J. Chem. Thermodynamics 69 (2014) 107–111

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J. Chem. Thermodynamics

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Vapor pressures and sublimation enthalpies of novel bicyclic heterocyclederivatives

0021-9614/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.jct.2013.09.042

⇑ Corresponding author. Tel.: +7 (4932)351545; fax: +7 (4932)336246.E-mail address: svb@isc-ras.ru (S.V. Blokhina).

Svetlana V. Blokhina a,⇑, Marina V. Ol’khovich a, Angelica V. Sharapova a, German L. Perlovich a,Alexey N. Proshin b

a Institute of Solution Chemistry, Russian Academy of Sciences, 1 Akademicheskaya Street, 153045 Ivanovo, Russiab Institute of Physiologically Active Compounds, Russian Academy of Sciences, 142432 Chernogolovka, Russia

a r t i c l e i n f o

Article history:Received 21 May 2013Received in revised form 23 September 2013Accepted 28 September 2013Available online 10 October 2013

Keywords:Novel bicyclic heterocycle derivativesVapor pressureSublimationThermodynamicsMolecular descriptors

a b s t r a c t

The vapor pressures of five novel bicyclic heterocycle derivatives were measured over the temperature341.15 to 396.15 K using the transpiration method by means of an inert gas carrier. From these resultsthe standard enthalpies and Gibbs free energies of sublimation at the temperature 298.15 K were calcu-lated. The effects of alkyl- and chloro-substitutions on changes in the thermodynamic functions havebeen investigated. Quantitative structure–property relationship on the basis HYBOT physico-chemicaldescriptors for biologically active compounds have been developed to predict the sublimation enthalpiesand Gibbs free energies of the compounds studied.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Saturated vapor pressure and sublimation thermodynamicparameters are among the most important characteristics of thesolid state of organic compounds. Sublimation enthalpy values re-flect the degree of molecular interactions in the crystalline stateand largely determine the solubility of a substance [1].

The objects of this research are new bicyclic heterocycle deriv-atives which can be viewed as agents to exhibit antitumoral, neu-rodegenerative and antioxidant properties [2–4]. The great interestof science to the substances of this class can be explained by theirhigh physiological activity similar to that of natural molecules, forexample alkaloids quinine and cytisine [5]. The main goal of thiswork is to study the influence of the substituents on the sublima-tion and fusion properties of the alkyl- and chlorine-derivatives [3-thia-1-aza-bicyclo[3.3.1]non-2-ylidene]-amine synthesized by us.This work continues our studies of crystal structure, sublimation,solubility and distribution of pharmaceutically relevant drug anddrug-like substances [6,7].

2. Experimental

2.1. Materials

We use five novel bicyclic heterocycle derivatives: (3-Methyl-phenyl)-[3-thia-1-aza-bicyclo[3.3.1]non-2-ylidene]-amine (1); (4-Ethyl-phenyl)-[3-thia-1-aza-bicyclo[3.3.1]non-2-ylidene]-amine(2); (4-Chloro-phenyl)-[3-thia-1-aza-bicyclo[3.3.1]non-2-ylidene]-amine (3); (3,4-Dihloro-phenyl)-[3-thia-1-aza-bicyclo[3.3.1]non-2-ylidene]-amine (4); (3-Chloro-4-methyl-phenyl)-[3-thia-1-aza-bicyclo[3.3.1]non-2-ylidene]-amine (5). The alkyl- and chloro-derivatives [3-thia-1-aza-bicyclo[3.3.1]non-2-ylidene]-amine weresynthesized as indicated in scheme 1.

To a stirred solution of 3,4-dichlorophenylisothiocyanate(2.04 g, 10 mmol) and (3-bromomethyl)piperidine hydrobromide(2.56 g, 10 mmol) in 30 ml of methanol a solution of sodiumbicarbonate (1.85 g, 22 mmol) in a minimal amount of waterwas added dropwise. When formation of the precipitate wasover, it was filtered and recrystallized from dioxane to yield2.1 g (70 percent) of the obtained compound as a white solid[8]. The purity of the bicyclic heterocycle derivatives arepresented in table 1. The X-ray molecular structure of thecompound 4 were determined and their molecular packing inthe crystals was revealed [9].

BrN H NH

BrNAr

S

S

N NAr

He Ha

HeHa

He

Ha

NaHCO3.HBrArNCS +

56

7

4

9

8

SCHEME 1. Scheme of synthesis of the bicyclic heterocycle derivatives studied.

TABLE 1Purity of the bicyclic heterocycle derivatives studied.

Chemical name Mass fraction

(3-Methyl-phenyl)-[3-thia-1-aza-bicyclo[3.3.1]non-2-ylidene]-amine >0.98(4-Ethyl-phenyl)-[3-thia-1-aza-bicyclo[3.3.1]non-2-ylidene]-amine >0.98(4-Chloro-phenyl)-[3-thia-1-aza-bicyclo[3.3.1]non-2-ylidene]-amine >0.98(3,4-Dihloro-phenyl)-[3-thia-1-aza-bicyclo[3.3.1]non-2-ylidene]-amine >0.98(3-Chloro-4-methyl-phenyl)-[3-thia-1-aza-bicyclo[3.3.1]non-2-ylidene]-amine >0.98

108 S.V. Blokhina et al. / J. Chem. Thermodynamics 69 (2014) 107–111

2.2. Apparatus and procedure

Temperatures and enthalpies of fusion of the compounds underinvestigation have been determined using a Perkin-Elmer Pyris 1DSC differential scanning calorimeter (Perkin-Elmer AnalyticalInstruments, Norwalk, Connecticut, USA) with Pyris software forWindows NT. DSC runs were performed in an atmosphere of flow-ing (20 cm3 �min�1) dry helium gas of high purity 0.99996 (massfraction) using standard aluminum sample pans and a heating rateof 2 K �min�1. The accuracy of weight measurements was0.005 mg. The DSC was calibrated with an indium sample fromPerkin–Elmer (P/N 0319–0033). The value determined for the en-thalpy of fusion corresponded to 28.48 J � g�1 (reference value28.45 J � g�1). The fusion temperature was 429.5 ± 0.1 K (deter-mined from ten measurements).

Sublimation experiments were carried out by the transpirationmethod. This method consists in passing a stream of an inert gasover a sample at the constant flow rate and temperature, the ratebeing low enough to achieve the saturation state of the gas withthe substance’s vapor. Then the vapor was condensed and the sub-limed quantity determined. The vapor pressure over the sample atthis temperature can be calculated from the amount of sublimedmaterial and the volume of the inert gas used.

Details of the technique are given in the literature [10]. The in-ert gas (nitrogen) from a tank flows through a column packed withsilica to adsorb any humidity from the gas. The stabilization of thegas temperature occurs in a thermostated water bath. The stabilityof the gas flow with precision better than 0.01 percent is realizedby use of a mass flow controller, MKS type 1259CC-00050SU. Theinert gas of constant temperature and velocity passes then to aglass tube, which is placed in a temperature controlled air bath.Three zones of the glass tube can be distinguished the starting zonefor stabilizing of the inert gas; the transitional zone in which thesublimation process occurs; ensuring slow sublimation of the sub-stance investigated; the finishing zone in which the inert gas to-gether with the sublimed substance is overheated by 4 to 5 K,controlled by a platinum resistance thermometer. The temperatureof the air thermostat is kept constant with a precision of 0.01 K bymeans of the temperature controller, PID type 650 H UNIPANequipped with a resistance thermometer. The finishing zone is cou-pled to a condenser built from glass helices, placed (outside thethermostat) located in a Dewar vessel filled with a liquid nitrogen.To avoid adsorption of water from the air, the condenser is con-nected to a vessel filled with CaCl2.

The equipment was calibrated using benzoic acid. The mea-sured value of the vapor pressure at this apparatus is 0.962 Pa atT = 317.15 K. This value is in good agreement with the literature

data: p = 0.99 Pa [11]. The standard value of the sublimation en-thalpy obtained was Dg

crHom = 90.5 ± 0.3 kJ �mol�1.This is in good

agreement with the value recommended by IUPAC(Dg

crHom = 89.7 ± 0.5 kJ �mol�1) [12].

From the experimentally determined pressure – flow rate rela-tionship, the optimal flow rate of 1.8 dm3 � h�1 has been deter-mined. At this flow rate the saturated vapor pressure isindependent of the flow rate and, thus, thermodynamic equilib-rium is achieved.

The saturated vapor pressures were measured five times at eachtemperature with the standard deviation of no more than 5 percent.Because the saturated vapor pressure of the investigated com-pounds is low, it may be assumed that the heat capacity changeof the vapor with temperature is so small that it can be neglected.

The amount of sublimed substance is determined by followingprocedure. The condensed substance is dissolved in a defined vol-ume of solvent Vsol. The determination of the mass of the substanceis based on the measuring of absorbance A of its solution by meansof CARY 1E UV–Visible Spectrophotometer, Varian. Knowing a va-lue of the extinction coefficient e (dm3 �mol�1 � cm�1) of the stud-ied compound dissolved in the solvent one can express theconcentration of the solution c (mol � dm�3) according to the Lam-bert–Beer law, by the following relation:

A ¼ ecl; ð1Þ

whereas the mass of sublimed substance is calculated from:

m ¼ cV solM; ð2Þ

where l is an absorbing path length; M is a molar mass of studiedsubstance. Considering that the vapor pressure of the substance isvery low, the ideal gas rule can be applied:

pVx ¼ nRT; ð3Þ

where Vx is a total inert gas volume at temperature T, of the mea-surement corrected with the thermal expansivity coefficient; R isthe gas constant; n = m/M is the number of moles of sublimed sub-stance. The Vx value is calculated from equation (4):

Vx=Vgas ¼ T=Tr; ð4Þ

where Tr is a temperature of the water thermostat, Vgas (dm3) is thegas volume at temperature Tr, calculated by equation (5):

Vgas ¼ mt; ð5Þ

where m (dm3/h) is a gas flow velocity; t (h) is the sublimation per-iod. Taking into account (2)–(5) we then obtain:

P ¼ cV solVgasRTr: ð6Þ

FIGURE 1. Molecular structure of bicyclic heterocycle derivatives studied.

TABLE 2The structural formula of the radical, purity, temperature and enthalpy of fusion andphysico-chemical descriptors (a,

PCa) of the bicyclic heterocycle derivatives studied.

Compound R- Tfus/K D1crHo

mðTfusÞ a RCa

1 398.5 ± 0.2 26.5 ± 0.5 28.67 5.88

2 394.6 ± 0.2 16.4 ± 0.5 30.05 4.60

3 Cl 411.8 ± 0.2 11.8 ± 0.5 28.77 6.12

4

Cl

Cl 370.9 ± 0.2 23.5 ± 0.5 30.69 6.52

5

Cl

406.6 ± 0.2 22.7 ± 0.5 30.60 6.23

S.V. Blokhina et al. / J. Chem. Thermodynamics 69 (2014) 107–111 109

The experimentally determined vapor pressure data are usuallypresented with the use of co-ordinates (lnP; 1/T):

lnðpÞ ¼ A� B=T: ð7Þ

The value of the sublimation enthalpy is calculated by the Clau-sius–Clapeyron equation:

DgcrH

omTÞ ¼ �R

@ðlnpÞ@ð1=TÞ

� �; ð8Þ

whereas the standard sublimation entropy at a given temperature Twas calculated from the following relation:

DgcrS

omðTÞ ¼

DgcrH

omðTÞ � Dg

crGomðTÞ

� �T

; ð9Þ

TABLE 3The experimental values of vapor pressures of the bicyclic heterocycle derivatives studied

1 2 3

T/K p/Pa T/K p/Pa T/K

353.15 0.0565 359.15 0.0109 368.15355.15 0.0721 362.15 0.0135 369.15357.15 0.0952 364.15 0.0175 371.15359.15 0.1112 366.15 0.0198 373.15361.15 0.1522 368.15 0.0254 374.15362.15 0.1678 371.15 0.0286 375.15364.15 0.2107 376.15 0.0303 376.15366.15 0.2637 374.15 0.0388 378.15368.15 0.3248 377.15 0.0541 380.15370.15 0.4227 380.15 0.0676 382.15372.15 0.5258 381.15 0.0759 384.15374.15 0.6715 382.15 0.0841 386.15376.15 0.8044 384.15 0.0843 388.15

386.15 0.1059 390.15389.15 0.1278 391.15

Standard uncertainty for temperature u(T) = 0.01 K; and relative standard uncertainty fo

with DgcrH

omðTÞ = �RT ln(p/p0), where p0 is the standard pressure of

p0 = 1.013 � 105 Pa.For experimental reasons sublimation data are obtained at ele-

vated temperatures. However, in comparison with effusion meth-ods, the temperatures were much lower, which makesextrapolation to room conditions easier. In order to further im-prove the extrapolation to T = 298.15 K, we estimated the heatcapacities (Co

p;mðcrÞ) of the crystals using the additive scheme pro-posed by Chickos et al. [13]. Heat capacity was introduced as a cor-rection for the recalculation of the sublimation enthalpy Dg

crHom

according to the equation:

DgcrH

omð298:15 kÞ=kJ �mol�1 ¼ Dg

crHomðTmÞ=kJ �mol�1 þ ½ð0:75

þ 0:15Cop;mðcrÞ=J � K�1 �mol�1Þ

� ðTm=K� 298:15 KÞ�=1000: ð10Þ

3. Results and discussion

The objects of investigation are five alkyl- and chlorine-deriva-tives of [3-thia-1-aza-bicyclo[3.3.1]non-2-ylidene]-amine. Thegeneral central fragment of molecules is given in figure 1. Thestructural formula of substituents, fusion temperatures and enthal-pies are shown in table 2. The thermo-physical studies, in whichDSC and thermo-gravimetric methods were applied, revealed thatin the interval from 298.15 K to the fusion temperature the synthe-sized substances are stable and that are no phase transitions occur-ring between them. As it follows from the data presented, thealkyl-substituted compounds have lower fusion temperatures thanthe chlorine-derivatives.

Table 3 presents experimental values of the saturated vaporpressures of the compounds studied at different temperatures. Ta-ble 4 gives the coefficients of the respective rectilinear correlationequations which characterize these correlations quite precisely.Figure 2 shows the temperature dependencies of the saturated va-por pressure of the substances. At the temperature of 376.15 K inthe descending order of their pressure, the compounds can be ar-ranged as follows: 1 > 4 > 3 > 2 > 5. As the obtained data show,the most volatile substance is the methyl-derivative. Extension ofthe alkyl chain and transition from the methyl-substitute to theethyl-one reduces the vapor pressure. Comparative analysis ofchlorine-containing substances allows us to conclude that intro-ducing an electron-acceptor chlorine atom as the second substitu-tent in the benzene ring increases the vapor pressure, whileintroducing an electron-donor methyl group decreases volatility.

.

4 5

p/Pa T/K p/Pa T/K p/Pa

0.0626 352.15 0.0115 376.15 0.03100.0704 353.15 0.0125 377.15 0.03390.0891 354.15 0.0159 378.15 0.03790.1094 355.15 0.0182 379.15 0.04050.1327 356.15 0.0226 381.15 0.05210.1490 357.15 0.0227 382.15 0.05740.1659 358.15 0.0256 383.15 0.06780.2065 359.15 0.0258 384.15 0.07600.2459 360.15 0.0291 386.15 0.09310.3128 361.15 0.0349 388.15 0.12390.3887 362.15 0.0400 390.15 0.15530.4737 363.15 0.0440 392.15 0.20060.5748 394.15 0.25210.7333 396.15 0.31090.8441

r pressure ur(p) = 0.05.

TABLE 4Coefficients Clausius–Clapeyron equation lnp = A � B/T (R P 0.9980).

Compound Trange, K A B

1 353.15–376.15 40.8 ± 0.1 15414 ± 1222 359.15–389.15 28.4 ± 0.3 11813 ± 1213 368.15–391.15 40.7 ± 0.4 16006 ± 1444 352.15–363.15 36.5 ± 0.5 14400 ± 1935 376.15–396.15 43.2 ± 0.6 17587 ± 220

FIGURE 2. Plot of vapor pressure ln(p/Pa) against reciprocal temperature of bicyclicheterocycle derivatives studied.

FIGURE 3. Relationship between the enthalpic and entropic terms of energy Gibbsof sublimation of bicyclic heterocycle derivatives studied.

110 S.V. Blokhina et al. / J. Chem. Thermodynamics 69 (2014) 107–111

In order to determine the relationship between the sublimationthermodynamic parameters and the molecular structure of thesubstances, we have extrapolated the saturated vapor pressure val-ues, using the equations obtained experimentally to the tempera-ture of 298.15 K. The thermodynamic parameters of sublimationprocesses are shown in table 5. The main criterion of sublimationspontaneity is the Gibbs free energy change. As Dg

crGomðTÞ values

rise, the saturated vapor pressure of the crystalline compoundsfalls. According to equation (9) the volatility goes down ifDg

crHomðTÞ increases and T Dg

crSomðTÞ decreases. As it has been deter-

mined, at the temperature of 298.15 K the saturated vapor pres-sures of the alkyl-substituted bicyclic compounds are greaterthan that of the chlorine-derivatives. The vapor pressure differenceof the substances results from the compensation effect that a com-peting effect of the enthalpy and entropy contribution to the Gibbsfree energy (figure 3). The cumulative effect of both contributionsdetermines the resulting sublimation Gibbs free energy of the sub-stances. With respect to the substances studied, reducing themobility of the crystal is caused by the increase in the molecularinteraction energy.

In addition we analysed the sublimation thermodynamicparameters of compounds with similar substitution, namely, thealkyl-derivatives (1,2) and chlorine-derivatives (3–5). The de-crease in vapor pressure in case of replacing the methyl group withan ethyl one is caused by the reduction of the sublimation entropy.

TABLE 5Thermodynamic parameters of sublimation of the bicyclic heterocycle derivatives studied

Compound DgcrHo

mðTÞ=kJ �mol�1

Tm/K Cop;mðcrÞ=

J �mol�1 � K�1

DgcrGo

m ð298:15Þ=kJ �mol�1

DgcrHo

m ð29

kJ �mol�1

1 128.2 ± 1.0 365.15 300.8 55.6 ± 0.8 131.0 ± 1.02 98.2 ± 1.0 374.15 327.7 56.4 ± 0.7 101.9 ± 1.03 133.1 ± 1.2 379.15 292.9 60.7 ± 0.9 136.7 ± 1.24 122.2 ± 1.0 357.15 312.6 58.1 ± 0.7 125.0 ± 1.15 146.2 ± 1.8 386.15 320.5 67.6 ± 1.0 150.5 ± 1.8

The decrease in volatility, that occurs when appearing while themethyl substituent is introduced to the structure of the chlorine-derivative, results from the enthalpy increase. In contrast, the risein volatility during the introduction of the second chlorine atom inthe benzene ring of the mono-chlorine-derivative is caused by thereduction of the enthalpy term.

Quantative structure–property relationship models on the basisof HYBOT physico-chemical descriptors [14] have been developedto predict sublimation enthalpies and Gibbs free energies of themolecular crystals studied. Molecular polarizability (mainly thebulk effect descriptor) a, the sum of H-bond acceptor factors inmolecule RCa and the sum of H-bond donor factors RCd were usedas descriptors in this model. These outlined descriptors describethe main interaction types in molecular crystals and have a strictphysical meaning. Besides, calculating sublimation enthalpiesand Gibbs free energies provide an opportunity to estimate the en-tropy term easily and, as a consequence, to predict all thermody-namic functions of the sublimation process. The substancesunder consideration are only capable of van der Waals interactionsand do not form hydrogen bonds in crystals. As the studied sub-stances are not hydrogen bond donors, their RCd descriptor isequal to zero. The calculated polarizability and RCa descriptor val-ues are shown in table 2. Thus, in this case Dg

crGomð298:15Þ values

depend on the sum of nonspecific interactions and weak chemicalforces of the acceptor type. The resulting correlation equations,where the dependent variables are sublimation enthalpies andGibbs free energies and the independent variables are polarizabil-ity and molecule acceptor abilities, are shown below [15]:

DgcrG

omð298:15Þcal ¼ ð39:8þ 1:4Þ þ ð2:03þ 0:05Þa

þ ð4:6þ 0:2ÞX

Ca; ð11Þ

DgcrH

omð298:15Þcal ¼ ð�0:5þ 1:6Þ þ ð1:37þ 0:06Þa

þ ð3:84þ 0:25ÞX

Ca: ð12Þ

.

8:15Þ= DgcrSo

m ð298:15Þ=J �mol�1 � K�1

DgcrGo

m ð298:15Þcalc=kJ �mol�1 DgcrHo

m ð298:15Þcalc=

kJ �mol�1

253.2 ± 1.9 56.6 129.0152.5 ± 4.6 58.9 119.0254.9 ± 2.5 60.8 130.4224.5 ± 2.2 61.5 127.9278.0 ± 4.8 67.9 134.8

FIGURE 4. Correlations between calculated and experimental values of enthalpy(j) and standard Gibbs free energy (d) of sublimation of bicyclic heterocyclederivatives studied.

S.V. Blokhina et al. / J. Chem. Thermodynamics 69 (2014) 107–111 111

Table 4 shows the calculated values of the thermodynamicfunctions, and figure 4 displays the correlations between the calcu-lated and experimental values of the sublimation enthalpies andGibbs free energies of the compounds. As it follows from the re-sults shown in figure 4, all thermodynamic characteristics can beappropriately approximated by the linear trends observed, charac-terized by the correlation coefficient R P 0.96. The mean deviationof the calculated thermodynamic parameters from the experimen-tal ones is ±3 kJ/mol, and the mean relative error of calculatingDg

crHomð298:15Þcal and Dg

crGomð298:15Þcal is ±0.69 kJ/mol. There is a

common dependence observed for all the compounds: with an in-crease in molecule polarizability and acceptor ability the substancecrystal lattice energy value rises, and the sublimation process be-comes more energy-consuming.

Equations (11) and (12) make it possible to evaluate the contri-butions of molecule polarizability and acceptor ability in the crys-tal lattice energy. The calculations have shown that thepolarizability contribution to the sublimation enthalpies and Gibbsfree energies is significantly larger than that of the molecule’sacceptor ability.

4. Conclusion

Differential scanning calorimetry was used to measure the tem-perature and molar enthalpy of five novel bicyclic compounds. Thetranspiration method with an inert gas was used to measure thesaturated vapor pressure of the compounds within the tempera-

ture range of 341.15 to 396.15 K. The influence of substituents onthe vapor pressure values were studied. From the temperaturedependencies of the vapor pressure of the molecular crystals, thestandard molar enthalpies and Gibbs free energies of sublimationat the temperature 298.15 K were derived. Quantitative struc-ture–property relationships using HYBOT physico-chemicaldescriptors for biologically active compounds was developed topredict sublimation enthalpies and Gibbs free energies. The molec-ular polarizability and acceptor ability contributions of the sub-stances studied to the enthalpy of sublimation were evaluated.

Acknowledgments

This work was supported by the grant of RFBR No. 12-03-000348-a, BioSol project No. 2010-1.1-234-069.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jct.2013.09.042.

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JCT 13-281