Raman spectroscopy of glyoxal oligomers in aqueous solutions

Article abstract of DOI:10.1016/j.saa.2012.09.050

Raman microscopy and Attenuated Total Reflection infrared spectroscopy were utilized to facilitate investigations of equilibria between various hydrated and oligomeric forms of glyoxal in aqueous glyoxal solution droplets. The assignment of spectra is obtained with the assistance of B3LYP density functional quantum chemical calculations of vibrational wavenumbers, Raman activities, and infrared intensities. Several forms of glyoxal derivatives with similar functional groups, e.g., hydroxyl and dioxolane rings, are found to be present. The absence of a Raman spectral peak corresponding to the vibrational carbonyl stretch provides evidence that both carbonyl groups of a glyoxal molecule become hydrated in solutions of a broad concentration range. The presence of bands corresponding to deformation vibrations of the dioxolane ring indicates that dihydrated glyoxal oligomers are formed in glyoxal solutions with concentrations of 1 M and higher. Under typical ambient temperature and humidity conditions, concentrated glyoxal solution droplets undergo evaporation with incomplete water loss. Our results suggest that formation of crystalline glyoxal trimer dihydrate from concentrated solutions droplets is hindered by the high viscosity of the amorphous trimer and requires dry conditions that could rarely be achieved in the atmosphere. However, crystallization may be possible for droplets of low initial glyoxal concentrations, such as those produced by evaporating cloud droplets.

Full text of DOI:10.1016/j.saa.2012.09.050

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 40–48
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
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Raman spectroscopy of glyoxal oligomers in aqueous solutions
⇑
⇑
Elena Avzianova , Sarah D. Brooks
Department of Atmospheric Sciences, Texas A&M University College Station, TX 77843-3150, United States
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
"
Glyoxal oligomers in water droplets
were studied by Raman and infrared
spectroscopy.
"
"
The interpretation of spectra was
assisted by B3LYP calculations.
Dihydrated glyoxal oligomers are
formed in solutions with
concentrations above 1 M.
Both carbonyl groups of a glyoxal
molecule are hydrated in aqueous
solutions.
OH
HO
O
OH
OH
O
O
+
HO
HO
OH
OH
OH
O
O
O
"
"
O
HO
Concentrated glyoxal droplets
evaporate with incomplete water
loss.
1400
1200
1000
800
600
Wavenumber/cm-1
a r t i c l e i n f o
a b s t r a c t
Article history:
Raman microscopy and Attenuated Total Reflection infrared spectroscopy were utilized to facilitate
investigations of equilibria between various hydrated and oligomeric forms of glyoxal in aqueous glyoxal
solution droplets. The assignment of spectra is obtained with the assistance of B3LYP density functional
quantum chemical calculations of vibrational wavenumbers, Raman activities, and infrared intensities.
Several forms of glyoxal derivatives with similar functional groups, e.g., hydroxyl and dioxolane rings,
are found to be present. The absence of a Raman spectral peak corresponding to the vibrational carbonyl
stretch provides evidence that both carbonyl groups of a glyoxal molecule become hydrated in solutions
of a broad concentration range. The presence of bands corresponding to deformation vibrations of the
dioxolane ring indicates that dihydrated glyoxal oligomers are formed in glyoxal solutions with concen-
trations of 1 M and higher. Under typical ambient temperature and humidity conditions, concentrated
glyoxal solution droplets undergo evaporation with incomplete water loss. Our results suggest that for-
mation of crystalline glyoxal trimer dihydrate from concentrated solutions droplets is hindered by the
high viscosity of the amorphous trimer and requires dry conditions that could rarely be achieved in
the atmosphere. However, crystallization may be possible for droplets of low initial glyoxal concentra-
tions, such as those produced by evaporating cloud droplets.
Received 10 July 2012
Received in revised form 12 September
2012
Accepted 20 September 2012
Available online 3 October 2012
Keywords:
Glyoxal
SOA
Raman spectroscopy
FTIR
DFT
Ó 2012 Elsevier B.V. All rights reserved.
Introduction
molecules are hydrated and involved in self-reactions to form ace-
tal oligomers [5]. Measured concentrations of glyoxal in solution
Glyoxal is produced in high yields in the atmosphere by the oxi-
dation of aromatic hydrocarbons and in smaller yields by reactions
of isoprene and terpenes [1–4]. Gas phase glyoxal molecules can be
scavenged by cloud droplets and fog. In the condensed phase, these
exceed concentrations predicted by Henry’s law [6,7]. The discrep-
ancy may be driven in part by the accessibility of water, since gly-
oxal forms hydrated species in solution, allowing higher solubility
of glyoxal than expected [6]. Furthermore, hydrogen bonding with-
in single glyoxal oligomers and between two oligomers and water
molecules may provide stabilization of condensed phase products.
Quantifying the formation of secondary organic aerosol (SOA)
products in evaporating cloud droplets is a challenge, since
⇑
Corresponding authors.
E-mail addresses: eavzianova@tamu.edu (E. Avzianova), sbrooks@tamu.edu
(S.D. Brooks).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.09.050

E. Avzianova, S.D. Brooks / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 40–48
41
reactions occurring in a water-restricted environment do not nec-
essarily proceed by the same mechanisms as those which occur
when water is readily available. Furthermore, products may de-
pend on concentrations of organic compounds and rates of evapo-
ration. Non-volatile products of SOA reactions remain in the
condensed phase after cloud droplets evaporate [8,9]. Condensed
phase product identification and yields for reactions involving
aldehydes, ketones, and organic acids are important for assessing
the contribution of heterogeneous organic chemistry to total atmo-
spheric aerosol loadings and thus, the anthropogenic and biogenic
aerosol impacts on clouds and climate.
and the resulting aerosol phase. From this point of view it is impor-
tant to determine the assignments of vibrations in Raman spectra
of glyoxal and its derivatives formed in water solutions under dif-
ferent conditions. Accurate peak assignments may be used to mon-
itor modifications of the structure by the environment when
physical factors such as temperature and water availability are var-
ied. Previous spectroscopic investigations of glyoxal oligomers in
aqueous solutions have focused on infrared (IR) spectroscopy
[10,18], in which assignment of the glyoxal spectral features is
complicated by interferences from water, which is a strong absor-
ber in the infrared. Since water shows low Raman activity, Raman
spectroscopy may be very suitable for investigation of equilibria in
glyoxal aqueous solutions. To the best of our knowledge Raman
spectra have been reported only for anhydrous glyoxal monomer
(CHOCHO) in liquid [19] and solid [20] phases, including polycrys-
talline glyoxal in N2-glyoxal matrix frozen at 5 K [21]. Also, photoa-
coustic Raman spectroscopy (PARS) and Coherent anti-Stokes
Raman spectroscopy (CARS) have been used to study glyoxal in
the vapor phase [22]. In order to fill the existing gap, we have re-
corded Raman spectra of glyoxal solutions of different concentra-
tions in water and deuterated water (D2O). The interpretation of
the spectra is supported by theoretical computations of vibrational
wavenumbers and Raman intensities obtained using Electronic
Structure Density Functional Theory (DFT).
Availability of water plays a key role in the glyoxal chemistry, as
illustrated by Reaction (1a) and (1b). In aqueous solution, one or
both aldehyde groups of the glyoxal can become hydrated [10].
O
OH
O
H2O
+
ð1aÞ
ð1bÞ
OH
O
HO
HO
OH
OH
O
OH
H2O
+
OH
Two monohydrates can then react through nucleophilic attack
of an OH group on the reactive carbonyl of the neighboring mole-
cule (Reaction (2)).
Experimental
OH
HO
HO
O
O
O
OH
Materials and methods
ð2Þ
2
Solutions of glyoxal monomer dihydrate were prepared by
hydrolyzing glyoxal trimer dihydrate (GTD) powder (92%, ICN
Biomedicals, Inc.) in deionized water (18
OH
OH
X ) overnight,
cmꢀ1
NMR data suggests that the majority of glyoxal dimers struc-
tures are most likely linked via five-membered 1,3-dioxalane rings
[11–13]. If the gem–diol formed in Reaction (2) becomes dehy-
drated, it can then react with a third glyoxal monohydrate, forming
the stable glyoxal trimer (which contains two dioxolane rings)
(Reaction (3)). According to calculations, the trimer is the thermo-
dynamic sink among all glyoxal derivatives involved in these reac-
tions [14].
followed by dilution with additional water. The concentrations of
glyoxal solutions varied from 0.1 M to 4.0 M. Deuterated water
(99.9 atom% D, Aldrich) was used for preparation of the D2O
solutions.
To examine the effect of glyoxal oligomerization on solution
composition, evaporation experiments were performed. To begin
an experiment, a 20–30 ll glyoxal solution droplet was placed on
the aluminum foil and exposed to the air. Drying experiments were
repeated using a range of droplet solution concentrations of 1.0–
4.0 M. An additional series of experiments was conducted using
deuterated water in place of the H2O. To eliminate exchange with
water molecules of air, evaporation of glyoxal solutions in D2O
droplets was conducted in a dessicator with silica gel.
Upon evaporation, aqueous concentrated glyoxal solutions
leave behind a thicker film than dilute solutions. The crystalliza-
tion behavior is likely caused by hindered transport of water in
highly viscous amorphous material. In order to remove the remain-
der of water from amorphous glyoxal obtained from concentrated
solutions, the samples were placed in a dessicator with silica gel
overnight.
OH
HO
O
OH
OH
O
O
+
HO
HO
OH
ð3Þ
OH
OH
O
O
O
O
HO
While it has been observed in our laboratory and others that
while oligomers form in low yields in dilute solutions, yields ap-
pear to be increased by evaporation at low relative humidities
[15]. Such observations may be justified by the fact that the dihy-
drate (formed in Reaction (1)) lacks the carbonyl group required
for the self-reactions and cannot react as monohydrates do (as
shown in Reactions (2) and (3)). Furthermore, water may play a
significant role in formation of amorphous solids, in which in-
creased hydrogen bonding and increased viscosity limit crystalliza-
tion [5]. Understanding the water-mediated equilibria of reactions
of aldehydes, ketones and other organic compounds is important
for assessments of the chemical and physical properties of atmo-
spheric aerosols [16,17].
Raman spectroscopy
Raman spectra were recorded over the range of 500–3500 cmꢀ1
using
a Thermo Fisher Scientific DXR Raman Spectrometer,
equipped with an Olympus BX microscope and a high resolution
CCD detector. A frequency doubled Nd:YVO4 diode pumped solid
state laser was used for excitation at 532 nm. The Raman apparatus
also features a motorized stage that moves automatically in the x
and y directions to image areas multiple areas within a single drop-
let. Raman spectra of glyoxal solutions in H2O and D2O in the range
of concentration between 1 and 4 M were recorded.
Spectroscopic studies in water-restricted environments may
provide insight into how water availability impacts the products
and yields of reactions of organic compounds in cloud droplets

42
E. Avzianova, S.D. Brooks / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 40–48
Infrared spectroscopy
In addition to the Raman measurement, Fourier Transform
Infrared Spectroscopy (FTIR) spectrum of aqueous glyoxal solu-
tions were recorded in the region between 2000 and 700 cmꢀ1
using a Nicolet Magna 560 spectrometer equipped with an Atten-
uated Reflectance (ATR) cell and an MCT detector for a subset of
solutions droplets. Standard spectra of solid glyoxal trimer dihy-
drate were recorded by pressing crystals onto the ZnSe crystal of
the ATR (Pike Technologies). Standard glyoxal solution spectra
were taken using the same conditions as the Raman spectra but
with a water droplet as the background and without any evapora-
tion delays.
Calculations
Density Functional Theory (DFT) calculations were performed
using Gaussian 03 at the B3LYP/6-311 + G(d,p) level of theory,
using an ultrafine pruned grid and tight optimization criteria
[23]. Geometry optimization, Raman shift (wavenumber), and Ra-
man scattering activity calculations were completed for mono-
and di-hydrated glyoxal monomer, mono- and di-hydrated dimers
of glyoxal, trimer glyoxal dihydrate, and their OD analogues. The
B3LYP method is less computationally expensive than traditional
correlated ab initio methods while providing a comparable accu-
racy in computing structural properties of large molecules, includ-
ing molecular geometry and vibrational wavenumbers [24]. The
theoretical Raman intensities IRi were calculated based on the Ra-
man scattering activities Si obtained from the Gaussian program
according to Eq. (1) [25],
Fig. 1. Raman spectra of 1–4 M glyoxal solutions in the 1800–500 cmꢀ1 region.
Black line (1): 1 M glyoxal solution. Green line (2): 2 M glyoxal solution. Red line
(3): 4 M glyoxal solution. Blue line (4): crystalline glyoxal trimer. (For interpretation
of the references to color in this figure legend, the reader is referred to the web
version of this article.)
performed for the following forms of glyoxal and its deuterated
analogues:
OH
HO
HO
HO
HO
O
O
HO
HO
OH
OH
O
O
O
O
OH
OH
4
IRi ¼ C ꢁ ðm0
ꢀ
miÞ ꢁ mꢀi 1 ꢁ Bꢀi 1 ꢁ Si
ð1Þ
ð2Þ
OH
I
II
III
Bi ¼ 1 ꢀ expðꢀh ic=kTÞ
m
where C is a constant (10ꢀ12), is the Bi is the temperature factor
which accounts for the intensity contribution of excited vibrational
states (Eq. (2)), m0 is the laser excitation frequency, mi is the fre-
quency of normal mode, k is the Boltzman constant, c is the speed
of light, h is the Planck constant, and T is the temperature.
DO
DO
OD
OD
OD
OD
DO
DO
O
O
O
O
DO
DO
OD
O
O
OD
IV
VI
V
Results and discussion
Fig. 1 shows Raman spectra of aqueous solutions, ranging from
1.0 to 4.0 M aqueous glyoxal, and crystalline glyoxal trimer. In this
concentration range, spectra have very similar features with a few
minor differences as discussed below. The largest differences are
observed between spectra of glyoxal solutions and crystalline gly-
oxal dihydrate timer. Evaporation under ambient conditions re-
quired extended periods of time and lead to the formation of
highly viscous, amorphous glass-like material. The Raman spec-
trum of amorphous glyoxal contains traces of water and is very
close to the spectrum of crystalline glyoxal trimer. Results of the
droplet drying experiments depended on solution concentration.
The spectra of initially 1 M glyoxal under evaporation are shown
in Fig. 2. Drying of glyoxal obtained from more dilute solutions
(initial concentrations lower than 0.5 M) resulted in solidification
of crystalline glyoxal trimer.
The structures were optimized at the B3LYP/6-311 + G(d,p) le-
vel of theory. The resulting geometries and wave functions corre-
sponding to energy minimum were used in the normal
coordinate analysis, as implemented in Gaussian, to calculate
wavenumbers and Raman activities. A relatively large basis set
was used to obtain accurate structural parameters and vibrational
wavenumbers. Since Raman intensities are known to be sensitive
to diffuse augmentation of the basis set, diffuse functions were
used in all calculations [26]. For instance, the systematic study re-
ported by Cheeseman and Frisch [27] showed that Raman Optical
Activity tensor invariants require basis sets with diffuse functions
to obtain reliable Raman activities. Calculated wavenumbers and
Raman activities with experimental data for hydrated/deuterated
glyoxal derivatives and water/deuterated water solutions are pre-
sented in Tables 1 and 2. Optimized structures and corresponding
Cartesian coordinates of all derivatives are now provided in Sup-
plementary Information (Figs. S1–S3 and Tables S1–S3). Compari-
son of calculated spectra of glyoxal derivatives with experimental
spectra of glyoxal solutions and glyoxal crystalline trimer dihy-
drate shows general qualitative agreement between the Raman
intensities (Fig. 5).
To facilitate assignment of glyoxal derivatives, spectra of gly-
oxal solutions in D2O were also obtained (Fig. 3). Also FTIR spectra
of crystalline glyoxal dihydrate trimer and glyoxal solutions in the
range of concentration between 1.0 and 4.0 M were recorded, as
shown in Fig. 4.
To complete the vibrational assignments of the experimental
spectra of glyoxal derivatives, quantum chemical calculations were

E. Avzianova, S.D. Brooks / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 40–48
43
Fig. 2. Raman spectra of glyoxal solutions upon evaporation. Black line (1): 1 M glyoxal solution. Green line (2): 1 M glyoxal solution after 20 min exposure on air. Red line
(3): 1 M glyoxal solution after 1 h exposure on air. Blue line (4): crystalline glyoxal obtained over silica gel. (For interpretation of the references to color in this figure legend,
the reader is referred to the web version of this article.)
Fig. 3. Raman spectra of glyoxal solution in the 1600–600 cmꢀ1 region. Black line (1): 2 M glyoxal solution in D2O. Red line (2): 2 M glyoxal solution. (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of this article.)
The primary difference between the spectra of glyoxal solutions
and that of crystalline glyoxal trimer is the water peak at
1640 cmꢀ1 (Fig. 1). During evaporation of glyoxal aqueous solu-
tions, the intensity of the 1640 cmꢀ1 band decreases significantly
due to water removal (Fig. 1). The drying is also accompanied by
changing intensities and shifting of bands in the region of CAC
and CAO stretching and CAH deformation of glyoxal derivatives
(1200–700 cmꢀ1). The spectra of heavy water glyoxal solutions
show the same behavior upon evaporation as H2O glyoxal solu-
tions, except that the scissoring bend of D2O lies in the region at
1200 cmꢀ1 [28], overlapping the CAC and CAO stretching region
(Fig. 3). Significant differences between spectra of aqueous and
heavy water glyoxal solutions appear mainly in spectral regions
around 1400–1300 and 1200–1000 cmꢀ1 (Fig. 3). Below we pro-
vide detailed assignments of Raman features in several spectral
regions.
Region 1500–1100 cmꢀ1. CAH bending vibrations occur in the
region 1400–1450 cmꢀ1 and CAH twisting vibrations give rise to
a band around 1300–1390 cmꢀ1 (Fig. 1) [29]. It is known that the
frequency of CAH deformations are often very sensitive to the de-
gree of vibrational coupling with other coordinate displacements.
The CAH bending vibration involves a large displacement of the
carbon atom, which couples with adjacent CAC stretching vibra-
tion, leading to splitting the CAH bending bands. In glyoxal dihy-
drates, CAH deformation vibrations extensively mix with CAC
stretching and dOAH in-plane deformation, resulting in massive
envelope of bands. In the spectra of solid glyoxal trimer in both
amorphous and crystalline forms this spectral feature has an inten-
sity maximum at 1330 cmꢀ1, whereas in liquid glyoxal solutions
the intensity maximum is shifted to 1353 cmꢀ1 (Fig. 2). According
to our calculations the spectral feature at 1353 cmꢀ1 is attributed
to CAH bending of monomer and dimer glyoxal gem–diol group

44
E. Avzianova, S.D. Brooks / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 40–48
solution and solid glyoxal have additional features at 1425 and
1280 cmꢀ1. After evaporation these features become more evident
on both sides of the envelope of bands (Fig. 2). These bands are
attributed to the CAH bending mode coupled with different OAH
deformation modes. Anderson and Bellamy [32] have studied the
spectra of gem–dihydroxy compounds produced by hydration of
carbonyl group and reported a similar splitting of CAH bending
bands. They showed that upon hydration aldehydic CAH deforma-
tion mode that occurs in the region of 1350–1370 cmꢀ1 disappears
and is replaced by a doublet at 1440–1420 and 1450–1410 cmꢀ1
and another peak at 1290–1280 cmꢀ1 due to coupling two differ-
ent dOH with adjacent CAH bending, confirming our assignment.
In glyoxal solutions in deuterated water, the corresponding dOD
vibration modes shifted to lower frequency and simplifying this
spectral region (Fig. 3). Due to decoupling of vibrations, the spec-
trum of deuterated glyoxal solution shows a distinct maximum
at 1368 cmꢀ1 corresponding to CAH wagging of monomer glyoxal,
in a good agreement with calculated wavenumber 1370 cmꢀ1 for
this mode (Table 2). The distinct band at 1423 cmꢀ1 (Fig. 3) in this
spectra corresponds to a vibration mode containing a high degree
of CAH bending related to gem–diol group as well as OACH bend-
ing of dioxolane ring.
Fig. 4. FTIR spectra of glyoxal solutions upon evaporation. Black line (1): 1 M
glyoxal solution. Red line (2): 4 M glyoxal solution. Blue line (3): crystalline glyoxal
obtained over silica gel. (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of this article.)
Region 1200–1100 cmꢀ1. CAOH stretching vibrations occur in
the region of 1000–1200 cmꢀ1 [29,33], giving rise to considerable
overlapping with CAC stretching vibration bands. Spectra of solid
glyoxal trimer shows band with medium intensity at 1175 cmꢀ1
(Fig. 1) assigned to linear CAC stretching mode with adjacent
dioxolane rings of glyoxal trimer mixed with CAH bending motion,
which is in a good agreement with predicted value of 1177 cmꢀ1
(Table 1). Features at lower frequencies are assigned to CAH defor-
mation vibrations of the dioxolan ring. A band at 1340 cmꢀ1 is
common for lone CAH bending [30]. For instance, in-plane meth-
yne CAH bending band for 2-propanol and its deuterated ana-
logues occur at 1340 cmꢀ1 [31]. Spectra of both liquid glyoxal
Table 1
Calculated wavenumbers and Raman activities of glyoxal derivatives.
Calculated
Observed
Assignment
I
II
III
1 M (H2O)a
Solidb
m
, cmꢀ1
m
, cmꢀ1
RA
m
, cmꢀ1
RA
m
, cmꢀ1
m
, RA
m
, cmꢀ1
589
761
3
1.1
639
762
610
758
823
1.1
2.4
OCObending (monomer)
OCObending (ring)
775
816
5.8
3.4
770
819
817
872
853
7
nCAC (mono-,dimer) + dOH
879
953
974
11
6.4
1.7
879
972
954
982
14.5
2.4
952
970.7
ring stretching
nCAOAC (r) + dCAC (linkage)
1001
17.8
1007
1050
3.4
1.5
1008
1042
1065
1002
1012
1027
1018
1050
6
4
1024
1.6
nCAO + CAOHbend ip
nCAOH
1060
1069
10.6
7.8
1068
1082
1103
1159
4.1
2
5
1066
1099
1089
1110
5
2
1102
1177
1203
1234
3.1
2.2
1.2
8
1
1110
1155 sh
nCAOH (gem–diol)
nCAClinkage (dimer) + ring str
nCAClinkage (trimer)
1160
1174
1185
1
1204
1223
1230
1.1
2
5
1278
1353
1283
1334
1421
CHbend + OHbend
1258
1280
5
6
1296
2.5
1324
5
1338
1376
3
4
1332
1365
1370
1377
1414
3
CAHbend (ring)
CAHbend (monomer, dimer)
3.3
3.3
3.7
1.1
1366
1379
1393
10
3.4
6
1402
1430
1465
3
3
2
CHbend + OHbend
1436
1469
3.6
2.8
1462
1473
1
2.6
a
1 M aqueous glyoxal solution.
Crystalline glyoxal trimer.
b

E. Avzianova, S.D. Brooks / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 40–48
45
for this mode. In aqueous glyoxal solutions with concentrations
from 0.5 to 1 M, no distinct bands are observable near this wave-
number, possibly due to interference by the strong band at
1110 cmꢀ1 which is assigned to CAOH stretching mode of gem–
diol group of dihydrated glyoxal. However, with increasing concen-
tration solution, a new peak at 1155 cmꢀ1 and a shoulder at
1175 cmꢀ1 arise. The band at 1155 cmꢀ1 is assigned to linear
CAC linkage stretching vibration coupling with ring stretching
deformation in dihydrated glyoxal dimer. After transition to solid
state, the peak at 1155 cmꢀ1 disappears and the 1175 cmꢀ1 feature
takes its place.
terated glyoxal solution show an aggregate of unresolved bands
with spectral features at 1180, 1138, 1112 and 1090 cmꢀ1
(Fig. 3). According to our calculation, the band at 1180 cmꢀ1 is
attributed to the
mCAC linear linkage of deuterated glyoxal dimer
hydrate with a theoretical wavenumber of 1183 cmꢀ1. The pre-
dicted value for
mCAO of gem–hydroxyl groups coupling with
dCH is 1136 and 1129 cmꢀ1 in deuterated glyoxal monomer hy-
drate and dimer hydrate, respectively (Table 2). Based on our cal-
culations, we assigned the band with wavenumber 1138 cmꢀ1 to
a m mCAO
CAO of gem–diol. A band at 1112 cmꢀ1 is assigned to a
of hydroxyl groups connected to dioxolane ring of glyoxal dimer
dideuterate, in a good agreement with predicted wavenumber of
1113 cmꢀ1 for this vibration mode. Unresolved peaks of lower
During evaporation, the band at 1110 cmꢀ1 is overlaid by a
strong band near 1099 cmꢀ1. According to calculations, CAO
stretching of the gem–diol glyoxal derivative appears at 1110
and 1103 cmꢀ1 for dihydrated monomer and dimer glyoxal,
respectively. For deuterated analogues, the corresponding pre-
dicted wavenumbers shift to 1130 and 1136 cmꢀ1 for the mono-
mer and dimer, respectively (Table 2 and Fig. 3). A similar blue
shift of CAO stretching vibration bands has been observed in IR
spectra of lactic acid and lactate where two bands at 1125 and
intensity at 1092 cmꢀ1 likely correspond to the
m
CAO of OD groups
connected to dioxolane ring of glyoxal trimer dideuterate. The cal-
culated wavenumber for this vibration mode is 1081 cmꢀ1
Region
1100–900 cmꢀ1. In the region between 1000 and
.
1100 cmꢀ1 spectra of aqueous glyoxal solutions feature a strong
broad band with peaks at 1062, 1050 and 1007 cmꢀ1 (Fig. 1). This
group of peaks are predominantly a CAO stretching mode mixed
with CAOH in-plane bending. The theoretical wavenumber for
the CAO stretching mode of hydroxyl groups connected to dioxo-
lane ring of glyoxal dimer is 1068 cmꢀ1 (Table 1). For glyoxal solu-
tion concentration of 1 M and higher, a medium band appears with
wavenumber 952 cmꢀ1. The of this band increased with increasing
of glyoxal concentration. Raman spectra of dimethyl substituted
1,3-dioxolanes show weak to medium bands at 955–947 cmꢀ1
due to ring breathing vibration [35,36]. In theory, there are
Raman-active vibrations are present at 953 and 954 cmꢀ1 for
1090 cmꢀ1, assigned to the
mCAO modes of the two forms, associ-
ated and free OH, move to 1160 and 1100 cmꢀ1 on deuteration
[34]. Also in the spectra of 1,1,1-trifluoro-2-propanol the
mCAO
band has been shown to shift from 1060 to 1110–1130 cmꢀ1 on
hydroxyl group deuteration [31].
The water O–D deformation vibration band at 1200 cmꢀ1 ap-
pears in the spectrum of deuterated water glyoxal solution cou-
pling with linear CAO and CAC linkage stretching modes in the
1155–1180 cmꢀ1 area and overlapping the region. Spectra of deu-
Table 2
Observed and calculated wavenumbers and Raman activities of deuterated glyoxal derivatives.
Calculated
Observed
Assignment
IV
V
VI
2 M (D2O)a
Solidb
m
, cmꢀ1
m
, cmꢀ1
RA
m
, cmꢀ1
RA
m
, cmꢀ1
RA
m
, cmꢀ1
554
793
2
5
537
748
5
2
615
764
4
8
629
762
609
OCObending (monomer)
OCObending (ring)
770
818
809
841
859
918.6
941
968
983
3
5
5
2
3
2
5
804
835
3
5
814
865
907
4
5
865
875
865
883
nCAC
939
2
969
4
2
3
972
989
984
994
10
15
989
997
1003
ring stretch + dOD
1008
1057
1028
1044
1
12
1044
1064
2
5
1041
1081
1102
1113
3
3
6
1092
1112
1138
nCAODring (trimer)
nCAODring (dimer)
nCAOD (monomer)
nCAClinkage (dimer)
1101
1119
1172
1113
1136
1183
2
2
3
1129
1201
2
4
1186
1308
4
7
1180
1200
1286
1297
1331
1357
1315
1345
1352
1358
4
2
5
3
1331
1368
1423
1361
1370
6
7
1355
1385
10
3
CAHwagging (monomer)
1380
1423
3
3
1404
1423
1
2
1424
1446
2
6
1423
CHbend + OCHbend (ring)
a
2 M glyoxal solution in heavy water.
Crystalline glyoxal trimer obtained after evaporation of heavy water solution.
b

46
E. Avzianova, S.D. Brooks / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 40–48
dihydrated glyoxal dimer and dihydrated glyoxal trimer, respec-
tively (Table 1). Neither theoretical, nor experimental spectra of
glyoxal monomer (CHOCHO) [21,22] show absorbance in this area.
According to this, we assign the band at 950 cmꢀ1 to ring stretch-
ing and conclude that the cyclic dihydrated glyoxal dimer is pres-
ent in detectable concentration even in 1 M solution.
(Fig. 1). During evaporation, the intensity of this peak decreases
significantly and new peaks of lower intensity appear, overlapping
over peak in this region. Resulting spectrum of crystalline glyoxal
shows few overlapping peaks of low intensity between 820 and
880 cmꢀ1. In contrast, the spectrum of deuterated glyoxal solution
contains a massive band with two distinct maxima at 875 and
865 cmꢀ1 and also a new medium-intensity feature containing sev-
eral overlapping bands with a maximum intensity at 814 cmꢀ1
(Fig. 3). Upon evaporation, the band at 814 cmꢀ1 disappears and
the intensity of the peak at 865 cmꢀ1 increases. The absence of a
strong 872 cmꢀ1 band in deuterated solutions suggests that this
vibration mode is be affected by OH motion.
In spectra of deuterated glyoxal solutions, the band at 952 cmꢀ1
is absent because the band corresponding to ring stretching in O–D
derivative of the dihydrated glyoxal dimer is shifted to a higher
wavenumber. A theoretical prediction for this vibration mode is
983 cmꢀ1 (Table 2). As shown in Fig. 4, experimental spectra of
deuterated glyoxal solutions have several bands in the region be-
tween 960 and 1010 cmꢀ1 that originate from ring stretching and
in-plane OD bending (dOD). Indeed, in long-chain alcohols, the
dOD peak appears at about 940 cmꢀ1 with an intensity ranging
from moderate to weak [37]. In deuterated gem–diols, a doublet
corresponding to dOD appears at two regions from 905 to 930
and from 913 to 966 cmꢀ1 [32].
The calculated wavenumber for
glyoxal monomer is 853 cmꢀ1. This mode involves strong degree
of dOH bending. The linear CAC of hydrated glyoxal dimer (hemi-
acetal-ring link) is predicted to arise at 879 cmꢀ1 (Table 1). For the
mCAC stretching of dihydrated
m
deuterated glyoxal monomer hydrate, the mCAC theoretical wave-
number is shifted to 793 cmꢀ1, indicating that CAC stretching and
Upon transition to the crystalline form of glyoxal trimer dihy-
drate, the intensity of a band at 950 cmꢀ1 decreases and the band
overlaps with a new strong band at 972 cmꢀ1 (Fig. 2). According to
our calculations, this new band could be assigned to CAOAC
stretching of rings involving bending of CAC linkage between
two dioxolane rings vibration mode, which has a predicted wave-
OD bending vibrations are coupled. Theoretical wavenumber for
the linear
2). The band at 865 cmꢀ1 in the theoretical spectra of deuterated
glyoxal monomer is assigned to CAO vibration mixed with dOD.
Because of strong coupling, no individual vibration corresponding
to the CAC linkage between two dioxolane rings appears in the
m
CAC of deuterated glyoxal dimer is 859 cmꢀ1 (Table
m
m
number of 982 cmꢀ1
.
calculated spectra of glyoxal trimer dihydrate or its deuterated
analogue.
Region 900–800 cmꢀ1. In the 900–800 cmꢀ1 region spectra of
water glyoxal solution feature the strong peak at 872 cmꢀ1
Barker et al. identified the strong peak at 830 cmꢀ1 as the
stretching of methyl-ring in highly polarized methyl substituted
1,3-dioxolanes [36]. In another study, the Raman spectrum of sin-
gle crystals of oxalic acid dihydrate contained showed medium to
strong peaks at 865 and 855 cmꢀ1 which were attributed to CAC
stretching vibrations [38]. For the deuterated analogue, this bands
shifted to 844–845 cmꢀ1 and the intensities increased. The same
authors attributed the medium-strong band at 844 cmꢀ1 to out-
of-plane OH bending (
deuterated oxalic acid dihydrate samples, the
appears at 812 cmꢀ1 [38]. In addition, Plath and Axson report a the-
c
OH) associated with water libration. In
c
OH bending band
oretical IR peak at 843 cmꢀ1 due to
mCAC coupling with dOH and a
second peak at 759 cmꢀ1 due to a decoupling CAC stretching of
glyoxylic acid monohydrate [33]. According to calculation and lit-
erature data, we attribute the band at 872 cmꢀ1 to a
m
CAC of dihy-
drated glyoxal monomer coupling with dOH with a possible minor
contribution of linear CAC of dihydrated glyoxal dimer.
m
Region 800–700 cmꢀ1. A broad band at 762 cmꢀ1 is observed in
the spectra of glyoxal solutions in H2O and D2O (Fig. 3). The band
shape arises due to multiple transitions. We assigned this band
to the OACAO bending of dehydrated glyoxal monomer on the ba-
sis of a theoretical value of 761 cmꢀ1 for in-plane OACAO bending
vibration (Table 1). Upon drying, the band at 762 cmꢀ1 shifted to a
higher wavenumber of 770 cmꢀ1 and increased in intensity. Since
the predicted wavenumber for the OACAO ring bending in glyoxal
trimer is 775 cmꢀ1, we attribute the band at 770 cmꢀ1 to OACAO
bending of 5-member dioxolan ring of glyoxal oligomers.
In the work by Loeffler et al. [10] FTIR has been used to investi-
gate equilibria between different forms of hydrated glyoxal in
evaporating droplets, but no theoretical confirmation of the spec-
tral peak assignments has been provided. To fill this gap, we col-
lected infrared spectra of aqueous glyoxal solutions and made
peak assignments with the aid of theoretically calculated vibra-
tional wavenumbers and insight from the Raman spectra. Fig. 4
shows FTIR spectra of 1–4 M glyoxal solutions obtained in our
study. The spectrum of 1 M glyoxal solution features a strong band
at 1070 cmꢀ1 with a shape suggesting that the band contains sev-
eral overlapping transitions. The calculated wavenumbers for
Fig. 5. Comparison of calculated and experimental Raman spectra of glyoxal
derivatives: (a) calculated monomer dihydrate, (b) calculated glyoxal dimer
dihydrate, (c) calculated glyoxal trimer dihydrate, and (d) experimental spectra of
glyoxal 1 and 4 M solutions and crystalline glyoxal trimer dihydrate.
m
CAO are 1050, 1088 and 1110 cmꢀ1 for the dihydrated glyoxal
monomer (Table 1). For the dihydrated glyoxal dimer the calcu-

E. Avzianova, S.D. Brooks / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 40–48
47
lated IR wavenumbers are 1050 and 1080 cmꢀ1. According to cal-
culations, we assigned this group of bands in IR spectra to a CAO
coupled with dCOH dihydrated glyoxal monomer, with a possible
contribution from the dihydrated glyoxal dimer. The spectrum of
1 M glyoxal solution also has low intensity peaks of at 1009 and
950 cmꢀ1. In the spectrum of 4 M glyoxal solution, the band at
1070 cmꢀ1 overlaps with a strong absorption at 1050 cmꢀ1, corre-
may be present in an amorphous state under a broad range of con-
ditions. In addition, the Raman and IR peak assignments deter-
mined in this study by combining experimental measurements
and theoretical calculations will be useful in future applications,
including heterogeneous chemistry of glyoxal in cloud droplets.
m
Acknowledgement
sponding to mCAO of the dihydrated glyoxal dimer. The bands at
1009 and 950 cmꢀ1 are more prominent in the spectrum of the
more concentrated solution. Our calculations predict a peak at
947 cmꢀ1 due to in-plane stretching of glyoxal trimer active in
IR. The observed IR absorbance at 950 cmꢀ1 has been attributed
to an asymmetrical CAOAC stretching of 5-membered dioxolane
ring.
The authors gratefully acknowledge financial support for this
research from Robert Welch Foundation.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2012.09.050.
Since the formation of acetals involves the interaction of a car-
bonyl with an OH group, glyoxal monohydrate plays a central role
in oligomerization of glyoxal in aqueous solutions [10,14]. The
equilibrium concentration of the monohydrate depends on the
water activity and is expected to increase upon droplet evapora-
tion. Since the stretching wavenumber of the carbonyl C@O groups
in Raman and IR spectra (1620–1800 cmꢀ1) is overlapped by a
strong OAH scissoring bend of liquid H2O at 1640 cmꢀ1, monitor-
ing the evolution of carbonyl in aqueous solutions is challenging.
On the contrary, in deuterated water solutions, the O–D scissoring
bend of liquid D2O is shifted to 1200 cmꢀ1 and the behavior of the
carbonyl can be monitored unobstructed. By conducting experi-
ments in D2O, we find that the carbonyl feature at 1600–
1800 cmꢀ1 is not observable in 1 M glyoxal solution. Furthermore,
it does not appear upon evaporation, when the equilibrium of
Reaction (1) is shifted to higher wavenumbers, thus suggesting
that in aqueous solutions glyoxal exists predominantly in dihy-
drated form. This is in agreement with findings of Malik and Joens
[39], who observed that less than 0.02% of the glyoxal molecules in
aqueous solution exist in the free dialdehyde form, 1.98% of the
molecules exist in monoaldehyde form, and approximately 98%
of the molecules are hydrated at both carbonyl groups.
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