What quantum chemical simulations tell us about the infrared spectra, structure and interionic interactions of a bulk ionic liquid.

The recently developed efficient protocol to explicit quantum mechanical modeling of the structure and IR spectra of liquids and solutions [Katsyuba et al., J. Phys. Chem. B, 2020, 124, 6664-6670] is applied to ionic liquid 1-ethyl-3-methyl-imidazolium tetrafluoroborate [Emim][BF4], and its C2-deuterated analog [Emim-d][BF4]. It is shown that the solvation strongly modifies the frequencies and IR intensities of both cationic and anionic components of the ionic liquids. The main features of the bulk spectra are reproduced by the simulations for cluster ([Emim][BF4])8, representing an ion pair solvated by the first solvation shell. The geometry of the cluster closely resembles the solid-state structure of the actual ionic liquid and is characterized by short contacts of all CH moieties of the imidazolium ring with [BF4]- anions. Both structural and spectroscopic analyses allow the contacts to be interpreted as hydrogen bonds of approximately equal strength. The enthalpies of these liquid-state H-bonds, estimated with the use of empirical correlations, amount to 1.2-1.5 kcal mol-1, while the analogous estimates obtained for the gas-phase charged species [Emim][BF4]2- and [Emim]2[BF4]+ increase to 3.6-3.9 kcal mol-1.

Fast Quantum Chemical Simulations of Infrared Spectra of Organic Compounds with the B97-3c Composite Method.

The ability of the quantum chemical computations to reproduce spectral positions and relative intensities of infrared (IR) bands for experimental vibrational spectra of organic molecules is assessed. The efficient B97-3c density functional approximation, routinely applicable to hundreds of atoms on a single processor, has been applied for the simulation of IR spectra for species containing up to 216 atoms. The results demonstrate that B97-3c, being much faster than the well-recognized hybrid functional B3LYP, offers similarly good quantitative performance in comparison to experimental data for relative IR intensities and fundamental frequencies (ν ≤ 2200 cm-1) for isolated molecules comprising from 3 to 21 first- or second-row atoms.

Leaching from Palladium Nanoparticles in an Ionic Liquid Leads to the Formation of Ionic Monometallic Species.

Molecular dynamics simulations and DFT calculations suggest that leaching of palladium species from Pd nanoparticles in ionic liquids does not involve "naked" Pd(0) atoms or neutral ArPdX species formed by oxidative addition of arylhalides. Instead, the ionic liquid contributes largely to leaching of ionic PdX- or PdAr+ species.

Quantification of Conventional and Nonconventional Charge-Assisted Hydrogen Bonds in the Condensed and Gas Phases.

Charge-assisted hydrogen bonds (CAHBs) play critical roles in many systems from biology through to materials. In none of these areas has the role and function of CAHBs been explored satisfactorily because of the lack of data on the energy of CAHBs in the condensed phases. We have, for the first time, quantified three types of CAHBs in both the condensed and gas phases for 1-(2'-hydroxylethyl)-3-methylimidazolium acetate ([C2OHmim][OAc]). The energy of conventional OH···[OAc](-) CAHBs is ∼10 kcal·mol(-1), whereas nonconventional C(sp2)H···[OAc](-) and C(sp3)H···[OAc](-) CAHBs are weaker by ∼5-7 kcal·mol(-1). In the gas phase, the strength of the nonconventional CAHBs is doubled, whereas the conventional CAHBs are strengthened by <20%. The influence of cooperativity effects on the ability of the [OAc](-) anion to deprotonate the imidazolium cation is evaluated. The ability to quantify CAHBs in the condensed phase on the basis of easier accessible gas-phase estimates is highlighted.

Conjugation in and optical properties of 1-R-1,2-diphospholes and 1-R-phospholes.

The strength of conjugation between the diene moieties of 1-R-1,2-diphospholes and 1-R-phospholes and exocyclic phenyl groups of these P-heteroles has been quantitatively characterized by the use of Raman activities of the bands of the phenyl substituents. It is shown that conjugation in both types of phospholes is very similar to the conjugation of phenyl groups with the diene system of cyclopentadiene. Introduction of substituents (-OMe, -C(═O)H, -NO2, -NMe2, and -CH═CH2) in the para-position of the phenyl groups of 1-R-1,2-diphospholes extends π-delocalization of exocyclic groups into the electronic system of the 1,2-diphosphole ring, producing bathochromic shifts of the absorption bands up to 63 nm. In contrast, hypsochromic shifts up to 40 nm can be achieved by introduction of SnMe3 or SiMe3 groups at the phosphorus(III) atom of the 1,2-diphosphole and concomitant increase of aromaticity of the P-heterole. Conjugation shifts the "centre of gravity" of the whole electronic absorption spectrum, whereas positions of separate absorption bands are not simply dependent on conjugation lengths.

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

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 Pd6 cluster, taken as a minimal-size model of the NPs, increases from ∼6 to ∼27 kcal mol(-1) in the order [PF6](-)≈ [BF4](-) < [Tf2N](-) < [OTf](-) < [Br](-)≪ [TFA](-). In contrast, the binding energy for all types of the [Bu(t)3PR](+) cations slightly varies at about ∼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 Pd6 cluster and various ion pairs, formed by the abovementioned counter-ions, follow the order found for the anions and vary from ∼24 to ∼47 kcal mol(-1). These values are smaller than the energy of addition of a Pd atom to a Pdn cluster (∼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 [Bu(t)3PR](+) cations to the cluster (up to 80%) relative to the case of 1-R-3-methylimidazolium cations (up to 40%).

How strong is hydrogen bonding in ionic liquids? Combined X-ray crystallographic, infrared/Raman spectroscopic, and density functional theory study.

Hydrogen bonding in ionic liquids based on the 1-(2'-hydroxylethyl)-3-methylimidazolium cation ([C2OHmim](+)) 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 [C2OHmim][PF6], by single crystal X-ray diffraction. The first quantitative estimates of the energy (E(HB)) and the enthalpy (-ΔH(HB)) of H-bonds in bulk ILs were obtained from a theoretical analysis of the solid-state electron-density map of crystalline [C2OHmim][PF6] and an analysis of the IR spectra in crystal and liquid samples. E(HB) for OH···[PF6](-) H-bonds amounts 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···[A](-) H-bonds follows the order: [PF6] (2.4 kcal·mol(-1)) < [BF4] (3.3 kcal·mol(-1)) < [Tf2N] (3.4 kcal·mol(-1)) < [OTf] (4.7 kcal·mol(-1)l) < [TFA] (6.2 kcal·mol(-1)). The formation of aggregates of self-associated [C2OHmim](+) cations is present in liquid [C2OHmim][PF6], [C2OHmim][BF4], and [C2OHmim][Tf2N], with the energy of the OH···OH H-bonds amounting to ~6 kcal·mol(-1). Multiple secondary interactions in the bulk ILs influence their structure, vibrational spectra, and H-bond strength. In particular, these interactions can blue-shift the stretching frequencies of the CH groups of the imidazolium ring in spite of red-shifting CH···[A](-) H-bonds. They also weaken the H-bonding in the IL relative to the isolated ion pairs, with these anticooperative effects amounting to ca. 50% of the E(HB) value.

Application of time-dependent density functional theory and optical spectroscopy toward the rational design of novel 3,4,5-triaryl-1-R-1,2-diphospholes.

Twenty 3,4,5-triaryl-1-R-1,2-diphospholes were studied within the framework of density functional theory (DFT) and experimentally by UV/vis spectroscopy to check their suitability for opto-electronic applications. Time-dependent DFT (TD-DFT) calculations employing the PBE0 hybrid density functional combined with moderately sized def-TZVP basis set were shown to excellently reproduce the experimental absorption spectra of various 1,2-diphospholes. Frontier molecular orbital analysis reveals that HOMO and LUMO are mainly localized on the diphosphole ring and, to some extent, on the aryl moieties. The HOMO-LUMO energy gap can be easily tuned by variation of substituents introduced in para-positions of the aryl moieties and, to a lesser extent, by modification of the R group at phosphorus atom. As a result, both position and intensity of the absorption bands with highest wavelength are strongly influenced by the above structural changes. The UV-spectra simulations reveal that the introduction of donor groups like para-OMe, para-NMe2, and para-N(H)Ph, which are in n-π conjugation with the aryl moieties, should result in absorption of visible light by the corresponding 1,2-diphospholes, thus making them promising candidates for new functional materials.

Is there a simple way to reliable simulations of infrared spectra of organic compounds?

To assess the ability of the quantum-chemical computations to reproduce the experimental relative intensities in the infrared (IR) spectra of both the gas- and condensed-phase systems, the hybrid DFT functional B3LYP has been applied to simulation of IR spectra for species containing from three to twelve first- or second-row atoms, both in the gas phase and in CCl4 solutions. The results demonstrate that B3LYP, combined with the highly compact double-ζ basis set 6-31+G* and "scaled quantum mechanics" techniques, offers excellent quantitative performance in the calculations of relative IR intensities and frequencies (ν ≤ 2200 cm(-1)) for the bands of vibrations of medium-size isolated molecules, whereas it produces unsatisfactory results for the solutions of the same species. Neither larger basis sets nor implicit treatment of the media effects improve the agreement of the simulated spectra with the condensed-phase experiment. At the same time, some preliminary results suggest that explicit modeling of media effects could offer better quality of the IR spectral simulations for the condensed-phase systems.

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 [BF(4)](-) 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 Pd-Pd 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-[BF(4)](-) binding. IR and Raman spectral simulations reveal that surface-enhanced Raman spectroscopy is much less sensitive to interactions of Pd-NPs with the [BF(4)](-) anion compared to interactions with [Mmim](+) cations. In contrast, IR spectroscopy is better suited to study the anion-metal interactions, whereas IR spectral manifestations of the cation-metal interactions are rather modest. The splitting of the strong ν(as)(BF(4)(-)) IR band into three components appears to be a convenient spectroscopic marker for Pd-[BF(4)](-) interactions, which is confirmed by actual spectra of Pd-NPs stabilised by [Emim][BF(4)]. The IR spectra and their assignment with quantum chemical computations suggest that both the anions and cations of [Emim][BF(4)] 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.

A remarkable anion effect on palladium nanoparticle formation and stabilization in hydroxyl-functionalized ionic liquids.

Pd nanoparticles (NPs) with a small size and narrow size distribution were prepared from the decomposition of Pd(OAc)(2) in a series of hydroxyl-functionalized ionic liquids (ILs) comprising the 1-(2'-hydroxylethyl)-3-methylimidazolium cation and various anions, viz. [C(2)OHmim][OTf] (2.4 ± 0.5 nm), [C(2)OHmim][TFA] (2.3 ± 0.4 nm), [C(2)OHmim][BF(4)] (3.3 ± 0.6 nm), [C(2)OHmim][PF(6)] (3.1 ± 0.7 nm) and [C(2)OHmim][Tf(2)N] (4.0 ± 0.6 nm). Compared with Pd NPs isolated from the non-functionalized IL, [C(4)mim][Tf(2)N] (6.2 ± 1.1 nm), it would appear that the hydroxyl group accelerates the formation of the NPs, and also helps to protect the NPs from oxidation once formed. Based on the amount of Pd(OAc)(2) that remains after NP synthesis (under the given conditions) the ease of formation of the Pd NPs in the [C(2)OHmim](+)-based ILs follows the trend [Tf(2)N](-), [PF(6)](-) > [BF(4)](-) > [OTf](-) > [TFA](-). Also, the ability of the [C(2)OHmim](+)-based ILs to prevent the Pd NPs from undergoing oxidation follows the trend [Tf(2)N](-) > [PF(6)](-) > [TFA](-) > [OTf](-) > [BF(4)](-). DFT calculations were employed to rationalize the interactions between Pd NPs and the [C(2)OHmim](+) cation and the various anions.

Ab initio and DFT predictions of infrared intensities and Raman activities.

Relative infrared (IR) intensities and relative Raman activities have been computed for vibrations of test molecules, including from two to nine heavy atoms, using second-order Moller-Plesset perturbation theory (MP2), and three hybrid density functionals (B3LYP, M05, and M05-2X). The basis set convergence of vibrational properties is discussed. Our results demonstrate that B3LYP offers the most cost-effective choice for the prediction of molecular vibrational properties, but the predictions of another two tested hybrid functionals are very similar and in very good agreement with experimental data. MP2 shows good performance for the IR intensities, whereas the quality of prediction of the relative Raman activities should be characterized as only moderate. B3LYP calculations of the relative IR intensities using highly compact Sadlej's Z3PolX basis set retain the high accuracy of the more CPU expensive Sadlej's pVTZ and much more expensive aug-cc-pVTZ calculations. Relative Raman activities are more sensitive to basis set effects and require at least Sadlej's pVTZ to obtain quantitative results.

A simple physical model for the simultaneous rationalisation of melting points and heat capacities of ionic liquids.

The analysis of potential energy surfaces of ion pairs within the framework of an anharmonic oscillator model allows rationalization and prediction of melting points (T(mp)) and heat capacities (C(p)) of ionic liquids (ILs) comprising di- and trialkylimidazolium or tetraalkylphosphonium cations and weakly coordinating BF(4), PF(6), or Tf(2)N anions. Multiple short contacts between the counterions are demonstrated to be typical for the imidazolium based ILs. Differences in the types of contacts result in moderate changes of melting points of the ILs, comparable with differences in T(mp) experimentally determined for the same crystal. The theoretical evaluation of IL heat capacities additionally requires a consideration of conformational behaviour of the corresponding cations. A similar conformational composition of 1-butyl-3-methylimidazolium hexafluorophosphate and tetrafluoroborate at ambient temperature is demonstrated by the combined DFT-vibrational spectroscopy approach. A rough proportionality of C(p) to 1/T(mp) of ionic liquids is suggested, provided that the conformational composition of the ILs does not change on crystal-to-liquid transition.

IR and NMR spectra, intramolecular hydrogen bonding and conformations of para-tert-butyl-aminothiacalix[4]arene in solid state and chloroform solution.

It is demonstrated that dissolution of aminothiacalix[4]arene in chloroform results in transformation of 1,3-alternate conformation, adopted in single-crystal and bulk polycrystalline solids, to the pinched-cone form. This conformer is stabilised by the intramolecular hydrogen bonds of two distal amino-groups acting as H-donors with another two amino moieties that appear as H-acceptors. The H-bonds cause quite small (ca. 10-20 cm(-1)) red shift of the IR bands of the NH(2) stretching vibrations, which suggests rather weak NHcdots, three dots, centeredN hydrogen bonding. This latter is sufficient to stabilize the pinched-cone conformation in the chloroform solution, but the energy gap between the pinched-cone and other conformations is small, and solid-state intermolecular forces easily overcome it, leading to realisation of the 1,3-alternate conformer. The comparison of the DFT computed and experimental vibrational and NMR spectra demonstrates good quality of present quantum-chemical computations, allows complete interpretation of the spectra and reveals simple IR and NMR spectroscopic markers of the conformers of aminothiacalix[4]arenes.

Revisiting ether-derivatized imidazolium-based ionic liquids.

A series of ether-derivatized imidazolium halides have been prepared and characterized. Contrary to literature reports, they are all crystalline solids and have melting points well above room temperature (50-100 degrees C). Single crystals of the imidazolium salts, obtained in situ by slow cooling from their molten state to room temperature, were analyzed by X-ray crystallography, revealing various anion-cation interactions in the solid state. Exchange of the halides with [Tf(2)N]- yielded room temperature ionic liquids with viscosities that are comparable to related 1-alkyl-3-methylimidazolium ionic liquids. Density functional theory combined with IR spectroscopy has been used to analyze the role of functionalization of the imidazolium side chain on the formation of the molecular and supramolecular structure of the compounds and its possible impact on their physical properties.

Application of density functional theory and vibrational spectroscopy toward the rational design of ionic liquids.

Density functional theory methods in combination with vibrational spectroscopy are used to investigate possible variants of molecular structure of the ion pairs of several imidazolium-based ionic liquids (ILs). Multiple stable structures are determined 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 liquid 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 association 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 separated and to regard the couple of the ions as an anharmonic oscillator. Such a model correlates the molecular structure of various ILs and their melting points without involving the energy of the interaction between the cations and anions but explains structure-melting point correlations on the grounds of quasy-elastic properties.