Correlation of Ionization Potentials and the Sums of Substituent Constants for Substituted Benzenes

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1 Correlation of Ionization Potentials and the Sums of Substituent Constants for Substituted Benzenes HARRY W. GIBSON Xerox Corporation, Rocl~ester Reserirch Center, 800 Phillips Rorirl, Webster, Nerv York Received January 23, 1973 Previously reported experimentally determined ionization potentials of substituted benzenes are well correlated by the sum of the para substituent constants, regardless of the orientation of the substituents. u+ is better than a for this purpose. Tri- through hexasubstituted benzenes are also correlated, but the slope and intercept are higher in accord with electrostatic theory. These results in support of recent MO calculations indicate that substituent constants are good measures of the energy of the highest occupied orbital. Les potentiels d'ionisation expirimentaux precedemment rapport& pour les benzknes substitues sont en bonne correlation avec la somme des constantes de substituant para. A cet effet, at est meilleur que a. La correlation pour les benzknes tri h hexa substituts a Cte egalement faite, la pente et I'ordonnCe B I'origine sont plus ClevCes ce qui est en accord avec la thcorie tlectrostatique. Ces rcsultats, qui concordent avec les recents calculs des OM, montrent que les constantes de substituants sont une bonne mesure de I'tnergie de I'orbitale occupee la plus haute. [Traduit par le journal] Can. J. Chem., (1973) The wide applicability of the Hammett and related equations to reaction kinetics and a variety of spectral and physical properties (1) has attracted the attention of molecular orbital theorists (2-6). These studies seem to place linear free energy equations of the Hammett type on a very sound theoretical basis. Ionization potentials have long been recognized as absolute measures of energies of the highest occupied molecular orbitals (7). The ionization potential is defined as the energy necessary to remove an electron completely from a molecule or atom in the ground state to produce a positively charged ion and a free electron. In a few instances ionization potentials (IP) for homologous series, e.g., p-substituted acetophenones (8), substituted toluenes (9) and monosubstituted benzenes (9), were correlated with the Hammett constants. No one, however, appears to have correlated all values for substituted benzenes with the Hammett equation. Such a correlation would demonstrate on an experimental basis the link between orbital energies and substituent constants. For such a correlation to exist the electron being removed must be from a n orbital involved in the aromatic ring. If a substituent itself contains localized electrons of comparable or lower ionization potential, the compound will not fit the general correlation. On this basis the substituent is affecting the reactive site, i.e., the n cloud, both by its resonance and inductive contributions. This would mean that o,,,, values should be used since they include contributions from both effects, whereas o,,,, values do not include resonance contributions. To a first approximation the effect of the disposition of substituents with regard to one another is expected to be small.' Indeed, for disubstituted benzenes the difference in ionization potentials of the three isomers is less than 0.2 ev (9-1 1) (see Table l), which is less than the deviation between the various methods used to obtain the data used herein.2 As shown in Table 2, para substituent constants do give better fits of the data (10, 11). Brown's o+ values (12), which were derived for electrophilic reactions, i.e., those involving generation of a positive charge on the aromatic nucleus as the result of attack by an electrophile, e.g., NO,', Br+, seem to give slightly better correlations as previously noted (8, 9). Figure 1 is a plot of ionization potential us. ~o+p,,il for 49 mono- and disubstituted benzenes (10, 11) excluding phenols, bromides, and 'One rather obvious exception is the case where steric interference of the two (or more) substituents causes non-coplanarity of the substituent and ring, leading to loss of resonance contribution. ZAIl substituted benzenes for which ionization potentials were available from refs. 10 and I I were used. The former are the "best" values from ref. IS. In many cases o+,,,, values are not available.

2 CAN. J. CHEM. VOL. 51, 1973 TABLE 1. Summary of data (a) Mono- and disubstituted benzenes --- Calculated IP (ev) Eq. 1 Eq. 2 H CH3 CzHs t-c4h, C-SH5 CHzCI CHO COOH COOCH, COCH, CN CI Br I OH OCH3 OCzHs SH NO2 NHz N(CH3)z NHCOCH3 tn-ch3 p-ch3 o-ch~ p-cho p-cooch3 nz-coch3 m-cn p-cn 0-C1 117-CI p-c1 nz-oh o-br ~n-br p-br 0-1 m-i P-1 tn-noz P-NO2 171-NHZ P-NHz o-n(ch3)z ITI-N(CH~)~ p-n(ch3)z H 0-CH3 tn-ch3 p-ch3 P-CF3 0-C1 m-c1 p-c1 p-br

3 GIBSON: ON IONIZATION POTENTIALS OF SUBSTITUTED BENZENES 3067 TABLE I. (Concl~ided) Calculated IP (ev) x Y C~para C~+p;lrn IP (ev)* Eq. 1 Eq. 2 F 0-F F P-F F 0-NHZ f F rn-nh F P-NHz NHz CHO NHz 0-C @t)/ N Hz nz-coch NHz p-coch NHz 0-NO NHz tn-noz NHz P-NO COCH, p-c COCHB tn-0ch COCH, p-och COCH, m-no COCH3 P-NOz C1 nz-c C1 p-c C1 0-C C1 p-oh COOCH3 P-NOZ COOCH, p-och N(CH3)z P-N(CH~)Z CHO P-NOZ 1.OO f CHO p-ci f CHO m-oh f COCH, tn-oh f COCH, p-oh f COCsH5 p-oh Br p-oh NO2 p-oh (b) Tri-, tetra-, penta- and hexasubstituted benzenes - - Calculated IP (ev) Compound Cop,,, CG+~.~~ IP (ev)* Eq. 3 Eq ,2,3-(CH,),C,H, t ,2,4-(CH3)3C6H t ,3,5-(CH3)3C,H t ,2,4,5-(CH3),C,H t ,2,3,4-FjCsHz f ,2,4,6-NH2(CH3)3C,Hz t ,2,4,5-NHz(CH,),C,Hz t CsF5H t (CH3)5C,H t CH~CGFS t ,~,~-[(CH,)~N](CH~)ZC~H~ ~,~,~-[(CH~)ZNI(CH~)ZC~H~ ,~,~-[(CH~)ZNI(CH~)ZC~H~ 'From ref. 10 unless otherwise specified; by photoionization unless otherwise specified. tfrom ref. I I. tby electron impact. llby charge transfer spectroscopy.

4 CAN. J. CHEM. VOL. 51, 1973 TABLE 2. Linear least squares correlation of IP' and Zo using various combinations of substituent constants (o) Series (number of points) Method-1 Slope Intercept Monosubstituted acetophenones (10) Monosubstituted fluorobenzenes (8) Monosubstituted toluenes (16) I *Meta ant1 pan-disubstit~~ted compounds only; ortho compounds are not considered in this table. Phenols iodides and bromides are ekcluded. Data from Table I. tmerhod 1: o.,,,, fdr both sibstituents used for meta compounds; o,,., for both substituents used for para compounds, c.s.. for,?i-nitroacetoplienone: ZCI = cr,,.,,(coc~,~ -I-,;= ! = MCIIIO~ 2: a,,,,, used resardless of substitut~on pattern, e.s?, for either iir- or,,-nitroacetophenone: Zo = o,,,,c~?c~~,, f op..,(~o2) = 0.50 $ = Merliod 3: of,.,,, used regardless of subst~tut~on'pattern. XCorrelation coefficient r = 1 for perfect fit. FIG. 1. Plot of experimental ionization potential us. sum of para electrophilic substituent constants (Zo+,,,,) for 49 mono and ortho-, nieta- and para-disubstituted benzenes. Slope 0.885, intercept 9.11, correlation coefficient iodides. The correlation coefficient is The standard deviation of the points from eq. 1 is 0.20 ev. Use of 28 values from the same data set obtained by photoionization gives a correlation coefficient of 0.958; electron impact data gives a correlation coefficient of for 18 points. While precisions of 0.05 ev or less are often quoted, inspection of literature data (10, 11) reveals that differences from laboratory to laboratory and method to method are at least 0.1 ev and quite often 0.2 ev. Therefore, this correlation is considered good in view of the fact that the ionization potentials were obtained by a variety of techniques and by a variety of investigators. No effort was made here to select "favorable" values. It is significant that nine ortho-disubstituted compounds are well correlated by use of Co+,,,,. For the ten such compounds included in the correlation, the standard deviation is 0.16 ev. This upholds the original premise that orientation effects in disubstituted benzenes, at least, are within the scatter of points from eq. 1 (see below, however). As one might anticipate bromo and iodo substituted aromatics do not always correlate; they have noticeably lower ionization potentials than expected from the Hammett constant values. If bromo and iodo compounds are included in the Co+p,ra correlation (total of 58 points) the correlation coefficient drops to The lower ionization potentials of the bromides and iodides may be due in part to their high polarizability, which could affect their electronic contribution more in solution than in the gas phase. The slopes are much lower than the other aromatics, that of the iodides being lower than bromides (slopes us , respectively; intercepts 8.89 us. 8.67, respectively). Ionization potential us. Cop,,, for 59 monoand disubstituted benzenes (excluding phenols, thiophenol, acetophenones, bromides, and iodides), yields a correlation that can be expressed as eq. 2.

5 GIBSON: ON IONIZATION POTENTIALS OF SUBSTITUTED BENZENES 3069 The standard deviation of the 59 points from this line is 0.26 ev and the correlation coefficient is Again the nine ortho-disubstituted benzenes correlate equally well with a standard deviation of 0.24 ev. Use of all data except that for thiophenol, phenols, and acetophenones (71 points) gives a slope of 1.42 and an intercept of 8.77, an equation only slightly different from eq. 2, but the correlation coefficient drops to Again this is partly due to the fact that bromides and iodides have lower than expected ionization potentials. Phenols (from examination of nine examples) seem to ionize at higher potentials than anticipated (1 3). This could be related to the unusual problems involved in the assignment of substituent constants in these systems (1). Or it may be that the ionization occurs from either a 7~ orbital of the benzene ring or a localized electron on the oxygen depending on the substituent, i.e., that the ionization potentials of the two orbitals are close. In contrast phenolic ethers are well correlated. Acetophenones seem to lose an electron from a different orbital than most of the aromatics since their ease of ionization (Table 1) is lower than expected from eq. 2 (8); the orbital involved could be on the carbonyl oxygen. We note, however, that the slope, i.e., sensitivity to substituent effect, is the same for acetophenones as for the other aromatics. (Acetophenones were not correlated by Co+ since o+ for COCH, is not available (I).) The NHCOCH, group also appears to lead to lower ionization potentials. The sulfhydryl group also falls in this category. A more rigorous test of the correlation of IP and Ca is provided by data for more highly substituted benzenes, although a limited number of such data are available (Table 1). The a+,,,, correlation for tri-through hexasubstituted benzenes is given by eq. 3 and is shown in Fig. 2. The standard deviation for the 13 points is 0.23 ev and the correlation coefficient is The slope and intercept are appreciably higher than for the mono- and disubstituted benzenes (eq. 1). This is in accord with the electrostatic model (9, 14) which predicts that placing groups with the same sign of a ortho to one another will result in a higher ionization potential than if X a+para FIG. 2. Plot of experimental ionization potentials us. sum of para electrophilic substituent constants (Co+,,,,) for tri- through hexasubstituted benzenes. Slope 1.10, intercept 9.73, correlation coefficient they are meta or para. Conversely, if the groups have oppositely signed o's, the ionization potential of the ortho isomer will be lower. This leads to a greater slope and intercept. The trithrough hexasubstituted benzenes are better fit by Cop,,, giving eq. 4; the standard deviation is 0.23 ev and the correlation coefficient These correlations of multi-substituted benzenes corroborate the linear relationship found in mono- and disubstituted benzenes, substantiate the general additivity of substituent effects even though the influence of dipolar effects modulates it and indicate a fundamental relationship between energies of highest occupied molecular orbitals and Hammett substituent constants. This correlation offers the possibility of assigning a and/or o+ values of substituents from the ionization potentials of a series of derivatives. For example o+ for CHO is not known (1); from 5 points it is estimated to be 0.50 f Preferably, all data would be gathered by the same technique on the same calibrated instrument. In conclusion a good general correlation between ionization potential and the sun? of substituent constants exists. This substantiates molecular orbital theory calculations which reveal the fundamental character of the Ha~nmett

6 3070 CAN. J. CHEM. VOL. 51, 1973 and related equations. Thus, substituent con- 7. J. c. LORQUET. Rev. Mod. ~hys. 32, 312 (1960). stants seem to provide a useful and convenient P. Susz. 8. ~i~~c~,";a~i7p.i~s~614'),and guide for prediction of ionization potentials for 9, G, T. CRABLE and G, L, K ~ J, Phys, ~ Chem. ~ 66, ~ substituted aromatic systems. Conversely ioni- 436 (1962). zation potentials may be used to calculate sub- 10. Handbook of Chemistry and Physics. Chemical stituent constants, especially o', which may not Rubber CO., 54th ed p. E-62ff. 11. V. I. VEDENEYEV, L. V. GURVICH, V. N. KONotherwise be available. DRAT'YEV, V. A. MEDVEDEV, and YE. L. FRAN- KURICH. Bond energies, ionization potentials and 1. C. D. RITCHIE and W. F. SAGER. Prog. Phys. Org. electron affinities. St. Martins Press, New York. Chem. 2, 323 (1964) H. H. JAFFE. J. Am. Chem. Soc. 76, 5843 (1954); 77, 12. H. C. BROWN. Adv. Phys. Org. Chem. 1, 35 (1963). 274 (1955). 13. S. PIGNATARO, A. FOFFANI, G. INNORTA, and G. 3. D. PETERS. J. Chem. Soc (1957). DISTEFANO. Z. Phys. Chem. (Frankfurt am Main), 49, 4. D. HOROWITZ. Stud. Cercet. Chem. 15, 927 (1967); 20 (1966). Chem. Abstr. 68, p (1968). 14. N. D. COGGESHALL. J. Chem. Phys. 32,1265 (1960). 5. J. KUTHAN and V. SKOLA. Z. Chem. 340 (1968); 15. lonizationpotentials, appearance potentialsand heats Chem. Abstr. 70, 14594d (1969). of formation of gaseous positive ions. U.S. Depart- 6. M. GODFREY. J. Chem. Soc. (B), 1540 (1971); Tet- ment of Commerce, National Bureau of Standards. rahedron Lett. 753 (1972). NSRDS-NBS 26, June 1969.

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