Role of Defects in Radiation Chemistry of Crystalline Organic

Role of Defects in Radiation Chemistry of Crystalline Organic Materials. 4. Crystal Structure Dependence of Vinyl-Type Radical Formation in Alkyne/n-A...
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J. Phys. Chem. 1994, 98, 10112-10118

10112

Role of Defects in Radiation Chemistry of Crystalline Organic Materials. 4. Crystal Structure Dependence of Vinyl-Type Radical Formation in Alkyneln-Alkane Mixed Crystals As Studied by ESR Spectroscopy Kaoru Matsuura and Hachizo Muto' National Industrial Research Institute of Nagoya, f Hirate-cho, Kita-ku, Nagoya 462, Japan Received: March 16, 1994; In Final Form: June 21, 1994@

An ESR study has been made on the migration and reactions of H atoms in 1-alkyne (0.5-7.0 mol %)/nCmHzm+2(m = 10-13) mixed crystals irradiated at 77 K in order to elucidate the alkane radiolysis. A vinyl type of radical (Rv; CHyC-CH2-R) was efficiently formed (-20%) in the orthorhombic mixed crystals having odd carbon numbers m but not in the triclinic crystals having even m. For pure n-alkanes, the terminaltype alkyl radical (RI; CH~-CHZ-CH~-) was efficiently formed (-25%) only in the former crystals in addition to penultimate (Rn; CH~-CH-CHZ-) and interior (Rm; -CHz-CH-CH2-) radicals. The yield of RVradical increases and the RI yield concomitantly decreases with increases in the solute concentration. The sum of their yields was almost constant, indicating the same precursor for both radicals. These results could be understood only by H atom reactions. The C-H bond dissociation of the excited molecules formed by holeelectron geminate recombination would occur more favorably for secondary -CH2- bonds than primary CH3- bonds having a large bond strength, resulting in RII and RIIIradical formation along with an equal amount of H atoms. The H atoms produced were usually converted to RIIand RJJIradicals by H abstraction from the secondary -CH2- bonds, as was observed in triclinic alkanes. In orthorhombic alkanes, where the molecules are packed with a wider molecular layer boundary than triclinic alkanes, some part (about half) of H atoms can escape to the boundary region and can migrate. They can abstract H atoms only from CH3 bonds to form RI, since only terminal CH3 groups are exposed to the regions in the orthorhombic alkanes. In 1-alkyne/orthorhombic alkane mixed crystals, the escaped H atoms can add only to the outer carbon atom of HC=C-CH2triplet bond, which is exposed to the boundary region, to form H~C==C-CHZ- vinyl-type radicals (no formation of HC=CH-CH2-). The present results reveal an important role of the layer boundary regions in the migration and reactions of H atoms and provide clear evidence for the H atom reaction mechanism in alkane radiolysis.

Introduction The elucidation of the primary process of radical formation is an important subject in the radiolysis of n-alkanes. A large number of works have been reported,'-* and the following two distinctive mechanisms have been proposed for the alkyl radical formation. (1) H atom reaction6'8b

RH, RH;+

- + - RH^* RH;+

e-

+ eRH' + H' RH2 + H' - RH' + H2

(1.1) (1.2) (1.3)

(2) ion-molecule reaction3"

+ RH, - RH' + RH,+ RH,+ + e- - RH' + H,

RH;+

(2.1)

(2.2) H atoms have never been detected in alkanes even at 4.2 K, except for methane.2.9 It is supposedly difficult for the H abstraction reaction to take place at cryogenic temperatures because of its large activation energy. Therefore the ion+ @

Renamed from Govemment Industrial Research Institute, Nagoya. Abstract published in Advance ACS Abstracts, September 1, 1994.

0022-365419412098-10112$04.50/0

molecule reaction mechanism (2) has been proposed for alkanes other than C&. The generation of H atoms only in CHq has been interpreted, as a special case, by the higher electron mobility than other alkane^.^ Namely, it is supposed that the germinate recombinations by return of electrons and C-H bond breakage (1.2) are faster than the ion-molecule reaction (2.1) in CH4. Whereas for H atom mechanism (l), it has been elucidated by the radiolysis of C2HdCH4 mixtures that H atoms can abstract H atoms from alkane (C2H6) molecules even at cryogenic temperatures (-20 K) but not from C&.loa The stable trapping of H atoms in CHq and no reaction of H atoms with CH4 in CzHdCH4 mixtures are interpreted by the large bond strength of CH4. The H atom reaction was also observed in the HI/CmH2,+2/Xe mixtures, where H atoms produced by photolysis abstracted H atoms from alkane molecules except for C h . l o b These reactions were well understood by the quantum tunneling effect and eliminated the problem of H atom reactions occurring at cryogenic temperatures?JO Solid alkanes have two kinds of crystal structures, which give rise to a large difference in formation of terminal-type radical (CHz-CH2CHz-), as is described later. The cause of the difference is also open to question in addition to the above radiolysis mechanism. We previously studied the radiolysis of CmH2m+2(1 mol %)/ C10Dz2 (m = 7-12) mixed crystals at 77 K.8b In the mixed crystals, the protiated solute radicals were efficiently formed, and the kinds of solute radicals depended on the difference of the chain lengths between the solute and the matrix molecules (Anc). Namely, the terminal radical (RI; CH~-CH~-CHZ-) 0 1994 American Chemical Society

J. Phys. Chem., Vol. 98, No. 40, 1994 10113

Radiation Chemistry of Crystalline Organic Materials was formed in the mixtures with shorter solute molecules, and the penultimate radical (Rn; CH3-CH-CH2-) and interior radical (Rm; CHz-CH-CHz-) were formed in the mixtures with longer solute molecules than the matrix molecules. Such efficient radical formations were not observed in the mixed crystals irradiated at 4.2 K. The efficient solute radical formations at 77 K were understood by the easier migration of D atoms at 77 K than at 4.2 K and by a tunneling H abstraction reaction with a large H/D isotope effect. The Anc dependence of the kinds of solute radicals could be reasonably understood by the D atom reaction in the molecular layer boundary regions of the crystalline alkanes. These results indicate that the hydrogen atom reaction mechanism 1 is preferable in solid alkane radiolysis. On the other hand, no efficient solute-radical formation was observed in protiated mixtures (CmH2m+2/C1d-122). In order to elucidate the radiolysis of usual protiated alkane crystals, we have presently studied the radiolysis of 1-alkyne/ n-alkane mixed crystals and the crystal structure dependence of the H atom addition reaction to alkyne molecules. The long-chain hydrocarbons with the number of carbons nc = 10-25 have layer structures.ll Those having odd and even number of carbons have orthorhombic and triclinic crystal structures, respectively, at low temperatures. Three types of alkyl radicals (RI, RII and RIII)are formed in the solids of pure n-alkanes by irradiation at 77 K. The formation of the terminal type of radical (RI; CH~-CHZ-CH~-) strongly depends on the crystal structures. It is efficiently formed in the odd alkanes (-25% of the total radical yield), whereas in the even n-alkanes RI is only a minor product. The difference in the RI radical formation can be explained by the following two distinct mechanisms: radical site transfer or migration and reaction of H atoms.8bs12The two mechanisms are related to the radiolysis mechanisms (1) and (2), as will be discussed in this report. It has been found in the present study that, in the alkyne/ alkane mixed crystals having orthorhombic structures, a vinyl type of radical (R,) is efficiently formed by addition of H atoms to the alkyne molecules but not in those having triclinic structures. A concomitant generation of R, and RI radicals was observed in the former mixed crystals, indicating the same precursor (namely, H atoms) for both radicals. A suppression of RI formation was observed for binary alkane mixed crystals having orthorhombic structures. These results are consistently understood by the reactions of the H atoms which have escaped to the layer boundary regions and by the difference in the molecular packing dependent on the crystal structures, c o n f i i ing the H atom reaction mechanism for the alkane radiolysis.

Experimental Section n-Alkanes of gas chromatographic standard grade (nc = 1013, >99.5%; Tokyo Kasei Kogyo Co. Ltd.) and 1-alkyne (nc = 10-13, >99%; Tokyo Kasei Kogyo Co. Ltd.) were used without further purification. The solutions of alkyneln-alkane and binary alkane mixtures were sealed off in sample tubes made of high-purity quartz after being degassed on a vacuum line. They were irradiated by 6oCo y-rays at 77 K with a dose of 5 kGy. The doses were low enough to prevent the dose saturation of radical formation. The setup of the cryostat, ESR measurements and the data treatment have been described elsewhere. l3 In the present study two methods were used to freeze the samples: slow cooling and rapid quenching to 77 K from the liquid phase. Both methods gave the same ESR spectra and results. Results Pure n-CloHzz and 1-Decyne(5.0 mol %)ln-C10Hz2. Figure l a shows the first-derivative absorption ESR spectrum observed

c-r

2.5 mT

'

V

\i

Figure 1. First-derivative ESR spectra observed for (a) n-ClaHzz and (b) 1-decyne(5.0mol %)ln-C1&2 mixed crystals irradiated at 77 K and (c) after illumination of infrared light (d > 690 nm). (d) Difference

spectrum (b) - (c). The species easily bleached by infrared light was assigned to the 1-decyne anion radical. The first-derivativeESR spectra observed for (e) n-Cl1H24 and (f) 1-undecyne(5.0 mol %)In-C11H24 mixed crystals irradiated at 77 K and (g) after illumination of infrared light (2 > 690 nm). (h) Difference spectrum (f) - (g). for the pure n-decane polycrystals y-irradiated at 77 K. Two kinds of alkyl radicals, the penultimate type (RII; CH3-CHCH2-) and the interior type (RIII; -CHz-CH-CHz-), are mainly formed, and the yield of the terminal type (RI; CH2CHz-CHz-) is low (nearly zero) in even alkanes having triclinic crystal structures as reported p r e v i ~ u s l y .Figure ~~ 1, b and c, are the spectra observed for 1-decyne (CHEC-CHZ-C~H~S; 5.0 mol %)/n-CloH22 mixed crystals irradiated at 77 K and after illumination of infrared light (A > 690 nm), respectively. The species which was easily bleached by infrared light was assigned to the 1-decyne anion (Figure Id). The ESR spectrum of the species was essentially the same as that of 1-hexyne anion.14 The anion decayed without being converted to other radicals. The ESR spectrum after photoillumination (Figure IC) was almost the same as that of pure n-CloH22 irradiated at 77 K (Figure la), indicating no formation of solute radicals other than the 1-decyne anion. The amount of the anion was about 6% (G 0.3) of the total radical yield. Pure n-CllHU and 1-Undecyne(5.0 mol %)/n-CllHU. Figure l e is the ESR spectrum of the pure n-undecane polycrystals y-irradiated at 77 K. It is known that RI radical is efficiently produced in the odd alkanes in addition to RII and Rm radi~als.4~J~ Figure 1, f and g, are the ESR spectra observed for 1-undecyne (CH=C-CH~-C~HI~;5.0 mol %)/n-C11&4 mixed polycrystals irradiated at 77 K and after illumination of infrared light, respectively. The undecyne anion radical was also formed (Figure lh) and was easily bleached by infrared light. The yield of 1-undecyne anion in this mixtures was slightly lower (about 2%; G 0.1) than the yield of 1-decyne anion in l-decyne/n-C1oH22 mixed polycrystals.16 In l-undecyneln-Cl2Hz6 mixed crystals having the same triclinic structures as C10H22, the yield of 1-undecyne anion was as much as the yield of 1-decyne anion in C10H22 matrix (-6%). The yield of the 1-decyne anion in n-ClIH24 was as low as that of the 1-undecyneanion in n-CI1Hw (-2%). Therefore, the difference of the anion yields in even and odd hydrocarbons should be ascribed to the difference in the crystal structures. The details will be discussed in a separate paper.16 Contrary to the case of

-

-

10114 J. Phys. Chem., Vol. 98, No. 40, 1994

Matsuura and Muto n

, I

I

./I

/I

h

c-

25mT

25mT

...'

.

.. ...........,

Figure 2. First-derivative ESR spectra observed for (a) n-C11H24 and (b) n-C13Hzs(5.0 mol %)/n-C11H24 mixed crystals irradiated at 77 K. The spectrum of (a) is composed of three kinds of alkyl radicals, CHzCHz-CgH19 (RI), CH~-CH-CHZ-C~HI~ (Ru), and CH3-CHz-CHCHZ-C~H~S (Rm), while that of (b) is mainly composed of RE and Rm. (c) Difference spectrum (a) - (b). (d) Simulated spectrum of Rr using the ESR parameters reported. (e) - (h) Integrated spectra of (a)-(d), respectively.

triclinic mixed crystals (1-decyne/n-decane), the ESR spectrum of the above orthorhombic mixed crystals (l-undecynelnundecane) after illumination (Figure lg) significantly differs from that obtained for the matrix alkane (pure n-undecane) irradiated at 77 K (Figure le), indicating efficient formation of solute radicals other than the anion. The solute radical was assigned to a vinyl type of radical (R,) as will be described later. The yields of RI and R, radicals were estimated from their component spectra obtained by the deconvolution of the ESR spectra of the n-undecane polycrystals and 1-undecyne/nundecane mixed crystals as follows. Figure 2a is the ESR spectrum of the pure n-C11H24 irradiated at 77 K (the same spectrum as Figure le), where three kinds of alkyl radicals, RI, RII, RIII,are formed. Figure 2b is the ESR spectrum observed for C13H28(5.0 mol %)lCilH24 mixed crystals irradiated at 77 K. In this binary n-alkane mixtures, the formation of RI is suppressed and Rn and R ~are I mainly formed. The reason will be described later. The component spectrum of RI (Figure 2c) was obtained by subtracting the spectrum in Figure 2b from that in Figure 2a. It could be well simulated using the reported ESR parameters for the terminal radical (CH~-CHZ-CHZ-) as shown in Figure 2d.15 Parts e-h of Figure 2 are the integrated spectra (absorption form) of Figure 2, a-d, respectively. The yield of RI was estimated from these integrated spectra to be 25 & 4% of the total radical amount. The spectrum of 1-undecyne(5.0 mol %)/C11H24 mixed crystals irradiated at 77 K (Figure 3a; the same spectrum as Figure Ig) was found to be composed of four kinds of radicals: RI, Rn, and RIIIand a vinyl-type radical (R,). The spectrum in Figure 3b, which is the same spectrum as that in Figure 2b and is mainly composed of RII and Rm, was subtracted from the spectrum in Figure 3a containing four kinds of radicals. The spectrum thus obtained is shown in Figure 3c. It is composed of RI and R, radicals. Further subtraction of the component spectrum of RI (Figure 3d) from that in Figure 3c was carried out, giving the component spectrum of the R, radical, which is shown in Figure 3e. The R, radical was assigned to a vinyl type of radical having HzC=C-CHzstructures by ESR spectral simulation using a set of proton hyperfine couplings: a(HzC=C-) = 3.0 and 5.8 mT and a(=C-CHz-) = 1.1 and 3.0 mT (Figure 30. It was found from the ESR parameters

x2

Figure 3. First-derivative ESR spectra observed for (a) l-undecyne(5.0 mol %)/n-C11H24 mixed crystals and (b) n-C13Hz8(5.0 mol %)/nCllH% mixed crystals irradiated at 77 K. The spectrum of (a) is composed of four kinds of radicals; CHZ-CH~-C~HI~ (RI), CH3-CHCH2-C&i17 (RE),and -CH2-CH-CH2(Rm) and a vinyl-type radical, C H Z = C - C ~ H Lformed ~, by H atom addition to 1-undecyne molecules. (c) Difference spectrum (a) - (b). (d) The first-derivative ESR spectrum of RI. (e) Difference spectrum (c) - (d). (0 Simulated spectrum of the vinyl-type radical, CHz=C-C9H19, using the ESR = 1.1 parameters a(HzC=C-) = 3.0 and 5.8 mT and n(%-CH2-) and 3.0 mT. (g)-(1) Integrated spectra of (a)-(Q, respectively.

that the other vinyl radical having HC=CH-CHz- structures was not formed. This result indicates that hydrogen atom is added only to the outer carbon atom of HCSC-CHz- triple bond. The yields of RI and R, radicals were estimated from their integrated ESR spectra (Figure 3g-1). The dependence of the R, and RI radical yields on the solute concentration was examined to elucidate the mechanism of radical formation in alkanes. Figure 4a shows an example of the ESR spectrum obtained for the mixtures containing a low concentration of 1-undecyne(O.5 mol %). The deconvolution of the spectrum and estimation of RI and R, radical yields were made in the same way as 1-undecyne(5.0 mol %)/n-C11H24 mixed crystals as is shown in Figure 4a-j. l-Dodecyne/n-C~zHz,jand l-Tridecyne/n-C13H28 Mixed Crystals. Thus, we found that the R, radical formation from the solute 1-alkyne strongly depended on the crystal structures: no formation in l-decyneln-CloHzz mixed crystals (triclinic) and efficient formation in l-undecyneln-Cl1H24 (orthorhombic). A similar experiment was carried out using n-ClzH26 and n-C13H28 matrices in order to ensure that this crystal structure dependence is also observed in other alkane matrices. The ESR spectra obtained for the latter two kinds of mixed crystals were essentially the same as those of the former two kinds of mixed crystals, respectively. Figure 5a shows the ESR spectrum obtained for l-dodecyne/n-ClzH~6. In this triclinic matrix R, was not formed. Only a small amount of 1-dodecyne anion was formed. On the other hand, a large amount of R, was formed in 1-tridecyne(5.0 mol %)/n-C13H28 mixed crystals (with orthorhombic structures) as shown in Figure 5b.

J. Phys. Chem., Vol. 98, No. 40, 1994 10115

Radiation Chemistry of Crystalline Organic Materials

:v . . .::. . . . ..,. . . . . . . . . . . .

.........

Figure 7. First-derivative ESR spectra observed for (a) n-CsHls(5.0 mol %)/n-C11H24 mixed crystals, (b) n-C14H30(5.0 mol%)/n-C11H24 mixed crystals, and (c) pure n-CllH24 polycrystals, irradiated at 77 K.

. 2.5 mT

.........

...

x 2.5

........................ ...................... ........ ....................... ............................. Figure 4. First-derivative ESR spectra observed for (a) l-undecyne(0.5 mol %)/n-C11H2~mixed crystals and (b) n-C13H2~(5.0mol %)/nCllH24 mixed crystals irradiated at 77 K. (c) Difference spectrum (a) - (b). The simulated ESR spectra of (d) the terminal type of alkyl radical, C H ~ - C H ~ - C ~ H I ~ and (e) the vinyl-type radical, CH2=C(RI), C9H19, respectively. (Q-0) Integrated spectra of (a)-(e), respectively.

increases and the RI yield concomitantly decreases with increasing the solute concentration in both mixed crystals. The sum of their yield is independent of the solute concentration and is almost constant (-25%) within the experimental errors (f4%). n-C,Hzn+2(5.0 mol %)/n-C1IHu(n = 8 and 14). Figure 7, a and b, are the spectra observed for n-CsHlg(5.0 mol %)lnCllH24 and n-C14H30(5.0 mol %)h-C11H24 binary alkane mixed crystals, respectively, irradiated at 77 K. The RI formation was greatly suppressed in both mixed crystals, as is easily seen from a comparison with the ESR spectrum of the pure n-undecane polycrystals (Figure 7c), where a large amount of RI (-25%) was formed as described before. Total Radical Yield. The total radical yield in the mixtures having orthorhombic crystal structures was estimated from the doubly integrated intensities using the procedures described e1~ewhere.l~The yield in each mixed crystals is the same as the yield in pure odd alkanes (n-CllH24 and n-Cl3H28) within experimental errors (&4%) and is independent of the solute concentration. This result indicates that the solute molecules do not act as a sensitizer for increasing the total radical amount and that the yield of the solute radicals balances the decrease of the yield of matrix radicals (Le., the decrease of RI radicals).

Discussion The present experimental results are summarized as follows. (1) In the n-alkanes having orthorhombic structures, the terminal Figure 5. (a) First-derivative ESR spectrum observed for l-dodecyne(5.0 mol %)/n-C12H26mixed crystals irradiated at 77 K. Dashed line is that observed after illumination of infrared light (1 > 690 nm). (b) The fust-derivative ESR spectrum observed for y-irradiated 1-tridecyne (5.0 mol %)/n-CI& mixed crystals after illumination of infrared light. (ai

Cd

i ;14,; ;

l! Solute concentration (mol%)

"

I

1

'

"

1 2 3 L 5, 6 7 Solute concentration (mol%)

Figure 6. Dependence of the yield of the terminal type of radical RI (0)and the vinyl-type radical, CH2=CH-C9H1gr ( 0 )and the sum of H2~ them (A) on the solute concentration in (a) l - u n d e ~ y n / n - C ~ ~and (b) l-tridecyneln-Cl3H28 mixed crystals.

Solute Concentration Dependence of the Yields of RI and R, Radicals. The yields of RI and R, radicals and their sum are plotted in Figures 6, a and b, for l-undecyne/n-C11H24 and l-tridecyneln-C13H28 mixed crystals, respectively, as a function of the solute concentration. It was found that the yield of R,

type of radical (RI) was efficiently formed (-25%) in addition to the penultimate radical (RII) and the interior radical (RIII). Whereas in those having the triclinic structures, only RII and RIIIwere formed. (2) In the I-alkyneln-alkane mixed crystals having orthorhombic structures, the vinyl type of radical (R"), H~C=C-CHZ-, was formed by addition of H atoms to the outer carbon atom of the HCEC-CH2- triple bond. Whereas in the mixed crystals having triclinic structures, neither RI nor R, radical was formed at the solute concentration lower than 7.0 mol %. (3) The R, yield in orthorhombic matrices increased and the RI yield concomitantly decreased with increasing the solute concentration. The sum of their yields did not depend on the solute concentration and was almost constant (-25%). (4) The formation of RI was suppressed in CnH2,+2/CllH24 (n = 8 and 14) mixed crystals. From these results, we concluded that in the n-alkane and the I-alkyneln-alkane mixed crystals having the orthorhombic structures both RI and R, radicals were formed via the H atom reaction mechanism as is discussed below. Crystal Structure Dependence of Radical Formation in Pure n-Alkane Polycrystals. Crystal Structures of Pure n-alkane. The long n-alkanes have layer structures and those having the odd and the even carbon numbers have orthorhombic and triclinic structures, respectively, as shown in Figure 8a and 8b." It is reported that the density is higher in even linear alkanes than in odd alkanes.17 The cross section per molecule projected along the molecular chain axis is estimated to be nearly

10116 J. Phys. Chem., Vol. 98, No. 40, 1994

Matsuura and Muto

structure

the molecular boundary regions. Some part of the H atoms radiolytically produced (a2) may abstract H atoms more easily from secondary -CH2- than from primary CH3- bonds to form RII and R ~ radicals I rather than RI. The other part of the hydrogen atoms may escape to the molecular layer boundary regions, especially in orthorhombic crystals which have wider boundaries. In the orthorhombic alkanes, they are forced to abstract hydrogen atoms from the terminal -CH3 bonds and producing RI radicals, since only CH3 groups are exposed to the boundary regions, even though the CH3 bond is stronger than the secondary -CH2- bond. The amount of H atoms escaped to the boundaries (Hes)can be estimated as follows. In reaction 2, alkyl radicals (mainly RII and RIII)and H atoms are reasonably supposed to be generated at a ratio of 1:l (neglect of the RI formation). Suppose the escaped H atoms abstract only the primary CH3 hydrogen atoms. Then the amount of the He, equals to the yield of R1 (-25% of the total radical yield). Thus, about half of the hydrogen atoms initially produced in reaction a2 escape to the boundary regions and produce RI (-25%) by H abstraction from the primary CH3bonds. Other H atoms may abstract the inner -CH2- secondary H atoms of alkane molecules to give RII and RIII before migrating to the boundary regions. Whereas, in the triclinic crystals, the H atoms produced are difficult to escape to the boundary regions, since the molecules are more tightly packed in the regions than in orthorhombic crystals. Even if H atoms can escape to the boundary regions, abstraction of H atoms occurs more easily from -CH2- (having a smaller bond strength) than from CH3- groups because -CH2- groups bonded to the CH3 are also exposed to the regions (Figure 8). Radical site transfer mechanism is less preferable to explain no formation of RI radical in even alkanes according to the following experimental results obtained for C & I ~ ~ - C H ~ C ~ / mixed C~~H crystals Z ~ irradiated at 77 K. A large amount of RI radicals were formed from the solute haloalkane molecules by dissociative electron attachment reaction and stably survived at 77 K. If R1 radicals were produced via homolytic C-H bond scissions of the excited molecules in pure even alkanes (triclinic), and if they were converted to Rn or Rm radical via the radical site transfer mechanism, the RI radical would also disappear in the above mixed crystals. However, it is not observed, indicating a less possible site migration. Crystal Structure Dependence of Radical Formation in 1-Alkyneln-Alkane Mixtures. The possible mechanisms of the efficient formation of vinyl radical observed can be classified into the following five cases, A-E. A. H atom reaction: S H' B. proton addition to alkyne anions: So- H+ (or RH3+) C. hole transfer and recombination with H-: So+ HD. proton transfer to alkyne and e- capture: S H+ (or RH3+) and eE. H atom abstraction by excited alkyne formed by energy transfer: S* RH2 where S denotes solute alkyne. Mechanisms S RH2* S H' (+RH') SH' RH' and RHz'+ S'RH' H+ SoRH' SH' are included in A and B, respectively. These mechanisms must also explain the experimental results (1)-(3), which are summarized in the beginning of the Discussion Section. A . HAtom Reaction. In the case of 1-alkyneln-alkanemixed crystals, the efficient formation of the vinyl-type radical only in orthorhombic crystals (results 2 and 3) can be also understood by a similar mechanism as the efficient RIformation. In the orthorhombic pure alkane crystals, about half of the produced hydrogen atoms (Hes) escape to the boundary regions and are forced to abstract H atoms from the CH3 bonds (a4), since only

a) orthorhombic structure

b) triclinic

Figure 8. Molecular packing modes in solid n-alkanes having (a) orthorhombic and (b) triclinic crystal structures. The former alkanes have a wider molecular layer boundary, where only CH3 groups are exposed. In the latter alkanes, the penultimate -CH2-(CH3) groups are also exposed.

the same each other from the X-ray crystallographic data.18 Therefore, the difference in the densities comes from the boundary regions between the layers. The lengths between the layers were found to be shorter in triclinic crystals ( r c e = 2.7 A) than in orthorhombic crystals (IC< = 3.1 A) from the X-ray data.18 Another difference between two kinds of crystal structures is that in the orthorhombic crystals only the terminal CH3- groups expose themselves to the boundary regions. Whereas, in the triclinic crystals -CH2- groups (penultimate carbon) bonded to the CH3 groups are also exposed to this regions (Figure 8). Radical Formation in Alkanes. The terminal type of radical (RI; CHz-CHz-CH2-) was produced only in orthorhombic crystals in addition to RII and R I radicals ~ and not in triclinic crystals (result 1). Two distinctive mechanisms are possible for this large crystal structure dependence of the RI radical formation. One is the intermolecular radical site transfer, and the other is the migration and reactions of H atoms in the molecular layer boundaries.8.12The crystal structure dependence of radical formation in 1-alkyneln-alkane mixed crystals and in n-alkane crystals can be reasonably and consistently understood only by the latter mechanism as follows. In the radiolysis of pure n-alkane polycrystals, the homolytic C-H bond scissions of the excited molecules, which are formed by the geminate recombination of holes and electrons, produce alkyl radicals (RH) and hydrogen atoms at a ratio of 1:l (a1 and a2). In the H atom reaction mechanism, the hydrogen atoms react with the matrix alkane molecules and produce the alkyl radicals (a3).

+ e- - RH^* - RH' + H' RH, + H' RH' + H,

RH;+

--.

(a2)

(a31

Rn and R I will ~ be more favorably formed than RI via the homolytic C-H bond scissions of the excited molecule (a2) and the hydrogen abstraction reaction (a3). This is because the secondary bond ( D O ~ ~ ~ ( - C H=~95 - ) kcaymol) is weaker than the primary C-H bond (Do298(-CH3)= 98 kcaymol). Thus, no formation of RI in triclinic crystals (result 1) can be understood without taking into consideration of intra- or intermolecular radical site transfer. No RI radical formation in branched alkanes may be understood by the same mechanism.19 On the other hand, the efficient formation of RI (-25%) in odd alkanes with orthorhombic crystal structures (result 1) is a special case and can be explained by the H atom reaction in

+

+

+ + + - + - ++

+

+

+

- -+

+

J. Phys. Ckem., Vol. 98, No. 40, 1994 10117

Radiation Chemistry of Crystalline Organic Materials CH3 groups are exposed to the regions as described in the previous section. In the alkyneln-alkanemixtures, the He, atoms can migrate for some distance to encounter the CGC triple bond of the alkyne molecule and easily add to the HCsC-CHztriple bond (a5) rather than abstract H atoms from CH3 groups having a large bond strength. The H atom addition to only the outer carbon atom of the triple bond (result 2) can be reasonably understood by the H atom migration and reaction at boundaries. The migration of He, atoms in orthorhombic crystals is easier because the width of the molecular boundary regions is larger in orthorhombic crystals ( r c q = 3.1 A) than in triclinic crystals (IC< = 2.7 A). Result 3, concomitant formation of the R1 and vinyl-type radicals (R,), indicates the same precursor for both radicals and is clear evidence for H atom reaction mechanism. In pure orthorhombic alkanes, He,

-

+ CH,-CH,-CH,

CH2-CH2-CHz-

(R,)

+ Hz

from an alkane molecule to the alkyne cation radical (c2). The ionization potential (IP) of C11Hz4 is not known. Since IPS of n-alkanes become lower as the chain length becomes longer and since the IPof C7H16 is reported to be 9.9 eV, it is estimated to be lower than 9.9 eV. The IP of I-undecyne is not known, either. The IPS of 1-alkynes are reported to be 10.36, 10.18 (or 10.34), and 10.39 eV for methylacetylene (propyne), ethylacetylene (1-butyne), and 1-pentyne, respectively.*l The IP of 1-undecyne may be about 10.3 eV because the IPS of I-alkynes are mainly determined by the CEC bond and independent of the chain length. Therefore, the hole-transfer mechanism from CllH24" alkane cation to 1-undecyne is less preferable than the H atom reaction mechanism. D . Proton Transfer to Alkyne Molecules and e- Capture RH,"

(a4) H,C=C+-CH,-R

In the orthorhombic 1-alkyneln-alkane crystals He,

-

+ HC=C-CH,-R

H,C=C-CH,-R

(R,)

(a5)

Whereas in triclinic crystals, even if hydrogen atoms can escape to the boundry regions, they cannot migrate long enough to encounter the C z C bond because of the higher potential barrier due to the narrower boundary. Moreover, atoms will abstract H atoms from the penultimate -CH2- groups, which are also exposed to the boundary region in triclinic mixed crystals, in the same way as in the triclinic pure alkanes. Other possible mechanisms can be excluded as follows. B . Proton Addition to Alkyne Anions

+ RH, - RH' + RH,+ HC=C-CH,-R + e- -HC=C-CH,-R*HCEC-CH,-R*- + RH,+ H,C=C-CH,-R' (or HC=CH-CH,-R*) + RH, RH;+

(bl) (b2)

RH,"

RH,

+ HC=C-CH,-R

+ HC=C-CH,-R*+

H,C=C-CH,-R'

-

RH,

(or HC=CH-CH,-R')

(cl)

+ RH+ (c2)

In this mechanism, the alkyne cation radical is assumed to be formed by hole transfer from the matrix alkane cation to the solute alkyne molecule (cl) following the ionization reaction (al). The R, radical might be formed by hydride (H-) transfer

+ H,C=C+-CH,-R (dl)

+ e- - H,C=C-CHz-R'

(d2)

-

+ eRH' + RH,+

RH;+

(d3)

+ RH, RH3+ + RH, - RH, + RH,' (H+ migration) RH,+ + e- - RH' + H,

(b3)

+ HC=C-CH,-R'+

RH'

Since the proton affinity (PA) of alkyne is large (PA for ethylene is 6.8 eVz2and that for alkyne must be larger because CGC triple bond is more electron-rich than the C=C double bond), proton transfer from alkane radical cation (hole) to the alkyne molecule should be considered. This mechanism requires either (i) the radiolytical production of RH2'+ closely located to alkyne molecule or (ii) proton migration over the long distance to the alkyne molecule. However, it would be difficult for process i to occur since the radiation-induced ionization takes place randomly in the crystalline lattice without any relation with the location of alkyne molecules. Process ii may also be difficult since geminate electron-hole (RH,'+) recombination occurs very fast and is completed before the proton can reach alkyne molecule in the ion-molecule reaction mechanism involving proton transfer (d3 -d6).

RH,

After ionization reaction a l , the ion-molecule reaction b 1 might occur between IU&*+and an alkane molecule to form an alkyl radical and a diamagnetic cation, RH,+. It might be possible that R, was formed by proton (H+) transfer from RH3+ to the 1-alkyne anion (b3). Such proton transfer to the anion radical was suggested in our previous studies for the irradiated polar organic compounds such as amino acidszoaor fumaric aciddoped succinic acid, which have acidic protons.z0b If it were the case for nonpolar alkanes, the RI yield would not depend on the solute alkyne concentration, since RI and R, are individually converted from hole (bl) and electrons (b2 and b3), respectively. Thus, result 3, concomitant formation of RI and R,, cannot be explained by this mechanism. C . Hole-Transfer Mechanism

-

+ HCEC-CH2-R

RH;+

(d4)

(d5) (d6)

A slow recombination of electrons with cationic species, H-C+=CH-R', in (d2) occurs only for free electrons. However, the free electron yield in alkane matrices (Gf < 0.1) is too low to explain the high yield of vinyl radical presently observed (Gvinyl= 1.0 at the solute concentration higher than 2.0 mol %). Even though these difficulties were excluded, the concomitant formation of R, and RI radicals cannot be explained by this mechanism combined with mechanism (1) H atom reaction (al-a3) or (2) ion-molecule reaction (d3-d6) as follows. In the combined mechanism of D and (2), since the production mode of alkyl radical by deprotonation in (bl), RH,'+ RHz RH' RH3+, and (dl) RH2" HC=C-R' RH' H,C=C+-R', are identical, the relative formation of RI, RII, and RIIIradicals are not affected via (bl) and (dl). Therefore, the decrease in the RI yield due to introduction of the solute alkyne, which balances the formation of R, radical (-20% at the solute concentration higher than 2.0 mol %), must be ascribed to reaction d6. Namely, reaction d6 must selectively produce the RIradical. The selective breakage of primary C-H

-

+

+

-.

+

+

10118 J. Phys. Chem., Vol. 98, No. 40, 1994

Matsuura and Muto

bond (-CH3) having the largest bond strength is difficult to take place as is discussed before, eliminating this combined mechanism. For the combined mechanism of D and (l), in the pure n-alkanes the alkyl radicals produced in process a2 are reasonably assumed to be RII and Rm from the bond strengths as described in the section of radical formation in alkanes. The radical yields via (a2) and (a3) are reasonably assumed to be 1:l (Y(RH, a2) = Y(RH, a3) = 50%). Since the yield of RI was about 25% in orthorhombic crystals, the yield of Rn and Rm via process a3 is assumed to be about Y(Rn,Rm,a2) = 25%. In the case of mixtures, the alkyl radical yield via (dl) is estimated to be Y(RH, d l ) = 20% from the same yield of vinyl radicals via (d2): Y(viny1, d2) = 20% at the solute concentration higher than 2.0 mol %. Therefore, the contribution of alkyl radicals via (a2) and (a3) were reduced by 0.4: Y(Rn,Rm,a2) = 30%, Y(R1,a3) = 15%, and Y(Rn,Rm,a3) = 15%. Thus, the reduction of RI yield by the introduction of alkyne is expected to be AY(R1) = 25 - 15 = lo%, even though RII and Rm were selectively produced via process dl: Y(Rn,Rm,dl) = 20%. Much lower reduction is expected if RI is produced via (dl), whereas the RI reduction observed was 20%. Thus, this mechanism cannot explain the observed results. E. H Atom Abstraction by Excited Alkyne Formed by Energy Transfer

+ e- -RH,* RH,* + H-CEC-R - RH, + HCEC-R* H-C=C-R* + FU-I, - H,C=C'-R + RH' FU-I,'+

(el) (e2)

(e3) As is described in the Results section, only the CH2=C'-R' type radical was formed by H addition to the end carbon atoms of alkyne molecule, and the other type C'H-CH-R' was not. In the mixed crystals, the guest alkyne molecules are aligned with nearly the same orientation with the matrix alkane molecules, and in the orthorhombic crystal structure the end carbon atom of alkyne is surrounded by the end CH3- groups of matrix molecules (Figure 8a). Therefore, from the crystal structure, the H abstraction would occur from the end carbon atom of a neighbor alkane molecule and would produce the RI radical C'H2-CH2-R. Therefore, it is difficult to explain the decrease of RI formation by this mechanism. Moreover, this mechanism can be excluded from the concomitant formation of R, and RI radicals in the similar way with the D and (1) combined mechanism. Suppression of the Formation of RI in CnH~n+2/CllH~. Formation of RI was suppressed in the CsHldCllH2a and C14H3d CllH24 binary mixed alkane crystals irradiated at 77 K. In the latter crystals where the solute molecule is longer than the matrix molecule, the penultimate -CH2- groups as well as the chain end CH3- of the solute molecule are exposed to the boundary regions.8a In the former mixed crystals with shorter solute molecules, the penultimate -CH2- groups of the matrix molecules which are located next to the solute molecules are exposed to the boundary regions. Therefore, in both mixed crystals, the hydrogen atoms escaped to the boundary regions can efficiently abstract the H atoms from -CH2- groups (of the solute or matrix molecules) with a smaller bond strength than from CH3- groups, resulting in the suppression of the R1 formation.

Concluding Remarks The crystal structure dependence in the alkane radiolysis was investigated using pure alkanes and 1-alkyne/n-alkane mixed

crystals having two kinds of crystal structures (orthorhombic and triclinic). The terminal-type alkyl radicals (RI) were efficiently formed only in the orthorhombic n-alkane crystals and not in the triclinic crystals. In the orthorhombic mixed crystals, efficient and concomitant formation of a vinyl-type of radical (R,) and RI radical was observed, contrary to no formation of the radicals in the triclinic mixed crystals. These results provide the strong evidence for the H atom reaction mechanism that both R, and RI radicals are formed from the same precursor, namely H atoms. The crystal structure dependence observed indicates an important role of molecular layer boundaries in the migration and reactions of H atoms. The estimation of the barrier for the H atom migration is under investigation with the aid of MO calculations in order to understand the crystal structure dependence of radical formation and the role of layer boundaries in the migration and reactions of H atoms.

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