Intramolecular Hydrogen Bond OH Research Articles

Substituent control of dynamical process for excited state intramolecular proton transfer of benzothiazole derivatives

Using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, in this work, the excited state intramolecular proton transfer (ESIPT) mechanisms of 2-(2′-hydroxyphenyl)benzothiazole (HBT) derivatives obtained by switching the ortho-position functional group of the hydroxyl group have been investigated. Analysis of bond parameters, bond energy, and infrared (IR) vibrational spectra show that hydrogen bonds are enhanced in the first excited state. The core-valence bifurcation (CVB) index and bond critical point (BCP) parameters are further applied to evaluate hydrogen bonds. The maximum absorption and emission peaks of molecules mainly originate from the transition of electrons from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). Exploring potential energy curves (PECs) and transition state (TS) energy profiles, we clarify the ultrafast ESIPT behavior of HBT and its derivatives are easier in the excited state. It is clarified that the HBT derivatives in the excited state with electron withdrawing group at the ortho-position weaken the intramolecular hydrogen bond OH···N, while are not conducive to the ESIPT process. And the electronic effect of the substituents can be employed to adjust the emission of the fluorophore from blue to near-infrared. The calculated results also show that the enol form is stable in the ground state, while the keto form is the main conformation in the excited state.

Lattice vibrations of the molecular crystal of photoreactive substance with defects

Lattice vibrations are studied theoretically by density-functional based tight-binding methods for the model structure of 2-(2'-hydroxyphenyl)benzoxazole crystal with defects. 2-(2'-hydroxyphenyl)benzoxazole is a photoreactive compound that exhibits excited state intramolecular proton transfer in the structure with an OH...N hydrogen bond. The unit cell of the model structure consists of two crystal unit cells where the molecules have the structure with intramolecular hydrogen bonds OH...N and one molecule is supposed to have a different orientation of the whole molecule or its fragment. The different orientation of the fragment forms the structure with an intramolecular hydrogen bond OH...O. It is calculated that defect caused by the different orientation of the molecule have a lower energy than the defect caused by the different orientation of the fragment. In the frequency region where the contribution of external vibrations of the molecules is significant, the vibrations mainly involve several molecules in the cell. In the region of internal vibrations there are modes, which are local vibrations of the defects. These local vibrations involve mainly motion of the atoms constituting the defect molecule. The number of local vibrations is larger for the defect that corresponds to the formation of the structure with the OH...O hydrogen bond than for the defect that corresponds to the different orientation of the whole molecule with the OH...N hydrogen bond. The internal vibrations of the defect molecule formed by the different orientation of phenol fragment in the lattice undergoes frequency shift in relation to the frequency of the modes of isolated molecule.

Coordination diversity in tin compounds with bis(benzoxazole)phenol as a polydentate ligand: Synthesis and crystal structure studies

The reaction of 2,6-bis(benzoxazolyl)-4-tert-butylphenol (HL) with [nBuxSnCl4−x] (x = 0, 1) in 1:1 stoichiometry yielded the tin coordination complexes [(HL)SnnBuxCl4−x] [x = 0 (1); x = 1 (2)]. Deprotonation of HL was performed using reagents having groups with high basicity such as nBuLi or [Sn{N(SiMe3)2}2]. These basic reagents prompted the coordination of the ligand in its anionic form, yielding the complexes [(thf)2Li(L)SnCl4] (3) and [(L)Sn{N(SiMe3)2}] (4), respectively. The molecular structure of HL displayed an intramolecular hydrogen bond OH—N and a planar arrangement of the bis(benzoxazolyl)phenolic system. In the molecular structures of both complexes containing HL an intramolecular hydrogen bond of NH—O type was also present. The coordination of the ligand in either neutral or anionic form is described by a κO,κN chelate mode toward Sn. All complexes displayed bis(benzoxazolyl)phenolic moieties close to planar; the least planar system was observed in 4 that was also studied by DFT methods.

The molecular structure and absorption spectrum of hydroxy substituted dibenzoylmethanatoboron difluoride in solution: A theoretical and experimental study

Electronic spectroscopy and quantum chemistry are used to study the structure and absorption spectra of the hydroxy substituted dibenzoylmethanatoboron difluoride (OHDBMBF2) in solutions. Introducing a hydroxy group in the diketonate moiety allows the dye to form intermolecular complexes with proton acceptors, such as solvents or analytes, thus making it a promising chemical sensor. Our calculations show that donor oxygen-containing solvents break the intramolecular hydrogen bond OH···Odik and form an intermolecular OH···Osolv bond thus disrupting the coplanarity of the dye and affecting the position and shape of its absorption bands. The spectra calculated with explicit solvent combined with polarizable continuum model (PCM) better agree with the experiment than those calculated only within PCM.

Theoretical DFT and matrix isolation FTIR studies of 2-(1,2,4-triazolyl)phenol isomers

The structure, isomerization pathways and vibrational spectra of the important heterocyclic 2-(1,2,4-triazolyl)phenol molecule were investigated by DFT calculations and matrix isolation FTIR spectroscopy. Among forty-five minima located on PES three isomers with intramolecular hydrogen bond OH⋯N, 2-TRP1, 1-TRP1 and 1-TRP2, are the most stable forms with the calculated abundance of 83.4%, 10.3% and 6.0%, respectively. The presented FTIR results allow identification and characterization of these species. Several hydrogen bond parameters such as OH bond distance, νOH wavenumber shift and occupancy of the antibonding σ∗(OH) orbital were found to be linearly related with the estimated interaction energy.

Assignment of photoelectron spectra of the 2-allylphenol with the help of electron propagator theory

Four low-lying conformers (Ia, Ib, IIa, IIb) of 2-allylphenol are studied by density function theory (B3LYP) with the aug-cc-pVDZ basis set. The conformer Ia having the intramolecular hydrogen bond OH---π is the most stable. The photoelectron spectrum (PES) of 2-allylphenol recorded by Kowshi et al. (2001) [14] are reassigned on the basis of the present electron propagator theory calculations. The results show that the strong ionization band at 10.01eV is contributed from the most stable conformer Ia, and the weak ionization band at 9.72eV is from the next stable conformer IIb.

Ab initio study on N,N-dimethylaminoethanol: Conformers and energies

All the possible rotamers of N,N-dimethylaminoethanol (DMAE) were fully optimized at the ab initio level using hybrid density functional B3LYP/aug-cc-pVDZ method. Twelve local minima for DMAE were found and their geometric parameters, dipole moments, intramolecular hydrogen bonds (HB) and relative energies of all conformers were determined. Their relative energies calculated at MP3 and MP4 (SDTQ) levels of theory further confirm that the gauche′-Gauche-gauche′ (g′Gg′) conformer having the intramolecular HB OH⋯N is the most stable. The vertical ionization energies of the conformers calculated with ab initio electron propagator theory in the P3/aug-cc-pVTZ approximation are in agreement with the experimental data from photoelectron spectroscopy. Combined with statistical mechanics principles, conformational distributions at various temperatures are computed and the temperature dependence of photoelectron spectra (PES) is interpreted. The intramolecular HB OH⋯N influences on the molecular electronic structures in the conformer g′Gg′ was exhibited by Natural bond orbital (NBO) analyzes.

Influence of OH⋯N and NH⋯O inter- and intramolecular hydrogen bonds in the conformational equilibrium of some 1,3-disubstituted cyclohexanes through NMR spectroscopy and theoretical calculations

The analysis of concentration effects in the 1H NMR data of cis-3-aminocyclohexanol ( ACOL) showed that its diequatorial conformer changes from 60% at 0.01 mol L −1 to 70% at 0.40 mol L −1 in acetone-d 6. A similar increase was also observed for the diequatorial conformer of cis-3-N-methylaminocyclohexanol ( MCOL), from 32% (CDCl 3 0.01 mol L −1) to 55% (CDCl 3 0.40 mol L −1). The increase in solvent basicity leads to a large stabilization effect for the diequatorial conformer of both compounds too. For ACOL, it changes from 47% (Δ G eqeq– axax = 0.06 kcal mol −1) in CCl 4 to 93% (Δ G eqeq– axax = −1.53 kcal mol −1) in DMSO, while for MCOL it goes from 7% (Δ G eqeq– axax = 1.54 kcal mol −1) in CCl 4 to 82% (Δ G eqeq– axax = −0.88 kcal mol −1) in pyridine-d 6. These results indicate that the intr amolecular hydrogen bonds (IAHB) OH⋯N and NH⋯O stabilize the diaxial conformers of these compounds in a non-polar solvent. For cis-3-amino-1-methoxycyclohexane ( ACNE) and cis-3-N-methylamino-1-methoxy-cyclohexane ( MCNE) no changes were observed in equilibrium with the variation of solvent polarity. These results indicate for the first time that the IAHB NH⋯O is not strong enough to stabilize the diaxial conformer of these compounds and that the conformation equilibria of the cis isomers of compounds ACOL and MCOL are influenced only by the IAHB OH⋯N. Moreover, the presence of a secondary amino group (93% of diaxial conformer in CCl 4) leads to an IAHB OH⋯N stronger than in primary and tertiary amino-derivatives (53 and 54% of diaxial conformer, respectively) for 1,3-disubstituted cyclohexanes. Values obtained from the theoretical data through the B3LYP functional are in agreement with the experimental results and indicate that the IAHB strength that influences the conformational equilibrium of these compounds is the IAHB OH⋯N. Thus, the IAHB NH⋯O do not stabilize the diaxial conformer of the cis isomer of compounds ACNE and MCNE showing that the diequatorial conformer will always be more stable than the diaxial conformer, independent of concentration or solvent.

Electron propagator theory study of 2-aminoethanol conformers

Thirteen low-lying conformers of 2-aminoethanol (AE) has been studied by ab initio calculations at B3LYP, MP2, MP3 and MP4 (SDTQ) levels of theory with aug-cc-pVDZ basis set. The gauche′–Gauche–gauche′ (g′Gg′) conformer having the intramolecular hydrogen bond (HB) OH ⋯ N is the most stable one. Vertical ionization energies of the two outer valence molecular orbitals for each conformer predicted with the electron propagator theory are in good agreement with the experimental data in the literature and the temperature dependence of photoelectron spectra (PES) is interpreted when taken into account the Boltzmann populations of the conformers. Natural bond orbital (NBO) analyses exhibit a remarkable influence on the molecular electronic structures by the strong intramolecular HB OH ⋯ N and the relative weak intramolecular HB NH ⋯ O, leading to the significantly lowered energy levels of the occupied lone-pair molecular orbitals.

Intramolecular and Intermolecular Hydrogen Bond Formation by Some Ortho-Substituted Phenols: Some Surprising Results from an Experimental and Theoretical Investigation

The effects produced by addition of various concentrations of the strong hydrogen bond (HB) acceptor, dimethyl sulfoxide (DMSO), on the OH fundamental stretching region of the IR spectra of several o-methoxy, o-nitro, and o-carbonyl phenols in CCl(4) are reported. In most of these phenols the intramolecular HB is not broken by the DMSO. Instead, the DMSO acts as a HB acceptor to the intramolecular HB forming a bifurcated intra/intermolecular HB. For o-methoxyphenols the bifurcated HBs are observed as new IR bands at much lower wavenumbers (Deltanu(OH) approximately -300 cm(-1)) than the band due to their intramolecular HB. The formation of bifurcated HBs and the large frequency shift of their OH bands in o-methoxyphenols are well reproduced by theoretical modeling. In contrast to the o-methoxyphenols DMSO has little effect (other than causing some broadening) on the intramolecular HB OH bands of o-nitro and o-carbonyl phenols, with the single exception of 2,4-dinitrophenol. In this case, but not for 2,4-diformylphenol, the intramolecular HB OH band decreases as the DMSO concentration increases and a new absorption grows in at lower wavenumbers, indicating that DMSO can break this intra-HB and form an inter-HB, a result well reproduced by theory. Although DMSO has little effect on the O-H stretching band of 2-nitrophenol, theory indicates extensive formation (90%) of bifurcated HBs with OH stretching bands at slightly higher wavenumbers (Deltanu(OH) approximately +20 cm(-1)) than that for the intramolecular HB OH group and 10% of a "simple" intermolecular HB in which the intramolecular HB has been broken. Theory also indicates that, with DMSO, 2-formylphenol also forms a bifurcated HB (Deltanu(OH) approximately +150 cm(-1)), whereas 2,4-diformylphenol forms both intermolecular HBs (Deltanu(OH) approximately -130 cm(-1)) and bifurcated HBs (Deltanu(OH) approximately +165 cm(-1)). The IR spectrum of 2-methoxymethylphenol shows that although an intramolecular HB conformer is dominant there is a small percentage of a "free" OH, non-HB conformer (2.1% in CCl(4), 1.5% in cyclohexane). These results are quantitatively reproduced by theory. We conclude that theory can provide important insights into the formation and structure of inter, intra, and bifurcated HBs, and into their OH stretching frequencies, that are not always revealed by IR studies alone.

Hydrogen bonds in dimers of 3-hydroxy-2-methyl-4-pyrone. AIM analysis

According to quantum topological analysis performed at the MP2/6-311++G** level the intramolecular hydrogen bond OH⋯O=C in 3-hydroxy-2-methyl-4-pyrone (maltol) is weak and has ionic nature. This bond is broken upon formation of H-complex of maltol with phosgene and with increase the polarity of medium. The chain dimer of maltol is stabilized by weak intermolecular bonds OH⋯O=C and CH⋯O=C of ionic character, whereas the cyclic dimer by two medium in strength intermolecular hydrogen bonds OH⋯O=C.

Matrix-isolation FT-IR and theoretical investigation of the competitive intramolecular hydrogen bonding in 5-methyl-3-nitro-2-hydroxyacetophenone

The paper presents the conformational, vibrational and hydrogen bond characteristics of 5-methyl-3-nitro-2-hydroxyacetophenone studied with the combined matrix-isolation FT-IR spectroscopic and theoretical (DFT/B3LYP/6-31++G ∗∗) technique. Theoretical calculations predict three stable conformations of the studied compound. Only two of these conformations could be identified experimentally using the matrix-isolation FT-IR technique. The conformation with the intramolecular hydrogen bond OH …O N has been found to be more stable than the conformation with the OH …O C type of hydrogen bond by 7.28 kJ/mol. The complete assignment of the experimental spectra could be performed based on the theoretical calculations including the normal coordinate analysis and isotopic substitution.

Structural changes caused by the conversion of 2-hydroxybenzamide (salicylamide) into the oxyanion

The structure of the salicylamide molecule and its oxyanion have been studied by IR spectra and quantum chemical calculations within the Onsager self-consistent reaction field (SCRF) model, using a density functional theory (DFT) method at the B3LYP/6-31++G** level. Structures of all conformational isomers of salicylamide and of its anion have been located, as well as their total and relative energies have been determined. According to the calculations the planar configurations with intramolecular hydrogen bonds OH… O C (molecule) and NH…O − (oxyanion) in the molecule and anion, respectively have been found to be the most stable. The molecule → oxyanion conversion leads to geometry changes in the whole species but the strongest structural variations take place near the oxyanionic centre. In agreement between theory and experiment, the conversion causes a 40 cm −1 decrease in the frequency of the carbonyl stretching band ν C O , 1.8-fold increase in its integrated intensity, strong intensity increases (4.2-fold) of the aromatic skeletal bands, and other essential spectral changes. The natural bond orbital (NBO) charges changes indicate that the oxyanionic centre bears nearly the 60% of the whole net negative charge.

IR spectral and theoretical characterization of intramolecular hydrogen bonds closing five-membered rings

A series of molecules with a common structural feature of a five-membered quasi-ring closed by an intramolecular hydrogen bond OH···OC interaction has been studied. In the series the compounds differ by the size of the main-ring which varies from 4 to 7 atoms. The molecules were isolated in low-temperature matrices and investigated by IR spectroscopy. The strength of the OH···OC interaction, depending on the size of the main ring, was spectroscopically revealed by the position of the infrared bands due to the stretching and torsional vibrations of the OH group. All the intermediate cases from very weak hydrogen-bond-like interaction to medium strength hydrogen bonding were observed, indicating that the strict boundary between the cases “hydrogen bond” and “no hydrogen bond” does not exist. This experimental conclusion is supported by theoretical calculations, performed at HF/6-31++G**, MP2/6-31 + G* and DFT(B3LYP)/6-31++G** levels.

(η 6-Phenyl)Cr(CO) 3- and (η 6-Phenyl)Co 4(CO) 90substituted metallocenylcarbinols of the iron subgroup: synthesis, structure and hydrogen bonding

The synthesis of α-metallocenylcarbinols (M = Fe, Ru, Os) with bulky organometallic substituents R in the carbinol group (R 1 = (η 6-Phenyl)Cr(CO) 3, R 2 = (η 6-Phenyl)Co 4(CO) 9) is reported. The steric hindrances caused by these R are shown to prevent the usual association of carbinols via intermolecular hydrogen bonds of the OH⋯O type. In the crystal, chelate cycles with intramolecular hydrogen bonds of the OH⋯M type are formed. The results of an X-ray diffraction study are in good agreement with the IR spectoscopic data, providing evidence of an additional weak OH⋯OC interaction present in R 1-substituted carbinols. In solution, no molecules with free (i.e. not bonded via intramolecular hydrogen bonds) OH groups are found. The relative stability of such chelate cycles is discussed.

Conformational analysis of the series of 3(4)-halogen-substituted caran-4(3)-ols and 3-methylnorcaran-4(3)-OLS by their spectra

As a rule, the equilibrium of the skewed twist conformera is observed for the 3(4)-halogen-substituted caran-4(3)-ols and 3-methylnorcaran-4(3)-ols. This is indicated by the simultaneous presence of the bands of νOH of the free OH groups and the OH groups bound by intramolecular hydrogen bonds. The steric interactions of the 7-R groups with the cis substituents at C3 and C4 and the intramolecular hydrogen bonds OH... Hal and OH... cyclopropane ring play a role in the realization of one or the other form. In the case of uniform substitution, the influence of the halogens on δνOH changes in the series F<Cl<Br.

A comparative study on the dynamics of epimeric 1-hydroxymethylquinolizidines: I. Conformational analysis of monomers and spectroscopic data for solid state, liquid state, and dilute solutions

A systematic comparative study of the conformational dynamics of lupinine and epilupinine, two 1-hydroxymethyl quinolizidine epimers, as a function of phase transition and of their protonation is presented. Lupinine and epilupinine have a trans-quinolizidine configuration and they differ only in the position of the CH2OH group. The axial position of this group allows the formation of an intramolecular hydrogen bond OH … N in lupinine, in contrast to epilupinine in which this group is in the equatorial position. Intrinsic conformational equilibria for both epimers have been defined. Despite the significant difference in the population of the preferred rotamers of lupinine and epilupinine, both epimers adopt very similar crystal structures, but in all other states and forms, e.g., pure liquids, solutions in inert and polar solvents, and protonated salts, they behave in drastically different ways.

Investigation of the dynamics of hydrogen bonding in ortho-substituted phenols

The 1H NMR spectra of phenols with intramolecular hydrogen bonds OH…O in solutions containing proton acceptors have been studied in the temperature range (200 to 110)K. The kinetic parameters of the reversible process of intramolecular ⇌ intermolecular hydrogen bond were determined.