Theoretical Study on the Interaction between Formaldehyde and Acetonitril

The optimization geometries and vibrational frequencies ofH2CO, CH3CN and 4 complexes acquired between H2CO and CH3CN have been calculated by using theab initiomethod and density functional method at the HF/6-311G(d,p), MP2/6-311G(d,p) and B3LYP/6-311G(d,p) levels. The non-minimum structures with negative vibrational frequencies are excluded. The lowest energy conformerofthese complexes is cyclic structurewith C-H…O and C-H…N hydrogen bonds on a common plane. No significant changes are observed in the geometries of the monomers in their complexed state. The most characteristic geometrical properties of the complex are the lengthening of the contacting C-X(X=O,N) bonds by 0.10.4 pm and the general shortening of the contacting C-Hbonds by 0.10.4 pmwith respect tothe monomers. The interaction energies of cyclic structure have been corrected by the basis set superposition error (BSSE) using the full Boys-Bernardi counterpoise correction scheme and zero point energy (ZPE). The corrected complex interaction energies of cyclic structure atHF/6-311G(d,p), MP2/6-311G(d,p) and B3LYP/6-311G(d,p) levels are -8.98, -8.33 and -7.84 kJ/mol, respectively. The interaction energy attheMP2 level ismore accurate and the inclusion of correlation effects is important for the description of hydrogen-bonding systems. Nevertheless, for the cases considered here, B3LYP calculations that give a good description of geometric, energetic and electronic properties as compared withMP2 calculation data can be used for investigations of hydrogen-bonded complexeswhere theMP2 level is not available. The interaction energy indicates that C-H…O and C-H…N are weak hydrogen bonds. Although differentmethods lead to different populations, one might expect that the changes of population both in the uncomplex and complex stateswould be similar in differentmethods. The results ofMulliken population analysis and natural bond orbital population analysis reveal that there is only a small charge-transfer in the process of forming the complex. The results of molecular interaction energy decomposition analysis showthat the electrostatic interaction plays an essential role in stabilizing the H2CO…CH3CN complex.