Characterization of benzazolo[3,2- α]quinolinium salts (BQS) using NMR techniques and applying novel NMR methods: Band-Selective and Pure Shift pulse sequences. Long-range proton-proton coupling studies.

Abstract

The proton (¹H) NMR spectra of the benzazolo[3,2-α]quinolinium salts (BQS, Figure 1) show similar coupling patterns in the corresponding proton (¹H) signals, and the proton-proton (¹H-¹H) vicinal (scalar) coupling constants values are close. In addition, the overlapping of some signals, and the presence of strong second order coupling effects, makes the chemical shifts (δ) assignments of the signals not straightforward. With respect to the carbon-13 (¹³C) spectra, very close signals are also observed, where, conventional two-dimensional (2-D) heteronuclear correlation experiments are not enough to help in the signal assignments process. The use of chemical shifts (δ) prediction software showed that the quality of the results is far from the expected for these cases and did not help in the NMR assignments. Figure 1. Benzazolo[3,2-α]quinolinium salt structure.<br /> <br /> That is why, the application of novel one- (1-D) and two-dimensional (2-D) NMR techniques was essential to be applied in this work. To attack the problem, we recurred to the applications, which are known as pure shift and band-selective constant-time (CT) approaches.<br /> <br /> In the calculation of the proton-proton (¹H-¹H) coupling constants, the determination of the value for some of the vicinal (³<em>J</em>) and long-range (ⁿ<em>J</em>, where n ≥ 4) coupling constants, which provides useful structural information, was not an easy task due to problems of line shape, spectral resolution or the implementation of new techniques for that purpose. Taking this into account, the task carried out in this area was to detect and assign all the proton-proton (¹H-¹H) vicinal and long-range couplings. The determination of experimental values of the long-range coupling that were not directly measurable was left for a future work.