1425
CHEMICAL LASERSPRODUCED FROM O(lD) ATOMREACTIONS
Chemical Lasers Produced from 0(ID) Atom Reactions. IV.
Competitive
Eliminations of HCl and HF from the Vibrationally Excited ClFHCOH, Cl,FCOH, and ClF,COH Molecules by M. C. Lin Chemistry Division, Naval Research Laboratory, Washington, D . C. 60.390 (Received December 17, 1971) Publication costs assisted by the Naval Research Laboratory
Chemical HC1 and HF laser emissions were observed from the competitive, four-centered unimolecular elimination reactions of chemically activated ClFHCOH, Cl,FCOH, and ClFzCOH molecules in an optical cavity. The HF emission was found to be significantlystronger and was observed in all three reactions; the HC1 emission, however, was detected only in the ClzFCOH system. This is attributable to a larger stimulated emission coefficient as well as a preferentially faster rate of elimination of HF than HC1. The vibrationally excited methanol molecule, which possesses at least 130 kcal/mol of internal energy, was generated by insertion of an O(lD) atom into a C-H bond of a chlorofluoromethane molecule. The O(1D) atom was produced from the flash-photolytic decomposition of 0 3 . The initial population ratios of the highest gain transitions have been estimated and compared with the values obtained from other 0 (ID) atom insertion-elimination reactions reported previously.
Introduction Recently, we have reported the observation of H F and HC1 laser emissions produced from the four-centered elimination reactions of chemically activated afluoro-' and a-chloromethanols,2 respectively. The vibrationally excited methanol molecule was generated by insertion of an O(lD) atom into a C-H bond of a halomethane molecule
O(lD)
+ CH,X4-,
+H,-iX4-,COHt
(I)
where X = F, C1 and n = 1, 2, 3. The vibrationally excited methanol thus produced possesses at least 130 kcal/mol of internal energy, judging by the exothermicity of the analogous reaction, O(lD) CH, + CH30H, AH" = -135 kcal/mol. On account of the large amount of excess vibrational energy ( E - eo 1 100 kcal/moll) and its small molecular size, the lifetime of the excited methanol is estimated to be as short as lo-" sec. Accordingly, reaction I is immediately followed by a rapid, four-centered elimination reaction (11), producing vibrationally excited hydrogen halide and carbonyl halide molecules
+
H,-,X4-,COHt
+HXt
+ H,-,X3-,COt
(11)
For n = 2, the subsequent elimination of the excited formyl halide molecule
HXCOt
HXt
+ CO
(111) was also found to take place and contributed partly to the laser oscillations. lv2 This rapid overall insertionelimination reaction is also manifested by the comparatively short appearance times of both HF and HC1 laser pulses (t,,, 5 4 psec1'2). --f
The initial population ratios of the highest gain transitions have been estimated for H F and HCl molecules produced in a series of insertion-elimination reactions. The population ratios were found to be less than In this work, we exunity for all cases amine the relative importance of the eliminations of HCI and HF competitively from an excited a,a-chlorofluoromethanol molecule.
Experimental Section The experimental apparatus has been described prev i o u ~ l y . ~A Suprasil laser tube (2.5-cm i.d., l-m length), fitted with NaCl windows at the Brewster angle, was positioned in an optical cavity formed by two 2.5-cm diameter, 3-m radius gold-coated mirrors at a separation of about 1.2 m; one of the mirrors had a l-mm coupling hole a t its center. Six 50-cm long Xe quartz flash lamps were placed coaxially around the laser tube in an alumina housing. The flash lamps were capable of delivering up to 5 kJ of energy with a pulse shape of about 3 psec rise time and 6-8 psec half width. Laser emissions were analyzed by passing the beams through a 50-cm Model 305 SllilP03 grating monochromator and were observed by an Au-Ge detector, maintained a t 77"K, in conjunction with a Tektronix Model 556 oscilloscope. Ozone generated from an ozonizer4 was collected in a silica gel trap maintained at -78" and was thoroughly (1) M. C. Lin, J . Phys. Chem., 75, 3642 (1971). (2) M. C. Lin, ibid., 76, 811 (1972). (3) L. E. Brus and M . C. Lin, ibid., 75, 2546 (1971). (4) H . Melville and B. G. Gowenlock, "Experimental Methods in Gas Reactions," Macmillan, London, 1964. The Journal of Physical Chemistry, V o l . 76, NO.10, 1972
1426
M. C. LIN
outgassed before use. CHzClF (du Pont), CHCI2F (Matheson) and CHCIF2 (Matheson) were subjected to repetitive degassings at 196" and were stored in glass bulbs. SFe (Matheson), which was transferred under high pressures, was used without further purification. The reaction mixture (Pt 5 400 Torr) was prepared in a 2-1. painted glass bulb; a sufficient time was always allowed for mixing before flashing. No significant drop in laser output was noticed when a 2-day-old mixture was flashed in comparison with a freshly made one. The decomposition of 0 3 a t room temperature under our conditions was therefore concluded to be unimportant; this is consistent with the half-life of 03 at room temperature, -lo6 sec (10 days). Both flashed and unflashed samples were analyzed in the same manner with a CEC-620 mass spectrometer. A liquid N2 trap was used to separate the condensable from the noncondensable fraction (e.g., H2C0 from CO) of the sample in order to avoid complications due to the cracking patterns of the former. The results of product analyses are similar to those reported previously in the CH,Fd-, and CH,C14-, flashes. l,z The conversions of halomethanes were found to be about 15-20%.
-
Results and Discussion The typical results obtained from flashing 30 Torr of 1: 1 :20 Os-halomethane (CH2ClF, CHCI2F, and CHC1F2,respectively)-SF$ mixtures are shown in Figure 1; a constant flash energy of 1.5 kJ was employed in all cases. Laser oscillations were detected in all three systems. The laser output (peak power) as a function of total sample pressure is given in Figure 2; the intensities of these three systems are comparable. The outputs reach their maxima a t about 40-50 Torr total pressure. A rapid decrease in power a t Pt 2 40 Torr was observed in the CHClF2 system, whereas only a slight drop was noticed in the CHClzF flashes. The output of the CH2ClF system, however, reaches its plateau value at Pt 2 40 Torr and begins to fall off at pressures greater than 90 Torr. Since the partial pressures of Os, halomethane, and SF6 are the same in all cases, the fast decrease in power in the CHClF2 system is probably attributable to a faster rate of relaxation of HX(v) by the CHCIFz molecule. The observed vibration-rotation transitions and their appearance times (in microseconds) are listed in Table I. The emissions from these three systems were found to be predominantly due to HF. The lasting sequence of the HF transitions is similar to those observed in the CH,F,-, (n = 1 and 2) systems in which the 2 -t 1 transitions have the highest gains and, accordingly, appear first. The appearance of 3 -t 2 transitions, which were absent in the CH,F,-, flashes, is probably attributable more to higher gains of the present systems, owing to the use of SF6as a diluent,2than to the difference in the dynamics of these unimolecular elimination reactions. The Journal of Physical Chemiatry, Vol. 78, N o . 10,1979
CH2ClF
lJ!.L-A ,
I"'
,
.
.
-t-. Figure 1. Total laser emission traces and the flash output a t 240 nm: ordinate, emission intensity (1 V/div); abscissa, time (2 psecldiv). In all cases, flash energy = 1.5 kJ, Pt = 30 Torr (03 :halomethane : SFe = 1 :1 :20).
CH2CI F
2 0
CHClrF
- 2
0
c
CHCIF2
:t8ka 5
2
01 0
IO 20 30 40 50 60 70
P (torr) Figure 2. The effect of total pressure on the laser intensity. Flash energy = 1.5 kJ, Oa:halomethane :SFe = 1 : 1 :20 for all cases except for the CH3Cl CHSF mixture, in which 0 3 : CH3Cl: CHaF :SE'B 1 : 0.5 :0.5 :20,
+
2
HC1 emissions were totally absent in both CHzClF and CHClF, flashes, and only one transition, P I O ( ~ ) , lasing weakly in the CHC12F system, was detected. The strong H F emission may result from a higher
CHEMICAL LASERSPRODUCED
FROM
1427
0 ('D) ATOMREaCTIONS
+ CHzCIF +CIFHCOHt H F t + HClCO' AH"1, -145 O(lD) + CH,ClF +CIFHCOHt HClt + HFCOt AHolb = -151 O(lD) + CHClZF ClZFCOHt HFt + ClzCOt AH",, = -154 O(lD) + CHClzF ClzFCOHt O(lD)
Table I: Observed H F and HC1 Vibration-Rotation Transit>ionsand Their Appearance Times in the CH2C1F, CHClJ?, and CHCIFz Systems'
+D
=
kcal/mol
(la)
---t
Lasing species
Yt
Transitions
om-'
CHzClF CHClzF CHClFl
kcal/mol
(lb)
kcal/mol
(2a)
AH'zb = -160 kcal/mol
(2b)
+ CHClF, +CIFzCOHt + HFt + CIFCOt AHo3, = -160 kcal/mol O(lD) + CHCIFz CIFzCOHt
(3a)
-3
3417.99* 3373.32
4.6d 5.2
5.0 5.6
3622.58 3577.52 3531.20 3483.71 3435.10
2.9 3.3 4.2 5.0 5.6
3.9 4.1 5.0 5.5 6.4
4.4 4.5 5.4 6.7 9.0
3693.64 3644.24 3593.89
4.3 4.5 5.5
4.6 5.2 7.1
5.1 6.5 9.7
2703.01"
8.8
a Flash energy = 1.5 kJ, Ptotal= 30 Torr (0s:halomethane: SF6 = 1:1:20). * Vacuum wavelengths reported by A. L. Mann, et al., J. Chem. Phys., 34, 420 (1961). 0 Vacuum wavelength observed by B. 0. Rank, et al., J. Mol. Spectrosc., 17,122 (1965). Appearance time of the individual laser pulse, psec.
-3
-+
HClt
4-CIFCOt
-+
O(lD)
+D
-3
HClt
4- FzCOt
AH03b
=
-168 kcal/mol
The thermochemical data for these reactions are given in Table 11; they were taken primarily from the JAKAF table^.^ The heats of formation of CHzCIF, CHClZF, and formyl halides were evaluated by assuming the heats of redistribution to be small and negligibk5 This assumption is probably valid and adequate for the present cases. For example, taking AH," 0 for the reaction CHzF2 CClzFz -t 2CHCIFz, then AHf" (CHC1F.J = [AHf" (CHZFz) AHt" (CCl,Fz)]/2 = -113.0 f 1.1 kcal/mol, using AHf" (CHZFZ)= -108.2 f 0.2 and AHt" (CClZFz) = - 117.8 f 2 kcal/mol.6 This is in good agreement with the experimental value, AHr" (CHC1F.J = -112.3 kcal/mol, determined by Edwards and Small.' However, it is not recommended that one employ the general scheme CX4 CHzYz + 2CHXzY to calculate AHf" (CHXzY) without correcting for the rearrangement energy.6 For instance, the heat of redistribution for the reaction CF4 CHzClz --t 2CHC1Fz becomes as high as +21 bcal/mol, which gives rise to an erroneous value of -123 kcal/mol for AHf" (CHClFJ. The errors in the heats of formation of ClFCO, HClCO, and HFCO are probably as much as A 5 kcal/mol for the omission of AH,". The exothermicities of the laser pumping reactions 1-3 are only slightly different from those of the CH,F4-, and CH,C14-, analogs; this is also the case for the H F laser transitions observed, as was mentioned earlier. The results given in Table I allow us to estimate the initial relative population, NO/Nu+ for the transition that has the highest gain. Assuming that H X deactivation is negligible before the first line appears and that the rotational-translational temperature is about 30OoK, then the observation that the Pzl(4) H F line reaches threshold first requires Nz/N1 2: 0.8 * 0.2 for
+
spontaneous emission coefficient (Einstein A coefficient) and a faster elimination rate of H F than HC1. These two factors can be demonstrated by comparing the laser emission intensity of the 1:0.5 :0.5 : 20 03-CH&1CH3F-SF6 flashes with that of the 1: 1: 20 03-CHzClFSF6 system under the same conditions as shown in Figure 2. The peak intensity of the 0.5 (CH3C1 CH3F) system is about a factor of 3 lower than that of the CHzCIF system at P, 2 30 Torr, and it decreases rapidly as pressure increases. Only H F laser emission was detected from the 0.5 (CH3C1 CH3F) flashes under the present conditions. Since HC1 lases in the 1: 1: 20 03-c H&-SF6 mixtures,z the absence of HC1 emission indicates that the gain of HC1 is only slightly below the threshold in the (3.5(CH&l CH3F) system. The rates of HC1 and H F formation in this system are expected to be nearly equal, because the rate-controlling process of these insertion-elimination reactions is probably the insertion rather than the elimination step. The inversion ratios, N1/No, of H F and HC1 produced from O(lD) CH3F and O(lD) CH3C1 reactions have been estimated to be 0.5l and 0.6,2 respectively; the inversion density should not, therefore, be an important factor in the present case either. Thus, the appearance of H F laser alone in the 0.5(CH3C1 CH3F) flashes is attributed to the higher A coefficient of HF. The fact that the laser outputs of the three systems are significantly higher than that of the 0.5 (CH3C1 CH3F)system can be concluded to be due to a preferentially faster rate of elimination of H F than HC1 from the vibrationally excited methanols formed in the following reactions.
+
+
+
+
+
+
+
(3b)
+
+
+
(5) D.R. Stull, Ed., "JANAF Thermochemical Tables," The Dow Chemical Co., Midland, Mich., 1960. (6) J. R. Lacher and H. A. Skinner, J . Chem. SOC.A , 1034 (1968). (7) J. W. Edwards and P. A. Small, Ind. Eng. Chem., Fundam., 4, 396 (1965). The Journal of Physical Chemistry, Vol. 76, N o . 10, 1978
1428
M. C.LIN
Table 11: The Heats of Formation (kcal/mol at 298'K) of Various Species Involved in the Laser Pumping Reactions' Species
O(lD) CHzClF CHClzF CHClFz HCl HF
AHf'zBs
Species
104.96
ClZCO FzCO ClFCO HClCO HFCO
- 66
-
69 -112.3" -22.0 -64.8
AHf'm
-52.6
- 153d - 102 -41 -90 -26.4
co
a Unless otherwise specified, all data obtained from ref 5. AHpo[O(lD)] = AHE'[O(~P)] 4- AE[O(lD) + O(3P)] = 59.6 45.3 = 104.9 kcal/mol. Reference 7. J. C. Amplett, J. R. Dacey, and G. 0. Pritchard, J. Phys. Chem., 75, 3024 (1971), and references cited therein.
I,
+
Table I11 : Comparison of the Estimated Population Ratios of the Highest Gain Transitions for Various O(1D) Atom Insertion-Elimination Reactions' Reactant
CHFs CHzFz CHsF CHCla CHzClz CHsCl CHzClF CHClzF CHClFz
Lasing species
HF HF HF HC1 HCl HCI HF HF HCl HF
Conclusion
v
NvlNV-i
-AHo
Ref
2 2
0.8 0.8 0.35 0.50 0.76 0.61 0.8 0.8 0.5 0.8
155 148 143 155 146 134 145 154 160 160
1 1 1 2 2 2 This work This work This work This work
1 1
2 1 2
2 1
2
a v is the vibrational quantum number of the upper laser level. The overall heat of reaction, AH', is in kcal/mol.
the H F molecules produced in reactions 1-3. This value is the same as t'hat obtained for the O(lD) CHF, and O(lD) CHzFz reactions. Similarly, the PI0 (8) line of HC1 lases alone, implying that the HCl molecule produced in reaction 2a has a population ratio N1/No I= 0.5 0.04. These values are compiled in Table I11 along with, for comparison, those values estimated for other insertion-elimination reactions studied previously. l , a The early appearance of laser pulse from the CHzClF flash (see Figure 1) is worth noting. It may result from the combination of the following two factors: (1) a faster rate of HCIFCOHt formation due t o a higher C-H bond density, and ( 2 ) the extra contribution of HFt from the elimination of vibrationally excited HFCO produced in reaction lb.
+
+
+ co
H F C O H~ F ~~
(4)
The first possibility is, in fact, supported by the observation of a slightly higher conversion of CHzClF (-20%) in comparison with that of CHClF, or CHCLF 4 was concluded to be contrib( ~ 1 5 % ) Reaction ~ uting a part of the laser emission from the CHzFz system. The Journal of Physical Chemistry, Vol. 76, N o . 10,1079
The absence of HCI emission in reaction 1 is, however, puzzling, since a significant amount of CO was detected in the noncondensable fraction of a flashed CHzCIF sample. The appearance of CO would imply that the subsequent elimination of HCICOt or HFCO? is important; this ~ o u l d ,in turn, indicate that the total concentration of HC1 is comparable to that of HF. A plausible explanation of its absence may be that the small population inversion created in reaction l b is offset by a dominant amount of IICl, produced from the elimination of HCICOt formed in reaction l a , which may not have a significant inversion because of a rather widespread internal energy in the HCICOt molecules. This in an interesting point which deserves further detailed investigation by employing a longer, temperature-controlled laser tube to pick up the low gain signals. Similar studies of the insertion of NF(1A)3and NH(lA) into hydrocarbons are also planned. The HF molecule is found to eliminate competitively with a faster rate than does HC1 from the vibrationally excited chlorofluoromethanol molecules. The data compiled in Table I11 indicate that (1) the H F molecule produced from the elimination of an excited ahalomethanol molecule containing two or three halogen atoms has the highest gain in the 2 -t 1 transition with a population ratio of N2/N1 = 0.8; (2) the HC1 molecule produced from an excited methanol containing three halogen atoms, however, has the highest gain in the 1 + 0 transition, the inversion ratio being 0.5; and (3) the H X (X = C1, F) molecule generated from this type of unimolecular elimination reaction, similar to few other elimination reactions studied,*-'l .always has partial inversion and possesses only a small fraction of exothermicity resulting from a random or near random distribution of reaction energy among various degrees of freedom. This conclusion, in turn, implies that the lifetime of the activated complex, (F. . H . * . R ) *, of the exothermic bimolecular abstraction reaction F RH + HFt R is much shorter than sec, the approximate time required for the reshuffling of vibrational energy within a molecule,l2 Complete population inversions of H F in both 2 + 1 and 1 + 0 transitions have been reported for this type of abstraction rea~ti0n.l~ Acknowbdgment. This work is partially supported by the Advanced Research Projects Agency under ARPA Order 660, which is gratefully acknowledged.
+
+
(8) P. N. Clough, J. C. Polanyi, and R . T. Taguchi, Can. J . Chem., 48, 2919 (1970).
(9) M. J. Berry and G. C. Pimentel, J . Chem. Phys., 53, 3453 (1970). (10) T. D. Padrick and G. C. Pimentel, ibid., 54, 720 (1971). (11) H. W. Chang, D. W. Setser, and M. J. Perona, J. Phys. Chem., 75, 2070 (1971). (12) J. D. Rynbrandt and B. S. Rabinovitch, ibid., 75, 2164 (1971). (13) W. H. Green and M. C. Lin, J . Chem. Phys., 54, 3222 (1971), and references quoted therein.
