FORMATE IONPYROLYSIS IN ALKALIHALIDEMATRICES
centration, one would expect this to have an effect on the diffusion coefficient. In contrast to their findings there was no noticeable systematic variation of the diffusion coefficient with concentration in the experiments reported here. However, if one assumes that the subhalide dissociates into Pb2+ ions and an electron pair with the latter moving as an entity, then the mobility of the subhalide as well as the diffusion coefficient would remain constant. This may clearly
1281
be seen by applying Klemm's equations to electron pairs instead of electrons. Further experiments are probably needed to resolve this problem.
Acknowledgments. The authors wish to thank Mr. Raymond J. Heus for designing the spectrophotometer furnace. They also wish to acknowledge the encouragement of and helpful discussions with Dr. Richard H. Wiswall.
The Kinetics of Formate Ion Pyrolysis in Alkali Halide Matrices132
by K. 0. Hartman and I. C. Hisatsune Department of Chemistry, Whitmore Laboratory, T h e Pennsulvania State University, University P a r k , Pennsylvania 16808 (Received .Vorember 16, 1965)
The pyrolysis of the formate ion isolated in KCl, KBr, KI, and XaBr matrices was investigated from 500 to 620" by infrared spectroscopy. The infrared spectra of these pressed disks after heating showed that the formate ion was distorted as it went into solid solution with the matrix. The COe valence angle in the distorted ion was estimated to be 136 f 10" from the isotopic frequency product rule. The decomposition reaction of the formate ion in solid solution with KBr was found to be second order in formate with a rate constant of 2.0 X 1Olo exp[(-50,700 & 3500)/RT] M-' see-l. Carbonate was obtained in 85% yield and traces of monomeric bicarbonate were also observed. The pyrolysis of formate-d showed a primary kinetic isotope effect, but no isotope effect was detected. Although the observed activation energy was independent of the matrix within experimental uncertainty, tthe rate constants were consistently higher in K I and NaBr matrices than in KBr and KC1 matrices.
Introduction Earlier we reported a kinetic study of the pyrolysis of calcium formate dispersed in a KBr matrix by infrared spectroscopy. I n the present investigation the pyrolysis of sodium formate in KCl, KBr, KI, and WaBr matrices was studied by the same experimental technique in order to test the reaction mechanism proposed earlier and to determine the effect of environmental changes on the decomposition kinetics of the formate ion. Numerous, although fragmentary, studies have been reported in the literature on the thermal decomposi-
tion of undiluted sodium and potassium formates. For example, Levi and Piva4 observed that the decomposition of sodium formate between 330 and 370" yielded an equimolar mixture of oxalate and carbonate. Above 500" they found the solid product to be ex(1) This work was supported by the National Science Foundation, Grant NSF-G17346, and by the Directorate of Chemical Sciences, Air Force Office of Scientific Research, Grant AF-AFOSR, 907-65. (2) Abstracted in part from the Ph.D. Thesis of K. 0. Hartman. (3) K. 0 . Hartman and I. C. Hisatsune, J . Phys. Chem., 69, 583 (1965). (4) M. G. Levi and A. Piva, Ann. Chem. Applicata, 5, 271 (1915); Chem. Abstr., 10, 2561 (1916).
V o l u m e '70, N u m b e r 4
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clusively carbonate. Freidlin5 confirmed these results and also showed that oxalate was the predominant solid product in the 390 to 430" temperature range. At a decomposition temperature below 330°, the main product reported by Takagis was carbonate. Takagi also reported this reaction to be second order in formate. Other decomposition products observed by previous investigators were CO and Hz. Freidlin5 postulated that formaldehyde was the precursor for these gases, and this molecule was in fact detected by T~yoda.~ I n our study the formate ion was diluted with different alkali halide salts, and the mixtures were pressed into disks. These disks were heated in the 500 to 620" temperature range, and the decomposition of the solute was followed by observing the changes in the infrared spectrum of the disk. Carbonate was found to be the principal reaction product, but traces of bicarbonate were also observed. The reaction was second order in formate in contrast with the first-order rate observed earlier in calcium formate3 decomposition. However, the kinetic results obtained in the present study were found to be consistent with the mechanism proposed before.
Experimental Section Optical grade KC1, KBr (powdered), and K I crystals were from Harshaw. Reagent grade NaBr from Fisher was used directly. Sodium formate was a Baker Analyzed reagent and was recrystallized from water. Matheson Coleman and Bell was the source of potassium formate, and this was used without, further purification. Sodium formate-d (99%) from Ilerck Sharp and Dohme of Canada and sodium formate-I3C (547,) from Bio-Rad Laboratories were also used directly. Sodium and potassium carbonates for intensity calibrations were Fisher reagent grade products. The technique and apparatus for pellet fabrication and heating, the infrared spectrometers, the procedure for determining the reaction stoichiometry, and the method of obtaining the kinetic data all have been described before. A conventional glass low-temperature cell with a cold finger adapted to hold a pressed disk was used to record the infrared spectrum of the sample at liquid nitrogen temperature. The KBr matrix Was used most extensively, and experimental results obtained with this matrix have the highest precision. I n solid solution studies and in kinetic runs. recrystallized sodium formate was used most often as the source of the formate ion since exactly the same results were obtained when potassium formate was used instead. Solute concentrations The Journal of Physical Chemistry
K. 0. HARTMAN A N D I. C. HISATSUNE
40d0' ' ' 3000 ''
' '
2000
1200
1600
800
400
FREQUENCY (cm-l)
Figure 1. The infrared spectrum of sodium formate in a KBr matrix: upper spectrum, before heating: and lower spectrum, after heating for 2 min at 500".
ranged from about 0.1 to 5.0 mg of solute saltlg of matrix.
Results A typical infrared spectrum of a KBr disk in which sodium formate had been dispersed by grinding is shown in Figure 1 (upper spectrum). Variation of the grinding time from 1 to 3 min or changing the pressure used to press the disk had no effect on the spectrum. A disk prepared by the freeze-dry method generally gave a sharper spectrum, but the frequencies of the bands in this spectrum were the same as those observed from a disk prepared by grinding a formate salt with the same cation as that of the matrix. Isotopic frequencies observed from sodium and potassium formates dispersed in KBr disks by grinding are summarized in Table I. Table I : Sodium and Potassium Formate Frequencies (cm-1) Observed in Unheated KBr Disks" Modeb
v((2-H)
v,(COz) v,dC-H) VdCOZ) (v, C-H 1 V,(COd
NaHW02
NaH13COz
KaDl?CO?
KH'ZCO?
2832 1606 1366 1366 1069 7i4
2818 1560 1365 1336 c 765
2131 1595 1014 1331 931 766
2810 1.590 1368 1343 C
764
Spectra recorded at 25'. * a: = antisymmetric stretch, bending, u = symmetric stretch, and w = out-of-plane uag. Not observed. a
p
=
The infrared spectra of sodium formate dispersed by grinding in KC1, KBr, KI, and NaBr matrices were the same as long as these disks were not heated. However, (5) L. Kh. Freidlin, J . A p p l . Chem. U S S R , 11, 975 (1938). (6) S. Tskagi, J . Chem. SOC.Japan, 60,625 (1939); Chem. Abstr., 36, 6401 (1942).
(7) R. Toyodit, ~ ~ ~Inst. i i Chem. . Res., K
~ O ~ nit., O
2 0 , 11 (1950).
FORMATE IONPYROLYSIS IN ALKALIHALIDE MATRICES
when such d i s h were heated for a short time at temperatures above about 240", they exhibited remarkable changes in their absorption spectra. An example of this heating effect on a typical KBr disk is shown in Figure 1. Here, the lower spectrum was obtained by heating at 500" for 2 min a KBr disk which gave initially the upper spectrum. I n this new spectrum the changes in relative intensities and frequency shifts of the original formate bands are apparent, and in addition two new strong bands have appeared where none were present before. These new bands, however, were demonstrated to originate from the same chemical species, which produced the remaining three strong bands in the lower spectrum, by band optical density correlations over a wide range of solute concentrations and by the observation that the new bands and the other three strong bands all decayed with the same rate when the disk was heated at higher temperatures. That the new spectrum was due to a decomposition product of the normal formate ion was ruled out also by the following observations. If a heated disk was allowed to stand for a few weeks at room temperature or if it was ground in air and then repressed, the original formate spectrum was partially regenerated. If the disk was dissolved in mater and freeze dried, the original spectrum was completely regenerated and the new bands disappeared. Reheating of the freeze-dried disk again yie!ded the new spectrum. Also, the new spectrum was generated even at 100" if the disk was heated for a few weeks. Thus, the new spectrum is still due to the formate ion, but one whose structure has been significantly altered by the matrix. The frequencies, together with their assignments, of isotopically substituted formate ions observed in heated KBr disks are eummarized in Table 11.
Table I1 : Frequencies (cm-1) of Isotopically Substituted Formate Ions in Heated KBr Matrix"
V(
Modeb
H'ZCOz-
H13C02 -
D12C02-
C-H)
2666.2 1632.9 1444.6 1350 4 752 3
2644.3 1590.7 1444.6 1329.5 745 0
1995.6 1621.9 1065 4 1327 7 744.8
VU(C0Z) up( C-H) vu( C02)
vp(C0J
' All spectra recorded at - 190'. Out-of-plane mode not observed.
* See footnote b of Table I.
Since sodium formate melts at 255O, the new spectrum may have been due to formation of a potassium bromide-sodium formate eutectic mixture. Indeed, a eutectic melting at 233" was observed when the
1283
mole fraction of sodium formate in KBr was 0.91. However, if a sample with this mole fraction was heated above 250", quenched to room temperature, mixed with more KBr, and pressed into a pellet, its spectrum was that of normal formate. None of the samples prepared from melts in which the mole fraction of sodium formate ranged from 0.85 to near 1.0 showed the shifted spectrum. On the other hand, KBr disks containing sodium formate in high dilution readily produced the shifted spectrum when they were heated for a few hours at 240". I n Table I11 the frequencies of HW02- in different heated matrices are listed. These same frequencies were obtained by heating either sodium formate or potassium formate in the matrices. Also, when a heated KBr-formate disk was diluted with KC1, pressed into a pellet, and then heated, its spectrum changed from that characteristic of the KBr matrix into one characteristic of the KC1 matrix. Table 111: HW02- Frequencies (em-') in Different Heated Matrices Modea
KClb
KBrb
KI
NaBrC
v(C-H)
2683 1643 1456 1357 755
2666.2 1632 9 1444 6 1350 4 752 3
2643 1617 1429 1344 753
2732 1633 1448 1363 763
VU(C0Z)
vpP(C-H
1
Va(CO2) va(CO2)
a See footnote b of Table I. Spectrum recorded at 25'.
Spectrum recorded at -190".
Prolonged heating of a formate-KBr disk at temperatures above 500" caused the solute to decompose, and carbonate was identified as the principal product in the matrix. The infrared spectrum of the carbonate produced by this decomposition was the same as that obtained from a heated KBr disk containing sodium or potassium carbonate. We also observed as a minor reaction product monomeric bicarbonate, whose infrared spectrum we have characterized and reported earlier.* Other weak infrared bands observed during the course of the pyrolysis reaction were due to trapped carbon dioxide and cyanate ion. The latter ion appeared as a common impurity in heated KBr disks containing solid chemical reagents.* Seither oxalate nor carbon monoxide was ever detected in our heated disks. However, our disks were generally much darker after heating than the previous calcium formate ~ a m p l e s ,and ~ hence CO was disproportionating into (8) D. L. Bernitt, K. 0. Hartman, and I. C. Hisatsune, J . Chem. Phys., 42, 3553 (1965).
Volume 70, Sumber 4 April 1966
K. 0. H A R T M A E ; AND I. C. HISATSUNE
1284
COz and carbon to a greater extent in the present studies. The minor product bicarbonate was observed during pyrolysis in KC1, KBr, and K I matrices but not in NaBr disks. The concentration of this product was usually maximum after the first heating cycle and remained the same or decreased with further heating. The pyrolysis of formate-d produced bicarbonate-d, which decreased after the initial heating, and eventually a small amount of unlabeled bicarbonate was formed. Heated disks ground in an oxygen atmosphere and then reheated gave slightly more bicarbonate than those ground in air. The stoichiometry of the decomposition reaction was analyzed by using the known initial concentrations of sodium formate and by determining the final carbonate concentrations from intensity calibration of its 8 8 0 - ~ m - ~nondegenerate, , out-of-plane wag infrared band. I n eight runs in which the initial sodium formate concentration ranged from 1.05 to 4.1 mg of solute/g of KBr, the mole ratio of formate to carbonate was 2.4 j= 0.23. This ratio corresponded to a carbonate yield of' 85 f 7%. When the initial solute concentration was less than 1.0 mg/g, the carbonate yield fell below 507,. However, in this case the relative amount of bicarbonate produced was higher even though the absolute concentration of this minor product was about the same as in higher concentration kinetic runs. Thus, the principal reaction at higher concentrations is
2HC02- = COS2-
+ H, + CO
The reaction order was determined from the slopes of log rate vs log concentration linear plots. The 752-cm-' formate band was used for this purpose, and from five samples an order of 2.0 i 0.1 with respect to formate was obtained. I n general, the reciprocal of the formate concentration was a linear function of time over two to three reaction half-lives. However, two high concentration samples followed a first-order rate law through the first 40 to 60% of the reaction and then a second-order rate through the remainder. The rate constants obtained from typical secondorder plots of the KBr data are listed in Table IV. The 752-cm -' formate band was used most extensively, and average deviations of the constants in two or three runs are also given in Table IV. The second-order rate constants for the decomposition in KCl, KI, and KaBr matrices are given in Table V. We have also listed here for comparison the calculated KBr constants for the corresponding temperatures. Only the 752-cm-1 band was used to study the kinetics in different matrxes. One KBr disk having a solute concentration of 27.4 mg/g was studied by the isothermal The Journal of Physical Chemistry
thermogravimetric analysis at 570". This study gave a second-order rate constant of 1.7 X M-' sec-' which agreed well with 1.8 X M-' sec-' calculated from the infrared data.
Table IV : Second-Order Rate Constants for HC02- Decomposition in a KBr Matrix r 10%
a
-
752 em-'
,bf-1 see-' 2666 om-'
880 em-'
O C
(H C O I - ) ~
(HCOz-)
(Cos*-)
497 f 2 512 & 1 533 f 1 578 i 3 614 f 2
7 . 1 f0 . 3 18 i 4 69 f 5 220 i 40 540 f 60
9.0 15 73 250 650
7 5 15 62 220 420
Temp,
Average value from two or three kinetic runs.
Table V : Second-Order Rate Constants for HCOz- Decomposition in Different 31atrices" Temp,
a
7 -
IO%,
M-1
see-l----
OC
KC1
KBr
KI
NaBr
501 i 1 548 f 2 576 f 2 595 f 2 600 zt 2
14 74
10 62 220 330 400
56 290
70 500 1940
780
1850
Rate constants from the 752-cm-1 formate band.
The Arrhenius activation energies and frequency factors calculated from kinetic data obtained in different matrices and with different infrared bands are summarized in Table VI. The 3.5-kcal/mole uncertainty in the activation energies is based on a 15% error in the reaction rate constants. The kinetic isotope effects were also investigated. The activation energy and frequency factors for formate-d decomposition in a KBr matrix are given in Table VI while the observed k H / k D ratios are listed in Table VII. The carbon isotope effect was examined by using a formate sample containing 54% carbon-13 and by recording the spectrum with the sample cooled to liquid nitrogen temperature where the lZC- and W-formate C-0 symmetric stretch bands were clearly resolved. However, a t 550" no kinetic isotope effect was observed, as both isotopic bands decayed a t the same rate.
Discussion The sodium and potassium formate fundamental frequencies observed initially in unheated pressed
FORMATE I O N
Table VI: Experimental Activation Energy and Frequency Factor
Frequency, cm-1
Activation energy, kcal/mole
Frequency factor, M-1 8ec-1
Potassium Bromide Matrix 50 7 + 3 5 752 (formate] 51 7 + 3 5 2666 (formate 1 880 (carbonate) 49 5 & 3 5 54 5 1 3 5 745 (formate-d)
2 0 x 4 1x 80 X 10 x
10'0 10'0 lo9 10'2
Different Matrices (752-Cm-' Formate Band) Matrix
53.0 =t 3 . 5 51.7 + 3 . 5 54.3 =!= 3.5
KCl KI NaBr
Table VII:
1 . 3 X 10" 2 . 2 x 10" 1 . 6 X 1012
Kinetic Isotope Effect ..Y/
Temp, OK
769 i:2 820 i:2 885 rir: 2 Temperature dependence a
1285
PYROLYSIS I N ALKALIHALIDE ATATRICES
Observed, this study.
Formate ion in KBra
1.80 f 0.55 1.52 f 0.45 1 . 2 4 f 0.40 exp[(i-3800 =IC 7000)/RT]
."U
Calcium formate in KBrb
2.25 1.69 1.24 exp(+7000/RT)
Calculated from data in ref 3.
disks (Table I) agree well with those reported by other investigat~rs.~Moreover, these frequencies are not markedly different from those of salts with other cat i o n ~ ~ bor. ' ~ from the infrared and Raman frequencies of the aqueous ion." Frequencies observed from our heated disks, however, are quite different from all previously reported values. Various experimental observations described in the Results section indicate that the new spectrum obtained from a heated disk must be due t o the exchange of the cation of the original formate salt and to subsequent diffusion of the formate ion into the matrix forming a solid solution. The final frequencies observed in potassium halide matrices depend on the anion of the matrix as shown in Table I11 and are significantly different from those of potassium formate (Table I). In particular, the new CH bond stretch infrared band appears 144 cm-1 to the lower frequency of the original position while the bending mode of the same bond is shifted 77 cm-1 to a higher frequency. Also, comparison of the frequencies in Tables I and I1 shows that the symmetric A1 modes are shifted to the red and the anti-
symmetric B1 modes are shifted to the blue in the solid solution spectra of all isotopic formate ions. The remarkable changes in the infrared spectrum produced by heating a formate-KBr disk suggest that the formate ion has become distorted as it formed a solid solution with the matrix. The shifted isotopic frequencies for the B1 modes, however, can still be predicted quite well by the CzV symmetry frequency product rule. Thus, the symmetry of the distorted ion appears to be CZv,the same as in sodium formate single crystal.12 The directions and magnitudes of frequency shifts in the CH bond stretching and bending mode infrared bands and the changes in relative intensities of the CO bond symmetric and antisymmetric stretch bands indicate that the CH bond has become weaker and the COz valence angle has increased in the distorted ion. Both of these structural changes are consistent with the interpretation that the p character of the carbon hybrid atomic orbitals, which form the u-bond frame of the ion, has changed. We expect that an increase in the p character of the CH bond carbon atomic orbital will cause the stretch and the bend frequencies to decrease and to increase, respectively. Conversely, a decrease in the p character of the CO bond carbon orbitals should produce a blue shift in the antisymmetric CO bond stretch infrared band and an increase in the COZ angle. The intensity of the antisymmetric stretch band should also increase relative to that of the symmetric mode. I n the limit when the carbon orbital is pure sp hybrid, the COZ group dipole change with respect to symmetric stretch will be zero, and this vibration becomes forbidden in the infrared. The COZ valence angle in the distorted formate ion can be calculated by equating the B1 class frequency product rule value to its theoretical expre~sion.'~ Different combinations of observed isotopic frequencies listed in Table I1 can be used for this purpose. With reasonable values of 1.09 and 1.25 .4, respectively, for the C-H and the C-0 bond distances, the best estimate of the valence angle was 136 =t 10". Here, the lower limit in the angle was obtained by using di(9) (a) R. M. Hammaker and J. P. Walters, Spectrochim. Acta, 20, 1311 (1964); (b) K. B. Harvey, B. A. Morron-, and H. F. Shurvell, Can. J . Chem., 41, 1181 (1963). (10) 3. D. Donaldson, J. F. Knifton, and S. D. Ross, Spectrochim. Acta, 20, 847 (1964). (11) K. Ito and H. J. Bernstein, Can. J . Chem., 34, 170 (1956). (12) W. H. Zachariasen, J . Chem. Phys., 1, 634 (1933). (13) G. -berg, "Infrared and Raman Spectra of Polyatomic Molecules, D. Van Nostrand Co., New York, N. Y., 1945, p 180. There is a typographical error in eq 11-218. The mass factor in the last term on the right side of this equation should be mxmy instead of mx2,
V o l u m e 70, Number 4
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rectly the observed product rule values in the calculation. The upper limit, on the other hand, was determined from corrected product rule values. Since there were deviations between the observed and calculated product rule values for the undistorted ion, it was assumed that similar deviations occurred in the distorted ion. Thus, deviations found in the normal ion were added to the experimental product rule values of the distorted ion for the same combination of isotopic frequencies. The geometry of the distorted ion estimated above together with the observed frequencies allows us to calculate the vibrational force constants which can be compared to those of the normal ion. Such calculations showed that our assignment of the shifted spectrum was reasonable and that the differences between the force constants of the normal and distorted ions were consistent with our interpretation of the changes in structure. Interestingly, we found that the CH bond stretch force constant decreased by 11% and the bend constant increased by 11% when the formate ion was distorted. Also, the CO bond stretch constant increased by 137, while the COZ angle bend constant showed a 25% decrease. The ease with which the formate ion diffused through our pressed disks, even at loo", was indeed surprising. Evidently, the activation energy associated with this diffusion process must be very much smaller than the decomposition activation energies listed in Table VI. These reaction activation energies observed in different matrices are the same within experimental uncertainty, and the frequency factors are also similar to those observed for many second-order reactions in the gas phase.14 In fact, one might deduce from these kinetic parameters that our decomposition is not occurring in the solid state but may be taking place in a liquid KBr-formate phase. However, this interpretation can be ruled out by the following arguments. The decomposition of undiluted formate occurs at a measurable rate at 300", but the half-life of this reaction in the disk at about 500" is 100 hr. Since the solute is dispersed in the matrix initially as crystallites, it is riot unreasonable to expect the concentration of the formate in the liquid KBr-formate phase, if it were produced at all, to be appreciable. Thus, such a large difference in the reaction half-lives of diluted and undiluted systems is not reasonable. Also, no shifted infrared spectrum was observed with samples prepared from high-concentration formate-KBr melts. The reaction half-life in the matrix at 500" is so long that we expect the KBr-formate liquid phase to be essentially in equilibrium with the surrounding solid KBr phase. The observed second-order rate is then unThe Journal of Physical Chemistry
K. 0. HARTMAN AND I. C. HISATSUNE
reasonable in view of the phase rule relationship between the two components at constant pressure and temperature. Although the activation energies obtained in different matrices were the same within the estimated uncertainty, the rate constants in K I and NaBr were higher than those in KC1 and KBr matrices as shown in Table V. If the same activation energy of 51 kcal/ mole is used for all matrices, then the frequency factors in KI and S a B r matrices are larger than those in KBr and KCl by a factor of about seven. Similarly, the entropies of activation suggest differences in the two sets of matrices. The activation entropy, based on a standard state of 1 ill, was -14 i 5 cal/deg mole in KBr and KC1 while it was - 11 i 5 cal/deg mole in K I and NaBr matrices. A reaction mechanism similar to that proposed earlier for calcium formate decomposition3 can satisfactorily account for the kinetic results obtained in the present investigation. I n this mechanism two formate ions react to form the following transition complex, which 0-
I .oI 1 Ha. .C=o I
O=C..
H
then decomposes into carbonate and formaldehyde. The inclusion of formaldehyde as the initial reaction product and as the source of CO and Hz is consistent with Freidlin's postulate5 and with Toyoda's observation.' The activation energy of 50.7 i 3.5 kcal/ mole observed here agrees well with 52.0 f 3.0 kcal/ mole obtained in calcium formate decomposition. We also observed a primary kinetic isotope effect in the present system, and the 3.8-kcal/mole temperature dependence of k H / h is, within experimental error, the same as that in calcium formate system as shown in Table VII. Although the differences are well within our estimated error, both the activation energy and the temperature dependence of the kinetic isotope effect are larger in calcium formate than in the present system. This is reasonable since both the C-H stretch frequency and its deuterium isotope shift are larger in calcium formate than in formate-KBr solid solution system. The yield of carbonate observed in the present study was generally low, and it was even more so when the (14) See, for example, A. A. Frost and R. G. Pearson, "Kinetics and Mechanism," John Wiley and Sons, Inc., New York, N. Y.,1953,P 101.
FORMATE ION PYROLYSIS IN ALKALIHALIDEMATRICES
formate in the matrix was in high dilution. The principal cause of this poor yield appears to be the side reaction which generated the bicarbonate ion. Since the maximum concentration of this by-product was observed after the first heating cycle of the disk and was about the same from one disk to another, oxygen and water trapped in the matrix may be involved in this side reaction. Observations in formate-d studies and in grinding experiments described in the Results section also support this interpretation. We suggested earlier that this side reaction may be the reverse of an unusual reaction encountered in the thermal reduction of bicarbonate to formate and that both of these reactions may involve the carbon dioxide anion free radi~a1.l~I n order to test this hypothesis, we examined the esr spectra of formate disks during and after the decomposition reaction. These disks generally showed a weak but sharp signal with g = 2.0028, and similar signals were observed in the heated bicarbonate disks. This signal disappeared when the disk was dissolved in water and then freeze dried again. The intensity of this esr signal also decreased markedly or disappeared when the disk was ground in air. However, neither formate-d nor formate enriched in carbon-
1287
13 changed this esr spectrum. Thus, this radical, although unidentified, does not appear to be a significant intermediate in the side reaction or in the thermal reduction reaction. This negative result still does not rule out the possibility that GOz- radical may be involved in these reactions, because in another investigation carried out in our laboratory16 we observed that the COZ- radical is not stable at the decomposition temperatures used in the present studies and also that this radical reacts with water to produce formate and bicarbonate ions.
Acknowledgments. We are grateful to Professor R. M. Hammaker for his generous gift of 13C-enriched sodium formate, to Dr. F. E. Freeberg for thermal gravimetric analysis, to Mr. E. F. Reichenbecher for scanning the esr spectra, and to Dr. D. L. Bernitt for force constant analysis. We are pleased to acknowledge the financial support from the National Science Foundation and the Air Force Office of Scientific Research. (15) I. C. Hisatsune and K. 0. Hartman, Science, 145, 1455 (1964). (16) K.0.Hartman and I. C. Hisatsune, t o be published.
Volume 70,Number 4 April 1066
