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ΔH1(t-C4H9) from dissociative electron capture measurements
International Journal of Mass Spectrometry and Ion Physics. 36 (1980) 249-25 1 Elsevier Scicntitic Publishing Company, Amsterdam - Printed in The Netherlands
249
Short Communication A_Hf(t-C,H,) FROM MEASUREMENTS
JIA-SHEN
WANG
Department (Received
l
DISSOCIATIVE
ELECTRON
CAPTURE
and J.L. FRANKLIN
of Chemistry,
Rice
University,
Houston,
Texas 77001 (U.S.A.)
14 April 1980)
The heat of formation of the t-Duty1 radical has been of interest to chemists for many years. Values of AIYf(t-C,H,) reported by several investigators vary horn 6.8 Cl] to 12.9 [2] keal mol-‘. Two principal methods have been employed: measurement of the activation energies of reversible steps in halogen abstraction from alkyl halides [1,3] and measurement of the ionization energy of the radical, this being deducted from the heat of formation of the corresponding carbonium ion [4-6]. The latter has been determined either from the appearance energy of the carbonium ion from a suitable compound or from the proton affinity of iso-butylene obtained preferably by equilibrium proton excahnge with a second compound. All of these methods require two measurements with the accompanying increased uncertainty. We have employed a method that has been developed and used in this laboratory to measure the heats of various ionic dissociation reactions. The average kinetic energy of a packet of ions of a given mass in a TOF mass spectrometer can be determined from the full width at half maximum (FWHM) of the mass peak of the ion in question [7] and by comparison with that of ions of known energy formed by other processes [8,9]. The method employed in this study is described in ref. 10. The kinetic energy of &he fragment ion E; is related to the average kinetic energy released in the dis-
sociation S; by the equation -7 =i =-..miL.--.-
(@lT) + _?!!I_._- Et H2i+??Z,
(1)
where mi and m, are the masses of the ion and neutral fragments, respectively. This equation assumes that two fragments only are formed. It has also been shown that _- - E&t et l
(2)
an
where
E: , t is the excess
vibrational
and translational energy, rz is the number
* Resent address: 93 Shadow Ridge Rd., Stanford, OOZO-7381/80~000~000/$02.25
@ 1980
CT 06905,
Elsevier Scientific
U.S.A. Publishing Company
of vibrational empirical Therefore,
modes
in the precursor ion before
dissociation,
constant_’ Thus, Et should increase linearly when Et is zero, czat is zero and 4 is {mi/(mi
with +
m,))
and cy is an
excess energy. - {(3/2)RT)_
The election energy at which C$ and hence S; is zero is thus the heat of the dissociation reaction_ In an earlier work using this method we determined the heats of the reactions CO+e+0-+C(3P)
(3)
NO
(4)
+ e -+ O- + N(*D)
respectively_ The known thermochemical to be 223.7 and 173-O kcal mol-‘, Thus, good agreement is obtained values are 222.9 and 171.5 kcal mol-‘_ between values determined by this method and known thermochemical results. It has been observed in many processes that ci and & increase linearly with electron energy over the greater part of the resonance as is predicted by (1) above. We have found this to be true for the reaction t-&H&l
+ e_’ + Cl- + t-&H9
(5)
as is shown in Fig_ 1. It wil’l be observed that both Ci and iTtare linear functions of electron energy and they both extrapolate to a thermal onset of kcal mol-‘. A second set of mea-0.39 eV, corresponding to AEXa = 3.0 surements (not shown) exhibited similar behavior and yielded BwR = -6.8 kcal mol-* _ Taking an average of -7.9 kcal mol-l for these two values, and
using A&It-C4H9Cl)
and arrf(Cl-)
values of -43.7
[11]
and -54.6
[12]
respectively, mf(t-C4Hr,) is computed to be 3.0 kcal mol-‘. kcal mol-I, This is lower than several values that have been published recently. In recent McLaughlin and Traeger [4], using photoionization, determined studies, and Houle and Beauchamp 153 and AJYf(t-CGH”,) to be 162.1 kcal mol-‘, using photoelectron spectroscopy, have determined Koenig et al. [S], 1,(t-C,H,) to be 6.7 and 6.85 eV, respectively_ Combining these ioniza-
1.0
r
ELECTRON
ENERGY.
t:V
Fig. 1. FZelativeintensity and kinetic energy of cl- as a function of electron energy_
251
\
tion potentials with the above AMf(t-C,HG) value yields AH,(t-C.+I&) to be 6.7 and 4.1 kcal mol-‘, respectively. Benson [ 33 gives Mf(t-CJ19) as 8.4 Thus, our value agrees reasonably well wit.h the lower value for kcal mol-l. Mdt-C.&I,) given above, but agrees only poorly with the two higher values. We cannot be certain which are the best results. However, since our value results from only a single kind of measurement, WC arc inclined to prefer it. It can be deduced from Fig. 1 that the ratio F&z& = 0.15, and a value of 0.20 was found in the other set of measurements quoted above. The quasiequilibrium theory predicts that ZJE:,~ = l/n and for t-C4H9C1 should be 0.028. Thus, the kinetic energy released is more than five times that predicted by theory. On the other hand, about 0.85 of the excess energy was distributed into vibration, as would seem reasonable in view of the fact that the t-C4H9 portion of the molecule would tend to change from a tetrahedral to a planar configuration as the Cl separated. Similar behavior has been observed in several similar processes. ACKNOWLEDGMENT
We express our appreciation port of this work.
to The
Robert
A. Welch
Foundation
for sup-
REFERENCES 1 2
J.A. Kerr, W. Tsang,
Cinem. Rev., Int. J. Chem.
66 (1966) 465. Kinet., 10 (1978)
821.
3 S.W. Benson, Thermochemical Kinetics, 2nd edn., Wiley, New York, 4 5 6 7 8 9 10 11 12
1976. R.G. McLaughlin and J.C. Tracger, J. Am. Chem. Sot., 101 (1979) 5791. F.A. Houle and J.L. Beauchamp, J. Am. Chem. Sot., 101(1979) 4067. T. Koenig, T. Balle and W. Snell, J. Am. Chem. Sot., 97 (1975) 662. J.L. Franklin, P.M. Hicrl and D.A. Whan, J. Chem. Phys., 47 (1967) 3148. M.A. Haney and J.L. Franklin, J. Chem. Phys., 48 (1968) 4093. P.W. Harland, J.L. Franklin and D.E. Carter, J. Chem. Phys., 58 (1973) 1430. J.-S. Wang and J.L. Franklin, Int. J. Mass Spectrom. Ion Phys., 36 (1980) 233. J.D. Cox and G. Pilcher, Thermochemistry of Organic and Organometilic pounds, Academic Press, New York, 1970. H.M. Rosenstock, K. Draxl, B.W. Steiner and J.T. Herron, J. Phys. Chem. Ref. 6 (1977) Suppl. 1, I 779.
ComData,
249
Short Communication A_Hf(t-C,H,) FROM MEASUREMENTS
JIA-SHEN
WANG
Department (Received
l
DISSOCIATIVE
ELECTRON
CAPTURE
and J.L. FRANKLIN
of Chemistry,
Rice
University,
Houston,
Texas 77001 (U.S.A.)
14 April 1980)
The heat of formation of the t-Duty1 radical has been of interest to chemists for many years. Values of AIYf(t-C,H,) reported by several investigators vary horn 6.8 Cl] to 12.9 [2] keal mol-‘. Two principal methods have been employed: measurement of the activation energies of reversible steps in halogen abstraction from alkyl halides [1,3] and measurement of the ionization energy of the radical, this being deducted from the heat of formation of the corresponding carbonium ion [4-6]. The latter has been determined either from the appearance energy of the carbonium ion from a suitable compound or from the proton affinity of iso-butylene obtained preferably by equilibrium proton excahnge with a second compound. All of these methods require two measurements with the accompanying increased uncertainty. We have employed a method that has been developed and used in this laboratory to measure the heats of various ionic dissociation reactions. The average kinetic energy of a packet of ions of a given mass in a TOF mass spectrometer can be determined from the full width at half maximum (FWHM) of the mass peak of the ion in question [7] and by comparison with that of ions of known energy formed by other processes [8,9]. The method employed in this study is described in ref. 10. The kinetic energy of &he fragment ion E; is related to the average kinetic energy released in the dis-
sociation S; by the equation -7 =i =-..miL.--.-
(@lT) + _?!!I_._- Et H2i+??Z,
(1)
where mi and m, are the masses of the ion and neutral fragments, respectively. This equation assumes that two fragments only are formed. It has also been shown that _- - E&t et l
(2)
an
where
E: , t is the excess
vibrational
and translational energy, rz is the number
* Resent address: 93 Shadow Ridge Rd., Stanford, OOZO-7381/80~000~000/$02.25
@ 1980
CT 06905,
Elsevier Scientific
U.S.A. Publishing Company
of vibrational empirical Therefore,
modes
in the precursor ion before
dissociation,
constant_’ Thus, Et should increase linearly when Et is zero, czat is zero and 4 is {mi/(mi
with +
m,))
and cy is an
excess energy. - {(3/2)RT)_
The election energy at which C$ and hence S; is zero is thus the heat of the dissociation reaction_ In an earlier work using this method we determined the heats of the reactions CO+e+0-+C(3P)
(3)
NO
(4)
+ e -+ O- + N(*D)
respectively_ The known thermochemical to be 223.7 and 173-O kcal mol-‘, Thus, good agreement is obtained values are 222.9 and 171.5 kcal mol-‘_ between values determined by this method and known thermochemical results. It has been observed in many processes that ci and & increase linearly with electron energy over the greater part of the resonance as is predicted by (1) above. We have found this to be true for the reaction t-&H&l
+ e_’ + Cl- + t-&H9
(5)
as is shown in Fig_ 1. It wil’l be observed that both Ci and iTtare linear functions of electron energy and they both extrapolate to a thermal onset of kcal mol-‘. A second set of mea-0.39 eV, corresponding to AEXa = 3.0 surements (not shown) exhibited similar behavior and yielded BwR = -6.8 kcal mol-* _ Taking an average of -7.9 kcal mol-l for these two values, and
using A&It-C4H9Cl)
and arrf(Cl-)
values of -43.7
[11]
and -54.6
[12]
respectively, mf(t-C4Hr,) is computed to be 3.0 kcal mol-‘. kcal mol-I, This is lower than several values that have been published recently. In recent McLaughlin and Traeger [4], using photoionization, determined studies, and Houle and Beauchamp 153 and AJYf(t-CGH”,) to be 162.1 kcal mol-‘, using photoelectron spectroscopy, have determined Koenig et al. [S], 1,(t-C,H,) to be 6.7 and 6.85 eV, respectively_ Combining these ioniza-
1.0
r
ELECTRON
ENERGY.
t:V
Fig. 1. FZelativeintensity and kinetic energy of cl- as a function of electron energy_
251
\
tion potentials with the above AMf(t-C,HG) value yields AH,(t-C.+I&) to be 6.7 and 4.1 kcal mol-‘, respectively. Benson [ 33 gives Mf(t-CJ19) as 8.4 Thus, our value agrees reasonably well wit.h the lower value for kcal mol-l. Mdt-C.&I,) given above, but agrees only poorly with the two higher values. We cannot be certain which are the best results. However, since our value results from only a single kind of measurement, WC arc inclined to prefer it. It can be deduced from Fig. 1 that the ratio F&z& = 0.15, and a value of 0.20 was found in the other set of measurements quoted above. The quasiequilibrium theory predicts that ZJE:,~ = l/n and for t-C4H9C1 should be 0.028. Thus, the kinetic energy released is more than five times that predicted by theory. On the other hand, about 0.85 of the excess energy was distributed into vibration, as would seem reasonable in view of the fact that the t-C4H9 portion of the molecule would tend to change from a tetrahedral to a planar configuration as the Cl separated. Similar behavior has been observed in several similar processes. ACKNOWLEDGMENT
We express our appreciation port of this work.
to The
Robert
A. Welch
Foundation
for sup-
REFERENCES 1 2
J.A. Kerr, W. Tsang,
Cinem. Rev., Int. J. Chem.
66 (1966) 465. Kinet., 10 (1978)
821.
3 S.W. Benson, Thermochemical Kinetics, 2nd edn., Wiley, New York, 4 5 6 7 8 9 10 11 12
1976. R.G. McLaughlin and J.C. Tracger, J. Am. Chem. Sot., 101 (1979) 5791. F.A. Houle and J.L. Beauchamp, J. Am. Chem. Sot., 101(1979) 4067. T. Koenig, T. Balle and W. Snell, J. Am. Chem. Sot., 97 (1975) 662. J.L. Franklin, P.M. Hicrl and D.A. Whan, J. Chem. Phys., 47 (1967) 3148. M.A. Haney and J.L. Franklin, J. Chem. Phys., 48 (1968) 4093. P.W. Harland, J.L. Franklin and D.E. Carter, J. Chem. Phys., 58 (1973) 1430. J.-S. Wang and J.L. Franklin, Int. J. Mass Spectrom. Ion Phys., 36 (1980) 233. J.D. Cox and G. Pilcher, Thermochemistry of Organic and Organometilic pounds, Academic Press, New York, 1970. H.M. Rosenstock, K. Draxl, B.W. Steiner and J.T. Herron, J. Phys. Chem. Ref. 6 (1977) Suppl. 1, I 779.
ComData,