Determination of the structure of 1,1′-diethyl-2,2′-carbocyanine iodide using NMR spectra and GIAO-HF/DFT calculations

The chemical shift assignments of pinacyanol (1,1-diethyl-2,2′-carbocyanine) iodide were obtained by means of 1-D (1H and 13C) and 2-D (1H–1H COSY, 13C–1H COSY, and HMBC) NMR and theoretical calculations (GIAO-HF and GIAO-DFT using the 6-31+G(d,p) basis set). An optimized structure, calculated using B3LYP/6-31G(d), of the stable conformer gave a twist angle of γ = 23.4° between the pairs of the quinoline planes, which was 0.4 kcal/mol lower than when γ = 0°. The (2E,12E) conformer having C2 symmetry, matched well with the calculated and experimental 13C and 1H NMR chemical shifts in DMSO-d6; GIAO-DFT provided better correlation (R2 = 0.997, 13C; R2 = 0.978, 1H) than GIAO-HF (R2 = 0.968, 13C; R2 = 0.949, 1H).