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The vacuum ultraviolet spectrum of bromosilane
Volume-38,‘number _-_
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3
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: : CHEMICAL PHYSICS LEmERS_..--
15 ,March 1976
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THE -ir;ACUUM ULTRAVIOLET
SPECTRUM
OF BROMOSILANE
G-C_ CAUSI$~, J-B_ CLARK and B.R. RUSSEiL aeparmenr of chemistry, North Texas State Universjty. Denton. Texas 96203. USA Received 18 November 1975
The vacuum ultraviolet absorption spectrum of bromosilrtne in the vapor phase is reported. Assignments are made using photoelectron data and oscillator strengths. The absorptions of this compound are related to absorptions of chlorosilane, methyl bromide and methyl chloride. The results of this work indicate the presence of drr-prr (Si-Br) delocalizations and concomitant stabilization of halogen nonbonding electrons.
1_ Introduction The spectral-characteristics of chlorosilane (SiH,Cl) have been the subject of Past research [ I] _The bromine derivative, however, has received little attention with respect to the energies of the excited states beyond the first ionization potential. The purpose of this letter is to present the vacuum ultraviolet spectrum of bromosilane (SiH3Br) in the region from 40 k!S to 80 ?& (250 nm to 125 nm). The first electronic absorptions for this compound are found within this region, and they are comprised of intravalent (e.g. u* + ltp, Si-Br).and Rydberg transitions associated with the most easily ionized electrons. They include: the highly -nonbonding spin-orbit split electrons of the halo?-n (e, 10.96 and 11.10 eV) and the Si-Br u bonding elecof trons (al, 12.85 eV) [2]. To aid in the assignments these absorptions, comparispns between SiH3Br and methyl bromide (CH3Br) as well as the chlorine analogs are presented. -4 vibrational progression has been observed in the spectrum and is discussed in a later section. This spectral study and comparisons which have been made support the importance of dn-plr interactions as suggested in previous works [3-61. The spectrum was obt@r,ed using a McPherson RS-225 l-meter vacuum ultraviolet spectrophoto-meter equipped with a 1200 iine/mm grating with a dispersion of 8.3 A/nim. The light.source was a Hintere@r hydrogen discharge lamp differentially pumped to allow windowless operaiion. LiF wifidbws were .‘ he*-
-. _-.
.:.
..
used with the 10 cm stainless steel sample cell. The sample was run in the vapor phase at pressures from ca. 0.1 to 0.6 torr. The SiH, Br was prepared and puri[7]. The fied by methods described in the literature purity was checked by IR [S] and mass spectral analysis. Due to the case of hydrolysis of this compound, it is highly pyrophoric, and a great deal of time and labor was involved in the vacuum fractionation. in addition to this purification, each sample was distilled immediately before the various recordings of the spectrum were taken.
2. Results
and discussion
of Sifi3 Br is shown in fig. I, and the band maxima and oscillator strengths for the four discrete bands which are observed in the spectrum. In the discussion which follows, “term vaiue” 19 J refers to energies calculated from the equation The spectrum
table 1 contains
T,, = IP - Fobs, where IP is the ionization band maxima. Oscillator using the equation
f =4;32 X IO-+dv.
(1) potential and vobs_is the strengths were calculated
(2)
A compilation of ionization potent&, absorption maxima and “te_m valu&” of compounds tb be com-
CH_EM~tiALPHYSKS LETTERS
Volumk 38, number 3
‘.I
15 March 19%
.
-_.
The first electronic absorption of SiHsBr has a. maximum at 19 1.Onm (52400 cm-l) and is the
Fig. 1. The gasphase vacuum ultraviolet absorption spectrum of bromosihne (SiH3Br).
Table 1 Observed absorption maxima and calculated os5llator strengths of bromosiiane (SiHsBr) Absorption maximaa)
Osci.llator strengti#
191.0 167.0 149.0 133.0
nm 52400 cm-’ 0.002 nm 59900 cm-t 0.017 run 67100 cm-t 0.10 nm 75200 cm-’ 0.48 ~---a) Band maxima are accurate to 20.2 nm. b) OscUntor strengths are accurate to * 10%.
Table 2 Summary of some reported ionization potentials (ev), band ma&a chlorosilane (SiHaCl)b), and methyibromide
and term
v&es
(cm”) of methylchloride (CHsCi)a),
Ionization
u* c ,zp (M-X)
Ot+l)s +-np
band maxima
term value
band m-&ma
term value
58100 -
33130
62580 -
28570
11.30 Il.32
SiH3Cl
11.6:
CH3Br
10.54
47mJ
10.86
50000
a) See refs. 111,181.
(cm-‘),
potential -_P--.~-CH3Cl
result of the ox + np (Si-Rr) transition. This weak, diffuse band is ea. 3600 cm-r higher in energy than the corresponding a* 6 np (C-Br) transition in CH3 Br E101. If dn-px (Si-Br) interactions are large, . this shift is reasonabfe. A stabilization of the nonbonding energy levels and a destabilization of the 19 + rzp (Si-E3r) level [3,6] would yieid a transition energy higher than in CH,Br. If this supposition is applied to SiHsCl, the results are similar but more drastic. The spectrum of SiH3Ci [l] shows no u*”+- np (Si-CI) transition in the expected region, which has been cakuiated to be ca. 62000 cm-l using “term values” (seti table 2) fcr CJ*+- np transitions from the spectral data of SiH3Br, CHsBr and CHsC1. In fact, the 4s + 3p molecular Rydberg transition is the first observed absorption in SiH,CI at ca. 67000 cm-r, and it appears that the u* + np (Si-Cl) band is buried in this band. This was observed si_mi.larlyin the dichlorosilanes [3] and may be the result of increased overlap population in the Si-Cl bond 3s predicted by Howell and Van Wazer [6] in an ab initio LCAO MO SCF calculation of the wavefunction of SiH,Q. In the calcuiation, both the vacant d orbit& of Si and those of Cl were included and were shown to have 3 considerable effect on the energies and pop&tions of the molecular orbitals of the ground state. It
37510 37590
(n+l)p-np
~-.-
band maxima
term value
71110 71780
20040 19530
67000
26810
74900
18910
56020 59160 A
28990 28430
66000 68660
19010 18930 -
b) See ref. [ZI_ .C)See refs. Il&l8I.
603
Volume38,‘nunib&3~.’ _-:,
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CHEMICAL
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PHYSlCS
is i&p&tit to point out; however, that the”suppoit -of this theoietical result is based on :Lmissing absorptioti; With this wegative evidence; there is room for doubt, but: t.$e blue-SIzift for &-+np (F&Cl) hti been rioted in many silylchlorides; and the in&ea& in _the energy and decrease in the “term value” for db: t np (Si-Br) would suaest that this is a pfausib!e &&on.
The next absorption in the spectrum of SiH$r assigned as 5&+4p with a maximum at 167.0 nm (59900
cm-l)
L5.hkrch
LETTERS-
1976
_-
is generally believed tb be associated with the delocalizatign of m&{fofd electrons. Following the &me reasoning as in the previous discussion, a comparison of SQCI and CM3Cl data yields an tinomalous result. In Sil-Q Cl, the “term value” of the 4p + 3p Rydberg is destabilized ca. 900 cm-1 compared to this transition in CH3CI. in general, the first Rydberg states of SiHs Br are stabilized, and those of SiH,CI are destabilized compared to their carbon analogs.
is
and has B “term value” with respect
to the averagecifirst ionization potential (I I .03 eV, e) of 29100 cm-I. This band corresponds to the B and C bands of CH, Br (56033 and 59 159 cm-l, respectively), but in.SiH3Br the spin-orbit splitting of this degenerate excited state is not seen with our resolu-tion. The “term value” of this transition is slightly higher than the “term vafue” of the 5s t- 4p molecukxr Rydberg of CH,Br (ca. 28700 cm-l). The blue-shift of the absorption wirh respect to CH3Br is primarily due to a stabilization of ca. 0.3 eV of the nonbonding electrons in the ground state. Again, an unusual observation occurs in the spectrum of SiH3 Cl where the
“term value” of the first Rydberg is 26600 cm-l as compared to 28600 cm-l for the first Rydberg in Cfi,Cl [I I]. Thinking of the “term vdue” as the ionization potential of the excited state, the first s Rydberg of SiHsCl appears to be destabilized ca. .2000 cm-l _As with the u* +- np (Si-Cl), there appears to be general destab~iz~tion of the lower excited states of SiH,Cl. Upon reviewing the various reported spectral values (see table 2) ior ail the compounds in question, this destabilization is unique to Siff3 Cl.
An intense structured absorption is observed in the and is thought to be comprised of Rydberg transitions and the cr* + o (Si-Br) intravafent transition. A Sd + 40 Rydberg transition associated with the first ionization potential should have a term value of ca. 13000 cm-l and lie in the region observed. A term value of 12600 cm-l for the 6s +4p Rydberg transition of the fhst ionization potential would also place additionai intensity in the region of 133.0 nm. The first Rydberg transition related to the second ionization potential, 5s + CI(Si-Br), should occur in this region and have a term value of ca. 28600 cm-l, again adding intensity to the 133.0 nm absorption. The oscillator stren,oth calculated for this band @f= 0.48) is much greater than expected even for muItipfe Rydberg transitions. Because of the strength of this band, the cr* +- (7(Si-Br) is thought to occur in the 133.0 nm region in addition to the Rydberg transitions. The accumulation of these excited states explains the unusually high intensity and irregular contour of this absorption. The vibrational progression noted fur this band (see table 3) begins at ca. 134.0 nm and has six resolved peaks spaced by ca. 505 cm-l. This excited state vibrational sequence would correspond,to either an region of 133.0 run
2.3. 7%~ 149.0 nnz band The Sp + 4p Rydberg transition for SiH3Br is found at 149.0 nm (ca. 67 100 cm-l) and has an associated
Table 3 Vibrational progression of the 133.0 nm band
“term value” of 2 19 00 cm-l. This value represents a
W3veIength Cnm)
Frequency (cm-‘)
A frequency (cm-’ )
stabilization of this Rydberg state oT ca. 3000 cm-’ as compared to the average first p “term values” in CH~Br. This band, like the 5s f 4p Rydberg, shows no vibrational structure or spin-orbit coupling. This .is npt unexpected as the photoelectron spectra [2] also show& very little structure, the absenci: of which
134.2 133.3 132.4 131.5 130.6 129.8
74520 75020 7.5530 76050 76570 77040
500 510 520 520 470
Volume 38, number 3
CHEMICALPHYSICSLETUTRS
increase of the ground state Si-Br stretching frequency (us) 430 cm-l or a decrease in the SiH3 symmetrical deformation (vi) of 930 cm-l [12,13]. It would seem unlikely that the Si-Br (~3) mode would increase that significantly in any of the excited states mentioned or that a vibrational progression (~2) associated with a Rydberg Ievel wouid be reduced by as much as 425 cm-l. However, large decreases in vibrAtional frequencies are not uncommon for antibonding states [ 143. On this basis, the observed vibrational piogression is assigned as several quanta of the vi, SiH3 symmetric deformation associated with the u* + G (SiBr) transition.
3. Conclusions The results and comparisons contained in this study suggest several basic conclusions. There is a general increase in transition energies for SiH, Br compared to CH,Br which is primarily the result of stabiIization of the nonbonding electrons of the halogen through delocalization to the silicon via dn-plr (SiBr) overlap in the ground state. This is also true for SiH,C!l as compared to CH3Cl. The major differences occur in the excited states, both Rydberg and intravalent . The Rydberg states of SiH,Br are stabilized as compared to those of CH3Br while there is a general destabilization of these states in SiH3CI as compared to CHsCl. If the Rydberg states are localized on the halogen, the Rydberg states of the bromine-containing compounds should he less stable relative to those of the chlorides as a result of the greater number of real precursors for bromine and, hence, less penetration to the core [ 151. There are at least two closely related explanations for the spectral observations. One reason is that the Rydberg orbital has a greater contribution from the SiH3 group in SiH3Cl than in SiH3 Br. Greater penetration to the silicon than the halogen would give less stable Rydberg states as is evidenced by the Rydberg states of the atoms [ 161, and in SiHsCl the Rydberg orbitals “see” more silicon than in SiH,Br. This is, in effect, the charge transfer argument of Robin [9]. Thus, just on a relative size consideration, the Rydberg orbitals of the chIorine compound necessariiy are associated more with the silicon atom than the corresponding bromide and are destabi-
15March 1976
l&d; The second iart of this rationalization
involves the delocalization of halogen a electron density to the silicon. The SiH3Br Rydbergs, if localized-more about bromide, would experience additional core stabilization as a resuit of the. d-ir-psr (Si-Br) interaction; whereas, the opposite could occur for the SiH3C1 Rydbergs if they arl localized about the silicon. The large blue-shift of the (T*f- np (Si-Cl) of SiH3C1 as compared to the several other compounds is the result of both the stabilization of nonbonding electrons and the d&tabilization of the antibonding state. This antibonding destabilization is brought about by chlorine vacant d orbital participation in the overlap population of the Si-CI o bond [6]. This increased population stabilizes the bonding orbital and destabilizes the antibonding orbital. This effect may be unique to the Si-CI bond as it is essentially absent in the Si-Br bond and Ge-CI bonds [&I 71. Finally, the spectrum of SiH3Br lacks the structure of CH3Br. Spin-orbit coupling is not evident, a result associated with the delocalization of the nonbonding electrons of the halogen; and only one vibrationa progression is found in the spectrum. This progression is thought to be associated with the r~* + u (Si-Br) transition and, by the enerr; spacing and shape of the absorption, indicates a relatively large change in the internuclear coordinates for this excited state.
Acknowledgement The authors thank the Robert A. Welch Foundation, Research Corporation and the NTSU Faculty Research Fund for their generous support of this work. Randall C. Rains, who aided in the preparation and purification of the compound, is also gratefully acknowledged.
References [l ] S. Bell and A.D. Walsh, Trans. Faraday SOC.62 (1966)
3005. [ 21 S. Cradock and R.A. whiteford. Trans. Faraday Sot. 67 (1971) 3425. [3] G-C. Causley and B-R. Russell, J. Electron Spectry., ?a be published. [4] D.C. Frost, F.G. H&ring, A. Katrib, R.A.N. McLean, I.E. Drake and N.Z.C. Westwood, Chem. Phys. Letters 13 (1971) 347;Csn. J. Chem. 49 (1971) 4033.
605
‘~oliime~38.nutibei :
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[Il]
161 J.&k tiowgllarqi J.R.van Wgzer, J. Am. Chem. Sot. 96 (1974j.1064. : .‘[7] .k.L.k&ud, in: -h~orgxnicsyntikis, Vol. 11, eb. W-L. Jolly @kGra~-I5ll, Xew Y&k; 1968) pp. 165. [SJ D-W. Mayo, N.E. Opitz end 3.S. Peak, 3. Chem- Phys. 23 1195.5)1344. 191M.B. Robin, Higher exkited &tcs ofpoIyatomic ni&&es, Vol. 1 (Academic Press, New York, 1974). ; flOj G.C. Causley arid B.R. RuskI.i, 3. Chem. Phys. 62 (1975) 848.
:
B.R. RUSSCU,LO. Edwards and J.W. Raysnoqd?, 4. Am. Chem. Sot. 95 (1973) 2129. f12I H. Burger, J. Cichon and A. Ruofi; Spectrochim. Acta 30A (1974) 223. [ 13 i J-L Duncan, Spectrochbn. Acta 20 (1964) i807, 1141 S.~Stokes and A.B.F. Duncan, 3. Am. Chem. Sac. 80 (1958j 6177. [ISI R.S. Mutiken, f. km. Chem. Sot. 86 (1964) 3183. [I&] C.E. Moore, N&l. Bur. Std. Circular467 (1958). [ 17 ] G.C. Causley and RR. Russ&, to bc’published. [18] A-W. P&s, H.J. Lempka. D-G, Strekts znd W.C. Price, Phil. Tmns. Roy. Sot. A268 (1970) 59.
.
-.
3
_
_‘ ,‘
I-‘
: : CHEMICAL PHYSICS LEmERS_..--
15 ,March 1976
.’ -_
THE -ir;ACUUM ULTRAVIOLET
SPECTRUM
OF BROMOSILANE
G-C_ CAUSI$~, J-B_ CLARK and B.R. RUSSEiL aeparmenr of chemistry, North Texas State Universjty. Denton. Texas 96203. USA Received 18 November 1975
The vacuum ultraviolet absorption spectrum of bromosilrtne in the vapor phase is reported. Assignments are made using photoelectron data and oscillator strengths. The absorptions of this compound are related to absorptions of chlorosilane, methyl bromide and methyl chloride. The results of this work indicate the presence of drr-prr (Si-Br) delocalizations and concomitant stabilization of halogen nonbonding electrons.
1_ Introduction The spectral-characteristics of chlorosilane (SiH,Cl) have been the subject of Past research [ I] _The bromine derivative, however, has received little attention with respect to the energies of the excited states beyond the first ionization potential. The purpose of this letter is to present the vacuum ultraviolet spectrum of bromosilane (SiH3Br) in the region from 40 k!S to 80 ?& (250 nm to 125 nm). The first electronic absorptions for this compound are found within this region, and they are comprised of intravalent (e.g. u* + ltp, Si-Br).and Rydberg transitions associated with the most easily ionized electrons. They include: the highly -nonbonding spin-orbit split electrons of the halo?-n (e, 10.96 and 11.10 eV) and the Si-Br u bonding elecof trons (al, 12.85 eV) [2]. To aid in the assignments these absorptions, comparispns between SiH3Br and methyl bromide (CH3Br) as well as the chlorine analogs are presented. -4 vibrational progression has been observed in the spectrum and is discussed in a later section. This spectral study and comparisons which have been made support the importance of dn-plr interactions as suggested in previous works [3-61. The spectrum was obt@r,ed using a McPherson RS-225 l-meter vacuum ultraviolet spectrophoto-meter equipped with a 1200 iine/mm grating with a dispersion of 8.3 A/nim. The light.source was a Hintere@r hydrogen discharge lamp differentially pumped to allow windowless operaiion. LiF wifidbws were .‘ he*-
-. _-.
.:.
..
used with the 10 cm stainless steel sample cell. The sample was run in the vapor phase at pressures from ca. 0.1 to 0.6 torr. The SiH, Br was prepared and puri[7]. The fied by methods described in the literature purity was checked by IR [S] and mass spectral analysis. Due to the case of hydrolysis of this compound, it is highly pyrophoric, and a great deal of time and labor was involved in the vacuum fractionation. in addition to this purification, each sample was distilled immediately before the various recordings of the spectrum were taken.
2. Results
and discussion
of Sifi3 Br is shown in fig. I, and the band maxima and oscillator strengths for the four discrete bands which are observed in the spectrum. In the discussion which follows, “term vaiue” 19 J refers to energies calculated from the equation The spectrum
table 1 contains
T,, = IP - Fobs, where IP is the ionization band maxima. Oscillator using the equation
f =4;32 X IO-+dv.
(1) potential and vobs_is the strengths were calculated
(2)
A compilation of ionization potent&, absorption maxima and “te_m valu&” of compounds tb be com-
CH_EM~tiALPHYSKS LETTERS
Volumk 38, number 3
‘.I
15 March 19%
.
-_.
The first electronic absorption of SiHsBr has a. maximum at 19 1.Onm (52400 cm-l) and is the
Fig. 1. The gasphase vacuum ultraviolet absorption spectrum of bromosihne (SiH3Br).
Table 1 Observed absorption maxima and calculated os5llator strengths of bromosiiane (SiHsBr) Absorption maximaa)
Osci.llator strengti#
191.0 167.0 149.0 133.0
nm 52400 cm-’ 0.002 nm 59900 cm-t 0.017 run 67100 cm-t 0.10 nm 75200 cm-’ 0.48 ~---a) Band maxima are accurate to 20.2 nm. b) OscUntor strengths are accurate to * 10%.
Table 2 Summary of some reported ionization potentials (ev), band ma&a chlorosilane (SiHaCl)b), and methyibromide
and term
v&es
(cm”) of methylchloride (CHsCi)a),
Ionization
u* c ,zp (M-X)
Ot+l)s +-np
band maxima
term value
band m-&ma
term value
58100 -
33130
62580 -
28570
11.30 Il.32
SiH3Cl
11.6:
CH3Br
10.54
47mJ
10.86
50000
a) See refs. 111,181.
(cm-‘),
potential -_P--.~-CH3Cl
result of the ox + np (Si-Rr) transition. This weak, diffuse band is ea. 3600 cm-r higher in energy than the corresponding a* 6 np (C-Br) transition in CH3 Br E101. If dn-px (Si-Br) interactions are large, . this shift is reasonabfe. A stabilization of the nonbonding energy levels and a destabilization of the 19 + rzp (Si-E3r) level [3,6] would yieid a transition energy higher than in CH,Br. If this supposition is applied to SiHsCl, the results are similar but more drastic. The spectrum of SiH3Ci [l] shows no u*”+- np (Si-CI) transition in the expected region, which has been cakuiated to be ca. 62000 cm-l using “term values” (seti table 2) fcr CJ*+- np transitions from the spectral data of SiH3Br, CHsBr and CHsC1. In fact, the 4s + 3p molecular Rydberg transition is the first observed absorption in SiH,CI at ca. 67000 cm-r, and it appears that the u* + np (Si-Cl) band is buried in this band. This was observed si_mi.larlyin the dichlorosilanes [3] and may be the result of increased overlap population in the Si-Cl bond 3s predicted by Howell and Van Wazer [6] in an ab initio LCAO MO SCF calculation of the wavefunction of SiH,Q. In the calcuiation, both the vacant d orbit& of Si and those of Cl were included and were shown to have 3 considerable effect on the energies and pop&tions of the molecular orbitals of the ground state. It
37510 37590
(n+l)p-np
~-.-
band maxima
term value
71110 71780
20040 19530
67000
26810
74900
18910
56020 59160 A
28990 28430
66000 68660
19010 18930 -
b) See ref. [ZI_ .C)See refs. Il&l8I.
603
Volume38,‘nunib&3~.’ _-:,
_-^
CHEMICAL
,.
PHYSlCS
is i&p&tit to point out; however, that the”suppoit -of this theoietical result is based on :Lmissing absorptioti; With this wegative evidence; there is room for doubt, but: t.$e blue-SIzift for &-+np (F&Cl) hti been rioted in many silylchlorides; and the in&ea& in _the energy and decrease in the “term value” for db: t np (Si-Br) would suaest that this is a pfausib!e &&on.
The next absorption in the spectrum of SiH$r assigned as 5&+4p with a maximum at 167.0 nm (59900
cm-l)
L5.hkrch
LETTERS-
1976
_-
is generally believed tb be associated with the delocalizatign of m&{fofd electrons. Following the &me reasoning as in the previous discussion, a comparison of SQCI and CM3Cl data yields an tinomalous result. In Sil-Q Cl, the “term value” of the 4p + 3p Rydberg is destabilized ca. 900 cm-1 compared to this transition in CH3CI. in general, the first Rydberg states of SiHs Br are stabilized, and those of SiH,CI are destabilized compared to their carbon analogs.
is
and has B “term value” with respect
to the averagecifirst ionization potential (I I .03 eV, e) of 29100 cm-I. This band corresponds to the B and C bands of CH, Br (56033 and 59 159 cm-l, respectively), but in.SiH3Br the spin-orbit splitting of this degenerate excited state is not seen with our resolu-tion. The “term value” of this transition is slightly higher than the “term vafue” of the 5s t- 4p molecukxr Rydberg of CH,Br (ca. 28700 cm-l). The blue-shift of the absorption wirh respect to CH3Br is primarily due to a stabilization of ca. 0.3 eV of the nonbonding electrons in the ground state. Again, an unusual observation occurs in the spectrum of SiH3 Cl where the
“term value” of the first Rydberg is 26600 cm-l as compared to 28600 cm-l for the first Rydberg in Cfi,Cl [I I]. Thinking of the “term vdue” as the ionization potential of the excited state, the first s Rydberg of SiHsCl appears to be destabilized ca. .2000 cm-l _As with the u* +- np (Si-Cl), there appears to be general destab~iz~tion of the lower excited states of SiH,Cl. Upon reviewing the various reported spectral values (see table 2) ior ail the compounds in question, this destabilization is unique to Siff3 Cl.
An intense structured absorption is observed in the and is thought to be comprised of Rydberg transitions and the cr* + o (Si-Br) intravafent transition. A Sd + 40 Rydberg transition associated with the first ionization potential should have a term value of ca. 13000 cm-l and lie in the region observed. A term value of 12600 cm-l for the 6s +4p Rydberg transition of the fhst ionization potential would also place additionai intensity in the region of 133.0 nm. The first Rydberg transition related to the second ionization potential, 5s + CI(Si-Br), should occur in this region and have a term value of ca. 28600 cm-l, again adding intensity to the 133.0 nm absorption. The oscillator stren,oth calculated for this band @f= 0.48) is much greater than expected even for muItipfe Rydberg transitions. Because of the strength of this band, the cr* +- (7(Si-Br) is thought to occur in the 133.0 nm region in addition to the Rydberg transitions. The accumulation of these excited states explains the unusually high intensity and irregular contour of this absorption. The vibrational progression noted fur this band (see table 3) begins at ca. 134.0 nm and has six resolved peaks spaced by ca. 505 cm-l. This excited state vibrational sequence would correspond,to either an region of 133.0 run
2.3. 7%~ 149.0 nnz band The Sp + 4p Rydberg transition for SiH3Br is found at 149.0 nm (ca. 67 100 cm-l) and has an associated
Table 3 Vibrational progression of the 133.0 nm band
“term value” of 2 19 00 cm-l. This value represents a
W3veIength Cnm)
Frequency (cm-‘)
A frequency (cm-’ )
stabilization of this Rydberg state oT ca. 3000 cm-’ as compared to the average first p “term values” in CH~Br. This band, like the 5s f 4p Rydberg, shows no vibrational structure or spin-orbit coupling. This .is npt unexpected as the photoelectron spectra [2] also show& very little structure, the absenci: of which
134.2 133.3 132.4 131.5 130.6 129.8
74520 75020 7.5530 76050 76570 77040
500 510 520 520 470
Volume 38, number 3
CHEMICALPHYSICSLETUTRS
increase of the ground state Si-Br stretching frequency (us) 430 cm-l or a decrease in the SiH3 symmetrical deformation (vi) of 930 cm-l [12,13]. It would seem unlikely that the Si-Br (~3) mode would increase that significantly in any of the excited states mentioned or that a vibrational progression (~2) associated with a Rydberg Ievel wouid be reduced by as much as 425 cm-l. However, large decreases in vibrAtional frequencies are not uncommon for antibonding states [ 143. On this basis, the observed vibrational piogression is assigned as several quanta of the vi, SiH3 symmetric deformation associated with the u* + G (SiBr) transition.
3. Conclusions The results and comparisons contained in this study suggest several basic conclusions. There is a general increase in transition energies for SiH, Br compared to CH,Br which is primarily the result of stabiIization of the nonbonding electrons of the halogen through delocalization to the silicon via dn-plr (SiBr) overlap in the ground state. This is also true for SiH,C!l as compared to CH3Cl. The major differences occur in the excited states, both Rydberg and intravalent . The Rydberg states of SiH,Br are stabilized as compared to those of CH3Br while there is a general destabilization of these states in SiH3CI as compared to CHsCl. If the Rydberg states are localized on the halogen, the Rydberg states of the bromine-containing compounds should he less stable relative to those of the chlorides as a result of the greater number of real precursors for bromine and, hence, less penetration to the core [ 151. There are at least two closely related explanations for the spectral observations. One reason is that the Rydberg orbital has a greater contribution from the SiH3 group in SiH3Cl than in SiH3 Br. Greater penetration to the silicon than the halogen would give less stable Rydberg states as is evidenced by the Rydberg states of the atoms [ 161, and in SiHsCl the Rydberg orbitals “see” more silicon than in SiH,Br. This is, in effect, the charge transfer argument of Robin [9]. Thus, just on a relative size consideration, the Rydberg orbitals of the chIorine compound necessariiy are associated more with the silicon atom than the corresponding bromide and are destabi-
15March 1976
l&d; The second iart of this rationalization
involves the delocalization of halogen a electron density to the silicon. The SiH3Br Rydbergs, if localized-more about bromide, would experience additional core stabilization as a resuit of the. d-ir-psr (Si-Br) interaction; whereas, the opposite could occur for the SiH3C1 Rydbergs if they arl localized about the silicon. The large blue-shift of the (T*f- np (Si-Cl) of SiH3C1 as compared to the several other compounds is the result of both the stabilization of nonbonding electrons and the d&tabilization of the antibonding state. This antibonding destabilization is brought about by chlorine vacant d orbital participation in the overlap population of the Si-CI o bond [6]. This increased population stabilizes the bonding orbital and destabilizes the antibonding orbital. This effect may be unique to the Si-CI bond as it is essentially absent in the Si-Br bond and Ge-CI bonds [&I 71. Finally, the spectrum of SiH3Br lacks the structure of CH3Br. Spin-orbit coupling is not evident, a result associated with the delocalization of the nonbonding electrons of the halogen; and only one vibrationa progression is found in the spectrum. This progression is thought to be associated with the r~* + u (Si-Br) transition and, by the enerr; spacing and shape of the absorption, indicates a relatively large change in the internuclear coordinates for this excited state.
Acknowledgement The authors thank the Robert A. Welch Foundation, Research Corporation and the NTSU Faculty Research Fund for their generous support of this work. Randall C. Rains, who aided in the preparation and purification of the compound, is also gratefully acknowledged.
References [l ] S. Bell and A.D. Walsh, Trans. Faraday SOC.62 (1966)
3005. [ 21 S. Cradock and R.A. whiteford. Trans. Faraday Sot. 67 (1971) 3425. [3] G-C. Causley and B-R. Russell, J. Electron Spectry., ?a be published. [4] D.C. Frost, F.G. H&ring, A. Katrib, R.A.N. McLean, I.E. Drake and N.Z.C. Westwood, Chem. Phys. Letters 13 (1971) 347;Csn. J. Chem. 49 (1971) 4033.
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161 J.&k tiowgllarqi J.R.van Wgzer, J. Am. Chem. Sot. 96 (1974j.1064. : .‘[7] .k.L.k&ud, in: -h~orgxnicsyntikis, Vol. 11, eb. W-L. Jolly @kGra~-I5ll, Xew Y&k; 1968) pp. 165. [SJ D-W. Mayo, N.E. Opitz end 3.S. Peak, 3. Chem- Phys. 23 1195.5)1344. 191M.B. Robin, Higher exkited &tcs ofpoIyatomic ni&&es, Vol. 1 (Academic Press, New York, 1974). ; flOj G.C. Causley arid B.R. RuskI.i, 3. Chem. Phys. 62 (1975) 848.
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B.R. RUSSCU,LO. Edwards and J.W. Raysnoqd?, 4. Am. Chem. Sot. 95 (1973) 2129. f12I H. Burger, J. Cichon and A. Ruofi; Spectrochim. Acta 30A (1974) 223. [ 13 i J-L Duncan, Spectrochbn. Acta 20 (1964) i807, 1141 S.~Stokes and A.B.F. Duncan, 3. Am. Chem. Sac. 80 (1958j 6177. [ISI R.S. Mutiken, f. km. Chem. Sot. 86 (1964) 3183. [I&] C.E. Moore, N&l. Bur. Std. Circular467 (1958). [ 17 ] G.C. Causley and RR. Russ&, to bc’published. [18] A-W. P&s, H.J. Lempka. D-G, Strekts znd W.C. Price, Phil. Tmns. Roy. Sot. A268 (1970) 59.