Georg Zundel

Intense depolarized Rayleigh scattering in Raman spectra of acids caused by large proton polarizabilities of hydrogen bonds

The depolarized Rayleigh scattering of liquid H2O and D2O and of aqueous solutions of HCl and DCl was studied as a function of concentration and temperature. This scattering is caused by time dependent changes of the anisotropy of the molecular polarizability. The scattering below 30 cm−1 is determined by a Lorentzian component. As shown in Ref. 1 with pure water, the scattering of this component is caused by the change of the molecular anisotropy of the polarizability due to breaking and formation of hydrogen bonds, whereby the mean lifetime of the hydrogen bonds determines the half‐width. This mechanism is confirmed by our results obtained with pure liquid H2O and D2O. With the acid solutions, the intensity of the scattering of this Lorentzian increases strongly with increasing acid concentration. This intensity increase is caused by the large anisotropic proton polarizability of the hydrogen bond in the H5O2+ groupings. Thus, this intense scattering is in addition to the intense continua in the IR spec...

Polarizability, proton transfer and symmetry of energy surfaces of carboxylic acid—N-base hydrogen bonds. Infrared investigations

Carboxylic acid—N-base systems are studied by i.r. spectroscopy. The position of the proton transfer equilibrium in OH ⋯ N ⇌ O–⋯ H+N bonds can be determined by considering the i.r. bands. Thus, in the case of pure 1 : 1 mixtures, 50 % proton transfer is observed in these bonds for ΔpKa= 2.3, i.e., if the pKa of the acid is 2.3 units smaller than that of the protonated base. It is shown that a double minimum energy surface is present in these bonds. An i.r. continuum indicates that these bonds are easily polarizable. Thus, B1H ⋯ B2⇌ B–1⋯ H+B2 bonds may be easily polarizable, too, when B1≠ B2. This polarizability is greatest for bonds with 50 % proton transfer, i.e., when the weights of the proton boundary structures OH ⋯ N and O–⋯ H+N are equal. With increasing degree of asymmetry of such bonds, the polarizability does not decrease appreciably within a very considerable ΔpKa region. A major decrease is found only when the ΔpKa becomes 3.5. Even when the proton is largely located at one side of the bond, as indicated by the bands, the polarizability is weaker but can still be observed and only decreases slowly with increasing asymmetry, as indicated by the continuum. The continua show band-like structures in the region 3200–1800 cm–1 which is also explained.The value ΔpKa= 2.3 for 50 % proton transfer is only valid when systems are considered with which the N-bases have additional NH groups. When no additional NH groups are present, 50 % proton transfer is obtained at ΔpKa= 4. Hence, changes of the structure of the molecules which alter their interaction characteristics with the environment, change the symmetry of the OH ⋯ N ⇌ O–⋯ H+N bonds, too.Acid—N-base 1 : 1 mixtures in which water molecules are present are also studied. Water shifts the proton transfer equilibrium, i.e., it increases the weight of the proton boundary structure O–⋯ H+N. In the presence of four water molecules per acid base pair, the ΔpKa is reduced from 2.3 to 0.9 (N-bases with additional NH groups) or from 4 to 2 (N-bases without additional NH groups). The reasons for this environmental influence on the energy surfaces are discussed. Finally, the dissociation of the acid—N-base bonds, which occurs with increasing water content in the systems is considered.

Proton conduction in bacteriophodopsin via a hydrogen-bonded chain with large proton polarizability

Abstract A Corey-Pauling-Kolthun molecular model of bacteriorhodopsin was built. This model shows that a largely structurally symmetrical hydrogen bonded chain asp, 6 tyr, glu may be formed. With regard to the total proton potential this chain shows very large proton polarizability and thus via this chain the positive charge can be conducted to the outside of the membrane via a Grotthus mechanism.

H 5 O 2 + and Other Easily Polarizable Hydrogen Bonds in Aqueous Solutions of H2SO4

Aqueous H2SO4 solutions are studied by IB-spectroscopy. The formation of easily polarizable hydrogen bonds is indicated by an intense IR continuum and the true degree of dissociation is obtained from changes of the anion bands. It is demonstrated that H502+ is of special importance in the hydrate structure network, since the hydrogen bond in this grouping is easily polarizable. With very concentrated solutions, easily polarizable acid-water hydrogen bonds are found. In these hydrogen bonds the lingering time of the proton is much larger at the water molecule than at the anion. A continuum observed with pure H2SO4 demonstrates that hydrogen bonds between sulfuric acid molecules are easily polarizable, too.

A proton pathway with large proton polarizability and the proton pumping mechanism in bacteriorhodopsin — Fourier transform difference spectra of photoproducts of bacteriorhodopsin and of its pentademethyl analogue

Abstract The first part of the photocycle of bacteriorhodopsin (BR) (BR 570 , K 630 , L 550 and M 412 ) is studied by difference Fourier transform-infrared (FT-IR) spectroscopy. These results are compared with the respective difference spectra of the pentademethyl analogue (BR a ). With the BR intermediates, the same bands are observed as in previous studies. This is especially true in the carbonyl region. With BR a , however, the behaviour of the intermediates is different from the respective intermediates of BR. The photocycle is interrupted before the L state and returns to BR a . In the case of BR, in the K intermediate, a very broad band is observed in the region 2800-2200 cm −1 . In the L intermediate, a continuous absorption is observed beginning at 2800 cm −1 and extending toward smaller wavenumbers over the whole region studied. This continuum vanishes with the formation of the M 412 intermediate and, instead, two very broad bands are found in the region 2700-2200 cm −1 . The broad band in the K state indicates that at least one strong hydrogen bond is formed in which the proton is not well localized. From the continuum observed in the L intermediate, it is concluded that a hydrogen-bonded chain is formed showing large proton polarizability caused by collective proton motion. This chain is discussed on the basis of a structural model based on literature data. The Schiff base—Asp 85 and Tyr 185—Asp 212 bonds show proton polarizability. With the transition to the intermediate M 412 , within both hydrogen bonds, the protons are shifted within these hydrogen bonds to Asp 85 and Asp 212, due to changes of the local electrical fields and to specific interactions arising due to conformational changes of the protein. In this way the negative charges in the neighbourhood of Arg 82 are neutralized and hence the proton potential at Arg 82 raised. Thus, the positive charge is shifted to the outside of the proton channel and released to the bulk water phase. The continuum is no longer observed in M 412 since the proton potentials in the chain are now asymmetrical. Hence, in the M 412 intermediate only broad bands are found, indicating strong but asymmetrical hydrogen bonds present in the active centre.

Theory of ir continua with polarizable hydrogen bonds. I. Aqueous solutions of strong acids

The wavenumber dependence of the continuous ir absorbance of strong aqueous acid solutions is calculated on the basis of SCF calculations on H5O2+. The two strongly coupled modes (νOH and νOO stretching vibrations) of the hydrogen bond in H5O2+ are treated exactly. A distribution of the local electrical field strength is taken into account since the hydrogen bond is easily polarizable. Furthermore, a distribution of OO bond lengths is used with these calculations. The calculated continua—wavenumber dependence, the absolute intensity, and the structure—are in close agreement with the ir continua observed with aqueous solutions of strong acids. The intensity of the calculated continuum decreases with increasing strength of the mean local electrical field at the polarizable hydrogen bonds, and it is independent of temperature over a very large temperature range, both of which are in very good agreement with experiment. The influence of the hydrogen bond lengths is studied, considering the system as a model f...

Proton transfer in and proton polarizability of hydrogen bonds: IR and theoretical studies regarding mechanisms in biological systems

Abstract The formation of AH⋯B⇌A − ⋯H + B bonds is an absolutely necessary precondition for all proton transfer processes. In the gas phase these equilibria are usually completely shifted to the left. These hydrogen bonds may show, however, large proton polarizability. Therefore, they interact strongly with their environments. Herewith, non-specific interactions (reaction field) as well as specific interactions are of importance. With regard to the strong interaction of these hydrogen bonds with their environments the thermodynamic quantities must be split into two components (intrinsic and extrinsic). The latter may shift these equilibria to the right. The theory of the AH⋯B⇌A − ⋯H + B bonds is described. It is shown that the proton polarizability of such bonds is two orders of magnitude larger than polarizabilities due to distortion of electron systems. With model systems it is demonstrated that many various hydrogen bonds between side chains of proteins and between side chains and phosphates, as well as hydrogen-bonded chains between side chains and several phosphates may show large proton polarizability. This is particularly true for all hydrogen bonds which may form in the active center of bacteriorhodopsin. The importance of such hydrogen bonds for mechanisms in active centers of enzymes is discussed. Finally, a simplified hypothetical picture for the mechanisms of the photocycle in bacteriorhodopsin is developed. It illustrates the great importance of such proton transfer processes via hydrogen bonds and hydrogen-bonded systems with large proton polarizability for the charge shifts and the conversion of energy into electrochemical energy, i.e., for the formation of the proton gradient at the purple membrane.

Calculated frequencies and intensities associated with coupling of the proton motion with the hydrogen bond stretching vibration in a double minimum potential surface

The influence of the coupling of the proton movement and the H bond stretching vibration in a double minimum potential energy surface on the energy levels, transitions, induced dipole moments and polarisabilities is calcualted ab initio as a function of an electric field for the H5O+2 system. The high polarisability of the hydrogen bonds remains to a large extent unchanged due to the coupling. New types of transitions occur, particularly when the tunnelling frequency and the frequency of the bond stretching vibration are comparable in size. Especially in this case numerous Fermi resonances occur due to the shift of the energy levels in the electric field, which leads to a considerable increase in the number of transitions. It is shown that the change of the frequencies of the transitions due to the induced dipole interaction of the bonds with fields from their environment is a decisive cause of the variety of energy level differences observed as a continuous absorption in the i.r. spectrum of such systems.

Homoconjugated hydrogen bonds with amidine and guanidine bases Osmometric, potentiometric and FTIR studies

Five very strong N bases, 1,5-diazabicyclo[4.3.0]non-5-ene (DBN), pK a =23.4; 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU),pK a =23.9; tetramethylguanidine (TMG), pK a =23.3; 2-phenyl-tetramethylguanidine (PhTMG), pK a =20.6; and 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD), pK a =24.97; have been studied by osmometric measurements which showed that they are monomeric in acetonitrile solutions. The constants of the formation of homoconjugated complexes were determined by potentiometric measurements. In the IR spectra of the semi-protonated complexes of DBN, DBU and TMG, the homoconjugated N + –H···NN ···H– + N hydrogen bonds cause broad band complexes in the region 3200–2500 cm -1 instead of the expected continua. This spectral peculiarity is discussed.

IR continua of hydrogen bonds and hydrogen-bonded systems, calculated proton polarizabilities and line spectra

Abstract Homoconjugated B + H·B⇌B·H + B hydrogen bonds cause intense continua in the IR spectra. Continua are also observed in the case of heteroconjugated (I) AH·B⇌A − ·H + B (II) bonds if the two proton limiting structures have noticeable weight. In this case, with the exception of extreme systems, the double minimum of the proton potentials is created by the interactions of these hydrogen bonds with their environments. Finally, structurally symmetrical or largely structurally symmetrical hydrogen-bonded chains or chains which are built up by the above mentioned hydrogen bonds cause continua by collective proton motion. Theory proves that all these three types of hydrogen bonds and hydrogen-bonded systems show so-called proton polarizabilities due to proton shifts. These proton polarizabilities are two orders of magnitude larger than usual polarizabilities due to distortion of electron systems, and with hydrogen-bonded chains they may be much larger still. The calculated line spectra of all these systems show many lines which shift as a function of the electrical field strength at the hydrogen bonds (simulating their environments). In the whole region some lines vanish, some arise with changes of the electrical field strength. The IR continua occur due to strong interaction effects of these hydrogen bonds or hydrogen-bonded systems with their environments which are caused by the large proton polarizability. Hydrogen bonds with large proton polarizability are of great significance with the electrochemistry of acid and base solutions. Furthermore, it is proved that a large number of hydrogen bonds and hydrogen-bonded systems occurring in biochemistry show large proton polarizability. Their hypothetical importance for biological functions is discussed. Finally, it is shown that at surfaces hydrogen bonds and hydrogen-bonded systems with large proton polarizability may be present and should be taken into account, when studying reactions at surfaces in which protons are involved.