Generation of C 7 H 7 + cations with isomerization reactions Kiew Shaphrang Kharnaior, Martina Devi, R.H. Duncan Lyngdoh ⇑ Department of Chemistry, North-Eastern Hill University, Shillong 793022, India article info Article history: Received 8 June 2016 Received in revised form 15 July 2016 Accepted 16 July 2016 Available online 18 July 2016 Keywords: C 7 H 7 + cation generation C 7 H 7 + cation isomerisation Hammond postulate B3LYP MP2 abstract The B3LYP and MP2 methods are used to study isomerization reactions of 15 C 7 H 7 + cations in gas phase, along with an MP2 study of the generation of 34 C 7 H 7 + isomers by ionic dissociation of their C 7 H 7 Br precursors. Ionic dissociation is endothermic, where dissociation facility does not correlate well with car- bocation stability, though various physicochemical factors facilitating dissociation facility correlate fairly well among themselves. The 15 C 7 H 7 + cations studied for isomerization include the benzyl, o-tolyl, m-tolyl, p-tolyl and 7-norbornadienyl cations, besides hypothetical higher energy species. 16 select concerted iso- merizations are considered, of which all but one proceed via well-defined transition states. 1,2 and 1,3 hydride shifts involve larger activation barriers than 1,2 CAC sigma bond shifts. By combinations among themselves, these one-step reactions lead to 7 multi-step schemes for conversion of various C 7 H 7 + cations to the most stable tropyl cation isomer. Some conversions involve alternative isomerization routes, of which the energetically more feasible one is identified. Transition states located are described with regard to their relative positions along the reaction coordinate in accordance with the Hammond postu- late linking transition state geometry to reaction energetics. Ó 2016 Elsevier B.V. All rights reserved. 1. Introduction The C n H n + series of carbocations present an interesting field of study on account of the unique stoichiometry. The isomers and rearrangement reactions for various C n H n + families with n = 3, 4, 5, 6 and 7 were recently studied using ab initio and DFT methods [1–3]. This computational study focuses on the family having n = 7. The C 7 H 7 + family has undergone much experimental study, especially in the context of mass spectroscopy, and also been sub- ject to computational investigation. The aspects of stability, geom- etry and charge distribution in a large number of C 7 H 7 + isomers were treated computationally in a previous study [3]. Other aspects of C 7 H 7 + chemistry like generation, rearrangement, dissoci- ation and reactivity have received attention from experimental and computational chemists. This work deals firstly with the genera- tion of numerous C 7 H 7 + isomers from the corresponding C 7 H 7 Br precursors in the gas phase, and then goes on to study reaction pathways for select one-step rearrangements involving some C 7 H 7 + cations. These concerted reaction steps are then combined among themselves to yield multi-step isomerizations of various C 7 H 7 + species to the tropyl cation isomer. The tropyl cation is the least energy isomer (global minimum) due to its aromaticity and lack of strain, being ranked as the lowest energy C 7 H 7 + isomer by numerous experimental and computational studies. In their ground states, all these C 7 H 7 + isomers are expected to be singlet closed shell systems. C 7 H 7 + species may in principle be formed from neutral C 7 H 7 X species (X = H, halide or other leaving group) by simple loss of the X – group. Such reactions in superacid medium may generate some of the lower energy C 7 H 7 + cations. Under the conditions of mass spectrometry, toluene and alkylbenzenes can ionize and then yield C 7 H 7 + species by loss of small groups like H atoms and CH 3 , NO 2 ,C 2 H 2 ,C 2 H 4 groups etc. [4–10]. These cations can be detected and can also undergo isomerization and further fragmentation. Known C 7 H 7 + species include the benzyl, tropyl, o-tolyl, m-tolyl, p-tolyl and 7-norbornadienyl cations [4,6–8,10–12]. Halotoluenes also undergo similar reactions [12,13]. Benzyl, m-tolyl and p-tolyl cations were produced from ionized benzyl bromide and m- and p-nitrotoluene [12]. The benzyl and tropyl cations predominate in another such product mixture [4]. Interconversion between iso- mers has also been an interesting topic of study, notably, between the benzyl and tropyl cations [5–7,9,14]. The C 7 H 7 + cation system is expected to be very reactive in gen- eral, but till date only limited experimental work has surfaced on the details of chemical reactivity of the various isomers of this system, notably the more well-known and lower energy isomers, viz., the tropyl, benzyl, o-, p- and m-tolyl cations [4,15–19]. The structures and isomerizations of C 7 H 7 + ions formed by fragmenta- tion of ethylbenzene, toluene and norbornadiene were studied by http://dx.doi.org/10.1016/j.comptc.2016.07.018 2210-271X/Ó 2016 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: rhdl@nehu.ac.in (R.H. Duncan Lyngdoh). Computational and Theoretical Chemistry 1091 (2016) 150–164 Contents lists available at ScienceDirect Computational and Theoretical Chemistry journal homepage: www.elsevier.com/locate/comptc