On prediction of OH stretching frequencies in intramolecularly hydrogen bonded systems

NH Stretching Frequencies of Intramolecularly Hydrogen-Bonded Systems: An Experimental and Theoretical Study

The vibrational NH stretching transitions in secondary amines with intramolecular NH···O hydrogen bonds were investigated by experimental and theoretical methods, considering a large number of compounds and covering a wide range of stretching wavenumbers. The assignment of the NH stretching transitions in the experimental IR spectra was, in several instances, supported by measurement of the corresponding ND wavenumbers and by correlation with the observed NH proton chemical shifts. The observed wavenumbers were correlated with theoretical wavenumbers predicted with B3LYP density functional theory, using the basis sets 6-311++G(d,p) and 6-31G(d) and considering the harmonic as well as the anharmonic VPT2 approximation. Excellent correlations were established between observed wavenumbers and calculated harmonic values. However, the correlations were non-linear, in contrast to the results of previous investigations of the corresponding OH···O systems. The anharmonic VPT2 wavenumbers were found to be linearly related to the corresponding harmonic values. The results provide correlation equations for the prediction of NH stretching bands on the basis of standard B3LYP/6-311++G(d,p) and B3LYP/6-31G(d) harmonic analyses, with standard deviations close to 38 cm⁻¹. This is significant because the full anharmonic VPT2 analysis tends to be impractical for large molecules, requiring orders of magnitude more computing time than the harmonic analysis.

Structural Studies of β-Diketones and Their Implications on Biological Effects

The paper briefly summarizes methods to determine the structure of β-diketones with emphasis on NMR methods. Density functional calculations are also briefly treated. Emphasis is on the tautomeric equilibria of β-diketones in relation to biological effects. Relevant physical parameters such as acidity and solubility are treated. A series of biologically active molecules are treated with respect to structure (tautomerism). Characteristic molecules or groups of molecules are usnic acids, tetramic and tetronic acids, o-hydroxydibenzoylmethanes, curcumines, lupulones, and hyperforines.

Quantitative Analysis of Infrared Spectra of Binary Alcohol + Cyclohexane Solutions with Quantum Chemical Calculations

Hydrogen bonding has profound effects on the behavior of molecules. Fourier-transform infrared spectroscopy is the technique most commonly used to qualitatively identify hydrogen-bonding moieties present in a chemical sample. However, quantitative analysis of infrared (IR) spectra is nontrivial for the hydroxyl stretching region where hydrogen bonding is most prominently expressed in organic alcohols and water. Specifically, the breadth and extreme overlap of the O-H stretching bands, and the order of magnitude variability of their IR attenuation coefficients complicates the analysis. In the present work, sequential molecular dynamics simulations and quantum mechanical calculations are used to develop a function to relate the integrated IR attenuation coefficient to the vibrational frequencies of hydroxyl bands across the O-H stretching region. This relationship is then used as a guide to develop an attenuation coefficient scaling function to quantitatively determine concentrations of alcohols in a hydrocarbon solution from experimental IR spectra by integration across the entire hydroxyl frequency range.

UV synchrotron radiation linear dichroism spectroscopy of the anti-psoriatic drug anthralin

Anthralin (1,8-dihydroxyanthrone, 1,8-dihydroxy-9(10H)-anthracenone), also known as dithranol and cignolin, is one of the most efficient drugs in the treatment of psoriasis and other skin diseases. The precise mode of biochemical action is not fully understood, but the activity of the drug is increased by the influence of UV radiation. In the present investigation, the UV absorption of anthralin is studied by synchrotron radiation linear dichroism (SRLD) spectroscopy on molecular samples partially aligned in stretched polyethylene, covering the near and vacuum UV regions with wavenumbers ranging from 23,000 to 58,000 cm–1(430–170 nm). The observed polarization spectra are well predicted by quantum chemical calculations using time-dependent density functional theory (TD–DFT). About a dozen spectral features are assigned to computed electronic transitions. The calculations support interpretation of the anomalous fluorescence of anthralin as a result of barrier-less excited state intramolecular proton transfer (ESIPT) to the tautomer 8,9-dihydroxy-1(10H)-anthracenone.

Hydrogen-Bond Structure and Dynamics of TADDOL Asymmetric Organocatalysts Correlate with Catalytic Activity

The catalytic efficiency of diol-based organocatalysts has been shown to strongly depend on the diols molecular structure including hydrogen-bonding, yet, the underlying molecular-level origins have remained elusive. Herein a study on the inter- and intramolecular hydrogen-bonding of two isomeric diol-based catalysts (TADDOLs) in solution is presented: 1-Naphthyl substituted TADDOL (1nTADDOL), which exhibits high catalytic efficiency, and 2-naphthyl substituted TADDOL (2nTADDOL), which is a poor catalyst. Using nuclear magnetic resonance and infrared spectroscopy, comparable hydrogen-bond strengths for both TADDOLs in solution were found, however, significantly slower bonding dynamics for 1nTADDOL. In aromatic solvents, 1nTADDOL forms less, but longer-lived, intermolecular OH⋅⋅⋅π bonds to solvent molecules, as compared to 2nTADDOL. Thus, rather than previously suggested differences in intermolecular hydrogen-bonding strengths, the results suggest that the hydrogen-bonding kinetics and entropies differ for both TADDOLs, which also explains their vastly different catalytic activities.

Tautomerism and stereodynamics in Schiff bases from gossypol and hemigossypol with N-aminoheterocycles

Gossypol, a natural male contraceptive, can be functionalized via dynamic imine linkages, whose tautomerism has been explored in detail.

NMR and IR Investigations of Strong Intramolecular Hydrogen Bonds

For the purpose of this review, strong hydrogen bonds have been defined on the basis of experimental data, such as OH stretching wavenumbers, νOH, and OH chemical shifts, δOH (in the latter case, after correction for ring current effects). Limits for O-H···Y systems are taken as 2800 > νOH > 1800 cm-1, and 19 ppm > δOH > 15 ppm. Recent results as well as an account of theoretical advances are presented for a series of important classes of compounds such as β-diketone enols, β-thioxoketone enols, Mannich bases, proton sponges, quinoline N-oxides and diacid anions. The O···O distance has long been used as a parameter for hydrogen bond strength in O-H···O systems. On a broad scale, a correlation between OH stretching wavenumbers and O···O distances is observed, as demonstrated experimentally as well as theoretically, but for substituted β-diketone enols this correlation is relatively weak.

Molecular reorganization of selected quinoline derivatives in the ground and excited states—Investigations via static DFT

Quinoline derivatives are interesting objects to study internal reorganizations due to the observed excited-state-induced intramolecular proton transfer (ESIPT). Here, we report on computations for selected 12 quinoline derivatives possessing three kinds of intramolecular hydrogen bonds. Density functional theory was employed for the current investigations. The metric and electronic structure simulations were performed for the ground state and first excited singlet and triplet states. The computed potential energy profiles do not show a spontaneous proton transfer in the ground state, whereas excited states exhibit this phenomenon. Atoms in Molecules (AIM) theory was applied to study the nature of hydrogen bonding, whereas Harmonic Oscillator Model of aromaticity index (HOMA) provided data of aromaticity evolution as a derivative of the bridge proton position. The AIM-based topological analysis confirmed the presence of the intramolecular hydrogen bonding. In addition, using the theory, we were able to provide a quantitative illustration of bonding transformation: from covalent to the hydrogen. On the basis of HOMA analysis, we showed that the aromaticity of both rings is dependent on the location of the bridge proton. Further, the computed results were compared with experimental data available. Finally, ESIPT occurrence was compared for the three investigated kinds of hydrogen bridges, and competition between two bridges in one molecule was studied.

Deuterium isotope effects on ¹³C-NMR chemical shifts of 10-hydroxybenzo[h]quinolines

Deuterium isotope effects on ¹³C-NMR chemical shifts are investigated in a series of 10-hydroxybenzo[h]quinolines (HBQ's) The OH proton is deuteriated. The isotope effects on ¹³C chemical shifts in these hydrogen bonded systems are rather unusual. The formal four-bond effects are found to be negative, indicating transmission via the hydrogen bond. In addition unusual long-range effects are seen. Structures, NMR chemical shifts and changes in nuclear shieldings upon deuteriation are calculated using DFT methods. Two-bond deuterium isotope effects on 13C chemical shifts are correlated with calculated OH stretching frequencies. Isotope effects on chemical shifts are calculated for systems with OH exchanged by OD. Hydrogen bond potentials are discussed. New and more soluble nitro derivatives are synthesized.