Evidence for strong intramolecular hydrogen bonding in some quinoline derivatives

Several analogues of quinoline-8-carboxamide were examined by ¹H NMR and IR in order to establish structural requirements for strong intramolecular hydrogen bonding.     

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.     

Preorganization-enhanced halogen bonding via intramolecular hydrogen bonding: a theoretical study

Structural Chemistry - Cationic and neutral halogen bonding (XB) donors use two types (I and II) of intramolecular hydrogen bonding (HB) to preorganize structures and increase the efficiency of...     

An infrared spectroscopic study of hydrogen bonding in ethyl phenol: A model system for polymer phenolics

A detailed analysis of the bands appearing in the OH stretching region of the infrared spectrum of ethyl phenol solutions is presented. In cyclohexane solutions, the band due to “free” (non-hydrogen-bonded groups) contains overlapping contributions from both monomeric and end-group species. Other assignments are made on the basis of whether the proton and oxygen in a particular OH group are both involved in hydrogen bonds (as “donors” and “acceptors”, respectively), or if only the proton is acting as a donor. The strongest band in the spectra obtained at the highest concentration of ethyl phenol is due to OH groups present in linear chains of hydrogen-bonded OH groups (as recognized in numerous other studies), but a band due to cyclic trimers has also been identified. The assignment of other modes is more uncertain and various possibilities are discussed. In toluene solutions, assignments are more complicated, because bands due to OH–π hydrogen bonds are observed instead of free groups. Finally, the data from cyclohexane solutions was used to calculate equilibrium constants capable of describing the distribution of species present. A new methodology for determining the equilibrium constant describing association in the form of dimers is described.     

Vapor pressure and intramolecular hydrogen bonding in fluorotelomer alcohols

Vapor pressure and aqueous solubility are important parameters used to estimate the potential for transport of chemical substances in the atmosphere. For fluorotelomer alcohols (FTOHs), currently under scrutiny by environmental scientists as potential precursors of persistent perfluorocarboxylates (PFCAs), vapor pressure is the more significant property since these compounds are only very sparingly soluble in water. We have measured the vapor pressures of a homologous series of fluorotelomer alcohols, F(CF2CF2)nCH2CH2OH (n = 2-5), in the temperature range 21-250 degrees C by three independent methods: (a) a method suitable for very low vapor pressures at ambient temperatures (gas-saturation method), (b) an improved boiling point method at controlled pressures (Scott method), and (c) a novel method, requiring milligram quantities of substance, based on gas-phase NMR, a technique largely unfamiliar to chemists and holding promise for studies of relevance to environmental chemistry. The concordant values obtained indicate that recently published vapor pressure data overestimate the vapor pressure at ambient temperature, and therefore the volatility, of this series of fluorinated compounds. It was suggested that substantial intramolecular -O-H...F- hydrogen bonding between the hydroxylic proton and the two fluorines next to the ethanol moiety was responsible for their putative high volatility. Therefore, we have used gas-phase NMR, gas-phase FTIR, 2D NMR heteronuclear Overhauser effect measurements, and high-level ab initio computations to investigate the intramolecular hydrogen bonding in fluorotelomer alcohols. Our studies unequivocally show that hydrogen bonding of this type is not significant and cannot contribute to and cause unusual volatility. The substantially lower vapor pressure at ambient temperatures than previously reported resulting from our work is important in developing a valid understanding of the environmental transport behavior of this class of compounds.     

Inter- and intramolecular hydroacylation of alkenes employing a bifunctional catalyst system

Based on a conceptually innovative bifunctional P,N ligand, an efficient protocol for the rhodium-catalyzed inter- and intramolecular hydroacylation of alkenes has been developed.     

Conformations of protonated gas-phase bradykinin ions: evidence for intramolecular hydrogen bonding

The post-source decay of bradykinin, Lys1-bradykinin, des-Arg1-bradykinin, des-Arg9-bradykinin and [D-Phe7]-bradykinin [M + H]+ ions was examined in order to assertain the influence of secondary structure on peptide ion dissociation. Fragment ions corresponding to the elimination of H2O and HN=C=NH are observed in the product ion mass spectra of Lys1-bradykinin and des-Arg1-bradykinin but not in the spectra of bradykinin or des-Arg9-bradykinin. Cleavage reactions at the Phe-Ser and/or Ser-Pro bonds are observed for all peptide [M + H]+ ions with the exception of des-Arg9-bradykinin. The product ions arising from the processes described above are rationalized in terms of the intramolecular solvation of the protonated guanidino groups of the arginines. The strongest intramolecular interaction appears to be a proton bridge between the guanidino groups of the N- and C-terminal arginines in bradykinin. In addition, increased abundances of fragment ions in the vicinity of Ser-Pro may be attributed to intramolecular solvation of the protonated C-terminal guanidino group by the Ser-Pro portion of the molecule. This self-solvation of the ionizing proton leads to a gas-phase peptide conformation that is supported by solution-phase NMR studies at elevated temperatures and in non-polar solvents but which is different from the conformation in polar solvents.     

Vibrational overtone spectra of chloroanilines--evidence for intramolecular hydrogen bonding in o-chloroaniline

The near infrared vibrational overtone absorption spectra of liquid phase aniline and chloroanilines are reported. The analysis of the observed CH and NH local mode mechanical frequency values shows that intramolecular hydrogen bonding occurs between NH2 group and chlorine atom in o-chloroaniline. This observation supports the conclusion drawn from microwave spectroscopic studies reported earlier.     

Studies of intramolecular hydrogen bonding in protonated and non-protonated hexahydropyridinobenzodioxins

The conformational stability of hexahydropyridobenzodioxin and related derivatives in both protonated and non-protonated forms have been investigated by means of ab initio molecular orbital methods as well as semi-empirical AM1 and PM3 methods. One of the cis conformers (cis2e) has been found to be most stable due to the formation of an intramolecular hydrogen bond, other conformers including the trans isomer cannot form this interaction but are of different stability because of the orientation of the polar oxygens and the nitrogen. The effect of the intramolecular hydrogen bonding on the stability of hexahydropyridobenzodioxin and its methylated derivatives has been examined using various basis sets levels. In protonated form, both the semi-empirical and ab initio calculations give excellent agreement in energetic order; however, different orderings of conformer stabilities are observed by different computational methods in non-protonated form. The results provide insight into the intramolecular hydrogen bonding in computational studies of biologically important molecules.     

Spectroscopic and quantum chemical study of cyclopropylmethylphosphine, a candidate for intramolecular hydrogen bonding

The properties of the novel compound cyclopropylmethylphosphine (C3H5CH2PH2) have been investigated by means of Stark-modulation microwave spectroscopy and high-level quantum chemical calculations. Spectra attributable to the three conformers of the molecule with a synclinal arrangement of the H-C-C-P atoms were recorded and assigned. The experimental rotational constants obtained for these conformers were found to be in good agreement with those generated by ab initio geometry optimizations at the MP2/aug-cc-pVTZ level of theory. An estimate of the relative energies of the three conformers with observable spectra, by means of relative intensity measurements, compared favorably with the results of G3 energy calculations performed for the molecule. In addition to the observation of ground-state rotational spectra for three conformers, spectra belonging to a number of vibrationally excited states were also assigned with the aid of radio frequency microwave double-resonance experiments. A tentative assignment of these excited-state spectra was proposed by appealing to the results of density functional theory vibrational frequency calculations performed at the B3LYP/6-311++(3df,2pd) level. The energetically preferred conformer of the molecule allowed a close approach between a hydrogen atom belonging to the phosphino group and the edge of the cyclopropyl ring. The possibility of the formation of an intramolecular hydrogen bond to electron density associated with so-called banana bonds is discussed.     

Dinuclear complexes of a pseudocalixarene macrocycle: structural consequences of intramolecular hydrogen bonding

A series of dinuclear complexes of a pseudocalixarene macrocycle H6L containing two 2,2'-methylenediphenol groups have been synthesised and structurally characterised. Using divalent metal ions, complexes containing a common hyperbolic paraboloid (saddle) M2(H4L)2+ core are formed. The structure is controlled by two strong O-H-O interactions resulting from metal ion-promoted monodeprotonation of the methylenediphenol units. The metal ions are located in a cleft within which neutral or anionic guests can bind. Use of trivalent metal ions leads to complete deprotonation of the phenol groups and loss of the saddle conformation.     

Evidence of intramolecular hydrogen bonding in a complex N,N-disubstituted formamide

Intramolecular hydrogen bonding has been determined to be the controlling factor in the cis—trans isomer ratio in N-(2-hydroxy-5-nitrobenzyl)-N-(1-methylthio-2-propenyl)formamide. Both ¹H NMR and i.r. show the presence of intramolecular hydrogen bonding in this compound.     

Tunable solvatochromic response of newly synthesized antioxidative naphthalimide derivatives: intramolecular charge transfer associated with hydrogen bonding effect

The solvatochromic behavior of two newly synthesized naphthalimide derivatives (I and II) which have potential antioxidative activities in anticarcinogenic drug development treatment, has been monitored in protic and aprotic solvents of different polarity applying steady-state and time-resolved fluorescence techniques. The compounds exhibit unique photophysical response in different solvent environments. The spectral trends do not appear to originate only from changes in the solvent polarity but also indicate that hydrogen bonding interactions and intramolecular charge transfer (ICT) influence the energy of electronic excitation of the compounds. Incorporation of an amino group at C(4) position of the naphthalimide ring in II makes it behave differently from I in terms of spectral characterization and fluorescence efficacy of the systems. The nonradiative relaxation process of the compounds is governed by medium polarity. The ground state geometry, lowest energy transition, and the UV-vis absorption energy of the compounds were studied using density functional theory (DFT) and time-dependent density functional theory (TDDFT) at the B3LYP/6-31G* level, which showed that the calculated outcomes were in good agreement with experimental data.