Paving the Way Toward New Push‐Pull Superbases in the Gas Phase by an Effective DFT Approach: The Case of a New Structurally Complex Molecule, 5‐Aminopyrrole‐2‐Amidine

Amidines, such as formamidines, acetamidines, and benzamidines {XYN–C(R)=NZ, R: H, Me, Ph}, belong to the family of Brønsted push‐pull organic N‐bases. Depending on substituents at N atoms (X, Y, and Z), they display gas‐phase proton basicities considerably stronger than that of NH 3 , but weaker than that of the bicyclic guanidine MTBD (7‐methyl‐1,5,7‐triazabicyclo[4.4.0]dec‐5‐ene). In our recent quantum‐chemical studies on simple tautomeric superbases, we noticed that some of their tautomeric forms can be employed as scaffolds for putative superbases. We focus the present study on APA , a new amidine, containing the Y‐conjugated aminopyrrole ring at the amidine functional C atom, a possible parent compound of a new series of 5‐amino‐1H‐pyrrole‐2‐carboximidamides (or 5‐aminopyrrole‐2‐amidines). For the analysis of its complex structure, tautomeric possibilities, and proton‐transfer equilibria, the DFT (density functional theory) method turned out to be effective. This method allows calculating with sufficient precision the gas‐phase energetic parameters for isomeric N‐containing π‐electron systems, comparable to those calculated at the ab initio G n levels, and which can be used to pave the way for an extension of the experimental gas‐phase basicity scale. Using DFT, the thermodynamics of all possible isomeric conversions was examined, and the energetically favored ones indicated. For selected isomers, the protonation sites, acid/base equilibria, micro‐ and macroscopic gas‐phase basicities were investigated. The calculated macroscopic gas‐phase basicity, which refers to the intrinsic strength of the investigated base, is very close to that of the superbasic bicyclic amidine DBN (1,5‐diazabicyclo[4.3.0]non‐5‐ene), experimentally investigated earlier by FTICR (Fourier transform ion cyclotron resonance) mass spectrometry.