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Properties of Cationic Pnicogen-Bonded Complexes F4–nHnP+:N-Base with F–P···N Linear and n = 0–3

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journal contribution
posted on 04.06.2015, 00:00 by Janet E. Del Bene, Ibon Alkorta, José Elguero
Ab initio MP2/aug′-cc-pVTZ calculations were performed to investigate the pnicogen-bonded complexes F4–nHnP+:N-base, for n = 0–3, each with a linear or nearly linear F–P···N alignment. The nitrogen bases include the sp3 bases NH3, NClH2, NFH2, NCl2H, NCl3, NFCl2, NF2H, NF2Cl, and NF3 and the sp bases NCNH2, NCCH3, NP, NCOH, NCCl, NCH, NCF, NCCN, and N2. The binding energies vary between −20 and −180 kJ·mol–1, while the P–N distances vary from 1.89 to 3.01 Å. In each series of complexes, binding energies decrease exponentially as the P–N distance increases, provided that complexes with sp3 and sp hybridized bases are treated separately. Different patterns are observed for the change in the binding energies of complexes with a particular base as the number of F atoms in the acid changes. Thus, the particular acid–base pair is a factor in determining the binding energies of these complexes. Three different charge-transfer interactions stabilize these complexes. These arise from the nitrogen lone pair to the σ*P–Fax, σ*P–Feq, and σ*P–Heq orbitals. The dominant single charge-transfer energy in all complexes is Nlp → σ*P–Fax. However, since there are three Nlp → σ*P–Feq charge-transfer interactions in complexes with F4P+ and two in complexes with F3HP+, the sum of the Nlp → σ*P–Feq charge-transfer energies is greater than the Nlp → σ*P–Fax charge-transfer energies in the former complexes, and similar to the Nlp → σ*P–Fax energies in the latter. The total charge-transfer energies of all complexes decrease exponentially as the P–N distance increases. Coupling constants 1pJ(P–N) across the pnicogen bond vary with the P–N distance, but different patterns are observed for complexes with F4P+ and complexes of the sp3 bases with F3HP+. These initially increase as the P–N distance decreases, reach a maximum, and then decrease with decreasing P–N distance as the P···N bond acquires increased covalent character. For the remaining complexes, 1pJ(P–N) increases with decreasing P–N distance. Complexation increases the P–Fax distance and 1J(P–Fax) relative to the corresponding isolated ion. 1J(P–Fax) correlates quadratically with the P–N distance.