Journal of Molecular Structure,
CD Elsevier Scientific
Publishing
30 (1976) 145-149
Company, Amsterdam
- Printed
in The Netherlands
AN AB INITIO STUDY OF INTERNAL HYDROGEN BONDING AND
CONFORMATIONAL PROPERTIES OF GL YCOLALDEHYDE
HARALD H. JENSEN and HARALD M<l>LLENDAL
Department of Chemistry, The University of Oslo, Blindern, Oslo 3 (Norway)
and EDWIN WISL<l>FF NILSSEN
Institute
of Biology
(Received
and Geology,
26 February
University
of Tromsø,
N-900l
Tromsø
(Norway)
1975)
ABSTRACT
Ab initio calculations have been made on glycolaldehyde
in order to investigate
structural and conformational
properties of molecules with weak intramolecular
hydrogen bonds. It is found that the structural parameters not belonging to the hydrogen
bond are well simulated by these computations
while a larger discrepancy exists for the
D-H bond length and the C-O-H
angle. Isomerization
energies are found to be too
small, and this disagreement
is in faet worse for the largest than for the smallest basis set.
INTRODUCTION
Glycolaldehyde has been studied at this institute by microwave [1], IR
[2] and Raman spectroscopy [2]. Theoretical calculations by the CNDOj2
method have also been performed [1]. The spectroscopic studies [1,2]
revealed that the preferred form of the free molecule is stabilized by an
intramolecular hydrogen bond similar to I of Fig. 1. No evidence was found
for the stable co-existence of additional forms by these spectroscopic
investigations indicating that the hydrogen-bonded
form is at least 1 kcal
mor' more stable than any other rotamer. The substitution structure showed
that the alcohol moiety of glycolaldehyde is considerably different from
that of methanol [3]. The most important differences are that the O-H
bond length is about 0.1 Å longer and the COH angle approximately 7°
smaller in glycolaldehyde than in the alcohol. These structural changes are
presumably largelya result of hydrogen bonding [1].
Recently, efficient computer programmes for ab initio molecular orbital
calculations have become available. This method has hitherto been very
Httle applied to internal hydrogen bonding problems and we thought it would
be interesting to investigate if these calculations can simulate the structural
changes resulting from such weak interactions. Furthermore, we wanted to
examine the conformational behaviour of the molecule in order to predict
the energies and conformations of high-energy forms. As will be shown, these
goals were partially achieved.
146
H
o
o
~
o
c
ko
H,H'
H,H'
c
H
I
C
C
H
~ J-<'H'
O
H
O
III
Fig. 1. Observed
(Il and Ill).
METHOD
conformation
(I) and possible
high-energy
forms of glycolaldehyde
OF CALCULATION
The calculations were carried out using the programme MOLECULE [4]
which solves the Roothan-Hall
equations for a Gaussian type basis. Most of
the results were obtained with a (7s3p/4s) basis contracted to (4,2/2) as
described in ref. 5. Calculations were also made with a (9s5p /4s) basis
contracted to (4,3/2) [6] as well as with the same basis adding a set of
p-functions to the hydroxyl group hydrogen atom. For the hydrogen atoms,
Huzinaga's exponents and coefficients [6] were utilized in all computations
with the exponents scaled with the factor 1.25.
RESULTS AND DISCUSSION
In glycolaldehyde, rotation about the C-C and C--o bonds gives rise to a
very complicated potential surface. A full calculation of this surface by ab
initio techniques is an extremely comprehensive undertaking beyond our
present computational facilities. Instead of this, we had to restrict ourselves
to performing calculations for a limited num ber of possible stable rotameric
forms. Three conformers, all with Cs symmetry, were considered to be of
special interest. The observed, energetically favoured form I of Fig. 1 was
selected to see how well ab initio calculations simulate small geometrical
changes presumably caused by internal hydrogen bonding. Rotamers Il and
III of Fig. 1 were selected because they are considered to be likely candidates
for stable high-energy forms of the molecule.
Computations were first made with the (7s3p/4s) basis. It was found that
extensive geometry optimalization was necessary to obtain a lower energy
for conformation I than for form Ill. The C=O, C-O, C-C and O-H bonds,
as well as the C-C=O, C-C-o
and C-O-H angles, were varied for the three
147
rotamers while the C-H bond lengths and the H-C-H, and C-C-H angles
were kept at the values determined experimentally for conformation I. The
geometric parameters were varied one by one and the corresponding energies
ca1culated. The energy minima for each parameter were then found assuming
a parabolie dependenee of the energy on the bond length or angle under
consideration. The results of these rather lengthy computations are collected
in Tables 1 and 2.
Inspection of Table 1 reveals that the calculated structural parameters
will simulate the micrawave findings with twa important exceptions, viz. the
o-H band length and the C-o-H
angle. While the other structural parameters
are calculated to be within about 0.01 Å and 1° of the experimental anes, the
computed O-H band length is approximately 0.06 Å shorter and the C-O-H
angle is off by more than 5°. Obviously, the structural changes brought about
TABLE 1
Structural
(7s3p/4s)
parametersa
basis
for various
Structural
parameter
Form I
exp.
C=O
1. 209
1.437
1.499
1.051
122.7°
111. 5°
101.6°
1.102
1.093
115.3°
109.2°
107.6°
C-O
C-C
O-H
LC-C=O
LC-C-O
LC-O-H
C-Hald
C-Hale
L C-C-Hald
LC-C-Hale
LH-C-H
conformers
of glycolaldehyde;
Form I
optimized
A
A
A
A
1.211
1.436
1. 509
0.989
122.1°
110.8°
106.9°
A
A
A
Å
calculations
made with
Form n
Form nI
optimized
optimized
1.204
1.436
1.509
0.982
123.4°
107.7°
109.5°
A
A
A
A
1. 209
1.441
1.505
0.981
121.9°
108.8°
108.8°
A
A
aThe last five structural parameters of this table were not optimized
but assigned the values appearing in the second column.
in the computations
T ABLE 2
Total energies in a. u. for the optimized
conformations
Rotamer
Basis
(7s3p/4s)
(9s5p/4s)
(9s5p/4s)a
alncIudingp-orbitals
-227.37644
-227.65306
-227.66705
on hydroxyl
group hydrogen
n
III
-227.36802
-227.37544
-227.65300
-227.66640
atom.
A
A
A
A
148
by weak intramolecular hydrogen bonding are badly reproduced by these
calculations.
Moreover , the stabilization energy of the hydrogen bond seems to be
underestimated too. As seen in Table 2, the energy difference between forms
I and nI is calculated to be only 0.63 kcal mor
l.
This is certainly too small
because of the IR and microwave results which strongly indicate that the I
conformation is favoured by at least 1 kcal mol-l.
In order to investigate the influence of basis enlargement, computations
with the (9s5p/4s) basis were then made. In this case the O-H bond length
and the C-O-H angle were optimized for the I and In forms with the other
structural parameters kept at the optimized values found with the (7s3p/4s)
basis. The results are indicated in Table 2. The energy difference between
these two forms is now found to be as low as 0.04 kcal mori and the
minimum energy geometries of the two rotamers are identical with the
results of the smaller basis set computations.
Finally, calculations were made with the larger basis including a set of
p-orbitals on the hydroxyl group hydrogen atom us ing the same structural
parameters as in the previous case. As before, the O-H bond length and the
C-O-H angle were optimized. This time the energy difference between
conformers I and nI was 0.41 kcal mol-I, which is certainly too low. The
O-H bond length was 1.010 Å and the C-O-H angle 104.0° differed about
0.04 Å and 2.5°, respectively, from their experimentally determined
counterparts. In conclusion it can be said that better isomerization energy is
found with the smallest basis set than in the other cases. Moreover, the
hydrogen bond geometry is unsatisfactorily reproduced by either basis set.
There are perhaps two reasons for the shortcomings of these computations.
Firstly, the d-orbitals which have been left out can be of importance.
Secondly, the correlation energy may play an important role for these weak
intramolecular interactions.
It is interesting to note that our results, to a certain extent, parallel re cent
findings for glyoxal. Elaborate calculations by Skancke and Sæbø [7] have
shown that better isomerization energies are found for this molecule with a
small basis than for large bases including even d-orbitals.
Lastly, a few words should be said about form n which is computed to be
5.28 kcal mol-I less stable than L The former conformer is not stabilized by
hydrogen bonding and there should also be more repulsion between the lone
pair electrons of the two oxygens than in rotamer L These two effects are
expected to result in a fairly high energy for the n conformation in agreement
with the present calculations.
149
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1
K.-M. Marstokk
(1973)
and H. Møllendal, J. Mol. Struct.,5 (1970)
2
H. Michelsen
3
R. M. Lees
4
5
J. AlmlPff, USIP Report 72-09,
B. Roos and P. Siegbahn, Theor.
6
S.
7
205;
259.
and
and
P. Klaboe,
J. Mol.
J. G. Baker,
J. Chem.
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Phys., 48 (1968)
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Acta,
Stockholm
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293.
5299.
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Huzinaga,
J. Chem.
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P. N. Skancke and S. Saebø, J. Mol. Struct., 28 (1975) 279.
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101;
16