Journal of Molecular Structure 567±568 (2001) 19±27
www.elsevier.nl/locate/molstruc
The structural and conformational properties of 2-methoxyfuran
as studied by microwave spectroscopy and quantum
chemical calculations q
J.A. Beukes, K.-M. Marstokk, H. Mùllendal*
Department of Chemistry, The University of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, Norway
Received 18 January 2000; accepted 4 February 2000
Abstract
The microwave spectrum of 2-methoxyfuran has been investigated in the 10.0±60.0 GHz spectral region at about 2358C.
One rotamer denoted Syn was assigned. This conformer is at least 3 kJ mol 21 more stable than any other form. Syn has a
symmetry plane (Cs symmetry). The methyl group is Syn to the nearest CyC bond of the ring. The dipole moment components
along the principal initial axes and the total dipole moment are (in units of 10 230 C mm): ma ˆ 3:62…16†, mb ˆ 5:98…3†, mc ˆ
0:0 (for symmetry reasons), and mtot ˆ 6:99…12†: Four vibrationally excited states belonging to three different normal modes
were assigned and their frequencies determined by relative intensity measurements. The barrier to internal rotation of the
methyl group is at least 14 kJ mol 21.
The microwave work has been assisted by quantum chemical computations at the MP2/6-31111G pp (frozen core), B3LYP/
cc-pVTZ and B3LYP/6-31111G pp levels of theory. These rather advanced calculations were found to predict rather different
conformations for a second conformer. q 2001 Elsevier Science B.V. All rights reserved.
Keywords: Microwave spectrum; Quantum chemical calculations; Conformations; 2-Methoxyfuran
1. Introduction
2-Methoxyfuran is the simplest furan ether. This
compound may display rotational isomerism. Three
typical rotamers, Syn, Skew and Anti are depicted in
Fig. 1. The methyl group is Syn (C7±O6±C4±C2
dihedral angle ˆ 08) to the C2±C4 bond in the Syn
conformation. This group is rotated about 1208 around
the C4±O6 bond in the Skew, and 1808 in the Anti
rotamer, respectively.
q
Dedicated to Professor Marit Trñtteberg on the occasion of her
70th birthday.
* Corresponding author. Tel.: 147-22-85-56-74; fax: 147-22-8554-41.
E-mail address: harald.mollendal@kjemi.uio.no (H. Mùllendal).
Several forces may be of importance for the conformational properties of the title compound. The lone
pair electrons of the O6 atom should have the right
symmetry in both Syn and Anti to be delocalized into
p electron system of the ring. This should lead to a
stabilization of both these rotamers.
The electron density is assumed to have a
maximum in the C2±C4 region. Repulsion between
these electrons and the lone pair electrons of O6 is
assumed to be another important effect. This interaction would stabilize Syn relative to Skew and Anti.
The lone pairs of O5 are also assumed to be of
importance for the conformational properties. A
minimum repulsive interaction between these lone
pairs and those of O6 should exist in Anti. Moreover,
0022-2860/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.
PII: S 0022-286 0(01)00531-2
20
J.A. Beukes et al. / Journal of Molecular Structure 567±568 (2001) 19±27
Fig. 1. The Syn, Skew and Anti conformers of 2-methoxyfuran. The Syn form was found in this work.
weak electrostatic attraction between the lone pairs of
O5 and the methyl group should stabilize Anti. The
aromatic delocalization of the electrons of the ring
may also in¯uence the conformational choices of the
2-methoxyfuran. The preferred form(s) must clearly
represent some sort of compromise between all these
interactions. The interesting conformational problem
presented by the title compound motivated the present
investigation.
No experimental studies of the conformational
properties of the 2-methoxyfuran have been
reported. However, several studies of related
compounds exist. Methyl vinyl ether, CH3OCHyCH2,
has been subject to a number of investigations by
spectroscopy and electron diffraction [1±9]. In this
compound the syn form has been found to be the
most stable conformation [1±8], while another
rotamer co-exists with a 4.8 kJ mol 21 higher energy
[1]. The conformation of this high-energy form is not
precisely known, but appears to be a near-anti form
[4,6].
One aromatic compound of some relevance to
J.A. Beukes et al. / Journal of Molecular Structure 567±568 (2001) 19±27
21
Table 1
Structure, rotational constants, dipole moment and energy differences of the two conformations of 2-methoxyfuran as calculated at the MP2/631111G pp, B3LYP/6-31111G pp and B3LYP/cc-pVTZ levels of theory. Atom numbering is given in Fig. 1
Conformer
Second form a
Syn conformation
MP2 b
B3LYP b
B3LYP c
MP2 b
B3LYP b
B3LYP c
Bond distance (pm)
C1±C2
C1±C3
C2±C4
C3±O5
C4±O5
C4±O6
O6±C7
C1±H8
C2±H9
C3±H10
C7±H11
C7±H12
C7±H13
143.8
136.6
137.4
137.3
134.8
134.0
142.5
108.1
107.8
107.8
108.8
109.5
109.5
144.0
135.4
136.5
137.9
134.9
133.7
142.7
107.9
107.7
107.5
108.8
109.5
109.5
143.7
135.0
136.1
137.8
134.8
133.6
142.4
107.6
107.4
107.7
108.6
109.3
109.2
143.5
136.9
136.9
136.8
136.2
134.1
143.6
108.1
107.9
107.9
108.9
109.3
109.5
143.8
135.5
136.0
137.8
136.1
133.7
143.8
107.9
107.7
107.6
108.8
109.1
109.4
143.4
135.1
135.9
138.1
135.4
133.6
143.0
107.7
107.4
107.3
108.6
109.0
109.1
Angle (8)
C2±C1±C3
C1±C2±C4
C1±C3±O5
C2±C4±O5
C3±O5±C4
C2±C4±O6
O5±C4±O6
C4±O6±C7
C2±C1±H8
C3±C1±H8
C1±C2±H9
C4±C2±H9
C1±C3±H10
O5±C3±H10
O6±C7±H11
O6±C7±H12
H11±C7±H12
O6±C7±H13
H11±C7±H13
H12±C7±H13
106.6
104.9
110.3
111.8
106.5
134.8
113.4
112.7
127.3
126.1
127.9
127.2
134.3
115.5
106.0
110.5
110.0
110.5
110.0
109.7
106.9
105.0
109.9
111.5
106.6
135.0
113.4
115.2
126.9
126.3
127.6
127.4
134.5
115.5
106.0
110.7
109.8
110.7
109.8
109.7
106.9
105.1
110.0
111.5
106.5
135.0
113.4
115.1
126.8
126.3
127.6
127.3
134.4
115.6
106.2
110.8
109.8
110.8
109.8
109.4
106.4
105.6
110.4
111.1
106.6
131.5
117.3
113.4
127.6
126.0
128.8
125.6
133.9
115.7
105.9
110.8
109.9
110.1
110.1
110.0
106.9
105.5
109.9
111.1
106.6
130.8
117.9
116.7
127.0
126.1
128.4
126.1
134.4
115.7
105.6
110.9
110.0
110.4
110.0
109.9
107.2
105.3
109.7
111.4
106.5
130.1
118.5
117.6
126.8
126.0
128.4
126.3
134.7
115.7
105.5
111.1
109.9
111.0
109.9
109.2
Dihedral angle d (8)
C4±C2±C1±C3
C4±C2±C1±H8
H9±C2±C1±C3
H9±C2±C1±H8
O5±C3±C1±C2
O5±C3±C1±H8
H10±C3±C1±C2
H10±C3±C1±H8
O5±C4±C2±C1
O5±C4±C2±H9
O6±C4±C2±C1
O6±C4±C2±H9
0.0
180.0
180.0
0.0
0.0
180.0
180.0
0.0
0.0
180.0
180.0
0.0
0.0
180.0
180.0
0.0
0.0
180.0
180.0
0.0
0.0
180.0
180.0
0.0
0.0
180.0
180.0
0.0
0.0
180.0
180.0
0.0
0.0
180.0
180.0
0.0
0.6
180.0
2179.8
20.5
20.7
180.0
178.7
20.7
20.3
180.0
175.7
23.9
0.6
179.9
2179.7
20.4
20.6
180.0
178.2
21.1
0.4
180.0
175.3
24.4
0.2
179.9
2179.8
20.1
20.2
180.0
179.4
20.4
20.1
179.9
178.6
21.4
22
J.A. Beukes et al. / Journal of Molecular Structure 567±568 (2001) 19±27
Table 1 (continued)
Conformer
Second form a
Syn conformation
MP2 b
B3LYP b
B3LYP c
MP2 b
B3LYP b
B3LYP c
0.0
180.0
0.0
180.0
0.0
180.0
180.0
60.8
260.8
0.0
180.0
0.0
180.0
0.0
180.0
180.0
60.9
260.9
0.0
180.0
0.0
180.0
0.0
180.0
180.0
60.8
260.8
0.5
2179.0
20.1
2176.7
126.7
257.5
2176.6
64.3
257.5
0.4
2178.3
0.0
2176.3
143.1
241.5
2176.8
64.1
258.0
0.1
2179.5
0.0
2178.9
. 171.9
29.4
2177.9
63.0
258.8
Rotational constants (MHz)
A
7552.4
B
2121.5
C
1674.0
7684.9
2089.0
1659.9
7713.5
2095.7
1665.4
7239.5
2128.2
1751.0
7435.9
2123.3
1713.7
7553.1
2163.6
1702.2
C4±O5±C3±O1
C4±O5±C3±H10
C3±O5±C4±C2
C3±O5±C4±O6
C7±O6±C4±C2
C7±O6±C4±O5
H11±C7±O6±C4
H12±C7±O6±C4
H13±C7±O6±C4
Dipole moment components e and total dipole moment (10 230 C m)
ma
2.83
3.27
mb
7.40
6.40
mc
0.0
0.0
m tot
7.92
7.18
Total energy (kJ mol 21)
f
2902370.26
b
c
d
e
f
0.04
1.28
4.72
4.91
1.25
2.12
3.03
3.90
2904944.37
2902368.15
2904.860.21
2.11
2.55
2.18
2.79
0.61
3.59
2904940.79
21
Energy difference (kJ mol )
0.0
a
2904862.76
3.28
5.38
0.0
6.30
0.0
0.0
3.58
See text.
6-31111G pp basis set.
cc-pVTZ basis set.
Measured from syn ˆ 08. Clockwise rotation corresponds to positive dihedral angle.
Along principal inertial axes.
Relative to Syn.
2-methoxyfuran, anisole (methoxybenzene), has been
studied by electron diffraction [10] and microwave
(MW) spectroscopy [11]. A syn rotamer was found
to be the preferred form of anisole [10,11].
Our methods of investigation have been MW
spectroscopy and quantum chemical calculations.
MW spectroscopy is ideal for investigating conformational equilibria in cases where polar conformers are
present because of its high selectivity and speci®city.
All conceivable rotamers of the title compound would
each possess a sizeable dipole moment, which is a
prerequisite for a MW spectrum. This makes 2methoxyfuran well suited for a MW conformational
investigation.
Advanced quantum chemical computations are
often found to be useful in predicting rotational
constants, dipole moments and energy differences
for the various conformers that are suf®ciently close
to the experimental ones to be really helpful starting
points in the spectral analysis. In addition, they may
give important information about rotamers that for
whatever reason have not been assigned by MW
spectroscopy. Such calculations are, therefore, of
interest in their own right as well.
2. Experimental
The sample used in this work was purchased from
Lancaster Synthesis, Ltd. UK. The compound was
speci®ed to be at least 97% pure and was used as
received. No impurities were detected in the MW
J.A. Beukes et al. / Journal of Molecular Structure 567±568 (2001) 19±27
spectrum that was studied using the Oslo Stark
spectrometer which is described in Ref. [12]. Most
measurements were made in the 10±39 GHz spectral
region. Selected parts of the 39±60 GHz region were
also investigated. The X-band brass absorption cell
was cooled to about 2358C during the experiments.
Radio frequency microwave double resonance experiments (RFMWDR) were carried out as described in
Ref. [13] using the equipment mentioned in Ref. [14].
The spectra were recorded at a pressure of a few Pa
and stored electronically using the computer programs
written by Waal [15]. The accuracy of the frequency
measurements is presumed to be better than
^0.10 MHz, and the resolution was approximately
0.5 MHz.
3. Results and discussion
3.1. Quantum chemical calculations
The gaussian 94 program package [16] running on
the IBM RS6000 cluster in Oslo was employed in all
the quantum chemical calculations. The 6-31111G pp
and the cc-pVTZ basis sets provided with the program
[16] were employed. Three different, rather high-level
computational schemes, viz. MP2/6-31111G pp,
B3LYP/6-31111G pp and B3LYP/cc-pVTZ, were
utilized, because we wanted to compare the results
obtained in different ways.
In the ®rst of these computational procedures, electron correlation was included using the second order
Mùller±Plesset (MP2) perturbation theory [17] with
frozen-core electrons [16]. In the second method,
density functional theory (DFT) calculations were
carried out employing the B3LYP procedure [18].
Full geometry optimization was made in all computations. The vibrational frequencies were calculated in
the B3LYP computations. All of them were found to
be positive. This is an indication that the conformation
in question indeed represents a minimum (is `stable')
on the energy hypersurface [19]. The vibrational
frequencies were not calculated in the MP2 calculations because of lack of resources. Several different
starting conformations of the methoxy group were
chosen in our re®nements. The computations always
converged to Syn in those cases where a near-syn
conformation was used as the starting point. Selected
23
results of the computations are shown in Table 1.
Atom numbering is given in Fig. 1.
Table 1 lists the results for the so-called second
form. This rotamer was found using a starting
geometry close to skew (the conformation-determining dihedral angle C7±O6±C4±C2 was given a
starting value of approximately 1208). The bond
distances and bond angles of the second form are
rather similar to the results obtained for Syn (see
Table 1). However, the conformations obtained in
these three calculations differ considerably, as can
be seen from the C7±O6±C4±C2 dihedral angle.
This angle is computed to be 126.78 in the MP2/631111G pp calculations. This corresponds to a skew
rotamer. The same dihedral angle is found to be
143.18 in the B3LYP/6-31111G pp procedure. This
corresponds to a conformation intermediate between
skew and anti. The dihedral angle became as large as
171.98 in the B3LYP/cc-pVTZ computations corresponding to a near anti form. In other words, the
C7±O6±C4±C2 dihedral angle vary by as much as
458 in these high-level calculations! The torsional
frequency around the O6±C4 bond was calculated to
be 38 cm 21 (not given in Table 1) in the B3LYP/631111G pp calculations, and 22 cm 21, respectively,
in the B3LYP/cc-pVTZ computations. It can be
mentioned for comparison that the same frequency
was calculated to be approximately 100 cm 21 for the
Syn form employing the same computational procedures. If the second form indeed exists, these calculations indicate that it might be rather ¯oppy.
All calculations predict relatively small energy
differences (2±4 kJ mol 21, respectively) between the
two conformers, with Syn as the preferred form.
3.2. MW spectrum and assignment of the ground
vibrational state of Syn
The quantum chemical computations (Table 1)
indicate that Syn is the preferred form. This rotamer
is also predicted to have a larger dipole moment than
the second form. Its largest dipole moment component
is calculated to lie along the b-inertial axis. 2-Methoxyfuran is predicted to have several low-frequency
normal modes. A dense spectrum of intermediate
intensity was thus foreseen for this conformer. This
was also observed.
Searches were ®rst made for the strong bQ-branch
24
J.A. Beukes et al. / Journal of Molecular Structure 567±568 (2001) 19±27
Table 2
Selected transitions of the MW spectrum of the ground vibrational state of the Syn conformer of 2-methoxyfuran
Transition
J 0K 021 ;K 011 Ã J 00 k 0021 ; k 0011
Observed frequency a
(MHz)
Observed2calculated frequency
(MHz)
21,2 Ã 10,1
32,1 Ã 31,2
50,5 Ã 40,4
62,4 Ã 61,5
71,7 Ã 61,6
74,3 Ã 64,2
82,7 Ã 81,8
92,8 Ã 91,9
121,11 Ã 120,12
153,12 Ã 144,11
183,16 Ã 174,13
224,18 Ã 223,19
264,23 Ã 255,20
288,21 Ã 279,18
328,25 Ã 319,22
12627.17
15987.35
18430.60
14438.84
24659.82
26608.05
25421.94
27502.77
30824.56
26549.33
19633.26
31041.37
28763.22
12436.51
29843.13
20.07
0.02
0.01
20.03
20.03
20.11
20.02
0.02
0.07
0.11
0.00
0.22
0.10
20.07
0.00
Coalescing K21-transitions b
188 Ã 179
2711 Ã 2810
3514 Ã 3613
4216 Ã 4315
4719 Ã 4818
4916 Ã 5015
5316 Ã 5217
6324 Ã 6423
7323 Ã 7224
7824 Ã 7725
9229 Ã 9130
28315.68
12017.73
15476.7
22969.41
26615.00
18027.64
18221.24
21604.81
15404.73
23979.51
20743.01
20.08
0.04
20.04
0.11
20.02
20.04
20.02
0.01
20.03
20.04
0.23
a
b
^ 0.10 MHz.
The K21 pair of transitions coalesce for high values of K21.
transitions using the rotational constants obtained in the
MP2 computations as the starting point because it is
expected [20] that this computational scheme produces
accurate rotational constants (and structures). These Qbranch transitions were soon identi®ed close to their
predicted frequencies. Their assignments were
con®rmed by Stark effect studies and their ®t to
Watson's Hamiltonian [21]. The a- and b-type R-branch
transitions were then searched for and soon assigned.
The assignments of several aR-lines were con®rmed by
RFMWDR experiments [13].
The assignments were next gradually extended to
include medium- and high-J transitions. A few
selected lines are listed in Table 2. 1 A total of about
1
The full spectra are available from the authors upon request.
570 transitions were ultimately assigned for the
ground vibrational state; 532 of which were used to
determine the spectroscopic constants (A-reduction, I r
representation [21]) shown in Table 3. Maximum
value of J was 90. Transitions involving even higher
values of J were searched for, but not identi®ed
presumably because they were too weak to allow
unambiguous assignments to be made. Only quartic
centrifugal distortion constants were employed in the
least-squares ®t because inclusion of sextic constants
yielded no signi®cant improvement of the ®t and
insigni®cant sextic constants.
It is seen in Table 3 that the following relation
between the principal inertial moments of inertia, Ia 1
Ib 2 Ic, has the value 3.350 843(23) £ 10 220 m 2 u.
This is close to (same units) 3.410 found for anisole
J.A. Beukes et al. / Journal of Molecular Structure 567±568 (2001) 19±27
25
Table 3
Spectroscopic constants (A-Reduction, I r representation [21]; uncertainties represent one standard deviation) of the ground and vibrationally
excited states of the Syn conformer of 2-methoxyfuran
Vibrational state
Ground vibrational
state bending vibration
1st ex. C4±O6 tors.
vibration
1st ex. in-plane
bending vibration
1st ex. Me tors.
vibration
1st ex. C4±O6 tors.
11 st ex. in plane
No. of transitions
Maximum value of J
RMS dev. a (MHz)
Av (MHz)
Bv (MHz)
Cv (MHz)
D J (kHz)
D JK (kHz)
D K (kHz)
d J (kHz)
d K (kHz)
(Ia 1 Ib 2 Ic) b (10 220 m 2 u)
532
92
0.067
7613.1668(14)
2111.50975(40)
1671.35814(32)
0.14203(89)
0.0357(59)
2.1805(19)
0.034619(24)
0.3981(97)
3.350843(23)
168
61
0.066
7588.3773(24)
2109.42386(67)
1673.12922(75)
0.1401(13)
0.033(15)
2.073(13)
0.03641(71)
0.291(37)
4.124453(83)
81
31
0.078
7648.0573(51)
2109.7517(12)
1669.1557(13)
0.1338(35)
0.135(21)
1.84(13)
0.0394(18)
0.289(68)
2.84848(19)
88
27
0.082
7601.7214(72)
2110.2892(24)
1671.8859(23)
0.1370(89)
0.129(27)
1.21(47)
0.0362(10)
0.390(43)
3.68467(18)
58
22
0.088
7617.069(96)
2107.7335(25)
1671.2108(25)
0.1434(96)
0.021(49)
2.73(87)
0.0331(25)
0.45(11)
3.71638(28)
a
b
Root-mean-square deviation.
Principal moments of inertia. Conversion factor: 505379.05 £ 10 220 m 2 u MHz.
[11] and 3.182 found for methyl vinyl ether (calculated from the entries of Table 1 in Ref. [5]). The last
two compounds have a heavy-atom skeleton and two
out-of-plane hydrogen atoms and thus Cs symmetry.
3.3. Vibrationally excited states
The ground state transitions were accompanied by
series of transitions presumably belonging to vibrationally excited states of Syn. Four excited states
belonging to three different normal vibrational
modes were assigned in the same manner as described
for the ground vibrational state lines. The RFMWDR
method [13] proved very useful in the assignment
procedure. The spectroscopic constants obtained for
these three excited states are listed in Table 3.
The most intense excited state (Table 3) has about
53% of the intensity of the ground vibrational state at
about 238 K. Its frequency was determined to be
106(15) cm 21 by relative intensity measurements
made largely as described in Ref. [22]. This should
be compared to 100 cm 21 found in the B3LYP/ccpVTZ calculations (not given in Table 1) for the
C4±O6 torsional mode. The fact that Ia 1 Ib 2 Ic
increases upon excitation (Table 3) is typical for an
out-of-plane vibration [23]. This increase of
0.8036 £ 10 220 m 2 u (calculated from the entries in
Table 3) can be used to obtain an estimate of the
said torsional fundamental, as described in Ref.
[24]. A value of 84 cm 21 is found in this manner.
The ®rst excited state of another fundamental
(Table 3) was found to have about 28% of the intensity of the ground vibrational state at approximately
238 K. A frequency of 214(25) cm 21 was determined
by relative intensity measurements [22]. This mode is
assumed to be the lowest in-plane-bending vibration
because Ia 1 Ib 2 Ic decreases from about 3.350 to
2.848 £ 10 220 m 2 u upon excitation, which is similar
to what was found for the corresponding mode in
methyl vinyl ether [5]. The B3LYP/cc-pVTZ value
for this fundamental was 239 cm 21.
The ®rst excited state of what is assumed to be the
torsional vibration of the methyl group was also
assigned and a frequency of 214(30) cm 21 determined
by relative intensity measurements, compared to
194 cm 21 found in the B3LYP/cc-pVTZ calculations.
Our main reason for this assignment is the change
found for Ia 1 Ib 2 Ic upon excitation. This change
is calculated from the entries in Table 3 to be
0.3339 £ 10220 m 2 u, almost the same (0.3066 £
10 220 m 2 u) as found in Ref. [5] for the corresponding
excited state of the methyl group torsional vibration in
methyl vinyl ether.
The transitions belonging to this excited state were
scrutinized for splitting caused by tunneling of the
methyl group. However, no such splittings were
26
J.A. Beukes et al. / Journal of Molecular Structure 567±568 (2001) 19±27
observed, and it is estimated that they must be smaller
than about 0.50 MHz. Model calculations using our
computer program MB10 [25] indicate that the barrier
to internal rotation of the methyl group is larger than
about 14 kJ mol 21. The barrier in methyl vinyl ether is
as high as 15.0 kJ mol 21 [8].
The fourth excited state shown in Table 3 is believed
to be a combination mode where both the C4±O6 and
the lowest in-plane-bending vibration are excited by one
quantum each. The rotational constants found for this
excited state agree well, but not completely with those
calculated from the relation X1 1 X2 2 X0, where X1
and X2 represent the rotational constants of the two
excited states, and X0 represents the ground vibrational
state. Relative intensity measurements yielded
330(50) cm 21 for this state.
3.4. Dipole moment
The dipole moment was determined in the standard
way [26] using Stark ®eld strengths in the 500±
1500 V cm 21 range. The results are shown in Table 4.
Comparison of the experimental total dipole
moment in this table with the calculated one in
Table 1 shows that the MP2 value is about 12% too
large, the B3LYP/6-31111G pp total dipole moment
is almost correct, whereas the B3LYP/cc-pVTZ value
is approximately 10% too low. The rather good agreement between the calculated and the observed rotational constants are additional evidence that Syn has
indeed been assigned, and not confused with Anti
which would also have a symmetry plane and two
out-of-plane hydrogen atoms and consequently a
similar value of Ia 1 Ib 2 Ic. However, the dipole
moments would be rather different according to the
B3LYP/cc-pVTZ computations (Table 1).
3.5. Searches for the second form
All the strongest lines of the spectrum have been
assigned to Syn. This conformer is thus undoubtedly
preferred by 2-methoxyfuran. However, the three
quantum chemical computations (Table 1) all predict
that a second form exists with a somewhat (2±
4 kJ mol 21) higher energy. This rotamer is predicted
to have a smaller dipole moment than Syn. Extensive
searches for it have been made. Two procedures were
used. In the ®rst of these, the rotational constants and
dipole moment components of this form given in
Table 4
Stark coef®cients and dipole moment of Syn of 2-methoxyfuran
(Uncertainties
represent
one
standard
deviation.
1
Debye ˆ 3.33564 £ 10 230 C m.)
Transition
uMu
Dn E 22/10 25 (MHz V 22 cm 2)
Observed
102,8 Ã 101,9
Calculated
10
9
8
7
2.09(3)
1.83(3)
1.61(3)
1.39(2)
2.16
1.88
1.64
1.42
92,7 Ã 91,8
9
8
7
6
5
4
2.30(3)
1.69(2)
1.08(2)
0.612(8)
0.188(3)
2 0.143(3)
2.23
1.61
1.06
0.577
0.174
2 0.156
82,6 Ã 81,7
8
7
6
5
4
2.39(3)
1.81(3)
1.23(2)
0.791(9)
0.421(6)
2.40
1.79
1.26
0.806
0.439
Dipole moment/10 230 (C m)
ma ˆ 3:62…16†
mb ˆ 5:98…3†
a
mc ˆ 0:0 a
mtot ˆ 6:99…12†
For symmetry reasons; see text.
Table 1 were used to predict its strongest transitions.
These lines were searched for amongst the unassigned
lines using ordinary Stark spectroscopy. The
RFMWDR method [13] was used in the second procedure. This method is very effective provided that the
compound has a signi®cant m a dipole moment component. This would be the case for near-anti conformations (see Table 1). Rather extensive RFMWDR
searches were performed, however, with negative
result. It is concluded from a study of the intensities
of unassigned lines which may or may not belong to
the second form, that Syn is at least 3 kJ mol 21 more
stable than any hypothetical second rotamer.
3.6. Structure
The observed (Table 3) and the calculated (Table 1)
rotational constants of Syn agree to within better than
1% in the case of the MP2 computations. A slightly
poorer agreement is seen for the B3LYP calculations.
It is believed that this is not fortuitous, but in fact
re¯ects that both the elaborate MP2 and as well as
J.A. Beukes et al. / Journal of Molecular Structure 567±568 (2001) 19±27
the B3LYP structures are rather accurate in this case.
The MP2 structure (Table 1) is suggested as aplausible structure for Conformer I, because it has been
shown [20] that such calculations are capable of
predicting accurate structures. It is expected that any
full experimental structure that is determined in the
future will be very close to the MP2 structure shown
in Table 1.
4. Conclusions
This study has demonstrated that gaseous 2-methoxyfuran prefers the Syn conformation which is at
least 3 kJ mol 21 more stable than any second rotamer.
The reason why Syn is preferred is presumably quite
complex. Delocalization of the lone pair electrons of
the oxygen atom into the p electron system of the ring
and repulsion between the same lone pair and the
electrons of the C2±C4 double bond are likely to be
the most important factors for the conformational
makeup of this compound.
Accurate predictions of the structure are found in
the MP2/6-31111G pp calculations, as well as in the
B3LYP/6-31111G pp and B3LYP/cc-pVTZ computations. The MP2 structure is suggested as a plausible
structure of the Syn rotamer.
Acknowledgements
Mrs. Anne Horn is thanked for the art work. This
work has received support from The Research Council
of Norway (Programme for Supercomputing) through
a grant of computer time.
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