CARBONYL-CARBONYL INTERACTIONS CAN BE COMPETITIVE WITH HYDROGEN-BONDS

The geometries and attractive energies of carbonyl-carbonyl interactio ns have been investigated using crystallographic data and ab initio mo lecular-orbital calculations. Analysis of crystallographic data for 90 49 carbon-substituted >C=O groups shows that 1328 (15%) form contacts with other >C=O groups, in which d(C...O) < 3.6 Angstrom. 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 instanc es) and (c) a highly sheared parallel motif (130 instances), which bot h involve a single short C O interaction. Together, these motifs accou nt for 945 (71%) of the observed interactions. Ab-initio-based molecul ar-orbital calculations (6-31G* basis sets), using intermolecular per turbation theory (IMPT) applied to a bis-propanone dimer model, yield an attractive interaction energy of -22.3 kJ mol(-1) for a perfect rec tangular antiparallel dimer having both d(C...O) = 3.02 Angstrom and a ttractive energies < -20 kJ mol(-1) over the d(C...O) range 2.92-3.32 Angstrom. These energies are comparable to those of medium-strength hy drogen 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) comp arable in strength to a C-H...O hydrogen bond and with the single d(C. ..O) again at 3.02 Angstrom.