J . Phys. Chem. 1993,97, 12718-12724
12718
Penning Ionization of CH3SCN, CH3NC0, and CHJNCS by Collision with He*(23S) Metastable Atoms Tibor Pasinszki,+Hideo Yamakado, and Koichi Ohno' Department of Chemistry, College of Arts and Sciences, The University of Tokyo, Komaba, Meguro-ku, Tokyo 153, Japan Received: September 8, 1993"
Penning ionization of CH3SCN, CH3NC0, and CH3NCS upon collision with He*(23S) metastable atoms was 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 pseudohalides investigated. In the studied energy range, the interaction potential was found to be repulsive around the methyl and attractive around the pseudohalide group. The ionization cross section of the "lone pair" CJ electrons localized on the nitrogen, oxygen, and sulfur atom in CH3SCN, CH3NC0, and CH3NCS, respectively, was found to be very enhanced in the Penning ionization electron spectroscopy.
potential is rather anisotropic. Vice versa, the anisotropy of the interaction potential can be deduced from the kinetic energy The interaction potential of molecular or atomic collisions has dependence of partial ionization cross sections. an important role in characterizing the nature of the interaction In Penning ionization electron spectroscopy (PIES), the kinetic process. It determines also the type and the extent of electron energies of electrons ejected by Penning ionization are analyzed.11 and energy transfer, or chemical reaction which can take place, Since the kinetic energy of ejected electrons can be correlated although its nature is usually not known either theoretically nor with the ionic states formed, the collision energy dependence of experimentally. the energy analyzed electron intensity yields the collision energy One kind of chemi-ionization is the Penning ionization, in which dependence of the partial ionization cross sections. a molecule (or atom) M collides with a metastable atom A* In our recent papers,12-15we have reported a novel technique having an excitation energy much larger than the lowest ionization including detection of state resolved Penning electrons as a function potential (IP) of the molecule. This ionization process yields the of the velocity of metastable atoms. Using this technique, we can ground-state atom A, one of the ionic states of the molecule Mi+, obtain the collision energy dependence of the partial ionization and an ejected electron e-:I cross section u(Ec) and hence information on the anisotropy of the interaction potential. A* M A + Mi+ e(1) Recently we found a relatively isotropic and attractive potential The measurements for the intensity of positive ions or electrons surface for He*(23S) + H2S and He*(23S) + H20,12 but the would give the total ionization cross section, UT. potential surface for He*(23S) + 0 2 was found to be very One of the most significant aspects of Penning ionization cross anisotropic.12 We found repulsive interaction potentials for He*sections is their dependence on the relative kinetic collision energy (23S) N2 and He*(23S) C02, and the repulsive walls for E,, because it reflects the details of the interaction poter~tial.~.~ end-on collisions were shown to be harder than those for side-on The total ionization cross section for various atoms and simple c01lisions.l~ The study of some saturated and unsaturated molecules collided with metastable atoms have been extensively hydrocarbons indicated that the interaction potentials between investigated in previous ~ears.3-I~ It has been observed that the the hydrocarbon molecule and He*(23S) atom were attractive ionization cross section increases with the increase of E, for the near the ?r orbital region, otherwise r e p u l ~ i v e . ~These ~ J ~ invescollisionof an Ar atomwith He*,3-7indicating that theinteraction tigations also suggest that the nature of an interaction potential potential is repulsive when He* approaches the Ar atom. On the surface strongly depends on the intramolecular surroundings of other hand, the ionization cross section decreases with the increase atoms or chemical groups. of Ec for a Hg atom with He*,2v3q7and the interaction potential In the present paper, we investigate the interaction potential is attractive. between He*(23S) atom and CH3SCN, CH3NC0, or CH3NCS In thecaseof a target atom, the interaction potential is isotropic molecule, focusing our attention mainly on the lone pair orbitals (due to the spherical symmetry of the atom), but if the target at the terminal position. system M is a molecule, there is an anisotropy of the interaction potential. In this case, it is difficult to obtain information on the 11. Experiment anisotropy from the study of total ionization cross section, because UT(E,)reflects only an average potential. If several final ionic The apparatus used in this work has been reported in previous states can be formed by the ionization of a molecular target with papers.l2-14 Themetastableatomsof He*(23S,21S)were produced a metastable atom, u@,) would then be the sum of the ionization by a discharge (dc) nozzle source, and the He*(2IS) component cross sections of the different ionic states (partial ionization cross was quenched by a water-cooled helium dc lamp. The kinetic section, u(Ec)). A given ionic state can originate from the energy of electrons ejected by collisional ionization were deterionization of a given molecular orbital. Since the molecular mined by a hemispherical electrostatic deflection type analyzer16 orbitals are (more or less) localized on a special part of the using an electron collection angle to the incident He*(2%) beam molecule, the u(Ec)functions will not be the same if the interaction axis of 90°. The energy resolution of the electron analyzer was estimated to be 40 meV from the full width at half-maximum t Permanent address: Department for Inorganic Chemistry, The Technical (FWHM) of t h e , Ar+(2P3p) peak in the He I ultraviolet University, H-1521 Budapest, Hungary. *Abstract published in Advance ACS Abstracts. November 15, 1993. photoelectron spectrum (UPS).
I. Introduction
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0022-3654/93/2097-12718$04.00/0
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Penning Ionization of CHsSCN, CHsNCO, and CH3NCS
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The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 12719
200 300 400 Time-Of -F1 ight/ps Figure 1. He*(23S) TOF spectrum for a detector to chopper wheel 100
separation of 504 mm.
In separate measurements, the metastable beam of He*(23S) was chopped by a mechanical chopper to produce a pulsed metastable beam. Figure 1 shows the time-of-flight (TOF) spectrum Z M ( ~ ) of He*(23S). The TOF signal was obtained by detecting emitted electrons from a stainless steel plate inserted into the collision cell. Time-of-flightof secondary electrons from the metal surface to the detector are negligibly short in comparison with TOF of the He* atoms. The efficiency of the secondary electron from a metal (stainless steel) plate was considered to be constant in the observed collision energy range." The signal at the very short TOF of nearly zero in Figure 1 was attributed to resonant photons from the discharge source. In the present study, two spectra with a low collision energy of about 98 meV on average (72-1 15 meV) and a high collision energy of about 223 meV on average (177-391 meV) were measured for each molecule, and the resolution of the analyzer was lowered to 250 meV (FWHM for He I UPS of Ar) in order to obtain higher counting rates of electrons. 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 ZE(~)was measured using the energy fixed mode of the electron analyzer (FWHM for He I UPS of Ar was 250 meV). Figure 2 shows typical timedependent signals of Penning electrons from sample gases as a function of the TOF of He*(23S) atoms. Since the time-resolved spectrum gives the electronintensity ZEas a function of thevelocity U M of He*(23S), the partial ionization cross section a(&) can be determined by the equations
(3) where c is a constant, U R is the relative velocity averaged over the velocity of the target molecule, k g is the Boltzmann constant, and T and m are the gas temperature and the mass of the target molecule, respectively. Finally, U(UR) is converted to a(&) by the relation
E, = puR2/2 (4) where p is the reduced mass of the system. Ultraviolet photoelectron spectra were measured by utilizing the He I resonance photons (21.22 eV) produced by a dcdischarge in pure helium gas. The electron spectra were obtained at an
0
100
200 T i me/ps
300
400
Figure 2. Time dependent signals of Penning electrons for He*(2%) + CH3SCN (B ionic state), as a function of the TOF of He*(23S), which travels SO4 mm from the chopper to the collision cell.
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 UPS data with those by Gardner and SamsonI7and Kimura et a1.18 111. Calculations
Due to our increased interest in ionization process of the terminal lone pair orbitals of the pseudohalideframe, interaction potential curves with the metastable atom approaching the pseudohalide molecule along the NCX frame were calculated using ab initio method. There is a well-known resemblance between He'(23S) and Li(22S). It has been shown19 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 interactions of various targets with He*(23S) and Li(22S) [see refs 2, 13, 20, and 21 and references cited therein]. Due to these findings and the wellknown difficulties associated with calculations for excited states, Li(22S) atom was used in the present study in place of He*(23S). Summing up, the CH3SCN Li, CH3NCO Li and CH3NCS + Li interaction potential curves were calculated with the Li atom moving along the CN, CO, and CS axis, respectively. The geometries of the neutral molecules, selected from the lite r a t ~ r e , were ~ ~ - used ~ ~ in the calculations. All the calculations in this work were carried out by using the Gaussian-90 quantum chemistry pa~kage.2~ A standard 6-31+G* basis set was used, and the correlation energy was partially allowed for by using second-order Mdler-Plesset perturbation theory (MP2). The full counterpoise (CP) method was used to correct for basis-set superposition errors (BSSE).
+
+
IV. Results Figures 3-5 show the He I ultraviolet photoeletron spectra and Penning ionization electron spectra of CH3SCN, CH3NC0, and CH3NCS. 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. Figures 6-8 show the collision-energy-resolved He*(23S) Penning ionization electron spectra of CHDCN, CHsNCO, and CH3NCS,respectively. In each figure, the low-collision-energy spectrum (98 meV) is shown by a solid curve and the high-collisionenergy spectrum (223 meV) is shown by a dashed curve. The
Pasinszki et ai.
12720 The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 Ionization Potent ial/eV IO 11 12 13 14 15 16 17 18 19 20 21 22
9 r
l
t
~
~
r
l
l
i
~
Ionization Potent ial/eV IO 11 12 13 14 15 16 17 18 19 20 21 22
9 i
t
,
l
13a'ln,)
3
4a'(n)
1
I
CH5CN
A 1
1
1
0
9
8
7
6
5
4
3
2
1
0
1 2 1 1 1 0 9
E l act ron Energy/eV
8 I 6 5 4 3 Electron Energy/eV
2
1
0
13aIn,)
5
1
,13a(ns)
3aW
5
La (n)
1
l k ( T T ) 12a 2a
0
9 8 7 6 5 4 3 E 1e c t r on Ene r gy/eV
2
1
J , W , Y ,,
0
1 1 1 0 9
8
6
7
5
12 13 14 15
16
,
,
,
3
2
1
0
Figure 5. He I UPS and He*(2%) PIES of CHpNCS.
Ionization Potential/eV 10 11
4
,
Electron Energy/eV
Figure 3, He I UPS and He*(2%) PIES of CHpSCN. 9
PIES
lla
CH3SCN He* PIES - - - - -.. :Ek-223meV :Ek- 98meV
17 18 19 20 21 22 I
...
8 7 6 5 4 3 2 Electron Energy/eV
1 1 1 0 9
1
0
1
1
1
1 1 1 0 9
1
!
1
1
1
,
1
.. ..
1
1
1
8 7 6 5 4 3 2 E l e c t r o n Energy/eV
1
0
Figure 6. Collision-energy-resolvedHe*(2%) Penning ionization electron spectra of CHpSCN (-, 72-115 meV, average 98 meV; -,177-391 meV, average 223 meV). High-resolutionspectra for the first band are shown in the insert (see text).
-
1
0
9
8
7
6
5
4
3
2
1
0
Electron Energy/eV Figure 4. He I UPS and He*(2%) PIES of CHpNCO.
relative intensities of the two spectra are normalized in the figures using the data of log u vs log E, plots. Table I lists thevertical ionizationpotentials (determined from the He I UPS)and the assignments of the observed bands. The peakenergy shifts in PIES measured with respect to the"nomina1" energy Eo (Eo = the difference between metastable excitation energy and target ionization potential) are also shown in Table I. The peak shift of 43a") and CH3(12a',2a'') bands in the PIES of CHpSCN were not determined due to the small relative intensity and strong mixing of the CH3(12a') and CH3(2a") bands and the hiding of the ~ ( 3 a " )band under the strong n~(13a') band. Figures 9-11 show the log u vs log Ec plots for CH3SCN, CH3NC0, and CH3NCS, respectively. The calculated electron density maps of the molecular orbitals are also shown in the figures. In the case of molecular orbitals with a' symmetry, the cutting plane for electron density maps is the symmetry plane of the molecule. For the electron density maps of orbitals with a" symmetry,two different cutting planes werechosen. These planes
CH3NCO He* PIES ..- - __. :&-220meV - :Ek- 97meV
1 1 1 0 9
7
8 7 6 5 4 3 2 E l e c t r o n Energy/eV
1
0
F i p e l . Collision-energy-resolvedHe*(23S) Penning ionization electron spectra of CHsNCO (-, 72-1 14 meV, average 97 meV, 175-368 meV, average 220 meV).
--
are perpendicular to the symmetry plane of the molecule; one of them contains the C(H,)-N or C(H3)S bond axis and the other contains the nearly linear NCX frame. The values of the slope m of log u vs log Ec plots as well as the calculated s, b / d , and d parameters (see below) are listed in Table I. The s, b / d , and d values were calculated only in cases when Iml > 0.1.
Penning Ionization of CHJSCN, CH3NC0, and CH3NCS
The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 12721
TABLE I: Band Assignments, Ionization Potentials, Peak Energy Shifts, and Obtained Parameter Values (See Text) molecule
band
orbital character
ionization potential/eV
peak energy shift/meV
m
S
CH3SCN
1 2 3 4
u(4a”) 1sa’) nN( 13a’) u(3a”) u( 14a‘) CH3( 12a’) CH3(2a”) a(1la’) r(3a”) u(12a‘) u( 1Oa’, 1a”) CH3(lla’,2aN) no( 8a? a(9a’) u(4a”, 15a’) a( 3a’’* 14a’) ns( 13a’) CH3(12a’,2a’’) a(1la’)
10.25 12.10 12.88 (13.1) 13.75 15.1 15.9 16.9 10.61 11.16 14.45 15.95 16.65 18.0 9.42 12.60 14.58 15.9 17.8
-100 f 25‘ 150 f 75 300 f 25
-0.01 1 0.088 -0.270
7.407
150 f 50
0.148 0.348 0.3 17 0.350
5
CH3NCO
CH3NCS
6 7 8 1 2 3*4 596 7 8 1,2 394 5
697 8 a
U(
-50 f 75 170 f 100 270 f 100
-0.207
180 f 75 100 f 100 150 f 50 Of75 250 f 25 300 f 75 75 f 25 -50 f 150 -50 f 150
bld
d
0.648 0.848 0.8 17 0.850
2.679 2.047 2.125 2.042
0.609
2.900
0.698 0.609
2.384 2.733
9.662
-0.064 0.109 -0.2 16 0.065 -0.162 -0.07 1 0.014 0.198 0.109
9.259 12.346
Determined at peak maximum. A shoulder with +200-meV peak energy shift can be observed too.
1.7
5
+ CH3SCN
CH3NCS He* PIES - - - _ _ _:Ek-223meV
t
i
c 0 Y
1 1 1 0 9
8 7 6 5 4 3 E l e c t r o n Energy/eV
2
1
0
Figure8. Collision-energy-resolvedHe*(23S)Penning ionization electron spectra of CH3NCS (-* 72-1 15 meV, average 98 meV; - - 177-391 meV, average 223 meV).
Figure 12 shows potential energy curves V ( R )obtained from the model potential calculations. R is the distance between the Li atom and N, 0, and S atoms, respectively. V. Discussion A. Assignmentof UPS and PIESBands. The UV photoelectron spectra of CH3NC0, CH3NCS, and CH3SCN have been extensively investigated p r e v i o u ~ l y , *but ~ ~the ~ assignments of some photoelectron bands are rather different from different studies. The quantum chemical calculationsusing different levels of theory and the analysisof the He I/He I1 band intensity ratios also gave different orderings in the orbital and photoelectron band ~equence.~~-~O Penning ionization electron spectroscopyis similar to ultraviolet photoelectron spectroscopy in which the kinetic energy of ejected electrons due to photoionization is analyzed, but the relative band intensities of PIES and UPS are very different, reflecting the difference in their ionization mechanisms.3’ In the Penning ionization process, an electron in a molecular orbital of target molecule (M) is transferred to the inner-shell orbital of metastable atom (A*), and the excited electron of A* is ejected.32 The probability of the electron transfer from M to A* largely depends on the spatial overlap between the orbitals of M and A*. Therefore the relativeband intensity of PIES reflects the electron distribution of individual molecular orbitals exposed outside the molecular surface (van der Waals surface)3*. Using this characteristic of PIES, we attempt to remove some previous contradictions and provide more definitive assignments.
cn 0
7 CH3(2a”)
8
o(lla’>
10
# 100
1000
Collision Energy/meV Figure9. Collision-energydependenceof partial ionization cross sections for CH3SCN collided with He*(2%).
In the photoelectron spectrum of CHzSCN, the assignments of third, fourth, and fifth bands (B, C, and D ionic states, respectively) originating from nN( 13a’), ?r(3a”), and ?r( 14a’) molecularorbitals28-30are ambiguous. In the PIES study of some nitriles, it was pointed out previously that the cross section for ionization of the “lone pair” cr electrons localized on the nitrogen atom was very enhan~ed.339~~ The third band of the Penning spectrum of CH3SCN is extremely enhanced (see Figure 3); therefore its assignment as the nN( 13a’) band should be unambiguous. Using the high-resolution UPS data,jOthe fourth band (observed only in UPS) and fifth band can be assigned to 743a”) and ?r( 14a’), respectively. The assignment of the CH3NCO spectrum in the 14-17-eV ionization potential region is also different in all the earlier papers.26927JO In this region (see Figure 4), PIES or UPS bands that originated from the ionizations of orbitals which consist of a symmetry-controlledmixing of methyl group orbitals, CH3(2a”, 1la’), with NCO group orbitals, ?r( la”,lOa’) and no(8a’), are expected. It was previously found in the PIES study of aldehydes and ketones that the band arising
Pasinszki et al.
12722 The Journal of Physical Chemistry, Vol. 97, No. 49, 1993
600
500 400 300 w 200 Q)
\
3
100
*
'0 -100 -200
-300 10
100 1000 Col1 i s i o n Energy/meV F i 10. Collision-energydependence of partial ionizationcrosssections for CH3NCO collided with He*(23S).
-400 -500 0
I
I
I
1
I
1
2
3
4
5
6
R/A Figure 12. Model potential curves V ( R )(A) for CH3SCN-Li; (0)for CH3NCO-Li; (0)for CH3NCS-Li. R is the distance between Li and N, 0, and S atoms, respectively. Li atom approaches the molecules along the C-N, C-O, and C S axes, respectively.
be expected. Indeed, a deep potential well has been obtained from the calculated potential curve, too (see Figure 12). If the metastable atom is a rare gas atom, the attractive interaction in the outgoing channel of ionization process is very weak, and the potential well depth e* of the interaction potential P can be estimated with the peak energy shift.36 The estimated potential well depth e* = 300 meV is in good agreement with the calculated value of ca. 340 meV from the potential curve. If the long-range attractive part of the interactionpotential P ( R )plays a dominant role, and its function form is the type of
x (3a",14a')-'
P ( R ) KS
(5)
a(&) can be represented2Jv8 by
1000 Col1 i s i o n Energy/meV 10
100
Fire11. Collision-energy dependence of partial ionizationcross sections for CH3NCS collided with He*(23S).
from the ionization of the lone pair u electrons was enhanced.35 Therefore the intense band at 16.65 eV can be attributed to an ionization from the no(8a') orbital. Recently14J5and also in this work (see below), we have found that the interaction potential between the metastable atom and the target molecule is repulsive around the methyl group. Therefore the slope of the log u vs log Ec plot for orbitals localized on the methyl group is expected to be positive. Only the slope of the band system at 15.95 eV is positive in the 14-17-eV region; therefore the 15.95-eV system is attributed to ionization from the CH3(2a",1 la') orbitals and the 14.45-eV system, from the a( la",lOa') orbitals. Our assignment of the CH3NCS spectrum (Figure 5 ) is in good agreement with earlier UPS investigation^.^^^^^^^^ B. CHfiCN. From the Penning spectrum of CHJSCN, the nN(13a') band at 12.88 eV is very intense in the spectral region investigated. This strong relative intensity suggests that the electron density of n~(13a') orbital is exposed strongly outside the molecular surface. The negative energy dependence of the a(&) function for thisstate (Figure 9) indicatesthat the potential energy surface is attractive around the nitrogen long pair. From the large peak energy shift (300 meV) of nN(13a') with respect to the normal energy, an attractive potential surface would also
a(Ec) a: EL2/' 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 plot of Figure 9. The s value obtained is listed in Table I. In contrast with the nitrogen lone pair, the potential surface around the methyl group is strongly repulsive, because the a(&) functions for the PIES bands that originated from the CH3( 12a') and CH3(2a") orbitals have large positive energy dependences (Figure 9). If the repulsive part of the interaction potentialgoverns the energy dependence, the interaction potential P ( R ) and the transition probability w(R) can be expressed3 as
w(R) = A exp(-bR)
(7)
and the a(&) can be derived3 as (9)
when the minor energy dependence of the first factor (In Ec/C)2 in eq 9 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
Penning Ionization of CH3SCN, CH3NC0, and CH3NCS (effectivedecay parameter of the electronictransition frequency) in eqs 7 and 8: m = ( b / 4 - (1 /2) (10) The electronic transition probability w(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 w(R) can be approximated with the value describing the decrease of target electron density related to the decay constant of target orbitals (see ref 13 and references cited therein). Since the asymptotic decay of every HartreeFock orbital was proved to be the same, except for the Be atom, and the asymptotic value of the orbital exponent was shown to be equal to (-2eHOMO)’/’1 where CHOMO is the orbital energy of the highest occupied M0,37J* the value of b is common for all ionic states of a given molecule and can be expressed as
6 = 2{2Z(M)}’/’ where I(M) is the lowest ionization potential. Combining eq 10 and eq 1 1, the effective hardness d of the repulsive potential wall in P ( R ) can be expressed as
d = 2{2Z(M)}’/2/(m + ’/’) The calculated d values are listed in Table I. The slope of the log u(E,) vs log E, plot for the band at 16.9 eV is very similar to those of CH3(2a”,12a’) bands. This PIES band originates from a u orbital of the CSCN frame which, according to the electron density map (Figure 9), has a large electron density on the methyl group and is exposed outside the molecular surface at nearly the same place as the CH3(2a”,l2a’) orbitals. The negative peak shift (-50 meV) also shows that the interaction potential is repulsive here. The positive energy dependence of u(E,) for the band that originated from a(14a’) orbital is a little surprising, because an attractivepotential surfacenear the a orbitalsis usually expected.*5 The interaction potential surface of CHoCN + HeS(23S) is also attractive near the TCN orbitals.39 Due to the a’ symmetry, the electron density of the a( 14a’) orbital is more concentrated and exposed outside near the symmetry plane of the molecule, and this would therefore be the most effectivedirectionfor metastable atoms. The bond angle at the sulfur atom is however nearly a right angle (99O 40); thus the methyl group (with its repulsive character) is situated close to the C-N fragment of the molecule, which can result in a repulsive molecular surface. The opposite side of the molecule, where the a(14a’) orbital is also situated, is expected to have an attractive character, although only an average character of the interaction potential can be concluded from the kinetic energy dependence of the a( 14a’) band intensity. The situation is nearly the same for the a( 1sa’) orbital too, which is shown by the similar energy dependence of u(EJ. The electron density of the a(4a”) orbital, due to the a” symmetry, is exposed outside, perpendicular to the symmetry plane of the molecule. Thereforethe repulsiveeffect of the methyl group is not so pronounced, which is demonstrated by the negative energy dependence of the a(4a”) band intensity. This band has an interesting shape, in particular a narrow band with a strong high-energy shoulder. In the UPS (see also refs 28 and 30), the u(4a”) band is very sharp, showing a strong adiabatic transition. Although, the vibrationalfine structureof PIES band is frequently different compared to those of UPS,this strong shoulder is not expected in the PIES. Supposing that the shoulder is due to the changing of vibrational fine structure, the large negative peak energy shift (-100 meV at the peak maximum), which indicates a repulsive interaction potential, is inconsistent with the negative energy dependence of u(E,). Further investigating this band structure, we have recorded the collision-energy-resolvedspectra of CH3SCN in the 9-12-eV ionization energy region again, but with higher resolution (Figure 6 ) . This shows that the kinetic
The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 12723 energy dependence of the intensity of the narrow peak and the shoulder is not the same, and the shoulder can be originated from a more attractive part of the interaction potential surface. According to the above discussions,it is clear that the interaction potential is repulsive around the methyl group and attractive around the nitrogen atom. The a(4a”) orbital is localized on the SCN fragment and has a nodal surface near the carbon atom; thus its electron density is exposed outside at the sulfur atom, near the repulsive surface, and at the nitrogen atom, where the interaction potential is attractive. The kinetic energy of electrons that originated from the ionization of such an orbital on the repulsive or attractive interaction potential surface would be similar, but the attractive or repulsive surface would result in a positive or negative peak energy shift, respectively. This is in good agreement with the observed -100- and +300-meV peak energy shift of the a(4a”) band maximum and shoulder, respectively. From the relative intensity of the narrow peak and high-energy shoulder of the a(4a”) band, it can be also concluded that the electron density of the u(4a”) orbital is much more exposed outside to the molecular surface at the sulfur atom. This is in good agreement with the earlier conclusion, that the r(4a”,l5a’) orbitals in the SCN group are not symmetric and tend to be localized on the sulfur atom.30 C. CHaCO. The band that originated from the oxygen lone pair in the PIES spectrum of CH3NCO (Figure 7) shows a large similarity to that of nN( 13a’) in the spectrum of CH3SCN. Both of them have a strong relative intensity in the spectrum, positive peak energy shift, and large negative collision energy dependence of u(E,). The slope of the log u(E,) vs log E, plot is also rather similar (see Table I). These show that the oxygen lone pair is strongly exposed outsidethe molecular surface, and the interaction potential surface is attractive around the lone pair similar to that of nitrogen. This is further supported by the model potential calculations,indicating a deep well when the He*(Li) approaches the oxygen atom along the C - O axis. The steepness of the attractive part of the interaction potential curve P ( R ) is rather similar to those of CH3SCN Li, which is shown by the similar s values (Table I) and the model potential curves (Figure 12), but the well depth is smaller than that of CH3SCN Li, which is in good agreement with the smaller peak energy shift of no(8a’). Not only around the oxygen lone pair but around the whole NCOgroup, the interaction potential is attractive,as demonstrated by the positive peak energy shifts and the negative energy dependence of u(E,) for the a( 1Oa’, 1a”) and u( 12a’”a’’) bands. The negative energy dependence of u(E,) for the r( 10a’”a’’) bands is smaller compared to those of the *(12”,3a”) bands, which can be well explained by the mixing of the a( 1Oa’”a’’) and CH3(l la’”’’) orbitals. In spite of the unexpected small slope of the log u vs log Ec plot, the positive energy dependence of u(E,) for the CH3(1la’,2a”) bands shows that the interaction potential is repulsivearound the methyl group. It is important to note that the CH3(1la’”’’) bands in the spectrum overlap with the no(8a’) band having opposite energy dependence of u(E,); therefore larger m and strong repulsive character of the interaction potential can be expected here. The band u(9a’) at 18.0 eV shows neither remarkable peak energy shift nor energy dependence of u(E,). This band can be attributed to an ionization of a u orbital, which is delocalized on, and exposed outside at both ends of, the CNCO frame (see Figure 10). D. CHaNCS. In the PIES spectrum of CH3NCS (Figure 8), the ns(l3a’) band originated from the sulfur lone pair orbital stands out in intensity in a similar way to those of nN( 13a’) and no(8a’) in the spectra of CH3SCN and CH3NC0, but this band does not show any collision energy dependence of u(Ec). The peak energy shift is also smaller than those of nN( 13a’) and (8a’); furthermore the potential well cannot be found on the
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Pasinszki et al.
12724 The Journal of Physical Chemistry, Vol. 97, No. 49, 1993
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calculated interaction potential curve of CH3NCS Li. These indicate that the character of the interaction potential is rather different to those of the nitrogen or oxygen lone pair around. Although the r( 14a’,3a”) and r( 15a”a’’) bands show negative energy dependence of u(Ec), the interaction potential is less attractive around the NCS group than around the NCO group. This is shown by the smaller absolute value of m for the r(14a’,3a”) and r(1Sa’,la’’) bands in the spectrum of CHSNCS. The smaller negative energy dependence of u(Ec)for the r( 14a’”a’’) bands compared to those of r( 15a”a”) can be explained again by the mixing of r(14a’,3a’’) with CH3(12a’”a’’) orbitals. Due to the positive collision energy dependence of u(Ec)and a small negative peak energy shift for the CH3( 12a”a”) bands, it can beconcluded again that the interaction potential is repulsive around the methyl group in this molecule too. The u(l la’) band at 17.8-eV ionization energy has a similar negative peak energy shift compared to that of CHI( 12a’”a”), but it shows a smaller positive energy dependence of u(Ec),notwithstanding that it is exposed outside the molecular surface at the methyl group according to the electron density map (Figure 11). There is, however, a remarkable difference between the u( 1la’) and CH3(12a‘,2a“) orbitals. The a( 1la’) orbital is exposed outside along the C-N axis, while the CH3(12a’,2a’’) orbitals are exposed perpendicular to the axis. This indicates that the interaction potential wall is harder when the He* atom approachesthe methyl group along the C-N axis.
VI. conclusiolls
International Foundation for a scholarship in support of this research. References and Notes (1) Penning, F. M. Natunvissenshaften 1927, 15, 818. (2) Niehaus, A. Adu. Chem. Phys. 1981,45,399. (3) Illenberger, E.; Niehaus, A. Z . Phys. B 1975, 20, 33. (4) Parr,T. P.; Parr, D. M.;Martin, R. M. J. Chem.Phys. 1982,76,316. ( 5 ) Peanelle, A.; Watcl, G.; Manus, C. J. Chem. Phys. 1975,62,3590. (6) Woodard, M.R.; Sharp, R. C.; Scely, M.; Muschlitz, E. E., Jr. J. Chem. Phvs. 1978.69.2978. (7) Appolloni,‘L.;Brunetti, B.; Hermanussen,J.;Vecchiocativi, F.;Volpi, G. G. J. Chem. Phys. 1987,87, 3804. (8) Allison, W.; Muschlitz,E. E., Jr. J . ElectronSpctrosc.Relat.Phenom. 1981, 23, 339. (9) Riola, J. P.; Howard, J. S.; Rundel, R. D.; Stebbmgs, R. F. J . Phys. B 1974, 7, 376. (10) Lindinger, W.; Schmeltekopf,A. L.; Fehsenfelt, F.C. J . Chem.Phys. 1974,61 2890. (11) kermAk, V. J . Chem. Phys. 1966.44, 3781. (12) Mitsuke, K.; Takami, T.; Ohno, K. J. Chem. Phys. 1989,91, 1618. (13) Ohno, K.; Takami, T.; Mitsuke, K.; Ishida, T. J . Chem. Phys. 1991, 94, 2675. (14) Takami, T.; Mitsuke, K.; Ohno, K. J. Chem. Phys. 1991, 95,918. (15) Takami, T.; Ohno, K. J . Chem. Phys. 1992,%, 6523. (16) Mitsuke, K.; Kusafuka, K.; Ohno, K. J. Phys. Chem. 1989,93,3062. (17) Gardner, J.L.;Samson, J.A.R.J. ElectronSpectrosc.Relat.Phenom. 1976,8, 469. (18) Kimura, K.; Katsumata, S.; Achiba, Y.; Yamazaki, T.; Iwata, S. Handbook of He1 PhotoelectronSpectra of Fundamental OrganicMolecules; Japan Scientific: Tokyo, 1981. (19) Rothe, E. W.; Neynaber, R. H.; Trujillo,S. M. J . Chem.Phys. 1%5, 42. . -, 3310. - - .-. (20) Hotop, H. Radial. Res. 1974, 59, 379. (21) Haberland, H.; Lee, Y. T.; Siska, P. E. Adv. Chem. Phys. 1981,45,
-”..
Aft7
The cross sections for ionization of the long pair u electrons localized on the N, 0,and S atoms in CHsSCN, CH3NC0, and CHBNCS,respectively, arevery enhanced in PIES. This indicates that the electron density of these lone pair orbitals are exposed strongly outside the molecular surface. Intensities of the nN(13a’) and no(8a’) bandsdecreasewith the increaseof thecollision energy. This indicates that the interaction potential is attractive if the He*(23S) atom approaches the molecules nearly along the SCN or NCO frame and is in good agreement with the deep well on the calculated interaction potential curves. In contrast, the sulfur lone pair band in the PIES of CH3NCS does not show collision energy dependence of the partial ionization cross section in the collision energy range investigated. The kinetic energy dependence of partial ionization cross sections reflects that the interaction potential for all of the investigated molecules with He*(23S)atom is strongly anisotropic. Theinteraction potential is repulsive around the methyl group and attractive around the pseudohalide group. The assignment of UPS was also reinvestigated. In light of our Penning spectroscopic results, it seems that the orderings in the ionic state sequence are different from that obtained by Hartree-Fock ab initio calculations using Koopmans’ theorem.
(22) Dreizler, H.; Rudolph, H. D.; Schlesser, M. Z . Narurforsch. 1970, 25a, 1643. (23) Koput, J. J. Mol. Spectrosc. 1986, 115, 131. (24) Koput, J. J. Mol. Spectrosc. 1986, 118, 189. (25) Frisch, M. J.; Head-Gordon, M.;Trucks, G. W.; Foresman, J. B.; Schlegel, H. B.;Raghavachari, K.; Robb, M.A,; Binkley, J. S.; Gonzalcz, C.; Defrees, D. J.; Fox, D. J.; Whiteaide, R. A.; Stcgcr, R.; Mclius, C. F.;Baker, J.; Martin, R. L.; Kahn, L. R.; Stewart, J. J. P.; Topiol, S.; Pople, J. A. Gaussian 90,Gaussian, Inc.: Pittsburgh, PA, 1990. (26) Eland, J. H. D. Philos. Trans. R. Soc. London, Ser. A 1970,268,87. (27) Cradock, S.; Ebsworth, E. A. V.; Murdoch, J. D. J . Chem. Soc., Faraday Trans. 2 1972,28, 86. (28) Neijren, B. J. M.; Dedange, C. A. J . Electron Spectrosc. Relat. Phenom. 1980,18, 179. (29) Andreocci, M. V.; B m a , M.; Furlani, C.; Piancastelli,M. N.;Cauletti, C.; Tarautelli, T. J. Chem. Soc., Faraday Trans. 2 1979, 75, 105. (30) Pasinszki, T.; Veszprtmi, T.; Fehtr, M.; Kovac, B.; KlasinE, L.; McGlynn, S. P. Int. J. Quantum Chem. 1992, 26, 44. (31) See recent review and references cited therein: Ohno, K.; Harada, Y. In Theoretical Models of Chemical Bonding, Parr 3; MaksiE, Z . B., Ed.; Springer-Verlag: Berlin-Heidelberg, 1991; pp 199-234. (32) Hotop, H.; Niehaus, A. Z . Phys. 1969, 228, 68. (33) Ohno,K.; Matsumoto,S.; Imai, K.; Harada, Y.J. Phys. Chem. 1984, 88,206. (34) CermBk, V.; Yencha. A. J. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 109. (35) Ohno, K.; Takano, S.; Mase, K. J. Phys. Chem. 1986, 90,2015. (36) Niehaus, A. Ber. Bunsen-Ges. Phys. Chem. 1973, 77,632. (37) Handy, N. C.; Marron, M. T.; Silverstone, H. J. Phys. Rev. 1969, 180. 180. ~~~
Acknowledgments. This work has been supported by a Grant in Aid for Scientific Research from the Japanese Ministry of Education, Science, and Culture. T.P. thanks the Matsumae
(38) -Morrell, M. M.; Parr, R. G.; Levy, M. J . Chem. Phys. 1975,62,549. (39) Ohno, K. To be published. (40)Pasinszki, T.; VeszprCmi, T.; Fchtr, M. Chem.Phys. Lett. 1992,189, 245.
