High pressure solid phases of benzene. III. Molecular packing analysis of the crystalline structures of C6H6

Abstract

With a computer simulated mechanical model for molecular packing analysis, the reconstitution and identification of the intermediate pressure-induced phase II of solid benzene C6H6 at 293 K, has been undertaken. The atom-atom intermolecular potential of the Buckingham type was generalized to account for short interatomic distances, especially under pressure. The model includes thermal motion and molecular deformation effects. Various crystal structures calculated in the pressure range of phase II and checked by their reticular distances and structure factors, are compared with the structure IIo proposed for this phase. Among them two possible monoclinic structures IIc and IIc′ have been evidenced by the calculation. Structure IIc has energy and enthalpy levels lower than that of phases Ic and IIIc, in the pressure range 0.5<P<1.0 GPa. This allows to suggest this stable monoclinic structure IIc for the real structure of the experimentally observed phase II. This structure IIc corresponds to the structure previously determined as metastable by Dzyabchenko and Bazilevskii [J. Struct. Chem. 26, 553 (1985)].

Calculation of crystal and molecular structures of carbon disulfide CS2

“…44 Inclusion of electrostatic interaction is necessary to get accurate crystalline parameters, and to stabilize the structure. [1][2][3][4] Coulombic interactions which only slightly modify the van der Waals potential curve at short distances have a distinct effect at long distances ͑Ͼ10 Å͒, as already outlined. 3,4 For CS 2 , as well as for C 6 H 6 and its chloro derivatives, the net atomic charges are small but not negligible.…”

“…[1][2][3][4] Coulombic interactions which only slightly modify the van der Waals potential curve at short distances have a distinct effect at long distances ͑Ͼ10 Å͒, as already outlined. 3,4 For CS 2 , as well as for C 6 H 6 and its chloro derivatives, the net atomic charges are small but not negligible. Several theoretical values of the net charge on the C and S atoms from q C ϭϪ2q S ϭ0.58e to lower values 0.308e and even 0.2234e were estimated as to reproduce the values of the electric quadrupole moment and dipole moment.…”

“…2 of Ref. 3. The minimum energy distance is assumed to be sϭr ei ϩr e j along the line A i A j , whatever the orientations of atoms A i and A j around the line A i A j ; r T ϭr A and r R are 1.85, 1.84 Å, and 1.88, 1.87 Å, for C and S, respectively, which is coherent with recommended values for van der Waals radii, r C ϭ1.86 Å and r S ϭ1.89 Å ͑not far from Bondi radii͒.…”

“…The atomic thermal vibration amplitudes which lead to an apparent increase of the atomic radius with temperature are calculated by using a simple empirical law. 1,3,4 This thermal effect is essential to calculate the thermal expansion coefficient of the lattice parameters as a function of T, which is an important characteristic of the method. As the pressure modifies the thermal motion, the isothermal compressibility V ( P)/V 0 is also a crucial sensor to test the validity of a realistic intermolecular potential.…”

“…[1][2][3][4] In the present calculation we extend this model to the reconstitution of crystal structure of solid carbon disulfide CS 2 to test its transferability to linear molecules. Like benzene, 5,6 carbon disulfide is known to exhibit pressureinduced phase transitions, [7][8][9][10][11][12][13][14][15][16][17][18] as shown on the tentative ( P-T) phase diagram proposed by Bolduan et al ͑Fig.…”