Matrix isolation infrared and density functional theoretical studies of organic silanones, (CH3O)2Si=O and (C6H5)2Si=O

Article abstract of DOI:10.1016/S0022-328X(98)00726-8

Transient organic silanones (CH₃O)₂Si=O (3) and (C₆H₅)₂Si=O (4) were generated by vacuum pyrolysis from 3,3-dimethoxy-6-oxa-3-silabicyclo[3.1.0]hexane (6) and its 3,3-diphenyl derivative (7), respectively, and after being trapped in argon cryogenic matrices at 12 K directly studied by IR spectroscopy. Vibrational assignments in the observed IR spectra of 3 and 4 have been made by comparison with the density functional theory B3LYP/6-311G(d, p) calculated harmonic frequencies and infrared intensities for these molecules and for the other silanones, H₂Si=O (1), (CH₃)₂Si=O (2), and CH₃(CH₃O)Si=O (5), studied earlier by matrix isolation techniques. The observed bands at 1247 cm^-1 in 3 and at 1205 cm^-1 in 4 were assigned to the Si=O stretching modes, which under the influence of the same substituents show the similar frequency shifts as ν (M=O) in ketones, phosphinoxides, and sulfoxides. Under the conditions of vacuum pyrolysis studied the diphenylsilanone 4 was found to be more thermodynamically stable than the dimethoxy derivative 3, while the latter indicated a higher kinetic stability towards cyclooligomerization than the silanones 2 and 4.

Full text of DOI:10.1016/S0022-328X(98)00726-8

Journal of Organometallic Chemistry 566 (1998) 45–59
Matrix isolation infrared and density functional theoretical studies of
organic silanones, (CH3O)2SiꢀO and (C6H5)2SiꢀO
Valery N. Khabashesku a,b,*, Zoya A. Kerzina b, Konstantin N. Kudin b, Oleg M. Nefedov b
a Department of Chemistry, Rice Uni6ersity, Houston, TX 77005-1892, USA
b Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 47 Leniskii prospekt, Moscow, 117913, Russian Federation
Received 4 January 1998
Abstract
Transient organic silanones (CH3O)2SiꢀO (3) and (C6H5)2SiꢀO (4) were generated by vacuum pyrolysis from 3,3-dimethoxy-6-
oxa-3-silabicyclo[3.1.0]hexane (6) and its 3,3-diphenyl derivative (7), respectively, and after being trapped in argon cryogenic
matrices at 12 K directly studied by IR spectroscopy. Vibrational assignments in the observed IR spectra of 3 and 4 have been
made by comparison with the density functional theory B3LYP/6-311G(d, p) calculated harmonic frequencies and infrared
intensities for these molecules and for the other silanones, H2SiꢀO (1), (CH3)2SiꢀO (2), and CH3(CH3O)SiꢀO (5), studied earlier
by matrix isolation techniques. The observed bands at 1247 cm−1 in 3 and at 1205 cm−1 in 4 were assigned to the SiꢀO stretching
modes, which under the influence of the same substituents show the similar frequency shifts as w (MꢀO) in ketones,
phosphinoxides, and sulfoxides. Under the conditions of vacuum pyrolysis studied the diphenylsilanone 4 was found to be more
thermodynamically stable than the dimethoxy derivative 3, while the latter indicated a higher kinetic stability towards
cyclooligomerization than the silanones 2 and 4. © 1998 Elsevier Science S.A. All rights reserved.
pounds, where they have been chemically trapped [8].
New routes to generation of silanones in the liquid
1. Introduction
phase have recently been reviewed [9].
The silicon analogues of ketones (silanones), R2SiꢀO,
The high polarity of the SiꢀO y-bond, estimated by
quantum chemical calculations [8,10–12], accounts for
the extreme reactivity of silanones towards dimerization
and trimerization, which proceed with no barrier, in
contrast to their carbon analogues. For this reason the
direct spectroscopic characterization of silanones as
monomers was only achieved in the 1980s. The inor-
ganic silanones F2SiꢀO and Cl2SiꢀO have been synthe-
sized in cryogenic matrices and their IR spectra was
recorded by Schno¨ckel [13,14]. Withnall and Andrews
reported the IR bands for matrix-isolated silanone 1,
and of its hydroxy derivatives, HO(H)SiꢀO and
(HO)2SiꢀO [15]. The H2SiꢀO species has also been
characterized in the gas phase by the chemiluminescent
emission spectrum in the visible region by Glinski et al.
[16], and recently, by its millimeter-wave rotational
spectrum [17].
are important intermediates in the combustion of
silanes [1], chemical vapor deposition processes employ-
ing the oxidation of silicon-containing precursors in
microelectronics industry [2], production of ceramic
powders [3], and of silicone-based materials [4].
Silanones have been suggested as transient species in
silicon thin film plasma deposition and etching technol-
ogy [5]. Evidence for the formation of the SiꢀO reactive
centers on the surface of mechanically activated quartz
has been obtained [6]. The parent silanone, H2SiꢀO (1),
is suggested to be of interest in interstellar chemistry [7].
The silanones are frequently expected to be formed in
the high-temperature or photochemically induced trans-
formations of the silicon–oxygen-containing com-
* Corresponding author. Tel.: +1 713 5278101; fax: +1 713
5238236.
0022-328X/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved.
PII S0022-328X(98)00726-8

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V.N. Khabashesku et al. / Journal of Organometallic Chemistry 566 (1998) 45–59
The first organic silanone studied directly by physico-
spectral analyses were performed on a Varian MAT 311
and an AEI MS 902 instruments at 70 eV ionization
energy. GC-MS analyses were done on a Finigan MAT
Incos 50 spectrometer using an RSL-200 capillary
column (0.25mm×30 m).
chemical methods was dimethylsilanone, (CH3)2SiꢀO
(2). In 1983 Arrington et al. [18] reported the generation
of 2 for further IR studies by an in situ cryogenic matrix
reaction of dimethylsilylene with O-atoms photochemi-
cally generated from several precursors, such as O3,
N2O, and C2H4O. However, this approach provides
extremely low yields of silanone, based on the observed
tiny IR peak at 1204 cm−1 attributed to 2 in [18] and
the recent results obtained by Almond [19] in attempt to
reproduce these experiments. The higher yields, suffi-
cient for the observation of the intense IR spectrum of
2 and its d6-analogue, were produced by vacuum pyrol-
ysis of 3,3-dimethyl-6-oxa-3-silabicyclo[3.1.0]hexane and
its 1,5-dimethyl and 3,3-bis(trideuteromethyl) deriva-
tives, respectively [20–23]. The observed strong band at
1210 cm−1 in the IR spectrum was assigned to the SiꢀO
stretching mode in 2. This was in excellent agreement
with the later work of Withnall and Andrews [24], where
2 and also the methylsilanone, CH3(H)SiꢀO, were gener-
ated by photooxidation of methylsilanes in argon ma-
trices. The IR frequencies for the heavier organic
silanones, such as (CH3O)2SiꢀO (3) and (C6H5)2SiꢀO (4),
were reported in our preliminary publication [25]. New
UV and IR absorptions, appearing after photoirradia-
tion of matrix-isolated dimethoxysilylene, (CH3O)2Si,
were tentatively assigned to methyl(methoxy)silanone,
CH3(CH3O)SiꢀO (5), by Maier et al. [26].
In a recent study Gole and Dixon [27] have assigned
a portion of the FTIR spectrum of an oxidized porous
silicon to the SiꢀO stretch of a surface-bound silanone-
based fluorofor groups, such as ROSiꢀO, where R is H,
SiH3, or hydrocarbon radical. In view of this work, and
continued matrix isolation efforts in search of alternative
routes to produce silanones for further UV and IR
detection, discussed in [19], we report here our matrix
isolation IR studies of organic radical substituted
silanones 3 and 4. The experimental data are supple-
mented by the density functional theory calculations
providing a basis for comparative vibrational analysis of
the spectra of the series of silanones 1–5.
2.2. Materials
2.2.1. 3,3-Dimethoxy-6-oxa-3-silabicyclo[3.1.0]hexane
(6)
This precursor was synthesized in 54% yield by the
epoxidation reaction of p-(methoxycarbonyl)perbenzoic
acid with 1,1-dimethoxy-1-silacyclopent-3-ene, which
was prepared in accordance with the described proce-
dure [28]. After distillation, 6 was isolated as a colorless
liquid, bp 113°C/90 Torr. According to GLC analysis
1
the purity of 6 was higher than 98%. H-NMR (CDCl3,
250 MHz): l 0.90 dd (2H, CH2), 1.13 d (2H, CH2), 3.36
m (2H, CH, J=1.1 Hz), 3.46 s (3H, CH3), 3.47 s (3H,
CH3). 13C-NMR (CDCl3, 250 MHz): l 11.08 (CH2),
50.51 (CH3), 55.05 (CH). IR (Ar matrix, 12 K): 3010 (m),
2950 (s), 2919 (m), 2851 (s), 1506 (m), 1465 (m), 1457
(m), 1421 (vw), 1396 (w), 1388 (vw), 1355 (vw), 1327
(vw), 1238 (w), 1195 (vs), 1187 (s), 1105 (m), 1095 (vs),
1082 (s), 1078 (s), 1034 (m), 968 (w), 936 (m), 927 (m),
857 (sh), 841 (m), 829 (w), 811 (m), 791 (s), 744 (w), 663
(vw), 590 (m), 515 (vw), 503 (vw), 500 (vw) cm−1. EIMS
(70 eV): m/z (relative intensity) 160 (3), 159 (14, M–H),
145 (53, M–CH3), 133 (19), 117 (100), 107 (28), 91 (68),
87 (42), 77 (34), 59 (74).
2.2.2. 3,3-Diphenyl-3-sila-6-oxabicyclo[3.1.0]hexane (7)
The precursor 7 was synthesized by the epoxidation of
1,1-diphenyl-1-silacyclopent-3-ene, prepared in 85%
yield according to a known procedure [29]. After recrys-
tallization from pentane the epoxide 7 was isolated in
1
94% yield with the purity higher than 98%. H-NMR
(CDCl3, 250 MHz): l 1.55 d (2H, CH2, Jgem=16 Hz),
1.85 d (2H, CH2), 3.70 m (2H,\CH), 7.5 m (10H, Ph).
13C-NMR (CDCl3, 250 MHz): 27.95 (CH2), 57.54 (CH),
127.54, 127.82, 129.34, 134.55, 135.0 (Ph). IR (Ar ma-
trix, 12 K): 3137 (vw), 3094 (w), 3076 (m), 3060 (m), 3008
(m), 2941 (m), 2927 (m), 2861 (w), 1610 (m), 1573 (w),
1490 (w), 1434 (s), 1408 (w), 1401 (m), 1385 (m), 1340
(vw), 1240 (m), 1183 (vs), 1159 (w), 1120 (s), 1111 (s),
1073 (w), 1022 (m), 1000 (m), 965 (m), 935 (s), 923 (s),
824 (s), 811 (vs), 792 (s), 756 (w), 740 (s), 732 (m), 722
(vs), 700 (vs), 667 (w), 590 (s), 554 (vs), 467 (s), 462 (m),
432 (vs) cm−1. EIMS (70 eV): m/z (relative intensity) 252
(10, M+), 225 (26), 209 (71), 208 (58), 183 (100), 181
(66), 174 (64), 164 (55), 163 (48).
For the matrix IR detection the silanones 3 and 4 were
generated by vacuum pyrolysis of 3,3-dimethoxy (6) and
3,3-diphenyl-6-oxa-3-silabicyclo[3.1.0]hexane (7). The
choice of these compounds as a precursors rested on a
proven successful utilization of their 3,3-dimethyl deriva-
tives for the generation and matrix IR observation of
dimethylsilanone 2 [20–23].
2. Experiment and calculations
2.1. General methods
2.3. Vacuum pyrolysis-matrix isolation spectroscopy
1H and 13C-NMR spectra of 6 and 7 were recorded on
The matrix isolation set-up used in present work was
Jeol FX 90 and Bruker WM-250 spectrometers. Mass
described elsewere [20–23]. The vapors of 6 and 7 were

V.N. Khabashesku et al. / Journal of Organometallic Chemistry 566 (1998) 45–59
47
passed through a resistively heated quartz pyrolyzer
(5–8 mm internal diameter, 50–90 mm length), coupled
to a vacuum shroud of the optical helium cryostat. The
pressure and temperature in the pyrolysis zone were
varied from 1 to 1×10−4 Torr and 500–950°C, respec-
tively. The pyrolysis products were cocondensed at
12–14 K with a large excess of argon (1:1000) on the
reflective copper plate, located at a distance of 50 mm
from the pyrolyzer orifice. The copper plate was cooled
with the help of a closed cycle CSW-208R refrigerator
from Air Products. The matrices were analyzed with
the LOMO IKS-24 infrared spectrometer in the range
of 400–4000 cm−1 using the reflection of the infrared
beam from the cold copper substrate surface.
The controlled warm-up experiments of deposited
matrices to diffusion temperatures 35–45 K, imperative
for identification of spectral absorptions of transient
species, were carried out with the help of a built-in
heater. The matrices from the pyrolyzate of 6 and 7
were also warmed up to room temperature under vac-
uum for collecting the final pyrolysis products into a
liquid nitrogen cooled trap. The collected condensates
were further examined by taking their IR spectra after
redeposition into argon matrices at 12 K.
Fig. 1. IR spectrum of matrix-isolated (Ar, 12 K) 3,3-dimethoxy-6-
oxa-3-silabicyclo[3.1.0]hexane (6).
species. The peaks in the 1050–1100 cm−1 spectral
range and at 1450 cm−1, characterizing the hexa(me-
thoxy)cyclotrisiloxane, [(CH3O)2SiO]3 (8), (denoted by
MS-label) were observed to grow in the spectrum of the
annealed matrix, while the band intensities of precursor
(6) and butadiene remained virtually unchanged, as
shown in Fig. 2b. When this matrix was warmed up to
room temperature and the products were collected into
a liquid N2-cooled trap and then redeposited into an
argon matrix, the IR spectrum of the new matrix did
not exhibit any of the six bands.
2.4. Calculations
The ground state optimized geometries for silanones
1–5 as well as their harmonic vibrational frequencies
and infrared intensities were computed at the density
functional B3LYP level of theory [30,31] with the 6-
311G(d, p) basis set of triple-zeta quality [32]. The
calculations were performed with the Gaussian 94 pro-
gram package [33]. The vibrational analysis of the
computed frequencies was carried out with the assis-
tance of a vibration visualization package and also on
the basis of potential energy distributions (PED) for
normal vibrational modes. The force constants in inter-
nal coordinates for the silanones 3 and 4 were calcu-
lated from the B3LYP/6-311G(d, p) second derivative
output values.
3. Results and discussion
3.1. IR spectra of pyrolysis products of epoxide (6)
In the IR spectrum of matrix-isolated pyrolysis prod-
ucts of (6), produced at temperature 800°C and pres-
sure 5×10−4 Torr in the reactor zone (Fig. 2a), six
new bands at 888, 1102, 1174, 1199, 1247, and 1471
cm−1 were observed along with the peaks of 1,3-buta-
diene [34] and those of unreacted precursor 6, whose
spectrum is given in Fig. 1. After matrix annealing from
12 to 40 K, a simultaneous decay of these six bands was
observed, indicating that they all belong to a transient
Fig. 2. IR spectra: (a) matrix-isolated (Ar, 12 K) pyrolysis (5×10−4
Torr, 800°C) products of 6. (b) The matrix whose spectrum is shown
under (b) after annealing from 12 to 40 K. Labels: BD, 1,3-butadiene;
MS, methoxycyclosiloxanes 8 and 9.

48
V.N. Khabashesku et al. / Journal of Organometallic Chemistry 566 (1998) 45–59
Fig. 3. IR spectrum (Ar, 12 K) of pyrolysis (10−1 Torr, 700°C)
products of 6. Labels: BD, 1,3-butadiene; MS-, methoxycyclosilox-
anes 8 and 9.
Fig. 4. IR spectrum (Ar, 12 K) of pyrolysis (10−3 Torr) products of
6 at elevated temperature of 930°C. Label: BD, 1,3-butadiene.
ments or pressure increase in the reaction zone and
agrees with the vacuum pyrolysis-MS studies of the
decomposition of 6 [40]. The formation of smaller
molecules at elevated pyrolysis temperatures can also be
explained by a secondary process involving the silanone
3, namely, the thermal dissociation according to
Scheme 1. One more reason for the assignment of the
six new IR features to the silanone 3 is also given by
very good agreement between the observed and calcu-
lated frequencies and infrared intensities for 3 (Table
1).
An increase of pressure in the pyrolysis zone from
5×10−4 to 10−1 Torr at the temperature 700°C, corre-
sponding to a dramatic increase of the number of
intermolecular collisions both in the gas phase and with
the hot walls of the reactor, led to a notable weakening
of the six new bands, growth of the peaks of te-
tra(methoxy)cyclodisiloxane, [(CH3O)2SiO]2 (9), and
trisiloxane 8, and appearance of the absorptions of
formaldehyde at 1748 cm−1 [35], methanol at 1035
cm−1 [36], dimethylether at 927 cm−1 [37], and of
silicon monoxide at 1226 cm−1 [38] (Fig. 3). The latter
small molecules along with the monomer silicon diox-
ide, exhibiting an absorption at 1419 cm−1 [39], were
also identified in the spectrum of pyrolyzate produced
from 6 at the elevated temperature (930°C) in the
reaction zone (Fig. 4).
Taking into consideration the observed formation of
butadiene as a ‘leaving group’ in the decomposition of
epoxide 6 and the expected similarity of this reaction
with the pyrolysis of 3,3-dimethyl analogue of 6, which
selectively led to the silanone, (CH3)2SiꢀO (2) after
elimination of butadiene [20–23], we believe that the
new six bands can be assigned to the dimethoxysilanone
3. This assignment allows to elucidate the accumulation
of cyclosiloxanes either during matrix warm-up experi-
3.2. IR spectra of matrix-isolated pyrolysis products of
3,3-diphenyl-6-oxa-3-silabicyclo[3.1.0]hexane (7)
In the spectrum of pyrolysis products of 7 at a
temperature of 820°C and pressure of 1×10−2 Torr in
the reactor (Fig. 6a), intense peaks of butadiene and
new bands at 479, 541, 825, 1205 cm−1, and the
absorptions at 700, 1120, and 1205 cm−1, have been
observed along with a weak band of unreacted precur-
sor 7, whose spectrum is given in Fig. 5. Annealing of
the matrix from 12 to 40 K brought about a remark-
able change to the spectrum (Fig. 6b), namely, the
depletion of all seven listed peaks with respect to ab-
sorptions of 1,3-butadiene and epoxide 7 and the
Scheme 1.

V.N. Khabashesku et al. / Journal of Organometallic Chemistry 566 (1998) 45–59
49
appearance of strong bands of hexaphenylcyclotrisilox-
3.3. Thermal and kinetic stability of silanones
ane, [(C6H5)2SiO]3 (10), identified by comparison with
the IR spectrum (Fig. 7) of matrix-isolated authentic
sample of 10, synthesized exclusively for this purpose.
The new peaks at 479, 541, 825 and 1205 cm−1 were
not observed in the spectrum of the pyrolyzate of 7
produced at the increased pressure of 10−1 Torr in the
The silanones 3 and 4 were generated in the present
work in the reactor of the same geometry as the one
utilized previously for production of dimethylsilanone 2
[20–23]. This allows a qualitative comparison of the
thermal and kinetic stability for the series of gas-phase
generated silanones 2–4 by following the conditions
(pressure and temperature) of their production as
monomers and of their decay via secondary oligomer-
ization or dissociation processes versus observed band
intensities in the IR spectra. It was noted that unlike
dimethyl- and diphenylsilanones, which get entirely
oligomerized at the pressures of 10−1 Torr in the
reactor, the dimethoxysilanone 2 under the same condi-
tions still survives as a monomer, exhibiting its IR bands
in the spectra. These data indicate a higher kinetic
stability of the silanone 3 as compared to its dimethyl
and diphenyl analogues, and also suggest that the ki-
netic stability of 3 is perhaps approaching that of
dichlorosilanone, Cl2SiꢀO, which was generated by the
gas-phase pyrolysis and observed to survive as a
monomer at pressures of about 1 Torr in the reactor,
according to matrix isolation work of Golovkin et al.
[43]. Based on ab initio studies of Kudo and Nagase
[10], the dimerization of H2SiꢀO (1) is predicted to
proceed with no barrier, which does not provide for an
explanation of the observed differences in the kinetic
stabilities of the larger molecules 2–4. In our opinion,
the latter can be related to the dipole moment change
within the silanone monomer series, which at the same
B3LYP/6-311G(d, p) level of theory are calculated to be
the smallest in dimethoxysilanone, 1.97 Db, (Chart 1)
reactor, while the bands at 700, 1120 and 1434 cm−1
,
overlapping with the peaks of precursor 7, were ob-
served to become weaker, and those of the trisiloxane 10
became more intense with respect to butadiene absorp-
tions (Fig. 8). The increase of the pyrolysis temperature
from 800 to 900–920°C at pressure 1×10−2 Torr in the
reaction zone led to an observation in the spectrum,
shown in Fig. 9, of seven weaker bands at 479, 541, 700,
825, 1120, 1205, and 1434 cm−1, absorption of the SiO
monomer at 1226 cm−1 [38], a strong band due to
benzene at 675 cm−1 [41], and also a shoulder at 708
cm−1, which very likely belongs to a phenyl radical [42].
The observed transformations of epoxide 7 and its
pyrolysis products are summarized in Scheme 2. We are
convinced that the most reasonable assignment of the
observed seven new IR bands is to a monomer of
diphenylsilanone 4, which forms a cyclotrimer 10 either
upon matrix warm-up or under increased pressure in
the pyrolysis zone in a gas phase where possibly a
transient cyclodimer of 4 is formed as well. The ele-
vated temperatures in the pyrolysis zone result in disso-
ciation of silanone 4 into SiO and phenyl radical along
with benzene, that is likely to originate from a sec-
ondary bimolecular reaction. The proposed assignment
also agrees with the calculations, that provide a good
match of computed and observed frequencies for 4
(Table 2).

50
V.N. Khabashesku et al. / Journal of Organometallic Chemistry 566 (1998) 45–59
and the largest, 4.87 Db, in diphenylsilanone (Chart 2)
and in dimethylsilanone, 4.61 Db (Fig. 10).
lanone 4 than for parent silanone 1, dimethyl- and
dimethoxysilanones, which were predicted to possess
the C2v symmetry. The planar structure with the C2v
symmetry for 4 was calculated to lie only by 1.14 kcal
mol−1 at 3-21G(d), and 1.37 kcal mol−1 at 6-
311G(d, p), higher in energy than the C2 equilbrium
geometry. However, computation of harmonic frequen-
cies for the C2v structure of 4 yielded a negative value
for the frequency of the torsion mode of phenyl groups
around the SiꢁC bonds and indicated that this structure
characterizes a transition state. Thus, the reduction of
the molecular symmetry in 4 is likely facilitated by the
repulsion of ortho-hydrogen atoms of neighboring
phenyl groups, causing their rotation around the Si–C
bonds until the total energy minimum is reached at a
dihedral angle ~, calculated to be 25.9 and 23.7° at
B3LYP/3-21G(d) and 6-311G(d, p), respectively (Chart
2). The same twist around the C–C bond is found in
the carbon analogue of 4, (C6H5)2CꢀO, giving a C2
It was also found that the fragmentation of diphenyl-
silanone 4 proceeds at the same temperatures (900–
920°C) in the reaction zone as the thermal destruction
of dimethyl derivative 2. This very likely indicates that
the silanones 2 and 4 have similar thermodynamic
stabilities, which at the same time are greater than that
of dimethoxysilanone 3.
3.4. Molecular geometries of silanones 1–5
The B3LYP/6-311G(d, p) optimized structures and
geometry parameters for the silanones 1, 2, and 5 are
given on Fig. 10, while those calculated for diphenylsi-
lanone 4 with the 3-21G(d) and 6-311G(d, p) basis sets
at the same level of theory, and for the dimethoxysi-
lanone 3 at the HF and DFT levels of theory, are
shown on Charts 1 and 2. The calculations yielded
structures with the lower symmetry (C2) for diphenylsi-

V.N. Khabashesku et al. / Journal of Organometallic Chemistry 566 (1998) 45–59
51
Table 1
Vibrations of dimethoxysilanone (3)
Mode Symmetry Frequencies (cm−1
)
PED (%) and assignment
HF/6-31G(d)a
B3LYP/6-
B3LYP/6-
Observed
31G(d)b
311G(d, p)b
1
2
3
4
5
6
b1
a2
b1
a1
a2
b2
60(1)c
64(0)
83(7)
125(7)
137(0)
172(37)
85(5)
89(0)
94(2)
133(5)
134(0)
180(30)
85(6)c
85(0)
91(1)
128(6)
140(0)
169(32)
94 COSiO symmetry in-plane bend
100 CH3 asymmetry torsion
100 CH3 symmetry torsion
75 COSi symmetry in-plane bend; 25 OSiO scissor
100 COSiO twist
79 COSi asymmetry in-plane bend; 21 OSiO in-plane
rock
7
8
9
a1
b1
b2
334(15)
383(132)
418(46)
349(10)
350(65)
417(24)
346(10)
357(74)
406(28)
68 OSiO scissor; 32 COSi; symmetry in-plane bend
98 OSiO wag
77 OSiO in-plane rock; 22 COSi asymmetry in-plane
bend
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
a1
b2
b2
a1
a2
b1
a1
b2
a1
b2
a1
b2
a1
a2
b1
b2
a1
b2
a1
a2
b1
710(53)
835(33)
1103(847)
1161(15)
1172(0)
1175(5)
1193(6)
1205(193)
1270(207)
1474(4)
1480(2)
1489(0.1)
1490(4)
1491(0)
1492(6)
2913(102)
2915(0.1)
2989(0.1)
2990(75)
2991(8)
724(39)
862(27)
1119(654)
1123(8)
1194(0)
1198(0.4)
1216(8)
1231(118)
1292(115)
1508(4)
1514(0.1)
1530(1)
1531(9)
1532(0)
1533(6)
3060(103)
3062(0.1)
3141(5)
3142(28)
3143(0)
716(41)
848(25)
1108(654)
1112(7)
1180(0)
1184(0.1)
1199(7)
1213(138)
1284(135)
1484(0.2)
1490(0.1)
1501(1)
1501(13)
1502(0)
1502(9)
87 Si-O symmetry str
888(w)
1102(vs)
62 Si-O asymmetry str; 38 C-O asym str
74 C-O asymmetry str; 23 Si-O; asym str
93 C-O symmetry str
93 CH3 rock ip
93 CH3 rock op
1174(w)
1199(s)
1247(m)
75 CH3 symmetry in-plane rock; 10 SiꢀO str
80 CH3 asymmetry in-plane rock
95 SiꢀO str
100 CH3 asymmetry deform
100 CH3 symmetry deform
95 CH3 asymmetry scissor deform
95 CH3 symmetry scissor deform
87 CH3 asymmetry scissor deform
87 CH3 symmetry scissor deform
100 CH3 asymmetry symmetry str
100 CH3 symmetry symmetry str
100 CH3 asymmetry asymmetry str
100 CH3 symmetry asymmetry str
100 CH3 asymmetry in-plane str
100 CH3 symmetry in-plane str
1471(w)
3036(101)
3037(0.1)
3116(5)
3116(30)
3117(0)
2992(45)
3144(51)
3117(53)
a Scaled by 0.9.
b Unscaled.
c Intensity (km mol−1).
structure instead of a C2v structure. It should be noted
that both geometries (C2 and C2v) in 4 reduce signifi-
cantly the degree of y-conjugation of the SiꢀO double
bond with the phenyl aromatic rings in 4, which is
reflected by the very small calculated Si–C bond short-
˚
ening (0.011 A) in 4 with respect to that in dimethylsi-
lanone (Fig. 10). This contrasts with the earlier
calculated significant shortening of the Si–C bond (by
˚
0.036 A) in the CꢀSi–CꢀC fragment of silacyclopenta-
1,4-diene, for which spectroscopic evidence for strong
y-conjugation has been obtained [44].
At the same (B3LYP) level of theory the SiꢀO bond
lengths in all studied silanones are found to be almost
˚
identical, lying within 1.525–1.535 A. For comparison,
the HF/6-31G(d) method yields lower values for parent
˚
silanone 1, 1.498 A [10], and dimethoxysilanone, 1.496
˚
A (Chart 1). However, according to Mulliken popula-
tion analysis the calculated total atomic charges on the
y-bonded Si and O atoms vary significantly within the
chosen series, depending on the electron donating
or withdrawing nature of the substituents. This is
clearly demonstrated by the increasing charge partition
Fig. 5. IR spectrum of matrix-isolated (Ar, 12 K) 3,3-diphenyl-6-oxa-
3-silabicyclo[3.1.0]hexane (7).

52
V.N. Khabashesku et al. / Journal of Organometallic Chemistry 566 (1998) 45–59
Fig. 8. IR spectrum (Ar, 12 K) of pyrolysis products of 7 at
temperature 820°C and increased pressure (10−1 Torr) in the reactor.
Labels: BD, 1,3-butadiene; PS, hexaphenylcyclotrisiloxane 10.
ability of the B3LYP/6-311G(d, p) method in predicting
the spectra of transient silenes, germenes, and ger-
manones has already been demonstrated in our most
recent studies [45]. In the present work we have ex-
tended the application of the same DFT method to
vibrational analysis of the spectra of silanones.
Fig. 6. IR spectra: (a) matrix-isolated (Ar, 12 K) pyrolysis (1×10−2
Torr, 820°C) products of 7. (b) The matrix whose spectrum is shown
under (b) after annealing from 12 to 40 K. Labels: BD, 1,3-butadiene;
PS, hexaphenylcyclotrisiloxane 10.
In the Table 3 the frequencies for the H2SiꢀO
molecule (1), previously calculated by the HF/6-31G(d)
and SCF TZ2P(f, d) methods, are compared with the
DFT spectral bands and infrared intensities computed
in the present work, and with the experimental IR
bands attributed to 1 by Andrews [15]. The unscaled
DFT values are in much closer agreement with the
experiment than the ab initio calculated frequencies
[10], and confirm the vibrational assignment of the two
yet observed bands in the spectrum of 1, given earlier
[15,46]. Therefore, we suggest that the DFT calculated
data provide better guidance for further experimental
detection of the other not yet observed spectral bands
in 1.
in the following direction: H2SiꢀOB(CH3)2SiꢀOB
(C6H5)2SiꢀO:CH3(CH3O)SiꢀOB(CH3O)2SiꢀO. The
B3LYP calculated dipole moments were found to
change in the following order: (C6H5)2SiꢀO\
(CH3)2SiꢀO\H2SiꢀO:CH3(CH3O)SiꢀO\(CH3O)2
SiꢀO. The valence angles at the double-bonded silicon
in 1–5 are calculated to be within 121.6–128.8°, thus
confirming the sp2 hybridization state of this atom.
3.5. IR spectra
A full assignment of IR bands of the silanones 3 and
4, observed in the present work, and of those tenta-
tively assigned to methyl(methoxy)silanone 5 in [26],
has been suggested by comparison with the DFT calcu-
lated vibrational spectra for the whole series of
silanones 1–5, including the potential energy distribu-
tions (PED) for each vibrational mode. The high reli-
The DFT calculations done in present work also
confirm the vibrational assignment of the observed
bands in dimethylsilanone 2, suggested in our previous
Fig. 9. IR spectrum (Ar, 12 K) of pyrolysis products of 7 at pressure
of 1×10−2 Torr and elevated temperature of 920°C in the reactor.
Label: BD, 1,3-butadiene.
Fig. 7. IR spectrum of matrix-isolated (Ar, 12 K) hexaphenylcyclo-
trisiloxane (10).

V.N. Khabashesku et al. / Journal of Organometallic Chemistry 566 (1998) 45–59
53
Scheme 2.
reports on basis of entirely empirical force field calcula-
tions [20–23]. The calculated and experimental frequen-
cies for 2 and their assignment are presented in Table 4.
The demonstrated excellent agreement between the the-
ory and experiment in the case of silanones 1 and 2
encouraged the use of the DFT method for vibrational
analysis of the observed spectra of the larger silanone
molecules 3–5.
with the experiment, while the B3LYP values are used
without further scaling.
The observed weak band at 888 cm−1 was attributed
to the antisymmetric Si–O stretching mode coupled
with the antisymmetric C–O stretching mode, in agree-
ment with the spectra calculations, PED values derived
from the B3LYP/6-311G(d, p) output data, and vibra-
tion animation. The strongest observed band at 1102
cm−1 matches very well the calculated high intensity
absorption of the antisymmetric C–O stretch coupled
with the Si–O stretch of the same symmetry in 5, which
is predicted to lie within 1103–1119 cm−1 depending
on the calculation method applied.
The
vibrational
assignment
for
methyl-
(methoxy)silanone 5, whose several IR bands have been
reported by Maier et al. [26] is given in Table 5. The
observed most intense feature at 1094.4 cm−1 is sug-
gested to be attributed to the C–O stretching mode, in
close agreement with the predicted value of 1103 cm−1
and the calculated highest intensity of this motion in 5.
To the CH3(O) in-plane rocking mode we attributed an
observed band at 1182.5 cm−1, which agrees with the
calculated frequency of 1199 cm−1. The observed band
at 1237.6 cm−1 is likely to belong to the SiꢀO stretch-
ing mode, according to the DFT predicted value of
1258 cm−1, although one would expect a much higher
intensity for the reported IR band in order to show a
better agreement with the calculated infrared intensity
of this mode in 5. A possible way to solve this dis-
crepancy will be to obtain an independent spectral data
for 5 by using a different method for generation of
silanone 5. The other three observed bands at 1453.9,
2851.1, and 2981.0 cm−1 are attributed to the deforma-
tion mode of the CH3 group attached to Si atom and to
the symmetric and antisymmetric CH stretching vibra-
tions of the CH3 moiety attached to O atom, respec-
tively, in agreement with calculated frequencies,
vibration animation and PED values (Table 5).
The dimethoxysilanone molecule (3) of C2v symmetry
has 30 fundamentals, among which 19 vibrations are IR
active in the 400–4000 cm−1 spectral region. We could
observe six bands in the IR spectrum. The other bands
are either of much lower intensity or overlapped by a
strong absorptions due to the other molecules present
in the matrix. In the Table 1 the frequencies of 3,
calculated with three different methods, are compared
with the observed IR spectrum. The HF frequencies are
scaled by a factor of 0.9 to allow better comparison
The CH3 rocking modes of the CH3O moiety in
dimethyl(dimethoxy)silane, (CH3O)2Si(CH3)2 [47], as
well as of the C(CH3)2 group in propane [41] are
located in the 1145–1198 cm−1 IR spectral region.
Therefore, we assigned an observed weak band at 1174
cm−1, and a strong band at 1199 cm−1 to the symmet-
ric and antisymmetric in-plane CH3 rocking modes in 5,
respectively, in agreement with the calculations (Table
1). The observed weak band at 1471 cm−1 was at-
tributed to the symmetric CH3 scissoring deformation
mode in 5 after comparison with the calculated fre-
quency, PED and computer animation of this vibra-
tional mode, and also after taking into account the
literature data on IR spectra of dimethyl(methoxy)
substituted silanes, where this mode is located in 1455–
1465 cm−1 [47].
As to the SiꢀO stretching mode, the assignment of
the observed medium intensity band at 1247 cm−1 in 3
was done on the basis of the well-documented spectral
data which show an increase in the frequency of the
MꢀO stretching mode in ketones, phosphinoxides, and
sulfoxides on going from dimethyl to dimethoxy deriva-
tives (Table 6). Our DFT calculations also predict the
increase of the w (SiꢀO) frequency under the influence
of the CH3O substituent in the silanones in the follow-
ing
direction:
(CH3)2SiꢀO
(1227
cm−1)B
CH3(CH3O)SiꢀO (1258 cm−1)B(CH3O)2SiꢀO (1284
cm−1) (Tables 1, 4 and 5).
A DFT method also provides a cost-effective tool for
computing the vibrational frequencies of larger

54
V.N. Khabashesku et al. / Journal of Organometallic Chemistry 566 (1998) 45–59
Table 2
Vibrations of diphenylsilanone (4)
Mode Symmetry Frequencies (cm−1
)
PED (%) and assignment
3-21G(d)a 6-311G(d, p)a
Observed
1
2
3
4
5
6
7
8
9
b
a
a
b
b
a
a
b
b
a
32(1)
41(0)
80(0)
100(3)
141(9)
167(0)
250(0)
263(15)
332(16)
381(2)
35(1)b
39(0)
79(0)
102(4)
143(12)
157(0)
239(0)
262(19)
326(18)
371(2)
82 Ph torsion ip
64 Ph torsion op, 19 CSiO bend; 17 CCSi bend
75 CCSiO symmetry out-of-plane bend
67 CCCSi out-of-plane bend; 21 CCSiꢀO out-of-plane bend
87 CCSi asymmetry in-plane bend
90 CSiC twist
47 Si–C symmetry str, 34 CCC bend
82 CSiO in-plane bend; 11 Si–C asymmetry str
67 CSiO out-of-plane bend; 26 CCCC bend
40 CCSi symmetry in-plane bend, 20 CSiO; bend, 18 CCCC bend, 15 Si–C
str
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
b
a
a
b
b
b
a
a
b
a
b
a
b
b
a
b
a
b
a
b
a
b
a
a
b
b
a
a
b
b
a
b
a
a
b
a
b
a
b
a
b
a
b
a
a
b
b
a
b
414(0)
424(0)
482(4)
491(40)
513(130)
656(0)
405(0)
411(0)
463(3)
478(46)
491(146)
631(0)
632(0)
693(1)
715(70)
720(7)
736(63)
758(6)
758(55)
873(2)
879(0)
948(1)
956(0)
100 CCCC ring out-of-plane bend
100 CCCC ring out-of-plane bend
4 CCCC ring out-of-plane bend
76 CCCC ring out-of-plane bend
45 CCCC bend, 22 CSiO in-plane bend; 10 Si–C str
95 CCCC ring symmetry in-plane bend
95 CCCC ring symmetry in-plane bend
91 CCC ring in-plane bend
84 CCCC out-of-plane bend
87 CCCC out-of-plane bend
84 CCC bend, 16 Si–C asymmetry str
66 CCCC out-of-plane bend, 34 HCCC out-of-plane bend
72 CCCC out-of-plane bend, 27 HCCC out-of-plane bend
100 HCCC out-of-plane bend
100 HCCC out-of-plane bend
92 HCCC out-of-plane bend
88 HCCC out-of-plane bend
74 HCCC out-of-plane bend; 20 CCCC bend
80 HCCC out-of-plane bend, 18 CCCC bend
93 CCC bend
479(m)
541(s)
658(0)
715(1)
735(104)
742(13)
755(54)
787(3)
787(29)
892(2)
899(0)
974(2)
982(0)
1028(1)
1029(0)
1030(12)
1035(0)
1054(0)
1056(0)
1066(10)
1066(1)
1107(0)
1112(1)
1124(3)
700(vs)
825(s)
998(1)
1004(0)
1013(7)
1015(0)
1021(0)
1022(0)
1046(1)
1049(1)
1096(1)
1101(2)
1104(3)
92 CCC bend
74 HCCC out-of-plane bend, 17 CCCC bend
60 HCCC out-of-plane bend, 30 CCCC bend
51 CC symmetry str, 25 CCC bend
56 CCC bend, 34 CC str
58 CC str, 14 HCC bend
41 CC str, 12 HCC in-plane bend; 11 CCC bend
86 CCC symmetry in-plane bend
82 CCC asymmetry in-plane bend, 12 Si–C asymmetry str
92 HCC asymmetry symmetry in-plane rock
90 HCC symmetry symmetry in-plane rock
100 HCC in-plane rock
88 HCC symmetry in-plane rock
99 SiꢀO str
74 CC asymmetry str
1155(159) 1132(181)
1120(s)
1235(0)
1235(0)
1249(13)
1251(2)
1263(65)
1292(0)
1298(4)
1387(6)
1392(9)
1487(14)
1490(21)
1540(1)
1542(3)
1600(1)
1601(1)
1619(7)
1620(13)
3193(2)
3195(0)
3198(1)
1186(0)
1186(0)
1209(10)
1213(5)
1220(107)
1305(0)
1311(4)
1354(3)
1359(4)
1460(14)
1464(25)
1514(0)
1514(0)
1607(1)
1610(0)
1630(8)
1630(18)
3166(1)
3167(0)
3173(1)
1205(m)
71 CC symmetry str
86 HCC asymmetry in-plane bend
88 HCC symmetry in-plane bend
80 HCC in-plane rock ip
1434(m)
78 HCC in-plane rock op
82 HCC symmetry in-plane rock ip
80 HCC symmetry in-plane rock op
41 CC asymmetry str, 33 CCC bend
42 CC symmetry str, 44 CCC bend
56 CC str
56 CC str
100 CH str
100 CH str
100 CH str

V.N. Khabashesku et al. / Journal of Organometallic Chemistry 566 (1998) 45–59
55
Table 2 (Continued)
Mode Symmetry Frequencies (cm−1
3-21G(d)a 6-311G(d, p)a
)
PED (%) and assignment
Observed
60
61
62
63
64
65
66
a
b
a
b
a
b
a
3200(0)
3206(7)
3207(1)
3215(14)
3216(21)
3226(29)
3226(6)
3175(4)
3180(12)
3182(2)
3188(14)
3189(21)
3195(36)
3196(10)
100 CH str
100 CH str
100 CH str
100 CH str
100 CH str
100 CH str
a B3LYP method.
b Intensity (km mol−1).
molecules, in particular, diphenylsilanone 4, and the
assistance in vibrational assignment of the observed
bands in 4. The molecule 4 has 66 normal vibrational
modes, among which 56 are predicted to be found in
the 400–4000 cm−1 spectral region. We could observe
seven bands, all located in the 400–1500 cm−1 region
of the IR spectrum of matrix-isolated 4. The rest of the
bands, such as those of the ꢀCH stretching modes, lying
within 3000–3100 cm−1, and of the other fundamen-
tals, were not detected since they are either too weak or
overlap the absorptions of the precursor 7 or 1,3-buta-
diene, present in the matrix.
The observed medium intensity band at 479 cm−1
was assigned to the phenyl ring out-of-plane bending
mode in agreement with the calculated (Table 2) fre-
quency (478–491 cm−1) and intensity of this mode. A
strong band at 541 cm−1 is attributed on basis of
calculations, computer animation of vibrational modes
and PED values to the phenyl ring bending mode
coupled with the Si–C stretch and the CSiO in-plane
bending vibration. The prominent peak, observed at
700 cm−1, is suggested to belong to the ring out-of-
plane CCCC bending mode. In the spectra of phenylsi-
lanes the ꢀCH out-of-plane bending mode is located in
˚
Fig. 10. Optimized B3LYP geometries of methyl(methoxy)silanone (5), parent silanone (1) and dimethylsilanone (2). Given are bond distances (A),
valence angles (°), atomic charges, and dipole moments (Db).

56
V.N. Khabashesku et al. / Journal of Organometallic Chemistry 566 (1998) 45–59
Table 3
Vibrations of silanone H2SiꢀO (1)
Mode
Symmetry
Frequencies (cm−1
)
Observed
PED (%) and assignmente
Reference [10]a
Reference [45]b
Present workc
1
2
3
4
5
6
b1
b2
a1
a1
a1
b2
787
812
1125
1356
2432
2433
759
802
1080
1356
2360
2362
705(69)f
710(66)
1022(58)
1216(71)
2229(45)
2245(164)
697
100 SiH2 in-plane rock
100 SiH2 wag
100 SiH2 scissor
100 SiꢀO str
100 SiH2 symmetry str
100 SiH2 asymmetry str
1202
a HF/6-31G(d).
b SCF TZ2P(f, d).
c B3LYP/6-311G(d, p).
d Reference [15].
e Present work
f Intensity (km mol−1).
the 690–840 cm−1 [48]. The calculations predict this
mode to be found in the silanone 4 at 758–787 cm−1 as
a high intensity band and to be coupled with the ring
out-of-plane CCCC motion (Table 2). The assignment of
the observed strong band at 825 cm−1 to this vibration
in 4 is in reasonable agreement with the considered above
grounds. The CCC in-plane bending modes in phenylsi-
lanes are found in the 1040–1130 cm−1 spectral region
[48]. The calculations predict these modes in the silanone
4 also to be located in the same region (Table 4).
Therefore, we assigned a strong band, observed at 1120
cm−1, to this type of vibration. The detected band at
1434 cm−1 is attributed to the in-plane CH rocking mode
in 4, in reasonable agreement with the calculations.
Table 4
Vibrations of dimethylsilanone (CH3)2SiꢀO (2)
Mode
Symmetry
Frequencies (cm−1
)
PED (%) and assignment
Calculateda
Observed
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
a2
b1
a1
b1
b2
a1
b2
a2
b1
b2
a1
a1
b2
a1
b2
a2
a1
b1
b2
a1
a2
b1
a1
b2
26(0)b
77(0.3)
223(0.02)
264(14)
292(34)
621(3)
100 CH3 symmetry torsion
100 CH3 asymmetry torsion
100 CSiC scissor
100 SiC2 wag
100 SiC2 in-plane rock
100 Si–C symmetry str
85 Si–C asymmetry str
96 CH3 symmetry rock
86 CH3 rock
50 CH3 asymmetry rock, 50 Si–C asymmetry str
81 CH3 symmetry in-plane rock
100 SiꢀO str
100 CH3 asymmetry deform rock
100 CH3 symmetry deform rock
96 CH3 deform
90 CH3 deform
95 CH3 deform
87 CH3 deform
93 CH3 str
93 CH3 str
100 CH3 str
100 CH3 str
98 CH3 str
98 CH3 str
686(10)
691(0)
657(w)
800(32)
810(128)
863(22)
1227(99)
1290(36)
1296(48)
1446(2)
1450(0)
1457(4)
1466(22)
3023(0.3)
3027(2)
3085(0)
3089(9)
3130(2)
3131(2)
770(m)
798(vs)
822(m)
1210(s)
1240(w)
1244(m)
a B3LYP/6-311G(d, p).
b Intensity (km mol−1).

V.N. Khabashesku et al. / Journal of Organometallic Chemistry 566 (1998) 45–59
57
Table 5
Vibrations of methyl(methoxy)silanone (5)
Mode
Symmetry
Frequencies (cm−1
)
PED (%) and assignment
Calculateda
Observedb
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
a%%
a%%
a%%
a%
a%
a%%
a%
a%
a%
a%%
a%
a%
a%%
a%
a%
a%
a%
a%%
a%
a%
a%%
a%
a%
42(0)c
72(0)
132(4)
100 CH3(Si) torsion
100 CH3(O) torsion
96 COSiO out-of-plane bend
74 COSi, 26 OSiO in-plane bend
99 CSiO in-plane scissor
95 CSiOO out-of-plane bend
82 OSiO, 17 COSi in-plane scissor
84 Si–C, 13 Si–O str
49 Si–O, 24 C–O, 15 Si–C str
93 CH3 out-of-plane rock
75 CH3 in-plane rock
148(16)
243(12)
304(37)
397(34)
664(17)
771(22)
781(23)
861(70)
1103(296)
1181(0)
1199(26)
1258(168)
1298(37)
1457(3)
1458(9)
1483(1)
1500(8)
1503(5)
3033(53)
3045(1)
3110(30)
3113(17)
3113(1)
3146(2)
1094.4 (1.00)
92 C–O str
93 CH3(O) out-of-plane rock
89 CH3(O) in-plane rock
97 SiꢀO str
1182.5 (0.26)
1237.6 (0.09)
99 CH3 in-plane deform. rock
96 CH3(Si) deformation
89 CH3(Si) deformation
100 CH3(O) deformation
96 CH3(O) deformation
87 CH3(O) deformation
100 CH3O CH symmetry str
100 CH3Si CH symmetry str
100 CH3O CH symmetry str
100 CH3O CH symmetry str
100 CH3Si CH symmetry str
100 CH3Si CH symmetry str
1453.9 (0.13)
2851.1 (0.09)
2981.0 (0.09)
a%%
a%
a%%
a%
a B3LYP/6-311G(d, p).
b Intensity (km mol−1).
c Tentatively assigned to 5 IR bands and their relative intensities, as given in ref. [26].
The observed medium intensity band at 1205 cm−1 is
located in the SiꢀO stretching mode region (1200–1300
cm−1), already identified for the other substituted
silanones (Table 6). Besides that, the frequency of the
SiꢀO stretch in 4 is expected to be shifted to a lower
value relative to that in dimethyl derivative by analogy
with the frequency change of the MꢀO (MꢀC, P, S)
stretching mode in ketones, phosphinoxides, and sul-
foxides (Table 6). This expectation also agrees with the
B3LYP/6-311G(d, p) calculations predicting high inten-
sities and a somewhat lower frequency value for 4 of C2
symmetry (1220 cm−1) as well as of C2v planar geome-
try (1210 cm−1) than for 2 (1227 cm−1). Therefore, the
assignment of the band at 1205 cm−1 to the SiꢀO
stretching mode in 4 seems to have no alternative.
(8.95 mdyn A−1) and the lowest for diphenyl substi-
˚
tuted derivative 4 (8.59 mdyn A−1). They all notably
˚
exceed the force constant of the Si–O single bond, e. g.
−1
˚
in (CH3)3SiOCH3 (4.6 mdyn
A
)
or in
(CH3)3SiOSi(CH3)3 (5.3 mdyn A−1) [47], pointing out
at a strong y-bonding between Si and O atoms in 1–5.
The SiꢀO force constant in dimethylsilanone 2, 8.75
˚
mdyn A−1, calculated in present work from B3LYP
˚
output data, is higher than our previously estimated
value of 8.32 mdyn A
−1
˚
from the force field approxi-
mation [20–23]. Since molecular geometry is not opti-
mized in this type of calculations, the latter force
constant value seems now somewhat underestimated.
Application of an empirical Siebert’s rule yielded the
SiꢀO bond orders in 1–5, lying within 1.65–1.72.
Among the series studied these values indicate a higher
y-bond order in dimethoxysilanone (1.712) and a lower
degree of SiꢀO double-bonding in diphenylsilanone
(1.658) than in the other silanones (Table 7). These
bond orders are consistent with the SiꢀO frequency
change in silanones 2–5, i. e. the higher bond order is
related to the higher frequency. The bond order in
parent silanone 1, being higher than in 2, but corre-
3.6. The SiꢀO force constants and bond orders in
silanones 1–5
The calculated force constants of the SiꢀO bond in
the internal coordinates for the silanones 1–5 are given
−1
˚
in Table 7. They range within 8.5–9.0 mdyn A
,
exhibiting the highest value for dimethoxysilanone 3

58
V.N. Khabashesku et al. / Journal of Organometallic Chemistry 566 (1998) 45–59
Table 6
Frequencies of the MꢀO (MꢀC, P, S, Si) stretching modes (cm−1) in the IR spectra of selected ketones, phosphinoxides, sulfoxides, and silanones
(C6H5)2CꢀO
1665
(C6H5)3PꢀO
1145
(C6H5)2SꢀO
1060
(C6H5)2SiꢀO
1205
(CH3)2CꢀO
1719
(CH3)3PꢀO
1161
(CH3)2SꢀO
1103
(CH3)2SiꢀO
1210
(CH3O)2CꢀO
1740
(CH3O)3PꢀO
1284
(CH3O)2SꢀO
1200
(CH3O)2SiꢀO
1247
Cl2CꢀO
1823
Cl3PꢀO
1292
Cl2SꢀO
1251
Cl2SiꢀO
1240
F2CꢀO
1928
F3PꢀO
1418
F2SꢀO
1333
F2SiꢀO
1309
Reference
[41,51]
CꢀO
PꢀO
SꢀO
SiꢀO
[49]
[50]
[13,14,20–23] and present work
Table 7
The calculated SiꢀO force constants (K) and bond orders (N) in silanones 1-5
H2SiꢀO
(CH3)2SiꢀO
8.75 [8.32]b
CH3O(CH3)SiꢀO
(CH3O)2SiꢀO
(C6H5)2SiꢀO
−1
˚
K (SiꢀO) (mdyn A
)
8.91
1.706
8.89
1.703
8.95
1.712
8.59
1.658
N (SiꢀO)
1.682 [1.45–1.53]a
a Refs. [20–23].
sponding to a lower frequency of the SiꢀO stretch than
in 2, provides an exception from this trend. This devia-
tion can be probably explained by the mixed nature of
the SiꢀO stretching vibration, which in 1 is strongly
coupled with the SiH2 scissoring mode. This is indicated
by the PED calculated values (Table 3) and computer
animation of vibrational modes. Apart from that, all
estimated bond orders in 1–5 seem to be in line with
the B3LYP/6-311G(d, p) optimized SiꢀO bond lengths.
This very likely conforms to the y-bond strengths,
slightly increasing in 1-5 in the following direction:
4B2B5ꢀ1B3 (Charts 1 and 2, Fig. 10).
highest w (SiꢀO) frequency value in the dimethoxysi-
lanone 3 is in line with the strengthening of SiꢀO
y-bond in this molecule due to a y-type electron dona-
tion from oxygen lone-pairs, which agrees with the
observed enhanced kinetic stability of 3 towards cy-
cloolygomerization as compared to the silanones 2 and
4.
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