J . Phys. Chem. 1987, 91, 6 120-6 127
6120
ARTICLES Benzene Ring Deformation and Rotational Isomerism in Terephthalaldehyde: A Study by Electron Diffraction and Molecular Orbital Calculations Charles W. Bock,'* Aldo Domenicano,*lbtcPhilip George,*ld Istvsin Hargittai,*le Gustavo Portalone," and Gyorgy Schultde Chemistry Department, Philadelphia College of Textiles and Science, Philadelphia, Pennsylvania 191 44, Department of Chemistry, Chemical Engineering and Materials, University of L'Aquila, I-671 00 L'Aquila. Italy, C N R Institute of Structural Chemistry, I-0001 6 Monterotondo Stazione, Italy, Biology Department, University of Pennsylvania, Philadelphia, Pennsylvania 191 04, Structural Chemistry Research Group, Hungarian Academy of Sciences, Pf. I 1 7, H-1431 Budapest, Hungary, and Chemistry Department, University of Rome "La Sapienza", I-00185 Rome, Italy (Received: July 31, 1986)
The molecular structure and rotational isomerism of terephthalaldehyde have been investigated by gas-phase electron diffraction at 418 K and by ab initio MO calculations with the 6-31G basis set. The experimental data indicate that the vapor is a nearly 1:l mixture of the trans and cis conformers. The MO calculations find the trans conformer to be more stable than the cis conformer by only 0.21 kcal mol-'. In the electron diffraction study the trans and cis conformers were assumed to have cz),and C,, symmetry, respectively, and to contain identical benzene rings and formyl groups. The benzene ring was constrained to Du, symmetry. The following geometrical parameters were obtained for the heavy-atom skeleton: ( rg(C-C)nn$ = 1.402 i 0.003 A, rg(C-CHO) = 1.487 i 0.004 A, rg(C=O) = 1.208 f 0.003 8, .K-C(CHO)-C = 120.5 f 0.4 , LC-C=O = 123.7 f 0.4'. The experimental data indicate that the two adjacent C-C-CHO angles differ by 0.7 i 0.5"; they also suggest that the central C-C bonds of the ring are some 0.01-0.02 8, shorter than the C,p-Co,h, bonds. The effect that the formyl group has on the ring geometry is much less pronounced than with other common substituents. In the MO calculations the two conformers were assumed to be planar. The calculated geometries of the heavy-atom skeletons are consistent with the experimental results. Also, the small "tilt" of the C-CHO bonds out of the Crpso-Clpso~ axis and the shortening bonds are well reproduced. In addition, the calculations of the central C-C bonds of the ring with respect to the Clpn-Cortho show that the inherent asymmetry of the substituent causes the ring deformation to be unsymmetrical with respect to the C,,.-C,, axis. Comparison with the calculated geometry of benzaldehyde indicates that the distortion of the benzene nucleus in the two conformers may be reproduced by superimposing separate distortions from each formyl group. The minute differences between the calculated geometries and those predicted by the superposition model suggest that the interaction of the formyl group with the benzene ring is less pronounced in terephthalaldehyde than in benzaldehyde. An attractive interaction appears to exist between the carbonyl of the substituent and the nearest aromatic proton.
Introduction Since their first observation by Keidel and Bauer in 1956,2 the structural effects of substitution on the carbon skeleton of the benzene ring have been the subject of extensive experimental work. The techniques that have been used to determine the small deviations from the reference geometry of benzene are X-ray crystallography, neutron crystallography, gas electron diffraction, microwave spectroscopy, and liquid crystal N M R spectroscopy. The structural information accumulated during more than two decades has been analyzed by Domenicano and c o - w o r k e r ~ , ~ - ~ leading to the recognition and interpretation of some important general trends. With most monosubstituted derivatives the symmetry of the benzene ring decreases from D6h to C20. Further lowering of the (1) Ja) Philadelphia College of Textiles and Science. (b) University of L'Aquila. (c) CNR Institute of Structural Chemistry. (d) University of Pennsylvania. (e) Hungarian Academy of Sciences. (f) University of Rome. (2) Keidel, F. A.; Bauer, S. H. J . Chem. Phys. 1956, 25, 1218. (3) Domenicano, A.; Vaciago, A.; Coulson, C. A. Acta Crystallogr., Sect. E 1975, 31, 221. (4) Domenicano, A,; Vaciago, A.; Coulson, C. A. Acta Crystallogr., Sect. E 1975, 31, 1630. ( 5 ) Domenicano, A.; Vaciago, A. Acta Crystallogr., Sect. E 1979, 35, 11x7
( 6 ) Domenicano,A,; Murray-Rust, P.; Vaciago, A. Acta Crystallogr., Sect. E 1983, 39, 457. (7) Domenicano, A,; Mazzeo, P.; Vaciago, A. Tetrahedron Lett. 1976, 1029.
(8) Domenicano, A,; Murray-Rust, P. Tetrahedron Lett. 1979, 2283
0022-3654/87/2091-6120$01.50/0
ring symmetry has been observed experimentally only with some highly unsymmetrical substituents, like the methoxy group.g The structural changes of the carbon skeleton involve bond distances as well as angles. They tend to be more pronounced at the place of substitution and depend markedly upon the CT and R electronic properties of the substituent. While some substituents with very high or low electronegativities (e.g., F, N2+,Li, SiMe,) cause deformations that may amount to several degrees for bond angles and to some hundredths of an angstrom for bond distances, substituents with intermediate electronegativities (e.g., COOH, COC1, COMe, CONH2) leave the geometry of the ring almost unchanged. In para-disubstituted benzene derivatives the deviation of the ring angles from 120' is interpreted quite well as arising from the superposition of separate effects from each substituent,8,i0 unless the electronic interaction between the ring and one substituent is modified by the presence of the More recently, ab initio M O calculations with full optimization of molecular geometry have been applied to the study of the (9) Di Rienzo, F.; Domenicano, A,; Portalone, G.; Vaciago, A. Presented at the Second Yugoslav-Italian Crystallographic Conference, Dubrovnik, Yugoslavia, May 31-June 3, 1976; Abstracts, p A102. (10) Norrestam, R.; Schepper, L. Acta Chem. Scand., Ser. A 1981, 35, 91.
(11) Colapietro, M.; Domenicano, A,; Marciante, C.; Portalone, G. Z . Naturforsch., E 1982, 37, 1309. (1.2) Colapietro, M.; Domenicano, A,; Portalone, G.;Schultz, Gy.; Hargittai, I. J . Phys. Chem. 1987, 91, 1728.
0 1987 American Chemical Society
Structure and Isomerism of Terephthalaldehyde structural effects of substitution in benzene derivative^.'^ A comprehensive study on 18 monosubstituted benzene derivatives has been carried out at the 6-31G level by Bock, Trachtman, and George.I4 Apart from confirming the general trends emerged from previous experimental studies, this study has shown that minute deviations of the carbon skeleton from CZUsymmetryhardly detectable by any experimental technique-are caused by all substituents that do not possess axial, or vertical mirror, symmetry. Moreover, it has shown that some of the C-H bond distances and C-C-H angles are also affected by substitution. A subsequent study on eight para-disubstituted benzene derivatives has confirmed the observation that substituent effects on the ring angles act independently and can be superimposed and has shown that the additivity rule also holds for other geometrical parameters, including C-H distances and C-C-H angles.15 Gas electron diffraction has been used extensively in our laboratories to determine the geometry of the benzene ring in 1,4d i s u b s t i t ~ t e dand ' ~ ~1,3,5-trisubstit~ted~~~~~ ~~~~ derivatives. These studies are aimed at measuring and interpreting the structural effects of different substituents, representing a wide range of electronic properties, in the absence of any solvent or crystal effect. In the present study experimental work has been paralleled by ab initio M O calculations at the 6-31G level, with the aim of (i) comparing the skeletal deformation of the benzene ring determined by calculations and experiment, (ii) studying the effect that the presence of the two formyl groups has on the C-H bond distances and C-C-H angles, (iii) comparing the geometries and energies of the trans and cis conformers of terephthalaldehyde, and (iv) comparing the geometry of terephthalaldehyde with that obtained for benzaldehyde by strictly similar calculation^.'^ The barrier to internal rotation of the formyl group in benzaldehyde has been studied by infrared,z6microwave,27and NMRZ
(13) Vincent, M. A.; Radom, L. J . Am. Chem. SOC.1978, 100, 3306. Pang, F.; Boggs, J. E.; Pulay, P.; Fogarasi, G. J . Mol.Struct. 1980, 66, 281. Boggs, J. E.; Pang, F.; Pulay, P. J. Comput. Chem. 1982, 3, 344. Von Nagy-Felsobuki, E.; Topsom, R. D.; Pollack, S.; Taft, R. W. J . Mol.Struct., Theochem 1982,88,255. Schaefer, T.; Wildman, T. A.; Sebastian, R. J . Mol. Struct., Theochem 1982.89, 93. Politzer, P.; Abrahamsen, L.; Sjoberg, P. J . Am. Chem. SOC.1984,106, 855. Konschin, H. J . Mol.Struct., Theochem 1983, 92, 173. Konschin, H. J. Mol.Struct., Theochem 1983, 105, 213. Konschin, H. J. Mol.Struct., Theochem 1984,110,267. Konschin, H. J . Mol. Strucf.,Theochem 1984, 110, 303. Konschin, H. J. Mol.Struct., Theochem 1984,110.31 1. Bock, C. W.; Trachtman, M.; George, P. Chem. Phys. 1985, 93,431. George, P.; Bock, C. W.; Trachtman, P. J . Mol.Struct., Theochem 1985,133, 11. Krygowski, T. M.; Hafelinger, G.; Schiile, J. Z . Naturforsch., B 1986, 41, 895. (14) Bock, C. W.; Trachtman, M.; George, P. J. Mol.Struct., Theochem 1985, 122, 155. (15) Bock, C. W.; Trachtman, M.; George, P. J . Mol.Struct., Theochem 1986, 137, 387. (16) Domenicano, A,; Schultz, Gy.; Kolonits, M.; Hargittai, I. J. Mol. Struct. 1979, 53, 197. (17) Schultz, Gy.; Hargittai, I.; Domenicano, A. J . Mol.Struct. 1980.68, 281. (18) Domenicano, A,; Schultz, Gy.; Hargittai, I. J. Mol.Struct. 1982, 78, 97. (19) Rozsondai, B.; Zelei, B.; Hargittai, I. J . Mol.Struct. 1982, 95, 187. (20) Colapietro, M.; Domenicano, A.; Portalone, G.; Schultz, Gy.; Hargittai, I. J . Mol.Struct. 1984, 112, 141. (21) Colapietro, M.; Domenicano, A.; Portalone, G.; Torrini, I.; Hargittai, I.; Schultz, Gy. J. Mol.Struct. 1984, 125, 19. (22) Domenicano, A.; Hargittai, I.; Portalone, G.; Schultz, Gy. Presented at the Seventh European Crystallographic Meeting, Jerusalem, Israel, Aug 29-Sept 3, 1982; Abstracts, p 155. (23) Schultz, Gy.; Hargittai, I.; Portalone, G.; Domenicano, A. Presented at the Eleventh Austin Symposium on Molecular Structure, Austin, TX, March 3-5, 1986; Abstracts, p 12. (24) Almenningen, A,; Hargittai, I.; Samdal, S.; Brunvoll, J.; Domenicano, A.; Lowrey, A. J . Mol. Struct. 1983, 96, 373. (25) Almenningen, A,; Hargittai, I.; Brunvoll, J.; Domenicano, A,; Samdal, S. J. Mol.Struct. 1984, 116, 199. (26) Silver, H. G.; Wood, J. L. Trans. Faraday Soc. 1964,60, 5 . Miller, F. A,; Fateley, W. G.; Witkowski, R. E. Spectrochim. Acta, Sect. A 1967, 23, 891. Campagnaro, G. E.; Wood, J. L. J. Mol.Struct. 1970, 6, 117. Durig, J. R.; Bist, H. D.; Furic, K.; Qiu, J.; Little, T. S. J. Mol.Struct. 1985, 129, 45. (27) Kakar, R. K.; Rinehart, E. A,; Quade, C. R.; Kojima, T. J . Chem. Phys. 1970, 52, 3803.
The Journal of Physical Chemistry, Vola91, No. 24, 1987
5
0
10
15
25
20
30
5,
h-1
6121
35
Figure 1. Molecular intensity curves for the two camera distances (E, experimental; T , theoretical). Also shown are the difference curves (experimental - theoretical).
D
__c
0
1
2
3
4 '
5
6
I
% I;A
Figure 2. Radial distribution curves (E, experimental; T, theoretical). The functions were calculated with an artificial damping factor equal to values were used in the 0.00 d s < 1.75 .k' e ~ p ( 4 . 0 0 2 ~theoretical ~~); region. The position of the most important distances is marked with vertical bars, whose height is proportional to the weight of the distances. Also shown is the difference curve (experimental - theoretical).
spectroscopies and found to be relatively high, about 5 kcal mol-' in the vapor phase and 6-8 kcal mol-' in the liquid phase and in solution. Evidence for the presence of both trans and cis conformers of terephthalaldehyde in an essentially 1:l ratio in different solvents has been obtained by dipole moment measurement^,^^ liquid crystal N M R spectra,30 and low-temperature N M R meas u r e m e n t ~ . ~ The ~ ~ ~N' M R data3' indicate that the barrier to rotation of the formyl group of terephthalaldehyde in solution is 6.8 kcal mol-]. Force field calculation^^^ yield equal energy for the trans and cis conformers when the dipolar energy is neglected; the calculated difference in dipolar energy is small, only 0.29 kcal mol-'. Two studies by ESR s p e c t r o s ~ o p yhave ~ ~ ~shown ~ ~ that also (28) Anet, F. A. L.; Ahmad, M. J . Am. Chem. SOC.1964, 86, 119. Drakenberg, T.; Jost, R.; Sommer, J. J . Chem. SOC.,Chem. Commun. 1974, 1011. Lunazzi, L.; Macciantelli, D.; Boicelli, A. C. Tetrahedron Lett. 1975, 1205. (29) Terephthalaldehyde has a dipole moment of 2.37 D in benzene; this has been attributed to the existence of two planar conformations in a nearly 1:l ratio (Barassin, J.; Queguiner, G.; Lumbroso, H. Bull.SOC.Chim. Fr. 1967, 4707. Klabuhn, B.; Clausen, E.; Goetz, H. Tetrahedron 1973, 29, 1153). It has been argued, however, that the value of the molar Kerr constant is not in keeping with the presence of substantial amounts of the cis conformer, and the presence of nonplanar conformations has been invoked (Gore, P. H.; Hopkins, P. A,; Le Ftvre, R. J. W.; Radom, L.; Ritchie, G. L. D. J . Chem. SOC.B 1971, 120). (30) Lunazzi, L.; Ticca, A,; Macciantelli, D.; Spunta, G. J . Chem. SOC., Perkin Trans. 2 1976, 1121. (31) Bernassau, J. M.; Drakenberg, T.; Liljenfors, T. Acta Chem. Scand., Ser. B 1977, 31, 836. (32) Maki, A. H. J . Chem. Phys. 1961, 35, 761. Stone, E. W.; Maki, A. H. J . Chem. Phys. 1963, 38, 1999.
6122
Bock et al.
The Journal of Physical Chemistry, Vol. 91, No. 24, 1987 09
09
2' "i A " ,
\\
\\
ne,
n5
1 ,42
cT " 7\ /c3\n3
'i
H6,
00
c7
"i
, " \
/H2
ne,
f /c3\n3
n5
"i
H4
I48
i'
\\
/H7
010
ns
I ,n2
, I /
7\
"i /c3\n3
'i
/\
H8
010
(a 1 (b) (C) Figure 3. Numbering of atoms in (a) benzaldehyde, (b) trans-terephthalaldehyde,and (c) cis-terephthalaldehyde.
the radical anion of terephthalaldehyde exists as a mixture of trans and cis conformers, in a 58:42 ratio. The present M O calculations indicate that the trans conformer is more stable than the cis conformer by only 0.21 kcal mol-'. The electron diffraction experiment provides evidence for the existence of both conformers in the gas phase at 418 K, with a transcis ratio close to 1:l.
Experimental Section Commercial terephthalaldehyde (Fluka, purum) was purified by crystallization from ethyl acetate. The electron diffraction photographs were taken in Budapest with a modified EG-lOOA apparatus,33using the so-called membrane nozzle system.34 The nozzle temperature was about 418 K. The accelerating voltage of the electron beam was 60 kV. The electron wavelength was calibrated with a TIC1 powder pattern (a = 3.84145 A)35and found to be X = 0.049098 A. Nozzle-to-plate distances of about 50 and 19 cm were used. The tracing of the plates and the reduction of the total intensities to molecular intensities were carried out as in ref 12. The ranges of the intensity data used in the analysis were 2.000 C s C 13.875 A-1 and 10.00 Q s Q 36.00 A-1, with data intervals As = 0.125 and 0.25 A-', respectively [s = 4aX-' sin (0/2), where 0 is the scattering angle]. The total experimental intensities are available as supplementary material (see paragraph at end of paper regarding supplementary material). The experimental molecular intensities are compared in Figure 1 with those calculated from the final refinement. Experimental and calculated radial distributions are presented in Figure 2. The atoms in the molecule are numbered as shown in Figure 3b,c. Analysis of the Experimental Data The analysis was carried out by applying the least-squares method to the molecular intensities, as in our previous studi e ~ . ' * J ~Intensities -~~ in the overlapping s regions were treated as independent data; unit weights were used throughout. Calculations were carried out on the IBM 3031 computer of the Hungarian Academy of Sciences and on the Univac 1100/82 computer of the University of Rome "La Sapienza," by using a modified version of the program by Seip and c o - w o r k e r ~ . The ~~ inelastic and the elastic scattering factors and phase shifts were taken from ref 37 and 38, respectively. (33) Hargittai, I.; Hernldi, J.; Kolonits, M. Prib. Tekh. Eksp. 1972, 239. Tremmel, J.; Kolonits, M.; Hargittai, I. J . Phys. E 1977, 10, 664. Hargittai, I.; Tremmel, J.; Kolonits, M . Hung. Sci. Insrrum. 1980, 50, 31. (34) Hargittai, I.; Hernadi, J.; Kolonits, M.; Schultz, Gy.Rev. Sci. Instrum. 1971, 42, 546. (35) Witt, W. 2.Naturforsch., A 1964, 19, 1363. (36) Andersen, B.; Seip, H. M.; Strand, T. G.; Stdevik, R.Acta Chem. Scand. 1969, 23, 3224. (37) Tavard, C.; Nicolas, D.; Rouault, M. J . Chim. Phys. Phys.-Chim. Biol. 1967, 64, 540. (38) Bonham, R. A.; Schafer, L. In International Tables f o r X-ray Crystallography; Kynoch: Birmingham, 1974; Vol. IV, Chapter 2.5.
During the first stages of the analysis the molecule was assumed to have the trans conformation. Later on, however, inspection of the radial distribution curve in the region r = 6.9-7.7 A showed that the 09.-010 peak was split, revealing the presence of both conformers in comparable amounts. Subsequent refinements were thus based on a model consisting of a mixture of the two species. The composition of the mixture was refined as an additional, independent variable. The trans and cis conformers were assumed to have and C, symmetry, respectively, and to contain identical benzene rings and formyl groups. The benzene ring was constrained to DZh symmetry. Eleven independent geometrical parameters are required to describe the molecular structure of terephthalaldehyde under the symmetry constraints adopted in the present analysis. We have chosen to define these parameters as follows: (i) the Cl-C2 bond distance; (ii) the difference between the Cl-C2 and C2-C3 bond distances, A(C-C); (iii) the C2-Cl-C6 bond angle; (iv) the difference between the Cl-C7 and Cl-C2 bond distances, A(CCHO); (v) the "tilt" of the Cl-C7 bond, A(C-C-C) = ~ c 6 Cl-C7 - K2-Cl-C7; (vi) the C7-09 bond distance; (vii) the Cl-C7-09 bond angle; (viii) the C2-H2 bond distance; (ix) the C3-C2-H2 bond angle; (x) the difference between the C7-H7 and C2-H2 bond distances, A(C-H); (xi) the Cl-C7-H7 bond angle. Parameters i through ix were refined simultaneously as independent variables. Parameters x and xi were varied stepwise, but allowing the other parameters to relax at each step by least-squares refinement. The difference A(C-H) = r(C7-H7) - r(C2-H2) (parameter x) was varied stepwise several times in the course of the analysis. It was found that the choice of A(C-H) had little effect on the geometry of the molecule and that the conditions of the analysis could alter the dependence of the R factor on A(C-H). The value of r(C7-H7) - 0.5 [r(C2-H2) r(C6-H6)] obtained by M O calculations for both conformers of terephthalaldehyde, +0.012 8,was eventually introduced as a constraint. Stepwise variation of the Cl-C7-H7 bond angle (parameter xi) invariably led to a minimum of R centered at about 121O . This value of LCl-C7-H7 is quite different from those obtained by MO calculations for terephthalaldehyde itself and other aromatic aldehydes: trans-terephthalaldehyde, 116.040;cis-terephthalaldehyde, 116.06'; ben~aldehyde,'~ 115.9,'; trans-m-chlorob e n ~ a l d e h y d e116.8O; , ~ ~ cis-m-chl~robenzaldehyde,~~ 116.4'. It is also at variance with the values obtained for two substituted benzaldehydes that have been studied with reasonable accuracy by X-ray crystallography, namely p-hydro~ybenzaldehyde,~~ 115 (1)O, and m-hydroxy-p-meth~xybenzaldehyde,~~ 114 (1)'. It was thus considered safer to fix LCl-C7-H7 at 1 16.0°.42
+
(39) Chiu, N. S.; Ewbank, J. D.; Askari, M.; Schafer, L. J . Mol. Struct. 1979, 54, 185. (40) Iwasaki, F. Acta Crystallogr., Sect. B 1977, 33, 1646. (41) Iwasaki, F. Chem. Let?. 1973, 227.
Structure and Isomerism of Terephthalaldehyde In addition to the geometrical parameters, 14 mean amplitudes of vibration I were refined as independent variables. Two of them were individual I values and 12 were coupled in blocks to other 1 values with constrained differences. The coupling scheme of the blocks is shown in Table I. Since spectroscopically calculated amplitudes were not available, the differences within each block were carefully chosen, also on the basis of previous experience with similar molecules. Moreover, the effect that alternative choices might have on the geometry and on the agreement between observed and calculated molecular intensities was thoroughly investigated. The remaining amplitudes were fixed at reasonable values. Our experience with spectroscopically calculated amplitudes in benzene derivatives is that they are not always reliable and should not be introduced blindly in the analysis of electron diffraction data. The lack of a reliable force field and a complete set of experimental frequencies, including the lowest ones, seems to be a serious obstacle for more accurate electron diffraction studies of these systems, especially when-as in the present c a s e t h e asymmetry of the substituent gives rise to a substantial increase in the number of independent amplitudes. The results of the final refinement, R = 0.0325, are given in Table I. Some of the geometrical parameters, viz., r(C7-09) and the mean of the aromatic C-C bond distances, are very well determined, as their values have proved to be virtually insensitive to the conditions of refinement, Le., to the choice of those variables that could not be refined and were thus either fixed or constrained.43 Most of the other parameters are reasonably well determined, in the sense that appreciable deviations from the values reported in Table I were only observed under extreme, unreasonable refinement conditions. The internal ring angle C2-Cl-C6 has shown a moderate sensitivity to the choice of the differences between some important amplitudes of vibration that were coupled in the refinement, namely, I(Cl-C7) - I(Cl-C2), I(09-Cl) - 1(Cld23), and I(09-C6) - l(Cl-..C4). For example, varying AI = I(Cl-C7) I(Cl-C2) from +0.002 to +0.010 A (compared to +0.006 A in Table I) causes LC2-Cl-C6 to decrease from 120.7' to 120.2'. Only two geometrical parameters should not be considered as accurately determined by the present experimental study: these are the C3-C2-H2 angle and the difference between the two nonequivalent C-C bond distances of the benzene ring, A(C-C). The angle C3-C2-H2 has been refined under the constraint of being equal to C5-C6-H6. This constraint is not substantiated, however, by the results of the M O calculations; see later on in this paper. Stepwise variation of an assumed difference between the angles C3-C2-H2 and C5-C6-H6 indicates that the experimental data do not contain enough information to allow a reliable determination of this parameter. Fortunately, all the other geometrical parameters were hardly affected by any reasonable choice of the difference LC3-C2-H2 - LC5-C6-H6. The difference between the two nonequivalent aromatic C-C bond distances, A(C-C) = r(Cl-C2) - r(C2-C3), has proved to be sensitive to the conditions of refinement. An extreme case may serve to exemplify this. Varying A1 = 1(09.-Cl) - I(Cl.-C3) from -0.012 to +0.004 A (compared to -0.004 A in Table I) causes A(C-C) to increase from 0.010 to 0.025 A. In no case, however, was A(C-C) found to vanish or to become negative, which means that the sign of this distortion is firmly established by the present experiment. An attempt was eventually made to ascertain whether the experimental data could reveal any twist of the formyl groups out of the ring plane. In this attempt the two substituents were considered as coplanar and their carbon atoms, C7 and C8, were constrained on the Cl-C4 axis. The plane of the substituents (42) Fixing the angle Cl-C7-H7 at 121.0' instead of 116.0' causes the
R factor to decrease from 0.0325 to 0.0313 but has only a limited effect on the other molecular parameters. The largest variations involve the angle C3-C2-H2, which increases by 0.5', and the angles C2-Cl-C6 and C1C7-09, which both decrease by 0.2-0.3O. (43) The space of such variables was sampled at more than 150 points in the course of the analysis.
The Journal of Physical Chemistry, Vol. 91, No. 24, 1987 6123 TABLE I: Final Molecular Parameters' from the Electron Diffraction Study Distances and Mean Amplitudes of Vibration atom pair Cl-C2 C2-C3 Cl-C7 C7-09 C2-H2 C7-H7 C1- * *C3 C 1. * *C4 C2. C 5 C2* C 6 C2. * .C7 C3. * .C7 C4. * .C7 C5.. .C7 C6. * *C7 C7. * *C8
multiplicity 4 2 2 2 4 2 4 1 2 2 2 2 2 2 2 1
09**.C1 0 9 . * .C2 0 9 - * *C3 09. * C 4 09-.*C5 0 9 . * *C6 0 9 . * .C8
2 2 2 2 2 2 2
10
1
C1** * H 2 C1. * *H3 C1** .H7 C2. * .H3 C2**.H5 C2** .H6 C2. .H7 C3-*.H7 C4. .H7 C5. * *H7 C6**-H7 C7. *H2 C7* *H3 C7. -H5 C7. * *H6 C7. * *H8
4 4 2 4 4 4 2 2 2 2 2 2 2 2 2 2
09-*.H2 0 9 . * *H3 0 9 - * .H5 0 9 . * *H6 0 9 . * *H7 09***H8
2 2 2 2 2 2
09-
e . 0
-
-
conformerc
trans cis
trans cis trans cis
trans cis
trans cis
(A)b key to coupling scheme
r,
1
1.406 (1) 1.388 ( 2 ) d 1.485 (2)d 1.2064 (6) 1.090 (3) 1.102 (3)d 2.417 ( 2 ) d 2.783 ( 5 ) d 2.809 (3)d 2.442 (3)d 2.496 ( 3 ) d 3.770 (2)d 4.268 (5)d 3.777 (2)d 2.506 (3)d 5.753 (4)d 5.753 (4)d 2.377 (3)d 3.614 (2)d 4.787 (2)d 5.042 (3)d 4.252 (4) 2.867 (4)d 6.503 (2)d 6.506 (3)d 7.375 (4)d 7.104 (7)d 2.183 (9)d 3.399 (7)d 2.203 (3)d 2.138 3.899 (4)d 3.438 (6)d 2.668 (5)d 4.053 (5)d 4.850 (6)d 4.618 (3)d 3.467 (4)d 2.728 (17)d 4.633 (9)d 4.642 2.743 6.312 (5)d 6.309 (6)d 3.933 (17)d 5.726 ( 8 ) d 4.916 (15)d 2.617 2.003 (4)d 6.905 (3)d 7.187 (4)d
0.0471 (8) 0.0471 0.0531 0.0378 (7) 0.0778 0.0778 0.052 (2) 0.072 (2) 0.072 0.052 0.067 (3) 0.068 (2) 0.086 (2) 0.068 0.067 0.083 (12) 0.083 0.048 0.072 0.068 (3) 0.082 (4) 0.096 0.104 0.102 (6) 0.102 0.090 (19) 0.105 0.120 (4) 0.098 (6) 0.150 0.120 0.095' 0.098 0.150' 0.145e 0.130' 0.120e 0.108 O.14Oc 0.120' 0.120' 0.140' 0.130e 0.130' 0.140e 0.130' 0.140e 0.165' 0.150 O.15Oe 0.135'
1
I 1
ii ii ii... 111
iv iv iii V
vi vii vi V
viii viii ... 111
vi ix X
vii iv xi xi xii xii... Xlll
xiv xiii ... Xlll
xiv
xiv
xiii
Other Molecular Parameters parameters
valuesf
parameters
valuesf
A(C-C)g A(C-CHO)" A(C-H)' LC2-Cl-C6 A(C-C-Cy
0.018 (3) 0.079 (2) 0.0 12' 120.5 (3) 0.7 (4)
LC6-Cl-C7 LC3-C2-H2 LCl-C7-09 LCl-C7-H7 trans:cis ratio
120.1 (2)d 118.8 (9) 123.7 (3) 1 16.0e 57:43 (6)
Least-squares standard deviations are given in parentheses as units in the last digit. bThe H.. .Hdistances are not shown; they have been given a fixed amplitude of 0.150 A. 'The conformer is indicated only for those distances that have been allowed to assume different values in the two conformers. Dependent parameter. e Assumed. (Bond distances are in angstroms; bond angles are in degrees. gA(C-C) = r(Cl-C2) - r(C2-C3). "A(C-CHO) = r(C1-C7) - r(Cl-C2). 'A(CH) = r(C7-H7) - r(C2-H2). 'A(C-C-C) = ~ C 6 - c l - C 7 - ~ c 2 - C l CI.
was rotated stepwise along that axis. At each step all molecular parameters were refined, including the conformer composition of
6124
The Journal of Physical Chemistry, Vol. 91, No. 24, 1987 ..
09
H7 .
A 1.48720.004 I
n5
~1.390~0.004
A An3 H7
A1.476
I HI
An important result of the present study is that the geometry of the benzene ring in terephthalaldehyde is only marginally different from that of unsubstituted benzene. The lack of an appreciable angular distortion is a consequence of the electronic properties of the formyl group. The internal angle at the ipso carbon, a parameter that is particularly sensitive to the u-inductive effect of the sub~tituent,~-~ is LC2-Cl-C6 = 120.5 f 0.4°.47 This value is intermediate between those obtained by electron diffraction for p-xylene,16 117.1 f 0.3', and p-difluorobenzene,'* 123.5 f 0 . l o , in accordance with the values of Taft's inductive parameter, oI, which are -0.08, 0.25, and 0.52 for the Me, CHO, and F substituents, r e s p e ~ t i v e l y . ~ ~ The mean length of the ring C-C bonds is ( r (C-C)~"&= 1.402 f 0.003 A, compared with 1.399 f 0.003 in unsubstituted benzene.49 The central C-C bonds of the ring are shorter than the Ciw-Corthobonds by some 0.01-0.02 A; it is unfortunate that the electron diffraction experiment is unable to provide a more accurate value for this difference. Differences of the same magnitude have been obtained by X-ray crystallography in several symmetrically para-disubstituted benzene derivatives where the electronic properties of the substituent are similar to those of the formyl group, e.g., p-dicyanobenzene,20 terephthalic acid,s0 terephthaloyl chloride,s' and terephthalic acid dimethyl ester.52 In VB terms, the shortening of the central C-C bonds of the ring that occurs in these systems may be attributed to contributions from polar canonical forms like
A
(a)
09
Bock et al.
11.363
A AH3
(b) Figure 4. Experimental and calculated molecular geometries of terephthalaldehyde (bond distances are in angstroms; bond angles are in degrees). (a) Experimental geometry by electron diffraction; bond distances are rg values. Total errors (estimated according to Hargittai and Hargittai&) are given as error limits. These errors take properly into account systematic effects of experimental nature, e.g., scale errors, but do not include effects caused by the constraints adopted in the refinement. Thus in the case of r(Cl-C2) and r(C2-C3) the errors given reflect only the uncertainty in the mean length of the ring C-C bonds; they do not take into account the fact that the value of A(C-C) = r(C1-C2) r(C2-C3) is affected by the conditions of the refinement (see text). The values of LCl-C7-H7 and of the difference between r(C7-H7) and r(C2-H2) have been assumed. (b) Geometry obtained by a b initio MO calculations a t the 6-31G level. The bond distances and angles of the trans and cis conformers (see Tables I1 and 111) have been averaged for a tramcis ratio of 57:43, imposing DZhsymmetry to the benzene ring.
the mixture. The results obtained indicate that a small twist of the formyl groups (up to about 15') is compatible with the experimental data. On the other hand, a structure with the formyl groups perpendicular to the plane of the ring is inconsistent with the experimental radial distribution of Figure 2, since it would require the 09-C2, 09.-C6 contributions to appear as a single peak at 3.2-3.3 A and the 0 9 4 2 3 , 0 9 4 3 contributions to coalesce into a peak at about 4.5 A.
Molecular Orbital Calculations The ab initio M O calculations on the trans and cis conformers of terephthalaldehyde were carried out in Philadelphia on a VAX 11/780 computer by using the Gaussian 82 program.44 The standard 6-3 1G basis set4swith gradient optimization was employed. The maximum forces on the distance and angle coordinates in the final optimization were less than 0.00045 hartree bohr-I and 0.000 45 hartree radian-', respectively. The molecule was assumed to be planar in both conformations. Results and Discussion Experimental Geometry. The bond angles and rgbond distances of terephthalaldehyde, based on the results of the final refinement, are shown in Figure 4a. (44) Binkley, J. S.; Frisch, M. J.; DeFrees, D. J.; Ragavachari, K.; Whiteside, R. A.; Schlegel, H. B.; Fluder, E. M.; Pople, J. A. Camegie-Mellon University, Pittsburgh, PA, 1983. (45) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257
The formyl group is slightly tilted in the plane of the ring, out of the Cl-C4 axis. The small difference between the C6-Cl-C7 and C2-Cl-C7 angles, 0.7 f 0.5', is well determined by the present experiment. We point out here that this difference has never become negative in the course of the analysis. Small differences of the same sign have been observed in those para- and meta-substituted derivatives of benzaldehyde that have been studied with reasonable accuracy by X-ray crystallography, 3.9', m-hydroxy-p-methoxynamely, p-hydro~ybenzaldehyde,~~ 0.7O. benzaldehyde:' 3.3', and 1,2-bis(p-f0rmylphenyl)ethane,~~ The C = O bond distance, rg = 1.208 f 0.003 A, is in excellent agreement with the corresponding bond distances in other simple aldehydes that have been studied accurately by gas-phase electron diffraction, like f ~ r m a l d e h y d e rg , ~ ~= 1.209 f 0.003 A, acetaldehyde,s4 rg = 1.209 f 0.003 A, and acrolein,ss rg = 1.2093 (6)
A.
The value of the C - C 4 angle, 123.7 f 0.4', compares well with the values obtained for acetaldehyde, 123.4°,s6 and acrolein, 124.7 (2)'. Values of 126.3 (3)O, 125.3 (2)', and 124.4 (4)', respectively, have been obtained by X-ray crystallography for the three substituted benzaldehydes referred to p r e v i o ~ s l y . ~ ~ ~ ~ ~ ~ ~ Calculated Geometry. The benzene ring geometries calculated (46) Hargittai, M.; Hargittai, I. J. Chem. Phys. 1973, 59, 2513. (47) Here and throughout this paper total errors are given as error limits; least-squares standard deviations are given in parentheses as units in the last digit. (48) These values are derived from the I9F NMR chemical shifts of meta-substituted fluorobenzenes (Taft, R. W.; Price, E.; Fox, I. R.; Lewis, I. C.; Andersen, K. K.; Davis, G. T. J. Am. Chem. Sot. 1963, 85, 709). (49) Schultz, Gy.; Kolonits, M.; Hargittai, I., unpublished work. A value of 1.399 f 0.001 A has been obtained by Tamagawa, K.; Iijima, T.; Kimura, M. J. Mol. Struct. 1976, 30, 243. (50) Bailey, M.; Brown, C. J. Acta Crystallogr. 1967, 22, 387. (51) Leser, J.; Rabinovich, D. Acta Crystallogr., Sect. B 1978, 34, 2253. (52) Brisse, F.; Perez, S. Acta Crystallogr., Sect. B 1976, 32, 2110. (53) Celikel, R.; Geddes, A. J.; Sheldrick, B. Cryst. Struct. Commun. 1978. 7. 683. (54)'Kato, C.; Konaka, S.; Iijima, T.; Kimura, M. Bull. Chem. SOC.Jpn. 1969. 42. 2148. (55) Traetteberg, M. Acta Chem. Scand. 1970, 24, 373. (56) Calculated from the values of ra(C-C), r,(C=O), and r,(C--O) given in ref 54.
Structure and Isomerism of Terephthalaldehyde TABLE II: Benzene Ring Geometry' in Benzaldehyde" and in Trans and Cis Conformers of Terephthalaldehyde, Calculated Using the 6-316 Basis Setb benztranscisparameters aldehyde terephthalaldehyde terephthalaldehyde 1.391, 1.3885 r(Cl-C2) 1.390, 1.3831 1.3859 1.3865 r(C2-C3) 1.3885 1.387, 1.3915 r(C3-C4) 1.3918 1.3948 r(C4-C5) 1.3910 1.3831 1.3803 r(CSwC6) 1.3836 1.3915 1.3948 1.3937 r(C6-Cl) 2.7733 2.7732 r(C1. *.C4) 2.7745 2.7799 2.7766 2.7746 r(C2. * C S ) 2.7799 2.78 1 5 2.784, r(C3*..C6) r(C2. 426) 2.40g5 2.41 1 2 2.41 1, 2.41 1 2 2.41 1, 2.410, r(C3...C5) 1.074, 1.0739 r(C2-H2) 1.074* 1.0713 1.0739 r(C3-H3) 1.0724 r(C4-H4) 1. O n 3 1.074, 1.0715 1.0728 r(CS-H5) r(C6-H6) 1.072] 1.0713 1.071, 2.4719 2.4604 r(H2***H3) 2.4611 2.4833 2.4730 2.4719 r(H5. eH6) LC6-Cl-C2 119.80 120.0, 120.0~ 119.98 120.2, LCI-C~-C~ 120.2~ 119.98 119.70 119.7, LC2-C3-C4 LC3-C4-CS 120.32 120.0~ 120.0, 120.2, 119.95 LC4-C5-C6 119.92 119.95 120.01 119.7, LCS-C6-C1 LCl-C2-H2 119.82 119.89 120.0, LC4-C3-H3 120.1, 119.16 120.0, LC3-C4-H4 119.83 LCS-C4-H4 119.85 LC4-CS-HS 120.03 119.89 119.07 LCl-C6-H6 118.99 119.16 119.07
-
-
'Internuclear distances are in angstroms; bond angles are in degrees. bThe values for benzene are r(C-C) = 1.388, A, r(Cl...C4) = 2.7765 A, r(C2.. C 6 ) = 2.404, A, r(C-H) = 1.0733 A, r(H2. *H3) = 2.4616 '4.14
by using the 6-31G basis set for ben~aldehyde'~ and for the trans and cis conformers of terephthalaldehyde are presented in Table 11; the formyl group geometries are given in Table 111. From Table I1 it can be seen that the Cipso-Corthobonds are consistently longer than the calculated C - C bond in unsubstituted benzene, 1.3883 whereas the central C-C bonds are shorter. There are, however, subtle differences in the lengths of the bonds related by the Cl--C4 axis. For instance, the Cipso-Corthobond on the side of the C=O bond is longer than that on the other side by about 0.004 8, in benzaldehyde and 0.006 8, in cis-terephthalaldehyde, whereas in the case of trans-terephthalaldehyde the two bonds have virtually equal length. These differences lead to an elongation of the ring in the direction of the C=O bond axis. The nonbonded C--C distances reveal this feature even more clearly. Relative to benzene, r ( C 3 . 4 6 ) is longer by about 0.005 A in benzaldehyde and 0.008 A in trans-terephthalaldehyde,while in cis-terephthalaldehyde, where the two C=O bonds have different orientations, both r(C3-C6) and r(C2-CS) are about 0.003 A longer than in benzene. The geometrical distortion of the benzene nucleus caused by the substituent in a monosubstituted derivative may conveniently be expressed in terms of deviations from the reference geometry of benzene, e.g., Aru = rii- r(C-C)benzene,ABi = Oi - 120' (where rij and Bi are the C-C bond distances and C-C-C angles of the ring). From the geometries of benzaldehyde and unsubstituted benzene calculated at the 6-3 1G level a complete set of Ar,. and ABi values is derived for the formyl group; see Table IV. 5-4 The distortional parameters of Table IV can be used to check whether the changes of the benzene ring geometry occurring in the two conformers of terephthalaldehyde may be interpreted as arising from the superposition of separate effects from each formyl (57) The 12 distortional parameters of Table IV are not geometrically independent, since only 10 parameters are required to define an irregular planar hexagon.
The Journal of Physical Chemistry, Vol. 91, No. 24, 1987 6125 group. For instance, the C2-C3 bond distance in trans-terephthalaldehyde is predicted to be r(C2-C3) = 1.3883 ArZ3 Ar56= 1.3818A; the calculated distance in Table I1 is 1.383] A. Similarly, the C2-C3-C4 angle in cis-terephthalaldehyde is predicted to be ~ c 2 - C 3 - C 4 = 120 A03 AB2 = 119.950;the calculated value is 1 19.980. Bond distances and angles of the benzene ring predicted for the two conformers of terephthalaldehyde are compared in Table V with those calculated using the 6-31G basis set. The predicted geometries are seen to reproduce well the calculated ones, as no difference exceeds O.0Ol6 A for bond distances and 0.06' for bond angles. Nevertheless, the small differences between the predicted and calculated geometries of Table V consistently indicate that the benzene ring of both conformers of terephthalaldehyde is slightly less distorted than implied by perfect additivity of distortional effects. Insofar as geometrical distortions are a consequence of electronic interactions between the ring and the substituent, it can be stated that the interaction of the formyl group with the benzene ring is somewhat less pronounced in terephthalaldehyde than in benzaldehyde. A larger variation of the ring-substituent interaction occurs with the amino group, as shown by a comparison of the experimental geometries of aniline and p-diaminobenzene.I2 There are also interesting differences between the formyl group geometries; see Table 111. The C = O bond in the two conformers of tere hthalaldehyde is shorter than in benzaldehyde by about while the C-CHO bond is longer by about 0.005 A. 0.002 Apparently the C 4 bond of terephthalaldehyde has more double bond character, and the C-CHO bond less double bond character, than the corresponding bonds in benzaldehyde. This indicates that the extent of conjugation of the formyl group with the ring is reduced when a second formyl group is introduced in the para position, in agreement with the conclusions derived from the analysis of the ring deformation. The asymmetry of the formyl group leads to an unsymmetrical attachment to the ring: the angle C6-Cl-C7 is greater than C2-Cl-C7 in benzaldehyde, and the same occurs with the corresponding angles of trans- and cis-terephthalaldehyde. The difference between the two adjacent C-C-CHO angles, which has been termed the "tilt" of the s~bstituent,'~ amounts to 0 . 7 4 9 ' in the three molecules considered. A striking feature involving both the formyl group and the ring is the contrast between the geometrical parameters of the aromatic C-H bond closest to the carbonyl and those of the other aromatic C-H bonds. The former C-H bond is consistently shorter than the others, especially in the case of trans- and cis-terephthalaldehyde, where the difference is about 0.003 A. Moreover, the angles that it makes with the two adjacent C-C bonds are unequal, in the sense that the C-H bond is bent toward the carbonyl. In the case of benzaldehyde the angle Cl-C6-H6 is smaller than C5-C6-H6 by 2.0°, compared to an average difference of only O.O4O for the other C-C-H angles. In the case of trans- and cis-terephthalaldehyde the corresponding difference is 1 .go. It appears that an attractive interaction exists between the carbonyl and the nearest aromatic proton.s8 A similar feature has been observed in cis-acrolein by MO calculations at the 4-31G leveLS9 Experimental versus Calculated Geometry. In order to allow a comparison of the calculated geometry of terephthalaldehyde with the experimental geometry of Figure 4a, the calculated bond distances and angles of the two conformers have been averaged for a tramcis ratio of 57:43, imposing Dlh symmetry to the
+
+
+
+
1,
~~
~
(58) The existence of an attractive interaction is also reflected in the H-H nonbonded distances around the ring; see Table 11. In benzaldehyde r(H2. -H3) is 2.4611A, hardly different from the benzene distance of 2.461, A. On the other hand r(H5-H6) is 0.01 1 A greater than in benzene, just as would be expected if H6 were drawn toward 09. In trans-terephthalaldehyde the structure is such that both H-H distances would be affected equally; accordin ly r(H2.-H3) and r(H5-H6) are both greater than in benzene by 0.010 In cis-terephthalaldehyde, r(H2.-H3) would be unaffected by any such interaction but r(H5-eH6) doubly affected; accordingly r(H2-H3 differs from the benzene value by -0.001 A, while r(H5-H6) is 0.022 greater. (59) George, P.; Bock, C. W.; Trachtman, M. J . Mol. Struct. 1980, 69,
ti.
183.
d
The Journal of Physical Chemistry, Vol. 91, No. 24, 1987
6126
Bock et al.
TABLE 111: Formyl Group Geometry" in Ben~aldehyde'~ and in Trans and Cis Conformers of Terephthalaldehyde, Calculated Using the 6-31G Basis Set transCIS~~
parameters
benzaldehyde
r(C 1-C7) r(C7-H7) r(C7-09) LC6-C 1-C7 LC2-c 1- c 7 LCl-C7-H7 LC 1-C7-09 L09-C7-H7 r(09. mH7) r(09.. mH6)
1.4713 1.0857 1.217, 120.4, 119.7, 1 15.9s 124.36 119.66 1 .9921 2.5768
-
parameters
terephthalaldehyde
r(C1-C7), r(C4-C8) r(C7-H7), r(C8-H8) r(C7-09), r(C8-0 10) LC6-Cl-C7, LC3-C4-C8 LC2-Cl-C7, LC5-C4-C8 LC 1-C7-H7, LC4-C8-H8 LCl-C7-09, LC4-C8-010 L09-C7-H7, L010-C8-H8 r(09-..H7), r(010...H8) r(09..-H6), r(OlO...H3)
1.4759 1.0846 1.2155 120.30 1 19.6, 1 16.04 123.9, 119.99 1.993, 2.5680
parameters
terephthalaldehyde
r(C 1-C7), r(C4-C8) r(C7-H7), r(C8-H8) r(C7-09), r(C8-0 10) LC6-Cl-C7, LC5-C4-C8 LC2-Cl-C7, LC3-C4-C8 LC 1-C7-H7, LC4-C8-H8 LCl-C7-09, LC4-C8-010 L09-C7-H7, L010-C8-H8 r(09. ..H7), r(010.. .H8) r(09...H6), r(010...H5)
1.475, 1.0846 1.2152 120.4, 119.52 1 16.06 123.9, 1199, 1.9925 2.572,
'Internuclear distances are in angstroms; bond angles are in degrees. TABLE I V Geometrical Distortions of the Benzene Nucleus Caused bv the Foimvl GrouD'
paramb
valuesc
paramb
valuesC
Ar12 Ar23
0.001, -0.00 18
Ah
-0.2, 0.25 -0.30 0.32 -0.0s 0.0,
b 3 4
Ar45 Ar56 Ar61
A02 A03 A04
-o.ooo8 0.002, -0.0047 0.0054
A05 A06
From the geometries of benzaldehyde and unsubstituted benzene, as obtained by ab initio MO calculations at the 6-31G level.I4 'Defined as ArI2 = r(C1-C2) - r(C-C), etc., A0, = LC2-Cl-C6 120°, etc., where r(C1-C2), LC2-Cl-C6, etc., refer to benzaldehyde, and r(C-C) refers to unsubstituted benzene. The atoms of the benzaldehyde molecule are numbered as shown in Figure 3a. 'Changes in bond distances are in angstroms;changes in bond angles are in degrees. TABLE V Geometry' of the Benzene Nucleus of Terephthalaldehyde, As Predicted by Superimposing Separate Distortions from Each Formyl Group and As Obtained by ab Initio MO Calculations at the 6-31G Level
parameters
pred values Trans Conformer
r(C1-C2), r(C4-C5) r(C2-C3), r(C5-C6) r(C3-C4), r(C6-Cl) LC2-Cl-C6, LC3-C4-C5 LCl-C2-C3, LC4-C5-C6 LC2-C3-C4, LC5-C6-C1
1.3929 1.38lS 1.3929 120.1, 120.1, 119.7,
Cis Conformer r(C1-C2), r(C3-C4) 1.389, 1 .384, r(C2-C3) r(C4-C5), r(C6-Cl) 1.3964 1.378, r(C5-C6) LC2-Cl-C6, LC3-C4-C5 120.12 LCl-C2-C3, LC2-C3-C4 119.95 LC4-C5-C6, LC5-C6-C1 119.93
calcd values 1.3918 1.383' 1.391, 120.0~ 120.2, 119.74 1.3885 1.3859 1.394s 1.3803 120.0, 119.9s 119.95
"Bond distances are in angstroms; bond angles are in degrees. benzene ring. The resulting geometry is shown in Figure 4b. The straightforward comparison of rBdistances obtained by gas-phase electron diffraction with re distances obtained by MO calculations is inappropriate, in view of their different physical meaning. Moreover, the calculated revalues depend on the basis set used and are affected by the neglect of electron correlation. It is legitimate, however, to compare differences between distances involving atoms of the same type, whereby most of the systematic effects cancel. In fact. the r,(C-C) values of Figure 4b are all smaller than the corresponding > & C k ) values of Ggure 4a, while the differences between C-C bond distances are reasonably well reproduced in the two parts of that figure. It is rewarding to note that calculations and experiment agree in indicating that the central C-C bonds of the benzene ring are somewhat shorter than the Cipo-Corthobonds. As regards bond the and calculated values of all angles not involving hydrogen atoms agree within experiY
-
TABLE VI: Changes in Expectation Energies" That Accompany the Cis Trans Conversion of Terephthalaldehyde*and GlyoxalC
terephthalaldehyde 6-31G
+o.ooo 33 -0.130 24 -0.12779 -0.257 70 +0.257 37 +0.127 46 -0.000 33 (-0.21)
glyoxal
glyoxal
4-31G +0.01073 -1.433 79 -1.339 73 -2.762 79 +2.754 10 +1.33104 -0.008 69 (-5.45)
6-31 lG** +0.008 66 -1.469 15 -1.37571 -2.836 20 +2.827 54 + 1.367 05 -0.008 66 (-5.43)
"Values are in au, except those in parentheses, which are in kcal mol-' (1 kcal mol-' = 0.001 5936 au). *The individual values of EK, V, V,,, Erep,V,, Eel=, and ETfor the trans conformer are 456.105 55, 611.61902, 456.187 16, 1523.911 73, -1979.88045, -912.15588, and -455.96872 au, respectively. c I n ref 59 the 4-31G basis set values were rounded off to four decimal figures. dAErep= AEK + AV,, + AV,,.
+
= AEK AV=
+ AV,,.
fAE, = AV,,
+ AE,,,,.
mental error. Also, the small tilt of the formyl group out of the C1-C4 axis is a common feature of calculations and experiment. The internal angle at the ipso carbon, C2-Cl-C6, is 120.5 f 0.4O from the electron diffraction experiment and 120.1O from the MO calculations. Discrepancies between the experimental and calculated values of this angle, amounting to about 2" in extreme cases, have been reported for various benzene derivative^.^^,^^,^' While in some cases experimental error cannot be ruled out, at least with fluorobenzene and cyanobenzene it would appear that if the discrepancies are due to inadequacies in the MO calculations, they are more likely to originate in the neglect of correlation energy than in basis set truncation.62 R o t a t i o n a l Isomerism. The M O calculations find the trans conformer of terephthalaldehyde to be more stable than the cis conformer by a mere 0.21 kcal mol-' (Table VI). This corresponds to an approximate transxis ratio of 56:44 at the temperature of the electron diffraction experiment. Of course, the question arises as to whether the use of more extended basis sets, including polarization functions, would cause a significant change in the small energy difference. Such a calculation was not feasible for terephthalaldehyde, but for the corresponding conformers of glyoxal, calculations at the 4-31G and 6-31 1G** levels find the energy difference to be the same to within 0.02 kcal mol-'; see Table VI. It is quite unlikely that for the terephthalaldehyde conformers, in which the formyl groups are much further apart, the energy difference would be much more basis-set dependent. For both molecules the cis trans conversion reaction is of the "repulsive-dominant exothermic" type.63 The values of AV=,
-
(60) Portalone, G.; Schultz, Gy.; Domenicano, A.; Hargittai, I. J . Mol. Struct, 1984, 118, 53, (61) Portalone, G.; Domenicano, A.; Schultz, Gy.; Hargittai, I. J . Mol. Struct., 1987, 160,97. (62) For fluorobenzene, calculations with the 6-31G, 6-31G*(SD), and 6-31G** basis sets give ipso angles that are smaller than the experimental value by 0.450,0.g9O, and 1.OIo, respectively. For cyanobenzene the corresponding differences are 1.740,1.4,", and 1.440. See Bock, C. W.; Trachtman, M.; George, P. J . Compur. Chem. 1985, 6, 592.
Structure and Isomerism of Terephthalaldehyde
....
*. 0
7'0
'
__
.
'
7'5
,
The Journal of Physical Chemistry, Vol. 91, No. 24, 1987 6127 erp. theor.,trans+CIs
'
rei)
Figure 5. Enlarged diagram of the 6.9-7.7-A region of the radial distribution of terephthalaldehyde, showing the experimental and three theoretical curves. The functions were calculated as described in the caption of Figure 2. The position of the 09-.010 and 09-H8 distances for the two conformers is marked with vertical bars, whose height is proportional to the weight of the distances. The theoretical curves correspond to refinements based on three different models: two consisting of only one conformer and the third consisting of a mixture of the two conformers, whose composition was refined as an independent variable.
AV,,, and AVengiven in Table VI show that the net driving force is a decrease in repulsive interactions, not an increase in attractive interactions. The major contribution to the decrease in A V , arises from the increase in the 09--010 internuclear separation, and the importance of repulsion between these two oxygen nuclei is further borne out by the following calculation. Rigid rotation of one of the formyl groups in the equilibrium trans structure to give a distorted cis structure results in an 09.-010 internuclear distance of 7.084 A, whereas in the equilibrium cis structure the distance (63) Allen, L. C. Chem. Phys. Lett. 1968, 2, 597. Goddard, J. D.; Csizmadia, I. G. Theor. Chim. Acta 1977, 44, 293. George, P.; Bock, C. W.; Trachtman, M.; Brett, A. M. Int. J . Quantum Chem. 1978, 13, 271.
is 7.099 A. Relaxation thus occurs with the oxygen nuclei moving 0.015 8, farther apart. The 6.9-7.7-A region of the experimental radial distribution, containing the contributions of the 09.-010 and 09-H8 distances, is shown in Figure 5. (Note that the vertical scale is blown up by about 8 times with respect to the horizontal scale, as compared with Figure 2). This region shows the existence of both conformers of terephthalaldehyde in the vapor phase. Essentially no information on the conformer composition is contained in the region of the curve below r = 6.9 A. A closer inspection of Figure 2 reveals how little is the relative contribution of conformationdependent interactions to the total electron scattering. In most refinements the conformer composition was treated as an independent variable. It proved to be rather insensitive to the conditions of the analysis as it was affected only by the choice of the difference 1(09~-010),,,-1(09-~01O),,,,,. Models based on a single conformer, though refining to fairly low R values, are definitely inferior to the model based on a mixture of the two conformers in reproducing the conformation-dependent portion of the radial distribution (Figure 5 ) . Moreover, they yield unreasonably large values for the 09-.010 amplitude. The value of the tramcis ratio from the final refinement is 57:43 (6). Statistical considerations would of course lead to the conclusion that the trans:cis ratio is 1:l within experimental error. We point out, however, that whenever the conformer composition of the mixture was allowed to refine as an independent variable, the trans conformer was always found to be slightly more abundant than the cis conformer. This suggests that the experimental value of the trans:cis ratio might be more accurate than implied by its formal precision. The present result is in agreement with the M O calculations and with the results of solution measurement^.^^-^* Acknowledgment. We express our appreciation to M5ria Kolonits for experimental work and to Clara Marciante and M5rta Kalicza for technical assistance. Registry No. Benzaldehyde, 100-52-7; terephthalaldehyde, 623-27-8; glyoxal, 107-22-2.
Supplementary Material Available: Two tables showing total experimental intensities (7 pages). Ordering information is given on any current masthead page.
