Vacuum ultraviolet absorption spectra of dichlorosilane, dichloromethylsilane and dichlorodimethylsilane

Journal of Electron Spectroscopy and Related Phenomena, 8 (1976) 7 l-80 @ Elsevier Scientific Publishing Company, Amsterdam - Printed in The

VACUUM ULTRAVIOLET DICHLOROMETHYLSILANE

G. C. CAUSLEY

Netherlands

ABSORPTION SPECTRA OF DICHLOROSILANE, AND DICHLORODIMETHYLSILANE

and B. R. RUSSELL

Chemistry Departmetit, North Texas State University, Den/on, Texas 76203 (U
ABSTRACT

The vapor phase vacuum ultraviolet absorption spectra of dichlorosilane, dichloromethylsilane and dichlorodimethylsilane are reported for the region from 40 000 to 83 000 cm- 1 (250-120 nm). Absorptions of these compounds are assigned as primariIy Rydberg excitations of chlorine valence non-bonded electrons. The u* + np(Si-Cl) transition for these compounds is the first observed absorption and is badly overlapped with the first. Rydberg absorptions. The first three ionization potentials of dichloromethylsilane were calculated to be 11.47, 11.82, and 12.28 eV using averaged term values and transition energies. The analysis of these spectra revealed that the first p and d molecular Rydberg states appear to be nearly degenerate and that these Rydberg levels are greatly destabilized with methyl substitution. The results of this study support the presence of strong d-p(Si-Cl) interactions. The strength of this effect is compared in dichlorosilane and dichlorodimethylsilane using an empirical relation. INTRODUCTION

The electronic absorption spectra of halogen-substituted alkanes and alkenes have been a continuing interest of this group ’ -4. The purpose of this paper will be the presentation of new data for some silicon analogs, dichlorosilane [H,SiCl,], dichloromethylsilane [CHsSiHCl,] and dichlorodimethylsilane [(CH,),SiCl,] concerning their electronic transitions in a region from 250 to 120 nm (40 000-83 000 cm- ‘). Of primary interest are the effects of the silicon and the effects of successive methy substitution on the first Rydberg transition energies. These transitions are thought to occur as the result of the excitation of electrons from the four linear combinations of chlorine np non-bonding molecular orbitals, which lie perpendicular to the Si-Cl bonding axes. These non-bonding orbital combinations are shown in Figure 1. The photoelectron ionization potentials for these molecular orbitaIs in H,SiCl,‘> 6

72

b;

al

M-Y

M-Y

Figure 1. The halogen non-bonding orbital combinations in CZ, symmetry for the dihalosilanes. TABLE

1

PHOTOELECTRON

IONIZATION

HdWXz Orbital

ReJ

bz bl

11.70 12.09 12.53 12.76

a2

ar

5

POTENTIALS

OF HzSiClz AND (CH&SiC12

(CHd Ref. 6

Ref.

11.64 12.06

10.99 11.52

&

zSiCh 7b

11.80

a All energies are given in eV. b The assignments given here are not those of the authors of Ref. 7.

and (CH,),SiC12’ are given in Table 1. It should be noted that in the case of (CH,); SiCl,, this report is in disagreement with the assignment of the 10.99~eV band as Q (Si-Cl) made by Green et a1.7. Based on the spectra contained in this study, the assignment of this band is non-bonding b,(Cl). Using these data and averaged term values4, many Rydberg assignments can be made for these compounds. The assignments wilt be discussed in later sections. Additionally, the energy of the u* c np(SiCl) intravalent transition is discussed and compared to similar bands in the spectra of carbon analogs. Finally, the anomalous stability of the non-bonding chlorine electrons will be discussed in terms of pn-dn interactions which were taken into account to explain the observed spectral characteristics. EXPERIMENTAL

The spectra were taken on a McPherson RS-225 1 meter vacuum ultraviolet spectrophotometer equipped with a grating having 1200 lines/mm. The light source

73 cm-l

x IO-3

i.A H2SiC12

Figure 2. The vacuum ultraviolet absorption silane (CH3SiHClz) and dichlorodimethylsilane

TABLE

(HzSiCla),

dichloromethyl-

2

BAND MAXIMA, ASSIGNMENTS CH3SiHC12 AND (CH&SiCL

HzSiCla

CH3SiHClz a

(CH3)2SiC12

spectra of dichlorosilane ((CHs)aSiClz).

AND

EXTINCTION

COEFFICIENTS

Maxima (cm -1)

Assignment

66 72 73 80 66 69 72 75 62 69 70 75

4s+w(bz) 4s+-np(bl) 4p3d+np(bz) 4p3d+np(az) 4s+-np(bz1 4s+np(bl) 4s+np(az+ar) 4p3d+np(bz) 4s+np(bz) 4w--np(br) 4s+np(az + al) 4p3d+np(bz)

230 200 (shoulder) 750 190 310 160 890 530 420 110 270 020

OF

H&iCl2,

& (!/J?U3k-CJK1)

a Although this molecule is of C, symmetry, CZ, notation for the configuration chlorine orbitals is used.

4 770 31200 16700 19 300 7 450 10400 12 loo IS200 6 060 9 360 9 180 18 500 of the non-bonding

74

was a windowless Hinteregger hydrogen discharge lamp. Samples were run in the gas phase at pressures of -70 - lo- 3 torr in a lo-cm stainless-steel cell with 2-mm LiF windows. Pressures were measured using an MKS Baratron capacitance manometer calibrated with a McLeod gauge for pressures from 1 - lo- 3-2,00 torr. The H,SiCl, was obtained from Silsco Inc., Dallas, Texas. The CH,SiHCl, and (CH,),SiCl, were obtained from Petrarch Systems. Samples were purified by methods previously described *. RESULTS

The spectra of the compounds are given in Figure 2. Table 2 contains the band maxima and extinction coefficients for the observed discrete absorptions. Although the spectra are given only from 200 to 120 nm, each compound was checked for absorptions to 400 nm at pressures up to 2.00 torr. There were no additional bands at lower energies found under these conditions. b* +- np (StCl) The a*+-np (Si-Cl) transitions are expected to be the lowest energy absorptions and by analogy with the carbon compounds expected to be weak and diffusel. The slowly rising contour to the red of the first major absorption in each of the spectra beginning at ca. 175 nm is assigned as the origin of this absorption. In the corresponding carbon analog of dichlorosilane, dichloromethane, this band at similar pressures extends from 200 to 167 nm (50 000-60 000 cm- ‘). Assuming the band is symmetric and the same width in the silanes as in dichloromethane, the cr*+np (Si-Cl) absorptions should extend from 175 to I50 nm (57 000-67 000 cm- ‘) and have a maximum at ca. 161 nm (62 000 cm-l). In H,SiCl,, no discrete maximum is observed at this energy, which is the result of the extensive overlapping with the first s Rydbergs. Owing to this difficulty, the origin of the absorptions will be considered instead of the band maxima. The term values of the origins of these bands in H,CCl, and H,SiCl, with respect to the first ionization potentials’ are 42 000 and 37 000 cm- ‘, respectively. If the G* +np transition is as broad and diffuse in H,SiCl, as in HzCC12, then this would correspond to a destabilization of the V* energy level of ca. 5 000 cm- ’ . The magnitude of this destabilization is dependent on the width of the absorption. If the width of this band is smaller in the dichlorosilanes, then the term values would be very similar to that observed for other compounds.

The spectrum is quite badly overlapped, but there are several characteristic discrete transitions which may be discussed. The first maximum (151 .O nm) is assigned as Rydberg in nature corresponding to the 4stnp for the b, in-plane, out-of-phase molecular orbital. This band gives rise to a term value of 28 100 cm- I. Using this term value and the photoelectron ionizations, the other 4s Rydberg transitions are

75 predicted at 144 nm (4stnp, bi, allowed), 137 nm (4stnp, a2, forbidden) and 134 nm (4scnp, a,, allowed). As can be seen in the spectrum, these 4s Rydberg transitions give rise to bands which underlie the second broad band which has a maximum at 135.6 nm. With respect to the first ionization, this band has a term value of 20 600 cm-l, a value which is generally thought as indicative of a first p Rydberg transition. Although this is a reasonable possibility, it is also possible that this band is a d Rydberg as is thought to be the case in SiH, 10-12. The last discrete maximum occurs at 124.7 nm and, owing to its sharpness and strength, is thought to be a Rydberg transition originating from the highly non-bonding a,(Cl) molecular orbital, With respect to this ionization, it has a term value of 20 900 cm- I. It would appear that both sharp bands, 135.6 and 124.7 nm, can be related to the most non-bonding molecular orbitals, b, and a, (see Figure 1), with nearly equal term values. It is felt that, because of the SiH, data and the lack of other discrete structure in the spectrum, the 3d and 4p Rydberg levels are nearly degenerate as in the silicon atom ’ 3.

In this spectrum, the first three bands are quite diffuse and are assigned as the first three 4s Rydberg transitions. As no photoelectron ionizations are available in the literature for this compound, term values cannot be directly calculated. Although this presents a problem, using the average decrease in the first s term value for monomethyl substitution in alkylhalides’ 2, the ionization potentials can be predicted. The reduced term value used to calculate ionization potentials is 26 200 cm- ‘. The predicted values are 11.47, Il.82 and 12.28 eV. With respect to the first predicted ionization, the sharper band at 132.4 nm has a term value of 17 200 cm-l. This value is reduced 3400 cm- ’ from that observed for the simiIar band (135.6 nm) in H,SiCl,. The very sharp 3d-4p Rydberg seen in H*SiCl, at 124.7 nm is not evident in this spectra. This is expected if there is a decrease in the term value, increasing the energy of this transition to a point outside the region of this investigation. (CH,)

2SiC12 Again, in the spectrum, three diffuse maxima and only one sharp band are observed. As related to the first ionization potential (10.99 eV), the first band at Rydberg 160.2 nm has a term value of 26 100 cm-l and is assigned as the 4stnp(b,) transition. The sharp band at 133.3 nm has a first ionization term value of 13 600 cm- ’ and is assigned as the 4p3dtn&b,) Rydberg transition. The substitution of a second methyl has reduced the term value, 3400 cm - I, from the term vaIue calculated for the CH,SiHC12 3d-4p Rydberg. The broad doublet structure (maxima at 144.3 and 142.3 nm) is assigned as primarily two 4s Rydberg transitions, where the maxima are related to the 4stnp(b,) and 4stnp(a1 +a,) Rydberg transitions, respectively. These maxima do not represent the true vertical transition energies as they are badly overlapping.

76 DISCUSSION

Several assertions can be made for these silanes based on the data currently available. The energy of the c*cnp(Si-Cl) transition may be increased. The energies of the non-bonding chlorine molecular orbitals are anomalously low for H,SiCl,, but these energies increase concomitantly with successive methyl substitution. Finally, the term value of the 4s decreased with methyl substitution, but not as great a decrease as found for the term values of the 4p and 3d Rydberg states. Any rationalization in support of the assignments will necessarily take into account these experimental results. --\ The major cause of so many unusual spectral features is the pn-d~ interaction between the non-bonding molecular orbitals of the chlorines and the vacant d orbitals of silicon. This interaction has been the subject of many papers. A recent work by Howell and Van Wazer ’ 4, who completed an ab initio LCAO-MO-SCF computation of H,SiCl concurs that this interaction plays a major role in the charge distribution of the ground state of this molecule; where, those orbitals which are in the z system undergo the greatest delocalization of charge and both n and 0 systems have greater overlap populations. The increased overlap population of (T states may result in the apparently high c*+np(Si-Cl) transition energy. Simultaneously, the non-bonding pn orbitals are stabilized as the a*(Si-Cl) levels are destabilized, resulting in higher energies for this transition in all of these silanes, although it was pointed out earlier that conclusion is questionable. The stabilization of the non-bonding levels in H,SiCl, is justifiable when the delocalization of the electrons into these orbitals is considered. The work of Frost et aL5 has accounted for this interaction using a modification of the semi-empirical approach used by Dixon et al. ’ 5, which was applied to the ionization potentials of the non-bonding electrons of the halogens in the halomethanes. Methyl substitution on the central atom, in general, brings about a greater electron density on that atom relative to the situation when only hydrogens are bonded. This substitution results in a decreased stabilization of the halogen non-bonding electrons owing to a decrease in the delocalizing ability of the silicon d orbitats which interact strongly with the methyl groups. The effect is discussed more fully in the next section. The term values found for the various Rydberg states of these silanes can be related to silicon atomic energy levels. As noted by Robin’*, the term values of Rydberg states are related to substituents in alkylated chromophores, not to the chromophore itself. Such is thought to be the case here, where the silane or methylated silane can be considered as a substituent to the dichloride two-center non-bonding chromophores and the Rydberg orbitals are highly delocalized because they are truly molecular Rydberg orbitals. This results in a Rydberg term value order of 4s greater than 36 slightly greater than 4p. The term values of the Rydberg states of these silanes fall into the regions predicted by Robin based on the silicon atom term values. The first s

77 TABLE TERM

3 VALUES

OF THE FlRST Term values

RYDBERG

4s

4p-3d

SiH4 b

27 500 28 100 25 300

22 000 20 600 16000

Sic14

(CHa)zSiClz Si(CH& b

OF SEVERAL

SILANES

a

Molecule

H2SiC12

STATES

26 100 2s 100

13600 13400

a Energiesare given in cm-l. b See ref. 12. Rydberg for the first ionization potential drops from 28 100 to 26 100 cm-l, while the first p-d Rydberg drops from 20 600 to 13 600 cm- I. The changes reflect the fact that the Rydberg of an alkyl substituted silane will have term values progressively more alkyl-like as substitution increases. This being the case, the Rydbergs should have decreasing term values with a lower limit of 22 000 cm- ’ for s, 13 000 cm- ’ for d and 16 000 cm-l for p. If the first p and d Rydberg are degenerate in H,SiCl,, this degeneracy is expected to break with successive methyl substitution. It is curious that this does not appear to be the case, but no more unusual than the fact that the apparent p-d Rydberg degeneracy is not broken upon going from silane to tetramethylsilanel” ll. Table 3 lists the term vaIues of the first s and p-d Rydbergs for a series of silanes. From these values it appears that the lower Rydberg term values of silanes are particularly sensitive to substituent groups, and that the p-d Rydberg level is more easily perturbed than the s. This may be the resuIt of the greater difference for d Rydberg orbital penetration to the silicon core vs. its penetration to the carbons. The equality of the energies of the p and d Rydbergs throughout the series is very difficult to understand. One possibility is that it is not a matter of degeneracy, but is simply a matter of intensity, where the 4p Rydbergs are weak at one end of the series and the 3d Rydbergs are weak at the other. CALCULATION

OF INTERACTION

PARAMETERS

In the work of Dixon et al. 15, non-bonding electron ionization potentials were assigned for the halomethanes using a pseudo one-electron HamiItonian. When p-pn(C-Cl) interactions were considered, excellent agreement was obtained. The caIculations of Frost et aL5, in which the same approach was used for some chlorosilanes with the addition of d-p(Si-Cl) interaction parameters, also gave excellent agreement (+ 0.07 eV) with experimental non-bonding electron ionization potentials. Since the bond lengths and angles of (CH3)2SiC12 are essentialIy unchanged from are known, the those of H2SiCI, I6 and the ionization potentials of (CH,)&Cl,

TABLE

4

INTERACTION

PARAMETERS

Zon state

Eigenvalue

2BZ

1 +

OF (CH&SiCl2

a

equation u

l/3(80 + 2/3x) - 2/3 y + -2y +

219 6 213 6 k oy + 413 6 l/3(/90 +- Z/J&J- 4/3 y + 16iY 6

2Bl

I-

2A2

I-+

2Al

I-

Molecule

I

Y

s

HzSiClz a (CH&SiClz

11.77 11.25

0.07 0.08

0.60 0.35

a See refs. 5 and IS. b In the equations, fla and on are proportional to the halogen-halogen overlap integrals with values of -0.38 and -0.05, respectively given in ref. 5. y and S are the p-pn(M-X) and d-p (M-X) interaction parameters and I relates to the net charge of the halogens.

\ 2-

-4p-3d -4p-3d -4s

-4p-3d -4s

-4s

T 4?

.6

$8 % WIOI 12 : 14-

Figure 3. A correlation diagram of some of the state energy levels of dichloresilane (H2SiCl& dichloromethylsilane {(CHs)SiHC12) and dichlorodimethylsilane ((CHs)2SiC12) derived from photoelectron ionization potentials and transition energies. See refs. 5-7.

interaction parameters of (CH,)2SiC1, were evaluated using the equations of Frost et al. The parameters found were I[Cl], y[p-pz(Si-Cl)] and G[d-p(Si-Cl).] The solution of this overdetermined system (four equations and three unknowns) was accomplished by least squares. Table 4 contains the equations of Frost et al. along with the parameters of H,SiCI, and those parameters of (CH,),SiCI, calculated for

79 which the error is + 0.14 eV. The results are a relatively large decrease in the I value for the ionization potentials, a very slight increase in p-pn(Si-Cl) interaction (7) and a decrease in d-p(Si-Cl) interaction (S}. All of these changes are expected. As the electron density on silicon builds up from methyl substitution, and likewise on the chlorines, then the 1 should decrease. The p-pi interactions should be slightly greater with the increased electron density in the cr orbitals of the proper symmetry to interact with the non-bonding orbitals on the chlorines. Finally, the d-p(Si-Cl) interaction is decreased primarily because of the d orbital interaction with the various carbon orbitalsi7, which essentially takes away from or averages out the stabilization of the non-bonding orbitals by the d orbitals. CONCLUSIONS

A correlation diagram of the known and predicted energy levels of some ground and excited states of these dichlorosilanes is contained in Figure 3. The results of this work support the importance of silicon d orbital participation in the stabilization of the halogen non-bonding molecular orbitals. Another point is the apparent sensitivity of the molecular Rydberg states of these molecules to substituent change, which is indicative of the importance of these substituents to term values. Additional data are needed which may shed light on those factors affecting the energies and intensities of transitions to Rydberg states, particularly the most stable of these. A case in point is the apparent anomalous near-degeneracy for the lowest p and d Rydbergs of the silanes. Finally, the application of empirically derived eigenvalue equations for H,SiCI, gives very reasonable predictions of interaction parameters in (CH,)2SiCI, which confirms that the d-p (Si-Cl) delocalization of chlorine nonbonding electrons plays a major role in the final energies of the states of these molecules. ACKNOWLEDGEMENTS

Support for this investigation by the Robert A. Welch Foundation, Research Corporation and the North Texas State University Faculty Research Fund is gratefully acknowledged. The authors wish to thank Paul R. Jones and John D. Scott for many helpful discussions. REFERENCES 1 2 3 4 5 6 7

B. R. Russell, L. 0. Edwards and J. W. Raymonda, .7. Amer. C’hsm. Sot., 95 (1973) 2129. J. D. Scott and B. R. Russell, Chem. Whys. Lett., 9 (1971) 375. John D. Scott and B. R. Russell, 1. Amer. Chem. Sot., 94 (1972) 2634. G. C. Causley and B. R. Russell, J. Chem. Phys., 62 (1975) 848. D. C. Frost, F. G. Herring, A. Katrib, R. A. N. McLean, J. E. Drake and N. P. C. Westwood, Can. J. Chem., 49 (1971) 4033. S. Cradock and R. A. Whiteford, Trans. Faraaizy Sot., 67 (1971) 3425. M. C. Green, M. F. Lappert, J. B. Pedley, W. Schmidt and B. T. Wilkins, J. Orgunometal. Chem., 31 (1971) c55.

8 9 10 11 12 13 14 15 16 17

A. A. Iverson and B. R. Russell, Spectrochim. Acta, Part A, 29 (1973) 715. A. W. Potts, I-I. J. Lempka, D. G. Streets and W. C. Price, Phil. Trans. Roy. SIX. London, Ser. A, 268 (1970) 59. A. G. Alexander, 0. P. Strausz, R. Pottier and G. P. Semeluk, Chem. Phys. Lett., 13 (1972) 608. Y. Harada, 3. N. Murrell and H. H. Sheena, Chem. Phys. Left., 1 (1968) 595. M. B. Robin, Higher Excited States of Polyatomic Molecubs, Vol. 1, Academic Press, New York, 1974. C. E. Moore, Nut. Bur. Stand. {U.S.) Circ., (1958) 447. James M. Howell and John R. Van Wazer, J. Amer. Chem. Sot., 96 (1974) 3064. R. N. Dixon, J. N. Murrell and B. Narayan, Mof. Phys., 29 (1971) 611. L. E. Sutton (Ed.), Tub&s ofInteratomic Distances, Chemical Society, London, 1958. A. E. Jonas, G. K. Schweitzer, F. A. Grimm and T. A. Carlson, J. Electron Spectrosc., 1 (1972) 29.