J. Phys. Chem. 1996, 100, 8681-8691
8681
Matrix-Isolation IR and UV Spectra of Si3H8 and Si4H10: Isomers and Conformers of Oligosilanes Bo Albinsson,1a Hiroyuki Teramae,1b Harald S. Plitt,1a Lisa M. Goss,1a Hubert Schmidbaur,1c and Josef Michl*,1a Department of Chemistry and Biochemistry, UniVersity of Colorado, Boulder, Colorado 80309-0215; NTT Basic Research Laboratories, Morinosato, Atsugi, Kanagawa 243-01, Japan; and Department of Inorganic Chemistry, Technical UniVersity Munich, D-85747 Garching, Germany ReceiVed: December 14, 1995X
Matrix-isolation IR and UV spectra of Si3H8, i-Si4H10, and the two conformers of n-Si4H10 have been recorded. A quantitative separation of the IR spectrum of n-Si4H10 into contributions from the anti and gauche forms was accomplished by a combination of matrix annealing and selective monochromatic photodestruction experiments. A qualitative separation of their UV spectra was achieved as well. The IR spectra of Si3H8, i-Si4H10, and the two conformers of n-Si4H10 have been assigned by comparison with results of ab initio calculations, which reproduce the frequencies and even the relative intensities quite well. The calculations predict dihedral angles ω of 180° and 57° for the anti and the gauche conformer of n-Si4H10, respectively, and confirm earlier predictions of nearly equal stability for an isolated molecule. In the matrix, the anti conformer is more stable. The conformational effects on the UV spectrum of n-Si4H10 are not those anticipated from simple models of the Sandorfy or ladder C type, in that it is primarily not the energy but the intensity of the low-energy excited singlet states that depends strongly on the SiSiSiSi dihedral angle ω. This result is interpreted in terms of data from 6-in-8 CASSCF 6-31G* calculations, which predict an avoided crossing between a strongly allowed σσ* B state and a very weakly allowed σπ* B state as ω changes, with the former lower in energy at 180° and the latter lower at 0°. Consequences for attempts to understand the effects of conformation on optical spectra of polysilanes are noted.
Introduction Spectroscopic properties of the saturated silicon hydrides, SinH2n+2, have been of considerable interest. Oligosilanes (n up to a dozen or two) and polysilanes (n very large), which contain chains of silicon atoms connected by single bonds, with the remaining valences saturated with hydrogen, are the silicon analogues of alkanes and polyethylene, respectively. Only the first two members of the series, SiH4 and Si2H6, are readily available commercially and have been studied in great detail. Here, we turn attention to trisilane, Si3H8, normal tetrasilane, n-Si4H10, and isotetrasilane, i-Si4H10 (Figure 1). A good understanding of the vibrational spectra of these lower oligosilanes, as a function of chain length, conformation, and branching, is likely to be useful in the investigation of the mechanism of chemical vapor deposition of SiH4 and of the structure of amorphous silicon and hydrogenated amorphous silicon, in which these units presumably occur as partial substructures.2 Low-resolution gas-phase IR and neat liquid Raman spectra of Si3H8,3-5 n-Si4H10,4-6 and i-Si4H104-6 were reported in the 1960s, and even most recent theoretical work on the force field for oligosilanes and polysilanes7,8 had only these early spectra to rely on. Although the general range of the characteristic vibrational frequencies is clear from this prior experimental work, little or nothing is known about details, such as conformational effects on the vibrational modes. The electronic spectra of oligosilanes and polysilanes reveal interesting information about their intriguing electronic structure and, in particular, σ delocalization. Gas-phase absorption spectra of Si3H8, n-Si4H10, and i-Si4H10 were first reported in a dissertation,5,9 and the spectrum of Si3H8 has more recently been remeasured over a larger wavelength range.10 The absorption X
Abstract published in AdVance ACS Abstracts, April 15, 1996.
S0022-3654(95)03731-2 CCC: $12.00
Figure 1. MP2/6-31G** optimized geometries of Si3H8, i-Si4H10, and n-Si4H10.
bands are broad and featureless, and assignments both to valence11 and to Rydberg10,12 transitions have been proposed. The electronic spectra of the much more stable fully alkylated derivatives are similar to those of the parent oligosilanes. They have been investigated by a large number of authors and show most interesting conformational properties.13 (These are the structures on which the initial observation of the then unexpected low-energy transitions were made.14) It appears desirable to investigate the unsubstituted parent structures in more detail, both because of their intrinsic importance and since they are much more tractable theoretically. © 1996 American Chemical Society
8682 J. Phys. Chem., Vol. 100, No. 21, 1996
Albinsson et al.
The structures of the two conformers of n-tetrasilane have been deduced from electron diffraction measurements combined with ab initio HF/6-31G* calculations with assumed dihedral angles of 60° and 180°.15 The photoelectron spectra of the linear oligosilanes up to n-pentasilane have also been measured16 and have attracted recent attention of theoreticians.11,12,17 Materials and Methods The matrix isolation experiments used nitrogen or argon gas (both from US Welding, 99.999% purity). Samples of n-Si4H10 and Si3H8 were obtained from the collection of the late Prof. Fehe´r (University of Cologne). They were contained in canisters that also contained other short-chain oligosilanes and hydrogen and, in the case of n-Si4H10, also a small amount of i-Si4H10. They were purified by vacuum distillation, mixed with the matrix gas in a ratio of about 1:1500, and slowly deposited on a polished CsI or CaF2 window, depending on whether IR or UV spectra were to be obtained. The flow rate was about 2 mmol/h. The deposition window was mounted on an oxygenfree copper sample holder which was attached to the second stage of a closed-cycle helium cryostat (Air Product Displex 202) and was maintained at 25 K during the deposition. The matrix held at 12 K was irradiated by the output from either a low-pressure iodine lamp (48 500 cm-1, 206.2 nm) or a lowpressure mercury lamp (54 080 cm-1, 184.9 nm) powered with a microwave generator (Microtron 200). Both lamps were home-built and had Suprasil quartz windows. A 1 in. airsaturated water filter was used with the iodine lamp to reduce the heat load on the sample and to block the unwanted 56 090 cm-1 (178.3 nm) radiation. The mercury lamp was used without a filter since tetrasilane does not absorb the unwanted intense 39 430 cm-1 (253.6 nm) line. Mid-IR and far-IR spectra were measured at 1 cm-1 resolution with Nicolet 800 and 20F FTIR spectrometers, respectively. The mid-IR spectrometer was equipped with a wide-range nitrogencooled MCT detector and the far-IR vacuum spectrometer with a liquid helium cooled Ge bolometer (Infrared Laboratories, Inc.). Vacuum-UV spectra were measured using a 1 m evacuated monochromator with MgF2 optics equipped with a deuterium lamp and a Hamamatsu R972 photomultiplier. The UV measurements were done with a constant slit width of 0.25 mm. Computations were performed on IBM RS6000-550 and 590 workstations. Hartree-Fock and MP2 calculations used the 6-31G**18 basis set and the Gaussian 92 program.19 A range of other basis sets were employed in HF calculations of the IR spectra, including both diffuse functions and larger polarized bases, up to HF/6-31+G(3df,2p), but MP2/6-31G** gave the best agreement with experimental observations. The MP2 calculations used the frozen-core approximation. The potential energy was calculated as a function of the SiSiSiSi dihedral angle ω by freezing ω at different values and optimizing all the other coordinates. Full optimization was performed at the resulting local minima. The calculation of the electronic excitation energies and oscillator strengths as a function of ω used the ground-state geometries optimized at the MP2/6-31G** level. The MOLCAS2 program20 was used with the 6-31G*
Figure 2. Calculated ground state potential energy of n-Si4H10 as a function of the SiSiSiSi dihedral angle ω. The displacement of the two curves is arbitrary.
basis set21 to obtain 6-in-8 CASSCF results and with the internal “polarized basis set”22 (for H, 6s4p contracted to 3s2p; for other atoms, 13s10p4d contracted to 7s5p2d) to obtain CIS results. Results We have investigated the IR and UV spectra of n-Si4H10 and the IR spectra of Si3H8 and i-Si4H10. The IR spectrum of n-Si4H10 was separated into contributions due to the anti and the gauche conformers. All spectral assignments are based on ab initio calculations. The material is organized as follows. (i) The ground-state potential energy of n-Si4H10 is calculated as a function of the silicon backbone dihedral angle ω, and two stable conformers with almost the same energy are found. (ii) The effects of UV irradiation on the UV and IR spectra of matrix isolated n-Si4H10 are examined. The results demonstrate the presence of two spectrally distinct conformers of n-Si4H10 and the formation of i-Si4H10 as the initially observed photoproduct. (iii) The existence of two conformers in the deposited matrix is further confirmed by matrix annealing, which causes the IR peaks of the more stable conformer to grow and those of the less stable conformer to shrink. It does not affect the peaks of i-Si4H10 significantly. (iv) The IR spectrum of matrix-isolated Si3H8 is measured and found to compare well with the calculated IR spectrum. (v) The matrix IR spectrum of n-Si4H10 is separated into contributions from two spectrally distinct conformers, and these are identified as anti and gauche by comparison with their calculated spectra. Detailed assignments of the vibrational and electronic spectra are considered in the Discussion section. Calculated Potential Energy Curve. Figure 2 shows the relative potential energy of n-Si4H10 as a function of ω. All coordinates except ω are optimized in the nonstationary points along the curve, and the geometry is fully optimized in the potential energy minima. The two curves were obtained with the 6-31G** basis set, using the HF and the MP2 (frozen core) method, respectively. The effects of variation in the zero-point vibrational energy along the curve are negligible. Both calculations predict a dihedral angle of 180° at the anti geometry and close to 60° at the gauche geometry (Table 1).
TABLE 1: Calculated Eneriges of Si3H8, i-Si4H10, and the Conformers of n-Si4H10 HF/6-31G** Si3H8 i-Si4H10 a-Si4H10 g-Si4H10 a
MP2/6-31G**
ω, deg
Etot, au
Erel,a kcal mol-1
ω, deg
Etot, au
Erel,a kcal mol-1
b b 180.0 67.0
-871.397 750 -1161.483 196 -1161.482 089 -1161.481 895
b -0.69 0.00 0.12
b b 180.0 56.8
-871.689 122 -1161.868 908 -1161.866 249 -1161.866 314
b -1.70 0.00 -0.04
Energy relative to a-Si4H10. b Not applicable.
IR and UV Spectra of Si3H8 and Si4H10
Figure 3. UV absorption spectrum of n-Si4H10 in N2 (12 K), as deposited (A) and after irradiation at 48 500 cm-1 (B) or 54 080 cm-1 (C), as indicated by arrows.
This is similar to what was calculated earlier17 and to what is found for the carbon analogue, n-butane,23 but the barriers separating the anti minimum from the gauche minimum, and the two gauche minima from each other, are smaller in n-tetrasilane, presumably due to the much longer Si-Si bond length compared to C-C. The energies of the anti and gauche forms of n-Si4H10 are very close to each other, with anti lower by 0.12 kcal/mol in the HF calculation and gauche lower by 0.04 kcal/mol in the MP2 calculation. The calculated dihedral angles and energies are collected in Table 1. The near identity of the two energies was noted previously in single-point MP2/ 6-31G* calculations on HF/3-21G* optimized structures of anti and gauche Si4H1017 and in an electron diffraction study.15 Photochemical Transformations. Observed UV and IR Spectra. The anti and gauche conformers are expected to be present in nearly equal amounts in room-temperature Si4H10 gas. Except for the statistical factor of 2 in favor of the racemic gauche form, present as a mixture of equal amounts of leftand right-handed helical conformers (Figure 1), the difference in entropy between the two rotamers is expected to have negligible influence on the free energy difference. Thus, unless perfect annealing occurs upon deposition, both conformers should be trapped in the inert gas matrix. Figure 3A shows the UV spectrum of n-Si4H10 in N2 matrix at 12 K. Two bands at 49 500 and 55 600 cm-1 are observed in the spectrum of the initially deposited material. Upon irradiation at 48 500 cm-1 (Figure 3B) the lowest energy peak decreases quickly, and absorption at higher as well as lower energy increases. Irradiation at 54 080 cm-1 (Figure 3C) induces similar changes, but the high-energy part of the spectrum decreases faster in this case. Clearly, the resolution in the UV spectra is far from what is needed to observe different conformers and photoproducts separately. Figure 4 shows the IR spectrum of n-Si4H10 isolated in a N2 matrix at 12 K as initially deposited (A), after irradiation with a mercury lamp (B), and after subsequent irradiation with an iodine lamp (C). Parent silanes show Si-H stretches in the region 2100-2200 cm-1 and SiH deformations below 1000 cm-1.24 Irradiation at 54 080 cm-1 destroys predominantly one
J. Phys. Chem., Vol. 100, No. 21, 1996 8683
Figure 4. IR absorption spectrum of n-Si4H10 in N2 (12 K), as deposited (A) and after irradiation at 54 080 cm-1 (B) and subsequently at 48 500 cm-1 (C).
set of peaks, and irradiation at 48 500 cm-1 predominantly another, demonstrating the intrinsic heterogeneity of this material. With irradiation at 48 500 cm-1, a third set of weak peaks grows in intensity, whereas with irradiation at 54 080 cm-1 the intensity of this set decreases. A GC-MS analysis of a purified n-Si4H10 sample shows two peaks with a molecular ion mass corresponding to Si4H10, indicating the presence of two different isomers: n-Si4H10 and i-Si4H10. Indeed, it has been noted earlier that spontaneous isomerization and chain cleavage occur thermally in these compounds upon storage.25 The purification procedure apparently removes all short-chain silanes but leaves a small impurity of i-Si4H10. The expected dominant photochemical reaction should involve a chain abridgement in which trisilane, Si3H8, and silylene, SiH2, are initially formed.13,26 However, in the matrix IR spectra of the photolyzed material no new peaks of either Si3H8 or SiH2 are observed. Instead, peaks which we associate with the impurity, i-Si4H10, grow in intensity. We therefore propose that the initially formed trisilane and silylene, located in close proximity to each other in the matrix cage, react immediately to form n-Si4H10 and i-Si4H10 by silylene insertion into a Si-H bond. The insertion reaction of silylenes into SiH bonds is known to occur with an activation energy close to zero.27 hν
n-Si4H10 {\} [Si3H8 + SiH2] f i-Si4H10 Matrix Annealing Experiments. Figure 5 shows a spectrum from an annealing experiment in which Si4H10 was deposited together with N2 at 25 K and thereafter annealed to 31 K. The spectrum changes as the temperature is raised, with one set of peaks increasing in intensity (labeled a) and another decreasing (labeled g). This again indicates that the deposited Si4H10 is heterogeneous and presumably is trapped as a mixture of related conformers a and g. The a conformer is more stable than the g conformer in this matrix, and the heating lowers the viscosity and permits transformation of g into a. A small amount of the g conformer remains after annealing to 31 K, showing that there is a distribution of activation energies for the transformation,
8684 J. Phys. Chem., Vol. 100, No. 21, 1996
Albinsson et al. and return to 25 K does not restore the original spectrum. This shows that a and g are not at equilibrium in the matrix. Comparison of Figures 4 and 5 reveals that the set of peaks observed to decrease fastest upon irradiation with the Hg lamp (54 090 cm-1) is identical with the set of peaks that decrease in the annealing experiment (labeled g). This is most obvious for some of the peaks in the deformation region, e.g., at 875.0, 873.1, 742.4, and 691.2 cm-1. Also, the peaks that decrease upon irradiation with the iodine lamp (48 500 cm-1) are the same as those that increase in the annealing experiment (labeled a), e.g., those at 869.2 and 655.8 cm-1. This shows that the a conformer predominantly absorbs the low-frequency light (48 500 cm-1) whereas the g conformer predominantly absorbs the high-frequency light (54 090 cm-1). It should be noted, however, that both conformers are destroyed by the 48 500 cm-1 light. In fact, in an experiment where Si4H10 was irradiated with a Zn lamp emitting at 214 nm (46 770 cm-1), predominantly the a set but also the g set of peaks decreased in intensity, showing that both conformers contribute to the red edge of the first absorption band. There are some peaks in the deposited spectrum of Si4H10 that do not change significantly in intensity upon annealing. This is most apparent for the band at 862.6 cm-1. These peaks also grow in intensity upon irradiation with the iodine lamp. We propose that these peaks are due to i-Si4H10. Table 2 lists the few that we observe together with their MP2/6-31G** calculated frequencies (scaled by 0.94) and intensities. This result is in accordance with the proposed photochemical transformation and also with the irradiation-induced changes in the vacuum-UV spectrum (Figure 3), where an increased absorption near 54 000 cm-1 is observed. In the UV spectra measured on gaseous i-Si4H10 and n-Si4H10, the first absorption maxima occur at 190 nm (52 600 cm-1) and 205 nm (48 800 cm-1), respectively.5 This would at least partly explain the changes seen in our UV spectrum upon photolysis. Furthermore, it would account for the fact that the IR bands that are assigned to i-Si4H10 increase upon irradiation at 48 500 cm-1
Figure 5. IR absorption spectrum of n-Si4H10 in N2 deposited at 25 K (- - -) and the difference resulting from subtracting this spectrum from a spectrum recorded after annealing to 31 K until no further changes were observed (s). The amount n-Si4H10 deposited is about 20 times higher in part C than in parts A and B.
probably related to the existence of a variety of energetically distinct sites in the matrix. The annealing process is irreversible, TABLE 2: Vibrations of i-Si4H10
obsd calcda
IRb
mode
ν˜ , cm-1
I, km mol-1
1 2, 3 4, 5 6 7 8, 9 10, 11 12 13, 14 15 16, 17 18, 19 20 21 22, 23 24 25, 26 27 28, 29 30 31, 32 33 34, 35 36
55.0 80.7 88.6 106.7 340.2 419.5 443.6 457.4 564.7 565.2 698.9 866.3 907.2 928.0 930.2 934.9 935.2 2138.8 2169.0 2174.7 2182.8 2183.2 2186.4 2187.4
0.0 0.4 1.3 2.6 1.0 16.4 0.7 0.0 0.2 30.7 110.1 496.7 69.4 0.0 10.3 115.4 76.7 63.4 119.6 12.7 6.1 0.0 203.5 343.3
a
assignment a2 e e a1 a1 e e a2 e a1 e e a1 a2 e a1 e a1 e a1 e a2 e a1
SiH3 torsion SiH3 torsion asym skeletal def sym skeletal def sym SiSi str asym SiSi str SiH3 rock SiH3 def SiH3 def SiH3 rock SiH def sym SiH3 def sym SiH3 def asym SiH3 def asym SiH3 def asym SiH3 def asym SiH3 def SiH str sym SiH3 str sym SiH3 str asym SiH3 str asym SiH3 str asym SiH3 str asym SiH3 str b
ν˜ , cm-1
int
Ramanc ν˜ , cm-1
100 352 429.7
w
IRc ν˜ , cm-1
380d 436
450 567 568.5 680.2 862.6 905.3
w m s m
693
570 694
895
903
931
m
932
937
2132
w
2116
2115
2146
2150
c
MP2/6-31G**, calculated wavenumbers are scaled by a factor 0.94. In nitrogen matrix. Raman spectrum of neat liquid and IR spectrum of gaseous sample, from ref 5. d We suspect that this peak was due to an impurity.
IR and UV Spectra of Si3H8 and Si4H10
J. Phys. Chem., Vol. 100, No. 21, 1996 8685 TABLE 3: Vibrations of Si3H8 obsd calcda
IRb
assignment
mode ν˜ , cm-1 I, km mol-1 1 2 3 4 5 6 7 8 9 10 11
Figure 6. IR spectrum of Si3H8: experimental (A, in Ar at 12 K) and calculated (B, MP2/6-31G**, frequency scaling factor 0.94).
even up to high conversions, whereas they only increase in the early stage of irradiation at 54 080 cm-1, after which they decrease, and the bands due to n-Si4H10 increase instead. At high conversions other competing photochemical transformations occur which give rise to IR and UV bands not accounted for by the simple chain abridgement reaction. These have not been examined in detail. Observed and Calculated IR Spectra of Si3H8. The IR spectrum of Si3H8 observed in Ar at 12 K is shown in Figure 6A, and the calculated IR spectrum is shown in Figure 6B (MP2/ 6-31G**, frequencies scaled by 0.94). The deposition of Si3H8 traps only a single species, as judged from annealing experiments and by comparison with the calculated spectrum. Most peaks above 400 cm-1 can be assigned by comparison with the calculations, and the results are presented in Table 3. At 720 cm-1 a single transition is calculated (SiH2 wag), and a group of at least three closely spaced peaks is observed. This is attributed to matrix site effects. A similar observation is made for the corresponding normal mode in g-Si4H10 but not in a-Si4H10 (see below). Observed and Calculated IR Spectra of a- and g-Si4H10. The identification of IR transitions in n-Si4H10 is more complicated than in Si3H8, due to three factors: (i) the larger size of the molecule, (ii) the intrinsic heterogeneity of the spectrum due to the presence of a mixture of conformers, and (iii) the presence in our sample of a second structural isomer, i-Si4H10. It was therefore necessary to separate the observed IR spectrum into contributions from the anti and gauche conformers and from the iso isomer. The fine agreement between the (scaled) calculated and observed spectra of Si3H8 suggests that the three contributions to the observed spectra of the Si4H10 isomers and conformers can be assigned by reference to calculations. The separation of the IR spectrum into three contributions was based on the presence of clearly distinguishable peaks of three species. It was achieved by forming linear combinations of the spectrum of the deposited matrix and of the two spectra obtained after irradiation with the different lamps at different stages of phototransformation. First, the peaks due to i-Si4H10 were removed by forming suitable linear combinations between spectra of the deposited matrix and the phototransformed samples, one at a time. This produces two linearly independent spectra which ideally are mixtures of only the a and g conformers. Second, these spectra are linearly combined, such that the peaks due to either a or g are made to disappear, leaving
b
76.6 94.4 96.4 304.1 382.3 409.3 430.3 460.9 559.9 590.5 698.6
0.0 1.6 0.08 25.6 0.8 0.0 25.7 5.2 9.3 11.0 0.0
a2 a1 b1 b1 a1 a2 b2 b2 a1 b1 a2
SiH3 torsion skeletal def SiH3 torsion SiH2 rock sym SiSi str SiH2 + SiH3 def asym SiSi str SiH3 rock SiH3 rock SiH3 rock SiH2 twist
12
720.0
341.2
b2 SiH2 wag
13 14
875.7 885.3
370.8 219.8
b2 sym SiH3 def a1 sym SiH3 def
15
917.8
14.2
16 17 18 19 20 21 22 23 24 25 26 27
930.9 932.4 934.4 941.3 2155.4 2166.2 2171.6 2176.5 2184.3 2186.9 2188.6 2188.9
0.0 41.6 102.0 87.9 95.5 40.6 121.4 23.3 0.0 99.6 186.8 325.1
a1 SiH2 sciss a2 b2 b1 a1 a1 b1 b2 a1 a2 b2 a1 b1
asym SiH3 def asym SiH3 def asym SiH3 def asym SiH3 def sym SiH2 str asym SiH2 str sym SiH3 str sym SiH3 str asym SiH3 str asym SiH3 str asym SiH3 str asym SiH3 str
Ramanc ν˜ , cm-1 int ν˜ , cm-1 109
392 468.3 466 564
561.7 613.0
698 710.3 712.1 718.2 872.6 879.5 931.6 935.3 943.0 946.0 951.6 2143.6 2149.8 2164.4 2177.7
s s s
873
m
920
m m m m m m m
2130
2147
2184 sh 2188.2 s
a MP2/6-31G**, calculated wavenumbers are scaled by a factor 0.94. In argon matrix. c Raman spectrum of liquid sample, from ref 5.
Figure 7. IR spectra of a-Si4H10 (black bars) and g-Si4H10 (white bars): experimental (A, in N2 at 12 K) and calculated (B, MP2/6-31G**, frequency scaling factor 0.94).
the spectra of the pure g and a conformers, respectively. This treatment relies on the assumption that the system is composed of three and only three spectrally distinct principal components, i.e., the a and g conformers of n-Si4H10, and i-Si4H10. Figure 7A shows an example of such a resolution. The deformation region, where the separation between peaks of the different species is the largest, is used for judging the correct weighting factors. The resolution is satisfactory, but some base line artifacts due to either matrix site effects or photoproducts other than i-Si4H10 cannot be avoided. The separated spectra of the a and g conformers obtained from selective phototransformation agree perfectly with the spectra obtained from the matrix annealing experiments, and
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TABLE 4: Vibrations of a-Si4H10 obsd calcda
IRb assignment
mode
ν˜ , cm-1
I, km mol-1
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
23.9 62.7 82.9 91.3 115.8 287.1 334.3 365.2 427.7 435.8 461.9 468.7 532.1 571.6 663.8 666.3 717.6 766.7 874.2 881.9 914.9 916.1 932.5 932.7 935.8 937.3
0.0 0.9 0.07 0.0 0.0 27.8 0.0 0.0 5.0 9.7 69.8 0.0 0.0 0.0 5.8 522.4 0.0 0.0 686.9 0.0 0.0 25.4 0.0 99.9 0.0 204.4
au bu au bg ag au bg ag bu au bu ag ag bg au bu bg ag bu ag ag bu bg au ag bu
dihedral torsion skeletal def SiH3 torsion SiH3 torsion skeletal def SiH2 + SiH3 rock SiH2 + SiH3 rock sym SiSi str asym SiSi str SiH2 + SiH3 rock SiH2 + SiH3 def sym SiSi str SiH3 def SiH2 + SiH3 rock SiH2 twist SiH2 wag SiH2 twist SiH2 wag sym SiH3 def sym SiH3 def SiH2 sciss SiH2 sciss asym SiH3 def asym SiH3 def SiH3 + SiH2 def SiH2 + SiH3 def
27 28 29 30
2152.5 2153.7 2160.9 2167.3
155.9 0.0 0.0 98.9
bu ag bg au
sym SiH2 str sym SiH2 str asym SiH2 str asym SiH2 str
31
2172.1
199.8
bu
sym SiH3 str
32 33 34 35
2175.0 2186.3 2186.8 2186.9
0.0 0.0 0.0 276.8
ag ag bg bu
sym SiH3 str asym SiH3 str asym SiH3 str asym SiH3 str
36
2188.6
356.4
au
asym SiH3 str
ν˜ , cm-1
int
Ramanc ν˜ , cm-1
127 320.7
w 377 (427)
454.3 477.4
vw w 467 540
655.8
vs
869.2
vs
921.0
w
933.7
m
938.4 2136.8 2142.4 (2141.2)
(652) (682) 714 744 870 900 (917) 930
s m s 2128
2151.9 (2149.0) 2167.3 (2166.0) 2169.5
s s 2146
2178.1 (2176.0) 2181.6 (2180.1) 2184.0
s s
a MP2/6-31G**, calculated wavenumbers are scaled with a factor 0.94. b In nitrogen matrix. Wavenumbers in parentheses are from annealing experiment in cases where these differ from values found in the separation based on the irradiation experiments. c Raman spectrum on liquid n-Si4H10 from ref 5. The observed frequencies are for a mixture of a- and g-Si4H10. Peaks that are believed to belong only to the gauche conformer are shown in parentheses.
we will now turn to the identification of these conformers and to comparison with the scaled calculated spectra. Figure 7B shows the calculated IR spectra of a- and g-Si4H10. The spectrum of the a conformer matches almost perfectly the calculated spectrum of the anti conformer, and the spectrum of the g conformer matches the spectrum calculated for the gauche conformer. Tables 4 and 5 collect the observed and calculated frequencies. Some of the peaks that either are covered by unidentified photoproducts or are too weak to be observed in the fairly low-concentration matrices suitable for irradiation experiments (e.g., the whole far-IR region) are identified as anti or gauche by the annealing experiment (Figure 5). Discussion Structure and Stability. The computed structures of Si3H8 and i-Si4H10 are similar to those reported earlier.17 The simple textbook-like conformational behavior of n-Si4H10 (Figure 2) contrasts with our recent findings for Si4Me10, which shows two anomalies (first, a twisted anti minimum at 162°, and second, an ortho minimum at 91° in addition to a gauche minimum at 53°).28,29 In the matrix, the anti conformer of n-Si4H10 is clearly more stable than the gauche conformer. This does not necessarily
contradict the results of the best computations for an isolated molecule, which give gauche as slightly more stable. Without electron correlation, the anti form is computed to be a little more stable. The more compact gauche form apparently benefits from a differential stabilization by intramolecular van der Waals attractions. In a matrix, intermolecular van der Waals attractions are likely to provide differential stability to the less compact anti form. The anti form may also enjoy a better fit into the matrix cavity. IR Spectra. In the following, we propose detailed assignments of the spectra of all four compounds. The description of the normal modes is based on an inspection of the calculated normal modes, and the contributions from the dominant internal coordinates are listed. The SiH stretches are well isolated from the other normal modes due to the large mass difference between Si and H and occur over a narrow frequency range (22002150 cm-1). They are calculated and observed in four groups: asymmetric SiH3 stretches, symmetric SiH3 stretches, asymmetric SiH2 stretches, and symmetric SiH2 stretches, in the order of decreasing frequency. For i-Si4H10 there is an additional SiH stretch of the tertiary Si-H bond which is calculated and observed at a somewhat lower frequency than the other SiH stretching modes.
IR and UV Spectra of Si3H8 and Si4H10
J. Phys. Chem., Vol. 100, No. 21, 1996 8687
TABLE 5: Vibrations of g-Si4H10 obsd calcda
IRb assignment
mode
ν˜ , cm-1
I, km mol-1
1 2 3 4 5 6
26.9 73.1 74.8 102.6 123.2 302.8
0.03 0.5 0.2 0.06 2.9 25.8
a a b a b b
dihedral torsion SiH3 torsion SiH3 torsion skeletal def skeletal def SiH2 + SiH3 rock
7
340.3
6.7
a
SiH2 + SiH3 rock
8 9 10 11 12 13 14 15 16
364.9 422.1 440.3 454.4 472.7 531.3 563.2 656.0 683.0
0.02 6.3 21.7 0.01 1.0 14.6 0.8 74.6 11.3
a b b a a b a b a
sym SiSi str asym SiSi str SiH2 + SiH3 def SiH2 + SiH3 def sym SiSi str SiH2 + SiH3 def SiH2 + SiH3 rock SiH2 twist SiH2 twist
17
701.5
395.6
b
SiH2 wag
18 19 20 21 22 23 24 25 26
750.1 880.8 881.6 905.3 920.7 931.4 932.5 934.3 939.5
80.5 135.8 424.0 69.2 0.7 96.7 18.7 143.8 27.1
a b a b a b a b a
SiH2 wag sym SiH3 def sym SiH3 def SiH2 sciss SiH2 sciss asym SiH3 def asym SiH3 def asym SiH3 def SiH2 + SiH3 def
27 28
2151.2 2155.8
65.8 110.1
b a
sym SiH2 str sym SiH2 str
29 30 31 32 33 34 35 36
2164.4 2167.4 2171.7 2175.1 2184.9 2185.9 2187.9 2189.0
67.2 81.0 43.1 46.6 265.5 28.3 303.3 67.6
a b b a b a b a
asym SiH2 str asym SiH2 str sym SiH3 str sym SiH3 str asym SiH3 str asym SiH3 str asym SiH3 str asym SiH3 str
Ramanc ν˜ , cm-1
ν˜ , cm-1
int
308.6 327.7 332.0 365.9
w
459.8
w
537.7
w
(467) (540)
658.4 664.6 685.1 691.2 694.1 742.4 873.1 875.0 903.9 917.1 934.7
m m
652 682
s
(714)
m s s w m s
(744)
941.1 2127.9 2134.0 2144 2146 2149.8 2153.5 2157 2163 2167 2172 2177
w m m m m m m s s s m s
2184
s
127 w w
(377) 427
870 900 917 930
2128
2146
a MP2/6-31G**, calculated wavenumbers scaled by a factor 0.94. b In nitrogen matrix. c Raman spectrum on liquid n-Si H 4 10 from ref 5. The observed frequencies are for a mixture of a- and g-Si4H10 and peaks that are believed to belong only to the anti conformer are shown in parentheses.
SiH deformations span a considerably larger frequency range, 1000-300 cm-1. In some cases, they are coupled to Si backbone vibrations. However, in most cases the calculations identify the SiH deformations as either asymmetric, symmetric, or rocking deformations of the SiH3 groups and scissoring, wagging, twisting, or rocking of the SiH2 groups. In cases where we were unable to describe the normal mode in terms of these simplified internal coordinates, Table 2-5 list them as SiH2 + SiH3 deformations. In most cases, our descriptions of the normal modes agree with those given by previous force fields based either on HF/6-31G7 or on scaled HF/6-31G**8 calculations. Si3H8. The 27 normal modes of Si3H8 span the irreducible representations 9a1 + 5a2 + 6b1 + 7b2 in the C2V point group. All normal modes are Raman active, and all but the a2 vibrations are IR active. Seven Si-H stretches are calculated to have nonzero IR intensities, and the six observed bands are assigned as shown in Table 3. The assignments are based on the calculated energy order and on a comparison of the predicted and observed
intensities. The grouping of the SiH stretches is particularly obvious for Si3H8 (cf. Table 3 and Figure 6). All theoretically predicted vibrational transitions between 1000 and 400 cm-1 with nonzero calculated IR intensities have been observed. The observed bands around 933 cm-1 (SiH2 scissoring) and 715 cm-1 (SiH2 wag) are split, presumably due to the matrix cage effect. The overall agreement between the observed and MP2/6-31G** calculated spectra is excellent. This result gives us confidence in the much more difficult task of assigning the observed peaks to the components of the n-Si4H10 mixture. At a lower level of theory it was not sufficient to scale the force constants with a single factor in order to bring both the stretches and the deformations of Si3H8 into agreement with the observed spectrum. i-Si4H10. The isomer, i-Si4H10, is of C3V symmetry. The 36 fundamental vibrational modes span the irreducible representations 8a1 + 4a2 + 12e. The a1 and e fundamentals are both Raman and IR active, and the four a2 fundamentals are inactive in both Raman and IR. We have not measured the matrixisolated IR spectrum of pure i-Si4H10 but have observed some
8688 J. Phys. Chem., Vol. 100, No. 21, 1996 of its vibrational transitions since it is present as an impurity in our sample of n-Si4H10 and is also formed as a photolysis product of n-Si4H10. In addition to our experimental observations and calculations, Table 2 also includes the previously reported5 low-resolution IR and Raman spectra of i-Si4H10 in the gas and liquid phase. The bands assigned to i-Si4H10 from the matrix IR spectra are present in the deposited mixture of Si4H10 and grow upon irradiation with 48 500 cm-1 light. The intensities of these bands do not change appreciably in the annealing experiments and are thereby distinguished from the bands due to anti and gauche Si4H10. Comparison with the calculated IR spectra provides further support for the assignments. In the Si-H stretching region, overlap with the much more abundant n-Si4H10 conformers precludes the observation of asymmetric and symmetric SiH3 stretches of i-Si4H10. A weak band at 2132 cm-1 is assigned to the SiH stretch (a1) of the tertiary Si-H bond. Five bands observed below 1000 cm-1 (3a1 + 2e) are assigned to SiH3 and SiH deformations, and one is assigned to the degenerate SiSi stretch (e). The agreement with the calculated IR spectrum is excellent, and most bands have been observed before in the room-temperature gas IR and neat liquid Raman spectra.5 Surprisingly enough, the strongest band observed in 862.6 cm-1 and calculated at 866.3 cm-1 (∼500 km/mol) has not been reported before. n-Si4H10. The spectral separation of the IR spectrum of Si4H10 into contributions of the gauche and anti conformers has been presented in Figure 7, and additional information is obtained from the annealing experiment (Figure 5). The photochemical spectral separation works well in the SiH deformation region and partly also in the SiH stretching region. However, in the latter the high-energy part of the spectrum of the gauche conformer is distorted, and here the information from the annealing experiment is more reliable. The observed and calculated spectra are compiled in Tables 4 and 5. The anti conformer is calculated to be of C2h symmetry and the gauche conformer of C2 symmetry. The 36 normal modes span the irreducible representations 11ag + 7bg + 8au + 10bu for the anti conformer and 19a + 17b for the gauche conformer. In the centrosymmetric anti conformer all gerade fundamentals are Raman active and IR inactive, and the ungerade fundamentals are IR active and Raman inactive. In the gauche conformer all fundamental vibrational transitions are symmetry allowed in both IR and Raman. In the SiH stretching region, the general features observed in the separated spectra are well reproduced by the calculations. Many bands seem to be split either by matrix site effects or as a consequence of Fermi resonance between the SiH stretching fundamentals and overtones or combination bands of SiH deformations. This precludes detailed assignments and discussion of individual band assignments. The general pattern already discussed still applies. In the SiH deformation region between 1000 and 600 cm-1, the difference in symmetry of the two conformers is apparent. Thus, only six bands are calculated to have nonzero intensity and five are observed for the anti conformer, whereas 12 are predicted for the gauche conformer and, of these, 10 are observed. Both allowed asymmetric SiH3 deformations of the anti conformer are observed, at 938.4 cm-1 (bu) and 933.7 cm-1 (au). The corresponding transitions occur at 934.7 (b) and 941.1 (a) cm-1 in the gauche conformer. Of the two SiH2 scissoring vibrations, only the bu mode is allowed and is observed at 921.0 cm-1 in the anti conformer. Both are observed in the gauche
Albinsson et al. form, at 917.1 (a) and 903.9 (b) cm-1. The symmetric SiH3 deformations of the anti conformer also span the ag and bu irreducible representations, and only the bu deformation is observed. It corresponds to the strongest band in the whole IR spectrum of n-Si4H10, located at 874.2 cm-1. In the gauche form, both bands are observed, at 875.0 (a) and 873.1 cm-1 (b). Since they are observed and calculated to lie so close together, we cannot be sure that their assignments as a and b are not interchanged. The two SiH2 wagging modes again span the ag and bu irreducible representations in the anti conformer and a and b in the gauche conformer. The allowed bu mode of the former is observed at 655.8 cm-1 and is intense. In the latter, the two wagging modes are observed at 742.4 cm-1 (a) and around 690 cm-1 (b). The high-energy wag is broader than the other bands, and the wagging transition at 690 cm-1 is clearly split into at least three components. This also happens for the wagging transition in Si3H8 but not in a-Si4H10. It seems that the SiH2 wagging transition is particularly sensitive to the matrix environment and that the more compact g-Si4H10 and Si3H8 fit into a multitude of different matrix sites, thereby creating the split absorption envelope. The allowed au SiH2 twisting deformation is calculated to be weak in the anti conformer and is not observed. Both twisting modes are allowed and observed for the gauche conformer, at 664.6 (a) and 658.4 cm-1 (b). Below 600 cm-1 the IR intensities are at least 1 order of magnitude weaker. It was therefore not possible to resolve the spectrum using the selective photolysis of conformers, due to excessive absorbance in the UV region. Instead, all assignments are based on the annealing experiments (Figure 5C) and comparison with the calculated spectra. Three transitions are observed in the anti and six in the gauche form between 600 and 200 cm-1, and agreement with the calculations is satisfactory except for the lowest energy rocking deformation. The calculations predict that this transition should be 15 cm-1 lower in the anti than in the gauche conformer, but the lowest SiH deformation in the former is observed at 320.7 cm-1 and in the latter at 308.6 cm-1. The SiSi stretching vibrations are not observed for the anti conformer, but possibly one of them is observed for the gauche conformer at 365.9 cm-1. These are all predicted to be weak, and other close-lying SiH deformations with higher intensities interfere with their assignment. Raman Spectra. The only Raman spectra available at this time are those reported previously for neat room-temperature liquids.4,5 Even though they have not been separated into contributions of individual conformers and are not directly comparable with our matrix-isolation data, they provide useful additional information and have been included in Tables 2-5. For example, the so-called LAM (longitudinal acoustic mode) Raman band reported at 127 cm-1 for n-Si4H10 is calculated to be strong, with predicted frequencies at 115.8 and 123.2 cm-1 for the anti and gauche conformers, respectively. It would be useful to perform a matrix-isolation Raman study analogous to the present IR study, particularly for the low-frequency region. Electronic States of n-Si4H10. On the basis of a classical discussion of several lines of experimental evidence,30 it is common to assing the lowest few transitions of oligosilanes observed in condensed media to predominantly valence excitations. In the limit of planar geometry for the silicon skeleton, the low-energy transitions then primarily involve promotions from the highest occupied MO of σ symmetry into one of the low-energy orbitals of σ or π symmetry and are labeled σSiSi13 σ*SiSi or σSiSi-π* The skeletal MOs σSiSi and σ*SiSi are SiH. delocalized in the Si backbone, and the latter contain a
IR and UV Spectra of Si3H8 and Si4H10 significant contribution from the symmetric combinations of the SiH orbitals. The π* SiH MOs are delocalized over the lateral bonds to substituents and are represented by the antisymmetric combination of the SiH antibonding orbitals. It is usual to continue the use of these symbols even at twisted chain geometries, although their lower symmetry prevents a strict distinction of σ and π orbitals. Indeed, the popular Sandorfy C model31 frequently used in the discussion of polysilanes13 is based on the assumption that the σSiSi and σ* SiSi orbitals preserve their identity at all chain conformations, and so is the more recent ladder C model.32 Calculations for the anti conformer33 showed that in an isolated Si4H10 molecule Rydberg states should occur in the same region as the lowest valence states and mix with them strongly. This does not necessarily clash with the experimental evidence referred to above, since in condensed phase spectral Rydberg transitions are broadened significantly, shifted to higher energies, and difficult to observe. Certainly, since the lowest transitions of the anti and gauche conformers of Si4H10 observed in our experiments both contribute to the red edge of the first absorption band, their excitation energies must be very close to each other. Since the first ionization potentials of the two conformers are believed to differ significantly (gauche about 0.25 eV above anti17), this adds to the arguments30 against a predominantly Rydberg assignment. Thus, we agree with the authors of ref 11 that it is reasonable to continue the long tradition of interpreting the observed states as predominantly valence in character. (For a different view, see refs 10 and 12.) According to standard notions,13 the first electronic excitation energy in oligosilanes is conformationally dependent and significantly lower in the all-anti form than in forms that contain gauche links. A CIS/6-31G** calculation28 of the first four valence excitation energies and oscillator strengths for Si4H10 as a function of the backbone dihedral angle ω suggested strongly that this is not so in n-tetrasilane and presumably in other oligosilanes: although the calculated energies of the lowest two transitions do show a weak dependence on ω, it is primarily not the energy but the computed intensity of the electronic transitions that is affected strongly by conformation: at ω ) 0° the lowest energy transition has virtualy no intensity, and at ω ) 180° it is by far the most intense, while its energy differs by less than 0.1 eV. Clearly, the present experimental results support the proposed reinterpretation. We have now repeated the CIS calculation with a larger polarized basis set22 and obtained very similar results. We have also performed a 6-in-8 CASSCF/6-31G* calculation, and the results were again rather similar, with a somewhat larger variation in the energies of the lowest two transitions and with a Au rather than a Bu state lowest at the anti geometry. We have therefore selected these moderately different results for presentation in Figure 8, even though it is uncertain that the Au below Bu order is correct. All told, it appears that the qualitative trends in the valence space are independent both of the size of the valence basis set and of the size of the CI space but that the quantitative aspects, such as the detailed shape of the state energy curves and even the ordering within the lowest pair of states, depend on the details of the calculation. Clearly, the present computational results are still only preliminary, and we are continuing our efforts to improve them by adding diffuse functions to the basis set and increasing the size of CI. We plan to publish the results separately. Since it now appears likely that the qualitative picture shown in Figure 8 will not change, we use it as a basis for discussing the observed spectral results. All of the calculations place transitions to two valence A states and two valence B states
J. Phys. Chem., Vol. 100, No. 21, 1996 8689
Figure 8. Calculated singlet excitation energies (A) and oscillator strengths (B) of Si4H10 as a function of the SiSiSiSi dihedral angle, ω (6-in-8 CASSCF/6-31G* at ground-state optimized MP2/6-31G** geometries).
into the region of interest. The excitation energies of transitions into the two lowest excited states, 2A and 1B, are calculated not to change much as a function of ω and those of transitions into the next two, 3A and 2B, hardly at all. The energy order within each A,B pair depends on the calculational details as well as on the value of ω and is certainly not predicted reliably. In contrast, the computed oscillator strengths vary remarkably as ω changes. At the planar anti geometry (ω ) 180°) the transition to the 1B(Bu) σSiSi-σ* SiSi state is intense, and the transition to the 2B(Bg) σSiSi-π* SiH state is forbidden by symmetry. At the other planar extreme, the syn geometry (ω ) 0°), the transition to the 1B(B1) σSiSi-π* SiH state is extremely weak, and the transition to the 2B(B2) σSiSi-σ* SiSi state is intense. For intermediate twisted geometries both the 1B and the 2B transitions are predicted to have fairly high intensities, with 1B being the stronger at ω larger than about 90°. The transitions to A states have much lower intensities, particularly when ω is larger than about 90°. This behavior has been attributed28 to avoided state crossing as ω is varied from 0° to 180°. It is worth describing in somewhat more detail, since its existence follows from a simple MO correlation diagram and is thus likely to survive any further refinements in the calculations, and since it is this avoided crossing that invalidates the traditional spectral interpretation in terms of conformationally dependent excitation energies and substitutes a new interpretation in terms of conformationally dependent transition intensities. Actually, there are two avoided crossings, one within the pair of B states and one within the pair of A states. The MO correlation diagram is shown in Figure 9. The trend in the bonding orbital energy is taken from HF calculations. The qualitative positioning of virtual orbital energies at 0° and 180° is taken from energies of the four lowest energy configurations that result from single-electron promotions from the highest occupied molecular orbital (HOMO). It does not agree with the ordering of the HF virtual orbitals, which approximate the energy levels of radical anion and disregard the presence of a hole in the HOMO. For the same HF orbital energy difference, a 1σπ* configuration is at a lower energy than a 1σσ*
8690 J. Phys. Chem., Vol. 100, No. 21, 1996
Figure 9. Properties of Si4H10 as a function of the SiSiSiSi dihedral angle, ω (schematic): thick lines, MO energies, thin lines, intended MO correlations. Contributions of Si atoms to the MOs are shown schematically (those of SiH bonds to σ* orbitals are omitted). Singleelectron promotions from the HOMO and symmetries of the resulting configurations are shown for ω ) 0°, 0° < ω < 180°, and ω ) 180°.
configuration, due to systematic differences in the two-electron exchange integrals, Kσπ* < Kσσ*. (The σ and π* orbitals avoid each other in space, while σ and σ* do not.) As noted previously for the bonding σSiSi orbitals,17 the order of virtual orbital energies agrees with intuition in both the σ* (a1 below b2 and bu below ag) and the π* (b1 below a2 and au below bg) case, as do the different energies of each orbital in the two conformational limits, given the number and position of nodal planes and the sign of the interaction elements (resonance integrals). The latter are negative for vicinal (both σ and π), geminal, and syn-periplanar interactions and are positive for anti-periplanar ones.13,34 The relative energies of σ* and π* orbitals within each a,b pair depend on the details of the calculation, but this is unimportant for the argument. The thin lines in Figure 9 show the intended correlation for both σ and π orbitals. This is the correlation that holds for σ orbitals in the Sandorfy C31 and ladder C32 models, in which both σ and π orbitals involving bonds to substituents are ignored. At intermediate twist angles, however, only a 2-fold symmetry axis (vertical in the formulas in Figure 9) is preserved in reality. Orbitals of σ and π symmetry can no longer be distinguished, and the intended crossings are avoided. As ω changes from 0° to 180°, the lowest antibonding a1 MO starts as a σ* orbital and gradually loses all σ character to become a pure π* orbital, au. The lowest antibonding b1 MO starts as a π* orbital and gradually becomes a pure σ* orbital, bu. The upper a and b orbitals undergo complementary changes. Because of the avoided crossings, the energies of the resulting MOs (Figure 9) do not change much as ω changes. The behavior of the energies of the four singly excited configurations shown in Figure 9 follows that of orbital energies. The four low-energy excited states result from the four configurations, with considerable mixing within the A symmetry and B symmetry configuration pairs at certain intermediate twist angles. The picture is somewhat more complicated than shown in Figure 9, in that at ω < 30° another singly excited configuration of A symmetry (A2 at ω ) 0°) drops in energy enough to dominate the 3A state. This is an excitation from the second highest occupied
Albinsson et al. MO, a σ orbital of b2 symmetry, into the lowest π* orbital (b1). This b2 orbital contains a single node and is strongly destabilized at small values of ω by a syn-periplanar interaction.17 In a σ delocalized system, oscillator strength is carried primarily by that one-electron σσ* excitation in which σ and σ* differ in a single node. At 0° twist, this is the a1 to b2 promotion, and it produces the higher energy B2 state. At 180° twist, this is the ag to bu promotion, responsible for the lower energy Bu state. At intermediate twist angles, the lower 1B and the upper 2B state share the oscillator strength, accounting for the results shown in Figure 8. The computational results shown in Figure 8, and their simple rationalization shown in Figure 9, do not find any support in early experimental results.13 The intense low-energy absorption of the anti form dominates ordinary absorption spectra of peralkylated tetrasilanes and other oligosilanes, and only the strongest peak of the gauche isomer was apparent at high energies in spectral studies designed to separate the contributions of the individual conformers.31 The first experimental indication that the conformer spectra differ in intensities rather than in excitation energies has been only very recently obtained for n-Si4Me10.29 The present experimental results for parent n-Si4H10s(i) the overlap of the UV spectra of the anti and gauche conformers at all wavelengths, with a shared red edge, and (ii) the predominant absorption into the anti conformer at lower energies and into the gauche conformer at higher energiessprovide a second independent piece of evidence for the above reinterpretation of the B states in tetrasilanes. They offer no evidence for the presence of the two computed low-energy A states. In view of their much lower calculated absorption cross sections at the gauche and particularly the anti geometry, and the anticipated spectral overlap, this is not surprising. Perhaps their existence could be detected by two-photon absorption spectroscopy. It now appears likely that the first singlet-singlet excitation in the twisted form of a tetrasilane, and perhaps longer oligosilanes, does not require particularly higher energy than that of the anti form and merely is less intense. Therefore, the Sandorfy C31 and ladder C32 models of oligosilane electronic structure should no longer be used for nonplanar conformations. The simplest Hu¨ckel-type model that they could possibly be replaced with is their as yet unparametrized ladder H analogue.32 Further, the origin of the segmentation of high-molecularweight polysilane chains into weakly coupled chromophores communicating by excitation energy transfer,13 originally deduced from fluorescence polarization measurements,34 is more complex than initially thought and needs to be reexamined. The interpretation proposed at first34 was based on the traditional beliefs concerning conformational effects on transition energies and on INDO/S calculations. These lead to significantly higher excitation energies for the gauche than the anti conformers, since they do not position correctly the relative energies of the σσ* and σπ* states and therefore fail to reveal their avoided crossing. Our unpublished calculations show that other semiempirical methods and also minimum basis set ab initio computations suffer from the same difficulty. The present results suggest strongly the possibility that conformational disorder along a polysilane chain does not modulate local excitation energy as much as it modulates the local nature of the excited state. Further progress in the understanding of optical properties of oligosilanes and polysilanes calls for a firm experimental identification of all the low-lying excited electronic states in a series of conformationally fixed model oligosilanes, for an improved ab initio computational treatment, and, if possible,
IR and UV Spectra of Si3H8 and Si4H10 for the development of a simple model of the ladder H type at the Hu¨ckel or Pariser-Parr-Pople level. Acknowledgment. This work was supported by USARO Grant DAAH04-94-G-0018, funded jointly with NSF/DMR, and by an NSF computer Grant CHE-9412767. B.A. is grateful to the Natural Science Research Council of Sweden for a fellowship. We are grateful to Prof. Veronica Vaida for permitting us to use her vacuum-UV spectrometer and to Dr. Michael Strem for assistance with the transport of highly sensitive materials. References and Notes (1) (a) University of Colorado. (b) NTT. (c) Technical University, Munich. (2) E.g.: Heintze, M.; Veprˇek, S. Appl. Phys. Lett. 1989, 54, 1320. (3) Spanier, E. J.; MacDiarmid, A. G. Inorg. Chem. 1962, 1, 432. (4) Fehe´r, F.; Fisher, H. Naturwissenschaften 1969, 51, 461. (5) Fehe´r, F. Forschungsbericht des Landes Nordrhein-Westfalen, Nr. 2632; Westdeutscher Verlag: Wiesbaden, 1977. (6) Gokhale, S. D.; Jolly, W. L. Inorg. Chem. 1965, 3, 946. (7) Cui, C. X.; Kertesz, M. Macromolecules 1992, 25, 1103. (8) Musgrave, C. B.; Dasgupta, S.; Goddard III, W. A. J. Phys. Chem. 1995, 99, 13321. (9) Freund, R. Ph.D. Dissertation, University of Cologne, Germany, 1973. (10) Itoh, U.; Toyoshima, Y.; Onuki, H. J. Chem. Phys. 1986, 85, 4867. (11) Crespo, R.; Piqueras, M. C.; Orti, E. Synth. Met. 1991, 43, 3457. (12) Stu¨ger, H.; Hengge, E.; Janoschek, R. Phosphorus, Sulfur Silicon Relat. Elem. 1990, 48, 189. (13) For a review, see: Miller, R. D.; Michl, J. Chem. ReV. 1989, 89, 1359. (14) Gilman, H.; Atwell, W. H.; Schwebke, G. L. J. Organomet. Chem. 1964, 2, 369. (15) Haaland, A.; Rypdal, K.; Stu¨ger, H.; Volden, H. V. Acta Chem. Scand. 1994, 48, 46. (16) Bock, H.; Ensslin, W.; Fehe´r, F.; Freund, R. J. Am. Chem. Soc. 1976, 98, 668.
J. Phys. Chem., Vol. 100, No. 21, 1996 8691 (17) Ortiz, J. V.; Mintmire, J. W. J. Am. Chem. Soc. 1988, 110, 4522. (18) Hariharan, P. C.; Pople, J. A. Chem. Phys. Lett. 1972, 66, 217. (19) Gaussian 92, Revision C: Frisch, M. J.; Trucks, G. W.; HeadGordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A.; Gaussian, Inc., Pittsburgh, PA, 1992. (20) Andersson, K.; Fu¨lscher, M. P.; Lindh, R.; Malmqvist, P.-A° ,; Olsen, J.; Roos, B. O.; Sadlej, A. J., University of Lund, Sweden, 1991. Widmark, P.-O. IBM, Sweden, 1991. (21) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54, 724. Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257. Gordon, M. S. Chem. Phys. Lett. 1980, 76, 163. (22) Sadlej, A. J. Collect. Czech. Chem. Commun. 1988, 53, 1995. Sadlej, A. J.; Urban, M. J. Mol. Struct. (THEOCHEM) 1991, 80, 234. (23) Smith, G. D.; Yoon, D. Y. J. Chem. Phys. 1993, 100, 649. (24) Bellamy, L. J. The Infrared Spectra of Complex Molecules, 3rd ed.; John Wiley & Sons: New York, 1975; Vol. 1, pp 374-383. (25) Veprˇek, S.; Schopper, K.; Ambacher, O. J. Electrochem. Soc. 1993, 140, 1935. (26) Steinmetz, M. G. Chem. ReV. 1995, 95, 1527. (27) Jasinski, J. M.; Becerra, R.; Walsh, R. Chem. ReV. 1995, 95, 1203. (28) Teramae, H.; Michl, J. Mol. Cryst. Liq. Cryst. 1994, 256, 149. (29) Albinsson, B.; Teramae, H.; Downing, J. W.; Michl, J. Chem. Eur. J., in press. (30) Robin, M. B. Higher Excited States of Polyatomic Molecules; Academic Press: New York, 1974; Vol. 1, p 305 ff. (31) Sandorfy, C. Can. J. Chem. 1955, 33, 1337. (32) Plitt, H. S.; Michl, J. Chem. Phys. Lett. 1992, 198, 400. Plitt, H. S.; Downing, J. W.; Raymond, M. K.; Balaji, V.; Michl, J. J. Chem. Soc., Faraday Trans. 1994, 90, 1653. (33) Balaji, V.; Michl, J. Polyhedron 1991, 10, 1265. (34) Klingensmith, K. A.; Downing, J. W.; Miller, R. D.; Michl, J. J. Am. Chem. Soc. 1986, 108, 7438.
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