Chemists engineer chemical reactions by taking steps to understand the fundamental mechanisms involved. Doing so allows chemists to improve yields of manufactured chemicals and do so at lesser cost to the environment. Low temperatures are useful for slowing the rates of reactions such that it becomes possible to see the reactive intermediates.
Isolating reaction intermediates
Isolation of reaction intermediates at low temperature using inert matrices to prolong the reaction lifetimes was originally developed in the 1950’s, notably by Pimentel1 (amongst others). More recently polymers and other matrices have been used to isolate reactive intermediates generated by photolysis. For instance, Calladine et al. have examined the activation of the C-H bond over metal catalysts at low temperatures using an alkane matrix.2
In order to perform spectroscopic measurements of these reactions, one needs to contain them in a temperature controlled environment. Specac’s Variable Temperature cell holder (“VT cell”) can be heated or cooled to operate at temperatures from -190 to 250 °C, which enables it to be used to probe reactions at both high and low temperature.
Applications of the VT cell
Investigating phosphorescence
The VT cell is highly versatile and can be used for far more than simply FTIR spectroscopy, and is suitable for use in Fluorescence, Raman and UV/vis spectroscopic experiments. Baklanova et al. have used the VT cell to study the thermal stability of the emission of doped NaLa9(GeO4)6, an IR phosphor.3 As you would expect, the phosphorescence of their compound decreased with temperature and they estimated that the emission would drop to 50% of its RT intensity at approximately 342 °C by extrapolating the trend line of the data collected up to 200 °C. They therefore concluded their compound’s thermal stability was excellent.
Safety of nuclear waste packaging
Boughattas et al. used the technique to assess the safety of nuclear waste packages exposed to very high temperatures, such as might be encountered in a transportation fire.4 In their study they heated samples of PVC, a common nuclear packaging material, in order to gain insights into the effect of the temperatures that might be encountered during a fire. For γ-irradiated samples, they observed a decrease in bands assigned to C-Cl species concurrent with an increase in bands assigned to unsaturated C-H and C=C stretches between 120-150 °C. The authors therefore concluded that the degradation of PVC proceeds with a dehydrochlorination chain reaction, with the formation of HCl gas as a by-product. This reaction occurs at a much greater temperature for samples of PVC that were not degraded by exposure to γ-radiation. Identification of reaction products at variable temperature can also be supplemented by probing the intermediates in reactions and other processes such as stereoisomer interconversions.
References
- E. Whittle, D. A. Dows and G. C. Pimentel, Matrix Isolation Method for the Experimental Study of Unstable Species, The Journal of Chemical Physics, 1954, 22, 1943.
- J. A. Calladine, S. B. Duckett, M. W. George, S. L. Matthews, R. N. Perutz, O. Torres and K. Q. Vuong, Manganese Alkane Complexes: An IR and NMR Spectroscopic Investigation, Journal of the American Chemical Society, 2011, 133, 2303–2310.
- Y. v Baklanova, O. A. Lipina, A. N. Enyashin, L. L. Surat, A. P. Tyutyunnik, N. v Tarakina, A. D. Fortes, A. Yu. Chufarov, E. v Gorbatov and V. G. Zubkov, Nd3+,Ho3+-Codoped apatite-related NaLa9(GeO4)6O2 phosphors for the near- and middle-infrared region, Dalton Transactions, 2018, 47, 14041–14051.
- I. Boughattas, E. Pellizzi, M. Ferry, V. Dauvois, C. Lamouroux, A. Dannoux-Papin, E. Leoni, E. Balanzat and S. Esnouf, Thermal degradation of γ-irradiated PVC: II-Isothermal experiments, Polymer Degradation and Stability, 2016, 126, 209–218.
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