Matrix-isolation FT-IR and DFT theoretical studies of the intramolecular hydrogen bonding in Mannich bases

FT-IR Ar-matrix isolated spectra were studied for dichloro- (Cl₂-MB) and tetrachloroderivatives (Cl₄-MB) of the ortho Mannich base. The spectra were analyzed based on the DFT calculated frequencies and intensities and compared with those recorded in CCl₄ solution in the region of the ν(OH) and ν(OD) vibrations. The matrix-isolated spectra are characterized by narrower ν(OH) and ν(OD) bands with much better resolved fine structure than in solution. The fine structure originates from the anharmonic coupling with the low frequency modes as well as from Fermi resonance. The ν(OD) band shapes can be reproduced exclusively by assuming the Fermi resonance with overtones and summation of the frequencies of modes into which the bridge atoms are involved. The frequency isotopic ratio (ISR) is for both compounds 1.33 while the half-width ratios are equal to 1.82 and 1.94, for Cl₂-MB and Cl₄-MB, respectively.     

LZnX complexes of tripodal ligands with intramolecular RN-H hydrogen bonding groups: structural implications of a hydrogen bonding cavity, and of X/R in the hydrogen bonding geometry/strength

Tripodal ligands N(CH2Py)3-n(CH2Py-6-NHR)n(R=H, n=1-3 L1-3, n=0 tpa; R=CH2tBu, n=1-3 L'1-3) are used to investigate the effect of different hydrogen bonding microenvironments on structural features of their LZnX complexes (X=Cl-, NO3-, OH-). The X-ray structures of [(L2)Zn(Cl)](BPh4)2.0.5(H2O.CH3CN), [(L3)Zn(Cl)](BPh4)3.CH3CN, [(L'1)Zn(Cl)](BPh4) 1', [(L'2)Zn(Cl)](BPh4)2'.CH3OH, and [(L'3)Zn(Cl)](BPh4)3' have been determined and exhibit trigonal bipyramidal geometries with intramolecular (internal) N-HCl-Zn hydrogen bonds. The structure of [(L'2)Zn(ONO2)]NO3 4'.H2O with two internal N-HO-Zn hydrogen bonds has also been determined. The axial Zn-Cl distance lengthens from 2.275 A in [(tpa)Zn(Cl)](BPh4) to 2.280-2.347 A in 1-3, 1'-3'. Notably, the average Zn-N(py) distance is also progressively lengthened from 2.069 A in [(tpa)Zn(Cl)](BPh4) to 2.159 and 2.182 A in the triply hydrogen bonding cavity of 3 and 3', respectively. Lengthening of the Zn-Cl and Zn-N(py) bonds is accompanied by a progressive shortening of the trans Zn-N bond from 2.271 A in [(tpa)Zn(Cl)](BPh4) to 2.115 A in 3 (2.113 A in 3'). As a result of the triply hydrogen bonding microenvironment the Zn-Cl and Zn-N(py) distances of 3 are at the upper end of the range observed for axial Zn-Cl bonds, whereas the axial Zn-N distance is one of shortest among N4 ligands that induce a trigonal bipyramidal geometry. Despite the rigidity of these tripodal ligands, the geometry of the intramolecular RN-HX-Zn hydrogen bonds (X=Cl-, OH-, NO3-) is strongly dependent on the nature of X, however, on average, similar for R=H, CH2tBu.     

Spectroscopic and computational studies of the intramolecular hydrogen bonding of 2-indanol

2-Indanol in its most stable form is stabilized by internal hydrogen bonding, which exists between the hydroxyl hydrogen atom and the pi-cloud of the benzene ring. A comprehensive ab initio calculation using the MP2/cc-pVTZ level of theory showed that 2-indanol can exist in four possible conformations, which can interchange through the ring-puckering vibration and the internal rotation of the OH group on the five-membered ring. A potential energy surface in terms of these two vibrational coordinates was calculated. Density functional theory calculations were used to predict the vibrational frequencies and to help in normal mode assignments. Fluorescence excitation spectra of 2-indanol confirm the presence of the four conformers in the electronic ground and excited states. The spectral intensities indicate that, at 90 degrees C, 82% of the molecules exist in its most stable form with the intramolecular hydrogen bonding. The other isomers are present at approximately 11, 5, and 3%. The MP2/6-311++G(d,p) calculation predicts a distribution of 70, 13, 9, and 8% at 90 degrees C, the experimental sample temperature.     

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     

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     

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.     

Antioxidant activity of o-bisphenols: the role of intramolecular hydrogen bonding

A kinetic and thermodynamic investigation on the antioxidant activity of 2,2'-methylenebis(6-tert-butyl-4-methylphenol) (2), 2,2'-ethylidenebis(4,6-di-tert-butylphenol) (3), and 4,4'-methylenebis(2,6-di-tert-butylphenol) (4) are reported. EPR studies of the equilibration between 3 or 4 and a reference phenol, and the corresponding phenoxyl radicals, allowed us to determine the O-H bond dissociation enthalpy (BDE) of the O-H bond as 81.2 and 81.1 kcal/mol in 3 and 4, respectively. Despite this similarity, the absolute rate constants for the reaction with peroxyl radicals, determined by autoxidation studies under controlled conditions, indicate that the o-bisphenols 2 and 3 behave as excellent antioxidants while the p-bisphenol 4 is less effective by a factor of 64 and 22, respectively. FT-IR spectroscopy and product studies suggest that the very good antioxidant activity of the o-bisphenols largely arises from both the reduced steric crowding about the hydroxyl group and the stabilization of the aroxyl radical due to the formation of an intramolecular hydrogen bond between the residual OH and the oxygen radical center.     

Effect of N-pyridyl substitution and hydrogen bonding on the deactivation of singlet excited 1,2-naphthalimide

Photophysical properties of 1,2-naphthalimide (1) and N-(4-pyridyl)-1,2-naphthalimide (2) as well as the effect of their hydrogen bonding with phenols have been studied in toluene. Fluorescence emission is the dominant energy dissipation pathway of the singlet excited 1. Introduction of the 4-pyridyl substituent into the imide moiety significantly accelerates the internal conversion due to the efficient vibronic coupling between close-lying S1 and S2 excited states, however, the rate of triplet formation exhibits negligible change. In contrast with the behavior of the corresponding substituted phenyl derivatives, 2 does not emit dual fluorescence because of the less extensive conjugation within the molecule. Fluorescence quenching with phenols takes place both in dynamic and static processes. Electron transfer is slower in the hydrogen bonded complex where phenols are linked to the pyridyl moiety due to the larger distance between the electron donor and acceptor components.     

Effect of hydrogen bonding on the intramolecular charge transfer fluorescence of 6-dodecanoyl-2-dimethylaminonaphtalene

The effect of hydrogen bonding on the intramolecular charge transfer (ICT) of 6-dodecanoyl-2-dimethylaminonaphtalene (laurdan) in neat and binary solvent mixtures has been investigated by using steady-state and time-resolved spectroscopic techniques. The different features of ICT emission of laurdan in methylcyclohexane–tetrahydrofuran and methylcyclohexane–ethanol are explained by the absence and presence of hydrogen bonded ICT. The presence of isosbestic point in absorption spectra of laurdan in methylcyclohexane–ethanol confirms the formation of 1:1 complex between laurdan and ethanol. The obtained data were used to determine the stoichiometric equilibrium constants. In protic rigid (77 K) the fluorescence spectra of laurdan show excitation wavelength dependence (the red-edge effect). Moreover, we reported the decay characteristics of laurdan molecule in locally excited (LE) and ICT state in methylcyclohexane–ethanol.     

Conformational landscape of platinum(II)-tetraamine complexes: DFT and NBO studies

The potential energy surfaces of chiral tetraamine Pt(II) coordination complexes were computed at the B3LYP/LANL2DZ level of theory by a systematic variation of two dihedral angles: C12–C15–C34–C37 (θ) and C24–C17–C31–C48 (ψ) employing a grid resolution of 30°. Potential energy surfaces calculated using density functional theory methods and Boltzmann-derived populations revealed strong preference for one diasteromer of each series studied. In addition, natural bond orbital analysis show that the minima are stabilized predominantly by a combination of electronic interactions between two phenyl groups, the phenyl groups and the Pt²⁺ ion, as well as with the amine groups. Additional experimental characterization of the diasteroisomers studied here is in progress and will permit further molecular modeling studies with the appropriate stereochemistry.     

Comparative study on the nonadditivity of methyl group in lithium bonding and hydrogen bonding

Quantum chemical calculations at the second‐order Moeller–Plesset (MP2) level with 6‐311++G(d,p) basis set have been performed on the lithium‐bonded and hydrogen‐bonded systems. The interaction energy, binding distance, bond length, and stretch frequency in these systems have been analyzed to study the nonadditivity of methyl group in the lithium bonding and hydrogen bonding. In the complexes involving with NH₃, the introduction of one methyl group into NH₃ molecule results in an increase of the strength of lithium bonding and hydrogen bonding. The insertion of two methyl groups into NH₃ molecule also leads to an increase of the hydrogen bonding strength but a decrease of the lithium bonding strength relative to that of the first methyl group. The addition of three methyl groups into NH₃ molecule causes the strongest hydrogen bonding and the weakest lithium bonding. Although the presence of methyl group has a different influence on the lithium bonding and hydrogen bonding, a negative nonadditivity of methyl group is found in both interactions. The effect of methyl group on the lithium bonding and hydrogen bonding has also been investigated with the natural bond orbital and atoms in molecule analyses. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Ab initio HF-SCF study of naphthazarin: Geometries,isomerism, hydrogen bonding,and vibrational spectrum

The structural properties and intramolecular hydrogen bonding of a series of structures of naphthazarin molecule were investigated by ab initio HF-SCF methods. The geometries of theC _2v ,C _2h ,D _2h , andC _s symmetry structures were optimized using split-valence basis sets. MP2/6-31G^*// HF/6-31G single-point energy calculations indicate that theC _2v isomer (5,8-dihydroxy-1,4-naphthoquinone) is the lowest energy structure of the molecule and that theC _2h symmetry one (4,8-dihydroxy-1,5-naphthoquinone), lying 37 kJ/mol above theC _2v form, is the other stable isomer of naphthazarin. At the HF/6-31G level, the intramolecular proton exchange between two equivalentC _2v structures is a two-step process where each proton can be independently transferred through an unsymmetrical potential having a 1,5-quinone intermediate, theC _2h symmetry structure, and two equivalent transition states ofC _s symmetry, with a barrier height equal to 38 kJ/ mol (MP2/6-31G^*//HF/6-31G). The study of naphthazarin molecule is flanked by a theoretical investigation on theC _2v andC _2h isomers of the parent naphthoquinone and dihydroxynaphthalene molecules. The SCF vibrational spectrum of the ground state of naphthazarin, harmonic frequencies, and infrared and Raman band intensities were computed at the HF/6-31G level. The results of the calculations are compared with the matrix isolation FT-IR spectroscopy measurements and with the infrared and Raman spectra of the crystal molecule.     

LCAO–MO studies on hydrogen bonding: The interaction between carbonyl and hydroxyl groups

Potential curves that show the energy dependence of hydrogen bonds between carbonyl and hydroxyl groups on the O? H bond length, on the distance between the molecules, and on the angle between the functional groups have been calculated with the CNDO /2 method. The results are presented for a small model system–formaldehyde/water—and for the dimer of formic acid. Good agreement is obtained with the available experimental data. The influence of the molecular geometry on the calculated results is discussed.