Since it was first reported by Olah et al. in 1963, [1] the long-sought tert-butyl cation has been extensively studied both experimentally and theoretically. [2][3][4][5][6][7][8][9][10][11] Of the possible structures of the tert-butyl cation, C 3v was first suggested in 1971 by Olah et al. [3] based on Raman and infrared spectroscopic studies. In another very important study (1989), Yannoni et al. [6] reported that the best fit for the central carbonmethyl carbon distance was between 1.46 and 1.47 , whereas the C-C + -C bond angle is 1208 according to nutation NMR spectroscopy. The X-ray studies (1993) on single crystals of tert-butyl-cation salts by Hollenstein and Laube [7] concluded in favor of a planar tetracarbon skeleton. The recent infrared studies carried out by Duncan and associates (2007) [10] deduced that only the C 1 and C s structures match the observed IR patterns.
In their pioneering paper in 1972, [4] Radom, Pople, and Schleyer reported the first ab initio study of the tert-butyl cation (and a great many other organic species). They considered both the C 3h and C 3v structures, with the latter lying about 1.4 kcal mol ร1 higher according to the STO-3G SCF method. In 1993, Sieber, Buzek, Schleyer, Koch, and Carneiro [8] computed three tert-butyl-cation stationary-point geometries, with C 3h , C 3v , and C s point groups, and reported that all three structures proved to be transition states according to the MP2/6-31G* method. At the higher MP4-
31G** level, they found the C 3h structure to be the lowest in energy after corrections for zero-point vibrational energy (ZPVE). Recently (2009), Rasul, Chen, Prakash, and Olah [11] used both DFT and second-order perturbation theory to optimize the three stationary points. At the MP2/6-31G** and B3LYP/cc-pVTZ levels, they found that two (or one) imaginary vibrational frequencies occurred for all three (C 3h , C 3v , and C s ) structures, although C s has the lowest energy after the ZPVE corrections. However, at the MP2/cc-pVTZ level, the C s struc-ture has no imaginary frequency, but the same energy as the C 3h structure.
The diversity of previous theoretical predictions suggests that further studies on this controversial system are needed. Herein we report a theoretical investigation of various structures of the tert-butyl cation, by using the reliable coupledcluster CCSD(T) methods in conjunction with large basis sets. All stationary-point structures have been fully optimized at each level of theory considered.
Structures optimized with the CCSD(T)/cc-pVQZ method are displayed in Figure 1, 2, and 3. The equilibrium structural parameters of the central carbon-methyl carbon distances r e A C H T U N G T R E N N U N G (C + ร C) are 1.4603 for the C 3h structure, 1.4632 for the C 3v structure, and 1.4584 plus 1.4607 for the C s structure. The distances between the methyl carbons r e A C H T U N G T R E N N U N G (C ร C) are 2.529 for C 3h , 2.533 for C 3v , and 2.524 plus 2.538 for C s , respectively. All three structures are in general agreement with the deductions from Yannonis NMR experi-[a] H. Feng,