A photoionization study of the energetics of the C7H7+ ion formed from C7H8 precursors

Intematkmal Journal of Mass Spectrometry 0 EIsevier Scientific Pubhshing Company,

A PHtiIONIZATION C,E31’; ION FORMED

J-C_ TRAEGER

and

(Received

3 November

STUDY OF THE EtiRGETICS FROM C,H, PRECURSORS *

R-G.

Departmeni of Physical 3083 (Austrafia)

and Ion Physics, 27 (1978) Amsterdam -Printed in The

319-333 Netherlands

319

OF THE

McLOUGHLIN Chemistry.

La Trobe

University,

Buncioom,

Victoria,

1977)

The photoionization efficiency curves for C,H$, formed from several CvHs precursors, have been measured in the energy range 9-13 eV_ By using extended signal-averag-big techniques it has been possible to greatly increase the sensitivity-of ion detection, hence minimiz+g the effect of kinetic shift for these fragmentation processesAs a result, cationic heats of formation have been obtained which are in good agreement with other thermochemical data_

INTRODUCPION

Since Meyerson and co-workers [l] first proposed that the gaseous C,H; ions formed from toluene and cycloheptatiene have the tropylium -strutture, numerous studies have been made to obtain structural‘information about the C,W, ion. The elucidation of the structures and reaction pathways involved in the unimolecular decomposition of CtHs isomers alone has taken on the dimensions of a classic problem in gas phase molecular energetits. McLafferty and Winkler [Z] ~ using colJ.kional activation mass--spectra, have shown that the bulk of the C,W, ions formed at. 70 eV from toluene and cycloheptatriene precursors do in fact have the tropylium structure, but at low electron energies nearly equivalent proportions pf benzyl and tropylium ions are formed, following a rapid equilibration between C,H’, ions with toluene and cycloheptatriene structures. Dunbar 13 3 ~has also proposed that such a dynamic equilibrium is cons&e&with ‘his photodissociation results for the toluene parent ion. However, he observed the fraction of benzyl.ion to decrease to zero near the photodissociation threshold. This he.interpr&ed in terms of the activation energy for dissociation being lower-for the cycloheptatriene form of C7wg than- the toluene form, More recpa&ly,-Dewar and Landman [4] have proposed that the easiest route for _H-loss from C,w8 for _ .: ~. k Presented New

Zealand;

in pa&-at the ANZSMS (Received date refers

1977 Conferen&;~Ja.&ary 25-28.1977. to re&eipt by- Journal -Editor;)

Weliirigton;

320

both tohxene and cycloheptatriene is via the cycloheptatriene molecular ion to tropylium_ Appearance potential f/W) measurements can be used in conjunction with other thermochemical data as a means of determinin g the structures of ions at threshold. There are, however, two major difficulties associated with this method of characterizing non-reacting ions. The first is that the ionization potential (IF’) and AP data on \vhlch calculated enthalpies of gaseous ions depend are often of poor quality. The most reliable methods of determining A.P’s employ either photoionization or energy-resolved electron beams. Of a more fundamental nature is the need to correct data for any excess energy. This is comprised of the reverse activation energy and the so-called “kinetic shift”, which represents an amount of energy in excess of threshold necessary for the ion to react sufficiently fast to be detected_ In the case of C,W, formed from toluene and cycloheptatriene there is a large kinetic shift associated with the decompositions which has resulted in previous overestimates of the A_P’s. This kinetic shift can be minimized by increasing the ion source residence. time 151 and/or increasing the detection sensitivity of the instrument_ In this report we have examined a series of C,H8 isomers, all of which give rise to a strong C,H= ion peak in their mass spectrh using a photoionization mass spectrometer_ By means of extended signal-averaging techniques [63, we have been able to examine the threshold regions of the C,H; and C,W* photoionization efficiency (PIE) curves at high sensitivity and energy resolution_ This has enabled us to obtain further evidence pertaining to the structure of the C,H, ion formed at threshold by the unimohzcular decomposition of the isomeric C& parent ions. EXPERIMENTAL

The present ionization efficiency data were obtained with a photoionization mass spectrometer recently constructed in this laboratory_ The photon source was the many-lined molecular hydrogen spectrum which was generated by a DC capillary discharge in a lamp of similar design to that described by Nicholson [7] _ Typical operating pressures were 16 torr with a currentregulated discharge [S] of 400 mA_ Because the optical system -was completely windowless, the lamp was differentially pumped between the entrance slit and the main chamber of the monochromator by a 4311 ejector oil diffusion pump; the monochromator was pumped by a 6-in oil diffusion pump which enabled a vacuum of better than 2 X lo-’ torr to be maintained under opemting conditions_ The monochromator was of the Seya-Namioka type with the light being dispersed by a 1200 groo_~/mm, l-m radius concave grating, The grating was mounted on a ball race znd rotated by means of a precision micrometer coupled to a sine drive mechanism [9] to give. a linear wavelength scale_ A 96-pole stepping motor (Philips PD20B) with suitable gearing produced

321

_-

wavelength increments of ~/QUA; backlash was
Fig. 1. Block

diagram of the computer?nass

spectrometer

system.

322 The normal data collection procedure for an ion of selected mass was to accumulate data at a number of preset wavelengths_ For each-wavelength, a concurrent measurement of the ion count rate and photon intensity was made, with typical count times of between 0.5 and 1.0 s per point per scanAfter each scan the data was checked for both overflow and underflow before being added to the resultant time-averaged curve, At the completion of.eve.ry 10 such sweeps the monochromator wavelength was automatically scanned down to a region where no light was generated by the hydrogen pseudocontinuum and a backgroxund measurement of stray photons and ions made; This was used to correct the raw data used in the calcuIation of the relative photoionization efficiency curves, The hydrocarbon compounds were obtained commercially and used without further purification_ GC-MS analysis indicaved no impurities of signiIicance in this study_ RESULTS

AND

DISCUSSION

CTH',tnolectilarions Toluene_ The adiabatic ionization potential (IP) determined from the photoionization efficiency (PIE) curve for toluene is 8.82 + 0.01 eV (Fig. 2) which is in excellent agreement with previous determinations (Table 1). The low intensity tail in the pre-tbzeshold region is attributed to transitio_ns from thermally-populated, excited vibrational levels of the neutral molecule_ The intensity and separation, 540 f 50 cm-‘, of these “hot bands” is consistent with excitation of the “ring-breathing” mode of molecular toluene which

-w

TOLUENE

-

Fig. 2. Photoionization

PHOTON

ENERGY

efficiency

kV)

-

curve for C,H$

. .

from tduene.

(A, V)

(V)

9.86

8,36 8,70 9,33 ‘I

11,22,

8,3!? 9,31

/

Photolonization

This work (eV)

‘I

” 1)0,23 ’ lo,66

:9.05

8,36 a,71 ,: L ‘9863 10.76 11,29

8,03 8,63 ‘9,46 10,38 lo,88

9,30 11,22 11,41 11,93

882

’

Photoelectron**

’ \,,,

:

‘.

,’

1,

8862‘[Se],8842[40], 8,42 [4X],8,67 [32] ,, ,, 8,96 [40], 8469[42] 9,42 [39], 9,62 [40] 9,78 [IlO],9,66 [42] 11,ll [39],10,90 [40] ) 11;60 [40], 11826[42],

lo,73 [39]

9,31 [39]

8,47-8866 [32], 8840[39], 8.20 [36]

!,

/

,/,I

‘\

,,

1

,,

,,

‘,

‘, ‘,

,,,,I, “,I

!‘: 1:

8,80-9,23 [32], 8.72 [13], 8.82 [33], 8182[34], 8,89 [36] ,‘,, 983[36] :: 9,24 [13],9,31 [33],9,13 [36], 9,3 (371 lo,90 [13], 11.21 [33], 11,20 [34], l:l.O [38], Ill6 [14] :‘, 11838[13], 11,40 [33], 11.4 [37], 11,3,[38] ,, ,,’ 11887[X3],12.0 [33]

Previousdotcrminntions(eV)

,

8’ ,’ ‘, * (A) P’adiabaticionizationenergy,(V) q verticalionizationenergy, I ,I “, **‘Dataobtained using’s 180: hemispherical’olectrostatic analyzer,with standardHe(I)/Hc(II)dischargelamp’@ separately,pumped ‘3, ionizationand analyzerchambers,

(? 1,&Heptadiyne 1st (A) ” ,‘(v) , 2nd (V)

,$!ycioheptatriene 1st (A) ,) (v) !nd (A) (v) ,?rd (A) ‘(V) 2,G.Norbornndieno ‘,lst I (A) (V) 2nd’ (A) (v) 3rd’ (A)

4th (V)

,‘,

2nd (A) (V) ijrd (A)

id

Toiuetie

Ionizationenergy *

C$Hsionlzationenergies

TABLE1

324

- occurs at 521cm-' [12]_ Tw o vibrational progressions are observed in the post-threshold region, one being in the“ring-breathing"modeandtheother in theringdeformationmode,1650-t 50~x1~',whichoccursat~l600cm-' in the neutral molecule [12]_ The previous assignment of the “ringmay breathing" mode asan “X-sensitive" modebyDebiesandRabz&.is[13~ have overestimated the coupling between the C-CI-& stretching mode and the “ring-breathing" mode, as force constant calculatiork]12] indicate that couplingonlyoccurstoaveryhmitedextentinthemolecule_ The onset ofthe second and third electronic states ofthemolecularion can be seen as distinct steps in the PI.. curve at9:31* O.OleVand11.22~ with our photoelectron data O-01 eV respectively, in good agreement (Table l), There is a broad band of peaks between lo-7 and11.2eVwhich is probably a series of unresolved preionizinglevels converging to the third electronicstate[14]_

Cycloheptatiene. The adiabatic IP for cycloheptatriene is not easily deter= 3). Unliketolueneitappearsthationization minedfromthePIEcurve(Fi,_ of cycloheptatrieneto the C,H', molecular ion in its groundelectronicstate results in a displacement of the potential minimum relative to the ground electronic state for the neutral molecule. Consequently, the vibrational overlap integral (F'rank~ondon factor) forthetransitions from the molecule ground electronic statetothecorrespondingstateoftheionisnon-zero for a large number ofvibrationallevels oftheion.Afurthercomplicationis thepresenceofhotbandsinthethresholdregion. Usingthe method of interpretation ofthresholddatadescribedby Guyon

.-.-* . _--

CYCLOHEPTATRIENE

8-c

a-6

8-B - PHOTON

9-o

9-L

S-2

ENERGY(cV)

s-6

s-6

c-

IO-0

-

Fig.3.PhotoionizationefficiencycurveforC+~~

frbmcycloheptatriene.

325

and Berkowitz 1151 we derive an adiabatic IP of8.29-+ O.OleV.However, if the O-O transition has a very small probabiliw then the value~we havzobtained will only representanupperlimit_WeestimatetheverticalIPtobe 8.525 O.OleV- which is in excellent agreement with ourPES value of .-I S-53 eV_ The vibrational progression observed in_thethreshold region oft&C& PIE curve(Fig. 3)has beenverified in our PES study, It appears that there are at least two progressions,one with a-spacing of 223 + 50cmy' and the. and otherwith~a spacing of 483+ 50cm:'_- By reference to the in&red Raman spectra for cycloheptatriene [16] we have assigned these two progressions to the totally symmetric ring deformation modes of the neutral I_ The.peakswhich occurinthe molecule which occur at223 and 421cmPIE curve between 8-7 and 9_1eV are probably due to preionization~of Rydberg series which converge on the second-electronic state oftheion at 9.222 O.OleV_ 2,5-norbornadiene_ The threshold region of the CIH', PIE curve for norbornadiene (Fig_4)is very similar in shape to that found for cycloheptatriene, There is a long vibrational progression which is particularly pronounced in the firstband of the photoelectron spectxum, This progression,whichhasa separation of381* 50cm-I, we have assigned tothe "wing-flapping"mode ofnorbomadienewhichoccursat423 cm-' intheneutimolecule[17]_An upper limit to the adiabatic IP is 8.35% O_OleV, although it is possibly lower than this because oftherelativelysm&O-Otransitionprobability_ Wehavedetermin e&the verticalIP tobe8.70* O.OleV,in goodagreementwith out PES value of 8.71 eV_ Thestep in the PIE curve commencingat

8-2

8-4

6-6

6-s

- p*TON

Fig_ 4. Photoionization

. .

s-o

9-t

ENERGY -.

(eV)

efficiency

9-L

9-6

48

-

curve for C,s

from 2,5-norbo&diene_

._

326

9.33eV is attributed to the onset of ,&ateoftheC,-~4 ion.

ionization to the second

electronic

1,6-hepfadiyne-

No parentmolecuiarionwas observedineithektheelectxon~ or photon-impact ionization mass spectrum of 1,6-heptadiyne, the C& fragment ion being the base peak, However, a field ionization (FI) mass spectrum ckarly indicated the existence ofaparentmolecularion Inaddition there was welkiefined vibrational structureinthe firstband ofthe photoelectronspectrum.Thisthereforeimpliesthationizationisnottakiug place to a dissociative region of the molecular ground electronic state but rather to a bound region, followed by a rapid vibrational predissociation [lS]_ The observation of a parent ion in the FI massspectrumweattribute to the population by FI ofaregion ofthemokcukriongroundstate not accessible by electron or photon impact. A similar observation has been madeforneopentane [19]_ The adiabatic ionization energy was foundto be9.85eV from ourPES data with a vertical value of 10_23eV_ The second vertical Ip was not as easiIy determined however, because of interference from the firstband-A valueof10_66eVwasobtained_

Toluene, cyclohepfatriene and 2,5-norbornadiene, Because of the similarity between the C& ikagmention PIEcurves fortoluene,cycloheptatrieneand norbomadiene (Figs_5-7) they will be discussed together_ All three curves

TQLUENE

10-S

IO-8

C,Hf

11-O -PHOTON

n-2

11-L

n-5

11-8

12-0

l2

ENERGY CeV) -

Fig. 5_ Photoionization efficiency curve threshold region on an expanded scale.

for

C&

from

toluene.

The

inset

shows

the

327

CY~LOHEPTATRIENE.

.

.-

c,Hf

. _ .- d

___

~_

_. . . . . . --

---

_....r

-.-

.--

-=---

--

d

S-2

9-L

S-6

S-8

-PHOTON

lo-2

104 ENERGY&V)

l(

lo-6

lo-4

-

Fig. 6_ Photoionization efficiency curve shows the threshold region on an expanded

for C,H$ scale.

from

cycloheptatriene.

The

inset

exhibit the samelowintensi~tailinthethreshoIdregionwhich,athigher energies,Ieadsintoasharplyrisingsection. The presence of a low intensity tail may be evidence for vibrational predksociation ClS] fromtbehighervibrationallevelsoftbemolecukuion ground electronic state, These higher vibrational levels must-be isolated with respect to vibrational relaxationon-themassspectrometertimescale

2.5 NORBORNADIENE

w M¶-o)r

.

C,H;

--

‘-

. . .. -* _.=.. : ~2. . .r

-_

~2*3¶-Lss c I .t .. ._...

--s-2

-94:

s-6

98 --

-

‘xl-0 PHOTON

.-

.a.-

ENERCY-(*a

._

11-2

NJ-2 -

Fig_ 7_ Pht&ionizati& efficiency--curve for C& from shows the thre&oId region 06 an expanded scale_ --.

2,5-norbomadiene:

-.

-1

The

inset

328

(-ps)_ The relatively low intensity of. this region- of the curves- can be attributed to the low values expected for the Frank-Condon (F-C) factors for transitions to the higher vibrational levels, where the displacement between molecule and ion ground electronic states is small, In addition, the kinetic factors involved in the dissociation process make the probability of dissociation at threshold energies small on the mass spectrometer time scale (kinetic shift) _ The sharply rising section of the three fragment curves correlates with the third electronic state of the molecular ion for cycloheptatriene and norbornadiene and with the fourth electronic state for toluene (Table 1) This may be the result of a vibronic relaxation of the excited state to a lower electronic state of the ion, probably the ground state_ In this case the increased C7H; photoionization efficiency could be the result of populating vibrational levels of the molecular ground state, above the dissociation limit, which are inaccessible via direct ionization_ The F-C factors for this vibronic relaxation process may differ substantially from those for direct ionization to the same levels of the ground state of the -ion_ They will also depend on the relative separation and displacement of the electronic states involved in the transition, and will be strongly dependent on the anharmonicities of the communicating vibrational modes: A large proportion of the transferred energy is therefore accepted initially by the most anharmonic vibrational modes, particularly the C-H stretching mode [20] _

The relaxation rate depends on the vibronic coupling between the quasidegenerate states which, to a first approximation, is given by the product of the electronic tram&ion moment, the vibrational overlap integral (F-C factor) and the fmal state degeneracy_ If the electronic energy gap between the states is so large that the vibrational levels of the lower (fmal) state form a quasi-continuum, then the degeneracy factor will be given by the vibrational state density_ In some cases the electronic transition may be symmetry-forbidden, causing the vibronic relaxation rate to be slow_ We consider that the experimental C,W, PIE curves are consistent with this approach_ The assignment of electronic states for the toluene molecular ion is B1, AZ, Al, B1, with the large increase in the C,w, curve corresponding to the symmetry-allowed BI-BI transitions_ The fact that we only see a small increase at 11.22 eV (Fig. 5) can be attributed to the symmetryforbidden nature of the A,-_B1 transitions_ Similarly for cycloheptatriene (Br, AZ, 23,) and norbornadiene (Bz, Al, B=), we are observing an increased production of C,H: ions corresponding to the onset of symmetry-allowed B,-B, and Bt-& relaxations respectivelyThe AP’s determined for the C,Hf fragment ions are 10.71 f 0.03 eV (toluene), 9-36 f 0.02 eV (cycloheptatriene) and 9-25 f O-03 eV (norbornadiene) These values are considerably lower than most previous determinations (Table 2), reflecting the relatively large kinetic shifts associated with the decompositions_ The observed AP for toluene, however, is in excellent agreement with the results of Gordon and Reid [ 51, who obtained a limiting

I’,, ‘I

$$oheptatrione 9,26 b,86 I

‘,a,:,

2,6~N&&nadicn~~ S’S,0[h4j ,I 94;7 1451 ” l,f;H~?ptidiyne

.‘,

$36

/, :

,,

lo,71 ,

43,90 [43]:]

.,

,’

’

:

11.99 [h’3]’

,,

Tolue-0”: 0’ j,’

,,‘I

This work ‘AH* jkcal,molW1) AP(C,H,*) (eV)

,‘Compound

~,CIH1+.Appearanca pdtentialsand heats of formation

TABLE2

,”

219,2 269,8

20786

20689

AH&,H,‘) (kcal mol”‘)

I.

’

I’

AH&H,*) (kcal niol-*)

11,80 [32] 232 207 10,70 [G] 11,66 [14] ’ 226 lO,l-lo,73 [32] : 225-239 610,O [36] (222 9,6 [32] 227 ,-

MC,H,*) (eV)

Previousdeterminations

8’

’

value of 10-70 eV at ion source residence times in excess of 900 ps_ Combining the heats of formation for the neutral species in the reaction C7Hs + kv + C,H; + I3 + e + (excess energy) with the observed AP's, and neglecting any excess energy contribution, results in heats of formation for C,H: of 206-g f O-7, 207-6 + O-5 and 219.2 t O-7 kcal mol-L from toluene, cycloheptatriene and norbomadiene respectively (Table 2) The kinetic energy release accompanying the loss of H from toluene and cycloheptatriene molecular ions has been measured from metastable studies [2lf to be 4.0 kcal mol-’ which probably represents an upper limit to the reverse activation energy. A recent MIND0 calculation 143 indicates that the reverse activation energy is indeed small, giving an estimate of only 1.4 kcal mol-‘_ Whilst no direct evidence is available, we consider that the reverse activation energy for the analogous process in norbornadiene is probably of a similar magnitude_ Lossing [22] has measured the LP for benzyl radi&l by mono-energetic electron impact to be G7.27 eV which is in good agreement with a recent PES value of 7.20 + O-03 eV [23]_ If A& (benzyl) is taken as 45 kcal mol-’ [24], we obtain AHt (benzyl ‘) = 211 kcal mol-’ which is lower than either a recent ab initio calculation of 217.1 kcal mol-’ [25] or a MIND0 calculation of 220-4 kcal mol-’ [26]_ It is also -lower than the value of 219 f 4 kcal mol-’ obtained from an ion cyclotron resonance study of the structure of C,W, ions [27]. It thus appears that, from an energetic view point, the C,l% ion formed at threshold for toluene and cycloheptatriene cannot have the benzyl structure_ The heat of formation for C,W, formed from norbomadiene is consistent with the upper values calculated for the benzyl ion, However, we cannot rule out the possibility that the C,H’, ion has a bridged structure, similar to that of the parent ion [2] _ It should also be noted that we may have underestimated the excess energy involved, in which case the calculated ZL& would be considerably lower, possibly approaching that for the qmuneizic tropylium ion_ Our calculated AHHr’s for C,H: ions formed from toluene and cycloheptatriene are not only self-consistent but also in good agreement with the theoretical calculation for tropylium ion by Abboud et al_ [25] (207-g kc&l mol-‘). Good agreement also exists with the ionization energy measurements for the tropyl radical [2S,29] (6.24 eV) which give a calculated value of 209 kcal mol-I_ However, the AH, (tropyl radical) used in this calculation could be somewhat lower than the estimate of 64.8 kcal mol-’ [30]. The recent MIND0 calculation of A& (tropylium) = 195.6 kcal mol-’ [ZS] isatvarian ce with all of the above results, Dewar and co-workers [ZS] have sugges@d that the difference between their results and those obtained from the tropyl radical Ip could be due to a large difference between the vertical and adiabatic values. Neither Thrush and Zwolenik [ZS] nor Elder and Parr [29 J observed vibrational structure in their data to support this proposaL

331

1.6 HEPTADIYNE

48

C,Hf

9-9 - PHOTON

100 10-l ENERGY (eY) -

Fig_ 8_ Photoionization efficiency curve for the threshold region on an expanded scale_

lo-3

lo-2

C,wy

from

1,6-heptadiyne.

The

inset

shows

1,6-heptadiyce. From the C,H’, PIE curue for 1,6-heptadiyne (Fig. 8) we estimate AP (C,W,) to be 9-85 i O-01 eV_ This value is in excellent agreement with the adiabatic Ip obtained from our PES study of 9.85 f 0.01 eV_ As discussed above, we believe that ionization of the molecule takes place to a predissociative region of a bound ground electronic state of the ion_ The fact that this state is bound, obtained from FI data, indicates that the displacement of the potential minimum of this state with respect to the neutral molecule is large. This is, in general, indicative of large structural changes in going from molecule to ion_ The stability of the fragmentation products appears to be much greater than that of the molecular ion, in which case the calculated AHf (C&) = 269.8 kcal mol-’ reflects the necessity to proceed to the fragment ion via the energetically unfavourable molecular ion, rather than a true AHt (C,H’,)_ It is interesting to note that, whereas 1,6-heptadiyne, l,‘?-octadiyne and 1,8-nonadiyne all undergo a very rapid fragmentation in the mass spectrometer (no parent ions), the homologous compound 1,5-hexadiyne has been shown to have an unusually slow rate of loss of e [31]. This would appear to be consistent with the formation of a very stable‘ cyclic ion for the three larger compounds, which is not energetically possible in the case of the hexadiyne, ACKNOWLEDGEXMENTS

The authors wish tothank Dr. P.J_ Derrick and Mr, M. Darcy for providing the FI mass spectrum of 1,6-heptadiyne; Mr_- E_ Nagy-Felsobuki for record-

332 the pho-loelectron spectra and Mr, ~A_ Graddon -who- performed the GC-MS anaty;es_ They are also greatly indebted to the members 6f the mechanical and electronic workshops for their technical assistance in the c&nstruction of the photoio nization mass spectrometer, and to Professor JD_ Morrison for his many constructive comments throughout the course of this project-

ing

REFERENCES P.N_ RyIander. S_ Meyerson and H-M. Grubb, J_ Am_ Chem. Sot-, 79 (1957) 84296 (1974) 5182. F-W_ LMcLafferty and J. Winkler, J. Am. Chem_ Sot., R-C. Dunbar, J. Am. Chem_ Sot., 97 (1975) 1382. M.J_S_ Dewar and D_ Landman, J_ Arn_ Chem. Sot_, 99 (1977) 2446s S.M_ Gordon and N-W_ Reid, Int_ J_ Mass Spectrom. Ion Phys., 18 (1975) 379_ R-G_ Dromey, J-D_ Morrison and J-C_ Traeger, Int_ J_ Mass SpectromIon Phys-, 6 (1971) 57_ Nicholson, AppL Opt_, 9 (1970) 1155. 7 A.J.C. J_ Phys_ E. in press. 8 J.C_ Traeger and G-W_ Haertel, Sci_ Light, 13 (1964) 45. 9 K_ Ando, Y_ Tanaka and J-C_ Larrabee, J_ Chem_ Phys., 39 (1963) 902_ 10 R-E_ Huffman, J_ Opt_ Sot_ Am_. 54 (1964) 6_ 11 J_A_R_ Samson, and H-J_ Bernstein, Can_ J_ Chem., 35 (1957) 911_ 12 J-K_ Wiimshurst Debies and J-W_ Rabahxis, J_ EIectron Spectrosc RelatPhenom-, 1 <1972/73) 13 TP_ 355_ and F-1. Vilesov, Khim. Vys. Energ., 2 (1968) 107_ 14 M.E. Akopyan and J_ Berkowitz, J_ Chem_ Phys_. 54 (1971) 1814_ 15 PM_ Guyon . 82 (1960) 1876_ 16 M-V_ Evans and R-C_ Lord, J_ Am_ Chem_ Sot, and N-A_ Kuebler, J_ Chem_ Phys_. 44 (1966) 2664_ 17 lM_B_ Robin Eiectronic Spectra of Polyatomic Molecuies, Van Nostrand, New York, 18 G, Henberg, 1966. pp_ 455_ Reckey, in R-M_ Elliott (Ed.), Advances in Mass Spectrometry, Vol. II, Perga19 H.D. mon, Oxford, 1963, p_ 1_ 20 E-W_ Schlag, S_ Schneider and SF_ Fischer, Ann_ Rev. Phys_ Chem., 22 (1971) 465_ 21 R-G_ Cooks, J-H. Beynon, M_ Bertrand and M_KI Hoffman, Org. Mass_ Spectrom-, 7 (1973) 1303_ Can. J_ Chem., 49 (1971) 357_ 22 F-P_ Lossing. 23 J-L_ Beauch-zunp, unpubiished results 24 D-M_ Golden and S-W_ Benson, Chem_ Rev_, 69 (1969) 125_ W-J_ Hehre and R-W_ Taft, J_ Am_ Chem_ Sot_, 98 (1976) 6072_ 25 J_M_ Abboud, J. Am_ Chem. Sot., 99 (1977) 372_ 26 C_ Cone, M-J-S_ Dewar and D_ Landman, private communication_ 27 J.A_ Jackson, and J-J_ Zwolenik. Discuss_ Faraday Sot., 35 (1963) 196. 28 B_A_ Thrush 29 F_A_ Elder and AC_ Parr* J_ Chem_ Phys., 50 (1969) 1027. 30 G_ Vincow, H-J_ Dauben, Jr., F-R_ Hunter and W-V_ Voiiand, J_ Am. Chem_ Sot., 91(1969)2823. 31 M-L_ Gross and R-J_ Aemi, J_ Am_ Chem_ Sot_, 95 (1973) 7875. 32 J-L_ Franklin, J-G_ Diiiard, H-M_ Rosenstock, J.T. Herron, K. Draxl and F-H_ Field, Ionization Potentials and Heats of Formation of Gaseous Positive Ions, NSRIX-NBS26, National Bureau of Standards, Washington, 1969_ 33 W-L. Stebbings and J-W. Taylor, Int- J. Mass Spectrom. Ion Phys., 9 (1972) 471_ 34 M.J.S Dewar and SD..Worley, J. Chem. Phys., 50 (1969) 654. and F;I_ Viiesov, Zh_ Fiz Khim_, 40 (1966) 125. 35 ME_ Akopyan

333 kD_ Baker, D-P_ May and D-W_ Turner, J. Chem- Sot. B., (1968) 22. M_ Kkssinger, Angew. Chem_, 84 (1972) 544. J-P_ Maier and D-W. Turner, J_ Chem. Sac-, Faraday Trans- 2.69 (1973) 196N_ Bodor_ M.J.S. Dewarand SD. Worlev. J. Am. Chem. SOL 92 (1970119. D_A_ De&ho and AJ. Yencha, J. Chem_Phys., 53 (1970) 4536. _ _ DA Dembo and M-A Ei-Sayed. J_ Chem_ Phys_, 52 (1970) 2622. P_ Bischof, J-A_ Ha&mall, E_ Heilbronner and V. Homung, Helv. Chim. Acta, 52 (1969) 3 745. 43 J-D_ Cox and G. Pilcher, Thermochemistry of Organic and Organometallic Compounds, Academic Press, New York, 1970_ 44 R_ Walsh and J-M_ Wells, J_ Chem_ Thermodye, 7 (1975) 149_ 45 S-W_ Benson and J.H. Buss, J_ Chem_ Phys_. 29 (1958) 546_ 36 37 38 39 40 41 42