With What Does It Mean the Organic Chemical Phenomena Are Solved?

Transcription

1 With What Does It Mean the Oganic Chemical Phenomena Ae Solved? Let me explain the meaning of this title quoting a simple example. Aniline has otho and paa oientation natue and nitobenzene has meta oientation one to an ectophilic substitution eaction (SE). Explanation of the text lev of oganic chemisty, using an aow ecton theoy, comes to show to Fig Resonance of aniline Resonance of nitobenzene Fig.. Oganic chemical expession of esonance in aniline and nitobenzene That is, since the unshaed ecton pai of the atom gathe at otho and paa positions accoding to the aows in aniline and π ecton density of those positions becomes lage, an ectophilic eagent is bought neaby those positions. On the othe hand, in nitobenzene, in ode fo π ectons of otho and paa positions to decease and those positions ae avoided by an ectophilic eagent fo those positions tend to be positivy chaged. Almost all oganic chemists ae satisfied with such explanation. Howeve, does it mean that one undestands these eactions tuly? Let's conside some points bow. () What these aows mean? () If the aows mean the oute to which the ecton movement is expessed, why ae they so? (3) Especially in aniline, why ectons flow into C with low ectonegativity fom with high ectonegativity? (4) What on eath is the cause of deciding atomic ectonegativity, and so on. The aow ecton theoy cannot explain anything of chemical phenomena at all. Owing to the spead of molecula obital calculation pogams, the ecton density in a molecule and the enegy of a eaction pocess can be easily calculated by oganic chemists as wl, and they can show validity of a eaction. Howeve, it is not in chage of the analysis of a phenomenon at all. The poblem is why it bings such a esult. Then, what kind of explanation is the eal explanation of a chemical phenomenon? This is the meaning of the title.

2 See the Fact that a Chemical Phenomenon Bongs to atual Phenomena A chemical phenomenon is egaded as one of the natual phenomena. obody may object this idea. If so, one should also explain a chemical phenomenon fom natual basic pinciples and this may be a natual conclusion. The paticle physics says that thee ae fou sots of foce which goven natue. In ode of stength, they ae stong foce, ectomagnetic foce, weak foce, and gavitational foce. Among them, the foce ated to chemical phenomena is only ectomagnetic foce. The ange which acts although stong foce is vey stong but its action is limited to a nuclea domain (0-5 m). The weak foce commits only at the time of collapse of a nucleus. Gavitational foce is about 0-40 time weak compaed with ectomagnetic foce, and can be disegaded complety. The atual Pinciples that Goven Chemical Phenomena The idea that the ectomagnetic foce is the complete basis to undestand chemical phenomena is not ight. In atoms and molecules, the wave chaacte of ectons plays impotant oles. The wave natue of a substance inevitably accompanies the Pauli uncetainty ation (pinciple). In addition, an ecton has the impotant chaacte: ementay paticles bong to eithe the goup to which only one can exist in one state (femion) o the goup called boson in which many paticles can exist in one state. Hee, a state efes to the stationay enegy state (it does not change in minute time) specified by all the coodinates including intenal ones. Fo example, if it is an ecton, they ae space coodinates and intenal coodinates (the spin, the coodinates of otation in classic undestanding). An ecton bongs to a femion. The question why an ecton is a femion is not a poblem which chemisty asks like the poblem why an ecton has an ectic chage: let's accept these as a fact. The femion chaacte of an ecton appeas concety in chemisty as the Pauli s exclusion pinciple. Afte all, the factos which goven the atomic and molecula wolds will be () the ectomagnetic foce, () the wave chaacte of an ecton (o the pinciple of supepostition), (3) the uncetainty ation, and (4) the Pauli s exclusion pinciple. Although 3 is a facto deived fom, a chemical phenomenon becomes easie to undestand by consideing and 3 sepaaty. The ecton s wave natue seves in undestanding the way of ecton distibution and expansion and shinkage of ecton distibution by obital inteactions. The uncetainty ation is convenient fo qualitative undestanding of the ationship between the ecton's existence ange and its kinetic enegy. How to Undestand Chemical Phenomena on atual Pinciples Using Molecula Obital Theoies Explanation based on the ectomagnetic foce, the wave natue of ectons, the uncetainty ation, and the Pauli s pinciple may be tue undestanding of chemical phenomena. ext, let us

3 conside the concete ways to undestand chemical phenomena fom those basic pinciples. This is compaativy easy. We use molecula obital theoies in ode to explain chemical phenomena. Just use the molecula obital theoies appopiaty. Let me explain using the Hatee-Fock (HF) method as a molecula obital theoy. Although thee ae flows who dislike this method because of not taking the ecton coation, I (tanuki 3) ) think it the exclent molecula obital theoy fo which the viial theoem is mateialized; this is basic equiement as a theoy when it is applied to the mechanical analysis. Besides, the defect that ecton coation is not taken into account is easily emoved by adding vaious ectonic states. Although it is a digession, it should be mentioned that the density functional theoies cannot be used fo the mechanical analyses of chemical phenomena. Although the details of the molecula obital theoy ae omitted, let me point out an impotant fact. The coefficient (c i ) of the expansion function of a molecula obital (ψ) is decided using the vaiation pinciple. E( c, clcilc c n i ) 0 Hee, E is the total ecton enegy. The point is that c is optimized to the total ecton enegy. It is neve optimized to the enegy of atom o between atoms. Although it is a digession, this shows that it is a mistake to divide the enegy of a molecule into those of atoms and between atoms. Moeove, thee is no theoetical guaantee on any kind of patitioning of enegy ad ecton distibution i8nto atoms ad between atoms, such as ecton density population, and bond ode, which have been cuently and taditionally pefomed. Using them also tuns into unsuitable application of a molecula obital theoy. () An Appopiate Enegy Patitioning The Hamiltonian (H ) of a system consists of opeatos of the kinetic enegy of ectons (T) and the potential enegy (V ). V is patitioned into opeatos of -ecton potential enegy (V e ) and -ecton potential enegy (V ee ). The expectation values of these opeatos ae theoetically appopiate. E H T V T V e Ψ V H Ψ T ee Ψ T e ee The fist, second, and thid tems of the ight-hand side of the lowe fomula of Eq.3 espectivy coespond to expectation values of the kinetic enegy of ectons, -ecton potential enegy, and -ecton potential enegy. 3

4 If the LCAO expansion is used (Eq. 4), each tem of Eq. 3 ae, in tems of atomic obitals ( χ ) and thei coefficients (c ), expessed as shown in Eqs. 5 to 7. n i i ψ c χ 4 Ψ T Ps χ ( ) () χ s () dτ 5 s e Q Ps χ ( ) χ s () dτ 6 s Q Q ee Ps Ptu ( s / tu) ( t / su) 7 s t u In these fomulas, P s is the bond-ode between atomic obitals χ and χ s while, (s/tu) is, ( s / tu) χ () χ s () χt () χu() dτdτ Z Although epeated, Eq. 5 gives the kinetic enegy of ectons which oiginate fom the uncetainty ation and Eq. 6 does the ectomagnetic (coulombic) enegy of ectons and nuclei. Eq. 7 coesponds to the inteaction enegy between ectons which includes simple coulombic enegy and the effect of avoiding the existence of the same spins by the Pauli s exclusion pinciple. The effect of the Pauli s exclusion pinciple is expessed by ( t / su) in Eq.7. The detail may be efeed to liteatues 5 to 8. Hee, let us make sue of some technical tems. Coulombic inteaction: the inteaction between two chages Electonic inteaction: the inteactions between two ectons including the Pauli s exclusion pinciple. Electon coation: the effect of ectons in which movement of a cetain ecton affects movement of othe ectons. Since ecton has a negative ectic chage, othe ectons move avoiding the ecton. The Hatee-Fock method does not include this effect, since the inteaction between ectons is taken in as the aveage value. As descibed in the "Poposition" of this HP, no infomation is acquied only by looking at the enegy value. By seeing the enegetic changes accompanying with changes of nuclea position, one can know what will paticipate in the chemical phenomenon fom the basic pinciples of natue. () Some Examples of Fundamental Analysis

5 The analyses of the chemical phenomena by such a view have appeaed fo many yeas, although thee ae not many examples. In 96, Ruedenbeg examined the kinetic enegy of ecton and the potential (nucleus inteaction is also included) enegy of the H -- H system as a function of intenuclea distance between H and H. 9) In conclusion, diffusion of the ecton to H fom H atom takes place at fist causing a fall of the kinetic enegy. At the same time a fall of the potential enegy happens in connection with it. This continues until the nuclea-nuclea epling Coulomb enegy and decease of the kinetic and one-ecton potential enegy ae balanced to each the equilibium distance. In this case, since the numbe of the ectons of the system is one, the effect by the Pauli s exclusion pinciple does not ente, but it is claified how the ectonic kinetic enegy which aises accoding to the uncetainty ation, and coulombic enegy ae coated. The poblem of conjugation of the aniline and nitobenzene shown at the beginning has aleady been dealt with. 8) Let me show the conclusion. The method is, as shown in Fig., to analyze the changes of the patitioned enegies when the substituents ae otated along the C- axis. θ X X A B X : H o O Fig.. Rotation of substituents along the C- axis. The conclusion is as follows. In an ecton-donative substituent goup like an amino goup, due to the ief of the kinetic enegy pessue of the unshaed ecton pai of the atom, the ectons flow into the benzene ing, causing a fall of the kinetic enegy. On the othe hand, in the ecton-withdawing substituent goup like nitobenzene, π ectons of the benzene ing move to the atom due to the stong potential of the atom of the nito goup. The authos have advocated that one calls the fome kinetic-enegy oigin and the latte, the potential-enegy oigin. The physical meaning was given to the conventional concept of ecton-donative and ecton-withdawing natues of substituents, and inconsistency of the concept in the chemisty that ectons flow into the small C atom fom the lage atom of ectonegativity in aniline is solved with the concept of ief of kinetic-enegy pessue. Kinetic-enegy pessue is the concept which Reudenbeg intoduced, and since the ecton confined in naow space has a lage kinetic enegy (due to the uncetainty pinciple), ectons have a tendency to spead. Although it is a concept sdom used, since it is moe diect than using the uncetainty ation, the use of kinetic-enegy pessue seems suitable fo an undestanding of chemical phenomena.

6 Poposition Since chemical phenomena bong to the natual phenomena, an undestanding fom the natual basic pinciples tuns into the fundamental undestanding. The factos which sway the wold of atoms and molecules ae () the ectomagnetic foce, () the wave chaacte of ecton (the pinciple of supeposition), (3) the uncetainty ation, and (4) Pauli s exclusion pincicle, and since those quantities ae calculable with molecula obital methods, one can undestand the cause of a chemical phenomenon fom those oles to obtain the tue undestanding. Moeove, the use of the concept of kinetic-enegy pessue is appopiate fo the chaacte in which the ecton tends to spead due to the uncetainty ation.. It is not contained in the text of oganic chemisty although the cause of deciding atomic ectonegativity is wl undestood.. Ration between the uncetainty ation and the wave natue of ecton may be efeed to Why Conjugation Takes Pace in the Poposition of this HP. 3. Takuki is a accoon-like mountain dog who sometime jokes human beings. Be awae! 4. Viial Theoem: the theoem that the atio of kinetic enegy (T) to the potential enegy (V) is - in the system in a stationay state (V/T-). This theoem is an impotant one which is mateialized in both classical and quantum mechanics. If this theoem is not theoetically fulfilled in a molecula obital theoy, such a theoy is not suitable fo the mechanical analysis of a phenomenon. 5. H.Ichikawa and Y. Ebisawa, J. Am. Chem. Soc., 07, 6 (985) 6. H. Ichikawa, Y. Ebisawa, K. Sameshima, Bull. Chem, Soc. Jpn., 6, 659 (988) 7. H. Ichikawa, Y. Ebisawa, A. Shigihaa, J. Phys. Chem., 9, 440 (988) 8. H. Ichikawa, K. Sameshia, J. Phys. Og. Chem., 3, 587 (990) 9. K. Reudenbeg, Rev. Mod. Phys., 34, 36 (96). See also, M.J. Feinbeg, K. Reudenbeg, J. Chem. Phys., 54, 495 (97)