NMR Spectroscopic and quantum chemical characterization of the (E )− and (Z )− isomers of the penta‐1,3‐dienyl‐2‐cation

Abstract

Dilute solutions of the (E )-and (Z )-isomers of pent-1,3-dienyl-2-cations (1) were obtained from reaction of 4-chloro-1,2-pentadiene (2) with SbF 5 in SO 2 ClF/SO 2 F 2 at À135°C using high-vacuum co-condensation techniques. The experimental NMR spectra of the mixture of the two isomers were compared with quantum chemical 13 C NMR chemical shift calculations at HF-SCF, MP2, CCSD and CCSD(T) levels using MP2/tzp geometries. Quantum chemical shift calculations were performed with a tzp basis (9s5p1d/5s3p1d) for carbon and a dz basis (4s/2s) for hydrogen using gauge-including atomic orbitals (GIAOs). The HF-SCF calculations deviate significantly for the positively charged carbon atoms of the allyl-type resonance system showing up to 40 ppm too deshielded values compared with the experimentally observed chemical shifts. The HF-SCF approach is therefore not sufficient in predicting satisfactorily the shielding tensors in this type of carbocation. Inclusion of electron correlation, however, allows an unequivocal assignment of the spectra of the Z-and E-isomers. The mean deviation between experimental and calculated NMR chemical shifts at the CCSD(T) level is 1.8 and 2.0 ppm for (Z)-and (E)-1, respectively. The dienyl cations (E/Z)-1 are the smallest vinyl cations ever generated as persistent species in superacidic solutions and observed by 13 C NMR spectroscopy. These carbocations were structurally fully characterized by advanced ab initio quantum chemical calculations of structure and NMR chemical shifts.

Benchmarking quantum mechanical calculations with experimental NMR chemical shifts of 2-HADNT

“…Accurate prediction of chemical shifts can be achieved through calculations using the coupled-cluster singles and doubles (CCSD) model augmented by perturbative correction for triple excitation (CCSD(T)) method [9][10][11], with a deviation of 1 ppm between the experimental chemical shifts and the calculated results for C-13. The accuracy can be further improved by the inclusion of zero-point vibrational correction in the Hartree-Fock (HF), MP2, CCSD, and CCSD(T) calculations of the chemical shifts except for the DFT calculations [11].…”

Computational Prediction of 1H and 13C NMR Chemical Shifts for Protonated Alkylpyrroles: Electron Correlation and Not Solvation is the Salvation

“…However, common to the large benchmark data sets that have been used in these studies is that cations are almost absent. In addition, results from the studies dedicated to organic cations have shown that accurate predictions of the chemical shifts pose a special challenge to the theoretical methods, sometimes demanding high‐level ab initio methods such as coupled cluster theory –,. In this report we present the results of a computational study of the chemical shifts or nuclear magnetic shielding constants for three protonated alkylpyrroles.…”

Ab initio calculations of NMR chemical shifts

“…The noniterative CCSD͑T͒ approach is found to perform better, relative to CCSDT, than the iterative CCSDT-n ͑n =1-3͒ approaches. 55 The HF-SCF calculations are found to be inadequate and inclusion of electron correlation is required to assign correctly the resonances of the two isomers. 53 Their results indicate that calculation of shielding polarizabilities is even more challenging than the calculation of shielding constants, with the second-order polarizabilities being especially affected by basis set and electron correlation effects.…”