J. Phys. Chem. 1993,97, 8595-8601
8595
Site Selectivity in the Proton-Transfer Reaction from Alkane Radical Cations to Alkane Molecules. Selective Formation of 1-Octyl Radicals by Proton Transfer from Octane Radical Cations to Pentane Molecules in ?-Irradiated I I - C ~ D ~ ~ / I I -Crystals C ~ H ~ at ~ 77 K Dominique Stienlet and Jan Ceulemans' Department of Chemistry, K.U.Leuuen, Celestijnenlaan 200- F, 8-3001 Leuuen, Belgium Received: February 17, 1993; In Final Form: May 14, 1993
The ESR spectrum obtained after y-irradiation of protiated octane (0.5 mol 76) in deuterated pentane contains distinct lateral lines not present in the spectrum of irradiated neat deuterated pentane. These lateral lines do not correspond with the spectrum of irradiated neat octane. Instead, by comparison with the spectrum of different isomeric octyl radicals, obtained by y-irradiation of the appropriate bromo- or chlorooctanes (1 mol 7%) in perdeuterated cis-decalin, the lateral lines may conclusively be attributed to 1-octyl radicals. The selective formation of 1-octyl radicals by y-irradiation of n-CsH18 in n-C5D12 is attributed to the proton-transfer reaction n-CsHls'+ n-C5D12 -,1-CsHlf n-C5D12H+, which is preceded by hole transfer from matrix cations to octane solute molecules. This attribution is supported by the observation that in pentane-dl2-0.5 mol 7% heptane, in which hole trapping by heptane is very weak, the formation of 1-heptyl radicals is negligibly small. Different alternative mechanisms for 1-octyl radical formation are discussed and discarded on various grounds. The results obtained support the view that the radical site in alkyl radicals formed by proton transfer from alkane radical cations to alkane molecules is related very strictly to the structure of the semioccupied molecular orbital of the parent cation.
+
+
Introduction A proper understanding of the ionic reactions of saturated hydrocarbons and of their characteristics is of fundamental importance in order to gain insight into ionic processes in general. Much information on ionic reactions may be obtained by mass spectrometry,but this technique generally fails to provide detailed structural information. An alternative to mass spectrometry is radiation chemistry, which can provide structural information much more easily but in which it is much more difficult to distinguish between the different processes occurring. By a combination of matrix isolation and spectroscopic techniques, using both electronic absorption1 and ESRZ spectroscopy, much information has been obtained on intrinsic properties of cationic species, mostly radical cations, derived from alkanes. Radical cations of cyclohexane,of various alkyl derivativesof cyclohexane, and of cis- and trans-decalin have also been studied by ESR spectroscopy.3 In addition to informationon intrinsic properties, information on specificreaction processes of alkane radical cations could also be obtained. The occurrence of specific dissociation reactions of alkane radical cations has been demonstrated by ESR ~tudies.~J Also, conclusive evidence for the occurrence of hydrogen/proton transfer between higher alkanes and their cations has been obtained by ESR spectroscopy.- Earlier indication for this process came from pulseradiolytic detectionof fast alkane radical cation decay in the radiolysis of liquid and glassy alkanes? The alkane solute concentrationdependence of the signal intensity of fluorescence-detectedmagnetic resonance (FDMR) spectra of alkane radical cations in liquid systems also is indicative for the occurrence in specific circumstances of hydrogen/proton transfer between alkane radical cations and alkane molecules.1° The ESR studies indicate that reaction l a
essentially occurs by proton transfer and not by hydrogen abstraction for higher alkanes. There are strong indicationsthat proton transfer from alkane radical cations to alkane molecules is characterized by a high degree of site selectivity. All experimental results which can unambiguously be attributed to this reaction process are in
agreement with a postulate, expressing such site selectivity. This postulate states:*aUThenature of alkyl radicals formed by proton transfer from alkane radical cations to alkane molecules is related to the structure of the semi-occupied molecular orbital of the parent cation. A high unpaired-electron density in a particular carbon-hydrogen bond leads to proton transfer from that site, giving rise to a particular alkyl radical." At present, the number of experimental results on this matter is very limited, however. Thermal conversion of alkane radical cations into alkyl radicals, which has been studied2quite extensively, cannot unambiguously be attributed to proton transfer from alkane radical cations to alkane molecules. Such studies yield diverging results on the relation between the electronic structure of the alkane radical cation and the radical site in the neutral radical formed. Also, in the thermal conversion of radical cations of alkyl derivatives of cyclohexane, no relation is observed between the electronic structure of the radical cation and the radical site in the cycloalkyl radical formed.3bJ All this quite plausibly results from the fact that processes other than proton transfer are often responsible for the transformation of alkane and cycloalkaneradical cations into (cyc1o)alkyl radicals by thermal conversion. Unambiguous evidence for a strict dependence on electronic structure of the site of proton donation in the proton-transfer reaction from alkane radical cations to alkane molecules is restricted at present to a study of hexane and octane in synthetic zeolites6 and of heptane and octane in CC13F.8 In the latter study, the effect of the conformation of the alkane radical cation (which affects the electronic structure of this species) on the site of proton transfer could also be demonstrated. The selection of proper experiments to study this interesting subject does not appear to be a straightforward matter either, because a number of stringent conditions have to be met. Most notably, the experiments selected must allow the specific study of the proton-transfer process, the information obtained being unperturbed by other reaction processes occurring during radiolysis. In the zeolite-alkane and CC13F-alkane systems, such informationwas obtainable because hole transfer occurs efficiently from the irradiated matrix to the alkane solute and because the alkane solute molecules are (at least partly) aggregated, allowing the proton-transfer reaction to occur.
0022-365419312097-8595$04.00/0 0 1993 American Chemical Society
Stienlet and Ceulemans
8596 The Journal of Physical Chemistry, Vol. 97, No. 33, 1993
Hole transfer is an important mode of energy transfer in the radiolysis of hydrocarbon mixtures as well. The Occurrence of hole transfer in such systems has clearly been established by direct observationof the electronic absorption spectrum of solute alkane radical cations upon irradiation of higher alkanes in lower alkane matrices. Radical cations of many higher alkanes are formed and trapped in pentane and 3-methylpentane matrices containing both the higher alkane and an electron scavenger, when y-irradiated at 77 K. The cations are characterized by a broad electronic absorption band in the visible and near-infrared region that shifts to longer wavelengths with increasing carbon number. Effects of branching on the spectral position are also observed.' Direct observation of positive hole transfer upon irradiation of alkane mixtures has been proven possible by pulse radiolysi~.~~ Hole trapping by higher alkanes, present at low concentration in lower alkane matrices, implies that the radical cations formed are situated next to a matrix alkane molecule. This raises the following question: will the solute alkane radical cations react, at least partly, with adjacent matrix alkane molecules by proton transfer? The main differencewith thezeolits-alkane and CC13Falkane studies is that the cation and molecule involved in the reaction have different chain lengths; i.e., a reaction of the following type may well occur:
RIH
+ RIIH'+
-
Pentane-d12
J Figure 1. First-derivative ESR spcctrum obtained after irradiation of neat deuterated pentane, to which COz was added as electron acceptor. Irradiation and measurement temperature, 77 K, absorbed dose, 7.5 X 10'9 eV-g-1.
Pentane-d12 + 0.5 mole% Octane
I
/I
20 G
H
RIH2++ RII*
If such proton transfer takes place, mixed alkane systems may offer an alternative way for studying site selectivity in the protontransfer reaction from alkane radical cations to alkane molecules. Such a study is experimentally feasible by ESR spectroscopy. Use of a deuterated alkane as matrix, which results in a considerable contraction of the ESR absorption of matrixassociated paramagnetic species, allows solute-associated alkyl radicals to be clearly detectable in the lateral regions of the obtained ESR spectra. This paper reports on such an investigation, in which solute (protiated) alkyl radicals formed by irradiation of protiated octane in pentane-&, containing C02 as electron acceptor, are studied by ESR spectroscopy with the aim of providing evidence for the Occurrence of proton transfer in irradiated mixed alkane systems and for the existence of site selectivity in this process. The results obtained firmly support the postulate stating that the radical site in alkyl radicals formed by proton transfer from alkane radical cations to alkane molecules is related very strictly to the structure of the semioccupied molecular orbital of the parent cation.
Experimental Section Products used in this study were pentane412 (98.2 atom % D) from Merck, Sharp and Dohme; octane and cis-decalin-dl8(99.5 atom % D) from Janssen Chimica; heptane and 1- and 2-bromooctane from Fluka; and 3-bromooctane and 4-chlorooctane from K&K. These products were of the highest purity commercially available and were used as received. All solutions were deoxygenated in cylindricalSuprasil tubes by bubbling with argon for 15 min, after which they were carefully sealed. The samples were irradiated with cobalt-60 y-rays at liquid nitrogen temperature, to a dose of 7.5 X lOI9 eV gl.The ESR analysis was largely as described before.5.' ESR spectra were recorded at 77 K with a Bruker ER2OOtt ESR spectrometer and accumulated at least 100 times with a BNC-12 minicomputer. Spectral enlargementswere obtained by increasing both the detector signal gain and the number of repetitive scans. Results
The ESR spectrumobtained after irradiation of neat deuterated pentane, to which COZwas added as electron acceptor, is shown in Figure 1. The ESR absorption only extends over a relatively
I1 I Figure 2. First-derivative ESR spectrum obtained after irradiation of 0.5 mol % octane in deuterated pentane, containing C02 as electron acceptor. Irradiation and measurement temperature, 77 K, absorbed dose, 7.5 X lof9eV&.
narrow spectral region, and no distinct ESR features appear to be present in the more lateral region of the spectrum. The ESR spectrum obtained after irradiation of deuterated pentane, containing 0.5 mol % protiated octane as well as C02, is shown in Figure 2. This spectrum contains a number of additional absorptions. First, a very intense and relatively sharp ESR absorption is clearly discernible near the center of the spectrum; obviously, this absorption is asymmetric with a pseudodoublet structure at the high-field side of the spectrum. Second, additional ESR absorptions are observed in the more lateral region of the spectrum. Spectral enlargement reveals that these consist of a "double-humped" curve as well as a relatively sharp and more intense hyperfine component, which is only partially resolved from the much stronger central absorption. The additional lateral absorptions do not correspond with the ESR absorption of irradiated neat octane reported earlier," either in appearance or in spectral spacing. In Figure 3, a comparison is made between the enlarged ESR spectrum of the irradiated pentane-d12+xtane-C02 system and the spectrum obtained by irradiation of 1-bromooctane in cis-decalin-dts. From this, it is evident that the lateral ESR features in these spectra correspond quite closely, both in appearance and in spectral spacing. ESR spectra obtained after irradiation of ~is-decalin-d~s, containing 2- and 3-bromooctane, are respectively shown in Figures 4 and 5 ; the ESR spectrum obtained after irradiation of 4chlorooctane in cis-decalin-dl8very closely matches the spectrum of irradiated cis-decalin-dls-3-bromooctane. All these ESR absorptions extend over a considerably wider spectral region than the ESR absorption of irradiated pentane-dlroctane-CO2 and cis-decalin-dls-1 -bromooctanesystems and have a quite different appearance.
Site Selectivity in a Proton-Transfer Reaction Pentane -d,2
+ 0.5 mole %
The Journal of Physical Chemistry, Vol. 97, No. 33, 1993 8597
Octane
cis-Decalin-d18
+ 1mole %
3-Bromoactane
20 G
M
Figure 5. First-derivative ESR spectrum obtained after irradiation of 1 mol 4% 3-bromooctane in cis-decalin-dls(0indicates a background signal). cis-Decalin-d18
+ 1mole YO
1 -Bromooctane
Irradiation and measurement temperature, 77 K absorbed dose, 7.5 X 1019 eV.g-1.
I
Pentane -d,2
+ 0.5 mole%
Heptane
20
G
c--(l
Figure 3. Comparison of the enlarged ESR spectrum obtained after irradiation of 0.5 mol 96 octane in deuterated pentane, containing COz as electron acceptor, with the spectrum obtained after irradiation of 1 mol W 1-bromooctane in cis-decalin-dig. Irradiation and measurement temperature, 77 K absorbed dose, 7.5 X 1019 eV-g-1.
cis-Decalin-d18
+ 1 mole
/ I
I
i
Figure 6. First-derivative ESR spectrum obtained after irradiation of
4% heptane in deuterated pentane, containing C02 as electron acceptor. Irradiation and measurement temperature, 77 K, absorbed dose, 7.5 X 1019 eV-g-1. 0.5 mol
Oh
2-Bromooctane
20 G
H
radicals. One involves C-D bond rupture in excited pentane molecules, n-C5D12*
+
C5Dl1'
+ D'
(2)
which are either formed directly by the ionizing radiation
-
n-CSD12
nC5Dj2*
(3a)
or formed by recombination of positive holes and electrons Figure 4. First-derivative ESR spectrum obtained after irradiation of 1 mol 96 2-bromooctanein cisdecalin-& (0 indicates a background signal). Irradiation and measurement temperature, 77 K, absorbed dose, 7.5 X 1019 eV-g-1.
The ESR spectrum obtained after irradiation of deuterated pentane, containing 0.5 mol % protiated heptane as well as C02, is shown in Figure 6. This spectrum is similar to the spectrum of irradiated pentane-dl4.5 mol % octane in that it contains a very intense and relatively sharp ESR absorption near the center of the spectrum. However, the additional ESR absorptions in the more lateral region of the spectrum are very weak.
Discussion 1. Assignment of the ESR Absorptionsand Mode of Formation of the Major Paramagnetic Intermediates. The ESR absorption obtained after irradiation of neat pentane-d12,to which COz was added as electronacceptor, can be attributed to deuterated pentyl radicals; trapped C02'- anions appear to be absent in the system. Two distinct mechanisms may account for the observed pentyl
n-C5D12
n-C5D,i+
+ e-
(3b)
A second mechanism involves deuteron transfer from and dissociation of pentane radical cations
and the subsequent neutralization with electrons of the diamagnetic cations formed C,D,,+
+ e--
+
CSDll*+ D2
C5Dll+ 6- C,Dl,'
(7) (8)
The relative importanceof these two mechanisms depends largely on whether extensive ionic transformations can occur before
8598
Stienlet and Ceulemans
The Journal of Physical Chemistry, Vol. 97, No. 33, 1993
neutralization. Recent experiments- have highlighted the role of such ionic transformations in the radiolysis of alkanes. A further source of pentyl radicals is provided by deuterium abstraction from pentane molecules by D atoms, formed according to the above two mechanisms. n-C,D12
+ D'
-
C,D,,'
+ D2
(9)
As a result of the deuteration, the pentyl radical spectrum is very much contracted, leaving the more lateral regions free of characteristic ESR absorptions. The addition of octane to pentane-dl&Oz systems results in the appearance of additional ESR absorptions upon irradiation. These absorptions can be attributed to two different paramagnetic species. (i) The intensenarrow and asymmetriccentral absorption can be attributed to C02*- anions, formed by electron attachment to C02. There is no splitting due to hyperfine interaction in this anion, but the (first-derivative) ESR absorption is asymmetric as a result of g-factor anisotropy.12J3 Interestingly, as noted above, this absorption is not present in irradiated neat pentaned12 (with added COZ), indicating that COz is not included in neat pentane crystals. Clearly, the structural disorder induced by inclusion of octane in pentane assists in the inclusion of COz in this system. (ii) The additional ESR absorptions in the more lateral region of the spectrum correspond quite closely, both in appearance and in spectral spacing, with the lateral absorption of irradiated cis-decalin-dl8-l-bromooctane systems, which can be attributed with certainty to 1-octylradicals.88 The formation of 1-octyl radicals in the latter system is due to dissociativeelectron attachment of radiation-produced electrons to 1-bromooctane. cisdecalin-dla
+ -
l-C,H,,Br
asdecalin-d,~c
e-
+ e-
l-C8H17*+ Br-
(IO)
(11)
In this experiment,cis-decalin is chosen because the low ionization energy of this compound with respect to that of most bromoalkanes precludes the occurrence of hole transfer to these compounds and thus the formation and stabilization of the corresponding radical cations, which are likely to interfere in the ESR analysis. Characteristic features in the 1-octyl radical spectrum, which make the spectrum easily recognizable, are a double-humped curve situated at both sides of the spectrum and relatively sharp intense hyperfine lines, which are more centrally situated. These features are predicted by a theoretical treatment of the hyperfine interactions in alkyl radi~a1s.l~Powder ESR spectra of 2- and 3-octyl radicals, obtained by irradiation of the corresponding brommtanes in cis-decalin-dl8,have a quite different appearance and extend over a considerably wider spectral region than the ESR absorption of irradiated pentane-dlmtane-COz. The powder ESR spectrum of 4-octyl radicals, on the other hand, is quite similar to that of 3-octyl radicals, as is evident from the ESR spectrum of irradiated cis-decalin-dl8-4-chlorooctane.On the basis of all this, the additional lateral absorption in the ESR spectrum obtained by irradiation of octane in pentane-d12, containing C02 as electron acceptor, can be attributed with certainty to 1-octyl radicals. The spectrum shows no indication of the presence of other isomeric octyl radicals, which should be easily detectable if present in the system. Indeed, the presence of an appreciable contribution of penultimate and interior octyl radicals would result in a flattening out of the double-humped structure and, at major contributions, in the appearance of an additional ESR band outside the spectrum of 1-octyl radi~a1s.l~ In the system under study, 1-octyl radicals clearly are predominant (as far as protiated alkyl radicals are concerned). 2. Mechanism of Formation of 1-Octyl Radicals upon Irradiation of the Pentane-d,&tan+COz System. Three major mechanisms can be envisaged for theselective formation of 1-octyl radicals upon irradiation of the pentane-dlroctane-COZ system, of which two must be rejected on different grounds.
a. SelectiveHydrogenAtom Abstraction by Deuterium Atoms. Someof thedeuterium atoms radiolytically produced incrystalline perdeuterated n-alkanes will abstract a deuterium atom from the matrix near the place where they are formed, but other deuterium atoms may escape to the molecular boundary regions (stacking planes), where the packing is looser relative to the other parts in the crystalline lattice. These deuterium atoms can migrate for some distance and abstract a hydrogen atom from a protiated solute molecule. Random abstraction preferentially takes place from protiated alkane solute molecules, because of the weaker C-H vs C-D bond strengths (A 8 kJ.mol-l). In the system under study, the reaction evidently would result in the formation of octyl radicals:
-
The mechanism outlined above is responsible for the effective and selective formation of solute alkyl radicals and for the dependence of it on chain-length differences in the radiolysis of n-CnHw2 (1 mol %)-n-CloD22 mixed crystals. Selective formation of alkyl solute radicals, the nature of these radicals depending on the chain-length difference between solute and matrix molecules, has indeed been found for n = 7-12, when such systems are irradiated at 77 K.l6 Chain-end solute radicals are only formed in mixed crystals with solute molecules that are shorter than the matrix (n < 10) and apparently are the only solute radicals in the case of n Q 8. The selectivity in their formation is attributed to the fact that some of the deuterium atoms, which are radiolytically produced, can escape to and migrate in the molecular boundary regions and that in mixed crystals with solute molecules shorter than the matrix molecules, especially in the case n d 8 in the n-CloDzz matrix utilized in that investigation, only the chain-end CHS groups of the solute molecules expose themselves to these regions. Secondary solute radicals, on the other hand, are formed selectively in the case of n 2 10. Their formation is due to the exposureof secondary C-H bonds (with lower bond strengths than primary C-H bonds), in addition to the exposure of methyl C-H bonds, to the molecular boundary regions. Although the above mechanism is perfectly acceptable for irradiated n-CnH2,,+2 (1 mol %)-n-CloDzz (n = 7-12) systems, containing no electron acceptor, it cannot account for the selective formationof 1-octyl radicals in the irradiated pentane-dl-taneCOz system studied in the present work. Indeed, as in the latter system the solute molecules are longer than the matrix molecules, selective formation of secondary octyl radicals would be anticipated on the basis of the selective hydrogen atom abstraction mechanism. The fundamental difference between the two types of systems is that in the system studied in the present work charge neutralization is greatly reduced by trapping of both electrons and positive holes. C02 n-C,D,,'+
+ e--
+ n-C,H,,
-
C0,'n-C,D12 + n-C,H18'+
(13) (14)
In contrast, in n-CnHtt+2 (1 mol %)-n-CloDzz systems with n = 7-12, hole trapping by solute alkane molecules may be expected to be negligibly small. Electronic absorption studies clearly indicate" that, although positive hole trapping by, for instance, octane in pentane is quite extensive, virtually no hole trapping occurs by heptane upon irradiation of pentane-heptane-COz, quite plausibly becausethe differencein ionization energy between pentane and heptane is too small for efficient stabilization of the hole on the heptane solute to occur. In line with this, no significant hole stabilization is expected to occur in irradiated n-CloD2r n-C12H~6(1 mol %) systems and, a fortiori, in the other n-CnHw2 (1 mol %)-n-CloDZ2(n = 7-12) systems.
Site Selectivity in a Proton-Transfer Reaction
The Journal of Physical Chemistry, Vol. 97, No. 33, 1993 8599
The blocking of the charge neutralization process in the system studied in the present work considerably reduces the amount of deuterium atoms formed by blocking D atom formation resulting from reaction 4 (followed by reaction 2) and, as a consequence, results in a considerable decrease in the formation of alkyl solute radicals by hydrogen atom abstraction. It results, on the other hand, in the appearance of a new and considerably more important formation mechanism for octyl radicals in which 1-octyl radicals are formed selectively. Quite naturally, hole trapping by octane puts the positive hole on octane radical cations and greatly increases the lifetime of the ionic species, thus making ionic transformations much more likely. It appears very reasonable therefore to accept that the selectiveformation of 1-octyl radicals in the irradiated pentane-dlpxtane-CO2 system results from a reaction of trapped octane radical cations. This view is strongly supported by the observation that in irradiated pentane-dlr heptane-COz, in which hole trapping by heptane is very weak," the formation of 1-heptylradicals is negligibly small. Two types of reactions can be envisaged for the selective formation of 1-octyl radicals in the irradiated pentane-dlz-octane-COz system under isothermal conditions, viz. unimolecular dissociation and ionmolecule reactions with the pentane matrix. b. Unimolecular Decomposition of Excited Octane Radical Cations. Two decomposition modes may be envisaged, which eventually could result in the formation of octyl radicals. These involve unimolecular deprotonation
and dissociation into alkyl carbenium ions and hydrogen atoms.
The deprotonation reaction (reaction 15) must be rejected on the basis of thermodynamic considerations. In the gas phase, this dissociation is endothermic by about 7.9 eV as calculated from (17) where DCH= 4.3 eV is the carbon-hydrogen bond dissociation energy and Ei(CnH~,,+2)rr 10 eV and Ei(H) = 13.6 eV are ionization energies. In contrast, dissociation into alkyl carbenium ions and hydrogen atoms is only endothermic by about 2.5 eV for primary bond scission, taking into account the ionization energies of hydrogen atoms and primary alkyl radicals.17 Consequently, of the two dissociation processes, reaction 16 must be considered much more likely than reaction 15. Dissociation into alkyl carbenium ions and hydrogen atoms cannot be a major sourcefor 1-octyl radical formation in irradiated pentane-dlpx$ane-COz systems either, however, and would not be expected to result in selective formation of 1-octyl radicals. First, the carbenium ion would still have to be neutralized
+e-
'IH1'l+
C8H17'
(18)
in the solid system in order for alkyl radicals to be observable in the ESR experiments, and such charge neutralization is greatly reduced in the system under study. Second, the dissociation still requires quite considerable excess energy (about 2.5 eV), and it appears unlikely that such an amount of excess energy would be transferred together with the hole in reaction 14. Highly excited pentane radical cations, which carry sufficient energy for dissociation, are likely to dissociate themselves (reaction 6), and pentane radical cations, which transfer their hole to octane molecules, are in all likelihood already considerably relaxed at the moment of transfer. Vibrational excitation energy in particular is lost very quickly. It may be important to point out here that highly mobile cations have been observed in irradiated cyclohexane, methylcyclohexane, and tram-decalin but not in irradiated aliphatic alkanes.188 The excess energy required for
thedissociation(reaction 16) cannot be derived from the difference in ionizationenergy between n-C5D12 and n-C8Hl8 either, because this difference only amounts to 0.48 eV.18bIn addition, it should be mentioned that the dissociation reaction is not expected to result in selective rupture at the chain-end position because of the quite extensive excitation energy required, which is likely to affect the electronic structure of the radical cation and, as a consequence, the unpaired-electron and (associated) positivehole distribution over this cation. Although in octane radical cations in theextendedconformation the two terminal C-H bonds that are in the plane of the C-C skeleton are selectively weakened in ground-state ions, a quite different situation may well exist for electronically excited cations. In such cations, the electron is removed from a lower-lying molecular ~ r b i t a l , which ~ ~ , ~ is~ probably characterized by a quite different distribution over the radical cation than the highest-occupied molecular orbital. c. Proton Tramfer from Octane Radical Cations to Pentaned,z Molecules. The selective formation of 1-octyl radicals indicates that octane radical cations, which are formed by hole transfer from matrix cations and are trapped in the solid matrix, may react by proton transfer with matrix molecules.
Conclusive evidence for the occurrence of proton transfer from alkane radical cations to alkane molecules of equal chain length has recently been obtained.- Very recently, proton transfer from heptane radical cations to octane molecules has also been observed.2' The reaction certainly requires much less excitation energy than the dissociation reactions (reactions 15 and 16), as proton transfer from alkane radical cations to alkane molecules is approximately thermoneutral.22 The proton-transfer reaction can easily account for the selective formation of 1-octyl radicals, which is observed experimentally. Indeed, consideringthat octane radical cations are in the extended conformation, proton transfer is expected to result in the exclusive formation of I-octyl radicals, because of the selective weakening of the two chain-end C-H bonds that are in the plane of the C-C skeleton and the presence of unpaired-electron and (associated) positive-hole density in these bonds. It has indeed been firmly established by ESR spectroscopy and INDO calculations that in n-alkaneradical cations in the extended conformation the unpaired electron is delocalized over the carbon-carbon a-bonds as well as the two in-plane chain-end carbon-hydrogen bonds.2 The ESR spectrum of octane radical cations in pentane, obtained by difference spectroscopy after illumination of irradiated pentane(3%) octane,23 indicates that such cations are indeed in the extended conformation. The spectrum contains two clearly observable absorptions that nicely match the lateral absorptions in the triplet spectrum of extended octane radical cations in CC12FCFzCl (the central absorption is obscured by an intense signal due to bleached trapped electrons). The presence of octane radical cations in other than the all-trans extended conformation, i.e., conformations in which hyperfine interactions are considerably larger, would result in ESR absorptions that extend over a considerably wider spectral region, which would thus easily be observable. Such absorptions apparently are not present in the difference ESR spectrum, obtained after irradiation of octane in pentane and subsequent illumination. 3. Relation between tbe Electronic Structure of Octane Radical Cations and the Radical Site in the Octyl Radicals Formed by Proton Transfer to Pentane Molecules. The ESR spectrum of octane radical cations in pentane, obtained by difference spectroscopy after illumination of irradiated pentane-( 3%) octane,z3 indicates that such cations are in the extended conformation, which appears quite reasonable. With such a structure, the imperfection in the crystal is largely limited to a small void at the end of the octane molecule (or radical cation). In the alternative structure, viz., with the surplus propyl group being
Stienlet and Ceulemans
8600 The Journal of Physical Chemistry, Vol. 97, No. 33, 1993 pushed into the intermolecular boundary layer resulting in a different (and much strained) conformation of octane, the imperfectionin the crystal must necessarily spread over a number of layers, and the energy of such a structure is undoubtedly much higher. As a matter of fact, reports of gauche conformers in mixed n-alkane crystals are limited to systems in which the chain length of the alkanes is very large compared to the surplus alkyl group that is to be accommodated in the intermolecular boundary layer, e.g., CI9/C2' systems. Effects of concentration on the relative contribution of gauche and all-trans conformers have been reported in such systems, but such effects extend over the full concentration range of the binary alkane system2*andcertainly do not suddenly appear at some very small concentration of the high-chain component. ESR spectra and INDO calculationsshow that in octane radical cations in the extended conformation the unpaired electron is delocalized over the C-C a-bonds as well as the two chain-end C-H bonds that are in the plane of the C-C skeleton.2 The results presented in this work clearly indicate that, in accordance with the electronicstructure, proton transfer to pentane molecules takes place selectively from chain-end sites, since 1-octyl radicals are formed selectively in the irradiated pentane-dlz-octane-CO2 system. The formation of 1-alkyl radicals is intuitively expected in the proton transfer from extended (all-trans) alkane radical cations to alkane molecules. The unpaired-electronand associated positive-hole density in the two in-plane chain-end C-H bonds clearly makes them prime candidates in proton-transfer reactions to neutral apolar molecules, reactions which are not likely to be affected by the overall charge distribution in the radical cation. The results obtained thus offer further support for the postulate which states that the radical site in alkyl radicals formed by proton transferfrom alkane radical cations to alkane molecules is related very strictly to the structure of the semioccupied molecular orbital of the parent cation. Previous experimental indicationfor such a relation came from an ESR study of irradiated hexane and octane in synthetic zeolites6 and in particular from a detailed study by ESR spectroscopyof heptyl and octyl radicals formed by irradiation of heptane and octane in CClpF.8 Studies of thermal conversion of alkane radical cations into alkyl radicals give rather diverging results on the relation between the electronicstructure of the alkane radical cation and the radical site in the alkyl radical formed.* Also, in the thermal conversion of radical cations of alkyl derivatives of cyclohexane, no relation is observed between the electronic structure of the radical cation and the radical site in the cycloalkyl radical f~rmed.~b.f All this quite plausibly results from the fact that processes other than proton transfer from (cyc1o)alkane radical cations to (cyc1o)alkane molecules are often responsible for the transformation of alkane and cycloalkane radical cations into (cyc1o)alkyl radicals by thermal conversion. As a matter of fact, charge neutralization appears a much more likely process for the thermal conversion of (cyc1o)alkane radical cations into (cyc1o)alkyl radicals. Not only is this process assisted by Coulombic attraction to bring the reacting entities together, but it also takes place with great efficiency, whereas proton transfer from alkane radical cations to alkane molecules is probably quite inefficient for alkane radical cations that (as in the thermal conversion experiments) have long returned to their ground state. The proton transfer from alkane radical cations to alkane molecules apparently requires some excitation energy, as is evidenced by the fact that an appreciable number of octane radical cations remain stabilizednext to pentane molecules over a very prolonged period of time at 77 K in the system under study" and by the fact that radical cations of higher alkanes may be observed at all in lower alkanes by matrix isolation spectroscopy.' In the isothermal concentration studies (zeolite alkane and CClpF-alkane) and in the present work in which octane is situated next to pentane molecules, proton transfer is assisted by the excess energy available as a result of the hole-transfer
process. Charge neutralization, on the other hand, is a quite unlikely process in these studies because the reacting entities are immobilized in the solid system. In thermal conversion studies, in contrast, the mobility of the reacting entities is likely to be assisted by phase transitions. There is as a consequence no fundamental contradiction between the observation of a dependence on electronic structure in isothermal concentration studies (zeolitealkane and CC13F-alkane) and in the present work and the absence of it in the thermal conversion studies. The only erroneous statement is the original postulateZbVcconcerning the dependence on electronic structure of deprotonation of alkane radical cations, because this may be accomplished by several distinct reactions, most notably proton transfer from alkane radical cations toalkanemolecules andneutraliition reactions (thelatter reactions not showing the same dependence on electronic structure). Asamatteroffact,arecent study oftheneutralization of octane radical cations by chloride ions indicates that the neutralization is governed to a large extent by the overall charge distribution in the radical cation.25 Finally, it should be pointed out that the selective formation of 1-octyl radicals by proton transfer from octane radical cations to pentane molecules does not necessarily require that all octane radical cations are in the extended (all-trans) conformation, bemuse proton transfer from this conformation is, in the system under study, in all likelihood much more efficient than proton transfer from the conformation in which the surplus propyl group would be packed into the intermolecular boundary layer. The argument is based on both thermodynamic and structural considerations. From proton affinity data obtained by ionequilibrium measurements,26it can be deduced that the proton affinity for protonation at a secondary C-H is considerablylarger than that for protonation at a primary C-H and that it also exceeds the proton affinity for C-C protonation. As the proton transfer from alkane radical cations to alkane molecules is approximatelythermoneutral,22these differencesin proton affinity may well be decisive with respect to the endo- or exothermicity and the occurrence of such proton transfer. When in the present system octane is in the extended conformation, it is quite clear that planar chain-end C-H bonds which are the site of proton donation comeintoclosecontact with both primary andsecondary C-D bonds of pentane. In contrast, in the alternative structure in which the surplus propyl group would be packed into the intermolecular boundary layer, planar C-H bonds only come into close contact with primary C-D bonds of pentane. It has already been observed that the presence of deuterated octane in heptane upon y-irradiation results in a substantial increase in the relative contribution of 1-heptyl radicals to the paramagnetic absorption, which has been attributed to similar effects.21
Conclusion Upon y-irradiation of octane in deuterated pentane containing CO2 as electron acceptor, part of the octane (solute) radical cations, formed by positive hole transfer from pentane (matrix) radical cations, react further by proton transfer to pentane molecules. Proton transfer from octane radical cationsto pentane molecules in pentane matrices results in the selective formation of 1-octyl radicals. The results obtained support the view that the radical site in alkyl radicals formed by proton transferfrom alkane radical cations to alkane moleculesis related very strictly to the structure of the semioccupied molecular orbital of the parent cation. Acknowledgment. We thank the national research fund (NFWO) of Belgium for financial support.
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