Fluorine-substituted cyclobutenes in the solid state: Crystal structures, vibrational spectra and mechanical and thermodynamic properties

Abstract

The crystal structures, infrared and Raman spectra, and mechanical and thermodynamic properties of four important fluorine-substituted cyclobutene derivatives in the solid state are investigated using first-principles solid-state methods based on density functional theory. These compounds are hexafluorocyclobutene (HFCB, C4F6), 1,3,3,4,4-pentafluoro-2-methoxycyclobut-1-ene (PFMCB, C4F5OCH3), 3,3,4,4-tetrafluoro-1,2-dimethoxycyclobut-1-ene [TFDMCB, C4F4(OCH3)2] and 1,2-dichloro-3,3,4,4-tetrafluorocyclobut-1-ene (DCTFCB, C4Cl2F4). Although some of the properties of the corresponding molecules in the gas phase have been studied, and the structures of the corresponding molecular crystals have been determined by refinement from X-ray diffraction data, their vibrational spectra and properties have not yet been reported. The computed crystal structures and X-ray diffraction patterns are in excellent agreement with their experimental counterparts. The infrared and Raman spectra are calculated from the computed crystal structures using density functional perturbation theory. The results are highly consistent with the corresponding spectra measured experimentally in the gas or liquid phases and, therefore, appropriate normal coordinate analyses of the theoretical results are employed to rigorously assign all the vibrational bands. The elasticity matrices of these materials are computed using the finite deformation technique and a complete set of relevant mechanical properties is determined. Their equations of state are also obtained. These compounds are shown to be weak, highly anisotropic materials displaying the negative Poisson's ratio phenomenon. The TFDMCB also exhibits the negative linear compressibility (NLC) phenomenon for external isotropic pressures in the range of 0.64–1.76 GPa. The computed minimum compressibility, found at P = 0.73 GPa, is substantial (k=−192.9 TPa−1). The NLC effect in TFDMCB is due to a collective rotation of the molecules within the crystal under increasing pressure. Finally, the thermodynamic properties of these materials are determined as a function of temperature using phonon calculations. The computed specific heats of HFCB, PFMCB, TFDMCB and DCMCB at T = 250 K are 127.5, 150.4, 169.9 and 134.8 J·K−1·mol−1, respectively, and corresponding entropies are 152.8, 173.3, 195.4 and 186.6 J·K−1·mol−1.