VoIwrke 60, mmiber 1 15 December 1978

ENRKHMEi%T OF GWBON43 IN TFIE i?RODUCTS OF THE BdIEI?PBOTON DISSOCIATION OF CFsX CO&lPOW?DS *

T%e separation factors f% the emiefiment of carbon-13 in the hexafluoroetI~~~e produced from the muIfiphoton dis- soeiation of CFsI, CF& &d CFsCI are reported as a function of C~2-Iaser line. The bromide-and chloride are signifi- cantly mere sekc%ive than the iodide both st room tempernture and at --SO°C_Thisisaconsequena:of~espectros- copy of tke individual molenitw.

I. Wroductiou

There is much current activity in the multiphoton decamposition of simple freons a~ a possible route to carbon-13 production. Enitiah~ it was shown that multiphoton dissociation of CCZIe irradiated on *he red edge of the ~4 t v1 f 9 band gave C2C14 and/ or C2C16 enriched in carbon-13 [1,2]_ An ‘bnrich- ment factor” of 7.30 was reported for irradiation at 979.7 cm- I_ Ritter and Freund preferenmy disso- ciated C-12 bearing CF$lz and measured the iso- topic composition of (a) the residual substrate and @) the products of reaction with various radical SCZXV- engers [31_ Products derived from dihalocarbenes and trihaIomet.hyf radicats were observed, lnitiaf sep- aration factors were not measured as the experiments were carried to high aonversioa. Subsequent work concentrates on carbon-12 enrichment in the products of the reaction of difluorocarbene with isobutene f41. however, ia Situ SfdieS of the d~GO~~O~~0~ of C!F$I!Iz have shown that the irrXal branching ra- tio of CF,CI + Cl or CF, + Cl2 is 32/l favuuring chlorine atom production fS]. This dichotomy has been discussed by Braun and Foulcher f6f who pos- tulated that thermal decomposition of CF# or its

reaction with chlorine atoms lead to CF2. Carbon- 12 depletion of CF$ was studied by Bittenson and Houston by measuring isotopic ratios in both the re- maining substrate and in the products 17 3. Low sekc- tivity factors were form room temperature, how- ever, much greater selec es were inferred from re- suits at -80°C. The dat again marred by the fact that experiments were carried to high convenians.

Obviously efficient Preparation of carbon-13 en- riched products or of carbon-13 depleted substrates will be based upon processes in whkh carbon-13 bearing molecules are selectively Photoly~d. Recent- ly LetoWtov and co-workers on CF$ [8f and our- selves on CF,Br f9f have demonstrated very signif% cant carbon-13 enrichment in the hexafluoroethane produced by the muhQ4roton dissociation of tbe sub- strate. Such direct measurements at low con~txsion~ are of obvious irrportance in determining the overall seiectivity of the m~ltip~oton diiociation with re- gard to assessing possible routes to carbo~43 synth& sis_ In. &is paper we report carbon-13 isotopic selec- tivity in the m~~photon dissociation of CF$Ii and make GIrect comparison of the primary a-vahres of the CFS,lE, CFsBr and CF$I systems at ambient tem- Perature and at -SO”&.

* Isntti as NRCC. 16942. ** NRCC Summer Student 1978,

16

Volume 60, number 1 CHE!!ICf?L PHYSICS IEITERS 15 December 1978

2. Experimenti

A line tunable TEA-CO, laser was used (Lumonics model 103). The central portion of the beam was passed through a 1.4 cm diameter aperture and focus- sed at the centre of the Pyrex reaction cell (5 cm diameter, 18 cm long with sodium chloride windows) by a 10 cm focal length germanium lens. The beam energy was measured leaving the cell by a calibrated pyroelectric detector_ Provision was made to cool the reaction cell by the passage of cold nitrogen gas through a helical coil inside the cell and the full length of it. An MKS O-l torr capacitance manom- eter measured the pressure in the cell_ CF,Cl (Matheson), CFsBr and CF31 (PCR Inc.) were puri- fied by low temperature distillation. AnaIysis by glc and mass spectrometry revealed no impurities, in par- ticular C2F6, C2F4 and CF3H were absent. Samples at a pressure of 0.2 torr were irradiated and the re- sultant mixture analysed by glc and mass spectrome- try. An iso-pentane slush bath (-160°C) prevented halogens from entering the mass spectrometer.

3. Results and discussion

C,F, was the only product detected and no evi- dence was found for its secondary decomposition.

This is to be expected because the reactions were carried to low conversions (<O-l %) and the least abundant isotopic species was being excited- Thus (a) the CzF6 concentration was low, both absolutely and when compared to the concentration of unex- cited CF3X mole&es, and (b) the average tib_ration- al “temperature’) of the inadiated. gas was low. This is a completely different situation to that eacounter- ed when the carbon-12 bearing substrate is decom- posed at the center of the absorption band: then the average vibrational temperature of the gas is high [lo] and secondary decomposition reactions are to be expected [11,12].

Values of Q were calculated from the peaks at m/e 119, 120, 121 in the mass spectrum of the prod- ucts (C2Fz ion is the most abundant in the mass spectrum of CzF6).

13/12 ratio in product CzF6

*= 13/12 ratio in initial CF,X *

Table 1 displays the measured (Y-v&ues for alI three systems at both room temperature and at -50°C. Three points are of note. As mentioned above, the seIectivity of the CF3Br and CF,Ci systems is mar- kedly greater than for the CF3I system. The selectivi- ties in the CF,Br and CF,CI systems have similar wavelength dependence and continue to increase as

Table 1 a-values observed in the multiphoton decomposition of CF3X compounds

CF3Cl

line a) L2

CF3Br

line a) c

CFsI

line a) d

+22-c -SOY +22” c -50°C +22Y -50°C

R<32) R(24) R(20) R(16) RN) R(10) R(8) R(6) R(4)

- 0.8 P(8) 3.2 8.6 PU6) 2.0 1.8 - 4.4 W12) 9.5 19.5 P(20) 4-o - 3.3 21 P(l6) 19.5 55 P(22) 5.7 4.4 6.5 31 W20) 38 107 KM) 5.9 -

18.8 60 W4) 56 120 P(26) 6.1 11.6 22-O - N26) 65 - P(28) 7.5 - 32.2 108 P(32) 8.0 14.0 33.8 - P<36) 8.8 8.6 45.5 - PC401 4.0 -

PW) R(38) b) E R(34) b) 8:s

$5 n-P-

R(30) b) 4.5 n-P-

a) 9-4 E band of CO2 laser. b, 10.4 or band of CO2 laser. C) np. = no products.

17

Volume 60, number 1 CHEbUcAL PHYgcs JJZTERS 1s December 1978

the photoiysis is carried out at progressively longer wavelengths on the red edge of tie absorption band_ However, the sefectivity for the CF3I system appears to show a maximum selectivity and increasing the waveIength of the photoIysis fine further leads to B decrease in selectivity_ Thirdly, all systems show a signifkantiy higher selectivity at low temperatures over that observed at room temperature.

The differences between the CF,I system and the o*&er two is most reasonably interpreted in terms of *he presence of the v2 + n3 comb&ration band_ In both CF3Br and CF3G this is to the biue side of the v,band : in CF,I it is to the red side and thus contributes a C-12 component to the absorption in *be region of least overIap between the C-22 and C- 13 contributions to the yI band.

It is not reasonable to attribute the entire effect of the increase of selectivity with decrease in temper- ature to the scrambling reaction,

13CF3 +‘12CF3X + t2CF3 f 13CF3X _ (1)

As we have previously discussed [9], tbis reaction will be SIOW for CF3Br and probably negligible for CF3Cl at room temperature. Decreasing the tempera- ture, then, should only have a marginal effect_ Also, the probable activation energies for the reaction are very dif%erent for the three hatides, yet the increase in setectivity is proportionately s&n&r for ah three. More probably the increase in selectivity is related to the narrowing of the rotational envelope with de- creasing tempemtiire. Although such narrowing is probably small overall for the temperature differences involved in the present experiments, its effect will be magnified in the band tails where the overlap between the C-12 component interferes with excitation of *be C-13 species. The observed narrowing will be similar for ah three species and is shown for CF,I in fig_ 1, which incidently displays the n2 + n3 combination band discussed earlier.

In conclusion then, the CF3I system is significantly Qss selective than either CF3Br or CF3CI, because, we believe, of the difference in position of the ~2 + v3 co&&&ion band. Selectiuities for decomposition

FREQUENCY /cm-’

Fig 1. kfrared absorption spectra of CFai at +22*C (solid lile) and -78°C (dashed iinej; 30 torr, 10 cm pathle~.

of the C-13 species are much larger at -50°C than at room temperature in ail three systems_ This is inter- preted in terms of narrowing of the rotational con- tour of the absorption band with secondary scram- bling reactions making only a minor contribution.

Referfmces

ELV. Ambe- Yu.k Gorokhov. vs. Letokhov and A.A. Pm-et.&& ETP Letters 22 (L97.S) 177, -XV’. Ambe, Yu.A.~Go~okhov, V.S. Letokhov, G.N. &fakaroT and A.A. PuretEkE, Phys Letters 56A (1976) 183. J.J. Ritter and S.M. Freund. J. C&m. Sot. Chem Commun (1976) 811. J.J. fitter, J. Am. Chem. Sot. 100 (3.978) 2441. 3.W. Hudgens, J. Chem. Phys. 68 (1978) 777. G. Foulcher and _W. Braun, J. Photochem. 8 (1978) 341. S. Bittenson and P.L. Houston, J. Chem. Phys. 67 (1977) 4819. R-V_ Ambartzumian, V.S. Letokhov, G-N. Makarov and A-k Pure&k& Paper N-8, Xth International Quantum Electronics Conference, Atlanta, Georgia, 29th May- 1st June, 1978. M. Gauthier, PA, Hackett, K Drouin, it PiIon and CT_ Wi!i& Can. J. Chem. 56 (1978) 2227. P.A. Hackett, M. Gauthier and C. willir, J. C&em. Phys. 69, No. 6 (1978), to be pub&%&. A. Gandini, private communication. P.A. Hackett, M. Gauthier and C. WiUis, unpublished data.