The structure of the phototransformation product of monothiodibenzoylmethane

Infrared (IR) and electronic spectroscopy studies were carried out for monothiodibenzoylmethane isolated in rare gas matrices, low-temperature glasses, polymer films, KBr tablets (IR) and liquid solutions. This molecule is known to undergo light-induced conversion, but the nature of the product has long been a subject of discussion. Comparison of the experimental IR spectra with theoretical predictions (B3LYP/cc-pVDZ) obtained for various possible structures shows that the photoproduct corresponds to a non-chelated –SH exo-rotamer of the (Z)-enethiol tautomeric form of monothiodibenzoylmethane. This conclusion is consistent with the experimentally determined energy of the activation barrier for the reverse ground-state process that brings the molecule back to the initial enolic, intramolecularly hydrogen-bonded structure.     

Determination of tautomeric phenotypes of β-thioxo esters and characterization of the tautomeric enethiolic constituents by means of 13C NMR spectroscopy

The ¹³C NMR spectra of 28 enethiolizable β-thioxo esters and 6 enethiolizable β-thioxo thioloesters have been recorded in order to establish the tautomeric phenotypes of these compounds. All compounds investigated are essentially enethiolic. The carbonyl-conjugated (Z)-enethiol form is the exclusive or predominant tautomer of open-chain β-thioxo esters and thioloesters, thioacylmalonates and medium-sized 2-alkoxycarbonylcycloalkanethiones. The carbonyl-conjugated (E)-enethiol form is identifiable for open-chain α-unsubstituted β-thioxo esters and thioloesters, and abundant for open-chain α-substituted β-thioxo esters. Non-conjugated enethiol forms [i.e. (Z)- and (E)-isomeric β,γ-unsaturated β-mercapto esters] are abundant tautomeric constituents of ω-substituted and higher 2-alkoxycarbonylcycloalkanethiones. The chemical shifts of the carbon atoms directly involved in the tautomeric change have been rationalized in terms of substituent screening contributions. Deuterium isotope effects on the central carbon atoms of selected deuterio-enethiolic compounds have been measured in order to depict the ester group rotamerism in CO-conjugated (Z)-enethiols. The abundance of the CO-conjugated (E)-enethiols, as well as the preferred population of the non-conjugated (Z)-enethiol form relative to the non-conjugated (E)-enethiol form, is rationalized in terms of the occurrence of a no-bond interaction between the lone-pair electrons of the enethiolic sulphur atom and the ‘chelating’ methylene hydrogen atoms of cis-alkyl groups.     

Superior Z→E and E→Z photoswitching dynamics of dihydrodibenzodiazocine, a bridged azobenzene, by S1(nπ*) excitation at λ = 387 and 490 nm

The ultrafast Z→E and E→Z photoisomerisation dynamics of 5,6-dihydrodibenzo[c,g][1,2]diazocine (1), the parent compound of a class of bridged azobenzene-based photochromic molecular switches with a severely constrained eight-membered heterocyclic ring as central unit, have been studied by femtosecond time-resolved spectroscopy in n-hexane as solvent and by quantum chemical calculations. The diazocine contrasts with azobenzene (AB) in that its Z rather than E isomer is the energetically more stable form. Moreover, it stands out compared to AB for the spectrally well separated S(1)(nπ*) absorption bands of its two isomers. The Z isomer absorbs at around λ = 404 nm, the E form has its absorption maximum around λ = 490 nm. The observed transient spectra following S(1)(nπ*) photoexcitation show ultrafast excited-state decays with time constants τ(1) = 70 fs for the Z and <50 fs for the E isomer reflecting very fast departures of the excited wave packets from the S(1) Franck-Condon regions and τ(2) = 270 fs (320 fs) related to the Z→E (resp. E→Z) isomerisations. Slower transient absorption changes on the time scale of τ(3) = 5 ps are due to vibrational cooling of the reaction products. The results show that the unique steric constraints in the diazocine do not hinder, but accelerate the molecular isomerisation dynamics and increase the photoswitching efficiencies, contrary to chemical intuition. The observed isomerisation times and quantum yields are rationalised on the basis of CASPT2//CASSCF calculations by a S(1)/S(0) conical intersection seam at a CNNC dihedral angle of ≈96° involving twisting and torsion of the central CNNC moiety. With improved photochromism, high quantum yields, short reaction times and good photostability, diazocine 1 and its derivatives constitute outstanding candidates for photoswitchable molecular tweezers and other applications.     

β-thioxo esters—II : Evidence for ester group rotamerism and pertubation of the intramolecular hydrogen-bonding in enethiolized 2-thioxo cycloalkanecarboxylic esters

Ethyl 2-thioxo cycloalkanecarboxylates of ring sizes 5–9 exist in CCl₄ solution in the tautomeric cis-enethiol form, whereas the 10–12 membered ring compounds exist as an equilibrium mixture of several tautomeric forms. On the basis of NMR and IR spectral data it is concluded that the cis-enethiols exist as equilibrium mixtures of two rapidly interconverting rotameric forms, one involving intramolecular H-bonding, the other having the ester group rotated through 180° so that the ester carbonyl group is unable to form an intramolecular H-bond, although still conjugated with the enethiolic double bond. The relative equilibrium concentrations at ～35° of the two rotameric cis-enethiol forms have been determined for all investigated compounds.     

Vibrational spectra of linear oligomers of carbonic acid: a quantum chemical study

Gas phase quantum chemical calculations of linear, hydrogen bonded oligomers of carbonic acid have been carried out to examine the feasibility for such species to be the building blocks of crystalline carbonic acid. Infrared and Raman vibrational spectra have been calculated and are compared against experimentally known spectra for two polymorphs of carbonic acid. The calculated anharmonic frequencies of the linear oligomer agree well with the experimental data for the centrosymmetric β-carbonic acid, rather than with that for the α polymorph. These calculations strongly suggest that β-carbonic acid should consist of one-dimensional hydrogen bonded carbonic acid molecules in the anti-anti conformation.     

Preparation and Characterization of Pure Thiosulfuric Acid

Pure thiosulfuric acid has not been prepared yet, although it is described in most textbooks of inorganic chemistry. Furthermore, no experimental evidence for the structure of thiosulfuric acid is known. Theoretical calculations predict the (SH)(OH) tautomer to be more stable than the (OH)(OH) tautomer. In this work we present the synthesis and spectroscopical characterization of pure thiosulfuric acid. X₂S₂O₃ (X = H, D) was obtained from the reaction of dry Na₂S₂O₃ with anhydrous HF at –60 °C. The experimental vibrational and NMR spectra together with quantum chemical calculations provide evidence for the predicted (SH)(OH) tautomeric structure.     

Experimental and theoretical UV characterizations of acetylacetone and its isomers

Cryogenic matrix isolation experiments have allowed the measurement of the UV absorption spectra of the high-energy non-chelated isomers of acetylacetone, these isomers being produced by UV irradiation of the stable chelated form. Their identification has been done by coupling selective UV-induced isomerization, infrared spectroscopy, and harmonic vibrational frequency calculations using density functional theory. The relative energies of the chelated and non-chelated forms of acetylacetone in the S0 state have been obtained using density functional theory and coupled-cluster methods. For each isomer of acetylacetone, we have calculated the UV transition energies and dipole oscillator strengths using the excited-state coupled-cluster methods, including EOMCCSD (equation-of-motion coupled-cluster method with singles and doubles) and CR-EOMCCSD(T) (the completely renormalized EOMCC approach with singles, doubles, and non-iterative triples). For dipole-allowed transition energies, there is a very good agreement between experiment and theory. In particular, the CR-EOMCCSD(T) approach explains the blue shift in the electronic spectrum due to the formation of the non-chelated species after the UV irradiation of the chelated form of acetylacetone. Both experiment and CR-EOMCCSD(T) theory identify two among the seven non-chelated forms to be characterized by red-shifted UV transitions relative to the remaining five non-chelated isomers.     

Solvent dependent excited state spectral properties of 4-hydroxyacridine: Evidence for only water mediated excited state proton transfer process

The possibility of ground and excited state proton transfer reaction across the five member intramolecular hydrogen bonded ring in 4-hydroxyacridine (4-HA) has been investigated spectroscopically and the experimental results have been correlated with quantum chemical calculations. The difference in the emissive behaviour of 4-HA in different types of solvents is due to the presence of different species in the excited state. In non-polar solvents, the species present is non-fluorescing in nature, whereas 4-HA molecule shows normal emission from intramolecularly hydrogen bonded closed conformer in polar aprotic solvents. In polar protic solvents like MeOH, EtOH, etc. (except water), a single broad emission band is attributed to the hydrogen bonded solvated form of 4-HA. However, in case of water, fluorescence from the tautomeric form of 4-HA is observed apart from emission from the solvated form. Emission from the tautomeric form may arise due to double proton transfer via a single water molecule bonded to 4-HA. Evaluation of the potential energy surfaces by quantum chemical calculations using density functional theory (DFT) and time dependent density functional theory (TDDFT), however, points towards the possibility of proton transfer—both intrinsic intramolecular as well as water mediated in the first excited state of 4-HA.     

Microwave spectrum, conformation and intramolecular hydrogen bonding of 2,2,2-trifluoroethanethiol (CF3CH2SH)

The microwave spectrum of 2,2,2-trifluoroethanethiol, CF3CH2SH, and of one deuterated species, CF3CH2SD, has been investigated in the 7-80 GHz spectral interval. The microwave spectra of the ground and three vibrationally excited states belonging to three different normal modes of one conformer were assigned for the parent species, and the vibrational frequencies of these fundamentals were determined by relative intensity measurements. Only the ground vibrational state was assigned for the deuterated species. The identified form has a synclinal arrangement for the H-S-C-C chain of atoms and the corresponding dihedral angle is 68(5) degrees from synperiplanar (0 degrees). A weak intramolecular hydrogen bond formed between the thiol (SH) group and one of the fluorine atoms is stabilizing this conformer. There is no evidence in the microwave spectrum for the H-S-C-C antiperiplanar form. The hydrogen atom of the thiol group should have the ability to tunnel between two equivalent synclinal potential wells, but no splittings of spectral lines due to tunneling were observed. The microwave work was augmented by quantum chemical calculations at the B3LYP/aug-cc-pVTZ and MP2/aug-cc-pVTZ levels of theory.     

Photochromism in p-methylbenzoylthioacetone and related β-thioxoketones

Irradiation of p-methylbenzoylthioacetone with UV or visible light brings about spectral changes characteristic for the photochromic behavior of β-thioxoketones. The nature of the initial species and of the photochromic product can be assigned based on spectral studies of the electronic and IR spectra in rare gas matrices combined with quantum chemical calculations. Additional arguments for the vibrational assignments are provided by using polarized light to induce the phototransformation, and by subsequent measurements of linear dichroism on partially aligned samples. Comparison with the results previously obtained for three related molecules: thioacetylacetone, p-methyl(thiobenzoyl)acetone, and monothiodibenzoylmethane reveals a common pattern of the photochromic reaction. In all four molecules, the initial species corresponds to an intramolecularly hydrogen-bonded enolic molecular structure, and the main photochromic product to an “open”, nonchelated enethiolic counterpart. On the basis of a computational TD-DFT study of ground and excited electronic states, possible mechanisms of the photochromic transformation are discussed.     

Identity double-proton transfer in (3Z)-3-hydroxy-1,4-di(quinolin-2-yl)but-3-en-2-one

Although there is a very fast (on the NMR timescale) double-proton transfer in (1Z,3Z)-3-hydroxy-4-quinolin-2-yl-1-quinolin-2(1H)-ylidenbut-3-en-2-one (the product of the condensation of ethyl oxalate with 2lithiomethylquinoline), it is the only species present in chloroform solution. Comparison of the product of condensation of ethyl oxalate with 2lithiomethyl derivatives of pyridine (recent studies) and quinoline (present studies) shows that benzoannulation considerably affects the tautomeric equilibrium. The observed changes are not only quantitative but also qualitative. Moreover, contrary to the proton transfer in the pyridine tautomers, this process is fast in the quinoline tautomers. Comparison of the experimental and ab initio/DFT GIAO-calculated (13)C and (15)N chemical shifts for the transition states in the proton-transfer reactions between (1Z,3Z)- 3-hydroxy-4-quinolin-2-yl-1-quinolin-2(1H)-ylidenbut-3-en-2-one and its tautomers support the theory that a concerted identity reaction takes place between the enolimine-enaminone and enaminone-enolimine tautomeric forms. As a consequence, the most stable tautomeric form, (1Z,3Z)-1,4-di(quinolin-2-yl)buta-1,3-diene-2,3-diol, is not present in the tautomeric mixture.     

Investigation of the structure and properties of the potentially tautomeric 5,7-dimethyl-6H-pyrrolo[3,4-d]pyridazines in the gas and aqueous phases using the AM1 and PM3/COSMO solvation method

The potentially tautomeric 5,7-dimethyl-6H-pyrrolo[3,4-d]pyidazines, 2H and 6H, and their fixed tautomeric forms were studied in order to predict the most stable form by the restricted Hartree-Foch approach using semiempirical PM3 and AM1 quantum chemical calculations at the SCF level in the gas and aqueous phases. Both methods predicted that the 6H form, 5, is more stable than the other forms, 1-4, in both gas and aqueous phases. The results obtained were found to be in agreement with the experimental data. Monoprotonated forms of 5,7-dimethyl-6H-pyrrolo[3,4-d]pyridazines were also examined. Proton affinity calculations predicted that the first protonations take place on the N6 atom in the 2H form and on the N2 atom in the 6H form, resulting in a common cation 13.     

Tautomeric properties and gas-phase structure of 3-chloro-2,4-pentanedione

The tautomeric properties of alpha-chlorinated acetylacetone, 3-chloro-2,4-pentanedione CH3C(O)-CHCl-C(O)CH3, have been investigated by gas electron diffraction (GED) and quantum chemical calculations (B3LYP and MP2 approximations with different basis sets up to cc-pVTZ). Analysis of the GED intensities resulted in the presence of 100(2)% enol tautomer at 269(8) K. The following skeletal geometric parameters (rh1 values) of the molecule, which possesses Cs symmetry, were derived: r(C=C) = 1.378(3) A, r(C-C) = 1.450(3) A, r(C=O) = 1.243(3) A, r(C-O) = 1.319(3) A, r(O-H) = 1.001(4) A, r(C-Cl) = 1.752(4) A, angleC-C=C = 121.3(1.0) degrees , angleC=C-O = 119.9(1.2) degrees , angleC-C=O = 119.1(1.2) degrees . Due to very small contributions of the keto tautomer in alpha-chlorinated acetylacetone and its parent species, the effect of alpha-chlorination on tautomeric properties cannot be derived from experimental data. Quantum chemical calculations (B3LYP/6-31G**, B3LYP/cc-pVTZ, and MP2/cc-pVTZ) predict that alpha-chlorination of acetylacetone has no pronounced effect on the tautomeric properties. On the other hand, similar calculations for 1-chloro-1,3-butanedion, ClC(O)-CH2-C(O)CH3, demonstrate that chlorination in one beta position destabilizes the enol tautomer. In both chlorinated species the enol form is strongly preferred.     

Tautomeric equilibria of 5-fluorouracil anionic species in water

It has long been postulated that rare tautomeric or ionized forms of nucleic acid bases may play a role in mispair formation. Therefore, ab initio quantum chemical investigations on the tautomeric equilibrium in 5-fluorouracil (5FU) and its anions (deprotonated from N1, AN1, and from N3, AN3) and their tautomeric forms in water were performed. The effect of the water as solvent was introduced using solute-solvent clusters (four water molecules). The influence of the water molecules on the tautomeric reactions between different forms was considered by multiple proton transfer mechanisms. We show that when a water dimer is located in the reaction site between the two pairs of N-H and C═O groups, the assistive effect of the water molecules is strengthened. All calculations of the solute-water complexes were carried out at an MP2 level of theory and supplemented with correction for higher order correlation terms at CCSD(T) level, using the 6-31+G(d,p) basis set. The ab initio calculated frequencies and Raman intensities of 5FU and its anions AN1, AN3, and dianion are in good agreement with the experimental Raman frequencies in aqueous solution at different pH. In order to establish the pH-induced structural transformation in the molecule of 5FU, further (1)H, (19)F, and (13)C NMR spectra in water solution for pH = 6.9-13.8 were acquired and the chemical shift alterations were determined as a function of pH. On the basis of NMR spectroscopic data obtained for 5FU in aqueous solution at alkaline pH, we suggest the existence of a mixture of the anionic tautomeric forms predicted by our theoretical calculations.     

Thin‐Film Properties of DNA and RNA Bases: A Combined Experimental and Theoretical Study

The structures of the DNA and RNA bases cytosine, uracil, and thymine in thin films with a nominal film thickness of about 20 nm are studied by using X‐ray photoemission spectroscopy (XPS) and Fourier‐transform infrared spectroscopy. The molecules are evaporated in situ from powder on a gold foil. The experimental results indicate that cytosine is composed of two energetically close tautomeric forms, whereas uracil and thymine exist in only one tautomeric form. Additionally, quantum chemical calculations are performed to complement the experimental results. The relative energies of the tautomeric forms of cytosine, uracil, and thymine are calculated using Hartree–Fock (HF), density functional theory (DFT), and post‐HF methods. Furthermore, the assignment of the XPS spectra is supported by using simple model considerations employing Koopmans ionization energies and Mulliken net atomic charges.     

Determination of the chelating site preferentially involved in the complex of lead(II) with caffeic acid: a spectroscopic and structural study

The complexation of lead(II) with mono-deprotonated caffeic acid in aqueous solution (pH = 6.50) has been investigated by UV-visible, fluorescence, and vibrational spectroscopies combined with quantum chemical calculations (DFT). The caffeate ion presents two chelating sites in competition: the carboxylate and the catechol functions. Electronic spectroscopies highlighted two different complexed forms with, respectively, 1:1 and 2:1 stoichiometry. The 1:1 complex predominates for low lead concentrations, even if the second complexed form appears before the first chelating site is fully occupied. Both spectroscopic data and calculations reveal that Pb(II) preferentially coordinates with the carboxylate function, in opposition with previous results found for the Al(III) complexation, where the catechol group presents the greater complexing power. The structural and vibrational modifications between the mono-deprotonated ligand and 1:1 complex engendered by the chelation are discussed. Water molecules have been added on the Pb ion to modify its coordination, and structures of Pb(H(2)CA)(H(2)O)(n)(+) with n = 0-4 were optimized. Calculations of theoretical frequencies have permitted us to propose a tentative assignment of infrared and Raman spectra of complexed species.     

Structure and IR Spectra of 3(5)-Aminopyrazoles and UV-Induced Tautomerization in Argon Matrix

The prototropic tautomerism in 3(5)-aminopyrazoles was investigated by matrix isolation infrared (IR) spectroscopy, supported by DFT(B3LYP)/6-311++G(d,p) calculations. In consonance with the experimental data, the calculations predict tautomer 3-aminopyrazole (3AP) to be more stable than the 5-aminopyrazole (5AP) tautomer (calculated energy difference: 10.7 kJ mol⁻¹; Gibbs free energy difference: 9.8 kJ mol⁻¹). The obtained matrix isolation IR spectra (in both argon and xenon matrices) were interpreted, and the observed bands were assigned to the tautomeric forms with help of vibrational calculations carried out at both harmonic and anharmonic levels. The matrix-isolated compound (in argon matrix) was then subjected to in situ broadband UV irradiation (λ > 235 nm), and the UV-induced transformations were followed by IR spectroscopy. Phototautomerization of the 3AP tautomer into the 5AP form was observed as the strongly prevalent reaction.     

Hydrogen bonding in acetylacetaldehyde: Theoretical insights from the theory of atoms in molecules

All the possible conformations of tautomeric structures (keto and enol) of acetylacetaldehyde (AAD) were fully optimized at HF, B3LYP, and MP2 levels with 6‐31G(d,p) and 6‐311++G(d,p) basis sets to determine the conformational equilibrium. Theoretical results show that two chelated enol forms have extra stability with respect to the other conformers, but identification of global minimum is very difficult. The high level ab initio calculations G2(MP2) and CBS‐QB3) also support the HF conclusion. It seems that the chelated enol forms have equal stability, and the energy gap between them is probably lies in the computational error range. Finally, the analysis of hydrogen bond in these molecules by quantum theory of atoms in molecules (AIM) and natural bond orbital (NBO) methods fairly support the ab initio results. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Experimental and theoretical evidence for U(C6H6) and Th(C6H6) complexes

The vibrational spectra of UBz and ThBz have been measured in solid argon. Complementary quantum chemical calculations have allowed the assignments of the vibrational spectra. According to the calculations, AcBz are stable molecules, as well as other species like BzAcBz and BzAc2Bz. Experimentally, there is no evidence for the sandwich compounds BzAcBz and BzAc2Bz due to the limitations in the reagent concentrations.     

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.