[CONTRIBUTION FROM
THE
RESEARCH AND DEVELOPXEST DIVISIOX, HUMBLE OIL A N D RIWXNISG COAWANY]
The Decomposition of Neopentane under Electron Impact BY F.W. LAMPE AND F. H. FIELD RECEIVED SEPTEMBER 27, 1958 Appearance potentials are reported for the formation by electron impact of these various ions from neopentane: t-CJIs’, C3H5+, CzH3+, GHh+, C2Hs+,CH3+. Where possible ionization processes and/or heats of formation are deduced. The heat of formation of t-CdHs+ is found t o be 179 kcal./mole, which disagrees with the heretofore accepted value. The heat of formation of t-butyl radical is found t o be about 2 kcal./mole, which agrees with published values within the limits of our experimental accuracy. The new AHf(l-C4He+)value is used to make a re-interpretation cf the previously published value of the appearance potential of t-butyl ion from 3,3-dimethyl-l-butene, and from the re-interpretation one can deduce for the quantities AHf(C2Hs) and D(C2H8-H) the values 68 and 107 kcal./mole, respectively. The evidence concerning the neopentane decomposition modes provided by metastable ions is considered, and it is found that the metastable ion evidence and appearance potential evidence for the reactions producing the mass 27 and 39 ions are not in agreement. A discussion of the possible origins of the disagreement is given.
We wish to report here the results we have obtained recently on the energetics of the importarit modes of decomposition of neopentane under electron impact. Experimental Appearance potentials were determined using the conventional vanishing current method’ in our Westinghouse Type LV mass spectrometer. The electron voltage scale was calibrated by means of krypton. The pertinent operating conditions were: electron current = 3 pamp., ion repeller = 1.9 v., ion accelerating voltage = 600 v., ionization chamber temperature = ca. 270°, minimum detectable ion current = 1 X IO+5 amp. Five or six replicate determinations of the ionization efficiency curves involved in each appearance potential determination were made, and the appearance potentials listed in Table I are derived by averaging the critical potentials obtained from the several replicate runs. Acceptable agreement between replicate measurements was obtained. AS a matter of interest, we also include in Table I the relative abundances (percentage of total ionization) of the several ions in the neopentane mass spectrum as calculatcd from the .\PI Catalog of Mass Spectral Data. TABLEI ~\I’l’IIARhXCE P O T E N T I A L S O F hTEOl’ENTANE I O N S hlass Rel. intensity (70) AP (e.v.)
57 41 39
20 27 15
40.6 1G.8 5 .4 15.6 6.4 2.6
10.83 13.13 17.08 13.81 17.95 13.14, 2O.I:7
Results and Discussion The results for the several ions will be discussed individually in the following paragraphs. Jf = 57, t-C4H,+, A = 10.83 v. = 250 Kcal./ mole.-The ionization reaction producing tC4HQ+ from neopentane is undoubtedly neoC5HI2 = t-C4Hg+ CH3 (omitting the electrons involved for ease of writing). Using this reaction the observed appearance potential corresponds to a value of 179 kcal./mole for the heat of formation of t-butyl ion. Our appearance potential is higher than the value of A(t-C4HQ+)= 10.29 v. reported several years ago by Stevenson.2 The t-butyl ion heat of formation corresponding to this latter appearance potential is 163 kcal./mole. However,
+
( I ) See Field and Franklin, “Electron Impact P h e n o m e n a and t h e Properties of C.aseous Ions,” Academic Press, Tnc., N e w York, N . Y.. 1967.
( 2 ) 13. 1’. S t e v e n s o n , Disc. Favnday Soc., 1 0 , 38 (1951l.
very recently Lossing and co-workers have made a direct measurement of the ionization potential of t-butyl radical, and they obtained3 the value I(t-C4HQ) = 7.42 v. = 171 kcal./mole. Values for hHf(t-C4HQ) in the range 3-6 kcal./mole have been reported,‘ which when combined with Lossing’s t-butyl ionization potential give AHf(tC4H9+) values in the range 174-177 kcal./mole. The agreement with the value derived from our neopentane appearance potential is satisfactory and leads us to think that Stevenson’s value is somewhat low. This new value of AfIf(t-C4HQ+) enables us to modify a distressingly high value of D(CzH3-H) which was reported some years ago.4 This value is obtained from the measured appearance potential (11.30 v.) of the t-butyl ion produced from 3,3dimethyl-1-butcne according to the reaction CoH12 = t-C4H9+ C2H3. The observed appearance potential when combiiied with the new value of AlI~(t-C4€€~f) gives for AIIf(CzHa)and D(C2Ha-H) the values 68 and 107 kcal./mole. respcctively, which we a t one time would have thought to bc inore reasonable than the original values of SI atid 119 kcal./mole which result from the use of 165 kcal./mole for AEIf(t-C4H9+). However, vcry recently Reed arid Thornley5 adduce evidence in favor of the old values from rneasurenients of appearance potentials of ions from formaldehyde, acrolein and acetaldehyde. Thus it is unfortunately true that the electron impact value for the heat of formation of vinyl radical is still uncertain. ~ l= f 41, C,H5+, A = 13.13 v. = 303 Kcal./ mole.-A tabulation of the allyl ion heats of fortnation shows that the values fall into two distinct groups: namely, those around 220 kcal./mole and those in the range 230-235 kcal./mole. The ions in the first group are formed by direct ionization of allyl radical or by the decomposition of allyl halides; whereas the ions in the higher energy group are formed from all the other compounds studied. We shall assume that the heat of formation value applicable to the C3H5+ion formed from neopentane is 230 kcal./mole, and in this case t h e reactions producing the inn with calculated heats of reaction closest to the observed appearance potential are
+
(3)
F,P Lossing anrl T. U. de Srtusn, TRIT,J C l l l R N A l F. €1. Field. J . Chrm. P h y s , 21, 1506 (lS1.53).
fT,)
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