Secondary limonene endo-ozonide: A major product from gas-phase ozonolysis of RR-(++)-limonene at ambient temperature

A 16 s old gas-phase ambient temperature and 1% relative humidity reaction mixture of ozone and R  -limonene (ca. 1:10) was sampled on XAD-2 resin followed by pressurized liquid extraction with dichloromethane at ambient temperature. Low temperature on-column injection and gas chromatography (GC) revealed equal amounts of diastereomeric secondary endo-limonene ozonides, in addition to 4-acetyl-1-methyl-cyclohexene (AMCH), 3-isopropyl-6-oxo-heptanal (IPOH), and endo-limonene mono-epoxides. The secondary endo-limonene ozonides began to decrease at extraction temperature above 150∘C and were absent at 200∘C. Their formation was unaffected by an increase of the relative humidity to 15%. The identification of the secondary limonene ozonides was confirmed by (1) unique consecutive losses of OH and H2O2H2O2, respectively, from the protonated quasi-molecular ion in GC-chemical ionization mass spectrometry mode (isobutane), in addition to high resolution mass determination of [M-OH] and [M-H2O2H2O2] ions in EI mode; (2) comparison of mass spectral data to that of synthesized secondary endo-limonene ozonides; and (3) oxidation of dimethyl sulfide to dimethyl sulfoxide and subsequent increase of IPOH.