Structural Analysis of Hepta-, Nona-, and Undeca-Cyclic Aromatic Hydrocarbons by NMR Spectroscopy

Abstract

Large condensed polycyclic aromatic hydrocarbons (LCPAHs) generally have distorted structures due to steric repulsion between neighboring hydrogen atoms. The distortion may influence their physicochemical properties such as electric conductivity, photoconductivity, and photochromism. Accordingly we have synthesized LCPAHs and analyzed their structure. In the present study, we synthesized an undecacyclic aromatic compound, dibenzo[a,rst]-dinaphtho[2,1,8-klm:1′,2′-o]pentaphene (DDPP) and determined the structure by use of NMR spectroscopy. To assign the ¹H- and ¹³C-NMR chemical shifts completely, we used the HMQC-TOCSY (Heteronuclear Multiple Quantum Coherence-Total Correlation SpectroscopY) method in which the original proton-carbon correlation is relayed to neighboring protons in the same spin-system. Furthermore, we compared the chemical shifts (δs) of the heptacyclic aromatic hydrocarbon, peropyrene, the nonacyclic, violanthrene B (VEB), and the undecacyclic, DDPP, and investigated correlations between their local structure and δs. For the protons in the bay region, the averaged δs decrease as molecular size increases: 9.23 ppm (hepta-) > 9.07 ppm (nona-) > 8.77 ppm (undeca-). Similar change in the δs is also found for the protons in the cove region: 9.37 ppm (nona-) > 8.81 ppm (undeca-). Thus, it is noted that the averaged δs for the protons in the cove region become downfield than those for the protons in the bay region. The findings are explained in terms of the anisotropic effect of the local magnetic field of benzene rings.

Keywords:

• condensation
• molecular orbital calculation
• nuclear magnetic resonance (NMR) spectra
• polycyclic aromatic hydrocarbons