Laser initiated thermal isomerization of methyl isocyanide

V&rrne 57. n-mnber3

LASER INITIATED

CHEMICAL

PHYSICS

1 Aug=st 1978

LFXTERS

THERMAL l?SOMERIZATION OF METHYJL ISOCYANIDE

DS- BETHUNE, J-R. LANKARD, M&T- LOY, J. ORS and PP. SOROKIN IBM ZiZronras J- W&on Research Center, Yorktown Heights. New York iOS98, USA Received16 April 1978 Revisedmanuscriptreceived12 May 1978

Applicationof singlepulsesof CC&a TEA laserradiationto a vapor of methyl isocyanide(CHaNC), with the lasertuned to coincidewith the frmdamentalfrequency~4 of this molecule, resultsin more than 50% conversionof the gas to its isomer methyl cyanide
We have observed that the application of single pulses of CO2 TEA laser radiation to a vapcr of methyl isocyanide (CH,NC), with the laser tuned to coincide with the fundamental frequency v4 of this molecule, results in more than 50% conversion of the gas to its isomer methyl cyanide (CH$X). The experiments, performed at room temperature at pressures 10-100 torr, are interpreted in terms of laser induced heating which is sufficient to trigger a thermal explosion_ The thermal isomerization of CHsNC to CH$N, known to be exothermic to about 14.7 kcal mol-l [I], has previously been suggested as an ideal reaction for testing thermal explosion theories f2] _This particular molecuIe has been the subject of extensive kinetic studies 131, as ares& of which it has emerged as a model unimoIecular reactant- Recently, an elegant experiment [4] was performed in which states of CH3NC having sufficient energy to effect iaomeriza-

tion were directly pumped by a cw dye laser. This experiment led to an unambiguous determination of the unimolectdar rate constant kc for the states selected_ In our experiment (fig.. l), the beam from a Tachisto CO2 TEA laser, tuned to the 942.4 cm-1 P(22) line, was passed through an external 3 mm aperture, then* through a 10 cm cell containing butane gas at various pressures which served as a convenient variable attenuator- A 10 cm f-Q_BaF2 lens then brought the beam to a focus at the center of an 18 cm long, 1 cm diameter glass sample cell fitted with NaCl windows at either

Fig. 1. Diagramof e?rperirnent.

end- This cell could be easily reftied with fresh CH:NC gas from an external manifold, and its contents could be transferred to an auxiliary ceJl for IR analysis_ A pair of windows (NaCl and Si), mounted at the center of the sample cell, transverse to its axrs, were used for viewing the infrared light emitted during the laser induced thermal isomerization- Various filter and photoconductive detector combinations were chosen to monitor IR fluorescence at the normal mode frequencies of CH3CN [S] . All of the CO2 laser beam transmitted through the sample cell was directed into a Scientech energy meter coupled to a chart recorder- Part of the input beam was also coupled onto a Ge : Au (77 K) detector to monitor the CO2 laser power for each shot. The CH3NC was prepared by the dehydration of N-methyl-formamide, following the procedure of Schuster et ai- [6] _ The compound was purified by 479

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CHEMICALPHYSICSLEITERS

through a 10 cm vigreauxcolumn and the 59-60 “C fraction was collected_The structurewas verified by proton NMR spectroscopy. IR spectraof unirradiatedgastaken at variouspressuresreveaIe6only known bands of CHzNC with no trace of those belonging to CH,CN, for example, the 2~~CH3 rock at 1041 cm-l. The basic resuit of our study is the observation that for a given pressureof CH,NC there exists a sharp threshold in CO2 laser power above which isomerization occurs_At powers below this threshold rio significant change in the IR spectrum was noted, even for thousandsof laser shots_ For shots below threshold no light was detected by the photoconductive detectors, which included a Cu I Ge (4 K) detector for viewing the long wavelen,* (23 0 cl) v8 bending modes of both CHsNC and CH,CN. As the laser power was gradually increasedby reducingthe butane pressurein the attenuator cell, a critical power was reached above which distillation

one laser shot would suddenly triger the isomerization, producing slowly rising (~1 ms to peak) but strong IR signafson the detectors and causing an irreversble change to appear in the IR absorption spec-

trum of the gas, a change readily interpretedas large scaIeisomerization_At 30 torr CHsNC, for example, the absorption correspondingto the ~2 @EC) stretching mode in the IR spectrum was reduced by a factor ~2 after a single, above critical ftig, and the characteristicv7 mode of CE3CN became dramaticallyapparent. Additional irradiationsby rhe laser produced no further measurablespectral changes of the gas in the sample cell_ AU features noted on the IR spectra were attributable to either CH3NC or to its isomer. The threshoid for thermal isomerization was observed to be pressuredependent, as shown in fig_ 2, where both the measuredthreshold energy applied at the entrance window of the cell, and the threshoIdenergy reaching the focus at the center of the cell (the latter deduced using measured transmissioncoefficients) are plottedTo model the laser inir_iatedthermal explosion, we assume that the reaction srarts at the focus of the laser beam. The laser pulse, which is tuned near the peak of the Q branch of the CH3NC (C-N) stretching mode (v~ = 944.6 cm-l) [7], deposits a certain amount of energy in the gas in a time corresponding to the laser puIse width (: 10-7 s) via singlephoton absorption md collisional reiaxation-In the region of the focus it may be assumed that a short time after 480

1 August1978

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PRESSURE

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F&. 2. Variationof threzold eneqy \kithpressure.Solid circksr thresholdenergiesmeasured at the entrancewindow of the cell. Open triangles:thresholdenergiesat the beam focus in the centerof the celI (deducedusingthe measuredtranmussion coefkients). The solid 2nd dashedcurvesare drawnfreehand throu& the experimentallydeterminedpoints.

the laser is fired a temperature T is realizedgiven by: T = To f AT exp(--r2/2&

,

(1)

with r. the beam radiusat the focus and To = 300 K. Standardchemical kinetics equations for thermal explosions now apply. One has cyllTjat=~V2T+Q,

(2)

where K is the thermal conductivity, cy is the heat capacity per unit volume, and Q is the rate of heat generation per unit volume given by: Q = enA exp(-E/RT)

,

(3)

where E and n are the heat of reaction and number denFity of CH3NC molecules, respectively,and A and E are the known [3] Arrheniusparametersfor the isomerization reaction_A thermal explosion occurs when the heat generatedby isomerization exceeds the heat !oss due to diffusion. Setting aT/a: = 0 at r = 0 in eq. (2) leads to the following threshold condition: -= 2#c r&A

exp [-E/R(T~ AT

f AT)] (4)

We may use eq. (4) to predict the temperaturerise AT needed for the thermal explosion to develop. Making

Voltlme 57. number3

CHEMICAL PHYSICS LETTERS

the assumption r. = 3.3 X 1O-2 cm, and using known values [3] for the &her parameters, viz., K = 6 X IO-5 cal/cm “C s, E = 2.$4 X lo-20 cal/molecule,tt = 1013-G s-t, E = 38.8 kcaljmol, one predicts AT to vary with pressure in the m&me, shown in fig_ 3. At mid-range pressures (40-8O(torr) a rise above ambient temperature of some SOO’K is evidently needed for the thermal explosion to/develop. Given that th& molar heat capacity is 14.5 cal/mol deg [3], one ea$ly calculates that to raise the temperature of the molycules in the focal region by =500 K requires, on thf average, that each molecule absorb 2.7 CO2 photF. If the absorption cross section per molecule in a @as traversed by a laser beam is independent of th+ beam intensity, the average number of photors abporbed per molecule from the beam is given by ‘PO, the product of photon fluence and molecular absorpti&n cross section. In the present instance, with u taken/to be the low intensity cross section 00, Cpomust at lkast be equal to 2.7, the average number of absorDed,bhotons deduced above. Frcm CO2 laser taken at power levels for isomerization, we obtain a section at the P(22) CO2 cm2. From fig. 2 we deduce, fcrthermo/e, that the threshold photon fluence at the higher pre+zsuresis about 1.6 X 1020 photons/cm2. - g to = 10 mJ of CO2 laser beam energy correspo +hn passing through the 3.3 X 1O-2 cm radius spot. Thus = 5k, clearly sufficient to provide the required *a0 number b f photons. Expectation of 2 closer corre-

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spondence between the two numbers is unwarranted, in view of the uncertainty involved in estimating r. . The ordinates Ei, and AT of figs. 2 and 3 are different, and one does not, a priori, know the relationship connecting them. An exception occurs at the higher pressures, where it is reasonable to assume that, due to collisional relaxation, the absorption cross section u is independent of laser intensity, i.e., equal to uo . In &is regime the rise in temperature is proportional to the applied laser energy_ Thus at the higher pressures the predicted pressure variation of the temperature rise needed to initiate thermal explosion should be the same as the observed variation of laser energy applied to the focal region at threshold. By comparing the curve of fig. 3 with the lower curve shown in fig. 2, one sees that this is so. At pressures less than 30 torr, the curves of fig. 2 rise much more steeply than does the one in fig_ 3. In this low pressure regime, the absorption cross section per molecule depends upon the laser intensity, being, in general, less than 00 because there is less co’%ional relaxation to prevent saturation. Accordingly, the photon fluence at threshold must be correspondingly higher. It is possible that the high temperatures (= 1800 K) reached in laser initiated thermal explosions of the type described here can be used to trigger even more exoenergetic rezctions in other molecular species mixed with CH3NC. For example, unimolecular HF elimination reactions leading to HF vii&rational chemical laser action can possibly be triggered by mixing small amounts of CH3NC with gases such as CH3NF2 [S] . We would like to thank R. Srinivasan for helpful discussions and encouragement and JJ. Wynne for a critical reading of the manuscript. This work was partially supported by the U.S. Army Research Office and the Office of Naval Research.

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of minimum temperature rise needed to initiate thermal e_xplosion as a function of pressure.

References 111NationalBureauof Standards, Kkbington, D-C.,Technical Note 270-3. 121H-0. Pritchard and BJ. Tyler, Can.J. Chem.51 (1973) 4001.

131F-W. Schneider and B.S. Rabinovitch, J. Am. Chem. Sot. 84 (1962) 4215.

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CfIEMICAL

PHYSICS LIsTrEm

[4] K.V. Reddy and M.J. Berry, Chem. Phys Letters 52 (1977) 111. [S] E-W. Parker, A-H. Nielsen and W.H. Fletcher. J. Mol. spectly. l(1957) 107.

[6] RX. Schuster, 3X. Scott 2nd J_ Casanova Jr., Org. Sym. 46 (1966) 75. [7] R.L. Will&q J. Chem. Fhys 2.5 (1956) 656. [S] TD. Padrick and G-C Pimentel, J. Fhys, Chem. 76 (1972) 3125.

MJ_ WOjcik and M. Falk, Band shapes of infrared absorption spectra of isotopically diluted X-H groups in hydrogenaonded crystals, Chem. Pkfs. Letters 56 (1978) 450. The sentence following eq. (1) should read: We assume the harmonic form of the potential energy for the effective X... Y vibration but do not specify the potential enew V’ for the X-H vibration.

The condition ho s kT in the fst line of the lefton page 452 should read Aa Z+ kT_

hand column

Jixeq. (S), b should be replaced by lb I and the third sentencein the following discussionshould read “at very high temperatures” instead of “at very low temperatures”.

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