4505
J . Phys. Chem. 1992, 96, 4505-4515 is most likely derived from the r-electron ring of the Pc. If the involvement of triplets is excluded on the basis of the very rapid decay, we believe that a two-photon excited singlet state is the most likely candidate for the subgap state. Further theoretical and experimental work is clearly required before an unambiguous assignment becomes possible.
Conclusions In conclusion, we have time- and wavelength-resolved the subpicosecond and few-picosecond relaxation mechanisms in FAlPc and ClAlPc thin films using transient pumpprobe spectroscopy. The exciton population created by 100-fs optical pulses rapidly relaxes to the bottom of the inhomogeneously broadened Q-band, bleaching the lower extrema r-r* transitions and, presumably, their associated vibronic transitions. In addition, induced absorption develops on the high-energy side of the Q-band as the absorption bleaching signal diminishes, indicating that some excitons relax into a subgap state. Different decay dynamics are observed for the absorption bleaching and induced absorption signals, providing additional evidence for the existence of a subgap state. Both signals exhibit rapid bimolecular decay at early times, indicating that, in agreement with previously published results, bimolecular processes are prominent decay mechanisms in these cyclic-molecule thin films. Lastly, the picosecondduration decay of the indicated subgap state suggests that the subgap state may possibly be identified as a two-photon singlet state, as opposed to a triplet state.
-
Recent studies on conjugated polymers have employed optical detection of magnetic resonance (ODMR)27and novel spin-dependent photomodulation (SDPM)l6 techniques to determine the spin of subgap excited states reached during the decay of the optical state. Similar ODMR and SDPM studies on Pc thin films could certainly help to provide an unambiguous assignment of the indicated subgap state as well as valuable information on the excited states that are most relevant in nonlinear optical processes. Interpretation of the experimental results obtained for these sublimed Pc thin films are complicated by their inhomogeneous nature. Thus, epitaxial Pc thin filmsu are the subject of our present optical characterization and theoretical ~tudies.4~ Acknowledgment. Support from the National Science Foundation (Grants ECS8911960 and CHE8921791) and the Optical Circuitry Cooperative of the University of Arizona is greatly appreciated. We are grateful to C. Arbour and J. P. Dodolet for preparation of the samples. V. S.Williams gratefully acknowledges the financial support of an IBM Fellowship during the term of this research. (44) Nebesny, K. W.; Collins, G. E.; Lee, P. A.; Chau, L.-K.; Danziger, J.; Osborn, E.; Armstrong, N. R. Chem. Mater. 1991, 3, 829. (45) Williams, V. S.;Sandalphon, Armstrong, N. R.; Mazumdar, S.; Peyghambarian, N. To be presented at the XVIII International Quantum Electronic Conference, Vienna, Austria, June 14-19, 1992. (46) Huang, T.-H.;Rieckhoff, K. E.; Voigt, E. M. J. Phys. Chem. 1981, 85, 3322.
Thermal Decompositlon of Isoxazole. Experimental and Modeling Study Assa Lifshitz* and Dror Wohlfeilert Department of Physical Chemistry, The Hebrew University, Jerusalem 91 904, Israel (Received: November 22, 1991; In Final Form: February 7, 1992)
The thermal decomposition of isoxazole was studied behind reflected shocks in a pressurized-driver single-pulse shock tube over the temperature range 850-1 100 K and overall densities of -3 X mol/cm3. Acetonitrile and carbon monoxide are the major decomposition products, followed by hydrogen cyanide, acrylonitrile, propionitrile, and acetylene. Methane, ethylene, and ethane are produced in smaller quantities. There is no effect of large quantities of toluene ([toluene]/[isoxazole] 10) on the rate of formation of acetonitrile and carbon monoxide, indicating that the latter are formed in a unimolecular process. It is suggested that this reaction channel in the decomposition of isoxazole involves a simultaneous N-O bond cleavage in the 1-2 position, a hydrogen atom shift from position 5 to 4, and a rupture of the C(4)-C(5) bond with the removal of carbon monoxide from the ring: ( 1 ) C3H30N CH3CN + CO. This process requires a very small N-O bond stretch which results in a very stiff transition structure. The rate constant for this reaction is kl = exp(-44 X 103/RT)s-I. Since several products are formed by free-radical reactions, it is suggested that the initiation of free radicals involves the same N-O bond cleavage as in reaction 1 but without hydrogen atom shift: (2) C3H30N CH2CN' HCO'. This is an endothermic reaction which proceeds at a much lower rate that reaction 1 .
-
-
Introduction Isoxazole is a five-membered ring compound containing, adjacent to one another in the ring, one nitrogen and one oxygen atom. It is isoelectronic with furan and pyrrole. Owing to the weak N - O bond in the molecule it is expected to be kinetically much less stable than both furan and pyrrole and thus decompose at much lower temperatures. Moreover, in view of the decreased symmetry in isoxazole compared to these two molecules, reaction channels which are equivalent in furan and pyrrole will lead to the production of different products in isoxazole. Whereas the decomposition of both furan and pyrrole has been thoroughly In partial fulfillment of the requirements for a Ph.D. Thesis to be submitted to the Senate of the Hebrew University by D.W.
-
+
investigated in the past,'-3 we are not aware of any study of isoxazole decomposition. As will be demonstrated later, some products which are obtained in the decomposition are due to unimolecular cleavage of the isoxazole ring. Some, such as methane and ethane, are produced by freeradical reactions. It is thus the purpose of this investigation to examine these reactions and compare the decomposition mechanism to those which determine the decomposition pattern in furan and pyrrole. (1) Lifshitz, A.; Bidani, M.; Bidani, S.J. Phys. G e m . 1986, 90, 5373.
(2) Lifshitz, A.; Tamburu, C.; Suslensky, A. J. Phys. Chem. 1989, 93, 5802. (3) Mackie, J. C.; Colket 111, M. B.; Nelson, P. F.; Esler, M. Inr. J . Chem. Kinef. 1991, 23, 733.
0 1992 American Chemical Society
4506 The Journal of Physical Chemistry, Vol. 96, No. 1 1 , 1992 TABLE I: Product Distribution in Percent for Representative Runs T5,K C5,mol/cm3 t , ms ISOX CH$N CO 2.19 96.48 1.48 868 2.83 X lo-' 1.55 2.12 90.49 1.27 5.40 891 2.98 X lo-' 2.17 85.84 918 2.63 X 4.58 8.19 79.11 2.16 8.90 9.13 932 3.28 X 2.10 74.74 946 3.77 X 11.43 9.92 2.10 37.38 23.44 991 3.58 X lo-' 30.19 2.08 23.52 29.31 1008 3.31 X 32.36 2.10 12.01 1031 3.18 X 49.49 23.53 1056 2.23 X lo-* 2.28 0.858 18.90 55.33 2.39 0.167 18.61 1080 2.50 X 53.60
Lifshitz and Wohlfeiler
HCN
C2H2
CH4
C2H4
C2H6
C2H3CN
0.321 0.401 0.989 2.02 2.42 5.29 8.27 7.13 7.04 14.24
0.136 0.079 0.154 0.290 0.400 0.932 0.392 1.815 2.685 2.620
0.010 0.027 0.041 0.072 0.085 0.266 0.449 0.687 1.565 2.097
0.002 0.006 0.034 0.073 0.066 0.233 0.386 0.564 1.441 2.081
0.001 0.008 0.022 0.013 0.046 0.053 0.072 0.129 0.098
0.008 0.037 0.148 0.290 0.525 2.142 2.503 2.537 1.210 3.518
-1
100
C2H5CN
0.381 1.72 2.09 0.58 2.08
.%
NPD
d
T = 1020 K
w 0 i-
#
lo/
a k
t
FI D
1
0.1 0.1
30
Figure 1. A gas chromatogram of a postshock mixture of 0.6% isoxazole in argon heated to 1020 K. The chromatogram is obtained on dual 2-m Porapak N columns using FID and NPD. Carbon monoxide which is analyzed separately is not shown in this figure. (The numbers on the peaks indicate relative attenuation factors.)
10
100
[CARBON]/S
I
IO 20 RETENTION TIME , ( m i n )
0
1
Figure 3. Nitrogen-carbon mass balance: a plot of the concentration of all the nitrogen containing species vs one-third the concentrations of all the products, each multiplied by the number of its carbon atoms. The 45O line drawn in the figure corresponds to a perfect mass balance. Within the limit of experimental scatter a nitrogenarbon atom balance is maintained.
.-cC. a
c .-
0
0 0
0.05 900
001 1
850
" 'I
900
'
I
I
950
1000 T,
I
I
1050
1100
950
1000
1050
Figure 4. Logarithm of the ratio of the GC peak areas of acetonitrile to that of isoxazole is plotted against reciprocal temperature for experiments with and without toluene. No difference is seen between the two series of experiments, suggesting a unimolecular formation of acetonitrile.
K
Figure 2. Distribution of reaction products as obtained in the postshock analyses for a reaction time of 2 ms. Acetonitrile and carbon monoxide are the major products.
Experimental Section Apporohr9. The decomposition of isoxazole was studied behind reflected shock waves in a pressurized-driver single-pulse shock tube made of 52 mm i.d. electropolished stainless steel tubing. Its driven section was 4 m long. Its driver, which had a maximum length of 2.7 m, could be varied in small steps in order to tune for the best cooling conditions. A 36-Ldump tank was connected to the driven section at 45O angle toward the driver, near the
diaphragm holder, in order to prevent reflection of transmitted waves. The driven section was separated from the driver by Mylar polyester film of thickness ranging from 0.75/1000 to 1/1OOO in. depending upon the desired shock strength. Prior to performing the experiments the tube was pumped down to approximately 2 X Torr and was then filled with 80-120 Torr of the reaction mixture. After the shock was fired, gas samples were withdrawn from the end block of the driven section and were analyzed by gas chromatography using a flame ionization and nitrogen phosphor detectors. A more detailed description of the single-pulse shock tube and its mode of operation have been described in detail in previous p~blications.~
The Journal of Physical Chemistry, Vol. 96, No. 11. 1992 4501
Thermal Decomposition of Isoxazole
1 .-E
without toluene
p
1
3'0
1000
C
2.5
a
FT---l -
c
fl
-
100
2.0 -
X
0
.-
v)
I
:
5 z.
10
1.5
-
" " " " " " " " " " " '
1.0
T, K Figure 5. Logarithm of the ratio of the GC peak areas of hydrogen cyanide to that of isoxazole is plotted against reciprocal temperature for experiments with and without toluene. The moderate effect of toluene suggests some contribution of a free-radical mechanism to the formation of HCN.
1000
1/T E3 (K.')
Figure 7. A plot of log ([CH3CN],/[isoxazole],J/r against reciprocal temperature. The straight line combining the points at the low temperature range gives the Arrhenius rate parameters for the unimolecular formation of acetonitrile which is the major reaction is isoxazole. E = 44 kcal/mol, A = 7.24 X 10" s-l.
1 2.0 -
1.5 -
1.0
900
950
1000
-
1050
T, K Figure 6. Logarithm of the ratio of the GC peak area of acetylene to that of isoxazole is plotted against reciprocal temperature for experiments with and without toluene. The relatively large suppression (factor of 3) of the rate by toluene suggests a large contribution of a free-radical mechanism to the formation of C2H2.
Reflected Shock Temperaturesand Densities. Reflected shock temperatures were calculated from the extent of conversion of Zchloropropane to propylene and hydrochloric acid. This internal standard served as a chemical thermometer in the present investigation. The amount of propylene formed in the decomposition of isoxazole under the condition of the experiment is completely negligible compared to the amount formed in the unimolecular decomposition of 2-chloropropane under the same conditions. The decomposition of 2-chloropropane is a simple unimolecular reaction with a preexponential factor A = 1013.8 s-', and an activation energy of E = 50.87 kcal/m~l.~The reflected shock temperatures were calculated using the following equation
where f is the reaction dwell time and x is the extent of decomposition defined as
x
=
[c3H61i/([c3H61t
+ [C~H~C~II)
(11)
and E and A are the activation energy and the preexponential factor of 2-chloropropane decomposition, respectively. (4) Tsang, W.; Lifshitz, A. Annu. Rev. Phys. Chem. 1990, 41, 559. ( 5 ) Tsang, W. In Shock Waues in Chemistry; Lifshitz, A., Ed.; Marcel Dekker, Inc.: New York, 1981.
Reflected shock densities were calculated from the measured incident shock velocities using the three conservation equations and the ideal gas equation of state. The latter were measured with two high-frequency pressure transducers placed 300 mm apart near the end plate of the driven section. A third transducer p l a d at the center of the end plate provided measurements of the reaction dwell times (approximately 2 ms) with an accuracy of approximately 5%. Cooling rates were approximately 5 X lo5 K/s. Materials and Analysis. Reaction mixtures containing approximately 0.6% isoxazole and 0.12% 2-chloropropane were prepared by injecting 178 p L of isoxazole and 35 p L of 2chloropropane into 12-L glass bulbs which were then filled with argon to 1 atm. Both the bulbs and the vacuum line were pumped down to better than Torr before the preparation of the mixtures. Isoxazole, listed as 99% pure, and 2-chloropropane, listed as better than 99% pure, were obtained from Aldrich Chemical Co. Inc. None of the reaction products were found in the original material. Argon was Matheson ultra-high-purity grade, listed as 99.9995%, and helium was Matheson pure grade, listed as 99.999%. All materials were used without further purification. Two series of gas analyses were performed on each postshock mixture. In the first series shocked samples were injected into the gas chromatograph (HP Model 5890) and were then equally
Lifshitz and Wohlfeiler
4508 The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 1.51
mu
\
TABLE II: First-Order Arrhenius Parameters for Product Formation E, E, compd log A, s-I kcal/mol compd log A, s-I kcal/mol isoxazole" 13.95 52 C2H2 12.79 54
t -0.5 0.90
0.95
1.00
1.05
1.10
'
CH$N HCN C2H3CN
co
C2H4 C2H6 CH4
16.18 12.35 12.66
72 58 56
First-order rate constant for the overall decomposition of isoxazole. 3.5
3.0
-'c
1.15
1 /T E3 (K.')
2.5
P
Y
Figure 9. A plot of log ([CzHz],/[isoxazole]o)/t against reciprocal temperature. Acetylene is formed by a freeradical process (see text). The Arrhenius parameters do not correspond to a unimolecular process.
1 .o '5
44 52 52 43
11.86 13.17 10.40 11.89
I 1 .o
. \ I
0.85
0.95
1.05
1.15
1 /T E3 (K.')
Figure 11. First-order rate constant for the overall decomposition of isoxazole, calculated from the relation ktOUl= -In { [isoxazole],/[isoxazole],)/t. Its value is ktOUI= iOi3.9s exp(-52 x 1 0 3 / ~ s-I. n ( H+ CH :N.)
CO -+85.16
r
1
-2.0~~""""""""'""'" 0.90
0.95
1.00
1.05
1.10
1.15
( CH,CN
+ CO )*
l / T E3, K.'
I
Figure IO. A plot of log ~[CzH,],/[isoxazole]o~/t against reciprocal temperature. Ethylene is not a major product in the pyrolysis.
in. X 2 m Porapak N columns. One divided between two column, connected to a flame ionization detector (FID), separated and quantitativelydetermined the reaction products without bound nitrogen. The second column, connected to a nitrogen phosphor detector, separated and quantitatively determined all the nitrogen-containing species. The initial columns' temperature of 35 OC was gradually elevated to 190 OC in an analysis which lasted about 35 min. In order to combine the GC peak areas in the two columns to one list, the area under the isoxazole peak in the FID column (Akm) was set equal to that in the NPD column (AimsNpD)and all the FID peaks were multiplied by the ratio (Av)/(Akm). In this way the sensitivities of all the products relative to isoxazole which were determined separately from standard mixtures in each column could be used without normalization. The purpose of the second series of analyses was to quantitatively determine the amounts of carbon monoxide in the postshock mixtures. This was performed on a Porapak N column connected to a FID. The carbon monoxide in the samples was reduced at 400 OC to methane prior to its detection using a Chrompak methanyzer with a carrier composed of 50% hydrogen and 50% argon. This analysis gave the ratio [C2H2] / [CO] . From this ratio and the known acetylene concentration obtained previously, the umcentration of CO could be calculated. The ratio [C,H,]/[CO] in a standard mixture of acetylene and carbon monoxide was determined periodically for calibration. A typical chromatogram of a shocked mixture of 0.6% isoxazole in argon heated to 1020 K is shown in Figure 1 for both detectors. The peaks drawn in broken lines are those of the internal standard and its decompo-
CH3CN+ CO
Figure 12. Energy diagram for the species in the decomposition of isoxazole. The reaction (CHpCN + CO)*= H' CHzCN' + CO' does not play any role in the initiation of free radicals in the system.
+
sition product and are not part of the isoxazole decomposition. GC peak areas were integrated with a Spectra Physics Model SP4200 computing integrator and were transferred to an IBM PC/AT for data reduction and graphical presentations. Results
Product Distribution. In order to determine the distribution of reaction products, tests were run with mixtures containing approximately 0.6% isoxazole in argon, covering the temperature range 850-1 100 K. Extensts of pyrolysis as low as few hundredths of 1% were determined. Details of the experimental conditions and the distribution of reaction products in several representative tests are given in Table I. The table shows the temperature behind the reflected shock
The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 4509
Thermal Decomposition of Isoxazole TABLE III: Renction Scheme for the Decomposition of Isoxnzole"
--
reaction
+
1. C3H3NF- CH3CN C o b 2. C;H;NO HCO + CH2CNb 3. C3H3N0 HCN CH2COb 4. C3H3N0 C2H2+ HCNOb 5 . Ar HCO- H CO Ar 6. Ar HCNO- HCN 0 Ar 7. H HCNO HCN + OH C2H3 HCNO 8. C3H3NO H 9. CjH3NO + CHj CH4 + (C3HZNO)' 10. C2HiNO H H, (CiH,NO)' 11. CH3CN H CH, + 'HCN 12. CHjCN CH3 CH4 + CH2CN H2 + CHZCN 13. CH3CN + H CH3CN 14. H CH2CN 15. C2H6 CH2CN CHlCN + C2H5 16. (CjH2NO)' C2H2 + NCO' 17. (C3H2NO)' CO CH2CNb 18. CH2CN CHZCN C4H4N2 19. C4H4N2 C2H3CN HCN 20. H C4H4N2 -t C2H4CN HCN 21. C2H4CN C2H3CN + H HCN C2H3 22. C2HJCN H C2HSCN 23. CH3 CH2CN 24. C2HSCN + H -+ C2H4CN + H2 C2H4CN + CH4 25. C2HsCN CH, C2HS + HCN 26. C2HSCN H 27. H + C2H4CN -C CIHSCN 28. CH3 + CH3 C2H5 H 29. CzH5 C2H4 H C2H6 30. CH3 + CH, C2H2 H Ar 31. C2H3 Ar
+ + +
+
+
+ +
+
+
+
+
+
-
+ + + +
+
--
-t
-- + --- + +
-+ -+ + +
+
+
--
+
+
- -+ + -
+
+
+ +
A 8.50 X 1.77 X 7.23 X 3.31 X 5.12 X 1.00 x 1.00 x 1.20 x 1.50 X
n
10" 10" 10l2 10" 1021 1016 1014
1014 10I2 5.00 x 1013 1.01 x 10" 5.00 X 10" 2.61 x 1013 4.86 X 10l2 1.05 x 1014 9.13 X 10" 7.08 X loi2 1.23 x 1014 1.14 x 1013 2.10 x 1013 9.30 x 1013 1.01 x 1013 2.45 x 1014 2.60 x 1014 3.00 X 10l2 5.01 x 1013 4.86 X 10l2 2.80 x 1013 4.80 X lo8 6.80 x 1014 3.00 X 1OIs
0 0 0 0 -2.14 0 0 0 0 0
0 0 0 0 0 0 0 -0.50
0 0 0 0 -0.50
0 0 0 0 0 1.19 -0.64 0
E 43 470 52 000 52 070 75 000 20 425 37 000 9 000 36 000 10000 9 000 2 000 9 000 8 000 0 19 470 40 000 43 470 0 68 000 2 000 38 400 2 000 0 8 000 9 000 2 000 0 13 593 37 205 0 32 030
k1(950 K) 8.50 X lo1 1.93 x 101 7.60 X loo 1.85 X lod 4.35 x 1010 3.08 x 107 8.50 X 10" 6.28 X los 7.51 x 109 4.25 X 10" 3.51 X 10I2 4.25 x 109 3.77 x 10" 4.86 X 10l2 3.49 x 109 5.74 x 102 7.08 X lo2 3.97 x 1012 2-59 x 10-3 7.28 X 10I2 1.36 x 105 3.51 X 10l2 7.95 x 1012 3.76 X 10I2 2.55 X 1OIo 1.74 x 1013 4.86 X 10l2 2.09 X 1OIo 4.64 x 104 8.45 X 10l2 1.29 X lo8
k,(950 K) 1.87 X 2.20 x 107 9.68 X 1.70 x 107 3.01 x 1014 3.28 x 1017 4.53 x 10-2 2.01 x 10'0 1.67 x 107 4.71 x 107 1.01 x 109 1.20 x 107 5.31 x 107 1.35 x 10-7 4.42 X 1OIo 6.04 x 107 1.98 x 10-7 1.35 X loo 4.51 x 104 2.96 X 10l2 5.86 X 10I2 3.05 X 10" 9.58 X lo-* 4.98 X 1O1O 6.77 x 109 7.55 x 109 1.44 x 10-9 1.37 x 1014 1.37 x 1013 1.22 x 10-4 3.70 X 10l6
AH,O(lOOO K) -23.8 51.7 -0.64 76.3 18.6 52.8 -5 1.4 35.6 -12.4 -11.2 -7.9 -13.4 -12.2 -94.0 8.46 36.0 -24.8 -67.0 39.1 1.11 38.0 1.41 -84.0 -4.21 -5.43 -6.35 98.0 11.6 38.0 -90.8 40.7
"Mro are expressed in units of kcal/mol. Rate constants are expressed as k = Ai"' exp(-E/RT) in units of cm3, s, mol, cal. bThisinvestigation.
Tsas calculated from the conversion of the internal standard, the overall density behind the reflected shock Cs(in units of mol/cm3), and the mole percent of the various reaction products in the mixture as obtained in the pastshock analyses. The concentration of isoxazole behind the reflected shock prior to decomposition (C,(isoxazole)o}is given by the percent of isoxazole in the unshocked mixture times C,. The percent of a given product in the total sample, as shown in the table, corresponds to its mole percent, (lOOC,/~C,),irrespective of the number of its carbon atoms and not including hydrogen and argon. Figure 2 shows the product distribution obtained in the postshock mixtures over the temperature range covered in this investigation. As can readily be seen, carbon monoxide and acetonitrile are the major reaction products. Hydrogen cyanide, acetylene, acetontrile, and propionitrile appear at lower concentrations. Methane, ethylene, and ethane are the lowest. Trace quantities of allene and methylacetylene are not shown. Carbon-NitrogenBalance. The balance of nitrogen vs carbon among the decomposition products is shown in Figure 3. The concentrations of all the nitrogen-containing products [C,] are plotted on a log-log scale against one-third of the concentrations of all the carbon-containing products 1/3C[Cc],each multiplied by the number of its carbon atoms nc. The 45' line in the figure represents a complete mass balance. As can be seen, within the limits of the experimental scatter, nitrogen balance is maintained. Inhibited Reaction. In order to assess the extent of free radical involvement in the production of acetylene and hydrogen cyanide, several tests were run in the presence of large excess of toluene ([toluene]/[isoxazole] 10). The latter is a free-radical scavenger. Its activity is based on the very stable benzyl radical which is formed by an attack of free radicals such as methyl group and hydrogen atom. It has been used successfully in single-pulse shock tube studies in the past.6 Whereas the production of molecules such as acrylonitrile, methane, ethane, etc. clearly involve free-radical processes, the production of acetylene and hydrogen cyanide, as will be discussed
-
( 6 ) Lifshitz, A.; Bidani,
M.;Bidani, S.J. Phys. Chem. 1986, 90, 3422.
later, can involve both free-radical reactions and unimolecular processes. The effect of toluene on acetylene and on hydrogen cyanide was compared to its effect on acetonitrile which is believed to be produced by a unimolecular process only, where no effects of free-radical scavengers are expected. The results of this series of experiments are shown in Figures 4-6 where the ratio [product]/[isoxazole] is plotted in arbitrary units on a logarithmic scale against the reflected shock temperatures for tests in mixtures containing 0.1% isoxazole 1% toluene in argon and 0.1% isoxazole without toluene. In both mixtures the concentration of the internal standard was 0.1%. Figure 4 shows no effect of toluene on the production of acetonitrile. Figure 5 shows a small effect on the production of hydrogen cyanide, and Figure 6 reveals a large effect on acetylene. Its production rate is reduced by more than a factor of 3, indicating that free-radical reactions play an important role in its production mechanism. Arrhenilro Parameters. In Figures 7-10, the rates of production of four representative products CH3CN, HCN, C2H2,and C2H4 defined as
+
rate(product,) = Cs(product,),/t
(111)
and divided by the initial isoxazole concentration, [ i ~ o x ]are ~, ploted (in units of s-l) against reciprocal temperature. By definition, these are first-order rate constants regardless of whether their production is unimolecular or not. However, they are true first-order rate constants only for products which are formed by unimolecular processes. As can be seen, in this particular presentation, the lines bend toward the high-temperature end. This behavior is mainly due to depletion of the reactant at high extents of reaction (high temperatures) but also may be due to further decomposition of the product (mainly by free-radical attack). Values of E in units of kcal/mol which are obtained from the slopes of these log k vs 1 / T lines and their corresponding preexponential factors are summarized in Table 11. All were obtained from the low-temperaturelow-conversion range in the figures where [isoxazole], N [isoxaz~le]~ and before curvatures in the lines begin to appear. The Arrhenius rate parameters for the production of acetonitrile, obtained in this manner, correspond to true first-order Arrhenius
-
-
Lifshitz and Wohlfeiler
4510 The Journal of Physical Chemistry, Vol. 96, No. I I, 1992 0.8 I
1
0.8
-0.8
2
1
I
900 K 1050KI
1
900K 1050 K
I 1
3
2
10
3
11
Reaction Number
Reaction Number
8
850 850
900
950
1000
1050
900
950
1000
1050
1100
1100
T, K
T, K Figure 13. Comparison between experimental and calculated mole percent of carbon monoxide (bottom) and the sensitivity analysis for its production (top). The line represents the best fit through the points which are calculated at 50 K intervals, for reaction times of 2 ms. Sensitivity analysis is shown at 900 and 1050 K. It gives the percent change in the concentration of carbon monoxide resulting from a factor of 3 increase in the rate constant. Only reactions which show an effect are listed.
parameters and indicate a very stiff transition structure for the major ring cleavage. Those for acetylene and other products do not correspond to true unimolecular rate parameters. Nevertheless, we find it quite useful to present the experimental results as first-order rate constants by calculating a pseudo-zero-order constant and dividing it by the initial reactant concentration. Figure 11 shows the first-order rate constant for the overall decomposition of isoxazole calculated from the relation: k,,,, = -In { [isoxazole],/ [isoxazole]o~/t.The value obtained is ktoul = 1013.95 exp(-52 X 103/RT) s-l, where R is expressed in units of cal/(K mol). This first-order rate constant does not imply that the pyrolysis of isoxazole obeys,under the conditions of the present experiments, a first-order reaction. It is a good way, however, to show the overall decomposition rate and its temperature dependence. As will be discussed later, the decomposition is composed of a large number of elementary reactions involving unimolecular dissociations as well as free-radical reactions.
Discussion Thermochemistry. Many arguments that will be presented in this section will be based upon the energetics of the various paths. Moreover, in the process of the computer simulation, values of k, are calculated from kf using the equilibrium constants of elementary reactions that appear in the scheme. For calculating the equilibrium constants the thermodynamicproperties of the species involved must be known and introduced as part of the input of the program. The computer program available to us can calculate a sensitivity spectrum with respect to variations (or rather uncertainty) in the thermodynamic properties of the species. Whereas the sensitivity analysis with respect to the rate constants gives the sensitivity of the system to variations in both the forward
Figure 14. Comparison between experimental and calculated mole percent of hydrogen cyanide (bottom) and the sensitivity analysis for its production (top). The line represents the best fit through the points which are calculated at 50 K intervals, for reaction times of 2 ms. Sensitivity analysis is shown at 900 and 1050 K. It gives the percent change in the concentration of hydrogen cyanide resulting from a factor of 3 increase in the rate constant. Only reactions which show an effect are listed.
and the reverse rate constants, the sensitivity to the values of ATPf and So gives the sensitivity to variations in k, only. In several test that were performed we found sensitivity only to species whose thermodynamic properties are very well known such as methyl group and hydrogen atom. The thermodynamic properties of all the species used in the simulation were taken from known literature S O U ~ C ~ S . ~ -For ~~*~ some species the properties are only estimated and are not very accurate. Since the simulation is not very sensitive to these values, we consider them as adequate. Reaction Mechanism. 1. Acetonitrile. The major products in isoxazole decomposition are acetonitrile and carbon monoxide. Their production rate is unaffected by suppression of free-radical reactions as was demonstrated with the experiments performed in the presence of toluene (Figure 4). It is thus reasonable to assume that these two species are obtained by unimolecular decomposition of isoxazole. This process takes place by a concerted N-0 bond cleavage, a hydrogen atom shift from position 5 to position 4, and a rupture of the C(4)4(5) bond with the removal of carbon monoxide from the ring:
rz
(4
(4)7-T: + q7-T c\ (5)
74 1 )
4I C;\?
+ cH3cN+co
u..N(*)
(1)
(7) Stull, D. R.; Westrum, Jr., E. F.;Sinke, G. C . The Chemical Thermodynamics of Organic Compounds; John Wiley & Sons: New York, 1969. (8) Tsang, W.; Hampson, R. F. J. Phys. Chem. Ref. Data 1986,15,1087. (9) (a) Pedley, J. B.; Taylor, R. D.; Kirby, S. P. Thermochemical Data of Organic Compounds;Chapman and Hall: London, 1986. (b) Stein, S. E.; Rukkers, J. M.; Brown, R. L. NIST-Stand. Ref. Data 1991, 25.
Thermal Decomposition of Isoxazole 0.8
0.4
h
-
The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 4511 0.8
900 K 1050 K
900 K 1050 K
C,H,CN
0.4
E. 0,
i;i
3
0.0
0.0
-0.4
-0.4
TD
v
% UJ
-
-0.8
-0.8
1
10
2
11
18
'
I 1
17
2
3
10 11
17 18 20 24
Reaction Number
Reaction Number
io m8 8
c
C
E P Q
-
E
0. 0.0' 850
900
950
1000
1050
1100
T, K
Figure 15. Comparison beween experimental and calculated mole percent of acetylene (bottom) and the sensitivity analysis for its production (top). The line represents the best fit through the points which are calculated at 50 K intervals, for reaction times of 2 ms. Sensitivity analysis is shown at 900 and 1050 K. It gives the percent change in the concentration of acetylene resulting from a factor of 3 increase in the rate constant. Only reactions which show an effect are listed.
A similar reaction was found in the decomposition of the isoelectronic molecule furan.' In its major decomposition reaction furan decomposes to methylacetylene (isoelectronic with acetonitrile) and carbon monoxide in practically the same unimolecular process. CH-CH
I/
CH,
\\
/CH
-
CH-$=CH
+
CO
0
The configurations of the transition states of these reactions in isoxazole and in furan are the same but there is a big difference in the distribution of their bond lengths. Whereas the Arrhenius parameters A and E for the reaction in furan are 1015.25 s-I and 77.5 kcal/mol, respectively, they are s-l and 43.5 kcal/mol in isoxazole. In order to allow for a hydrogen atom shift from the 5- to the 4-position in the ring, position 4 must be "sensitized". In other words, it should acquire a partial free-radical character. This is accomplished by stretching the 1-2 bond in the ring. In furan the stretch required for such a shift is big. It therefore results in a very loose transition state and a high activation energy, close to the bond strength. Owing to the presence of a nitrogen atom adjacent to the oxygen in the ring the stretch in isoxazole is much smaller. The extent of elongation required to "sensitize" position 4 in the isoxazole ring to allow for a migration of a hydrogen atom to that position is very small. This results in a stiff transition state and a very low activation energy. In view of the decreased symmetry in isoxazole compared to furan, the 1-2 and the 1-5 bonds are not equivalent but on the other hand there is no H atom on the nitrogen to allow migration from the 2-to the 3-position to produce different products. Thus CH$N and CO are the only products resulting from this type of reaction.
T, K
Figure 16. Comparison between experimental and calculated mole percent of acrylonitrile (bottom) and the sensitivity analysis for its production (top). The line represents the best fit through the points which are calculated at 50 K intervals, for reaction times of 2 ms. Sensitivity analysis is shown at 900 and 1050 K. It gives the percent change in the concentration of acrylonitrile resulting from a factor of 3 increase in the rate constant. Only reactions which show an effect are listed.
2. Hydrogen Cyanide. Another important product in the pyrolysis is HCN. This product can be obtained directly from the ring by a process very similar to reaction 1. It is initiated by cleavage of the N - O bond and hydrogen atom migration from position 5 to position 4. The only difference is the rupture of the C(4)-C(3) bond rather than the C(5)-C(4) bond. This will lead to the production of hydrogen cyanide and ketene: (4) CH-
(4
CH (3)
/I \ \ v - - - + \ h cs>cy /N"
NCH-CH
\\
N(*)
C+
0''
4)
-
(3)
A similar reaction has been observed in the pyrolysis of furan CH-CH
//
CH,
\\
/CH
-
C2H2
+
CH2CO
0
where it has been clearly established that the formation of acetylene and ketene was indeed unimolecular.I In order to verify whether such a process occurs in isoxazole, we tried first to verify the presence of ketene in the postshock mixtures and then to examine the results of the pyrolysis in the presence of toluene. However, owing to the very high reactivity of ketene, it reacts with water absorbed on the walls of the glass bulbs and elsewhere to produce acetic acid, which also sticks to the walls and is thus hard to analyze. In order to overcome this difficulty we collected the postshock mixtures, in several exper-
-
-
4512 The Journal of Physical Chemistry, Vol. 96, No. 11, 1992
A
0.4
Lifshitz and Wohlfeiler
I
1050 K
900 K 1050K
-".O
-0.8 1
2
1
10 11 17 18 23 24 26 30
2
9 10 11 12 13 18 23 25 30
Reaction Number
Reaction Number
-
8
lo
I
C,H,CN "
I m
3
m
1 :
m
1 : 0.1 :
/ 0.1 900
950
1000
1050
1100
T, K Figure 17. Comparison between experimental and calculated mole percent of propionitrile (bottom) and the sensitivity analysis for its production (top). The line represents the best fit through the points which are calculated at 50 K intervals, for reaction times of 2 ms. Sensitivity analysis is shown at 900 and 1050 K. It gives the percent change in the concentration of propionitrile resulting from a factor of 3 increase in the rate constant. Only reactions which show an effect are listed.
iments, in bulbs containing a few Torr of methyl alcohol. The latter reacts instantaneously with ketene and forms methyl acetate which was clearly identified on the MSD by its mass spectrum. No quantitative data on the concentration of ketene could be obtained but its presence has been qualitatively proven. This observation, together with the results obtained in the presence of toluene where not a very large effect on the production rate of HCN was obtained, clearly indicates that a portion of hydrogen cyanide is formed by a unimolecular cleavage of the ring similar to the one that occurs in furan. As will be shown later in this discussion, the additional contributionto the formation of hydrogen cyanide comes from hydrogen atom attack on acetonitrile,*Owhich is the major reaction product. 3. Free-Radical Reactions. Except for acetonitrile, carbon monoxide, and hydrogen cyanide which we believe are formed by unimolecular reactions from isoxazole, the production of all the other products is, in one way or another, associated with freeradical reactions. The question that thus arises is what would be the first step that leads to the production of free radicals. One should bear in mind that the temperature range covered in these series of experiments is relatively low and the activation energies involved (Table 11) are small. This excludes the possibility that hydrogen atom ejection from the ring, a process that requires at least 95-105 kcal/mol, would be the step by which free radicals are first generated in the system. (The exact C-H bond strength in isoxazole is not known.) It seems therefore that another process must take place in order to initiate the production of free radicals. Acetonitrile and carbon monoxide which are formed in the decomposition of isoxazole are thermally hot by the sum of the (10) Lifshitz, A.; Moran, A.; Bidani, S. In?.J . Chem. Kinet. 1987,19,61.
T, K Figure 18. Comparison between experimental and calculated mole percent of methane (bottom) and the sensitivity analysis for its production (top). The line represents the best fit through the points which are calculated at 50 K intervals, for reaction times of 2 ms. Sensitivity analysis is shown at 900 and 1050 K. It gives the percent change in the concentration of methane resulting from a factor of 3 increase in the rate constant. Only reactions which show an effect are listed.
AHl0 and El which is approximately 67 kcal/mol. We have examined the possibility that the thermal excitation of these products would be enough to further dissociate to (CH3CN + CO)* H' CH2CN' + CO before losing its excitation energy by collisions. A similar mechanism has been suggested for the decomposition of ethylene oxide' where the thermally excited acetaldehyde which is formed by the isomerization of ethylene oxide possess enough energy to directly dissociateto (CH3CHO)* C2HS* HCO'. Even if we assume that all the thermal excitation is acquired by the acetonitrile alone this process in isoxazole is still short of some 27 kcal/mol and thus cannot take place. The energy diagram for this process is shown in Figure 12. It is therefore suggested that the initiation step for free radicals is a step similar to reaction 1 except that there is no migration of a hydrogen atom from position 5 to position 4 in the ring, namely, rupture of the N-O bond followed by elimination of HCO rather than CO:
-
-
+
+
CH-CH
// CH\o,N
\\
-
CH2CNo
+
HCO.,
AW = 51.7 kcaWmol
(2)
This reaction is endothermic with an endothermicity roughly equal to the activation energy for the formation of acetylene and methane. We assumed that the activatioin energy for this reaction is equal to its endothermicity, namely, E = 52 kcal/mol. Since this will require a larger stretch of the N-O bond in the ring, it will be associated with a looser transition state. The best fit to ( 1 1 ) Lifshitz, A.; Ben Hamou, H. J . Phys. G e m . 1983, 87, 1782.
.
Thermal Decomposition of Isoxazole 1.0
0.5
I
tl
-
-
The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 4513 0.8
I
t
900 N 1050
1
I
D 900K
r
C,",
0.0
1050K
I
-
-0.5
-1.o -1.0 1 2
-0.4
-
-0.8
I
9 10 11 18 23 24 28 28 29 3
1
2
Reaction Number
3
10 11 13 18 23 30
0
Reaction Number
t 1
0.1
E
*
0*01 0. 001 850
900
950
1000
1050
0
2
0 P 0 0
\\
-
C2H2
+
HCNO
0.01
8
0.001
+
~
-
850
1100
(4)
is very unfavorable and even if it does take place it will have no or negligible contribution to the overall production of acetylene. First, rupture of the C-O bond must compete with the rupture of a N-O bond which is several tens of kilocalories weaker. In addition, the reaction isoxazole C2H2 HCNO is -76 kcal/mol endothermic. The exclusion of this channel as an important channel in the production of acetylene is strongly supported by the suppression of its production by toluene (Figure 6). It is therefore suggested that the production of acetylene must be associated with freeradical reactions. Whereas the picture regarding the initiation of free radicals is clear, their involvement in the production of acetylene must still
-
:
E
our entire experimental results required a preexponential factor of the order of 1013. This will be demonstrated later when the computer simulation of the pyrolysis will be discussed. Once HCO is formed it will instaneously decompose to CO and H and chain reactions will be initiated. An equivalent reaction to reaction 2 would be a rupture of the C-O bond to produce NO' and C3H3*.First, we do not believe that such a reaction will be of any importance because of the C-O vs N-O bond strengths. Second, C3H3'is a very stable species and can hardly initiate a chain reaction under the condition of the present experiment. 4. Acetylene. It can be shown by kinetic and thermochemical considerations that reaction 4, which is similar to reaction 3, //
0.1
-
T, K Figure 19. Comparison between experimental and calculated mole percent of ethylene (bottom) and the sensitivity analysis for its production (top). The line represents the best fit through the points which are calculated at 50 K intervals, for reaction times of 2 ms. Sensitivity analysis is shown at 900 and 1050 K. It gives the percent change in the concentration of ethylene resulting from a factor of 3 increase in the rate constant. Only reactions which show an effect are listed.
CH-CH
L
i=
-
.
-
900
-
950
-
-
.
1000
-
-
-
1050
-
.
.
-
-
-
1100
T, K Figure 20. Comparison between experimental and calculated mole percent of ethane (bottom) and the sensitivity analysis for its production (top). The line represents the best fit through the points which are calculated at 50 K intervals, for reaction times of 2 ms. Sensitivity analysis is shown at 900 and 1050 K. It gives the percent change in the concentration of ethane resulting from a factor of 3 increase in the rate constant. Only reactions which show an effect are listed.
be clarified. As the pyrolysis proceeds methyl radicals are produced by the reactionlo H' CH3CN ---* CH3' HCN (1 1) which via abstractions, attachments, displacements, and recombinations will produce methane, ethane, and ethylene. These products as well as acetylene are normally formed when organic compounds are exposed to high temperatures. The relative concentration of acetylene which is small at low temperatures increases as the temperature goes up. At temperatures above 1300 K its concentration exceeds those of the other products. However, the temperatures in these experiments are too low to allow for the observed quantities of acetylene where methane, ethane, and ethylene serve as its precursors. It is therefore believed that acetylene is not formed from the other hydrocarbons and another free-radical mechanism is operative. We suggest that acetylene is formed from isoxazole by a unimolecular reaction, but not before the latter has either lost a hydrogen atom by abstraction or gained one by an attachment to become an unstable intermediate: H' C3HjNO H2 (C3H2NO)' (10)
+
+
-
-
+
+
H' + C3H3NO ---* (C3H4NO)' (loa) The decomposition of one of these two unstable intermediates can lead to the production of acetylene. The radical obtained in reaction 10 will lead, upon its decomposition, to the production of C2H2and CNO, whereas the radical obtained in reaction 10a will produce C2Hjand HCNO, followed by C2H3' Ar C2H2+ H' + Ar
+
-
.
-
-
-
I
-
4514 The Journal of Physical Chemistry, Vol. 96, No. 11, 1992
t
"*"
m
900K 1050W
-0.4
-"." 1
2
3
Reaction Number
CI
t Q
2
10
Q
9.
Q 0
E
T, K Figure 21. Comparison between experimental and calculated mole percent of acetonitrile(bottom) and the sensitivity analysis for its production (top). The line represents the best fit through the points which are calculated at 50 K intervals, for reaction times of 2 ms. Sensitivity analysis is shown at 900 and 1050 K. It gives the percent change in the concentration of acetonitrile resulting from a factor of 3 increase in the rate constant. Only reactions which show an effect are listed.
Both HCNO and CNO are very unstable species. They are much less stable than their isomers HNCO and NCO. (AHf0298(HCNO)= 38.4 kcal/mol)12 whereas AH:298(HNCO) = -24.9 kcal/mol).12 Assuming that the attachment of a hydrogen atom to isoxazole is approximately 40 kcal/mol exothermic (similar to attachment of H to ethylene), then, in view of the instability of both CNO and HCNO, reactions 10 and 10a would be endothermic with AHro= -80 kcal/mol and thus very improbable. The only way that C2H2can be eliminated from the ring with a more favorable thermochemistry is by an internal isomerization to the NCO structure, where O(1) migrates and attaches itself to C(3). Such a process will be much simpler in (C3H2NO)'than in (C3H4NO)'. This is supported also by the computer modeling. Reaction Scheme. On the basis of the arguments raised above we have constructed a reaction scheme which describes the decomposition of isoxazole. The scheme contains 25 species and 31 elementary reactions. It is listed in Table 111. The first three columns in the table give the three parameters A, n, and E for the forward rate constants corresponding to the reactions as they are listed in the table. The rate constants are given as k = AT" exp(-E/RT) in units of cm3, mol, s. Column 4 gives the values of the forward rate constants as calculated from the rate parameters at a temperature of 950 K and column 5 shows the value for the reverse rate constants calculated from kf and the equilibrium constants of the reactions, also at 950 K. Column 6 gives the heat of reaction at 1000 K. The scheme is composed of unimolecular dissociations of the reactant molecule, unimolecular (12) Miller, J. A.; Bowman, C. T. Prog. Energy Combust. Sci. 1989, 15, 287.
Lifshitz and Wohlfeiler dissociations of radical intermediates, abstractions, and recombinations. Rate parameters for the various unimolecular ring openings were taken from the experimental Arrhenius plots as described previously. They are all characterized by stiff transition states and low activation energies. Arrhenius parameters for other reactions were taken from literature sources, mostly from the NIST-Kinetic Data Base13and from Warnatz's compilations14 and were varied within the limits of their reported uncertainties. Parameters for reactions which could not be found in available compilations were determined by comparison with similar reactions for which the rates are known. The activation energy for reaction 2 was taken as roughly equal to its endothermicity and the preexponential factor was determined according to the best fit of the data. The value of 1.77 X 1013s-I is in perfect accord with a preexponential factor that can be expected for such a dissociation. Computer Modeling. Figures 13-21 show comparisons between mole percent of several products as obtained in the analyses of the postshock mixtures and in the calculations using the reaction scheme given in Table 111. The points in the figures are experimental and the solid lines are the best fit through calculations at 50 K intervals, for reaction times of 2 ms. The figures show also the sensitivityanalyses for the formation of these products calculated at 900 and 1050 K again for reaction times of 2 ms. They show, on a logarithm scale, the change in concentratioin of a given product due to a factor of 3 increase in the forward (and reverse) rate constants. The figures concentrate on reactions that have the most influence on the production rates of these species. Concentrations of products which are formed by unimolecular decompositions are sensitive to fewer steps than concentrations of those which are associated with free-radical reactions. Thus, carbon monoxide (Figure 13) and acetonitrile (Figure 21) are mainly sensitive to reaction 1. An increase in the rate of reaction 2 which is the initiation reaction for free radicals increases only very slightly the concentration of CO (due to HCO CO + H) but decreases considerably, particularly at high temperatures, the concentration of CH3CN. The latter as a product of the highest concentration is attacked by hydrogen atoms: H + CH3CN CH3 + HCN. HCN which is produced by unimolecular reaction 3 and by free radicals is strongly affected by both reactions 2 and 3. The rates of all the products which are associated with free radicals are enhanced and practically controlled by reaction 2. On the other hand, as can be seen in Figures 13-21, for most of these products reaction 1 is inhibiting, particularly at high temperatures. This is simply a result of two competing parallel reactions. At high temperatures the concentration of the reactant is more scarce and the free-radical reactions are more important. The general agreement for most of the products as shown on the figures seems to be satisfactory except for ethane where there is a serious discrepancy at high temperatures. The reason for this discrepancy is unclear. The question that always arises in studies of this type is to what extent the combination of rate parameters used in the study is unique. Is there another combination with realistic rate parameters that can produce the same modeling results? In systems with relatively large number of products the interaction between the various elementary steps limits the freedom of arbitrary variation of rate parameters. Nevertheless, some degrees of freedom are still left and the correctness of the rate parameters must be supported by comparisons with other systems.
-
-
Conclusion Production rates and distribution of products obtained in the thermal decomposition of isoxazole are successfully simulated with (1 3) Westly, F.; Herron, J. T.; Cvetanovic, R. J.; Hampson, R. F.; Mallard, W. G. NIST-Chemical Kinetics Data Base. (14) Warnatz, J. In Combustion Chemistry; Gardner, W. C., Jr., Ed.; Springer-Verlag: New York, 1984; p 197.
J . Phys. Chem. 1992,96, 4515-4521 a kinetic scheme containing 31 elementary reactions and 25 species. A very important feature in the dissociation of isoxazole is the very low preexponential factors and activation energies. These low preexponential factors indicate stiff transition structures in the unimolecular dissociations of the reactant molecule. It results from a minimal stretch of the N-O bond in the transition state.
4515
Acknowledgment. This research was supported by a grant from G.I.F., the German-Israeli Foundation for Scientific Research and Development. We thank Professor P. Roth who served as the cooperative investigator for this research for his advice and encouragement. Registry No. Isoxazole, 288-14-2.
Vlslble Raman and Near-Infrared Fourier Transform Raman Characterization of Adsorbed 4,4’-Cyanine D.L.Akins,* J. W.Macklin: and H.-R. Zbu Department of Chemistry, The City College of The City University of New York, New York. New York 10031 (Received: October 4, 1990)
Raman spectra obtained using several visible excitation wavelengths and a dispersive monochromator and FT-Raman spectra obtained using a CW Nd:YAG laser at 1.064 pm (9395 cm-I) and a FTIR spectrometer are combined to gain enhanced detail about the structural character of 4,4‘-cyanine adsorbed at various potentials onto a silver electrode from solutions of high and low pH. Spectral information is interpreted to indicate that the most prevalent substituents in the electrode adsorbate are polycrystallite and interfacial aggregate molecules: at high pH, the adsorbate is mainly polycrystalline material, while at low pH, the polycrystallite is solubilized and the aggregate exists in principally its protonated form. Also, for near-IR excitation in the low-pH system, the absolute increase in intensity of Raman bands over that for the high-pH system is interpreted as indicating that aggregation enhancement is a principal enhancement mechanism at both high and low pH.
I. Introduction Earlier papers from this laboratory have detailed the types of information that can be deduced through studies of Raman scattering by adsorbed, aggregated cyanine dyes. Theory as well as experiments has provided insight on the determinationof relative dipole moments of the excitonic state’ and intermolecular dipoledipole interaction energies: as well as the existence and nature of molecular conformers within aggregate structure^.^ Most of our experimental studies have focused on the dyes adsorbed onto a polished silver electrode in an electrochemical cell and dealt with lI1’-diethyl-2,2’-cyanineiodide or the chloride (hereinafter referred to as 2,2’-cyanine), which is the prototypical cyanine dye spectral sensitizer. We have gained, as a result of our varied studies, a better understanding of theoretical roots of enhanced Raman scattering by aggregated cyanines4as well as some measure of spectroscopic assignment of spontaneous fundamental and higher-order (i.e., overtone and combination) Raman band^.^,^
In the present paper, we utilize variables such as pH, excitation frequency, and surface potential to gain insight into the structure of aggregated l,l’-diethyl-4,4’-cyanineiodide, referred to hereinafter as 4,4’-cyanine and with its molecular structure depicted in Figure 1 (without the halide shown), drawing from the knowledge garnered from similar studies of 2,2’-cyanineO3q5This paper specifically describes the use of Raman spectra obtained using visible radiation in combination with FT-Raman spectra obtained using a CW Nd:YAG laser at 1.064 pm (9395 cm-l) to gain enhanced detail about the structural character of 4,4’cyanine adsorbed at various potentials onto a silver electrode from solutions of high and low pH. We find that the Raman spectrum of 4,4’-cyanine adsorbed onto a polished silver surface exhibits many of the same characteristics as found for 2,2’-cyanine: specifically, surface potential effect on apparent surface pH, resonance Raman scattering at crystal and solution monomer absorption frequencies, the utility of overtone and combination bands for correlation of fundamental vibrational modes with a single chemical species, and the existence of an enhanced Raman ‘Department of Chemistry, University of Washington, Seattle, WA 98195.
scattering which is attributable to “aggregation enhancement”. This latter effect has been associated with an increase in the scattering species’ polarizability, upon aggregate formation, due to a size increase and, possibly, near-resonance contributions in the Albrecht B term associated with the small energy spacings between the molecular aggregate state and other with the exclusion of surface-enhanced Raman scattering (SERS).3.4 Spectral information is interpreted to indicate that the most prevalent substituents in the adsorbate are the polycrystallineand molecular aggregated states of the dye. Section I1 provides a discussion of the experimental techniques, procedures, and conditions used to acquire Raman spectra of 4,4‘-cyanine. Analyses of our spectroscopic results are provided in section 111. 11. Experimental System and Procedures Both dispersive Raman and FT-Raman instruments have played a prominent role in the present study. The dispersive Raman instrumentation consisted of a SPEX 1404double monochromator with photomultiplier detection (RCA 31034) and has been thoroughly described in earlier publications from this laboratory.1-6 The near-IR excited Raman measurements were conducted with a Bomem DA 3.16 FT-Raman instrument. In these latter measurements, Raman scattering was collected in a backscattering configuration in which an on-axis ellipsoidal mirror (supplied with a hole allowing passage of incident excitation) reflected and focused the scattered radiation in a 90’ direction into the entrance aperture of the interferometer. The incident, unpolarized near-IR radiation at 1.064 pm was supplied by a TEMm, CW 4-W, Quantronix Model 114 Nd:YAG laser; usually about 1 W of power was focused through the ellipsoidal mirror onto the sample. A CaF, beam splitter was used in the interferometer stage, and a liquid N2 cooled indium-galliumarsenide (InGaAs) photodiode (1) (2) (3) (4) (5) (6)
Akins, D. L.; Akpabli, C. K.;Li, X.J . Phys. Chem. 1989, 93, 1977. Akins, D. L.; Lombardi, J. R. Chem. Phys. Lert. 1987, 136, 495. Akins, D. L.; Macklin, J. W. J . Phys. Chem. 1989, 93, 5999. Akins, D. L. J . Phys. Chem. 1986, 90, 1530. Akins, D. L.; Macklin, J. W.; Zhu, H.-R. J . Phys. Chem. 1991,95,793. Gu, B.;Akins, D. L. Chem. Phys. Leu. 1984, 105, 263.
0022-3654f 92f 2096-45 15%03.00f 0 0 1992 American Chemical Society
