J. Phys. Chem. 1995,99, 14678-14685
14678
Penning Ionization of CH3CN and CH3NC by Collision with He*(23S) Metastable Atoms Tibor Pasinszki Department of Inorganic Chemistry, Technical University of Budapest, H-1521 Budapest, Hungary
Hideo Yamakado and Koichi Ohno* Department of Chemistry, Faculty of Science, Tohoku University, Aramaki, Aoba-ku, Sendai 980-77, Japan Received: June 14, 1995; In Final Form: July 29, 1995@
Collision of CH3CN and CH3NC with He*(23S) metastable atoms has been studied by collision-energy resolved Penning ionization electron spectroscopy. Collision energy dependence of the partial ionization cross sections indicates that the interaction potentials are strongly anisotropic between He*(23S) and the molecules investigated. In the studied energy range, the interaction potential is attractive if the metastable atom approaches thepseudohalide group along the CCN or CNC frame, but it is repulsive around the methyl group. Model potential curves for the collision of CH3CN and CHsNC with Li(22S) atoms have been calculated and the computational strategy discussed. The quantum chemical calculations and the spectroscopic investigations predict the existence of stable C2H3NLi radicals, and their structure is characterized using the MP2/6-31+G** method.
I. Introduction In Penning ionization,‘ a molecule M collides with a metastable atom A* having an excitation energy larger than the lowest ionization potential (IP) of the molecule to yield the ground-state atom A, one of the ionic states of the molecule Mi+, and an ejected electron e-:
M
+ A* ‘Mif + A + e-
(‘1
In Penning ionization electron spectroscopy (PIES),*the kinetic energy of the electron ejected is analyzed. The exact kinetic energy of the electron E k released in the Penning ionization process is equal to the difference between two potential energy curves, the incoming M A* (V*, interaction potential energy curve) and the outgoing Mif A (V,+), and depends on the intemuclear distance between the metastable atom and molecule when ionization occurs
+
+
where EAis the excitation energy of the metastable atom, and IPi is the ionization potential of the molecule. This effect is responsible for the shift of the PIES bands AE,(R) with respect to the band positions observed in ultraviolet photoelectron spectroscopy (UPS), for a broadening of the PIES peak, and holds basic information about the interaction potential energy curve. AEi(R) is negative if the interaction potential is attractive and positive if this potential is repulsive. Since the PIES bands originate from the ionization of more or less localized molecular orbitals, the anisotropy of the interaction potential can also be concluded from the PIES band shifts. These band shifts, however, cannot be determined in the case of broad andor overlapping bands. Another significant aspect of the Penning ionization process that can be used to derive information about the interaction potential is the dependence of the ionization cross section on the relative kinetic collision energy Ec.3-4The total ionization cross section DT(E,) for various atoms and simple molecules colliding with metastable atoms has been extensively investi~~~
‘Abstract published in Advance ACS Abstracts, September 1, 1995.
0022-365419512099- 14678$09.0010
gated in previous years.4-” In the case of a target atom, where the interaction potential is isotropic, the study of the total ionization cross section is sufficient, but in the case of a molecule, where the interaction potential is anisotropic, only an “average” potential can be deduced from the collision energy dependence of CT(E,). If several ionic states form in the ionization of a molecule, the total ionization cross section is the sum of the ionization cross sections of different ionic states (partial ionization cross section ~ ( E J ) Since . a given ionic state originates from the ionization of a given molecular orbital localized on a special part of the molecule, the u(E,) functions reflect the anisotropy of the interaction potential. In our recent papers,I2-l7 we have reported a novel technique which includes detection of state resolved Penning electrons as a function of the velocity of metastable atoms as well as the first results of a study on the interaction potential between simple molecules and metastable helium atom He*(23S). It has been shown earlierI8 that the shape of the velocity dependence of the total scattering cross section of He*(23S) by He, Ar, and Kr is very similar to that of Li(22S). Interaction potential well depths and the location of potential wells have also been found to be very similar for interaction of various targets with He*(23S) and Li(22S) (see refs and note 3, 14, 19, and 20). This similarity between He*(23S) and Li(22S) is used to compare the computationally much more feasible M(mo1ecule)-Li potential curves with the experimental M-He*(23S) results. In addition, this similarity also means that if an attractive interaction potential is observed between the molecule and the He*(23S) metastable atoms, there must be at least one energy minimum on the Li-M energy hypersurface. (The interaction potential curve is only a cut of the energy hypersurface and thus does not necessarily run through a minimum on the energy surface.) The Li-M complex corresponding to this minimum is probably not stable in most cases having a low barrier to rupture of the Li-M bond. The stability of the Li-M complex, however, can also be estimated using the He*(23S) Penning spectrum. The exact kinetic energy of the Penning electron is determined by the ingoing (V*) and outgoing (Vi+) channels, but if the metastable atom is a rare gas atom, the attractive interaction in the outgoing channel is very weak, and thus the potential well depth E* of the attractive interaction potential can be estimated 0 1995 American Chemical Society
Penning Ionization of CH3CN and CH3NC with the peak energy shift (see ref 21). This means that the negative peak energy shift in the PIES is a measure of the stability of the Li-molecule complex. This very close link between PIES and lithium chemistry has never been discussed in detail, and, for the first time, we have applied Penning spectroscopy to predict the stability and structure of lithium organic radicals. From a chemical point of view it is especially interesting how the character of the interaction potential depends on the type of chemical group. Our study on some saturated and unsaturated hydrocarbons has indicated that the interaction potential is attractive near the n orbital region, otherwise it is r e p ~ l s i v e . ’ ~The . ’ ~ interaction potential between He*(23S) and CH3NC0, CH3NCS, or CH3SCN has also been found to be attractive around the pseudohalide n orbitals and repulsive around the methyl group.17 A specially strong attractive potential with a large negative peak shift (note that reverse definition of the peak shift, i.e. “positive”, has been used in our previous paperI7)has been observed in the terminal nitrogen and oxygen “lone electron pair” regions of CH3SCN and CH3NCO, and a further investigation of this effect was also an objective of this work. In the last decade the computational power applied to solve chemical problems has increased tremendously. This has resulted in the use of more sophisticated methods as well as more sophisticated chemical models. The interaction potential is usually calculated using ab initio methods to assist the experimental results. Due to the large amount of computational work, the geometry of the target molecule is kept fixed, and thus the calculations provide interaction potential curves for an idealized stationary structure. This type of calculation also neglects the possible dependence of the interaction potential on the velocity of the metastable atom (i.e., relaxation of the geometry by the approach of a slow metastable atom). The molecules, however, are in motion, and ionization can occur at any geometry determined by the total energy hypersurface. In this work we investigate for the first time the effect of molecular vibrations and geometry relaxation on the interaction potential. 11. Experiment
The apparatusI2 used in this work and the modifications to the metastable atom source and metastable atom detectioni4have been reported in previous papers. Metastable atoms of He*(23S,2’S) were produced by a discharge nozzle source, and the He*(2IS) component was quenched by a water-cooled helium dc lamp. In the Penning ionization electron spectra, contributions of photoelectron signals due to He I resonance photons from the helium discharge were found to be negligible and estimated to be much less than 1% of the total electron signal. The kinetic energies of electrons ejected by Penning ionization were determined by a hemispherical electrostatic deflection type analyzerI3 using an electron collection angle 90” to the incident He*(23S) beam axis. The energy resolution of the electron analyzer was estimated to be 40 meV from the full width at half-maximum (FWHM) of the Ar+(2P3,2) peak in the He I ultraviolet photoelectron spectrum. For the measurements of collision energy dependences of Penning ionization cross sections, a chopper disk with 2 mm wide slits was placed between the metastable atom source and the quench lamp and rotated at a frequency of about 400 Hz in order to produce a pulsed metastable beam. In order to determine the collision energy dependence of the partial ionization cross section, the time-dependent spectrum of Penning electrons for a given ionic state of the molecule was measured using the energy fixed mode of the electron analyzer. The resolution of the analyzer in these experiments was lowered
J. Phys. Chem., Vol. 99, No. 40, I995 14679 to 250 meV (FWHMfor He I U P S of Ar) in order to obtain higher electron counting rates. The time-of-flight (TOF) spectrum h ( t ) of He*(23S) was obtained by detecting emitted electrons from a stainless steel plate inserted into the collision cell. The time-of-flight of secondary electrons from the metal surface to the detector is negligibly short in comparison with the TOF of the He* atoms. The ionization efficiency of the secondary electrons from the metal plate was considered to be constant in the observed collision energy range. Since the timeresolved spectrum gives the electron intensity as a function of the velocity V M of He*(23S), the partial ionization cross section u(E,) can be determined by the equations
(4) where c is a constant, V R is the relative velocity averaged over the velocity of the target molecule, kB is the Boltzmann constant, and T and m are the gas temperature and the mass of the target molecule, respectively. Finally, ~ ( Y R )is converted to u(EJ by the relation
E = pv:/2
(5)
where p is the reduced mass of the system. Selecting a small energy window from the pulsed metastable beam, two spectra with a low collision energy of about 96 meV on average (72- 114 meV) and 100 meV on average (76- 123 meV) and a high collision energy of about 216 meV on average (176-394 meV) and 252 meV on average (200-488 meV) were measured for CH3NC and C H F N , respectively. As in the case of energy fixed mode, the resolution of the analyzer was lowered to 250 meV (FWHM for He I U P S of Ar) to obtain higher electron counting rates. Ultraviolet photoelectron spectra were measured by utilizing the He I resonance line (21.22 eV) produced by a discharge in pure helium gas. The electron spectra were obtained at an ejection angle of 90” with the same electron energy analyzer employed in the PIES measurements. The transmission of the electron energy analyzer was determined by comparing our U P S data of 0 2 , CO, Nz, and some hydrocarbons with those of Gardner and Samson22and Kimura et al.23 The CH3NC sample for the investigations was synthesized from AgCN and CH31using a literature method.24 CH3CN was a commercial product (Wako, purity 98.0%).
III. Calculations The calculation of the interaction potential between a molecule and a He*(23S) atom is a very complex task, which involves the calculation of a highly excited He-M system. Since there are well-known resemblances between He*(23S) and Li(2*S) (see the Introduction part of the paper), all of the difficulties associated with the calculations for excited states can be avoided by calculating the interaction potential between the molecule and a Li(22S) atom. In this work the potential curves between the molecule and Li(22S)atom were calculated using the MW6-31fG** method. All electrons were included in the correlation energy calculations (i.e., “full”) and the full counterpoise (CP) method25was used to correct for the basis-set superposition errors (BSSE). The curves, unless otherwise indicated, were obtained by proceeding along the given reaction coordinate and optimizing all remaining degrees of freedom. The structure of the CH3CNLi and CH3NCLi complexes were fully optimized and then harmonic
Pasinszki et al.
14680 J. Phys. Chem., Vol. 99, No. 40, 1995 r
5001 2
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\\
al
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m
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180
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Figure 1. CNC bending potential function of CH3NC (curve a) and CNC bending potential function of a Li atom perturbed CH3NC (curve b; Li-N distance is 2.5 A; Li-N axis is perpendicular to the pseudohalide (NC) frame).
4.0
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He*(Li) (curve a) and a fast He*(Li) (curve b).
Ionization Potential/eV 11
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Figure 3. Interaction potential curves between CH3NC and a slow
Li
0
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21 1
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PIES
NC in three different geometries which occur during the CNC deformational motion.
A. Results. Figure 1 shows the CNC bending potential function of CH3NC (curve a) and the CNC bending potential function of a Li atom perturbed CH3NC (curve b). The Li-N distance in the latter case is 2.5 A, and the Li-N axis is perpendicular to the pseudohalide (NC) frame. Figure 2 shows the interaction potential curves between a fast He*(Li) (i.e., geometry of the molecule is frozen during the Penning process) and CH3NC in three different geometries which occur during the CNC deformational motion. The geometries of the molecule were obtained by fixing the CNC angle at 180.0" and 180.0 f 9.5' and optimizing all other bond lengths and angles. The corresponding points on Figure 1 (curve a) are marked with asterisks. The Li approaches the nitrogen atom perpendicular to the pseudohalide frame. Figure 3 shows the interaction potential between CH3NC and a fast He*(Li) (Le., geometry of the molecule is frozen; the
7
-
7a1 (nN)
Figure 2. Interaction potential curves between a fast He*(Li) and CH3-
IV. Results and Discussion
-
6 5 4 3 2 E l e c t r o n Energy/eV
4.5
vibrational frequencies were calculated at the equilibrium geometry using numeric second derivatives. All of the calculations in this work were performed using the Gaussian-9026 quantum chemistry package.
20 1
1.2
9
8
/I
7
6 5 4 3 2 1 0 E l e c t r o n Energy/eV Figure 4. He I UPS and He*(2%) PIES of CH3CN.
equilibrium geometry of the molecule is kept fixed in the calculation of the interaction potential curve) and a slow He*(Li) (Le., geometry of the molecule is relaxing during the approach of the He*(Li); geometry of the molecule is optimized at each point of the curve). The Li approaches the nitrogen atom perpendicular to the pseudohalide frame. Figures 4 and 5 show the He I ultraviolet photoelectron spectra and Penning ionization electron spectra of CH3CN and CH3NC, respectively. The electron energy scales for PIES are shifted relative to those of U P S by the difference in the excitation energies, 21.22-19.82 = 1.40 eV. Figures 6 and 7 show the collision-energy-resolved He*(23S) Penning ionization electron spectra of CH3CN and CH3NC, respectively. In each figure, the low-collision-energyspectrum is shown by a solid curve, and the high-collision-energy spectrum is shown by a dashed curve. The relative intensities
Penning Ionization of CH3CN and CH3NC
J. Phys. Chem., Vol. 99, No. 40, 1995 14681
I o n i z a t i o n P o t e n t i a l /eV
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CH3NC
1
He* (23s) PIES
7a1(nc)
........
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:E,-21 6meV
- :Ec- 96meV
4l 5e 1 1 1 0 9
8 7 6 5 4 3 E l e c t r o n Energy/eV
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1 9
PIES
8
7 6 5 4 3 2 E l e c t r o n Energy/eV
7
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Figure 7. Collision-energy-resolved He*(2%) Penning ionization electron spectra of CH3NC (-, 72- 114 meV, average 96 meV; - -, 176-394 meV, average 216 meV).
7ai (nc)
9
8
E l e c t r o n Energy/eV
1
10
6
He*(23S) + CH3CN
1
0
ot
d
Figure 5. He I UPS and He*(23S) PIES of CH3NC.
3
CH3CN
7a1(nN)
He* (23S)PIES ........
:E, -252meV
I , IIIJ,II , 8
IO
100
4 111,111
I
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C o l l i s i o n Energy/meV 9
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7 6 5 4 3 2 E l e c t r o n Energy/eV
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Figure 6. Collision-energy-resolved He*(23S) Penning ionization electron spectra of CH3CN (-, 76-123 meV, average 100 meV; - -, 200-488 meV, average 252 meV).
of the two spectra are normalized in the figures using the data of log (T vs log Ec plots. Figures 8 and 9 show the log u vs log Ec plots for CH3CN and CH3NC, respectively. The calculated electron density maps of the molecular orbitals are also shown in the figures. The values of the slope m of log 0 vs log Ec plots as well as the calculated s, b/d, and d parameters (see below) are listed in Table 1. Figures 10 and 11 show potential energy curves V*(R) obtained from the model potential calculations; the equilibrium geometry of the molecule is kept fixed in the calculation of interaction potential curves. R is the distance between the Li atom and a given atom of CH3CN or CH3NC. Table 1 lists the vertical ionization potentials (determined from the He I UPS) and the assignments of the observed bands. The peak energy shifts in PIES measured with respect to the “nominal” energy EO (EO= the difference between the metastable excitation energy and target ionization potential) are also shown in Table 1.
Figure 8. Collision-energy dependence of partial ionization cross sections for CH3CN collided with He*(23S). The contour plots show electron density maps for respective orbitals. The solid lines in the maps indicate a simple estimation of the repulsive surface from spheres of van der Waals radii. The most effective directions are indicated by arrows.
Tables 2 and 3 show the calculated equilibrium geometry and harmonic vibrational frequencies of CH3CNLi and CH3NCLi. B. The Calculated Interaction Potential. When a metastable atom approaches a molecule, depending on the relative velocity of the metastable atom, the geometry of the molecule may change in this process. If the speed of the vibrational motion of the molecule is negligibly slow compared to the relative velocity of the matastable atom, the geometry of the molecule found by the metastable atom does not change in the Penning process. The possible geometries of the molecule that occur in the internuclear motion are determined in this case by the total energy hypersurface of the molecule. Since the interaction potential is not the same at different geometries, the experimental data reflect a “vibrationally” averaged potential. In the model potential calculations, however, the interaction potential is usually calculated for a given structure, and the averaging according to the intramolecular motion would involve a very large number of calculations. Thererfore we have decided to investigate this effect. The CNC deformational potential curve of CH3NC can be seen on Figure 1 (curve a). The zero-
Pasinszki et al.
14682 J. Phys. Chem., Vol. 99, No. 40, 1995 He”(23S)
+ CH3NC
c
1
IO 100 1000 Collision Energy/meV Figure 9. Collision-energy dependence of partial ionization cross
sections for CH3NC collided with He*(23S). The contour plots show electron.density maps for respective orbitals. The solid lines in the maps indicate a simple estimation of the repulsive surface from spheres of van der Waals radii. The most effective directions are indicated by arrows.
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V 2 3 4 5 Li-N/Li-C distance (angstrom)
Figure 10. Model potential curves V*(R) for CHXN-He*(Li) as a function of the Li-N or Li-C distance.
point vibrational level obtained from the calculations is also indicated. Due to the vibrational motion, the largest deviation from the equilibrium structure at this vibrational level is ca. f 9 . 5 O of CNC bond angle. The geometries corresponding to the equilibrium position and these two deformed structures are marked ‘with asterisks in Figure 1, and the interaction potential curves for these three geometries are shown in Figure 2. Since curves a and c represent the largest distortion from the equilibrium geometry, all of the other interaction potential curves could be obtained at different “frozen” structures which must be between these two limits. This shows that an “averaging” over the vibrational motion would yield a curve running really close to curve b which is obtained at the equilibrium geometry (see Figure 2). If the speed of the vibrational motion is fast compared to the velocity of the metastable atom, the geometry of the molecule is continuously relaxing during the approach of the metastable
2
3
4
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Li-NILi-C distance (angstrom) Figure 11. Model potential curves V*(R) for CH3NC-He*(Li) as a function of the Li-N or Li-C distance. atom. This effect on the interaction potential curve can be seen in Figure 3. In this case the geometry of the molecule has to be optimized at each point of the calculation for the interaction potential curve. This method of calculation, however, still does not take care of the “vibrational” effect which is quite difficult in this case. The CNC deformational potential curve of CH3NC if the Li atom is 2.5 A far from the nitrogen atom of the molecule (perpendicular to the pseudohalide frame) can be seen on Figure 1 (curve b). The deformational vibrational motion is govemed by this curve at this point, but ionization can occur at any distance between the molecule and metastable atom. However, the probability of ionization is exponentially increasing by decreasing the distance. Figures 2 and 3 show the “vibrational” and “relaxation” effect, when the Li atom approaches perpendicular to the central atom of the CNC frame. We have chosen to display this case, because these effects have been found to be the largest in this case. If the Li atom approaches from a different direction, these effects are much smaller. The CH3CN and CH3NC molecules are known as rigid molecules (Le., small amplitude vibrational motions), which explains the small vibrational and relaxation effect, and we can conclude in general that these effects can be neglected in the case of rigid molecules. The largest effect, however, has been found when the approach of the Li atom is coupled with the largest amplitude vibration (Le., CNC and NCC deformation), which predicts that the investigation of floppy molecules may hold many surprises where these effects are more important. C. PIES Investigations. The UPS spectrum of CH3CN27-30 and CH3NC28q33as well as the PIES spectrum of CH3CN29-32 have been earlier investigated. The downfall of Koopmans’ theorem has also been disc~ssed,2~.~* and thus detailed discussion of the assignment of the spectra shown in Figure 4-7 is not necessary. The assignment with the experimentally observed band positions can be found in Table 1. From the Penning spectrum of CH3CN and CH3NC, it can be seen that the n ~ ( 7 a 1band ) at 13.14 eV and the nc(7a1) band at 11.21 eV are very intense in the spectral region investigated. This strong relative intensity shows that the electron density of the lone pair orbitals is exposed strongly outside the molecular surface. The large negative peak energy shift of these bands (Table 1) and the negative energy dependence of the o(EJ function for these states (Figures 8 and 9) indicate that the interaction potential is attractive around the nitrogen and carbon
Penning Ionization of CH3CN and CH3NC
J. Phys. Chem., Vol. 99, No. 40, 1995 14683
TABLE 1: Band Assignments, Ionization Potentials (eV), Peak Energy ShifP (meV), and Obtained Parameter Values (See Text) molecule band orbital character ionization potential peak energy shift m S b/d d CH3CN 12 x(2e) 12.22 -420 f 100 -0.247 8.101 3 n~(7ad 13.14 -370 f 25 -0.401 4.992 475 CHdle) 15.8,b16.3b 100 f 150 0.147 0.647 2.931 6 uc-d6a1) 17.5 CH3NC 1 nd7a1) 11.21 -300 f 25 -0.087 22.934 23 N2e) 12.35 -225 f 100 0.133 0.633 2.867 0.446 0.946 1.920 435 CHdle) 16.2,b16Ab 175 f 150 UC-N(~~I) 18.3 6 a Peak Energy shift is negative if the AEi(R) in eq 2 is negative (Le., attractive interaction potential). Band maxima of Jahn-Teller distorted *E band.
TABLE 2: Calculated Equilibrium Structures of CH3CNLi and CHaCLi" CH3CNLi CH3NCLi C-C/C-N 1.457 1.427 C-N/N-C 1.174 1.179 C-H 1.OS6 1.085 N-LVC-Li 1.963 2.149 CCN/CNC 180.0 180.0 CNLi/NCLi 180.0 180.0 HCC/HCN 109.5 108.8 - 139.83320 tot. energy -139.78679 4094 (2797) baniefl 3068 (2542) 10.16 9.45 PC Ad 158.4814 159.4498 B 4.1959 4.0352 C 4.1959 4.0352 Bond lengths in A, bond angles in deg, total energy in atomic units; calculated at the MP2/6-31+G** level. Barrier to the rupture of N-Li or C-Li bond (in cm-I); values in parentheses were obtained by the CP correction for BSSE. Dipole moment in Debye. Rotational constants in GHz; isotopes: (2-12, N-14, H-1, Li-7. TABLE 3: Calculated Harmonic Vibrational Frequencies (cm-l) and IR Intensities (Wmol) of CH3CNLi and CHfiCLi" CH3CNLi CH3NCLi assignment freq IR int freq IR int and description 3256 (e) 0.03 3272 (e) 0.29 Yg CH3 as. Str 3148 (al) 0.37 3158 (al) 9.14 V I CH3 sym. str 2299 (al) 13.94 2233 (al) 48.90 v2 CN/NC str 1517 (e) 10.90 1537 (e) 11.05 v7 CH3 as. def v3 CH3 sym. def 1455 (al) 4.99 1498 (al) 3.76 1091 (e) 2.94 1178 (e) 0.0005 vg CH3 rock v4 C-C/C-N str 44.89 964(al) 6.16 974 (ai) v9 CCNlCNC def 47.61 429 (e) 2.94 335 (e) v5 N-LVC-Li str 9.41 419 (a]) 3.42 319 (al) v10 CNLi/NCLi def 149 (e) 55.33 118 (e) 70.03 a
Calculated at the MP2/6-31+G** level.
lone pairs. This is in good agreement with the calculated potential curves (Figures 10 and l l ) , which show a deep potential well when the metastable atom approaches the pseudohalide group along the CN frame. In the case of a helium metastable atom, the attractive interaction in the outgoing channel of the ionization process is very weak (see Introduction part of the paper), and the potential well depth E* of the interaction potential V* can be estimated from the peak energy shift. The estimated potential well depth E* = 370 meV for CH3CN and E* = 300 meV for CH3NC is in good agreement with the calculated values of ca. 352 and 314 meV, respectively, from the potential curves. If the long-range attractive part of the interaction potential V*(R) plays a dominant role, and its function form is of the type
u(E,) can be r e p r e ~ e n t e d ~by, ~ , ~
o(EC) a
(7)
The s value determines the steepness of the attractive part of the interaction potential curve and can be obtained from the slope m of the log u vs log Ec plots of Figures 10 and 11. The s values obtained are listed in Table 1. In contrast to the nitrogen and carbon lone pair regions, the other end of the molecules, around the methyl group, is very repulsive as indicated by the large positive energy dependences of the u(Ec)functions corresponding to the CH3 orbitals (Figures 10 and 11). If the repulsive part of the interaction potential govems the energy dependence, the interaction potential P ( R ) and the transition probability w(R) can be expressed4 as
w(R) = A exp(-bR)
(8)
P ( R ) = C exp(-dR)
(9)
and u(Ec)can be derived4 as
If the minor energy dependence of the f i s t factor (In EJQ2 in eq 10 is neglected, the positive slope m of the log u(Ec)vs log Ec plots can be related roughly to the two parameters d (effective steepness or hardness of the repulsive potential wall) and b (effective decay parameter of the electronic transition frequency) in eqs 8 and 9
m = (b/d) - (1/2)
(11)
The electronic transition probability o ( R ) involves the interaction of target electrons with the 1s electron of the He* atom. Since the target orbital is more diffuse, the parameter b in o( R ) can be approximated with the value describing the decrease of target electron density related to the decay constant of target orbitals (see ref 14 and references cited therein). Since the asymptotic decay of every Hartree-Fock orbital has been proven to be the same and the asymptotic value of the orbital exponent has been shown to be equal to (-~EHoMo),'/~ where €HOMO is the orbital energy of the highest occupied molecular ~ r b i t a l ,the ~ ~value , ~ ~ of b is common for all ionic states of a molecule and can be expressed as
b = 2(2Z(M)}1/2
(12)
where Z(M) is the lowest ionization potential. The effective hardness d of the repulsive potential wall in V ( R )can thus be obtained from the slope m of the log (T vs log E, plots as
The calculated d values are listed in Table 1.
14684 J. Phys. Chem., Vol. 99,No. 40, 1995 From the slope m of the log u vs log Ec plots and from the peak energy shifts (Table 1) it can be concluded that the character of the interaction potential in the x orbital region is intermediate between the repulsive character of the methyl group and attractive character of the terminal lone pair regions. According to the collision energy dependence of a(&), this region is repulsive in the case of CH3NC and attractive in the case of CH3CN, which is surprising at first sight. It is also surprising that in the case of CH3NC, the negative peak energy shift of n(2e) predicts an attractive and the positive collision energy dependence of u(Ec) a repulsive interaction potential. The localization of the x orbitals, however, is slightly different (see Figures 8 and 9, obtained from the calculations) in these two molecules. The calculations clearly show that the ionization of n orbitals can occur on the repulsive and attractive potential surface too (see, e.g., curves b and c on Figures 10 and 11). Thus the peak energy shift and the collision energy dependence of u(E,) reflect only an average character of the interaction potential around the pseudohalide group. Since the nitrogen p orbitals have the largest linear coefficient in the n molecular orbitals of both molecules, the highest electron density of n orbitals is localized closer to the very repulsive methyl group in the case of CH3NC, which explains the difference of the collision energy dependence of u(Ec)for these two molecules. The “inconsistency” obtained from the peak shift and collision energy dependence of u(Ec)in the case of CH3NC shows that the average character of the interaction potential is reflected differently in the peak shift than in the collision energy dependence of u(Ec). This inconsistency might be a good indicator of an orbital delocalized on the attractive and repulsive surface. The calculations show that the interaction potential is very similar around CH3CN and CH3NC (see Figures 10 and 1l), which is now in good agreement with the experiment. D. CH3NCLi and CH3CNLi Radicals. The PIES investigations and the calculated potential curves show that the interaction potential is most attractive around the terminal nitrogen and carbon lone pair region. The large negative YZN(7a1) and nc(7al) peak energy shift and the deep well on the calculated potential curves (curve a on Figures 10 and 11) indicate the existence of stable Li-M radicals (see the Introduction part of the paper). To this end the geometry of the M-Li system has been fully optimized at the MP2/6-31G** level, and the obtained structures for CH3NCLi and CH3CNLi can be seen in Table 2. Both radicals have a linear frame and C3v symmetry. The bond between the methyl pseudohalide and Li is a coordinative dative bond, indicated by the negative charge on the lithium atom, -0.22 and -0.59, for CH3CNLi and CH3NCLi, respectively. This charge also indicates that there is electron back-donation from the Li atom to the pseudohalide, which is realized by the delocalization of the lithium 2s unpaired electron on the pseudohalide group. This latter effect is much more important in the case of CH3CNLi, as shown by the calculations. The calculated barrier to the rupture of Li-N or Li-C bond is quite high, 4094 and 3068 cm-’. These values decrease to 2797 and 2542 cm-’, respectively, if BSSE is taken into account by the full CP correction, but still remarkably high. These radicals are predicted to be stable by the PIES/ab initio methods, but their generation and especially their isolation is expected to be a difficult task, because these molecules, like many open shell systems, are probably very reactive. Their generation and study seem most feasible in the gas-phase or in an inert solid matrix. To assist with future identification, the calculated dipole moments, rotational constants, vibrational frequencies, and infrared intensities are given in Tables 2 and 3.
Pasinszki et al.
V. Conclusions The investigations show that the interaction potential between a He*(23S)metastable and C H F N or CH3NC is very anisotropic having a repulsive potential around the methyl group and an attractive potential in the pseudohalide terminal lone electron pair region. The ionization cross section of the terminal lone pair u electrons is very enhanced in PIES. This indicates that the electron density of these lone pair orbitals are exposed strongly outside the molecular surface. It seems that this is a characteristic feature of the pseudohalides since the same enhancement has also been observed in the case of C H W N , CH3NC0, and CH3NCS.I7 Interaction potential curves for the collision of Li(PS) with CH3CN and CH3NC have been calculated and are in good agreement with the experiment. The effect of molecular vibrations and the relative velocity of the metastable atom on the interaction potential have also been discussed. The calculations show that in the case of rigid molecules the “vibrational” and “relaxation” effects on the interaction potential can be neglected, and thus the model potential calculations can be carried out keeping the geometry of the molecule simply “frozen”. For the first time, we have shown the advantage of Penning spectroscopy as a tool for searching for stable lithium-organic radicals. The structure of the predicted stable CH3CNLi and CH3NCLi radicals has been investigated using the MP2/63 1+G** method.
Acknowledgment. We thank the Japanese Ministry of Education, Science, and Culture for a Grant in Aid (No. 03403004) for Scientific Research and the Hungarian Scientific Research Found for grant (OTKA No. T007424) in support of this work. Dr. Takami’s preliminary work on CH3CN is gratefully acknowledged. References and Notes (1) (2) (3) (4)
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