Dipole Moment Values from Single-Solution
Maximo Bar6n1 and Enrique Sanchez Arevalo Departamento de Fisica, Facultad de Ciencias Exactas y Naturales-Pabellon 1, Ciudad Universitaria-Nuiiez, 1428-Buenos Aires, Argentina I n two recent u a o e n (I. . . 2 .) a n eauation was uronosed that provides a very kmple procedure'to obtain vapoi-phase diuole-moment values for liauids from sinele-solution measurements:
where r = permittivity, n = refractive index, N z = numher density (molecules/mL), a = molecular radius in the Onsager cavity, calculated from a; = 3 M z / 4 ~ N ~ dNzA, = Avogadro's number, d = density, M = molecular weight, k = Boltzmann constant, and T = absolute temperature. The subindices 1,2, and 12 refer to solvent, solute, and solution, respectively. Equation 1results ( I ) from the linear behavior of mixture permittivities in highly dilute solutions, wherein polarizabilities may be taken as additive. Molecules can then be considered as immersed in a sea of solvent, and Onsager's cavity model is applicable. This is represented in eq 1 by (nz 2)(2r1+ 1)/3(2rl+ ni) = t h e dipole moment within the cavity and q(n: 2)/(2rl+ nD = external moment of the immersed dipole. Originally used for compounds that are liquids a t room temperature, eq 1is now extended to solids a s shown in the present work.
Density The candidate substances (0.5-0.7 g) were ground in an agate mortar to a few microns in grain size and pressed into a pellet, in vacuum under 10 atm pressure, using an IR KBr pellet die. The diameter and thickness of the pellet were then measured with a Palmer caliper to k0.01 mm, and the volume was calculated. With the weight measured to fO.OOO1 g the density is readily obtained. Most solids are not friable and can he pressed to compact and firm pellets. This procedure is a simple and convenient alternativeto the crystal flotationmethod (3). Refractive Index Refractive index can be calculated rearranging eq 2 for molar refractivity
+
+
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where M = molecular weight and d = density, and with RD obtained from bond refractions (4). If the density of the candidate substance is known, this is a simple alternative to other procedures (5). A few test calculations showed that the values of n so obtained differ less than 1%from those measured experimentally. The success of this procedure is due to the fact that the distortion polarization of polar molecules is relatively small compared to its orientetion polarization.
Data and Cornparatlve Results
d2 Compound
g/mL
5
h
1,2,4,5-tetrachlorobenzene
1.8165 1.8165 2.0361 1.4080 1.44695 1.30760 1.5898 1.2285 1.5865
45.52
1.69180
55.46 58.03 26.87 32.38 40.47 29.34 42.48
1.41535 1.1894 1.1694 1.3241 1.4428 1.5401
30.91 54.92 54.92 31.72 30.91 34.48
hexachlarobenrene anthraquinone benzoquinane benmic acid 2.8dichlorobenzaldehyde re~orcinol 2.4.5-trichlorophenol
d~
w2
lo@
M(D) this work
M(D) it.
Solvent
g/mL
(I
1.72361' 1.71400 1.63723 1.61100 1.65442 1,56830 1.60762
0.079 0.21 0.32 0.054 0.168 0.286 0.158 0.0065 0.378
CCll CaHe CsHe CCl, CeHs CaHe C8Ha C8Hs CeHs
1.5848 0.87361 0.87361 1.5848 0.87361 0.87361 0.87361 0.87361 0.87361
2.2279 2.2729 2.2729 2.2279 2.2729 2.2729 2.2729 2.2729 2.2734
2.2288 2.2757 2.2748 2.2286 2.2760 2.2783 2.2779 2.2759 2.2837
0.06 0.05 0.1 0.26 0.64 0.78 1.40 2.14 2.12
0 0 0.2 0.27 0.50 0.76 1.42 2.05 2.40
1.64445 1.62275 1.62275 1.60459 1.64445 1.64783
0.0345 0.333 0.340 0.077 0.023 0.019
CsHB CCl, CaH6 CsH6 CaHs CBHa
0.87361 1.5848 0.87361 0.87361 0.87361 0.87361
2.2729 2.2279 2.2729 2.2729 2.2729 2.2729
2.2748 2.2508 2.2984 2.2820 2.2733 2.2786
2.48 2.84 3.06 3.04 4.65 5.04
2.53 3.07 3.07 3.07 4.42 5.11
X
2
(7)
Vector
addn. nicotinic acid benzophenone nicotinarnide ioonieotini~acid phthalic anhydride
644
Journal of Chemical Education
Permittivity Solutions of 0.00005-0.005 wlw conc. were prepared and the permittivity was measured as previously reported (6).All experimental and calculated results are listed in the table. toeether with results u from solutiou measurements reported in the literature (7).
Conclusion The d i ~ o l moment e values calculated with ea 1from sinare in good with re. gle.solut&n s u l k obtained through conventional procedures.
-
Acknowledament Valuable suggestions by the referee are gratefully appreciated, Literature Cited
4. Yogel, A. J.: cresswd~,W. T.: ~ e f f rG. ~ M.; . hieester. J. J. them. sac.1952.514. 5. Partington, J. R. An Advonesd Treolsr on Physical Chemistry: Longmans: Landan,
1962: v01.4, P 3s. 6. Ban%. M.An,&c. Q u i m . A w n t . 1976.64.383. 7. McClellan, A. L. Tables of Erprrimentol Dipole Momanfa; Rahars Enterprires: El corrito, CA, 1071: vok.I and 2.
Volume 65 Number 7
July 1988
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