The analysis of the high-resolution 1H NMR spectrum of indene

Abstract

The long-range coupling constants in unsaturated systems are still a subject of interest because of their sensitivity to conformational and other structural changes (1). Proton magnetic resonance studies in this field, however, usually do not enter into a complete analysis of the spectra avoiding the tedious work necessary to measure extremely high-resolved spectra and to analyze very complex spin systems. At this time even the spectra of simple unsaturated or aromatic compounds have seldom been analyzed if the resulting spin system involves more than six protons. We started our studies of complex proton magnetic resonance spectra with an analysis of the ‘H NMR spectrum of styrene (2). In this note we report the complete analysis of the A&MWXYZ eight-spin system of indene, which would appear to be one of several compounds related to styrene by a fixed orientation of the vinyl and phenyl moieties. Indene has been the subject of several proton magnetic resonance studies. Elleman and Manatt (3) observed a long-range coupling between one of the aromatic ring protons and those of the five-membered ring, which could be identified as the coupling between the protons 3 and 7 (4). Douris and Mathieu (5) tried to explain the ‘H NMR spectrum as a superposition of an ABCD and an AKXz spectrum. They could detect some long-range couplings, but they could not determine the magnitudes or signs. The ABCD spin system of the aromatic protons of 1,1,3-d,indene has been analyzed by Luzikow et al. (6), who were able to assign the protons by means of proton selectively decoupled 13C NMR spectra. The FT NMR spectrum of distilled indene in a 2 M solution in acetone-d6/TMS (98:2) was measured at 25°C using a Bruker WH-270 spectrometer. The solution was thoroughly degassed by several freeze-pump-thaw cycles and sealed under vacuum in a 5-mm sample tube. In order to get a high digital resolution, the spectrum was recorded in three separate parts: the aromatic protons were observed with a spectral width of 400 Hz, the olefinic protons with a spectral width of 130 Hz (filter width: 140 Hz), and the methylene protons with a spectral width of 80 Hz (filter width: 100 Hz) resulting in digital resolutions of 0.024, 0.008, and 0.010 Hz/Pt, respectively. The best linewidth was 0.05 Hz. The spectral analysis was performed with the program PANIC (7) on a Bruker Aspect 2000 computer. In