SiH stretching frequencies in chlorodisilanes and ab initio calculations of geometry, stretching frequency, vibrational intensity and Mulliken and King effective atomic charges

Journal

of Molecular Journal

of

MOLECULAR STRUCTURE

Structure 376 (1996) 289-303

SiH stretching frequencies in chlorodisilanes and ab initio calculations of geometry, stretching frequency, vibrational intensity and Mulliken and King effective atomic charges’ D.C. McKeanaT*, M.H. Palmer”, “Department

of Chemistry, bC.L.R.C.

M.F. Guestb

Universit~~ of Edinburgh, West Mains Road, Edinburgh Daresburjj Laboratory, Warrington, WA4 4AD, tiK

EH9 3JJ, UK

Received 6 July 1995; accepted in final form 4 October 1995

.4bstract Infrared frequencies in the vSiH region observed in mixtures of chlorodisilanes are assigned by the joint use of substituent effects, determined earlier in SizHSCl and l,l-Si2H,C12, and ab initio calculations for 1.2~Si2H4C12, 1,1,2Si2H,C1, and SiH,SiC1~, which together with 1,1,2,2-Si2HzC14 were identified as being present. t&iH frequencies for all the staggered conformers are predicted using harmonic local mode theory. Ab initio geometries were obtained for the above three compounds at the HF/6-3lG* level, and for both conformers of I,2-Si2H4C12 at HFitzvp and MPZ/tzvp levels. Alpha or gauche chlorine substitution shortens both SiH and SiCl bonds, the former reflecting vSiH changes. Trans chlorine substitution shortens Sic1 bonds slightly. but has no effect on SiH bonds. The local mode behaviour of vSiH vibrations extends to both infrared and Raman intensities. Ab initio-based atomic polar tensors are used to obtain dipole derivatives, dp./dr, for the SiH and SiCl bonds and King effective atomic charges, 6, for all atoms. The changes in the latter are compared with changes in the Mulliken atomic charges and a close connection is found in the case of gauche chlorine substitution.

1. Introduction Two recent papers [1,2] have demonstrated the potential of the combined use of isolated SiH stretching frequencies, measured in partially deuterated SiH-containing compounds, together * Correspondmg

author.

’Dedicated to Professor James E. Boggs on the occasion of his 75th birthday in grateful recognition of his global role in the encouragement and support of quantum-mechanical applications in every sphere of chemistry, including the present theme of the close relationship between CH and SiH stretching frequencies and bond lengths, to which he has made extensive

contributions himself, e.g. Refs. [2,4,6 81. 0022-2860/96/$15.00 0 1996 Else&r SSDI 0022.2860(95)09088-6

with harmonic local mode (HLM) force field calculations, for the analysis of the z&H and vSiD regions of isotopomers of these compounds. Information is obtained about the stretching force constants (f) of individual SiH bonds and the way these vary with substitution and conformation [2-51, and also of the stretch-stretch interaction constants between bonds sharing a common silicon atom ( fd) or on adjacent silicons ( L!. f,‘i for gauche- and trans-related SiH bonds. ‘a The ’ ’a’constints ’ obtained in this way for S&H&l and l,l-Si2H4C12, two of the molecules studied in Ref. [2], have been nicely confirmed by scaled ab initio force fields (SAIFF) treatment of the

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290

D.C. McKean etal.~Journal~~Moiecular S~rucrure 376 (I9961289 303

complete harmonic vibrational spectrum for these two molecules [6,7] and Si2H, [8]. The method used to prepare these chlorocompounds, the reaction of boron trichloride with disilane, yields mixtures of products of varying volatility. In the less volatile fractions of the undeuterated samples there occurred an additional 18 peaks in the vSiH region not attributable to SizH,Cl (hereafter 1) or 1,I-Si2H,C1, (hereafter 11). In the deuterated samples an additional five z&H peaks were found. The assignment of these peaks provided a fitting challenge for an application of the principles which proved successful in elucidating the spectra of 1 and 11. Use of the latter has enabled us to identify the presence of two conformers each of 1,2Si,H4C12 (hereafter 12T,G), 1,1,2-S&HsCls (hereafter 112CJ12Ci) and 1,1 ,2,2-SizHzC14 (hereafter 1122T,G) and also of 1, 1,l-SiH, SiCls (hereafter 111).We are able to predict all the SiH stretching frequencies of each of the above compounds and also of 1~1,I ,2,2-Si,HCl, (hereafter 11122) which should be of interest in view of the recent report of the preparation of pure samples of all these compounds [9].* The only prior vibrational data available are some infrared bands for 12 and 112 in earlier work by Hollandsworth and Ring [lo]. In addition to the successful identification of nearly all the unknown bands, the availability of new ab initio treatments of the geometries and vibrational properties of these compounds has enabled us to explore the way in which bond lengths change with chlorine substitution and also the effects of the latter on the electrical properties of these molecules which determine the fundamental band intensities. While intensities at an HF level at least are often far different from those measured experimentally, there is increasing evidence that changes from one molecule to another observed experimentally are well reproduced ab initio providing the level and basis set are kept constant throughout [6.7].

For 1 and 11: connections were found between Mulliken atomic (qa) and King effective (&a) atomic charges, the latter being defined by

(,2= {P, .P,} where P, is the ab initio-derived atomic polar tensor [6,7]. These connections extend to the compounds 12, 112 and 111 studied here. The data from the ab initio calculations of course have a bearing on the question of conformer abundance in the 12 and 112 compounds, since complete sets of vibrational frequencies are available as well as the absolute energies in the equilibrium states. These will be discussed in an accompanying paper [l 11.

C M-’ ’ Our

predictions for the gas phase should be particularly useful since the Raman data to be reported from this study [9] will apparently be for the liquid phase only.

Fig. 1. Infrared spectrum in the uSiH region of 1,2-Si2H& containing a little 1,1.2-SizH,C13: vapour pressure z 5 Torr in an 11.7cmcell.

D.C. McKean

2. Experimental and experimental

et a/./Journal

of Molecular

Structure

376 (1996)

291

289-303

Table 1 Infrared features (cm-‘) observed in the uSiH region of chlor-

results

The method of preparation having been discussed previously [2,6], we need mention here only that the composition of the products varied from one run to another, possibly reflecting the condition of the vacuum system. In one later preparation, a sample was obtained which was almost pure 12, containing just a little 112 (see Fig. 1). This sample was stable at room temperature, in contrast to earlier reports of the spontaneous rearrangement of 12 to 11 [12]. Infrared spectra were obtained as before on a Nicolet 7199 spectrometer at a resolution of 0.5cm-‘. Fig. 2 shows a typical spectrum of a mixture of higher chlorodisilanes which contained 1122, 112, 111 and 12 compounds. Table 1 lists the wavenumbers of all features seen in the vSiH region which could not be ascribed to

odisilanes other than those due to Si2HsCIor I,I-Si2H4C12 Undeuterated 2204.4Q 2201.9q 2199.9Q 2197.6sh 2192.5Q 2191.8sh 2190.2sh 2186.9Q 2186.4Q

samples 1122T ll22G 112c, 112C, hot? 12T 1 112c, 112c, 12G

Deuterated samples 2204.5 Q 1122T

2199.2Q 2188.9Q

112c1 112c,

2184.5q 2184.1sh 2181.6q 2179.0q 2177 sh 2175.0q 2172.9Q 2169.7q 2165.7q,A

12G 112c, 111 112c, ? 112c, 111 ? 12G

2184.6sh 2181.4q

112c, 112c,, +c, ?

either 1 or 11. The assignments the discussion below.

given follow from

3. Ab initio calculations These

were carried out using the program [13]. In addition to the standard 6-31G* basis used for the 12, 112 and 111 compounds, 12 was also treated with a tzvp basis set. In the latter, for the silicon and chlorine atoms, the (12~9~) primitive set of GTOs of Veillard [14] is recontracted [6s3slslslsls/5plplplplp], with one exponent repeated in the 6s/3s contracted functions [15] (L& = 0.388, &, = 0.619). For the hydrogen atom, the (4s) set of GTOs is contracted to [3sls] with &, = 1.00 (Huzinaga [16]). Calculations were performed at both HF and MP2 levels with this basis set for 12. In the version of GAMES used here, the orbital exponents of the wave functions differed slightly from those in GAUSSIAN 90, the program employed in the earlier work on Si2H6. 1 and 11 [2,6-g]. Since the analysis of small changes in bond length and atomic charge across a series of molecules requires complete uniformity of treatment, we have repeated the calculations of geometry and atomic charges for the above molecules using GAMESS. In every case, refinement was continue{ until bond length changes were less than 0.0001 A. GAMESS

2220

22n0

2iH0 CM-’

2iGo

Fig. 2. Infrared spectrum m the vSiH region of a mixture of chlorodlsilanes. gas phase.

292

D.C. MC&WI el al.:Journnl o/‘Molecuiar

3.1. Assignments efSects

A: predictions from substituent

The key to understanding the influence of chlorine atoms on either SiH or Sic1 bonds is the identification of the numbers of chlorine atoms lying alpha (vicinal). gauche or trans to the bond concerned. This is most readily seen from Newman diagrams for the staggered conformers likely to be present. Fig. 3 shows such diagrams for the 12 and 112 compounds, each H or Cl atom being labelled with a superscript x or xx depending on whether it is N to one or two chlorines. and with subscripts including g and t to indicate the numbers of gauche and trans chlorines. We now seek to predict the effects of these chlorines from our previous experience with the 1 and 11 compounds. Table 2 shows the v”SiH values determined in 1 and 11 [2], together with

%

“i

“9”

x Ht

Stru~rure 376 (1996) 289-303 Table 2 u”SiH values Si2H,Cl,

and

substituent

S&H&l

effects

in Si2H,CI

and

l,l-

1.1~Si2HJC12

Bond

I? (cm-‘)

S, a (cm-‘)

Bond

d” (cm-‘)

S, b (IX-‘)

SiHX SiH SiHB

2176.5 2174.3 2160.6

sx 13.9 s, 11.7 s, -2.0

SiHXx SiH SiHiF

2191.8 2179.7 2172.3

S” 15.3 ss 5.4< s, -2.0d

a S, = #SiH, - 2162.6 cn-’ (Y” for S&H, [l]). b S = &iH in l,l-Si,H,CI, - v’“SiH, in Si2H,Cl. ’ dr2S = l;.l cm-‘, based on S&H,. d OrSgg+ S, = 9.7 cm-‘, based on Si2H6.

the substituent effects Si which measure the changes in vi’ due to substitution of a chlorine atom in an CY (S”), gauche (S,) or trans (S,) position. The Si values are not constant: Sx rises slightly from 1 to 11, whereas Ss falls substantially.3 An alternative way of interpreting the data for 11, which refers the vi’ values directly to S&He, gives 2Ss = 17.1 cm ‘, S, + S, = 9.7 cm ‘. For the prediction of II” in the other chlorodisilanes, we add to the js value for the SiH,Cl group in 1 (2176.5 cm-‘), or the corresponding value for the SiHCl* group in 11 (2191.8 cm-‘), the appropriate Sp or S, increment. Thus for the 12 conformers, the solitary T frequency is given by v”SiHi = 2176.5 + 11.7 = 2188.2, while for the G conformer, where two types of SiH bond are present, as illustrated in Fig. 3, we have PSiH:

= 2176.5 + 11.7 = 2188.2cm-’

u’“SiHf = 2176.5 - 2.0 = 2174Scm-’ I Cl&41

112c,

Ix Hgg(4)

112Cl

Fig. 3. Newman diagrams for the conformers of 1,2-SiZH,Cl,, point groups Cz (12G) and CZh (12T). and of 1,1,2-Si2H,C13, point groups C, (112C,) and C, (112C,). The superscripts x or xx indicate that the atom concerned is cy to one or two chlorine atoms; the subscripts g or t indicate the numbers of gauche or tram chlorines.

For 11122, we take 2Ss and S, values from 11 in Table 2 and obtain v”SiHC$, = 2191.8 + 17.1 - 2.0 = 2206.9cm-‘.

3A lack of strict additivity of S, values for CH bonds has been previously noticed in chloroethanes [17], where a rule of “diminishing returns” seems to apply.

293

D.C. McKean et al.!Journal of h4olecular Structure 376 11996) 289-303

In a similar way for 111, we get v’“SiH,,,

A standard HSiH angle of 108.88” was assumed in each case. The resulting da frequencies appear under (1) in Table 3.

= 2162.6 + 17.1 - 2.0 = 2177.7cm-‘.

The values of &iH predicted in this way are listed under (1) in Table 3. These of course refer to the frequencies expected in the partially deuterated species with only one SiH bond present. Frequenties for the undeuterated molecules were then derived from the v%iH values via a harmonic local mode calculation, assuming constants similar to those in Ref. [2], namely, fd = 0.0023, f,’ = O.O074,f,’ = -0.0024 mdyn Angstrom-‘.

Table 3 Comparison of predicted and observed Compd.

12T

z&H

3.2. Assignments

Table 4 includes all the data relevant to the SiH stretching transitions obtained from the HF/6-31G* treatments, i.e. frequencies, infrared and Raman intensities. Inclusion of some of the partially deuterated species enables us to see how well the assumption of separability of the high SiH

vSiH values for chlorodisilanes

(cm-‘)

vSiH (~&a)(cm-‘)

Bond

%r(l)n

Qr(2?

“ob,’

SYtIl

Qr(lY

5mb

“ob,’

Hi

2188.2

(2185.5)

(2184.5)

a, ‘k

bu

2195.2 2187.3 2187.0 2183.0

2193.5 2185.3 2184.8 2178.4

2192.5 (2184.3) (2183.8) (2177.4)

a” a’ a’ a”

2191.0 2187.2 2 177.4 2169.6

2187.4 2184.7 2175.5 2165.3

2186.4 2184.5 (2175.1) 2165.7

2191.8 2189.3 2181.1

2190.0

2186.9 21X4.1 2175.0

ag 12G

H;. H:

112c,

H; H;,t

2188.2 2174.5

2184.9 2171.6

(2184.4) (2171.6)

2189.X 2186.2

2182.4 2185.9

2184.6 2181.4

:: a’

112Cl

111

H& H;,t

2203.5 2193.6 2186.2

2196.3 2197.0 2188.9

2199.2 2188.9 2181.4

H ggt

2177.7

2179.7

(2178.6)

H;:

2204.2 2194.3 2184.6 e a1

1122T

B: ub initio predictions

%

2208.9

1122G

Hz:

2201.5

1112

%!I

2191.6

11122

H41

2206.9

2204.5

(2198.4)

2180.6 2171.2

2199.9 2190.2 217Y.O 2183.2 2172.5

2181.6 2172.9

aP

2209.9 2108.0

2204.4 (2204.6)

a’ a”

2204.4 2198.6

2201.9 (2194.8)

a’ a”

2194.7 2188.3

bu

’ Frequency predicted from #SiH data for 1 and 11. b Frequency predicted from HF/6-31G* force field, scaled by O.Y064(SiH”), 0.8992 (SiH’“) or 0 9086 (SIH in 111). ’ Frequency observed in this work. In parentheses, values estimated from the HLM calculations in Table 5 for 12T. 12G, 112C,, 112C, and 111, or withf,’ = -0.001 (llZZT),$ = 0.0089 (1122G). d Based on the u’%H predlctions under (I) using the HLM constantsf,’ = 0.023,fd = 0.0074 andh’ = -0.0024 mdyn k’ [Z]. HSiH angle assumed = IOR.88”.

294

D.C. McKean

Table 4 vSiH frequencies (cm-‘) and intensities lations for chlorodisilanes Molecule

Sym. species or bond

S&H, l-d,, l-d,

C/6

2365.1

C/5

2388.6

Z/5

2401.3 2385.4 2369.3 2388.5

C/4

2404.2

X/4

2420.0 2411 Sl 2410.4 2403.3 2411.2

SiH’ SiH, SiH,

11 lZT-da

Wa

a, b, a, b,

IX-da

12C-d,

112C,-d,

Ill-d,,

2162.6’

AC

159.3

139.0

2176.5’ 2174.fj6 216O.la 2172.5

170.4 148.4 172.6 163.0 141.5

140.4

2193.5’ 21RS.i’ 2184.8’ 2178.4’ 21X5.5

324.3 0.n 0.0 23X.7 140.8

0.0 177.0 366.6 0 0 135.9

C/4

2410.5 2395.8 2403.2

2184.9’ 2171.6’ 2178.2

143.3 171.7 157.5

C/3

2428.4 2416.2 2405.5 2416.7

C/3

2421.0 2411.6 2416.7

C/3

2443.0 2426.0 2411.9 2421.1

C/3

2442.5 2423.9 2414.9 2427.1

2196.3 h 2197.0’ 2188.9’ 2194.1

127.7 121.5 140.6 129.9

C/3

2402.9 2391.2 2399.0

2183.2’ 2172.5l 2179.6

3n1.4 78.1 126.5

SiHxX SiH$,

xx

SiH, SiHX SiHff

c a,

376 (1996)

289-303

2190.0’

2182.4h 2185.9’ 2184.7

187.0 156.9 99.1 147.7

Molecule

Sym. species or bond

Wa

Pb

AC

Cd

Ill-d,

SiH,,,

2399.0

2179.6’

129.5

134.5

Cd

134.5

C/4

a’ a”

Structure

calcu-

173.6

289.5 0.1 243.8 86.8 155.1

112C,-d,,

llZC,-da

iib

2187.4’ 2184.7’ 2175.5’ 2165.3’ 2178.2

SiHi SiH:

of Moiecular

Table 4 Continued from HF/6-3lG*

2413.3 2410.3 2400.1 2388.9 2403.2

b a a b

a’

112C s -d-. /

et al./Journal

10.9 266.2 235.9 78.4 147.9

216.9 96.9 157.0 156.9

154.7 146.6 149.3 144.5 112.0 129.5 128.7

178.9 230.1) 136.3

* Unscaled frequency (cm-‘) from the ab mitio calculation. Data for S&H,, 1 and 11 are from GAUSSMNYUand are greater than the C~AMESS values by 6-7 cm-‘. b Scaled frequency (cm-‘). ’ Infrared intensity in km mol-‘. For S&H,, l-da and 11 the A values from the GAMESS calculation are corresponding respectively, 177.9, 161.8 and 143.1 km mol-‘. d Raman cross-section in A4 amu-‘. e Scale factor 0.9141 [S]. ’Scale factor 0.9064. a Scale factor 0.9117 (average of values for SiHt and SiH, in 1 I’31). h Scale factor 0.8992. ’Scale factor 0.9086 (avcragc 11

of values for SiH,,

and SiH,, m

[71X

stretching motions from those elsewhere in the molecule is obeyed. Thus the sum of the calculated isolated frequencies always equals precisely the sum of the do frequencies, the so-called sum rule for such frequencies. Both infrared and Raman intensities show a pleasing additivity per number of SiH bonds present, e.g. in the comparison of the intensity per SiH bond. However, for the present purpose of assigning the observed bands, we have first to look for scale factors to apply to the raw ab initio frequency data. Suitable scale factors can be found from our earlier treatments of 1 and 11 with the same 6-31G* basis set [6,7]. In these works, each ab initio SiH stretching force constant F,, was multiplied by the square of a factor si in order to fit exactly the appropriate v%H value. The factors obtained are therefore suitable for scaling ab initio SiH stretching frequencies directly, without the need for a full vibrational treatment. However, a difficulty then emerges that slightly different scale factors are required for diKerent types of SiH bond. While the values of si for the two kinds of bond in the SiHs groups of 1 and 11 arc csscntially identical, that for the SiHX or SiHXX bond is significantly different. Where only one type of bond is present in the molecule, as in 12T and 111, or where there are two bonds with very similar scale factors, as in 12G, there is no problem. However, in the 112

D.C. McKean et al./Journnl

of Molecular

conformers, except for the A” species of 112C,, the vibrations in the do species involve varying amounts of stretching of bonds with differing Si values, in circumstances where the coupling in the do species is enormously sensitive to the exact viSSiH values of the bonds concerned. In such a situation it is quite useless to attempt the prediction Table 5 Harmonic

local mode (HLM)

treatments

of scaled dO frequencies if high accuracy is to be achieved. Scaling is only feasible therefore in the 12 and ill-L& compounds and for the A” species of 112C,, the results for which are included in Table 3, in the prediction columns (2). The agreement between the scaled ab initio results and those from substituent

for chlorodisilanes

12G Mode

12T Obs

Scaled 6-3 1G*” v

e”

f’ I

HSiH

b 0.0 -0.1 0.1 0.1 0.0 -0.2

2187.4 2184.7 2175.5 2165.3 2184.9 2171.5

JP

295

Structure 376 (1996, 289-303

Mode b

v

c,

2186.4 2184.5

0.0 0.0 (2175.1) 0.0 (2184.4) (2171.6)

2165.7

2.7361 2.7032 0.0238 0.0096 (O.OY 109.643

Scaled 6-3lG*”

Obs

v

ub

2193.5 2185.3 2184.8 2178.4 (2185.5)

2192.5 (2184.3) (2183.8) (2 177.4) (2184.5)

2.1379

2.7350(l) 2.7030(l) O.O259(2)d (0.0096)e (O.O)e

0.0226 0.0091 ~0.0010 109.885

1 12c, U&s

e,

Mode

2199.9 2190.2 2179.0 2199.2 2188.9

0.0 0.0 0.0 0.0 0.0 (2181.2)

v, a’ v,> a” vz a’ #SiH; &iH;,

2.7720(l) 2.7463(l) 2.7268( 1) 0.0241(l) 0.0089(2) 110.652

i,“” .rd:

’ Scale factor 0.9064. b eu = v,,, - u,,,,. In parentheses, the calculated value of v. ’ Force constants in mdyn k’, angles in degrees. d Iff,’ is constrained to 0.0238, vi is predicted at 2183.5 cm-‘, ’Constrained. ‘ Scaled ab initio value 0.0248 mdyn A-‘.

v&z*

t”

2186.9 2184.1 2175.0 2184.6 2181.4

U&r

-0.2 0.2 -0.2 0.2 0.3

2112.9 2181.6

2.7349(4) 2.7267(8)

2.7212

0.0246(4) 0.0088(3) 110.167

v’“SiHi at 2183.7 cm-’ and u%iH:

0.0264’ 110.602

at 2172.4 cm

’

296

D.C. McKenn et a/./Journal

effects is good and confirmed a number assignments already made from the latter. 3.3 Assignments

C: harmonic

of Molt~ulrrr

of

locul mode

calculations

The final step in assigning the observed spectra involved HLM force field treatments. These took slightly different forms depending on the molecule and data available. For 12G, with only two secure vSiH values in the de species, and no Y” bands identified, the procedure was first to perform a HLM treatment on the scaled ab initio frequencies. As shown in Table 5, this yielded _fz = 0.0238, 1,’ = 0.0096 mdyn A- ’. ft’ was very small and needed to be constrained to zero. With all three interactions constrained to the above values, the two observed frequencies were then fitted and one V” frequency was predicted to lie at 2183.6 cm-‘, very close to a very weak feature at 2184.5 cm-] in the spectrum (Fig. 1). The force field quoted in Table 5 gives an exact fit to these three frequencies with f,’ changed to 0.0259 mdyn A-‘. The treatment of the scaled ab initio frequencies of 12T was similar. However, the higher symmetry makes refinement unnecessary and allowsf,’ to be determined as being very small (-0.001 mdyn A-‘). The solitary of observed frequency

j .

.~”

Structure 376

(lYY6)

2192.5 cm-’ is then adequate to allow exact prediction of the remaining unobserved ones, without further refinement. For 112Ct, all frequencies were observed except for one of the #SiH ones. vSiHi,. A completely empirical HLM treatment was therefore possible. The values of fi, 0.0241. A-’ (Table 5) agree and J’;. 0.0089 mdyn excellently with those derived from the ab initiobased treatments for 12G and 12T. 112C, could be treated in the same way, with five observed data. Although the overall fit was slightly poorer, the values of Ji and &’ are pleasingly close to the others, leaving little doubt about the essential correctness of the assignments. The estimates of Y” in 12 and 112 then permit an interpretation of the spectrum seen in the first overtone region of 12 (Fig. 4, Table 6). While this spectrum is that of the undeuterated material, coupling between the individual bonds at the fundamental level is so small that at the first overtone level local mode behaviour should be complete. Table 6 shows that values of x, obtained on this assumption are within the range normally found for SiH bonds. Slightly more consistent results are obtained if the lower value of v”SiHi of 2183.6 cm-’ for 12G is adopted, as indicated in parentheses in Table 6. For 111, the two observed frequencies, in conjunction with an ab initio HSiH angle, give fd = 0.0264, in good agreement with the scaled ab initio value of 0.0248 mdyn A-‘. The consistency of our force field results in Table 5 makes us confident that our predictions of the unobserved frequencies are good to better than into the 1 cm ~I. These estimates are incorporated “observed” columns of Table 3, in parentheses, and together with the observed data show us to Table 6 Local mode analysis

Fig. 4. Infrared

spectrum

of 1,2-Si2H4C12 in the 2vSiH region.

289-303

of SiH stretching

uOhr (mm’)

Assignment

4296.0 Q 4293.0 Q 4288.6 q 4271 A?

2#SiHX 2zYSiH” Zvl’SiH”, 2zPSiH” ,r

v%iH 12T 12G 112C 12G

1

(a~-‘)~

2184.5 2184.4(2183.7) 2181.4 2171.6(2172.4)

a In parentheses, the value obtained = 2 x YIS~ 2lP.

b2x,

overtones -xc a.b (cm-‘) 36.5 37.9(37.2) 37.1 36.1(36.9)

forf,’ = 0.0238 mdyn A-‘.

D.C. MeKcan

er u/./Journal

of Molrculur

what extent the predictions in the columns under (1) and (2) have been successful. Use of substituent effects, under (1) appears to give frequencies greater than the true ones by 45 cm-’ for 12,112 and 1122T, about 2 cm-’ greater for 1122G, but less by about 1 cm-’ for 111. The predictions (1) for 1112 and 11122 are therefore unlikely to bc in error by more than 2-3 cm-‘. The success of the ab initio predictions is more varied, ranging from very good for the 12T,G and 111 compounds, to good for 112C,, but somewhat poorer for two of the 112Ct ljis values, for SiHi, and SiH:, respectively. Part of the difficulty is due to the fact that there are small differences in vSiH for the same molecule between the GAMESS and GAUSSIAN 90 calculations, so that scale factors based on the latter are not exactly appropriate for application to the former. The values of these ab initio calculations tends therefore to lie more in the assistance they provide in determining the interaction constants of the

Table 7 Ab lnitio Bond/angle

geometnes of

I,~-SI~H,CI,

HF/6-3lG*

Datuln

12T SiCI, SiSi Cl&i SiH”

2.0?98 2.3480 IO8I.4718 009

CISiH, H,SiH, H,SiSi CISiSiH, H,SiSiH g

107.905 109.885 Ill.492 61.612 56.777

12c SiCl, SiSi SiH” SiHf ClSiSi H,SiSi H,SiSi II,SiH, H,SiCI H,SiCI ClSiSiCl CISiSiH, ClSiSiH,

2 0742 2.3502 1.4718 I .4743 110.008 Ill.lRX ‘III 13’ 109 643 107.437 108.342 -68.700 51.321 173.062

3 IX

4 19 20

conformers LKI

HFitzvp

MP:/tzvp

2.0868 2.3618 107.645 1.4725

2.0786 2.3436 107.905 I .47 IX

107.799 110.261 111.576 61.9’8 56.163

108.078 110.092 111.277 61.590 56.820

2.0816 2.3654 1.4727 1.4748 109.460 11 I .293 I IO.414 109.998 107.413 108.153 -68.658 50.838 173.297

2.0734 2.3468 1.4719 I .4739 109.249 1’0.993 110.553 lOY.YlO 107.835 108.212 -66.347 52.900 175.137

S~rucrure 376 (1996) 289-303

297

HLM force held than in estimating absolute magnitudes of frequencies. Nevertheless, both are useful. Looking now at the chemical significance of the results, we see that the prime reason for the low value of Y” in 111 is the absence of an cy chlorine atom, together with a diminishing gauche effect of chlorine. which it shares with the SiH,, bond in 11. The highest Y” value should appear in 1122T. slightly greater than that in 11122. It will be interesting to discover if fluorine substitution has similar effects: the close resemblance between the effects of chlorine and bromine in 1 and 11 [2] leaves little doubt that the same behaviour as for chlorine will be found in bromodisilanes generally.

4. Ab initio geometries The equilibrium bond lengths and angles determined ab initio are listed in Tables 7 (12T,G), 8 (112C,,C,) and 9 (111). Revised data based on GAMESS for SiZHh, 1 and 11 are collected in Table 10, while the charge information for the 12 and 112 compounds is given in Table 11. Variations in geometry with respect to level and basis set for the 12 conformers are much as might be expected. Only in 12G are there significant changes in bond angle, notably about 2’ amongst the dihedral angles. The MPZjtzvp bond lengths are closer to the HFjh-31G* ones than to the HF/tzvp set. In so far as we are primarily interested in changes in bond lengths, these are virtually unaffected by choice of basis set or level as seen for example in the SiH;I SiH; difference for 12G. In order to describe in detail this sort of difference, we attach a datum number to each of the Sic1 and SiH bond lengths in Tables 7710, and in Tables 12 and 13 show the differences in rSiC1 and rSiH respectively due to substitution of chlorine in LY,gauche or tram positions, as derived by subtractions specified by the data numbers in these Tables. Each subtraction gives the elfect of a single o, gauche or tram chlorine substitution. Amongst the &Sic1 values in Table 12 there are marked shortenings associated with Q: or gauche chlorine and a modest one associated with a trans chlorine atom. The only slight departure from a

298

D.C. McKum

Table 8 HF/6-3lG*

geometries

et ul.;Jourrrul of’Molecuh-

289-303

for 1,l ,2-Si,H3C13 conformers

112c,

IlZC,

Bondiangle

Datum

SiCll(5) SiCl,,(4)

2.0599 2.0657

SiSi SiH:‘(l) SiH;,(7)

2.3499

1.4686 110.470 110.334

H( l)SiSi H(7)SiSi

111.919 109.528

Cl(S)SiCl(6) Cl(5)SiIl(l) Cl(4)SiH(7)

109.bl8 107 265 108.566

H(7)SiH(8) H( I)SiSiCI(4) H(I)SiSiH(7) Cl(S)SiSiC1(4) C1(5)SiSiH(7) Cl(S)SiSiH(8)

no

Band/angle

5 6

1.4719

Cl(4)SiSi Cl(5)SiSi

SiC1;(6) slcl:(s) SiCl,,(7) SiSi SiHiX( I) SiH&(4)

21 22

S$,(8) Cl(S)SiSi Cl(6)SiSi Cl(7)SiSi H( l)SiSi H(4)SiSi H(X)SiSi Cl(S)SiCl(b) Cl(5)SIII(l) CI(G)SiH( 1) C1(7)SiH(4) Cl(7)SiH(X) II(4)SiH(S) H(l)SiSiH(4) H( l)SiSiCl(7) H( l)SiSiH(8) C1(5)SiSiH(4) Cl(S)SiSlCl(7) CI(S)SiSiH@) Cl(6)SiSiH(4) C1(6)SiSiC1(7) Cl(6)SlSlH(8)

110 167 180.0 60.462 60.639 -58.898 -179.823

high degree of consistency derives from the Sic1 length in 111, datum 10, which is therefore 0.004A longer than might have been expected. The changes in rSiH in Table 12 are remarkably consistent. Again, alpha or gauche chlorine shortens the bond concerned. However, trans chlorine has a negligible effect. Looking at the Table 9 HFib-31G*

Srructur~ 376 (19961

geometry

Bond

Length

sIc1”” SiSi SIH,,,

2.0566 2.3448 1.4746

(A)

for l,l,l-SI~H,CI, Datum 10 26

no.

Angle

(2)

HSiH HSiSi ClSiCl ClSiSi

110.602 108.316 107.826 I Il.070

Datum

no

2.0597 2.0659 2.0714 2.3476 1.4661 I .4699 1.4714 107.760 110.420 108.878 114.195 110.643 109.715 109.264 107.495 107.593 108.730 108.161 110.652 -173.094 -53.657 64.537 67.566 - 172.997 -54.803 51.704 67.734 -174.072

SiSi lengths in Tables 7-19, there is a steady shortening of about 0.003 A per chlorine in the sequence Si2H6-l-11-111, where chlorine is progressively attached at one end only, but, where substitution occurs at both ends, as in the 12 and 112 compounds, the length is sensitive to conformation. The shorter SiSi bond is found where there are two chlorines trans to each other. While the changes in the lengths of the SK1 and SiSi bonds should be readily detectable in electron diffraction or microwave investigations, those in the SiH bonds will normally be at or below the limits of experimental accuracy. The best way to handle the experimental data would then be to constrain the differences in the SiH lengths of whatever

299

D.C. McKcan etal.lJournalofMolecuIar Srrucmre376 (1996)289-303 Table

10

HF/6-31G*

data for Si2H6, Si2HsCI

Molccllle

Bond/atom

Si,HG

SiSi SiH Si‘Si Sic1 SiHX SiH SiH: Si”“Si SiW SiH”” SiH @z SiH,,

Si,H,CI

I,l-Si,H,Q

and l,l-Si2H4CIZ. Datum

0.0

0.0

-0.353

0.0

-0.103

0.0

-1.177

from which db/dr

no.

11

12 13 14 2 15 16 17

a Mulliken atomic charge. ’ King effective atomic charge. ’ Misquoted in Ref. [6]. The C~AVSSIAN HF/6-31G* -0.426

recalculated

using GAMESS re (‘Q

4a (e)”

E, (e?

2.3535 1.4804 2.3501 2.0818 1.4719 1.4752 1.4780 2.3474 2.0667 1.4655 1.4728 1.4749

0.356 -0.119 0.523”, 0.370 -0.367 -0.115 -0.116 -0.126 0.622”” 0.397_ -0.325 ’ -0.099 -0.106 -0.116

1.791 0.585 2.380”, 1.688 1.315’ 0.609 0.551 0.586 2.903”“, 1.698 1.333 0.622 0.524 0.558

polar tensor for Cl should

have been

0.158

= ~ l.l87e,

directed

7.6” off the bond direction.

Table 11 Mulliken atomic

(qd) and King effective atomic

Atom

Datum

no.

The tensors

(f,) charges in 1,2-dichloro

for the two silicon atoms

and 1,1,2-trichlorodisilanes,

4a (c)

5, (c)

Atom

Datum

3 18

0.530 -0.342 -0.094

2.372 1.327 0.576

12G Six C’, H; W

5

0.623 0.545 -0.289

2.867 2.340 1.291

Cl,, H:

6 21

-0.311 -0 088

1.220 0.620

Hi,

22

-0.096

0.585

10 26

0.663 0.425 -0.264 -0.099

3.298 1.732 1.337 0.534

12T Si” Cl, Hi

112C, Six’ Six Cl;

111 Six” Si CP L a Using wwss.

11x, Six’ Six Cl; Cl; Cl,, HP HL Hit

no.

should

be transposed

from HF/6-31G*

calculation?

qa (e)

L (c)

4 19 20

0.530 -0.330 -0.095 -0.105

2.355 1.268 0.578 0.610

7 8 9 23 24 25

0 625 0 543 -0 28X -0.302 -0.323 -0.077 -0.086 -0 093

2.880 2.370 1.290 1.34x 1.285 0.591 0.553 0.580

D.C. McKean et al./Journd

300 Table 12 Changes in Sic1 bond lengths, for chlorine atoms due chlorodisilanes” .4. Alpha chlorine substitution

qf Molecular

Y,and atomic charges, qcl and &,, to chlorine substitution in

Data numbers

al,

2-l 8-3 l-4 5-4 IO-2

PO.0149 PO.0139 -0.0145 -0.0143 -n.o11s

kx

(A)

(e)

AL (c)

0.043 0.040 0.042 0.041 0.043

0.00x 0.021 0.022 0.023 0 003

Structure 376 (1996) 289-303

130 km mol-’ in Ill-d2 should be compared with that of 173 km mol-’ for the SiH, bond in 1 for the efrect of two gauche chlorines. The data relevant to the effect of LY chlorine on the vSiH infrared intensity are less complete, but those available show a smaller and more variable diminution Table 13 Changes in SiH bond lengths. I, and atomic charges, qH and &, for hydrogen atoms due to chlorine substitution in chlorodiailanesa A. dlpha chlori>ze substitution

B. Gauche chlorine substitutmn Data numbers

Al, (A)

Aqg (c)

A<, (c)

4-1 h-4 5-2 7-2 Y-3

~0.0088 -0.01)x5 -0.00x2 -0.0084 -0.0084

0.020 0.019 0.01x 0.019 0.019

PO.058 -0.04x -0 044 -0 042

-0.043

c‘. Tmns chlorine substitutmn Data numbers

Ar, (A)

Aq, cc)

AC, (c)

3-I 8-2 9-4

PO.0032 -0.0022 -0.0028

0.008 o.ons 0.007

0.001 0 014 0017

’ From

HF/6-3IG*

calculatlona

kind are being calculated, found here.

using WMFSS.

r s* r cl) r a, r,,,

etc., to those

5. Electrical properties We consider first the vSiH infrared intensities in Table 4, which reflect the dipole derivative dLL,dr for the SiH bond. The diminution from a value of around 175 km mol-’ per bond in S&H6 due to gauche chlorine which was previously found in 1 in the 12, 112 and 111 and 11 [6,7], continues compounds.4 Thus the value for the SiHi bond is down to 141 (lZT,av) or 143 (12G) km mol-’ and a second gauche chlorine, as for SiH& in 112C1, reduces it further (122 km mol-I). The value of 4 The

GAUSSIANw and GAMESSintensities for the same molecule.

are virtually

Data numbers

Al, (A,

Aqx (e)

A<, (e)

12~11 15-12 18-13 19-13 20 14 21 20 22-17 23-19 24-16 25-17

-0.0061 -0.0060 -0.0056 -0.0056 ~0.0059 ~0.0057 O.OOSl -0.0057 -0.0051 -0.0056

0.013 0.017 0.013 0.012 0.013 0.017 0.01 I 0.018 0.011 0.014

0.023 0.010 0.025 0.027 0.023 0.010 0.028 0.013 0.030 0.023

E. Gauche chlorine substitution Data numbers

Ar, (A,

Aqs (e)

At,

13-11 16-13 17-14 26-17 18-12 19-12 22-20 23-15 24419 24-18 25-20

~0.0030 PO.0024 -0.0032 -0.0024 -0.0025 -0.0025 -0.0024 -0.0022 -0.0019 -0.0019 -0.0029

0.012 0.010 0.011 0.008 0.012 0.01 I 0.009 0.012 0.009 0.008 0.012

0.034 0.028 -0.030 -0.023 -0.032 -0 030 -0 025 -0 027 -0 025 -0.023 po.o3o

Aq, (e)

%I (e)

(e)

C. Tram ddorine substitution Data numbers

Ar, iA)

il~ll 17-13 26-16 21-E 22-1x ??-I9

-omo2

n.ooi

0.002

-0.0004

0.000

0.006 0.01 I 0.002

-0.0004

-0.002

o.ono3

fl.001

o.nool

o.no9

0.002

II-18

-o.u004

-no02 -0.00 I o.on I

25-19

-0.0004

0.002

0.0001

identical

‘IFrom

HFi6-3 lCi* calculations

with GAMESS.

o.nn7 0.004

D.C. McKean et al.jJournal Of Molecular Structure 376 (1996) 289-303

than that of gauche chlorine. The effect of tram chlorine is negligible. Since the infrared intensities of all the SiH stretching vibrations concerned, except for those in SizHs, involve small contributions from the rotation of a permanent dipole moment, it is preferable to examine the dipole derivatives dpLjdr themselves for specific electronic effects, dp/dr being obtained from the appropriate column of the atomic polar tensor, once one axis has been aligned with the bond direction. The polar tensor is calculated with respect to space-fixed axes and so there is no contribution to dp/dr from the permanent dipole moment. (The extension of the bond is not, of course, a true vibration, since it will in general rotate the inertial axes, but the permanent moment does not move in this distortion). Calculating dp/dr also enables us to examine the effects of chlorine substitution on the Sic1 bond, from the stretching of which there are no Table 14 Bond dipole derivatives dp/dr in some chlorodisilanes. HF/6-3lG* calculations with GAMES

from

A. Sic/

Molecule

Atom

-dp;dr

(e)

Off-bond angles (“)

1

Cl

1.202

1.3

11 12r 12G 111 112c.

Cl”

CL

1.215 1.203 1.147 1.223 1.175 1.102

6.3, 0.3 8.0 8.4, 1.2 3.7 7.5. 0.2 9.4

Molecule

Atom

-dpL:dr (e)

Off-bond an&s (“)

Si2He

H

0.412

7.3

1

H”

11

H, H, H””

0.394 0.373 0.410 0.368 0.339 0.374 0.357 0.357 0.393 0.343 0.369 0.362

1.3, 7.6, 7.3 1.9 8.9 6.4, 2.6, 5.3, 4.5, 91 2.1 1.3,

Cl, Cl, CP Cl;,

B. SiH

12T 12G 111 112c.

H,, Ha Hi Hi H; H ,p Hit

6.3 2.9

5.2 5.1 6.1 4.5

8.0

301

highly localised group frequencies available for intensity study comparable with the SiH ones. Table 14 contains the HF/6-31G* values of d/I/dr for the Sic1 and SiH bonds for all the molecules studied except 112Ci. In addition the off-bond angles are quoted, to show that departure from the bond dipole model is minor, the direction of dpidr being in all cases close to the bond direction. Examination of the dp/dr values for the SiH bond shows the trends noted in the raw intensities, only more clearly. The identical values for the Hi bonds in 12T and 12G are pleasing. The trans effect of chlorine is very clearly negligible and the modest lowering by LYchlorine is well defined. For the Sic1 bond there is the same definite lowering of dp/dr due to gauche chlorine and a negligible effect of a trans one. However, o chlorine raises dp/dr slightly by about 1O/o. We have not previously examined ab initio calculations of Raman intensity for local modetype vibrations for SiH bonds and are unaware of any experimental data that might indicate whether variations calculated in this way are signilicant. The incomplete data listed in Table 4 are not enough to establish any clear trend in themselves, whether or not they bear any relation to future experimental observations. The data for Ill-d, and -dz together provide a solitary example of good additivity per bond. We turn now to consider the King effective charge, &. As the square root of the sum of the squares of all the elements in the polar tensor, &, provides a global measure of the infrared intensities of all the bands, as well as including rotational contributions [18]. Table 11 lists the values both of E, and of the Mulliken atomic charge, qa, both derived from the HF/6-31G* calculations using GAMESS. Pleasing consistencies are at once obvious. Thus both kinds of quantity are virtually identical for the H”p atoms in 12T and 12G, and for the Cl; and H$ atoms in 112C, and 112C1. In Tables 12 and 13 we can see the changes in these quantities for the Cl and H atoms respectively, due to chlorine substitution in a, gauche or trans positions. In parts A of these Tables we see that for both chlorine and hydrogen, CEchlorine makes the atom concerned more positive, from the

302

D.C. Mch'eanetal.~Jotrmal of Mdecttlar Structure 376 (19961 289-303

Mulliken charge, qa, and at the same time slightly increases the unsigned King effective charge, I,, although the latter effect is somewhat variable. For the chlorine atom, &-t will be dominated by the r&Cl intensities, reflecting d!r/dr, the Sic1 bending bands being in general much weaker. The rise in
6. General discussion Overall, our ab initio calculations show a remarkable uniformity in the changes they predict in both bond length and the two kinds of atomic charge, Mulliken and King, the changes being highly predictable from the number of chlorine atoms substituted and the types of position into which they are inserted. cy effects of halogen on bond lengths have of course been much studied both experimentally and theoretically and interpreted in terms of a balance between electrostatic and anomeric or negative hyperconjugation effects [20]. In little of this work have the electrical properties been considered. Mulliken atomic charges are reported for fluorosilanes with large basis sets [21] and King effective charges in these molecules with SCF,%-31G calculations [22]. The latter show that xu(= tn. 33”‘) and dn/dr,,n do not decrease regularly from SiH4 to SiHF3, the largest value occurring with SiH3F. Assuming this to derive from the anomeric effect in the latter, the same effect is clearly much less marked in Si,H,Cl. A current study of Huorodisilanes will be of interest here [23]. We are not aware of any comparable theoretical treatment of the gauche effects of halogen revealed in the present work or indeed any awareness of their generality. It remains to be seen whether a model employing a purely through-space electrostatic effect is capable of explaining what the ab initio method has detected.

Acknowledgment We thank Alison L. McPhail for several of the spectra, the Chemistry Department, University of Aberdeen, for laboratory facilities and the E.P.S.R.C. for a grant for the purchase of a work station on which the ab initio calculations were performed.

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[7]

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[12] J.E. Drake and N. Goddard, [13]

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