Formation of a Tunneling Product in the Photorearrangement of o ‐Nitrobenzaldehyde

The photochemical rearrangement of o ‐nitrobenzaldehyde to o ‐nitrosobenzoic acid, first reported in 1901, has been shown to proceed via a distinct ketene intermediate. In the course of matrix isolation experiments in various host materials at temperatures as low as 3 K, the ketene was re‐investigated in its electronic and vibrational ground states. It was shown that hitherto unreported H‐tunneling dominates its reactivity, with half‐lives of a few minutes. Unexpectedly, the tunneling product is different from o ‐nitrosobenzoic acid formed in the photoprocess: Once prepared by irradiation, the ketene spontaneously rearranges to an isoxazolone via an intriguing mechanism initiated by H‐tunneling. CCSD(T)/cc‐pVTZ computations reveal that this isoxazolone is neither thermodynamically nor kinetically favored under the experimental conditions, and that formation of this unique tunneling product constitutes a remarkable and new example of tunneling control.