Rao, C. N. R.

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Indian Journal of ChemistryVol. 14A, March 1976, pp. 147-149Photoelectron Spectra of C-Nitro & N-Nitro Compoundsc. N. R. RAO*Department of Chemistry, Indian Institute of Technology, Kanpur 208016Received 4 October 1975; accepted 31 October 1975Photoelectron spectra of aliphatic nitro compounds have been studied and assignments madeon the basis of molecular orbital calculations and electronic absorption spectra. Substituenteffects on PES bands due to the nitro group are discussed. PES of dimethylnitramine anddimethylnitrosamine are compared.PHOTOELECTRON spectra (PES) is the bestavailable technique for the direct determi-nation of molecular energy levels'. Ionizationenergies obtained from PES are related to ~heorbital energies by Koopmans' theorem. Orbitalenergies can in turn be calculat~d by m?lecularorbital methods and such calculations provide thebasis for the interpretation of the PES of molecules.Photoelectron spectra characteristic of varioustypes of chromophoric gro,":ps have been .re:portedin recent years>". In particular, th~ van~tlOn ?fionization energies of the lone pan orbitals 111organic halides and carbonyl compounds have beenexamined in some detail1,4,5. In the present paper,we have investigated the PES of aliphatic ~itrocompounds with specific reference .to the or~:lltalsof the nitro group. The PES of nitro and nitrosogroups in similar compounds have also been ~om-pared. The PES characteristic of these functlO.nalgroups are discussed in t~e light of .molecular orbitalcalculations and electronic absorption spectra.Materials and MethodsAll the compounds were purified by fractionation.PES were recorded on a Perkin-Elmer PS 18 photo-electron spectrophotometer and also with a instru-ment built at the University of Oxford.Results and DiscussionPES of nitroalkanes and halonitroaklanes - Mole-cular orbital calculations of Mcllwen" and Katoet aU have shown that the ordering of the occupiedenergy levels of the nitro group in nitromethane ISa1(n*), b2(n), a8("2) and ~1("1)' The two non-bondingorbitals essentially consist of oxygen Zp". and. ZP.,.functions and the coefficients for the atomic orbitalsof nitrogen are negligibly small. Furthermore,the two non-bonding orbitals appear to be nearlydegenerate. A schematic representation of. thefirst three occupied orbitals are shown 111 FIg. 1.CNDOj2 calculations, however, sho~ the a2(,,)orbital to be the highest occupied or~Ital followedby the al(n) orbital. The a2(,,) or~Ital. has non-bonding character while the n orblta~ IS due tothe bonding combination of oxygen orbitals.The electronic absorption spectrum of CHaN02shows a characteristic n-7t* transition around 270*Jawaharlal Nehru Fellow.yFig. 1 - Schematic representation of the N02 group orbitalsin nitroalkanes [Ordering of energy levels is shown in theinset]nm and a 7t-7t*transition around 200 nm". A banddue to an n*-7t* transition is not distinctly seen,but is expected to be close to the n-7t* band" (calc.280 nm). Based on the electronic spectrum, there-fore, we would expect the n orbital to be the highestoccupied orbital followed by the 7t orbital.PES of nitro methane shows two bands at 11·35and 11·85 eV (Fig. 2) and the remaining bands areat much higher energy (>14 eV). The 11·35 and11·85 eV bands are assigned to the ionization ofnand 7t orbitals respectively. The second bandat 11·85 eV is very sharp indicating the non-bondingcharacter of the orbital. The bands at higherenergies (> 14 eV) obviously involve e-ionizationand are difficult to assign to specific orbitals.The 11·35 and 11·85 eV bands of CHaNOz areshifted when the alkyl substituent is varied. Thus,in C2HsN02, these bands appear at 11·15 and 11·60eV. Both the nand 7t ionization energies of theN02 group in simple nitroalkanes give linear rela-tions when plotted against the ionization energiesof the corresponding alkyl iodides (Fig. 3). Alkyliodides, it may be recalled, give sharp peaks dueto the ionization of the lone pair orbital of the iodineatom and the IE values are therefore very reliable-.To rationalise the substituent effects of alkyl groupson the nand r. ionization energies of nitroalkanes,the aliphatic substituent constants, 0'*, of Taft"147INDIAN J. CHEM., VOL. 14A, MARCH 19761\" (a2)I\...l_<>__ 1 ..-:,::' ••.1 .J.! Ij8 IGi4 12 e\'Fig. 2 -- Photoelectron spectra of nitro compounds, RNO.Fig. 3 - Correlation of nand 7t ionization energies of theNO. group of uitroalkanes (circles and triangles respectively)with (a) ionization energies of alkyl iodides, RI, (b) aliphaticsubstituent constants, cr* [In the orcIer of decreasing cr*values, the points refer to R = CH3• C.H5, n-C.H., i-C.H,and t-C.Ho]were plotted against nand 7t ionization energies.The plots are indeed linear, the slopes being nearlythe same. Both the nand 7t ionization energiesof the N02 group decrease with the increasing electrondonating power of the alkyl substituent. Thesetrends are in the expected direction and similarvariation of the n ionization energy with alkylsubstituents has been observed earlier in the caseof aliphatic ketones",In order to further substantiate the assignmentof the PES bands of the N02 group in nitroalkanes,the spectra of CC13N02 and CF3N02 have beenrecorded (Fig. 2). Since CCla and CF3 groups areboth highly electron-withdrawing in nature (a* ~2·6), we would expect the PES bands of the N02group to appear at considerably higher energies.PES of CCl3N02 shows bands due to the ionizationof the Cl lone pair orbitals just as in CHCla, butthe bands due to the N02 group can still be seenaround 12·4 and 12·75 eV (Fig. 2). The spectrumof CFaN02, on the other hand, clearly shows the148I I L~L-.--L--~16--.L---,L.4---'--"""12:-------10 eVFig. 4 - Photoelectron spectrum N,~-dimethylnitramine,(CH.).NNO.bands due to nand 7t orbitals around 12·6 and 12·8eV respectively, the former appearing as a shoulderon the latter. It appears that the "perfluoroeffect" is slightly greater on the n orbital than onthe 7t orbital. In conjugated molecules like buta-diene, substitution by fluorine mainly affects abands leaving the 7t bands unchanged-". The nand 7t ionization energies of CCl3 and CF3 derivativesdo not fall on the linear a* plot of simple nitroalkanes(Fig. 3) just as in the case of the n bands of aliphaticketones", This is because, the interaction of thehalogen atoms with the N02 group is not properlyrepresented by the a* constants which accountonly for the inductive or field effects.PES of nitrosamines and nitramines - When anelectron donating substituent like NH2 is directlyattached to the N02 group, the n-7t* transition isshifted only slightly, since the non-bonding electronsof N02 are almost entirely located at the oxygenatoms. Close to the n-7t* transition, we wouldexpect to see an intramolecular charge transferband involving charge transfer from NH2 to N02•PES of N,N-dimethylnitramine, (CHa)2NN02' showsevidence for such effects (Fig. 4). The PES bandsat 10·8 and 11·8 eV are similar to the nand 7t bandsof CHaN02• The additional sharp band at 9·75 eVis most likely due to the n orbital of the N(CHahgroup just as in methyl amines [n band of CH3NH2,9·8 eV; (CH3)2NH, 9·0 eV]. It is instructive tocompare the PES of (CH3hNN02 with that of N,N-dimethylnitrosamine, (CH3)2NNO, where the non-bonding orbital of the nitroso group, unlike thatof the N02 group, contains contributions from boththe oxygen and nitrogen atomic orbitals. Beforeproceeding to discuss the PES of (CH3)2NNO, weshall briefly examine the energy levels of the NOgroup.Molecular orbital calculations show that in theC-nitroso group, the highest filled orbital is then* orbital arising from the antibonding combinationof the lone pair orbitals of Nand ° atoms. Thisis followed by the n orbital due to the correspondingbonding combination of the atomic orbitals. The7t orbital of the NO group lies below the n orbital.Electronic absorption spectra'v-" of nitrosoalkanesare consistent with this energy level scheme andthe n*-7t*, n-7t* and 7t-7t*bands appear in the orderof increasing energy (absorption maxima around700, 290 and 220 nm respectively). PES of nitroso-RAO: PHOTOELECTRON SPECTRA OF ALIPHATIC NITRO COMPOUNDS· A ~I A I\ )\n1 J V. \I IV 1\9.93 ~.OS 7.97 ~.vn"J11 10 9eVFig. 5 - Photoelectron spectrum of N,N-dimethylnitrosa-mine, (CH3bNNO. [In the inset, the three characteristicbands of the nitro so group in t-nitrosobutane are shown]alkanes-" show that ionization energy increases inthe order n*, nand TC.H an electron donating group (D) is directlylinked to the NO group and D has low ionizationenergy and can conjugate with the NO group, thenintramolecular charge transfer can occur!", Then*-TC* transition is shifted to the blue and D-TC*transition can occur at lower energies than then-TC* transition. Accordingly, in (CH3)2NNO, then*-TC* transition is at 380 nm while the D-TC* transi-tion is at 230 nm. The n-TC* transition is not seenabove 200 mn. Generally, greater the tendencyof electron transfer from D to TC*, larger is theblue shift of the n*-TC* transition. A similar situ-ation is also found in n-TC* transitions of aliphaticcarbonyl compoundsw-t''. PES of (CH3)2NNO showsa band at 9·1 eV probably due to the n* orbitalionization (Fig. 5). The strong sharp band centredaround 9·7 eV has a shoulder around 10 eV. Thesetwo ionization energies are likely to correspondto the donor, N(CH3b lone pair orbital and n(NO)orbital.The appearance of donor levels in between then* and n levels of the NO group is also seen innitrosobenzene. The PES spectrum of nitro so-benzene'? shows a band due to the ionization ofthe n* (NO) orbital at 8·9 eV followed by two bandsdue to the a2 and bI levels of aromatic TC system.The electronic absorption spectrum of nitroso-benzene'? shows the n*-TC* transition in the visibleregion followed by two TC-TC* transitions at lowerwavelengths. Clear evidence for the n level of NOis not seen in either the PES or the electronicabsorption spectrum. In nitrobenzene, on theother hand, PES bands due to ionization of thenand TC levels of the N02 group appear at higherenergies than those due to the TC levels of thearomatic'", The n-TC* transition of nitrobenzeneis indeed masked by the intense TC-TC* transitionof the aromatic system-".AcknowledgementThe author is thankful to Dr A. F. Orchard andDr J. C. Green for providing some of the spectraand to the Jawaharlal Nehru Memorial Fund for afellowship.References~1. TURNER, D. 'IN., BAKER, C, BAKER, A. D. & BRUNDLE,C. R., Molecular photoelectron spectroscopy (Wiley Inter-science, New York), 1970 .,2 ELAND, J D. H., Photoelectron spectroscopy (Butterworths,London), 1974.,,3. RAO, C. N. R., Indian J. Chem., 13 (1975), 950.4. COCKSEY, B. J., ELAND, J. D. H. & DANBY, C J., J." chem . Soc., B (1971), 790.\ 5. SWEIGART, D. A. & TURNER, D. Vi., J. Am. chem, Soc.,94 (1972), 5592.'6. McEwEN, K. L., J. chem, Phys., 32 (1960), 1801., 7. KATO, H., YONEZAWA, T., MOROKuMA, K. & FUKUI, K.,Bull. chem, Soc. Japan, 37 (1964), 1710.8. RAO, C. N. R., in The chemistry of nitro and nitroso groups,'" edited by H. Feuer (Interscience, New York), 1969,Chapter 2.9. TAFT, R. w., DENO, N. C. & SKELL, P. S., Ann. Rev.phys. Chem., 9 (1958), 287.,10. BRUNDLE, C. R. & ROBIN, M. B., in Determination oforganic structures by physical methods, edited by F.Nachod & J. J. Zuckerman (Academic Press, NewYork), 1971.,.11. RAO, C. N. R. & BHASKAR, K. R., in The chemistry ofnitro and nitroso groups, edited by H. Feuer (Inter-science, New York), 1969, Chapter 3.,12. BHUJLE, V. V., WILD, U. & RAO, C. N. R., J. chem. Soc.Faraday Trans. II (1974), 1761..13. 'NAGNIERE, G., in The chemistry of nitro and niirosogroups, edited by H. Feuer (In terscience, New York),1969, Chapter 1.•.14. EGDELL, R., GREEN, J. C. & RAO, C. N. R., J. chem, Soc.Faraday Trans. II (1976), in print.15. TANAKA, M., TANAKA, J. & NAGAKURA, S., Bull. chem,Soc. Japan, 39 (1966), 766."16. RAO, C. N. R., Ultraviolet and visible spectroscopy, (Butter-worths, London), 1975.-,17. EGDELL, R., GREEN, J. C. & RAO, C. N. R., Chem . Phys.Letts., 33 (1975), 600.149

Photoelectron Spectra of C-Nitro & N-Nitro Compounds

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Photoelectron Spectra of C-Nitro & N-Nitro Compounds

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