Time-resolved photoacoustic calorimetry
Time—resolved photoacoustic calorimetry is an experimental method that provides both the dynamics and energetics of reactive intermediates that are produced in photochemical reactions. With the present apparatus, dynamics on the timescale spanning 10 nanoseconds to 50 microseconds can be resolved. The enthalpy sensitivity is 1.0 kcal/mole. This technique has been applied to the hydrogen atom abstraction reaction of t—butoxy radical with phenol. Also, the strain enthalpy of 1—phenyl—trans—cyclohexene has been measured. INTRODUCTION Photochemists have long recognized the necessity of having reliable thermochemical data that can be used in the formulation of reaction mechanisms. Unfortunately, it has been almost impossible to obtain thermochemical information for reactive intermediates with lifetimes on the nanosecond or microsecond timescales. For example, it has been extremely difficult to discuss the effect a substituent may have on the stability of a radical or carbene, or how the medium may effect the energetics of an ionic intermediate. During the past two years, we have been developing the experimental method of time—resolved photoacoustic calorimetry whereby both the energetics and dynamics for photochemically generated reactive intermediates can be obtained on timescale spanning 10 nanoseconds to 50 microseconds (ref. 1). Similar methods have been developed in the laboratory of Bras— lavsky (ref. 2). In this report, we will discuss the most recent developments, and present two chemical problems to which the technique has been applied.