13528
J. Phys. Chem. 1995,99, 13528-13536
Methanol Production from Methane in Lithium-Doped Argon Matrices by Photoassisted, Dissociative Electron Attachment to N20 J. Mark Par&,* Larry E. Hoover, David B. Pedersen, and D. Dawn Patterson Department of Chemistry, Trent UniversiQ, Peterborough, Ontario, Canada K9J 7B8 Received: February 20, 1995; In Final Form: June 22, 1995@
Broad-band, UV-visible light irradiation of matrices formed by co-condensation of Li atomic vapor (0.010.2 mol % Li) with argon containing N20 and C& results in production of methanol at rates which are much greater than the corresponding process with no Li present, as measured by FTIR spectroscopy. The threshold wavelength for methanol production is observed to change from about 220 nm, with no alkali metal present, to between 350 and 400 nm with Li present. The rate of growth of methanol, in irradiation time, is linearly related to the rate of depletion of N20, indicating that methanol formation is associated with production of a reactive form of atomic oxygen during N20 decomposition. The rate of methanol production is strongly dependent on the concentration of N20, reaching a maximum at 0.4% N20, above and below which the rate decreases. The primary reaction is interpreted as an electron transfer from Li to N20, initially within a weakly interacting, Li-N20 complex which is observed spectroscopically at the time of sample formation. This transfer is believed to form NzO-, which decomposes to form 0- initially. 0-, or O('D) formed from 0- following a photodetachment process, then formally inserts into the C-H bond of methane to yield methanol. Regeneration of Li atoms via electron transfer from product anions or 0- to Li' is proposed to account for the high yield of methanol formed. Extended irradiation generates the known formaldehydewater complex, due to O('D) or 0- reaction with methanol.
Introduction Photoinduced electron transfer between donor and acceptor pairs in low-temperature, rare gas matrices has been shown to be a convenient means by which ionic species may be generated for spectroscopic investigation.'.2 Alkali metals are ideal electron donors for the formation of anionic species in cryogenic matrices, due to their low vaporization temperatures and low ionization potentials. Kasai has demonstrated the utility of electron transfer from Na atoms to molecular electron acceptors such as tetracyanoethylene,diborane, and furan, in the formation of their anionic counterparts or decomposition products.Ia Recently, similar visible light-induced electron transfer from Na to a series of Ar matrix-isolated ketonesIb and sulfones and sulfonatesIChas been successfully demonstrated by ESR spectroscopy. The dynamics of photoassisted charge transfer have been examined in some detail by several groups. Fajardo and A~karian~ have , ~ examined both localized and delocalized charge-transfer reactions between Xe and C1 in rare gas solids. Localized states are found to adequately describe charge transfer between Xe and electron acceptor contact pairs. Excitations involving long-range charge transfer from Xe to C1 are excitonic in nature, consisting of a localized electron on the acceptor atom and a delocalized hole centered on the Xe atoms of the matrix. This picture has been confirmed in the work of Kunz et al. through the observation of extensive Rydberg series in the excitation spectra of C1 and H atoms isolated in rare gas (Rg) Kr and Xe crystals associated with the charge-transfer species Cl-Rg+ and H-Rg+.5 Photoinduced reactions involving alkali metals and N20 have been studied in the matrix isolation environment by Milligan and Jacox.2 They have observed that codeposition of Na or Cs metal with dilute Ar:N20 gas mixtures resulted in partial
* To whom all correspondence should be addressed. @
Abstract published in Advance ACS Abstracts, August 1, 1995.
depletion of N20 upon irradiation with a medium-pressure mercury arc lamp, with concomitant production of N202- and other oxides. These workers postulated initial formation of a weakly interacting, and spectroscopically undetected, chargetransfer complex between N20 and the alkali metal involved, M
+ N 2 0-M
*N20
which undergoes photoassisted electron transfer upon irradiation with light of wavelengths 250 nm). Reagent and product relative concentrations were measured in arbitrary units from areas under the following selected absorption bands: 3024 cm-' (CH& 2222.2 cm-' (N20), 1030 cm-' (CH30H), and 1735 cm-' (CH20/H20).10-12 Other absorption features associated with these molecules were also used to check for vibrational mode-specific anomalies. No significant differences were found for results obtained with these alternate absorptions.
Results General Spectroscopic and Photochemical Features. Li vapor (0.01-0.2 mol %) was co-condensed with argon gas containing 1% N20 and 5% C&. Before irradiation, the major IR absorption features were due to.matrix-isolated N20 (2222.2, 1282.9, 588.2 cm-I) and C& (3024, 1304 cm-').'' A portion of the deposition spectrum is given in Figure 1. Minor features at 2247.2 and 2263 cm-I (broad) were observed (Figure 2A), which appear very close to the V I N-N vibrational mode of N20 and which are not present in spectra of samples in which no Li is present. The dependence of these features on the concentration of Li in the samples is illustrated in Figure 3, where the area of both features may be seen to increase in a nonlinear fashion with Li concentration. Subsequent experiments showed that these features also grew in intensity with the concentration of N20 in the sample (see below). We therefore attribute these new absorptions to a Li-NZO complex formed on deposition. Such a complex has been postulated previously by Milligan and Jacox.2 No other significant features were observed in the deposition spectra, even at relatively high loadings of Li metal. Li Concentration Effects on Photochemistry. Matrices containing 0.4 or 1.0% N20, 5% C b ,and 0.01-0.2% Li were irradiated with broad-band UV-visible light in order to observe the effect of metal concentration on product yield. Except for
Parnis et al.
13530 J. Phys. Chem., Vol. 99, No. 36, 1995
-.I
+
2280
20 2260
2240
2220
Wavenumber /cm-'
0
Figure 2. Portions of spectra between 2285 and 2200 cm-l of a matrix containing 1% N20, 5% CH4, and 0.12% Li (A) on deposition and following 10 (B), 30 ( C ) ,and 120 min (D) of broad-band UV-visible irradiation. Also given are scaled spectra in the same region for a matrix containing 1% methanol and 1% N20 in argon (E) as well as spectrum A reduced 20-fold (F).
0 12 2247 cm'
0 10
.-3
008
a
F
E
r
% 008 . m t E
2
004
0 02
ow 0 00
0 05
0 10
0 15
0 20
Li mole %
Figure 3. Graphical plots of the variation in the area of the 2247 and 2263 cm-' absorptions of the LVN20 complex formed on matrix deposition, with the concentration of Li in the sample. Matrix composition was Ar containing 1% NzO, 5% CHI, and 0.0-0.18 mol % Li. The solid curves are second-order spline fits.
the metal loading level, all matrices were prepared in the same manner and contained the same amount of gas condensed over the same time period, 0.60 cm3 min-' gas flow over 1 h. As has been previously observed in analogous experiments in which no Li was present,* the major photochemical product was methanol, as shown by the growth of absorptions at 3661.0, 2950.7, 1471.9, 1332.6, and 1030.4 cm-' and others.I0 Figure 1 is an illustration of the major spectroscopic changes observed on irradiation over a 120 min period, in which the decrease in Y:! of N20 and the increase in the C-0 stretching mode of
20
40
60
80
100
120
140
irradiation timel minutes
Figure 4. Plots of total methanol yield, as measured by the area under the absorption at 1030 cm-' in arbitrary units, versus the length of exposure to broad-band, UV-visible light irradiation for argon matrices containing 1% N20, 5% CHI, and 0.0-0.18 mol % Li metal. Li metal concentrations are noted in the figure. Solid curves are for clarity only.
methanol are clearly seen. Figure 4 illustrates the influence of Li concentration on the rate of production of methanol following various irradiation periods, which clearly illustrates that the rate of methanol production increases with Li concentration. Weak spectral features associated with oxides of nitrogen such as cis(N0)2, NO, and ON-NO2 were also ~ b s e r v e d . ' ~ - ' ~ Absorptions present on deposition at 2247.2 and 2263 cm-' were rapidly depleted following UV-visible light irradiation, typically disappearing after 30 min of irradiation. Figure 2A-D illustrates the changes in these features during 120 min of broadband W-visible irradiation. No obvious linear relationship could be developed between the rate of loss of these absorptions and the rate of growth of features due to methanol, indicating that the disappearance of the features at 2247.2 and 2263 cm-' is not directly correlated with the growth of methanol. A new absorption centered at 2234 cm-' was observed (Figure 2AD), whose rate of growth was linearly related to that of the methanol absorption at 1030.4 cm-I. The feature at 2234 cm-' is at the same wavenumber as a major absorption present in the deposition spectrum of an Ar matrix containing methanol and N20 (see below), which is most likely due to a binary complex of N20 and methanol. Spectra of extensively irradiated samples also showed growth of spectroscopic features at 1735, 1498, and 1173 cm-' and others (see Figure 5 ) , which correspond closely to those assigned by Nelander to the matrixisolated binary complex of formaldehyde and water.'* No evidence for the production of uncomplexed formaldehyde was seen. The influence of Li concentration on the rate of production of methanol during broad-band UV-visible light irradiation is illustrated in Figure 6. Here the methanol yield is plotted versus Li concentration following a brief, 10 min irradiation period, during which depletion of N20 was below 10% for all samples. A linear increase in the rate of methanol production with Li concentration is apparent. A distinct downward curvature is obvious in the plots of methanol yield with irradiation time (Figure 4). This occurs when there is significant conversion of methane to methanol,
Methanol Production from Methane
J. Phys. Chem., Vol. 99, No. 36, 1995 13531
0
Methanol
0.8
0
.1
0 Formaldehyde/water
0.2
rioo
1600
1400
1200
1000 0.0
Wavenumber /cm-'
0
Figure 5. Portions of spectra between 1800 and lo00 cm-I of a matrix containing 1 % NzO, 5% CH4, and 0.2% Li (A) on deposition and (B, C) following 30 and 120 min broad-band UV-visible irradiation, respectively. Features associated with methanol and the formaldehydewater binary complex are annotated with an M and an F, respectively.
0.30
1
2
3
4
5
N20 consumption I arbitrary units
Figure 7. Plots of total methanol yield versus NzO consumed during broad-band, UV-visible light irradiation of argon matrices containing 1% N20 and 5% CHI, for various Li loading levels between 0.02 and 0.2 mol %. Also shown is the total yield of the formaldehyde-water
complex with N20 depletion for the same experiments.
{
1
0 00
-7
0 05
~
0 10
0 15
0 20
LI mole %
Figure 6. Plot of the total yield of methanol, in arbitrary units, versus the mole percent Li content, following brief (10 min) irraditaion with broad-band UV-visible light. Conditions are as noted in Figure 1.
suggesting a decrease in efficiency of the process, due to consumption of reagents or the onset of a secondary reaction in which methanol is consumed. In order to establish to what extent the decreasing rate of methanol production was due to a decrease in the rate of NzO consumption, plots of the total yield of methanol versus the total amount of N20 consumed throughout the entire irradiation period for various Li loading levels were prepared. Figure 7 is a graph of the same data shown in Figure 4 in this form, in which the total methanol yield after a given irradiation time interval is given in terms of the amount of N20 consumed in that same time period. Plots from experiments involving different concentrations of Li in the matrix have been overlaid. The curves in Figure 7 show that a linear relationship exists between depletion of N20 and produc-
tion of methanol during most of the total irradiation period, indicating that the two processes are directly correlated and that most of the nonlinearity in Figure 4 is due to a decrease in the rate of Nz0 consumption. As well, the concentration of Li in the matrix does not seem to have a measurable effect on the degree of correlation between the consumption of N20 and the production of methanol, since plots at different Li concentration levels exhibit virtually identical forms. The persistence of downward curvature in portions of the plots of methanol yield versus N20 consumption indicates that the efficiency of methanol production from N20 decreases during the latter stages of the irradiation periods. This is most likely due to increased competition from another process which consumes the active form of oxygen atoms formed following dissociation of N20. As noted above, spectra of extensively irradiated samples revealed spectroscopic features which have been assigned to the binary complex of formaldehyde and water. In Figure 7 is also shown the integrated area of the formaldehyde-water complex absorption at 1735 cm-' versus the total decrease in the integrated area of the N2O absorption at 2222 cm-I, for samples containing several different loadings of Li. The growth of the formaldehyde-water complex features, as a function of the amount of NzO consumed, shows a marked induction period, the end of which can clearly be seen to correlate with the onset of the decrease in the efficiency of methanol production. This strongly suggests that the two processes are linked and that production of the formaldehydewater complex is a significant contributor to the decrease in the efficiency of methanol formation at relatively high methanol concentrations. Consumption of N20 was found to exceed consistently the theoretical maximum that would be attainable if a 1:1 stoichiometric reaction between Li and N20 were occumng. For example, it was observed in several runs that 2-3 times as many moles of N20 were consumed per mole of Li, with the highest recorded value being approximately 5 mol of N20 per mole of Li in the sample. This effect is attributable to the presence of
13532 J. Phys. Chem., Vol. 99, No. 36, 1995
Parnis et al. 0.10
0.08
0.6 -
‘E 3 0.5
.a
-
t
3
0.06
t. ,m
g
L
0.4
.
.
-
2
-ZI
-
3
‘E. 0.3-
2
0
5
0.04
5
m
E
0.2-
0.1
180
0.02
-
60
30 10 2
0.0 -
0.00
200
250
300
350
400
>450
composite irradiation range limits I nm 0.0
0.2
0.4
0.6
0.8
1.0
1.2
N20 mole %
Figure 8. Plots of methanol yield versus the N20 concentration (mole percent) of argon matrices containing 5% CH4 and 0.06 mol % Li, following broad-band, UV-visible light (’230 nm) irradiation for various time intervals between 2 and 180 min, as noted.
a catalytic cycle involving multiple transfers of electrons whose original source is Li, as discussed below. N2O Concentration Effects. The influence of N20 concentration on methanol production efficiency was examined by preparing a series of samples containing 5.0% C&, 0.06 mol % Li, and 0.1-1.0% N20. The most notable effect, on deposition, was the increase in the intensity of the absorptions at 2247.2 and 2263 cm-I on increase of the concentration of N20, between 0.1% and 1.0%. Recall that these absorptions have been assigned above to a Li-N20 complex. Figure 8 illustrates variations in methanol yield with N20 concentration, over a series of interrupted irradiation periods totaling 180 min. The results clearly show that the rate of methanol production is greatest when the concentration of N20 is about 0.4%. In a series of experiments in which the concentration of N20 was raised to 2%, it was observed that virtually no N20 depletion occurred, nor was any methanol formed. No other photochemical processes appear to occur in this concentration regime. Therefore, it would appear that at concentrations of N20 above 1%, the efficiency of the methanol-producing process is greatly diminished by a deactivation mechanism that does not involve an alternate chemical reaction. Wavelength Dependence in Methanol Production. The wavelength dependence of the production of methanol in argon matrices containing 1% N20,5% C&, and 0.09 or 0.18 mol % Li was examined. Samples were irradiated for 30 min with light of progressively shorter wavelengths by employing cutoff filters, i.e., >400, >350, >300, and >250 nm, or by irradiation with the unfiltered lamp, >200 nm. The brief time intervals chosen ensured that N20 consumption was sufficiently low (-=lo% over the entire irradiation period) such that reagent depletion was not an important factor. Incremental methanol yields following each irradiation period were measured by subtracting the total methanol previously formed during prior irradiations from the methanol yield measured after the irradiation period in question. In order to estimate the yield of methanol formed by light within a given composite irradiation wavelength range, the corrected yield obtained above was further
Figure 9. Graphical representation of the corrected yield of methanol, following a 30 min irradiation period, of an argon matrix containing 1% N20,5% CH4, and 0.18 mol % Li, versus the composite irradiation range employed. See text for discussion.
adjusted to remove the amount of methanol formed by light within all previously measured ranges. For example, the incremental yield of methanol following irradiation with light of >300 nm was adjusted by subtracting the previously obtained incremental yield of methanol following irradiation with light of >350 nm, to obtain an estimate of the composite incremental yield of methanol for light between 300 and 350 nm. Figure 9 shows the corrected yield of methanol generated in a 30 min interval of broad-band irradiation within various composite wavelength ranges, calculated in the manner described above, for a sample containing 0.18 mol % Li. The cutoff wavelength at which production of methanol is first observed with Li present lies between 350 and 400 nm, as compared to the cutoff point for methanol production with no Li present which lies below 230 nm. The influence of N20 concentration on this cutoff wavelength was not studied. Secondary Reactions. In order to explore further the secondary reactions of methanol with the product of N20 dissociation in the presence of Li metal, a matrix was prepared containing 1% N20, 1% methanol, and 0.06% Li in Ar. In addition to features associated with unperturbed N20 and methanol, the deposition spectrum showed a strong absorption at 2234 cm-I (Figure 2E) not found in the deposition spectra of argon matrices containing N20, Li, and C&. Note that this feature, which may be attributable to a methanolm20 binary complex, was observed to grow linearly with methanol in the N20/methane studies described above. Broad-band irradiation of this sample yielded spectral features due to the formaldehydewater complexI2 as well as oxides of nitrogen as noted above. The relative yield of the nitrogen oxides was high with respect to that expected based upon the analogous experiments described above involving methane, presumably due to decreased efficiency of the reaction of the active form of oxygen with methanol compared to that with methane.
Discussion Initial Formation and Decomposition of N20-. The various observations made in the present study are consistent with a photoassisted electron transfer from lithium atoms to N20 to form N20-,
Methanol Production from Methane
M
+ N,O + hv (A < 400 nm) - M+ + N,O-
Milligan and Jacox2 have proposed such a step in their mechanism to account for the production of N202- following the irradiation of argon matrices containing N20 and Na or Cs metal, as noted above. The lack of observation of matrixisolated N20- here, which has also been noted in the previous work of Milligan and Jacox2 as well as Andrews et a1.,16cannot be due to inherent thermodynamic instability, since N2O- is a known, stable, gas-phase species" and N20 is known to have a positive adiabatic electron affinity.' It is therefore likely that N2O- is unstable under the experimental conditions employed in this work, with respect to either spontaneous dissociation immediately following its formation or photolytic decomposition. A detailed discussion of the energetics of this process appears later in this paper. The fact that much more N20 is depleted and much more methanol is produced than there is Li present in the matrix indicates that the electron transferred in this first step would have to be catalytic. The overall mechanism would have to include a step that generates N20- from photoinduced electron transfer to N20 from anions produced at intermediate steps in the reaction, such as the methanol anion, CH3OH-, or 0-, or from Li atoms re-formed following the recombination of Li+ and an electron. Indeed, stoichiometric electron transfer from Li atoms present on deposition to N20 must be a relatively minor contributor to the formation of N20-, given the excess of N20 with respect to Li in the matrix. Since ionization potentials of molecular anions are, in general, much less than the ionization potential of Li, it would appear that the wavelength threshold for electron transfer from a molecular anion such as CH30H- should be at a much longer wavelength than predicted for Li N20. The absence of any obvious change in the rate of methanol growth during the initial stages of the irradiations, during which all Li would have been consumed in a 1:1 stoichiometricallybalanced process, indicates that the rate of N20- formation is rapid with respect to subsequent steps leading to the formation of methanol, irrespective of the source of the electron involved. It is likely that, if formed, N20- would decompose spontaneously to yield N2 and 0- with considerable kinetic energy.
+
N,O-
-
N,
+ 0-
The vertical electron affinity of N20-, about -215 kJ mol-',' is much greater than D(N2-O-) = 42 & 10 kJ mol-', indicating that N20-, once formed in the gas phase, would have about 160 kJ mol-' of internal energy in excess of that required to rupture the N - 0 bond. Chantry has measured the kinetic energy of 0- formed following dissociative electron attachment to N20, finding the peak value to be about 37 kJ mol-', which leads to a total kinetic energy for both 0- and N2 of 54 kJ mol-'.I8 Thus, there is clearly a good deal of excess energy to be disposed of in order to stabilize N20- formed by dissociative electron attachment to N20. Despite the fact that N20- would appear to be highly energized upon formation, it is possible that it is nevertheless rapidly stabilized by efficient energy-transfer interactions with the matrix. The lack of observation of N20- in this and other work2 suggests that, if formed, N20- must be highly unstable with respect to photolysis at the wavelengths employed in this work, Le., 200-350 nm, yielding the same products. N20-
+ hv -.N, + 0-
Both these processes have been previously suggested in the
J. Phys. Chem., Vol. 99, No. 36, 1995 13533 related work of Milligan and Jacox.2 As discussed above, dissociative attachment of electrons to N2O is a well-known source of gas-phase and such a process may occur spontaneously in Ar matrices. The lack of observation of N20in this and other studies of electron transfer to N20 in matrices makes it difficult to distinguish between a process involving direct dissociative attachment to N20 and one involving formation of stabilized N20- followed by its photodecomposition. Indeed, one cannot discount the possibility that initial formation of N20- results in the formation of O('D) and a solvated electron, a decomposition route that may be possible in the matrix, despite the lack of observation of O(lD) in gasphase dissociative attachment to N20. Reactions of 0- and O(lD) with Methane. 0- has been shown20-22to react in the gas phase with methane to form OHand CH3': 0-,69'8$'9
0-
+ CH, - OH- + CH,'
Direct production of methanol from 0- and CHq via an associative detachment mechanism
0-
+ CH, - CH,OH + e-
is not believed to be a favored gas-phase process,21although it would appear to be the more direct route to the product observed in the present work. We do not observe production of OH- or CH3 radicals in the matrix at any time and suggest that if 0-is the major species that reacts with methane, a very rapid matrixcage-induced recombination process is most likely, OH-
+ CH,' - CH,OH-
The methanol anion thus formed is suggested here to then undergo rapid electron transfer, either spontaneously or under irradiation, to Li+, which would be close by and have the most favorable electron affinity of all matrix constituents present:
CH,OH-
+ Li+ (+hv) -CH,OH + Li
Subsequent photoassisted charge transfer from the newly formed Li atom to another N20 molecule would ensure a catalytic process in which many N20 molecules could be consumed for each Li atom present. This would also allow a large amount of methane to be converted to methanol, much more than could be generated via a 1:1 stoichiometric reaction between Li and N20. As an altemative, it is possible that an associative detachment process involving Li+ as an electron acceptor is active,
0-
+ CH, + Li+ - CH,OH + Li
in which there would be no formation of CH30H-. We are unable to distinguish between these two mechanisms, which are essentially identical from the point of view of product analysis in irradiation time. It is possible to propose a radically different mechanism for the formation of methanol from 0- and methane, which involves formation of the first excited state of atomic oxygen, O(lD), as produced in a photodetachment process, 0-
+ hv - O('D) + e-
or a dissociative attachment process, as noted above,
N,O
+ e-
-
N,
+ o('D) + e-
13534 J. Phys. Chem., Vol. 99, No. 36, 1995 Either process would be expected to be followed by direct insertion of O('D) into the C-H bond of methane
O('D)+ CH,
-
CH,OH
The first photodetachment process is know to have a gas-phase threshold of 330.9 kJ m ~ l - ' , ' ~which . ~ ~ is in accord with the theoretical threshold for this process, which corresponds to a photon of wavelength 361 nm or shorter. It is difficult to accurately estimate the wavelength at which this process would occur in the matrix, especially since it may involve Li' as an acceptor of the free electron so formed. The threshold for photodetachment of 0- to form O('D) in Ar is probably close to that given above and would therefore be in accord with the threshold for formation of methanol in this work of e400 nm. The kinetics and dynamics of reactions of O('D) with alkanes have been studied in great detail.24 It is generally accepted that the initial interaction leads to insertion into a C-H bond, yielding the primary product alcohol which decomposes in the gas phase to form alkyl and hydroxyl radicals. We have recently shown that this process leads to the formation of the stabilized alcohol product in Ar matrices.8 The reaction of O('D) with methane is about 600 kJ mol-' exothermic and would be expected to occur readily upon formation of O('D) from 0- in the presence of methane. It is clear that there are at least two very different mechanistic approaches which could account for production of neutral methanol from methane, following formation of N20-. We find that we reach the same conclusions as Milligan and Jacox;2 it is not possible here to discern whether 0- or O('D) is the major actor in the formation of methanol or whether both are involved. Direct Interactions and Reactions between Li and NzO. The absence of a significant oxygen atom transfer reaction between Li and N20 during matrix condensation is in accord with the known activation energy barrier of 12.2 kJ mol-' for the oxygen atom-transfer reaction to form LiO and N2.25 No significant growth of any features associated with lithium oxides was observed under any of the conditions employed in the present study, suggesting that they do not play a significant role in the observed conversion of methane to methanol. Note also that the features associated with methanol showed no significant change when Li was present in the matrix, indicating little interaction between these potentially reactive species. Evidence is seen for a weakly bound, photoactive complex of Li with N20, as manifest by the blue-shifted absorptions in the region of the N-N stretching mode of N20, which increased in intensity with Li and N20 concentration. The disappearance of these features during the early stages of irradiation did not appear to correspond with a significant increase in the methanol yield, since only a small fraction of the total methanol yield was produced during the time that these features disappeared, and there is no evidence of bimodality in the plots of the increase in the methanol absorption area versus decrease in N20. This would appear to rule out the participation of such binary complexes as are present on deposition as the primary source of oxygen atoms or anions for methanol production from methane, since it would be expected to show a growth and decay behavior which is linked to the rate of production of methanol. We consider this complex to be due to a weak, charge-transfer interaction between Li and N20, such as has been suggested by Milligan and Jacox.2 Its rapid depletion during the initial stages of the irradiation is as expected for a complex which acts as a source of catalytic electrons. The electron is likely passed from subsequently formed product anions, such as 0or CH30H-, to neighboring Li' ions, which would then undergo subsequent charge transfer to unperturbed N20, the source of
Parnis et al. most of the oxygen atoms for formation of methanol. We may therefore have observed part of the spectrum of the weak charge transfer complex implicated by Milligan and Jacox in their earlier work.2 The nonlinearity of its growth with Li concentration, in experiments in which a fixed excess of N20 is present, is likely due to the onset of dimerization of Li which is expected to occur at higher concentrations of Li in the matrix. As Liz is a closed-shell species and Li is a radical, it is reasonable to propose that it would interact much less strongly with N20 than Li . ! Energetics Considerations in the Formation of NzO-. The results presented in this work are in accord with a localized electron transfer between Li and N20. Following the work of McCaffrey et a1.26and Kunz et ala5on charge-transfer processes in solid Xe matrices, the threshold energy for photoinduced electron transfer from Li to N20 in Ar matrices may be estimated as
Here the threshold energy is determined by the difference between the ionization potential of Li (520 kJ m01-I)~' and the vertical electron affinity of N20, less the self-energies of the ions involved and the Coulombic attraction energy of the ion pair formed. 6 is the dielectric constant of Ar. Electron transfer is predicted following absorption of a photon of energy greater than the value of AE. Note the absence of a '12 factor in the Coulombic stabilization energy term, which is appropriate for cases where a delocalized excitonic hole state is formed such as with charge transfer from Xe to C1 in Xe matrices. The present conditions would appear to preclude the formation of such an excitonic state, due to the low concentration of Li in the matrix and the high ionization potential of Ar. In the absence of sufficient energy to ionize Ar, the positive charge would be expected to be localized on one Li+ center. The absence of excitonic progressions in the excitation spectra of WXe pairs in Kr and Ar matrices has been accounted for in a similar manner.5 However, the charge transfer may initially involve the formation of a delocalized Rydberg-like excited state of Li+, since the first two Rydberg-state transitions for Li atoms in solid argon are predicted to occur at about 335 and 291 nme2* The use of the vertical electron affinity of N20 is appropriate since the bent geometry of the N20- anion formed is significantly different from that of its neutral precursor. Thus, while the adiabatic electron affinity of N20 is 21 f 10 kJ mol-', the vertical electron affinity is about -215 kJ mol-'.' Schwentner and Chergui28 give an estimate of 1.15 eV for p+, the hole polarization energy for cations in argon matrices. Kunz et aL5 give an estimate of 1.5- 1.6 eV for p - , the polarization energy for halogen ions in Xe and Kr. Using these values and the threshold energy equation given above, we obtain a wavelength threshold value of 330-340 nm for R = 7 A, which is in reasonable accord with the observation of the onset of methanol production of 350-400 nm in the present work. As an alternative to this mechanism, one might propose that the initial ionization step is simply photoionization of Li to form an Ar-solvated electron, which localizes on N20 following its formation. The energy requirement for such a process is estimated to be 4.4 eV, the calculated ionization energy for Li atoms in argon matrices,28which corresponds to an ionization onset wavelength of about 280 nm. The less satisfactory agreement between this value and the observed threshold for methanol production suggests that, while such a process may contribute at higher energies to the formation of N20-, it cannot alone account for the observed wavelength dependence of the process.
Methanol Production from Methane
Formaldehydelwater Complex Formation. The observed decrease in the rate of N20 consumption in irradiation time, and its ,closely correlated decrease in the rate of methanol production, appear to be due to an effect which is linked to the decrease in N20 concentration in the matrix as the photochemical process proceeds. A possible explanation is that the efficiency of the electron-transfer process decreases with N20 consumption due to the increased distance between electron donors and N20. This may be restated as a decrease in the absorptivity of the electron transfer donor/acceptor pair, which results in a decrease in the rate of photon absorption and a corresponding decrease in the rate of formation of N20-. The nonlinear dependence of the rate of methanol production on N20 concentration during the latter stages of the reaction suggests that when significant amounts of methanol are present in the matrix, reaction of 0- or O(’D) with methanol becomes a significant process. This would be expected to lead to the formation of an intermediate species which decomposes or rearranges, perhaps with the loss of an electron, to yield ultimately formaldehyde and water. The latter two species are formed in a manner that precludes their trapping as unperturbed and separated species, such that the known formaldehyde-water binary complexI2 is the major and only significant secondary product. Possible intermediate species are the methanediol, CH2(0H)2, or its anion, formed by insertion of O(lD) or 0into a C-H bond of methanol, or a hydroxymethyl radical and a hydroxyl radical or anion, formed following hydrogen atom abstraction from methanol by 0-. Direct abstraction of H from methane by O(lD) is not believed to occur to any significant Observation of the formaldehyde-water complex indicates that a subsequent radical-radical recombination process followed by decomposition to form this complex would have to occur if the abstraction process were occurring. N20 Concentration Effect. The observed dependence of the rate of methanol growth on N20 concentration (Figure 8) in otherwise similar matrices is puzzling. The increase in methanol growth rate in the 0.1-0.4 mol % N20 concentration regime appears to be due to an increase in the efficiency of photon absorption by the matrix sample. This may be due to a higher Coulomb stabilization energy arising from the closer proximity of the ions in the product ion pair, better excitedstate orbital overlap between the Li/N20 charge-transfer pairs, or simply an increase in the concentration of the initially formed Li/N20 complex, the suggested initial source of electrons. Recall that absorptions due to this complex were observed to increase in intensity with N20 concentration up to 1%. Above 0.4% N20, the methanol growth rate rapidly diminishes with increased N20 concentration, until the reaction is effectively stopped at 2% N20 (not illustrated). We have not been able to detect any other products formed during irradiations of matrices containing 2% N20 and in fact observe no decrease in absorptions due to N20 and C b following very long irradiation periods. These observations suggest that the initial electron transfer step is effectively quenched or reversed by an N20 concentration-dependent effect. This conclusion is paradoxical from the point of view of the efficiency of the electron transfer, since such a process should become more thermodynamically favored as the interion distance of the electron transfer donor/ acceptor pair is decreased, due primarily to an increase in the Coulombic attraction force between the two ions formed. Thus, it seems unlikely that the electron-transfer process is quenched. We propose that, at high N20 concentrations, electron transfer to N20 from Li does occur but that the ion pair produced is unstable with respect to the reverse electron transfer when the distance between the ions formed becomes small. Kasai has
J. Phys. Chem., Vol. 99, No. 36, 1995 13535 suggested that it is the presence of Ar between the ion pair constituents, and the resulting stabilization due to ionhnduceddipole interactions, which prevents exoergic electron trasfer reactions from occurring spontaneously in Ar matrices.Ia At larger distances associated with more dilute N20 matrices, the presence of Ar between the ions is believed to generate a kinetic barrier to spontaneous reverse electron transfer. When the concentration of N20 becomes too great in the matrix, there may be insufficient Ar between the ions to prevent spontaneous reverse electron transfer, yielding the initial reactants and no net change, as is observed in the present work. Such a proposal would require that the decomposition of N20- to form 0- be much slower than the reverse electron transfer suggested to occur at higher N20 concentrations. This would support the view that N20- is formed as a thermodynamically stable but photolytically unstable species, following the initial electron transfer to N2O.
Summary Photolytic generation of methanol from methane in Ar matrices containing Li and N20 is proposed to occur via a UVlight-induced electron transfer from Li to N20 to form N20-. A weak ground-state interaction between Li and N20 is suggested by the observation of a Li-N20 complex which decays rapidly during the initial stages of the reaction. N20is believed to decompose spontaneously or, following photoexcitation, to form N2 and 0-. The 0- thus formed, or O(lD) produced via photodetachment from 0-, then reacts with methane to form methanol. An electron is believed to be transferred to a nearby Li+ ion in the process, thereby regenerating the initial electron donor and allowing significant amounts of N20 to be consumed, despite the limited amount of Li available in the matrix. Formation of the formaldehyde/water complex during the latter stages of the reaction occurs as a result of secondary reactions of methanol with 0- or O(’D). The wavelength cutoff of 350-400 nm for the process is in reasonable accord with the predicted threshold for formation of a Li+iN20- ion pair in an Ar matrix. The unusual dependence of the rate of methanol production, in irradiation time, with N20 concentration in the matrix is interpreted as arising from a reversible, redundant electron transfer from Li to N20. This is believed to occur when there is an absence of significant stabilization from Ar polarization interactions, due to the ions being formed in too close proximity.
Acknowledgment. The support of the Natural Sciences and Engineering Research Council’s Operating Grants and Undergraduate Summer Research Awards Programs is gratefully acknowledged. We kindly acknowledge J. G. McCaffrey for helpful discussions regarding the photophysics of charge-transfer processes. References and Notes (1) (a) Kasai. P. H. Acc. Chem. Res. 1971.4.329. (b) KoDDe, R.: Kasai. P. H. J. Phys. Chem. 1994, 98, 12904. (c) Kasai, P. H. J. Am. Chem. SOC. 1991, 113, 3317. (2) Milligan, D. E.; Jacox, M. E. J . Chem. Phys. 1971, 55, 3404. (3) Fajardo, M. E.; Apkarian, V. A. J. Chem. Phys. 1986, 85, 5660. (4) Fajardo, M. E.; Apkarian, V. A. J. Chem. Phys. 1988, 89, 4102. (5) Kunz, H.; McCaffrey, J. G.; Chergui, M.; Schriever, R.; Unal, 0.; Stephanenko, V.; Schwentner, N. J. Chem. Phys. 1991, 95, 1466. (6) (a) Caledonia, G. E. Chem. Rev. 1975, 75, 333. ( b ) Warman, J. M.; Fessenden, R. W.; Bakale, G J. Chem. Phys. 1972, 57, 2702. (7) Hopper, D. G.; Wahl, A. C.; Wu, R. L. C.; Tieman, T. 0. J. Chem. Phys. 1976, 65,5474. (8) Pamis, J. M.; Hoover, L. E., Fridgen, T. D., Lafleur, R. D. J. Phys. Chem. 1993, 97, 10708.
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