Singlet鈥揟riplet and Triplet鈥揟riplet Transitions of Asphaltene PAHs by Molecular Orbital Calculations

详细信息    查看全文

• 作者：Yosadara Ruiz-Morales ; Oliver C. Mullins
• 刊名：Energy & Fuels
• 出版年：2013
• 出版时间：September 19, 2013
• 年：2013
• 卷：27
• 期：9
• 页码：5017-5028
• 全文大小：596K
• 年卷期：v.27,no.9(September 19, 2013)
• ISSN：1520-5029

文摘

Two important features of the molecular structure of asphaltenes remain unresolved; the size distribution of the asphaltene polycyclic aromatic hydrocarbons (PAHs) and the number of PAHs per asphaltene molecule. The relatively small molecular weight of asphaltenes restricts the PAH size; if there are several PAHs per asphaltene molecule, they must be rather small. Optical spectroscopy especially when coupled with molecular orbital (MO) calculations is an excellent probe of asphaltene PAH populations and thus asphaltene molecular architecture. Previously, singlet鈥搒inglet transitions for asphaltenes were analyzed using both experiment and MO theory. Here, we describe MO calculations performed to treat triplet鈥搕riplet transitions from the ground triplet state for 103 PAHs. Qualitative comparisons with corresponding triplet鈥搕riplet transition measurements for asphaltenes are discussed. In addition, spin-forbidden transitions between the singlet and the triplet states, corresponding to phosphorescence, are calculated and discussed in terms of the probability of intersystem crossing of PAHs in asphaltenes and crude oils. Conclusions obtained here are consistent with the corresponding study of singlet鈥搒inglet transitions and support the model of a single, relatively large PAH per asphaltene molecule as the predominant asphaltene molecular architecture: the island model. This is consistent with a most probable asphaltene PAH of seven fused aromatic rings (7FAR) with a width of four to ten fused aromatic rings (4FAR-10FAR). This molecular architecture is a central feature of the Yen鈥揗ullins model of asphaltene nanoscience.