Formaldehyde oxime <--> nitrosomethane tautomerism

Ozonation of Gabapentin in Water─Investigating Reaction Kinetics and Transformation Mechanisms of a Primary Amine Using Isotopically Labeled Ozone

Aliphatic amines are abundant micropollutants in wastewater treatment plant effluents. In order to mitigate such micropollutants, ozonation is one of the most commonly employed advanced treatment processes. Current research regarding ozone efficiency is heavily focusing on reaction mechanisms of different contaminant groups, including structures with amine moieties as reactive sites. This study analyzes pH-dependent reaction kinetics and pathways of gabapentin (GBP), an aliphatic primary amine with an additional carboxylic acid group. The transformation pathway was elucidated applying a novel approach using isotopically labeled ozone (18O) and quantum chemistry calculations. While the direct reaction of GBP with ozone is highly pH-dependent and slow at pH 7 (13.7 M-1 s-1), the rate constant of the deprotonated species (1.76 × 105 M-1 s-1) is comparable to those of other amine compounds. Pathway analysis based on LC-MS/MS measurements revealed that ozonation of GBP leads to the formation of a carboxylic acid group and simultaneous nitrate formation, which was also observed in the case of the aliphatic amino acid glycine. Nitrate was formed with a yield of approximately 100%. Experiments with 18O-labeled ozone demonstrated that the intermediate aldehyde does most likely not include any oxygen originating from ozone. Furthermore, quantum chemistry calculations did not provide an explanation for the C-N scission during GBP ozonation without ozone involvement, although this reaction was slightly more favorable than for respective glycine and ethylamine reactions. Overall, this study contributes to a deeper understanding of reaction mechanisms of aliphatic primary amines during wastewater ozonation.

Reaction of Amino Acids with Ferrate(VI): Impact of the Carboxylic Group on the Primary Amine Oxidation Kinetics and Mechanism

Ferrate (Fe(VI)) is a novel oxidant that can be used to mitigate disinfection byproduct (DBP) precursors. However, the reaction of Fe(VI) with organic nitrogen, which is a potential precursor of potent nitrogenous DBPs, remains largely unexplored. The present work aimed to identify the kinetics and products for the reaction of Fe(VI) with primary amines, notably amino acids. A new kinetic model involving ionizable intermediates was proposed and can describe the unusual pH effect on the Fe(VI) reactivity toward primary amines and amino acids. The Fe(VI) oxidation of phenylalanine produced a mixture of nitrile, nitrite/nitrate, amide, and ammonia, while nitroalkane was an additional product in the case of glycine. The product distribution for amino acids significantly differed from that of uncarboxylated primary amines that mainly generate nitriles. A general reaction pathway for primary amines and amino acids was proposed and notably involved the formation of imines, the degradation of which was affected by the presence of a carboxylic group. In comparison, ozonation led to higher yields of nitroalkanes that could be readily converted to potent halonitroalkanes during chlor(am)ination. Based on this study, Fe(VI) can effectively mitigate primary amine-based, nitrogenous DBP precursors with little formation of toxic halonitroalkanes.

Insights into Nitrosoalkane Binding to Myoglobin Provided by Crystallography of Wild-Type and Distal Pocket Mutant Derivatives

Nitrosoalkanes (R-N═O; R = alkyl) are biological intermediates that form from the oxidative metabolism of various amine (RNH₂) drugs or from the reduction of nitroorganics (RNO₂). RNO compounds bind to and inhibit various heme proteins. However, structural information on the resulting Fe-RNO moieties remains limited. We report the preparation of ferrous wild-type and H64A sw Mb^II-RNO derivatives (λ_max 424 nm; R = Me, Et, Pr, iPr) from the reactions of Mb^III-H₂O with dithionite and nitroalkanes. The apparent extent of formation of the wt Mb derivatives followed the order MeNO > EtNO > PrNO > iPrNO, whereas the order was the opposite for the H64A derivatives. Ferricyanide oxidation of the Mb^II-RNO derivatives resulted in the formation of the ferric Mb^III-H₂O precursors with loss of the RNO ligands. X-ray crystal structures of the wt Mb^II-RNO derivatives at 1.76-2.0 Å resoln. revealed N-binding of RNO to Fe and the presence of H-bonding interactions between the nitroso O-atoms and distal pocket His64. The nitroso O-atoms pointed in the general direction of the protein exterior, and the hydrophobic R groups pointed toward the protein interior. X-ray crystal structures for the H64A mutant derivatives were determined at 1.74-1.80 Å resoln. An analysis of the distal pocket amino acid surface landscape provided an explanation for the differences in ligand orientations adopted by the EtNO and PrNO ligands in their wt and H64A structures. Our results provide a good baseline for the structural analysis of RNO binding to heme proteins possessing small distal pockets.

Carbodicarbenes and Striking Redox Transitions of their Conjugate Acids: Influence of NHC versus CAAC as Donor Substituents

Herein, a new type of carbodicarbene (CDC) comprising two different classes of carbenes is reported; NHC and CAAC as donor substituents and compare the molecular structure and coordination to Au(I)Cl to those of NHC-only and CAAC-only analogues. The conjugate acids of these three CDCs exhibit notable redox properties. Their reactions with [NO][SbF₆ ] were investigated. The reduction of the conjugate acid of CAAC-only based CDC with KC₈ results in the formation of hydrogen abstracted/eliminated products, which proceed through a neutral radical intermediate, detected by EPR spectroscopy. In contrast, the reduction of conjugate acids of NHC-only and NHC/CAAC based CDCs led to intermolecular reductive (reversible) carbon-carbon sigma bond formation. The resulting relatively elongated carbon-carbon sigma bonds were found to be readily oxidized. They were, thus, demonstrated to be potent reducing agents, underlining their potential utility as organic electron donors and n-dopants in organic semiconductor molecules.

Azodioxy compounds as precursors for C-radicals and their application in thermal styrene difunctionalization

An atom-economic thermal α,β-difunctionalization of various styrenes with readily prepared azodioxy compounds is reported. Mechanistic studies reveal that the starting azodioxy compounds can thermally be cleaved to the corresponding C-nitroso compounds, which under these thermal conditions further homolyze to generate reactive C-radicals along with the persistent NO radical. In the presence of a styrene, C-radical addition with subsequent nitrosylation followed by tautomerization is occurring, resulting in an overall styrene β-alkylation-α-oximation reaction.

Free chlorine formation in the process of the chlorine dioxide oxidation of aliphatic amines

Chlorine dioxide (ClO₂) is commonly used as an alternative disinfectant to chlorine because it has a high bactericidal effect and may produce limited concentrations of halogenated disinfection byproducts (DBPs). However, previous studies have reported that free available chlorine (FAC) was produced when ClO₂ reacted with some compounds, such as phenol, leading to the formation of halogenated DBPs. In this study aliphatic amines was found to react rapidly with ClO₂ to form significant amount of FAC and its related DBPs. This study investigated the formation of FAC when ClO₂ reacts with six model aliphatic amines (including primary amines, secondary amines and tertiary amines). FAC was formed immediately as ClO₂ was added to the precursor solution. The maximum yield of FAC even reached 45% (based on consumed ClO₂) when ClO₂ reacted with 20 μM methylamine at a dose of 10 μM, which is close to a realistic maximum dose (about 0.8 mg/L) in the U.S.. The reactivity of amines to result FAC follows the sequence tertiary amines < secondary amines < primary amines. It was verified that the addition of aliphatic amines may enhance the formation of FAC during ClO₂ oxidation in actual water samples. Organic chloramines and other chlorinated DBPs, such as cyanogen chloride, were detected when ClO₂ was used as the sole oxidant of real water samples. This study demonstrated that chlorine-related byproducts may also be formed in the presence of organic amines during ClO₂ treatment.

Theoretical study of water-assisted reaction between methane and nitrosonium cation

The water-assisted reaction between CH4 and NO+ has been studied employing post Hartree–Fock and density functional theory methods. Two reaction pathways were considered: (i) a frontside NO+ attack on the C–H bond of methane with retention of configuration and (ii) a backside attack with inversion of configuration. The first pathway leads to a thermodynamically more favorable hydrate of N-protonated nitrosomethane, and the second one leads to its O-protonated isomer. The catalytic effect of a single water molecule is expressed in decreasing the activation energy for the elementary step by about a factor of two for the frontside mechanism and by a factor of four to five for the backside one when compared with the calculated literature data on water-free methane nitrosation. The combination of the activation strain model of reactivity and the energy decomposition analysis reveals that the activation barriers are largely determined by the relative stability of the termolecular reactant complexes. The crucial factor that stabilizes these complexes comes from the electrostatic attraction. The catalytic effect of the key water molecule is decreased with introduction of additional one or two explicit water molecules, which form a coordinate O→N bonding with NO+. On the contrary, an additional water molecule hydrogen bonded to the key catalytic water molecule enhances the catalytic effect.

Nitriles as main products from the oxidation of primary amines by ferrate(VI): Kinetics, mechanisms and toxicological implications for nitrogenous disinfection byproduct control

Ferrate (Fe(VI)), a promising water treatment oxidant, can be used for micropollutant abatement or disinfection byproduct mitigation. However, knowledge gaps remain concerning the interaction between Fe(VI) and dissolved organic matter structures, notably primary amines. This study investigated degradation kinetics and products of several aliphatic primary amines by Fe(VI). Primary amines showed appreciable reactivity toward Fe(VI) (2.7-68 M-1s-1 at pH 7-9), ranking as follows: benzylamine > phenethylamine > phenylpropylamine > methylamine ≈ propylamine. Nitriles were the main oxidation products of the primary amines, with molar yields of 61-103%. Minor products included aldehydes, carboxylic acids, nitroalkanes, nitrite, nitrate, and ammonia. The buffering conditions were important. Compared to phosphate, borate enhanced the reactivity of the amines and shifted the products from nitriles to carbonyls. An evaluation of the effect potency of some cyano-compounds by an in vitro bioassay for oxidative stress response and cytotoxicity suggested that non-halogenated nitriles are unlikely to pose a significant threat because they were only toxic at high concentrations, acted as baseline toxicants and did not cause oxidative stress, unlike halonitroalkanes or halonitriles. The formation of non-halogenated nitriles is preferable to the formation of nitroalkanes arising from the ozonation of primary amines (other than amino acid N-terminals) because, during chlorination, nitriles remain unreactive while nitroalkanes lead to potent halonitroalkanes.

Three decades of unveiling the complex chemistry of C-nitroso species with computational chemistry

This review revisits the complex reactivity of C-nitroso derivatives through the synergistic combination of computational and synthetic organic chemistry, with an emphasis on the rationalization of mechanisms and selectivities. 

P(III)/P(V)-Catalyzed Methylamination of Arylboronic Acids and Esters: Reductive C-N Coupling with Nitromethane as a Methylamine Surrogate

The direct reductive N-arylation of nitromethane by organophosphorus-catalyzed reductive C-N coupling with arylboronic acid derivatives is reported. This method operates by the action of a small ring organophosphorus-based catalyst (1,2,2,3,4,4-hexamethylphosphetane P-oxide) together with a mild terminal reductant hydrosilane to drive the selective installation of the methylamino group to (hetero)aromatic boronic acids and esters. This method also provides for a unified synthetic approach to isotopically labeled N-methylanilines from various stable isotopologues of nitromethane (i.e., CD₃NO₂, CH₃¹⁵NO₂, and ¹³CH₃NO₂), revealing this easy-to-handle compound as a versatile precursor for the direct installation of the methylamino group.

Diaryliodonium Salt-Mediated Intramolecular C-N Bond Formation Using Boron-Masking N-Hydroxyamides

Intramolecular aromatic C-N bond formation reactions using electron-rich aromatic tethered boron-masking N-hydroxyamide as substrate were realized. These new C-N bond formation reactions involve the in situ generation of a diaryliodonium salt by treatment with hypervalent iodine, deborylation by base treatment, spontaneous N → O acyl migration, cyclization, reductive elimination, elimination of benzoic acid, and tautomerization to indole formation. Hereby, we obtained highly functionalized electron-rich indoles and quinoline in practical yields.

Amidoximes and Oximes: Synthesis, Structure, and Their Key Role as NO Donors

Nitric oxide (NO) is naturally synthesized in the human body and presents many beneficial biological effects; in particular on the cardiovascular system. Recently; many researchers tried to develop external sources to increase the NO level in the body; for example by using amidoximes and oximes which can be oxidized in vivo and release NO. In this review; the classical methods and most recent advances for the synthesis of both amidoximes and oximes are presented first. The isomers of amidoximes and oximes and their stabilities will also be described; (Z)-amidoximes and (Z)-oximes being usually the most energetically favorable isomers. This manuscript details also the biomimetic and biological pathways involved in the oxidation of amidoximes and oximes. The key role played by cytochrome P450 or other dihydronicotinamide-adenine dinucleotide phosphate (NADPH)-dependent reductase pathways is demonstrated. Finally, amidoximes and oximes exhibit important effects on the relaxation of both aortic and tracheal rings alongside with other effects as the decrease of the arterial pressure and of the thrombi formation

Reactions of aliphatic amines with ozone: Kinetics and mechanisms

Aliphatic amines are common constituents in micropollutants and dissolved organic matter and present in elevated concentrations in wastewater-impacted source waters. Due to high reactivity, reactions of aliphatic amines with ozone are likely to occur during ozonation in water and wastewater treatment. We investigated the kinetics and mechanisms of the reactions of ozone with ethylamine, diethylamine, and triethylamine as model nitrogenous compounds. Species-specific second-order rate constants for the neutral parent amines ranged from 9.3 × 10⁴ to 2.2 × 10⁶ M^-1s^-1 and the apparent second-order rate constants at pH 7 for potential or identified transformation products were 6.8 × 10⁵ M^-1s^-1 for N,N-diethylhydroxylamine, ∼10⁵ M^-1s^-1 for N-ethylhydroxylamine, 1.9 × 10³ M^-1s^-1 for N-ethylethanimine oxide, and 3.4 M^-1s^-1 for nitroethane. Product analyses revealed that all amines were transformed to products containing a nitrogen-oxygen bond (e.g., triethylamine N-oxide and nitroethane) with high yields, i.e., 64-100% with regard to the abated target amines. These findings could be confirmed by measurements of singlet oxygen and hydroxyl radical which are formed during the amine-ozone reactions. Based on the high yields of nitroethane from ethylamine and diethylamine, a significant formation of nitroalkanes can be expected during ozonation of waters containing high levels of dissolved organic nitrogen, as expected in wastewaters or wastewater-impaired source waters. This may pose adverse effects on the aquatic environment and human health.

Direct Reductive N-Functionalization of Aliphatic Nitro Compounds

The first general protocol for the direct reductive N-functionalization of aliphatic nitro compounds is presented. The nitro group is partially reduced to a nitrenoid, with a mild and readily available combination of B₂ pin₂ and zinc organyls. Thereby, the formation of an unstable nitroso intermediate is avoided, which has so far severely limited reductive transformations of aliphatic nitro compounds. The reaction is concluded by an electrophilic amination of zinc organyls.

Contrasting C- and O-Atom Reactivities of Neutral Ketone and Enolate Forms of 3-Sulfonyloxyimino-2-methyl-1-phenyl-1-butanones

The mechanisms of intramolecular cyclization of 3-sulfonyloxyimino-2-methyl-1-phenyl-1-butanones (1) under basic (DABCO and t-BuOK) and acidic (AcOH and TFA) conditions were investigated by means of experimental and computational methods. The ketone, enol, and enolate forms of 1 can afford different intramolecular cyclization products (2, 3, 4), depending on the conditions. The results of the reaction of 1 under basic conditions suggest intermediacy of neutral enol (DABCO) and anionic enolate (t-BuOK), while the results under acidic conditions (AcOH and TFA) indicate involvement of neutral ketones, which exhibit reactivities arising from both the oxygen lone-pair electrons (O atom reactivity) and carbon σ-electrons (C atom reactivity). The neutral enol in DABCO afforded 2H-azirine 4. On the other hand, the products (isoxazole 2 and oxazole 3) generated from the ketone form and from the enolate form are the same, but the reaction mechanisms are apparently different. The results demonstrate ambident-like reactivity of neutral ketone in the 3-sulfonyloxyimino-2-methyl-1-phenyl-1-butanone system.

Revisiting oxime–nitrone tautomerism. Evidence of nitrone tautomer participation in oxime nucleophilic addition reactions

Oxime–nitrone tautomerism takes place through a biomolecular mechanism. Participation of nitrone tautomer in nucleophilic addition reactions is evidenced by the first time.

A Database of Formation Enthalpies of Nitrogen Species by Compound Methods (CBS-QB3, CBS-APNO, G3, G4)

Accurate thermochemical data for compounds containing C/H/N/O are required to underpin kinetics simulation and modeling of the reactions of these species in different environments. There is a dearth of experimental data so computational quantum chemistry has stepped in to fill this breach and to verify whether particular experiments are in need of revision. A number of composite model chemistries (CBS-QB3, CBS-APNO, G3, and G4) are used to compute theoretical atomization energies and hence enthalpies of formation at 0 and 298.15 K, and these are benchmarked against the best available compendium of values, the Active Thermochemical Tables or ATcT. In general the agreement is very good for some 28 species with the only discrepancy being for hydrazine. It is shown that, although individually the methods do not perform that well, collectively the mean unsigned error is <1.7 kJ mol(-1); hence, this approach provides a useful tool to screen published values and validate new experimental results. Using multiple model chemistries does have some drawbacks but can produce good results even for challenging molecules like HOON and CN2O2. The results for these smaller validated molecules are then used as anchors for determining the formation enthalpies of larger species such as methylated hydrazines and diazenes, five- and six-membered heterocyclics via carefully chosen isodesmic working reactions with the aim of resolving some discrepancies in the literature and establishing a properly validated database. This expanded database could be useful in testing the performance of computationally less-demanding density function methods with newer functionals that have the capacity to treat much larger systems than those tested here.

Theoretical in-Solution Conformational/Tautomeric Analyses for Chain Systems with Conjugated Double Bonds Involving Nitrogen(s)

Conformational/tautomeric transformations for X=CH–CH=Y structures (X = CH₂, O, NH and Y = NH) have been studied in the gas phase, in dichloromethane and in aqueous solutions. The paper is a continuation of a former study where s-cis/s-trans conformational equilibria were predicted for analogues. The s-trans conformation is preferred for the present molecules in the gas phase on the basis of its lowest internal free energy as calculated at the B97D/aug-cc-pvqz and CCSD(T)_CBS (coupled-cluster singles and doubles with non-iterative triples extrapolated to the complete basis set) levels. Transition state barriers are of 29–36 kJ/mol for rotations about the central C–C bonds. In solution, an s-trans form is still favored on the basis of its considerably lower internal free energy compared with the s-cis forms as calculated by IEF-PCM (integral-equation formalism of the polarizable continuum dielectric solvent model) at the theoretical levels indicated. A tetrahydrate model in the supermolecule/continuum approach helped explore the 2solute-solvent hydrogen bond pattern. The calculated transition state barrier for rotation about the C–C bond decreased to 27 kJ/mol for the tetrahydrate. Considering explicit solvent models, relative solvation free energies were calculated by means of the free energy perturbation method through Monte Carlo simulations. These calculated values differ remarkably from those by the PCM approach in aqueous solution, nonetheless the same prevalent conformation was predicted by the two methods. Aqueous solution structure-characteristics were determined by Monte Carlo. Equilibration of conformers/tautomers through water-assisted double proton-relay is discussed. This mechanism is not viable, however, in non-protic solvents where the calculated potential of mean force curve does not predict remarkable solute dimerization and subsequent favorable orientation.

A DFT study of the role of the Lewis acid catalysts in the [3 + 2] cycloaddition reaction of the electrophilic nitrone isomer of methyl glyoxylate oxime with nucleophilic cyclopentene

The mechanism and stereoselectivity of the BH₃-catalysed 32CA reaction between C-methoxycarbonyl nitrone and cyclopentene has been studied using MPWB1K/6-31G(d) computational level.

Competing intramolecular vs. intermolecular hydrogen bonds in solution

A hydrogen bond for a local-minimum-energy structure can be identified according to the definition of the International Union of Pure and Applied Chemistry (IUPAC recommendation 2011) or by finding a special bond critical point on the density map of the structure in the framework of the atoms-in-molecules theory. Nonetheless, a given structural conformation may be simply favored by electrostatic interactions. The present review surveys the in-solution competition of the conformations with intramolecular vs. intermolecular hydrogen bonds for different types of small organic molecules. In their most stable gas-phase structure, an intramolecular hydrogen bond is possible. In a protic solution, the intramolecular hydrogen bond may disrupt in favor of two solute-solvent intermolecular hydrogen bonds. The balance of the increased internal energy and the stabilizing effect of the solute-solvent interactions regulates the new conformer composition in the liquid phase. The review additionally considers the solvent effects on the stability of simple dimeric systems as revealed from molecular dynamics simulations or on the basis of the calculated potential of mean force curves. Finally, studies of the solvent effects on the type of the intermolecular hydrogen bond (neutral or ionic) in acid-base complexes have been surveyed.

DFT study on the oxygen transfer mechanism in nitroethenediamine based H2-receptor antagonists using the bis-dithiolene complex as the model catalyst for N-oxide reductase enzyme

Nitroethenediamine is an important functional unit, which is present in H2-receptor antagonists. These drugs show low bioavailability due to the bacterial degradation caused by the N-oxide reductase type of enzymes present in the human colon. Quantum chemical studies have been carried out to elucidate the mechanism of metabolic degradation of nitroethenediamine in the active site of N-oxide reductase. Three different pathways have been explored for the N-oxide bond cleavage by the model system, Mo(IV) bis-dithiolene complex [Mo(OMe)(mdt)2](-), (where mdt=1,2-dimethyl-ethene-1,2-dithiolate) using B3LYP/6-311+G(d,p) and M06/6-311+G(d,p) Density Functional Theory methods. The oxygen atom transfer from the nitrogen atom of nitroethenediamine to the Mo(IV) complex, involves simultaneous weakening of the N-oxide bond and the formation of Mo-O bond through a least motion path. During this transfer, Mo center is converted from a square pyramidal geometry to a distorted octahedral geometry, to facilitate the process of oxygen atom transfer. The energy barrier for the oxygen atom transfer from the imine tautomer has been estimated to be 25.9kcal/mol however, the overall reaction has been found to be endothermic. On the other hand, oxygen transfer reaction from the nitronic acid tautomer requires 30.5kcal/mol energy leading to a highly exothermic metabolite (M-1) directly hence, this path can be considered thermodynamically favorable for this metabolite. The alternative path involving the oxygen atom transfer from the enamine tautomer requires comparatively a higher energy barrier (32.6kcal/mol) and leads to a slightly endothermic metabolite. This study established the structural and energetic details associated with the Mo(IV) bis-dithiolene complex that catalyzes the degradation of nitroethenediamine based drug molecules.

Mechanisms of the reaction between polyhalogenated nitrobutadienes and electron-deficient anilines: computational modeling

Nitro-substituted polyhalogenated butadienes are valuable synthetic precursors for polyfunctionalized bioactive heterocyclic compounds. Recently, a new reaction between 2-nitroperchloro-1,3-butadiene and electron-deficient anilines producing the Z stereoisomers of a variety of allylidene arylhydrazines has been reported. Although the formation of a chlorinated nitrile oxide intermediate was proved by trapping it with appropriate alkenes via 1,3-dipolar cycloaddition, the details of the overall mechanism remained unclear. The elucidation of the mechanism is important for a better understanding of polyhalogenated nitrobutadiene chemistry. We proposed six reaction paths for the formation of allylidene arylhydrazine, starting from 2-nitroperchloro-1,3-butadiene and para-nitro aniline, and generated the potential energy profiles with the DFT/B3LYP/6-31+G(d,p) method. To include the solvent effect, single-point energy calculations were carried out at the B3LYP/6-31+G(d,p) level by the polarizable continuum model with tetrahydrofuran, as used in the experimental study. The Gibbs activation energies of the rate-determining steps of each mechanism were defined. Taking into account the downhill nature of the overall potential energy profile, Paths 5 and 6 which proceed via extrusion of p-nitrophenylisocyanate and the formation of chlorinated nitrile oxide were chosen as plausible mechanisms. Results also provide insights into the chemistry of nitrile oxides, oximes, oxazete, and nitroso compounds as well as S(N)Vin reactions.

MOLECULAR STRUCTURE, VIBRATIONAL ASSIGNMENTS, CONFORMATIONAL STABILITY, GROUND AND EXCITED STATE HYDROGEN-BONDING ANALYSIS OF 2-NITROSO VINYL AMINE

In the present work, a detailed conformational study is performed using several computational methods, including density functional theory (DFT) (B3LYP), MP2 and G2MP2 on 2-Nitroso vinyl amine (NVA) in order to determine the stability order of conformers and the various possibilities of intramolecular hydrogen bonding (HB) formation. Four conformers exhibit HB . This feature, although not being the dominant factor in energetic terms, appears to be of foremost importance to define the geometry of the molecule. According to our theoretical results, oximimine conformers are more stable than the corresponding nitrosoamine and nitrosoimine analogues. Theoretical calculations show the following order for intramolecular HB strength in the conformers of title compound: [Formula: see text]

The nature of intramolecular HB has been investigated by means of the Bader theory of atoms in molecules (AIM) and natural bond orbital (NBO) analysis. Also, Harmonic Oscillator Model of Aromaticity (HOMA) index as a geometrical indicator of a local aromaticity are investigated. The influence of the solvent on the stability order of conformers and the strength of intramolecular HB is considered using the Onsager reaction field model. The calculated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies confirm that charge transfer occur within the molecule. Further verification of the obtained transition state structures were implemented via intrinsic reaction coordinate (IRC) analysis. Calculations of the 1 H NMR chemical shift at GIAO/B3LYP/6–311++G** levels of theory are also presented. The excited-state properties of intramolecular HB in H -bonded systems have been investigated theoretically using the time-dependent density functional theory (TD-DFT) method. The complete vibrational assignment for three H -bonded conformers has been made on the basis of the calculated potential energy distribution (PED).

Desmotropy, polymorphism, and solid-state proton transfer: four solid forms of an aromatic o-hydroxy Schiff base

The Schiff base derived from salicylaldehyde and 2-amino-3-hydroxypyridine affords a diversity of solid forms, two polymorphic pairs of the enol-imino (D1 a and D1 b) and keto-amino (D2 a and D2 b) desmotropes. The isolated phases, identified by IR spectroscopy, X-ray crystallography, and (13)C cross-polarization/magnetic angle spinning (CP/MAS) NMR spectroscopy, display essentially planar molecular conformations characterized by strong intramolecular hydrogen bonds of the O-H⋅⋅⋅N (D1) or N-H⋅⋅⋅O (D2) type. A change in the position of the proton within this O⋅⋅⋅H⋅⋅⋅N system is accompanied by substantially different molecular conformations and, subsequently, by divergent supramolecular architectures. The appearance and interconversion conditions for each of the four phases have been established on the basis of a number of solution and solvent-free experiments, and evaluated against the results of computational studies. Solid phases readily convert into the most stable form (D1 a) upon exposure to methanol vapor, heating, or by mechanical treatment, and these transformations are accompanied by a change in the color of the sample. The course of thermally induced transformations has been monitored in detail by means of temperature-resolved powder X-ray diffraction and infrared spectroscopy. Upon dissolution, all forms equilibrate immediately, as confirmed by NMR and UV/Vis spectroscopy in several solvents, with the equilibrium shifted far towards the enol tautomer. This study reveals the significance of peripheral groups in the stabilization of metastable tautomers in the solid state.

UNDERSTANDING THE ROLE OF WATER IN PROMOTING E-ISOMER PRODUCTION AND PHOTOCHROMISM OF SOLID SCHIFF BASE: A DFT AND TD-DFT STUDY

The formation mechanism and photochromic property of a novel solid state Schiff base compound (E-isomer, C 21 H 18 I 2 N 3 O 2) derived from 4-aminoantipyrine and 3,5-diiodosalicylaldehyde is investigated in this work using density functional theory at the B3LYP/6-31l++G** level with and without solvent water presence. Our computational results show that the conversion in the gas phase is unlikely to occur and the solvent water is involved in the reaction to decrease the barrier height. We considered one to three water molecule participations in the water-catalyzed mechanism to produce the E-isomer. Time-dependent density functional theory (B3LYP/6-311+G*) is employed to examine the photochromic property of the molecule. A large Stokes shift between absorption and emission has been attributed to the intramolecular H-transfer ( O – H ⋯ N to O ⋯ H – N ) following photoexcitation of the enol species, suggesting that the keto form of the photoproduct structure plays the key role, in agreement with the experimental finding of stereoselectivity. Conceptual DFT reactivity indices are employed to rationalize the findings from the present study.

EXPLORING THE POTENTIAL ENERGY SURFACE OF THE CATALYZED ISOMERIZATION OF NITROSOMETHANE TO FORMALDOXIME

The mechanism of the isomerization of nitrosomethane to formaldoxime catalyzed by neutral molecule ( H 2 O and HCOOH ) has been investigated at the level of B3LYP/6-311+G**. Calculated results indicate that the rearrangement from nitrosomethane to more stable trans-formaldoxime can proceed via two different reaction channels, but the favorable reaction pathway catalyzed by water and formic acid is different from the one in the catalyst-free reaction. It is more favorable that the tautomeric reaction involves the formation of cis-formaldoxime and a subsequent rotation about the N – O bond to form trans-formaldoxime in the catalyzed reaction. The activation energy of rate-determining step was reduced from 197.9 kJ/mol to 138.7 kJ/mol in the water-catalyzed reaction and 79.6 kJ/mol in the formic acid-catalyzed reaction, respectively, due to the catalysis of hydroxylic groups, but the catalysis of more acidic hydroxyl group in the latter system has been shown to be more efficient.

Isomerization of nitrosomethane to formaldoxime: energies, geometries, and frequencies from the parametric variational two-electron reduced-density-matrix method

The isomerization of nitrosomethane to trans-formaldoxime is treated with the parametric variational two-electron reduced-density-matrix (2-RDM) method. In the parametric 2-RDM method, the ground-state energy is minimized with respect to a 2-RDM that is parameterized to be both size extensive and nearly N-representable. The calculations were performed with an efficient version of the 2-RDM method that we developed as an extension of the PSI3 ab initio package. Details of the implementation, which scales like configuration interaction with single and double excitations, are provided as well as a comparison of two optimization algorithms for minimizing the energy functional. The conversion of nitrosomethane to trans-formaldoxime can occur by one of two pathways: (i) a 1,3-sigmatropic hydrogen shift or (ii) two successive 1,2-sigmatropic hydrogen shifts. The parametric 2-RDM method predicts that the reaction channel involving two sequential 1,2-shifts is about 10 kcal/mol more favorable than the channel with a single 1,3-shift, which is consistent with calculations from other ab initio methods. We computed geometric parameters and harmonic frequencies for each stationary point on the reaction surfaces. Transition-state energies, geometries, and frequencies from the 2-RDM method are often more accurate than those from traditional wave function methods of a similar computational cost. Although electronic-structure methods generally agree that the 1,2-shift is more efficient, the energy ordering of the reactant nitrosomethane and the 1,2-shift intermediate formaldonitrone is unresolved in the literature. With an extrapolation to the complete-basis-set limit the parametric 2-RDM method predicts formaldonitrone to be very slightly more stable than nitrosomethane.

Synthesis and tautomerization study of pseudonitrosites to 1,2-nitroximes

4-R-Substituted 2-nitro-1-nitrosoethylbenzenes (R = H, CH3, OCH3, Cl, F) have been synthesized under solvent-free conditions and the mechanism of their tautomerization to 2-isonitroso-1-nitro-2-phenylethanes have been investigated by 1H NMR spectroscopy.Key words: tautomerization, pseudonitrosite, mechanism, 2-nitro-1-nitrosoethylbenzene, 1H NMR spectroscopy

Ab initio molecular dynamics study of the keto-enol tautomerism of acetone in solution

We have studied the keto-enol interconversion of acetone to understand the mechanism of tautomerism relevant to numerous organic and biochemical processes. Applying the ab initio metadynamics method, we simulated the keto-enol isomerism both in the gas phase and in the presence of water. For the gas-phase intramolecular mechanism we show that no other hydrogen-transfer reactions can compete with the simple keto-enol tautomerism. We obtain an intermolecular mechanism and remarkable participation of water when acetone is solvated by neutral water. The simulations reveal that C deprotonation is the kinetic bottleneck of the keto-enol transformation, in agreement with experimental observations. The most interesting finding is the formation of short H-bonded chains of water molecules that provide the route for proton transfer from the carbon to the oxygen atom of acetone. The mechanistic picture that emerged from the present study involves proton migration and emphasizes the importance of active solvent participation in tautomeric interconversion.