a-Electron Distortions in Aromatic Systems
199
Phenolic-OH Torsional Frequency as a Probe for Studying -Electron Distortions in Aromatic Systems William G. Fateley,* Department of Chemistry, Kansas State University, Manhattan, Kansas 66506
Gerald L. Carlson, Mellon Institute of Science, Carnegie-Mellon University, Pittsburgh, Pennsylvania 752 13
and Freeman F. Bentley Air Force Materials Laboratory/LPH, Wright-Patterson Air Force Base. Ohio 45433 (Received July 25, 7974) Publication costs assisted by Kansas State University
The phenolic-OH torsional frequencies for a large number of substituted phenols have been obtained. For the monosubstituted phenols, the shift of the torsional frequency from its position in phenol itself is found to be directly related to the electron-donating or -withdrawing power of the substituent group. This has led to the derivation of a new parameter, Awt, which is suggested to be a direct measure of the effect of a substituent on the r-electron density of the aromatic ring, and Aut values are presented for a variety of substituent groups in the ortho, meta, and para positions. For multisubstituted phenols, i t is found that the torsional frequency can often be predicted using the Aut value for each substituent. This indicates that, in the absence of steric effects, the inductive and/or mesomeric effects of substituents are additive until the ring becomes almost fully substituted.
Introduction A large amount of effort encompassing many types of physicochemical techniques has gone into the measurement of substituent effects on the r-electron system in aromatic compounds. The goal of this work has been the determination of electronic interactions in aromatic molecules for the purpose of correlating structure and reactivity and predicting properties and reactions rates. An excellent review of this topic has been given by Katritzky and Topsome1 In 1967, Miller, Fateley, and Witkowski2 examined the effect of para substituents on the CHO torsional barrier in benzaldehyde and attempted to correlate the barrier potential ( V z )with the double bond character of the C-CHO bond, and, hence the electron donor or acceptor properties of the 4 substituent. Although these authors concluded that there was no correlation of the torsional barrier with the Hammett c constants of the substituent groups, Campagnaro and Wood3 later showed that if substituents with large donor or acceptor powers were used, a correlation with various Hammett and Taft c parameters, and in particular with 19F shielding parameters, could be derived. These workers also proposed that a similar correlation could be made for 4-substituted phenols. In a previous publication,4 it was shown that the effect of 4 substituents on the barrier to internal rotation about the C-0 bond in phenol as determined by far-infrared studies of the torsional frequencies was in excellent agreement with results obtained by ab initio molecular orbital calculations. This work showed definitively that substituents which are a-electron donors lower the observed barrier while r-electron acceptors raise the barrier, and that the phenolic-OH torsional frequency is a direct measure of the double bond character of the phenolic C-0 bond. In the present study, we have measured the phenol-OH torsional frequency in a large number of substituted phe-
nols. The results obtained suggest that the torsional frequency can be used as a sensitive probe for measuring the effect of substituents, including multiple substitution, on the a-electron density in aromatic systems.
Experimental Section The phenol samples studied were obtained from a variety of sources. Commercially available phenols were purchased in the, highest available purity, and in most cases were confirmed by comparison of their infrared spectra with published reference spectra. A large number of phenol samples were also made available to us through the courtesy of Dr. A. W. Baker of the Dow Chemical Co. In general, the purity of the samples was probably better than 95%; in a few cases purification by distillation or recrystallization was required. Low-frequency infrared spectra were obtained with a Digilab FTS-14 Fourier transform spectrometer. Both 3(600-100-~m-~range) and 6-p (450-75-cm-l range) beam splitters were employed in conjunction with a globar source. A resolution of 4 cm-l and a triangular apodization function were generally used. Spectra were obtained using dilute (0.01-0.06 M ) cyclohexane solutions and polypropylene or polyethylene cell windows. Path length was generally 5 mm; for a few samples of very low solubility, a 10-mm path was required. Spectra of the analogous phenol-OD compounds were obtained by deuteration of the sample directly in the cy&hexane ~ o l u t i o n . ~ For a few compounds whose torsions fell above 500 cm-1, isooctane was used in place of cyclohexane as the solvent. Results Validity of Torsional Frequencies Measured in Cyclohexane Solution. It is well known that phenols are strongly associated in the liquid and also undergo hydrogen bonding The Journal of Physical Chemistry, Vol. 79, No. 3, 1975
W. G . Fateley, G. L. Carlson, and F. F. Bentley
200
TABLE I: Comparison of Cyclohexane Solution and Vapor Phase Torsional Frequencies for Phenols OH torsional frequency, cm-'
Phenol Phenol
p- Fluorophenol p- Chlorophenol nz- Fluorophenol nz- Chlorophenol m- Bromophenol m- Methylphenol m-CF, 0-Fluorophenol (cis) 0-Chlorophenol (cis) a
Cyclohexane solution
3 10 280 303 318 312 312 312 317 366 396
Vapora
310 2 80 302 318.5 312.5 314 311 315.5 379 407
References 9-10,
with many solvents. With'cyclohexane, there seems little possibility of hydrogen bonding with the solvent, but solute-solvent interactions have been reported. von Keussler6 has shown that phenol in cyclohexane at a concentration of 3X M is completely monomeric, but is 52% associated a t a concentration of 0.15 M. Dearden and Forbes' noted a shift between the ultraviolet spectra of phenol vapor and 2.3 X M cyclohexane solutions which they attributed to solute-solvent interaction because phenol should be completely monomeric at this concentration. Furthermore, Evansa found that the positions of some of the infrared bands of phenol are dependent on the degree of association. To determine whether the association of phenols at the concentrations required for this study would influence the torsional frequency, a dilution study on phenol in cyclohexane solution was carried out. Over the concentration range 0.02-0.001 M , which is the lower limit of detection with a 1-cm path length, the torsional frequency did not shift by more than 1 cm-l. This study indicates that association effects at the concentrations employed should be negligible. To test the effect of interaction of phenols with cyclohexane, we have also compared the torsional frequencies of several phenols with their available gas-phase frequenc i e ~ . ~ -This l l comparison is given in Table I. In the case of meta- and para-substituted phenols, the solution frequencies are in excellent agreement with the vapor. For the orthohalophenols, a vapor solution shift is found which is reflecting a change in the strength of the intramolecular hydrogen bond9 and such a shift is not unexpected. Both of these studies give confidence that by employing cyclohexane solutions, we can obtain valid torsional frequency data for the isolated, monomeric phenol molecule free of association and solvent effects. It should, however, be noted that this is not true for many other solvents. For example, Green, et a1 q,12a have pointed out that the torsional frequency for p - fluorophenol shifts from -283 cm-l in cyclohexane to 290 cm-l in CS2 and to 350 cm-l in benzene. These observations are in agreement with the conclusion of Woolley and Hepler12b that cyclohexane is a (nearly) inert solvent while there is considerable specific interaction between phenol and benzene. Presentation of Data. Although complete far-infrared spectra were obtained for all of the phenols studied as well The Journal ot Physical Chemistry, Voi. 79, No. 3, 1975
as the analogous phenol-OD compounds, because of the large number of compounds included, only the torsional frequencies are presented. Torsional frequency data for 45 monosubstituted phenols are given in Table 11. The data are presented both as the torsional frequency and as the shift (Aut) from the unsubstituted phenol torsion at 310 cm;l. Torsional frequencies for di-, tri-, tetra-, and pentasubstituted phenols are given in mini print.22 The assignment of the torsional frequencies was generally straightforward. The phenolic-OH torsion is usually the strongest band in the far-infrared solution spectrum and can be readily verified by its disappearance on deuteration of the 0-H group. In a few cases, the 0-Htorsion was overlapped by other intramolecular bands, but these were considerably weaker and presented no particular problem in identifying the torsion. For phenol itself and several of the substituted phenols, including all of the meta-substituted phenols, the 0-D torsion undergoes Fermi resonance with another mode in the vicinity of 235 cm-l and appears as two bands of about equal intensity. The torsion for o-cyanophenol-OH also is split, presumably due to Fermi resonance, and its actual position was deduced from the frequency of the 0-D torsion. Additional torsional frequency data for a few substituted phenols not available in this study were obtained from the 1iterature.l3-l5 Some of these frequencies were measured a t relatively high concentrations and in solvents other than cyclohexane and probably are somewhat influenced by solvent and/or association effects.
Discussion Monosubstituted Phenols. It has been shown from both theory18 and experimentlg that the phenol molecule is planar. The planar structure is favored because in this configuration the maximum delocalization of the p-type lone pair electrons on the oxygen atom can occur. The extent of this delocalization determines the energy difference between the planar and orthogonal forms, and hence the barrier to internal rotation (and the torsional frequency) of the 0-H group. For phenol, the torsional frequency in cyclohexane solution is 310 cm-l yielding a barrier ( V 2 ) of 1240 cm-l (3.54 kcal/mol). Substitution a t the 4 position of the phenol ring results in changes in the V2 barrier, and the direction and magnitude of these changes has been accurately predicted by ab initio molecular orbital calculations from the electronwithdrawing or -donating power of the substituent groupe4 For a a-donating group, a-electron donation by OH decreases, the double bond character in the C-0 bond decreases, and the barrier (and the torsional frequency) decreases. Conversely, a x acceptor will cause an increase in the 0-H torsional frequency. Thus, we are proposing that the phenolic-OH torsional frequency is a sensitive probe for determining the effect of a 4 substituent on the electron density at the 1 position of the aromatic ring. The Aut values listed in Table I1 are then proposed as a parameter which measures the electron-donating (Aut negative) or -withdrawing (Aq positive) power of the substituent group relative to hydrogen in phenol itself. T o be strictly correct, we should use the change in the V2 barrier as our parameter; however, in the case of para-substituted phenols where the barrier must be twofold, the torsional frequency exactly parallels the barrier and we use the frequency or change in frequency for convenience.
n-Electron Distortions in Aromatic Systems
201
TABLE 11: Torsional Freauencies of Monosubstituted Phenols in Csclohexane Solution Torsional freq (ut), cm" Concn, Phenol
Concn,
M
OH
Phenol
0.01
310
0
P a r a substituted p-Nitro p- CHO p- Cyano P-CF3
Satd Satd Satd 0.02
350 349 343 334
+40 +39 +33 +24
0.02 Satd Satd 0.015 0.03 0.015 0.01 0.01 Satd 0.03 Satd
313 310 307 303 303 301 298 280 269 268 266
+3 0
-12 -3 0 -4 1 -42 -44
Satd
321
+11
m-Fluro
0.01
318
+8
m- Hydroxyl m- Methoxy
Satd 0-06
318 317
+8
p- Iodo p- Phenyl p-CH3S p- Chloro p-Bromo p-tert-Butyl p-Methyl p- Fluor o p-Methoxy p-n-Butoxy p-Hydroxyl Meta substituted m- Nitro
OD
Awts
-3
-7 -7 -9
+7
Phenol
Torsional freq (4,Cm-l
NI
OH
m-Methyl
0.01
312
+2
m-CHO m- Phenyl
Satd 0.03
312 311
+2 +1
m- tert- Butyl
0.03
308
-2
0.80 0.07 0.70 0.70 0.03
713' 675 537c 53Y 432 386 428 388 411 396 361 395 361 383 319 ?
Ortho substituted 0-CHOB o-Nitro O-CH,Se o-C,H,Se o-Ethoxy, cis trans o-Methoxy, cis trans 0-Hydroxy o-Chloro, cis trans o-Bromo, cis trans o-Phenyl, cis trans 0-Cyano, cis
m-CF3
0.01
317
+7
m-cyano
Satd
316
+6
m- Iodo
0.02
313
+3
m- Chloro
0.008
312
+2
trans o-Iodo, cis trans 0-Fluoro, cis trans 0-t e&- Butyl, trans cis 0-Isopropylf o-CF,
m-Bromo
0.006
312
+2
0-Methyl
0.015 Satd 0.008
0.02 0.03 Satd
{%343
0.01
378 345 366 332 307 277 ? 301d 300
0.01
297
0.02 0.01 0.02
OD
Awta
+403 +365 +227 +227 +122 +76 +118 +78 +111 +86 +51 +85 +51 +73 240 ? +9 290 -+72 261 285 246 269 227
+33 +68 +35 +56 +22 -3 -33g
{E 224
-9 -10 -13
a Awt = cotsubst phenol - a t p h e n o l . * Bracketed frequencies indicate Fermi resonance pairs. CS2 solution values. Polyethylene matrix. e Reference 15. f Reference 14. g Reference 16.
In the case of the meta-substituted phenols, the presence of the meta substituent removes the twofold symmetry of the ring and introduces additional terms in the potential function for internal rotation. However, these additional terms have been shown to be very smallll and the torsional frequency again parallels the Vz barrier to a good approximation. For the ortho-substituted phenols, the barrier becomes considerably more complicated. Steric effects and intramolecular hydrogen bonding result in the existence of cis and trans isomers and perhaps even nonplanarity of the 0-H group in the case of large bulky groups. In these cases, the V I .potential term becomes appreciable9J6 and the correlation between the Vz barrier and the torsional frequency should no longer hold. However, we have found empirically that the Aut values for the ortho substituents is still a useful parameter as will be shown below. In cases where the torsional frequencies for both the cis and trans forms could be observed, Aut values for both forms are given.
Campagnaro and Wood3 pointed out that the torsional parameters they obtained for 4-substituted benzaldehydes and for a limited number of 4-substituted phenols correlated well with the various Hammett type parameters for the substituents and particularly with the I9F shielding parameter derived from nmr data on fluorobenzene. The comparison of our Awt values with appropriate Hammett u values and with available 19F shielding parameters is given in Table 111. Examination of the data in Table I11 shows no good correlation between our Awt values and the 19F parameters. Although the correlation for the most powerful electron-withdrawing substituents is reasonable (e.g., NOz and OH for para substituents; NO2 and t - Bu for meta substituents), there are many substituents for which the agreement is very poor, and it would appear that the conclusions reached by Campagnaro and Wood are not substantiated by our data. The lack of agreement between our Aut substituent values and the corresponding Hammett substituent conThe Journal of Physical Chemistry, Vol. 79, No. 3, 1975
W . G.
202
Fateley, G. L. Carlson, and F. F. Bentley
TABLE 111: Comparison of A u t (cm-1) Values with Substituent Constants for Substituted Phenolsa P a r a substituents Substituent hut, cm-l
Sch unUa
19 F,,,
CHO CN
+40 +39 +33
+1.24b +1.03b f0.88'
-9.4
CF, I
+24 +3
C6H5
0 0
NOz
H
CH,S
-3
Meta substituents (l9FpWa - stit19 Fmeta) uent A u t , cm-l
-9.3 -9.2
-5.8 -8.1 -6.4
NO2 F OH
+0.54 +0.30
-5.1 +1.5
t3.0 +3.9
tO.O1 0
t2.9 0
+0.21
Dmeta
Ortho substituents Substit19Fmeta uent
A u t , Cm"'
+11 +8 +8
+0.71 +0.34 +0.10
-3.5 -1.3
CHO NO2 CH,O
OCH, CF,
+7 +7
f0.14 +0.47
-1.1 -2.1
C1
CN I
+6 +3
+0.61 t0.35
-2.8 -2.4
{ Zt"
Br
{
c1
+2
+0.37
-2.0
CBH,
-1.3
Uoorthob
t0.75 +1.24
+403 c +365 c
r:: t"
0
+0.68
z t"
+0.70
{2;
0
c
c1
Br t-BU CH, F
CH,O OH a
-7 -7
-9 -12
-30
+0.23 +0.27 -0.20
+3.1 +2.5
-0.17 +0.06 -0.1lb -0.37
+5.4 +6.8 +11.5 +10.8
+5.1 +4.8
+0.39
+2 +2 +2
Br CH,
CHO
-0.07
+0.36
-2.3 $1.2 -1.3
{ Zt" { Zt"
I
F +4.2 +9.8 +12.6 +12.1
C6H5
+1
t-BU
-2
+0.06 -0.10
t-BU
-3 t
-41 CH, -13 -44 values from ref 20.19Fvalues from ref 21. * Indicates special values for phenols. c and t refer to cis and trans forms.
A
w
~
+ ) (Awt)Fpara ~ ~ ~
The Journal of Physical Chemistry, Vol. 79, No. 3, 1975
~
+0.54 -0.52 -0.13
Since o- chlorophenol has cis and trans isomeric forms, it should also be possible to use (Aut)&rtho values for the cis and trans forms to predict both torsional frequencies for the 2-chloro-4-fluorophenol molecule: cis form utCalCd = 310 (+86) (-30) = 366 cm-l; trans form utCalCd = 310 (+51) (-30) = 331 cm-I. The observed torsional frequencies for this molecule are 370 (cis) and 336 (trans) cm-l. Thus in the case of a substituted phenol free of steric effects, we find good agreement between observation and prediction. A similar calculation has been carried out for all of the phenols givenzzand the calculated torsional frequencies are compared with the observed. For many of the phenols, the agreement between calculation and observation is quite good. One problem which has not yet been resolved is the calculation of the torsional frequency for 2,6 or 3,5 disubstitution. To illustrate, consider the following examples. ( 1 ) 2,6-Dibromophenol ortho - mtPheno1 + (Aw,),, Here we would not know whether to add the Aut contribution of one Br, both Br's, or the cis Aut value for one Br and the trans of the other. Since the observed torsion is at 395 cm-1, in order to bring our calculated value into agreement we only can use the Aut value (+85 cm-l) of a single cis bromine substituent. This same result appears to hold for all cases of symmetrical 2,6 disubstitution. ( 2 ) 2-Methyl-6-tert-butylphenol.In this case, it would appear that we should combine the contribution of the 2methyl group with the trans Aut value of the tert- butyl group because steric factors would favor the 0-H being trans to the bulkier group:
stants may not be unexpected. The Aut parameter is reflecting the effect of a ring substituent on the a-electron density at the 1 position of the aromatic ring of the phenol molecule while the Hammett r constant is a measure of the substituent effect on the ionization of the proton from the -OH group in benzoic acid or phenol. On the'other hand, the 19Fshielding parameter depends upon electron density in the very close vicinity of the 19F nucleusz1 and should perhaps correlate better with the Aut values. I t is, however, not the purpose of this paper to resolve the relationship between substituent effects on a-electron density in the aromatic ring and on reactivity constants. We feel, and calculation has shown,4 that the new parameter we are presenting is an accurate measure of the effect of substituents on the a-electron density in the aromatic ring. Multisubstituted Phenols. In order to study the effect of multiple substitution on the electron density in the aromatic ring, the phenolic-OH torsional frequency was measured for a large number of substituted phenols.zz The results of this study indicate that, in the absence of steric effects, the net effect of several ring substituents can be predicted from the A u t value for each substituent. If the substituents exert linear mesomeric and inductive effects, i.e., if each substituent in a given position always influences the a-electron density with the same magnitude, then it should be possible to use the Aut values in Table I1 to predict the torsional frequencies and barriers for multisubstituted phenols. For example, the torsional frequency of 2-chloro-4-fluorophenol should be the linear combination of the A u t values of the 2-chloro and 4-fluOrO substituents. T o predict the torsional frequency for this molecule, it is only necessary to add these Aut values to the torsional frequency of phenol: = utPheno1 + (
4-0.63
+
+
WtCalCd
~
~
~
+
+
= UtPheno1
+
- 310
ortho 4
(-13)
A
4-
(AWt)t-Bu
o r t h o , t rans
(-3) = 294 cm"
*-Electron Distortions in Aromatic Systems
203
The observed value is 297 cm-1 which would indicate that only the methyl contribution should be used, but the accuracy of measurement is probably not sufficient to determine this unequivocally. Thus in these cases, the calculated values22 are given as the range of frequencies for one or both contributions. ( 3 ) 3,5-Dimethylphenol and 3-Chloro-5-Methoxyphen01. As in example 2 above, it is difficult to decide whether contributions from both the 3 and 5 substituents should be included. However, the Aut values for most meta substituents are so small that they have only a small effect on the calculated value. It also gppears that the influence of an 0- nitro group on the phenolic-OH torsional frequency (Aut = 365 cm-l) is so large that it generally is the dominant factor in determining the torsional frequency regardless of other substituents on the ring. It can be seen from the data in miniprint material that most of the multisubstituted phenols which contain a 2-nitro substituent have a torsional frequency near 675 cm-l. This is presumably because of the strength of the intramolecular hydrogen bond formed by the OH rotor to the o-nitro group. It is noted, however, that para groups with large Aut values, e.g., F and NO2, do exert their influence on the observed frequency of the 2,4-dinitro- and 2nitro-4-fluorophenols. Steric inhibition of resonance also appears to manifest itself in the position of the observed torsion. Examples of this are WtObSd
3-Trifluoromethyl-4-nitrophenol
3,5- Dimethyl- 4-nitrophenol 3,6-Di- tert-butyl- 4-nitrophenol
344 331 329
utCa1cd
357 352-354 345
The raticlnale here is that the substituents in the 3 and 5 positions prevent complete coplanarity of the NO2 group, and thus inhibit its full contribution to the torsional frequency. The agreement between the observed and calculated torsional frequencies becomes poorer as the ring becomes
more fully substituted. The reason for this is not fully understood at this time, and the unavailability of appropriate model compounds has prevented further study. The data for tetra- and pentafluorophenol are representative of this situation: utCa1Cd
2,3,5,6- Tetrafluorophenol Pentafluorophenyl
374
344
utobsd
348 316
(The utCalCd values include only one o-fluoro and one mfluoro contribution.) The point of interest is that the utobsd values for the two molecules differ by almost the same amount as the calculated values (30 cm-l) which is the contribution of the 4-flUOrO group. Thus some combination of substitution in the 2, 3, 5, and 6 positions must be causing the descrepancy between calculation and observation. A variety of di- and trifluorophenols would be very useful in gaining insight into the effects of a large number of substituents on the electron density in the ring, but we have been unable to locate a source of these compounds. The important consideration in studies dealing with multisubstituted phenols is the ability of the torsional frequency to depict the electronic character of the molecule. If the factors contributing to the position of the torsional frequency can be rationalized, agreement between the calculated and observed values in multisubstituted molecules may be achieved. This should lead to a better understanding of the chemical properties of such systems. Conclusion The accumulation of a large amount of data on torsional frequencies in substituted phenols has suggested that the phenolic-OH torsional frequency can be used as a sensitive probe for measuring substituent effects on r-electron densities in aromatic rings. A new substituent parameter, Aut, has been proposed, and its potential usefulness in gaining information on structural effects on electronic distribution in molecules and of the effects of these distributions on various observed properties has been suggested. The Journal of Physical Chemistry, Vol. 79, No. 3, 1975
K. Tabayashi, 0. Kajimoto, and T. Fueno
204
Acknowledgment. Partial support for this work was provided by the United States Air Force, Wright-Patterson Air Force Base, Contract No. F 33615-71-C-1157, and Carnegie-Mellon University. One of the authors (W.G.F.) acknowledge NSF Grant No. 1275 in support of this research. The authors are also grateful to Dr. A.W. Baker of the Dow Chemical Co. for providing some of the samples used in this study. Miniprint Material Available. Full-sized photocopies of the miniprinted material from this paper only or microfiche (105 X 148 mm, 24X reduction, negatives) containing all of the miniprinted and supplementary material for the papers in this issue may be obtained from the Journals Department, American Chemical Society, 1155 16th St., N.W., Washington, D. C. 20036. Remit check or money order for $3.00 for photocopy or $2.00 for microfiche, referring to code number JPC-75-199. References and Notes (1) A R. Katritzky and R. D. Topsom, Angew. Chem., int. Ed. Engl., 9, 87 (1970). (2) F. A. Miller, W. G. Fateley, and R. E. Witkowski, Spectrochim Acta, Sect. A, 23, 891 (1967).
(3) G. E. Campagnaro and J. L. Wood, J. Mol. Structure, 6, 117 (1970). (4) L. Radom, W. J. Hehre, J. A. Pople, G. L. Carlson, and W. G. Fateley. J. Chem. Soc., Chem. Commun., 308 (1972). ( 5 ) G. L. Carlson, W. G. Fateley, and F. F. Bentley, Spectrochim. Acta, Sect. A, 28, 177 (1972). (6) V. von Keussler, Z.Electrochem., 58, 136 (1954). (7) J. C. Dearden and W. F. Forbes, Can. J. Chem., 37, 1294 (1959). (8) J. C. Evans, Specfrochim.Acta, 16, 1382 (1960). (9) G. L. Carlson, W. G. Fateley, A. S.Manocha, and F. F. Bentiey, J. Phys. Chem., 76, 1553 (1972). ( I O ) W. G.Fateley, F. A. Miller, and R. E. Witkowski, Technical Documentary Report No. AFML-TR-66-408, Jan 1967. (11) A. S.Manocha, G. L. Carlson, and W. G. Fateley, J. Phys. Chem., 77, 2094 (1973). (12) (a) J. H. S. Green, D. J. Harrison, and W. Kynaston, Specfrochim. Acta, Sect. A, 27, 2199 (1971); (b) E. M. Woolley and L. G. Hepler, J. Phys. Chem., 76, 3058 (1972). (13) J. H. S. Green, D. J. Harrison, and W. Kynaston, Spectrochim. Acta, Sect. A, 28, 33 (1972). (14) R. J. Jakobsen and J. W. Brasch, Spectrochim.Acta, 21, 1753 (1965). (15) R. A. Nyquist, Spectrochim.Acta, 19, 1855 (1963). (16) G. L. Carlson and W. G. Fateley, J. Phys. Chem., 77, 1157 (1973). (17) H. D. Bist, J. C. D. Brand, and D. R. Williams, J. Mol. Specfrosc., 24, 402 (1967). (18) W. J. Hehre, L. Radom, and J. A. Pople, J. Amer. Chem. Soc., 94, 1496 (1972). (19) T. Kojima, J. Phys. SOC.Jap., 15, 284 (1960). (20) G. B. Barlin and D. D. Perrin in “Elucidation of Organic Structures by Physical and Chemical Methods,” Part I, Wiley-lnterscience, New York, N.Y., 1972, Chapter IX, pp 638-639. (21) R. W. Taft, Jr., J. Phys. Chem., 64, 1805 (1960). (22) See paragraph at end of text regarding miniprint material.
Thermal Dissociation of Cyanogen Bromide in Shock Waves K. Tabayashi, 0. Kajimoto, and T. FuenoI Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka,Osaka 560, Japan (Received September 4, 1973; Revised Manuscript Received August 22, 1974)
The thermal dissociation of cyanogen bromide diluted in argon was studied behind incident shock waves over the temperature range 2200-3600’K. The course of the dissociation was followed for 0.2-1% BrCN by monitoring the CN (0-1) violet absorption centered a t 4216 A. The initial slopes of absorption gave the bimolecular dissociation rate constants which are represented by k = [1.84 X 1012/1.5!]T1/2(E/RT)1.5 exp(-E/RT) cm3 mol-l sec-l, with E = D O(Br-CN) = 81.6 kcal/mol. The overall absorption profile was somewhat complex at lower temperatures, indicating the occurrence of a rapid homorecombination of CN to form C2N2 as well as chain reactions involving Br and CN as chain carriers. Plausibility of the overall reaction scheme assumed was checked by computer integrations of a set of relevant rate equations. I t was confirmed that the initial slope for the time-concentration curves of CN radicals is least affected by the various subsidiary reactions.
I. Introduction Kinetics of the bimolecular dissociation of cyanogen bromide in shock waves BrCN
+
M
-
Br
f
CN
f
M
(1)
has already been investigated by three groups of workers. 1-3 Patterson and Greenel first determined the dissociation rate of BrCN in argon, by monitoring the CN (0-1) B2Z+ X2E+ emission a t temperatures between 2600 and 4200’K. They obtained second-order rate constants k 1 which were fitted by a simple collisional expression having the activation parameter E , = 90.5 kcal/mol. Kayes and
-
The Journal of Physical Chemistry, Voi. 79, No. 3, 1975
Levitt2 reinvestigated profiles of the same CN emission in greater detail and showed that, over the temperature range 2000-4000’K, their values of K 1 are nearly two orders of magnitude greater than those reported by Patterson and Greene. Further, the temperature coefficient ( E a) of k 1 was found to be 1.5RT less than the bond dissociation energy, D 0 (Br-CN) = 82 kcal/mol. Recently, Clark, Dove, and Finkelman3 studied the BrCN decomposition in neon and krypton, by means of the time-of-flight mass-spectrometric determination of the BrCN concentrations. The lzl values which they suggested for the BrCN-Kr system a t 2100-2900’K were of the same order of magnitude as those obtained by Kayes and Levitt
