J. Phys. Chem. 1991, 95, 5142-5149
5742
4. Summary and Conclusions The ionization potentials, dipole moments, and geometries of the series of Ar-DCV compounds were calculated by using AM 1 methods and are in good agreement with the limited data available in the literature. The ground-state geometry of the Ar-DCV compounds, in particular, the dihedral angle (e) between the plane of the aromatic ring and DCV acceptor group apparently has a strong influence on the ground- and excited-state properties of the compounds. Each of the Ar-DCV compounds displays a low-energy absorption band which is assigned to an intramolecular ~r, ~ r charge transfer transition. The ICT assignment is supported by the fact that the energy of the band correlates well with the first ionization potential of the aromatic donor. Interestingly, the energy of the ICT absorption is influenced little by solvent polarity, which may be due to the comparatively high polarity of the molecules in the ground state. A highly solvatochromic, lowquantum yield ICT fluorescence emission is observed from four of the Ar-DCV compounds. The ICT fluorescence is observed regardless of the excitation wavelength, indicating that excitation into the LE manifold results in rapid interconversion to the ICT manifold. Preliminary evidence suggests that the fluorescent state may be a minimum-overlap charge-transfer (TICT) state.
-
Solvent Polarity Figure 8. Energy level diagram for intramolecular charge transfer ex-
cited states. Alternatively, excitation into the low-energy ICT band initially populates 'ICTW which exists in a solvent sphere that is unrelaxed with respect to the electronic structure of the ICT excited state. Rapid (nonradiative) relaxation of the solvent environment results in formation of the emissive state, 'ETmhd. However, note that the predominant pathway for decay of 'ICTEbd in the Ar-DCV compounds is via nonradiative decay as indicated by the low fluorescence quantum yields. The lack of a significant solvent dependence in the position of the ICT absorption band indicates that solvent polarity influences the energies of the ground state and 'ICTF4 in a parallel manner. This is very likely due to the fact that the ground state of the Ar-DCV compounds is relatively polar and the extent of charge separation in the ground state is a function of solvent polarity. By contrast, the energy of the ICT fluorescence is highly solvent dependent, which indicates that '1CTEhd is stabilized to a greater extent than the ground state in polar solvents.
Acknowledgment. We thank Dr. N. Malhotra for assistance with the MOPAC calculations. Registry No. Ph-DCV, 2700-22-3; NapDCV, 2972-83-0; Phe-DCV, 12731-15-8; Ant-DCV, 55490-87-4; Pyr-DCV, 27287-82-7. Supplementary Material Available: Figure showing the calculated dependence of AHf on the dihedral angle (e) for the Ar-DCV compounds (2 pages). Ordering information is given on any current masthead page.
Penning Ionization Electron Spectroscopy of Haiogenotoiuenes: p-CHsC6H4X(X = Ci, Br, I )
0-,
m-, and
Shoji Fujisawa,+Isao Oonishi,* Departments of Chemistry and Biomolecular Science, Faculty of Science, Toho University, Miyama, Funabashi-shi, Chiba 274, Japan
Shigeru Masuda, Koichi Ohno, and Yoshiya Harada* Department of Chemistry, College of Arts and Sciences, The University of Tokyo, Komaba, Meguro-ku, Tokyo 153, Japan (Received: October 5, 1990; In Final Form: March 6, 1991)
He*(2%) Penning ionization electron spectra (PIES) and He I ultraviolet photoelectron spectra (UPS) were measured to study the electronic structures and the orbital reactivities of halogenotoluenes; 0-, m-, and p-CH3C6H4X(X = CI,Br, I). On the basis of the feature of PIES, which provides direct information on the electron distribution of individual molecular orbitals, all the bands in UPS wereassigned. The relative reactivity of the orbital upon electrophilic attack can be related to the relative intensity of the band in PIES. It was found that the reactivities of the n and T orbitals depend on the electronic effect due to the size of the halogen p orbitals and the degree of the n-T interaction and also on the steric effect due to the methyl and halogeno substituents shielding some orbitals from the impact of metastable atoms. The reactivities of the u orbitals were also discussed in terms of the steric shielding effect.
-.
Introduction In Penning ionization (A* + M A + M+ + e-), an excited metastable atom (A*) collides with a target molecule (M) yielding an ionic state of the molecule (M+) and the ground state of the atom (A) together with an eject4 electron in a continuum state (e-), The energy analysis of ejected electrons provides a Penning
'Department of Chemistry. *Departmentof Biomolecular Science. 0022-3654/9 112095-5742$02.50/0
ionization electron spectrum (PIES).' In this process, A* attacks a molecular orbital of M, from which an electron is extracted into the lowest vacant orbital of the excited atom.2 Therefore, the probability of Penning ionization depends on the Spatial Overlap between the molecular orbital of M and the vacant orbital of A*. Since an outer orbital exposed outside the repulsive (van der V. J . Chcm. Phys. 1966, 44, 3781-3786. (2) Hotop, H.; Niehaus, A. Z.Phys. 1969, 228,68-88. (1) t e r m l k ,
0 199 1 American Chemical Society
*
~
The Journal of Physical Chemistry, Vol. 95, No. 15, 1991 5143
PIES of Halogenotoluenes Waals) surface of M can be more largely overlapped with the A* orbital than an inner orbital localized inside the surface, the former orbital gives a stronger PIES band than the lattera3 For example, PIES bands due to T and lone-pair orbitals having large distributions are generally enhanced compared with those due to u orbitals. Namely, the band intensities in PIES reflect the spatial electron distributions of individual molecular orbital^.^^^ Moreover, when a bulky substituent such as a methyl group or a halogen atom is introduced into a molecule M, molecular orbitals mainly due to the unsubstituted part of M can be protected from the impact of A* and the intensities of the corresponding bands in PIES become weak. Thus, Penning ionization electron spectroscopy is considered to be a powerful technique for studying stereochemical properties of molecule^.^*^ From a chemical point of view, the Penning ionization process is regarded as an electrophilic reaction of an excited atom A* with a molecule M; A * acts upon a molecular orbital of M as an electrophilic reagent and extracts an electron into the vacant orbital of A*. In this respect, the intensity of a band in PIES is connected with the reactivity of the corresponding orbital in an electrophilic attack by A*: Molecular orbitals extending outside the repulsive surface can readily react with A* and give a stronger band in PIES. Thus, PIES is very useful for understanding the reactivities of individual molecular orbitals as well as for probing the spatial distributions of the orbitals. When we apply PIES to the study of a solid surface, at which molecules are arranged with a definite orientation, we can selectively observe a particular part of a molecular orbital exposed outside the surface. We have already measured the PIES of various solid samples and obtained useful information about the geometrical orientation of molecules and the electron distribution of individual molecular orbitals from the analysis of the band intensity.&* Previously we took up mono ha loge no benzene^^ and dichlorobenzenes’O as a series on the study of halogeno derivatives of benzene; on the basis of the characteristics of PIES, we assigned each band in the ultraviolet photoelectron spectra (UPS) of these compounds and also investigated the stereochemical properties and reactivities of individual molecular orbitals. In the present study, we have extended our work to groups of halogenotoluenes, 0-, m-, and p-CH3C6H4X (X = CI, Br, I). Particularly, we aim to observe the steric shielding effects of the methyl group upon the A orbitals of the benzene ring and the n orbitals of the halogen atom and also to study changes in the reactivities of orbitals due to the A-n interaction. Experimental Section
The samples were obtained commercially. The apparatus used for the measurements of He* (2%) PIES and He I UPS were previously described in detail.” The helium metastable atoms, 2% (19.82 eV) and 2 ’ s (20.62 eV), were produced by coldcathode discharge with a discharge current of 50 mA and a voltage of 240 V.I2 A water-cooled helium discharge lam was used to quench 2% atoms. The H e I resonance line (584 21.22 eV) for UPS was produced by dc discharge in pure He gas. Electron spectra were obtained by means of a hemispherical analyzer at an ejection angle of 90’ with respect to the metastable atom beam or the photon beam. The relative band intensities of the spectra were
AI
(3) Ohno, K.;Mutoh, H.; Harada, Y . J . Am. Chem. Soc. 1983, 105, 4555-4561. (4) Ohno, K.;Matsumoto, S.;Harada, Y . J . Chem. Phys. 1984,81,4447. (5) Ohno, K.; Fujisawa, S.:Mutoh, H.; Harada, Y . J . Phys. Chem. 1982, 86, 440-44 1.
(6) Ohno, K.; Harada, Y . Theoretical Models of Chemical Bonding, Part
3; MaksiE, 2.B., Ed.; Springer: Berlin (to be published). ( 7 ) Harada, Y . Surf. Sci. 1985, 158,455. (8) Harada, Y . ;Ozaki, H. Jpn. J . Appl. Phys. 1987, 26, 1201. (9) Fujisawa, S.;Ohno, K.; Masuda, S.;Harada, Y.J . Am. Chem. Soc. 1986, 108, 6505-651 1. (10) Fujisawa, S.;Oonishi. 1.; Masuda, S.;Ohno. K.; Harada, Y. To be
published. (11) Harada, Y.;Ohno, K.; Mutoh, H. J . Chem. Phys. 1983, 79,
TABLE I: Observed and Calculated Ionization Potentials for o -CMorotoluene IPLlbJd/eV IPald/eV MO characteP 1 2 3 4 5 6 7 8 9 10 11 12 13 14
8.84 9.35 11.09 11.53 12.03 12.24 12.70 13.60 13.82 14.13 14.38 15.35 15.64 16.56
8.92 9.22 12.13 12.58 13.44 13.72 14.14 14.84 15.55 15.99 15.99 17.29 17.87 18.88
6a”( q) 5a”(r2) 27a’(n,) 4a”(nL) 26a’ 25a’ 3a”(rI) 24a’ 23a’ 2a” 22a’ 21a’ 20a’ 19a‘
‘The symbol nIlor n L indicates the MO predominantly due to the chlorine 3p A 0 distributed parallel or perpendicular to the benzene ring.
TABLE 11: Observed and Calculated Ionization Potentials for m -Chlorotoluene IP-/eV IPaM/eV MO characte14 1 2 3 4 5 6 7 8 9 10 11 12 13 14
8.96 9.37 11.18 11.34 12.05 12.54 12.95 13.66 13.82 14.13 14.43 15.26 15.63 16.60
8.94 9.23 12.14 12.42 13.52 13.61 14.45 15.01 15.67 15.95 15.95 17.18 17.86 19.09
6a”(*3) 5a”(r2) 27a’(ni) 4a”(nL) 26a’ 25a’ 3a”(rr) 24a’ 23a’ 2a” 22a’ 21a‘ 20a’ 19a’
“The symbol ns or nI indicates the MO predominantly due to the chlorine 3p A 0 distributed parallel or perpendicular to the benzene ring.
TABLE Ilk Observed and Calculated Ionization Potentials for p-Chlorotoluene IPobJd/eV IPdd/eV MO characteP 1 2 3 4 5 6 7 8 9 10 11 12 13
14
8.72 9.49 11.17 11.34 12.02 12.54 12.96 13.56 13.92 14.24 14.56 15.29 15.61 16.51
8.77 9.42 12.12 12.35 13.40 13.72 14.47 15.04 15.26 15.93 16.38 17.52 17.88 18.62
6a”( ~ 3 ) Sa”(r2) 27a‘(ni) 4a”( nI) 26a’ 25a’ 3a”(rI) 24a’ 23a’ 2a” 22a’ 21a’ 20a’ 19a’
‘The symbol nI or nI indicates the MO predominantly due to the chlorine 3p A 0 distributed parallel or perpendicular to the benzene ring.
calibrated by using the transmission efficiency curve of the spectrometer.) Since the measured PIES contained a small number of UPS signals due to the H e I resonance line produced in the metastable source, pure PIES were obtained by subtraction of appropriately scaled UPS from measured PIES. Calculations
Ab initio molecular orbital calculations for 0-,m-, and pCH3C6H4Clwere performed by using library program GSCFZ’)
3251-3255.
(12) Aoyama, M.; Masuda, S.;Ohno, K.;Harada, Y.;Mok, C. Y.;Huang, H. H.; Lee, S.Y . J . Phys. Chem. 1989, 93, 1800-1805.
(1 3) Kosugi, N. Program oscn. -ram The University of Tokyo: Tokyo, 1981.
Library, The Computer Center,
Fujisawa et al.
5744 The Journal of Physical Chemistry, Vol. 95, No. 15, 1991
I
12
10
8 Ek/eV
12
10
I
'
I
8
I
~
I
6
I
6 I
l
I
12
2
4
10 12
4
'
I
'
l
~
He1 UPS
#
l
6
I
'
#
l
2
4
Ek/eV 8
10
I
I
,
8
6 I
4
'
I
'
I
He1 UPS
3
I'; I
8
10
12 I P/eV
14
16
1
,
8
18
,
I
12
IO
I
I
I
I
16
11
18
I P/eV
Figure 2. He*(Z'S) PIES and He I UPS of m-chlorotoluene.
Figure 1. He*(2%) PIES and He I UPS of o-chlorotoluene. TABLE I V Observed Ionization Potentials for o-Bromotoluene IP-/eV MO charactef I 2 3 4 5 6 7 8 9 10
II 12 13 14
8.83 9.40 10.48 11.13 I I .84 12.07 12.35 13.34 13.55 13.86 14.24 15.07 15.56 16.50
a"(*J a"(*2) a'(n a"(n,)
CI
a"(n2) 2
a'
a' a"(*,) a' a' a' a''
1
12
1
1
8
10
1
1
6
1
1
2
4
Ek/eV
8
10
12 I
'
I
'
6 I
4
'
I
~
I
3
a'
1'
a' a'
He1 UPS
"The symbol ny or n, indicates the MO predominantly due to the bromine 4p A 0 distributed parallel or perpendicular to the benzene ring.
TABLE V Observed Ionization Potentials for m-Bromotolwne IP-/eV MO charactep
IO
8.78 9.29 10.50 10.96 11.72 11.99 12.48 13.33 13.65 13.84
11
14.18
a"
12
15.00 15.61 16.54
a'
I 2 3 4 5 6 7 8 9
13
14
8
a"(*3) a"(d a'(nd a"(n,)
10
12
14
16
18
I P/eV
Figure 3. He*(23S) PIES and He I UPS of p-chlorotoluene. TABLE VI: Observed Ionization Potentials for p-Bromotolwne IP-/eV MO charactep
a'
a' a"(*J a' a' a' a'
a'
1 2 3 4
8.68 9.45 10.49 10.91
5
11.72
6 7
12.04 12.40 13.16 13.29 13.63 14.31 15.20 15.50 16.38
8
"The symbol n! or n, indicates the MO predominantly due to the bromine 4p A 0 distributed parallel or perpendicular to the benzene ring.
with a M-680computer at the Computer Center, The University of Tokyo; a 4-31G basis set" was used.
9 10 11 12
13 14
a"(*3) a"(d
1"
a'Yn,) a' a' a"(d a' a' a" a' a' a' a'
0-,
"The symbol nI or n, indicates the M O predominantly due to the bromine 4p A 0 distributed parallel or perpendicular to the benzene ring.
(14) Ditchfield, R.;Hehrc, W. J.; Pople, A. J . Chrm. Phys. 1971, 51, 124-1 28.
m-,and p-CH3C6H4X (X = CI, Br, I). For convenience of comparison, the electron energy scales for the PIES are shifted
Results Figures 1-9 show the He* (2%) PIES and He I UPS of
PIES of Halogenotoluenes
The Journal of Physical Chemistry, Vol. 95, No. 15, 1991 5145
12
8
10
6
4
Ek/eV 12
8
10
I
’
I
’
6
I
’
He1
3
1
8
He1
4
10
12
14
8
12
I
Ek/eV
I
I
~
I
I
6
@
J
’
‘
I
I
,
8
l2
lo 12
4
I
,
I
2
4
8
10
I
I
6
I
UPS
I P/eV Figure 6. He*(2%) PIES and He I UPS of p-bromotoluene.
18
16
^ I
10
’
UPS
I P/eV Figure 4. He*(2%) PIES and He I UPS of o-bromotoluene.
12
4
I
~
I
I
Ek/eV
10
I
I
,
8
I
I
,
,
,
6 6 I
I
2
4 4
~
I
’
I
I
He1 UPS
He1 UPS
1
I
I J , W , 8
10
I
12
,
1P/eV
I
14
I
I
16
,
I
18
8
10
12
14 1P/eV
16
18
Figure 5. He*(Z3S) PIES and He I UPS of m-bromotoluenc.
Figure 7. He*(23S) PIES and He I UPS of o-iodotoluene.
TABLE VII: Observed Ioniution Potentials for o-Iodotoluene IP-/eV MO charactee 1 8.56 a”(4 2 9.16 a”(4 3 9.61 “1 4 10.44 a”(n,) 5 11.20 a’ 6 11.33 a‘ l 12.00 a”(rl) 8 13.05 a’ 9 13.15 a’ IO 13.54 a’ 11 14.06 a” 12 14.86 a’ 13 15.36 a’ 14 16.33 a’ “The symbol nl or n, indicates the MO predominantly due to the iodine 5p A 0 distributed parallel or perpendicular to the benzene ring.
TABLE VIII: Observed Ionization Potentials for m-Iodotolwne IP-/eV MO charactep 1 8.55 a”(*3) 2 9.14 a”(%) 3 9.62 a’(nl) 4 10.33 a”(n,) 5 11.30 a’ 6 11.66 a’ l 12.05 a’’(* L 1 8 12.31 a’ 9 13.16 a’ 10 13.42 a’ 11 13.68 a” 12 14.15 a’ 13 15.41 a‘ 14 16.40 a’ “The symbol nl or nl indicates the M O predominantly due to the iodine 5p A 0 distributed parallel or perpendicular to the benzene ring.
relative to those for the UPS by the difference in the excitation energies, 21.22 - 19.82 eV = 1.40 eV. The assignments of UPS
bands and the observed and calculated values of ionization potentials for chlorotoluenes are tabulated in Tables 1-111. Tables
Fujisawa et al.
5746 The Journal of Physical Chemistry, Vol. 95, No. 15, 1991
He*(23S)PIES
o, 14
12
10
Ek/eV
12 I
6
8 I
6
8
10 ’
4
’
I
’
I
4 ’
I
. i
;:
He1 UPS
J >10Y 8 I
Y’
I
14
12
1
1
I
16
He1 UPS
I
18
I P/eV
1 P/eV
Figure 8. He*(2%) PIES and He I UPS of m-iodotoluene.
Figure 9. He*(23S) PIES and He I UPS of p-iodotoluene.
TABLE IX Observed Ionization Potentials for p-Iodotoluene IP,M/CV MO charactef
and other phenyl compounds,16 the A bands appear around IP = 9-10 eV ( r 3and r2),and 13-14 eV ( A ~ )and , they should be enhanced in the PIES owing to their character of wide distribution outside the molecular surface. Accordingly, we can assign bands 1, 2, and 7 to the 6a”(~3),5a”(n2) and 3a”(r1) orbitals, respectively. In general, the band due to the nl,orbital gives a sharp structure in UPS, and the band due to the nL orbital shows a strong intensity in PIES.9*’0Therefore, we can assign band 3 to the 27a’(nl,)orbital and band 4 to the 4a”(nL) orbital, respectively. In Figures 1-3 the n bands are stronger than the ?r bands in the PIES. This can be explained as follows. The A bands are mainly due to the 2p orbitals of carbon atoms, while the n bands are predominantly due to the 3p orbitals of chlorine atoms. Therefore, the n orbitals are further extended outside the repulsive molecular surface, and more strongly interact with He* to give larger band intensities in the PIES. The assignments for the remainder, i.e. for the u bands, are listd in Tables 1-111. Among them, band 14 is significantly stronger in the PIES. This is because the u orbital (19a’) corresponding to this band has C-H bond character of the benzene ring and hence its electron distribution extends outside the repulsive surface along the C-H bonds, as can be seen in Figures 1-3. Such a tendency is also found in benzene) and its derivati~es?*’~J’ 2. Bromotoluenes and Iodotolwoes. The UPS bands of bromotoluenes (e, m-, and pCH3C6H4Br)and iodotoluenes (e, m-, and pCH3C6H41)can also be assigned by use of the characteristics of the PIES and the results of the calculation (Tables IV-IX). Applying the same discussion as in the case of 0-, m- and pchlorotoluenes, we can assign bands 1.2, and 7 to the r3,?r2, and r 1 orbitals, and bands 3 and 4 to the nIIand nL orbitals, respectively. All the remaining bands are due to u orbitals. Among the u bands, band 14 originating from the C-H bonds of the benzene ring is enhanced in PIES as in the case of chlorotoluenes. B. Relative Reactivities of Orbitals with Metastable Atoms. As was described in the Introduction, the Penning ionization process can be interpreted as an electrophilic,reaction of a metastable atom with a molecule. Thus, the relative reactivities of individual orbitals upon such electrophilic attack can be studied from the relative intensities of the bands in PIES. For example, from the analysis of the n-band intensities, we can discuss the difference in reactivity due to the size of the halogen atom, the difference
I 2 3 4
5 6 7 8 9 10 11 12 13 14
8.45 9.34 9.57
10.25 1 1.27
a”(r3)
a”(4 a‘(nu) a’’ (n 1
a‘
12.10
a’ a’’(r,)
12.41 13.10 13.56 14.16 14.98 15.36 16.14
a” a’ a’ a’ a’
11.38
a‘ a’
OThe symbol nHor n L indicates the MO predominantly due to the iodine 5p A 0 distributed parallel or perpendicular to the benzene ring. IV-IX list the observed ionization potentials of bromotoluenes and iodotoluenes obtained from the UPS together with their assignments. Figures 10-12 shows the shapes of the occupied molecular orbitals of chlorotoluenes. In the figures, the orbital coefficients are shown by circles or couples of ellipses whose areas are proportional to the squares of the coefficient of the atomic orbitals. The solid and broken circles indicate the s orbital and the outsf-plane p orbital, respectively, while the couples of ellipses show the in-plane p orbital. The thick and thin curves indicate the lobes with positive and negative signs, respectively. Discussion A. Assignments of UPS Bands. 1. 0 - , m-,and p-Chlorotoluenes. In halogenotoluenes, 0-, m-, and p-CH3CbH4X( X = CI, Br, I), extended orbitals are the r orbitals of the benzene ring, the A), rZ,and r1orbitals, and the lonepair orbitals of the halogen atom, the n,,and nL orbitals distributed parallel and perpendicular to the benzene ring, respectively. Since these orbitals are exposed to the outside of the repulsive molecular surface, they are expected to react with He* in high probability and to provide strong bands. Utilizing the characteristics of PIES and also referring to the band assignments of the UPS of tolueneI5 and monochloroben~ene,~ we have assigned all the bands in the UPS of chlorotoluenes. In Figures 1-3, bands 1-4, 7, and 14 are enhanced in PIES relative to the other bands. In monochlorobenzene,9 toluene,Is
(! 5) Asbnnk, L.; Fridh, C.; Lindholm, E.Chem. Phys. fett. 1972, IS, 567.
Debics, T. P.; Rabalais. J. W.J. Electron Spcctrosc. 1973, I , 355. Kobayashi, T.; Nagakura, S.Bull. Chem. Soc. Jpn. 1974,47, 2563.
(16) Kimura, K.; Katsumata, S.;Achiba, Y.;Yamazalti, T.; Iwata, S. Handbook of He I Photoelectron Spectra of Fundamental Organic Molecules; Japan Scientific Societies Press: Tokyo, 1981. (17) Aoyama, M.;Masuda, S.;Ohno, K.;Harada, Y.;Mok, C. Y.; Huang, H. H.; Lee. S.Y . J. Phys. Chem. 1989, 93, 5414-5418.
The Journal of Physical Chemistry, Vol. 95, No. IS, 1991 5141
PIES of Halogenotoluenes
Figure 10. Shapes of the occupied molecular orbitals of o-chlorotoluene listed in Table 1.
6 a"(x3)
20d
5 a"(7r2)
19a'
Figure 11. Shapes of the occupied molecular orbitals of m-chlorotoluene listed in Table 11.
*-e ***
1.
3 a'1m 1
24a'
23a'
2 a"
22a'
21 a'
Figure 12. Shapes of the occupied molecular orbitals of p-chlorotoluene listed in Table Ill.
in the interaction between the n and ?r orbitals, and also the stereochemical properties of the molecular orbitals such as a steric shielding effect due to the methyl group and the halogen atom. In this section, we will mainly discuss these points. 1. Reactivities of n Orbitals with Metastable Atoms. As can be seen in Figures 10-12, in para derivatives @-CH3C6H4CI, pCH3C6H4Br,and pCH3C6H41),the ?r2 orbital does not mix with
the other orbitals, especially with the n orbitals of the halogen atom. In addition, the r2orbital is scarcely shielded against an attack of He* by the methyl group and the halogen atom. From these reasons, the intensity of the u2band was taken as a reference to examine the realtive intensity of the n bands in the para derivatives; the relative intensity is defined as [(I(n,) + I(nL)!/ 2]/1(r2), where I represents the intensity of each band put in
Fujisawa et al.
5748 The Journal of Physical Chemistry, Vol. 95, No. 15, 1991 TABLE X Relative Bud Imtdtkr h PIES CH&H,CI 0
m
P
0
CH3C6H,Br m
+ I(nL))/21/I(rd I(nl)/l(/(wd + I ( r N 2 1
1.16 i 0.08 1.06 f 0.12 1.56 f 0.06 1.53 f 0.19 1.18 f 0.04 1.68 f 0.04 I(rd/I(d 0.62 f 0.06 I(d/I(r2) 1.41 f 0.09 [xI(u)/9]/[(xI(r) + xI(n))/5] 0.57 f 0.03 0.57 f 0.01 0.51 f 0.03 0.53 f 0.01 0.46 f 0.03 I(I(nl)
parentheses. As indicated in Table X, the relative intensities of the n orbitals increase in the order, pCH3C6H4C1< pCH3C6H4Br < p-CH3C6H41. This finding is explained as follows: the size of the halogen lone-pair orbital, i.e. the extension of the orbital outside the repulsive surface, increases in the order, CI(3p) < Br(4p) < I(Sp), and hence the interaction with He* also increases in this order. In fact, it has already been found that the band intensity of the halogen lone-pair orbital becomes stronger in the order, F(2p) < CI(3p) < Br(4p) < I(Sp), in the PIES of monohalogenobenzenes C6H,X (X = F, CI, Br, I).9 Next we compare the relative intensity of the nil band among chloro derivatives, among bromo derivatives, and also among iodo derivatives. Table X shows the intensity of the nIIband with respect to an average of the intensities of the r2and 7r3 bands, I(nl,)/ [(I(r2) f(x3))/2]. As can be seen in the table, in all the compounds (CH3C6H4X,X = CI, Br, I), the relative intensity of the nIIband is weaker for the ortho derivatives than those for the meta and para derivatives. This is because in ortho derivatives the nIlorbital is shielded from the attack of metastable atoms by the methyl group located in close proximity to the halogen atom. Further, the steric shielding effect should be stronger for chloro derivatives, because their nllorbitals are much smaller than those of bromo and iodo derivatives. In fact, the nli band of oCH3C6H4Ciis significantly weaker in the PIES compared to those of o-CH3C6H4Brand o-CH3C6H41(Table X). Such a shielding effect of the ortho substituent upon the halogen ne orbital has been found in the case of o-dichlorobenzene,I0 2. Reactivities of r Orbitals. The intensities of the A bands vary considerably among them; most A ' bands (band 7) are stronger compared to the a3 and 1r2 (bands 1 and 2) and the intensity ratio of band ?r, to r3is found to vary among the compounds. Table X shows the relative intensities of the r3and A' bands with respect to that of r 2 , I ( T ~ ) / I ( A and ~) I ( ~ ) / ( I ( A ~ ) , for pCH3C6H4CI,pCH3C6H4Br,and pCH3C6H41. The 7r2band is taken as a reference again, because in the para derivaties the corresponding orbital does not interact with the n orbital and also it is not effectively shielded by other orbitals. We can find from Table X that the relative reactivity of the 7r3 orbital increases and that of A' decreases on going from pCH3C6H4CIto pCH3C6H41. As will be shown below, these intensity changes can be interpreted in terms of the interaction between the A and n orbitals and of the steric shielding effect due to the methyl and halogeno substituents. First we discuss the effect of the interaction between the A and n orbitals. Figure 13 shows a correlation diagram for the A and n orbitals of the para derivatives. As shown in the figure, the r3 and rlorbitals of the benzene ring mix with the halogen p,, orbital, and as a result the degeneracies of the A (7r3 and r 2 ) and halogen p (p, and p,,) orbitals are removed. Generally, the degree of the interaction between the 7r3 and pv orbitals depends on the energy difference between them, 6,. On the other hand, that between the A' and p,, orbitals is determined by their energy difference, h2. In Figure 14. 6, is approximated to be the difference in the observedf 1P between the nIIand uz orbitals, and b2 to be that between the A , and nil orbitals. The position of the xI orbital before conjugation is estimated on the assumption that the energy separation between the A' and u2orbitals is 2.86 eV, which is the difference in the IP between the la2,(*) and lel,(*) orbitals of benzene.'* In Figure 14 the 61 value becomes smaller, and hence the halogen p,, character for the x 3 orbital stronger on going from
P
0
CH3C6HJ m
P
1.45 f 0.06 1.68 f 0.06 1.32 f 0.14 1.52 f 0.08 0.75 f 0.02 1.18 f 0.04 0.36 f 0.01 0.42 i 0.02 0.38 f 0.01
1.63 i 0.09 1.54 f 0.06 1.19 f 0.09
0.78 i 0.07 0.35 f 0 . 0 3
+
(18) Asbrink, L.; Edquist, 0.;Lindholm,E.; Selin, L. E. Chcm. fhys. &rr. 1970, 5, 192-194.
+I
-
Xl
Figure 13. Correlation diagram for
9
10 -
T
/--w3
----_.---
-T- - -
and n orbitals.
,-~-~;,~"=0.30
>
: -
11-
12-
13
t
66$
_......../'
CI
Br
I
Figure 14. Correlation diagram for T and n orbitals of p-CH3C6H4CI, p-CH3C,H4Br,and p-CH3C6H41.
CH3C6H4Cl to CH3C6H41.This explains the increase in the relative intensity of the 7r3 band (Table X),because the reactivity of the 7r3 orbital upon metastables increases with the increase in the character of the halogen p,, orbital having a large distribution outside the molecule. On the other hand, in Figure 14 the 6, value increases from CH3C6H4CIto CH3C6H41,which corresponds to the decrease in the halogen p,, character for the xIorbital. The changes in the intensities of the r3and ?rl bands caused by the interaction between the A and n orbital is also observed in the case of mono ha loge no benzene^.^ Next we deal with the steric shielding effect of the methyl and halogeno substituents in para derivertives. In contrast to the case of the r2orbital, the x3 and xIorbitals have electron distributions at the positions of substitution. Therefore, these orbitals are expected to be much more shielded by the substituents from metastable atoms than the 1r2orbital, giving weaker bands in the PIES. The fact that the intensity ratio, I ( q ) / I ( x 2 ) is, not more than unity, in spite of the mixing of the halogen p,, orbital into 7r3, indicates this steric shielding effect (Table X). In the case
J. Phys. Chem. 1991, 95, 5149-5153
of the rIorbital (Table X), the effect of the p mixing is stronger than that of steric shielding for CH3C6H4d and CH3C6H4Br ( I ( q ) / I ( z 2 )= 1.41 and 1.18, respectively). However, the small I(r,)/l(r2) value of 0.78 for CH$&I shows that the shielding effect is predominant in this case owing to the effect of the large iodine atom and the weak u-n interaction. 3. Reactivities of u Orbitals. The intensities of the u bands also vary among the compounds. As shown below, this indicates that the shielding effect is largely affected by the size and position of the halogen atom. Table X shows the ratio of the average intensity of the u bands to that of the T and n bands, I(u)/I(u,n), where I(u) denotes the average value of the intensities of the nine u bands, CI(u)/9, and I(r,n) denotes that of the three u and two n bands, [XI(*) CI(n)]/5. As is seen in Table X, the above ratio decreases in the order Cl derivatives > Br derivatives > I derivatives. This is because the size of the halogen lonepair orbital becomes larger in the order, Cl(3p) C Br(4p) C I(Sp), and hence the shielding effect on the u orbitals becomes more significant from chloro to iodo derivatives. The position of the halogen atom also influences the u band intensity; the u band in the para derivatives is much smaller than that in the ortho and meta derivatives (Table X). In the para derivatives, the methyl group and the halogen atom are separated from each other so that both of them effectively shield the u orbitals from the attack of metastable atoms. On the other hand,
+
5149
in the ortho derivatives, the methyl group and halogen atom partly counteract their shielding effects on each other because their positions of substitution are adjacent. Thus, the shielding effect increases, and hence the relative intensity of the u bands decreases in the order o > m > p. Such a shielding effect is also found in dichlorobenzenes.1°
Conclusion In Penning ionization electron spectroscopy (PIES), information is directly provided on the electron distributions of individual molecular orbitals of the molecule. On the basis of this unique feature of PIES, all the bands in the He I spectra of halogenotoluenes, 0-, m-, and p-CH3C6H4X (X = C1, Br, I), have been assigned. The band intensities in PIES are closely related to the reactivities of individual molecular orbitals. From the analysis of the band intensities of the halogenotoluenes, the reactivities and stereochemical properties of the n, u, and u orbitals have been investigated. It was found that the orbital reactivities depend on the size of the halogen lone-pair orbitals, the interaction between the u and n orbitals, and also on the steric shielding effect of some orbitals against the attack of metastable atoms. Registry No. o-CH3C6H4CI,95-49-8; m-CH3C&CI, 108-41-8; p CH$6H,CI, 106-43-4; o-CH3C6H4Br,95-46-5; m-CH3C6H4Br,59 1 17-3; p-CH3C6H4Br, 106-38-7; o-CH3C6H41,61 5-37-2; m-CH3C6H41, 625-95-6; p-CH,C,H,I, 624-31-7.
-
Absorption Spectrum of the Solvated Electron in Ammonia and Amines Halina Abramczyk* and Jerzy Krob* Institute of Applied Radiation Chemistry, Technical University, 93-950 &kif, Wrbblewskiego 15, Poland (Received: October IO, 1990)
The absorption spectra of solvated electrons in ammonia, methylamine, and trimethylamine are calculated in terms of the theory presented in our previous paper' and compared with the experimental data. The theory reproduces very well the experimental absorption spectra, their bandwidths, and the band shapes with the characteristic asymmetric tail on the high-frequency side. The role of the H bond in stabilization and spectroscopic properties of the solvated electron in amines are discussed.
Introduction The nature of the solvated electron in ammonia and amines has been the subject of many e ~ p e r i m e n t a Pand ' ~ theoretical" ( I ) Abramczyk, H. J . Phys. Chem., in press. (2) Dye, J. L.; Debacker, M. G.; Dorfman, L. M. J . Chem. Phys. 1970, 52, 6231, (3) Jou, F.-Y.; Freeman, G. R. Can. J . Chem. 1979.57, 591. (4) Olinger, R.; Schindmolf, U.;Gaathon, A,; Jortner, J. Ber. Bunsen-Ges. Phys. Chem. 1971, 75,690. ( 5 ) Olinger, R.; Schindewolf, U. Ber. Bunsen-Ges.Phys. Chem. 1971, 75, 693. (6) Jou, F.-Y.; Freeman, G. R. J . Phys. Chem. 1981,85,629, 633. (7) Dcalaire, J. A.; Barouin, J. R. Can. J . Chem. 1979, 57, 2013. (8) Gavlas, J. F.; Jou. F. Y.;Dorfman, L. M. J . Phys. Chem. 1974, 78, 263 I . (9) Rubinstein, G.; Tuttle, T. R.; Golden, G. J . Phys. Chem. 1973, 77, 2872. (IO) Farhataziz; Perkey, L. M.; Hentz, R. R. J . Chem. Phys. 1974, 60, 4383. ( I I ) Seddon, W. A.; Fletcher, J . W.; Sopchyshyn, F. Ch. Can. J . Chem. 1978, 56, 839. (12) Seddon, W. A.; Fletcher, J. W.; Sopchyshyn. F. C. Chem Phys. 1976, 15, 377. (13) Fletcher, J . W.; Seddon, W. A.; Jevcak. J.; Sopchyshyn, F. C. Chem. Phys. Lett. 1973, 18, 592. (14) Lin. D. P.; Kevan, L. J . Chem. Phys. 1971, 55, 2629.
0022-3654191/2095-5149302.50/0
studies, molecular dynamics simulations,3w32and ab initio calc u l a t i ~ n s . ~ ~Ammonia -~~ is one of the most frequently studied ~~
~~
( I S ) Farhataziz; Perkey, L. M. J . Phys. Chem. 1975, 79, 1651. (16) Jou, F.-Y.; Freeman, G. R. Can. J . Chem. 1982,60, 1811. (17) Huppcrt, D.; Rentzepis, P. M.; Struve, W. S. J . Phys. Chem. 1975, 79, 2850. (18) Shida, T.; Iwata, S.; Watanabe, T. J . Phys. Chem. 1972, 76, 3683. (19) Haberland, H.; Langosch, H.; Schindler, H. G.; Worsnop, D. R. Ber. Bunsen-Ges Phys. Chem. 1984,88, 270. (20) Stupak, C.; Tuttle, T. R., Jr.; Golden, S.J . Phys. Chem. 1984,819, 3804. (21) Stupak, C.; Tuttle, T. R., Jr.; Golden, S.J . Phys. Chem. 1982,86, 327. (22) Tuttle, T. R., Jr.; Golden, S.;Lwenje, S.;Stupak, C. J. Phys. Chem. 1984, 88, 38 11. (23) Ogg, R. A. J . Am. Chem. Sor. 1946.68, 155. (24) Kestner, N. R.; Jortner, J. J . Phys. Chem. 1973, 77, 1040. (25) Fueki, K.; Feng, D. F.; Kevan, L. J. Am. Chem. Soc. 1973,95,1398. (26) Kajiwara, G.; Funabashi, K.; Naleway, C. Phys. Rev. A 1971,6,808. (27) Kestner, N. R. In Electron-Solvent and Anion Solvent Interactiom; Kevan, L., Webster, B., Us.; Elscvier: Amsterdam, 1976; Chapter 1. (28) Huang, J. T.; Ellison, F. 0. Chem. Phys. Lett. 1974, 28, 189. (29) Jortner, J. Be?. Bunsen-Ges. Phys. Chem. 1971, 75, 696. (30) Barnett, R. N.; Landman, U.; Nitzan, A. J . Chem. Phys. 1989, 91, 5567. (31) Sprik, M.; Klein, M. L. J . Chem. Phys. 1988.89, 1592. (32) Sprik, M.; Klein, M. L. J . Chem. Phys. 1989, 91, 5665. (33) Newton, M. D. J . Phys. Chem. 1975, 79, 2795.
0 1991 American Chemical Society
