Reversible cyclomerization and long range hydrogen abstraction by electron impact of 7‐coumarinyl diesters ROOC(CH 2 ) n COOR

The mass spectra of 4‐methyl‐7‐coumarinyl and 7‐coumarinyl diestes ROOC(CH 2 ) n COOR ( n = 2‐12) have ben studied by appearance potential measurements, deuterium labelling and by comparison with suitable reference compounds such as the mised diestes ROOC(CH 2 ) n COOR′ (R=4‐methyl‐7‐coumarinyl and R′ = methyl and phenyl) and 3.4‐dihydro‐4‐methyl‐coumarinyl diestes. Observations on the fragment ions of m / e 324, produced from the 7‐coumrinyl diestes and their photocyclomers, by elimination of the central bridge as OCCH(CH 2 ) n–2 CHCO, demonstrate the existence and reversible formation of cyclomeric molecular ions. A stable bound system between the coumrin end groups is formed only at high internal energies by expulsion of a hydrogen atom, followed by elimination of the central bridge from the [MH] + ion. It is also shown that the lifetime of the open form molecular ions decreases remarkably for chain lengths with n larger than 6.