Journal of Molecular Structure 695–696 (2004) 163–169
www.elsevier.com/locate/molstruc
Intramolecular hydrogen bonding in (1-fluorocyclopropyl)methanol as
studied by microwave spectroscopy and quantum chemical calculationsq
H. Møllendala,*, A. Leonovb, A. de Meijereb
b
a
Department of Chemistry, The University of Oslo, P.O. Box 1033 Blindern, NO-0315 Oslo, Norway
Institut für Organische Chemie der Georg-August-Universität Göttingen, Tammannstrasse 2, D-37077 Göttingen, Germany
Received 2 October 2003; accepted 11 November 2003
Dedicated to Brenda and Manfred Winnewisser on the occasion of their 65th and 70th birthdays
Abstract
(1-Fluorocyclopropyl)methanol has been studied by microwave spectroscopy in the 12 – 61 GHz spectral region. The rotational spectra of
the ground and of four vibrationally excited states belonging to three different normal modes of one rotamer have been assigned. Most other
cyclopropylmethanol derivatives prefer conformations stabilized by an internal hydrogen bond with the pseudo-p electrons along the edges
of the ring. This is not the case for the title compound. The conformer assigned in this work has an internal hydrogen bond formed between
the fluorine atom and the hydrogen atom of the hydroxyl group. This rotamer is at least 4 kJ/mol more stable than any other form of the
molecule. It is pointed out that electrostatic interaction between the O– H and C –F bond dipoles can largely explain the conformational
preference of this compound. The microwave work has been assisted by gas-phase infrared spectroscopy and quantum chemical calculations
made at the MP2/6-311þþ G** and B3LYP/6-311þ þG** levels of theory.
q 2003 Elsevier B.V. All rights reserved.
Keywords: (1-Fluorocyclopropyl)methanol; Rotamer; Quantum chemical calculations
1. Introduction
The laboratory in Oslo has had a long-standing interest in
the way intramolecular hydrogen (H) bonding influences the
structural and conformational properties of free molecules.
Recent examples and surveys are found in Refs. [1 – 11].
Our H bond studies are now extended to include (1fluorocyclopropyl)methanol (FCP).
Our reasons for choosing FCP are as follows: IR studies
in the late sixties showed that the pseudo-p electrons along
the edges of the cyclopropyl ring [12] can act as
proton acceptor for intramolecular H bonds [13,14].
Subsequent microwave (MW) studies have in the cases
of cyclopropropylmethanol (C 3H 5CH 2OH) [15,16],
1-cyclopropropylethanol (C3H5CH(OH)CH3) [17], trans2-methylcyclopropropylmethanol (CH3C3H4CH2OH) [18]
and 2-bicyclopropylidenylmethanol (C3H4yC3H3CH2OH)
q
Supplementary data associated with this article can be found at doi: 10.
1016/S0022-2860(03)00836-6
* Corresponding author. Tel.: þ 47-22-85-5674; fax: þ47-22-85-5441.
E-mail address: harald.mollendal@kjemi.uio.no (H. Møllendal).
0022-2860/$ - see front matter q 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2003.11.046
[1] confirmed the classical IR findings. Modern quantum
chemical calculations also indicate that internal H bonding
is a predominating force in these compounds [1,10,17– 20].
The situation in FCP is more complicated than in these
four compounds referred to above because there are two
possible acceptors for H bonds, viz. the pseudo-p electrons
as well as the fluorine atom.
Five rotameric forms that can be distinguished by
spectroscopy may be envisaged for the title compound.
They are drawn in Fig. 1 and given Roman numerals for
reference. Atom numbering is indicated on Conformer I.
These five rotamers can perhaps best be characterized by
reference to the F5 – C3 –C4 – O6 and C3 – C4 –O6 –H13
chains of atoms. The F– C – C –O link has an antiperiplanar
conformation in conformers I and II, and a synclinal
(‘gauche’) orientation in the remaining three rotamers. The
C – C –O – H chain of atoms is antiperiplanar in I and III,
and þ or 2 synclinal in the three other conformers.
Internal H bonding may stabilize two of the five
rotamers. H bonding to the pseudo-p electrons occurs in
Conformer IV, whereas the fluorine atom is the H
bond acceptor in V. A competition between these two
164
H. Møllendal et al. / Journal of Molecular Structure 695–696 (2004) 163–169
Fig. 1. The five possible rotamers of (1-fluorocyclopropyl)methanol. Atom numbering is indicated on Conformer I. Relative MP2/6-311þþ G** energies are
indicated, see text.
comparatively weak acceptors thus exists in this compound.
Will the H atom of the hydroxyl group prefer the pseudo-p
electrons as it indeed does in the other cyclopropylmethanols referred to above, will it prefer the fluorine atom, or is
there no specific preference? This question prompted the
present investigation.
MW spectroscopy assisted by gas-phase infrared (IR)
spectroscopy, and high-level quantum chemical calculations
have been the methods used for this study. MW spectroscopy has a unique resolution, IR spectroscopy provides
insight into the nature of the H-bonding interaction through
its O – H stretching vibration, whereas the quantum
chemical calculations may provide rather accurate values
for a number of parameters not readily available from the
MW experiment.
2. Experimental
FCP was synthesized and purified as described in Ref.
[21]. The Oslo Stark spectrometer used here is described
briefly in Ref. [22]. The MW spectrum of FCP in the
12– 61 GHz spectral region was taken at room temperature,
or at about 2 15 8C. Lower temperatures which would have
increased the intensities of the spectral lines could not be
used owing to insufficient vapor pressure. The 12– 40 GHz
region was investigated much more exhaustively than the
rest of the spectrum (40 – 61 GHz). Radio frequency
microwave frequency double resonance (RFMWDR) experiments were carried out as described in Ref. [23] using the
equipment mentioned in Ref. [24]. The spectrum was
recorded and stored electronically using the programs
H. Møllendal et al. / Journal of Molecular Structure 695–696 (2004) 163–169
written by Waal [25] and Grønås [26]. The accuracy of the
spectral measurements is better than ^ 0.10 MHz.
The gas-IR spectrum was taken at a pressure of roughly
200 Pa using a Bruker Fourier-transform spectrometer
model IFS 88 equipped with at gas cell. The path length
was 2.4 m and the resolution 2 cm21.
3. Results
3.1. Quantum chemical calculations
The quantum chemical calculations have been made
using the GAUSSIAN 98 program package running on the HP
Superdome in Oslo [27]. The 6-311þ þ G** basis set
provided with the program was used. Møller-Plesset second
order perturbation calculations (MP2) [28], as well as
density functional theory (DFT) calculations using Becke’s
three parameter hybrid functional (B3LYP) [29] were
carried out. It is assumed that both these procedures
model both the structure and the internal H bonding rather
well.
The structures of the five rotamers of FCP shown in Fig. 1
were fully optimized. The vibrational frequencies were
calculated for each rotamer. No negative frequencies were
found for any of them. This indicates that the conformers of
Fig. 1 are indeed minima on the potential energy
hypersurface.
The structures found in the MP2 and B3LYP calculations
were very similar. Only the MP2 results are thus listed in
Table 1. Inspection of the structural parameters in Table 1
reveals that there is nothing unusual or unexpected about the
structures of the five forms.
The MP2 relative energy differences obtained after
correcting for zero-point vibrational effects are shown in
Table 1, as well as in Fig. 1. The corresponding B3LYP
results were quite similar (6.9, 8.2, 8.0, 6.2 and 0.0 kJ/mol,
respectively, for the five conformers). It is seen that
Conformer V which is stabilized by an internal H bond
involving the fluorine atom is predicted to be the preferred
form. Interestingly, this rotamer is predicted to be as much
as 5.3 kJ/mol more stable than Conformer IV in the MP2
calculations, where and internal H bond involving the
pseudo-p electrons exists. The B3LYP result was similar
(6.2 kJ/mol), as already noted.
The MP2 dipole moment components along the principal
inertial axes which are also listed in Table 1 were about 10%
larger than the B3LYP counterparts. This is rather typical.
3.2. MW spectrum and assignment of the ground
vibrational state
A fairly weak and dense spectrum with absorption lines
occurring every few MHz throughout MW region was
observed at 2 15 8C. This is not surprising given that
the predicted rotational constants of the five conformers
165
(Table 1) are comparatively small and that each rotamer has
about six normal vibrational modes below about 500 cm21
(B3LYP result not shown in Table 1) rendering a low
population in each quantum state (Application 1).
The quantum chemical calculations above indicate that
Conformer V should be the most stable form of the
molecule. Its a-type R-branch transitions should be among
the strongest ones in the spectrum. These lines were first
searched for and soon found. Their assignments were
confirmed by their Stark effects, fit to Watson’s Hamiltonian
(A-reduction I r -representation [30]), and their RFMWDR
patterns. The assignments were then gradually extended to
include b-and c-type lines of higher and higher values of the
J quantum number. Ultimately, more than 700 transitions
were assigned with a maximum value of J ¼ 76: Lines with
even higher values of J were searched for but were not
found, presumably because of insufficient intensity. The
majority of the intermediate- and high-J lines were Qbranch lines. The high-J b- and c- type R-branch lines have
much lower intensities. All five quartic as well as three
sextic centrifugal distortion constants had to be used in
order to get a root-means-square of the fit comparable to the
experimental uncertainty of ^ 0.10 MHz. The spectroscopic
constants obtained from 699 transitions are listed in Table 2.
The full spectra of the ground and of vibrationally excited
states are available at doi:10.106/S0022-2860(03)00836-6.
It is seen in Table 2 that the experimental rotational
constants agree to within a few MHz from the MP2
rotational constants (Table 1). Unfortunately, the dipole
moment of this rotamer could not be determined experimentally owing to low intensities of the low-J lines
normally used for this purpose. However, from the
intensities of the assigned lines it can be concluded that
ma . mb < mc ; and that these components have roughly the
values calculated for Conformer V (Table 1). This is taken
as corroborative evidence that Conformer V has indeed been
assigned and not confused with III and IV, both of which
are predicted (Table 1) to have quite similar rotational
constants but very different dipole moment components.
3.3. Vibrationally excited states
The ground-state lines were accompanied by several
series of weaker lines with similar Stark and RFMWDR
patterns. These transitions are assigned as vibrationally
excited states of Conformer I. The torsion around the C3 –
C4 bond is the lowest fundamental according to the B3LYP
calculations. Its frequency is predicted to be 104 cm21. The
strongest excited state is assigned as the first excited state of
this mode. About 550 lines were assigned, 520 of which
were used to determine the spectroscopic constants shown
in Table 2. Relative intensity measurements made as
described in Ref. [31] yielded 92(15) cm21, compared to
the B3LYP value of 104 cm21.
It is also assumed that the second excited state of this
mode has been assigned, as seen in Table 2. Relative
166
H. Møllendal et al. / Journal of Molecular Structure 695–696 (2004) 163–169
Table 1
Structures, rotational constants, principal-axis dipole moment components and relative energies of the five conformers of (1-fluorocyclopropyl)methanol as
predicted in the MP2/6-311þþ G** calculations
Conformer
Ia
Bond length (Å)
C1 –C2
C1 –C3
C2 –C3
C3 –C4
C3 –F5
C4 –O6
C1 –H7
C1 –H8
C2 –H9
C2 –H10
C4 –H11
C4 –H12
O6–H13
Bond angle (deg)
C2 –C1 –H7
C2 –C1 –H8
C1 –C2 –H9
C1 –C2 –H10
C1 –C3 –C4
C2 –C3 –C4
C1 –C3 –F5
C2 –C3 –F5
C3 –C4 –O6
C3 –C4 –H11
C3 –C4 –H12
C4 –O6–H13
Dihedral angleb,c (deg)
H7–C1 –C3 –F5
H8–C1 –C3 –F5
H9–C2 –C3 –F5
H10–C2 –C3 –F5
C1 –C3 –C4–O6
C2 –C3 –C4–O6
F5 –C3– C4–O6
F5 –C3– C4–H11
F5 –C3– C4–H12
C3 –C4 –O6–H13
Rotational constants (MHz)
A
B
C
II
1.533
1.489
1.489
1.505
1.384
1.423
1.083
1.084
1.084
1.083
1.098
1.098
0.960
III
1.532
1.493
1.487
1.511
1.385
1.420
1.084
1.084
1.084
1.084
1.097
1.097
0.961
IV
1.528
1.496
1.491
1.494
1.381
1.424
1.084
1.083
1.803
1.084
1.098
1.098
0.961
V
1.529
1.492
1.498
1.499
1.380
1.419
1.084
1.803
1.803
1.085
1.099
1.093
0.962
1.530
1.492
1.490
1.500
1.390
1.420
1.084
1.084
1.083
1.084
1.092
1.098
0.962
117.6
116.7
116.7
117.6
123.0
123.0
116.1
116.1
107.5
108.6
108.6
107.4
117.8
116.8
116.8
117.5
123.7
123.0
115.4
115.9
112.0
118.7
109.0
107.4
118.7
116.5
116.6
118.7
121.6
121.7
115.4
115.6
108.6
108.4
108.4
107.6
118.4
116.4
116.6
118.0
121.9
121.4
115.4
115.1
112.9
107.9
109.6
106.9
118.7
116.5
116.4
118.7
122.8
122.8
115.8
115.4
112.4
109.0
108.9
106.5
2145.7
0.3
20.3
145.7
37.8
237.8
180.0
59.1
259.1
180.0
2146.4
0.1
0.5
146.4
35.1
240.8
177.3
53.1
264.7
278.6
2145.3
0.0
0.5
145.1
145.5
71.6
271.8
167.4
49.3
160.9
2146.1
20.7
0.1
146.2
151.8
78.0
264.4
171.8
53.6
250.7
2145.5
20.2
20.6
144.5
152.1
76.8
265.2
176.7
58.3
56.2
5313.1
3188.0
2413.3
5283.9
3123.8
2388.4
5636.3
2890.3
2467.9
5666.9
2861.3
2453.0
5736.2
2862.1
2447.3
Dipole moment component (D)d
ma
mb
mc
2.02
0.00e
0.00e
0.40
1.05
1.28
0.35
2.16
2.02
1.96
2.78
0.02
1.46
0.81
0.84
Relative energye (kJ/mol)
DE
5.8
7.7
7.4
5.3
0.0
a
Atom numbering as given in Fig. 1.
Dihedral angle of a synperiplanar arrangement of four atoms is defined to be zero.
c
Clock-wise orientation of the dihedral angle is defined to be positive.
d
1 D ¼ 3.3356 £ 10230 C m.
e
Energy difference corrected for zero-point vibrational energy relative to Conformer V. The total energy of this conformer corrected for zero-point
vibrational energy is 2868 469.13 kJ/mol.
b
H. Møllendal et al. / Journal of Molecular Structure 695–696 (2004) 163–169
167
Table 2
Spectroscopic constants of the ground and vibrationally excited states of Conformer V of (1-fluorocyclopropyl)methanol
Vibrational state
Ground
First ex. torsiona
Second ex. torsiona
Lowest bend
Second lowest bend
A (MHz)
B (MHz)
C (MHz)
DJ (kHz)
DJK (kHz)
DK (kHz)
dJ (kHz)
dK (kHz)
FJK (Hz)
FK (Hz)
fJ c (Hz)
No of transition in fit
Maximum value of J
Rms deviation (MHz)
5743.1050(17)
2859.4360(14)
2445.1540(14)
0.5055(77)
3.34724(90)
0.947(13)
0.072402(51)
0.8174(14)
0.01799(50)
20.807(53)
0.003906(69)
699
76
0.083
5728.1209(22)
2863.1846(18)
2443.1823(18)
0.5063(92)
3.37266(98)
0.774(17)
0.077727(62)
0.9078(15)
0.02032(62)
20.882(71)
0.0004685(83)
520
76
0.086
5722.5036(68)
2866.0024(48)
2442.7171(48)
0.554(27)
4.2390(74)
1.404(29)
0.09249(52)
0.464(14)b
5730.0962(75)
2856.8876(64)
2444.5215(64)
0.560(29)
3.398(14)
1.27(10)
0.07191(57)
0.890(16)b
5746.7520(82)
2860.2738(61)
2445.0571(61)
0.482(30)
3.809(16)
1.86(12)
0.08272(75)
0.160(20)b
a
b
c
111
46
0.131
104
46
0.108
103
32
0.126
A-Reduction I r representation [30]. Uncertainties represent one standard deviation.
Torsion around the C3 –C4 bond; see Fig. 1.
All sextic constants preset at zero for this vibrational state.
Further sextic constants preset at zero.
intensity measurements yielded 180(30) cm21 for this
excited state. The rotational constants deviate somewhat
from linearity upon excitation (Table 2). This may indicate
that the C3 –C4 torsion is not very harmonic [32].
The second lowest vibrational fundamental is a bending
mode according to the B3LYP calculations. Its frequency
was calculated to be 207 cm21. The first excited state of this
mode is assumed to be assigned as indicated in Table 2.
Relative intensity measurements yielded ca. 200 cm21 for
this fundamental.
The third lowest normal mode is also a bending vibration
according to the B3LYP computations which predict
306 cm21 for this vibration. It is assumed that this state
too has been assigned. Its spectroscopic constants are found
in Table 2. Relative intensity measurements yielded ca.
280 cm21 for this vibration.
conclusions are drawn for the three remaining conformers
(I – III) as well, because these three rotamers are all
predicted to be quite polar (Table 1).
3.5. Structure
It has been found that MP2 calculations with large basis
sets generally predict accurate structures [33]. The 6311þ þ G** basis set used here is actually quite large. It is
therefore assumed that the very good agreement between the
calculated (Table 1) and the experimental rotational
constants (Table 2) is not fortuitous, but indeed reflects
that the MP2 structure of Conformer V in Table 1 is actually
close to the equilibrium structure.
3.4. Searches for further conformations
4. Discussion
A total of about 1600 transitions were assigned as
described above. All the strongest lines in the 12 – 40 GHz
spectral interval, the majority of the transitions with
intermediate intensities and many weak lines were assigned.
Futile attempts were first made to assign Conformer IV,
which was predicted by theoretical calculations to be the
second most stable conformer. Conformer IV is predicted to
have a substantially higher dipole moment than that of the
identified form (V). The fact that no comparatively strong
lines in this spectral region remain unassigned coupled with
the fact that IV is predicted to be considerably more polar
than V, leads us to conclude that the former rotamer is
considerably less stable than the identified form. It is
concluded that Conformer V it at least 4 kJ/mol more stable
than the other H-bonded alternative, Conformer IV.
This estimate is considered to be conservative. Similar
The nature of the intramolecular H bond in Conformer V
warrants attention. The geometry of this bond is characterized by the non-bonded distance between the fluorine atom
(F5) and the H atom (H13), which is calculated from the
structure in Table 1 to be 2.51 Å. This distance is roughly
the same as the sum of the van der Waals radii of H (1.2 Å)
and F (1.35 Å), totaling 2.55 Å [34]. Moreover, the
F5· · ·H13 – O6 angle is 103.78, far from 1808, which is the
ideal angle for H bond interaction. The comparatively long
H· · ·F distance and the non-linearity of the F· · ·H – O atoms
show that there must be very little covalent bonding
involved in this H bond.
Electrostatic forces are obviously predominating in this
case since the two very polar bonds C3 – F5 and O6 –H13
are about 6.58 from being parallel. The associated bond
dipoles are thus nearly anti-parallel. This stabilizes
168
H. Møllendal et al. / Journal of Molecular Structure 695–696 (2004) 163–169
Norway (Program for Supercomputing) through a grant of
computer time. For the group in Göttingen, this work was
supported by the Fonds des Chemischen Industrie.
References
Fig. 2. The gas-phase IR spectrum in the region of the O– H stretching
vibration. This vibration has a maximum at 3656 cm21.
Conformer V in an ideal manner making it at least 4 kJ/mol
more stable than other forms of this compound.
The idea that electrostatic stabilization is the most
important effect in this case is supported by the IR spectrum.
This spectrum in the O-H stretching region is shown in
Fig. 2. This vibration has an absorption maximum at
3656 cm21. This frequency is a little red-shifted (26 cm21)
relative to methanol whose O –H stretching vibration falls at
3682 cm21 [35]. It is interesting to compare this to the
corresponding findings for 2-bicyclopropylidenylmethanol
[1] the preferred form of which is stabilized by internal H
bonding to the pseudo-p electrons. The O –H stretching
vibration is red-shifted by about 42 cm21 relative to
methanol in the latter compound [1]. It is also considerably
broader and more intense than that of the title compound
(Fig. 2). The increased red-shift and enhanced intensity of
the O –H stretching band in 2-bicyclopropylidenylmethanol
are evidence that the H bonds are different in the two
compounds. It is also inferred that covalent forces are more
important in the H bond in 2-bicyclopropylidenylmethanol
than in FCP.
Acknowledgements
Anne Horn is thanked for her excellent assistance. This
work has received support from the Research Council of
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Synthesis (1977) 189) 1-(1-fluorocyclopropyl)ethanone. (1) Oxidation
to 1-fluorocyclopropanecarboxylic acid: NaOBr, NaOH, 10 C, 1.5 h,
83%. (2) Conversion of the acid to the acid anhydride: (COCl)2,
CH2Cl2, 40 C, 4 h, 51%. (3) Reduction of the acid anhydride to FCP:
LiAlH4, Et2O, 0 C, 1 h, 72%. The sample was purified by preparative
GC. Spectral data for FCP: 1H NMR (250 MHz, CDCl3): d ( ¼ 0.63–
0.72 (m, 2H, Cpr-H), 1.00–1.13 (m, 2H, Cpr-H), 2.41 (bs, 1H, OH),
3.81 (d, 3J HF ¼ 22 Hz, 2H, CH2OH). 13C NMR (62.9 MHz, CDCl3):
d ¼ 9.07 (d, 2J CF ¼ 11.8 Hz, CH2, 2C, C-20 , 3 0 ), 65.93 (d,
2
J CF ¼ 22.4 Hz, CH2, C-1), 79.86 (d, 1J CF ¼ 216.9 Hz, C, C-10 ).
MS (70 eV, CI, NH3), m=z (%): 198 (4) [2M þ NH4]þ, 125 (67)
[M þ NH4 þ NH3]þ, 108 [M þ NH4]þ.
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