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Li+, Na+, K+ and Be2+ bonds—IR continua and cation polarizabilities of these bonds
MOLSTR 10943
Journal of Molecular Structure 511–512 (1999) 19–33 www.elsevier.nl/locate/molstruc
Li 1, Na 1, K 1 and Be 21 bonds—IR continua and cation polarizabilities of these bonds G. Zundel* Bruno Walterstr. 2, 5020 Salzburg, Austria Received 1 December 1998; accepted 11 January 1999
Abstract Not only hydrogen bonds but also Li 1, Na 1, K 1 and Be 21 bonds may show cation polarizabilities because of fluctuation and shifts of the cations. These polarizabilities are indicated by continua in the far infrared region. The electron density at the acceptor groups can be changed by the substituents. With increasing electron density the Li 1 ions in the 2OLi 1…ON O 2 … 1 O Li ON bonds shift to the acceptor. The same is true for Na 1 bonds. The fluctuation amplitude of K 1 in potassium bonds is very small. The wavenumber regions are compared in which the continua in H 1, Li 1, Na 1 and K 1 bonds occur. There are also cation polarizabilities observed because of collective cation fluctuation in two or four cation bonds. Finally, the fluctuation of one H 1, one Li 1 and one Na 1 in multi-minima potentials are discussed with crown ether systems. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Cation polarizability; IR continua; Electron density; Cation bonds
1. Introduction
2. Homoconjugated B 1M…B O B…M 1B bonds
If one dissolves, for instance, LiClO4 in acetonitrile, one observes in the far infrared (FIR) region, the so-called ion motion band [1–10]. In polar solvents this ion motion band is caused by the vibration of the cations in a cage of solvent molecules. For a LiClO4 solution in acetonitrile this ion motion band is observed at about 405 cm 21. Fig. 1 shows that the ion motion band at 405 cm 21 vanishes when molecules which form cation bonds are added to LiClO4 solutions. If these cation bonds show cation polarizabilities continua appear in the FIR region. As for hydrogen bonds, these cation polarizabilities occur due to fluctuation and shifts of the cations in these bonds [11].
Now homoconjugated B 1M…B O B…M 1B bonds, where M 1 Li 1 or Na 1 will be discussed. First, a comparison of the properties of these cation bonds with those of polarizable hydrogen and deuteron bonds will be made. For Li 1 bonds one can compare the dependence of the continua on the length and strength of the Li 1 bonds with the bond length dependence of easily polarizable hydrogen bonds. Fig. 2(A) and (B) shows the spectra of the Li 1 salts of the compounds 2 and 3, respectively. FIR continua are observed, indicating that the Li 1 ion fluctuates in the N 1Li…N O N…Li 1N bonds [12]. These continua begin at about 450 cm 21 and extend toward smaller wavenumbers. They indicate that the homoconjugated Li 1 bonds show large Li 1 polarizability.
* Tel.: 1 43-662-642311; fax: 1 43-662-64231176.
0022-2860/99/$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(99)00138-6
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Fig. 1. FT-IR spectra of 0.1 mol dm 23 chloroform solutions at 298 K: (– –), (Bu)4N 1 2:1 complex; (—), Li 1 1:1 complex. The spectrum of the pure LiClO4 solutions is given for comparison (-·-·-) (taken from Ref. [13]).
Such continua correspond to the continuum observed for H5O21. A typical double minimum Li 1 potential is present. Fig. 1 shows a continuum caused by a shorter and already relatively strong 2OLi 1…O 2 O O…Li 1O 2 bond, present in the Li 1 salts of dialcoholate molecules [13]. The continuum begins at about 380 cm 21 and shows a broad band-like structure with a maximum at about 100 cm 21. The wavenumber dependence of its intensity is comparable to that observed for the SO 1H…OS O SO…H 1OS hydrogen bonds in solutions of strong acids in dimethylsulfoxide. Fig. 3(A) shows the FT-IR spectra observed for very strong and short O 2…Li 1… 2O bonds formed between two carboxylate groups (compound 1). These bonds are completely formed, since the ion motion band observed in a pure LiClO4 dioxane solution has vanished (not shown). The continuum begins at about 250 cm 21 and is much more intense at smaller wavenumbers. These continua are caused by
intramolecular Li 1 bonds formed by two carboxylate groups (compound 1) [14]. The continua shown in Fig. 3(B) are caused by intermolecular NOLi 1…ON bonds formed between NO groups of compound 2 [15]. These Li 1 continua may correspond to the continua caused by the very strong homoconjugated hydrogen bonds formed between carboxylic acid and carboxylate groups. The appearance of these continua demonstrates that all these Li 1 bonds show Li 1 polarizability caused by Li 1 fluctuation. The integrated intensity of the continua decreases and thus, the Li 1 polarizability decreases within this series of compounds. Fig. 4 shows the results for intramolecular O 2…Na 1… 2O bonds formed by two carboxylate groups (compound 1) [13]. These bonds are fully formed, since the Na 1 ion motion band, observed in pure NaClO4 solutions, has completely vanished. The continuum indicates the Na 1 polarizability of these bonds. The continua caused by polarizable Na 1 bonds are
Fig. 2. IR spectra of acetonitrile solutions of the free bases (– –), and of a 1:1 mixture of the base and LiClO4 (—): (A) compound 2; (B) compound 3 (taken from Ref. [12]).
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Fig. 3. FT-IR spectra of solutions in a mixture of dioxane and chloroform (ratio 1:4). (A) (– –) compound 1 (0.1 mol dm 23); (—), Li 1 complex (1:1) of compound 1 (0.1 mol dm 23); (-·-·-), (0.2 mol dm 23). (B) (-·-·-), compound 2; (– –), LiClO4:N oxide (compound 2) ratio 1:2 (0.1 mol dm 23); (0.1 mol dm 23) (taken from Refs. [13] and [14]).
observed at smaller wavenumbers when compared to those of Li 1 bonds. Furthermore, the integrated intensity of these continua is lower, and hence, their cation polarizability smaller. The shift towards smaller wavenumbers is caused by the larger mass of the Na 1 ions when compared to the Li 1 ions. The smaller polarizabilities of the Na 1 bonds when compared to the Li 1 bonds are due to the smaller fluctuation amplitudes of Na 1 compared with Li 1.
3. Heteroconjugated A 2M 1…B O A 2…M 1B In the following discussion of heteroconjugated A 2M 1…B O A 2…M 1B bonds shall be made, where M 1 Li 1, Na 1 or K 1. Also these heteroconjugated Li 1 or Na 1 bonds may show large cation
Fig. 4. FT-IR spectra of solutions in a mixture of dioxane and chloroform (ratio 1:4) in the region 250–25 cm 21. (– –), compound 1 (0.1 mol dm 23); (—), Na 1 complex (1:1) of compound 1 (0.1 mol dm 23); (-·-·-) (0.2 mol dm 23) (taken from Ref. [13]).
polarizabilities due to fluctuation and shifts of the cations within them. Let us first consider the IR spectra of the family of Li 1 compounds shown in Scheme 1 [15]. Within this series of compounds the electron density at the O atom of the NO group increases from the compound with the substituent R NO2 to that with the substituent R OC2H5. Fig. 5 shows the carbonyl stretching vibrations of these compounds as a function of the substituent R. The decrease of the n (CyO) band at higher wavenumbers and the increase of the n as(CO22) band at lower wavenumbers demonstrates that within this series from the R NO2 to the R OC2H5 compound in the (I) O 2Li 1…ON O O 2…Li 1ON (II) bonds, the weight of limiting structure I decreases and that of structure II increases. In the compound R NO2 the weight of the polar structure is very small, since only the n (CyO) band is observed. For the compound R OC2H5 n (CyO) and n as(CO22) have comparable intensity. Thus, in this system both limiting structures have almost comparable weight. Thus, the equilibrium is shifted as a function of the electron density at the O atoms of the NO
Scheme 1.
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Fig. 5. Carbonyl region of the FT-IR spectra of acetonitrile solutions of the Li 1 salts of ((4R)-2-pyridyl-N oxide) acetic acids, (—), –NO2; (– –), –Br; (-·-·-), –H; ( · · · · ·), –CH3; (—), –OC2H5 (taken from Ref. [15]).
Fig. 6. FT-IR spectra of chloroform solutions of the Li 1 salts of ((4R)-2-pyridyl-N oxide) acetic acids (—), and, for comparison, the tetrabutylammonium salts (– –): (A) –NO2; (B) –Br; (C) –H; (D) –CH3; (E) –OC2H5 (taken from Ref. [15]).
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Fig. 7. FT-IR spectrum of a solution in a chloroform:acetonitrile mixture (ratio 1:2): (– –), tetrabutylammonium salt of one of the compounds; (—), 1:1 complexes formed with LiAuCl4; (· · · · · ·), pure solution of LiAuCl4 (taken from Ref. [17]).
acceptor groups. Furthermore, the shift of the n (CyO) band from 1708 to 1687 cm 21 proves that the nature of the non-polar structure also changes in this series of compounds. Fig. 6 shows the spectra taken in the FIR region. For the compound R NO2 an intense continuum is observed in the region 425–25 cm 21. Its intensity decreases, however, strongly toward smaller wavenumbers. Hence, the double minimum Li 1 potential is still relatively asymmetrical. In the cases of R Br and R H, continua are observed, beginning at 350 cm 21 and extending with large intensity down to 25 cm 21. These are the systems with a double minimum Li 1 potential which is on the average largely symmetrical. The bonds now show the largest Li 1 polarizability. In the case of R OC2H5 only a broad band with a maximum at about 90 cm 21 is observed. Thus, for all compounds, except the last, the (I) O 2Li 1…ON O O 2…Li 1ON (II) bonds show large Li 1 polarizability. The Li 1 ions can easily be shifted within these bonds by local electrical fields. With the next family of systems, the four substituted 2-diethylaminomethylphenolate-N oxides, almost the same change of the wavenumber-dependent intensity distribution of the continua is observed as a function of the substituents [16]. These (I)
Scheme 2.
Fig. 8. FT-IR spectra in the region 1800–1500 cm 21 of the sodium complexes (—), and tetrabutylammonium salts (– –), of ((4R)6tert-butyl-2-pyridyl-N oxide) acetic acid in CH3CN solutions. (A) R –NO2; (B) R –Br; (C) R –H; (D) –CH3 (taken from Ref. [18]).
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Fig. 9. FT-IR spectra in the region 300–25 cm 21 of the sodium complexes (—), and the tetrabutylammonium salts (– –), of ((4R)6-tert-butyl-2-pyridyl-N oxide) acetic acids in CHCl3 solutions. (A) R –NO3; (B) R –Br; (C) R –H; (D) R –CH3 (taken from Ref. [18]).
O 2Li 1…ON O O 2…Li 1ON (II) bonds show also large Li 1 polarizability if a double minimum Li 1 potential is present (Scheme 2). Fig. 7 shows an example of the N-(1,6-dihydro-6oxopyridine-2-yl)-amino acid family of systems. The same spectral feature is observed for all compounds [17]. In all systems an intense continuum demonstrates that all these (I) O 2Li 1…N O O 2…Li 1N (II) bonds show large Li 1 polarizability. The continuum is independent of the substituents present, since they do not change the electron density, neither at the carboxylic acid O nor at the N atom. Let us compare these Li 1 bonds with the results of hydrogen bonds. The shift of the equilibria observed in Li 1 bonds as a function of the electron density change at the acceptor O atom corresponds to the
shift observed in hydrogen bonds as a function of the DpKa. Let us now consider heteroconjugated Na 1 bonds and their Na 1 polarizabilities (Fig. 8). The (I) O 2Na 1…ON O O 2…Na 1ON (II) bonds were studied for this family (formula) of systems [18]. The bands in the carbonyl region are shown in Fig. 8. As a function of the substituents, n (CyO) decreases and n as(CO22) increases. This result shows that the equilibrium is shifted from the left- to the right-hand side as a function of the substituents. A double minimum Na 1 potential is present. Fig. 9 shows the FIR spectra of the compounds. For the compound R NO2 a broad band caused by these bonds is observed in the region 150–50 cm 21. With compound R Br a second band arises, since the sodium limiting structure II gains weight. For compound R H, i.e. if both limiting structures have considerable weight, both bands have comparable intensity. In the case of the compound R CH3, both bands fuse to a continuous absorption, observed in the region 200–25 cm 21. This spectral feature suggests that with the (I) COO 2Na 1…ON O COO 2…Na 1ON (II) bonds the band at lower wavenumbers is observed if the Na 1 ions are present at the carboxylic group (limiting structure I), whereas the band at higher wavenumbers indicates the presence of the Na 1 ions at the O atom of the NO groups (limiting structure II). In the case of the compounds R NO2, R Br and R H, the residence time of the Na 1 ions at the donor and acceptor groups of the bonds is longer than the time which is necessary for the absorption of the FIR quanta. For the compound R CH3 the residence time is, however, comparable to the time necessary for the absorption of FIR quanta, since both bands have the tendency to fuse to one broad absorption. The mass of the Na 1 ions is much larger than that of the Li 1 ions, and hence, the fluctuation frequency of the Na 1 ions is much smaller. Thus, with this Na 1 system one is either below or, for the most symmetrical system, at the coalescence point. If one approaches the coalescence point the increase of the integrated intensity of the band complex indicates that the Na 1 bonds gain more and more polarizability in this series of complexes. However, this Na 1 polarizability is much smaller than the Li 1 polarizability of heteroconjugated Li 1 bonds.
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Fig. 10. Far IR spectra of: (– –), Mannich base tetrabutylammonium salt, (—), its 1:1 mixture with KClO4 and, for comparison, (· · · · · ·), a KClO4 solution (taken from Ref. [19]).
Let us now consider heteroconjugated K 1 bonds [19]. If one adds KClO4 to a solution of this Mannich base, the K 1 Mannich base complex is formed completely. This is shown by the fact (Fig. 10) that the ion motion band, observed in the pure KClO4 solution at 142 cm 21, vanishes completely. Now a broad band is observed in the region 140–75 cm 21. Compared with the integrated intensity of the Na 1 continuum the integrated intensity of this band is much smaller. Thus, the K 1 polarizability of these K 1 bonds is small. This result is understandable, since the fluctuation amplitudes of the K 1 ions are small. 4. Wavenumber regions with H 1, D 1, Li 1 and Na 1 bonds In the case of typical double minimum bonds let us compare the wavenumber regions in which the continua corresponding to H 1, D 1, Li 1 and Na 1 bonds are observed. For hydrogen bonds the continua are observed in the region below 3000 cm 21; for deuteron bonds in the region below 2300 cm 21; while for Li 1 bonds they are found in the FIR region below 450 cm 21, and finally for Na 1 bonds they are observed below 200 cm 21. This pronounced shift is related to the mass of the fluctuating cations. Furthermore, the integrated intensities of these continua decrease within this series of cations. With increasing mass of the cations the energy levels are deeper in the potential wells. Hence, the fluctuation amplitudes of the cations become smaller and thus,
the polarizabilities of the cation bonds decrease. The K 1 bonds are an extreme limiting case.
5. Cation polarizabilities due to collective cation motion For hydrogen-bonded systems particularly large proton polarizabilities may be observed due to collective proton motion in hydrogen-bonded chains. Cation polarizabilities due to collective cation motion were also recently observed in systems with two Li 1 or Na 1 bonds, respectively. Here the electron density at the phenolic O atom is changed by the substituent R. Fig. 11 shows the far infrared region of these diLi 1-salts [20]. In this series of compounds a band with a wing toward higher wavenumbers changes to a continuum which extends from 350 to 25 cm 21. With further decreasing electron density at the phenolic O atom the continuum vanishes again. These results can be explained by considering these three Li 1 limiting structures which correspond to the three minima present in the Li 1 potential. If the electron density at the phenolic O atom is high, the Li 1 ions fluctuate between the structures I and II. If the three minima, corresponding to these three limiting structures are energetically comparable the fluctuation is the fastest and the Li 1 polarizability due to collective Li 1 motion the highest. A three-minima Li 1 potential is realized. Finally, if the electron density at the phenolic O atom is the smallest only structure III is realized. The Li 1 ions are localized at the O
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Fig. 11. FT-IR spectra of chloroform solutions of 2:1 complexes of lithium tetrachloraurate and tetrabutylammonium salts of 2,6-bis(diethylaminomethyl)-(3R, 4R)-phenol di-N oxides (—). For comparison, the spectra of chloroform solutions of the corresponding tetrabutylammonium salts (· · · · · ·) and a LiAuCl4 acetonitrile solution (– –), are given. (A) R –4-tert-C4H9; (B) R –4-C6H5; (C) R –4-Cl; (D) R –4-COOCH3; (E) R –4-NO2; (F) R –3,4-(NO2)2 (taken from Ref. [20]).
atoms of the NO groups. The Li 1 polarizability has vanished. Fig. 12 shows that analogous results are obtained for the di-Na 1 ion systems (Scheme 3) [21]. The continuum begins at about 200 cm 21 and extends to 25 cm 21. The continuum is, however, much more sensitive to the electron density at the phenolic O atom when compared to the di-Li 1 systems. This result is again understandable, since the mass of the Na 1 ion
is greater and therefore the energy levels of Na 1 are deeper in the potential wells than those of Li 1. Also, the collective Li 1 and Na 1 fluctuation in fourminima cation potentials, present in the compounds, shown in the formulae, results in large Li 1 or Na 1 polarizability of these systems [22]. This result is demonstrated by the IR spectra in Fig. 13. For the Li 1 complex an intense continuum is observed, but a continuum is also found for the Na 1 complex.
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Thus, large cation polarizabilities occur not only due to collective proton motion, but also due to collective motion of Li 1 or of Na 1 ions, respectively. Let us now compare the continua observed with polarizable Li 1 to the results obtained with Be 21 complexes. The molecular mass of Be 21 is comparable to that of Li 1. Therefore, we studied the following Be 21 complexes: Compounds with four carboxylate groups [23], complexes with four N oxide groups [24], and finally, the Be 21 complex of the gossypole molecule (see Scheme 4) [25]. With all Be 21 complexes, almost the same spectral feature is observed. Therefore, in Fig. 14 only the infrared spectrum is shown for a Be 21 N oxide complex. A very broad absorption is found, extending in all cases from about 1250 to 500 cm 21. Furthermore, one can show that all four O atoms of the four NO groups are influenced to the same extent by the cations. The molecular mass of Be 21 is comparable to that of Li 1. The absorption of the Be 21 bonds is, however, found at much higher wavenumbers. Thus, the shape of the Be 21 potential must be completely different. Due to the large affinity of the Be 21 ions to the acceptor O atoms, the Be 21 potential has a very steep slope at the O atoms. Furthermore, the potential must be much narrower than with the Li 1 bonds. Thus, the Be 21 potentials in which the Be 21 ions fluctuate are four minima potentials with very small barriers or perhaps potentials without barrier. Only with a Be 21 potential of this type it can be explained that the broad absorption is shifted toward higher wavenumbers, compared with the continua caused by Li 1 bonds with large Li 1 polarizability. Now cation polarizabilities caused by collective fluctuations and shifts of two or four cations will be discussed. The reason of proton or cation polarizabilities can also be the fluctuation of one cation in multiminima potentials. Such systems can be realized with crown ethers, adding HAuCl4, LiClO4 or NaClO4 in the ratio 1:1 to their solutions.
Fig. 12. FT-IR spectra of chloroform solutions of 2:1 complexes of sodium tetrachloraurate and tetrabutylammonium salts of 2,6bis(diethylaminomethyl)-(3R, 4R)-phenolate di-N oxides (—). For comparison, the spectra of the corresponding tetrabutylammonium salts (– –), and a NaAuCl4 acetonitrile solution (· · · · · ·) are given. (A) R –4-C6H5; (B) R –4-Cl; (C) R –4-COOCH3; (D) R –4-NO2; (E) R –3,4-(NO2)2 (taken from Ref. [21]).
6. Crown ethers Fig. 15 shows the systems with one proton present in the crown. The system with the smallest crown (Fig.15(a)) shows a continuum in the region 3500– 1000 cm 21, which is particularly intense in the range
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Fig. 13. FT-IR spectra of 1:4 mixtures of the pentacene tetrakisbutylammonium salt of 1,11,12,13,14-pentahydroxypentacene (A) with LiAuCl4 (—), and a (– –), pure LiAuCl4 solution; (B) with a NaAuCl4 (—), and (– –), pure NaAuCl4 solution (taken from Ref. [22]).
3000–1600 cm 21. This result demonstrates that the large proton polarizability of this system is due to the fluctuation of the proton in a five-minima potential, with the minima at the O atoms [26]. A fiveminima potential occurs in a circular arrangement.
The proton is delocalized in the whole ring. This result is illustrated in the scheme. It represents the five proton limiting structures. The dots in this scheme indicate the minima of the proton potential. Fig. 15(b) shows the spectrum of complex 2. No
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Scheme 3.
Scheme 4.
continuum is observed, the proton is localized at the N atom. This complex shows no proton polarizability. The spectrum of complex 3 is shown in Fig. 15(c). A continuum is observed in the region 3500– 1200 cm 21 which is particularly intense at higher wavenumbers and shows two band-like structures at about 2260 and 1750 cm 21. For complex 4 (Fig. 15(d)) a continuum is observed in the region 3600–900 cm 21. A relatively intense band is superimposed on the
continuum at about 1700 cm 21. The continua observed for the complexes 3 and 4, respectively, demonstrate that these complexes show large proton polarizability due to fluctuation of the proton in six- or eight-minima proton potentials. A comparison of the spectra of the complexes 1, 3 and 4 shows that in this series of complexes the intensity of the continua shifts toward higher wavenumbers. Further, band-like structures are found. These changes occur, since the barriers in the multi-minima potentials increase in this series of complexes. Furthermore, the arising bands prove that an increase of the barriers favors Fermi resonance effects. In the following we discuss the complexes with the Li 1 ions [26]. Fig. 16 shows the corresponding IR spectra. For comparison, the spectrum of a pure LiClO4 solution is given as the dashed line. For all solutions of the complexes, the ion motion band, observed in the pure LiClO4 solution at 405 cm 21,
Fig. 14. FT-IR spectra (region 1300–1500 cm 21) of 0.2 mol dm 23 acetonitrile solutions. (– –), di-N oxide, (—), 2:1 di-N oxide: Be(AuCl4)2 complex, (-·-·-), pure Be(AuCl4)2 solution (taken from Ref. [24]).
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Fig. 15. FT-IR spectra of (– –), crown ethers and (—), their 1:1 HAuCl4 complexes: (a) crown ether 1; (b) crown ether 2; (c) crown ether 3; (d) crown ether 4 (taken from Ref. [26]).
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has vanished completely, indicating complete complex formation. In the spectra of the 1:1 complexes 1–3 intense continua are observed in the FIR region, as expected if Li 1 ions tunnel in Li 1 bonds. This result demonstrates that the Li 1 ions tunnel in these complexes in five or six multi-minima Li 1 potentials, leading to a large Li 1 polarizability of the systems. This result is illustrated in Figs. 1–3. The dots at the circles are the positions of the minima in the Li 1 multi-minima potentials. In contrast to the 1:1 complexes with a proton, these minima are located between two O
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˚ , of atoms. This is due to the larger diameter, 1.36 A the Li 1 ions. Therefore, the Li 1 ions interact with two O atoms, resulting in potential minima between the O atoms. The locations of the minima were estimated on the basis of CPK models of the complexes. With crown ether 4 no continuum is observed. Instead, a band with substructure is found in the region 360–260 cm 21. This band is due to the vibration of the Li 1 ions in O…Li 1…O bonds. In this large crown the Li 1 ion is obviously localized. For all complexes a weak broad band is observed at about 90 cm 21 which is probably caused by a skeleton vibration of the crown.
Fig. 16. FT-IR spectra of (· · · · · ·), crown ethers and (—), their 1:1 LiClO4 complexes. For comparison, the spectrum of a pure LiClO4 solution (– –), is given: (a) crown ether 1; (b) crown ether 2; (c) crown ether 3; (d) crown ether 4 (taken from Ref. [27]).
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Fig. 17. FT-IR spectra of (· · · · · ·), a crown ether with six O atoms in the crown and (—), their 1:1 NaClO4 complex. For comparison, the spectrum of a pure NaClO4 solution (– –), is given (taken from Ref. [26]).
Finally, a discussion of 1:1 Na 1 complex [26]: Fig. 17 shows the spectrum of a 1:1 Na 1 complex of the crown ether with six O atoms in the crown. A continuum in the region 225–50 cm 21 is observed. This continuum indicates Na 1 tunneling in a six-minima sodium potential. In this structure a six-minima Na 1 potential occurs. The dots at the circle show the positions of the minima of the Na 1 potential. Thus, this complex shows Na 1 polarizability due to tunneling of the Na 1 ion within this structure. In the other complexes bands are observed instead of continua. The Na 1 ions are much heavier than the Li 1 ions (atomic mass, Li 1 7 and Na 1 21). Therefore, they fluctuate slowly. To summarize we can say [26]: These crown ether complexes demonstrate clearly that also the motion of one proton or one Li 1 or one Na 1 ion, respectively, in a multi-minima potential may cause proton or cation polarizabilities. Such polarizabilities are strongly dependent on the type of the cations present and on the size of the crown. Similar results are obtained with calixarenes [27]. Furthermore, we have recently shown that also the Li 1 channel in gramicidines cause intense continua, demonstrating their large Li 1 polarizability [28]. We have learned that there are not only hydrogen bonds that show polarizabilities. Li 1 and Na 1 bonds also show Li 1 and Na 1 polarizabilities. For intramolecular heteroconjugated Li 1 bonds, as for hydrogen bonds, the 2OLi 1…ON O 2O…Li 1ON equilibrium can be shifted if the electron density at the NO group is changed by the substituents. The same is true with
the Na 1 bonds for which one is already below or near the coalescence point. The cation polarizability decreases in the series H 1, 1 D , Li 1, Na 1, K 1. Also systems with two or four Li 1 or Na 1 bonds may show cation polarizabilities because of collective cation motion. The Be 21 ion may fluctuate in a very narrow, probably four minima potential or a potential without barrier. Therefore, the absorbance of such systems is shifted toward higher wavenumbers. For crown ethers we have seen that one H 1, one Li 1 or one Na 1 may fluctuate in multi-minima potentials. Due to this fluctuation these systems show large cation polarizability, which depends strongly on the kind of the cations present and on the size of the crown. References [1] J.C. Evans, G.Y. -S, Lo, J. Phys. Chem. 69 (1965) 3223. [2] W.F. Edgell, A.T. Watts, J. Lyford, W.M. Risen Jr, J. Am. Chem. Soc. 88 (1966) 1815. [3] W.F. Edgell, J. Lyford, R. Wright, W.M. Risen Jr, A. Watts, J. Am. Chem. Soc. 92 (1970) 2240. [4] B.W. Maxey, A.I. Popov, J. Am. Chem. Soc. 89 (1967) 2230. [5] B.W. Maxey, A.I. Popov, J. Am. Chem. Soc. 91 (1969) 20. [6] T.L. Buxton, J.A. Caruso, J. Phys. Chem. 77 (1973) 1882. [7] W.J. McKenney, A.I. Popov, J. Phys. Chem. 84 (1970) 535. [8] J.L. Wuepper, A.I. Popov, J. Am. Chem. Soc. 91 (1969) 4352. [9] A.I. Popov, Pure Appl.Chem. 41 (1975) 275. [10] A. Hourdakis, A.I. Popov, J. Solution Chem. 6 (1977) 299. [11] G. Zundel, B. Brzezinski, J. Olejnik, J. Mol. Struct. 300 (1993) 573.
G. Zundel / Journal of Molecular Structure 511–512 (1999) 19–33 [12] B. Brzezinski, G. Zundel, J. Chem. Phys. 81 (1984) 1600. [13] B. Brzezinski, G. Zundel, R. Kra¨mer, J. Phys. Chem. 92 (1988) 7012. [14] B. Brzezinski, G. Zundel, R. Kra¨mer, Chem. Phys. Lett. 146 (1988) 138. [15] B. Brzezinski, J. Olejnik, G. Zundel, R. Kra¨mer, Chem. Phys. Lett. 156 (1989) 213. [16] B. Brzezinski, G. Zundel, J. Chem. Soc. Faraday Trans. 87 (1991) 1545. [17] B. Brzezinski, J. Spychala, K. Golankiewicz, G. Zundel, J. Chem. Soc. Faraday Trans. 88 (1992) 1391. [18] B. Brzezinski, J. Olejnik, G. Zundel, Chem. Phys. Lett. 167 (1990) 11. [19] B. Brzezinski, A. Jarczewski, G. Zundel, J. Mol. Liquids 67 (1995) 15.
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[20] B. Brzezinski, H. Maciejewska, G. Zundel, J. Phys. Chem. 96 (1992) 6911. [21] B. Brzezinski, H. Maciejewska-Urjasz, J. Olejnik, G. Zundel, J. Chem. Soc. Faraday Trans. 89 (1993) 1211. [22] B. Brzezinski, G. Zundel, J. Phys. Chem. 98 (1994) 2271. [23] B. Brzezinski, G. Zundel, J. Phys. Chem. 94 (1990) 4772. [24] B. Brzezinski, G. Zundel, Chem. Phys. Lett. 178 (1991) 135. [25] B. Brzezinski, St. Paszyc, G. Zundel, J. Mol. Struct. 246 (1991) 45. [26] B. Brzezinski, A. Rabold, G. Zundel, J. Phys. Chem. 99 (1995) 8519. [27] B. Brzezinski, F. Bartl, G. Zundel, J. Phys. Chem. 101 (1997) 5611. [28] F. Bartl, B. Brzezinski, B. Ro´zalski, G. Zundel, J. Phys. Chem. B 102 (1998) 5234.
Journal of Molecular Structure 511–512 (1999) 19–33 www.elsevier.nl/locate/molstruc
Li 1, Na 1, K 1 and Be 21 bonds—IR continua and cation polarizabilities of these bonds G. Zundel* Bruno Walterstr. 2, 5020 Salzburg, Austria Received 1 December 1998; accepted 11 January 1999
Abstract Not only hydrogen bonds but also Li 1, Na 1, K 1 and Be 21 bonds may show cation polarizabilities because of fluctuation and shifts of the cations. These polarizabilities are indicated by continua in the far infrared region. The electron density at the acceptor groups can be changed by the substituents. With increasing electron density the Li 1 ions in the 2OLi 1…ON O 2 … 1 O Li ON bonds shift to the acceptor. The same is true for Na 1 bonds. The fluctuation amplitude of K 1 in potassium bonds is very small. The wavenumber regions are compared in which the continua in H 1, Li 1, Na 1 and K 1 bonds occur. There are also cation polarizabilities observed because of collective cation fluctuation in two or four cation bonds. Finally, the fluctuation of one H 1, one Li 1 and one Na 1 in multi-minima potentials are discussed with crown ether systems. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Cation polarizability; IR continua; Electron density; Cation bonds
1. Introduction
2. Homoconjugated B 1M…B O B…M 1B bonds
If one dissolves, for instance, LiClO4 in acetonitrile, one observes in the far infrared (FIR) region, the so-called ion motion band [1–10]. In polar solvents this ion motion band is caused by the vibration of the cations in a cage of solvent molecules. For a LiClO4 solution in acetonitrile this ion motion band is observed at about 405 cm 21. Fig. 1 shows that the ion motion band at 405 cm 21 vanishes when molecules which form cation bonds are added to LiClO4 solutions. If these cation bonds show cation polarizabilities continua appear in the FIR region. As for hydrogen bonds, these cation polarizabilities occur due to fluctuation and shifts of the cations in these bonds [11].
Now homoconjugated B 1M…B O B…M 1B bonds, where M 1 Li 1 or Na 1 will be discussed. First, a comparison of the properties of these cation bonds with those of polarizable hydrogen and deuteron bonds will be made. For Li 1 bonds one can compare the dependence of the continua on the length and strength of the Li 1 bonds with the bond length dependence of easily polarizable hydrogen bonds. Fig. 2(A) and (B) shows the spectra of the Li 1 salts of the compounds 2 and 3, respectively. FIR continua are observed, indicating that the Li 1 ion fluctuates in the N 1Li…N O N…Li 1N bonds [12]. These continua begin at about 450 cm 21 and extend toward smaller wavenumbers. They indicate that the homoconjugated Li 1 bonds show large Li 1 polarizability.
* Tel.: 1 43-662-642311; fax: 1 43-662-64231176.
0022-2860/99/$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(99)00138-6
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Fig. 1. FT-IR spectra of 0.1 mol dm 23 chloroform solutions at 298 K: (– –), (Bu)4N 1 2:1 complex; (—), Li 1 1:1 complex. The spectrum of the pure LiClO4 solutions is given for comparison (-·-·-) (taken from Ref. [13]).
Such continua correspond to the continuum observed for H5O21. A typical double minimum Li 1 potential is present. Fig. 1 shows a continuum caused by a shorter and already relatively strong 2OLi 1…O 2 O O…Li 1O 2 bond, present in the Li 1 salts of dialcoholate molecules [13]. The continuum begins at about 380 cm 21 and shows a broad band-like structure with a maximum at about 100 cm 21. The wavenumber dependence of its intensity is comparable to that observed for the SO 1H…OS O SO…H 1OS hydrogen bonds in solutions of strong acids in dimethylsulfoxide. Fig. 3(A) shows the FT-IR spectra observed for very strong and short O 2…Li 1… 2O bonds formed between two carboxylate groups (compound 1). These bonds are completely formed, since the ion motion band observed in a pure LiClO4 dioxane solution has vanished (not shown). The continuum begins at about 250 cm 21 and is much more intense at smaller wavenumbers. These continua are caused by
intramolecular Li 1 bonds formed by two carboxylate groups (compound 1) [14]. The continua shown in Fig. 3(B) are caused by intermolecular NOLi 1…ON bonds formed between NO groups of compound 2 [15]. These Li 1 continua may correspond to the continua caused by the very strong homoconjugated hydrogen bonds formed between carboxylic acid and carboxylate groups. The appearance of these continua demonstrates that all these Li 1 bonds show Li 1 polarizability caused by Li 1 fluctuation. The integrated intensity of the continua decreases and thus, the Li 1 polarizability decreases within this series of compounds. Fig. 4 shows the results for intramolecular O 2…Na 1… 2O bonds formed by two carboxylate groups (compound 1) [13]. These bonds are fully formed, since the Na 1 ion motion band, observed in pure NaClO4 solutions, has completely vanished. The continuum indicates the Na 1 polarizability of these bonds. The continua caused by polarizable Na 1 bonds are
Fig. 2. IR spectra of acetonitrile solutions of the free bases (– –), and of a 1:1 mixture of the base and LiClO4 (—): (A) compound 2; (B) compound 3 (taken from Ref. [12]).
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Fig. 3. FT-IR spectra of solutions in a mixture of dioxane and chloroform (ratio 1:4). (A) (– –) compound 1 (0.1 mol dm 23); (—), Li 1 complex (1:1) of compound 1 (0.1 mol dm 23); (-·-·-), (0.2 mol dm 23). (B) (-·-·-), compound 2; (– –), LiClO4:N oxide (compound 2) ratio 1:2 (0.1 mol dm 23); (0.1 mol dm 23) (taken from Refs. [13] and [14]).
observed at smaller wavenumbers when compared to those of Li 1 bonds. Furthermore, the integrated intensity of these continua is lower, and hence, their cation polarizability smaller. The shift towards smaller wavenumbers is caused by the larger mass of the Na 1 ions when compared to the Li 1 ions. The smaller polarizabilities of the Na 1 bonds when compared to the Li 1 bonds are due to the smaller fluctuation amplitudes of Na 1 compared with Li 1.
3. Heteroconjugated A 2M 1…B O A 2…M 1B In the following discussion of heteroconjugated A 2M 1…B O A 2…M 1B bonds shall be made, where M 1 Li 1, Na 1 or K 1. Also these heteroconjugated Li 1 or Na 1 bonds may show large cation
Fig. 4. FT-IR spectra of solutions in a mixture of dioxane and chloroform (ratio 1:4) in the region 250–25 cm 21. (– –), compound 1 (0.1 mol dm 23); (—), Na 1 complex (1:1) of compound 1 (0.1 mol dm 23); (-·-·-) (0.2 mol dm 23) (taken from Ref. [13]).
polarizabilities due to fluctuation and shifts of the cations within them. Let us first consider the IR spectra of the family of Li 1 compounds shown in Scheme 1 [15]. Within this series of compounds the electron density at the O atom of the NO group increases from the compound with the substituent R NO2 to that with the substituent R OC2H5. Fig. 5 shows the carbonyl stretching vibrations of these compounds as a function of the substituent R. The decrease of the n (CyO) band at higher wavenumbers and the increase of the n as(CO22) band at lower wavenumbers demonstrates that within this series from the R NO2 to the R OC2H5 compound in the (I) O 2Li 1…ON O O 2…Li 1ON (II) bonds, the weight of limiting structure I decreases and that of structure II increases. In the compound R NO2 the weight of the polar structure is very small, since only the n (CyO) band is observed. For the compound R OC2H5 n (CyO) and n as(CO22) have comparable intensity. Thus, in this system both limiting structures have almost comparable weight. Thus, the equilibrium is shifted as a function of the electron density at the O atoms of the NO
Scheme 1.
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Fig. 5. Carbonyl region of the FT-IR spectra of acetonitrile solutions of the Li 1 salts of ((4R)-2-pyridyl-N oxide) acetic acids, (—), –NO2; (– –), –Br; (-·-·-), –H; ( · · · · ·), –CH3; (—), –OC2H5 (taken from Ref. [15]).
Fig. 6. FT-IR spectra of chloroform solutions of the Li 1 salts of ((4R)-2-pyridyl-N oxide) acetic acids (—), and, for comparison, the tetrabutylammonium salts (– –): (A) –NO2; (B) –Br; (C) –H; (D) –CH3; (E) –OC2H5 (taken from Ref. [15]).
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Fig. 7. FT-IR spectrum of a solution in a chloroform:acetonitrile mixture (ratio 1:2): (– –), tetrabutylammonium salt of one of the compounds; (—), 1:1 complexes formed with LiAuCl4; (· · · · · ·), pure solution of LiAuCl4 (taken from Ref. [17]).
acceptor groups. Furthermore, the shift of the n (CyO) band from 1708 to 1687 cm 21 proves that the nature of the non-polar structure also changes in this series of compounds. Fig. 6 shows the spectra taken in the FIR region. For the compound R NO2 an intense continuum is observed in the region 425–25 cm 21. Its intensity decreases, however, strongly toward smaller wavenumbers. Hence, the double minimum Li 1 potential is still relatively asymmetrical. In the cases of R Br and R H, continua are observed, beginning at 350 cm 21 and extending with large intensity down to 25 cm 21. These are the systems with a double minimum Li 1 potential which is on the average largely symmetrical. The bonds now show the largest Li 1 polarizability. In the case of R OC2H5 only a broad band with a maximum at about 90 cm 21 is observed. Thus, for all compounds, except the last, the (I) O 2Li 1…ON O O 2…Li 1ON (II) bonds show large Li 1 polarizability. The Li 1 ions can easily be shifted within these bonds by local electrical fields. With the next family of systems, the four substituted 2-diethylaminomethylphenolate-N oxides, almost the same change of the wavenumber-dependent intensity distribution of the continua is observed as a function of the substituents [16]. These (I)
Scheme 2.
Fig. 8. FT-IR spectra in the region 1800–1500 cm 21 of the sodium complexes (—), and tetrabutylammonium salts (– –), of ((4R)6tert-butyl-2-pyridyl-N oxide) acetic acid in CH3CN solutions. (A) R –NO2; (B) R –Br; (C) R –H; (D) –CH3 (taken from Ref. [18]).
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Fig. 9. FT-IR spectra in the region 300–25 cm 21 of the sodium complexes (—), and the tetrabutylammonium salts (– –), of ((4R)6-tert-butyl-2-pyridyl-N oxide) acetic acids in CHCl3 solutions. (A) R –NO3; (B) R –Br; (C) R –H; (D) R –CH3 (taken from Ref. [18]).
O 2Li 1…ON O O 2…Li 1ON (II) bonds show also large Li 1 polarizability if a double minimum Li 1 potential is present (Scheme 2). Fig. 7 shows an example of the N-(1,6-dihydro-6oxopyridine-2-yl)-amino acid family of systems. The same spectral feature is observed for all compounds [17]. In all systems an intense continuum demonstrates that all these (I) O 2Li 1…N O O 2…Li 1N (II) bonds show large Li 1 polarizability. The continuum is independent of the substituents present, since they do not change the electron density, neither at the carboxylic acid O nor at the N atom. Let us compare these Li 1 bonds with the results of hydrogen bonds. The shift of the equilibria observed in Li 1 bonds as a function of the electron density change at the acceptor O atom corresponds to the
shift observed in hydrogen bonds as a function of the DpKa. Let us now consider heteroconjugated Na 1 bonds and their Na 1 polarizabilities (Fig. 8). The (I) O 2Na 1…ON O O 2…Na 1ON (II) bonds were studied for this family (formula) of systems [18]. The bands in the carbonyl region are shown in Fig. 8. As a function of the substituents, n (CyO) decreases and n as(CO22) increases. This result shows that the equilibrium is shifted from the left- to the right-hand side as a function of the substituents. A double minimum Na 1 potential is present. Fig. 9 shows the FIR spectra of the compounds. For the compound R NO2 a broad band caused by these bonds is observed in the region 150–50 cm 21. With compound R Br a second band arises, since the sodium limiting structure II gains weight. For compound R H, i.e. if both limiting structures have considerable weight, both bands have comparable intensity. In the case of the compound R CH3, both bands fuse to a continuous absorption, observed in the region 200–25 cm 21. This spectral feature suggests that with the (I) COO 2Na 1…ON O COO 2…Na 1ON (II) bonds the band at lower wavenumbers is observed if the Na 1 ions are present at the carboxylic group (limiting structure I), whereas the band at higher wavenumbers indicates the presence of the Na 1 ions at the O atom of the NO groups (limiting structure II). In the case of the compounds R NO2, R Br and R H, the residence time of the Na 1 ions at the donor and acceptor groups of the bonds is longer than the time which is necessary for the absorption of the FIR quanta. For the compound R CH3 the residence time is, however, comparable to the time necessary for the absorption of FIR quanta, since both bands have the tendency to fuse to one broad absorption. The mass of the Na 1 ions is much larger than that of the Li 1 ions, and hence, the fluctuation frequency of the Na 1 ions is much smaller. Thus, with this Na 1 system one is either below or, for the most symmetrical system, at the coalescence point. If one approaches the coalescence point the increase of the integrated intensity of the band complex indicates that the Na 1 bonds gain more and more polarizability in this series of complexes. However, this Na 1 polarizability is much smaller than the Li 1 polarizability of heteroconjugated Li 1 bonds.
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Fig. 10. Far IR spectra of: (– –), Mannich base tetrabutylammonium salt, (—), its 1:1 mixture with KClO4 and, for comparison, (· · · · · ·), a KClO4 solution (taken from Ref. [19]).
Let us now consider heteroconjugated K 1 bonds [19]. If one adds KClO4 to a solution of this Mannich base, the K 1 Mannich base complex is formed completely. This is shown by the fact (Fig. 10) that the ion motion band, observed in the pure KClO4 solution at 142 cm 21, vanishes completely. Now a broad band is observed in the region 140–75 cm 21. Compared with the integrated intensity of the Na 1 continuum the integrated intensity of this band is much smaller. Thus, the K 1 polarizability of these K 1 bonds is small. This result is understandable, since the fluctuation amplitudes of the K 1 ions are small. 4. Wavenumber regions with H 1, D 1, Li 1 and Na 1 bonds In the case of typical double minimum bonds let us compare the wavenumber regions in which the continua corresponding to H 1, D 1, Li 1 and Na 1 bonds are observed. For hydrogen bonds the continua are observed in the region below 3000 cm 21; for deuteron bonds in the region below 2300 cm 21; while for Li 1 bonds they are found in the FIR region below 450 cm 21, and finally for Na 1 bonds they are observed below 200 cm 21. This pronounced shift is related to the mass of the fluctuating cations. Furthermore, the integrated intensities of these continua decrease within this series of cations. With increasing mass of the cations the energy levels are deeper in the potential wells. Hence, the fluctuation amplitudes of the cations become smaller and thus,
the polarizabilities of the cation bonds decrease. The K 1 bonds are an extreme limiting case.
5. Cation polarizabilities due to collective cation motion For hydrogen-bonded systems particularly large proton polarizabilities may be observed due to collective proton motion in hydrogen-bonded chains. Cation polarizabilities due to collective cation motion were also recently observed in systems with two Li 1 or Na 1 bonds, respectively. Here the electron density at the phenolic O atom is changed by the substituent R. Fig. 11 shows the far infrared region of these diLi 1-salts [20]. In this series of compounds a band with a wing toward higher wavenumbers changes to a continuum which extends from 350 to 25 cm 21. With further decreasing electron density at the phenolic O atom the continuum vanishes again. These results can be explained by considering these three Li 1 limiting structures which correspond to the three minima present in the Li 1 potential. If the electron density at the phenolic O atom is high, the Li 1 ions fluctuate between the structures I and II. If the three minima, corresponding to these three limiting structures are energetically comparable the fluctuation is the fastest and the Li 1 polarizability due to collective Li 1 motion the highest. A three-minima Li 1 potential is realized. Finally, if the electron density at the phenolic O atom is the smallest only structure III is realized. The Li 1 ions are localized at the O
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Fig. 11. FT-IR spectra of chloroform solutions of 2:1 complexes of lithium tetrachloraurate and tetrabutylammonium salts of 2,6-bis(diethylaminomethyl)-(3R, 4R)-phenol di-N oxides (—). For comparison, the spectra of chloroform solutions of the corresponding tetrabutylammonium salts (· · · · · ·) and a LiAuCl4 acetonitrile solution (– –), are given. (A) R –4-tert-C4H9; (B) R –4-C6H5; (C) R –4-Cl; (D) R –4-COOCH3; (E) R –4-NO2; (F) R –3,4-(NO2)2 (taken from Ref. [20]).
atoms of the NO groups. The Li 1 polarizability has vanished. Fig. 12 shows that analogous results are obtained for the di-Na 1 ion systems (Scheme 3) [21]. The continuum begins at about 200 cm 21 and extends to 25 cm 21. The continuum is, however, much more sensitive to the electron density at the phenolic O atom when compared to the di-Li 1 systems. This result is again understandable, since the mass of the Na 1 ion
is greater and therefore the energy levels of Na 1 are deeper in the potential wells than those of Li 1. Also, the collective Li 1 and Na 1 fluctuation in fourminima cation potentials, present in the compounds, shown in the formulae, results in large Li 1 or Na 1 polarizability of these systems [22]. This result is demonstrated by the IR spectra in Fig. 13. For the Li 1 complex an intense continuum is observed, but a continuum is also found for the Na 1 complex.
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Thus, large cation polarizabilities occur not only due to collective proton motion, but also due to collective motion of Li 1 or of Na 1 ions, respectively. Let us now compare the continua observed with polarizable Li 1 to the results obtained with Be 21 complexes. The molecular mass of Be 21 is comparable to that of Li 1. Therefore, we studied the following Be 21 complexes: Compounds with four carboxylate groups [23], complexes with four N oxide groups [24], and finally, the Be 21 complex of the gossypole molecule (see Scheme 4) [25]. With all Be 21 complexes, almost the same spectral feature is observed. Therefore, in Fig. 14 only the infrared spectrum is shown for a Be 21 N oxide complex. A very broad absorption is found, extending in all cases from about 1250 to 500 cm 21. Furthermore, one can show that all four O atoms of the four NO groups are influenced to the same extent by the cations. The molecular mass of Be 21 is comparable to that of Li 1. The absorption of the Be 21 bonds is, however, found at much higher wavenumbers. Thus, the shape of the Be 21 potential must be completely different. Due to the large affinity of the Be 21 ions to the acceptor O atoms, the Be 21 potential has a very steep slope at the O atoms. Furthermore, the potential must be much narrower than with the Li 1 bonds. Thus, the Be 21 potentials in which the Be 21 ions fluctuate are four minima potentials with very small barriers or perhaps potentials without barrier. Only with a Be 21 potential of this type it can be explained that the broad absorption is shifted toward higher wavenumbers, compared with the continua caused by Li 1 bonds with large Li 1 polarizability. Now cation polarizabilities caused by collective fluctuations and shifts of two or four cations will be discussed. The reason of proton or cation polarizabilities can also be the fluctuation of one cation in multiminima potentials. Such systems can be realized with crown ethers, adding HAuCl4, LiClO4 or NaClO4 in the ratio 1:1 to their solutions.
Fig. 12. FT-IR spectra of chloroform solutions of 2:1 complexes of sodium tetrachloraurate and tetrabutylammonium salts of 2,6bis(diethylaminomethyl)-(3R, 4R)-phenolate di-N oxides (—). For comparison, the spectra of the corresponding tetrabutylammonium salts (– –), and a NaAuCl4 acetonitrile solution (· · · · · ·) are given. (A) R –4-C6H5; (B) R –4-Cl; (C) R –4-COOCH3; (D) R –4-NO2; (E) R –3,4-(NO2)2 (taken from Ref. [21]).
6. Crown ethers Fig. 15 shows the systems with one proton present in the crown. The system with the smallest crown (Fig.15(a)) shows a continuum in the region 3500– 1000 cm 21, which is particularly intense in the range
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Fig. 13. FT-IR spectra of 1:4 mixtures of the pentacene tetrakisbutylammonium salt of 1,11,12,13,14-pentahydroxypentacene (A) with LiAuCl4 (—), and a (– –), pure LiAuCl4 solution; (B) with a NaAuCl4 (—), and (– –), pure NaAuCl4 solution (taken from Ref. [22]).
3000–1600 cm 21. This result demonstrates that the large proton polarizability of this system is due to the fluctuation of the proton in a five-minima potential, with the minima at the O atoms [26]. A fiveminima potential occurs in a circular arrangement.
The proton is delocalized in the whole ring. This result is illustrated in the scheme. It represents the five proton limiting structures. The dots in this scheme indicate the minima of the proton potential. Fig. 15(b) shows the spectrum of complex 2. No
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Scheme 3.
Scheme 4.
continuum is observed, the proton is localized at the N atom. This complex shows no proton polarizability. The spectrum of complex 3 is shown in Fig. 15(c). A continuum is observed in the region 3500– 1200 cm 21 which is particularly intense at higher wavenumbers and shows two band-like structures at about 2260 and 1750 cm 21. For complex 4 (Fig. 15(d)) a continuum is observed in the region 3600–900 cm 21. A relatively intense band is superimposed on the
continuum at about 1700 cm 21. The continua observed for the complexes 3 and 4, respectively, demonstrate that these complexes show large proton polarizability due to fluctuation of the proton in six- or eight-minima proton potentials. A comparison of the spectra of the complexes 1, 3 and 4 shows that in this series of complexes the intensity of the continua shifts toward higher wavenumbers. Further, band-like structures are found. These changes occur, since the barriers in the multi-minima potentials increase in this series of complexes. Furthermore, the arising bands prove that an increase of the barriers favors Fermi resonance effects. In the following we discuss the complexes with the Li 1 ions [26]. Fig. 16 shows the corresponding IR spectra. For comparison, the spectrum of a pure LiClO4 solution is given as the dashed line. For all solutions of the complexes, the ion motion band, observed in the pure LiClO4 solution at 405 cm 21,
Fig. 14. FT-IR spectra (region 1300–1500 cm 21) of 0.2 mol dm 23 acetonitrile solutions. (– –), di-N oxide, (—), 2:1 di-N oxide: Be(AuCl4)2 complex, (-·-·-), pure Be(AuCl4)2 solution (taken from Ref. [24]).
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Fig. 15. FT-IR spectra of (– –), crown ethers and (—), their 1:1 HAuCl4 complexes: (a) crown ether 1; (b) crown ether 2; (c) crown ether 3; (d) crown ether 4 (taken from Ref. [26]).
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has vanished completely, indicating complete complex formation. In the spectra of the 1:1 complexes 1–3 intense continua are observed in the FIR region, as expected if Li 1 ions tunnel in Li 1 bonds. This result demonstrates that the Li 1 ions tunnel in these complexes in five or six multi-minima Li 1 potentials, leading to a large Li 1 polarizability of the systems. This result is illustrated in Figs. 1–3. The dots at the circles are the positions of the minima in the Li 1 multi-minima potentials. In contrast to the 1:1 complexes with a proton, these minima are located between two O
31
˚ , of atoms. This is due to the larger diameter, 1.36 A the Li 1 ions. Therefore, the Li 1 ions interact with two O atoms, resulting in potential minima between the O atoms. The locations of the minima were estimated on the basis of CPK models of the complexes. With crown ether 4 no continuum is observed. Instead, a band with substructure is found in the region 360–260 cm 21. This band is due to the vibration of the Li 1 ions in O…Li 1…O bonds. In this large crown the Li 1 ion is obviously localized. For all complexes a weak broad band is observed at about 90 cm 21 which is probably caused by a skeleton vibration of the crown.
Fig. 16. FT-IR spectra of (· · · · · ·), crown ethers and (—), their 1:1 LiClO4 complexes. For comparison, the spectrum of a pure LiClO4 solution (– –), is given: (a) crown ether 1; (b) crown ether 2; (c) crown ether 3; (d) crown ether 4 (taken from Ref. [27]).
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Fig. 17. FT-IR spectra of (· · · · · ·), a crown ether with six O atoms in the crown and (—), their 1:1 NaClO4 complex. For comparison, the spectrum of a pure NaClO4 solution (– –), is given (taken from Ref. [26]).
Finally, a discussion of 1:1 Na 1 complex [26]: Fig. 17 shows the spectrum of a 1:1 Na 1 complex of the crown ether with six O atoms in the crown. A continuum in the region 225–50 cm 21 is observed. This continuum indicates Na 1 tunneling in a six-minima sodium potential. In this structure a six-minima Na 1 potential occurs. The dots at the circle show the positions of the minima of the Na 1 potential. Thus, this complex shows Na 1 polarizability due to tunneling of the Na 1 ion within this structure. In the other complexes bands are observed instead of continua. The Na 1 ions are much heavier than the Li 1 ions (atomic mass, Li 1 7 and Na 1 21). Therefore, they fluctuate slowly. To summarize we can say [26]: These crown ether complexes demonstrate clearly that also the motion of one proton or one Li 1 or one Na 1 ion, respectively, in a multi-minima potential may cause proton or cation polarizabilities. Such polarizabilities are strongly dependent on the type of the cations present and on the size of the crown. Similar results are obtained with calixarenes [27]. Furthermore, we have recently shown that also the Li 1 channel in gramicidines cause intense continua, demonstrating their large Li 1 polarizability [28]. We have learned that there are not only hydrogen bonds that show polarizabilities. Li 1 and Na 1 bonds also show Li 1 and Na 1 polarizabilities. For intramolecular heteroconjugated Li 1 bonds, as for hydrogen bonds, the 2OLi 1…ON O 2O…Li 1ON equilibrium can be shifted if the electron density at the NO group is changed by the substituents. The same is true with
the Na 1 bonds for which one is already below or near the coalescence point. The cation polarizability decreases in the series H 1, 1 D , Li 1, Na 1, K 1. Also systems with two or four Li 1 or Na 1 bonds may show cation polarizabilities because of collective cation motion. The Be 21 ion may fluctuate in a very narrow, probably four minima potential or a potential without barrier. Therefore, the absorbance of such systems is shifted toward higher wavenumbers. For crown ethers we have seen that one H 1, one Li 1 or one Na 1 may fluctuate in multi-minima potentials. Due to this fluctuation these systems show large cation polarizability, which depends strongly on the kind of the cations present and on the size of the crown. References [1] J.C. Evans, G.Y. -S, Lo, J. Phys. Chem. 69 (1965) 3223. [2] W.F. Edgell, A.T. Watts, J. Lyford, W.M. Risen Jr, J. Am. Chem. Soc. 88 (1966) 1815. [3] W.F. Edgell, J. Lyford, R. Wright, W.M. Risen Jr, A. Watts, J. Am. Chem. Soc. 92 (1970) 2240. [4] B.W. Maxey, A.I. Popov, J. Am. Chem. Soc. 89 (1967) 2230. [5] B.W. Maxey, A.I. Popov, J. Am. Chem. Soc. 91 (1969) 20. [6] T.L. Buxton, J.A. Caruso, J. Phys. Chem. 77 (1973) 1882. [7] W.J. McKenney, A.I. Popov, J. Phys. Chem. 84 (1970) 535. [8] J.L. Wuepper, A.I. Popov, J. Am. Chem. Soc. 91 (1969) 4352. [9] A.I. Popov, Pure Appl.Chem. 41 (1975) 275. [10] A. Hourdakis, A.I. Popov, J. Solution Chem. 6 (1977) 299. [11] G. Zundel, B. Brzezinski, J. Olejnik, J. Mol. Struct. 300 (1993) 573.
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