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Remarkable Differences in Spin Couplings for Various Self‐Paired Dimers of Ring‐Expansion‐Radicalized Uracil: A Basis for the Design of Magnetically Anisotropic Assemblies
Author(s) -
Zhao Peiwen,
Bu Yuxiang
Publication year - 2018
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.201701068
Subject(s) - crystallography , chemistry , antiferromagnetism , diradical , dimer , spin (aerodynamics) , coupling (piping) , ferromagnetism , molecular physics , condensed matter physics , materials science , atomic physics , singlet state , physics , excited state , organic chemistry , metallurgy , thermodynamics
The spin‐coupling properties of a series of radicalized uracil (rU) dimer diradicals with different H‐bonding modes is examined. Each rU has four double H‐bonding sites [the amide units: two at the Watson–Crick face (upper site WC 1 and lower site WC 2 ), a Hoogsteen site (HO), and a minor‐groove site (MI)], and ten homogeneous dimers (rU‐rU) can self‐pair with well‐defined diradical characters and comparable stability to the native U dimers. More interestingly, all these dimers exhibit distinctly different spin‐coupling characters (ferromagnetic (FM) versus antiferromagnetic (AFM) and large‐ versus small‐magnitude spin couplings), indicative of remarkable magnetic‐coupling anisotropy of rU. This observation originates from the fusion of a cyclopentadienyl radical to U, which leads to uneven spin‐density distribution. In rU, the fused five‐membered radical ring can spin‐polarize to the edge in the minor groove, and thus dimerization of rU leads to different H‐bonded structures with remarkably different magnetic couplings. The calculated larger magnetic coupling constants J are 1003.7 and 540.2 cm −1 for the WC 2 ‐HO and MI‐HO H‐bonding modes between rU, which exhibit considerably large FM couplings, the MI‐MI, WC 1 ‐WC 2 and WC 2 ‐WC 2 modes show moderate FM couplings ( J =0.4–77 cm −1 ), and the other modes exhibit moderate or weak AFM couplings. These observations indicate that the HO and MI sites are favorable spin‐coupling sites. In addition, the H‐bond lengths and electronic structures of the H‐bonding sites, proton transfer, and extra H‐bonding interaction with the surroundings can also affect the magnetic couplings of the base pairs. Clearly, the unique magnetic coupling anisotropy of rU provides a promising application basis for the design and assembly of bio‐inspired anisotropically magnetic membranes and even magnetism‐tunable building blocks for novel magnetic nanoscale devices.