## Abstract

Ab initio density functional theory (DFT) calculations are reported for the chromium sandwich structure Cr_{n}R_{2}, where n = 7 and R is the aromatic hydrocarbon hexabenzocoronene (C_{42}H_{18}). This system is remarkable in that the structure of the chromium sites strongly resemble those in chromium bis-benzene C_{r1}(C_{6}H _{6})_{2}, as judged by geometry and charge density properties. The electron localization function of the sandwich shows a hexagonally arrayed set of V(C, Cr, C) valence basins about each chromium atom with modification due to local site symmetry. This system satisfies an extension of the 18-electron rule to components of a conjugated molecular system. This idea is explored further by examining the electronic and geometric properties of the series Cr_{n},R_{2}, where n and R are given by n = 1, benzene C _{6}H_{6} as reference; n = 2, biphenyl (C_{6}H _{5})_{2}; n = 3, triphenylene C_{18}H_{12}; n = 3, coronene C_{24}H_{18}; and n = 4, dibenzopyrene C _{24}H_{14}. On the basis of electron counting and ring isolation, all the sandwich structures in this series could satisfy the extension of the 18-electron rule, with the exception of coronene, which was deliberately included. The DFT calculations predict spin-paired ground states for some but not all of the sandwich structures, implying that the Cr-ring interactions at work require understanding at a deeper level. Thus, while sandwiches with n = 1, n = 2, n = 4 and n = 7 have spin paired singlet ground states and appear to satisfy the rule, those with n = 3 (triphenylene, coronene) have antiferromagnetic singlet ground states and do not. This is attributed to nonuniformity in the electronic charge density of the rings of the isolated hydrocarbons and to a reduction of symmetry from D_{3h} to C _{2v} with a concomitant spin-charge density change in the sandwiches.

Original language | English |
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Pages (from-to) | 2034-2042 |

Number of pages | 9 |

Journal | Journal of Physical Chemistry A |

Volume | 112 |

Issue number | 10 |

DOIs | |

Publication status | Published - 2008 Mar 13 |