摘要:

Bond Distortions in Dibenzo[a,g]-s-indacene and Its Congeners$ Masahiro Kataoka*a and Azumao Toyotab aTohoku College of Pharmacy Komatsushima 4-4-1 Aoba-ku Sendai 981 Japan bDepartment of Chemistry Faculty of Education Yamagata University Yamagata 990 Japan Semiempirical MO calculations predict that dicyclopenta[a,g]- dicyclopenta[a,h]- dibenzo[a,g]- dicyclohepta[a,g]- and dicyclohepta[a,h]-s-indacene undergo no bond distortions retaining the full molecular symmetry groups whereas dibenzo[a,h]-s-indacene suffers a pseudo Jahn¡¾Teller bond distortion from C2v to Cs. Cata-condensed non-alternant polycyclic p systems have attracted much interest theoretically and experimentally. Theoretically Toyota and Nakajima1 have examined the energetically most favourable ground-state geometrical structures with respect to C0C bond lengths of pentaleno- [1,2-b]- and [1,2-a]-pentalene azuleno[1,2-b]- and azuleno- [1,2-a]-azulene.The results show that the pentalenopenta- lenes undergo a pseudo Jahn¡¾Teller e€ect,2 exhibiting a marked double-bond ¢çxation in the peripheral carbon skeleton whereas the azulenoazulenes su€er no molecular- symmetry reduction. Experimentally the diaryl derivatives of azuleno[1,2-b]azulene have been synthesized by Toda et al.,3 Nitta et al.4 prepared azuleno[1,2-a]azulene and Stowasser and Hafner5 the tetra-tert-butyl derivative of pentaleno[1,2-b]pentalene. Their physical properties are in good agreement with theoretical prediction.1 Recently Zhou et al.6 synthesized a new 20 p-electron system tetraiododi- benzo[a,g]-s-indacene. Its parent molecule dibenzo[a,g]-s- indacene 3 is a cata-condensed non-alternant pentacyclic hydrocarbon (Fig.2). Analogous non-alternant hydro- carbons are dicyclopenta[a,g]- 1 dicyclopenta[a,h]- 2 dibenzo[a,h]- 4 dicyclohepta[a,g]- 5 and dicyclohepta[a,h]-s- indacene 6. In this paper we examine the energetically most favour- able ground-state geometrical structures with respect to C0C bond lengths of molecules 1¡¾6 by use of the symmetry rule1,7a and the semiempirical SCF MO method. On the basis of the geometrical structures calculations of the magnetic susceptibilities of 1¡¾6 are carried out to examine the e€ects of bond distortion on the magnetic properties. We ¢çrst examined the stability of the most symmetrical structures of compounds 1¡¾6 using the symmetry rule for predicting bond distortions of conjugated hydrocarbons if the energy gap between the ground (c0) and the ¢çrst excited singlet state (c1) of a molecule is smaller than the critical value about 1.2 eV the molecule should be distorted into a less symmetrical structure through an unsymmetrical nuclear deformation.The symmetry of the bond distortion is identical with that of the lowest excited singlet state. The distribution of the two centre components of transition density r01 indicates the actual type of distortion. When the lowest excited singlet state corresponds to a one-electron transition between molecular orbitals fi and fj r01 is given by Z2fifj. The symmetries and energy gaps of the lowest excited singlet states for the fully symmetrical structures were calcu- lated by use of the Pariser¡¾Parr¡¾Pople-type SCF MO CI method with the variable bond-length technique.1,7 The results are summarized in Table 1.J. Chem. Research (S) 1998 278¡¾279$ Fig. 1 Distributions of the transition density r01 over the molecular skeleton (top) and bond lengths (in A ) in the C2v structure of compound 4 Table 1 Energy gaps and symmetries of the first excited singlet states of compounds 1¡¾6 with the highest molecular symmetries Molecule Energy gap (point group) (E1¢§E0)/eV Symmetry 1 (C2h) 1.98 Bu 2 (C2v) 2.02 A1 2.36a B2 a 3 (C2h) 2.50 Ag 2.71a Bu a 4 (C2v) 0.88 B2 5 (C2h) 2.24 Bu 6 (C2v) 2.34 B2 aEnergy gap and symmetry of the second excited singlet state. Fig. 2 Predicted C0C bond lengths (in A ) and molecular symmetry groups for compounds 1¡¾6 $This is a Short Paper as de¢çned in the Instructions for Authors Section 5.0 [see J.Chem. Research (S) 1998 Issue 1]; there is there- fore no corresponding material in J. Chem. Research (M). *To receive any correspondence. 278 J. CHEM. RESEARCH (S) 1998 Table 2 Calculated magnetic susceptibilities (DK) of compounds 1¡¾6 Molecule DK/DKbenzene 1 ¢§0.13 2 ¢§0.13 3 1.61 4 (C2v) ¢§1.54 4 (Cs) 0.26 5 2.86 6 2.90 %In compound 4 (Cs) no convergence was obtained for iterative self- consistent procedure when the original ! parameter of 1.0 was used. By use of ! a 0.9 self-consistency was achieved. From Table 1 it is predicted that compound 4 should su€er the molecular symmetry reduction from C2v to Cs through a b2 nuclear displacement because the energy gap is smaller than the critical value about 1.2 eV. The transition density r01 shown in Fig.1 is located essentially on the s-indacene-like moiety suggesting that the bond alternation should take place primarily in this region. In the C2v structure (Fig. 1) a ¢çrst-order double-bond ¢çxation is seen on both sides of the six-membered rings. On account of the large energy gaps the other molecules are not expected to undergo any bond distortions. In 2 and 3 the lowest excited singlet state is totally symmetric. We thus examine the second excited singlet states of these molecules whose energy gaps and symmetries are also listed in Table 1. We see immediately that the energy gaps are larger than the critical value. Using the starting distorted geometries suggested by the transition densities we optimized the distorted structures. We then calculated the stabilization energy de¢çned as the di€erence in total energy between the fully symmetrical and the distorted structure.Here the total energy is assumed to be the sum of the p-electron and s-bond energies the latter being calculated by use of the harmonic-oscillator model8 with a force constant of 714 kcal mol¢§1 A E ¢§2. Fig. 2 shows the molecular-symmetry groups and C0C bond lengths of the most favourable structures of com- pounds 1¡¾6. As expected 4 su€ers the pseudo Jahn-Teller distortion from C2v to Cs. The reduced structure shows that in one of the six-membered rings the bond lengths are nearly equalized as in benzene itself while in the other six-membered ring a strong double-bond ¢çxation exists. The stabilization energy is estimated to be 2.3 kcal mol¢§1. On the other hand the other molecules su€er no molecular- symmetry reduction.In view of the distribution of C0C bond lengths 1 and 2 can be said to have the delocalized benzene-like and two bond-alternated fulvene- like regions. Similarly 5 and 6 have the delocalized benzene-like and two bond-alternated heptafulvene-like regions and 3 has the p-quinodimethane-like and two benzene-like regions. We next examine the magnetic properties of compounds 1¡¾6 by using the modi¢çed London¡¾Hoarau method in the framework of the Wheland¡¾Mann-type SCF MO approxi- mation.9 % Calculated magnetic susceptibilities DK (in units of DKbenzene) are shown in Table 2 where positive values indicate diamagnetism and negative values paramagnetism. The results show that 1 and 2 should be very weakly para- magnetic whereas 5 and 6 should be diamagnetic.Although these four molecules have both the delocalized and the bond-alternated regions the former group di€ers markedly from the latter in magnetic susceptibility. Compound 3 is calculated to be diamagnetic. Partial evidence for this is provided by 1H NMR spectra of derivatives of 3.6 Since 4 is predicted to undergo the pseudo Jahn¡¾Teller e€ect the magnetic susceptibilities for the C2v and Cs struc- tures were calculated. Interestingly the symmetrical (C2v) structure exhibits a paramagnetic susceptibility while the less symmetrical (Cs) structure exhibits a weak diamagnetic susceptibility. Received 8th January 1998; Accepted 29th January 1998 Paper E/8/00144H References 1 A. Toyota and T. Nakajima Tetrahedron 1981 37 2575. 2 R. F. W. Bader Mol. Phys. 1960 3 137; R. G. Pearson J. Am. Chem. Soc. 1969 91 4947; L. S. Bartell J. Chem. Educ. 1969 45 754. 3 T. Toda N. Shimazaki T. Mukai and C. Kabuto Tetrahedron Lett. 1980 21 4001. 4 M. Nitta K. Nishimura and Y. Iino Tetrahedron Lett. 1993 34 2157. 5 B. Stowasser and K. Hafner Angew. Chem. Int. Ed. Engl. 1986 25 466. 6 Q. Zhou P. J. Carroll and T. M. Swager J. Org. Chem. 1994 59 1294. 7 (a) T. Nakajima Fortschr. Chem. Forsch. 1972 32 1; T. Nakajima A. Toyota and S. Fujii Bull. Chem. Soc. Jpn. 1972 45 1022; (b) M. Kataoka and T. Nakajima J. Chem. Soc. Perkin Trans. 2 1986 1529. 8 L. C. Snyder J. Phys. Chem. 1962 66 2299. 9 H. Yamaguchi and T. Nakajima Bull. Chem. Soc. Jpn. 1974 47 1898; J. Hoarau Ann. Chim. 1956 1 544; G. W. Wheland and D. E. Mann J. Chem. Phys. 1949 17 264. J. CHEM. RESEARCH (S) 1998 279

ISSN:0308-2342    

DOI:10.1039/a800144h

出版商:RSC    

年代:1998

数据来源: RSC