MP2, CCSD(T), and Density Functional Theory Study of the 2-Butyl Cation: New Insight into the Methyl- and Hydrogen-Bridged Structures

Abstract

Using the MP2, CCSD(T), and DFT (B3LYP) methods, the structures and energies of the 2-butyl cation (C4H9(+)) were calculated. Energetically, the C-C hyperconjugated structure 1 and hydrogen-bridged structure 2 were found to be almost identical at all levels. The (13)C NMR chemical shifts of 1 and 2 were computed by the GIAO-CCSD(T) method using different geometries. On the basis of calculated relative energies and calculated (13)C NMR chemical shifts, an equilibrium involving 1 and 2 (in a 50:50 ratio) seemed likely responsible for the experimentally observed (13)C NMR chemical shifts in superacid solutions at -80 °C. However, on the basis of computed and experimental frequencies the hydrogen-bridged structure 2 is most likely responsible for the experimentally observed frequencies in the solid state at -125 °C.

Competition of Secondary versus Tertiary Carbenium Routes for the Type B Isomerization of Alkenes over Acid Zeolites Quantified by Ab Initio Molecular Dynamics Simulations

“…The second part of the profile, ranging from ξ = 2.76 to ξ = 2.16 (the secondary carbenium ion), exhibits significant variations with T , which can be explained by the relative entropic stabilization of free forms (carbenium ions), with respect to the bound forms (π-complexes). A similar conclusion was reported by Cnudde and co-workers when comparing the π-complexes, alkoxides, and carbenium ions formed by pentene in H-ZSM-5. − The stabilization of the cationic form has a significant impact on the value of the term Δ A ξ ref,R →ξ* constituting a major contribution to the free energy of activation (see eq ): its value decreases from 76.6 ± 5.4 kJ/mol to 55.7 ± 6.4 kJ/mol as T increases from 300 K to 500 K. We note that the secondary cation has a classical structure (see Figure , as well as Section SVIII.2 in the Supporting Information) in agreement with the fact that nonclassical hydrido-bridged structures , are only found at very low temperatures …”

“…A similar conclusion was reported by Cnudde and co-workers when comparing the π-complexes, alkoxides, and carbenium ions formed by pentene in H-ZSM-5. 26−28 The stabilization of the cationic form has a significant impact on the value of the term ΔA ξ ref,R →ξ* constituting a major contribution to the free energy of activation (see eq 1): its value decreases from 76.6 ± 5.4 kJ/ mol to 55.7 ± 6.4 kJ/mol as T increases from 300 K to 500 K. We note that the secondary cation has a classical structure (see Figure 1, as well as Section SVIII.2 in the Supporting Information) in agreement with the fact that nonclassical hydrido-bridged structures 63,64 are only found at very low temperatures. 65 The third part of the profile corresponds to the monomolecular transformation between the secondary (ξ = 2.16) and tertiary (ξ = 0.66) cations by the type B isomerization.…”

A comprehensive benchmark investigation of quantum chemical methods for carbocations

“…Transition state structures have also been added. Several carbocations in the list are classics and have been subjected to previous computational studies, for instance: ethyl cations ( 1A–1B ), 70 2-propyl cations ( 2A ), 71 2-butyl cations ( 3B–3E ), 72 and cyclobutylmethyl cations ( 5F , 5G ). 73 Carbocations have been divided into five groups of isomers for energy comparison, covering up to a ca.…”

Computational1H and13C NMR in structural and stereochemical studies

“…A number of papers appeared mainly from Olah's group [ 286–293 ] together with those by Shubin, et al, [ 294–301 ] deal with the calculation of 13 C NMR chemical shifts of carbocations and charged heterosystems. Herewith, we will only give the most illustrative examples and will not discuss the aspects of computational NMR of charged systems in full, since this and related topics are thoroughly reviewed elsewhere.…”