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NMR Spectroscopic and quantum chemical characterization of the ( E )− and ( Z )− isomers of the penta‐1,3‐dienyl‐2‐cation
Author(s) -
Siehl HansUllrich,
Müller Thomas,
Gauss Jürgen
Publication year - 2003
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.662
Subject(s) - chemistry , chemical shift , carbocation , ab initio , computational chemistry , nmr spectra database , carbon 13 nmr , resonance (particle physics) , nuclear magnetic resonance spectroscopy , quantum chemical , atomic orbital , spectral line , stereochemistry , molecule , organic chemistry , electron , atomic physics , physics , quantum mechanics , astronomy
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 −135 °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. Copyright © 2003 John Wiley & Sons, Ltd.