7772
J . Phys. Chem. 1989, 93, 7172-7780
ARTICLES Molecular Structures, Electronic Properties, and Energetics of Fluorinated Allenes and Isomeric Acetylenest David A. Dixon* and Bruce E. Smart* Central Research and Development Department, Experimental Station, E . I . duPont de Nemours & Company Inc., Wilmington, Delaware I9880 (Received: November 8, 1988; In Final Form: May 26, 1989)
The geometries, energies, and various electronic properties of the allenes CH2=C=CH2, CH,=C=CHF, CHF=C=CHF, CH2=C=CF2, CHF=C=CF2, CF2=C=CF2 and the acetylenes CH3C=CH, CH3CrCF, CF3C=CH, and CF3C=CF have been calculated at the SCF level with a double-{ basis set augmented by polarization functions on carbon. Final energies and properties were calculated with a fully polarized double-{ basis set. Final energies were determined at the MP-2 level. The C=C and C-F bond lengths in the allenes decrease with increasing fluorine substitution. The CCC bond angles in CH,=C=CHF, CHF=C=CHF, and CHF=C=CF2 all deviate about 1’ from collinearity. A series of isodesmic reaction enthalpies was calculated to derive heats of formation of all the fluorinated allenes and acetylenes and to evaluate their isodesmic stabilities. The effects of fluorination on the thermodynamic stabilities of allenes, acetylenes, and ethylenes are compared. Mulliken charge distributions, dipole moments, and ionization potentials were calculated for each molecule. The properties of the HOMO and LUMO’s in the various fluorinated allenes and acetylenes are discussed.
Introduction
The molecular properties of fluorocarbons are important in developing an understanding of their chemistry. Owing to a variety of experimental problems, however, it is very difficult to determine heats of formation of fluorocarbons and there are little thermodynamic data for many classes of fluorocarbons.’ Furthermore, good structural information on even simple fluorocarbons can be hard to obtain because of the lack of stable isotopes for fluorine (microwave) and the similarity of C-F and C=C bond lengths (electron diffraction). It has been shown recently that ab initio molecular orbital theory can provide reliable structural and energetic information on fluorocarbons if adequate-size basis sets (augmented double-f) are used.2 Thus, the theoretical methods combined with available experimental data can be used to extend the experimental data on fluorocarbons. Besides structural and energetic information, theory also provides the wave function that can be used to predict reactivity and gain qualitative insights into the bonding in fluorocarbons. There has been considerable recent research on fluoroallenes, especially directed toward elucidating the factors that control regioand stereoselectivities in their [2+213, [4+2],4 and 1,3-dipolars cycloaddition reactions. For example, biradical [2+2] cycloadditions of CH2=C=CHF or CH2=C=CF2 occur predominantly on their fluorinated a-bonds to give products that reflect thermodynamic ~ o n t r o lwhereas ,~ concerted cycloadditions usually occur exclusively on their nonfluorinated a-bonds in accordance with frontier orbital c o n t r ~ l . ~Despite -~ the extensive studies of fluoroallene chemistry,6 there are little accurate experimental structural data and no experimental heats of formation of any fluorinated allenes. Consequently, nothing is known about their thermodynamic reactivities. As part of our detailed theoretical survey of fluorocarbons, we have calculated with an augmented double-f basis set the geometries, energies, and several electronic properties of the allenes: C H 2=C=C H 2, C H 2=C=C H F, CH*=C=C F2, CH F=C=CHF, CHF=C=CF2, and CF2=C=CF2. Heats of formation were obtained from computed enthalpies of suitable isodesmic ‘Contribution N o . 4928
0022-3654/89/2093-7772$01.50/0
reactions. To compare properties, we also studied the isomeric acetylenes: CH3C=CH, CH3C=CF, CF3C=CF, and CF3C= CF. Calculations on CH2=C=CH2, CH2=C=CF2, and CF2=C=CF2 at the 4-31G level have been reported,4d but only data on orbital energies were given. Calculations of the structure and energy of CH2=C=CHF with larger basis sets appeared while this work was in progress.’,* Calculations
The initial calculations were performed with the HONDO program p a ~ k a g eon ~ ~an. ~IBM Model 308 1 computer. The geom-
(1) Smart, B. E. In Molecular Structures and Energetics; Liebman, J. F., Greenberg, A,, Eds.; VCH: Deerfield Beach, FL, 1986; Vol. 3, Chapter 4. (2) (a) Dixon, D. A. J . Phys. Chem. 1988, 92, 86. (b) Dixon, D. A,; Fukunaga, T.; Smart, B. E. J . Am. Chem. SOC.1986,108,4027. (c) Dixon, D. A. J . Phys. Chem. 1986, 90, 54, 2038. (d) Dixon, D. A,; Fukunaga, T.; Smart, B. E. J . Am. Chem. SOC.1986, 108, 1585. (3) (a) Dolbier, W. R., Jr.; Seabury, M. J. J . Am. Chem. SOC.1987, 109, 4393. (b) Dolbier, W. R., Jr.; Seabury, M . J. Tetrahedron Leu. 1987, 28, 1491. (c) Dolbier, W. R., Jr.; Wicks, G. E. J . Am. Chem. Soc. 1985, 107, 3626. (d) Dolbier, W. R., Jr.; Burkholder, C. R. J . Org. Chem. 1984, 49, 238 I. (4) (a) Dolbier, W. R., Jr.; Burkholder, C. R.; Piedrahita, C. A. J . Fluorine Chem. 1982, 20, 637. (b) Dolbier, W. R., Jr.; Burkholder, C . R. Tetrahedron Lett. 1980, 21, 785. (c) Dolbier, W. R., Jr.; Piedrahita, C. A,; Houk, K. N.; Strozier, R. W.; Gandour, R. W. Tetrahedron Letr. 1978, 2231. (d) Domelsmith, L. N.; Houk, K . N.; Piedrahita, c . A,; Dolbier, W. R., Jr. J . A m . Chem. SOC.1978, 100, 6908. (5) (a) Dolbier, W. R., Jr.; Wicks, G. E.; Burkholder, C. R. J . Org. Chem. 1987, 52, 2196. (b) Dolbier, W. R., Jr.; Burkholder, C. R.; Wicks, G . E.; Palenik, G . J.; Gawron, M. J . Am. Chem. SOC.1985, 107, 7183. (c) Dolbier, W. R., Jr.; Burkholder, C. R. Isr. J . Chem. 1985, 26, 115. (d) Blackwell, G. B.; Haszeldine, R. N.; Taylor, D. R. J . Chem. SOC.,Perkin Trans. I 1983, I.
(6) Reviews: (a) Pasto, D. J. Tetrahedron 1984,40,2809. (b) Smart, B. E. In The Chemistry of Functional Groups, Supplement D, Part 2; Patai, S . , Rappoport, 2..Eds.; Wiley: New York, 1983; Chapter 14, p 603. (c) Taylor, D. R. Chem. Rea. 1967. 67. 3 17. (7) Ogata, T.; Fujii, K.; Yoshikawa, M.; Hirota, F. J . Am. Chem. SOC. 1987, 109, 7639. (8) Furet, P.; Matcha, R. L.; Fuchs, R. J . Phys. Chem. 1986, 90, 5571.
0 1989 American Chemical Society
Fluorinated Allenes and Isomeric Acetylenes etries were optimized at the S C F level with use of gradient techniquesIoa* in the following symmetries: H2CCCH2, D2d; HFCCCH2, C,; F2CCCH2, Chi HFCCCHF, C2h; FZCCCHF, C,; F2CCCF2,D2d;and the substituted methyl acetylenes, C3". The basis set is of double-{quality in the valence space, and the carbon basis set is augmented by a set of d polarization functions on carbon (DZ+D,), giving a final basis set of the form ( 9 ~ 5 p l d / 9~5p/4~)/[3~2pld/3~2p/2s] in the order C/F/H. The exponents and coefficients are from Dunning and Hay.'I This basis set gives good structures for a variety of fluorocarbons at the S C F level, including a proper description of the C-F bond lengths2 Force fields for determining zero-point energies (ZPE)were calculated analyticallylMqcwith the program GRAD SF.'^ Final calculations were done with the above basis set augmented by p polarization functions on H and d polarization functions on F." This basis set is denoted as DZ+P. Correlation corrections were included at the level of MP-2I3for the valence electrons. Results and Discussion Geometries. The geometries for the allenes and acetylenes are given in Tables I and I I and are compared to experiment where available. The theoretical bond distances are slightly shorter than the experimental values as would be expected for good SCF calculations. For the C=C double bonds, the correction factor between theory and experiment (0.004 A) is somewhat smaller than that found in the fluoroethylenes (0.016 A);2bsee text below. Both C-F and C=C distances are predicted to decrease as the number of substituted fluorines increases, exactly as found for the fluoroethylenes.2b The predicted decrease in r(C=C) going from allene to perfluoroallene is 0.01 8 A, exactly that predicted for the difference in ethylene and tetrafluoroethylene at the SCF level.2b The experimental results on the allenes are also in agreement with this trend, although the observed variation is somewhat smaller. Furthermore, if there are unequal numbers of fluorines on the substituted carbons, as in CH2=C=CHF, CH2=C=CF2, and CHF=C=CF2, then the carbon with the most fluorines is predicted to have the shorter C=C bond. As expected, this difference in bond len ths is most pronounced for CH2=C=CF2 (Ar(C=C) = 0.012 ). The experimental results also indicate that this trend is present, although Ar(C=C) is too small to be statistically significant. The "experimental" structures for the fluorinated allenes are all determined by microwave spectroscopy. Thus, there are not enough experimental data to fit the number of unique structural parameters except for CH2=C=CHF and CH2=C=CF2 where five isotopic species were studied, but even here, the experimental uncertainties in some parameters are not small. It is therefore not surprising that we find our best agreement between theory and experiment for CH2=C=CH2, CH2=C=CHF, and CH2=C=CF2 where the structures were determined by highresolution vibration-rotation spectroscopy. For example, the calculated bond angles are in excellent agreement with experiment as would be expected. For CH2=C=CF2, the calculated value for r(C-F) is 0.007 8, shorter than found experimentally, in
x
(9) (a) Dupuis, M.; Rys, J.; King, H . F. J . Chem. Phys. 1976,65, 11 1. (b) King, H . F.; Dupuis, M.; Rys, J. National Resource for Computer Chemistry Software Catalog, Lawrence Berkeley Laboratory, USDOE: Berkeley, CA, 1980; Vol. I , Program QH02 (HONDO). ( I O ) (a) Komornicki, A.; Ishida, K.; Morokuma, K.; Ditchfield, R.; Conrad, M. Chem. Phys. Lett. 1977,45, 595. (b) McIver, J. W.,Jr.; Komornicki, A. Chem. Phys. Lett. 1971, IO, 303. (c) Pulay, P. In Applications of Electronic Srructure Theory; Schaefer, H . F., 111, Ed.; Plenum Press: New York, 1977; p 153. (d) King, H . F.; Komornicki, A. In Geometrical Derivatives of Energy Surfaces and Molecular Properties; Jorgenson, P., Simon, J. S., Eds.; NATO Advanced Study Institutes Series, Series C.; Reidel: Dordrecht, The Netherlands, 1986; Vol. 166, p 207. (e) King, H . F.; Komornicki, A. J. Chem. Phys. 1986,84. 5645. ( I I ) Dunning, T. H . , Jr.; Hay, P. J. I n Methods of Electronic Structure Theory; Schaefer, H. F . , Ill, Ed.; Plenum Press: New York, 1977; Chapter 1.
( I 2) GRADSCF is an ab initio gradient program system designed and written by A . Komornicki at Polyatomics Research. (13) (a) Maller, C . ; Plesset, M. S. Phys. Reu. 1934, 46, 618. (b) Pople, J. A.; Binkley, J. S.; Seeger, R. f n t . J. Quantum Chem., Symp. 1976, 10, 1.
The Journal of Physical Chemistry, Vol. 93, No. 23, 1989 7113 TABLE I: Geometry Parameters for Allenes parameter"
calcdb
H ,C=C=C H , 1.304 ( I ,308)
exptl
1.007 118.3
1.308 ( 0 ) c 1.087 118.2 (2)
1.298 (1.302) 1.303 (1.307) 1.343 (1.354) 1.073 1.077 118.0 112.7 124.9 122.4 179.1
1.301 (4)d 1.309 (4) 1.360 (6) 1.083 (2) 1.086 (2) 118.4 (4) 113.8 (8) 124.3 (3) 121.9 ( 5 ) 178.2 (6)
r(C-F) r(C-H) B(FCF) B(HCH)
1.289 (1.293) 1.301 (1.305) 1.316 (1.327) 1.077 109.4 117.8
1.302 (1 2)L 1.306 (2) 1.323 (11) 1.086 (3) 110.2 ( I O ) 117.8 (2)
r(C=C) r(C-F) r(C-H) B(HCF) 8(HCC), B(FCC) 8(CCC)
1.296 (1.300) 1.334 ( I ,345) 1.074 112.4 125.0 122.6 178.9
1.306.8 1.336 1.086 114.7 121.1 124.2 180.0
r(C=C) r(C-H) B(HCH)
r(G=C2)
r(c2=c3)
r(C-F) r(C I - H ) r(C3-H) 8(HCH) B(HCF) 8(C2C,H) ~(C~CIF) 8(CCC)
r(C,=C,)
r(c2=c3)
H
\ ..J ,c3 =c, =c; F.
F
8(CK3F) S(CCC)
1.287 (1.291) 1.294 (1.298) 1.309 (1.320) 1.329 (1.340) 1.074 109.6 112.2 125.1 122.7 179.2
r(C=C) r(C-F) 8( FC F )
F,C=C=CF2 1.286 (1.290) 1.304 (1.315) 109.1
r(C,=C2)
r(c2=c3) 4Cr-F) r(C3-F) r(C-H) 8(FCF) B(FCH) 8(C2C3H)
1.2998 1.30Ih 1.317 1.317' [ 1.0861J 110.1 112.6 [ 121.I]' 126.3 [ 180.01j
OBond distances in angstroms and bond angles in degrees. bCorrected values in parentheses, +0.004 8, for r(C=C) and +0.011 8, for r(C-F). See text. CMaki, A. G.; Toth, R. A. J . Mol. Spectrosc. 1965, 17, 136. dReference 7. CDurig, J. R.; Li, Y. S.; Tong, C . C . ; Zens, A. P.; Ellis, P. D. J . Am. Chem. SOC.1974, 96, 3805. /Ellis, P. D.; Li, Y. S.; Tong, C . C . ; Zens, A. P.; Durig, J. R. J. Chem. Phys. 1975, 62, 131 1. COgata, T.;Ando, B.-I. J. Mol. Spectrosc. 1986, 118, 70. hr(C2=C3) - r(C,=C2) = 0.002 8, assumed. jr(C,-F) = r(C,-F) assumed. jAssumed values.
excellent agreement with what is found in the fluoroethylenes.2b We note that the structure of CF2=C=CF2 has not been determined experimentally. The structures of C H F = C = C H F and CHF=C=CF2 are not determined precisely and include a number of assumed parameters. In fact, in the original paper on the microwave spectrum of
7774 The Journal of Physical Chemistry, Vol. 93, No. 23, 1989 TABLE 11: Geometry Parameters for Acetylenes
Darameter"
calcd
r(C=C) r(C-C) r(CI-H) r(C3-H) iXHCH)
HCI=C2-C,H, 1.192 1.476 1.059 1.082 108.7
r(C=C) r(C-C) r(C-F) r(C-H) B(HCH)
FC-C-CHI 1.181 1.478 1.282 1.082 108.7
r(C=C) r(C-C) r(C-F) r(C-H) h'(FCF)
HCEC-CF, 1.185 1.474 1.323 1.061 107.7
r(C=C) r(C-C) r(CI-F) r(Cr-F) B(FCF)
exptl I .206b 1.459 1.056 1.105 110.2
Dixon and Smart TABLE III: Total Energies (DZ+P/MP-2), Zero-Point Energies, and Heats of Formation for Allenes, Acetylenes, and Ethylenes
molecule ~~~
ZPE"/ enereviau kcalimol Allenes
CH,=C=CH, CH,=C=CHF CHF=C=CHF CH2=C=CF2 CHF=C=CF, CF,=C=CF*
-116.270330 -215.296989 -314.327839 -314.332437 -413.365969 -5 12.405 402
HC=CH HC=CF FC=CF CHICGCH CH,C=C F CF3C=CH CF,C=CF
-77.083259 -176.094128 -275.094 294 - 1 16.276673 -215.285 705 -41 3.399 879 -5 12.409 463
C H2=C H, CH2=CHF C H 2=C F2 C H F=C F2 CF2=CF2
-78.322 182 30.9 (30.9) -177.359821 26.8 -276.402255 22.4 (22.1) -375.419807 18.1 -474.447 264 13.4 ( I 3.3)
33.4 (33.3) 29.6 25.6 25.3 21.2 16.6
AHr" b /
kcal /mol (45.6 f 0.3) 7.1 -32.1 -35.3 -77.2 -123.8
Acetylenes
1.201' 1.464 1.335 1.056 107.5 ( I O )
FCI=C2-C,F3 1.176 1.474 1.266 1.322 107.5
Bond distances in angstroms and bond angles in degrees. Harmony. M. D.: Laurie, V. W.; Kuczkowski, R. L.; Schwendeman, R. H.; Ramsay, D. A,; Lovas, F. J.; Lafferty, W. J.; Maki, A. G. J . Ph,vs. Chem. Ref, Data 1979, 8, 619. 'Shechan, W. F., Jr.; Schomaker, V . J . A m . Chem. Soc. 1952, 7 4 , 4468. CHF=C=CHF (Table I, footnotefl, no detailed structure is reported. For CHF=C=CF2, Ar(C=C) was set at 0.002 8,the C-H distance was set at 1.086 8 (from CH2=C=CF2), and O(CCH) was fixed at 121.1O (from CH2=C=CF2). As the theoretical results show, B(CCH) is expected to be significantly larger. (The hydrogen position does not significantly affect the moments of inertia so that any error in its assignment should not affect the other experimental values.) Also, it was assumed that r(C,-F) = r(C2-F), and theory clearly predicts a substantial difference of 0.02 8 in the two values. This accounts for the experimental value being close to the average of the theoretical C-F values. These results illustrate the difficulty in experimentally determining the structures and also show that theory is probably doing a better job of describing the structural parameters, especially if the bond lengths are scaled by constant increments. For the C=C bonds, we take a scaling factor of +0.004 8 from the allene result, and for the C-F bonds, a scale factor of +0.011 %, from previous work on the fluoroethylenes2bis used. We prefer the scaled bond lengths and the theoretical bond angles listed in Table I . Notably, our scaled theoretical structure for CH,=C=CHF agrees quite well with that calculated by Ogata and c o - ~ o r k e r s , ~ using a 6-3 IC* basis set with correlation corrections at the MP-2 level (r(C,=C,) = 1.309 8,r(C2=C3) = 1.312 8, r(C-F) = 1.361 8). Finally, it is interesting that the allenes are not required by symmetry to have a collinear C=C=C moiety; in fact, our calculations show a slight deviation from collinearity of about l o for CH2=C=CHF, CHF=C=CHF, and CHF=C=CF2. This has been confirmed experimentally for CH2=C=CHF.7 Comparison of the calculated values for the fluoroallenes with those calculated for the fluoroethylenesZbshows that the allenic C=C double bonds are consistently shorter. The values for r(C-F) for CH2=C=CHF and CH2=C=CF2 are predicted to be slightly longer than those in the corresponding ethylenes. However, for CHF=C=CHF and CF2=C=CF2, the situation is reversed and the allenic C-F bonds are slightly shorter. For CHF=C=CF,, the results are mixed with the unique allenic C-F bond being shorter and the equivalent C-F bonds being longer than the ones in CHF=CF2. The fluoroacetylenes show 2 similar behavior (Table 11).
16.6 (16.2) 12.7 (12.2) 8.7 (8.7) 33.9 (33.8) 30.0 20.7 (20.4) 16.7
(54.5 f 0.2) 25.8 (25.5)' 3.7d (5 f 5 ) e (44.6 f 0.5) 17.4 -98.2 -126.1
Ethylenes (12.5 f 0.3) (-33.2 f 0.4) (-80.1 f 0.8) (-1 17.2 f 2) (-157.9 f 0.4)
DZ+D, calculated values multiplied by 0.90. Experimental values in parentheses were calculated from vibrational frequency data: ( I ) Shimanouchi, T. Tables of Molecular Vibrational Frequencies; National Standard Reference Data Series (United States, National Bureau of Standards); NSRDS-NBS 39; U.S. Government Printing Office: Washington, DC, 1972. (2) Shimanouchi, T. J . Phys. Chem. Ref. Data 1977, 6, 1027. *Experimental data in parentheses, unless noted otherwise; from: Pedley, J. B.; Naylor, R. D.; Kirby, S. P. Thermochemical Data of Organic Compounds, 2nd ed.; Chapman and Hall: London, 1986. CKloster-Jensen,E.; Pascual, C.; Vogt, J. Helv. Chim. Acta 1970, 53, 2109. A value of 30 f 15 kcal/mol' also has been reported. dFrom 2CH=CF CFECF CHECH ( A E = 6.7 and AH" = 6.6 kcal/mol) or C H = C F CH,=CHF CF=CF + C H 2 = C H 2 ( A E = 23.5 and AH" = 23.6 kcal/mol) with use of AHf"(CH=CF) = 25.8 kcal/mol. CStull,D. R.; Prophet, H. JANAF Thermochemical Tables, 2nd ed.; National Standard Reference Data
-
+
+
-
Series (United States, National Bureau of Standards); NSRDSNBS37; U S . Government Printing Office: Washington. DC; 1971. Experimentally, only the structure of methylacetylene is known. The theoretical results show that fluorine substitution at the acetylenic carbon decreases r(C=C). Substitution of three fluorines in the methyl group also leads to a decrease in r(C=C) although the change is about 50% that of substitution of F at a triply bonded carbon. The C-C single bond is little affected by fluorine substitution at either carbon. The C-F bond lengths in the CF, group are little affected by substitution at the acetylenic carbon. However, the C-F bond at the acetylenic carbon is strongly affected by whether there is a CH, or a CF, group at the other carbon. Substitution of CF, for CH, leads to a dramatic shortening in r(C-F). Heats of Formation. The total energies (DZ+P/MP-2) of the fluoroallenes and fluoroacetylenes are given in Table 111. (Total energies with the DZ+D, basis set at the SCF and MP-2 levels, and the DZ+P/SCF energies, are given in Table VI1 of the Appendix.) There are several possible isodesmic reactions that can be used to determine the unknown allene and acetylene heats of formation. We selected simple ones involving only allenes, acetylenes, and ethylenes with not more than two reactants and products to minimize errors. The calculated isodesmic reaction energies ( A E ) were converted to enthalpies (AH'). The AZPE contributions to convert AE's to AHO's were based on the calculated ZPE's from Table 111. The AHf' of CH2=C=CHF and CH2=C=CF2 are best determined from isodesmic reactions 1 and 2. All M ' s and Me's are in kilocalories/mole. (For comparison, the A E s for reactions 1-21 at the DZ+D,/SCF, DZ+D,/MP-2, and DZ+P/SCF levels are given in Table VI11 of the Appendix. With few exceptions, the results at all four levels differ by less than 2 kcal/mol.) These
The Journal of Physical Chemistry, Vol. 93, No. 23, 1989 7775
Fluorinated Allenes and Isomeric Acetylenes CH2=C=CHF
+ CH,=CH2
AHo = -7.2
AE = -6.9, CH2=C=CF2
kcal/mol and other theoretical estimates.l We employ the theoretical value in the following calculations. For FC=CCH3, we can write two isodesmic reactions, 1 1 and 12, which both give F C r C C H 3 HCECH HC=CCH3 + HCECF ( 1 1)
+ CH,=CHF
+
CH2=C=CH2
+ CH2=CH2
+
+ CH2=CF,
CHZ=C=CH,
2)
AE = -1 1.3, AHo = -1 1.7 reactions should give good AHFO’s based on our previous experience with the fluoromethanes and fluoroethylenes.ksb From the known = 7.1 AHro’s (Table I l l ) , we obtain AH?(CH,=C=CHF) kcal/mol and AHro(CH2=C=CF2) = -35.3 k ~ a l / m o l . ’ ~The isomerization reaction 3 gives AH? (CHF=C=CHF) = -32.1 kcal/mol. C H 2=C=C F2 C H F=C=CH F (3) +
AE = -1.2,
CH2=C=CH2
+
2CH,=C=CF2
(4)
AHo = -1.4
+ CHF=C=CF,
+
CH2=C=CF2
FC=CCH,
+ H2C=CH,
AH?(FC=CCH3) = 17.4. For AH?(HC=CCF,), we can use reactions 13 and 14 which give AHfo(HC=CCF3) = -98.4 H C E C C F ~ CH2=CH2 HC=CH CH,=CHCF3 ( 1 3)16 AE = -6.5, AHo = -6.4
+
+ CH,=CHCH,
+ CH,=C=CHF
AE = -9.2,
(5)
FC=CCF3
+ CH,=CH,
(6)
AE = 6.8, AHo = 7.4 CH2=C=CF,
+ CF,=C=CF2
+
2CHF=C=CF,
(7)
A E = 3.7, AHo = 4.2 C H 2=C=C H 2
+ C F2-C-C
F2 CHF=C=CF, +
+ CH2=C=CHF
(8)
AE = 8.0, AHo = 8.8 and reaction 5 gives -78.1 kcal/mol, which average to -77.2 kcal/mol. Reactions 6-8 give -123.6, -123.3, and -124.5 kcal/mol, respectively, for AH?(CF,=C=CF,) with an average value of -123.8 kcal/mol. Reaction 9 can be used to check AHrO(CH,=C=CHF), and it gives AH? = 8.1 kcal/mol, which is within experimental uncertainty to the value obtained above from reaction I . CH2=C=CH2
+ CHI=C=CF2
-
2CHZ=C=CHF
(9)
AE = 5.5, AHo = 6.0 The evaluation of the AHfo’s of the fluoromethylacetylenes involves similar isodesmic reactions. We calculate AHfo(HC= CF) from reaction I O since an appropriate experimental check is available. Reaction IO gives AH?(HF=CF) = 25.8 kcal/mol HCZCF
+ CH2kCH2
-
HC=CH
AE = -16.8,
+ CH,=CHF
(IO)
AHo = -17.0
-
A E = -8.5,
AHro(CHF=C=CF2) and reactions 6-8 for AHfo(CF2=C= = -76.3 kcal/mol CF,). Reaction 4 gives AH?(CHF=C=CF,) 2CH,=C=CF,
+
+ CH2=CHCF3
( 1 4)16
AHo = -9.1
-
+ HCzCH
-
-
kcal/mol from the former and -97.9 kcal/mol from the latter, which average to -98.2 kcal/mol. Similarly for AHfo(FC= CCF,), reactions 15-17 give -125.9, -126.2, and -126.2 kcal/mol, respectively, and an average value of AHf’ = -1 26.1 kcal/mol. FCECCF~ + C H ~ E C H C H , FCzCCH, CH2=CHCF3 ( I5)l6 FC=CCF3
+ CF2=C=CF2
H C E C C H ~ + H,C=CHF (12) AHo = -18.5
+
AE = -18.3,
AE = 4.3, AHo = 4.3
CH,=C=CH,
AHo = -1.5
HC=CCH,
We tend to find significant errors in calculating AHO’s for reactions that involve CF,=CF, (and by extension CHF= so we use the above AHro’sfor CH2=C=CHF and CF2),2b,15 CH2=C=CF2 with isodesmic reactions 4 and 5 to obtain
+ CHF=C=CF2
AE = -1.5,
HC=CCF3
AE = 2.9,13 AHo = 3.2
CH2=C=CHF
-
+
+
A H o = -8.3
HC=CCF3
AE = -0.8, +
A E = -17.6,
+ HC=CF
(16)
AHo = -0.7 HCrCCF3
+ CH,=CHF
(1 7)
AHo = -17.7
As a check on the consistency of the above AHfo’s,we use the isomerization reactions 18-21 where the values in parentheses are our calculated AHO’s from the AHro’s in Table 111. The CH,=C=CH, 4 HC=CCH3 AE = -4.0, AHo = -3.5 (-1.1 f 0.8) (18) CH,=C=CHF FC=CCH3 AE = 7.5, AHo = 7.9 (10.3) (19) CHF=C=CF, -+ HCgCCF3 AE = -21.3, AHo = -21.8 (-21.0) (20) CF,=C=CF, ---* FC=CCF3 AE = -2.5, AHo = -2.4 (-2.3) (21)
-
agreement between the two is good. If one takes a correction factor of 2.4 kcal/mol from reaction 18 whose AHo is known experimentally and applies it to reaction 19, the agreement becomes even better with a corrected AHo of 10.4 kcal/mol. Finally, it is interesting to compare our theoretical allene AHfo’s with those estimated by group additivity schemes. The most commonly used empirical methods for fluorocarbons are those developed by Benson” and Dolbier.I8 Benson’s popular method, which accurately estimates AHP’s for many organic molecules, works reasonably well with perfluorinated and relatively nonpolar polyfluorocarbons but fails with most polar fluorocarbons.’.I8 Dolbier’s method, which complements Benson’s, uses different fluorine-containing-group contribution values that accurately reproduce AHHfo’s of most partially fluorinated alkanes and alkenes
in exceJlent agreement with both the experimental value of 25.5
-
(14) The reaction CH2=C=CH2 + 2CH2=CHF CH2=C=CF2 + 2CH2=CH2 (AE = 8.3 and AHo = 8.4 kcal/mol) gives AHIo(CH2=C= CF2) = -37.4 kcal/mol. ( I 5) The difference between the calculated and average experimental AHO’s for CH2=CH2 CF2=CF2 2CH2=CF2 is 7 . 2 kcal/mol. AHfe(CH2=C=CF2) = -42.1 kcal/mol from the reaction CH2=C=CH2 C F 2 = C F 2 CH2=C=CF2 + CH2=CF2 (AE = -10.7 and AHo = -9.9 kcal/mol) similarly differs by 6.8 kcal/mol from the value obtained with reaction 2.
+
-
+
-
(16) Total E = -414.649 153 (DZ+P/MP-2), -414.407591 (DZ+D,/ MP-2), -413.766572 (DZ+P/SCF), and -413.723 148 au (DZ+D,); ZPE = 35.1 kcal/mol; and AHl’ = -146.8 f 1.6 kcal/mol (Table 111, footnote b) for CH2=CHCF,. For CH2=CHCH,, total E = -117.511 310 (DZ+P/ MP-2). -117.460320 (DZ+D,/MP-2), -117,098499 (DZ+P), -1 17.087218 au (DZ+D,/SCF); ZPE = 48.2 kcal/mol; and AHro = 4.8 f 0.1 kcal/mol. ( 1 7) Benson, S. W. Thermochemical Kinerics, 2nd ed.: Wiley: New York, 1976. ( 1 8) (a) Dolbier, W. R., Jr.; Medinger, K. S. Tetrahedron 1982, 38, 241 1 . (b) Dolbier, W . R., Jr.; Medinger, K. S.; Greenberg, A,; Liebman, J. F. Tetrahedron 1982, 38, 2415.
7776
The Journal of Physical Chemistry, Vol. 93, No. 23. 1989 CHF=C=CF2
TABLE IV: Estimated and Theoretical Heats of Formation allene C H 2=C=C H F C H 2=C=C F2 C H F=C=CH F C FI=C=C H F C F2=C=C F2
AHfo/kcal/mol Dolbier' Bensonb 2. I -47.5 -42.6 -92.2 -141.8
+ CH,=CH2
-
CH,=C=CH,
+ CF,=CHF
(22)
AH' = -6.9
theoryd
1 .o -38.5 -44.8 -84.3 -123.8
Dixon and Smart
7.1 -35 3 -32.1 -77.2 -123.8
CF,=C=CF,
+ CH,=CH2
+
CH,=C=CH,
+ CF,=CF2 (23)
AHo = -1.0 A comparison of reactions 5 , 6 , and 8 with the corresponding
"Cd-(H)(F) = -38.4 and Cd-(F)2 = -88.0 kcal/mol from ref 18; C,(=C=) and Cd-(H), from ref 17. bC, = 34.2, cd-(H)2 = 6.3, = -79.0 (revised)' from Cd-(H)(F) = -39.5 (revised): and C,-(F), ref 17. CRevised according to currently accepted best values for AHfo(CH2=CHF) = -33.2 kcal/mol and AHfo(CF2=CF,) = -157.9 kcal/mol (Table 111, footnote b ) . Benson's group values of -37.6 and -77.5 kcal/mol for Cd-c(H)(F) and Cd-c(F),, respectively, in ref 17 were based on AH,0(CH2=CHF) = -31.6 kcal/mol and AHfo(CF2=CF2) = -155 kcal/mol. dFor comparison, the respective values derived from isodesmic reaction energies computed at the DZ+D,/ S C F level (see text and Table VI11 in the Appendix) are 6.9. -35.8. -33.1, -77.0, and - 1 21.7 kcal/mol.
ones for ethylenes, 24-26, reveals significant differences in the effects of fluorination. The AH"s for the allene reactions are similar (4.3-8.8 kcal/mol) and are all moderately endothermic, but those for the ethylenes are more variable and are all exothermic. Particularly striking are the different W ' s for reactions
but is restricted to molecules that have no neighboring (vicinal) fluorine substituents. Thus, it might be anticipated that Dolbier's method would be more accurate for the monofluoro- and difluoroallenes, but Benson's method would work better with trifluoro- and tetrafluoroallene. The results are given in Table IV. Surprisingly, except for AHfo(CH2=C=CHF), the Dolbier group value estimates are all very far from theory, although CHF=C=CHF is correctly estimated to be less stable than CH2=C=CF,. The Benson estimates unexpectedly are in good agreement with theory for both AHfo(CH2=C=CF2) and AH?(CF2=C=CF2), but they differ substantially on AHf'(CHF=C=CF2) and on the relative stability of the difluoroallene isomers. These comparative results reemphasize the shortcomings of group additivity schemes for estimating fluorocarbon AHr0's.I Group contributions that are derived from one class of molecules often are not applicable to other system^;'^^*^ that is, the group additivity principle fails in general for fluorocarbons. Ab initio molecular orbital theory is a more reliable technique for estimating AH:s' of fluorocarbons when basis sets of adequate size are used and vibrational corrections are included. lsodesmic Stabilities. The isodesmic reactions used to derive AHp's also provide insight into the relative stabilities of the various fluorinated allenes and acetylenes. Reaction 1 shows that substitution of a fluorine in allene is destabilizing by 7.2 kcal/mol relative to substitution of one fluorine in ethylene. Since fluorine stabilizes ethylene by ca. 3.3 kcal/mol,',21it follows that a fluorine substituent destabilizes allene by ca. 3.9 kcal/mol. Fuchs and co-workers8 also have studied the effect of a fluorine substituent on allene. Using geometries optimized with the 6-31G basis set and energies determined with 6-31G, 6-31G**, and 6-31 1G** basis sets,22they obtained values of -6.6 (6-31G), -7.0 (6-31G**), and -6.9 (6-31 IC**) kcal/mol for AE of reaction 1 in excellent agreement with our result. After examining several other substituents, they concluded that allene is destabilized relative to ethylene by n-donors and o-acceptors of which F is a prime example. I , 1-Difluorination is even more destabilizing relative to its effect in ethylene (reaction 2), but substitution by three fluorines is comparatively less destabilizing (reaction 22; AH' from AHfo's in Table I l l ) and perfluorination is only I kcal/mol more destabilizing in allene than in ethylene (reaction 23).23
CH,=CH,
~~~
~~
~
~
~
(19) A familiar example is the different C(F),(C) group values that are required, depending u on whether the CF, group is bonded to CH,, CF,, or (CF2),CF3 ( n > O).'.2g (20) Liebman, J. F.; Dolbier, W. R., Jr.; Greenberg, A. J . Phys. Chem. 1986, 90, 394. (21) (a) Greenberg, A.; Stevenson, T. A. In Molecular Sfrucfuresand Energerics; Liebman, J. F.; Greenberg, A,, Eds.; VCH: Deerfield Beach, FL, 1986; Vol. 3, Chapter 3. (b) Greenberg, A,; Stevenson, T. A. J . Am. Chem. soc. 1985, 107,3488. (22) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A . Ab lnitio Molecular Orbifal Theory; Wiley-Interscience: New York, 1986.
CH2=CH2
+ CHF=CF,
+
CH,=CF,
+ CH,=CHF
(24)
AH' = -8.6
+ CF,=CF2
CH,=CH,
+ CF,=CF2
4
AHo = -14.8 CHF=CF,
2CH2=CF,
+ CH,=CHF
(25) (26)
AH' = -5.0 6 and 25 where the allene reaction is endothermic (7.4 kcal/mol), but the ethylene one is highly exothermic (-14.8 kcal/mol). This large difference arises because, as noted above, I , 1-difluorination in ethylene compared to allene is quite stabilizing, but tetrafluorination in allene is only slightly more destabilizing than in ethylene [AHo(25) - AH'(6) = 2AH0(2) - AH0(23)]. Another contrast in the properties of fluorinated allenes and ethylenes is the marked difference in the stability of difluoroethylene isomers compared with difluoroallene isomers. Reaction 3 shows that CHF=C=CHF is only 3.2 kcal/mol less stable than CH2= C-CF,, but trans-CHF=CHF is 8.6 kcal/mol less stable than CH2=CF2.2bThese results on the allene-ethylene differences are consistent with the hypothesis that fluorine substitution at the 3-position in the allenes is not strongly affected by substitution at the I-position, whereas substitutions at the two carbons in the ethylenes are strongly coupled. The effects of fluorination on a triple bond are revealed in reactions 10 and 13. The F substituent destabilizes a triple bond by 17.0'kcal/mol compared to its effect on a double bond, that is, F overall destabilizes a triple bond by ca. 13.7 (17.0-3.3) kcal/mol. A CF3 group also destabilizes a triple bond relative to a double bond by 6.4 kcal/mol, according to reaction 13. Since a CF3 substituent is estimated to destabilize a double bond by ca. 3 kcal/mol,' CF3 destabilizes a triple bond by ca. 9.6 kcal/mol, or ca. 4 kcal/mol less than F. These results compare to previous estimates that F destabilizes a triple bond by 13.7' and 19.22' kcal/mol, and CF3 destabilizes it by 12.1' and 11.4 k ~ a l / m o l . ~ ~ Reaction 16 indicates that the effects of F and CF, on a triple bond are almost exactly additive. A comparison of the AHo%summarized in reaction 27 shows that the destabilizing effect of F on a triple bond is essentially independent of substituents on the neighboring carbon, with the notable exception of a second F substituent. XCECF
+ CH,=CH,
AHo = -17.0 (X
-t
XCSCH
+ CH,=CHF
(27)
H), -18.5 ( X = CH3), -18.2 (X = CF,), -23.9 (X = F)
(23) For different conclusions about the effects of fluorination in allenes versus ethylenes, see ref I , p 172-173. The analysis in this work, however, was based on fluoroallene AHfo'sestimated from Dolbier group values and, therefore, is flawed. (24) These stabilization or destabilization energies, of course, are not absolute terms but in the context of this work are defined with reference to the corresponding alkanes. That is, the stabilization energy of substituent X in C H S X or CH2=C=CHX is the difference between the W ' s of complete hydrogenation of the substituted compound and the corresponding parent hydrocarbon (X = H). Within experimental error, this is equivalent to equating stabilization energy of X to AHD o f the isodesmic reaction RX + CH,CH, RH + CH3CH2X.
-
T h e Journal of Physical C h e m i s t r y , V o l . 93, No. 23, I989
Fluorinated Allenes and Isomeric Acetylenes
7777
TABLE V: Electronic Properties of Allenes property"
HzC=C=CH,
H,C=C=CHF
H,C=C=CF,
0.00 -0.27 0.14 0.13, 0.14 -0.28 10.34
0.12 -0.34 -0.34 0.14
10.28 (e) (9.69)b
11.03 -3.40 -4.60 0.84 0.93 1.68 2.03 (1.76,' 1.979
-4.39 (e) 0.77 (e) 1.54
HFC=C=CHF
HFC=C=CF2
-0.13 0.19 0.19 0.14 -0.26 11.06
-0.28 0.22 0.63 0.14 -0.25, -0.23 11.23
1 1.06
1 1.49
-3.60 -3.61 0.99 0.99 1.98 2.05 (1.77)'
-3.3 1 -3.98 1.os 1.06 2.14 1.73 (1.49)h
-0.15 -0.24 0.58 0.15 -0.24 10.52 (9.7 9)C 1 1.49 (1 1.26)c -3.04 -5.06 0.93 1 .oo 1.93 2.36 (2.07)g
F2C=C=CF2 -0.43 0.65 0.65 -0.22 11.67 (e) (10.88)d -3.73 (e) 1.13 (e)
2.26
" Atom charges q in electrons; orbital energies in electronvolts with experimental values in parentheses. All properties calculated with the D Z + P basis set except for the charge analysis of the H O M O which is calculated with the DZ+Dc basis set. bBaker, C.; Turner, D. W. Chem. Commun. 1969, 480. 'Reference 4d. dSchmidt, H.; Schweig, A. Angew. Chem., Int. E d . Engl. 1973, 12, 307. eOgata, T.; Yoshikawa, M.; Fujii, K. Chem. Lett. 1985, 1797. ,Table I , footnotef BTable I, footnote e. "'Table I, footnote g. Reactions 18-21 give absolute relative stabilities of isomeric allenes and acetylenes. Both theory and experiment show that propyne is slightly more stable than allene. From the relative destabilizing effects of F on an allene versus a triple bond (3.9 versus 13.7 kcal/mol), CH3C=CF is expectedly less stable than CH,=C=CHF by ca. IO kcal/mol (reaction 19). A CF3 group on a triple bond, however, is much less destabilizing than three fluorines on an allene (reaction 20). Theory predicts that CF3C=CF and CF2=C=CF2 are close in energy, with the acetylene being slightly more stable.25 There are no quantitative experimental data on the relative stabilities of the isomeric fluoroallenes and acetylenes, but two studies indicate that CF2=C=CF2 in fact is more stable than CF3C=CF. Banks and co-workers2(' reported that, in the gas phase over CsF, CF2=C=CF2 does not rearrange to CF3C=CF below 165 "C within the limits of detection, but CF3C=CF at 95 "C (30-min contact time) produces a 5.3:l mixture of CF3C=CF/CF,=C=CF2 (93% material balance), and a 153.8 mixture (87% material balance) at 165 "C. Recently, it has been found that infrared multiphoton excitation of tetrafluorocyclopropene produces about a 5:l ratio of CF2=C=CF2/CF3C=CF independent of pressure, pulse energy, and conversion, but it is uncertain how close this is to the true thermodynamic ratio.27 Although our ab initio AHfo's indicate that C F 3 C r C F is about 2 kcal/mol more stable, it must be recognized that the precision of our calculated values is no better than the net experimental uncertainties in AHfo's of the reference compounds in the isodesmic reactions that are used to derive the unknown AHF's. For CF3C=CF, this uncertainty is conservatively f 2 kcal/mol. a-Bond Dissociation Energies. It is that there is a qualitative inverse relationship between the reactivities of fluorinated ethylenes in thermal, biradical [2+2] cycloadditions and their a-bond dissociation energies (D,") as defined by Benson:" D," = DH'(CC-X) - DH"(CC-X). The reactivity of allenes in thermal [2+2] cycloadditions increases in the order CH2= C=CH2 < CH2=C=CF2 < CF2=C=CF2,6C and one might expect a similar inverse relationship to D,",but the experimental data needed to derive fluoroallene D,' values are not available. We have found previously, however, that within experimental error the AHO's of isodesmic reactions involving fluorinated ethylenes (25) This result conflicts an earlier prediction that CF2=C=CF2 should be >20 kcal/mol more stable than CF3C=CF. This prediction, however, was based on an estimated AHfo(CF2=C=CF2) that our present work shows is in error by ca. 20 kcal/mol; seeref 23. (26) (a) Banks, R. E.; Barlow, M. G.; Davies, W. D.; Haszeldine, R. N.; Taylor, D. R.J . Chem. SOC.C 1969, 1104. (b) Banks, R. E.; Barlow, M. G.; Mullen, K. J . Chem. SOC.C 1969, 1131. (27) Friedrich, H. B.; Burton, D. J.; Tardy, D. C. J . Phys. Chem. 1987, 91, 6334. (28) (a) Pickard, J . M.; Rcdgers, A. S. J . Am. Chem. SOC.1977, 99, 695. (b) Wu, E.-C.; Rodgers, A. S . J . Am. Chem. SOC.1976, 98, 6112. ~
(for example, reaction 25) are identical with the differences bethat is, tween the reactant and product ethylene D," AH" = -AD,".If we assume this identity also holds for reactions 2 and 23, then from the experimental D," values of 59.1, 62.8, and 52.3 kcal/mo128for CH2=CH2, CH2=CF2, and CF2=CF2, respectively, D,"(CH2=C=CH2) - D,'(CF2=C=CF2) = 7.8 kcal/mol and D,"(CH2=C=CH2) - D,"(CH2=C=€F2) = 8.0 kcal/mol. Thus, D,"(CH2=C=CF2) is essentially the same as D,'(CF2=C=CF2). (Use of the reaction CF2=C=CF2 CH2=CH2 CH2=C=CF2 CH2=CF2 (AH" = -4.1 kcal/mol) gives a similar AD," of 0.4 kcal/mol). The order of D,"'S, CF2=C=CF2 < CH2==C=CH2, indeed is in accord with the relative biradical reactivities of the allenes, although this analysis predicts CF2=C=CF2 and CH2=C=CF2 should have similar reactivities. We can estimate the individual D," values for the fluorinated allenes as follows. Following Benson, D,".(CH2=C=CH2) = DHo(CH2=CHCH2-H) - DHo(CHZ=CCH,-H) = AHfO( C H 2 4 H C H 2 ' ) AHf"(CH24CH3) - AHHf"(CH24HCH3) - AHt(CH2=C=CH2). From the values AHy(CH2=CHCH2') = 39.1 f 1.529 (allyl radical), AHfo(CH2=CCH3) "= 60,30 1Hfo(CH2=CHCH3) = 4.8 f 0.1, and AHf"(CH2=C=CH2) = 45.6 f 0.3 kcal/mol, D,"(CH2=C=CH2) is estimated to be about 49 kcal/mol. From this value for D,O(CH2=C=CH2) and the AD," values above, then D,"(CH2=C=CF2) "= D,'(CF2= 41 kcal/mol. Note that our derivation implicitly C=CF2) defines D,' for an allene as the difference in energy between the allene and biradical 1, not 2, and in this sense differs from Benson's
+
+
+
+
=
2
1
"instantaneous" a-bond energy. By our definition, C H 2 = C = C F 2 has only one a-bond energy. Although a a-bond energy of 41 (29) McMillen, D. F.; Golden, D. M. Annu. Rev. Phys. Chem. 1982, 33, 493. (30) Following a reviewer's suggestion, AHfo(CHzCCH3)can be estimated from the following two equations assuming AH(qxn) = 0 kcal/mol (equal bqnd strengths): C6H5' + CH2=C,HCH3 CH2==€CHj + C6H6 and CH,=CH + CHz=CHCH, CH2=CCH3 H2C=CH2, which yield AHf' = 64.0 f 1 and 55.7 f 1 kcal/mol, respectively, with AHf0(C6HJ') = 79 f 1 kcal/mol and AHfo(CzH3') = 63.4 f 1 kcal/mol. The radical AHfO's are from: Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J . Phys. Chem. Ret. Data, Suppl 1 1988.17. The average of the two values for AHfo(CH2CCH3)is 60 f 2 kcal/mol. This value is in good agreement with the value of 58.1 kcal/mol reported by: Franklin, J. L.; Dillard, J . D.; Rosenstock, H. M.; Herron, J. T.; Draxl, K.; Field, F. M. National Standard Reference Data Series (United States, National Bureau of Standards); NSRDS-NBS 26; U S . Government Printing Office: Washington, DC, 1969.
-
-
+
7778 The Journal of Physical Chemistry, Vol. 93, No. 23, 1989 kcal/mol for CF2=C=CF2 may seem unrealistically low, it is not all that inconsistent with the known properties of the allene. Tetrafluoroallene is extraordinarily reactive toward addition of free radicals; it very rapidly homopolymerizes under autogeneous pressure at 20 “C, and it even slowly cyclodimerizes at 20 0C.6c931 Electronic Properties. Various electronic properties of the fluoroallenes are summarized in Table V. The calculated dipole moments compared with experiment are about 15% too high as would be expected with this DZ+P basis set. The order of increasing dipole moments, p(CHF=C=CF,) < p(CH,=C= CHF) p(CHF=C=CHF) < K(CH,=C=CF,), is correctly reproduced by theory. Thus, we prefer the more recently measured value of p = 1.76D for HFC=C=CH2 to the older one of 1.97 D. The Mulliken atomic charges for the carbons show some interesting variations. The charges on the hydrogens are approximately constant, +O. 13 to +O. 15e. The fluorine charges are all negative and range in magnitude from -0.28e for H F C = C = C H 2 to -0.22e for F2C=C=CF2. The more negatively charged fluorines are found when only one fluorine atom is bonded to a carbon. The charge on the central carbon shows a large variation from +O. 12e in allene to -0.43e in perfluoroallene. Thus, as fluorines are substituted, the central carbon becomes progressively more negative. Terminal carbons with two hydrogens are very negative; those with one H and one F are neutral and those with two fluorines are quite positive. Since the terminal carbon group charges (sum of the terminal carbon charge and the charges of the atoms bonded to the terminal carbon) are not zero for two bonded hydrogens (negative) or for two bonded fluorines (positive), the results clearly demonstrted that fluorine substitution does push negative charge to the central carbon. This fluorine a-donating effect is also clearly seen in the related fluoroethylenes, CH2= C H F and CH2=CF2.32 The ionization potentials (IP) of allene, 1,I-difluoroallene, and perfluoroallene are known. The adiabatic I P S increase by 1.2 eV from allene to perfluoroallene. The calculated IP’s from Koopmans’ theorem show a larger increase of 1.4 eV. We predict that the IP of I , 1 -difluoroallene should be 0.24 eV higher than that of allene, whereas experimentally the difference in the adiabatic IP’s is only 0.1 eV. These results are very similar to those found for I P S calculated at the 4-31G level.4d The calculated IP’s are all larger than the adiabatic IP’s. The calculated results do predict that the I P S should increase with increasing fluorination. Interestingly, the IP of CHF=C=CHF is predicted to be 0.5 eV higher than that for its isomer, CH,=C=CF2. The NHOMO eigenvalues do not vary in the same manner. For the allenes with nondegenerate HOMO’S, the NHOMO IP increases with increasing fluorine substitution, except for CHF=C=CHF where the value is less than that of H2C=C=CF2 and HFC=C=CF2 and more like that in H2C=C=CHF. For H FC=C=CHF, the HOMO and NHOMO IP’s are essentially degenerate, which is consistent with the result that the two Korbitals do not interact and that each terminal carbon has a fluorine bonded to it. The energies of the LUMO’s do not show a regular variation with increasing fluorine substitution. The highest energy LUMO is for the hydrocarbon, as expected, and the most available LUMO is found for H,C=C=CF,, which also has the least available NLUMO. The regiochemistry of concerted cycloadditions to the allenes will be largely controlled by the location of the HOMO and of the LUM0.4 Diagrams for the four orbitals (HOMO, NHOMO, LUMO, NLUMO) are shown in Figures 1-3 for the asymmet-
Dixon and Smart
HOMO
NHOMO
d
C
=
(31) Banks, R. E.; Haszeldine. R. N.; Taylor, D. R. J . Chem. SOC.C 1965,
918. (32) Bock, C. W.; George, P.; Mains, G. J.; Trachtman, M. J . Chem. Soc., Perkin Trans. 2 1979, 814. (33) (a) Halgren, T. A.; Lipscomb, W. N. Proc. Natl. Acad. Sci. U.S.A. 1972, 69, 652. (b) Halgren, T.A.; Lipscomb, W . N. J . Chem. Phys. 1973, 58. 1569. (34)Jorgensen, W. L.; Salem, L. The Organic Chemist’s Book of Orbitals; Academic Press: New York, 1973.
\
F -NLUMO
CHFCCH2
Figure 1. Highest occupied and lowest unoccupied molecular orbitals for fluoroallene. Orbitals calculated with a minimal STO basis set at the PRDDO level33 at the optimum geometries reported in the text. The form of the orbitals is the same as that found with the larger basis set. Orbital plots were done with the Jorgenson-Salem plotting program34 as implemented a t Du Pont by D. A. Pensak and F. Van-Catledge. Key: (a) HOMO; (b) NHOMO; (c) LUMO; (d) NLUMO. a
%---NHOMO
HOMO
LUMO
NLUMO
CF2CCH2
Figure 2. Highest occupied and lowest unoccupied molecular orbitals for 1,l-difluoroallene. See Figure 1 caption. 0
b
C
d
NHOMO
-
LUMO
CF2CCHF
Figure 3. Highest occupied and lowest unoccupied molecular orbitals for trifluoroallene. See Figure 1 caption.
rically substituted allenes. The HOMO for FHC=C=CH2 is predominantly localized in the C,-C2 region. Thus, the HOMO becomes localized toward the carbon bonded to the fluorine. As shown, the HOMO has about equal populations on C2 and C l and is somewhat delocalized onto the fluorine with 0.15e in the outof-plane p lone pair. The NHOMO is of course localized in the C,-C3 region and is slightly polarized toward C3. There is very little contribution from the in-plane F lone pair. The LUMO,
The Journal of Physical Chemistry, Vol. 93, No. 23, 1989 7779
Fluorinated Allenes and Isomeric Acetylenes TABLE VI: Electronic Prowrties of Methvlacetvlenes CH3C=CH 0.14 -0.37 -0.36 0.16, 0.14 10.52 (e) (10.36)b -5.53 (e) 0.86 (e) 0.99 0.78 (0.781)d
CH~CECF CF3CICH -0.12 -0.01 0.81 -0.36 -0.24 0.24 0.19 0.14 -0.25 -0.19 12.30 (e) 10.78 (e) ( I 1 .96)c -3.99 (e) -5.88 1.00 (e) 0.89 (e) 0.94 0.8 1 2.57 1.66 (2.36)d
TABLE VII: Total Energies (au) CF3C=CF -0.33 0.82 0.39 -0.16, -0.24 12.51 (e) -4.86 (e) 1.06 (e) 0.76 1.70
"Atom charges 9 in electrons; orbital energies in electronvolts with experimental values in parentheses. bRosenstock, H. M.; Draxl, K.; Steiner, B. W.; Herron, J . T. J . Phys. Chem. ReJ Data, Suppl. 1 1977, 6, 110. 'Bieri. G.; Heilbronner, E.; Hornung, V.; Kloster-Jensen, E.; Maier, J. P.; Thommen, F.; Niessen, W. v. Chem. Phys. 1979, 36, I . dNelson, R. D., Jr.; Lide, D. R.. Jr.; Maryott, A. A. National Standard Reference Data Series (United States, National Bureau of Standards); U S . NSRDS-NBS IO: US. Government Printing Office: Washington, DC, 1967.
in contrast, is localized in the C2-C3 a*-orbital, and the NLUMO is localized in the C,-C2 a*-orbital. For F2C=C=CH2, the HOMO is still localized in the CI-C2 bonding region and is both more highly polarized and more delocalized. The charge on C2 is increased by O.le to 0.93e, as compared to HFC=C=CH2, whereas the C, charge decreases by 0.2e to 0.65e. The HOMO delocalizes by 0.12e to each of the fluorine p out-of-plane lone pairs. The decrease at C , accounts for the increase at C2 and the delocalization to the second fluorine. The NHOMO is again localized in the C2
