ASSOCIATION OF N-H COMPOUNDS
Association of N-H Compounds.
181
'8'
11.
Infrared Spectroscopic
Investigation of the Self-Association of 3,5-Dimethylpyrazole in Benzene and in Carbon Tetrachloride
by S. N. Vinogradov3and Martin Kilpatrick Chemistry Department, Illinois Institute of Technology, Chicago 16, Illinois
(Received August 09, 1963)
The self-association of 3,5-dimethylpyrazole in C6H6 and in cc14 was investigated over the temperature range 4 to 60" by means of absorption intensity measurements of the N-H stretching vibration band assigned to monomers. The results were found to be consistent with the formulation that 3,5-dimethylpyrazole self-associates to cylic dimers and trimers only, in agreement with the recent work of Rossotti and in disagreement with that of Lorenzelli and Mirone. The following heats of association were obtained: in benzene, AH20 = -8.5 kcal./mole and AHSO= -11.6 kcal./mole; in CC14, AH20 = -9.2 kcal./mole and AH30 = -13.9 kcal./mole.
+ monomer cyclic dimer cyclic dimer + monomer linear trimer n-mer + monomer - (n + 1)-mer
-
Unlike the 0-H . . O hydrogen bond, the N-H N hydrogen bond has been studied very little, even in s ~ l u t i o n . There ~ has been a paucity of quantitative hydrogen data on the thermodynamics of N-H..*K bond formation. A previous investigationb showed, as the result of the cryoscopic study of the self-association of 3,5-dimethylpyrazole in CaH6 over the range 0.02 to to 0.12 m, that the compound self-associated rather strongly to dimers and trimers with Pm = 2.5 and.1830 = 2500 a t 5 . 5 O . e Since the completion of the present work, reports by two independent investigators on the self-association of 3,5-dimethylpyrazole in CC1, studied by infrared absorption spectroscopy have disagreed on the mode of the self-association, Mirone and Lorenzelli' found that a plot of integrated intensity of the fundamental N-H stretching vibration band a t 3478 cm.-' US. concentration exhibited a plateau between 4 and 15 X M . The result was explained by assuming that below 5 X M , 3,5-dimethylpyrazole self-associated exclusively to cyclic dimer' with P 2 0 = 1.7 x lo3 l./mole; for concentrations higher than 2 X M , Mirone and TJorenzelli used the Kempter and Mecke modela to calculate the over-all association equilibrium constant, K = 770 l./mole. The authors proposed the following three equilibria to explain their results over the whole concentration range
monomer
Rossotti and his collaborators have demonstrated rather convincingly in a careful investigation of the self-association of pyrazole and some of its derivatives including 3,5-dimethylpyraaole, in CCl,, that these Taken in part from the dissertation presented by S. N. Vinogradov t o the faculty of the Graduate School of the Illinois Institute of Technology in 1959 in partial fulfillment of the requirements for t h e degree of Doctor of Philosophy. (2) Presented before t h e Division of Physical and Inorganic Chemistry, 132nd National Meeting of the American Chemical Society, Chicago, Ill., September, 1957. (3) Department of Biochemistry, Yale University, New Havpn, Conn. (1)
(4)
(5) (6)
(7) (8)
G . C. Pimentel and A. L. McClellan, "The Hydrogen Bond," W. H. Freeman and Co., San Francisco, Calif.. 1960, Chapter VII. N. E. White and M . Kilpatrick, J. Phys. Chem., 59, 1044 (1955). T h e terminology followed is the one of F. J. C . Itossotti and H. Itossotti, "The Determination of Stability Constants," ,MeGrsw-Hill Rook Co., Inc., New York, N.Y., 1961. P. Mirone and V. Lorenselli, Ann. chim. (Rome), 49, 59 (1959). H. Kempter and It. Mecke, 2. p h y s i k . Chem., B46, 229 (1940); I. Prigogine and It. Defay, "Thermodynamique Cheniique," Desocr, Liege, 1950, p. 449.
Volume 68, Number I
January, 1964
S.N. VINOGRADOV AND MARTIN KILPATRICK
182
compounds self-associate to cyclic dimers and trimers only over the concentration range to lou2 174.9 In the case of 3,5-dimethylpyrazole the equilibrium = 45,900 constants were Pz0 = 285 A 4 l./mole and f 2800 1.2/mole2at 19'.
Experimental All measurements were performed on a Perkin-Elmer Model 21 double-beam spectrophotometer equipped with a CaFz prism. Demountable nickel cells were fitted inside a copper block placed between the source and monochromator housings. Both sample and reference cells were maintained to better than 0.1" in the range 0-70'. Rolled optical grade AgC1, 1 mm. thick (Harshaw Chemical Co.), was used as window material. The range of cell thicknesses available was 0.05 to 1.2 em. 3,5-Dimethylpyrazole (Eastman Kodak) was crystallized twice from water and dried under vacuum. It melted a t 106.4-107.0' and was used interchangeably with a sample left by White.lo The literature values range from 106.5 to 107.0°.6~10Eastman Kodak Spectrograde CaHBand CC14 were used without further purification. The water content was estimated as 2.5 x 10-6M. In the case of CC1, as solvent, the transmission a t 3475 f 10 cm.-l, the N-H stretching vibration band maximum, was measured over the concentration range 0.0007 to 0.1 m a t eight temperatures from 277 to 328'K. In the case of C6H6, due to its appreciable absorption, the cell thickness range was restricted to 0.05-0.4 cm. and the transmission a t 3430 f 10 cm. was measured over the concentration range 0.002 to 0.14 m at five temperatures from 286 to 330°K. All solutions were made up by weighing. The peak heights of the monomer X-H band were measured a t slit widths of 60,80, and 100 p in cc14 and 100,120, and 200 p in C6He. The spectral slit widths were computed from the formulas 8' = 0.188 3.OF(s) for Cc14 and 8' = 0.178 f 2.9F(s) for C6H611where 8 is the mechanical slit width in p . All our calculations were performed on the IBM 650 computer of the Armour Research Foundation. The ~ eB, where E Is the molal abdata were plotted as 1 / us. sorptivity and B is the stoichiometric molality, and fitted by the method of least squares to a polynomial of the type
+
n-q
f(X)
=
C
a,X"
n=O
with q = 1, 2, 3, and 4.1* This treatment of infrared data has been described in detail elsewhere.la It IS based on the assumption, consistent with the findings of The Journal of Physical Chemistry
Rossotti19that either all polymers are cyclic or that the end N-H groups of linear polymers (if they exist) do not absorb at the same wave length as the N-H groups of monomeric molecules.
Results and Discussion Figure 1 shows a typical plot of I/Eus. EB. The term involving p40 was negative a t all temperatures in both solvents, indicating that over the concentration ranges
-EI 0.05
0.04
0.03
6.02
0.01
c -2 t. 4 0
I
3
2
4
€0
Figure 1. 3,5-Dimethylpyrazole in CCl,, data at fixed slit width 60 ,u; plot of l/c: us. E B : 0, 227°K.; 0, 326°K.
investigated, 3,5-dimethylpyrazole self-associates to dimers and trimers, in agreement with the conclusion of Rossottig and contrary to the findings of Mirone and Loren~elli.~Table I gives the P20 and P30obtained from the foregoing treatment of the data taken a t the mechanical slit width of 60 y in CC14 and 100 P in CeHe (spectral slit widths of -12 and 19 cm.-1, respectively, assuming F ( s ) = 0.5). It is evident that 3,5-dimethylpyrazole is much less associated in CeH6than in CC1,. ~~
D. M. W. Anderson, J. L. Duncan, and F. J. C. Rossotti, J . Chem. Soc., 140, 4201 (1961). (10) N. E. White, Ph.D. Thesis, University of Pennsylvania, 1954. (11) R. Barnes, R. S. McDonald, V. Z. Williams, and R. F. Kinnaird, J . A p p l . Phys., 16, 77 (1945); V. Z. Williams, Rev. 9ci. Instr., 19, 135 (1948). (12) IBM program 6.0.006. (13) S. N. Vinogradov, Can. J . Chem., 41, 2179 (1963).
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Table I : The AIonorner- TXnier and Xionomer-Trirrier Association 1~:quilit)riumConstants of 3,5-Dirriethylpyraaole
277 28 I 286 2% 208 310 31 s 314
480
87 66
200 I20
20 1ti
i!N)
x
108 450
SO
280
ti 8 110 94
3 4 5 3
61 4i
2 2
321
326 328 331
34
6 2
40
3 2
34
1 9
In benzrne thr average of data at 100, 120, and 200 1.1 LUI= ~ ~-8.5 arid A l l s a = - 11.6 kcal./mole. In C U 4 the average hrats of association calculated from the data a t 60 and 80 1.1 were = -9.2 and AZlso = -- 13.9 kcal. ,/mole. It is interesting to note that in CCl, the strengths of the hydrogen bonds in the dimer, trimer, and the overall change in heat of association for the process
gnw
dimer
+ monomer
4trimer
(1)
are all equal, '/zAHzo = -4.0! 1/3AH30 = -4.6, and A H S O - AN20 = -1.7 kcal./mole, while in CeHa the trend, 1/2AJ12~ = -4.3, l / 3 A N S a = -3.9, and All30 AH*" = -3.0 kcal.,'mole, must be due to the solvation by CsH6,but it is unclear why the rffect would be so proriounccd for process I .
Equally unclrar are the reasons for thr quantitative discrepancy t)c.twern t h r equilihriurn constants of this irivcstigation arid those of tlossotti' (correcting for the difference in units) a i d the very considerable difTerrnce i n the modes of self-association of 8,3-dimc.thylpyrnx~)le arrived at by the two forcyying investigations arid by Alironcl arid I,orcnxelli.7 I t is possiblr that the resolutioris of thc spectrophotorricters used could play a role siricr in all three cases the spectral slit widths werr riot less than one-fifth the apparrnt half-band width of the riarroiv N-TI stretching vibration, 8 coridi tion which was fouiid experimentally arid theorrtically nccrssary to climinatc the effect of slit width on absorption hand intrnsity. l 4 We consider that although thr equilibrium constants obtained from our d a b rannot tw considrrcd as reliable in view of the aforcmcntioncd discrrpancies arid the dosirability f o r furthrr work, the A H v a l u c ~ohtainrd should hr reliable t,o f 10yo. They seem to fall within the raiigr of values of A I I for S -H . . . S hydrogen bondirig6: pyrrole-pyiidine, 3.8 kcal./mole (calorimetry) , I 5 3.8 kcal. / m o k (infrared spectroscopy), l a arid 4.3 kcal.1 mole (r1.m.r.)1 7 ; aniline arid N-methylariilinc with 2mrthylpyridinr, 8.0 and 1.8 kral./mole, respectively (calorimetry) I"; iridolc -pyridine, 3.55 kcal./mole (infrared spectroscopy).1Y G. Pidot. I
