(Bicaret) in Ordinary and Heavy Water at ... - ACS Publications

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Standard Volumetric Properties of Tetra‑N‑ethylglycoluril (Bicaret) in Ordinary and Heavy Water at Temperatures from (278.15 to 318.15) K and Ambient Pressure Evgeniy V. Ivanov,* Elena Yu. Lebedeva, and Vladimir K. Abrosimov Laboratory of Thermodynamics of Solutions of Non-electrolyte and Biologically Active Substances, G.A. Krestov Institute of Solution Chemistry of the Russian Academy of Sciences, 1 Akademicheskaya Straße, Ivanovo, 153045, Russian Federation ABSTRACT: The densities of solutions of bicaret (2,4,6,8-tetraethyl-2,4,6,8tatraazabicyclo[3.3.0]octane-3,7-dione or tetra-N-ethylglycoluril) in ordinary (H2O) and heavy (D2O) water were measured using the Anton Paar DMA 5000 M high-precision vibrating tube densimeter. All experiments were carried out between 278.15 K and 318.15 K at pressure to be ca. 0.1 MPa. The solution molality ranged from approximately (0.01 to 0.2) mol·(kg of solvent)−1 for each of H/D isotopically distinguishable systems compared. The temperaturedependent apparent molar volumes as well as expansibilities (down to infinite dilution) of the solute were derived from density data. The behavior of D2O− H2O solvent isotope effects on the considered quantities was described taking into account the structural features of H2O and D2O media. The obvious relationship between D2O−H2O solvent isotope effects on the standard molar volume of bicaret and standard molar enthalpy of its dissolution in water was discovered. One of the points of this work is a comparison of the obtained volumetric characteristics with those for tetra-N-methylglycoluril (mebicar), using the previous data.

1. INTRODUCTION The molecules of a general formula (CH)2[(NR)2CO]2 where R ≡ H or Alk, consisting of two close-to-planar five-membered heterocycles, are known as the alkyl-N-substituted glycoluril derivatives (see Figure 1). At present, there is some ambiguity in

The alkyl-N-substituted glycolurils have found plenty of useful applications in the field of construction of self-assembling molecular entities, mainly, supramolecular coordination-bonded systems,8−12 as well as the preparation of pharmaceuticals.4,13−19 Among the N-alkyl derivatives of glycoluril, the tetra-Nsubstituted compounds are considered to be the most active.20 Herewith it is assumed that their activity decreases markedly with the decrease in the number and bulk of N-alkyl substituents. The psychotropic drugs mebicar (2,4,6,8-tetramethylglycoluril) and albicar (2,6-dimethyl-4,8-diethylglycoluril) are well-known tranquilizers and antidepressants.4,13,15 Another known representative of the specified fully N-alkylated bioactive compounds, bicaret (2,4,6,8-tetraethylglycoluril), is much less studied.16,21−23 Unlike most glycolurils including albicar, the conformationally similar mebicar and bicaret are achiral compounds due to symmetry of the alkyl-N-substituted positions in a molecule structure (Figure 1). Because of this, both glycolurils compared have certain geometric features of the molecular bicyclic framework. In particular, owing to the cis-fusion of the heterocyclic rings with the formation of a ∼119° dihedral angle between them, these compounds adopt a conformation of “halfopen book” or “gull-wing” shape.17,20,24 However, the alkyl substituents of bicaret have a more bulk-branched structure compared to that of mebicar. When the specified glycolurils are subjected to dissolution in water, this leads to the fact that the molecules of bicaret are hydrated more strongly than those of its tetra-N-methylated analogue.25 Herewith, unlike the mebicar

Figure 1. A simplified molecular structure of the tetra-N-alkylated glycoluril.

naming of these compounds. So, the CAS name of unsubstituted glycoluril (R ≡ H), a title compound of the given series, is tetrahydroimidazo[4,5-d]imidazole-2,5(1H,3H)-dione.1−3 At the same time this naming does not reflect the fact of the existence of contiguous carbon atoms [>C(H)−C(H) 0.98

analysis method final mass fraction purity purification method initial mass fraction purity

d

(for which the standard molar enthalpy of solution, ΔsolHo2, is positive),26−28 the hydration of bicaret (where ΔsolHo2 < 0) is treated as a prevailingly hydrophobic process.25 One would expect that the existing differences in the hydration behavior of mebicar and bicaret will be reflected on the peculiarities of structure-packing transformations in the aqueous environment. The latter should be manifested in the volumetric properties of both a solute and a binary solution on the whole. In this context, the experimental approach based on a combination of such two “structure-non-perturbing” methods as the highprecision densimetry and the D2O− H2O solvent isotope substitution is proven to be rather informative.29−45 By virtue of the quantum nature of H/D isotope effects,46−49 the given approach allows one to establish (at the molecular level) the role of hydrogen-bonding and hydrophobic effects in the structureforming process, which are manifested in the volume-related characteristics. The standard (apparent or partial at infinite dilution) molar ∞ o o volumes, Vo2(≡V∞ ϕ,2 ≡V̅ 2 ), and expansibilities, Ep,2 = (∂V2/∂T)p, are among the most often determined thermodynamic characteristics of the aqueous solutions in a wide temperature range. This fact is caused by their independence from the solute−solute interactions, that is, from the composition effect.50−52 Previously,42,53 we have studied the temperature-dependent behavior of Vo2 and Eop,2 for mebicar as a solute in H2O and D2O. This tetra-N-alkylated derivative demonstrates some interesting features of the hydration behavior, being connected primarily with a superposition of hydrophobic and hydrophilic mechanisms (with the latter predominating). Seen in this light, the volumetric properties of dissolved (hydrated) bicaret was not studied hitherto. Besides, taking account of the recently obtained data on ΔsolHo2,25 it appears fairly interesting to assess the influence of further lengthening the peripheral hydrocarbon chains of mebicar, to form bicaret, on the structure-packing changes in H/D isotopically distinguishable aqueous media. To fill this gap, we report here the results of densimetric measurements and derived from them the data on Vo2 and Eop,2 for bicaret in ordinary (H2O) and heavy water (D2O) within the temperature range between 278.15 K and 318.15 K (with a step of 10 K) and at the pressure of p = (99.6 ± 0.4) kPa. Within the scope of this study, we have discussed also the corresponding D2O−H2O solvent isotope effects (hereinafter, IEs or δ), comparing them with the similar results obtained previously for mebicar.42

IR-spectra,e DSC peak profilef density,h electrical conductivityi density,h electrical conductivityi

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DOI: 10.1021/acs.jced.5b00154 J. Chem. Eng. Data 2015, 60, 2079−2089

Journal of Chemical & Engineering Data

Article

Table 2. Comparison of Measured Densities for Pure Heavy Water, ρ1,D/(g·cm−3), with the Literature Values at Different Temperatures and Pressure p = 0.1 MPa (Values in Italics Correspond to the Predicted 100 % Deuterosubstitution in Heavy Water Molecules)a x(D2O) data source this work P. Scharlin and K. Steinbyb E.V. Ivanov and V. K. Abrosimovc I.B. Rabinovichd T.-L. Chang and J.-Y. Chiene T.-S. Chang and L.-H. Tungf A.I. Shatenshteing G.S. Kellh F.J. Millero, R. Dexter and E. Hoffi average-weighted value

temperature, T/K

mass fraction

278.15

288.15

298.15

308.15

318.15

0.9992 1.0 0.9977 1.0 0.9995 1.0 1.0 1.0 1.0 1.0 1.0 0.9988 1.0 1.0

1.105526 1.105611 1.105585 1.105828 1.10559 1.10564 1.10557 1.10549 1.10555 − 1.10562 1.105531 1.105658 1.10562

1.105771 1.105857 1.105715 1.105961 1.10585 1.10590 1.10585 1.10577 1.10583 1.10589 1.10587 1.105751 1.105879 1.10587

1.104364 1.104450 1.104264 1.104511 1.10443 1.10448 1.10445 1.10440 1.10440 1.10446 1.10445 1.104362 1.104491 1.10445

1.101634 1.101720 1.101492 1.101740 1.10172 1.10177 1.10168 1.10172 1.10169 1.10172 1.10173 1.101631 1.101760 1.10173

1.097843 1.097929 1.097560 1.097807 1.09792 1.09797 1.09787 1.09792 1.09790 1.09792 1.09794 1.097864 1.097993 1.09792

The expanded uncertainties, ± U, do not exceed U(ρ1,D) = 2·10−5 g·cm−3 (1·10−5 in the case of our study), U(T) = 0.01 K, and U(p) = 0.5 kPa (level of confidence = 0.95). bReference 41. cReference 45. dReference 46. eReference 57. fReference 58. gReference 59. hReferences 60, 61. i Reference 62. a

later, in subsection 3.2).42−45 In our case, the bicaret molality ranged from ca. 0.01 to ca. 0.195 mol·(kg H2O)−1 in the ordinary (protiated) aqueous media. The caqm,2 values ranged up to ca. 0.185 mol·(55.51M1,D)−1 in the deuterated aqueous media. The finite values of caqm,2(m) in both binary systems compared were close to the bicaret solubility in H2O and D2O at T = 278.15 K. The overall uncertainty in the molality/aquamolality was estimated to be less than ± (3·10−5) mol·per kg of H2O or per (55.51M1,H(D))−1, respectively. 2.3. Densimetric Measurements. The densities of water isotopologues, ρ1,H(D), and solutions of bicaret, ρs(caqm,2), in them were measured using an Anton Paar DMA 5000 M oscillating Utube high precision densimeter provided with automatic viscosity correction and operated under the static mode. According to the setup Instruction Manual, the temperature repeatability should be 0.001 K. However, the temperature of the measuring cell was kept to be constant up to ±0.01 K during the experiments. All measurements were carried out within p = (99.6 ± 0.4) kPa. The apparatus was calibrated with dry air and freshly prepared water (Table 1) just prior to each series of ρs measurements. The temperature-dependent data on density of ordinary water, ρ1,H, were assumed to be those of the IAPWS Formulation 1995.66 Herewith the quality of a “local” H2O was checked densimetrically by comparing with the “ultra pure water” density standard (Anton Paar, Austria). According to the analysis results, the examined sample of water differed from the reference one of no more than by ± (5·10−6) g·cm−3. To fill the densimetric cell with solution (of volume of ca. 1.0 cm3), the self-flowing mode without contact with the atmosphere moisture was realized. Every solution sample was slightly preheated above the measurement temperature to prevent the possible bubbleforming (a visually observed effect). Then, the solution was cooled step-by-step before making the density serial measurements. Under such conditions, triplicate measurements of ρs(caqm,2) were reproducible to within ± (5·10−6) g·cm−3. Taking into account the influence of all possible factors,67,68 the overall uncertainty of the measured ρs(caqm,2) did not exceed statistically ± (1.0·10−5) g·cm−3. The apparatus design and experimental procedure are detailed elsewhere.69

the bicaret sample was stored in a light-proof vacuum desiccator over P2O5. The deionized water was twice distilled (initially from a KMnO4 dilute solution) up to a specific conductivity (κ) of 1.3 × 10−6 S·cm−1. Heavy water with κ = 1.2·10−6 S·cm−1 was used as such (Table 1). The additional D/H analysis of heavy water was made densimetrically with an apparatus described below (in subsection 2.3). The deuterium content (up to ± 0.01 atom % D) was tested using the procedure46,56 based on the comparison of obtained density (ρ1,D) data with those for “reference” heavy water containing 100 atom % D at different temperatures (see Table 2). One can see in Table 2 our ρ1,D values are in good agreement with literature data as a whole, given the deuteriumatom content in heavy water. Herewith the Kell’s data,60,61 which are closest to the average-weighted ρ1,D for absolutely deuterated heavy water, were chosen as reference ones at calculating xD. The residual H2O content in heavy water was taken into account in the calculation of the D2O molar mass (M1,D) while preparing the H/D isotopically distinguishable solutions. A conversion to molar mass for each component (M1,H, M1,D, or M2) was based on the 2011 IUPAC table on atomic weights of the elements.63 2.2. Preparation of Solutions. All the isotopically distinguishable aqueous solutions were prepared by weighting under airtight conditions using an analytical balance (LLB-200G model, Russia) with an uncertainty of ± (5·10−5) g. The bicaret content was expressed in the form of both molality, m/(mol·(kg H2O)−1), for protiated aqueous solutions only and aquamolality, caqm,2, for both isotopically distinguishable systems. caqm,2 is defined by the number of moles of a solute per approximately 55.51 (more precisely, 55.50843) moles of D2O or H2O.44,64,65 The factor of 55.51 being roughly equal to the number of H2O moles in 1 kg of water is introduced in order for caqm,2 and m in aqueous (H2O) solutions to coincide numerically. Applying the caqm,2 scale is necessary to allow the proper comparison of solution densities, ρs, and apparent molar volumes of a solute, Vϕ,2, at the same (rounded) concentrations in the presence of equal numbers of H2O and D2O molecules. Moreover, noteworthy is that the employing of the given scale allows reducing an error in the definition of Vϕ,2 at high dilutions (see 2081



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Table 3. Experimental Densities, ρs/(g·cm−3), and Smoothed Apparent Molar Volumes, Vϕ,2/(cm3·mol−1), for Solutions of Bicaret in Ordinary and Heavy Water at Different Temperatures, T/K, and molalities, m/(mol·(kg solvent)−1, and at p ≈ 99.6 kPaa 278.15 K ρs

caqm,2b

m 0 0.025423 0.040867 0.054117 0.073280 0.090782 0.11045 0.12881 0.13302 0.14158 0.15898 0.17602 0.19482

0

0.022463 0.026754 0.041437 0.058624 0.072017 0.088470 0.10106 0.10538 0.11894 0.13781 0.15649 0.16640

0.024970 0.029740 0.046062 0.065167 0.080055 0.098345 0.11234 0.11714 0.13222 0.15319 0.17396 0.18497

288.15 K Vϕ,2

ρs

298.15 K ρs

Vϕ,2

0.999965 1.000826 1.001347 1.001791 1.002443 1.003034 1.003705 1.004327 1.004476 1.004767 1.005352 1.005942 1.006574

220.374 220.245 220.132 219.972 219.823 219.658 219.503 219.467 219.395 219.248 219.105 218.947

0.999101 0.999914 1.000412 1.000829 1.001447 1.002012 1.002636 1.003233 1.003371 1.003637 1.004202 1.004754 1.005356

1.105814 1.105871 1.106063 1.106297 1.106481 1.106715 1.106891 1.106963 1.107159 1.107441 1.107730 1.107884

219.524 219.481 219.328 219.148 219.006 218.833 218.698 218.652 218.507 218.304 218.102 217.994

1.105993 1.106040 1.106185 1.106373 1.106519 1.106705 1.106847 1.106904 1.107060 1.107291 1.107522 1.107647

Water + Bicaret 0.997047 222.343 0.997822 222.223 0.998292 222.117 0.998695 221.966 0.999282 221.827 0.999814 221.672 1.000415 221.526 1.000978 221.493 1.001112 221.426 1.001360 221.288 1.001894 221.154 1.002415 221.006 1.003000 Heavy Water + Bicaretc 221.824 1.104537 221.782 1.104573 221.640 1.104688 221.472 1.104835 221.341 1.104947 221.179 1.105099 221.054 1.105215 221.011 1.105259 220.876 1.105384 220.687 1.105573 220.498 1.105759 220.398 1.105859

308.15 K

318.15 K

Vϕ,2

ρs

Vϕ,2

ρs

Vϕ,2

224.284 224.169 224.073 223.933 223.805 223.662 223.527 223.496 223.434 223.308 223.183 223.047

0.994035 0.994779 0.995227 0.995612 0.996170 0.996686 0.997256 0.997794 0.997915 0.998165 0.998675 0.999166 0.999719

226.172 226.069 225.980 225.854 225.738 225.608 225.487 225.459 225.402 225.288 225.176 225.052

0.990216 0.990931 0.991363 0.991738 0.992273 0.992767 0.993315 0.993833 0.993947 0.994195 0.994673 0.995157 0.995680

228.020 227.926 227.844 227.727 227.619 227.499 227.387 227.362 227.309 227.203 227.099 227.985

223.925 223.887 223.755 223.600 223.479 223.330 223.215 223.176 223.052 222.877 222.704 222.611

1.101767 1.101798 1.101884 1.101999 1.102087 1.102205 1.102291 1.102331 1.102429 1.102582 1.102726 1.102805

225.952 225.918 225.806 225.672 225.568 225.439 225.340 225.306 225.198 225.047 224.896 224.816

1.097944 1.097967 1.098032 1.098121 1.098187 1.098281 1.098353 1.098381 1.098457 1.098573 1.098693 1.098758

227.929 227.901 227.800 227.681 227.587 227.471 227.382 227.351 227.255 227.120 226.984 226.913

a The uncertainties are U(ρs) = 1.0·10−5 g·cm−3, U(Vϕ,2) = 0.02 cm3·mol−1, U(m) = 3·10−5 mol·kg−1, U(T) = 0.01 K, U(p) = 0.4 kPa (level of confidence = 0.95). bAquamolalitiy: mol·(55.51M1,H(D))−1; in H2O-aqueous solutions, m and caqm,2 coincide numerically. cDensity values at m(caqm,2) = 0 are presented in Table 2.

Table 4. Regression Coefficients, ai, and Standard Errors of the Fit, σρ, from eq 1 for the H/D Isotopically Distinguishable Binary Systems Compared at Different Temperatures and p = (99.6 ± 0.4) kPaa D2O + Bicaretb

H2O + Bicaret T/K

103a0

103a1

103a2

103σρ

103a0

103a1

103a2

103σρ

278.15 288.15 298.15 308.15 318.15

999.9650 (0.0027) 999.1012 (0.0027) 997.0475 (0.0032) 994.0353 (0.0017) 990.2160 (0.0021)

33.749 (0.062) 31.958 (0.061) 30.436 (0.073) 29.167 (0.038) 28.111 (0.049)

1.001 (0.305) 0.802 (0.302) 0.471 (0.358) 0.010 (0.189) −0.312 (0.241)

3.47 3.44 4.07 2.15 2.74

1105.5262 (0.0017) 1105.7700 (0.0018) 1104.3632 (0.0019) 1101.6343 (0.0025) 1097.8433 (0.0016)

11.322 (0.040) 8.761 (0.045) 6.775 (0.047) 5.181 (0.061) 3.883 (0.040)

7.721 (0.205) 7.540 (0.229) 7.174 (0.240) 6.298 (0.311) 5.758 (0.204)

2.20 2.46 2.58 3.34 2.19

a Units: a0 and σρ, g·cm−3; a1, g·(55.51M1,H(D))·mol−1·cm−3; a2, g·(55.51M1,H(D))2·mol−2·cm−3. In parentheses, standard fitting deviations at the 95 % confidence level for each of calculated values of ai are given. bComputed including the density data from Table 2.

together with the standard errors of the fit, σρ, for each of the temperatures chosen are presented in Table 4. The values of σρ were defined by52,70

3. RESULTS AND DISCUSSION 3.1. Experimental Density of Solutions. The densities of pure water H/D isotopologues and solutions of bicaret in them at each of temperatures are summarized in Table 3. To calculate the concentration-dependent Vϕ,2 as well as other volume-related characteristics of the isotopically distinguishable aqueous solutions at the same (rounded) caqm,2 values, the measured ρs(caqm,2) were fitted using a least-squares polynomial regression to 2 ρs = a0 + a1caqm,2 + a 2caqm,2

⎡ 1 σρ = ⎢ ⎣n − k

1/2

⎤

∑ (ρcalcd − ρexptl )2 ⎥⎦

(2)

In eq 2, k is the number of ai coefficients used in eq 1, and n represents the number of direct experimental concentrationdependent ρs values at each temperature taken (see in Table 3). As it is mentioned above, we have not found any literature data on ρs and other volumetric characteristics of aqueous bicaret to verify our results at least for one of the temperatures. Hence we

(1)

The number of coefficients ai in eq 1 was determined by applying an F-test70,71 at the 95% confidence level. The ai quantities 2082



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Table 5. Standard Molar Volumes, Vo2, Coefficients v22 of eq 6 and Values of Packing Density, d, for the (Water + Bicaret) and (Heavy Water + Bicaret) Systems at Different Temperatures, T/K, and p ≈ 99.6 kPaa H2O + Bicaret T/K 278.15 288.15 298.15 308.15 318.15

H2O → D2O

D2O + Bicaret

Vo2

v22

d

Vo2

v22

d

δVo2

δv22

d

220.558 222.544 224.467 226.337 228.172

−8.43 −7.90 −7.29 −6.60 −6.09

0.6307 0.6250 0.6197 0.6146 0.6096

219.780 222.061 224.143 226.142 228.101

−9.64 −8.98 −8.27 −7.16 −6.42

0.6329 0.6264 0.6206 0.6151 0.6098

−0.78 −0.48 −0.32 −0.19 −0.07

−1.21 −1.08 −0.98 −0.56 −0.33

0.0022 0.0014 0.0009 0.0005 0.0002

Units: V, cm3·mol−1; v22, cm3·(55.51M1,H(D))·mol−2. The standard uncertainties of fitting do not exceed σV = 0.01 cm3·mol−1, σv = 0.02 cm3· (55.51M1,H(D))·mol−2, σd = 0.0001 (if the Vvdw,2 value is taken to be exactly the same for both H/D isotopically distinguishable systems).

a

believe that values listed in Tables 3 to 5 (see above and below) are reported for the first time. 3.2. Apparent and Standard Molar Volumes. In Table 3, the smoothed Vϕ,2 values are listed, too. For computing Vϕ,2(caqm,2), the formula expressing the volume of a binary solution, Vs, in the aquamolality scale was applied42−45 Vs(caqm,2) =

55.51M1,H(D) + M 2caqm,2 ρs

* = V1,H(D) + Vϕ ,2caqm,2

(3)

where V*1,H(D) = 55.51M1,H(D)/ρ1,H(D) is the solution volume when caqm,2 → 0. In accordance with the inferences,64,72−74 the excess volume of a solution per 55.51 mol of H2O or D2O, VEs (caqm,2) can be expressed in the form * VsE(caqm,2) = Vs(caqm,2) − V1,H(D) − V 2ocaqm,2

(4)

In turn, at fairly low solution concentrations, the VEs value can be expanded in a power virial series of caqm,272,75 VsE(caqm,2)

= v22caqm,2 +

2 v222caqm,2

+ ...

Figure 2. Volume-related virial coefficients of a solute−solute pairwise interaction for solutions of mebicar (circles)42 and bicaret (squares) in H2O (solid lines) and D2O (dashed lines) as a function of temperature. For mebicar, the mean-weighted half-width of 95 % confidence interval of the quantity considered does not exceed 1.0 cm3·(55.51M1,H(D))· mol−2, at worst.

(5)

Putting the right-hand side of eq 4 into eq 5 (instead of VEs ) yields the following expression * Vs(caqm,2) − V1,H(D) = Vϕ ,2caqm,2 2 3 = V 2ocaqm,2 + v22caqm,2 + v222caqm,2 + ...

enhanced on going from ordinary to heavy water. Here, the point is that the water (H2O or D2O) molecules, occupying larger volumes in the hydration cosphere than in the solvent bulk, are more structured in the former case. Taking into account the fact that the hydrogen-bonding is stronger in heavy water than in ordinary water,46,47,77 it is clear why the v22 coefficient has a more negative value in the D2O medium. In harmony with this observation are positive enthalpic virial coefficients, h22, and the corresponding solvent IEs.25,78,79 Previously,25 the h22 quantities were estimated by fitting the enthalpies for dilution, ΔdilHm2 , of H/D isotopically distinguishable aqueous solutions of bicaret with the polynomial expansion based on the McMillan−Mayer formalism.76,78−81 The positive sign at the h22 coefficient shows that the homotactic interactions between bicaret molecules are prevailingly hydrophobic in nature. It means that the solute molecules in H/D isotopically distinguishable aqueous media should experience some repulsion, although the overlapping of cospheres with the same hydration type should promote association. However, such an overlapping of cospheres in the case of hydrophobically hydrated solute leads to considerable enhancement of clathrate-formation in the nearest vicinity of its nonpolar groups, a factor that facilitates the separation of hydrated solute molecules.82 It is

(6)

Coming from the formalism of the McMillan−Mayer solution theory,76 the virial volumetric coefficients v22 and v222 in eq 5 represent the contribution of pair and triple aggregates, being formed by the solute molecules, to the VsE quantity.73,74 According to eq 6, the same goes for Vϕ,2. In our case, the statistical analysis (based on the F test)70,71 showed that the contribution of term v222 to Vϕ,2(caqm,2) is negligible when the experimental data are described adequately using eq 6. The Vo2 and v22 values obtained in such a way are collected in Table 5. Note that the specified approach (eqs 3 to 6) was successfully employed previously44,45,64,74 when the volume- and interactionrelated properties of H/D isotopically distinguishable aqueous solutions of 1,1,3,3-tetramethyl-2-urea and its cyclic derivatives, 1,3-dimethyl-2-ethyleneurea and 1,3-dimethyl-2-propyleneurea, were subjected to study. In all these cases, both v22 and δv22(H2O → D2O) = v22(D2O) − v22(H2O) values were found to be negative, decreasing in magnitude as the temperature rises. As follows from the data of Table 5 and Figure 2, a similar situation is typical for aqueous bicaret, too. According to conclusions,72 this means that the solute under consideration has a prevailingly structure-making effect on the aqueous surroundings, which is 2083



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likely that for bicaret, the latter effect prevails over the former. This is supported by the increase in h22 at the deuteration of solvent molecules.25 Seen in this light, most interesting is the fact that the v22 values for mebicar are negative too,42,53 although they are somewhat smaller in magnitude compared to those for bicaret (see Figure 2). So, in the former case, v22/{cm3·(55.51M1,H(D))·mol−1} was found to be equal to −(6.2 ± 0.8) and −(3.3 ± 0.5) in H2O and D2O, respectively, at T = 298.15 K.42 However, similar to the sign inversion of δh22(H2O → D2O) on going from mebicar to bicaret,25,79 the simultaneous introduction of each of the compared glycoluril-derivatives into the ordinary and heavy water leads to radically different solvent IEs on v22. In the case of solute bicaret, the corresponding IE is negative, whereas for solute mebicar, quite the opposite situation is observed: δv22(H2O → D2O) > 0 over the whole temperature range being considered (Figure 2). Herewith a situation with δv22 for mebicar is in contrast to the current opinion35,41,83 that positive thermodynamic quantities in the binary D2O-containing systems are more positive while the negative quantities are more negative than those in the protiated aqueous systems. Such an inconsistence in the distribution of IEs has been found only in a few works,84,85 including ours, but it was not yet subject to a reasonable explanation. In our opinion, here, this fact can be related to the above-mentioned predomination of hydrophilic effects in the solute hydration as it is in the case of aqueous urea where δv22 > 0 and δh22 < 0 also.37,78 On the other hand, it must be recognized that the deduction of structure-related changes in aqueous media from experimental measurements of only volumetric or another thermodynamic property has well-known limitations:31 some properties may indicate structure-making whereas others may be more consistent with structure-breaking effects. The conclusions from a single characteristic will primarily depend on the extent to which the property is sensitive to long-range effects.31 Therefore, volume-related interaction characteristics (such as the v22 values) cannot be the sole criterion of a solute hydrophobicity or hydrophilicity. A combination of two or more thermodynamic properties including the corresponding solvent IEs that we try to obtain in this work must be considered. Table 5 contains also the values of packing density, d = Vvdw,2/ Vo2, which represents the fraction of Vo2 occupied by the intrinsic volume of a solute.52,86 The latter quantity is expressed in terms of the van der Waals volume per 1 mol, Vvdw,2 = NAvvdw,2, where NA is the Avogadro constant. According to the method87 that assumes possible intersections of the van der Waals spheres of the valence nonbonded atoms, a molecule volume vvdw,2 was calculated as the sum of volumes of the C, N, O, and H atomic increments that depend on the structural features of the environment of each atom in the organic “overloaded” molecule. Herewith it is postulated that Vvdw,2, being found to be approximately 139 cm3·mol−1 {vvdw,2 = (231.0· ± 2.3)·10−24 cm3}87 for bicaret and approximately 101 cm3·mol−1 {vvdw,2 = (168.2· ± 1.7)·10−24 cm3}87 for mebicar at T = 298.15 K, is independent of T over the temperature range under study. Since the value of Vvdw,2 is taken to be unaltered (exactly the same) for both the H/D isotopically distinguishable systems being considered, the data on parameter d (Table 5) and the corresponding IEs (Figure 3) can be treated as quite reliable. Recall also that the D2O−H2O solvent IEs on Vo2 are dominated by changes in water−water interactions (H compared to D) in binary isotopically distinguishable systems.33,46 Most of these changes concern a difference between the librational and

Figure 3. Packing density parameters for solutions of mebicar (circles)42 and bicaret (squares) in H2O (solid lines) and D2O (dashed lines) as a function of temperature.

hindered-translational motions as well as “volumes” of molecules in the liquid phases of H2O and D2O.42,88,89 Herewith a decrease in the zero-point vibrational energy of water molecules during their deuteration brings about the formation of more strong hydrogen bonds in an aqueous medium.46,47,90 When the Vo2 quantities are combined with the corresponding solvent IEs, all of the above volume-isotope changes are part of the values of δd(H2O → D2O) depicted in Figure 3. As follows from the data in Figure 3, the temperature dependence of a quantity considered (d) for each of H/D isotopically distinguishable aqueous solutions of bicaret is close to a linear function. Similar dependences for aqueous mebicar are far from linearity and the derivative (∂d/∂T)p takes decreasingly negative values when the temperature increases. Upon going from mebicar to bicaret, the fraction of free volume (expressed as Vvdw,2/Vo2) in the structural packing of a glycoluril-containing “hydration complex” increases by (2.5 to 3.2) %. At that these “volume increments” are greatest at lower temperatures, excluding T = 318.15 K. Such a tendency to the structureloosening in H/D isotopically distinguishable aqueous solutions of bicaret can serve as a confirmation of the above findings about the more bulk-branched structure of this tetra-N-substituted glycoluril. This is due primarily to the fact that the terminal methyl groups of bicaret alkyl radicals are sited under the “wings” of the folded molecular frames in gauche orientations toward C− N bonds connected with a C−C bridge of the bicyclic compound considered.24 A similar packing effect is manifested at the D2O-by-H2O substitution too, although to a much lesser extent (Figure 3). Suffice it to say that the packing density values in H2O and D2O differ from each other by a factor of 1.005 for solute mebicar at T = 288.15 K and of 1.003 for solute bicaret at T = 278.15 K, with formation of the denser structure in the case of a glycoluril solution in heavy water. That is, the introduction of additional CH2 groups on the N,N,N′,N′-sited positions of a mebicar molecule, to form a bicaret molecule (see Figure 1), results in a relatively looser packing of the hydration complex when each of them is placed in the D2O medium. The nature of this effect is not entirely clear as yet. One can suggest that it is in agreement 2084



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respect to (T − θ) where θ = 298.15 K is a mean-weighted temperature69,93

with the possibility of mebicar molecules to form hydrogen bonds with water molecules not only through carbonyl oxygens, but also via the HC−CH bridging group (according to the millimeter-absorption and 13C NMR spectroscopic studies).91,92 Herewith the formation of such CH···OD/OH-bonds in the H/ D isotopically distinguishable aqueous solutions of bicaret seems to be sterically hindered. As we mentioned above, the replacement of H2O with D2O is accompanied by a marked strengthening of “heterocomponent” hydrogen bonds (by 1.0 kJ·mol−1 and more)46,77,90 with the formation of the more densely packed but more temperaturesensitive aqueous structure that surrounds a bicaret (mebicar) molecule. The largest changes in δVo2 at low temperatures (see Figure 4) are probably due to joint effects of hydrophobic

Ep , ϕ ,2(caqm,2) = Epo,2 + (∂v22 /∂(T − θ ))p caqm,2

(7)

Figure 5 illustrates the trend of changing the quantity in question with increasing caqm,2 for both (H2O + bicaret) and

Figure 5. Apparent molar expansibilities of solute bicaret in H2O (solid lines) and D2O (dashed lines) as a function of the solution aquamolality at temperatures: squares, T = 278.15 K; circles, T = 298.15 K; triangles, T = 318.15 K.

(D2O + bicaret) at three reference temperatures. It can be seen that a simultaneous increase in both the bicaret content and temperature causes the oppositely directed IEs on Ep,ϕ,2. In the former case, the structural packing of a hydration complex in D2O becomes increasingly expansible compared to that in H2O. In the latter case, a difference in compactness of H/D isotopically distinguishable structures becomes decreasingly appreciable, confirming our previous inferences (based on the enthalpyisotope effects)25 about weakening of a hydrophobic constituent of bicaret hydration with increasing solute aquamolality and temperature. The temperature-dependent values of Eop,2 are depicted in o Figure 6. To calculate E p,2 , the V 2o−T functions were approximated by the quadratic equation42,69

Figure 4. D2O−H2O solvent isotope effects on the standard molar volume of mebicar (circles)42 and bicaret (squares) in water. The meanweighted half-width of 95 % confidence interval of the quantity considered for mebicar does not exceed 0.15 cm3·mol−1.

hydration of the solute molecules and ability of them to form stronger D-bonds with the aqueous surroundings. Each of the specified IEs gives a negative contribution to Vo2.30,42,43 With an increase in temperature, the structural (vibrational) differences between water H/D isotopologues become less pronounced.46,88−90 As a consequence, the positive IE caused by a faster volume expansion of the spatial network of D-bonds in heavy water becomes larger. Figure 4 shows that the δVo2 value for mebicar decreases (in magnitude) with temperature more rapidly than does the corresponding value for bicaret. In the region close to T ≈ 310 K, the temperature dependences of δVo2(H2O → D2O) for both compared glycoluril-derivatives intersect at the point ca. −0.2 cm3·mol−1. Hence the solvent IE on the considered quantity for bicaret hypothetically crosses the zero-axis, where the volume effects of solvation in H2O and D2O are identical, at a higher temperature than in the case of mebicar. The above speculations could explain largely the revealed differences in volume properties of aqueous bicaret and mebicar. The additional information can be excerpted from the analysis of derivatives of Vϕ,2 and Vo2 with respect to temperature. 3.3. Apparent and Standard Molar Expansibilities. The apparent molar expansibilities of the solute, Ep,ϕ,2(caqm,2), in H2O and D2O were derived by differentiating eq 6 (without v222) with

V 2o(T ) = b0[ ≡ V 2o(θ )] + b1(T − θ ) + b2(T − θ )2

(8)

It follows from eq 8 that Epo,2(T ) = (∂V 2o/∂(T − θ ))p = b1 + 2b2(T − θ )

(9)

Resulting from such approximation, the quantities desired at T = θ = 298.15 K are (0.1902 ± 0.0008) cm3·mol−1·K−1 for bicaret in ordinary water and (0.2072 ± 0.0047) cm3·mol−1·K−1 for bicaret in heavy water. From the data presented in Figure 6, it is conspicuous that, unlike the aqueous mebicar, the temperature has a slight effect on the structure of a bicaret hydration complex. The fact of larger decrease in Eop,2 of mebicar with increasing temperature may be primarily due to disruption of the H/D-bonded local structures being formed at the expense of both proton-acceptor groups and the above-named additional proton-donor (glyoxalic) those of 2085



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or from one temperature to another, the “structural pattern” of such interrelation will appreciably vary. As seen in Figure 7, such a correlation dependence for mebicar is a close-to-linear function which is sloping toward a decrease of

Figure 6. Standard molar expansibilities of mebicar (circles)42 and bicaret (squares) in H2O (solid lines) and D2O (dashed lines) as a function of temperature. The mean-weighted value of a 95 % confidence interval half-width of the quantity considered for mebicar does not exceed 0.01 cm3·mol−1·K−1. Figure 7. Correlations between the temperature-dependent changes in D2O−H2O solvent isotope effects on the standard molar enthalpy of hydration and standard molar volume for mebicar (circles, from T = 288.15 to T = 318.15 K)27,42 and bicaret (squares, from T = 278.15 to T = 318.15 K)25 in water. The points are distributed with a temperature step of 10 K.

the solute molecules. On the other hand, the temperature dependence of Eop,2 for each of solutes compared can be used to extract useful information on the capability of the solute to promote or destroy the water structure, based on the thermodynamic relation proposed by Hepler94 (∂C po,2/∂p)T = −T (∂ 2V 2o/∂T 2)p = −T (∂Epo,2 /∂T )p

negative δVo2(H2O → D2O) and positive δΔhydrHo2(H2O → D2O) values in magnitude with increasing temperature. The similar relationship for bicaret is an upgoing curve, for which the “slope” is opposite in direction to the previous case. Both relationships convincingly confirm the above conclusion that the volume- and enthalpy-isotope changes are manifested to a large extent at low temperatures when the aqueous environment is the most structurally pronounced. Meanwhile, IEs on Vo2 and ΔhydrHo2 for aqueous mebicar cross the zero-axis values in the closely spaced temperature regions near T = 318.15 K. This fact indicates that, in the given temperature range, the mebicar molecules have identical influence on H2O and D2O that manifests in the mutual compensation of structure-packing (factually, entropic) and energy-related (enthalpic) effects. In the case of aqueous bicaret, it is evidence that the enthalpy-isotope hydration effect, δΔhydrHo2, should be considered allowing for the additional contribution at δV2o(H2O → D2O) ≈ 0. This contribution is caused presumably by a stronger solvent−solvent interaction in heavy water due to the bicaret hydrophobic hydration as well as configurational differences in the molecular packing of H/D isotopically distinguishable systems under comparison.

(10)

where Cop,2 is the standard (limiting partial, or at infinite dilution) molar isobaric heat capacity of the dissolved component. On the basis of such an approach, the structure-breaking and structuremaking solutes should have a negative and positive sign at the derivative (∂Eop,2/∂T)p, respectively. The values of (∂Eop,2/∂T)p were computed by 2-fold differentiating Vo2 (the right-hand side of eq 6) with respect to (T−θ). From here, the following values of (∂Eop,2/∂T)p have been derived (in cm3·mol−1·K−2): −(0.5 ± 0.1)·10−3 and −(1.0 ± 0.8)·10−3 for bicaret in H2O and D2O, respectively, as well as −(3.8 ± 0.4)· 10−3 and −(4.5 ± 0.3)·10−3 for mebicar in the same water H/Disotopologues over the whole temperature range under study.42 Seen in the light of the specified classification,94 aqueous mebicar is considered to be a prevailingly structure-breaking solute. As for the (H2O + bicaret) and (D2O + bicaret) systems, the considered derivative is smaller by several folds in magnitude, approaching to zero value. It can serve as evidence that neither hydrophilic nor hydrophobic effects do not have a predominating influence on the hydration process in infinitely dilute solutions in question. It should be borne in mind also that the sign at (∂E∞ p,ϕ,2/∂T)p is not a sufficient condition to identify the enhancement of the solvent structure or its destroying.52 3.4. Interrelation between Volume- and EnthalpyIsotope Effects of Dissolution. Given the structure-dependent nature of thermodynamic hydration characteristics,42−45,51 we can suggest that there is an obvious interrelation between the volumetric (δVo2) and enthalpic (δΔhydrHo2 ≡ δΔsolHo2) IEs of dissolution (hydration) of a tetra-N-methylated glycoluril in the aqueous medium. Certainly, upon going from mebicar to bicaret

4. CONCLUDING REMARKS The inference to be drawn from the above-discussed results is that the revealed packing changes that occur upon dissolution (hydration) of bicaret are directly related both to structural features of the studied solute and to differences in properties of water H/D isotopologues. Because of the more pronounced ability of heavy water to form hydrogen-bonded molecular aggregates (including hydration complexes), the compactness of 2086



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Korotkova, G. V.; Lapshina, L. V.; Kulik, A. F. The chemistry of bicyclic bisureas. 2. N-alkylation of bicyclic bisureas. Bull. Acad. Sci. USSR, Div. Chem. Sci. 1979, 28, 1222−1227. (6) Kravchenko, A. N.; Maksareva, E.Yu.; Belyakov, P. A.; Sigachev, A. S.; Chegaev, K.Yu.; Lyssenko, K. A.; Lebedev, O. V.; Makhova, N. N. Synthesis of 2-monofunctionalized 2,4,6,8-tetraazabicyclo[3.3.0]octane3,7-diones. Russ. Chem. Bull., Int. Ed. 2003, 52, 192−197. (7) Rezaei-Seresht, E.; Tayebee, R. Synthesis of glycoluril derivatives catalyzed by some heteropolyoxometalates. J. Chem. Pharm. Res. 2011, 3, 103−107. (8) Pryor, K. E.; Rebek, J., Jr. Multifunctionalized glycolurils. Org. Lett. 1999, 1, 39−42. (9) Rowan, A. E.; Elemans, J. A. A. W.; Nolte, R. J. M. Molecular and supramolecular objects from glycoluril. Acc. Chem. Res. 1999, 32, 995− 1006. (10) Kölbel, M.; Menger, F. M. Materials based on glycoluril. Adv. Mater. 2001, 13, 1115−1119. (11) Sliwa, W.; Matusiak, G.; Peszke, J. Glycolurils. Heterocycles 2004, 63, 419−443. (12) Svec, J.; Dusek, M.; Fejfarova, K.; Stacko, P.; Klán, P.; Kaifer, A. E.; Li, W.; Hudeckova, E.; Sindelar, V. Anion-free bambus[6]uril and its supramolecular properties. Chem.Eur. J. 2011, 17, 5605−5612. (13) Val’dman, A. V.; Zaikonnikova, I. V.; Kozlovskaya, M. M.; Zimakova, I. E. A study of the spectrum of psychotropic action of mebicar. Bull. Exp. Biol. Med. 1980, 89, 621−624. (14) Kostyanovsky, R. G.; Kadorkina, G. K.; Lyssenko, K. A.; Torbeev, V.Yu.; Kravchenko, A. N.; Lebedev, O. V.; Grintselev-Knyazev, G. V.; Kostyanovsky, V. R. Chiral drugs via the spontaneous resolution. Mendeleev Commun. 2002, 12, 6−8. (15) Prokopov, A. A.; Kostebelov, N. V.; Berlyand, A. S. Excreation of albicar from the rat organism. Pharm. Chem. J. 2002, 36, 170−171. (16) Prokopov, A. A.; Berlyand, A. S.; Kazantseva, O. N. Experimental pharmacokinetics of bicaret. Pharm. Chem. J. 2003, 37, 132−135. (17) Atavin, E. G.; Golubinsky, A. V.; Kravchenko, A. N.; Lebedev, O. V.; Vilkov, L. V. Electron diffraction study of molecular structure of mebicar. J. Struct. Chem. 2005, 46, 417−421. (18) Lenev, D. A.; Lyssenko, K. A.; Kostyanovsky, R. G. The chiral drug Albicar: Resolution of its racemate via complexation with BINOL. New. J. Chem. 2010, 34, 403−404. (19) Ryzhkina, I. S.; Kiseleva, Yu.V.; Mishina, O. A.; Timosheva, A. P.; Sergeeva, S.Yu.; Kravchenko, A. N.; Konovalov, A. I. Correlations between the self-organization, physicochemical properties, and biological activity of Mebicar in dilute aqueous solutions. Mendeleev Commun. 2013, 23, 262−264. (20) Dekaprilevich, M. O.; Suvorova, L. I.; Khmelnitskii, L. I. 1,6Dimethyltetrahydroimidazo [4,5-d]-imidazole-2,5(1H,6H)-dione monohydrate. Acta Crystallogr. 1994, C50, 2056−2058. (21) Prokopov, A. A.; Berlyand, A. S.; Kazantseva, O. N. Study of bicaret metabolism in rats. Pharm. Chem. J. 2002, 36, 520−522. (22) Prokopov, A. A.; Berlyand, A. S.; Kazantseva, O. N. Quantitative analysis of bicaret in biological objects by gas chromatography. Pharm. Chem. J. 2002, 36, 628−630. (23) Prokopov, A. A.; Berlyand, A. S.; Kazantseva, O. N. Bioaccessibility of bicaret studied in vitro and in vivo. Pharm. Chem. J. 2003, 37, 170−173. (24) Pletnev, V. Z.; Miklhailova, I.Yu.; Sobolev, A. N.; Galitskii, N. M.; Verenich, A. I.; Khmelnitsky, L. I.; Lebedev, O. V.; Kravchenko, A. N.; Suvorova, L. I. Three-dimensional structure of psychotropically active N-polytetraalkyl derivatives of 2,4,6,8-tetraazabicyclo[3.3.0]octanedione-3,7 series in crystal revealed by X-ray analysis. Russ. J. Bioorg. Chem. 1993, 19, 671−681. (25) Ivanov, E. V.; Batov, D. V.; Gazieva, G. A.; Kravchenko, A. N.; Abrosimov, V. K. D2O−H2O solvent isotope effects on the enthalpies of bicaret hydration and dilution of its aqueous solutions at different temperatures. Thermochim. Acta 2014, 590, 145−150. (26) Abrosimov, V. K.; Smirnov, V. I.; Ivanov, E. V.; Lebedev, Yu.A. Enthalpy of solution of tetramethyl-bis-urea (mebicarum) in light and heavy water and methanol isotopomers at 298.15 K. Russ. J. Phys. Chem. 2005, 79, 1878−1880.

heterocomponent structures arising in this solvent is higher than in ordinary water. The data obtained in this work show that, unlike that of the aqueous mebicar, the hydrophobic constituent of bicaret hydration is predominating. Herewith the D2O−H2O solvent isotope effect on standard molar volume of mebicar decreases (in magnitude) with temperature more rapidly than does the corresponding value for bicaret. In the region close to T ≈ 310 K, the compared temperature dependences for both glycoluril-derivatives intersect at a point of ca. −0.2 cm3·mol−1. That is, the isotope effect on the considered quantity for bicaret hypothetically crosses the zero-axis, where the volume effects of solvation in H2O and D2O are identical, at the higher temperature than in the case of mebicar. Also, it has been shown that the volume- and enthalpy-isotope effects of hydration for each of tetra-N-methylated glycolurils are thermodynamically related. But, if both these H/D isotope effects for aqueous mebicar cross the zero-axis values near T = 318.15 K, the negative enthalpy−isotope hydration effect for bicaret at the same temperature should be considered allowing for the additional contribution in the case of zero volume-isotope effect. In our opinion, this contribution is caused presumably by a stronger solvent−solvent interaction in heavy water due to the bicaret hydrophobic hydration and configurational differences in the molecular packing of H/D isotopically distinguishable systems under comparison as well.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. Tel.: +7 4932 351859. Fax: +7 4932 336246. Funding

The given work was supported by the Russian Foundation for Basic Researches (Grant No. 13-03-00716-a). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Drs. A.N. Kravchenko and G.A. Gazieva (Laboratory of Nitrogen-Containing Compounds, N.D. Zelinsky Institute of Organic Chemistry of the RAS, Moscow, Russia) for the synthesis implementation and assistance in the provision of a bicaret sample. All the densimetric measurements were carried out using the equipment of the “The Upper-Volga Regional Centre of Physicochemical Researches” (located at the G.A. Krestov Institute of Solution Chemistry of the RAS, Ivanovo, Russia).

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REFERENCES

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