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This work extends our earlier quantum chemical studies on the gas-phase basicity of very strong N-bases to two series of nitriles containing the methylenecyclopropene and cyclopropenimine scaffolds with dissymmetrical substitution by one or two electron-donating substituents such as Me, NR2, N=C (NR2)2, and N=P (NR2)3, the last three being strong donors. For a proper prediction of their gas-phase base properties, all potential isomeric phenomena and reasonable potential protonation sites are considered to avoid possible inconsistencies when evaluating the energetic parameters and associated protonation or deprotonation equilibria B + H+ = BH+. More than 250 new isomeric structures for neutral and protonated forms are analyzed. The stable structures are selected and the favored ones identified. The microscopic (kinetic) gas-phase basicity parameters (PA and GB) corresponding to N sites (cyano and imino in the cyclopropenimine or in the substituents) in each isomer are calculated. The macroscopic (thermodynamic) PAs and GBs, referring to the isomeric mixtures of favored isomers, are also estimated. The total (pushing) substituent effects are analyzed for monosubstituted and disubstituted derivatives containing two identical or two different substituents. Electron delocalization is examined in the two π–π conjugated transmitters, the methylenecyclopropene and cyclopropenimine scaffolds. The aromatic character of the three-membered ring is also discussed. [ABSTRACT FROM AUTHOR]

The geometry-based HOMA (Harmonic Oscillator Model of Aromaticity) descriptor, based on the reference compounds of different delocalizations of n- and π-electrons, can be applied to molecules possessing analogous bonds, e.g., only CC, only CN, only CO, etc. For compounds with different heteroatoms and a different number of CC, CX, XX, and XY bonds, its application leads to some discrepancies. For this reason, the structural descriptor was modified and the HOMED (Harmonic Oscillator Model of Electron Delocalization) index defined. In 2010, the HOMED index was parameterized for compounds with C, N and O atoms. For parametrization, the reference molecules of similar delocalizations of n- and π-electrons were employed. In this paper, the HOMED index was extended to compounds containing the CP, CS, NN, NP, PP, NO, NS, PO, and PS bonds. For geometrical optimization of all reference molecules and of all investigated heterocompounds, the same quantum–chemical method {B3LYP/6-311+G(d,p)} was used to eliminate errors of the HOMED estimation. For some tautomeric systems, the Gn methods were also employed to confirm tautomeric preferences. The extended HOMED index was applied to five-membered heterocycles, simple furan and thiophene, and their N and P derivatives as well as for tautomeric pyrrole and phosphole and their N and P derivatives. The effects of additional heteroatom(s) in the ring on the HOMED values for furan are parallel to those for thiophene. For pyrroles, aromaticity dictates the tautomeric preferences. An additional N atom in the ring only slightly affects the HOMED values for the favored and well delocalized NH tautomers. Significant changes take place for their rare CH forms. When intramolecular proton-transfer is considered for phosphole and its P derivatives, the PH tautomers seem to be favored only for 1,2,3-triphosphole/1,2,5-triphosphole and for 1,2,3,5-tetraphosphole. For other phospholes, the CH forms have smaller Gibbs energies than the PH isomers. For phosphazoles, the labile proton in the favored form is linked to the N atom. The PH forms have smaller HOMED indices than the NH tautomers but higher than the CH ones. [ABSTRACT FROM AUTHOR]