J . Phys. Chem. 1989, 93, 5414-5418
5414
states is relatively large and solvent effects on the TI state are small in this molecule. The present results clearly show that the T I states of both 1,2-NQ and 9,lO-PQ are influenced by solvent polarity, though the luminescent properties are significantly different from each other. Lim3' proposed that vibrational coupling between closelying 3 n ~ and * 3 ~ via~the*out-of-plane bending mode causes potential energy distortion in the excited states (proximity effect), leading to a remarkable increase in the nonradiative relaxation rate of the TI state. However, no phosphorescence is observed for 1,2-NQ in EtOH, while the TI state is assigned to the almost pure AT* state. This fact seemed to suggest that the proximity effect is not the main factor for the nonphosphorescenceof 1,2-NQ. This suggestion is supported by the result of 9,lO-PQ emits phosphorescence in every matrix while 3 n ~ * - 3 interaction ~~* is observed. It has been that in aromatic a-dicarbonyl (31) Lim, E. C. J . Phys. Chem. 1986,90, 6770. (32) Morantz, D. J.; Wright, A. J. C. J . Chem. Phys. 1971,54, 692. (33) Arnett, J. F.; McGlynn, S. P. J . Phys. Chem. 1975,79, 626.
molecules photorotamerization occurs in the excited state; that is, for benzil, although stable conformation in the ground state is a skew structure (dihedral angle between two carbonyls, 0 N goo), it is a trans-planar form (0 N 180O) in the S, and T, states. Therefore, the vanishing of luminescence in 1,2-NQ considered to be ascribed to the shallow potential surface in the excited state rather than to a proximity effect. Two carbonyl groups in 1,2-NQ can easily move toward the opposite out-of-plane sites because of the loose molecular structure, while they are fixed by two benzene rings in 9,lO-PQ. Acknowledgment. We are grateful to Professors J. Higuchi and M. Yagi for their help in the spectral simulation. The present work was partially supported by Grant-in-Aids for Scientific Research No. 62470001 and 63540324 from the Japanese Ministry of Education, Science and Culture. Registry No. 1,2-NQ, 524-42-5; 9,10-PQ, 84-1 1-7. (34) Roy, D. S.;Bhattacharyya, K.; Bera, S. C.; Chowdhury, M. Chem. Phys. Lett. 1980,69, 134.
Penning Ionization Electron Spectroscopy of Group IVB Trimethylphenyls: (CH,),MCBH, (M = C, Si, Ge, Sn, Pb) Masahide Aoyama, Shigeru Masuda, Koichi Ohno, Yoshiya Harada,* Department of Chemistry, College of Arts and Sciences, The University of Tokyo, Komaba, Meguro- ku, Tokyo 153, Japan
Mok Chup Yew, Huang Hsing Hua, and Lee Swee Yong Department of Chemistry, National University of Singapore, Kent Ridge, Singapore 051 1 (Received: December 1.5, 1988)
He*(2%) Penning ionization electron spectra (PIES) and He I photoelectron spectra (UPS) of group IVB trimethylphenyls (CH3),CC6H5,(CH3)$iC6H5, (CH3)sG&6H5,(CH3)$nC6H5, and (CH3),PbC6H5were measured to study the spatial electron distributions of individual molecular orbitals. The relative band intensities in PIES were analyzed on the basis of ab initio MO calculations, and all the UPS bands were assigned to molecular orbitals. Except for (CH3),CC6H5,most bands in the PIES and UPS can be interpreted in terms of the superpositionsof the correspondingspectra of metal tetramethyls and benzene, showing that interaction between the metal trimethyl and phenyl moieties is weak. The phenyl A orbitals give strong bands in PIES, because they are widely distributed outside the molecule and easily interact with metastables. With increasing size of the central atom, the relative intensity of the M ns band decreases in PIES, although the spatial expanse of the M ns atomic orbital increases. This indicates that the electron densities on the surrounding moieties, which diminish on going from (CH3)$C6H5 to (CH3)3PbC6H,,contribute most to the band intensity as in the case of (CH3)4M.
Introduction From a chemical point of view, the spatial distribution of molecular orbitals is of great interest since it is one of the most important factors determining the reactivity of molecules. Recent studies of Penning ionization electron spectroscopy have revealed that the relative band intensity of the spectrum reflects the distribution of individual molecular orbitals. In Penning ionization' of molecules M (M A * M+ + A + e-), metastable rare gas atoms A* attack occupied orbitals of M, from which an electron is transferred to the inner-shell orbital of A* in association with an electron emission from the outer shell of A* into a continuum state.2 Kinetic energy analyses of ejected electrons provide Penning ionization electron spectra (PIES), which are similar in many respects to UV photoelectron spectra (UPS).' Recent studies, however, have revealed that occupied molecular orbitals whose electron distributions are extending
+
-
( I ) Penning, F. M. Narurwissenshafren 1927,1 5 , 818. (2) Hotop, H.; Niehaus, A. Z . Phys. 1969,228, 68-88 (3) CermPk, V. J . Chem. Phys. 1966,44, 3781-3786.
0022-3654/89/2093-5414$01.50/0
outside the molecule give strong bands in PIES.4,5 This characteristic of PIES has been understood in terms of the electronexchange mechanism in which exterior electron distributions of molecular orbitals are selectively probed by incoming metastable atoms. Such a nature of PIES provides a valuable means for the assignment of UV photoelectron spectra, the studies of the spatial (4) Ohno, K.; Mutoh, H.; Harada, Y. J . Am. Chem. SOC.1983, 105, 4555-4561. (5) Ohno, K.; Matsumoto, S.; Harada, Y. J . Chem. Phys. 1984, 81, 4447-4454. (6) Kajiwara, T.;Masuda, S.;Ohno, K.; Harada, Y. J . Chem. Soc., Perkin Trans. 2 i988,507-5 1 1 . (7) Ohno. K.; Fuiisawa, S.; Mutoh, H.; Harada, Y. J . Phys. Chem. 1982, 86, 440-441. ( 8 ) Fujisawa, S.; Ohno, K.; Masuda, S.; Harada, Y. J . Am. Chem. SOC. 1986,108, 6505-651 1. (9) Munakata, T.; Kuchitsu, K.; Harada, Y. Chem. Phys. Lert. 1979,64, 409-412. Munakata, T.; Ohno, K.; Harada, Y.; Kuchitsu, K. Chem. Phys. Letr. 1981,83, 243-245. (IO) VeszprBmi, T.;Bihatsi, L.; Harada, Y.; Ohno, K.; Mutoh, H. J. Organomet. Chem. 1985,280, 39-43. (11) Harada, Y.; Ohno, K.; Mutoh, H. J Chem. Phys. 1983, 79, 3251-3255.
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 14, 1989 5415
PIES of Group IV Trimethylphenyls Me Me-t-Me
~ e (23s) * PIES
A
8
1 2 1 0
Me Me-%-Me
6
4 2 Ek/eV
1
0
8
6
4 E k/eV
He1 UPS
' P7
He1 UPS
12.13
5 16 - 1 1
,u, ,:Le 3 4
I
12
8
10
8-12
!
12
,
14 ,
16
18
J
10
I P/eV
Figure 1. He*(2%) PIES and He I UPS of (CH3)$C6H5.
distribution of molecular orbita1s,"I2 and also those of the outermost layer of solid s ~ r f a c e s . ' ~ Since the relative reactivity of orbitals can be probed by utilizing electrophilic attacks of metastable rare gas atoms, the application of the PIES technique to various organic molecules is highly interesting. In a preceding paper,I4 we have taken up the group IVB tetramethyls, (CH3),,M ( M = C, Si, Ge, Sn,Pb), and found useful information on the electron distribution and the reactivity of individual molecular orbitals. For example, on passing from (CH3)& to (CH3)4Pb,the relative reactivity of the uMCand M ns orbitals upon the electrophilic attack of metastable atoms decreases as the interaction between the central atom and the methyl groups become weaker. In this paper, we have extended the previous work to the group IVB trimethylphenyls, (CH3),MC6H5 ( M = C, Si, Ge, Sn,Pb). Distefano et al. observed the first ionization potentials (IP) of these compounds.15 Further, Bischof et al. have measured the H e I UPS for M = C, Si, Ge, and Sn and have assigned bands appearing in the lower IP regions of the spectra.16 Using the features of PIES mentioned above, we have established the assignment of all the bands observed in the H e I UPS. With increasing size of the central atom the group IVB trimethylphenyls show systematic changes in the electronic structure as well as in the reactivity of molecular orbitals as was found for the group IVB tetramethyls. Experimental Section The sample of (CH3)3PbC6H5was synthesized by a published method." The other samples were obtained commercially. He*(23S) PIES and He I UPS were measured with the electron spectrometer described elsewhere."J4 Since the PIES contains (12) Ohno, K.; Imai, K.; Matsumoto, S.;Harada, Y. J. Phys. Chem. 1983, 87, 4346-4348. Ohno, K.; Matsumoto, S.;Imai, K.; Harada, Y. J . Phys. Chem. 1984,88,206-209. Ohno, K.; Takano, S.;Mase, K. J . Phys. Chem. 1986,81, 2183. Ohno. K.; Imai, K.; Harada, Y. J . Am. Chem. SOC.1985, 207, 8078. Ohno, K.; Ishida, T.; Naitoh, Y.; Izumi, Y. J . Am. Chem. SOC. 1985. 107. 8082. (13) Harada, Y. SurJ. Sci. 1985, 128,455-472. Harada, Y.; Ozaki, H. Jpn. J . Appl. Phys. 1987, 26, 1201-1214. (14) Aoyama, M.; Masuda, S.; Ohno, K.; Harada, Y.; Mok, C. Y.; Huang, H. H.; Lee, S.Y. J . Phys. Chem. 1989, 93, 1800. (15) Distefano, G.; Pignataro, S.;Ricci, A.; Colonna, F. P.; Pietropaolo, D. Ann. Chim. 1974, 64, 153-157. (16) Bischof, P. K.; Dewar, M. J. S.;Goodman, D. W.; Jones, T. B. J . Organomet. Chem. 1974,82, 89-98. (17) Krause, E.; yon Grosse, A. Die Chemie der Metall-Organishen Verbindungen; Borntraeger: Stuttgart; FRG, 1937; p 384.
14
12
16
I P/eV Figure 2. He*(23S)
PIES and He I UPS of (CH3),SiC6H5. Me MeGeMe
A
J
11
9
7
5
11
13
15
3 Ek/eV
8 -
9
17 1 P/eV
Figure 3. He*(2%) PIES and He I UPS of (CH3),GeC6H5.
a small number of UPS signals due to the H e I resonance line produced in the metastable source, we obtained pure PIES by the subtraction of appropriately scaled UPS from the measured PIES. The UPS component in the measured PIES is at most 5% for the 12Zg+band in N2+.The He I resonance line (584 A, 21.22 eV) was produced with dc discharge in pure helium gas. The electron energy spectra were obtained for electrons ejected at an angle of 90' with respect to the metastable helium beams or the photon beams, by means of a hemispherical analyzer. The energy resolution was about 30 meV fwhm. The relative band intensity of the spectra was calibrated by using the transmission efficiency curve of the ~pectrometer.~ Calculations Ab initio molecular orbital calculations for (CH3)3MC6H5(M = C , Si, Ge) were performed by using a library program GSCFZ at the computer center of the University of Tokyo.'" The 4-31G
5416 The Journal of Physical Chemistry, Vol. 93, No. 14, 1989 Me
TABLE I: Observed and Calculated Vertical Ionization Potentials of (CH~~CCSHS band IP(obsd)/eV IP(calcd)/eV MO character‘
Me-SnMe
He* (Z3S) P I E S
Aoyama et al.
n3 n2
8.80 9.12 10.78 1 1.20 1 1 .60b 1 1.90
1
12
2 3 4 5 6
9
11
5
7
!}
8.64 8.93 12.10 12.19 12.49 13.33 13.50
{ !::::
-12.7
10 11
3
E k/eV
13.82
He1 UPS
14 15
13-17
{
14.75 15.00
16} 17 18 19
-15.3
{
16.55 17.45
14.50 14.96 15.84 16.01 16.52 16.72 17.05 17.17 18.96 20.26
9a”( r 3 ) 8a”( rz) 28a‘ 27a‘ 7a” 6a”(rI) 26a‘ 25a’ 5a” 24a’ 4a” 23a’ 22a’ 3a” 21a’ 20a’ 19a’ 8a’(aCH) 17a’(C 2s)
“The r, and r2orbitals are related to the benzene el orbitals with and without electron distribution at the position of substitution. Shoulder .
13
11
9
17
15
I P/eV Figure 4. He*(2%) PIES and He I UPS of (CH3),SnC6HS
TABLE II: Observed and Calculated Vertical Ionization Potentials of (CH3)fiiC6H5 band IP(obsd)/eV IP(calcd)/eV MO character‘
He*(23S) PIES
ni
n2
8.94 9.24b 10.20 10.55 10.92b 11.57 11.96 12.77
1 2 3 4 5 6 7 8
Me Me Pb Me
1314
-13.15
11 12 13
12
10
8
6
13.80
-
4 Ek/eV
He1 UPS
16 17 18 19
8.89 9.01 11.14 11.26 11.32 13.24 13.49 14.32 14.57 14.80 14.84 15.26 15.35 15.64 15.95 16.58 16.74 17.49 19.21
I
14.45 15.53 16.55
9a” (ir,) 8a”( r2) 32a’ 31a’ 7a” 30a’ 6a”(rl) 29a’ 5a” 28a‘ 4a” 27a‘ 3a”(pseudo T) 26a’ 25a‘ 24a’ 23a’ 22a‘ (Si 3s) 21a’(aCH)
“The r, and rz orbitals are related to the benzene el* orbitals with and without electron distribution at the position of substitution. bShoulder. TABLE 111: Observed and Calculated Vertical Ionization Potentials of ( C H ~ ~ G ~ ~ H S band IP(obsd)/eV IP(calcd)/eV MO character“
8
10
12
14
I 2 3 4 5 6 7
16 IP/eV
Figure 5. He*(2%) PIES and He I UPS of (CH3),PbC6H,.
basis functions were used for the carbon and hydrogen atoms.19 For Si and Ge, we employed the following split valence sets: 3-21G2’ for Si, and the (33321/3321/3) set derived from the (3333/333/3) set by Huzinaga et aL2I for Ge. Results Figures 1-5 show the transmission-corrected He*(23S) PIES and He 1 UPS of (CH3)3CC6Hj1(CH3)3SiCsHj, (CH3)3GeC6H5, (18) Kosugi, N. Program GSCFZ;Program Library; The Computer Center, The University of Tokyo: Tokyo, Japan, 1981. (19) Ditchfield, R.;Hehre, W. J.; Pbple, J. A. J . Chem. Phys. 1971, 54, 724-728. (20) Pietro, W. J.; Francl, M. M.; Hehre, W. J.; DeFrees, D. J.; Pople, J. A.; Bidkley, J. S. J. Am. Chem. Soc. 1982, 104, 5039-5048. (21) Andzelm, A.; Huzinaga, S.; Klobukowski, M.; RadzbAndzelm, E.; Sakai, Y.;Tatewaki, H. Gaussian Basis Ser for Molecular Calculations; Elsevier: Amsterdam, 1984.
1;
11 12 13 14
1
;:)
8.95 9.20 9.80 10.01 1O.4Ob 11.40b 11.85 -13.0
19
{ :::::
13.72 13.92 -14.2
17 18
8.88 9.01 10.88 10.98 1 1.05 13.24 13.50 14.32
15.70 16.60
{
14.86 15.22 15.31 15.57 15.97 16.63 16.73 17.52 19.20
12a”( r,) 11a“(r2) 38a‘ 37a‘ 1Oa” 36a‘ 9a”(rl) 35a’ 8a” 34a‘ 7a” 33a’ 6a”(pseudo r) 32a‘ 31a‘ 30a’ 29a’ 28a’(Ge 4s) 27a‘(ocd
“The r, and r2orbitals are related to the benzene elg orbitals with and without electron distribution at the position of substitution. bShoulder.
The Journal of Physical Chemistry, Vol. 93, No. 14, 1989 5417
PIES of Group IV Trimethylphenyls
TABLE I V Obsetved Vertical Ionization Potentials of (CH3)8nCa5
band 1 2 3 4 5 6 7
,%
11 12
IP(obsd)/eV 8.75 9.15
MO character’ 1Sa”(*,) 14a”(ir2) 44a’ 43a’ 13a”
9.456
9.72 10.056 11.30b
]
13
11.85
-13.0 13.60
14 \
42a’
{
12a”( ?rl)
41a‘
(A)
(6)
Figure 7. Schematic diagram of the 30a’ (A) and 29a’ (B) orbitals of (CH&%C~HS.
1Oa”
39a‘ 9a”(pseudo 7r) ( 38a‘
..
18 19
15.40 16.32
34a’(Sn 5s) 33a’(ocH)
(A)
(B)
Figure 8. Schematic diagram of 3e0 orbitals of benzene.
“he 7r3 and r2 orbitals are related to the benzene elg orbitals with and without electron distribution at the position of substitution.
position as the 3al, (ucH) band of benzene (at 16.84 eV), and its band shapes for PIES and UPS are very similar to those in b e n ~ e n e . This ~ band is rather eminent in the PIES since the corresponding orbital having C-H character extends outside the molecule and easily interacts with metastable atoms (see Figure 6). According to the results of the calculation, bands 3, 4, and 5 correspond to the 4t2 bands of nwpentane. In fact, their observed I P values (10.78, 11.20, and 11.60 eV) are close to those for b neopentane (10.90, 11.41, and 11.9 eV).I4 Bischof et al. have not assigned these bands clearly. Figure 6. Schematic diagram of the 18a‘ (uCH) orbital of (CH3)3CC6H6. Band 19 is mainly due to the C 2s orbital of the central carbon. (CH3)3SnC6Hs,and (CH3)3PbC6HS.The electron energy scales This band is located at about the same I P position as the C 2s for PIES are shifted relative to those for UPS by the difference band of neopentane and shows strong intensity in the PIES. A in the excitation energies, 21.22-19.82 eV = 1.40 eV, so that the similar enhancement of the C 2s band has been observed in many correspondence between PIES and UPS bands may be seen easily. other molecules such as benzene, cyclopropane, toluene, etc.” As Tables I-V list the observed vertical ionization potentials (IP) we have described in a preceding paper,I4 strong intensity of the of ( C H ~ ~ C C ~(HC SH,~ ) ~ S ~ C(CH3)3GeC6H~, ~HS, ( C H ~ ~ S ~ C ~ H SC , 2s band in PIES cannot be accounted for by a simple indeand (CH3)3PbC6HSobtained from the UPS together with their pendent-particle model of Penning ionization based on the electron assignments to respective M O s . The symmetry of M O s is transfer from the target molecule to the metastable helium atom. classified based on the point group C,. For (CH3)3CC6HS, 2. ( C H ~ ) ~ S ~ C(CH3)3GeC6Hs, ~HS, ( C H ~ ) ~ S H C ~and H S ,(c(CH3)3SiC6Hs, and (CH3)3GeC6HS,the calculated I P S via H3)3PbC6H,.Since the interaction between the central atom and Koopmans’ theoremz2 are also shown in Tables 1-111. the phenyl group is much weaker in (CH3)3SiC6H5,(CH3)3GeC6HS,(CH3)3SnC6HS,and (CH3)3PbC6HSthan in (CH3)3CC6HS, Discussion the PIES and UPS of these compounds can be interpreted as the 1 . ( C H 3 ) 3 C C a s We . will discuss the results of (CH3)3CC6Hs superpositions of those of benzene and (CH3)4M (M = Si, Ge, first, since the interaction between the central atom and the Sn,and Pb) except for the cases of bands 6 and 8 as described surrounding moieties is much stronger in this compound than in below (see Figures 2-5). Especially in PIES, this spectral feature the other ones. Bischof et al. assigned lower ionization bands in of superposition is prominent, since the character of the wave the 8-12-eV region of UPS using first-order perturbation theory.I6 function is reflected directly to the relative band intensity. As shown in Table I, we could assign all the bands in UPS on Bands 1, 2, 7, and 19 in Figures 2-4 and bands 3, 4, 7, and the basis of ab initio MO calculations and the comparison between 19 in Figure 5 are related to the 7r3, ?r2, al,and ucHorbitals of the PIES and UPS. In the following the reasoning for the present the benzene ring for the same reasons as described for assignment will be described in some detail. (CH3)3CC& They are almost at the same position as in From analogy with benzene having the r3and r2barids at IP benzene, the relative intensities of the a bands are strong in the = 9.25 eV (degenerate) and the band at 12.38 eV,23three A PIES, and the shapes of the uCHbands are similar to those of bands of (CH3)3CC6Hs are expected to appear in the I P region benzene both in the PIES and UPS. The present assignments 8-13 eV. Since the PIES in Figure 1 shows three strong bands of the A bands are in agreement with those by Bischof et al. for in this region (at IP = 8.8, 9.1, and 11.9 eV), we assigned them the Si, Ge, and Sn compounds.16 We will discuss the positions to the 7r3, rz,and r 1orbitals. The enhancement of A bands due of the 7r3 and x 2 bands in (CH3)3PbC6HSin subsection 3. to their large spatial distribution has already been observed in the On the basis of the results of the calculation, bands 3, 4, and PIES of many other aromatic molecules, such as benzene, toluene, 5 in (CH3)3MC6Hs(M = Si, Ge, Sn) are correlated to the 5tz, ~ , ~assignment of the A bands is the anthracene, and so f ~ r t h . The 7t2, 9t2 orbitals of (CH3)4Mhaving uMc bonding character. This same as that by Bischof et a1.I6 assignment is in line with that by Bischof et al.; they ascribed two We can assign band 18 at 16.55 eV to the orbital having ucH of these bands (3 and 4) to orbitals with uMC character. In character in the benzene ring since it appears at about the same (CH,)3PbC6H,, bands 1, 2, and 5 correspond to the 12t2orbitals of (CH3)4Pbsince, as is seen in (CH3)4Pb,2s*26 two of the UMC Shoulder.
(22) Koopmans, T. Physica (Amsrerdom) 1933, 1, 104. (23) Kimuta, K.;Katsumata, S.; Achiba, Y.; Yamazaki, T. Monograph Series of Research Institute of Applied Electricity, Hokkaido University, No. 25, 1978.
(24) Masuda, S.;Aoyama, M.; Ohno, K.; Harada, Y., to be submitted for publication.
5418 The Journal of Physical Chemistry, Vol. 93, No. 14, 1989
Aoyama et al.
TABLE V Observed Vertical Ionization Potentials of (CH&PbCIH, band 1 2 3 4 5 6 7
IP(obsd) /eV 8.54 8.77 9.20 9.486 9.90 11.55 1 1.90
12 13 14 15
MO charactera 54a‘ 53a’ 2 1a” ( H 2 ) 2Oa”(~,) 19a” 52a’ 18a”(~,) 51a’ 17a”
12.8 13.61 13.90 14.25
15a”(pseudo 48a’ 41a‘
H)
t
18:
18 19
15.65 16.50
44af(Pb 6s) 43a‘(~)
‘The H , and 7r2 orbitals are related to the benzene elg orbitals with and without electron distribution at the position of substitution. bShoulder.
orbitals are destabilized owing to spin-orbit interaction and have higher energies than the r3and r2orbitals of the benzene ring. According to our a b initio calculation, bands 6 and 8 are due to orbitals of types A and B in Figure 7, which are related to the 3ezgorbitals of benzene shown in Figure 8. These orbitals are degenerate in benzene, but in (CH3)3SiC6HS,(CH3)3GeC6H5, (CH3)3SnC6HS,and (CH3)3PbC6HSthe B-type orbital has lower energy than the A-type one, because, as can be seen from Figure 7, it is influenced much more by the introduction of the group (CH3)3M into the benzene ring. Thus, bands 6 have lower I P values than bands 8 by 1-1.5 eV in (CH3)3SiC6Hs,(CH3)3GeC6H5,(CH3)3SnC6H5,and (CH3)3PbC6HS(Figures 2-5). Bischof et al. also assigned bands 6 to the A-type orbital. Band 18 is attributed to the M ns orbital as in the case of (CH3)4M ( M = Si, Ge, Sn, and Pb; n = 3, 4, 5, and 6 , respectively).14 As is easily seen from Figures 1-5, the relative intensity of the M ns bands in the PIES becomes weaker on going from the carbon to the lead compound, although the M ns orbital becomes larger and easier to interact with the metastable atom. This can be interpreted like the case of (CH3),M; the M ns orbital is shielded to a large extent by the surrounding methyl and phenyl groups and it is the electron density on these groups that actually contributes to the intensity of the M ns band. As is observed in (CH3)4C,the relative intensity of the C 2s band of (CH3)3CC6H5 is particularly large. The value, however, cannot be compared directly with those for the other compounds, because the methyl groups also contain the carbon (M) atoms and the interaction between the C 2s orbital and the metastable atom is extraordinarily strong, as was described before.14 In Tables 11-V, the assignments of the other bands have been made on the basis of the results of the calculations and the comparison of the PIES and UPS. 3. Orbital Correlation among (cH3),Cc6HS,( C H ~ ) & C ~ H S , (CH3),CeC6H5,(CH3),SnC6H5,and (cH3)3Pbc6H5.Figure 9 shows a correlation diagram for the observed (vertical) I P values of (CH3)3MC6H5(M = C, Si, Ge, Sn, and Pb). The energy levels of the three uMC orbitals (corresponding to bands 3,4, and 5 for M = C, Si, Ge, Sn and bands 1, 2, 5 for M = Pb) rise as the central atom becomes larger because the energies of the M ns/np (n = 2-6) orbitals increase and the destabilization of the uMC orbitals due to the interaction with the methyl and phenyl orbitals becomes more effective. The 1r3and r2orbitals, which are degenerate in benzene, are split in these compounds. The position of the r2orbital changes little on going from benzene to (CH3),PbC6HSsince it has no electron distribution at the point of
I70
m
Figure 9. Correlation diagram for the observed (vertical) IP values of (CH3),MC6H5(M = C, si, Ge, Sn, and Pb). The 7r3 and 1r2 orbitals are related to the benzene el, orbitals with and without electron distribution at the position of substitution.
21 -
LI HMH
Me MeMMe
P
Me
Me MMW
/’1 v
231
Figure 10. Correlation diagram for the observed (vertical) IP values of the M ns orbitals of MH4 (M = C, Si, Ge, and Sn), (CH3)4M, and (CH3),MC6HS (M = C, Si, Ge, Sn, and Pb).
substitution. In the Si, Ge, and Sn compounds, the energy separation of the 7r3 and r2orbitals becomes larger as the size of the central atom increases, since the energy levels of the T and uMC orbitals become closer causing stronger interaction between them. In (CH3)3CC6H5,the energy separation is irregularly large since the interaction between the central atom (carbon!) and the phenyl moiety is much larger than for the other compounds. In (CH3)3PbC6HS,as was described before, two of the uMc orbitals have higher energies than the phenyl r3and r2orbitals. Owing to the interaction with these uMC orbitals, the energy level of the x 3 orbital goes down below that of the 7r2 orbital. The IP positions of the ?rl and UCH orbitals do not change much, indicating a small interaction of these orbitals with the substituent orbitals. In ( C H ~ ) ~ C C ~the H Sstrong , interaction among the 2s atomic orbitals of the central, methyl, and phenyl carbon atoms leads the energy level of the C 2s orbital to a much deeper position compared to those of the M ns orbitals of (CH3)3MC6HS( M = Si, Ge, Sn, and Pb) as in the case of (CH3)4M. The IPSof the M ns orbitals ( M = Si, Ge, Sn, and Pb) do not decrease monotonically down the group but show discontinuities at Ge and Pb. This is due to the advent of the filled d shell at Ge and that of the filled f shell at Pb. This tendency is also seen in (CH3)4M14and MH?’ (Figure 10). Acknowledgment. We express our thanks to Mr. T. Ishida for his help in the calculations. , (Ck&SiC6H5, 768-32-1; Registry NO. ( C H ~ ) , C C ~ H S98-06-6; (CH,),GeC6H5, 1626-00-2; ( C H ~ ) & I C ~ H934-56-5; S, (CH3)3PbC6Hs, 19040-53-0, ~
(25) Evans, S.;Green, J. C.; Joakim, J. P.; Orchard, A. F.; Turner, D. W.; Maier, J. P. J. Chem. SOC., Faraday Trans. 2 1972, 68, 905.
~~
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