A characterization study on 2,6-dimethyl-4-nitropyridine N-oxide by density functional theory calculations

This study deals with the identification of a title compound, 2,6-dimethyl-4-nitropyridine N-oxide by means of theoretical calculations. The optimized molecular structures, vibrational frequencies, corresponding vibrational assignments, thermodynamic properties and atomic charges of the title compound in the ground state were evaluated using density functional theory (DFT) with the standard B3LYP/6-311G(d,p) method and basis set combination for the first time. Theoretical vibrational spectra were interpreted with the aid of normal coordinate analysis based on scaled density functional force field. The results show that the optimized geometric parameters (bond lengths and bond angles) and vibrational frequencies were observed to be in good agreement with the available experimental results. Based on the results of comparison between experimental results and theoretical data, the chosen calculation level is powerful approach for understanding the molecular structures and vibrational spectra of the 2,6-dimethyl-4-nitropyridine N-oxide. Moreover, we not only simulated frontier molecular orbitals (FMO) and molecular electrostatic potential (MEP) but also determined the transition state and energy band gap. Based on the investigations, the title compound is found to be useful to bond metallically and interact intermolecularly. Infrared intensities and Raman activities were also reported.     

Crystal structure, spectroscopic, and theoretical investigations of excited-state proton transfer in the doubly hydrogen-bonded dimer of 2-butylamino-6-methyl-4-nitropyridine N-oxide

The crystal structure of 2-butylamino-6-methyl-4-nitropyridine N-oxide (2B6M) was resolved on the basis of X-ray diffraction. Solid 2B6M occurs in the form of a doubly hydrogen-bonded dimer with squarelike hydrogen-bonding network composed of two intra- (2.556(2) A) and two intermolecular (2.891(2) A) N-H...O type hydrogen bonds. The molecule thus has both a protonable and a deprotonable group that led us to investigate the possibility of an excited-state proton transfer (ESIPT) reaction in different solvents by means of experimental absorption, steady state, and time-resolved emission spectroscopy. The results were correlated with quantum mechanical TD-DFT and PM3 calculations. Experimental and theoretical findings show the possibility of an ESIPT reaction in polar solvents. It is demonstrated that in particular the emission spectra of 2B6M are very sensitive to solvent properties, and a large value of the Stokes shift (about 8000 cm(-1)) in acetonitrile is indicative for an ESIPT process. This conclusion is further supported by time-resolved fluorescence decay measurents that show dual exponential decay in polar solvents. Vertical excitation energies calculated by TD-DFT reproduce the experimental absorption maxima in nonpolar solvents well. The majority of electronic transitions in 2B6M is of pi --> pi* character with a charge shift from the electron-donating to the electron-accepting groups. The calculations show that, due to the charge redistribution on excitation, the acidity of the amino group increases significantly, which facilitates the proton transfer from the amino to the N-oxide group in the excited state.     

A multinuclear 1H, 13C, and 15N magnetic resonance study of ten 4-nitropyridine N-oxides

The ¹H, ¹³C, and ¹⁵N NMR chemical shifts of ten 4-nitropyridine N-oxide derivatives are assigned. The shielding of the pyridine ring nitrogen is sensitive to ring substitution through inductive effects, steric effects by ortho-substituents, and the possibility for electron delocalisation (resonance energy). In solution, 3-ethylnitrosoamino-4-nitropyridine N-oxide has two tautomers. The proposed reason is the steric crowding between vicinal 4-nitro and 3-ethylnitrosoamino groups, causing a disturbance to amino nitrogen that can delocalize its lone pair to the oxygen atom of the nitroso group.     

UV/Vis Spectroscopic Evaluation of 4-Nitropyridine N-Oxide as a Solvatochromic Indicator for the Hydrogen-Bond Donor Ability of Solvents

The potential of 4-nitropyridine N-oxide to act as a solvatochromic indicator of the hydrogen-bond donor ability of solvents has been evaluated. A linear free-energy relationship has been established that is predominantly dependent on the Kamlet-Taft alpha parameter of the solvent. In comparison to the previously reported results obtained for pyridine N-oxide, 4-nitropyridine N-oxide possesses a solvatochromic effect that is located in the long wavelength ultraviolet region (lambda = 330-355 nm) of the spectrum, making it a viable probe for hydrogen-bond donation assessment.     

Electronic spectra and structure of methyl derivatives of 4-nitropyridine N-oxide

The UV spectra of seven methyl derivatives of 4-nitropyridine N-oxide in ethanol have been examined. The electronic spectra were calculated by a modified INDO method. Transition energies, intensities and assignments were compared with UV spectra. Spectroscopic manifestations of intramolecular interaction indicate that methyl groups modify the electronic interaction between the N-oxide and NO₂ groups mainly through a steric strain.Department of Organic Chemistry, University of Economics, Pl-53342 Wroclaw, Poland. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 786–793, June, 2000.     

Crystal Structure of [Cd（pom）2Cl2]

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Facile one-pot direct arylation and alkylation of nitropyridine N-oxides with Grignard reagents

Facile arylation and alkylation of nitropyridine N-oxides were developed through the reactions of Grignard reagents with nitropyridine N-oxides. For the same 4-nitropyridine N-oxide, arylation occurred at the 2- (or 6-) position, whereas alkylation occurred at the 3-position in an adjustably site-selective manner. The cooperative action of the two groups was discovered in the reactions of 3-nitropyridine N-oxides. This protocol can find wide applications in building various pyridine compounds as illustrated in total syntheses of Emoxipin and Caerulomycin A and E.     

The first total synthesis of novel human chymase inhibitor SPF32629A

The first total synthesis of SPF32629A, a novel human chymase inhibitor, has been accomplished from the readily available precursor 4-nitropyridine N-oxide, following an efficient strategy with a sequence of eight straightforward steps.     

Effect of alkyl substituents on excited state intramolecular proton transfer dynamics of jet-cooled bis(benzoxazolyl)phenoles

Structural factors affecting the dynamics of the excited state intramolecular proton transfer (ESIPT) are studied for alkyl derivatives of 2,5-bis(2-benzoxazolyl)phenol. Two fluorescence bands with equal decay times are observed in solution, while only one--emitted by the phototautomer--in supersonic jet. All evidence indicates the existence of a potential barrier in the S(1) state. Upon deuteration of the OH group the laser induced fluorescence (LIF) excitation spectra become much sharper as a result of slowing down the proton transfer reaction. Two conformers (rotamers) of each compound in the ground state were detected using hole burning technique. With a help of theoretical calculations three vibrations were identified as the most active ones in reducing the distance between two heavy atoms, N and O, involved in H-bond formation. The widths of (0,0) transitions in LIF excitation spectra decrease with increasing size or number of alkyl substituents at terminal aromatic rings. The corresponding calculated rate constants of ESIPT reaction ( approximately 10(12) s(-1)) decrease approximately three times upon the substituent effect. In contrast, model compound 2,5-bis(2-benzoxazolyl)-4-methoxyphenol (BBMP) with OCH(3) parasubstituent in central ring slows down the ESIPT reaction to such an extent that double, primary and phototautomeric, fluorescences coexist.     

ESIPT-regulated Mechanoresponsive Luminescence Process byIntroducing Intramolecular Hydrogen Bond in NaphthalimideDerivatives#br#

Herein, two compounds, 4-2′-hydroxybenzylidenehydrazinyl-N-butyl-1,8-naphthalimide(BN-1) and 4-benzylidenehydra-zinyl-N-butyl-1,8-naphthalimide(BN-2), were synthesized to explore the hydrogen bonding effect on mechanoresponsive luminescent(MRL). The results showed that compound BN-1 exhibited strong emission in solution and solid-state compared with compound BN-2. After grinding, the emission intensity of compound BN-1 sharply decreased by as much as 15 times with an obvious red-shift from 552 nm to 577 nm. The control compound BN-2, by contrast, did not change so much before and after grinding. Single crystal analysis suggests that BN-1 molecule formed strong intramolecular interaction via ―N=N···H―O hydrogen bond with a distance of 0.2632 nm. An excited-state intramolecular proton transfer(ESIPT) based fluorophore featured this intramolecular hydrogen bond. The intramolecular hydrogen bond as well as other intermolecular interactions can rigidify the molecular conformation of compound BN-1 in solid-state, and thus suppress the nonradiative pathways, resulting in strong emission. These intra- and intermolecular interactions were destroyed by mechanical stimuli, accompanied by molecular conformation change that decreases the luminescence and blocks the ESIPT process. The MRL process was also demonstrated by scanning electron microscopy and powder X-ray diffraction. The molecular stacking mode changed from crystalline to a disordered amorphous state after grinding.     

Intramolecular proton-transfer processes starting at higher excited states: a fluorescence study on 2-butylamino-6-methyl-4-nitropyridine N-oxide in nonpolar solutions

This article describes the exceptional photophysics of 2-butylamino-6-methyl-4-nitropyridine N-oxide (2B6M). It is known from the literature that this compound may undergo excited-state intra- or intermolecular proton-transfer reactions. In nonpolar solvents, 2B6M exhibits an unusual fluorescence behavior: there is a substantial difference between the relative band intensities of the excitation and absorption spectra. Furthermore, in emission two bands are observed, and their relative intensities depend on the excitation wavelength, thus violating the Kasha-Vavilov rule. It is the objective of this research to interpret these results. For this purpose, steady-state fluorescence excitation and emission spectra in the liquid state were recorded and quantum yields were determined for the two types of emission. In addition, absorption spectra were measured at room temperature and under low-temperature conditions. Finally, fluorescence lifetimes of the emitting species were determined using the time-correlated single photon counting technique. The results suggest that in the liquid state only one (monomeric) ground state species dominates, which can emit via two different pathways (from the normal and the tautomeric excited state). The excitation spectra point at two different internal proton-transfer processes, one starting at the S1 state and one starting at the S2 state. On the basis of the measured lifetimes and fluorescence quantum yields, a kinetic scheme was completed that can quantitatively explain the observations.     

Isocyanate-, isothiocyanate-, urea-, and thiourea-substituted boron dipyrromethene dyes as fluorescent probes

Boron dipyrromethene dyes (Bodipy) bearing a meso-phenyl substituent carrying a variety of functional groups can be prepared under mild conditions. A single-crystal X-ray structure determination for the 3,5-dinitrophenyl compound shows the phenyl ring to be almost orthogonal (dihedral angle 84 degrees) to the plane of the Bodipy core, with one nitro group almost coplanar with the ring and the other tilted by approximately 21 degrees. Nitro substituents at the 3-, 4-, and 5- positions of the phenyl group are readily reduced to the corresponding amino groups and then converted to isocyanato, isothiocyanato, urea, thiourea, and some polyimine derivatives, the last providing additional functionality (phenazine and pyridylindole units) suitable for chelation of metal ions. All compounds are redox active, the electron-transfer processes being assigned on the basis of comparisons with model compounds. Their fluorescence properties are sensitive to the phenyl group substituents. The Bodipy unit excited state appears to be a strong reductant (Eo approximately -1.4 V) and a modest oxidant (Eo approximately +1.0 V). Quenching processes in the nitro and phenazine derivatives appear to involve intramolecular photoinduced electron transfer.     

The Reaction of N-Oxide with Ethyl Bromoacetate

The reaction of 4-methoxypyridine 1-oxide with ethyl bromoacetate was found to produce a high yield of ethyl glyoxylate, trimethylamine N-oxide and pyridine 1-oxide also afforded the glyoxylate but in lower yield in the reaction, and 4-nitropyridine 1-oxide was found to be inactive with bromoacetate. Pyridine 1-oxide and trimethylamine N-oxide exhibit the same stoichiometry, while 4-methoxypyridine 1-oxide follows a different course.     

Multinuclear 1H, 13C and 15N NMR study of some substituted 2-amino-4-nitropyridines and their N-oxides

1H, 13C and 15N NMR chemical shift assignments based on pulsed field gradient selected PFG 1H,X (X = 15C and 15N) HMQC and HMBC experiments are reported for three 4-nitropyridine N-oxides and four 4-nitropyridines. It was found that an ortho effect of a methyl group inhibits the deshielding effect of the 4-nitro group and that this effect and the so-called back donation is influenced by electronegativity and position of substituents in the multisubstituted pyridine N-oxides. The shielding effect of N-oxide group is most pronounced in the 15N NMR chemical shifts of the studied compounds. This effect is further modified by methylamino, methylnitramino, 5- or 3-methyl and 4-nitro groups. Among them the 4-nitro group exerts the highest influence on the shielding effect of the N-oxide functionality. Experimental 1H, 13C and 15N NMR chemical shifts and GIAO/DFT theoretical calculations are consistent with each other and supported by the reactivity on nucleophilic substitution, the UV spectral and the dipole moment data.     

Synthesis and Crystal Structure of 3-Nitrophthalic Acid·3-methyl-4-nitropyridine N-Oxide Adducts

The title compound, a 1:1 molecular adduct of 3-nitrophthalic acid and 3-methyl-4-nitropyridine N-oxide (PPOM), has been synthesized and characterized by X-ray single-crystal structure analysis. It crystallizes in triclinic, space group P-1 with a = 7.6076(15), b = 7.8180(16), c= 14.546(3) (A), α= 93.90(3), β = 97.21(3), γ= 114.43(3)°, C14H11N3O9, Mr = 365.26,Z=2,V=774.6(3) (A)3, Dc = 1.566 g/m3, u(MoKα) = 0.134 mm-1, F(000) = 376, R = 0.0538 and Wr = 0.1460 for 885 observed reflections (I＞2σ(I)). The protons of the carbonylic acids in the molecule are not transferred and the O-H…O and C-H…O hydrogen bonds form zigzag chains in the molecule.     

Analyse en coordonnees normales des vibrations de la 4-nitropyridine N-oxyde

A 51-parameter force field is established for the vibrations of the 4-nitropyridine N-oxide (NPO) and the 4-nitropyridine-d₄ N-oxide (NPO-d₄) molecules. The mean error between the 72 observed and calculated frequencies is 4.8 cm⁻¹ for the in-plane vibrations and 2.1 cm⁻¹ for the out-of-plane vibrations. The NCA calculations confirm our previous assignments for the NPO spectra, but four frequencies were permuted for NPO-d₄. The force field calculations were extended to fit the vibrational frequencies of NPO in solution in CCl₄, MeOH and H₂O. The fact that a donor solvent favours one resonant form of NPO is clearly demonstrated by the variations of five force constants.     

Synthesis of 2-Azastilbene Derivatives with Intramolecular Charge Transfer

The condensation reaction of 2-cyanomethyl-5-nitropyridine with aromatic aldehydes has been carried out with the aim of preparing 2-azastilbene derivatives having intramolecular charge transfer. The yield of the condensation products can be increased if the reaction is carried out in the medium used for obtaining the starting 2-cyanomethyl-5-nitropyridine without separating or purifying it. The electronic absorption spectra of the compounds show a charge-transfer band, the energy of which increases and the intensity falls with lowering of the electron-donor properties of the substituent in the 4-position. Introduction of the heteroatom into the acceptor part when changing from the stilbene to the 2-azastilbene system is accompanied by a decrease in the energy and increase in the intensity of the charge-transfer electronic transition.     

Synthesis of pyridine N-oxide-SbCl5 complexes and their intramolecular and oxygen-transfer reaction

Mixing with equimolar solutions of pyridine N-oxide or its homologs and SbCl₅ in CCl₄ deposited 1:1 complexes as colorless crystals in high yield. On thermolysis, these complexes underwent intramolecular oxygen transfer to give selectively the corresponding 2-pyridone derivatives. N,N-Dimethylaniline N-oxide and SbCl₅ also gave a crystalline 1:1 complex which on thermolysis yield o-dimethylaminophenol in good yield.     

Chemistry of heterocyclic N-oxides and related compounds

The dehydrogenating activity of 4-nitropyridine N-oxide and picolinic, nicotinic, isonicotinic, and quinolinic acid N-oxides was studied in the case of the reaction with the Hantzsch ester.See [1] for communication VIII.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1651–1653, December, 1977.     

Reactions of excited states of phenoxazin-3-one dyes with amino acids

The interaction with amino acids of the excited states of the N-oxide resazurin and its deoxygenation product resorufin, has been studied in aqueous solution at pH 7.5. Steady-state and time-resolved studies show that the fluorescence is quenched by amino acids. Complexation of the dyes in the ground state with aromatic amino acids was also observed. The singlet quenching is attributed to electron transfer from the amino acids to the excited dye based on the dependence of the bimolecular rate constants with the ionization potential of quenchers. Flash photolysis experiments allowed determination of the quenching rate constants for the triplet deactivation of dyes by several amino acids, as well as the characterization of the transients formed in the process. These data show that the triplet is also deactivated by an electron transfer process. However, the deactivation of the N-oxide dye by tryptophan can be described by a hydrogen atom transfer. The protolytic dissociation constants of the dye radical ions are reported. The irradiation of rezasurin in the presence of amino acids leads to deoxygenation of the dye to give resorufin. This process involves the triplet excited state of resazurin and is efficient only in the presence of amino acids containing the -SH group.