J . Phys. Chem. 1989, 93, 3068-3077
3068
-
i . 2- j
HIS x 1 1 1
30
40
1
t He'(Z3S)
50
60
80
70
90
100
e
.
I10
120
130
I40
160
E l e c t r o n E l e c t i o n Angle / deq
Figure 10. Comparison of the observed angular distributions (symbols) of Penning electrons emitted from H2S in collision with He*(2 3S) and those determined by model calculation (line graphs). Average relative kinetic energies: 0 , 59 meV; 0 , 141 meV. In calculation, the internal angular distributions involving s-, p-, and d-wave contributions are used. Set of parameters: 5 , = 0.6, cos 6, = 1.0, t2 = 1.0, cos 62 = -1.0 (see
caption of Figure 7). The relative kinetic energies are 60 meV (-), 140 meV and 40 meV (--). (-a),
angle with respect to the vector of the initial relative velocity reaches the lowest value. Furthermore, the behavior of the angular distribution curve at Ek = 60 meV is found to be essentially determined by this region. These findings can be explained by the so-called rainbow effect; Le., the deflection angle of the collision trajectory reaches its most negative value owing to the long-range intermolecular attraction.26 Incomplete agreement of the experimental distribution at = 59 meV to the theoretical one at Ek = 60 meV is attributable to the effect of collision with Ek20 eV below the A* feature) in order to minimize errors in the extrapolation to the region of continuum normalization. Reproducibility tests on independent spectra indicate a precision (and thus relative accuracy) of 1 5 % . The CH3CN, CH3SCN, and CH3NCS samples were obtained commercially (CH3CN and CH,SCN, Aldrich, analytic grade 99+%; CH3NCS, Fluka, 99+%), while the CH,NC sample was synthesized according to standard procedures and purified prior
-
-
(37) Johnston, A. R.; Burrow, P. D. J . EIectronSpectrosc. 1982, 25, 119. (38) Hitchcock, A. P.; Beaulieu, S . ; Steel, T.; Stohr, J.; Sette, F. J . Chem. Phys. 1984, 80, 3927. ( 3 9 ) Newbury, D. C.; Ishii, I.; Hitchcock, A. P. Can. J . Chem. 1985, 64, 1145.
(40) Hitchcock, A. P.; Ishii, I. J . Electron Spectrosc. 1987, 42, 11. (41) Henke, B. L.; Lee, P.; Tanaka, T. J.; Shimabukuro, R. L.; Fujikawa, B. K . At. Data Nucl. Data Tables 1982, 27, I .
I
1
_-
I
3 E L E C T R O N ENERGY
5
7
IeVI
Figure 1. Electron transmission spectra of CH3CN,CH3NC, CH3SCN, and CH3NCS recorded with the high rejection mode (approximate total scattering cross section). The vertical bars are the attachment energies, taken as the vertical midpoints of the minimum and maximum of the differentiated signal.
to spectral a c q u i s i t i ~ n . ~The ~ solid methyl isothiocyanate (mp = 36 "C) was melted for transfer and degassing but the vapor above the solid was used for spectral acquisition. The purity of the sample in the ISEEL spectrometer was monitored during spectral acquisition with a quadrupole mass spectrometer.
3. Results and Discussion 3.1. Electron Transmission Resonances. The electron transmission spectra of the title molecules in the 0-7-eV range are shown in Figure 1. The bars above the spectral features locate the vertical attachment energies, that is, the negative of the electron affinities. The AE values are given in Table I together with the proposed assignments. Correlations throughout this molecular series in the attachment energies (AEs), photoelectron ionization potentials (IEs), and core excitation term values (TVs) are presented in Figure 2. The ET spectrum of CH3CN displays an intense resonance at 2.82 eV, assigned to electron capture into the degenerate x * , - ~ MO. The energy determined in this work is in good agreement with that of 2.86 eV reported by Jordan and Burrow.25 The x * , - ~ AE measured in methylpr~pyne~~ is slightly higher, consistent with the stabilizing effect caused by replacement of a C H group by a nitrogen atom as previously noted in aromatic compounds.44 Two weaker resonances are observed in the ETS of CH3CN at 5.7 and 6.8 eV. These structures could be associates with capture into cr* MO's, but in this energy range core-excited shape resonances (Le., two particle processes) could also occur. Previous vibrational energy loss studies of C-H-containing species2J1suggest that a*,-- shape resonances may contribute around 6 eV. A u*CH character of the resonances around 6 eV in CH3CN and CH3NC is indicated by the strong excitation of C-H stretching vibrations in the corresponding vibrational excitation resonances." The ET spectrum of CH,NC (Figure 1) is very similar to that of CH3CN, with the A* resonance located at the same energy within experimental accuracy. This finding is consistent with CH3N+=C- and CH3C=N valence bond structures for these molecules, in agreement with the structure4sand photoelectron data.46 The higher energy structures in the ETS of CH,NC are (42) Corey, E. J . Org. Synth. 1966, 46, 75. (43) Ng, L.; Baiaji, V.;Jordan, K. D. Chem. Phys. Lett. 1983, 101, 171. (44) Modelli, A.; Burrow, P. D. J . Electron Spectrosc. 1983, 32, 263. (45) Structure Data of Free Polyatomic Molecules; Landolt-Bornstein New Series 11; Hellwiege, K.-H., Hellwiege, A. M., Eds.; Springer: Berlin,
1988; Vol. 7.
EELS of CH3CN, CH3NC, CH3SCN, and CH3NCS similar in shape but occur at slightly lower energies than in CH3CN. The present results are in good agreement with the production of negative fragment ions at 3 eV and in the 6-12-eV energy range, in both acetonitrile and methyl isocyanide.26 This indicates that the resonance (and probably also the two resonances at higher energies) observed in the ET spectra follow a dissociative decay channel. In methyl thiocyanate a low-energy u*cs resonance is expected, associated with the -SCH3 group and there should be a sizeable stabilization of the empty MO’s through mixing with the l,~~ empty S3d and u*cs orbitals, as observed in t h i o b e n z e n e ~ ~and t h i o k e t o n e ~ . ~The ~ , ~bent ~ C-S-C geometry reduces the symmetry of CH3SCN to C, and lifts the degeneracy of the a*CNMO. Both the a’(a*) component lying in the C-S-C plane and the perpendicular a ” ( a * l component can be stabilized by interactions with S 3d orbitals of appropriate symmetry. The a’ orbital can be further stabilized by hyperconjugation with u*cs orbitals, while the a” can be destabilized by charge-transfer interaction with the filled sulfur lone pair orbital. The ETS of CH3SCN displays three distinct resonances. The lowest energy feature (0.87 eV) is ascribed to electron capture into the a’ molecular orbital, of mixed and u*cs character. The second resonance (2.37 eV) is assigned to electron capture into the a”(r*CN) orbital. The third resonance (3.6 eV) is associated with the a’ symmetry partner to the lowest energy a’ unoccupied MO. The a”(a*,-J resonance lies 0.45 eV lower than the a* resonance in CH3CN. In agreement with previous res u l t ~ , ~the~ stabilizing * ~ ~ . ~ ~effect of a*/S 3d mixing dominates the opposing effect of sulfur lone pair/a* mixing. Even considering that it is partly due to sulfur lone pair/a”(r*) interaction ~ eV) ~ in CH3SCN is very large the a’-a” splitting of the P * (1.5 when compared, for example, to the stabilization (ca. 0.5 eV) of the a*CO anion state of thioacetone produced by a*co/u*cs mixing.48 The large a*/u*interaction in CH3SCN appears to be associated with a very small energy gap between the interacting u* and a* fragment orbitals. To substantiate this, we note that ~ in (CHJ2S is located at 3.3 eV,30and the -CN the u * resonance group exerts a large stabilizing inductive effect (0.6 eV on the A2(a*) anion state in b e n z ~ n i t r i l e ~ ~Thus, ) . in the absence of a * C N / f f * a mixing, the u*cs orbital in CH3SCN would lie around 2.7 eV, very close to the orbital energy in CH3CN. Our spectral interpretation is further supported by ab initio calculations on the ground state of HSCH2CN,49which indicate a very strong a’(a*)/a’(u*cs) mixing, giving rise to a’/“’ a* splitting similar to those observed in the ETS of CH3SCN. Analogous u / a interaction is expected to stabilize the occupied a’(acN) orbital of CH3SCN. However, the a”(acN) M O is also strongly stabilized by mixing with the sulfur lone pair orbital, thus reducing the energy separation of the a’ and a” filled a orbitals. In agreement, the peak assigned to a ionization in the photoelectron spectrum of CH3SCNSodisplays a maximum at 12.9 eV (at 0.7 eV higher ionization energy than in CH3CN; see Figure 2) and a high-energy shoulder, instead of two resolved peaks. The probable range of the unresolved a’-”’ a splitting in CH3SCNSo is indicated by the hatching in Figure 2. A description of the electronic structure of CH3NCS is less straightforward than for the other three molecules. Although the N C bond length (1.21 AI8) is only 0.05 8, longer than that of the C N triple bond in CH3CN, the bent geometry at the nitrogen atom (1 40’) and the C S bond length (1.60 A, only slightly longer than that typical of a C=S double bond) suggest that the CH3N=C=S valence bond structure (which implies nondegenerate energies and different localization properties for the a‘ and a” a* (46) Brant, P. J. Elecfron Spectrosc. 1984, 33, 153. (47) Turner, D. W.; Baker, C.; Baker, A. D.; Brundle, C. R. Molecular Photoelectron Spectroscopy; Interscience: London, 1976. (48) Olivato, R.; Guerrero, S. A,; Modelli, A,; Granozzi, G.; Jones, D.; Distefano, G. J . Chem. Soc., Perkin Trans. 2 1984, 1505. (49) Guerra, M.; Distefano, G.; Jones, D.; Modelli, A,, manuscript in preparation. (50) Andreocci, M. V.; Bossa, M.; Furlani, C.; Piancastelli, M. N.; Caueltti, C.; Tarantelli, T. J . Chem. Soc., Faraday Trans. 2 1979, 75, 105. (Data on the empty orbitals was communicated privately.)
The Journal of Physical Chemistry, Vol. 93, No, 8, 1989 3071 CH$N
AE
4‘1 2
68
57 -
CH3NC CH3SCN CHSNCS 2 55 -
oS*x * 2 8 2 %I-
281
-
2 37 0.87 -
40
183, 0.81a, -
..3.3 .....
* ct
9 37
a,a
IO
a
b
C
iev)
Figure 2. Orbital correlation diagram for CH3CN, CH3NC, CH3SCN, and CH3NCS based on experimental ionization energies (IE) from PES (a, ref 47; b, ref 50; c, ref 5 2 ) , ISEELS term values (TV) and ETS attachment energies (AE). The core excitation term values are the average of the C 1s and N 1s term values. The hatched regions indicate the energy ranges of unresolved orbitals.
orbitals) is a better model of the ground state than CH3N+=
c-s-.
The ET spectrum of CH3NCSdisplays three resonances at 0.8 1, 1.83, and 4.0 eV. The AEs of the first two resonances are somewhat higher than those of a r*c4 MO destabilized by nitrogen lone pair/a* interaction (AE = 0.5 eVS1 in N,N-dimethylthioformamide and thioacetamide) and of the r*,-- M O in (t-Bu),C=NH (AE 1.29 eVsl), respectively. The destabilization of the first two anion states of methyl isothiocyanate with respect to the (a*cs)and ( T * ~ anion ~ ) states in the reference molecules cited above is consistent with the reduced N C bond length which increases the nitrogen lone pair/a’r*cs overlap and the antibonding character of the a“r*cN orbital. Similarly the reduction of the C S bond length with respect to CH3SCN can account for the destabilization of the u*cs resonance and for the corresponding stabilization of the bonding ucs orbital (see Figure 2). The present assignment for the ETS of CH3NCS is supported by ab initio and CNDO c a l c u l a t i ~ n swhich , ~ ~ predict a very small energy separation ( ~ 0 . 1eV) between the a’ and a” filled orbitals of CH3NCS but a large energy separation ( ~ 0 . eV) 7 between the empty a* orbitals, with a’ below a”. Moreover, the calculated wave-functioncoefficientss0indicate that the a”a* M O has mainly carbon and nitrogen character, while the a’a* can be described as primarily a a*cs M O mixed in an antibonding manner with the nitrogen lone pair orbital and in a bonding manner with u*CN orbitals. The different nature of the two “a*”orbitals is also consistent with the significantly different relative intensities of the first two resonances in the E T spectra of CH3SCN and CH3NCS. 3.2. Inner-Shell Excitation Spectra. Wide range C 1s oscillator strength spectra of CH3CN, CH3NC, CH3SCN, and CH3NCS are presented in Figure 3 while expansions of the discrete structure along with the results of curve fitting are plotted in Figure 4. Similar presentations of the N 1s spectra are given in Figure 5 (long range) and Figure 6 (curve fit analysis of the discrete structure). The energies, term values (TV = IP - E), and proposed assignments of the C 1s and N 1s spectral features for each molecule are listed in Tables 11-V. The S 2p and S 2s spectra of CH3SCN and CH3NCS are presented in Figure 7 on an asrecorded basis and as the converted oscillator strength in the S
-
(51) Modelli, A.; Jones, D.; Rossini, S.; Distefano, G. Tetrahedron 1984, 40, 3251. (52) Cradock, S.; Ebsworth, E. A. V.; Murdoch, J. D. J . Chem. Soc.,
Faraday Trans. 2 1972, 68, 86.
3072 The Journal of Physical Chemistry, Vol. 93, No. 8,1989
CH3NCS
I . . . . I , . . . I , . . . I . . . . I . . . . I
280
290
300 310 Energy (eV)
320
i 0
330
Figure 3. Oscillator strengths for C 1s excitation in acetonitrile, methyl isocyanide, methyl thiocyanate, and methyl thioisocyanate derived from dipole-regime electron energy loss spectroscopy (2.5 keV final electron energy, 2' scattering angle). The conversion from the raw data involves subtraction of an extrapolation of the lower energy continua (see Figure 7); multiplication by a kinematic correction factor (which tilts the spectrum by 5-10% over the 60-eV range plotted); and normalization to calculated atomic Is ionization oscillator strengths at 25 eV above the IP, scaled to the number of atoms. The solid line at the begihing of the hatching indicates the ionization thresholds determined by XPS. The numbered vertical bars indicate the positions of the features determined via curve fitting. TABLE II: Energies ( E , eV), Term Values (TV, eV), and Proposed As.dgnments of Fentures in the C 1s and N 1s Spectra of Acetonitrile
(CH,W
c 1s proposed assignt CN CHS
TV 1 2 sh 3 4 5 6 sh
CN IP CH3 IPb I 8
E (fO.l) 286.90 (8)' 288.1 289.07 289.8 290.8 291.5 292.44 292.98 297.2 308.0 (8)
CN
CH3
5.5 3.4 2.6 1.6
Hitchcock et al.
286
288
290
292
Energy (eV)
Figure 4. Expansions and curve fits for the discrete C Is excitations in CH3CN, CH3NC, CH3SCN, and CH3NCS. Crosses, data; heavy line, total fitted curve; lighter solid line, components. A slight asymmetry on the low-energy side of the first peak has been fitted by an additional Gaussian. I " " I ' " ' I " " I ' ' ' " "
I
6
I 23156
A
4
ir*
4.9 4.0 3.2 2.2 1.5
3p
3s u*CH
3P u*C< 4p
u*C
