Boron trifluoride activated enaminocarbonyl compounds: the influence of intermolecular interactions on the ab initio determination of structures and vibrational spectra

The structures of the BF3–enaminoaldehyde adducts 3a, b have been determined by quantum‐chemical calculations employing Hartree‐Fock, Møller‐Plesset and density functional methods. The length of the coordinative BO bond ranges between 1.59 and 1.62 Å. The BSSE‐free energy of the BO bond ranges between −14.7 and −16.9 kcal/mol. Taking into account hydrogen bonds and dispersion forces between two molecules of 3a, b in the dimeric structure 4 reduces the BO bond length significantly (BO: 1.52–1.57 Å) thereby reproducing the BO bond length found in the x‐ray structure of 1 (BO: 1.49 Å) with much better accuracy. Using the calculated IR and Raman spectra of the dimers 4 as reference, the BO band in the experimental spectra can be assigned. Our approach seems to be of value in discussions comparing structural features gained from gas phase ab initio calculations and solid state x‐ray structures involving strong dipolar structures which allow for various intermolecular interactions. Copyright © 2004 John Wiley & Sons, Ltd. Boron trifluoride activated enaminocarbonyl compounds: The impor tance of intermolecular van der Waals and hydrogen bonds in the ab initio determination of structures and vibrational spectra. The structures of BF3–enaminoaldehyde adducts have been determined allowing for dispersion forces and hydrogen bonds employing Hartree‐Fock, Møller‐Plesset and density functional methods. The determined structures are useful for description of Lewis acid activated enaminocarbonyl compounds.