Theoretical Study of Structure,Energetic, Hydrogen Bonds Strength and Intramolecular Proton Transfer of 2‐(2‐R (ROH,NH2, SH) Phenyl (or Pyridyl)) Benzoxazoles (or Benzothiazoles)

The ground‐ and excited‐state intramolecular proton transfer processes of 2‐(2‐R (R?OH, NH₂, SH) phenyl (or pyridyl)) benzoxazoles (or benzothiazoles) are investigated by the DFT methods. The calculated results indicate that in the ground state there is a high correlation (R=0.9950) between the proton transfer barrier and the intramolecular hydrogen bonds (IMHB) strength. The increase of the strength of IMHB in the proton transfer processes leads to a larger barrier contributions. Intramolecular proton transfer process pathway is along with the minimal difference of change value in the IMHB angle. In the excited‐state, there is a similar relationship between the IMHB and the barrier.     

Conformational analysis,tautomeric preference,intramolecular hydrogen bonding,and solvent effect on dinitrosamine: A quantum chemical study

HF, B3LYP, and MP2 methods with the standard basis set, 6‐311++G(d,p), were used to study various aspects of dinitrosamine. These results were compared with the outcomes of G2 and CBS‐QB3 methods. First, the conformational analysis and characterization of equilibrium conformations, especially global minima, were performed. On the basis of relative energies, we found that the dinitroso tautomers are more stable than the nitroso‐hydroxy (NH) ones. This preference is well‐interpreted in terms of tautomerization process and nitrosamine resonance. Furthermore, the nature of O? H···O intramolecular hydrogen bond (IMHB), in chelated forms of NH (NH‐11 and NH‐13) was comprehensively studied to evaluate the effect of hetero atoms (N) on the characteristic of IMHB systems. According to the results of isodesmic reaction method, the hydrogen bond energy of NH‐11 is greater than the malonaldehyde (MA) and NH‐13, whereas the electron density analysis and energy‐geometry correlation methods clearly predict that the hydrogen bond of NH‐11 is weaker than the MA. Additionally, the geometrical, atoms in molecules (AIM) and natural bond orbital's (NBO) parameters also emphasize on the MA as a chelated form with the strongest hydrogen bond. Finally, the solvent effects on the relative stability of selected dinitrosamine conformers are evaluated by different continuum (polarizable‐continuum model, isodensity polarizable continuum model, and self‐consistent isodensity polarizable continuum model), discrete and mixed solvent models. Theoretical results readily show that the potential energy surface of dinitrosamine, especially global minima, is strongly affected by the solvent. © 2012 Wiley Periodicals, Inc.     

Studies of intramolecular hydrogen bonds (IMHB): crystal and molecular structure of 2-(2′-hydroxy-phenyl)imidazoles

The molecular and crystal structure of 2-(2′-hydroxyphenyl)imidazole (2) and 1-methyl-2-(2′-hydroxyphenyl)imidazole (5) have been determined by X-ray analysis. Compound (2) presents a strong intramolecular hydrogen bond (IMHB) responsible for the planarity of the molecule. In both compounds the molecules form chains through N---H…O (compound 2) and O---H…N hydrogen bonds (compound 5) but giving rise to the same packing mode. Ab initio calculations (6–31G**) have been carried out on both compounds in order to study the effect of the IMHB on the structure.     

Structure and isomerization of heterapentalene compounds containing hypervalent bonds with central nitrogen,phosphorus, or arsenic atom

The possibility for a specific type of isomerization (electromorphism) to occur in conjugated bicyclic organic compounds containing Group V elements was studied by the ab initio (RHF/6-31G**, MP2(full)/6-31G**), and DFT (B3LYP/6-31G**) methods. Compounds 2 (X = N, P, As) were found to exist in a monocyclic planar form with intramolecular donor-acceptor N...O coordination (X = N) and as aromatic heterapentalene structures with hypervalent O--X--O bonds (X = P, As). According to calculations, no isomerization of planar heteroaromatic structures into pyramidal ones occurs. The strength of the O--X--O hypervalent bond and the aromaticity of heterapentalene structures 2 with ten -electrons increase on going from X = N to X = P. Correct estimation of these effects requires the inclusion of electron correlation.     

Structural,electronic properties and intramolecular hydrogen bonding of substituted 2-[(E)-imino methyl] benzenethiol in ground and first excited state by quantum chemical methods

Quantum chemical calculations of geometric structure, the intramolecular hydrogen bond, harmonic vibrational frequencies, NMR spin–spin coupling constants, and physical properties such as chemical potential and chemical hardness of the 2-(E)-imino methyl benzenethiol and its nineteen derivatives were carried out using density functional theory (DFT/B3LYP/6-311++G**) method in the gas phase and the water solution. Furthermore, the topological properties of the electron density distributions for S–H···N intramolecular hydrogen bond have been analyzed in terms of the Bader’s theory of atoms in molecules (AIM). Natural bond orbital (NBO) analysis also performed for better understanding the nature of intramolecular interactions, the results of analysis by quantum theory of AIM and NBO method fairly supported the DFT results. Besides, MEP was performed by the DFT method. On the other hand, the aromaticity of the formed ring has been measured using several well-established indices of aromaticity such as nucleus-independent chemical shift, harmonic oscillator models of the aromaticity, para-delocalization index, average two-center indices, and aromatic fluctuation index. Also, the excited-state properties of intramolecular hydrogen bonding in these systems have been investigated theoretically using the time-dependent DFT method.     

DFT computational studies on rotation barriers,tautomerism, intramolecular hydrogen bond,and solvent effects in 8‐hydroxyquinoline

Intramolecular hydrogen binding interactions in 8‐hydroxyquinoline, both in its zwitterionic tautomer and in the rotamer without the intramolecular hydrogen bond (IHB), have been computed using the B3LYP and MPW1K density functionals. The rotation of the O? H bond and intramolecular proton transfer reactions were studied theoretically. The following theory levels have been applied: B3LYP/6‐31G(d,p), B3LYP/6‐311++G(d,p), MPW1K/6‐311++G(d,p), and MPW1K/6‐311++G(2d,3p)//MPW1K/6‐311++G(d,p). Natural bond orbital (NBO) analysis has also been carried out. The effect of medium (benzene, chloroform, tetrahydrofuran, 1,2‐dichloroethane, acetone, water) was simulated using the self‐consistent reaction field (SCRF) method within the framework of the polarizable continuum model (PCM), at the MPW1K/6‐311++G(d,p) level. The evolution of geometry, relative energies, heights of rotation (around the O? H bond) and tautomerization barriers, IHB energies, and ΔG(solv) have been systematically investigated. The results obtained have shown the failure to neglect some changes of the above characteristics in polar media with respect to the gaseous phase. The series of stability of the forms under study in the gaseous phase remains the same in solution. Thus, in spite of the important role of the solvent electrostatic effects, the intrinsic stability of those species overcomes the solvent effects. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Substituent effect on intramolecular hydrogen bonding in 2‐hydroxybenzaldehyde

The effect of some substituents on intramolecular hydrogen bonding of 5‐X‐2‐hydroxybenzaldehyde (5‐X‐2‐HBA) has been studied by B3LYP and MP2 methods using 6‐311++G** and AUG‐cc‐PVTZ basis sets. The relationship between hydrogen bond energy E_HB and electron donation (or withdrawal) of substituents has been investigated. An approximately good linear relationship has been detected between Hammett coefficients and hydrogen bond formation energy (R² = 0.98). Herein, population analysis has been performed by atoms in molecules (AIM) and natural bond orbital (NBO) methods. The results of AIM and NBO analyses are in a good agreement with calculated energy values. Furthermore, correlation between ring aromaticity and hydrogen bonding has been investigated by nucleus‐independent chemical shift (NICS) at GIAO/B3LYP/6‐311++G** level of theory. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

DFT based computational study on the excited state intramolecular proton transfer processes in o-hydroxybenzaldehyde

Potential energy (PE) curves for the intramolecular proton transfer in the ground (GSIPT) and excited (ESIPT) states of o-hydroxybenzaldehyde (OHBA) were studied using DFT-B3LYP/6-31G(d) and TD-DFT-B3LYP/6-31G(d) level of theory, respectively. Our calculations suggest the non-viability of ground state intramolecular proton transfer in this compound. Excited states PE calculations support the ESIPT process in OHBA. The contour PE diagram and the variation of oscillator strength along the proton transfer co-ordinate support the dual emission in OHBA. Our calculations also support the experimental observations of Nagaoka et al. [S. Nagaoka, U. Nagashima, N. Ohta, M. Fujita, T. Takemura, J. Phys. Chem. 92 (1988) 166], i.e. normal emission of the title compound comes from S(2) state and the red-shifted proton transfer band appears from the S(1) state. ESIPT process has also been explained in terms of HOMO and LUMO electron density of the enol and keto tautomer of OHBA and from the potential energy surfaces.     

The effect of substitution on the intramolecular hydrogen bonding in 3‐hydroxy‐propenethial

The intramolecular hydrogen bond strength of 3‐hydroxy‐propenethial (HPT) as well as the fluoro, chloro, bromo, and methyl derivatives were investigated at the B3LYP/6‐311++G^** level of theory. Solvent‐based calculations (in water) for HPT and derivatives were also carried out. The nature of the intramolecular hydrogen bond existing within the molecular under investigation has been studied by means of the Bader theory of atoms in molecules (AIM) that is based upon the use topological properties in terms of the electron density. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Donor-acceptor interaction and intramolecular proton transfer in aminopolyenes

The influence of donor and acceptor substituents at chain termini on the geometry of the chain and charge distribution on atoms was studied for the ground and lower triplet electronically excited state of model ω-dimethylaminopolyene molecules (CH₃)₂N(CH=CH) _n CH=C(CN)₂, n = 1–3. Calculations were performed by the B3LYP/6-31+G** method. The influence of substituents on bond lengths and the amplitude of deviations from the equilibrium carbon-carbon bond length in unsubstituted polyenes increased as the conjugation chain grew longer. The deviations of the effects of both donor and acceptor groups from additivity, however, decreased. In the lower triplet electronically excited state of the molecule, the effect of substituents on changes in C-C bond lengths along the chain was not damped. The section of the potential energy surface for intramolecular proton shift from the donor amino to the acceptor nitrile group in “cyclic” (cis) conformers of the H₂N-CH=CH-CN and H₂N-CH=CH-CH=CH-CN molecules was analyzed. The structure of the reaction transition state and the height of the barrier to proton transfer were calculated.     

Substituent effect on structure,electron density,and intramolecular hydrogen bonding in nitroso‐oxime methane

Density functional calculations with Beck's three‐parameter hybrid method using the correlation functional of Lee, Yang, and Parr (B3LYP) were carried out for investigation of the intramolecular hydrogen bond strength in Nitroso‐oxime methane and its derivatives. Also, vibrational frequencies for them were calculated at the same level of theory. The π‐electron delocalization parameter (Q) and as a geometrical indicator of a local aromaticity, the geometry‐based harmonic oscillator measure of aromaticity index has been applied. Additionally, the linear correlation coefficients between substituent constants and selected parameters in R position have calculated. The obtained results show that the hydrogen bond strength is mainly governed by the resonance variations inside the chelate ring induced by the substituent groups. The topological properties of the electron density distributions for O? H ··· O intramolecular bridges have been analyzed in terms of the Bader theory of atoms in molecules (AIM). Correlations between the H‐bond strength and topological parameters have been also studied. The electron density (ρ) and Laplacian (?²ρ) properties, estimated by AIM calculations, show that O ··· H bond have low ρ and negative (?²ρ) values (consistent with covalent character of the HBs), whereas O? H bond have positive (?²ρ) Furthermore, the analysis of hydrogen bond in this molecule and its derivatives by quantum theory of natural bond orbital (NBO) methods fairly support the ab initio results. Natural population analysis data, the electron density, and Laplacian properties as well as υ(O? H) and γ(O? H) were further used for estimation of the hydrogen bonding interactions and the forces driving their formation. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

On the fluorescence of methyl salicylate: the significance of its IMHB

The two forms of methyl salicylate bearing an intramolecular hydrogen bond (IMHB) are responsible for the three fluorescence emissions produced by this compound on electronic excitation in inert media. Whereas the form possessing an IMHB between its hydroxyl group and ether oxygen undergoes no excited state intramolecular proton transfer (ESIPT) in its first excited electronic state, that with an IMHB involving the carbonyl oxygen exhibits ESIPT with near-unity efficiency. Whereas the former species exhibits standard photophysical behaviour, the latter species exhibits two fluorescence emissions from the same electronic excited state; a photophysical scheme is proposed, which brings together all the available photophysical evidence for methyl salicylate in inert media.     

Synthesis and Theoretical Investigation of a 1,8‐Bis(bis(diisopropylamino)cyclopropeniminyl)naphthalene Proton Sponge Derivative

We report herein the synthesis and characterization of a new proton sponge derivative, 1,8‐bis(bis(diisopropylamino)cyclopropeniminyl)naphthalene 4 (DACN), as well as its bis‐protonated counterpart 6 . A crystal structure of 6 is presented, along with variable temperature ¹H NMR data on the BF₄^? salt ( 6?BF₄ ). DFT calculations were performed to investigate the structure of the monoprotonated species 7 and to gain insight into the structural and electronic nature of all three species. The proton affinity (PA) of 4 , calculated at the B3LYP/6‐311G++(d,p)//B3LYP/6‐31G(d,p) level, taking into account thermal corrections from the B3LYP/6‐31G(d,p) method, was 282.3 kcal mol^?1, while its pK_a was estimated at 27.0. NICS calculations were performed to examine the changes in aromaticity within these systems upon each successive protonation. Lastly, homodesmotic reaction schemes were used in order to estimate the factors contributing to the strong PA predicted for 4 .     

Ab initio and DFT studies on 1-(thionitrosomethylene) hydrazine: conformers, energies, and intramolecular hydrogen-bond strength

In the current study, we present an intramolecular HB, molecular structure, π-electrons delocalization and vibrational frequencies analysis of 25 possible conformers of 1-(thionitrosomethylene) hydrazine by means of DFT (B3LYP), MP2 methods in conjunction with the 6-311++G** and augmented correlation-consistent polarized-valence triple-zeta basis sets and G2MP2 theoretical level. The influence of the solvent on the stability order of conformers and the strength of intramolecular hydrogen-bonding was considered using the Tomasi’s polarized continuum model. Statistical analyses of quantitative definitions of aromaticity, nucleus independent chemical shift, harmonic oscillator model of aromaticity, aromatic fluctuation index, and the π-electron delocalization parameter (Q) as a geometrical indicator of a local aromaticity, evaluated for this conformers. Further verification of the obtained transition state structures were implemented via intrinsic reaction coordinate (IRC) analysis. Calculations of the ¹H NMR chemical shift at GIAO/B3LYP/6-311++G** levels of theory are also presented. The calculated highest occupied molecular orbital (MO) and lowest unoccupied MO energies show that charge transfer occur within the molecule. Hydrogen-bond energies for H-bonded conformers were obtained from Espinosa method and the natural bond orbital theory and the atoms in molecules theory were also applied to get a more precise insight into the nature of such H-bond interactions.     

Ground- and excited-state proton transfer and rotamerism in 2-(2-hydroxyphenyl)-5-phenyl-1,3,4-oxadiazole and its O/"NH or S"-substituted derivatives

The intramolecular proton-transfer process, rotational process, and optical properties of 2-(2-hydroxyphenyl)-5-phenyl-1,3,4-oxadiazole (HOXD) and its O/"NH"- and O/"S"-substituted derivatives, 2-(2-hydroxyphenyl)-5-phenyl-1,3,4-triazole (HOT) and 2-(2-hydroxyphenyl)-5-phenyl-1,3,4-thiadiazole (HOTD), respectively, have been studied. DFT (B3LYP/6-31+G**) single-point energy calculations were performed using HF- and DFT-optimized geometries in the ground state (S0). TD-B3LYP/6-31+G** calculations using CIS-optimized geometries were carried out to investigate the properties of the first singlet excited state (S1) and first triplet excited state (T1). The computational results revealed that a high-energy barrier inhibits the proton transfer from cis-enol (Ec) to keto (K) form in S0, whereas the proton transfer in S1 can take place through a very-low-energy barrier. The rotation between Ec and trans-enol (Et) can occur in S0 through a low-energy barrier, whereas it is prohibited because of the high-energy barrier in S1 for each of the three molecules. The vertical excitation energies were calculated using the TD-B3LYP/6-31+G** method based on the HF- and CIS-optimized geometries. Absorption and fluorescence wavelengths of HOT show a hypsochromic shift (6-15 nm) relative to HOXD, while those of HOTD show a bathochromic shift (21-29 nm). The phosphorescence wavelength of HOTD shows a significant bathochromic shift relative to that of HOXD.     

Excited state intramolecular proton transfer in 5-hydroxy flavone: A DFT study

Potential energy surfaces (PES) for the ground and excited state intramolecular proton transfer (ESIPT) processes in 5-hydroxy-flavone (5HF) were studied using DFT-B3LYP/6-31G(d) and TD-DFT/6-31G(d) level of theory, respectively. Our calculations suggest the non-viability of ground state intramolecular proton transfer (GSIPT) in 5HF. Excited states PES calculations support the existence of ESIPT process in 5HF. ESIPT in 5HF has been explained in terms of HOMO, LUMO electron density of the enol and keto tautomer of 5HF. PES scan by phenyl group rotation suggests that the twisted form, i.e., phenyl group rotated by 18.7° out of benzo-γ-pyrone ring plane is the most stable conformer of 5HF.     

Proton-controlled photoisomerization of 1-(2-pyridyl)-2-(2-quinolyl)ethylene

Quantum-chemical calculations (B3LYP/6-31G*) predict the formation of intramolecular hydrogen bond (IMHB) in the monoprotonated Z-isomer of 1-(2-pyridyl)-2-(2-quinolyl)ethylene (2P2Q), with this bond stabilizing the isomer relative to its E-counterpart. An experimentally observed increase in the quantum yield of trans-cis photoisomerization (φ_tc) by more than an order of magnitude (from 0.033 to 0.42 in acetonitrile) on passing from the neutral to the monoprotonated form of 2P2Q can be associated with IMHB, which manifested itself in the spectral properties of the Z-isomer. The IMHB breaks in the diprotonated form, and the value of φ_tc decreases back to the initial value. In addition to the photoisomerization, the photoreduction and photoaddition reactions of solvent molecules have been observed in an ethanol solution of 2P2Q.     

A comparative study of open‐close and related rotamers methods to evaluate the intramolecular hydrogen bond energies in 3‐imino‐propen‐1‐ol and its derivatives

MP2 study of O? H…N intramolecular hydrogen bond (IMHB) in 3‐imino‐propen‐1‐ol and its derivatives were performed and their IMHB energies were obtained using the related rotamers and open‐close methods. Also the topological properties of electron density distribution and charge transfer energy associated with IMHB were gained by quantum theory of atoms in molecules and natural bond orbital theory, respectively. The computational results reveal that the related rotamers method energies are well correlates with geometrical parameters, topological parameters at hydrogen bond and ring critical points, integrated properties, proton transfer barrier and charge transfer energy of O? H…N unit. Surprisingly, it was found that the open‐close hydrogen bond energies cannot represent good linear correlations with these parameters. Consequently, we extrapolate a number of equations that can be used in estimation of O? H…N IMHB energy in complex biological systems. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

An SCF and CI Study of the 1,3 Shift in the HX?CHY⇌XCH?YH Isoelectronic Series: X,YCH2, NH,and O

Ab initio molecular orbital calculations at SCF level with the 3-21G, 6-31G, and 6-31G** basis sets and CI level with the 6-31G basis set have been carried out for an isoelectronic series HX? CH?Y and X?CH? YH, where X, Y can be CH₂, NH, and O. Optimized structures (3-21G and 6-31G**) for both tautomers and the 1,3 hydrogen shift transition states are reported. The relative stabilities of the isomers and the barriers of the 1,3 shift are discussed in terms of proton affinities and bond orders. It is shown that both the relative stabilities of the tautomers and the relative barrier heights can be explained qualitatively using simple proton affinity arguments and that the barrier heights are quantitatively related to bond orders.