Infrared and Raman spectra, ab initio calculations and conformational studies of ethyl iodosilane

Article abstract of DOI:10.1016/S0022-2860(02)00480-5

Ethyl iodosilane (CH₃CH₂-SiH₂I) was synthesized for the first time. Infrared spectra were recorded in the vapour, amorphous and crystalline solid phases in MIR and FIR regions. Additional MIR spectra of the compound isolated in argon and nitrogen matrices were obtained at 5 K. Raman spectra of the liquid, excited by argon and by Nd³⁺ YAG lasers, were recorded at room temperature including polarization measurements. The spectra were studied in an extended temperature range 173-353 K and a ΔH value of 1.2 ± 0.3 kJ mol^-1 was obtained with gauche being the low energy conformer. Spectra of the amorphous and crystalline solids were obtained at liquid nitrogen temperature. Ethyl iodosilane exists in an equilibrium between anti and gauche conformers, in the vapour, liquid and amorphous states. After careful annealing the amorphous solid on a cold Cu finger (Raman) or on a CsI or Si window (infrared) to 160 K a partly crystalline solid was formed. A number of IR and Raman bands were reduced in intensity after annealing, although they did not vanish completely. From comparison between the observed and calculated vibrational modes it was apparent that the gauche conformer was present in the crystal. The sample was mixed with argon and nitrogen in a ratio 1:1000, deposited on a window at 5 or 10 K and annealed to temperatures between 5 and 36 K (argon) and 5-30 K (nitrogen). IR bands attributed to the anti and gauche conformers were reduced and increased in intensities, respectively. Thus, the gauche conformer was the low energy conformer in the matrices and probably also in the vapour phase. Ab initio calculations were performed at the RHF/3-21 G* and 6-311G* B3LYPs and gave optimized geometries, IR and Raman intensities and vibrational frequencies for the anti and gauche conformers. An enthalpy difference of 0.9 kJ mol^-1 was obtained from the calculations with gauche being the low energy conformer. After scaling, a reasonably good agreement between the experimental and calculated wavenumbers for the anti and gauche conformer was obtained. The spectra of ethyl iodosilane were closely related to those of the corresponding ethyl fluoro, ethyl chloro and ethyl bromosilane.

Full text of DOI:10.1016/S0022-2860(02)00480-5

Journal of Molecular Structure 644 (2003) 105–118
www.elsevier.com/locate/molstruc
Infrared and Raman spectra, ab initio calculations
and conformational studies of ethyl iodosilane
Valdemaras Aleksaa,b,1, David L. Powella,2, Alytis Gruodisa,1, Karl Hasslerc,
c
b
b
a,
*
Reinhard Hummeltenberg , Klaus Herzog , Reiner Salzer , Peter Klaeboe ,
Claus J. Nielsena
aDepartment of Chemistry, University of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, Norway
b
¨ ¨
Institut fur Analytische Chemie, Technische Universitat Dresden, D-01062 Dresden, Germany
c
¨ ¨
Institut fur Anorganische Chemie, Technische Universitat Graz, Stremayrgasse 16, A-8010 Graz, Austria
Received 5 August 2002; accepted 9 September 2002
Abstract
Ethyl iodosilane (CH3CH2–SiH2I) was synthesized for the first time. Infrared spectra were recorded in the vapour, amorphous
and crystalline solid phases in MIR and FIR regions. Additional MIR spectra of the compound isolated in argon and nitrogen
matrices were obtained at 5 K. Raman spectra of the liquid, excited by argon and by Nd3þ YAG lasers, were recorded at room
temperature including polarization measurements. The spectra were studied in an extended temperature range 173–353 K and a
DH value of 1.2 ^ 0.3 kJ mol21 was obtained with gauche being the low energy conformer. Spectra of the amorphous and
crystalline solids were obtained at liquid nitrogen temperature. Ethyl iodosilane exists in an equilibrium between anti and gauche
conformers, in the vapour, liquid and amorphous states. After careful annealing the amorphous solid on a cold Cu finger (Raman)
or on a CsI or Si window (infrared) to 160 K a partly crystalline solid was formed. A number of IR and Raman bands were reduced
in intensity after annealing, although they did not vanish completely. From comparison between the observed and calculated
vibrational modes it was apparent that the gauche conformer was present in the crystal. The sample was mixed with argon and
nitrogen in a ratio 1:1000, deposited on a window at 5 or 10 K and annealed to temperatures between 5 and 36 K (argon) and 5–
30 K (nitrogen). IR bands attributed to the anti and gauche conformers were reduced and increased in intensities, respectively.
Thus, the gauche conformer was the low energy conformer in the matrices and probably also in the vapour phase.
Ab initio calculations were performed at the RHF/3-21 G* and 6-311G* B3LYPs and gave optimized geometries, IR and
Raman intensities and vibrational frequencies for the anti and gauche conformers. An enthalpy difference of 0.9 kJ mol21 was
obtained from the calculations with gauche being the low energy conformer. After scaling, a reasonably good agreement between
the experimental and calculated wavenumbers for the anti and gauche conformer was obtained. The spectra of ethyl iodosilane
were closely related to those of the corresponding ethyl fluoro, ethyl chloro and ethyl bromosilane.
q 2002 Elsevier Science B.V. All rights reserved.
Keywords: Infrared spectra; Ethyl iodosilane; Raman spectra; Matrix isolation spectroscopy; Conformations; Ab initio calculations
*
Corresponding author. Tel.: þ47-2285-5678; fax: þ47-2285-5441.
E-mail address: peter.klaboe@kjemi.uio.no (P. Klaeboe).
Permanent address: Department of General Physics and Spectroscopy, Vilnius University, Vilnius 2734, Lithuania.
1
2
Permanent address: Department of Chemistry, The College of Wooster, Wooster, OH 44691, USA.
0022-2860/03/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved.
PII: S0022-2860(02)00480-5

106
V. Aleksa et al. / Journal of Molecular Structure 644 (2003) 105–118
1. Introduction
In our laboratories, a series of halogenated silanes,
disilanes and trisilanes have been synthesized. They
were investigated by infrared and Raman spec-
troscopy and some compounds have also been studied
by gaseous electron diffraction. We are interested in
comparing the thermodynamic and spectroscopic
results for various halosilanes and the differences
caused by the various halogen substituents. Various
silanes with conformational equilibria containing one
[1–3], two [4–7] and three [8,9] silicon atom have
been investigated. It is of particular interest to
compare the vibrational spectra and the conformation-
al equilibria in molecules with C–C, Si–C and Si–Si
central bonds, to elucidate the differences between the
carbon and silicon atoms, the bonds and the barrier to
internal rotation.
Fig. 1. The anti and gauche conformers of ethyl iodosilane (EISI).
expected and are illustrated in Fig. 1. The spectra and
conformations of EISI are very similar to those of the
related compounds: ethyl fluorosilane [10], ethyl
chlorosilane [11] and ethyl bromosilane [12] which
have recently been studied. A preliminary account of
the results for EISI and the related molecule ethyl
diiodosilane has already been reported [13].
Ethyl iodosilane (CH3CH2–SiH2I), later to be
abbreviated EISI, was synthesized and the infrared
and Raman spectra were investigated. Raman spectra
of the liquid were recorded and polarization ratios
were obtained. Raman spectra were recorded at
different temperatures to obtain the enthalpy differ-
ence between the conformers. Raman spectra of EISI
were recorded using both visible (argon laser,
frequency doubled Nd3þYAG laser) and near IR
laser (Nd3 þ YAG), and spectra of the crystal,
obtained by different cooling techniques, were
observed.
2. Experimental
2.1. Sample preparation
Halogenated silanes are sensitive towards moisture
and, in the case of iodosilanes, also towards oxygen.
All reactions and manipulations therefore were
carried out in an inert atmosphere of nitrogen using
standard Schlenk techniques. All solvents were
distilled from potassium in a nitrogen atmosphere
prior to use. The purities of the compounds were
1
checked with H and 29Si-NMR spectroscopy (sol-
The vapour, amorphous and crystalline samples
of EISI were recorded in the middle (MIR) and far
infrared (FIR) regions. Infrared matrix isolation
technique was employed to obtain spectra of the
compound trapped in argon and nitrogen matrices.
With this technique, neighbouring bands of differ-
ent conformers, which overlap in spectra of the
vapour and liquid, can frequently be separated, due
to the narrow band widths observed in the matrix
spectra. Also, by appropriate annealing of the
matrices in the temperature range 25–38 K, the
conformational barriers can often be estimated in
the matrices.
utions in C6D6), elemental analysis as well as gas
chromatography coupled with mass spectrometry.
Ethyliodosilane, EtSiH2I (EISI) was prepared by
protodearylation of ethylmesitylsilane (which in
turn was prepared by reduction of ethylmesityldi-
chlorosilane) with liquid hydrogen iodide in a
sealed glass tube. 0.9 g (21.9 mmol) of EtMeSiH2
or 4.5 g (21.2 mmol) of EtPh2SiH was placed in a
thick-walled glass tube. With the aid of a vacuum
line, a tenfold excess of hydrogen iodide HI was
then condensed into the tube which was then sealed
in vacuo. The tube was kept at 235 8C for one
week, then opened cautiously. The excess of
hydrogen iodide was condensed off with the aid
of a vacuum line and the liquid residue consisting
of the desired iodosilane and mesitylene or benzene
was fractionated. In this way, 3.76 g (20.2 mmol,
The conformational energies, the structure, the
force constants and the Raman and infrared intensities
were calculated by ab initio quantum chemical
methods. Two staggered conformations of EISI are

V. Aleksa et al. / Journal of Molecular Structure 644 (2003) 105–118
107
92%) of EISI was obtained as a colourless liquid;
b.p.760 mmHg ¼ 90–95 8C; 29Si-NMR: d ¼ 240.4 ppm
spectrometer from Jobin Yvon was employed, using a
frequency doubled Nd3þ:YAG laser resulting in a
green line at 532 mm. An FT-Raman spectrometer
from Bruker was also used, excited by an Nd3þ:YAG
laser operating at 1.064 mm. The liquid was recorded
in a capillary at room temperature using each of the
four spectrometers and polarization measurements
were made with argon laser excitation. The tempera-
ture dependent measurements were carried out with a
device consisting of a copper block on which the
capillary with the substance is mounted. The copper
block was placed in an evacuated chamber equipped
with quartz windows and is cooled with liquid
nitrogen from a small Dewar vessel. It can also be
heated resistively, and with a small thermocouple the
temperature is monitored and controlled. Thus,
temperatures above ambient were obtained using the
same setup, and temperatures between 353 and 173 K
were employed. Band areas were used in the van’t
Hoff plots and the convolutions were made with the
Peakfit program package.
1
against TMS, J(SiH) ¼ 266.6 Hz.
2.2. Infrared spectral measurements
The spectra in the MIR were recorded on the
following Fourier transform spectrometers: a Bruker
model IFS-88 (4000–450 cm21), a Bruker model 66
(4000–450 cm21) and a Perkin–Elmer model 2000
(4000–450 cm21). Spectra in the FIR were covered
by a Bruker IFS-113v vacuum spectrometer (600–
50 cm21). Beamsplitters of Ge substrate on KBr were
employed in the MIR, and beamsplitters with 3.5 and
12 mm thickness of Mylar were used in the FIR. The
vapour was studied in cells with KBr (10 cm) and
polyethylene windows (20 cm) in MIR and FIR,
respectively. The amorphous and crystalline solids
were deposited on a CsI window in the MIR and on a
wedge shaped window of silicon in the FIR; both
cryostats were cooled with liquid nitrogen.
The sample was diluted with argon or nitrogen
(1:1000) and deposited on a CsI window at 5 K of a
Displex cryostat from APD (model HS-4) with a three
stage cooling system. Independent matrix spectra
were obtained with a closed cycle cryostat from
Leybold (model RGD 210) deposited at 10 K,
employing a similar technique. The spectra of the
unannealed matrices were first recorded. Sub-
sequently, the matrices were annealed to temperatures
between 15 and 20 K to remove site effects. Then they
were annealed in steps of 3–5 K to a maximum of
36 K in argon and 32 K in nitrogen, in periods from
10 min to 1 h. Higher temperature measurements are
not feasible since the inert gases have too high a
pressure in the cryostat and the matrices turn ‘soft’
followed by diffusion of the solute. After each
annealing the window was recooled to 5 or 10 K
and the spectra were recorded.
Spectra of the crystal were obtained in a capillary
tube of 2 mm inner diameter surrounded by a Dewar,
cooled by gaseous nitrogen evaporated from a
reservoir [14]. Independently, the vapour of EISI
was condensed on a copper finger at 78 K. An
amorphous phase was first formed and after annealing
to ca. 145–165 K and recooling to 78 K, a partly
crystalline phase was obtained. The presence of a
crystal was apparent from frequency shifts of the
bands relative to the amorphous phase and sharpening
of the bands. Certain bands belonging to the high
energy conformer were reduced in intensity (but did
not completely disappear). However, we did not
observe any clear-cut cases of lattice modes in the low
frequency region.
3. Results
2.3. Raman spectral measurements
3.1. Infrared spectral results
The Raman spectra of EISI were obtained on a
single channel, triple monochromator spectrometer
Dilor RTI-30 and on a multichannel Dilor XY 800
spectrometer, excited by argon ion lasers (Spectra-
Physics model 2000 and Carl Zeiss, Jena Model
ILA20-l), using the 514.5 mm line. An additional
Infrared vapour spectra of EISI in the 1300–
475 cm21 region are given in Fig. 2. A vapour
spectrum in the region 600–80 cm21 at full pressure
appears in Fig. 3. As is apparent from these figures,
some bands had well resolved rotational fine structure

108
V. Aleksa et al. / Journal of Molecular Structure 644 (2003) 105–118
in the vapour, liquid or crystalline states. They were
recorded in argon and in nitrogen matrices at 5 or
10 K in the ratios (1:1000). Infrared spectra of EISI in
unannealed nitrogen matrices and in matrices
annealed to 32 K are given in Fig. 7 (1140–
800 cm21). Spectra in argon matrices have been
omitted since it was observed that spectra of the
annealed matrices in argon had considerably higher
background than those recorded of the unannealed
matrices. Much higher scattering from the annealed
than the unannealed matrices has been observed also
in various other halogenated silanes, making the
interpretation more difficult [15,16]. In Fig. 7 the
ordinate scale has been changed for the annealed
matrices, making the curves more similar to those of
the unannealed curves.
Fig. 2. MIR spectrum of EISI as a vapour in a cell with 10 cm path
and full pressure (15 Torr) in the range 1300–475 cm21
.
Supposedly, the conformational equilibrium of the
vapour phase is maintained when the gas mixture is
quickly frozen on the CsI window at 5 or 10 K,
provided that the barrier to conformational equili-
brium is high enough to prevent conversion. Small
spectral changes occurred both in the argon and
nitrogen matrices when the samples were annealed to
temperatures below 20 K, indicating minor site effects
in the matrices. At higher annealing temperatures
(20–34 K) changes were observed both in the argon
and nitrogen matrix spectra which were correlated
with conformational changes in EISI.
with possible A, B, C or hybrid structure as described
in Table 1.
The infrared spectra of the amorphous and
annealed solids at 78 K are presented in the regions
3000–2000 cm21 (Fig. 4), 1300–500 cm21 (Fig. 5)
and the region (600–50) cm21 (Fig. 6). Some bands
present in spectra of the amorphous solid (solid line)
vanished or were reduced in intensities in the spectra
of the crystal after annealing (dotted line): 1005, 900,
845, 763, 684, 427, 382 and 231 cm21. They are
generally the same bands as those disappearing in the
Raman spectra of the crystal (see below) and therefore
belong to the high energy conformer of the liquid.
The infrared spectra of EISI in matrices at 5 or
10 K display much sharper bands than those recorded
It can be clearly seen from the observed spectra
that the bands at 1006, 905, 839 and 806 cm21 (values
in nitrogen) and those at 906, 852, 767, 688, 643, 555
and 408 cm21 (values in argon) are reduced in
intensity after annealing. On the other hand the
bands at: 1026, 987, 939, 873, 837, 818, 732, 648, 548
and 517 cm21 (nitrogen) and 1023, 978, 867, 765,
734, 682, 648, 517 and 412 cm21 (argon) are
enhanced after annealing.
In most cases the bands being reduced in intensity
in one or both matrices after annealing were the same
as those being reduced in intensity after crystal-
lization. Moreover, the bands being enhanced mostly
corresponded with those remaining in the crystal.
Accordingly, the same conformer which was present
in the crystal was also the more stable conformer in
the matrices and probably also in the vapour. The
matrix bands which increase or decrease in intensities
after annealing are marked with arrows pointing
upwards or downwards in Table 1. The fact that
Fig. 3. FIR spectra of EISI as a vapour in a cell with 20 cm path and
pressure 15 Torr in the range 600–80 cm21
.

V. Aleksa et al. / Journal of Molecular Structure 644 (2003) 105–118
109
Table 1
Infrared and Raman spectral data for ethyl iodosilane, CH3CH2SiH2I (EISI)
Infrared
Raman
Liquid
Interpretation
Vapour
298 Kb
Ar-matrix
N2-matrix
Solid
Amorphous Crystalline Anti
Gauche
Amorphous Crystal
165 Ka
78 Kb
36 Ka
5 Kb
32 Ka
5 Kb
165 Ka
78 Kb
78 Kb
298 Kb
78 Kb
3112 vw
3102 vw
3046 vw
2977 s
3101 vw,br 3101 vw
2975 s
2970 s
2945 m
2971 s
2971 s
2956 vs
2929 m
2959 s
2959 m, D?
2929 m, P
2918 vw, D
2904 m, P
2956 m
2930 m
2920 m
2954 m
2924 s
2921 m
A00n1 A0n1 n1n2
2931 m
2945 m
2931 m
A00n3
2920 vw, sh 2918 vw
2919 m,sh 2920 m
n3
n4
2891 m
2885 m
2894 w
2886 m
2887 m, sh
2885 m
2901 w
2884 w
2870 s
2905 vw
2884 vw, sh (p) 2882 s, P
2883 m
2868 s
2882 m
2868 s
A0 n4
A0 n5
2873 s
2872 s, P
2850 vw
n5
n6
n7
2843 vw
2839 w
2196 vs
2190 vs
2186 vs
2850 vw
2850 vw
2186 vs R
2183 vs Q
2180 vs P
2179 vs R
2174 vs Q
2170 vs P
2197 s
2180 w, sh
2168 vs
2188 vs
2179 vs
2168 vs
2175 vs
2180 m, D
A00n6
A0n7
2172 vs
2178 vs
2169 vs
2137 w,sh
1456 m
2161 vvs, P
2163 vs
1454 w
2167 w,sh 2168vw
2152 w,sh? 2152 w,sh?
1469 m
2137 w,sh
1465 vw
1456 m
2123 w, bd
1464 w
1471 w
A00n8
A0n9
n8
n9
1465 w Q
1458 w
1465 m
1462 m
1454 m, D
1454 w
1417 w
1419 w
1412 m
1410 m
1383 w
1245 m
1235 w
1232 w
1229 vw
1225 vw
1419 m
1401 m
1419 m
1401 m
1412 w Q
1407 w
1411 m
1402 m, D
1377 w, P?
1401 w
1378 w
1401 w
1378 w
A0n10
A0n11
n10
n11
1385 vw
1383 m
1243 w
1368 w
1371 m
1232 w Q
1220 vw
1236 w #
1228 m #
1231 m
1222 s
1232 s
1220 s
1226 s, P
1231 s
1231 w
A00n12
A0n13
n12
n13
1223 w, sh, D 1222 m
1220 vvw
1086 vw
1090 m,br 1075 m, br # 1090 w
1090 vw
1070vw
1070 w
1031 w R
1028 w Q
1023 w P
1013 m R
1029 m #
1026 w "
1023 w
1027 m
1023 w "
1025 s
1022 s
1022 w, P
1024 w
1024 w
n14
1010 m #
1006 s #
983 w
980 s
p
1007 s
981 w
978 m "
973 m
970 s
1006 m #
987 w
1005 vw (p)
975 s
1002 w,P
972 vw,sh
962 m, P
A0n14
A00n15
1006 m P
976 s Q
974 s
986 vw, P
976 m
973 s, sh
974 m
967 m
n15
n16
977 w
969 s
969 s Q
964 s
965 vs
966 s
964 m
957 m
953 m
947 s
959 w
954 w,sh
954 w,sh
A0n16
946 s
944 s #
939 s #
945 s
(continued on next page)

110
V. Aleksa et al. / Journal of Molecular Structure 644 (2003) 105–118
Table 1 (continued)
Infrared
Raman
Interpretation
Vapour
Ar-matrix
N2-matrix
Solid
Liquid
Amorphous Crystalline Anti
Gauche
Amorphous Crystal
165 Ka
36 Ka
5 Kb
32 Ka
5 Kb
165 Ka
78 Kb
298 Kb
937 s
78 Kb
78 Kb
298 Kb
78 Kb
943 s
938 s
936 s
926 vs
898 w
928 vs
929 m, D
925 m
925 w
A0n17
n17
931 m
929 m
906 m #
876 s
930 m #
900 vvw
882 m R
877 m P
852 w, sh
905 w #
873 m #
877 m
900 w (p)
867 s "
852 vw #
838 vs
868 m
845 w
870 m
845 vvw (p)
854 vw
839 m #
837 m "
832 vw
832 vs
819 vs
829 m
808 vs
828 m,sh
804 vs
827 vs R
822 vs Q
817 vs P
806 m, sh
818 vs "
815 w
817 vw,sh
802 w,br
817 vvw
n18
804 w, br
797 w, br
807 vs
806 vs #
802 vvw (p) A0 n18
759 vvw(p) A00 n19
729 w
791 vw
779 vw
767 m "
765 m "
738 m
788 w
774 w
768 w
776 w
763 w
729 vs
763 vw(p)
759 vw
759 vvw
729 m
766 m
736 m #
732 m "
721 m
690 w
738 s R
733 s Q A
728 s P
734 m "
732 m
729 m
725 s
726 m P
n19
700 vw
698 vw
688 m #
682 m "
648 m #
643 w #
p
p
686 w
679 w
643 w
639 w
686 w
683 m
647 m
642 w
622 vw
613 vw
685 m
677 m, P
683 w
640 s
A0n20
684 m
n20
n21
647 w, sh
643 m
643 m
636 m
637 vs, P
625 w, sh
640 m
A00n21
612 vw
588 w
573 w,P
555 w #
548 w #
536 w
535 m R
530 m Q
527 m Q
523 m P
486 w,sh
448 vw
436 vw
418 w
532 m
523 s
532 w
531 s
527 w, P
518 vvw?
532 m
532 w
n22
517 w "
488 w
496 m
426 vw
498 m
500 vvw
424 vvw?
500 vvw
425 vvw
392 vvw
A00n22
439 m
427 vw (p)
413 w
412 s "
408 s #
406 w
392 m
395 m
381 m
394 m
382 w (p)
394 vw
377 s, P
392 vw
377 w
385 s
377 vw (p) A0n23
359 s
343 m, sh
355 s
338 vs
342 vs
340 vs, P
335 vs
335 vs
n23
349 s

V. Aleksa et al. / Journal of Molecular Structure 644 (2003) 105–118
111
Table 1 (continued)
Infrared
Raman
Interpretation
Vapour
298 Kb
Ar-matrix
N2-matrix
Solid
Liquid
Amorphous Crystalline Anti
Gauche
Amorphous Crystal
165 Ka
78 Kb
36 Ka
5 Kb
32 Ka
5 Kb
165 Ka
78 Kb
298 Kb
78 Kb
78 Kb
270 w
264 w
233 w
225 w
275 m
279?
268 m P
274 m
274 m
n24
p
;
p
231 vw
228 s, P
215 vvw?
198 vw
229 w
;
A0n24
A00n25
212 vw
190 vw
147 m, br
212 vw
190 vvwp
146 w
n25
n26
138 vw
115 vw
65 vw?
136 w, D
92 w
115 w, P
60 vw
A0n26
A00n27
72 w
70 w
75 vw
65 vw
74 vvw
61 vw
58 vw
n27
Abbreviations: s, strong; m, medium; w, weak; v, very; sh, shoulder, P, polarized; D, depolarized; P, Q, R, rotational fine structure; asterisks
denote bands disappearing or diminishing in intensity in the crystal; # " denote bands in the matrices with diminishing and increasing
intensities, respectively, after annealing.
a
Annealing temperature.
b
Recording temperature.
the matrix bands never vanished after annealing
suggests that the enthalpy difference in the matrices
(and probably also in the vapour) was low. A new
equilibrium was observed in the matrices in the range
5–34 K in which the conformational equilibrium was
shifted in the direction of gauche after annealing
compared to the equilibrium value in the vapour at
room temperature. The fact that the intensity vari-
ations occur after annealing in the temperature range
28–34 K suggests that the barrier to internal rotation
in EISI is approximately 8–10 kJ mol21, using the
curves given by Barnes [17]. Some of the IR bands
which change intensities after annealing have not
been assigned to fundamentals, but are probably due
to combination bands or overtones. In other cases
matrix bands changing intensities were attributed to
overlapping anti and gauche bands n12, n13 and n15.
solid disappear in the spectrum of the crystal: 1002,
759, 683 and 229 cm21. These bands are attributed to
the second conformer which is absent in the crystal
and are marked with asterisks in Table 1. In most
cases the corresponding bands were also reduced in
intensities in the infrared spectra after annealing.
By comparing with the calculated wavenumbers
obtained from the ab initio calculations after
3.2. Raman spectral results
Raman spectra of the liquid in the ranges 3000–
2800 and 2600–50 cm21 are given in Fig. 8, and a
spectrum of the annealed crystalline solid at 78 K is
presented in Fig. 9. The following bands present in
Raman spectra of the liquid and of the amorphous
Fig. 4. MIR spectra of EISI in the range 3000–2000 cm21 at 78 K,
solid curve, unannealed, amorphous solid; dashed curve, partly
crystalline solid, annealed to 165 K.

112
V. Aleksa et al. / Journal of Molecular Structure 644 (2003) 105–118
Fig. 7. MIR spectra of EISI in a nitrogen matrix (1:1000) recorded at
10 K, before annealing (solid line) and after annealing to 32 K
Fig. 5. MIR spectra of EISI in the range 1300–500 cm21 at 78 K,
solid curve, unannealed amorphous solid; dashed curve partly
crystalline solid, annealed to 165 K.
(dashed line) in the region 1180–800 cm21
.
from a frequency doubled Nd3þ:YAG laser. However,
satisfactory Raman spectra with the argon laser line
including polarization measurements were obtained
with the multichannel spectrometer, requiring less
exposure time. The spectra of the amorphous and
crystalline solid and spectra in the low frequency
range below 200 cm21 were observed with the triple
monochromator of the Dilor instruments. Irradiation
with the Nd3þ:YAG laser at 1.064 mm did not give
free iodine and good Raman spectra of the liquid were
appropriate scaling, it will be shown below that the
vanishing bands belong to the anti conformer.
Accordingly, the bands present in the crystal should
belong to the gauche rotamer. However, many of the
fundamentals of one conformer overlap those of the
other. The anti and gauche bands overlap more in
silanes than in ethanes which is probably due to lower
conformational barriers, relatively weak interaction
between the end groups and the longer Si–C distance
in the silanes compared to the C–C distance in ethanes.
The samples of EISI were slowly discolored by
free iodine when irradiated with the 514.5 mm
radiation of an argon ion laser or the 532 mm emission
recorded with the FT technique above 200 cm21
.
Variable temperature measurements of the liquid
(353–173 K) revealed that some bands were
reduced in intensity compared to neighbouring
Fig. 6. FIR spectra of EISI in the range 600–50 cm21 at 78 K, solid
curve, unannealed amorphous solid; dashed curve partly crystalline
solid, annealed to 165 K.
Fig. 8. Raman spectra of EISI as a liquid at ambient temperature,
excited by an argon ion laser.

V. Aleksa et al. / Journal of Molecular Structure 644 (2003) 105–118
113
Fig. 11. Van’t Hoff plot of the band pair 377*/340 cm21 in the
temperature range 353–192 K.
I
anti=Igauche; and it is assumed that the DH is constant
Fig. 9. Raman spectra of EISI as a partly crystalline solid at 78 K,
annealed to 165 K.
with temperature. The data were plotted as van’t
Hoff plots in Figs. 10–12. The values obtained were
1.18, 1.22 and 1.33 kJ mol21 giving an average of
1.2 ^ 0.3 kJ mol21 with gauche being the low
energy conformer.
bands. This was interpreted as a displacement of the
conformational anti–gauche equilibrium with tem-
perature. The bands becoming weaker at low
temperatures were the same as those vanishing or
being reduced in intensities after crystallization, and
to be discussed below these bands belonged
undoubtedly to the anti conformer. The correspond-
ing gauche bands were enhanced at low tempera-
tures revealing that gauche was the low energy
conformer. Among the conformer pairs observed,
the Raman pairs 677/727, 377/340 and
228/268 cm21 were chosen, in which bands in
the numerator belong to anti and those in the
denominator to gauche. The intensities of each band
pair were fitted to the equation: 2ln K ¼ DH=RT 2
DS=R where K is the ratio of the integrated areas of
The observed infrared and Raman spectral data
are listed in Table 1. As is apparent, Raman bands
in the amorphous phase at 1002, 683, 377 and
229 cm21 vanished partly in the spectra of the
crystal and have been marked with asterisks in
Table 1. In spite of careful annealing the ‘crystal
phase’ undoubtedly was mixed with amorphous
material leading to incomplete disappearance of the
high energy conformer bands. The same results were
observed in the related ethyl bromosilane [12].
These partly vanishing bands in the spectra of EISI
have been attributed to the high energy conformer
which turned out to be anti.
Fig. 10. Van’t Hoff plot of the band pair 677*/726 cm21 in the
temperature range 270–170 K.
Fig. 12. Van’t Hoff plot of the band pair 228*/268 cm21 in the
temperature range 353–173 K.

114
V. Aleksa et al. / Journal of Molecular Structure 644 (2003) 105–118
3.3. Quantum chemical calculations
and C–Si stretches and the skeletal deformations are
highly mixed. Obviously, the vibrational modes for
the anti conformer, separated into symmetry species
A0 and A00 are more localized than those of the gauche
conformer in which all the modes belong to the same
species.
HF, DFT (B3LYP) and MP2 calculations were
carried out using the GAUSSIAN98 program [18]
employing the 3-21G* and LanL2DZ basis sets. The
minima on the potential surface were found by
relaxing the geometry; the geometrical parameters
obtained with the different methods were essentially
identical and they are not presented here for the sake
of brevity.
4. Discussion
The calculations on EISI all favour gauche as the
low energy conformer in agreement with the results
for ethylfluoro- [10], ethylchloro- [11], and ethyl-
bromosilane [12]. The differences between the
calculated thermal corrections to the enthalpy
for the gauche and anti conformations were
negligible.
4.1. Conformations
It can be seen from Table 1 that the vibrational
bands at: 1010 (band center), 768, 686, 385 and
233 cm21 (IR vapour bands) vanish or are reduced
in intensity during crystallization both in the
infrared and Raman spectra. In addition, the
Raman bands at 797 and 573 cm21 and the infrared
band at 900 cm21 seem to disappear in the crystal.
Many of these bands also diminish in intensities
after annealing in the IR spectra of the argon and
nitrogen matrices.
3.4. Normal coordinate calculations
Analytical H–F force constants were derived for
each of the two conformers in EISI, using the
MP2/LanL2DZ basis set. The calculated ab initio
force constants were transformed from Cartesian to
symmetry coordinates, derived from a set of valence
coordinates. The ab initio calculated wavenumbers
are invariably larger than the experimental values.
In order to make a complete assignment of the
observed infrared and Raman bands, a normal
coordinate analysis with scaled force constants was
carried out. Reasonably good agreement between the
experimental and calculated wave numbers was
achieved by using scaling factors of 0.95 for the
stretching and bending modes above, and 1.0 for the
modes below, 400 cm21. The infrared intensities,
Raman scattering cross sections and Raman polar-
ization ratios were calculated and the data are listed
in Tables 2 and 3.
Neither the infrared vapor contours, Raman
polarization data nor the ab initio calculated
energies could answer the following question: do
the bands vanishing in the crystal spectra belong to
the anti or the gauche conformer? While the
calculated energies for the anti and gauche rotamers
derived from the ab initio calculations have large
uncertainties, the force constants and the wave
numbers after appropriate scaling usually give a
good agreement with those of the observed funda-
mentals. Within the group frequency regions for the
CH3 and CH2 stretching and deformation vibrations,
the calculated wave numbers for the anti and
gauche fundamentals overlap nearly completely
(Tables 2 and 3) except for n3 and n4. Below
1200 cm21 there are nine instances in which the anti
and gauche fundamentals are separated more than
10 cm21, and in four cases (n19, n22, n23 and n24)
the shifts are larger than 30 cm21. Obviously, the
observed anti/gauche band pairs are most likely to
appear at these frequencies.
The potential energy distribution (PED) in Tables 2
and 3 is expressed in terms of the symmetry
coordinates. The normalized symmetry coordinates
are identical to those recently given in Ref. [13] for
ethyl bromosilane and have been constructed from a
set of valence coordinates. Only PED terms larger
than 10% have been included in Tables 2 and 3 and
the largest contribution described in terms of valence
coordinates. The C–H and Si–F stretching modes are
reasonably well localized, but the CH3 rock, the C–C
The following observed IR and/or Raman pairs
(wavenumbers from IR spectra of the vapour, except
for n25): 1006*/1028, 940/900*, 822/806*, 768*/733,
686*/679, 530/486*, 385*/355, 270/233* and
215/198* cm21. The bands with asterisks vanished in

V. Aleksa et al. / Journal of Molecular Structure 644 (2003) 105–118
115
Table 2
Observed and calculated normal modes of vibration in the gauche conformer of ethyl iodosilane (EISI)
No.
Freq.a
IR
R
r
Obs.b
PEDc
Description
1
2971
2967
2939
2878
2871
2168
2148
1464
1460
1406
1391
1271
1228
1020
994
32.5
25.1
0.61
0.64
0.75
0.09
0.05
0.75
0.06
0.75
0.73
0.65
0.68
0.25
0.75
0.65
0.74
0.45
0.74
0.53
0.45
0.74
0.46
0.61
0.27
0.48
0.28
0.68
0.75
2971
2971
2920
2891
2872d
2183
2174
1471
1465
1412
1385
1232
1220
1028
976
30 S1 þ 64 S17
68 S1 þ 26 S17
85 S18
CH3 antisymmetric stretch
CH3 antisymmetric stretch
CH2 antisymmetric stretch
CH2 symmetric stretch
CH3 symmetric stretch
SiH2 antisymmetric stretch
SiH2 symmetric stretch
2
42.5
2.2
53.2
94.8
170.9
78.1
88.0
216.8
12.4
14.8
11.4
2.0
3
4
12.5
39.9
80.7
88.2
9.4
91 S3
5
95 S2
6
100 S19
7
100 S4
8
47 S5 þ 46 S20
47 S5 þ 46 S20
90 S6
CH3 antisymmetric deformation
CH3 antisymmetric deformation
CH2 scissors
CH3 symmetric deformation
CH2 wag
9
8.8
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
5.0
7.9
90 S7
6.4
2.2
79 S8
6.4
7.8
59 S21 þ 29 S22
56 S10
CH2 twist
13.7
72.9
1.1
8.5
CH3 rock
3.2
36 S21 þ 33 S22
80 S9
CH3 rock
954
9.1
969
C–C stretch
920
121.9
266.5
48.2
1.7
27.4
7.2
937
98 S11
SiH2 def
808
822
63 S12 þ 16 S22
17 S12 þ 19 S23 þ 31 S24
52 S23þ27 S24
63 S13
SiH2 wag
726
10.7
21.0
20.0
10.7
18.6
5.9
733
CH2 rock
656
679
SiH2 twist
623
3.5
643
Si–C stretch
SiH2 rock
513
19.4
18.3
2.8
530
79 S25
308
355
65 S14
Si–I stretch
262
270
19 S14 þ 54 S15
17 S15 þ 62 S26
60 S16
C–C–Si bend
CH3 torsion
225
0.7
1.1
225
121
0.6
4.2
138
C–Si–I bend
SiH2I tors
63
0.2
1.7
65
84 S27
a
b
c
d
Frequencies from MP2/LanL2DZ calculations have been scaled by 0.95.
IR vapour phase values except when noted.
For definition of symmetry coordinates, see Ref. [1].
Raman liquid values.
the infrared and/or Raman spectra. In all cases the
bands marked with asterisks were correlated with the
anti, the other bands with the gauche rotamer.
Particularly convincing were the band pairs 768*/733
(n19), 530/486* (n22), 385*/355 (n23) and 270/233*
(n24) predicted from the MP2 calculations to give
shifts: 51, 37, 46 and 37 cm21, respectively, compared
is low since an equilibrium is attained at the annealing
temperatures of 32–37 K.
Thus, the gauche conformer remains in the crystal
and is also the low energy conformer in the liquid and
in the matrices. The same conclusion was drawn from
the results of the ab initio calculations in which all the
basis sets predicted the gauche conformer to have the
lower energy. Also, in both the two related com-
pounds ethyl chlorosilane [12] and ethyl bromosilane
[13] gauche was the low energy conformer and was
also present in the crystal.
to the experimental values: 41, 44, 30, and 34 cm21
.
As reported above the intensity variations observed
in the IR spectra of EISI in the argon and nitrogen
matrices after annealing supported the assignments
into anti and gauche conformers. In most cases arrows
pointing upwards (signifying increased intensity after
annealing) correspond with gauche fundamentals
while arrows pointing downwards fit with anti
bands. The energy difference between the conformers
4.2. Spectral assignments
The assignments of the infrared and Raman spectra
of EISI to the anti and gauche conformers appear in

116
V. Aleksa et al. / Journal of Molecular Structure 644 (2003) 105–118
Table 3
Observed and calculated normal modes of vibration in the anti conformer of ethyl iodosilane (EISI)
No.
Freq.a
IR
R
r
Obs.b
PEDc
Description
1
2967 A000
2967 A
2949 A00
2897 A0
2868 A0
2169 A00
2150 A0
1462 A00
1459 A0
1415 A0
1390 A00
1270 A00
1229 A
1012 A00
1006 A0
945 A0
46.2
36.6
1.7
9.1
0.75
0.57
0.75
0.14
0.01
0.75
0.05
0.75
0.74
0.54
0.61
0.28
0.75
0.75
0.46
0.44
0.69
0.50
0.75
0.19
0.75
0.75
0.35
0.75
0.30
0.70
0.75
2971
2971
2931
2885
2872d
2183
2174
1471
1465
1412
1385
1232
1220
1006
981
57 S17 þ 43 S18
99 S1
CH3 antisymmetric stretch
CH3 antisymmetric stretch
CH2 antisymmetric stretch
CH2 symmetric stretch
CH3 symmetric stretch
SiH2 antisymmetric stretch
SiH2 symmetric stretch
CH3 antisymmetric deformation
CH3 antisymmetric deformation
CH2 scissors
2
98.2
86.9
84.2
135.4
79.9
202.1
19.3
11.9
12.8
1.0
3
43 S17 þ 57 S18
99 S3
4
12.0
42.9
76.6
77.4
12.9
5.4
5
100 S2
6
100 S19
7
100 S4
8
93 S20
9
93 S5
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
5.0
95 S6
4.9
94 S7
CH3 symmetric deformation
CH2 wag
2.8
1.6
80 S8
1.1
8.0
61 S21 þ 28 S22
44 S21 þ 35 S22 þ 16 S24
17 S8 þ 66 S10
78 S9
CH2 twist
10.4
79.9
7.7
4.5
CH3 rock
5.2
CH3 rock
16.3
24.8
14.0
7.5
946
C–C stretch
923 A00
115.9
297.3
21.8
16.4
3.3
937
90 S11
SiH2 deformation
SiH2 wag
796 A
806
92 S12
775 A000
652 A00
622 A
768
34 S22 þ 16 S23 þ 38 S24
82 S13
CH2 rock
14.9
23.8
5.7
686
Si–C stretch
639
70 S23 þ 20 S24
15 S24 þ 69 S25
54 S14 þ 31 S15
83 S26
SiH2 twist
478 A00
352 A0
217 A00
212 A0
103 A0
59 A00
12.4
23.2
0.2
486
SiH2 rock
15.8
0.2
385
Si–I stretch
233
198d
CH3 torsion
3.6
10.6
6.3
47 S14 þ 39 S15
24 S15 þ 71 S16
90 S27
C–C–Si bend
1.3
115
C–Si–I bend
0.2
0.6
65
SiH2I torsion
a
b
c
d
Frequencies from MP2/LanL2DZ calculations have been scaled by 0.95.
IR vapour phase values except when noted.
For definition of symmetry coordinates, see Ref. [1].
Raman liquid values.
Tables 1–3. For the sake of similarity, the funda-
mentals of both the anti and gauche conformers have
been numbered consecutively, instead of the conven-
tional numbering of the modes in the anti conformer
belonging to species A0 before those of A00.
mode is much stronger in Raman than the out-of-phase
mode. Both fundamentals have overlapping anti and
gauche bands as predicted from the calculations.
The CH3 deformation and the CH2 deformation,
wagging and twisting modes (n8–n13) are expected in
the range 1470–1200 cm21. In agreement with
the results of the calculations all these modes will
have coinciding anti and gauche bands. As is apparent
from Table 1 they are attributed to mostly medium
intense IR and Raman bands in the expected range.
The C–C stretching mode (mixed with CH3 rock) n14
is assigned to the Q-branch at 1028 cm21 (gauche )
and the band center at 1009 cm21 of a band with
B-contour (anti ). The latter IR band diminished in
intensity in the crystal and the Raman band vanished.
Of the two CH3 rocking modes, n15 is tentatively
The five hydrogens belonging to the CH3 and CH2
groups should give rise to five C–H stretching
fundamentals n1–n5 of each conformer in the range
2980–2870 cm21. Three of the anti and gauche
fundamentals apparently overlap while n3 and n4 are
split into two conformer pairs. With two Si–H bonds
we expect two fundamentals around 2200 cm21 for
each conformer. Theout-of-phase and the in-phase Si–
H stretches give rise to intense, prominent Q-branches
at 2183 and 2174 cm21, respectively, both having well
resolved R and P branches. As expected the in-phase

V. Aleksa et al. / Journal of Molecular Structure 644 (2003) 105–118
117
attributed to the bands at 981 (anti ) and 976 cm21
(gauche ) and n16 to the bands at 969 (gauche ) and
946 cm21 (anti ) although the latter lacks Raman
components. The SiH2 scissor (n17) was tentatively
the matrix data support the assignments. For both the
modes n23, involving predominantly Si–I stretch, and
n24 (CH3 torsion) the crystal data are conclusive and
these pairs were employed for two of the van’t Hoff
plots (Figs. 11 and 12). The assignments of n25 and n26
are tentative, since the IR and Raman bands are weak
and no matrix spectra were available in this low
frequency range. Finally, the SiH2I torsion (n27) is
tentatively assigned to the IR and Raman bands around
65 cm21 for both conformers.
assigned to overlapping modes at 937 cm21
.
All the modes n18–n26 have been assigned to anti
and gauche pairs. The calculations give large splittings
for most of these modes (as large as 46 cm21 for n23)
except for n20 and n21, in which the shifts were only 4
and 1 cm21, respectively. In the conformer pairs n18,
n
19 and n20 the anti components are reduced or vanish
The fundamental frequencies assigned to the more
stable gauche conformer in EISI are compared with the
corresponding fundamentals in the two related com-
pounds ethyl chlorosilane [11] and ethyl
bromosilane [12] in Table 4. It appears that most of
in the crystal, and the gauche component is enhanced
and the anti reduced in intensity in the matrix spectra
after annealing. In the conformer pairs n21 and n22, the
crystal data are not conclusive, but at least in n21
Table 4
Observed frequencies (cm21) for the gauche conformer of ethyl halosilanes (X ¼ Cl, Br, I) in the vapour phase
Vib. No.
Fundamental
X ¼ Cla
X ¼ Brb
X ¼ Ic
n1
CH3 antisymmetric stretch
CH3 antisymmetric stretch
CH2 antisymmetric stretch
CH2 symmetric stretch
CH3 symmetric stretch
SiH2 antisymmetric stretch
SiH2 symmetric stretch
CH3 antisymmetric deformation
CH3 antisymmetric deformation
CH2 symmetric deformation
CH3 symmetric deformation
CH2 wag
2979
2972
2928
2905
2885
2186
2180
1468
1458
1411
1392
1242
1234
1031
972
2978
2978
2940
2896
2892
2187
2179
1466
1466
1415
1387
1235
1225
1028
978
2971
2971
2920
2891
2872d
2183
2174
1471
1465
1412
1385
1232
1220
1028
976
n2
n3
n4
n5
n6
n7
n8
n9
n10
n11
n12
n13
n14
n15
n16
n17
n18
n19
n20
n21
n22
n23
n24
n25
n26
n27
CH2 twist
C–C stretch
CH3 rock
CH3 rock
981
970
969
SiH2 deformation
SiH2 wag
952
950
937
862
843
822
CH2 rock
744
736
733
SiH2 twist
693
662
679
C–Si stretch
657
645
643
SiH2 rock
562
549
530
Si–X stretch
522
414
355
CCSi bend/CSiX
CH3 torsion
283
276
222d
270
202
225
CSiX bend/CCSi
SiH2X/CCSi torsion
158
140
76d
138
83
65
a
See Ref. [11]
b
c
d
See Ref. [12]
This work.
Frequencies from Raman spectrum of the liquid.

118
V. Aleksa et al. / Journal of Molecular Structure 644 (2003) 105–118
[5] T. Johansen, K. Hagen, K. Hassler, G. Trekautz, R. Stølevik,
J. Mol. Struct. 509 (1999) 237.
the vibrational modes do not vary much between the
three compounds. An exception is n23 involving
predominantly SiX stretch and to some extent n26
CSiX bend/CSi bend which obviously are strongly
depending upon the mass of the halogen. Since the SiX
stretch and bend make contributions to many modes, it
is expected that most modes are displaced towards
lower wavenumbers going from Cl to Br to I. The Si–I
stretch n23 agrees well with earlier results for 1,2-
diiodosilane, 1,1,2,2-tetraiodosilane [19] and 1,1,2-
triiodosilane [5]. Furthermore, with a few exceptions
the same low frequency vibrational modes n17–n26
form conformer band pairs in the three compounds [5,
19]. The enthalpy difference between the two con-
formers were 0.65 ^ 0.19 for ethyl fluorosilane [10],
0.93 ^ 0.13 for ethyl chlorosilane [11] and
1.67 ^ 0.3 kJ mol21 for ethyl bromosilane [12] com-
pared with 1.24 ^ 0.3 kJ mol21 for EISI.
¨
[6] A. Jahn, K. Schenzel, I. Zink, K. Hassler, J. Raman Spectrosc.
29 (1998) 841.
[7] S.L. Hinchley, B.A. Smart, C.A. Morrison, H.E. Robertson,
D.W.H. Rankin, R. Zink, K. Hassler, J. Chem. Soc., Dalton
Trans. 14 (1999) 2303.
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29 (1998) 377.
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29 (1998) 1055.
[10] J.R. Durig, Y.E. Nashed, J. Tao, G.A. Guirgis, J. Mol. Struct.
560 (2001) 39.
[11] T.A. Mohamed, G.A. Guirgis, Y.E. Nashed, J.R. Durig, Struct.
Chem. 10 (1999) 333.
[12] G.A. Guirgis, Y.E. Nashed, J. Tao, D.L. Powell, A. Gruodis,
V. Aleksa, P. Klaeboe, C.J. Nielsen, J. Mol. Struct. (2002) in
press.
[13] R. Hummeltenberg, K. Hassler, in: N. Auer, J. Weiss (Eds.),
Organosilicon Chemistry IV, From Molecules to Materials,
Wiley–VCH, Weinheim, 2000.
[14] F.A. Miller, B.M. Harney, Appl. Spectrosc. 24 (1970) 291.
[15] V. Aleksa, P. Klaeboe, C.J. Nielsen, G.A. Guirgis, J. Mol.
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Acknowledgements
[16] V. Aleksa, P. Klaeboe, C.J. Nielsen, V. Tanevska, G.A.
Guirgis, Vibr. Spectrosc. 17 (1998) 1.
The authors are grateful to Anne Horn, Oslo for
valuable assistance. V. Aleksa received a grant in
Dresden from the Konferenz der Deutschen Aka-
demien der Wissenschaften and A. Gruodis was
supported by Det Norske Videnskaps-Akademi,
Oslo.
[17] A.J. Barnes, J. Mol. Struct. 113 (1984) 161.
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A.D. Daniels, K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi,
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