Photoinduced amino–imino tautomerism of 2-aminopyridine in a low-temperature argon matrix

On Prototropy and Bond Length Alternation in Neutral and Ionized Pyrimidine Bases and Their Model Azines in Vacuo

In this review, the complete tautomeric equilibria are derived for disubstituted pyrimidine nucleic acid bases starting from phenol, aniline, and their model compounds-monosubstituted aromatic azines. The differences in tautomeric preferences for isolated (gaseous) neutral pyrimidine bases and their model compounds are discussed in light of different functional groups, their positions within the six-membered ring, electronic effects, and intramolecular interactions. For the discussion of tautomeric preferences and for the analysis of internal effects, recent quantum-chemical results are taken into account and compared to some experimental ones. For each possible tautomer-rotamer of the title compounds, the bond length alternation, measured by means of the harmonic oscillator model of electron delocalization (HOMED) index, is examined. Significant HOMED similarities exist for mono- and disubstituted derivatives. The lack of parallelism between the geometric (HOMED) and energetic (ΔG) parameters for all possible isomers clearly shows that aromaticity is not the main factor that dictates tautomeric preferences for pyrimidine bases, particularly for uracil and thymine. The effects of one-electron loss (positive ionization) and one-electron gain (negative ionization) on prototropy and bond length alternation are also reviewed for pyrimidine bases and their models.

Alkynyl Ligands as Building Blocks for the Preparation of Phosphorescent Iridium(III) Emitters: Alternative Synthetic Precursors and Procedures

Alkynyl ligands stabilize dimers [Ir(μ-X)(3b)₂]₂ with a cis disposition of the heterocycles of the 3b ligands, in contrast to chloride. Thus, the complexes of this class—cis-[Ir(μ₂-η²-C≡CPh){κ²-C,N-(C₆H₄-Isoqui)}₂]₂ (Isoqui = isoquinoline) and cis-[Ir(μ₂-η²-C≡CR){κ²-C,N-(MeC₆H₃-py)}₂]₂ (R = Ph, ^tBu)—have been prepared in high yields, starting from the dihydroxo-bridged dimers trans-[Ir(μ-OH){κ²-C,N-(C₆H₄-Isoqui)}₂]₂ and trans-[Ir(μ-OH){κ²-C,N-(MeC₆H₃-py)}₂]₂ and terminal alkynes. Subsequently, the acetylide ligands have been employed as building blocks to prepare the orange and green iridium(III) phosphorescent emitters, Ir{κ²-C,N-[C(CH₂Ph)Npy]}{κ²-C,N-(C₆H₄-Isoqui)}₂ and Ir{κ²-C,N-[C(CH₂R)Npy]}{κ²-C,N-(MeC₆H₃-py)}₂ (R = Ph, ^tBu), respectively, with an octahedral structure of fac carbon and nitrogen atoms. The green emitter Ir{κ²-C,N-[C(CH₂^tBu)Npy]}{κ²-C,N-(MeC₆H₃-py)}₂ reaches 100% of quantum yield in both the poly(methyl methacrylate) (PMMA) film and 2-MeTHF at room temperature. In organic light-emitting diode (OLED) devices, it demonstrates very saturated green emission at a peak wavelength of 500 nm, with an external quantum efficiency (EQE) of over 12% or luminous efficacy of 30.7 cd/A.

Acetylide ligands have been employed as building blocks to prepare orange and green iridium(III) phosphorescent emitters, with an octahedral structure of fac carbon and nitrogen atoms. In OLED devices, the emitter Ir{κ²-C,N-[C(CH₂^tBu)Npy]}{κ²-C,N-(MeC₆H₃-py)}₂ demonstrates very saturated green emission at a peak wavelength of 500 nm, with a luminous efficacy of 30.7 cd/A.

FTIR spectroscopic evidence for new isomers of 3-aminopyrazine-2-carboxylic acid formed in argon matrices upon UV irradiations

The UV-induced photochemistry and molecular structure of 3-aminopyrazine-2-carboxylic acid were studied in argon matrices by Fourier-transform infrared spectroscopy and B3LYP/6-311++G(2d,2p) calculations. Out of seventeen possible isomers of this molecule located on the singlet potential energy surface the most stable one, APA1 comprising intramolecular O-H···N and N-H···O hydrogen bonds, was detected experimentally in the matrix after deposition. Two new conformers APA2 and APA3 were generated upon irradiation with λ = 280 nm by trans/cis-COOH isomerization and at λ = 360 nm by COOH group rotamerization, respectively, whereas an amino-imino tautomerization leading to IPA1 and IPA2 structures occurred at λ = 305 nm. The reverse reactions were also observed upon irradiation of the matrices at 265, 230 and 400 nm. Simultaneously with the photoisomerizations, a cleavage of the pyrazine ring along with CO₂ elimination was observed leading to the formation of carbodiimide and cyanamide derivatives.

Effect of a Solid-Hydrogen Environment on UV-Induced Hydrogen-Atom Transfer in Matrix-Isolated Heterocyclic Thione Compounds

To shed more light on the mechanisms of UV-induced hydrogen-atom-transfer processes in heterocyclic molecules, phototautomeric thione → thiol reactions were investigated for thione compounds isolated in low-temperature Ar as well as in n-H₂ (normal hydrogen) matrices. These studies concerned thione compounds with a five-membered heterocyclic ring and thione compounds with a six-membered heterocyclic ring. The experimental investigation of 2-thioimidazole and 3-thio-1,2,4-triazole (thione compounds with a five-membered heterocyclic ring) revealed that for the compounds isolated in solid n-H₂ only trace amounts of thiol photoproducts were photogenerated; even though for the same compounds isolated in the solid Ar matrix, the thione → thiol photoconversion was nearly total. In contrast to that, for 3-thiopyridazine and 2-thioquinoline (thione compounds with a six-membered heterocyclic ring) isolated in solid n-H₂, the UV-induced thione → thiol conversion occurred with the yield reaching 25–50% of the yield of the analogous process observed for the same species isolated in solid Ar. The obtained experimental results allow us to conclude that the dissociation–association mechanism nearly exclusively governs the phototransformation in thione heterocycles with high barriers for tautomerization (such as thione compounds with a five-membered ring), whereas the strictly intramolecular hydrogen-atom shift contributes to the mechanism of hydrogen-atom transfer in thione heterocycles with lower barriers (such as thione compounds with a six-membered ring).

Solvent polarity dependent excited state hydrogen bond effects and intramolecular double proton transfer mechanism for 2-hydroxyphenyl-substituted benzo[1,2-d:4,5-d']bisimidazole system

In this work, we probe into the photo-induced excited state hydrogen bonding interactions and excited state proton transfer (ESPT) behaviors for a representative benzo[1,2-d:4,5-d']bisimidazole derivative (i.e., 2-hydroxyphenyl-substituted benzo[1,2-d:4,5-d']bisimidazole (HPBB)) compound. In view of aprotic solvents with different polarities, cyclohexane (CYH), dichloromethane (DCM) and acetonitrile (MeCN) solvents are considered. Analyzing hydrogen-bond geometrical parameters, infrared (IR) vibrational spectra, Mayer bond order and predicting hydrogen bonding energy (E_(HB)), we verify dual hydrogen bonds of HPBB are strengthened in S₁ state. Particularly, in nonpolar solvent, the enhanced excited state hydrogen bonds become more obvious. The intriguing charge redistribution and frontier molecular orbitals (MOs) reveal hydrogen bonding acceptance ability of acceptor moieties becomes stronger, which plays a crucial role in capturing hydroxyl proton via photoexcitation. To check and explore ESIPT mechanism, we present the solvent polarity dependent asynchronous excited state intramolecular double proton transfer (ESIDPT) mechanism. That is, nonpolar solvent promotes excited state intramolecular single proton transfer (ESISPT) process for HPBB, while polar solvent contributes to ESIDPT behavior with the primary single proton-transfer product in S₁ state. This work not only makes a rational attribution to experimental phenomena, but also clarifies detailed excited state behaviors for HPBB and presents regulating ESIPT mechanism via solvent polarity.

Combinations of Tautomeric Forms and Neutral-Cationic Forms in the Cocrystals of Sulfamethazine with Carboxylic Acids

The cocrystals of sulfamethazine with different acids, namely, 2-mercaptophenylcarboxylic acid, 2,6-pyridinedicarboxylic acid, 4-(4-hydroxyphenylazo)phenylcarboxylic acid, 3-(4-hydroxyphenyl)propanoic acid, and 4-(phenyl)phenylcarboxylic acid, are studied here. Each has distinct notable supramolecular features. The pyrimidin-2-amine unit of the sulfamethazine provided unique examples of cocrystals in which amidine and imidine forms or neutral and protonated forms of sulfamethazine are observed in 2:2 ratios. Hence, this study provides avenues to explore cocrystals with tautomeric forms together in a cocrystal and also neutral and protonated cocrystal partners as apparent multicomponents in cocrystals. Among the cocrystals, three of them have the amidine form of the sulfamethazine in respective self-assembly. The cocrystal of 2-mercapto-phenylcarboxylic acid with sulfamethazine has the amidine form and it has the distinction of having S-H···π interactions. The cocrystal of sulfamethazine with 2,6-pyridinecarboxylic acid is a rare example of a 1:1 cocrystal of sulfmethazine with dicarboxylic acid. It has methanol molecules as a solvent of crystallization. Sulfamethazine forms a hydrated cocrystal with 4-(4-hydroxyphenylazo)-phenylcarboxylic acid that has conventional R 2 2(8) synthons of amidine hydrogen-bonding with carboxylic acid. The phenolic part of the acid component is anchored to the water molecule and provides a robust self-assembly. The hydrated cocrystal of sulfamethazine with 3-(4-hydroxyphenyl)propanoic acid (2:2 cocrystal) has two independent molecules of sulfamethazine, one in amidine form and the other in imidine form. It has two neutral carboxylic acids anchored through complementary hydrogen bonds and also has two water molecules of crystallization. The cocrystal of sulfamethazine with 4-(phenyl)phenylcarboxylic acid is also a 2:2 cocrystal. It is a di-hydrate, which has a neutral and protonated form of sulfamethazine. The neutral form is hydrogen-bonded to a neutral carboxylic acid, whereas the protonated form is charge-assisted hydrogen-bonded to the corresponding carboxylate anion.

Detailed theoretical investigation of excited-state intramolecular proton transfer mechanism of a new chromophore II

In the present work, TDDFT has been used to investigate the excited state intramolecular proton transfer (ESIPT) mechanism of a new chromophore II [Sensors and Actuators B: Chemical. 202 (2014) 1190]. The calculated absorption and fluorescence spectra agree well with experimental results. In addition, two types of II configurations are found in the first excited state (S1), which can be ascribed to the ESIPT reaction. Based on analysis of the calculated infrared (IR) spectra of O-H stretching vibration as well as the hydrogen bonding energies, the strengthening of the hydrogen bond in the S1 state has been confirmed. The frontier molecular orbitals (MOs), Hirshfeld charge distribution and the Natural bond orbital (NBO) have also been analyzed, which displays the tendency of the ESIPT process. Finally, potential energy curves of the S0 and S1 states were constructed, demonstrating that the ESIPT reaction can be facilitated based on the photo-excitation.

Tautomerization, molecular structure, transition state structure, and vibrational spectra of 2-aminopyridines: a combined computational and experimental study

Background

2-amino pyridine derivatives have attracted considerable interest because they are useful precursors for the synthesis of a variety of heterocyclic compounds possessing a medicinal value. In this work we aim to study both structural and electronic as well as high quality vibrational spectra for 2-amino-3-methylpyridine (2A3MP) and 2-amino-4-methylpyridine (2A4MP).

Results

Møller–Plesset perturbation theory (MP2/6-31G(d) and MP2/6-31++G(d,p) methods were used to investigate the structure and vibrational analysis of (2A3MP) and (2A4MP). Tautomerization of 2A4MP was investigated by Density Functional Theory (DFT/B3LYP) method in the gas phase. For the first time, all tautomers including NH → NH conversions as well as those usually omitted, NH → CH and CH → CH, were considered. The canonical structure (2A4MP1) is the most stable tautomer. It is 13.60 kcal/mole more stable than the next (2A4MP2). Transition state structures of pyramidal N inversion and proton transfer were computed at B3LYP/6-311++G(d,p). Barrier to transition state of hydrogen proton transfer is calculated as 44.81 kcal/mol. Transition state activation energy of pyramidal inversion at amino N is found to be 0.41 kcal/mol using the above method. Bond order and natural atomic charges were also calculated at the same level. The raman and FT-IR spectra of (2A3MP) and (2A4MP) were measured (4000–400 cm⁻¹). The optimized molecular geometries, frequencies and vibrational bands intensity were calculated at ab initio (MP2) and DFT(B3LYP) levels of theory with 6-31G(d), 6-31++G(d,p) and 6-311++G(d,p) basis sets. The vibrational frequencies were compared with experimentally measured FT-IR and FT-Raman spectra.

Conclusion

Reconsidering the vibrational analysis of (2A3MP) and (2A4MP) with more accurate FT-IR machine and highly accurate animation programs result in new improved vibrational assignments. Sophisticated quantum mechanics methods enable studying the transition state structure for different chemical systems.

Electronic supplementary material

The online version of this article (doi:10.1186/s40064-015-1363-2) contains supplementary material, which is available to authorized users.

Role of topological charge stabilization in protomeric tautomerism

Protomeric tautomerism is analyzed in view of the topological charge stabilization rules. Based on Hückel molecular orbital considerations and modern DFT calculations, it was found that the branching of amino or hydroxyl groups significantly contributes to the stability of major species through the first- and second-order perturbations with respect to the isoelectronic hydrocarbon. While amino-imino tautomerism is almost completely dominated by topological charge stabilization, hydroxyl-oxo tautomerism is affected by changes in the resonance integral of C-O/C═O bonds. Nevertheless, apart from side effects such as hydrogen bonds or solvent effects, a quantitative preference rule for the prediction of the tautomeric stability can be developed using topological π-electron energetics. As well as the analyses of simple bases, applications to complex or extended systems are exemplified analyzing purine bases, polyguanide, and polyuret. The present approach can be useful in conjunction with chemical intuition that comes from conventional valence bond theory.

Multiple hydrogen bonding in excited states of aminopyrazine in methanol solution: time-dependent density functional theory study

Aminopyrazine (AP) and AP-methanol complexes have been theoretically studied by using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The excited-state hydrogen bonds are discussed in detail. In the ground state the intermolecular multiple hydrogen bonds can be formed between AP molecule and protic solvents. The AP monomer and hydrogen-bonded complex of AP with one methanol are photoexcited initially to the S2 state, and then transferred to the S1 state via internal conversion. However the complex of AP with two methanol molecules is directly excited to the S1 state. From the calculated electronic excited energies and simulated absorption spectra, we find that the intermolecular hydrogen bonds are strengthened in the electronic excited states. The strengthening is confirmed by the optimized excited-state geometries. The photochemical processes in the electronic excited states are significantly influenced by the excited-state hydrogen bond strengthening.

An amino–imino resonance study of 2-amino-4-methylpyridinium nitrate and 2-amino-5-methylpyridinium nitrate

The contributions of the amino and imino resonance forms to the ground-state structures of 2-amino-4-methylpyridinium nitrate, C6H9N2+·NO3−, and the previously reported 2-amino-5-methylpyridinium nitrate [Yan, Fan, Bi, Zuo & Zhang (2012).Acta Cryst.E68, o2084], were studied using a combination of IR spectroscopy, X-ray crystallography and density functional theory (DFT). The results show that the structures of 2-amino-4-methylpyridine and 2-amino-5-methylpyridine obtained upon protonation are best described as existing largely in the imino resonance forms.

DFT studies on one-electron oxidation and one-electron reduction for 2- and 4-aminopyridines

Quantum-chemical calculations {DFT(B3LYP)/6-311+G(d,p)} were performed for all possible tautomers (aromatic and nonaromatic) of neutral 2- and 4-aminopyridines and their oxidized and reduced forms. One-electron oxidation has no important effect on the tautomeric preference for 2-aminopyridine. The amine tautomer is favored. However, oxidation increases the stability of the imine NH tautomer, and its contribution in the tautomeric mixture cannot be neglected. In the case of 4-aminopyridine, one-electron oxidation increases the stability of both the amine and imine NH tautomers. Consequently, they possess very close energies. As major tautomers, they dictate the composition of the tautomeric mixture. The CH tautomers may be considered as very rare forms for both neutral and oxidized aminopyridines. A reverse situation takes place for the reduced forms of aminopyridines. One-electron reduction favors the C3 atom for the labile proton for both aminopyridines. This may partially explain the origin of the CH tautomers for the anionic states of nucleobases containing the exo NH₂ group.

Five isomers of monomeric cytosine and their interconversions induced by tunable UV laser light

Photoisomerization processes involving five isomers of cytosine were induced by narrowband tunable UV irradiation of matrix-isolated monomers of the compound. Irradiation of an argon matrix containing cytosine monomers with UV λ = 313 nm laser light resulted in syn↔anti photoisomerizations between the two imino-oxo forms, whereas the substantially more populated amino-hydroxy and amino-oxo forms stayed intact. Subsequent irradiation with shorter-wavelength UV λ = 311 nm laser light led to two concomitant phototautomeric processes consuming the amino-oxo isomer: (i) an oxo → hydroxy hydrogen-atom transfer photoprocess converting the amino-oxo form into the amino-hydroxy tautomer; (ii) amino → imino hydrogen-atom transfer converting the amino-oxo form into the imino-oxo isomers. The UV-induced phototransformations, together with mutual conversions of the two amino-hydroxy conformers induced by irradiation with narrowband NIR light, allowed positive detection and identification of the five isomeric forms of monomeric cytosine. This is the first experimental observation of all five low-energy isomers of cytosine.

Photoinduced Amino−Imino Tautomerism: An Infrared Study of 2-Amino-5-methylpyridine in a Low-Temperature Argon Matrix

The photoreaction of 2-amino-5-methylpyridine was investigated by matrix-isolation infrared spectroscopy and DFT calculation. Photoinduced reversible amino (N=C-NH(2))-imino (NH-C=NH) tautomerism was found between 2-amino-5-methylpyridine and 5-methyl-2(1H)-pyridinimine; the amino tautomer changes to the imino tautomer by UV irradiation (340>lambda>or= 300 nm) and the reverse change occurs by longer-wavelength light irradiation (420>lambda>or= 340 nm). The results of the CASSCF calculation revealed that the amino-imino tautomerism proceeds in vibrational relaxation process from electronic excited state to the ground state. The IR spectra of 2-amino-5-methylpyridine in the T(1) state and 5-methyl-2-pyridinamino radical were also obtained by UV irradiation (lambda>or= 300 nm).