The impact of the π-electron conjugation on 15N, 13C and 1H NMR chemical shifts in push–pull benzothiazolium salts. Experimental and theoretical study

✍ Scribed by Peter Hrobárik; Branislav Horváth; Ivica Sigmundová; Pavol Zahradník; Olga L. Malkina

• Publisher: John Wiley and Sons
• Year: 2007
• Tongue: English
• Weight: 319 KB
• Volume: 45
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.2074

✦ Synopsis

Abstract

The ^15^N as well as ^13^C and ^1^H chemical shifts of eight push–pull benzothiazolium iodides with various π‐conjugated chains between dimethylamino group and benzothiazolium moiety have been determined by NMR spectroscopy at the natural‐abundance level of all nuclei in DMSO‐d~6~ solution. In general, the quaternary benzothiazolium nitrogen is more shielded [δ(^15^N‐3) vary between − 241.3 and − 201.9 ppm] with respect to parent 3‐methylbenzothiazolium iodide [δ(^15^N‐3) = − 183.8 ppm], depending on the length and constitution of the π‐conjugated bridge. A larger variation in ^15^N chemical shifts is observed on dimethylamino nitrogen, which covers the range of − 323.3 to − 257.2 ppm. The effect of π‐conjugation degree has a less pronounced influence on ^13^C and ^1^H chemical shifts.

Experimental data are interpreted by means of density functional theory (DFT) calculations. Reasonable agreement between theoretical and experimental ^15^N NMR chemical shifts was found, particularly when performing calculations with hybrid exchange‐correlation functionals. A better accord with experiment is achieved by utilizing a polarizable continuum model (PCM) along with an explicit treatment of hydrogen‐bonding between the solute and the water present in dimethylsulfoxide (DMSO). Finally, ^13^C and ^1^H NMR spectra were computed and analysed in order to compare them with available experimental data. Copyright © 2007 John Wiley & Sons, Ltd.