High-resolution proton magnetic resonance spectra of polynuclear heterocycles. Part III. Electronic spectra and high-resolution proton magnetic resonance of acenaphtho[1,2-b]thiophen and acenaphtho[1,2-c]thiophen
Electronic spectra of ethanol solutions of acenaphtho[1,2-b]thiophen (II) and acenaphtho[1,2-c]thiophen (III) are reported and assigned by analogy with the spectrum of fluoranthene (I). From 1H n.m.r. spectra of solutions of (II) and (III) in carbon tetrachloride and [2H]chloroform measured at 60, 100, and 220 MHz, iterative analyses of the ABC spin system of (III) and the overlapping ABC systems of (II) have been made. Chemical shifts were assigned with the aid of HMO calculations of ortho bond orders, pij. Spectra have been correlated with HMO calculations of energy differences, mn–mp, charge densities, ρi, and bond orders, pij; there is a good agreement with pij values derived from coupling constants. Shieldings relative to (I), based on HMO-calculated charge densities, enable Δχ and ring-current effects to be estimated in the isosteres (II) and (III). Currents in the thiophenoid rings (D) of (II) and (III) are smaller than that in the benzenoid ring of (I), and the current in the central rings (C) appears to be negligible.