A selected ion flow tube study of the reactions of H 3O +, NO + and O 2[rad]+ with a series of sesquiterpenes

Abstract

Product ion distributions (PIDs) for the reactions of seven sesquiterpenes (aromadendrene, β-caryophyllene, α-cedrene, α-humulene, isolongifolene, longifolene and δ-neoclovene) with H 3O +, NO + and O 2 + reagent ions have been determined in a selected ion flow tube (SIFT) instrument operated at 1.4 mbar and 297 K. The H 3O +/sesquiterpene reactions mainly proceed by non-dissociative proton transfer (C 15H 25 +, m/ z 205), except for the reactions of H 3O + with β-caryophyllene and α-humulene which are characterized by more pronounced fragmentation. Two groups of fragment ions of which the components are separated by CH 2 units, are generally observed. The NO +/sesquiterpene reactions mainly result in the charge transfer product (C 15H 24 +, m/ z 204). The NO +/sesquiterpene charge transfer yield is generally larger than the protonated sesquiterpene yield for the corresponding H 3O +/sesquiterpene reactions. A small amount of association product (C 15H 24·NO +, m/ z 234) is found for the sesquiterpenes aromadendrene, β-caryophyllene, longifolene and δ-neoclovene. O 2 +/sesquiterpene reactions are found to proceed mainly by dissociative charge transfer, resulting in severe fragmentation. The collisional rate constants for these reactions were calculated using the parameterized theory of Su and Chesnavich, taking into account polarizabilities and electrical dipole moments of the sesquiterpenes as derived from B3LYP/aug-cc-pVDZ quantum chemical calculations. Experimental rate constants for the reactions of NO + and O 2 + with the sesquiterpenes were obtained relatively to the calculated H 3O +/sesquiterpene rate constants. Absolute H 3O +/sesquiterpene rate constants were obtained for β-caryophyllene, α-cedrene, α-humulene and longifolene. Large but systematic discrepancies between experimentally determined rate constants and calculated collision rate constants suggest the need for a re-evaluation of sesquiterpene saturation vapor pressure values. In view of possible detection of sesquiterpenes in moist samples, the influence of water vapor on the ion chemistry was investigated and the reaction rate constants of H 3O +·H 2O and NO +·H 2O with sesquiterpenes were obtained relative to those of bare H 3O + and NO +, respectively.