NMR spectroscopy and ion pairing: Measuring and understanding how ions interact

Abstract

Pulsed gradient spin-echo (PGSE) diffusion and Overhauser NMR data together with density functional theory (DFT) calculations afford a qualitative estimation of the amount of ion pairing, as well as insight into the structures of a variety of inorganic, organic, and organometallic salts.

Evidence of a Donor–Acceptor (Ir–H)→SiR3Interaction in a Trapped Ir(III) Silane Catalytic Intermediate

“…For the measures carried out in d 5 -PhCl, the D cation /D anion ratio for [2] [BArF 24 ] was found to be only slighly higher than the theoretical value of 1 for an ideal tight ion pair model (analytical D cation /D anion = 1.11 (1), 2D-DOSY plot D cation /D anion = 1.1), whereas for [2][BF 4 ] the ratio was closer to 1 (D cation /D anion = 1.01(1), 2D-DOSY plot D cation /D anion = 1.0). 23 In CD 2 Cl 2 , a slightly more polar solvent, the qualitative values of D cation /D anion ratios extracted from 2D-DOSY plots were 1.2 and 0.9 for [2] [BArF 24 ] and [2] 24 ] at 298 K in CD 2 Cl 2 displayed no mutual through-space correlation between the signals of the two ions, which indicates that within the time scale of NMR spectroscopy the ion pair is indeed dissociated according to Pregosin's criteria. 23 These results indicate that the ion pairings in the latter BArF 24 − and BF 4 − salts are not largely different in a solvent of moderate polarity such as d 5 -PhCl and that ion-pair dissociation is only slightly enhanced in a more polar solvent such as CD 2 Cl 2 because the values of the D cation /D anion ratio remain close to 1.…”

“…23 In CD 2 Cl 2 , a slightly more polar solvent, the qualitative values of D cation /D anion ratios extracted from 2D-DOSY plots were 1.2 and 0.9 for [2] [BArF 24 ] and [2] 24 ] at 298 K in CD 2 Cl 2 displayed no mutual through-space correlation between the signals of the two ions, which indicates that within the time scale of NMR spectroscopy the ion pair is indeed dissociated according to Pregosin's criteria. 23 These results indicate that the ion pairings in the latter BArF 24 − and BF 4 − salts are not largely different in a solvent of moderate polarity such as d 5 -PhCl and that ion-pair dissociation is only slightly enhanced in a more polar solvent such as CD 2 Cl 2 because the values of the D cation /D anion ratio remain close to 1. Static DFT calculations of idealized ion pair dissociation enthalpies 24 26 suggest that the slightly more favored dissociation of BArF 24 − should be entropy grounded in comparison to BF 4 − (Figure 2).…”

“…This difference in Coulombic contribution is consistent with the larger delocalization of the charge density mostly at the F atoms in the anion in comparison to BF 4 − . 23 The dispersion and orbital interaction energy terms contribute respectively to 3% and 13% of the attractive term ∑ΔE attractive in [2] Figure 3; cf. Figure S37 in the Supporting Information).…”

Heteronuclear NOE Spectroscopy of Ionic Liquids

“…[14] Several groups were already able to demonstrate the usefulness of NOE spectroscopy for the investigation of ion pairs, particularly for metal complexes in solution. [15][16][17][18][19][20] Consequently, the next step was the investigation of ILs in solution through NOE spectroscopy. Dupont was the first to apply homonuclear H,H HOESY NMR spectroscopy on ionic liquids in organic solvents to study ion pairing.…”

Pulsed Gradient Spin-Echo NMR Studies of the Interactions of Platinum Complexes

“…[10][11][12] It has been used to probe aggregation, ion pairing, and reaction mechanisms of metal complexes; [13][14][15] yet, its use to study PCs is limited. Examples of recent and future applications are presented below.…”