Hydrogen Bond Vibrations Research Articles

Structure and Hydrogen-Bond Vibrations of Water Complexes of Azaaromatic Compounds: 7-(3′-Pyridyl)indole

Complexes with water have been studied in the regime of supersonic jet isolation for 7-(3'-pyridyl)indole, a bifunctional molecule possessing hydrogen bond donor and acceptor groups. Two rotameric forms, syn and anti, are possible, of which only the former is able to form cyclic hydrogen bonds with protic solvents. Infrared-induced ion depletion spectroscopy was used to obtain vibrational patterns for 1:1 and 1:2 complexes in the hydrogen bond stretching region. The analysis of the spectra, supported by DFT calculations, revealed that for both stoichiometries the dominant forms correspond to cyclic, doubly or triply hydrogen-bonded species. The frequencies of NH...O, OH...N, and OH...O stretching vibrations were compared with the literature data to assess the strength of single vs multiple hydrogen bonds. Several new assignments and reassignments were proposed.

Waiting Time Dynamics in Two-Dimensional Infrared Spectroscopy

We review recent work on the waiting time dynamics of coherent two-dimensional infrared (2DIR) spectroscopy. This dynamics can reveal chemical and physical processes that take place on the femto- and picosecond time scale, which is faster than the time scale that may be probed by, for example, nuclear magnetic resonance spectroscopy. A large number of chemically relevant processes take place on this time scale. Such processes range from forming and breaking hydrogen bonds and proton transfer to solvent exchange and vibrational population transfer. In typical 2DIR spectra, multiple processes contribute to the waiting time dynamics and the spectra are often congested. This makes the spectra challenging to interpret, and the aid of theoretical models and simulations is often needed. To be useful, such models need to account for all dynamical processes in the sample simultaneously. The numerical integration of the Schrodinger equation (NISE) method has proven to allow for a very general treatment of the dynamical processes. It accounts for both the motional narrowing resulting from solvent-induced frequency fluctuations and population transfer between coupled vibrations. At the same time, frequency shifts arising from chemical-exchange reactions and changes of the transition dipoles because of either non-Condon effects or molecular reorientation are included in the treatment. This method therefore allows for the disentanglement of all of these processes. The NISE method has thus far been successfully applied to study chemical-exchange processes. It was demonstrated that 2DIR is not only sensitive to reaction kinetics but also to the more detailed reaction dynamics. NISE has also been applied to the study of population transfer within the amide I band (CO stretch) and between the amide I and amide II bands (CN stretch and NH bend) in polypeptides. From the amide I studies, it was found that the population transfer can be used to enhance cross-peaks that act as structural markers for beta-sheet structure in proteins. From the amide I/II investigation, it was found that the amide II band and the hydrogen-bond stretch vibration are important parts of the relaxation pathway for the amide I vibration. With the development of simple approximations, it becomes possible to apply the NISE method even to very big systems, such as the OH stretch of bulk water, which can only be described well when large numbers of coupled vibrations are taken into account.

Crystal chemistry of synthetic Co- and Ni-analogues of natrochalcite the shortest known hydrogen bonds among mineral-type compounds. Part II: Spectroscopic studies

Four new representatives of the natrochalcite structure type s.s. – A + Me 2+ 2 ( X 6+ O 4 ) 2 [(H 2 O)(OH)], monoclinic, space group C 2/ m , Z = 2 – with A + = Na or K, Me 2+ = Co or Ni, and X 6 + = S or Se, were structurally characterised in Part I of this study. These compounds feature an extraordinary hydrogen bonding system, based on formal H 3 O − 2 units with extremely strong intramolecular hydrogen bonds. In particular, the Na-sulphates of Co and Ni exhibit the shortest known oxygen donor–acceptor distances hitherto reported for minerals or mineral-type phases at ambient conditions (and with fully occupied donor/acceptor positions), i.e. 2.429 and 2.420 A , respectively. In the present part, the nature of these formal H 3 O 2 − units is investigated by powder- and polarised single-crystal FTIR spectroscopy. The single-crystal spectra are characterised by a strictly polarised, extremely broad and asymmetric absorption band, caused by the O–H stretching vibration of the strong intramolecular hydrogen bond. In the sulphates the maximum of this band is located around 1000 cm −1 , shifted to slightly higher wavenumbers in the selenates, while the stretching modes of the longer terminal hydrogen bonds are found around 3400 cm −1 . The additional occurrence of various bending modes, e.g. at ~1640 cm −1 , strongly indicates the assignment of relevant absorption bands to distinct H 2 O and OH − groups. This also confirms the non-centrosymmetric configuration of the formal H 3 O 2 − unit and the presence of a split hydrogen position along the strong hydrogen bond, even at O···O distances as short as 2.42 A . In addition, polarised optical absorption spectra of the two sodium cobalt compounds are presented. The spectra are interpreted in terms of a pseudotetragonal crystal field, compressed along the short octahedral H (2) O–Co–OH (2) axis. The extracted crystal field and interelectronic repulsion parameters indicate the presence of comparatively strong crystal fields and a rather high ionic character of the Co–O bonds in these natrochalcite-type compounds. Due to the strong contribution of the close H 2 O/OH − ligands along the pseudotetragonally compressed axis, the spectroscopic tetragonal field compression exceeds the respective geometric compression of the CoO 6 polyhedra, in agreement with the high position of water molecules in the spectrochemical series of ligands.

Crystal growth, characterization, structural characteristics and second harmonic generation in N-benzoyl glycine crystals

Optical quality single crystals of N-benzoyl glycine (NBG) with good morphology of size 15 mm × 3 mm × 3 mm have been grown by slow evaporation solution growth technique using acetone–water (60:40) as solvent. The grown crystals were characterised by elemental analysis, Fourier transform infrared spectroscopy, FT-Raman spectroscopy and powder X-ray diffraction techniques. From thermal studies it was found that the NBG crystal is stable up to 188 °C. The OH and NH groups involving in strong hydrogen bonding could be responsible for the improved mechanical and thermal properties of NBG. The crystal structure of NBG is stabilized due to strong O–H…O and N–H…O hydrogen bond interactions. The second harmonic generation (SHG) efficiency of NBG is two times greater than that of urea. The main contribution to the SHG in NBG results from the hydrogen bond interactions that appeared as very intense IR bands of the hydrogen bond vibrations in the FTIR and FT-Raman spectrum of NBG.

Impact of Methyl Rotor in the Excited State Level Mixing of Doubly Hydrogen-Bonded Complexes of 2-Pyridone

We have presented in this paper the laser-induced fluorescence excitation and resolved fluorescence spectra of five 1:1 hydrogen-bonded complexes of 2-pyridone (2PY) with formic acid (FA), acetic acid (AA), propanoic acid (PA), formamide (FM), and acetamide (AM). The resolved fluorescence spectra, measured following excitation to different single vibronic levels of the dimers indicate that the intermolecular hydrogen bond vibrations undergo mixing with a number of intramolecular modes of the 2PY moiety in the excited state. A comparison of the emission spectral features of these dimers clearly indicates that the methyl groups belonging to the AA and AM moieties spectacularly accelerate the vibrational energy redistribution (IVR) in the 2PY moiety. On the other hand, although the molecular size of PA is bigger than AA, the spectral features of the 2PY-PA dimer bear signatures of a slower IVR rate compared to those of 2PY-AA. We propose that hyperconjugation of the methyl group with the cyclic hydrogen-bonded network involving AA and AM is responsible for the observed phenomenon.

Theoretical IR spectral density of H-bond in liquid phase: Combined effects of anharmonicities, Fermi resonances, direct and indirect relaxations

The bandshape of hydrogen bond vibrations in liquids is studied theoretically, taking into account the anharmonic coupling between the high-frequency stretching vibration, X − H → … Y (double well potential), and low-frequency lattice phonons, X ← − H … Y → (Morse potential), as well as Fermi resonances between states involving the X − H → … Y stretching and overtones or combinations of some internal modes. The relaxation of the fast mode (direct damping) and of the H-bond bridge (indirect damping) is incorporated by aid of previous results [N. Rekik, B.Ouari, P. Blaise, O. H. Rousseau, J. Mol. Struct. (Theochem) 687 (2004) 125–133]. The approach has been constructed in the framework of the linear response theory for which spectral density is obtained by Fourier transform of the autocorrelation function of the dipole moment operator of the fast mode. Numerical results show that mixing of all these effects results in a broad and complicated structure. The indirect damping favourites have a more sensitive smoothing of the full line shapes whereas the direct one entrains a broadening of the spectra.

Anharmonic interactions and infrared bandshape of the hydrogen bond vibration of potassium hydrogen (deuterium) maleate crystals

The spectral profiles of the hydrogen bond vibrations in the potassium hydrogen maleate (KHM) crystal and its deuterated analogue (KDM) were studied theoretically taking into account the Fermi resonance (FR) effect and strong coupling between the hydrogen bond vibration and lattice phonons. In addition, ab initio calculations of the frequencies and intensities of the H(D)-maleate ion for both C 2v and C s symmetry were made. It was found that C s symmetry has to be used and that practically all intense bands have a contribution from the OH vibration. It was shown that, taking into account all the factors: ab initio results, FR and anharmonicity of vibrations, the experimental spectra for both KHM and KDM crystals can be explained adequately. Anharmonic constants were obtained as a result of fitting theoretical spectra to the experimental ones.

Laser Induced Fluorescence Spectroscopy of a Mixed Dimer between 2-Pyridone and 7-Azaindole

We report here the laser induced fluorescence excitation (FE) and dispersed fluorescence (DF) spectra of a 1:1 mixed dimer between 7-azaindole (7AI) and 2-pyridone (2PY) measured in a supersonic free jet expansion of helium. Density functional theoretical calculation at the B3LYP/6-311++G** level has been performed for predictions of the dimer geometry and normal mode vibrational frequencies in the ground electronic state. A planar doubly hydrogen-bonded structure has been predicted to be the most preferred geometry of the dimer. In the FE spectrum, sharp vibronic bands are observed only for excitation of the 2PY moiety. A large number of low-frequency vibronic bands show up in both the FE and DF spectra, and those bands have been assigned to in-plane hydrogen bond vibrations of the dimer. Spectral analyses reveal Duschinsky-type mixing among those modes in the excited state. No distinct vibronic band structure in the FE spectrum was observed corresponding to excitations of the 7AI moiety, and the observation has been explained in terms of nonradiative electronic relaxation routes involving the 2PY moiety.

N, N′-Dimethylpiperazine mono-betaine complex with two molecules of 3-iodobenzoic acid and water studied by X-ray diffraction, DFT, FTIR and NMR methods

N, N′-Dimethylpiperazine mono-betaine ( N-carboxymethyl- N, N′-dimethylpiperazinium inner salt, MBPZ) forms a crystalline complex with two molecules of 3-iodobenzoic acid (3IBA). The complex crystallizes as a monohydrate, MBPZ(3IBA) 2H 2O. The complex has been characterized by X-ray diffraction and FTIR, Raman, 1H and 13C NMR spectroscopies. The crystals are monoclinic, space group P2 1/ c, with a = 6.7810(17), b = 38.391(4), c = 10.0542(16) Å, β = 90.024(18)°. The piperazinium ring has a chair conformation with the N +CH 2COO − substituent in the axial and both methyl groups in the equatorial positions. The 3IBA molecules interact with the carboxylate group of N +CH 2COO − and N′ atom through the O–H···O and O–H···N hydrogen bonds of the lengths 2.550 and 2.695 Å. The water molecules link MBPZ(3IBA) 2 complexes by the O–H···OOCCH 2N + hydrogen bonds of 2.850 and 2.793 Å into infinite chains. The MBPZ(3XBA) 2H 2O (X = I, Br, Cl), MBPZ(3IBA) 2 complexes and MBPZ have been analyzed by the B3LYP/MidiX level of theory. The conformers with the axial CH 2COO − substituent have lower energies than these with the equatorial CH 2COO − one. The bands at 3433, 2790, 2450, 1915 cm −1 assigned to the ν(OH) vibration of the various hydrogen bonds, and two bands attributed to the ν(C O) (1680 cm −1) and ν as(COO) (1660 cm −1) vibrations in the FTIR spectrum confirm the structure of the title complex. The 1H– 13C HMBC spectrum has been analyzed in order to assign the carbon-13 chemical shifts.

Absolute Rate Calculations. Proton Transfers in Solution

The reaction path of the intersecting-state model is used in transition-state theory with the semiclassical correction for tunneling (ISM/scTST) to calculate the rates of proton-transfer reactions from hydrogen-bond energies, reaction energies, electrophilicity indices, bond lengths, and vibration frequencies of the reactive bonds. ISM/scTST calculations do not involve adjustable parameters. The calculated proton-transfer rates are within 1 order of magnitude of the experimental ones at room temperature, and cover very diverse systems, such as deprotonations of nitroalkanes, ketones, HCN, carboxylic acids, and excited naphthols. The calculated temperature dependencies and kinetic isotope effects are also in good agreement with the experimental data. These calculations elucidate the roles of the reaction energy, electrophilicity, structural parameters, hydrogen bonds, tunneling, and solvent in the reactivity of acids and bases. The efficiency of the method makes it possible to run absolute rate calculations through the Internet.

Hydrogen-bond vibrations in the S1↔S spectra of a nucleobase pair analog: A mixed dimer between 2-pyridone and formamide

The vibronically resolved electronic spectra for S(1)<-->S(0) transitions of a mixed dimer between 2-pyridone (2PY) and formamide have been measured in a supersonic free jet expansion using laser-induced fluorescence spectroscopy. Quantum chemistry method at different levels of theory has been used to optimize the geometries of the dimer for the S(0) and S(1) electronic states and also to calculate the normal vibrational modes. Assignments for the vibronic bands observed in the dispersed fluorescence spectrum of the 0(0) (0) band have been suggested with the aid of the ground state frequencies calculated by density functional theoretical method. Spectral analysis reveals that electronic excitation causes extensive mixing of the low-frequency intermolecular vibrational modes of the dimer with some of the intramolecular modes of the 2PY moiety. This spectral behavior is consistent with the complete active space self-consistent field theoretical prediction that with respect to a number of geometrical parameters the dimer geometry in S(1) is significantly distorted from the geometry of the S(0) state.

Theoretical IR Spectra for Water Clusters (H2O)n (n = 6−22, 28, 30) and Identification of Spectral Contributions from Different H-Bond Conformations in Gaseous and Liquid Water

The vibrational IR spectra in the O-H stretching region are computed for water clusters containing 6-22, 28, and 30 molecules using quantum-chemical calculations (B3LYP and an augmented basis set). For the cluster with 20 molecules, several different structures were studied. The vibrational spectrum was partitioned into contributions from different molecules according to their coordination properties. The frequency shifts depend on the number of donated/accepted H-bonds primarily of the two molecules participating in the H-bond, but also of the surrounding molecules H-bonding to these molecules. The frequencies of H-bonds between two molecules of the same coordination type are spread over a broad interval. The most downshifted hydrogen-bond vibrations are those donated by a single-donor 3-coordinated molecule where the H-bond is accepted by a single-acceptor molecule. The H-bonded neighbors influence the downshift, and their contribution can be rationalized in the same way as for the central dimer. Single donors/acceptors cause larger downshifts than 4-coordinated molecules, and the least downshift is obtained for double donors/acceptors. This result is at variance with the conception that experimental liquid water spectra may be divided into components for which larger downshifts imply higher H-bond coordination. A mean spectral contribution for each coordination type for the donor molecule was derived and fitted to the experimental liquid water IR spectrum, which enabled an estimation of the distribution of H-bond types and average number of H-bonds (3.0 +/- 0.2) in the liquid.

Molecular structure, hydrogen bonding, basicity and spectroscopic properties of 3-hydroxypyridine betaine hydrochloride monohydrate

The effect of hydrogen bonding, inter- and intramolecular electrostatic interactions on the structure of 3-hydroxy-pyridine betaine hydrochloride monohydrate (1-carboxymethyl-3-hydroxypyridinium chloride monohydrate), 3-HO-PBH·Cl·H 2O, has been studied by X-ray diffraction, 1H and 13C NMR and FTIR spectroscopies, and by the B3LYP/6-31G(d,p) calculations. In the crystal, the Cl − anion is connected with protonated betaine via the hydrogen bond, O C O H⋯Cl − = 2.993(2) Å and with neighboring H 2O molecules via the 3.111(1) and 3.578(1) Å bonds, while the 3-OH group interacts with water molecule by the hydrogen bond of 2.566(2) Å, forming an aggregate along the [b] direction. The water molecule additionally forms hydrogen bonds of 2.888(3) Å to the C O bond of O C OH group. On recrystallization the 1:1 complex slowly converts into the 2:1 complex, bis(3-hydroxy-pyridine betaine) hydrochloride, [bis(1-carboxymethyl-3-hydroxypyridinium) chloride], [(3-HO-PB) 2H·Cl]. The geometries of 3-HO-PBH·Cl·H 2O in the gas phase (vacuum), DMSO and water solutions have been optimized by the B3LYP/6-31G(d,p) level of theory using the COSMO model. Good linear correlations between 13C and 1H experimental chemical shifts and GIAO/ B3LYP/6-31G(d,p) calculated magnetic isotropic shielding tensors ( σ) have been obtained. The FTIR spectrum of the 1:1 complex shows a broad and intense absorption in the 3100–2500 cm −1 region due to the stretching vibration of hydrogen bonds between the Cl − anion and COOH, OH substituents and H 2O molecules, and the νC O band at 1739 cm −1. The spectrum of the 2:1 complex shows an additional broad absorption in the 1900–800 cm −1 region due to the O H·O hydrogen bonds between COO groups.

A phase-matchable nonlinear optical material N-(3-nitrophenyl)phthalimide: Synthesis, crystal growth and characterization

The N-(3-nitrophenyl)phthalimide (N3NP) single crystal of size 4 mm×1 mm×0.5 mm, sufficient enough to confirm the existence of phase-matching direction in the crystal using the Kurtz and Perry powder second-harmonic generation (SHG) measurements, was grown by slow evaporation solution growth technique using DMF solvent. Powder XRD, FTIR, CHN analysis and H 1NMR technique were employed to characterize N3NP single crystal. The single crystal XRD data revealed the noncentrosymmetric crystal structure of N3NP, which is an essential criterion for SHG. The main contribution to the SHG results comes from the strong hydrogen bonds, which also appear as intense IR bands of hydrogen bond vibrations in spectral characteristics. From DT/TG analysis, the N3NP was found to be thermally stable up to 246 °C. The four C–H······O hydrogen bonds, which hold the molecules of N3NP in three-dimensional crystal lattice, are responsible for the higher thermal stability. The UV–Vis optical absorption spectrum of N3NP shows the lower optical cut off at 370 nm and was transparent in the visible region. The N3NP is a phase-matchable NLO crystal. This crystal can be used as an efficient frequency doubler and optical parametric oscillator due to its high SHG conversion efficiency.

Understanding the Influence of Polymorphism on Phonon Spectra: Lattice Dynamics Calculations and Terahertz Spectroscopy of Carbamazepine

Rigid molecule atomistic lattice dynamics calculations have been performed to predict the phonon spectra of the four polymorphs of carbamazepine, and these calculations predict that there should be differences in the spectra of all four forms. Terahertz spectra have been measured for forms I and III, and there are clearly different features between polymorphs' spectra, that are accentuated at low temperature. While carbamazepine adopts the same hydrogen bonded dimers in all of its known polymorphs, the calculations show that differences in packing arrangements of the dimers lead to changes in the frequency ranges for each type of hydrogen bond vibration, giving a physical explanation to the observed differences between the spectra. Although the agreement between calculated and observed spectra does not allow a definitive characterization of the spectra, it is possible to make tentative assignments of many of the observed features in the terahertz region for the simpler form III; we can only make some tentative assignments of specific modes in the more complex spectrum of form I. While harmonic rigid molecule lattice dynamics shows promise for understanding the differences in spectra between polymorphs of organic molecules, discrepancies between observed and calculated spectra suggest areas of improvement in the computational methods for more accurate modeling of the dynamics in molecular organic crystals.

Structural characteristics and second harmonic generation in l-threonine crystals

l-threonine, an organic material with interesting non-linear optical properties, has been grown in solution and characterized by X-ray diffraction, FT infrared spectral, Raman spectral, UV VIS spectral and thermal techniques. The structural and spectral characteristics show that strong hydrogen bonds appearing in l-threonine have an influence on the stabilization of the crystal structure. The main contributions to the second harmonic generation in threonine results from the hydrogen bond and also from the vibrational part due to very intense IR bands of the hydrogen bond vibrations. The OH and NH involved in strong hydrogen bonds are responsible for the higher thermal stability of l-threonine compared to other organic non-linear optical materials. l-threonine is thermally stable up to 270 °C.

Electrostatic DFT Map for the Complete Vibrational Amide Band of NMA

An anharmonic vibrational Hamiltonian for the amide I, II, III, and A modes of N-methyl acetamide (NMA), recast in terms of the 19 components of an external electric field and its first and second derivative tensors (electrostatic DFT map), is calculated at the DFT(BPW91/6-31G(d,p)) level. Strong correlations are found between NMA geometry and the amide frequency fluctuations calculated using this Hamiltonian together with the fluctuating solvent electric field obtained from the MD simulations in TIP3 water. The amide I and A frequencies are strongly positively correlated with the C=O and N-H bond lengths. The C=O and C-N amide bond lengths are negatively correlated, suggesting the solvent-induced fluctuations of the contribution of zwitterionic resonance form. Sampling the global electric field in the entire region of the transition charge densities (TCDs) is required for accurate infrared line shape simulations. Collective electrostatic solvent coordinates which represent the fluctuations of the 10 lowest amide fundamental and overtone states are reported. Normal-mode analysis of an NMA-3H(2)O cluster shows that the 660 cm(-1) to 1100 cm(-1) oscillation found in the frequency autocorrelation functions of the amide modes may be ascribed to the two bending vibrations of intermolecular hydrogen bonds with the amide oxygen of NMA.

Vibrational coupling in carboxylic acid dimers

The vibrational level splitting in the ground electronic state of carboxylic acid dimers mediated by the doubly hydrogen-bonded networks are investigated using pure and mixed dimers of benzoic acid with formic acid as molecular prototypes. Within the 0-2000-cm(-1) range, the frequencies for the fundamental and combination vibrations of the two dimers are experimentally measured by using dispersed fluorescence spectroscopy in a supersonic jet expansion. Density-functional-theory calculations predict that most of the dimer vibrations are essentially in-phase and out-of-phase combinations of the monomer modes, and many of such combinations show significantly large splitting in vibrational frequencies. The infrared spectrum of the jet-cooled benzoic acid dimer, reported recently by Bakker et al. [J. Chem. Phys. 119, 11180 (2003)], has been used along with the dispersed fluorescence spectra to analyze the coupled g-u vibrational levels. Assignments of the dispersed fluorescence spectra of the mixed dimer are suggested by comparing the vibronic features with those in the homodimer spectrum and the predictions of density-functional-theory calculation. The fluorescence spectra measured by excitations of the low-lying single vibronic levels of the mixed dimer reveal that the hydrogen-bond vibrations are extensively mixed with the ring modes in the S1 surface.

Anharmonic midinfrared vibrational spectra of benzoic acid monomer and dimer

Anharmonic vibrational calculations for the benzoic acid monomer and dimer in the mid-IR regime (500-1800 cm(-1)) are reported. Harmonic frequencies and intensities are obtained at the DFT/B3LYP level of theory employing D95(d,p) and cc-pVTZ basis sets. Anharmonic corrections obtained from standard perturbation theory lead to redshifts of 1%-3%. In almost all cases, the resulting frequencies deviate by less than 1% from previous measurements [Bakker et al., J. Chem. Phys. 119, 11180 (2003)]. Calculated intensities are in qualitative agreement with the absorption experiment, with the cc-pVTZ values being superior to the D95(d,p) ones for a few modes of the dimer. The antisymmetric out-of-plane bending mode of the dimer, which is strongly blueshifted with respect to the monomer frequency, represents a remarkable exception: The harmonic frequencies obtained for the two basis sets differ notably from each other, and the anharmonically corrected frequencies deviate from the experimental value by 8% [D95(d,p)] or 3% (cc-pVTZ). Nonperturbative calculations in reduced dimensionality reveal that the relatively small total anharmonic shift (few tens of cm(-1)) comprises of partly much larger contributions (few hundreds of cm(-1)) which are mostly canceling each other. Many of the individual anharmonic couplings are beyond the validity of second-order perturbation theory based on cubic and semidiagonal quartic force constants only. This emphasizes the need for high-dimensional, nonperturbative anharmonic calculations at high quantum-chemical level when accurate frequencies of H-atom vibrations in double hydrogen bonds are sought for.

Possible mechanisms of dielectric relaxation of liquid water and calculation of the temperature dependence of the complex permittivity of water

The spectra of the complex dielectric permittivity and absorption of water (H2O) in the frequency range 0–1000 cm−1 are calculated for a wide temperature interval. Using the method of autocorrelation functions, the dielectric response of dipoles rotating in potential wells of three types is found. The majority of dipoles (about 90%) rotate in a deep and comparatively narrow potential well, whose profile resembles an upside-down hat. Such a potential models a molecular structure with strongly bent and/or broken hydrogen bonds. The hat model describes the complex permittivity in the low-frequency (Debye) range and in the range 300–1000 cm−1. The remaining dipoles (∼10%) execute harmonic vibrations of two types: rotational vibrations about the equilibrium direction of a hydrogen bond and translational vibrations along this direction. These types of motion yield the dielectric response in the frequency range 10–300 cm−1. This response is described by the Lorentz lines in terms of the harmonic oscillator model and the truncated parabola model. The hat–harmonic oscillator–truncated parabola composite model provides good agreement with experimental spectra. The lifetimes of the three types of motion considered are about 10, 0.2, and 0.05 ps, respectively. They characterize (i) tetrahedral translations of molecules accompanied by their rotations, (ii) librations of dipoles in the hatlike potential well, and (iii) elastic interactions of hydrogen-bonded molecules. Based on data of independent methods of investigation, it is concluded that the temperature 300 K is a singular point with respect to the properties of liquid water.