CHI-WINGTSAOAND JOHNW. ROOT
308
A New Primary Process in the Ultraviolet Photolysis of
Methyl Iodide. The Direct Photolysis to :CHI by Chi-wing Tsao and John W. Root* Department of Chemistry, University of California, Davis, California 96616
(Received July 26 1971)
Publication costs assisted by the U.8. Air Force Ofice of Scientific Research
The molecular elimination of Hz has been shown to contribute to the primary photodissociation mechanism of CHJ at ultraviolet wavelengths shorter than 3145 3t: 20 A. From these results upper limiting values of 94.2 =k 0.6 and 91.3 rt 1.7 kcal/mol were calculated for the 29SOK heat of formation of :CHI and the carbon-hydrogen bond dissociation energy Do(CHI-H), respectively.
Introduction For many years the primary photochemical process in methyl iodide has been recognized to bel+ CHgI
+ h~
---it
CH,
+I
(1)
The continuous absorption band for methyl iodide occurs in the ultraviolet from about 3600 8 with the first absorption maximum at about 2600 8. At short wavelengths two new primary processes become energetically possible.
+ Hz CHsI + hv + :CH2 + H I cH3I i- hv + :CHI
(2)
(3)
The occurrence of reaction 2 in alkanes is well known. Mahan and Mandal demonstrated the formation of H2 in the vacuum ultraviolet photolysis of CH4 and concluded that it was produced primarily from an intramolecular p r o ~ e s s . ~Herzberg suggested process 2 in CHaCl from a theoretical point of view.6 I n recent matrix-isolation studies of the vacuum ultraviolet photolyses of CHaCl and CHsF the spectra of :CHC1 and :CHF were identified.6,’ It was suggested in this latter work that both : CHCl and : CHF resulted from primary processes such as (2)) but no attempts were made to detect the postulated molecular elimination of H2. The occurrence of reaction 3 at 2288 8 has been verified recently in photolysis experiments with tritium-labeled methyl iodide.8 The singlet methylene ( I : CHT) was detected through the well-known olefin addition and carbon-hydrogen bond insertion reactions. During the course of our hot methyl radical research program, we suspected that molecular hydrogen was being formed via primary process 2 in the ultraviolet photolysis of CHJ. Another series of experiments was therefore initiated with the tritium-labeled methyl iodide in a dual effort to confirm the existence of CHzTI The Journal of Physical chemistry, Vol. 76,No.8,1978
photodissociation channel 2 and to measure its photochemical threshold energy.
Experimental Section Broad spectrum ultraviolet radiation was produced from a Hanovia Type 929B009U, 2500-W Xe-Hg lamp. The collimated light was predispersed by a quartz prism and then concentrated upon the entrance slit of a Bausch and Lomb monochromator. The entrance and exit slits of the monochromator were so adjusted that a reasonably intense beam of light emerged from the monochromator with an effective bandwidth of 40 A (full width half-maximum). The apparatus and a representative light spectrum are shown on Figures 1 and 2. The quartz optical elements and the light source were carefully aligned by means of a Spectra Physics He-Ne gas laser. The laser beam was passed through the center of each optical element and finally struck the lamp 2 mm below the tip of the tungsten anode. Using this particular optical system it was found that further reductions of the output bandwidth were impractical because of severe intensity losses. Also, the undesirable shoulder at short wavelengths could not be eliminated. The light intensity incident upon the photolysis cells was monitored either by means of an Eppley thermopile and a Keithley nanovoltmeter or a photomultiplier tube and a Keithley electrometer. Except for the quartz windows the reaction cells were constructed from ordinary Pyrex glass and Kontes high (1) R. D. Doepker and P. Ausloos, J . Chem. Phys., 41, 1865 (1964). (2) R.D. Schultz and A. A. Taylor, ibid., 18, 194 (1960). (3) G. M. Harris and J. E. Willard, J . Amer. Chem. Soc., 76, 4678 (1950). (4) B.H . Mahan and R . Mandal, J . Chem. Phys., 37, 207 (1962). (5) G. Herzberg, “Electronic Spectra of Polyatomic Molecules,” Van Nostrand, Princeton, N. J., 1967, p 448. (6) M. E. Jacox and D. E. Milligan, J . Chem. Phys., 53, 2688 (1970). (7) M. E. Jacox and D. E. Milligan, ibid., 50, 3525 (1969). (8) (a) G. W. Mutch and J. W. Root, unpublished results: (b) C. C . Chou, P. Angelberger, and F. 8. Rowland, J. Phys. Chem., 75, 2536 (1971).
309
ULTRAVIOLETI?HOTOLYSIS OF CH,I vacuum O-ring stopcocks. Suprasil quartz windows were cemented onto the Pyrex body by means of epoxy glue. The sample cells and vacuum apparatus were absolutely grease free, and connections to the vacuum system were made by means of O-ring ball and socket joints. The photolysis cells were designed to allow their direct incorporation into the flow system of the gas chromatography analytical apparatus. The entire vacuum gas transfer apparatus was fabricated from copper and brass in order to avoid the hydrogen exchange problems often encountered in photochemical recoil experiment^.^ The brass vacuum valves were Hoke Type 411M, 4B, and the gas pressures were monitored by Hastings gauges in the micron range and by a diaphragm gauge in the Torr range. With the present apps:ratus ten reaction cells can be filled at once with identical gas mixtures. For each analysis the reaction cell contents were flushed through a series of liquid nitrogen temperature traps into the gas chromatography column. The eluted tritium-labeled products H T and CHIT were detected by an internal gas flow proportional counter.1° To ensure that, no CHzTI was allowed to contaminate the analytical gas chromatography column, a short stripper column consisting of a 5-cm section of Linde Type 5A molecular sieve packed in glass tubing was inserted into the flow stream ahead of the analytical column. The latter column consisted of a 20-ft section of Linde Type 5A molecular sieve. The retention times for H T and CHaT were 7 and 23 min, respectively, at a helium flow rate of 85 cc/min NTP. Signal pulses generated in the counter passed successively through an Ortec scintillation preamplifier provided with a high-voltage standoff capacitor, a linear amplifier, a discriminator, and a digital output device. The chemicals employed in these experiments included the following: He, J. T. Baker high purity grade; 02, Matheson research grade; and CHZTI, International Chemical and Nuclear Corp., 25 mCi/mmol. In a typical experiment nine photolysis cells were filled simultaneously with 350 1 pressure of the labeled methyl iodide, 20 Torr 02,and 180 Torr He. The cells containing the reactants were allowed to stand in the dark overnight to ensure complete mixing. One cell from each set was kept in the dark throughout the experiment as a blank run, while the others were irradiated one by one a t preselected ultraviolet wavelengths. Because of the low intensity of our narrow bandpass light source, several of the runs required photolysis times as long as 12 hr. Following the irradiations, the reaction mixtures were analyzed as quickly as possible in order to minimize any dark reactions,
Results The results that we wish to report are summarized in Table I. The reported H T yields represent the signals obtained after corrections for counter background
4 3
Figure 1. Schematic drawing of the photolysis apparatus. (A) High-pressure Hg-Xe lamp, (B) focusing lens, (C) predispersion prism, (D) focusing lens, (E) variable entrance slit, (F) concave mirror, (G) diffraction grating, (H) variable exit slit, (I)exit lens, (J) photolysis cell, and (K) thermopile or photomultiplier detector.
0
.-$ 4 .I.’
$ 2 0 3580 3600 3620 3640 3660 Wavelength
(A)
3680
Figure 2. A typical output spectrum of the Bausch and Lomb monochromator a t 3650 d.
and for the small activities from the blank runs. Absolute quantum yields were not, measured in these experiments; instead, the data were normalized by means of the following formula
N Y=kIOCt
(4)
in which Y is the relative yield of HT; L is an arbitrary constant; N is the net observed H T activity; I ois the total light intensity falling on the entrance window of the photolysis cell; C is the concentration of the methyl iodide; and t is the duration of exposure to the ultra(9) (a) R. M. Martin and J. E. Willard, J . Chem. Phys., 40, 2999 (1964); (b) It. M. Martin and J. E. Willard, ibid., 40,3007 (1964). (10)J. K.Lee, E. K. C. Lee, B. C. Musgrave, Yi-Noo Tang, J. W. Root, and F. 8. Rowland, Anal. Chem., 34, 741 (1962). The Journal of Physical Chemistry, Vol. 76, No. 8 , 1976
310
CHI-.WINO TSAO AND JOHNW. ROOT
violet light, Due to uncontrolled variations in the experimental conditions, the data show fluctuations from run to run. The general trends indicated by these results were quite reproducible, however, and support several conclusions.
Table I : Reaction Yields of H T and CHsT" HT
CHRI, Expt
I I1
I11
a
A
A,
c
2804 2967 3025 3025 3130 3230 3340 3025 3165 3200 3340
450 450 450 350 350 350 350 300 300 300 300
(normalized counts)
CHaT (normalized counts)
1336 f 17 1052 f 16 1197 f 22 807 f 17 991 f 23
18 0 18
767 292. 243 270 405 840 720 120 300 300 700
...
... ... ...
0
78 f 3 258 z t 20
I . .
... ...
0 0 0
Time, min
...
He = 180 Torr; 02 = 20 Torr in all the experiments.
Discussion The experimental results pointed to three unambiguous conclusions. (1) From experiments I1 and I11 the threshold wavelength for molecular H T formation was determined to occur in the wavelength interval between 3130 and 3165 A. Since the effective band width of the light source is 40 d, full width at) half-maximum, our best estimate for this threshold is 3145 f 20 d. (2) The total amount of H T produced did not fluctuate appreciably within each set of experiments. (3) I n all these experiments the CHaT yields were small compared t o the yields of HT. There are three possible mechanistic paths (5-7) by which H T could be formed in the CHzTI ultraviolet photolysis experiments CHzTI
+ hv
:CHI
+ HT AH = 4.0
CHzTI
f 0.2 eV
(5)
+ hv -% CHzI + T" A H = 4.5
T" + CHJ
4
CHzTI
HT
+ CHzI 111
AH
=
0.07 eV (6a) (6b)
C
+ hv -
f
+ HI :CH2 + TI
:CHT
4.12
f 0.04
+ M a:CHT + l\iI ':CHT + +H T + COz TI + hv +T" + I T" + CHaI +H T + CHzI l:CHT
4
0 2
The Journal of Physical Chemistry, Vol. '76,No. 8, 19'72
(7a) (7b)
eV (74 (74 (74 (7f)
The approximate thermodynamic threshold energy for process I is 4.0 f 0.2 eV based upon a recent electron impact measurement of the heat of formation of :CHI." The uncertainty assigned to the enthalpy change for process 5 represents an estimate, since no accuracy limits were specified in the original reference. The thermodynamic threshold for process I1 is 4.50 f 0.07 eV, corresponding to single photon absorption at 2754 The actual photodissociation threshold would probably occur at shorter wavelengths than the 2754 8 value. From recent tabulations of the standard heats of formation of CHJ, :CH2, and H I the thermodynamic threshold for process 111 is calculated as 4.12 0.04 eV.13J4 Our measured threshold for H T formation from the ultraviolet photodissociation of CHzTI is 3145 f 20 d. A 3145 A photon has an energy of 3.94 eV, which is insufficient for the initiation of either photodissociation process I1 or 111, and we conclude that the observed H T must have come from process I. Because of the possibility for uncertainties in the thermochemical data, we have also carried out a series of kinetics experiments to substantiate the conclusions derived from the threshold measurements. It has been demonstrated experimentally that triplet methylene reacts very efficiently with oxygen to produce molecular hydrogen as in reaction 7d.15 No significant changes in our H T yields were observed whet otherwise identical samples were photolyzed a t 3130 A with and without added oxygen scaven6er. Additional experiments were carried out at 3130 A in which the samples contained added ethylene. No labeled cyclopropane was detected from the well-known olefin addition scavenging reaction 8
*
a : CHT
+ CzH4
c-C~H~T
(8)
The half-pressure for collisional stabilization of the triplet addition product from reaction 8 is about 170 Torr, so that a substantial fraction of the c - C ~ H ~ T would have survived unimolecular isomerization to C3H6Tunder our experimental conditions. Therefore reaction 7d involving triplet :CHT did not contribute significantly to the HT yields at 3130 i , S a Furthermore, molecular H T is not an expected product from singlet :C H T reactions, and we conclude that process I11 can be ruled out as a source of molecular H T at wavelengths longer than 3100 A. One other possible source for molecular H T might (11) J. J. D e Corpo and J. L. Franklin, J . Chem. Phys., 54, 1886 (1971). (12) (a) D.M.Golden and S. W. Benson, Chem. Rev., 69, 126 (1969); (b) S. Furuyama, D. M . Golden, and S. W. Benson, Int. J. Chsnt. Kinet., 1, 283 (1969). (13) 5. W.Benson, "Thermochemical Kinetics," Wiley, New York, N.Y., 1968. (14) W.A. Chupka and C. Lifshitz, J . Chem. Phys., 48, 1109 (1968). (15) R.L. Russell and F . S. Rowland, J . Amer. Chem. Soc., 90, 1671 (1968).
ULTRAVIOLET PHOTOLYSIS OF CHsI involve some unspecified reaction of hot CHzT radicals with Oz. This mechanism is eliminated by the absence of H T in the experiments carried out at wavelengths longer than 3130 A. Furthermore, the H T yields were not affected by the presence or absence of 02. Unimolecular decomposition of the hot CHzT radicals t o form either HI’or T atoms is not energetically possible at 3145 8. By this process of elimination, we conclude that the observed molecular H T yield a t 3145 8 must have resulted from inolecular elimination photodissociation mechanism I. Herzberg has advanced a theoretical interpretation for photodissociation processes of this kind.5 According to his model the CHsI would be excited t o an attractive upper state corresponding t o Hz :CHI. This mode for the unsymmetrical, direct dissociation is depicted in Figure 3. Our observation in Table I that the molecular H T yield is essentially constant a t energies immediately above threshold requires that the onset of the upper state vibrational continuum be located directly above the potential minimum for the CHJ ground state. The heat of formation for :CHI can readily be estimated from this unsymmetrical direct dissociation model. From. our experimental threshold for reaction 5 the heat of formation for :CHI is calculated as 94.2 f 0.6 kcal/mol. This is an upper limiting value because of the likelihood that the :CHI contains vibrational excitation. This calculated heat of foxmation for :CHI is in excellent agreement with the independent result of 95 kcal/mol obtained recently by De Corpo and Frankli.n.’l Based upon our threshold value and the known CHJ-H bond dissociation energy,12we calculate a 298’K Do(C:HI-H) value of 91.3 A 1.7 kcal/mol.
+
31 1 I
5,
P W
Internuclear
Distance
Figure 3. Approximate two-dimensional representation of the dissociation process involved in the photodissociation of CHJ.
Summary The existence of a new primary photodissociation process for methyl iodide has been established. This process leads to molecular HZand :CHI with a threshold corresponding to ultraviolet absorption at 3145 f 20 A. Based upon Herzberg’s unsymmetrical direct photodissociation model, upper limiting values of 94.2 f 0.6 and 91.3 f 1.7 kcal/mol were calculated for the 298°K heat of formation of :CHI and the carbon-hydrogen bond dissociation energy Do(CHI-H), respectively.
Acknowledgment. The authors gratefully acknowledge generous financial support from the U. S. Air Force Office of Scientific Research.l6 We would also like t o thank Mr. G. W. Rhtch of this laboratory for his assistance withsome of the experiments. (16)
This work has been supported under AFOSR Contract AF-
AFOSR-68-1493.
The Journal of Physical Chemistry, Vol. 76,No. 8,1072
