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

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Summary

This study uses solid-state NMR to analyze the bonding in π-[tetrathiafulvalene⋅⋅⋅tetracyanoethylene] (TTF⋅⋅⋅TCNE) polymorphs. The results reveal unusually long intra-dimer bonds, indicating significant electronic structure changes in both TTF and TCNE components.

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Area of Science:

  • Solid-state chemistry
  • Materials science
  • Spectroscopy

Background:

  • Investigating the electronic structure and bonding in charge-transfer complexes is crucial for understanding their properties.
  • The π-[TTF⋅⋅⋅TCNE] system exhibits complex interactions, including abnormally long intra-dimer bonds, which require detailed analysis.
  • Previous studies on homo-dimers provide benchmarks for comparison with hetero-dimer systems.

Purpose of the Study:

  • To analyze the principal values of the 13C chemical shift tensor for β and δ polymorphs of π-[TTF⋅⋅⋅TCNE].
  • To understand the formation of unusually long intra-dimer bonds in singlet π-[TTFδ+ ⋅⋅⋅TCNEδ- ].
  • To compare 13C tensor data with homo-dimers (π-[TTF]2 2+ and π-[TCNE]2 2-) to elucidate changes in electronic structure.

Main Methods:

  • Solid-state Nuclear Magnetic Resonance (NMR) spectroscopy was employed to study the 13C chemical shift tensor.
  • Analysis focused on principal values of the 13C chemical shift tensors for both TTF (tetrathiafulvalene) and TCNE (tetracyanoethylene) moieties.
  • Data from β and δ phases of π-[TTF⋅⋅⋅TCNE] were compared with existing data for π-[TTF]2 2+ and π-[TCNE]2 2- homo-dimers.

Main Results:

  • The TCNE ethylenic 13C shift tensors indicate TCNE oxidation states of -0.46 (β phase) and -0.73 (δ phase).
  • TTF sites in the β phase show differences primarily in ethylenic π-electrons compared to benchmarks.
  • The δ phase exhibits significant deviations at both ethylenic and CH sites, suggesting alterations in both π-electrons and σ-bonds, supporting long bond formation.

Conclusions:

  • The NMR data supports the formation of long bonds at nitrile and CH sites in both hetero-dimer phases (β and δ), a phenomenon not observed in homo-dimers.
  • The study highlights how electronic structure variations influence the 13C chemical shift tensor principal values in TTF and TCNE systems.
  • Solid-state NMR is a powerful tool for probing subtle bonding changes and electronic structures in organic charge-transfer complexes.