A computational (DFT, MP2) and GIAO NMR study of substituent effects in benzenediazonium mono- and dications

Abstract

15N and 13C NMR chemical shifts were computed by GIAO-DFT and GIAO-MP2 for a series of p-substituted benzenediazonium mono- and dications in order to probe the electronic effects of the substituents on the diazonium moiety. Optimized geometries and N/N vibrational frequencies werealso considered for comparison. The GIAO-DFT derived15N chemical shifts correlate more closely with the experimental values as compared to GIAO-MP2. Energyminimizations at the B3LYP/6-311+G(2d,p), M062X-6-311+G(2d,p), MP2/6-311+G(2d,p), G2(MP2), and CBS-Q levels were carried out. Relative dication stability order HCO+ > HOMe+ > HN(Me) 2+ > HOH+ > HCN+ > HNO 2+ was derived from isodesmic proton transfer reactions. The Nβ-protonated dications were less stable than the corresponding p-R+ dications. Among the regioisomeric N β-protonated dications (with R = -F, -Cl, and -CN), those with the R group in the para position were preferred. For the regioisomeric, ring-protonated benzenium-diazonium dications, the meta-protonated dications were more favored (by DFT and MP2). Influence of the counterion and solvent on the computed 15N NMR chemical shifts in PhN2+ X- were also assessed. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.