Deciphering unusually large modulations in two related organic hydroxy channel structures

The incommensurately modulated structures of two related organic hydroxy-channel compounds [(R)-1-(1,3-dihydroxypropan-2-yl)-3-(octan-2-yl)urea, 1; 1-(1,3-dihydroxypropan-2-yl)-3-octylurea, 2] have been determined and compared to the structure of a non-modulated reference compound [(R)-bis-3-(octan-2-yl)urea, 3] lacking hydroxy-channel forming groups. The origin of the modulation is explored through the competition between strong orthogonal urea-urea and hydroxy-hydroxy interactions, though this competition may not be a prerequisite for modulation. Notably, the modulation in these two compounds is unusually pronounced, as shown by the first-order satellite reflections exhibiting intensities comparable to those of the main reflections. This observation is contextualized by comparing the relative intensity differences between main and satellite reflections in a benchmark set of 117 modulated structures. Additionally, an alternative graphical method is introduced to visualize structural modulation within the basic unit cell. This method combines a conventional ball-and-stick representation of the average structure with Lissajous curves that map the spatial modulation onto the unit cell. The curves are color-coded according to the modulation t-parameter, enabling immediate visualization of in-phase and out-of-phase individual modulation displacements.