Dissociative electron capture in trifluoroacetaldehyde, pentafluoropropionaldehyde and heptafluorobutyraldehyde

73 hernatiomd fowmi of Mass -.@ FEkcvier ScicntiEc

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DISSOCIATIVE ELECTRON CAPTURE IN HYDE, PENTAFLUOROPROPIONALDEHYDE BUTYRALDEHYDE

TRTFLUOROACETALDEAND HEPTAFLUORO-

P_ \V_ HARLAXD* AND J_ C_ J_ THYME* Chemiscr_r Department,

Ekfinburgh Unirersity,

Ednburgh.

(Great

Britain)

(Received 3 March 1975)

The formation of negative ions by CF,CHO, C2F5CH0 and n-&F&HO as a resuit of electron bombardment has been studied as a function of electron energy and various processes sugested to explain their formarion.

As Part of a programme [I-6] concerned with the formation of negative ions by fluorinated mokcules as a consequence of their bombardment with low energy electrons, we have examined CF,CHO, CIFsCHO and n-C3F7CH0. No previous study of these molecules appears to have been reported_

The data were obtained using a Bendix time-of-fli&t mass spectrometer, model 3015_ The experimental details have been described previousIy [l-6]. The electron ener,oy scale was calibrated using the formation of the O- ion from sulphur dioxide as the reference, the actual calibration point being the maximum of the fkst resonance peak at 49 eV [7, S]. All mass spectra were measured using the same conditions as far as possible, the ion source pressure beins ca_ 5 - IO+ torr- The electron trap current was kept constant by automatic regulation, * P-t

address: CSIRO, Division of Applied Physics. Chippendale, New South Watts, Australia 20% w Communications shouId be addressed to this author at: c/o Foreign and CommonwxIth Office, Outward Bag Room, King Charies Street, London SWIA 2AH.

74 RESULTS AND DISCtflgOX Mass

spectra of rJze$‘uoroaJd?eJt_wJes

CF,CHO_ This moleculeis an abundantsource of negativeions both at 10 and 70 eV although, BScan be seen from Table I, the W-Ospectrashow substantial differences,most notabIyin the abundkcezof other ions relativeto F-_ The ratio of F- to total ions variesfrom 0.28 at 10 eV to 0.86 at 70 eV_ In both spectraFis veryabundantalthoughin the low energyspectrumC2F,- is the most abundant ion. At 70 eV the CzF2- intensityfalls by a factor of SO,presumabIyindicating that ion formation by ion-pairreactionis much fess favouredthan ion formation by dissociativecaphue- A similar(thoughsomewhatless marked)patternis noted for the CF,- and CF,C’ ionsTABLE: .XEG.XTXVE IO?*‘ ~~15 lOR

0OH’

FCFCOFCF,CFCOC,F, CF,CF=COCF=CHOCF,CCf,CH CF,CO-

CF,CHO-

SPECZ-I-RA OF

CF,CHO

AT 10 Cv

Ret_ IZIL (IO cV) 63

ASD

70 Cv

izef_ Jnr_ (70 CV) 45

9

3

!MI 2 2 4 20 1000 900 s 2 159 11

Ioiw 3 (1 5
20 72 3 2 4 -cl 2

<1

“Mixed+’ ions, that is ions containingC, H, F; C, F, 0; and C, F, 0 and H are observedbut usuallyin low abundance; in the i0 eV range, however,appreciable yields of C?FO- and CLF,H’ are observed refiectingthe preferential formationof srichfmgment ions by dissociativecapture-At 70 eV (but not at the lower energy) a parent ion is observed; this must be formed as a consequenceof secondaryekctron capture,the secondaryekctrons beingformed by such positive ionization processes as CF,CIIO - CF3CtCHO-r-e-_ Similar behaviour has been reportedby Thynne for hexafiuoroacetone[9]. An interestingfature of both spectra is the sequence of ion intensity CF,- > CF,’ > CF-_ Formation of CF&Hat IO eV (but not at 70 eV)

wouId suz*est that it is the dissociative capture reaction, CF,CHO --, CF&H+ 0, which is responsible for ion formation and not the ion pair reaction CFSCHO 4 CF3CH’ + O’_ On the other hand the absence of CF,COat the lower enera? suz~ests that that ion is formed almost entirely at 70 eV by the reaction, CF,CHO + CF,CO-tH*_ No H- ion was detected in either spectrum_ C&sCXU_ As with CF,CHO substantial differences are noted between the 10 and 70 eV spectra of C=F,CHO, as shown in Table 2 A detailed analysis of relative ion intensities is complicated by the large number of ions present in both spectra, although neither I-I- nor parent ions were observed at either energy_ TABLE XEGATWE

1 IOS

LIXSS

SPECTSM

OF

C=F&HO

IOR

Rd

0OHFCiCFGC:FCOFCFrCFCOCzF:CFJC=F:OCzF,CrFaC,F,OLSF-r_ C=F5C,F,OCsFsO-

3s 5 1000 3 3 65 3 25 42 39 5 57-t IS 554 95 25 171 IS5 456 5

AI- 10 Cv

hr. (f0 cV)

ASD

70 CL'

ReL IJIL (70 eV) 3 i 1 196 tl
The F’ ion is the most abundant ion only at 10 eV; at 70 eV the major ion is C,F,O’, possibiy formed by the ion-pair reaction invoking HF* formation, atthough the abundance of the negative ion at 10 eV makes it clear that dissociative capture processes must also be responsibIe for ion formation_ (The analogous ion CF&O-/CF&HO was of much lower abundance)_ An interesting feature of the spe&rt.un is the ion at rzz/e= 39; this could be either the rearrangement ion F,H’ or the ion C,‘; on balance t-he Iatter is more probable since formation of F,H’ would require extensive rearrangement to occur and the CS - ion fias been observed previously in the sptctra of various hydrocarbons.

76 “Mixed” ions arefound in considerabre number, the tendency for retention of the oxygen atom ixathe ionic form being marked. There is a broad tendency for the mixed ions containing an even number of atoms, e-g_ C,F,O-, to be more abundant than the ions containing an odd number of atoms. In the case of the fluorocarbon ions several interesting features are apparent_ Two sequences of ion intensity for C2FX- ions are noted, These are: at 10 eV, CtFs - > CzFs- > C,F,> C,F,> C2F-; and at 70 eV, C2F,- > C,F,> C1F3- > C,F- > C,F,-_

Tn addition the intensities of all of these ions are much greater at 10 eV than at 70 eV, reflecting the grearet importance of dissociative capture reactions at the

Iower energy_ In the case of the tow energy spectrum the ionic sequence refkcts the pattern which might have been expected from a simpIe over-al1 picture of the

likely stabiIity of the ions_ The CF3- and C2F5- ions are noted in both spectra but their relatively Iow akmdance at 70 eV (as in the case of CF,-/CF,CHO) suzmts that stzch ion-pair reactions as C&F&HO + CF,-+CF,CHO’ and C2F5CH0 + C,F,+CHO+ are of minor significance_ G&CNO_ The large number of ions (ca. 40) formed both at IO and 70 eV makes extensive analysis impracticabIe; several patterns of ion formation may however be identified in TabIe 3_ As in the cztseof C2F,CH0 the F- ion is the most abundant ion only in the !nw enem spectrum; at 70 eV the principal ion is C,F,O-, i-e_ the ion formed by fess of hydrogen fluoride from the parent. A simiiar conclusion was drawn from the perfluoropropionaldehyde work above_ Observation of the HF’ and Fz - ions in both spectra is clear evidence for the occurrence of rearrangement reactions. As with C2FsCH0, an ion at m/e = 39 is noted; the likely identity of the ion would seem to be CBm2particuIarIy in view of the abundance of other C,F,ions_ Interesting (aJthough not completely explicable) sequences are noted for both the C,F,and C,F, - ionic families C,F,ions are considerably more abundant at 70 eV than at 10 eV; in geneni the odd-numbered ions are more abundant than those with even numbers (as with C,F,CHO) the sequence being, at IO eV; C,F,> C2Fz- > C,F- > C,F,> C2FZ- and, at 70 eV, C,F- > CLF3- > CLF2- > C,F,> C,F,-_ NotabIe in these series are the completely opposite positions in the C,F,- sequence occupied by C,F- and C,F,- in the 70 eV spectra of C,F&HO and C,F,CHO_ We have no explanation for this observation_ Not surprisingiy, in view of the known tendency for multiple-carbon atom negative ions to be formed [lo], there is more of a tendency for C,F,ions to be formed than either C2F,- or CF,-; ions such as C4F6- and C4Fz- are ako formed. Apart from the much Iower relative ionic abundances at I9 eV the pattern of C,F,rormation .is remarkably similar at both Iow and high energies, the

77 TABLE

3

STGATWE10X ion o-

OHFC;-

HNS

SPECTRA

n-C&CHO

OF

Rcl. IIN.. (JO cV) 107

2 IO00

ReJ. Jnr. (70 cV) 97

1 347 7

tl

CF-

AT fO &‘ ASIB 70 Cv

F=CSCZFCOFCF: -

1

6

2 IO < I I
2 29 11 2 4

CFCOC=Fz-

‘1

CFSC,F:CF:COC,F,CaF=CsFsC,F,CJF30-

? < I
12 1 I 5 ‘, 7 I 1-t II 2 7


I II

C,FaCzFrC,FaOCsFs -

CaF,O C,F,-

C*F, -

C,f,C,F,O C,FsO-

1

1

--‘I

1 27 r, 1

z 3

17

2

6 1000 18

sequence at 70 eV being CBFs- > C,Fa- > C,Fh- > C,fs> C,F,- > C,F2-In the case of the CF,- ions, the ions tend to be more abundant when x is an odd number,It is also noticeablethations containingan evennumberof carbon atoms tend to be Iess abundant than those with an odd numberof carbon atoms, The “mixed”’ ions containingoxygen are of interestand, in common with CIFsCHO, the (parent-I-IF)ion is remarkably abundant in the high enemy spectrum, su_gestirtg that it may well be formed by the intramolecuiarehmination of HF”, The perfiuorobutyryl ion is much iess intense,partict&&rly at 10 cV. The marked decrease in intensityof the ions C3F,CO’, C,F,COand CF,CO(which are formed by successiveC-C bond ruptures) may be seen in Table 3_

CF&HO_ A iarge number of dissociative capture ions were formed in the energy range O-10 eV; those ions detected with intensities suitabIe for further investigation were O-, F-, CF:,-, CF,‘. C2Fz-, C2F,- and CF,CO’_ The experimental and deconvoluted ionization efficiency curves at low and high electron energies are shown in Figs_ 1 and 2; the appcarancc potentials are given in Table 4.

L 2

CORRECTED

3

4

5

ELECTRON

EhiRCv

45 6

_

7

Cevl

6

7

6

7

CORRECTED

8

8

9

ELECTRON

10

If

ENERGY

(ev)

Fis_ I_ CF,CHOLow ckcfron energy ionization t!Zici~~ curves for F- (e), CFz-
CF,-

(0).

C:F,-

AIthough the heat of formation of trifiuoroacetaldehyde is not known directIy a value of -S-2&0.5 eV may be estimated for A& (CF,CHO) using the group additivity r&s compiled by Benson et al. II I J. This estimate is used in all subsequent thermochemicaJ calculations_ (i) O-_ Two resonances were noted for this ion, the first, at 7_2f0_2 having a much lower cross-section (ca- 15 times) than that at 8.2+0_2 eV_ CF,CHOte

eV

*

O-+CF,CH

0)

+

O-+CF,+CH

(2)

If reaction (1) is responsl%ie for 0:

formation then, using a value! of I-47 eV

79 TABLE4 APPEARAXCE

FOR THE

P0TESftAl.S

SEGATIVE

0-

-

CFs-

C:Fz_

C:F,CF,CO

RESOXAMZE

SY

PEAK StAXIMA

-

(nf)

ASD

PEAL

HALF

WIDTH!5

(tv)

(IS

Cv)

CF,CHO

72-_LO' 82&0_1 3_6%0.1 6.6&O-3 --_7_5 3.350.1 4.6&O-3 3.5&0_1 62&0_2 zs.0 7_1&0.1 -zr723 6-S&0.1 rr-8.0

F-

CF,

(A),

iO.XS FORSGD

3.s*o.1

7_7503 9.3&IO.2 4350.1

1.1;0.2 l.lfr0.2

6_77f02 ?.OfO.l

O_S~&I

4.4+x1 7.2502

o.sio.l

s.sio1 7_3&0_1

1_0~0.1

45~0.1

0.9*0.1

for the electron affinity of oxygen 1121 in conjunction with the reiation D(CF,CH-0) 6 A(0’) f E(O), we find that D(CF,CH-O) < S-750-3 eV_ This may be compared with a value of ca_ S-1 eV which may be deduced the-ochemically for the analogous bond in acetaidehyde, D(CH,CH-0) if it is asslrnled that D(CH,-CH?) 2: D(CH,CH-CHCH,)_ Since D(CF,-CH) = 3-7 eV then reaction (2) cannot contribute to the major rcsonancc at S_-. 3 eV_ A more probable explanation would be the occurence of reaction (1) with a release of 1 et’ of ewess ener,oy into translational or vibrational modes (ii) F-_ The diagra ms show two major resonances for this ion_ The calculated minimum enthalpy requirements 4H,i, for possible responsibte reactions (s)-(6) are shown. The similarity of the calculated vaIue for reaction (4) with the observed value su~qcsts that reaction (4) is responsible for the initial ion formation. AHmi, (eV) CF,CHO+e

=-I-S

(3)

--+ F-+CF,+CHO

36

(3)

+ F-tCF+CO+H

4-9

(5)

--, F-+CF&H+O

7-o

(6)

--, F- -f-CF,CHO

(iii) C.=-_ Several inconsistent values have been reported for the electron affinity of CF2 varying from 0.6 to 2.6 eV. On the basis of several reactions in

80 pertiorocyclobutane leading to CF,formation, Harland and Thynne have suggested 1131an acceptabJe vaiue to be E(CF2) < 1.3+O.S eV. CF,CHO+e+

CF,-tC0tHF

(71

+ CF,-+F;CHO

(9

Reactions (7) and (S) have a minimum entJtaJpyrequirement of 1-I and 5.7 eV respectively_ Reaction (7) would therefore appear to be responsible for the two resonance processes observed for the CF,ion, appreciable amounts of Jcinetic~vibrationaIenergy being reieased in both cases- It would seem likely that much of the excess energy is rckased to form vibrationaity excited CO and HF since both of these molecules are known 1141to have vtbrational states st ener$es up to 1-2 eV above their ground state(iv) CF,__ AHmi” (eV) CF,CHO+e

-+ CF,-tCH0

1.2

(9)

-* CF,‘tCO+H

2.5

(JO)

+ CF,-i-CH-J-0

102

(JJ)

None of the three experimentaltlly-observedresonances corresponds at al1 cioseiy to the c.$cuJated minimum enthalpy values suggesting that ion formation occurs with a cGnsiderabIe release of excess energy_ De Corpo and Franklin [IS] have measured the appearance potentials and kinetic energies of the ionic products resulting from several dissociative electron capture processes in poiyatomic moJecules using a Bendix time-of-flight spectrometer- They reported the following correlation of excess energies at onset with the translational energies of the negative ions: z = O-42 = E*/NZc

CA1

where P is the total excess enerq of the system which may be determined from the appearance potential and the heat of reaction, cI the total translational energy and N the number of de,of freedom of the parent molecule_ If we apply relation ]A] to reaction (9) where 7? (at 3.8 eV) is 2.6 eV, then Zc = O-4 eV; this Jeaves22 eV of excess energy which may bc divided between the fragments as excitation ener_9y_Since excited states of negative ions are not common we may assume that most cf the excess energy sees into the CHO fragment as vibrational energy_ Since D(H-CO) ‘Y 1 eV then it is likely that the formyJ fragment decomposes vibrationally to CO-t-H_ Reaction (10) would therefore appear to be responsible for CF3 - ion formation. vibrationaily excited CO being produced during both resonance processes-

81 (v) cF,co-_ CF,CHO+e

--+ CF&O’tHF

(13)

--+ CF,CO-tH+F

03)

The CF2CO- exhibits a six&e sharp resonance onsetting at 3.8&0.1 eV and for which reaction (12) or (13) must be responsible- Lack of thermochemical data precludes a detailed analysis; however some _&dance to the likely reaction responsible may be obtained if we assume that D[F-CF,CO) N D(H-CH2CO) ‘Y 2 eV. Reactions (12) and (13) have minimum enthalpy requirements of 0.4 eV - E(CF,CO) and 62 eV - E(CF,CO)_ In the latter &caseE(CF2CO) is ca. 2.4 eV; in the former at least 3-4 eV of excess energgywould be released_ In view of the sharpness of the resonance peak w-eare inclined to attribute resonance to reaction (13)_ The iack of thermochemical information regarding the radicals CF& and CF,C hinders an analysis of the resonance processes responsible for CF&and CF,C’ formation aithoush reactions (14) and (15) would seem probable. CF,CHO+e

+ CF&-+O+H

04)

--, CF,C-+F+O;H

09

Our data in conjunciion with reactions (13) and (IS) wouId suggest the heats of formation of CF&and CF,C- to be -6.6&O-6 eV and -6_7&O_6 eV respectively_

CzF,CHO_ The negative ions detected and studied at Iow electron energies were F-, CF,-, C,F5-, and C,F,CO-_ Our experimental data are shown in Table 5 and the typical ionization efficiency curves for F-, CF3-, and C2F5-, before and after deconvolution, are shown in Figs. 3a and 3b_To avoid confusion in the Fieres the CIF,COionization curve has not been included: the ion exhibited a broad resonance peak onsetting at 0 eV together with three smaiIer resonances, the detaiIs of which are inchrded in Table 5_ Using Benson’s additivity rules 1113the heat of formation of C,F&HO was estimated to be - 12_4_tO.5eV_ Figures 3a and 36 show that, within experimental error, all ions exhibit resonances onsetting at ca. 3 eV and 6 eV. These may arise from the decomposition of metastable molecular negative ion states at these energies_ C.F&HOte

(3 eV) + C,F,CHO’*

C,F,CHO+e

(6 eV) --+ CIF5CHO-*

--, F’, CFX-, C,FS-, --, F-, CF,-,

C2Fs-,

C,F,COC,F,CO-

Similar bchaviour has been observed for perfiuorocarbon motecular negative ions, notably C3F6 [S] and c_vcIo-C,F, [13].

82

CoI?aECTED

EEcT3os

EsEaGr

(cv)

Fig_ 3_ CrF,CHO_ Ioniationcfficicncy curwx for F- (0). &convoIution,~b)nfter &convoIu~ion. TABLE

CF,-

(@I and C2FI’

(&I:

(uj before

5

FCFsC,F,c,r*co-

2-9 *.o.z 5_7*.0_2 3.0402 5.9*0.2 3.450.1 6.2fO.Z 0.0 3_opo.3 X9&0.3 7-75543

4.4 * 0.1 7.9 2 0.2 4.350~2 zs_0 4_2&02 7_2-_LO.1 1.1*0.2 6_7&O_2 5.7502

L320.1 24 _L0.2 1.3+0.1 -2s 0_9&0.1 IA&O_2 O.S~O.2

(i) F-. Two resonrtnce peab were noted for this ion; the similarity of the observed (2_9f0-2 eV) and calculated (3.O_tO.S eV) appearance potentiaIs strongIy suggests that reaction (16) is responsible for the first ionization processCLF,CHOte

-+ E’+CZFStCHO

(16)

The second resonance peak which onsets at 5.7&0.2 eV is unusually broad (width at half-peak-height = 2_4-+0_2 eV)_ Thus it may be composed of several

83 closely overlappins resonances which have not been resolved by deconvolution, In view of the number of fluorine atoms which may be removed from the moJecuJe it would not be surprising if several differect reactions did occur in this ener_g range_ (ii) CF,-. Us-mg a value of LO&-0.2 eV for E(CF,) 116, 171 the minimum enthaJpy requirements of reactions (17) and (IS) which are propsed to account for CF3 - ion formation, are 3_S$-O-6 and 4_8+0_6 eV; these are in reasonable accord with the observed vaJuesof 3_0%0_2 and 5_9+0.2 eV, with - 1 eV of excess energy release in the Jattcrcase_ C,F,CHO+e

+ CF,-tCF,+CHO

(17)

-

WI

CF,-+CF+CO-+H

(iii) C,F,-_ C2F,CHO+c

--+ C,F,-+CHO

1-O

v9

-

C,F, -tCOtH

2.1

WI

4

C,F,-

9-S

(W

;CH+@

Neither of the observed appearance potentials (3-4 and 6.2 eV) is in reasonable accord with the values calculated on the basis of the most rmonable reactions JikeJylo be responsible for ion formation_ A comparable discrepancy was noted for CF,-jCF,CHO formation_ It seems most probable that reaction (20) is responsible for both ionization processes, with the Jikelihood that vibrationally excited CO is produced during the reaction_ (iv_) C&CO-. If reaction (22) is responsible for C,F,COion formation at 0 eV then the heat of formation AH, (C=FJcO-) may be estimated to be ca_ -9-6 eV_ If this value is used in reaction (23) the enthalpy requirement may be cakulated to be ca_ 5-9 eV_ This is in exceilent accord with the observed vaiue of 5.9sfsO.3 eV, and we therefore tentatively assip reactions (22) and (23) to these energiesC,F&HOte

-+ C,F,CO-+HF

(22)

+ C.F,CO-

(23)

cHF

The other resonances at 3-O&0.3 and 7_7+0.3 eV may weJJbe due to these reactions in which excess ener_gycontributions are invoived. C3F7CIfO- On the basis of the observed ion intensities the dissociative electron capture cross-section for heptafluorobutyraldehyde is an order of mawtude lower than that for trifJuoroaceta!dehyde_F- and C,F-,- were the only ions formed with sufficient intensity to permit experimental investigation under the

TABLE 6

ZU

l0n

F’

I-Z&O-Z

=3-O 0.9 io.1 ,uZ-0 0.9&O-I

CFsC:F,-

a750.1

Cd=,CsF,,CO-

2

broad

4

CWS?ECTED

6 ELECTRON

8 ENERGY

lo

12 te’d

Fig- 4- C&CHO. Ionization efficiency curves for F’ VOIUtiOn, (b) after dcconvohtion.

(0)

and CIFz’

(A): (0)bcforc ckcon-

same conditions as with CF,CHO and &F&HO_ Our data for these ions are shown in Table 6 and Figs_ 4a and 4b_ Using higher electron mrapcurrents than and C3F&Owere also observed to be tonned by usuaf (0. I /A) CFI -, CIF,dissociative resonance processes_ Apmce potential data for these ions are also included in Tabk 6. QN,(C,F,CHO) was calculated using additivity ruks [ll] tobe --t6_6&0_6 eV. The superposition of resonances onsetting at ca. 3 eV suggests, as for C,F&HO, the decomposition of a metastabfe mokcular negative ion state at this energy.

(i) F-. Reaction (241, for which Al-&, (eV) is 25 eV, would seem to be the most likeIy reaction to account for F- ion formation at 3_2-1_0_1eV_ The second resonance peak, onsetting at 5.4_tO.l eV, is shown after deconvolution to be a double peak with further unresolved resonances evident in the trailing edge of -. the peak A large number of reactions could be written to fuhiil the enerGe requirements imposed by this broad peak_ Reaction (25) would however seem most likely to explain the increase in ion current at 5.4tO.1 eV since AH,i, is also 54 eV_ C,F,CHO+e

-+ F’+C,F,+CHO

(23)

+ F-+CF,+C,F,+CHO

(25)

(ii) CF,‘_ If the CF3’ appearance potential at l_9rfO.2 eV corresponds to reaction (26) then, using E(CF,) = 2_0+0_1 eV, D(CF,-C,F,CHO) may be estimated to be 3-9 +OA eV_ This may be compared with values of 4-3 +0_2 and 4_2fO_2 eV estimated for D(CF,-C,F,) [IS ] and D(CF,-C,F,CN) 1191rcspcctively. C,F,CHO+e

-P CF2- tC,F,CHO

(26)

--, CF,-+XF,+CHO

(27)

The reasonable accord between the observed and caIcuIated values for A(CF,-) of 6.5 and X9_i-O-6 eV respectively suzest that reaction (27) occurs in this energy rage_ (iii) CzFs-_

if E(C,F,) is assumed to be 2.2 eV [17, IS] then, for reaction (2S), Aff,i, (eV) = 32 eV; this reaction is therefore considered to be responsible for C,F5- formation at ca. 3 eV. C,F,CHO+e

+ C,Fs’+CFLtCHO

PSI

(iv) C,F,‘_ A single xesonance onsetting at 3_3fO_1 eV is noted for this ion_ If reaction (29) is responsible then approximately I_3 eV of excess energy is released in the reaction- As in the CISC?of CF3-/CF,CHO ad CzF5’/C2F,CH0, production of the fluoroaihq4 ion is associated with formation of vl%rationally excited CO. C,F#ZHO+e

+ C,F,‘+COtH

(29)

(v) C,F&OY_ As in the case of the analogous ion from CIFsCHO, this ion WLSobserved at 0 eV, presumably formed by an intramolecular reaction involving HF elimination. On the basis of reaction (30) AH~C,F,CO’) may be estimated to be - 13-S eV_ C,F&HOte

3 C,F&O-t HF

(30)

86

The following values (in eV) for heats of formationhave been used in our caIcufations:F O_S, HF -2.8, CF 2-5 [2f f. CH 6-2, CF= - f_SfO_I [21], CF3 -4.9&W QI], C,F, -2_2+0_7, C,F, -2.0 [X3], C,F, -6.7&G_1 1233,C,F5 -92 [Z?JzC,F,; - I I_3 031, C,F, - 13-3 1231,0 26, H 2.26, HCO -0.2, CO -1-15, CF3CH0 -S_2+0_5 (this work), CZF5CH0 - 12.4+0_5 (this work), C3FTCH0 - 16-620-6 (this worJc’)_ Except where indicated aII V&KS are taken

fro,rnRf_ 20_

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