Since I was talking about microwave heating of reactions here the other week, I wanted to mention this correspondence in Angewandte Chemie. Oliver Kappe is the recognized expert on microwave heating in chemistry, and recently published an overview of the topic. One of the examples he cited was a report of some Friedel-Crafts reactions that were accelerated by microwave heating. The authors did not take this very well, and fired back with a correspondence in Ang. Chem., clearly feeling that their work had been mistreated in Kappe's article. They never claimed to be seeing some sort of nonthermal microwave effect, they say, and resent the implication that they were.
Kappe himself has replied now, and seems to feel that Dudley et al. are trying to have things both ways:
In their Correspondence, Dudley and co-workers have suggested that we attempt to impugn their credibility by associating their rationalization for the observed effect with the concept of nonthermal microwave effects. This is clearly not the case. On the contrary, we specifically state in the Essay that “The proposed effect perhaps can best be classified as a specific microwave effect involving selective heating of a strongly microwave-absorbing species in a homogeneous reaction mixture (”molecular radiators).“ As we have already pointed out, our Essay was mainly intended to provide an overview on the current state-of-affairs regarding microwave chemistry and microwave effects research. Not surprisingly, therefore, out of the incriminated 22 uses of the word ”nonthermal“ in our Essay, this word was used only twice in reference to the Dudley chemistry, and in both of these instances in conjunction with the term ”specific microwave effect“.
The confusion perhaps arises since in the original publication by Dudley, the authors provide no clear-cut classification (thermal, specific, nonthermal) of the microwave effect that they have observed. In fact, they do not unequivocally state that they believe the effect is connected to a purely thermal phenomenon, but rather invoke arguments about molecular collisions and the pre-exponential factor A in the Arrhenius equation (for example: “Chemical reactions arise from specific molecular collisions, which typically increase as a function of temperature but also result from incident microwave irradiation”). Statements like this that appear to separate a thermal phenomenon from a microwave irradiation event clearly invite speculation by non-experts about the involvement of microwave effects that are not purely thermal in nature. This is very apparent by the news feature in Chemistry World following the publication of the Dudley article entitled: “Magical microwave effects revived. Microwaves can accelerate reactions without heating”
Based on his own group's study of the reaction, Kappe believes that what's going on is local superheating of the solvent, not something more involved and/or mysterious. His reply is a lengthy, detailed schooling in microwave techniques - why the stated power output of a microwave reactor is largely meaningless, the importance (and difficulty) of accurate temperature measurements, and the number of variables that can influence solvent superheating. The dispute here seems to be largely a result of the original paper trying to sound coy about microwave effects - if they'd played things down a bit, I don't think this whole affair would have blown up.
But outside of this work, on the general topic of nonthermal microwave reaction effects, I side with Kappe (and, apparently, so do Dudley and co-authors). I haven't seen any convincing evidence for microwave enhancement of reactions that doesn't come down to heating (steep gradient, localized superheating, etc.)