The n-butonium cation (n-C4H11+): The potential energy surface of protonated n-butane

The structure and energetics of the n-butonium ion, the protonated form of n-butane, were computed at the MP4SDTQ(fc)/6-311++G**//MP2(full)/6-31G** le vel. Eleven stable structures were found for the n-butonium ion, following the stability order 2-C-n-butonium > 1-C-n-butonium > 2-H-n-butonium > 1-H- n-butonium. The transition states for intramolecular bond-to-bond rearrange ment and for decomposition of the carbonium ions into the van der Waals com plexes were also calculated. The H-n-butonium and the 1-C-n-butonium ions a re higher in energy than the van der Waals complexes 13, 14, and 15. The va n der Waals complexes between the isopropyl cation plus CH4 and the tert-bu tyl cation plus H-2 are the most stable C4H11+ species. It was concluded th at the 1-H-n-butonium ion prefers to undergo intramolecular rearrangement t o the 1-C-n-butonium ion, whereas the 2-H-n-butonium ion prefers to decompo se into the van der Waals complex of the sec-butyl cation plus H-2. The cal culated proton affinity of n-butane (156.7 kcal/mol) agrees well with the e xperimental value of 153.7 kcal/mol. The C4H11+ (b) species, formed upon th e gas-phase reaction between C2H5+ and ethane, was confirmed to be the 2-C- n-butonium cation, and the C4H11+ (a) species was confirmed to be the 2-H-n -butonium cation, as proposed by Hiraoka and Kebarle (Can. J. Chem. 1980, 5 8, 2262-2270). The experimental activation energy of 9.6 kcal/mol was compa red with the value of 12.8 kcal/mol, computed for the reaction 11 --> 5 thr ough the transition state 21.