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

Abstract The [C 4 H 6 O] +˙ ion of structure [CH 2 CHCHCHOH] +˙ ( a ) is generated by loss of C 4 H 8 from ionized 6,6‐dimethyl‐2‐cyclohexen‐1‐ol. The heat of formation Δ H f of [CH 2 CHCHCHOH] +˙ was estimated to be 736 kJ mol −1 . The isomeric ion [CH 2 C(OH)CHCH 2 ] +˙ ( b ) was shown to have Δ H f ,  761 kJ mol −1 , 54 kJ mol −1 less than that of its keto analogue [CH 3 COCHCH 2 ] +˙ . Ion [CH 2 C(OH)CHCH 2 ] +˙ may be generated by loss of C 2 H 4 from ionized hex‐1‐en‐3‐one or by loss of C 4 H 8 from ionized 4,4‐dimethyl‐2‐cyclohexen‐1‐ol. The [C 4 H 6 O] +˙ ion generated by loss of C 2 H 4 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 [CH 2 CHCHCHOH] +˙ and [CH 2 C(OH)CHCH 2 ] +˙ ions with that of the above daughter ion. It is further concluded that prior to their major fragmentations by loss of CH 3 ˙ and CO, [CH 2 CHCHCHOH] + ˙ and [CH 2 C(OH)CHCH 2 ] +˙ do not rearrange to their keto counterparts. The metastable ion and collisional activation characteristics of the isomeric allenic [C 4 H 6 O] +˙ ion [CH 2 CCHCH 2 OH] +˙ are also reported.