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The geometries and attractive energies of carbonyl–carbonyl interactions have been investigated using crystallographic data and ab initio molecular-orbital calculations. Analysis of crystallographic data for 9049 carbon-substituted >C=O groups shows that 1328 (15%) form contacts with other >C=O groups, in which d(C...O) < 3.6 Å. Three common interaction motifs are observed in crystal structures: (a) a slightly sheared antiparallel motif (650 instances) involving a pair of short C...O interactions, together with (b) a perpendicular motif (116 instances) and (c) a highly sheared parallel motif (130 instances), which both involve a single short C...O interaction. Together, these motifs account for 945 (71%) of the observed interactions. Ab-initio-based molecular-orbital calculations (6-31G** basis sets), using intermolecular perturbation theory (IMPT) applied to a bis-propanone dimer model, yield an attractive interaction energy of −22.3 kJ mol−1 for a perfect rectangular antiparallel dimer having both d(C...O) = 3.02 Å and attractive energies < −20 kJ mol−1 over the d(C...O) range 2.92–3.32 Å. These energies are comparable to those of medium-strength hydrogen bonds. The IMPT calculations indicate a slight shearing of the antiparallel motif with increasing d(C...O). For the perpendicular motif, IMPT yields an attractive interaction energy of −7.6 kJ mol−1, comparable in strength to a C—H...O hydrogen bond and with the single d(C...O) again at 3.02 Å.
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