Probing Molecular Interactions between ... - ACS Publications

Subscriber access provided by University of Otago Library

Article

Probing Molecular Interactions between Ammonium-based Ionic Liquids and N,N-Dimethylacetamide: A Combined FT-IR, DLS and DFT Study Pannuru Kiran Kumar, Anjeeta Rani, Lukman O. Olasunkanmi, Indra Bahadur, Pannuru Venkatesu, and Eno E. Ebenso J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b07535 • Publication Date (Web): 14 Nov 2016 Downloaded from http://pubs.acs.org on November 14, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry B is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Probing Molecular Interactions between Ammonium-based Ionic Liquids and N,N-Dimethylacetamide: A Combined FT-IR, DLS and DFT Study Pannuru Kiran Kumara, Anjeeta Ranib, Lukman O. Olasunkanmia,c, Indra Bahadura∗ and Pannuru Venkatesub∗, Eno E. Ebensoa a

Department of Chemistry, School of Mathematical and Physical Sciences, Materials Science Innovation & Modelling (MaSIM) Research Focus Area, Faculty of Agriculture, Science and Technology, North-West University (Mafikeng Campus), Private Bag X2046, Mmabatho 2735, South Africa b Department of Chemistry, University of Delhi, Delhi 110 007, India c Department of Chemistry, Faculty of Science, Obafemi Awolowo University, Ile-Ife, 220005 Nigeria

ABSTRACT The present study investigates the effects of alkyl chain length of the cationic head-group of some ammonium-based ionic liquids (ILs) (having the same anion) on the interaction between the ILs and N,N-dimethylacetamide (DMA). The molecular interactions between the studied ILs, tetraethylammonium hydroxide (TEAH), tetrapropylammonium hydroxide (TPAH) and tetrabutylammonium hydroxide (TBAH) and their binary mixtures with N,N-dimethylacetamide (DMA) were studied using the Fourier transform infrared spectroscopy (FT-IR) technique, dynamic light scattering (DLS) experiment, and quantum chemical calculations. It was observed from both experimental FTIR analysis and theoretical studies that the strength of intermolecular interactions such as hydrogen bonding, ion-ion pair interactions and induced dipole interactions between the ILs and DMA depend on the alkyl chain length of the IL cation head-group. The interaction of DMA with IL is energetically favourable and occurs via direct interactions between the IL anion and DMA carbonyl oxygen. The results further revealed that the shorter the alkyl chain length of the cationic head-group of the ILs, the stronger the interaction with DMA molecule, such that the strength of interactions between the ILs and DMA follows the order TEAH > TPAH > TBAH. This trend can be attributed to the increased self-organized aggregation with increasing alkyl chain length of the IL cation.

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 35

INTRODUCTION Recently, ionic liquids (ILs) are considered as a unique class of novel solvents that are intensely being used in many promising applications due to an amazing and wide variety of desirable properties.1-5 ILs have been emerging with significant awareness from research and industry communities for a modern scientific of novel applications. The knowledge of molecular interactions of the ILs and their mixtures with molecular solvents are essential for many of chemical industry applications.6-8 An interesting aspect is that a slight variation in the constituent ions of ILs can lead significant differences in their physicochemical properties, which are known to show tunable properties that excellent thermal stability, the liquid state with wide temperatures, immeasurable of electrochemical window and so on.9-12 Subsequently, the addition of ILs to the molecular solvents has been found to strongly affect various properties of ILs that includes density, viscosity and conductivity. Moreover, ILs are vastly used as possible alternative for harmful conventional molecular solvents in the many scientific and industrial applications because of the recyclability and eco-friendly nature of ILs.13,14 Despite, ILs have been hugely used in acid catalyst for alkylation reaction and pharmaceutical applications such as separation and crystallization.15-17 The acetamide compounds are very motivating molecules because of consisting the -C=O and -NH- groups, which are of fundamental significance as models for biomolecules purposes in the chemical laboratories, elucidating intermolecular interactions in the biomolecules as well as are the vital solvents for practical purposes. In this context, N,N-dimethylacetamide (DMA) is highly polar compound and highly soluble in a variety of polar and non-polar solvents and readily suitable solvent to explore dipole-dipole or solvent-solvent interactions in the mixture.1822

Moreover, it is most principally used for polymer dissolution in man-made fiber production

industry because it possesses a very common in nature donor-acceptor -CO-NH- peptide bond and shows the property of self-assembly by hydrogen bond.23,24 Furthermore, DMA is huge fascinating considerable attention to form ion-pair interaction with other components in the mixture because of it has more electronegative atoms such as oxygen and nitrogen.25 ILs belong to a class of neoteric solvents that is made up of bulky, asymmetric organic cations and inorganic or organic anions to create a nonaqueous solvating medium and these ILs are easily mingling with polar solvents.6,26,27 The physicochemical properties of ILs with 2 ACS Paragon Plus Environment

Page 3 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


molecular solvents are desperately used to understand the characterization of molecular interactions between the unlike molecules in the mixtures.28-30 Among various families of ILs, ammonium-based ILs are identified and intense interest to display their wide range of applications in chemical and biochemical processes.1,5-7 An adequate information of molecular interactions and mechanism of ammonium-based ILs with polar solvents are essentially required to design chemical and biotechnological processes. In this regard, in recent years, numerous research

articles

on

ammonium-based

ILs

with N,N-dimethylformamide (DMF),31-33

dimethylsulfoxide (DMSO),34-38 N-Methyl-2-pyrrolidone (NMP),39-41 and with water42,43 by our research group have focused on the obtaining molecular interactions and structural effects of these solvents with the ILs. To the best of our knowledge, the available literature data on the structural conformation and molecular interactions between ILs and DMA are very limited.44-47 More so, the molecular interactions between ammonium-based ILs and DMA have not been widely explored. The present work was carried out to understand the nature of intermolecular interactions of ammonium-based ILs such as tetraethylammonium hydroxide (TEAH), tetrapropylammonium hydroxide (TPAH) and tetrabutylammonium hydroxide (TBAH) with DMA. In this regard, Fourier transform infrared spectroscopy (FT-IR) technique, dynamic light scattering (DLS) measurements, and density functional theory (DFT) calculations have been employed in order to get insight into the molecular interactions between the ions of ammonium-based ILs and DMA solvent. In the overall, the data have been successfully employed to gain better understanding of the strength of molecular interactions between ions of ammonium family ILs and DMA as well as studied the effect of alkyl chain length of IL with DMA on the molecular interactions in the mixtures. EXPERIMENTAL SECTION Materials. The three ammonium-based ILs such as TEAH, TPAH and TBAH are synthesized in our laboratory and the detailed procedure elucidated in our prior paper.40 DMA is purchased from Sigma Aldrich Co. USA. All three ILs had low levels of water (below 70 ppm) as analyzed by a Karl Fischer titrator from Analab (Micro Aqua Cal 100) after purified the chemicals. The different mole fractions of the samples were prepared with an accuracy of ±0.0001 g by



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 35

weighing the components using a Mettler Toledo balance for all measurements. The estimated uncertainty on the mole fraction was found to be less than 5 × 10−4. Fourier Transform Infrared Spectroscopy (FTIR) Characterization of Ammonium-based ILs with DMA Mixtures. To unveil the interaction between DMA and ILs at different compositions, we examined the changes in mid IR bands. Infrared spectra of binary mixtures of DMA and ILs at various compositions were recorded in the range of 4000 cm-1 to 500 cm-1 at 25 0

C by using a thermo scientific FTIR spectrometer. All the samples were pre-equilibrated for 1h.

Samples were held with ZnSe windows by using 15 µm path length teflon spacers in an IR cell. 256 scans were performed at 2 cm-1 resolution and averaged. Omnic software was used to analyse FTIR spectra. The error in the band position was not exceeded 1nm. Dynamic Light Scattering (DLS) Measurements. The dynamic light scattering measurements were performed by Zetasizer Nano ZS90 (Malvern Instruments Ltd., UK) with He-Ne laser (4 mW) at a scattering angle of 90o and a fixed wavelength (633 nm) and emitting vertically polarized light. The instrument was equipped with an inbuilt thermostatic sample chamber for maintaining temperature equal to 25 oC in an airtight quartz cell where sample was loaded. The instrument detection range was from 0.1 nm to 10 µm. All samples were filtered through 0.45 µm pore size PTFE milipore syringe filter. Specifically, by analyzing the scattered light fluctuations, DLS size distributions are obtained. The scattered light fluctuations are related to the size of particle, viscosity and refractive index. Therefore, the values of refractive index, dielectric constant and viscosity were also taken into consideration for all binary mixtures. Each measurement consisted of three subsequent individual runs and averaged of three concordant reading. From the time dependent fluctuations in the scattering intensity of light, this instrument measured the translational diffusion coefficient. The hydrodynamic diameter (dH) was attained from diffusion coefficient by means of well-known Stokes-Einstein equation by the instrument software and all data were further evaluated by plotting using origin 9 software (OriginLab Corp., Northampton, MA, USA). Quantum Chemical Studies Geometry optimizations and frequency calculations were performed on the studied ILs and their corresponding IL-DMA complexes in the gas phase. The Density Functional Theory (DFT) 4 ACS Paragon Plus Environment

Page 5 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


method was employed for all the calculations. The adopted model comprises the Becke 3parameter exchange functional together with the Lee-Yang-Parr correlation functional (B3LYP),48,49 used in combination with the 6-31+G(d,p) basis set. The adopted functional is a widely used and famous functional that produces satisfactory geometry and electronic parameters with moderately inexpensive computational resources.50,51 It has also been successfully used to obtain optimized geometries of ILs.52,53 The Pople type double-zeta polarized basis set, 6-31+G(d,p) was used for all the calculations. The adopted basis set adds diffuse functions and d-type polarization functions on non-hydrogenic orbitals, and also p-type polarization functions on hydrogenic orbitals. The model (B3LYP/6-31+G(d,p)) used in this work has been reported to produce comparable results (at 1% level of closeness) to B3LYP/6-31++G(d,p) and nearly the same geometry parameters as MP2/6-31++G(d,p) for similar systems.54 However, an attempt was made to utilize some higher level models in the present work, but these exercises, which might eventually be redundant, were found to be too expensive for our current computational gadgets. All the optimized structures were confirmed to correspond to true energy minima by the absence of imaginary frequency in the force constant calculations. Though the work of Davies et al.,54 provided some explanations on anion-cation interactions between some trimethylammonium, tetramethylammonium, and trimethylethylammonium, and selected anions (including hydroxide ion), a more robust analysis of these interactions using the natural bond orbital (NBO) theory was left out. Therefore, NBO analyses were also carried out in the present work, using the default NBO construct in Gaussian 09 to gain further insights into the nature of cation-anion interactions within the IL systems as well as the intermolecular interactions between the IL and DMA molecules. Donor-acceptor interaction energies were derived by solving the second order Fock matrix and the salient second order energy (E2) values were recorded for each system. The E(2) associated with the delocalization from a donor (i) to an acceptor (j) was calculated as:55,56

E (2) = ∆Eij = −ni where σ i F σ j

2

σi F σ j ε j − εi

2

= − ni

Fij2 ∆ε ij

(1)

or Fij2 is the off diagonal Fock matrix element between the i and j NBOs,



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 35

ε i and ε j are the energies of i and j NBOs, and ni is the occupancy or population of the i (donor) NBO. Initial geometries and molecular modeling of the ILs and their complexes with DMA were modelled with GaussView 5.0., while all quantum chemical calculations were carried out using the Gaussian 09 software suite (version D.01).57

RESULTS AND DISCUSSION The results are delineated on the molecular interactions in the binary mixtures of different alkyl chain length of ammonium hydroxide-based ILs and DMA at atmospheric pressure and at room temperature. Molecular Interaction between IL and DMA by FTIR Spectroscopic Analysis. To obtain thriving information about the interaction between IL and DMA, FTIR has been successfully employed for binary mixtures of IL and DMA with varying composition. Figure 1 shows FTIR spectra of binary mixtures of IL and DMA with different composition in the range of 3600-3100 cm-1 where the broad band arising from the O-H stretching of IL can be identified. This broad band between 3500-3400 cm-1 may be attributed to the interaction of DMA to the IL as the absorbance and position of IR bands change significantly as function of increasing xIL. The IL and DMA molecules may be bonded to a slightly different extent, thereby, resulting into a broad band. However, this band is not symmetric and exhibits additional shallow band at the low frequency site (Figure 1). This shallow located at lower wavenumber is very difficult to analyze as it is overlapped due to the contribution from both overtone frequency of the C=O stretching vibration (νC=O) of a DMA molecule at ~3283 cm-1 and stretching vibration from very strongly bonded O-H group of IL.58 In case of TEAH, as can be seen clearly in Figure 1a, the band at 3490 cm-1 is red shifted by 70 cm-1 with increase in xIL from 0.1 to 0.5 (DMA rich region) which may put emphasis on the strong H-bonding interaction between carbonyl group of DMA and anion of IL which increases with augment in the concentration of IL. Undoubtedly, the ion-dipole interactions between DMA and IL are more favorable as compared to the dipole-dipole interactions in DMA


Page 7 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


solvent. Moreover, hydrophobic interactions between DMA molecules also decrease, thereby, solvent clusters are breaking down. Furthermore, at xIL > 0.5, there is shifting of band towards higher wavenumber (O-H stretching band position moves from 3420 to 3455 cm-1 in Figure 1a), indicating decreased interaction between DMA and IL. In addition, the absorbance is also decreased after increase in xIL > 0.5 which may be attributed to the formation of self-organized IL clusters at these higher concentrations leading to decreased number of O-H stretching vibrations. These aggregates in IL rich region result from increased inter-ionic interaction between ions of ILs.59 Consequently, this enhanced 3D network of IL results in the reduced DMA and IL interaction, hence, increased stretching wavenumber. Accordingly, it can be stated here that two type of clusters are formed; one in DMA rich region and second in IL rich region. As there is increase in the size of alkyl chain length in cation of ILs, there is appearance of O-H stretching band vibration at higher wavenumber (Figures 1a-c) which may be due to the increased hydrophobic effects of large alkyl groups of IL causing a decreased interaction of DMA with IL. The corresponding band, therefore, is located at 3524 cm-1 in case of TBAH and shifts to 3425 cm-1 resulting from the increased interaction with DMA as a function of xIL (0.1 to 0.5 in Fig. 1c). The DMA and IL interactions varies in order; TEAH> TPAH> TBAH as is obvious from Figure 1. Figure 2 depicts the variation of peak position of aforementioned bands arising from DMA bonded to IL (peaks around 3490, 3500 and 3524 cm-1 for TEAH, TPAH and TBAH at xIL ≈ 0.1, respectively, as shown in Figures 1 a-c). With increase in the xIL, the wavenumber of corresponding band decreases first and then slightly increases after xIL > 0.5. In case of high concentration of TPAH and TBAH, there is increase in hydrophobic interactions between DMA and IL alkyl groups as well as between IL-IL alkyl groups which escort to the almost constant band frequencies. Furthermore, these large sized cations may be getting aggregated into big cluster with irregular 3D network of cation and anion due to strong hydrophobic attractions between large alkyl groups of IL. Thereby, these ILs exhibit somewhat lower frequency O-H vibrations at their extreme concentrations as compared to that of TEAH in which compact and regular 3D packing of ions appreciably diminish interaction with DMA. The bands ranging 2800-3100 cm-1 did not show any huge change in the peak position with change in the 7 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 35

composition of mixtures of IL and DMA (Figure 1). In addition, this region is very complex to analyze due to the presence of large number of alkyl groups in both DMA as well as IL. The behavior of C=O stretching modes of DMA can also be well explained on the basis of solvent-solvent and solute-solvent interactions. A strong band at ~1652 cm-1 is due to the C=O stretch of DMA.60 For pure DMA and mixtures with lower mole fraction of IL, this band is broad that may be due to the variation in the interaction of carbonyl of DMA (Figure 3). In Figures 3 ac, the peak values shown for pure DMA and various mixtures with lower mole fraction of ILs, are average values for the broad band intended for sake of more clarity. Apparently, there may be dipole-dipole interaction between DMA molecules or weak H-bonding or ion–dipole interaction between ions of IL and carbonyl of DMA. In fact, ion–dipole between DMA and IL interactions are stronger and contribute mainly with change in xIL from 0.1 to 0.5. Therefore, νc=o shifts to lower wavenumber side and band becomes narrow as a function of increasing xIL. The extent of interaction between DMA and IL varies from IL to IL depending upon the size of alkyl groups in the cation of IL. In comparison, the IR bands are located at higher wavenumbers for large sized IL pointing towards less affordable interactions with DMA and follow the order as TBAH>TPAH>TEAH. At very high concentration of IL (Figures 3 a-c), there are enhanced ion-ion interactions in ILs and IL clusters are formed. Thereby, the IL and DMA interaction decreases and C=O stretching frequency increases. In general, the stretching wavenumbers for C=O bonds (Figure 3) are found to be in same order in different ILs as for O-H stretching modes in Figure 1. Nonetheless, there is simultaneous appearance of new bands at 1580, 1580 and 1582 cm-1 at xIL≈ 0.1 of TEAH, TPAH and TBAH, respectively, as can be effortlessly speculated in Figures 3 a-c. The intensity of original band at ~1652 cm-1 decreases. The appearance of low frequency band may be accredited to the very strong interaction between anion of IL and DMA. This interaction is increased as a function of IL concentration. Thus, there is increase in intensity as well as shifting of band towards lower wavenumbers (~1568, 1570 and 1575 cm-1 correspondingly for TEAH, TPAH and TBAH) as a function of increasing IL concentration till 0.5 (Figure 3). The lowest frequency band for TEAH may be bestowed to its greater tendency to interact with DMA. Furthermore, at xIL > 0.5, the strength of this band lessened which may put stress on the absence of any strong interaction between DMA and IL. Certainly, it can be 8 ACS Paragon Plus Environment

Page 9 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


observed again here that a very similar trend is established for stretching vibrations of O-H as well as C=O bond. Alternatively, the wavenumber for bending/deformation IR bands follows the opposite trends in comparison to the stretching vibrations. Two bands observed at 1412 and 1397 cm-1 are corresponding to N(CH3) and C(CH3) deformations of DMA61, respectively (black spectra in Figure 4). With increase in the mole fraction for all ILs, there is increase in deformation frequency which again supports the earlier results comprising the strong interaction between DMA and IL at xIL ≤ 0.5 (shown in Figure 4). However, at xIL > 0.5, the bands at lower wavenumbers indicate decreased interactions between DMA and IL. At extreme higher concentrations of IL, C(CH3) deformation frequency decreases to more extent in case of TBAH (1397 cm-1 for TEAH, 1394 cm-1 for TPAH and 1383 cm-1 for TBAH at xIL ≈ 0.90). This can be dictated here that the large sized alkyl group of cation in TBAH escort the less or no chances of the interactions to the DMA and in turn, offers easier C(CH3) deformation modes. Furthermore, the intensity of N(CH3) deformation is totally vanished due to roughly no contribution of N(CH3) bending modes at very high xIL. Additionally, C-CH3 rocking modes60 at 1014 cm-1 shift in the direction of the higher wavenumber as is shown in Figure 5, which again designate the increased DMA and IL interactions. TBAH acquiring large alkyl groups produces less significant change in the band position of the respective band due to poor interaction with DMA (Figure 5c). At xIL ≈ 0.5, there is appearance of another peak towards lower wavenumber by 10, 5 and 4 cm-1 for TEAH, TPAH and TBAH in comparison to pure DMA, respectively, which articulates about paramount change in the solvent property. At higher xIL ≈ 0.90, bands reallocate to the further lower wavenumbers (at 1004 cm-1 for TEAH, at 1007 for TPAH cm-1 and 1007 cm-1 for TBAH in Figure 5). Figure 6 represents the above obtained results more distinctly. The observed red shift may be ascribed to the reduced contact with DMA due to the formation of IL cluster, thus, resulting lower rocking vibration energy. The steric hindrance by the large alkyl groups in TBAH and TPAH may be assigned to be the reason behind the manifestation of IR bands at higher wavenumbers in comparison to TEAH and decreased DMA interaction with IL (TBAH or TPAH).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 35

Characterization of Aggregate size in DMA-IL mixtures by using DLS. The DLS measurements have been carried out in order to analyze the size of aggregates formed due to DMA-IL and IL-IL interactions. The size of aggregates signifies the strength of the interactions between DMA and IL as a function of their varying composition. Figure 7 represents particle size distribution of binary mixtures of DMA and IL at different compositions. Figure 7 does not include pure DMA due to insufficient number of counts for pure solvent during DLS measurement. In case of TEAH, the two intensity distribution peaks appear at its lower concentration, one may be corresponding to DMA-IL interactions and other one for IL-IL interactions (Figure 7a). From the number distribution of particles (Figure 7d), it may be stated that DMA-IL interactions are main contributor and large size of aggregates are negligible at lower concentration of IL. The IL-IL interactions may be weakening to a large extent due to the stronger ion-dipole interaction between DMA and IL which is well supported by the FTIR results. The dH values for smaller aggregates increase till xIL ≤ 0.5 which may put emphasis on the increased DMA-IL interactions (Table 1). There is no doubt that large size aggregates are also increased in size at xIL = 0.5, however, their numbers are very less as can be seen clearly in Figures 7a and d. Afterwards, the increasing mole fractions of IL (xIL ≥ 0.5) does not show any peak consequent to the smaller size aggregates, however, large aggregates appear at much bigger size which may be depicting highly increased IL-IL interactions which result in diminishment of DMA-IL interactions. All these results are found to be in well agreement with the FTIR results where we have stated about two types of clusters, one predominantly present in DMA rich region and another in IL rich region. On the other hand, TPAH and TBAH illustrate intense increase in the IL-IL hydrophobic interactions as there is appearance of extremely large size aggregates (exposed in Figures 7b,c and Table 1). The peaks corresponding to small size aggregates is more or less smaller than that for TEAH which represent decline in interaction between DMA and IL due to the large alkyl groups of cations in TPAH and TBAH which increase highly favourable hydrophobic interaction between IL-IL. In case of TPAH and TBAH, the peak related to small aggregates are not appearing even at xIL = 0.5 because the hydrophobic interactions between the large sized butyl groups is occurring to an extent that may be escorting to more or less no interaction between DMA and IL. The number particle size distributions provide a clear picture of both type of aggregates in binary mixtures of DMA with TPAH and TBAH. Even at lower xIL (0.1 and 0.25) 10 ACS Paragon Plus Environment

Page 11 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of TBAH, large aggregates are present as can be speculated from Figure 7c which portrays appreciable existence of IL-IL interactions, however, these aggregates are not in considerable numbers (Figure 7f). It is mandatory to mention here that average size of aggregates changes in replicate measurements which may put stress on insufficient stability of aggregates. However, all measurements retain trend followed with increasing size of alkyl chain length of side chains of IL. The dH of large aggregates follow the trend as; TEAH < TPAH < TBAH. Quantum Chemical Studies: Cation-anion interactions within the ILs and IL-DMA intermolecular interactions Gas phase optimized structures of TEAH, TPAH and TBAH are shown in Figure 8. The dotted arrows indicate the strongest possible bonds (bond length and donor-acceptor direction) between the OH- (anion) of the IL and their respective cations. The results in Figure 8 showed that there are strong H-bond like interactions between the OH- ion of the ionic liquids and the near-by pendant H-atoms of the non-polar C-H of the alkyl groups. These interactions can be considered to be anion-induced dipole interactions. The OH- anion is interacting strongly and directly with three H-atoms of the alkyl groups directed outward from the cationic head-group. Similar observations have been reported in literature.54 The strongest of such bonds based on the OH---H bond length is 1.754 Å in TEAH, 1.775 Å in TPAH, and 1.879 Å in TBAH. This suggests that the order of the cation-anion interactions in the IL molecules is TEAH > TPAH > TBAH. The range of OH----H bond lengths obtained in this study is in good agreement with previously reported results on similar systems.54 The OH----H-C anion-induced dipole interactions are as a result of donor-acceptor exchanges between the lone pair of the OH- and the antibonding orbitals the corresponding C-H bond. The intramolecular cation-anion interactions within the ILs are therefore dependent on the alkyl chain length of the cationic head-group. Salient hyperconjugation interaction energies for the H-bonds are listed in Table 1. The magnitudes of E(2) in Table 1 also revealed that the strongest cation-anion interaction was found in the TEAH molecule. The direction of coulomb interactions is such that the antibonding (BD*) orbitals of the non-polar C-H bonds are receiving charges from the lone pair (LP) of the OH- ion. The optimized structures of the IL-DMA complexes are shown in Figure 9. The graphical images of the optimized structures suggest that the DMA molecule is interacting with the ILs via the anionic OH-. Selected second order perturbation energies that describe the strength of orbital 11 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 35

interactions between DMA and the ILs are listed in Table 2. The results in Table 2 showed that the interactions between DMA and TEAH enhances intramolecular interactions within the IL. This is reflected in the significant values of E(2) for interactions involving the OH- anion of TEAH and the quaternary N-atom (the N-C antibonding orbitals). Such interaction was not noticeable in the isolated TEAH molecule. A very large value of E(2) (749.47 kJ/mol) obtained for the LP(2)O46→BD*(1)O30-H31 interaction between the lone pair (LP) orbital of the carbonyl oxygen of DMA and antibonding (BD*) orbital of OH- is an indication that the prevalent interaction between TEAH and DMA occurred via the anion of the IL. Therefore, DMA could be a bond making solvent for TEAH. Based on the values of E(2) (Table 2) for LP orbitals of the carbonyl oxygen atom of DMA and BD* orbitals of O-H- (IL anion), it can be inferred from Table 2 that the strength of interactions in the IL-DMA systems follows the order TEAH (749.47 kJ/mol) > TPAH (10.78 kJ/mol) > TBAH (9.99 kJ/mol). This trend also agrees with experimental observations. In order to gain further insight into the donor-acceptor relationships between the IL molecules and DMA molecule, the frontier molecular orbital (FMO) energies of the molecules were employed. The energies of the highest occupied molecular orbitals (EHOMO) and lowest unoccupied molecular orbitals (ELUMO) were obtained for the individual ILs and DMA. EHOMO is a measure of the tendency of a molecule to donate its HOMO electron to an appropriate orbital of an accepting specie, while ELUMO is a measure of the tendency of a molecule to accept charges from the appropriate orbital of a donor specie into its LUMO. The FMO energies have been successfully used to describe possible donor-acceptor relationships for different systems and relative reactivity of molecules. The FMO energies for the studied molecules are listed in Table 3. The results in Table 3 showed that TPAH has the highest EHOMO, while TEAH has the lowest ELUMO. This suggests that under the same conditions, TPAH has better tendency of donating electrons into an accepting orbital than TEAH and TBAH, while TEAH has the highest propensity to accept electrons from occupied orbital of a donor molecule or specie. More importantly, the values of the energy gaps (1∆ELUMO-HOMO and 2∆ELUMO-HOMO) in Table 3 were used to elucidate the most favourable direction of donor-acceptor relationships between the ILs and DMA. The values of 1∆ELUMOHOMO

are generally lower than those of 2∆ELUMO-HOMO, which suggests that the prevailing

interactions between DMA and IL molecules involved charge donation from DMA to the IL 12 ACS Paragon Plus Environment

Page 13 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


molecule. This however does not preclude possible retro-donation from IL molecules to DMA. The trends of the values of 1∆ELUMO-HOMO and 2∆ELUMO-HOMO respectively are TPAH < TBAH < TEAH and TEAH < TPAH < TBAH. Interestingly, the order of the values of 2∆ELUMO-HOMO, which interprets to the favourability of IL-DMA interactions in the order TEAH > TPAH > TBAH agrees with experimental observations. While this might be co-incidental, it suggests that the disposition of the studied ILs towards interactions with DMA molecules may be related to the tendency of the ILs to accept electrons from DMA molecule during donor-acceptor interaction. The interaction energy (∆E) between each IL molecule and DMA was calculated in order to predict the strength of interactions in the IL-DMA complexes and the results are listed in Table 4. The results in Table 4 revealed that the trend of interaction energy (∆E) for the IL-DMA systems is TEAH > TPAH > TBAH, which is in agreement with experimental observations. These results suggest that the interaction between DMA molecules and the studied ILs decrease with increasing hydrophobic (alkyl) chain length.

CONCLUSIONS This study provides the information about the molecular interactions between ions of ammonium-based ILs and DMA at different composition of IL by using FT-IR and DFT. FTIR study reveals that DMA causes the H-bond weakening between the ions of IL as a function of increasing concentrations of ILs, however, higher concentration of IL (xIL ≥ 0.5) results in the decreased interaction between DMA and ions of IL. FTIR band analysis also shows that large sized IL has less affordable interactions with DMA as compared with small sized IL. The DLS study confirmed two types of clusters in binary mixtures of DMA and IL, one in DMA-rich region and second in IL-rich region. The conformational properties of ILs and DMA mixtures are mainly ruled by the strength of the bonding interactions between the ions of ILs and DMA molecules, and aptitude of self-assembly of IL-IL and DMA-DMA molecules. The extent of interaction between DMA and IL varies from IL to IL depending upon the size of alkyl groups in the cation of IL. Quantum chemical calculations provide complementary information and support the experimental findings that the interactions energies of TEAH, TPAH and TBAH with DMA decrease with increase in alkyl chain length of the cations of ILs following the trend TEAH > TPAH > TBAH. These results provide important information for a deeper understanding of the 13 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 35

characterization of molecular interactions of ILs with polar solvent and may shed more light on the applications of the ILs. AUTHOR INFORMATION *e-mail: [email protected]; [email protected]; Tel:+91-11-27666646-142; Fax: +91-11-2766 6605 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS Authors are grateful for the support from University of Delhi (R & D Grant No. RC/2015/9677) and DU-DST Purse Grant (Grant No. CD/2015/1534). L.O.O. acknowledges North-West University for granting him Post-Doctoral fellowship.


Page 15 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


REFERENCES (1) Jha, I.; Venkatesu, P. Endeavour to simplify the frustrated concept of protein-ammonium family ionic liquid interactions, Phys. Chem. Chem. Phys., 2015, 17, 20466-20484, Perspective. (2) Son, C. Y.; McDaniel, J. G.; Schmidt, J. R.; Cui, Q.; Yethiraj, A. First-principles united atom force field for the ionic liquid BMIM+BF4–: An alternative to charge scaling, J. Phys. Chem. B 2016, 120, 3560-3568. (3) Couadou, E.; Jacquemin, J.; Galiano, H.; Hardacre, C.; Anouti, M. A Comparative study on the thermophysical properties for two bis[(trifluoromethyl)sulfonyl]imide-based Ionic liquids containing the trimethyl-sulfonium or the trimethyl-ammonium cation in molecular solvents, J. Phys. Chem. B 2013, 117, 1389-1402. (4) Kumar, A.; Venkatesu, P. Overview of the stability of α-chymotrypsin in different solvent media. Chem. Rev., 2012, 112, 4283-4307. (5) Bahadur, P.;

I.;

Letcher,

Ramjugernath,

D.

T.

M.;

Excess

Singh, molar

S.;

Redhi,

volumes

of

G. binary

G.;

Venkatesu,

mixtures

(an

ionic liquid + water): A review. J. Chem. Thermodyn., 2015, 82, 34-46. (6) Govinda, V.; Venkatesua, P.; Bahadur, I. Molecular interactions between ammoniumbased ionic liquids and molecular solvents: current progress and challenges, Phys. Chem. Chem. Phys., 2016, 18, 8278-8326, (7) Kumar, A.; Meena, B.; Venkatesu, P. Exploring the structure and stability of amino acids and glycine peptides in biocompatible ionic liquids, RSC Adv., 2016, 6, 18763-18777. (8) Podgorsěk, A.; Jacquemin, J.; Pad́ ua, A.A.H.; Gomes, M.F. C. Mixing enthalpy for binary mixtures containing ionic liquids, Chem. Rev. 2016, 116, 6075-6106. (9) Greaves, T. L.; Drummond, C. J. Protic ionic liquids: Evolving structure-property relationships and expanding applications, Chem. Rev., 2015, 115, 11379-11448. (10) Pinkert, A.; Ang, K. L.; Marsh, K. N. Pang, S. Density, viscosity and electrical conductivity of protic alkanolammonium ionic liquids, Phys. Chem. Chem. Phys., 2011, 13, 5136-5143. (11) Gonfa, G.; Bustam, M. A.; Muhammad, N.; Khan, A. S. Evaluation of thermophysical properties of functionalized imidazolium thiocyanate based ionic liquids, Ind. Eng. Chem. Res., 2015, 54, 12428-12437. 15 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 35

(12) An, J. H.; Kim, J. M.; Chang, S. M.; Kim, W. S. Application of ionic liquid to polymorphic design of pharmaceutical ingredients. Cryst. Growth Des., 2010, 10, 30443050. (13) Stevanovic, S.; Podgoršek, A.; Pádua, A. A. H.; Gomes, M. F. C. Effect of water on the carbon dioxide absorption by 1-alkyl-3-methylimidazolium acetate ionic liquids, J. Phys. Chem. B 2012, 116, 14416-14425. (14) Tang, Y.; Chi, X.; Zou, S.; Zeng, X. Facet effects of palladium nanocrystals for oxygen reduction in ionic liquids and for sensing applications, Nanoscale 2016, 8, 5771-5779. (15) Shan, W.; Yang, Q.; Su, B.; Bao, Z.; Ren, Q.; Xing, H. Proton microenvironment and interfacial structure of sulfonic-acid functionalized ionic liquids, J. Phys. Chem. C 2015, 119, 20379-20388. (16) Amarasekara, A. S.; Owereh, O. S. Synthesis of a sulfonic acid functionalized acidic ionic liquid modified silica catalyst and applications in the hydrolysis of cellulose. Catal. Commun., 2010, 11, 1072-1075. (17) Sekhon, B. S. Ionic liquids based active pharmaceutical ingredients, Ars Pharm., 2013, 54, 37-44. (18) Sekhar, G. C.; Venkatesu, P.; Rao, M. V. P. Excess molar volumes and speeds of sound of N,N-dimethylacetamide with chloroethanes and chloroethenes at 303.15 K, J. Chem. Eng. Data 2001, 46, 377-380. (19) Fan, X. H.; Chen, Y. P.; Su, C. S. Density and viscosity measurements for binary

mixtures of 1-ethyl-3-methylimidazolium tetrafluoroborate ([Emim][BF4]) with dimethylacetamide, dimethylformamide and dimethyl sulfoxide, J. Chem. Eng. Data 2016, 61, 920-927. (20) Wu, J. Y.; Chen, Y. P.; Su, C. S. Density and viscosity of ionic liquid binary mixtures of 1-n-butyl-3-methylimidazolium

tetrafluoroborate

with

acetonitrile,

N,N-

dimethylacetamide, methanol and N-methyl-2-pyrrolidone, J. Solution Chem., 2015, 44, 395-412. (21) Weerachanchai, P.; Wong, Y.; Lim, K. H.; Tan, T. T. Y.; Lee, J. M. Determination of solubility parameters of ionic liquids and ionic liquid/solvent mixtures from intrinsic viscosity, Chem Phys Chem., 2014, 15, 3580-3591.


Page 17 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(22) Shekaari, H.; Kazempour, A.; Khoshalhan, M. Schiff base ligands and their transition metal complexes in the mixtures of ionic liquid + organic solvent: a thermodynamic study, Phys. Chem. Chem. Phys., 2015, 17, 2179-2191. (23) Alcalde, R.; García, G.; Trenzado, J. L.; Atilhan, M.; Aparicio, S. Characterization of amide-alkanediol intermolecular interactions, J. Phys. Chem. B 2015, 119 , 4725-4738. (24) Sekhar, G. C.; Rao, M. V. P.; Prasad, D. H. L.; Kumar, Y. V. L. R. Excess molar enthalpies of N,N-dimethylacetamide with substituted benzenes at 298.15 K, Thermochimica Acta 2003, 402, 99-103. (25) Krakowiak, J.; Koziel, H.; Grzybkowski, W. Apparent molar volumes of divalent transition metal perchlorates and chlorides in N,N-dimethylacetamide, J. Mol. Liq., 2005, 118, 57-65. (26) Araque, J. C.; Hettige, J. J.; Margulis, C. J. Modern room temperature ionic liquids, a simple guide to understanding their structure and how it may relate to dynamics, J. Phys. Chem. B, 2015, 119, 12727-12740. (27) Wang, C.; Mahurin, S. M.; Luo, H.; Baker, G. A.; Li, H.; Dai, S. Reversible and robust CO2 capture by equimolar task-specific ionic liquid-superbase mixtures, Green Chem., 2010, 12, 870-874. (28) Attri, P.; Baik, K. Y.; Venkatesu, P.; Kim, I. T.; Choi, E. H. Influence of hydroxyl group position and temperature on thermophysical properties of tetraalkylammonium hydroxide ionic liquids with alcohols, Plos One 2014, 9, e86530-14. (29) Freemantle, M. An introduction to ionic liquids, RSC Publishing, Cambridge, UK, 2010. (30) Gonzalez, ́ E. J.; Domínguez, A; Macedo, E. A. Physical and excess properties of eight binary mixtures containing water and ionic liquids, J. Chem. Eng. Data 2012, 57, 21652176. (31) Attri, P.; Venkatesu, P.; Kumar, A. Temperature effect on the molecular interactions between ammonium ionic liquids and N,N-dimethylformamide, J. Phys. Chem. B 2010, 114, 13415-13425. (32) Attri, P.; Venkatesu, P.; Hofman, T. Temperature dependence measurements and structural characterization of trimethyl ammonium ionic liquids with a highly polar solvent, J. Phys. Chem. B 2011, 115, 10086-10097. 17 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 35

(33) Attri, P.; Reddy, P. M.; Venkatesu, P.; Kumar, A.; Hofman, T. Measurements and molecular interactions for N,N-dimethylformamide with ionic liquid mixed solvents, J. Phys. Chem. B 2010, 114, 6126-6133. (34) Govinda, V.; Reddy, P. M.; Attri, P.; Venkatesu, P.; Venkateswarlu, P. Influence of anion on thermophysical properties of ionic liquids with polar solvent, J. Chem. Thermodyn., 2013, 58, 269-278. (35) Govinda, V.; Attri, P.; Venkatesu, P.; Venkateswarlu, P. Thermophysical properties of dimethylsulfoxide with ionic liquids at various temperatures, Fluid Phase Equilib., 2011, 304, 35-43. (36) Govinda, V.; Attri, P.; Venkatesu, P.; Venkateswarlu, P. Evaluation of thermophysical properties of ionic liquids with polar solvent: A comparable study of two families of ionic liquids with various ions. J. Phys. Chem. B 2013, 117, 12535-12548. (37) Govinda, V.; Reddy, P. M.; Bahadur, I.; Attri, P.; Venkatesu, P.; Venkateswarlu, P. Effect of anion variation on the thermophysical properties of triethylammonium based protic ionic liquids with polar solvent, Thermochimica Acta 2013, 556, 75-88. (38) Govinda, V.; Attri, P.; Venkatesu, P.; Venkateswarlu, P. Temperature effect on the molecular interactions between two ammonium ionic liquids and dimethylsulfoxide, J. Mol. Liq., 2011, 164, 218-225. (39) Kavitha, T.; Attri, P.; Venkatesu, P.; Rama Devi, R. S.; Hofman, T. Thermophysical properties for the mixed solvents of N-methyl-2-pyrrolidone with some of the imidazolium-based ionic liquids, J. Mol. Liq., 2014, 198, 11-20. (40) Kavitha, T.; Attri, P.; Venkatesu, P.; Rama Devi, R. S.; Hofman, T. Influence of alkyl chain length and temperature on thermophysical properties of ammonium-based ionic liquids with molecular solvent. J. Phys. Chem. B 2012, 116, 4561-4574. (41) Kavitha, T.; Attri, P.; Venkatesu, P.; Rama Devi, R. S.; Hofman, T. Temperature dependence measurements and molecular interactions for ammonium ionic liquid with Nmethyl-2-pyrrolidone, J. Chem. Thermodyn., 2012, 54, 223-237. (42) Govinda, V.; Vasantha, T.; Khan, I.; Venkatesu, P. Effect of the alkyl chain length of the cation on the interactions between water and ammonium-based ionic liquids: experimental and COSMO-RS Studies, Ind. Eng. Chem. Res., 2015, 54, 9013-9026.


Page 19 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(43) Umapathi, R.; Attri, P.; Venkatesu, P. Thermophysical properties of aqueous solution of ammonium-based ionic liquids, J. Phys. Chem. B 2014, 118, 5971-5982. (44) Shekaari, H.; Zafarani-Moattar, M. T. Volumetric properties of the ionic liquid, 1-butyl3-methylimidazolium tetrafluoroborate, in organic solvents at T = 298.15 K, Int J Thermophys., 2008, 29, 534-545. (45) Gadzurić, S.; Tot, A.; Zec, N.; Papović, S.; Vranes, M. Volumetric properties of binary mixtures of 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate with N-methylformamide,

N-ethylformamide,

N,N-dimethylformamide,

N,N-dibutyl

formamide and N,N-dimethylacetamide from (293.15 to 323.15) K, J. Chem. Eng. Data 2014, 59, 1225-1231. (46) Vranes, M.; Tot, A.; Zec, N.; Papović, S.; Gadzurić, S.; Volumetric properties of binary mixtures of 1-butyl-3-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate with N-methylformamide,

N-ethylformamide,

N,N-dimethylformamide,

N,N-dibutyl

formamide and N,N-dimethylacetamide from (293.15 to 323.15) K, J. Chem. Eng. Data 2014, 59, 3372-3379. (47) Zafarani-Moattar, M. T.; Shekaari, H. Volumetric and compressibility behavior of ionic liquid, 1-n-butyl-3-methylimidazolium hexafluorophosphate and tetrabutylammonium hexafluorophosphate in organic solvents at T = 298.15 K, J. Chem. Thermodyn., 2006, 38, 624-633. (48) Becke, A. D. Density-functional exchange-energy approximation with correct asymptotic

behavior. Phys. Rev. A 1988, 38, 3098-3100.

(49) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density. Phys. Rev. B 1988, 37, 785–789. (50) Xu, X.; Goddard, W. A. The X3LYP extended density functional for accurate descriptions of nonbond interactions, spin states, and thermochemical properties. Proc. Natl. Acad. Sci. USA. 2003, 101, 2673–2677. (51) Georgieva, I.; Trendafilova, N. Bonding analyses, formation energies, and vibrational properties of M-R2dtc complexes (M = Ag(I), Ni(II), Cu(II), or Zn(II)). J. Phys. Chem. A 2007, 111, 13075–13087. (52) Umebayashi, Y.; Mitsugi, T.; Fukuda, S.; Fujimori, T.; Fujii, K.; Kanzaki, R.; Takeuchi, M.; Ishiguro, S. I. Lithium ion solvation in room-temperature ionic liquids involving 19 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 35

bis(trifluoromethanesulfonyl) imide anion studied by Raman spectroscopy and DFT calculations. J. Phys. Chem. B 2007, 111, 13028-13032. (53) Castner, E. W. Jr.; Wishart, J. F.; Shirota, H.; Intermolecular dynamics, interactions, and solvation in ionic liquids. Acc. Chem. Res. 2007, 40, 1217–1227. (54) Davies, A. S.; George, W. O.; Howard, S. T. Ab initio and DFT computer studies of complexes

of

quaternary

nitrogen

cations:

trimethylammonium,

tetramethylammonium, trimethylethylammonium, choline and acetylcholine with hydroxide, fluoride and chloride anions. Phys. Chem. Chem. Phys. 2003, 5, 4533–4540. (55) Ravikumar, C.; Joe, I. H.; Jayakumar, V. S. Charge transfer interactions and nonlinear optical properties of push–pull chromophore benzaldehyde phenylhydrazone: A vibrational approach. Chem. Phys. Lett. 2008, 460, 552–558. (56) Chocholousova, J.; Spirko, V.; Hobza, P. First local minimum of the formic acid dimer exhibits simultaneously red-shifted O–H---O and improper blue-shifted C H---O hydrogen bonds. Phys. Chem. Chem. Phys. 2004, 6 , 37–41. (57) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, Revision D.01; Gaussian, Inc.: Wallingford CT, 2009. (58) Takamuku, T.; Matsuo, D.; Tabata, M.; Yamaguchi, T.; Nishi, N. Structure of aqueous mixtures of N,N-dimethylacetamide studied by infrared spectroscopy, X-ray diffraction, and mass spectrometry, J. Phys. Chem. B 2003, 107, 6070-6078. (59) Marium, M.; Rahman, M. M.; Mollah, M. Y. A.; Susan, Md. A. B. H. Molecular level interactions in binary mixtures of 1-ethyl-3-methylimidazolium tertaflouroborate and water, RSC Adv., 2015, 5, 19907-19913. (60) Verbovy, D. M.; Smagala, T. G.; Brynda, M. A.; Fawcett, W. R. A FTIR study of ionsolvent interactions in N,N-dimethylacetamide, J. Mol. Liq., 2006, 129, 13-17. (61) Dwivedi, A. M.; Krimm, S.; Mierson, S. Vibrational force field and normal mode analysis of N,N-dimethylacetamide, Spectrochimica Acta 1989, 45A, 271-279.


Page 21 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. FTIR spectra of O-H stretching vibration for the binary mixtures of DMA with ILs; (a) TEAH, (b) TPAH and (c) TBAH at different mole fraction of IL; 0.0 (black), 0.1 (red), 0.25 (green), 0.5 (blue), 0.75 (cyan), 0.90 (pink) and 1.0 (yellow).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 35

3540 3520 3500 3480 3460 3440 3420 0.0

0.2

0.4

0.6

0.8

1.0

Figure 2. Variation of peak position of O-H stretching vibration for the binary mixtures of DMA with ILs; TEAH (red), TPAH (green) and TBAH (blue) as a function of mole fraction of IL.


Page 23 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. FTIR spectra of C=O stretching vibration for the binary mixtures of DMA with ILs; (a) TEAH, (b) TPAH and (c) TBAH at different mole fraction of IL; 0.0 (black), 0.1 (red), 0.25 (green), 0.5 (blue), 0.75 (cyan), 0.90 (pink) and 1.0 (yellow).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 35

Figure 4. FTIR spectra of N(CH3) and C(CH3) deformations vibration of DMA for the binary mixtures of DMA with ILs; (a) TEAH, (b) TPAH and (c) TBAH at different mole fraction of IL; 0.0 (black), 0.1 (red), 0.25 (green), 0.5 (blue), 0.75 (cyan), 0.90 (pink) and 1.0 (yellow).


Page 25 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1014

(a) TEAH

1015 1016

1004 1004

1017

1004 1004

1018

1014

(b) TPAH

1014 1016 1007 1007

1017

1009

(c) TBAH

1014 1015 1015 1014

1025

1015

1010

1007 1006

1005

995

Wavenumber (cm-1)

Figure 5. FTIR spectra of C(CH3) rocking mode vibration for the binary mixtures of DMA with ILs; (a) TEAH, (b) TPAH and (c) TBAH at different mole fraction of IL; 0.0 (black), 0.1 (red), 0.25 (green), 0.5 (blue), 0.75 (cyan), 0.90 (pink) and 1.0 (yellow).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 35

1016

1012

1008

1004 0.0

0.2

0.4

0.6

0.8

1.0

Figure 6. Variation of peak position of C(CH3) rocking mode vibration for the binary mixtures of DMA with ILs; TEAH (red), TPAH (green) and TBAH (blue) as a function of mole fraction of IL.


Page 27 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40

40 35

30

30 25

20

20 15

10

10 5

0

0 0

100 200 300 400 500

1000

0

10000

100 200 300 400 500

1000

10000

0

100 200 300 400 500

1000

10000

0

100 200 300 400 500

1000

10000

40

40

30

30

20

20

10

10

0

0 0

100 200 300 400 500

1000

10000

40

40

30

30

20

20

10

10

0

0 0

100 200 300 400 500

1000

10000

Figure 7. The intensity size distribution and number size distribution of particles of binary mixtures of DMA with (a,d) TEAH, (b,e) TPAH and (c,f) TBAH, respectively, at 25 oC with varying mole fraction of IL ( xIL): 0.1 (red), 0.25 (green), 0.5 (blue), 0.75 M (cyan) and 1 (pink).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 35

TEAH

TPAH

TBAH Figure 8. Gas phase optimized structures of the studied ILs showing the strongest H-bonds in each molecule.


Page 29 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TEAH-DMA

TPAH-DMA

TBAH-DMA Figure 9. Gas phase optimized structures of IL-DMA complexes



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 35

Table 1. Second order perturbation energy for the cation-anion interactions within the IL systems Donor-acceptor orbitals interactionsa TEAH LP(1)O30→BD*(1)C5-H6 LP(2)O30→BD*(1)C9-H10 LP3O30→BD*(1)C23-H24

E(2) (kJ/mol) 39.62 64.81 102.30

TPAH LP(1)O26→BD*(1)C5-H6 LP(2)O26→BD*(1)C8-H9 LP(3)O26→BD*(1)C20-H21

37.91 57.11 98.03

TBAH LP(1)O54→BD*(1)C2-H4 27.99 LP(1)O54→BD*(1)C15-H18 9.58 LP(1)O54→BD*(1)C28-H30 11.34 LP(2)O54→BD*(1)C2-H4 12.26 LP(2)O54→BD*(1)C28-H30 39.12 LP(3)O54→BD*(1)C2-H4 22.01 LP(3)O54→BD*(1)C15-H18 52.84 LP(3)O54→BD*(1)C28-H30 21.92 a A→B implies that A is the donor orbital and B is the acceptor orbital


Page 31 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Second order perturbation energy (E2) and the interacting orbitals for the IL-DMA complexes Interacting units

Within the IL (TEAH): cation-anion interactions

IL anion (OH-)-DMA interactions

Donor-acceptor orbitals interactions

E(2) (kJ/mol)

TEAH-DMA BD(1)N1-C16→BD*(1)O30-H31 LP(1)O30→BD*(1)N1-C9 LP(2)O30→BD*(1)N1-C2 LP(2)O30→BD*(1)N1-C9 LP(2)O30→BD*(1)N1-C16

11.33 13.17 16.97 29.09 24.58

BD(1)O30-H31→BD*(1)C41-O46 BD(1)C41-O46→BD*(1)O30-H31 LP(1)O46→BD*(1)O30-H31 LP(2)O46→BD*(1)O30-H31

24.08 30.10 23.45 749.47

IL cation-DMA interactions

LP(1)O46→BD*(2)C17-O22

7.61

Within the IL (TPAH): cation-anion interactions

LP(2)O42→BD*(1)C2-H5 LP(2)O42→BD*(1)C12-H15 LP(3)O42→BD*(1)C12-H15

10.16 12.50 122.64


BD(2)C53-O54→BD*(1)O42-H43 LP(2) O54→BD*(1)O42-H43

17.18 10.78

IL cation-DMA interactions

LP(1)O54→BD*(1)C6-H10

8.69

TPAH-DMA

TBAH-DMA Within the IL (TPAH): cation-anion interactions


BD(1)O54-H55→BD*(1)C15-H18 LP(1)O54→BD*(1)N1-C41 LP(2)O54→BD*(1)N1-C15 LP(2)O54→BD*(1)N1-C41 LP(2)O54→BD*(1)C15-H18

15.55 16.97 11.16 12.41 31.48

BD(1)O54-H55→BD*(1)C65-O66 BD(1)O54-H55→BD*(2)C65-O66 BD(1)C65-O66→BD*(1)O54-H55 BD(2)C65-O66→BD*(1)O54-H55 LP(1)O66→BD*(1)O54-H55

40.67 26.21 47.02 59.94 9.99



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 35

Table 3. Frontier molecular orbital energies for the studied IL molecules and DMA Molecule TEAH TPAH TBAH DMA 1

EHOMO (eV) -4.1828 -4.0938 -4.1284 -6.6922

ELUMO (eV) -0.4892 -0.4269 -0.3467 -0.1012

1

∆ELUMO-HOMO (eV) 4.0816 3.9926 4.0272 -

2

∆ELUMO-HOMO (eV) 6.2030 6.2653 6.3455 -

∆ELUMO-HOMO is the difference between the ELUMO of DMA and EHOMO of IL, while 2∆ELUMO-HOMO is the difference between the ELUMO of IL and EHOMO of DMA


Page 33 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 4. Total and interaction energies for the studied ILs and their IL-DMA complexes EIL (x106) EIL-DMA (x106) ∆E** (kJ/mol) (kJ/mol) (kJ/mol) TEAH -1.1747 -1.9307 256.1482 TPAH -1.5875 -2.3435 196.0243 TBAH -2.0004 -2.7563 130.9119 **∆E = EIL + EDMA - EIL-DMA; where EDMA = -7.5576 x 105 kJ/mol

IL Molecule



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 35

TOC Graphic


Page 35 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Probing Molecular Interactions between Ammonium-based Ionic Liquids and N,N-Dimethylacetamide:: A Combined FT FT-IR and DFT Study P. Kiran Kumara, Anjeeta Ranib, Varadhi Govindaa,b, Alo Oluyemi Oludarea, Indra Bahadura∗ and Pannuru Venkatesub∗, Eno E. Ebensoa a

Department of Chemistry, School of Mathematical and Physical Sciences, Materials Science Innovation & Modelling (MaSIM (MaSIM)) Research Focus Area, Faculty of Agriculture, Science and Technology, North-West West University (Mafikeng Campus), Private Bag X2046, Mmabatho 2735, South Africa

b

Department of Chemistry, University of Delhi, Delhi 110 007, India

ACS Paragon Plus Environment

Probing Molecular Interactions between ... - ACS Publications

Recommend Documents