J. Phys. Chem. 1982, 86, 443-447
spectra of secondary electrons emitted from n-C4Hw thin film. They observed that the spectral features associated with the high density-of-states regions of the conduction bands of crystalline n-C4Hw are smeared out sharply at the bulk melting point, Le., the conduction band features are only observed for crystalline samples. Their results suggest that the cyclopentane and cyclododecane samples deposited on the metal surface in situ under vacuum at -80 K glassy to some extent. A more detailed investigation of the conduction band structure of alkanes, alkenes, and aromatics and of the dynamic processes of molecules in the films of these compounds is now in progress.6 Another characteristic difference of the transmission spectra between C5,12and C, cycloparaffmsis that a broad negative peak appearing at -14 eV for cyclopentane is considerably smaller than those for CW cycloparaffms and is almost missing of cyclododecane. This broad negative peak is observed for all compounds of alkanes, alkenes, ethers, alcohols, and ketones in our experiments with the exception of only two compounds. One exception is ice2
(6) K. Hiraoka and M. Nara, to be submitted for publication in J. Phys. Chem.
443
and the other is cyclododecane. The transmission spectra of these compounds are similar in that they are markedly structureless compared to those of other compounds. This may be owing to the more or less glassy structure of films of these two compounds. Because the broad negative peak is due to the ionization of molecules in a film,2i3 the scattering processes of incident and ejected electrons in crystalline solids must be quite different from those in glassy solids. When the reflection coefficient of electrons at the film-vacuum interface ( R ) is small, a discernible negative peak is expected in the transmission spectra. In other words, the larger the reflection coefficient is, the less prominent the negative peak becomes. The increase of the value of R in glassy solids may be due to the efficient energy loss of electrons in the solids, Le., the mean free path of electron-phonon scattering is much shorter in glassy solids than in crystalline solids. In the case of V,, I 0, after an electron cascades to the bottom of the conduction band by losing its kinetic energy by a severe electron-phonon scattering in glassy solid, it can hardly escape to the vacuum due to the energy barrier (-Vo) and to the surface potential of the solida3This may account for the absence of a broad negative peak in the transmission spectra of cyclododecane and ice.
ARTICLES Group Additivity Parameters for the Estimation of Thermochemical Properties of Gaseous and Liquid Nltrlles James Y. Chu, Tam T. Nguyen, and Kelth D. Klng" Department of Chemical Engineering, University of Adale&, Adele&, In F h d FOrm: JU/y 27, 1981)
South Austral& 5001 (Received: April 2, 1981;
Contributions of the CN-containing groups in.the estimation of thermochemical properties (AH?,So,and Cpo) of gaseous and liquid nitriles according to the group additivity method are assessed. Taking into account the more recent data, existing group values are revised and some new group contributions are evaluated for the gas phase. Group contributions are derived for the fist time for these compounds in the liquid state. In general, the data are quite self-consistent, but further measurements of AHf0 are needed for a better evaluation of significant next-to-nearest-neighborinteractions in unsaturated nitriles and polycyano compounds. Introduction A group additivity scheme for the estimation of thermochemical properties (AH,',So, C ") of compounds in the ideal gas state was proposed by benson and Buss' in 0022-3654/82/2086-0443$01.25/0
1958. Some 10 years later the procedure was extended and applied to a large variety of molecules by Benson and (1) Benson, S. W.; Buss, J. H.J. Chem. Phys. 1968,29, 546.
0 1982 American Chemical Society
444
The Journal of Physical Chemistry, Vol. 86, No. 4, 1982
Chu et ai.
TABLE I: Group Contributions to AHf",,, (kcal/mol), Sozy8 (cal/(mol K)), and C p o T (cal/(mol K)) for Gaseous Nitriles
c~' T group
so
Hf02ss
300K
400K
500K
600K
800K
1OOOK
40.2' 19.8miy .-2.9"'Y 28.3°,y
11.4f 10.8u8y
13.6 12.9
15.3 14.5
16.9 15.8
19.3 17.2
20.5 18.9
37.P
10.3'
12.0
13.4
14.6
16.4
17.6
34.94 20.5'9Y - 2.8'
10.5wry 10.OX'Y
11.7 11.5
12.5 12.6
13.2 13.3
14.3 14.3
15.2 15.0
198
22.5a 25.8'3 2 9.8'1 >' 70.1d3N 1 1 4. j e r Y 22.gf 25.ggsY 37.2h (81.2
y
63.8.'~~
35.8k34 --3.gz (SIZ O.lb'
(-3.7)C ringcompound correctionsaa
ring-compound correctionsm
--32.1
27.2
25.8
23.1
54.7
3.9
27.4
-2.0
26.3
'
Average from alkanenitriles.1,8-12 From isobutyronitrile.8 From pivalonitrile.8 From 1,1,2,2-cyclopropanetetra~ a r b o n i t r i l e and ' ~ ringcorrection value for cyclopropanecarbonitrile. e From l , l , l - t r i ~ y a n o e t h a n e . f~ From ~ 3-butenonitrile and trans-3-pentenonitrile.'" g From l,4-dicyanobut-2-yne.l3 Average of three values for a ~ r y l o n i t r i l e . ~ * ~ ~Esti,'' mates from measurements for tricyanoethyleneI6 and t e t r a ~ y a n o e t h y l e n e ' ~and + assuming that the cis CN-CN correction is given by A ( A ( AHf)) for the series tetracyanoethylene, tricyanoethylene, trans-1,2-dicyanoethylene,acrylonitrile (cyanoFrom b e n ~ o n i t r i l e . ~ Average of six values from alkanenitriles.7+-2' From ethylene). From dicyanoacetylene. l 7 O ~From dimethylmalononitrile.24 P Average value from a c r y l ~ n i t r i l e ' ~ Jand ~*'~ isobutyronitrile.22 From p i ~ a l o n i t r i l e . ~ truns-crotononitrile.22 4 Average value from d i ~ y a n o a c e t y l e n e ' +and ~ ~ l - ~ y a n o p r o p y n e . ~From ~ benzonitrile.28 DeterAverage value from p r ~ p i o n i t r i l e ' *and '~ mined by fitting predicted value to observed value for cis-crotononitrile. 2 2 n-butyronitrile,' From i s ~ b u t y r o n i t r i l e . ~" Average from acrylonitrile.'4Js From l-cyanopropyne." From benzoFrom single source of data. nitrile.7 Assigned by fitting predicted values t o observed values for cis-crotononitrile" and From alkanenitriles. c c Esticis-l-butenyl cyanide.'O aa Assigned by fitting predicted values t o observed values. mate from 1,l,l-tricyanobut-3-ene.l 3 2*8~16329
co-workers2and shown to reproduce the thermochemical properties to essentially within experimental uncertainty. The present report is concerned with the contributions of CN-containinggroups in the estimation of thermochemical properties of gaseous and liquid nitriles. Group values now in use for the nitrile compounds in the gas phase were compiled in the above review article by Benson and coworkers2over 10 years ago; the extant experimental data at that time were particularly sparse and few of the possible group contributions could be evaluated and crosschecked. There has been a substantial increase of experimental data since then, and a reassessment at this time seems warranted. Our interest in the thermochemistry of nitriles stems from recent studies3in this laboratory of the kinetics and mechanism of the thermal decomposition of selected nitriles. The relationship between thermochem-
istry and kinetics and the importance of the former in helping to understand the latter has been treated in detail by bnson.4 The aim of this report is twofold: (i) to revise the existing group values and add new types of group contributions for gaseous nitriles, and (ii) to evaluate group values for the first time for the liquid compounds. The literature up to mid-1980 was surveyed.
(2) Benson, S. W.; Cruickshank, F. R.; Golden, D. M.; Haugen, G . R.; O'Neil, H. E.; Rodgers, A. S.; Shaw, R.; Walsh, R. Chem. Reu. 1969,69,
Results The basic concept and application of the group additivity rules for estimation of thermochemical data has been well d ~ c u m e n t e d . ' The ~ ~ ~nomenclature ~ and the general presentation adopted here are the same as in the previous report^.^^^ Individual group values contributing to heats of formation (AHHfOB8),entropies (Som8),and heat capacities (Cpor)of nitriles were evaluated. For gaseous nitriles the estimation of CN-containing groups was accomplished with the use of the listed group values for gaseous hydrocarbon~.~ Shaw,6 and Luria and Bensons have dem-
279. (3) King, K. D.; Goddard, R. D. Int. J. Chem. Kinet. 1981,13,755; J . Phys. Chem. 1978,82,1675; Int. J. Chem. Kinet. 1978,10,453; J. Phys. Chem. 1976,80,546; Int. J.Chem. Kinet. 1975,7,837; J. Am. Chem. SOC. 1975,97,4504; Int. J . Chem. Kinet. 1975, 7,109. King, K. D. J . Chem. SOC.,Faraday Trans. 1 1978, 74, 912.
(4) Benson, S. W. "Thermochemical Kinetics"; Wiley: New York, 1976. (5) Shaw, R. J . Chem. Eng. Data 1969, 14,461. (6) Luria, M.; Benson, S. W. J. Chem. Eng. Data 1977, 22, 90.
The Journal of Physical Chemistry, Vol. 86, No. 4, 1982
Thermochemical Properties of Nitriles
TABLE 11: Group Contributions to
445
AH^^^^^ (kcal/mol), So2,,(cal/(mol K)), and CpoT (cal/(mol K ) ) for Liquid Nitriles C,"
group C-(H 1 1 (C )(CN) C-(H)(C),(CN) C-(C),(CN 1 C-(H),(Cd)(CN) cd-(H)(cN) C-( H),(CB)(CN) CB-(CN) Ct-(CN) C-(C),(CN), cd-(c)(cN) cis alkyl-CN alkyl-CN gauche CN-CN gauche ortho correction
AHfo,, 15.2a 20.lb 25.3c9u 15. 5d 29.3e 17.5f9u 29.9g 60.3h,'
so
250K
270K
290 K
300K
25.2'
18.4'
18.7
19.0
19.1 19.gm 16.9"~'
16.9
16.7
16.4
18.3
27.9Olu 19.1
28.0 19.7
27.9 20.3
298
-3.1i1'
5.Sk9" 17.2p
17.7
18.1
310K
330K
350K 19.4
- 2.20 0.8' ( 2.5js 0.1
ring-compound correctionsU
AH;
group
198
- 7.4
LLCN 22.8 U
C
Cp0298
-7.0
N
DCN
OCN
27.6
2.5
4.5
-9.2
-0.9
-14.0
a Average of 1 0 values from alkanenitriles and alkanedinitriles.8"2qm F r o m t w o determinations for isobutyronitrile.8vy From 3 - b ~ t e n o n i t r i l e ' and ~ * ~trans-3-penten0nitrile.'~e Average of values from a c r y l ~ n i t r i l e , ~ * ' ~ From pivalonitrile.8 ~" o-t~lunitrile.~' truns-crotononitrile,lo*M and trans-2-pentenonitrile.lo f From benzylnitrile.% g From b e n ~ o n i t r i l e ~and From dicyanoacetylene. From p r ~ p i o n i t r i l eand ~ ? ~glutaronitrile.*l J From p i v a l ~ n i t r i l e . ~ ~From dimethylmaloniO~ From dimethyltrile.24 From propionitrile18 and g1utaronitrile.l' From isobutyronitrile.8 From p i v a l ~ n i t r i l e . ~ m a l ~ n o n i t r i l e . ~p ~From a ~ r y l o n i t r i l e . ' ~ * ~ 'From cis-crotononitrile'O and cis-2-pentenonitrile." Average of three values Estimate from 1 , 2 - d i ~ y a n o e t h a n e . ~Assigned to be the same as that calculated for 0-xylene.? from alkane nit rile^.^^'^^^^ Ring corrections were assigned by fitting predicted values to the observed values;6 for Cpo a value of 19.9 was obtained for the group C-(H)(C),(CN) from isobutyronitrile.8 ' From single source of data. Q
onstrated that heat capacities of liquids (mainly hydrocarbons) can be accurately estimated by using the group additivity approach. We have applied the method not only to the heat capacities of liquid nitriles but also to AH; and So. For liquid nitriles, the required hydrocarbon group values for AHfo and So were derived from liquid-hydrocarbon data tabulated by Stull, Westrum, and Sinke,' and the hydrocarbon group values for Cpo were taken from the report by Luria and Benson? When more than one value for a CN-containiig group were available, these values were averaged. This process did not include any group values which seem questionable in the sense that they differ significantly from other values obtained for the same group. The group values obtained in this work are tabulated in Tables I and 11. Group additivity does not take into account next-to-nearest-neighbor interactions, such as cis effects or ring strain. Structural corrections for these effects are also given in Tables I and 11. For example, ring corrections were determined from the difference between the experimental results and the predicted values based on group additivity applied to noncyclic compounds. Comparison of observed and estimated thermochemical properties of some gaseous and liquid nitriles is presented in Tables I11 and IV. This is not a comprehensive tabulation of all species considered but rather a selective one which is illustrative of the use of the whole range of evaluated nitrile group contributions. The higher members (7) Stull, D. R.; Westrum, E. F.; Sinke, G. C. 'The Chemical Thermodynamics of Organic Compounds"; Academic Press: London, 1970.
of a homologous series usually show the same agreement between estimated and observed values as the lower members since no new groups are involved. For example, in the alkanenitrile series, heats of formation of octanenitrile, decanenitrile, undecanenitrile, and tetradecane(8) Hall, H. K.; Baldt, J. H. J. Am. Chem. SOC. 1971,93, 140. (9) Evans, F. W.; Skinner, H. A. Trans. Faraday SOC.1959,55, 255. (10) Konicek, J.; Prochazaka, M.; Krestanova, V.; Smisek, M. Collect. Czech. Chem. Commun. 1969,34, 2249. (11) Lebedeva, N. D.; Katin, Yu. A. Zh.Fiz. Khim. 1973, 47, 1620. (12) Stridh, C.; Sunner, S.; Svensson,Ch. J. Chem. Thermodyn. 1977, 9, 1005. (13) Frankel, M. B.; Adolph, A. B.; Wilson, E. R.; McCormick, M.; McEarchen M. Adv. Chem. Ser. 1965, No. 54, 108. (14) Finke, H. L.; Messerly, J. F.; Todd. S. S. J. Chem. Thermodyn. 1972. 4. 359. (15)'Davis, H. S.; Wiedman, 0. F. Znd. Eng. Chem. 1945, 37, 482. (16) Boyd, R. H. J. Chem. Phys. 1963,38, 2529. (17) Armstrong, F. T.; Marantz, S. J. Phys. Chem. 1960, 64, 1776. (18) Weber, L. A.; Kilpatrick, J. E. J. Chem. Phys. 1962, 36, 829. (19) Duncan, E. E.; Ja&, G. J. J. Chem. Phys. 1955,23,434. (20) Wulf, C. A,; Westrum, E. F. J. Phys. Chem. 1963,67,2376. (21) Clever, H. L.; Wulf, C. A.; Westrum, E. F. J.Phys. Chem. 1965, 69, 1983. (22) Wyss, H. R.; Gunthard, H. H. Helu. Chim. Acta 1961,44, 625. (23) Westrum, E. F.; Ribner, A. J. Phys. Chem. 1967, 71, 1216. (24) Ribner, A.; Westrum, E. F. J. Phys. Chem. 1967, 71, 1208. (25) Halverson, F.; S t a " , R. F.; Whalen, J. J. J. Chem. Phys. 1948, 16, 808. (26) Stull, D. R. "JANAF Interim Thermochemical Data"; Dow Chemical Company: Midland, MI, 1960-1966. (27) Tubmo, R.; Dellepiene,G.; Zerbi, G. J. Chem. Phys. 1969,50,621. (28) Green, J. H. S. Spectrochim. Acta 1961, 17, 607. (29) Daly, L. H.; Wiberley, S. E. J. Mol. Spectrosc. 1958, 2, 177. (30) Kharash, M. S. J. Res. Natl. Bur. Stand. (US.)1929, 2, 359. (31) Moffat, J. B. J. Chem. Eng. Data 1968, 13, 36; 1969, 14, 215. (32) Boyd, R. H.; Guha, K. R.; Wuthrich, R. J . Phys. Chem. 1967,71, 2187.
446
The Journal of Physical Chemistry, Vol. 86, No. 4, 1982
Chu et al.
TABLE 111: Comparison of Heats of Formation ( L M ~ 'Entropies , ~ ~ ) , (Soz98), and Heat Capacities (C,",) with Those Estimated bv Using Group Additivitya A Hf"
compd CH,CH,CN
n-~;~,d~ i-C,H,CN n-C,H,CN CH,C(CN), (CH3 )ZC(CN)Z (CH,),CCN NC(CH,),CN NC(CH,),CN H,C=CHCN trans-CH,CH=CHCN cis-CH,CH=CHCN trans-NCCH=CHCN (NC),C=C(CN), CH,=CHCH,CN NC(CH,),CN NCCH=C(CN), NCC= CCN CH,C=CCN NCCH,C=CCH,CN CH,=CHCH,C(CN), c-C,H,CN c-C,H2-1,1,2,2-(CN), c-C,H,CN c-C,H,CN c -C HI CN C,H,CN
of Gaseous Nitriles
S"
CPO
obsd
est
Ab
ref
obsd
est
Ab
ref
obsd
est
Ab
ref
12.3 7.5 5.4 2.7 104.4
12.3 7.5 5.5 2.5
0 0 0.1 0.2
8 8 8 10 13
68.8 77.8 74.9
68.5 77.9 74.9
0.3 -0.1 0
18 7 7
17.3 23.2 23.1
17.5 23.0 23.1
-0.2 0.2 0
19 7 7
- 0.8 40.9 35.7 43.9 35.8 32.0 81.3 171.4 37.7
-0.8 40.2 35.2 43.5 35.6 32.1 74.4 172.4 37.8
0 0.7 0.5 0.4 0.2 -0.1 6.9 -1.0 -0.1
8 7 11 14 22 10 32 13 10
83.4 79.6 88.1
83.4 79.6 88.5
0 0 -0.4
24 23 21
65.9 71.4
64.9 73.5
1.0 -2.1
14 22
15.3
15.4
-0.1
25
79.1
79.1
0
20
123.9 127.5
123.4 127.6
0.5 -0.1
16 17
69.4 68.6
68.5 69.5
0.9 -0.9
26 27
19.8
19.8
0
27
107.0 115.3 43.2 162.5 34.2 10.0 - 0.9 52.3
106.9 d e e e e e e
0.1
13 13 7 13 7 7 7 9
10.1
70.7
0
28
C
76.7 76.7 0 28 26.2 26.2 0 7 AHf' in kcal/mol; S o and C P o in cal/(mol K). A = observed - estimated. ' Sole source of data for the group The difference between the group value and the observed value was used t o estimate the vinyl-CN gauche C-(C)(CN),. interaction. e Ring corrections for these cyclic compounds were determined from the difference between the group value and the observed value. The estimated value therefore must necessarily be equal t o t h e observed value, a
TABLE IV: Comparison of Heats of Formation AH^",,,), Entropies (Saz9*), and Heat Capacities ( C p o T )of Liquid Nitriles with Those Estimated by Using Group Additivitya compd
obsd
CH,CH,CN CH,CH,CH,CN NCCH,CH,CN NC(CH,),CN CH3(CH2)6CN (CH,),CHCN (NC),CCH,CH=CH, (CH3)2C(CN)2 (CH,),CCN trans-CH,CH=CHCH,CN CH,=CHCH,CN (CN),C=CHCN CH,=CHCN trans-CH,CH=CHCN cis-CH,CH=CHCN trans-C,H,CH=CHCN cis-C,H,CH=CHCN C,H,CN o-C6H5(CH3)(CN) NCCsCCN C,H,CH,CN
3.7 -1.4 32.9 24.4 -25.7 -3.3 110.3 - 9.5 19.3 28.1 108.2 35.2 24.1 22.7 17.9 17.2 39.0 30.9 120.6
A Hf" est Ab
3.6 -1.7
0.1 0.3
e 25.9 -26.1 -3.1
f
-1.5 0.4 -0.2
19.0 23.2
0.3 -0.1
34.5 25.2 23.1 19.0 16.8 39.7 31.0
0.7 -1.1 -0.4 -1.1 0.4 -0.7 -0.1
32.0
So
cpoT
ref
obsd
est
Ab
ref
8 9 30 30 12 8 13
45.4
45.5
-0.1
18
28.6,,
57.2
57.0
0.2
21
44.9 55.5
44.9' 55.5'
24 23
8 10 10 16 8 10 16 10 10 9 30 17
obsdd
estd
Ab
ref
27.8,,
0.8
18
38.5, 43.9,,,
38.2, 45.5,
0.3 -1.6
20 21
46.5,,, 43.0,,
46.5,,, 43.0,,,'
0
24 23
26.1,,
25.5,,
0.6
14
30
AHf" in kcal/mol; S o and CPo in cal/(mol K). A = observed - estimated. ' Sole source of data for a particular group. Therefore the estimated value must necessarily be equal t o the observed value. The temperature is specified as a subscript. e The difference between the group value and the observed value was used t o estimate the CN-CN gauche interaction. There are insufficient data t o determine both the group C-(C)(CN), and the vinyl-CN gauche interaction.
nitrile have been measured12(not listed in Tables I11 and IV), but in terms of the group additivity scheme the only difference between these compounds and n-butyronitrile is the number of C-(C),(H), groups involved. Also, some thermochemical data are available for CH3CN,33,34 CH2(33) Pilcher, G.,unpublished work. (34) Crowder, G.A.;Cook, B. R.J.Phys. Chem. 1967, 71,914.
(CN)2,32935 and C(CN),,36 but these nitriles are not amenable to the Benson and Buss group additivity scheme.' They are best treated either as unique groups2or by bond additivity.l (35) Halverson, F.; Francel, R. J. J. Chem. Phys. 1949, 17,694. (36) Barnes,D.S.;Mortimer, C. T.; Mayer, E. J.Chem. Thermodyn. 1973, 5, 481.
Thermochemical Properties of Nitriles
The Journal of Physical Chemistry, Vol. 86, No. 4, 1982 447
Discussion The group additivity values presented in this report are based on considerably more experimental data than were available previously, but there still is a small amount of data compared with that available for hydrocarbons and most other classes of compounds. Many of the group values listed in Tables I and I1 have been derived from a single source of data. Tables I11 and IV highlight the distribution of the extant data and indicate areas where more measurements, particularly entropy and heat capacity, are needed. There is generally good agreement between estimated and observed values with the deviations for most compounds being f1.5 kcal/mol in AHfo,f l cal/(mol K) in So, and fl.O cal/(mol K) in Cpo. The largest deviations occur for unsaturated nitriles and polycyano compounds. Group additivity seems to work very well for the liquid state, at least for the limited amount of data available. For crotononitrile, CH3CH=CHCN, the cis isomer is considerably more stable than the trans isomer and therefore a stabilizing cis alkyl-CN correction was evaluated. In contrast, the cis isomers of alkenes are less stable than the trans isomers.2 The extra stability of cis nitriles is probably due to a combination of the small steric requirements of the CN group and a strong dipole-dipole attraction between CN and the methyl (or alkyl) group. Clearly, because the CN group is highly there should be a large destabilizing cis CN-CN interaction. There has been no measurement of hHf0(cis-NCCH= CHCN), and the results for tricyanoethylene and tetracyanoethylene are insufficient to enable determination of both a cis CN-CN correction and a value for the group Cd-(CN),. A measurement of AHf0(H2C=C(CN),) would be useful. A rough estimate of the cis CN-CN correction may be obtained by assuming that it is equal to the second-order differences in AHf' in the series (CN)2C= C(CN)2,NCCH=C(CN)2, trans-NCCH=CHCN, H2C= CHCN. This yields a value of -5 kcal/mol (average of 4.9 and 5.2) for the gas phase (there are unsufficient data for the liquid phase) which seems reasonable when compared with the stabilizing cis alkyl-CN correction of -3.5 kcal/mol. There are no measured data to enable evaluation of cis corrections for So and C p o . In view of the large cis interactions in nitriles, stabilizing alkyl-CN and destabilizing CN-CN gauche interactions of magnitude similar to the cis effects might be expected. However, analysis of the data on alkanenitriles reveals that gauche interactions between alkyl substituents and the CN group are destabilizing and small (see Tables I and 11, and
also the work of Stridh, Sunner, and Svensson12). On the other hand, a stabilizing vinyl-CN gauche interaction of -3.7 kcal/mol is indicated, but this is based on one measurement only (see Table I). Likewise, a single measurement of AHf' (NCCH2CH2CN)indicates a destabilizing CN-CN gauche interaction of 2.5 kcal/mol for the liquid phase (the gas-phase value is ca. 8 kcal/mol, which is based on an estimated AHfo(NCCH2CH2CN)of ca. 53 kcal/ mol; this is not shown in the tables). Clearly, more evidence is required with respect to gauche interactions. In the case of fumaronitrile, trans-NCCH=CHCN, the difference between observed and estimated values for AH? is rather large and warrants some comment. The experimental measurement is a modern value from combustion calorimetry,32and it appears to be reliable. It comes from the same laboratory as the measurements for malononitrile,32tricyanoethy1ene,l6and tetracyanoethylene,'6 and the hHfo's in the series acrylonitrile, fumaronitrile, tricyanoethylene,tetracyanoethylene are quite self-consistent. If the difference between observed and estimated values for fumaronitrile is real, then this indicates a large destabilizing interaction consistent with that found for succinotrile, NCCH2CH2CN. A stabilizing effect is expected from a-electron conjugation of CN groups with the carbon-carbon double bond, but, because of the high polarity of the CN groups, the dipoledipole repulsion still is large even for the trans configurati~n~~ and this could account for the difference. Previous evidence has shown that problems may arise with the use of group additivity to predict AH? for very polar compound^.^,^,^ In the Benson and Buss scheme' the CN group is treated as a monovalent unit (or pseudoatom) so that particular low molecular weight compounds cannot be decomposed into more than one group; e.g., CH3CN represents the unique group C-(CN)(H),. Moffat31considered nitriles by taking the carbon atom of the nitrile group as a polyvalent atom of ligancy 2 so that CH3CN becomes two groups, C,-(N,)(C) and C-(H),(C,) where C, and N, refer to the triple-bonded carbon and nitrogen atoms, respectively. This procedure allows some nitriles to be treated by group additivity where they might otherwise have to be treated by bond additivity. The Moffat scheme seemed to work well for some polycyanoacetylenes and polycyanoethylenes (but not for acrylonitrile and fumaronitrile), but this apparent success is almost certainly fortuitous because cis and trans corrections were assumed to be of the same sign and magnitude as originally proposed by Benson and Buss' for alkenes. We have not tested the Moffat scheme on the data in the present compilation.
(37)Cumper, C.W.N.; Dev, S.K.;Landor, S.R.J. C h m . SOC.,Perkin Tram. 2,1973,537.
Acknowledgment. This work was supported by the Australian Research Grants Committee.
