Chemiluminescence in the Reaction of Mn Atoms with CF4 - The

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J. Phys. Chem. 1996, 100, 14584-14591

ARTICLES Chemiluminescence in the Reaction of Mn Atoms with CF4 Dale L. Herbertson† and Martin R. Levy* Department of Chemical and Life Sciences, UniVersity of Northumbria, Ellison Building, Ellison Place, Newcastle upon Tyne NE1 8ST, UK ReceiVed: March 19, 1996; In Final Form: May 20, 1996X

Translational excitation functions have been determined for production of several MnF* statessb5Π, c5Σ+, d5Π, and (most probably) e5Σ+sin the reaction of a laser-ablated beam of Mn atoms with gaseous CF4. Although all observed channels show high initial thresholds, ∼200-300 kJ mol-1, reaction appears to be due to excited Mn atoms rather than the ground state, a6S. The reagent species appears to be either the first or third metastable level, a6DJ or a4DJ. Analysis of the energy dependences, in terms of a multiple line-ofcenters model [Levy, Res. Chem. Kinet. 1993, 1, 163], indicates that at relatively low energies, a common process is responsible for b5Π and c5Σ+ formation, involving a ∼14% forward shift in reaction transition state as collision energy increases. Quite separate processes, without transition state shifts, lead to production of MnF*(d5Π) and of MnF*(e5Σ+)/“blue” emission at relatively low energies and to enhanced c5Σ+ production at high energies. It is possible that enhanced production of MnF*(e5Σ+) and perhaps the d5Π state from ∼650-700 kJ mol-1 derives from the depletion of MnF*(b5Π, c5Σ+). Despite the undoubted negative CF4 electron affinity, it seems likely that avoided ionic-covalent curve crossings at least play a role in the b5Π/ c5Σ+ production channel.

Introduction Some five to six different band systems contribute to the electronic spectroscopy of the manganese monohalides, MnX.1-6 Until recently, only the UV system had been definitively assigned, as A7Π f X7Σ+.3 Now, however, the MnF bands at ∼690 and ∼505 nm have been characterized as c5Σ+-a5Σ+ and d5Π-a5Σ+, respectively,4,5 with the ∼825 nm system being attributed to b5Π-a5Σ+, by analogy with MnH.7 Quite separate experiments8 have demonstrated that the ∼495 nm MnF system is d5Π-X7Σ+, from which it can be deduced9 that the a5Σ+ state excitation is only ∼7 kJ mol-1. A ∼450 nm system, observed first for MnCl in 19392 but never reinvestigated, has been detected in this laboratory as chemiluminescence from the reactions Mn + SnCl4,10 SiCl4,9 SF6,11 F2,12 and Cl212 and has been tentatively assigned9 as e5Σ+-a5Σ+, again by analogy with MnH.7 Such a large number of electronic states of relatively low excitation offers an interesting opportunity to investigate the degree to which different, and energetically close, reaction potential surfaces interact. Indeed, that was a major motivation behind the Mn atom reaction studies listed above. In those experiments, laser vaporization was employed to generate a fast pulsed Mn atom beam containing significant quantities of metastable atomic states (a6DJ, z8PJ, a4DJ, ...) in addition to the ground state (a6S). Before interacting with the reagent molecules, the atoms were velocity-separated by time-of-flight, allowing translational excitation functions for chemiluminescence in the different band systems to be determined. The investigations so far published comprise Mn + SnCl4, SiCl4, SF6. For these, the exoergicity of the fully ground-state † Present address: Heaton Manor School, Jesmond Park West, Newcastle upon Tyne NE7 7DP, UK. * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, August 1, 1996.

S0022-3654(96)00827-1 CCC: $12.00

reaction is respectively about +94, -37, and about +33 kJ mol-1 respectively, requiring reagent electronic or translational excitation for any chemiluminescence to be observed. Collision energy-dependent emission from the c5Σ+, d5Π, and e5Σ+ states of MnCl* or MnF* has been observed in all three cases, with the b5Π state also being found for Mn + SiCl4 and SF6 and the A7Π state detected for Mn + SF6 alone. From the measured thresholds, and other considerations, metastable Mn atoms do seem to be the reagent species, but nonetheless, all chemiluminescent channels show significant excess barriers. The analysis of these excitation functions has been significantly enhanced by the development, by one of us,13 of a “multiple line-of-centers” model (MLC):

σ(ET) ≈ ∑σk(1 - Ek/ET)

(1)

In this k ) 0, 1, 2, ..., only σ0 is necessarily positive, and each term applies from its threshold Ek. If eq 1 holds, then conversion of the data to the yield function form

Y(ET) ) ETσ(ET) ≈ ∑σk(ET - Ek)

(2)

should generate multilinear plots, allowing the thresholds Ek and relative values of σk to be extracted. Indeed, it was the observation of such multilinearity in a number of reactions that led to the development of the MLC model. The fact that such a simple empirical model can adequately represent the functionality of data from a wide range of reactions13 suggests a deeper, if approximate, underlying truth. In particular, a significant proportion of reactions with positive thresholds E0 are quite satisfactorily represented by a two-term version of eq 1, in which σ1 < 0. We have rationalized such “simple fall-off” in terms of competition between (a) reaction, with probability P0 at internuclear distance R0 and line-of-centers © 1996 American Chemical Society

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energy E0, and (b) depletion, with additional probability Pd, once the system has sufficient line-of-centers energy (gE1) to penetrate to internuclear distance Rd:

σ(ET) ≈ πP0R02(1 - E0/ET) - πP0PdRd2(1 - E1/ET)

(3)

This depletion could be an alternative reactive or inelastic process, or simply “recrossing” back to the entrance channel, by reflection from the repulsive wall of the potential surface.14 The yield function derived from eq 3,

Y(ET) ≈ πP0R02(ET - E0) - πP0PdRd2(ET - E1)

(4)

will clearly show an initial linear rise, then a reduced slopesperhaps even zero slopesfrom E1. Comparison of the slopes of the two linear regions yields

|σ1/σ0| ) Pd(Rd/R0)2

(5)

from which a lower limit to either Pd or Rd/R0 is obtained,9-11,13 since Pd e 1.0 and Rd is not expected to be substantially larger than R0. Within the terms of this approach, more complex multilinear yield functions, in which the slope increases at some collision energy below the depletion threshold, suggest increases in reaction probability. This could be caused by the opening up of parallel reaction channels or by participation of more than one reagent state. However, there is a particular type of “complex” yield function, observed in a number of reactions,9-13 where, with increasing collision energy, the slope of the plot actually becomes negative (very rapid fall in the excitation function), although it usually levels off to near-zero gradient at even higher energies. For a three-term functionality of this type,

Y(ET) ≈ σ0(ET - E0) + σ1(ET - E1) + σ2(ET - E2) (6) we have -σ1 ≈ σ0 + σ2, and the sharp negative slope region is E1 < E T < E 2 . This behavior has been explained10,13 in terms of an unseen forward shift in the reaction transition state, from R0 to a new value Rs, at energy Es followed at higher energies by depletion at internuclear distance Rd, which here is of magnitude comparable to Rs. The forward shift is only revealed once its line-of-centers functionality outstrips that of the initial process, i.e., at higher energies still, when the slope of the yield plot changes from negative to near zero. That means that above E2, we can write eq 6 as

Y(ET) ≈ σs(ET - Es) + σ1(ET - E1)

(7)

where σs ) (σ0 + σ2) ) πP0Rs2 and σ1 ) -πP0PdRd2 as before. Assuming, for simplicity, that Pd ≈ 1.0, we then get Rd/R0 ≈ |σ1/σ0|1/2 and

Rs/R0 ≈ (1 + σ2/σ0)1/2

(8)

A necessary requirement for the interpretation to be correct is that the shift takes place before the onset of depletion, i.e., Es e E1, where, from eqs 6 and 7,

Es ≈ (σ0E0 + σ2E2)/σs

(9)

In Mn + SnCl4,10 the MnCl*(e5Σ+, d5Π, c5Σ+) yield functions are all found to be of the “complex” form, with transition state shifts of ∼9-22%. In addition the e5Σ+ and c5Σ+ channels show enhancement of reaction probability before the onset of depletion. Each excitation/yield function is, however, quite

distinct. In Mn + SiCl4,9 only the c5Σ+ channel shows a clear shift, ∼10-13%, and here, depletion of this state is accompanied by enhancement of d5Π production. Surprisingly, b5Π state production has the highest initial threshold and e5Σ+ the lowest. In Mn + SF6,11 all five chemiluminescent channels are of the “complex” type, but the interchannel interaction is taken a stage further. Three common processes, with different thresholds but all attributed to Mn*(a6DJ), appear to contribute to MnF*(c5Σ+, d5Π, e5Σ+) production, and it seems that the process with the highest threshold may also be responsible for enhanced A7Π production. The relatively low threshold A7Π process appears to be due to Mn*(z8PJ). Again, the b5Π channel shows the highest initial threshold but a low transition state shift, only ∼4%. In contrast, the two lower threshold processes leading to MnF*(c5Σ+, d5Π, e5Σ+) display shifts of ∼13-25%, while the Mn*(z8PJ) f MnF*(A7Π) functionality implies a shift of ∼26-36%. Such behavior has been rationalized9,11,12 in terms of an ionic-covalent curve crossing model, in which it is assumed that the observed chemiluminescent channels derive only from excited ionic potentials. Simple Magee-type calculations15 indicate that the “inner crossings”16 with such curves will only occur at very short internuclear distances, requiring near C3V approach geometry in both Mn + SiCl4 and Mn + SF6 in order for the Mn to penetrate close enough. In this picture, therefore, the reagent molecule behaves as a “soft sphere”. At high relative velocities, however, the target molecule does not have time to adjust to the arrival of the incoming Mn atom, so the vertical electron affinity becomes more important, with the result that the relevant crossings are forced out to higher energies and greater internuclear distances, i.e., a shift to “hard sphere”-type behavior. The commonality in the c5Σ+, d5Π, and e5Σ+ excitation functions in Mn + SF6 has been ascribed to a reduced symmetry requirement at short internuclear distances, leading to mixing of the relevant potential surfaces. The present study of Mn + CF4 allows us to examine whether such an interpretation applies in a much more extreme situation. The fully ground-state reaction,

Mn + CF4 f MnF + CF3 ∆H°298 ) 118 ( 14 kJ mol-1 17,18 is the most endothermic of all the polyhalide reactions we have so far investigated. In addition, as discussed below, the electron affinity of CF4 is likely to be significantly negative, shifting many of the anticipated “inner crossings” with excited ionic curves to much higher energies. Thus, reaction, quite apart from chemiluminescence, is likely to be strongly disfavored, with high barriers and without any mechanism that could lead to a forward transition state shift. Indeed, the accepted wisdom of the inertness of CF4 initially suggested to us that chemiluminescence might be absent altogether: we know of no other reports of metal atom + CF4 reaction dynamics. Experimental Section The experimental configuration has already been described.19,20 As before, the Mn target was of electrolytic quality and the laser-produced pulsed beam was monitored, at a distance x ) 282 mm from the target, by the long-lived Mn*(z8PJ f a6S) emission. CF4 (BDH) was bled from the lecture bottle into the reaction chamber to maintain a standing pressure of ∼0.013-0.13 Pa, as measured by a Penning gauge. MnF chemiluminescence was detected at the same point as the Mn beam emission and isolated into the different band systems by means of the same filter and photomultiplier combinations as

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Figure 1. (a) Excitation function σ(ET0) and (b) derived yield function Y(ET0) ) ET0σ(ET0) for production of blue MnF chemiluminescence in Mn + CF4. In each case the point-to-point resolution represents 0.2 µs time delay, and the curves shown correspond to the best fit, by the multiple line-of-centers approach (MLC), to Y(ET0), assuming four contributing terms. Noise at low collision energies in (a) reflects the small beam and luminescence signals at long delay times. The comparable negative-going data at such times are not shown.

employed for Mn + SnCl4 10 and Mn + SF6:11 330-380 nm (“UV”, A7Π f X7Σ+), 417-490 nm (“blue”, e5Σ+ f a5Σ+), 470-510 nm (“green”, d5Π f a5Σ+/X7Σ+), 590-690 nm (“red”, c5Σ+ f a5Σ+), and >780 nm (“IR”, b5Π f a5Σ+). Again, unlike Mn + SiCl4,9 no significant luminescence was found in the 470-490 nm region, so the blue and green ranges can be considered as 417-470 and 490-510 nm, respectively. In this beam-gas configuration, where the nominal collision energy ET0 ) 1/2µ(x/t)2, the excitation function σ(ET0) is obtained by comparing averaged chemiluminescence and beam signals for each delay time t while correcting for metastable radiative decay and the different detection sensitivities.9,11,19 Results and Analysis Chemiluminescence was observed in the blue, green, red, and IR regions, but notsin contrast to Mn + SF6 11sin the UV (A7Π f X7Σ+). The derived excitation functions, up to ET0 ) 2500 kJ mol-1, are displayed in panels a of Figures 1-4 and represent the result of averaging over ∼17 300, 16 300, 9200, and 8600 laser shots, respectively. Table 1 lists crude estimates of the relative peak/plateau cross sections, taking into account the differing photomultiplier sensitivities, filter transmissions, and CF4 pressures required. Owing to difficulties in maintaining a steady pressure, however, the values in the table should be treated with some caution. In addition, the data in the figures at ET0 g 2000 kJ mol-1 may not be completely reliable; such

Herbertson and Levy

Figure 2. (a) Excitation function and (b) derived yield function for production of green MnF chemiluminescence. Again, the curves correspond to the best MLC fit to Y(ET0), assuming four contributing terms.

energies correspond to the rising edge of the beam pulse, i.e., Mn atom flight times < 26 µs, where we have reason to believe that there may be incomplete cascading into the Mn*(z8PJ) state, which is used to monitor the beam. The data sets all show high thresholds quite close together in the range ∼200-300 kJ mol-1 but significant differences thereafter. The blue excitation function (Figure 1a) peaks at ∼500 kJ mol-1, then starts to rise again at ∼650 kJ mol-1, and again at ∼1800 kJ mol-1. The green data (Figure 2a) rise from an initial threshold at ∼300 kJ mol-1 to a plateau at ∼1100 kJ mol-1 and then display a further rise from ∼2000 kJ mol-1. The red (Figure 3a) and infrared (Figure 4a) data show initial thresholds close to 300 kJ mol-1, but the IR data peak earlier and fall off much more rapidly. The corresponding yield function plots for all the above excitation functions, Y(ET0) ) ET0σ(ET0), are shown in panels b of Figures 1-4. These transformed data sets have been analyzed using the MLC approach described in the Introduction, and the curves displayed in the figures represent the best fits obtained by nonlinear least squares regression21 and employing the minimum number of parameter pairs consistent with the data. Both the data and the fits in fact display some threshold curvature because of the spread of collision energies at nominal value ET0 in our beam-gas configuration. The function that has been used to model the data is therefore not eq 1 or 2 but the much more complicated transformed version.9,20 The parameters of the analysis are listed in Table 2, and the back-calculated fits to the original data are displayed in panels a of Figures 1-4. In all cases χ2 , 1.0, the correlation coefficients r > 0.996, and the degree of interparameter

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Figure 3. (a) Excitation function and (b) derived yield function for production of red MnF chemiluminescence. Curves correspond to the best MLC fit to Y(ET0), assuming only three contributing terms.

Figure 4. (a) Excitation function and (b) derived yield function for production of infrared MnF chemiluminescence. Curves correspond to the best MLC fit to Y(ET0), assuming four contributing terms.

correlation is quite modest. Thus, for each excitation function, the different line-of-centers terms appear to be quite distinct. For the blue data (Figure 1b), the yield function clearly shows three production processes and one depletion, as suggested by the excitation function. The depletion term, σ1, appears to be numerically larger than σ0, although the uncertainties would allow the two to be equal. Indeed, if we assume unit depletion efficiency, then from eq 5 we get Rd/R0 ≈ 1.11 ( 0.10, which is not inconsistent with “simple” depletion. Both the σ2 and σ3 terms are too large for the sort of “complex” depletion originating from a transition state shift (eqs 6 and 7), but a contribution from such a possibility cannot be excluded; the necessary secondary rise (slope change of 0.16 ( 0.15) could be either hidden in the noise of the data or (if small) outside the measured energy range. Indeed, with its low depletion threshold and high depletion probability, this initial feature is more akin to those Mn-halide excitation functions that show a transition state shift than to those that do not.9-11 The threshold at ∼670 kJ mol-1 suggests the opening up of a new reaction channel, perhaps involving a different reagent state, as does the threshold at ∼1811 kJ mol-1. However, as pointed out earlier, the σ3 term could be an artifact due to the rising edge of the beam pulse. The green data (Figure 2b) likewise show three production processes and one depletion. Here, the negative term, σ2, is numerically smaller than either of the preceding terms, so in this case there is clearly no outward transition state shift. The σ1 term indicates either an increase in reaction probability or the opening up of a different reaction channel, again perhaps involving a different reagent state. However, even if depletion from E2 is exclusively associated with the E0 production channel,

TABLE 1: Crude Estimates of Relative Cross Sections (at Peak or Plateau) emission

ET0/kJ mol-1

σ/arb units

blue green red IR

500 1500 860 660

1.25 3.0 1.0 6.5

its relative probability Pd is likely to be less than 100%; substituting σ2 and σ0 from Table 2 into eq 5, with Pd ) 1.0, leads to Rd/R0 ) 0.88 ( 0.05, which seems rather a large degree of inward penetration for depletion given that reaction is already likely to be occurring at short internuclear distances (see below). As with the blue chemiluminescence, the E3 threshold suggests a further reaction channel, but again, we cannot exclude the possibility of an artifact. The back-calculated excitation function in Figure 2a appears to underestimate the data in the region of E1. We therefore repeated the nonlinear regression in this case but attempted to fit the excitation function instead. In the other channels, such a process had a minimal effect, generally increasing χ2 and the parameter uncertainties but yielding optimized parameter values insignificantly different from those displayed in Table 2. However, in this case the optimum values of E1 and E2 were changed from 708 ( 7 and 1284 ( 20 kJ mol-1 to 685 ( 11 and 1328 ( 65 kJ mol-1, respectively, the other parameters being largely unaffected. The precise derived values of E1 and E2 therefore need to be treated with some circumspection here. The analysis of both the red and infrared data (Figures 3b and 4b, respectively) indicates an initial production threshold at ∼301 kJ mol-1 and depletion beginning at ∼641-653 kJ

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Herbertson and Levy

TABLE 2: Parameters of Analysis: σk/Arbitrary Units; Ek/kJ mol-1 emission

k

0

1

2

3

blue (e5Σ+-a5Σ+) green (d5Π-a5Σ+) red (c5Σ+-a5Σ+) infra-red (b5Π-a5Σ+) “red extra” (c5Σ+-a5Σ+)

σk Ek σk Ek σk Ek σk Ek σk Ek

0.73 ( 0.04 257 ( 6 0.467 ( 0.015 322 ( 6 1.63 ( 0.04 301 ( 4 1.79 ( 0.02 301 ( 2 0.79 ( 0.03 742 ( 11

-0.89 ( 0.15 514 ( 17 0.778 ( 0.018 708 ( 7 -0.61 ( 0.06 641 ( 22 -1.03 ( 0.06 651 ( 10 -0.84 ( 0.03 1349 ( 18

0.96 ( 0.15 670 ( 15 -0.362 ( 0.016 1284 ( 20 -1.17 ( 0.05 1039 ( 12 -1.39 ( 0.06 902 ( 8

0.86 ( 0.03 1811 ( 14 1.27 ( 0.05 2157 ( 11

mol-1. However, the degree of that depletion differs, and, as the figures indicate, the red data subsequently show only an additional fall while the IR data show an additional fall plus a secondary rise. If we regard the behavior as quite distinct, then from eq 5 the red data imply Rd/R0 ≈ 1.04, with initial depletion probability of ∼0.34 from ∼641 kJ mol-1. The IR data, on the other hand, suggest a reaction transition state shift. From the derived parameters and eqs 8 and 9, we find that Rs/R0 ) (1 + σ3/σ0)1/2 ) 1.14 ( 0.01, with Es ) 570 ( 15 kJ mol-1, i.e., below the first depletion threshold, as expected. The sum of the two IR depletion processes, σ1 + σ2, slightly exceeds σs ) σ0 + σ3, so it seems that Rd/Rs ≈ 1.02. The initial depletion probability, corresponding to the threshold E1 ≈ 708 kJ mol-1, is given by the ratio σ1/(σ1 + σ2) ) 0.43 ( 0.03. Nonetheless, the close agreement in the first two thresholds for these channels suggests very strongly that there must be some underlying feature in common. Since the IR data fall off more rapidly, that implies that an additional component must be contributing to the red data. To reveal that, we have weighted the IR data in Figure 4a by the inverse ratio of the two σ0 values, i.e. by 1.63/1.79, and subtracted each weighted σ(ET0) value from the corresponding datum in Figure 3a. The result, to the same scale as in Figure 3a, is shown in Figure 5a, with the corresponding yield function in Figure 5b. A “simple depletion” MLC expression was sufficient to fit the data to the same quality as in the other cases. The parameters, as “red extra” in Table 2, indicate Rd/R0 ≈ 1.03. Such a result illustrates the need for caution generally in analyzing measured excitation functions. Even in apparently simple situations, such as in Figure 3, more than one process may be contributing, although we cannot assume that in the absence of additional evidence. In the present case, the coincidence of the initial threshold and the simple functionality of the “deconvoluted” red data are strong arguments in favor of a common red/IR process. However, the precise form of both the “deconvoluted” data and fit in Figure 5 must also be regarded with some skepticism in view of the noise on the two original data sets. Indeed, small differences in the region 600900 kJ mol-1 could easily result in the derived initial threshold moving up or down. Because of the noise in Figure 3, the depletion at ∼650 kJ mol-1 and the production at ∼740 kJ mol-1 are not resolved, and it could even be that these two thresholds are identical, although we argue below that this is unlikely. The normalization of the σk values given in Table 2 is that required for a fit to each of the excitation functions displayed, i.e, panels a of Figures 1-5. To reproduce the measured crude relatiVe peak/plateau cross sections, given in Table 1, a different scaling factor would have to be applied to each set of parameters, namely ∼0.93, ∼1.0, ∼0.28, and ∼1.69 for the blue, green, red, and infrared regions, respectively. This suggests that the σ0 values are very approximately in the ratio 1.5:1:1:7. On this scale, the σ0 value for the “deconvoluted” high-threshold red component in Figure 5a is ∼0.5.

0.53 ( 0.02 1479 ( 18

Figure 5. (a) “Excitation function” derived by subtracting the estimated contribution of the infrared functionality (Figure 4a) from the λ ) 590690 nm data (Figure 3a). (b) “Yield function” derived from the data in panel a. In each case the normalization is the same as in Figure 3, and the curve corresponds to the best MLC fit to Y(ET0), assuming two processes.

Discussion As with Mn + SnCl4, SiCl4 and SF6,9-11 it has not proved possible to measure well-resolved emission spectra. Nonetheless, as discussed before,9,11 MnF2* emission can be excluded as the origin. In addition, the only other possible molecular product species that are known to emit in this region, CF3*(12E′,22A2′′),25-28 have state energies far too high (∼8 eV by ab initio calculation28) to be consistent with the measured initial thresholds. In any case the known CF3* band profile, continuous from ∼420 to ∼750 nm, would require the same collision energy dependence in the blue, green, and red regions. As far as Mn* atomic transitions are concerned, none is known for the IR, red, and green regions, which could be excited with the given thresholds from any metastable state of Mn likely to be significantly present in the beam.22-24 For the blue channel, however, the e6DJ f z6PJ (∼446 nm), z4DJ f a4DJ

Reaction of Mn Atoms with CF4 (445-450 nm), and y4PJ f a4DJ (424-431 nm) transitions are a possibility, since the minimum thresholds would be ∼358, ∼267, and ∼280 kJ mol-1 from a6DJ, a4DJ, and a4DJ atoms, respectively. We nonetheless regard this as unlikely, since emission that could be consistent with such processes has been absent in every other Mn-halide case studied: Mn + SnCl4,10 SiCl4,9 SF6,11 CCl4,29 GeCl4,30 TiCl4,30 S2Cl2,31 SO2Cl2,31 F2,12 Cl2,12 and ICl.12 In all these systems, except for SF6, we have found strong collision-induced Mn* z6PJ f a6S emission at ∼403 nm. Only in Mn + SiCl4, F2, Cl2, and ICl has another atomic emission been found, the e8S f z8PJ and/or z4FJ f a4DJ transition at 479.3 and 475.9 nm, respectively. Given that the latter is missing in the present study, it is doubtful that any of the aforementioned 424-450 nm atomic transitions should be present. In addition, the rapid depletion observed in the blue excitation function at ∼514 kJ mol-1 is quite uncharacteristic of collision-induced atomic emission12,13,19,29-32 and much more like a reactive chemiluminescence excitation function. On this basis, the initial blue production process, if not the two higher ones, seems likely to correspond to MnF*(e5Σ+). Considering the wavelength ranges of the different filters, we can therefore be satisfied that the green, red, and IR emission processes originate from MnF in the d5Π-a5Σ+/X7Σ+, c5Σ+a5Σ+, and b5Π-a5Σ+ band systems, respectively, while the blue ∼257 kJ mol-1 process, at least, derives from MnF*(e5Σ+). The results therefore indicate (i) a common b5Π/c5Σ+ production process with threshold ∼301 kJ mol-1, (ii) a separate c5Σ+ process with apparent threshold ∼742 kJ mol-1, although we cannot exclude the possibility that this coincides with the b/c depletion threshold at ∼650 kJ mol-1, (iii) two d5Π processes with thresholds ∼320 and ∼685-710 kJ mol-1, plus a possible third process at ∼2160 kJ mol-1, and (iv) an e5Σ+ process with threshold ∼257 kJ mol-1 and two possible higher processes at ∼670 and ∼1810 kJ mol-1. Although the b/c channel does not have the lowest threshold, it is the predominant process detected with only a small proportion of the flux leading to MnF*(c5Σ+). The close agreement, within narrow experimental error, between the b/c depletion threshold at ∼650 kJ mol-1 and the blue production threshold at ∼670 kJ mol-1 suggests that the two may be connected. Indeed, when we make a crude absolute comparison, applying the scaling factors given above to the blueσ2 and b5Π-σ1 parameters in Table 2 (adding in an appropriate proportion for the c5Σ+ state), we obtain values of about 0.9 and -1.9, respectively. By the same calculation, σ1 for the d5Π process at ∼685-710 kJ mol-1 becomes about 0.8. Considering the crudity of the comparison, and the aforementioned ambiguity about the d5Π threshold value, these figures are not inconsistent with a substantial part of the b/c depletion being due to enhanced MnF*(e5Σ+ and/or d5Π) production. Similar behavior was observed in Mn + SiCl4, where enhanced formation of MnCl*(d5Π) at high energies appeared to be due to depletion of the c5Σ+ state, and in Mn + SF6, where depletion of MnF*(b5Π) seemed to lead to increased production of the c, d, e, and (perhaps) A states. However, in contrast to those observations, the enhancement in the present case is not in turn depleted at higher energies still. In the absence of further information, therefore, we must reserve judgement on the origin of these present enhancement processes. Although the unique c5Σ+ channel (“red extra”) does have similar Ek values to E1 and E2 of the d5Π excitation function, the substantial difference in relative σk values makes it unlikely that these two correspond to the same process.

J. Phys. Chem., Vol. 100, No. 35, 1996 14589 TABLE 3: Reagent and Product State Excitations4,5,8,9,24 (Energies in kJ mol-1) Mn state 6S

a a6DJ z8PJ a4DJ

excitation

MnF state

excitation

0 207 ( 4 222 ( 2 281 ( 2

X7Σ+

0 ∼7 ∼153 ∼180 242 ∼273 341

a5Σ+ b5Πi c5Σ+ d5Π e5Σ+ A7Π

Table 3 lists the excitation energies of both the different MnF* states detected and the possible reagent states. By energy balance, we have

E0 ) D0(CF3-F) - D0(MnF) + Ei(MnF*) - Ei(Mn*) + Eb - Ev,r(CF4) (10) where Ei represents electronic excitation, Ev,r(CF4) is that part of the reagent vibrational-rotational excitation that is used to promote reation, and Eb is the remaining excess barrier over the endothermicity. As with Mn + SnCl4, SiCl4, and SF6,10-12 we can ignore the Ev,r term since all the yield functions show initial linear behavior, i.e., only a single mode (translation) contributes at the transition state.32 This leads to

Ei(Mn*) g Ei(MnF*) + ∆H - E0

(11)

Substituting E0 from Table 2 and Ei(MnF*) from Table 3, we find that Ei(Mn*) g ∼134 ( 15, 38 ( 15, -3 ( 15, and -30 ( 15 kJ mol-1 for production of MnF* in the e5Σ+, d5Π, c5Σ+, and b5Π states, respectively. Thus, ground-state Mn(a6S) atoms could in principle be the progenitors of the b5Π and c5Σ+ product states in the common lower-threshold process and of the c5Σ+, d5Π, and e5Σ+ states in the higher-threshold processes. However, for the lower-threshold processes leading to MnF*(d5Π,e5Σ+), excited atoms are definitely required. By comparison with the previous investigations of Mn + SnCl4, SiCl4, SF6, the most likely excited reagent species might be expected to be the a6DJ state. Production of all the observed quintet states from z8PJ atoms is spin-forbidden, and indeed, the only Mn-halide cases so far studied where z8PJ atoms have been implicated are the spin-allowed A7Π channels of Mn + SF6, F2, and Cl2,11,12 a process clearly absent here. However, generation of the observed chemiluminescence would be spinallowed from the third excited state, Mn*(a4DJ), and it is arguable that any inherent barrier could be smaller here than for the a6DJ state. As shown in ab initio calculations on FeCO, CuCO, and NiCO,33-34 repulsive interactions may be alleviated by 3d-4s hybridization, provided the orbitals involved contain a single electron of opposite spin; one empty sd hybrid then points toward the ligand while the other doubly occupied hybrid points in the other direction. In Mn, both the a6DJ and a4DJ states are predominantly ...3d64s1, so repulsion could well be lower for the latter, singlet-coupled, case. In principle, higher metastable states of Mn could also be involved, but this seems unlikely: (a) the populations in the beam will fall with increasing excitation; (b) the next three metastable levels, a4G, a4P, and b4D (∼302, 326, and 363 kJ mol-1, respectively) are all overwhelmingly ...3d54s2 in character,24 i.e., greater repulsion would be expected from the doubly occupied 4s orbital. The observed thresholds give further evidence in favor of excited atoms as the reagent species in all cases. Substitution of the a6DJ excitation into eq 9 indicates a substantial excess barrier over the endothermicity for d5Π and e5Σ+ production, respectively 169 ( 15 and ∼73 ( 15 kJ mol-1. If a4DJ atoms

14590 J. Phys. Chem., Vol. 100, No. 35, 1996 are the reagent, then the barriers are ∼74 kJ mol-1 higher. On the other hand, for common formation of MnF*(b5Π,c5Σ+) from Mn(a6S) atoms, the results would suggest an essentially zero excess threshold over the c5Σ+ asymptote. This appears to be inconsistently small, especially since, as discussed below, ground-state atoms should be much less reactive. Excess thresholds have proved to be a consistent feature of these Mnhalide reactions, and as noted previously,9,11 the only cases where ground-state atoms have definitively been shown to be involvedsthough always in conjunction with a6DJ reactionsare Mn + F2, Cl2.12 We therefore conclude that either a6DJ or a4DJ atoms are also the progenitors of the common b5Π/c5Σ+ process. In principle, ground-state atoms could still be responsible for the separate c5Σ+ process at ∼742 kJ mol-1 and for the upper production thresholds in the d5Π and e5Σ+/blue channels, but here again, a6DJ/a4DJ atoms must be preferred, since only a single process contributes to the common b5Π/c5Σ+ channel. Indeed, given that this is the dominant process observed, it is probable that the same Mn state is involved in all cases. Despite the lower initial threshold for the e5Σ+/blue channel, it is difficult to see why any one reagent species should be restricted to just one or two more excited product states. As pointed out in the Introduction, chemiluminescence in Mn + SnCl4, SiCl4, SF6 has been rationalized9,11,12 in terms of an ionic-covalent curve crossing model in which the emitting states derive only from excited ionic potentials. The crossings with these curves are estimated to occur at very short internuclear distances, which can only be achieved if the Mn approaches the central atom (Sn, Si, S) along or close to a C3 axis. In this geometry, the excited covalent and ionic curves are split according to the spatial orientation of the odd d electron in Mn*(...3d64s, a6DJ) and Mn+*(...3d6, a5DJ). Further splittings occur as the collision complex moves from C3V to Cs. It was concluded that, in Mn + SiCl4, each emitting state derives from a different ionic curve, but that in Mn + SF6, the c5Σ+, d5Π and e5Σ+ states are formed in common processes, suggesting a breakdown in the strict C3V requirement, possibly due to vibrational motion. The forward transition state shifts observed in these reactions were rationalized within this curve-crossing model as arising from a lack of time, at high collision energies, for the geometry of the target molecule to shift to that of the anion. In the present reaction, any involvement of ionic-covalent curve crossings in the entrance channel is expected to be problematic owing to the apparently large negative electron affinity of CF4. Indeed, there seem to be no specific determinations of this quantity, since the two lowest CF4- potentials are ˜ 2A) and F(2P) + CF3-, repulsive, dissociating to F- + CF3(X respectively.35,36 For the former, lower channel, the F- appearance potential is ∼4.5 eV, while the F- + CF3 asymptote lies ∼1.95 eV above the bottom of the CF3-F potential well. Regarding the latter as the negative of the adiabatic electron affinity, we find that a simple Magee-type calculation15 yields a maximum crossing radius, for the ground-state ionic channel, of ∼2.0 Å for Mn*(a6DJ) + CF4 and ∼1.5 Å for Mn(a6S) + CF4. Of course, this simple electron jump model is only a crude approximation, regarding both ions as point charges. Nonetheless, given the C-F equilibrium bond length of 1.323 Å37 and Mn, F atomic radii of 1.18 and 1.35 Å, respectively,38 it is clear that the crossing radii will lie will within the “hard sphere” repulsion region. Even if, as proposed for Mn + SiCl4, SF6,9,11 the Mn atom approaches toward C along a C3 axis, the system should start to experience repulsion at much greater distances, ∼2.7 Å.

Herbertson and Levy For the excited ionic potentials that would correspond to the observed luminescent products, the relevant curve crossings will clearly be shifted even further into the repulsive “hard sphere” region. In such circumstances, a contribution from ground-state atoms seems extremely unlikely, except perhaps at the very highest energies achieved in this experiment. Even Mn*(a6DJ or a4DJ) atoms are likely to require substantial collision energy. Nonetheless, since the various MnF* states observed can all be regarded as Mn+*F-, crossings with ionic curves must be reached at some point in the reaction. In general this means that the critical crossing must be reached before the system has penetrated very far into the exit channel. The experiments do not provide direct evidence for such curve crossings. Nonetheless, the forward transition state shift observed for the common b5Π/c5Σ+ process is suggestive of such behavior. As noted above, a similar shift could be occurring for the lowest-threshold process in the blue/e5Σ+ channel, and there is a similarity to the SiCl4 case in that this most highly excited state has the lowest initial threshold. The major difference in the present case, apart from the (for the most part) higher crossing energies, seems to be that the b5Π and c5Σ+ potentials are intimately mixed, with b5Π flux predominating. This could result from the higher degree of penetration and distortion needed to reach the relevant curve crossings, with the system moving off the C3 approach axis so that the Mn starts to insert into the C-F bond. As in Mn + SF6, vibrational motion in the polyatomic fragment in the exit valley could also play a role here. Conclusions The reaction between CF4 and a laser-ablated beam of Mn atoms yields chemiluminescence from at least three, and most probably four, different MnF* states in the collision energy range ET ) 0-2500 kJ mol-1. Analysis of the excitation functions, by the multiple line-of-centers approach, indicates that (1) the b5Π and c5Σ+ product states are predominantly formed on a common potential surface with threshold of ∼301 kJ mol-1 and with a forward transition state shift of ∼14% at line-of-centers energy ∼570 kJ mol-1, (2) depletion of product on this surface, above ∼650-700 kJ mol-1, is not inconsistent with enhanced production of the e5Σ+, and perhaps d5Π, states of MnF*, and (3) separate processes, with thresholds ∼257, ∼322, and ∼742 kJ mol-1, lead to production of MnF*(e5Σ+) (probably), MnF*(d5Π), and MnF*(c5Σ+), respectively. The Mn state involved in all cases appears to be either the first or third metastable level, a6DJ or a4DJ. On theoretical grounds, the latter might be expected to exhibit lower barriers to reaction. Despite the undoubted negative CF4 electron affinity, the forward transition state shift observed for the common b5Π/ c5Σ+ process is suggestive of an ionic-covalent curve crossing mechanism in which the C-F bond has insufficient time at high collision energies to stretch to the distance typical of the anion. The common b5Π/c5Σ+ functionality is attributed to a high degree of penetration and distortion at the critical crossing, with the system moving off the C3 approach axis so that the Mn starts to insert into the C-F bond. Acknowledgment. We thank the UK Science and Engineering Research Council (now the Engineering and Physical Sciences Research Council) for the equipment grant that led to this research and unknown referees for helpful comments. References and Notes (1) Hayes, W. Proc. Phys. Soc., London 1955, 68A, 1097, and references therein.

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