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Hydrogen bonding in protic and aprotic amide mixtures: Low-frequency Raman spectroscopy, small-angle neutron scattering, and molecular dynamics simulations
Journal of Molecular Liquids 238 (2017) 518–522
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Hydrogen bonding in protic and aprotic amide mixtures: Low-frequency Raman spectroscopy, small-angle neutron scattering, and molecular dynamics simulations Kenta Fujii a,⁎, Mari Yoshitake a, Hikari Watanabe b, Toshiyuki Takamuku c, Yasuhiro Umebayashi b a b c
Graduate School of Sciences and Technology for Innovation, Yamaguchi University, 2-16-1 Tokiwadai, Ube, Yamaguchi 755-8611, Japan Graduate School of Science and Technology, Niigata University, 8050 Ikarashi, 2-no-cho, Nishi-ku, Niigata 950-2181, Japan Department of Chemistry and Applied Chemistry, Graduate School of Science and Engineering, Saga University, Honjo-machi, Saga 840-8502, Japan
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Article history: Received 13 April 2017 Received in revised form 1 May 2017 Accepted 5 May 2017 Available online 10 May 2017 Keywords: Hydrogen-bonding interactions Amide solvents Low-frequency Raman spectroscopy Molecular dynamics simulations
a b s t r a c t The hydrogen-bonding interactions in protic and aprotic amide solvent mixtures, i.e., formamide (FA) and N,Ndimethylformamide (DMF), were investigated via low-frequency Raman spectroscopy, small-angle neutron scattering (SANS) experiments, and molecular dynamics (MD) simulations. In a neat amide system, the low-frequency Raman spectra R(ν)s were well reproduced by the corresponding S(ν) spectra derived from the MD simulations. The observed peaks in R(ν)s at around b 200 cm−1 were assigned to the intermolecular interactions, particularly in terms of the hydrogen-bonding network formation and its dimensionality in the liquid state. The SANS experiments for the FA–DMF mixtures demonstrated that the FA molecules forming an extended three–dimensional hydrogen-bonding structure in the neat system interacted with DMF molecules through the hydrogen bonds in the mixtures over the whole range of solvent compositions, resulting in a homogeneous mixing state. Additionally, the R(ν) spectra for the mixtures were represented by the corresponding S(ν) spectra. From the R(ν) and S(ν) spectra of the FA–DMF mixtures, we found that (1) the Raman band at around 110 cm−1 mainly originates from the libration mode of amide molecules in the chain-like hydrogen-bonded structure and (2) the higher frequency band (approximately 200 cm−1) was attributed to the libration of the FA molecule restricted by the three-dimensional hydrogen-bonded network, which remained even in the DMF-rich compositions. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Formamide (FA) is a typical amide solvent with two proton donor sites at the − NH2 group and one proton acceptor site at the C_O group, that forms a strong liquid structure through intermolecular \\NH ⋯ O_C\\ hydrogen bonding [1–7]. The FA molecule has been often regarded as a simple model unit in peptide chains; thus, the hydrogen-bonding behaviors of amide molecules are an important issue in biochemistry for understanding the physicochemical properties of proteins [8–12]. The FA molecule can form a cyclic dimer and a chain-like structure because NH hydrogen locates at both cis and trans to the C_O group. Indeed, the hydrogen-bonding network in the crystalline state is constructed from these two types of structures [13]. To date, the intermolecular interactions of FA in the liquid state have been studied, e.g., the liquid structure based on X-ray diffraction reported by Ohtaki et al. implied that liquid FA comprises a chain-like structure and a distorted cyclic dimer structure to form two-dimensional networks ⁎ Corresponding author. E-mail address: [email protected] (K. Fujii).
http://dx.doi.org/10.1016/j.molliq.2017.05.017 0167-7322/© 2017 Elsevier B.V. All rights reserved.
containing the two types of structures [1–3]. On the other hand, according to theoretical studies by means of molecular dynamics (MD) simulations, the liquid FA structure is regarded as a three-dimensional NH⋯O hydrogen-bonding network and the cyclic dimers that are the dominant component of the crystalline phase play only a minor role in the network structure [5–7]. N-Methylformamide (NMF) with NH hydrogen trans to the C_O group forms only a chain-like structure in the liquid state, which has been established from several technics such as vibrational spectroscopy, dielectric relaxation, X-ray scattering, and molecular orbital/molecular dynamics simulations [4,14–22], as well as the studies in FA system as mentioned above. Thus, the dimensionality in the hydrogen-bonding network is higher for FA than for NMF. Indeed, the boiling and melting points and the viscosity are higher for FA than for NMF [23]. We reported the hydrogen-bonding structure in liquid NMF and its binary mixtures with aprotic amide N,N-dimethylformamide (DMF), which is a less structured solvent due to the dipole–dipole interactions [7,16]. We found that (1) the DMF molecules rupture the chain-like structure of NMF in the mixtures and (2) the DMF and NMF molecules are homogeneously mixed through hydrogen bonds over the entire range of solvent compositions in the mixture. On the other hand, it
K. Fujii et al. / Journal of Molecular Liquids 238 (2017) 518–522
has been established that water with a three-dimensional hydrogenbonding network is heterogeneously mixed with conventional organic solvents, such as alcohols, acetonitrile, and dioxane [24–26]. This is due to the presence of self-aggregated water clusters in the mixtures. This study investigates the effect of the hydrogen-bonding network dimensionality on the mixing state and the intermolecular interactions in binary solvent mixtures. The liquid structure and dynamics in the amide mixtures of protic FA (extended hydrogen-bonding network) and aprotic DMF (without hydrogen bonding) were studied using low-frequency Raman spectroscopy and MD simulations. The experimental results were discussed comparing with those of the mixtures of protic NMF (one-dimensional chain-like hydrogen bonding) and DMF. In addition, small-angle neutron scattering (SANS) experiments were performed for the FA–DMF mixtures to clarify the mixing state, homogeneous or heterogeneous, at the mesoscopic level.
2. Experimental 2.1. Materials FA and NMF were dried over 3 Å molecular sieves for several weeks, distilled at 328 K under reduced pressure, and stored in a dark bottle with a P2O5 drying tube. DMF was dried over 4 Å molecular sieves for several weeks and distilled at 303 K under reduced pressure. The water content was checked by Karl Fischer titration to be b 200 ppm. All materials and solutions were treated in a glove box under an argon atmosphere.
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2.4. MD simulations MD simulations for neat solvents (FA, NMF, and DMF) and their binary mixtures (FA–DMF and NMF–DMF) were performed for an NTP ensemble (298 K and 1 atm) with 256 molecules using the Materials Explorer 5.0 program (Fujitsu, Japan). The simulation time was 500 ps for all the systems. The system was equilibrated for the first 300 ps with an interval of 0.2 fs, and the data collected at every 0.1 ps during 300–500 ps were analyzed to calculate the velocity autocorrelation and the atom–atom pair correlation functions. In this research, we performed the simulation using the five-point OPLS model to simply describe the intermolecular interaction dynamics in the complicated binary mixtures [29]. In the OPLS model, the hydrogen of the aldehyde group is not considered and the C–H group is treated as the united atom. Non-bonded intermolecular interactions are represented by a sum of the Lennard-Jones and Coulombic terms. The velocity autocorrelation function Cα(t) was calculated for the α-sites of the amides (α = O, C, N, and H), and Ccenter(t) was also calculated for the center of mass for the solvent molecule. The average Cα(t) is given by the following equation: C α ðt Þ ¼ bvðt Þ vð0ÞN 1 Nα ¼ Σ vα ðt þ t i Þ vα ðt i Þ Nα 1
ð2Þ
where vα(t) and Nα denote the velocity of particle α at time t and the number of particles, respectively. The corresponding power spectra were calculated using the following equation:
2.2. Low-frequency Raman spectroscopy Raman spectra in the region of 10–400 cm−1 were measured using a dispersion Raman spectrometer (NR-1100; JASCO, Japan) with an argon ion laser operating at 514.5 nm. The optical resolution was 2.0 cm−1, and the laser power was 500 mW. Neat DMF (or FA) in a vessel was mixed with neat FA (or DMF) using an auto-burette, and spectral data were accumulated at each titration point. The R(ν) spectra were obtained from measured I(ν) according to the following equation: RðνÞ ≈ IðνÞðν0 −νÞ−4 ν½1− expð−hcν=kT Þ
ð1Þ
Sα ðvÞ ¼
1 π
Z
∞ 0
C α ðt Þ cosvtdt:
ð3Þ
The summation over all Sα(ν)s, i.e., Σ Sα(ν), gives a total power spectrum, Stotal(ν). The Stotal(ν) was compared with the experimental R(ν) in terms of the frequency position. Here, note that the Stotal(ν) is not suitable to directly compare to the experimental R(ν) in intensity. However, the Stotal(ν) was useful for comparing with the R(ν) in frequency (i.e., time scale of the corresponding motions).
where ν0 and ν (cm−1) represent the frequencies of the irradiated laser light and the Raman shift, respectively, and other parameters are physical constants or quantities of the usual meanings.
2.3. SANS Sample solutions were prepared by mixing deuterated DMF(d7) with undeuterated FA to obtain a high contrast of scattering for DMF. The SANS measurements for the mixtures of DMF mole fraction xDMF = 0.1–0.9 were performed at 298 K using the SANS-U spectrometer installed on the JRR-3 reactor (JAEA, Tokai, Japan) [27]. A sample-to-detector distance of 2 m was employed to cover the momentum transfer q (= 4πλ− 1sinθ, where λ and 2θ denote the wavelength of neutron beams (7 Å) and the scattering angle, respectively) from 0.02 Å to 0.15 Å. The transmission was measured with a 3He detector located at the beam stopper position. SANS profiles corrected for background, using an empty cell, were normalized with respect to the scattering of polyethylene as a secondary standard material. The SANS profiles thus obtained were further corrected for incoherent scattering to obtain the scattering intensities I(q). The incoherent scattering intensities were estimated according to the procedure reported in the literature [28].
Fig. 1. Low-frequency Raman spectra R(ν)s observed for neat formamide (FA), Nmethylformamide (NMF), and N,N-dimethylformamide (DMF).
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Fig. 2. Molecular dynamics (MD)-derived power spectra of (a) center of mass and (b) sum of Sα(ν)s (α = O, C, N, and H atoms) for FA, NMF, and DMF.
3. Results and discussion 3.1. Neat amide solvents Fig. 1 shows the R(ν) spectra observed for neat FA together with neat NMF and DMF reported in a previous study [16]. The protic NMF shows two bands at around 60 and 110 cm− 1, whereas aprotic DMF shows only a single band at around 60 cm−1. FA exhibited a minor peak at around 50 cm−1 and two major peaks at around 110 and 200 cm−1. Fig. 2a and b show the corresponding power spectra Scenter(ν)s and Stotal(ν)s, respectively, for FA, NMF, and DMF calculated by the current MD simulations. In general, Scenter(ν)s were assigned to the translation mode of the molecules because they were calculated for the center of mass of the molecules. Contrast, in Fig. 2b, Stotal(ν)s were calculated as a sum of the Sα(ν) spectra for the position α = O, C, N and H atoms; thus, the libration and/or rotation modes might be a better assignment, although the translation mode is also included in Stotal(ν). Comparing the calculated S(ν) with the experimental R(ν) in the NMF system, the two Raman bands (60 and 110 cm−1) in the R(ν) spectrum were well reproduced in Stotal(ν) than in Scenter(ν), indicating that these bands mainly originated from the libration of the hydrogen-bonded NMF. Stotal(ν) in the DMF system gave a single band that was nearly similar to that of the corresponding Scenter(ν), indicating that the solvent without hydrogen bonding shows only a single band in the range b 100 cm− 1. Thus, we found that (1) the higher frequency band at around 110 cm−1 corresponds to the libration of the hydrogen-bonded solvent molecules and (2) the lower frequency band at around 60 cm−1
is mainly attributed to the translation and/or libration modes of the solvent molecules accommodated in a cage formed by its neighbors. In the FA system, R(ν) in Fig. 1 exhibits three broad peaks at around 50, 110, and 200 cm−1. These bands could be reproduced by the corresponding Stotal(ν); therefore, the 110 and 200 cm−1 bands in R(ν) were well explained in terms of intermolecular librations. According to a series of theoretical studies by Torii et al. [4,15,30], Raman bands originating from hydrogen-bonded intermolecular interactions are observed at around 200 and 100 cm−1 for FA and only at around 100 cm−1 for NMF. Therefore, we expected that this difference might be related to the structuredness or dimensionality of the hydrogen-bonding network, i.e., FA is a three- or two-dimensional structure, whereas NMF is a one-dimensional (chain like) structure. To confirm the structural dimensionality in hydrogen-bonded FA, the spatial distribution functions (SDF) were evaluated from the current MD data in the FA system (Fig. 3). The clouds in the SDF indicate the isoprobability surface of the O, C, and N atoms within the FA molecule around a central FA molecule. In each case, the distributions are the atomic density of 0.015 (atom/Å3) in the distance range 0 Å–10 Å. As shown in Fig. 3, the O atoms align with the two N\\H bonds of the central FA molecules. Thus, it is plausible that cyclic dimers, which are the dominant component in the crystalline phase, are not dominant in liquid-phase FA. The probability distributions of the N atoms were extensively found around the O atoms of the central FA, implying that the NH⋯O hydrogen bonds are relatively rambled or distorted. The C atoms were three-dimensionally distributed around the central FA, i.e., the hydrogen-bonding network was formed with a wide variety of hydrogen-bond geometries. Hence,
Fig. 3. Spatial distribution functions (SDFs) of O, N, and C atoms in the FA molecules around a central FA molecule obtained from the MD simulations for neat FA.
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we concluded that liquid FA has a continuous rambling three-dimensional structure based on extended hydrogen-bonding interactions, which is consistent with the previously reported results [5–7]. The three-dimensional structure of the FA molecule is hydrogenbonded at three sites: two at the NH2 group and one at the C_O group. This differs from the chain–like structure, where the FA molecule is hydrogen bonded to only one hydrogen atom and one oxygen atom, like NMF molecule. The libration energies of the FA molecule in the three-dimensional structure might be higher than that in the chainlike structure. It is thus plausible that the 100 cm−1 band can be assigned to the libration of the hydrogen-bonded FA molecule in the chain-like structure. In addition, the 200 cm−1 band can be assigned to that of the restricted FA molecule by the three hydrogen bonds in the three-dimensional networked structure. 3.2. FA–DMF mixtures It is well known that SANS spectra provide information about the mixing state and/or miscibility in binary solvent mixtures, i.e., homogeneous or heterogeneous mixing at the mesoscopic level. Indeed, the SANS measurements have been widely applied to several aprotic solvent–D2O mixtures (or deuterated aprotic solvent–H2O mixtures), providing general knowledge regarding heterogeneous mixing in such aqueous binary mixtures [16,31–34]. Fig. 4 shows the SANS profiles observed for the solvent mixtures of deuterated DMF (DMF(d7)) and normal FA at the DMF mole fractions xDMF = 0.1–0.8. It is clear that for all xDMF, there were no scatterings over the entire q range examined herein. This result strongly suggests that DMF molecules do not appreciably self-aggregate, i.e., DMF and FA molecules are homogeneously mixed at any solvent composition. According to 1H NMR and molar volumes for FA–DMF mixtures [35], the mixture was almost ideally mixed over the entire range of solvent composition, which was consistent with the SANS result. Fig. 5 shows the R(ν) spectra obtained from the FA–DMF mixtures, 0 (neat FA) b xDMF b 1 (neat DMF). R(ν) for neat FA (xDMF = 0) drastically changed with increasing DMF content, indicating a new type of intermolecular interaction in the mixtures. The 110 cm−1 band in neat FA, which originates from the chain-like hydrogen-bonded structure, gradually shifted to the lower frequency side with increasing xDMF. This implies that the DMF molecules rupture the chain-like structure of selfaggregated FA to form FA–DMF hydrogen bonds, similarly found in the NMF–DMF system [16]. The peak at 200 cm−1 in neat FA monotonically decreased in intensity with increasing xDMF but remained even at high DMF content: xDMF = 0.9. In addition, no or little peak shifting
Fig. 4. Small-angle neutron scattering (SANS) profiles observed for the FA–DMF(d7) mixtures.
Fig. 5. R(ν) spectra observed for the FA–DMF mixtures.
around 200 cm−1 was found over the entire xDMF range. Thus, it is expected that the higher-dimensional (two or three) FA structure is stably formed in the mixtures for all solvent compositions. Fig. 6 shows MDderived Stotal(ν)s obtained for the FA–DMF mixtures with changing xDMF. The corresponding Stotal(ν)s for the NMF–DMF mixtures are also shown in the Supporting Information (Fig. S1). We found that in both FA–DMF and NMF–DMF systems, the variation of Stotal(ν) with changing xDMF reproduced that of the experimental R(ν) quite well. Stotal(ν)s in the FA–DMF mixtures showed three broad bands at around 30, 120, and 200 cm−1, and those in the NMF–DMF mixtures showed two bands at around 30 and 120 cm−1. Focusing on the FA–DMF system, the higher frequency bands for neat FA (110 and 200 cm−1) decreased in intensity with increasing xDMF. This indicated that the libration of FA molecules restricted to one- and higher-dimensional hydrogen-bonded networks, respectively, disappear in their mixtures. Instead, the FA molecules interact with the DMF molecules through hydrogen bonds. The hydrogen-bonded DMF with FA was indeed reproduced by the MD simulations. Fig. 7 shows the power spectra SO-DMF(ν)s calculated for the O atoms of the DMF molecules in the FA–DMF mixtures. The band intensity at around 120 cm−1 in the SO-DMF(ν) increased with increasing FA
Fig. 6. MD–derived Stotal(ν)s obtained for the FA–DMF mixtures.
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Fig. 7. MD-derived SO-DMF(ν)s calculated for the O atoms of DMF molecules in the FA–DMF mixtures.
content. This was due to the formation of FA–DMF hydrogen bonds and the rupture of chain-like FA–FA hydrogen bonds. 4. Conclusion Intermolecular interactions particularly focusing on hydrogenbonding interactions in neat amides (FA, NMF, and DMF) and their amide mixtures were investigated by both experimental and theoretical approaches. The R(ν) spectra obtained from low-frequency Raman spectroscopy were well represented by the corresponding theoretical S(ν) spectra for neat amides and their mixture systems. The SANS experiments for the FA–DMF mixtures revealed that DMF molecules do not self-aggregate and are dispersed in the mixtures at all DMF mole fractions xDMF. This result suggests that the mixing state is ideally homogeneous. The R(ν) spectra indicated that the FA molecules are hydrogen-bonded with DMF over the entire range of solvent composition. We conclude that (1) the band at around 110 cm−1 in the mixtures mainly corresponds to the libration mode of the DMF and FA molecules restricted in the chain-like hydrogen-bonded structure and (2) the band at around 200 cm−1 mainly corresponds to the libration mode of the FA molecule in the extended three-dimensional hydrogen-bonded network. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.molliq.2017.05.017. Acknowledgments This research has been financially supported by Grant-in-Aids for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology (No. 15K17877 to K.F.). The SANS experiments were carried out under the Joint-use Research Program for Neutron Scattering, Institute for Solid State Physics (ISSP), the University of Tokyo, at the Research Reactor JRR-3, JAEA (Proposal No. 4548). References [1] H. Ohtaki, N. Katayama, K. Ozutsumi, T. Radnai, The structure of liquid formamide studied by means of X-ray diffraction and NMR at high temperatures and high pressures, J. Mol. Liquids 88 (2000) 109–120. [2] H. Ohtaki, S. Itoh, Has liquid Formamide a linear-chain structure or ring-dimer structure? Z. Naturforsch. A 40 (1985) 1351–1352. [3] M. Miyake, O. Kaji, N. Nakagawa, T. Suzuki, Structural analysis of liquid formamide, J. Chem. Soc. Faraday Trans. 2 (81) (1985) 277–281. [4] H. Torii, M. Tasumi, Low-wavenumber vibrational dynamics of liquid formamide and N-methylformamide: molecular dynamics and instantaneous normal mode analysis, J. Phys. Chem. A 104 (2000) 4174–4181.
[5] E. Tsuchida, Ab initio molecular-dynamics study of liquid formamide, J. Chem. Phys. 121 (2004) 4740–4746. [6] Y.P. Puhovski, B.M. Rode, Structure and dynamics of liquid formamide, Chem. Phys. 190 (1995) 61–82. [7] Y.P. Puhovski, L.P. Safonova, B.M. Rode, Molecular dynamic simulations of a liquid formamide and N,N-dimethylformamide with new quantum mechanical potential, J. Mol. Liquids 103 (2003) 15–31. [8] K.P. Sagarik, R. Ahlrichs, A test particle model potential for formamide and molecular dynamics simulations of the liquid, J. Chem. Phys. 86 (1987) 5117–5126. [9] A. Barth, Infrared spectroscopy of proteins, Biochim. Biophys. Acta 1767 (2007) 1073–1101. [10] H. Torii, Amide I vibrational properties affected by hydrogen bonding out-of-plane of the peptide group, J. Phys. Chem. Lett. 6 (2015) 727–733. [11] A.K.M. Gomes, W.A. Alves, Catalytic activity of the Co (II)/Co (III) couple towards peptide bond cleavage and formation of molecular oxygen adducts in protic amide solutions, Vib. Spectrosc. 89 (2017) 57–61. [12] B. Wurm, C. Baar, R. Buchner, J. Barthel, Dielectric relaxation spectroscopy of ion– solvent interactions in formamide and N-methylformamide, J. Mol. Liquids 127 (2006) 14–20. [13] J. Ladell, B. Post, The crystal structure of formamide, Acta Cryst 7 (1954) 559–564. [14] H. Ohtaki, S. Itoh, B.M. Rode, The structure of liquid N-Methylformamide by means of X-ray diffraction and Ab initio LCGO–MO–SCF calculations, Bull. Chem. Soc. Jpn. 59 (1986) 271–276. [15] H. Torii, T. Tatsumi, T. Kanazawa, M. Tasumi, Effects of intermolecular hydrogenbonding interactions on the amide I mode of N-methylacetamide: matrix-isolation infrared studies and ab initio molecular orbital calculations, J. Phys. Chem. B 102 (1998) 309–314. [16] K. Fujii, T. Kumai, T. Takamuku, Y. Umebayashi, S. Ishiguro, Liquid structure and preferential solvation of metal ions in solvent mixtures of N,N-dimethylformamide and N-methylformamide, J. Phys. Chem. A 110 (2006) 1798–1804. [17] K. Fujii, Y. Umebayashi, R. Kanzaki, D. Kobayashi, R. Matsuura, S. Ishiguro, Thermodynamic aspects of metal–ion complexation in the structured solvent, Nmethylformamide, J. Solut. Chem. 34 (2005) 739–753. [18] O.F. Nielsen, D.H. Christensen, O.H. Rasmussen, Low-frequency vibrational spectra of some hydrogen-bonded amides, J. Mol. Struct. 242 (1991) 273–282. [19] O.F. Nielsen, C. Johansson, D.H. Christensen, S. Hvidt, J. Flink, S.H. Hansen, F. Poulsen, Collective vibrational effects in hydrogen bonded liquid amides and proteins studied by isotopic substitution, J. Mol. Struct. 552 (2000) 71–80. [20] F. Hammami, M. Bahri, S. Nasr, N. Jaidane, M.D. Oummezzine, R. Cortes, Short-range order in liquid N-methylformamide HCONHCH3, as studied by X-ray scattering and density functional theory calculations, J. Chem. Phys. 119 (2003) 4419–4427. [21] I. Skarmoutsos, J. Samios, Molecular dynamics of cis/transN-methylformamide liquid mixture using a new optimized all atom rigid force field, Chem. Phys. Lett. 384 (2004) 108–113. [22] J. Barthel, R. Buchner, B. Wurm, The dynamics of liquid formamide, Nmethylformamide, N,N-dimethylformamide, and N,N-dimethylacetamide. A dielectric relaxation study, J. Mol. Liquids 98 (2002) 51–69. [23] Y. Marcus, The Properties of Solvents, John Wiley & Sons Ltd., New York, 1998. [24] N. Nishi, S. Takahashi, M. Matsumoto, A. Tanaka, K. Muraya, T. Takamuku, T. Yamaguchi, Hydrogen-bonded cluster formation and hydrophobic solute association in aqueous solutions of ethanol, J. Phys. Chem. 99 (1995) 462–468. [25] T. Takamuku, D. Matsuo, A. Yamaguchi, M. Tabata, K. Yoshida, T. Yamaguchi, M. Nagao, T. Otomo, T. Adachi, Small-angle neutron scattering study on aggregation in acetonitrile–D2O and acetonitrile–D2O–NaCl mixtures, Chem. Lett. 29 (2000) 878–879. [26] T. Takamuku, A. Nakamizo, M. Tabata, K. Yoshida, T. Yamaguchi, T. Otomo, Largeangle X-ray scattering, small-angle neutron scattering, and NMR relaxation studies on mixing states of 1, 4-dioxane-water, 1, 3-dioxane-water, and tetrahydrofuranwater mixtures, J. Mol. Liquids 103 (2003) 143–159. [27] H. Iwase, H. Endo, M. Katagiri, M. Shibayama, Modernization of the small-angle neutron scattering spectrometer SANS-U by upgrade to a focusing SANS spectrometer, J. Appl. Crystallogr. 44 (2011) 558–568. [28] M. Shibayama, T. Matsunaga, M. Nagao, Evaluation of incoherent scattering intensity by transmission and sample thickness, J. Appl. Crystallogr. 42 (2009) 621–628. [29] W.L. Jorgensen, C.J. Swenson, Optimized intermolecular potential functions for amides and peptides. Structure and properties of liquid amides, J. Am. Chem. Soc. 107 (1985) 569–578. [30] H. Torii, M. Tasumi, Intermolecular hydrogen bonding and low-wave-number vibrational spectra of formamide, N-methylformamide, and N-methylacetamide in the liquid state, Int. J. Quant. Chem. 70 (1998) 241–252. [31] T. Takamuku, A. Yamaguchi, D. Matsuo, M. Tabata, M. Kumamoto, J. Nishimoto, K. Yoshida, T. Yamaguchi, M. Nagao, T. Otomo, Large-angle X-ray scattering and small-angle neutron scattering study on phase separation of acetonitrile− water mixtures by addition of NaCl, J. Phys. Chem. B 105 (2001) 6236–6245. [32] T. Kusano, K. Fujii, M. Tabata, M. Shibayama, Small-angle neutron scattering study on aggregation of 1-alkyl-3-methylimidazolium based ionic liquids in aqueous solution, J. Solut. Chem. 42 (2013) 1888–1901. [33] K. Sadakane, A. Onuki, K. Nishida, S. Koizumi, H. Seto, Multilamellar structures induced by hydrophilic and hydrophobic ions added to a binary mixture of D2O and 3-methylpyridine, Phys. Rev. Lett. 103 (2009) 167803. [34] K. Sadakane, H. Seto, H. Endo, M. Shibayama, A periodic structure in a mixture of D2O/3-methylpyridine/NaBPh4 induced by solvation effect, J. Phys. Soc. Jpn. 76 (2007) 113602. [35] M. Cezary, K.J. Wojciech, L. Skulski, A. Kolasiński, Internal structures of liquid N,Ndimethylformamideformamide binary mixtures, Phys. Chem. Liq. 34 (1997) 181–187.
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Hydrogen bonding in protic and aprotic amide mixtures: Low-frequency Raman spectroscopy, small-angle neutron scattering, and molecular dynamics simulations Kenta Fujii a,⁎, Mari Yoshitake a, Hikari Watanabe b, Toshiyuki Takamuku c, Yasuhiro Umebayashi b a b c
Graduate School of Sciences and Technology for Innovation, Yamaguchi University, 2-16-1 Tokiwadai, Ube, Yamaguchi 755-8611, Japan Graduate School of Science and Technology, Niigata University, 8050 Ikarashi, 2-no-cho, Nishi-ku, Niigata 950-2181, Japan Department of Chemistry and Applied Chemistry, Graduate School of Science and Engineering, Saga University, Honjo-machi, Saga 840-8502, Japan
a r t i c l e
i n f o
Article history: Received 13 April 2017 Received in revised form 1 May 2017 Accepted 5 May 2017 Available online 10 May 2017 Keywords: Hydrogen-bonding interactions Amide solvents Low-frequency Raman spectroscopy Molecular dynamics simulations
a b s t r a c t The hydrogen-bonding interactions in protic and aprotic amide solvent mixtures, i.e., formamide (FA) and N,Ndimethylformamide (DMF), were investigated via low-frequency Raman spectroscopy, small-angle neutron scattering (SANS) experiments, and molecular dynamics (MD) simulations. In a neat amide system, the low-frequency Raman spectra R(ν)s were well reproduced by the corresponding S(ν) spectra derived from the MD simulations. The observed peaks in R(ν)s at around b 200 cm−1 were assigned to the intermolecular interactions, particularly in terms of the hydrogen-bonding network formation and its dimensionality in the liquid state. The SANS experiments for the FA–DMF mixtures demonstrated that the FA molecules forming an extended three–dimensional hydrogen-bonding structure in the neat system interacted with DMF molecules through the hydrogen bonds in the mixtures over the whole range of solvent compositions, resulting in a homogeneous mixing state. Additionally, the R(ν) spectra for the mixtures were represented by the corresponding S(ν) spectra. From the R(ν) and S(ν) spectra of the FA–DMF mixtures, we found that (1) the Raman band at around 110 cm−1 mainly originates from the libration mode of amide molecules in the chain-like hydrogen-bonded structure and (2) the higher frequency band (approximately 200 cm−1) was attributed to the libration of the FA molecule restricted by the three-dimensional hydrogen-bonded network, which remained even in the DMF-rich compositions. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Formamide (FA) is a typical amide solvent with two proton donor sites at the − NH2 group and one proton acceptor site at the C_O group, that forms a strong liquid structure through intermolecular \\NH ⋯ O_C\\ hydrogen bonding [1–7]. The FA molecule has been often regarded as a simple model unit in peptide chains; thus, the hydrogen-bonding behaviors of amide molecules are an important issue in biochemistry for understanding the physicochemical properties of proteins [8–12]. The FA molecule can form a cyclic dimer and a chain-like structure because NH hydrogen locates at both cis and trans to the C_O group. Indeed, the hydrogen-bonding network in the crystalline state is constructed from these two types of structures [13]. To date, the intermolecular interactions of FA in the liquid state have been studied, e.g., the liquid structure based on X-ray diffraction reported by Ohtaki et al. implied that liquid FA comprises a chain-like structure and a distorted cyclic dimer structure to form two-dimensional networks ⁎ Corresponding author. E-mail address: [email protected] (K. Fujii).
http://dx.doi.org/10.1016/j.molliq.2017.05.017 0167-7322/© 2017 Elsevier B.V. All rights reserved.
containing the two types of structures [1–3]. On the other hand, according to theoretical studies by means of molecular dynamics (MD) simulations, the liquid FA structure is regarded as a three-dimensional NH⋯O hydrogen-bonding network and the cyclic dimers that are the dominant component of the crystalline phase play only a minor role in the network structure [5–7]. N-Methylformamide (NMF) with NH hydrogen trans to the C_O group forms only a chain-like structure in the liquid state, which has been established from several technics such as vibrational spectroscopy, dielectric relaxation, X-ray scattering, and molecular orbital/molecular dynamics simulations [4,14–22], as well as the studies in FA system as mentioned above. Thus, the dimensionality in the hydrogen-bonding network is higher for FA than for NMF. Indeed, the boiling and melting points and the viscosity are higher for FA than for NMF [23]. We reported the hydrogen-bonding structure in liquid NMF and its binary mixtures with aprotic amide N,N-dimethylformamide (DMF), which is a less structured solvent due to the dipole–dipole interactions [7,16]. We found that (1) the DMF molecules rupture the chain-like structure of NMF in the mixtures and (2) the DMF and NMF molecules are homogeneously mixed through hydrogen bonds over the entire range of solvent compositions in the mixture. On the other hand, it
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has been established that water with a three-dimensional hydrogenbonding network is heterogeneously mixed with conventional organic solvents, such as alcohols, acetonitrile, and dioxane [24–26]. This is due to the presence of self-aggregated water clusters in the mixtures. This study investigates the effect of the hydrogen-bonding network dimensionality on the mixing state and the intermolecular interactions in binary solvent mixtures. The liquid structure and dynamics in the amide mixtures of protic FA (extended hydrogen-bonding network) and aprotic DMF (without hydrogen bonding) were studied using low-frequency Raman spectroscopy and MD simulations. The experimental results were discussed comparing with those of the mixtures of protic NMF (one-dimensional chain-like hydrogen bonding) and DMF. In addition, small-angle neutron scattering (SANS) experiments were performed for the FA–DMF mixtures to clarify the mixing state, homogeneous or heterogeneous, at the mesoscopic level.
2. Experimental 2.1. Materials FA and NMF were dried over 3 Å molecular sieves for several weeks, distilled at 328 K under reduced pressure, and stored in a dark bottle with a P2O5 drying tube. DMF was dried over 4 Å molecular sieves for several weeks and distilled at 303 K under reduced pressure. The water content was checked by Karl Fischer titration to be b 200 ppm. All materials and solutions were treated in a glove box under an argon atmosphere.
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2.4. MD simulations MD simulations for neat solvents (FA, NMF, and DMF) and their binary mixtures (FA–DMF and NMF–DMF) were performed for an NTP ensemble (298 K and 1 atm) with 256 molecules using the Materials Explorer 5.0 program (Fujitsu, Japan). The simulation time was 500 ps for all the systems. The system was equilibrated for the first 300 ps with an interval of 0.2 fs, and the data collected at every 0.1 ps during 300–500 ps were analyzed to calculate the velocity autocorrelation and the atom–atom pair correlation functions. In this research, we performed the simulation using the five-point OPLS model to simply describe the intermolecular interaction dynamics in the complicated binary mixtures [29]. In the OPLS model, the hydrogen of the aldehyde group is not considered and the C–H group is treated as the united atom. Non-bonded intermolecular interactions are represented by a sum of the Lennard-Jones and Coulombic terms. The velocity autocorrelation function Cα(t) was calculated for the α-sites of the amides (α = O, C, N, and H), and Ccenter(t) was also calculated for the center of mass for the solvent molecule. The average Cα(t) is given by the following equation: C α ðt Þ ¼ bvðt Þ vð0ÞN 1 Nα ¼ Σ vα ðt þ t i Þ vα ðt i Þ Nα 1
ð2Þ
where vα(t) and Nα denote the velocity of particle α at time t and the number of particles, respectively. The corresponding power spectra were calculated using the following equation:
2.2. Low-frequency Raman spectroscopy Raman spectra in the region of 10–400 cm−1 were measured using a dispersion Raman spectrometer (NR-1100; JASCO, Japan) with an argon ion laser operating at 514.5 nm. The optical resolution was 2.0 cm−1, and the laser power was 500 mW. Neat DMF (or FA) in a vessel was mixed with neat FA (or DMF) using an auto-burette, and spectral data were accumulated at each titration point. The R(ν) spectra were obtained from measured I(ν) according to the following equation: RðνÞ ≈ IðνÞðν0 −νÞ−4 ν½1− expð−hcν=kT Þ
ð1Þ
Sα ðvÞ ¼
1 π
Z
∞ 0
C α ðt Þ cosvtdt:
ð3Þ
The summation over all Sα(ν)s, i.e., Σ Sα(ν), gives a total power spectrum, Stotal(ν). The Stotal(ν) was compared with the experimental R(ν) in terms of the frequency position. Here, note that the Stotal(ν) is not suitable to directly compare to the experimental R(ν) in intensity. However, the Stotal(ν) was useful for comparing with the R(ν) in frequency (i.e., time scale of the corresponding motions).
where ν0 and ν (cm−1) represent the frequencies of the irradiated laser light and the Raman shift, respectively, and other parameters are physical constants or quantities of the usual meanings.
2.3. SANS Sample solutions were prepared by mixing deuterated DMF(d7) with undeuterated FA to obtain a high contrast of scattering for DMF. The SANS measurements for the mixtures of DMF mole fraction xDMF = 0.1–0.9 were performed at 298 K using the SANS-U spectrometer installed on the JRR-3 reactor (JAEA, Tokai, Japan) [27]. A sample-to-detector distance of 2 m was employed to cover the momentum transfer q (= 4πλ− 1sinθ, where λ and 2θ denote the wavelength of neutron beams (7 Å) and the scattering angle, respectively) from 0.02 Å to 0.15 Å. The transmission was measured with a 3He detector located at the beam stopper position. SANS profiles corrected for background, using an empty cell, were normalized with respect to the scattering of polyethylene as a secondary standard material. The SANS profiles thus obtained were further corrected for incoherent scattering to obtain the scattering intensities I(q). The incoherent scattering intensities were estimated according to the procedure reported in the literature [28].
Fig. 1. Low-frequency Raman spectra R(ν)s observed for neat formamide (FA), Nmethylformamide (NMF), and N,N-dimethylformamide (DMF).
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Fig. 2. Molecular dynamics (MD)-derived power spectra of (a) center of mass and (b) sum of Sα(ν)s (α = O, C, N, and H atoms) for FA, NMF, and DMF.
3. Results and discussion 3.1. Neat amide solvents Fig. 1 shows the R(ν) spectra observed for neat FA together with neat NMF and DMF reported in a previous study [16]. The protic NMF shows two bands at around 60 and 110 cm− 1, whereas aprotic DMF shows only a single band at around 60 cm−1. FA exhibited a minor peak at around 50 cm−1 and two major peaks at around 110 and 200 cm−1. Fig. 2a and b show the corresponding power spectra Scenter(ν)s and Stotal(ν)s, respectively, for FA, NMF, and DMF calculated by the current MD simulations. In general, Scenter(ν)s were assigned to the translation mode of the molecules because they were calculated for the center of mass of the molecules. Contrast, in Fig. 2b, Stotal(ν)s were calculated as a sum of the Sα(ν) spectra for the position α = O, C, N and H atoms; thus, the libration and/or rotation modes might be a better assignment, although the translation mode is also included in Stotal(ν). Comparing the calculated S(ν) with the experimental R(ν) in the NMF system, the two Raman bands (60 and 110 cm−1) in the R(ν) spectrum were well reproduced in Stotal(ν) than in Scenter(ν), indicating that these bands mainly originated from the libration of the hydrogen-bonded NMF. Stotal(ν) in the DMF system gave a single band that was nearly similar to that of the corresponding Scenter(ν), indicating that the solvent without hydrogen bonding shows only a single band in the range b 100 cm− 1. Thus, we found that (1) the higher frequency band at around 110 cm−1 corresponds to the libration of the hydrogen-bonded solvent molecules and (2) the lower frequency band at around 60 cm−1
is mainly attributed to the translation and/or libration modes of the solvent molecules accommodated in a cage formed by its neighbors. In the FA system, R(ν) in Fig. 1 exhibits three broad peaks at around 50, 110, and 200 cm−1. These bands could be reproduced by the corresponding Stotal(ν); therefore, the 110 and 200 cm−1 bands in R(ν) were well explained in terms of intermolecular librations. According to a series of theoretical studies by Torii et al. [4,15,30], Raman bands originating from hydrogen-bonded intermolecular interactions are observed at around 200 and 100 cm−1 for FA and only at around 100 cm−1 for NMF. Therefore, we expected that this difference might be related to the structuredness or dimensionality of the hydrogen-bonding network, i.e., FA is a three- or two-dimensional structure, whereas NMF is a one-dimensional (chain like) structure. To confirm the structural dimensionality in hydrogen-bonded FA, the spatial distribution functions (SDF) were evaluated from the current MD data in the FA system (Fig. 3). The clouds in the SDF indicate the isoprobability surface of the O, C, and N atoms within the FA molecule around a central FA molecule. In each case, the distributions are the atomic density of 0.015 (atom/Å3) in the distance range 0 Å–10 Å. As shown in Fig. 3, the O atoms align with the two N\\H bonds of the central FA molecules. Thus, it is plausible that cyclic dimers, which are the dominant component in the crystalline phase, are not dominant in liquid-phase FA. The probability distributions of the N atoms were extensively found around the O atoms of the central FA, implying that the NH⋯O hydrogen bonds are relatively rambled or distorted. The C atoms were three-dimensionally distributed around the central FA, i.e., the hydrogen-bonding network was formed with a wide variety of hydrogen-bond geometries. Hence,
Fig. 3. Spatial distribution functions (SDFs) of O, N, and C atoms in the FA molecules around a central FA molecule obtained from the MD simulations for neat FA.
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we concluded that liquid FA has a continuous rambling three-dimensional structure based on extended hydrogen-bonding interactions, which is consistent with the previously reported results [5–7]. The three-dimensional structure of the FA molecule is hydrogenbonded at three sites: two at the NH2 group and one at the C_O group. This differs from the chain–like structure, where the FA molecule is hydrogen bonded to only one hydrogen atom and one oxygen atom, like NMF molecule. The libration energies of the FA molecule in the three-dimensional structure might be higher than that in the chainlike structure. It is thus plausible that the 100 cm−1 band can be assigned to the libration of the hydrogen-bonded FA molecule in the chain-like structure. In addition, the 200 cm−1 band can be assigned to that of the restricted FA molecule by the three hydrogen bonds in the three-dimensional networked structure. 3.2. FA–DMF mixtures It is well known that SANS spectra provide information about the mixing state and/or miscibility in binary solvent mixtures, i.e., homogeneous or heterogeneous mixing at the mesoscopic level. Indeed, the SANS measurements have been widely applied to several aprotic solvent–D2O mixtures (or deuterated aprotic solvent–H2O mixtures), providing general knowledge regarding heterogeneous mixing in such aqueous binary mixtures [16,31–34]. Fig. 4 shows the SANS profiles observed for the solvent mixtures of deuterated DMF (DMF(d7)) and normal FA at the DMF mole fractions xDMF = 0.1–0.8. It is clear that for all xDMF, there were no scatterings over the entire q range examined herein. This result strongly suggests that DMF molecules do not appreciably self-aggregate, i.e., DMF and FA molecules are homogeneously mixed at any solvent composition. According to 1H NMR and molar volumes for FA–DMF mixtures [35], the mixture was almost ideally mixed over the entire range of solvent composition, which was consistent with the SANS result. Fig. 5 shows the R(ν) spectra obtained from the FA–DMF mixtures, 0 (neat FA) b xDMF b 1 (neat DMF). R(ν) for neat FA (xDMF = 0) drastically changed with increasing DMF content, indicating a new type of intermolecular interaction in the mixtures. The 110 cm−1 band in neat FA, which originates from the chain-like hydrogen-bonded structure, gradually shifted to the lower frequency side with increasing xDMF. This implies that the DMF molecules rupture the chain-like structure of selfaggregated FA to form FA–DMF hydrogen bonds, similarly found in the NMF–DMF system [16]. The peak at 200 cm−1 in neat FA monotonically decreased in intensity with increasing xDMF but remained even at high DMF content: xDMF = 0.9. In addition, no or little peak shifting
Fig. 4. Small-angle neutron scattering (SANS) profiles observed for the FA–DMF(d7) mixtures.
Fig. 5. R(ν) spectra observed for the FA–DMF mixtures.
around 200 cm−1 was found over the entire xDMF range. Thus, it is expected that the higher-dimensional (two or three) FA structure is stably formed in the mixtures for all solvent compositions. Fig. 6 shows MDderived Stotal(ν)s obtained for the FA–DMF mixtures with changing xDMF. The corresponding Stotal(ν)s for the NMF–DMF mixtures are also shown in the Supporting Information (Fig. S1). We found that in both FA–DMF and NMF–DMF systems, the variation of Stotal(ν) with changing xDMF reproduced that of the experimental R(ν) quite well. Stotal(ν)s in the FA–DMF mixtures showed three broad bands at around 30, 120, and 200 cm−1, and those in the NMF–DMF mixtures showed two bands at around 30 and 120 cm−1. Focusing on the FA–DMF system, the higher frequency bands for neat FA (110 and 200 cm−1) decreased in intensity with increasing xDMF. This indicated that the libration of FA molecules restricted to one- and higher-dimensional hydrogen-bonded networks, respectively, disappear in their mixtures. Instead, the FA molecules interact with the DMF molecules through hydrogen bonds. The hydrogen-bonded DMF with FA was indeed reproduced by the MD simulations. Fig. 7 shows the power spectra SO-DMF(ν)s calculated for the O atoms of the DMF molecules in the FA–DMF mixtures. The band intensity at around 120 cm−1 in the SO-DMF(ν) increased with increasing FA
Fig. 6. MD–derived Stotal(ν)s obtained for the FA–DMF mixtures.
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Fig. 7. MD-derived SO-DMF(ν)s calculated for the O atoms of DMF molecules in the FA–DMF mixtures.
content. This was due to the formation of FA–DMF hydrogen bonds and the rupture of chain-like FA–FA hydrogen bonds. 4. Conclusion Intermolecular interactions particularly focusing on hydrogenbonding interactions in neat amides (FA, NMF, and DMF) and their amide mixtures were investigated by both experimental and theoretical approaches. The R(ν) spectra obtained from low-frequency Raman spectroscopy were well represented by the corresponding theoretical S(ν) spectra for neat amides and their mixture systems. The SANS experiments for the FA–DMF mixtures revealed that DMF molecules do not self-aggregate and are dispersed in the mixtures at all DMF mole fractions xDMF. This result suggests that the mixing state is ideally homogeneous. The R(ν) spectra indicated that the FA molecules are hydrogen-bonded with DMF over the entire range of solvent composition. We conclude that (1) the band at around 110 cm−1 in the mixtures mainly corresponds to the libration mode of the DMF and FA molecules restricted in the chain-like hydrogen-bonded structure and (2) the band at around 200 cm−1 mainly corresponds to the libration mode of the FA molecule in the extended three-dimensional hydrogen-bonded network. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.molliq.2017.05.017. Acknowledgments This research has been financially supported by Grant-in-Aids for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology (No. 15K17877 to K.F.). The SANS experiments were carried out under the Joint-use Research Program for Neutron Scattering, Institute for Solid State Physics (ISSP), the University of Tokyo, at the Research Reactor JRR-3, JAEA (Proposal No. 4548). References [1] H. Ohtaki, N. Katayama, K. Ozutsumi, T. Radnai, The structure of liquid formamide studied by means of X-ray diffraction and NMR at high temperatures and high pressures, J. Mol. Liquids 88 (2000) 109–120. [2] H. Ohtaki, S. Itoh, Has liquid Formamide a linear-chain structure or ring-dimer structure? Z. Naturforsch. A 40 (1985) 1351–1352. [3] M. Miyake, O. Kaji, N. Nakagawa, T. Suzuki, Structural analysis of liquid formamide, J. Chem. Soc. Faraday Trans. 2 (81) (1985) 277–281. [4] H. Torii, M. Tasumi, Low-wavenumber vibrational dynamics of liquid formamide and N-methylformamide: molecular dynamics and instantaneous normal mode analysis, J. Phys. Chem. A 104 (2000) 4174–4181.
[5] E. Tsuchida, Ab initio molecular-dynamics study of liquid formamide, J. Chem. Phys. 121 (2004) 4740–4746. [6] Y.P. Puhovski, B.M. Rode, Structure and dynamics of liquid formamide, Chem. Phys. 190 (1995) 61–82. [7] Y.P. Puhovski, L.P. Safonova, B.M. Rode, Molecular dynamic simulations of a liquid formamide and N,N-dimethylformamide with new quantum mechanical potential, J. Mol. Liquids 103 (2003) 15–31. [8] K.P. Sagarik, R. Ahlrichs, A test particle model potential for formamide and molecular dynamics simulations of the liquid, J. Chem. Phys. 86 (1987) 5117–5126. [9] A. Barth, Infrared spectroscopy of proteins, Biochim. Biophys. Acta 1767 (2007) 1073–1101. [10] H. Torii, Amide I vibrational properties affected by hydrogen bonding out-of-plane of the peptide group, J. Phys. Chem. Lett. 6 (2015) 727–733. [11] A.K.M. Gomes, W.A. Alves, Catalytic activity of the Co (II)/Co (III) couple towards peptide bond cleavage and formation of molecular oxygen adducts in protic amide solutions, Vib. Spectrosc. 89 (2017) 57–61. [12] B. Wurm, C. Baar, R. Buchner, J. Barthel, Dielectric relaxation spectroscopy of ion– solvent interactions in formamide and N-methylformamide, J. Mol. Liquids 127 (2006) 14–20. [13] J. Ladell, B. Post, The crystal structure of formamide, Acta Cryst 7 (1954) 559–564. [14] H. Ohtaki, S. Itoh, B.M. Rode, The structure of liquid N-Methylformamide by means of X-ray diffraction and Ab initio LCGO–MO–SCF calculations, Bull. Chem. Soc. Jpn. 59 (1986) 271–276. [15] H. Torii, T. Tatsumi, T. Kanazawa, M. Tasumi, Effects of intermolecular hydrogenbonding interactions on the amide I mode of N-methylacetamide: matrix-isolation infrared studies and ab initio molecular orbital calculations, J. Phys. Chem. B 102 (1998) 309–314. [16] K. Fujii, T. Kumai, T. Takamuku, Y. Umebayashi, S. Ishiguro, Liquid structure and preferential solvation of metal ions in solvent mixtures of N,N-dimethylformamide and N-methylformamide, J. Phys. Chem. A 110 (2006) 1798–1804. [17] K. Fujii, Y. Umebayashi, R. Kanzaki, D. Kobayashi, R. Matsuura, S. Ishiguro, Thermodynamic aspects of metal–ion complexation in the structured solvent, Nmethylformamide, J. Solut. Chem. 34 (2005) 739–753. [18] O.F. Nielsen, D.H. Christensen, O.H. Rasmussen, Low-frequency vibrational spectra of some hydrogen-bonded amides, J. Mol. Struct. 242 (1991) 273–282. [19] O.F. Nielsen, C. Johansson, D.H. Christensen, S. Hvidt, J. Flink, S.H. Hansen, F. Poulsen, Collective vibrational effects in hydrogen bonded liquid amides and proteins studied by isotopic substitution, J. Mol. Struct. 552 (2000) 71–80. [20] F. Hammami, M. Bahri, S. Nasr, N. Jaidane, M.D. Oummezzine, R. Cortes, Short-range order in liquid N-methylformamide HCONHCH3, as studied by X-ray scattering and density functional theory calculations, J. Chem. Phys. 119 (2003) 4419–4427. [21] I. Skarmoutsos, J. Samios, Molecular dynamics of cis/transN-methylformamide liquid mixture using a new optimized all atom rigid force field, Chem. Phys. Lett. 384 (2004) 108–113. [22] J. Barthel, R. Buchner, B. Wurm, The dynamics of liquid formamide, Nmethylformamide, N,N-dimethylformamide, and N,N-dimethylacetamide. A dielectric relaxation study, J. Mol. Liquids 98 (2002) 51–69. [23] Y. Marcus, The Properties of Solvents, John Wiley & Sons Ltd., New York, 1998. [24] N. Nishi, S. Takahashi, M. Matsumoto, A. Tanaka, K. Muraya, T. Takamuku, T. Yamaguchi, Hydrogen-bonded cluster formation and hydrophobic solute association in aqueous solutions of ethanol, J. Phys. Chem. 99 (1995) 462–468. [25] T. Takamuku, D. Matsuo, A. Yamaguchi, M. Tabata, K. Yoshida, T. Yamaguchi, M. Nagao, T. Otomo, T. Adachi, Small-angle neutron scattering study on aggregation in acetonitrile–D2O and acetonitrile–D2O–NaCl mixtures, Chem. Lett. 29 (2000) 878–879. [26] T. Takamuku, A. Nakamizo, M. Tabata, K. Yoshida, T. Yamaguchi, T. Otomo, Largeangle X-ray scattering, small-angle neutron scattering, and NMR relaxation studies on mixing states of 1, 4-dioxane-water, 1, 3-dioxane-water, and tetrahydrofuranwater mixtures, J. Mol. Liquids 103 (2003) 143–159. [27] H. Iwase, H. Endo, M. Katagiri, M. Shibayama, Modernization of the small-angle neutron scattering spectrometer SANS-U by upgrade to a focusing SANS spectrometer, J. Appl. Crystallogr. 44 (2011) 558–568. [28] M. Shibayama, T. Matsunaga, M. Nagao, Evaluation of incoherent scattering intensity by transmission and sample thickness, J. Appl. Crystallogr. 42 (2009) 621–628. [29] W.L. Jorgensen, C.J. Swenson, Optimized intermolecular potential functions for amides and peptides. Structure and properties of liquid amides, J. Am. Chem. Soc. 107 (1985) 569–578. [30] H. Torii, M. Tasumi, Intermolecular hydrogen bonding and low-wave-number vibrational spectra of formamide, N-methylformamide, and N-methylacetamide in the liquid state, Int. J. Quant. Chem. 70 (1998) 241–252. [31] T. Takamuku, A. Yamaguchi, D. Matsuo, M. Tabata, M. Kumamoto, J. Nishimoto, K. Yoshida, T. Yamaguchi, M. Nagao, T. Otomo, Large-angle X-ray scattering and small-angle neutron scattering study on phase separation of acetonitrile− water mixtures by addition of NaCl, J. Phys. Chem. B 105 (2001) 6236–6245. [32] T. Kusano, K. Fujii, M. Tabata, M. Shibayama, Small-angle neutron scattering study on aggregation of 1-alkyl-3-methylimidazolium based ionic liquids in aqueous solution, J. Solut. Chem. 42 (2013) 1888–1901. [33] K. Sadakane, A. Onuki, K. Nishida, S. Koizumi, H. Seto, Multilamellar structures induced by hydrophilic and hydrophobic ions added to a binary mixture of D2O and 3-methylpyridine, Phys. Rev. Lett. 103 (2009) 167803. [34] K. Sadakane, H. Seto, H. Endo, M. Shibayama, A periodic structure in a mixture of D2O/3-methylpyridine/NaBPh4 induced by solvation effect, J. Phys. Soc. Jpn. 76 (2007) 113602. [35] M. Cezary, K.J. Wojciech, L. Skulski, A. Kolasiński, Internal structures of liquid N,Ndimethylformamideformamide binary mixtures, Phys. Chem. Liq. 34 (1997) 181–187.