Regiochemical Substituent Switching of Spin States in Aryl(trifluoromethyl)carbenes

Although aryl(trifluoromethyl)diazirines have achieved great popularity in photoaffinity labeling applications, the properties of the corresponding carbenes have not been as widely explored. Here, low-temperature matrix-isolation spectroscopy and reactivity studies indicate that in contrast to m-methoxyphenyl(trifluoromethyl)carbene and most known aryl(CF3)carbenes, the para isomer is a ground-state singlet rather than triplet. DFT calculations support these results as well as the notion that the p-CH3O group stabilizes the singlet carbene via resonance. These results may have relevance to the wide range of substituted aryl(CF3)diazirines in photoaffinity applications.