Krishna, Bal

Sharma, Suresh C.

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NotesAn Improved IOC-m Technique: Part V-Ionization Potentials & Heats of Formationof Some Organic MoleculesSURESH C. SHARMA* & BAL KRISHNADepartment of Chemistry, University of AllahabadAllahabad 211002Received 19 December 1974; accepted 19 September 1975Ionization potentials and heats of formation of someorganic molecules have been calculated using animproved IOC-CJ)technique. The results have beencompared with some previously calculated and experi-mental values.WE report here the ionization potentials and heatsof formation of some organic molecules usingan improved IOC-<ll technique- in which the overlapin the ee-technique has been included. The netcharge on atom r has been taken as that resultingwhen overlap charges are distributed equally overall the atoms involved. The following form thecoulomb integrals has been used for the presentwork- (Eq. 1):H" = rJ.r +<lll(1-1/2 7 (Prj s,+t» Sjr)1 ... (1)where all the symbols have their usual significance.Resonance integrals have been calculated using theself-consistent ~ method":Hr. = ~o exp(0·55Prs-0·3666) ... (2}where u.;» the resonance integral at iteration Nand Prs is the bond order at iteration (N-1).The method of calculation and parameters usedhave been described in our previous papers! of thisseries. For calculations on heterocyclic molecules,appropriate changes in the o: and ~ parametershave been made using the definition of Whelandand Pauling! and taking h = 0·5 and K = 1·0 (ref.5-7). For the calculation of heats of formation wehave used the energies of various types of bonds asgiven by Dewar and Gleicher".A comparison of the ionization potentials of somealternant hydrocarbons, as calculated by the presentmethod with those calculated using various otherTABLE 1 - IONIZATION POTENTIALS OF SOME ALTERNANT HYDROCARBONSMolecule Ionization potential, eVPresent Dewar SCF-MO IOC-w Simplified Experimenta.l Referencework et al» method-e techniquett IOC-wtechniquetsDibenz-[1,2: 5,6]-anthracene 7-87 7·84 8·070 7·75 7·97 7·42 13Dibenz-[1,2: 7,8]-anthracene 7·93 7·88 8·193 7·92 8·15 7·42 13Picene 7·61 8·157 7·78 7·99 7·62 13Perylene 7·55 7·724 7·35 7-19 7-15 14Coronene 7·88 7·976 7·80 7·95 7·50 13Hexatriene 8·36 8·43 8·43 8·79 8'23 15Octatetraene 8·02 7·89 7·80 15Stilbene 8·01 7·89 7·91 7'95 14Dibenz-[1,2: 3,4]-anthracene 7·94 8·098 7·81 8·02Benz-[4,5]-pyrene 7·87 7·86 8'024 7·77 7·87Benz-[1,2]-pyrene 7·64 7·56 7·747 7·47 7·73Dibenz-[1,2: 4,5]-pyrene 7·69 7·841Pentacene 7·35 7-17 7'54Pentaphene 7·83 7·72 7·97Dibenz-[3,4: 5,6]-phenanthrene 7·94 7·84 8·02Hexahelicene 7·93 7'72 8·UuBenz-[1,12]-perylene 7-71 7·57 7·78Ovalene 7'51 7·25 7'51TABLE 2 -- HEATS OF FORMATION OF SOME NITROGEN CONTAINING HETEROCYCLIC MOLECULESMolecule -e-Bond energy Heat of formation (eV)(eV)Present work Dewar and Simplified IOC-w Expl. valuest?Harget-e techniqne?Pyridine 7·890 51·148 51·88 52·042 51'78Pyridazine 7·897 45·137 45'59Pyrimidine 7'325 44·343 46·86 45'825 46'99Pyrazine 7·772 44'790 46·32 45·784 46·44Quinoline 13·829 85·022 85'32 86·049 85-18Isoquinoline 13·175 84'369 85'32 85'794*Author to whom correspondence should be made at Naval Chemical & Metallurgical Laboratory, Bombay 400023.435INDIAN J. CHEM., VOL. 14A, JUNE 1976methods=P, given in Table 1, shows that the valuesobtained by us are very close to those obtained byDewar et al.1I and using IOC-w techique-t, butgenel1i.llylower than those obtained using semi-empiricalSCF-MO method-" and simplified version of IOC-wtechnique-s. For molecules for which experimentalvalues 'of vertical ionization potentials are available th efirst two methods give very encouraging results, whilethe latter two generally give high values. Since, in thecalculations molecular geometry has been assumedto remain unchanged, these values are the so calledvertical ionization potentials and therefore, should beessentially higher than the spectroscopic values1S-15•Table 2 reports the heats of formation of somenitrogen containing heterocyclic molecules. Ourresults are in fair agreement with the experimentalvalues1' although these are not as good as thoseobtained by Dewar and Harget-t,References1. (a) SHARMA, S. C., SRIVASTAVA, A. K. & KRISHNA, B.,]. chem. Soc. Faraday Trans. II, 71 (1975), 168; 172.2. KRISHNA, B. & GUPTA, S. P.,]. Am. cbem. Soc., 92 (1970),7247.3. BOYD, G. V. & SINGER, N., Tetrahedron, 22 (1966), 3383.4. WHELAND, G. W. & PAULING, L., ]. Am. chem, Soc., 57(1935), 2086.5. MURRELL, J. N., Malec. Phys., 1 (1958), 384.6. BROWN, R. D., ]. chem, s«; (1956), 272.7. SRIVASTAVA, A. K. & KRISHNA, B., Indian]. pure appl.Phys., 11 (1973), 246.8. DEWAR, M. J. S. & GLEICHER, G. J.,]. chem, Phys., 44(1966), 759.9. DEWAR, M. J. S., HASHMALL, J. A. & VERNIER, C. G.,]. Am. chem, s«., 90 (1968), 1953.10. DEWAR, M. J. S., HASHMALLL, J. A. & TRINA]STIC, N.,]. Am. chem, Soc., 92 (1970), 5555.11. GUPTA, S. P., Quantum mechanical evaluation of bondparameters in some molecules, D. Phil, thesis, Universityof Allahabad, Allahabad, 1970, 34.12. SRIVASTAVA, A. K., Studies in theoretical evaluation ofsome bond parameters of some molecules, D. Phil. thesis,University of Allahabad, Allahabad, 1973, 50, 52.13. MASTEN, F. A., ]. chem. Phys., 24 (1956), 602.14. BRIEGLEB, G. & CZEKELLA, Z., Z. Elecktrochem.,63 (1959), 6.15. PRICE, 'V. C. & WALSH, A. D., Proc. R. s«.. London A,185 (1945), 182.16. DEWAR. M. J. S., Molecular orbital theory of organicchemistry (McGraw-Hill, New York), 1969, 382.17. DEWAR, M. J. S. & HARGET, A. J., Proc. R. Soc., LondonA, 315 (1970), 457.An Improved IOC-m Technique: Oxidation &Reduction Potentials & Charge TransferEnergies of Some Conjugated HydrocarbonsSURESH C. SHARMA* & BAL KRISHNADepartment of Chemistry, University of AllahabadAllahabad 211002Received 24 February 1975; accepted 25 August 1975The polarographic half-wave oxidation and reductionpotentials of conjugated hydrocarbons and chargetrans~energies of their complexes with iodine have been calcu-latedfrom the values of ionization potentials and electronafllnities calculated using a recently described improved10e-co technique. The results are compared and foundto be in good agreement with the experimental values.*Present address: Naval Chemical & MetallurgicalLaboratory, Bombay 400023.436COR&ELA TIONS between the polarographic half-. wtllre reduction potentials (-El/2 red.) ofconjugated hydrocarbons and the lowest unoccupiedHMO energies have been described by variousworkers='. Similar correlations have also beenobserved+s between polarographic half-wave oxida-tion potentials (El/2 ox.) and energies of highestoccupied HMOs. Dewar et al.7,8 have obtainedexcellent correlations between reduction potentialsand electron affinities and oxidation potentials andionization potentials using a variable ~-semi-empiricalSCFMO method for hydrocarbons as well as hetero-molecules. Pysh and Yang' have calculated ion-ization potentials of some aromatic hydrocarbonsfrom these correlations and measured values ofoxidation potentials. The following relations canbe derived theoretically between oxidation andreduction potentials and ionization potentials (1) andelectron affnities (A )6,7:E1/2(OX.) =ml +e ...(1)and-El/2(red.) =m'A -l-C" ... (2)In a series of charge transfer complexes betweena single acceptor and a series of polynuclear hydro-carbons if the interactions between the donor andacceptor are small, the transition energy, AECT forthe first charge transfer (CT) band should be givenbye,AEcT = 1-A -l-const. . .. (3)1is the ionization potential of hydrocarbon and A,the electron affinity of acceptor molecule. For agiven acceptor molecule Eq. (3) becomes:AETc = 1 -l-const,and a linear plotand 1:MCT =m"l +e" ... (5)On the basis of above correlations, we have cal-culated, in the present paper polarographic half-wave oxidation and reduction potentials of someconjugated molecules and charge transfer transitionenergies of some iodine-hydrocarbon complexesfrom the values of ionization potentials and electronaffinities, obtained using an improved IOC-w tech-nique". Method of calculation and some of theresults have already been reported9,lo. The para-meters m and e have been obtained by theoreticallycalculating the slope and intercept respectively ofa plot betweenE1/2(oX.) (on Y-axis) and 1(on X-axis).Similarly parameters m', C', mil and C" can alsobe obtained. Here, we have obtained, m = 0·90,e = -5·88, m' = -0·86, C' = 2·07,m" = 1·032andC" = -5·098.Polarographic half-wave oxidation and reductionpotentials of some conjugated hydrocarbons andcharge transfer transition energies of some iodine-hydrocarbon complexes have been compared withrespective experimental values6,ll,12 in Table 1. Forcomparison we have used the values which refer tomeasurements done under identical experimentalc?nditions for a large number of hydrocarbons,SInce, although the relative values of oxidation and. .. (4)can be obtained between AECT

An Improved IOC-ω Technique: Part V - Ionization Potentials & Heats of Formation of Some Organic Molecules

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An Improved IOC-ω Technique: Part V - Ionization Potentials & Heats of Formation of Some Organic Molecules

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