Proton Transfer Dynamics in Acetylacetone: A Mixed Quantum-Classical Simulation of Vibrational Spectra

New inorganic inhibitors derived from cefotaxime to enhance corrosion resistance of mild steel in 3% NaCl

To overcome the threat of corrosion and its cost, a new Schiff base was prepared and utilized to synthesize inorganic inhibitors to enhance corrosion resistance and reduce current density. The Schiff base was obtained from the interaction of cefotaxime with acetylacetone, while 1H NMR and IR spectra were used to confirm the preparation. Moreover, FeIII, CoII, NiII and CuII metal salts were reacted with the Schiff base to give the corresponding complexes. Meanwhile, the non-ionic behavior of the observed complexes in solutions was proved from the conductance results. In addition, the octahedral geometry and the postulated structure of complexes were determined from CHNM%, IR spectroscopy, UV-visible spectra, and TGA analysis. Also, the energy of molecular orbitals (HOMO and LUMO) and other quantum mechanics parameters were calculated using the DFT method. The observed results indicated the reactivity of metal complexes and their ability to donate electrons more than the Schiff base. Furthermore, the corrosion rate of a steel sample under various concentrations of inhibitors was calculated by a potentiodynamic polarization test. The obtained data displayed that metal complexes declined the corrosion rate more than the Schiff base; therefore, the binding between the metal ion and the Schiff base improved the inhibition efficiency.

Validation of potential energy distribution by VEDA in vibrational assignment some of β-diketones; comparison of theoretical predictions and experimental vibration shifts upon deutration

The harmonic vibrational frequencies of the cis-enol forms of some of β-diketones with different substitution in beta position, vis. H, CH3, and Ph ring, as the symmetric and asymmetric molecules, were calculated using density functional theory (DFT) at the B3LYP/6-311++G(d,p) level of theory. The results of DFT calculations were used to obtain the potential energy distribution (PED) by VEDA software. The PED results compared with the Gauss View animation, as our reassignments, and the experimental IR shifts upon deuteration of hydrogen in the OH and CHα. According to our study, the PED contributions, Gauss View animation and observed shifts show similar results for most of the bands which are not coupled with the OH and/or CHα bending, such as asymmetric and symmetric CH₃ stretching and in-plane deformations, CH₃ rocking vibrations and 8a, 19b, 9a, 15, 18a, and 12 motions of the phenyl ring. The largest discrepancies were observed in the 1700-1000 cm^-1 region, likely due to the coupling with the OH and CHα in-plane bending vibrations, such as νaC = C-C = Ο, νsC = C-C = Ο and δOH. Furthermore, the calculated PED contributions by VEDA software do not well define the vibrational contributions to those groups in the molecule that are directly involved in the intramolecular hydrogen bond and the observed failure of the VEDA procedure is possibly due to inappropriateness of the default options.

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.

Double deuterated acetylacetone in neon matrices: infrared spectroscopy, photoreactivity and the tunneling process

Open enol conformers of double deuterated acetylacetone are produced by UV and IR irradiation, allowing the analysis of the tunnelling effect between them.

Proton conduction in Mo(vi)-based metallo-supramolecular polymers

High proton conduction was observed in a Mo(vi)-based metallo-supramolecular polymer with carboxylic acids at 95%RH. The integration of OH groups into the polymer was analysed using FTIR spectroscopy and found to be crucial for the proton transport in the polymer.

Pharmacophore-based database mining for probing fragmental drug-likeness of diketo acid analogues

A number of the structurally diverse chemical compounds with functional diketo acid (DKA) subunit(s) have been revealed by combined online and MoStBiodat 3D pharmacophore-guided ZINC and PubChem database screening. We used the structural data available from such screening to analyse the similarities of the compounds containing the DKA fragment. Generally, the analysis by principal component analysis and self-organizing neural network approaches reveals four families of compounds complying with the chemical constitution (aromatic, aliphatic) of the compounds. From a practical point of view, similar studies may reveal potential bioisosteres of known drugs, e.g. raltegravir/elvitegravir. In this context, it seems that mono-halogenated aryl substructures with para group show the closest similarity to these compounds, in contrast to structures where the aromatic ring is halogenated in both ortho- and para-locations.

Car-Parrinello simulation of hydrogen bond dynamics in sodium hydrogen bissulfate

We studied proton dynamics of a short hydrogen bond of the crystalline sodium hydrogen bissulfate, a hydrogen-bonded ferroelectric system. Our approach was based on the established Car-Parrinello molecular dynamics (CPMD) methodology, followed by an a posteriori quantization of the OH stretching motion. The latter approach is based on snapshot structures taken from CPMD trajectory, calculation of proton potentials, and solving of the vibrational Schrodinger equation for each of the snapshot potentials. The so obtained contour of the OH stretching band has the center of gravity at about 1540 cm(-1) and a half width of about 700 cm(-1), which is in qualitative agreement with the experimental infrared spectrum. The corresponding values for the deuterated form are 1092 and 600 cm(-1), respectively. The hydrogen probability densities obtained by solving the vibrational Schrodinger equation allow for the evaluation of potential of mean force along the proton transfer coordinate. We demonstrate that for the present system the free energy profile is of the single-well type and features a broad and shallow minimum near the center of the hydrogen bond, allowing for frequent and barrierless proton (or deuteron) jumps. All the calculated time-averaged geometric parameters were in reasonable agreement with the experimental neutron diffraction data. As the present methodology for quantization of proton motion is applicable to a variety of hydrogen-bonded systems, it is promising for potential use in computational enzymology.

Luminescence and Raman Spectra of Acetylacetone at Low Temperatures

Raman spectra of acetylacetone were recorded for molecules isolated in an argon matrix at 10 K and for a polycrystalline sample. In the solid sample, broad bands appear superimposed on a much weaker Raman spectrum corresponding mainly to the stable enol form. The position of these bands depends on the excitation wavelength (514.5 and 488.8 nm argon ion laser lines were used), sample temperature, and cooling history. They are attributed to transitions from an excited electronic state to various isomer states in the ground electronic state. Laser photons have energies comparable to energies of a number of excited triplet states predicted for a free acetylacetone molecule (Chen, X.-B.; Fang, W.-H.; Phillips, D. L. J. Phys. Chem. A 2006, 110, 4434). Since singlet-to-triplet photon absorption transitions are forbidden, states existing in the solid have mixed singlet/triplet character. Their decay results in population of different isomer states, which except for the lowest isomers SYN enol, TS2 enol (described in Matanović I.; Doslić, N. J. Phys. Chem. A 2005, 109, 4185), and the keto form, which can be detected in the Raman spectra of the solid, are not vibrationally resolved. Differential scanning calorimetry detected two signals upon cooling of acetylacetone, one at 229 K and one at 217 K, while upon heating, they appear at 254 and 225 K. The phase change at higher temperature is attributed to a freezing/melting transition, while the one at lower temperature seems to correspond to freezing/melting of keto domains, as suggested by Johnson et al. (Johnson, M. R.; Jones, N. H.; Geis, A; Horsewill. A. J.; Trommsdorff, H. P. J. Chem. Phys. 2002, 116, 5694). Using matrix isolation in argon, the vibrational spectrum of acetylacetone at 10 K was recorded. Strong bands at 1602 and 1629 cm(-1) are assigned as the SYN enol bands, while a weaker underlying band at 1687 cm(-1) and a medium shoulder at 1617 cm(-1) are assigned as TS2 enol bands.

Transition state theory can be used in studies of enzyme catalysis: lessons from simulations of tunnelling and dynamical effects in lipoxygenase and other systems

The idea that enzyme catalysis involves special factors such as coherent fluctuations, quantum mechanical tunnelling and non-equilibrium solvation (NES) effects has gained popularity in recent years. It has also been suggested that transition state theory (TST) cannot be used in studies of enzyme catalysis. The present work uses reliable state of the art simulation approaches to examine the above ideas. We start by demonstrating that we are able to simulate any of the present catalytic proposals using the empirical valence bond (EVB) potential energy surfaces, the dispersed polaron model and the quantized classical path (QCP) approach, as well as the approximate vibronic method. These approaches do not treat the catalytic effects by phenomenological treatments and thus can be considered as first principles approaches (at least their ability to compare enzymatic reaction to the corresponding solution reactions). This work will consider the lipoxygenase reaction, and to lesser extent other enzymes, for specific demonstration. It will be pointed out that our study of the lipoxygenase reaction reproduces the very large observed isotope effect and the observed rate constant while obtaining no catalytic contribution from nuclear quantum mechanical (NQM) effects. Furthermore, it will be clarified that our studies established that the NQM effect decreases rather than increases when the donor-acceptor distance is compressed. The consequences of these findings in terms of the temperature dependence of the kinetic isotope effect and in terms of different catalytic proposals will be discussed. This paper will also consider briefly the dynamical effects and conclude that such effects do not contribute in a significant way to enzyme catalysis. Furthermore, it will be pointed out that, in contrast to recent suggestions, NES effects are not dynamical effects and should therefore be part of the activation free energy rather than the transmission factor. In view of findings of the present work and our earlier works, it seems that TST provides a quantitative tool for studies of enzyme catalysis and that the key open questions are related to the nature of the factors that lead to transition state stabilization.

Experimental and Theoretical UV Characterizations of Acetylacetone and Its Isomers

Cryogenic matrix isolation experiments have allowed the measurement of the UV absorption spectra of the high-energy non-chelated isomers of acetylacetone, these isomers being produced by UV irradiation of the stable chelated form. Their identification has been done by coupling selective UV-induced isomerization, infrared spectroscopy, and harmonic vibrational frequency calculations using density functional theory. The relative energies of the chelated and non-chelated forms of acetylacetone in the S0 state have been obtained using density functional theory and coupled-cluster methods. For each isomer of acetylacetone, we have calculated the UV transition energies and dipole oscillator strengths using the excited-state coupled-cluster methods, including EOMCCSD (equation-of-motion coupled-cluster method with singles and doubles) and CR-EOMCCSD(T) (the completely renormalized EOMCC approach with singles, doubles, and non-iterative triples). For dipole-allowed transition energies, there is a very good agreement between experiment and theory. In particular, the CR-EOMCCSD(T) approach explains the blue shift in the electronic spectrum due to the formation of the non-chelated species after the UV irradiation of the chelated form of acetylacetone. Both experiment and CR-EOMCCSD(T) theory identify two among the seven non-chelated forms to be characterized by red-shifted UV transitions relative to the remaining five non-chelated isomers.

H-bonded complexes between acetylacetone and two molecules of methanol: HF and DFT level study

Five stable H-bonded complexes (supersystems) between acetylacetone and two methanol molecules were investigated at the B3LYP and HF levels of theory using the 6-311G** and 6-11++G** basis sets. The most stable complex was found as the one with the highest relative bonding and interaction energies. All vibrational frequencies resulting from calculations with the 6-311++G** basis set were compared with the recorded IR spectrum of acetylacetone/methanol mixture in a molar ratio 1:2.

Theoretical IR spectroscopy based on QM/MM calculations provides changes in charge distribution, bond lengths, and bond angles of the GTP ligand induced by the Ras-protein

The GTPase Ras p21 is a crucial switch in cellular signal transduction. Fourier transform infrared (FTIR) spectra of the substrate guanosine triphosphate (GTP) show remarkable changes when it binds to the enzyme. The reduced band widths indicate that the flexible GTP molecule is guided by the protein into a preferred conformation. The delocalized phosphate vibrations of unbound GTP become localized. The frequency shifts show an electron movement toward beta-phosphate, which probably contributes to catalysis by reducing the free activation energy. To quantify these qualitative observations we performed QM/MM molecular dynamics simulations of Ras.GTP and GTP in water. The triphosphate part of GTP was treated quantum mechanically using density functional theory (DFT). Vibrational spectra were calculated in harmonic approximation with an average deviation of 3% from the experimental frequencies. This provides a high confidence in the computational results as vibrational spectra are highly sensitive to conformation and charge distribution. As compared to GTP in water, Ras-bound GTP shows a shift of negative charge of approximately 0.2 e toward the beta-phosphate from gamma-phosphate and from alpha-phosphate due to the positive charge of the magnesium ion, to a lesser extent of Lys-16, and surprisingly without any effect of the P-loop backbone. Magnesium and Gly-13 twist and bend the gamma-O-beta bonds such that the crucial bond is stretched before cleaving.

Infrared Spectroscopy of the Intramolecular Hydrogen Bond in Acethylacetone: A Computational Approach

The intramolecular hydrogen bond in the enol-acethylacetone (ACAC) is investigated by performing reduced-dimensional quantum calculations. To analyze the shared proton vibrations, two sets of coordinates were employed: normal mode coordinates describing the motion in the vicinity of the most stable configuration, and internal coordinates accounting for the double minimum proton motion. It is proved that the extreme broadness of the OH-stretch band in ACAC is a consequence of the coexistence of two enol-ACAC structures: the global minimum and the transition state for rotation of the distal methyl group. Further, a ground-state tunneling splitting of 116 cm(-1) is found, and it is shown that the inclusion of the kinematic coupling is mandatory when treating large-amplitude proton motion. In the OH-stretch direction a splitting of 853 cm(-1) was predicted.