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May 21, 2015 - 3-5-8 Saiwai-cho, Fuchu, Tokyo 183-8509, Japan. ABSTRACT: Near-infrared (NIR) absorption spectrum meas- urements over a frequency ...
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Dangling OH Vibrations of Water Molecules in Aqueous Solutions of Aprotic Polar Compounds Observed in the Near-Infrared Regime Naoya Sagawa and Toshiyuki Shikata* Division of Natural Resources and Eco-materials, Graduate School of Agriculture, Tokyo University of Agriculture and Technology, 3-5-8 Saiwai-cho, Fuchu, Tokyo 183-8509, Japan ABSTRACT: Near-infrared (NIR) absorption spectrum measurements over a frequency range from 4000 to 12000 cm−1 were employed to investigate the effects of the presence of solute compounds to vibrational modes of water molecules in aqueous solutions of some aprotic hydroneutral polar compounds with large dipole moments, such as nitro compounds and nitriles. The obtained NIR spectra for the aqueous solutions were decomposed into three components: free water, solute, and water molecules affected by the presence of solutes. Newly determined NIR spectra of affected water molecules were well-described with at least four absorption modes observed at 7040, 6850, 6450, and 5640 cm−1 for both the nitro compounds and nitriles. The highest frequency mode at 7040 cm−1 possessing the strongest intensity was assigned to the first stretching overtone of affected water hydroxy (O−H) groups, which are nonhydrogen bonded to other water molecules and dangling. The second highest frequency mode at 6850 cm−1 was assigned to the first stretching overtone of affected water O−H groups hydrated to other (free) water molecules. The third mode at 6400 cm−1 was attributed to a combination mode of the fundamental stretching of O−H and the first overtone of the O−H bending mode of the affected water molecules. The lowest frequency mode at 5640 cm−1 was assigned to the combination mode of the fundamental O−H stretching mode, the fundamental O−H bending mode, and the hindered rotational (libration) mode of the affected water molecules. Because absorption intensities of the third and lowest frequency modes for water molecules affected by the solutes depended on the sizes of alkyl groups of polar solutes, these two modes possibly result from the contribution of hydrophobic hydration effects.

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INTRODUCTION Although many standard chemistry textbooks1−5 classify all polar compounds and polar groups as hydrophilic, whereas nonpolar compounds are classified as hydrophobic, certain typical polar molecules bearing large dipole moments neither behave as hydrophilic compounds nor dissolve into water at all. Of course, such a classical clear-cut classification is comprehensive and effective for some kinds of polar compounds possessing a donor number (DN)6−8 higher than the critical value of 18, such as ethers (DN = 19−20), alcohols (DN = 19−20), and amides (DN = 24). The donor number, DN, proposed by Gutmann,7,8 is a quantitative measure of the strength of a molecule as a Lewis base and is quite useful to discuss the ability for a solute molecule to form a hydrogen bond to solvent molecules (water molecules with DN = 18, in this case). Alcohols possessing hydroxy (OH) group(s) dissolve into water at relatively high concentrations. The hydration number per OH group (nH) has been evaluated experimentally using dielectric spectroscopic methods.9,10 The nH value for OH groups isolated from other OH groups without forming intramolecular hydrogen bonds has been reported to be 5 at 25 °C, and the nH value decreases with increasing temperature.10 The water solubilities of compounds bearing ether groups (−CH2OCH2−) and amide groups (−C(O)NH−) are also © 2015 American Chemical Society

rather high. The nH value of ether groups has been evaluated to be ca. 4 below the room temperature and decreases with increasing temperature also via dielectric methods,11,12 and the nH value ca. 6 for amide groups of low mass compounds.13 The positive nH values for these compounds definitely reveal that these polar groups with DN values higher than 18 are hydrophilic because of the formation of hydrogen bonds between the polar groups and water molecules. There exist other types of water-soluble compounds possessing highly polar groups with dipole moments greater than 3.0 D. Although nitro compounds, such as nitro methane (MeNO2) and nitro ethane (EtNO2) bearing a nitro group (NO2) with the large dipole moment of 3.4 D, but a low DN value of 3−4, are water-soluble, 1-nitropropane (PrNO2) only slightly larger than EtNO2 possesses a low water solubility of 0.17 M.14 Very recently, dielectric spectroscopic measurements have revealed that the hydration number of the NO2 group is nH = 0.14 Then, the relatively high water solubility of MeNO2 and EtNO2 are not caused by the hydration effect of the NO2 group. The low DN of 3−4 for NO2 groups does not lead to Received: March 25, 2015 Revised: May 10, 2015 Published: May 21, 2015 8087

DOI: 10.1021/acs.jpcb.5b02886 J. Phys. Chem. B 2015, 119, 8087−8095

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The Journal of Physical Chemistry B the formation of hydrogen bonds to water OH groups at all.6,14 The NO2 group essentially has only dipolar interactions with water molecules. Small nitriles like cyanomethane (acetonitrile, MeCN), cyanoethane (propionitrile, EtCN), and 1-cyanopropane (valeronitrile, n-butyronitrile, PrCN) bearing highly polar nitro groups (−CN) with a dipole moment of 3.7 D and DN = 12−14 are also water-soluble. Dielectric spectroscopic measurements have demonstrated that the hydration number for the CN group is nH = 0 as well as nitro compounds.14 Then, the water solubilities of MeCN, EtCN, and PrCN are not assisted by the hydration behavior caused by the formation of strong hydrogen bonding as in the case of nitro compounds. Consequently, it might be concluded that nH values are not controlled by the magnitude of the dipole moment of a polar group but the ability for hydrogen bond formation to water molecules qualified by the value of donor number, DN. These aprotic dipolar compounds or functional groups possessing dipole moments larger than 3.0 D and nH = 0 like the NO2 and CN group have been classified into a new category called hydroneutral compounds or groups.14,15 Number of experimental methods to determine the hydration number of solute molecules in aqueous solution have been proposed, such as nuclear magnetic resonance (NMR) techniques,16 neutron scattering experiments,17,18 ultrasound interferometry,19 Raman scattering (RS),20−23 extremely low-frequency depolarized Raman scattering (LFRS),24,25 infrared (IR) absorption,6 near-infrared (NIR) absorption,26 and dielectric spectroscopic measurements.9−15 However, different techniques have provided rather different hydration numbers for the same solute. The reason for differences in reported nH values obtained by distinct techniques is a difference in physical meaning of nH depending on the techniques.26 Dielectric spectroscopic techniques to precisely determine the hydration numbers of solute molecules dissolved in water are performed in an extremely high frequency range up to, for example, 50 GHz.9−15 Because the dielectric relaxation strength of free-water molecules in aqueous solutions is exactly evaluable in the frequency range, the amount of water molecules hydrated to solute molecules can be precisely determined. If the water molecules hydrated to examined solute molecules have a shorter hydration lifetime than the rotational relaxation time (τW) of free water, ca. 8 ps at 25 °C, the hydration number for the examined solute would be evaluated to be nH = 0 via dielectric spectroscopic techniques. In general, RS, IR, and NIR spectrum measurements provide molecular vibrational information which is obtained in a timescale much shorter than τW like snapshots. Water molecules hydrated to solute molecules with hydration lifetimes shorter than τW (in other words, affected by the presence of the solute molecules) show RS, IR, and NIR spectra, which are different from those of the pure liquid state of water. Then, the existence of water molecules affected by the presence of solute molecules is precisely detected, and the number of water molecules affected by the solutes should be handled as an affected number, naff, separately from the value of nH due to a difference in physical meanings.26 Recently, physical meaning of the hydrophobic hydration of alkyl chains in aqueous solution observed at relatively high temperatures has been discussed based on the results obtained by RS techniques.22 Moreover, thermodynamics of the hydrophobic hydration has been discussed as a function of temperature.23 The concept of dangling OH groups, which means OH groups without

hydrogen bond formation to certain hydrophilic solute molecules or other water molecules, is quite important to discuss the hydration state not only for hydrophobic alkyl chains but also for hydroneutral molecules like nitro compounds and nitriles. In this study, hydration behavior of several nitro compounds: MeNO2, EtNO2, and PrNO2, and nitriles: MeCN, EtCN, and PrCN, in aqueous solution was investigated in detail using NIR spectroscopic techniques over a frequency range from 4000 to 12000 cm−1 at room temperature of 25 °C. The combination mode of symmetric and asymmetric (or simply the first overtone of) O−H stretching mode for water molecules in the pure liquid state observable in a frequency range from 5500 to 7500 cm−1 was remarkably altered by the presence of both the nitro compounds and nitriles in a systematic manner with a concentration change. Therefore, characteristic NIR spectra for water molecules affected by the presence of the nitro compounds and nitriles were newly determined precisely in the frequency range. Then, quantitative comparison between NIR spectra for the free and affected water molecules was performed to understand alternation of O−H vibrational characteristics of affected water molecules. Moreover, assuming the density of affected water is identical to that of the pure liquid water, the naff values for the examined compounds were also evaluated at 25 °C. Then, the naff values for both the nitro compounds and nitriles were compared with the relationship nH = 0 determined via dielectric techniques. In addition to a water molecule bearing one dangling OH group, another water molecule possessing two dangling OH groups, which is similar to a free water molecule in the vapor state except for being hydrogen boded to two other water molecules schematically depicted in Figure 1, were necessary to understand the newly obtained characteristic NIR spectra for the affected water molecules by the presence of the examined solutes.

Figure 1. Schematic depiction representing two kinds of molecules affected by the presence of a solute molecule. (a) A molecule bearing a nonhydrogen-bonded (dangling) OH group hydrogen-bonded one to another water molecule and (b) a molecule bearing two dangling OH groups.

water water and a water

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EXPERIMENTAL SECTION Materials. Nitromethane, MeNO2, (>98%) was purchased from Tokyo Chemical Industry Co. Ltd. (Tokyo). Nitroethane, EtNO2 (>95%), cyanomethane, MeCN, (>99.5%) and cyanoethane (EtCN) (>98%) were purchased from Wako Pure Chemical Industries, Ltd. (Osaka). 1-Nitropropane, PrNO2 (>98%), and 1-cyanopropane, PrCN, (>98%) were purchased 8088

DOI: 10.1021/acs.jpcb.5b02886 J. Phys. Chem. B 2015, 119, 8087−8095

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An NIR spectrum for pure solute, MeNO2, (ABSS) at 25 °C is also plotted in Figure 2. The combination band of C−H fundamental stretching and bending modes (νCH + δCH) was observed at 4200 cm−1. A strong signal at 4460 cm−1 was assigned to the second overtone of an N−O stretching mode (3νNO) because the fundamental (asymmetric) stretching mode of N−O is usually found at ca. 1560 cm−1. The small signal observed at 5250 cm−1 was attributed to the combination mode of νCH and the first overtone of C−H bending of a methyl group, 2δCH; νCH + 2δCH. The first overtone of C−H stretching, 2νCH, was clearly observed at 5950 and 6050 cm−1, symmetric and degenerated stretching mode, respectively. The combination mode of 2νCH + δCH, was found at 7000−7400 cm−1. Moreover, the second overtone of C−H stretching mode, 3νCH, was barely observed at ca. 8700 cm−1.27,28 It is likely that absorption intensity for MeNO2, ABSS, in the examined dilute aqueous solutions is not too large to disturb the quantitative analysis of ΔABS in the frequency range of 5500−7500 cm−1, which is a frequency range for mentioned water O−H vibration modes. If solute MeNO2 has no water molecules affected by the presence of itself or, in other words, all the water molecules existing in the solution possess the same NIR spectrum as that of the pure liquid water, the volume fraction of the pure liquid water component, f, is given by the relationship f = 1 − 103VS c, where VS means the partial molar volume of a solute molecule, MeNO2 in this case. Assuming MeNO2 has no affected water molecules, a concentration reduced absorbance difference, ΔABS × c−1, was calculated for aqueous MeNO2 solutions using the experimentally determined value of VS = 49.42 cm3 mol−1 (Figure 3a). The fact that sharp two positive small peaks attributed to 2νCH of C−H stretching modes in MeNO2 molecules were clearly observed in ABS × c−1 spectra irrespective of c revealed that NIR spectra determined in this study was highly quantitative. However, a distinctive sharp, negative peak was found at 6700 cm−1 in the ΔABS × c−1 spectrum shown in Figure 3a. Although magnitude of the negative peak in ΔABS × c−1 was quantitative, the presence of negative absorption is not logical. Then, we might easily conclude that the simple assumption that there exist no water molecule affected by the presence of MeNO2 was invalid in the range from 5500 to 7500 cm−1 where the first overtone of O− H stretching modes are observable for water molecules in the pure liquid state. Even if only positive peaks appear in ΔABS × c−1 spectra of the frequency range, the conclusion is not altered, because there is no distinctive NIR absorption for MeNO2 in the frequency range except for 2νCH mode at 5950 and 6050 cm−1. Essentially, the same results as seen in Figures 2 and 3a were obtained also in other nitro compounds. Consequently, there exist water molecules possessing O−H vibration modes affected by the presence of the nitro compounds examined in aqueous solution irrespective of molar masses. Moreover, O−H stretching modes of water molecules affected by nitro compounds observed in the frequency range from 5500 to 7500 cm−1 have considerably different spectra from that of water molecules in the pure liquid state seen in Figure 2. To analyze an NIR spectrum of water molecules possessing the affected O−H vibrations in the frequency region of 5500− 7500 cm−1 more quantitatively, here we consider the affected water layer surrounding the nitro compounds with a volume fraction of 103 mVS c, where m is a constant dependent on the size of the affected water layer and will be used to quantify the value of naff. In the layer, the affected water molecules have an

from Kanto Chemical Co. Inc. (Tokyo). All of the purchased chemicals were used without further purification. Highly deionized water with a specific resistance higher than 18 MΩ cm obtained by a Direct-Q 3UV system (Millipore-Japan, Tokyo) was used as the solvent for the aqueous sample solution preparation. The concentrations, c, of the aqueous solutions of the nitro compounds ranged from 0.30 to 1.0 M for MeNO2, 0.20 to 0.60 M for EtNO2, and 0.05 to 0.15 M for PrNO2. In the case of aqueous solutions of nitriles, the values of c were altered in the range from 0.5 to 1.5 M for MeCN and EtCN and from 0.20 to 0.40 M for PrCN. Methods. NIR absorption spectra of sample solutions were recorded over a frequency (wavenumber, WN) range from 4000 to 12000 cm−1, using an MPA FT-NIR Analyzer (Bruker, Madison) at 25 °C in accuracy of ±0.1 °C. A quartz sample cell with optical path length of 2.0 mm was used for all the measurements. Prior to each measurement, back ground was taken with the vacant quartz cell. Density measurements for all of the aqueous sample solutions were carried out using a digital density meter, DMA4500 (Anton Paar, Graz), to determine the partial molar volumes of the solute molecules at the same temperature used for the NIR measurements.

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RESULTS AND DISCUSSION NIR Spectra for Aqueous Solution of Nitro Compounds. Concentration, c, dependencies of NIR absorption spectra, absorbance (ABS) vs WN, for aqueous MeNO2 solutions are shown in Figure 2 as typical examples. The

Figure 2. Frequency, WN, dependencies of absorption spectra, ABS, in the NIR regime for aqueous MeNO2 solutions at several c values and 25 °C. Absorption spectra for pure liquid water, ABSW, and pure MeNO2, ABSS, were also plotted.

spectrum at c = 0 M represents that for pure water (ABSW). Since signals were saturated due to strong absorbance higher than 3 in the frequency ranges, ∼ 4200 and 4900−5200 cm−1, quantitative analysis was impossible in the frequency ranges. A saturated region in a frequency range lower than 4200 cm−1 is attributed to the fundamental stretching mode of water O−H groups (νOH), and the other saturated region of 4900−5200 cm−1 to the combination of νOH and water O−H bending modes (δOH); νOH + δOH.27 However, absorbance difference, ΔABS (= ABS − f ABSW), was evaluable for all the solutions in frequency regions except for the saturated ones, where a factor f means the volume fraction of a free water component possessing the same NIR spectrum as the pure liquid water in sample solutions. 8089

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Figure 3. (a) Dependencies of concentration reduced absorbance difference, ΔABS × c−1, for aqueous MeNO2 solutions on WN at several c values and 25 °C. (b) WN dependencies of modified concentration reduced absorbance difference, ΔABS* × c−1, for aqueous MeNO2 solutions calculated from the spectra seen in (a), assuming m = 0.38. A green solid line represents the best fit curve obtained by the summation of Gauss-type absorption functions summarized in Table 1

Table 1. Parameters Necessary to Reproduce the Modified Concentration Reduced Absorbance Difference, ΔABS* × c−1, with the Summation of Gauss-Type Absorption Functions [Ij(WN) = Ij/((2π)1/2σj) × exp{−[(WN−νj)2]/(2σ2j )}] for Each Aqueous Solution of Nitro Compounds Examined

*

Peak frequency for an absorption mode j.

**

Half width value for an absorption mode j.

***

Area intensity for an absorption mode j.

simply described with the summation of Gauss-type absorption functions as NIR absorption spectra usually observed. Then, we chose the value of m = 0.38 and determined the ΔABS* × c−1 seen in Figure 3b as ABShydW for MeNO2, which is describable with the summation of Gauss-type absorption functions. The most significant point found in the ΔABS* × c−1 spectrum for MeNO2 seen in Figure 3b is the presence of the relatively sharp largest absorption signal at 7055 cm−1. This major vibration mode assigned to the first overtone of water O−H stretching, 2νOH, affected by the presence of MeNO2 is observed at a frequency substantially higher than that of the absorption peak, ca. 6900 cm−1 (cf. Figure 2), of the pure liquid water forming dense hydrogen bond networks. Moreover, it has been well-known that O−H stretching vibration modes

NIR absorption spectrum (ABShydW) substantially different from that of water molecules in the pure liquid state. Such consideration is essentially the same as that proposed by McCabe and Fisher.29 Then, we define a modified concentration reduced absorbance difference, ΔABS* × c−1 (= [ABS − {1−103(1 + m)VS c}ABSW − α ABSS] × c−1); α is a proportional constant to convert ABSS into the spectrum at each c value, which corresponds to the quantity of ABShydW at c = 1.0 M. Figure 3b shows frequency dependence of ΔABS* × c−1 for MeNO2 without negative values calculated, assuming m = 0.38. The ΔABS* × c−1 was still independent of the c values and quantitative. If the value of m = 0.35 is assumed, a negative peak disappears. However, the frequency dependencies of the spectra calculated in an m range, 0.35 < m < 0.37, cannot be 8090

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Figure 4. WN dependencies of ΔABS* × c−1 for (a) EtNO2 and (b) PrNO2 calculated assuming m = 0.27 and 0.5, respectively. A green solid line represents the best fit curve obtained by the summation of Gauss-type absorption functions summarized in Table 1.

and also a hindered rotational (libration) mode (νWL ∼ 600 cm−1) of the affected water molecules: νOH + δOH + νWL,30,31 and the combination mode of νOH + 2δOH, respectively, for the affected water molecules. A small mode observed at 7380 cm−1 was assigned to the combination mode of 2νOH + νWL for the affected water molecules. The additional two vibrational modes found at 5620 and 7380 cm−1 are observed also in the ABSW spectrum for the pure liquid water as a small but distinctive bump and shoulder as seen in Figure 2. However, the mode at 6450 cm−1 is not clearly recognized in the ABSW spectrum because of the presence of the strong 2νOH mode of water O− H strongly hydrogen bonded to other water molecules located at a frequency close to 6450 cm−1. The number of water molecules affected by the presence of one MeNO2 molecule, naff, was roughly calculated assuming the density of water in the affected water layer around MeNO2 to be identical to the value of the pure liquid water via the relationship naff = m VS × VW−1, where VW means the molar volume of water at 25 °C, ca. 18.0 cm3 mol−1. Then, the value of naff = 1.04 was obtained for MeNO2. Consequently, this small naff value close to unity reveals that one water molecule bearing one dangling OH group with dipolar interaction to the NO2 group exists in the vicinity of a MeNO2 molecule. Essentially the same procedure was used to analyze ABS spectra for aqueous solutions of EtNO2 and PrNO2, and the obtained ΔABS* × c−1 spectra assuming m = 0.27 and 0.50, respectively, are shown in Figure 4 (panels a and b). Because the first overtone of C−H stretching, 2νCH, for CH2 groups observed at 5900 and 6000 cm−1 in aqueous solutions slightly moved toward a higher frequency side in addition to those of CH3 groups for both the EtNO2 and PrNO2, the results of subtraction were not perfect and a portion of the ΔABS* × c−1 spectra in a frequency range near 6000 cm−1 scattered a little. However, it is not doubtful that the essential frequency dependence of the ΔABS* × c−1 spectrum for EtNO2 was the same as that of MeNO2. Moreover, the affected number of water molecules, naff, was calculated to be 0.99 using the experimental value of VS = 65.8 cm3 mol−1 for EtNO2, which is essentially the same as MeNO2. On the other hand, all the vibration signals observed at 5600, 6430, 6840−7070, and 7320 cm−1 in the ΔABS* × c−1 spectrum for PrNO2 seem to be almost twice as large as those for MeNO2 and EtNO2 as summarized in Table 1 as the values of area intensities, Ij, of each vibration mode. This observation well corresponds to the fact that the naff value of 2.28 was calculated via the experimental quantity, Vs = 65.8 cm3 mol−1 for PrNO2 in aqueous solution. Moreover, the frequency dependence of the major absorption signal observed at 6840− 7070 cm−1 for PrNO2 is markedly different from those for

including overtones shift toward a lower frequency side (red shift) when hydrogen bonds are formed between OH groups. Then, the marked blue shift for the overtone of O−H vibration mode observed in the ΔABS* × c−1 spectrum seen in Figure 3b clearly reveals that water molecules affected by the presence of MeNO2 do have nonhydrogen-bonded and dangling OH groups. If one looks at the ΔABS* × c−1 spectrum carefully, the major signal is consisting of a sharp larger component at 7035 cm−1 and a small one at 6860 cm−1, which is a usual frequency for the first overtone of O−H stretching mode forming hydrogen bonding in the pure liquid water. The Gauss-type absorption parameters of each constituent component necessary for the best fit curve shown as a green solid line in Figure 3b are summarized in Table 1. The absorption intensity (I5) of the signal at 7055 cm−1 was about twice as large as that (I3) at 6860 cm−1. These observations reveal that water molecules affected by the presence of MeNO2 have each of hydrogen bonded and dangling OH group as schematically depicted in Figure 1. The frequency dependency of the ΔABS* × c−1 spectrum seen in Figure 3b is considerably different from that of aqueous solutions of poly(ethylene oxide)s and crown ether samples consisting of hydrophilic ether groups, which showed a strong major peak at 6900 cm−1 and another broad one at 6500 cm−1 assigned to the first overtone of O−H stretching, 2νOH, hydrogen boned to oxygen atoms of other water molecules and those of solute ether groups.26 According to the previous study using dielectric spectroscopic technique, MeNO2 molecules behave as a hydroneutral compound and have no hydrated water molecules possessing hydration lifetime longer than relaxation time, τW, of water molecules in the pure liquid state.26 Then, water molecules affected by the MeNO2 do not have strong interaction to form a hydrogen bond leading to stable hydration but a weak dipolar interaction between the dangling OH group and a polar −NO2 group bearing a large dipole moment of 3.4 D. The presence of the dangling OH has been clearly confirmed in aqueous solutions of dipolar solutes such as MeCN and acetone based on the result of RS measurements demonstrating additional fundamental O−H stretching mode at 3450−3500 cm−1, meaning the presence of dangling OH around MeCN.20 The ΔABS* × c−1 spectrum for MeCN will be discussed in detail later. In the ΔABS* × c−1 spectrum for MeNO2 seen in Figure 3b, two (or three) weak additional absorption signals are recognized at 5620, 6450 (and 7380) cm−1 attributed to water O−H vibration modes affected by the presence of MeNO2 except for the 2νCH signals of C−H stretching at 5900 and 6050 cm−1. The vibration modes observed at 5620 and 6450 cm−1 were assigned to the combination mode of νOH, δOH 8091

DOI: 10.1021/acs.jpcb.5b02886 J. Phys. Chem. B 2015, 119, 8087−8095

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Figure 5. (a) Dependencies of concentration reduced absorbance, ABS × c−1, for aqueous MeCN solutions on WN at several c values and 25 °C. (b):WN dependencies of ΔABS* × c−1 for MeNO2 calculated from the spectra seen in (a), assuming m = 0.38. A green solid line represents the best fit curve obtained by the summation of Gauss-type absorption functions summarized in Table 2.

Table 2. Parameters Necessary to Reproduce the ΔABS* × c−1 with the Summation of Gauss-Type Absorption Functions, Ij(WN), for each Aqueous Solution of Nitriles Examined

MeNO2 and EtNO2, which have a distinctive shoulder at ca. 6860 cm−1. The meaning of the disappearance of the shoulder in the major signal is the presence of an additional absorption at ca. 6950 cm−1. Constituent components necessary to reproduce the absorption signals summarized in Table 1 clearly demonstrate that the increase of the intensity of the absorption at 7070 cm−1 by twice and the new appearance of an additional signal at 6950 cm−1, which is never attributed to an absorption signal of the first overtone of O−H stretching hydrogen bonded to other water molecules or solute molecules due to a high frequency as the bands. A possible speculation for the appearance double absorption peaks at 6950 and 7070 cm−1 for water molecules affected by the presence of PrNO2 is that the compound bearing a little larger propyl group holds another affected water molecule near the propyl group with the two dangling OH groups free from hydrogen bond formation, showing the first overtones of symmetric and asymmetric O−H

stretching at the frequencies, in addition to the affected water molecule located near the NO2 group commonly found in smaller nitro compounds due to strong dipolar interaction. The additional affected water molecule existing near the propyl group is not a completely free water molecule like that in the gaseous state, and its oxygen atom should be hydrogen bonded by other water molecule OH groups. Then, the situation of the additional affected water molecule setting its two OH groups face to hydrophobic alkyl groups without forming special intermolecular interaction can be schematically described as in Figure 1b. Such an intermolecular interaction between water and hydrophobic group seen in Figure 1b could be recognized as hydrophobic hydration. If the nitro compounds bearing larger alkyl groups than PrNO2 had water solubility higher than reported, one more water molecule possessing two dangling OH groups showing the first overtone of OH stretching mode at 6950 and 7070 cm−1 will be found as a, so-called, 8092

DOI: 10.1021/acs.jpcb.5b02886 J. Phys. Chem. B 2015, 119, 8087−8095

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Figure 6. Dependencies of ΔABS* × c−1 for (a) EtCN and (b) PrCN on WN calculated assuming m = 0.32 and 0.4, respectively. A green solid line represents the best fit curve obtained by the summation of Gauss-type absorption functions summarized in Table 2.

hydrophobically hydrated water molecule. Davis et al.22 have also reported the presence of hydrohobically hydrated water molecules around alkyl groups of relatively long alcoholic compounds like 1-heptanol in a temperature range higher than 80 °C. NIR Spectra for Aqueous Solution of Nitriles. Assuming MeCN has no affected water molecules, a concentration reduced absorbance difference, ΔABS × c−1, was calculated for aqueous MeCN solutions using the experimentally determined value of VS = 47.6 cm3 mol−1 (Figure 5a). Sharp two positive peaks at 5850 and 6000 cm−1 attributed to 2νCH of C−H stretching modes in a methyl group were clearly, quantitatively observed. A distinctive large negative peak was found at 6750 cm−1 in the ΔABS × c−1 spectrum as well as in MeNO2 (cf. Figure 3a). Then, we might conclude again that the simple assumption that there are no water molecules affected by the presence of MeCN was invalid. There exist water molecules possessing O−H vibration modes affected by the presence of MeCN in aqueous solution. Figure 5b shows frequency dependence of ΔABS* × c−1 for MeCN without negative values calculated assuming m = 0.38. The frequency dependencies of the spectra calculated in an m range, 0.35 < m < 0.37, cannot be simply described with the summation of Gauss-type absorption functions. Then, we chose the value of m = 0.38, and the obtained ΔABS* × c−1 seen in the figure as ABShydW for MeNO2, which is describable with the summation of Gauss-type absorption functions as summarized in Table 2. A striking point found in the ΔABS* × c−1 spectrum is the discovery of a major absorption peak at 7035 cm−1 with a shoulder at 6860 cm−1, which are assigned to the first overtone of O−H stretching of a dangling and hydrated OH groups to other water molecules as well as in the case of nitro compounds above. The number of affected water molecules, naff, by the presence of MeCN was calculated to be naff = 1.01 via the values of m = 0.38 and experimental VS = 47.6 cm3 mol−1, which is perfectly identical to that of MeNO2. Because the shape of the major signal is not completely the same as that for MeNO2, the O−H vibrational condition for the water molecules affected by MeCN is only slightly altered by a difference in the intermolecular interaction. Consequently, the O−H vibrational condition for water molecules affected by MeCN is essentially identical to that by MeNO2, because these solutes are both hydroneutral compounds bearing DN values lower than 18. It is worth noting here that the obtained naff value of 1.01 for MeCN in this study reasonably agrees with the number of perturbed water molecules by the presence of MeCN determined by using RS techniques with multivariate curve resolution (MCR).20 Then, we might conclude that our procedure used for NIR data analysis in this study is adequate

and the results obtained are well-consistent with those by RS techniques with MCR. The reason for the scattered data observed in a frequency range from 5700 to 6100 cm−1 in the ΔABS* × c−1 for MeCN in aqueous solution is a slight frequency shift in the second overtones of C−H stretching modes, 2νCH, from those of pure liquid state as observed also in nitro compounds discussed above. Although the absorption signal found at 5600 cm−1 possessed scattering complication, we treated and analyzed it as a relatively broad single absorption signal attributed to the combination mode of νOH + δOH + νWL for water molecules affected by the presence of MeCN. Smaller absorption signals than the peak at 5600 cm−1 observed at 6400 and 7450 cm−1 were assigned to two combination modes of νOH + 2δOH and 2νOH + νWL, respectively, for water molecules affected by the presence of MeCN, since the observed peak frequencies were perfectly identical to those found in aqueous solutions of nitro compounds discussed above. The ΔABS* × c−1 for EtCN obtained assuming m = 0.32 is shown in Figure 6a. Except badly scattered data in a frequency range from 5600 to 6100 cm−1 due to the slight frequency shifts of 2νCH modes for methyl and methylene groups, frequency dependencies of other absorption signals seem perfectly identical to those of MeCN seen in Figure 5b. Moreover, the value of naff = 1.08 calculated from the experimental VS = 60.8 cm3 mol−1 was identical to that of MeCN. These observations reveal that EtCN holds one water molecule with a dangling and hydrogen-bonded OH to other water molecules as well as MeCN. Figure 6b shows the ΔABS* × c−1 for PrCN, assuming m = 0.5. Except badly scattered 2νCH modes for methyl and methylene groups in a frequency range from 5600 to 6100 cm−1, frequency dependencies of other absorption signals are similar to those for MeCN (Figure 5b) and EtCN (Figure 6a). However, the magnitude of each absorption mode for PrCN was almost twice as large as those of MeCN and EtCN as summarized in Table 2 as the value of area intensity, Ij, for each mode. In relation to this observation, the value of naff = 1.80 for PrCN calculated via the experimental value of VS = 80.8 cm3 mol−1 was almost twice the values of MeCN and EtCN. These behaviors observed in aqueous solution of PrCN strongly propose that PrCN holds a hydrophobically hydrated water molecule with two dangling OH groups near its propyl group in addition to a water molecule with one dangling OH in the vicinity of its CN group as in the case of PrNO2. Because unfortunately the presence of a shoulder in the major absorption signals at 6860 cm−1 for MeCN and EtCN was not so striking as discovered in both MeNO2 and EtNO2 (cf. Figure 3b and Figure 4a), the addition of the first overtones 8093

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of symmetric and asymmetric O−H stretching, 2νOH, at 6910 and 7055 cm−1 for two OH groups free from hydrogen bond formation is not as clear as in the case of the nitro compounds. However, the constituent absorption signals necessary to reproduce the experimental ΔABS* × c−1 summarized in Table 2 clearly shows the necessity of the contribution of the first overtones of symmetric and asymmetric O−H stretching, 2νOH, for hydrophobically hydrated water molecules near the propyl group of PrCN. Although we used the standard NIR spectroscopic methods in this study, there is a straightforward way to detect spectral changes in fundamental O−H stretching modes directly using IR spectrometers equipped with a newly developed sophisticated broadband infrared source.32,33

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CONCLUSIONS

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AUTHOR INFORMATION

REFERENCES

(1) For example, Gilbert, T. R.; Kirss, R. V.; Foster, N.; Davies, G. Chemistry, 2nd ed.; W. W. Norton & Company: New York, 2009; Chapter 10. (2) Vollhardt, K. P. C.; Schore, N. E. Organic Chemistry: Structure and Function, 5th ed.; W. H. Freeman & Company: New York, 2007; Chapter 8. (3) McMurry, J.; Castellion, M. E.; Ballantine, D. S.; Hoeger, C. A.; Peterson, V. E. Fundamentals of General, Organic, and Biological Chemistry, 6th ed.; Pearson Education: Upper Saddle River, 2009; Chapter 18. (4) Holum, J. R. Fundamental of General, Organic, and Biological Chemistry, 6th ed.; John Wiley and Sons: New York, 1998; Chapter 21. (5) IUPAC. Compendium of Chemical Terminology (the ″Gold Book″), 2nd ed; McNaught, A. D., Wilkinson, A., Eds.; Blackwell Scientific Publications: Oxford, 1997. XML on-line corrected version: http:// goldbook.iupac.org; Nic, M.; Jirat, J.; Kosata, B., Eds.; 2006. Updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook. (6) Gojło, E.; Gampe, T.; Krakowiak, J.; Stangret, J. Hydration of Aprotic Donor Solvents Studied by Means of FTIR Spectroscopy. J. Phys. Chem. A 2007, 111, 1827−1834. (7) Gutmann, V. Empirical Parameters for Donor and Acceptor Properties of Solvents. Electrochim. Acta 1976, 21, 661−670. (8) Gutmann, V. The Donor-Acceptor Approach to Molecular Interactions; Plenum Press: New York, 1978. (9) Satokawa, Y.; Shikata, T. Hydration Structure and Dynamic Behavior of Poly(vinyl alcohol)s in Aqueous Solution. Macromolecules 2008, 41, 2908−2913. (10) Shikata, T.; Okuzono, M. Hydration/Dehydration Behavior of Polyalcoholic Compounds Governed by Development of Intramolecular Hydrogen Bonds. J. Phys. Chem. B 2013, 117, 2782−278. (11) Shikata, T.; Takahashi, R.; Sakamoto, A. Hydration of Poly(ethylene oxide)s in Aqueous Solution As Studied by Dielectric Relaxation Measurements. J. Phys. Chem. B 2006, 110, 8941−8945. (12) Shikata, T.; Okuzono, M.; Sugimoto, N. TemperatureDependent Hydration/Dehydration Behavior of Poly(ethyleneoxide)s in Aqueous Solution. Macromolecules 2013, 46, 1956−1961. (13) Ono, Y.; Shikata, T. Contrary Hydration Behavior of NIsopropylacrylamide to its Polymer, P(NIPAm), with a Lower Critical Solution Temperature. J. Phys. Chem. B 2007, 111, 1511−1513. (14) Sagawa, N.; Shikata, T. Are All Polar Molecules Hydrophilic? Hydration Numbers of Nitro Compounds and Nitriles in Aqueous Solution. Phys. Chem. Chem. Phys. 2014, 16, 13262−13270. (15) Shikata, T.; Okuzono, M. Are All Polar Molecules Hydrophilic? Hydration Numbers of Ketones and Esters in Aqueous Solution. J. Phys. Chem. B 2013, 117, 7718−7723. (16) Ishihara, Y.; Okouchi, S.; Uedarira, H. Dynamics of Hydration of Alcohols and Diols in Aqueous Solutions. J. Chem. Soc., Faraday Trans. 1997, 93, 3337−3342. (17) Mason, P. E.; Neilson, G. W.; Enderby, J. E.; Cuello, G.; Brady, J. W. Neutron Diffraction and Simulation Studies of the Exocyclic Hydroxymethyl Conformation of Glucose. J. Chem. Phys. 2006, 125, 224505. (18) Impey, R. W.; Madden, P. A.; McDonald, I. R. Hydration and Mobility of Ions in Solution. J. Phys. Chem. 1983, 87, 5071−5083. (19) Burakowski, A.; Gliński, J. Hydration Numbers of Nonelectrolytes from Acoustic Methods. Chem. Rev. 2012, 112, 2059− 2081. (20) Perera, P.; Wyche, M.; Loethen, Y.; Ben-Amotz, D. SoluteInduced Perturbations of Solvent-Shell Molecules Observed Using Multivariate Raman Curve Resolution. J. Am. Chem. Soc. 2008, 130, 4576−4577. (21) Perera, P. N.; Fega, K. R.; Lawrence, C.; Sundstrom, E. J.; Tomlison-Phillips, J.; Ben-Amotz, D. Observation of Water Dangling OH Bonds around Dissolved Nonpolar Groups. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 12230−12234. (22) Davis, J. G.; Gierszal, K. P.; Wang, P.; Ben-Amotz, D. Water Structural Transformation at Molecular Hydrophobic Interfaces. Nature 2012, 491, 582−585.

Vibration behavior of water O−H groups affected by the presence of typical aprotic hydroneutral solute molecules such as nitro compounds and nitriles was carefully investigated in a near-infrared regime at 25 °C. Because the ability for both nitro compounds and nitriles to form hydrogen bonds to water molecules is not high enough because of their lower donor numbers than the critical value of 18, only one water molecule is located near nitro and cyano groups as an affected water molecule by the solute without forming hydrogen bonding but with dipolar intermolecular interaction due to large dipole moments of these groups. A water molecule affected by the nitro and cyano groups possess one dangling hydroxy group without hydrogen bonding, which demonstrates the first overtone of O−H stretching at the characteristic high frequency of 7035−7070 cm−1. On the other hand, the other hydroxy group of the affected water molecule forms a hydrogen bond to other water molecules and shows the first overtone of O−H stretching at 6810−6850 cm−1. Although nitro compounds and nitriles bearing small methyl and ethyl groups hold one affected water molecule near their nitro and cyano groups, in the case of 1-nitropropane and 1-cyanopropane bearing a slightly larger propyl group, one more affected water molecule is additionally held in the vicinity of the propyl group as a hydrophobically hydrated water molecule. Both hydroxy groups of the additional affected water molecules are dangling ones and demonstrate the first overtones of symmetric and asymmetric O−H stretching at 6910−6950 and 7055−7070 cm−1, respectively.

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Article

ACKNOWLEDGMENTS

T.S. is indebted to Prof. M. Takayanagi of Tokyo University of Agriculture and Technology (TUAT) for his kind permission to use an MPA FT-NIR Analyzer installed in Research Center for Frontier Plant Factory of TUAT and his fruitful discussion on the data obtained in this study. This work was partially supported by JSPS Grant-in-Aid for Scientific Research (B) no. 26288055. 8094

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DOI: 10.1021/acs.jpcb.5b02886 J. Phys. Chem. B 2015, 119, 8087−8095