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The electron affinities of Si2H2, Si2H3, and Si2H4
Volume 192, number 5,6
CHEMICALPHYSICSLETTERS
15 May 1992
The electron affinities of Si2H2, Si2H3, and SiEH4 J o s e f Kalcher a n d Alexander F. Sax InstitutJ~r Theoretische Chemie, Universitiit Graz, Mozartgasse 14, A-8010 Graz, Austria
Received 24 January 1992
The three title compounds are found to form stable negativelycharged states. Si2H2exhibits two anion states (2As and 2Bs) correspondingto adiabatic electronaffinities (EAs) of 1.76 and 1.23 eV. Si2H3has two stable anion states ( tA and 3A) associated with EAs of 2.20 and I. 17 eV. Disilene shows a remarkablyhigh adiabatic EA of 1.03 eV, but this electron attachment energyis very sensitiveto the out-of-planebending of the molecule.
1. Introduction Multiply bonded silicon hydrides have frequently been investigated during the last decade. The main theoretical interest focused on the similarities versus dissimilarities with respect to the corresponding carbon hydrides. Especially, disilyne and disilene, Si2H2 and SiEH4, have attracted much attention [ 1-14]. Disilyne has been shown to have a doubly bridged C2v singlet ground-state structure and the most stable triplet state corresponds to the disilavilylidene isomer, which is effectively planar but exhibits an extremely shallow out-of-plane bending potential. Recently SiEH2 has been detected in the gas phase and the structural parameters, derived from the microwave spectrum are found to be in good agreement with the predicted di-bridged geometry [ 15 ]. Although the experimental studies on the heavily substituted disilenes indicate planar geometries, and thus mimick ethylene-like structures, the theoretical investigations on the Si2H4 parent provide evidence that the unsubstituted species has a significantly nonplanar C2h ground-state structure. The barrier to planarity, however, is rather small, so that large substituents may easily enforce planarity upon the disilene moiety [ 9,10 ]. Most studies on the multiply bonded silicon hydrides performed hitherto have concentrated on the neutral species and were devoted to determining the ground-state structures or to estimating the corresponding singlet-triplet splittings. In a previous study
on disilavinylidene we could show that this species has a pronounced propensity to attach a surplus electron and to form a stable anion [ 16 ]. Several successive investigations on various silicon hydrides led us to the assumption that even "closed-shell" multiply bonded silicon hydrides may form stable anions. This study tackles the problem of electron affinities (EA) of such multiply bonded Si2Hn systems, which is of interest since ethene as well as acetylene are found to have only short-lived resonances and do not form bound negatively charged states.
2. Computational details In all calculations we employed the pseudopotential for silicon which was developed in our laboratory [ 9,17 ]. The basis set for silicon is the (3s, 3p) set described in ref. [9 ], augmented by two s-type (0.04, 0.015), two p-type (0.035, 0.012), and two d-type (0.35, 0. l ) functions. The hydrogen basis set consists of the (4s) Huzinaga set, augmented by one s-type (0.05) and one p-type (0.45) function. All calculations were performed with the program system MOLEKEL [ 18,19 ], which has been modified for the silicon pseudopotential. The geometry optimizations, which have been carried out with our analytical MCSCF gradient program, were completed when the residual forces were smaller than l0 -5 hartree/bohr. Harmonic vibrational frequencies were computed subsequently by numerical dif-
0009-2614/92/$ 05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
451
Volume 192, number 5,6
CHEMICALPHYSICSLETTERS
ferentiation of the analytical gradients, in order to establish the optimized stationary points as minima. The differential correlation energies were evaluated by taking into account all singly and doubly substituted configurations with respect to the corresponding reference wavefunctions. The results calculated within this approach are denoted by CI (SD). The unlinked cluster corrections were estimated by two methods. On the one hand we used the Langhoff and Davidson formula in the extended form as given by Bruna et al. [20], the results of which we denote by CI (SDQ), and on the other hand we adopted the CEPA-1 method [21]. Since both approaches yield almost identical results, we are sure that errors in the electron affinities due to size consistency are smaller than 3%. The reference wavefunctions used for the electron correlation calculations and the geometry optimizations were MCSCF expansions of the so-called CAS (n, m) type, where all configuration state functions that result from distributing n electrons among m active orbitals are taken into account.
3. Results and discussion
3.1. SieH7 The neutral Si2H2 species has been considered in previous theoretical investigations and two minima located on the singlet energy hypersurface. The most stable isomer has been assigned a doubly bridged C2v structure [ 1,3,4 ], which has been confirmed by microwave spectra [15]. The second minimum has been claimed for the ~A~ disilavinylidene C2v. The electron affinity of the disilavinylidene species has been treated in ref. [ 16 ]. Since this system exhibits an empty valence orbital, its rather pronounced electron affinity of more than 1.7 eV is not surprising. This special bonding situation in singlet Si=SiH2 allows even a stable 4A" (C~) excited anion state [ 16 ]. In a more refined CAS-SCF study it could be shown that trans bent CEh disilyne (~As) represents a further minimum on the singlet energy hypersurface [ 14 ]. The strongly bent structure, which exhibits an H-Si-Si bond angle of 123.9 °, gives rise to a lowlying vacant ag orbital. This orbital seems well suited to accept a surplus electron and thus form a stable 452
15 May 1992
Table 1 Geometry data of the (Czh) 2Ag and 2Bgstates of Si2H7 (distances in pro, angles in deg)
Si-Si Si-H H-Si-Si
2Ag
2Bg
220.2 152.1 107.7
232.6 153.9 96.4
Table 2 Adiabatic electron affinities Too (in eV) of multiply bonded silicon hydrides
H2Si=SiH~- (2A8)(C2h) H2Si=SiH- (IA) (CI) H2Si=SiH- (3A)(CI) HSi=SiH- (2As)(C2h) HSi=SiH- (2lg) (C2h) Si=SiH~" a) (2B2)(C2v) Si=SiH~ ") (4A") (C2v)
CI(SD)
CI(SDQ)
CEPA
0.79 2.01 0.95 1.49 1.01 1.58 0.68
1.00 2.16 1.12 1.72 1.23 1.71 0.64
1.03 2.20 1.17 1.76 1.23 1.73 0.66
a) Taken from ref. [ 16]. anion. Table 1 collects the most salient features of the negatively charged disilyne species. The Si-Si distance of the anion ground state is rather long, whereas the H-Si-Si angle is only 107.7 °. The corresponding geometry parameters in the ~A~ ground state of the neutral species are 8 pm smaller and 16 ° larger. The lengthening of the central bond in the anion is not surprizing, since the a s orbital is antibonding and moreover the attachment of the extra electron also increases the nonlinearity. As can be seen from table 2, the adiabatic electron affinity, calculated at the CEPA level of theory, is 1.76 eV and thus almost as large as the value found for the 2B2 state of the disilavinylidene anion. Since the latter exhibits a stable excited 4A" state which is at least 0.66 eV stable against loss of the electron, the question arose whether the disilyne anion might also form a stable excited state. The optimization of the 2Bg state (C2h) and the corresponding analysis of the harmonic vibrational spectrum revealed that it is also a minimum. The comparison of the collected geometry parameters in table 1 shows that the Si-Si distance in the anion excited state is 12 pm larger compared to the 2Ag anion ground state. This long central bond can be rationalized by the fact that the additional electron resides in the "antibonding 7t-
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CHEMICAL PHYSICS LETTERS
type" orbital. The adiabatic EA of disilyne to form the 2Bg excited state is calculated as 1.23 eV at both the CI(SDQ) and CEPA levels of theory and, thus, only 0.5 eV smaller than that of the most stable anionic state. This electron attachment capability to form either anion state depends, however, rather sensitively upon the non-linearity of the molecule and disappears completely as the H-Si-Si angle gets large. Concomitantly, we find a significantly negative electron affinity for the linear (Dooh) 2I-ls anion state at the CAS(5, 4) and CI(SDQ) levels of theory. It is worth noting that the most stable singlet structure, the di-bridged (C2v) disilyne conformer, is devoid of any propensity to bind an additional electron.
3.2. s~M~
Since Si2H3 owns a half-filled molecular orbital, it is not surprising that it can easily accomodate a further electron to form a stable anion. Expectedly, the lowest negatively charged (C~) ~A state is characterized by a rather pronounced adiabatic EA of 2.03 eV. The optimized geometry of Si2H~- is not very different from that of the neutral Si2H3 species (see ref. [ 14 ] ). Thus we note only two significant changes, i.e. the Si-Si bond lengthens to about 229 pm, the dangling Si-H bond is 3.7 pm longer and the corresponding H-Si-Si angle is reduced to 92.4 °, as can be expected from the strong localization of the surplus electron in the isocentric lone pair. This geometry should, however, not be taken too seriously, since the barrier to planarity of the ~A anion state is n o larger than 2 kJ/mol. Thus Si2H~- in its ground state can be termed a "floppy" anion. The next higher anion state, 3A, has been found stable too, and the corresponding value for the adiabatic EA amounts to 0.9 eV at the CI(SDQ) and CEPA levels of approximation. The geometry differs significantly from that of neutral as well as the ~A anion species, since all Si-H bonds are elongated and range from 151.7 to 152.2 pm, and on account of the fact that the additional electron occupies the "antibonding n-type" orbital, the central Si-Si bond is 239.2 pm and thus even longer than in disilane. The triplet state exhibits a somewhat larger barrier to
15 May 1992
planarity and we find the (Cs) 3A" state some 10 kJ/ mol above the (Cl) 3A minimum.
3.3. SieH7 The disilene molecule has been investigated at various levels of theory [9-14] and all the investigations performed with larger basis sets and including correlation effects agree that the ground state of Si2H4 is non-planar and shows a C2h structure. In contrast to C2H4, which provides no bound anion state, disilene does attach a surplus electron, which resides in the "anti-bonding" a s orbital and yields the 2Ag state. The anion retains C2h symmetry (table 3), the geometry, however, differs very strongly from that of neutral disilene. The Si-H bond distances are optimized as 151.1 pm and are only some 2 pm longer than in the uncharged species, but the Si-Si bond length in the anion turns out to be no less than 16 pm larger. Concomitantly, the out-of-plane angles O of the Sill2 groups with respect to the Si-Si moiety are some 65 °, which is almost the same as found for Sill7 [22] and which corresponds to almost twice the value in Si2H4 [ 14 ]. The adiabatic EA calculated at our highest level of theory is calculated as 1.03 eV, which turns out to be remarkably high. The linear synchronous transit potential (LST) energy curves depicted in fig. 1 show that the electron attachment propensity of disilene decreases quite strongly with decreasing O. Due to this behavior, the vertical attachment energy, evaluated for O= 35 °, is only 0.3 eV, i.e. only 30% of the adiabatic electron affinity, whereas the vertical detachment energy is found as 1.43 eV (see table 4). A further interesting quantity is the value of the out-of-plane angle O for which EA reaches zero. Since we have optimized the planar D2h disilene anion and found its energy significantly above that of disilene, the existence of a stable planar anion can be ruled out. From the LST Table 3 Geometry data of the 2Agstate of Si2H~- (distances in pm, angles in deg) Si-H Si-Si H-Si-Si oop z. @
151.1 235.0 100.0 64.5
453
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CHEMICAL PHYSICS LETTERS
15 May 1992
t h o u g h b o t h states are a s s o c i a t e d w i t h CI structures, the c o r r e s p o n d i n g barriers to planarity are q u i t e small. Si2H4 has b e e n f o u n d to attach a surplus elect r o n to f o r m the (C2h) 2Ag a n i o n , the g e o m e t r y o f w h i c h d e v i a t e s m u c h m o r e f r o m p l a n a r i t y t h a n that o f neutral disilene. T h e e l e c t r o n b i n d i n g force varies strongly, h o w e v e r , w i t h t h e t r a n s - b e n d i n g a n d e v e n v a n i s h e s for small o u t - o f - p l a n e angles.
-10.05 -ID.D6 -10.07 -10.08 -lO.Og -lO.lO -10.1!
-10.12
........
i ......
I0
,,,
,,b,,
20
30
4D
50
60
70
Acknowledgement
® Fig. 1. Linear synchronous transit (LST) potential energy curves for (a) Si2H4 and (b) Si2H~-. The horizontal axis denotes the out-of-plane angle O, which is defined as 180 ° - ~, where q~ is the angle between the Si-Si bond and the bisector of the Sill2 group. The vertical axis denotes the energy in au. The dotted segment of the anion potential curve for 0 ° ~
1.03 eV 1.43 eV 0.30 eV 19 °
analysis we can infer t h a t O > / 2 0 ° m u s t h o l d for the a n i o n to be stable. T h e s e findings c o u l d be i m p o r t a n t for the substituted, sterically c o n g e s t e d Si2X4 species, since m a n y o f these disilenes h a v e b e e n f o u n d to be o n l y slightly t r a n s - b e n t or e v e n planar. Since larger substituents d i s f a v o r a p u c k e r e d Si2X4 m o i e t y a n d p l a n a r o r o n l y slightly b e n t disilene a n i o n s exhibit significantly shorter d i s t a n c e s S i - S i t h a n e v e n (C2h) SizH4, we expect t h a t such Si2X~- a n i o n s s h o u l d be less stable t h a n Si2H~- itself.
4. Concluding remarks All i n v e s t i g a t e d Si2Hn species exhibit r a t h e r large a d i a b a t i c e l e c t r o n affinities. Si2H2 has two stable anion states, b o t h o f w h i c h e x h i b i t C2h s y m m e t r y , w h e r e a s the d i - b r i d g e d Czv singlet g r o u n d state does not b i n d an a d d i t i o n a l electron. F o r Si2H~- we f i n d two b o u n d negatively c h a r g e d states, IA a n d 3A. A1454
T h e c o m p u t e r t i m e m a d e a v a i l a b l e on the V A X 6000-410 at the E D V Z o f the K a d - F r a n z e n s - U n i v ersit~it G r a z is gratefully a c k n o w l e d g e d .
References [ 1 ] L.C. Snyder, Z.R. Wassermann and J.W. Moskowitz, Intern. J. Quantum Chem. 21 (1982) 565. [ 2 ] H. Lischka and H.-J. K6hler, J. Am. Chem. Soc. 105 (1983) 6646. [3] J. Kalcher, A.F. Sax and G. Olbrich, Intern. J. Quantum Chem. 25 (1984) 543. [4] J.S. Binkley, J. Am. Chem. Soc. 106 (1984) 603. [ 5 ] S. Koseki and M.S. Gordon, J. Phys. Chem. 92 (1988) 364. [6] H. Lischka and H.-J. K6hler, Chem. Phys. Letters 85 (1982) 467. [ 7 ] H.-J. KShler and H. Lischka, J. Am. Chem. Soc. 104 ( 1982 ) 5884. [8] K. Krogh-Jespersen, J. Am. Chem. Soc. 107 (1985) 537. [ 9 ] A.F. Sax, J. Comput, Chem. 6 ( 1985 ) 469. [ 10] G. Olbrich, Chem. Phys. Letters 130 (1986) 115. [ 11 ] H. Teramae, J. Am. Chem. Soc. 109 (1987) 4140. [12] D.A. Hrovat, H. Sun and W.T. Borden, J. Mol, Struct. THEOCHEM 163 (1988) 51. [ 13] K. Somosundram, R.D. Amos and N.C. Handy, Theoret. Chim. Acta 70 (1988) 393. [ 14] A.F. Sax and J. Kalcher, J. Mol. Struct. THEOCHEM 208 (1990) 123. [15] M. Bogey, H. Bolvin, C. Demuynck and J.L. Destombes, Phys. Rev. Letters 66 ( 1991 ) 413. [16] J. Kalcher and A.F. Sax, Chem. Phys. Letters 133 (1987) 135. [ 17 ] R. Janoschek, A.F. Sax and E.A. Halevi, Israel J. Chem. 23 (1983) 58. [ 18 ] H.-J. Werner and W. Meyer, J. Chem. Phys. 74 ( 1981 ) 5794. [ 19] H.-J. Werner and E.-A. Reinsch, J. Chem. Phys. 76 (1982) 3144. [ 20 ] B.J. Bruna, S.D. Peyerimhoff and R.J. Buenker, Chem. Phys. Letters 72 (1980) 278. [21 ] W. Meyer, Intern. J. Quantum Chem. 5 ( 1971 ) 341. [22] J. Kalcher, Chem. Phys. 118 (1987) 273.
CHEMICALPHYSICSLETTERS
15 May 1992
The electron affinities of Si2H2, Si2H3, and SiEH4 J o s e f Kalcher a n d Alexander F. Sax InstitutJ~r Theoretische Chemie, Universitiit Graz, Mozartgasse 14, A-8010 Graz, Austria
Received 24 January 1992
The three title compounds are found to form stable negativelycharged states. Si2H2exhibits two anion states (2As and 2Bs) correspondingto adiabatic electronaffinities (EAs) of 1.76 and 1.23 eV. Si2H3has two stable anion states ( tA and 3A) associated with EAs of 2.20 and I. 17 eV. Disilene shows a remarkablyhigh adiabatic EA of 1.03 eV, but this electron attachment energyis very sensitiveto the out-of-planebending of the molecule.
1. Introduction Multiply bonded silicon hydrides have frequently been investigated during the last decade. The main theoretical interest focused on the similarities versus dissimilarities with respect to the corresponding carbon hydrides. Especially, disilyne and disilene, Si2H2 and SiEH4, have attracted much attention [ 1-14]. Disilyne has been shown to have a doubly bridged C2v singlet ground-state structure and the most stable triplet state corresponds to the disilavilylidene isomer, which is effectively planar but exhibits an extremely shallow out-of-plane bending potential. Recently SiEH2 has been detected in the gas phase and the structural parameters, derived from the microwave spectrum are found to be in good agreement with the predicted di-bridged geometry [ 15 ]. Although the experimental studies on the heavily substituted disilenes indicate planar geometries, and thus mimick ethylene-like structures, the theoretical investigations on the Si2H4 parent provide evidence that the unsubstituted species has a significantly nonplanar C2h ground-state structure. The barrier to planarity, however, is rather small, so that large substituents may easily enforce planarity upon the disilene moiety [ 9,10 ]. Most studies on the multiply bonded silicon hydrides performed hitherto have concentrated on the neutral species and were devoted to determining the ground-state structures or to estimating the corresponding singlet-triplet splittings. In a previous study
on disilavinylidene we could show that this species has a pronounced propensity to attach a surplus electron and to form a stable anion [ 16 ]. Several successive investigations on various silicon hydrides led us to the assumption that even "closed-shell" multiply bonded silicon hydrides may form stable anions. This study tackles the problem of electron affinities (EA) of such multiply bonded Si2Hn systems, which is of interest since ethene as well as acetylene are found to have only short-lived resonances and do not form bound negatively charged states.
2. Computational details In all calculations we employed the pseudopotential for silicon which was developed in our laboratory [ 9,17 ]. The basis set for silicon is the (3s, 3p) set described in ref. [9 ], augmented by two s-type (0.04, 0.015), two p-type (0.035, 0.012), and two d-type (0.35, 0. l ) functions. The hydrogen basis set consists of the (4s) Huzinaga set, augmented by one s-type (0.05) and one p-type (0.45) function. All calculations were performed with the program system MOLEKEL [ 18,19 ], which has been modified for the silicon pseudopotential. The geometry optimizations, which have been carried out with our analytical MCSCF gradient program, were completed when the residual forces were smaller than l0 -5 hartree/bohr. Harmonic vibrational frequencies were computed subsequently by numerical dif-
0009-2614/92/$ 05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
451
Volume 192, number 5,6
CHEMICALPHYSICSLETTERS
ferentiation of the analytical gradients, in order to establish the optimized stationary points as minima. The differential correlation energies were evaluated by taking into account all singly and doubly substituted configurations with respect to the corresponding reference wavefunctions. The results calculated within this approach are denoted by CI (SD). The unlinked cluster corrections were estimated by two methods. On the one hand we used the Langhoff and Davidson formula in the extended form as given by Bruna et al. [20], the results of which we denote by CI (SDQ), and on the other hand we adopted the CEPA-1 method [21]. Since both approaches yield almost identical results, we are sure that errors in the electron affinities due to size consistency are smaller than 3%. The reference wavefunctions used for the electron correlation calculations and the geometry optimizations were MCSCF expansions of the so-called CAS (n, m) type, where all configuration state functions that result from distributing n electrons among m active orbitals are taken into account.
3. Results and discussion
3.1. SieH7 The neutral Si2H2 species has been considered in previous theoretical investigations and two minima located on the singlet energy hypersurface. The most stable isomer has been assigned a doubly bridged C2v structure [ 1,3,4 ], which has been confirmed by microwave spectra [15]. The second minimum has been claimed for the ~A~ disilavinylidene C2v. The electron affinity of the disilavinylidene species has been treated in ref. [ 16 ]. Since this system exhibits an empty valence orbital, its rather pronounced electron affinity of more than 1.7 eV is not surprising. This special bonding situation in singlet Si=SiH2 allows even a stable 4A" (C~) excited anion state [ 16 ]. In a more refined CAS-SCF study it could be shown that trans bent CEh disilyne (~As) represents a further minimum on the singlet energy hypersurface [ 14 ]. The strongly bent structure, which exhibits an H-Si-Si bond angle of 123.9 °, gives rise to a lowlying vacant ag orbital. This orbital seems well suited to accept a surplus electron and thus form a stable 452
15 May 1992
Table 1 Geometry data of the (Czh) 2Ag and 2Bgstates of Si2H7 (distances in pro, angles in deg)
Si-Si Si-H H-Si-Si
2Ag
2Bg
220.2 152.1 107.7
232.6 153.9 96.4
Table 2 Adiabatic electron affinities Too (in eV) of multiply bonded silicon hydrides
H2Si=SiH~- (2A8)(C2h) H2Si=SiH- (IA) (CI) H2Si=SiH- (3A)(CI) HSi=SiH- (2As)(C2h) HSi=SiH- (2lg) (C2h) Si=SiH~" a) (2B2)(C2v) Si=SiH~ ") (4A") (C2v)
CI(SD)
CI(SDQ)
CEPA
0.79 2.01 0.95 1.49 1.01 1.58 0.68
1.00 2.16 1.12 1.72 1.23 1.71 0.64
1.03 2.20 1.17 1.76 1.23 1.73 0.66
a) Taken from ref. [ 16]. anion. Table 1 collects the most salient features of the negatively charged disilyne species. The Si-Si distance of the anion ground state is rather long, whereas the H-Si-Si angle is only 107.7 °. The corresponding geometry parameters in the ~A~ ground state of the neutral species are 8 pm smaller and 16 ° larger. The lengthening of the central bond in the anion is not surprizing, since the a s orbital is antibonding and moreover the attachment of the extra electron also increases the nonlinearity. As can be seen from table 2, the adiabatic electron affinity, calculated at the CEPA level of theory, is 1.76 eV and thus almost as large as the value found for the 2B2 state of the disilavinylidene anion. Since the latter exhibits a stable excited 4A" state which is at least 0.66 eV stable against loss of the electron, the question arose whether the disilyne anion might also form a stable excited state. The optimization of the 2Bg state (C2h) and the corresponding analysis of the harmonic vibrational spectrum revealed that it is also a minimum. The comparison of the collected geometry parameters in table 1 shows that the Si-Si distance in the anion excited state is 12 pm larger compared to the 2Ag anion ground state. This long central bond can be rationalized by the fact that the additional electron resides in the "antibonding 7t-
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type" orbital. The adiabatic EA of disilyne to form the 2Bg excited state is calculated as 1.23 eV at both the CI(SDQ) and CEPA levels of theory and, thus, only 0.5 eV smaller than that of the most stable anionic state. This electron attachment capability to form either anion state depends, however, rather sensitively upon the non-linearity of the molecule and disappears completely as the H-Si-Si angle gets large. Concomitantly, we find a significantly negative electron affinity for the linear (Dooh) 2I-ls anion state at the CAS(5, 4) and CI(SDQ) levels of theory. It is worth noting that the most stable singlet structure, the di-bridged (C2v) disilyne conformer, is devoid of any propensity to bind an additional electron.
3.2. s~M~
Since Si2H3 owns a half-filled molecular orbital, it is not surprising that it can easily accomodate a further electron to form a stable anion. Expectedly, the lowest negatively charged (C~) ~A state is characterized by a rather pronounced adiabatic EA of 2.03 eV. The optimized geometry of Si2H~- is not very different from that of the neutral Si2H3 species (see ref. [ 14 ] ). Thus we note only two significant changes, i.e. the Si-Si bond lengthens to about 229 pm, the dangling Si-H bond is 3.7 pm longer and the corresponding H-Si-Si angle is reduced to 92.4 °, as can be expected from the strong localization of the surplus electron in the isocentric lone pair. This geometry should, however, not be taken too seriously, since the barrier to planarity of the ~A anion state is n o larger than 2 kJ/mol. Thus Si2H~- in its ground state can be termed a "floppy" anion. The next higher anion state, 3A, has been found stable too, and the corresponding value for the adiabatic EA amounts to 0.9 eV at the CI(SDQ) and CEPA levels of approximation. The geometry differs significantly from that of neutral as well as the ~A anion species, since all Si-H bonds are elongated and range from 151.7 to 152.2 pm, and on account of the fact that the additional electron occupies the "antibonding n-type" orbital, the central Si-Si bond is 239.2 pm and thus even longer than in disilane. The triplet state exhibits a somewhat larger barrier to
15 May 1992
planarity and we find the (Cs) 3A" state some 10 kJ/ mol above the (Cl) 3A minimum.
3.3. SieH7 The disilene molecule has been investigated at various levels of theory [9-14] and all the investigations performed with larger basis sets and including correlation effects agree that the ground state of Si2H4 is non-planar and shows a C2h structure. In contrast to C2H4, which provides no bound anion state, disilene does attach a surplus electron, which resides in the "anti-bonding" a s orbital and yields the 2Ag state. The anion retains C2h symmetry (table 3), the geometry, however, differs very strongly from that of neutral disilene. The Si-H bond distances are optimized as 151.1 pm and are only some 2 pm longer than in the uncharged species, but the Si-Si bond length in the anion turns out to be no less than 16 pm larger. Concomitantly, the out-of-plane angles O of the Sill2 groups with respect to the Si-Si moiety are some 65 °, which is almost the same as found for Sill7 [22] and which corresponds to almost twice the value in Si2H4 [ 14 ]. The adiabatic EA calculated at our highest level of theory is calculated as 1.03 eV, which turns out to be remarkably high. The linear synchronous transit potential (LST) energy curves depicted in fig. 1 show that the electron attachment propensity of disilene decreases quite strongly with decreasing O. Due to this behavior, the vertical attachment energy, evaluated for O= 35 °, is only 0.3 eV, i.e. only 30% of the adiabatic electron affinity, whereas the vertical detachment energy is found as 1.43 eV (see table 4). A further interesting quantity is the value of the out-of-plane angle O for which EA reaches zero. Since we have optimized the planar D2h disilene anion and found its energy significantly above that of disilene, the existence of a stable planar anion can be ruled out. From the LST Table 3 Geometry data of the 2Agstate of Si2H~- (distances in pm, angles in deg) Si-H Si-Si H-Si-Si oop z. @
151.1 235.0 100.0 64.5
453
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15 May 1992
t h o u g h b o t h states are a s s o c i a t e d w i t h CI structures, the c o r r e s p o n d i n g barriers to planarity are q u i t e small. Si2H4 has b e e n f o u n d to attach a surplus elect r o n to f o r m the (C2h) 2Ag a n i o n , the g e o m e t r y o f w h i c h d e v i a t e s m u c h m o r e f r o m p l a n a r i t y t h a n that o f neutral disilene. T h e e l e c t r o n b i n d i n g force varies strongly, h o w e v e r , w i t h t h e t r a n s - b e n d i n g a n d e v e n v a n i s h e s for small o u t - o f - p l a n e angles.
-10.05 -ID.D6 -10.07 -10.08 -lO.Og -lO.lO -10.1!
-10.12
........
i ......
I0
,,,
,,b,,
20
30
4D
50
60
70
Acknowledgement
® Fig. 1. Linear synchronous transit (LST) potential energy curves for (a) Si2H4 and (b) Si2H~-. The horizontal axis denotes the out-of-plane angle O, which is defined as 180 ° - ~, where q~ is the angle between the Si-Si bond and the bisector of the Sill2 group. The vertical axis denotes the energy in au. The dotted segment of the anion potential curve for 0 ° ~
1.03 eV 1.43 eV 0.30 eV 19 °
analysis we can infer t h a t O > / 2 0 ° m u s t h o l d for the a n i o n to be stable. T h e s e findings c o u l d be i m p o r t a n t for the substituted, sterically c o n g e s t e d Si2X4 species, since m a n y o f these disilenes h a v e b e e n f o u n d to be o n l y slightly t r a n s - b e n t or e v e n planar. Since larger substituents d i s f a v o r a p u c k e r e d Si2X4 m o i e t y a n d p l a n a r o r o n l y slightly b e n t disilene a n i o n s exhibit significantly shorter d i s t a n c e s S i - S i t h a n e v e n (C2h) SizH4, we expect t h a t such Si2X~- a n i o n s s h o u l d be less stable t h a n Si2H~- itself.
4. Concluding remarks All i n v e s t i g a t e d Si2Hn species exhibit r a t h e r large a d i a b a t i c e l e c t r o n affinities. Si2H2 has two stable anion states, b o t h o f w h i c h e x h i b i t C2h s y m m e t r y , w h e r e a s the d i - b r i d g e d Czv singlet g r o u n d state does not b i n d an a d d i t i o n a l electron. F o r Si2H~- we f i n d two b o u n d negatively c h a r g e d states, IA a n d 3A. A1454
T h e c o m p u t e r t i m e m a d e a v a i l a b l e on the V A X 6000-410 at the E D V Z o f the K a d - F r a n z e n s - U n i v ersit~it G r a z is gratefully a c k n o w l e d g e d .
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