Silicon-Based Polymer Science - American Chemical Society

30 Ground States and Ionization Energies of Polysilane Oligomers Downloaded by TUFTS UNIV on June 20, 2017 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1990-0224.ch030

J. V. Ortiz1 and J. W. Mintmire2 Department of Chemistry, University of New Mexico, Albuquerque, NM 87131 Chemistry Division, Naval Research Laboratory, Washington, DC 20375

1

2

Ab initio calculations were performed on the structures and vertical ionization energies of small silane chains. Optimizations of Hartree-Fock double ζ plus polarization geometry were supplemented by correlated calculations on ground-state and vertical ionization energies. Energy differences between minima on the ground-state surface were insignificant, and rotation barriers were ~ 1 kcal/mol. Vertical ionization energies were more dependent on rotations about single bonds. For tetrasilane, the experimental photoelectron spec trum showed a mixture of rotational minima. Molecular orbital stud ies of pentasilane gave similar results. Parametrized calculations on longer chains confirmed the qualitative trends observed in the ab initio studies. Simple molecular orbital ideas, regardless of whether an atomic or bond function basis is used, suffice to explain the con formational dependence of vertical ionization energies.

INTEREST IN THE UNUSUAL

spectral and chemical properties of polysilanes has been intensified by the emergence of several technological applications (1-9). Absorbance in the UV range by molecules with no ΤΓ, d, or lone-pair electrons (10-11) is an especially provocative feature. Bond orbital models (12) suggest that there are similarities between these saturated chains and ττ-conjugated polymers such as polyacetylene. Derealization of particle and hole states is suggested by the ESR (electron spin resonance) spectra of stable radical anions (13-16) and low-lying ionization energies (17-23). The conformational dependence of elementary excitation energies may be re sponsible for bathochromic shifts that accompany the phase transitions of substituted polysilanes in solution (24-32). 0065-2393/90/0224-0551$06.00/0 © 1990 American Chemical Society

Zeigler and Fearon; Silicon-Based Polymer Science Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

Downloaded by TUFTS UNIV on June 20, 2017 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1990-0224.ch030

552

SILICON-BASED POLYMER SCIENCE: A COMPREHENSIVE RESOURCE

Much spectroscopic and theoretical work has concentrated on the con formational dependence of ground-, excited-, and ionic-state energies. Pa rametrized computational studies have treated the ground- and excited-state energies of oligomers (33-45) and the bands of infinite polymers (46-47). Until now, few ab initio studies have appeared on the electronic structure of any system larger than disilane (48-57). Thorough ab initio calculations on small silanes are a valuable precursor to studies of longer chains (58, 59). Two kinds of calculations are presented in this chapter: geometric optimization to obtain accurate structures and relative energies of rotational isomers and calculations of vertical ionization energies, which can be compared with photoelectron spectra and which disclose aspects of electronic structure that relate spectra to molecular ge ometry. The calculated electron-binding energies for tetrasilane were used to adjust a parametrized method, which was then applied to large polysilane chains. The conformational dependence of the energy of the highest occupied orbital is explained in terms of a simple bond orbital model.

Experimental Procedures At the optimized HF/3-21G* geometries, single-point Hartree-Fock and MBPT(2) total energies were calculated with the 6-31G* basis (60-69). Electron propagator theory (EPT) (70-75) in the quasiparticle outer valence approximation (OVA) was used to calculate vertical ionization energies with relaxation and correlation correc tions to Koopmans's theorem (76). Calculations on disilane and trisilane produced excellent results. HF/3-21G* Si-Si and Si-Η distances of 2.345 and 1.48 À, respectively, are in agreement with results of previous studies and experiment (48-57). Bond angles are close to 110°. Results of EPT-OVA with the 6-31G* basis on the first vertical ionization energy of each symmetry lie within 0.2 eV of experimental results (17, 18). Sorting out the effects of dihedral-angle changes can begin with this reliable model.

Results and Discussion

anti,

gauche,

n-Tetrasilane. Optimizations of HF/3-21G* geometry were performed on the sequence ecl-120 (eclipsed 120°), and which describes a steady decrease of the Si-backbone dihedral angle from 180° to 120° to 60° to 0°. For all four cases, the optimized Si-Si distances are about 2.35 Â, and the S i - Η distances are about 1.48 Â. Si-Si-Si angles are close to 111°. Dihedral angles, even when not constrained by symmetry, are near 60°, -60°, and 180°. All of the ground-state total-energy differences, regardless of basis set or correlation treatments, give essentially the same result. For example, MBPT(2) calculations with the 6-31G* basis favor the conformation over the conformation by just 0.04 kcal/mol. The barrier between these two conformers is 0.6 kcal/mol, but the barrier between the two forms is 1.2 kcal/mol. Steric factors will dominate considerations of the relative energies of substituted rotamers.

anti

gauche gauche


30.

ORTIZ & MINTMIRE

Ground States and Ionization Energies

553

The three highest levels correspond to molecular orbitals (MOs) with nearest neighbor bonding interactions along the Si backbone. Two MOs are symmetric with respect to C rotation (the symmetry operation that is con served for all four structures), and the other is antisymmetric (Figure 1). Both of the MOs with symmetry label a (symmetric) are destabilized in the anti form, but the MOs with symmetry label b (antisymmetric) are stabilized. Simple MO notions suffice to explain these trends. For MOs with symmetry label b, the two terminal bonding regions are out of phase. These regions approach each other as one passes from right to left on the abscissa, and the one-electron level is destabilized. For MOs with symmetry label a, bonding relationships exist in each terminal Si-Si region, and symmetry requires that these two regions be in phase. Because the total energies for anti and gauche conformers are virtually the same, the photoelectron spectrum will depend on the vertical ionization energies of both isomers. The results for both the anti (9.27, 10.46, and 10.73 eV) and gauche (9.49, 10.05, and 10.81 eV) conformers are in reason able agreement with the maxima (17,18) at 9.62, 10.3, and 10.85 eV. Quan titative errors in results of calculations using Koopmans's theorem are large, but a similar dependence of energy on dihedral angle is produced with the 3-21G* basis. The mainflawsof results obtained by using Koopmans's theo rem are the absolute energies and the relative displacements of the final states, but the dependence on dihedral angles is represented well.


2

n-Pentasilane. Optimizations of HF/3-21G* geometry on the antianti and gauche-gauche conformers of the unbranchedfive-Sichain produce bond lengths, angles, and dihedral angles that do not vary significantly from those of the smaller chains. At this level, the anti-anti form is lower in energy by 0.16 kcal/mol, but the basis set and correlation improvements could reverse this order. Because results obtained by using Koopmans's theorem give a reasonable qualitative description of how vertical ionization energies change with con formation in tetrasilane, the 3-21G* canonical orbital energies of pentasilane can perform a similar service in comparing the anti-anti and gauche-gauche conformers. The four highest occupied MOs are predominantly Si-Si nearest neighbor σ-bonding orbitals (Figure 2). Each symmetry label (a and b) has 1-4 bonding and antibonding cases. (In this notation, 1-2 refers to a single bond function, 1-3 refers to adjacent-bond functions, and 1-4 refers to nextto-adjacent-bond functions.) For symmetry label a, an avoided crossing exists between the two structures. The lower a level is 1-4 antibonding for the anti-anti conformer but becomes bonding for the gauche-gauche conformer. If the noncrossing rule is disregarded in following the transformation from the anti-anti conformer to the gauche-gauche conformer, the 1-4 bonding level for symmetry level a declines from -11.33 to -11.92 eV, and the other a level ascends from -11.54 to -10.74 eV. For b symmetry, the lower level is 1-4 antibonding and ascends from -11.74 to -11.34 eV. Finally, the Zeigler and Fearon; Silicon-Based Polymer Science Advances in Chemistry; American Chemical Society: Washington, DC, 1989.



554

Figure 1. HOMO s of tetrasilane represented as linear combinations of Si-Si bond functions.


30.

ORTIZ & M I N T M I R E

Ground States and Ionization

Energies

E C L - 0


H,

H,

Si

Si H

2

H,

Figure 1.—Continued.


556


^ ^2

ι -g

^1 ήS

0

CO


Is

.s* o?

S

•JL «

CO

Q

g« •S .s ^ ϋ "δ ^

gto^ to 8* to

S

1*I §11 •ÎS

©

to S co Ο £ ° SX CD

w. ϋ

» 3

SS

§j

ϋ 3 to ce

•δ SS

α

bo

Ι| •s

Silicon-Based Polymer Science - American Chemical Society

Recommend Documents