259
Journal of Molecular Structure, 16 (1973) 259-270
i&JElsevier Scientific Publishing Company, Amsterdam
- Printed
in The Netherlands
MICROW AVE SPECTRA OF ISOTOPIC GLYCOLALDEHYDES, SUBSTITUTION STRUCTURE, INTRAMOLECULAR
HYDROGEN BOND
AND DIPOLE MOMENT
K.-M.
MARSTOKK
Department
AND HARALD
of Chemistry,
M0LLENDAL
The University
of Oslo, Blindern,
Oslo 3 (Norway)
(Received 6 November 1972)
ABSTRACT
The microwave spectra of 13CH2OH-CHO, CH2OH-13CHO, and
CH2OH-CH180 are reported and have been used in combination with previously
published data on other monosubstituted glycolaldehydes to determine the substitution structure of the molecule as r(C~O) = 1.209 A, r(C-O) = 1.437 A,
r(C-C) = 1.499 A, r(O-H) = 1.051 A, r(C-Hald) = 1.102 A, r(C-Hale) = 1.093
A, r(O... H) = 2.007 A, r(O... O) = 2.697 A, L(C-C~O) = 122°44',
L(C-C-Hald) = 115°16',
L(C-C-O) = 111°28',
L(C-O-H) = 101°34',
L(C-C-H.1J = 109°13',
L(H-C-H) = 107°34', L(O-H. . . O) = 120°33',
L(H. . . O~C) = 83°41', and L (O-H, C~O) = 24°14'. The intramolecular
hydrogen bond and the other structural parameters are discussed and compared
to related molecules. The dipole moment is redetermined to be Jla = 0.262:1:0.002
D, Jlb = 2.33:1:0.01 D, and Jltot = 2.34:1:0.01D. Relativeintensity measurements
yielded 195:1:30cm-l for the c-c torsional fundamental and 260:1:40 cm-l for
the lowest in-plane skeIetal bending mode. Computations performed by the
CNDO/2 method correctly predict the observed cis hydrogen-bonded conformer
to be the energetically favoured one and in addition yield some indication of the
existence of at least two other non-hydrogen-bonded forms of higher energy.
INTRODUCTION
The structural properties of glycolaldehyde in various states of aggregation
have in recent years been the subject of several investigations. Michelsen and
Klaboe1 studied the molecule in the vapour and crystalline phases and as a melt
260
using infrared and Raman spectroscopy. Thom2 has maae solid state X-ray
diffraction studies. His work is now being continued by Dr. Berit F. Pedersen 3.
Proton magnetic resonance examinations of glycolaldehyde in solution have been
carried out by several workers4-6. Moreover, Zeeman studies 7 and mass spectrometric8 investigations have been reported.
In the vapour phase glycolaldehyde has been shown by classical methods9
as well as by infraredt, microwave spectroscopic10,11, and mass spectrometric8
techniques to be a monomeric et-hydroxyaldehyde. The infrared1 and microwave
studies10, 11 revealed that the free molecule has a planar HOC-CHO skeleton
with two out-of-plane hydrogens, and that the carbonyl and hydroxyl groups are
cis to each other allowing a five-member intramolecular hydrogen bond to be
formed. It was shown that the observed conformation is the dominating one1°, but,
unfortunately, the rotational constants of glycolaldehyde and four deuterated
species of the molecule were found to be insufficient to uniquely determine a
complete molecular ro structure11. This was probably caused by the fact that no
rotational constants of heavy-atom isotopic species were available for the structure
determination. In the present work, the spectra of 13CH2OH-CHO, CH2OH13CHO, and CH2OH-CH180 are presented, and the rotational constants of these
isotopic species have been used in combination with four previously determined
sets of rotational constants to determine the complete rs structure. This accurate
structure is considered to be of interest because the et-hydroxyaldehyde group is
a constituent of many molecules. Several of these compounds are important biochemicals, e.g., the carbohydrates or "sugars" of which glycolaldehyde is the
simplest possible member.
EXPERIMENTAL
CH2OH-CH180 was produced by dissolving commercial glycolaldehyde in
about 20 per cent enriched 180-water. After several days the water was distilled
off at reduced pressure. The mass spectrum indicated that glycolaldehyde had been
about 15 per cent enriched in 180. The microwave spectrum showed that only the
oxygen of the carbonyl group had been appreciably exchanged with 180. The
existence of CH2180H-CHO was not detected in the microwave spectrum despite
careful searching, and we feel quite sure that concentrations
exceeding 5 % of the
total of this species would have been identified if present. Unfortunately, no
economically feasible route to its synthesis is known to the authors. The
13CH2OH-CHO and CH2OH-13CHO species were measured in natural abundance (about l %). A conventional microwave spectrometer described briefly in
ref. 10 and a Hewlett-Packard 8460 A spectrometer were employed. Measurements
were performed at room temperature in the 12.4-36.3 GHz spectral region.
261
MICROW A VE SPECTRA AND ASSIGNMENTS
The assignment of the strong spectrum of CHzOH-CH180 was made in a
way analogous to that reported for the main 10 and the deuterated species11. The
spectrum is shown in Table l, and the derived molecular constants obtained by
least squares fitting the measured frequencies to Watson's eight parameters formula12 using the perturbation expression and computer programme MB071O,
are presented in Table 2.
TABLE 1
MICROWAVE
SPECTRAOF 13CH2OH-CHO,
Transition
CH2OH-'3CHO
Observed
freq.
(MHz)
13CH2OH-CHO
O
O
O
1
O
1
1
O
1
2
1
1
3
O
3
3
3
1
4
1
3
5
1
4
7
1
6
8
1
8
1
1
2
2
3
4
4
5
7
8
1
1
1
2
1
O
2
2
2
2
1
O
2
O
2
4
2
3
5
6
23049.88"
13203.87
32895.99
35063.83
17745.89
35871.56
31874.22
30992.77
32844.81
36194.18
CH2OH-'3CHO
O
O
O
1
O
1
1
O
1
2
O
2
2
1
1
2
1
2
3
O
3
3
1
2
3
1
3
4
O
4
4
1
3
4
2
3
5
O
5
5
1
4
6
O
6
6
1
5
6
3
4
8
1
7
1
1
2
2
2
3
3
3
4
4
4
5
5
5
6
6
7
8
1
1
1
1
2
O
1
2
O
1
2
1
1
2
1
2
2
2
1
O
2
1
O
3
2
1
4
3
2
4
4
3
5
4
5
6
23183.93"
13334.85
33033.02
15025.39
35503.12
23245.00
17815.65
33838.45
35688.61
21958.14
32301.92
29895.80
27660.07
31374.90
34967.4 7
31498.37
27761.12
36222.39
ANDCH2OH-CH'80
Obs.-calc.
(MHz)
Centr.
corr.
(MHz)
-0.09
-0.02
-0.05
0.15
-0.07
0.12
-0.06
0.12
0.08
-0.02
-0.10
-0.13
0.06
0.12
-0.10
-0.08
0.03
0.01
-
-
0.06
0.10
0.12
0.13
1.03
1.07
0.33
0.53
2.47
0.89
0.01
5.59
2.11
0.32
4.19
0.02
14.64
4.85
262
TABLE 1 (continued)
Transition
CHzOH-CH'80
O
O
1
O
1
O
2
O
2
1
2
1
3
O
3
1
3
1
4
2
5
1
6
O
6
1
6
3
8
1
8
3
12
6
12
6
12
5
14
7
14
7
14
6
a
:1:0.25MHz.
O
1
1
2
1
2
3
2
3
3
4
6
5
4
7
5
6
7
7
7
8
9
1
1
2
2
2
3
3
3
4
5
5
6
6
7
8
9
11
11
13
13
13
15
1
1
1
1
2
O
1
2
O
1
2
1
2
2
2
2
7
7
4
8
8
5
1
O
2
1
O
3
2
1
4
4
3
5
4
5
6
8
5
4
10
6
5
10
ObservfJd
freq.
(MHz)
(MHz)
Centr.
corr.
(MHz)
22865.52b
13308.62
32422.46
14900.37
35660.02
22051.87
17517.04
34051.49
34124.11
27459.99
31512.75
33570.38
31439.34
24138.11
35374.16
25033.62
28995.11
29004.08
34035.29
31717.86
31719.75
34830.33
-0.01
0.11
-0.08
0.04
-0.02
0.02
0.05
0.07
-0.06
0.09
0.14
-0.11
0.10
0.00
-0.10
-0.14
-0.08
-0.07
0.10
0.13
-0.03
-0.03
0.01
0.03
0.03
0.07
0.19
0.69
0.27
0.28
1.52
4.51
1.12
4.46
0.82
- 13.06
3.99
7.03
9.78
9.71
- 54.27
18.24
18.21
-103.56
Ob,. --ca/c.
-
b :1:0.10 MHz.
TABLE 2
MOLECULARCONSTANTSFOR ISOTOPIC GLYCOLALDEHYDES
Species
Number of transitions
a
(MHz)
CHzOH-CHoa
49
0.089
CHzOD-CHOb
65
0.071
CHzOH-CDOb
38
0.068
A
B
C
18446.410:1:0.026
6526.042 :1:0.008
4969.274:1:0.012
27.39698 :1:0.00004
77.46748 :1:0.00009
101. 70017 :1:0.00025
- 27.14:1:0.54
-73.22:1:0.72
88.3 :I: 1.4
5.648 :1:0.096
-3.40:1:0.12
3.13671 :1:0.00027
17490.806:1:0.015
6499.754:1:0.006
4882.969:1:0.008
28.89381 :1:0.00002
77.75306:1:0.00007
103.49762:1:0.00017
- 23.83 :1:0.61
-65.04:1:0.48
84.2 :I: 1.1
4.87 :1:0.13
-4.137:1:0.094
3.14920:1:0.00019
17151.310:l:0.Q31
6362.975 :1:0.021
4779.018 :1:0.010
29.46576 :1:0.00006
79.42443 :1:0.00021
105.74893 :1:0.00022
- 23.4 :I: 1.8
- 54.79 :1:0.66
61.4 :I: 1.3
8.28 :1:0.43
-2.32:1:0.11
3.14129:1:0.00031
(MHz)
(MHz)
(MHz)
la
(uA Z)
lb
(uAZ)
le
(uA Z)
dJ
(kHz)
dJK
(kHz)
dK
(kHz)
dWJ X 106
dWK X 106
la+lb-Ie
(uAZ)
Conversion factor 505376 MHz uAz. The uncertainties represent one standard deviation.
a Taken from ref. 10.
b Taken from ref. 11.
263
The assignment of the two 13C species, present in a total of about l per cent
concentrations, was quite difficult for the two following main reasons. Firstly, the
molecule has a fairly dense microwave spectrum with absorptions occurring every
few MHz throughout the entire microwave region. Secondly, only b-type transitions have sufficient intensity to be observed. Because of this, all intense transitions
depend heavily on the rotational constant A, whose absolute value is much more
difficult to estimate accurately a priori than in the case of B and C. In order to
assign the weak absorptions originating from the 13C species accurate estimates
of the rotational constants were essential to avoid excessive searching in the
spectrum. By considering reasonable structural models it was found that the
rotational constant C could be predicted within :t 3 MHz for both 13C species.
Plausible values of B were then chosen at intervals of 3 MHz and the corresponding
A's obtained from Ia+lb-Ie = 3.135 uÅ2 (refs. 10 and 11), which was expected
to hold within better than 0.5 % because the substitutions take place in the symmetry plane. The resulting sets of rotational constants were then used with Kraitchman's equations13 to calculate the substitution coordinates for the two carbon
atoms. From these coordinates and those already available for the other
substituted atoms, the C~O, C-Hald' C-C, and C-Hale bond lengths were computed and compared to the expected values of approximately 1.21 Å, 1.10 Å, 1.50
Å, and 1.09 Å, respectively. This procedure proved very useful and the B's and
A's could be predicted within :t 10 MHz and :t75 MHz, respectively. A search
was then made for low J transitions which were found within the expected frequency ranges. The assignments of these transitions were confirmed by their
CHDOH-CHOb
55
0.077
13CH2OH-CHO
10
CH2OH-'3CHO
18
0.086
CH2OH-CH'80
22
0.082
16987.977 ::1::0.031
6385.521 ::1::0.007
4843.832::1::0.017
29.74907 ::1::0.00005
79.14402::1::0.00009
104.33389 ::1::0.00037
-29.9::1:: 1.3
- 58.35 ::1::0.80
18126.88::1::0.10
6487.47::1::0.07
4923.03 ::1::0.05
27.87614::1::0.00015
77 .90029 ::1::0.00084
102.6554 ::1::0.0010
18259.524::1::0.051
6472.328::1::0.017
4924.556::1::0.020
27.67738::1::0.00008
78.08254::1::0.00021
102.62362 ::1::0.00042
-10.0::1:: 1.8
-313::1::53
-433::1::113
18087.032::1::0.026
6242.805 ::1::0.009
4778.491 ::1::0.012
27.94133 ::1::0.000
80.95331 ::1::0.000
105.76052::1::0.000
-17.1::1::1.6
-268::1::26
-396::1::64
73.4::1:: l. 7
6.38 ::1::0.28
3.45 ::1::0.30
43::1::9
-3.54::1::0.17
3.1210::1::0.0013
3.13630::1::0.00047
4.15::1::0.27
35.2::1::5.2
3.13413::1::0.000
---
264
intensities, characteristic modulation, Stark effects, which we were able to resolve
in a few cases, as well as their positions in the spectrum, and in the case of CH2OH13CHO, their fit to Watson's centrifugal distortion formula with the results shown
in Tables l and 2. Owing to several overlapping lines originating from the main
species, not enough transitions belonging to l3CH2OH-CHO were measured to
allow ameaningful centrifugal distortion analysis to be carried out for this species.
The rotational constants given in Table 2 were obtained by taking into consideration the probable centrifugal distortion contribution of the measured lines. Due
to the weakness of the l3C lines the spectral accuracy is no better than :!:0.25 MHz.
ENERGlES OF THE TWO LOWEST FUNDAMENTAL
MODES
Michelsen and Klaboel have made tentative assignments for gaseous glycol-
aldehyde of fifteen fundamental frequencies located above 400 cm-1. We have
now perforrned relative intensity measurements of the two lowest fundamentals
assigned in ref. 10. A value of 195:!:30 cm-l was obtained for the c-c torsional
mode of species a" and 260:!:40 cm -1 for the lowest in-plane skeietal bending
mode of species a'. These values were derived by averaging several relative intensity measurements perforrned on low J transitions following closely a procedure
prescribed by Esbitt and Wilson14.
The lowest in-plane fundamental of 260:!:40 cm -1 is in disagreement with
the observed frequency of 550 cm -1 suggested for this mode in the infrared workl.
A rough value of the C-C torsional fundamental may be calculated from
(J) (cm-l)
= 67.5/A (ref. 15) where A = I/+l+Ibv+l_Ic"+l-(Iav+lbv-IcV).
Substituting a value of 0.408 UA2 for A taken from ref. 10 a torsional mode of
165 cm-l is found. There is agreement between this rough estimate and 195:!:30
cm -1, obtained above by the relative intensity method.
The rotational constants of the second excited state of the c-c torsional
mode have now been determined as A = 18480.99:!:0.15MHz, B = 6439.60:!:O.10
MHz, and C = 4961.09:!:0.1O MHz. By comparing these rotational constants
to those of the ground and first vibrationally excited states of the same mode it is
seen that they vary almost linearly, indicating an essentially harmonic C-C torsional
frequency. In agreement with this, the intensities of the rotational transitions of
successively excited states of this mode were seen to decrease steadily.
DIPOLE MOMENT
Due to improved equipment we are now able to perform measurements of
the dipole moment more accurately than previously. A redetermination of the dipole
moment of glycolaldehyde has therefore been undertaken. A d.c. voltage was
265
app1ied 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 %. The electric field was calibrated using the OCS
1--+2 transition with Jlocs = 0.71521 D (ref. 16). Stark coefficients of the
00,0 --+11,1 at 23415.72 MHz, the 21,1 --+2z,0 at 35903.29 MHz, and the
40,4 --+41.3 transitions at 22143.02 MHz were used to determine the dipole
moment. Stark splitting rangings from about 5 MHz to about 35 MHz were
measured. The 00,0 --+11,1 and 40,4 --+41,3 transitions were found to have the
usua1 second order Stark effect within the measurement accuracy, whereas a
marked fourth order contribution was found for the IMI = 1 and IMI = 2 Stark
lobes of the 21,1 --+2z.0 transition. The second order coefficients for this transition
were determined by plotting !wjEZ versus EZ. The measured second order Stark
coefficients and their standard deviations are given in Tab1e 3. The theoretical
second order Stark coefficients were calculated by the computer programme
MB04 (ref. 17) using the Golden-Wilson formula18.
TABLE 3
STARK COEFFICIENTS
AND DIPOLE MOMENT OF GLYCOLALDEHYDE
The uncertainties represent one standard deviation. The standard deviations of the dipole moment
components obtained from the least squares fit were half the values given in this table.
Transition
!1V/E2 (IMHz/(V/cm)21 xlO5)
Observed
Calculated
00,0
-+ 11.1
M=O
2.54::1:0.04
2.53
21.1
-+ 22.0
M=l
M=2
1.36 ::1:0.03
3.62::1:0.07
1.34
3.64
40.4
-+ 41.3
M=3
M=4
1.48 ::1:0.02
2.60::1:0.02
1.47
2.61
!l. = 0.262::1:0.002
D
flb = 2.33
D
fltot = 2.34
::1:0.01
::1:0.01 D
A least squares fit using a diagonal weight matrix was performed. The
weights were chosen to be (J-z, where (Jis the experimentally determined standard
deviation of the second order Stark coefficients appearing in Table 3. From the fit,
Jla = 0.262 D and Jlb = 2.325 D with standard deviations of 0.001 D and 0.005 D,
respectively, were obtained. However, by taking into account possible systematic
errors, these standard deviations are probab1y too small, possibly by a factor of 2.
As the final result, Jla
=
0.262::1::0.002 D, Jlb = 2.33 ::1::0.01D, and Jltat = 2.34::1::0.01
D are given. The uncertainties quoted represent one standard deviation. The high
accuracy of Jla results mainly from the Stark effect of the 21.1 --+2z,0 transition
266
being strongly dependent on this dipole moment component. The present result
is at variance with our previous one10 now considered to be inaccurate. Calculations perforrned by the CNDOj2 (complete neglect of differential overlap)
method19, 20 yielded 2.45 D for the dipole moment in reasonable agreement with
the present experimental value.
STRUCTURE
Determination of the molecular structure was perforrned by the familiar rs
method21 which is ideally suited for glycolaldehyde because no atom is closer to
a principal axis than about 0.5 Å. The substitution coordinates presente d in Table 4
were, with the exception of the coordinates of 0(1) of Fig. 1, calculated employing
the effective moments of inerti a in Kraitchman's equations13 for a non-planar
asymmetric rotor using CH2OH-CHO as the parent molecule. The a and b
coordinates of 0(1) were computed from Lmpi = Oand Lmjbj = O,respectively.
The standard deviations shown in Table 4 are derived from the standard deviations
of the rotational constants. The resulting substitution structure presented in Table 5
was computed from the Cartesian coordinates of Table 4. The standard deviations
appearing in Table 5 were derived from the standard deviations of the substitution
coordinates shown in Table 4 and are thus only a measure for the precision with
which they are determined. A model of the molecule projected in the a-b principal
axes plane is shown in Fig. 1.
TABLE 4
SUBSTITUTION
COORDINATES
FOR GLYCOLALDEHYDE
CH2OH-CHO is the parent molecule. The uncertainties represent one standard deviation calculated from the standard deviations of the rotational constants.
Atom'
O(Ob
0(2)
C(l)
C(2)
H(l)
H(2,2')
H(3)
Principal
axes
a (A)
b (A)
-1.3586::1:0.0004
1.3388 ::1:0.0001
-0.6841 ::1:0.0003
0.8049 ::1:0.0003
-0.5485::1:0.0004
-0.9804::1:0.0002
1.3857 ::1:0.0001
0.5612::1:0.0005
0.5487 ::1:0.0001
-0.7072::1:0.0005
-0.5367::1:0.0003
1.2308 ::1:0.0001
-1.2811 ::1:0.0001
-1.4733 ::1:0.0001
c (A)
0.0
0.0
0.0
0.0
0.0
::1:0.8818 ::1:0.0002
0.0
'L.m,a;b, = 0.0297 uA2
1,0-1a' = -0.502
uA2
lo°-lb'
= 0.409 uA2
leo-le'
, See Fig. 1 for notation.
b
Coordinates calculated from 'L.m,aj= Oand 'L.mjb;= o.
= -0.410 uA2
267
b
0(1)
0(2)
a
H (3)
Fig. 1. Projection of stable conformer of glycolaldehyde in the a-b principal axes plane.
TABLE 5
SUBSTITUTION
STRUCTURE
OF GLYCOLALDEHYDE
The uncertainties represent one standard deviation. They are only a measure for the precision
with which the operationally defined substitution structure is obtained. See text.
Bonded
C~O
C-O
C-C
O-H
1.2094::1::0.0003
Å
1.4366::1::0.0007
Å
1.4987
Å
1.0510::1::0.0005
Å
C-H.ld
C-H.le
1.l021
Å
::1::0.0004
::1::0.0003
1.0930::1::0.0003
Å
L c-c~o
L C-C-HOld
L c-c-o
L C-O-H
L C-C-Haie
LH-C-H
LH-C-O
122°44'::1::2'
L O-H, . . O
L H . . . O-C
L O-H, C-O'
120°33'
115°16'
::1::2'
111°28'::1::2'
101 °34' ::1::2'
109°13'::1::1'
107°34'
::1::2'
109°39'::1::
I'
Non-bonded
O...H
0",0
2.0069::1::0.0004
Å
2.6974::1::0.0004
Å
::1::2'
83°41'::1::1'
24°14'
::1::2'
o Angle between the O-H and C-O bonds.
Possible deviations of the rs structure from the equilibrium structure were
estimated by increasing all substitution coordinates of Table 4 by (jx = 0.0012/x.
This empirical relation was obtained by Costain22 from available experimental
data. The estimated difference of the rs bond lengths from the equilibrium structure
are -0.003 Å, -0.003 Å, -0.003 Å, +0.002 Å, +0.001 Å, and -0.001 Å in the
268
C~O, C-O, C-C, O-H, C-Hald' and C-Hale bonds, respectively. The substitution
structure should therefore represent the equilibrium structure within about 0.005 Å.
Another effect that may influence the structure determination of glycolaldehyde deserves comment. Crystal studies2 3,24 have shown that for "strong"
X-H, . . y hydrogen bonds the X . . . y distance increases by between 0.001 Å to
0.04 Å upon replacement of H by D, but no change or a small contraction has
been observed for "weak" hydrogen bonds. Arecent microwave study of gauche
2-chloroethano125 has shown that there is probably no, or very little, change in
the pertaining O... Cl distance when hydrogen is replaced by deuterium.
Although the data for glycolaldehyde is a bit incondusive with regard to this
isotope effect, it is expected to be small because of the relative "weak" hydrogen
bond the observed conformation possesses. In keeping with this, no unusual effects
have been detected in the structural parameters of the molecule.
If the rs structure of Table 5 is compared to the partial ro structure ofref. 1]
it is seen that they agree within approximately 0.03 Å and 3°, or better.
As shown in Table 4 Iao-Ias
and Ieo-Ies are both negative presurnably
mainly as a result of the low frequency in-plane bending fundamental at
260:1::40cm-l.
DISCUSSION
It is evident from the microwave data that the most stable conformation of
glycolaldehyde is the one shown in Fig. 1. This form of the molecule is stabilized
by a five member intramolecular hydrogen bond which is very non-linear with
LOH' . . O = 120°33', r(H' . . O) = 2.007 Å, and r(O . . . O) = 2.697 Å. The
latter two non-bonded distances are about 0.6 Å and 0.1 Å, respectively, shorter
than the sums of the pertinent van der Waals' radii26. The angle between the H-O
and C~O bonds is 24°14' from being parallel which would probably be the most
favourable arrangement if the hydrogen bond was solely electrostatic in origin25.
LH' . . O = C is 83°41' or about 37° smaller than the pro bable direction of the
sp2-hybridized lone electron pair of the carbonyl oxygen. LCOH is about 7°
smaller and r(O-H) is approximately 0.1 Å longer than the corresponding structural parameters of methano127. The simultaneous O-H lengthening and the
decrease of LCOH of glycolaldehyde as compared to methanol result in a doser
proximity of the hydroxyl proton and carbonyl oxygen in the former molecule.
The rather long O-H bond is difficult to explain from a purely electrostatic model
of the hydrogen bond and probably means that some covalent character must be
invoked. However, this weak covalent effect does not manifest itselfto a noticeable
degree in the structural parameters of the carbonyl group, e.g., the rs values for the
C~O bond length and the C-C~O angle are very dose to the corresponding ro
parameters of acetaldehyde28, and there is really no significant difference.
269
The C-C and C-Raid bond lengths of glycolaldehyde are also very dose to
their counterparts in acetaldehyde28. A difference is found, however, in the C-o
bond length being 1.4246:t0.0024 Å in methano127 and 1.437 Å in glycolaldehyde,
but the latter value is not too different from the rs length of 1.431:t 0.003 Å found
in trans ethano129.
The strength of the hydrogen bond of glycolaldehyde and the stability of
the stable cis conformation of Fig. l is of considerable interest. Almenningen
et al. 30 have in arecent electron diffraction study evaluated the energy of the
.:I
hydrogen bond of gauche 2-chloroethanol to be 3.8 (+2.0, -1.0) kcal mole-1.
The O-R bond is about 0.04 Å shorter in this molecule25 than in glycolaldehyde,
and it is therefore probable that the energy of the hydrogen bond is higher in
glycolaldehyde than in gauche 2-chloroethanol. A hydrogen bond energy of 3-6
kcal mole-1 is therefore suggested for glycolaldehyde.
Conformers other than the cis form of Fig. 1 have not been found experimentally1O. In order to obtain indications of the existence of additional rotamers
and their energies the semi-empirical molecular orbital CNDO /2 method 19,2o
has been exploited. Glycolaldehyde constitutes a difficult problem for such calculations because rotation around the C-o and C-C bonds each may produee
distinct rotamers. In addition, other geometrical parameters of the molecule may
change as the dihedral angles are varied. A complete mapping of the resulting
potential surface is thus an extremely lengthy process and has only partially been
carried out. Our calculations showed that the method correctly predicts the
observed conformation to be the energetically favoured form of the molecule.
Indications were obtained that conformers I and Il of Fig. 2 are stable and of
1.4kcal mole-1 and 1.6kcal mole-1, respectively,higher energythan the observed
cis form. If these indications were correct, there should be approximately 10 %of
I and 7 %of n, respectively, present at room temperature if Boltzmann distribution
is followed and a statistical weight of 1 is assumed for each of them. Our micro-
o
C
/
H,H'
o
c
c
c
H
H
o
Il
Fig. 2. Possible high energy forms of glycohildehyde. CNDOj2 computations indicate that I is
possibly about 1.4 kcal mole - 1 and Il app~oximately 1.6 kcal mole - " respectively, less stable
than the observed conformation. Both forms have planar HOC-CHO skeletons with two out-ofplane hydrogens and are not stabilized by hydrogen bonds.
270
wavestudyl o stronglytends to refute, although not completelyrule out, such high
concentrations of other forms than the identified one. Although the CNDOj2
proposal of the possible existence of stable rotamers such as I and Il seems
pro bable, it is felt that this method yields too low an energy for them possibly
because the method underestimates the strength of the hydrogen bond.
ACKNOWLEDGEMENT
Fil.Kand. Hasse Karlsson and Professor Carl Lagerkrantz of the University
of Gothenburg (Sweden) are thanked for making available the commercial
spectrometer used in part of this work. Financial support from the Norwegian
Research Council for Science and the Humanities is gratefully acknowledged.
REFERENCES
l
2
3
4
5
6
7
8
9
10
Il
12
13
14
H. MICHELSEN AND P. KLABOE, J. Mol. Structure, 4 (1969) 293.
E. THOM, personal communication,
1968.
B. F. PEDERSEN, personal communication,
1972.
G. C. S. COLLINS AND W. O. GEORGE, J. Chem. Soc. (B), (1971) 1352.
C. I. STASSlNOPOULOUAND C. ZIOUDROU, Tetrahedron, 28 (1972) 1257.
B. PEDERSEN, personal communication,
1972.
J. H. S. WANG AND W. H. FLYGARE, J. Chem. Phys., 53 (1970) 4479.
E. F. H. BRITTAIN, W. O. GEORGE AND G. C. S. COLLINS, J. Chem. Soc. (B), (1971) 2414.
N. P. MCCLELAND, J. Chem. Soc., (1911) 1827.
K.-M. MARSTOKK AND H. MØLLENDAL, J. Mol. Structure,
5 (1970) 205.
K.-M. MARSTOKK AND H. MØLLENDAL, J. Mol. Structure, 7 (1971) 101.
J. K. G. WATSON, J. Chem. Phys., 45 (1966) 1360.
J. KRAITCHMAN, Amer. J. Phys., 21 (1953) 17.
A. S. ESBITT AND E. B. WILSON JR, Rev. Sei. Instrum., 34 (1963) 901.
15 D. R. HERSCHBACH AND V. W. LAURIE,J. Chem. Phys., 40 (1964) 3142.
16 J. S. MuENTER, J. Chem. Phys., 48 (1968) 4544.
17 K.-M. MARSTOKK AND H. MØLLENDAL, J. Mol. Structure, 4 (1969) 470.
18 S. GOLDEN AND E. B. WILSON JR, J. Chem. Phys., 16 (1948) 669.
19 J. A. POPLE AND D. L. BEVERIDGE, Approximate
Molecular Orbital Theory, McGraw-Hill,
New York, 1970.
20 J. A. POPLE AND G. A. SEGAL, J. Chem. Phys., 43 (1965) S136.
21 C. C. COSTAIN, J. Chem. Phys., 29 (1958) 864.
22 C. C. COSTAIN, Trans. Amer. Crystallogr. Assoc., 2 (1966) 157.
23 A. R. UBBELHODE AND K. Y. GALLAGHER, Acta Crystallogr.,
8 (1955) 71.
24 R. E. RUNDLE, J. Phys. Radium, 25 (1964) 487.
25 R. G. AZRAK AND E. B. WILSON JR, J. Chem. Phys., 52 (1970) 5299.
26 L. PAULING, The Nature o/the Chemical Band, Cornell University Press, Ithaca, N.Y., p. 260.
27 R. M. LEES AND J. G. BAKER, J. Chem. Phys., 48 (1968) 5299.
28 R. W. KILB, C. C. LIN AND E. B. WILSON JR, J. Chem. Phys., 26 (1957) 1695.
29 J. P. CULOT, Fourth Austin Symposium
on Gas Phase Molecular Structure, The University
of
Texas at Austin, Austin, Texas, 1972, p. 55.
30 A. ALMENNINGEN, O. BASTIANSEN, L. FE\!-,NHOLTAND K. HEDBERG, Acta Chem. Scand., 25
(1971) 1946.
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