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Short- and long-range liquid structures of [CnmIm+][TFSA-] with n = 2, 4, 6, 8, 10, and 12 have been studied by high-energy x-ray diffraction (HEXRD) and small-angle neutron scattering (SANS) experiments with the aid of MD simulations. Observed x-ray structure factor, S(Q), for the ionic liquids with the alkyl-chain length n > 6 exhibited a characteristic peak in the low-Q range of 0.2-0.4 Å -1, indicating the heterogeneity of their ionic liquids. SANS profiles IH(Q) and ID(Q) for the normal and the alkyl group deuterated ionic liquids, respectively, showed significant peaks for n = 10 and 12 without no form factor component for large spherical or spheroidal aggregates like micelles in solution. The peaks for n = 10 and 12 evidently disappeared in the difference SANS profiles ΔI(Q) [=ID(Q) - IH(Q)], although that for n = 12 slightly remained. This suggests that the long-range correlations originated from the alkyl groups hardly contribute to the low-Q peak intensity in SANS. To reveal molecular origin of the low-Q peak, we introduce here a new function; x-ray structure factor intensity at a given Q as a function of r, SQpeak(r). The SQpeak(r) function suggests that the observed low-Q peak intensity depending on n is originated from liquid structures at two r-region of 5-8 and 8-15 Å for all ionic liquids examined except for n = 12. Atomistic MD simulations are consistent with the HEXRD and SANS experiments, and then we discussed the relationship between both variations of low-Q peak and real-space structure with lengthening the alkyl group of the CnmIm. [ABSTRACT FROM AUTHOR]

The solvation structure of 1,3-butanediol (1,3-BD) in aqueous binary solvents of acetonitrile (AN), 1,4-dioxane (DIO), and dimethyl sulfoxide (DMSO) at various mole fractions of organic solvent x OS has been clarified by means of infrared (IR) and ¹H and 13C NMR. The change in the wavenumber of O-H stretching vibration of 1,3-BD in the three systems suggested that water molecules which are initially hydrogen-bonded with the 1,3-BD hydroxyl groups in the water solvent (x OS = 0) are more significantly replaced by organic solvent molecules in the order of DMSO ≫ DIO > AN. This agrees with the order of the electron donicities of the organic solvents. The ¹H and 13C chemical shifts of 1,3-BD also revealed the most remarkable replacement of water molecules on the hydroxyl groups by DMSO. In contrast to the DMSO system, the O-H vibration band of 1,3-BD in the AN and DIO systems suggested the formation of the intramolecular hydrogen bond between the two hydroxyl groups of 1,3-BD above x OS = ~0.9. To further evaluate the intramolecular hydrogen bonding of 1,3-BD in AN-water binary solvents, molecular dynamics (MD) simulations and NMR experiments for spin-lattice relaxation times T 1 and ¹H-¹H nuclear Overhauser effect (NOE) were conducted on 1,3-BD in the AN system. These results showed the intramolecular hydrogen bond within 1,3-BD in the AN-water binary solvents in the high AN mole fraction range of x AN > 0.9. Especially, the pair correlation functions g(r) of the OH-O interactions of 1,3-BD obtained from the MD simulations indicated that the intramolecular hydrogen bond remarkably increases in the AN solvent as the x AN rises to the unity. [ABSTRACT FROM AUTHOR]

• The BF 4 − anion and in particular its F atoms, is, as expected, located close to the hydrogen atoms of the C153. • The cation, in particular its ring hydrogen atoms, are located close to the F and O atoms of the C153. • The PC solvent molecules are located in the direct environment of the H atoms of C153. In this work we report the results of the molecular dynamics (MD) simulation of the effect of the mixture composition of 1-butyl-3-methylimidazolium terafluoroborate (BmimBF 4) and propylene carbonate (PC) on the solvation structure of coumarin 153. For this purpose, the local structure around C153 was characterized using distance and angle descriptors of the C-H...A configuration. They are based on the calculation of the nearest neighbor radial distributions between the C or H atoms of one component of the solution (C153, PC, Bmim+) and the electronegative A atoms (F, O) of the other ones. We also considered the combined distribution functions that include the radial and the orientation between the ring of either the cation or the solvent and that of C153. Our results show that in neat ionic liquid, the BF 4 − anion and in particular its F atoms, is, as expected, located close to the hydrogen atoms of the C153, while the cation, in particular its ring hydrogen atoms, are located close to the F and O atoms of the C153 and its ring has an almost the above/below parallel orientation with respect to the C153 lactone ring. In neat PC, its O atoms are located in the direct environment of the H atoms of C153 and concomitantly, its ring has the above/below parallel orientation with respect to the C153 rings. By comparing the distance and angle descriptors values of the C-H...A configuration, with a threshold hydrogen bond distance 0.30 nm and an angle of 151°, we were able to rank the C-H...A interactions as weak hydrogen bonds. Furthermore, in the excited state, noticeable changes in charge values on the H, F and O atoms of C153, occurs and as a consequence, the radial distribution of the anion, cation and the solvent around these atoms of C153 is more affected. At the mixture composition around 0.20 mol fraction of BmimBF 4 , the behavior of the distance angle descriptors of the solvation of C153, indicates a cross over between the situation where solvation is dominated by the ions to that dominated by the solvent molecules. In the excited state of C153, the distance descriptor values are slightly decreasing with respect to those in the ground state while the angle descriptor values remain almost the same. [ABSTRACT FROM AUTHOR]

The hydrogen-bonding interactions in protic and aprotic amide solvent mixtures, i.e., formamide (FA) and N , N -dimethylformamide (DMF), were investigated via low-frequency Raman spectroscopy, small-angle neutron scattering (SANS) experiments, and molecular dynamics (MD) simulations. In a neat amide system, the low-frequency Raman spectra R ( ν )s were well reproduced by the corresponding S ( ν ) spectra derived from the MD simulations. The observed peaks in R ( ν )s at around < 200 cm − 1 were assigned to the intermolecular interactions, particularly in terms of the hydrogen-bonding network formation and its dimensionality in the liquid state. The SANS experiments for the FA–DMF mixtures demonstrated that the FA molecules forming an extended three–dimensional hydrogen-bonding structure in the neat system interacted with DMF molecules through the hydrogen bonds in the mixtures over the whole range of solvent compositions, resulting in a homogeneous mixing state. Additionally, the R ( ν ) spectra for the mixtures were represented by the corresponding S ( ν ) spectra. From the R ( ν ) and S ( ν ) spectra of the FA–DMF mixtures, we found that (1) the Raman band at around 110 cm − 1 mainly originates from the libration mode of amide molecules in the chain-like hydrogen-bonded structure and (2) the higher frequency band (approximately 200 cm − 1 ) was attributed to the libration of the FA molecule restricted by the three-dimensional hydrogen-bonded network, which remained even in the DMF-rich compositions. [ABSTRACT FROM AUTHOR]

The intermolecular interactions in mixtures of ionic liquids (ILs) of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (C n mimTFSA) and dimethyl sulfoxide (DMSO) have been elucidated by measurements of refractive indexes, densities, and ATR-IR spectra as a function of the alky chain length ( n = 2–12) of C n mim + and DMSO molar fraction x DMSO . The excess quantities of refractive indexes and molar volumes showed that in all C n mimTFSA + DMSO systems the packing of IL and DMSO is the loosest around 1:3 molar ratio of IL to DMSO. However, the significant alkyl chain dependence was not observed in both quantities. This suggests that the hydrogen bonding of the three imidazolium hydrogen atoms with DMSO molecules mainly contributes to the loose packing. From the ATR-IR spectra measured for C n mimTFSA + DMSO mixtures, the excess molar absorbance in the wavenumber ranges of the imidazolium ring C H vibrations and the S O vibrations of TFSA − anion and DMSO was depicted as a function of DMSO content. The excess molar absorbance of the C H vibrations revealed that the three imidazolium C 2,4,5 H sites, particularly the C 2 H, are progressively hydrogen-bonded with DMSO as the molar fraction increases to x DMSO ≈ 0.6. The S O vibrations of TFSA − anion suggested that TFSA − anions are eliminated from the imidazolium rings with increasing x DMSO to the same. Hence, TFSA − anions are replaced by DMSO around the imidazolium rings with increasing DMSO content. However, the hydrogen bonding between them is not significantly influenced by the cation's alkyl chain. The partial molar absorbance of the S O vibrations of DMSO showed that DMSO molecules self-aggregate in the mixtures above x DMSO ≈ 0.6. However, the aggregation of DMSO molecules may be suppressed with the longer alkyl chain of C n mim + . [ABSTRACT FROM AUTHOR]

Description of the local microscopic structure in ionic liquids (ILs) is a prerequisite to obtain a comprehensive understanding of the influence of the nature of ions on the properties of ILs. The local structure is mainly determined by the spatial arrangement of the nearest neighboring ions. Therefore, the main interaction patterns in ILs, such as cation-anion H-bond-like motifs, cation-cation alkyl tail aggregation, and ring stacking, were considered within the framework of the nearest-neighbor approach with respect to each particular interaction site. We employed classical molecular dynamics (MD) simulations to study in detail the spatial, radial, and orientational relative distribution of ions in a set of imidazolium-based ILs, in which the 1-butyl-3-methylimidazolium (C4mim+) cation is coupled with the acetate (OAc-), chloride (Cl-), tetrafluoroborate (BF4 -), hexafluorophosphate (PF6 -), trifluoromethanesulfonate (TfO-), or bis(trifluoromethanesulfonyl)amide (TFSA-) anion. It was established that several structural properties are strongly anion-specific, while some can be treated as universally applicable to ILs, regardless of the nature of the anion. Namely, strongly basic anions, such as OAc- and Cl-, prefer to be located in the imidazolium ring plane next to the C-H2/4-5 sites. By contrast, the other four bulky and weakly coordinating anions tend to occupy positions above/below the plane. Similarly, the H-bond-like interactions involving the H² site are found to be particularly enhanced in comparison with the ones at H4-5 in the case of asymmetric and/or more basic anions (C4mimOAc, C4mimCl, C4mimTfO, and C4mimTFSA), in accordance with recent spectroscopic and theoretical findings. Other IL-specific details related to the multiple H-bond-like binding and cation stacking issues are also discussed in this paper. The secondary H-bonding of anions with the alkyl hydrogen atoms of cations as well as the cation-cation alkyl chain aggregation turned out to be poorly sensitive to the nature of the anion. [ABSTRACT FROM AUTHOR]

1 H NMR measurements were conducted on aqueous mixtures of ethanol (EtOH) and 2,2,2-trifluoroethanol (TFE) at 25.0 and −10.0 °C with varying alcohol mole fraction x alc . At 25.0 °C the TFE hydroxyl proton and water protons in TFE–water mixtures could not be individually observed on the NMR time scale due to the fast exchange between both protons. By cooling at −10.0 °C, the peaks of the alcohol hydroxyl and water protons became distinguishable in both aqueous EtOH and TFE mixtures over the entire x alc range. The alcohol hydroxyl and water protons in both alcohol mixtures at −10.0 °C were gradually deshielded with decreasing x alc , suggesting the increase in the number of the hydrogen bonds in the mixtures. In particular, the NMR results showed that the hydrogen bonds between the alcohol hydroxyl group and water strengthen in both mixtures below x alc ≈ 0.2. Moreover, the water structure by hydrogen bonding among water is enhanced in the mixtures below the same mole fraction. To clarify the contribution of the alcohol hydroxyl hydrogen and oxygen atoms in the hydrogen bonding for both mixtures, molecular dynamics (MD) simulations were made on aqueous EtOH and TFE systems at −10.0 °C. The hydrogen bonding abilities of the EtOH hydroxyl hydrogen and oxygen atoms are comparable with each other. The ability of the TFE hydroxyl hydrogen is higher than EtOH, while that of the TFE oxygen is much lower than EtOH. The present results suggested that the higher hydrophobicity of TFE compared to EtOH is mainly attributed to the two lone pairs of the oxygen with the lower electron donicity. [ABSTRACT FROM AUTHOR]

Abstract: Large angle X-ray scattering (LAXS) experiment for 1-ethyl-3-methylimidazolium tetrafluoroborate [EMI+][BF4 −] ionic liquid was carried out at 298 K, to reveal the closest ion–ion interaction in the ionic liquid. The intra-molecular atom–atom correlations based on the molecular geometries in crystals were subtracted from the total pair correlation function to yield the inter-molecular pair correlation function. In the inter-molecular pair correlation function, peaks of 3.4, 4.4 and 5.5 Å were successfully extracted as the closest ion–ion interactions. Molecular dynamics simulations based on the effective pair potentials were also performed to ascribe the peaks found in the experimentally evaluated inter-molecular pair correlation function at an atomistic level. The X-ray interference function derived from MD simulations and thus Fourier transferred X-ray weighted pair correlation function were reasonably agreement with those obtained by the experiment, the extracted peaks in the experimental inter-molecular pair correlation function can be appropriately attributed to the C (EMI+) - F (BF4 −) atom–atom correlations. In addition, artificial MD simulations employed rigid models for the nonplanar and the planar EMI+ isomers were performed to obtain further insight into the effect of the conformational isomerism on the ion–ion interactions in the ionic liquid. It turned out that the ion–ion interaction, namely the anion orientation around the cation, depends on the conformational isomerism of the cation from the artificial MD simulations. [Copyright &y& Elsevier]

Abstract: Large angle X-ray scattering of the N-butyl-N-methylpyrrolidinium bis-(trifluoromethanesulfony) amide (P14 +TFSA−) ionic liquid was measured at 298 K. The total interference function i LAXS(s) and the total pair correlation function G LAXS(r) were successfully obtained. The i LAXS(s) was analyzed in terms of the intra-molecular geometries of the conformational isomers of the P14 + and TFSA− ions based on the crystal structures, the inter-molecular correlation function G inter LAXS(r) and the inter-molecular radial distribution function as the form D inter LAXS(r)−4πr 2 ρ 0 were evaluated. With D inter LAXS(r)−4πr 2 ρ 0, the inter-molecular correlation peaks at 3.5, 4.5 and 5–6 Å evidently appeared as the short range interaction, while considerable long range interactions were found as the broad peaks at 10 and 15 Å. Molecular dynamics simulations of the ionic liquid based on the effective pairwise potentials were carried out. The X-ray total interference function derived from the MD simulations i MD(s) was in agreement with the experimental one. According to the inter-molecular partial atom–atom correlation functions from MD simulations, the observed peaks at 3.5, 4.5 and 5–6 Å can be predominantly ascribed to the closest atom–atom correlations between the P14 + and TFSA– ions such as C···O and C···F. In the inter-molecular partial atom–atom correlation function between terminal methyl carbons of the butyl chain in the P14 + ion, a peak at 4.0 Å was found as the inter-molecular correlation, suggesting that the ionic liquid shows an alkyl chain aggregation, which is similar to that observed for 1-alkyl-3-methylmidazolium ionic liquids. [Copyright &y& Elsevier]

To clarify the unique solvation properties of 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) on a biomolecule of L-phenylalanine (Phe), the solvation structure of Phe in HFIP-water mixed solvents at 70 mmol dm−3 has been observed over the entire range of HFIP mole fraction x HFIP using infrared (IR), 1H and 13C NMR techniques with a help of molecular dynamics (MD) simulation. The solvation structure of Phe in the HFIP solvents has been compared with that in 2-propanol (2-PrOH)-water mixed solvents and the structure of L-leucine (Leu) in both alcohol-water mixed solvents previously reported. The results from IR, NMR, and MD simulations showed that water molecules hydrogen-bonded with the Phe carboxylate group are gradually replaced by HFIP with increasing x HFIP. In contrast, the replacement of water molecules by HFIP on the Phe aminium group does not easily take place. These findings arise from the high electron acceptability and the very low electron donicity of the HFIP hydroxyl group due to the electron drawing of the six fluorine atoms. In the 2-PrOH solutions, the replacement of water on the carboxylate group by 2-PrOH less easily occurs with increasing x 2-PrOH compared to HFIP because of the lower electron acceptability of 2-PrOH. The solvation for the Phe hydrophilic parts is similar to that of Leu. The most significant difference between HFIP and 2-PrOH was observed for the solvation of the Phe phenyl group. The hydrophobic hydration shell around the phenyl group is collapsed with increasing alcohol content in both alcohol solutions. Instead of water, the HFIP fluorine atoms significantly interact with the phenyl hydrogen atoms, whereas 2-PrOH molecules do not markedly approach the hydrogen atoms. The conformational change of Phe molecule against the dihedral angle of Cγ-Cβ–Cα-COO− takes place with increasing HFIP content, but does not with the increase in 2-PrOH. This is attributed to the steric hindrance between the large phenyl group and the carboxylate group remarkably solvated by HFIP molecules. • Instead of water, HFIP molecules are hydrogen-bonded with the Phe carboxylate group. • HFIP molecules do not easily hydrogen-bonded with the Phe aminium group. • The hydrophobic hydration shell around Phe is collapsed with the increase in HFIP. • The specific interaction acts between the phenyl group and HFIP, but not 2-PrOH. • The conformational change of Phe occurs with increasing HFIP content. [ABSTRACT FROM AUTHOR]