The intramolecular hydrogen bond in 2-hydroxy-benzamides

The two crystal structures of 5-chloro-2-hydroxy-benzamide and 2-hydroxy-N,N-diethyl-benzamide were determined by X-ray diffraction at 100 K. The intramolecular and intermolecular hydrogen bonds were found in these structurally similar 2-hydroxy-benzamides. Analysis of the hydrogen bonding was carried out on the basis of X-ray data, infrared spectra, and DFT calculations. Disruption of the intramolecular hydrogen bonding in the solid state by a steric effect is shown. Conformational analysis and potential energy calculations as functions of the turning angle around the C_aryl–C_alkyl bond were conducted. The values obtained for the HOMA index indicate mutual compensation of the amide and hydroxyl groups (due to the high degree aromaticity of the phenyl ring).     

Quantum chemical investigation of low-temperature intramolecular hydrogen transfer reactions of hydrocarbons

The B3LYP functional was evaluated as a method to calculate reaction barriers and structure-reactivity relationships for intramolecular hydrogen transfer reactions involving peroxy radicals. Nine different basis sets as well as five other MO/DFT and hybrid methods were used in comparing three reactions to available experimental data. It was shown that B3LYP/6-311+G(d,p) offers a good compromise between speed and accuracy for studies in which thermodynamic and kinetic data of many reactions are required. Sixteen reactions were studied to develop structure-reactivity relationships to correlate the activation energy with the heat of reaction. As long as no structural heterogeneities were present in the transition state ring, a simple Evans-Polanyí relationship was shown to capture the activation energy as a function of heat of reaction for reactions in the 1,5-hydrogen shift family. For peroxy radicals undergoing self-abstraction of a hydrogen atom in the 1,5-position, the activation energy was calculated as E(a) (kcal mol(-1)) = 6.3 + Delta H(rxn) (kcal mol(-1)). For reactions with a carbonyl group embedded in the ring of the transition state, the activation energy of peroxy radicals undergoing self-abstraction was correlated as E(a) (kcal mol(-1)) = 18.1 + 0.74 Delta H(rxn) (kcal mol(-1)). The impact of the size of the transition state ring on the activation energy and pre-exponential factor was also probed, and it was shown that these effects can be described using simple nonlinear and linear fits, respectively.     

C~6~0SiH~2的结构和电子光谱的量子化学研究

用INDO系列方法研究了C~6~0SiH~2的两种结构: 一是SiH~2加在两个六元环之间的键上形成C~2~v构型; 另一是SiH~2加在一个五元环和一个六元环之间的键上形成C~s构型。从总能量和LUMO-HOMO能级差看, C~6~0SiH~2的稳定结构应是C~2~v构型, 其中桥C(15)-C(30)的键长为0.1508nm, 键序为0.9369, 说明不开环, 形成类环丙烷结构。文中计算了两种构型的电子吸收光谱和NMR谱, 此类计算是基于对C~6~0SiH~2的等电子体C~6~0O和C~6~0CH~2的研究之上, 且后两者的研究结果与实验相一致。     

Two-dimensional model of intramolecular hydrogen bond

One- and two-dimensional vibrational problems were solved to determine the states of H and D in the intramolecular hydrogen bond of malonic dialdehyde. Within the one-dimensional approach the model potential (barrier height 51 kJ/mol) satisfied with the IR and microwave spectroscopy data. For the two-dimensional problem an approach to evaluation of eigenvalues with high accuracy based on the Ritz method was developed. Within the two-dimensional approximation the barrier height was taken to be 57 kJ/mol. An introduction of the second dimension was found to give rise to the vibrational non-adiabatic effects.     

Nature of the chemical bond of hydrogen and oxygen atoms with PT(100) surface: Quantum chemical calculation and disappearance potential spectra

Electronic structures of clean, hydrogen covered, and oxygen covered Pt(100)-(1×1) surface have been calculated. Both absorbates form surface subzones localized below the metal conduction band but overlapping partially with. Furthermore, the local density of states (LDOS) reveals a peak of the resonant state on the absorbed atom which is narrower for hydrogen than for oxygen. The comparison of LDOS on absorbed and Pt atoms shows that subzone surface states are responsible for the covalent component of the chemical bond between absorbed and platinum atoms, while resonant states make the ionic contribution. The obtained LDOS were used to calculate disappearance potential spectra. Theoretical spectra are well consistent with experimental ones.     

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.     

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.     

Influence of molecular electronic properties on the IR spectra of dimeric hydrogen bond systems: polarized spectra of 2-hydroxybenzothiazole and 2-mercaptobenzothiazole crystals

We have studied the polarized IR spectra of the hydrogen-bonded molecular crystals of 2-hydroxybenzothiazole (HBT) and 2-mercaptobenzothiazole (MBT). The crystal structure of 2-hydroxybenzothiazole was determined by X-ray diffraction. The polarized spectra of the crystals were measured, in the frequency ranges of the ν_N-H and ν_N-D bands, at room temperature, and at 77 K. In both systems an extremely strong H/D isotopic effect in the spectra was observed, involving reduction of the well-developed ν_N-H band fine structure to a single prominent ν_N-D line only. The two ν_N-H bands were also shown to exhibit almost identical properties, band shapes, temperature and dichroic properties included. The spectra were quantitatively reconstituted, along with the strong isotopic effect, when calculated using the ‘strong-coupling’ theory, assuming the centrosymmetric dimers of HBT or MBT to be the structural units responsible for the crystalline spectral properties. The similarity of the spectra of the two crystalline systems was considered to be a result of longer-distance couplings between the proton vibrations in the dimers, via the aromatic ring electrons. When investigating the ‘residual’ ν_N-H band shapes for crystals isotopically diluted by deuterium, we observed some ‘self-organization’ effects in the spectra, indicating the energetically favored presence of two identical hydrogen isotopes in each hydrogen bond dimer.     

Quantum chemical study of the intramolecular transfer of hydrogen in o- methylacetophenone and 1- alkylanthraquinones

The geometries and formation enthalpies of reagents, products, and triplet intermediates in the reaction of photoenolization of o- methylacetophenone and 1- methylanthraquinone are calculated by semiempirical quantum chemical methods. It is shown that the triplet intermediate is a triplet Σπ- biradical. The geometries and formation enthalpies of the transition state of the thermal transfer of the hydrogen atom in 9- hydroxy- 1,10- anthraquinone- 1- methide are calculated. The MNDO semiempirical methods give significantly exaggerated activation enthalpies in the thermal transfer of the hydrogen atom. Translated fromZhumal Strukturnoi Khimii, Vol. 38, No. 4, pp. 645–654, July–August, 1997.     

Effect of the intramolecular hydrogen bond on the electronic structure of organic molecules with a planar quasicycle

The electronic structure of the following organic molecules is studied using the HF/6-311G(d,p) method: malonic dialdehyde, acetylacetone, thiomalonic aldehyde, 2-XC₆H₄NH₂ aniline derivatives, 2-XC₆H₄OH phenol derivatives, 2-XC₆H₄SH thiophenol derivatives (X = CHO, COOH, COO^?, NO, NO₂, OH, OCH₃, SH, SCH₃, F, Cl, Br), 8-hydroxyquinoline, 8-mercaptoquinoline, tropolone. It is found that the intramolecular hydrogen bond (IHB) leads to a local electronic redistribution in the quasi-cycle, and above all to the electron density transfer among the immediate participants of IHB — from the hydrogen atom to the proton-acceptor atom. When the IHB of the S-H?O type forms, the electron density mainly decreases on sulfhydryl hydrogen atom and increases on sulfur atom.     

Molecular structure of dipivaloylmethane and the intramolecular hydrogen bond problem

Molecular structure of dipivaloylmethane was investigated by X-ray electron diffraction at 24°C. The C_2v and C_s geometrical models involving an intramolecular hydrogen bond are considered. The C_2v model with enol hydrogen lying symmetrically relative to the oxygen atoms has several advantages over the classical model of the enol tautomer. Translated from Zhumal Struktumoi Khimii, Vol. 41, No. 1, pp. 58-66, January–February, 2000     

Quantum chemical study of the electronic structure and nature of the chemical bond for crystal modifications of metastable titanium dicarbide

A study was carried out on the electronic energy structure and nature of the chemical bond for a series of crystal modifications (like CaC₂, ThC₂, CsCl, or CaF₂) of metastable titanium dicarbide. The TiC₂ phase may be obtained as a film on the CaC₂ surface, and a metal-semiconductor heterojunction is formed. The possibilities of altering the electronic properties of this heterosystem are discussed. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 35, No. 5, pp. 289–294, September–October, 1999.     

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.     

Proton tunnelling in the intramolecular hydrogen bond of 9-Hydroxyphenalenone

Inelastic neutron scattering spectra of the 9-hydroxyphenalenone and ring deuterated analogue reveal an intense band at 91 cm⁻¹ with all characteristics anticipated for a tunnelling the transition of a (quasi)symmetric double minimum potential for proton transfer along the intramolecular hydrogen bond. This frequency is compared to those previously reported for similar systems.     

Influence of intramolecular hydrogen bond strength on OH-stretching overtones

Vapor-phase OH-stretching overtone spectra of 1,3-propanediol and 1,4-butanediol were recorded and compared to the spectra of ethylene glycol to investigate the effect of increased intramolecular hydrogen bond strength on OH-stretching overtone transitions. The spectra were recorded with laser photoacoustic spectroscopy in the second and third OH-stretching overtone regions. The room-temperature spectra of each molecule are dominated by two conformers that show intramolecular hydrogen bonding. Anharmonic oscillator local-mode calculations of the OH-stretching transitions have been performed to aid assignment of the different conformers in the spectra and to illustrate the effect of the intramolecular hydrogen bonding. The hydrogen bond strength increases in the order ethylene glycol, 1,3-propanediol, and 1,4-butanediol. The overtone transitions of the hydrogen-bonded hydroxyl groups are more difficult to observe with increasing intramolecular hydrogen bond strength. We suggest that the bandwidth of these transitions increases with increasing hydrogen bond strength and with increasing overtone and furthermore that these changes are in part responsible for the lack of observed overtone spectra for complexes.