537
IR MATRIXISOLATION SPECTRUM OF CHa RADICAL salts of each cation were examined and no anion effect was observed. Infrared spectra of the tetraalkylammonium perchlorate likewise did not indicate any shift for the three bands. The magnitude of the shift is Table 11: Frequency Shifts of Three Pyridine Skeletal Vibrations Produced by Alkali Metal Salts
Pyridine Na+ solutions NHI+ solution Li+ solution
1581 ., . 1590 $9 1592 $11 1597' >$12
992 . . . 996 $4 999 $7 1003 '$11
603 . . . 611 +8 614 $11 620 $17
"Assignments of L. Corrsin, B. J. Fax, and R. C. Lord, J . Chem. Phys., 21,1170 (1953). 'The shifted 1581-cm-' band lies too close to the 1598-cm-1pyridine band to be resolved; consequently, only a single band at 1597 cm-1 is observed.
N a + < "49 < Li+ and should give an indication of the relative bond strength of the three ions with the pyridine molecules. Finally, the interaction of pyridine with alkali metal salts has also been demonstrated by the formation of solid solvates with lithium chloride. lo Likewise, the
low conductances of alkali metal salts in pyridine have been explained as being due to an interaction between the alkali metal ions and pyridine. l1 On the basis of the above evidence it seems reasonable to conclude that the alkali metal cations are solvated in pyridine solutions and that the infrared bands listed in Table I are due to the vibrations of the cations in the solvent cage. This observation does not preclude the formation of solvent-separated ion pairs in pyridine solutions as demonstrated by conductometric measurements.11112 In most cases, however, electrostatic interaction between solvated ions does not seem to influence noticeably the frequencies of the solventmetal ion vibrations. It is possible that in the case of sodium iodide, the anion may enter into the primary solvation shell of the cation.
Acknowledgment. The authors gratefully acknowledge the support of this work by a grant from the National Science Foundation. (10) H. Brusset and S. Halut-Desportes, Bull. SOC.Chim. Fr., 469 (1967). (11) D. S. Burgess and C. A, Kraus, J. Amer. Chem. Soc., 70, 706 (1948). (12) H. C.Mandel, W. M. McNabb, and J. F. Hazel, J . Electrochem. Soc,, 102,263 (1955).
Infrared Matrix Isolation Spectrum of the Methyl Radical Produced by Pyrolysis of Methyl Iodide and Dimethyl Mercury by Alan Snelson IIT Research Institute, Chicago, Illinois 60616
(Received March 12, 2969)
The matrix isolation technique has been used t o trap methyl radicals formed by the gas-phase pyrolysis of methyl iodide, &-methyl iodide, and dimethyl mercury. The three observed infrared-active vibration frequencies of the CH, radical assuming a planar structure, point group DBh, are v 2 = 617 cm-l, ~3 = 3162 om-', and ~4 = 1396 om-'. The corresponding frequencies for CDa are vz = 463 cm-', Y8 = 2381 cm-', and ~4 = 1026 cm-l. The infrared-inactive frequency V I is calculated a t 3044 and 2153 om-' for CH, and CD,, respectively, using the UBFF potential for an analysis of the vibrational modes. Some observations on the mechanisms of decomposition of methyl iodide and dimethyl mercury under the present experimental conditions are made, and a tentative vibrational assignment for the radical HgCH, is given.
Introduction The results of three investigations aimed a t trapping methyl radicals in rare gas matrices have recently been published. I n the first of these, Pimentel and Andrews' attempted to form methyl Edicals by reaction of either methyl. iodide or bromide with lithium
atoms during the deposition of the matrix.2 Two absorption bands were Observed in an argon matrix a t 730.3 and 1383 em-' which were assigned to the (1) L.Andrews and G. C. Pimentel, J. Chem. Phys., 47,3637 (1967). (2) w. L.8. Andrews and G. c. PimenteI, ibid., 44,2527 (1966). Volume 7d8Number 9 February 6, 1970
538
ALANSNELSON
methyl radical with Dah symmetry. The lower frequency was assigned to the out-of-plane bending mode from observations of the isotopic frequency shifts using deuterium and carbon-13 substituted species. The agreement between the observed and calculated frequency shifts was excellent. In the second investigation, Milligan and Jacox3 attempted to form methyl radicals by the vacuum-ultraviolet photolysis of methane in argon and nitrogen matrices. From the ultraviolet spectrum of the photolyzed methane, absorption features were observed at about 1500 in excellent agreement with Herzberg’s4 assignment for one of the CH3 radical’s electronic transitions. An absorption band at 611 cm-l in the argon matrix was assigned to the out-of-plane bending mode of the CHI radical, and again, frequency shifts due to isotopic substitution with deuterium and carbon-13 supported this assignment. The obvious disagreement between the two reported values for the out-of-plane bending mode of the CH3 radical prompted Pimentel and Tan5 to reinvestigate their alkyl halide plus alkali metal system. This time sodium and potassium were used in addition to lithium atoms to reduce the methyl halides during ,entrapment in the inert gas matrices. This study indicated that the band at 730 cm-l previously assigned to the methyl radical could be best assigned to a new molecular species, a methyl alkali halide. This interpretation throws some doubt on the value of the higher frequency a t 1383 cm-l previously assigned to a doubly degenerate in-plane bending mode of the methyl radicaln2 Milligan and Jacox attributed their inability to locate the frequencies for the two infrared-active in-plane bending and stretching modes to low extinction coefficients. I n a,n effort to observe the three infrared-active vibration frequencies of the methyl radical an attempt was made to adapt the matrix isolation technique to the trapping of radicals produced by the low-pressure pyrolysis of suitable organic compounds. Although this type of experiment has been tried by several workers before2 without success, the results of some recent studies on the matrix isolation spectrum of lithium fluoride in this laboratorye suggest that this is now possible. I n the earlier attempts, matrix dilutions of several hundred parts of inert gas to one of the trapped species were used. The study on lithium fluoride demonst>ratedthat efficient isolation of a reactive material in a molecular beam can only be achieved at dilution ratios of about 80,000 parts of matrix to one of the reactive molecule. There is little doubt that the primary cause of failure in the earlier studies was that of poor isolation.
The original Knudsen cell furnace was removed from the cryostat. In its place an attachment for the pyrolysis of suitable organic vapors was added. This is shown in Figure 1. The pyrolysis reactor consisted of a platinum tube about 1.8 in. long with an id of about 0.090 in. It was heated resistively. No attempt was made to achieve temperature uniformity along the length of the pyrolysis zone. The temperature was measured at the center of the pyrolysis zone with an optical pyrometer. To achieve better heat transfer between the reactor and the reactant, the pyrolysis zone was packed with a small amount of platinum gauze. One end of the tube was mounted in a water-cooled copper plate and was placed about 1.5 in. from the matrix isolation window in the cryostat. The other end was attached to a gas inlet system. The material to be pyrolyzed was fed into the reactor, mol/hr. The usually a t a rate of about 1-5 X amount of material which is described as being deposited on the window in Figures l, 2, and 3 is simply the number of moles of the given species which were bled into system via the pyrolysis tube during the course of the experiment. This rate was controlled by interposing a slow fixed leak between the gas storage system and the pyrolysis reactor and maintaining a suitable pressure differential across the leak. The matrix gas flow rate was controlled by a needle valve and monitored by measuring the pressure differential on a dibutyl phthalate manometer. In all experiments flow rates of about 0.05 mol/hr were used, and neon was used as the matrix material since experience has shown that this gas produces the most transparent matrices. Liquid helium was used as the refrigerant. Although the experimental conditions used in the present study were sufficient to obtain fairly good isolation of the reactive species, they were definitely not good enough to prevent a small amount of reaction occurring in the trapping process.
Experimental Seetion The matrix isolation cryostat used in this study has been described elsewhere,6 and only the modifications new to the present arrangement are described.
(3) D. E. Milligan and M. E. Jaoox, J . Chem. Phys., 47,6146 (1967). (4)G.Herzberg, Proc. Roy. Soc., A262, 291 (1961). (6) L.Y.Tan and G. C . Pimentel, J . Chem. Phys., 48,6202 (1968). (6) A. Snelson, J . Phys. Chem., 74,1919 (1969).
The Journal of Physical Chemistry
JETS TOR INTRODUelNC INBI 6AS PLAIINUR PYROLYSIS 1UBE
IO CAS STORAGE SYSTEL ’\
INSULATING S L W E
’ ,/ iiouii N I I A O G ~CUO O L ~ Dsniiin
I \
CESIUM lODlSI WINDOW
Figure 1. Pyrolysis attachment to mabrix isolation cryostat.
IR MATRIXIBOLATION SPECTRUM OF CHa RADICAL
539 any pretreatment. The maximum stated hydrocarbon impurities in the sample on a volumetric basis were ethylene 575, methane 1%, and propane plus propylene 2%. Research grade neon (Matheson Co.) was used for the matrix gas and was passed through a liquid nitrogen cooled trap prior to injection into the matrix isolation cryostat. Infrared spectra were recorded on a Perkin-Elmer 621 spectrophotometer. The calibration of the instrument was checked against atmospheric water and carbon dioxide bands. Reported frequencies are believed accurate to 1 cm-'.
(b) C.H.
0.4 -
.
30
J/
0
n
I
, P I
I
0
B
IC)
-
CH,
1
*
b
~:I.~xIo"
0.8 1.1.5 hrs.
Infrared Spectra. Results
0.4 -
Figure 3. Matrix isolation spectra in neon of (a) pure CDJ, 8.8 x 10-8 moles deposited in 3 hr, and (b) pyrolyzed CDd, 1.1 X 10-4 mol deposited in 6 hr. Approximate fraction decomposed, 0.20.
The pyrolysis of methyl iodide is known to occur by schism of the carbon-iodine bond.7 At the pressures existing in the pyrolysis reactor used in this study of approximately atm, some of the radicals undergo reaction through gas-phase collision before escaping from the reactor and being trapped in the matrix. These reactions can lead to the formation of stable products methane and ethane, etc. For this reason, it was deemed advisable to examine the matrix isolation spectra not only of pure methyl iodide and its deuterated derivatives before pyrolysis, but also that of methane and ethane. In this way the effect of the matrix environment on the spectra of the various compounds could be observed, allowing more positive identification of the products of the pyrolysis reaction. The important regions in the spectra of C&I, CD& Hg(CHs)z, CHI, and C2H6,are shown in Figures 2, 3, and 4. Frequencies are listed in Table I. Absorbance values were calculated for all lines so that relative intensity considerations in addition to frequency values could be used in making assignments for the pyrolysis products. With the exception of five bands in the spectrum of CH& all the matrix frequencies were found to correspond with the previously reported gas phase values.s-16 It was found possible to assign these five "excepted" bands in the CHJ spectrum to combination modes rather than to impurity bands. I n general, the matrix frequencies were lower than the reported gas-phase
Methyl iodide (Matheson Coleman and Bell) reagent grade was distilled once prior to introduction to the gas storage system. It was out-gassed thoroughly using standard vacuum line techniques. The &methyl iodide (Volk Radiochemical Company) was used directly after thorough out-gassing. Dimethyl mercury (Alpha Inorganics) was thoroughly out-gassed and distilled under high vacuum using liquid nitrogen M a coolant. The initial condensate was discarded, and the middle fraction retained for use. Methane (Matheson Co. research grade) with a stated purity of 99.99% was used without any pretreatment. Ethane (Matheson Co. technical grade) was used without
(7) 8. W. Benson, "The Foundations of Chemical Kinetics," McGraw-Hill Book Co., Inc., New York, N. Y., 1960. (8) W. T.'King,I. M. Mills, and B. Crawford, J. Chem. Phys., 27, 465 (1967). (9) F. D. Verderame and E. R.Nixon, ibid., 45,3476 (1966). (10) G. Heraberg, "Infrared and Raman Spectra of Polyatomic Mole cules," D. Von Nostrand Co., Inc., Princeton, N. J., 1960. (11) L.G.Smith, J . Chent. Phgs., 17,139 (1949). (12) A. Cabana, G. B. Savitsky, and D. F. Hornig, ibid., 39, 2942 (1963). (13) F.H. Fraver andG. E. Ewing, ibid.,48,781 (1968). (14) H. S.Gutowsky, ibid., 17,128 (1949). (16) A. B. Kittila, Ph.D. Thesis, University Microfilms Inc., Ann Arbor, Mich., No. 667056. (16) D. R. J. Boyd, H. W. Thompson, and R. L. Williams, Discussions Faraday Soc., 9,164 (1950).
0.
' (dl Pyrolysed CH,I I
n:I.i'5~10"
'
I
I
a
Figure 2. Matrix isolation spectra in neon of (a) pure CHJ, 2.7 X 10-4 mol; (b)pure CaHs,2.6 X lo-' mol; (0) pure C&, 1.5 x 10-6 mol; and (d) pyrolyzed CHJ, 1.75 X lo-' mol. Approximate fraction decomposed 0.25. 1.2
If
(a) CD,I
t
Oa8 0.4 -
e
9
h I CT
o
-1
0.4 -
On2/
I
I
I
m
(b) Pyrolysed Coal
,I;,In ilkL l
i
,
400 600 800
,
1000 1200
~~
bjkp
?,
2004 2200 2400
1
cm-1
Volume 74, Number 9 February 6 , 1870
540
ALANSNELSON
Table I: Observed Frequencies (cm-1) for CH& CDaI, CH4, CzH4, CzHs, and Hg(CH& Trapped in a Neon Matrix --CHsI-
a b c d e f
g h i xa x" xa
r-CDsI-
3055 2965 2855 2830' 1435 1409' 1249 883 532 2496' 2320' 2135'
a b c
d e f g h
d e f g h i
CHI
0
C
j
1 a b
--C*H4--7
.----C2H#-.----.
2267 2154 2080 1045 1169 945 655 499
2979 2947 2921 2888 2857 2839 2747 1467 1372 819
Hg (CHs)e-----
7 -
a k n
3099 1437 948
b m
3015 1305
a b c d x x e
CHC
f g h i
2988 2943 2906 2828 2380 1720 1472
j
k 1
1442 1430 1375 1337 1199 784 549
3015 1305
a The frequencies designated with an x are not shown in Figures 1, 2, and 3. * These frequencies were not included in Crawford's assignment for CHaI. 'These frequenciesresult from CZH4 and CHd impurities in the CgH6.
Table I1 : Frequency Assignment for the Pyrolysis Productsa of Methyl Iodide (cm-1)
0.4
Frequency, om-1
0.2
0
-I
0.4 0.2
0 cm-1
3162 3055 3015 2979 2965 2948 g 2921 h 2888 i 2855 j 2830 a
Figure 4. Matrix isolation spectra in neon of (a) pure Hg(CH&, 4.7 X low6mol deposited in 2.5 hr, and (b) pyrolyzed H[g(CHa)g,8.3 X 10+ mol deposited in 4 hr. Approximate fraction decomposed, 0.83.
values by about 5 cm-l, and there was little evidence of the matrix environment causing splitting of absorption bands due to the trapped molecule occupying different sites in the matrix lattice. The only previous matrix studies involving any of these species involved the trapping of CH, in matrices of argon, krypton, and x e n ~ n . ~In~ these , ~ ~ matrices, both CH, infrared absorption bands showed considerable structure which was satisfactorily interpreted as being due to hindered rotation of CH4 in the matrix cage. The lack of any structure for these same bands in a neon matrix suggests that hindered rotation is not occurring in this matrix. Pyrolyzed C H J , C D J , and Hg(CH&. Methyl iodide and &-methyl iodide were pyrolyzed a t temperatures between 1200 and 1400". The spectra are shown in Figures 2d and 3b, and the frequencies listed in Tables I1 and 111. Five experiments were made with methyl iodide using somewhat different flow rates through the pyrolyzing zone. In this way it was The Journal of Physical ChElni8tTy
Frequency, cm -1
k
a b c d e f
CI
\ uo OI
Species
1
m n o p q
r s t
Species
1467 1435 1409 1398 1305 948 883 819 614 532
Pyrolysis temperature of 1300"and a neon matrix.
Table 111: Frequency Assignment for the Pyrolysis Productsa of da-Methyl Iodide (cm-1) Frequency, cm-1
a 2381 b 2297 c 2258 d 2236 e 2225 f 2154 g 2005 h 2080 i 993 a
Species
Frequency, om -1
j
k 1 m n o p q
Species
1045 1026 996 945 717 655 499 463
Pyrolysis temperature of 1350" and a neon matrix.
possible to use absorption intensity considerations to identify bands belonging to the same molecular species. For methyl iodide it was found possible to account for all absorption bands, with the exception of three, in terms of the species CHJ, CH4, C2H6, and C2H4. Relative absorption intensities of bands assigned to these species were in agreement with those
IR MATRIXISOLATION SPECTRUMOF CH3 RADICAL observed previously in the spectra of the pure compounds. The most intense of the three unassigned bands occurred at 617 cm-l, with two much weaker features at 1395 and 3162 cm-l. The band at 617 was split into two components wit,h a separation of 6 cm-I. It was difficult to discriminate if splitting occurred in the band a t 1398 cm-l due to the close proximity of a methyl iodide band at 1409 cm-l. No splitting occurred in the band a t 3162 cm-l. Four pyrolysis experiments were made on ds-methyl iodide. Absorption bands assigned to CDI, CzDa, and CzD4 were made using the gas-phase frequencies listed by Hereberglo and comparison with the comparable spectra of the hydrogenated species. Again three absorption bands were observed which could not be assigned to the above stable hydrocarbons; these occurred a t 2381, 1026, and 463 cm-I. The lowfrequency band at 463 cm-' exhibited a splitting of 7 cm-l, but the two higher frequencies appeared as singlets. Dimethyl mercury was pyrolyzed a t temperatures between 1200 and 1400". A typical spectrum is shown in Figure 3b and the frequencies listed in Table IV. Of twenty-seven absorption bands, nineteen can be assigned between the species Hg(CH&, CH4, CZHB, and CzH4. Of the remaining eight, three a t 617, 1396, and 3162 cm-I had similar relative intensities as bands with the same frequencies in the spectrum of pyrolyzed CH3I. The band at 617 cm-l showed splitting of 6 cm-l. The two higher frequencies appeared as singlets.
Table IV : Frequency Assignment for the Pyrolysis Products" of Hg( CH& Frequenoy, cm-1
a b c d e f g
h i j
k 1 m n
3162 3099 3062 3015 2979 2947 2921 2906 2888 2857 2839 2828 2747 1467
Species
CHa C2H4 HgCHa CHI CzHa CzHe CZH6 Hg(CHa)z CZH6 CeHe CZH6 Hg(CHa)r CZH6 CzHe
Frequency, cm-1
o p q r s t
u v w x y z 8'
1450 1437 1396 1372 1305 1245 1030 948 819
784 614 570 549
Species
HgCHs C2H4 CHa CZH8 CHI HgCHa HgCHa CzHd CZH6 Hg(CHs)z CHa HgCHa Hg(CH8)a
Pyrolysis temperature of 1300" and a neon matrix.
I n Table V, the intensities of the absorption bands at 617, 1396, 3162, 463, 1023, and 2381 cm-1 obtained from the various pyrolysis experiments are shown. Because of the relatively large difference in absorption
54 1 intensity between the low- and two higher-frequency bands in the three sets of pyrolysis experiments, accurate intensity measurements were difficult. For the data quoted in Table V, the automatic slit programmer of the spect,rometer was disengaged and the slit opening maintained at a fixed setting during the intensity measurements. No intensities for the band a t 1396 cm-I in the pyrolysis of CH31 are listed since the close proximity of a methyl iodide band precluded meaningful measurements. The precision of the intensity measurements is believed to be about * 5 % for the high-frequency low-intensity bands and somewhat better for the more intense low-frequency bands. The values for the ratios of band intensities listed in Table V are consistent, within the precision of the measurements, with an assignment for the bands a t 617, 1396, and 3162 cm-I to a single molecular species and those a t 463, 1023, and 2381 cm-' likewise to a single molecular species.
Discussion Methyl Radical. I n the spectra of both pyrolyzed methyl iodide and d3-methyl iodide, two sets of absorption bands are observed at 617, 1398, and 3162 cm-*, and 463, 1026, and 2381 cm-l, respectively, which cannot be assigned t o any of the expected stable pyrolysis products of the reactants. The appearance of three absorption bands in the spectra of pyrolyzed dimethyl mercury with similar frequencies and relative intensities as those observed in the pyrolysis of methyl iodide is evidence that the same compound is responsible for both sets of absorption bands and that it consists of carbon and hydrogen only. The frequencies of the most intense absorption bands at 617 cm-l (pyrolyzed CH3I) and 463 cm-l (pyrolyzed CD31) are in close agreement with those assigned by 1iIilligan and Jacox3 to the out-of-plane bending modes of CH3, 611 cm-' and CD3, 463 cm-*. The latter frequencies were recorded for the radicals trapped in a nitrogen matrix and hence exact agreement cannot be expected with those obtained in the present investigation for which a neon matrix was used. Milligan and Jacox have concluded from the existing data on the methyl radical and from their own experiments that the methyl radical is most probably planar, belonging to the point group Dah. Such a molecule has, in addition to the out-of-plane bending mode, two infrared-active in-plane vibration modes. If the interpretation of the present experimental data is correct, these modes should correspond to the two higher frequencies observed at 1398 and 3162 cm-' and 1026 and 2381 cm-l in the CHaI and CD3I pyrolysis studies. Evidence in favor of this assignment may be obtained from a comparison of the observed and calculated isotopic shifts of these lines. Using the nomenclature of Herzberg, the ratio ~ 3 ~ 2 v ~ ~ Z / vbased $ ~ v ~on ~ ~ the atomic masses is calculated at 1.824. The H, and Volume 74, Number d February 6,19'70
542
ALANSNELSON
Table V : Experimental Absorption Intensities and Intensity Ratios for Absorption Bands Assigned to the CHs and CDs Radicals Expt no.a
Log Io/I for peak at: 617 om-’
1396 om-1
3162 cm-1
0.379 1.192 0.627 0.625 0.431
0.039 0.121 0.066
0.043 0.131 0.075 0.062 0.050
1 2 3
4 5
, 6
7 8
. I .
...
ABlr/Ala07
9.72 9.85 9.50 I . .
1026 cm-1
2381 cm-1
0.203 0.226 0.187
0,024 0.026 0.021
0.033 0.038 0.031
Ad6a/AlOZ8
... ...
Intensity ratios AP68/ASZSl
8.44 8.64 9.04
A8lGZ/Ala08
1.10 3. .08 1.14
8.81 9.10 8.36 8.47 8.63
...
Log Io/Z for peaks at:
463 om-1
Intensity ratioaA817/A8182
5.78 6.00 6.11
A2881/AlOZO
1.37 1.44 1.48
a Expt no. 1, 2, and 3 apply to CHa from the pyrolysis of Hg(CH#, and no. 4 and 5, to CH3 from the pyrolysis of CH& log Io/lfor the absorption band a t N om-1.
DZ superscripts indicate the frequencies of the hydrogenated and deuterated samples, respectively. The observed value for this ratio is 1.810 in good agreement with the calculated value considering that r13 anharmonic corrections have been applied to be observed frequencies. I n calculating this observed ratio of the frequencies, the value of vqH2 = 1396 cm-l was used, derived from the pyrolysis of dimethyl mercury. The value obtained from the methyl iodide pyrolysis was complicated by the proximity of the v 3 vg combination band of CHd and was believed t o be less reliable. Further support for the present assignment might be obtained by attempting to observe the spectra of the species CHzD and CHD2, since for these radicals all six fundamental vibration modes are ir active. Unfortunately, at the time of the investigation, this was not possible. The author believes that such a study would probably encounter difficulties with low absorption intensities of some of the in-plane vibration modes. Although, in principle, weak absorption bands can be observed by performing experiments with long deposition times, practical experience shows that a t the high matrix dilutions used in these experiments, the optical transparency of the matrix becomes severely reduced after about 8 hr of deposition. In Table VI the results of applying the Urey-Bradley force field to the in-plane vibrations of CH3 and CD3 are given. To simplify the calculations, the approximation for the interaction constants given by Shimanouchi,l7 F’ = 0.1F, was used and values of K , H,and F , the in-plane bond stretching, bond bending, and nonbonded interaction constants calculated. Shimanouchill7after applying the UBFF t o a variety of molecules containing carbon-hydrogen bonds found that K ranged between 3.9 and 4.8 X dyn cm-l. The value of K = 5.2 X dyn cm-l obtained for the CR3 radical is somewhat larger than Shimanouchi’s typical values, but is not unreasonable considering the the bond length given by Herzberg14from an analysis of the rotational
+
T h s Journal. of Physical Chemistry
b
AN =
Table VI : Force Constants and Frequencies for CH3and CDaa -----CHS-------. Frequencies, cm-l Obsd Calcd Vl Y2
v3
u4
617 3162 1396
3044 617 3161 1398
-CDaFrequencies, cm-1 Obsd Calod
463 2381 1026
2153 476 2362 1025
a K = 5.2 X lo6dyn ern-’, H = 0.315 X lo6 dyn cm-1, F = 0.100 dyn cm-1, and F’ = 0.1F and P y = 0.179 X lo6dyn cm-1.
spectrum of CD3, at 1.079 A, is shorter than that found in methane, 1.091 A, and ethylene, 1.086 A. The inplane bending constant H = 0.315 X E06 dyn cm-l and the interaction constant F , = 0.1 X lob dyn cm-l are in the same range as for other carbon-hydrogen molecules. The effect of varying F’ by &loo% in terms of F was found to have a negligible effect on the calculated values of K , H , and F. Considering that anharmonic effects were not taken into account, the agreement between the observed and calculated values of v3 and v4 for CH3 and CD3 is satisfactory. The outof-plane bending constant F , = 0.179 X lo5dyn cm-1 was calculated from the observed frequency for CH3. The rather poor agreement between the observed and calculated values of this frequency for CDs using the above value of F , can probably be attributed t o anharmonic effects. The ratio of the in-plane t o out-ofplane bonding force constants of CH3, K I F , = 29, is considerably larger than the same ratios for other XY, ions and molecules which have been studied, values of latter ranging from 3.7 to 9.8.l* This relatively low out-of-plane bending frequency is almost certainly a reflection of the small amount of energy required for the CH3radical to assume a nonplanar configuration. (17) T. Shimanouchi,J . Phys. Chem., 17,849 (1949). (18) G. J. Janz andY. Mikawa, J . Mol. Spectrosc., 1, 92, 1960.
IRMATRIXISOLATION SPECTRUM OF CH3 RADICAL Pimentel, et d.,l have reported a value of v4 = 1383 cm-1 for the methyl radical. However, in a later publication they showed that the absorption bands previously assigned to the methyl radical were almost certainly due to a compound of the type CHaMX where M is an alkali metal and X a halogen atom. Comparison of this assignment for v4 with that derived from this study is therefore not valid. Pyrolysis Reactions. I n the pyrolysis of methyl iodide (Figure 2d) the only products formed based on the infrared spectroscopic analyses were CH4, CzH4, C2H6, and CH3. No absorption bands were observed which could not be assigned to these species. Although no detailed pyrolysis study of methyl iodide has been made, Bensonlg has predicted the products to be CH4, CzH4,and Iz, methane being formed in the reaction CH3 CH31 = CH4 CHzI and ethylene from 2CH2I = (C2H412)*= CzH4 12. If Benson’s predictions are correct, the presence of ethane in the reaction products of pyrolysis in this study must be assigned to polymerization of CH3 radicals occurring in the matrix. However, estimates of the amount of ethane formed by polymerization in the matrix under the conditions of these experiments are lower by a factor of 5 than those actually observed. There appears to be a possibility in the methyl iodide pyrolysis that reactions of the type CH3 CH3I = C2H6 I or CH3 CHa M = C2H6 M are occurring. Two pyrolysis experiments were made in which both the matrix gas and methyl iodide were passed through the reactor at about 1300” and then frozen on the matrix window. I n these experiments the amount of methane formed was smaller by a factor of five compared to when methyl iodide alone was passed through the pyrolysis zone. No ethylene was observed a t all. This observation is consistent writh the reaction steps proposed by Benson for the formation of methane and ethylene. The fact that no absorption bands were observed which could be assigned to the intermediate CHJ suggests that either its concentration is too low, thus precluding its observation spectroscopically, or that its lifetime under the experimental conditions is too short. The former possibility is not likely, since if sufficient CHJ is formed to yield a measurable quantity of ethylene by bimolecular collision, it would be expected that enough would fail to react to permit trapping and observation. The pyrolysis of dimethyl mercury (DMM) in this investigation gives the products CH,, CZH4, C2H6, and CH3 and some other specie or species. This products distribution is based on analysis of the infrared spectra of the pyrolysis reaction. Reference to previous studies on the pyrolysis of DMM shows that a t moderate pressures, and temperatures of 300400”, a typical product distribution might be CH4, 50%; C2H4, 7%; C2H6, 30%; C3H8, 12%, with the remaining 1% consisting of other hydrocarbons.20 I n the present study, the ratio of C2He:CH4is about 2 : 1. This is markedly
+ +
+
+
+
+
+
+
543 different from the “typical product distribution’’ a t 300-400° but the observation has been made that a t higher temperatures the ethane production is enhanced. I n this study it was not possible to estimate the relative amount of ethylene to methane or ethane, since no calibration experiments were performed with pure ethylene. According to Kallend, et u Z . , ~ O the major reactions leading t o methane and ethane formation at moderate pressures are CH3 CH3HgCH3 = CH, CH2HgCH3, Ilg CH3 followed and CH3 CH2HgCH3 = CZH, by C2Hj CHaHgCHs = CZH6 CHzHgCH3. This scheme accounts for the formation of larger quantities of methane than ethane, but in the present study more ethane than methane was produced. Thus at the low pressure and the high temperatures used in this study ethane must be formed by some additional process, possibly from the recombination of methyl radicals, 2CH3 = CZHO, or by a radical abstraction reaction, CH3 CH3HgCH3 = C2H6 HgCH3. Kallend, et U Z . , ~ ~ have reviewed the available data of the DMM pyrolysis and conclude the initial reaction can be represented by CH3HgCH3 = 2CH3 Hg, but suggest that it probably occurs in two steps, CH3HgCH3 = HgCH3 CH3 and HgCH3 = Hg CH3. They have inferred from kinetic data that the decomposition of HgCH3 occurs with zero activation energy and hence its lifetime is short. That no absorption bands with similar intensity as the strongest of the methyl radical bands occur which can be assigned to HgCH3 is evidence in favor of short lifetime for HgCH3; alternatively, it could indicate that HgCH3 does not exist, and that the initial decomposition reaction is not a two-step process. In the analysis of the infrared spectrum of pyrolyzed DMilil, five weak absorption bands were observed which could not be assigned to an expected hydrocarbon species. Tentatively, these bands are assigned t o the radical HgCH3, which, assuming the same structure as methyl iodide, will have six infrared active absorption bands. Comparison of the frequencies listed in Table IV for HgCH3 with those of methyl iodide given in Table I indicate the following assignment to be reasonable: vl = 3062 cm-l, ug = 1450 cm-l, u2 = 1245 crn-l, V6 = 1030 cm-’, and u3 = 570 cm-l. The unobserved frequency would be expected in the region of the ethane and methane intense absorption bands a t 3000 cm-I. The origin of the HgCH3 radicals, in view of the low intensity of the absorption bands assigned to the species, can most likely be attributed to reaction of methyl radicals and mercury atoms during the trapping process, with possibly some contribution from a short-lived radical CH3Hg.
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Conclusion The matrix isolation technique can be used to trap (19) S. W. Benson, J. Chem. Phys., 38,1945 (1963).
(20)A. S. Kallend and J. H. Purnell, Trans. Faraday Soc., 60, 93 (1966). Volume 74, hlurnber 3 February 6, 19YO
G.R. UNDERWOOD, D. JURKOWITZ, AND S. C. DICKERMAN
544 free radicals formed by the pyrolysis of suitable chemical compounds. This has been demonstrated in an investigation of the pyrolysis of methyl iodide and dimethyl mercury resulting in the infrared spectrum of the methyl radical being characterized. This type of experiment should be of great use, not only as a means of observing the spectra of free radicals but also to obtain data on the primary reaction products in thermal decomposition reactions. It should be emphasized that the primary object of this study was to demonstrate the feasibility of trapping free radicals formed in pyrolysis reactions using the matrix isolation technique. In the present study, the pyrolysis reactor, although effective, was rather crude in design since the true pyrolysis temperature and surface area of the reactor were ill defined. If this reactor were replaced by the (‘very low pressure pyrolysis reactor” as described by Benson, et u Z . , ~ ~ and the matrix isolation technique used in a quantitative mode as has been recently demonstratedjBkinetic data on the primary dissociation reactions could be obtained. In some cases the matrix isolation method has advan-
tages over the mass spectrometer as a means of analysis, since with the latter the detection and quantitative measurement of free radicals is fraught with difficulties.22 Another area where the pyrolysis method in conjunction with the matrix isolation technique could be useful would be in the study of reactions occurring between free radicals and neutral molecular species. This could be achieved as described by Benson, et U Z . , ~ by ~ allowing the radical source in addition to a second molecular species to pass through the pyrolysis zone. Alternatively, the molecular species could be added to the matrix gas and reaction allowed to proceed during the trapping of the radical species. Acknowledgment. The author gratefully acknowledges the support of the Air Force Office of Scientific Research in funding this study. (21) 8.W.Benson and G. N. Spokes, J. Amer. Chem. SOC.,89, 2626 (1967). (22) F. P. Lossing, “Mass Spectrometry,” C. A. McDowell, Ed., McGraw-Hill Book Co., Inc., New York,N. Y., 1963,p 442.
Structure and Conformations of Free Radicals. 11. Radical Ions from Nitrophenyl Aromatic Hydrocarbons1 by Graham R. Underwood, Don Jurkowitz, and S. Carlton Dickerman Department of chemistry, New York University, University Heights, Bronx, N w York 10468 (Received July 11, 1969)
The radical anions and cations of a large number of nitrophenylben~enes,-naphthalenes, and -anthracenes have been studied by epr spectroscopy. The experimental data were compared with McLachlan SCF spin densities calculated by assuming various angles between the nitrophenyl ring and the remaining planar part of the molecule. It was found that the p-nitrophenyl derivatives were the only ones which could be expected to give meaningful results, and the apparent angles obtained appear reasonable in comparison with other techniques.
Recent interest in studies concerned with the nonplanarity of phenyl-substituted anthracenes and related compounds2-9 has stimulated us to examine the radical anions and cations derived from nitrophenyl-substituted benzene, naphthalene, anthracene, and ferrocene. The reason for this interest has been severalfold. Basically, however, the work has been centered around the fact that even in the simplest system-biphenyl-there is expected t o be some degree of nonplanarity between the two aromatic rings due to steric repulsion between the two pairs of ortho hydrogens. This nonplanarity is reflected in those physical and chemical properties The Journal of Physical Chemistry
which are dependent on the distribution and molecular orbital energy levels of the .rr-electron cloud. Thus, for (1) For paper I, see G. R. Underwood and R. S. Givens, J . timer. Chem. SOC.,90,3713 (1968). (2) L. 0.Wheeler, K. 9. V. Santhanam, and A. J. Bard, J . Phys. Chem., 73,2223 (1967). (3) A.J. Bard, K. 8.V. Santhanam, J. T. Maloy, J. Phelps, and L. 0. Wheeler, Discussions Faraday Soc., 167 (1968). (4) H. Suzuki, Bull. Chem. SOC.Jap., 32, 1340,1360 (1969),and refer-
ences cited therein. (5) J. N. Murrell and H. C. LonguebHiggins, PTOC.Phys. SOC.,A68, 601 (1955). (6) J. N.Murrell, J . Chem. SOC., 3779 (1956).
