Conformations,Chemical Reactivities and Spectroscopic Characteristics of Some Di-substituted Ketenes:An ab initio Study

摘要

A systematic study of the structure,energetics and spectral characteristics of substituted aminoketenes R(NH_2)C=C=O(R=H,CH_3,NH_2,OH,OCH_3,CH=CH_2,C ident to CH,CN,CHO,NO,NO_2)which are highly reactive and transient intermediates in synthesis has been conducted by ab initio calculations at the MP2/6-31G*//MP2/6-31G* level.Twenty four stable isomers of the eleven substituted aminoketenes having dihedral angles phi_(NH_2)approx 120 deg and 60 deg have been identified and their optimized geometries and energies obtained.Electrostatic and steric effects on the molecular geometries have been analyzed.While the pi-acceptor groups lead to planar conformations,the electron-donor groups give rise to non-planar conformations.Isodesmic substituent stabilization energies relative to alkenes have been calculated and correlation with group electronegativities established.Role of induction effect by the substituent groups and resonance effects in charge distribution in the molecules has been analyzed.An analysis of the asymmetric stretching frequencies and intensities of the C=C=O group shows that affect of non-pi acceptor substituents on the frequency is determined by the field effect(F)and resonance effect(R)parameters,the calculated intensities I(km/mol.)are correlated to group electronegativities chi of the substituents by the relationship I=640.2-100.1 chi(r=0.92).The pi-acceptor substituents increase the intensity which may be explained in terms of their delocalizing effect on the negative charge at the C_(beta)atom.