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J. Phys. Chem. 1982, 86,440-441
specific ions for fragmentation studies using IR MPD. Thus, we can follow the fragmentation of molecular ions through several steps, obtaining unambiguous information in each instance. Isotope labeling will be very useful in such experiments.
Acknowledgment. The authors are indebted to F. Niertit and his electronic shop for invaluable assistance and to F. Kong and H. Reisler for the loan of the COz TEA laser. This research was supported by the National Science Foundation.
Application of Penning Ionization Electron Spectroscopy to Stereochemistry. Steric Shielding Effect of Methyl Groups on Penning Ionlzation in Substituted Anilines Kolchl Ohno,' ShoJI FuJIsawa,+Hldekl Mutoh, and Yoshlya Harada Department of Chembtry, ColEsge of General Educatlon, me University of Tokyo, Meguro-ku, Tokyo 153, Japan (Received: November 24, 198 1; In Final Form: December 14, 198 7)
The Penning ionization electron spectra resulting from thermal collision of He*(23S)metastable atoms with aniline and its methyl derivatives were compared to the respective UV photoelectron spectra at 584 A. The relative populations of the ionic states differ appreciably for the Penning ionization depending on the numbers and positions of the methyl groups, whereas they are nearly constant for the corresponding photoionization. This observation indicates that the presence of a bulky group causes a steric shielding effect on the electronic interaction in such a manner that depends on the type of the ionized electron (i.e,, nonbonding or r ) . Thus Penning ionization electron spectroscopy has potential value in the study of stereochemistry as well as in the assignment of different ionization bands to the type of the ionized electrons.
Evidence is observed for a steric shielding effect of methyl groups on Penning ionization probabilities. This finding indicates that a molecular orbital can be "protectedn from the impact of incoming species by a bulky group in the target molecule and that Penning ionization electron spectroscopy has potential value for the study of stereochemistry. Further, the observation of the steric effect enables us to establish the assignment of ionization bands. Penning ionization is known to be one of the most important types of chemical reactions in aerospace. When a molecule meets with an excited atom with an enough energy, an electron-exchange process may take place to yield a Penning electron and an ionic state of the molecule. Analysis of the kinetic energy distribution of the ejected electrons provides a Penning ionization electron spectrum (PIES), which is similar in many respects to a UV photoelectron spectrum (UPS).I However, a remarkable systematic difference has been observed in the relative intensities of PIES and UPS bands in addition to the well-known small shifts in their band positions; for aelectron systems, such as ethylene and benzene, the PIES bands due to ionization from P orbitals are markedly enhanced with respect to those related to u orbitals.24 This finding is interpreted in terms of the spatial electron distribution of the relevant orbital; orbitals "exposedn to the outside of molecules interact with a metastable atom more effectively to yield larger intensities in PIES. Recently, we have applied this characteristic of PIES to the assignment of deep P bands in the UPS of naphthalene and anthracene.6 When methyl groups are introduced in a conjugated system, various effects may be expected on the cross sections for Penning ionization. A steric shielding effect, if it exists, may reduce the cross sections. Inductive effects 7 Department of Chemistry, Faculty of Science, Toho University, Funabashi, Chiba 274, Japan. 0022-365418212086-0440~0 1.2510
or resonance effects can cause significant changes in the cross sections through modification of the electron distribution of the orbitals. In order to know how methyl groups affect the Penning ionization probabilities, a comparative study of the PIES and UPS was made for aniline and its methyl derivatives, N-methylaniline, NJV-dimethylaniline, 2-methylaniline (o-toluidine), 3-methylaniline (m-toluidine), 4-methylaniline (p-toluidine), 2,6-dimethylaniline (2,6-xylidine), 2,4-dimethylaniline (2,4-xylidine), 3,5-dimethylaniline (3,5-xylidine), and 2,4,6-trimethylaniline (mesidine). The electron spectrometer used in the present study will be described elsewhere.6 The helium metastable atoms (2%, 23S)were produced by impact of 60-eV electrons. A quench lamp was used to eliminate 'S atoms, and almost pure (more than 95%) 3Satoms were introduced into the interaction chamber. The He I resonance line used for UPS was produced by dc discharge of pure helium gas. Figure 1 shows the observed PIES and UPS. As for the UPS peaks for aniline, the first two ionization bands have been assigned to ring a orbitals (a3and 1r2) which correspond to the lel, orbitals of benzene and the next band to the nonbonding orbital (n) involving the nitrogen lone pair.' The first three bands for the other compounds are , n bands. The location of also assigned as the 7r3, P ~and the deepest a band (al)has not been determined in earlier (1) H. Hotop, E.Kolb, and J. Lorenzen, J. Electron Spectrosc. Relat. Phenom, 16, 213 (1979), and references cited therein. (2) T. Munakata, K. Kuchitsu, and Y. Harada, Chem. Phys. Lett., 64, 409 (1979). (3) T. Munakata, K. Kuchitau, and Y. Harada, J.Electron Spectrosc. Relat. Phenom., 20, 235 (1980). (4) T.Munakata, K. Ohno, and Y. Harada, J. Chem. Phys., 72,2880 (19NO). ,-.-.,
(5) T. Munakata, K. Ohno, Y. Harada, and K. Kuchitsu, Chem. Phys. Lett., 83, 243 (1981). (6) Y. Harada, K. Ohno, and H. Mutoh, to be published. (7) T. P. Debies and J. W. Rabalais, J. Electron Spectrosc. Relat. Phenom., 1, 355 (1972/73).
0 1982 American Chemical Society
The Journal of Physical Chemistry, Vol. 86, No. 4, 1982
Letters
441
1 P /ev 8
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Flgure 1. He*(2%) Penning ionization electron spectra (PIES, left) and He I photoelectron spectra (UPS, right) of N,Ndimethylaniline(top), aniline (middle), and 2,4,&trimethylaniline (bottom).
work. The assignment of the alband can be made by the present result, since the alband as well as the a2,a3,and n bands are enhanced with respect to the u bands in the PIES; a detailed report in this respect will be made in a separate papers6 Figure 2 shows the ratio of the integrated intensities, I(n)/I(q), for the UPS and PIES. In the UPS the first three bands have similar intensities for all the compounds studied in this work; the intensity ratios are nearly constant I(n)/I(a2)= 1.0 f 0.10 and I(aJ/I(a&= 1.05 f 0.15. On the other hand, the corresponding PIES band intensities exhibit a remarkable disparity; the two a bands have similar intensities (1(a3)/I(a2) = 0.85 f 0.10), whereas the relative intensities of the n bands with respect to the a bands, I(n)/I(a), show a systematic change depending on the numbers and the positions of methyl groups. This observation can be explained in terms of the steric shielding effect of methyl groups on the Penning ionization cross sections. The introduction of a methyl group to the ring reduces the PIES intensities of the a bands, because the cross sections for the a orbitals become smaller by the shielding effect of a bulky group. Further methylation at the ring emphasizes this effect to yield larger values for I(n)/I(a). On the other hand, upon methylation at the amino group, the intensity of the n band decreases to give smaller values for I(n)/I(a), because the methyl group at the nitrogen atom shields the nonbonding orbital more effectively than the a orbitals. (8) H. Mutoh, K. Ohno, and Y. Harada, to be published.
Flgure 2. The integated intensity ratios for the n and a2bands. Closed circles are for PIES and open circles for UPS. The compounds studied are shown at the top of the figure. The poskions of methylation at the ring are shown beside the circles for mono- and dlmethylaniiines.
Some other effects can be ruled out as possible causes to explain the observed tendency in the PIES intensities. (i) Approximate constancy of I(n)/I(a2)over 24-, 2,6-, and 3,5-dimethylanilines indicates that steric hindrance to rotation of the amino group does not give appreciable changes in the PIES intensities. (ii) Inductive or resonance effects of methyl groups may increase the size of the n or a orbitals. However, changes in the cross sections due to such effects occur in the opposite direction with respect to the steric shielding effect. Therefore, the resulting tendency in the intensity ratio should be opposite to the observed one. (iii) The a3and n orbitals involve a resonance interaction between one of the degenerate e,, orbitals of the benzene ring and the nitrogen 2p orbital. Modification of this interaction by methyl substitution may lead to changes in I(a3)and I(n), whereas for the noninteracting a2orbital little change is expected. This effect is, however, found to be unimportant, since the observed ratios of I(a3)/I(a2) are nearly constant for all the compounds. The arguments given above lead to the conclusion that the presence of methyl groups in substituted anilines causes a steric shielding effect on Penning ionization probabilities. Thus, Penning ionization electron spectroscopy has its important application in the study of organic compounds, because it provides useful information about the spatial electron distribution and the stereochemical environment of a particular molecular orbital as well as a powerful method for assigning ionization bands to molecular orbitals.
Acknowledgment. The authors thank Professor K. Mutai and Dr. T. Munakata for their helpful discussions.
