Abstract
Photoionization mass spectrometry was used to investigate the dynamics of ion-neutral complex-mediated dissociations of the n-pentane ion (1). Reinterpretation of previous data demonstrates that a fraction of ions 1 isomerizes to the 2-methylbutane ion (2) through the complex CH3CH+CH ·3 CH2CH3 (3), but not through CH3CH+CH2CH ·3 CH3 (4). The appearance energy for C3Hin +7 formation from 1 is 66 kJ mol−1 below that expected for the formation of n-C3H +7 and just above that expected for formation of i-C3H +7 . This demonstrates that the H shift that isomerizes C3H +7 is synchronized with bond cleavage at the threshold for dissociation to that product. It is suggested that ions that contain n-alkyl chains generally dissociate directly to more stable rearranged carbenium ions. Ethane elimination from 3 is estimated to be about seven times more frequent than is C-C bond formation between the partners in that complex to form 2, which demonstrates a substantial preference in 3 for H abstraction over C-C bond formation. In 1 → CH3CH+CH2CH3 + CH3 by direct cleavage of the C1–C2 bond, the fragments part rapidly enough to prevent any reaction between them. However, 1 → 2 → 4 → C4H +8 + CH4 occurs in this same energy range. Thus some of the potential energy made available by the isomerization of n-C4H9 in 1 is specifically channeled into the coordinate for dissociation. In contrast, analogous formation of 3 by 1 → 3 is predominantly followed by reaction between the electrostatically bound partners.
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Traeger, J.C., Hudson, C.E. & McAdoo, D.J. A photoionization study of the ion-neutral complexes CH3CH +CH ·3 CH2CH3] and CH3CH2CH+CH ·3 CH3 in the gas phase: Formation, H-transfer and C—C bond formation between partners, and channeling of energy into dissociation. J Am Soc Mass Spectrom 7, 73–81 (1996). https://doi.org/10.1016/1044-0305(95)00591-9
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DOI: https://doi.org/10.1016/1044-0305(95)00591-9