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A Raman spectral study of the kinetics of deuterium-hydrogen exchange on the formate anion at elevated temperatures and pressures
SPECTROCHIMICA ACTA PART A ELSEVIER
Spectrochimica Acta Part A 52 (1996) 1695 1701
A Raman spectral study of the kinetics of deuterium-hydrogen exchange on the formate anion at elevated temperatures and pressures Richard J. Bartholomew, Wendy J. Stevenson, Donald E. Irish* Guelph-Waterloo Centre for Graduate Work in Chemistry, Department of Chemistry, University of Waterloo, Waterloo, Ont. N2L 3G1, Canada
Received 15 February 1996; revised 7 June 1996; accepted 7 June 1996
Abstract
Raman spectra of the hydrogen-deuterium exchange reaction occurring in the H C O O - -D20 system at elevated temperatures and pressures are reported. The rate constants at four temperatures have been measured and from these an activation energy of around 170 kJ m o l - 1 has been calculated. Exchange also takes place in the D C O O - -H20 system. The rate constants at four temperatures indicate an activation energy of 93 kJ mol 1. Keywords: Activation energy; Formate anion; HCOO
-D20 system; Hydrogen-deuterium exchange; Raman
spectroscopy; Rate constant
I. Introduction
When bonded to the carbon atom of a carbonyl group, hydrogen atoms may undergo exchange with deuterium atoms in neutral, liquid DEO at temperatures near 200°C [1-7]. Early experiments were conducted by holding samples in sealed tubes at a selected temperature and for a fixed time, removing the solvent, burning the dried salt and determining the deuterium oxide content of the resulting water or the salt. Formates and acetates were found to follow first-order kinetics [4-7]. Activation energies and volumes were determined and mechanisms were proposed. Moti* Corresponding author.
vated by the quest for procedures for the production of labelled organic compounds, the exchange process was also studied by N M R spectrometry [8-12]. Recently, in the course of measuring the Raman spectra of formate ion in light and heavy water [13], we observed spectral changes which clearly resulted from the reaction H C O 0 - + D 2 0 -* D C O 0 - + H O D
(1)
R a m a n spectroscopy probes the process at the molecular level and thus it was felt that in situ measurements might further elucidate the mechanism. F r o m the initial study [13] it was observed that the high-frequency component of the Fermi resonance doublet, arising from the vI ( C - H
0584-8539/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved PH S0584-8539(96)01765-5
1696
R.J. Bartholomew et al. / Spectrochimica Acta Part A 52 (1996) 1695 1701
Table t Vibrational modes of aqueous NaHCOOa Description
Position/cm 1 5.0 mol dm 3 NaHCOO H20 5.0 mol dm 3 NaHCOO D20 5.5 mol dm ~ NaDCOO D20
OCO scissoring C H out-of-plane bend Symmetric C-O stretch C H in-plane bend Asymmetric C-O stretch Fermi resonance doublet
764 1063 1350 138l b 2733, 2819
768 1060 1350 1379 1597 2727, 2818
759 915 1324 1025 1587 2028, 2113
~Assignments after Ref. [15]. bWeak, partially masked by v2 (H20). stretch) and the 2v5 ( C - H in-plane bend), decreases from 2818 c m - ~ at 49°C to 2802 cm ~ at 239°C. (For the calculated unperturbed v° (C H stretch) vibration, the shift is from 2783 to 2757 c m - 1 . ) This 26 cm ~ decrease suggests some weakening of the C - H bond as the temperature increases, thereby facilitating the exchange. In this paper, we report results of the study of the kinetics of both process (1) and the complementary reaction DCOO
+ H20-* H C O O
+ HOD
(2)
Excitation was achieved by an argon ion laser operating at 514.5 nm. The mechanical slit width was set at 200 lain. In Tables 2 4 we have provided error estimates for the calculated rate constants and activation energies. The errors given for the rate constants are 95% confidence intervals for slopes of the In (1 + R) vs. t graphs (see below) as calculated from regression analysis. The errors for the activation energies are estimated from the propagation of the relative error in the rate constants.
2. Experimental
3. Results and discussion
Sodium formate was supplied by Aldrich and had a stated purity of greater than 99%. Survey spectra of deuteroformate were collected for solutions of sodium deuteroformate (M.S.D. Laboratories; above 99%). It was from these spectra that the band positions given in Table 1 were taken. However, potassium deuteroformate (obtained from Aldrich; above 99%) was used for the kinetic measurements. The change from sodium to potassium is not expected to have any significant effect on the spectrum of the deuteroformate anion. No further purification was performed on any of the salts. Solutions were prepared with either deionized Milli-Q water or D20 (M.S.D. Laboratories; 99.9% deuterium oxide). The details of the high-pressure furnace, the R a m a n spectrometer and the data manipulation procedures have been previously described [13,14].
The spectra of H C O O H20 and D C O O D20 in the C O stretching region are presented in Fig. 1. As reported previously [13], spectra of these two systems possess some important differences, most notably in the C O stretching region and in the band positions of the Fermi resonance doublet. The band positions and assignments are listed in Table 1 for convenience. The band from the C - O stretch shifts downwards about 25 cm 1 (from 1350 to 1324 cm 1) on deuterium substitution. The Fermi resonance doublet also shifts by an amount consistent with deuterium substitution [13]. R a m a n spectra were collected from 5.0 mol dm 3 N a H C O O - D z O at temperatures of 176, 201, 224, 250 and 274°C and a pressure of 10 MPa (see Figs. 2 4). Spectra were obtained sequentially on the same sample; approximately 5.5
R.J. Bartholomew et al./ Spectrochimica Acta Part A +52 (1996) 1695-1701 Table 2 Regression results for 5.0 mol dm 3 N a H C O O Temp./°C
D20
Regression line
Solution A 190 202 212 222
Correlation
106k,/s i
6t+0.168 5t+0.142 5t+0.206 5t+0.345
0.969 0.981 0.993 0.993
5.0_+1.3 18_+3.7 38_+5.3 95_+21
10 6l 4- 0.158 10-st 4-0.176 10 5t4-0.189 10 5t+0.279
0.996 0.992 0.992 0.998
5.8 _+ 0.27 17 +_ 2.1 45 + 6.4 81 _+ 9.9
Correlation
105k'/s
0.987 0.981 0.953 0.985
2.6 3.0 4.3 4.8
ln[l+R)=5.024x10 In(l+R)=l.830x10 ln(l+R)=3.792x10 ln(l+R)=9.541x10
Solution B 192 202 212 221
In (1 + R) In (1 + R) In (1 + R) in (1 4-R)
= = = -
5.794 1.664 4.507 8.099
Table 3 Regression results for 2.3 mot dm 3 D C O O Temp./°C
Equation of line
213 218 223 227
In ( I 4- R) In (1 + R ) In ( l + R) In (1 + R)
= = = =
2.580 2.993 4.326 4.752
x x x x
H20
x 10 - 5t - 0.003 x 10 ~t4-0.064 x 1 0 5t+0.104 x 10-5t+0.390
h elapsed between the start of the spectrum at 201°C and the start of the spectrum at 274°C. At 201°C and above, the C - O stretching band of DCOO is evident at about 1325 cm ~ (Fig. 2). Concurrent with this change are spectral changes in other regions. Between 2675 and 3025 cm 1 the bands from the Fermi resonance doublet occur. For HCOO D20, one-half of the doublet is obscured by the bands of D 2 0 , but the visible half shows a decrease in intensity with increasing temperature (Fig. 3). Between 1925 and 2225 cm - 1 where the corresponding doublet from DCOO - D 2 0 o c c u r s , a doublet begins to grow (Fig. 4). These three pieces of evidence collectively support the reaction suggested above. As Fig. 5 clearly shows, at 200°C, the 1324 cm ~ band increases in intensity relative to the 1350 cm + L band over a period of 300 min. Furthermore, the rate of isotope exchange increases with increasing temperature. If a rate equation of the form - d[HCOO dt
]
= k[HCOO
1697
][D20 ] = k'[HCOO
+ 0.59 + 0.45 _+0.78 _+ 0.82
is virtually constant) is assumed, an expression relating 11325/11349, k' and t can be derived. Eq. (3) implies pseudo-first-order kinetics. Hamann and Linton [7] have confirmed the validity of this assumption. Under mass balance (assuming Kb for HCOO is very small) (where [D20 ]
Cr = ~ + fl
(4)
where ~ = [ H C O O ] a n d f l = [ D C O O tensity ratio R is defined as follows R - 63:5 It 349
J/~/¢ J~
/~ ~
c~-
~
]. An in-
(5)
(assuming the molar scattering coefficients J~ and
J/¢ are equal). Using the rate law above d~
dt
-
k'a
]
it follows that
(3)
In (1 + R) = k't
(6)
(7)
R.J. Bartholomew et al. / Spectrochimica A eta Part A 52 (1996) ! 695-1701
1698
Table 4 Regression results for Arrhenius plots Solution
Regression line
E=/
Correlation
1)
(kJ mol HCOO - D 2 0 (A) HCOO- - D 2 0 (B) DCOO- - H 2 0
I n k ' = -20.31 x 103(1/T)+31.77 Ink' = -20.24 x 103(1/T)+31.58 In k' = - 11.15 x 103(I/T)+ 12.57
Hence, a plot of In (1 + R) versus t should be linear with a slope of k'. For the analogous reaction of DCOO - + H20 --* HCOO - + HOD, an identical expression may be derived but with R defined as 11349/11325. To test this hypothesis, a kinetic study of this exchange at about 200°C was undertaken. The experiment was performed on two separately prepared solutions of HCOO - D 2 0 , A and B, both 5.0 mol d m - 3 . Raman spectra were collected at equal intervals of 45 min in the region 1250-1460 cm 1. The In (1 + R) vs. t plot is indeed linear (Fig. 6). Time is measured from the beginning of the first spectrum, not from the beginning of the reaction, and thus the points do not pass through the origin. From these data, k' was determined to be 1.8 x 10 -5 s -1 for solution A and 1.7 x 10 5 s 1 for solution B. Hamann and Linton [7] have reported a rate constant for this reaction (at 200°C and 30 MPa) of 1.47 x 10-5 s-1. Considering the differences in the techniques, this dis-
a
-0.994 -0.993 -0.975
169 _+ 34 168 + 34 93 _+ 19
crepancy does not appear to be too great. Directly analogous experiments were performed on the D C O O - - H 2 0 system at four different temperatures (213, 218, 223 and 227°C). The kinetics results are summarized in Table 3 and Fig. 7. Direct comparison of the rate constants for H C O O - - D 2 0 and D C O O - - H 2 0 should only be carried out with extreme caution. These two systems have different ionic strengths and, in general, the ionic strength affects the rate constant of ionic reactions. However, Spinner [16] has reported that reaction (1) is not affected by alkali concentration. The activation energies subsequently calculated are comparable because they depend neither on concentration nor ionic strength. A problem with bandfitting was sometimes encountered when the concentration (at a given time) of HCOO was low. There is a weak shoulder in the high-frequency side of the C - O stretch of H C O O - . It arises from the C - H inplane bend. Obviously, if the C - O stretch is
b
t~ c
_=
J 1250
1350
1450
1255
I 1296
1337
I 1378
f
i
1419
1460
Raman Shift / cm -1
Raman Shift / cm -1
Fig. 1. C O stretching region of (peak a) 5.5 tool dm -3 N a D C O O - D 2 0 and (peak b) 5.0 tool dm -3 N a H C O O - H 2 0 at ambient temperature (normalized).
Fig. 2. Spectra of 5.0 mol dm -3 NaHCOO-D20 (C O stretching region) at (peak a) 176°C, (peak b) 224°C, and (peak c) 274°C.
1699
R.J. Bartholomew et al. / Spectrochimica Acta Part A 52 (1996) 1695-1701
c ID
(I)
_~
a
c b
2675
2800
2925
1250
1320
1390
1460
Raman Shift / cm -1
Raman Shift / cm -1
Fig. 3. Spectra of 5.0 mol dm -3 NaHCOO-D20 (C H stretching region) at (peak a) 176°C, (peak b) 224°C, and (peak c) 274°C.
Fig. 5. Spectra of 5.0 mol dm 3 NaHCOO D20 (C-O stretching region) at (peak a) 0 min, (peak b) 135 min, and (peak c) 315 min (202°C). i
present, so must the C - H in-plane bend be present. Unfortunately, it was found the bandfitting routine gave clearly erroneous results when the intensity of the C - O stretching band was weak. In these cases, the results were not used in any further analysis. The rate constants for this exchange process were measured at four temperatures for the two different solutions and these results are summarized in Table 2. Unfortunately, the temperature range is rather narrow. Below 190°C, the reaction is so slow that the experiment is impractical. Above 220°C, the reaction proceeds too quickly to be followed with this spectrometer. From these
results, Arrhenius plots (Fig. 8) were constructed and the activation energies determined. Values are 169 kJ mol-~ for solution A, 168 kJ mol-~ for solution B and 93 kJ m o l - 1 for DCOO - H 2 0 . Hamann and Linton [7] reported rate constants at four different temperatures between 160 and 200°C for reaction (1). From their results, an activation energy of 148 kJ m o l - 1 may be calculated. As with the rate constants, this value and our value do not appear greatly different. The analytical expressions for the Arrhenius plots and the correlation coefficients are summarized in Table 4. This exchange reaction was also observed for a more concentrated solution (9.6 mol dm -3) of 0.54
i
i
+ 0.33 (::
1925
2025
2125
2225
Raman Shift / cm -1
0.12 0
I
I
6500
13000
19500
Time / s e c .
Fig. 4. Spectra of 5.0 mol dm 3 NaHCOO_D20 (C D stretching region) at (peak a) 176°C, (peak b) 224°C, and (peak c) 274°C.
Fig. 6. Plot of In (1 + R ) vs. time for 5.0 mol dm -3 HCOO - D 2 0 isotope exchange. Solution A, 202°C; solution B, 201°C.
1700
R.J. Bartholomew et al./ Spectrochimica Acta Part A 52 (1996) 1695-I701 1.80
1.35 nr
+ y..E 0.90
g
0.45
I 15000
30000
reactions involve different activated complexes and therefore different mechanisms. Possibly the rate determining step is not C - H (D) bond scission. In the reactions as studied here the D20 and H20 have dual roles: they act as both solvent and reagent. There is then the possibility of a large solvent isotope effect contributing to the large difference in the activation energies. Hamann and Linton [7] have suggested the following simple, concerted mechanism for this exchange reaction:
Time /sec.
Fig. 7. Plot of In (1 + R ) vs. time for 2.3 tool dm -3 DCOOH20 at 227°C.
N a H C O O - D 2 0 but no kinetics measurements were made at this concentration. Notwithstanding the rather large errors (approximately 15 20%), the activation energies for these two systems are quite different and this is somewhat surprising. If the mechanism of the reaction were the same for both systems and if the rate determining step were C - H (D) bond scission (assuming the C H (D) scission is rate-determining does not seem unreasonable in light of the strength of the bond), one would expect to see a primary isotope effect. In other words, the DCOO - H 2 0 should have a higher activation energy because of the lower zero point energy of the C - D bond. However, this is clearly not the case. Therefore, it could be inferred that the two -9.1
i " ~
--e----
: Solution 'A'
---~--
: Solution 'B'
-10.3
..E = -11.5
HCO0- / 020
-12.7 1.99
= 2.08
10 "~ / T
~
2.17
/ K-t
Fig. 8. Arrhenius plots for the isotope exchange; In k' against 103/T.
0~, ,,H @;C + /O--D 0 D
~o~C".H'"'~O--D ] *
""d
O\ @~C, 0
]
H D
+
"O--D
This proposal is based on the predicted value for A V~o. Their reasoning was as follows. With this mechanism A V*o should be very small because the solvating D20 molecule is already held close to the formate ion by electrostatic forces and forming the transition state should not lead to greater electrostriction of the solvent. They did, in fact, observe only a small value for A Vo. If it is assumed that both reactions (1) and (2) follow the same mechanism, our results indicate that the [DCOO '"H20] ~ complex is considerably more stable than the [ H C O O - ' " D 2 0 ] ++complex. The structural difference between these two activated complexes is quite small and it is hard to see how such a small structural change could account for such a large change in the activation energy. Therefore, this large difference in activation energy strongly hints that either the two reactions follow different mechanisms or there is a very strong solvent isotope effect.
R.J. Bartholomew et al. / Spectrochhnica Acta Part A 52 (1996) 1695 1701
Acknowledgements T h i s w o r k was s u p p o r t e d by g r a n t s f r o m the Natural Sciences a n d Engineering Research C o u n c i l o f C a n a d a . T h i s r e s e a r c h was s t i m u l a t e d by results f o u n d by R i c h a r d Visser, an u n d e r g r a d u a t e w o r k i n g in o u r l a b o r a t o r y . W e w o u l d also like to t h a n k D r S.D. H a m a n n f o r p r o v i d i n g a r e p r i n t o f his p a p e r [7].
References [l] K.F. Bonhoeffer, Trans. Faraday Soc., 34 (1938) 252. [2] L.D.C. Bok and K.H.Geib, Z. Elektrochem., 44 (1938) 695. [3] L.D.C. Bok and K. Geib, Z. Phys. Chem., Abt. A, 183 (1939) 353. [4] L.D.C. Bok and M.D. Cohen, J. S. Aft. Chem. Inst., 4 (1951) 37.
1701
[5] L.D.C. Bok and L.G. Mitchell, J. S. Afr. Chem. Inst., 4 (1951) 51. [6] L.D.C. Bok and L.B. Petters, J. Chem. Soc., (1952) 1524. [7] S.D. Hamann and M. Linton, Aust. J. Chem., 30 (1977) 1883. [8] J.G. Atkinson, J.J. Csakvary, G.T. Herbert and R.S. Stuart, J. Am. Chem. Soc., 90 (1968) 498. [9] J.G. Atkinson, D.W. Cillis and R.S. Stuart, Can. J. Chem.. 47 (1969) 477. [10] J.G. Atkinson. M.O. Luke and R.S. Stuart, Chem. Cornmum, (1969) 283. [11] P. Bdlanger, J.G. Atkinson and R.S. Stuart, Chem. Commun., (1969) 1067. [12] N.H. Werstiuk and C. Ju, Can. J. Chem., 67 (1989) 5. [13] R.J. Bartholomew and D.E. Irish, Can. J. Chem., 71 (1993) 1728. [14] D.E. Irish, T. Jarv and C.I. Ratcliffe, Appl. Spectrosc., 36 (1982) 137. [15] K. Ito and H.J. Bernstein, Can. J. Chem., 34 (1956) 170. [16] E. Spinner, paper presented at the Natl. Conf. of the Organic Div. of the Royal Australian Chemical Institute, Brisbane, 24 28 August, t975 (as quoted in [7]).
Spectrochimica Acta Part A 52 (1996) 1695 1701
A Raman spectral study of the kinetics of deuterium-hydrogen exchange on the formate anion at elevated temperatures and pressures Richard J. Bartholomew, Wendy J. Stevenson, Donald E. Irish* Guelph-Waterloo Centre for Graduate Work in Chemistry, Department of Chemistry, University of Waterloo, Waterloo, Ont. N2L 3G1, Canada
Received 15 February 1996; revised 7 June 1996; accepted 7 June 1996
Abstract
Raman spectra of the hydrogen-deuterium exchange reaction occurring in the H C O O - -D20 system at elevated temperatures and pressures are reported. The rate constants at four temperatures have been measured and from these an activation energy of around 170 kJ m o l - 1 has been calculated. Exchange also takes place in the D C O O - -H20 system. The rate constants at four temperatures indicate an activation energy of 93 kJ mol 1. Keywords: Activation energy; Formate anion; HCOO
-D20 system; Hydrogen-deuterium exchange; Raman
spectroscopy; Rate constant
I. Introduction
When bonded to the carbon atom of a carbonyl group, hydrogen atoms may undergo exchange with deuterium atoms in neutral, liquid DEO at temperatures near 200°C [1-7]. Early experiments were conducted by holding samples in sealed tubes at a selected temperature and for a fixed time, removing the solvent, burning the dried salt and determining the deuterium oxide content of the resulting water or the salt. Formates and acetates were found to follow first-order kinetics [4-7]. Activation energies and volumes were determined and mechanisms were proposed. Moti* Corresponding author.
vated by the quest for procedures for the production of labelled organic compounds, the exchange process was also studied by N M R spectrometry [8-12]. Recently, in the course of measuring the Raman spectra of formate ion in light and heavy water [13], we observed spectral changes which clearly resulted from the reaction H C O 0 - + D 2 0 -* D C O 0 - + H O D
(1)
R a m a n spectroscopy probes the process at the molecular level and thus it was felt that in situ measurements might further elucidate the mechanism. F r o m the initial study [13] it was observed that the high-frequency component of the Fermi resonance doublet, arising from the vI ( C - H
0584-8539/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved PH S0584-8539(96)01765-5
1696
R.J. Bartholomew et al. / Spectrochimica Acta Part A 52 (1996) 1695 1701
Table t Vibrational modes of aqueous NaHCOOa Description
Position/cm 1 5.0 mol dm 3 NaHCOO H20 5.0 mol dm 3 NaHCOO D20 5.5 mol dm ~ NaDCOO D20
OCO scissoring C H out-of-plane bend Symmetric C-O stretch C H in-plane bend Asymmetric C-O stretch Fermi resonance doublet
764 1063 1350 138l b 2733, 2819
768 1060 1350 1379 1597 2727, 2818
759 915 1324 1025 1587 2028, 2113
~Assignments after Ref. [15]. bWeak, partially masked by v2 (H20). stretch) and the 2v5 ( C - H in-plane bend), decreases from 2818 c m - ~ at 49°C to 2802 cm ~ at 239°C. (For the calculated unperturbed v° (C H stretch) vibration, the shift is from 2783 to 2757 c m - 1 . ) This 26 cm ~ decrease suggests some weakening of the C - H bond as the temperature increases, thereby facilitating the exchange. In this paper, we report results of the study of the kinetics of both process (1) and the complementary reaction DCOO
+ H20-* H C O O
+ HOD
(2)
Excitation was achieved by an argon ion laser operating at 514.5 nm. The mechanical slit width was set at 200 lain. In Tables 2 4 we have provided error estimates for the calculated rate constants and activation energies. The errors given for the rate constants are 95% confidence intervals for slopes of the In (1 + R) vs. t graphs (see below) as calculated from regression analysis. The errors for the activation energies are estimated from the propagation of the relative error in the rate constants.
2. Experimental
3. Results and discussion
Sodium formate was supplied by Aldrich and had a stated purity of greater than 99%. Survey spectra of deuteroformate were collected for solutions of sodium deuteroformate (M.S.D. Laboratories; above 99%). It was from these spectra that the band positions given in Table 1 were taken. However, potassium deuteroformate (obtained from Aldrich; above 99%) was used for the kinetic measurements. The change from sodium to potassium is not expected to have any significant effect on the spectrum of the deuteroformate anion. No further purification was performed on any of the salts. Solutions were prepared with either deionized Milli-Q water or D20 (M.S.D. Laboratories; 99.9% deuterium oxide). The details of the high-pressure furnace, the R a m a n spectrometer and the data manipulation procedures have been previously described [13,14].
The spectra of H C O O H20 and D C O O D20 in the C O stretching region are presented in Fig. 1. As reported previously [13], spectra of these two systems possess some important differences, most notably in the C O stretching region and in the band positions of the Fermi resonance doublet. The band positions and assignments are listed in Table 1 for convenience. The band from the C - O stretch shifts downwards about 25 cm 1 (from 1350 to 1324 cm 1) on deuterium substitution. The Fermi resonance doublet also shifts by an amount consistent with deuterium substitution [13]. R a m a n spectra were collected from 5.0 mol dm 3 N a H C O O - D z O at temperatures of 176, 201, 224, 250 and 274°C and a pressure of 10 MPa (see Figs. 2 4). Spectra were obtained sequentially on the same sample; approximately 5.5
R.J. Bartholomew et al./ Spectrochimica Acta Part A +52 (1996) 1695-1701 Table 2 Regression results for 5.0 mol dm 3 N a H C O O Temp./°C
D20
Regression line
Solution A 190 202 212 222
Correlation
106k,/s i
6t+0.168 5t+0.142 5t+0.206 5t+0.345
0.969 0.981 0.993 0.993
5.0_+1.3 18_+3.7 38_+5.3 95_+21
10 6l 4- 0.158 10-st 4-0.176 10 5t4-0.189 10 5t+0.279
0.996 0.992 0.992 0.998
5.8 _+ 0.27 17 +_ 2.1 45 + 6.4 81 _+ 9.9
Correlation
105k'/s
0.987 0.981 0.953 0.985
2.6 3.0 4.3 4.8
ln[l+R)=5.024x10 In(l+R)=l.830x10 ln(l+R)=3.792x10 ln(l+R)=9.541x10
Solution B 192 202 212 221
In (1 + R) In (1 + R) In (1 + R) in (1 4-R)
= = = -
5.794 1.664 4.507 8.099
Table 3 Regression results for 2.3 mot dm 3 D C O O Temp./°C
Equation of line
213 218 223 227
In ( I 4- R) In (1 + R ) In ( l + R) In (1 + R)
= = = =
2.580 2.993 4.326 4.752
x x x x
H20
x 10 - 5t - 0.003 x 10 ~t4-0.064 x 1 0 5t+0.104 x 10-5t+0.390
h elapsed between the start of the spectrum at 201°C and the start of the spectrum at 274°C. At 201°C and above, the C - O stretching band of DCOO is evident at about 1325 cm ~ (Fig. 2). Concurrent with this change are spectral changes in other regions. Between 2675 and 3025 cm 1 the bands from the Fermi resonance doublet occur. For HCOO D20, one-half of the doublet is obscured by the bands of D 2 0 , but the visible half shows a decrease in intensity with increasing temperature (Fig. 3). Between 1925 and 2225 cm - 1 where the corresponding doublet from DCOO - D 2 0 o c c u r s , a doublet begins to grow (Fig. 4). These three pieces of evidence collectively support the reaction suggested above. As Fig. 5 clearly shows, at 200°C, the 1324 cm ~ band increases in intensity relative to the 1350 cm + L band over a period of 300 min. Furthermore, the rate of isotope exchange increases with increasing temperature. If a rate equation of the form - d[HCOO dt
]
= k[HCOO
1697
][D20 ] = k'[HCOO
+ 0.59 + 0.45 _+0.78 _+ 0.82
is virtually constant) is assumed, an expression relating 11325/11349, k' and t can be derived. Eq. (3) implies pseudo-first-order kinetics. Hamann and Linton [7] have confirmed the validity of this assumption. Under mass balance (assuming Kb for HCOO is very small) (where [D20 ]
Cr = ~ + fl
(4)
where ~ = [ H C O O ] a n d f l = [ D C O O tensity ratio R is defined as follows R - 63:5 It 349
J/~/¢ J~
/~ ~
c~-
~
]. An in-
(5)
(assuming the molar scattering coefficients J~ and
J/¢ are equal). Using the rate law above d~
dt
-
k'a
]
it follows that
(3)
In (1 + R) = k't
(6)
(7)
R.J. Bartholomew et al. / Spectrochimica A eta Part A 52 (1996) ! 695-1701
1698
Table 4 Regression results for Arrhenius plots Solution
Regression line
E=/
Correlation
1)
(kJ mol HCOO - D 2 0 (A) HCOO- - D 2 0 (B) DCOO- - H 2 0
I n k ' = -20.31 x 103(1/T)+31.77 Ink' = -20.24 x 103(1/T)+31.58 In k' = - 11.15 x 103(I/T)+ 12.57
Hence, a plot of In (1 + R) versus t should be linear with a slope of k'. For the analogous reaction of DCOO - + H20 --* HCOO - + HOD, an identical expression may be derived but with R defined as 11349/11325. To test this hypothesis, a kinetic study of this exchange at about 200°C was undertaken. The experiment was performed on two separately prepared solutions of HCOO - D 2 0 , A and B, both 5.0 mol d m - 3 . Raman spectra were collected at equal intervals of 45 min in the region 1250-1460 cm 1. The In (1 + R) vs. t plot is indeed linear (Fig. 6). Time is measured from the beginning of the first spectrum, not from the beginning of the reaction, and thus the points do not pass through the origin. From these data, k' was determined to be 1.8 x 10 -5 s -1 for solution A and 1.7 x 10 5 s 1 for solution B. Hamann and Linton [7] have reported a rate constant for this reaction (at 200°C and 30 MPa) of 1.47 x 10-5 s-1. Considering the differences in the techniques, this dis-
a
-0.994 -0.993 -0.975
169 _+ 34 168 + 34 93 _+ 19
crepancy does not appear to be too great. Directly analogous experiments were performed on the D C O O - - H 2 0 system at four different temperatures (213, 218, 223 and 227°C). The kinetics results are summarized in Table 3 and Fig. 7. Direct comparison of the rate constants for H C O O - - D 2 0 and D C O O - - H 2 0 should only be carried out with extreme caution. These two systems have different ionic strengths and, in general, the ionic strength affects the rate constant of ionic reactions. However, Spinner [16] has reported that reaction (1) is not affected by alkali concentration. The activation energies subsequently calculated are comparable because they depend neither on concentration nor ionic strength. A problem with bandfitting was sometimes encountered when the concentration (at a given time) of HCOO was low. There is a weak shoulder in the high-frequency side of the C - O stretch of H C O O - . It arises from the C - H inplane bend. Obviously, if the C - O stretch is
b
t~ c
_=
J 1250
1350
1450
1255
I 1296
1337
I 1378
f
i
1419
1460
Raman Shift / cm -1
Raman Shift / cm -1
Fig. 1. C O stretching region of (peak a) 5.5 tool dm -3 N a D C O O - D 2 0 and (peak b) 5.0 tool dm -3 N a H C O O - H 2 0 at ambient temperature (normalized).
Fig. 2. Spectra of 5.0 mol dm -3 NaHCOO-D20 (C O stretching region) at (peak a) 176°C, (peak b) 224°C, and (peak c) 274°C.
1699
R.J. Bartholomew et al. / Spectrochimica Acta Part A 52 (1996) 1695-1701
c ID
(I)
_~
a
c b
2675
2800
2925
1250
1320
1390
1460
Raman Shift / cm -1
Raman Shift / cm -1
Fig. 3. Spectra of 5.0 mol dm -3 NaHCOO-D20 (C H stretching region) at (peak a) 176°C, (peak b) 224°C, and (peak c) 274°C.
Fig. 5. Spectra of 5.0 mol dm 3 NaHCOO D20 (C-O stretching region) at (peak a) 0 min, (peak b) 135 min, and (peak c) 315 min (202°C). i
present, so must the C - H in-plane bend be present. Unfortunately, it was found the bandfitting routine gave clearly erroneous results when the intensity of the C - O stretching band was weak. In these cases, the results were not used in any further analysis. The rate constants for this exchange process were measured at four temperatures for the two different solutions and these results are summarized in Table 2. Unfortunately, the temperature range is rather narrow. Below 190°C, the reaction is so slow that the experiment is impractical. Above 220°C, the reaction proceeds too quickly to be followed with this spectrometer. From these
results, Arrhenius plots (Fig. 8) were constructed and the activation energies determined. Values are 169 kJ mol-~ for solution A, 168 kJ mol-~ for solution B and 93 kJ m o l - 1 for DCOO - H 2 0 . Hamann and Linton [7] reported rate constants at four different temperatures between 160 and 200°C for reaction (1). From their results, an activation energy of 148 kJ m o l - 1 may be calculated. As with the rate constants, this value and our value do not appear greatly different. The analytical expressions for the Arrhenius plots and the correlation coefficients are summarized in Table 4. This exchange reaction was also observed for a more concentrated solution (9.6 mol dm -3) of 0.54
i
i
+ 0.33 (::
1925
2025
2125
2225
Raman Shift / cm -1
0.12 0
I
I
6500
13000
19500
Time / s e c .
Fig. 4. Spectra of 5.0 mol dm 3 NaHCOO_D20 (C D stretching region) at (peak a) 176°C, (peak b) 224°C, and (peak c) 274°C.
Fig. 6. Plot of In (1 + R ) vs. time for 5.0 mol dm -3 HCOO - D 2 0 isotope exchange. Solution A, 202°C; solution B, 201°C.
1700
R.J. Bartholomew et al./ Spectrochimica Acta Part A 52 (1996) 1695-I701 1.80
1.35 nr
+ y..E 0.90
g
0.45
I 15000
30000
reactions involve different activated complexes and therefore different mechanisms. Possibly the rate determining step is not C - H (D) bond scission. In the reactions as studied here the D20 and H20 have dual roles: they act as both solvent and reagent. There is then the possibility of a large solvent isotope effect contributing to the large difference in the activation energies. Hamann and Linton [7] have suggested the following simple, concerted mechanism for this exchange reaction:
Time /sec.
Fig. 7. Plot of In (1 + R ) vs. time for 2.3 tool dm -3 DCOOH20 at 227°C.
N a H C O O - D 2 0 but no kinetics measurements were made at this concentration. Notwithstanding the rather large errors (approximately 15 20%), the activation energies for these two systems are quite different and this is somewhat surprising. If the mechanism of the reaction were the same for both systems and if the rate determining step were C - H (D) bond scission (assuming the C H (D) scission is rate-determining does not seem unreasonable in light of the strength of the bond), one would expect to see a primary isotope effect. In other words, the DCOO - H 2 0 should have a higher activation energy because of the lower zero point energy of the C - D bond. However, this is clearly not the case. Therefore, it could be inferred that the two -9.1
i " ~
--e----
: Solution 'A'
---~--
: Solution 'B'
-10.3
..E = -11.5
HCO0- / 020
-12.7 1.99
= 2.08
10 "~ / T
~
2.17
/ K-t
Fig. 8. Arrhenius plots for the isotope exchange; In k' against 103/T.
0~, ,,H @;C + /O--D 0 D
~o~C".H'"'~O--D ] *
""d
O\ @~C, 0
]
H D
+
"O--D
This proposal is based on the predicted value for A V~o. Their reasoning was as follows. With this mechanism A V*o should be very small because the solvating D20 molecule is already held close to the formate ion by electrostatic forces and forming the transition state should not lead to greater electrostriction of the solvent. They did, in fact, observe only a small value for A Vo. If it is assumed that both reactions (1) and (2) follow the same mechanism, our results indicate that the [DCOO '"H20] ~ complex is considerably more stable than the [ H C O O - ' " D 2 0 ] ++complex. The structural difference between these two activated complexes is quite small and it is hard to see how such a small structural change could account for such a large change in the activation energy. Therefore, this large difference in activation energy strongly hints that either the two reactions follow different mechanisms or there is a very strong solvent isotope effect.
R.J. Bartholomew et al. / Spectrochhnica Acta Part A 52 (1996) 1695 1701
Acknowledgements T h i s w o r k was s u p p o r t e d by g r a n t s f r o m the Natural Sciences a n d Engineering Research C o u n c i l o f C a n a d a . T h i s r e s e a r c h was s t i m u l a t e d by results f o u n d by R i c h a r d Visser, an u n d e r g r a d u a t e w o r k i n g in o u r l a b o r a t o r y . W e w o u l d also like to t h a n k D r S.D. H a m a n n f o r p r o v i d i n g a r e p r i n t o f his p a p e r [7].
References [l] K.F. Bonhoeffer, Trans. Faraday Soc., 34 (1938) 252. [2] L.D.C. Bok and K.H.Geib, Z. Elektrochem., 44 (1938) 695. [3] L.D.C. Bok and K. Geib, Z. Phys. Chem., Abt. A, 183 (1939) 353. [4] L.D.C. Bok and M.D. Cohen, J. S. Aft. Chem. Inst., 4 (1951) 37.
1701
[5] L.D.C. Bok and L.G. Mitchell, J. S. Afr. Chem. Inst., 4 (1951) 51. [6] L.D.C. Bok and L.B. Petters, J. Chem. Soc., (1952) 1524. [7] S.D. Hamann and M. Linton, Aust. J. Chem., 30 (1977) 1883. [8] J.G. Atkinson, J.J. Csakvary, G.T. Herbert and R.S. Stuart, J. Am. Chem. Soc., 90 (1968) 498. [9] J.G. Atkinson, D.W. Cillis and R.S. Stuart, Can. J. Chem.. 47 (1969) 477. [10] J.G. Atkinson. M.O. Luke and R.S. Stuart, Chem. Cornmum, (1969) 283. [11] P. Bdlanger, J.G. Atkinson and R.S. Stuart, Chem. Commun., (1969) 1067. [12] N.H. Werstiuk and C. Ju, Can. J. Chem., 67 (1989) 5. [13] R.J. Bartholomew and D.E. Irish, Can. J. Chem., 71 (1993) 1728. [14] D.E. Irish, T. Jarv and C.I. Ratcliffe, Appl. Spectrosc., 36 (1982) 137. [15] K. Ito and H.J. Bernstein, Can. J. Chem., 34 (1956) 170. [16] E. Spinner, paper presented at the Natl. Conf. of the Organic Div. of the Royal Australian Chemical Institute, Brisbane, 24 28 August, t975 (as quoted in [7]).