Specifics of photoinduced excited-state intramolecular proton transfer in o-tosylaminobenzoic and o-acetylaminobenzoic acids

In the ground state, o-tosylaminobenzoic and o-acetylaminobenzoic acids exist in the form of two rotamers with intramolecular hydrogen bonds N-H...O=C (cis) and N-H...O(OH)-C (trans). In nonpolar solvents, the formation of dimers with hydrogen bonding between carboxyl groups takes place. Efficient barrierless excited state intramolecular proton transfer (ESIPT) occurs along the N-H...O=C hydrogen bond upon excitation of o-tosylaminobenzoic acid. The efficiency of ESIPT in o-acetylaminobenzoic acid is lower because of the low acidity of the substituted amino group.     

A (salicylidene)aniline derived Schiff-base adduct of methyltrioxorhenium(VII)-Cis-and trans-coordination of the ligand

A (salicylidene)aniline derived Schiff-base adduct of methyltrioxorhenium(VII), displaying both a cis- and a trans-arrangement of the Schiff-base ligand to the Re bound methyl group has been synthesized and described. Despite displaying some significant differences in the solid state (e.g. Re-O (Schiff base) bond distance, Re-C bond distance and several bond angles), the two configurations are indistinguishable in solution, obviously interchanging quickly.     

Conformational analysis of new 14-membered ring diketal dilactam macrocycles: molecular mechanics, liquid and solid state NMR studies

A conformational study of new diversely substituted 14-membered diketal dilactam macrocycles was conducted by NMR spectroscopy in liquid and solid states, molecular mechanics calculations and, for one compound, a previous X-ray analysis. The results obtained by the different techniques show that the conformations depend closely on whether the molecules are chiral or achiral and on the stereochemistry of the ketal OMe groups. In achiral compounds, the most stable conformation of each compound has, in both the liquid and solid states, the two NH-CO links positioned perpendicular to the macrocycle plane, lending to the trans-7,7′-OMe macrocycles 6b and 7b a rectangular [3434]-type structure. In contrast, in chiral compounds, the most stable conformations are not the same in the liquid and solid phases. In the liquid state the conformations are set by the presence of one or two N₄-H?O_1′, N_4′-H?O₁ intramolecular hydrogen bonds that position the amide group parallel to the macrocycle plane, whereas in the solid state the amide moieties again adopt a perpendicular position which can be stabilized, when the 3-R substituent is not too bulky, by intermolecular N-H?OC bonds between parallel sheets, and exceptionally, in the cis-7,7′-OMe-3,3′-Ph compound 1c, by a π-π stacking effect between the phenyl groups.     

Trans-cis octahedral interconversion pathway in diorganotin compounds

Using structural data from bis(bidentate)diorganotin compounds in the Cambridge Structural Database a potential pathway for trans-cis interconversion is envisaged with nondissociative Sn-donor bonds and retaining metal coordination number 6. C-Sn-C bond angles in the range 180-145° correspond to skewed trapezoid bipyramidal geometry for 6- and 5-membered O,O′ chelates; geometries that resemble the transition state of the trans-cis pathway starts forming at about C-Sn-C 134°. cis-Diorganotins explored in this work have C-Sn-C bond angles in the range 102-110°; it is the statistically favored configuration for diphenyltins. The proposed trans-cis conversion pathway is deduced from a series of geometries associated with decreasing the C-Sn-C bond angle and shows 2 weakly (secondary) bound chelating atoms lengthening their bonds until near the transition state and later strengthening; they end up cis to each other and opposite to the organic groups. Conversely, the other 2 (primary) donors shorten their bonds until the transition state is reached and later lengthen; they end up trans to each other. The entire transformation from trans to cis configuration occurs with relative rotation of 3 bonds.     

The structural and theoretical study of 1H-3,5-di-phenyl-1,2,4-diazaphosphole in the solid state

The N-, P-containing five-membered heterocyclic compound 1H-3,5-di-phenyl-1,2,4-diazaphosphole (1) was prepared in good yield and has been structurally characterized. 1H-3,5-Di-phenyl-1,2,4-diazaphosphole (1), crystallizing in two unexpected cyclic dimers with N-H?N hydrogen bonds, presents in the solid state a dynamic proton disorder implying a dynamic equilibrium within both dimers. The conformations of the phenyl rings, the disorder of the NH protons, and the intermolecular hydrogen bond of several 1,2,4-diazaphospholes (1-5) in the solid state have been rationalized by DFT [B3LYP/6-311++G(d,p)] calculations.     

The molecular structure of trans- and cis-bicyclo-[4.3.0]nonane in the gas phase,studied by electron diffraction and molecular mechanics

The structure and conformations of trans- and of cis-bicyclo[4.3.0]nonane have been studied in the gas phase. Molecular mechanics calculations applying the force field of Ermer and Lifson were used to obtain geometrical constraints, vibrational amplitudes and perpendicular vibrational corrections. The vibrational parameters were corrected for the large amplitude motion of the five-membered ring. The refinement for the trans-isomer confirms completely the predictions of the force field calculations. Although a stable solution could not be obtained for the cis-compound there is no contradiction between experiment and model calculations. The cyclohexane ring in both isomers is found to have a distorted chair conformation. In the cis-isomer it is flattened along the junction and more twisted in the other part. For the trans-compound the reverse is true.The following structural parameters r_g, r_α-structure) are put forward, (a) trans-compound: C₂-symmetry, _r(C-C)_av = 1.536 Å. Average bond angle and average torsion angle in the cyclohexane ring are 110.2° and 58.1°, respectively. The connection angle, defined as the angle between the planes bisecting C6-C1-C5-C9 and C2-C1-C5-C4, is 180°. (b) cis-compound: no symmetry, r(C-C)_av = 1.536 Å. Average bond and torsion angles in the cyclohexane ring are 112.2° and 52.3°, respectively. The connection angle is 124.8°.A comparison is made with structures of related compounds.     

Solid state and solution conformation of 2-pyridinecarboxylic acid hydrazides: a new structural motif for foldamers

X-Ray crystallography and NMR show a strong preference for trans conformers of N′-phenyl or N′-(2-pyridyl) 2-pyridinecarboxylic acid hydrazides, stabilized by an NHN_pyr. intramolecular hydrogen bond both in the solid state and in solution. This allows us to extrapolate that oligomers of this unit should adopt extended linear conformations.     

Theoretical study of IR spectra of paraphenylenediamine

Normal coordinate analysis of the paraphenylenediamine (1,4-diaminobenzene, PPD) molecule has been carried out and complete interpretation of the vibrational spectrum is given for both trans and cis isomers. The reliable force field and electro-optical parameters of PPD have been determined by refinement in order to fit the experimental wavenumbers and intensities of PPD molecule. The initial force field parameters of PPD were refined from the corresponding parameters of aniline molecule. The initial values of bond dipole moments of the molecule were calculated by MINDO/3 method. The combination of the calculated IR spectra of trans and cis isomers of PPD is found to reproduce the experimental IR spectrum of solid PPD, satisfactorily, indicating that PPD exists as a mixture of both conformations.     

Infrared and raman spectra,and stereochemistries of 1:2 trimethylphosphine complexes of group V trihalides,MX3 · 2P(CH3)3 and MX3 · 2P(CD3)3 (where M = P or As and X = Cl or Br); spectra of 1,2-bisdimethylphosphinoethane and chelate complexes with group v trihalides

The 1 : 2 trimethylphosphine (deuterated and non-deuterated) adducts of the Group V trihalides MX₃ (where M = P or As and X = Cl or Br) are prepared and examined by vibrational spectroscopy. The forced cis-configuration complexes MX₃ · bdpe where bdpe is the bidentate ligand 1,2-bisdimethylphosphinoethane are also prepared and examined spectroscopically. Comparison of the Raman and IR spectra of these complexes shows the monodentate adducts to be covalent monomers having a trans-stereochemistry in the solid state. Normal coordinate calculations in C_2v symmetry (trans-stereochemistry) are performed for all monodentate adducts reported.     

Etude du mecanisme de photolyse des complexes cis- et trans-[PtCl2(C2H4)(4-CH3C5H4N)]. Influence de la concentration et de la longueur d'onde

Photodimerization, photoisomerization and photosubstitution quantum yields are measured for cis- and trans-[PtCl₂(C₂H₄)(4-CH₃C₅H₄N)], at various concentrations and wavelengths. Dissociation of the platinumethylene bond o?curs with a quantum yield nearly unity when the cis-compléx is irradiated in the charge transfer bands 5d → π^*(C₂H₄). Dissociation is also observed, but with a lower efficiency, at longer wavelengths. A cis → trans-photoisomerization reaction, probably via a low energy dd excited state is observed at 313,366 and 405 nm, with a constant quantum yield.     

The conformations of allylic alcohols in solution from FT-i.r. OH stretching vibration measurements

The distribution of conformations of allylic alcohols in CCl₄ differs from that in the vapour phase and from ab initio calculations. FT-i.r. measurements of the OH stretching vibrations show two peak maxima which can be resolved by band splitting techniques. The predominant conformations of allylic alcohols are intramolecularly hydrogen bonded, with a conformation gauche (G or G′) with respect to rotation about the CO bond and eclipsed (E or E′) with respect to rotation about the CC bond. In contrast with vapour phase data and ab initio calculations, no other hydrogen bonded conformations have been identified.For primary alcohols, the shoulder to the higher wavenumber side can be unequivocally assigned to conformations in which the OH is free, with a trans (T) conformation with respect to rotation about the CO bond.The secondary allylic alcohols exhibit no bands that can be attributed to free OH conformations except in the case of severe steric interaction in the eclipsed (E) conformation.In accordance with our previous work on solvent interactions with the OH group, the contrast with the vapour state assignments are interpreted in terms of an interaction of the solvent with the OH proton which destabilizes the hydrogen bond.     

Reversible assembly of enantiomeric helical polymers: from fibers to gels

A novel class of stereocomplexes is described by the interaction of helically complementary poly(phenylacetylene)s (PPAs) carrying an α-methoxy-α-trifluoromethylphenylacetamide pendant group. The formation of the stereocomplex requires the presence of cis amide bonds on the external crest of the polymer to provide efficient cooperative supramolecular hydrogen bonding between matching enantiomeric helical structures. The interlocking of the chains gives rise to supramolecular fiber-like aggregates that, at higher concentrations, result in gels. The modification of the cis–trans amide conformation at the pendant groups allows the controlled formation and cleavage of the stereocomplex due to a dramatic change between the intermolecular and intramolecular hydrogen bond interactions.     

Heterobimetallic [Ru(II)/Fe(II)] complexes: On the formation of trans- and cis-[RuCl2(dppf)(diimines)]

The synthesis and characterization of trans/cis-[RuCl₂(dppf)(diimines)], dppf = 1,1′-bis(diphenylphosphino)ferrocene; diimines = 2,2′-bipyridine (trans/cis-(1)), the new complexes with 4,4′-dimethyl-2,2′-bipyridine (trans/cis-(2)) and 1,10-phenanthroline (cis-(3)) are presented. The complexes were synthesized using two routes and the trans/cis-isomer formation is dependent upon conditions and the precursor applied. The trans-isomer (kinetic) readily isomerizes to the cis-isomer (thermodynamic) when exposed to light (fluorescent) and this process was followed by cyclic voltammetry and UV-vis. The electrochemical studies on these complexes reveal that Fe^(III)/Fe^(II) couples are insensitive to the isomer (trans/cis) formed, but the Ru^(III)/Ru^(II) couples are dependent on the isomer. Transfer-hydrogenation reactions for reduction of acetophenone were conducted using complexes cis-(1) and cis-(2) and the results are compared with that obtained for similar complexes. X-ray structure for cis-(3) are presented and discussed.     

Hydrogénolyse en phase liquide sur Pd/C des époxydes du carvomenthène et du limonène

Hydrogenolysis over Pd/C of cis and trans epoxides of carvomenthene and limonene give a mixture of hydrocarbons, secondary and tertiary alcohols, and ketones in proportions dependent upon the nature of the starting material. In the limonene epoxides, the extracyclic double bond plays an important role in the opening of the oxirane ring through a common unsaturated tertiary alcohol intermediate by double bond migration, hydrogenation of which leads to the products. For the carvomenthene epoxides the results are similar to those found in the 4-t-butyl series with competition between cis addition and trans addition of hydrogen. The presence of the isopropenyl group leads to slower reaction rates in comparison with t-butyl analogues.     

Degradation and cis-to-trans isomerization of poly[(2,4-difluorophenyl)acetylene]s of various initial molecular weight: SEC,NMR, DLS and EPR study

Molecular weight (MW) and configurational stabilities of a series of high-cis poly[(2,4-difluorophenyl)acetylene]s (PdFPhA, M_w from 2.2 × 10⁴ to 9.7 × 10⁵) exposed to the atmosphere and diffuse daylight are reported. Polymers dissolved in THF-d₈ undergo simultaneous cis-to-trans isomerization and degradation under formation of a bimodal MW distribution. Degradation and isomerization rates are tightly connected and decrease with decreasing MW of the parent PdFPhAs. This finding may be interesting for the functional applications of substituted polyacetylenes. The partly aged PdFPhAs contain two separable fractions: higher-MW high-cis macromolecules containing a low amount of unpaired electrons and lower-MW deeply isomerized cis/trans macromolecules enriched in the unpaired electrons content. The difference in MW of these fractions reflects the various degradation rates of isomerized and high-cis macromolecules. Only a slow degradation without changes in the configurational structure proceeds if PdFPhA is allowed to age in the solid state. The suppression of the cis-to-trans isomerization reflects the suppression of motion of the polymer chains segments in the solid state.     

Relative stability,potential barrier and intensity parameters of the trans and cis isomers of fluoroacetyl fluoride: An ab initio study

Ab initio calculations have been performed to determine molecular geometries, relative stabilities and the potential barrier of the fluoroacetyl fluoride molecule. The calculations indicate the existence of two stable conformations, trans and cis, in agreement with experimental studies. The potential barrier presents a maximum around 80° and an energy difference of ⋍ 17.5 kJ mol⁻¹ with respect to the most stable conformer (trans). These calculations also confirm that this conformer is more polar than the cis form, with the total dipole moment oriented along the carbonylic bond. The intensity parameters of the trans and cis isomers in terms of their atomic polar tensors have been analysed using the “charge-charge flux-overlap” model. The calculations predict similar atomic polar tensors for both forms, suggesting practically equivalent electronic structures in the ground state.     

Synthesis of macrocyclic polyethers via Ru complex-catalyzed metathesis cyclization and their use as the ring component of rotaxanes

Ring closing metathesis of the vinyl group-terminated oligoethers catalyzed by RuCl₂(CHPh)(PCy₃)₂ yielded macrocyclic polyethers containing vinylene group. The ¹H and ¹³C NMR spectra indicated the presence of CC double bond (trans/cis = ca. 80:20). The obtained 23-membered cyclic ether reacted with benzyl(anthrylmethl)ammonium hexafluorophosphate to produce the pseudo-rotaxane as colorless crystals. X-ray crystallography revealed N–H⋯O hydrogen bonds and stacking of the aromatic planes between the host and guest molecules, which stabilized the rotaxane structure in the solid state.     

Resonance Assisted Hydrogen Bonding Phenomenon Unveiled through Both Experiments and Theory: A New Family of Ethyl N-Salicylideneglycinate Dyes

Extensive experimental and theoretical investigations are reported on the nature of resonance-assisted hydrogen bonding phenomenon (RAHB) and its influence on photophysical properties of the newly designed dyes differing in donor–acceptor properties, namely ethyl N-salicylideneglycinate ( 1 ), ethyl N-(5-methoxysalicylidene)glycinate ( 2 ), ethyl N-(5-bromosalicylidene)glycinate ( 3 ) and ethyl N-(5-nitrosalicylidene)glycinate ( 4 ). All compounds are thermochromic in the solid state and they contain a typical intramolecular O−H⋅⋅⋅N hydrogen bond formed between the hydroxyl hydrogen atom and the imine nitrogen atom, yielding the enol form in the solid state. It is unveiled, that the magnitude of RAHB effect fine tunes the strength of the O−H⋅⋅⋅N bonding and accordingly the relative populations of the enol, cis-keto and trans-keto forms leading to variation of the photophysical properties of 1 – 4 . It is determined, that the electron-withdrawing NO₂ in 4 amplifies the most RAHB effect causing the breaking of the O−H⋅⋅⋅N hydrogen bond and accordingly formation of the dominant cis-keto isomer in both the solid state and EtOH. To this end, the UV/Vis spectra of 1 – 3 in EtOH revealed the exclusive presence of the enol form, while the prevalent contribution of the cis-keto form was found for 4 . Furthermore, only compound 4 is emissive in the solid state in ambient condition due to dual emission arising from the cis-keto* and trans-keto* forms, while 2 was found to be highly emissive in EtOH. It is revealed qualitatively and quantitatively, based on the ETS-NOCV charge and energy decomposition scheme and the EDDB population-based method, that RAHB is strongly a non-local phenomenon based on electrons pumping or sucking through both the π- and σ-channels, which accordingly exerts chemical bonding changes at both the phenyl ring and predominantly a distant O−H⋅⋅⋅N area.     

First separation and characterization of cis and trans 1,2-bisaryloxy perfluorocyclobutanes

4-Bromotrifluorovinyloxybenzene (4-Br-TFVE) undergoes cyclodimerization upon heating at 150 °C affording cis and trans isomers of 1,2-bis(4-bromophenoxy)hexafluorocylcobutane. Stereoisomers were separated by selective crystallization, confirmed by single crystal X-ray, and further characterized by NMR. Remarkable difference in the solid state structures include aryl to aryl dihedral angles of 12° for the trans isomer and 88° for the cis isomer. Polymers containing roughly equal amounts of cis and trans fused perfluorocyclobutyl (PFCB) rings should be expected to have low crystallinity due to the marked difference in the two unit cells as is observed.     

Conformational analysis of N-methyl-m-fluoroaniline by microwave spectroscopy

Conformational analysis of N-methyl-m-fluoroaniline has been performed by low resolution microwave spectroscopy. Two rotational isomers, corresponding in a near-planar configuration to the m-fluorine being either cis or trans with respect to the amino hydrogen, have been detected. The energy difference is found to be 270 ± 70 cal mol^?1, the cis isomer being the more stable. Ab initio calculations indicate a barrier height for the internal rotation of the HNCH₃ group around the C_ph—N bond of 9.04 kcal mol^?1.