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Building Blocks for Ionic Liquids: Vapor Pressures and Vaporization Enthalpies of N‑Functionalized Imidazoles with Branched and Cycloalkyl Substituents Sergey P. Verevkin,*,†,‡ Ksenia V. Zaitseva,‡ Alexander D. Stanton,§ Michelle S. Hindman,§ A. Christopher Irvin,§ and Jason E. Bara*,§ †

Department of Physical Chemistry and Department “Science and Technology of Life, Light and Matter”, University of Rostock, D-18059 Rostock, Mecklenburg-Vorpommern, Germany ‡ Department of Physical Chemistry, Kazan Federal University, 420008 Kazan, Russia § Department of Chemical & Biological Engineering, University of Alabama, Tuscaloosa, Alabama 35487-0203, United States S Supporting Information *

ABSTRACT: The imidazole structure offers a versatile means of developing molecules with controlled/tunable physicochemical properties that have significant utility in many applications and can be further derivatized to form ionic liquids. In the literature, the vast majority of studies on structure−property relationships in these types of molecules are devoted to linear (e.g., n-alkyl) substituents. However, imidazoles with branched or cycloalkyl groups are equally accessible through convenient synthetic methods − yet there are essentially no reports on the physical properties of such compounds in the literature. Here, the absolute vapor pressures of branched and cycloalkyl derivatives of imidazole have been determined as a function of temperature by the transpiration method. The standard molar enthalpies of vaporization were derived from the temperature dependences of vapor pressures. The measured data sets were successfully checked for internal consistency by comparison with vaporization enthalpies of the parent species, and a group contribution method is put forth by which the vaporization enthalpies of imidazoles, and imidazolium-based ILs, with alkyl groups in any configuration can be rapidly predicted. potential to create perhaps more than 106 unique species and over 1012 binary mixtures.4,5 A great deal of effort has been expended to develop understandings of the structure−property relationships arising from systematic variation of functional groups in ILs. However, much less work has been devoted to understanding the structure−property performance relationships in imidazole derivatives. Imidazole molecules are certainly interesting due to their tunable physical properties.6−13 Yet, imidazoles are not chemically inert and can be used as bases, nucleophiles, and ligands.14,15 Furthermore, many imidazoles are found within natural products (e.g., histidine, carbonic, anhydrase enzyme) and can have biological and pharmaceutical activities.16−18 Thus, tuning of imidazole molecular structure to control physical properties can also offer simultaneous control of chemical reactivity. With regard to prior reports of structure−property relationship studies on ILs and imidazoles, there has been a great tendency to focus almost exclusively on linear substituents (e.g., n-alkyl, oligo(ethylene glycol), nitrile-terminated alkyl, etc.) appended to the nitrogen atom(s) of the five-membered ring. We and others have previously studied the influence of a number of different functionalities on the properties of ILs and imidazoles, including the following: vapor pressure,9,10,19−25 enthalpy of vaporization, viscosity,13,26−28 density, pKa,29 and

1. INTRODUCTION In recent years, the term “designer solvent” has been applied to substances such as ionic liquids (ILs) and related compounds wherein the thermophysical properties of the liquid can be “tuned” via rational selection of one or more substituents synthetically attached to a heterocyclic core.1−3 Imidazoliumbased ILs, and the N-functionalized imidazoles from which they are derived, are perhaps the most readily tailored species, as the imidazolium cation can be modified at each atom within the 5-membered ring (Figure 1, left), while the neutral imidazole molecules can be functionalized at four positions (Figure 1, rght).

Figure 1. General structures of imidazolium cations (left) and neutral imidazoles (right) illustrating the possible points of functionalization.

For imidazolium-based ILs, the most straightforward modifications are performed at the nitrogen atom(s). Typically, 1-methylimidazole (R = Me; X, Y, Z = H) is used as a precursor for the synthesis of ILs and is then reacted with any number of possible reagents (e.g., ethyl bromide, benzyl chloride, etc.) to form the 1,3-difunctionalized imidazolium salt. Subsequent ion exchange reactions may then be performed to replace halides with molecular anions such a bistriflimide ([NTf2]), tetrafluoroboate ([BF4]), dicyanamide ([N(CN)2]), and many others. These functionalizations (and mixtures of ILs) have the © 2015 American Chemical Society

Received: Revised: Accepted: Published: 9850

April 29, 2015 September 18, 2015 September 21, 2015 September 21, 2015 DOI: 10.1021/acs.iecr.5b01599 Ind. Eng. Chem. Res. 2015, 54, 9850−9856

Article

Industrial & Engineering Chemistry Research CO2 absorption capacity/selectivity.8,11,13,30 However, only a few studies have considered the effects of branched or cyclic isomers on the thermophysical properties of ILs and imidazoles.31,32 This is an area worth exploring given the applications of imidazoles with branched or cycloalkyl groups. Imidazoles with these types of functional groups have found significant utility for forming N-heterocyclic carbenes or as the basis for pharmaceuticals or other bioactive molecules.33−40 Here, we report on the vapor pressures and enthalpies of vaporization for a group of seven imidazole derivatives containing branched or cycloalkyl groups (Figure 2).

purified by multiple fractional distillations under reduced pressure. For each sample, the degree of purity was determined by gas chromatography (Hewlett-Packard 5890 Series II) with flame ionization detector. No impurities greater than 5 × 10−4 mass fraction were detected in samples used in the study. 2.2. Vapor Pressure Measurements: Transpiration Method. Absolute vapor pressures of imidazole compounds were measured using the method of transpiration.43,44 About 0.5 g of the sample was mixed with small glass beads and placed in a U-shaped saturator. A well-controlled N2 stream was passed through the saturator at a constant temperature (Ti ± 0.1 K), and the transported material was collected in a cold trap. The amount of condensed sample was determined by GC analysis using an external standard (n-octane, C8H18). Vapor pressures (pi) at each temperature (Ti) were calculated from the amount of the product collected within a defined interval eq 1, assuming gas ideality and the validity of Dalton’s Law applied to the N2 stream saturated with the substance i pi = mi ·R ·Ta/V ·Mi ;

V = VN2 + Vi ;

(VN2 ≫ Vi )

(1)

where R = 8.314462 J·K−1·mol−1; mi is the mass of the condensed compound, Mi is the molar mass of compound i, and Vi is its volume contribution to the gaseous phase. VN2 is the volume of the carrier gas, and Ta is the ambient temperature of the flowmeter. The N2 gas flow rate was measured with the HP Agilent soap film flow meter (model 0101-0113). The volume of the carrier gas VN2 was determined from the flow rate and the time measurement. Data for each imidazole compound (1−7) collected by this method are presented in Table 1.

Figure 2. Structures of branched and cycloalkyl-functionalized imidazoles (1−7) studied in this work.

It was found that trends in vapor pressure and enthalpy of vaporization associated with the size and configuration of the alkyl substituents appended to the imidazole-based species examined in this work are consistent with structure−property relationships observed in molecular solvents and ILs with similar functionalities. Furthermore, the experimental results were used to develop group contribution values for estimation of vaporization enthalpies of both imidazoles and imidazolium based ILs with alkyl groups in any configuration (linear, branched, and/or cyclic).

3. RESULTS AND DISCUSSION 3.1. Vapor Pressures of Alkylimidazoles from the Transpiration Method. Vapor pressures measured at different temperatures were fitted with eq 2 as applied in a prior work43

2. EXPERIMENTAL SECTION 2.1. Materials. With the exception of N-tert-butylimidazole, all other imidazoles were prepared according to methods outlined by Bara where a sodium imidazolate intermediate is alkylated by a corresponding (R-Br) organobromine compound (Scheme 1a).41 N-tert-Butylimidazole was prepared via the

R ·ln p /p° = a +

⎛T ⎞ b + Δgl C p◦, m·ln⎜ ⎟ T ⎝ T0 ⎠

(2)

where a and b are adjustable parameters, T0 (in K) is an arbitrarily chosen reference temperature (which has been chosen to be 298.15 K), and p° = 1 Pa. The Δg1C°p,m term is the difference of the molar heat capacities (in J·K−1·mol−1) of the gaseous and the liquid phases, respectively. Values of Δg1Cp,m ° were calculated according to the procedure developed by Chickos and Acree.45 The latter were based on values C°p,m (l, 298.15 K) estimated by the group-contribution method.46 Compilation of the heat capacity values is given in Table 2. No vapor pressures of the imidazole compounds studied here were found elsewhere in the literature for comparison. 3.2. Enthalpy of Vaporization of Alkyl-Imidazoles from the Transpiration Method. Vapor pressures of alkyl-imidazoles were studied as close as possible to the reference temperature (298.15 K). Vaporization enthalpies were derived from the temperature dependence of vapor pressures using eq 3 and are reported in Table 2.

Scheme 1. a. Synthesis of Imidazoles (1−3, 5−7) via the Imidazolate Method and b. Synthesis of N-tertButylimidazole (4) via the Radziszewski Method

Radziszewski reaction from glyoxal, formaldehyde, ammonia, and tert-butylamine (Scheme 1b).42 All compounds were low viscosity liquids at ambient temperature. 1H NMR data were consistent with published values, and our 1H NMR data and spectra images for N-substituted imidazoles (1−3, 5−7) are provided as Supporting Information. Postsynthesis and isolation, all compounds were further carefully

Δgl Hm◦(T ) = −b + Δgl C p◦, m·T

(3)

3.3. Correlation of Vaporization Enthalpies. A valuable check for internal consistency of the experimental vaporization enthalpies is a comparison of the Δg1Hm° (298.15 K) values of these imidazoles with branched and cyclic functionalities 9851

DOI: 10.1021/acs.iecr.5b01599 Ind. Eng. Chem. Res. 2015, 54, 9850−9856

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Industrial & Engineering Chemistry Research Table 1. Results from Measurements of the Vapor Pressure (pi) of Each Imidazole Compound (1−7) Using the Transpiration Method Ta (K)

V(N2)c mb (mg) (dm 3)

gas-flow (dm3/h)

pd (Pa)

Table 1. continued Ta (K)

Δg1Hm° (T) (kJ·mol−1)

(pexp-pcalc)e (%)

ln(p /Pa) =

N-isopropylimidazole: Δg1H°m (298.15 K) = (58.4 ± 0.3) kJ·mol−1

ln(p /Pa) = 283.4 286.5 289.4 292.4 295.3 298.2 303.1 304.5 306.0 309.0 312.0 314.1 316.0 318.0 320.0 321.0 323.0

ln(p /Pa) = 289.6 292.6 295.6 298.3 301.2 302.5 303.2 306.0 307.1 308.1 309.2 310.0 311.3 311.4 313.3 314.2 316.6 317.1 319.1 319.1 319.2 320.4 322.1 323.2 324.0 324.4 326.1

310.3 314.3 316.8 317.0 320.1 324.3 324.5 326.3 328.4 330.2

288.11 78832.2 68.6 ⎛ T /K ⎞ ⎟ − − ln⎜ ⎝ 298.15 ⎠ R R· (T , K) R

1.57 7.03 4.30 5.04 0.6 0.83 2.79 4.20 6.62 0.3 0.88 2.29 4.29 8.51 0.9 0.82 1.68 4.21 10.89 0.2 0.89 1.43 4.29 13.80 0.6 0.80 1.06 4.23 16.97 −2.6 0.30 0.268 1.06 24.51 −3.2 0.93 0.741 2.02 28.32 0.7 0.70 0.498 1.07 31.02 −1.8 0.49 0.269 1.08 40.17 2.1 0.68 0.306 1.08 49.53 1.7 0.93 0.372 1.49 56.73 0.5 0.78 0.266 0.94 64.34 −0.6 1.10 0.325 1.08 74.82 0.8 1.00 0.261 0.98 84.70 −0.2 1.06 0.261 1.08 90.13 −0.5 1.15 0.246 0.98 103.6 0.2 N-isobutylimidazole: Δg1H°m (298.15 K) = (62.2 ± 0.3) kJ

59.39 59.18 58.98 58.77 58.58 58.37 58.04 57.95 57.84 57.64 57.43 57.29 57.15 57.01 56.88 56.81 56.67 mol−1

ln(p /Pa) = 303.0 305.4 308.2 309.3 311.3 312.2 314.2 314.2 316.8 317.2 320.2 320.3 323.1 325.5 329.1 331.5 334.3 337.5 340.4 343.3

311.6 87515.5 76.9 ⎛ T /K ⎞ ⎟ − − ln⎜ ⎝ 298.15 ⎠ R R· (T , K) R

1.41

3.93

4.21

298.4

1.34

2.97

4.35

301.7 304.4 307.6

0.86 0.87 0.84

1.45 1.18 0.872

3.00 3.08 3.08

7.11

1.2

64.78

8.99

0.0

64.57

11.98 14.84 19.39

0.9 −0.5 −0.4

64.32 64.11 63.86

pd (Pa)

311.6 87515.5 76.9 ⎛ T /K ⎞ ⎟ − − ln⎜ ⎝ 298.15 ⎠ R R· (T , K) R

304.6 84451.2 75.5 ⎛ T /K ⎞ ⎟ − − ln⎜ ⎝ 298.15 ⎠ R R· (T , K) R

9852

0.52 0.55 0.53

3.88 3.11 2.46

3.6 62.08 −0.4 61.70 −0.1 61.47 −2.6 61.25 −0.8 61.03 −3.6 60.80 1.6 60.58 −0.8 60.36 −2.0 60.13 3.1 59.90 1.4 59.68 2.2 59.45 0.7 59.22 −2.2 59.00 0.5 58.77 −0.4 58.54 = (59.8 ± 0.3) kJ mol−1

280.1 81243.7 71.8 ⎛ T /K ⎞ ⎟ − − ln⎜ ⎝ 298.15 ⎠ R R· (T , K) R

1.28 6.99 0.26 1.17 0.78 2.87 0.85 2.84 0.51 1.47 0.92 2.47 1.07 2.55 0.79 1.90 0.78 1.57 1.23 2.37 0.90 1.42 0.85 1.33 0.69 0.886 0.71 0.776 0.69 0.591 0.75 0.554 0.82 0.513 0.72 0.369 0.86 0.369 0.79 0.278 N-cyclopentylimidazole:

ln(p /Pa) = 305.0 308.0 310.8

Δg1Hm° (T) (kJ·mol−1)

(pexp-pcalc)e (%)

296.4 3.19 5.48 4.57 11.57 301.4 2.23 2.62 4.57 16.85 304.4 1.65 1.51 4.54 21.63 307.3 1.62 1.22 4.57 26.43 310.3 4.66 2.72 4.54 33.91 313.3 2.06 0.989 4.57 41.37 316.2 2.68 0.983 4.54 53.92 319.2 0.95 0.289 1.02 65.35 322.2 5.05 1.25 1.18 79.81 325.2 1.95 0.374 1.02 103.28 328.2 4.55 0.725 1.18 124.19 331.2 2.10 0.272 1.02 152.69 334.2 2.35 0.255 1.18 182.36 337.2 2.57 0.238 1.02 214.14 340.2 3.54 0.265 1.18 264.87 343.2 3.24 0.204 1.02 314.57 N-cyclopropylmethylimidazole: Δg1Hm° (298.15 K)

303.9 85076.7 76.9 ⎛ T /K ⎞ ⎟ − − ln⎜ ⎝ 298.15 ⎠ R R· (T , K) R

295.7

gas-flow (dm3/h)

0.88 0.747 1.99 23.75 −1.7 63.65 0.87 0.528 1.75 32.90 −0.4 63.35 0.87 0.449 1.42 39.32 −1.4 63.16 1.51 0.748 1.69 40.65 0.0 63.14 1.48 0.578 2.04 51.73 0.8 62.90 1.18 0.335 1.12 69.99 1.1 62.58 1.25 0.357 1.07 70.09 −0.7 62.56 1.01 0.254 1.01 80.12 0.1 62.42 1.27 0.273 1.09 93.00 0.4 62.26 1.31 0.251 1.00 105.33 0.7 62.13 N-tert-butylimidazole: Δg1H°m (298.15 K) = (61.9 ± 0.4) kJ mol−1

ln(p /Pa) =

0.76 3.424 4.59 4.42 0.3 62.81 0.88 3.007 4.51 5.85 1.1 62.57 0.84 2.25 3.97 7.50 −0.1 62.34 0.86 1.81 3.96 9.55 1.5 62.14 0.82 1.34 2.60 12.07 0.6 61.91 1.11 1.68 1.58 13.16 −1.4 61.81 1.69 2.43 1.80 13.91 −1.1 61.76 0.80 0.920 3.25 17.49 −0.3 61.55 0.95 0.990 1.80 19.30 0.1 61.46 0.97 0.945 1.80 20.50 −1.0 61.39 0.79 0.687 1.96 22.80 0.6 61.30 0.94 0.777 1.79 24.06 −0.1 61.24 0.80 0.607 2.02 26.08 −1.5 61.14 0.88 0.648 2.59 27.01 0.7 61.13 1.01 0.656 1.79 30.71 −0.3 60.99 0.84 0.507 2.03 33.17 0.5 60.92 0.73 0.375 1.00 38.96 −1.1 60.73 0.82 0.404 1.05 40.21 −1.5 60.69 1.27 0.537 1.43 47.08 −0.1 60.54 0.67 0.287 0.98 46.85 −0.5 60.54 0.80 0.332 1.37 47.83 1.0 60.53 0.85 0.327 1.03 51.47 −0.2 60.44 0.77 0.264 0.99 58.12 −0.5 60.31 0.94 0.294 1.18 63.21 0.4 60.22 0.82 0.243 1.00 67.39 0.9 60.16 0.92 0.271 1.02 68.34 0.2 60.13 1.00 0.255 1.02 78.34 1.8 60.00 N-sec-butylimidazole: Δg1Hm° (298.15 K) = (64.6 ± 0.3) kJ mol−1

ln(p /Pa) =

V(N2)c mb (mg) (dm 3)

4.07 3.70 4.11 4.54 4.10 5.48 1.14 5.99 4.11 6.98 4.50 7.39 4.50 8.43 4.07 8.38 4.50 9.98 4.07 10.40 4.05 12.76 4.45 12.75 1.11 15.55 1.11 18.34 1.11 23.29 1.11 27.09 1.14 31.87 1.11 39.17 1.11 47.33 1.11 56.46 Δg1Hm° (298.15 K) =

0.6 2.6 0.1 1.0 1.5 0.6 −0.7 −1.3 −2.2 −0.9 −1.1 −1.9 −1.0 −0.4 0.2 0.0 −1.2 0.0 1.6 2.3 (70.6 ± 0.5)

59.49 59.32 59.12 59.04 58.90 58.83 58.69 58.69 58.50 58.47 58.26 58.25 58.05 57.88 57.62 57.45 57.24 57.01 56.81 56.60 kJ mol−1

315.6 93551.5 76.9 ⎛ T /K ⎞ ⎟ − − ln⎜ ⎝ 298.15 ⎠ R R· (T , K) R 4.16 4.15 4.16

2.42 3.19 3.90

3.2 3.7 −0.6

70.10 69.87 69.65

DOI: 10.1021/acs.iecr.5b01599 Ind. Eng. Chem. Res. 2015, 54, 9850−9856

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Industrial & Engineering Chemistry Research

(Figure 2) with the Δg1H°m (298.15 K) values of similarly shaped alkanes or alkylbenzenes given in Table 3. For example, N-isopropylimidazole can be taken as a comparable homomorph to isobutane or isopropylbenzene. This comparison is presented in Figure 3, which illustrates that vaporization enthalpies of these N-substituted imidazoles are compared against analogous homomorph molecules,22,47,48 bearing the same alkyl moiety in the molecular structures. The data for the two classes of compounds exhibit a strong linear relationship with a correlation coefficient (R2 = 0.97−0.98), confirming the reliability of the experimental results measured in this work. 3.4. Prediction of Vaporization Enthalpies of AlkylImidazoles. Vaporization enthalpies of the imidazoles measured in this work can now be applied for development of group-additivity parameters sufficient for prediction of N-alkyl-imidazoles with an alkyl substituent of any structure. In our recent work we suggested a set of parameters for calculation of vaporization enthalpies of aliphatic amino-compounds.49 However, in the case of the substituted imidazoles (Im) examined here, three additional parameters are required: CH2− (Im)(C), CH-(Im)(C)2, and C-(Im)(C)3 in order to take into account branching of the C atom attached to the N(1)-atom of the imidazole ring. These parameters are presented in Table 4. For the sake of simplicity we decided to define the imidazole moiety as a single parameter, which has been derived by subtraction of the CH3-(C) contribution from the experimental enthalpy of vaporization Δg1H°m (298.15 K) = (55.0 ± 0.3) in kJ·mol−1 of N-methylimidazole.25 Experimental enthalpies of vaporization of imidazoles with branched and cyclic functionalities (1−7) given in Table 2 together with those reported for N-(n-alkyl)imidazoles previously24 have been used to derive groupadditivity parameters listed in Table 4. In order to take into account specific contributions to the vaporization enthalpy from three-, five-, and six-membered aliphatic rings contained within the structures of certain imidazoles (5−7) studied in this work, we calculated the appropriate “cyclic”-corrections (see Table 4) by using data for ethylcyclopropane, methylcyclopentane, and ethylcyclohexane provided in Table 3. Collection of the groupadditivity parameters listed in Table 4 is sufficient to predict a wide variety of alkyl-substituted imidazoles with acceptable uncertainty on the level of about ±(1−2) kJ·mol−1 according to the comparison given in Table 5. Having these data and method available can be useful for the design of ILs and chemical engineering process calculations.7,13,50 3.5. Prediction of Vaporization Enthalpies of Imidazolium Based ILs. ILs have admittedly negligible vapor pressures at ambient temperatures and very large vaporization enthalpies, yet the experimental determination of these vaporization

Table 1. continued Ta (K)

V(N2)c mb (mg) (dm 3)

ln(p /Pa) =

gas-flow (dm3/h)

pd (Pa)

(pexp-pcalc)e (%)

315.6 93551.5 76.9 ⎛ T /K ⎞ ⎟ − − ln⎜ ⎝ 298.15 ⎠ R R· (T , K) R

312.7 0.26 1.02 1.97 4.64 313.6 0.57 2.01 4.15 5.06 316.9 0.53 1.50 4.10 6.37 317.8 0.59 1.52 4.15 6.92 318.8 0.28 0.688 1.97 7.44 321.2 0.59 1.17 4.13 9.10 321.5 0.49 0.930 1.93 9.41 324.2 0.75 1.16 4.09 11.73 324.8 0.68 0.996 1.93 12.44 327.9 0.62 0.706 1.93 15.79 328.3 0.93 1.02 4.09 16.52 330.9 0.74 0.652 2.16 20.43 334.0 0.44 0.323 0.97 24.61 336.9 0.68 0.405 1.01 30.09 340.1 0.60 0.273 0.82 39.46 343.2 1.10 0.407 1.14 48.81 346.1 0.95 0.292 1.14 58.67 N-cyclohexylmethylimidazole: Δg1Hm° (298.15 K)

ln(p /Pa) = 314.0 316.6 323.0 323.1 326.0 319.9 329.0 325.8 330.8 334.9 340.9 343.8 343.9 346.8 349.9 352.8

0.45 0.44 0.35 0.45 0.43 0.53 0.49 0.42 0.43 0.65 0.42 0.41 0.43 0.54 0.42 0.44

6.12 4.61 2.12 2.64 2.01 4.11 1.83 1.96 1.36 1.47 0.589 0.458 0.469 0.487 0.299 0.249

Δg1Hm° (T) (kJ·mol−1)

0.4 69.51 1.4 69.44 −3.3 69.19 −2.4 69.12 −3.4 69.04 −2.7 68.86 −1.7 68.83 −1.1 68.62 0.1 68.58 0.0 68.34 1.4 68.31 3.0 68.11 −1.3 67.87 −2.2 67.65 2.2 67.40 2.0 67.16 0.7 66.94 = (77.5 ± 0.4) kJ mol−1

331.6 102448.8 83.7 ⎛ T /K ⎞ ⎟ − − ln⎜ ⎝ 298.15 ⎠ R R· (T , K) R 3.46 4.54 3.54 4.52 4.46 4.52 4.76 4.52 4.52 4.62 1.96 1.96 2.01 1.95 1.00 1.00

1.12 1.43 2.52 2.55 3.24 1.95 4.07 3.23 4.80 6.65 10.67 13.46 13.78 16.86 21.23 26.47

0.01 0.00 0.00 0.03 −0.02 0.04 −0.11 0.02 −0.05 −0.12 −0.17 −0.03 0.14 −0.04 0.18 0.51

76.17 75.95 75.41 75.41 75.16 75.67 74.91 75.18 74.76 74.42 73.92 73.67 73.66 73.42 73.17 72.92

147.9 147.1 145.4 145.5 144.6 146.4 143.7 144.7 143.3 142.3 140.8 140.2 140.3 139.5 138.8 138.2

a Saturation temperature (u(T) = 0.1 K). bMass of transferred sample m condensed at T = 243 K. cVolume of nitrogen (u(V) = 0.005 dm3) used to transfer m (u(m) = 0.0001 g) of the sample. dVapor pressure at temperature T calculated from the m and the residual vapor pressure at T = 243 K estimated by iteration. eThe combined standard uncertainty of vapor pressures measurements are at the level of 2−3% (see ref 43). Uncertainties of vaporization enthalpies are expressed in this table as standard deviations.

Table 2. Compilation of Data on Enthalpies of Vaporization Δg1Hm° at Tav and at 298.15 K for N-Alkyl Substituted Imidazoles compound

T-range (K)

N-isopropylimidazole [4532-96-1] N-isobutylimidazole [16245-89-9] N-sec-butylimidazole [20075-29-0] N-tert-butylimidazole [45676-04-8] N-cyclopropylmethylimidazole [717908-74-2] N-cyclopentylimidazole [71614-58-9] N-cyclohexylmethylimidazole [71621-00-6]

283.4−323.0 289.6−326.1 295.7−330.2 296.4−343.2 303.0−343.3 305.0−346.1 314.0−352.8

Δg1Hm° (Tav) (kJ mol−1)

° a (−Δg1Cp,m ° )b Cp,m (J·K−1 mol−1)

± ± ± ± ± ± ±

223.2(68.6) 255.1(76.9) 255.1(76.9) 249.7(75.5) 235.3(71.8) 255.2(76.9) 281.1(83.7)

58.1 61.4 63.5 60.4 58.1 68.6 74.6

0.2 0.1 0.2 0.3 0.2 0.4 0.3

Δg1Hm° (298.15K)c (kJ mol−1) 58.4 62.2 64.6 61.9 59.8 70.6 77.5

± ± ± ± ± ± ±

0.3 0.3 0.3 0.4 0.3 0.5 0.4

a

The molar heat capacities of liquid at constant pressure calculated by the group additivity.46 bEstimated using the procedure developed by Chickos ° . and Acree.45 cDerived using eqs 2 and 3 with the molar heat capacity difference, Δg1Cp,m 9853

DOI: 10.1021/acs.iecr.5b01599 Ind. Eng. Chem. Res. 2015, 54, 9850−9856

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Industrial & Engineering Chemistry Research

Table 3. Experimental Vaporization Enthalpies of Selected Alkanes and Alkylbenzenes, Used As Homomorphs for Comparison with Alkyl-Imidazoles (in kJ·mol−1) branched alkanes (Majer and Svoboda)47

Δg1Hm° (298.15 K)

substituted benzenes (Verevkin)48

± ± ± ± ± ± ±

isopropylbenzene isobutylbenzene sec-butylbenzene tert-butylbenzene cyclopropylmethylbenzene [1667-00-1] cyclopentylbenzene [700-88-9] cyclohexylmethylbenzene [4410-75-7]

isobutane 2-methylbutane 2-methylbutane neopentane ethylcyclopropane methylcyclopentane ethylcyclohexane

20.0 25.2 25.2 22.4 26.2 31.8 40.6

0.2 0.2 0.2 0.2 0.5 0.2 0.4

Δg1Hm° (298.15 K) 45.2 47.9 48.5 47.7 52.4 56.3 67.4

± ± ± ± ± ± ±

0.2 0.2 0.2 0.2 0.5 0.5 0.5

Table 5. Comparison of Experimental and Calculated Enthalpies of Vaporization Δg1H°m for n-Alkyl- and Branched/ Cyclic (This Work) Imidazoles at 298.15 K in kJ·mol−1 compound

Δg1H°m(exp.)

Linear 61.1 ± 69.1 ± 73.1 ± Branched N-isopropylimidazole 58.4 ± N-isobutylimidazole 62.2 ± N-sec-butylimidazole 64.6 ± N-tert-butylimidazole 61.9 ± N-cyclopropylmethylimidazole 59.8 ± N-cyclopentylimidazole 70.6 ± N-cyclohexylmethylimidazole 77.5 ± N-(n-propyl)-imidazole N-(n-pentyl)-imidazole N-(n-hexyl)-imidazole

Figure 3. Comparison of vaporization enthalpies Δg1H°m (298.15 K) in kJ·mol−1 of N-alkyl substituted imidazoles derived in this work with those for the similarly shaped molecules: alkanes (●); alkylbenzenes (○), and ILs (Δ) available in the literature.22,47,48

parameters

a

value 6.3356 4.5256 1.2456 −2.6956 1.00 −2.00 −5.80

parameters cyclic-correction 3-cycle 5-cycle 6-cycle imidazole unity Im cation [imidazolium]+ anion [NTf2]

Δ

0.2 0.5 0.2

60.6 69.6 74.1

0.5 −0.5 −1.0

0.3 0.3 0.3 0.4 0.3 0.5 0.4

59.4 63.6 63.9 61.9 60.0 70.9 79.4

−1.3 −1.4 0.7 0.0 −0.2 −0.3 −1.9

Table 6. Comparison of Experimental and Calculated Enthalpies of Vaporization Δg1Hm° for n-Alkyl- and Branched Series [Cnmim][NTf2] at 298.15 K in kJ·mol−1

Table 4. Parameters for the Calculation of Enthalpies of Vaporization, Δg1Hm° , of Alkyl Substituted Imidazoles and Imidazolium Based ILs at 298.15 K (in kJ·mol−1) alkane-chain CH3-(C)a CH2−(C)2 CH-(C)3 C-(C)4 CH2-(Im)(C) CH-(Im)(C)2 C-(Im)(C)3

Δg1H°m(add.)

IL Linear [n-C3mim][NTf2] [n-C4mim][NTf2] [n-C5mim][NTf2] [n-C6mim][NTf2] Branched [iso-C4mim][NTf2] [sec-C4mim][NTf2] [cyclo-C3-CH2-mim][NTf2] [cyclo-C5-mim][NTf2] [cyclo-C6-CH2-mim][NTf2]

value 5.5 6.2 5.9 48.7 78.522 28.222

CH3-(C) ≡ CH3-(Im) ≡ CH3-(imidazolium) . +

Δg1Hm° (exp.)

Δg1Hm° (add.)

Δ

127.1 131.2 134.7 140.0

± ± ± ±

0.919 0.819 0.919 0.919

124.9 129.4 133.9 138.4

2.2 1.8 0.8 1.6

131.4 128.4 131.9 133.3 142.0

± ± ± ± ±

1.657 1.657 1.657 1.857 2.457

127.9 128.2 135.3 129.6 143.5

3.5 0.2 −2.0 2.3 −1.5

In the current study we extended this set with three additional parameters CH2-(Im)(C), CH-(Im)(C)2, and C-(Im)(C)3 taken from imidazoles with branched and cycloalkyl substituents. Comparison of the experimental and predicted values for ILs is given in Table 6. As it can be seen the new set of parameters allows to predict vaporization enthalpies at the general level of about ±(1−2) kJ·mol−1. The only one outlier with large deviation of 3.5 kJ·mol−1 was observed in Table 6 for [iso-C4mim][NTf2]. However, a similar deviation is also observed for vaporization enthalpies of similarly shaped isopropylbenzene in comparison to sec-butylbenzene.48 It seems to be that a special small correction is required in the case of isopropyl substituted homomorphs in order to improve estimation. Thus, the set of increments in Table 4 allows prediction of vaporization enthalpies for alkyl-substituted imidazoles and imidazolium-based cations with the [NTf2] anion, with an alkyl chain of an arbitrary length and configuration. The same

enthalpies is a challenging, yet important task.19−22,51−55 Thus, prediction of ILs vaporization enthalpies is highly desired. In our recent work,19 we noticed that for the [Cnmim][NTf2] family the -(CH2)- contribution is close to, but somewhat smaller than, those that are observed for molecular compounds.19 In this context, it is interesting to test whether the three additional parameters CH2-(Im)(C), CH-(Im)(C)2, and C-(Im)(C)3 (see Table 4) developed for alkyl-functionalized imidazoles are also valid for prediction of IL vaporization enthalpies. For this purpose, we used experimental data just recently published for the [Cnmim][NTf2] family with n-alkyl groups19 in combination with data for imidazoles with branched alkyl substituents in this work. These data are collected in Table 6. A preliminary set of group-contributions required for estimation of vaporization enthalpies of ILs was published recently.22 9854

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(6) Shannon, M. S.; Irvin, A. C.; Liu, H.; Moon, J. D.; Hindman, M. S.; Turner, C. H.; Bara, J. E. Chemical and Physical Absorption of SO2 by N-Functionalized Imidazoles: Experimental Results and Molecularlevel Insight. Ind. Eng. Chem. Res. 2015, 54, 462−471. (7) Bara, J. E.; Moon, J. D.; Reclusado, K. R.; Whitley, J. W. COSMOTherm as a Tool for Estimating the Thermophysical Properties of Alkylimidazoles as Solvents for CO2 Separations. Ind. Eng. Chem. Res. 2013, 52, 5498−5506. (8) Shannon, M. S.; Tedstone, J. M.; Danielsen, S. P. O.; Hindman, M. S.; Bara, J. E. Properties and Performance of Ether-Functionalized Imidazoles as Physical Solvents for CO2 Separations. Energy Fuels 2013, 27, 3349−3357. (9) Garist, I. V.; Verevkin, S. P.; Bara, J. E.; Hindman, M. S.; Danielsen, S. P. O. Building Blocks for Ionic Liquids: Vapor Pressures and Vaporization Enthalpies of 1-(n-Alkyl)-benzimidazoles. J. Chem. Eng. Data 2012, 57, 1803−1809. (10) Garist, I. V.; Verevkin, S. P.; Samarov, A. A.; Bara, J. E.; Hindman, M. S.; Danielsen, S. P. O. Building Blocks for Ionic Liquids: Vapor Pressures and Vaporization Enthalpies of Alkoxy Derivatives of Imidazole and Benzimidazole. Ind. Eng. Chem. Res. 2012, 51, 15517− 15524. (11) Shannon, M. S.; Hindman, M. S.; Danielsen, S. P. O.; Tedstone, J. M.; Gilmore, R. D.; Bara, J. E. Properties of alkylbenzimidazoles for CO2 and SO2 capture and comparisons to ionic liquids. Sci. China: Chem. 2012, 55, 1638−1647. (12) Turner, C. H.; Cooper, A.; Zhang, Z.; Shannon, M. S.; Bara, J. E. Molecular Simulation of the Thermophysical Properties of NFunctionalized Alkylimidazoles. J. Phys. Chem. B 2012, 116, 6529− 6535. (13) Shannon, M. S.; Bara, J. E. Properties of Alkylimidazoles as Solvents for CO2 Capture and Comparisons to Imidazolium-Based Ionic Liquids. Ind. Eng. Chem. Res. 2011, 50, 8665−8677. (14) Liu, H.; Bara, J. E.; Turner, C. H. Tuning the Adsorption Interactions of Imidazole Derivatives with Specific Metal Cations. J. Phys. Chem. A 2014, 118, 3944−3951. (15) Liu, H.; Bara, J. E.; Turner, C. H. DFT study on the effect of exocyclic substituents on the proton affinity of 1-methylimidazole. Chem. Phys. 2013, 416, 21−25. (16) De Luca, L. Naturally occurring and synthetic imidazoles: Their chemistry and their biological activities. Curr. Med. Chem. 2006, 13, 1− 23. (17) Anderson, E. B.; Long, T. E. Imidazole- and imidazoliumcontaining polymers for biology and material science applications. Polymer 2010, 51, 2447−2454. (18) Yao, K.; Wang, Z.; Wang, J.; Wang, S. Biomimetic materialpoly(N-vinylimidazole)-zinc complex for CO2 separation. Chem. Commun. 2012, 48, 1766−1768. (19) Verevkin, S. P.; Zaitsau, D. H.; Emelyanenko, V. N.; Yermalayeu, A. V.; Schick, C.; Liu, H.; Maginn, E. J.; Bulut, S.; Krossing, I.; Kalb, R. Making Sense of Enthalpy of Vaporization Trends for Ionic Liquids: New Experimental and Simulation Data Show a Simple Linear Relationship and Help Reconcile Previous Data. J. Phys. Chem. B 2013, 117, 6473−6486. (20) Zaitsau, D. H.; Yermalayeu, A. V.; Verevkin, S. P.; Bara, J. E.; Stanton, A. D. Structure−Property Relationships in Ionic Liquids: A Study of the Influence of N(1) Ether and C(2) Methyl Substituents on the Vaporization Enthalpies of Imidazolium-Based Ionic Liquids. Ind. Eng. Chem. Res. 2013, 52, 16615−16621. (21) Zaitsau, D. H.; Yermalayeu, A. V.; Emel’yanenko, V. N.; Verevkin, S. P.; Welz-Biermann, U.; Schubert, T. Structure-property relationships in ILs: A study of the alkyl chain length dependence in vaporisation enthalpies of pyridinium based ionic liquids. Sci. China: Chem. 2012, 55, 1525−1531. (22) Zaitsau, D. H.; Fumino, K.; Emel’yanenko, V. N.; Yermalayeu, A. V.; Ludwig, R.; Verevkin, S. P. Structure-Property Relationships in Ionic Liquids: A Study of the Anion Dependence in Vaporization Enthalpies of Imidazolium-Based Ionic Liquids. ChemPhysChem 2012, 13, 1868−1876.

procedure could be easily adjusted for the prediction of vaporization enthalpies of IL with any functionalization (e.g., CF2, OH, CN, etc.) of the side group, provided that appropriate additive parameters can be obtained from molecular compounds of similar structure. Currently we are working on extension of the same procedure for prediction vaporization enthalpies for the 2-alkyl-substituted imidazolium based ILs as well as for the pyridinium, pyrrolidinium, and ammonium based ILs.

4. CONCLUSIONS In the current work, we have applied the transpiration method to measure the highly pure samples of imidazoles with branched and cyclic alkyl substituents. Simple group-additivity procedures have been developed for prediction of enthalpies of vaporization of alkyl-substituted imidazoles and imidazolium based ILs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b01599. 1 H NMR data for imidazole compounds 1−3, 4−7 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Phone: 49-381-498-6508. Fax: 49-381-498-6524. E-mail: [email protected]. To whom correspondence concerning the experimental measurements and data evaluation should be addressed. (S.P.V.). *Phone: 1-205-348-6836. Fax: 1-205-348-7558. E-mail: jbara@ eng.ua.edu. To whom correspondence concerning the synthetic part and discussion should be addressed (J.E.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Ksenia V. Zaitseva gratefully acknowledges a research scholarship from Deutsche Akademische Austauschdienst (DAAD). This work has been partly supported by the Russian Government Program of Competitive Growth of Kazan Federal University. Jason E. Bara acknowledges partial support for this work provided by ION Engineering, LLC; U.S. Department of Energy − National Energy Technology Laboratory (DEFE0005799); and U.S. Department of Energy SBIR/STTR Program (DE-SC0010227). This manuscript is dedicated to the memory of Alexander “Xander” D. Stanton.

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