VAPORIZATION OF SOMEAMINE-TYPE PERCHLORATES
fit in to the normal hydrogen-bonded structure of the water.9127 Franks and Smithz8explained the increase in volume of sodium dodecyl sulfate solutions below the critical miscible concentration by the loss of structural water on forming dimers. This is in agreement with the present interpretation of the behavior of the tetraalkylammonium ions. In the case of the fluorides, the present model predicts an additional gain in structure on dilution since the fluoride is a structure-making ion.8 The chloride and bromide would be expected to give structurebreaking contributions since these ions are structure breakers.8 On this basis, the order for (PL of F- > C1- > Br- > I- is correctly predicted. Some unpublished calculations of Wu and Friedmanzgindicate that in the 1-3 m region overlap will be approximately
2155
++,
proportional to concentration and the - -, and - overlap will all make significant contributions. The calculation explains the large contribution of the overlap of two tetraalkylammonium ions to the heat of dilution and also explains the rough linearity of (PL us. m plots in this concentration range for the higher members of the series where the overlap is the main contribution to the value of (PL.
+
Acknowledgment. The authors wish to acknowledge the help of Mr. J. W. Jackson, Mr. C. J. Twardowskii, Mr. J. R. Ward, and Mr. D. E. Werner in constructing the dewar calorimeter used in this work. (27) L. A. D’Orazio and R. H. Wood, J . Phys. Chem., 67, 1435 (1963). (28) F. Franks and J. T. Smith, ibid., 68, 3581 (1964). (29) See ref 11, footnote 31.
Vaporization of Some Amine-Type Perchlorates’
by J. L. Mack and G . B. Wilmot Research and Development Department, Naval Ordnance Station, Indian Head, Maryland (Received December 6,1966)
$0640
Ammonium perchlorate, hydroxylammonium perchlorate, methylammonium perchlorate and hydrazine perchlorate have been shown by the cold matrix isolation-infrared spectroscopic technique to vaporize by dissociation; Le., RNH&104 -.t RNHz(g) HCIOl(g). The relative vaporization temperatures of these salts can be correlated with the pK values of the bases in aqueous solution. Evidence is presented that this proton transfer reaction plays an important role in the thermal decomposition of hydrazine diperchlorate and guanidinium perchlorate.
+
The proton-transfer mechanism has been postulated to play an important role in the thermal decomposition of ammonium p e r c h l ~ r a t e . ~While ~ ~ it has long been known that this mechanism is also involved in the vaporization of the ammonium halide^,^ Galwey and Jacobs2 have proposed that, since perchloric acid is an exceptionally strong acid, the sublimation of ammonium perchlorate may proceed without dissociation to give an ammonium perchlorate vapor species.
However, by the matrix isolation-infrared spectroscopic method, we were able to show that the vapori(1) This work was funded under the Foundational Research Program of the Director of Naval Laboratories. (2) A. K. Galwey and P. W. M. Jacobs, J . Chem. SOC.,837 (1959). (3) R. D. Schultz and A. 0. Dekker, “Sixth Symposium (International) on Combustion,” Reinhold Publishing Corp., New York, N. Y.,1957,p 618. (4) W. R. Rodebush and J. C. Michalek, J . A m . Chem. SOC.,51, 748 (1929).
Volume 71, Number 7 June 1067
2156
zation of ammonium perchlorate proceeds via a protontransfer reaction to yield ammonia and perchloric acid as vapor species.6 Recently, Heath and Majer8 have shown mass spectrometrically that sublimation proceeds via dissociation. Cassel and Liebman' and Inami, et a1.,8 have given indirect evidence for vaporization by dissociation. We describe here the further application of the matrix-isolation method to the determination of the vapor species of a variety of amine perchlorates. The matrix-isolation method was chosen for these studies since it is a direct method whereby vapor species are trapped without further reaction in a frozen matrix of some inert gas for identification by infrared spectroscopy. This method was considered to be more promising for the detection of a weakly associated complex than mass spectrometry where complete dissociation of a complex or free base by electron impact is a possibility.
Experimental Section Compounds. Ammonium perchlorate (AP) was taken from clear portions of single crystals grown in aqueous solution by slow isothermal evaporation a t 34" from once-recryst,allized reagent grade ammonium perchlorate. Hydrazine diperchlorate (HP,) of 99% purity was used as received. Hydrazine perchlorate (HP) was synthesized in situ by thermal decomposition of hydrazine diperchlorate in an evacuated Pyrex effusion cell at temperatures up to 135". Hydroxylammonium perchlorate (HAP) and guanidinium perchlorate (GP) were twice recrystallized from ethanol-chloroform solutions. Methylammonium perchlorate (MAP) was synthesized by the addition of dilute perchloric acid to an aqueous solution of methylamine. The water was pumped off and the product twice recrystallized from ethanol-chloroform solution. Spectroscopic Method. The matrix isolation techniq~e~> has ~ Ofound wide application to the spectroscopic study of reactive species. The technique has been elaborated by Weltner and Warn" and LinevskylZ for the determination of the spectra of the high-temperature species of a wide variety of compounds and reactions. Our methods and equipment were similar to those of these latter workers. A few hundred milligrams of the salt was placed in a Pyrex cell 2.0-cm X 1.2-cm diameter. One end was closed and the other open with :a 7-mm diameter orifice facing an infrared transmitting window cooled to 7-10°K. The cell was heated resistively and the temperature monitored by a copper-constantan thermocouple in contact with the external wall of the cell. The temperature required to deposit in 30 min sufficient material for spectral The Journal of Physical Chemiatry
J. L. MACKAND G. B. WILMOT
observation was determined by a separate vacuum vaporization experiment. The cell was heated to this temperature and the effusing species together with a large excess of nitrogen were condensed onto the cold window. The window was cooled in a metal cryostat of the type usual for spectral studies.l' Ratios of matrix gas to active species in the range 500-1500 were used. At the conclusion of the deposition, the window was rotated 90" about a vertical axis into the infrared beam. The spectrum was recorded by a double-beam grating spectrophotometer over the range 250-4000 cm-l. The matrix-isolated spectra of ammonia and perchloric acid were recorded for comparison purposes. The ammonia deposit was obtained by premixing the matrix gas and ammonia in the ratio 700:l and depositing the mixture over a 30-min period. Anhydrous perchloric acid was prepared by the method of Smithla and distilled into a calibrated capillary tube for volume determination. The capillary was opened to a tube directed toward the cold window onto which the nitrogen matrix gas simultaneously impinged. The flow rate of the perchloric acid was controlled by a thermostated bath and by the length and diameter of the connecting tube. The HClOJN2 ratio was 1 :600.
Results and Discussion From a comparison of the spectra of matrix-isolated ammonia and perchloric acid with that of the matrixisolated vapor species from AP (Figures 1 and 2), it is clear that the principal vapor species of AP at this temperature (180") are perchloric acid and ammonia. The spectrum of the vapor species from HAP a t 120" (Figure 1) clearly shows perchloric acid and the bands at 3640, 1604, 1364, 1132, and 893 cm-' agree well with the hydroxylamine gas-phase bands of Giguere and Liu14 a t 3656, 1605, 1125, 1115, and 896 cm-I if small matrix shifts are permitted. It should be noted that the spectrum of perchloric acid contains impurity bands a t 2855 (HCl), 2350 (COZ), 1525 (unidentified), (5) J. L. Mack, A. S. Tompa, and G. B. Wilmot, Spectrochim. Acta, 18, 1375 (1962). (6) G. A. Heath and J. R. Majer, Trans. Faraday SOC.,60, 1783 (1964). (7) H.M.Cassel and I. Liebman, J . Chem. Phys., 34, 343 (1961). (8) S. H. Inami, W. A. Rosser, and H. Wise, J . Phys. Chem., 67, 1077 (1963). (9) E.Whittle, D.A. Dows,and G. C. Pimentel, J . Chem. Phgs., 22, 1943 (1954). (10) E.D.Becker and G. C. Pimentel, ibid., 25, 224 (1956). (11) W. Weltner, Jr., and J. R. W. Warn, ibid., 37, 292 (1962). (12) M.J. Linevsky, ibid., 34, 587 (1961). (13) G. G. Smith, J . Am. Chem. SOC.,75, 184 (1953). (14) P. A. Giguere and I. D. Liu, Can. J . Chem., 30, 948 (1962).
VAPORIZATIONOF SOMEAMINE-TYPE PERCHLORATES
2157
1
FREQUENCY
(CM")
Figure 1. (a) Spectrum of matrix-isolated hydroxylammonium perchlorate vapor species; (b) spectrum of matrix-isolated perchloric acid.
and 662 cm-l (COz). The ammonia spectrum contains impurity bands at 3727 (HZO), 2350 (C02), 1600 (H20), 662 (C02), and 612 cm-l (instrumental). For the AP spectrum, except for three very weak, unidentified bands in the 1100-1200-~m-~ region, the only bands found other than ammonia and perchloric acid were shown to be weak or moderate intensity bands of H20, NzO,and C 0 2 . COZis a frequent impurity in our trapping experiments and N20 and HzO are wellknown low-temperature decomposition products of AP.16 Besides the bands of hydroxylamine and perchloric acid, the spectrum of the HAP vapor products showed only N20 and H 2 0 in relatively small amounts which can arise from the decomposition of hydroxylamine.l6 Although the spectra of hydrazine perchlorate and methylammonium perchlorate showed evidence of more extensive thermal decomposition, the major species were shown to be the free bases and perchloric acid.
The free bases were identified from the matrix-isolated spectra of hydrazinell and methylamine.'s Hydrazine diperchlorate had a relatively high vapor pressure at low temperatures (50-100") and the vapor species proved to be mainly perchloric acid and water. Upon prolonged heating to 135O, evolution of water and perchloric acid ceased, leaving a residue of HP. The water was an impurity in the very hygroscopic HPz and the salt was thus shown to decompose to give perchloric acid vapor and HP. The dissociation was catalyzed by the water since the vaporization temperature increases as the water is driven off. Although the spectrum of the vapor products of (15) L. L. Biroumshaw and B. H. Newman, Proc. Boy. SOC.(London), A227, 115 (1954). (16) I(.A. Hofmann and F. Kroll, Chem. Ber., B57. 937 (1924). (17) E. Catalano, R. H. Sanborn, and J. W. Frazer, J. Chem. Phye., 38,2266 (1963). (18) J. R. Durig, private communication.
Volume 71,Number 7 June 1967
J. L. MACKAND G. B. WILMOT
2158
FREQUENCY ('34')
FREQUENCY (CM') Figure 2. (a) Spectrum of matrix-isolated ammonium perchlorate vapor species; (b) spectrum of matrix-isolated ammonia.
guanidinium perchlorate showed large amounts of perchloric acid, no guanidine was detected. However, in addition to other products, ammonia in roughly a 1 : l mole ratio with perchloric acid and cyanamide was detected. There was some evidence for condensation products of cyanamide. These products support the decomposition mechanism of GP proposed by Glasner and MakovkyIg C(NH2:)3C104+NHzCN
NH&N
+ NH3 + HClOi
*condensation products
Recently we have trapped the vapor species from several guanidinium salts.*O The entire series studied follows the vaporization temperature-acid strength relationship discussed below. For those salts which vaporize at the lowest temperatures, complete vaporization to guanidine and free acid was found with no evidence for decomposition. Salts vaporizing at intermediate temperatures gave both guanidine and decomposition products, while, at the highest temperature, GP gave The Journal of Physicid Chemistry
only guanidine decomposition products and HC104. Thus, it appears that proton transfer is the initial step for both vaporization and decomposition and, a t the highest temperature, the decomposition reaction predominates with the free guanidine breaking down as fast as it is formed. By the matrix isolation-infrared spectroscopic method, direct evidence has been obtained in this study that AP, HAP, MAP, and H P vaporize by proton transfer RNHaC104
RNH2(g)
+ HC104(g)
and that the proton-transfer reaction also is important in the decomposition of G P and HP2. While, in nearly all cases, weak unidentified bands were found, the intensities of these bands increased more rapidly with increasing temperature than those of the free base and (19) A. Glasner and A. Makovky, J. Chem. Soc., 182 (1953). (20) J. L. Mack, presented at 163rd National Meeting of the American Chemical Society, Miami Beach, Fla., April 1967.
VAPORIZATION OF SOMEAMINE-TYPE PERCHLORATES
1:
11
Pb 9
Vaporization Temprmture (OC)
Figure 3. Vaporization temperature vs.. pK. (pK. values from H. K. Hall, Jr., J . Am. C h . Soc., 79,5441 (1957) and N. F. Hall and M. R. Sprinkle, &id., 54,3469 (1932)).
perchloric acid and thus could not be assigned to a complex RNHrC104 vapor species. Such a vapor species would have a lower heat of vaporization and hence increase less rapidly with temperature than the dissociation products. By adjusting the temperature required to produce a given intensity of the perchloric acid spectrum in a given time, it is possible t o define a “vaporization temperature” for each compound. This temperature is a tenperature at which the compound has a vapor pressure of roughly torr. Figure 3 shows the rela-
2159
tionship between this temperature and the pK values of the free bases in aqueous solution for several of the perchlorates studied. Although no guanidine was found in the vapor products of GP, the vaporization temperature of GP is included in Figure 3 since proton transfer appears to be the controlling step for the decomposition of GP and no HCIOd decomposition was noted. It is evident that there is a correlation between proton-transfer equilibria in solution (pK) and those in vaporization (vapor pressure). The relative stabilities with respect to vaporization follow the order of the pK values in every case except methylammonium perchlorate which has a higher vapor pressure than this relation would predict. A similar relation between the acid strength and volatility exists. Thus, Markowitz and Boryta2I find sublimation temperatures of the ammonium halides to be in order NH&I < NH4Br < NHJ and we find for some guanidinium (G) salts the order GOH < G2C03 < GCl < GBr < GC104.20 The volatilities of the salts in both series decrease with increasing acid strength. This relationship, while not of high accuracy, may be of value in predicting the relative volatilities of new amine salts and, where the decomposition also proceeds v i a proton transfer, in estimating the relative thermal stabilities with respect to degradation. (21) M. M. Markowitz and D. A. Boryta, J. Phys. Chem., 66, 1477
(1962).
Volums 71, Number 7 June 1867
