COMMUNICATIONS TO THE EDITOR
3642 Chemical Lasers Produced from O(lD) Atom Reactions. 11. A New Hydrogen Fluoride Elimination Laser from the O(lD)
+ CH,FI-,
Table I : Observed HF Vibration-Rotation Transitions and Their Appearance Times in t h e CHF,, CHtFz and CHEFSystems0 Transitions
u,
cm-1
CHFs
CHzFz
3.09 3.3 5.3
2.8 3.2 3.2
CHsF
( n = 1, 2, and 3) Reactions' Publication costs assisted by the Naval Research Laboratory
3622.58b 3577.52 3531.20
S i r : Chemical HF elimination lasers produced from 3693.64 5.2 4.4 chemically activated CF3CHaZaand CH3NF22bhave 3644.24 5.4 6.4 4.8 been discovered recently by Pimentel and coworkers. 3593.89 6.7 6.4 4.8 The vibrationally excited molecules are believed to be 3542.21 7.4 6.8 5.2 3489.67 8.0 6.6 principally formed by radical-radical recombination reaction^.^ In this communication, we wish to report a a Flash energy = 1.5 kJ, Ptotal= 30 Torr, and 03: CH,Fa-,: H e new type of H F elimination laser produced from the = 1:3:10, 1:1.5:10, a n d 1 : l : l O f o r n = 1 , 2 , and3,respectively. These values are those reported b y D. E. Mann, et al., J . Chem. decomposition of chemically activated a-fluorinated Phys., 34, 420 (1961). I n the present study, the laser transitions alcohols, mainly methanols. The vibrationally excited were identified with a 50-cm Model 305 SMP03 monochromator, methanol molecule was generated by insertion of an using 250-p slits, which provided a resolution of about fO.5 cm-1 O(lD) atom into a carbon-hydrogen bond of a fluoroin t h e spectral region of interest. More details of t h e detecting methane molecule in a manner similar to the insertion of system can be found in ref 10. c Appearance time of the individual laser pulse in psec. a CHZ('A1) radical into a C-H b0nd.I The apparatus employed in this work is essentially the same as that used in our recent study of the O(lD) between the observed transitions and the exothermiciCaOz ('E,+) reaction, in which some 40 vibrationrotation transitions in CO laser emission were o b ~ e r v e d . ~ ties of these reactions, as will be discussed later. O3 was found to be essential t o the laser action. No The concentrations of 03 to fluoromethanes were adlaser oscillation was detected when the three fluorojusted such that the relative probabilities of O('D) methane-helium mixtures were flashed in the absence attack on C-H bonds were statistically equal in the 0 3 under the same conditions; nor was laser emission of three cases. Thus, the mixtures of the following comobserved when KO2 was used instead of 03. NO2 was positions were used: O s :CH,FI-,: He = 1:3: 10, also found to be a less efficient O(lD) atom source than 1:1.5: 10, and 1: 1: 10 for n = 1, 2, and 3, respectively. O8 for the O(lD) C302 reaction.' Mass spectroInert gases such as He and Ar were found to be helpful metric analyses of the flashed samples indicate the in enhancing the lasing, as was found in the case of the presence of FZCO, HFCO, and HzCO in the CHF3, O(lD) C302reaction. This effect is believed to be and CH3F systems, respectively. Large attributed to the lowering of rotational t e m p e r a t ~ r e . ~ " * CH2F2, ~ amounts of CO and COz were also detected in the last The total emission traces obtained from flashing 30two systems. The folloming reactions are proposed to Torr mixtures of the three fluoromethanes are shown in account for our present observations Figure 1. I n all cases, a constant flash energy of 1.5 kJ was employed. No significant increase in laser output was observed when a higher energy was (1) This work is partially supported by the Advanced Research Projects Agency under ARPA Order 660. used. A decrease in flash energy to 0.6 kJ, however, (2) (a) M. J. Berry and G. C. Pimentel, J . Chem. Phys., 49, 5190 pronouncedly reduced the laser intensity and delayed (1968); (b) T. D. Padrick and G. C. Pimentel, ibid., 54, 720 (1971). the appearance time to as long as 10 psec. Among (3) M . C. Lin, unpublished work. On the basis of the results recently obtained by Ross and Shaw ( J . Phys. Chem., 75, 1170 (1971)) and by these three systems, CHzFz was found t o have the Bumgardner, et al. (Chem. Commun., 1079 (1968)),we have calculated highest output and CHF, the lowest. The observed the rate constant ratio for CH3 + NZF4 and CHs + NFz to be 1.0 X 10-2, at 300°K, with the aid of the RRKM theory. In the same calvibration-rotation transitions and their appearance culations, we also found the critical energy for the elimination of H F times (in psec) are listed in Table I. The transitions from CHaNFz to be 38.0 kcal/mol, which agrees exactly with the observed in the CHF, and CH2Fzsystems are similar; value arrived at by employing the RRK theory, assuming S = 9 (see Ross and Shaw), Since the maximum amount of energy the CHsNFz both Av = 2 -+ 1 and 1 + 0 are present. However, molecule can attain in the CHa + N2F4 reaction is only 40 kcal/mol, only 1-+ 0 transitions were detected in the CH3F flashes. the m a x i m u m rate of decomposition of CHaNFzt produced from CHs + NtF4 is about 10-2 times as fast as that of CHsNFzt produced from I n all cases, no transitions higher than 2 1 were CHa + NF2. Both factors would make the CH3 + NzFi reaction a observed. The appearance times given in Table I less likely laser-pumping reaction. (4) See,-for example, G. Paraskevopoulos and R. J. Cvetanovi6, J . indicate that in boih CHF, and CH,K Systems, the 2 + Chem. Phys., 5 2 , 5821 (1970). 1 transitions have higher gain than 1 4 0, similar to the (6) M . C . Lin and L. E. Brus, ibid., 54, 5423 (1971), referred t o as F R H reactions.6 In the case of CH3F, however, nartI. .-- - ~ only the 1 + 0 transition has high enough gain to over(6) w. H. Green and hi. C. Lin, ibid., 54, 3222 (1971). (7) M. C . Lin and L. E. Brus, unpublished work. come the threshold. There is a qualitative correlation
+
+
+
--+
+
The Journal of Physical Chemistry, Vol. 76, No. $3,1071
COMMUNICATIONS TO THE EDITOR
3643
O a L O ( ' D ) 4-Oz('A)
O(lD)
+ CHFa -+ CF30Ht
(AH1'
O(lD)
+ CHzFz
--f
-+ HFt
=
+ CH3F
- 155 kcal/mol) (1)
CHFzOHt -+ HFt
-+ CHzFOHt -+
(AH3'
+ HFCO
- 148 kcal/mol) (2)
(AHz"
O('D)
+ FzCO
=
HFt
+ HzCO
-143 kcal/mol)
(3)
The heats of formation of various species are taken from either Benson's books or from the JANAF table^.^ The heats of formation of these methanols, however, are not known. The vibrationally excited methanols formed in the above reactions possess about 130 kcal/mol of excess internal energies. This can be envisaged from the analogous reaction
O('D)
+ CHI
-+
CH30Ht (AFL'
= - 135 kcal/mol)
(4) Thus, the newly born excited fluoromethanol molecule has at least 100 kcal/mol of excess energy above the critical barrier for the four-centered elimination reaction, which is probably much less than 30 kcal/mol because of the instability of the a-fluorinated alcohols. The lifetime of the excited methanol is expected to be of the order of lo-" sec. I n view of this high excess energy and short lifetime, the deactivation of excited molecules cannot occur within the period of laser oscillation under our present conditions. The early appearance of laser pulses, in comparison with that of the F-atom abstraction laser produced in the flash photolysis of N2F4-RH mixtures,'O also indicates that the O(lD) insertion-elimination reaction is very fast; it is probably much faster than the insertion of CHz('A1) into a C H bond, which is known to have a rate constant of about 1 X 1Ol2 cm3/mol sec.ll An attempt to make an H F elimination laser from the CH2(lA1) CH.Fc. reactions failed. Both CH2N2 and CH&O were used a8 the CHz radical source in these experiments.IP COz laser emission was not detected when either CHaNz or CHzCO was flash-photolyzed in the presence of COZ and He. A similar attempt to use C20('A) instead of CHz('A1) by flashing the mixture of CaOz and CHFs with He as a diluent also failed to produce laser oscillation. l2 Our present results given in Table I also allow us to estimate the initial relative population, No/NO-', for the transition that has the highest gain. Assuming H F deactivation is negligible and that the rotationaltranslational temperature is about 300"K, the observation that PZl(4) reaches threshold first (see Table I), in both CHFI and CHZFZsystems, implies that N z / N 1 0.8 0.2, according to our gain calculations. Similarly, for the H F molecule produced in reaction 3, a value of N I / N o 0.35 0.1 can be deduced.
+
*
F;:
-
*
Figure 1. The total emission traces. Ordinate: emission intensity (0.5 V/division); abscissa: time (2 psec/division): a, CHFs mixture (Oa:CHFa:He= 1:3:10); b, CHIFl mixture (OI:CHIF1:He= 1:1.5:10); and o, CHsFmixture (0a:CHaF:He = 1:l:lO). Inallcases,flashenergy = 1.5kJ and Phw = 30 Torr.
Recently, Clough, Polanyi, and TagnchiI3 measured the relative populations of HF produced in the following reactions CHa
+ CFI
+
---t CHaCFat -+ HFt
(AHs'
Hg*(aP)
+ CHzCFz -69 kcal/mol)
+ CHZCFZ Hg CHZCFZ~ -+ Hg + HFt + CHCF (AHGO -64 kcal/mol) -+
F;:
(5) (6)
N z / N 1 ratios were determined to be 0.39 and 0.36 for reactions 5 and 6, respectively. These values are about a factor of 2 lower than our estimate of -0.8 0.2 for (8) S. W. Benson. "Thermochemical Kineti-," Wiley, New York, N. Y..1968. (9) D.R. Stull. Ed., "JANAF Themoohemioal Tables." The Dow
Chemical Co.. Midland, Mioh., 1960. (IO) L. E. Brus and M. C. Lin, J . Phys. Chem., 75, 2546 (1971). (11) W. Brsun, A. M. Bass, and M. Pilling, J . Chem. Phys., 52,5131 (1970). (12) These experiments were carried out with the oooperation of Dr. L. E. Brus. (13) P. N. Clouph, J. C. Polanyi, and R. T. Tepuchi. Can. J . Chm., 48, 2919 (1970).
The Joumd of Phyakal Chemistry, Vol. 76, No. 8.9.1971
COMMUNICATIONS TO THE EDITOR
3644 reactions 1 and 2. It is to be noted that the exothermicities of reactions 5 and 6 are also about half as much as those of reactions 1 and 2. In contrast to reaction 5, in which the 1 --+ 0 transition has the highest gain,2a,13 we observed a higher gain for 2 1 than for 1 -+ 0 in both reactions 1 and 2. Our estimated value OI N1/No = 0.35 f 0.1 for reaction 3 compares closely with the ratio Nl/No = 0.35 f 0.02 determined by Padrick and PimentelZbfor H F produced from the CH3NFzt elimination reaction. If our conclusion3 is correct, then the exothermicity of the reaction, --+
CH3
+ NFz + CH3NFzt + 2HFt + HCN (AH," =
- 141 kcal/mol)
(7)
is comparable to AH,". The main difference between the two reactions is that reaction 7 produces two HF m~lecules,probably in a consecutive manner. Another interesting aspect of our present results is the observation of both CO and COzin the flashed mixtures of O3 CHzFzand 0 3 CH,F. They were, however, absent in the flashed O3 CHF3samples. CO may be formed by the decomposition of vibrationally excited formaldehydes, which possess large fractions of exothermicity in reactions 2 and 3 because of their greater numbers of degrees of freedom.2be13,14
+ +
+
+ CO +H + FCO
HFCO 4 HF
H2C0-+ H2 +H
+ CO
+ HCO
(AHss" = 0 kcal /mol
(sa)
(AHsb" = 87 kcal/mol
(8b)
(AH9,'
1.3 kcal/mol)
(9a)
( A h , " = 87 kcal/mol)
(9b)
=
Both FCO and HCO may undergo further decomposition into CO. Other possibilities such as a radicalinduced decomposition cannot be ruled out, however. COzis probably formed by the secondary reactions such
The Journal of Physical Chemistry, Vol. 76, No. $3, 1971
+ +
+
+
+
as 0 CO M COz M and OZt(lA) CO --LCO2 O. No CO or COZlaser emission was detected in these systems. The fact, that no significant amount of CO or COz appeared in the flashed 0 3 CHF3sample is consistent with the extraordinarily high stability of F2CO; the first C-F bond dissociation energy of FzCO is known to be as high as -137 kcal/mol.15 The elimination reaction (8a) has been shown to take place rather readily at low temperatures;16 its occurrence may account Cor the comparatively stronger laser output observed in the CHZFZsystem (Figure 1). It would be worthwhile to flash-photolyze the HFCO molecule in the vacuum-uv region to further substantiate this conclusion. We are currently extending the investigation of O( 'D) atom reactions in the following ways: (1) further study of hydrogen halide elimination reactions of many other vibrationally excited LY and @ halogenated alcohols, including the simultaneous, competitive elimination of different hydrogen halide molecules from an excited molecule; (2) detailed study of both pressure and temperature effects by employing the "equal-gain technique'lz to determine the exact population ratios; and (3) construction of a low-pressure, fast-flow setup to study the ir chemiluminescence of various ir-active products formed in the O(lD) atom reactions. --+
+
(14) H. W. Chang, D. W. Setser, and M. J. Perona, J. Phys. Chem., 75, 2070 (1971). (15) The value D(FC0-F) % 137 koal/mol was arrived a t by taking AHsO(FC0) cz -34 i 5 kcal/mol reported by H. Henrici, M. C. Lin, and S. H. Bauer, J. Chem. Phys., 52, 5834 (1970). (16) G. Fisher and A . S. Buchanan, Trans. Faraday Soc., 60, 378 (1964).
PHYSICAL CHEMISTRY BRANCH NAVALRESEARCH LABORATORY WASHINGTON, D. C. 20390 RECEIVED JULY 28, 1971
M. C. LIN
