The Effect of High Pressure on Polymorphs of a Derivative of Blatter's Radical: Identification of the Structural Signatures of Subtle Phase Transitions

Abstract

The effect of pressure on the α and β polymorphs of a derivative of Blatter's radical, 3-phenyl-1-(pyrid-2-yl)-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl, has been investigated using single-crystal X-ray diffraction to maximum pressures of 5.76 and 7.42 GPa, respectively. The most compressible crystallographic direction in both structures lies parallel to π-stacking interactions, which semiempirical Pixel calculations indicate are also the strongest interactions present. The mechanism of compression in perpendicular directions is determined by void distributions. Discontinuities in the vibrational frequencies observed in Raman spectra measured between ambient pressure and ∼5.5 GPa show that both polymorphs undergo phase transitions, the α phase at 0.8 GPa and the β phase at 2.1 GPa. The structural signatures of the transitions, which signal the onset of compression of initially more rigid intermolecular contacts, were identified from the trends in the occupied and unoccupied volumes of the unit cell with pressure and in the case of the β phase by deviations from an ideal model of compression defined by Birch-Murnaghan equations of state.

Figure 1

(a) The Blatter’s radical derivative 3-phenyl-1-(pyrid-2-yl)-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl (1). (b) Diagram depicting the numbering scheme used in this work.

Figure 2

Formation of the different π-stacking interactions via a 2₁ screw in 1α and lattice translation in 1β.

Figure 3

Strain axes X₂ and X₃ viewed along the largest eigenvalue X₁ in (a) polymorph 1α and (b) polymorph 1β. Molecules are labeled as in Tables 1 and 2. Void distributions visualized using Mercury with a probe radius of 0.75 Å and a grid spacing of 0.3 Å are shown in (c) for polymorph 1α and (d) polymorph 1β. The distribution of the voids in 1α (b) is more uniform, whereas the voids are aligned approximately in the direction of X₂ in 1β (d).

Figure 4

Change in unit cell dimensions for (a) 1α and (b) 1β across the pressure series.

Figure 5

(a) Raman spectra of 1α as a function of pressure. (b) Trends with respect to pressure in the wavenumbers of modes near 1600 and 1000 cm^–1. (c) and (d) show analogous plots for 1β.

Figure 6

(a) Linear fits to the variation of the network volume with pressure of 1α above and below 0.8 GPa. (b) Analogous fits for the 1β phase above and below 2.12 GPa. Error bars lie within the symbols.

Figure 7

Relative change to the centroid distance for each interaction with pressure for (a) 1α and (b) 1β.

Figure 8

Voids (also shown in Figure 3d) in the regions between the gray reference molecule and the blue and red molecules in interactions D and F, respectively.