2085
J. Phys. Chem. 1980, 84, 2085-2087
TABLE I: Comparison of Nitromethane and Reaction-Product Spectra ~
nitromethane assignment3
reaction product obsda
obsda
possible assignments
3200 3080 VVB NH,', acid OH 2900 1705 sh acid carbonyl 1610 s ionized acid carbonyl, v N=N, v C=N, amide carbonyl vaNO,
1565s
s a CH,
1430sh
tis CH,
1406s 1370s
1450 m 1420 sh
L
L
vSNO,
2hl
2WO 1& d WAVENUMBERS A NlTROMtTHANE 8. REACTION PROOUCT
36W
32W
28W
o
Flgure 1. Infrared spectra of neat nitromethaneand of reaction product.
Spectra are obtained (a) at initial loading of the liquid nitromethane, (b) of the polycrystalline solid formed at high pressure, (c) of the reaction product(s) immediately after heating and still under pressure, and (d) of the product after releasing the pressure and removing the gasket. In the latter case, the sample is a thin smear between the diamond anvils (Figure 1B). The reaction has been repeated with ten samples of nitromethane under various conditions. Pressure has ranged from -20 to n.50 kbar; maximum temperature has ranged from 100 to -160 "C; gasket material has included Inconel, molybdenum, and platinum; samples included pure nitromethane, nitromethane doped with 1% ethylenediamine, and nitromethane-d3. From these experiiments, the following preliminary conclusions are drawn: (a) Reaction occurs very slowly at temperatures between 100 and 130 "C (only partially complete after several hours) and relatively rapidly at 150 "C (essentially complete in -10-20 min). (b) Gasket material has no effect on the reaction. (c) Reaction does not occur in the liquid state; apparently, the effect of pressure is to maintain the crystalline phase until the necessary temperature has been reached. (d) C 0 2 is produced during the reaction. (e) Trace amounts of amines appear to increase the rate of the reaction but are not necessary for the reaction, Observed absorptions are listed in Table I for nitromethane and for the reaction product. The shape and intensity of the broad, complex absorpt,ionnear 3000 cm-l in the product spectrum is most characteristic of N-H absorption; however, 13uchmaterials nearly always have moderate to strong absorption near 1550 cm-I arising from the N-H deformation. Similarly, acids which have intense, broad absorption near 3000 cm-l also have strong absorptions near 1300 and 900 cm-l associated with the C-0 stretch and the 0-H deformation. Absence of absorptions in these regions makes difficult any assignment of the three major bands in the product. A computer search of infrared reference spectra has been performed by Dr. Clara Craver of Chemir Laboratories. A very close match of absorption band frequencies was found with the spectrum of ammonium oxalate. Because there are significant differences in band shapes and relative intensities between the two spectra, it is not possible to make a positive identification. The similarities strongly indicate that the reaction product must be chemically similar to ammonium oxalate. 0022-3654/8O/2084-2085$0 1.OO/O
1310 w CH, rock 1100 m vClN 9 2 0 ~ 775 W b
sSNO,
ionized acid (?)
654s
a s = strong; m = medium; w = weak; sh = shoulder; Does not appear in all samVVB = very, very broad. ples.
Further experiments are in progress to better characterize the product and to determine the range of experimental conditions under which it forms. Production of larger amounts of material would be highly advantageous; however, equipment is not available at this laboratory to produce the necessary pressure at the high temperature required. I am confident that the further series of diamond-cell experiments presently under way will permit a reliable characterization of the product(s). Observation of this reaction is a most significant development in our explosives program, since an understanding of the mechanism of this reaction will give insight into the behavior of nitro compounds under shock conditions. References and Notes (1) C. Weir, E. Lippincott, A. Van Valkenburg, and E. Bunting, J. Res. Natl. Bur. Stand., Sect. A , 63, 55 (1959). (2) J. Barnett, S. Block, and G. Piermarini, Rev. Sci. Instrum., 44, 1 (1973). (3) A. Wells and E. Wilson, Jr., J. Chem. Phys., 9, 314 (1941). Naval Surface Weapons Center White Oak Laboratory Silver Sprlng, Maryland 209 10
J. W. Brasch
Received: Januety 7, 1980
Chernilumlnescent Reactions of OpF. 3. Reactlon with Magnesium Atoms
Sir: The reaction of magnesium with F2has been investigated by using both chemiluminescence1 and molecular-beam2techniques. This process liberates 70 kcal/ mol, which is insufficient for population of the lowest lying excited electronic state of MgF, the A(211) state lying at 79.5 kcal/mol. Hence, the reaction was observed to produce no A X emission under the single-collision conditions of the crossed-beam experiment.2 A laser-induced fluorescence (LIF) analysis indicated that the energy lib-
-
-
@ 1980 American Chemlcal Society
2086
The Journal of Physical Chemistry, Vol. 84, No. 10, 1980
Communications to the Editor
TO TRAP A N 0 PUMP
PRESSURE TAP
O;A~F,
SOLID
1
I
250
M~POWDER,
I n WX-
THERMOCOUPLE’
I1
300
I 350
I
I
I
400 WAVELENGTH lnmi
450
500
550
+
Figure 2. Emission spectrum produced by the Mg 02F reaction for a relatively high Mg flow rate (-5 X mmol/s).
CJ liY
A r OR N2OICO A,
r
+
Figure 1. Apparatus used for observation of the Mg 0,F reaction. Mg atoms were generated by evaporation of the metal owder, and 02F was produced by thermal decomposition of solid O, AsFB The
P -.
P ~ ) in the reaction zone proportion of electronicaliy excited ~ g * ( 3 ~atoms could be enhanced by addition of a CO/N20 mixture or by excitation of Mg by Ar” (3P,,,) metastables created by passage of argon through a microwave discharge.
erated is efficiently channeled into vibrational excitation of the MgF producta3 Studies of this and similar alkaline earth-molecular halogen reactions have shown, however, that the cross sections for production of electronically excited metal halides are considerably enhanced (by an order of magnitude or more) by excitation of the metal atom reagent to the 3P or lD metastable state^.^^^ In this communication we present observations of the reaction of magnesium atoms with gas-phase 0 2 Fradicals. In contrast to the F2case, the Mg + 02Freaction should release approximately 92 kcal/mol, more than sufficient for direct production of the A(211) state. The objective of the present experiments was to measure the spectra and yields of the emissions produced for various conditions and, from these results, to deduce the mechanisms responsible for production of the excited species. The experiments required an apparatus incorporating a calibrated source of magnesium atoms compatible with the technology developed in our laboratory for the generation of 02F.6 The cell constructed for this purpose is shown in Figure 1. As in earlier experiments, gas-phase 02Fradicals were produced by the thermal decomposition of solid 02’ASFB. The solid rested on two course-fritted disks as shown in the figure. Argon (Matheson, 99.999%) was passed through the frits to carry the decomposition products of the salt into the reaction zone. Magnesium was generated by evaporation of the pure metal (AlfaVentron, 50 mesh powder, 99.8%) in the presence of an argon carrier. Magnesium flow rates were calibrated by allowing the atoms to condense on a removable foil cylinder lining the inner wall of the cell. The flow rate for a given oven temperature was calculated from the weight of the metal accumulated on the foil for a measured run time.
The remainder of the apparatus was quite similar to that used in previous ~ t u d i e s .Emission ~ spectra were recorded with a 0.3m McPherson monochromator, a cooled GaAs photomultiplier tube (RCA C31034), and a PAR photoncounting apparatus. Photon yields were measured by calibrating this system with techniques described previ0us1y.~The pressure in the system was measured with an MKS Baratron capacitance manometer, and the flow rates of diluent and carrier gases were monitored with Tylan mass flowmeters. The interaction of Mg atoms with 02Fgenerated in the apparatus produced a bright blue-white flame. The intensity of the flame increased as the flow rate of either Mg or 0 2 F increased and was quenched when the O2+AsF6solid was consumed or allowed to cool. When the Mg powder was removed from the oven, the intensity of the flame was greatly reduced, although not completely eliminated. The residual emission is apparently the result of an interaction between the hot diluent passed through the oven and either of the 02+AsF6-decomposition products (02F or AsF5). The full emission spectrum of the flame was recorded with the apparatus described above, and the results are presented in Figure 2. The spectrum shown represents a typical result for a relatively high Mg flow rate (-5 X mmol/s). The blue-white appearance of the flame is caused by a “continuous” emission spanning the region from 300 nm to >700 nm. No structure was evident in this feature to within the resolution of our apparatus (- 1nm, limited by the signal-to-noise ratio). The shoulder shown on the rise of the emission near 310 nm was reproducible and was found to be a spurious feature attributable to the residual emission noted above. The intensity of the broad-band emission rose with increasing flow of both Mg and 0,F; it was much more sensitive to the Mg flow rate, however. As the Mg flow rate was increased for a fixed (moderate) 02Fflow, three intense bands appeared in the vicinity of 360 nm. These were easily identified as the Av = 1,0, and -1 sequences of the MgF A(211) X(2Z+)transition. A much less intense fourth band near 375 nm was observed for high Mg flow rates, possibly attributable to the Av = -2 sequence. For a fixed 0 2 F flow rate, the intensity of the A X bands increased sharply for increasing Mg flow. For a fixed Mg flow rate, the A X intensity diminished as the flow rate of 0 2 F increased. These results can be explained by a mechanism consisting of the following reactions:
-
-
-
Mg(g) + O2F
+
MgF’ + 0
2
(1)
The Journal of Physical Chemistry, Vol. 84, No. 16, 1980 2087
Communications to the E:ditor
-
MgF' -I- O2F MgF* + M
MgF2* + 0
(2)
2
MgF*(A2n) + M
(3)
The source of the broad-band emission is suggested to be electronically excited MgF, (reaction 2). Emission from this species has been reported previously only once, from the molecular beam study by Engelke.2 The excited molecules in that case were thought t o be produced by a novel Mg2-F2 four-center reaction. The spectrum reported was a broad-band emission (with some features resolved) in general agreement with the present results, if the onset shown in Figure 2 is corrected for the spurious feature noted above. In the mechanism we propose, both MgF2* and MgF*(A211)are produced by reactions of a long-lived precursor (MgF*)generated by the initial Mg + 02F interaction. In view of the LIF results noted above, it seems likely that this precursor is vibrationally excited ground state MgF. In the present experiments, the geometry of the apparatus was such that the cone of sight of the monochromator encompassed the majority of the flame volume. Hence the intensity changes caused by variation of the reagent flow rates represent changes in concentrations integrated over the entire flame volume. Under these conditions the formation rate of the MgF* intermediate is determined solely by the flow rate of the limiting reagent, magnesium. The intensity of the MgF A X emission is proportional to the rate of reaction 3 and, hence, increases with increasing magnesium flow. The A X intensity decreases with increasing 02Fflow since the rate of formation of MgF*is constant, but the proportion of the intermediates removed by process 2 increases. The intensity of the broad-band (MgF2) emission is determined by this proportion as follows:
-
I(MgF2)
-
IkdO2.Fl/(k2[OzFI + k3[MI))[MgFt1 (4)
Hence this emission intensifies with increases in the flow rates of either Mg or 02F. We note that k3[M] must be nonzero for I(MgF2*)to vary at all with changes in the 02F flow rate. The fact that this variation was observed to be small suggests that k3[M] < k2[02F]. The source of the MgF*(A2n) molecules in the system is clouded by the presence of transitions from excited Mg atoms in the spectrum. Atomic emission lines were observed at 518,457,383, and 285 nm, corresponding to Mg d3S1 3 3 P ~33P1 , 3%0, 3 3 D ~ 3 3 P ~and , 3lP1 3lSo transitions, respectively. These features were observed only when the Mg flow rate was high and the 02Fflow rate low. It seems probable that the excited atoms are produced by energy transfer from vibrationally excited MgF formed in reaction 1, analogous to the mechanism producing excited alkali metal atoms in alkali-molecular halogen flames? Multiple encounters would be required for generation of the 3S1, 3 D ~and , 'P1 states. Clearly, the appearance of these lines suggests the possibility that MgF*(A211)is producedl by a reaction between metastable Mg*(3P~)and O:!F. A inumber of experiments were performed to test this hypothesis. First, Mg*(3PJ)was generated in the mixing zone by using both discharge" and chemical generation8 techniques. In the latter case, Mg*(3P~)is produced hy the reaction of Mg('So) with a mixture of N2O and C0.8 No enhancement of the MgF A X emission was noted in either case, although the Mg* 'P1 'SO emission had been intensified by orders of magnitude. In addition, experiments were performed to test the pressure dependence of the MgF A X emission. If MgF*(A211) is produced by a collision-induced process involving a ptecursor (as suggested by oq 3) rather than
-
- -
-
-
-
-
by reaction with Mg*, the A X intensity should increase with increasing pressure. This was found to be the case; the intensity rose by 165% for a 26% pressure increase caused by added argon. CO was found to be still more efficient at stimulating the production of the A state. The A X intensity rose by nearly an order of magnitude for a 25% pressure rise caused by addition of CO. Addition of either Ar or CO did not enhance the Mg* 3P1 'SO intensity. Hence, the data obtained are, on the whole, consistent with a mechanism in which the initial Mg(lSo) + 02Freaction produces a long-lived MgF* intermediate, likely a vibrationally excited ground state molecule. Electronically excited species may then be produced by three different paths as follows:
-
Mg t OzF
M/
Mg*
i
MgF
t 02
(5)
jopF \:c.co MgF2'
MgF*(A211)
It is clear from the spectrum (Figure 2) that production of Mg* is a minor route. Photon yields (with respect to the Mg flow rate) were measured for the production of MgF2* and MgF*(A211)to determine the relative importance of these processes. The magnitude of the MgF*(A211)yield was small, varying in the range 10-5-104. The dominant path for production of electronically excited species would appear to be reaction of MgFl with 0 2 Fto form MgF2*. Although data for the yield of the broadband emission were quite scattered and were obscured further by the spurious features noted above, the limiting yield in this case appeared to approach for low Mg flow rates. The results of these experiments are similar to those obtained in previous studies of 02Freactions in the sense that, although electronically excited fluorides are produced, they appear to be generated indirectly by multistep mechanisms. In every case studied, it seems probable that the intermediate is a vibrationally excited ground state m ~ l e c u l e .This ~ general result underscores the idea that 02F reactions are bimolecular processes which are dynamically similar to three-body recombination mechanisms. The O2 moiety serves as a "third body" present in every collision, enabling the reactions to proceed at a fast rate. Acknowledgment. This work was performed under Contract F29601-78-C-0039with the Air Force Weapons Laboratory, Air Force Systems Command, United States Air Force, Kirtland Air Force Base, New Mexico. We are grateful to Dr. A. T. Pritt, Jr., for several helpful discussions.
References and Notes (1) D. J. Eckstrom, S. A. Edelsteln, and S. W. Benson, J. Chem. Phys., 60, 2930 (1974). (2) F. Engelke, Chem. Phys., 30, 279 (1979); 44, 213 (1979). (3) F. Engelke, Chem. Phys. Lett., 65, 564 (1979). (4) A. Brinkmann and H. Telie, J . Phys. B, 10, 133 (1977). (5) R. D. Coombe and R. K. Horne, J . Phys. Chem., 82, 2484 (1978); 83, 2435, 2805 (1979). (6) See, for example, M. C. Moulton and D.R. Herschbach, J. Chem. Phys., 44, 3010 (1966). (7) G. Taub and H. P. Brolda, J . Chem. Phys., 65, 2914 (1976). (8) D. J. Benard, W. D. Slafer, and P. H. Lee, Chem. Phys. Lett., 43, 69 (1976). Rockwell International Science Center Thousand Oaks, California 9 1360
Received: March 4, 1980
R. D. Coombe* R. K. Home
