Kinetics and Mechanism of Transformations of Aliphatic Alcohol Molecules Under Multiphoton IR Excitation

The main channels for transformations of C2H5OH, 
 iso-CH3H7OH and n-C3H7OH molecules 
 under IR multiphoton excitation are the monomolecular reactions of dehydration (1), and Cα−Cβ 
 bond breaks (2). The ratio of the yields of reactions (1) and (2), γ, is determined by the molecules' structure, radiation energy density, the pressure of alcohol vapors and inert gas. The direction of the alcohol molecules' transformations under IR MPE, under thermal equilibrium conditions, differs from pyrolysis, when the chain processes of C2H5OH and C3H7OH molecules dehydrogenation, and CH4 splitting-out from n-C3H7OH molecules are developing. Reactions of H and CH3 radicals with alcohol molecules under laser initiation do not lead to the development of chains due to the high rate of H atoms diffusion from reaction zone and effective 
recombination of CH3 radicals.


INTRODUCTION
Under pulsed laser IR irradiation of organic molecules a reaction can proceed simultaneously by several channels characterized by different energy barriers. Two main cases may be distinguished" 1. The reaction runs by molecular elimination mechanism.
2. The reaction results from the chemical bond's dissociation and consequent transformations of radicals.
The ratio of the yields of these processes depends on several factors including the height of energy barriers, peculiarities of molecule structure, as well as radiation energy density and gas pressure. The first path is typical of laser chemistry, as will be shown below; the second is similar to the chain processes beginning. Of special interest to laser chemistry is studying the problem of the relation of the two paths, which is directly related with the problem of selectivity. We have studied this problem for some aliphatic alcohols, RIRzCHOH, differing by the carbon chain length and structure, under varying pressure and radiation energy density. The RIRzCHOH molecules can undergo the monomolecular reaction of dehydration RaRzCHOH H20 + CH2 CH2 (1) as well as C-C bond break RaR2CHOH----> R + RzCHOH (2) The latter is capable of initiating the chain processes 1-3 by pattern 1. The direction of transformations is determined by the ratio of the rates of primary reactions (1) and (2), and by the probability of chain development.
The major processes involved in pyrolysis of these alcohols (800-900 K) are chain dehydrogenation of CzHsOH and iso-C3H7OH, and detachment of CH4 with n-C3H7OH. Pulsed laser induced reactions go under specific conditions of collisionless processes with a localized reaction zone. These conditions hinder chain propagation and there-fore, such transformations can be expected to follow mechanisms other than pyrolysis. Our purpose is to investigate general reaction patterns for compounds of a given class and determine the dependence of reaction directions on molecular structures. EXPERIMENTAL A pulsed transverse-discharge CO2 laser (Z'pulse 180 ns, Epulse 0.06 J) operating at the 9R(16) 1050 cm -1 line, the resonance frequency of C-O stretching vibrations, was employed. Laser energy density was varied using BaF2 lenses with focal distances of 5.0, 12.5 and 25.0 cm. Alcohol vapours were irradiated in a quartz cell with BaF2 windows (1 5.0, d 1.5 cm). In some experiments they were diluted with an inert gas such as helium or nitrogen. The role played by radicals in the formation of reaction products was studied using mixtures of alcohol vapours with 02 as radical scavenger. Low oxygen partial pressures (<100 Pa) were used to minimize oxygen effects on excitation of alcohol molecules during a pulse. Reaction products were analyzed after irradiation by gas chromatographing with Polysorb-1 or 13 X molecular sieves as adsorbents. The compounds detected were: C2H4, CH3CHO, C2H6, and CH4 for C2HsOH; C3H6, C2H4, C2H6, CH4, C3H8, C4H10, CH3CHO and CH3CHzCHO for n-C3HyOH; C3H6, C2H6, CH4, CH3CHO, CH3COCH3, C2H4, and C4H6 for iso-C3H7OH.

RESULTS AND DISCUSSION
The dependences of reaction product yields q(X) on alcohol vapour pressure P are shown in Figure 1. The yields q(X) are given per unit vapour pressure and one pulse. The transition from collisionless conditions to collision ones occurs at 30-50 Pa. Independence of q(X) from P and the absence of products formed in reactions between molecules and radicals (3) (e.g. CH4 with C2HOH) have been used as criteria of collisionless conditions. The absence of products from reactions between radicals and molecules is indicative of only insignificant heating of the reaction zone after a pulse, for the activation energy of such reactions is equal to 40 kJ/mol. The additon of 02 to alcohol 15 I0.  vapours results in a decrease of q(CH4) and q(C2H6) for C2H5OH; q(C2H6) for iso-C3HyOH; and q(C2H6), q(C3H8) and q(C4Hlo) for n-C3HvOH (typical results are represented in Figure 2). This is indicative of the participation of radicals in the formation of these products.  Table I. Consideration of product compositions and 02 effects on product yields shows that collisionless reactions of alcohol n.-CH,OH --"* C, H5 + CHzOH The occurrence of reactions (21) and (22) follows from the product ratio b(CH3CHO) > qS(CH4) observed for n-C3HTOH and iso-C3HTOH under IR MPE. Radicals Calls, CHaCHaOH, and CH3CHOH obtain excitation energy required for reactions (20) to (22) either in the dissociation of excited alcohol molecules or as a result of IR MPE of radicals formed during a laser pulse (the rocking vibration frequency of the CH3 group in radical Call5 and the stretching vibration frequency of the C-O bond in radicals CHaCHaOH and CH3CHOH are in resonance with the 1050 cmlaser radiation frequency). Radicals CH3 and Call5 formed in the dissociation of C-C bonds (reactions (16) to (19)) participate in recombination reactions which occur after laser pulses. This is substantiated by a decrease in q(C2H6), qS(C3H8) and q(C4Hlo) at oxygen pressures below 100 Pa, for the time between molecule collisions is then greater than the pulse time" CH3 + CH3 + M (or wall) C2H6 + M (or wall) CH3 + C2Hs + M (or wall) -----> C3Hs + M (or wall) The yield from CC# bond dissociation reactions, q(C-C#), is equal to twice the q(C2H6) value for C2HsOH and iso-C3H7OH; and for n-C3H7OH, q(Ca-C#) q(C2H4) + q(C3H8) + 2q(C4Hlo) q is the elimination or C-C bond dissociation reaction to the total product yield ratio.
and q(Ctr-C,) 2tp(C2H6) + qS(C3H8). Table II contains relative contributions from various IR MPE-induced reactions to the total decomposition yield. One can see that the process is dominated by dehydration reaction (1) and Ca-C/ bond dissociation reaction (2). Under IR MPE conditions, the major decomposition path is dehydration for C2HsOH and iso-C3H7OH and Ca-Ct bond dissociation for n-C3HTOH.
The rate constants of IR MPE-induced reactions are listed in Table  III. These are calculated from reaction product yield dependences on PHe,rq2 (typical results are shown in Figure 3) using the equation 4 k ZoP1/2 (25) where k is the reaction rate constant, Zo the specific frequency of deactivating collisions, P1/2 the inert gas (He or N2) pressure corresponding to a twofold decrease in the reaction product yield.
One can see from Table III that for C2HsOH and iso-C3HTOH, the dehydration rate constant exceeds the rate constant of C-C bond dissociation, while the latter reaction is faster than the former one in n-C3H7OH. The pre-exponential factors and activation energies for dehydration (1) and C-C bond dissociation (2)   ASsym.n-C3HTOH Rln2, and ASsym.iso-c3HTOH Rln6). Reaction (1) activation energies for n-and iso-C3H7OH were also estimated from the E value for CzHsOH, with taking into consideration of the observation that the activation energy for molecular elimination of HX is practically independent of the chain length in n-alkyl radicals and increases by ca. 15 kJ/mol upon a-methylation of carbon. 6'1 An analysis of Table IV leads one to conclude that the difference of threshold Ca-C/ bond dissociation and dehydration energies is far larger for C2H.sOH and iso-C3HTOH than it is for rI-C3H7OH. As a result dehydration should play a far more important role in transformations of C2H.sOH and iso-C3H7OH than with n-C3H7OH, for the A I/A2 ratio is practically the same for all the molecules. This conclusion is in agreement with experimental results (Tables II and III). It should be noted that although the activation energy of Czr-C , bond cleavage in n-C3H7OH is fairly high (E%,_% > Eco_%) this reaction makes a considerable contribution to IR MPE-induced decomposition of n-C3H7OH, probably because of a larger pre-exponential factor value, Acr.c/Ac,_c ]0'66. The structure of alcohols is thus in large part responsible for their transformations.
The dehydration to Ca-C bond dissociation reaction yield ratio, V, depends on the density of absorbed radiation energy. Table V contains values observed at various energy densities. When the focal distance is varied a decrease in energy density enhances low-energy dehydration reaction and conversely, increasing energy density favours the C,-C bond dissociation reaction. Absorbed energy density can also be varied by varying the laser radiation frequency if it affects radiation absorption cross section. According to our data 12 decreasing the cross section of absorption by C3HsOH on passing from Vl 1050 to v2 3650 cm -1 causes a change in the direction of molecular trans- formations" at 1050 cm -, decomposition of C3HsOH proceeds via two channels, that is dehydration and C-C bond cleavage, whereas at 3650 cm -1 dehydration is only observed. This is a consequence of different competition conditions for the low-and high-energy channels at different radiation energy densities. In fact, excitation of molecules above the dehydration threshold, E, to levels that should be reached for C-C bond dissociation to occur, E2, is more probable at higher radiation energy densities, that is at the radiation frequency corresponding to the higher absorption cross section value.
A comparison of data on alcohol vapour pyrolysis and IR laser photolysis under collisionless conditions shows that the two processes follow essentially different directions: while pyrolysis mainly goes as chain reactions of dehydrogenation of C2HsOH and iso-C3H7OH and the elimination of CH4 from n-C3H7OH, the principal paths of alcohol decomposition in IR photolysis are monomolecular reactions of dehydration and C-C bond dissociation whose relative importance depends on the alcohol structure and absorbed radiation energy density.

Collisional conditions
Gas heating caused by collisions results in an increase in the fraction of hot molecules compared with collisionless processes discussed above. This favours high-energy C-C bond dissociation reactions (Table   VI). Heating in collisions is sufficient for the occurence of reactions between radicals and molecules with activation energies of about 40 kJ/mol and radical dissociation reactions with activation energies of 90 / 120 kJ/mol (Table VII). The attainment of a constant dehydration to C--C# bond dissociation product yield ratio with increasing alcohol pressure is indicative of the occurrence of reactions under thermal equilibrium conditions at the same temperature (Table VI).
The effective temperature T. in the reaction zone 14   From Pn.C3HTOH 250 Pa and at higher pressures, the product yield from reactions involving C,-C bond dissociation is practically equal to (C:zH4) under collisional conditions (the yield of radical C2H5 recombination products is smaller than 10% of (C2H4), and the contribution of radicals C2H5 to the formation of C2H6 (reaction (3)) is also only insignificant compared with decomposition reaction (20): the rate of the decomposition of radicals C2H5 is an order of magnitude higher than the raction (3) rate even at 700 K (k3 1011 exp(-40000/RT)13)).
The yield of Cr-C, bond dissociation products in n-CaHTOH under collisional conditions is equal to the sum q(CH4) / 2b(CEH6). As radicals C2H5 are mainly consumed in reaction (20) rather than (3) the major contribution to the formation of C2H6 comes from reaction (7).  Dimolecular reaction rate constants are given in cm3mol is-1, and monomolecular ones in s-1.
The major processes under thermal equilibrium conditions are the same as in the absence of collisions, that is molecular dehydration and Ca-C bond dissociation reactions. Reactions of radicals with molecules which occur in collisional IR MPE of vapours do not initiate chain processes. In fact, the principal pyrolysis products formed in reactions (4) and (5), that is CH3CHO for CzHsOH, CH3COCH3 for iso-C3HyOH, and CH4 for n-C3H7OH, are only obtained in quantities below 15% of the total product yield. Under collisional IR MPE conditions, the ratio of products from reactions (4) and (5) which may be chain propagation steps to those from reaction (2) capable of chain initiation, that is q(CH3CHO)/q(Ca-C) for CzHsOH, q(CH3-CHO)Ap:(C-C) for n-C3HyOH, and p(CH3COCH3)hP(CCt) for iso-C3HyOH, is below unity. This evidences an only ineffective consumption of radicals H or CH3 in reactions between radicals and molecules.
The chain propagation mechanisms is the same for all alcohols: one step is the dissociation of an alcohol radical with an about 100 kJ/mol activation energy (reactions (4a) and (4b)), and the other one is radical recovery in the interaction of active centres H or CH3 with initial alcohol molecules (reactions (5a) and (Sb)). Hydrogen atoms can participate in reaction (5a) or escape from the reaction zone by diffusion. Table VIII  where a is the degree of conversion of RIR2CHOH in reaction (1), Vc the cell volume, v the mean thermal velocity of R1R2CHOH molecules, and ores the gas kinetic cross section of R1RECHOH. Table VIII shows that the rate of hydrogen atom diffusion from the reaction zone is comparable with or exceeds the reaction (5a) rate.
Diffusion of H atoms from the hot zone to the cold one results in chain termination because the dissociation of RIR2CHOH radicals (reaction (4a)) is hindered in that zone its activation energy being fairly high, of about 100 kJ/mol. The participation of CH3 radicals in chain propagation reaction (29) involving n-CaH7OH is reduced because of effective recombination of these radicals (reaction (7)). In fact depending on alcohol vapour pressure, IR MPE of n-C3H7OH is characterized by q(CEH6) values higher than or comparable with q(CH4) (Figure 1). The high recombination probability for radicals CH3 is the result of the high concentration of these particles. The quadratic law for CH3 recombination follows from a linear dependence of/3 on the initial n-CaH7OH concentration ( Figure 4) where fl is the ratio of CH4 formation rate to the square root of C2H6 formation rate. The CH4 and C2H6 formation rates are calculated by dividing the concentrations of these particles in the reaction zone by the reaction time taken to be equal to the time of tion is only a weak one over a narrow pressure range 15 and therefore, it can be ignored. The straight line shown in Figure 4 has a slope of 3 of H atoms from the reaction zone and recombination of CH3 radicals whose high concentration makes the latter process capable of competing with chain propagation reactions.

CONCLUSIONS
Monomolecular dehydration and Ca-C bond dissociation reactions are the major processes initiated by IR MPE of C2H5OH, n-C3H7OH, and iso-C3H7OH under both collisionless and collisional conditions.
Conversely, pyrolysis of the alcohols is dominated by chain reactions of dehydrogenaton of C2H5OH and iso-C3H7OH and splitting off of cn4 from n-C3HTOH. The dehydration to CCtbond dissociation rate ratio is determined by the structure of alcohol molecules and either absorbed radiation energy density under collisionless conditions or effective reaction zone temperature under thermal equilibrium conditions. Absorbed radiation energy.densities and temperatures being the same the contribution from the low-energy dehydration reaction to the total yield of alcohol conversion products is far higher for C2H5OH and iso-C3H7OH than for n-C3H7OH. This is explained by a larger difference of threshold energies for the bond cleavage and dehydration reactions characteristic of the first two alcohols. Increasing radiation energy density and temperature enhances C-C bond dissociation reactions, while decreasing them favours the dehydration.