title: Determining the Conformational Landscape of σ and π Coupling Using para-Phenylene and "Aviram-Ratner" Bridges.
abstract: The torsional dependence of donor-bridge-acceptor (D-B-A) electronic coupling matrix elements (H(DA), determined from the magnetic exchange coupling, J) involving a spin SD = 1/2 metal semiquinone (Zn-SQ) donor and a spin S(A) = 1/2 nitronylnitroxide (NN) acceptor mediated by the σ/π-systems of para-phenylene and methyl-substituted para-phenylene bridges and by the σ-system of a bicyclo[2.2.2]octane (BCO) bridge are presented and discussed. The positions of methyl group(s) on the phenylene bridge allow for an experimentally determined evaluation of conformationally dependent (π) and conformationally independent (σ) contributions to the electronic and magnetic exchange couplings in these D-B-A biradicals at parity of D and A. The trend in the experimental magnetic exchange couplings are well described by CASSCF calculations. The torsional dependence of the pairwise exchange interactions are further illuminated in three-dimensional, "Ramachandran-type" plots that relate D-B and B-A torsions to both electronic and exchange couplings. Analysis of the magnetic data shows large variations in magnetic exchange (J ≈ 1-175 cm(-1)) and electronic coupling (H(DA) ≈ 450-6000 cm(-1)) as a function of bridge conformation relative to the donor and acceptor. This has allowed for an experimental determination of both the σ- and π-orbital contributions to the exchange and electronic couplings.
authors.affiliation: †Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, United States.
authors.forename: Daniel E
authors.initials: DE
authors.lastname: Stasiw
authors.affiliation: †Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, United States.
authors.forename: Jinyuan
authors.initials: J
authors.lastname: Zhang
authors.affiliation: †Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, United States.
authors.forename: Guangbin
authors.initials: G
authors.lastname: Wang
authors.affiliation: ‡Department of Chemistry, The University of New Mexico, MSC03 2060, 1 University of New Mexico, Albuquerque, New Mexico 87131-0001, United States.
authors.forename: Ranjana
authors.initials: R
authors.lastname: Dangi
authors.affiliation: ‡Department of Chemistry, The University of New Mexico, MSC03 2060, 1 University of New Mexico, Albuquerque, New Mexico 87131-0001, United States.
authors.forename: Benjamin W
authors.initials: BW
authors.lastname: Stein
authors.affiliation: †Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, United States.
authors.forename: David A
authors.initials: DA
authors.lastname: Shultz
authors.affiliation: ‡Department of Chemistry, The University of New Mexico, MSC03 2060, 1 University of New Mexico, Albuquerque, New Mexico 87131-0001, United States.
authors.forename: Martin L
authors.initials: ML
authors.lastname: Kirk
authors.affiliation: §Department of Chemistry, University of South Florida, 4202 East Fowler Avenue, CHE 205, Tampa, Florida 33620-5250, United States.
authors.forename: Lukasz
authors.initials: L
authors.lastname: Wojtas
authors.affiliation: †Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, United States.
authors.forename: Roger D
authors.initials: RD
authors.lastname: Sommer