Ionisation Energies of Pyridine N-Oxides determined by Photoelectron Spectroscopy BY JOHN P.MAIER*AND JEAN-FRAN~OISMULLER~ Physikalisch-Chemisches Institut der Universitat, Klingelbergstrasse 80, CH-4056 Basel, Switzerland Received 2nd May, 1974 High resolution He1 photoelectron spectra of twelve pyridine N-oxides are presented. The exceptional separation of the first four, or more, bands throughout the series allows assignments of these in relation to the proposed scheme for pyridine N-oxide. These are discussed in view of the vibrational detail discernible, variations in the relative intensities of the bands with He11 and He1 radiation, especially in the chloro derivatives, and substitution trends. The correlation of the assign- ments affords a probe for substituent effects on the ground and first two excited states from the well defined changes in the corresponding ionisation energies.The latter are considered in terms of dipole perturbations due to the substituents. The linear regressions observed for the para-substitutedderivatives between the first three ionisation energies and dipole moments, basicities and reactivity constants are discussed. Heterocyclic N-oxides have aroused much chemical interest with the realisation of their importance as synthetic reagents, catalysts, ligands and the biological activity associated with some of the naturally occurring species.' These have instigated many studies in the photochemical field,2 for example, and have been subject to a broad spectrum of physico-chemical measurements.' U.V.spectroscopy in particular has been pr~rninent.~ Most studies have been on the neutral molecules though the anion 0 X X (2; 4-CH3) (8 ; 3-CH3)(3; 4-CN) (9; 3-CN) (4; 4-OCH3) (10; 2-CH3) (5 ; 4-C1) (11 ; 2-CN)0\ 0 (6 ; 4-WH3)dI (7; 4-NO2) 0 QX. CI (12) t permanent address : FacultC des Sciences, UniversitC de Metz, he du Saulcy, 57000 Metz, France. 1991 1992 IONISATION ENERGIES OF PYRIDINE N-OXIDES radicals were examined by u.v., e.s.r. and p~larography,~ and some cation radicals by e.~.r.~ We have embarked on the study of the electronic structure of the N-oxides of aza-aromatic compounds by high resolution photoelectron (p.e.) spectroscopy. In this contribution we consider the p.e.spectra of the pyridine N-oxides (1)-(12). In contrast to the p.e. spectra of most aromatic systems where the conglomeration of bands particularly with substituents usually limits the assignment,6 the p.e. spectra of the pyridine N-oxides show a clear separation of the first four (or more) bands. This affords a probe for the effect of substituents on the ground (%), first excited (i)and two higher excited ionic states, associated with the ionisation of electrons of the oxygen " lone-pairs " of n and B symmetry and of the ring n-orbitals respectively. EXPERIMENTAL The p.e. spectra were recorded on a spectrometer based on the design of Turner.' The analyser is a 10 cm radius n/J2 cylindrical condenser and the source of excitation is a d.c.capillary discharge with conditions adjusted for output of HeIa(21.22 eV) or HeIIa(40.80 eV) radiation. The He1 spectra were all recorded with high resolution, 0.02 eV or better, full width at half-height for 5 eV electrons. When He11 radiation was used some resolution was sacrificed in favour of flux. The ionisation energy (i.e.) scale was calibrated in situ with the rare gases, with naphthalene and with the photoelectrons obtained by the ionisation of xenon by the HeIP(23.09 eV) radiation. The samples were either commercial products (l), (2), (3), (4), (7), (8), (10) or were pre- pared from the corresponding substituted pyridines by oxidation with hydrogen peroxide as described in the literature (9,(9).8 C2H5]Pyridine N-oxide was prepared from t2H5]pyridine by oxidation with 30 % D202(in D20)and deuteroacetic acid.The product [2H5]-(1) was extracted from the aqueous phase while any remaining [2H5]pyridine was contained in the dichloromethane phase. Conipound (6) was prepared from (5) by the method described except that pure liquefied dimethylamine was employed. Attempted synthesis of (11) (2-CN) by oxidation of 2-cyanopyridine under the conditions prescribed,1° consistently resulted in 2-carboxy pyridine N-oxide. Compound (11) was prepared however from 2-cyanopyridine (1.2 cm3) when oxidised with 90 % hydrogen peroxide (1.5 cm3) in glacial acetic acid (3.6 cm3) at 70°C for 45min. After extraction and two vacuum sublimations -0.9 g of (11) was acquired,m.p. = 120"Cinaccord with that given in ref.(10). Pentachloro-pyridine N-oxide (12) was kindly provided by Prof. H. Suschitzky." All the samples were purified prior to use and checked for chemical purity. In the p.e. spectra of the N-oxides studied at the lower temperatures (=20-100°C) some variation was noted in the relative intensities of the two most intense vibrational components as well as the breadth of the second component in the first two bands (cf. fig. 3). How-ever, no shift in the i.e. of their maxima was detected. For compound (1) the i.e. values quoted were reproduced on three different n/J2 p.e. spectrometers. In view of the extreme hygroscopic nature of the N-oxides these variations may well be associated with sample pressure fluctuations. On the low i.e.tail of the first three bands there is evidence of vibra- tional hot bands (fig. 3). RESULTS The He1 p.e. spectra are shown in fig. 1-2a. The bands below i.e. -13 eV are exceptionally well separated. It will be shown that in compound (1) these 4 bands are attributable to nand ap-type orbitals localised essentially on the oxygen and to the ring n-orbitals related to the elspair in benzene.6 When substituents are introduced, additional band(s) may be seen in this region due to the n-orbitals of the substituent (X), X = N(CH3)2, OCH3 or o-lone-pairs, X = CN, or both, as is the case for X = C1, NO,. Nevertheless, the bands are always sufficiently distinct to permit clearcut J. P. MAIER AND J.-F. MULLER i i6 78 9 1011 12 13 14 15 16 17 1 rg 2 21 678-10 I1 12 13 14 15 16 17 18 1920210 $I1691011 121 ~~ 910 I1 12 13 1415 I6 17 18 19 2021 6 78 9 10 I1 1213 1415 16 1718 192021 6 7 8 91011 12131415 16171819202 6 7 8 9 10 If 12 13 14 15 16 17 It3192021 (4 (b) i.e./eV FIG.1.-He1 photoelectron spectra of the pyridine N-oxides (a)(l),(9,(12) and pentachloropyridineand (b)the pyridine N-oxides (2), (S), (9) and (3).IONISATION ENERGIES OF PYRIDINE N-OXIDES II1,718 1920 9 1011 1213 14 15 0It 7 8 9 10 II 12 13 14 15 16 17 18192021 6 7 8 9 10 II 12 I3 I4 I5 16 17 18 1920; 6 7 8 9 1011t2 131415161718192021 6 7 6 9 1011 1213141516 17\8192021 6 7 8 9 10 II 1213 1415 16 17 18 192021 6 7 8 9 10 I1 12 13 14 15 I6 !7I8192021 (4 (6) i.e./eV FIG.2.-(u) He1 photoelectron spectra of the pyridine N-oxides (7), (4) and (6) and (b)He I1 photo-electron spectra of compounds (l), (5), (12) and pentachloropyridine.J. P. MAIER AND J.-F. MULLER h FIG.3.-Expansion of the first three bands in the He1 photoelectron spectra of compounds (l), r2Hs1-(l),(5) and (3). assignments. In table 1 are presented the experimental vertical i.e. values and the propounded assignments are summarised. For convenience C, symmetry represent- ations are used in part for the molecules with the coordinate system as shown in fig. 4. The listed vertical i.e. values are k0.02 eV when the vibrational fine structure is distinct or kO.05 eV for the maxima of the broader bands. In fig. 2(b)are reproduced the He11 p.e.spectra of compounds (l), (9,(12) and of pentachloropyridine. Also shown is the He1 spectrum of the latter [fig. l(a)] for comparison. Although the He1 spectrum has been published l2 no i.e. values were given ; the present spectrum is well-resolved. IONISATION ENERGIES OF PYRIDINE N-OXIDES Vibrational fine structure is abundant under high resolution conditions on the first three bands (fig. 1,2) and in fig. 3 are shown the expanded sections of these in the pe. spectra of compounds (I), [2H5]-(1),(5) and (3) 2s examples. The suggested analyses TABLE (1)-(I 2) 1.-IONISATIONENERGIES(5V) OF THE PYIUDINE N-OXIDES compound I1 a(xO:) I2(00 :) 13(znz> 1.1 15 16 (1)(2) (44333) (4) (4-ocH3) (3) (4-CN) (5)(4-CO (6) (4-N(CH3)z) (7) (4-NOd (8) (3-CH3), [(lo) (2-CH3)J (9) (3-CN), [(I 1) (2-CN)] (12) (--CIS) = 8.38 8.12 7.74 8.95 8.42 7.21 9.03 8.20 L8.211 8.93 18.961 8.72 9.22 9.02 8.78 9.74 9.31 8.65 9.80 9.02 l8.991 9.68 l9.761 9.60 10.18 11.59 (nbl) 10.07 11.18 (A,) 9.97 10.50 10.84 11.58 (nbl) 10.44 11.14 9.03 (dl) 9.92 (ZQ) 10.81 11.26 9.80 l9.791 11.20 [I 1.391 10.56 [10.49] 1 1.90 10.87 11.52 13.0 (~bl) 12.5 12.08 12.18 * (ON:) 11.87 11.62 (dl) 11.46 12.60 12.50 b (ON:) 12.0 13.8 13.3 13.7 13.0 13.5 12.5 12.3 13.35 13.18 13.4 The numbering of the bands i in fig.1,2 corresponds to the index i of the ioilisation eaergies Ii. a For nomenclature of assignment see text : b vibrational spacing 1850 cm-1, c i.e. values of pentachloropyridine : 11-9.44, Z2-10.28, 13-10.60, 14-11.38, 1,-11.65 eV ; d i.e.value of triniethylainine N-oxide : 11-8.25, 12-12.6 eV. for the discernible progressions are indicated. In table 2 are given the corresponding ionic frequencies. It must also be pointed out that the vibrational analyses proposed (fig. 3) for the third bands are more tentative. We have also studied the p.e. spectrum of compound [2H,]-(1) to compare with that of (1). As can be seen in fig. 3, there are no clear changes in the vibrational pattern and any CH mass dependence is not 2.-vIBRATIONAL FREQUENCIES (WAVENUMBER/Cm-’) EXCITED IN THE GROUNDTABLE (k), FIRST (X)AND SECOND (ii)IONIC STATES OF THE PYRIDINE N-QXIDES -pyridine N-oxide % x B 520 1250, 810, 520 480 520 1250, 810, 520 1450, 520, 280 480 1210, 810, 360 400 1210, 818,400 1210, 500, 280 1290 1290 400 1210, 810, 360 1370, 320 400 1210, 810, 320 520 1210 perceptible within these experimental limits.Then if we consider only the 11 totally symmetric fundamentals (in C2v)of compound (l), as is probably reasonable in view of the Franck-Condon factors, it is possible to attribute the most prominent vibra- tional progression, 1250cm-l, on the second band to the N-0 stretching frequency while the 520 cm-l frequency, discernible on the bands, is allotted to a ring vibration.’ The ground molecular state frequencies are 1243 and 544 cm-1 re~pective1y.l~ It is notable that substitution in most instances does not markedly affect the vibrational features observed in compound (1) and this fits well with the assignments.DISCUSSION We first assign the bands in the p.e. spectrum of cornpound (1) which correspond N-to the ionisation processes leaving the radical cation in the ionic states g,A, B etc. The photoelectrons generating these bands can be considered as originating from the respective molecular orbitals (MOs) by means of which the closed-shell parent species is described. l4 Pyridine N-oxide (I), may be considered as a monosubstituted benzene where the J. P. MAIER AND J.-F. MULLER 1997 N--8 replaces the C-H group. Consequently the degeneracies of the MOs are reiiiovsd by interaction with the MOs of the N-0 group. In the p.e. spectra of (CH3)3N0 (i.e. values in footnote d to table 1) and CF3N0 l5 the first ionisation process (i.e.values of 8.25 and 10.40 eV respectively) relate to the degenerate oxygen lone-pairs while the subsequent i.e. in (CH3)3N0 is ~12.6eV. Thus we anticipate that two additional bands to the two in the p.e. spectrum of benzene (i.e. = 9.25 eV) will be present in the low i.e. region in the p.e. spectrum of compound (1). Indeed, four bands are observed [fig. l(a) @@I] 12 eV. From comparison of the below i.e. i.e. data of the lone-pairs in alcohols, ketones l6 and halogenoacetylenes l7 with (CH3), or CF3,and CH, groups in the p position the first i.e. values of (CH&NO and CF3N0both give the oxygen lone-pair basis in the 9-93 eV range for the environment in compound (1). Then the second band 0in the spectrum of (1) [fig.l(a)] can be associated with the oxygen a-lone-pair, 00 : (b2) (cf. fig. 4). By comparison, the mean of the i.e. values leading to singlet and triplet states on ejection of photoelectrons of the oxygen lone-pair in nitroxide radicals is also ~9 eV." The dominant vibrational progression observed on the second band 0(fig. 3) and attributed to the N-0 stretching vibration is also then reasonable. 0.6 9 0.85 0.5 I 0.3 7 12 0.49 0.2 8 0.41 . 0-52 c2v no:(3bJ a 0: (b2) na2 42bl) WJl) ie/eV 8.38 9.22 10.18 11.59 N 13.0 band 0 0 8 @ 8 FIG.4.-Qualitative representation of the five MQs of pyridine N-oxide associated with the first five bands in the p.e. spectrum [fig. l(a)]. The coefficients shown were calculated by the MINDO/2 procedure. The assignment of the other three bands in this region 0, @ follows directly.0, The symmetric (w.r.t. xz plane fig. 4) nbl-orbital of the benzene el, pair is stabilized in preference to the antisymmetric component, za2,as the latter has a nodal plane at the point of substitution. The oxygen p,-lone-pair can only interact with the symmetric component and thus the first band @ is associated with the out-of-phase combination (n3bl), while the fourth band @ with the in-phase (7~26,) and band @ with the 71a2 MO. These are depicted qualitatively in fig. 4.* The lowest n-MO in compound (1) in benzene) is also of bl symmetry but lies much deeper M 13 eV. Furthermore, the composition of the MO connected with the first ionisation process is heavily biased in favour of the oxygenp, basis. The coefficients of the oxygen, p, and p,,, orbitals in the two highest occupied MOs are calculated by MIND0/2 or CND0/2 as 0.7 and 0.86 respectively.Thus we may refer to these as oxygen nand a lone-pairs (fig. 4). The highest occupied MO characteristics have also been predicted previously by calculations, e.s.r. studies of the radicals, and electronic spectra to be of n-type with a predominating contribution from the oxygen p, A0.3-5 A more detailed discussion * In fig. 4 are also shown the coefficients of the eigenfunctions as calculated by MIND0/2. The CND0/2 values are very similar. IONlSATION ENERGIES OF PYRIDINE N-OXIDES of the p.e. spectroscopic data of compound (1) in relation to these studies is more appropriate in conjunction with the p.e.spectra of the N-oxides of the azabenzenes.lg We also note here that an empirical linear correlation between the i.e. of the na2 in monosubstituted benzenes (4X)and their experimental dipole moments (p)exists : i.e. na2(4X) = 0.19,u+9.35 (correlation coefficient Y = 0.946, degrees of freedom IZ = 14). From the dipole moment of compound (l), 4.130 D,20the predicted i.e. of the nu2 MO is then 10.13 eV, supporting our assignment of band 0,i.e. = 10.18 eV. This also suggests that the inductive stabilisation of the N-0 group is comparable to the cyano group, for example. The proposed assignment for compound (1) is further supported by the trends on substitution and the HeII p.e. spectra of (l), (5) and (12) [fig.2(b)]. Fluorination is a useful procedure for distinguishing n and CT bands in the p.e. spectra as the CT states are appreciably more stabilized than the n.16 However, substitution by chlorine can provide an alternative when He11 radiation is available. The decrease in ionisation cross-section of the chlorine 3p A0 in passing from 58.4 to 30.4 nm wavelength of incident radiation is greater than that of 2p AOS.~' This suffices for the effect to be discernible in the He1 and HeII p.e. spectra of molecular systems and the relative changes in intensity of the bands are, qualitatively, a reflection of the contribution of the C13p AO(s) basis in the MO in question. The He11 p.e. spectra of the chlorine containing species [fig. 2(b)] show consider- differences from the He1 spectra (fig.1) in the relative intensity of the bands.* The most drastic changes occur for the bands in the 12-13 eV energy region. These bands are associated with the MOs predominantly of chlorine character, chlorine " lone-pairs ". In the He1 and HeII spectra of compound (1) comparatively small changes are seen in the relative intensities of the first four bands, while in the spectra of (5) changes are already apparent. Bands 0,@ and 0decrease in intensity in relation to bands Q and 0.This is in accord with the assignment for compound (1) as the nbl MOs associated with these bands mix with the chlorine 3p, A0 in (5). In addition, in the He11 spectra of compounds (1) and (5), intensities of bands C2J (w.r.t.0)are enhanced somewhat in agreement with 02plC2p ionisation cross-section variation for MOs of 02p and C2p composition respectively.22 In the He11 spectrum of pentachloropyridine [fig. 2(b)] the first band is depressed in intensity relative to bands 0and 0.This agrees with the published assignment l2 to the nu2 MO which has a larger chlorine 3p character than the n2b1M0 (band 0or 0).In the HeII spectrum of compound (12) the intensity drop of band @ suggeststhat the sequence of MOs of (1) for the first three bands still holds. Band @ (and possibly 0)may also be correlated with the n2bl (and nbl) MOs of compound (1) in view of the HeII/HeI intensity variation. This would indicate that the three i.e. values of compound (12) have increased in respect to those of (1).For the first two bands, this stabilisation is reasonable due to the dominant dipole influence, as the electron density is largely concentrated around the oxygen. This is already apparent in the increase of the first three i.e. values of compound (5) with respect to those of (I). However, the large influence on the nu2 MO in compound (12) is anomalous, as conjugative interaction should be considerable in comparison to chlorobenzenes.6* The alternative interpretation is then to reverse the assignments of bands Q and a,notwithstanding the intensity changes and a surprising magnitude of the stabilisation * The increase in intensity of bands above i.e.a 15 eV is, to a large extent, due to the discrimination of the analyser against slow photoelectrons when He1 radiation is used.J. P. MAIER AND J. F. MULLER (z1.6 eV) of the a0: band. However, the Franck-Condon profile of the band @ of compound (12) does resemble band Q of (1). SUBSTITUENT CONSEQUENCES In the p.e. spectra of the pyridine N-oxides (2)-(12) the separation of the first group of bands remains. Additional bands, in the region of the first four bands (<13 eV) in the p.e. spectrum of compound (1) are readily identified by comparison to the p.e. spectra of substituted benzenes. 23 In the para-derivatives the consequences of the nodal differences of the bl and a2n-MOs are again evident. Thus in compounds (4) and (6) [fig. 2(a)] the additional band is due to the low i.e. of the basis of the lone-pair electrons of oxygen and nitrogen 10.2 eV and 8.7 eV re~pectively,~~ which mix considerably with the zb, MOs.Bands @ and @ in the spectrum of compounds (4) and (6) respectively are ascribed to MOs which have the largest contribution from these lone-pairs, in compound (7) the bands @Iand @ [fig. (2a)I can be attributed to the MOs characteristic of the nitro group while in compounds (3), (9), (11) bands @ are those of the nitrogen, lone-pair of the cyano group.6 The p.e. spectra of the chlorine con- taining species (5) and (12) have been described previously. In the substituted pyridine N-oxides the bands can be allotted to MOs within the model for compound (1) (fig. 4), as is summarised in table 1. t 130 -120 -2 110 -d calculated i.e./eV FIG.5.-Correlation between the p.e.spectroscopic i.e. values and the MIND0/2 i.e. values for the .rr-bands, and in the insert of the oxygen sigma lone-pair bands, of the pyridine N-oxides. The p.e. spectra of the two meta-derivatives (8) and (9) suffice to confirm the assignment. The i.e. values of bands 0and 0in compounds (2) and (8), or (3) and (9), barely differ in contrast to bands 0(table 1). The latter however lie z0.2-0.3 eV to lower i.e. for the meta-compounds. This destabilisation shows the contribution of through-space overlap of the deeper-lying n-MOs of the substituent with the z-MO of the ring [nna2 in compound (l)] absent in the para-species. For ortho-derivatives (10) and (1 l), the p.e. spectra show a great deal of similarity to those of the correspond- ing meta-compounds.In the higher i.e. region there are some slight differences. These trends are also observed in disubstituted benzenes.25 For most of the molecules studied, we have carried out semi-empirical SCF calculations to compare the eigenvalues with the i.e. values observed. The relative IONISATION ENERGIES OF PYRIDINE N-OXIDES order of the n-levels were best represented by the MIND0/2 procedure.* In all cases the n-MO sequences suggested (table 1) are in accord with the calculations. In fig. 5 is shown a least-squares linear correlation between the p.e. spectroscopic i.e. values (<12 eV) and the MIND0/2 i.e. values (Koopmans’ values) for the n-MOs. The insert shows the a0 : correlation.In table 3 are given the analysis of the re- gressions (1) and (2). Extrapolating of regression (1) yields an i.e. -13 eV for the deepest 7t-band of compound (1) (fig. 1 band 0)from the calculated MIND0/2 value. TABLE 3.-COMPILATION OF THE REGRESSIONS Ij = ax+ 6, SUBJECTED TO A LEAST-SQUARES ANALYSIS AND DISCUSSED IN THE TEXT. Ii ARE THE I.E. VALUES OF THE para-SUBSTITUTED PYRIQINE N-OXIDES degreesof a 6 correlation coefficient(r) freedom relation (n) number 1.03f0.04 -0.982 0.39 0.987 0.90k0.15 -0.29k0.03 0.30+ 1.50 9.31k0.14 0.935 -0.968 -0.20+ 0.02 9.94k 0.07 -0.980 -0.17f0.02 -0.33+0.02 10.95+0.07 8.51 k0.04 -0.974 -0.992 -0.22+ 0.03 9.37k0.05 -0.974 -0.17+0.03 10.42k0.07 -0.929 1.15+ 0.09 8.20+ 0.05 0.985 0.75k0.03 8.40+ 0.03 0.995 0.78+ 0.08 9.18k0.04 0.974 0.49+ 0.07 9.22+ 0.06 0.949 0.66f0.10 10.28+ 0.05 0.946 0.40+ 0.09 10.39+ 0.07 0.891 1.43 0.14 -4.90+ 1.26 0.978 1.55k0.29 -7.69k 2.99 0.922 1.12f 0.1 1 -2.35k 1.15 0.976 a r-i.e.values below m 12 eV ; excludes compound (3) (lack of convergence of MIND0/2) and compound (5) ; 6 in compound (6) read 14(.rraz); C PKa values of compound (3) not found. In table 4are shown the changes of i.e. of the a0 :and 7ta2bands inpara-substituted pyridine N-oxides and included are the changes observed in the i.e. of the na2 bands in the corresponding benzenes. The magnitude of the changes of the a0 : bands [cf. i.e.(na,)] are somewhat surprising for substituents as far removed.With electron TABLE4.-DIFFERENCES BETWEEN THE I.E. VALUES OF 00 :AND OF 7142 BANDS IN para-SUBSTI-TUTED PYRIDINE N-OXIDES AND THE DIFFERENCE INTO THESE I.E.VALUES IN PYRIDINE N-OXIDE THE I.E. VALUES OF na2 BANDS OF MONOSUBSTITUTED BENZENES TO THAT IN BENZENE pyridine N-oxides ~ ~~~~ benzene X Ai.e. (00 :)lev Ai.e. (naz)/eV Ai.e. (naZ)/eV NO2 0.58 0.63 1.10 CN 0.52 0.66 0.90 c1 0.09 0.26 0.44 CH3 -0.20 -0.11 -0.25 OCH3 -0.44 -0.21 -0.04 “CH3)2 -0.61 -0.35 -0.25 * Standard geometry and N-0 bond length of 1.26 8, (MIND0/2 and CND0/2 minimum energy value) were used. The coplanar conformation was predicted but the potential minimum was shallow for deviation up to m20”above, or below, the ring plane of the 0 atom.J. P. MAIER AND J.-F. MULLER donating substituents especially, a more pronounced destabilisation of the a0: MOs than the nu2 MOs is evident. The smaller changes of the nu2 bands of the N-oxides with electron withdrawing substituents than are observed with the correspond- ing benzene are perhaps a consequence of dipole-dipole interactions between the substituent and the N-0 gr0up.t The inductive effects of substituents have been interpreted by a short range and a long range term which can be represented through a change in a Coulomb integral of the adjacent atom, and the interaction of the dipole of the substituent with the remaining atoms not attached respectively. It was found that the i.e. values associated with the no:, 00: and nuz MOs give a good linear regression with the dipole moments of the m01ecules.~~ For the sake of conciseness the details of relations (3), (4), (5) and other linear relations (least squares analyses) obtained are collected in table 3 and in fig.6(a)are shown the regressions.If one represents the long-range inductive term by 26 where E is the dielectric constant of the framework, ri is the vector position of atom i with respect to the point dipole of the group attached at positionj, then by expansion of this term about a common point of origin (r) for all the substituents, a linear regression of the form observed [relations (3)-(5) table 31 is obtained from the first term of the expansion. With monosubstituted benzenes and para-substituted pyridine N-oxides the slopes of the i.e.(na2) against dipole regressions are 0.19 (given earlier) and 0.18 [table 3 relation (5)]respectively. For a distance of x1.4A, from the atom of the ring where the substituents are attached, as the common point, the observed slopes yield dielectric constants E w1.6 and w1.4 respectively for the sigma electron framework.In halogenobenzenes the values E = 2.28 28 and E = 2.95 26 were obtained, where the centre of the C-X bond was taken for the origin. The dependence of the first two i.e. values on the dipole moments is in accord with the concentration of the electron density around the oxygen in the two MOs and with the small electron density at the point of thepara-substituent. The short-range inductive term and the destabilisation as consequence of overlap are not so important. The latter terms are, however, significant in benzenes and a linear dependence of the first i.e.on the dipole moments is not obtained. We also note that the slopes of the regressions (3), (4) (table 3) differ. Furthermore, with the approximations made, the perturbation of the oxygen (00:) i.e. values is estimated to be nearly the same for para and meta substituents, whereas a factor of two or so larger with ortho. Indeed the a0: i.e. values of compounds (2) and (8), and (3) and (9) are very similar, but so are those of compounds (10) and (1 1) respectively. Nevertheless, this is reasonable as in compound (4) the proximity of the substituents may result in the destabilisation of the a0: MO by interaction with the deeper lying n-MOs of the cyano group, while the anticipated changes in compound (10) are small.The first two i.e. values also yield good regressions [relations (6),(7)] with the pK, values,29 as .does the i.e. (na2)(8); though it is not as good. This is reasonable since protonation occurs at the oxygen. In addition, these three i.e. values correlate very well with a[fig. 6@)]and a+reactivity constants,30 relations (9)-(14). It could be argued that the nO : i.e. correlates better with o+ whereas a0: i.e. and na2 i.e. with 0, in harmony with the differences in the derivation of the o and a+values. The analogy between pyridine N-oxide and the phenoxide anion is evident. It is worth noting that t For the electron donating substituents, the differences are not so significant as the i.e.values of the ra2 bands in the benzenes are not well defined. IONISATION ENERGIES OF PYRIDINE N-OXIDES 110 -lo5 -no -95 -90 -85 -80 -75 -70 7 ao 10 20 30 LO SO 60 70 11.0 10.0 .% @.I 9.0 8.0 7.0 ! 1 I 1 1 I* -0.9 -05 -01 .3 07 1.1 U (6) FIG.6.-Correlation between the first three i.e. values of the pyridine N-oxides and (a)dipole mom-ents, (b)reactivity constants. the significantly greater gradient of relations (9) [or (lo)]than of (1 l), (1 3) [or (12), (14)] suggests a more pronounced influence of the substituents on the ground ionic state than on the excited ionic states. This is also shown independently on the reactivity indices, by the internal relationships between these three i.e.values, relations (15)-( 17). In contrast the i.e. values of the bands correlated with the n(2bJ in compound (I) do not yield significant linear correlation with any of the parameters mentioned above. J. P. MAIER AND J.-F. MULLER These correlations of the i.e. values with other experimental entities confirm that the pyridine N-oxides provide an exceptional model for the study of substituent effects on the i.e. values of an aromatic system. Hitherto, such extensive p.e. data were available only for substituted benzenes.6* 23 However, in the p.e. spectra of pyridine N-oxides the changes are accurately discernible and available also for the first two (or more) excited ionic states of known characteristics in the whole series.We thank Ciba-Geigy S.A., F. Hoffmann-La Roche & Cie S.A. and Sandoz S.A. for their financial support. J. P. M. thanks the Royal Society for a Research Fellow-ship. This work is Part 73 of project no. 2.823.73 of the Schweizerischer Nationalfonds zur Forderung der wissenschaftlichen Forschung. Part 72 is ref. (3 1). 'see E. Ochiai, Aromatic Amine Oxides (Elsevier, Amsterdam, 1967) and A. R. Katritzky and J. M. Lagowski, Chemistry of the Heterocyclic N-oxides (Academic Press, London, 1971), for leading references and general reviews. G. G. Spence, E. C. Taylor and 0. Buchart, Chem. Rev., 1970, 70, 231. M. Yamakawa, T. Kubota and H. Akazawa, Theor. Chim. Acta, 1969, 15,244, and references therein.T. Kubota, K. Nishikida, H. Miyazuki, K. Iwatami and Y. Oishi, J. Amer. Chem. SOC., 1968, 90, 5080 and references therein. K. Nishikida, T. Kubota, H. Miyazaki and S. Sakata, J. Magnetic Resonance, 1972,7,260, and references therein. D. W. Turner, A. D. Baker, C. Baker and C. R. Brundle, Molecular Photoelectron Spectroscopy (Wiley-Interscience, 1970, London). D. W. Turner, Proc. Roy. SOC.A, 1968, 307, 15. E. Ochiai, J. Org. Chem., 1953, 18, 534. A. R. Katritzky, J. A. T. Beard and N. A. Coats, J. Chem. SOC.,1959, 3680. lo V. Boekelheide and W. L. Lehn, J. Org. Chem., 1961, 26,428. G. E. Chivers and H. Suschitzky, Chem. Comm., 1971, 28. l2 J. N. Murrell and R. J. Suffolk, J. Electron Spectr., 1973, 1, 471. i3 V. I. Berezin, Opt.Spectr. (U.S.S.R.),1965, 18, 119 and references therein. l4 T. Koopmans, Physica, 1934, 1, 104. l5 P. J. Carmichael, B. G. Gowenlock and C. A. F. Johnson, J.C.S. Perkin IZ, 1973, 1853. M. B. Robin and N. A. Kuebler, J. Electron Spectr., 1972,1, 13 ; C. R. Brundle, M. B. Robin, N. A. Kuebler and H. Basch, J. Amer. Chem. Soc., 1972, 94, 1451. l7 E. Heilbronner, V. Hornung, J. P. Maier and E. Kloster-Jensen, in preparation. 1. Morishima, K. Yoshikawa and T. Yonezawa, Chem. Phys. Letters, 1972, 16, 336. l9 T. Kubota, J. P. Maier and J. F. Muller, to be published. 2o R. D. Brown, F. R. Burden and W. Garland, Chem. Phys. Letters, 1970, 7,461. W. C. Price, A. W. Potts and D. G. Streets, Electron Spectoscopy, ed. D. A. Shirley (North- Holland, Amsterdam, 1972), p.187. 22 A. Schweig and W. Thiel. Mol. Phys., 1974, 1, 265. 23 A. D. Baker, D. P. May and D. W. Turner, J. Chem. SOC.By1968, 22. 24 J. P. Maier, Helv. Chim. Acta, 1974, 57, 994. 25 J. P. Maier and D. W. Turner, unpublished data. 26 B. Narayan and J. N. Murrell, Mol. Phys., 1970, 19, 169. 27 A. L. McClellan, Tables ojExperimenta1 Dipole Moments (W. H. Freeman and Company, San Francisco, 1963). 28 D. G. Streets and G. P. Ceasar, Mol. Phys., 1973, 26, 1037. 29 D. D. Perrin, Dissociation Constants of Organic Bases in Aqueous Solution (Butterworth, London, 1965), and supplement. 30 P. R. Wells, Linear Free Energy Relationships (Academic Press, London, 1968). 3' G. Bieri, E. Heilbronner, E. Kloster-Jensen, A. Schmelzer and J. Wirz, Helv. Chim. Acta, 1974, 57, 1265. NOTE ADDED IN PROOF A communication by the authors on the assignment of the p.e. spectra of the pyridine N-oxides has been published. Tetrahedron Letters, 1974, 2987, instigated by a paper (M.A. Weiner and M. Lattman, Tetrahedron Letters, 1974, 1709) dealing with the p.e. spectra of some 4-substituted pyridine N-oxides, where, however erroneous assignments were made.