J. Phys. Chem. 1989, 93, 1800-1805
1800
Penning Ionizatlon Electron Spectroscopy of Group I V B Tetramethyl Compounds: (CH,),M (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: June 10, 1988)
He*(2%) Penning ionization electron spectra (PIES) and He I photoelectron spectra (UPS) were measured to study the valence electronic structures and reactivities of group IVB tetramethyl compounds: (CH3)4C,(CH3)4Si,(CH,),Ge, (CH3)4Sn, and (CH3)4Pb. On the basis of ab initio MO calculations and the character of PIES, we have established the assignment of the UPS bands. The bands in UPS and PIES are classified into three groups, which are due to the uMC, uCH,and M ns orbitals. With increasing size of the central atom, the relative band intensity of PIES for the uMC and M ns orbitals is found to decrease. This can be interpreted by the electron distributions on the methyl groups for these orbitals, which diminish on going from (CH3)4Cto (CH,),Pb, as the interaction between the central atom and the methyl groups become weaker.
Introduction In Penning ionization, electrons are ejected by collisions between target molecules T and metastable atoms A*:’ T A* T+ A eThe ionization can be considered as an electrophilic reaction,2 in which an electron in a moleculer orbital of T is transferred into the vacant orbital of the metastable atom and its excited electron is e j e ~ t e d . The ~ probability of the electron transfer is essentially determined by the spatial overlap between a molecular orbital +i and the vacant orbital of the metastable atom xa. Since the metastable atom can approach the repulsive molecular surface, the degree of the relative overlapping of +iwith can be estimated by the distribution of dioutside the “molecular surface”. Thus, an outer orbital distributed outside the molecular surface has a large probability of electron transfer and, therefore, gives a strong band in the Penning ionization electron spectrum (PIES).2 To evaluate the spatial expanse of the molecular orbital +i outside the molecular surface, we have introduced the concept of the exterior electron density (EED).2,4 The EED is defined as an integrated electron density of each molecular orbital $ias follows:
+
-
+ +
The integration is taken over the region fl outside the repulsive molecular surface, which is approximated by the van der Waals radii of the atoms in the molecule. Our previous work has revealed that the intensity of a band in PIES is basically proportional to the EED value of the relevant molecular 0 r b i t a 1 . ~ * ~With , ~ the introduction of a bulky group some molecular orbitals can be protected from the impact of metastables and become less reactive to met as table^.^,' Therefore, the analysis of the relative band intensity of PIES provides information about the steric shielding effect on the molecular orbital in addition to that about the distribution of the orbital. ( I ) Penning, F. M. Naturwissenschaften 1927, 15, 8 1 8 . Cermlk, V. J. Chem. Phys. 1966, 44, 3781-3786. (2) Ohno, K.; Mutoh, H.; Harada, Y . J. Am. Chem. Soc. 1983, 105,
4555-4561. (3) Hotop, H.; Niehaus, A. 2.Phys. 1969, 228, 68-88. (4) Ohno, K.; Matsumoto, S.; Harada, Y . J. Chem. Phys. 1984, 81, 4447-4454. ( 5 ) Kajiwara, T.; Masuda, S.;Ohno, K.; Harada, Y. J . Chem. Soc., Perkin Trans. 2, in press. (6) Ohno, K.; Fujisawa, S.; Mutoh, H.; Harada, Y. J. Phys. Chem. 1982, 86, 440-44 1. (7) Fujisawa, S.; Ohno, K.; Masuda, S.; Harada, Y. J. Am. Chem. Soc. 1986, 108. 6505-651 1 ,
0022-3654/89/2093-1800$01.50/0
The above features of PIES have been successfully applied to the assignments of bands in UV photoelectron spectra (UPS) of various organic molecules and also to the study of the spatial electron distribution of individual molecular o r b i t a l ~ . ~ , ~ - ~ ~ Furthermore, PIES allows us to investigate the electron distribution of individual orbitals exposed outside solid surfaces. This character of PIES has been used to probe the geometrical orientation and electronic state of molecules at the outermost surface layer.12J3 In this paper we have taken up the group IVB tetramethyl compounds, because they are among the most basic compounds in organometallic chemistry. With increasing size of the central atom they show systematic changes in the electronic structure as well as in reactivity. Although two papers have been published on the UPS of the group IVB tetramethyl c o m p o u n d ~ , ’ ~there J~ is a discrepancy in the assignment of the observed bands between them. Using the features of PIES mentioned above, we have established the assignment and also found useful information on the electron distribution and the reactivity of individual molecular orbitals. Experimental Section He*(23S) PIES and He I UPS were measured by use of the electron spectrometer described elsewhere.I0 A high-intensity metastable atom source was constructed for the present study. This source is of the same type as used by Hotop et a1.I6 Figure (8) Munakata, T.; Kuchitsu, K.; Harada, Y. Chem. Phys. Lett. 1979, 64, 409-412. Munakata, T.; Ohno, K.; Harada, Y.; Kuchitsu, K. Chem. Phys. Lett. 1981, 83, 243-245. (9) Veszprhi, T.; Bihatsi, L.; Harada, Y.; Ohno, K.; Mutoh, H. J. Organomet. Chem. 1985, 280, 39-43. (10) Harada, Y.; Ohno, K.; Mutoh, H . J . Chem. Phys. 1983, 79, 3251-3255. ( 1 I ) 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. (12) Munakata, T.; Ohno, K.; Harada, Y. J. Chem. Phys. 1980,72,2880. Ohno, K.; Mutoh, H.; Harada, Y. Surf. Sci. 1982, 115, L128-L132. Harada, Y.; Ozaki, H.; Ohno, K. Phys. Reu. Lett. 1984, 52, 2269-2272. Harada, Y.; Ozaki, H.; Ohno, K.; Kajiwara, T. Surf. Sci. 1984, 147, 356-360. Ozaki, H.; Harada, Y. J. Am. Chem. SOC.1987,109,949-950. Ozaki, H.; Harada, Y.; Nishiyama, K.; Fujisawa, S. J. Am. Chem. SOC.1987, 109, 950-951. ( 1 3 ) Harada, Y. Surf. Sci. 1985, 158, 455-472. Harada, Y.; Ozaki, H. Jpn. J . Appl. Phys. 1987, 26, 1201-1214. (14) Evans, S.; Green, J. C.; Joakim, J, P.; Orchard, A. F.; Turner, D. W.; Maier, J. P . J. Chem. Sor., Faraday Trans. 2 1972, 68, 905. ( 1 5 ) Jonas, A. E.; Sweitzer, G. K.; Grimm, F. A,; Carlson, T. A. J. Electron Spectrosc. 1972173, 1, 29-66. (16) Hotop, H.; Kolb, E.; Lorenzen, J. J. Electron Spectrosc. 1979, 16, 2 13-243
0 1989 American Chemical Society
PIES of Group IVB Tetramethyl Compounds repeller
discharge source
The Journal of Physical Chemisiry, Vol. 93. No. 5. 1989 1801
TABLE I: Observed and Calculated Vertical Ionization Potentirls of (CH,),C and EED Values band IP(obsd)/eV IP(calcd)/eV
'
quench lamp
y x 401
10
12
16 18 I P/eV
14
'
EEW
12.20
4t2
2.89 (8.68)
13'95
13.59 14.78
It, le
2.34 (7.03) 2.87 (5.73)
15.20 17.89
16.46 19.89
31, 4a,
2.50 (7.49) 1.92
1095)
B,
11.40) 11.86' 12.50
B2
BI
collimating skimmer
Figure 1. Schematic diagram of the cold-cathode metastable source.
sym
A ..
} 14.35'
C
a EED values are for one orbital. For the degenerate orbitals having t or e symmetry, the sum of EED values for two or three orbitals is shown in parentheses. 'Shoulder.
TABLE II: Observed and Calculated Ionizatiom Potentials of (CHMi band IP(obsd)/eV A 10.23 11.63 Il.0ob BI -12.85
z}
C
}
-14.00 15.63
IP(calcd)/eV
sym
EED'
11.25
St*
2.18 (6.55)
14.26 14.86 (15.43 16.86
It1 le 4tl
2.53 (7.58) 3.05 (6.09) 3.36 (10.07) 1.64
'EED values are for one orbital. For the degenerate orbitals having t or e symmetry, the sum of EED values for two or three orbitals is
shown in parentheses. bShoulder.
TABLE III:
A
0
inns
)
EED Spectrum
&sized
1
Observed and Calculated Ionization Potentials of
8
6
4
EkkV
2
Figure 2. He'(2'S) PIES, He I UPS, and synthesized EED spectrum of (CHI)&. I shows a schematic diagram of the metastable source. The helium metastable atoms, 2's (19.82 eV) and 2's (20.62 eV), were obtained by a cold-cathode discharge with the discharge current of 50 mA and voltage of 240 V. For the measurements of the He*(2%) PIES, He8(2'S) atoms were quenched with a watercooled helium discharge lamp. The intensity of He'(2IS) beam was monitored with a Faraday cup and was typically 1 X 1Olo atom ssl. Since the measured PIES contains 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% far the l22: band in N2*.The He I resonance line (584 A, 21.22 eV) was produced with a 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 hand intensity of the spectra was calibrated using the transmission efficiency curve of the spectrometer?
Calculations Ab initio molecular orbital calculations for (CH,),M ( M = C, Si, Ge) were performed using library program GSCF2 at the computer center of the University of Tokyo." The 4-31G basis (17) Kosugi, N. Program GSCF2. Program Library: The Computer Center, The University of Tokyo: Tokyo. Japan. 1981.
'EED values are for one orbital. For the degenerate orbitals having t or e symmetry, the sum of EED values for two or three orbitals is shown in parentheses. 'Shoulder.
TABLE I V
Observed Vertical Ionization Potentials of (CH,)Sn
band A
IP(obsd)/eV 9.35' 1 9.65 10.05'
)
sym 9t2
-13.55 C
14.78
7a1
'Shoulder. functions were used for the carbon and hydrogen atoms.18 Since the 4-31G basis set has not been devised for Si and Ge, we employed the following split valence sets: the 3-21G set19for Si and the (33321/3321/3) set derived from the (3333/333/3) set by Huzinaga et al." for Ge. The exterior electron densities (EED)2,4were calculated with our program to analyze the relative band intensity of PIES. The repulsive molecular surfaces were approximated by composition of the spheres with the van der Waals radii of the atoms (RH= 1.20A,Rc= 1.70A,Rsi=2.10A,andRGc=2.10A)inthe molecules. (18) Ditchfield. R.: Hebre, W. I.: Poplc, I. A. J . Chem. Phys. 1971, 54, 724-728. (19) Pietm, W. 1.: Francl, M.M.:Hehre, W. I.; DeFrees. D. I.: Pople. 1. A.; Binkley, I. S. 3. Am. Chcm. Sm. 1982. 104, 5039-5048. (20) Andzelm, A.: Huzinaga, S.: Klobukowski, M.: Radzia-Andulm, E.: Sakai. Y.:Tatcwaki, H. Gaussian Basis Sers for Molecular Colculoriom: Elsevier: Amsterdam, 1984.
1802 The Journal of Physical Chemistry, Vol. 93, No. 5, 1989 (CH&SI
Aoyama et al.
,*,
UPS
10
14
12
16 18 I P/eV
10
12
14
16 18 I P/eV
PIES
1 Synthesized EED Spectrum
0
8
6
4
Ek/eV
2
Figure 5. He*(23S) PIES and He I UPS of (CH3),Sn. (CH3)h
Pb 2tr
1
0
8
6
4
E k/eV
Figure 3. He*(23S) PIES, He I UPS, and synthesized EED spectrum of (CH3),Si.
8
He1 UPS
12
10
14
16
18
I P/eV /B \J ,
He'(23S)PIES
i 10
12
14
16 18 1P/eV
(3
A
~ $ ( 2 3 s )PIES
1
2
1
0
8
4 2 Ek/eV Figure 6. He*(23S) PIES and He I UPS of (CH3),Pb 6
TABLE V Observed Vertical Ionization Potentials of (CH3),Pb band IP(obsd)/eV sym 1
0
8
6
4
Ek/eV
A
2
;:;;) 9.80
Synthesized EED Spectrum
B
4)
-13.50
B3
C
Figure 4. He*(23S) PIES, He I UPS, and synthesized EED spectrum of (CH3),Ge.
Results Figures 2-6 show the transmission-corrected He*(2,S) PIES and He I UPS of (CH,),C, (CH,),Si, (CH,),Ge, (CH3),Sn, and (CH,),Pb. The electron energy scales for PIES are shifted relative to those for the UPS by the difference in the excitation energies, (21.22 - 19.82) = 1.40 eV. For (CH,),C, (CH3),Si, and (CH3),Ge, synthesized EED spectra are also shown in Figures 2-4. These spectra were obtained from Gaussian-type bands with area proportional to the respective EED values. The width of each peak comprising EED spectra was taken to be 1.O eV except band C
15.20
12t*
{2
llt, gal
in (CH3),Si and (CH,),Ge, where the widths are 0.5 eV. The peak positions were adjusted to those for the observed spectra. Tables I-V list the observed vertical ionization potentials (IP) of (CH3),C, (CH3)4Si,(CH3),Ge, (CH,),Sn, and (CH,),Pb obtained from the UPS together with their assignments to respective MO's. The symmetry of MO's are classified on the basis of the point group Td. For (CH3)4C, (CH3)4Si,and (CH3)4Ge, the calculated I P S via Koopmans' theorem2' and the EED values are also shown in Tables 1-111. Discussion
Assignments of UPS Bands. ( 1 ) Neopentane. The UPS of neopentane (Figure 2) is quite characteristic in comparison with the other group IVB tetramethyl compounds. It has five wellresolved bands, labeled A, B,, B,, B,, and C. These bands have (21) Koopmans, T.Physica (Amsterdam) 1933, 1 , 104.
PIES of Group IVB Tetramethyl Compounds
The Journal ofPhysica1 Chemistry, Vol. 93, No. 5, 1989 1803
I P/eV been assigned to relevant molecular orbitals in terms of the constituent localized bond orbitals: band A corresponds to the (CH,),C (CH~)L,SI (CH,),Ge 4t2 orbital having C C bonding character; bands B1, B2, and B3 correspond to the 1tl, le, and 3t2 orbitals, respectively, all of which are C H bonding; band C corresponds to the 4al orbital having C 2s ~ h a r a c t e r . ~ This ~ , ~assignment ~,~~ is in agreement with ours based on the ab initio M O calculations (Table I). To confirm the above assignment, we compared the observed PIES of neopentane with the synthesized PIES based on the EED model (Figure 2). Within the simple model for Penning ionization, the relative intensity of each band in PIES is to be basically proportional to the corresponding EED value. For bands A and B,-B3, the synthesized PIES agrees quite well with the observed one, supporting the present assignment (Figure 2). Band C is surprisingly enhanced in PIES and cannot be explained by the EED value, Le., the spatial expanse of the C 2s orbital outside the molecular surface. A similar enhancement of the C 2s band has been observed in many other molecules such // as benzene, cyclopropane, and toluene.23 Conversely, we can apply 201 4al - ’ this effect to the assignment of the C 2s band in UPS. The strong Figure 7. Correlation diagram for the calculated energy levels of (Cintensity of the C 2s band in PIES can not be accounted for by H3)4C,(CHJ4Si, and (CH3)4Ge. a simple independent-particle model of Penning ionization based on the electron transfer from the target molecule to the metastable Figure 7 shows a correlation diagram for the calculated energy He atom. To explain this effect the interactions among many levels of (CH3)4C,(CH3),Si, and (CH3),Ge. With increasing size electrons contained in the united molecule-He system should be of the central atom, it shows the following tendencies, which are taken into account upon ionization. This will be described in a also found experimentally for the group IVB tetramethyl comsubsequent paper. pounds (Figures 2-6). As is seen from Figure 2, bands A and BI-B3 are split into two 1. As the central atom M becomes larger, the energy level of or three subbands owing to the Jahn-Teller e f f e ~ t . ’ ~It . ’ is ~ worth the uMC (t2) orbitals goes up because the energies of the M ns/np noting that the relative intensity of the separated bands in the PIES ( n = 2-6) orbitals increase and also the stabilization of the uMC is not the same as that in the UPS (see band A), although the orbitals due to the interaction with the methyl orbitals becomes energy separations are almost the same. less. ( 2 ) (CH3),Si,(CH3)4Ge,(CH3)4Sn,and (CH3),Pb. According 2. The width of band B due to the energy splitting of the C H to the ab initio calculations, the sequence of the orbitals is not bonding orbitals becomes less broad with increasing size of the changed in (CH3)4Si and (CH3)4Gefrom that in neopentane, central atom, because the splitting is originated from the perwhich suggests that the assignments of bands A, B, and C for these turbation caused by the central atom, which becomes weaker as two compounds are the same as that for neopentane (Table I1 and the energies of the M ns/np ( n = 2-6) orbitals increase. 111). Further, Figures 3 and 4 show that the observed PIES agree 3. In neopentane, the strong interaction among the 2s atomic well with the synthesized EED spectra, which also supports the orbitals of the central and the methyl carbon atoms leads the level above assignments. From analogy with these assignments, we can of the a, orbital to a much deeper position compared to those of take the same orbital ordering for (CH3)4Snand (CH3),Pb. This the other compounds, where the position of the a l does not change will be shown to be reasonable in the next section from the conmuch with increasing size of the central atom. sideration of the electron distribution of the molecular orbitals. Comparison of PIES among Tetramethyl Compounds. At first In Figure 6, band A for (CH3),Pb is split owing to spin-orbit we will discuss the relative band intensity of the PIES of neointeraction into two distinct bands, the first of which is further pentane in connection with the electron distribution of individual split into two subbands on account of the Jahn-Teller e f f e ~ t . ~ ~ . ] ~ molecular orbitals. In this respect, band C should be excluded Again, it is worthwhile to note that the relative intensity of the because, as mentioned before, it shows a particularly strong inseparated bands in the PIES is not the same as that in the UPS, tensity, which cannot be explained by a simple model for Penning although the energy separations are almost the same. ionization. Band A is due to the 4t2 orbitals having C C character, The above assignments for the group IVB tetramethyl comwhile bands B,, B,, and B3 correspond to the orbitals with C H pounds are the same as those by Evans et aI.,l4 whereas Jonas et character. We may think at first sight that band B1, B2, or B3 al.15 have assigned band C in the Si, Ge, Sn, and Pb compounds should be stronger than band A, because the orbitals with C H to the e orbital, though that in neopentane to the a l orbital. They character are considered to be distributed further outside than claimed that the a , orbital shifts to the lower IP region on going the 4t2 orbitals with C C character. Figure 2, however, shows a from the carbon to the lead tetramethyl compound and that band reverse tendency. This indicates that the 4t2 orbitals have a fairly B includes the band due to the a , orbital in the Si, Ge, Sn, and large amount of electron distribution on the hydrogen atoms. In Pb compounds. They gave two main reasons for it; one is based order to study this effect in more detail we have calculated the on their semiempirical calculations using the CND0/2, MINDO, atomic population of EED (APEED) defined as follows. The and SCCMO methods and the other is the presence of a band exterior region outside the molecular surface can be devided into related to the a, orbital which they observed in the lower IP side subspaces by means of the distance from the repulsive surface (van of band B in the UPS of (CH3)4Pb. However, no such band is der Waals surface) of each atom (Figure 8). Let us consider found in the UPS of (CH3)4Pbmeasured by Evans et al. as well the distances from a point P in the exterior region to the surfaces as in the one measured by us (Figure 6). This “a, band” may of atoms. If the distance from P to the surface of atom a, R,, be due to some impurity. Band C, which is rather vague in the is the smallest in comparison with the distances Rb, R,, ..., P is UPS, appears distinctly in the PIES of (CH3)4C,(CH3)4Si,and assigned to subspace 9,. Thus, the exterior region R is divided (CH3)4Ge (Figures 2-4). Therefore, band C is not considered into subspaces, a,, R b , ..., as shown in Figure 8. The APEED for to be due to the Jahn-Teller splitting of the degenerate orbitals the ith MO and atom a is defined by the integral over the subspace belonging to band B . R,:
’$1
(22) Kimura, K.; Katsumata, S.; Achiba, Y.; Yamazaki, T. Monogr. Ser. Res. Inst. Appl. Electr., Hokkaido Uniu. 1978, no. 25. ( 2 3 ) Masuda, S.; Aoyama, M.; Ohno, K.; Harada, Y . Unpublished results.
(APEED): = J,ldi(r)12 dr Table VI shows the APEED for the equivalent orbitals with t2
1804
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989
Aoyama et a]. TABLE IX: APEED of ( C H B ) ~ M( M = C, Si, Ce) Summed for the Three Equivalent Orbitals with t2 Symmetry"
compound (CH,),C (CH3),Si (CH,),Ge
APEED C(Me)b 1.68
M
0.22 1.29
2.13 2.28
1.44
H(Me)' 5.59 6.65
EED 1.49 10.07 10.03
6.31
"The 3t2, 4t2, and 6t2 orbitals for (CH,),C, (CH,),Si, and (CH,),Ge, respectively. bThe APEED value for the four methyl carbon atoms. 'The APEED value for the twelve methyl hydrogen atoms. I
I
Figure 8. Subspaces R,, Rbr and Q, by means of the distances R,, Rb, and R, from the repulsive surfaces of atoms a, b, and c. TABLE VI: APEED of (CH3)4M (M = C, Si, Ce) Summed for the Three Equivalent Orbitals with t2 Symmetry"
comDound (CH3)IC (CH,),Si (CH,),Ge
M 0.02 0.06 0.20
APEED C(Melb 2.17 2.73
H(MelC 6.49 3.76
2.49
3.21
EED 8.68 6.55 5.90
"The 4t2, 5t2, and 7t2 orbitals for (CH,),C, (CH,),Si, and (CH,),Ge, respectively. bThe APEED value summed for the four methyl carbon atoms. CTheAPEED value summed for the twelve methyl hydrogen atoms.
0
M 0.01
0.20 0.25
APEED C(Me)b 1.32 1.58
H(Me)' 5.70 5.80
EED 7.03
1.62
5.58
7.45
7.58
"The I t , orbitals for all three compounds. bThe APEED value for the four methyl carbon atoms. 'The APEED value for the twelve methyl hydrogen atoms. TABLE VIII: APEED of (CH3)4M ( M = C, Si, Ce) Summed for the Two Equivalent Orbitals with e Symmetry"
compound (CH3)IC (CH,),Si (CH,),Ge
M
0.01 0.49 0.50
APEED C(Me)b 1.68 1.66 1.92
1 C
SI
Ge
Sn (C%4
Pb=M
M
Figure 9. Ratio of the intensity of band A to that of band B.
TABLE VII: APEED of (CH3)4M (M = C, Si, Ce) Summed for the Three Equivalent Orbitals with tl Symmetry"
compound (CH,)4C (CH,),Si (CH,),Ge
.
02 1
O6
","
t
C
Sn
Pb=M M Figure 10. Ratio of the intensity of band C to that of band B. SI
Ge
(CH3)4
H(Me)' 4.04 3.94 3.52
EED 5.73 6.09 5.94
"The le, le, and 2e orbitals for (CH3),C, (CH3),Si, and (CH,),Ge, respectively. *The APEED value for the four methyl carbon atoms. 'The APEED value for the twelve methyl hydrogen atoms. symmetry: the 4t2, 5t2, and 7t2 orbitals for (CH3)4C,(CH3),Si and (CH3)4Ge,respectively. In the case of neopentane the table shows that the contributions of the central and the methyl carbon atoms to the EED value (8.68) are only 0.02 (0.2%) and 2.17 (25%), respectively, while that of the hydrogen atoms is 6.48 (75%). This indicates that the electron distribution on the hydrogen atoms contributes most of the intensity of band A in PIES. The situation is the same for bands B,, B,, and B3 (Tables VIIIX). From Tables VI-IX we can find that in neopentane the APEED of the hydrogen atoms for the 4t2 orbital is larger than those for the It,, le, and 3t2 orbitals. This explains the abovementioned fact that band A is stronger than band B,, B,, or B,. To compare the relative band intensity of PIES among the group IVB tetramethyl compounds, we have chosen the intensity of band B for each compound as a standard because the latter intensity is almost independent of the central atom as shown below. Band B is related to the t l , e, and t2 orbitals. As for the t, and e orbitals, they are not mixed with the s and p orbitals of the central atom because of their symmetries. On the other hand, the t2 orbitals interact with the orbitals of the central atom, but
the mixing is to a very small extent for (CH3),Si and (CH,),Ge according to our ab initio calculations. In neopentane, the mixing is more than that in the other two compounds, buf it is still not very large. This is shown by the EED values of the orbitals related to band B, which are 20.24, 23.74, and 23.42 for M = C, Si, and Ge, respectively. Figure 9 shows the ratio of the intensity of band A to that of band B, Z(A)/Z(B) for M = C, Si, Ge, Sn, and Pb. The relative intensity of band A becomes weaker on passing from the carbon to the lead compound, showing the contraction of the electron distribution of the uMCorbitals. This is in accordance with the trend that the sum of the EED for the uMCorbitals related to band A becomes smaller on going from (CH3),C to (CH3),Ge (Tables 1-111). Next we consider this effect in terms of the APEED. In Table VI the APEED of the central atom increases and that of hydrogen atoms decreases as the central atom becomes larger. This means that the decrease in the relative intensity of band A (Figure 9) is explained in terms of the electron distribution on the hydrogen atoms. The electron distribution on the hydrogen atoms for the uMC orbitals decreases with increasing size of the central atom, because the interactions between the ns/np (n = 2-6) orbitals of the central atom and the localized uCH bond orbitals decrease in this order. The relative intensity of band C, Z(C)/Z(B), also becomes weaker on going from the carbon to the lead tetramethyl compound (Figure 10). According to the calculation, band C is mainly due to the s orbital of the central atom, namely, the C 2s,
J. Phys. Chem. 1989, 93, 1805-1812
TABLE X: APEED of (CH3),M (M = C, Si, Ce) for the Orbitals with a, Symmetry'
compound (CH3)PC (CH3),Si (CH3)4Ge
M 0.00
0.03 0.07
APEED C(Me)b 0.74 0.7 1 0.55
H(Me)' 1.18
0.91 0.67
EED 1.92 1.64 1.29
'The 4al, 5al, and 6a, orbitals for (CH,),C, (CH,),Si, and (CH,),Ge, respectively. bThe APEED value for the four methyl carbon atoms. CTheAPEED value for the twelve methyl hydrogen atoms.
Si 3s, Ge 4s, Sn 5s, and Pb 6s orbitals. If C band were purely due to the M ns orbital, the ratio Z(C)/Z(B) should increase because the M ns orbital becomes larger and easier to interact with the metastable atom with increasing size of the central atom. In Figure 10 the Z(C)/Z(B) for neopentane is particularly large. The value 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. As for the other compounds, Table X shows that the APEED for the methyl carbon and hydrogen atoms diminish steadily on passing from the carbon to the germanium compound, while that for the central atom M is negligible (and gradually increases). These facts clearly indicate that the M ,ns orbital is shielded by the four methyl groups to a large extent and it is the electron density on the methyl carbon and hydrogen atoms that actually contributes to band C. To confirm this effect, we have studied the steric shielding effect of the methyl groups on the PIES of
1805
(CH3)nSiHcn (n = 0-4). With increasing number of the methyl group, the Si 3s band in PIES becomes weaker in agreement with the above finding. Concluding Remarks
In Penning ionization, an electron in a molecular orbital is transferred into the hole of a metastable atom and its excited electron is ejected. The relative intensity of the bands in PIES reflects the relative reactivity of the orbital upon electrophilic attack, which gives information about the spatial distribution of the orbital electron density. Using this characteristic feature of PIES together with ab initio M O calculations, we assigned all the UPS bands in group IVB tetramethyl compounds, (CH,),M ( M = C, Si, Ge, Sn, and Pb). From the analysis of the APEED and the relative intensity of the observed bands in PIES, we have found that it is the electron density of the methyl groups that actually contributes to the uMC and M ns bands; the reactivity of Penning ionization for each orbital is governed by the electron distribution of the orbital in the outermost region of the molecules. The present method using PIES and APEED will be useful for the study of the electron distribution of other molecular orbitals, especially those shielded by some substituents.
Acknowledgment. We express our thanks to Professor H. Hotop and Dr. M.-W. Ruf, Universitat Kaiserslautern, for informing us of the details of their cold-cathode metastable source. Thanks are also due to T. Ishida for his help in the calculations. Registry No. (CH,),C, 463-82-1; (CH3),Si, 75-76-3; (CH,),Ge, 865-52-1;(CH,),Sn, 594-27-4; (CH,),Pb, 75-74-1;He*(2%), 7440597.
Collisional Energy Transfer in the Three-Channel Thermal Decomposition of 1-Chloro-2,2,3,3-tetrafluorocyclobutane Warren S. Staker and Keith D. King* Department of Chemical Engineering, University of Adelaide, S . A . 5001, Australia (Received: June 13, 1988)
The three-channel thermal decomposition of 1-chloro-2,2,3,3-tetrafluorocyclobutane has been investigated over the temperature range 915-1225 K by using the technique of very low pressure pyrolysis (VLPP). The three individual channels are the elimination of hydrogen chloride, the formation of tetrafluoroethylene and chloroethylene, and the formation of 1,l-difluoroethylene and l-chloro-2,2-difluoroethylene.Extrapolated high-pressure rate coefficients (s-l) for the separate channels are 1013.2*0.3 exp(-271 f 8 kJ mol-'/RT), 10'5.3*03 exp(-287 f 8 kJ mol-'/RT), and 1015.3*0.3 exp(-287 f 8 kJ mol-I/RT), respectively, which are consistent with previous studies of related compounds and the predicted effects of chloro and fluoro substitution. With the technique of pressure-dependent VLPP, the average downward energy transferred from the reactant to the bath gas upon collision, (AEdown),has been obtained for the bath gases Ne, Kr, COz, CH,, CF,, C2H4, and CH,CF, at ca. 1048 K. The values (cm-I) are 445 (Ne), ca. 500 (Kr), 735 (CO,), 665 (CH,), 1370 (CF,), 920 (CZH,), and 1190 (CH,CF,). The data are found to be insensitive to the choice of the energy-transfer probability function (four functions values for the bath gases CF4 and CHzCFzwith those for CHI were tested). A comparison is made between the (AEdOwn) and C2H4,respectively, with a discussion of the effect of resonance.
Introduction
The measurement of the pressure and temperature dependence of the rate coefficient for a single unimolecular reaction in general provides data that are insensitive to the functional form of the probability (per gas kinetic collision) of the energy transfer from energy E'to energy E, P(E,E?. However, pressure dependences of thermal rate coefficients for unimolecular decomposition that occurs via a multiple-channel mechanism should be sensitive to the assumed functional form of P(E,E?,' especially when the channels are separated by small differences in critical energy ~~
~
(1) Chow, N.; Wilson, D. J. J . Phys. Chem. 1962, 66, 342.
(approximately the same as the average downward collisional energy transfer, ( AEdm))., Pressure-dependent very low pressure pyrolysis (VLPP) is a well-established technique for such In these experiments the temperature-dependence of the rate coefficients for each channel is measured over a wide range of temperatures at a sufficiently low pressure so that only gas/wall (2) King, K. D.; Nguyen, T. T.; Gilbert, R. G. Chem. Phys. 1981,61,221. (3) Nguyen, T. T.; King, K. D.; Gilbert, R. G.J . Phys. Chem. 1983.87,
494.
(4) Gilbert, R. G.; King, K. D. Chem. Phys. 1980, 49, 367. (5) Gilbert, R. G.; Gaynor, B. J.; King, K. D. Int. J . Chem. Kinef. 1979, 11, 317.
0022-3654/89/2093-1805$01.50/00 1989 American Chemical Society
