IR laser-induced decomposition of dialkoxyalkanes

Volume 86, number 1

CHEMICALPHYSICSLETTERS

5 February 1982

IR LASER-INDUCED DECOMPOSITION OF DIALKOXYALKANES James E. KUDER and Douglas R. HOLCOMB Celanese Research Company, Summit, New Jersey 07901, USA Received 2 October 1981; in final form 13 November 1981

Infrared multiphoton excitation of dimethoxymethane and related compoundsresults in initiaUy very high conversions with subsequent failure of the reaction to go to completion. This behavior is attributed to a free radical chain decomposition with inhibition of further reaction by coUisionaldeactivation or trapping of intermediates by reaction products.

1. Introduction

/

The infrared laser photochemistry of one- or twocarbon halogenated compounds has probably been studied more thoroughly than that of any other class of organic compounds. Much of the motivation for these studies comes from their promise of selective routes for isotope separation [1 ]. In addition, since the single-photon vibrational spectroscopy of haloalkanes and haloalkenes is generally well understood and since they absorb strongly at frequencies corresponding to the output of the CO2 laser these compounds are well suited to the study of the details of infrared multiphoton laser photochemistry. Two pathways typically account for the products formed following multiphoton excitation, simple bond cleavage [2] I I -C-X+nhu~-C. + X. I I

OCH3

/

OCH3

OCH3

01METHOXYMETHANE (METHYLAL) CH~C,/' 3 OCH3

/ \ CH3 0 C H

OCH5

ACROLEINDIMETHYLACETAL

/ OCH2CH3

CH3CH \ 3

2,2-DIMETHOXYPROPANE

OCH2CH 5 1, 1-DI ETHOXYETHANE (ACETAL)

Fig. 1. Structures of compounds discussed in text. replacement of X by OR moieties and might be expected to undergo similar multiphoton-induced decomposition. In this communication, the IR laserinduced reaction of four dialkoxy compounds shown in fig. 1 is described.

and elimination [3] I I -C-Ct I H X

2. Experimental

/

+ nhv-*~C=C

+ HX.

By comparison, the IR laser chemistry of the analogous alkoxy compounds has received relatively little attention, although these too absorb strongly in the region 900-1100 cm -1 . The alkoxy compounds may be regarded formally (and sometimes in practice) as being derived from the halogenated compounds by

The alkoxy compounds were obtained from Aldrich Chemical Company and were distilled immediately prior to use. Sample cells were constructed from 30 mm Pyrex O-ring joints and had a path length of 10 cm. The cells were provided with a Teflon needle valve side arm and a second side arm which terminated in a glass-to-metal transition for connection to the pressure monitoring equipment. KC1 windows were 11

Volume 86, number 1

CHEMICALPHYSICSLETTERS

5 February 1982

used. The laser was a Lumonics 203-2 multigas TEA laser whose output frequency was measured with an Optical Engineering model 16A spectrum analyzer and whose output energy was obtained using a factorycalibrated Lumonics model 20D pyroelectric detector. The course of the reaction was followed by means of IR absorption spectroscopy (accuracy +5%) and by changes in pressure using a Baratron pressure head (accuracy +-1%). In all cases, the pulse rate was sufficiently low that no noticeable temperature rise occurred during the course of an experiment.

Table 1 The effect of laser frequency on the photolysis of dimethoxymethane (5 Tort)

3. Results

a) (Energy through evacuated cell) - (energy through filled cell).

Laser frequency (cm-l )

laser energy (J/pulse) incident absorbed a) % conversion 500 pulses 2000 pulses

1052

937

4.7 1.1

4.3 0.3

65 72

32 42

3.1. Dimethoxymethane The simplest of the dialkoxyalkanes is dimethoxymethane (DMM) whose FTIR absorption spectrum is shown in fig. 2. This compound has two absorption bands corresponding to the output of the CO 2 laser, one at 1054 cm -1 (OCO asymmetric stretch) and one at 936 cm -1 (CH 3 rock) [4]. Initial studies using 5 Torr samples and monitoring the reaction by IR spectroscopy showed that unfocused irradiation in either of these absorption bands results in efficient decomposition of DMM. The major products, identified by their absorption spectra, are methane (3020 and 1306 cm -1) [5] and methyl formate (1754 and 1207 cm -1)

i

I

o.6

i

i

i

CH2 (OCH312

~ 0.4 0.2

1200

~ I

L I

1lO0 lO00 WAVENUMBER

900

Fig. 2. Absorption spectrum of dimethoxymethane in the region 850-1200 cm-1 , sample pressure 5 Tort. Path length 10 cm. 12

[6]. No visible light emission accompanied the irradiation, implying that the products arise from a multiphoton process rather than dielectric breakdown. The effect of laser frequency is shown in table 1. First, upon comParison of the percent conversions after 500 and 2000 pulses at the two frequencies, it is seen that the conversion is about twice as great with 1052 cm -1 excitation than with 937 cm - I excitation, while the incident laser energy per pulse did not greatly differ. It is seen from fig. 2, however, that the ratio of single-photon absorption cross sections at 1052 and 937 cm -1 is 2.2 and that the corresponding ratio of absorbed laser energies (table 1) is 3.7. Thus it may be concluded that the effect of excitation frequency is due to the difference in efficiency of energy absorption rather than mode-specific chemistry. A second point is that the reaction is initially fast and then becomes inefficient. This will be considered further below, but in the present context the result is to blur the difference in reaction efficiency at the two exciting frequencies with long irradiation times. The effect of fluence was examined in a series of experiments in which the beam intensity was attenuated by means of KCI disks placed in the beam path and in which the pressure of DMM initially at 2.0 Torr was monitored. The pressure changes after 5 and 100 pulses are given in table 2. In addition to increasing extent of reaction with increasing absorbed energy, it will be noted that the ratio AP(100 pulses)/ AP(5 pulses) decreases with increasing absorbed energy. In other words, at high fluence the greatest pressure change occurs on the first few pulses, while with

Volume 86, number 1

CHEMICAL PHYSICS LETTERS

Table 2 Change in pressure of DMM (2 Tort) as a function of absorbed energy Absorbed energy (J/pulse) 0.1 0.4 0.5 0.9 1.3 1.4

Pressure change (Tort) 5 pulses

100 pulses

0.00 0.08 0.25 2.18 3.36 4.22

0.01 0.29 0.75 3.18 3.92 4.48

decreasing fluence the response tends to become more linear. The pressure change at 5 pulses shown in table 2 may be used to obtain a threshold value of ~0.4 J/ pulse absorbed energy for the initiation of reaction. With a reaction volume of 70 cm 3 and 1052 cm - 1 radiation, this corresponds to a threshold of 3.9 photons per molecule or an apparent activation energy o f ~-12 kcal/mole. Clearly this is insufficient to initiate bond scission and must therefore represent a mean value for the system. The effect o f added inert gas is seen in comparison of the second and third entries of table 3. When a 0.2 Torr sample of DMM was irradiated in absence and presence of argon, the ratio of the final to initial par. tial pressure was lower in case of the sample containing added inert gas. This effect, which is probably due to collisional deactivation, may be part of the reason why conversion per pulse is initially high and then rapidly decreases, since with increasing decomposition of DMM the sample pressure and thus collisional fre-

5 February 1982

quency increases. It may further be noted that since the reaction products do not absorb at the irradiating frequency, the reaction zone of the sample is cooled more than would be the case with the same pressure of absorbing and therefore activated molecules. It is also evident that if the only products formed were methane and methyl formate, then the final pressure should approach twice the initial pressure, while in fact it is exceeded. As noted in the first column of table 3, when a 20 Torr sample of DMM was irradiated at 1052 cm - 1 , the sample reached an essentially steady-state final pressure of 52.2 Torr after 100 pulses. At this point, the partial pressure of DMM was 9.2 Tort as judged by its IR absorption and, in addition to methane (9.4 Torr) and methyl formate (1.8 Torr), the following compounds not observed previously with lower initial pressure of DMM were recognized by characteristic absorption bands; carbon monoxide (2140 cm -1 ), methanol (1034 c m - 1 ), ethylene (949 cm - 1 ) and acetylene (729 c m - 1 ) . It is possible that some of these products arise from decomposition of methyl formate, but if so this must be as a result of collisional rather than direct excitation since methyl formate is transparent at 1052 cm - 1 , and when a 2 Torr sample of HCO2CH 3 was irradiated at 1052 cm -1 there was no change in IR absorption or in sample pressure.

3.2. Related compounds Three additional dialkoxy compounds were subjected to 1052 cm - 1 laser irradiation. These three (acrolein dimethyl acetal, 1,1-diethoxyethane and 2,2dimethoxypropane) all have absorption bands which

Table 3 Effect of sample pressure on the efficiency of DMM decomposition (final pressure is after 100 pulses with 1052 cm -1 radiation) Initial pressure (Tort)

final pressure (Tort) energy absorbed (J/pulse) a)

Pinitial/Pfinal percent conversion

20.0

0.203

52.2 5.4 2.61 54

0.540 0.2 2.66 53

2.00 (0.20 DMM + 1.80 argon) 2.150 0.2 1.75 b) 33

a) Average of first three pulses. b) After subtracting partial pressure of argon. 13

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i

i

i

CHEMICAL PHYSICS LETTERS i

Ill

i

I

i

i

i

i

i

i

5

4

~3 t~J

1

o o

0

0

13

[3

ACROLEIN DIMETHYLACETAL

~

1,1-DIETHOXYETHANE

~

2 , 2 - DIMETHOXYPROPANE

~'

I

I

I

I

2

tl

6

8

IIA

DIMETHOXYMETHANE

I

I

I

S

I

I0 20 40 60 NUMBER OF PULSES

I

I

80

I

I00

Fig. 3. Changes in sample pressure with irradiation of compounds discussed in text.

overlap the laser output at 1052 cm -1 . The observations are summarized in fig. 3 and table 4, where the observations for dimethoxymethane are shown for comparison. A common feature of the photolysis of the four dimethoxy compounds is the very high conversion (60-80%) on the very first pulse, with very little change thereafter.

4. Discussion

By analogy with the haloalkanes it might have been possible a priori to postulate the formation of HCO2H, CH3CH 3, H2CO, CH3OCH, CH3OH and

5 February 1982

CH3OCH=CHOCH 3 in addition to CH4 and HCO2CH 3 as products arising from CH2(OCH3) 2 by molecular elimination and bond scission-recombination. It seems likely, however, that the products observed result from the initial cleavage of the O - C H 3 bond followed by radical recombination and disproportionation reactions. This is in contrast to the electron impact-induced fragmentation of dimethoxymethane in which C H 3 0 - C H 2 scission is favored [7,8]. It is relevant to compare the results of the laserinduced decomposition of DMM with earlier studies of its thermal decomposition. Molera and co-workers [9,10] found in the range 459-520°C that DMM affords as major products CH4, HCO2CH3, CO and H 2 as well as smaller amounts of CH3OH and CO 2. The kinetic scheme proposed by these workers for DMM pyrolysis will not be reproduced in detail except to say that it involves a long radical chain process with O - C H 3 cleavage as the initiation step and propagation steps involving abstraction of H atoms from the central CH 2 group of DMM by intermediate radicals. It was also found that the decomposition of DMM was inhibited by nitric oxide, ethylene and propane. If the same reaction scheme holds for the laser-induced decomposition of DMM, then the observations of the present study may be explained. Multiphoton excitation of DMM results in its fragmentation with the initiation of a long radical chain process consuming additional DMM molecules. The failure of the reaction to go to completion with extended irradiation times is a result of two factors: collisional deactivation of excited DMM and interruption of the radical chain process by scavengers such as ethylene and acetylene formed as products. Although not examined in

Table 4 Results of irradiating dialkoxy compounds (100 pulses, 1052 cm -1 excitation)

energy absorbed (J/pulse) a) initial pressure (TOE) final pressure (TOE) percent conversion b) products identified b)

a) Initial pulse,

14

Dimethoxymethane

2,2-dimethoxypropane

Acrolein dimethyl acetal

1,1-diethoxyethane

1.4 2.05 6.53 62 CH4, HCO2CH3, CHaOH, CO, C2H4, C2H2

1.0 2.11 3.70 63 CO, CH4, CHaOH

0.9 2.00 5.80 76 CH4, CHaOH, C~H4, C2H2, carbonyl compound

0.7 2.00 5.12 72 CH4, C2H4, C2H2, carbonyl compound

b) From IR absorption spectra.

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CHEMICAL PHYSICS LETTERS

detail, it is evident that the related dialkoxy compounds undergo similar radical decomposition pathways with inhibition of the reaction by products of dissociation. The failure to observe CO 2 in the present study and the presence of C2H 4 and C2H 2 (suggestive of carbene intermediates) indicate that additional reaction manifolds are possible with laser excitation of DMM and the other dialkoxyalkanes.

References [1] J.B. Marling and I.P. Herman, Appl. Phys. Letters 34 (1979) 439. [2] D.F. Dever and E. Grunwald, J. Am. Chem. Soc. 98 (1976) 5055; E. Grunwald, K.J. Olszyna, D.F. Dever and B. Knisbkowy, J. Am. Chem. Soe. 99 (1977) 6515;

5 February 1982

G.A. Hill, E. Grunwald and P. Keehn, J. Am. Chem. Soc. 99 (1977) 6521. [3] A. Gandini, Can. J. Chem. 55 (1977) 4045; F.M. Lussier and J.I. Steinfield, Chem. Phys. Letters 50 (1977) 175; W.C. Danen, W.D. Munslow and D.W. Setzer, J. Am. Chem. Soc. 99 (1977) 6961. [4] K. Nukada, Speetrochim. Acta 18 (1962) 745. [5 ] G. Herzberg, Infrared and Raman spectra of polyatomic molecules (Van Nostrand, Princeton, 1945). [6] J.S. Byme, P.F. Jackson, K.J. Morgan and N. Unwin, J. Chem. Soc. Perkin Trans. II (1973) 845. [7] M.J. Rix, A.J.C. Wakefield and B.R. Webster, Chem. Commun. (1966) 748. [8] K. Hirota and J. Takezaki, Bull. Chem. Soc. Japan 41 (1968) 76. [9] M.J. Molera, J. Femandez-Biarge, J. Centeno and L. Arevalo, J. Chem. Soc. (1963) 2311. [10] M.J. Molera, J. Centeno, L. Arevalo and J. Orza, J. Chem Soc. (1963) 5009.

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