The high resolution He 584 Å photoelectron spectra of three diazanaphthalenes and some of their fluorine derivatives are presented. The qualitative model that is used frequently in the discussion of lone-pair level splittings is examined

Beerlage, M.

Feil, D.

Ham, D.M.W. van den

Meer, D. van der

English

van den Ham, D.M.W.

van der Meer, D.

NARCIS 

Photoelectron spectra of fluorine substituted diazanaphthalenes: “Even cases”

University of Twente Research Information

Journal of Electron Spectroscopy and Related Phenomena, 7 (1975) 33-43 @ Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands PHOTOELECTRON SPECTRA OF FLUORINE SUBSTITUTED DIAZA- NAPHTHALENES: “EVEN CASES” D. M. W. VAN DEN HAM, M. BEERLAGE, D. VAN DER MEER and D. FEIL Chemical Physics Lab~rutory, Twente University of Technobgy, P.U.B. 217, Enschede (The Netherlands) (Received II February 1975) ABSTRACT The high resolution He 584 A photoelectron spectra of three diazanaphthalenes and some of their fluorine derivatives are presented. The qualitative model that is used frequently in the discussion of lone-pair level splittings is examined. INTRODUCTLON In previous contributions we reported the photoelectron (PE) spectra of some fluorine-substituted diazanaphthalenesl and diazabenzenes’. Partial or total fluorine substitution of these compounds proved to be an excellent experimental method to analyse the PE spectra of the parent compounds. Fluorine substitution not only shifted 0 and n bands relative to n bands (the perfluoro effectj), but also influenced the level splitting between the two “lone pair” bands that occur in all these spectra. In the case of symmetrical diazanaphthalenes where the nitrogen centres are separated by an odd number of intervening G bonds (odd case), this level splitting and the influence of substituted fluorine on it, could be explained within a qualitative interaction model. This so-called through space and through bond model, however, fails to explain the ordering of the symmetrical and antisymmetrical lone-pair levels of those diaza-compounds where the nitrogen centres are separated by an even number of cr bonds (even case). Failure in this respect means that the qualitative model and the extended Hiickel model (EHT) predict different level orderings. To find out the more realistic model, we studied the diazines and their fluorine derivatives2. It turned out that the EHT results corresponded with the experimental results. In this contribution we collected additional data for the “even cases”. Pre- viously reported assignments of the PE spectra of 1,8- and 2,7_diazanaphthalene were confirmed. On basis of these data and the corresponding EHT calculations the through bond interaction model was critically examined. 34 METHODS AND RESULTS The spectra were recorded on a Vacuum Generators spectrometer. All photoionizations have been brought about by irradiation with a He 584 i% source. The spectra were calibrated with argon. The spectra are presented in Figures 1-3; those of the unsubstituted compounds and of hexafluoroquinazoline have been published before4, but have been added for reasons of comparison. The fluorine-containing compounds were synthesized by conversion of the corresponding chlorine derivatives with potassium fluoride or caesium fluoride. Details of these syntheses will be published eIsewhere5. All compounds have been purified prior to use, either by sublimation or by preparative GLC. ICNlZATlON POTENTIAL ( e.v: ) Figure 1. Photoelectron spectra of the 1,8-naphthyridines. 35 DISCUSSION Analysis of the spectra The assignment of PE bands to either rc or n ionization processes usually relies in part on the observation that bands that arise from the same type of orbital have comparable band shapes, and in part on the correlation between experimental 20 15 10 Cl IONIZATION POTENTIAL (eV) Figure 2. Photoelectron spectra of the 2,7_naphthyridines. 36 1 20 I I I- 15 iQ 8 Ic1\IIZATlC& POTENTIAL (e.V 1 Figure 3. Photoelectron spectra of the quinazolines. TABLE 1 I EXPERIMENTAL AND CALCULATED TC IONIZATION POTENTIALS Quinazoline 2-Fluoroquinazoline 4-Fluoroquinazoline 2,4-Ditluoroquinazoline Hexafluoroquinazoline 1 ,&Naphthyridine 2,7-Difluoro-1,Gnaphthyridine Heuaffuoro-l,&naphthyridine 2,7_Naphthyridine 1,3,6,8-Tetrafluoro-2,7_naphthyridine Hexafluoro-2,7-naphthyridine 761 n2 m 5t4 9.08 9.08 11.30 9.15 I 9.90) (11.50) 9.22 (10.00) 11.30 12.25 9.30 10.15 11.50 12.35 9.43 10.41 11.86 (12.45) ( 9.40) ( 9.40) 11.33 9.26 10.05 11.71 12.11 (10.01) (10.01) 11.95 12.50 8.98 ( 9.90) (10.98) 9.55 10.45 11.60 13.05 9.50 10.75 11.85 13.22 IP (n) Hiickel (eV) a _ m m? n3 n4 8.98 10.04 11.39 9.20 10.25 11.75 9.04 10.08 11.52 12.33 9.14 10.17 11.56 12.39 9.85 10.59 11.72 12.66 9.37 9.12 11.39 9.40 10.11 11.39 12.39 10.03 10.40 11.78 12.55 8.72 10.31 11.05 9.37 10.72 11.20 12.88 9.78 10.72 11.49 12.91 a u = 6.50 eV; fl = 3.22 eV. 38 TABLE 2 EXPERIMENTAL AND CALCULATED “LONE-PAIR” IONIZATION POTENTIALS (eV) Compound Quinazoline Difluoroquinazoline Tetrafluoroquinazoline 2,4-Difluoroquinazoline Hexafluoroquinazoline 1 ,I?-Naphthyridine 2,7-Difluoro-1,8-Naphthyridine Hexafluoro-1,8-Naphthyridine 2,7-Naphthyridine 1,3,6,X-Tetrafluoro-2,7-Naphthyridine Hexafluoro-2,7-Naphthyridine Band nl n2 nl n2 n1 n2 nl n2 nl n2 nl n2 nl n2 nl n2 nl n2 nl n2 nl n2 Exp. Ca Ic . MIEffM Symmetry - --- 9.5 11.52 10.7 12.43 9.9 12.02 11.5 13.27 10.0 12.15 11.3 12.85 10.15 12.63 11.6 13.68 10.9 13.87 12.3 15.03 9.20 11.31 S 10.1 12.42 A 10.2 12.39 s 10.95 13.43 A 11.05 24.05 S 11.6 14.81 A 9.35 11.57 A 10.1 12.21 S 11.1 13.52 A 11.6 14.25 S 11.3 14.03 A 12.0 14.99 s and calculated ionization potentials. The application of the band shape criterion, however, is often made questionable by partial or total overlap of two or more bands. This di!Eculty can be evaded by gradually increasing the amount of substituted fluorine in a molecule, Stepwise substitution permits the fate of each band to be followed. Bands that are hidden under others in the parent compounds, may appear separately as a consequence of the substituent effects and vice versa. Using this experimental tool, one is able to assign most of the bands with reasonable reliability to ionization from either 7~ or n orbitals. We have designated our assignments in the spectra (Figures l-3) and collected the measured vertical ionization potentials in Tables 1 and 2. When overlap of bands made accurate measurements impossible, the data were estimated and appear between brackets in the tables. Correlation of experimental und cabdated ionization potentials In previous work” 2 we demonstrated that the effect of fluorine substitution on 7~ ionization potentials could be described with Hiickel calculations. The same parameter set [EN = EC -t o-7&; &-(F) = gC + 0.6j&; aF = ac t 3.0&c; &, = pee; P CF = 0.7&,] worked equally well with the compounds presented here. The cal- culated and experimental data of Table 1 are plotted in Figure 4. We consider this 39 Figure 4. Correlation of experimental versus calculated 3c ionization potentials. Figure 5. Correlation of experimental and calculated “lone-pair” ionization potentials. nice correlation as an affirmation of our assignment of bands to ionizations from 71: orbitals. For the description of lone pair bands in heteroaromatic molecules, EHT calculations give reasonably good results. However, this method is not able to cal- culate correctly the influence of increasing fluorine substitution on the lone pair 1P’s. An important reason for this failure is that this method does not allow for redistribution of charges. The modified and iterative version of EHT (MIEHM) as developed by J. Spanget-Larsen 6, copes with this shortcoming of EHT. The results of this calculation method were very satisfactory in the series of Auorine-substituted diazabenzenes. This prompted us to use the same calculation method in this work. The results of the calculation and the experimental data are collected in Table 2. 40 Figure 5 shows the correlation between calculation and experiments. In view of the fact that non-isomeric molecules are considered, the results are good. Through bond interaction in a localized orbital model The effect of through bond coupling between the lone pair orbitals and the symmetry adapted localized orbitals (SLO) of the d skeleton can be estimated by the general perturbation expressions 1 and 2: AE(Ns) = c H(N,, 9” symm i HW - W) A,!Z(N,) = H(N& .ntisgm j E(N,) - E(j) W is the matrix element that couples the (1) (2) interacting SLO’s; NA and N, denote the antisymmetrical (n, - nz) and symmetrical (nl + n2) SLO’s that are constructed from the sp’ hybrids that contain the lone pair electrons. In the initial approach the contributions to the summation were assumed to be roughly the same and only their number was taken into account. It was pointed out that for the “odd cases” the occupied SLO’s of “S” symmetry outnumber those of the “A” type. As a consequence the N, level is destabilized most. Exactly the same reasoning should apply for the “even cases”. For molecules as pyrimidine (1,3- diazabenzene) and 2,7_naphthyridine, however, EHT calculations predict the reverse ordering of lone pair levels. For 1,8-naphthyridine the qualitative model and EHT agree again. We investigated whether the controversy between both models could be removed by taking into account all factors (energy differences and values of matrix elements) of expressions 1 and 2. To estimate the influence of the denominator we transformed the occupied orbitals of EHT into symmetry localized orbitals by means of a procedure, described by Magnasco and Perico ‘. Although localization never was perfect, the resulting orbitals closely resembled the SLO’s that can be constructed from sp* hybrids. For the odd cases the orbital scheme paralleled expectation: all the energy expectation values associated with the SLO’s being roughly equal and lower than those of N, and N,; the S type outnumbered the A type. For even cases the situation is different. A large energy between the energy expectation values of the SLO’s: bA(b, - (Figure 6). gap (5 eV) was found b2) and bs(bl + b2) This splitting brings an A level very close to the N, level. If the matrix elements between b, and N, are also considerable, the combined effects in nominator and denominator of eyn. (2) can result in a larger raising of the N, level. We will estimate the importance of these matrix elements by calculating the ratio U(N,, bA)‘/H(Ns, bs)’ for the I ,3 and 1,8 cases, assuming these matrix elements to be proportional to the corresponding overlap elements of the overlap matrix. 41 ?Jzf-l? ’ bl , 3 4 P OVERLAPS I-6 0.12 I- 7 -013 I- 8 -0.04 3-8 0.06 3-7 0.12 Figure 6. The 1,3 and the 1,8 case. Figure 7. Overlap of sp2 hybrids. For the 1,3 case, the relevant orbitals, expressed as linear combinations of sp’ hybrids (Figure 7), take the forms: Ns = c,(l + 2) N, = c,(l - 2) b, = ck(5 + 6 + 7 + 8) bA = c;(5 + 6 - (7 + 8)) (3) (4) (5) (6) and for the 1,8 case: Ns = c,(3 + 4) (7) N, = c,(3 - 4) (8) and (5) and (6), where cs, cA etc. are normalization constants. Using the sp’-sp2 overlap data of Hoffmann’ (Figure 7) we find for the 1,3 case : S(N,, bA)’ w tick” - 0.34 SW,, bS)2 x c’sck2 - 0.01 and H(N,, bA)‘/WNs, bS)2 = S(NA, b..#/S(N,, bs)2 x 34 for the 1,8 case: SWAT bd2 x c:c: * 0.01 SF,, W2 M c’SC;~ . 0.36 and N(N,, b,)2/N(Ns, bs)2 w l/36 Summing up, we see that for the 1,3 case the relatively large factor N(N,, bJ2 and the small difference ENA - EbA contribute strongly in the perturbation sum (2), thus favouring the level ordering NA above N,, whereas in the 1,8 case this term will be insignificant, due to the small value of H(N,, bA)2. In the latter case the level ordering N, above N, becomes more likely. However, all these considerations are qualitative, only trends have been weighed out. In the next section we shall show that there are severe objections against the use of the naive model. 42 Through bond interaction and the variation treatment The qualitative through bond interaction model is based on the assumption that the interaction energies can be estimated with a second order perturbation approximation. Although this approach seems successful in some cases (previous section) we felt obliged to check the validity of this assumption. MIEHM calculations have been shown to be in good agreement with photo- electron results (Figure 5). As the level ordering for the lone pair combinations is the same in the iterative as in the non-iterative method, we used EHT calculations as a reference. A version of EHT was developed which permits the use of a sp2 hybrid basis set and allows the Hamilton matrix to be diagonal&d, based on any group of basis orbitals. In this way “fragment” MO’s can be constructed that resemble the bond orbitals of the qualitative model. Interaction between two groups of fragment orbitals can be accounted for either by second order perturbation theory or by diagonalizing the Hamilton matrix in which all the basis orbitals, that are used in the construction of the “fragment”, take part. This method for comparison of variation and perturbation theory will be illustrated in the case of 1,3_didehydrobenzene, a model compound that bears close relationship to pyrimidine. The four hybrid atomic orbitals that constitute the bonds b, and bz (Figure 6), were combined as to form MO’s for the corresponding fragment. The same was done for the n, and n2 hybrids to give the practically degenerate NA and N, combinations_ Interaction between the two groups of “fragment” MO’s was calculated by eqns. (1) and (2) and also by the described variational method. This procedure was repeated to calculate the interaction of other “fragment” MO’s with the NA and Ns combinations. The results are listed in Table 3. As can be seen from this table, the results of both calculation methods are proportional. This corres- TABLE 3 COMPARISON OF THE THROUGH-BOND SPLITTING Atb = (NA-Ns) (eV), CALCULATED IN A VARIATION AND A PERTURBATION MODEL No. of the L‘fragment”a A total A througkpace A through-bond variation perturbation 1 0.87 0.29 0.58 3.49 2 -0.36 0.29 -0.65 -3.95 3 0.53 0.29 0.24 1.31 4 0.26 0.29 -0.03 -0.28 5 0.08 0.29 -0.21 -1.21 6 0.29 0.29 0.00 -0.01 I-t2 0.08 0.29 -0.21 -0.07 1+2+3 0.00 0.29 -0.29 0.17 1+2+3+4 f5 0.37 0.29 -0.08 -0.07 Total molecule 0.33 0.29 0.04 -0.07 s The numbers of the fragments correspond to Figure 6. 43 pondence is remarkable in view of the fact that the large values that result from the calculation do not warrant the use of second order perturbation terms (1) and (2). This correspondence disappears, however, when two or more “fragments” are allowed to interact simultaneously with the “N” combinations. The perturbation method prescribes additivity of the interaction terms, but the variation method does not confirm this (Table 3). The mutual interaction of “fragments”, when acting on the N combinations, can only be accounted for by including higher order perturbation terms, a conclusion already suggested by the size of the contributions. As no simple qualitative model corresponds either with variation or with higher order perturbation theory, we have to conclude that only solutions of the Hamilton matrix can predict the order of the energy levels in this type of molecules. REFERENCES 1 D. M. W. van den Ham and D. van der Meer, J. Electron Specfrosc., 2 (1973) 247. 2 D. M. W. van den Ham, D. van der Meer and D. Feil, J. Ekcfron Spectrosc., 3 (1974) 479. 3 C. R. Brundle, M. B. Robin and N. A. Kuebler, J. Amer. Chem. Sot., 94 (1972) 1466. 4 D. M. W. van den Ham and D. van der Meer, Chem. Phys. Left., 15 (1972) 549. 5 D. M. W. van den Ham, J. Fiuorine Chem., in press. 6 J. Spanget-Larsen, Quantum Chemistry Program Exchange, 1975, No. 246. 7 V. Magnasco and A. Perico, .7. Chem. Phys., 47 (1967) 971. 8 R. Hoffmann, A. Imamura and W. Hehre, J. Amer. Chem. Sot., 90 (1968) 1499. 

Photoelectron spectra of fluorine substituted diazanaphthalenes:“Even cases”

Fiuorine Chem.,

Quantum Chemistry Program Exchange,

https://doi.org/10.1016/0368-2048(75)80053-3

Photoelectron spectra of fluorine substituted diazanaphthalenes: “Even cases”

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University of Twente Research Information