Rationalization of Solvation and Stabilization of Palladium Nanoparticles in Imidazolium-Based Ionic Liquids by DFT and Vibrational Spectroscopy

A combined DFT/vibrational spectroscopy approach is used to determine the interactions of the 1,3-dimethylimidazolium ([Mmim]+) and 1-ethyl-3-methylimidazolium ([Emim]+) cations and [BF4]- anions with each other in the liquid state and with Pd nanoparticles (NPs) immersed in ionic liquids (ILs) composed of these ions. Formation of aggregates of the counter-ions in the liquid does not have a strong influence on the interaction energy of the IL components with the palladium clusters used to model NPs, which is smaller than the energy of addition of a Pd atom to the cluster. Stronger PdPd interactions in comparison to the interactions of the palladium cluster with the IL suggest kinetic stabilisation of Pd-NPs in 1,3-dialkylimidazolium tetrafluoroborates rather than thermodynamic stabilisation. Moreover, the palladium clusters interact more strongly with the anions than with the cations, and this suggests an important role of the anions in formation and stabilisation of Pd-NP in ILs. At the same time, binding between an isolated Pd atom and [Mmim]+ cation is stronger than Pd[BF4]- binding. IR and Raman spectral simulations reveal that surface-enhanced Raman spectroscopy is much less sensitive to interactions of Pd-NPs with the [BF4]- anion compared to interactions with [Mmim]+ cations. In contrast, IR spectroscopy is better suited to study the anionmetal interactions, whereas IR spectral manifestations of the cationmetal interactions are rather modest. The splitting of the strong ?as(BF4-) IR band into three components appears to be a convenient spectroscopic marker for Pd[BF4]- interactions, which is confirmed by actual spectra of Pd-NPs stabilised by [Emim][BF4]. The IR spectra and their assignment with quantum chemical computations suggest that both the anions and cations of [Emim][BF4] interact with the Pd-NP surface in the IL. The cation ring orientation close to the surface normal appears to be the dominant interaction. The anion is bound to the surface through either two or three fluorine atoms.

How Strong Is Hydrogen Bonding in Ionic Liquids? Combined X-ray Crystallographic, Infrared/Raman Spectroscopic, and Density Functional Theory Study

Paul Joseph Dyson, Zhaofu Fei, Gabor Laurenczy, Emilia Paunescu, Rosario Scopelliti, Ning Yan

Hydrogen bonding in ionic liquids based on the 1-(2'-hydroxylethyl)-3-methylimidazolium cation (C(2)OHmim) and various anions (A) of differing H-bond acceptor strength, viz. hexafluorophosphate PF6, tetrafluoroborate BF4, bis-(trifluoromethanesulfonimide) Tf2N, trifluoromethylsulfonate OTf, and trifluoroacetate TFA, was studied by a range of spectroscopic and computational techniques and, in the case of [C(2)OHmim][PF6], by single crystal X-ray diffraction. The first quantitative estimates of the energy (E-HB) and the enthalpy (-Delta H-HB) of H-bonds in bulk ILs were obtained from a theoretical analysis of the solid-state electron-density map of crystalline [C(2)OHmim][PF6] and an analysis of the IR spectra in crystal and liquid samples. E-HB for OH center dot center dot center dotPF6 H-bonds amounts to similar to 3.4-3.8 kcal.mol(-1), whereas weaker H-bonds (2.8-3.1 kcal-mol(-1)) are formed between aromatic C2H group of imidazolium ring and the PF6 anion. The enthalpy of the OH center dot center dot center dotA H-bonds follows the order: [PF6] (2.4 kcal.mol(-1)) < [BF4] (3.3 kcal.mol(-1)) < [Tf2N] (3.4 kcal-mol(-1))

American Chemical Society2013

Solvation and stabilization of palladium nanoparticles in phosphonium-based ionic liquids: a combined infrared spectroscopic and density functional theory study

Ning Yan

Analysis of infrared spectra of palladium nanoparticles (NPs) immersed in the tri-tert-butyl-R-phosphonium-based ionic liquids (ILs) demonstrates that both cations and anions of the ILs interact with the NPs. According to quantum-chemical simulations of these interactions, the binding energy of anions to the Pd-6 cluster, taken as a minimal-size model of the NPs, increases from similar to 6 to similar to 27 kcal mol(-1) in the order PF6 approximate to BF4 < Tf2N < OTf < Br < TFA. In contrast, the binding energy for all types of the (Bu3PR)-P-t cations slightly varies at about similar to 22 kcal mol(-1) only moderately depending on the choice of the R moiety (n-pentyl, 2-hydroxyethyl, 2-methoxyethyl, 2-ethoxy-2-oxoethyl). As a result, the energies of interaction between a Pd-6 cluster and various ion pairs, formed by the abovementioned counter-ions, follow the order found for the anions and vary from similar to 24 to similar to 47 kcal mol(-1). These values are smaller than the energy of addition of a Pd atom to a Pd-n cluster (similar to 58 kcal mol(-1)), which suggests kinetic stabilization of the NPs in phosphonium-based ILs rather than thermodynamic stabilization. The results are qualitatively similar to the trends found earlier for interactions between palladium clusters and components of imidazolium-based ILs, in spite of much larger contributions of the London dispersion forces to the binding of the (Bu3PR)-P-t cations to the cluster (up to 80%) relative to the case of 1-R-3-methylimidazolium cations (up to 40%).

Royal Soc Chemistry2014

Application of Density Functional Theory and Vibrational Spectroscopy Toward the Rational Design of Ionic Liquids

Paul Joseph Dyson, Ana Vidis

D. functional theory methods in combination with vibrational spectroscopy are used to investigate possible variants of mol. structure of the ion pairs of several imidazolium-based ionic liqs. (ILs). Multiple stable structures are detd. with the anion positioned (a) near to the C2 atom of the imidazolium ring, (b) between N1 and C5, (c) between N3 and C4, and (d) between C4 and C5. Chloride and bromide anions in vacuum also occupy positions above or below the imidazolium ring, but in the condensed state these positions are destabilized. In comparison with the halides that almost equally occupy the positions (a-d), tetrafluoroborate and hexafluorophosphate anions strongly prefer position (a). The position and the type of the anion influence the conformation of the side chains bound to the imidazolium N1 atom, which are able to adopt in vacuum all usual staggered or eclipsed conformations, although in the liq. state some of the conformations are present only as minor forms if at all. Vibrations of the cations depend both on the conformational changes and on the assocn. with the anion. The formation of the ion pairs influences mainly stretching and out-of-plane vibrations of the imidazolium C-H groups and stretching vibrations of the perfluoroanions. Other modes of the ions retain their individuality and practically do not mix. This allows "interionic" vibrations to be sepd. and to regard the couple of the ions as an anharmonic oscillator. Such a model correlates the mol. structure of various ILs and their m.ps. without involving the energy of the interaction between the cations and anions but explains structure-m.p. correlations on the grounds of quasy-elastic properties.

2007