6608
J. Phys. Chem. A 2010, 114, 6608–6612
Microwave and Quantum Chemical Study of Allyldifluorosilane (H2CdCHCH2SiF2H)
Harald Møllendal,*,† Svein Samdal,† Gamil A. Guirgis,‡ and Charles J. Wurrey§
Centre for Theoretical and Computational Chemistry (CTCC), Department of Chemistry, UniVersity of Oslo,
P.O. Box 1033 Blindern, NO-0315 Oslo, Norway, Department of Chemistry and Biochemistry, College of
Charleston, Charleston, South Carolina 29424, Department of Chemistry, UniVersity of MissourisKansas City,
Kansas City, Missouri 64110
ReceiVed: March 4, 2010; ReVised Manuscript ReceiVed: April 14, 2010
The microwave spectrum of allyldifluorosilane (H2CdCHCH2SiF2H) has been investigated for the first time
in the 28-80 GHz spectral interval at a temperature of -30 °C. The spectrum of the ground vibrational state
of one conformer characterized by an anticlinal orientation for the CdC-C-Si chain of atoms and a synclinal
conformation for the CsCsSisH link has been assigned. This rotamer was found to be at least 2 kJ/mol
more stable than further rotameric forms. The spectroscopic investigation has been augmented with quantum
chemical calculations employing the MP2 and B3LYP methods using the 6-311++G(3df,3pd) basis set. The
theoretical predictions are generally in good agreement with the experimental results.
Introduction
Recently, there has been great interest in the syntheses and
investigation of the spectroscopic, conformational, and structural
properties of silicon-containing compounds. Recent examples
include F3SiCH2CH2SiF3,1 cyclopropylmethylsilane,2,3 ClCH2SiH3,4 Cl2CHSiH3,4 ClCH2SiF3,4 Cl2CHSiF3,4 diethylsilane,5
diethyldifluorosilane,5 vinyl silyl fluoride,6 H3SiCH2SiH2CH3,7
disilabutane,8 silacyclopentane,9 and ethylmethylfluorosilane.10
In this work, we focus on the little-investigated conformational and structural properties of compounds possessing
the difluorosilane (-SiF2H) group, with allyldifluorosilane
(H2CdCHCH2SiF2H) as an example. Previous work on
molecules with the difluorosilane group includes spectroscopic investigations of difluorosilane (SiH2F2),11-13 methyl
difluorosilane (H3C-SiF2H),14,15 and vinyldifluorosilane
(H2CdCHSiF2H).16 Only the last-mentioned of these compounds is capable of displaying rotational isomerism. This
property can perhaps be best envisaged by reference to the
hydrogen atom of the difluorosilane group. The SisH bond
is synperiplanar (sp; CdCsSisH dihedral angle ) 0°) with
the vinyl group in one form, and anticlinal (ac; approximately
120° from synperiplanar) in the second rotamer. An infrared
study of this compound dissolved in krypton yielded an
energy difference of 1.42(14) kJ/mol, with the sp form as
the more stable.17 Only the sp rotamer was found in a
microwave (MW) investigation of the gaseous species, and
this form was shown to be at least 3 kJ/mol more stable than
the ac conformer.16 In the present work, these studies are
extended to include the synthesis and MW study augmented
with quantum chemical calculations of a novel member
of this class of compounds, namely, allyldifluorosilane
(H2CdCHCH2SiF2H).
Studies of other allylsilane compounds such as the parent
allylsilane (H2CdCHCH2SiH3)18-20 and allyltrifluorosilane
(H2CdCHCH2SiF3)21 have been reported. It was concluded in
* To whom correspondence should be addressed. Tel: +47 2285 5674.
Fax: +47 2285 5441. E-mail: harald.mollendal@kjemi.uio.no.
†
University of Oslo.
‡
College of Charleston.
§
University of MissourisKansas City.
these studies that there are two rotameric forms of these
compounds, which can be characterized by the CdCsCsSi
dihedral angle. The preferred form of these two compounds has
an ac orientation of this angle, which has been determined to
be 106.8(11)° in allylsilane.19 There is also considerable
evidence that a high-energy sp conformer coexists with the ac
rotamer in these two compounds.18-20
The conformational problem posed by the title compound
is more complicated than for H2CdCHCH2SiH3 and
H2CdCHCH2SiF3 because of the asymmetry of the SiF2H group.
Two dihedral angles, namely, the CdCsCsSi and CsCsSisH
dihedral angles, can now be conveniently used to describe the
conformational properties of H2CdCHCH2SiF2H. Five rotamers
may exist as minima on the conformational energy hypersurface.
These forms are sketched in Figure 1 and atom numbering is
given on the conformer denoted I. The CdCsCsSi chain of
atoms has an +ac conformation (roughly +120°) in I, II, and
III depicted in Figure 1, and a sp conformation in IV and V.
The CsCsSisH link is +synclinal (+sc) (about +60°) in I
and V, antiperiplanar (ap) (roughly 180°) in II and IV, and -ac
in III. A mirror image conformation exists for each of I, II,
III, and V.
A successful investigation of a delicate conformational
equilibrium such as the one presented by H2CdCHCH2SiF2H
requires experimental methods possessing high resolution. MW
spectroscopy meets this requirement because of its superior
accuracy and resolution, making this method especially well
suited for conformational studies of gaseous species. The
spectroscopic work has been augmented by high-level quantum
chemical calculations, which were conducted with the purpose
of obtaining information for use in assigning the MW spectrum
and investigating properties of the potential-energy hypersurface.
Experimental Section
Synthesis. The allyldifluorosilane sample was prepared by
fluorination of the corresponding dichlorosilane using a freshly
sublimed sample of antimony trifluoride without the use of a
solvent. Allyldichlorosilane was obtained as a byproduct from
the chlorination of allylsilane by tin tetrachloride. A manuscript
describing this reaction is in preparation. The progress of the
10.1021/jp101950z  2010 American Chemical Society
Published on Web 05/26/2010
Allyldifluorosilane
J. Phys. Chem. A, Vol. 114, No. 24, 2010 6609
Figure 1. Models of H2CdCHCH2SiF2H with atom numbering. The MW spectrum of conformer I was assigned. I was found to be at least 2
kJ/mol more stable than the other rotameric forms. MP2 and B3LYP calculations predict I to be favored by ∼2-10 kJ/mol relative to II, III, and
IV. The theoretical calculations predict that V is a transition state.
fluorination reaction was monitored by taking samples every 5
min for infrared spectroscopy to observe the disappearance of
the dichloro compound. The final sample was purified by trapto-trap distillation three times. The identity of the compound
was checked by NMR and infrared spectroscopy.
Microwave Experiment. The spectrum of H2CdCHCH2SiF2H was studied in the 28-80 GHz frequency interval by
Stark-modulation spectroscopy using the microwave spectrometer of the University of Oslo. Details of the construction and
operation of this device have been given elsewhere.22,23 This
spectrometer has a resolution of about 0.5 MHz and measures
the frequency of isolated transitions with an estimated accuracy
of ≈0.10 MHz. The experiments were performed at about -30
°C by cooling the MW cell with dry ice in an attempt to increase
the intensity of the spectrum, which was recorded at a pressure
of roughly 10 Pa.
Quantum Chemical Methods. The present ab initio and
density functional theory (DFT) calculations were performed
employing the Gaussian 03 suite of programs,24 running on the
Titan cluster in Oslo. Electron correlation was taken into
consideration in the ab initio calculations using Møller-Plesset
second-order perturbation calculations (MP2).25 Becke’s threeparameter hybrid functional26 employing the Lee, Yang, and
Parr correlation functional (B3LYP)27 was employed in the density functional theory (DFT) calculations. The 6-311++G(3df,3pd)
wave function, which is of triple-ζ quality and augmented with
diffuse functions, was used in both MP2 and B3LYP calculations. Geometry optimizations with no symmetry restraints were
performed on the stationary points found for H2CdCHCH2SiF2H
employing the default convergence criteria of Gaussian 03.
Results and Discussion
Quantum-Chemical Calculations. The energies, structures,
rotational and centrifugal distortion constants, harmonic vibrational frequencies, and dipole moments were calculated for
conformers I-IV by employing both the MP2 and B3LYP
procedures. Only positive vibrational frequencies were calcu-
lated for each of these conformers, which are therefore minima
on the potential energy hypersurface. Interestingly, one negative
vibrational frequency associated with the torsion about C4-C6
bond was calculated for V in both the MP2 and B3LYP
calculations, which therefore implies a first-order transition state.
Calculations of the anharmonic frequencies are rather costly,
and these constants were calculated only for I (whose MW
spectrum was assigned) using the B3LYP procedure.
The MP2 structures of I-IV are shown in Table 1, while
the corresponding B3LYP structures are collected in Table 1S
in the Supporting Information. The rotational constants calculated from the MP2 structures are shown in Table 2, together
with Watson’s A-reduction quartic centrifugal distortion constants,28 the components of the dipole moment along the
principal inertial axes, and the energy differences relative to
the energy of the global minimum conformer, which turned out
to be I. The energy differences have been corrected for zeropoint vibrational energies. Corresponding B3LYP parameters
are listed in the Supporting Information, Table 2S. The MP2
and B3LYP structures of the transition state V are displayed in
Table 3S. This rotameric form has an electronic energy that is
13.0 kJ/mol above the energy of I in the MP2 calculations, and
11.8 kJ/mol above I in the B3LYP calculations. The harmonic
and anharmonic vibrational frequencies of this conformer are
listed in Table 4S.
Both the MP2 (Table 2) and B3LYP (Table 2S) calculations
predict that I is the preferred form of the molecule by ∼2-3
kJ/mol relative to the two other CdCsCsSi ac conformers
(II and III), and about 10 kJ/mol more stable than the
CdC-C-Si sp rotamer (IV). The high energy of the
CdCsCsSi sp arrangement relative to the ac conformation is
reminiscent of the findings made for H2CdCHCH2SiH318-20 and
H2CdCHCH2SiF3.21
The structures in Tables 1 and 1S reveal that the bond lengths
and bond angles are rather similar. A notable difference is seen
for the C1dC4sC6sSi9 dihedral angles, which are less by ∼5°
6610
J. Phys. Chem. A, Vol. 114, No. 24, 2010
Møllendal et al.
TABLE 1: MP2/6-311++G(3df,3pd) Structures of Four
Conformers of H2dCHCH2SiF2H
Ia
II
C1-H2
C1–H3
C1–C4
C4–H5
C4–C6
C6–H7
C6–H8
C6–Si9
Si9–H10
Si9–F11
Si9–F12
Bond Length (pm)
108.2
108.2
108.0
108.0
133.6
133.5
108.5
108.4
149.8
149.9
109.1
109.3
109.2
109.2
185.1
185.1
146.3
146.4
159.9
159.9
159.9
159.6
H2–C1–H3
H2–C1–C4
H3–C1–C4
C1–C4–H5
C1–C4–C6
H5–C4–C6
C4–C6–H7
C4–C6–H8
C4–C6–Si9
H7–C6–H8
H7–C6–Si9
H8–C6–Si9
C6–Si9–H10
C6–Si9–F11
C6–Si9–F12
H10–Si9–F11
H10–Si9–F12
F11–Si9–F12
Angles
117.7
121.3
121.1
118.9
124.5
116.5
111.5
110.8
109.3
107.9
109.1
108.2
113.4
110.6
109.1
108.2
108.2
107.2
H2–C1–C4–H5
H2–C1–C4–C6
H3–C1–C4–H5
H3–C1–C4–C6
C1–C4–C6–H7
C1–C4–C6–H8
C1–C4–C6–Si9
H5–C4–C6–H7
H5–C4–C6–H8
H5–C4–C6–Si9
C4–C6–Si9–H10
C4–C6–Si9–F11
C4–C6–Si9–F12
H7–C6–Si9–H10
H7–C6–Si9–F11
H7–C6–Si9–F12
H8–C6–Si9–H10
H8–C6–Si9–F11
H8–C6–Si9–F12
a
(deg)
117.7
121.3
121.0
119.0
124.4
116.5
111.0
110.8
111.8
107.4
107.8
108.0
114.5
109.2
109.7
107.9
107.8
107.5
Dihedral Angle (deg)
179.8
179.6
2.8
1.8
–0.7
–0.4
–177.7
–178.2
–138.6
–134.1
–18.4
–15.0
100.7
105.6
44.4
48.1
164.5
167.2
–76.3
–72.3
–61.5
174.9
176.8
53.8
59.1
–63.8
176.4
52.8
54.6
–68.4
–63.0
174.0
59.3
–62.9
–62.5
175.9
179.9
58.3
III
IV
TABLE 2: MP2/6-311++G(3df,3pd) Parameters of
Spectroscopic Interest of Four Conformersa of
H2CdCHCH2SiF2H
Ib
108.2
108.0
133.5
108.5
149.9
109.2
109.2
185.1
146.4
159.7
159.9
108.2
108.0
133.5
108.5
150.3
109.6
109.6
184.8
146.4
159.8
159.8
117.7
121.2
121.0
118.7
124.6
116.7
111.1
110.9
110.1
107.7
108.8
108.1
114.0
109.1
110.2
108.0
108.1
107.1
117.0
122.5
120.5
118.2
126.1
115.7
109.7
109.7
118.5
104.7
106.7
106.7
112.3
110.9
110.9
107.7
107.7
106.9
179.7
2.1
–0.5
–178.1
–139.4
–19.7
100.0
42.9
162.7
–77.7
50.0
–70.9
171.7
–72.0
167.1
49.7
171.3
50.4
–67.0
180.0
0.0
0.0d
180.0
122.8
–122.8
0.0
–57.2
57.2
180.0
180.0
59.4
–59.4
55.7
–64.9
176.0
–55.7
–176.4
65.9
The MW spectrum of this conformer was assigned.
in the MP2 structures of IsIII compared to the corresponding
B3LYP structures.
Comparison with experimental findings for similar molecules
is warranted. There is an accurate estimate of the equilibrium
structure of H2SiF2,13 where the Si-F and Si-H equilibrium
bond lengths are 157.59(20) and 146.14(20) pm, respectively.13
The theoretical Si-F bond lengths (Tables 1 and 1S) are
approximately 2 pm longer than this estimate,13 while the
theoretical Si-H bond length agrees to within 0.8 pm, or better.
The FsSisF equilibrium angle13 is 107.88(20)°, slightly larger
than the theoretical values (same tables). There is also an
accurate MW structure of CH3SiHF2.15 The Si-F and Si-H
bond lengths in this compound are 158.0(8) and 147.1(10) pm,
A
B
C
∆J
∆JK
∆K
δJ
δK
µa
µb
µc
∆E
5664.5
1614.2
1400.5
II
Rotational Constants
4457.7
1853.1
1647.8
III
(MHz)
5703.7
1675.8
1413.9
IV
4044.3
1991.6
1965.2
Quartic Centrifugal Distortion Constantsc (kHz)
0.680
1.82
1.01
0.874
2.34
–0.979
0.479
5.90
6.46
6.31
4.06
–5.97
0.0137
0.130
0.234
–0.028
1.95
2.03
2.96
–62.9
–6.2
–2.8
–0.4
Dipole Moment (10–30 C m)
2.9
–5.4
–2.1
–4.3
6.4
1.9
Energy Differencee (kJ/mol)
0.0
2.7
2.2
7.4
–2.5
0.0d
9.8
a
Minima on the potential energy hypersurface; see text. b The
MW spectrum of this conformer was assigned. c A-reduction.28 d For
symmetry reasons. e Relative to conformer I and corrected for the
zero-point energy. Electronic energy of conformer I: -1591 800.51
kJ/mol.
respectively, while the F-Si-F angle is 107.1(5)°.15 These
experimental values are much closer to their counterparts in
H2SiF213 than to the present theoretical results.
The quartic centrifugal distortion constants predicted by the
two methods vary considerably (Table 2 and 2S). This is not
surprising since they depend on the second derivative at the
minima of the potential-energy hypersurface. The B3LYP dipole
moments (same tables) are generally somewhat smaller than
their MP2 counterparts, which is typical.
Microwave Spectrum and Assignment of Conformer I.
This rotamer is predicted to be the preferred form of the
molecule by ∼2-3 kJ/mol relative to the other conformers
in both the MP2 and B3LYP calculations (Tables 2 and 2S).
I is a prolate asymmetric rotor (Ray’s asymmetry parameter29
κ ∼ -0.90), with µa ≈ 6 × 10-30 C m as its major dipole
moment component (Tables 2 and 2S). Pile-ups of aR-branch
transitions separated by approximately B + C ∼ 3.01 GHz
(Table 2) would therefore be expected to occur in the 40-80
GHz spectral range. A comparatively strong a-type spectrum
was not expected despite a sizable µa since as many as seven
normal vibrations are calculated to be below 500 cm-1 (Table
4S; Supporting Information). The anharmonic frequencies of
the two lowest fundamentals, which are the torsions about the
C4-C6 and C6-Si9 bonds, are predicted (same table) to be as
low as 56 and 59 cm-1, respectively. This, combined with the
fact that the rotational constants are relatively small, means that
the partition function is large at -30 °C, rendering a low
population in each quantum state and consequently a relatively
weak spectrum.
Survey spectra revealed several crowded aR pile-up regions
separated by about 3.1 GHz, in accord with these predictions.
Lines were also observed in the spectral intervals between the
pile-ups, but they were much less intense than those found in
the pile-ups. Unidentified impurity lines were also encountered.
Pairs of aR-lines with identical K-1 g 4 - 6 coalesce, because
κ ∼ -0.9. These transitions are modulated at relatively low
Stark fields, which facilitated their assignments. The MP2 values
of the ∆J and ∆JK centrifugal distortion constants were also
Allyldifluorosilane
J. Phys. Chem. A, Vol. 114, No. 24, 2010 6611
TABLE 3: Spectroscopic Constantsa,b of the Ground
Vibrational State of Conformer I of H2CdCHCH2SiF2H
A/MHz
B/MHz
C/MHz
∆J/kHz
∆JK/kHz
∆K/kHz
δJ/kHz
δK/kHz
ΦJ/Hz
ΦJKd/Hz
rmse
no. transitionsf
5737.3(51)
1592.307(65)
1384.115(66)
0.6555(61)
2.598(15)
6.46c
0.0137c
1.95c
0.0272(48)
-0.117(13)
1.510
295
a
A-reduction, Ir-representation.28 b Uncertainties represent one
standard deviation. c Fixed at this value in the least-squares fit.
d
Further sextic centrifugal distortion constants preset at zero.
e
Root-mean-square deviation of a weighted fit. f Number of
transitions used in the fit.
useful in this respect. No unambiguous assignment could be
made for lines with K-1 e 3 because they have slow Stark
effects and could not be completely modulated. Overlapping
with excited-state lines and spectral weakness were other reasons
for not obtaining unambiguous assignments for these transitions.
A total of 295 aR-transitions with 9 e J e 26 were ultimately
assigned and fitted employing Watson’s A-reduction Irrepresentation Hamiltonian,28 using Sørensen’s program Rotfit.30
b- and c-type lines were searched for, but not found, presumably
because the corresponding dipole moment components are much
smaller than µa, which is in accord with the calculations (Table
2). The quartic centrifugal distortion constants ∆K, δJ, and δK
of this near-prolate rotor could not be determined from the aRlines, and they were therefore held fixed at the MP2 values
(Table 2) in the weighted least-squares fit. Inclusion of two
sextic constants, ΦJ and ΦJK, were necessary to obtain a
satisfactory fit. The spectrum of the ground state is shown in
Table 5S in the Supporting Information, and the spectroscopic
constants are listed in Table 3. The uncertainties given in Table
5S were used as weights in the least-squares fit. It is seen from
Table 3 that an accurate value for the A rotational constant was
not obtained from these a-type transitions, whereas the B and
C rotational constants have been obtained with high accuracy.
Comparison of the theoretical (Table 2) and experimental
(Table 3) spectroscopic constants is in order. It is seen from
these two tables that the rotational constants of I and III are
not very different. The differences between each of the
experimental (Table 3) and MP2 rotational constants (Table 2)
are less than about 1.4% in the case of I, whereas differences
as large as 5.3% are seen for III (the B rotational constant). It
has been claimed that MP2 structures are close to equilibrium
structures provided a sufficiently large basis set has been used
in the calculations.31 This is one evidence that the spectrum
shown in Table 5S indeed belongs to I, and not to III.
The dipole moment components of the two forms provide
additional evidence that I has not been confused with III. µb is
calculated to be roughly 80% of the size of µa in III, but only
45% in I (Table 2). The former rotamer should therefore exhibit
a relatively much stronger b-type spectrum than I, but no such
spectrum was observed. The fact that the two theoretical
methods both predict that I is the global minimum is consistent
with the experimental findings.
Further Conformers. It is seen from Tables 2 and 2S that
the remaining conformers II-IV each have at least one dipole
moment component of the same order of magnitude as µa of I.
Spectra of similar intensities should therefore exist provided
they were present with the same concentration as I. This is
clearly not the case. Intensity considerations led us to conclude
that any rotamer other than I would have been assigned provided
its spectrum was roughly one-third as intense as that of I. This
makes it possible to estimate that I is at least 2 kJ/mol more
stable than any further rotamer.
Discussion
There are probably several reasons why conformer I is
preferred. This global minimum has an ac conformation for the
C1sC4sC6sSi9 chain of atoms, just as observed for the
preferred forms of H2CdCHCH2SiH318-20 and allyltrifluorosilane H2CdCHCH2SiF3.21 This conformational preference is
perhaps a result of repulsion between the silyl or trifluoro silyl
groups and the vinyl group in the sp conformation of these two
compounds. It is suggested that a similar interaction makes
conformers I-III more stable in our case by ∼10 kJ/mol (Table
2 and 2S) compared to IV.
There are some structural differences between rotamers I-III
that may be responsible for the preference of I over II and III.
One of these is the position of H10 (Figure 1) of the difluoro
silyl group in I. This atom is closer to the π-electrons of the
C1dC4 double bond in this conformer than in II or III. The
nonbonded distances between this atom and C1 and C4 atoms
are 325 and 366 pm according to the MP2 calculations,
compared to the sum, 290 pm, of the van der Waals radii of H,
120 pm, and the half-thickness of an aromatic molecule.32 It is
therefore likely that this nonbonded interaction will have a weak
stabilizing effect on conformer I compared to II and III, where
a similar interaction is not possible.
The interaction between the fluorine atoms of the difluoro
silyl group and the π-electrons of the vinyl moiety is probably
repulsive in nature. Both the very electronegative fluorine atoms
are directed away from the π-electrons in I, whereas one fluorine
atom is directed toward the double bond in II and III. It is
therefore suggested that the weak attraction between the H10
atom and the π-electrons combined with the more favorable
orientation of the fluorine atoms in I makes this conformer at
least 2 kJ/mol more stable than any other rotamer.
Acknowledgment. We thank Anne Horn for her skillful
assistance and Alexey Konovalov for recording the spectrum.
The Research Council of Norway (Program for Supercomputing)
is thanked for a grant of computer time. Also, support from the
Summer Undergraduate Research Forum (SURF) at the College
of Charleston is gratefully acknowledged.
Supporting Information Available: Results of the B3LYP/
6-311++G(3df,3pd) calculations (bond lengths and angles,
rotational constants, distortion constants, dipole moments,
vibrational frequencies) and the microwave spectra. This material is available free of charge via the Internet at http://
pubs.acs.org.
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