Quantum Chemical Calculations on α-Substituted Ethyl Cations:  A Comparison between B3LYP and Post-HF Methods

Abstract

Alkyl cations of the form CH3C(+)HR have been investigated using high-level quantum chemical methods to study the influence of α-substituents R (R = H, CH3, CHCH2, C⋮CH, F, and Cl) on cation geometries and relative energies with respect to the neutral precursors CH3CH2R. The results of density-functional B3LYP computations with a variety of basis sets were compared with MP2, MP4, QCISD(T), and CBS-Q model chemistry results and benchmarked against experimental data. The results show that geometrical features are already accurately described using B3LYP/6-311G(d,p) or MP2/6-311G(d,p). For a systematic study of the energetics of α-substitution on alkyl cations, B3LYP/6-311+G(d,p) and MP2/6-311+G(d,p) computations form a useful compromise between accuracy (average deviation within 1 kcal/mol of the experimental error) and computational efficiency. The electronic structures of these species and their precursors CH3CH2R were studied using both natural bond orbital (NBO) and Atoms-in-molecules (AIM) analyses. These analyses clearly show that the electron-donating power of α-substituents at (partially) positively charged carbon atoms does not correlate well with the thermochemical stabilization of cations provided by such substituents.

RETRACTED: Aqueous Room Temperature Mono-Dehydration of Sugar Alcohols Using Functionalized Yttrium Oxide Nanocatalysts

“…The calculations were carried out over Gaussian 09 package (Frisch et al, 2013). Full geometry optimizations were performed by using the HF method, and RB3LYP functional was selected because it was suitable for discussing the reaction profile (van Alem et al, 1998).…”

Hyperconjugation in Carbocations, a BLW Study with DFT approximation

“…Because it involves bond breaking/forming, the bridged cation (1c) needs a priori a higher level of computation than the hyperconjugated system (1a) (van Alem et al, 1998). However, all the correlated levels converged to similar energy differences, within a few kcal/mol.…”

Uncommon hydrogen bonds between a non-classical ethyl cation and π hydrocarbons: a preliminary study

“…Interestingly, a large number of theoretical studies of the ethyl cation and its hyperconjugation effect have been reported, including the classic work done by Pople [40][41][42] and others [43][44][45]. However, Zuilhof et al [46] and Carneiro et al [47] analyzed the molecular geometry of ethyl cation using the MP2(FULL)/6-31G(d,p) and B3LYP/6-311G(d,p) calculations. At these two levels of theory, the structure of the ethyl cation was characterized by a C 2v point group, although two distinct results for the distance between C1 and C2 carbons (see Fig.…”

“…These RC1-C2 distances lie midway between the experimental data of 1.34 and 1.53 Å for ethene (2a) and ethane (2c) [48], respectively. On the basis of these results [49], we have assumed that a C1-C2 bond length of 1.38 Å is an appropriate parameter for evaluating the hyperconjugation effect of the ethyl cation [46][47][48][49].…”