Solid-State 13C NMR Evidence for Long Multicenter Intradimer Bonding in Zwitterion-like Structures

The principal values of the C-13 chemical shift tensor for the beta and delta polymorphs of pi-[TT....TCNE] (TTF=tetrathiafulvalene; TCNE=tetracyanoethylene) have been analyzed to understand the abnormally long intra-dimer bonding of singlet pi-[TTF delta+....TCNE delta-]. These structures possess 12 intradimer contacts <3.40 angstrom, with the shortest intra pi-[TTF...TCNE] separations involving 2-center (2c) C-S and 3c C-C-C orbital overlap contributions between the [TTF](delta+) and [TCNE](delta-). This solid-state NMR study compares the [TTF....TCNE] C-13 tensor data against previously reported pi-[TTF](2) (2+) and pi-[TCNE](2) (2-) homo-dimers to determine how the tensor principal values change as a function of electronic structure for both TTF and TCNE moieties. In the beta and delta phases of [TTF....TCNE], the TCNE ethylenic C-13 shift tensors predict TCNE oxidation states of-0.46 and-0.73, respectively. The TTF sites are less similar to benchmark C-13 data with the beta-phase differing primarily in the ethylenic pi-electrons. The delta form differs significantly from the homo-dimer data at all principal values at both the ethylenic and CH sites, indicating changes to both the pi-electrons and sigma-bonds. In both hetero-dimer phases, the NMR changes supports long bond formation at nitrile and CH sites not observed in homo-dimers.