Substituent Effects on the Strength of the Intramolecular Hydrogen Bond of Thiomalonaldehyde (original) (raw)
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High-Level ab Initio Calculations on the Intramolecular Hydrogen Bond in Thiomalonaldehyde
Journal of Physical Chemistry A, 1997
High-level ab initio calculations, in the framework of the G2(MP2) theory, have been carried out on the different tautomers of thiomalonaldehyde (TMA). These calculations are compared with those obtained using density functional theory methods, namely B3LYP, with extended basis sets. In general the enethiol tautomers of TMA are 5-10 kcal/mol more stable than the corresponding enol analogues, with the only exception being the Z-enol (E1) and the Z-enethiol (T1) hydrogen-bonded species, which are the global minima of both series. At the G2(MP2) level both tautomers are nearly degenerate, the enethiol T1 being 0.2 kcal/mol more stable than the enol E1. Electron correlation effects stabilize preferentially the enol form, while the ZPE corrections go in the opposite direction, due essentially to the differences between S-H and O-H stretching frequencies. As a consequence, when the hydrogen atom involved in the intramolecular hydrogen bond (IHB) of both tautomers is replaced by deuterium, the stability order is reversed and E1 is predicted to be more stable than T1. An analysis of these IHBs in terms of the topological characteristics of the electron charge density and of the shifts of the S-H and O-H vibrational frequencies reveals that the HB in E1 is much stronger than in T1. The existence of this IHB results in an increase of the electron delocalization which enhances the stability of tautomer E1. At the G2(MP2) level two open-chain rotamers, namely T4 and T7, are predicted to be within an energy gap smaller than 0.5 kcal/mol with respect to the global minimum. The use of continuum and discrete-continuum models indicates that both open-chain enethiols and enols are significantly stabilized by solute-solvent interactions, and they should predominate in aqueous solution. B3LYP/6-311+G(3df,2p) relative stabilities are in excellent agreement with G2(MP2) values.
Structural Chemistry, 2020
In the present work, a detailed investigation of synergistic effects between the intramolecular hydrogen bond (IMHB) and πelectron delocalization (π-ED) of 3-hydroxy prop-2-en thial (HPT) and its halogenated derivatives was performed. For this purpose, at first, the π-ED in the enol form of the benchmark systems by various aromaticity indices such as λ, λ́, HOMA, NICS, PDI, ATI, and FLUπ were evaluated. On the other hand, the strength of IMHB by various descriptors such as energetical, geometrical, spectral, topological, and molecular orbital parameters was also estimated. For better understanding the nature of the synergistic phenomenon, we examined and compared the linear relationships between the π-ED indices with the HB descriptors. Our results show that the geometrical indicators have the best linear relationships with all of the mentioned HB parameters. Also, according to their absolute linear correlation coefficients, the following order is concluded: λ́> λ > HOMA > FLUπ > ATI > NICS (1) > PDI > NICS (0) Finally, the synergistic effect between the π-ED and IMHB from the position and nature point of views is discussed. These results clearly show that the synergistic effect of R1 derivatives is negative, while the corresponding effects of R2 and R3 ones are positive. Moreover, the synergetic effects also depend on the nature of substitutions especially their electronegativity values (F > Br > Cl). Keywords 3-hydroxy prop-2-en thial. π-ED. IMHB. Synergistic effects * A. Nowroozi
Journal of Molecular Structure: THEOCHEM, 1991
We carried out an ab initio study of various derivatives of malonaldehyde by using the 3-21G basis and full optimization in order to analyse the influence of substituents on the features of the intramolecular hydrogen bond. The results show the hydrogen bond to be strengthened by electron-releasing and weakened by electron-withdrawing substituents. The estimated hydrogen-bond energies of all the compounds studied are closely related to the corresponding 0. * *O bond distances. We also studied the intramolecular proton transfer by tunnelling and found its rate to be lower than that in the parent compound (malonaldehyde). This lowering is attributed to the fact that the substituents raise the energy barrier.
In this work, the structure of (E)-2-{[2- (hydroxymethyl) phenylimino]methyl}-5-methoxyphenol was characterized by X-ray single crystal diffraction technique, infrared spectroscopy, and quantum chemical computational methods as both experimental and theoretically. The compound crystallizes in the triclinic space group P1 with a ¼ 9.4601 (5) Å , b ¼ 11.7273 (7) Å , c ¼ 12.4400 (8) Å , a ¼ 88.179 (5) , b ¼ 71.442 (4) , c ¼ 84.977 (5) , and Z ¼ 4. X-Ray study shows that both enol-imine and keto-amine tautomeric forms coexist in the asymmetric unit as two independent molecules. The molecular geometry was also optimized at the B3LYP/6-311G(d,p) level by using density functional theory started from the crystallographically achieved parameters of molecule. From the optimized geometry of the molecule, molecular electrostatic potential was evaluated, frontier molecular orbitals and natural bond orbital analysis were performed, and vibrational frequencies were computed theoretically. The polarizable continuum model calculations starting from the optimized geometry were also carried out in both gaseous and solution phase to investigate the energetic behavior and dipole moment of the title compound with the same level of theory.
Chemical Physics, 2007
The proton transfer in malonaldehyde and in some of its derivatives have been considered in order to study the interrelation between the reaction barrier and the p-delocalization in the quasi-ring. A set of simple and mostly common substituents having different properties in resonance effect according to values of substituents constants were chosen in order to simulate the influence of substitution in position 2 or in position 1 (or 3) of malonaldehyde on the quasi-aromaticity and H-bonding. The following substituents have been taken into consideration: NO, NO 2 , CN, CHO, F, H, CH 3 , OCH 3 , OH, and NH 2 . Our results show that when the substituent is attached at position 2 of the quasi-ring, the resonance effect predominates over the field/inductive effect which leads to changes in H-bonding and quasi-aromaticity of the ring motif, while in the case of 1(3) substitution the field/inductive effect is significantly more effective influencing the HB strength, and thus, the proton transfer barrier. Somehow counterintuitively, for the 1(3) substituted systems, the most stable isomer is the one having the weakest HB and lower aromaticity. The reason for this surprising behaviour is discussed.
Journal of Computational Chemistry, 2010
The inequivalence of substitution pair positions of naphthalene ring has been investigated by a theoretical measurement of hydrogen bond strength, aromaticity, and excited state intramolecular proton transfer (ESIPT) reaction as the tools in three substituted naphthalene compounds viz 1-hydroxy-2-naphthaldehyde (HN12), 2-hydroxy-1-naphthaldehyde (HN21), and 2-hydroxy-3-naphthaldehyde (HN23). The difference in intramolecular hydrogen bond (IMHB) strength clearly reflects the inequivalence of substitution pairs where the calculated IMHB strength is found to be greater for HN12 and HN21 than HN23. The H-bonding interactions have been explored by calculation of electron density q(r) and Laplacian ! 2 q(r) at the bond critical point using atoms in molecule method and by calculation of interaction between r* of OH with lone pair of carbonyl oxygen atom using NBO analysis. The ground and excited state potential energy surfaces (PESs) for the proton transfer reaction at HF (6-31G**) and DFT (B3LYP/6-31G**) levels are similar for HN12, HN21 and different for HN23. The computed aromaticity of the two rings of naphthalene moiety at B3LYP/6-31G** method also predicts similarity between HN12 and HN21, but different for HN23.
Journal of Molecular Structure, 2015
The effects of various substituent groups on the hydrogen bond energy in the 3,3'-dihydroxy-4,4'-[5-methyl-1,3-phenylenebis(nitrilomethylidyne)]-bis-phenol (L(CH 3)pnp) molecules is one of the factors controlling intramolecular proton transfer process in the molecule's functional group. In this work, we focused on the influence of different groups on this phenomenon, into the framework of the atom in molecules (AIM) theory. In addition, the calculations of transition state were performed to evaluate the proton transfer's energy barrier of the proton transfer in the Schiff base molecule Lpnp. Investigation the effect of the electron withdrawing groups including, CHO, CN, CF 3 ,NO 2 , COOH, COCH 3 , F, Cl And that of electron donating species consisting of NH 2 , OH, C 2 H 5 , CH 3 on the covalent nature of the intramolecular hydrogen bond shows different results: decrease in the electron withdrawing ability reduces the covalent nature whereas, electron donating substituents increase it. The topological properties such as the bond critical point (BCP), ring critical point (RCP), the delocalization index DI, and the integrated properties of the interatomic surface δ (O, H), δ (N, H) and H (G+V) can all be considered as indicators to determine the strength of the intramolecular hydrogen bond.
Theoretical study of intramolecular proton transfer reactions in some thiooxalic acid derivatives
Journal of Physical Chemistry A, 2002
In this study, further development of the Golden Rule (GR) approach is presented for the dynamics of intramolecular (IM) hydrogen atom and proton transfer (PT) in solution. The IM modes are treated following the procedure reported earlier which simplifies drastically the problem of evaluating the multidimensional transfer integrals. The polar solvent is treated as a dielectric continuum with classical Debye spectrum. In the most typical case of relation between the parameters involved, the rate constant is expressed as a product of two almost independent terms: the "pure" tunneling rate of the same transfer but without any reorganization effects taken into consideration, and a suppressing tunneling factor of Levich-Dogonadze type in a generalized form. Two major effects are present: the promoting effect of the IM vibrations symmetrically coupled to the reaction coordinate, and the suppressing effect resulting from the final reorganization of both the molecule and solvent. This approach is applied to the hydrogen atom and proton transfer in the photochemical cycle of 5,8-dimethyl-1-tetralone (DMT) observed by Grellmann and co-workers in a polar protic solvent (EPA). This compound exhibits typical non-Arrhenius temperature and isotope dependence of the rate of triplet enolization. The kinetic curves of the ground-state reketonization reaction are close to Arrhenius, with significantly higher slopes than for typical intramolecular PT reactions. Semiempirical quantum-chemical calculations at AM 1 level were carried out to study the relative stability, structure, and charge distribution of all states involved in the photochemical cycle, including the effects of solvation in a polar H-bonding solvent. Two rotamers E1 and EII for the enol form were located corresponding to different positions of the H atom of the hydroxyl group. In ground state the first is more stable in both the gas phase and polar protic solvent modeled by water. Therefore, the reketonization reaction is treated as one-step tunneling from the rotamer E1 to the keto form, i.e., without activated rotational equilibration E1 -EII proposed by Grellmann and co-workers in an earlier study. Calculations of the rate constants were performed for both the direct and reverse reaction. Standard AM1 output (structural and force field data) was used as input, and good agreement with the available kinetic experiments was reached for both compounds. The high slope of the kinetic curve of this reaction is attributed to the additional activation energy resulting from the final reorganization of the low-frequency oscillators, mainly those from the solvation layer.
Journal of Molecular Structure: THEOCHEM, 2008
The conformational isomerism of substituted (substituents = OR and SR, R@H and Me) acetaldehydes and thioacetaldehydes is described in terms of intramolecular interactions, namely hydrogen bonding (when R@H), hyperconjugation involving the carbonyl or the thiocarbonyl system, and classical effects (steric and electrostatic interactions). 3D potential energy surfaces were obtained by scanning both XACAC@Y (a) and RAXACAC (/) dihedral angles (X/Y@O and S) and used to identify local and global minima. Geometry optimization and NBO calculations, including determination of NLMO steric energies and deletion of hyperconjugative interactions, were then performed in order to find the governing factors for these conformational equilibria. Hyperconjugative contribution for hydrogen bonding showed to be more important for thioaldehydes, while OAH showed to be a better proton donor than SAH; however, hydrogen bonding also appeared to be as of electrostatic nature. Overall, orbital interactions, particularly those involving the p * system, and classical factors (steric and electrostatic effects) drive the conformational isomerism of the title compounds.