Experimental Charge Density Analysis and Electrostatic Properties of Crystalline 1,3-Bis(Dimethylamino)Squaraine and Its Dihydrate from Low Temperature (T = 18 and 20 K) XRD Data

Abstract

Multipolar refinements of structural models fitting extensive sets of X‐ray diffraction(XRD) data from single crystals of 1,3‐bis(dimethylamino)squaraine [SQ, C8H12N2O2] and itsdihydrate [SQDH, C8H12N2O2∙2H2O], collected at very low T (18 ± 1 K for SQ, 20 ± 1 K for SQDH),led to an accurate description of their crystal electron density distributions. Atomic volumes andcharges have been estimated from the experimental charge densities using the Quantum Theory ofAtoms in Molecules (QTAIM) formalism. Our analysis confirms the common representation (in theliterature and textbooks) of the squaraine central, four‐membered squarylium ring as carrying twopositive charges, a representation that has been recently questioned by some theoreticalcalculations: the integrated total charge on the C4 fragment is estimated as ca. +2.4e in SQ and +2.2ein SQDH. The topology of the experimental electron density for the SQ squaraine molecule ismodified in the dihydrated crystal by interactions between the methyl groups and the H2Omolecules in the crystal. Maps of the molecular electrostatic potential in the main molecular planesin both crystals clearly reveal the quadrupolar charge distribution of the squaraine molecules.Molecular quadrupole tensors, as calculated with the PAMoC package using both Stewart andQTAIM distributed multipole analysis (DMA), are the same within experimental error.

Anharmonic Thermal Motion Modelling in the Experimental XRD Charge Density Determination of 1-Methyluracil at T = 23 K

“…They are virtually the same, with a coefficient of variation equal to 0.02, and their average value, when multiplied by Z = 8, reproduces the unit cell experimental volume to within 0.06% (see Table 1 ). The quantitative accuracy of the latter estimate is comparable with those obtained in previous molecular crystals EDD studies [ 56 , 57 , 58 ].…”