Various local minimum structures of an aniline complex with carbon monoxide; DFT and MP2 calculations

Theoretical calculations by DFT and MP2 (with 4-31G*, 6-31G**, and 6-31++G**) give various local minima of an aniline-CO complex with binding energies of about 10-20 kJ/mol, DFT giving the larger energy. Two different types of complexes, ''CO on phenyl'' (CO on the 2H side of NH2, C-s symmetry), and ''CO towards NH2'' (with the hydrogen bond of NH-CO, C-1 symmetry), show almost the same binding energy with a slightly stable hydrogen-bond structure. CO in the latter structure is located almost perpendicularly to the phenyl ring because of the two different interactions of the hydrogen bond and pi-pi interaction to phenyl, Vibrational frequencies of the complex are also obtained for several local minimum structures. The zero-point energies decrease the binding energies by ca, 3 kJ/mol, Frequency shifts of the NH2 wagging modes in the hydrogen-bond complex are positive and large both by DFT and MP2. The frequency shifts of the other modes are zero or very small while those of the CO stretching mode are negative in the "CO on phenyl" structure by DFT and MP2 and almost zero (by DFT) or positive (by MP2) in the hydrogen-bond complex. The DFT calculation gave a higher electron transfer between CO and aniline and a larger binding energy and frequency shifts. The calculated frequency shifts in the NH symmetric and antisymmetric stretching mode of the hydrogen-bonded aniline-CO, - 5 and - 6 cm(-1), respectively, by MP2 support the experimentally observed shifts, - 6 and - 11 cm(-1), respectively; only those frequencies are reported presently. Another frequency shift calculated here will be used to determine more accurately the structure of the aniline-CO complex, if the appropriate laser as a light source for the experiments is available. (C) 1999 Elsevier Science B.V. All rights reserved.