Spectroscopic studies of glycolaldehyde

Journal of Molecular StrucZure

Elsevier Publishing Company,

SPECTROSCOPIC

HARALD

MICHELSEN

Amsterdam.

STUDIES

AND

PETER

Printed in the Netherlands

293

QF GLYCOLALDEHYDE

KLABOE

Department of Chemistry, University of Oslo, Blindern, Oslo 3 (Norway)

(Received April 1% 1969)

ABSTRACT

The infrared spectrum of glycolaldehyde in the vapour phase, as a melt, in solution and as a crystalline solid were recorded between 4000 and 200 cm-l. Raman spectra were obtained of the solid glycolaldehyde, of a supercooled liquid and of a saturated aqueous solution. The spectra have been interpreted in terms of monomeric molecules in the vapour phase at 95”. In the crystalline state glycolaldehyde forms dimers having a centre of symmetry, probably of l&dioxane structure. As a melt at lOO”, as a supercooled liquid at 30” or when dissolved in water or polar organic solvents, glycolaldehyde apparently exists as an equilibrium between monomer and dimer molecules, the latter predominating.

INTRODUCTION

Glycolaldehyde is the lowest hydroxyaldehyde and therefore the simplest possible “sugar”. The molecule is of considerable biochemical interest and, upon condensation with amines, various natural products are formed’. Several authors have investigated glycolaldehyde in solution and it was reported in a classical study2 that the compound first dissolves in water as dimers. However, the freezing point depression in water increased with time and after 24 h the data suggested monomeric molecules 2. Other workers reported that when glycolaldehyde was dissolved in formic acid, methanol, ethanol, acetonitrile or acetone, the apparent molecular weight diminished with time. Again, this was interpreted as a transition from dimeric to monomeric molecules, the kinetics being of the tist order3. It was shown by a dilatometric method that, for glycolaldehyde4 as well as for the related dihydroxyacetone’, the transition from dimers to monomers was catalysed by bases as well as by acids. Emil Fischer’s had already proposed that the glycolal~ehyde dimer had a dioxane structure. Later, an unstable heterocyclic four-membered ring3 as well as J. Mol. Structure, 4 -(1969) 293-302

294

H. MICHELSEN,

P. KLABOE

an ethylene oxide structure’ were proposed. Summ~rbell and Rochen8 prepared compounds which were identical with derivatives of dimeric glycolaldehyde from dioxadiene, and they concluded that the structure was 2,!Gdihydroxydioxane. More recently, Spgth and Rascbikg reported that the dimeric glycolaldehyde is no pure substance, but consists of a mixture of molecules, including the two stereoisomers of 2,5-dihydroxy-1,~ioxane and 2-hydroxymethyl-5-hydroxy-1,3-dioxolane. fn this department, Mollendal lo has initiated a microwave study of glycolaldehyde and some of its deuterated analogues. The rotational constants have been determined”, revealing that the vapour consists of monomeric molecules in which the OCCOH skeleton is planar, probably stabilized by intramolecular hydrogen bonding. Furthermore, an X-ray crystallographic study of the crystalline compound has recently been startedl’. The preliminary results” indicate a centrosymmetrical space group with two molecules in the unit cell, and the molecules seem to have a symmetry centre. In addition to these structural determinations of glycolaldehyde in the vapour and crystalline states we felt it would be of interest to investigate this compound in its various states of aggregation by spectroscopic techniques. Therefore, we have recorded the infrared and Raman spectra of glycofaldehyde under various conditions, and some ultraviolet spectra have been obtained in addition. By these methods a correlation between the molecular structure of glycolaldehyde in the vapour, as a melt, as a supercooled liquid, when dissolved in various solvents and in the crystalline state has been attempted.

EXPERIMENTAL

(a) Chemicals ~ly~olaldehyde from Fluka was dried over P,Os in vacuum. The solvents (acet-on&rile, pyridine, methanol and ethanol) were of the highest commercial grade and were not purified further.

The infrared spectra were recorded with a Perkin-Elmer model 225 spectrometer. A 9 cm high temperature cell from RIIC equipped with KBr windows and a thermocouple was empfoyed for the vapour study (Fig. 1). The metal cell was heated and evacuated prior to transferring the sample in order to remove adsorbed moisture. Spectra of melted glycoialdehyde were obtained at 100’ with a Perkin-Eimer high temperature liquid cell, having KBr windows. The supercooled liquid was studied as a capillary between K33rplates at ambient temperature. Saturated solutions of glycolaldehyde in pyridine and acetonitrile gave reasonabiy good infrared spectra, while the solubility in conventional solvents like carbon tetrachloride or & Mol. Structure,4

(1969) 293-302

SPECTROSCOPIC

STUDIES

OF GLYCOLALDEHYDE

295

carbon disulphide was much too low. Infrared spectra of solid glycolaIdehyde as KBr and ICIpellets and in a nujol mull were scanned from 4000-200 cm- ’ (Fig. 2). Raman spectra were recorded with the aid of a Cary 81 spectrometer equipped with a Spectra Physics No_ 125 helium-neon laser_ The solid sample was studied as a pressed disc, but less background scattering was obtained when the powdered sample was placed in a Pyrex tube with a flat bottom (Fig 3). The supercooled liquid as well as a saturated aqueous solution were examined in the standard silica capillary cells. Because of the low solubility in organic solvents (methanol, acetonitrile and pyridine) we obtained no satisfactory Raman spectra of glycolaldehyde in these solvents. Ultraviolet spectra were recorded with a Beckman DK-1 and a Cary 14 spectrometer. The solid was studied as a nujol mull between silica plates, and ca. 0.02 M solutions in water and ethanol were examined in 1 cm silica cells.

RESULTS

AND

DISCUSSION

(a)

Vapour The infrared spectrum of glycolaldehyde vapour at 95” is shown in Fig. 1

Fig.

1. The infrared spectrum of glycolaldehyde

vapour at 95” in a 9 cm cell having KBr windows.

and the observed absorption frequencies are in TabIe 1. The number of observed bands and the vapour phase band contours agree with the assumption of monomeric molecules at this temperature. In particular, the strong absorption band at I753 cm-l with A-type contours reveals the existence of a free carbonyl group characteristic of the monomeric molecule. The following principal moments of inertia were caIculated from the microwave investigationlo: 45.5011, 128.6114 and 168.9048 x 10e4’ g cm’, indicating a structure with CS symmetry_ Accordingly, the 18 fundamental frequencies will divide into 12 of species A’ and 6 of species A”. Since the axis of largest moment of inertia should be perpendicular to the symmetry plane, the A” fundamentals should have C-type contours, and those of species A’ should have contours which J_ Mol. Structure, 4 (1969) 293-302

296

Ii. MICHELSEN,

P. KLABOE

are hybrids of. type A and B. By extrapolation from the Badger and Zumwalt curvesl’ tie calculated the PR separations 22, 18 and 22 cm-’ for the Al, B and C type bands, respectively. No reliable assignment of the fundamentals can be based only upon these criteria, However, the spectrum is closely related to the one reported for acetaIdehydei3_ We believe that 15 fundamentals (12 A’ -r-3 A”) were observed above 400 cm-’ while 3 A” were unobserved. A highly tentative assignment, indicating the proposed atomic motions, is listed in Table 1. TABLE 1 VIBRATIONAL

SPECTRA

OF GLYCOLALDEHYDE

IN THE VAPOUR

AND

THE

Vapowl

Solid

Infrared

Infrareda

cm-l

.I

Assignment

3585 3565 3546

mb m m

Y

vw,sh

2920

2880

m

2835

S

(OH) A’

v&H)

v,(CHI)

3’&332)

2810 2717 2696 1764 1753 1742 1468 1440 1410 1376

1

A’

I

3410

vs

2998 29700

m

28820

STATES

Raman

crnwx

295oc

A”

CRYSTALLINE

m m

&m-l

I

3001 2975 2954 2944 2903

S

2868

W

1456

S

in W VW S

W

A

S W

1 R Q P

W VS

v(C=O)

vs

A’

vs m,b m,b m,b

m

1359 1 1299

W

1282 R 1273 Q 2268 1266 1258 P I

m

1146

w,sh

&rO=-kz~A’ &,dCH) A

1456~ 1395

m,sh w

k&H)z

1364

S

A’

W

1290 m vw m m

J. Moi._Srr~cture,

~(‘-0)

m

1356

ln

1274

S

1241

w,sh

w,sh

A'

&.dCHz)

4 <1969) 293-302

1393

A”

1265

S

1238 1141

S

w,sh

SPECTROSCOPIC

1117 R 1112 ill0 1097 P I

871 86I 859 845 762 752 7.50 743 746 743 738 568 550 535 I

297

S

1107

m

108I

m

1039

vs

s W

Y(C-6)

A’

s

m m m 916

STIJDIES OF GLYCOLALDEIIYDE

&COH) A’

W

IQ98

m

1077

S

1020 1012

m,sh

911 899

S

S S

908 901

m w,sh

820

vs

711

S

s S S

&C--C=O)

A’

S

870

S

810

S

W

m W W

&dCHd

(hot bands)

A”

602

m,b

W

562 551

W W

VW VW

S(C-c-0)

A’

532 426

s s

VW

424 393 265 237

5 w,sfi

m

m W

m

e Bands observed from nujoi mull, except when marked. tl s = strong, m = medium, w = weak, v = very, sh = shoulder, b = broad. c Bands observed from ICI pellet.

Certain features of the gfycoiatdehyde infrared vapour spectrum might be mentioned. Firstly, I@-ES) at 3505 cm-’ might be compared with that of methaIX&~ and ethanol15 at 3637 and 3660 cm-‘, respectively, under low pressure. The somewhat tower frequency in glycolaldehyde indicatesi 6 a weak intramolecular hydrogen bond to-the carbonyl group, stabilizing a planar stru&ure. Moreover, a progression of sharp peaks with approximately 2 cm’ 1 spacing was observed in J. Mid. Structmv~ 4 (l969) 293302

H. MICHELSEN,

298

P. KLABOE

the 1272 cm- ’ band and particularly in the 750 cm-’ band, probably caused by “hot bands” of the type v+vt-vl, v+2v,-2v, etc. in which v, is the unobserved, low frequency torsional mode. (b) Solid The infrared spectrum of glycoIaldehyde in a KI pellet is shown in Fig. 2, the Raman spectrum of the crystailine powder is given in Fig. 3 and the frequencies

t 1ow

I

I

3ooo

I

I

0

2000

l600 FREOUENCY

Fig. 2. The infrared spectrum of glycolaldehyde mull (350-200 cm-‘).

1200

I

I

I

600

unl

(CM-‘)

in a KI disc (above 350 cm-‘)

and in a nujol

FREQUENCY (CM-‘)

Fig. 3. The Raman spectrum of crystalline glycolaldehyde.

are listed in Table 1. It appears that the infrared spectra of glycolaldehyde in the vapour phase and in the crystalline state are very different, and non-equivalent molecular structures in the two states of aggregation are verified. No trace of a carbonyl stretching band could be detected in the solid state spectra, and the few infrared frequencies which were common to the vapour and the solid seem accidental. Moreover, among the 24 infrared and the 24 Raman frequencies listed in Table 1, only nine instances of correspondence were observed. Among these, three corresponding frequencies were situated in the CH, stretching and two in 3. Mol. Structure, 4 (1969) 293-302

SPECTROSCOPIC

STUDIES

299

OF GLYCOLALDEHYDE

the CH2 deformation or wagging region and they are characteristic group fre-

quencies fairly independent of the molecular configuration. Thus, only four remaining infrared bands with Raman counterparts were observed. Therefore, the present spectroscopic data can hardIy be interpreted except in terms of a molecule with a symmetry centre, giving the well known mutual exclusion of ir&?ared and Raman frequencies. The proposed &* 2,5-dihydroxy-I+dioxane structure for the solid glycolaldehyde seems quite probable and agrees with the missing carbonyl stretching band and the proposed symmetry centre. Very little spectroscopic work has been reported on the 1,rldioxane derivatives, but the present data agree well with the dioxane spectra”-lg. It is interesting to note that as many infrared-Raman coincidences due to accidental degeneracy are observed in l+dioxane as in the solid glycolaldehyde. The present data for solid glycolaidehyde can definitely not be interpreted as a mixture of the dihydroxydioxane and a five-membered cyclic diether as previously proposedg. The ultraviolet spectra obtained as an absorption of solid giycolaldehyde in a nujol mull, revealed an absorption band at 256 rnp, indicating an n + n*

transition of a carbonyi group. A small amount of monomeric molecules may be present in the solid, although not observable in the vibrational spectra. From the forthcoming X-ray study”, the definite structure of crystalline glycolaldehyde will be established.

When heated to approximately 96” gtycolaldehyde melts to a viscous liquid,

which only crystahizes after prolonged storage (1-2 days) at room temperature. The infrared spectrum of the melt recorded at 100” (Fig. 4) was practically identical to that of the supercooled Iiquid at ambient temperature. The spectra had very broad absorption bands and appeared quite different from those of the vapour and of the crystalline solid. Raman spectra of the supercooled liquid gave a fairly strong background scattering, the observed infrared and Raman frequencies are listed in Table 2. Some infrared bands of the liquid had Raman counterparts, but most of

, I4ooo

3ooo

, zoo0

600 FREQUENCY

Fig.

4. The infrared

spectrum

1200 KU-‘)

of melted gfycolaldehyde

at 100”. J. Mol. Siructure,

4

(1969)293-302

l-l. MICHELSEN,

300 TABLE

2

VIBRATIONAL

Melt

P. KLABOE

SPECTRA

(row)

OF GLYCOLALDEHYDE

Liquid

Solution

Roman

Infrared

AS

A MELT,

SUPERCOOLED

LIQUID

AND

IN SOLUTION

fit ferpretatio+

Infrared

Raman

Pyridf ne cm-l

I

3440 2937

vs,bb s

2882 1742 1732 1710

don-’

I

2938

s,b

s w,sh s w,sh

2894

s,b

1468 1420

w,sh s,b

I459

1371

s,b 1340

1267 1215 I147 I058 990

s,b

1267 1235 1219

s,b s,b

cm-l

I

Aceionitrfle I cm-’

3175

vs

3460

2875 1743 1730

m m

1.590

W

1265 1248

if45 m

~I054

s

N

w

999

m

880

w,sh

910 890

m m

854 824

s

853

m

773

w

668

w

857

m

809

w

771 744

w

w

990

s

980 960

m

895 870

m m

m

s 810 767

m m

M H-bonded D D D M

I458

VS

1387

W

1348 1303 1272 1248 1225 1150 1108 1053

W W W&l m

DIM

D M DIM M

W W

M D/M

YS

M

W

995

W

882

W

D

850 822

w,sh

M

775

S

D M

m 612

430

VW? ws?

1105

s m m,sh

S

vw?

s,b

m

~560 532

m

s

878

2947 2903 2890 1710

I

VS

s

vw? vw? m

Wafer Acm-’

m

VW? w? w?

492 435

w s

8 M, monomer and D, dimer. b For abbreviations, see footnotes to Table 1.

J. Moi. Structure, 4 (1969) 293-302

D

SPECTROSCOPIC

STUDIES

301

OF GLYCOLALDEHYDE

the infrared or Raman bands were single. The infrared-Raman pairs were frequently observed also in the vapour, and the single ones in the solid spectra (Table I), strongly indicating that monomer and dimer molecuIes are present in the melt_ Certain new infrared or Raman bands in the melt suggest that other species might be present as well. It seems quite probable that although the monomeric molecules have a symmetry plane (cis) in the vapour”, the strong intermolecular hydrogen bonding in the iiquid may favour conformers obtained by rotation around the C-C or C-0 bonds. The extreme broadness of the infrared and Raman bands in the melt, strongly suggest the existence of various ill-defined conformers. Another possible interpretation of the melt and liquid spectra might include the dimers as postulatedg, but in that case several new bands should appear. Moreover, when the melt recrystallizes, the spectra are identical with the previous crystal spectra. The melting and crystallization are therefore reversible processes, and an interconversion between two dimers of entirely different structure seems improbable. Because of the reported 3*4 slow dimer-monomer conversion of gIycoIaIdehyde in solution, we made extensive spectral studies in solution, although the solvents were severely limited by the low solubility. Infrared frequencies in acetonitrile and pyridine solutions, and Raman shifts of glycolaldehyde in water are listed in Table 2. Although many of the frequencies agree with those observed in the melt, the spectra had a “normal” appearance and the bands were not excessively broad. Monomers and dimers are apparently in equilibrium in solution. The infrared bands around 1720 cm-’ clearly reveal carbonyl groups in acetonitrile and pyridine solutions. No corresponding Raman band was observed in aqueous solution. However, the ultraviolet spectra of aqueous and alcoholic solutions clearly demonstrated carbonyl absorption and the Raman carbonyl band-of glycolaldehyde evidently has a low intensity. The band intensities may suggest a preponderance of dimers. The solvents were all proton acceptors and we expect solutesolvent hydrogen bonding increasing in the order acetonitrile, pyridine and water. Since the monomeric as well as the dimeric molecules have hydroxyl groups and can form hydrogen bonds to the solvent mofecules, it is not clear how the solvent will affect the monomer-dimer equ~ibrium. We were not able to detect any changes with time of the ultraviolet absorption band at 256 rnp in aqueous solution. Therefore, the reported3*4 slow dimermonomer conversion in solution was not supported by the present work. Neither did the addition of hydrochloric acid to the aqueous solutions result in any noticeable change in the carbonyl absorption. ACKNOWLEDGEMEWl’

Financial support from the Norwegian Research Council for Science and the Humanities is gratefully acknowledged. L

Mol. Strucrure, 4 (1969) 293-302

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302

P. KLABOE

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J. Mol. Strucrure, 4 (1969) 293-302

San