Volumetric properties of tetraphenylporphyrin and some of its alkoxy and tert-butyl derivatives in tetrachloromethane solutions

Journal of Molecular Liquids 121 (2005) 27 – 34 www.elsevier.com/locate/molliq

Volumetric properties of tetraphenylporphyrin and some of its alkoxy and tert-butyl derivatives in tetrachloromethane solutions W. Zielenkiewicz*, G.L. Perlovich1 Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/55, 01-224 Warsaw, Poland Available online 29 September 2004

Abstract Densities, apparent molar volumes, and partial molar volumes, V 20, of tetraphenylporphyrin, H2TPP; meso-tetra-4-alkoxyphenyl porphyrin derivatives, H2T(4-OCn H2n+1)PP (where n=4, 6, 8, 10, 12, 16); meso-tetra-4-tert-butylphenyl porphyrin, H2T(4-tBu)PP; meso-tetra-3,5-ditert-butylphenyl porphyrin, H2T(3,5-tBu)PP, in tetrachloromethane (TCM) solutions at 25 8C were determined. For TCM and benzene solutions of the compounds examined, relationship DK=K 2
K 1 was established as a function of van der Waals volume V 2W, where K 2=V 2W/V 20 and K 1=V 1W/V 1* represent the packing coefficients of the solute and the solvent (V 1* is molar volume of solvent), respectively. The resulting correlations were compared with the correlations established for members of various homologous series like n-alkanes, cycloalkanes, crown-ethers, cage structures. The DK-value was found to be closely related to the geometry of the solute molecule. Relationships are also given which relate V 20 with the number n of the structural repeating units (methylene groups) in compounds H2T(4-OCn H2n+1)PP, n-alkanes, cycloalkanes, and crown-ethers. An attempt was made to interpret the solvation properties of the solutions of individual compounds in the homologous series and also the similarities and differences between the homologous series. D 2004 Elsevier B.V. All rights reserved. Keywords: Volumetric properties; Tetraphenylporphyrin; Tetrachloromethane solutions

1. Introduction Partial molar volumes, V 20, of a solute in the solution, as determined by density measurements, have been interpreted by various methods. Simple additivity schemes have been used, considering the V 20 value to be the sum of the atomic volumes of constituent atoms or functional groups [1–4] present in the molecule. In other models, the V 20 value has been correlated with the structural parameters of the molecule studied including the van der Waals volume, V 2W, of the solute. For example, Terasawa et al. [5] considered that the partial molar volumes can be expressed by a linear function of the van der Waals volumes and that the increments of partial molar volume, dV 20, of the repeating structural units in the * Corresponding author. Tel.: +48 22 6324 389; fax: +48 22 6325 276. E-mail address: [email protected] (W. Zielenkiewicz). 1 Permanent address: Institute of Solution Chemistry, Russian Academy of Sciences, 153045 Ivanovo, Russia. 0167-7322/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2004.08.023

homologous series of the compounds studied represent additive values. This assumption is not always satisfied for large molecules endowed with conformational flexibility and having a complex structure. For compounds existing in various conformational states, Lee and Richards [6,7] have proposed a model assuming a correlation to exist between V 20 and volume accessible to the solvent and the solvent excluding volume. However, each increase in the size of the large molecule causes essentially a change in the number of its conformational states and renders the determination of its distribution function difficult. A few scores years ago, for the description of the properties of molecular crystals, Kitaigorodsky [8] introduced a new term—the packing coefficient—defined as the ratio of the van der Waals volume of the molecule to its volume. This term was later applied by King [9] to analyze the volumetric properties of electrolyte solutions. In this case, the packing density of the solute was defined as the ratio of the van der Waals volume V 2W to the partial molar volume V 20

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of the solute which characterizes the effective volume of the solute in the solution. Instead, in the work [10], the parameters V 20/V 2W and (V 20–V 2W)/V 20 have been introduced to analyze the changes in the volumetric properties in the transition from the solid to the liquid phase in pure organic compounds. An attempt to correlate, in the same work, V 20 with several physicochemical parameters (e.g. refraction index, dielectric constant, Hildebrand’s solubility parameter) for macromolecules C60, 18-crown-6-ether and cryptand222 solutions in various solvents failed. In the present study, examination and interpretation of the volumetric properties of tetraphenylporphyrin and its alkoxy and tert-butyl-derivatives are based on the packing phenomena of the solvent and the solute. The packing coefficients of the solute and the solvent were introduced into consideration. Let us assume that the packing coefficient of the solvent molecule placed in an infinitely diluted solution can be expressed as K2 ¼ V2W =V20

ð1Þ

whereas the packing coefficient of the solvent is defined as: K1 ¼ V1W =V1 4

ð2Þ

where V1* is the molar volume of the solvent. Packing coefficient K 1 characterizes the intermolecular interactions, topological structure and population of molecule’s conformational states. For infinitely diluted solutions, K 2 is a measure of averaged local packing of the solute molecule within the solvent molecules. Packing coefficient K 2 is the function of the geometry, topology and conformational state both of solute and solvent molecules, solute– solvent interactions and the topology coupling between solute and solvent molecules. Additionally, let us introduce the quantity: DK ¼ K2
K1

ð3Þ

which is the difference between the packing properties of the solute in the solvent and the solvent alone. This quantity is particularly useful in comparing the volumetric properties of the compounds investigated in various solvents, because it enables the properties of the solution to be analyzed with reference to those of the solvent and thus makes it possible to study the quantities considered in a mutual system of coordinates. In this study, the above-defined parameters were used to analyze the solvating properties of the compounds investigated. According to the topic considered, DK is examined as a function of different variables. To describe comparatively the volumetric properties of the solute in various solvents, the relationship DK=f(K 1) is used. To consider the volumetric properties of compounds representing members of a given homologous series in a selected solvent use was made of the relationship DK=f(V 2W). The value of DK is closely related to the geometry of the solute molecule. As will be demonstrated below, this fact results in a non-

monotonic course of the function. Therefore, an attempt is made to explain how changes in DK are related to the increments of the partial molar volumes, dV 20, corresponding to the structural unit repeating in the molecules of the homologous series H2T(4-OCn H2n+1)PP (n=1–12). Analysis of the solvation properties of the compounds investigated was expanded to include the packing coefficients and the DK for compounds of the following homologous series: alkanes, cycloalkanes, crown-ethers, and cage structures. Relationships were determined between dV 20 and the number n of the structural repeating units occurring in the homologous series considered. Interpretation of the properties of the solvation shells was approached for compounds of a given homologous series and results obtained for the series examined were compared. This study is a continuation of our earlier works [11,12] on the volumetric properties of porphyrin molecules dissolved in various solvents, intended to establish the regularities governing the processes of solvation of conformationally flexible molecules. In the present work, partial molar volumes were determined for tetraphenylporphyrin and some of its alkoxy and tert-butyl derivatives in tetrachloromethane (TCM) as solvent. TCM was selected as solvent because it has been often used [1,2,13–17] in the studies on various classes of compounds to establish the effect of various factors like the size of molecule, molecular structure, and conformational states on the dV 20-value. In this way, it is possible to compare the properties determined in the present work with those evaluated from literature data.

2. Materials and methods Free base tetraphenylporphyrin, H2TPP, meso-tetra- 4tert-butylphenyl porphyrin, H2T(4-tBu)PP; meso-tetra-3, 5-ditert-butylphenyl porphyrin, H2T(3,5-tBu)PP; and meso-tetra-4-alkoxyphenyl derivatives of porphyrin, H2T(4OCn H2n+1)PP (n=4, 8, 10, 12, 16) were synthesized by the well known methods [18–20]. The structural formulas of the compounds investigated are shown in Fig. 1. All the compounds were thoroughly purified by repeated preparative chromatography (using chloroform–benzene) and repeated recrystallizations. Absorption spectra of these substances were found to conform with those reported in the literature. The compounds were dried under vacuum at 70 8C to a constant weight. Tetrachloromethane (TCM) (POCh, bp 76 8C, pure p.a., N99.6%), was used as received. Densities were measured with an Anton Paar DMA 60/602 digital densimeter. The temperature stability, obtained by a Hetoterm Model CB-7 thermostat, was F0.002 8C. All solution densities were measured relative to that of pure water. The following protocol was used for each measurement. Before a sample was injected into the density cell, the cell was washed five times with benzene, two times with acetone and once with alcohol and blown dry for 3 min with

W. Zielenkiewicz, G.L. Perlovich / Journal of Molecular Liquids 121 (2005) 27–34

29

0.01% and to within 0.1% depending on the absolute V 20value.

3. Results and discussion

Fig. 1. The structural formulas of tetraphenylporphyrin and some substituted tetraphenylporphyrins.

compressed air filtered through cotton and calcium chloride. The calibration constant of the densimeter was determined by using the known densities of dry air [21] and water [22] at least twice a day. The density measurements of each substance at all concentrations were carried out on the same day. The apparent molar volumes V / were calculated from solution densities d as V/ ¼ M =d
ð1000=mÞð1=d0
1=d Þ

ð4Þ

where d 0 is the density of pure TCM, m—the molality, and M—the molar weight of the solute. The uncertainty of the densimetric measurements is F210
5 g cm
3. The values of apparent molar volumes extrapolated at infinite dilution are assumed to be identical with the partial molar quantities V 20 since V/ ¼ V20 þ Av m

ð5Þ

To analyze the increments to the partial molar volumes (dV 20) per the structural repeating unit in compounds representing members of various homologous series, the Terasawa [5] equation was used: V20 ¼ a0 þ a1 V2W

ð6Þ dV 20-values

easier to compare between In this study, to make simple and topologically complex porphyrin molecules, the Terasawa equation was used in the following form [13]: V20 ¼ b0 þ b1 n

ð7Þ

where n is the number of structural repeating units. The van der Waals volumes of the molecules studied were estimated by using Kitaigorodsky’s [8] and Gavezzotti’s [23,24] data. For benzene at d 0=0.87356, V 1*=89.40 and K 1 =54.2%; for TCM d 0 =1.58439, V 1 * =97.08 and K 1=53.9%. The calculated K 1-values are accurate to within

The experimental results for H2TPP; H2T(4-OCn H2n+1) PP (where n=4, 8, 10, 12, 16); H2T(4-tBu)PP; H2T(3,5-tBu) PP are collected in Table 1, including molality m, density d of the tetrachloromethane solution, and the apparent molar volume V /. The partial molar volumes (V 20), the van der Waals volumes (V 2W), and the packing coefficients of the molecules investigated (K 2) are given in Table 2. The molecule H2T(4-OCn H2n+1)PP in Fig. 1 represents free base H2TPP to which four alkoxy chains are attached. All the five moieties of the molecule are conformationally mobile; the conformations are affected by the length of the alkoxy chains. This effect should also be evident in the volumetric properties of the molecule. Formerly we have analyzed the effect of conformational modifications of the H2TPP molecule on the volumetric properties in benzene as solvent [11,12]. It was of interest to see the effect, if any, of the introduction of alkoxy substituents into the position of p-phenyl moieties of the H2TPP molecule and whether the resulting changes are similar to those observed in other homologous series of organic compounds. For this purpose, solutions of n-alkanes, cycloalkanes, and crown ethers in TCM and solutions of n-alkanes, fatty acids and n-alkanols in benzene were studied to establish the relationships V 20=f(n), where n is the number of structural repeating units and DK=f(V 2W). Interesting conclusions can be drawn from the course of the function DK=f(V 2W). To define them DK-values were evaluated from the reported partial molar volumes V 20, calculated van der Waals volumes and packing coefficients K 2 for n-alkanes, crown ethers, cycloalkanes, cage structures and aromatic hydrocarbons in tetrachloromethane solutions (Table 3). The DK=f(V 2W) values thus obtained were plotted for cycloalkanes, crown ethers, and n-alkanes in TCM solutions (Fig. 2) and for n-alkanols, fatty acids and n-alkanes in benzene solutions (Fig. 3, Table 4). The data in Fig. 2 show that as the alkyl chain in Cn H(2n+2) is increased in length by introducing more methylene groups (-CH2-), the DK-values increase from negative values at nb8 to positive at nN8. Cycloalkanes and crown ethers in TCM solutions are seen to exhibit a similar course of DK-values. Similarly, as the number of the repeating units (n) in the classes of the compounds examined is increased, the function DK=f(V 2W) follows an asymptotic course. For alkanols and fatty acids in benzene solutions [2, 25], the relationship DK vs. V 2W is seen to follow a similar course (Fig. 3). For alkanols, DK-values are seen to vary considerably but there are too few data to ascertain the entire range of changes of the function DK=f(V 2W), as the chain length is n=5, the asymptotic value

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Table 1 Densities and apparent molar volumes of some derivatives of tetraphenylporphyrin in tetrachloromethane solution as a function of molality at 25 8Ca m 103

d

V/

H2TPP 4.76500 3.97550 3.56300 2.98210 2.61140 2.00780 1.59000 – –

505.5 505.6 505.7 505.8 505.7 505.3 505.9 – –

a

18.935 16.619 14.376 8.9669 6.9815 5.2311 3.6336 2.8802 2.1047

1.57467 1.57558 1.57639 1.57761 1.57817 1.57832 1.57883 1.57936 1.57987 1.58025

1179.8 1179.0 1178.6 1177.6 1177.2 1176.8 1176.7 1176.7 1176.3 1175.5

14.2250 9.2747 8.2093 7.1782 6.1832 5.2064 4.3283 3.4409 2.5625 –

1.56375 1.56684 1.56907 1.57077 1.57287 1.57458 1.57636 1.57783 1.57936 1.58079

775.4 775.0 773.2 772.5 771.5 770.7 769.7 769.1 768.3 767.9

15.0900 12.9050 11.1400 9.4393 7.5378 6.0685 4.7673 3.5039 2.2910

d

V/

760.3 761.5 760.9 759.5 759.4 758.6 758.3 758.8 757.9

1.35580 1.15270 0.94640 0.75232 0.62808 – – – –

1.58327 1.58344 1.58361 1.58377 1.58387 – – – –

1041.5 1040.7 1040.5 1040.4 1041.8 – – – –

H2T(4-OC16H33)PP

1.56859 1.57395 1.57524 1.57639 1.57749 1.57858 1.57956 1.58055 1.58153 –

1308.9 1310.4 1304.9 1303.9 1303.6 1302.7 1301.9 1301.1 1300.3 –

1.57128 1.57314 1.57470 1.57618 1.57783 1.57911 1.58024 1.58134 1.58240

1025.9 1025.9 1024.1 1023.1 1022.2 1021.3 1020.8 1020.0 1018.6

9.06450 8.54170 8.00080 7.23360 6.52020 5.83670 5.34920 4.74490 4.26340 3.86980

1.57188 1.57256 1.57333 1.57439 1.57538 1.57632 1.57700 1.57783 1.57850 1.57904

1557.1 1558.4 1556.5 1555.5 1554.3 1553.7 1552.6 1552.2 1551.1 1551.0

H2T(3,5-tBu)PP

Units: d, gd cm3, V /, cm3 mol
1; m, mold kg
1 (d 0=1.58439); accuracy of the densimetric measurements is F210
5.

Table 2 The partial molar volumes, V 02 at 25 8C, van der Waals volumes, V W 2 , and packing coefficients, K 2, tetrachloromethane solutions of H2TPP and H2T(4-OCn H2n+1)PPa H2T(4-tBu)PP

V 02

RMSD (V 02)b

VW 2

K2

H2TPP 4-OC4H9 4-OC8H17 4-OC10H21 4-OC12H25 4-OC16H33 4-tBu 3,5-tBu

505.8 757.9 1041.0 1172.8 1298.5 1545.5 766.0 1017.8

0.3 0.3 1.3 0.3 1.3 0.6 0.2 0.3

357.0 540.5 702.1 782.9 863.6 1025.2 544.1 725.0

70.6 71.3 67.4 66.8 66.5 66.3 71.0 71.2

b

m 103 H2T(4-OC8H17)PP

1.57555 1.57656 1.57762 1.58017 1.58110 1.58193 1.58268 1.58303 1.58340

has not yet been achieved. For fatty acids, data have been available only for n=12–18. Over these lengths of the hydrocarbon chains, DK has achieved an asymptotic value, and K aN0. The considerable changes observed in the initial course of the DK=f(V 2W) function can be explained in terms of

a

V/

H2T(4-OC12H25)PP

H2T(4-tBu)PP 34.889 29.524 25.847 22.967 19.427 16.541 13.546 11.059 8.4834 6.0646

d

H2T(4-OC4H9)PP 1.58299 1.58322 1.58334 1.58351 1.58362 1.58380 1.58392 – –

H2T(4-OC10H21)PP 9.92670 8.99970 8.16990 6.92500 6.35310 6.20350 5.67870 5.13150 4.61150 4.23010

m 103

3
1 Units: V 02, cm3 d mol
1; V W 2 , cm d mol ; K 2, %. Root mean square deviation, cm3d mol
1.

reorganization of the solvation shell. On the other hand, dV 20(-CH2-) in the asymptotic course of the function considered at high n is caused only by an increment in the partial molar volume to the repeating unit with no reorganization to occur in the building structure of the solvation shell. The monotonically increasing DK function with the increasing n is presumably due to an essential reorganization of the solvation shells, rather than due to the increment of partial molar volume to the repeating unit. Taking this into account, we have attempted to carry out the analysis of the increments of partial molar volumes for different classes of compounds by using the knowledge about the character of change of the function DK=f(V 2W). For these cases, the experimental data were fitted by the regression Eq. (7); next the analysis of statistical meaning of each experimental point was carried out. Results of the regression analysis are presented in Table 5. From examination of Table 5, it follows that in the TCM solution of n-alkanes, the increasing branch of the function DK=f(V 2W) corresponds to value dV 20(-CH2-)=16.53F0.08

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31

Table 3 The partial molar volumes, in tetrachloromethane solutions of different classes of substances, their van der Waals volumes, V W 2 , and packing coefficients, K 2a V 02

RMSD (V 02)b

VW 2

K2

116.2c 132.3c 149.3c 166.0c 181.8c 198.9c 216.29d 231.6e 248.78d 265.33d 283.29d 298.6e 364.5e 398.3e 499.2e 565.8e

0.3 0.6 0.3 0.4 0.4 0.3 0.34 – 0.30 0.44 0.22 – – – – –

58.6 68.7 78.7 88.9 99.0 109.1 119.2 129.3 139.4 149.5 159.6 169.7 210.1 230.3 290.9 331.3

50.4 51.9 52.5 53.6 54.5 54.9 55.1 55.8 56.0 56.3 56.3 56.8 57.6 57.8 58.3 58.6

85.02d 157.59d 196.28d 234.75d 273.68d 311.85d

0.04 0.02 0.07 0.17 0.72 0.50

45.6 91.3 114.1 136.9 159.7 182.5

53.6 57.9 58.1 58.3 58.4 58.5

95.67f 109.84f 123.05f 135.16f 164.67f 197.55f 247.64f 262.46f

– – – – – – – –

51.2 61.5 71.6 81.8 102.3 122.8 153.4 163.7

53.5 56.0 58.2 60.5 62.1 62.2 61.9 62.4

Cage structures Norbornane cis-Decalin trans-Decalin Bicyclohexyl Adamantane OHMIh

112.58g 154.33g 159.60g 188.21f 139.93g 141.92g

0.06 0.04 0.02 0.05 0.06 0.03

64.7 95.4 95.4 115.9 88.6 88.6

57.5 61.8 59.8 61.6 63.3 62.4

Aromatic hydrocarbons Benzene Naphthalene Anthracene

89.90d 124.57d 158.17d

0.02 0.14 0.42

48.4 74.0 107.0

53.9 59.4 67.7

Substances n-Alkanes n-Pentane n-Hexane n-Heptane n-Octane n-Nonane n-Decane n-Undecane n-Dodecane n-Tridecane n-Tetradecane n-Pentadecane n-Hexadecane n-Eicosane n-Docosane n-Octacosane n-Dotriacontane

(C5) (C6) (C7) (C8) (C9) (C10) (C11) (C12) (C13) (C14) (C15) (C16) (C20) (C22) (C28) (C32)

Crown ethers Dioxane 12-Crown-4 15-Crown-5 18-Crown-6 21-Crown-7 24-Crown-8 Cycloalkanes Cyclopentane Cyclohexane Cycloheptane Cyclooctane Cyclodecane Cyclododecane Cyclopentadecane Cyclohexadecane

a b c d e f g h

(C5) (C6) (C7) (C8) (C10) (C12) (C15) (C16)

Fig. 2. The dependence of DK vs. V W 2 for cycloalkanes, crown ethers and nalkanes in TCM solutions (n—the number of repeating units).

occur: the first three points deviate from the regression straight line. For crown ethers in TCM solution, the first experimental point deviates (Fig. 4). For n-alkanols, fatty acids and n-alkanes in benzene solution, analogous analysis was not carried out, because there were insufficient experimental points for each class of compounds. The regression coefficients of Eq. (7) for these substances are given in Table 5. For n-alkanes, the value dV 20(-CH2-) in benzene is larger than that in TCM solutions. This fact appears to be explicable in terms of stronger van der Waals interactions of the solute with TCM than with benzene. The functions DK=f(V 2W) for H2T(4-OCn H2n+1)PP in benzene and TCM solutions are presented (Fig. 5) in the

3
1 Units: V 02. cm3d mol
1; V W 2 , cm d mol ; K 2, %. 3 Root mean square deviation, cm d mol
1. Ref. [1]. Ref. [13]. Ref. [14]. Average values from all data presented in Ref. [15]. Ref. [15]. Tricyclo[5,2,1,0/2.6]decane.

cm3d mol
1, whereas the asymptotic branch of this function to dV 20(-CH2-)=16.67F0.03 cm3d mol
1. For cycloalkanes in TCM solution, an analogous tendency was found to

Fig. 3. The dependence of DK vs. V W 2 for n-alkanols, fatty acids and nalkanes in benzene solutions (n—the number of repeating unit).

32

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Table 4 The partial molar volumes, in benzene solutions of different classes of a substances, their van der Waals volumes, V W 2 , and packing coefficients, K 2 N

Substances

n-Alkanes 1 n-Pentane (C5) 2 n-Hexane (C6) 3 n-Heptane (C7) 4 n-Octane (C8) 5 n-Nonane (C9) 6 n-Decane (C10) 7 n-Dodecane (C12) 8 n-Pentadecane (C15) 9 n-Octadecane (C18)

V 02 117.0b 133.8b 150.3b 167.6b 184.4b 201.8b 235c,d 286c,d 336c,d

n-Alkanols 10 Methanol 11 Ethanol 12 n-Propanol 13 n-Butanol 14 n-Pentanol Fatty acids 15 Lauric (C12) 16 Tridecanoic (C13) 17 Myristic (C14) 18 Pentadecanoic (C15) 19 Palmitic (C16) 20 Heptadecanoic (C17) 21 Stearic (C18) a b c d

j(V 02)

41.7b 61.2b 77.1b 94.6b 111.1b

0.1 0.5 0.3 0.3 0.3 0.3 159.6

VW 2

K2

58.6 68.7 78.4 88.9 99.0 109.1 129.3 55.8 189.9

50.1 51.3 52.2 53.0 53.7 54.1 55.0

21.7 31.8 41.9 52.0 62.1

52.0 52.0 54.3 55.0 55.9

129.1 139.2 149.3 159.3 169.4 179.5 189.6

56.4 56.4 56.8 56.9 57.0 57.0 56.9

0.2 0.2 0.2 0.3 0.4

229c,d 247c,d 263c,d 280c,d 297c,d 315c,d 333c,d

56.5

Units: V 02. cm3 mol
1; K 2, %. Ref. [2]. Approximated values at 25 8C. Ref. [25].

form of curves ascertained by approximation of the data by a power function (smooth and dashed lines). This approximations does not include the data obtained for the solutions

H2T(4-OCn H2n+1)PP in benzene with the number of -CH2groups n=2–4, 7, and n=4 in TCM. Since the compounds H2T(4-OCn H2n+1)PP examined contain alkyloxy chains, let us consider the course of the function DK=f(V 2W) first for the substitutions before we proceed with the compounds themselves. Let us suppose that the alkoxy chains in positions para to the phenyl fragments in H2TPP which solvate the solvent molecules are approximately equivalent to the n-alkane molecules. Examination of the dependence DK vs. V 2W for n-alkanes (Figs. 2 and 3) shows that, at less than about 8 units, in benzene as well as in TCM solutions, the values DK are lower than zero. This fact suggests that the solute molecules influence on the packing of the pure solvent. However, the increase in the chain length to more than 8 repeating units leads to the opposite effect: the packing coefficient of these molecules, K 2, becomes higher than K 1. Let us assume that for the alkoxy chains of H2T(4-OCn H2n+1)PP at nb8, there are energetically and geometrically favorable conditions for an essential change in the conformational state of the whole porphyrin molecule, since the solvation region near the substitute has a bfriableQ structure with plenty of free volume at disposal. The results of reorganization of the conformational state solute and the structure of solvent give rise to extremum values of DK (very low or very high). It is interesting to note that, at nN8, the values DK for H2T(4-OCn H2n+1)PP attain asymptotic values. This is very likely to be associated with the geometrical factor following an increase in V 2W. Furthermore, if nN8, the van der Waals volumes of the substituents become comparable with solvation shell for H2TPP {V 2W(H2TPP)c4d V 2W (-OC8H17)}. For n-alkanes with nN8, DKN0, which corresponds to a greater packing than that at nb8. These

Table 5 The b 0 and b 1 coefficients values from the correlation Eq. (7) of some substance classes at 25 8C Compound Tetrachloromethane n-Alkanes Cn H2n+2

Cycloalkanes Cn H2n

Crown Ethers (-O-CH2-CH2-)n H2T(4-OCn H2n+1)PP

Benzene n-A1kanes Cn H2n+2 Fatty acids n-A1kanols Cn H2n+1OH H2T(4-OCn H2n+1)PP a b

Interval for n

b0

b1

sa

r

mb

5–10 11–32 5–32 5–7 8–16 5–16 4–8 2–8 4–16 8–16

33.4F0.6 32.2F0.6 32.5F0.3 27.4F1.7 5.1F2.5 16.0F2.9 3.3F0.4 7.5F1.6 508F15 541F8 d(m)=(15.73F0.15)

16.53F0.08 16.67F0.03 16.65F0.02 13.7F0.3 16.1F0.2 15.3F0.3 38.59F0.06 37.9F0.3 65.5F1.4 62.9F0.6

0.33 0.69 0.57 0.39 1.33 3.03 0.19 1.37 12.30 3.8

0.9999 0.9999 0.9999 0.9997 0.9997 0.9990 0.9999 0.9998 0.9993 0.9999

6 10 16 3 5 8 5 6 5 4

5–18 12–18 1–5 1–16 1–16

32.6F0.2 22F2 25F1 535F16 551F6 d(m)=(16.20F0.14)

16.87F0.02 17.3F0.1 17.2F0.3 66.4F1.9 64.80F0.56

0.29 0.72 0.99 27.4 6.49

0.9999 0.9998 0.9995 0.9967 0.9999

9 7 5 10 6

Standard deviation of the fitting in units of cm3d mol
1. Number of experimental points.

W. Zielenkiewicz, G.L. Perlovich / Journal of Molecular Liquids 121 (2005) 27–34

Fig. 4. The dependence of V 02 vs. n for crown ethers and cycloalkanes at 25 8C in TCM solutions.

considerations indicate that there are conditions for stabilization of the conformational state of the porphyrin molecules and for the absence of conspicuous deviations in the course of the function DK=f(V 2W). The correlation coefficients of Eq. (7) for H2T(4OCn H2n+1)PP in TCM and benzene solutions are presented in Table 5. These are the coefficients for all experimental n values, as also for n values lying on the approximated curve {dV 20 (-CH 2 -) are given in brackets}. For H 2 T(4OC n H 2n +1 )PP in TCM solution, the dV 20 (-CH 2 -) is 15.7F0.2 cm3d mol
1 and this value is less than for nalkanes. The analogous value for the porphyrin molecule in the benzene solution dV 20(-CH2-) is 16.2F0.1 cm3d mol
1, and this value is less than that for n-alkanes. On the basis of the calculations, it can be concluded that the dV 20(-CH2-) in TCM is less than in benzene solutions. This behavior is

Fig. 5. The dependence of DK vs. V W 2 for H2T(4-OCn H2n+1)PP at 25 8C in benzene and TCM solutions.

33

probably due to van der Waals interactions of the solute with TCM stronger than those in benzene solutions. Introduction of the t-Bu substituents into the phenyl moieties of the H2TPP molecule (as compared with alkoxy substituents of the same van der Waals volumes) essentially results in the values K 2. It is not surprising, because the compact and symmetric form of t-Bu substitutes promotes: (a) closer packing of the solute molecules into the TCM molecules; (b) the efficiency of the van der Waals interactions with the TCM molecules. A comparative analysis of the volumetric properties of the different cyclic compounds (bicycloalkanes, tricycloalkanes, aromatic hydrocarbons) with H2TPP in TCM solutions was carried out in the coordinate system DK vs. V 2W (Fig. 6). As can be seen from Fig. 6, for the aromatic hydrocarbons, the increase in the number of condensed benzene fragments has led to the strongest increase in the value DK, from 0 (for benzene) to 13.8 (for anthracene). Probably, the reason for this behavior is the essential increase of nonspecific interactions between the conjugated electron systems of the solute molecule and the solvent molecule. For bicycloalkanes, the topological structure of the molecule is the principal moment regarding the fine building of the molecule into the general structure of the solvent. For example, the cis- and trans-decalins have different values DK due to the different compaction of the molecules (provided equal conditions of nonspecific solute– solvent interactions). The tricycloalkanes have more compact topological structure of the molecules, therefore, they are packed closer to the solvent molecules than bicycloalkanes. The regularity in the DK behavior is observed only

Fig. 6. The dependence of DK vs. V W 2 for aromatic hydrocarbons (1— benzene; 2—naphthalene; 3—anthracene), tricycloalkanes (4—adamantane; 5—OHMI), bicycloalkanes (6—norbornane; 7—cis-decalin; 8— trans-decalin; 9—bicyclohexyl) and H2TPP at 25 8C in TCM solutions.

34

W. Zielenkiewicz, G.L. Perlovich / Journal of Molecular Liquids 121 (2005) 27–34

for tricycloalkanes. Adamantane and OHMI have equal V 2W but DK (adamantane)NDK (OHMI). From the inspection of Fig. 6, it follows that the molecule H2TPP possesses a particular solvation shell different from that of the compounds considered. Probably, the structure of this solvate shell is the result of the several reasons: (a) nonspecific interactions of the conjugated electron macrosystem of the porphyrin core with the solvent molecules; (b) nonspecific interactions of the phenyl fragments with solvent molecules; and (c) the characteristic topological structure and the conformational flexibility of the H2TPP molecule.

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