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Ring Strain and Other Factors Governing the Basicity of Nitrogen Heterocycles - An Interpretation by Triadic Analysis

Radić, Nena; Despotović, Ines; Vianello, Robert (2012) Ring Strain and Other Factors Governing the Basicity of Nitrogen Heterocycles - An Interpretation by Triadic Analysis. Croatica Chemica Acta, 85 (4). pp. 495-504. ISSN 0011-1643

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M06-2X/6-311++G(2df,2pd)//M06-2X/6-31+G(d) computations were employed to investigate the intrinsic gas phase basicity of strained nitrogen heterocycles involving aziridine, azetidine, pyrrolidine and piperidine, together with their N-methyl and N-phenyl derivatives, NR(CH2)(n) (n = 2-5; R = H, Me and Ph). Basicity constants were compared with the corresponding acyclic counterparts, NR(CH3)(2) (R = H, Me and Ph), and were, based on triadic analysis, resolved into contributions mirroring features of both initial base and the final protonated form as well as their interplay, thus offering quantitative insight into the obtained results. In general, the N-methyl derivatives provided strongest bases investigated here, and, within each group of molecules, the basicity increased on going from three- to six-membered rings, consistent with a decrease in ring strain, with four-membered systems already surpassing or coming close to the basicity of the acyclic gauge molecule. Calculated basicities were found in a very good agreement with available experimental data, except for N-methylazetidine, where a remarkable discrepancy was revealed, suggesting that this system should be experimentally reassessed and its gas-phase basicity parameters revised. Triadic analysis showed different behaviour of individual contributions governing basicities, both among and within distinct families of molecules. It also convincingly demonstrated that, if a proper and a quantitatively accurate interpretation of observed basicity trends is desired, one should not rely only on concepts such as localized reactive hybrid orbitals (RHO) or thus derived nitrogen electron-donating ability (T. Ohwada et al., J. Org. Chem. 69 (2004) 7486), which take into account only properties of the initial base in question, but rather consider protonation reaction in its entirety.

Item Type: Article
Uncontrolled Keywords: Baeyer ring strain; protonation; substituent effect; superbases; triadic formula; paradigm shifting implications; gas-phase basicities; electrophilic aromatic-substitution; bond-dissociation energies; hydride affinities; theoretical determination; electrostatic energies; molecular-structure; proton affinities; organic-molecules
Subjects: NATURAL SCIENCES > Chemistry
Divisions: Division of Organic Chemistry and Biochemistry
Project titleProject leaderProject codeProject type
Broenstedove i Lewisove kiseline i baze u kemiji i biokemiji[237604] Robert Vianello098-0982933-2932MZOS
Depositing User: Robert Vianello
Date Deposited: 11 Oct 2013 08:27
DOI: 10.5562/cca2121

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