Electron propagator and coupled-cluster calculations on the photoelectron spectra of thiouracil and dithiouracil anions

Electron affinities, vertical electron detachment energies, and isomerization energies of 4-thiouracil, 2-thiouracil, and 2,4-dithiouracil and their valence anions have been calculated with ab initio electron propagator and other many-body methods. Anions in which protons have been transferred to the C5 from the N1 or N3 ring positions have been considered, but the canonical forms are most stable for the 4-thiouracil and 2,4-dithiouracil anions. Electron affinities of 0.61, 0.26, and 0.87 eV have been determined for 4-thiouracil, 2-thiouracil, and 2,4-dithiouracil, respectively. Electron propagator calculations on the canonical anions yield vertical electron detachment energies that are in close agreement with experimental peaks at 1.05, 3.21, and 3.32 eV for 4-thiouracil and at 1.4 eV for 2,4-dithiouracil.     

Association of Cu2+ with uracil and its thio derivatives: a theoretical study.

The structures and relative stabilities of the complexes between Cu2+ and uracil, 2-thiouracil, 4-thiouracil, and 2,4-dithiouracil were investigated by B3LYP/6-311+G(2df,2p)//B3LYP/6-31G* DFT calculations. In those systems in which both types of basic centers, that is, a carbonyl and a thiocarbonyl group, are present, association of Cu2+ with the oxygen atom is systematically favored, in contrast to what was found for the corresponding Cu+ complexes. This can be understood by considering that association of Cu2+ is immediately followed by oxidation of the base, which accumulates the negative charge at the oxygen atoms. Similarly, for 2,4-dithiouracil the most basic site for Cu+ attachment is the sulfur atom at the 4-position, while for association of Cu2+ it is sulfur at the 2-position. In contrast, differences between uracil-Cu+ and uracil-Cu2+ complexes are very small, and in both cases the oxygen atom at the 4-position is the most basic. Cu2+ binding energies are about 4 and 1.2 times larger than Cu+ binding energies and proton affinities, respectively. Uracil- and thiouracil-Cu2+ complexes are thermodynamically unstable but kinetically stable with respect to their dissociation into uracil*+ + Cu+ or thiouracil*+ + Cu+. The Cu2+ binding energies vary with the difference between the second ionization potential of the metal and the first ionization potential of the base. regardless of the reference acid (H+, Cu+, Cu2+) the basicity trend is 2,4-dithiouracil > 4-thiouracil > 2-thiouracil > uracil.     

Theoretical Studies on the Binding of Cd~（2＋） to Uracil and Its Thio-derivatives

The interaction of Cd2+ with uracil,2-thiouracil,4-thiouracil and 2,4-dithiouracil have been investigated by the density functional theory(DFT) calculations.For uracil and 2,4-dithiouracil,where the two basic sites are the same,Cd2+ attachment to the heteroatom at position 4 is preferred.However,for the systems where both types of basic centers,a carbonyl and a thiocarbonyl groups,are present,Cd2+ association with sulfur is favorable.The enhanced stability of these enolic and thiol forms comes from Cd2+ interaction with two basic sites simultaneously,which thereby triggers a significant aromatization of the ring.More significantly,the Cd2+ binding energy with uracil and its thio-derivatives is larger than the tautomerization barriers connecting the diketo-like forms with the corresponding enolic-like tautomers.Consequently,when associated with Cd2+,all tautomers are energetically accessible and should be observed in the gas phase.     

Theoretical studies on the properties of uracil and its dimer upon thioketo substitution

The influences of thioketo substitution on the properties of uracil monomer and dimer and their interactions with Zn²⁺ have been systematically investigated at the B3LYP/6-311+G^*level of theory. Those properties include the structural characteristics, acidities, ionization potentials, and singlet–triplet energy gaps of SU monomers and their dimers, where SU=2-thiouracil, 4-thiouracil, and 2,4-dithiouracil, respectively. Computational results suggest that thioketo substitution leads to an increase in the acidities of the N-H groups for both uracil and its dimer, where the N₁–H group is still the most acidic site relative to that of N₃–H group. However, the opposite behaviors are true for the ionization potentials and the singlet–triplet energy gaps of uracil monomer and its dimer, suggesting that thiouracils are more susceptible to radiation damage relative to the unsubstituted uracil. For uracil and 2-thiouracil, the corresponding triplet excited-state geometries are predicted to be highly nonplanar compared with the planar geometries of the ground state as well as 4-thiouracil and 2,4-dithiouracil upon triplet excitation. As a rule, the intermolecular H-bonds involving the sulfur atom directly have been influenced more significant than those the oxygen atom directly involved for U::U and SU::SU base pairs upon ionization and excitation. Additionally, Zn²⁺ binding is expected to lead to an increase in the stability of U::U and SU::SU base pairs.     

THE ELECTRONIC SPECTROSCOPY OF PYRIMIDINES: THE EFFECT OF COVALENTLY BONDED SULFUR ON THE PHOSPHORESCENCE AND ABSORPTION SPECTRA

Abstract— The phosphorescence of uracil, 2-thiouracil, 2 ,4-dithiouracil and 2-mercaptopyrimidine was studied at 77 K. 2-Thiouracil and 2,4-dithiouracil showed phosphorescence quantum yields of 0.65 and 0. 1 , respectively. The emitting triplet states of these compounds have been assigned as the ³(π, π*) type. The enhancement of spin-orbit coupling through the involvement of the 3d orbitals on sulfur has been invoked in describing emission characteristics of thiouracils.     

Gas-phase deprotonation of uracil-Cu2+ and thiouracil-Cu2+ complexes

The deprotonation of Cu2+ complexes with uracil, 2-thiouracil, 4-thiouracil, and 2,4-dithiouracil has been investigated by means of B3LYP/ 6-311+G(2df,2p)//6-31G(d) calculations. The most stable [(uracil-H)Cu]+ and [(thiouracil-H)Cu]+ complexes correspond to bidentate structures in which Cu interacts with the deprotonated ring-nitrogen atom and with the oxygen or the sulfur atom of the adjacent carbonyl or thiocarbonyl group. For 2- and 4-thiouracil derivatives, the structures in which the metal cation interacts with the thiocarbonyl group are clearly favored with respect to those in which Cu interacts with the carbonyl group. This is at variance with what was found to be the most stable structure of the corresponding Cu2+ complexes, where association to the carbonyl oxygen was always preferred over the association to the thiocarbonyl group. The [(uracil-H)Cu]+ and [(thiouracil-H)Cu]+ complexes can be viewed as the result of Cu+ attachment to the uracil-H and thiouracil-H radicals formed by the deprotonation of the corresponding uracil+* and thiouracil+* radical cations. As a matter of fact their relative stability is dictated by the intrinsic stability of the corresponding uracil-H and thiouracil-H radical and by the fact that, in general, the N3-deprotonated site is a better electron donor than the N1. In all complexes, the bonding of Cu both to nitrogen and sulfur and to nitrogen and oxygen has a significantly large covalent character.     

A Unified Experimental/Theoretical Description of the Ultrafast Photophysics of Single and Double Thionated Uracils

Photoinduced processes in thiouracil derivatives have lately attracted considerable attention due to their suitability for innovative biological and pharmacological applications. Here, sub-20 fs broadband transient absorption spectroscopy in the near-UV are combined with CASPT2/MM decay path calculations to unravel the excited-state decay channels of water solvated 2-thio and 2,4-dithiouracil. These molecules feature linear absorption spectra with overlapping ππ* bands, leading to parallel decay routes which we systematically track for the first time. The results reveal that different processes lead to the triplet states population, both directly from the ππ* absorbing state and via the intermediate nπ* dark state. Moreover, the 2,4-dithiouracil decay pathways is shown to be strongly correlated either to those of 2- or 4-thiouracil, depending on the sulfur atom on which the electronic transition localizes.     

Influence of thioketo substitution on the properties of uracil and its noncovalent interactions with alkali metal ions: threshold collision-induced dissociation and theoretical studies

Experimental and theoretical studies are carried out to determine the influence of thioketo substitution on the properties of uracil and its noncovalent interactions with alkali metal ions. Bond dissociation energies of alkali metal ion-thiouracil complexes, M(+)(SU), are determined using threshold collision-induced dissociation techniques in a guided ion beam mass spectrometer, where M(+) = Li(+), Na(+), and K(+) and SU = 2-thiouracil, 4-thiouracil, 2,4-dithiouracil, 5-methyl-2-thiouracil, and 6-methyl-2-thiouracil. Ab initio electronic structure calculations are performed to determine the structures and theoretical bond dissociation energies of these complexes and provide molecular constants necessary for thermodynamic analysis of the experimental data. Theoretical calculations are also performed to examine the influence of thioketo substitution on the acidities, proton affinities, and A::SU Watson-Crick base pairing energies. In general, thioketo substitution leads to an increase in both the proton affinity and the acidity of uracil. 2-Thio substitution generally results in an increase in the alkali metal ion binding affinities but has almost no affect on the stability of the A::SU base pair. In contrast, 4-thio substitution results in a decrease in the alkali metal ion binding affinities and a significant decrease in the stability of the A::SU base pair. In addition, alkali metal ion binding is expected to lead to an increase in the stability of both single-stranded and double-stranded nucleic acids by reducing the charge on the nucleic acid in a zwitterion effect as well as through additional noncovalent interactions between the alkali metal ion and the nucleobases.     

Photoelectron spectroscopy of hydrated adenine anions

We report the observation of hydrated adenine anions, A(-)(H(2)O)(n), n=1-7, and their study by anion photoelectron spectroscopy. Values for photoelectron threshold energies, E(T), and vertical detachment energies are tabulated for A(-)(H(2)O)(n) along with those for hydrated uracil anions, U(-)(H(2)O)(n), which are presented for comparison. Analysis of these and previously measured photoelectron spectra of hydrated nucleobase anions leads to the conclusion that threshold energies significantly overstate electron affinity values in these cases, and that extrapolation of hydrated nucleobase anion threshold values to n=0 leads to incorrect electron affinity values for the nucleobases themselves. Sequential shifts between spectra, however, lead to the conclusion that A(-)(H(2)O)(3) is likely to be the smallest adiabatically stable, hydrated adenine anion.     

Photoelectron spectrum of valence anions of uracil and first-principles calculations of excess electron binding energies

The photoelectron spectrum (PES) of the uracil anion is reported and discussed from the perspective of quantum chemical calculations of the vertical detachment energies (VDEs) of the anions of various tautomers of uracil. The PES peak maximum is found at an electron binding energy of 2.4 eV, and the width of the main feature suggests that the parent anions are in a valence rather than a dipole-bound state. The canonical tautomer as well as four tautomers that result from proton transfer from an NH group to a C atom were investigated computationally. At the Hartree-Fock and second-order Moller-Plesset perturbation theory levels, the adiabatic electron affinity (AEA) and the VDE have been converged to the limit of a complete basis set to within +/-1 meV. Post-MP2 electron-correlation effects have been determined at the coupled-cluster level of theory including single, double, and noniterative triple excitations. The quantum chemical calculations suggest that the most stable valence anion of uracil is the anion of a tautomer that results from a proton transfer from N1H to C5. It is characterized by an AEA of 135 meV and a VDE of 1.38 eV. The peak maximum is as much as 1 eV larger, however, and the photoelectron intensity is only very weak at 1.38 eV. The PES does not lend support either to the valence anion of the canonical tautomer, which is the second most stable anion, and whose VDE is computed at about 0.60 eV. Agreement between the peak maximum and the computed VDE is only found for the third most stable tautomer, which shows an AEA of approximately -0.1 eV and a VDE of 2.58 eV. This tautomer results from a proton transfer from N3H to C5. The results illustrate that the characteristics of biomolecular anions are highly dependent on their tautomeric form. If indeed the third most stable anion is observed in the experiment, then it remains an open question why and how this species is formed under the given conditions.     

On the unusual stability of valence anions of thymine based on very rare tautomers: A computational study

We characterized anionic states of thymine using various electronic structure methods, with the most accurate results obtained at the CCSD(T)/aug-cc-pVDZ level of theory followed by extrapolations to complete basis set limits. We found that the most stable anion in the gas phase is related to an imino-oxo tautomer, in which the N1H proton is transferred to the C5 atom. This valence anion, aT(c5)(nl), is characterized by an electron vertical detachment energy (VDE) of 1251 meV and it is adiabatically stable with respect to the canonical neutral nT(can) by 2.4 kcal/mol. It is also more stable than the dipole-bound (aT(dbs)(can)), and valence anion aT(val)(can) of the canonical tautomer. The VDE values for aT(dbs)(can)and T(val)(can) are 55 and 457 meV, respectively. Another, anionic, low-lying imino-oxo tautomer with a VDE of 2458 meV has a proton transferred from N3H to C5 aT(c5)(n3). It is less stable than aT(val)(can) by 3.3 kcal/mol. The mechanism of formation of anionic tautomers with the carbons C5 or C6 protonated may involve intermolecular proton transfer or dissociative electron attachment to the canonical neutral tautomer followed by a barrier-free attachment of a hydrogen atom to C5. The six-member ring structure of the anionic tautomers with carbon atoms protonated is unstable upon an excess electron detachment. Within the PCM hydration model, the low-lying valence anions become adiabatically bound with respect to the canonical neutral; becomes the most stable, being followed by aT(c5)(nl), aT(c5)(n3), aT(can), and aT(c5)(nl).     

Inorganic-organic hybrid compounds based on octamolybdates and multidentate N-donor ligand: syntheses, structures, photoluminescence and photocatalysis

Six inorganic-organic hybrid compounds, namely, [Cu(2)(2,4'-tmbpt)(2)(β-Mo(8)O(26))(H(2)O)(2)]·7H(2)O (), [Cu(2,4'-tmbpt)(γ-Mo(8)O(26))(0.5)(H(2)O)]·H(2)O (), [Co(2,4'-Htmbpt)(2)(γ-Mo(8)O(26))(H(2)O)(2)] (), [Zn(2,4'-Htmbpt)(2)(γ-Mo(8)O(26))(H(2)O)(2)] (), [Ni(2,4'-tmbpt)(α-Mo(8)O(26))(0.5)(H(2)O)]·2.5H(2)O () and [Ag(2,4'-Htmbpt)(β-Mo(8)O(26))(0.5)] (), have been synthesized under hydrothermal conditions (2,4'-tmbpt = 1-((1H-1,2,4-triazol-1-yl)methyl)-3-(2-pyridyl)-5-(4-pyridyl)-1,2,4-triazole). The structures of compounds have been determined by single-crystal X-ray diffraction analyses and characterized by infrared spectra (IR), elemental analyses, powder X-ray diffraction (PXRD) analyses and thermogravimetric analyses (TGA). Compound shows a 3D (3,4)-connected framework constructed by the 2D Cu(ii)-organic fragments and [β-Mo(8)O(26)](4-) anions. Compound exhibits a 2D layer structure based on Cu(ii)-organic chains and [γ-Mo(8)O(26)] chains. The layers are extended into a 3D supramolecular framework by hydrogen-bonding interactions. Compounds and are isostructural, and display 1D chain structures. The chains are further interlinked by hydrogen-bonding interactions to form 3D supramolecular architectures. Compound shows a 3D framework based on the 2D Ni(ii)-organic fragments and [α-Mo(8)O(26)](4-) anions. In compound , the 1D chains constructed by the Ag(i) ions, 2,4'-Htmbpt ligands and [β-Mo(8)O(26)](4-) anions are extended by hydrogen-bonding interactions into a 2D supramolecular layer. Each layer threads into the adjacent layers, yielding a 2D → 3D interdigitated structure. Moreover, the photoluminescent properties of and , the optical band gaps of , and the photocatalytic properties of have also been investigated.     

Anti-HIV Active Naphthyl Analogues of HEPT and DABO

 5-Isopropyl-6-naphthyl uracil and 5-isopropyl-6-naphthyl-2-thiouracil were alkylated to give N-1-(ethoxymethyl and methylthiomethyl) uracil and S²-cyclohexyl-thiouracil, respectively. 5-Ethyl-6-naphthyl uracil and 5-ethyl-6-naphthyl-2-thiouracil afforded N-1-(ethoxymethyl, methoxy-methyl, methylthiomethyl, acetoxyethoxy methyl and hydroxyethoxy methyl) uracil and S²-((2,2- diethoxyethyl), methoxycarbonylmethyl, ethoxycarbonylpropyl, methylthiomethyl, ethoxymethyl, methyl and cyclohexyl)-thiouracil upon alkylation.     

Anti-HIV Active Naphthyl Analogues of HEPT and DABO

Summary.  5-Isopropyl-6-naphthyl uracil and 5-isopropyl-6-naphthyl-2-thiouracil were alkylated to give N-1-(ethoxymethyl and methylthiomethyl) uracil and S²-cyclohexyl-thiouracil, respectively. 5-Ethyl-6-naphthyl uracil and 5-ethyl-6-naphthyl-2-thiouracil afforded N-1-(ethoxymethyl, methoxy-methyl, methylthiomethyl, acetoxyethoxy methyl and hydroxyethoxy methyl) uracil and S²-((2,2- diethoxyethyl), methoxycarbonylmethyl, ethoxycarbonylpropyl, methylthiomethyl, ethoxymethyl, methyl and cyclohexyl)-thiouracil upon alkylation. Received September 25, 2001. Accepted (revised) December 3, 2001     

Polysulfide anions II: structure and vibrational spectra of the S4(2-) and S5(2-) anions. Influence of the cations on bond length, valence, and torsion angle

The influence of the cations on bond length, valence, and torsion angle of S4(2-) and S5(2-) anions was examined in a series of solid alkali tetra- and pentasulfides by relating their Raman spectra to their known X-ray structures through a force-field analysis. The IR and Raman spectra of BaS4.H2O and the Raman spectra of (NH4)2S4.nNH3, gamma-Na2S4, and delta-Na2S5 are presented. The similarity of spectra of gamma-Na2S4 with those of BaS4.H2O suggests similar structures of the S4(2-) anions in these two compounds with a torsion angle smaller than 90 degrees. The variations of SS bond length, SSS valence angle, and dihedral angle of Sn2- anions are related to the polarization of the lone pair and electronic charge of the anion by the electric field of the cations. A correlation between the torsion angle and the SSS valence angle is shown as that previously reported between the length of the bond around which the torsion takes place and the dihedral angle value. These geometry changes are explained by the hyperconjugation concept and the electron long-pair repulsion.     

Subtle role of polyatomic anions in molecular construction: structures and properties of AgX bearing 2,4'-thiobis(pyridine) (X(-) = NO(3)(-), BF(4)(-), ClO(4)(-), PF(6)(-), CF(3)CO(2)(-), and CF(3)SO(3)(-))

Studies on the subtle effects and roles of polyatomic anions in the self-assembly of a series of AgX complexes with 2,4'-Py(2)S (X(-) = NO(3)(-), BF(4)(-), ClO(4)(-), PF(6)(-), CF(3)CO(2)(-), and CF(3)SO(3)(-); 2,4'-Py(2)S = 2,4'-thiobis(pyridine)) have been carried out. The formation of products appears to be primarily associated with a suitable combination of the skewed conformers of 2,4'-Py(2)S and a variety of coordination geometries of Ag(I) ions. The molecular construction via self-assembly is delicately dependent upon the nature of the anions. Coordinating anions afford the 1:1 adducts [Ag(2,4'-Py(2)S)X] (X(-) = NO(3)(-) and CF(3)CO(2)(-)), whereas noncoordinating anions form the 3:4 adducts [Ag(3)(2,4'-Py(2)S)(4)]X(3) (X(-) = ClO(4)(-) and PF(6)(-)). Each structure seems to be constructed by competition between pi-pi interactions of 2,4'-Py(2)S spacers vs Ag.X interactions. For ClO(4)(-) and PF(6)(-), an anion-free network consisting of linear Ag(I) and trigonal Ag(I) in a 1:2 ratio has been obtained whereas, for the coordinating anions NO(3)(-) and CF(3)CO(2)(-), an anion-bridged helix sheet and an anion-bridged cyclic dimer chain, respectively, have been assembled. For a moderately coordinating anion, CF(3)SO(3)(-), the 3:4 adduct [Ag(3)(2,4'-Py(2)S)(4)](CF(3)SO(3))(3) has been obtained similarly to the noncoordinating anions, but its structure is a double strand via both face-to-face (pi-pi) stackings and Ag.Ag interactions, in contrast to the noncoordinating anions. The anion exchanges of [Ag(3)(2,4'-Py(2)S)(4)]X(3) (X(-) = BF(4)(-), ClO(4)(-), and PF(6)(-)) with BF(4)(-), ClO(4)(-), and PF(6)(-) in aqueous media indicate that a [BF(4)(-)] analogue is isostructural with [Ag(3)(2,4'-Py(2)S)(4)]X(3) (X(-) = ClO(4)(-) and PF(6)(-)). Furthermore, the anion exchangeability for the noncoordinating anion compounds and the X-ray data for the coordinating anion compounds establish the coordinating order to be NO(3)(-) > CF(3)CO(2)(-) > CF(3)SO(3)(-) > PF(6)(-) > ClO(4)(-) > BF(4)(-).     

Interaction of Ca2+ with uracil and its thio derivatives in the gas phase

The structures and relative stabilities of the complexes formed by uracil and its sulfur derivatives, namely, 2-thio-, 4-thio, and 2,4-dithio-uracil when interacting with Ca(2+) in the gas phase have been analyzed by means of density functional theory (DFT) calculations carried out at the B3LYP/6-311++G(3df,2p)//B3LYP/6-31+G(d,p) level. For uracil and 2,4-dithiouracil, where the two basic sites are the same, Ca(2+) attachment to the heteroatom at position 4 is preferred. However, for the systems where both types of basic centers, a carbonyl or a thiocarbonyl group, are present, Ca(2+)-oxygen association is favored. The most stable complexes correspond to structures with Ca(2+) bridging between the heteroatom at position 2 of the 4-enol (or the 4-enethiol) tautomer and the dehydrogenated ring nitrogen, N3. The enhanced stability of these enolic forms is two-fold, on the one hand Ca(2+) interacts with two basic sites and on the other triggers a significant aromatization of the ring. Besides, Ca(2+) association has a clear catalytic effect on the tautomerization processes which connect the oxo-thione forms with the enol-enethiol tautomers. Hence, although the enol-enethiol tautomers of uracil and its thio derivatives should not be observed in the gas phase, the corresponding Ca(2+) complexes are the most stable species and should be accessible, because the tautomerization barriers are smaller than the Ca(2+) binding energies.     

DFT study of molecular geometries and vibrational characteristics of uracil and its thio-derivatives and their radical cations

DFT calculations at the B3LYP/6-311++G** level have been carried out to study the vibrational characteristics of the neutral molecules, anionic and cationic radicals of uracil, 2-thiouracil and 4-thiouracil. In the U molecule, CC bond loses its double bond character and magnitude of the CC stretching frequency decreases significantly as a result of radicalization. Frequency for the in-plane deformation mode of CO increases when a sulfur atom is substituted for the oxygen atom at the site C₂ in the uracil molecule but decreases when a sulfur atom is substituted for the oxygen atom at the site C₄. The magnitude of both the N–H stretching frequencies decreases in all the radical cations as compared to their neutral molecules. Radicalization leads to significant changes in the magnitudes and intensities corresponding to some of the normal modes for all the three cases. Removal of an electron leads to decrease in the electronic charge mainly from the sulfur atom in the case of 2-TU and 4-TU, whereas it is distributed over the sites N₁, C₅, O₈ and O₁₀ in case of the U molecule.     

Extent of M2 δ to ligand π-conjugation in neutral and mixed valence states of bis(4-isonicotinate)-bis(2,4,6-triisopropylbenzoate) dimetal complexes (MM), where M = Mo or W, and their adducts with tris(pentafluorophenyl)boron

The reaction between W(2)(T(i)PB)(4), where T(i)PB = 2,4,6-triisopropylbenzoate, and 2 equiv of 4-isonicotinic acid (nicH) yields the compound W(2)(T(i)PB)(2)(nic)(2), 2, and T(i)PBH. Compound 2 is related to the previously reported molybdenum analog, Mo(2)(T(i)PB)(2)(nic)(2), 1. Compounds 1 and 2 react with 2 equiv of B(C(6)F(5))(3) in THF to form the adducts M(2)(T(i)PB)(2)(nic-B(C(6)F(5))(3))(2), 1B (M = Mo) and 2B (M = W), which have been crystallographically characterized as solvates M(2)(T(i)PB)(2)(nic-B(C(6)F(5))(3))(2)·2THF n-hexane. Compounds 1 and 2 are intensely colored due to M(2) δ to π* MLCT transitions, and upon complexation with B(C(5)F(5))(3) to give 1B and 2B, these bands shift to lower energy and gain in intensity. Each compound shows two one-electron ligand-based reductions with a ΔE(1/2) = 120 (1), 300 (1B), 440 (2), and 650 mV (2B). The larger ΔE(1/2) values for the tungsten compounds reflect the greater orbital mixing of the metal 5d-based M(2) δ and the nic π* LUMO. Reduction of solutions of 1B and 2B with (C(5)Me(5))(2)Co leads to the anions 1B(-) and 2B(-), which have been characterized spectroscopically by electron paramagnetic resonance (EPR) and UV-vis-NIR absorption. The EPR spectra of 1B(-) and 2B(-) are consistent with ligand-based (i.e., organic) radicals. The electronic spectra contain low-energy narrow charge resonance (IVCT) bands at 3800 (1B(-)) and 4500 cm(-1) (2B(-)), consistent with fully delocalized mixed valence radical anions. The results are compared with electronic structure calculations and with the spectral features of the metal-centered delocalized mixed valence radical cations [(Bu(t)CO(2))(3)M(2)](2)-μ(2)-(O(2)C-CO(2))(+), to which they are remarkably similar, as well as with other organic-based mixed valence systems.     

Unimolecular photochemistry of 4-thiouracils

Unimolecular phototautomeric reactions in 4-thiouracil, 1-methyl-4-thiouracil and 6-aza-4-thiouracil were studied using the matrix-isolation technique combined with infrared absorption spectroscopy. For monomers of these compounds, isolated in solid argon at 10 K, an intramolecular proton-transfer photoreaction was observed. Upon UV (lambda > 345 nm) irradiation, the initial oxo-thione forms of 4-thiouracils were converted into the corresponding oxo-thiol tautomers. The photogenerated oxo-thiol isomers were identified by comparing their experimental IR spectra with the spectra theoretically calculated at the DFT(B3LYP)/6-311++G(2d,p) level. Good agreement between the observed and predicted pattern of spectral bands allowed a reliable identification. This is the first report on experimental observation of isomeric forms of 4-thiouracils other than the canonical oxo-thione tautomers.