Study on the uv light‐mediated fragmentation of 7‐(2,4,6‐trialkylphenyl‐)7‐phosphanorbornene 7‐oxides in alcohols: New evidences on the mechanism

Preparative‐scale photolyses of the title triisopropylphenyl‐substituted compounds 1 and 4 at 254 nm, in the presence of alcohols, led to aryl‐H‐phosphinates 3 in good yields. Reaction of the tri‐tert‐butylphenyl derivative 5 was, however, accompanied by side reactions. The results of two series of competitive reactions are consistent with a mechanism involving a five‐coordinate adduct 8 of the alcohol on the P=O group of the phosphanorbornene oxides. © 2001 John Wiley & Sons, Inc. Heteroatom Chem 12:6–9, 2001     

A theoretical study on the excited‐state intramolecular proton transfer mechanism of 4′‐dimethylaminoflavonol chemosensor

In this work, density functional theory (DFT) and time‐dependent density functional theory (TDDFT) methods are used to explore the excited‐state intramolecular proton transfer (ESIPT) mechanism of a novel system 4′‐dimethylaminoflavonol (DAF). By analyzing the molecular electrostatic potential (MEP) surface, we verify that the intramolecular hydrogen bond in DAF exists in both the S₀ and S₁ states. We calculate the absorption and emission spectra of DAF in two solvents, which reproduce the experimental results. By comparing the bond lengths, bond angles, and relative infrared (IR) vibrational spectra involved in the hydrogen bonding of DAF, we confirm the hydrogen‐bond strengthening in the S₁ state. For further exploring the photoexcitation, we use frontier molecular orbitals to analyze the charge redistribution properties, which indicate that the charge transfer in the hydrogen‐bond moiety may be facilitating the ESIPT process. The constructed potential energy curves in acetonitrile and methylcyclohexane solvents with shortened hydrogen bond distances demonstrate that proton transfer is more likely to occur in the S₁ state due to the lower potential barrier. Comparing the results in the two solvents, we find that aprotic polar and nonpolar solvents seem to play similar roles. This work not only clarifies the excited‐state behaviors of the DAF system but also successfully explains its spectral characteristics.     

A structural and theoretical study of the intermolecular interactions in 8‐hydroxyquinolinium‐7‐carboxylate monohydrate

In the title compound, C₁₀H₇NO₃·H₂O, the zwitterionic organic molecules and the water molecules are connected by N—H...O and O—H...O hydrogen bonds to form ribbons, and π–π stacking interactions expand these ribbons into a three‐dimensional net. The energies of these hydrogen bonds adopt values typical for mildly weak interactions (3.33–7.75 kcal mol⁻¹; 1 kcal mol⁻¹ = 4.184 kJ mol⁻¹). The total π–π stacking interactions between aromatic molecules can be classified as mildly strong (energies of 15.3 and 33.9 kcal mol⁻¹), and they are made up of multiple constituent π–π interactions between six‐membered rings. The short intermolecular C—H...O contact between two zwitterionic molecules is nonbonding in character.     

A TD‐DFT study on the cyanide‐chemosensing mechanism of 8‐formyl‐7‐hydroxycoumarin

Proton transfer (PT) and excited‐state PT process are proposed to account for the fluorescent sensing mechanism of a cyanide chemosensor, 8‐formyl‐7‐hydroxycoumarin. The time‐dependent density functional theory method has been applied to investigate the ground and the first singlet excited electronic states of this chemosensor as well as its nucleophilic addition product with cyanide, with a view to monitoring their geometries and spectrophotometrical properties. The present theoretical study indicates that phenol proton of the chemosensor transfers to the formyl group along the intramolecular hydrogen bond in the first singlet excited state. Correspondingly, the nucleophilic addition product undergoes a PT process in the ground state, and shows a similar structure in the first singlet excited state. This could explain the observed strong fluorescence upon the addition of the cyanide anion in the relevant fluorescent sensing mechanism. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2011     

A study on the ability of 2,5‐dihydro‐ and 2,3,4,5‐tetrahydro‐1h‐phosphole oxides,as well as 7‐phosphanorbornene 7‐oxide derivatives to undergo uv light‐mediated fragmentation‐related phosphinylation of methanol

Reactivity of the title P‐heterocycles ( 1‐14 ) in the photoinduced fragmentation‐related phosphinylation of methanol was found to be influenced by the extent of ring strain and the UV absorption at 254 nm. The 7‐phosphanorbornene oxides ( 7‐14 ) are universal precursors due to their ring strain, no matter if they are UV‐active or not at 254 nm. The easily available 2,5‐dihydro‐1H‐phosphole oxides can be applied only in case of 1‐phenyl substitution that enhances the absorption at 254 nm. The ring strain of representative P‐heterocycles ( 5‐8 ) was evaluated by HF/6‐31G* and B3LYP/6‐31+G* calculations. UV spectra of compounds 5‐8 were interpreted by ZINDO/S and MNDO‐d calculations. The new precursors ( 11‐14 ) made possible the extension of the phosphinylations.     

A high‐accuracy theoretical study of the CHnP Systems n = 1–3

We have performed high‐level electronic structure computations on the most important species of the CH_nP systems n = 1–3 to characterize them and provide reliable information about the equilibrium and vibrationally averaged molecular structures, rotational constants, vibrational frequencies (harmonic and anharmonic), formation enthalpies, and vertical excitation energies. Those chemical systems are intermediates for several important reactions and also prototypical phosphorus‐carbon compounds; however, they are often elusive to experimental detection. The present results significantly complement their knowledge and can be used as an assessment of the experimental information when available. The explicitly correlated coupled‐cluster RCCSD(T)‐F12 method has been used for geometry optimizations and vibrational frequency calculations. Vibrational configuration interaction theory has been used to account for anharmonicity effects. Basis‐set limit extrapolations have been carried out to determine accurate thermochemical quantities. Electronic excited states have been calculated with coupled‐cluster approaches and also by means of the multireference configuration interaction method. © 2013 Wiley Periodicals, Inc.     

A theoretical study on the conformation of 3‐phosphinoxido‐ and 3‐phosphono‐1,2,3,4,5,6‐hexahydrophosphinine 1‐oxides

The title compounds ( 2 and 4 ) obtained by the diastereoselective hydrogenation of the corresponding 1,2,3,6‐tetrahydrophosphinine oxides ( 1 and 3 ) were subjected to a detailed quantum chemical study. The possible chair conformers were calculated at the HF/6‐31G* level of theory, according to which, the 1‐phenyl‐3‐P(O)Y₂‐substituted products ( 2 ) exist in the trans₁ form, in which all substituents are equatorial. At the same time, the 1‐ethoxy‐3‐dialkylphosphono compounds ( 4 ) adopt the cis conformations, in which the 1‐ethoxy group is axial and the 3‐P(O)(OR)₂ moiety is equatorial. The major diastereomer ( 4–1 ) is cis₃, in which the 5‐methyl group is axial, while the minor one is cis₁ with an equatorial methyl substituent. It is noteworthy that the rotational position of the exocyclic P(O)Z₂ function affected the energy content of the chair conformer to a high extent. The possibility of the involvement of the twist conformers was also considered. © 2005 Wiley Periodicals, Inc. Heteroatom Chem 16:520–524, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20148     

1,3‐Dipolar cycloadditions of Stone–Wales defective single‐walled carbon nanotubes: A theoretical study

The presence of Stone‐Wales defects in single‐walled carbon nanotubes (SWNTs) not only leads to new interesting properties, but also provides opportunities for tailoring physical and chemical properties, and expands their novel potential applications. With a two‐layered ONIOM method, 1,3‐dipolar cycloadditions (1,3‐DCs) of a series of 1,3‐dipoles (azomethine ylide, nitrone, nitrile imine, nitrile ylide, nitrile oxide, and methyl azide) with Stone‐Wales defective SWNTs have been investigated theoretically for the first time. The calculated results demonstrate that the bond c , rather than the previously focused central bond a , exhibits the highest chemical reactivity among the defective sites. More interestingly, bond c is even more reactive thermodynamically and kinetically than the perfect C? C bond in SWNTs, suggesting the feasibility of utilizing 1,3‐DC reactions to separate and purify perfect and defective SWNTs. The reactivity order for nonequivalent bonds in defective sites is different from that of [1+2] cycloaddition, indicating that the reactivity order for nonequivalent bonds depends on the kind of the chemical reactions. Except azomethine ylide, nitrile ylide and nitrile imine are found to be good candidates for 1,3‐DCs upon Stone‐Wales defective SWNTs. The SW‐ A and SW‐ B defective SWNTs show different chemical reactivity toward nitrile ylide, making it possible to purify and separate the SW‐ A and SW‐ B defective SWNTs. The SWNT diameters are found to moderately influence the 1,3‐DC reactivity of both perfect and Stone‐Wales defective SWNTs, implying that Stone‐Wales defective SWNTs with different diameter would be separated experimentally through 1,3‐DC chemistry. The above 1,3‐DC reactivity can be well understood in terms of the distortion/interaction theory, which means that instead of frontier molecular orbitals interaction energy, the distortion energy controls the chemical reactivity. © 2013 Wiley Periodicals, Inc.     

Experimental and theoretical study of the intramolecular C–H···N and C–H···S hydrogen bonding effects in the 1H and 13C NMR spectra of the 2‐(alkylsulfanyl)‐5‐amino‐1‐vinylpyrroles: a particular state of amine nitrogen

In the ¹H NMR spectra of the 1‐vinylpyrroles with amino‐ and alkylsulfanyl groups in 5 and 2 positions, an extraordinarily large difference between resonance positions of the H_A and H_B terminal methylene protons of the vinyl group is discovered. Also, the one‐bond ¹J(C_β,H_B) coupling constant is surprisingly greater than the ¹J(C_β,H_A) coupling constant in pyrroles under investigation, while in all known cases, there was a reverse relationship between these coupling constants. These spectral anomalies are substantiated by quantum chemical calculations. The calculations show that the amine nitrogen lone pair is removed from the conjugation with the π‐system of the pyrrole ring so that it is directed toward the H_B hydrogen. These factors are favorable to the emergence of the intramolecular C–H_B???N hydrogen bonding in the s‐cis(N) conformation. On the other hand, the spatial proximity of the sulfur to the H_B hydrogen provides an opportunity of the intramolecular C–H_B???S hydrogen bonding in the s‐cis(S) conformation. Presence of the hydrogen bond critical points as well as ring critical point for corresponding chelate ring revealed by a quantum theory of atoms in molecules (QTAIM) approach confirms the existence of the weak intramolecular C–H???N and C–H???S hydrogen bonding. Therefore, an unusual high‐frequency shift of the H_B signal and the increase in the ¹J(C_β,H_B) coupling constant can be explained by the effects of hydrogen bonding. Copyright © 2013 John Wiley & Sons, Ltd.     

A theoretical study on three conformational structures of 2,6‐distyrylpyridine

Ab initio methods at the levels HF/cc‐pVDZ, HF/6‐31G(d,p), MP2/cc‐pVDZ, and MP2/6‐31G(d,p), as well as methods based on density functional theory (DFT) employing the hybrid functional B3LYP with the basis sets cc‐pVDZ and 6‐31G(d,p), have been applied to study the conformers of 2,6‐distyrylpyridine. Bond distances, bond angles, and dihedral angles have been calculated at the B3LYP level. The calculated values were in good agreement with those measured by X‐ray diffraction analysis of 2,6‐distyrylpyridine. The values calculated using the Hartree‐Fock method and second‐order perturbation theory (MP2) were inconsistent. The optimized lowest‐energy geometries were calculated from the reported X‐ray structural data by the B3LYP/cc‐pVDZ method. Three conformations, A, B, and C, were proposed for 2,6‐distyrylpyridine. Calculations at the three levels of theory indicated that conformation A was the most stable structure, with conformations C and B being higher in energy by 1.10 and 2.57 kcal/mol, respectively, using the same method and basis function. The same trend in the relative energies of the three possible conformations was observed at the two levels of theory and with the different basis sets employed. The reported X‐ray data were utilized to optimize total molecular energy of conformation A at the different calculation levels. The bond lengths, bond angles, and dihedral angles were then obtained from the optimized geometries by ab initio methods and by applying DFT using the two basis functions cc‐pVDZ and 6‐31G(d,p). The values were analyzed and compared. The calculated total energies, the relative energies of the molecular orbitals, the gap between them, and the dipole moment for each conformational structure proposed for 2,6‐distyrylpyridine are also reported. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

A theoretical study for the regioselective Diels–Alder reaction of 5,6‐fulleroid with strained anti‐Bredt olefins

Regioselectivity of Diels–Alder reaction of the anti‐Bredt olefin of fulleroid, the 5,6‐methylene‐bridged fullerene derivative, is theoretically clarified by DFT calculations. In transition state calculations, the addition of a noncyclic diene 2,3‐dimethyl‐1,3‐butadiene (DMBD) at the anti‐Bredt olefin has considerably lower activation energy than those at the other sixteen olefins, while the reaction of a rigid cyclic diene cyclohexadiene (CHD) has similar activation energy at both anti‐Bredt and another nontwisted olefin. The changes in strain and interaction of the reactants are estimated along its reaction coordinate by activation strain model. Noncyclic diene shows asynchronous C? C bond formation with a significant gain of interaction energy, while the rigid cyclic diene shows a large growth of strain energy canceling out the interaction energy between diene and fulleroid.     

Functionalization of diamond (001)‐2 × 1 surface by cycloaddition of 1,3‐cyclohexadiene: A theoretical study

Density functional theory calculations have been used to study [4 + 2] and [2 + 2] cycloaddition reaction between 1,3‐cyclohexadiene and diamond (001)‐2 × 1 surface. The calculations revealed four possible reaction pathways for 1,3‐cyclohexadiene with the surface dimmers of diamond. Full geometry‐optimized structures were obtained for all products, including intradimer and interdimer reaction products. These results were analyzed both in terms of the total energy values and the detailed optimized geometries. We found that the intradimer [4 + 2] product is energetically favored over the other products, and the barrier to intradimer [4 + 2] addition is lower than the other additions, so the intradimer [4 + 2] product is expected to be the dominant product on the surface. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

A theoretical study on electronic structure and structure–activity properties of novel drug precursor 6‐acylbenzothiazolon derivatives

In this work, electronic properties and structure–activity relationship (SAR) parameters of 20 novel drug precursor 6‐acylbenzothiazolon derivatives with analgesic activity have been investigated theoretically by performing Austin Model‐1 (AM1) and DFT‐B3LYP/6‐31G (d) calculations with the aim to correlate the properties of each substance—particularly electronic properties and SAR parameters—with the biological interactions that are linked to their pharmacological effects. Their molecular properties were related to the biological activity of these drug precursor molecules. The relationship between octanol–water partition coefficient (log P) and each of the SAR parameters [E_LUMO–HOMO, molecular volume (V_m), ionization potential (IP), electron affinity, electronegativity (χ), chemical hardness (η), chemical softness (S), electrophilic index (ω), and molar refractivity] present linear correlation except for IP and χ. This result suggests that there are future prospects for designing or developing new drugs based on the correlation between the theoretically calculated parameters. According to AM1 calculation, the values of heat of formation of 6‐acylbenzothiazolon derivatives are negative (exothermic), which shows that these molecules are thermodynamically stable. E_LUMO–HOMO energy levels of the studied molecules are 4–5 eV, which also indicate that they are kinetically unstable. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

A halide‐free pyridinium‐substituted η3‐cycloheptatrienide–Pd complex

We report the synthesis and characterization of a novel 4‐(dimethylamino)pyridinium‐substituted η³‐cycloheptatrienide–Pd complex which is free of halide ligands. Diacetonitrile{η³‐[4‐(dimethylamino)pyridinium‐1‐yl]cycloheptatrienido}palladium(II) bis(tetrafluoroborate), [Pd(C₂H₃N)₂(C₁₄H₁₆N₂)](BF₄)₂, was prepared by the exchange of two bromide ligands for noncoordinating anions, which results in the empty coordination sites being occupied by acetonitrile ligands. As described previously, exchange of only one bromide leads to a dimeric complex, di‐μ‐bromido‐bis({η³‐[4‐(dimethylamino)pyridinium‐1‐yl]cycloheptatrienido}palladium(II)) bis(tetrafluoroborate) acetonitrile disolvate, [Pd₂Br₂(C₁₄H₁₆N₂)₂](BF₄)₂·2CH₃CN, with bridging bromide ligands, and the crystal structure of this compound is also reported here. The structures of the cycloheptatrienide ligands of both complexes are analogous to the dibromide derivative, showing the allyl bond in the β‐position with respect to the pyridinium substituent. This indicates that, unlike a previous interpretation, the main reason for the formation of the β‐isomer cannot be internal hydrogen bonding between the cationic substituents and bromide ligands.     

A theoretical study on camptothecin–AM1 treatment

Camptothecin (irinotecan), a new cytotoxic agent having antitumor activity and especially used in the treatment of advanced colorectal cancers is considered for AM1–RHF type semiempirical quantum chemical treatment together with some analogs of camptothecin. The structural properties and energetics of these molecules have been compared. The structures arising from monoprotonation of camptothecin at various sites are analyzed from thermodynamic point of view.     

Kinetics and mechanism of the oxidation of 4‐methyl‐3‐thiosemicarbazide by acidic bromate

The oxidation of 4‐methyl‐3‐thiosemicarbazide (MTSC) by bromate and bromine was studied in acidic medium. The stoichiometry of the reaction is extremely complex, and is dependent on the ratio of the initial concentrations of the oxidant to reductant. In excess MTSC and after prolonged standing, the stoichiometry was determined to be H₃CN(H)CSN(H)NH₂ + 3BrO₃^? → 2CO₂ + NH₄⁺ + SO₄^2? + N₂ + 3Br^? + H⁺ (A). An interim stoichiometry is also obtained in which one of the CO₂ molecules is replaced by HCOOH with an overall stoichiometry of 3H₃CN(H)CSN(H)NH₂ + 8BrO₃^? → CO₂ + NH₄⁺ + SO₄^2? + HCOOH + N₂ + 3Br^? + 3H⁺ (B). Stoichiometry A and B are not very different, and so mixtures of the two were obtained. Compared to other oxidations of thiourea‐based compounds, this reaction is moderately fast and is first order in both bromate and substrate. It is autocatalytic in HOBr. The reaction is characterized by an autocatalytic sigmoidal decay in the consumption of MTSC, while in excess bromate conditions the reaction shows an induction period before autocatalytic formation of bromine. In both cases, oxybromine chemistry, which involves the initial formation of the reactive species HOBr and Br₂, is dominant. The reactions of MTSC with both HOBr and Br₂ are fast, and so the overall rate of oxidation is dependent upon the rates of formation of these reactive species from bromate. Our proposed mechanism involves the initial cleavage of the C? N bond on the azo‐side of the molecule to release nitrogen and an activated sulfur species that quickly and rapidly rearranges to give a series of thiourea acids. These thiourea acids are then oxidized to the sulfonic acid before cleavage of the C? S bond to give SO₄^2?, CO₂, and NH₄⁺. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 237–247, 2002     

Synthesis of 7‐Azido‐3‐Formylcoumarin – A Key Precursor in Bioorthogonally Applicable Fluorogenic Dye Synthesis

Coumarins represent an important group of natural products and a common part of various drugs and fluorescent dyestuffs. Herein, we present the synthesis of a coumarin that can serve as a key starting material in the design and synthesis of bioorthogonally applicable fluorogenic dyes. The synthesis of 7‐azido‐3‐formylcoumarin started from 7‐diallylaminocoumarin. This allyl protected aminocoumarin is otherwise hard to obtain by conventional methods but was conveniently accessed in good yields by a sequential, Wittig‐reaction–UV isomerization process. This sequential approach was studied in more details and applied for the synthesis of a series of substituted coumarins even in one‐pot.     

Reactions of 9,9′‐bibenzonorbornenylidene sulfoxides with TMSOTf: Anomalous pinacol‐type rearrangement of thiirane 1‐Oxides

syn‐9,9′‐Bibenzonorbornenylidene sulfoxide 8b underwent pinacol‐type rearrangement to form 9 , together with a mixture of thiiranes 4a and 4b by reaction with TMSOTf in CH₂Cl₂ at room temperature. The rearrangement of anti‐sulfoxide 8a proceeded more slowly giving a mixture of 9, 4a , and 4b . © 2009 Wiley Periodicals, Inc. Heteroatom Chem 20:29–34, 2009; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20507     

Combined experimental–theoretical NMR study on 2,5‐bis(5‐aryl‐3‐hexylthiophen‐2‐yl)‐thiazolo[5,4‐d]thiazole derivatives for printable electronics

Four 2,5‐bis(5‐aryl‐3‐hexylthiophen‐2‐yl)thiazolo[5,4‐d]thiazole derivatives have been synthesized and thoroughly characterized. The extended aromatic core of the molecules was designed to enhance the charge transport characteristics, and solubilizing hexyl side chains were introduced on the thiophene subunits to enable possible integration of these semiconducting small molecules in printable electronics. Complete elucidation of the chemical structures by detailed one‐dimensional/two‐dimensional NMR spectroscopy is described, providing interesting input for chemical shift prediction software as well, because limited experimental data on these types of compounds are currently available. Furthermore, theoretical calculations have assisted experimental observations—giving support for the chemical shift assignment and providing a springboard for future screening and predictions—demonstrating the benefits of a coordinated theoretical–experimental approach. Copyright © 2012 John Wiley & Sons, Ltd.