Structure and formation of gaseous [C4H6O]+˙ ions: 1—The enolic ions [CH2C(OH)-CHCH2]+˙ and [CH2CH-CHCH(OH)]+˙ and their relationship with their keto counterparts

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Abstract
The [C4H6O]+˙ ion of structure [CH2CHCHCHOH]+˙ (a) is generated by loss of C4H8 from ionized 6,6‐dimethyl‐2‐cyclohexen‐1‐ol. The heat of formation ΔHf of [CH2CHCHCHOH]+˙ was estimated to be 736 kJ mol−1. The isomeric ion [CH2C(OH)CHCH2]+˙ (b) was shown to have ΔHf,  761 kJ mol−1, 54 kJ mol−1 less than that of its keto analogue [CH3COCHCH2]+˙. Ion [CH2C(OH)CHCH2]+˙ may be generated by loss of C2H4 from ionized hex‐1‐en‐3‐one or by loss of C4H8 from ionized 4,4‐dimethyl‐2‐cyclohexen‐1‐ol. The [C4H6O]+˙ ion generated by loss of C2H4 from ionized 2‐cyclohexen‐1‐ol was shown to consist of a mixture of the above enol ions by comparing the metastable ion and collisional activation mass spectra of [CH2CHCHCHOH]+˙ and [CH2C(OH)CHCH2]+˙ ions with that of the above daughter ion. It is further concluded that prior to their major fragmentations by loss of CH3˙ and CO, [CH2CHCHCHOH]+˙ and [CH2C(OH)CHCH2]+˙ do not rearrange to their keto counterparts. The metastable ion and collisional activation characteristics of the isomeric allenic [C4H6O]+˙ ion [CH2CCHCH2OH]+˙ are also reported.