm3 in size (see photo, right).
The Raman spectra of the diamondoids contain 2 regions of interest, a low energy region (400-1500 cm-1) due to stretching of the C-C cage, and a high energy region (2700-3100 cm-1) due to C-H stretches and bends. An example of one of the vibrations from each of these 2 regions can be seen below in the spectrum of 1212 pentamantane.
Some interactive spectra can be seen here, but spectra and structures for all of the diamondoids we have studied are below. The Raman spectra were taken from powder samples or single crystals using a 488 nm laser. The 3D structures of the molecules can be downloaded as mol files by clicking on the image. The Raman data can also be downloaded as an Excel file (x,y plot), or as an interactive JCamp spectrum (requires the Chime plug-in).
- J. Filik, J.N. Harvey, N.L. Allan, P.W. May, J.E.P. Dahl, S. Liu, and R.M.K. Carlson, "Raman Spectroscopy of Diamondoids", Spectrochimica Acta A: Mol. Biomol. Spect. 64 (2006) 681-692.
- J. Filik, J.N. Harvey, N.L. Allan, P.W. May, J.E.P. Dahl, S. Liu, R.M.K. Carlson, "Raman spectroscopy of nanocrystalline diamond: An ab initio approach", Phys. Rev. B. 74 (2006) 035423 pp.1-10.
- N.C. Polfer, O. Pirali, Y. Ueno, R. Maboudian, P.W. May, J. Filik, J.E. Dahl, S. Liu, R.M.K. Carlson, "Infrared spectroscopic investigation of higher diamondoids", J. Mol. Spectr. 238 (2006) 158-167.
- J.N. Hart, P.W. May, N.L. Allan, J.E Dahl, Shenggao Lui, Robert M Carlson, J.L. Adcock, "Vibrational analysis of per-fluorinated-triamantane", Chem. Phys. Letts.. 460 (2008) 237-240.