IMINOPROPADIENONES, RN=C=C=C=O - THEORY AND EXPERIMENT

Ab initio molecular orbital calculations at the MP2/6-31G level have been used to examine the structures and infrared spectra of a new clas s of compounds, the iminopropadienones, RN=C=C=C=O (R = H, CH3, and Ph ). The agreement between calculated and experimental IR spectra of PhN CCCO, Ph(15)NCCCO, and PhNCC(13)CO is excellent. Inclusion of electron correlation is found to be important for the correct prediction of th e relative intensities of the cumulenic stretching vibrations. All thr ee iminopropadienones are predicted to have a slightly bent NCCCO back bone (angle CCC approximate to 176 degrees). As with carbon suboxide, these cumulenes are calculated to have an extremely flat CCC bending p otential. The parent compound, HNCCCO (4a) is calculated to be thermod ynamically stable toward dissociations into (i) HNCC + CO, (ii) HNC CCO, (iii) HN + CCCO, and (iv) H. + NCCCO.. Rearrangement of 4a to the more stable cyanoketene isomer requires a sizeable barrier of 402 kJ mol(-1) [G2(MP2)]. The calculated stability of 4a is consistent with i ts experimental observation in neutralization-reionization mass spectr um. The adiabatic ionization energy and heat of formation of HNCCCO ar e predicted to be 9.51 eV and 175 kJ mol(-1), respectively.