J . Chem. SOC., Faraday Trans. I, 1984, 80, 553-561 Phenol-Amine Hydrogen Bonds with Large Proton Polarizabili ties Position of the OH. - . N * O - - * .H+N Equilibrium as a Function of the Donor and Acceptor BY GUNNAR ALBRECHT AND GEORG ZUNDEL* Institut fur Physikalische Chemie der Universitat Miinchen, D-8000 Miinchen 2, Federal Republic of Germany Received 9th May, 1983 1 : 1 mixtures of octylamine + chlorophenols and pentachlorophenol + aromatic amines have been studied in CC1, solution using infrared spectroscopy. It has been shown by conductivity measurements that no charged species are present in these systems. Therefore only the association constants K,, and the equilibrium constants, KpT, of proton transfer in the equilibrium OH * . - N 0- . . * H+N need be considered. Linear relations exist between log K, and the ApK, values for both families of systems.Log KPT increases with both systems in proportion to ApK,, i.e. the Huyskens equation is valid. This increase is caused by a decrease in the enthalpy term Ah$ due to increasing acidities of the donors or (in the other family of systems) increasing basicities of the acceptors. In addition to this direct effect, an increase in the size of the negative A 3 values due to the increasing interaction of these hydrogen bonds with their environment is responsible for this shift in the equilibrium. ApKS,O% amounts to 3.6 in the case of the family containing the aliphatic amine and 1.6 in the case of the family containing the aromatic amines, since with the latter the charge is spread over a more extended region if the proton approaches the N atom.The intensity of the continuum (and thus the proton polarizability) is largest if both proton-limiting structures OH . . . N and 0- * * H+N have almost the same probability. INTRODUCTION 0- - - H+N in solutions were first studied by Barrow and Yerger' using infrared spectroscopy. They studied such equilibria with carboxylic acid + amine systems in chlorinated hydrocarbons. Phenol + amine systems in CD,CN were investigated in ref. (2), and poly-L-lysine films with various phenols by Kristof and ZundeL3 It was shown in these studies2?, that if both proton-limiting structures have noticeable probability these hydrogen bonds show a large proton p~larizability~-~ due to proton motion, as indicated by the occurrence of continua in the infrared spectra.Furthermore, analogous intramolecular OH - - - N f 0- - - H+N bonds have been studied by various groups7-l0 with Mannich bases using U.V. and i.r. spectroscopy. Huyskens and Zeegers-Huyskensll have shown that the following equation is valid for the proton-transfer equilibrium constant KPT (1) where ApK, is the pK, of the protonated base minus the pK, of the acid and < and 6 are constants which are different for each family of systems. A family is a set of chemical compounds which possess the same donor and acceptor groups. The 553 Proton-transfer equilibria OH - - - N log Kp, = CApK, - 6554 PHENOL-AMINE HYDROGEN BONDS compounds have different ply, values because they possess different substituents ; however, these substituents show similar interaction properties with their environments.S is a function of the solvent in which the hydrogen bonds are present. It has been shown by various group^^^-^^ that the dipole moment of such OH * - N f 0- * * H+N bonds increases by 8-12 D* if the polar structure is realized. The influence of the environment on such hydrogen bonds is determined by the interaction of the hydrogen-bond dipole with the reaction field induced in the solvent, as demonstrated by Fritsch and ZundeP for phenol + amine systems in various solvents. In addition, specific interactions of these hydrogen bonds with their environments may influence the position of the proton-transfer equilibrium.l6* l7 Thus the thermodynamic properties determining the equilibrium are influenced by the properties of the donors and acceptors, and particularly by the interaction of these hydrogen bonds with their environments.Thus, as discussed in ref. (1 7), AH," + AH; AS; +AS; + R lnly,, = - RT whereby A= and A g are determined by the acidity and basicity of the donor and acceptor, and AG and A% by the interaction of the hydrogen bonds with their environments. In ref. (2) it was not taken into account that the proton-transfer equilibria AH - BAA-. 7 . . H+B are one stage in a sequence of various equilibria: Ka KPT Kd Kh AH+B$(AH * * * B A----H+B)SA-+H+B AH+B (AH.. . A - e - A . . . HA)+(B+H - - - B e B - - - H+B). (3) All these equilibria will be considered in this study. Furthermore, with two families of systems we will consider the influence of the acidity of the donor and the basicity of the acceptor on A%.Furthermore, we will compare a family of compounds having an aliphatic amine with one containing various aromatic amines. EXPERIMENTAL The substances were purchased from Fluka AG (Switzerland), EGA and Merck (Federal Republic of Germany). In every case, substances with the highest degree of purity available were used. CC1, (for spectroscop ) from Merck, Darmstadt (Federal Republic of Germany) dichloro-substituted phenols were dried over P,O,. All solutions were prepared in a water-free glove box. The concentrations of the donors and acceptors in the solution were 0.1 mol dm-3. For the i.r. investigations, cells with NaCl windows were used (layer thickness 0.2 or 0.5 mm). The solvent bands were compensated by a cell of variable layer thickness filled with pure solvent in the reference beam.The temperature of the samples was 298 K: The spectra were taken with a spectrophotometer (model 325, Bodenseewerk Perkin-Elmer, oberlingen, Federal Republic of Germany). It was flushed with dry and C0,-free air. The equilibria were determined by evaluating the bands corresponding to the donors or acceptors as described in the next section. The sigmoid curves shown later in fig. 3(b) and 4(b) are calculated from the curves in fig. 3(a) and 4(a) using the relation was used and was dried over 3 w molecular sieves, as were the pyridines. The mono- and 100 KPT percentage proton transfer = ~ KPT + 1 The pK, values were taken from ref. (1 8) and (19). * 1 D x 3.3356 x 10-lo C m.G.ALBRECHT AND G. ZUNDEL 555 0 4000 3200 2400 2000 1600 1200 800 wavenum ber/cm-' Fig. 1. (a) Infrared spectra of (-) 2,3,4-trichlorophenol+ octylamine in CCl,, concentration 0.246 mol dmV3, layer thickness 0.528 mm; (----) 2,3,4trichlorophenol in CCl,, concentra- tion 0.246 mol dm-3, layer thickness 0.528 mm. (b) Infrared spectra of (-) 3,5-lutidine +pentachlorophenol in CCl,, concentration 0.05 mol dm-3, layer thickness 0.5 mm; (- - - -) pentachlorophenol, concentration 0.05 mol/dm3, layer thickness 0.5 mm. The absorbance of the continuum was evaluated at ca. 1800 cm-l, i.e. in a region in which no bands are present. The values were referred to 1 mol dm-3 of absorbing hydrogen bonds in a reference layer of 1 cm thickness. RESULTS AND DISCUSSION Fig.1 shows i.r. spectra for examples of both families of systems, octylamine+ chlorophenols and pentachlorophenol+ aromatic amines. In the i.r. spectra of both examples intense continua are found, indicating that hydrogen bonds with large proton polarizabilities are formed between phenols and amines. As we shall see in the following, with these systems, both proton-limiting structures of the proton-transfer equilibria OH - 0 - N g 0 - * * * H+N have a significant probability. To clarify whether all species in the above-mentioned sequence of equilibria must be considered, we first performed conductivity measurements. For all systems studied, these measurements showed that the conductivity of the solutions is (within the limits of experimental error) not higher than the conductivity of the pure solvents.This result excludes the presence of all charged species. Thus we can limit our studies to the formation constant, K,, of OH * - N S O - - - - H+N complexes, and to the constant, KpT, describing the position of the proton-transfer equilibria within these hydrogen bonds. These data are summarized in table 1.556 PHENOL-AMINE HYDROGEN BONDS Table 1. Data for the association and the proton-transfer equilibria ~~ ~~ ~~ ~ ~ 1 2 3 4 5 6 7 8 donor acceptor K, ApK, KPT pGo% 6 4-chlorophenol 3-chlorophenol 3,4-dichlorophenol 3,5-dichlorophenol 2,4-dichlorophenol 2,3-dichlorophenol 2,3,4-trichlorop hen01 2,4,5-trichlorophenol 2,3,5-trichlorophenol 2,4,6-trichlorophenol pentachlorophenol 41 1.28 0.0 52 1.68 0.01 76 2.03 0.01 86 2.40 0.05 36 2.75 0.82 octylamine 39 2.94 0.63 58 3.68 1.30 85 3.93 2.51 62 4.22 2.51 86 4.66 5.90 - 133 5.91 3.6 0.92 3.29 pyridine 97 0.51 3-picoline 283 0.96 pentachlorophenol 2097 1.36 2,4-lutidine 185 1.96 1.73 2,4,6-collidine 227 2.76 6.8 1 1.6 0.75 1.22 ASSOCIATION EQUILIBRIA The association constants, K,, are determined from the integrated absorbance of the stretching vibration observed in the region 3610-3520 cm-l of non-hydrogen- bonded OH groups of phenols (see fig.1). This band is calibrated using the same band in the solution of the pure phenols. The K, values are given in table 1, column 3. They are shown in fig. 2 as a function of ApK, for both families of systems. With both families, two linear relations between log K, and ApK, are found, one in systems of ortho-substituted phenols and another for other phenols. When a donor-acceptor combination of the same ApK, is compared, log K, is smaller if a chlorine atom is present in the ortho-position of the phenol [fig. 2(a)], since with these phenols intermolecular association is reduced by the formation of intramolecular OH - * * C1 hydrogen bonds.Furthermore, with donor-acceptor combinations having the same ApK,, log K, is smaller if a CH, group is present in the ortho-position [points 4 and 5 in fig. 2(b)], since with these amines the intermolecular association may be slightly hindered sterically . PROTON-TRANSFER EQUILIBRIA The constants KPT for proton transfer in the OH - N e 0 - - - H+N equilibria are determined as follows: For octylamine + chlorophenol systems the concentration of the polar structure 0- - - - H+N was determined from the integrated absorbance of the antisymmetrical bending vibrations of the NHZ group at ca.1620 cm-l [see fig. 1 (a)]. This band was calibrated from the integrated absorbance of the corresponding band for the octylamine + pentachlorophenol complex. With this system, all protons are present on the amine molecule. This is demonstrated by the fact that the phenol band at 1185 cm-l is no longer observed in this system, whereas the phenolate band at 1210 em-' is very intense. The overall concentrations of the phenol + amine complexes were obtained from the respective K, values.G. ALBRECHT AND G. ZUNDEL 557 2 . 0 1 . 5 0 1 2 3 A PK, Fig. 2. Logarithm of the association constants K, plotted as function of ApK,.(a) Systems comprising octylamine + substituted phenols : (1) 4-chlorophenol, (2) 3-chlorophenol, (3) 3,4-dichlorophenol, (4) 3,5-dichlorophenol, (5) 2,4-dichlorophenol, (6) 2,3-dichlorophenol, (7) 2,3,4-trichlorophenol, (8) 2,4,5-trichlorophenol, (9) 2,3,5-trichlorophenol, (10) 2,4,6-trichloro- phenol, (1 1) pentachlorophenol. (b) Systems comprising pentachlorophenol + substituted pyridines : (1) pyridine, (2) 3-picoline, (3) 3,5-lutidine, (4) 2,4-lutidine, (5) 2,4,6-collidine. For the pentachlorophenol + aromatic amine systems, the concentration of the non-polar OH N structure was determined from the band at 1185 cm-l [see fig. l(b)]. This band, which shows large 6(OH) character, was calibrated with the pentachlorophenol + pyridine system. With this system, all protons are present on the chlorophenol molecule, as demonstrated by the fact that the phenolate band at 1215 cm-l is no longer observed. The overall concentrations of the phenol+amine complexes were obtained from the K, values.The KPT values of both families of systems are given in table 1, column 5. For the octylamine+chlorophenol systems, log KPT is shown in fig. 3(a) and the percentage proton transfer in fig. 3(b) as a function of ApK,. The corresponding plots for the chlorophenol+aromatic amine systems are shown in fig. 4(a) and (b). Fig. 3(a) and 4(a) show that the Huyskens relation eqn (1) is valid to good approximation for both families of systems. The intersections of the curves with the abscissa in the figures gives a value of Apeo% of 3.6 for the family of systems with the.aliphatic amine and 1.6 for the family with the aromatic amines.These values are given in table 1, column 6 and the respective < and 6 values, also obtained from these figures, in columns 7 and 8. The A% values are usually positive and relatively large, since in the gas phase the AH * - - BAA- T- - - - H+B equilibrium usually lies to the left. In liquids these equilibria become more or less strongly shifted to the right-hand side since 6% is overcompensated by A%, a term arising from the interaction of the hydrogen bonds with their environments. This term is always negative and relatively large, shifting the equilibria to the right-hand side. In addition, this term overcompensates the influence of the term ASO, which is negative with such equilibria'~ lo* 2ov 21 due to the increasing558 2 0 .1 1 I I I 1 - U L 5 0 , - 2 2 5 1 5 - 6,” pa (c) - - 2 - 2 a 3 4 s ‘, - b9 ‘\ 10 11 - s m 41 \ =--*- - 5 - 1 1 1 I 1 1 , PHENOL-AMINE HYDROGEN BONDS 2 0 . I I I 1 8 1 1 I I - 05,- 7 15 - yrl ( d , - 21 E * 1- g > g 10 : A 4 j 4 sip -; 2 \ ee,, 11 1 8-- - --@ - 5 - I l l l l l l l l l - 0 1 2 3 4 5 6 7 order in the solvent around the polar structure, which also shifts the equilibrium to the left-hand side. All these viewpoints are discussed in detail in ref. (1 7). In the octylamine + chlorophenol family, Kp,increases on going from 4-chlorophenol to 2,4,6-trichlorophenol (table 1, column 5). This increase in KPT is caused by a decrease in AI$ due to increasing acidity of the phenols.In addition, if the equilibrium becomes increasingly shifted to the right-hand side, the magnitude of the negative A% value also increases, since the interaction of the hydrogen bonds with their environments increases. By this effect, the equilibrium is also shifted to the right-hand side. Thus, increasing acidity of the acceptors favours a shift of the equilibria to the right-hand side, directly via 6% and indirectly via A%. With the pentachlorophenol + aromatic amine family, KPT increases on going from 3-picoline to 2,4,6-collidine (table 1, column 5). This increase in KPT is caused by a decrease in A G due to the increasing basicity of the hydrogen-bond acceptors. In addition, if the equilibrium becomes increasingly shifted to the right-hand side the magnitude of the negative A% value also increases.Hence, owing to this indirect effect, the equilibrium is also shifted to the right-hand side on increasing the basicity of the acceptors. ApK5,OX is the ApK, value at which both proton-limiting structures OH - - - N .C- 0- - - H+N have the same probability. For the family of systems containing an aliphatic amine, ApGo% is 3.6 and for systems containing the aromatic amines its value is 1.6.G. ALBRECHT AND G. ZUNDEL 559 1 0 - I - 2 0 1 2 3 APK, 100 80 2 6 0 E 40 h 5 m G 0 U g 20 0 - 1 0 1 2 3 4 APKa Fig. 4 (a) Log KPT of systems comprising pentac lorophenol+ substituted pyridines ploll:d as a function of ApK, : (1) pyridine, (2) 3-picoline, (3) 3,5-lutidine, (4) 2,4-lutidine, (5) 2,4,6-collidine.(b) Percentage proton transfer of the same systems plotted as a function of ApK,. (c) and (d) Absorbance of the continuum for the same systems plotted as function of ApK, and log KpT, respectively. This result demonstrates that the aromatic character of the amines favours proton transfer to the amines, i.e. A% decreases. This result is understandable since aromatic character leads to the positive charge being delocalized. In fig. 3(c) and ( d ) and 4(c) and ( d ) the absorbance of the continua is shown as a function of ApK, and log KPT. These figures show that the i.r. continua are most intense, and thus that the proton polarizability of these hydrogen bonds is largest in the region in which both proton-limiting structures have similar weight. This result is understandable, since in this case the fluctuation of the proton is largest.With the octylamine +chlorophenol family, however, a small shift of the intensity maximum of the continuum towards log KPT < 0 is observed. This result can be explained by the following considerations. The proton polarizability is determined by the proton potential and hence by the enthalpy term. The position of the equilibrium, however, is determined by the free energy, and thus by both enthalpy and entropy terms. With such proton-transfer equilibria, A 9 is negative8-l09 20, 21 owing to the large degree of order in the solvent around the polar proton-limiting structure. Thus this entropy term shifts the equilibrium to the left-hand side. Hence the proton potential is, on average, already symmetrical (AH0 = 0) before both limiting structures have the same probability (AGO = 0).This consideration may explain the slight shift in the intensity maximum of the continuum towards KPT < 1.560 PHENOL-AMINE HYDROGEN BONDS CONCLUSIONS With the two families of systems, octylamine + chlorophenols and pentachloro- phenol + aromatic amines, no charged species is found in CC1, solutions. The hydrogen bonds formed between the phenols and amines show large proton polarizabilities, as indicated by continua in the i.r. spectra. Linear relations exist between the logarithm of the complex formation constants, log K, and ApK, (pK, of the protonated base minus pK, of the acid). Log K, increases with increasing ApK,. These relations are different, however, when substituents are present in the ortho position.With the chlorophenols the formation of intramolecular OH - - - C1 bonds reduces complex formation. The same is true with methyl substituents in the ortho position of the N-bases, owing to steric hindrance. Log KpT, the logarithm of the OH - - * N +O- - - - H+N proton-transfer equilibrium constant, increases with both families of systems in proportion to the ApK, values, i.e. the Huyskens equation is valid. This increase in log KPT is caused by a decrease in the enthalpy term A%, resulting from the increasing acidity of the donors, or in the other family of systems, from the increasing basicity of the acceptors. This direct effect of the acidities and basicities is enhanced by an indirect effect, since the amounts of the negative A% values increase, too, due to the increasing interaction of the hydrogen bonds with their environment, if the probability of the polar structure increases.The ApK, value at which both proton-limiting structures have the same probability (ApK5,Ox) is 3.6 for the family containing the aliphatic amine and 1.6 for that containing the aromatic amines. 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