COMPARATIVE CUMULENE SUBSTITUENT EFFECTS - KETENES, ALLENES, DIAZOMETHANES, DIAZIRINES, AND CYCLOPROPENES BY AB-INITIO MOLECULAR-ORBITAL CALCULATIONS

Ab initio calculations of the effect of substituents on the structures and energies of ketenes (1), diazomethanes (3), diazirines (4), allenes (5), and cyclopropenes (6) reveal that the isodesmic substituent stabilization energies of 1, 3, and 5 relative to alkenes are correlated with group electronegativities with slopes of -15.1, -10.6, and -5.8, respectively, at the HF/6-31G*+ZPVE//HF/6-31G* level, corresponding to a successive decrease in polar character of the cumulenes in this order. Isomerization energies of 3 to 4 show that the relative stabilities of substituted diazomethanes and diazirines are dominated by the stabilization of diazomethanes by electropositive substituents. The energetics of the isomerization of allenes 5 to cyclopropenes 6 are affected by a general greater stability of allenes due to the ring strain of cyclopropenes, a greater stabilization of allenes by electropositive substituents, and aromaticity/antiaromaticity effects in which cyclopropenes are favored by electronegative substituents and disfavored by electropositive substituents. A comparison of charges calculate by the Mulliken, Natural Bond Orbital (NBO), and Bader methods reveals similar trends in the charges from all three methods, with good correlations between the Mulliken and NBO charges. The best correlations of charges with substituent electronegativity are found for the NBO-derived charges. The Bader charges do not correlate well with Mulliken and NBO charges, nor with electronegativity, and give very large absolute atomic charges in many cases. The calculated cumulene IR asymmetric stretching vibrations are correlated with measured values for ketenes and allenes, but for diazomethanes the calculations are of limited diagnostic utility.