title: Close relation between quantum interference in molecular conductance and diradical existence.
abstract: An empirical observation of a relationship between a striking feature of electronic transmission through a π-system, destructive quantum interference (QI), on one hand, and the stability of diradicals on the other, leads to the proof of a general theorem that relates the two. Subject to a number of simplifying assumptions, in a π-electron system, QI occurs when electrodes are attached to those positions of an N-carbon atom N-electron closed-shell hydrocarbon where the matrix elements of the Green's function vanish. These zeros come in two types, which are called easy and hard. Suppose an N+2 atom, N+2 electron hydrocarbon is formed by substituting 2 CH2 groups at two atoms, where the electrodes were. Then, if a QI feature is associated with electrode attachment to the two atoms of the original N atom system, the resulting augmented N+2 molecule will be a diradical. If there is no QI feature, i.e., transmission of current is normal if electrodes are attached to the two atoms, the resulting hydrocarbon will not be a diradical but will have a classical closed-shell electronic structure. Moreover, where a diradical exists, the easy zero is associated with a nondisjoint diradical, and the hard zero is associated with a disjoint one. A related theorem is proven for deletion of two sites from a hydrocarbon.
authors.affiliation: Department of Chemistry and Chemical Biology, Cornell University, Ithaca, NY 14853;
authors.forename: Yuta
authors.initials: Y
authors.lastname: Tsuji
authors.affiliation: Department of Chemistry and Chemical Biology, Cornell University, Ithaca, NY 14853; rh34@cornell.edu.
authors.forename: Roald
authors.initials: R
authors.lastname: Hoffmann
authors.affiliation: Nano-Science Center and Department of Chemistry, University of Copenhagen, 2100 Copenhagen Ø, Denmark.
authors.forename: Mikkel
authors.initials: M
authors.lastname: Strange
authors.affiliation: Nano-Science Center and Department of Chemistry, University of Copenhagen, 2100 Copenhagen Ø, Denmark.
authors.forename: Gemma C
authors.initials: GC
authors.lastname: Solomon
keywords: determinants; diradicals; molecular conductance; nonbonding orbitals; quantum interference