BRUCEM. PAVAAND FREDE. STAFFORD
4628
Spectra of Gaseous Protio- and Deuteriooxalic Acids1 by Bruce M. Pava and Fred E. Stafford Department of Chemistry and The Materials Reaearch Center, Northwestern University, Evanston, Illimis 60601 (Received July 1 , 1968)
The ir spectra from 400 to 4000 cm-l were obtained for gaseous oxalic acid and oxalic acid-&. These are consistent with a chelated structure for the gaseous molecules. The carbonyl stretching frequencies of the dicarbonyl species X(C0)ZY correlate with the electronegativities of X and Y, as do the C=O stretching frequencies in the XCOY system.
Introduction I n the spectra of a-alkoxy and a-keto carboxylic acids in dilute solutions, two OH stretching frequencies appear in the 3400-3600 cm-' region. The higher frequency band was assigned to the free form; it appears between 3500 and 3550 cm-l. The lower frequency band, 3400-3500 cm-', was assigned to the proton-chelated form. It was found that the lower frequency band appears only when a five-membered internally hydrogen bonded ring can be formed.2 The ir spectra of pyruvic acid (PA), methylpyruvic acid (MPA), dimethylpyruvic acid (DMPA), and trimethylpyruvic acid (TMPA) have been studied also in the gas phase at approximately 100". Two bands due to OH stretching modes were found in the 34003600 cm-' region. The higher frequency band always appeared between 3560 and 3575 cm-I. The lower frequency band appeared between 3440 and 3470 cm-l. I n the spectrum of trimethylpyruvic acid-d, two OD stretching bands were observed at 2559 and 2636 cm-I. Again, the higher frequency bands were assigned to the nonchelated structure; the lower frequency bands were assigned to the chelated structure3 (Table I). Table I : The OH Absorption Frequencies of Gaseous Pyruvic Acid, Methylpyruvic Acid, Dimethylpyruvic Acid, and Trimethylpyruvic Acid at 100"" Acid
PA MPA (I
DMPA TMPA Reference 3.
----Frequency, Nonchelsted
3573 3574 3570 3560
cm-1-
Chelated
3465 3463 3458 3441
Oxalic acid also has a proton-accepting group a to the carboxyl group. Although the condensed phase has been extensively studied,4,6particularly with respect to hydrogen bonding, only electron diffraction resultse are reported for the vapor. This lack of data is probably due to the thermal decomposition of oxalic The Journal of Phyeical Chemistry
acid. The purpose of this work is to determine the ir spectrum of gaseous oxalic acid to help deduce its structure.
Experimental Section The anhydrous oxalic acid was purchased from the Fisher Chemical Co. The anhydrous deuterated oxalic acid was prepared by repeatedly dissolving oxalic acid in DzO and evaporating under vacuum. Because of the toxicity of oxalic acid, precautions were taken to keep it out of the air and off the body. Spectra were obtained using a 1-m mullite cell with KBr windows. A side arm permitted the cell to be evacuated and refilled with argon. A Thermac, R. I. Controls, Minneapolis, Minn., was used to regulate the temperature and a Beckman IR-9 spectrometer wm used to obtain the spectra. All spectra were taken in single-beam operation with the internal chopper.' A cycle consisted of recording spectra a t room temperature, next a t a series of selected higher temperatures, and then again a t room temperature. Every experiment was recycled a t least once. Data and Results The spectra of gaseous oxalic acid and oxalic acid-& were first observed a t approximately 105'. At higher temperatures, white deposits sometimes coated the ends of the cell. Decomposition, at approximately 140°,was evidenced by the appearance of bands due to carbon monoxide and formic acid as well as by the reduction in intensity of the bands that were assigned to oxalic acid. The frequencies of the observed bands are in Table I1 and Figure 1. (1) Supported by the U. S. Army Research Office, Durham, N. C., and the National Science Foundation through an undergraduate research grant. (2) M.Oki and M. Hirota, Nippon Kaoaku Zasahi, 81, 855 (1960). (3) A. Schellenberger, W. Beer, and G. Oehme, Spectrochim. Acta, 21, 1345 (1965). (4) R.G.Delaplane and J. A. Ibers, J . Chem. Phya., 45, 3451 (1966), and earlier references. (5) J. Reynolds and S. Sternstein, ibid., 41, 47 (1964). (6) S. Shibata and M. Kimura, Bull. Chem. SOC.F p . , 27,485 (1964). (7) S. M.Chackalackal and F. E. Stafford, J . Anter. Chem. SOC.,88, 723 (1966).
INFRARED SPECTRA OF GASEOUS PROTIOAND DEUTERIOOXALIC ACIDS
I o
4000
1
8
I
1
3000
1
1
~
"
"
zoo0
"
"
"
4629
'
J I500
loo0
Figure 1. The ir spectra of oxalic acid and oxalic acid42 (bg, background; gc, grating change). No features were observed between 1000 and 400 cm-1. Note the scale change a t 2000 cm-l.
Table I1 : I r Absorption Frequencies of Gaseous Oxalic Acid and Oxalic Acid-dz, 110-140"" 0
--Frequency, 3485 sh
2575 sh
1830 vs 1810 1325 s
1800 vs 1820 vs} 1220 vs
vsl
1240 br a
om-1-F
Assignments
1900
-
1800
-
O-H(D) str C=O str
... ...
c
sh, sharp; vs, very strong; s, strong; br, broad; str, stretch.
6
1700 I
4.0
Only one OH and only one OD stretching band were found for oxalic acid and oxalic acid-ch. The observed OH and OD stretching frequencies correspond in shape, location, and relative intensity to those assigned to the proton-chelated structure of pyruvic acid3 (Table 1). For compounds of the type XCOY (X, Y = F, OH, C1, CH3,H, etc.), it is known that the carbonyl stretching frequency8-lo varies systematically with the electronegativityll of X and Y, as discussed by Kagarise.lz This relationship is shown by the open circles in Figure 2. That this correlation can be extended successfully to symmetric molecules of the type X(CO),X is shown by the filled circles. I n addition, the frequencies for the two carbonyls of CH3(C0)20Hcorrelate well if each is treated individually using, respectively, the electronegativities of CH3 and OH (half-filled circles). The frequencies shown in Figure 2 are for oxalic acid and ~ chloride,14 oxalic acid-dz, as well as for g l ~ o x a l , 'oxalyl and methyl oxalate. A very weak feature which may be a Q branch some-
I
3.0
I-
2.0
Electronegativity
Figure 2. Carbonyl frequency (cm-1) us. average electronegativity11 for gaseous XCOY (open circles), X(C0)zX (filled circles), and CHa COCOOH (half-filled circles); see the text. Points for isotopically substituted molecules (e.g., -OH and -OD) are connected by vertical lines and demonstrate the weakness of using frequencies for such a correlation. See the text. The data are from Nakamoto? Bellamye (pp 135, 155, 165-67, 179), and Herzberg.lo Data concerning the X(C0);Y systems are from B. D. Saksena and R. E. Kagarise, J . Chem. Phys., 19, 987 (1951), R. K. Harris, Spectrochim. Acta, 20, 1129 (1964), and this work, (8) K. Nakamoto, "Infrared Spectra of Inorganic and Coordination Compounds," John Wiley & Sons, Inc., New York, N. Y., 1963. (9) L. J. Bellamy, "The Infrared Spectra of Complex Molecules," John Wiley & Sons, Inc., New York, N. Y., 1968. (10) G. Herzberg, "Infrared and Raman Spectra," D. Van Nostrand Co., Inc., Princeton, N. J., 1945. (11) A. L. Allred, J . Inorg. Nucl. Chem., 17, 215 (1961). (12) R. E. Kagarise, J. Amer. Chem. Soc., 77, 1377 (1955). (13) R. K. Harris, Spectrochim. Acta, 20, 1129 (1964). (14) B. D. Saksena and R. E. Kagarise, J. Chem. Phys., 19, 987
(1951). Volume 7.9, Number 18 December 1988
4630 times appeared in the oxalic acid spectrum a t 1817 cm-' between the two prominent branches of the C=O :stretching band. Low resolution and strong back,ground, however, prevented assigning it to the oxalic acid spectrum. Another similar feature appeared in the oxalic acid-dz spectrum a t 1808 cm-'. This band also was too weakly resolved to be assigned to the spectrum of oxalic acid-dz.
Discussion The OH stretching bands of oxalic acid and oxalic acid-4 are consistent with a predominantly chelated structure. Although only one absorption frequency was found for the OH stretch in oxalic acid, a weaker one at a higher frequency could have been hidden by the water vapor background. In the oxalic acid-& spectrum, however, this background was at a minimum, and the intensity of the second band, if present, was 501, or less of the chelated band intensity. The assumption of a chelated structure for the two oxalic acids helps explain the planarity of the gaseous oxalic acid molecule deduced by Shibata and Kimura from electron diffraction data.6 Electron resonance structures, as invoked to explain the planarity of glyceraldehyde, oxalyl chloride, and butadiene, also help to explain this planar structure. I n Figure 2, the three vertical lines connect points for isotopically substituted molecues in which X and Y are H, F and D, F; H, OH and D, OH; and H, H and D, D. Quite clearly the CO frequency depends on the mass as
The Journal of Physical Chemistry
BRUCEM. PAVA AND FRED E. STAFFORD well as the electronegativity of X and Y . Overend and SchereP have shown that the Urey-Bradley C=O force cpstants are sensibly constant a t 12.7 f 0.1 mdyn/A, as are the corresponding C=O bond distances for Cl&O, F&O, and Br2C0. They have also offered an explanation for the observed variation of frequency. Subsequently k ( C 0 ) in HzCO has been reportedI6 to be 12.6 f 0.2 m d y n j i . Shirk and Pimentel have shown that k ( C 0 ) in HCO, FCO, and ClCO also are relatively insensitive to the electronegativity of the attached atom. The case of the XzCO thus seems to be qualitatively different from those of XaPO1' and XS02Y,18where the -PO and -SOz frequencies seem to be insensitive to the masses of X and Y and where the change in MO force constants can be attributed to the coming into play of the 3d orbitals.
Acknowledgments. The assistance of Dr. Brian G. Ward and Mr. Robert Delaplane is gratefully acknowledged. Acquisition of the infrared spectrometer was made possible by a National Science Foundation Institutional Facilities Grant. (15) J. Overend and J. R. Scherer, J. Chem. Phys., 3 2 , 1296 (1960). (16) J. 5. Shirk and G. C. Pimentel, J . A m r . Chem. Soc., 90, 3349 (1968). (17) 5. M.Chackalackal and F. E. Stafford, ibid., 88, 4823 (1966),
and references cited therein. (18) M. Spoliti, S. M. Chackalackal, and F. E. Stafford, ibid., 89, 1092 (1967),and references cited therein.
