Solid-State NMR spectra and long, intra-dimer bonding in the pi-[TTF](2)(2+) (TTF = tetrathiafulvalene) dication
Merrill D Halling1, Joshua D Bell, Ronald J Pugmire
1Department of Chemistry, University of Utah, 315 S. 1400 E. Rm 2124, Salt Lake City, Utah 84112-0850, USA.
Abstract:
The (13)C chemical-shift tensor principal values for TTF and pi-[TTF](2)(2+) (TTF = tetrathiafulvalene) dimer dications have been measured in order to better understand the electronic structure and long intradimer bonding of these TTF-based dimer structures. The structure of pi-[TTF](2)(2+) is abnormal due to its two C-C and four S-S ca. 3.4 A intradimer separations, which is less than the sum of the sulfur van der Waals radii, and has a singlet (1)A(1g) electronic ground state. This study of TTF and [TTF](2)(2+) was conducted to determine how the NMR chemical-shift tensor principal values change as a function of electronic structure. This study also establishes a better understanding of the interactions that lead to spin-pairing of the monomeric radical units. The density functional theory (DFT) calculated nuclear shielding tensors are correlated with the experimentally determined principal chemical-shift values. The embedded ion method (EIM) was used to investigate the electrostatic lattice potential in [TTF](2)(2+). These theoretical methods provide information on the tensor magnitudes and orientations of their tensor principal values with respect to the molecular frame. The experimental chemical-shift principal values agree with the calculated quantum mechanical chemical-shielding principal values, within typical errors commonly seen for this class of molecular system. Relatively weak Wiberg bond orders between the two [TTF](+) components of the dimer dication correlate with the long bonds linking the two [TTF](+) monomers and substantiate the claim that there is weak multicenter bonding present.
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