A photoionization study of the ion-neutral complexes CH3CH (+)CH 3 (·) CH 2CH 3] and CH 3CH 2CH (+)CH 3 (·) CH 3 in the gas phase: Formation, H-transfer and C-C bond formation between partners, and channeling of energy into dissociation.

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.