J . Phys. Chem. 1986, 90, 964-973
964
GENERAL PHYSICAL CHEMISTRY Proton Potentials and Proton Polarizability in Carboxylic Acid-Trimethylamine Oxlde Hydrogen Bonds as a Function of the Donor and Acceptor Properties: IR Investigations Ulrich Bohner and Georg Zundel* Physikalisch- Chemisches Institut der Universitat, 0-8000Miinchen 2, West Germany (Received: March 13, 1985)
+
Various carboxylic acid trimethylamine oxide (TMAO) systems are studied in acetonitrile solution by IR spectroscopy as a function of the ApK, (pK, of the protonated amine oxide minus pK, of the acid). It is shown that only 1:l acid-base complexes must be taken into account whereas the other species can be neglected. In the case of the four systems with lower ApK,, broad single minimum proton potentials are present, having probably a hump in the potential curve at the acceptor. In the case of the four systems with higher ApK,, typical double minimum potentials occur in the OH-.ON F= -O-H+ON bonds whereby the weight of the polar limiting structure increases with increasing ApK,. This means that the proton is more frequently present at the N-oxide than at the carboxylic acid and the well at the acceptor becomes deeper with increasing = 2.76. The ApK,. The Huyskens relation is valid and both proton-limiting structures have the same weight at ApKaSosb change in shape of the proton potential is caused by increasing acidity of the acid, and especially by the interaction of the dipole of the hydrogen bond with the reaction field induced by it in the solvent. IR continua indicate that all these hydrogen bonds show large proton polarizability if their proton potential is not too asymmetrical. The wavenumber-dependent intensity distribution of the IR continua changes characteristically as a function of the ApK,. With systems with smaller ApK, continua are found in the region 1500-700 cm-I with largest intensity around 1000 cm-’, whereas with systems with higher AK, the continua extend over the whole region below 3000 cm-’ and show a bandlike structure in the region 2700-2200 cm-l, as expected with longer hydrogen bonds with double minimum. The intermolecular hydrogen bond stretching vibration confirms that the hydrogen bonds become weaker if, with increasing ApK,, the shape of the potential becomes a characteristic double minimum. This change in shape of the potential as a function of the ApK, is additionally confirmed by the different behavior with deuteration of the low and high ApK, systems. Furthermore, the influence of the reaction field on the shape of the potential is different for the H and the respective D systems.
I. Introduction It is known that in a long hydrogen bond between relatively weak acids (donors and protonated acceptors with large pK,’s), for instance, in OH--N == O--H+N phenol N base’-5 or carboxylic acid + N base6-8 bonds, double minimum proton potentials may be present (see ref 9, Figure 5, 0-0distance of 2.6 or 2.7 A). Of course, such double minima occur only in a ApK, (pK, of the protonated base minus pK, of the acid) region in which the degree of the asymmetry of the proton potential is not too If the hydrogen bonds become shorter (donors and acceptors with lower pK,), the double minimum changes to a potential having instead of a second minimum a humplike shape at the donor or acceptor (see ref 9, Figure 5,O-O distance of 2.5 A). The residence time at this side of the hydrogen bond becomes shorter but the proton may still be present there. With very short hydrogen bonds broad more or less symmetrical single minimum proton potentials are present; examples are the homoconjugated -F-H+-F ‘ @ I 2 or -O-H+-O- carboxylic acid + carboxylate,’0s12
+
and the heteroconjugated O--H+-ON bonds between trifluoroacetic acid and N - 0 ~ i d e s . l ~All these hydrogen bonds may show a large proton polarizability caused by proton motion if the proton potentials are not too a s y m m e t r i ~ . ~ , ~ J ~ J ~ Caused by their large proton polarizability these hydrogen bonds interact strongly with their environments. Hydrogen bonds with this property cause intense continua in the IR ~ p e c t r and a ~ ~ ~ ~ ~ ~ Rayleigh wings in the Raman spectra.I6 With these interaction effects the interaction of the dipole of the hydrogen bonds’x2 with the reaction field induced by this dipole in the solvent is of particular i m p 0 r t a n ~ e . l ~ In the case of heteroconjugated hydrogen bonds, AH-eB + A-.-H+B, the double minima usually occur only due to the interaction with their environment^.'^^'^ Thus the shape of the (10) I. Olovsson and P. G. Jonsson, “The Hydrogen Bond-Recent Developments in Theory and Experiments”, P. Schuster, G. Zundel, and C. Sandorfy, Eds., Vol: 11, North Holland, Amsterdam, 1976. (1 1) G. C. Pimentel and A. L. McClellan, Annu. Reo. Phys. Chem., 22, 347 (1971). (12) D. Hadii and
(1) H. Ratajczak and L. Sobczyk, J . Chem. Phys., 50, 556 (1979). (2) R. Nouwen and P. Huyskens, J . Mol. Srrucr., 16, 459 (1973). (3) G . !Zundel and A. Nagyrevi, J . Phys. Chem., 82, 685 (1978). (4) G. Albrecht and G. Zundel, J . Chem. SOC.,Faraday Trans. 1.80, 553 1984). ( 5 ) W. Kristof and G. Zundel, Biophys. Sfrucf. Mech., 6, 209 (1980). (6) G. M Barrow, J . A m . Chem. SOC.,78, 5802 (1956). (7) S . L. Johnson, and K. A. Rumon, J . Phys. Chem., 69, 74 (1965). (8) R. Lindemann and G. Zundel, J . Chem. SOC.,Faraday Trans. 2,73, 788 (1977). (9) R. Janoschek, E. G . Weidemann, H . Pfeiffer, and G . Zundel, J . Am. Chem. Soc., 94, 2387 (1972).
0022-3654/86/2090-0964$01.50/0
S.Bratos in “The Hydrogen Bond-Recent Developments in Theory and Experiments”, P. Schuster, G. Zundel, and C. Sandorfy, Eds., Vol. 11, North Holland, Amsterdam, 1976. (13) (a) B. Brycki and M. Szafran, J . Chem. SOC.,Perkin Trans. 2, 223 (1984); (b) U.Bohner and G. Zundel, J . Chem. SOC.,Faraday Trans. I , 81, 1425 (1985). (14) E. G.Weidemann and G . Zundel, Z . Naturforsch. A, 25,627 (1970). (1 5) G. Zundel in “The Hydrogen Bond-Recent Developments in Theory and Experiments”, P. Schuster, G. Zundel, and C. Sandorfy, Eds., Vol. 11, North Holland, Amsterdam, 1976. (16) W. Danninger and G. Zundel, J . Chem. Phys., 74, 2769 (1981). (17) J. Fritsch and G. Zundel, J . Phys. Chem., 85, 556 (1981). (18) J. Fritsch and G. Zundel, J . Phys. Chem., 88, 6295 (1984).
0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 5, 1986 965
IR Study of Carboxylic Acid-TMAO Systems
O
I
7
m
0.7 1.0
--]
I I
Or----
1
" _It
I
i0.2 m 0, 8 0.
%
07 1.0
t--
l
i
1
'
1
c m
0,
$0.
n m
""%tis??
wave number cm -1
Figure 1. IR spectra of trimethylamine oxide (TMAO) of various carboxylic acids (---), and of the respective acid-base complex (-) in CD,CN; spectral range 3750-2750 and 1900-1300 cm-I; concentration 0.1 mol/dm3; layer thickness 96 pm: (a) TMAO (---), CH,COOH (---), CH3COOH + TMAO (-); (b) CHjOCH2COOH (---), CH3OCH2COOH + TMAO (-); (c) CsHSOCH2COOH (---), C6HSOCH2COOH TMAO (-); (d) C1CH2COOH(---), CICH,COOH + TMAO (-); (e) NCCH2COOH (---), NCCH2COOH + TMAO (-); (f) HC=CCOOH (---), HC=CCOOH + TMAO (-); (g) C12CHCOOH (---), C1,CHCOOH + TMAO (-); (h) CF,COOH (---), CF3COOH TMAO (--). (-.-a),
+
+
proton potential is mainly determined by the interaction of these hydrogen bonds with their surroundings. The reaction field is oriented on the mean charge distribution in the hydrogen b ~ n d . ' ~ . ~ With the double minimum proton potential the mean charge distribution has two centers of gravity; hence, the double minimum character increases due to the influence of the reaction field. In contrast to this behavior, with a more or less symmetrical flat single
minimum potential, the mean charge distribution has only one center of gravity. This well becomes deeper due to the influence of the reaction field, i.e., the single minimum character increases. Hence, it is of great interest to study families of systems21122 in which the proton potential may change as a function of the donor property from a broad more or less symmetrical single minimum to a double minimum potential well. With regard to
(19) G. Zundel and J. Fritsch in 'Chemical Physics of Solvation", Vol. 11, R. R. Dogonadze, E. KZlmBn, A. A. Kornyshev, and J. Ulstrup, Eds., Vol. 11, Elsevier, Amsterdam, 1986, Chapter 2. (20) H.Pfeiffer, G. Zundel, and E. G. Weidemann, J. Phys. Chem., 83,
(21) P. Huyskens and Th. Zeegers-Huyskens, J . Chim.Phys., 61, 81 (1964). (22) Th. Zeegers-Huyskens and P. Huyskens in 'Molecular Interactions", H. Ratajczak and W. J. Orville-Thomas, Eds., Vol. 11, Wiley, London, 1980, Chapter I, p 1.
2544 (1979).
966 The Journal of Physical Chemistry, Vol. 90, No. 5, 1986
Bohner and Zundel
Figure 2. (a) IR spectra of CF,COOH Bu4N+CF3C0C(---), and Bu4N+[(CF3CO2),H]-(-) in CD,CN; concentration0.1 mol/dm3; layer thickness 96 wm. (b) IR spectra of (CH3)3N0(---), (CH3),NOH+C104-(-), and [(CH3)3N0]2H+C10;(-.-.) in CD3CN; concentration 0.1 mol/dm3; layer thickness 96 wm. (-.-a),
the pK, values, a behavior like this could be expected for the family of systems trimethylamine oxide (TMAO) with various aliphatic carboxylic acids in acetonitrile. 2. Results and Discussion In Figure 1 the IR spectra (OH stretching and carbonylcarboxylate stretching vibration region) of all studied systems are shown. From the acetic acid trimethylamine oxide (TMAO) to the trifluoroacetic acid + TMAO systems the ApK, increases (ApK, = pK, of the protonated N-oxide minus the pK, of the respective carboxylic acid). The spectra of the respective pure acids are shown for comparison in Figure 1, too. To obtain information on the protontransfer equilibria OH-ON + O--.H"ON, it is necessary to investigate the appearance of all species which could be present in the solutions and their concentrations. Hence, the following equilibria must be taken into account:
+
K,
OH
+ O N + [OH-ON 0- + H'ON
KP
Kd
+ O-*-H+ON] + Kh
+
[OH-*O- + -O**.HO]
OH + N O
[NO+H*-ON + NO*-H+ON] (1) The concentration of the charged species was estimated by conductivity measurements (Table I) and by comparison of the spectra with spectra of solutions in which the different types of pure charged species are present; examples are given in Figure 2. The specific conductivities increase by up to two orders of magnitude for the acid-base solutions compared with solutions of pure components. This conductivity can either be caused by free anions and cations or by complexes with homoconjugated hydrogen bonds. For solutions of pure trifluoroacetic (Figure 2a, dashed line) the v,,(-C02-) band is observed at 1694 cm-'. With all systems except trifluoroacetic acid + TMAO (causing the largest con-
TABLE I: Specific Conductivities li (IOd R-' em-' ) at 298 K"
donor RCOOH, R
acceptor (CH3)jNO
pKab 4.65 4.76 3.57 3.16 2.87 2.47 1.89 1.35 0.52
ApKa
IAH
1, 7.5
~AHB
-0.11 3.0 185 CH3 CH,OCH, 1.08 3.5 252 1.49 7.1 C6HSOCH2 233 1.78 6.6 CICHz 182 2.18 NCCH2 8.8 I67 HC=C 2.76 5.5 204 3.30 7.0 121 C12CH 4.13 2.8 477 CF3 "Specific conductivity of the pure solvent CH3CN is I = 0.5 (10" R-i cm-I). bThe pK, value of (CH,),NO was taken from C. Klofutar, F. Krasovec, and M. KuSar, Croat. Chem. Acta, 40, 23 (1968); the remaining pKa values were from E. P. Serjeant and B. Dempsey, "Ionization Constants of Organic Acids in Aqueous Solution", IUPAC Data Series No. 23, Pergamon Press, Oxford, 1978.
ductivity), no band of free anions is found. In this system a very weak shoulder of vaS(-CO,) due to free. anions is observed at 1694 cm-' (Figure lh). An estimation, comparing the integrated absorbances of this band in Figures l h and 2a, yields a concentration of less than 3% of the initial acid concentration. An intense characteristic band of the homoconjugated carboxylic acid + carboxylate groupings is observed, for instance, in the case of the trifluoroacetic acid + trifluoroacetate system at 1745 cm-' (Figure 2a, solid line). No indication of such a band is found in the respective acid-base system in Figure 1h. Hence, this species can also be neglected. Thus, the increased conductivity of the trifluoroacetic acid + TMAO system is caused by the free anions and by the protonated N-oxide species. Whether the protonated N-oxide is present as free species or as homoconjugated cation cannot be decided from the spectra since the characteristic bands of these species, for
The Journal of Physical Chemistry, Vol. 90, No. 5, 1986 967
IR Study of Carboxylic Acid-TMAO Systems TABLE 11: Data and Results with the RCOOH
+ TMAO Systems
v(C=O), cm-'
R
ApK,
pure acid"
1:l complex
CH3 CH3OCH2 C,HSOCHz CICHI NCCHZ HC=C CIZCH CF3
-0.11 1.08 1.49 1.78 2.18 2.76 3.30 4.13
1754, 1725 1763, 1746 1769, 1746 1758, 1739 1762, 1746 1744, 1127 1765, 1752 1792
1690 1688 1691 1693 1706 1706 1737 1792
v,,(COc), cm-I, 1:l complex
1643 1629 1642 1665
Av,b
AG0n,
cm-'
% ' PT
KpT
log KpT
kJ mol-'
64 75 78 65 56 21
0 2.4 7.7 16.2 24.4 53.0 74.4 92.9
0 0.03 0.08 0.19 0.32 1.13 2.91 13.07
-1.61 -1.08 -0.71 -0.49 0.05 0.46 1.12
9.1 6.2 4.1 2.8 -0.3 -2.7 -6.4
28 0
"The band at higher wavenumbers is due to the monomeric that at lower due to the dimeric species. In the case of the R = ClCH2 system a third band at 1792 cm-' is observed. The assignment of this band could not be clarified. bv(C=O) of the pure monomeric acid minus v(C==O) of the 1:l complex. numbers (Table 11, column 4). This becomes particularly clear instance, the band at 1522 cm-I of the free cations (Figure 2b, solid line), are masked. if Av(C=O), Le., the differences of the absorption maxima in the In summary we can state that for the trifluoroacetic acid pure acid solutions and the complexes, are considered (Table 11, TMAO system the concentration of charged species is less than column 6, Figure le-h). This result indicates the decreasing 3% of the initial concentration. Comparison of the conductivities double bond character of the C=O group. Thus, the influence of the other systems with that of the trifluoroacetic acid TMAO of the proton in the -COOH--ON structure on the donor group system (Table I) demonstrates that the concentration of charged decreases from the CF3COOH + TMAO to the NCCHzCOOH + TMAO system, showing a progressive loosening of the proton species in the other systems must be lower. The deuterated systems from the -COOH group, which indicates a decrease of the barrier show the same behavior. For the deuterated trifluoroacetic acid TMAO system the dissociation of the O--D+ON structure is, in the double minimum proton potential. For the four systems with smaller ApK, only one band is found however, considerably enhanced (see section 2.4). Information on the formation of the acid-base complex, Le., at about 1690 cm-I. The intensity of this band slightly decreases with increasing ApK,, Le., from the CH3COOH TMAO to the on the K, is obtained from the O H stretching vibration of the acids. In Figure 1, the dashed lines show that with solutions of pure acids CICHzCOOH TMAO system. The result that only one band this band is found with maximum in the range 3300-2900 cm-l. is found demonstrates that single minimum potentials are present The fact that this band is broad and shifted toward smaller in these systems. The shift of v(C=O) from 1706 to about 1690 wavenumbers shows that the acid molecules are either dimers or cm-I, proceeding from the NCCHzCOOH + TMAO system with bonded to the acetonitrile molecules. Comparison of the solid with double minimum to the systems with single minimum, shows that the dashed lines in Figure 1 shows that this band has vanished the proton is still a little less tightly bound to the carboxylic acid completely in the donor-acceptor solutions. This result demongroup compared to systems with double minimum proton potentials. strates that with all systems complex formation is almost 100%. Thus, K, is very high. Hence, almost complete formation of the The comparison with theoretically calculated proton potentials acid-base complexes can be assumed in the following. In suggests that probably a humplike shape in the proton potentials agreement with this result, it was recently shown by M a ~ t n e r ~ ~ occurs at the acceptor group [see ref 9, Figure 5, 0-0 distance for many systems that the stability of hydrogen bonds with proton of 2.5 A]. The fact that with such potentials v,,(-C02-) is no transfer is relatively large. longer observed is understandable since with such potentials the 2.1. The Acid-Base Hydrogen Bond. Figure 1 shows the residence time of the proton at the anion is too short. If this carbonyl-carboxylate stretching vibration region (1800-1600 residence time is, however, very short, v,,(-C02-) of the polar cm-I). The absorption maxima of the bands are summarized in structure should no longer be observed. This conclusion is conTable 11, columns 4 and 5. The additional shoulder at 1694 cm-', firmed by a great number of results on homoconjugated hydrogen only observed in the trifluoroacetic acid + TMAO system, has bonds, B'H-B B-.H+B or A H - X * -A.-HA. For this type already been discussed in the preceding paragraph. of hydrogen bond the fluctuation of the proton is always so fast For systems with larger ApK, (pK, of the protonated base minus that in the IR spectra one cannot distinguish between the protonated and the nonprotonated g r o ~ p s . * ~ - ~ ~ pK, of the acid) two bands are observed. One band at higher wavenumbers shifts with increasing ApK, from 1706 to 1792 cm-'. Thus, in summary, the discussion of the v(C=O) and v,,(C02-) Its intensity, however, strongly decreases from the NCCH,COOH bands shows that with the systems with higher ApK, a double TMAO to the CF3COOH TMAO system. The second band minimum potential is present. With decreasing ApK, the OH-ON * O---H+ON equilibrium shifts more and more to the left-hand is found at smaller wavenumbers with maximum in the region 1625-1665 cm-I (Table 11, column 5). The intensity of this band side. In addition, the proton is more and more loosened from the increases in this series of systems. carboxylic acid group, Le., the barrier in the double minimum is The band at higher wavenumbers is caused by the C=O lowered. If the ApK, decreases further, only a single minimum stretching vibration of the carboxylic acid group in the nonpolar proton potential is present which probably still has, however, a proton-limiting structure, OH-ON, whereas the band at lower hump at the acceptor group. Under these conditions the residence wavenumbers is due to the antisymmetrical stretching vibration time of the proton at the acceptor group is too small to observe a carboxylate band. These ApK,-dependent changes of the proton of the carboxylate ion in the polar proton-limiting structure, O-.-H+ON. Thus, the obtained results demonstrate that with potential are confirmed later on by considering the IR continua, these systems characteristic double minimum proton potentials the intermolecular hydrogen bond stretching vibration, and in are present in the OH-ON F= -O-.H+ON bonds, whereby the particular deuteration experiments. In the following, the position of the OH-ON + O-.-H+ON intensity change shows that this equilibrium shifts from the NCCH2COOH + TMAO to CF3COOH TMAO systems in equilibria will be determined quantitatively from the v(C=O) and favor of the polar structure. Within this series, in the opposite the v,,(C02-) bands. It is, however, necessary to calibrate these direction, Le., from the CF,COOH + TMAO to the NCCH2CO O H + TMAO system, the v(C=O) vibration of the nonpolar (24) D. Schioberg and G. Zundel, Z . Phys. Chem. (Frankfurf am Main), proton-limiting structure shifts, however, toward smaller wave102, 169 (1976).
+
+
+
+
+
+
+
+
(23) M. Meot-Ner (Mautner), J . Am. Chem. SOC.,106, 1257 (1984).
(25) J. Fritsch and G. Zundel, J . Chem. SOC.,Faraday Trans. 1,77,2193 (1981). (26) B . Brzezinski and G. Zundel, Chem. Phys. Lefr., 95, 458 (1983).
968 The Journal of Physical Chemistry, Vol. 90, No. 5, 1986
3 for the system NCCH2COOH + TMAO. With increasing base.acid ratio, V, the v(C=O) band of OH-ON at 1706 cm-’ dec I m e s , whereas the band at 1643 cm-’ caused by a superposition of va,(C02-) of the free anions and of the O-.-H+ON structure increases. Hence, with this procedure the base concentration is an additional experimental parameter. It allows a fit of the experimentally determined absorbance of the bands as a function of the initial acid and base concentrations. With thi\ fit the unknown absorptivities 6 and equilibrium constants KpT and KB are parameters and hence these quantities are obtained. r h i s function can be derivated from the above equilibria (see Experimental Section). If v(C=O), the band at higher wavenumbers, is evaluated, this equation is
0.2 -
Ec=o = t c = ~ d ( ( [ K ~- c~CA(KB ~ - 1)12 + ~CA’(KB- l)\”’ [ K B ~-B%A(KB- 1)1)/[2(1 + KPT)(KB- 111 (3)
6 0.4 n L
where E- is the integrated absorbance of v(C=O), cBthe initial base concentration, cA the initial acid concentration, and d the layer thickness. If, however, the band at lower wavenumbers is evaluated, one has to take into account that this band is a superposition of v,,(COz-) of O--H+ON and of v,,(COJ of the free anions. In this case the equation has the following form
0 v)
n
m
0.7 1.01.5 a, 1850
E,
- 2cA(KB - 1)1)/[2(1 + l/KPT)(KB - ‘11 + t d ( K B c B - ([KBcB - ~CA(KB - 1)12 ~ C A ~ ( -K 1)]”2)/[2(Kj3 B - I)] (4)
+
I! 0
1700
wave number cm-“
+
bands. The easiest way for this calibration would be to compare their intensities with those of the respective bands of the pure acid or anion solutions of known concentrations. A necessary supposition of this procedure would be, however, identical absorptivitie: of the acid-base system bands and the respective calibration bands. The compared bands have, however, different positions (cf. Table 11, columns 3 and 4) and therefore probably also different absorptivities. Thus, such a simple procedure is not possible, here. To obtain quantitative results we proceeded therefore as follows: The KPT
+ E, = c,d(([KBcB - ~CA(KB - ])I2 + 4CA2(KB- 1)11’*-
lKBCB
Figure 3. IR bands in the range 1850-1550 cm-I of the system NCCHzCOOH TMAO in CH,CN with variable base-acid ratio V; constant acid concentration 0.1 mol/dm’; layer thickness 96 Km: (1) V = 1.00; (2) V = 1.25; (3) V = 1.50; (4) V = 1.67; (5) V = 2.00; (6) V = 2.50; (7) V = 3.00; (8) V = 5.00.
[AH-B F= A-.-H+B]
Bohner and Zundel
+B
KB
[B+H-*.BF= B.-H+R]
+ A(2)
equilibrium determined by KB is shifted to the right with variation of the base:acid ratio, V, if one adds base to the 1:l solution at constant initial acid concentration. This fact is illustrated in Figure
where E, and cp are the integrated absorbance and the absorptivity of v,,(CO,-) of O--H+ON, and E, and t, the respective quantities of the free anion. Examples of both fits are shown in Figure 4. TI . obtained KpT, log KpT, and AGopT(298K) values are given 111 Table 11, columns 8-1 0. Furthermore, the percent proton transfer values, %PT, are given in column 7 . They are obtained from the KpT values, using the relation %PT =
1OOKpT
KPT
+
No values are given for the acetic acid + TMAO system (system 1) since with variation of V no noticeable change of the u(C=O) band occurs, indicating that Km, %PT, and KB are almost zero. log KpT and %PT are shown as a function of the ApK, (pK, of the protonated N-o> de minus pK, of the acids) in parts a and b of Figure 5 . In thi.. (gure, the ApK, regions in which double minimum potentials no longer occur are shown only as dashed lines. The linear relation obtained in Figure Sa demonstrates that the Huyskens relation22 log KpT = EApK, - 6
(6)
is valid for this family of systems. A family of systems is char-
--.50 h
E
.--, LO 2
-
h4 30
20 10
r.-
LJ
P
d
!
0
2
3
5
7 2 3 L 5 V V Figure 4. Integral absorbance of bands in the carbonyl-carboxylate region for the systems RCOOH + TMAO, as a function of base-acid ratio, V. (0)experimental values (-) best fit; (a) CICH,COOH + TMAO system; (b) NCCHZCOOH+ TMAO system. 7
L
IR Study of CarbJxylic Acid-TMAO Systems
,
100 YoPT
I
I
8060-
L
-
1
0
1
2
3
4
5
LO
-
20
-
0
c“.’
L-49
I
I
I
Figure 5. (a) log KPT.vs. ApK, for the systems RCOOH + TMAO in CH3CN. (b) %PT vs. ApK, for the same systems: (0)experimental values (-) best fit; (1) R = CH,; (2) R = CH,OCH,; (3) R = C6H50CH2;(4) R = CICH,; (5) R = NCCH,; (6) R = HCEC; (7) R = C12CH;(8) R = CF,.
acterized as follows: In a family, the chemical compounds possess the same donor and acceptor groups. These compounds, however, have different pK, values due to different substituents, but these substituents show similar interaction properties with their environments. From the linear regression in Figure 5a we obtained E = 0.86, 6 = 2.39, and ApKaSo”/” = 2.76. The latter value is the ApK, value at which both proton-limiting structures have the same weight, Le., KPT = 1 . The sigmoid curve in Figure 5b through the experimental points is the transformation of the straight line in Figure 5a, using eq 5 and 6. 2.2. I R Continuum and Proton Potential in the OH-.ON @-.H+ON Bonds. As already mentioned in the Introduction, continua in the IR spectra indicate the large proton polarizability of hydrogen bonds, Le., polarizabilities caused by proton moti o n ? ~ ’ ~In- Figure ~~ 6 the spectra of all studied systems are summarized, whereby in each figure the spectrum of the pure TMAO solution is given for comparison by dotted lines. It is known from e ~ p e r i m e n t a l l ~as, ~well ~ - ~as~ from theoretical results that short, strong, hydrogen bonds with a small barrier in the proton potential or with broad potential wells (see ref 9, Figure 5, 0-0 distance of 2.5A) cause continua in the region 1500-700 cm-I, which are most intense around 1000 cm-I. When hydrogel? bonds with a large proton polarizability become longer and the barrier in the potential increases the continua extend from 3000 cm-’ toward smaller wavenumbers over the whole region. Further information is obtained by comparison with the theoretically obtained line spectra, shown in Figure 7 (taken from ref 29). Considering these line spectra, one has to take into account that due to their large proton polarizabilities these hydrogen bonds are already considerably polarized. Therefore, the line spectra of the hydrogen bonds at which fields of 5 X lo6 to 5 X lo7 V/cm are present must be considered. Then, this figure shows that, with short hydrogen bonds, the high intensity around 1000 cm-’ is mainly caused by the fundamental transition. With increasing bond length and barrier, more intensity is expected at higher wavenumbers, and especially with increasing asymmetry of the potential the transition 0-2, near 2500 cm-I, gains more intensity. On the basis of these literature data29the intensity distributions of the continua in Figure 6 are understandable. The intense continua in the region 1500-700 cm-’ with maximum at about 1000 cm-I, observed with the systems shown in Figure 6a, indicate short hydrogen bonds with a large proton polarizability, having potentials without barrier, whereby a hump at the acceptor group may be present. Figure Ab shows that the intensity of this continuum shifts with increasing ApK, toward higher wavenumbers and the intensity (27) A. Hayd, E. G . Weidemann, and G . Zundel, J . Chem. Phys., 70,86 (1979). (28) B. Brzezinski and G. Zundel, J . Mol. Srrucr., 72, 9 (1981). (29) R. Janoschek, E. G. Weidemann, and G . Zundel, J . Chem. SOC., Faraday Trans. 2, 69, 505 (1973).
TABLE 111: Deuterated Systems v(C=O), cm-’ of donor
the 1:l complex
RCOOD(H), R
H
D
CH3 CH30CH2 C6HSOCH2 ClCH2 NCCH, HC=C C12CH CF,
1690 1688 1691 1693 1706 1706 1737 1792
1682 1661 1658 1654 1752 1730 1766
of the bandlike structure at about 2500 cm-’ increases. Figure 7 shows that this change in the intensity distribution is expected with increasing hydrogen bond length, Le., increasing barrier within the proton potentials. Here the bandlike structure at about 2500 cm-’ is mainly caused by the 0-2 transition. Figure 6c shows that when the proton is preferentially present at the acceptor, the intensity of the continuum at lower wavenumbers vanishes and the band in the region 2700-2200 cm-’ gains intensity, whereby the transition 0-1 and 0-2 contribute to this bandlike structure of the continuum. In summary, we can state that the continua observed demonstrate that these hydrogen bonds show large proton polarizability, if they are not too asymmetrical. For systems with small ApK,, shorter hydrogen bonds with single minimum proton potentials with broad well are present. With increasing ApK,, longer hydrogen bonds with double minimum proton potentials occur and the barrier in the double minimum rises. Furthermore, with increasing ApK, the deeper well present at the acceptor is lowered. 2.3. Hydrogen Bond Stretching Vibration in the Far-IR. These results are confirmed by far-IR measurements. Figure 8 shows the spectra for two cases: in Figure Sa a system with a single minimum proton !>otential is shown, whereas in the system shown in Figure 8b a double minimum proton potential is present. In the first case the hydrogen bond stretching vibration is found at 243 cm-l, in the second at 180 cm-’. When the harmonic oscillator approximation of two point masses was used, for the system acetic acid TMAO k = 116 N m-I, and for the system trifluoroacetic acid + TMAO k = 87 N m-’ were obtained. Thus, the strength of the hydrogen bonds is less if instead of a broad single minimum a double minimum occurs in the hydrogen bonds. This change in bond strcngth as function of the shape of the potential becomes understandable, considering the reaction field effects in section 2.5. 2.4. Changes with Deuteration. Figure 9 shows selected spectra of deuterated samples, whereby the solid lines are the acid-base solutions and the dashed ones are the pure acid solutions. In Table I11 data for nondeuterated and deuterated samples are compared. For the four systems with smaller ApK,, v(C=O) of the OD-ON structure is shifted toward smaller wavenumbers com-
+
970 The Journal of Physical Chemistry, Vol. 90, No. 5, 1986
Bohner and Zundel I
I
r
-
t
wave number cm
Figure 6. IR spectra of the systems RCOOH + TMAO in CD3CN;concentration 0 3 mol/dm3,layer thickness 96 wm (a) (---) CH,COOH + TMAO, CH30CH2COOH TMAO, (-) C,H,OCH,COOH TMAO, TMAO; (b) (---) CICH2COOH + TMAO, (-*-.-) NCCHZCOOH + TMAO, (--) HCZ~~ECCOOHTMAO (-) TMAO, (c) (---) C12CHCOOH + TMAO, (-) CF3COOH TMAO, TMAO ( - e - * - )
+
+
+
pared with the respective band in the OH.-ON structure, and this shift increases with increasing acidity of the donors (Table 111, columns 2 and 3). This result demonstrates that the double bond character of the C=O bond decreases with deuteration of the carboxylic acid group. This decrease is more pronounced the larger the acidity. Hence, in these cases the influence of D+ on the carboxylic acid residue is less than that of H+. This is understandable because potentials with broad single minimum are present in these systems. For these potentials D+ is not as strongly attached to the carboxylic group as H', since the D energy levels are lower and therefore deeper in the potential and hence the deuteron influences the carboxylic group less strongly than the proton. With these D systems intense continua are also observed (Figure 9a-c), indicating that the deuteron polarizabilities are also large. With systems with larger ApK,, a v(C=O) band of the nonpolar structure as well as v,,(-C02-) of the polar structure are observed. For the trifluoroacetic acid + TMAO system the v(C=O) band (at about 1692 cm-I) has almost completely vanished with deuteration. In all cases the intensity of v,,(-CO,-) of the nonpolar structure increases considerably with deuteration. Thus, in deuterated systems the transfer equilibrium is strongly shifted in favor of the O--D+ON structure compared with the position of
(*.e)
+
(*e*)
the respective equilibrium in the H systems. In the trifluoroacetic acid + TMAO system (Figure 9e) a second band arises at 1690 cm-I, indicating remarkable dissociation of the O--D+ON structure. Such an effect is, however, only observed with this deuterated system. The shift of equilibrium in favor of the polar structure with the D systems indicates that the deeper well of the potential is already at the acceptor, since under this condition the residence time of the deuterons at the acceptor is higher than that of the protons. Thus, the reaction field lowers the deeper well at the acceptor much more in the D than in the H case, resulting in a shift in favor of the polar structure in the D case. In contrast to the behavior of systems with a broad single minimum, the v(C=O) band of the nonpolar structure shifts toward higher wavenumbers with deuteration demonstrating the larger double bond character of v(C=O). This result is understandable because for the nonpolar structure of the double minimum the D is more strongly attached than the H to the carboxylic residue. For systems with double minima the continua are considerably less intense with the deuterated systems (Figure 9, d and e), indicating that the deuteron polarizabilities are much smaller than the proton polarizabilities. These isotope effects show that these hydrogen or deuterium bonds are already strongly polarized by
The Journal of Physical Chemistry, Vol, 90, No. 5, 1986 971
IR Study of Carboxylic Acid-TMAO Systems 0-0
0-0 = 2.5 A
I
f
1
3000
1000
-
5000 1000 wave number cm-l
2.6 A
The influence of the reaction field on the shape of the proton potential is, however, different with single and double minimum proton potentials, as already mentioned in the Introduction. The reaction field is directed on the mean charge density distribution. For a broad single minimum potential, one has only one maximum in the charge density distribution, whereas with double minima two maxima occur. Thus, the reaction field in the first case deepens the single minimum well, increasing the strength of the hydrogen bonds. With the double minimum, it lowers both, but preferentially the deeper well, and increases the barrier, thus decreasing the strength of the hydrogen bonds. With this sequence of systems a sudden change from the single to the double minimum character of the potentials takes place. This fact is due to the different influence of the reaction field on the proton potential for broad single and for double minimum potentials.
SI.41dWcm
?
3000
5000
Figure 7. Relative absorption intensities of the transitions in O+H--O
== O.-H+O hydrogen bonds at various 0-0 distances, electric field strength, and temperatures: ( 0 )0 K, (m) 100 K, (A) 200 K, (-) 300 K,
(X) 01
400 K (taken from ref 29). a
I
I
1
01
1
Figure 8. Far-IR bands of the systems: (a) (-) CH,COOH + TMAO, c = 0.5 mol/dm3; CH,COOH, c = 0.5 mol/dm3; (---) TMAO, c = 0.1 mol/dm3; (b) (-) CF3COOH + TMAO, c = 0.5 mol/dm’, (---.) CF,COOH, c = 0.5 mol/dm3; (---) TMAO, c = 0.1 mol/dm’; solvent: CH,CN; layer thickness 96 pm. (-.-e)
their environments, since only under these conditions is the deuteron polarizability smaller than the proton p o l a r i ~ a b i l i t y . ~In~ the CF,COOD TMAO system, Figure 9e, a continuum is no longer found. Instead of it a broad band with maximum at about 1850 cm-’ is observed. This result proves that the deuteron polarizability has almost vanished in this system. It was shown by Sokolov and Savel’ev3’ that hydrogen bonds with potentials with low barrier are shortened upon deuteration and those with high barrier become longer. These theoretical results3’ agree very well with our experimental findings. In summary, we can state that the deuteration experiments also confirm the change of shape of the potential as a function of the ApK, and of the environmental (reaction field) effects. 2.5. Reaction Field and Proton Potential. For double minimum proton potentials the interaction of the hydrogen bonds with their environments increases with increasing weight of the polar structure O--H+ON. This increase occurs since this interaction effect is mainly determined by the interaction of the dipole of the hydrogen bond with the reaction field induced by it in the solvent.”
+
(30) R. Janoschek, A. Hayd, E. G . Weidemann, M. Leuchs, and G. Zundel, J . Chem. Soc., Faraday Trans. 2, 14, 1238 (1978). (31) N. D. Sokolov and V. A. Savel’ev, Chem. Phys., 22, 383 (1977).
3. Conclusions In acetonitrile solutions of 1:1 mixtures of various carboxylic acids with trimethylamine oxide (TMAO) hydrogen bonds formation is almost 100%. For the first four systems (smaller ApK,) broad single minimum potential wells which have probably a humplike shape at the acceptor occur in the O--.H+-ON hydrogen bonds. With systems with a higher ApK,, double minimum proton potentials occur in the OH-ON -O-H+ON bonds. This pronounced change of shape of the potential is caused by the increasing acidity of the acid, and especially by the interaction of the dipole of the hydrogen bond with the reaction field induced by it in the solvent. The strength of this reaction field increases with increasing transfer of the proton to the acceptor. Its influence on the shape of the potential is different with broad single minimum and with double minimum proton potentials. With single minima the reaction field increases the single minimum character and shortens the hydrogen bonds, whereas with double minima it increases the double minimum character and stretches the hydrogen bonds. This change in shape of the potential is demonstrated by the positions and half-width of the v(C=O) and the v,,(-COz-) bands, by the change of the intensity distribution of the continua observed in the IR spectra, by the intermolecular hydrogen bond stretching vibration in the far-IR, and, finally, by deuteration effects. For systems with double minimum proton potentials the weight of the nonpolar proton-limiting structures OH-ON decreases and that of the polar -O-.H+ON increases with increasing ApK,, Le., in the following series of systems: NCCH2COOH + TMAO, HCzCOOH + TMAO, C1,CHCOOH + TMAO and CF,COOH + TMAO. The Huyskens equation log KPT= (ApK, - 6 is valid for this family of systems. Both proton-limiting structures have the same weight for ApKaso%= 2.76. Intense IR continua indicate that all these hydrogen bonds show a large proton polarizability caused by proton motion if they are not too asymmetrical. 4. Experimental Section The substances were purchased from Merck (Federal Republic of Germany) and Fluka (Switzerland). In every case, substances of the highest degree of purity available were used. Acetonitrile and acetonitrile-d3 (for spectrosco y) from Merck, Darmstadt (FRG) were used and dried over 3- molecular sieves. The acids were dried with Pz05 and then distilled twice under reduced pressure in a nitrogen atmosphere. If deuterated acids were not commercially available they were prepared as follows: The acids were dissolved in C H 3 0 D and the solvent was removed. This procedure was repeated several times. After that the liquid D acids were distilled under nitrogen and the solid D acids dried in vacuo. Trimethylamine oxide is commercially available only as the dihydrate (CHJ3NO.2H20. The water molecules were removed as follows: The dihydrate was dried at 351 K under reduced pressure for several days over PzOsand then dissolved several times in absolute CzHSOH,which was dried with 3-A molecular sieves, and the solvent was removed. After that, the substance was sublimed at 400 K under reduced pressure. IR spectra, analysis,
w
972
The Journal of Physical Chemistry, Vol. 90, No. 5, 1986
Bohner and Zundel
Figure 9. IR spectra of RCOOD + TMAO systems in CD3CN; concentration 0.3 mol/dm3; layer thickness 96 wn: (a) (---) CH,COOD, (-) CH3COOD TMAO, (-.-*-) TMAO, (b) (---) CH,OCH&OOD, (-) CH,OCH,COOD + TMAO; (c) (---) CICH,COOD, (-) ClCH2COOD TMAO; (d) (---) HC=CCOOD, (-) HC=CCOOD TMAO; (e) (---) CF3COOD, (-) CF3COOD + TMAO.
+
+
+
and melting point showed that the very hygroscopic amine oxide was then free of water. All preparations and transfer of solutions were done in a water-free glovebox under a nitrogen atmosphere. The concentrations of the donor and the acceptors were in the range 0.1-0.5 mol dm-3. For the IR investigations, cells with silicon windows were used. Because of the high reflectivity of this material, wedge-shaped layers were applied to avoid interference pattern superposed on
the spectra. The mean layer thickness of the layer used was determined to be 96 hm, as described in ref 8. The solvent bands were compensated by an adjustable layer of solvent in the reference beam. In the regions of the strongest solvent bands, the energy loss was too high to obtain any information. In the spectra these regions are indicated as dotted lines. The spectra were plotted with a Model 325 spectrophotometer, Bodenseewerk Perkin-Elmer, Uberlingen, West Germany. It was flushed with dry and C0,-free air. The sample temperature was 298 K.
The Journal of Physical Chemistry, Vol. 90,No. 5, 1986 973
IR Study of Carboxylic Acid-TMAO Systems ?;he absorbances taken from the spectra had to be corrected, taking into account the fact that with wedge-shaped layers the Lambert-Beer law is no longer valid. The necessary corrections were made as described in ref 8. With this corrected scale, the integral absorbances of the bands used for the evaluations were determined. The far-IR spectra were recorded with a FTS-20 FT-IR spectrophotometer from Digilab, Cambridge, MA, under the same conditions as described above. The equilibria were determined as described in section 2 by evaluating bands in the carbonyl-carboxylate stretching vibration region as functions of the acceptor concentrations and by fitting the experimental values to the theoretical equations, eq 3 and 4. The derivation of these theoretical equations are given in the following: The theoretical functions eq 3 and 4 for the evaluation of the systems are derived as follows from the equilibria (eq 2):
the concentration and constants are as follows: (initial acid concentration) cA = c, c, ci
(8) + + (initial base concentration) (9) cB = c, + cp + c b + 2ch (concentration of the 1:l acid-base complex) cc = c, + cp KpT
KB =
= cp/c, chcj
-
(proton-transfer constant) (formation constant of BHB’A-)
(10) (1 1) (12)
cccb
and c, =
[sz+ 4cAz(KB - 1)]1’2- s
( + -iPT)
2 1
(18)
(KB-1)
with
s = KBCB - 2CA(KB - 1) The integrated absorbance Ec=o of u(C=O) of the nonpolar proton limiting structure is
Ec=o = tc=gdC,
(19)
Inserting eq 17 into eq 19 one obtains as result (eq 3): Ec=o = tc=od(([K~C~ - 2cA(KB - 1)12 + 4cA2(KB [KBCB- ~ C A ( K-B1)1)/[2(1 + KPT)(KB- 111
E- is now a function of C, (initial acid concentration), cB(initial base concentration), KpT(proton-transfer constant), KB (formation constant of,BHB+A-), and eC4 (absorptivity of If C~ is varied, the unknown parameters K p T , KB and t c d result from the fit to the experimental points. If the band at lower wavenumbers is evaluated one has to take into account that this band is a superposition of u,,(COz-) of the polar proton-limiting structure O-.-H+ON and v,,(CO2-) of the free anion. In this case the following relation is valid: E,
+ Ei = (cPcp+ c,c,)d
(20)
From the combination of the eq 8-10, 12, and 13 and by elimination of cb, ch, and c, the following relation for the equilibrium concentration ci of the free anion is obtained:
and from electroneutrality reasons ch = ci Combining eq 7, 8, 10, 12, and 13 and eliminating the unknown variables cb, ch and ci yields the following equation for the equilibrium concentration cc of the 1: 1 acid-base complex:
cc = (([KBCB - 2cA(KB - I)]’
Inserting eq 18 and 21 into eq 20 yields (eq 4)
+ 4cAz(KB - 1))1’2- [KBCB 2cA(KB - 1)1)/[2(KB - l)1 (14)
Combination of eq 10 and 1 1 yields cn = cc/(l
cp = cc/(
1
+ KPd
(15)
+ -i) KPT
Inserting eq 14 in eq 15 or eq 16 one gets the equilibrium concentrations of the both proton-limiting structures of the 1:l acid-base complex:
The absorptivity ti can be determined from a pure solution of a salt with the respective anion A-. If cB is varied again, the constants K p T , KB, and t, result from the fit to the measured points.
Acknowledgment. We thank the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie for providing the facilities for this work. Registry No. CH,COOH, 64-19-7; CH,OCH,COOH, 625-45-6; C6HsOCH2COOH, 122-59-8; CICH,COOH, 79-1 1-8; NCCH,COOH, 372-09-8; HCECCOOH, 47 1-25-0; CI,CHCOOH, 79-43-6; CFJOOH, 76-05-1; TMAO, 1184-78-7.
