1166
J. Phys. Chem. 1083, 8 7 , 1166-1169
Electron and Organic Radical Anion Solvation. Pulse Radiolysis of Tetrahydrofuran and Its Solutions of N-Methylacetamide or Pyrrolidone T. H. Tran-Thi and A. M. Koulkes-Pujo' Laboratoire Associ6 au CNRS (LA 331). mpartemenf de Physico Chimle, Cen-Saciay, 9 1 19 1 Gif sur Yvette Cedex, France (Received: July 22, 1982; In Final Form: October 20, 1982)
The competition between electron solvation and presolvated electron capture by an amide solute (linear or cyclic),N-methylacetamide (NMA) or pyrrolidone (Pyr), was studied in tetrahydrofuran (THF) by using the pulse radiolysis technique. A simple competition law was found, [OD,-]-l being proportional to the scavenger concentration for both NMA and Pyr. Evidence of dry organic anion formation was shown, but stabilization occurred only for NMA- giving rise to a transient absorbing solvated species, NM$-. A relationship between [ODNM&-]-l and was established, which takes into account both the competition between electron solvation and presolvated electron capture by NMA, and the competition between NMA- solvation and its reaction with NMA. A mechanistic scheme of the electron attachment process in the liquid phase is discussed and compared with that in the dense gas phase.
Introduction Electron attachment to the same molecular species in both the gas phase and the liquid phase remains a largely unexplored field, especially in the case of large, polar, organic molecules. Nevertheless, the formation of the parent negative ions of polyatomic molecules is often the important step in many of their chemical reactions. A study of electron reactivity with four linear and cyclic amides, N-methylacetamide (NMA), dimethylacetamide (DMA), pyrrolidone (Pyr), and N-methylpyrrolidone (NMPyr), has been performed in the gas phase with a view to elucidating the problems related to the effect of the nature and density of the environment on the electron attachment process.' In the dilute gas phase we have shown, by mass spectrometry, the existence of different dissociative attachment processes at thermal energy, giving rise to [NMA(-H)]-, [DMA(-H)]-, [Pyr(-H)]-, [NMPyr(-H)]-, and dimer anions [(NMA),(-H)]- and [(Pyr),(-H)]-. In electron swarm experiments where the attaching gas (amide) is mixed in very small proportion (1/105) with a nonattaching gas, the stabilizing effect of high pressure (500-8000 torr) on the formation of the amide anions has been shown for the four compounds. These results lead us to assume the existence of similar attachment processes in the liquid phase, where the stabilization of transient anions would occur through nonreactive collisions with the solvent molecules. However, no evidence of NMA- or Pyranions has ever been obtained in protic or dipolar aprotic solvents such as propanol, glycerol, water,, dimethylformamide, DMA, and NMPyr3 probably because of their high reactivity with these media, so that the stabilization process does not occur. Since tetrahydrofuran (THF), a polar aprotic solvent, is often used in anionic polymerizations as a nonreactive solvent, one may ask if transitory solvation of the amide anions could occur in THF. Furthermore, THF is a hydrogen-bond acceptor (though only a weak donor) and stabilization of organic anions could involve such interactions. The existence of hydrogen bonding has been shown in the binary mixtures of THF with formamide, N-methylformamide, dimethylformamide, me(1)T. H. Tran-Thi, L. G. Christophorou, A. M. Koulkes-Pujo, D. L. McKorkle, and J. G. Carter, work in progress. (2) T. H. Tran-Thi, A. M. Koulkes-Pujo, and J. Sutton, Journees d'Etude sur la Chimie des Radiations, Louvain-la-Neuve, Belgique, June 1982, communication. (3) T. H. Tran-Thi and A. M. Koulkes-Pujo, Tihany Symp. Radiat. Chem., Proc. 5 , in press.
thylpropionamide, and butylacetamide, by S ~ r i . ~ These different considerations have led us to study the electron attachment reaction and the electron and anion solvation processes in NMA-THF and Pyr-THF media, using the pulse radiolysis technique coupled with fast absorption spectrophotometry (visible and near-infrared).
Experimental Section NMA ultrapure reagent grade from Merck was purified by using the Pucci, Vedel, Tremillon m e t h ~ d .Pyr ~ from Merck was bidistilled under reduced pressure of argon. THF p.a. from Uvasol, refluxed over CuCl, was distilled over sodium metal, in the presence of benzophenone. The pulse radiolysis experiments were carried out with a Febetron 707 delivering 204s (base) pulses of 1.8-MeV electrons, with doses from 20 to 50 krd. The analyzing beam came from a high-voltage pulsed xenon source (XBO 450) emitting a continuum between 220 and 830 nm and more or less intense emission lines with maxima at 940, 1020,1120,1300,and 1520 nm. Between 600 and 1500 nm, a Huet monochromator (blazed for 1.0 pm) coupled with a silicon photodiode (BPY 13 A) or a germanium photodiode (JlGLD),depending on the region explored, was used to measure the intensity of the transmitted light at each wavelength. Appropriate filters were used to avoid second-order diffraction contributions. The output electrical signal was amplified with a new type of low-frequency amplifier developed by Anitoff.6 A bias circuit with variable voltage (-5.3, -7.5, -15 V) was incorporated for the polarization of the fast-response photodiodes. A more detailed description and the circuit scheme of this fastresponse (5 ns) and low-noise (4 dB) amplifier will be published.6 Optical densities of irradiated solutions (normalized to a constant dose) were measured as a function of time, at each wavelength studied. Values of initial optical densities were obtained by extrapolation to time t = 0 corresponding to the end of the pulse.
-
Results and Discussion The spectra of the transient absorbing species formed during the radiolysis of pure THF and its mixtures with NMA are reported in Figure 1. In pure THF, the con-
s. K. Suri, Thermochim. Acta, 39,325-8 (1980). (5) Pucci, Vedel, and Tremillon, J.Electroanal. Chem., 22, 253 (1969). (6) 0. Anitoff, Reu. Sci. Instrum., submitted for publication.
(4)
0022-3654/83/2087-1166$0 1.50/0 0 1983 American Chemical Society
The Journal of Physical Chemistry, Vol. 87, No. 7, 1983
Electron and Organic Radical Anion Solvation
U
1
3
2
1167
(NMA) M
Flgure 3. Rates of disappearance of ea- and NMA,- vs. NMA concentration. Figure 1. Absorption spectra of transient species formed in irradiated pure THF and its solutions containing NMA.
[
OOfI v c PUR
-+-
('t.
/
Figure 2 shows the initial optical spectra of each species A and B, for different NMA concentrations. We have attributed the absorption, ODo (B), to electrons solvated by THF which disappear slowly in the medium with a rate equal to 2.3 X lo6 9-l independent of the NMA concentration (Figure 3). We attribute the second absorption band, with a Maximum at 1200 nm, to a solvated anionic species coming from NMA- formed by the capture of presolvated electrons scavenged by the molecules of NMA. For [NMA] 2 3.98 M, THF-solvated electrons were no longer seen, but the absorption of NMA, remained. Addition of the strong acid, HC102NMA (4.35 X M), suppressed this NM&absorption band. This was attributed to anion capture by H+ as described by the following reaction: N W -
Flgure 2. Absorption bands of ea- and NMA, in irradiated pure THF and its solution of NMA obtained by deconvolution of spectra in Figure 1.
tinuous absorption band has been attributed to electrons solvated by THF molecules. Its maximum lies at 2120 nm.' Addition of NMA (0.52 M) decreased the initial optical density and produced a shift of the maximum toward the visible region. This phenomenon increased with increasing NMA concentration (1.12 and 3.87 M) as shown in the figure. Meanwhile, oscilloscope traces such as Figure 1 clearly indicated the existence in the solution of two transient species of greatly different lifetimes (A and B) and therefore the absorption band shift may be only apparent. The disappearance-kinetic curves of the two transient species In (OD) = f ( t )established at different wavelengths show two lines with distinct slopes. Extrapolation to time t = 0 from the second slope gives the initial optical density of the longer-lived species B which disappears in the medium in a monotonic way. Extrapolation to t = 0 from the slope of the first line after correction for the contribution of B gives a value of the initial optical density ODo (A). (7)F. Y. Jou and L. M. Dorfman, J. Chem. Phys., 58, 4715 (1973). (8)R.K.Wolff, M. J. Bronskill, and J. W. Hunt, J. Chem. Phys., 53, 4211 (1970). (9)K. Y. Lam and J. W. Hunt, Int. J. Radiat. Phys. Chem., 7,317 (1975).
+ H+
-
NMAH.
In NMA-THF mixtures, N W - disappears with pseudofirst-order kinetics at a rate depending on the total NMA concentration. The rate constant of disappearance of NM&- by reaction with NMA calculated from the slope of Figure 3 gives I Z N m - + N m = 5.1 X lo6 M-' s-'. Thus, in THF, NM&- is more reactive toward NMA than is the solvated electron. The decrease of the initial optical density of the THFsolvated electrons as the NMA concentration increases shows up a competition between presolvated electron scavenging by NMA and solvation of electrons by THF. Furthermore, increasing the NMA concentration decreases the initial absorption of NMA;. We account for this in terms of the reaction with NMA of the presolvated NMAcoming from a transient resonant state NMA*-. As in the gas phase (electron swarm experiments),' NMA*- can autoionize or lose its excess energy, leading to NMA- which can then either be stabilized through nonreactive collisions or react with the scavenger solute. In the light of our results, we propose the following mechanism of formation and disappearance of electrons created in excess in NMA-THF mixtures: THF
-
pulsed electrons
THF+ + THF e-
-
+ THF+
or e-
THF+ + e-
THF(-H)* + THF(+H)+ THF* (or THF)
-
+ THF(-H)* e- + nTHF
THF(-H)e;
(1)
(2)
(3a) (3b) (4)
1188
Tran-Thi and Koulkes-Pujo
The Journal of Physical Chemistty, Voi. 87,No. 7, 1983
e-
-
t NMA t NMA*-
e,-
+ NMA
-
+ THF NMA; + NMA e,-
(5)
NMAb\
N M b
products
products
(6)
products
(7)
-
products
(8)
On the time scale of these experiments, the most important reactions are reactions 3-5; the slow reactions 6-8 can be neglected. If we assume that reaction 3 is negligible (which will be shown later) and apply the quasi-stationary-state theory to our mechanistic scheme, the following relation between the solvated electron yield G, and the initial electron yield G,- was established:
G,; = G , - v ~ / ( v+~ v ~ ( N M A ) ) or
[Gel-]-' = [G,-]-'[l
+ v~(NMA)/v~]
(1)
v4 being the solvent trapping frequency and v5 the effective encounter frequency of e- and NMA, as proposed by Hamill et al.IO G,; (the solvated electron yield for 100-eV energy absorption) being proportional to the normalized optical density OD,;, eq I becomes
[ODe$-]-'= C[G,]-'[l
+ v~(NMA)/v~]
IO0
(11)
C being the proportionality constant. The relationship between [OD -I-' measured at 1500 nm and NMA concentration is veri?ied, thus supporting the hypothesis of the existence of a simple competition between the electron solvation and electron capture by NMA. The slope of the line (Figure 4) gives v5/v4 = 4.9, which is representative of the efficiency of NMA in scavenging presolvated electrons. Presolvated electron capture by solutes at sufficiently high concentration is a relatively well-known phenomenon in different polar media such as water and alcohols."10 However, such a simple relationship ha^ never been shown in these media. In aqueous solutions of Cd2+,Wolff and al.* explained the lower values of GeWcompared to GCd+in terms of spur reactions of dry electrons occurring prior to solvation. The following relation given by Hamill et al.,I1 which also fits Hunt's data,s is applicable to all media in which dry electron reactions may occur:
G,- is the total dry electron yield (reaction l), Go,,- the initial yield of all dry electrons which survive from reaction 3 and become solvated (reaction 4), G,- the observed yield of solvated electrons, v 4 / v 5 the reaction frequency ratio which takes into account the competition between dry electron capture and electron solvation processes, and (S) the solute concentration. As Razem and Hamill have pointed out, the relation of Wolff and al.8
(10)D.Razem and W. H. Hamill, J. Phys. Chem., 81, 1625 (1977). (11) T.Sawai and W. H.Hamill, J. Phys. Chem., 74, 3914 (1970).
50
(SOME)
1,o
3.0
4.0
Figure 4. Reciprocal of es- optical density vs. NMA or pyrrolidone concentration.
does not accurately represent the dry electron scavenging model, but the C37notion facilitates the comparison of solute efficiencies (C37being the concentration of scavenger (S) necessary to reduce the ratio [e;]/[e;], to 0.37). Using our measured optical densities for pure THF and for its NMA solutions, the experimental results fit Hamill's relation (eq 111),giving 0.96 for the ratio G0,;/G,-. This high value implies that almost all dry electrons are solvated and Go%,; Ge-. Therefore, the simpler relation (eq I) may be applied in this case. The dry electron in THF would be longer-lived than in alcohols or water. On the other hand, despite the difference in the competition scheme, using Hunt's equation, we find a value of C,, equal to 0.72 M which places NMA in the range of quite good presolvated electron scavengers, the best being potassium chromate (C,7 = 0.20 M) and cadmium perchlorate (C37= 0.35 M) in aqueous media or carbon tetrachloride (C37 = 0.14 M) in ethan01.~ A second relation between the solvated anion yield GW- and the solvated electron yield was also established
-
or r
or
[ODo (NMA[)]-'
=
r
For two given NMA concentrations, one can calculate v ~ ~ from I v eq ~ V. The constant values obtained (V5b/v5a = 6.6 f 1.0) support the dry electron model and the hypothesis of the existence of a simple competition between the anion solvation process and anion capture by NMA. Moreover, the value of vBb/vBavery close to v5/v4 suggests that the competition between anion solvation and capture by a given solute in THF is independent of the anion size (e- or NMA-). NMA appears to be as good a scavenger for presolvated anions as for presolvated electrons, which may explain why in pure NMA no NM&- was ever seen. A similar kinetic scheme of the formation of S- anion by electron capture by a scavenger solute S in ethanol has been proposed by Razem and Hamilllo without showing however the importance of anion solvation:
The Journal of Physical Chemisity, Vol. 87, No. 7, 1983 1169
Electron and Organic Radical Anion Solvation
e- t S
&
es-
-1
S*-
t S
2
S-
product
-31 1 -IS + ier, Si IS
Formation of a collision pair (e;$) was suggested to account for the much less than diffusion-controlled reaction rate constant observed in these media. The authors suggested that the inefficient scavenging of e; by S may be due in part to slow formation of S- (reaction -3) and in part to ionization of S- (reaction 3). On the other hand, it was assumed that formation of S- via dry electron scavenging would be a more efficient channel, if the electron remains firmly bound to the molecule. In our case, formation of (e[,NMA) could not occur as the rate constant of disappearance of e; is independent of NMA concentration. Moreover, we had the indirect proof of dry NMAformation, NMA- being the precursor of the transient absorbing NMA; anion. In the case of THF-Pyr mixtures, the spectra of the transient absorbing species formed during radiolysis show a continuous absorption similar to that observed in pure THF, in the 700-1500-nm region. The kinetic decay curves established at different wavelengths show the disappearance of a unique absorbing species which we identified with THF-solvated electrons. The initial intensity of the absorption band decreased with increasing Pyr concentration, indicating a presolvated electron capture process in competition with electron solvation. As in the THF-NMA mixtures, [OD,(e;)]-l is proportional to [Pyr] (Figure 4). The experimental values of v5/v4 = 0.28 and C3, = 5.1 M show that Pyr is a less efficient presolvated electron scavenger than NMA. No other absorption was observed which might have been attributed to the Pyr- anion. It seems that stabilization of Pyr- does not occur, contrary to what was found in the case of "MA-. The low efficiency of Pyr for presolvated electron scavenging in THF-Pyr solutions might be attributed to the existence of molecular association. Self-association of Pyr molecules by H bonding in CC1412J3and dioxane13leads to the formation (12) S. Blanchard and A. M. Tistchenko, private communication. (13) C. Y. S. Chen and C. A. Swenson,J. Phya. Chem., 73,2999 (1969).
of cyclic and linear dimers, the two forms being in equilibrium in the solution. Self-associationin the cis position blocks the most electroattractive sites of the amide and would thus disfavor the electron attachment process. Moreover, the eventual association of THF-Pyr molecules would lead to a loop structure which is also unfavorable for an electron attachment process. Pyr could be a good dry electron scavenger but the free Pyr attaching molecules would be in very small concentration which increases the C3,value.
Conclusion Electron attachment processes in the dilute gas phase depend essentially on the intrinsic properties of the reactant (electron affinity) and the electron energy. In the liquid phase, the reaction is strongly influenced by the solvating medium and depends on the privileged interactions existing between the solvent and reactant molecules. However, the very first steps in the anion formation mechanism appear to be the same in both the dense gas and liquid phases via a resonant state and a dry anion. The stability and thus the reactivity of such anions are strongly affected by their proximate environment. Thus, the NMA- as well as the Pyr- anion are stabilized in dense gas phases. In the liquid phase the NMA- anion is solvated by THF which is composed of cyclic molecules, but is unstable in other protic or dipolar aprotic solvents such as propanol, glycerol, water, dimethylformamide, DMA, and NMPyr. In all these media, as well as in pure Pyr, Pyr; was never seen. In this case, the choice of a linear molecule (a linear ether, for example) as a stabilizing medium would be probably more fruitful. These results show once more how important it is to obtain a better understanding of the influence of solute-solvent and solute-solute interactions on the electron attachment process and anion stabilization. This should allow a more judicious choice of reactants and the appropriate solvating medium for a given reaction (synthesis or polymerization). Acknowledgment. We are indebted to J. Sutton for fruitful and stimulating discussions and F. Brunet for providing us with purified THF. Registry No. N-Methylacetamide, 79-16-3; pyrrolidone, 616-45-5; tetrahydrofuran, 109-99-9.
