J. Phys. Chem. 1993,97, 1019-1024
1019
Ab Initio MO Studies on S i a h SiHJGeHs, and GezHs Radical Anions as Prototypes of Polymer Anions with Silicon and Germanium Backbones Tsukasa Tada' and Reiko Yoshimura Toshiba Research d Development Center, 1 Komukai Toshibacho, Saiwai- Ku, Kawasaki- City, Kanagawa 21 0. Japan Received: October 12, 1992
Disilane, silylgermane, and digermane radical anions have been studied by ab initio molecular orbital theory, using the unified level of basis set as prototypes of polymer anions with Si and Ge backbones. These prototype anions have the same kind of minimum geometry (C2hor Czh-like symmetry) with an unpaired electron occupying the S i S i , Si-Ge, or Ge-Ge us antibonding orbital with a large Si or Ge s diffuse orbital contribution. Little Si 3d or Ge 4d orbital participation is found for these anions. All these anions can exist as kinetically bound anions with positive first vertical ionization potential values. The stability of an unpaired electron is in the order Si2H6-< SiH3GeH3- < Gel&-. The origin of this stability difference is also discussed. These results suggest that polymer anions with Si and Ge backbones have the same kind of molecular and electronic structures with no d r - d r conjugation and that their stability is in the order polysilane anion < Si-Ge copolymer anion < polygermane anion.
1. Introduction
Organopolysilanes that are soluble in conventional organic solvents were first synthesized by West and his group.' Subsequently, syntheses of organopolygermaneswere reported by the same group.' Since the success of these polymer syntheses, polysilanes,polygermanes, and Si-Ge copolymers have attracted considerable attention as so-called u-conjugated polymers as compared with conventional polymers with C-C sequences. They have somedistinguished properties, whichare different from those of the conventional C backbone polymers, and these properties have been extensively studied.3 One of these properties is that relatively stable polymer anions are generated by exposing polysilanes and polygermanes to ionizing radiation." It was reported that both aliphatic and aromatic polysilane anions did not decay within 150 nsa4 The half-life of polysilastyrene anions was reported to be 30 ps at room temperat~re.~ Poly(dibutylgermane) radical anions were also reported to be observed by pulse radiolysis studies.6 However, such stable polymer anions have not been observed for analogous aliphatic hydrocarbon polymers. In this respect, polysilanes and polygermanes make a sharpcontrast toconventional C backbone polymers. However, the molecular and electronic structures of these stable polymer anions are not fully understood so far. In a previous paper,' we reported the molecular and electronic structures of the disilane (Si2H6) radical anion as a prototype of polysilane anions. This anion was shown to be a bound anion with a positive first vertical ionization potential (VIP) value. It was also shown to have a C2h minimum geometry with an unpaired electron occupying a Si-Si antibonding b, orbital with a large Si s diffuse orbital contribution, suggesting that polysilane anions also have the same kind of molecular and electronic structures. This paper reports the results of our further studies on the same series of radical anions, digermane (Gt2H6) and silylgermane (SiHjGeH3)anions, together with the disilane anion,as prototypes of polymer anions with Si and Ge backbones. The differences and similarities of these three anions have been investigated. Investigations on these prototype anions provide insight into the molecular properties of polymer anions with Si and Ge backbones. From this point of view, our attention has been focused mainly on the following three aspects. (1) What are the molecular and electronicstructuresof these prototype radical anions? (2) What are the differences and similarities among these radical anions in termsof their stability or their electronic structures? (3) What OO22-3654/58/2097-1019$04.OO/O
kind of molecular and electronic structures are expected for polymer anions with Si and Ge backbones?
2. Metbod of Calculation Ab initio molecular orbital calculations were performed by using the GAUSSIAN 82,8 86,9 and 9010 programs. The determination on the stationary points of the three prototype anions and their neutral parent molecules was carried out by using analytical gradient techniques" at the Hartrce-Focklevel. Vibrational frequency analyses were carried out to ensure true minimum geometries. The minimum geometries were further reoptimized at the MP2I2J3(with all electrons correlated) level. Energy quantities, such as adiabatic electron affinity (EA) and the first VIP, werecalculated at the MP4SDTQI2J3(frozen core) level on the MP2 geometries. To compare the MP4SDTQ values with other post-Hartrec-Fock calculated values, the QCISD(T)14 and SDCI level calculations were also carried out on the MP2 geometries. The calculated ( S Z ) values for radical anions are in the range 0.75-0.79, indicating that there exists no serious spin contamination in our UHF and UHF based post-HartreeFock level calculations. Charge distributions were estimated on the basis of the Mulliken population analysis.I5J6 In the previous paper,' the 6-3 1++G(2d,p)17 basis set was employed for the calculation of the disilane anion. To compare the three prototype radical anions, however, a unified level of basis sets must be employed. From this point of view, we employed the double-{ quality basis set containing Si, Ge s and p diffuse functions throughout this work. For Si and H atoms, DunningHuzinaga16 (1 ls7p/6s4p) and (4s/2s) were employed, respectively. One set of d-polarization functions with an exponent of ad(Si) = 0.519and one set of p-polarization functions with the exponent of u,(H) = 1.0i8were added to the (1 ls7p/6s4p) and (4s/2s), respectively. For a Ge atom, the basis set used is the OlbrichZocontractionof Dunning primitive basis (13s9pSd/ 7sSp3d). One set of d-polarization functions with an exponent of ad(Ge) = 0.2519was added to this basis set. For both Si and Ge atoms, single s and one set of p diffuse functions were added to these double-{ quality basis set and their exponents were optimized for disilane and digermane radical anions, respectively. The optimized exponents are u,(Si) = 0.029, up(Si) * 0.030, a,(Ge) = 0.026, and u,(Ge) = 0.029. In the Dunning double-l; basis set, only one set of p diffuse functions was reported for a Si atom for the calculation of Si-containing anionic systems.'*
8 1993 American Chemical Society
1020 The Journal of Physical Chemistry, Vol. 97, No. 5, 1993 H Hi
(a)
(d)
C2h
C2v
H 3,423h
1.507hv$’
Si
\H
H// H
(b)
D3d
H
H
I
H
I
,iy09.1*
H0
H
(C) C 2 h
H H 127.6’fsi
H’
. H
I
I
sk115.2’
\H
H
H
(9) c s Figure 1. Stationary points of disilane radical anion (UHF/DZPD geometries).
This is due to the fact that the diffuse functions were optimized for an isolated Si atom. In the caseof thedisilane anion, however, a large Si s diffuse orbital participation is found in our previous calculation^.^ We assert, therefore, that the exponents of the diffuse functions should be optimized for molecular systems instead of isolated atoms for the molecular calculations. So the employed double-lquality basis set in this work is designated as (12s8pld/7s5pld) forSi, (14slOp6d/8~6p4d)forGe,and (4slp/ 2slp) for H atoms. This basis set is denoted as DZPD for convenience in this paper. 3. S t . t i ~ ~ rPoints y and Minimum Geometries of the Prototype Anions For the disilane anion, one additional stationary point with C, symmetry has been found besides the previously reported six stationary point^.^ So the disilane anion has seven stationary points with one D3d, one D3h, two C2hr two C,, and one C, symmetries. The calculated geometries (UHF/DZPD) at these seven stationary points are shown in Figure 1. All these seven kinds of structures have negative SOMO (singly occupied molecular orbital) eigenvalues. As shown in this figure, the seven stationary points exist on the potential energy surface along the SiH3rocking vibrations. The additional C, structure corresponds to an unsymmetrically distorted C, structure. However, an unsymmetrically distorted C2h structure is not found as a stationary point. As regards the previously reported six stationary points of the disilane anion, nearly the same geometries are obtained as those calculated by the 6-31++G(2d,p) basis set. The DZPD result provides somewhat longer S i S i bond distances than does theprevious6-31++G(2d.p) result. Thisdifferencecanbemainly attributed to the difference in the polarization functions for a Si
Tada and Yoshimura atom. As reported in the previous paper, only one C2h structure (a) has all real vibrational frequencies. Another c 2 h structure (c) has two imaginary vibrational frequencies (a, 1671’and b, 405i cm-I). One C, structure (d) also has two imaginary frequencies (a2 1081’and b2 2353 cm-I). Another Cb structure (f) has three imaginary frequencies (a2 59i. 340i, and bl 137i cm-I). The D3d structure (b) has four imaginary frequencies (doubly degenerate e, 127i and doubly degenerate e, 303i cm-I). The D3hstructure (e) has five imaginary frequencies (al” 27i, doubly degenerate e’ 124i, and doubly degenerate e” 302icm-I). On the other hand, the C, structure (8) has one imaginary frequency (a” 202i cm-I) along the SiH3 rotation about the S i S i axis. Considering the SiH3 rotation of the disilane anion, it is expected that the c 2 h minimum energy structure is distorted by this SiH3 rotation and comes back to the original C2, structure through a transition state. The C, structure (8) is considered to correspond to this transition state. The DZPD potential energy surface, therefore, has the same imaginary frequency number for eachstationarypoint asdoes the6-3 l++G(Zd,p) potentialenergy surface except for the C, structure (f). This C, structure has one additional imaginary frequency (b2) in the case of the 6-31++G(2d,p) basis set. Considering that the 6-31++G(d) potential energy surface also has the same three imaginary frequencieson the C2, ( f )stationary point, this differencebetween the DZPD and the 6-3 l++G(Zd,p) potential energy surfaces can also beattributed to thedifference in the polarization functions for a Si atom. As is expected, the same seven kinds of stationary points with one D3d, one D3h, two Czh, two C,, and one C, symmetries are also found in the digermane anion, indicating that the digermane anion has qualitatively the same kind of potential energy surface as does the disilane anion. The calculated geometries (UHF/ DZPD) at the seven stationary points are shown in Figure 2. The digermaneanion at all these stationary points has negative SOMO eigenvalues. Vibrational frequency analyses indicate that its minimum structure is only one C2h structure (a) among the seven stationary points, like the disilane anion. Another C2h structure (c) has two imaginary frequencies (a, 134i and b, 3241 cm-1). One structure (d) has two imaginary frequencies (a2 95i and b2 217i cm-I). Another C, structure (f) has four imaginary frequencies (a2 Sli, 309i, bl 119i and b2 130i cm-I). The D3d structure (b) has four imaginary frequencies (doubly degenerate e, 108i and doubly degenerate e, 2871’cm-I). The D3h structure (e) has five imaginary frequencies (a1” 32i, doubly degenerate e’ 102i, and doubly degenerate e” 2871 cm-I). The C, structure (g) has one imaginary frequency (a” 162icm-I). This C, structure also corresponds to a transition-state structure during the GeH3 rotation about the Ge-Ge axis. The number of imaginary frequenciesin the C, structure (f) is different from that of disilane anion in the DZPD potential energy surface but is the same as that of disilane anion on the 6-31++G(2d,p) potential energy surface. On the other hand, the silylgermane anion has six stationary points (four C, and two C3, structures) with negative SOMO eigenvalues. The calculated geometries (UHF/DZPD) at these six stationary points are also shown in Figure 3. Like other prototype anions, only structure a, corresponding to the C2h structure (a) in the disilane and digermane anions, has all real vibrational frequencies. Structure c, corresponding to one of the C2h structures in the remaining two prototype anions, has two imaginary frequencies (a” 149i, 3673 cm-I). One C3, structure (b), corresponding to the Djd structure of other prototype anions, has four imaginary frequencies (a couple of doubly degenerate e 117i and 2971cm-I). Another C3, structure (e), corresponding to the D3hstructure of other prototype anions, has five imaginary frequencies (al 27i and a couple of doubly degenerate e 113i, 2971 cm-1). Structures d and f correspond to the C, structure (8) in the disilane and digermane anions, each having one imaginary
Ab Initio MO Studies on Si2H6, SiHjGeH3, and Ge2H6
The Journal of Physical Chemistry, Vol. 97, No. 5, 1993 1021
H i.%iA
\ 99.0'
H
Y
H
I llyJ%H
'> 113.9'
128.4-f'
H
H/
128.6-
\H
Figure 3. Stationary points of silylgermane radical anion (UHF/DZPD geometries).
H/~
