Hydrogen-bond basicity of push-pull α,β-unsaturated enaminoketones (original) (raw)
Related papers
2018
The stability of two stable cis-enol forms in two categories of β-diketones, including parasubstituted of trifluorobenzoylacetone (X-TFBA) and 1-aryl-1,3-diketone malonates (X-ADM, X: H, NO2, OCH3, CH3, OH, CF3, F, Cl, and NH2) has been obtained by different theoretical methods. According to our results, the energy difference between the mentioned stable chelated enol forms for the titled compounds is negligible. The theoretical equilibrium constants between the two stable cis-enol of the mentioned molecules are in excellent agreement with the reported experimental equilibrium constant. In addition, the effect of different substitutions on the intramolecular hydrogen bond strength has been evaluated. The correlation between Hammett para-substituent constants, σp. with the theoretical and experimental parameters related to the strength of hydrogen bond in p-X-TFBA and p-X-ADM molecules also investigated by means of density functional theory calculations. The electronic effects of para-substitutions on the intramolecular hydrogen bond strength were determined by NMR and IR data related to intramolecular hydrogen bond strength, geometry, natural bond orbital results, and topological parameters. These parameters were correlated with the Hammett para-substituent constants, σp. Good linear correlations between σp and the several parameters related to the hydrogen bond strength, in this study were obtained.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2009
IR Fourier and 1 H NMR spectra of -dimethylaminoacrolein (DMAA) were investigated in various pure solvents. Quantum chemical calculations by the method AM1 also was carried out to evaluate relative energy and dipole moment of each conformer. On the basis of NMR and IR-spectra we showed that the (DMAA) presented in solutions as equilibrium of two conformers, (E-s-Z) (E-s-E). Constant of this equilibrium, K eq = C(E-s-E)/C(E-s-Z), depended strongly on the total (DMAA) concentration: ln K eq = ln K eq0 + a(1 − e −bC total). Besides, (E-s-Z) conformer of the (DMAA) was more polar and more stable than the (E-s-E) conformer. Correlation of the out-of-phase˜ (C O) and in-phase˜ (C C) vibrations with solvatochromic parameters of Kamlet, Abbot, and Taft (KAT) revealed that the main contribution to the shift of the out-of-phase˜ (C O) vibrations of the both conformers made solvent's hydrogen bond acceptor (HBA) (ˇ) term, whereas hydrogen bond donor (HBD) acidity (˛) term influenced predominantly on the shift of the in-phase˜ (C C) vibrations of the conformers. Moreover, influence of these dominated terms was more pronounced for the (E-s-Z) conformer in comparison with the (E-s-E) conformer, hence the first conformer was more polarized than the last. Investigations of the enthalpies of the (E-s-Z) (E-s-E) equilibrium in carbon tetrachloride, 1,4-dioxane and their mixtures showed that these enthalpies depended predominantly on the solvent's atomic and electronic polarization and dispersive interactions.
Studies on amphiprotic compounds. 3. Hydrogen-bonding basicity of oxygen and sulfur compounds
The Journal of Organic Chemistry, 1988
The hydrogen bonding (HB) basicity parameters, &, for various "monomeric" alcohols and thiols, as well as for a variety of carbonyl and thiocarbonyl amphiprotic bases, have been determined. Also determined were the HB basicity parameters, @, for a series of aprotic carbonyl and thiocarbonyl bases (@, = B for these materials). The analysis of the data for a set of 62 compounds provides the basis for comparison of structural effects on the HB basicity of oxygen and sulfur bases. In general, the HB basicity of sulfur bases is lower than that of their oxygen homologues. Within families, structural effects on the basicity of oxygen and sulfur bases are proportional. Our results show the contributions from polarizability and Aeld/inductive and resonance effects to the basicity of these compounds. Also some differential contributions from steric hindrance and hybridization changes have been singled out. The relative stability of cyclic 1:l complexes involving amphiprotic thioamides and 4-chlorophenol has been evaluated. I. Introduction Recent studies have shown the usefulness of the empirical parameters ** (dipolarity-polarizability), a (hydrogen-bonding acidity), and j3 (hydrogen-bonding basicity) for the quantitative analysis of solvent effects on chemical reactivity: solubility? and spectral (UV-vis, IR, EPR) and biological proper tie^.^ We have pointed out' that the determination of j3 values for "monomeric" self-associating compounds, p, , requires the use of special techniques. This is so because the use of solvatochromic indicators'& dissolved in the bulk liquids provides a measure of the "averagen basicity of "monomers" and "oligomers". Consider equilibrium 1, describing the hydrogen-bonding (HB) association between a nonamphiprotic HB base, B, and a proton donor, H-A, in dilute solution in an inert solvent, S. K, and K, respectively stand for the equilib-(1) rium constants for this reaction expressed in liters/mole and in mole fraction units. I t is found'Job that (2) where a, b, and c are constants and j3 and p respectively stand for the HB basicity parameter and the molecular dipole moment of the base. For nonamphiprotic bases, j3,
Conventional and Unconventional Intramolecular Hydrogen Bonding in some Beta-diketones
2017
This study presents our view of unconventional and conventional intramolecular hydrogen bonds (HBs) for some beta-diketones theoretically and experimentally. According to our results, the groups such as Phenyl and t-But in beta positions increase and CF3 group decrease IHB strength, respectively. For better understanding of the substitution effects, the compounds with similar and different substitutions compared to each other experimentally and theoretically. Comparison between theoretical and experimental results, EHB and OH, show, by adding one substitution, these parameters change, which by adding another similar substitution, these changing approximately doubled. This conclusion achieved for DMPD, with phenyl and t-But groups in beta positions. Whereas, TFBA, with phenyl and CF3 groups, and TFDMHD, with CF3 and t-But groups in beta positions, don’t follow this achievement. The 1HNMR chemical shifts for the stable cis-enol forms of the mentioned compounds have been calculated at...
Substituent Effects on the Strength of the Intramolecular Hydrogen Bond of Thiomalonaldehyde
The Journal of Organic Chemistry, 1999
The effect of CH 3 , NH 2 , OH, and F substituents on the intramolecular hydrogen bond (IHB) of thiomalonaldehyde (TMA) was analyzed through the use of B3LYP density functional theory calculations. The geometries of the C1-, C2-, and C3-susbtituted enol and enethiol tautomers were optimized at the B3LYP/6-31G(d) level while their final energies were evaluated using a 6-311+G-(3df,2p) basis set expansion. In general C1-substitution strengthens the IHB of the enolic tautomer, while C3-substitution strengthens the IHB of the enethiolic form. These changes are related with an enhancement of the intrinsic acidity of the OH and the SH groups, respectively. Important cooperative effects are also present when the substituent can form an additional IHB with either the oxygen atom or the sulfur atom of TMA. However, the trends observed in the relative stabilities of the enol and the enethiol tautomers do not follow the changes observed in the strength of the IHB. C1-substitution specifically stabilizes the enethiol form, while C3-substitution stabilizes preferentially the enol tautomer. When substitution takes place at the central carbon atom, the enethiol tautomer is predicted to be slightly more stable than the enol counterpart. Substituent effects on the proton-transfer energy barrier are dramatic, and the interconversion between the enolic and the enethiolic forms of the C1-and the C3-substituted derivatives is barrier-free. In contrast, C2-substitution leads to an increase of the barrier.
Journal of The American Chemical Society, 1975
Hydrogen-bonded dimers involving first-and second-row hydrides have been studied theoretically with ab initio molecular orbital methods, using a 431G basis set. Certain generalizations about H-bonded dimers found in a previous stu-dyZa of first-row dimers (those involving "3, H20, and HF) are supported by this study; others require modification. In addition to studying the dependence of H-bond energy and properties on the row of the periodic table, we examine the dependence of H-bond energies on the "hybridization" of the electron donor, including HCN, H2C0, H2CS, HNC, and HCP as electron donors. We have also studied ionic H bonds, ''P" H bonds, and H-bonded trimers in an attempt to relate their properties to those of the more conventional H-bonded dimers. Can a C-H bond be an effective H-bond proton donor? We attempt to answer this question by examining the proton donor ability of CH4 and CHF3. Electrostatic potentials turn out to facilitate our understanding of H-bond energies and structures, being more useful than Mulliken populations in rationalizing H-bond energies. Finally we address ourselves to the question of predicting dimer H-bond energies from the monomers involved. Using a very simple algebraic model, we are able to predict the H-bond energy of a total 144 H-bonded complexes, using as a basis our theorctical calculations on 25 complexes.
Multiple Hydrogen-Bond Accepting Capacities of Polybasic Molecules: The Case of Cotinine
Journal of Physical Chemistry A, 2004
The hydrogen-bond (HB) basicities of the carbonyl oxygen and pyridine nitrogen of the cotinine molecule, a long-lived metabolite of nicotine, have been measured in carbon tetrachloride and estimated in water. For the first time, the equilibrium constants of two coexisting 1:1 associations of a phenol on the basic centers of a bifunctional compound have been measured individually. The sum of these individual equilibrium constants closely corresponds to the global experimental constant obtained by the classical IR method based on the measurement of the free phenol OH absorption intensity. The solvation of the cotinine amide group has been examined in various mixed acetonitrile-water solutions revealing the presence of di-and tri-hydrogen-bonded carbonyl groups in pure water. Independently, the accepting strengths of the two sites of cotinine have been calculated from linear correlations between the pK HB scale and the electronic energy of the reaction of hydrogen fluoride complexation on substituted pyridines and carbonyl model compounds using density functional theory calculations at the B3LYP/6-31+G** level. The agreement between the calculated and the experimental individual equilibrium constants of cotinine is well inside the experimental error. The knowledge of the individual acceptor strengths of cotinine in carbon tetrachloride enables the calculation of the octanol-water partition coefficient, this estimation exactly fits the experimental data. Contrary to the order of basicity measured by the pK a scale, the HB basicity of the carbonyl group appears to be 1.6 pK units greater than the HB basicity of the pyridine moiety in water.
Journal of Molecular Structure, 2011
Complete conformational analyses of all possible keto and enol forms, molecular structure, intramolecular hydrogen bonding (IHB), and vibrational frequencies of 5,5-dimethyl hexane-2,4-dione (DMHD, also known as acetylpinacolin and pivalylacetone), were investigated by means of ab initio calculations and IR and Raman spectroscopies. The results are compared with those of acetylacetone (AA) and 2,2,6,6-tetramethyl-3,5-heptanedione (TMHD). The energy differences between three stable E1, E2, and E3 chelated enol forms are negligible. Comparing the calculated and experimental band frequencies and intensities suggests the coexisting of these three conformers in comparable proportions in the sample. The vibrational frequencies of DMHD and its deuterated analogue were also clearly assigned. According to the theoretical calculations, the stable cis-enol conformers of DMHD have an average hydrogen bond strength of 16.6 kcal/mol, calculated at the B3LYP/6-311++G** level, which is about 0.7 kcal/mol stronger than that of AA. This enhancement in the IHB strength is also consistent with the experimental results of the band frequency shifts of OH/OD and O⋯O stretching and OH/OD out of plane bending frequencies. The theoretical calculations and spectroscopic results indicate that the IHB strength of DMHD is between those of AA and TMHD.► Substitution of bulky groups (such as t-butyl) in the β-positions of acetylacetone increases the enol content in the sample. ► Both theoretical and experimental data indicate that the hydrogen bond in DMHD is stronger than that in acetylacetone. ► More than one enol conformer exists in the sample.