Gas-phase energetics of organic free radicals using time-resolved photoacoustic calorimetry
Introduction
Time-resolved photoacoustic calorimetry (TR-PAC) is a recent technique that is being established as a tool for the determination of bond dissociation enthalpies in solution [1], [2]. It represents a development of the classical (non-time-resolved or static) PAC, already a well-known technique in the same area [3], but which does have some limitations. In both cases the experimental strategy usually involves the cleavage of the bond of interest through a suitable reaction which, in the case of classical PAC, has to be very fast (typically with an overall duration in the nanosecond time scale). TR-PAC not only eliminates this requirement, but it also provides kinetic information in addition to the enthalpic data. In this work we used time-resolved photoacoustic calorimetry to determine the O–H bond dissociation enthalpy of phenol in several solvents, as a general example of the procedure, and to illustrate the advantages of the time-resolved vs. the classical version of the technique. A crucial part of the study of bond dissociation enthalpies is the relation of the experimentally determined solution results with gas-phase values. Its importance for hydrogen-bonding molecules, and for phenol in particular, has been pointed out, and several related methods to deal with this issue have been presented [4], [5]. However, we recently found that the validity of those methods is not general, and that more sophisticated models, making use of computational chemistry calculations (CC), are needed to afford accurate results [6], [7]. In the following sections we will analyze the combined TR-PAC/CC approach in this context. The experimental setup and general procedure will be presented, and the algorithms used to yield thermodynamic data will be described in some detail, so as to discuss the main assumptions and approximations of the method.
Section snippets
Materials
Benzene (Aldrich), acetonitrile (Aldrich) and carbon tetrachloride (Aldrich) were of HPLC grade and used as received. Di-tert-butylperoxide (Aldrich) was purified according to a literature procedure [8]. tert-Butanol (Merck) was dried over calcium hydride, fractionally distilled, and kept in a glove box over calcium. Phenol (Aldrich, +99%) was sublimed in vacuum and kept under nitrogen prior to use. ortho-Hydroxybenzophenone (Aldrich) was recristalized twice from an ethanol-water mixture.
Results and discussion
The general strategy for determining the R–H bond dissociation enthalpy in an organic molecule using photoacoustic calorimetry, is illustrated in Scheme 1. A tert-butoxy radical generated from the photolysis of di-tert-butylperoxide (reaction 5) abstracts a hydrogen atom from the substrate (phenol in this example), yielding the corresponding radical (phenoxy in reaction 6). Reaction 7 represents the net process.
The slower process in Scheme 1 is the hydrogen abstraction with the tert-butoxy
Summary and final comments
Like all other techniques used to probe the energetics of transient molecules, photoacoustic calorimetry also has its virtues and its problems. However, a fairly large number of PAC-derived thermochemical results have become available, most in excellent agreement with values derived from other techniques [3]. There has been an enduring effort to improve the reliability of photoacoustic calorimetry. Nevertheless, several questions keep being raised, the most common regarding the use of di-tert
Acknowledgements
We thank Dr. Manuel Minas da Piedade (FCUL) and Dr. Hermı́nio Diogo (Instituto Superior Técnico, Lisboa) for assistance with the reaction–solution calorimetry experiments. This work was supported by Fundação para a Ciência e a Tecnologia (FCT), Portugal (POCTI/35406/QUI/1999). C.F.C. and P.M.N. thank FCT for a Ph.D. (SFRH/BD/6519/2001) and a post-doctoral grant (SFRH/BPD/11465/2002), respectively.
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