Intermolecular Interactions in N,N-Dimethylacetamide without and with

Subscriber access provided by University of South Dakota

B: Liquids, Chemical and Dynamical Processes in Solution, Spectroscopy in Solution

The Intermolecular Interactions in N,N-dimethylacetamide without and with LiCl Studied by Infrared Spectroscopy and Quantum Chemical Model Calculations Nikolay Kotov, Vladimír Raus, and Ji#í Dybal J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b05569 • Publication Date (Web): 04 Sep 2018 Downloaded from http://pubs.acs.org on September 10, 2018

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 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 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.

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 27 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

The Intermolecular Interactions in N,N-Dimethylacetamide without and with LiCl Studied by Infrared Spectroscopy and Quantum Chemical Model Calculations Nikolay Kotov,* Vladimír Raus and Jiří Dybal Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsky Sq. 2, 162 06 Prague 6, Czech Republic Abstract The mixture of LiCl and N,N-dimethylacetamide (DMAc) is an important laboratory-scale solvent for cellulose. However, the mechanism of cellulose dissolution in DMAc/LiCl could not be fully established due to the limited knowledge about the interactions between DMAc and LiCl. To address this issue, we studied neat DMAc and DMAc/LiCl mixtures by ATR FTIR spectroscopy and quantum chemical model calculations. Based on the calculations, we newly assigned the bands at 1660 and 1642 cm–1 in the ν(C=O) region of the spectra to DMAc monomeric and dimeric structures. The latter are presumably stabilized by the C– H···O=C weak hydrogen bonds that prevail in both neat DMAc and DMAc/LiCl mixtures. The analysis of the concentrated (7.9 wt% of LiCl) DMAc/LiCl mixture revealed that only about half of DMAc molecules interact directly with LiCl. The resulting average stoichiometry of about 2.8:1 (DMAc:LiCl), indicating the predominance of [(DMAc)2–LiCl] and [(DMAc)3–LiCl] complexes, was found to be temperature independent. Conversely, the stoichiometry was considerably temperature sensitive for the diluted DMAc/LiCl mixture (2.6 wt% of LiCl), indicating that further DMAc molecules can be incorporated into the primary solvation shell of LiCl at higher temperatures. These results highlight the dynamic character of the DMAc/LiCl system that needs to be considered when studying the cellulose dissolution mechanism.

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 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 27

Introduction Cellulose is an abundant natural polymer that is used in numerous fields, including cosmetic, pharmaceutical, food, or textile industries. In many applications, dissolution of cellulose is the key processing step. However, the peculiar character of its semi-crystalline structure restricts cellulose dissolution in the majority of common solvents.1,2 Nevertheless, several cellulose solvents were developed for both industrial3–5 and laboratory use.2,6 A concentrated solution of LiCl in DMAc is a popular cellulose solvent on the laboratory scale, finding applications in the chemical modification7–10 and analysis11,12 of the polymer. Despite the wide adoption of this solvent in cellulose chemistry, the general mechanism of cellulose dissolution in DMAc/LiCl is surprisingly still a subject of ongoing scientific debate. It was proposed that the chloride anions of LiCl penetrate into the cellulose structure, establishing hydrogen bonds with cellulose hydroxyl groups and replacing thus the hydrogen bonds between adjacent cellulose chains.1,7,13 The bulky [(DMAc)x–Li+] complexes14 then follow the anions into the cellulose structure, serving as spacers disrupting the cellulose chain packing. However, recent studies continue speculating about the actual role of LiCl in the mechanism where either it is thought to have no direct interactions with cellulose chains13 or lithium is presumed to establish hydrogen bonds with cellulose hydroxyl groups2 or even contribute to eliminating the hydrophobic interactions between the cellulose chains.1 Furthermore, it still remains unclear how many DMAc molecules are bound by LiCl and if this number changes at different conditions. Vibrational spectroscopy is a useful tool for studying intermolecular interactions between solutes and solvents. In case of amides, the intensive C=O stretching vibration (ν(C=O)) can be conveniently used to obtain information about the environment of the molecule. For example, the ν(C=O) region is frequently utilized for identification of the secondary structure of proteins.15–18 Furthermore, this band has been also employed for assessing the polymersolvent interactions.19–24 In 1968, Chalapathi and Ramiah25 were the first to assign the infrared and Raman frequencies of two typical amides – N,N-dimethylformamide (DMF) and DMAc. Later, multiple research groups performed more detailed investigations of the ν(C=O) region of DMF, DMAc, and other amides such as formamide and N-methylformamide, contributing significantly to our understanding of this band composition.26–33 In these studies, authors mainly employed theoretical methods and Raman spectroscopy while infrared (IR) spectroscopy was used less frequently. Importantly, the assignment of Raman bands was 2 ACS Paragon Plus Environment

Page 3 of 27 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


extended also to the IR bands. Despite the fact that Raman and IR spectroscopies are complementary techniques, the origin of the bands in the respective spectra could be sometimes different owing to the different origin of the studied molecule’s dipole moment and polarizability. Therefore, this simple approach to the band assignment cannot be considered universal, particularly if the band is influenced by effects such as dipole coupling or, importantly, hydrogen bonding. For example, this is the case in proteins where the C– H···O weak hydrogen bonds between the carbons of the peptide backbone and the electron withdrawing C=O groups are known to contribute considerably to the stabilization of protein secondary structures.34–36 Furthermore, the C–H···O=C hydrogen bonds were reported to stabilize dimeric forms of formamide, N–methylacetamide, acetamide, and DMF.31,33,37–39 However, the ability of DMAc to form weak hydrogen bonds giving rise to DMAc-DMAc dimers has not been considered in the relevant literature, possibly leading to incorrect interpretation of the intermolecular interactions within this solvent. In this work, neat DMAc and DMAc/LiCl mixtures with various concentration of LiCl were investigated by infrared spectroscopy at different temperatures and by quantum chemical calculations. The combination of these two methods provided deeper insight into the character of DMAc-DMAc interactions and also into the stoichiometry of DMAc-LiCl complexes.

Materials and Methods Materials N,N-Dimethylacetamide (DMAc) (99.5%, Acros Organics) and CCl4 (p.a., Penta, Czech Republic) were dried over activated molecular sieves and kept under argon. Lithium chloride (LiCl) (99%, Sigma-Aldrich) was dried at 120 °C in vacuum overnight and then kept under argon.

Preparation of DMAc/LiCl solutions To a flask containing a magnetic stirring bar and the known amount of dried LiCl, the calculated volume of dried DMAc was added through a syringe and the suspension was stirred intensively until all LiCl dissolved (ca. 3 hours). The solutions were kept under argon afterwards.



Page 4 of 27

Table 1. Compositions of the DMAc/LiCl mixtures. Sample 1 2 3 4

Weight fraction of LiCl (%) 0.2 0.8 2.6 7.9

Molar fraction of LiCl (%) 0.4 1.6 5.2 15.0

Attenuated-Total-Reflectance (ATR) FTIR spectroscopy A Thermo Nicolet Nexus 870 FTIR spectrometer purged with dry air and equipped with a liquid-nitrogen-cooled MCT (mercury-cadmium-telluride) detector was used for obtaining ATR FTIR spectra. All the spectra were acquired using the Golden Gate single reflection ATR accessory (Specac Ltd.) equipped with a diamond internal reflection element. The spectra acquisition parameters were: resolution 4 cm–1, 128 scans. When performing the temperature dependence investigations (from 5 to 60 °C, increment 5 °C), a fresh sample aliquot was placed onto the ATR FTIR accessory pre-heated to the desired temperature, equilibrated for 1 minute, and subsequently the ATR FTIR spectrum was acquired. Note that for temperatures above 45 °C, the amount of scans was decreased from 128 to 8 scans to at least partially offset evaporation of the sample from the diamond that was taking place for neat DMAc and diluted DMAc/LiCl mixtures. In the present experimental set-up, samples were placed on the ATR accessory by a syringe and the accessory was exposed to the ambient atmosphere for a few seconds before it was covered by a metal cap. Since both DMAc and LiCl are highly hygroscopic, certain amount of water was absorbed during this time. To assess the amount of water in the samples, the region of O–H stretching vibrations in the respective IR spectra were studied and compared with a calibration curve (data not shown), indicating water content of 0.1–0.2 wt%, which we considered negligible. For the analysis of the region of ν(C=O) vibrations in the acquired ATR FTIR spectra, the spectra were normalized to the most intensive band at ca. 1645 cm–1. Then, the second derivative spectra (OMNIC™ software ver. 8.3.103, Savitzky-Golay algorithm, number of points set to 7, the polynomial order set to 3) were obtained to identify the peak maxima (1660 cm–1 and 1639 cm–1). For spectra deconvolution, the acquired ATR FTIR spectra were fitted with the Voigt function using the Peak Resolve routine of the software, which also


Page 5 of 27 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


allowed assessing the integral intensity of the bands. Note that the position of the band at 1660 cm–1 had to be fixed before fitting in order to avoid artifacts (temperature-dependent shifts that were not observed in the second derivative spectra). Conversely, the position of peak at 1639 cm–1 was not fixed as temperature sensitivity was expected (theoretical calculations), and deformed band profiles that did not correspond well to the original spectra were obtained upon fixing of the peak position. Also note that this peak has probably a complex composition, and so the fitting method (sensitive to the peak full width at half maximum) provided the peak positon of 1642 cm–1; this wavenumber was used for identifying the band throughout the manuscript. The same approach to data processing was used also for the spectra of DMAc/LiCl mixtures.

DFT calculations The calculations were done at the DFT level of theory with the B3LYP exchange correlation functional and the semi-empirical dispersion correction GD3BJ.40 In order to verify the reliability of the DFT results, the stable structures were re-optimized at the Møller-Plesset (MP2) level of theory. All calculations were performed with the 6-311+G(d,p) basis set employing the Gaussian 09 program package.41 Vibrational frequencies of the normal modes were calculated at the same level of theory as the geometry optimizations. It was confirmed that the optimized geometries were true energy minima on the potential energy surface since no imaginary frequencies were obtained as a result of frequency calculations. In addition, the Boys and Bernardi counterpoise correction was carried out in the calculations of the stabilization energies of the dimers (the difference of the energy of two monomers and the dimer) in order to take the basis set superposition error42 into account.

Results and Discussion In order to correctly identify any new intermolecular interactions established between DMAc and LiCl, we started by investigating the structure of neat DMAc. For this purpose, the infrared spectra of neat DMAc were acquired within a wide temperature range. Then, DMAc/LiCl mixtures of the selected concentrations (see Table 1) were studied in an analogous way. Based on the spectra analysis, the most probable structures prevalent in these liquids were calculated via the quantum chemical model calculations on the DFT level of theory.



Page 6 of 27

Neat DMAc

Figure 1. The region of ν(C=O) vibrations in the ATR FTIR spectra of neat DMAc (solid lines) at different temperatures with (A) the respective second derivative spectra (dash dot lines) and (B) the deconvoluted spectra after the curve fitting (dashed lines). ATR FTIR spectra of neat DMAc were measured in the temperature range of 5 to 60 ºC (with a 5 °C increment) to investigate how the intermolecular interactions within the liquid change with temperature. For the sake of clarity, only the spectra collected at 5, 20, and 60 ºC are presented in Figure 1. The band at the wavenumber 1645 cm–1 in the ν(C=O) region of the spectra has an asymmetric profile (Figure 1, solid lines), which is in agreement with literature.13,32 At higher temperatures, the band gradually shifts to higher-wavenumbers: from 1645 cm–1 at 5 °C to 1647 cm–1 at 60 °C. This observation is rather unusual, as the stretching 6 ACS Paragon Plus Environment

Page 7 of 27 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


vibrations of C=O groups are expected to shift to the lower wavenumbers owing to the increase in the bond length with temperature. In order clarify the nature of the observed temperature-dependent changes in the IR spectra, we obtained the respective second derivative spectra of the 1700–1600 cm–1 region (Figure 1A, dash dot lines). In this way, we revealed that the band at 1645 cm–1 is composed of the two bands, at 1660 cm–1 and at 1639 cm–1 (Figure 1A), which is in agreement with literature.32 Figure 1A shows that the maximum of the higher wavenumber peak at 1660 cm–1 exhibits no shift with temperature whereas the lower wavenumber peak shifts by 6 cm–1 from 1639 cm–1 at 5 °C, to 1645 cm–1 at 60 °C. Because both the bands are attributed to ν(C=O) vibrations, their different behavior could not be attributed to the temperature effects such as the change in the refractive index of DMAc (estimated to be ca. 0.022 in the temperature range studied)43,44 or to the temperature-induced weakening of the bond. To explain this peculiar temperature behavior, the original spectra were deconvoluted into their fundamental components (Figure 1B, dashed lines). The best fitting results for ν(C=O) region at various temperatures are displayed in Figure 1B. They are in a good agreement with the experimental spectrum as well as with the second derivative spectra even though the band at lower wavenumber is located at a higher wavenumber (1642 cm–1 instead of 1639 cm–1). The performed deconvolution reveals that, in addition to a gradual blue-shift of the band at 1642 cm–1 up to 1645 cm–1 at 60 °C, the I(1660)/I(1642) integral intensity ratio gradually increases with temperature (for visualization of this trend see Figure S1). Previously, using IR and Raman spectroscopies, Katsumoto et al.32 thoroughly investigated the origin of the peaks in the ν(C=O) vibration region of DMAc diluted with other solvents. They suggested that the transition–dipole coupling between two adjacent DMAc molecules could give rise to two vibrations of the pair, the out-of-phase and in-phase vibrations, detectable in the isotropic and anisotropic components of the Raman spectra at ca. 1660 and 1645 cm–1, respectively. Further, they assumed that DMAc cannot establish hydrogen bonds and extended the proposed assignment to the bands found in the ν(C=O) region of the IR spectrum, supporting this approach by comparing the anisotropic component of the Raman spectrum with the respective IR spectrum. The temperature behavior of the bands at 1660 and 1642 cm–1 reported in the present work, however, does not lend support to the assignment proposed by Katsumoto et al. If these bands were to be assigned to the out-of-phase and in-phase ν(C=O) vibrations, then the observed changes of the I(1660)/I(1642) integral intensity ratio with temperature would indicate that the out-of-phase vibration of C=O groups in the coupled 7 ACS Paragon Plus Environment


Page 8 of 27

DMAc molecules is enhanced by the thermal motion and, concurrently, the in-phase vibration in these molecules is restricted. DMAc molecules are rather small and the temperature is rather high; therefore, it is unlikely for such specific ordering to occur in this system. Furthermore, the peak attributed to the out-of-phase vibration of the C=O group should have higher intensity in the IR spectrum than the peak of the in-phase vibration since the former should result in a larger change of the dipole moment of the molecule. Consequently, different

I(1660)/I(1642) intensity ratio should be observed than that found by the deconvolution procedure (Figure 1). Therefore, different assignment of these bands in the IR spectrum of neat DMAc has to be proposed.

Figure 2. The optimized structure of the most stable dimer of DMAc calculated at the B3LYP-GD3BJ/6-311+G(d,p) level of theory. The interatomic distances are in Å. Oxygen atoms are in red, hydrogens ˗ white, and nitrogens ˗ blue. In order to get deeper insight into the intermolecular interactions of DMAc and interpret the observed behavior of the complex C=O stretching band, we performed quantum chemical model calculations of the possible DMAc dimers. The calculations showed that DMAc molecules are able to form various stable dimeric structures. The structure of the most stable dimeric complex where two DMAc molecules are oriented antiparallel to each other is presented in Figure 2. The calculated stabilization energies ∆E for the displayed structure are: 3.19 kcal/mol (B3LYP/6-311+G(d,p)), 9.88 kcal/mol (B3LYP-GD3BJ/(6-311+G(d,p)) and 7.22 kcal/mol (MP2/6-311+G(d,p)). These values are not much different for other possible dimeric structures. Moreover, the obtained values indicate that contributions of dispersion energy, included in the MP2 and B3LYP-GD3BJ methods, take a significant role in the intermolecular interactions. High values of the calculated stabilization energy support the 8 ACS Paragon Plus Environment

Page 9 of 27 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


existence of stable dimers in liquid DMAc. Additionally, calculations of the vibrational modes performed at the B3LYP/6-311+G(d,p) level of theory reveal that frequency of the C=O stretching modes of DMAc in dimers and monomers should differ by 10 cm–1, with the respective vibrational bands of dimers found at lower wavenumbers than that of monomers. These findings are in a good agreement with the experimental data, and we thus propose that the band at 1660 cm–1 can be assigned to the ν(C=O) vibrations of DMAc monomers and the band at 1642 cm–1 to the ν(C=O) vibrations of DMAc dimers that are stabilized by C– H···O=C weak hydrogen bonds established between the two adjacent molecules. Such interactions have already been proposed to play role in intermolecular interactions of other amides, N-methylacetamide45 and DMF.27,33,37 The proposed assignment of the ν(C=O) bands aligns well with the observed temperatureinduced changes seen in the ATR FTIR spectra of neat DMAc (Figure 1). The changes in the

I(1660)/I(1642) intensity ratio thus reflect the population of molecules in the particular state, i.e., a slightly increasing number of the DMAc monomers at the expense of the dimers that get disrupted when temperature increases. Moreover, because IR absorption coefficients of the DMAc monomers and dimers are very close according to our calculations, the I(1660)/I(1642) ratio clearly indicates the prevalence of the dimeric structure in liquid DMAc at all the temperatures studied. Note that concurrently with the change in the band intensity ratio, the profile of the band at 1642 cm–1 slightly changes with temperature, resulting into the shift of the peak position to higher wavenumbers. Since the results of quantum chemical model calculations indicate possibility of existence of more than two dimeric structures, we attribute this blue-shift to the change in population of different dimeric structures of similar energy. Additionally, the complex band composition resulting from the presence of several dimeric structures is probably the reason for the observed difference in the peak position in the second derivative (1639 cm–1; Figure 1A) and in the deconvoluted (1642 cm–1; Figure 1B) spectra. In their report, Katsumoto et al. also studied how dilution of DMAc with CCl4 affects the ν(C=O) region in the IR spectrum of DMAc.32 They observed that the dilution results in an increase in the I(1660)/I(1642) intensity ratio, which they related to a change in the dielectric constant of the mixture and also to an unexpectedly complex solvation effect of CCl4. Since we proposed the alternative assignment of the ν(C=O) bands at 1660 and 1642 cm–1, we decided to reproduce the experiment and diluted DMAc with CCl4 to 0.28 wt% (0.49 mol%). (Figure S2). In the ν(C=O) region (Figure S2B), we indeed detected a substantial increase in 9 ACS Paragon Plus Environment


Page 10 of 27

the I(1660)/I(1642) intensity ratio. In addition, the position of the band at 1660 cm–1 showed similar insensitivity to dilution as to temperature changes. Based on our assignment of these bands, we attribute this observation to the increasing concentration of DMAc monomers at the expense of the dimeric structures in the highly diluted solution. Importantly, we also observed additional changes in the ν(C–H) vibrations region (Figure S2A) where bands at 3014 and 2932 cm–1 blue-shifted by 6 cm–1. These bands have been previously assigned to asymmetric and symmetric C–H stretching vibrations of NCH3 in DMAc, respectively.25,46 Because these shifts are concurrent with the changes in the ν(C=O) region, they should be related to the same phenomenon. The detected blue-shift could then be associated with the breakage of the C–H···O=C weak hydrogen bonds between NCH3 and C=O groups in DMAc dimers which, in turn, leads to strengthening of the respective C–H bonds. In general, the formation of weak hydrogen bonds can be associated with blue- or red-shift of the corresponding vibrational bands.47–49 Our data indicate that in the present system the latter variant is taking place.

DMAc/LiCl mixtures Having assessed the structure of neat DMAc, we set out to investigate by IR spectroscopy how the structure of DMAc changes upon addition of LiCl. After dissolution of LiCl, clear changes in the ATR FTIR spectrum of the solvent were detected not only in the ν(C=O) region (Figure 3A), which has already been reported by other groups,13,14,46 but also in the regions of C–CH3 and N–(CH3)2 stretching vibrations (ν(C–CH3) and ν(N–(CH3)2; Figures 3B and 3C). In their fundamental work, Chalapathi and Ramiah proposed that in neat DMAc the vibrations of the C=O group are coupled with the vibrations of the N–CH3 groups and also noted that a strong interaction between the modes of stretching vibrations of the C–CH3 and N–(CH3)2 groups takes place.25 In their assessment of the DMAc/LiCl spectrum, other researchers largely ignored changes occurring in the low-wavenumber region of the DMAc spectrum, possibly due to the low intensity of these bands.13,14,32 Nevertheless, we decided to include this region into our investigation as these bands might provide additional information about DMAc-LiCl interactions (Figure 3).


Page 11 of 27 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. ATR FTIR spectra of DMAc/LiCl solutions of various concentrations: (A) the ν(C=O) region, (B) the ν(C–CH3) region, and (C) the ν(N–(CH3)2) region. All the spectra were acquired at laboratory temperature and offset for clarity. Note that each figure section (A, B, and C) has its own absorbance scale; intensities of the ν(C–CH3) and ν(N–(CH3)2) were scaled by ca. 15 and 12 times, respectively. In agreement with literature, the shift of the ν(C=O) maximum from 1645 cm–1 to the lower wavenumbers is observed due to the formation of [(DMAc)x-LiCl] complexes with increasing LiCl concentration (Figure 3A).13,14,46 Moreover, in the respective ATR FTIR spectra, the bands at 966 and 745 cm–1 appeared (Figures 3B and 3C). With increasing LiCl concentration, the intensity of these bands increased at the expense of the intensity of the nearby bands at 958 and 735 cm–1, assigned to the ν(C–CH3) and ν(N–(CH3)2) of neat DMAc.25 This concentration dependence implies that the growing bands correspond to the ν(C–CH3) and ν(N–(CH3)2) of the DMAc molecules interacting with LiCl. The blue-shift then reflects the strengthening of the C–C and N–C covalent bonds of DMAc molecules stemming from the attraction of the C=O group oxygen to the lithium atom of LiCl.



Page 12 of 27

Figure 4. The ν(C=O) region of DMAc in DMAc/LiCl mixtures of different concentrations: solid lines – the original ATR FTIR spectra, dashed lines – the respective deconvolution spectra. All the spectra were acquired at the laboratory temperature and offset for clarity. Figure 4 shows how the ν(C=O) region develops with increasing concentration of LiCl in DMAc/LiCl mixtures. Peak deconvolution revealed a new peak at 1629 cm–1 in addition to the original peaks at 1660 and 1642 cm–1 previously observed in the spectrum of neat DMAc. The intensity of the peak at 1629 cm–1 increased with increasing LiCl concentration while the intensities of the peaks at 1660 and 1642 cm–1 decreased proportionally. Based on this observation, we assigned the peak at 1629 cm–1 to the ν(C=O) of the DMAc molecules interacting with LiCl and the bands at 1660 and 1642 cm–1 to the DMAc molecules (monomers and dimers, respectively) not interacting with LiCl. Apparently, the DMAc dimers prevail over monomers also in DMAc/LiCl mixtures (Figure 4). It needs to be highlighted that the proposed assignment of the ν(C=O) bands is extremely important as it allows us to easily estimate the proportion of DMAc molecules involved in the [(DMAc)x-LiCl] complex formation. This is possible because the molar absorption coefficients differ only negligibly for the respective C=O groups according to our theoretical calculations. Note that the ν(C–CH3) and ν(N–(CH3)2 bands could be theoretically also used for this assessment, but the calculations show that for those vibrations the molar absorption coefficients differ significantly for the interacting and non-interacting DMAc molecules. 12 ACS Paragon Plus Environment

Page 13 of 27 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


Subsequently, with the knowledge of molar composition of the mixture, we can easily calculate the average stoichiometry of the [(DMAc)x-LiCl] complex. For instance, only about half of DMAc molecules are in a direct interaction with LiCl in the most concentrated DMAc/LiCl mixture of this study (15 mol% of LiCl). This might be rather surprising for such a highly concentrated mixture (the saturation limit is 16 mol%50), but such behavior has been previously observed also for other mixtures of salts and amides, e.g., formamide, Nmethylformamide, or DMF.29 Furthermore, since the molar ratio of the components in this mixture is 5.7:1 (DMAc:LiCl), the actual average stoichiometry of the complex is about 2.8:1. It follows that this sample most likely involves the mixture of [(DMAc)2-LiCl] and [(DMAc)3-LiCl] complexes with the latter being dominant. To further investigate how the LiCl presence influences DMAc and what is the structure of DMAc-LiCl complexes in the concentrated DMAc/LiCl solutions, the two mixtures with the highest LiCl content (2.6 and 7.9 wt% LiCl) were investigated by ATR FTIR at temperatures from 5 to 60 °C with the 5 °C increment. The selected concentrations of LiCl are relevant from the practical point of view as they are utilized for cellulose dissolution6,7,51 or its characterisation.11,12 For the sake of clarity, only the ATR FTIR spectra acquired at temperatures 5, 20, and 60 ºC are presented in Figure 5 (7.9 wt% LiCl) and in Figure 6 (2.6 wt% LiCl); the deconvoluted spectra are included for the ν(C=O) region (Figures 5A and 6A).



Page 14 of 27

Figure 5. ATR FTIR spectra of 7.9 wt% DMAc/LiCl measured at 5, 20, and 60 °C (solid lines): (A) the ν(C=O) region and the respective deconvoluted spectra after the curve fitting (dashed lines), (B) the ν(C–CH3) region, and (C) the ν(N–(CH3)2) region. Note that each figure section (A, B, and C) has its own absorbance scale; intensities of the ν(C–CH3) and ν(N–(CH3)2) were scaled by ca. 15 and 12 times, respectively. All the spectra were offset for clarity.


Page 15 of 27 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 6. ATR FTIR spectra of the 2.6 wt% DMAc/LiCl mixture measured at 5, 20, and 60 °C (solid lines): (A) the ν(C=O) region and the respective deconvoluted spectra after the curve fitting (dashed lines), (B) the ν(C–CH3) region, and (C) the ν(N–(CH3)2) region. Note that each figure section (A, B and C) has its own absorbance scale; intensities of the ν(C–CH3) and ν(N–(CH3)2) were scaled by ca. 15 and 12 times, respectively. All the spectra were offset for clarity. In the ATR FTIR spectra of both the mixtures, the bands attributed to the DMAc molecules interacting with LiCl (1629, 966, and 745 cm–1) and to the non-interacting DMAc (1660, 1642, 958, and 735 cm–1) were detected at all temperatures; no new bands were found in the spectra. The bands exhibited no shifts with the sole exception of the peak at 1642 cm–1 where a small shift (within 3 cm–1) is attributed to the change in the concentration of DMAc dimers as discussed above. Strikingly, considerable differences in the sensitivity of the I(1629)/I(1642+1660) band intensity ratio towards temperature were observed for the two mixtures studied. In the concentrated mixture (7.9 wt% of LiCl), the intensity of the key bands was virtually constant at all the temperatures (Figures 5, S3, S4, and S5). This clearly implies that the average stoichiometry of the DMAc-LiCl complexes did not change with temperature (at least in the temperature range studied), remaining at about 2.8:1 (DMAc:LiCl)



Page 16 of 27

as calculated above. Conversely, in the diluted mixture (2.6 wt%, i.e., 5.2 mol% of LiCl), the intensity of the bands at 1629, 966, and 745 cm–1 increased with temperature while that of the bands at 1660, 1642, 958, and 735 cm–1 decreased (Figures 6, S3, S4, and S5). To illustrate the impact of these changes, we calculated the average stoichiometry of the DMAc-LiCl complexes at different temperatures using the same approach as introduced above. As can be seen in Figure 7, the average stoichiometry changes from ca. 2.2:1 at 5 °C, over ca. 3.3 at 45 °C, to 4.5:1 at 60°C. Note, however, that the values for the last three temperatures of the range (50, 55, and 60 °C) are artificially inflated to an unknown extent by DMAc evaporation from the ATR crystal that is more pronounced for the diluted mixtures (see Materials and Methods). Considering the development of the dependence at lower temperatures, stoichiometry of 3:1 to 4:1 appears to be more realistic at the higher temperatures. Therefore, in the dilute DMAc/LiCl mixture, LiCl is probably present in the form of the [(DMAc)2-LiCl] and [(DMAc)3-LiCl] complexes, with an unknown contribution of the [(DMAc)4-LiCl] complex at higher temperatures. It is noteworthy that at room temperature (ca 20 °C), the average stoichiometry of the DMAc-LiCl complexes is very similar in both the mixtures studied (the intersection of the dependencies in Figure 7).

Figure 7. Change of the average stoichiometry of the DMAc-LiCl complexes with temperature.


Page 17 of 27 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 8. Optimized structures of the most stable complexes of LiCl with (A) two and (B) three solvating molecules of DMAc calculated at the B3LYP-GD3BJ/6-311+G(d,p) level of theory. The interatomic distances are in Å. Oxygen atoms are in red, hydrogens ˗ white, and nitrogens ˗ blue. To provide further support to the experimental data, we performed DFT calculations for the structures involving 1 to 4 DMAc molecules interacting with LiCl. The calculation suggested that LiCl might form stable complexes with 1 to 4 molecules of DMAc. However, the complex with 1 DMAc molecule did not form a sufficiently closed structure and thus was not further considered. Further, the calculations revealed that the formation of stable [(DMAc)4LiCl] complex requires dissociation of LiCl into Li+ and Cl–. Solvation of the dissociated Cl– would then involve two additional DMAc molecules. As a result, differences in characteristic vibrations of DMAc molecules in bulk and in the primary solvation shells of Li+ and Cl– should be seen. However, this was not observed in the acquired ATR FTIR spectra, which implies that the [(DMAc)2-LiCl] and [(DMAc)3-LiCl] complexes are the prevalent structures. For these complexes, the stabilization Gibbs free energy ∆G was calculated with respect to the free LiCl ion pair and free DMAc molecules, being 31.3 kcal for [(DMAc)2-LiCl] (Figure 8A) and 33.6 kcal/mol for [(DMAc)3-LiCl] (Figure 8B), respectively. DFT calculations of the vibrational frequencies then showed that the vibrations ν(C=O), ν(C–CH3), and ν(N–(CH3)2) should be shifted in the complexes with respect to the free DMAc molecule by –57, +10, and +23 cm–1 for [(DMAc)2-LiCl] and by –52, +8, and +19 cm–1 for [(DMAc)3-LiCl]. The calculated frequency shifts for both structures are in a good agreement with the experimental data where, upon addition of LiCl to DMAc, the bands shifted from 1660 (ν(C=O) of



Page 18 of 27

monomers) to 1629 cm–1 (–31 cm–1) for ν(C=O), from 958 to 966 cm–1 (+8 cm–1) for ν(C– CH3), and from 735 to 745 cm–1 (+10 cm–1) for ν(N–(CH3)2). These theoretical results indicate that we can hardly distinguish complexes with two or three DMAc molecules by the characteristic vibrations of DMAc in ATR FTIR spectra. Nevertheless, both the theoretically predicted stoichiometries are in a good agreement with the experimental data, especially for the concentrated DMAc/LiCl mixture. Current literature contains conflicting views about the stoichiometry of the DMAc-LiCl complexes, particularly with regard to the mechanism of cellulose dissolution. For example, in recent works, one2,52 or four molecules of DMAc13 have been proposed to interact with LiCl, affording the [DMAc-LiCl] or [(DMAc)4-Li+Cl–] complexes, respectively. Moreover, these studies propose that the interactions between cellulose chains and positively charged moieties/macrocations of the solvent are necessary for efficient dissolution of the biopolymer. Importantly, our results indicate that in concentrated DMAc/LiCl mixtures exploited for cellulose dissolution, ([(DMAc)3-LiCl]) and ([(DMAc)2-LiCl]) are the prevalent complexes, which should result in the different spatial arrangement, charge distribution, or complex size than those suggested in literature. When studying neat DMAc, we addressed also the changes in the high wavenumber region of the spectrum. Based on these data, we proposed that the (red-shifting) C–H···O=C weak hydrogen bonds is responsible for the stabilization of DMAc dimers. Since the added LiCl is supposed to interact with the C=O group, we wanted to verify if this interaction impacts on the weak hydrogen bond stability. We found that the band at 3014 cm–1, previously associated with the formation of DMAc dimers, exhibited further red-shift by 2 cm–1 with increasing LiCl concentration (Figure S6). This might reflect certain strengthening of the intermolecular hydrogen bonding in the dimeric structures. To lend support to this hypothesis, we performed quantum chemical model calculations assessing the probable interactions between two DMAc molecules, one in a direct interaction with LiCl and the other one non-interacting (Figure S7). The obtained results indicated that the interaction of DMAc with lithium induces a shift of the charge density in this DMAc molecule, making the NCH3 hydrogens more prone to establishing the C–H···O=C weak hydrogen bonds with the adjacent free DMAc molecule. In other words, the interaction with lithium might lead to formation of more stable DMAc dimeric structures. It is currently unclear how these enhanced interactions contribute to the


Page 19 of 27 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 behavior of the saturated DMAc/LiCl mixture, particularly, to the large proportion of DMAc molecules not involved in the direct interaction with LiCl.

Conclusions Structures adopted by neat DMAc and DMAc/LiCl were investigated by ATR FTIR spectroscopy within the 5–60 °C temperature range, and the results were complemented by the quantum chemical model calculations on the DFT level of theory. In the ATR FTIR spectra of neat DMAc, the complex band at 1645 cm–1 was shown to consist of two peaks at 1660 and 1642 cm–1, newly attributed to the stretching vibrations of the C=O groups of DMAc monomers and dimers, respectively. Moreover, the observed diminishing of the band attributed to the DMAc dimers at elevated temperatures and, especially, at high dilution with CCl4 is also supporting this assignment. Importantly, the dimeric structure was found to be dominant in neat DMAc as well as in bulk DMAc within the DMAc/LiCl mixtures. We propose that weak hydrogen bonds play an important role in stabilizing these dimeric structures and the behavior is thus similar to that of other amides. After dissolution of LiCl in DMAc, the bands attributed to DMAc molecules interacting with LiCl appeared in the respective ATR FTIR spectra, indicating that [(DMAc)x-LiCl] complexes exist in the mixtures. Although existence of such complexes is not generally disputed, the current literature lacks a consensus about the structure of these complexes. Our results showed that in the concentrated DMAc/LiCl mixture (i.e., setting relevant for cellulose dissolution) the [(DMAc)2-LiCl] and [(DMAc)3-LiCl] complexes are the dominant structures. The average stoichiometry showed negligible changes with temperature, despite the fact that only about half of the present DMAc molecules interacted directly with LiCl, with the other half being obviously unavailable for the complex formation. On the other hand, in the diluted DMAc/LiCl mixture, free DMAc molecules were available, and so the complex stoichiometry was temperature-dependent, with the complexes involving more DMAc molecules formed at higher temperatures. Based on these results, we propose that the future models of cellulose dissolution will need to take into account the observed dynamic character of the DMAc/LiCl system.



Page 20 of 27

AUTHOR INFORMATION

Corresponding Author E-mail: [email protected].

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

Supporting Information Figure S1. Peaks in the ν(C=O) vibrations region in ATR FTIR spectrum of neat DMAc at various temperatures obtained by deconvolution and the subsequent curve fitting of the respective normalized region. Figure S2. Regions of (A) ν(C–H) and (B) ν(C=O) vibrations in ATR FTIR spectra of neat DMAc (black solid line) and DMAc/CCl4 (0.49/99.51 mol%; pink solid line) acquired at laboratory temperature (ca. 20 °C). The second derivative spectrum of neat DMAc is displayed for comparison (offset black dash dotted line). The displayed spectra underwent baseline and ATR corrections. The spectrum of DMAc/CCl4 (pink) is scaled ca. 50 times owing to the low intensity of the bands due to dilution. Figure S3. Temperature dependence of the integral intensity of the peak at 1660 cm–1 (based on the calculated deconvoluted spectra of the respective normalized region of ν(C=O) in ATR FTIR spectra). Figure S4. Temperature dependence of the integral intensity of the peak at 1642 cm–1 (based on the calculated deconvoluted spectra of the respective normalized region of ν(C=O) in ATR FTIR spectra).


Page 21 of 27 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 S5. Temperature dependence of the integral intensity of the peak at 1629 cm–1 (based on the calculated deconvoluted spectra of the respective normalized region of ν(C=O) in ATR FTIR spectra). Figure S6. Region of ν(C–H) in ATR FTIR spectra of neat DMAc (black solid line) and of DMAc/LiCl 7.9 wt% mixture (violet solid line) at 20 °C. The respective second derivative spectra are also displayed for comparison (offset dash dotted lines). The displayed spectra underwent baseline and ATR corrections. Figure S7. Optimized structure of the (DMAc)2-LiCl complex with one of the DMAc molecules forming a dimeric structure with another DMAc molecule from the second solvation sphere, calculated at the B3LYP-GD3BJ/6-311+G(d,p) level. The interatomic distances are in Å. Oxygen atoms are in red color, hydrogens – white, and nitrogens – blue. ACKNOWLEDGMENT The authors are grateful to the Czech Science Foundation for financial support (Grant No. 1703810S). We wish to thank Dr. Michal Bláha for his valuable comments on the manuscript. We also thank Mr. Štěpán Adamec for his assistance with sample preparation and Mrs. Miroslava Brunclíková for her help with infrared measurements.



Page 22 of 27

References (1)

Lindman, B.; Karlström, G.; Stigsson, L. On the Mechanism of Dissolution of Cellulose. J. Mol. Liq. 2010, 156 (1), 76–81.

(2)

Medronho, B.; Lindman, B. Competing Forces during Cellulose Dissolution: From Solvents to Mechanisms. Curr. Opin. Colloid Interface Sci. 2014, 19 (1), 32–40.

(3)

Rosenau, T.; Potthast, A.; Sixta, H.; Kosma, P. The Chemistry of Side Reactions and Byproduct Formation in the System NMMO/Cellulose (Lyocell Process). Prog. Polym. Sci. 2001, 26 (9), 1763–1837.

(4)

Perepelkin, K. E. Lyocell Fibres Based on Direct Dissolution of Cellulose in NMethylmorpholine N-Oxide: Development and Prospects. Fibre Chem. 2007, 39 (2), 163–172.

(5)

Klemm, D.; Heublein, B.; Fink, H.-P. P.; Bohn, A. Cellulose: Fascinating Biopolymer and Sustainable Raw Material. Angew. Chemie - Int. Ed. 2005, 44 (22), 3358–3393.

(6)

Striegel, A. Theory and Applications of DMAC/LICL in the Analysis of Polysaccharides. Carbohydr. Polym. 1997, 34 (4), 267–274.

(7)

McCormick, C. L.; Callais, P. A.; Hutchinson, B. H. Solution Studies of Cellulose in Lithium Chloride and N,N-Dimethylacetamide. Macromolecules 1985, 18 (12), 2394– 2401.

(8)

Ramos, L. A.; Morgado, D. L.; El Seoud, O. A.; da Silva, V. C.; Frollini, E. Acetylation of Cellulose in LiCl-N,N-Dimethylacetamide: First Report on the Correlation between the Reaction Efficiency and the Aggregation Number of Dissolved Cellulose. Cellulose 2011, 18 (2), 385–392.

(9)

Heinze, T.; Liebert, T. Unconventional Methods in Cellulose Functionalization. Prog. Polym. Sci. 2001, 26 (9), 1689–1762.

(10)

Raus, V.; Štěpánek, M.; Uchman, M.; Šlouf, M.; Látalová, P.; Čadová, E.; Netopilík, M.; Kříž, J.; Dybal, J.; Vlček, P. Cellulose-Based Graft Copolymers with Controlled Architecture Prepared in a Homogeneous Phase. J. Polym. Sci. Part A Polym. Chem. 2011, 49 (20), 4353–4367.

(11)

Potthast, A.; Radosta, S.; Saake, B.; Lebioda, S.; Heinze, T.; Henniges, U.; Isogai, A.; Koschella, A.; Kosma, P.; Rosenau, T.; et al. Comparison Testing of Methods for Gel 22 ACS Paragon Plus Environment

Page 23 of 27 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


Permeation Chromatography of Cellulose: Coming Closer to a Standard Protocol. Cellulose 2015, 22 (3), 1591–1613. (12)

Striegel, A. M.; Timpa, J. D. Gel Permeation Chromatography of Polysaccharides Using Universal Calibration. Int. J. Polym. Anal. Charact. 1996, 2 (3), 213–220.

(13)

Zhang, C.; Liu, R.; Xiang, J.; Kang, H.; Liu, Z.; Huang, Y. Dissolution Mechanism of Cellulose in N,N -Dimethylacetamide/Lithium Chloride: Revisiting through Molecular Interactions. J. Phys. Chem. B 2014, 118 (31), 9507–9514.

(14)

Balasubramanian, D.; Shaikh, R. On the Interaction of Lithium Salts with Model Amides. Biopolymers 1973, 12 (7), 1639–1650.

(15)

Hayashi, T.; Mukamel, S. Vibrational−Exciton Couplings for the Amide I, II, III, and A Modes of Peptides. J. Phys. Chem. B 2007, 111 (37), 11032–11046.

(16)

Surewicz, W. K.; Mantsch, H. H. New Insight into Protein Secondary Structure from Resolution-Enhanced Infrared Spectra. Biochim. Biophys. Acta - Protein Struct. Mol. Enzymol. 1988, 952, 115–130.

(17)

Miller, L. M.; Bourassa, M. W.; Smith, R. J. FTIR Spectroscopic Imaging of Protein Aggregation in Living Cells. Biochim. Biophys. Acta - Biomembr. 2013, 1828 (10), 2339–2346.

(18)

Andrew Chan, K. L.; Kazarian, S. G. Attenuated Total Reflection Fourier-Transform Infrared (ATR-FTIR) Imaging of Tissues and Live Cells. Chem. Soc. Rev. 2016, 45 (7), 1850–1864.

(19)

Šturcová, A.; Dybal, J.; Zhigunov, A.; Kotov, N.; Braunová, A. The Effect of Micellization-Induced Deprotonation on the Associative Behavior of a Carboxyl Modified Pluronic P85. Soft Matter 2014, 10 (40), 8011–8022.

(20)

Abidi, N.; Cabrales, L.; Hequet, E. Fourier Transform Infrared Spectroscopic Approach to the Study of the Secondary Cell Wall Development in Cotton Fiber. Cellulose 2009, 17 (2), 309–320.

(21)

Kazarian, S. G.; Vincent, M. F.; Bright, F. V.; Liotta, C. L.; Eckert, C. A. Specific Intermolecular Interaction of Carbon Dioxide with Polymers. J. Am. Chem. Soc. 1996, 118 (7), 1729–1736.

(22)

Sedlák, M.; Koňák, C.; Dybal, J. Heat-Set Poly(Ethylacrylic Acid) Nanoparticles:



Page 24 of 27

Combined Light Scattering, Calorimetric, and FTIR Study. Macromolecules 2009, 42 (19), 7439–7446. (23)

Liao, L.-F.; Lien, C.-F.; Shieh, D.-L.; Chen, F.-C.; Lin, J.-L. FTIR Study of Adsorption and Photochemistry of Amide on Powdered TiO2: Comparison of Benzamide with Acetamide. Phys. Chem. Chem. Phys. 2002, 4 (18), 4584–4589.

(24)

Maeda, Y.; Nakamura, T.; Ikeda, I. Change in Solvation of Poly( N , N Diethylacrylamide) during Phase Transition in Aqueous Solutions As Observed by IR Spectroscopy. Macromolecules 2002, 35 (27), 10172–10177.

(25)

Chalapathi, V. V.; Ramiah, V. K. Normal Vibrations of N, N-Dimethylformamide and N, N-Dimethylacetamide. Proc. Indian Acad. Sci. - Sect. A 1968, 68 (3), 109–122.

(26)

Mortensen, A.; Nielsen, O. F.; Yarwood, J.; Shelley, V. Vibrational Spectra of Mixtures of Isotopomers of Formamide. Anomalies in the Carbonyl Stretching Region. J. Phys. Chem. 1994, 98 (20), 5221–5226.

(27)

Borrmann, H.; Persson, I.; Sandström, M.; Stlhandske, C. M. V. The Crystal and Liquid Structures of N,N-Dimethylthioformamide and N,N-Dimethylformamide Showing a Stronger Hydrogen Bonding Effect for C–H · · · S than for C–H · · · O †. J. Chem. Soc. Perkin Trans. 2 2000, 0, 393–402.

(28)

Bukowska, J. Intermolecular Coupling of the CO Stretching Vibrations in Electrolyte Solutions of Carbonyl Compounds. J. Mol. Struct. 1986, 143, 309–312.

(29)

Bukowska, J. Raman and Infrared Studies of Interactions between Amides and Ions. J. Mol. Struct. 1983, 98 (1–2), 1–10.

(30)

Perelygin, I. S.; Osipov, V. S. Investigation of the Ion-Molecular Interactions in Dimethylformamide Medium by the Method of Infrared Spectroscopy. J. Appl. Spectrosc. 1985, 43 (6), 1346–1350.

(31)

Perelygin, I. S. Band Shape in Vibrational Spectra and Interactions of Polar Molecules in Liquids. J. Struct. Chem. 1997, 38 (2), 218–226.

(32)

Katsumoto, Y.; Tanaka, T.; Ozaki, Y.; Hosoi, S. Effects of Dipole Interaction and Solvation on the CO Stretching Band of N,N-Dimethylacetamide in Nonpolar Solutions: Infrared, Isotropic and Anisotropic Raman Measurements. Vib. Spectrosc. 2009, 51 (1), 119–124.


Page 25 of 27 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


(33)

Zhang, C.; Ren, Z.; Liu, L.; Yin, Z. Modelling Hydrogen Bonds in N N Dimethylformamide. Mol. Simul. 2013, 39 (11), 875–881.

(34)

Derewenda, Z. S.; Lee, L.; Derewenda, U. The Occurence of C–H · · · O Hydrogen Bonds in Proteins. J. Mol. Biol. 1995, 252 (2), 248–262.

(35)

Fabiola, G. F.; Krishnaswamy, S.; Nagarajan, V.; Pattabhi, V. C—H...O Hydrogen Bonds in β-Sheets. Acta Crystallogr. Sect. D Biol. Crystallogr. 1997, 53 (3), 316–320.

(36)

Park, H.; Yoon, J.; Seok, C. Strength of C α −H···OC Hydrogen Bonds in Transmembrane Proteins. J. Phys. Chem. B 2008, 112 (3), 1041–1048.

(37)

Vargas, R.; Garza, J.; Dixon, D. A.; Hay, B. P. How Strong Is the Cα −H···O=C Hydrogen Bond? J. Am. Chem. Soc. 2000, 122 (19), 4750–4755.

(38)

Cato, J. M. A.; Majumdar, D.; Roszak, S.; Leszczynski, J. Exploring Relative Thermodynamic Stabilities of Formic Acid and Formamide Dimers – Role of LowFrequency Hydrogen-Bond Vibrations. J. Chem. Theory Comput. 2013, 9 (2), 1016– 1026.

(39)

Adhikari, U.; Scheiner, S. Preferred Configurations of Peptide–Peptide Interactions. J. Phys. Chem. A 2013, 117 (2), 489–496.

(40)

Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A Consistent and Accurate Ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H-Pu. J. Chem. Phys. 2010, 132 (15), 154104.

(41)

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.

(42)

Boys, S. F.; Bernardi, F. The Calculation of Small Molecular Interactions by the Differences of Separate Total Energies. Some Procedures with Reduced Errors. Mol. Phys. 1970, 19 (4), 553–566.

(43)

Pacák, P. Refractivity and Density of Some Organic Solvents. Chem. Pap. ZVESTI 1991, 45, 227–232.

(44)

Wohlfarth, C. Refractive Index of N,N-Dimethylacetamide. In Optical Constants. Condensed Matter; Lechner, M. D., Ed.; Springer, Berlin, Heidelberg, 2017; Vol. 50, p 151.



(45)

Page 26 of 27

Whitfield, T. W.; Martyna, G. J.; Allison, S.; Bates, S. P.; Vass, H.; Crain, J. Structure and Hydrogen Bonding in Neat N-Methylacetamide: Classical Molecular Dynamics and Raman Spectroscopy Studies of a Liquid of Peptidic Fragments†. J. Phys. Chem. B 2006, 110 (8), 3624–3637.

(46)

Durgaprasad, G.; Sathyanarayana, D. N.; Patel, C. C.; Randhawa, H. S.; Goel, A.; Rao, C. N. R. Normal Vibrations of N,N-Dimethylacetamide. Spectrochim. Acta Part A Mol. Spectrosc. 1972, 28 (12), 2311–2318.

(47)

Sato, H.; Dybal, J.; Murakami, R.; Noda, I.; Ozaki, Y. Infrared and Raman Spectroscopy and Quantum Chemistry Calculation Studies of C–H⋯O Hydrogen Bondings and Thermal Behavior of Biodegradable Polyhydroxyalkanoate. J. Mol. Struct. 2005, 744–747 (19), 35–46.

(48)

Hobza, P.; Havlas, Z. Blue-Shifting Hydrogen Bonds. Chem. Rev. 2000, 100 (11), 4253–4264.

(49)

Joseph, J.; Jemmis, E. D. Red-, Blue-, or No-Shift in Hydrogen Bonds: A Unified Explanation. J. Am. Chem. Soc. 2007, 129 (15), 4620–4632.

(50)

Potthast, A.; Rosenau, T.; Buchner, R.; Röder, T.; Ebner, G.; Bruglachner, H.; Sixta, H.; Kosma, P. The Cellulose Solvent System N,N-Dimethylacetamide/Lithium Chloride Revisited: The Effect of Water on Physicochemical Properties and Chemical Stability. Cellulose 2002, 9 (1), 41–53.

(51)

Khaliq, Z.; Kim, B. C. Molecular Characterization of Thermoreversibility and Temperature Dependent Physical Properties of Cellulose Solution in N,NDimethylacetamide and Lithium Chloride. Macromol. Res. 2016, 24 (6), 547–555.

(52)

Gross, A. S.; Bell, A. T.; Chu, J.-W. Preferential Interactions between Lithium Chloride and Glucan Chains in N , N -Dimethylacetamide Drive Cellulose Dissolution. J. Phys. Chem. B 2013, 117 (12), 3280–3286.


Page 27 of 27 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 Graphic


Intermolecular Interactions in N,N-Dimethylacetamide without and with

Recommend Documents