Energy, phase, and temperature effects on hydrogen atom abstraction

Chem. , 1981, 85 (5), pp 564–569. DOI: 10.1021/j150605a022. Publication Date: March 1981. ACS Legacy Archive. Cite this:J. Phys. Chem. 85, 5, 564-56...
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564

J. Phys. Chem. 1881, 85, 564-569

the OH + halocarbon reactions. In a vertical series in Figure 2, the reactivity varies in a regular fashion according to the nature of X. For example, in the CHX3series the reactivity increases as CHF, < CHF2Cl< CHFC12< CHC1,. The regularity in this and analogous series allows rate constants to be estimated, or at least bracketed, in cases where no experimental values C~ be exare available. The value of log ~ C H ( C H ~ ) Fwould pected to lie about midway between the values of log ~ c H ( c ~ and ) F ~log ~ C H ( C Hc12by analogy with the rate constants for CH2F2,CH2F61,and CH2C12. Likewise, rough

estimates can be made for the rate constants for CH3CFC12, CH(CH3)2C1,and CH(CH3)2F. Although the exact magnitudes of the various steric and electronic effects which contribute to the difference in the observed reactivities are not known, the rate constants appear to follow, in a qualitative sense, a regular pattern when displayed as in Figure 2. Acknowledgment. We thank M. E. Bednas for the mass-spectrometric analyses and Dr. H. Magid of Allied Chemical Corp. for supplying us with a sample of CH2FC1.

Energy, Phase, and Temperature Effects on Hydrogen Atom Abstraction Reaction from Isobutane by H Atoms at 4 and 77 K Tetsuo Miyazaki, Akihiro Wakahara, Toyoakl Kimura, and Kenji Fueki Department of Synthetic Chemistry, Facuky of Engineering. Nagoya University, Chikusa-ku, Nagoya 464, Japan (Received: June 9, 1980; In Final Form: September 15, 1980)

Hydrogen atom abstraction reaction from isobutanes, such as HC(CH3)3,DC(CHs),, or HC(CD3),, by H atoms has been studied in the solid phase at 4 and 77 K by ESR spectroscopy. In order to obtain H atoms with different initial kinetic energies, we performed the photolysis of HI with 254 and 229 nm, and the dissociative electron attachment to HI in radiolysis was also utilized. The relative rate constants for the abstraction reaction from a primary H atom ( k l ~and ) that from a tertiary H atom ( k 3 ~were ) estimated from the yields of isobutyl and tert-butyl radicals produced. The H atom with high energy abstracts preferentially the primary H atom of isobutane, while that with low energy abstracts the tertiary H atom. The ratio klH/kSHdepends remarkably upon phase and temperature. For example, klH/k3H for the reaction of H atoms, produced the 254-nm photolysis of HI at 77 K, exceeds 11in the crystalline state, while k l ~ / k is 3 ~only 0.15 in the glassy state. The ratio k l ~ / k 3 ~ for the reaction of H atoms, produced by the 229-nm photolysis of HI in the glassy state, exceeds 11at 4 K, while the ratio amounts only to 0.21 at 77 K. The phase and temperature effects on the abstraction reaction by the H atoms are interpreted in terms of the different efficienciesof the energy loss of H atoms with excess kinetic energies. The energy loss efficiency decreases in the following order: glass at 77 K > crystal at 77 K > glass at 4 K > crystal at 4 K.

Introduction The reaction of the H atom is very important in chemical kinetics as well as in the radiation chemistry of organic compounds. We can easily investigate the solid-state reaction of the H atom by irradiation with ultraviolet light or y-rays. The H atom produced in the solid alkane at low temperature reacts selectively. Miyazaki et a1.l reported that the H atoms, produced by the radiolysis of neopentane or the photolysis of HI, react selectively with the solute alkane in neopentane-alkane mixtures at 77 K. The ratios of the rate constants, k(H + solute)/k(H + neo-C5H12),for the abstraction reaction from the solute to that from neopentane are -500-1000. The ratio increases with the decrease in the kinetic energy of the H atom." (1)(a) T. Wakayama, T. Miyazaki, K. Fueki, and Z. Kuri, J. Phys. Chern., 77,2365(1973);(b)T.Miyazaki and T.Hirayama, ibid., 79,566 (1975);(c) T. Miyazaki, K. Kinugawa, M. Eguchi, and S. M. L. Guedes, Bull. Chem. SOC.Jpn., 49,2970 (1976);(d) T. Miyazaki, K. Kinugawa, and J. Kasugai, Radiat. Phys. Chem., 10,155 (177);(e) K. Kinugawa, T. Miyazaki, and H. Hase, ibid., 10,341 (1977);(0T. Miyazaki, J. Kasugai, M. Wada, and K. Kinugawa, Bull. Chem. SOC.Jpn., 51,1676(1978);(g) K. Kinugawa, T. Miyazaki, and H. Hase, J. Phys.Chem., 82,1697(1978); (h) T.Miyazaki, N. Goshima, K. Fueki, Y. Aratono, and E. Tachikawa, Radiat. Phys. Chem., 16,561(1980);(i) T.Miyazaki, S. M. L. Guedes, and K. Fueki, Bull. Chern. SOC.Jpn., 53, 1813 (1980).

Willard et a1.2 found that, when glassy 3-methylalkane undergoes y irradiation or H-atom attack at 77 K, radicals are formed predominantly by the rupture of a secondary C-H bond rather than weaker tertiary or more abundant primary bonds. When isobutane is exposed to y irradiation or H-atom attack at 77 K, the isobutyl radical is predominantly formed in the crystalline state, while the tert-butyl radical is formed in the glassy state., The cause of this phase effect was not elucidated in the previous work. In order to obtain further information on the selective hydrogen atom abstraction reaction, we have quantitatively studied the phase effect on the reaction of the H atom with isobutanes, such as i-C4Hl0,i-C4D9H,and i-C4H@, at 4 and 77 K. The H atom produced by the photolysis of HI has initially excess kinetic energy. When the wavelength of (2)(a) S.Adita and J. E. Willard, J. Am. Chem. SOC.,88,229(1966); (b) D.J. Henderson and J. E. Willard, ibid., 91,3014 (1969); (c) T. Ichikawa and N. Ohta, J. Phys. Chem., 81,560 (1977). (3)(a) T. Miyazaki, T. Wakayama, K. Fueki, and Z. Kuri, Bull. Chem.

SOC.Jpn., 42,2086 (1969);(b)T.Wakayama, T. Miyazaki, K. Fueki, and

Z. Kuri, J.Phys.Chem., 74,3584 (1970);(c) Y.Saitake, T. Wakayama, T. Kimura, T. Miyazaki, K. Fueki, and Z. Kuri, Bull. Chern. SOC.Jpn., 44,,301 (1971); (d) T. Wakayama, T. Miyazaki, K. Fueki, and Z. Kuri, zbid., 44, 2619 (1971).

0022-3654181 /2085-0564$Ol.25/0 0 1981 American

Chemical Society

Hydrogen Atom Abstraction Reaction from Isobutane

The Journal of Physical Chemistry, Vol. 85, No. 5, 1981 585

light is varied in the photolysis, H atoms with different kinetic energies are produced. It will be reported in this work that the selectivity in the hydrogen atom abstraction reaction from isobutane by H atom depends remarkably upon the temperature, phase, and initial kinetic energy of the H atom.

Experimental Section Isobutane (i-C4H10), supplied by the Takachiho Shoji Co., was more than 99.7 mol % pure. At least 95% of isobutane-2-dl (i-C4HgD)was labeled with the deuterium atom at the tertiary position.lb 2-Methylpropane1,1,1,3,3,3-d6 (i-C4D9H),supplied by Merck Sharp and Dohm, Canada Ltd., had a stated isotopic purity of 98 mol %. Hydrogen iodide was the same as used before.3d Methylcyclohexane (MCH) was purified by passing through a column of molecular sieves supplied by Nakarai Chemicals Ltd. UV illumination was performed by a homemade cadmium lamp or a low-pressure mercury lamp with a Toshiba UV-25 filter which transmits the longer light than 240 nm and cuts off the light of 185 nm. The emission spectra” of the light from the lamps used in the experiment were measured with an emission spectrometer which consisted of a monochrometer and a photomultiplier. The photolysis of HI with the low-pressure mercury lamp with UV-25 filter was caused by a strong emission line at 254 nm. Though the cadmium lamp emits two strong lines of 326 and 229 nm, the photolysis of HI was caused by the line a t 229 nm. This is because the photolysis of HI does not occur upon the irradiation with the cadmium lamp with a UV-29 filter which cuts off the light of 229 nm and transmits only 326 nm. y irradiations were done by Co-60 at total doses of 0.43 Mrd at 77 K and 0.15 Mrd at 4 K. The UV-illumination and y-irradiation times are 2 and 0.5 h, respectively. Since the butyl radicals do not decay by the storage of the sample for 5 h at 77 K, the radicals do not decay during the illumination and the irradiation. In order to seal a thermocouple into the glass of an ESR tube, we connected a small chip of tungsten-nickel wire to a chromel-gold (0.07% Fe) thermocouple. There was no appreciable difference in a thermal voltage between a normal thermocouple and the thermocouple containing a chip of tungsten-nickel wire. The temperature of the sealed sample reaches 4 K after 15-min immersion of the sample in liquid helium. In order to completely cool a sample at 4 K, we kept the sealed sample in liquid helium for 1 h before the irradiation. The temperature of the sample did not change during the illumination. The free radicals were measured at 77 K with a JES3BX ESR spectrometer at a microwave power level of 0.2 mW. Figure 1shows that the microwave power level used did not result in saturation of the signals of butyl radicals. The isobutyl radicals produced by the photolysis of HI at 77 K in the crystalline state saturate at the lowest power among the experimental conditions in Figure 1. The power sturation of tert-butyl radicals is approximately the same as that of the isobutyl radicals. The simulated ESR spectrum of a mixture of butyl radicals was obtained with a FACOM M-200 computer at the Nagoya University Computation Center. Results Figure 2 shows typical ESR spectra of irradiated isobutanes. The calculated spectral patterns for butyl radicals are shown below the experimental spectra. Calculated spectrum a shows the pattern of the CH3CH(CH3)CH,radical whose splitting constants of the CY proton and the /3 proton are 21 and 38 G, respectively. Calculated spec-

/+

i

i 1

m

2

Flgure 1. Microwave power saturation behavior of Isobutyl radicals formed by the 254-nm photolysis of HI (0.2 mol %). ESR measure(4 mol %)-HI ments were performed at 77 K: (0) i-C,H,,-MCH mixture irradiated in the glass at 4 K (A) i-C4Hlo-HI mixture lrradhted in the crystal at 4 K ( 0 )I-C4Hl0-MCH (4 mol %)-HI mixture irradiated in the glass at 77 K; (V)IG4HI0-HI mixture irradiated in the crystal A, 0 , and V)coinclde at the at 77 K; (0)points where all marks (0, same positions.

trum b shows the pattern of the (CH3)3C.radical whose splitting constant of the 0 proton is 23 G. Calculated spectrum c shows the pattern of the CD3CH(CD3)CD2radical whose splitting constants of the CY deuteron and the /3 proton are 3.2 and 38 G, respectively. Calculated spectrum d shows the pattern of the (CD3)&-radical whose splitting constant of the /3 deuteron is 3.5 G. Calculated spectrum e shows the pattern of the CH3CD(CH3)CH2. radical whose splitting constants of the a! proton and the /3 deuteron are 21 and 5.9 G, respectively. Pure isobutane is polycrystalline at 77 and 4 K, while isobutane containing 4 mol % methylcyclohexane is glassy at these temperature^.^^^^ When the isobutane-MCH (4 mol %)-HI (0.2 mol % ) mixture was irradiated by UV light or y-rays, the formation of methylcyclohexyl radical was not detected by ESR spectroscopy. Figure 2A shows an ESR spectrum of the UV-illuminated i-C4Hlo-MCH (4 mol %)-HI (0.2 mol %) mixture in the glassy state at 4 K. The same spectrum was obtained in the UV illumination of the polycrystalline iC4Hlo-HI (0.2 mol %) mixture a t 4 K. Only i-C4Hgradicals are produced in both the glassy and polycrystalline state at 4 K. When pure i-C4H,o is y-irradiated in the polycrystalline state at 4 and 77 K, only i-C4H9radicals are produced. The y irradiation of polycrystalline i-C4H10 containing HI (0.2 mol %), however, produces t-C4Hgradicals in addition to i-C4H9radicals (cf. Figure 2 B). When an i-C4Hlo-HI (0.2 mol %) mixture is illuminated by W light in the polycrystalline state at 77 K, only i-CJ19 radicals are formed (cf. Figure 2C). On the contrary, the UV illumination of the i-C4Hlo-MCH (4 mol %)-HI (0.2 mol %) mixture in the glassy state at 77 K produces a large amount of t-C4Hgradicals in addition to i-C4H9radicals, as shown in Figure 2D. The spectra ascribed to t-C4Hgand i-C4H9radicals are indicated by the spectral patterns of b and a, respectively. The phase effect on the formation of i-C4H9and t-C4Hg radicals was not observed at 4 K. There is some doubt that, when the i-C4Hl0-MCH-HI mixture is cooled a t 4 (4) The terms “glass” or “glassy state” used in this paper mean only that the solid is in the transparent state.

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The Journal of Physical Chemistry, Vol. 85,No. 5, 1981

I

I

I

,

-a

Mlyazakl et al.

b

- C

A

d

Flgure 2. ESR spectra of butyl radicals produced in the reaction of H atoms with isobutane In the solid phase. ESR measurements were performed at 77 K (concentrations of the solutes are expressed by mol % in parentheses): (A) photolysis of i-C4HIo-MCH(4)-HI(O.2) wlth 254-nm light in the glass at 4 K; (B) y radlolysls of I-C,H,o-HI(0.2) In the crystal at 4 K; (C) photolysis of I-C4Hlo-HI(0.2) wlth 254-nm llght In the crystal at 77 K; (D) photolysis of i-C,Hlo-MCH(4)-HI(0.2) with 254-nm light In the glass at 77 K; (E) simulated spectrum of a mixture of CH3CH(CH3)CH2. (57.5%) and (CH3)3C-(42.5%) radicals; (F) photolysis of i-C4D,H-MCH(4)-HI(0.2) with 254-nm light in the glass at 4 K; (0) photolysis of i-C4DDBH-HI(0.2)with 254-nm light in the crystal at 77 K; (H) simulated spectrum of a mixture of CH3CH(CH3)CH2*(70%), (CD3)3C. (17%), and CD3CH(CD3)CD,. (13%) radicals; (I) photolysis of iC,D9H-MCH(4)-HI(O.2) with 254-nm llght in the glass at 77 K; (J) photolysis of i-C,HgD-HI(0.2) with 229-nm light in the crystal at 77 K; (a-e) spectral patterns of CH3CH(CH3)CH2-,(CH3),C., CD3CH(CD3)CD2.,(CD3)3C., and CH3CD(CH3)CH2. radicals, respectively.

K, the mixture may become polycrystalline, resulting in the formation of i-C4H9radical. The possibility, however, is eliminated by the following facts. First, the i-C4HI0MCH-HI mixture is precooled at 4 K before the irradiation at 77 K. The t-C4H9radicals are formed remarkably in the precooled sample. Second, the i-C4Hl0-MCH (4 mol %)-HI (0.2 mol % ) mixture is in the transparent solid state even at 4 K. Figure 2E shows an example of the simulated spectrum of a mixture of i-C4H9radical (57.5%) and t-C4H9radical (42.5%).The typical ESR spectrum of an isobutyl radical

or a tert-butyl radical, obtained experimentally, was used in the simulation of the mixed spectrum of the two radicals. The mixing of the two spectra was performed with a computer. Figure 2E is the simulated spectrum best fitted to Figure 2D. Figure 2F shows aspectrum of the glassy i-C,$&-MCH (4 mol %)-HI (0.2 mol %) mixture irradiated with 254-nm light at 4 K. The spectrum is ascribed to a mixture of CD3CH(CD3)CD2.and (CD3)&. radicals. The hyperfine splitting is caused by the deuterons of the radicals. A similar spectrum was obtained also in the UV illumination

The Journal of Physical Chemistry, Vol. 85,No. 5, 1981 567

Hydrogen Atom Abstraction Reaction from Isobutane

of a polycrystalline i-C4D9H-HI (0.2 mol 5%) mixture at 4 K. Figure 2G shows a spectrum of the i-C4D9H-HI (0.2 mole %) mixture irradiated with 254-nm light at 77 K. The spectrum is ascribed to a mixture of i-C4Hg,(CD3)& and CD3CH(CD3)CD2.radicals. Since i-C4D9Hcontains 2% iX4H10 as an impurity, i-C4H9radical is due to the i-C4Hio impurity. The formation of the i-C4H9radical will be discussed further in the later section. Figure 2H shows a simulated spectrum of a mixture of i-C4H9 (70%), (CD3)3C-(17%), and CD3CH(CD3)CD2. (13%) radicals. The spectrum is the one best fitted to Figure 2G. Figure 21 shows a spectrum of the glassy i-C,D&I-MCH (4 mol %)-HI (0.2 mol %) mixture irradiated with 254-nm light at 77 K. The remarkable formation of t-C4Dgradical is observed, as compared with Figure 2, F and G. Figure 25 shows a spectrum of the i-C4H$-HI (0.2 mol %) mixture irradiated with 229-nm light at 77 K. The spectrum is ascribed to CH3CD(CH3)CH2.radical.

Discussion Relative Rate Constants of Hydrogen Atom Abstraction Reaction from Isobutane by H Atom. When an isobutane-HI mixture is irradiated with light of 254 or 229 nm, H atoms produced by the photolysis of HI react with isobutane to form butyl radicals by the following reactions.

HI~Y-H+I

H

-

+ CH3CH(CH3)2

H

+ CD3CH(CD3I2 kw

or H

H2 + CH3CH(CH3)CH2

- + - + -+ -+ - + kaH

or

kin

+ CH3CD(CH&

kaD

H2

klD

CH3C(CH3),

HD

H2

klH

CD3CH(CD3)CD2

CD3C(CD3),

H2

HD

CH3CD(CH&H2

CH3C(CH3),

(1)

(2) (3)

(4)

(5)

(6) (7)

There is a possibility of the photoinduced isomerization of a tert-butyl radical to an isobutyl radical (reaction 8)

-

t-C4Hg i-C4Hg

hv

i-C4H9

AH

(8)

t-CdHg

(9) or the thermal isomerization of an isobutyl radical to a tert-butyl radical (reaction 9). The possibility of these radical conversions can be dismissed from the fact that the isomerized butyl radicals were not formed in the photolysis of the partially deuterated isobutanes, such as (CD3)3CH and (CH3)3CD,where the occurrence of the isomerization can be detected by ESR spectroscopy. Since the amounts of i-C4H9and t-C4H9radicals do not change during the storage of the sample at 77 K for 44 h,9b we can neglect the possibility that a t-C4H9radical may be produced by the hydrogen atom abstraction reaction from isobutane by an i-C4H9radical (reaction 10). i-C4H9+ i-C4HI0 i-C4HIo+ t-C4Hg (10) +

Therefore, the amounts of the butyl radicals, produced in the photolysis of the isobutane-HI system, represent

TABLE I: Relative Yields of Butyl Radicals in the Radiolysis of Isobutanea ~

~~

irradiation t e m p sample

phase

77K

4K

i-C4Hlo i-C,H,,-HI(0.2) i-C,H,,-MCH(B) i-C,H,,-MCH( 4)-HI( 0.2)

crystal crystal glass glm

1.0

1.0 1.2 0.9 1.2

a The amounts o f radicals, measured

1.2

1.1 1.5

at 77 K,were ob-

tained b y double integration of the first-derivative ESR s i p a l of butyl radicals. The yields represent the total amounts of i-C,H, and t-C,H, radicals. The yield of t h e pure isobutane system is normalized to 1 . Concentrations of t h e solutes are expressed by m o l % in parentheses. MCH represents methylcyclohexane.

the relative efficiencies of reactions 2-7. The relative rate constants per one C-H bond can be estimated by eq 11, (11) k l H / h H = y9[i-c4Hd / [t-C4H91 where is the ratio of the number of tertiary hydrogen atoms to that of primary hydrogen atoms in an isobutane molecule. The amounts of i-C&IBand t-C4Hgradicals were estimated by the comparison of the experimental ESR spectrum with the best-fit simulated spectrum. The error of the amounts of the butyl radicals is 10%. H atoms and butyl radicals are produced by the y radiolysis of isobutane at 77 K (reaction 12). The H atoms i-C4H10 *+H + C4H9 (12) react also with isobutane as shown in reactions 2-7. It was reported previously that reaction 12 is not affected upon the addition of electron scavengers, such as N20, SFe, and C2H61,suggesting that the neutralization process plays a minor role in the production of H atoms.5 The absence of effects of electron scavengers has also been reported in the radiolysis of solid ne~pentane.~JTable I shows that the yield of butyl radicals increases by 20-3070 upon the addition of HI to the isobutane. The increase is due to the reaction of H atoms produced by dissociative electron attachment to HI8 (reactions 13-15), where H’means the HI + e- H’ I(13) H’ i-C4H10 H2 i-C4H9 (14) H2 + t-C,Hg (15) H atom produced by the dissociative electron attachment. Thus, the increment of the butyl radical yields represents the occurrence of reactions 14 and 15. k l ~ / k 3for~ H’ can be estimated from the increment of the butyl radical yields and eq 11. The ratios of these rate constants are summarized in Tables 11-IV. When H atoms are produced by the photolysis of HI in the i-C4HI0glass at 4 K, they abstract the primary H atom to produce only i-C4H9radicals, as shown

-

+

--

+ +

(5) T. Wakayama, T.Kimura, T. Miyazaki, K. Fueki, and 2. Kuri, Bull. Chem. SOC.Jpn., 43,1017 (1970). Hz yields in the radiolysis of solid isobutane are also not affected by the addition of electron scavengers, such as NzO and SF,,(cf. ref 3b). Though the yields of solvent radical and Hz are not reduced by the addition of ordinary electron scavengers, the yields decrease upon the addition of CClb This effect may be due to the scavenging of excitation energy by CCl, (cf. T. Miyazaki, Znt. J. Radiat. Phys. Chem., 8, 57 (1976)). (6) M. Kato, Y. Saitake, T. Miyazaki, and 2.Kuri, Bull. Chem. SOC. Jpn., 46,2004 (1973). The yields of cyclohexyl radicals in the radiolysis of neopentane-cyclohexane mixtures at 77 K do not decrease upon the addition of NzO and COz which do capture electrons. (7) M. Iwasaki, H.Muto, K. Toriyama,M. Fukaya, and K. Nunome, J. Phys. Chem., 83, 1590 (1979). Since the effect of scavengers on the total radical yields is shown, the definitive conclusion of the scavenger effect cannot be obtained. (8) P.H. Kasai, Acc. Chem. Res., 4, 329 (1971).

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TABLE 11: Ratio (k,H/k3H).per Atom of Rate Constants for Hydrogen Atom Abstraction Reaction from i-C,H,, b y H Atom source of H atom condition

229-nm photolysisa

264-nm photolysisa

DEAC

0.17 >11 >11 >11 0.06 >11 0.03 >11 211 0.15 0.01 0.21 H atom is produced by the photolysis of HI with 229H atom is produced b y the photolysis of HI n m light. with 254-nm light. H atom is produced by the diasociative electron attachment of HI. The values are estimated by the assumption that the solvent radical yields are not affected by the addition of the electron scavengers (see text). If it is assumed that the solvent radical yields decrease by 15% upon the addition of the electron scavengers, the values of 4 K crystal, 4 K glass, 7 7 K crystal, and 77 K glass are changed as 0.38, 0.2, 0.1,and 0.03, respectively. The sample in the glassy state contains methylcyclohexane a t 4 mol %. 4K 4K 77 K 77 K

crystal glassd crystal glassd

TABLE 111: Ratio ( k l D / k 3 H ) per Atom of Rate Constants for Hydrogen Atom Abstraction Reaction from i-C,D,H ((CD,),CH) b y H Atom source of H atom condition

229-nm photolysis“

254-nm photolysisa

K crystal 0.6 0.6 K glassc 0.6 0.6 0.09d 0.OSe K crystal K glassc 0.03f 0.02f a H atom is produced by the photolysis of HI with 229nm light. H atom is produced by the photolysis of HI with 254-nm light. The sample in the glassy state contains methylcyclohexane a t 4 mol %. The ratio ( k l H / k J H ) of rate constants for the primary hydrogen atom abstraction from impurity i-C4Hlaand the tertiary hydrogen atom abstraction from i-C,D,H by H atom is 16. e The ratio ( k l H / k 3 H ) of rate constants for t h e primary hydrogen atom abstraction from impurity i-C4Hlaand the tertiary hydrogen atom abstraction from i-C,D,H b y H atom is 22. The ratio ( k l H / k 3 H ) of rate constants for the primary hydrogen atom abstraction from impurity i-C4Hloand the tertiary hydrogen atom abstraction from i-C,D,H b y H atom is 1.4. 4 4 77 77

in Figure 2A. The minimum yield for the detection of t-C4H9radical in this experiment is 1% of the total radical yield. Thus, klHII23H for this system in Table I1 is expressed as 111 X 99). Energy Dependence of Rate Constants for Hydrogen Atom Abstraction Reaction by H Atom. Tables 11-IV show that the ratio of rate constants for hydrogen atom abstraction reaction from isobutane by H atom depends remarkably upon the source of H atoms, such as 229- or 254-nm photolysis of HI and dissociative electron attachment to HI. For example, the ratio klH/k3H in the i-C4H10 glass at 77 K decreases in the order of the 229-nm photolysis, the 254-nm photolysis, and the dissociative electron attachment (cf. Table 11). The 229- or 254-nm photolysis of HI in the gas phase produces the H atoms with 20-42 kcal/mol or 33-55 kcal/mol, respectively? The photolysis in the solid phase may be complicated by the occurrence of the cage effect on the dissociation of HI. Since the photon energy of 229 nm is higher than that of 254 nm, it is assumed here that (9) R. D. Clear, S. J. Riley, and K. R. Wilson, J . Chem. Phys., 63,1340 (1975), and references cited therein.

TABLE IV: Ratio ( k l H / k r D ) per Atom of Rate Constants for Hydrogen Atom Abstraction Reaction from I-C,H,D ((CH,),CD) by H Atom source of H atom condition 4K 4K 77 K 77 K

crystal glassc crystal glassc

229-nm photolysisa

254-nm photolysisa

2 11

211 211

211 1.4

a11

a11

I

0.63

H atom is produced by the photolysis of HI with 229n m light. H atom is produced by the photolysis of HI with 254-nm light. The sample in the glassy state contains methylcyclohexane at 4 mol %.

the initial energy of the H atoms produced by the 229-nm photolysis is higher than that by the 254-nm photolysis in the solid phase. The electron affinity of I atom and the bond energy of HI are 76 and 70 kcal/mol, respectively.1° Since the ionic radius (2.75 A) of I- (ref 11)is approximately the same as that (2.8-3.0 A) of e- (ref 12), a solvation energy of e- due to induced polarization may be approximately equal to the solvation energy which would be available to I- on the time scale of the dissociative capture rea~ti0n.l~Then the H atom produced by the dissociativeelectron attachment has approximately an excess kinetic energy of 6 kcal/mol,14 which is much lower than the initial energy (55-20 kcal/mol) of H atom produced by the ultraviolet photolysis of HI. Thus the initial energy of H atoms produced by the three methods may decrease in the order of the 229-nm photolysis, the 254-nm photolysis, and the dissociative electron attachment. klH/kaH (or klHlk3D) in the same solid state decreases with the decrease in the initial energy of H atom (cf. Tables I1 and IV). The activation energy for hydrogen atom abstraction by the H atom from the tertiary C-H bond of i-C4H1o is reported as 4.7 (ref 15) and 6.8 (ref 16) kcal/mol. Since the bond dissociation energy (98 kcal/mol) of the primary C-H bond of isobutane is approximately the same as that of ethane,” it is assumed here that the activation energy for hydrogen atom abstraction by the H atom from the primary C-H bond of isobutane is roughly equal to that (8.6 (ref 15) and 9.5 (ref 18) kcal/mol) from the C-H bond of ethane. Thus, it is expected that the H atom with very low energy abstracts a hydrogen atom preferentially from the tertiary C-H bond, while the H atom with high energy abstracts a hydrogen atom from the primary C-H bond as well as that from the tertiary C-H bond. The ratio klH/k3H is related to the energy of H atoms. (10) V. I. Vedeneyev, L. V. Gurvich, V. N. Kondratyev, V. A. Medvedev, and Ye. L. Frankevich, “Bond Energies, Ionization Potentials,and Electron Affinities,” translated in English by. W. C. Price, Edward Arnold Ltd., London, 1966. (11) G. Stein and A. Treinin, Tram. Faraday SOC.,55, 1086 (1959). (12) T. Kimura. K. Fueki. P. A. Naravama. . and L. Kevan. Can. J. Chern:,55, 1940 (i977). ‘ (13) According to Born’s equation the polarization energy (P) is expressed as P = -[ea/(2r)](l - l/c), where rand care the radius of the ion and the dielectric constant, respectively. The polarization energies induced by the ions with the same radius amount to the same value. (14) When an electron-capturereaction in the solid phase is caused by a mobile quasi-free electron, the H atom produced by the dissociative electron attachment has an excess energy of V , (an energy of quasi-free electron state, 10 kcal/mol) in addition to 6 kcal/mol (cf. K. Funabashi and J. L. Magee, J. Chem. Phys., 62,4428 (1975) and D. Grand and A. Bernas, J. Phys. Chem., 81, 1209 (1977)). (15) K. Yang, J. Phys. Chem., 67, 562 (1963). 60, 1236 (16) R. R. Baldwin and R. W. Walker, Trans. Faraday SOC., (1964). (17) J. A. Kerr, Chem. Reu., 66, 466 (1966). (18) T. Kagiya, Y. Sumida, T. Inoue, and F. S. Dyachkovskii, Bull. Chem. SOC.Jpn., 42, 1812 (1969).

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Hydrogen Atom Abstraction Reaction from Isobutane

Phase and Temperature Effects on Hydrogen Atom Abstraction Reaction by H Atom. Tables 11-IV show the phase and temperature effects on the rate constants for the abstraction reaction by H atoms, produced by the photolysis with 229- or 254-nm light or by the dissociative electron attachment of HI. The most pronounced phase effects on the rate constants are observed at 77 K. For example, when an H atom is produced by the photolysis of HI with 254-nm light in the iC4H10 matrix at 77 K, the H atom in the crystalline state abstracts preferentially the primary H atom, that is, klH/kBH 111,while the H atom in the glassy state abstracts the tertiary H atom, that is, k l ~ / k a ~0.15. A possible reason for the phase effect is that the hydrogen atom abstraction reaction itself is affected by the different distortion of a potential-energy surface between the crystal and the glass and by the difference in the proximity of tertiary vs. primary bonds to the HI in the cages surrounding the HI in the two phases. This possibility, however, may be excluded by the following reasons. First, the H atoms produced by the photolysis at 4 K abstract a primary H atom from i-C4H10 in both the glassy and crystalline states (cf. Tables 11-IV). Second, the H atom with a low initial energy, produced by the dissociative electron attachment, abstracts preferentially a tertiary H atom from i-C4Hlo even in the crystalline state at 4 and 77 K (cf. Table 11). The remarkable phase effect on the rate constants at 77 K may be attributed to differences in the “efficiency of energy loss” by the H atoms. Both energy loss and abstraction require collisions. If one assumes that the initial H atom energy is the same in both the crystalline and glassy systems, the difference in product yields must reflect a difference in the probability of reaction relative to energy loss for collisions of the same energy. Tables 11-IV show that the rate constant depends also upon the temperature. For example, when H atom is produced by the 229-nm photolysis of HI in the glassy state, the ratio of the rate constants, klH/kaH,exceeds 11 at 4 K, while the ratio is only 0.21 at 77 K (cf. Table 11). The difference also may be due to the “efficiency of energy loss” by the H atoms at the two temperatures. Since information on the energy loss of hot H atoms in the solid phase is quite scanty at present, we cannot discuss the detailed mechanism of the energy loss in the isobutane matrix at low temperature. The energy loss may be affected by phonon vibrations, the rigidity of the solid, and

The Journal of Physical Chernlstty, Vol. 85, No. 5, 1981 569

so on, which are closely related to the temperature and the

phase of the solid. It is expected that the H atoms with lower energy abstract more preferentially the tertiary H atom because of the lower activation energy for the abstraction reaction. Tables I1 and I11 show that the ratios of the rate constants for the H atom, produced by the dissociative electron attachment in the i-C4H10 matrix and by the photolysis in the i-Cp& matrix, decrease in the following order: crystal at 4 K > glass a t 4 K > crystal at 77 K > glass a t 77 K. The H atoms in the glass at 77 K abstract most efficiently the tertiary H atom because of the most effective energy loss. Thus, the efficiency of the energy loss of the hot H atom may decrease in the following order: glass at 77 K > crystal at 77 K > glass a t 4 K > crystal at 4 K. Figure 2, G and H, shows that the i-C4Hgradical is one of the main products in the photolysis of crystalline iC4D&-HI mixtures at 77 K. This result gives interesting information on the energy of mobile H atoms in the crystalline i-C4D&I at 77 K. The H atom produced by the photolysis migrates through the matrix and reacts selectively with an i’C4H10 impurity, resulting in the formation of i-C4H9radical. This reaction is a familiar reaction of the H atom in alkane mixtures at 77 K.l For example, when a D atom is produced by the radiolysis of crystalline n-CloDzzcontaining a small amount of n-CloHzeat 77 K, the D atom migrates through the matrix and reacts selectively with n-Cl$In.lf When H atoms are produced by the radiolysis of i-C4H10 or by the photolysis of HI in the i-C4H10-C3H8(2 mol %) mixture at 77 K, most of the mobile H atoms react selectively with C3H8,resulting in the formation of C3H7radical.laid The important fact in Figure 2G is that the mobile H atom abstracts an H atom of the primary C-H bonds of i-C4H1p If the mobile H atom may lose ita energy effectively during its migration, the H atom should abstract a tertiary H atom of i-C4H,* The H atom in the crystalline i-C4DgH may migrate through the matrix without losing efficiently ita energy and thus abstract the primary H atom of i-C4H1p The migration of H atoms with excess energies has also been suggested in the neopentane matrix at 77 K.” Acknowledgment. We thank Dr. E. Tachikawa of the Japan Atomic Energy Institute for a supply of i-C4D9H. We thank Mr. Y. Fujitani of Nagoya University for help on the experiment at 4 K. We also thank Mr. T. Imura of Nagoya University for making a sealed thermocouple.