247
Journal of Molecular Structure, 18 (1973) 247-256
~ Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
MICROWAVE SPECTRUM OF METHOXYACETIC ACID:
ASSIGNMENT OF THE HYDROGEN-BONDED ROTAMER
K.-M.
MARSTOKK
Department
AND HARALD
of Chemistry,
M0LLENDAL
The University
of Oslo, Blindern,
Oslo 3 (Norway)
(Received 26 February 1973)
ABSTRACT
Microwave spectra of CH3OCHzCOOH and CH3OCHzCOOD are
reported. One conformation has been assigned. This form of the molecule has
a planar HCOCCOOH skeleton with four out-of-plane hydrogens. A weak
five-membered intramolecular hydrogen bond is forrned between the hydroxyl
proton and the ether oxygen thus stabilizing the plan ar form. Absolute intens ity
measurements and arguments based on a few reasonable assumptions have been
used to show that the assigned rotamer is present at concentrations amounting
to between 10 and 30 per cent of the total. Other forms are not identified. Seven
vibrationally excited states were assigned and attributed to the three lowest
torsional modes. The dipole moment was determined to be Pa = 4.72::1:0.04D,
Pb = 0.15::1:0.02D, and Ptotal= 4.72::1:0.04D.
INTRODUCTlON
For a molecule as complicated as methoxyacetic acid, CH3OCHzCOOH,
rotation around the three C-O and the one C-C bonds may give rise to a large
num ber of distinct conformers. Figure l shows four arbitrarily selected forms
which are interchangeable by rotation about the C-C and hydroxyl bonds with
the CH3OCHz- gro up fixed. Conformation I differs from the other three rotamers
in that an intramolecular hydrogen bond is formed between the hydroxyl hydrogen
atom and the ether oxygen atom whereas hydrogen bond stabilisation is not
possible in the other three cases. Rotamers other than l-IV may of course be
possible.
The conformational properties of methoxyacetic acid have previously been
studied by Oki and Hirota 1. U sing infrared spectroscopy they found that in dilute
248
carbon tetrachloride solutions two rotamers exist, one being a hydrogen-bonded
form while the second was shown to have a normal carboxyl group with the
hydroxyl hydrogen atom between the carboxyl group oxygens as in forms Il-IV
of Fig. l. The enthalpy difference between these two conformers was found to
favour the non-hydrogen-bonded form by 0.76 kcal mole -1, but an unambiguous
entropy difference could not be obtained 1.
The present work was undertaken to study the conforrnational behaviour
of free methoxyacetic acid, the simplest member of a large family of (X-oxyacids.
Particular attention was paid to the influence of intramolecular hydrogen bonding
on the se molecular properties. We have been able to assign one rotamer with an
intramolecular hydrogen bond similar to form I in Fig. 1. There is also considerable evidence in the microwave spectrum for the coexistence of one or more additional forms left unassigned.
EXPERIMENT
AL
Methoxyacetic acid purum was purchased from Fluka AG, Buchs, Switzerland. The commercial product was purified by gas chromatography before use.
CH3OCH2COOD was produced by direct exchange with 99 per cent D20 in
the cell. Fast scan broadband spectra in the 26.5-40 GHz range were recorded on
a Hewlett-Packard 8460A spectrometer and extensive studies were carried out in
the 21.9-32 GHz spectral region on a conventional microwave spectrometer
described in ref. 2. Other spectral ranges were also surveyed briefly. Measurements
were made at room temperature or at about + 5°C, the molecule has toa low a
vapour pressure to allow a microwave study at lower temperatures. Measurements
V
"1h"
00
"I
CH3 "H
8"°°
I
"~,.
VCH3
o
I
H
8"60°
Il
SH3
"(j)""'
Ho
I
H
8"120°
!Il
Jj)"o
I
H
8"180°
IV
Fig. 1. Newman projections offour possible conformations ofCH,OCH2COOH.
The projections
are viewed along the C-C bond. The four rotamers are interchangeable by rotation around the
C-C and hydroxyl bonds.
249
Werecarried out with apparent vapour pressures between 10 and 60 microns. The
compound decomposed slightly in the brass sample cells to give methanol as one
of the decomposition products.
MICROWAVE
SPECTRUM
AND ASSIGNMENT
OF THE GROUND
VIBRATIONAL
STATE
Preliminary rotational constants of several pro bable rotational isomers of
methoxyacetic acid were computed by combining structural parameters taken
from related compounds and varying the appropriate dihedral angles. Typical
structural parameters used are listed in Table 1. All pro bable rotamers were found
to be near-symmetric prolate rotors. Calculations perforrned by the CNDOj2
method 3.4 yielded small energy differences between the individual rotamers.
These were typically about 0.5 kcal mole-l. Of the four farms shown in Fig. l,
rotamer I was computed to be a few hundred calories more stable than the others.
TABLE I
PLAUSIBLE STRUCTURAL
PARAMETERS", OBSERVED AND PREDICTED
ROTATIONAL
CONSTANTS
KRAITCHMAN'S COORDINATES OF THE HYDROXYL HYDROGEN OF METHOXYACETIC ACID
CO
C-Oethe<
C-OhYd,Oxy!
C-C
O-H
C-H
1.228 Å
1.415 Å
1.317 Å
1.510 Å
1.030 Å
1.093 Å
L CCO
LCOC
LCCO
LCOH
LCCH
LHCH
LOCH
Rotational constants (MHz)
CH3OCH2COOH
Ao
Bo
Co
Observed
9409.08
2088.817
1748.884
Calculated
9811.257
2087.266
1759.363
CH3OCH2COOD
Ao
Bo
Co
Kraitchman's
laHI
[bHI
8999.9
2088.627
1734.114
9432.750
2087.256
1746.788
coordinates of the hydroxyl hydrogen
Observed
0.143 Å
1.566 Å
a Not a deri ved structure.
See text.
Calculated
0.033 Å
1.441 Å
124.0°
112.0°
110.0°
104.0°
109.5°
109.5°
109.5°
AND
N
V1
O
T ABLE 2
MOLECULAR
CONSTANTS FOR CH3OCH2COOH'
AND CH3OCH2COODb
Conversion facto r 505376 UA2 MHz. The uncertainties represent one standard deviation. For the attributions
Vibrational
Number
state
of transitions
Ground'
First ex. C-C tors.'
31
0.100
a (MHz)
Second ex. C-O tors.'
20
to the fundamental
First ex. C-C torso+.first ex. C-O tors.'
7
0.225
7
0.129
0.343
AvCMHz)
Bv(MHz)
Cv(MHz)
I,(uA 2)
Ib(uA2)
Ic(uA2)
I,+Ib-Ic(uA2)
9409.08
::!:0.79
9384.9
::!:1.2
9173.4
::!:5.0
9309.8
::!:4.6
2088.817
::!:0.006
2090.167
::!:0.008
2091.309
::!:0.037
2091.096
::!:0.024
1748.884
::!:0.006
1753.505
::!:0.008
1754.582
::!:0.037
1756.712
::!:0.024
55.091
::!:0.030
54.284
::!:0.027
Second ex. C-C tors.'
Third f'X. C-C tors.'
53.7115 ::!:0.0045
53.8499 ::!:0.0069
241.9436 ::!:0.0007
241.7873 ::!:0.0009
241.6553 ::!:0.0044
241.6800::!:0.0028
288.9706 ::!:0.001O
288.2090::!:0.0013
288.0321 ::!:0.0061
287.6829 ::!:0.0039
6.6845 ::!:0.0047
7.4282::!:0.0071
13
0.141
8
0.189
First ex. C-O tors.'
8.714
::!:0.030
First ex. CF/3 tors.'
13
0.189
8.281
::!:0.027
Groundb
6
0.261
9
0.121
9355.2
::!:1.6
9331.5
1:2.5
9337.0
::!:2.0
9325.7
::!:5.1
8999.9
2092.124
::!:0.01O
2094.735
::!:0.017
2090.145
::!:0.015
2088.348
::!:0.031
2088.627
::!:0.01O
1758.097
::!:0.01O
1763.044
::!:0.017
1752.039
::!:0.014
1748.055
::!:0.030
1734.114
::!:0.012
54.158
::!:0.015
54.126
::!:0.012
54.192
::!:0.030
54.0209 ::!:0.0093
::!:1.2
56.1535 ::!:0.0077
241.5612::!:0.0012
241.2601 ::!:0.0020
241.7899 ::!:0.0018
241.9980 ::!:0.0036
241.9657 ::!:0.0012
287.4563 ::!:0.0017
286.6497 ::!:0.0029
288.4502::!:0.0024
289. 1O76::!:0.005 1
291.4318::!:0.0020
8.1258 ::!:0.0095
8.768
::!:0.015
7.466
::!:0.012
7.082
::!:0.030
modes, see text.
6.6874:1: 0.0080
251
The calculated dipole moments and their components along the principal axes of
the different forms were found to differ considerably. Conformer I, for example,
was computed to have a dipole moment of about 5 D predominantly along the
principal a-axis, whereas IV was calculated to have approximately a dipole moment
of 3 D mainly along the b-axis. Forms Il and III were computed to have roughly
dipole moments of 2 D and 2.5 D, respectively, with sizeable components along
all three principal inerti al axes.
Fast scan broadband spectra of methoxyacetic acid revealed one remarkably
weak series of the characteristic a-type R-branch high K-l pileups at about
every B + C. A preliminary value of B + C was immediately determined to be about
3.86 GHz, dose to the expected value for the hydrogen-bonded form as shown in
Table 1. High resolution spectra were then studied for the J = 5 -> 6, 6 -> 7,
and 7 -> 8 transitions. The characteristic Stark effects and positions in the spectrum
as well as the relatively high intensities of the K-l = Oand K-l = l lines led to
a rapid assignment of the ground vibrational state lines. The lines are weak,
narrow and unsplit and were fitted to a rigid rotor spectrum with the results shown
in Table 2. The list of measured frequencies is available from the authors or from
the microwave Data Center at the National Bureau of Standards, Washington,
D.C., D.S.A., where it has been deposited. A Watson eight parameterS, first-order
centrifugal distortion analysis of the se lines was carried out and showed that
centrifugal distortion is very small for these transitions. Indeed, significant values
of the five determinable centrifugal distortion coefficients could not be obtained.
As shown in Table 2, la+ lb - le = 6.6845:t 0.0047 uÅ 2. This is dose to
similar values found in molecules possessing a symmetry plane and four out-ofplane sp3-hybridized hydrogen atoms6. For a completely rigid molecule of this
type la+ lb - le is about 6.40 uÅ 2. The increase of approximately 0.28 uÅ 2 found
for methoxyacetic acid is probably caused by the low-frequency torsional modes 7.
VIBRATIONAL
SATELLITE SPECTRA
The ground vibrational state lines were accompanied by a rich and, compared to the ground state transitions, intense satellite spectrum. Most of these
R-branch absorption lines occur at higher frequencies with respect to the ground
state transitions. All satellites are narrow and unsplit to within a resolution of
better than 0.7 MHz and fit a rigid rotor spectrum. As shown in Table 2, these
vibrationally excited states have been assigned to three different torsional fund a-
mentals. These assignments have been made because la+ lb - le increases upon
excitation. The torsions around the C-C and the H2C-O bonds are expected to
be the two lowest vibrational modes. On the basis of the microwave data it is
difficult to distinguish between these two fundamentals
and the assignments
presented in Tables 2 and 3 are thus tentative and may be interchangeable.
252
TABLE 3
STARK COEFFICIENTS
AND DIPOLE MOMENT OF METHOXYACETIC
ACID
The uncertainties represent one standard deviation. The standard deviations of the dipole
moments and its components along the principal axes obtained from the least squares tit were
half the values given in this table.
Transition
ilvfE2[MHzf(Vfcm)2]
Observed
Calculated
-2.60
-2.12
-1.93
51.4-+61.5
M = O
-2.60::1::0.06
61,6-+71.7
M = O
-2.10::1::0.08
M = O
-1.95
61,5-+71,6
x 106
::1::0.06
fl- = 4.72::1::0.04 D
flb = 0.15::1::0.02 D
flto!_l = 4.72::1::0.04D
Attempts to make high-precision relative intensity measurements to determine
these fundamental frequencies proved futile, because of the low absolute intensities
of the lines. However, a crude estimate of the frequencies has been obtained from
the expression7: ro (cm-l) = 67.5/b., where b. = laY+1+lbY+1-I/+1-(laY+
IbY- leV).In this way, 91 cm -1 and 86 cm -1 were found for the C-C and C-O
fundamentais, respectively. These values are probably accurate to within 30
per cent.
The intensities of the successively excited states of these two torsional
modes were found to decrease steadily. The variations of the corresponding
rotational constants are also fairly regular, and it is therefore concluded7 that the
equilibrium form essentially has a plan ar HCOCCOOH skeieton.
The first excited state of the methyl torsion is also believed to be assigned
(Tab le 2). From the variation of the inerti al defect, a frequency of 170 cm -1 was
calculated. Within the expected uncertainty limits, this value agrees with 203 cm-1
calculated for the lowest torsional mode in dimethylether8 where a barrier to
internal rotation of 2.72 kcal mole -1 has been determined9. The barrier to methyltorsion in methoxyacetic acid should thus be of the same order of magnitude as
in dimethylether. No splittings could be resolved for the very weak transitions
assigned to this mode indicating a barrier height of more than l kcal mole - 1.
The above assignments included all the strongest lines present in the spectrum. There were, however, a num ber of very weak lines of uncertain origin left
unaccounted for. These might belong to further conformers, but despite a thorough
search no assignments could be made. It was very unfortunate that the low
volatility of the compound prohibited a study at very low temperatures where
absorption line intensities are much enhanced enabling assignments to be made
with greater ease.
-
253
H.H,bi
o
..
a
H
Fig. 2. Projection of assigned rotamer of methoxyacetic
MICROWAVE
SPECTRUM
OF
CH3OCH2COOD
acid in the a-b principal axes plane.
AND THE CONFORMATIONOF
METHOXY-
ACETIC ACID
The microwave spectrum of CH3OCH2COOD was studied to obtain
additional information about the structural and conforrnational properties of
the acid. A search was made initially for the low K-l a-type R-branch transitions
which were found within a few MHz of the predicted frequencies. The derived
spectroscopic constants are listed in Table 2.
The value of la + lb - le found for the deuterated speeies is almost identical
with the main species counterpart, confirming the assumption of a symmetry
plane. Without substitution of the hydrogens in the methyl group, its exact conformation cannot be determined but it must have either the conformation shown
in Fig. 2, or a form where the methyl group is rotated through 60° in order to
satisfy the symmetry plane condition. The methyl group conformation indicated
in Fig. 2 is similar to that found in dimethyletherl o.
The rotational constants of the main and the deuterated speeies furnish toa
little information for a detailed determination of bond lengths and angles. The
molecular mode! of Table l produces rotational constants which are dose to the
experimental values. The hydroxyl hydrogen coordinates determined from
Kraitchman's equationsll also show dose agreement to the molecular model
coordinates (Table l).
DIPOLE MOMENT
The microwave spectrum was too weak to allow a quantitative study of
individual Stark lobes, the normal method for the determination of dipole
moments and an alternative way was used. Calculation of the Stark coefficients
showed that for most K-l = l transitions the M-independentparts of the Stark
254
effect were much larger than the M-dependent ones. Therefore, at relatively small
field strengths, all Stark components will coincide for these lines, yielding sufficient
intensity to make quantitative measurements feasible. The M = OStark coefficients
were determined in this way for the transitions reported in Table 3. Stark splittings
in the 2-8 MHz range were measured by application of a d.c. voltage between
the Stark septum and the cell, with the modulating square wave voltage superimposed. The d.c. voltage was measured with a digital voltmeter having an
accuracy of 0.025 per cent. The electric field in the cell was calibrated using the
OCS l -> 2 transition with flocs = 0.71521 D (ref. 12). The observationswere
fitted by the least squares method employing a diagonal weight matrix with the
inverse squares of the uncertainties quoted in Table 3 as weights. The standard
deviations obtained in this way were doubled to take into account possible
systematic errors. The CNDOj2 result was 5.07 D in fair agreement with the
experimental value of 4.72::1::0.04D.
ABSOLUTE INTENSITY
AND THE CONFORMATIONAL
EQUILIBRIUM
It was shown in the infrared work1 that two forms of methoxyacetic acid
exist in dilute carbon tetrachloride solution. The unusually low intensities of the
microwave absorption lines indicate that a similar situation is probably present
in the gaseous state. In fact, a study of the absolute intensities of microwave transitions may in principle allow the determination of the fraction of the molecules
belonging to a particular rotamer. This information may the n be used to deduce
the thermodynamic functions of a conformational equilibrium. In the case of
methoxyacetic acid the following procedure was employed to obtain a crude, yet
quantitative, estimate of the fraction of molecules belong to the assigned form.
The peak absorption coefficient rx(in cm -1 ) of any asymmetric-top spectrum
line is given approximately by13
rx = 3.85xlO-14
2 2Å.
V flg (J F (ABC )t g e-EJ</kT
Tt(ilv)1
v
l
where Fv is the fraction of molecules in the particular vibrational state under
observation, A, B, and C are the rotational constants, T is the absolute temperature, v is the absorption frequency, flgis the dipole moment component (expressed
in Debye units) along the g principal inerti al axis, Å.is the line strength of the
transition under study, El,. is the rotational energy of the lowest state involved
in the transition, k is Boltzmann's constant, (ilV)l is the line breadth in MHz at
T = 300 oK and P = l torr, (J is the symmetry num ber, and gl is the reduced
nucIear weight.
The assigned conformation of methoxyacetic acid has Cs symmetry and
then (J, gl = l. Furthermore, for the a-type ilJ = + 1 transitions occurring in
--
255
the accessible spectral region e-EJ</kT= l to a good approximation.
v, Jlg and T
are also determined accurately. ()( was measured for the 61.5 --+ 71, 6 ground
vibrational state transition at ro om temperature and found to be (7::!:2) x 10- 8
cm - 1. The speetrometer was calibrated against values reported for OCS absorption lines 14. The rather large uncertainty arises from the use of a speetrometer
not designed for making accurate intensity measurements. As no measurements
could be conducted the line breadth (AV)l was estimated to be 8 times the di pole
moment15. Fo consists of two terms, viz. the fraction of molecules belonging to
the assigned form multiplied by the fraction resulting from the ordinary Boltzmann
distribution among the individual vibrational states of this particular rotamer.
Assuming harmonic vibrational modes, illi' the latter part of Fo is given by16:
F~ = il (l-e-hWilkT)
(2)
i
for Cs molecules with the fundamental vibrational frequencies dividing into the
non-degenerate a' and a" speeies. No normal coordinate analysis has been reported
for this molecule and an exact value for F~ cannot be computed. However, the
two lowest modes, each around 90 cm - 1, completely dominate in F~. Assuming
reasonable values for the rest of the fundamental frequencies expected to occur
below 700 cm -1 and varying the two lowest torsional modes within liberallimits,
it was found that the assigned conformation is present at concentrations between
10 and 30 per cent of the total. Most estimates fell in the 10-20 per cent range.
If there is essentially one additional form (e.g. another rotamer with a
symmetry plane) with a statistical weight of l, the energy difference is calculated
to be between0.5 and 1.3 kcal mole-1, favouring the unassigned rotamer. A non
planar form with a statistical weight of 2 is likewise found to be more stable by
0.1-0.9 kcal mole -1. In conclusion it can be said that these estimates compare well
with the more accurate and less indirect infrared value1 of 0.76 kcal mole -1.
DISCUSSION
The possibility of forming a hydrogen bond between the hydroxyl hydrogen
atom of the carboxyl group and the ether oxygen atom of methoxyacetic acid is
probably the most important reason why the assigned conformation is realized.
This five-membered hydrogen bond arrangement is entirely plan ar. The electron
cloud of the hydroxyl bond thus overlaps equally with both, essentially Sp3hydridized, lone electron pairs of the ether oxygen. Non-planar conformations
where the ether and acid parts of the molecule are twisted to allow a maximum
overlap between only ane of the lone electron pairs of the ether oxygen and
hydroxyl hydrogen atom do not appear to contribute to a noticeable extent.
Models of these forms indicate that there are probably no steric conditions prohibiting their formation and their low populations cannot be ascribed to this
cause.
-~-
256
With the model of Table l, a number of the geometrical parameters of the
hydrogen bond have been calculated as r(O'''O) = 2.49 Å, r(O"'H) = 1.71 Å,
LO-H.'.O = 129°.The former two distances are about 0.3 and 0.9 Å, respectively,
shorter than the sums of the pertinent van der Waals' radii 1 7. The angles between
the O-H and the two C-O bonds of the ether part of the molecule were computed
to be 146° and 34°, respectively, and the angle between the former bond and the
bisector of the COC angle was found to be 56°. These values are hopefully correct
within a few degrees and a few hundredths of an ångstr6m.
With only one of the rotamers assigned, little can be said about the exact
shape of the potential function governing the conformational equilibrium. The
low frequency out-of-plane fundamentals suggest that one barrier maximum
occurs at rather low values of the torsional angle () with an energy not expected
to exceed 2 kcal mole -1 above the identified form.
ACKNOWLEDGEMENT
We thank fil. kand. Hasse Karlsson of the University of Gothenburg,
Sweden, for measuring the broad band spectra and Miss Gerd Teien for gas
chromatography of the samples.
REFERENCES
1 M. OKI ANDM. HIROTA,Bull. Chem. Soc. Jap., 33 (1960) 119; ibid., 34 (1961) 374; ibid., 36
(1963) 290.
2 K.-M. MARSTOKKANDH. MØLLENDAL,J. Mol. Structure, 5 (1970) 205.
3 J. A. POPLEAND D. L. BEVERIDGE,Approximate Mo/ecu/ar Orbita/ Theory, McGraw-HiII,
New York, 1970.
4 J. A. POPLEANDG. A. SEGAL,J. Chem. Phys., 43 (1965) S136.
5 J. K. G. WATSON,J. Chem. Phys., 45 (1966) 1360.
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New York, 1967.
7 D. R. HERSHBACH
ANDV. W. LAURIE,J. Chem. Phys., 40 (1964) 3142.
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Amer. J. Phys., 21 (1953) 17.
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14 P. KISLlUK AND C. H. TOWNES,Mo/ecu/ar Microwave Spectra Tab/es, National Bureau of
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15 W. GORDY, W. V. SMITHAND R. F. TRAMBARULO,Microwave Spectroscopy, Wiley, New
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