J. Phys. Chem. 1995, 99, 4078-4085
4078
Excited State Photochemistry of Iodoalkanes Philip L. Ross and Murray V. Johnston* Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 1971 6 Received: August 24, 1994; In Final Form: December 12, 1994@
Unimolecular photodissociation of iodoalkanes in the C3-CS size range is presented. The photolysis products are photoionized with coherent vacuum ultraviolet radiation and analyzed by time-of-flight mass spectrometry. With 248 nm excitation, the only primary reactions observed are those involving the C-I bond. For primary iodoalkanes, C-I bond cleavage predominates. For secondary and tertiary iodoalkanes, a competitive pathway leading to the elimination of hydrogen iodide becomes more favorable as the number of P-hydrogen atoms increases. With 193 nm excitation, primary reactions involving C-I, C-C, and in certain cases C-H bond cleavages are observed. As the molecular size increases, C-C bond cleavage occurs farther away from the C-I bond. Ground state (2P3/2)and excited state PI/^) iodine atoms are produced by C-I bond cleavage at both wavelengths, but 248 nm excitation shows a much higher propensity for excited state iodine atom production. The hydrogen iodide produced by 248 nm photodissociation is internally excited since it can be photoionized with radiation 0.7 eV below the ground state ionization energy. These results are consistent with rapid, selective dissociation of the C-I bond at 248 nm which yields nonstatistical energy partitioning among the products. This selectivity persists as the molecular size increases. In contrast, excitation at 193 nm permits bond cleavage throughout the molecule and is consistent with a more random partitioning of the absorbed energy prior to dissociation.
below. It was not conclusive whether the reaction occurred by a concerted mechanism or by stepwise elimination of hydrogen and iodine atoms followed by recombination.
Introduction Many small molecules are known to photodissociate directly from an excited electronic state. Depending upon the nature of the excited state, preferential cleavage at the chromophore may be observed. If similar processes occur in large molecules, then site-selective bond cleavage may be achieved as long as competing reaction channels do not interfere. Haloalkanes are of particular significance in this respect owing to the lability of the carbon-halogen bond. In fact, methyl iodide photodissociation stands as a benchmark from both the~reticall-~ and experimental4-I3 perspectives. Experimental work has focused mainly on excitation of the A band, a broad featureless absorption extending from 210 to 310 nm. Dissociation from the excited state yields exclusively cleavage of the C-I bond, depicted in reaction 1 below. CH,-I
-
CH,'
+ I'
(CH,),CH-I
CH3CH2CH,I
(1)
* Author to whom correspondence
should be addressed. Abstract published in Advance ACS Abstracrs, March 1, 1995.
0022-365419512099-4078$09.00/0
CH2=CH-CH3
+ HI
(2)
More recently, unimolecular photodissociation of C2H51, C3H71, and a series of deuterated analogs was studied using 193, 222, and 248 nm e x ~ i t a t i o n . ' ~ Selective -~~ deuteration was used to identify the sites of hydrogen atom production during photodissociation. It was concluded that the hydrogen atom was produced by dissociation of an intermediate alkyl radical and that at least two photons were required for the overall process. One possible reaction sequence is shown in reactions 3 and 4 below for 1-iodopropane.
The iodine atom can be produced in either the ground 2P3/2state, denoted as I, or the PI/^ excited state, denoted as I*. For small (Cl -C3) iodoalkanes and perfluorinated iodoalkanes, the I*/I branching ratio is dependent on both molecular ~ t r u c t u r e ' ~ - ' ~ and photodissociation ~ a v e l e n g t h . ~ The , ' ~ quantum yield for I* formation is typically greater than 70%. The photochemical selectivity of C-I bond cleavage in small molecules may not be achieved in larger molecules if competing pathways, such as C-C bond cleavage or intramolecular rearrangements, become important. Some evidence for primary reactions other than C-I cleavage was reported in early liquid and gas phase photolysis experiments.I8 In this work, formation of HI was observed from precursors such as iodoethane, 2-iodopropane, and tert-butyl iodide. The reaction was thought to occur by a P-hydrogen elimination to produce the corresponding alkene, as shown for 2-iodopropane in reaction 2 @
-
CH,CH,CH,'
-
-
CH3CH,CH,'
+ I'
(3)
CH,CH=CH,
+ H'
(4)
Previous studies of iodoalkane photodissociation have been limited in several ways. In the condensed phase and highpressure gas phase experiments, the primary photodissociation pathways were inferred from the distribution of stable molecules produced by secondary unimolecular and bimolecular reactions. The complexity of these secondary reactions precluded the study of larger molecules. In the unimolecular gas phase experiments, detection of atomic products (I, I*, H) has been emphasized. Reaction pathways leading to other products have not been investigated. Both the unimolecular and high-pressure studies have focused primarily on excitation of the A band. Iodoalkanes also exhibit a sharp B band in the 190-200 nm region,24which marks the beginning of a Rydberg series. Unlike the A band, the B band initially populates a bound state which subsequently couples to a dissociative state. It is unclear whether excitation of the B band results in a similar photochemical selectivity for C-I bond dissociation.
0 1995 American Chemical Society
Excited State Photochemistry of Iodoalkanes
J. Phys. Chem., Vol. 99,No. 12, 1995 4079
TABLE 1: Relative Intensities of PDPI-MS Fragments of Iodoalkanes from 248 nm Photodissociation and 9.68 eV Photoionization fragment, mlz
1-iodopropane 2-iodopropane I-iodobutane 2-iodobutane 2-methyl-2-iodopropane 1-iodo-2-methylpropane 1-iodopentane 2,2-dimethyl- 1-iodopropane 1-iodohexane
2
12 22 4
70 42 20
9 3
3 4
4 49 27 100 65
3
5 84 12 11
3
4
22" PIb 2 25 4
35 65 13
5 18 9
83 59 100
11 10 2 15 37 9
100
100 100 100 58 11 100
46" PIb PIb PIb
7
35 20
100
37" PIb
7
35
6"
100 79
a Photoionization signal at this mlz is subtracted from the total signal to obtain the PDPI signal. Photoionization signal is observed at the same mlz. No PDPI signal is detected above the noise in the photoionization signal.
Observation of competing photochemical reactions in large molecules requires a universal detection method. Recently, we used photodissociation-photoionization mass spectrometry (PDPI-MS) to identify the products of alkene22and alkar~ethiol~~ photodissociation. In these experiments, molecular photodissociation was performed with an ultraviolet laser pulse in the source region of a time-of-flight mass spectrometer. After a short time delay, the photodissociation products were softly ionized with coherent vacuum ultraviolet radiation and mass analyzed. For alkanethiols, photodissociation with 248 nm radiation yielded enhanced S-H bond cleavage, while photodissociation with 193 nm radiation gave a larger contribution of C-S and C-C bond cleavage. This behavior was opposite of what would be expected based upon ground state bond energies but was consistent with the excited state electronic configurations which showed a greater contribution from U*SH at 248 nm than 193 nm. Excited state photochemistry leading to enhanced S-H bond cleavage was observed for molecules up to c5. The goal of this work is to study the unimolecular photochemistry of iodoalkanes in the C3-Cg size range. Primary and secondary reaction pathways are examined as a function of molecular structure, molecular size, and photodissociation wavelength. The results are discussed from the perspective of achieving photochemical selectivity of C-I bond cleavage in large molecules.
Experimental Section The experimental setup for photodissociation-photoionization (PDPI) has been described in detail p r e v i ~ u s l y .Sample ~~ vapor was introduced into the mass spectrometer through a room temperature molecular leak. Ultraviolet radiation at 193 nm (6.42 eV, 620 kJ/mol) or 248 nm (5.0 eV, 518 kJ/mol) from an excimer laser was focused to a 4 mm2 spot size in the source region of a linear time-of-flight mass spectrometer. For the iodoalkanes, excimer laser pulse energies on the order of 1 mJ were found to give adequate photodissociation yields. These pulse energies corresponded to irradiances on the order of 2 MW/cm2. The irradiance of the photodissociation pulse was such that typically only 1% of the parent molecules were photodissociated. This low irradiance minimized the possibility of multiphoton excitation effects. Indeed, we found no evidence for nonlinear behavior among the PDPI fragments: the intensities of the PDPI fragments relative to each other were independent of excimer laser irradiance. Since it is highly probable that I and I* were formed only by a one-photon dissociation of the parent molecule, the irradiance dependence suggests that other fragments were also formed simply by a one photon dissociation. Although multiphoton processes have
been observed in previous studies with i o d o a l k a n e ~ , l ~the -~~ laser irradiances used in our work were at least 1 order of magnitude lower. Background signal produced from the excimer laser pulse by multiphoton ionization of the pump oil was rejected through application of a high-voltage pulse to the source region repeller plate. After these ions were eliminated, the neutral photodissociation products and any remaining undissociated parent molecules were photoionized with a pulse of coherent vacuum ultraviolet radiation and accelerated into the time-of-flight mass analyzer. The minimum delay time between the photodissociation and photoionization pulses (ca. 1 ps) was determined by the rise time of the high-voltage pulse. In this work, the delay time was varied from 1 to 50 ps to check for metastable dissociation processes. No significant changes in the relative PDPI signal intensities were observed. The absolute intensities, however, showed a decrease with long delay times as the fragments migrated from the "field-of-view" of the vacuum ultraviolet beam. Photoionization wavelengths of 118.2 nm (10.49 eV), 121.4 nm (10.21 eV), and 128.1 nm (9.68 eV) were generated by nonresonant frequency tripling of ultraviolet radiation produced from a Nd:YAG pumped dye laser system.26 Physical separation of the ultraviolet and vacuum ultraviolet beams was accomplished by off-axis focusing of the ultraviolet radiation into the frequency-tripling With this configuration, the vacuum ultraviolet beam was focused to the center of the source region while the ultraviolet beam passed approximately 1 cm below the center. Thus, the high-voltage pulse and optical setup used in this experiment allowed us to selectively detect ions generated from the vacuum ultraviolet beam alone. This was verified by evacuating the cell to eliminate vacuum ultraviolet generation. Each mass spectrum was averaged over approximately 500 laser pulses. The peak intensities were measured relative to the most intense PDPI peak in order to correct for variations in the laser pulse energies and spatial overlap of the excimer and vacuum ultraviolet beams. The dynamic range of the data system was sufficient to detect PDPI peaks 2 orders of magnitude smaller than the most intense PDPI peak. The data in Tables 1 and 2 represent the mean peak areas calculated from at least three averaged spectra. Relative standard deviations were typically f 2 % of the base PDPI peak area. All compounds were obtained from Aldrich (Milwaukee, WI) and were used without further purification. Samples were deaerated prior to study by several freeze-pump-thaw cycles.
Ross and Johnston
4080 J. Phys. Chem., Vol. 99, No. 12, 1995
TABLE 2: Relative Intensities of PDPI-MS Fragments of Iodoalkanes from 193 nm Photodissociation and 9.68 eV Photoionization fragment, m/z CH3, C2H3, C2H5, C3H3, C3H.5, C 3 h r C3H7. C4H7, C4Hs. C4H9, CsH9, CsHlo. C ~ H I I ,I, C2H41, C3H61, C4H81, CsHioI, 15 27 29 39 41 42 43 55 56 57 69 70 83 127 155 169 183 197 1-iodopropane 2-iodopropane 1-iodobutane 2-iodobutane 2-methyl-2-iodopropae 1-iodo-2-methylpropane 1-iodopentane 2.2-dimethyl- 1-iodopropane 1-iodohexane 1-iodooctane a
5 10 4 7
4 11
15 20 7 10 8 33 6 32 83
18 44 4 4
100 100 4 18 22 7 68 5 40 24
9 19
PI" PI0 100 100
12
7 100 60 12 72 100
7 100 55
9 44 100 37
PI" PI" PIG
PI" 100 27
8 25 56
15 100 35
67
18 48 18 61 65 22 55 24 59 62
5 4
4 40
11
38 12
27 13
8 14
7
Photoionization signal is observed at the same m/z. No PDPI signal is detected above the noise in the photoionization signal.
R
PI
PI
I
PI
\
Figure 1. PDPI mass spectra of (A) 1-iodopropane, (B) 1-iodobutane, and (C) 1-iodohexane using 248 nm photodissociation and 9.68 eV photoionization. Peaks labeled PI are observed in the photoionization mass spectrum of the corresponding parent molecule. Peaks labeled with an asterisk (*) are impurity or background ions.
ReSUltS Photoionization Mass Spectra. Since the ionization energies of the iodoalkanes studied are on the order of 9.2 eV, these compounds are efficiently ionized with 9.68-10.49 eV radiation. Therefore, the parent molecule photoionization spectrum is superimposed on the PDPI spectrum. The only major ionic fragmentation observed following photoionization of the parent molecule is cleavage of the C-I bond to produce the corresponding alkyl cation. The relative Signal intensity of the alkyl cation increases with increasing photoionization energy. Even with 9.68 eV radiation, the signal intensity is large enough to interfere with PDPI peak detection at the same mlz. 248 nm Photodissociation. Figure 1 shows PDPI mass spectra of 1-iodopropane, 1-iodobutane, and 1-iodohexane using 248 nm photodissociation and 9.68 eV photoionization. Ions produced by photoionization of the undissociated parent molecule are indicated with an asterisk. Relative peak areas in the
spectra of these and other primary iodoalkanes are summarized in Table 1. The major photodissociation pathway observed for all primary iodoalkanes is cleavage of the C-I bond, as indicated by the intense signal corresponding to photoionized iodine atoms at mlz 127. The ionization energy of the ground state (2P3,2) iodine atom is 10.45 eV, while that of the excited (*P,,2) iodine atom (I*) is 9.5 eV. Therefore, the mlz 127 signal observed with 9.68 eV photoionization corresponds only to I*. Alkyl radicals produced by C-I bond cleavage are also photoionized, but these ions cannot immediately be distinguished from alkyl cations produced by photoionization of the parent molecule. In some cases, the PDPI peak areas can be determined by subtracting the contribution due to photoionization of undissociated parent molecules from the total peak area. PDPI peak areas measured in this way are indicated in Table 1. Various low-mass hydrocarbon fragments are present in the PDPI spectra of 1-iodobutane and larger compounds. These fragments arise from secondary processes, either neutral decomposition of the alkyl radical prior to photoionization or ionic decomposition of the photoionized alkyl radical. Figure 2 shows the PDPI mass spectra of 2-iodopropane, 2-iodobutane, and 2-methyl-2-iodopropane using 248 nm photodissociation and 9.68 eV photoionization. Relative peak areas in the spectra of these compounds are also summarized in Table 1. The signal intensity at mlz 127 indicates that C-I bond cleavage leading to I* formation occurs with secondary and tertiary iodoalkanes as well. However, the mlz 127 signal intensity is less than that observed with primary iodoalkanes, suggesting that I* formation is less efficient with the secondary and tertiary compounds. In fact, mlz 127 is no longer the base PDPI peak for 2-iodobutane and 2-methyl-2-iodopropane. Cleavage of the C-I bond also produces an alkyl radical giving mlz 43 and 57 ions after photoionization for the C3 and C4 compounds, respectively. Unfortunately, this signal is superimposed on the ion current produced by ionic decomposition of the parent molecules. Since ionic decomposition of secondary and tertiary iodoalkanes is much more efficient than that of the primary compounds, the PDPI signal intensity at this mlz cannot be accurately determined. Secondary and tertiary iodoalkanes also give a product at mlz 128 which corresponds to photoionized HI. This shows that a photodissociation pathway leading to HI elimination (reaction 2) competes with simple C-I bond cleavage at 248 nm. Varying the ion source pressure from 1 x to 2 x lo-' Torr or the delay time between the photodissociation and photoionization pulses from 1 to 50 ps had no effect on the relative peak area of mlz 128. These observations suggest that HI is produced by a unimolecular reaction. Since the ionization energy of ground state HI is 10.39 eV, the HI produced by this reaction must have a considerable amount of internal energy,
Excited State Photochemistry of Iodoalkanes
J. Phys. Chem., Vol. 99, No. 12, 1995 4081
m/z
20
40
60
80
100
120
140
160
180
m/z
Figure 2. PDPI mass spectra of (A) 2-iodopropane, (B) 2-iodobutane, and (C) 2-methyl-2-iodopropane using 248 nm photodissociation and 9.68 eV photoionization. Peaks labeled PI are observed in the photoionization mass spectrum of the corresponding parent molecule. Peaks labeled with an asterisk (*) are impurity or background ions.
at least 0.7 eV, to be ionized with 9.68 eV radiation. No signal corresponding to photoionized HI is observed with 248 nm photodissociation of primary iodoalkanes, even with 10.49 eV photoionization, which is above the ground state ionization energy. If the HIf/I+ signal intensity ratio is used to estimate the branching ratio for HI elimination relative to C-I bond cleavage, Table 1 shows that 2-methyl-2-iodopropane has the largest branching ratio for HI elimination. Interestingly, 2-methyl-2iodopropane also has the greatest number of ,&hydrogen atoms. Various hydrocarbon ions are observed in the PDPI spectra of secondary and tertiary iodoalkanes. These ions arise from secondary ionic and/or neutral fragmentation of the primary products. The primary products can either be alkyl radicals produced by C-I bond cleavage or alkenes produced by HI elimination. In general, the secondary and tertiary compounds exhibit larger signal intensities of ions corresponding to the loss of one or two hydrogen atoms from the primary products. For example, the signal intensities of mlz 41 and 42 are much higher for 2-iodobutane than l-iodobutane. 193 nm Photodissociation. Figure 3 shows comparative 193 and 248 nm PDPI spectra for l-iodobutane using 9.68 eV photoionization. Peak areas from the 193 nm spectra of this compound and others are summarized in Table 2. The most notable difference between the spectra in parts a and b of Figure 3 is the decreased signal at mlz 127 using 193 nm radiation. Tables 1 and 2 show that a similar wavelength dependence is observed for all of the iodoalkanes studied. This result may indicate that C-I cleavage is less efficient at 193 nm. However, this conclusion is not supported by other features of the PDPI spectra. For example, an intense peak at mlz 41 is observed in the 193 nm PDPI spectrum of l-iodopropane. This ion corresponds to C3H5+ and shows that a large number of three-
Figure 3. PDPI mass spectra of 1-iodobutane using (A) 193 nm and (B) 248 nm photodissociation with 9.68 eV photoionization. Peaks labeled PI are observed in the 9.68 eV photoionization mass spectrum of l-iodobutane. Peaks labeled with an asterisk (*) are impurity or background ions.
carbon fragments are still formed as primary photodissociation products. In fact, this ion could be formed by secondary loss of two hydrogen atoms from the propyl radical produced by C-I bond cleavage. Similar ( R - 2)+ ions ( R = mass of the alkyl radical) are predominant in the 193 nm PDPI spectra of all compounds except for 2-methyl-24odopropane and neopentyl iodide, where only the ( R - 1)' ion is observed. No PDPI signal corresponding to the intact alkyl radical is detected above the noise in the photoionization signal at the same mlz. Photodissociation with 248 and 193 nm radiation can produce I* and I, but only I* is photoionized with 9.68 eV radiation. Therefore, the lower intensity of mlz 127 in the 193 nm PDPI spectrum of l-iodopropane most likely arises from a lower branching ratio for I* formation. No ion current corresponding to photoionized HI (mlz 128) is observed with 193 nm photodissociation and 9.68 eV photoionization. However, 9.68 eV radiation will only photoionize intemally excited HI. Photoionization with 10.49 eV radiation does produce a weak signal at mlz 128 which corresponds to ground state HI. This ion is observed for all compounds studied and usually has a signal intensity less than 1% of the base PDPI peak. The spectrum in Figure 3a also shows an ion at mlz 155. This ion corresponds to the photoionized iodoethyl radical and indicates that C-C bond cleavage also occurs at 193 nm. Figure 4 shows the PDPI spectra of l-iodopentane, l-iodohexane, and l-iodooctane using 193 nm photodissociation and 9.68 eV photoionization. In these spectra, C-C bond cleavage is indicated by the presence of a series of ions corresponding to photoionized iodoalkyl radicals. Progressively larger radicals are observed as the molecular size increases. l-Iodopentane gives products corresponding to C2 and C3 iodoalkyl radicals, 1-iodohexane gives products corresponding to C2-C4 iodoalkyl radicals, and l-iodooctane gives products corresponding to CzC5 iodoalkyl radicals. Cleavage of the C-C bond is summarized in reaction 5:
Ross and Johnston
4082 J. Phys. Chem., Vol. 99, No. 12, 1995
m/z
20
40
60
80
100
120
140
160
180
200
220
240
m/z
Figure 4. PDPI mass spectra of (A) 1-iodopentane,(B) 1-iodohexane, and ( C ) 1-iodooctane using 193 nm photodissociation and 9.68 eV
photoionization. Peaks labeled PI are observed in the photoionization mass spectrum of the corresponding parent molecule.
where y = 4, 5, ... and the range of x values for a given y are ..., y - 2. Ions corresponding to the alkyl radical products of reaction 5 are also observed. For example, C ~ H S + and C3H7+ are observed in the PDPI spectrum of 1-iodopentane. No peaks are observed which correspond to CH2I' or M - CHi, where M = mass of the parent molecule. This suggests that C-C bond cleavage does not occur at terminal C-C bonds along the hydrocarbon chain. Also unique to the 193 nm PDPI spectra of 1-iodopropane, 2-iodopropane, and 2-iodobutane are peaks at ( M - l)+,where M = parent molecule mass. These peaks appear as a lowintensity shoulder on the parent photoionization peak, and therefore are not included in the tables. They suggest that the parent molecule can also photodissociate by C-H bond cleavage, but the site of hydrogen atom dissociation cannot be determined. It should be noted that C-H bond dissociation has also been observed with 193 nm photodissociation of methyl iodide.28 PhotoionizationEnergy Dependence. Photoionization with 10.21 and 10.49 eV radiation gives relatively little additional information on the photodissociation products. As mentioned previously, photoionization with 10.49 eV radiation does reveal a small amount of ground state HI produced by all compounds with 193 nm photodissociation. Otherwise, photoionization with higher energy radiation yields considerably more ionic fragmentation of the photoionized products than 9.68 eV radiation and is not as useful for product identification. However, the 10.21 and 10.49 eV spectra can be used to distinguish the relative yield of ground and excited state iodine atoms. Excited state iodine atoms are produced by all compounds at both 193 and 248 nm. While only excited state atoms are photoionized
x = 2, 3,
Figure 5. PDPI mass spectra of 1-iodobutaneusing 193 nm photo-
dissociation and (A) 10.21 eV and (B) 10.49 eV photoionization. Peaks labeled PI are observed in the photoionization mass spectra of 1 -iodobutane. with 9.68 and 10.21 eV radiation, both ground and excited state atoms are photoionized with 10.49 eV radiation. Figure 5 shows comparative 193 nm photodissociation spectra of 1-iodobutane taken with 10.21 and 10.49 eV photoionization. The intensity of mlz 127 relative to other PDPI fragments is significantly larger with 10.49 eV photoionization than with 10.21 eV photoionization. In contrast, the mlz 127 signal intensity change is much less when photodissociation is performed with 248 nm radiation. These observations suggest that a greater fraction of the iodine atoms produced by photodissociation with 193 nm radiation are in the ground state than those produced with 248 nm radiation. Quantifying the branching ratio for I* formation at each wavelength is difficult since the photoionization cross sections are not known, the vacuum ultraviolet pulse energies are different at each photoionization energy, and the relative overlap of the photodissociation and photoionization beams may not be the same. However, a qualitative indication of the relative yields of I and I* as a function of molecular structure and photodissociation wavelength can be established by calculating the ratio of the mlz 127 signal intensity to the signal intensity of all other PDPI ions at each photoionization energy. This process is illustrated below with the spectra of 1-iodobutane in Figure 5. In Figure Sa, the ratio of the mlz 127 signal intensity to that of the sum of all other major PDPI ions (mlz 29, 41, 55, 155) is 0.20. This value is related to the number of excited state iodine atoms produced by 193 nm photodissociation. In Figure 5b, the ratio of the mlz 127 signal intensity to that of the sum of all other PDPI ions is 1.0. This value is related to the total number of ground and excited state iodine atoms produced by 193 nm photodissociation. The change in this ratio when going from 10.21 to 10.49 eV photoionization is given by the quotient R193 = 0.2011.0 = 0.20 for 193 nm photodissociation of 1-iodobutane. In a similar way, R248 can be calculated from the 10.21 and 10.49 eV photoionization spectra using 248 nm photodissociation. Rl93 and R248 are related (but not equal) to
Excited State Photochemistry of Iodoalkanes
J. Phys. Chem., Vol. 99, No. 12, 1995 4083
TABLE 3: Propensity for I* Formation with 248 and 193 nm Photodissociation 1-iodopropane 24odopropane
1-iodobutane 2-iodobutane 2-methyl-2-iodopropane 1-iodo-2-methylpropane 1-iodopentane neopentyl iodide 1-iodohexane a
0.34
0.29 0.081
1.1 0.16
0.19 0.12
0.049
0.034 0.22 0.24 0.23 0.27
1.3
1.4
1.4 1.2 5.4
4.5 4.2 5.8 1.3 1.4
6.4 5.8 5.2
20
Note: R(248) and R(193) are defined in the text.
the branching ratio for I* formation at 193 nm as long as the photoionization cross sections of the other PDPI ions do not change appreciably between 10.21 and 10.49 eV.29 This is a reasonable assumption for most of the photodissociation products. Photodissociation yields primarily alkyl and iodoalkyl radicals whose ionization energies are more than 1 eV below that of the photoionization radiation. In these cases, the photoionization cross sections are relatively independent of the intemal energy of the neutral precursor. This is crucial since the intemal energy will change as a function of molecular structure and photodissociation wavelength. The only exception is HI. In this case, 10.21 eV is below the ground state ionization energy, and the photoionization cross section strongly depends upon internal excitation. For this reason, the HI signal intensity is excluded from these calculations. Table 3 gives values for R193 and R248 that have been determined from the PDPI spectra of all iodoalkanes studied. Also given is the quotient R248/R193, which indicates how the I* branching ratio for each compound changes when going from 248 nm to 193 nm. Table 3 shows that R24dR193is greater than 1 for all compounds studied. This suggests that excited state iodine atoms are uniformly produced more efficiently at 248 nm than at 193 nm.
Discussion
C-I Bond Cleavage. Both the A and B band absorptions in small iodoalkanes involve promotion of a nonbonding 5p electron of the iodine atom. Excitation in the A band is known to access a directly dissociative u* orbital. In iodomethane, the A band consists of transitions to three states,13 labeled by Mulliken2* as 3Ql, 3Qo, and 'Q1. At 248 nm, the absorption intensity is dominated (75%) by a parallel t r a n ~ i t i o n ~to * ~the ,'~ 3Qostate, which correlates asymptotically to formation of CH3 and I* (2P~,2).Experiments have shown that C-I dissociation in CH3I takes place on the order of s following excitation to the 3Qo state." Curve crossing to the 'QI state, which correlates with formation of I ('P3/2), can also occur from the 3Q0state. The B band absorption excited at 193 nm is thought to be the first in a series of Rydberg transitions that do not directly involve C-I molecular orbitals. Larger iodoalkanes are thought to undergo similar transitions. This assumption is supported by the measured absorption spectra of a number of such compounds.24 In particular, the location and shape of the B band is similar for 1- and 2-iodopropane, tert-butyl iodide, and 1-iodohexane, each exhibiting a maximum at 200 nm. Photoionized I* is observed at each photodissociation wavelength for every iodoalkane studied. Thus, C-I bond predissociation uniformly plays a significant role in the photodissociation process. However, the propensity for I* formation depends upon molecular structure and photodissociation wavelength. It is most strongly favored for primary iodoalkanes at 248 nm. Under these conditions, no competing photodissocia-
tion channels are detected, and this selectivity persists as the molecular size increases. Secondary and tertiary iodoalkanes exhibit two competing channels at 248 nm, one to form J* and I by C-I bond predissociation and one to eliminate internally excited HI. Primary iodoalkanes exhibit competing photodissociation channels at 193 nm that correspond to C-C bond cleavage at various locations along the hydrocarbon chain. Each of these competing channels coincides with a decrease in the I* signal intensity, suggesting that predissociation becomes less efficient. HI Elimination. Formation of HI by reaction 2 is observed with 248 nm photodissociation of 2-iodopropane, 2-iodobutane, and 2-iodo-2-methylpropane. Several characteristics of this reaction are consistent with a 1,2 elimination. Elimination of HI is enhanced as the number of P-hydrogen atoms increases. The highest photoionized HI signal intensity is observed in 2-iodo-2-methylpropane, which has nine P-hydrogen atoms. Significant HI signal intensities are observed in 2-iodobutane and 2-iodopropane, with five and six P-hydrogen atoms, respectively. All other compounds are primary iodoalkanes which contain two or fewer P-hydrogen atoms and do not yield significant HI signal intensities at 248 nm. Secondary fragmentation is also consistent with a 1,2 elimination. For example, 1-iodobutane yields an intense signal at mlz 29. This ion corresponds to C2H5+, which is a favored secondary product of the n-butyl radical produced by C-I bond cleavage. In contrast, 2-iodobutane yields an intense signal at mlz 41, which is a favored secondary product of the C4H8 alkene produced by HI elimination. The HI signal intensity is strongly dependent on both the photodissociation and photoionization wavelengths chosen for the experiment. It is most intense with 248 nm photodissociation and 9.68 eV photoionization. Since the ground state ionization energy of HI is 10.39 eV, the HI photoproduct must be formed with at least 0.7 eV of internal energy to be photoionized with 9.68 eV radiation. With 193 nm photodissociation, the HI signal intensity is weak and is only observed by photoionization with 10.49 eV radiation. This suggests that the HI produced contains little if any intemal excitation. Thus, both I and HI photoproducts follow a similar trend: formation of the intemally excited species is more strongly favored at 248 nm than at 193 nm. C-C Bond Cleavage. Molecular dissociation by C-C bond cleavage, reaction 5, is observed only at 193 nm. For linear molecules, C-C bond cleavage occurs at all positions along the hydrocarbon chain except at the ends. For example, 1-iodopentane gives products that correspond to dissociation at the C2-C3 and C3-C4 bonds but not at the C I - C ~and c 4 - C ~ bonds. For 1-iodooctane, dissociation is observed as remote from the iodine atom as the c(j-c7 bond. Since C-C bond cleavage occurs throughout the molecule, at least partial energy randomization must occur prior to dissociation. This contrasts with C-I bond cleavage where dissociation occurs through a localized excited electronic state and the excess energy is released preferentially as electronic excitation of the iodine atom. The failure to observe products corresponding to cleavage of a terminal C-C bond in linear iodoalkanes can be rationalized on the basis of relative bond energies. For halocarbons, a(CC) bond energies are expected to be significantly larger than other C-C bond energies as a result of the electron-withdrawing effect of the halogen atom. Unfortunately, calculation of the a(C-C) bond energy in a linear iodoalkane would require an estimate of AWf for the iodomethyl radical, which is not known. However, the a(C-C) bond energies for analogous bromo- and chloroalkanes are approximately 13 Mlmol higher than for
4084 J. Phys. Chem., Vol. 99, No. 12, 1995
intemal C-C bonds.27 Terminal C-C bond cleavage is also thermodynamically hindered in hydrocarbon chains: the -CH2CH3 bond energy is approximately 28 kJ/mol higher than an intemal -CH2-CH2- bond.27 Accordingly, C-C bond cleavage is suppressed when either a methyl or iodomethyl radical is the expected product. This also explains why C-C bond cleavage is not a major photodissociation pathway in molecules such as 1-iodopropane, 2-iodopropane, and 2-methyl-2-iodopropane. The suppression of terminal C-C bond cleavage relative to intemal C-C bond cleavage is also observed with alkanethiols. As with the iodoalkanes, C-C bond cleavage in alkanethiols occurs at each location in the molecule unless a methyl radical is the expected product. Unlike the iodoalkanes, C-C bond cleavage in alkanethiols is observed with both 193 and 248 nm radiation. Although C-C bond cleavage in iodoalkanes is thermodynamically feasible at 248 nm, it also requires energy redistribution following absorption of a photon. Energy redistribution does not occur since the molecule predissociates. At 193 nm, randomization of the intemal energy can compete favorably with direct dissociation of the C-I bond since a different electronic transition is involved. Secondary Reactions. With both 248 and 193 nm photodissociation, C-I bond cleavage leaves a considerable amount of internal energy to be distributed among the products. Taking 248 nm photodissociation of 1-iodopropane as an example, cleavage of the C-I bond to produce ground state iodine atoms and propyl radicals requires 239 kJ/mol, leaving 243 kJ/mol among the products.27 The majority of the iodine atoms produced by C-I bond cleavage at 248 nm are actually in the 2P1/2state, which consumes an additional 0.93 eV (90 kJ/mol). Some energy may be taken up in recoil of the iodine atom following dissociation. In 266 nm photolysis studies of l-iodopropane, Riley and WilsonI6 determined that roughly 50% of the excess energy following C-I bond cleavage goes into recoil of the iodine atom, with the remaining 50% going into intemal excitation of the propyl radical. At 266 nm, this excess energy corresponds to 60 kJ/mol. Assuming that all of the additional energy available from a 248 nm photon relative to a 266 nm photon goes into internal excitation of the propyl radical, the excess energy for 248 nm photodissociation can be up to 93 kJ/mol. For 193 nm photodissociation, the excess energy can be up to 230 kJ/mol. These energies are sufficient to cause secondary fragmentation of the propyl radical. The presence of numerous small mass fragments in iodoalkane PDPI spectra shows that secondary fragmentation does occur with both 193 and 248 nm excitation. If the primary product is an alkyl radical, these secondary reactions generally take the form of H2 loss, C-C bond cleavage, or H atom loss and require approximately 70, 90, and 220 kJ/mol, respect i ~ e l y .Of ~ ~these, only the first two reactions are expected at 248 nm on the basis of the photodissociation energetics discussed previously. All three reactions are possible at 193 nm. Therefore, the observation of secondary fragmentation in the PDPI spectra of iodoalkanes is not at all surprising. At 248 nm, the photoionized alkyl radical is often detected above the photoionization background. At 193 nm, the photoionized alkyl radical can no longer be detected and a secondary product corresponding to loss of H2 becomes dominant. Thus, secondary fragmentation is more extensive at 193 nm than at 248 nm.
Conclusions The photodissociation pathways of simple iodoalkanes depend strongly on excitation wavelength and molecular structure. At 248 nm, the products are determined by excited state photo-
Ross and Johnston chemistry of the C-I bond. For primary iodoalkanes, 248 nm excitation causes exclusive dissociation of the C-I bond to produce an alkyl radical and an iodine atom. The quantum yield for formation of an excited state (2P1,2) iodine atom is close to 1. The selectivity of this reaction persists as the molecular size increases. Although secondary and tertiary iodoalkanes undergo C-I bond cleavage to produce an excited state iodine atom, they also exhibit a competing channel which results in the elimination of hydrogen iodide. The branching ratio for hydrogen iodide formation correlates with the number of ,&hydrogen atoms in the molecule, suggesting that it is produced by a 1,2 elimination. The photoionization energy dependence of the hydrogen iodide product indicates that it is formed with a considerable amount of internal energy, at least 0.7 eV. At 193 nm, C-I bond cleavage is observed, but the quantum yield for formation of an excited state iodine atom is much lower. In all cases, hydrogen iodide is observed as a minor product and the photoionization energy dependence indicates little or no intemal excitation. Molecules larger than iodopropane exhibit nonselective C-C bond cleavage. As the molecular size increases, products are observed which correspond to C-C bond cleavage progressively further away from the C-I bond. However, C-C bond cleavage does not occur at terminal bonds where the expected product is a methyl or iodomethyl radical. In certain cases, direct C-H bond cleavage is observed as well. The 248 and 193 nm results illustrate the competition between selective and nonselective photochemistry in a molecule. At 248 nm, a dissociative state is directly populated. Rapid dissociation of the C-I bond limits the number of possible reaction pathways and causes nonstatistical energy partitioning among the products. This selectivity is maintained as the molecular size increases. In contrast, a bound state is initially populated at 193 nm. At least partial energy randomization can occur before dissociation and a wider variety of reaction pathways are observed.
Acknowledgment. This research was supported by the National Science Foundation under Grant No. CHE9300644. References and Notes (1) (a) Guo, H. Chem. Phys. 1992,96, 2731. (b) Guo, H.; Lao, K. Q.; Schatz, G. C.; Hammerich, A. D. J. Chem. Phys. 1991, 94, 6562. (2) Sundberg, R. L.; Imre, D.; Hale, M. 0.;Kinsey, J. L.; Coalson, R. D. J. Phys. Chem. 1986, 90, 5001. (3) Shapiro, M. J . Phys. Chem. 1986, 90, 3644. (4) Fan, Y . B.; Donaldson, D. J. J. Chem. Phys. 1992, 97, 189. (5) Penn, S. M.; Hayden, C. C.; Carlson-Muyskens, K. J.; Crim, F. F. J. Chem. Phys. 1988, 89, 2909. (6) Black, J. F.; Powis, I. Chem. Phys. 1988, 125, 375. (7) Hess, W. P.; Naaman, R.; Leone, S. R. J. Phys. Chem. 1987, 91, 6085. (8) Hess, W. P.; Kohler, S. J.; Haugen, H. K.; Leone, S. R. J. Chem. Phys. 1986, 84, 2143. (9) Knee, J. L.; Khundar, L. R.; Zewail, A. H. J. Chem. Phys. 1985, 83, 1996. (10) Van Veen, G. N. A.; Baller, T.; De Vries, A. E.; Van Veen, N. J. A. Chem. Phys. 1984, 87, 405. (11) Barry, M. D.; Gony, P. A. Mol. Phys. 1984, 52, 461. (12) Imre, D.; Kinsey, J. L.; Sinha, A,; Krenos, J. J. Phys. Chem. 1984, 88, 3956. (13) Hunter, T. F.; Kristjansson, K. S. Chem. Phys. Lett. 1978,58,291. (14) Comes, F. J.; Pionteck, S. Chem. Phys. Lett. 1978, 58, 616. (15) Comes, F. J.; Pionteck, S. Chem. Phys. Lett. 1976, 42, 558. (16) Riley, S. J.; Wilson, K. R. Discuss. Farad. SOC.1972, 53, 132. (17) Kasper, J. V. V.; Parker, J. H.; Pimentel, G. C. J. Chem. Phys. 1965, 43, 1827. (18) (a) Sammes, P. G . In The Chemiszry of the Carbon Halogen Bond Patai, S., Ed.; John Wiley and Sons: London, 1973; pp 751-753. (b) Gillis, H. A.; Williams, R. R.; Hamill, W. H. J. Am. Chem. SOC. 1971, 83, 17. (19) Brum, J. L.; Deshmukh, S.; Wang, 2.; Koplitz, B. J. Chem. Phys. 1993, 98, 1178.
Excited State Photochemistry of Iodoalkanes (20) Brum, J. L.; Deshmukh, S.; Koplitz, B. J. Phys. Chem. 1991, 95, 8676. (21) Deshmukh, S.; Brum, J. L.; Koplitz, B. Chem. Phys. Lett. 1991, 176, 198. (22) Van Bramer, S. E.; Ross, P. L.; Johnston, M. V. J. Am. SOC.Mass Spectrom. 1993, 4 , 65. (23) Ross, P. L.; Johnston, M. V. J. Phys. Chem. 1993, 97, 10725. (24) (a) Robin, M. B. Higher Excited States ofPolyaromic Molecules; Academic Press: New York, 1974; Vol. 1, pp 155-172. (b) Boschi, R. A.; Salahub, D. R. Mol. Phys. 1972, 24, 289. (c) Lao, K.; Person, M. D.; Chou, T.; Butler, L. J. J. Chem. Phys. 1985 89, 3463. (25) Van Bramer, S. E.; Johnston, M. V. Anal. Chem. 1990, 62, 2639. (26) Van Bramer, S . E.; Johnston, M. V. Appl. Spectrosc. 1992, 46, 255. (27) N. I. S. T. Standard Reference Database 19A, Positive Ion Energetics, Version 2.01, Gaithersburg, MD, 1994.
J. Phys. Chem., Vol. 99, No. 12, I995 4085 (28) Continetti, R. E.; Balko, B. A,; Lee, Y. T. J. Chem. Phys. 1988, 89, 3383. (29) It is also possible that some mlz 127 signal intensity arises from HI photoionization followed by fragmentation to 1.' Formation of It by photoionization with 10.21 or 10.49 eV radiation requires that the HI photoproduct contain over 3 eV internal excitation. This process can be ruled out with 193 nm photodissociation since none of the compounds studied produce internally excited HI (see text). Although unlikely, this process cannot be ruled out in all cases with 248 nm photodissociation since a few compounds do produce internally excited HI. This internal excitation would have to include electronic excitation to the Ill state in order to achieve the energy needed. If this process does occur, then it would cause smaller values of R248 and R248/R193 to be obtained. However, the basic conclusion from Table 3, that 193 nm yields less I* than 248 nm, is unaffected. (30) Mulliken, R. S. J. Chem. Phys. 1940, 8, 382. JF942266W
