768
J. Phys. Chem. 1986, 90, 768-771
Dipole Moment of Bis(acetylthioethyi) Ether Evaristo Riande,* Julio GuzmHn, Instituto de Plcisticos y Caucho (CSIC),Juan de la Cierva 3, Madrid, Spain
Maria P. Tarazona, and Enrique Saiz Departamento de Quimica Fisica, Facultad de Ciencias, Universidad de Aka16 de Henares, Alcalci de Henares, Madrid, Spain (Received: June 25, 1985)
Mean-squared dipole moments of bis(acetylthioethy1)ether (BTE) were determined from measurements of dielectric constants and refractive indices in benzene solutions at several temperatures. The results were ( p 2 ) = 4.12 D2at 30 O C and d(ln (p2))/dT = 6.0 X lo4 K-'. Comparison of these experimental results with the values calculated with the rotational isomeric states model indicates that trans states about CH2-0 bonds are preferred by ca. 0.9 kcal/mol over the alternative gauche states; however, trans and gauche states about either S-CHI or CH2-CH2 bonds are almost equivalent in conformational energy. Comparison of this analysis with a similar one performed earlier for the molecule of thiodiethylene glycol dibenzoate, TDB, suggests that the permutation of the sulfur and the labeled oxygen (0')atoms in the O=C-0'-CH2-CH2-S residue does not change the rotameric population about CH2-CH2bonds, although it strongly affects the polarity of the resulting compound, since the mean-squared dipole moment of TDB is roughly twice that of BTE.
Introduction The critical interpretation of the conformational-dependent properties of macromolecules has been successfully used to obtain a better understanding of the relationship between structure and properties in polymers.] Of great importance in these studies is the knowledge of the rotational states of the skeletal bonds and their relative energies. For this purpose, information obtained from spectroscopic and thermodynamic measurements on small molecules having structural features similar to those of the chain molecules under investigation is frequently The comparison of theoretical and experimental values of configuration-dependent properties has also proved to be a reliable way to obtain information on the values of the conformational energies4 Thus, the rotational isomeric state analysis of the experimental dipole moments of two oligomers (x = 1 and x = 2) of poly(oxymethy1ene) (POM) shows that the gauche states about the OCHz-CHzO segment have an energy ca. 1.4 kcal/mol lower than that of the alternative trans states.',4 The high preference for gauche states of acetalic skeletal bonds was latter confirmed in studies of dipole moments and their temperature coefficient carried out on p ~ l y f o r m a l s . ~It* ~should be pointed out, however, that a quantitative explanation of the strong preference for gauche states in acetalic bonds was not provided by quantum mechanical calculations of the conformational energies of small molecules analogues of p~ly(oxymethylene).~ A smaller but still pronounced preference for gauche states about CH2-CHzbonds occurs in poly(ethy1ene oxide) (POE). The conformational energy associated with these states is ca. -0.4 kcal/mol with respect to the corresponding trans state. The schematic substitution of alternating oxygen by sulfur atoms leads to poly(thiodiethy1ene glycol) (FTDG),' an alternating copolymer of ethylene oxide and ethylene sulfide. Gauche states about CH2-CH2in FTDG bring the sulfur and oxygen atoms into close proximity and the energy of these states rises to ca. 0.4 kcal/mol over the alternative trans rotational isomer. The nature of the local chain environment greatly influences the rotameric population about the CH2-CHz bonds in these two cases. For example, the gauche population about OCOCHz-CHzO and OCOCH2-CHzS bonds, determined by ' H N M R spectroscopy,8 was found to be (1) Flory, P. J. "Statistical Mechanics of Chain Molecules"; Interscience: New 1969. . . York. . . (2) Mizushima. S . "Structure of Molecules and Internal Rotations"; Academic Press: New York, 1954.
(3) Volkenstein, M. V. "Configurational Statistics of Polymeric Chains"; Interscience: New York, 1963. (4) Abe, A.; Mark, J. E. J. Am. Chem. SOC.1976, 98, 6468. (5) Riande, E.; Mark, J. E. Macromolecules 1978, 11, 956. (6) Riande, E.; Guzmin, J.; Saiz, E. Polymer 1981, 21, 465. (7) Riande, E.; Guzmln, J. Macromolecules 1979, 12, 952.
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TABLE I: Summarv of Dielectric Results T,'C d(e - ej)/dw d (n2 - n,2)/dw 30 40 50 60
2.16 2.08 1.98 1.90
0.049 0.054 0.061 0.066
(/A'),
D2
4.11, 4.17, 4.17, 4.205
0.9 (Le. gauche preferred by ca. -0.9 kcal/mol) and 0.66 (Le. trans preferred by about 0.1 kcal/mol), respectively, indicating that the presence of the carbonyl group reduces the value of the conformational energy about CHz-CHz bonds by ca 0.3-0.5 kcal/mol in the two cases. This work studies how the permutation of the sulfur and the residue labeled oxygen (0')atoms in the O==C-O'-CHz-CH2-S influences the conformational energies about both the CH2-CH2 and the OCX-CHzCHz (where X represents either 0 or S atoms) bonds. For this purpose, the dipole moment and its temperature coefficient for bis(acetylthioethy1) ether (BTE) was measured. The results were critically analyzed by statistical mechanics procedures and the conclusions were then compared with those obtained from a similar analysis reported elsewhere8 for the molecule thiodiethylene glycol dibenzoate (TDB).
Experimental Part Materials and Procedure. The synthesis of bis(acetylthioethy1) ether was carried out by reaction of acetyl chloride and bis(mercaptoethyl) ether at room temperature in chloroform-pyridine solution. The crude product was washed several times with distilled water in order to eliminate the pyridine chlorhydrate formed in the reaction. Finally, it was distilled in a nitrogen atmosphere (bp 135 "C (3 mm)). The 'H N M R spectrum of the compound was recorded at room temperature with a Varian EM-390 apparatus working at 90 MHz. Deuterated chloroform and tetramethylsilane were used as solvent and internal reference, respectively. The spectrum presents two triplets, centered at 3.1 and 3.6 ppm, and a singlet at 2.4 ppm, corresponding to the resonances of thiomethylene, oxymethylene, and methyl protons, respectively. The areas of the peaks are in the ratio 1:l:l S , indicating the purity of the product. Dielectric constants of solutions of the compound in benzene were measured with a capacitance bridge (General Radio, Type 1620 A) operating at 10 kHz, using a three-terminal platinum cell. Calibration of the cell was carried out at 30, 40, 50, and 60 O C using benzene (Merck), cyclohexane (Merck), and carbon tetrachloride (Merck), all of known dielectric c o n ~ t a n t . ~ .The '~ (8) San Romin, J.; Guzmin, J.; Riande, E.; Santoro, J.; Rico, M. Macromolecules 1982, IS, 609.
0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 5, 1986 769
Dipole Moment of BTE
TABLE I 1 Geometry” bond lengths, 8,
‘P
rii
a N
P
c-c c*-c c*-o* c *-s s-c c-0
5
5
C
bond angles, deg
cc*s c*sc cco
1.53 1.50 1.22 1.73 1.82 1.43
COC
cc*o*
115 100 114 110 121
“Taken from ref 13.
3 -
3
c)
0
TABLE 111: Parameters for the Lennard-Jones Potential’ atom
1
van der Waals radius,
C 0 H C* O* S
0
2
1 102
3
w
Figure 1. Concentration dependence of the increment in dielectric constant and squared index of refraction for benzene solutions of the BTE molecule at 30 (0) and 60 OC ( 0 ) .
1.8 1.6 1.3 1.8 1.6 1.9
A
no. eff electrons
polarizn,
5
0.93
7
0.70 0.42
0.9 5
A3
1.23
7
0.84
13.5
2.39
“Taken from ref 16-18.
Table I. The value of the temperature coefficient of the dipole moment d(ln (p2))/dT, estimated by plotting the natural logarithm of the dipole moment against temperature, amounts to 6.0
x 10-4 K-I. Theoretical Analysis
A L L L ! J . J i l , Figure 2. The molecule of bis(acetylthioethy1) ether shown in its planar all-trans conformation. First/second-order interactions are shown above/below the skeleton of the molecule. The positive direction of the dipole moments is represented by arrows and the orientation of wt is defined by the angle p.
solvents had a purity exceeding 99.5% as received and were further purified with Linde Type 4A molecular sieves. Increments of the refractive indices of the solutions were determined at the temperatures of interest with a Chromatix KHX laser differential refractometer working at 632.8 nm. Results. Differences between the static dielectric constant of the solutions c and that of benzene e,, for different weight fractions of solute w ,were measured at 30, 40, 50, and 60 “C. In order to obtain the values of d(c - el)/dw in the limit w 0, values of A6 = t - e l , at each temperature, were plotted as a function of w. Illustrative plots of this kind at two extreme temperatures are shown in Figure 1. Results pertaining to different temperatures are given in the second column of Table I. Values of d(n2 - n12)/dwin the limit w 0, obtained from plots similar to those shown in Figure 1, are indicated in the third column of Table I. The mean-square dipole moment ( h 2 )of BTE was obtained by means of the equation of Guggenheim and
-
-
(h2)=
(27kTM)(4qdVA)-’(q + 2)-2{d(e - e,)/dw - d(n2 - n12)/dw]
where k is the Boltzmann constant, Tis the absolute temperature, is the density of the solvent, M is the molecular weight of the solute, and N A is Avogadro’s number. Values of the dipole moment at several temperatures are shown in the fourth column of p
(9) Timmermans, J. ’Physic0 Chemical Constants of Pure Organic Compounds”; Elsevier: Amsterdam, 1965, Vol. 1 and 2. (10) Landolt, H. H. “Landolt-Bornstein: Zahlenwerte und Funktionen aus Physik, Chemie, Astronomie, Geophysik und Technik”; Springer-Verlag: Berlin, 1959; Vol. 2, Part 6. (1 1) Guggenheim, E. A. Trans. Faraday SOC.1949, 45, 714. 1951, 47, 513. (12) Smith, J. W. Trans. Faraday SOC.1950, 46, 394.
Geometry and Energy Parameters. Figure 2 shows the molecule of BTE in its planar all-trans conformation. Bond lengths and bond angles used in the present calculations were taken from the literaturei3 and are summarized in Table 11. Among the five different kinds of bonds in the molecule (Le. bonds a through e in Figure 2), the rotations over bonds a modify only the orientation of methyl groups and therefore have no effect on the results of the calculation. Bonds of type b were assumed to be trans (Le. as shown in Figure 2) due to the delocalization of the C*=O* double bond. Bonds CH2-0 like e of BTE appear in many p o l y ~ x i d e sand ~ ~it~ has ~ ~ been proven that the t states of these bonds are preferred by ca. 0.9-1.2 kcal/mol over g* rotational isomer which are placed at flOOO. We represent by E , the energy of g vs. t states of bond e and use a value Ed, = 0.9 kcal/mol in all the calculations, since, as it will be shown below, this energy has a very minor effect on the results of the calculated dipole moments. Rotations over bonds d involve interactions of the S atom with other atoms or group of atoms and should be noticeably different from those of similar bonds in esters. Something similar is expected from bonds c since the bond length S-C is much larger than that of the 0-C bond. Some conformational energy calculations were performed for these two kind of bonds (Le. c and d) in order to obtain an estimation of their first-order energies E,, and E,,,; the results obtained for these two energies will be optimized later by comparison of the experimental and theoretical values of the dipole moment of the BTE molecule. Nonbonded interactions between atoms separated by more than two bonds were computed with a 6-12 Lennard-Jones function using the parameters summarized in Table 111. Threefold intrinsic torsional potentials were used for rotations over S-C and C-C bonds. A barrier V, = 2.8 kcal/mol was used for the C-C bond.19 The barrier for the S-C bond within the thioester group was not found in the literature; however, it can be estimated from the value reported for the S-C bond in thioethersIs (Le. 1.8 kcal/mol) and (1 3) Sutton, L. E., Ed. “Tables of Interatomic Distances and Configuration in Molecules and Ions”; The Chemical Society: London: 1958, Spec. Pub. No. 11: 1965. SDec. Pub. No. 18. (14) RiandqrEE.; Guzmin, J.; Tarazona, M. P.; Saiz, E. J . Polym. Sci., Polym. Phys. Ed. 1984, 22, 917. (15) Riande, E. J . Polym. Sci., Polym. Phys. Ed. 1977, 15, 1397. (16) Brant, D. A,; Miller, W. G.; Flory, P. J. J . Mol. Biol. 1977, 23, 1397. (17) Flory, P. J.; Sundararajan, P. R.; de Bolt, L. C. J . Am. Chem. SOC. 1974, 96, 5015.
770 The Journal of Physical Chemistry, Vol. 90, No. 5, 1986
Riande et al. CHART I
c
3.0
L
-a2 ” 1 \
W
2.0
1.0
TABLE IV. Variation of the Dipole Moment of the BTE Molecule with the Parameters” Used in the Calculations
parameter
103d(ln ( f i 2 ) )/d(parameter)
E,
286.4 353.0 -55.6 -14.2 0.00 -0.01 -0.45 0.45 360.3 1123.2
Ed Ed’
0.0 Figure 3. First-order interactions for rotations over bonds S C H 2 (c) and CH2-CHI (d). The second part of the curves (Le. q5 > 180’) is symmetric with the one shown here.
the analysis presented by Flory et aLzowho estimated the barrier for the 0-C bond in the carboxylic ester group to be about 1.0 kcal/mol from the value of 1.8 reported for the 0-C bond in ethers; thus a value of V, = 1.O kcal/mol seems to be a reasonable estimate for this barrier. At any rate, the exact value of this barrier is not critical, since the rotational isomers appear very close to the staggered positions for which the torsional potential is zero, and besides, the results of all the energy calculations will be used only as a starting value for an optimization. Dipole-dipole interaction energies were computed by assigning partial charges to the ether (qo = -0.32, qc = 0.16) and thioester (qc. = 40. = 0.22) groups so as to reproduce the dipole moments of pc = 1.07, ht = 1.4 D (see below); a value e = 3.0 was used in these calculations as the effective dielectric constant. The results of these calculations are summarized in Figure 3 where one can see that the three staggered positions over bonds c and d are almost equivalent in energy. Specifically, E,, = 0.05 with 4 = O,fl15 is obtained for bond c and E; = 0.1 with 4 = 0,*120 for bond d. The same kind of energy functions were used to estimate the second-order interactions arising in g’g’ conformations. The values obtained were E , = 1.6, E; = 5.3, and EWtI= 4.1, all in kcal/mol. The parameter w is due to interactions between the 0 of the ether group and the C*of the thioester, and the low value of this energy is due to the length of the C*-S and S-CH, bonds; the other two parameters represent interactions S 4 H 2 and CH,-CH,; both the S atom and the C H 2 group are bulky and therefore produce large energies. The values of the energies for the second-order interactions thus obtained may have uncertainties of some tenths of kcal/mol; however, the values indicated above will be used throughout all the calculations, since a variation of * O S kcal/mol in any of these energies does not produce any substantial modification of the dipole moment of the BTE molecule. The statistical weight of any given conformation was represented by a Boltzmann factor of its energy. Thus, statistical weight matrices for each pair of consecutive bonds of the molecule are given in Chart I. Dipole Moments. Two dipole moments ple each having a magnitude of 1.07 D and the direction of the 0-CH, bonds were assigned to the ether part of the m ~ l e c u l e . ’The ~ dipole moments (18) Riande, E.; GuzmBn, J.; Saiz, E.; de Abajo, J. Macromolecules 1981, 1 4 , 608. (19) Yoon, D. Y.; Suter, U. W.; Sundararajan, P. R.; Flory, P. J. Macromolecules 1915, 8, 184. (20) Saiz, E.; Hummel, J. P.; Flory, P. J.; Plavsic, M. J . Phys. Chem. 1981, 85, 3211.
E, Ed
Eu#> P T Ne fit
“The “main set” used for this calculation was E , = -0.2; E,‘ = 0.0; E,” = 0.9; E, = 1.6; E’, = 5.3; E,” = 4.1, all in kcal/mol: /3 = 100’; T = 30 OC; fit = 1.4; fie = 1.07 D.
3.5 1 1 ’ -0.6 -0.4 -0.2
,
\ ,
1-0.5
0.0 a2
0.4 Eg, I kcal mOi’
Figure 4. Mean-square dipole moment ( r 2 )(solid lines) and its tem-
perature coefficient 103d(ln (fi2)/dT(broken lines) of the BTE molecule as function of the first-order energies E , and E;. The values used for the remaining parameters were E / = 0.9; E, = 1.6; E, = 5.3; E; = 4.1, all in kcal/mol. T = 30 OC. Bond angles of Table I1 and rotational isomers placed at O , f 1 1 5 O ; 0,f120°; and O f l O O O for bonds c, d, and e, respectively. of each of the thioester residues were represented by gt for which a magnitude of2’ 1.4 D (the dipole moment of the S-ethyl thioacetate measured in benzene solution at 25 “C) was used. The direction of p, is given by the angle /3 that it makes with the prolongation of the CH3-C* bond. Flory et aL20have proven that, in the case of carboxylic esters, fl = 123”. In the case of the thioester group, given the smaller electronegativity of the S atom as compared with the 0 atom, a somewhat smaller value of fl should be expected. Applying Flory’s procedure20 to reproduce the dipole moment of S-p-chlorophenyl thioacetate2’ (g = 2.26 (21) McClellan, A. L. “Tables of Experimental Dipole Moments”; Rahara
Enterp.: El Cerrito, CA, 1974; Vol. 2.
J . Phys. Chem. 1986, 90, 771-774 D in benzene at 30 "C) from the dipole moment of chlorobenzene21 ( p = 1.59 D in benzene at 25 "C) and S-ethyl thioacetate2' gives a value of p = 100" which will be used in the present study. Calculations. The mean-squared dipole moment ( p 2 ) of the BTE molecule was calculated with standard procedures of the The variation of ( p 2 ) with the matrix multiplication parameters used in the calculation is summarized in Table IV; these variations were computed for small increments of each parameter from the values indicated as "main set". As Table IV shows, the most critical of the conformational energies are E,, and E,. Figure 4 shows the variation of ( p 2 ) and its temperature coefficient with these two energies; as one can see, the experimental value of ( w 2 ) is reproduced with Ed = 0.0 f 0.1 and E , = -0.2 0.2 kcal/mol; moreover, the value 103d(ln ( p 2 ) ) / d T 0.5 obtained with these energies is in very good agreement with the experimental result of 0.6. Consequently, the values of the energies associated with gauche states about S-CH, and CH2-CH2 skeletal bonds in the BTE molecule calculated from semiempirical potential functions are in satisfactory agreement with the values established for these parameters by the statistical analysis of the dipole moment and its temperature coefficient for this compound. This analysis suggests that the gauche states about CH2-CH2bonds which bring oxygen and sulfur atoms into close proximity have almost the same energy as the alternative trans state. A similar conclusion was obtained in an earlier works from the critical interpretation of the ' H N M R spectrum and dipole moments of thiodiethylene glycol dibenzoate (TDB). Therefore, the permutation of the sulfur and the labeled oxygen (0')atoms in the O=C-0'-CH2-CH2-S residue apparently does not change the rotameric population about CH2-CH2 bonds. It changes, however, the conformational energy E , associated with g states about the OCX-CH2CH2 bonds; the value of E , with respect to the corresponding t state decreases from ca. 0.5 kcal/mol for X = 0 to ca. -0.2 kcal/mol for X = S. The polarity of BTE is significantly lower than that of TDB. Thus, the mean-square dipole moment corresponding to BTE (4.1 D2) is roughly half the value of this quantity for TDB (8.5 D2).
*
(22) Flory, P. J. Macromolecules 1974, 7, 381.
77 1
The reason for this difference is the high sensitivity of ( p 2 ) to both pt and pe dipole moments and to the conformational energy E,. As Table IV shows, ( K ~ is) very sensitive to the contributions pt and pe corresponding to the dipole moments of the ester/ thioester and thioether/ether groups, respectively, and specially to p,. Both contributions are larger in the case of TDB than in BTE. Thus, in the case of TDB, these contributions p, = 1.81; pe = 1.21 D, corresponding respectively to ester and thioether residues, whereas in the case of BTE the residues are a thioester and an ether for which pt = 1.41; pe = 1.07 D, respectively. Since ( p 2 ) increases with pe and specially with p,, both contributions tend to give a higher mean-square dipole moment for TDB than for BTE. A simple way of evaluating the incidence of this effect is to calculate the dipole ratio E, = ( p 2 ) / x p o 2with xpo2 = 2(p: p:). The result for TDB (0,= 0.90) is about 38% higher than that for BTE (D, = 0.65); therefore, the combined effect of both p, and pe accounts for about 68% of the difference in mean square dipole moment of these compounds. It can be seen in Figure 2 that, when the molecule of BTE is in the all-trans conformation, the dipole moments p, of the two thioester residues are both roughly parallel to the vector obtained by addition of the two pe contributions. This positive correlation between all the contributions is diminished when either the CH2-CH, or CH2-S bonds are placed in a g state. Consequently, as Table IV shows, ( p 2 ) increases with both E , and E; (Le. with decreasing population of g states about these two bonds). As pointed out above, E; has the same value for both TDB and BTE molecules; however, E , N 0.5 and -0.2 kcal/mol for TDB and BTE, respectively; the contribution of the COX-CH2 bonds to the total polarity of the molecule will then be larger for TDB (X = 0) than for BTE (X = S). Thus, the effect of the conformational energy E,, adds up to that of the contributions p, and pe explained above, and the net result is that ( p 2 ) for TDB is roughly twice that for BTE.
+
Acknowledgment. Thanks are due to Mr. D. Delgado for his technical assistance. This work was supported by the CAICYT through Grant No. 513/83. Registry No. BTE, 65079-30-3.
Temperature and'Density Study of the Rayteigh Line Shape of Fluid N,O T. W. Zerda, X. Song, and J. Jonas* Department of Chemistry, School of Chemical Sciences, University of Illinois, Urbana, Illinois 61 801 (Received: August 29, 1985)
The Raman v l depolarized Rayleigh line shapes of N 2 0 are investigated at pressures varied from 8 bar to 2 kbar and over temperature range 298 to 373 K. Rotational, collision-induced,and cross-term contributions to the band shape and second moments are discussed and compared to previous results for COz, OCS, and CSz.
Introduction In recent studies of the depolarized Rayleigh scattering (DRS) of the linear molecules C02,1-5CS2,6-9and OCS,loJ'the line (1) A. De Santis and M. Sampoli, Mol. Phys., 51, 1 (1984). (2) A. De Santis and M. Sampoli, Phys. Lett. A , 101, 131 (1984). (3) H. Versmold and U. Zimmermann, Mol. Phys., 50, 65 (1983). (4) H. Versmold, Mol. Phys., 43, 383 (1981). (5) R. C. H. Tam and A. D. May, Can. J . Phys., 61, 1571 (1983). (6) B. Hegemann, K. Baker, and J. Jonas, J . Chem. Phys., 80, 570 (1984). (7) B. Hegemann and J. Jonas, J . Chem. Phys., in press. (8) P. A. Madden and T. I. Cox, Mol. Phys., 43, 287 (1981). ( 9 ) T. I. Cox and P. A. Madden, Mol. Phys., 39, 1487 (1980).
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shapes appeared to be sensitive to pressure and temperature changes. For all of these molecules the central part of the DRS spectra narrows with increasing density or decreasing temperature. In agreement with current t h e o r i e ~the ~ , ~observed effects were attributed to a diffusional type of rotational relaxation. The changes in the exponentially decaying DRS wings were explained in terms of collision-induced effects among which the dipole-ind u d dipole interaction (DID) was assumed to be the predominant mechanism. In the far wings of the DRS spectra of CS2 and OCS (IO) B. Hegemann and J. Jonas, J . Phys. Chem., 88, 5851 (1984) (11) B. Hegemann, Thesis, University of Illinois, 1984.
0 1986 American Chemical Society
