β
-ionone, a model compound of carotenoids
ring structure, was investigated by FT-IR spectroscopy in a
low-temperature argon matrix as well as using B3LYP/6-311+
+G(d,p) and MP2/6-311++G(d,p) quantum-chemical calcu-
lations. The spectrum of matrix-isolated
E
-
β
-ionone was
analyzed and attributed to six conformers of the compound.
Then, matrix-isolated
E
-
β
-ionone was submitted to UV
irradiation using either a broadband source (with di
ff
erent
cuto
fffi
lters) or a narrowband laser/MOPO system (at various
wavelengths). Upon 240 nm narrowband irradiation, the
formation of both
Z
-
retro
-
γ
-ionone and
Z
-
β
-ionone was
observed, the reactant and the photoproducts being in a
photostationary equilibrium. Under these conditions, the matrix environment was found to hamper subsequent reactions of
Z
-
retro
-
γ
-ionone and
Z
-
β
-ionone, so that this last species could be observed directly for the
fi
rst time. Furthermore, the formation of
Z
-
retro
-
γ
-ionone was shown to occur directly via an intramolecular [1,5] H-atom shift and thereby, under the constraints imposed
by the matrix con
fi
nement, the conformations assumed by this photoproduct were found to be strictly determined by those
initially assumed by the reactant molecules. Broadband irradiation resulted in the completion of the reaction (disappearance of
the reactant) and the sole observation of
Z
-
retro
-
γ
-ionone. These results imply that under these conditions the
Z
-
β
-ionone is
unstable, very likely decaying to additional conformers of
Z
-
retro
-
γ
-ionone, as re
fl
ected in the broader bands due to this
photoproduct observed in the infrared spectra of the broadband irradiated matrix.