Dipole moment,isomerization, and intramolecular hydrogen bond in some benzene derivatives (computations)

The dipole moment vectors of some benzene monoderivatives are calculated to test the method used in an attempt to explain the dipole moments of o-, m- and p-substituted phenols (X-C₆H₄-OH; X = -OH, -F, -NO₂, -CHO, -COOH, -CH₃). The vectors seem to include contributions from a mixture of planar conformers whose populations obey a Boltzmann-type distribution. The calculated intramolecular interaction moment is proposed as a simple criterion indicating the strength of the intramolecular hydrogen bond in o-phenols.     

Reinvestigation of intramolecular hydrogen bond in malonaldehyde derivatives: An ab initio,AIM and NBO study

The RAHB systems in malonaldehyde and its derivatives at MP2/ 6‐311++G(d,p) level of theory were studied and their intramolecular hydrogen bond energies by using the related rotamers method was obtained. The topological properties of electron density distribution in O? H···O intramolecular hydrogen bond have been analyzed in term of quantum theory of atoms in molecules (QTAIM). Correlations between the H‐bond strength and topological parameters are probed. The results of QTAIM clearly showed that the linear correlation between the electron density distribution at HB critical point and RAHB ring critical point with the corresponding hydrogen bond energies was obtained. Moreover, it was found a linear correlation between the electronic potential energy density, V(r_cp), and hydrogen bond energy which can be used as a simple equation for evaluation of HB energy in complex RAHB systems. Finally, the similar linear treatment between the geometrical parameters, such as O···O or O? H distance, and Lp(O)→σ*_OH charge transfer energy with the intramolecular hydrogen bond energy is observed. © 2010 Wiley Periodicals, Inc., Int J Quantum Chem, 2011     

DFT studies, vibrational spectra and conformational stability of 4-hydroxy-3-methylacetophenone and 4-hydroxy-3-methoxyacetophenone

The FT-IR and FT-Raman spectra of 4-hydroxy-3-methylacetophenone (HMA) and 4-hydroxy-3-methoxyacetophenone (HMOA) have been recorded. The total energy calculations of HMA and HMOA were tried for various possible conformers. The spectra were interpreted with the aid of normal coordinate analysis based on density functional theory (DFT) using standard B3LYP/6-31G* and B3LYP/6-311+G** methods and basis sets combinations for the most optimized geometries. Normal coordinate calculations were performed with the DFT force field corrected by a recommended set of scaling factors yielding fairly good agreement between observed and calculated frequencies. On the basis of the comparison between calculated and experimental results, assignments of fundamental modes were examined.     

Conformational behaviour of 3-phenyl- and 3-carbomethoxy-substituted cyclopropene derivatives

According to MNDO calculated heats of formation for various 3-substituted cyclopropenes the π-acceptor methoxycarbonyl as well as the π-donor group phenyl both prefer a non-bisected orientation with respect to the three-membered ring. Temperature dependent NMR chemical shifts seem to confirm this behaviour for the free molecules. In the crystalline state, however, 1,2,3-triphenylcyclopropene (3e) and 3-isopropyl-1,2,3-triphenylcyclopropene (3f) both adopt a conformation with a bisected orientation of the 3-phenyl groups.     

Inter- and intramolecular hydrogen bond in methyl 2-hydroxy-9H-1-carbazole carboxylate: effect of solvents and acid concentration

The photo-physics of methyl 2-hydroxy-9H-1-carbazazole carboxylate (MPCC) in different solvents and cyclohexane-trifluoroethanol (TFE) mixtures has been studied by means of absorption, fluorescence, fluorescence excitation spectra, time dependence spectrofluoremetery and AM1 semi-empirical quantum mechanical calculations. Only one small Stoke’s shifted fluorescence band is observed under all the environments, indicating that the geometry of the molecule is not changed much on excitation to the first singlet state (S₁) and excited state intramolecular proton transfer (ESIPT) is not viable both in the ground (S₀) and S₁ states at the room temperature. AM1 calculation shows that the ESIPT is still endothermic in S₁ state. Single exponential decay is observed in the fluorescence from MPCC in all the solvents except acetonitrile and methanol. This suggests that in these two solvents, at least two different conformers are present in the S₀ state, whose absorption spectra are not different from each other. Spectral characteristics of MPCC in cyclohexane as a function of TFE have shown a slight blue shift in the λ_max^ab, decrease in the ε_max, red shift in the λ_max^fl and decrease in the φ_fl. This suggests that intermolecular hydrogen bonding is playing a major role in the deactivation of the fluorescence intensity than the intramoleuclar hydrogen bonding (IHB). Spectral properties of MPCC were also studied as a function of acid–base concentrations. pK_a values for different prototropic equilibriums were determined in S₀ and S₁ states and discussed.     

Vibrational assignment, structure and intramolecular hydrogen bond of 4-methylamino-3-penten-2-one

The molecular structure, intramolecular hydrogen and vibrational frequencies of 4-methylamino-3-penten-2-one were investigated by a series of density functional theoretical (DFT) calculations and ab initio calculation at the post-Hartree-Fock (MP2) level. Fourier transform infrared and Fourier transform Raman spectra of this compound and its deuterated analogue were clearly assigned. The calculated geometrical parameters show a strong intramolecular hydrogen bond with a N...O distance of 2.622-2.670 A. This bond length is about 0.02 A shorter than that in its parent, 4-amino-3- penten-2-one which is in agreement with spectroscopic results. Furthermore, the conformations of methyl groups with respect to the plane of the molecule and with respect to each other were investigated.     

New stable neutral radical with intramolecular hydrogen bonding: synthesis and characterization of 2,5,8-tri-tert-butyl-7-hydroxy-6-oxophenalenoxyl

A new stable neutral radical with intramolecular hydrogen bonding, 2,5,8-tri-tert-butyl-7-hydroxy-6-oxophenalenoxyl, was synthesized from the corresponding dihydroxyphenalenone and isolated as a stable solid under air atmosphere at room temperature. The structure was unequivocally determined by means of IR spectra, ESR/ENDOR techniques, and DFT calculations.     

Simulation of intramolecular hydrogen bond dynamics in manzamine A as a sensitive test for charge distribution quality

Subtle balance of inter- and intramolecular hydrogen bond strength in aqueous solutions often governs the structure and dynamics of molecular species used as potential drugs and in supramolecular applications. In silico molecular dynamics study of water solution of manzamine A has been performed with different atomic charges in order to investigate the influence of charge distribution choice on predicting qualitative and quantitative features of the simulated systems. Various well known charge schemes (MK-ESP, RESP, Mulliken, AMI-BCC, Gasteiger-Hückel, Gasteiger-Marsili, MMFF94, and Dynamic Electronegativity Relaxation - DENR) led to qualitatively different pictures of dynamic behavior of the intramolecular hydrogen bond. The reported calculation framework represents a relatively rare case where differences in charge distributions lead to noticeable differences in simulated properties, thus providing a useful test case for force field and charge distribution development, provided high quality experiments are conducted to use as references.     

The competition between the intramolecular hydrogen bond and π‐electron delocalization in trifluoroacetylacetone—A theoretical study

Conformational study of trifluoroacetylacetone was carried out using the HF, B3LYP, and MP2 methods with the 6‐31G(d, p) and 6‐311++G(d, p) basis sets. All of the results show that the chelated enol structures (E11 and E31) have extra stability with respect to the other forms and one of them (E11) is global minimum. The energy gap between the chelated forms is in the range 0.7–5.9 kJ mol^?1. Theoretical calculations show that this compound has an asymmetric double minimum potential energy surface which is in contrast with the electron diffraction result. Moreover, the computational results predict that due to the withdrawing effect of CF₃ group, hydrogen bond in trifluoroacetylacetone is weaker than the acetylacetone. Because of the more stability of E11, it is expected that the hydrogen bond energy in E11 is greater than the E31, but at all of the computational levels with most extended basis set the converse results were observed. These results clearly show that the hydrogen bond is not a superior parameter in conformational preference and the contribution of resonance is probably greater than the hydrogen bond. Finally, the analysis of this system by quantum theory of atoms in molecules and natural bond orbital methods fairly support the ab initio results. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Vibrational assignment, structure and intramolecular hydrogen bond study of 3-amino-1-phenyl-2-buten-1-one

Fourier transform infrared and Fourier transform Raman spectra of 3-amino-1-phenyl-2-buten-1-one and its deuterated analogue were recorded in the regions 400-4,000 and 150-4,000 cm(-1), respectively. Furthermore, the molecular structure and vibrational frequencies of title compound were investigated by a series of density functional theoretical, DFT, and ab initio calculations at the post-Hartree-Fock (MP2) level. Although, the calculated frequencies are generally in agreement with the observed spectra but the DFT results are in much better quantitative agreement with the observed spectra than the MP2 results. The observed wavenumbers were analyzed and assigned to different normal modes of vibration of the molecule. The calculated geometrical parameters show a strong intramolecular hydrogen bond with a N...O distance of 2.621-2.668 A. This bond length is shorter than that of its parent, 4-amino-3-penten-2-one (with two methyl groups in the beta-position), which is in agreement with spectroscopic results. The topological properties of the electron density contributions for intramolecular hydrogen bond in 3-amino-1-phenyl-2-buten-1-one and 4-amino-3-penten-2-one have been analyzed in term of the Bader theory of atoms in molecules (AIM). These results also support the stronger hydrogen bond in the title compound with respect to the parent molecule.     

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.     

Microwave spectra of isotopic glycolaldehydes,substitution structure,intramolecular hydrogen bond and dipole moment

The microwave spectra of ¹³CH₂OH-CHO, CH₂OH-¹³CHO, and CH₂OH-CH¹⁸O are reported and have been used in combination with previously published data on other monosubstituted glycolaldehydes to determine the substitution structure of the molecule as r(CO) = 1.209 Å, r(C-O) = 1.437 Å, r(C-C) = 1.499 Å, r(O-H) = 1.051 Å, r(C-H_ald) = 1.102 Å, r(C-H_alc) = 1.093 Å, r(O β H) = 2.007 Å, r(O β O) = 2.697 Å, ∠(C-CO) = 122°44', ∠(C-C-H_ald) = 115°16', ∠(C-C-O) = 111°28', ∠(C-O-H) = 101°34', ∠(C-C-H_alc) = 109°13', ∠(H-C-H) = 107°34', ∠(O-H β O) = 120°33', ∠(H β OC) = 83°41', and ∠(O-H, C0) = 24°14'. The intramolecular hydrogen bond and the other structural parameters are discussed and compared to related molecules. The dipole moment is redetermined to be μ_a = 0.262 ±0.002 D, μ_b = 2.33 ± 0.01 D, and μ_tot = 2.34 ± 0.01 D. Relative intensity measurements yielded 195 ± 30 cm^?1 for the C-C torsional fundamental and 260±40 cm^?1 for the lowest in-plane skeletal bending mode. Computations performed by the CNDO/2 method correctly predict the observed cis hydrogen-bonded conformer to be the energetically favoured one and in addition yield some indication of the existence of at least two other non-hydrogen-bonded forms of higher energy.     

对映-贝壳杉烯二萜分子内氢键对6, 7位羟基化学反应性影响研究

对映-贝壳杉烯二萜分子在激烈条件下进行乙酰化反应, 反应发生在半缩醛羟基, 而非仲羟基上; 该反常现象的生产可能是由于6位仲羟基与15位羰基之间形成了分子内氢键, 本文根据^1H NMR变温实验观察分子在不同温度时6位和7位羟基质子的化学位移变化值△δ/T(Hz.K^-^1), 证明了上述假定。     

Microwave spectrum,conformation, intramolecular hydrogen bond,and dipole moment of pyrrole-2-carboxaldehyde

Microwave spectra of C₄H₃NH-CHO, C₄H₃ND-CHO, and C₄H₃NH-CH¹⁸O are reported. The stable form of the molecule is demonstrated to be planar with the N-H and C-O bonds in a cis conformation. Other forms of the molecule are at least 1 kcal mol^?1 less stable. The H(1) · O distance is 2.592±0.006 Å. Six vibrationally excited states were attributed to the C-C torsional mode, the symmetrical, and the antisymmetrical aldehyde group deformation vibrations. Relative intensity measurements yielded 151±11 cm^?1, for the first frequency, 210±17 cm^?1 for the second, and 270±38 cm^?1 for the last mode. The dipole moment was determined to be μ_a = 2.47 ±0.02 D, μ_b = 0.16±0.06 D, and μ_tot = 2.48 ±0.02 D, respectively.     

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.     

Copper-catalyzed intramolecular C-N bond formation: a straightforward synthesis of benzimidazole derivatives in water

A straightforward, efficient, and more sustainable copper-catalyzed method has been developed for intramolecular N-arylation providing the benzimidazole ring system. With Cu(2)O (5 mol %) as the catalyst, DMEDA (10 mol %) as the ligand, and K(2)CO(3) as the base, this protocol was applied to synthesize a small library of benzimidazoles in high yields. Remarkably, the reaction was exclusively carried out in water, rendering the methodology highly valuable from both environmental and economical points of view.     

Localized orbital studies of hydrogen bonding: III. The intramolecular hydrogen bond in 4-methylimino 2-pentanone

Geometry optimizations using the INDO molecular orbital method are carried out on the three possible isomeric forms of 4-methylimino- 2-pentanone. It is found that the molecule strongly prefers a structure containing a hydrogen-bonded chelate ring. Of the two possible isomers containing an intramolecular hydrogen bond, only the structure with the chelated proton closer to the oxygen corresponds to an energy minimum, although the energy surface connecting these two isomers is very flat. The results are interpreted using experimental and calculated NMR properties and energy localized molecular orbitais.