Journal of Molecular Structure,
(9 Elsevier Scientific Publishing
30 (1976) 137-144
Company, Amsterdam
- Printed
in The Netherlands
MICROW AVE SPECTRA OF ISOTOPIC GL YOXYLIC ACIDS,
STRUCTURE AND INTRAMOLECULAR HYDROGEN BOND
INGRID CHRISTIANSEN, K.-M. MARSTOKK and HARALD MQ>LLENDAL
Department of Chemistry, The University of Oslo, Blindern, Oslo 3 (Norway)
(Received 24 February 1975)
ABSTRACT
Microwave spectra of CH1 8 OCOOH, CHOCl8 OOH, CHOC018 OH, 13CHOCOOH
and CH013 COOH are reported and have been used in combination
with data on
CHOCOOH and CHOCOOD to determine the molecular
1.174 ! 0.006 A, r(C=O)acid = 1.203 ! 0.006 A, r(C-O)
1.535!
0.005 A, r(o-H)
= 0.948 ! 0.004 A, r(C-H) =
126.2!
0.3°, LCC-O = 112.5 ! 0.4°, LCC°ald. = 123.7
and
LHCC
= 114.6
! 1.0°.
Important
non-bonde
structure as r(C=O)ald. =
= 1.313 ! 0.010 A, r(C-C) =
1.104 ! 0.010 A, LOCO =
! 0.4°, LCOH = 107.8 1: 0.1°
d parameters
are:
r(O
. . . H) =
2.139!
0.008 A, r(O. . . O) = 2.698 1: 0.005 A, LO-H.
. . O = 116.4 1: 0.1°,
LH. . . O=C = 79.7 1: 0.2° and the angle between the hydroxyl and the carbonyl bonds
is 16.1 ! 0.4°. The hydrogen bond geometry implies that covalency plays a minor role
in this case and the short bond lengths of the carbonyl groups and the long carboncarbon bond indicates that conjugation
is of little importance
in this molecule.
INTRODUCTION
The conformational behaviour of free glyoxylic acid has previously be en
the subject of IR [1] and microwave [2] spectroscopic investigations. These
studies established that the preferred form of the molecule is at least 1 kcal
mori more stable than any other rotamer. The stable conformer was
found to be planar with the two carbonyl groups trans to each other, and a
weak five-membered intramolecular hydrogen bond is formed between the
carboxyl group hydrogen atom and the carbonyl group oxygen atom.
In the present work five additional isotopic species of the molecule have
been investigated in order to determine a complete structure. There are
several reasons why this was done. Firstly, very few complete structures
of gaseous molecules possessing internal hydrogen bonds have been
determined. Accordingly, little is known about the exact geometrical
requirements and consequences of this type of weak interaction. More
structural data thus seem to be needed for a better understanding of this
bond. Secondly, glyoxylic acid is the smallest prototype of an important
series of a-carbonyl acids. To our knowledge no complete structure of any
gaseous species of this family has previously been reported although the
138
chemically and biologically important pyruvic acid has been studie d by
microwave spectroscopy [3, 4] . It is expected that the gross structural
features are carried over from glyoxylic acid to other members of the
a-carbonyl acids and may help to improve our understanding of their
properties and chemical reactions.
EXPERIMENT AL
A mixture of CH18 OCOOH, CHOC18OOH and CHOCOl8OH was
obtained by dissolving commercial glyoxylic acid monohydrate in about
20% enriched 18O-water. After about two weeks the water was distilled
off at reduced pressure. The remaining acid-hydrate was then pumped at
room temperature by an oil diffusion pump until all the hydrated water
was removed. This proeess to ok approximately six weeks. Judged by the
absolute intensities of the absorption lines, the three above-named spe eies
had each been about 15% enriched in
18
O. No search was made for species
containing more than onel8 O-atom. The 13CHO COOH and CHOl3 COOH
speeies were measured in natural abundance (about 1%). A conventional
speetrometer described briefly in ref. 5 was utilized. Measurements were
performed at room temperature in the 22-40 GHz spectral region.
MICROWAVE SPECTRA AND ASSIGNMENTS
The assignments of the strong low-J R-branch lines of the 180-species
were easily made because their positions in the spectrum could be predicted
quite well. The high-J Q-branch transitions proved to be much more
difficult to assign because of their weakness and the large num ber of
molecular speeies present simultaneously. The assignments of these
transitions were made by fitting likely candidates by trial and error to
I
l
'
W -- WJ,T (A ' B , e ) + 4I T I aaaa < Pa 4 > + 41 T I b b b b < P b 4 >
+1 4
T I cccc
<P c 4 >+
using the computer
11
4T
aab b
programme
<p a 2 p b 2+ p b 2 p a 2 >
(1)
described in ref. 2. In this manner about 25
absorption lines were determined for each of the 18 O-species. A list of
frequencies of these, as well as the 13 C-species, is available from the authors
upon request or from the Microwave Data Center, Molecular Spectroscopy
Section, National Bureau of Standards, Washington, D.C. 20234, U.S.A.,
where it has been deposited. The results of the least-squares fit to eqn. (1)
are found in Table 1.
The assignments of the 13 C-species lines were restricted to the strongest
of the a-type R-branch transitions which were found close to their predicted
spectral positions. Owing to their weakness, overlapping normal species
excited state lines and Stark lobes, their frequencies could not be determined
more accurately than within :t 0.75 MHz. This is the reason for the rather
T ABLE 1
Molecular
constants
for isotopic
glyoxylic
acids
Speeies
CHOCOOHa
CHOCOODa
CH'80COOH
CHOC'800H
CHOCO'8 OH
13CHOCOOH
CH013COOH
Number of
transitions
a (MHz)
49
22
22
26
29
5
5
0.082
0.071
0.16
0.10
0.15
1.00
0.87
A (MHz)
10966.813
:<:0.015
10422.262
:<:0.136
10965.74
:<:0.27
10737.525
:<:0.092
10264.66
:<:0.13
10855.4
:<:8.9
10975.1
:<:9.5
B (MHz)
4605.988
:<:0.003
4600.668
:<:0.004
4349.089
:<:0.010
4414.702
:<:0.005
4579.313
:<:0.007
4573.43
:<:0.34
4591.17
:<:0.26
C (MHz)
3242.092
:<:0.004
3190.305
:<:0.010
3112.754
:<:0.025
3126.948
:<:0.010
3165071
:<:0.018
3219.38
:<:0.22
3236.77
:<:0.24
la (u A2)
46.08230
:<:0.00006
109.72151
lb(uA2)
:<:0.00007
155.87960
lc(uA2)
T' aaaa(kHz)
:<:0.00020
-39.7
:<:5.8
48.49005
:<:0.00063
'109.84839
:<:0.00009
158.40993
:<:0.00050
-138.6
:<:12.2
46.0868
:<:0.0011
116.20271
:<:0.00027
162.3566
:<:0.0013
-70.5
:<:48.0
47.06634
:<:0.00040
114.47567
:<:0.00013
161.61958
:<:0.00052
-47.2
:<:19.0
49.23455
:<:0.00060
110.36066
:<:0.00017
159.67288
:<:0.00091
46.048
:<:0.040
110.503
:<:0.008
110.076
:<:0.006
156.980
:<:0.011
156.136
:<:0.011
-87.8
:<:23.0
T' bbbb(kHz)
-3.31
:<:0.16
-6.45
:<:0.55
-6.2
:<:1.3
-2.69
:<:0.54
-5.11
:<:0.93
T' cccc(kHz)
1.8
:<:0.4
-6.5
:<:1.0
-5.0
:<:2.6
1.3
:<:1.1
-2.8
:<:2.1
-36.79
:<:0.76
-23.3
:<:2.6
-47.2
:<:4.8
-29.5
:<:2.3
-24.9
:<:2.5
T' aabb (kHz)
46.555
:<:0.038
,...
lc-la-lb
(..
R 2\
0.07578
+0000')')
0.07149
+ O 0007
0.0671
+00017
0.077 56
+ f\ OOOl:7
0.07765
+ o
00077
0.078
+OOAO
0.013
+OOAt)
CI:J
140
large standard deviations of the rotational
shown in Table 1.
constants
of these isotop es as
STRUCTURE
Determination of the molecular structure was made largely by the usual
substitution method [6, 7]. The B and C rotational constants were
employed in Kraitchman's equations for a planar molecule [8] to determine
all but the b-coordinate of the carboxyl group carbon atom and the
coordinates of the carbonyl group hydrogen atom. The structural parameters
of H(l) were determined.by minimizing differences of moments of inertia
in away resembling the one proposed by Nosberger et al. [9]. In this
manner the C(l)-H(l)
bond length and C-C-H angle were determined as
shown in Table 3. Attempts were then made to determine the b-coordinate
of C(2) by requiring that 'L,mibi ==O or 'L,miaibi ==O. This, however, led to
unacceptable structures for the carboxyl group. Small values of the
b-coordinate were then employed in a systematic way and the geometry of
the carboxyl group as well as the first and second moments were computed.
A value of -0.035 Å was finally accepted because this yields areasonable
geometry and small values for the first and second moments. The results of
these calculations appear in Table 2.
TABLE 2
Substitution
coordinates
Atom
for glyoxylic
Principal
acid
axes
a(A)
b (A)
I 1.8218
0(1)
0(2)
0(3)
C(1)
C(2)
H(l)
H(2)
:I:
0.0082:1: 0.0100
-0.7365:1:
0.0016
1.2775 :I:0.0009
-0.6359
:I:0.0019
-0.0350
:I:0.0100
-1.7400
1.5582 :I:0.0008
0.0007
-1.5489:1:
0.0008
-0.5584:1:
0.0021
0.8407 :I:0.0014
-0.5713
:I:0.0021
0.8700
0.3477 :I:0.0035
~ miai = -0.1066
u A ~ mibi = 0.5505 u A ~ miaibi = -0.0796 u A' la ° - las = 0.924 u A'
(2.0%) lb o-lb S = -0.052 u A 2 (0.5%) le o-le S = 0.948 u A2 (0.6%)
CHOCOOH
is the parent
molecule.
For derivation
of uncertainties,
see text.
The empirical equation, OX ==0.0012jx, as proposed by Costain [7] was
used to determine the uncertainties of all coordinates of Table 2 except the
two small b-coordinates of 0(1) and C(2), respectively, and the coordinates
of H(l). For the latter atom, the fitting procedure led to an uncertainty of
:t0.01 Å for the C-H bond length and :t1° for the C-C-H angle. In the case
141
of the heavy atoms, the uncertainties in their b-coordinates were estimated
as :t0.01 Å. This estimate was made by considering its impact on structural
parameters and the first and second moments. The uncertainty limits
as well as the first and second moment equations are found in Table 2.
It is seen that 'L.miai and 'L.miaibi are very dose to zero, while 'L.mibi differs
somewhat from this value. Presumably, this is largelya result of the small
coordinates of the two heavy atoms. Furthermore, it is found that the
substitution coordinates reproduce the observed moment of inertia well
in the case of lb and le while a large r difference is found in la' This was to
be expected because the changes in the A rotational constant upon vibrational excitation are much larger than for B and C as found in ref. 2.
The structure of Table 3 was calculated from the Cartesian coordinates
of Table 2. The uncertainties of the structural parameters of the former
table were derived from those of the latter using the formula for propagation
of errors [10]. The Table 3 structure should be considered as the best
possible approximation to the equilibrium structure and the uncertainties
as three times one standard deviation [6, 7] .
TABLE 3
Structure
of glyoxylic
acid
Bonded
C=O( 1)
C=0(2)
C-O
C-C
C-H
o-H
1.174:!:0.006A
1.203 :!: 0.006
1.313:!:
0.010
1.535 :!: 0.005
1.104:!:
0.010
0.948
:!: 0.004
Å
Å
Å
Å
Å
LCC=O(l )
LCC=0(2)
LOCO
LCOH
LHCO
LHCC
LCCO(3)
123.7:!:
0.4°
LOH . . . 0(1)
LH . . . O(l)C
LO-H, c=oa
116.4 :!:0.1°
79.7 :!:0.2°
16.1 :!:0.4°
121.3 :!: 0.5°
126.2 :!: 0.3°
107.8:!:
0.1°
121. 7 :!: 0.3°
114.6 :!: 1.0°
112.5 :!: 0.4°
Non-bonded
O(l)...H
0(1). . . 0(3)
2.139:!: 0.008 Å
2.698 :!:0.005 Å
For derivation of uncertainties,
see text.
a Angle between the o-H and C=°ald. bonds.
Two effects that may influence the hydroxyl bond length deserve
comment. On substitution of hydrogen by deuterium the O-H bond
length may shrink by about 0.003 Å [11]. Moreover, crystal studies
[12, 13] have shown that for strong X-H. . . y hydrogen bonds the
X' . . y distance increases by between 0.001 Å and 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 spectroscopic
study of gauche 2-chloroethanol
[14] has shown that there is no detectable
142
change in the pertinent O . . . CI distance when hydrogen is replaced by
deuterium. The latter effeet is thus not expected to influence the structure
determination noticeably because of the "weak" bond the observed conformation possesses. The former bond-shrinkage effect, however, is
more serious, but model calculations indicate that the uncertainty limit of
the hydroxyl bond length appearing in Table 3 is realistic.
DlSCUSSION
It is evident from the microwave data that the most stable conformation
of glyoxylic acid is the one shown in Fig. 1. This form of the molecule is
stabilized by a five-membered intramolecular hydrogen bond which is
charaeterized by being very non-linear with LO-H. . .0(1) = 116.4 ::!: 0.1°.
The non-bonded distances r(H . . . 0(1)) = 2.139 ::!: 0.008 Å, and
r(O(l) . . .0(3)) = 2.698:!: 0.005 Å are about 0.5 Å and 0.1 Å, respectively,
shorter than the sum of their corresponding van der Waal's radii [15]. The
angle between o-H and C=O bonds is 16.1 ::!:0.4° from being parallel
which would probably be the most favourable arrangement if the hydrogen
bond were solely eleetrostatic in origin [14]. The H . . . O=C angle is
79.7 ::!:0.2° or about 40° small er than the probable direetion of the
. Sp2-hybridized electron pair of the carbonyl group. The O-H distance of
0.948 ::!:0.004 Å is surprisingly short. In faet, this value is nearly the same as
that of methanol, being 0.9451 ::!:0.0034 Å [16]. This finding is contrary
to what has been found in all other gaseous molecules with internally
hydrogen-bonded hydroxyl groups [17,18] where considerably longer
bonds exist. For example, in the heavy-atom gauche conformations of
2-amino-[19] and 2-chloroethanol [14] which possess relatively weak
internal hydrogen bonds, hydroxyl group bond lengths of 1.139 ::!:0.001 Å
b
'1'
H(2)
0(3)
I
0(1)
-70
~
Fig. 1. Projection
H(l)
of stable rotamer
of glyoxylic
acid in the a-b
principaI
axes plane.
143
and 1.0077 :!:0.0030 Å, respeetively, have been determined by the substitution method. In salicyl aldehyde [20] and 2-nitrophenol [21] which
contain relative ly strong intramolecular hydrogen bonds the O-H bond
lengths are about 1.04 Å and 1.0O:!: 0.02 Å, respeetively. In glycolaldehyde
[22] and oxalic acid [23] the O-H bonds are in fact about 0.1 Å longer than
in glyoxylic aeid, although the hydrogen bond geometries are very similar
in all of these compounds. It is not only the hydroxyl bond length which
is peculiar, the C-o-H angle is also rather unexpectedly large. The value
of 107.8 :!:0.10 is about 60 larger than in e.g., glycolaldehyde [22]. A
large C-O-H angle leads to a larger 0(1) . . . H(2) distance and will
bring the hydroxyl and carbonyl bonds in a more parallel position. The
short o-H bond length, the small H(2) . . . 0(1)C(1) angle and the faet
that the O-H and C=O bonds are nearly parallellead us to conclude that
there is little covalency involved in this bond and that the forces responsible
for the stabilization of the observed hydrogen bonded conformation are
mainly eleetrostatic in origin [14] .
The carboxyl group conformation of glyoxylic acid is unusual. In most
carboxylic acids the hydroxyl bond is rotated through 1800 from the form
found in glyoxylic acid. It is interesting to note that the O-H length is
about 0.02 Å shorter in glyoxylic .leid than in formic aeid [24] while the
C-o-H
bond angles are remarkably similar, viz. 107.8 :!:0.10 and 106.32 :!:
1.000 [24], respectively. The O=C-O angles are also very similar in these
two acids.
The remainder of the structure is charaeterized by the long C-C bond and
rather short carbony l and C-O bonds while the bond angles are quite
normal. The C-C bond length of 1.535 :!:0.005 Å is almost exaetly the
mean of its two counterparts in glyoxal [25] and oxalic acid [23]. Introduction of the electronegative hydroxyl group as substituent to the carbonyl
group thus leads to an increase of about 0.01 Å on going from glyoxal to
glyoxylic acid and to another increase by approximately the same amount on
pro gressing on to oxalic acid. The lon g carbon--carbon
bonds in glyoxal [25]
glyoxylic acid, oxalic aeid [23], oxalyl chloride [26] and bromide [27] and
2,3-butanedione
[28] show that conjugation plays a minor role in these
compounds. In the case of glyoxal this is in accord with elaborate ab initio
calculations by SæbØ and Skancke [29].
Another noteworthy feature of the structure is the short aldehyde group
carbonyl bond length of 1.174 :!:0.006 Å. In a num ber of acid halides short
carbonyl bonds in the 1.16-1.19 Å range have been determined [26, 27,
30] while in the case of acid cyanides such as, e.g., acetyl cyanide [31] a
long carbonyl bond of 1.226 :!:0.005 Å has been found. This shows that the
carbonyl bond length is dependent on its substituents in a complex manner.
Glyoxylic acid is seen to resemble the acid halides more than the acid
cyanides.
In the carboxyl group a rather short C-o bond of length 1.313 :!:0.010 Å
is found. The corresponding length is 1.339 :!:0.002 Å in oxalic aeid [23]
144
which presumably has the same carboxyl group conformation as glyoxylic
acid. In formic acid [24] the C-O bond is also longer than in this molecule,
viz. 1.343 :t 0.010 Å. The carbonyl group of the carboxyl group is 1.203 :t
0.006 Å which is very close to 1.208 :t 0.001 Å in oxalic acid [23] and
1.202:t 0.010 Å in formic acid [24].
ACKNOWLEDGEMENT
Cand. real. Per Kolsaker is thanked for his interest and synthetic work.
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