Journal of Molecular Structure 780–781 (2006) 111–114
www.elsevier.com/locate/molstruc
Microwave spectrum and conformation of n-propyltrifluorosilane
Anne Horna, Harald Møllendala,*, Gamil A. Guirgisb
a
Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, NO-0315 Oslo, Norway
Department of Chemistry and Biochemistry, College of Charleston, Charleston SC 29424, USA
b
Received 31 March 2005; revised 22 April 2005; accepted 22 April 2005
Available online 6 September 2005
Dedicated to Jean Demaison on the occasion of his retirement
Abstract
The conformational properties of gaseous n-propyltrifluorosilane (CH3CH2CH2SiF3) have been investigated by microwave spectroscopy
and high-level quantum chemical calculations. The microwave spectrum was investigated in the 20–62 GHz spectral range at a temperature
of K78 8C. The spectra of the ground vibrational state and three vibrationally excited states of one conformer having an antiperiplanar
conformation of the C–C–C–Si chain of atoms were assigned. No evidence for the existence of the synclinal (gauche) conformer was seen in
the microwave spectrum. It is concluded that the synclinal form is at least 3.5 kJ/mol less stable than the antiperiplanar conformer in the gas
phase. Density functional theory calculations have been performed for the system mainly to predict the effects of centrifugal distortion. The
G3 quantum chemical method has been used to test the ability of this method to predict the energy difference between the synclinal and
antiperiplanar conformers.
q 2005 Elsevier B.V. All rights reserved.
Keywords: n-Propyltrifluorosilane; Microwave spectroscopy; Antiperiplanar onformation; Conformational equilibrium; Centrifugal distortion
1. Introduction
The infrared and Raman spectra of n-propyltrifluorosilane (PSI) in the three states of aggregation was reported 2
years ago [1]. The spectroscopic work was supported by ab
initio calculations at various levels of theory. It was
concluded in this study that two rotameric forms, namely
C–C–C–Si antiperiplanar (ap) and synclinal (sc) (gauche),
exist for this compound in the gas phase and in solution,
while only the antiperiplanar form was found in the
crystalline state. The energy difference between the sc and
ap conformers was determined to be 2.14(22) kJ/mol in the
neat liquid, and 1.62(17) kJ/mol in liquid krypton, with ap
being the more stable [1].
Microwave (MW) spectroscopy represents a fourth
independent way to investigate the conformational composition of PSI in the gas phase. This method is more specific
than infrared and Raman spectroscopy, owing to its high
* Corresponding author. Tel.: C47 2285 5674; fax: C47 2285 5441.
E-mail address: harald.mollendal@kjemi.uio.no (H. Møllendal).
0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2005.04.048
resolution. Moreover, comparatively small energy differences between conformers can be accurately determined by
MW spectroscopy, making it an ideal method for
conformational studies.
It was necessary to extend the quantum chemical study of
the previous work [1] to calculate the effects of centrifugal
distortion, in order to facilitate the assignment of the MW
spectrum. Density functional theory calculations were
therefore carried out for this purpose.
The energy difference between sc and ap is a focal point
of this investigation. The G3 quantum chemical method [2],
which is expected to predict energy differences accurately,
was employed in order to test this method’s ability to predict
the energy difference between sc and ap.
2. Experimental
The compound used in this work was prepared and
purified as described in Ref. [1]. The major features of the
Stark microwave spectrometer are described in Ref. [3].
Radiofrequency microwave double resonance (RFMWDR)
experiments were carried out as outlined in Ref. [4], using
the equipment mentioned in Ref. [5]. The spectra were
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A. Horn et al. / Journal of Molecular Structure 780–781 (2006) 111–114
recorded at dry-ice temperature (K78 8C) and at a pressure
of a few Pascal. Overlapping transitions occur frequently in
this spectrum and the accuracy of the spectral measurements
is therefore estimated to be no better than G0.15 MHz.
J =15 14
J = 13 12
J = 14 13
Intensity
J = 12 11
3. Results
3.1. Quantum chemical calculations
Accurate predictions of the quartic centrifugal distortion
constants of Watson [6] were found to be very helpful in
assigning the MW spectrum, as outlined below. Predictions
of these constants were made using the GAUSSIAN 03 suite of
programs [7]. Previous experience suggests that density
functional theory (DFT) calculations often yield vibrational
force fields and centrifugal distortion constants that are quite
reliable. The DFT functional of Becke et al. (B3LYP) [8]
was used in conjunction with the 6-311G(3df,2pd) basis set.
The spectroscopic constants obtained from these calculations are listed in Table 1. The energy difference between
the two rotamers was found to be 3.23 kJ/mol at this level of
theory, after correcting for zero-point energy differences,
with ap being the more stable conformer.
The energy difference between the sc and ap conformers
was central to this investigation. The G3 method [2] is
renowned for predicting comparatively accurate energy
differences. The energy difference between sc and ap was
calculated to be 1.68 kJ/mol using this method, with ap
being the more stable. Previous values [1] include Hartree–
Fock 6-31(d) (2.83 kJ/mol), and second order Møller–
Plesset (MP2) [9] calculations using the 6-31G(d), 6-311C
G(d,p) and 6-311CG(2d,2p) basis sets, resulting in energy
differences of 0.92, 1.96 and 1.65 kJ/mol, respectively.
Table 1
B3LYP/6-311(3df,2pd) predictions of rotational constants (MHz), quartic
centrifugal distortion constants (kHz)a, dipole moments (10K30 C m)b and
energy differences (kJ/mol) of the antiperiplanar and synclinal conformers
of n-propyltrifluorosilane (CH3CH2CH2SiF3)
Conformer
Antiperiplanar
Synclinal
A
B
C
DJ
DJK
DK
dJ
dK
ma
mb
mc
DE
3670.4
1127.2
1114.6
0.0915
3.00
K2.25
0.000331
K1.00
8.15
2.21
0.0c
0.0
3301.8
1383.0
1321.6
0.317
2.59
K2.17
0.0338
K2.16
7.13
3.99
1.97
3.23
a
b
c
Watson’s A-reduction [6].
Conversion factor: 1 DZ3.3356!10K30 C m.
By symmetry.
28000
30000
32000
34000
Frequency /MHz
Fig. 1. Broad-band microwave spectrum of n-propyltrifluorosilane (CH3CH2CH2SiF3) in the 25.5–36 GHz spectral range revealing the existence of
the antiperiplanar conformer. The J quantum numbers involved in the
transitions are shown above each pile-up.
3.2. Microwave spectrum and assignments
The B3LYP rotational constants in Table 1 show that
both ap and sc are nearly prolate asymmetrical tops with
Ray’s asymmetry parameter k [10] being K0.99 and
K0.93, respectively. The largest component of the electric
dipole moment lies along the a-principal inertial axis in both
cases. The values of ma are predicted to be similar for ap and
sc, as listed in Table 1. A MW broadband spectrum was
expected to be very simple, consisting mainly of pile-ups of
lines separated by about 2.2 GHz in the case of the ap
conformer, and by approximately 2.7 GHz in the case of the
sc form. However, only pile-ups of the ap rotamer could be
observed, as illustrated in Fig. 1, which shows the broadband spectrum in the 26.5–36 GHz spectral range. Broadband spectra in other spectral regions displayed a similar
pattern.
The absence of pile-ups belonging to the sc rotamer
was unexpected, since the enthalpy difference between sc
and ap is only 1.62(17) kJ/mol in liquid krypton, and it
was assumed that a similar value would be observed in
the gas phase. An enthalpy difference of 1.6 kJ/mol
should lead to approximately 60% ap and 40% sc at dryice temperature (K78 8C), if a Boltzmann distribution is
assumed and the statistical weight of sc is 2 relative to
ap. It is thus concluded from these survey spectra that the
enthalpy difference between the two forms must be
considerably higher than 1.6 kJ/mol. It is felt that sc
would certainly have been seen in the broadband
spectrum, provided 20% of the gas belonged to this
form. This would lead to an enthalpy difference between
the two rotamers of more than 3.5 kJ/mol, with ap as the
more stable, assuming Boltzmann distribution and
A. Horn et al. / Journal of Molecular Structure 780–781 (2006) 111–114
113
Table 2
Spectroscopic constants for the antiperiplanar conformer of n-propyltrifluorosilane (CH3CH2CH2SiF3)
Vib. State
Ground
1st ex. –H2C–CH2– tors.a
2nd ex. –H2C–CH2– tors.a
1st ex. C–Si tors.a
Ab (MHz)
B (MHz)
C (MHz)
DJ (kHz)
DJKc (kHz)
No of trans.
Rmsd (MHz)
3803
1138.1593(78)
1130.9053(80)
0.0919(17)
3.5868(90)
94
0.164
3803
1140.711(30)
1130.809(33)
0.0919(37)
3.582(14)
32
0.174
3803
1140.763(74)
1132.794(77)
0.1022(40)
3.587(31)
29
0.147
3803
1137.961(71)
1129.267(76)
0.0855(33)
3.444(16)
51
0.159
a
b
c
d
Tentative assignment of normal vibration; see text.
Preset at this value; see text.
The further quartic constants DK, dJ and dK preset at the values given in Table 1.
Root-mean-square deviation.
the statistical weight of sc being 2 relative to ap. This
value for the enthalpy difference is larger than any of the
theoretical calculations referred to above.
Ten of the normal vibrational modes have frequencies
lower than 500 cmK1 [1]. The pile-ups of ap therefore have
a very crowded and complicated fine structure caused by
several well-populated vibrationally excited states. The
effects of centrifugal distortion become more important as
the values of J and KK1 increase. When J becomes larger
than about 20, several KK1 lines are well resolved, because
of centrifugal distortion. This adds to the complexity of the
spectrum, but at the same time offers the possibility of a
detailed assignment.
The first assignments of individual KK1 lines were
facilitated by the use of the B3LYP centrifugal distortion
constants displayed in Table 1. RFMWDR experiments
were also useful for this purpose. A total of about 100
transitions were assigned for the ground vibrational state.
Only transitions having KK1O1 were unambiguously
identified. The KK1Z0 and KK1Z1 transitions could not
be definitely assigned, owing to their slow Stark-modulation
patterns and the fact that they were often overlapped by
other lines.
The constants B0 and C0 of the ground vibrational state
were accurately determined from these KK1O1 aR-lines,
whereas the value of the A0 rotational constants will have a
large uncertainty, because this constant depends little on the
KK1O1aR-lines.
An estimate of the value of the A0 rotational constant
has been obtained in the following manner: The ac
conformer has a symmetry plane with two fluorine atoms
and six hydrogen atoms lying out of the symmetry plane.
The relationship DZIaCIbKIcZ2!Smiz2i is valid for a
completely rigid molecule possessing a symmetry plane
[11]. In this equation, Ia, Ib and Ic are the principal
moments of inertia, mi is the mass of atom i lying out of
the symmetry plane and zi is the corresponding out-ofplane coordinate. The effective value of D derived from
its moments of inertia of the ground vibrational state
is 120.5!10K20 u m2 for CH2aCH–SiF3 [12], which has
two out-of-plane fluorine atoms. The corresponding value
is 9.4!10K20 u m2 for propane (CH3CH2CH3) [13],
which has six out-of-plane hydrogen atoms. The effective
value of D in our case was assumed to be approximately
130.0!10K20 u m2 for the title compound, which is
the sum of the two aforementioned quantities. The A0
rotational constant was calculated to be z3803 MHz
using this value for D and the experimental values of B0
and C0. The A0 rotational constant was fixed at 3803 MHz
in the least-squares refinement.
It is not possible to determine DK, dJ and dK from
the spectrum. These centrifugal distortion constants
were kept fixed at the B3LYP values shown in
Table 1. The spectroscopic constants (A-reduction,
Ir-representation [6]) obtained from 92 transitions are
shown in Table 2. Sørensen’s program ROTFIT [14] was
used in the fitting procedure. The spectra of the ground
state and of the vibrationally excited states are available
at doi:10.1016/j.molstruc.2005.04.48.
The differences between the values of the B0 and
C0 rotational constants (Table 2) and the B3LYP
predictions of the approximate equilibrium rotational
constants (Table 1) are C0.96 and C1.44%, respectively. There is also reasonable agreement between the
experimental and B3LYP values of the DJ and DJK
centrifugal distortion constants (C0.3 and C16.2%,
respectively).
The spectra belonging to three vibrationally excited
states were assigned, as shown in Table 2. Two of these
have been tentatively attributed to successively excited
states of the –H2C–CH2– torsional vibration, while the third
has been tentatively assigned to the first excited state of the
C–Si torsion. The B3LYP vibrational frequencies are 71 and
43 cmK1, respectively, for these two normal vibrations.
Quantitative relative intensity measurements in order to
determine the vibrational frequencies of the two vibrational
modes, were not possible in the present case owing to the
crowded nature of the spectrum. The A rotational constants
of the excited states was arbitrarily fixed at the same value
as for the ground vibrational state (3803 MHz) in the leastsquares fits.
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A. Horn et al. / Journal of Molecular Structure 780–781 (2006) 111–114
4. Conclusions
References
The MW spectrum confirms the previous findings,
i.e. that the PSI prefers the ap conformation. However, the
energy difference between this form and the sc rotamer is
much higher in the gas phase than in either the neat liquid or
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Acknowledgements
We are grateful to the Research Council of Norway
(Programme for Supercomputing) for a grant of computer
time. Dr George C. Cole is thanked for his thorough reading
of the manuscript and for his many suggestions and
corrections.
Supplementary Material
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.molstruc.2005.
04.048