1332
W. D. GOOD,D. R. DOUSLIN, ASD J. P. MCCULLOUGH
Vol. 67
1,2-BIS-DIFLUOROA&~IN0-4!-RiIETHYLPENTANE HEATS OF COMBUSTION, FOR&IATION, AND VAPORIZATION ; VAPOR PRESSURE ; AND N-F THERXOCHERPICAL BOYD ESERGY' BYW. D. GOOD,D. R. DOUSLIX, ASD J. P. RICCULLOUGH Contribution 11-0.119 f r o m the Thermodynamics Laboratory of the Bartlesville Petroleum Research Center, Bureau of illines, U . S . Department of the Interior, Bartlesville, Okla. Received January 12, 1963 The heat of combustion of 1,2-bis-difluoroamino-4-methylpentane was measured by rotating-bomb calorimetry, and the vapor pressure was measured between -20 and +20" with an inclined-piston gage. Experimental techniques suitable for studying compounds of this class were developed. The results were used t o calculate the following thermochemical data in kcal. mole-' a t 298.15"K.: standard heat of formation of the liquid, -60.09; heat of vaporization, 10.51: and standard heat of formation of the gas, -49.58. The S-F thermochemical bond energy in this compound was found to be 67 kcal. mole-', about the same as the N-F thermochemical bond energy in NF3 and NzF4 but significantly less than that in perfluoropiperidine.
I n continuing studies of organic fluorine compounds, the Federal Bureau of Mines has made accurate thermodynamic measurements on 1,2-bis-difluoroamino-4methylpentane. CHs-CH-CH,-CH-CH,
The heat of combustion of a purified sample was measured in a rotating-bomb calorimeter, and the vapor pressure was measured with a recently developed inclined-piston gage. The heat of vaporization and the standard heats of formation in the liquid and gas states a t 298.15'K. were computed from the experimental results. The S-F thermochemical bond energy in this compound was compared with that in SF3,NzF4,and perfluoropiperidine, the only other compounds for which the K-F bond energy has been determined accurately. This report also describes the purification of the sample used in the experiments and new techniques developed for combustion calorimetry and vapor-pressure measurements on compounds of this class.
Experimental A. Material and Purification.-A sample of 1,2-bis-difluoroamT no-4-methylpentane was provided through the courtesy of Dr. R. W. Walker, Rohm and Haas Co., Huntsville, Alabama. The material was purified by a preparative-scale g.1.c. method by T. C. Davis of the Bureau's Laramie (Wyo.) Petroleum Research Center. About 2-g. samples were charged t o the purification unit, and fractions of about 1.5 g. were trapped and sealed in individual glass ampoules under nitrogen. The purified sample was analyzed on a 0.25-in. by 40-ft. analytical g.1.c. column, operated a t 120" and packed with 20 parts of Dow Corning 550 silicone oil per 100 parts of 30-42 mesh firebrick. The chromatogram showed one major component and three minor components. Analysis by a microhydrogenation technique2 showed that the major component was 1,2-bis-difluoroamino-4-methylpentane and that the minor components were isomeric C6H,zNzF4compounds and a CjH182F4 compound. From these results, it was concluded that the sample was 99.8 (probably F~ mole % C6H12N2F4,with about 0.2 mole % C ~ H M N Z 1,2-bis-difluoroamino-3-methylbutane).Also, the sample was F~ other shown to contain about 4.5 mole yo of C ~ H I Z N Zisomers than 1,2-bis-difluoroamino-4-methylpentane. These isomeric impurities (probably including some components with un(1) This research was supported b y the United States Bir Force and t h e Advanced Research Projects Agency through the Air Force Officeof Scientific Research of the Air Research and Development Command under Contract No. CSO 59-9, ARPA Order No. 24-59, Task 3. Reproduction i n whole or in part is permitted for a n y purpose of t h e United States Government. (2) C. J. Thompson, H. J. Coleman, C. C. Ward, and H. T. Rail, Anal. Chem., 32, 424 (1960).
branched carbon skeletons) would not cause significant error in the experimental results. A small correction for the C6HloN2F4 impurity was applied in calculating the heats of combustion and formation. B. Safety Precautions.-Compounds of this type are shock and spark sensitive. Consequently, the sample was always handled in 2-g. or smaller quantities, and it was possible in all the experimental procedures to provide shielding equipment adequate to prevent dangerous exposure of the investigator to an explosion. The samples were handled by tongs behind explosion shields while the investigator's hands and arms were protected by gauntlets. An ionizing radiation source was used to avoid accumulation of static electricity. Pipets were used instead of groundglass syringes for transferring liquid samples to avoid frictional effects that might cause detonation. The compound probably is toxic and was handled as if it were. C. Calorimetric Technique. Apparatus.-The rotating-bomb calorimeter, laboratory designation BMR-111, is similar to that previously de~cribed.~The modified platinum-lined bomb, Pt-5, internal volume 0.347 l., also has been d e ~ c r i b e d . ~ Units of Measurements and Auxiliary Quantities.-For consistency with related published data, results reported herein are based on the 1951 International Atomic Weights: and fundamental constantse and the definitions: 0°C. = 273.15"K.; 1 cal. = 4.184 (exactly) joules. The laboratory weights had been calibrated a t the National Bureau of Standards. For reducing weights in air to in vacuo, converting the energy of the actual bomb process to the isothermal process, and reducing to standard states, the following estimated values, all for 298.15' K., were used for the difluoroamino compound: density, 1.15 g. rnl.-'; specific heat, 0.4 cal. deg.-l g.-l; and ( ~ E / ~ P ) T , -0.007 cal. atm.-' g.-'. The paraffin oil, polyester film, and fuse material have been Calibration.-The energy equivalent of the calorimetric system, &(calor.),was determined by combustion of benzoic acid (National Bureau of Standards standard sample 39h, which was certified to evolve 26,434 d= 3 j . g,-' when burned under specified conditions). Five calibration experiments gave the value, &(calor.) = 4030.98 f 0.19 cal. deg.-l (mean and standard deviation). Procedures.-The calorimetric and analytical procedures generally were those previously described for organic fluorine c0mpounds.3~~However, ignition of the undiluted difluoroamino compound in the bomb resulted in detonation or other violent and incomplete reaction, as evidenced by unburned carbon in the products. Smooth and complete combustion reactions were obtained when the sample was diluted with a paraffin oil in such proportion that 58 to 75% of the evolved energy came from the diluent. Even such mixtures did not burn completely in (3) W. D. Good, D. W. Scott, and G. Waddington, J. Phys. Chem., 60, 1080 (1956). (4) W. D. Good, D. R. Douslin, D. W. Scott, A. George, J. L. Lacina, J. P. Dawson, and G. Waddington, zbzd., 63, 1133 (1959). (5) E. Wichers, J. A m . Chem. Soc., 74, 2447 (1952). Use of the recently adopted unified atomic weight scale (Chem. Eng. Areus, Nov. 20, 1961, p. 42) would not change t h e results significantly. (6) F. D. Rossini, F. T. Gucker, Jr., H. L. Johnston, L. Pauling, and G. W . Vinal, J. Am. Chem. Soc., 74, 2699 (1952).
June, 1963
THERMODYNAMIC PROPERTIES
several experiments, data from which were rejected in computing the final results. To prepare a diluted sample, the appropriate quantity of nonUSBN-P3a, empirical formula volatile paraffin was placed in a previously weighed bag of polyester film, and the mass of oil plus bag was determined. A sample of the difluoroamino compound was then introduced by a pipet, the bag was sealed, {he components were mixed, and the filled bag was weighed. I n this way, the mass of each component of the combustion iiample, including container, was determined accurately, To obtain a perfect closure of the bag, these operations were carried out so that no liquid touched the edges t o be sealed, and it was necessary to leave a small air bubble in the neck of the bag. For the comparison experiment^,^ the combustion example was paraffin oil (AEc"/M = 10984.12 cal. g.-l), and enough aqueous were added to the bomb to produce final states H F and "03 essentially the same as those obtained in the experiments with the difluoroamino compound. D. Vapor Pressure Measurements.-The Bureau of Mines recently developed an apparatus, t o be described elsewhere, for accurate vapor pressure measurements in the range 0.01-40 mm., a range in which previous devices were not satisfactory for many thermodynamic purposes. It was especially designed for studies of sensitive or high-boiling substances, and its first application other than tests with water was in the measurements reported here. The instrument, an inclined-piston gage, is a novel adaptation of the piston-cylinder principle long used in dead-weight gages a t high pressures. In these experiments, a one-gram sample was contained in a glass bulb immersed in a thermostat. The sample was thoroughly outgassed by cycles of freezing in liquid nitrogen, evacuation, and thawing. It was necessary to distil several small portions from the bulb before reproducible vapor pressure results could be obtained on the remaining sample. This procedure probably removed a small quantity of volatile impurity and residual dissolved nitrogen. The precision of the measurements %'as about 10.001 mm., and the estimated absolute accuracy (not including the effect of impurities) ranges from f O . O O 1 mm. at the lowent pressures t o f 0 . 0 2 mm. a t the highest.
Results Calorimetric Results.-Seven satisfactory combustion experiments and corresponding comparison experiments mere performed. Data for a single pair of combustion and comparison experiments, selected as typical of all experiments, are given in Table I. The final results of all experiments are given a t the bottom of Table I. The values of A E c " / N in this table refer to the idealized combustion reaction 1.
+ 802(g) + 196H20(1) = 6C02(g) 3-K2(g) + 4[HF.50H201(1) (1)
C6HI2N2F4(soln. in oil)
In computing the results, the heat of mixing of the substance and paraffin oil was neglected. This heat effect should be small enough to be included in the stated uncertainty interval, for no significant trend was noted in the results for samples of varying composition. Analysis of the products of the combustion reactions indicated that the H F formed represented 99.9 f 0.3y0 of that to be expected from reaction of pure CeHI2N2F4. This result is confirming evidence for the purity of the sample and the assumed stoichiometry of the combustion reaction. Vapor Pressure and Heat of Vaporization.-The results of vapor-pressure measurements in the range -20 to +20° are given in Table 11. These results are represented by the Antoine equation log p
=
6.88576 - 1427.201/(209.946
+ t)
1313
O F l,a-BrS-DIFLUOROAM11VO-4-METHYLPERTTAR'E
(2)
where p is in mm. and t is in "C. Equation 2 does not represent the experimental results within their precision (see Table 11),but it is satisfactory for most purposes.
TYPICAL PAIR
OF
TABLE I COMBUSTION AND COMPARISON EXPERIMENTS" Combustion experiment
m( compound), g. m(auxi1iary oil), g. m(polyester), g. (at n'(HzOj, mole nl(HF), mole nl(H?J03), mole At,, deg.
Comparison experiment
0.53218 ,41913 ,07774 (52) ,55344
% rel. hum.)
&(calor.)(-At,), cal. &(coni.)(- At,), tal.' AE,,,., cal. AE, cor. to st. states, cal. AE, cor. for impurity A E f d w ("Oa), Cal. - mAEe"/M(fuse), cal. - mAEc'/M(oil), cal. -mAEc'/M(polyester), cal.
2 00401 -8081.1 -27.0 0.7 6.6 -0.5 14.2 5.1 4603.8 424.7
mAEc"/ill( compound), cal. AEc'/M(compound), cal. g.-l
- 3053.5 - 5757.7
0 7128!5
.5459!1 ,01131 .00101 1.93876 - 7818, Ob -25.6 O.!j
6 ,0 1.4 5.7 7830 0
AEc'/M, cal. g.-l, all results: -5748.2, -5739.9, -5732.5,
- 5734.8, - 5731.4, -.5737, -5734.6.
AEc"/M, cal. g.-l, mean and std. dev.: -5737.0 f 2.2. The symbols and terminology are, except as noted, those of W. N. Hubbard, D. W. Scott, and G. Waddington, "Experimental Thermochemistry," F. D. Rossini, Ed., Interscience Publishers, Inc., New York, N. Y., 1956, Chapter 5, pp. 75-128. Value used t o determine &(calor.)for the corresponding combus&f(Cont.)(25' t,ion experiment. t:l(Cont.)(t, - 25')
tf
+
+ Atcorr ).
The Clapeyron equation and eq. 2 were used to compute the standard heat of vaporization, AHvo2,,.,, = 10.51 f 0.05 kcal. mole-I. TABLE I1 VAPOR
PRESSURE O F 1,2-BIS-DIFLUOROAMINO-4-METHYLPENTAKE
t,
oc.
- 20.000 -15.000 -10.000 - 5.000 0.000 5.000 10.000 15.000 20.000
+
p(obsd.), mm.
0.236 .364 ,560 ,843 1.235 1.743 2.466 3.478 4 812
p(obsd.1
- p(calcd.), mm. 0.000 - .003 .000 .007 .010 - ,019 - .028 .002 ,036
+ + + +
Heat of Formation.-Table I11 lists derived thermochemical data, including the heat of formation. The values of AEc' and AHco apply to the idealized reaction, eq. 1. The values of heat of formation, AHf", refer to equation 3 and mere computed from AHc", AHvO, and heat of formation data for carbon dioxide,? water,? and aqueous HF.? 6C(c, graphite)
+ 6Hsk) + N&) + 2F2(g)
=
C B H I Z Y ~or F .g) ~ (3) Discussion of Results The thermodynamic data given herein are the first accurate values reported for any organic difluoroamino compound, a new class8 of much theoretical interest. The only other compounds containing N-F bonds for (7) F. D. Rossini, D. I). Wagman, W. H. Evans, S. Levine, and T. Jaffe, "Selected Values of Chemical Thermodynamic Properties," Natl. Bur. Std. Circ. 500, 1952. (8) R. C. Petry and J. P. Freeman, J . Am. Chem. Soc., 83,3912 (1961).
AXDREWD. LIEHR
1314
Vol. 67
TABLE I11 THERMOCHEMICAL DATAFOR 1,2-~IS-DIFLUOROAXINO-4-METHYLPENTAKE IN KCAL.MOLE-^ AT 298.15"K. AEc" Liquid -1079.54 & 0.80" AHc" Liquid -1080.13 f 0.80° AHfo Liquid - 60.09 AHv' 10.51 =I=0.05 AHfo Gas - 49.58 a Uncertainty is the uncertainty interval equal t o twice the final "over-all" standard deviation.
for which
which accurate thermochemical data are available are ?;F3,9 ITeF4,10and perfluoropiperidine."l The S-F thermochemical bond energy, E(K-F), in the last three compounds is reported to be G6,9 GG,l0 and 7311 kcal. mole-', respectively. Although bond energy calculations are arbitrary for complex organic compounds, a reasonable estimate can be made of E(K-F) in 1,2-bisdifluoroamino-4-methylpentane for comparison with the foregoing values. Consider the following hypothetical reaction
where the E's are thermochemical bond energies. From this relationship, the heats of formation of the difluoroamino compound and 2-methylpentane,12and a consistent set of bond energies [E(H-H) = 104.20413; E(F-F) = 37.80614; E(N=;?;) = 226.O9Oi6; E(C-H) = 99.297, from the heat of formation of methane' and the heat of atomization of carboni6; E(K-H) = 93.4, from the heat of formation of ammonia'; and E(C-N) = 67.41, from the heat of formation of dimethylamine16], the N-F bond energy in 1,2-bis-difluoroamino-4methylpentane was calculated to be E(K-F) = 67 kcal. mole-l. This value is about the same as those for NF3 and XzF4,but it is significantly less than that for perfluoropiperidine.
+
CHz-CH-CHz-CH-CH3(g)
I
1
XFZ NFZ
I
CHs CH~-CHZ--CH~-CH-CH~(~)
I
+ Hz(g) = + Kz(g) + 2F2(g).
CH3 ( 9 ) G. T. Armstrong, S. Marantz, a n d C. E'. Coyle, J . Am. Chem. Soc., 81, 3798 (1959). (10) G . T. Armstrong, S. N a r a n t a , a n d C. F. Coyle, Natl. Bur. Std., Report No. 6584 (1961). (11) W. D. Good, S. S. Todd, J. F. JIesserly, J. L. Lacina, J. P. Dawson, D. W. Scott, a n d J. P. McCullough, J . PAYS.Chem., 67, 1306 (1963).
(12) "Selected Values of Physical a n d Thermodynamic Properties of Hydrocarbons a n d Related Compounds," American Petroleum Institute Research Project 44, Carnegie Press, Pittsburgh, Penna., 1953. (13) A. G. Gaydon, "Dissociation Energies," Chapman and Hall, London, 1953. (14) W.H. Evans, T. R. Nunson, a n d D. D. Wagman, J . Res. Nafl. Bur. Std., 68, 147 (1955). (15) L. Brewer a n d A. W. Searcy, Ann. Rev. Phyis. Chem., 7 , 259 (1966). (16) W. H. Johnson, I. Jaffe, and E. J. Prosen, J . Res. A'afl. Bur. Sfd., 6SA, 71 (1961).
THE THREE ELECTRON (OR HOLE) CGBIC LIGAKD FIELD SPECTRTW BY AXDREWD. LIEHR~ Bell Telephone Laboratories, Inc., Murray Hill, Arelo Jersey, and Xellon Institute, Pittsburgh 13, Pennsylvania Received January 14, 1963 The theoretical dependence of the energy with respect to the ligand field coulombic (Dq), spin-orbit ( p ) , and electron correlation parameters ( B and C) for kd3z7( k = 3, 4, 5), transition metal systems is applied to a variety of fresh experimental situations, and a number of surprising new results are obtained. First, it is found that the use of the "exact" ligand field energy level scheme to interpret the spectral features of a number of carefully chosen test systems introduces fundamental alterations in the established theory of nephelauxetic shifts [that is, the displacements of the energy levels due to reductions in the magnitude of the electron correlation parameter B as compared with its free ion value]. Second, the experimental rule of systematic variation of spectral frequency with alternant ligands [the so-called spectrochemical variation] is subject t o strict reservations in crystalline media. Third, the effects of next nearest neighbor polarizations are important in fixing the magnitude of the ligand field coulombic parameter Dq. And fourth, an unambiguous determination of all the ligand field parameters can only be accomplished upon experimental resolution of tbe large number of distinct spin forbidden multiplets permitted by the exact theory. Sample systems are discussed to illustrate each of these results, and precautionary words are spoken to dampen some of the wild enthusiasm currently prevalent in the use of elementary ligand field concepts.
Introduction The most general set of secular equations for a l ~ d , ~ , ' (k = 3, 4, 5 ) , transition metal complex in a cubic environs which is allowed by the fundamental approxi-
mations of ligand field theory has recently been derived by Ei~enstein,~ and subsequently has been utilized by him to interpret the magnetic and optical properties of &ReC16 and IrF6. Concomitantly, Weakliem,4Raaah,
(1) This paper was originally scheduled t o appear under t h e joint by-line of Andrew D. Liehr, James Ferguson. and Darwin L. Wood, and was so referenced in previous advertisemenw of t h e author. T h e author most firmly believes t h a t t h e by-line should still so read. However, Drs. Ferguson a n d Wood believe otherwise and have most firmly pressed upon t h e author t h e d u t y of changing this by-line t o its present form. T h e work repoited in this article is a direct result of over two years of close collaborative effort with Drs. Wood a n d Ferguson, a n d hence, is in nowise t h e sole production of t h e author. Indeed, most of t h e figures a n d tables here recorded a n d much of t h e text are joint preparations of all three of us. Thus,
in reading and referencing this work please be sure t o associate it most intimately with t h e several works of Drs. Ferguson a n d Wood o n the experimental and interpretive aspects of t h e three electron a n d hole ligand field problem (ref. 28a, 37m, 38f,and 39d). This paper was presented, in part, by t h e three of us a t t h e Symposia on Molecular Structure a n d Spectroscopy, Ohio State Unilersity, Columbus, Ohio, June, 1961 (A. D. L.),and 1962 (A. D. L., J. F., and D. L. IT.), (2) Mellon Institute, Pittsburgh 13, Penna. (3) J. C. Eisenstein, J . Chem. Phgs., 33, 1887 (1960); 33, 1530 (1960); 34, 1628 (1961) [Errata, ibid., 35, 2246 ( l Q G l ) ] .
