Electronic structures, intramolecular interactions, and aromaticity of substituted 1-(2-iminoethylidene) silan amine: a density functional study

The intramolecular hydrogen bond, molecular structure, π electrons delocalization, and vibrational frequencies in 1-(2-iminoethylidene) silan amine and its derivatives have been investigated by means of density functional method with 6-311++G** basis set, in gas phase, water, and carbon tetrachloride solutions. The obtained results showed that the hydrogen bond strength is mainly governed by resonance variations inside the chelate ring induced by the substituent groups. Furthermore, the topological properties of the electron density distributions for N–H···N intramolecular hydrogen bond were analyzed in terms of the Bader's theory of atoms in molecules. On the other hand, the aromaticity of the ring formed is 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, aromatic fluctuation index, and π-fluctuation aromatic index. Natural population analysis data, the electron density and Laplacian properties, as well as γ(NH) and ν(NH) were further used for estimation of the hydrogen bonding interactions and the forces driving their formation.     

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     

The analysis of structural and electronic properties for assessment of intramolecular hydrogen bond (IMHB) interaction: a comprehensive study into the effect of substitution on intramolecular hydrogen bond of 4-nitropyridine-3-thiol in ground and electronic excited state

The hydrogen bond strength, molecular geometry, π-electron delocalization, and physical properties such as dipole moment, chemical potential, and chemical hardness of 4-nitropyridine-3-thiol and its 29 derivatives have been studied by means of density functional method with 6-311++G** basis set in gas phase and water solution. Also, the excited-state properties of intramolecular hydrogen bonding in these systems have been investigated theoretically using the time-dependent density functional theory method. The HOMA, NICS, PDI, ATI, FLU, and FLU_π indices as well-established aromaticity indicators have been examined. Natural bond orbital analysis is also performed for better understanding the nature of intramolecular interactions. The electron density and Laplacian (?² ρ) properties, estimated by AIM calculations, indicate that H···O bond possesses low ρ and positive ?² ρ values, which are in agreement with electrostatic character of the HBs, whereas S–H bond has covalent character. Numerous correlations between topological, geometrical, and energetic parameters are also found.     

Theoretical study of substituents effects on characteristics of resonance-assisted hydrogen bond in (Z)-(thionitrosomethylene)hydrazine and its derivatives in ground and electronic excited state

The molecular geometry, intramolecular hydrogen bond strength, vibrational frequencies, ¹H NMR chemical shift, and nuclear quadrupole resonance parameters of ¹⁴N, ³⁵S and ²H atoms and several well-established indices of aromaticity in (Z)-(thionitrosomethylene)hydrazine molecule and its derivatives were studied by density functional theory method. The results of calculations were obtained at B3LYP/6-311++G** level of approximation on model species, with the resonance-assisted hydrogen bonds. A set of simple and mostly common substituents having different properties in resonance effect according to values of substituents constants were chosen to simulate the influence of substitution in R position of title molecule on the quasi-delocalization and H-bonding. The following substituents have been taken into consideration: F, Cl, NO₂, OCH₃, OCF₃, SCH₃, SH, and OH. The excited-state properties of intramolecular hydrogen bonding in substituted systems have been investigated theoretically using the time-dependent density functional theory method. Also, the possible charge transfer and the topological properties of investigated molecule and its derivatives were studied by means of natural bond orbital and atoms in molecules (AIM) theory. The energy of the N–H···S interactions studied here was found medium in strength ( \( E_{\text{HB}}^{*} \)  = ?36.5 to ?45.3 kJ mol^?1). The electron density (ρ), Laplacian (?² ρ) properties and the total electron energy density (HC), estimated by AIM calculations, indicate that H···S bond possesses low ρ, positive ?² ρ and HC < 0 which are in agreement with partially covalent character of HB.     

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     

Density functional theory and atoms-in-molecules investigation of intramolecular hydrogen bonding in derivatives of malonaldehyde and implications for resonance-assisted hydrogen bonding

A density functional theory (DFT) and atoms-in-molecules (AIM) analysis has been applied to the intramolecular hydrogen bonding in the enol conformers of malonaldehyde and its fluoro-, chloro-, cyano-, and nitro-substituted derivatives. With the B3LYP/6-311++G(2d,p) method, good agreement between the DFT geometries and published experimental structures has been found. The donor-acceptor distance was also varied in a series of constrained optimizations in order to determine if energetic, structural, and topological trends associated with intermolecular hydrogen bonding remain valid in the intramolecular case. At very short donor-acceptor distances (<2.24 A), the hydrogen is symmetrically located between donor and acceptor; at distances longer than this, the hydrogen bonding is no longer symmetric. The AIM methodology has been applied to explore the topology of the electron density in the intramolecular hydrogen bonds of the chosen model systems. Most AIM properties for intramolecular hydrogen bond distances longer than 2.24 A show smooth trends, consistent with intermolecular hydrogen bonds. Integrated AIM properties have also been used to explore the phenomenon of resonance-assisted hydrogen bonding (RAHB). It is shown that as the donor-acceptor distance is varied, pi-electron density is redistributed among the carbon atoms in the intramolecular hydrogen bond ring; however, contrary to prior studies, the integrated atomic charges on the donor-acceptor atoms were found to be insensitive to variation of hydrogen-bonding distance.     

Theoretical study of structure, bonding, and aromaticity of borazyne and B-substituted borazynes

The density functional theory (DFT) is used to study the geometries, and electronic structures of triplet and singlet of borazyne and B-substituted of borazyne. The aromaticity of these systems is analyzed in the light of nucleus-independent chemical shift (NICS), average of two-center indices (ATI). These methods show increasing of aromaticity in deactivating groups. The relation between electron density in ring critical point (RCP) and NICS(1.0) is observed. The most important interaction in these molecules has been investigated by natural bonding orbital method (NBO).     

Intramolecular hydrogen bonding in structural conformers of 2‐amino methylene malonaldehyde: AIM and NBO studies

The molecular structure and intramolecular hydrogen bond energies of 44 conformers of 2‐Amino methylene malonaldehyde were investigated at MP2 and B3LYP levels of theory using the standard 6‐311++G** basis set and AIM and NBO analysis. The calculated geometrical parameters and conformational analysis in gas phase show that the closed ring via intramolecular hydrogen bonded conformers of this compound are more stable than the other ones. Hydrogen bond energies for H‐bonded conformers were obtained from the related rotamers method (RRM) and Schuster method, and also the nature of H‐bonding of them has been investigated by means of the Bader theory of atoms in molecules, which is based on topological properties of the electron density. Delocalization effects can be identified from the presence of off diagonal elements of the Fock matrix in the NBO basis. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Intramolecular hydrogen bonding in chemoselective synthesized 2-substituted pyrrole stable phosphorus ylide: GIAO, AIM, and NBO approaches

The chemoselectivity of geometrically ylide compounds is often hard to assign from experimental techniques, particular system with intramolecular hydrogen bonding (IHB) are even more challenging. Herein, theoretical calculations were performed to investigate whether theoretical results would provide consistent evidence for the existence of IHB to confirm experimental data and to evaluate strength of the N–H···O IHB from geometrical synthesized 2-substituted pyrrole stable phosphorus ylide (dimethyl 2-(1H-pyrrol-2-yl)-3-(triphenylphosphoranylidene) butanedioate in a single chemoselective compound. Topological parameters at the bond critical points (BCP) of intramolecular hydrogen bonds from Bader’s atoms in molecules (AIM) theory and Winhold’s natural bond orbital (NBO) calculations were analyzed at the B3LYP/6-311++g** level in details. A series of gage-including atomic orbital chemical shift (GIAO c.s.) calculations at the HF and DFT levels of theory were carried out to assign the ¹H NMR chemical shifts. The best prediction of the experimental ¹H NMR values was obtained at the mPW1PW91 levels using the 6-31G** basis set. Theoretical results, in agreement with the experimental data, were confirmed the N–H···O IHB was caused the deshielding of the proton to lower field. The barriers in double bond and single bond rotation were theoretically estimated in detailed and the AIM and NPA approaches were confirmed the loss of charge of the hydrogen atom involving in intramolecular N–H···O hydrogen bonding. The geometrical and topological parameters from AIM and NBO analyses were indicated the medium N–H···O IHB.     

Unraveling weak interactions in aniline-pyrrole dimer clusters

Weak intermolecular interactions in aniline-pyrrole dimer clusters have been studied by the dispersion-corrected density functional theory(DFT) calculations. Two distinct types of hydrogen bonds are demonstrated with optimized geometric structures and largest interaction energy moduli. Comprehensive spectroscopic analysis is also addressed revealing the orientation-dependent interactions by noting the altered red-shifts of the infrared and Raman activities. Then we employ natural bond orbital(NBO)analysis and atom in molecules(AIM) theory to have determined the origin and relative energetic contributions of the weak interactions in these systems. NBO and AIM calculations confirm the V-shaped dimer cluster is dominated by N.H···N and C.H···π hydrogen bonds, while the J-aggregated isomer is stabilized by N.H···π, n→π* and weak π···π* stacking interactions.The noncovalent interactions are also demonstrated via energy decomposition analysis associated with electrostatic and dispersion contributions.     

Intramolecular hydrogen bond in 3‐imino‐propenylamine isomers: AIM and NBO studies

The molecular structure and intramolecular hydrogen bond energy of 18 conformers of 3‐imino‐propenyl‐amine were investigated at MP2 and B3LYP levels of theory using the standard 6‐311++G** basis set. The atom in molecules or AIM theory of Bader, which is based on the topological properties of the electron density (ρ), was used additionally and the natural bond orbital (NBO) analysis was also carried out. Furthermore calculations for all possible conformations of 3‐imino‐propenyl‐amin in water solution were also carried out at B3LYP/6‐311++G** and MP2/6‐311++G** levels of theory. The calculated geometrical parameters and conformational analyses in gas phase and water solution show that the imine–amine conformers of this compound are more stable than the other conformers. B3LYP method predicts the IMA‐1 as global minimum. This stability is mainly due to the formation of a strong N? H···N intramolecular hydrogen bond, which is assisted by π‐electrons resonance, and this π‐electrons are established by NH2 functional group. Hydrogen bond energies for all conformers of 3‐imino‐propenyl‐amine were obtained from the related rotamers methods. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Inequivalence of substitution pairs in hydroxynaphthaldehyde: A theoretical measurement by intramolecular hydrogen bond strength,aromaticity, and excited‐state intramolecular proton transfer reaction

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 ρ(r) and Laplacian ?²ρ(r) at the bond critical point using atoms in molecule method and by calculation of interaction between σ* 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. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010     

Intramolecular hydrogen bond, molecular structure and vibrational assignment of tetra-acetylethane. A density functional study

The intramolecular hydrogen bond, molecular structure and vibrational frequencies of tetra-acetylethane have been investigated by means of high-level density functional theory (DFT) methods with most popular basis sets. Fourier transform infrared and Fourier transform Raman spectra of this compound and its deuterated analogue were recorded in the regions 400-4000 cm(-1) and 40-4000 cm(-1), respectively. The calculated geometrical parameters of tetra-acetylethane were compared to the experimental results of this compound and its parent molecule (acetylacetone), obtained from X-ray diffraction. The O...O distance in tetra-acetylethane, about 2.424A, suggests that the hydrogen bond in this compound is stronger than acetylacetone. This conclusion is well supported by the NMR proton chemical shifts and O-H stretching mode at 2626 cm(-1). Furthermore, the calculated hydrogen bond energy in the title compound is 17.22 kcal/mol, which is greater than the acetylacetone value. On the other hand, the results of theoretical calculations show that the bulky substitution in alpha-position of acetylacetone results in an increase of the conjugation of pi electrons in the chelate ring. Finally, we applied the atoms in molecules (AIM) theory and natural bond orbital method (NBO) for detail analyzing the hydrogen bond in tetra-acetylethane and acetylacetone. These results are in agreement with the vibrational spectra interpretation and quantum chemical calculation results. Also, the conformations of methyl groups with respect to the plane of the molecule and with respect to each other were investigated.     

β‐Aminoacrolein: An ab initio,AIM and NBO study

The molecular structure and the intramolecular hydrogen bonding of β‐aminoacrolein and its simple derivatives were investigated at the MP2 and B3LYP levels of theory using the standard 6‐311++G(d, p) basis set. The “atoms in molecules” or AIM theory of Bader which is based on topological properties of the electron density (ρ), was used. Additionally, an analysis of the critical points was performed to study the nature hydrogen bonding in these systems. Natural bond orbital (NBO) analysis was also carried out for to better comprehend the nature of the intramolecular interactions in β‐aminoacrolein and its derivatives. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Density functional theory calculations and vibrational spectral analysis of 3,5-(dinitrobenzoic acid)

The FT-IR and Raman spectra of 3,5-dinitrobenzoic acid (DNBA) have been recorded and analyzed. The equilibrium geometry, various bonding and harmonic vibrational wavenumbers have been calculated with the help of density functional theory (DFT) method. Most of the vibrational modes are observed in the expected range. Mulliken population analysis shows the interactions C-N-O?H-C and C-O?H-C. The most possible interaction is explained using natural bond orbital (NBO) analysis. The strengthening and polarization of the CO bond increases due to the degree of conjugation. HOMO-LUMO energy and the thermodynamic properties are also evaluated.     

Ab initio/DFT and AIM studies on dual hydrogen-bonded complexes of 2-hydroxypyridine/2-pyridone tautomerism

The second-order M?ller-Plesset (MP2) and density functional theory (DFT) calculations have been carried out to investigate the structures and stabilities of hydrogen (H-) bonded 2-hydroxypyridine (2HP)/2-pyridone (2PY) dimeric forms as well as 2HP-2PY complexes. The results on single-point counterpoise (CP) correction of these complexes were compared against CP-optimized correction. The nature of the intermolecular contacts in the sense of normal H-bond or blue-shifting H-bond was determined on the basis of harmonic vibrational, atom-in-molecule (AIM), and natural bond orbital (NBO) analysis. A blue-shifting C-H...N H-bond was found and NBO analysis revealed a slight decrease in the population of the contacting sigmaC-H* antibonding orbital as the primary reason of the C-H contraction. Good correlations have been established between the interaction energies and the H-bond distances versus other characteristic H-bond parameters.     

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.     

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     

Intramolecular S···O chalcogen bond in thioindirubin

Results of X-ray diffraction study and quantum-chemical calculations revealed that molecular conformation of thioindirubin molecule creates suitable conditions for formation of intramolecular C–H···O and S···O interactions. Analysis of molecular electrostatic potential (MEP) demonstrates existence of two areas of positive MEP (σ-holes) in the outermost part of the sulfur atom on the continuation of the lines of the C–S bonds. One of these σ-holes is oriented toward region of negative MEP around the oxygen atom of carbonyl group. Such situation corresponds to formation of σ-hole or chalcogen bond. Existence of both types of bonding interactions is confirmed by topological analysis of electron density distribution using “Atoms in Molecules” (AIM) theory. Energies of the C–H···O hydrogen bond and the S···O σ-hole bond derived from AIM and NBO theories are very close.