IRRADIATION OF ~-HEPTADECANE
The activation energies and constants were obtained from the graphs of the temperature dependence of the rate constant (Figure 5 ) . Table I11 lists these values.
3203
Activation energies were also calculated from the slopes of plots of In (1 - 4 1 - s)2 against 1 / T . These were in agreement with the values of Q listed in Table 111.
On the Irradiation of n-Heptadeeane
by R. Salovey and W. E. Falconer Bell Telephone Laboratories, Incorporated, Murray Hill, New Jersey
(Received May 10, 1966)
Radiolyses of n-heptadecane in the pure state and in the presence of iodine are examined and compared with those of n-hexadecane. I n general, similar irradiation behavior is noted. However, differences are observed in radiolyses of the respective crystalline solids. Lower cross-linking yields and an unsymmetrical distribution of low molecular weight products result from irradiated solid n-heptadecane as compared to solid n-hexadecane. These differences are associated with poorer molecular coupling in the orthorhombic structure of odd n-alkanes than in the triclinic habit of even homologs. The end-linking yield in solid n-heptadecane is larger than in solid n-hexadecane. The combination of heptadecyl radicals yields at least 35 and 80% of the tetratriacontane isomers on irradislr tion of pure n-heptadecane in solid and liquid states, respectively.
Introduction Detailed studies of the irradiation behavior of nhexadecane1i2 lead to conclusions on the radiolysis of long-chain paraffins. To explore the generality of these results, the next higher homolog (containing an odd number of carbon atoms), n-heptadecane, was examined. There is an even-odd alternation in the crystallographic properties of normal paraffins3 Even nalkanes of carbon numbers 8 through 24 crystallize in a triclinic modification, with layers of parallel molecules obliquely inclined to layer planes drawn through chain ends. Just below their melting points, odd linear paraffns between C9 and C35 assume rotational crystalline structures in which molecules rotate about their long axes. At lower temperatures, these odd nalkanes crystallize in orthorhombic habit with molecular axes normal to “end plane^."^ Radiolyses of solid and liquid n-hexadecane are sensitive to physical state. Cage recombination of
radical fragments from main-chain scission in the crystalline solid was adduced to explain differences in product yields in different states. Since molecular packing in solid paraffins is altered by crystalline modification, it is of interest to examine the effect of crystal state on radiation behavior. The importance of crystalline modification in n-hexadecane radiolysis was indicated by the observation that cross linking was affected by the addition of specific impurities which cause n-hexadecane to assume an ort,horhombicform. 1,5 Another reason for studying the importance of crystal habit is to ascertain the validity of considering nhexadecane as a polyethylene analog. Linear poly-
(1) R. Salovey and W. E. Falconer, J. Phus. Chem., 69,2345 (1965). (2) W.E.Falconer and R. Salovey, J . C h m . Phys., 44,3151 (1966). (3) A. I. Kitiagorodskii, “Organic Chemical Crystallography,” Consultants Bureau, New York, N. Y., 1961. (4) Yu. V. Mnyukh, J . Phys. Chem. Solids, 24, 631 (1963). (5)A. M U e r and K. Lonsdale, Acta Cryst., 1, 129 (1948).
Volume 70,Number 10 October 1966
R. SALOVEY AND W. E. FALCONER
3204
Table I : Yields of Low and Intermediate Molecular WeighPProducts in the Radiolysis of Solid n-Heptadecane (4.0 Mrads) Carbon no.
I
Pure
+l.Z%
I
0.041 0.032
0.038 0.046 0.046 0.042 0.034
0.034
9 10 11 12 13
0.027 0.024 0.026 0.032 0.028
0.029 0.023 0.025 0.031 0.029
0.028 0.022 0.024 0.030 0.027
14 15 16 17 17 n-17
0.045 0.053 0.018"
0.044 0.055 0"
0.045 0.056 0.024"
4 5 6 7 8
0,039 0.046
Hydrocarbons +0.6% I
0.044
0.034
0.033
ZG(Ci-CieY
0.51
0.47
0.51
zG(intermediate)"*"
0.05
0.05
0.04
ethylene is a semicrystalline polymer whose stable crystalline modification is orthorhombic16 and low molecular weight odd alkanes may be preferable polyethylene prototypes to n-hexadecane. Experimental Details Materials. Petroleum derived, 99% minimum purity, olefin-free n-heptadecane was obtained from the Humphrey Chemical Co. The melting point of the normal paraffin is 22". Baker and Adamson reagent iodine was used without further purification. Irradiation. Aliquots (0.5 ml) of pure n-heptadecane or of solutions of iodine in n-heptadecane were placed in thin-walled (0.5 mm) Pyrex glass tubes, degassed by repeated freeze-pump-thaw cycles and sealed a t -10-6 torr. Samples were exposed to a beam of l-Mev electrons from a van de Graaff generator to an absorbed dose of 4 Mrads. n-Heptadecane was irradiated on a thennostated hollow copper block either as a solid at 4" or in the liquid state a t 24". Radiation yields are expressed as G values, the number of molecules formed for 100 ev of energy absorbed. The dosimetry was calibrated by measuring the evolution of hydrogen from liquid cyclohexane, which G value was taken to be 5.3 a t 4 M r a d ~ . ~ , ' Analysis. Gas chromatography with flame ionizaThe Journal of Physical Chemistry
0.31 0.07 0.013
0.315 0.10 0.013
0.044
0.034
14 X average value
+1.2% I
0.037 0.044 0.044
AV C&is
' Reduced precision because of impurities.
-1ododkanes+0.6% I
+ 2 X C I yield. ~
Estimate from most reliable data, excluding
tion detection was used to analyze the irradiated samples. *
Results and Discussion Chromatograms of electron-irradiated n-heptadecane resemble those previously found for irradiated nhexadecane.lv2 Thus three groups of products may be distinguished : low molecular weight linear paraffins and olefins eluting before the parent hydrocarbon, intermediate hydrocarbon products of carbon number between 17 and 34, and isomers of tetratriacontane, the C34paraffins. The irradiation of n-heptadecane containing dissolved iodine yields additional products analogous to those in the scavenged radiolyses of n-hexadecane.2 I n the liquid state, new peaks in the low molecular weight region correspond to l-iodoalkanes. For both solid and liquid irradiations, three prominent chromatographic peaks in the intermediate region are assigned to iodoheptadecanes. Radiation yields of low and intermediate molecular weight products are tabulated for the solid-state (6) P. H. Geil, "Polymer Single Crystals," Interscience Publishers, Inc., New York, N. Y., 1963. (7) R. H. Schuler and A. 0. Allen, J. Am. Chem. Soc., 77, 507 (1955).
IRRADIATION OF n-HEPTADECANE
3205
Table 11: Yields of Low and Intermediate Molecular Weight Products in the Radiolysis of Liquid n-Heptadecane (4.0 Mrads) Carbon Pure
Hydrocarbons +0.6% I
4 5 6 7 8
0.091 0.117 0.130 0.122 0.101
0.061 0.075 0.085 0.076 0.059
0.056 0.062 0.072 0.062 0.049
9 10 11 12 13
0.082 0.070 0.069 0.073 0.065
0.047 0.035 0.035 0.037 0.035
0.037 0,029 0.030 0.031 0.025
14 15 16 17 17
0.070 0.066 0,021"
0.042 0.042 0.003"
0.033 0.032 0"
no.
-1odoalkanes+l.Z%
n-17
AV Cs-Cis
0.071
0,039
0.031
ZG(CI-CIG)~
1.03
0.55
0.43
ZG(intermediate)a9c
0.29
0.07
0.06
a Reduced precision because of impurities. &I.
* 14 X
average value
radiolysis of n-heptadecane in Table I and for the irradiation of liquid n-heptadecane in Table 11. Radiation yields of tetratriacontanes are listed in Table 111. All results are averages of repeated analyses; pure paraffins were irradiated and analyzed at least in triplicate, and scavenged radiolyses were in duplicate.
Table 111: Yields of Tetratriacontane in the Radiolysis of n-Heptadecane (4.0 Mrads) State
G(1inear dimer)
Additive
G (dimer)
0 . 6 % iodine 1.2y0 iodine
1.13 0.75 0.732
0.043 0.042 0.042
0. 6y0iodine 1.2'3$ iodine
1.63 0.368 0.307
0
Solid
Liquid
0.0045
0
Low molecular weight hydrocarbons identified in Tables I and I1 are linear paraffins and olefins. Sormal alkanes and alkenes with the same number of carbon atoms were not resolved. Branched hydrocarbons would have eluted significantly earlier than linear isomers and were not detected.
+ 2 X Cle yield.
I
+0.6%
I
+1.2%
I
0.077 0.060 0.052 0.054
0.072 0.066 0.054 0.056
0.062 0.057 0.048
0.063 0.057 0.046
1.11 0.51 0.152
1.16 0.61 0.176
Estimate from most reliable data excluding
By injecting samples into the chromatograph immediately upon opening sealed irradiation tubes, products as low as Cb were determined. However, the derivation of concentrations from peak heights becomes increasingly unreliable below c8 (cf. Figure 1 in ref 2). Moreover, in the scavenged radiolysis of the liquid, peaks from c8 to C4 are not resolved from iodoalkanes. Consequently in the low molecular weight region, the accuracy of radiation yields decreases from C8 to C4, and the most reliable hydrocarbon yields are Cg to CI5. Only these will be discussed; lower hydrocarbon yields are included for cautioned comparison. In the irradiation of hexadecanej2yields for each of the Cg to (3x4 hydrocarbons were approximately equal with an average G = 0.036 for solid and G = 0.073 for liquid radiolyses. Yields of each of the Cg to C15hydrocarbons from the irradiation of liquid n-heptadecane are also nearly equal with an average G value of 0.071. I n the solid radiolysis, however, an unmistakable minimum yield is observed at Clo with G = 0.024 as compared to Cls with G = 0.053. These products are linear paraffins and olefins from main-chain scission. A preferential depletion of certain scission fragments by secondary cleavage may account for these results. When chain scission produces fragments of comparable size, excess Volume 70, Number 10
October 1966
3206
energy is not removed by a small particle and further fragmentation may ensue, creating an unsymmetrical product distribution. That this is observed in solid n-heptadecane but not in n-hexadecane suggests that excess energy associated with energetic fragmentation of’a C-C bond may be more effectively dissipated in the triclinic than in the orthorhombic crystal modification. Because some product yields are higher in n-heptadecane than in n-hexadecane, it is further inferred that primary scission is enhanced by poorer coupling of heptadecane molecules in the orthorhombic habit. Relatively lower yields of CMhydrocarbons are a consequence of the reduced probability of terminal relative to internal carbon-carbon scission.*,* Yields of low molecular weight products from irradiated solid n-heptadecane are unaffected by the presence of 0.6 or 1.2 mole yo iodine. This is consistent with the view that these products result from molecular However, chromatograms of products eluting before n-heptadecane in the irradiated liquid are considerably altered by the presence of iodine. The average radiation yield of each linear hydrocarbon is reduced from 0.071 to 0.039 with 0.6% iodine and to 0.031 by 1.2% iodine. As with hexadecane irradiation, about half of this region in the liquid results from radical reactions, largely disproportionation of radical fragments from main-chain scission with n-heptadecyl radicak2 In addition to the reduction in yield of linear hydrocarbons, a series of peaks corresponding to 1-iodoalkanes is observed. Intermediate hydrocarbon products, comprising carbon numbers C18 to were not specifically identified. Total radiation yields for the entire group, excluding iodoheptadecanes which elute in this region, were estimated using the same calibration as for low molecular weight products. Allowance was made for C18 and C I S impurities. Since this region is characterized by a series of similar peaks, an average peak height calculated from the most reliable chromatographic data was multiplied by the total number of major peaks in the intermediate region. In the radiolysis of solid n-heptadecane, intermediate hydrocarbons are produced with a G value of 0.05. The yield for liquid n-heptadecane irradiation is 0.29. Corresponding values for n-hexadecane radiolysis are very similar, 0.05 and 0.31, respectively.2 I n the presence of iodine, intermediate region yields are unaffected in the solid but reduced in the liquid irradiation to approximately solid-state values. Intermediate region hydrocarbons in irradiated liquid n-heptadecane are largely combination products of radical fragments from main-chain scission with heptadecyl radicals. * The yield of CI to c16 l-iodoalkanes may be related The Journal of Physical Chemistry
R. SALOVEYAND W. E. FALCONER
to the scavenged portion of low and intermediate products in the irradiated liquid. Most of the radical fragments from main-chain scission react with heptadecyl radicah2 In solid n-heptadecane, as in solid n-hexadecane,2 radical reactions leading to intermediate and low molecular weight products are suppressed by cage recombination of radical fragments from mainchain scission, and no l-iodoalkanes below C17are anticipated or found. Yields of tetratriacontane from irradiated n-heptadecane measure the cross-linking reaction and are referred to as G(dimer). For irradiated liquid n-heptadecane the radiation yield of total dimer (Table 111) is very close to the cross-linking yield in n-hexadecane liquid (G = 1.7).2 However, whereas no linear “dimer” resulted from irradiation of liquid n-hexadecane, about 0.3% of the total dimer from liquid n-heptadecane was linear tetratriacontane. The cross-linking yield of solid n-heptadecane is, however, considerably different from the analogous reaction in irradiated solid nhexadecane. The radiation yield of dimer, 1.13, is lower than for n-hexadecane where G = 1.6. Moreover, the proportion of linear dimer is almost 4% as compared to 1% in n-hexadecane radiolysis.’O The enhancement of linear dimer is related to crystal structure, and was demonstrated in doped n-hexadecane radiolysis. Although there is closer molecular packing in the triclinic structure of even n-paraffins than in the orthorhombic modification of odd homologs, layers of odd n-paraffins are rectangular whereas even n-paraffins have an oblique layer s t r ~ c t u r e . ~This difference in end-group packing is reflected in the efficiency of the end-linking reaction to form linear dimer; that is, terminal radicals apparently combine more readily in the rectangular structure. Poorer molecular packing of crystalline n-heptadecane compared to n-hexadecane may account for the reduced yield of total dimer. The presence of iodine reduces cross-linking yields in the irradiation of n-heptadecane (Table 111). For solid n-heptadecane the yield of tetratriacontane is reduced 35% by the addition of 1.2% iodine. The comparable figure for the suppression of dimer in liquid radiolysis is 80%. In the irradiation of solid and (8) A. V. Topchiev, “Radiolysis of Hydrocarbons,” Elsevier Publishing Co., Amsterdam, 1964. (9) L. Kevan and W. F. Libby, J. Chem. Phys., 39, 1288 (1963). (10) A ratio of 8% linear dimer/total dimer for n-hexanes~llis consistent with 1% for this ratio in n-hexadecane. Assuming primary C-H rupture is half as probable as secondary C-H scission, and neglecting differences in disproportionation, random combination gives 7 and 1% for linear dimer/total dimer for C B and Cis paraffins, respectively; that is, the fraction of linear dimer for these triclinic crystalline solids is that predicted by random cross linking, whereas in the orthorhombic c17 it is enhanced by a factor of 4. (11) H. Widmer and T. Gaumann, Heh. Chint. Acta, 46,944 (1963).
IRRADIATION O F
3207
n-HEPTADECANE
liquid n-hexadecane it was shown that dimer suppression by iodine was 40 and 70%, respectively.2 The fraction of dimer formed by the combination of parent alkyl radicals is at least as great as the portion scavenged. I n addition, dimer formation by the combination of proximate radical pairs may be unaffected by iodine. The radiation yield of linear tetratriacontane is not affected by iodine in the solid state but is completely suppressed in the irradiated liquid. We suggest that most of the dimer is formed in a ‘Lhot”hydrogen atom mechanism. Heptadecyl radicals and “hot” hydrogen atoms are produced by carbonhydrogen bond scission. These hydrogen atoms carry excess kinetic energy and abstract readily from neighboring heptadecane molecules to form close pairs of heptadecyl radicals. In the crystalline solid, these combine to form dimer. As a result, 65% of the total dimer and all of the linear dimer are unaffected by reactive solutes. Indeed, it is possible that the fraction of dimer scavenged in the crystalline solid may be the result of post-irradiation reactions of trapped radicals, e . g . , on melting prior to analysis.12 In the irradiated liquid, radical pairs diffuse apart and react with iodine so that most of the dimer can be suppressed. These hypotheses are consistent with previous studies.I3 The reduction in dimer yield in the presence of iodine scavenger is effected by reaction of heptadecyl radicals with iodine to produce iodoheptadecanes. Three discrete peaks assigned to these materials elute with iritermediate molecular weight products (Tables I anti 11). The smallest and last of these peaks to elute corresponds to 1-iodoheptadecane. The other two peaks correspond to unresolved nonterminal iodoheptadecanes. 1-Iodoheptadecane accounts for 3% of total iodoheptadecanes in the solid and 9% in the liquid. In the solid, primary heptadecyl radicals combine more efficiently prior to reaction with iodine. Since 16.7% of the hydrogen atoms in n-heptadecane are terminal, but only 9% 1-iodoheptadecane is formed in the liquid, it is inferred that the probability of secondary C-H ,scission is about twice that of primary rupturea8 The radiation yield of iodoheptadecanes should correspond to the reduction in radiolytic products involving heptadecyl radicals if all the scavenging occurs by combination with iodine. However, the radiation yield of iodoheptadecanes is inadequate to account for all the scavenged radicals in irradiated n-heptadecane. It is inferred that in n-heptadecane radiolysis some dimer may be suppressed by iodine without the formation of iodoheptadecane. This may indicate some physical quenching or disproportionation reactions with iodine. A possible complication may involve
changes in the crystal structure of n-heptadecane in the presence of iodine. The solute may introduce sufficient instability so that the solid assumes the rotational crystalline state at irradiation temperatures.4 This could lead to unexpected behavior in the scavenged radiolysis of the solid, such as the observation of 1iodoheptadecane. From studies on the irradiation of aggregates of lamellar chain-folded crystals of polyethylene, it was concluded that cross linking primarily involves fold regions. ’ 6 A radiation sequence of C-H rupture, radical migration, and combination at the fold is inferred. This necessitates little cross linking in the crystal lattice. The distribution of dimer in irradiated crystalline n-hexadecane’ and n-heptadecane is apparently not in accord with this mechanism. I n fact, a rather random distribution of isomers of dimer is observed in the irradiated solid paraffins. The enhancement in linear dimer yield in the radiolysis of orthorhombic n-heptadecane relative to triclinic nhexadecane may indicate some alteration of irradiation mechanism associated with radical migration and specificity of cross-linking site. Since, the stable crystal structure of polyethylene is orthorhombic,6 its irradiation behavior may be more closely related to n-heptadecane. Further, the presence of folds in polyethylene may significantly perturb the irradiation mechanism. l4t
Conclusions The irradiation behavior of n-heptadecane is similar to that of n-hexadecane’J and of n-hexane. l 1 Product yields on irradiation of liquid n-heptadecane are almost identical with radiation yields from liquid nhexadecane. However, differences are observed in radiolyses of the crystalline solids. The cross-linking yield in irradiated solid n-heptadecane (1.13) is lower than in n-hexadecane (1.6).2 An unsymmetrical distribution of low molecular weight hydrocarbons results from the irradiation of solid n-heptadecane, whereas the yields of each of the low molecular weight products from irradiation of the even homolog are very nearly equal. These differences may be associated with reduced density of molecular packing in the orthorhombic structure of odd n-paraffins as compared to even nparaffins in the triclinic m~dification.~Poorer molecular coupling in solid n-heptadecane may hinder cross 99
(12) (13) (14) (15)
W. E. Falconer and R. Salovey, unpublished results. For other references, see ref 2. T. Kawai and A. Keller, Phil. Mug., 12, 673 (1965). R. Salovey, J . Polymer Sei., 61, 463 (1962).
Volume 70, Number 10 October 1086
3208
ALANSNELSON
linking but enhance both primary main chain scission and secondary cleavage of scission fragments. The proportion of linear dimer produced on irradiation of solid n-heptadecane (4%) exceeds that for solid nhexadecane (1%). Differences in end linking are a
consequence of variations in the environment of chain ends in the two crystalline modifications. Iodoheptadecane yields from scavenged radiolyses of n-heptadecane are lower than corresponding products from nhexadecane, particularly for solid-state irradiation.
Infrared Spectra of Some Alkaline Earth Halides by the Matrix Isolation Technique
by Alan Snelson I I T Research Institute, Cfiicago, Illinois
60616
(Received February d8, 1966)
The infrared spectra of beryllium fluoride, beryllium chloride, magnesium fluoride, calcium fluoride, strontium fluoride, and barium fluoride were observed in matrices of neon, argon, and krypton over the wavelength region 2.5 to 50 p. For beryllium fluoride and magnesium fluoride the asymmetric stretching and the bending frequencies were observed, while for beryllium chloride the asymmetric stretching mode only was observed. These molecules are assigned a linear configuration. There is a discrepancy between the experimentally determined entropy of magnesium fluoride and that calculated using the new frequency assignment. For each of the remaining fluorides of calcium, strontium, and barium, the symmetric and asymmetric stretching modes were observed. These molecules are assigned a nonlinear configuration.
Introduction At the present time there is considerable uncertainty concerning molecular configurations and spectroscopic constants of many inorganic dihalide gas-phase species existing at high temperature. This uncertainty largely reflects the experimental difficulties in determining these parameters for high-temperature systems. Brewer, et al.,' reviewed the existing data on internuclear distances, vibrational frequencies, and molecular geometries of the inorganic dihalides. The paucity of experimental data is such that of the 72 compounds considered, a complete vibrational assignment was available for 3, a partial vibrational assignment for 16, and estimated values only for the remainder. Several experimental studies on group I1 dihalides indicate that some of the molecular parameters in The Journal of Physical C h i s t r y
Brewer's tabulation may be in error. Hildenbrand and Theard21s used vapor pressure and mass spectrographic data to determine the gaseous entropies of all the alkaline earth difluorides and some of the chlorides. In all cases the experimentally determined entropies were larger than those tabulated by Brewer. In some cases the discrepancy was of the order of 5 to 6 eu. Hildenbrand2 suggested that the values assigned to the vibrational constants of the molecules, together
(1) L. Brewer, G. R. Somayajula, and E. Braokett, Chem. Rev., 111, 63 (1963). (2) D. L. Hildenbrand. Aeronutronio Report No. U-3183, June 30,
1965. (3) D. L. Hildenbrand and L. P. Theard; J . Cfiem. Pfiys., 42, 3230 (1965).
