The vibrational spectra of germane and silane derivatives—III

SpectrochbicaActa,Vol. 26A,pp. 1697to 1706.PergsmonPress1970.Printedin NorthernIreland

The vibrational spectra of germane and silane derivatives-III* A reassessment of the assignments in monosilylphosphineand monosilylarsine J. E. Department

DRAKE

and C.

RIDDLE

of Chemistry, The University of Windsor, Windsor Ontario, Canada (Received11 JuZy 1969)

A-t-The infrared spectra of SiDsPD, and SiDsAsD, have been recorded. The spectra of SiHePH, and SiH&H, have been reported previously, but a normal coordinate caloulation involving all four speoiesleads to a reassignmentof the deformation regions of these molecules. Comparison with the methyl- and monogermyl- analogues supports the proposals.

accounts of the vibrational spectra of methylphosphine, [2] methylarsine, [3] monogermylphosphine [4] and monogermylarsine [5] hsve appeared. LINTON snd NIXON [6] have reported the infrared spectrum of monosilylphosphine, SiB,PH,, identifying six of the fifteen fundamentals positively ltnd a further seven tentatively. Independent confirmation has been provided by an account of the liquid-phase Raman spectrum [7]. For monosilylsrsine, SiH.&H,, the infrared spectrum of both gas and solid, snd the Reman of the liquid, have been reported identifying ten fundamentals [S1. We have prepared the perdeutero derivatives, SiDaPD, and SiD&D,, and recorded their spectra. A normal coordinate calculation, similar to those undertaken on related molecules, [Z-5] indicates that although the stretching regions of the spectra were correctly assigned previously, the deformation regions were not. The reassignment is strengthened by a comparison of the PH, and ASH, deformation regions with those in the methyl- and monogermyl-analogues. DETAILED

EXPER~ENTAL SiPD, snd SiAsD, were prepared by passing SiD, and either PD, or AsD, through a silent electric discharge ss described elsewhere [9]. Trap-to-trap distillations on a conventional greaseless vacuum line separated the required products from Si,D, snd S&D, to yield samples whose aH isotopic purity was estimated to be >98%.

* For Part II, see Ref. [I]. [l] J. E. -

and C. RIDDLE, J. Chum. Sot. A, 2114 (1969). [2] J. A. LANNON and E. R. NIXON, &mtmchitn. A&Q 88A, 2713 (1907). [3] A. B. HARVEY and M. I(. WILsON, J. Chewa. P&a. 44, 3635 (1966). [4] K. 116.MAOKAY, K. J. SVJXYON, S. R. STOBART,J. E. D~~KIEand C. RIDDLE,SpectrochGn. A&z $%A, 926 (1969). [5] J. E. Dm, C. R~DLE, K. M. MACXAY, K. J. S~TON and 8. R. STOBART, S~ro&k. Acta 25A, 941 (1969). [S] H. R. LINT~N and E. R. NIXON, Spe&o&kn. Act+ 11,146 (1969). [7] J. S~SON, Thesis, University of Southampton (1967). [S] J. E. DaagIE and J. SIMPsON,&wctroch~~. Aota MA, 981 (1968). [Q] C. RIDDIZ, Thesis, University of Southampton (1969). 1697

1698

J. E. DRAKE and C. RIDDLE

Infrared spectra were recorded on a Perkin-Elmer 337 spectrometer (4000-400 cm-l) using sampling cells fitted with KBr windows. Raman spectra were recorded on neat liquid samples using a Cary 81 laser Raman spectrometer with helium/neon source. Both molecules belong to the C, point group and their expected geometry is indicated in Fig. 1, with the internal coordinates used. The vibrational modes are divided into those symmetric (A’) or antisymmetric (A”) with respect to the molecular plane of symmetry. The axis of least inertia, in these asymmetric top molecules, is close to the Si-M (M = P, As) bond thus resembling the symmetry axis of the hypothetical symmetric tops “SiH&H9”. The intermediate axis lies out of the

Fig. 1. The internal coordinate descriptions for monosilylphosphine end -arsine.

plane and that of greatest inertia in the symmetry plane, at right angles to the Si-M bond. It follows that the A’ vibrations involve a dipole change along the axis of least or greatest- inertia whilst the A” involve change along the intermediate axis. Six of the A’ modes should appear as type-8 bands, with P, Q, R branches, related to parallel (al) modes in the symmetric top analogue. The other three should appear as type-C bands, displaying a series of Qx sub-branches of spacing 2(A - 8), which arise from splitting of the degenerate perpendicular (e) modes of “SiHSMH3”. The A" modes, which should appear as type-B bands, also arise from this loss of degeneracy and the three associated with the SiHa moiety (i.e. vii, y12 and yi4) are expected to lie close to the corresponding type-C bands (i.e. Ye, y5 and ~arespectively). The fundamentals of SiH,PH, are numbered according to HERZBERQ’S convention [lo] and those of SiH,AsH, by analogy, thus facilitating comparison with monogermyl-phosphine and -arsine. The rotational parameters in Table 1 were calculated from parameters assumed in the light of available data on related species, viz. : symmetry

SiH,PH,:

Si-P,

H$H,

93’28’;

[lo] G.

HERZBERG,

2.243 A;

Si-H,

1.480 A;

P-H,

1.424 A;

H&k,

SigH 97” 30’. Infrared and Raman Spectra. Vsn Nostrnnd (1946).

H&P,

109”28’;

The vibrational spectra of germane and &lane derivatives-III

1699

Table 1. Rotational paranwters (cm-l)

SiH,PH a

A B c 2(A - 8)

1.707 0.188 0.188 3.04 17.7

A”P. R

SiD,PD,

SiH&sH,

SiD,AsD,

0.855 0.101 0.181 1.39 16.4

1.378 0.126 0.126 2.60 14.5

1.062 0.108 0.108 1.91 13.5

Table 2. The vibrational spectrum of monosilylphosphine SiD,PD,

SiH,PH, celculstod

IR (gas)’

Calcul&ed

IR (g.=)

3350, 3082. 2@85,2960, VW 2812, 2794

Assignment end aombinetion bends PH impurity

overtone

3100 2002 vyw 2312 VW 167l(v10) lSSS(v,)

16811118

2321

2312

VI and %‘lo SiH impurity

2160 VW 2196 ( 2189

(2190)

1688 min

2176

2169 Q 2168 PI 2178 R

% VI1

1648

VII

1336(2v&, 1215(2u,) ~ 109O(v* + VP)

1370, 1390, 1360, 1140

-807 775 680 673 663 682

1074 943

1072 920

939 914 908.6 904 min 901 896.5 I 844 760 734 R 723 Q 711 P

726 616 ( 674 468

Si-As,

2.3408;

1.48OA;

771

*4

663

%

670

Vlf

disilane impurity? 643

%a?

546 463 ( 430 419

1460

Si-H,

overtone end combination benda SiI&D,_, dot,

706

636/618 w 663 R 645 Q 636 P I

766

464

I

VW c VW R Q vs P1 w, sh

702, 699, w, sh

896

621

&H.&H,:

1692 { 1681

AS-H,

1.5208;

HCH,

H&?&s,

109’28’; &H, 90’; S&H, 94”. An ICL 1907 computer w&sused for the calculations with the program ‘SOTONVIB’ described by BEATTIEet al. [ 111. The observed and calcul&ed frequencies are listed for each molecule (Tables 2, 3) with their tignments. The Potential Energy Distributions (PEDs) amongst the force constants (Table 4) are listed in Tables 5 and 6. [ll]

I. R.

BEATTIE,

N. F. CHEETHAM

and D. E.

ROUERS,

to be published.

J. E. DR~~JEand C. RIDDLE

1700

Table 3. The vibrational spectrum of monosilylarsine SiH&H,

SiD,AsD, Assignments

IR(ga.s)*”

2157.5 2107.5

C&T.

IR(g=) 1663 Q br 1664 Q B

2167 2160 2164 2120

1527 Q 1090 br 796 A w 680-730 br 701 Q 689 w

1100 940-960 br 946 -935

891

-706

1686 1566 1674 1511

sh dep “8 pol sh Bpol

IR(aolid) 1568 s 1535 m 1562 s 1496 m 1484 m

668

586

696 680

600 R 493 Q m 484 P I -

-

366

732 sh 684 m dep

696 sh 670 m

656 m pol

638 s

Motion

1 2 3 4 5 0 7 8 9 10

11 12 13

:

t ; Y 6 rr ;;I

BY" tY w'

600 w

-

453 w 429 w 355 vs pol

SiH,PH 2

SiH&sH,

2.737 2.036 3.069 0.423 0.561 0.701 0.670 0.019 0.036 0.138 -.0.016 0.108 0.364*

2.674 1.770 2.631 0.443 0.610 0.660 0.616 0.039 0.066 0.086 0.059 -

Units are: mdyn/A for stretching, mdyneA for bending, and mdyn for stretch-bend constants. All unlisted force constants were held at zero. * see text.

QQW,) 67W,)

1676(v,,) 664

VO

496

Vl9

490

V?

433 427

VO V14

346

V9

?

Table 4. Force constant listinga Force constant

1567 1542 1667 1609 1609

620 br, w

* Reman band.

Number

CA?.

78OW

694

664

-480 357*

668 R SSOQ 8 652 P 1 560 w

892

R(liquid)

The vibrational spectra of germane and s&me derivatives-111

1701

Table 5. Description of the fundamcntala of monosilylphosphine

Vl

PH stretch

V2

8iH asym. stretch

VS

SiH sym. stretch

V4

PH, scissors

v5

SiH, asym. def.

VI

SiH, sym. def.

V?

PH, wag

V8

SiH, rock

VB

SiP stretch

VlO

PH stretch

Vll

SiH stretch

C-2

SiH, esym. def.

V18

PH, twist

V14

SiH, rock

I A’(

A”

Potential energy distribution amongat the force constanta*

Conventional description

Mode

97(3) 97(3) 101(l) 100(l) 99(l) 98(l) 97(7) 94(7) 69(4) + 25(5) 81(4) + u(5) 52(4) + 31(5) + Q(6) + 7(10) 12(2) + 35(4) + 33(5) + Q(6) + 6(10) lO(3) + 13(4) + 21(S) + 84(6) - 7(Q) - 16(13) E(3) + 17(4) + 28(5) + 72(6) - 7(Q) - 12(13) Q(4) + 82(5) + 16(B) - ll(10) 12(2) + S(4) + 76(5) + lO(6) - lO(10) 96(2) 70(2) + ~(5) + 18(6) 99(3) 98(3) 101(l) 100(l) 82(4) + il(5) 84(4) + Q(5) ll(4) + 83(6) + 6(10) - 8(13) lO(4) + 83(6) + 6(10) - Q(13) 7(4) + 93(S) + 24(e) - 21(10) 96(6) + 23(6) - 21(10) Not observed or calculated

(

\V16 * Contributions of <6% SiH,PH,, SiD,PD,.

omitted;

force constant number refers to list in Table 4; listed in order

For SiH,PH, and SiH3AaH, simple valence foroe fields (SVFFs), involving seven force con&ants, were derived by comparison with related molecules. The calculati frequencies arising from these force fields, whose associated PEDs provided our assignments, were improved by iterative adjustments, using the conventional least-squares refinement as described previously [l]. After confirming that rewnable transfer to SiD,PD, and &D&D, could be aohieved, interaction terms were introduced, w for monogermyl-phosphine [4] and -amine [6], in an ‘Overlay’ oalculation. These interactions, in turn, were : (i) the stretch-stretch term, f,, to account for the difference between Y*and vs but otherwise having little effect; (ii) the angleangle terms, fsy, connecting the MH, wagging mode, v,, with the SiH8 symmetric deformation mode, r6; and (iii) the stretch-angle terms ft, required to fit the PH, deformation modes v, and v13. The internal torsion, vU, was not observed or taken into account in the calculations. A maximum of 28 experimental frequencies was thus available for each molecule. RESULTS

The of the The calculationa is necessary.

AND

DISCUSSION

mociated with stretching modes well characterthe independence these modes only brief

J. E. DRMCEand C.

1702

RIDDLE

Table 6. Desoription of the fumlam~tals of monosilylaraine Mode

A’

Potential energy dietribution amongat the force con&ante*

Vl

AsH stretoh

lOO(3) lOO(3)

Vl

SiH asym. stretch

VI

SiH sym. stretch

98(l) 101(l) 101(l)

V4

ASH, scissors

( v6 VO

SiH, sym. def. &H,

%

SiH, rook

“b

SiAs stretch

weg

ASH stretch

VU

SiH stretch

~1s

SiH, asym. def.

V11

ASH, twist

VU

SiH, rock

\VlS

97(l) 36(4) + lO(6) + 60(7) 8(4) + 90(7) 46(4) + 7(6) -t 47(7) 61(4) + 26(6) + 8(7) 47(4) + 34(6) + 6(6) 69(4) + 20(6) 12(4) + 21(6) + 86(6) - 13(9) lO(4) + 28(6) + 76(6) - 16(9) 8(4) + 86(6) + lO(6) + 7(9) - 6(10) 6(4) + 78(6) -f 21(e) + 9(9) - 9(10) 98(2) 92(2) lOO(3) lOO(3) 101(l) 101(l) 91(4) + 8(6) 93(4) + 6(6) lOO(6) 99(6) 8(4) + 92(b) 6(4) + 92(6) Not observed or oalculated

SiH, asym. def.

5

%o

A”

Conventional desmiption

torsion

l Contribution8 of
6(11)

form constant number refer8 to list in Table 4; listed in order

The PH and ASH, y1 and yrO,and SiH, ve, v3 and vll stretching vibrations Both SiH,PH, and SiD3PDs exhibit a broad feature with poorly resolved fine structure in the region of or and r10which consequently are not distinguished. In SiH8PH, a single intense band envelope was observed [6] in the SiH stretching region, with a prominent Q-branch at 2169 cm-l. In SiD,PD, the type-d band assigned to the symmetric stretch, y3, is centred at 1657 cm-l. The high frequency side of the band envelope contains a series of Qn sub-branches surrounding a central minimum at 1688.5 cm-l, assigned to vi1 (Table 7). A maximum for the predicted near-degenerate asymmetric stretch, Ye,may lie at 1687 cm-l. The sub-band separation (1.31 cm-l) is in reasonable agreement with the calculated value (1.39 cm-l). For the amine the five ASH and SiE fundamentals all lie within one band envelope with 6ne structure again apparent on the high frequency wing. Three Q-branches may be discerned in SiD,AsD, corresponding to the A’ modes vi, vz and y3. The SiP and &As stretching vibrations, Ye On deuteration the SiP stretch in SiHsPH, moves from 464 to 426 cm-l, similar to the shift noted for the corresponding methylphosphines [2]. In the amine this mode remains constant around 366 cm-l. Our assignment of the deformation modes differs from the earlier work and is consequently presented in detail.

The vibrational spectra of germane and silane derivatives-III

1703

Table 7. Rotational etructure in the region of vz, va aud vzl for SiD*PD, Band No.

Frequency

Band No.

1 2 3 4 6 6 7 Minimum 8

1678.0 1679.4 1680.8 1682.1 1683.6 1686.0 1686.8 1688.6 1690.6

10 11 12 13 14 16 16 17

9

Frequency

Bend No.

Frequency

1692.0 1693.3 1694.7 1696.0 1697.4 1698.8 1699.8 1601.2 1602.6

18 19 20 21 22 23 24

1603.6 1604.8 1606.0 1607.3 1608.7 1609.9 1611.1

Average sub-band separation = 1.31 cm-l. Calculated 2(A - B) = 1.39 cm+.

The PH, deformation modq v4, v, and vlB The PH, ‘scissors’ mode, v~, occurs at 1092,

1072 and 1073 cm-i in CE,PH,, [2] SiHsPH,, [6] and GeH,PH, [4] respectively. In CD,PD, and GeD,PD, the analogous bands are at 789 and ~770 cm-l. In SiD,PD, we assign va as the C-type band at 775 cm-l. As might be expected from its constancy the calculations demonstrate the independence of vp. The wagging vibration, v,, occurs at 730 and 701 cm-l in CH,PH, [2] and GeH,PH,[4] respectively. Prom the PEDs (Table 6) it is clear that, as previously suggested, [4] this mode should be assigned to the A-type band at 723 cm-l in SiH,PHB. This conflicts with the earlier assignment of the band as vs, the SiH, rocking vibration [S]. In the deuteride, the PD, wagging mode occurs at 546 cm-l. The calculations indicate only minor mixing of v, with other modes. The twisting vibration, Q, is expected to be weak and has proved difllcult to assign in related molecules [2-b]. A weak band at 760 cm-l in SiH,PH,, lying close to v,, could be via. The calculations reproduce this frequency to within 8 cm-l although an usually high value of the interaction force constant fty’ (Table 4) is required. If, in the iterative procedure, frr’ is held at zero and no weight is given to v13 a calculated value for vi8 of 720 cm-’ results. In either case, v, and vn,, are coincident in the calculations on the deuteride.

The SiH, deformations, v5, vs and viz, for mmmdylphosphine This region of the spectrum is distinctive in related molecules [4, 51 consisting of a strong type-A band, assigned to the symmetric deformation, vg, lying to low frequency of a broader feature containing the expected near degenerate, asymmetric modes, v5 and vi2. In Sil&PH, the strong low-frequency feature comprises 4 sharp peaks with a central minimum at 904 cm-l. This was assigned by LINTONand NIXON[6] to vlg but we prefer to place vg here. The calculations support the reassignment although some mixing between vg and vs is indicated. The abnormal band shape associated with va may arise from Fermi resonance with 2v,, (2 x 464 = 908 cm-l). The calculations place v5 and via close together at 943 and 939 cm-i, in the high frequency band as in related molecules. That the low frequency feature in the deuteride is type-A may not be significant as the PEDs indicate a reversal of the relative positions of vs and vs.

1704

J. E. DRAKE and C. RIDDUZ

The SiH, rocking vibrations, vg and vlp,for monosi~y~phmphine

Having changed the previous assignment of these modes a reassessment is required. The weak features at 621 and -460 cm-l in SiH,PH* and SiI&PD, may contain rocking vibrations as the calculations indicate. Similar regions are more definitely assigned to the corresponding modes in monosilylarsine as well as many other silyl compounds [ 121. The ASH, deforwudon modes, v,, v, and vls

The ASH, scissors mode, v~, like the PH,, is independent. In CH&H, [3] and GeH&H, [5] it is found at 973 and 961 cm-l respectively moving to 710 and 682 cm-l on deuteration. In SiH,AsH, [8] it has been assigned at 946 om-1 and in SiD,AsDs there is a band at 689 cm-l. The calculations indicate that, although in SiD,AsD, the mode is again unmixed, in SiH,AsH, it is fully mixed with the near coincident SiH, asymmetric deformation mode, vg. As with the phosphine, our calculations indicate that the previous assignments of v, and vls should be interchanged with those of vg and vi4. The wagging mode, v,, found at 674 and 646 cm-l in CH,AsH, [3] and GeHsAsH, [6] respectively is now placed at 664 cm -l. The shift on deuteration is to 493 cm-l, compared with ~463 cm-l in the methyl- and monogermyl-analogues. The twisiting mode, v,a, may lie at ~705 cm-l in SiH&sH,, an observation strengthened by the calculated value. Equally well, an overtone of the skeletal stretch, vg (2 x 357 = 714 cm-l) may contribute to this band. In the deuteride v19 is coincident with v, from the calculations, and may be obscured by it. The calculations were performed without the interaction terms fi, which were shown to have minimal effect. The SiHs ckformatiom, v,, a+,and v14for nwm8ilylarsine

These modes were all assigned to the strong absorption centred at 891 cm-l in SiHaAsHs, [S]. We assign ve here but via is calculated to lie at 947 cm-l, corresponding to a minimum at 436 cm-l, and as mentioned above v, is mixed with v, at -950 cm-i. In SiD&D, vs is the strong type-A band at 660 cm-l mixed with va. The asymmetric modes, v, and vi%,lie with v,. The Si& rod&g

vibrationa,

ve and vr4,for mono8ilylami7w

These modes are observed as weak features. The calculations indicate that as expected they are essentially independent modes lying close together at 586 and -430 cm-l for SiH&sHH,and SiD,AsD, respectively. General relatkmahip The assignments for monosilyl-phosphine and -amine, although incomplete, are now in line with those of their monogermyl analogues [4, 51. The MH, deformation frequencies have caused some difficulty in the past. That the assignments of v, and v, are now in little doubt may be illustrated by two Tables. In Table 8 we compare the observed frequencies of these two modes and the associated force [12] B. J. AYLE~, A&L Inorg. C&m. Rad~ockwn. la, 249 (1969).

The vibrational spectra of germane and silane derivatives-III

1705

Table 8. The MH, soissors, Q, and wagging, v,, modea in M’H,MH, molecules M’H,

CH,

SiH,

GeH,

WH,

0.68 1092 789

0.07 1072 775

0.66 1073 778

Q(H) v,(D)

0.65 973 710

0.02 946t 689

0.62 961 682

z$i, v,(D)

0.78 730 523

0.70 723 646

0.70 701 623

0.76 674 463

0.65 664 493

0.54 646 464

Q(H) 0) 4bH,

YhHI

v,(H) v,(D)

* Force constants are in mdyn d, frequencies in om-l. t V~in SiH&sH, is fully mixed with the near coincident vs. Table 9. The MH, scissorsmode in M’H,MH, and related vibrations MHrl

E deformation

M&M”%*

M’H&% MH, scissor

PHS

1121

(%PHa SiH,PH, GeH,PH,

*sHs

1005

CJ+%AsH, SiH&sH, GeH&sH,

E deformation

1092 1072 1073

CH,SiHH, siH,siH, G&&H,

980 940 -9037

973 946 961

CHsGeH’, SiH,GeH, GeH,GeH,

900 ssst 879

*M” =SiwhenM =PandM” =GewhenM t Deduced from quoted spectrum13.

=As

constants for the methyl-, monosilyl-, and monogermyl species. The scissors motion, vp, described almost entirely by H-M-H angle bending (Id) shows little variation. The wag, Y,, described by SiM-H angle bending (j’,) is a less pure mode but nonetheless shows consistent force constant and frequency trends. In Table 9 we extend a correlation suggested by HARVEY and WILSON[3]. The MH, scissors mode, v,, is compared with the related modes in MH, and in related symmetric top molecules (e.g. SiH&H, is compared with SiH,GeHs). Once again our data for monosilylphosphine and -arsine exhibits a consistent p&tern within the groups of molecules presented. In Part II of this series [l] we indicated some of the limitations of the calculations. It is worth stressing that the exact geometry of the molecules is unknown. [13] E. J. SPANIER and A. G. MA~DIAR~, 5

Iwg. Chem. 2,216 (1963).

1706

J. E. DBAKZIend C. RIDD~

In methylphosphine, the fit for the CH, deformations was very susceptible to changes in the value (-2’) of E,the angle between the M’-M bond (M’ = C, Si, Ge) in the and the symmetry axis of the M’H, group. We have aesumed Eto be zero, EUJ monogermyl species [4, 61 because no accurate value is known. Acknowkx-&m~-We are indebted to Dr. D. E. ROUERSof Southampton University, England for allowing ua to use hip program in the oaloulations. One of us (C. R.) thanks the S.R.C. for the award of a maintenmncegrant.