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Crystal structure, vibrational and optic properties of 2-N-methylamino-3-methylpyridine N-oxide - Its X-ray and spectroscopic studies as well as DFT quantum chemical calculations.

The crystal and molecular structure and physicochemical properties of 2-N-methylamino-3-methylpyridine N-oxide (MA3MPO) have been studied. MA3MPO was synthesized from 2-amino-3-methylpyridine by several steps to form colorless crystals suitable for crystallographic analysis. The data reveal that MA3MPO crystallizes in the monoclinic space group P21/n. The studied compound contains a nearly flat triply substituted pyridine skeleton whose structure is stabilized by an intramolecular N–H⋅⋅⋅O hydrogen bond. The N-oxide molecules are connected together by weak C–H⋯O hydrogen bonds, an acceptor of which is the oxygen atom from the N-oxide group. This leads to creation of two-dimensional network of hydrogen bonds. Its IR, Raman, UV–Vis and luminescence spectra have been measured and analyzed on the basis of DFT and NBO quantum chemical calculations in which the B3LYP/6-311++G(d,p) approach was applied. The distribution of the electron levels in the studied compound has been analyzed in terms of the possibility of its participation in the ligand-to-lanthanide ion energy transfer.


Introduction
Pyridine derivatives belong to the exceptional class of heterocyclic compounds due to the variety of their appearance in nature, and their applications in different areas. They form a prosthetic group of many enzymes that play a significant role in living organisms. They are present in vitamins, such as pyridoxine and niacin, as well as in natural alkaloid nicotine. Simple pyridine derivatives appear as metabolites from bacteria and fungi [1]. Nikkomycins [2], piericidins [3], collismycins [4] belong to this class of compounds. New bioactive bacteria metabolites and new natural vitamin B6 analogues were obtained from terrestrial Streptomyces sp. and characterized [5]. Due their biological activity, they form basic units in thousands of drugs, pharmaceuticals, antibiotics, insecticides and medically significant substances. Pyridine derivatives also possess anti-inflammatory [6,7], antiviral [8,9], anticancer [10e14], antimicrobial activity [15,16], antidiabetic [17,18], osteogenic [19,20], antihypertensive [21] and perfumery applications [22].
Very prospective applications of these compounds originate from their antiviral activity. It was found that they are active against HIV-1, HIV-2 and human cytomegalovirus (HCMV) as well as they show antifungal properties [23e30]. Pyridine N-oxides also show inhibiting properties against several coronaviruses which suggests the necessity of investigating them pre-clinically in the future as a potential new class of antiviral drugs [31].
The pyridine N-oxide derivatives form unique class of compounds are products of the pyridine oxidation. They are used frequently as oxidizing reagents in organic syntheses [32,33]. Another huge potential of pyridine N-oxides and their derivatives is related to the possibilities of complexation of metal ions by the oxygen atom of the N-oxide group. The potential mode of action may include complexation of metal ions by the oxygen atom of heterocyclic N-oxides [34,35].
The present paper is a continuation of our studies on 2aminopyridine derivatives [36e40] and their N-oxides [41,42]. Herein, we present the X-ray single crystal structure of 2-Nmethylamino-3-methylpyridine N-oxide (MA3MPO) and the results of IR, Raman, UVeVis and luminescence spectra, complemented by an analysis on the basis of the DFT and NBO quantum chemical calculations. Application of the density functional theory for assignment of the IR and Raman bands to the respective normal modes of vibrations is particularly valuable and credible when the input data for these calculations use the structural parameters derived from the XRD studies, as this was done in this work. This allowed for evaluation of vibrational and electron energy levels of the studied compound, their assignment to respective transitions between them, electron density of the atoms characterized by Mulliken population parameters derived from MPA analysis and non-bonding orbitals (NBO). Quantum chemical calculations were performed using the Gaussian 03W program and the B3LYP method broadly applied in chemistry. As in our earlier papers, the B3LYP/6-311þþG (d,p) approach, that allows for comparison of the former [36e42] and present results, was used.
All these studies were undertaken to characterize the structural and spectroscopic properties of this kind of heterocyclic compounds, hoping that they can be used for complexation of the lanthanide ions which may form light emitting systems. The Noxide group substituted to the pyridine ring can act effectively as an electron donor and electron acceptor at the same time. For these reasons, these compounds appear as synthetic intermediates in the field of heterocyclic chemistry, protecting groups, auxiliary agents, catalysts and as complexing ligands of d-and f-metal ions.

Synthesis
2-N-Methylamino-3-methylpyridine N-oxide (MA3MPO) was synthesized from 2-amino-3-methylpyridine by means of several reactions shown in Fig. 1. The synthesis method leading to the 2chloro-3-methylpyridine N-oxide was described earlier [43]. The last step of this synthesis was nucleophilic substitution of the chlorine atom in the 2 position by the methylamine group. 2-Chloro-4-nitropicoline N-oxide reacts with the primary amines under reflux in methanol solution to give the 2-N-alkylaminopyridine derivatives. Lack of nitro substituent in the 4-position of the 2-chloro-3-methylpyridine N-oxide causes that substitution of chlorine with methylamine in the methyl solution does not take place. The use of an aqueous solution of methylamine and heating the reaction mixture to 120 C gives the expected product.
The reaction was carried out as follows: 5 g of 2-chloro-3methylpyridine N-oxide and 10 g of 40% aqueous solution of methylamine were dissolved in 50 cm 3 of water. The mixture was placed in the autoclave and was heated for 5 h at 120 C. Excess of methylamine and water was distilled off and allowed to crystallize. The result of this reaction, a colourless crystalline substance, was obtained in 63% yield. Its melting point equal to m p ¼ 106 C (379 K).
The composition of the obtained compound was established by microanalysis giving the following stoichiometry: calculated C e 60.84%; N e 20.28%; H e 7.24%, found C e 60.40%; N e 19.99%; H e 7.25%.

Single crystal X-ray diffractions
A block-shaped colourless crystal of MA3MPO suitable for single-crystal X-ray diffraction analysis was obtained by recrystallization from a methanol/ethanol diffusion solvent system in the air at room temperature. Crystallographic measurement was performed on a Kuma KM4-CCD automated four-circle diffractometer with graphite monochromatized Mo Ka radiation at 110(2) K using an Oxford Cryosystems cooler. The CRYSALIS program system was used for data collection, cell refinement and data reduction [44]. The structure was solved by direct methods using the SHELXS-97 program [45], and refined on F 2 by full-matrix least squares with anisotropic thermal parameters for all non-H-atoms using SHELXL-97 [46]. All H-atoms were initially located in difference Fourier maps, and in the final refinement cycles were treated as described below. All C-bound H atoms were placed in the calculated positions, with CeH ¼ 0.95e0.98 Å, and refined with the riding model with U iso (H) ¼ 1.2U eq (C) for CH or 1.5U eq (C) for CH 3 . The N-bonded H atom was refined freely, with U iso (H) ¼ 1.2U eq (N). The figures were made using the DIAMOND program [47]

IR and Raman studies
IR spectra were measured using the Nicolet iS50 FT-IR (Thermo Scientific) spectrometer equipped with an Automated Beamsplitter exchange system (iS50 ABX containing DLaTGS KBr detector and DLaTGS Solid Substrate detector for mid-IR and far-IR regions, respectively), built-in all-reflective diamond ATR module (iS50 ATR), Thermo Scientific Polaris™ and HeNe laser as an IR radiation source. The mid-IR spectra were collected in the 4000-100 cm -1 range in Nujol and Fluorolube mulls and KBr pellets. These spectra were compared to those recorded using the attenuated total reflection (ATR) technique with the advanced ATR correction software. This algorithm is a part of OMNIC™ 6.2 program attached to the Thermo Scientific Nicolet™ FT-IR spectrometer. The index of refraction was taken 1.5 characteristic for majority of organic materials. The ATR corrected IR spectra are identical to those obtained from transmission measurements [48]. The resolution of these measurements was 2.0 cm -1 .
Raman spectra (RS) in the 4000e80 cm -1 range were measured in back scattering geometry with a FT Bruker 110/S spectrometer. The resolution was 2.0 cm -1 . The YAG:Nd 3þ laser operating at wavelength 1064 nm was used as an excitation source.

UVeVis and luminescence studies
Room temperature electron absorption spectra were measured in the 200e1500 nm spectral range using a Cary-Varian 5E UV-VISnear-IR spectrophotometer. The reflectance spectra were recorded with a Praying Mantis diffuse reflectance accessories. In these measurements, the base line was first recorded for the Al 2 O 3 powder and, next, this line was subtracted from the obtained spectra for particular powder samples. The reflectance spectra were transformed into the absorption mode.
Excitation spectra were measured on a SSF-01 spectrofluorometer, equipped with PTI Felix software and ASOC 10 arrangement. The excitation source was a 150 W steady xenon lamp. Excitation spectra were corrected for the excitation light intensity.
Emission spectra were recorded with a grating spectrograph (Princeton Instr. Model Acton 2500i) coupled to a streak camera (Hamamatsu Model C5680) operating in the 200e1100 nm spectral range with temporal resolution of 20e100 ps.

Quantum chemical calculations
Geometry optimization of the studied compound was performed using Gaussian 03 program package [49], starting from Xray geometry and applying the B3LYP approach [50e52] with the 6-311þþG(d,p) basis set [53,54]. Its molecular structure and atomic description is presented in Fig. 2. Due to weak intermolecular interactions between the components of the unit cell, and the lack of factor group splitting in the experimental spectra, the calculations were performed for the monomeric molecule. The scaling factors were used to correct the evaluated wavenumbers for vibrational anharmonicity and deficiencies inherent to the used computational level. In the Potential Energy Distribution (PED) procedure the BALGA program was used [55]. The ChemCraft program was used for visualisation of the vectoring graphics of the normal modes [56]. The mean square deviation [57] between the experimental and calculated unscaled wavenumbers for MA3MPO was 7.7 cm -1 for the IR and 7.3 for the Raman spectra. The scaling of the calculated wavenumbers improves this result to 2.3 cm -1 for the IR and 2.7 for the Raman spectra. 0.9529 scaling factor was used for the range 3500e2500 cm -1 and 0.978 for the range 2499e0 cm -1 of the spectra. The theoretical Raman intensities were recalculated using the ChemCraft computer program. Table 1 lists selected geometrical parameters: bond lengths, bond angles and torsional angles. Experimental and calculated intra-and intermolecular hydrogen bonds as well as short contacts are compared in Table 2. The 2-N-methylamino-3-methylpyridine N-oxide (MA3MPO) crystallizes in the monoclinic space group P2 1 /n (Z ¼ 4), with one symmetry-independent molecule in the asymmetric unit (Fig. 2). The pyridine ring in the crystal is almost planar (rms deviation of fitted atoms ¼ 0.006 Å). Additionally, the methyl and the N-oxide groups lie almost within the pyridine ring plane. A slight distortion from this plane is observed for the Nmethylamine group with torsion angles N1eC2eN2eC7 ¼ 161.82 (9) and C3eC2eN2eC7 ¼ À21.05 (16) ( Table 1).

Crystal structure
The NeO bond length of 1.3342 (11) Å is longer in comparison to the previously studied 2-alkylaminopyridine N-oxides, for which this value is within the range of 1.301(2)-1.318(2) Å [41,42,58e60]. Elongation of the NeO bond distance is most likely related to the involvement of the oxygen atom as a multiple acceptor for one intra-and three intermolecular hydrogen bonds ( Table 2). Conformation of the MA3MPO molecule is stabilized by the intramolecular NeH/O [2.5266 (14) Å] hydrogen bond, between the hydrogen atom of the amine group and the oxygen atom of the NO group. Similar intramolecular hydrogen bond has been observed previously in these kinds of derivatives, and can be classified as medium strong [41,42,58e60].
The N-oxide molecules are connected together by three types of weak CeH/O hydrogen bonds. The first is created between the pyridine hydrogen and the oxygen atom from the N-oxide group  Table 2). This leads to creation of a two-dimensional network of hydrogen bonds in which the oxygen atom of the Noxide group of each molecule occurs as a multiple acceptor (Fig. 3).  Table 3 lists the observed and calculated wavenumbers together with their assignment to the respective normal modes based on PED data.

Vibrational data
The X-Ray data obtained for the studied compound reveal that the amino and the N-oxide groups are nearly coplanar with the pyridine ring plane. Therefore, the molecular structure of this compound could be described by the C s point group symmetry with the plane formed by pyridine ring, N]O bond and CeNeH(CeH) bonds of the N-methylamine group. 20 atoms of this molecule give rise to 60 of freedom that contain 3 rotational A' þ 2A 00 modes, 3 translational 2A' þ A 00 modes and 54 internal 35A' þ 19A 00 modes. The internal modes could be further subdivided into 18A' þ 2A 00 stretching and 17A' þ 17 A 00 bending modes. Because A' and A" modes are both IR and Raman active, 54 bands should be observed in both spectra. This is close to the number of bands seen in the IR spectrum (46 bands) and the Raman one (38 lines). Comparing these data, it is obvious that the Davidov splitting does not appear in our case and molecular C s (and C 1 ) symmetry is sufficient in our considerations. Although the medium strong intramolecular NeH,,,O HB (D,,,,A 2.527 Å) appears in the monomeric molecule (Fig. 2, Table 2), it does not influence the number of observed bands.

Pyridine ring and CeH vibrations
The assignment of the bands to the respective vibrations of pyridine rings was based on the nomenclature introduced by Urena et al.

Methyl groups vibrations
The assignment of the methyl group vibrations was based on the DFT calculations presented here and the experimental data derived from our previous studies of other pyridine derivatives [36e42].
Three fundamental stretching n as (CH 3 ) bands are within the range of 2886e3005 cm -1 and those of n s (CH 3 ) are within the range of 2820e2876 cm -1 . Bending vibrations appear within the range of 1430e1484 cm -1 for d as (CH 3 ) modes and 1384-1422 cm -1 for d s (CH 3 ) modes. The other bands for the methyl groups' vibrations are observed in the following ranges: 956-1059 cm -1 for r(CH 3 ) rocking; 1240-1244 cm -1 for n(f-CH 3 ); 680-690 cm -1 for g(f-CH 3 ); 283-340 cm -1 for d(f-CH 3 ), and 120-230 cm -1 for t(CH 3 ). In principle, the asymmetric and symmetric vibrations of the ring methyl group and attached to the amino group, described in Table 3 as (CH 3 )a and (CH 3 )b, lie within the same ranges, although the latter show a slight shift towards higher energy.

NeH vibrations
The assignment of the IR and Raman bands to the vibrations of the amino-group usually requires more attention. The vibrations of the C f -N-H,,,O moiety are described by 12 of freedom that includes 3 translations and 3 librations of the whole unit. Three of them refer to stretching vibrations and three correspond to the bending motions. However, due to the formation of the cyclic unit from the N-oxide and NeH groups, the number of modes in which the NeH bond is involved is significantly higher. The following,  Table 2 Intra-and intermolecular hydrogen bonds and short contacts (exp. and calc.) in MA3MPO.   [41,42].
Such broad wavenumber range of these vibrations also follows the engagement of the oxygen atom of this group in the hydrogen bond. In derivatives containing a planar nitro group at the 4position, the NeO bond of 1.301e1.305 and 1.315 Å length, mainly due to the return donation of the NO and NO 2 substituents, has the double-bond character. In the molecule analyzed, the bond length of 1.334 Å indicates that the NeO bond is an almost single bond, and therefore its stretching vibration band should appear at lower wavenumbers. The second reason for the NeO bond elongation is the intramolecular interaction between the oxygen atom of the -NO and the hydrogen atom of the -NHCH 3 groups located at vicinal positions of the pyridine ring. For the above reasons, one should expect a shifting of this band toward red.
The characteristic stretching vibration of this bond is usually located within the range of 1200e1320 cm -1 . For the compound studied here, this vibration appears as a strong IR band and as a medium Raman band at 1212 cm -1 . Our theoretical calculations, presented in this paper, predict this vibration within the range of 1317 -1212 cm -1 These values confirm the participation of the Noxide group in medium strong intramolecular hydrogen bond as found in X-Ray studies. The comparison of selected geometrical parameters of the N-oxide intramolecular hydrogen bond and the respective wavenumbers of MA3MPO with 2-N-alkylamino-3nitropyridine N-oxides [41,42] are presented in Table 4.

Intra-and intermolecular interactions
The complex contour in the 3200-3500 cm -1 range of the IR Table 3 The proposed assignments of the IR and Raman bands to respective normal modes on the basis potential energy distribution (PED In-plane vibrations: n e stretching; d e bending; r e rocking; out-of-plane vibrations: g e torsional; u e wagging; t e twisting; f e pyridine ring. Used scaling factor: fsc ¼ 0,9529 (4000e2000 cm À1 ) and 0,978 (2000e30 cm À1 ); a e pyridine methyl group; b e N-methylamine methyl group. Percentage of the intensity (IR;RS) is given in brackets.
spectrum and at about 3271 cm -1 of the Raman spectrum corresponds to the n(NH) vibrations of the amino group. It contains two components because two types of NeH bond probably appear in this molecule: engaged and non-engaged in the HB-s. The D$$$A distance of the former system was determined in the crystal structure as 2.527 Å and angle 120 . Its theoretical value is close to experimental and equals 2.539 Å and 118 , respectively, and the visualisation contains an intramolecular hydrogen bond in the calculated molecule.
The n(NH) vibrations are shifted from the theoretical values calculated for the isolated molecule at 3271 cm -1 (with scaling factor). Such shift agree with the HB theory described in the review by Bratos et al. [63]. The spectacular calculated stretching vibration of the O/H hydrogen bond, shown in Fig. 5 was found at 390 cm -1 . On the basis of the PED contributions, the experimental weak bands at 391 (396) cm -1 should be assigned to the stretching O/H vibration.
The X-ray studies propose the existence of additional CeH/O short contacts (c.a. 3.40 Å) between the oxygen atom of the N-oxide group and CH pyridine ring. The IR and Raman spectra and the quantum chemical calculations do not confirm the existence of such interactions between the adjacent molecules.

Electronic properties of the studied compound
Two p-electron units compose the structure of the studied compound: pyridine ring and the N-oxide group. The latter unit can act both as an electron-donor and electron-acceptor in relation to the pyridine ring. The N-methylamino unit, situated at a position adjacent to the N-oxide group, is a typical electron-donor substituent.
Electron density of the atoms in this compound is characterized by Mulliken population parameters derived from the MPA analysis and non-bonding orbitals (NBO). Atomic charges and NBO data calculated using these procedures, are summarized in Table 5. The results of these calculations will be used in the discussion regarding the MA3MPO electron states.
While analyzing the MPA and NBO data, the following conclusions can be made: -The negative values of Mulliken and NBO parameters were assigned for carbon atoms of the methyl groups, attached to the pyridine ring and amine group. The lower negative charge on the methyl group at C3 carbon of the pyridine ring originates from the electron-withdrawing effect of the pyridine ring (see Table 5). Table 4 Comparison of selected geometrical parameters of the N-oxide intramolecular hydrogen bond, and the respective wavenumbers of MA3MPO with 2-N-alkylaminopyridine Noxides [41,42]   -Similar behavior should be noted in the case of charges on nitrogen atoms of the amino group, for which the NBO charge (À0.63651 e) is significantly higher than Mulliken value (À0.127876 e).
-The hydrogen atom of the methylamino group, interacting with the HB oxygen of N-oxide group, exhibits the highest positive charge (þ0.42298 e) among other hydrogen atoms of the studied compound e this confirms the existence of HB interaction between these units.
For such system, the existence of eight mesomeric structures is possible which is presented in Fig. 6. It should be noted that the charge distribution of the MA3MPO molecule, i.e. the induction and mesomeric effects of substituents in pyridine-methylamino skeleton are properly reflected by the theoretical data. While analyzing the calculated Mulliken and NBO charges and mesomeric structures (see Table 5 and Fig. 6), it can be stated that among the presented structures, (7) and (8) are mostly favored.

Electron energy levels
It is expected that the studied compound should give rise to the p / p * electron transitions usually observed within the range of The conjugated molecules exhibit a small energy gap between the highest occupied molecular orbital HOMO and the lowest unoccupied molecular orbital LUMO. This is the result of a significant degree of intramolecular charge transfer (ICT) from the end capping electron-donor groups to the efficient electron-acceptor groups through p-conjugated path. HOMO level, lying at À5.67 eV, is localized on (lone pair) orbital of the oxygen atom of the NO group, whereas the LUMO level of the energy À1.19 eV is p* orbital localized on the pyridine ring. Electron transfer occurs from the electron-donor oxygen atom of the NO group to the electronwithdrawing part of pyridine ring. The HOMOeLUMO energy gap in MA3MPO, calculated at the B3LYP/6-311þþG(d,p) level, equals to 4.48 eV. Such HOMOeLUMO energy gap originates from the large stabilization of the LUMO orbital due to the strong electronacceptor ability of the pyridine ring.

Natural bond orbital analysis
NBO analysis has been performed for MA3MPO at the DFT/ B3LYP level using the NBO 3.1 program that is part of the Gaussian 03 package. It was expected that these evaluations would allow for  characterization of the role of the intramolecular hydrogen bond in stabilizing the structure and delocalization of electron density within the molecule. One of the methods for determination of energy and some properties of hydrogen bonds is the natural bonded orbitals method (NBO) [65e68]. The existence of the intramolecular hydrogen bond in the MA3MPO molecule is confirmed by the NBO presented in Table 6. It follows from the parameters characterizing interaction between the oxygen lone pair of the N-oxide group and NeH antibonding orbital of the amino group (LP (2) O2/s*N2eH2). Although the energetic contribution (6.55 kcal/mol) of hyperconjugative interactions is not significant, the E (2) value has chemical significance, and can be used as a measure of the intra-molecular electron delocalization in the molecule. The medium binding the strength of an intramolecular hydrogen bond is also confirmed by the high value of electron density (ED ¼ 0.03610 e) on the s*NeH orbital ( Table 6). While analysing the energies that follow from the second-order perturbation theory of Fock matrix strong intra-molecular hyperconjugative interactions are seen. The E (2) (donor / acceptor) energies are significant for the p * (C2eN1) bond and the values shown in Table 5 Table 6). Higher interaction energy E (2) is observed for electron donation from lone pair of the amino nitrogen atom LP(1) N2 to the antibonding acceptor p*(C2 e N1) (E (2) ¼ 46.29 kcal/mol).

UV eVIS absorption spectra
The electron absorption spectrum of the studied compound was recorded at room temperature in the solid state (Nujol mull). The spectrum is shown in Fig. 7. The contour observed within the range of 200e400 nm has been deconvoluted into nine Gaussian components with the maxima at 215, 238, 248, 261, 283, 318, 349, 364, 389 nm. The assignment of these wavelengths to the respective electron transitions was made on the basis of time depended TD-DFT method. The results of these evaluations are listed in Table 7 in which the energies of ten singlet and triplet states, oscillator strength and main contributions are summarized. The singlet states were localized at the following wavenumbers: S 1 32363; S 2 35840 S 3 36232; S 4 38168; S 5 40161; S 6 41494; S 7 42735; S 8 43290; S 9 43860 and S 10 44643 cm -1 and those of triplet states: at T 1 20161; T 2 28409 and T 3 29762 cm -1 . The respective electron transitions should be assigned to the following wavelength ranges: p / p * at 190e220 nm; n(O) / p * at 230e255 nm; n(N pyridine ) / p * at 280e300 nm and n(N amine ) / p * at 320e420 nm. The calculated energies of electron transitions are in good agreement with those reported in our earlier papers on other pyridine derivatives [42,69e72].

Emission spectra
The emission spectrum of the studied compound was measured in the solid state. It contains a very broad contour with the maximum at about 580 nm (Fig. 8). It is clearly shifted into red in relation to the S 0 / S 1 absorption transition. The observed contour probably originates from T 1 ,T 2 / S 0 emission that may proceed by the inter-system crossing S 1 / T 1 ,T 2 transition. Room temperature fluorescence is generally not observed from excited triplet states, however, having in mind the calculated energies of the Molecular Orbitals, we believe that such character of this transition is reasonable. According to the DFT calculations, the energies of the T 1 and T 2 states are 20161 cm -1 and 28409 cm -1 , respectively, i.e. are close to those expected for the emission from the triplet state. The small energetic splitting between these levels probably causes overlapping of these bands in the form of broad T 1 -T 2 contour. Besides, their red shift agrees with the expectations because both intra-and inter-molecular hydrogen bonds significantly affects the excited state properties. The excited-state hydrogen bonding influences nonadiabatic processes like internal conversion, Atom numbering is the same as presented in Fig. 1. LP ¼ lone electron pair orbital. The table presents the E (2) values greater than 5 kcal/mol, E (2) e denotes the hyperconjugative interactions, E (j) e E (i) is the energy difference between donor (i) and acceptor (j) orbitals, F(i,j) is the Fock matrix element between i and j NBO orbitals. intersystem crossing, intramolecular charge transfer and photoinduced electron transfer [73e79]. It should be noted that the Stokes shift for the studied sample is close to 8.250 cm -1 . Fig. 9 represents the system of ten singlet and three triplet states of MA3MPO.
The theoretical DFT calculations and experimental data show that for the studied compound the energy of the singlet states appears within the range of 309 nm (32,360 cm -1 ) e 224 nm (44,645 cm -1 ), and for the triplet states within the range of 490 nm (20,160 cm -1 ) e 330 nm (29,765 cm -1 ). These energy levels fit well those of europium(III) and terbium(III) ions, forming complexes for molecular imaging and sensing applied in chemical, biological and biomedical sciences [73e79].

2-N-methylamino-3-methylpyridine N-oxide (MA3MPO) is a
new compound for which we expect several prospective applications. It crystallizes in monoclinic P2 1 /n structure, and conformation of the MA3MPO molecule is stabilized by the intra-molecular NeH/O (2.527 Å and 120 ) hydrogen bond, between the hydrogen atom of the amine group and the oxygen atom of the N-oxide group. The respective geometrical parameters (2.539 Å and 118 ) obtained in the DFT quantum chemical calculations are close to experimental values. 2. The experimental NeO bond length (1.334 Å) is slightly longer than the calculated distance (1.298 Å) as well as those reported earlier for 2-alkylaminopyridine N-oxides (1.301e1.318 Å). This effect follows from the participation of this bond in the HB interactions. The X-ray studies also postulate the existence of weak CeH/O contacts between the N-oxide as acceptors and the H atoms of the pyridine and the methyl groups as donors. However, the IR and Raman spectra as well as the quantum chemical calculations do not confirm the presence of such interactions. 3. The quantum-chemical calculations conducted for isolated MA3MPO molecules locate the lowest singlet state assigned to the n / p* (CT) transitions overlapped with some contribution of the p / p* intramolecular charge transfer (ICT). The energy gap between these electron levels forms a specific sequence that may be applied for the ligand to lanthanide ion energy transfer. The depopulation mechanism of the excited states for the studied pyridine N-oxide is proposed. 4. The N-oxide group substituted to the pyridine ring can act effectively as an electron donor and an electron acceptor at the same time. For these reasons, the studied compound appears as a synthetic intermediate in the field of heterocyclic chemistry, protecting group, auxiliary agent, catalysts, and as complexing ligands for d-and f-metal ions. In particular, the MA3MPO could be used as a complexing ligand of europium and terbium ions in the luminescent probes of biological and medical applications. Table 7 Experimental and theoretical electronic transitions (with only major contribution) and oscillator strength for MA3MPO (H denotes the HOMO and L the LUMO orbitals).