Nov., 1963
TRANSITION AND FUSIOX OF TRIETHYLENEDIAMINE
residual disorder in crystal I1 and justifies assumptions made in the entropy of transition calculation. Summary From the evidence presented, we conclude that the transition from crystal I1 to crystal I is from an ordered phase to a plastically crystalline phase. The analysis of the entropy and transition shows that some freedom of molecula,r rotational reorientation must be allowed to account for the observed entropy increment. This conclusion is supported by the low value for the entropy of melting, which indicates crystal I to be highly disordered. Comparison of the measured enthalpy of melting with this value obtained by van de T'loed7 from freezing point depressions indicates that care must be taken in the interpretation of data obtained by that method. If relatively large amounts of the second component are used, the formation of a solid solution coulcl lead t o a crystal structure other than that of the plastic crystal phase and a resulting enthalpy of melting corresponding to the melting of an ordered phase. The existence of an ordered phase by solid solution formation is supported by the accord between the entropy of melting, from freezing point depressions, 8.5 cal./mole-°K., and the sum of the observed entropies of transition and melting, 9.0 cal./ mole-OK. The postulated molecular rotational reorientation in crystal I is supported by dielectric dispersion data of Clemett aiid DaviesZ1who conclude (21) C . Clemett and
M. Davies, J . C h e n . Phgs., 3%,316 (1960).
2381
that rotation in the solid involves the entire molecule and is a cooperative phenomenon involving adjacent molecules. Future studies suggesl ed by this research concern the heat capacities of malo- and glutaronitriles which contain one less and one more CH2 group, respectively. Malonitrile is a solid a t room temperature having the mechaiiical properties associated with a plastic crystal. Glutaronitrile exists in two solid modifications, a inetastable phase (formed by rapid cooling to -60') which undergoes an irreversible transition t o a stable form a t -40°.22 The dimorphism has also been noted by van de Vloed7 who reported melting temperatures and entropies of melting derived from cryoscopic constants. Thermal measurements on glutaronitrile are presently in progress. Acknowledgment.-The authors thank Professor C. E. Nordmaii for making accessible unpublished X-ray diffraction data aiid Professor G. J. Janz of Rensselaer Polytechnic Institute for unpublished preliminary results on the gas phase thermodynamic functions of succinonitrile and Dr. Elfreda Chang for assistance in the evaluation of the data. C. A. W. acknowledges the support of the Institute of Science and Technology of the University of Michigan in the form of a postdoctoral fellowship. The partial financial support of the U. S. Atomic Energy Commission Division of Research is greatly appreciated. (22) I. Matusbara, zbzd., 35, 373 (1961).
HEAT CAPACITIES AND THERMODYNAMIC PROPERTIES OF GLOBULAR MOLECULES. VII. TRANSITION AND FUSION OF TRIIETHYLENEDIAMINE~ BY JOHX C. TROWBRIDGE AXD EDGAR F. WESTRUM, JR.~ Department of Chemistry, University of Michigan, A r m ArboP, Michigan Received M a y 23, 1963 The heat capacity of 1,4-diazabicyclo[2,2,2]octane has been determined from 300 to 450°K. by adiabatic calorimetry. A sharp transition a t 351.08'K. associated with the transformation to the plastically crystalline phase involves an entropy increment of 7.19 cal./(mole OK.). The triple point occurs a t 433.1"K. with an entropy increment 4.10 cal./(mole OK.) and thus confirms the classification of this substance as a plastic crystal. Thermodynamic functions have been computed from the primary thermal data.
Introduction I n a previous paper of this series3.the low temperature heat capacity of triethylenediamine, l14-diazabicyclo[2,2,2]octane, was reported, revealing the onset of a transition just beyond the high temperature limit (350'K.) of the cryotkat. This investigation coiiceriis studies on the same material a t higher temperat'ures and presents evidence coiicerning the nature of the "rotator" or as designated by Timmermans4 the "plastically-crystalline" phase. Experimental Preparation of Sample.-The triethylengdiaiiiirie saniyle used in t,his investigation was that previously used in t,he low tempera-.
(1) This work w a s supported i n part by the Division of Research of the United States Atomic Energy Commission. submitted by J. C. T. in partial fulfillment of the requirements for the Ph.D. degree of the Horace H, Reckham School of Graduate Studies of the University of Michigan. ( 2 ) To whom correspondence concerning this paper should be directed. (3) 8. 8 . Chang and Z. F. Westrum, Jr., J . Phys. Cham., 64, 1551 (1960). ($1 J . Timmevmans, J . c h i w . ph~/.s., 36, 331 (1938); I d chinr. B e l g c , 16, 178 (1951); cf. J . P h y s . Chem. Solids, 18, 53 (1961).
ture measurement^.^ Because of the hygroscopicity of the sample, the calorimeter was loaded in the anhydrous nitrogen atmosphere of a drybox. Although crystals used for the calorimetry were in the form of transparent hexagonal platelets approximately 1 cm. in diameter, they were fused in the process of these measurements. The fractional melting experiments reported later in this paper give a further indication of the high purity of this material. Microchemical analysis of the triethylenediamine sample indicated the following composition: C, 64.39; H, 10.81; and X , 24.75 icalcd.: C, 64.24; H, 10.78; and N, 24.96, for CsHizNz). Silver Calorimeter.-The calorimeter used in this study (laboratory designation W-22) is machined from two solid silver cylinders and shown in Fig. 1 . It is provided with an entrant, axial well ( G ) rontaining the 2.50-ohm Karma wire heater and J,eeds and Northrup capsule-type resistance thermometer (H). The t,herniometer is gripped in a threaded beryllium-copper collet arrangement tightened by being forced into the slightly conical bore of the heater sleeve. The heater sleeve is itself conical and is held in a mating hole within the well of the calorimeter by means of a fine screw thread. The thermal equilibration spool (1) ensures that the leads achieve the surface temperature of the calorinieter. Six vertical radial vanes machined as an integral piece of the thermometer well portion facilitated thermal equili-
2382
JOHN C. TROWBRIDGE AND EDGAR F. WESTRUM, JR.
-25 --20
--I5
r-IO
-- 5 -
-
-0 CM Fig. 1.-Cross-sectional diagram of Mark IV intermediate temperature thermostat: (A) guard shield ring; (B) calorimeter suspension collar; ( C ) primary radiation shield; (D) calorimeter closure assembly; (E) guard shield; ( F ) calorimeter assembly; (G) thermometer-heater well; (H) thermometer; (I) thermal equilibration spool; (J)lead bundle; (K) adiabatic shield. bration. All seams of the calorimeter are force-fitted and sealed vacuum-tight by silver alloy brazing. For expeditious loading and unloading of this calorimeter, a vacuum-tight closure (D), operable a t room temperature and not involving weight adjustments of solder, etc., is desirable. To meet these requirements, a gold gasket is forced by screw threads against a circular knife edge to provide a suitable seal to a 1-em. diameter inlet tube. A suitable jig within a cyIinder which can be evacuated to mm. is used to close this seal in the presence of vacuum or low pressures of inert gases for thermal conduction. Further details concerning the demountable thermometerheater assembly, the nature of the seal for the sample space, and the mechanism for closing this seal under vacuum may be found elsewhere? Intermediate Temperature Thermostat.-The Mark IV adiabatic thermostat depicted in Fig. 1 in schematic cross section is designed for the study of thermal properties of substances and systems over the temperature range 250 t o 550°K. The calorimeter (F) is suspended by a fine alloy wire from the collar (B) on adiabatic shield (K) which consists of a hemispherical top spun from 1.3-mm. copper silver-alloy brazed to a cylindrical middle section. The open lower end of the adiabatic shield is closed with a closely fitting copper plate held in place by six machine screws. All exterior surfaces of the adiabatic shield are wound with Fiberglas-insulated wire. The bundle of lead wires ( J ) is tempered by the guard shield ring (A) and is laid in a helical groove along the exterior of the adiabatic shield. Those wires which lead t o the calorimeter and the heater windings make good thermal contact with the adiabatic shield by being firmly cemented to the respective copper members with Formvar. The ( 5 ) J. C. Trowbridge, "Thermodynamic Functions and Phase Transitions including Fusion for Perchloric Acid, Pentserythrityl Fluoride and Triethylenediamine," Doctoral Dissertation, University of Michigan, University Microfilms.
Vol. 67
adiabatic shield is suspended by three alloy Rites from the guard shield (E) which is also provided with heater and control thermocouples on portions of each of its three sections. The ends from 'ndrical section may be withdrawn by removing small macrews. The top end IS rigidly supported from the top the radiation shield (C) which is fixed to a 2.0-cm. diameter stainlee steel tube with 0.25-mm. wall supporting the entire assembly within the cylindrical brass can evacuated by highspeed vacuum pumps. Another radiation shield not shown in Fig. 1 surrounds the shield (C) and reduces somewhat the electrical power required to maintain the highest temperatures. A liquid nitrogen tank (not shown) was used for achieving lower temperatures. In operation, the temperature differential between the adiabatic shield and calorimeter is maintained a t less th&n &0.001", and the guard shield is held about 1O below the calorimeter temperature. Adiabaticity is maintained by a three-channel automatic shield control device described elsewhere.8 Separate channels are used to control the main and bottom portions of the adiabatic shield and the guard shield. Calorimetric Procedure.-The technique of the calorimetric measurements was in general similar to that described in the cryogenic studies previously r e p ~ r t e d with , ~ the exception that a Rubicon double, thermal-free potentiometer, together with a photoelectric galvanometer, was used for the microvolt potential measurements. All potential resistance, mass, and time measurements were referred to standards or devices calibrated by the National Bureau of Standards. A platinum resistance thermometer (laboratory designation A-7), also calibrated by the National Bureau of Standards, was used to define the temperature scale. The calorimeter was loaded in an anhydrous nitrogen atmosphere with 70.3256 g. (zn vacuo) of triethjdenediamine. The calorimeter was heated during the loading process to fuse the sample and permit a greater quantity t o be used. After filling, the calorimeter was evacuated and filled with purified helium a t 10 cm. pressure a t 300°K. t o provide better thermal contact between sample and calorimeter. The heat capacity of the empty calorimeter-heater-thermometer assembly was determined in a separate series of measurements and represented approximately 24% of the total normal observed heat capacities. The calorimetric system was calibrated with a Calorimetry Conference sample of synthetic sapphire.8
Results Heat Capacity.-The experimental heat capacity determinations are presented in Table I in chronological order, in terms of the thermochemical calorie defined as exactly 4.1840 abs. j . The ice point is taken as 273.15"K. and the molecular weight of triethylenediamine as 112.172 g. Corrections to the observed values of A H I A T were applied to adjust for the analytically determined curvature correction, when applicable, and for the sublimation (or vaporization) of the sample a t higher temperatures. These correctioris have magnitudes of approximately 0.01 and 0.03% as maximum values, respectively. The basic logic of the vaporization correction is patterned after that devised by Hoges but was adapted for digital computer application.5 The approximate temperature increments used in the measurements usually can be inferred from the adjacent mean temperatures in Table I. These data are considered to have a probable error of approximately 0.0870 over the entire range on the basis of the statistical analysis of the deviations from the smoothed curve calibration runs with synthetic sapphire and 011 several separate series of runs on the heater-therinom(6) H. G. Carlson, "Thermodvnamic Properties of Methyl 8lcoho1, 2Methy1-2,5-dimethylthiopheneand 9-Methylfuran," Doctoral Thesis, University of Michigan, Ann Arbor, hlichigan, 1963; U. S. iZtomic Energy Commission Report TID-15153, 1962. (7) E. F. Westrum, Jr., J. B. Hatcher, and D. Ti-. Osborne, J . Chem. PRZ(5.. 21, 419 (1953). (8) G.T.Furukawa, T. B. Douglas, R. E. McCoskey. and D. C. Ginnings, J . Res. 'VaVatZ. Bur. Std., 67, 67 (1956). (9) H. J. Hoge, zbzd., 36, 111 (1946).
TRANSITION AKD :FUSION OF TRIETHYLENEDIAMISE
s o v . , 1963
eter-calorimeter assembly. The smoothed heat capacities a t saturation obtained by a digital compui er f,t of the experimental data (which accords excellen1,ly with values read from curves on large-scale plots) are presented in Table 11. TABLE I HEATC.WACITYOF TRIETHYLENEDIAMINE [Ynits: cal., mole, OK.] -
~
T
CS
T
37.06 38.74 40.65 42.86 45.50
397.65 407.03 416.34 423.61 428.66 4'29.60 430.54 431.64 432.44
Series I
302.43 311.66 321.34 331.10 340.79
Transition runs A
358.51 368.74 378.97 389.42 400.02 410.64 421.28
50.58 51.46 52.23 53.16 54.20 55.19 56.35 Fusion runs B 427.62 57.51 Series I1
320.03 326.63 334.36 341.64 347.60
--Transition 1
348.34 351.06 351.13 352.21 350.58 351.09 351.16 352.04 350.69 351.86 350.67 364.44
Series I11
344.76 348.09 349.94
50.16 50.31 50.40 51.00 51.67 52.46 53.19
354.02 357.49 365.80 376.83
50 J
0
5
t /
0
Fusion runs J Series VI1
433.66 434.19 434.54 435.07 438.21 441.91 443.80 445.83 448.01
57.23 57.26 57.26 57.47 57.70 57.96 58.19 58.36 58.64
299.89 36.44 303.33 37.14 308.09 37.94 31.1.43 38.66 319.33 40.23 328.56 42.09 338.18 44.82 AHt run H
Transition runs K
C80
-
This Research
Series VI
Series VI11
-Fusion T
60
49.86 50.E14 51.516 52.20
Series IV
runs-AT
46.38 47.74 47.88
C.
L
Series J'
57.37 57.24 5T.78
Transition runs E Enthalpy run F AHf run G
40.09 41.62 43.62 45.64 47.83
Transition runs C
355.29 358.30 359.55 363.97 372.45 381.18 389.34
53.95 54.83 55.81 56.33 56.56 56.96 56.76 57.91 64.55
:Fusion runs D
434.59 436.25 438.94
T
C.
2383
383.49
52.70
Series IX Fusion runs L
runsAT
T.'K.
Fig. 2.-Heat capacity of tiiethylenediamine: 0: data froin cryogenic adiabatic calorimeter of Chang and tI-estruni3; C , from this research. TABLEI1 THERMODYNAMIC FUNCTIOKS OF TRIETHYLENEDIAJIISE (C6HI2S2:1 mole = 112.172g.; units: cal., mole, "K.j T
C,
SO
Ho
- H""
-(Go
- Hoo)/T
Crystal I1
298.15" 300 320 340 351.08
36.49 36.80 40.35 45.13 (48.80)
37.67 37.90 40.38 42.96 44.43
351.08 360 380 400 420 432.98
(49.90) 50.62 52.34 54.17 56.04 (56.90)
51.61 52.90 55.69 58.42 61.11 62.84
432.98 440 450
(57.16) 57.83 58.79
66.95 67.86 69.17
5624.8 5592.6 6363.1 7214.7 7723.3
19.14 19.25 20.50 21.74 22.43
Crvstal I
10244 10707 11737 12802 13904 14636
22.43 23.16 24.80 26.42 28.00 29.04
16412 16817 17400
29.04 29.64 30.50
Liquid
e."
Runs A Runs B 5.433 (97.22) 429.82 6.414 (104.3) 433.01 0.018 454 0.058 13400 .131 7030 433.71 1.405 (524.5) 2.279 (288.1) Runs C Runs D 301 432.79 0.207 175 0.970 .lo2 15800 433.61 1.416 (1291) ,156 3010 (157.5) Runs J 1.762 Runs E 432.38 1.212 (515.8) 432.99 0.015 52500 0.528 97.8 434.48 2.973 (203.2) Runs L 1.826 (1413) Runs IC (Cf.Table 111) 0.300 82.1 27.23 (98.10)
a T'alues in parentheses involve finite temperature incremer ts in regions of high curvature.
Enthalpy of I1 + I Transition.-The sharp and seerniiigly first-order transition at 351.08"K. is characterized by apparent heat capacities as high as 15,800 cal./(mole"K.) and thermal equilibrium within an hour after energy input. If the lattice contributions to the
a
From Chang and Westrum.3
heat capacity (as depicted by the dashed ( w v o ill Fig. 2 ) are deducted from the apparent heat capacity, a transitional enthalpy increment of 2524 cal /mole and a corresponding entropy increment of 7.19 cal.1 (mole O K . ) are obtained. These probably represent minimal values since a maximum value of lattice heat capacity probably has been taken. Fusion Transition and Purity of the Sample.-Fusion occurs at 432.98"K. with an enthalpy increment of 1776 cal./mole and an entropy increment of 4.10 cal./ (mole OK.) upon subtraction of the lattice contributions. hIolal heat capacities as high as 58,000 cal./(mole OK.) were observed in the fusion regioii and thermal equilibrium was readily attained within 1 hi. The near absence of premelting permitted applicafor the detion of the method of Tunnicliff and terminations of sample purity using the data listed in Table 111. The linearity of the melting temperature us. reciprocal fraction melted (118') implies absence of (10) D. D. Tunnicliff and H. Stone, Anal. Chern., 27, 73 (1955).
2384
JOHN C. TROWBRIDGE AND EDGAR E'. WESTRUM, JR.
solid solutioii formation and indicates 0.003 mole yo liquid-soluble solid-insoluble impurity in this sample. Extrapolation of this plot also furnishes the true triple point temperature of this sample and that of the pure substance as 432.98 and 432.9goK., respectively.
-
TABLE 1%' ENTHALPY AXD ENTROPY INCREMENTS FOR TRIETHYLENEDIAMINE [Units: cal., mole, OK.] No. of
TABLE 111 FRACTIONAL FUSION OF TRIETHYLENEDIAMINE [Units: cal., mole, OK.]
T
AT
430 46 432.94 432.97 432.98 432.98 433.54
4.953 0.054 ,009 ,007 .002 1.048
AHeacess
Fusion 69.13 7530 42530 58230 1690 70 37
1/F
runs L 60.9 29.153 468.5 3.791 856.7 2.074 1294.2 1 3 7 3 1749.1 1.016 1776.7
Triple point: this sample pure compound
Tfinal
432.909 432.964 432.973 432.980 432.982 434.030
Series 11(C) Runs A Series I11 (E) Run R Runs K
7 4 5 1 2
-
H'sas ?'final
Tinitial
H"a4a
Transition 357.66 338.18 3011.1 353.35 345.62 (3008.3)" 3010.9 354.29 342.96 3010.6 378.06 350.54 3010.9 Average 3010.9
-
H'aao
Series I1 (D) Runs B Run G Runs J Runs L
Discussion The pseudo-rotational transitioir observed a t 351.08OK. is found at a temperature approximately 4' higher than previously reported by Farkas, et The higher temperature is attributed to the careful purification of the sample and its handling in the anhydrous nitrogen atmosphere of the drybox to prevent contamination by moisture and carbon dioxide. The fractional melting studies attest to the results achieved. The crystal I1 phase of triethylenediamine stable below 351.08OK. has been subjected to room temperature X-ray analysis by Wada, et al.,la who also made .
runs
H 4 '4po
Thermodynamic Functions.-Molal values of the heat capacity a t saturation (Cs),the entropy at saturation (S,'), the enthalpy increment (H" - Hoe), and the free energy function [ ( G O - H o O ) / T have ] been computed by numerical quadrature of the heat capacity plus values associated with phase transitions and are tabulated a t selected temperatures in Table 11. The normal heat capacity values and the thermodynamic functions are considered to have a probable error of less than 0.1%. In order to make the tables internally consistent and to permit interpolation, the tabulated values occasionally include more digits than are justified by the probable error. The entropy and free energy functions do not include contributions from nuclear spin and isotope mixing and are hence practical values for chemical thermodynamic purposes. As a further test of the precision of the calorimetry and the trend of the heat capacity curve in the transition regions, enthalpy increments, each entirely spanning the respective transition regions or a portion of the crystal I region (solid phase stable immediately below the melting point), were made and compared with heat capacity-type determinations as shown in Table IT. Excellent agreement was obtained for transitions and enthalpies over heat capacity regions. The listed enthalpy changes in these regions are a direct summation of the energies used while entropy increments have been integrated from heat capacity data except in regions of extremely high heat capacities where the process is assumed to be isothermal.
1
Designation
Fusion
(1,000) (432.98) (0.000) (432.99)
~
Vol. 67
~
(11) A. Farkas, G. A. Mills, W. E. Emer, and J . B. Maerker, Ind. Ens. Chem., 51, 1299 (1959). (12) A. Farkrts, G. A. Mills, W. E . Erner, and J. B. Maerker. J . Chem. Ens. Data, 4, 334 (1959).
~
9 4 1 3 6
440.81 440.84 435.97 439.97 434.03
429.98 426.61 429.34 431.78 427.99 Average
2348.4 (2352.9)" 2350.1 2348.2 2349.8 2349.1
-
8'866
SO846
8.569 (8.561)" 8.568 8.568 8.568 8.568 8'440
-
So48G
5.423 (5.433)" 5.428 5.423 5.427 5.425
Crystal I H04ao
-
Ho86S
Series I Series I1 Run F
7
426.60 353.36 4010.4 431.09 352.91 4009.8 429.38 352.77 4014.3 Average 401I.5 Numerical quadrature of smoothed curve: 4013.1 a Rejected from average by Chauvenet's criterion.
8
'
16 1
differential thermal studies on the transition first reported by Farkas, et a1.,I2 and on fusion. The structure of crystal I is not yet available. These data, combined with the information obtained from this research, still are not adequate to account for the magnitude of the entropy increment of the solid-solid transition. According to Guthrie and iUcCullough, l4 molecular orientations that are energetically favorable usually will be among those for which the approximate symmetry elements of the molecule are aligned with the symmetry elements of the lattice. Thus, for favorable orientations the effective point symmetry of the molecule will be a sub-point group of the crystal symmetry a t the lattice site, and the number of possible distinguishable orientations can be estimated from symmetry considerations. Using this theory an entropy term of only R In 8 [Le., 4.13 cal./(mole OK.)] might be anticipated for triethylenediamine, whereas a value of 7.19 cal./ (mole OK.) was found experimentally. The calculated value would arise from assuming a molecular symmetry ~ finding the most likely distinguishable orienof D Band tations of the molecule a t an o h lattice site. With this assignment the threefold molecular axis could be aligned with each of the four threefold axes of the cubic lattice in two different ways differing by a 60' rotation about the molecular axis. Hendrickson's concludes from molecular orbital calculations that the bicyclo [2,2,2]octane molecule would be energetically most favorably arranged in a slightly twisted conformation allowing the hydrogen atoms on the ethylene groups to be situated a t positions of mini~
(13) T. Wada, E. Kishida, Y. Tomiie, H. Suga, 9. Seki, and I. Nitta, Bull. Chem. SOC.Japan, 33, 1317 (1960). (14) G.B. Guthrie and J. P. McCullough, J. Phys. Chem. Solzda, IS 53 (1961). (15) J. B. Hendriokson, Chem. Eng. News, 39, 47,40 (1961).
Nov., 1963
HEATCAPACITY AND THERMODYNAMIC PROPERTIES OF NbB1.963
mum potential energy rather than a t the less favorable D3h configuration. This twisting reduces the molecular symmetry to D3 which leads to several interesting ramifications. Wada, et aZ.,13reported the space group of the crystal 11 phase to be C26h, which if true would preclude the twisted form in this phase region, but twisting could take place a t the trarisition temperature and exist in the crystal I transition entropy increment. This would increase the transitional entropy contribution to R In 16 [;.e., 5.51 cal./(mole OK.)] since both a right-handed and left-handed twist would be equally probable. However, X-ray analysis distinguishes conclusively only with difficulty between C26h and C66,the latter of which is the space group for Ds molecular symmetry in this case. If the true molecular symmetry is D3 a t room temperature, right-handed and left-handed forms may exist in the crystal I1 phase without producing much strain. Since no thermal anomalies have been observed below room temperature, it is possible that a residual entropy may exist a t extremely low temperatures in the amount R In 2. Availability of spectral data would permit an independent evaluation of the entropy and resolution of any ambiguity. The entropy values reported by Wada, et CLZ.,'~ for the crystal I1 + I transition and fusion do not agree with those of this research. They reported the increments to be 6.62 cal./(mole OK.) a t 353.0°K. and 3.3 cal./ (mole OK.) a t 434.3"K., respectively, in contrast to corresponding increments from this study of 7.19 cal./ (mole OK.) a t 351.09OK. and 4.10 cal./(mole OK.) a t 432.98OK., respectively. Their values are based on sublimation and vapor pressure data. Experience has shown that discord between such data and the more accurate calorimetric values often obtains16unless pressure data of high excellence are used. Entropies of transition of other hoinologous bicyclic (16) E.g., cf. data on transition hexafluorides reported b y E. F. Westrum, Jr., summarily in J. Chem. Educ., 89, 443 (1962).
2385
compounds have also been reported. Bicyclo [2,2,1]heptane14 and 3-azabicyclo [3,2,2]n0nane'~have been studied thermally yielding transitional entropies of 7.53 cal./(mole OK.) a t 131.7OK. and 11.63 cal./(mole OK.) a t 297.7S°K., respectively. Both compounds would be expected to have identical C2, molecular symmetry and are presumed to be in the plastically crystalline region above their transitions. Therefore, symmetry consideratiom alone will not account for the magnitudes of the anomalies. Work presently underway in this Laboratory to investigate the fusion thermal effects of 3-azabicyclo [3,2,2]nonane may provide some insight into the mechanism of the solid-state transition. Re~ ~ ezo-28 cent unpublished studies by Kolesov, et ~ 1 . on cyanobicyclo[2,2,1]heptane give an enthalpy of transition of 7.97 cal./(mole OK.) a t 237.7OK. and an entropy of melting of 2.34 cal./(mole OK.) a t 300.27OK. The authors consider that the family of which bicyclooctane is the prototype and triethylenediamine a member provide unusually fertile opportunities to study plastically crystalline behavior in t e r m of availability, opportunities to modify the symmetry and ligands. More studies by X-ray diffraction and infrared and Raman spectroscopy of crystals I and I1 would be obvious desiderata. Acknowledgment.-The authors thank Dr. G. T. Furukawa for helpful advice concerning the design of the automa,tic shield control apparatus and the closure on the W-22 calorimeter, and Drs. G. A. Mills and A. Farkas and Houdry Process Corporation for the triethylenediamine sample. The generous assistance of H. G. Carlson i~ the construction of the calorimeter and of Dr. B. H. Justice in the numerical analysis of the data is recognized. The authors are indebted to the United States Atomic Energy Commission for facilities and funds, and J. C. T. is grateful to the Monsanto Chemical Company for a research fellowship. (17) C. M. Barber and E. F. Westrum, Jr , J. Phgs. Chem., 61, 2373 (1963). (181 V. P. Kolesov, E. A. Seregin, and S. M. Skuratov, Luginin Thermochemistry Laboratory, iMoscow State University, personal oommunication.
NbB,.,,, : THE HEAT CAPACITY AND THERMODYNAMIC PROPERTIES FROM 5 TO 350'K. BY EDGARF. WESTRUM, JR., Department of Chemistry, Universilg of Michigan, A n n Arbor, Michigan AND
GERALD A. CLAY
Arthur D. Little, Inc., Cambridge, Massachusetts Received M a y $8,1963
The heat capacity of a zone-melted nonstoichiometric NbBl.gm has been measured by adiabatic cryogenic calorimetry and found t o have a normal sigmoid temperature dependence without transitions or thermal anomalies. Values of the heat capacity a t constant pressure (ep), the entropy (So),the enthalpy function ( H O H'o), and the Gibbs free energy function ( - [ao H 0 o ] / T ) are 11.42, 8.91, 5.433, and 3.478 cal./(g.f.m. OK.), respectively, a t 2!38.15"K.
-
Introduction Increasing demand far thermodynamic data on carbides, borides, and related compositions of the group IV elements as a consequence of technological developments in nuclear reactors, missiles, and other high tem-
-
perature applications of refractory materials has Prompted the present endeavor to P r m ~ r ereliable thermodynamic data in the cryogenic range on one of these materials, nonstoichiometric NbB,. 963, by adiabatic calorimetry.
