Journal of the Korean Physical Society, Vol. 59, No. 5, November 2011, pp. 31923200

Infrared and Raman Spectroscopic Studies of Tris(trimethylsilyl)silaneDerivatives of (CH3)3Si)3Si-X [X = H, Cl, OH, CH3, OCH3, Si(CH3)3]:Vibrational Assignments by Hartree-Fock and Density-functional Theory

Calculations

Bong Hyun Boo

Department of Chemistry, Chungnam National University, Daejeon 305-764, Korea

(Received 3 February 2011)

IR and Raman spectra were measured to elucidate the vibrational structures oftris(trimethylsilyl)silane derivatives [((CH3)3Si)3Si-X, X = H, Cl, OH, CH3, OCH3, and Si(CH3)3]in the fundamental state. Hartree-Fock (HF) and density-functional theory (DFT) calculationswere carried out to study the molecular structure and the vibrational spectra. In the IR spectra,two scaling factors of 0.978 and 0.917 were applied to the low (below 2300 cm1) and the high(above 2300 cm1) energy fundamental frequencies giving rms deviations of 44.0 and 20.4 cm1,respectively. In the Raman spectra, however, a uniform scaling factor of 0.900 was applied to thewhole spectra, yielding a rms deviation 40.0 cm1. Comparison between the experimental and thesimulated IR and Raman spectra of the tris(trimethylsilyl)silane derivatives were made. We clearlyobserve the Si-H stretching fundamental at 2052 and 2048 cm1 in the IR and the Raman spectra,respectively. The intensities for the vibrations are relatively intense compared with those of theC-H stretching fundamental. The skeletal vibrations involving Si-Si are found to have relativelyweak intensities in the IR and the Raman spectra. Pure Si-O stretching at 611 cm1 in the IRspectrum is clearly observed to have an intense absorption both in the IR and the Raman spectra.The strong IR intensity is due to the electronegativity difference between the corresponding twoatoms.

PACS numbers: 82.50.FvKeywords: Tris(trimethylsilyl)silane derivatives, IR, Raman, HF, DFT, Vibrational assignmentDOI: 10.3938/jkps.59.3192

I. INTRODUCTION

Polysilane consisting of Si-Si central bonds and Si-Hand Si-C terminal bonds has received considerable at-tention due to its spectroscopic properties [1]. Owingto the higher polarizability of the Si atom in comparisonwith C and H atoms, the normal modes of the vibrationsinvolving displacements of Si atoms have been found tohave significantly enhanced Raman intensity [25]. Cost-effective density-functional theory (DFT) calculationsare predominantly used for vibrational assignments ofmolecules [617]. On the other hand, peak assignment ofRaman spectrum is accomplished by using the Hartree-Fock (HF) method. Skeletal vibrational spectroscopy in-volving the Si-Si backbone have been used to researchtargets because IR and Raman spectroscopies are impor-tant tools for analyzing polysilanes [18]. Displacementsof electron-rich atoms give rise to intense Raman scatter-

E-mail: bhboo@cnu.ac.kr; Tel: +82-42-821-6551; Fax: +82-42-821-8896

ing owing to the high polarizability of the correspondingatoms. The major goals to achieve in the present studyare (1) to synthesize various polysilanes, (2) to measurethe IR and Raman spectra, (3) to reproduce reasonableIR and Raman spectra by using fundamental vibrationalpeaks and their corresponding intensities obtained withDFT method and Lorentzian statistics which can be ap-plied to the simulated vibrational spectrum, (4) to elu-cidate the effect of the skeletal Si-Si vibrations on theRaman intensity, and finally (5) to investigate group fre-quencies and their intensities involving Si-X (X = H,Cl, OH, CH3, OCH3, and Si(CH3)3) experimentally andtheoretically, where refers to ((CH3)3Si)3.In our previous work, we measured IR and Raman vi-

brational spectra of (Me3Si)3SiX, where Me = CH3and X = H,Cl,OH,CH3,OCH3,Si(CH3)3; then, we as-signed the vibrational peaks with the aid of the AM1semiempirical method [18]. In the present work, how-ever, we analyzed the vibrational spectra with the aid ofab-initio and DFT calculations and clarified whether ornot DFT and HF calculations would reproduce reason-

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Infrared and Raman Spectroscopic Studies of Tris(trimethylsilyl)silane Bong Hyun Boo -3193-

ably the IR and the Raman spectra. These methods pro-vide straightforward assignments of the group frequen-cies corresponding to the terminal Si-H, Si-C, and Si-Obonds, as well as the Si-Si central bonds. This studymight help to clarify the presence of impurities formed inthe syntheses with the aid of only IR and Raman spec-troscopies. In many cases, the presence of impuritiescontaining Si-O bonds hampers the straightforward as-signments of polysilanes.

II. EXPERIMENTAL AND THEORETICALMETHODOLOGIES

The ground-state equilibrium geometries and the en-ergies were probed by using the Kohn-Sham DFT [19].Beckes three-parameter exchange functional [20,21] andthe gradient-corrected Lee-Yang-Parr correlational func-tional (B3LYP) [22, 23] were used with the 6-31G andthe 6-31G(d) basis sets. For each structure, completesets of harmonic vibrational frequencies were evaluatedby using the analytical second-derivative techniques us-ing the HF/6-31G(d) and the B3LYP/6-31G(d) meth-ods for the geometries optimized with the B3LYP/6-31Gmethod. The harmonic vibrational frequencies were alsoused to determine whether a given structure was a localminimum on the potential energy surface or not. Notethat inner-shell electrons were excluded in the electron-correlation calculation. All the ab-initio and DFT cal-culations were carried out with the Gaussian 03 packagesuite [24].

III. RESULTS AND DISCUSSION

The simulated IR and Raman spectra mimic the corre-sponding experimental spectra. A one-to-one correspon-dence was observed between the theories and the exper-iments. This enabled straightforward assignments of thevibrational fundamentals of the tris(trimethylsilyl)silanederivatives. Surprisingly, all the IR and Raman lineswere assigned unequivocally via these relatively simpleDFT and HF calculations. Fortunately, all the funda-mentals were observed without any overlap of neighbor-ing fundamentals.Figures 1 and 2 present the IR and the Raman spec-

tra of SiH, respectively, together with their simulatedspectra. We clearly observe the Si-H stretching funda-mental at 2052 and 2048 cm1 in the IR and the Ramanspectra, respectively. The intensities of the Si-H fun-damental vibration are relatively strong compared withthose of the C-H stretching fundamentals. This spectro-scopical phenomenon may arise from the fact that theelectronegativity of Si is lower than that of C, resultingin a change of the bond polarity. The silicon atom is

Fig. 1. Infrared spectra of (Me3Si)3SiH: (a) experimentand (b) B3LYP/6-31G(d)//HF/6-31G.

Fig. 2. Raman spectra of (Me3Si)3SiH: (a) experimentand (b) HF/6-31G(d)//HF/6-31G.

more electron-rich than the C atom, ending up with in-creased polarization. Contrary to expectation, the skele-tal vibrations involving Si-Si are found to have relativelyweak intensities in the IR and the Raman spectra.

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Fig. 3. Infrared spectra of (Me3Si)3SiCl: (a) experimentand (b) B3LYP/6-31G(d)//HF/6-31G.

Fig. 4. Raman spectra of (Me3Si)3SiCl: (a) experimentand (b) HF/6-31G(d)//HF/6-31G.

Figures 3 and 4 depict the IR and the Raman spec-tra of SiCl, respectively, together with their simulatedspectra. As expected, the fundamental vibrations in-volving the main framework are quite similar to thoseof SiH. The Si-Cl stretching vibration is not observedin the IR and the Raman spectra because of limitations

Fig. 5. Infrared spectra of (Me3Si)3SiOH: (a) experimentand (b) B3LYP/6-31G(d)//HF/6-31G.

Fig. 6. Raman spectra of (Me3Si)3SiOH: (a) experimentand (b) HF/6-31G(d)//HF/6-31G.

of both spectrometers. However, the fundamentals aretheoretically evaluated. The scaled fundamentals for theSi-Cl stretching are 368 and 355 cm1 in the IR and theRaman spectra, respectively.The IR and the Raman spectra of SiOH are pre-

sented in Figs. 5 and 6, respectively, together with their

Infrared and Raman Spectroscopic Studies of Tris(trimethylsilyl)silane Bong Hyun Boo -3195-

Fig. 7. Infrared spectra of (Me3Si)3SiMe: (a) experimentand (b) B3LYP/6-31G(d)//HF/6-31G.

Fig. 8. Raman spectra of (Me3Si)3SiMe: (a) experimentand (b) HF/6-31G(d)//HF/6-31G.

simulated spectra. This compound is very interestingbecause the molecule shows both free and H-bonded O-H vibrations. We presume that the spectroscopic phe-nomenon arises from the intermolecular properties thatthe molecule exists in two forms, as a free molecules and

Fig. 9. Infrared spectra of (Me3Si)3SiOMe: (a) experimentand (b) B3LYP/6-31G(d)//HF/6-31G.

Fig. 10. Raman spectra of (Me3Si)3SiOMe: (a) experi-ment and (b) HF/6-31G(d)//HF/6-31G.

a dimer formed via intermolecular hydrogen bonding.Figures 7 and 8 present the IR and the Raman spectra

of SiMe, respectively, together with their simulatedspectra. In this molecule, two types of Si-Me bondingsexist. However, the molecule does not exhibit any char-

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Fig. 11. Infrared spectra of (Me3Si)4Si: (a) experimentand (b) B3LYP/6-31G(d)//HF/6-31G.

acteristic fundamental vibrational frequencies.The IR and Raman spectra of SiOMe are presented

in Figs. 9 and 10, respectively, together with their sim-ulated spectra. As expected, the C-H stretching funda-mental corresponding to the -OMe group is clearly ob-served at 2892 cm1. Why do we observe the strongabsorption of the C-H stretching in the methoxy group?We assume that the oxygen atom donates an electronto the methoxy carbon atom, resulting in an enhance-ment of the C-H bond polarity. Pure Si-O stretching at611 cm1 in the IR spectrum is clearly observed. The in-tense absorption is reflected in the electronegativity dif-ference between the corresponding two atoms. Also, weobserve an intense Raman line corresponding to the Si-O vibration. The intense scattering may be reflected inthe enhanced polarizability involving the electron cloudof the silicon atom.Figures 11 and 12 present the IR and the Raman spec-

tra of Si(SiMe3)4, respectively, together with their simu-lated spectra. The optimized molecular geometry quiteresembles a Td structure. Therefore, only T2 funda-mental vibrations are expected to be observed. How-ever, almost all the fundamentals are observed in the IRand the Raman spectra. This presumably indicates thatthe Si(SiMe3)4 molecular structure deviates from a Tdframework owing to the steric hindrance.

Fig. 12. Raman spectra of (Me3Si)4Si: (a) experiment and(b) HF/6-31G(d)//HF/6-31G.

IV. CONCLUSIONS

The simulated IR and Raman spectra mimic the corre-sponding experimental spectra. A one-to-one correspon-dence is observed between the theories and the exper-iments. This enabled unequivocal and straightforwardassignments. We clearly observe the Si-H stretching fun-damental at 2052 and 2048 cm1 in the IR and the Ra-man spectra of SiOH, respectively. The intensities forthe vibrations are relatively intense compared with thoseof the C-H stretching fundamental. The skeletal vibra-tions involving Si-Si are found to have relatively weakintensities in the IR and the Raman spectra. The scaledfundamentals for Si-Cl stretching are 368 and 355 cm1in the IR and the Raman spectra SiCl, respectively.Pure Si-O stretching at 611 cm1 is clearly observed tohave the intense absorption both in the IR and the Ra-man spectra of SiOMe. The strong IR intensity isdue to the electronegativity difference between the cor-responding two atoms. The intense Raman line may bedue to the enhanced polarizability involving the electroncloud of the silicon atom. The optimized molecular struc-ture of Si(SiMe3)4 resembles the Td structure. However,both the almost all the fundamentals are observed in theIR and Raman spectra. This presumably indicates thatthe SiSiMe3 molecular structure deviates from a Tdgeometry.

Infrared and Raman Spectroscopic Studies of Tris(trimethylsilyl)silane Bong Hyun Boo -3197-

Table 1. IR and Raman frequencies of tris(trimethylsilyl)silane.

Approximate type IR (Rel. int., %) Raman (Rel. int.,%) Literatureb

of modea

Experiment B3LYP/ Experiment HF/ IR (Rel. int.) Raman (Rel. int.)

6-31G(d) 6-31G(d)

//HF/6-31G //HF/6-31G

(Scaled with two (Scaled with a uniform

factorsb) factor of 0.900)

Antisym C-H str 2955 (34) 2953 (17) 2948 (34) 2946 (100) 2956 (s) 2951 (vs)

Sym C-H str 2893 (17) 2879 (9.9) 2892 (100) 2884 (97) 2900 (m) 2897 (vs)

Si-H str 2052 (20) 2086 (14) 2048 (13) 1976 (9.8) 2051 (s) 2049 (vs)

C-H bending 1439 (9.1) 1456 (3.3) 1434 (1.3) 1440 (12) 1443 (m) 1443 (vw)

1404 (12) - 1404 (2.6) - 1400 (w) 1402 (m)

Sym C-H bending 1250 (38) 1253 (11) 1258 (2.7) - 1245 (vs) 1254 (w)

- 1242 (3.0) - - 1239 (m)

Antisym C-H bending 841 (100) 831 (100) 868 (2.6) 844 (1.0) 838 (vs) 865 (w)

838 (2.3) - - 833 (m)

Si-H bending, 752 (22) 729 (8.8) 744 (2.6) - 745 (m) 743 (m)

Antisym C-H bending

Antisym Si-C str, 691 (23) 633 (12) 686 (6.7) - 686 (s) 685 (s)

Si-H bending

Sym Si-C str, 613 (34) 566 (6.4) 624 (12) 626 (5.2) 636 (w) 623 (vs)

Si-H bending

Antisym Si-Si str 450 (6.5) - 444 (2.2) 409 (1.1) 445 (s) 444 (m)

aStr refers to stretching. bTwo scaling factors of 0.978 and 0.917 were applied to the low (below 2300 cm1) and the high(above 2300 cm1) energy fundamental frequencies, respectively.

Table 2. IR and Raman frequencies of tris(trimethylsilyl)chlorosilane.

Approximate type IR (Rel. int., %) Raman (Rel. int.,%)

of modea

Experiment B3LYP/ Experiment HF/

6-31G(d) 6-31G(d)

//HF/6-31G //HF/6-31G

(Scaled with two (Scaled with a uniform

scaled factorsb) factor of 0.900)

Antisym C-H str 2955 (61) 2954 (11) 2946 (36) 2946 (94)

Sym C-H str 2897 (43) 2883 (6.2) 2886 (100) 2887 (100)

Antisym 1400 (34) 1453 (3.6) 1396 (3.6) 1444 (14)

Sym Si-C-H bending 1250 (83) 1255 (11) 1232 (5.4) -

Antisym Si-C-H bending 837 (100) 830 (100) 826 (3.6) 846 (1.3)

Antisym 745 (37) 707 (1.7) 734 (5.5) -

Anitsym Si-C str, 691 (43) 636 (3.1) 678 (15) 634 (5.2)

antisym Si-C-H bending

Sym Si-C str 625 (26) 568 (2.0) 614 (73) 558 (6.6)

Antisym Si-Si str 486 (44) 431 (0.9) 474 (4.4) 435 (1.0)

aStr refers to stretching. bTwo scaling factors of 0.978 and 0.917 were applied to the low (below 2300 cm1) and the high(above 2300 cm1) energy fundamental frequencies, respectively.

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Table 3. IR and Raman frequencies of (trimethylsilyl)silanol.

Approximate type of modea IR (Rel. int., %) Raman (Rel. int.,%)

Experiment B3LYP/6-31G(d) Experiment HF/6-31G(d)

//HF/6-31G //HF/6-31G

(Scaled with two (Scaled with a uniform

factorsb) factor of 0.900)

Pure O-H str 3661 (13) 3729 (5.0) 3664 (8.3) 3706 (11)

H-bonded O-H str 3433 (16, very broad) - - -

Antisym C-H str 2955 (37) 2952 (16) 2936 (49) 2944 (100)

Sym C-H str 2893 (20) 2878 (8.8) 2880 (100) 2884 (98)

Antisym 1400 (13) 1457 (3.2) 1390 (9.0) 1441 (13)

H-C-H bending

Sym Si-C-H bending 1250 (42) 1253 (9.9) 1222 (12) -

Antisym Si-C-H bending 841 (100) 831 (100) 854 (8.5) 842 (1.4)

Antisym 748 (43) 688 (2.8) 727 (10) -

Si-C-H bending

Antisym Si-C str, 691 (24) 633 (3.3) 666 (17) 625 (4.1)

antisym Si-C-H bending

O-H bending 625 (15) 516 (16) 602 (39) 555 (4.9)

Antisym Si-Si str, 471 (7.8) 404 (5.6) 440 (7.6) 445 (0.70)

O-H bending

aStr refers to stretching. bTwo scaling factors of 0.978 and 0.917 were applied to the low (below 2300 cm1) and the high(above 2300 cm1) energy fundamental frequencies, respectively.

Table 4. IR and Raman frequencies of (trimethylsilyl)methylsilane.

Approximate type of modea IR (Rel. int., %) Raman (Rel. int.,%)

Experiment B3LYP/6-31G(d) Experiment HF/6-31G(d)

//HF/6-31G //HF/6-31G

(Scaled with two (Scaled with a uniform

factorsb) factor of 0.900)

Antisym C-H str 2951 (45) 2953 (22) 2944 (56) 2944 (100)

Sym C-H str 2893 (31) 2878 (13) 2886 (100) 2884 (94)

Antisym 1400 (19) 1456 (4.0) 1400 (6.4) 1440 (13)

H-C-H bending

Sym Si-C-H bending 1250 (43) 1254 (12) 1234 (7.6) -

Antisym Si-C-H bending 837 (100) 833 (100) 832 (6.3) -

Antisym 783 (41) 753 (21) 736 (7.4) -

Si-C-H bending

Antisym 741 (21) 690 (3.1) - -

Si-C-H bending

Anitsym Si-C str, 6914 (30) 633 (5.2) 678 (21) 626 (4.3)

antisym Si-C-H bending

Sym Si-C str 621 (21) 568 (4.2) 612 (31) 556 (3.9)

Antisym Si-Si str - - 444 (4.3) 416 (1.0)

aStr refers to stretching. bTwo scaling factors of 0.978 and 0.917 were applied to the low (below 2300 cm1) and the high(above 2300 cm1) energy fundamental frequencies, respectively.

Infrared and Raman Spectroscopic Studies of Tris(trimethylsilyl)silane Bong Hyun Boo -3199-

Table 5. IR and Raman frequencies of (trimethylsilyl)methoxysilane.

Approximate type of modea IR (Rel. int., %) Raman (Rel. int.,%)

Experiment B3LYP/6-31G(d) Experiment HF/6-31G(d)

//HF/6-31G //HF/6-31G

(Scaled with two (Scaled with a uniform

factorsb) factor of 0.900)

Antisym C-H str in 2955 (45) 2952 (16) 2942 (49) 2946 (86)

the silyl groups

Antisym C-H str in 2897 (29) 2927 (6.8) 2884 (100) 2885 (100)

the -OMe group

Sym C-H str in 2820 (18) 2878 (11) 2812 (14) -

the silyl groups

Antisym 1400 (14) 1455 (3.0) 1398 (7.6) 1440 (12)

H-C-H bending

Sym Si-C-H bending 1250 (40) 1253 (10) 1234 (8.8) -

Antisym O-C-H bending 1169 (11) 1144 (3.9) - -

C-O-Si str 1080 (53) 1076 (28) 1072 (5.1) 1029 (0.84)

Antisym Si-C-H bending 841 (100) 831 (100) 824 (6.3) 843 (1.2)

Antisym Si-C str 702 (35) 635 (4.6) 674 (17) 628 (4.2)

Pure Si-O str 625 (14) 600 (4.6) 608 (32) 579 (2.7)

Antisym Si-Si str 475 (8.6) 435 (2.1) 452 (4.0) -

aStr refers to stretching. bTwo scaling factors of 0.978 and 0.917 were applied to the low (below 2300 cm1) and the high(above 2300 cm1) energy fundamental frequencies, respectively

Table 6. IR and Raman frequencies of tetrakis(trimethylsilyl)silane.

Approximate type of modea IR (Rel. int., %) Raman (Rel. int.,%)

Experiment B3LYP/6-31G(d) Experiment HF/6-31G(d)

//HF/6-31G //HF/6-31G

(Scaled with two (Scaled with a uniform

factorsb) factor of 0.900)

Antisym C-H str 2955 (38) 2953 (16) 2942 (66) 2945 (100)

Sym C-H str 2893 (28) 2879 (9.7) 2886 (100) 2885 (99)

Antisym 1400 (16) 1458 (2.2) 1394 (13) 1444 (9.8)

H-C-H bending

Sym Si-C-H bending 1246 (36) 1252 (7.9) 1232 (16) -

Antisym Si-C-H bending 833 (100) 830 (100) 826 (14) 844 (1.2)

Antisym 750 (13) 715 (0.81) 734 (18) 745 (0.67)

Si-C-H bending

Anitsym Si-C str, 687 (24) 629 (3.5) 674 (34) 625 (4.9)

antisym Si-C-H bending

Sym Si-C str 621 (20) 571 (3.2) 616 (35) 561 (4.6)

Antisym Si-Si str 460 (3.8) 383 (0.0) 442 (13) 414 (1.1)

aStr refers to stretching. bTwo scaling factors of 0.978 and 0.917 were applied to the low (below 2300 cm1) and the high(above 2300 cm1) energy fundamental frequencies, respectively.

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ACKNOWLEDGMENTS

This study was financially supported by the researchfund (for the Development of Distinguished Scientists ofRegional Universities in Republic of Korea) of NationalResearch Foundation in 2011 (Grant Number: 2011-0011564), which is gratefully acknowledged.

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