Properties of cationic pnicogen-bonded complexes F<sub>4-</sub><i><sub>n</sub></i>H<i><sub>n</sub></i>P<sup>+</sup>:N-base with H–P···N linear and <i>n</i> = 1–4
Creators: name: Alkorta, Ibon
name: Elguero, José
name: Bene, Janet E. Del
Publisher: name: Taylor & Francis
Year: 2015
Subjects: subject: Plant Biology
subject: FOS: Biological sciences
subject: FOS: Biological sciences
subject: Biological Sciences
subject: Ecology
subject: Chemistry
subject: Earth and Environmental Sciences
subject: Biochemistry
subject: Biophysics

description: ABSTRACT<i>Ab initio</i> MP2/aug'-cc-pVTZ calculations have been carried out to investigate the pnicogen-bonded complexes F<sub>4-</sub><i><sub>n</sub></i>H<i><sub>n</sub></i>P<sup>+</sup>:N-base, for <i>n</i> = 1–4, each with a linear or nearly linear H<sub>ax</sub>–P···N alignment. The sp<sup>3</sup>-hybridised nitrogen bases include NH<sub>3</sub>, NClH<sub>2</sub>, NFH<sub>2</sub>, NCl<sub>2</sub>H, NCl<sub>3</sub>, NFCl<sub>2</sub>, NF<sub>2</sub>H, NF<sub>2</sub>Cl, and NF<sub>3,</sub> and the sp bases are NCNH<sub>2</sub>, NCCH<sub>3</sub>, NP, NCOH, NCCl, NCH, NCF, NCCN, and N<sub>2</sub>. Binding energies increase as the P–N distance decreases, with an exponential curve showing this relationship when complexes with sp<sup>3</sup> and sp hybridised bases are treated separately. However, the correlations are not as good as they are for the complexes F<sub>4-</sub><i><sub>n</sub></i>H<i><sub>n</sub></i>P<sup>+</sup>:N-base for <i>n</i> = 0–3 with F–P···N linear. 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 stabilise these complexes, namely N<sub>lp</sub>→σ*P–H<sub>ax</sub>, N<sub>lp</sub>→σ*P–F<sub>eq</sub>, and N<sub>lp</sub>→σ*P–H<sub>eq</sub>. Unlike the corresponding complexes with F–P···N linear, N<sub>lp</sub>→σ*P–H<sub>ax</sub> is not always the dominant charge-transfer interaction, since N<sub>lp</sub>→σ*P–F<sub>eq</sub> is greater in some complexes. N<sub>lp</sub>→σ*P–H<sub>eq</sub> makes the smallest contribution to the total charge-transfer energy. The total charge-transfer energies of all complexes increase exponentially as the P–N distance decreases in a manner very similar to that observed for the series of complexes with F–P···N linear.Equation-of-motion coupled cluster singles and doubles (EOM-CCSD) spin–spin coupling constants <sup>1p</sup>J(P–N) across the pnicogen bond vary with the P–N distance, but different patterns are observed which depend on the nature of the acid, and for some acids, on the hybridisation of the nitrogen base. <sup>1p</sup>J(P–N) values for complexes of F<sub>3</sub>HP<sup>+</sup> 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. <sup>1p</sup>J(P–N) for complexes with H–P···N linear and those with F–P···N linear exhibit similar distance dependencies depending on the number of F atoms in equatorial positions and the hybridisation of the base. Complexation may increase, decrease, or leave the P–H<sub>ax</sub> distance unchanged, but <sup>1</sup>J(P–H<sub>ax</sub>) always decreases relative to the corresponding isolated ion. Decreasing <sup>1</sup>J(P–H<sub>ax</sub>) can be related to decreasing intermolecular P–N distance.