IR spectra and phototautomerism of matrix isolated 4-oxopyrimidine

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).

Evidence of IR-Induced Chemistry in a Neat Solid: Tautomerization of Thiotropolone by Thermal, Electronic, and Vibrational Excitations

Thiotropolone isolated in argon and xenon matrices (as monomers) or in a neat solid (as the crystalline or amorphous state) at low temperature was found to exist only in the thione-enol form. Visible light irradiation (λ ≥ 400 nm) leads to thione-enol → thiol-keto tautomerization in matrices and under neat solid conditions at 15 K. The assignment of the IR spectra of the two thiotropolone tautomers (thione-enol and thiol-keto) was carried out with the support of B3LYP/6-311+G(2d,p) computations. The thiol-keto form generated in situ in a neat solid was found to tautomerize back to the thione-enol upon annealing up to 100 K. Gaussian-4 (G4) computations estimate that such a tautomerization process has an energy barrier of ∼25 kJ mol^-1, which is consistent with the observations. Moreover, it was found that narrowband IR irradiation of the thiol-keto form in a neat solid, at the frequency of its CH stretching overtones/combination modes, also induces tautomerization to the thione-enol form. Such a result constitutes an important demonstration of vibrationally induced chemistry under neat solid conditions.

Photochemical Generation of Benzoazetinone by UV Excitation of Matrix-Isolated Precursors: Isatin or Isatoic Anhydride

Benzoazetinone was photochemically generated by UV irradiation of isatin isolated in low-temperature Ar matrixes. Upon UV (λ = 278 nm) excitation of isatin, monomers of the compound underwent decarbonylation and the remaining part of the molecule adopted the benzoazetinone structure or the structure of its open-ring isomer α-iminoketene. The same products (benzoazetinone and α-iminoketene) were generated by UV (λ = 278 nm) induced decarboxylation of matrix-isolated monomers of isatoic anhydride. Photoproduced α-iminoketene appeared in the low-temperature matrixes as a mixture of syn and anti isomers. Photoproducts generated upon λ = 278 nm irradiation of matrix-isolated isatin were subsequently exposed to λ = 532 nm light. That irradiation resulted in the shift of the α-iminoketene–benzoazetinone population ratio in favor of the latter closed-ring structure. The next irradiation at 305 nm caused the shift of the α-iminoketene–benzoazetinone population ratio in the opposite direction, that is, in favor of the open-ring isomer. Neither benzoazetinone nor its α-iminoketene open-ring isomer was generated upon UV (λ = 278 nm) irradiation of phthalimide isolated in Ar matrixes. Instead, the UV-excited monomers of this compound underwent such phototransformations as oxo → hydroxy phototautomerism or degradation of the five-membered ring with release of HNCO and CO. The efficiency of these photoconversions was low.

UV-promoted radical formation, and near-IR-induced and spontaneous conformational isomerization in monomeric 9-methylguanine isolated in low-temperature Ar matrices

Three low-energy isomers of 9-methylguanine, the amino-oxo (AO) form and two amino-hydroxy (AH1 and AH2) conformers, were trapped from the gas phase into low-temperature argon matrices. The AH1 and AH2 isomers, differing in the orientation of the OH group, were found to transform into each other upon excitation with near-IR light. The population of the AO form of the compound was not changed upon any near-IR irradiation of the matrix samples. Using monochromatic near-IR light, generated by a frequency-tunable laser source, it was possible to selectively induce the AH1 → AH2 or AH2 → AH1 conversion. Photoreversibility of this conformational transformation was then demonstrated. Exposure of matrix-isolated monomers of 9-methylguanine to broadband near-IR light also led to conformational conversions within the amino-hydroxy tautomeric form; the final stage of this process was always the same photostationary state independent of the initial ratio of AH1 and AH2 populations. Spontaneous conformational conversion, transforming the higher-energy AH2 form into the lower-energy AH1 isomer, was observed for matrix-isolated monomers of 9-methylguanine kept in the dark. The mechanism of this process must rely on quantum tunneling of the light hydrogen atom. Irradiation of matrix-isolated 9-methylguanine with UV laser light at λ = 288 or 285 nm led to a substantial consumption of the two AH forms, while the amount of AO isomer remained unchanged. On the other hand, a decrease in the population of the AO isomer occurred upon excitations at shorter wavelengths, λ = 280 or 275 nm. The spectral changes observed after UV-irradiation suggest the generation (and stabilization in the matrix) of a radical species, resulting from the photocleavage of the O-H or N1-H bonds, in the AH or AO isomer, respectively.

Unraveling the Mechanisms of the Excited-State Intermolecular Proton Transfer (ESPT) for a D-π-A Molecular Architecture

Precise revealing the mechanisms of excited-state intermolecular proton transfer (ESPT) and the corresponding geometrical relaxation upon photoexcitation and photoionization remains a formidable challenge. In this work, the compound (E)-4-(((4H-1,2,4-triazol-4-yl)imino)methyl)-2,6-dimethoxyphenol (TIMDP) adopting a D-π-A molecular architecture featuring a significant intramolecular charge transfer (ICT) effect has been designed. With the presence of perchloric acid (35 %), TIMDP can be dissolved through the formation of a HClO₄ -H₂ O-OH(TIMDP)-N(TIMDP) hydrogen-bonding bridge. At the ground state, the ICT effect is dominant, giving birth to crystals of TIMDP. Upon external stimuli (e.g., UV light irradiation, electro field), the excited state is achieved, which weakens the ICT effect, and significantly promotes the ESPT effect along the hydrogen-bonding bridge, resulting in crystals of [HTIMDP]⁺ ⋅[H₂ O]⋅[ClO₄ ]^- . As a consequence, the mechanisms of the ESPT can be investigated, which distorted the D-π-A molecular architecture, tuned the emission color with the largest Stokes shift of 242 nm, and finally, high photoluminescence quantum yields (12 %) and long fluorescence lifetimes (8.6 μs) have achieved. These results not only provide new insight into ESPT mechanisms, but also open a new avenue for the design of efficient ESPT emitters.

UV-induced hydrogen-atom transfer and hydrogen-atom detachment in monomeric 7-azaindole isolated in Ar and n-H2 matrices

Upon UV excitation, the N1H form of 7-azaindole isolated in an Ar matrix transforms into N7H, C3H tautomers and the 7-azaindolyl radical; whereas only C3H and 7-azaindolyl radical products are photogenerated in solid H₂ environment.

Protonation effect on the electronic properties of 2-pyridone monomer, dimer and its water clusters: a theoretical study

The CC2 (second order approximate coupled cluster method) has been applied to investigate protonation effect on electronic transition energies of 2-pyridone (2PY), 2-pyridone dimer, and micro-solvated 2-pyridone (0-2 water molecules). The PE profiles of protonated 2-pyridone (2PYH(+)) as well as monohydrated 2PYH(+) at the different electronic states have been investigated. The (1)πσ∗ state in protonated species (2PYH(+)) is a barrier free and dissociative state along the O-H stretching coordinate. In this reaction coordinate, the lowest lying (1)πσ∗ predissociates the bound S1((1)ππ∗) state, connecting the latter to a conical intersection with the S0 state. These conical intersections lead the (1)ππ∗ state to proceed as predissociative state and finally direct the excited system to the ground state. Furthermore, in presence of water molecule, the (1)πσ∗ state still remains dissociative but the conical intersection between (1)πσ∗ and ground state disappears. In addition, according to the CC2 calculation results, it has been predicted that protonation significantly blue shifts the S1-S0 electronic transition of monomer, dimer, and microhydrated 2-pyridone.

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.

A computational study on the mechanism of intramolecular oxo-hydroxy phototautomerism driven by repulsive pi sigma* state

Potential energy (PE) surfaces of the lowest excited states of the 4(3H)-pyrimidinone/4-hydroxypyrimidine system were investigated with the aid of the CC2 and CASSCF methods of the electronic structure theory. These studies resulted in identification of a low-lying pi sigma* state, which is dissociative with respect to the stretching of the N-H or O-H bonds in the oxo and hydroxy structures of the compound, respectively. After initial excitation to the lowest local n pi* and/or pi pi* singlet states, the system can access the PE surface of the pi sigma* state by crossing a low barrier. It was computationally demonstrated that the system should evolve on the PE surface of the repulsive pi sigma* state toward a broad seam of intersection with the PE surface of the ground state. At the intersection, the nonadiabatic transition to the ground electronic state takes place and the system can either evolve to a minimum of the initially excited tautomer or to the ground-state minimum of the other tautomer. The steps listed above provide a mechanism of photoinduced dissociation-association (PIDA) phototautomerism, experimentally observed for a number of monomeric molecules, structurally similar to 4(3H)-pyrimidinone/4-hydroxypyrimidine. This mechanism describes a new class of intramolecular phototautomeric reactions driven by a repulsive pi sigma* state.

Systematic Effect of Benzo-Annelation on Oxo−Hydroxy Tautomerism of Heterocyclic Compounds. Experimental Matrix-Isolation and Theoretical Study

Oxo-hydroxy tautomerism and phototautomerism of 2-quinolinone, 1-isoquinolinone, 3-hydroxyisoquinoline, 2-quinoxalinone, and 4-quinazolinone were studied using the matrix-isolation technique. These compounds contain a benzene ring fused with a heterocyclic ring of 2-pyridinone, 2-pyrazinone, or 4-pyrimidinone. It turned out that direct attachment of a benzene ring to a heterocycle leads to a very pronounced increase of the relative stability of oxo tautomers (in comparison with the tautomerism of the parent compounds 2-pyridinone, 2-pyrazinone, and 4-pyrimidinone). The only exception concerns 3-hydroxyisoquinoline, where fusion with a benzene ring enforces rearrangement of the double- and single-bond system in the oxo tautomer. This destabilizes substantially the oxo form with respect to the hydroxy tautomer. The ratios of population of the oxo and hydroxy tautomers observed in Ar matrixes correspond to the tautomeric equilibria of the compounds in the gas phase. These equilibria were well reproduced by theoretical calculations carried out at the QCISD and QCISD(T) levels. The combined experimental and theoretical results reveal links between aromaticity and tautomerism. Moreover, a UV-induced phototautomeric reaction transforming the oxo forms into the hydroxy tautomers was observed for all (except 3-hydroxyisoquinoline) studied compounds. This photoeffect allowed separation of the IR spectra of the tautomers in question.

Dimerization and Double Proton Transfer-Induced Tautomerism of 4(3H)-Pyrimidinone in Solution Studied by IR Spectroscopy and Quantum Chemical Calculations

The tautomerism and dimerization of 4(3H)-pyrimidinone (4(3H)Pyr) in carbon tetrachloride (CCl4) and chloroform (CHCl3) solutions were investigated using IR spectroscopy and quantum chemical calculations. The observed IR spectra in the NH and OH stretching regions clearly revealed the predominance of the keto tautomer in both solvent systems. The enol form only exists in a very small proportion in the CCl4 solution. The tautomeric constant for the two monomers KT[OH/NH] = 0.012 and DeltaE = 2.62 kcal/mol were estimated at 25 degrees C. This result was supported by the self-consistent reaction field/polarizable continuum (SCRF/PCM) calculation at the MP4(full, SDQ)/aug-cc-pVDZ level, which predicted DeltaE = 3.06 kcal/mol in CCl4. In the C=O stretching region, two bands were observed, suggesting the coexistence of two keto structures at equilibrium. The calculated IR spectra indicated that the bands at 1711 and 1675 cm(-1) arise from the keto monomer and keto-keto (KK) ring dimer, respectively. At elevated temperature, the populations of both the keto and enol monomers increased for the CCl4 solution. The present study revealed that the keto <--> enol tautomerization does not occur in the isolated monomer molecule. The double proton transfer (DPT) reaction in the KK ring dimer presumably plays a substantial role in the population increase of the enol monomer. To our knowledge, this may be the first observation of the tautomerization in a model base pair via the temperature-induced ground-state DPT reaction under a nonpolar liquid environment reported so far. This tautomerism can serve as a mimic circumstance for the spontaneous mutations induced by proton transfer in the DNA base pairs.

UV-Induced Oxo → Hydroxy Unimolecular Proton-Transfer Reactions in Hypoxanthine

Monomers of hypoxanthine isolated in low-temperature Ar matrixes were studied using Fourier transform infrared spectroscopy. Two most stable tautomeric forms of hypoxanthine: oxo-N(9)-H and oxo-N(7)-H as well as a very small amount of the minor hydroxy-N(9)-H tautomer were observed in Ar matrixes directly after their deposition. UV irradiation of the matrixes induced conversion of the oxo-N(9)-H and oxo-N(7)-H tautomers of the compound into the hydroxy-N(9)-H and hydroxy-N(7)-H forms, respectively. Upon exposure of the matrixes to the UV (lambda > 270 nm) light, the oxo-N(9)-H --> hydroxy-N(9)-H phototautomeric reaction dominated strongly over the oxo-N(7)-H --> hydroxy-N(7)-H phototransformation. The latter phototautomeric reaction occurred effectively when matrix-isolated hypoxanthine was irradiated with shorter-wavelength (lambda > 230 nm) UV light. Thanks to this wavelength dependency, it was possible to clearly distinguish the oxo --> hydroxy photoreaction within the N(9)-H tautomers from the analogous phototautomeric process within the N(7)-H tautomers. All of the observed isomers of hypoxanthine (substrates and products of the photoreactions) were identified by comparison of their IR spectra with the spectra calculated at the DFT(B3LYP)/6-31++G(d,p) level of theory.

UV-induced generation of rare tautomers of allopurinol and 9-methylhypoxanthine — A matrix isolation FTIR study

Monomers of allopurinol and 9-methylhypoxanthine were studied using the matrix isolation technique combined with Fourier transform infrared spectroscopy. The oxo tautomeric forms of both compounds were found to dominate in freshly deposited low-temperature argon matrices. For 9-methylhypoxanthine, a small amount of the hydroxy tautomer was also detected in an Ar matrix before any irradiation. Upon exposure of the matrices to the UV (lambda>230 nm or lambda>270 nm) light, a proton transfer photoreaction converting the oxo forms of both compounds into the corresponding hydroxy tautomers occurred. Generation of conjugated ketenes as minor photoproducts was also observed. For 4(3H)-pyrimidinone (a model compound for both allopurinol and 9-methylhypoxanthine), photoreversibility of the UV-induced oxo --> hydroxy transformation was experimentally proven by direct observation of the back hydroxy --> oxo photoreaction. The substrates (oxo tautomers) and products (hydroxy tautomers) of the observed phototransformations were identified by comparison of their IR spectra with the spectra theoretically predicted at the DFT(B3LYP)/6-31++G(d,p) level. The IR bands in the experimental spectra were assigned to the calculated normal modes.

Mechanochromism of Piroxicam Accompanied by Intermolecular Proton Transfer Probed by Spectroscopic Methods and Solid-Phase Changes

Structural and solid-state changes of piroxicam in its crystalline form under mechanical stress were investigated using cryogenic grinding, powder X-ray diffractometry, diffuse-reflectance solid-state ultraviolet-visible spectroscopy, variable-temperature solid-state (13)C nuclear magnetic resonance spectroscopy, and solid-state diffuse-reflectance infrared Fourier transform spectroscopy. Crystalline piroxicam anhydrate exists as colorless single crystals irrespective of the polymorphic form and contains neutral piroxicam molecules. Under mechanical stress, these crystals become yellow amorphous piroxicam, which has a strong propensity to recrystallize to a colorless crystalline phase. The yellow color of amorphous piroxicam is attributed to charged piroxicam molecules. Variable-temperature solid-state (13)C NMR spectroscopy indicates that most of the amorphous piroxicam consists of neutral piroxicam molecules; the charged species comprise only about 8% of the amorphous phase. This ability to quantify the fractions of charged and neutral molecules of piroxicam in the amorphous phase highlights the unique capability of solid-state NMR to quantify mixtures in the absence of standards. Other compounds of piroxicam, which are yellow, are known to contain zwitterionic piroxicam molecules. The present work describes a system in which proton transfer accompanies both solid-state disorder and a change in color induced by mechanical stress, a phenomenon which may be termed mechanochromism of piroxicam.

Spectroscopic identification of 2,4-pyrimidinedithiol; an experimental matrix isolation and ab initio Hartree-Fock and density functional theory study

2,4-Pyrimidinedithiol (the dithiol form of 2,4-dithiouracil) was generated by UV (lambda > 335 nm) irradiation of the dithione form of 2,4-dithiouracil isolated in low-temperature argon or nitrogen matrices. The IR and UV spectra of the photoproduct are reported. The dithiol form of 2,4-dithiouracil was identified by comparison of the experimental spectrum with the spectrum theoretically predicted for this form at the HF/6-31 (d,p) and DFT(B-3LYP)/6-31 (d,p) levels. This comparison resulted also in assignment of the bands observed in the IR spectrum of 2,4-pyrimidinedithiol to the theoretically predicted normal modes.

X-ray Crystal Structures and ab Initio Calculations on the Photochemically Formed Dewar Isomers of the 4(3H)-Pyrimidinone Derivatives

The structures of the 5-oxo-2,6-diazabicyclo[2.2.0]hex-2-enes (imine Dewar pyrimidinones) 2a-h have been studied by X-ray diffraction analysis and ab initio calculations at the HF/6-31G(d,p) and MP2/6-31G(d,p) levels of theory. The crystal structures of 1-alkyl-3-tert-butyl-6-methyl imine Dewar pyrimidinones 2a-c have been determined at low temperature. The X-ray diffraction studies revealed that both the 2-azetidinone and dihydroazete rings are almost planar, and their eight bond angles are nearly 90 degrees (81-100 degrees ). The C1-N2(=C3) bond distance in the dihydroazete ring is longer due to the bond angle strain by ca. 0.06 Å than that of a typical straight-chain imine molecule. Full geometry optimizations on the imine Dewar isomers (2a and 2d-h) show deviations between the HF and MP2 geometries. The MP2 structure of the 3-tert-butyl-1,6-dimethyl imine Dewar pyrimidinone 2a is compared with those of the X-ray diffraction. The results of the calculations are found to be in good agreement with the X-ray diffraction data. The full geometry optimizations are necessary with the inclusion of electron correlation for the highly strained molecules. The ab initio calculations of the 2-azetidinone 9, 3,4-dihydroazete 11, and N-ethylidenemethylamine 12 have been carried out at the HF and MP2/6-31G(d,p) levels of theory to compare with the structures, orbital hybridization, bond orders, and charge distributions of the Dewar pyrimidinones 2. The theoretical results reveal that the MP2 C1-N2 bond distance of the Dewar pyrimidinone 2a is consistent with the abnormally elongated X-ray C1-N2 bond distance. The electronegative N2 and N6 atoms in the Dewar pyrimidinones 2 give the great positive charge on the C1 atom. The smaller bond angles (ca. 90 degrees ) in the Dewars 2 than typical sp(2) and sp(3) bond angles (ca. 120 and 109 degrees ) increase p character in the endocyclic bonds and decrease p character in the exocyclic bonds.