Reversible Interconversion between Singlet and Triplet 2-Naphthyl(carbomethoxy)carbene
datasetposted on 16.03.1999, 00:00 by Zhendong Zhu, Thomas Bally, Louise L. Stracener, Robert J. McMahon
Photolysis of methyl α-diazo-(2-naphthyl)acetate at 450 nm yields primarily the triplet ground state of 2-naphthyl(carbomethoxy)carbene (3NCC), which was characterized by its UV/visible, IR, and ESR spectra as well as by trapping with O2 and CO. Bleaching of the weak visible bands of 3NCC (λ > 515 nm) leads to the disappearance of the triplet ESR spectrum and to a new optical spectrum with broad bands in the near-UV region. The spectra of 3NCC can be almost fully recovered in the dark at 12 K. The same process also occurs upon irradiation at 450 nm, with the exception that the recovery of 3NCC is accompanied by the formation of small amounts of 2-naphthyl(methoxy)ketene and 2-(2-naphthyl)propiolactone. The observed optical and IR spectra of the intermediate that is formed reversibly from 3NCC are in full accord with quantum chemical model calculations for the singlet state of the same carbene 1NCC, but not with the tricyclic cyclopropene derivative observed previously for the parent 2-naphthylcarbene or the 1-(2-naphthyl)-2-methoxyoxirene. Exploration of the potential energy surfaces for singlet and triplet NCC show that a barrier between the two states is created by a pronounced change in conformation of the carbomethoxy group, which may furthermore be hindered in a solid matrix environment.