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Effect of Methoxy Substituents on the Structural and Electronic Properties of Fluorinated Cyclobutenes: A Study of Hexafluorocyclobutene and Its Vinyl Methoxy Derivatives by XRD and Periodic DFT Calculations

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journal contribution
posted on 02.04.2009, 00:00 by Leonardo Lo Presti, Arkady Ellern, Riccardo Destro, Bruno Lunelli
The effect of the methoxy substituent on the structure, crystal packing, and electrostatic properties of hexafluorocyclobutene (C4F6) was investigated in the solid-state with DFT-B3LYP calculations. Full geometry optimizations were done for the parent compound and its two vinyl methoxy derivatives C4F5OCH3 and C4F4(OCH3)2, starting from the structures obtained by single-crystal X-ray diffraction at low temperature. A full topological analysis, followed by the calculation of several electrostatic properties, was performed on the periodic electron density using the quantum theory of atoms in molecules. Eventually, the cohesive energies of the three crystals were estimated. In the cyclobutene plane, the methoxy substitution yields a significant electronic rearrangement involving the π-electrons. The solid-state (periodic) results agree with those obtained by gas-phase calculations on C4F6 and its derivatives at a comparable level of theory. It was found that the substitution of one or two vinylic fluorine atoms with the OCH3 group considerably influences the molecular dipole moment, which undergoes an enhancement in both the solid and the gas phase as large as 200% and 235% for C4F5OCH3 and C4F4(OCH3)2, respectively, with respect to that calculated for C4F6. The charge rearrangement due to the substituents provides a significant electrostatic contribution to the lattice energy, and in turn it can be related to the change in the observed crystal packing on going from C4F6 (space group P21/c) to both of its derivatives (space group P1̅). It is also shown that the dispersion energy significantly contributes to the lattice stability in all three compounds. Since the DFT calculations, in the limit of large separations, entirely miss the dispersion term, this was estimated by applying a recently proposed dampening function to the semiempirical atom−atom C6 R−6 potentials in the mainframe of Spackman’s energy decomposition scheme for Mulliken multipoles.