1188
J. Phys. Chem. 1983, 87, 1188-1191
reasonable when one considers that the proton is vibrating in a highly asymmetric potential as a result of probable electron densities in 1* and that energy flow from normal modes in the region of a highly anharmonic potentials is especially rapid. This situation, interestingly, is similar, in principle, to polyatomic molecules undergoing unimolecular thermal isomerization with an energy content comparable to the “barrier height” for reaction. Other Mechanisms. It was recently proposed by Makagaki et a l l 2 that an additional intermediate, which they labeled “X” is involved in the 1* - 2* isomerization. I* -* “X” -.+ 2* This proposal resulted from an interpretation of their data of the transient spectroscopy of HBT at room temperature in 1:l methylcyclohexane/isopentane,where 2* undergoes a rapid radiationless decay.” These authors envoked the “X”intermediate to explain the discrepancy between the rapid excited-state radiationless decay indicated by transient absorption measurements (- 50 ps) and the slower decay (-500 ps) they observe from emission measurements. Recent lifetime measurements for HBT at the same temperature and in the same solvent, however, show the fluorescence lifetime of HBT to be -100 ps.5b Our observations verify this latter result.
Considering the inherent inaccuracy of the transient absorption measurement, which was apparently not analyzed by deconvolution methods of the instrument response function, it is likely that transient absorption and emissions are, in fact, in agreement within experimental error. The “X” intermediate is, therefore, an unnecessary modification to the Cohen and Flavian model.
Conclusion and Summary The excitation and time-resolved fluorescence spectroscopy of HBT in argon at 12 K is highly consistent with the mechanism photochemical proposed by Cohen and Flavionsc as modified recently by Barbara et al.5b The unresolvably rapid rate of ESIPT in HBT suggests that ESIPT may be a special type of VR for this molecule. A simple analysis suggests that the rate of ESIPT exceeds the rate of other types of VR in electronically excited HBT by at least an order of magnitude. Acknowledgment. Acknowledgment is made for partial support of this research to the following: the donors of the Petroleum Research Fund, administered by the American Chemical Society, the Research Corporation, and the Graduate School of the University of Minnesota. P.F.B. thanks Professor A. Weller for helpful discussions. Registry No. 2-(2-Hydroxyphenyl)benzothiazole,3411-95-8.
Involvement of Amine Protons in n -Butylamine-Cresol Hydrogen Bonding Akram AI Awar, Mary Codd, Norman Pratt, and Ronald M. Scott’ Department of Chemistty,Eastern Michigan University, Ypsilanti, Michigan 48 197 (Received: March 12, 1982; I n Final Form: September 29, 1982)
Studies of the hydrogen bonding of cresols with n-butylamine by difference spectroscopy in the infrared range reveal participation of protons from both the phenols and the amines. A cyclic structure for the interaction is proposed.
Introduction Phenol-amine hydrogen bonding has been extensively studied, particularly by means of infrared spectrophotometry, NMR spectrometry, and calorimetry.’ Attention has been focused on the influence of phenol acidity and amine basicity, on solvents and solvation effects, and on steric hindrance. In all these investigations it has generally been assumed that the interaction consists solely of the attraction of the unshared electrons of the amine nitrogen for the phenolic proton. We have studied a variety of aspects of this system in our laboratory. In a recent calorimetric study2 it was observed that the enthalpy changes when triethylamine and 0-or p-cresol undergo hydrogen bonding were very similar. However, for the interactions between n-butylamine and p-cresol the enthalpy change was significantly larger, while with o-cresol it was sharply lower (Table I). Values for (1) (a) Fritsch, J.; Zundel, G. J. Phys. Chem. 1981, 85, 557. (b) Pimente], G. c.; McClellan, A. L. “The Hydrogen Bond”; W. H. Freeman: San Francisco, 1960. (c) Vinogradov, S. N.; and Linnell, R. H. ‘The Hydrogen Bond”; Van Nostrand-Reinhold: New York, 1970. (2) Kogowski, G.; Scott, R. M.; Filisko, F. J . Phys. Chem. 1980, 84, 2262. 0022-3654/83/2087-1188$0 1.50/0
TABLE I: Previously Reported Parametersu of
Cresol-Amine Hydrogen Bonding enthalpy change, phenol amine kcalimol o-cresol o-cresol p-cresol p-cresol a
log Keq
triethylamine
-7.35 t 0.35 1.67 -5.20 2 0.24 1.90 -7.61 t 0.36 1.78 -8.61 I0.41 1.78 Enthalpy changes are from ref 2 and log K,, values
n-butylamine triethylamine n-butylamine
from ref 3.
log K for these same reactions reveal a less dramatic pattern (Table I).3 Previous studies in this laboratory have indicated that the base strengths of these two amines 0- and p-cresol in nonpolar solvents are ~ i m i l a r . Since ~ are very similar in acid strength, and since the inert solvent cyclohexane was used in all these studies, the character of the hydrogen bond itself was suspected to be the cause of the patterns of enthalpy change observed. It is the purpose of this research to investigate this possibility. (3) Farah, L.; Wilson, D.; Giles, G.; Ohno, A.; Scott, R. M. J . Phys. Chem. 1979,83, 2455. (4) Reyes, A.; Scott, R. M. J . Phys. Chem. 1980, 84, 3600.
0 1983 American Chemical Society
The Journal of Physical Chemistry, Voi. 87, No. 7, 1983
n -Butylamine-Cresol Hydrogen Bonding
Experimental Section Materials. Triethylamine and n-butylamine (Eastman reagent grade) were distilled before use and stored in dark bottles under an inert, dry gas. 0- and p-cresol were supplied by Dow Chemical Co. and were purified by sublimation. Carbon tetrachloride (Mallinckrodt Spect AR) and cyclohexane (Eastman Spectra ACS) were used without further purification. Infrared Spectrophotometry. Absorption spectra were taken on a Beckman DK-2A spectrophotometer from 2500 to 3500 nm (4000-2960 cm-l). Cresol stock solutions were prepared by weight in carbon tetrachloride. Difference infrared spectra were obtained by using four matched 1-cm silica cells. Two cells in the reference path contained cresol solution and 20 pL of pure amine in 2 mL of carbon tetrachloride, respectively. In the sample path one cell contained 20 pL of pure amine in 2 mL of cresol solution, while the other cell contained only carbon tetrachloride. Cresol solutions ranged from 0.002 to 0.015 M in concentration. Spectra were taken immediately after mixing to avoid problems with the slow reaction between carbon tetrachloride and the amines. In such spectra we observe only those infrared-absorbing structures which are altered as cresols and amines interact. Those structures which are eliminated are presented as minima and those which are formed appear as maxima. UV-Visible Spectrophotometry. The absorption spectra from 260 to 315 nm were determined by using matched silica cells in a Beckman DK-2A, the temperatures of both sample and reference cells being maintained at 25 "C. All solutions were prepared by weight in cyclohexane. Cresol concentrations ranged from 2 X to 5X M and cells of 0.1 mm, 1 mm, 1 cm, and 10 cm were used as required. For each cresol concentration spectra were recorded without amine and with four or five different amine concentrations. The method of Rose and Drago5was used to calculate equilibrium constants from the absorbances of cresol in the presence of the various amine concentrations. The array of equilibrium constants obtained from each calculation was subjected to Chauvenet's criterion to reject extreme values, and the remaining values were averaged. Data were taken and calculated at three different wavelengths for each experimental procedure. Results and Discussion A preliminary study was performed to ascertain that self hydrogen bonding by the cresols was not present to a significant degree. On the sample side one cell contained undiluted cresol at each of the concentrations studied and the other pure carbon tetrachloride. On the reference side each cell contained the cresol solution diluted 1:l with carbon tetrachloride. The infrared spectra showed no features whatever. The spectra for the interaction by hydrogen-bond formation of p-cresol with triethylamine (Figure 1)show the disappearance of the 0-H stretch absorption at 2750 nm. As the concentration of p-cresol is increased, the changing depth of the minima at 2750 nm reflects increased hydrogen bonding. Correspondingly the peak at 3250 nm, representing the hydrogen-bonded 0-H stretch, increases as absorbance at 2750 nm decreases. The n-butylamine-p-cresol spectrum is quite different (Figure 2). As before, the disappearance of the free hydroxyl group is indicated by a minimum at 2750 nm. In addition, there is a minimum at 2900 nm. By examination ( 5 ) Rose, N. J.; Drago, R.
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c
2500 3000 WAVELENGTH (nml Flgure 2. Infrared difference spectra of p -cresol and n-butyaimine. See Figure 1 for cresol concentrations.
of the spectrum of n-butylamine and in a separate experiment in which the formation of a hydrogen bond between n-butylamine and dimethyl sulfoxide was observed
1190 o.6
The Journal of Physical Chemistty, Vol. 87, No. 7, 1983
AI Awar et ai.
n-butylamine and o-cresol reflects the partial interference of the ortho substituent with the dual hydrogen-bond arrangement. Three models for the arrangement of cresols and amines consistent with the observed data were then postulated. In the first, the bonding involves either the hydroxyl proton or the amine proton, an equilibrium existing between the two forms (eitherlor model). In the second the nitrogen and oxygen are in close proximity, each with unshared electrons attracting the proton of the other and generating simultaneous nonlinear hydrogen bonds (dimer model). In the third the cresol and amine molecules form an alternating copolymer array, each hydroxyl donating a proton to the amine on one side and accepting a proton from the amine on the other side (copolymer model).
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See Figure 1 for cresol concentrations.
by the difference technique, it was established that the minimum at 2900 nm is due to the disappearance of free N-H. The peak representing the hydrogen-bonded hydroxyl group appeared as in the study with triethylamine, but centered at 3200 nm rather than 3250 nm, and with a shoulder near 3100 nm. These experiments were repeated, o-cresol being substituted for p-cresol. Triethylamine and o-cresol produced spectra virtually identical with those in Figure 1. However n-butylamine and o-cresol (Figure 3), while producing a pattern qualitatively like that of Figure 2, differed in that the minimum for the disappearance of free amine was relatively less intense than was observed with p-cresol. It is our interpretation that the proton of the phenolic hydroxyl group and the proton of the primary amine were each involved in a hydrogen bond. The hydroxyl proton was attracted to nonbonded electrons of the amine nitrogen, and the amine proton similarly was associated with nonbonded electrons of the phenol oxygen. The lesser involvement of the amine protons in the interaction with o-cresol was assumed to be the result of steric hindrance by the o-methyl group. The similar values obtained for the enthalpy change of o-cresol and of p-cresol forming a hydrogen bond with triethylamine reflect the similarity of bonding and of acid strength of the two cresols. The imposition of the single ortho group does not significantly affect hydrogen-bond formation, as previously noted.3 The much higher enthalpy change displayed when n-butylamine and p-cresol undergo hydrogen bonding is the result of the formation of two hydrogen bonds. The fact that the enthalpy change does not roughly double implies that each of the two bonds formed is weaker than is the single hydrogen bond formed between the cresols and triethylamine. This is supported by the smaller shift of the peak for the hydrogen-bonded hydroxyl group from the free-hydroxyl position in the n-butylamine interactions. The lower enthalpy change for
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The eitherlor model explains the disappearance of both free-hydroxyland free-amine stretching absorbances. The observation that the o-cresol-n-butylamine pair displays less involvement of the amine proton than does the p cresol-n-butylamine pair represents a shift in equilibrium because the amine-proton hydrogen bond is more difficult to form in the presence of the o-methyl group. However, given that the equilibrium constants for the cresol-amine interactions are high enough to infer relatively complete pairing and are approximately the same in magnitude, this model predicts that the increased involvement of the amine proton would be accompanied by a noticeably decreased involvement of the hydroxyl proton. Selecting one cresol concentration and comparing the absorbances (depths of minima) for free hydroxyl group in each of the four spectra, one finds them to be approximately the same. Furthermore, the model does not provide an explanation of the enthalpy-change values. These objections were considered serious enough to warrant dropping this model from further consideration. The dimer model requires a geometry different from that usually encountered in phenol-amine hydrogen b ~ n d i n g The . ~ amine approaches the hydroxyl group from the unsubstituted side of the ortho-substituted phenol in a bond involving the hydroxyl proton only, but it must approach along the axis of the oxygen bond to the ring in the dimer model. In this case interference by the ortho substituent is likely, distorting and weakening the bonding pattern. The proposed nonlinear hydrogen bonds correlate with the reduced shift in the position of the peak for hydrogen-bonded hydroxyl when compared to the corresponding peak when triethylamine is employed. A reasonable explanation of the enthalpy data is provided by this model. The copolymer model seems intuitively the more attractive when looking at models since linear hydrogen bonds are possible. However, this model fails to explain the reduced shift of the hydroxyl absorbance on hydrogen bonding. Evidence of a three-molecule phenol-amine complex has been presented.6 This complex consists of (6) Clotman, D.; Van Lerberghe, D.; Zeegers-Huyskens, T. Spectrochim. Acta, Part A 1970, 26, 1621.
n -Butylamine-Cresol Hydrogen Bonding
TABLE 11: Equilibrium Constants for p-Cresol-n-Butylamine Hydrogen Bonding at Varying Concentrations individual log K,, value from pairs of amine concns p-cresol lowest highest av log K,, concn concn concn, M 1.69 1.79 f 0.04 1.66 2.2 x 1.98 1.78 2.1 x 1.90 t 0.02 1.70 1.77 1.79 i. 0.02 4.8 x 2.07 1.83 1.85 t 0.015 2.5 X 1.90 1.70 1.84 f 0.04 2.0 x 1.98 1.78 t 0.10 2.10 2.0 x Dimer assumption.
a phenol hydrogen bonded to a second phenol which in turn is hydrogen bonded to triethylamine. Although not the arrangement that we propose, this work does support the concept of an oligomeric phenol-amine structure in solution maintained by hydrogen bonding. On the basis of this analysis we favored the dimer model. In spite of the attractiveness of forming linear hydrogen bonds in the copolymer model, it may be that the concentrations of the reactants are too low to favor a multimolecular complex. A further test that would distinguish between the dimer and copolymer models is provided by observing the effect of altering p-cresol concentration on the equilibrium mixture. The dimer and copolymer models involve different equilibrium expressions: dimer Keg = (complex)/((cresol)(amine)J copolymer
Keg = (complex)/((cresol)n(amine)nl The equilibrium constants as determined for a 1000-fold range of p-cresol concentration are reasonably uniform when calculated by using the dimer expression (Table 11). Furthermore, the method of calculation generates an equilibrium constant from each possible combination of different amine concentrations and corresponding absorbances. Thus, for an experiment involving five additions of amine, 10 values are generated for the equilibrium constant. Comparing constants generated by low and high amine concentrations reveals no amine concentration ef-
The Journal of Physical Chemistry, Vol. 87, No. 7, 1983
1191
fect. The extreme values, those from the lowest two concentrations and those from the highest two, are recorded in Table 11. These are at greater variance with the average values than most of the other values obtained since they are based on small absorbance differences, but serve to illustrate that there is no simple trend as amine concentration is increased as would be the case if the calculation were based on an incorrect equilibrium constant expression. The combination of several phenols with diethylamine has been studied by means of cryoscopic and dielectric measurements by Ratajczak.' On the basis of these studies a linear A2B2structure is proposed for the hydrogen-bonded complexes resulting from weakly acid phenols, and a cyclic AzB2structure is hypothesized for protontransfer complexes of strongly acidic phenols. These studies were done at higher concentration and lower temperature than ours. In fact, if the trend displayed for formation of the AzBz complex with increasing concentration were extrapolated back to the concentrations described in this report, an AB complex would be predicted to predominate. We conclude that our evidence supports the existence of a cresol-n-butylamine hydrogen-bonded structure that can be described as a dimer wherein both the amine and the hydroxyl protons are involved in nonlinear hydrogenbond structures. This model was presented as the structure of alcohol and of phenol dimers based on the position of the infrared frequency for the hydrogen-bonded 0-H stretch and NMR data.&1° Summary Evidence is presented to support the existence of a complex involving 0-or p-cresol and n-butylamine in which one molecule of each species is joined together by two nonlinear hydrogen bonds, one utilizing the hydroxyl proton and the other an amine proton. Registry No. o-Cresol, 95-48-7; p-cresol, 106-44-5; triethylamine, 121-44-8; n-butylamine, 109-73-9. (7) Oszust, J.; Ratajczak, H. J. Chem. SOC.,Faraday Trans. 1 1981,77, 1209, 1215. ( 8 ) Van Thiel, M.; Becker, E. D.; Pimentel, G. C. J. Chem. Phys. 1957, 27, 95. (9) Liddel, V.; Becker, E. D. Spectrochim. Acta 1957, 10, 70. (10)Becker, E. D.; Liddel, U.; Schoolery,J. N. J. Mol. Spectrosc. 1958, 2, 1. (11) Coggeshall, N. D.; Saier, E. L. J. Am. Chem. SOC.1951, 73,5414.
