A Gaussian 80 (6-21G) study of the species SiHn (n = 1–4) and SiHm+(m = 1–5)

Journal of Molecular Structure, THEOCHEM Elsevier

Science

Publishers

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A GAUSSIAN 80 (6-21G) STUDY OF THE SPECIES SiH, (n = l-4) SiHA (m = 1-5)

AND

Some comments on the electron impact mass spectrum of silane

DEIRDRE

POWER,

PAUL

Chemistry Department, (Received

22 August

BRINT

and TREVOR

SPALDING

University College, Cork (Eire)

1983)

ABSTRACT Gaussian 80 (6-21G) calculations were performed in both H-F and CI modes on the species SiH, (n = l-4) and SiH& (m = l-5). In general the agreement between calculated and experimentally determined properties such as the structures of SiH, molecules and the ionisation potentials of SM, were very good. The present results were compared with those previously published. The production of the ions SiH: and SiH; in the electron impact mass spectrum of silane was also analysed. INTRODUCTION

To date no comprehensive theoretical study has been undertaken of the structure, bonding, and reactivity of all silicon hydrides SiH, and their corresponding ions, even though there is now substantial data available for correlation between experiment and theory. Such a study would be valuable in understanding the reactions of the neutral molecules SiH,, where n = 1-4, and the ionization of SiH4 and its subsequent fragmentation reactions. These latter aspects have been extensively investigated by photoelectron spectroscopy [ 1, 21 and mass spectrometry [ 31. Further interest arises from studies of related silicon hydride compounds such as organosilanes H,SiCH, [4], divalent silicon compounds, silylenes, which are formal analogues of carbenes [ 51, and multiply bonded compounds [ 61 especially with the siliconcarbon double bond as found in silaethylene [7]. A number of the neutral an ionic hydride species of the types SiH, and SiH’, have been treated by theoretical methods previously. However the quality of the calculations has varied and included ab initio and semiempirical types with both fixed or optimised geometries. Even in the ab initio calculations different quality basis sets have been employed, thus making it very difficult to combine and correlate results. The availability in the Gaussian 80 programme of a small split-valence shell Gaussian representation of atomic orbit& with good inner shell representation (6-21G) allows economical ab initio calculations with analytical optimisation of geometry to be performed on these molecules. 0166-1280/84/$03.00

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This basis set is not of as good quality as the double-zeta set used by Goddard [4], and lacks d orbital polarisation functions, but it approaches full doublezeta quality and is certainly better than the single-zeta basis sets. Its application and accuracy in molecular calculation is well documented. In this paper the electronic structure and bonding of the molecules in their ground electronic states and in some cases also in their excited states are investigated. The geometries and various thermochemical data are compared with experimental values to test the applicability of the 6-21G basis set. Since both the H-F (Hartree-Fock) calculations and the CI calculations (configuration interaction with all double substitutions from the HartreeFock reference determinant) were performed it is possible to compare these two sets. The ion reactions occurring after ionisation of the SiH4 were investigated in an attempt to account for certain observations in the photoelectron spectrum and mass spectrum of silane. CALCULATIONAL

METHODS

The Gaussian 80 suite of programmes of Pople and co-workers [S] was used for all calculations with the 6-21G basis set of Hehre and co-workers

[91. Geometries were optimised at the single configuration Hartree-Fock level (unrestricted H-F where applicable), using the Bemy optimisation procedure. Configuration interaction (CI) calculations were performed using the optimised geometries and involved all double substitutions. Appearance potentials (AP) were calculated without recourse to empirical data from AP(SiH,‘) = E,(SiHi)

- E,(SiH,)

- (4 - n) E,(H)

where ET(X) refers to the calculated total energy of the species X. This obviously ignores any kinetic energy imparted to the species on formation. Such contributions, though small, may be significant in a comparison of the calculated and experimental (mass spectrometric) AP values. RESULTS

AND DISCUSSION

Table 1 contains the calculation and experimentally derived structural parameters for the ground states of the species SiH, (n = l-4) and ions together with the calculated total energies of the species. In general the agreement between calculated and experimental geometries for the neutral molecules is good. This encourages confidence in the results for the ions but unfortunately there are no experimental values for comparison. The results from the CI calculations performed on the optimised Hartree-Fock 6-21G geometries are far superior in all cases to the corresponding H-F calculations, illustrating the importance of configuration mixing in these species. Hence, further discussion will concentrate on the CI results.

SiH, SiH, SiH, SiH SiH: SiH: SiH: SiH+

148.0 151.6 152.0 -

(pm)

[25] [371

[lo]

Bond length

Experimental

Species

structures

Tel C,, C,, C mv -

148.9 148.9 153.4 154.4 14661240.3 146.6 147.9 151.7

(pm)

Bond length

data

Td C 3Y c N C C; C 8” C Z” C “v

Symmetry

of these species

120.0/91.4 120.0 121.0 -

109.5 111.7 93.7 -

(deg.)

II-Si-H angle

and total energies

Calculated

and SiH& species

Symmetry

of SiH,

110.6 113,141 92.08 [25] -

(deg. f

H-Si-H angle

data

and experimental

1

Calculated

TABLE

-291.25790 -290.63517 -290.03405 -289.43852 -290.85344 -290.35082 -289.72781 -289.16890

-289.11140

CI

-291.16666 -290.56070 -289.96461 -289.38784 -290.78082 -290.27926 -289.67314

H-F

ET (a.u.)

84

Neutral

molecules

Monosilane, SiH,, has a tetrahedral geometry with a Si-H bond length of 148.0 pm, as determined by Boyd using high resolution infrared spectroscopy [lo]. The 6-21G Hartree-Fock calculation gave a Si-H bond length of 148.9 pm, which agrees well with the experimental value, and a tetrahedral geometry. A recent ab initio molecular orbital calculation by Hopkins and Lien [ll], which employed a double-zeta basis set with optimised geometries, gave a Si-H bond length of 148.3 pm and ET value of -291.18027 a.u. which compares with our Hartree-Fock value of -291.16666 a.u. and CI value of -291.25790 a.u. A more recent calculation by Pople and coworkers [ 121, using a 3-21G* basis set with d polarisation function gave ET -289.78426 a.u. and a Si-H bond length of 147.5 pm. The radical SiH, has been detected with ESR in a krypton matrix at 4.2 K by Gordy and co-workers [ 13, 141. They report that the hyperfine structure caused by 2gSi showed that it is pyramidal and not planar like CH,. They determined the percentage s character in the Si-H bond and predicted a bond angle of 110.6”. Milligan and Jacox [15] studied the IR spectrum arising from the vacuum ultraviolet photolysis of silane. They concluded from the appearance of the stretching modes that the three silicon hydrides SiH, SiH, and SiH, give rise to vibrations which occur at frequencies significantly lower than silane and that the Si-H bonds are lengthened by the removal of one or more hydrogen atoms from the silane molecule. Walsh’s [ 161 rules for tetraatomic hydride molecules predict that the ground state AH3 molecules containing not more than six valence electrons should be planar and molecules containing seven or eight electrons should be pyramidal. Since SiH, contains seven valence electrons a pyramidal structure is expected. The 6-21G optimised calculation supports all the above observations, with the calculated geometry C3” and a H-Si-H angle of 111.7”, which is very close to the angle of 110.6” calculated by Gordy and co-workers [13, 141. The trend of increasing Si-H bond lengths predicted by Milligan and Jacox [ 151 is found in the calculations for SiH, and SiH but the Si-H bond lengths in SiH, and SiH, are equal. Wirsam [ 171 has described the results of ab initio calculations which give an accurate description of both the ground and excited state potential curves for the species SiH3, SiH; and SiHi. His SCF calculations for the ground states show that SiH3 and SiH; have C& geometry and the SiHf ion has &, geometry. Not all the published data from molecular orbital calculations agree with the present work. Jordan [ 181 describes a semiempirical quantitative theory of the ground state for different H-Si-H angles in SiH,. His results show that the geometry is trigonal planar. Recently Bell et al. [19] apparently have assumed that SiH3 has a planar geometry with a Si-H bond length of 147.7 pm. Interest in SiH2 (“silylene”) stemmed initially from its relationship to carbenes, CR2, and the possibility of silylene intermediates in particular has

85

attracted considerable attention [ 20-221. Silylenes have been directly detected in flash photolysis experiments. Although a number of careful spectroscopic studies [23--251 have been reported for SiH2, no transitions involving triplet electronic states have been identified. It would therefore seem reasonable to assume that the ground state of SiHz is a singlet. However, Dubois et al. [23] have cautioned against precluding the possibility of a triplet ground state. Skell and Goldstein [26] in 1964 demonstrated that dimethyl silylene, SiMe,, had a singlet ground state but it was not until 1974 that SiHz was proved conclusively to have a singlet ground state by Zeck et al. [ 271 when they used a nuclear recoil technique to study SiH2. Two states of SiH, were studied, the ground state ‘A, and the excited state 3B1. Dubois [25], using a flash photolysis technique, established that the ‘A, ground state has an angle of 92.1” and a Si-H bond length of 151.6 pm. The calculated geometry had an angle of 93.7” and bond length 153.4 pm, agreeing well with experiment. Meadows and Schaefer [ 281, using a very large basis set, found that the ‘A, state had an angle of 93.5” with a bond length of 150.9 pm. The structure of 3B1 SiH2, though not known from experiment, has been the subject of some theoretical calculations. Jordan’s semiempirical studies [lS] predicted a bond angle of 137.8”. Wirsam’s [29] ab initio calculated structure for triplet silylene had a bond angle of 123.5” and a Si-H bond length of 155 pm. Meadows and Schaefer [28] reported that the Si-H bond length was 147.1 pm and that the bond angle was 117.6”. The present work is in very good agreement with the results of Meadows and Schaefer, giving a bond angle of 118.8” and a bond length of 148.5 pm. Dubois [25] determined the geometry of the excited singlet ‘B, experimentally. It has a bond angle of 123” and a bond length of 148.7 pm. Meadows and Schaefer calculated the parameters to be 123.5” and 146.8 pm respectively. Bell et al. [30] constructed a graph of the values of the heats of formation estimated using a bond energy scheme of SiH, vs. n. A smooth curve was obtained for the values n = 0, 1, 3 and 4, but the heat of formation of the singlet SiH, was some 11.85 kcal mol-’ more stable than that predicted from the curve. They concluded that the difference of ca. 11.9 kcal mol-’ corresponded to the ‘A1-3B1 separation of SiH,. This separation has not been determined experimentally but the difference in the heats of formation of the ‘A, and 3B1 states has been calculated to be 13.98 (Wirsam [29] ) or 9.95 (Kasdam et al. [31] ) kcal mol-‘. Using semiempirical calculations, Jordan [18] determined that the 3B, state lay 46 kcal mol-’ above the ground state. A later study by Wirsam [17] reported the separation to be 4.8 kcal mall’. More recently, Meadows and Schaefer [ 281 found the separation to be 18.6 kcal mol-’ but they then reduced this value to ~10 kcal of a semiempirical correction. This correction was simimol-’ on application lar to that necessary to reconcile the calculated singlet-triplet splitting for CH, with the experimental result of Lineberger and co-workers [ 321. In the present work, the separation is calculated to be 1.94 kcal mol-’ if the H-F

86

results are used and 10.4 kcal mol-’ from the CI results. The latter is in excellent agreement with Meadows and Schaefer’s work [ 281. Several spectroscopic studies of the SiH molecule have been reported [33-361 and there has been much interest recently in the detection of the radio frequency spectra of this molecule among interstellar species. The calculated bond length of 154.4 pm agrees well with the experimental value of 152 pm [37]. Ions

One of the first ion-molecule reactions to be studied in detail was the formation of the methanium ion, CH: in the bimolecular reaction of CH: with methane [38, 391 CH,’ + CH4 -+ CH; + CH3

(1)

One of the differences between silane and methane is the facile formation of the methanium ion via reaction (1) and the absence of the analogous silanium ion, SiH: in mass spectrometric studies of silane via SiH,’ + SiH4 + SiH: + SiH3

(2)

The failure to observe reaction (2) has been attributed to the absence of significant amounts of SiH,’ as a primary ion formed by electron impact on SiH4 [40]. The SiH: ion has, however, been observed by many workers via other reactions. Beggs and Lampe [41] reported the appearance of SiH: in silane-methane mixtures and proposed a termolecular reaction to account for its formation CH: + SiH4 + M + SiH: + CH3 + M

(3)

Cheng and Lampe [42] using a tandem mass spectrometric technique showed that SiH: is formed in bimolecular reactions of SiH4 with NH:, C,H& CzH6+ and C3Hl ions. Formation of the silanium ion was also observed by Sefcik et al. [ 431 via reactions of the type CH: + SiH4 + SiH: + CH4

(4)

Initial theoretical work on SiH: was carried out by Hartmann et al. [44] who considered only two possible geometries, CQVand D,,.,. They found the latter to be 1.9 kcal mol-’ more stable than the former. Sefcik et al. [45] studied the formation of the silanium ion experimentally in detail using deuterated reagents and their results indicated that the silanium ion does not have D3h symmetry. Instead they favoured a C, structur,e, similar to that calculated [46] for the methanium ion CH: which has been experimentally verified [43]. They carefully did not preclude the possibility of silanium ion having a CJV geometry and suggested that the CsV geometry should be included when SiH: was examined theoretically. We have studied the C, and C,, geometries in detail at both the H-F and CI levels. The results are

87 HO

\

Ho, SI+~

Hb-Hc

CS

C3”

E TOT = -291.50387

au.

E TOT = -291.50554

0.”

Bond lengths (pm): SI-Ho= 146.6, Sl-Hb’ 213.7, Hb-H,= 74.2

Bond lengths (pm): S%H,=l46.5, SI-H~,~= 244.0, Hb-H, = 74.8

Bond angles. Ho-SI-H,

Bond angles: H,~S1-H,~l20.0~ Ho+-Hb= Hb-Z&H,= 17.7’

= 119.8: Ha-St-Hb=91.9’

97.3:

H,,-SI-H,~~~.~”

Fig. 1. Structures of SiH: ions.

summarised in Fig. 1. The calculations indicate that the C, structure was lower in energy than the Csv structure by 0.9 kcal mol-’ (H-F) or by 1.1 kcal mall’ (CI). Very recently, Schleyer et al. [47] have studied the structure and energetics of SiH:. These workers had the facility of using an MP4/6-31G* basis set which was unavailable for the present work. The lowest energy geometries for SiH: were C, and C3”, with C, being lower by 7.2 kcal mol-’ (MP4/6-31G** 6-31G*) or 2.47 kcal mol-l (6-31G* 6-31G*) respectively. The 6-21G calculation gave somewhat longer Si-H,,, bond lengths (Fig. 1) being 244 pm compared to 194 pm for the C,, and 214 pm compared to 208 pm for the Csv geometries. Further, the 6-21G calculations showed the total energies of these systems to be remarkably insensitive to Si-H bond length changes. In their discussion of the photoelectron spectrum of silane, Potts and Price [ 21 have shown that three states of SiH,’ were observed in the 11.514.0 eV region. These states are all produced by the removal of an electron from the t2 orbital of SiH4. An ion in tetrahedral geometry with the electronic state t2,5:2T is found between two DZd Jahn-Teller distorted states. The t2 orbital splits under this distortion to e + b, and the two states are e4 b :‘B at lower energy and e 3b ‘:‘E at higher energy. From their photoelectron spectrum Potts and Price determined the adiabatic ionization of ‘B as 11.60 eV, the vertical ionization of ‘T, at 12.82 eV and the vertical ionization of ‘E at 13.4 eV. The vibrational structure of the ‘B band was used to estimate a H-Si-H bond angle of 140”. The total energy of SiH4+was calculated in a number of different geometries and several states with slightly different energies were found. We therefore restricted further calculations to selected structures. These were a Td structure with Si-H bond lengths retained at the SiH4 value, i.e., 148.9 pm designated Td*, optimised Td, DZd, Czv, and free varied Csv which corresponded to the weak complex (SiH:* * 0H). Figure 2 shows the structures of the Td, Dzd (‘B), Czv, and C& SiHz ions; Table 2 summarises the energies. Perfect agreement between the calculated

88 H

H,

HO

D2d

G Bond

lengths

St-H

= 158.1

Bond

angles:

H-51-H

(pm

1.

SI-H

= 109.5”

Fig. 2. Structures

C2”

= 152.5

C3”

SI-Ho=

160.9,

SI -H,

S-H,,=

146.6

SI-H,=240.3

a = 14o.op

H,-SI-H,

@=969’

H,-SI-Hb=

113.8:

Hb-S~-Hb=

122.9’

= 64.7:

= 146.6,

Ho-SI-H,

= 119.9:

H,-SI-H~=

91.4”

of SiH: ions.

and experimental adiabatic IP of the 2B state was found. The adiabatic ionisation from 2T, was 12.18 eV, lower by 0.46 eV than the experimental vertical ionisation. The calculated bond angle of the DZd structure agrees well with Potts and Price’s estimate, although this may not be very significant as the energy of the molecule is fairly insensitive to this parameter in the region of 140”. Potts and Price showed that Dzd was the only (Jahn-Teller) distorted state of SiHl to be observed spectroscopically, even though this was one of a number of possible geometrical distortions. We investigated Czv distortions and the lowest energy one is reported in Table 2. This state has an energy 0.36 eV lower than the lowest energy ionisation observed by Potts and Price. Another state, the Csv (SiH: * * * H) complex, has an even lower energy, being 0.21 eV below the Czv state. The “complex” consisted essentially of a planar SiH: ion loosely bound to a hydrogen atom 240 pm from the Si atom along the C3 axis. There are no experimental data available on the geometry of SiH: but Walsh’s rules [16] predict that the lowest energy state should be planar since it has six valence electrons, also a Dsh planar triangular geometry is expected TABLE

2

Calculated

states

of SiH:

Symmetry

Bond length (pm)

(iii) (iv) (v)

Dzd (‘B) c zv C 3”

148.9 158.1 152.5 146.4/160.9 146.61240.3

aPhotoelectron spectroscopically troscopically measured adiabatic

ET (CI) (a.u.) -290.803709 -290.810167 -290.832369 -290.845761 -290.853442

measured vertical IP = 12.82 IPDzd = 11.60 eV [ 21.

Ip (eV) (ET ion-E,

SiH,)

12.36 12.1ga 11.5gb 11.21 11.00 eV. bPhotoelectron

spec-

89

by comparison with the known CH: ion. However, calculations performed by Wirsam [17] on SiH: produced a non planar geometry (C&, Si-H bond length 152.9 pm, H-Si-H bond angle 115.7”). Hopkins and Lien [ll], using a high quality basis set, found the expected planar Dsh geometry with Si-H bond lengths of 146 pm. The present work agrees completely with the latter giving bond lengths 146.6 pm and bond angles 120.0”. The electronic structure can be succinctly described as sp’ hybridised silicon with an unoccupied orbital of pure p character in the C3 axis. No experimental or calculated data are reported in the literature for the geometry of SiH:. The V-shaped structure was calculated to have a Si-H bond length of 147.9 pm and a bond angle of 121.0”. Like the previous species there was no experimental bond length available for SiH’. The calculated value is 161.7 pm. Ionisation

and fragmentation

reactions

of SiH, in the mass spectrometer

There have been several reports of the mass spectrum of silane and in some papers IP and AP data were published. These data were generally used to analyse the fragmentation reactions of the SiH,’ molecular ion. Unfortunately there are many discrepancies in the literature. Perhaps the most obvious example concerns SiH,‘. Morrison and Traeger [3] reported that they did not detect the SiH,’ ion and therefore could not measure the IP of SiH4 mass spectrometrically, i.e., via the reaction SiH4 + e- + SiH,’ + 2e-

(5)

Saalfeld and Svec [48], and Neuert and Clasen [ 491 claimed to have observed very low intensity, but measurable, amounts of SiH,’ and report the IP as 11.4 eV and 12.2 eV respectively. Morrison and Traeger pointed out that the relatively high IP found by Neuert and Clasen probably referred to the AP of the ion 2gSiHf (vide infra) whilst the low value of Saalfeld and Svec may have been due to an unspecified impurity. Saalfeld and Svec’s value is 0.2 eV lower than the adiabatic IP measured later by photoelectron spectrosCOPY ]I, 21. While it seems possible that their sample could have been impure, it seems highly improbable that they were observing lower energy states such as those of C,, or C,, symmetry (Table 2) which Potts and Price did not find. The calculated values for the AP’s of these states were 11.00 eV and 11.21 eV, respectively, whereas the lowest state (DZd) observed by photoelectron spectroscopy was at 11.6 eV and the Td state was at 12.2 eV. According to the value obtained by Neuert and Clasen (AP = 12.2 eV) they did not observe any of the C&, Czv or Dzd states but only the T, state. However, it should be noted in this discussion that errors in the mass spectrometric measurement of IP’s and AP’s are commonly accepted to be ?O.l-20.2 eV, which is quite large compared with the energy separation of the states of SiH& Since even the most basic evidence for the appearance or non-appearance of SiH4+is con-

90

tradictory, it is very difficult to provide an unambiguous interpretation the results concerning SiHi. Fortunately, the experimental AP evidence production of the ions SiHl and SiH: is much more consistent. The formation of SiHi must occur via the reaction SiH4 + e- -+ SiH: + H’ + 2eExperimentally measured AP values for reaction [3,49] and 11.8 eV [48]. The formation of SiH: mass spectrometrically tion (7) or (8)

of

for

(6) (6) are reported

to be 12.2

could occur by either reac-

SiH4 + e- -+ SiH: + Hz + 2e-

(7)

SiH4 + e- + SiH: + 2H’ + 2e-

(8)

Two sets of appearance potentials have been reported. Morrison and Traeger observed SiH: formation at 11.8 and 16.2 eV, whereas Saalfeld and Svec obtained values of 12.1 and 16.5 eV. The results of Neuert and Clasen (14.5 and 18.6 eV) would appear to be much too high. Morrison and Traeger assigned their lower AP to reaction (7) and the higher AP to (8). Thus it may be seen that the formation of SiH: from (6) and SiH: from (7) occur with approximately the same AP, i.e., 11.8-12.2 eV, according to both Morrison and Traeger, and Saalfeld and Svec. This situation is unusual in the mass spectroscopy of simple inorganic molecules where, in general, even-electron species predominate. In the present case the ion SiH: is an evenelectron species and SiH,’ is an odd-electron ion. There are two possible reactions to give SiH’ SiH4 + e- -+ SiH’ + Hz + H’ + 2eSiH, + e- + SiH’ + 3H’ + 2e-

(9) (10)

Various values for process (9) have been reported, e.g., 14.7 eV [3], 16.1 eV [48] and 14.5 eV [49]. The large disparity precludes an unambigous interpretation of energetics of (9). Saalfeld and Svec were the only workers to report an AP for reaction (10). Their value was 20.4 eV. The three possible reactions producing the Si’ ion are given below (11-13) SiH, + e- + Si’ + 2Hz f 2e-

(11)

SiH4 + e- + Si’ + H, + 2H’ + 2e-

(12)

SiH, + e- + Si’ + 4H’ 2e -

(13)

Saalfeld and Svec, and Morrison and Traeger observed AP’s for all three processes. Saalfeld and Svec’s values of 11.7, 16.4, and 20.8 eV may be compared with Morrison and Traeger’s of 13.3 eV, 18.5 eV, and 23.4 eV for reactions (ll), (12) and (13), respectively. Once again the disparity in the results makes further comment on these reactions very difficult. For further discussion of the mass spectrum of silane we will concentrate

91

on the results obtained by Morrison and Traeger [3] for the SiH& SiH: and SiH: ions. Their work is the most recent and they used a deconvolution method to obtain AP’s which is far superior to the earlier methods. Their failure to observe the SiHi ion in the mass spectrometer is consistent with the fact that a totally optimised calculation on SiHi fails to find a geometry in which all the hydrogens could be considered to be satisfactorily bonded to the silicon atom. Instead, the calculation suggests a species (C& H,Si’* . . H), which is in the process of spontaneous dissociation. It may therefore be reasonable to assume that when silane is ionised vertically, the ion formed decays within the time scale of the mass spectrometer, i.e., lo6 s, to fragmented products. The same argument could be applied to the state with Cav geometry at 0.21 eV higher than the C3” ion, but with possibly different fragments being formed, i.e., SiH: and Hz. Thus upon electron impact SiH4 is vertically ionised into a vibrationally excited Dzd (*B) geometry state. The potential energy curve for the Dzd (*B) state interacts with those of the Csv and Czv geometry states and the SiHi ions formed at about 12 eV spontaneously decompose to SiHl and SiH: ions via reactions (6) and (7). In Morrison and Traeger’s work reaction (7) giving the SiH: ion was more favourable with an AP 11.8 eV compared to 12.2 eV for SiHl by reaction (6). An increase in the ionising energy above 12 eV gives more excited Dzd ions and eventually SiHl in Td, and Dzd (*E) geometries. Decomposition of these ions leads to other SiH: and SiH: ions. CONCLUSION

The data presented represent the most comprehensive study to date of the species SiH, (n = l-4) and SiHL (m = l-5). Generally, the agreement between calculated and the experimentally determined structure of the neutral molecules is good. Likewise, the calculated and photoelectron spectroscopically determined IP’s of SiH4 agree well, This leads to confidence in the calculated data for the ionic species. However, the extension of the basis set to, e.g., the recently published 6-31G** [47] will provide more accurate results. ACKNOWLEDGEMENT

One of us (D. P.) wishes to acknowledge Education in the Republic of Ireland.

the support of the Department

of

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