13597
J. Phys. Chem. 1995,99, 13597-13599
Alignment and Orientation Effects for the Indirect Ionization of CH3Cl in the 1111) Rotational State by Fast Electron Impact T. Kasai,* T. Matsunami, H. Takahashi, T. Fukawa, H. Ohoyama, and K. Kuwata Department of Chemistry, Faculty of Science, Osaka University, Toyonaka, Osaka 560 Japan Received: March 8, 1995@ The steric opacity function, which includes alignment and orientation effects, for the indirect ionization process of CH3Clf has been obtained and analyzed from the study of fast electron impact of the oriented CH3C1 molecule with the well-defined orientation of the IJKM) = 1111) state. On the basis of a particle collision approximation for the projectile electron and the target molecular orbital electrons, both orientation and alignment dependences have been clarified by the ab initio calculations of the HOMO electron densities of the molecule. A good correlation between the site-specific ionization cross section and the cross section of the spatial distribution of 2e HOMO electrons of CH3C1 was confirmed.
Introduction Since electron impact ionization is a highly sensitive detection of molecules, this method is most widely used in mass spectrometry. Clarification of mechanisms of molecule ionization is one of the most fundamental subjects in chemical reaction. A molecular-level argument of its stereodynamical aspect, however, is lacking. On a simple intuitive basis, it is expected that ionizing efficiency of molecule in electron or particle collision should depend on sites of a target molecule, because different molecular sites consist of different molecular orbitals. Though valence electronic structure of molecules can be viewed by binary (e,2e) coincidence spectro~copy,’-~ its ionizing processes need to be investigated from various viewpoints. There has been only slight experimental evidence that clarifies site-specific ionizations. Recently, a study of the steric effect in direct ionization of oriented CH3C1 by electron bombardment has been carried out, in which the CH3 end attack produces more CH3Clf ions than the C1 end attack: In contrast, our study on fast electron impact of the same molecule in the 1111) single rotational state has led to the conclusion that CH3C1+ ions are also produced via a long-lived excited CH3C1, where more CH3Clf ions were produced for the electron attack at the C1 end than at the CH3 end.5 We refer to this process via a long-lived excited CH3C1 as indirect ionization in this paper. To clarify more quantitatively what is really going on, it is desirable to represent such site-specific processes in the form of a steric opacity function (viz., the reaction probability as a function of orientation of the target molecule). Interpretation of the obtained steric opacity function needs to be done next. In this study, we first derive the steric opacity function for the indirect ionization process of CH3Cl by utilizing the experimentally well-defined orientation of the IJKM) = 1111) state, where J, K , and M stand for three quantum numbers of rotation for the symmetric top molecule.6 On the basis of a particle collision approximation for the fast electron impact,’ we investigated a possible cause of the steric effect that includes both orientation and alignment dependencies, in conjunction with the spatial distribution of the HOMO electron densities of CH3C1. Experimental Results Table 1 lists the normalized CH3C1+ ion yields at the detector for the C1 end (Z+O) and the CH3 end orientation (Lo). @
Abstract published in Advance ACS Abstracts, August 15, 1995.
TABLE 1: Orienting Field Dependences of the Normalized Yields of C H & P strength of the orienting field (EN cm-I)
C1 end orientation (I+o)
CH3 end orientation (1-O)
0 10 20 35
1.005 5 0.004 1.025 f 0.007 1.042 f 0.012 1.052 rt 0.015
0.998 f 0.005 0.998 f 0.003 0.992 f 0.008 0.970 f 0.006
Each figure of the table was obtained after accumulation of 3600 signal pulses. Z+O is the normalized ion yield for the C1 end orientation as compared with the signal for random orientation. I-o for the CH3 end orientation is similarly defined.
In eqs 1 and 2, the signal intensity for the positive polarity of the orienting field at the field strength of E V cm-’, Z+O(E), corresponds to the ion yield for the C1 end orientation of methyl chloride, and the signal intensity for the negative polarity, Z-O(E), corresponds to the ion yield for the CH3 end orientation in the electron impact. For the positive polarity of the orienting field
I+’(@ = Z+(E,on)/Z+(E,off)
(1)
For the negative polarity of the orienting field
The terms “on” and “off” in parentheses indicate the operating condition of the 50-cm guiding field located between the 2-m hexapole and the orienting field. The guiding field was kept “on” when the 1111) state of CH3C1 orientation was required, and it was turned “off” to have zero field when CH3C1 in the 1111) state was required to be randomized in the orienting field in the electron impact. In this experimental procedure, we are able to obtain the equal number of the randomized CH3C1 molecules all the time in the orienting field. Thus the divisions of Z*(E,on) by Z*(E,off+) can cancel out any possible artifact caused from the orienting field. As a result, Z+O(E) and Z-O(E) become the quantity that represents only steric effects. The experimental values in Table 1 appear to be small, but the vertical gap between Z+O(E) and Z - O ( E ) becomes larger as E is increased. At zero orienting field (Le., E = 0 V cm-I), I+O(E) and I-O(E) give 1.005 f 0.004 and 0.998 f 0.005, indicating no steric asymmetry as expected. When E is increased, Z+O(E) increases and Lo(@ decreases on the contrary. These tendencies
0022-365419512099-13597$09.00/0 0 1995 American Chemical Society
Kasai et al.
13598 J. Phys. Chem., Vol. 99, No. 37, 1995
[-=GI
e'
e'
CH3 end
1.5
I
- -
CH3C1+
6 \
I ? h F.
8
v
loo
b
0.5
1
-1
0
cos Y Figure 2. Steric opacity function for the indirect CHsCl+ formation by 700 eV electron impact. The experimental values of Z+O = 1.052 f 0.015 and Z-O = 0.970 f 0.006 at E = 35 V cm-I were used (see the text and Table 1).
-1
-0.5
0
case
0.5
1
Sideways
Figure 1. Orientational distribution of the 11 11) rotational state of CH3C1. The molecule is oriented parallel to the electric field when cos 8 = 1, and it is oriented antiparallel when cos 8 = -1.
qualitatively reflect the existence of effects of molecular orientation;the C1 end attack of the fast electron produces more CH3Cl+ ion at the detector and the CH3 end attack produces less number of the ion.5 Bulthuis and co-Workers have studied the electric field dependence of reactivity of state-selected and oriented methyl halides8 They point out that the degree of orientation falls off rapidly at lower field strengths; therefore, the E field dependence of reactivity for the 1111) state of CH3Cl in the present study should be checked carefully. It was found within the experimental error of f0.006 for Z+O-Z-O that the field dependence of Z+O-Z-O for E = 0, 10, 20, and 35 V cm-I (see Table 1) appeared to be similar to that of the calculated orientation of the first Legendre moment ( P I ) ,reported by Bulthuis et al. In obtaining the steric opacity function, we thus chose Z+O and Z-o for the orienting field of E = 35 V cm-I, since the calculated ( P I )tends to be saturated.8 Therefore, the orientational distribution shown in Figure 1 can be utilized for the analysis with the 1111) state of CH3C1.8*9 The orientational distribution of the 1111) state can be expressed only with the orientation and the alignment term, these two coefficients, 01/00 and 0 2 / 0 0 , of the Legendre polynomials for the steric opacity function of eq 3 can be determined from the experimental values of Z+O and Z-O, by using the relations of (4) and ( 5 ) : 8 7 9
n = 0, 1, and 2
COS y)/ao = ~ ( a n / a o ) P n ( cyo) s
(3)
n
where y denotes the angle of the electron attack with respect to the molecular axis. y = 0" corresponds to the CH3 end attack and y = 180" corresponds to the C1 end attack: o,/ao= I +
a2/a0 = 5(I,O
0
-I-
0
+ ILO- 2)
(4)
(5) Figure 2 shows the steric opacity function, thus obtained, for the indirect ionization of CH3Cl at E = 35 V cm-'; Z+O - Z-o
(a)
CI end
(c)
CH3 end
Figure 3. Schematic pictures of typical p-type molecular orbitals. The cross sections of HOMOS in the C1 end attack (a), the sideways attack (b), and the CH3 end attack (c).
+
= 0.082 f 0.016 and 5(Z+O Z-o - 2) = 0.11 f 0.08. As shown in Figure 2, the C1 end attack gives a larger probability of ionization than the CH3 end attack (viz. the orientation effect), and the sideways attack gives the smallest probability which indicates the effect of molecular alignment.
Computational Results and Discussion It would be appropriate to assume that the fast electron with energy of 700 eV behaves as a particle. Therefore, a classical picture of binary collision of the impinging electron with an orbital electron of the molecule may be useful for interpreting the steric effect in the present site-specific ionization process. Because a typical total ionization cross section in high-collision energy region is expected to be much less than a few A2.loFor indirect processes such as molecular excitation to Rydberg states, the ionization cross section would be smaller." We may, therefore, assume in this experiment that the electron collides with CH3Cl with small impact parameters. Figure 3 shows schematic pictures of typical p-type of highest occupied molecular orbitals of the target molecule, indicated as HOMO1 and HOM02. As we will see later, the degenerated 2e HOMO of CH3Cl gives almost the same character as these
Indirect Ionization of CH3Cl CI
1 0.25 - 1 0
‘2 0.2
c
J. Phys. Chem., Vol. 99, No. 37, 1995 13599 c1
-I
Axial
-8 -6 -4 -2 0 2 4 6 8 r(x,y) / a.u.
a
c
0.251 -
0.2
-
CI
Sideways
-8 -6 -4 -2 0 2 4 6 8 z I a.u.
Figure 4. Cross sections of a 2e HOMO electron density in the axial and sideways directions. The r ( x y ) and z distances are measured from the C1 atom (left) and from the C atom (right), respectively. The broken line stands for an assumed threshold electron density, above which the projectile electron collides with the orbital electron with the unit probability.
p-type MOs, whose densities are mostly localized at the C1 end of the molecule. Suppose the projectile electron impinges on an orbital electron when it passes through a dense areas of the p globes, where a part of the energy of the incident electron is transferred to the target molecule and the molecule is excited to a long-lived precursors state. Then the long-lived state is ionized at the detector forming CH3Cl+. This is the scheme of the indirect ionization mentioned before. Parts a and c in Figure 3 represent the cross sections of the degenerated p globes of HOMOs in the axial direction in the C1 end attack and in the CH3 end attack, respectively. Part b represents the cross section in the sideways attack. If the binary electron collision takes place wherever the p electron density is higher than some threshold value, it becomes conceivable that the impinging electron in the C1 end attack faces a larger cross section of the HOMO electron cloud than in the CH3 end attack, because nonreactive methyl group stands in the way as an obstacle against the incoming electron. This situation in collision geometry may explain the orientation dependence of the CH3C1+ formation we observed. Similarly, if we compare the cross sections of HOMOs in part a (or c) with part b, the alignment dependence of the CH3Cl+ formation would be concluded, because the latter gives a smaller cross section of HOMO electron cloud than the former. Before reaching a conclusion, one must obtain the exact molecular orbitals of CH3Cl. We calculated the electron densities of the HOMO electrons, for it is commonly seen that HOMO electrons play an important role in collisional energy transfer reactions such as Penning ionization.I2-I4 Figure 4 shows the cross section of a 2e electron density viewed in the axial direction of the molecule (the z axis), and the one in the sideways direction perpendicular to the z axis. These computations were carried out by the same ab initio RHF-SCF MO method we used in previous studies,15*16where the HuzinagaDunning primitive Gausian basis sets of 9s5p(/3s2p) extended by the polarization functions for all atoms were employed. For convenience in further discussions, the computed density was integrated over the azimuthal angle from 0 to 2n; therefore, the radial distance r becomes a unique parameter in the axial view, i.e., r = (x2 y2)Ii2. The origin of r coincides with the molecular z axis. As for the sideways view, we set the carbon atom at z = 0 and the chlorine atom at 3.5 au along the z axis. The broken line indicates an example of threshold electron
+
density, above which the projectile electron is assumed to collide with the orbital electron with the unit probability. On the basis of such particle collision approximation, Figure 4 appears to suggest that the axial approaches would give a larger cross section than the sideways approach, and this difference in the cross section of the 2e electron cloud accounts for the alignment dependence (head-on vs sideways) of the ionization we experimentally observed. In return, the orientation dependence (heads vs tails) could be explained in a similar manner again: the electron attack at the C1 end becomes more favorable than the CH3 end attack, since the CH3 group stands as an obstacle against the approaching electron from the rear side of the molecule, while the 2e electrons are localized preferentially at the C1 atom. Recently, in a similar study of the K CF3Br reaction, a clear site specificity has been discovered in the K+ ion formation.” The harpooning electron of the K atom was found to be preferentially transferred to the Br end of the molecule around 5-eV collision energy. In photoionization, the anisotropic ejection of the photoelectrons from oriented CH3I also has been observed.18 It is therefore likely that the site-specific processes are a direct reflection of the spatial distribution of the relevant molecular orbitals in many cases.
+
Acknowledgment. The authors thank the Computer Center, Institute for Molecular Science, Okazaki for the use of NEC SX-3R. References and Notes (1) Minchinton, A.; Cook, J. P. D.; Weigold, W. Chem. Phys. 1987, 113, 251. (2) Coplan, M. A.; Moore, J. H.; Tossell, J. A. 2. Nutu$orsch. 1993, 48a, 358. (3) Drukarev, G. F. In Collisions of Electrons with Atoms and Molecules; Plenum Press: New York, 1987; Chapter 7. (4) Aitken, C. G.; Blunt, D. A.; Harland, P. W. J . Chem. Phys. 1994, 101, 11074. (5) Kasai, T.; Matsunami, T.; Fukawa, T.; Ohoyama, H.; Kuwata, K. Phys. Rev. Lett. 1993, 7, 3864. (6) Townes, C. H.; Schawlow, A. L. In Microwave Spectroscopy; Dover: New York, 1975. (7) (a) Mott, N. F.; Massey, H. S. W. In The Theory of Atomic Collisions; Oxford University Press: London, 1965; Chapter 16. (b) Brooks, P. R.; Jones, E. M. J. Chem. Phys. 1966,45, 3449. (c) Beuhler Jr., R. J.; Bemstein, R. B.; Kramer, K. H. J. Am. Chem. Soc. 1966, 88, 5331. (d) Beuhler Jr., R. J.; Bemstein, R. B. J. Chem. Phys. 1969, 51, 5305. (e) Marcelin, G.; Brooks, P. R. J. Am. Chem. Soc. 1975, 97, 1810. (8) Bulthuis, J.; Milan, J. B.; Janssen, M. H. M.; Stolte, S. J . Chem. Phys. 1991, 94, 7181. (9) (a) Choi, S. E.; Bemstein, R. B. J. Chem. Phys. 1986,95, 150. (b) Stolte, S.; Chakravorty, K. K.; Bernstein, R. B.; Parker, D. H. Chem. Phys. 1982, 71, 353. (10) de Heer, F. J.; Inokuti, M. In Electron Impact Zonizution;Miirk, T. D.; Dunn, G. H., Eds.; Springer-Verlag: Wien, 1985, Chapter 7. (11) Tarr, S. M.; Schiavone, J. A.; Freund, R. S. J. Chem. Phys. 1981, 74, 2869. (12) Hotop, H.; Niehaus, A. 2. Phys. 1981, 228, 68. (13) Miller, W. H.; Morgner, J. J. Chem. Phys. 1977, 67, 4923. (14) Ohono, K.; Mutoh, H.; Harada, Y. J. Am. Chem. Soc. 1983, 105, 4555. (15) Takahashi, H.; Ohoyama, H.; Kasai, T.; Kuwata, K.; Nakano, M.; Yamaguchi, K. Chem. Phys. Lett. 1994, 224,445. (16) Takahashi, H.; Ohoyama, H.; Kasai, T.; Kuwata, K.; Nakano, M.; Yamaguchi, K. Chem. Lett. 1994, 11, 1985. (17) Xing, G.; Kasai, T.; Brooks, P. R. J. Am. Chem. Soc. 1994, 116, 742 1. (18) Kaesdorf, S.; Schonhense,G.; Heinzmann, U. Phys. Rev. Lett. 1985, 54, 885.
JP950661W
