MOLECULAR IMAGING OF PETROLEUM ASPHALTENES BY SCANNING-TUNNELING-MICROSCOPY - VERIFICATION OF STRUCTURE FROM C-13 AND PROTON NUCLEAR-MAGNETIC-RESONANCE DATA

Scanning tunneling microscopy (STM) was used to verify the molecular s tructure of Maya asphaltene which had been derived from combined C-13 and proton nuclear magnetic resonance (NMR) experiments. Petroleum asp haltenes are known to contain large polynuclear aromatic centers with aliphatic sidechains. Average molecular models of Maya asphaltenes wer e derived using studies which included combined proton and C-13 NMR da ta to determine total aromatic carbon content and the ratio of periphe ral to internal aromatic ring carbons. These parameters permitted esti mating the average number of aromatic rings per condensed cluster. The se Maya asphaltenes were imaged by scanning tunneling microscopy (STM) in a dilute solution of tetrahydrofuran on highly oriented pyrolytic graphite. The sizes and structures of the asphaltenes as observed by S TM are in reasonable agreement with these average molecular models. We observed asymmetric structures whose largest dimension averaged 10.4 Angstrom +/- 1.9 Angstrom from 24 separate images. The condensed ring portions of three representative NMR derived molecular models yielded an average dimension of 11.1 Angstrom +/- 1.4 Angstrom.