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 its dihydrate [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 and charges have been estimated from the experimental charge densities using the Quantum Theory of Atoms in Molecules (QTAIM) formalism. Our analysis confirms the common representation (in the literature and textbooks) of the squaraine central, four-membered squarylium ring as carrying two positive charges, a representation that has been recently questioned by some theoretical calculations: the integrated total charge on the C4 fragment is estimated as ca. +2.4e in SQ and +2.2e in SQDH. The topology of the experimental electron density for the SQ squaraine molecule is modified in the dihydrated crystal by interactions between the methyl groups and the H2O molecules in the crystal. Maps of the molecular electrostatic potential in the main molecular planes in both crystals clearly reveal the quadrupolar charge distribution of the squaraine molecules. Molecular quadrupole tensors, as calculated with the PAMoC package using both Stewart and QTAIM distributed multipole analysis (DMA), are the same within experimental error.
Highlights
In his extensive review on the reactivity of squaric acid, Schmidt named “squaraines” the1,3-disubstituted squaric acid derivatives containing delocalized bonds
For the full-matrix least-squares refinement of the SQ and SQDH low-T structures, multipolar scattering factors were used as implemented in the VALRAY code [27,28,29], to model the asphericities in the electron density distribution [21]
In all our Electron Density Distribution (EDD) studies so far [44], we have found that reliable modelling of H atoms requires anisotropic displacement parameters (ADPs), and poles up to the quadrupole level [45,46]
Summary
In his extensive review on the reactivity of squaric acid, Schmidt named “squaraines” the1,3-disubstituted squaric acid derivatives containing delocalized bonds. “while the technological uses of squaraines are extensive and a large number of squaraines have been synthesized and found to be useful, very little is known about the fundamental properties of squaraines,”. This statement was made many years ago [7], but it appears to be still valid: a simple search of the SciFinder database [8], finds no references containing both the “fundamental properties” and “squaraines” keywords, out of 2303 positive hits for the “squaraines” keyword alone.
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