Symmetric and asymmetric proton transfer from heptane and octane radical cations to heptane molecules in γ-irradiated n-C7H16-n-C8H18-2-C6H 13CL crystals: Structural disorder in mixed alkan

Article abstract of DOI:10.1039/b206335m

A study has been made of the isomeric composition of secondary chloroheptanes formed upon γ-irradiation at 77 K and subsequent melting of heptane containing 1 mol% 2-chlorohexane and various concentrations of octane. It is observed that the relative impor

Full text of DOI:10.1039/b206335m

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Symmetric and asymmetric proton transfer from heptane and
octane radical cations to heptane molecules in ꢀ-irradiated
n-C7H16–n-C8H18–2-C6H13Cl crystals: structural disorder in mixed
alkane crystals
2
Adelheid Demeyer and Jan Ceulemans*
Department of Chemistry, K.U. Leuven, Celestijnenlaan 200-F, B-3001 Leuven, Belgium.
E-mail: Jan.Ceulemans@chem.kuleuven.ac.be; Fax: ϩ32-16-32.79.91
Received (in Cambridge, UK) 1st July 2002, Accepted 20th August 2002
First published as an Advance Article on the web 3rd October 2002
A study has been made of the isomeric composition of secondary chloroheptanes formed upon γ-irradiation at
77 K and subsequent melting of heptane containing 1 mol% 2-chlorohexane and various concentrations of octane.
It is observed that the relative importance of 2-chloroheptane increases as a result of the presence of octane in the
crystallites. This increase is attributed to selective proton transfer from ground state heptane radical cations to
penultimate C–H bonds in heptane molecules. The proton transfer is induced by (partial) dislocation of heptane
molecules adjacent to octane solute molecules, which brings the penultimate heptane C–H bonds into close contact
with planar chain-end C–H bonds in heptane radical cations. Proton transfer from extended all-trans octane radical
cations (on which the positive hole temporarily resides) to heptane molecules may also contribute to the observed
effect. The results have important implications with respect to the molecular packing in binary n-alkane crystals.
In binary n-alkane crystals, in which the shorter component is predominant, molecules of the longer component
cannot be fully accommodated in one molecular layer of the crystal, even with a chain length mismatch of only
one methylene unit; instead (at least partial) dislocation of adjacent molecules of the shorter component takes place.
Such dislocations do not extend indefinitely over the crystal, however, and crystal order is restored by squeezing,
deformation and changes in conformation of the appropriate molecules, including molecules of the shorter
component.
has also been observed in asymmetric proton transfer, viz.,
in the proton transfer from heptane radical cations to decane
Introduction
Information on donor site selectivity in symmetric proton
transfer from n-alkane radical cations to n-alkane molecules
has been obtained from EPR studies on the nature of the alkyl
radicals in irradiated SF6–n-alkane (upon annealing),1 zeolite–
n-alkane2 and CCl3F–n-alkane3,4 systems at cryogenic temper-
atures. The experiments point to a strict relationship between
the electronic structure of the n-alkane radical cations and the
site of proton donation (intrinsic donor site selectivity). In fact,
all experimental data that can unambiguously be attributed to
the proton-transfer process indicate that such transfer takes
place selectively from planar C–H bonds, i.e., C–H bonds
coplanar to the planar or planar part of the C–C skeleton, in
n-alkane radical cations in the extended all-trans and gauche-at-
C2 conformation, respectively, because of the high unpaired-
electron and positive-hole density in those bonds. An EPR
study on the nature of the octyl radicals formed by γ-irradi-
ation of pentane-d12–octane at 77 K has confirmed the donor
site selectivity with respect to the (asymmetric) proton transfer
from extended all-trans octane radical cations to pentane
molecules.5
Information on (intrinsic) acceptor site selectivity in sym-
metric proton transfer from n-alkane radical cations to
n-alkane molecules has been gathered from γ-irradiated frozen
CCl3F–n-alkane solutions by gas chromatographic analysis of
the appropriate chloroalkanes after melting.6,7 The proton
acceptance appears restricted to C–H bonds at secondary
carbon atoms (no proton transfer to C–C bonds nor to C–H
bonds at primary carbon atoms), with a preference for the
penultimate position and mutually equal (but considerably
lower relative to the penultimate site) transfer to the different
interior positions. Selective transfer to penultimate C–H bonds
molecules in γ-irradiated heptane–decane–1-chloroheptane
crystals, by chromatographic analysis after melting.8
Crystals are structurally highly organized and as a con-
sequence selectivity with respect to the site of proton donation
may lead to selectivity with respect to the site of proton accept-
ance; (structurally determined acceptor site selectivity). This
may lead to additional selectivity with respect to the site
of proton acceptance in irradiated mixed n-alkane crystals.
Structurally determined acceptor site selectivity in γ-irradiated
heptane–decane–1-chloroheptane has been observed with
respect to protonation of the inner positions of decane and can
be explained on the basis of structural diagrams that depict the
packing in heptane crystals containing decane molecules.8
Another system, viz. γ-irradiated heptane–octane–1-chloro-
hexane, is also of great interest because, in this system, penulti-
mate C–H bonds in octane, which for reasons of energetics have
the greatest propensity to act as proton acceptors, are also
structurally greatly favored over the interior C–H bonds as pro-
ton acceptors from planar chain-end C–H bonds in the heptane
radical cations. The combined preference for the penultimate
C–H bonds due to structural and thermodynamic factors in
this system results in very high (almost exclusive) acceptor
selectivity with respect to these bonds.9
All experiments on structurally determined acceptor site
selectivity at present relate to asymmetric proton transfer from
matrix radical cations to solute molecules. In order to demon-
strate the universality of the phenomenon and the correctness
of the explanation presented, it appeared of great interest to
extend the experimental evidence to symmetric proton trans-
fer from matrix radical cations to matrix molecules. This is
accomplished in the present work by studying the effect of the
DOI: 10.1039/b206335m
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concentration of octane on the isomeric composition of the
secondary chloroheptanes formed by γ-irradiation of n-C7H16–
n-C8H18–2-C6H13Cl crystals at 77 K and subsequent melting.
The results obtained are of further interest to solid-state chem-
ists because they also provide information on the packing in
mixed n-alkane crystals and on the crystal imperfections that
originate from the chain-length mismatch due to the presence
of a longer-chain (solute) n-alkane in a mixed n-alkane crystal.
Experimental
Products used in this study were heptane (≥99.7%) from
Rathburn, octane (>99.0%) from Janssen Chimica and 2-chloro-
hexane (>97%) from Fluka. Heptane and octane were purified
by passage through a column containing silica gel, which had
been activated immediately prior to use by heating at 250 ЊC for
24 h. The activation procedure removed possible non-paraffinic
impurities in the alkanes that might have a disproportionate
effect on the radiolytic process.
Fig. 1 Relevant part of the chromatograms showing the analysis of
the secondary chloroheptanes formed upon γ-irradiation at 77 K and
subsequent melting of heptane containing 1 mol% 2-chlorohexane and
various concentrations of octane. The number above the arrows
indicates the position of the chlorine atom in the chloroheptane
isomers.
Solutions, prepared by conventional techniques, were con-
tained in large (11 mm i.d.) cylindrical tubes with a narrow
orifice made of borosilicate glass. Argon was bubbled through
the solutions for 30 min to remove oxygen. After deoxygen-
ation, samples were carefully sealed and rapidly cooled to 77 K
by immersion in liquid nitrogen. All samples were irradiated
with 60Co γ-rays to a dose of 7.5 × 1019 eV gϪ1. The procedure
used in the chromatographic analyses was largely as described
before,9b except that no internal standards were added because
no absolute yield values were determined. The separations were
performed at a column temperature of 30 ЊC. Quantitative data
(relative yields) were obtained on the basis of peak heights and
(adjusted) retention times according to a method developed
previously.10a Individual peak heights for 2- and 3-chloro-
heptane were obtained by deconvolution, in which the (real)
peak height of the adjacent overlapping peak at the elution
maximum of the other was subtracted from its (apparent)
maximum value and vice versa until conversion to within (on
average) 3% occurred (seven iterations). Peak widths and
shapes required for this process were derived from the elution
profile of 4-chloroheptane, which was enlarged in accordance
with the linear variation of peak width with (adjusted) reten-
tion time.10b,c Because of the partial overlap of the peaks of
2- and 3-chloroheptane, quantitative values (even of relative
yields) are prone to some error. The discussion can, however, be
solidly based on the evolution of relative yields with octane
concentration, which especially for 2- vs. 3-chloroheptane gives
a quite straightforward indication because of the nearness in
chromatographic elution time and radiolytic yield at low octane
concentration. In fact, a major observation (viz., the increase
of the yield of 2- vs. 3-chloroheptane with increasing octane
concentration) can be discerned directly on the respective
chromatograms.
ably lower formation of 4-chloroheptane, which can (at least
partly) be explained on purely statistical grounds. With increas-
ing concentration of octane, the importance of 2-chloro-
heptane increases with respect to that of 3-chloroheptane as is
quite evident from Fig. 1; already at 1 mol% octane the relative
importance of the two isomers is reversed. The effect is further
illustrated in Fig. 2, in which the relative yield of the two
Fig. 2 Ratio of the radiolytic yield of 2- to 3-chloroheptane (based
on non-deconvoluted peak heights) vs. octane concentration, upon
γ-irradiation at 77 K and subsequent melting of heptane containing
1 mol% 2-chlorohexane and various concentrations of octane.
products (based on non-deconvoluted peak heights) is plotted
as a function of the concentration of octane. An increase in
importance is also observed for 2- vs. 4-chloroheptane, as is
evident from Fig. 3 in which the ratio of the yield of 2- to
4-chloroheptane (based on non-deconvoluted peak heights) is
shown as a function of the concentration of octane. Similar
conclusions are obtained from graphs in which the relative yield
of the respective products (based on deconvoluted peak
heights) is plotted as a function of the concentration of octane,
but the observed effects are considerably more pronounced,
especially with respect to the yield of 2- vs. 3-chloroheptane
(see Figs. 4 and 5).
Results
The relevant part of the chromatogram showing the analysis of
the secondary chloroheptanes formed upon γ-irradiation at
77 K and subsequent melting of heptane containing 1 mol%
2-chlorohexane and various concentrations of octane, is shown
in Fig. 1. Different radiolysis products are present in the
chromatographic region where 1-chloroheptane elutes, which
interfere with the analysis of this product; also, data on the
chain-end isomer are not essential to the discussion as prim-
ary C–H bonds are not effective as proton acceptors from
ground-state alkane radical cations6,7 and its analysis was there-
fore not pursued. With respect to the secondary chloro-
heptanes, the results at zero octane concentration indicate
about equal formation of 2- and 3-chloroheptane (with a small
bias towards the formation of 3-chloroheptane) and consider-
Discussion
Mode of formation of chloroheptanes in ꢀ-irradiated heptane–2-
chlorohexane
The interaction of the ionizing radiation in the heptane–2-
chlorohexane system mainly takes place with the major
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Electrons formed in the ionization process that escape the
coulomb attraction of the counter radical cations can migrate
through the crystal and have a high likelihood of reacting with
2-chlorohexane, present as additive, by dissociative electron
attachment [eqn. (2)].
Ϫ
2-C6H13Cl ϩ eϪ
2-C6H13 ϩ Cl
(2)
ؒ
The heptane radical cations formed in the ionization process
are usually substantially excited and may be transformed into
other cationic species, viz. into carbenium ions by dissociation
[eqn. (3)]
ϩ
ؒ
ϩ
ؒ
n-C7H16
*
C7H15 ϩ H
(3)
and into carbonium ions by proton transfer to heptane
molecules [eqn. (4)].
Fig. 3 Ratio of the radiolytic yield of 2- to 4-chloroheptane (based
on non-deconvoluted peak heights) vs. octane concentration, upon
γ-irradiation at 77 K and subsequent melting of heptane containing
1 mol% 2-chlorohexane and various concentrations of octane.
n-C7H16 ϩ* ϩ n-C7H16
C7H15 ϩ C7H17
(4)
ϩ
ؒ
ؒ
No site selectivity is to be expected in these two reactions,
because of the electronic excitation energy on the radical
cation. Heptane radical cations that are not transformed by the
above processes before substantial relaxation takes place, may
contribute to positive-hole migration in the irradiated system.
In γ-irradiated neat heptane crystals, positive-hole migration
can take place by electron transfer from heptane molecules to
adjacent heptane radical cations, the transfer process repeating
itself in successive steps [eqn. (5)].
ϩ
n-C7H16 ϩ ϩ n-C7H16
n-C7H16 ϩ n-C7H16
(5)
ؒ
ؒ
The migration is driven by the coulomb attraction of chlor-
ide ions trapped in the vicinity and, if not interfered with, will
result in the encounter of these cationic and anionic species
leading to charge neutralization.
There are three distinct neutralization processes that may
lead to the formation of chloroheptanes in the γ-irradiated
heptane–2-chlorohexane system under study. For two of these
neutralization and chloroheptane formation is delayed until the
sample is warmed and melted after the irradiation, viz. the
neutralization of heptyl carbenium and heptyl carbonium ions
[eqns. (6) and (7)]
Fig. 4 Ratio of the radiolytic yield of 2- to 3-chloroheptane (based
on deconvoluted peak heights) vs. octane concentration, upon
γ-irradiation at 77 K and subsequent melting of heptane containing
1 mol% 2-chlorohexane and various concentrations of octane.
C7H15ϩ ϩ ClϪ
C7H17ϩ ϩ ClϪ
C7H15Cl
(6)
(7)
C7H15Cl ϩ H2
as these cationic intermediates as well as chloride ions are
trapped in the crystalline system at 77 K. Positive-hole
migration takes place in the irradiated system at 77 K and
neutralization of heptane radical cations by chloride ions
therefore occurs during γ-irradiation. The exact nature of this
neutralization process is unknown at present and is currently
under study in our laboratory.
Origin of the increase in the prominence of 2-chloroheptane
relative to other secondary chloroheptanes upon addition of
octane
Fig. 5 Ratio of the radiolytic yield of 2- to 4-chloroheptane (based
on deconvoluted peak heights) vs. octane concentration, upon
γ-irradiation at 77 K and subsequent melting of heptane containing
1 mol% 2-chlorohexane and various concentrations of octane.
The results obtained indicate that addition of octane to the
heptane–2-chlorohexane system results in a considerable
increase in the prominence of 2-chloroheptane relative to other
secondary chloroheptane isomers. This can be attributed to
site-selective proton transfer from heptane radical cations in
their electronic ground state to heptane molecules, with either
the radical cation or the neutral molecule being dislocated in
the crystal due to the presence of octane [eqn. (8)].
component, which is heptane, and results in its excitation and
ionization [eqns. (1a) and (1b)].
(1a)
(1b)
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ϩ
n-C7H16 ϩ ϩ n-C7H16
1-C7H15 ϩ 2-C7H17
(8)
ؒ
ؒ
In this process, proton transfer takes place selectively from
planar chain-end C–H bonds in heptane radical cations to pen-
ultimate C–H bonds in heptane molecules. The reaction
undoubtedly also contributes to the previously observed
increase in the prominence of 1-heptyl radicals due to the pres-
ence of octane in γ-irradiated n-C7H16–n-C8D18.9 Details on the
dislocations induced by octane solute molecules in n-C7H16–n-
C8H18 crystals and on the site selective proton transfer they
induce are presented below.
In n-alkane crystals (in the low temperature phase), alkane
molecules are in the extended all-trans conformation as indi-
cated by X-ray diffraction11 and infrared spectroscopy.12 This
also applies to alkane radical cations formed as a result of ion-
ization by ionizing radiation in n-alkane crystals, as has been
demonstrated by fluorescence detected magnetic resonance
(FDMR) experiments.13 EPR and FDMR experiments as well
as theoretical calculations show that in extended n-alkane rad-
ical cations the unpaired electron and associated positive hole is
delocalized over the C–C σ-bonds as well as the two chain-end
C–H bonds that are in the plane of the C–C skeleton.13,14 There
is now very substantial evidence that proton donation (at least
to neutral alkane molecules) takes place from such planar C–H
bonds,1–5 which appears quite reasonable in view of the weaken-
ing of these bonds and the presence of positive-charge (hole)
density on the respective hydrogen atoms. Structural diagrams
clearly show that in neat heptane crystals the planar chain-end
C–H bonds only come into close contact with primary C–H
bonds in other heptane molecules. We will now consider the
consequences if, in mixed heptane–octane crystals, the presence
of octane effects (at least partial) dislocation of adjacent hep-
tane molecules. With such dislocation, planar chain-end C–H
bonds in heptane radical cations would be in close contact with
secondary C–H bonds at the penultimate position in adjacent
heptane molecules as is evident from Figs. 6 and 7. Due to the
much higher proton affinity for protonation at a secondary than
at a primary C–H bond (see Fig. 8), proton transfer from hep-
tane radical cations (that are in their electronic ground state) to
heptane molecules would therefore be substantially enhanced,
simply as a result of the physical presence of octane in the
Fig. 7 Structural diagram showing the packing in heptane crystals
containing octane molecules in the extended all-trans conformation, in
which the chain-length mismatch causes a methyl group of octane to
protrude at the left hand side into the intermolecular boundary region;
(packing without strain or deformation and with full dislocation in
adjacent layers). Potential sites of proton transfer from planar chain-
end C–H bonds in heptane radical cations to penultimate C–H bonds in
heptane and octane molecules are indicated by solid arrows; potential
transfer sites from octane radical cations are indicated by dashed
arrows; ᭹ specifically indicates transfer to heptane molecules. The
potential sites for asymmetric proton transfer are indicated for the sake
of completeness only, their importance appearing rather minor.
system. In this transfer process, the penultimate position is
strongly favored for proton acceptance over the other second-
ary positions for structural reasons (see Figs. 6 and 7); it is also
the favored site with respect to proton acceptance on energetic
grounds.6–8 The transfer process would therefore result in select-
ive formation of heptane carbonium ions that are C–H proton-
ated at the penultimate position, neutralization of which by
chloride ions results in the selective formation of 2-chloro-
heptane. This hypothetical reaction scheme correlates nicely
with the observed increase in the prominence of 2-chloro-
heptane relative to the other secondary chloroheptane isomers
as a result of the presence of octane in the system, which indi-
cates its relevance. The results thus provide evidence for (at least
partial) dislocation of heptane molecules adjacent to octane
solute molecules in the crystal.
The structural diagrams shown in Figs. 6 and 7 indicate that
the proton transfer from heptane radical cations to heptane
molecules, induced by the physical presence of octane, takes
place predominantly (perhaps exclusively) along the a-axis of
the crystal. Due to the presence of octane, heptane molecules
(and heptane radical cations on passage of the positive hole)
are shifted by about one methylene unit, bringing planar chain-
end C–H bonds in heptane radical cations into direct contact
with secondary (penultimate) C–H bonds in heptane molecules
above and/or below the radical cations along the a-axis of the
crystal (with the single or dual transfer potential depending
on whether the reacting heptane molecule or heptane radical
cation is shifted in the crystal). Potential proton-transfer sites
between heptane radical cations and heptane molecules do not
appear to be created along the b- and c-axis of the crystal by the
presence of octane. The structural diagrams further indicate
that the dislocation of heptane molecules and the associated
selective proton transfer to C–H bonds at the C2 position
would for an ‘unstrained’ crystal occur regardless of the fact
that octane is in the extended all-trans or gauche-at-C2 con-
formation. The presence of octane puts considerable strain in
the crystal, however, which can (to some extent) be relieved by
partial squeezing and deformation of appropriate molecules. It
is evident that this process is more easily effected with octane
molecules that are in a gauche-at-C2 conformation than with
fully extended octane molecules; the displacement of adjacent
heptane molecules and the associated proton transfer will be
Fig. 6 Structural diagram showing the packing in heptane crystals
containing octane molecules in the extended all-trans and gauche-at-C2
conformation, in which the chain-length mismatch causes a methyl
group of octane to protrude at the right hand side into the inter-
molecular boundary region; (packing without strain or deformation
and with full dislocation in adjacent layers). Potential sites of proton
transfer from planar chain-end C–H bonds in heptane radical cations
to penultimate C–H bonds in heptane and octane molecules are
indicated by solid arrows; potential transfer sites from octane radical
cations are indicated by dashed arrows; ᭹ specifically indicates transfer
to heptane molecules.
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Such a process is restricted to octane radical cations in the
extended all-trans conformation; planar C–H bonds in octane
radical cations in the gauche-at-C2 conformation are structur-
ally quite similar to planar chain-end C–H bonds in (fully
extended) heptane radical cations and are most certainly not in
direct contact with secondary C–H bonds in adjacent molecules
(see Fig. 6). Also, the reaction appears largely limited to octane
radical cations that extend ‘at the back side’ in the inter-
molecular boundary region because of unfavorable directional
matching of the appropriate bonds in the alternative mode of
packing (see Figs. 6 and 7). Asymmetric proton transfer from
solute alkane radical cations to matrix molecules has been
observed in γ-irradiated n-C5D12–n-C8H18 crystals by EPR
spectroscopy.5 The importance of the process appears critically
dependent on positive-hole trapping by the solute alkane
molecule. Hole trapping by the solute alkane greatly increases
the lifetime of the solute radical cation in solid systems and
apparently makes the proton-transfer process much more
effective. This is indicated by the fact that in γ-irradiated
pentane-d12–heptane (at 0.5 mol% heptane), in which hole trap-
ping by heptane is very weak, the corresponding proton-
transfer process is not important. Clearly, the site selective
proton transfer from ground-state alkane radical cations during
the migration process of the positive hole does not take place at
every opportunity that would appear structurally favorable, i.e.,
formation of a radical cation in which planar chain-end C–H
bonds are in close contact with secondary C–H bonds in
adjacent alkane molecules. The actual occurrence of the pro-
cess in all likelihood depends critically on a suitable matching
of the phase of the vibrations of the (donor and acceptor) C–H
bonds involved. In the present n-C7H16–n-C8H18–2-C6H13Cl
system, hole trapping on octane radical cations is not expected,
in view of the limited difference in ionization energy between
heptane (9.93 eV) and octane (9.80 eV).16 An electronic absorp-
tion study of γ-irradiated heptane, containing 1 mol% 1-chloro-
hexane and 1 mol% octane, gave no indication of the presence
of trapped octane radical cations.9b Asymmetric proton trans-
fer from octane radical cations to heptane molecules is thus not
enhanced by hole trapping on octane radical cations in the
present system and is thus restricted to the accidental passage
of the hole on octane in its migration process through the
irradiated system, as it is with heptane in the symmetric transfer
process. Its occurrence and relevance to the formation of
2-chloroheptane should not be disregarded, however, especially
at the higher octane concentrations, in view of the documented
increase of all-trans vs. gauche-at-C2 conformers of the long-
chain component with increasing concentration of this
component, which appears to be very considerable.15
Fig.
8
Scheme establishing the energetics of pentacoordinated
heptane carbonium ions protonated at either primary or secondary
C–H bonds. The enthalpy of formation20 of the neutral alkane and
the heterolytic bond strengths21 were taken from the literature. The
enthalpy of formation of hydride ions was calculated from the
commonly available bond strength of H2 and electron affinity of
hydrogen atoms. The enthalpy changes associated with the dissociation
of primary and secondary C–H protonated heptanes were assumed
equal to those for the lower members of the alkane series and were
taken from the literature.22
affected accordingly, i.e., they are likely to be more extensive in
the vicinity of a fully extended octane molecule. Both fully
extended and gauche-at-C2 conformers of octane may be
expected to be present in the system studied, by analogy with
the structure of the binary solid nonadecane–henicosane, which
was investigated by IR spectroscopy using partially deuteriated
samples.15 Such a study has shown that the percentage of
henicosane in the gauche-at-C2 conformation increases with
decreasing henicosane concentration; at for instance 10 mol%
henicosane in nonadecane, about 40% of the henicosane mol-
ecules are in the gauche-at-C2 conformation. It is to be noted
that in the present heptane–octane system, the gauche-at-C2
conformation appears structurally excluded when the protrud-
ing alkyl group is at the ‘front’ of the molecule (i.e. at the
left hand side of the molecule as depicted in the structural
diagrams), because of the physical strain this would put on the
crystal (see Fig. 7).
Molecular packing in binary n-alkane crystals
The results obtained provide important information with
respect to the packing of alkane molecules in binary n-alkane
crystals and the disordering effects of the chain-length mis-
match. Asbach et al. have introduced two structure models
for the crystal imperfections in the border regions between the
layers in binary n-alkane crystals,17 which are characterized,
respectively, by the creation of voids (packing type A, longer
component predominant) and by the presence of end-gauche
conformers of the longer component (packing type B, shorter
component predominant). For intermediate compositions two
additional packing schemes have been proposed by Maroncelli
et al., involving (i) the interleaving of chains at the layer surface
(packing type C) and (ii) the pairwise packing of all-trans con-
formers of the short component with end-gauche conformers of
the long component (packing type D).15 In view of the range of
concentrations of octane in the systems under investigation,
packing type B is anticipated to apply largely in the present
study. With such a packing scheme the question arises as to
whether molecules of the longer component can be packed fully
The observed increase in prominence of 2-chloroheptane
with increasing octane concentration relative to other second-
ary chloroheptanes is partially due to asymmetric proton trans-
fer from octane radical cations to heptane molecules, which
occurs in parallel to the symmetric proton-transfer process
described above [eqn. (9)].
n-C8H18 ϩ ϩ n-C7H16
1-C8H17 ϩ 2-C7H17
(9)
ϩ
ؒ
ؒ
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in one molecular layer of the crystal (the width being deter-
mined by the shorter component), with the excess alkyl group
forced into a suitable conformation, and this without any dis-
location of adjacent molecules of the shorter component. The
structural diagrams shown suggest that such packing without
dislocation of molecules in adjacent layers appears improb-
able. Also, the results of Maroncelli et al. indicate that even at
relatively small concentrations of the long-chain component
(e.g., between 5 and 10 mol%) a substantial fraction of the long-
chain molecules in mixed n-alkane crystals are in the extended
all-trans conformation,15 which makes packing fully within one
molecular layer especially problematic. The experimental
results of the present study clearly point to dislocation of
molecules of the shorter component (in this case heptane) in
adjacent layers of the crystallites, as is indicated by the increase
in the formation of 2-chloroheptane relative to that of the other
secondary chloroheptanes by the physical presence of octane.
There must thus be a fair amount of interleaving even at low
concentrations of the long-chain component. It is likely, how-
ever, that the dislocations do not extend indefinitely through the
crystallites, which would result in unequal (and substantial)
shifting of the different rows of molecules and thus in complete
elimination of structurally related site selectivity in the proton-
transfer process. In fact, unlimited interleaving would destroy
the layer structure of the crystallites, whereas X-ray diffraction
studies show mixed n-alkane crystals to be packed in layers
analogous to neat n-alkanes, with the long spacing varying
smoothly with composition.17,18 The layer structure of the
crystallites can be preserved by partial squeezing, deformation
and changes in conformation of appropriate molecules, thus
allowing the crystal defect to be eliminated and crystal order to
be restored in layers (and rows) adjacent to a chain-length
mismatch. It is evident that the deformation can be most
easily effected in a gauche conformation. It is interesting in this
regard that the IR study of partially deuteriated nonadecane–
henicosane mixed crystals has shown gauche-at-C2 conformers
of nonadecane (the short-chain component) to be present,
whereas such conformers are virtually absent in neat nona-
decane crystals.15,19 On the basis of all this, it can be concluded
that the molecular packing in mixed n-alkane crystals at low
and medium concentrations of the long-chain component
has characteristics that correspond to both packing type B
(deformation, accompanied by formation of end-gauche con-
formers of the long- and also of the short-chain component)
and packing type C (interleaving of chains at the layer surface,
though only partial and restricted to the immediate vicinity of
the chain-length mismatch).
Electron, positive-hole and neutral-radical scavengers generally
act in the strict sense of the word in that they effectively (though
not necessarily fully) cleanse the system of the specific inter-
mediate by transforming it into a stable entity or a (less
reactive) intermediate associated with the scavenger. Proton
scavengers also can act as scavengers in the strict sense if they
interfere with protonated molecules of the system under
investigation. The role of octane in the present system is con-
siderably more complicated, however, and can hardly be
described as a simple proton scavenger. For a proper under-
standing it will be useful to outline the triple action it exercises.
(i) With respect to the formation of 2-chlorooctane9b it acts as a
proton scavenger (proton acceptor), but the proton is not trans-
ferred from protonated heptane molecules but from heptane
radical cations [eqn. (10)].
n-C7H16 ϩ ϩ n-C8H18
1-C7H15 ϩ 2-C8H19
(10)
ϩ
ؒ
ؒ
As a result of this scavenging action, only the ionic part of
the reactivity is transferred to the scavenger, whereas the radical
reactivity remains on a species associated with the system. (ii)
With respect to its effects on the isomeric composition of
secondary chloroheptanes discussed in the present paper its
action is double in nature: (a) it acts as a temporary positive-
hole scavenger and the resulting octane radical cations may act
as proton donors to heptane molecules; (b) it dislocates heptane
molecules in the crystallites and brings penultimate C–H bonds
in these molecules into direct contact with planar chain-end
C–H bonds in heptane radical cations, causing proton transfer
to heptane molecules to take place from heptane radical cations
in their electronic ground state. The latter action is quite
intriguing (and quite novel) as octane exercises this action
without interfering directly with the radiolytic process at all;
its action in this regard is purely physical in nature.
Acknowledgements
Financial support from the Fund for Scientific Research of
Belgium (F.W.O.-Vlaanderen) is gratefully acknowledged.
References
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1880
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View Article Online
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