Molecular Orbital Theory of Orientation in Aromatic, Heteroaromatic, and Other Conjugated Molecules Kenichi Fukui, Teijiro Yonezawa, Chikayoshi Nagata, and Haruo Shingu Citation: The Journal of Chemical Physics 22, 1433 (1954); doi: 10.1063/1.1740412 View online: http://dx.doi.org/10.1063/1.1740412 View Table of Contents: http://aip.scitation.org/toc/jcp/22/8 Published by the American Institute of Physics Articles you may be interested in A Molecular Orbital Theory of Reactivity in Aromatic Hydrocarbons The Journal of Chemical Physics 20, 722 (2004); 10.1063/1.1700523 MO-Theoretical Approach to the Mechanism of Charge Transfer in the Process of Aromatic Substitutions The Journal of Chemical Physics 27, 1247 (2004); 10.1063/1.1743986 An Extended Hückel Theory. I. Hydrocarbons The Journal of Chemical Physics 39, 1397 (2004); 10.1063/1.1734456 Self-consistent molecular orbital methods. XX. A basis set for correlated wave functions The Journal of Chemical Physics 72, 650 (2008); 10.1063/1.438955 Electron density, Kohn–Sham frontier orbitals, and Fukui functions The Journal of Chemical Physics 81, 2862 (1998); 10.1063/1.447964 Gaussian basis sets for use in correlated molecular calculations. I. The atoms boron through neon and hydrogen The Journal of Chemical Physics 90, 1007 (1998); 10.1063/1.456153 CONJUGATION OF PHENYL RADICALS 1433 provided comparisons are made only between closely methods. Commercial samples were purified by re­ related compounds. This observation suggests that the crystallization or distillation. Unless otherwise indi­ central atom is strongly involved in the transition cated, the spectra were determined in solution in concerned. Although the data presented here are commercial 95 percent ethanol which was transparent insufficient to make a decision on this point, it is not above 210 m,u. A Beckman DU spectrophotometer and unlikely that the end absorption corresponds to a 1 em quartz cells were used throughout. The concentra­ 7 dissociation process, as Milazzo has proposed. tions of the solutions were so chosen as to obtain EXPERIMENTAL optical densities between 0.3 and 0.7 at the absorption All compounds were either obtained from commercial maxima, and no optical density data below 0.15 were sources or prepared in this laboratory by known used. THE JOURNAL OF CHEMICAL PHYSICS VOLUME 22, NUMBER 8 AUGUST, 1954 Molecular Orbital Theory of Orientation in Aromatic, Heteroaromatic, and Other Conjugated Molecules KENICHI FUKUI, TEIJIRO YONEZAWA, CHIKAYOSHI NAGATA, AND HARUO SHINGU Faculty of Engineering, Kyoto University, Kyoto, Japan (Received March 31, 1953) As to the LCAO MO treatment of the orientation problem in chemical reactions of 7r-electron systems, the frontier electron concept which has been previously introduced by the authors for explaining the reactiv­ ities in aromatic hydrocarbons is subjected to an extension in the sense that the frontier orbitals are specified according to the type of reaction. Thus, fltndamental postulates relating to the reactivities of 7r-electron systems are set up, which are believed to include general principles involved in the mechanism of both sub­ stitution and addition of electrophilic, nucleophilic, or radical type. On the basis of these postulates it is possible to predict the position of attack in conjugated molecules in all the three types of substitution as well as addition in a consistent manner. There is a nearly perfect agreement between the theoretical con­ clusions and experimental results hitherto reported. 1. INTRODUCTION tion method," is related to the calculation of the differ­ NE of the recent successes achieved by quantum ence in unsaturation energy of the hypothetical transi­ O mechanics in the field of physical organic chem­ tion complexes. istry is the development of the theory in explaining the Prominence of the latter method should be admitted orienting effect of substituents in aromatic molecules. in that it is capable of explaining the reactivity in all the The quantitative treatments of this problem so far three types, i.e., electrophilic, nucleophilic, and radical reported may be roughly classified into two groups. type, of substitution from a unified standpoint, whereas The one,l which may be called the "static method," is by means of the former method it is difficult to account based on the hypothesis that the position of higher (or for the point of attack in the radical substitution, and lower) calculated density of total 7r electrons is more in this respect the "free valency method"5 serves as easily attacked by electrophilic (or nucleophilic) re­ filling up that defect of the static method. agents. The other,2 which may be called the "localiza- In this paper it is shown that another unifying method can also be established on the basis of the ''frontier 1 E. Hiickel, Z. Physik 72, 312 (1931); Z. physik. Chern. B35, 163 (1937); Z. Elektrochem. 43, 827 (1937); G. W. Wheland and electron" concept which has been previously introduced L. Pauling, J. Am. Chern. Soc. 57, 2086 (1935); T. Ri, Rev. Phys. by some of the present authors in the case of electro­ Chern. Japan 17, 1, 16 (1943) (in Japanese); M. J. S. Dewar, philic substitution in condensed aromatic hydro­ Trans. Faraday Soc. 42, 764 (1946); H. C. Longuet-Higgins and a C. A. Coulson, Trans. Faraday Soc. 43, 87 (1947); C. A. Coulson carbons. On treating the orientation problem in sub­ and H. C. Longuet-Higgins, Proc. Roy. Soc. (London) A191, 39; stituted atomatic and other related molecules, it is A192, 16 (1947); A. Pullman, Rev. sci. 86, 219 (1948); A. Pullman and J. Metzger, Bull. soc. chim. France 1948, 1021 (1948); H. C. found that the concept should naturally be extended in Longuet-Higgins and C. A. Coulson, J. Chern. Soc. (London) the sense that the frontier orbitals are specified according 1949, 971 (1949); C. Sandorfy, Bull. soc. chim. France 1949,615 to the type of reaction. Thus, we are led to set up the (1949); H. C. Longuet-Higgins, J. Chern. Phys. 18, 283 (1950); Orgel, Cottrel, Dick, and Sutton, Trans. Faraday Soc. 47, 113 fundamental postulates, in the light of which the position (1951); J.-I. F. Alonso, Compt. rend. 233, 403 (1951); L. SzabO, of attack by electrophilic, nucleophilic, and radical Compt. rend. 233, 625 (1951). reagents can be predicted not only for substitution but 2 G. W. Wheland, J. Am. Chern. Soc. 64, 900 (1942); H. C. Longuet-Higgins, J. Chern. Phys. 18,283 (1950); M. J. S. Dewar, also for addition in an excellent agreement with ex­ J. Chern. Soc. (London) 1949, 463 (1949); C. A. Coulson, Re­ periment. search 4,307 (1951); M. J. S. Dewar, J. Am. Chern. Soc. 74, 3355, 3357 (1952). 3 Fukui, Yonezawa, and Shingu, J. Chern. Phys. 20, 722 (1952). 1434 FUKUI, YONEZAWA, NAGATA, AND SHINGU Types of Attacking Reagent 1. In case of the reaction with an electrophilic re­ 1. Electrophilic 2. Nucleophilic 3. Radical agent, that position is more susceptible to attack which ,', -.-li!-::~ -. '" has the higher density of the two electrons occupying the ," • • ", -0--0- • 0--::: highest molecular orbital in the ground state. -0---0- --0----0- --0---0- 2. In case of the reaction with a nucleophilic reagent, (a) that position is more susceptible to attack which would -0---0- -0-:--0-- -0--0- have the higher density of the two electrons assumed to -0---0- -0--0- ----0--0- occupy the lowest unoccupied orbital of the ground state. -0-----0- -0--0- -0---0- 3. In case of the reaction with a radical reagent, that position is more susceptible to attack which would have the higher density of the two electrons, one occupying the highest orbital and the other occupying the lowest unoccu­ ,. ;!: -0---*-::: pied orbital of the ground state. As a consequence of these postulates we can classify • 0--;:'-0--0- -0---0-- the frontier electrons and orbitals for various types of --0--0- (b) -0--0- -0--0- reactants and reagents, which is illustrated in Fig. 1, --0--0--- --0--0- including the cases of even and odd 1r-electron systems. -0---0-- -0---0- In the latter case where the 1r-electron system is a 0: electron in the ground _tat• • : frontier electron radical, the above postulates should be somewhat modi­ *: trontier orbital fied as shown in Fig. 1 (b). Also in the case of an excited FIG. 1. Frontier electrons and frontier orbitals. (a) Even ,,"-electron state of the 1r-electron system, e.g., in the case of a system. (b) Odd 1r-electron system. diradical state, the principle involved in the postulates may be correspondingly applicable. 2. FUNDAMENTAL POSTULATES As a general characteristic of chemical activation it 3. ELECTROPHILIC SUBSTITUTION may be assumed that in the vicinity to the transition Substitution in Heterocyclic Compounds state the electrons in the reagent as well as those in the reactant molecule will be delocalized and a transfer of For the quantum-mechanical treatment of reactivity electrons will occur between the reagent and the re­ in heterocyclic compounds several parameters repre­ actant. From the standpoint of the frontier electron senting the influence of heteroatom as to the Coulomb concept we suppose that the frontiers are most sus­ and exchange integrals have been introduced by several ceptible to the electron transfer and that this transfer authorsl,2 from some empirical or theoretical arguments. of electrons, which may serve to lower the energy of the activated complex, will form the essential part of the .149 .534 .235 .322 .057 electronic interaction in question. Two electrons should 364 N 1 2.013 be characterized in this connection as frontiers in that 5 3 .454 3.157 they are most closely related to the formation of a 4 4 0.214 .177 3q~ covalent u bond between the 1r-electron system and the reagent, and these two are provided either from the PYRIDINE QUINOLINE ISOQUINOLINE highest occupied orbital in the 1r-electron system to the reagent in an electrophilic reaction, or from the reagent to the lowest empty orbital in the 1r-electron system in 3r a nucleophilic reaction, and in a radical reaction one 4 3 .276 .058 co·5.005 3 .5'15 ~'724 6.