J. Phys. Chem. 1992, 96, 1284-1288
1284
Study of the Ca and Sr Metal Imides
+ HN3 Reactions:
Indirect Evidence for the Formation of the
Jing Chen and Paul J. Dagdigian* Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland 21 21 8 (Received: August 12, 1991; In Final Form: October 8, 1991)
The reaction of Ca and Sr atoms with HN3 has been studied in a beam-gas configuration. Emission from electronically excited me@ atoms (IPand 'P) and the metal hydrides (A2Uand B2Z+)was observed. Additional molecular chemiluminmce was also seen and assigned to production of electronically excited MOH. By the deliberate addition of oxygen-containing molecules, the formation of MOH* was confirmed as being due to a secondary reaction involving O2impurity in the HN3 samples. In laser fluorescence experiments, ground-state MOH and MN3 were detected. An unsuccessful search for CaNH laser fluorescence excitation was also conducted. All of the observed emissions can be explained as arising from secondary reactions of the metal imide (MNH) formed in the M + HN3 primary reaction. A kinetic model is presented to explain these observations, and it is concluded that the M + HN3 reaction proceeds mainly by formation of MNH, rather than MN3. Bounds on the M-NH bond dissociation energy are also derived.
Introduction Hydrazoic acid (HN,) is an interesting reagent because of its extremely small dissociation energy. Its stability is conferred by the fact that dissociation to the lowest energy fragments, NH(X32;-) N2(X1Z8+),is spin-forbidden.' There are two chemically distinct decomposition channels for HN,:
+
HN,
--
NH H
+ NZ,
+ N3,
AJY = +45 kJ/mol
AH = +373 kJ/mol
(1) (2)
Either of the two pathways has been found operative in reactions of HN3. For example, the F + HN, reaction24 occurs most likely by hydrogen atom abstraction, yielding HF with some vibrational excitation and the azide radical N3. By contrast, the reaction of HN, with H atoms appears to proceed with cleavage of the HN-N2 bond to yield the products NH2 + N2.5 It is interesting to speculate about the interaction of hydrazoic acid with metal atoms. In the present study, we investigate such reactions involving the alkaline earth metal atoms Ca and Sr. There are two possible reactive pathways M + HN3 MN3 + H (3) +
+
MNH
+ N2
(4)
where M = Ca, Sr. We can reasonably ignore an alternate pathway to reaction 3 involving cleavage of the H-N3 bond, namely the formation of MH N3, because of the small bond energies of the metal hydrides? The Ca and Sr azides have been previously spectroscopically characterized by Brazier and Bernath.7 The alkaline earth metal imides (MNH) have not yet been observed but are isoelectronic with the well-known alkaline earth metal oxides.&l0 Very recently, the first example of a metal imide,
+
(1) Alexander, M. H.; Werner, H.-J.; Hemmer, T.; Knowles, P. J. J. Chem. Phys. 1990,93, 3307. (2) Habdas, J.; Wategaonkar, S.;Setser, D. W. J . Phys. Chem. 1987,91, 451.
( 3 ) Sloan, J. J.; Watson, D. G.; Wright, J. S. Chem. Phys. 1979, 43, 1; 1981, 63. 283. (4) Coombe, R. D.; Pritt, A. T. Jr. Chem. Phys. Lett. 1978,58,606. Pritt, A. T. Jr.; Coomk, R. D. Int. J. Chem. Kinet. 1980,12,741. Pritt, A. T. Jr.; Patel, D.; Coombe, R. D. Int. J . Chem. Kinet. 1984, 16, 977. (5) Kajimoto, 0.; Kawajiri, T.; Fueno, T. Chem. Phys. Lett. 1980,76,315. ( 6 ) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular Structure, Val. IV. Constants of Diatomic Molecules; Van Nostrand Reinhold: 1979. . . .- New . York. ... (7) Brazier, C: R.; Bernath, P. F. J . Chem. Phys. 1988, 88, 2117. (8) Field, R. W. J. Chem. Phys. 1974,60, 2400. (9) Marks, R. K.; Schweda, H. S.; Gottscho, R. A.; Field, R. W. J . Chem. Phys. 1982, 76,4689. (IO) Norman, J. 8.; Cross, K.; Schweda, H. S.; Polak, M.; Field, R. W. Mol. Phys. 1989, 66, 235.
namely YNH, has been identified through observation of several electronic band systems in the visible region." In this paper, we present the results of our investigation of reactions 3 and 4, including indirect evidence for the formation of MNH. In addition to laser fluorescence detection of the metal azides formed in reaction 3, we have observed emission from electronicallyexcited metal hydrides and atoms in the wavelength range of 420-800 nm with pund-state M + HN, mctants. The formation of these species can be explained through subsequent reactions of MNH with HN3. Additionally, we have seen molecular emission in the green and orange, whose carrier we have identified as MOH. On the basis of a number of different types of experiments, we conclude that the electronically excited metal hydroxide arises from the rapid reaction of MNH with oxygencontaining molecules, in particular the small O2impurity in our hydrazoic acid samples, as well as with oxygen-containing molecules deliberately introduced into the reaction chamber. We have also attempted unsuccessfully to detect the metal imides directly by laser fluorescence excitation. Apprratus Thew experiments were performed in a beam-gas configuration approaching singlecollision conditions, using an apparatus which has been described in detail p r e ~ i o u s l y . ~A~ 'near-effusive ~ beam of alkaline earth metal atoms in the ground ns2 'S state was generated in a high-temperature oven housed in a differentially pumped source chamber. The typical pressure in the crucible (orifice diameter 0.15 cm) containing the metal atoms was ca. 6 Torr, at a temperature of 1200 K for Ca and somewhat lower for Sr. In so"experiments, the metal atoms were excited to their metastable nsnp 3P and ns(n - l)d ID states by a dc discharge of ca. 20 V,as previously described in detail.I4 The metal atoms passed into the reaction chamber through a narrow slot 0.2 X 0.8 cm2which was 2.5 cm from the crucible orifice. Reagent gas was introduced into the reaction chamber, and typical pressures in the source and reaction chambers were 1 2 X lw and 1 2 X 10-4 Torr, respectively. As a small beam collimator was employed to minimize leakage of the reagent gas into the source chamber, the possibility of having reactions with the molten metal inside the oven crucible was prevented. Chemiluminescence in the reaction chamber was observed through a 5 cm focal length fused silica lens with a magnification (11) Simard, B.; Balfour, W. J.; Vasseur, M.; Hackett, P. A. J . Chem. Phys. 1990, 93,4481. (12) Chen, J.; Quiaones, E.; Dagdigian, P. J. J . Chem. Phys. 1990, 93, 403 3. (1 3) Campbell, M. L.; Dagdigian, P. J. Faraday Discuss. Chem. Soc. 1987, 84, 127. (14) Irvin, J. A.; Dagdigian, P. J. J . Chem. Phys. 1980, 73, 176.
0022-3654/92/2096-1284$03.00/00 1992 American Chemical Society
Study of the Ca and Sr + HN, Reactions
CaOH B + X A'
r
-I
CaOH A --t X A b 1 0 -1
(b)
400
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1285
,
A
400
J\ I
JW,
500 600 wavelength (nm)
500
700
600
800
wavelength (nm)
Figure 2. Chemiluminescence spectra observed in the reaction of Sr atoms with hydrazoic acid. The pressure in the reaction chamber was 1.6 X lo4 Torr.
1
700
8
Figure 1. Chemiluminescence spectra observed in the reaction of Ca atoms with (a) hydrazoic acid and (b) tert-butyl hydroproxide. The pressures (in units of lo4 Torr) of the reagent in the reaction chamber were 1.5 and 4.8, respectively. The various atomic and molecular emission featura are identified.
6
4 2
factor of 2.2. The observation zone was 10 cm downstream of the beam collimating slot with a volume of approximately 0.1 X 1.4 X 4.4 cm3. The emission was dispersed with a 0.25-m spectrometer (Jarrell-Ash 82-410, unit magnification, slits 500 gm wide and 2.3 cm high, spectral band-pass 1.7 nm fwhm), detected by a cooled RCA C31034-02 photomultiplier coupled to a photon counting system (Mechtronics 5 11 amplifier/discriminator and 777 ratemeter). The analog output of the ratemeter was amplified and acquired under computer control, and the spectra were stored magnetically for later analysis. Wavelength calibration of the spectrometer was verified using Ca atomic lines at 422.7 and 657.3 nm. The absolute photon yield was measured by comparison with known chemiluminescencecross section of the Ca('S) + N 2 0 CaO* N2 reaction.15 The nonemitting ground-state species were detected by laser fluorescence excitation using a XeCl excimer laser pumped dye laser (Lambda Physik EMGlOlMSC and FL3002) with a typical line width of 0.2 cm-'. The diameter of the laser beam in the reaction chamber was 0.5 cm,and the laser pulse energy was kept below 1 CJ to avoid saturation effects. The fluorescenceemission was imaged with a telescope onto a photomultiplier (EM1 9816B). The signal was directed to a gated integrator, whose output was collected under computer control. Hydrazoic acid was synthesized by two different methods: the reaction16of stearic acid and sodium azide at 400 K or the addition of H,PO, to NaN, at room temperature. The deuterated version (DN,) was prepared by the latter method with D,PO,. The purity of the samples was checked with IR and/or mass spectroscopy, and H20, COz, and O2impurities were found. The isotopic purity of DN, was found to be approximately 80%.
+
-
RtYdtS While the M-N3 and M-NH bond energies are not known, we can still speculate on the likely exothermicities of reactions 3 and 4. The azide moiety is a pseudohalide, and the M-N, bond energy would be expected to be similar to that of a corresponding alkaline earth metal halide, most likely the chloride, since the A-X and B-X transition energies of the azides' and chlorides6 are quite similar. Equating the M-N3 bond energies with the known" dissociation energies of CaCl and SrCl, we find that the AH of reaction 3 is moderately negative, -21 and -28 kJ/mol for M =
0 0
1
2
3
Figure 3. Dependence of the emission intensities of the various atomic and molecular features on the pressure of added hydrazoic acid in the reaction chamber for the Ca HN3system. The curves have been drawn through the experimental data to aid the eye.
+
Ca and Sr, respectively. This would allow production of the metal azide in its ground electronic state only. It is somewhat harder to estimate the exothermicity of reaction 4. Clearly, the M-NH bond energy will be significantly less than that of the corresponding isoelectronic oxide1*since the electron a f f ~ t of y NH is much less than that of the oxygen atom: EA(NH) = 0.381 f 0.014 eVI9 and EA(0) = 1.462 f 0.003 eV.20 If we were to assume that the MNH and MO bond energies were equal, then the exothermicity of reaction 4 would be ca. 350 kJ/mol, allowing production of electronically excited MNH states with occupied metal-centered npr or npu orbitals. The experiments described below indicate that this does not occur; we present evidence to suggest that the M-NH bond energy is of the order of 200 kJ/mol, approximately one half of the corresponding MO dissociation energy. Figures l a and 2 present the chemiluminescence spectra obtained when a metal atom beam impinges on a sample of hydrazoic acid introduced into the reaction chamber at relatively low pressures. Emissions from electronically excited atoms (nsnp 'P and ,P1) and the metal hydride (A211 and B2Z+) are straightforwardly identified. The production of excited atoms in reactions involving ground-state atoms is surprising and must involve efficient secondary reactions reflecting the energetic nature of HN,. On the basis of our previous experience with beam studies of chemiluminescent alkaline earth metal r e a c t i o n ~ , ' ~weJ ~would ~~~ not have expected secondary reactions to be important at pressures of less than ca. lod4Torr. Also evident in these spectra are molecular emission features near 555 and 600-625 nm for the Ca reaction and somewhat further to the red (600and 6 5 0 4 7 0 nm) for Sr. The wavelengths of these bands are consistent with the assumption that these transitions correspond to 4pa, 4 p i ~ 4su transitions of the nonbonding metal-centered electron, as has been seen in many
-
~
(15) Irvin, J. A.; Dagdigian, P. J. J . Chem. Phys. 1981, 74, 6178. (16) Krakow, B.;Lord, R. C.; Neely, G. 0. J. Mol. Specrrosc. 1968,27, 148. (17) Kleinschmidt, P. D.; Hildenbrand, D. L. J . Chem. Phys. 1978, 68, 2819.
4
HN3 pressure (104Torr)
HN3 pressure (104Torr)
~~
(18) Pedley, J. B.;Marshall, E. M.J . Phys. Chem. Ref. Dara 1983, 12, 967. (19) Engelking, P. C.; Lineberger, W. C. J. Chem. Phys. 1976,65,4323. (20) Hotop, H.; Lineberger, W. C. J. Phys. Chem. ReJ Dora 1975,4,539. (21) Yuh, H.-J.; Dagdigian, P. J. J. Chem. Phys. 1984,81, 2375. Furio, N.; Campbell, M. L.; Dagdigian, P. J. J . Chem. Phys. 1986, 84, 4332.
1286 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992
alkaline earth metal c o m p o ~ n d s . 7 ~In~ an J ~effort ~ ~ ~to determine the carrier of these emission signals, the dependence of the intensities as a function of the pressure of hydrazoic acid introduced into the reaction chamber was measured for the Ca reaction and is shown in Figure 3. It can be seen that these features at 555 and 600425 nm are the only ones exhibiting an apparently linear pressure dependence. One would be thus tempted to conclude that these emission features are due to the formation of CaNH in reaction 4, since the wavelengths of these bands are similar to the arc band t r a n s i t i o n ~ ~ in J ~the 9 ~ ~isoelectronic CaO molecule. However, such an assumption would lead to a higher than expected Ca-NH bond energy. If this emission were due to MNH, then we would expect to observe these transitions by laser fluorescence excitation of ground-state molecules formed either by radiative cascade or directly in reaction 4. We searched in the wavelength range 520-680 nm for laser fluorescence features which could be identified with MNH as the carrier. No signals assignable to MNH were observed; however, the known A-X and B-X band systems’ of the metal azides were observed near 615 and 585 nm, respectively, for Ca and near 630 and 600 nm for Sr. In addition, relatively strong bands at the wavelengths of the molecular emission features of interest were observed and were assigned as A-X and B-X bands of CaOH and SrOH.25926This assignment was unambiguous since individual rotational lines were resolved, except at band heads. We are thus led to assign the emission features as due to the formation of electronically excited MOH. We also prepared electronically excited CaOH from another source, namely the reaction of electronically excited Ca atoms with rert-butyl hydroperoxide: Ca* + 1-BuOOH CaOH* + r-BuO (5)
-
Formation of &OH* is energetically allowed in this reaction. The chemiluminescence spectrum from reaction 5 is shown in Figure lb. This CaOH* spectrum is virtually identical with that observed for the Ca*(’P) HOOH reaction.27 While the resolution of our chemiluminescence spectra is not high, the wavelengths of the CaOH bands in Figure l b match well with the corresponding bands in Figure la. The differing relative intensities of the A-X Au = 0 and +1 sequences probably reflect the differing degree of vibrational excitation in the CaOH*. Our CaOH* emission spectrum in Figure l a has a very similar profile to that observed by Wormsbecher et al.2*in a flow system. Chemiluminescence from CaOH* and other species was also seen by Brazier and Bemath7 in their spectroscopic study of the alkaline earth metal azides, in which samples of HN3were allowed to react with a flow of metal atoms. The presence of MOH* in this system must be connected with the presence of oxygen-containing impurities, such as C02, 02, and H20. However, such an excited species cannot be formed in a bimolecular elementary reaction of these molecules with ground-state metal atoms but must arise from some secondary process; these emission features are not observed in the absence of HN3. The role of such oxygen-containing molecules was confirmed in separate experiments in which the molecules HzO, COz, 02,NO, and N 2 0 were deliberately admitted to the Ca + HN3 system. The CaOH* emission intensity was found to be enhanced, to various degrees, by the addition of 02, NO, and N20,
+
(22) Rice, S.F.; Martin, H.; Field, R. W. J . Chem. Phys. 1985,82, 5023. (23) Fernando, W. T. M. L.; Ram, R. S.;OBrien, L. C.; Bernath, P. F. J . Phys. Chem. 1991, 95,2665. (24) Pearse,R. W. B.; Gaydon. A. G. The Identification of Molecular Spectra, 4th ed.; Chapman and Hall: London, 1976. (25) Nakagawa, J.; Wormsbecher, R. F.; Harris, D. 0.J . Mol. Spectrosc. 1983,97, 37. Hilborn, R. C.; Qingshi, 2.; Hams, D. 0.J . Mol. Spectrosc. 1983, 97, 73. (26) Bemath, P. F.; Kinney-Nielsen, S.Chem. Phys. Lett. 1984, 105,663. Bernath, P. F.; Brazier, C. R. Astrophys. J . 1985,288, 373. Hailey, R. A,;
Jarman, C. N.; Fernando, W. T. M. L.; Bernath, P. F. J . Mol. Spectrosc. 1991. - - ,147. -
40.
(27) Oberlander, M. D.; Kampf, R. P.; Parson, J. M. Chem. Phys. Lett. 1991, 176, 385. (28) Wormsbecher, R. F.; Trkula, M.; Martner, C.; Pcnn, R. E.; Harris, D.0. J . Mol. Spectrosc. 1983, 97, 29.
Chen and Dagdigian
2
1
zE
l
J
r’ J.
I
/
I
/
0l
I 0
1
2
3
4
5
oxidant pressure (1f l T o r r )
Figure 4. Dependence of the intensity of the CaOH B
6
7
-
X emission on the pressure of added oxygen-containing molecules at a constant hydrazoic acid pressure of 1.2 X Torr for the Ca HN, system. The curves have been drawn through the experimental data to aid the eye.
+
while no such effect was found with H 2 0 or C02. For the former molecules, the CaOH* chemiluminescence spectra had mentially identical profiles. The 555-nm emission intensity vs added oxidant pressure is presented in Figure 4. These data suggest that electronically excited CaOH(A,B) can be formed by the reactions CaNH + O2 CaOH(A,B) + NO (6) CaNH
--
+ NO
CaOH(A,B)
+ N2
(7)
Discussion of the complicated dynamics of these reactions is deferred to the next section. The nonlinear pressure dependence for N 2 0 oxidant implies that the formation of CaOH in this case is more complicated. Further evidence that the metal hydroxide is formed by reactions 4 and 6 comes from studies with DN3. Unfortunately, the isotope shiftZ5J6of the CaOH/CaOD bands is too small to be observable in the chemiluminescence spectra. We did observe ground-state CaOD by laser fluorescence detection when DN3 was employed. However, the degree of deuteration in this product was significantly less than in the DN3 sample employed; the observed CaOD to CaOH ratio was found to be 1:3 from the laser fluorescence spectra. This presumably reflects the fact that ground state hydroxide can be formed by mechanisms other than reactions 4 and 6. From the observed increase in the CaOH* signal when a known pressure of O2was added to the reaction chamber, it was possible to estimate the amount of O2 impurity in the hydrazoic acid samples. With HN3made by the stearic acid method, the &OH* signal without added O2is equivalent to the signal increase caused by addition of 0.15 X Torr of oxygen at a HN3 pressure of Torr. Thus, the content of O2in these HN, samples 1.2 X is estimated as 12%, assuming that O2 is the appropriate oxygen-containing impurity. The pressure was measured by an ion gauge, and no correction was made to take account of differing ion gauge calibration factors. The HN3generated by H3P04save onethird the CaOH* signal as the previous sample under identical conditions; this implies that the percentage of oxygen in this sample is ca. 4%. The derived fractional oxygen content of these samples seems quite high; however, the differing estimated values for HN3 synthesized by different procedures show that the oxygen does not come solely from the background gas in the chamber. In principle, another mechanism can be proposed to explain the formation of electronically excited MOH* in these systems, namely that first €IN3reacts with O2to form nitrous acid (HONO), which then reacts with a metal atom to form MOH*. Both of these reactions are strongly exothermic. However, this mechanism is contradicted by the fact that HN3 does not react with 02,or even 03,at an appreciable rate at room temperature. For H N 3 / 0 2mixtures deposited in solid nitrogen, nitrous acid is observed to be produced only upon irradiation of lightsz9 The (29) Baldcschwieler, J. D.; Pimentel, G. C. J . Chem. Phys. 1960.33, 1008.
Study of the Ca and Sr
+ HN3 Reactions
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1287
248-nm photolysis of gas-phase HN,/O3 mixtures has also been studied.m In this caw, chemiluminescence initiated by the photolytic production of O(lD) from O3 is ohserved. In neither experiment, was significant reaction in room-temperature gas mixtures reported. Our observation of m / e = 32 (0,)in mass spectra of our HN3 samples, but not m/e = 30 (NO from the decomposition of HONO), is consistent with the lack of reaction of HN3 with 0,impurity in our samples.
Discmarion In contrast to the formation of MOH*, the production of electronically excited metal atoms and metal hydrides in this reaction system can be straightforwardly explained through the presence of secondary reactions involving hydrazoic acid. Both of these excited species can be produced through very exothermic reactions of MN3 or MNH with HN3: MN3 HN3 M('P) 3N2 + H (8)
+
+ 3N2 M(3P,'P) + 2N2 + H2 MH(A,B) + 2N2 + H
+
MNH
+
+
+HN3
-
+
MH(A,B)
(9)
(10) (11)
The metal azide could also be converted from the imide in a secondary reaction: MNH + HN3 -C MN3 + NH2 (12) Amidogen (NH,) was actually observed throu@ laser fluorescence excitation of (O,lO,O)-(O,O,O) band of the A-ft transition near 570 nm in both the Ca and Sr reactions, although no attempt was made to quantify the NH2 concentration relative to the other species detected by laser fluorescence excitation. It is intemting to note that the set of reactions 3 and 8, as well as reactions 4 and 10, constitute the metal-atom-catalyzed decomposition of hydrazoic acid: 2HN3 3N2 + 2H (3 + 8)
TABLE k Estimnted Relative Rate Constants for Formation of secoadvy Emittiar Species in tbe Ca HNI System
+
reagent HN,
0 2
NO a
excited product
relative rate constant
Ca('P) Ca(V) CaH(A) CaH(B) CaOH(A,B) CaOH(A,B)
(1)' 63 2.4 0.24
45 8
uncertainty,
96 100 20 20 25 25
Rate constants normalized to this value.
the residence time in the detection zone so that the latter must be estimated in order to derive the relative rate of formation of this state. In addition, only one spin-orbit component ( J = 1) of Ca('P) radiates to the ground state. With a reasonable assumption concerning the average laboratory velocity of Ca(3P), its residence time can be estimated. In Table I, we present our derived relative rate constants for the various assumed secondary reactions of CaNH leading to electronically excited species. The most uncertain value is that for production of Ca(3P);nevertheless, the production of Ca(3P) and CaOH* appear to be the most efficient processes. In the above analysis, we have assumed that reactions 8 and 9 can be ignored, we consider below the validity of this assumption. To estimate whether CaN3 or CaNH is the main product of the Ca HN3reaction, we compare the ground-state populations of CaOH and CaN3, as measured by laser fluorescence detection. In this analysis, we ignore the secondary pathway (reaction 12) for the formation of CaN,. In a steady-state approximation, we can determine the CaNH concentration from (d/dt)[CaNH] = k4[Ca][HN3]- kd[CaNH] = 0 (13)
+
Because H2 is formed in the latter scheme, the metal atom in reaction 10 can be more highly excited than in reaction 8. In beam studies of the present type, secondary reactions are usually strongly suppressed compared with the primary steps, in contrast to studia in flow systems, where chemiluminescence due to multistep mechanisms is often prominently observed.28 It is thus surprising that MOH* is seen here even though its formation apparently requires secondary reactions with impurities. However, this MOH* chemiluminescence signal is small compared with that found for many bimolecular chemiluminmt alkaline earth metal reactions. We have compared our CaOH* signal with CaO* formed in the Ca('S) N20, whose chemiluminescence cross section we measured previou~ly.'~From the ratio of integrated CaOH* and CaO* signals (taking account of the detection wavelength sensitivity) normalized to the same HN3 (stearic acid method, no additional 0,added) and N 2 0 pressures, we calculated that the chemiluminescence cross section for the former reagent would equal 0.02 A2, if we assumed that CaOH* arose from a bimolecular reaction. We consider below a kinetic scheme for the Ca system in order to show that quite reasonable reaction cross sections can be invoked for the various steps to explain quantitatively the magnitude of the observed emission signals. If the observed emissions are well represented by the secondary reactions 6-1 1, it is straightforward to compare the formation rates of MH*, MOH* and M* from reactions 6,7, 10, and 11, which involve the MNH intermediate. The emission intensities corresponding to production of CaH(A,B) and Ca('P) may be directly compared to estimate relative chemiluminescence cross sections since these excited states have short radiative lifetimes. However, the radiative lifetime3' of Ca(3P,) is long compared to
where the last term represents the loss due to flyout from the detection zone and can be estimated as u/d, where u is the CaNH velocity and d is the dimension of the reaction zone. (Loss of CaNH through secondary reactions is negligible by comparison.) In similar manner, the steady-state concentrations of CaOH and CaN3 can be derived. Their ratio is found to equal [CaOH] k4k6'[021 -=(14) ICaN3I k3kd where k6, is the reaction rate constant for formation of CaOH in both ground and excited states. From eq 14, it is possible to estimate k3/ko. We find by laser fluorescence detection in their respective A-X Au = 0 sequences that [CaOH]/[CaN3] = 10 at a HN3 pressure of lo4 Torr,taking into account that only 1/3 of the calcium hydroxide appears to be formed by reactions 4 and 6, as indicated by the experiments with DN3. Assuming a CaNH velocity of 1V cm/s and a rate constant kq of 10-l0cm3molecule-' s-' and taking the O2density as 12%of HN,, we estimate k3/k4 to equal 3 X 10-4. While these assumed parameters could be in error, it nevertheless is clear that CaNH is the main product of the Ca HN3 reaction. As a corollary, reactions 8 and 9 also probably do not play a significant role in this system. Similar conclusions apply to Sr + HN3. Although the absolute photon yield of CaOH* has been measured by comparison with the known Ca + N 2 0 chemiluminescence cross s e c t i ~ n ,the ' ~ rate constants k4 and k6 cannot be determined independently. Nevertheless, the product of these rate constants can be estimated to be of the order of ( cm3 Thus, the observation of CaOH* in a multistep mechanism at low pressures (ca. IO4 Torr) can be explained semiquantitativelyif both reactions proceeded with near-gas-kinetic rate constants. It is surprising that reaction 6, involving complicated nuclear dynamics, should have such a large cross section. A similarly complicated dynamics also applies to the formation of electronically excited CaO in the Ca(lS) + HOOH reaction.,'
(30) Ongstad, A. P.;Coombe, R. D.; Neumann, D. K.; Stech, D. J. J . Chem. Phys. 1989, 93, 549.
1265.
2HN3
--
3N2 + H2
(4
+ 10)
+
+
(31) Husain, D.; Schifino, J. J . Chem. Soc., Faraday Trans. 2 1983, 79,
J . Phys. Chem. 1992, 96, 1288-1293
1288
Finally, we speculate on the strength of the Ca-NH bond. From the apparent occurrence of reaction 4, this bond energy must be greater than 45 kJ/mol. Reaction 4 is not sufficiently exothermic to allow production of electronically excited CaNH since such chemiluminescence,if it occurred, would be expected to be stronger than that from the secondary reactions yielding CaOH*. Unfortunately, our lack of knowledge of CaNH electronic states does not allow us to derive an upper bound to D(Ca-NH), although its energy level structure might be reasonably expected to be similar to that of the isoelectronicCaO molecule.8-10 Other bounds can be derived from the observed secondary products. From the fact that reaction of CaNH with N 2 0 does not appear to yield CaOH* in a single bimolecular step, we conclude D(Ca-NH) 2 154 kJ/mol. The formation of CaH(B) in reaction 9 implies that D(Ca-NH) I 203 kJ/mol. Both of these bounds assume that the CaNH formed in reaction 4 is not carrying
significant amounts of internal energy. It is hoped that the present report on the Ca,Sr + HN3kystems and our indirect evidence concerning the metal imides wlll stimulate further work on this class of metal compounds. Perhaps it will be possible to observe fluorescence excitation of the imides directly in a jetaoled sample, in which the spectral features due to the possible interfering species will be considerably narrowed. Acknowledgment. We appreciate a conversation with P. F. Bernath and C. R. Brazier, who suggested that the additional molecular emitter could be the metal hydroxide, and discussions with S.Rosenwaks. This work has been supported by the Air Force Office of Scientific Research under contract no. F4962088-C-0056 and by the National Science Foundation under grant no. CHE-9020727. We also acknowledge travel support from the U.S-Israel Binational Science Foundation.
Laser-Initiated Chain Reactions and Microexplosions In Solid Solutions of Simple Alkenes and Chlorine Thomas J. Tape, Jr., Peter M. IUigmann, C. Patrick Collier, Mikbai A. Ovchinnikov, and Charles A. Wight**+ Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 (Received: August 13, 1991; In Final Form: October 8, 1991)
Solid-state photochlorination reactions of ethene and propene have been initiated by pulsed ultraviolet laser photolysis. Quantum yields and product branching ratios have been determined by Fourier transform infrared s p e c t ” p y . At 48 K, the photochemical quantum yield for the ethene/C12system is 116 f 15. The reaction forms predominantly the anti conformer of 1,2dichlomthane. At 50 K, the propene/C12 system yields the largest quantum yields reported to date in the solid state, 740 f 120; the anti conformer of 1,2dichloroprope is the predominant product in this case. At 10 K, the reactions of both systems are characterhd by a sudden burst of reactivity (a microexplosion) after exposure to a cumulative laser fluence of only 0.9 mJ/cm2 at 337 nm. As much as 67% of the reactants are umverted to products during the microexplosion. A new technique has been developed which utilizes microexplosions for determining relative IR band intensities for conformational isomers in the solid state.
Introduction The focus of several previous reports from this laboratory has been to characterize the chemical reactivity of disordered solids. The two primary classes of reactions we have studied are cationic photopolymerization reactions and free-radical chain chlorination of saturated hydrocarbons. The photopolymerization reactions allow us to ~sse8sthe effects of orientational disorder on the ability of chain reactions to propagate in solid materials.’-’ Modeling of the photochlorination reactions has likewise afforded a better understanding of the effects of site disorder on chain reactions in binary solid solutions at low temperature.e8 In this study, we have extended our investigation of solid-state reactions to include a third class of chain reactions, addition of molecular chlorine to carbon-carbon double bonds in simple alkenes. In liquid solution, the addition of the chlorine across a double bond occurs exclusively by trans addition.”’ The stereochemistry of these reactions is usually determined by chlorination of substituted alkenes wherein the formation of diastereomeric or meso products is used to infer the reaction mechanism. In the solid state, detection of the nascent conformational isomer of the dichloroalkaneproduct can be used to deduce the stereochemistry of halogen addition since neighboring molecules prevent rotation about carbon-carbon single bonds. This method permits mechanistic studies of unsubstituted alkenes and is therefore a powerful and general technique. Photolysis of crystalline solids of ethene/C12mixtures has been reported to yield predominantly the anti conformer of 1,2-di+Alfred P. Sloan Research Fellow, 1990-92.
0022-3654/92/2096-1288$03.00/0
chloroethane via trans addition.12 However, the thermally activated reaction in crystalline solids of ethene/C12 is reported to produce the gauche adduct via syn addition.13 The present work was initiated to determine whether photochemical addition of C1, to simple alkenes in a disordered solid proceeds via syn addition, trans addition, or a mixture of the two reaction pathways. Benderskii and coworkers have shown that, at low temperature, the addition of C12to ethene in glassy solids can occur in sudden bursts during the photolysis p e r i ~ d . ~Several ~ ~ ’ mechanisms have (1) Mansueto, E. S.;Ju, C. Y.; Wight, C. A. J. Phys. Chem. 1989, 93, 2143. (2) Mansueto, E. S.;Wight, C. A. J . Am. Chem. SOC.1989, 111, 1900. (3) Mansueto, E. S.;Wight, C. A. J . Photochem. Phorobiol., A, in press. (4) Sedlacek, A. J.; Mansueto, E. S.;Wight, C. A. J. Am. Chem. SOC. 1987, 109,6223. ( 5 ) Wight, C. A.; Kligtnann, P. M.; Botcher, T. R.; Sedlacek, A. J. J . Phys. Chem. 1990, 94, 2487. (6) Sedlacek, A. J.; Wight, C. A. J. Chem. Phys. 1988,88, 7418. (7) Sedlacek, A. J.; Wight, C. A. h e r Chem. 1988, 8, 155. (8) Sedlacek, A. J.; Mansueto, E. S.;Wight, C. A. J. Phys. Chem. 1988, 92, 2821. (9) Poutsma, M. L. J . Am. Chem. SOC.1%5,87,2161. (10) Poutsma, M. L. J . Am. Chem. SOC.1965,87,2172. (1 1) Koch, J. K.Free Radicals; Wiley: New York, 1973; Vol. 11, p 185. (12) Titov, V. A.; Filippov, P. G.; Misochko, E. Ya.; Usmanov, R. D.; Bendersku, V. A. Dokl. Akud. Nuuk. SSSR 1986,290, 1414. (13) Kimelfeld, I. M.; Lumer, E. V.; Shwcdchikov,A. P.Chem. Phys. Lett. 1973, 21, 429. (14) Benderskii, V. A.; Ovchinnikov, A. A.; Philippov. P. G. Reacf.Solids 1988,4, 287. (1 5) Benderskii, V. A.; Misochko, E. Ya.; Philippov, P. G. Khim. Fiz. 1985, 4 , 409.
0 1992 American Chemical Society
