Spectral shifts of the n → π* and π → π* transitions of uracil based on a modified form of solvent reorganization energy

According to our recent studies on the nonequilibrium solvation, the solvent reorganization energy is found to be the cost of maintaining the residual polarization P', which equilibrates with the extra electric field E(ex). On the basis of this solvent reorganization energy and the well-established equilibrium solvation energy, a novel and reasonable expression for the spectral shift of the electronic absorption spectra is proposed in this work. Furthermore, the two lowest transitions of uracil in aqueous solution are investigated as test cases with the TDDFT/6-311++G** method. The obtained spectral shift is 0.48 eV for n → π* transition and -0.14 eV for π → π* transition, agreeing well with available experimental results. The contributions to the shift are discussed and the electrostatic plus polarization components are found to be crucial for the electronic absorption spectra of uracil in aqueous solution.     

Spectral shift of the n → π* transition for acetone and formic acid with an explicit solvent model

In our recent work, a new form of the electrostatic solvation energy for the nonequilibrium polarization has been derived by introducing the method of constrained equilibrium state in the framework of continuous medium theory. Up until now, the idea of the constrained equilibrium state method has not been introduced into the explicit solvent model by others; therefore this nonequilibrium energy form was further equivalently extended to the explicit solvent model in this work based on the discrete representation of the solvent permanent charges and induced dipoles. Making use of this expression in explicit solvent model, we modified the nonequilibrium module in the averaged solvent electrostatic potential/molecular dynamics program to implement numerical calculations. Subsequently, the new codes were applied to study the solvatochromic shifts of the n → π* absorption spectra for acetone and trans-formic acid in aqueous solution. The calculation results show a good agreement with the experimental observations. When our results of spectral shift are compared with those achieved directly from the continuum model, it can be seen that both the explicit solvent model and continuum model derived based on the constrained equilibrium approach can give reasonable predictions. The hydrogen bond effect was also discussed and deemed to be a dominant contribution to the spectral shift by calculating the n → π* absorption spectra of acetone-water complexes.     

Kinetics of the hydrogen abstraction CHO + Alkane → HCHO + Alkyl reaction class: an application of the reaction class transition state theory

The kinetics of the hydrogen abstraction at alkanes by formyl radicals is investigated using the reaction class transition state theory (RC-TST) approach combined with the linear energy relationship (LER) or the barrier height grouping (BHG). The rate constants of a reaction in this class can be estimated through those of the reference reaction, CHO + C₂H₆, which are obtained from rate constants of the reaction that involves the smallest species, namely CHO + CH₄, using the explicit RC-TST scaling. The thermal rate constants of this smallest reaction are evaluated at the canonical variational transition state theory (CVT) with the corrections from the small-curvature tunneling (SCT) and hindered rotation (HR) treatments. Our analyses indicate that less than 40% systematic errors, on the average, exist in the predicted rate constants using both the LER approach, where only reaction energy is needed, and the BHG approach, where no additional information is needed; while comparing to explicit rate calculations the differences are less than 60%. Contribution to Mark S. Gordon 65th Birthday Festschrift Issue.     

The total π-electron energy as a problem of moments: application of the Backus–Gilbert method

The total π-electron energy problem can be formulated as a classical problem of moments. This observation allows us to apply general methodologies developed in the field of moment’s theory to solve the total π-electron energy problem. In the present article, we apply the Backus–Gilbert method to obtain analytical expressions for the total π-electron energy in terms of its spectral moments.AMS subject classification: 30E05, 05C50, 92E10, 78M05     

Intramolecular 2pπ* → 3dπ charge transfer in the excited state of phenyldisilane studied by picosecond and nanosecond laser spectroscopy

Intramolecular 2pπ^* → 3dπ charge transfer in the excited state of phenyldisilane occurs very rapidly (<10 ps) both in MP at 293 K and in EPA glass at 77 K. At room temperature a long-lived 425 nm transient (which is assigned to a rearranged intermediate) is produced with a rise time of 30 ps, showing that the transient formation proceeds via the ¹(2pπ,3dπ) CT state.     

Solvent effect in an “isolated” solvated molecule in a free jet: Intramolecular charge-transfer state of solvated 9,9'-bianthryl

The fluorescence spectra of bare and solvated 9,9'-bianthryl were recorded in a free Jet. In the case of bare bianthryl, only the fluorescence from the locally excited (LE) state was observed, even on excitation to a high vibronic state. In contrast, the excitation of the 0-0 band of 9,9'-bianthryl solvated by acetone gave fluorescence from the charge-transfer (CT) state. The ratio of the intensity of the CT fluorescence to that of the LE fluorescence increases with increasing degree of solvation. Solvation by cyclohexane did not give CT fluorescence.     

nσ* and πσ* excited states in aryl halide photochemistry: a comprehensive study of the UV photodissociation dynamics of iodobenzene

A recent review (Ashfold et al., Phys. Chem. Chem. Phys., 2010, 12, 1218) highlighted the important role of dissociative excited states formed by electron promotion to σ* orbitals in establishing the photochemistry of many molecular hydrides. Here we extend such considerations to molecular halides, with a particular focus on iodobenzene. Two experimental techniques (velocity mapped ion imaging (VMI) and time resolved infrared (IR) diode laser absorption) and electronic structure calculations have been employed in a comprehensive study of the near ultraviolet (UV) photodissociation of gas phase iodobenzene molecules. The VMI studies yield the speeds and angular distributions of the I((2)P(3/2)) and I*((2)P(1/2)) photofragments formed by photolysis in the wavelength range 330 ≥λ≥ 206 nm. Four distinct dissociation channels are observed for the I((2)P(3/2)) atom products, and a further three channels for the I*((2)P(1/2)) fragments. The phenyl (Ph) radical partners formed via one particular I* product channel following excitation at wavelengths 305 ≥λ≥ 250 nm are distributed over a sufficiently select sub-set of vibrational (v) states that the images allow resolution of specific I* + Ph(v) channels, identification of the active product mode (ν(10), an in-plane ring breathing mode), and a refined determination of D(0)(Ph-I) = 23,390 ± 50 cm(-1). The time-resolved IR absorption studies allow determination of the spin-orbit branching ratio in the iodine atom products formed at λ = 248 nm (?(I*) = [I*]/([I] + [I*]) = 0.28 ± 0.04) and at 266 nm (?(I*) = 0.32 ± 0.05). The complementary high-level, spin-orbit resolved ab initio calculations of sections (along the C-I bond coordinate) through the ground and first 19 excited state potential energy surfaces (PESs) reveal numerous excited states in the energy range of current interest. Except at the very shortest wavelength, however, all of the observed I and I* products display limiting or near limiting parallel recoil anisotropy. This encourages discussion of the fragmentation dynamics in terms of excitation to states of A(1) total symmetry and dissociation on the 2A(1) and 4A(1) (σ* ← n/π) PESs to yield, respectively, I and I* products, or via non-adiabatic coupling to other σ* ← n/π PESs that correlate to these respective limits. Similarities (and differences) with the available UV photochemical data for the other aryl halides, and with the simpler (and more thoroughly studied) iodides HI and CH(3)I, are summarised.     

Development of an on-line automated sample clean-up method and liquid chromatography–tandem mass spectrometry analysis: application in an in vitro proteolytic assay

Fluorescence detection has been a method of choice in industry for screening assays, including identification of enzyme inhibitors, owing to its high-throughput capabilities, excellent reproducibility, and sensitivity. Occasionally, inhibitors are identified that challenge the fluorescence assay limit, necessitating the development of more sensitive detection methods to assess these compounds. For data mining purposes, however, original assay conditions may be required. A direct method transfer to highly sensitive and specific LC-MS-based methods has not always been possible due to the presence of MS-incompatible neutral detergents and non-volatile salts in the assay matrix. Utilizing an in vitro proteolytic screening assay for the serine protease hepatitis C virus (HCV) nonstructural (NS) 3 protease as a test case, we report the development of an automated sample clean-up procedure implemented on-line with liquid chromatography–tandem mass spectrometry (LC-MS/MS) analysis to complement fluorescence detection. Ion exchange and peptide microtraps were employed to remove MS-incompatible assay matrix components. Three protease inhibitors were used to validate the MS/MS method. Comparable potencies were achieved for these compounds when assessed by fluorescence and MS/MS detection. Furthermore, four-fold less enzyme could be utilized when employing the MS/MS method compared to fluorescence detection. The longer analysis time, however, resulted in reduced sample capacity. The potency of our designed HCV NS3 protease inhibitors are thus routinely evaluated using a continuous fluorescence-based assay. Only pertinent inhibitors approaching the fluorescence assay sensitivity limit are subsequently analyzed further by LC-MS/MS. This methodology allows us to maintain a database and to compare results independent of the detection method. Despite the relatively slow sample turnaround time of this LC-MS approach, the versatility of the automated on-line clean-up procedure and sample analysis can be applied to assays containing reagents which were historically considered to be MS incompatible.     

Ultrafast excited state dynamics of spirilloxanthin in solution and bound to core antenna complexes: Identification of the S∗ and T(1) states

Ultrafast excited state dynamics of spirilloxanthin in solution and bound to the light-harvesting core antenna complexes from Rhodospirillum rubrum S1 were investigated by means of femtosecond pump-probe spectroscopic measurements. The previously proposed S? state of spirilloxanthin was clearly observed both in solution and bound to the light-harvesting core antenna complexes, while the lowest triplet excited state appeared only with spirilloxanthin bound to the protein complexes. Ultrafast formation of triplet spirilloxanthin bound to the protein complexes was observed upon excitation of either spirilloxanthin or bacteriochlorophyll-a. The anomalous reaction of the ultrafast triplet formation is discussed in terms of ultrafast energy transfer between spirilloxanthin and bacteriochlorophyll-a.     

Synthesis and spectroscopic studies on charge-transfer molecular complexes formed in the reaction of imidazole and 1-benzylimidazole with σ- and π-acceptors

The spectrophotometric characteristics of the solid charge-transfer molecular complexes (CT) formed in the reaction of the electron donors imidazole (IML) and 1-benzylimidazole (BIML) with the σ-acceptor iodine and π-acceptors 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), tetracyanoethylene (TCNE) and 2,3,5,6-tetrachloro-1,4-benzoquinone (CHL) have been studied in chloroform at 25 °C. These were investigated through electronic and infrared spectra as well as elemental analysis. The results show that the formed solid CT-complexes have the formulas [(IML)2 I]I3, [(IML)(DDQ)], [(IML)2(TCNE)5] and [(IML)(CHL)] for imidazole and [(BIML) I]I3, [(BIML)(DDQ)2], [(BIML)(TCNE)2] and [(BIML)(CHL)2] for 1-benzylimidazole in full agreement with the known reaction stoichiometries in solution as well as the elemental measurements. The formation constant KCT, molar extinction coefficient ?CT, free energy change ΔG0, CT energy ECT and ionization potential Ip have been calculated for the CT-complexes [(IML)2 I]I3, [(IML)(DDQ)], [(IML)(CHL)], [(BIML) I]I3, [(BIML)(DDQ)2], [(BIML)(TCNE)2] and [(BIML)(CHL)2].     

Validation and application of a novel UHPLC–MS/MS method for the measurement of furanodienone in rat plasma

A sensitive ultra-high performance liquid chromatography–tandem mass spectrometry (UHPLC–MS/MS) method was established to analyze furanodienone in rat plasma. In the process of chromatographic separation, selected reaction monitoring transitions for furanodienone and patchouli alcohol (internal standard, IS) were m/z 231.1 → 83.2 and m/z 205.1 → 95.1, respectively. Great linearity of furanodienone in plasma samples was found in the corresponding concentration range (r > 0.995). Intra- and inter-day precisions (RSD, %) were <11.3% in plasma, and the accuracy (RE, %) was within ±10.7%. This method was used to the furanodienone study on rat pharmacokinetics after a single oral dose of 10 mg/kg of furanodiene. The results indicated that the maximum observed plasma concentration was 52.4 ± 19.1 ng/ml at 1.2 ± 0.7 h with an elimination half-life of 2.2 ± 0.7 h. The obtained data indicated that furanodienone could be moderately distributed and eliminated.     

Iron porphyrin-modified electrodes: influence of the method of modification on the stability and electroactivity in oxidation of sulfite or hydrogensulfite in ethanol–water solutions

The aim of this work is to study four types of modification of a glassy carbon electrode by Fe(III)-tetrakis(p-tetraaminophenyl)porphyrin and determine the influence of the method of immobilization of the complex on glassy carbon in electrocatalytic properties for the sulfite and hydrogensulfite oxidation in ethanol–water. The first modification was deposition of a drop of solution containing the porphyrin on a glassy carbon electrode and evaporation of the solvent (dry-drop method). The second method was immersion of the electrode at 54°C in a solution of dimethylformamide containing the porphyrin for 2 h. The third method consisted of the same heating treatment but after formation of a chemical bond of 4-aminopyridine on the glassy carbon surface, which acts as an axial ligand for the first layer of porphyrin. The fourth method involves electropolymerization of the porphyrin on the electrode surface. Important differences in stability, the potential where the oxidation wave begins and selectivity of the electrode to sulfite or hydrogensulfite were observed. The behavior of the polymer-modified electrode is different in water compared to ethanol–water.     

Submillimeter-wave spectroscopy of DCO+ in the excited vibrational states: does the Stark effect cause anomalies in the (02(2)0) state?

The lowest two rotational transitions of (02(2)0) were not detected in previous investigations. This nonobservation was ascribed to the Stark broadening caused by the electric field in a hollow cathode discharge and an extended negative glow discharge. However, rotational lines of symmetric-top ions such as CH(3)CNH(+) and SD(3)(+) were observed in extended negative glow discharges with no such Stark effect. Also, no anomalies were observed for similar lines for HCN and HNC produced in an extended negative glow discharge. In the present investigation, we extended the measurements of DCO(+) up to 800 GHz. The DCO(+) ions were produced in an extended negative glow discharge in a gas mixture of D(2) and CO (a couple of millitorr each) in Ar buffer ( approximately 12 mTorr). The measurements were made mostly at liquid nitrogen temperature. Our observations confirmed that the lowest rotational lines in (02(2)0) within our frequency coverage, J=4-3, were too weak to be detected. However, a most notable result obtained in the present investigation is that the J=5-4 and J=6-5 lines of (02(2)0) and the J=5-4 line of (04(2)0) have been detected in induced emission. This observation implies that the previous nonobservation of low-J lines in (02(2)0) may not be due to the Stark effect. The l-type splitting in (03(3)0) has been observed for the J=9-8 transition and higher. However, the splittings for the J=7-6 and J=8-7 lines that are expected to be large enough have not been resolved. The reason for this "narrowing" has been unexplained at the present stage. The population inversion suggests that, initially, DCO(+) is formed predominantly in stretching vibrational states, and, subsequently, the energy transfer to bending vibrational states takes place through collisional relaxation processes.     

Interference effects in 1<Emphasis Type="Italic">s</Emphasis> → π* resonance excitation processes of the no molecule

A ¹Π → X ¹Σ⁺ fluorescence in the NO⁺ molecular ion observed after Auger decay of the 1s ^?1 π* resonances of the N*O molecule and NO* was studied theoretically. The energies and probabilities of the transition between the vibrational levels of the electronic states, determining the excitation and Auger decay of the resonances of the nitrogen monoxide molecule and further radiation-induced decay of the NO⁺ molecular ion were calculated by the first principles method. Multiplet splitting of the resonances of N*O and NO* and interference of the amplitudes of excitation of the molecule through various vibrational levels of the intermediate resonance explain the observed dependences of the intensity of A ¹Π(υ′) → X ¹Σ⁺(υ″) fluorescence on the excitation radiation energy. The discrepancies between the calculated and experimental integrated intensities of fluorescence point to the necessity of studying cascade processes determined by radiation transitions in NO⁺, including dipole transitions with a changed net spin.     

Development and validation of an LC–MS/MS method for the quantification of artificial sweeteners in human matrices

Artificial sweeteners are widely used as substitutes for sugar. The sweeteners are generally considered safe, however their whereabouts during pregnancy and lactation and the effect on child development are poorly explored. There is a need for new tools to measure these substances during pregnancy and lactation. Here, we describe the development and validation of a sensitive liquid chromatography–tandem mass spectrometry method for the simultaneous quantification of acesulfame, cyclamate, saccharin and sucralose in human plasma, umbilical cord blood, amniotic fluid and breast milk. The samples were prepared by protein precipitation and separated on a Luna Omega Polar C₁₈ column (2.1 × 50 mm, 1.6 μm). Electrospray ionization in negative mode and multiple reaction monitoring were used to monitor the ion transitions. The validated concentration ranges were from 1 to 500 ng/ml (10–500 ng/ml for sucralose). Interassay precisions were all ≤15% and the accuracies were within ±15%. Stability, linearity, dilution integrity, carryover and recovery were also examined and satisfied the validation criteria. Finally, this analytical method was successfully applied on spiked samples of plasma, umbilical cord blood, amniotic fluid and breast milk, proving its suitability for use in clinical studies on artificial sweeteners, including during pregnancy and lactation.     

Hydrogen bonding and reactivity of water to azines in their S1 (n,π*) electronic excited states in the gas phase and in solution

A unified picture is presented of water interacting with pyridine, pyridazine, pyrimidine, and pyrazine on the S(1) manifold in both gas-phase dimers and in aqueous solution. As (n,π*) excitation to the S(1) state removes electrons from the ground-state hydrogen bond, this analysis provides fundamental understanding of excited-state hydrogen bonding. Traditional interpretations view the excitation as simply breaking hydrogen bonds to form dissociated molecular products, but reactive processes such as photohydrolysis and excited-state proton coupled electron transfer (PCET) are also possible. Here we review studies performed using equations-of-motion coupled-cluster theory (EOM-CCSD), multireference perturbation theory (CASPT2), time-dependent density-functional theory (TD-DFT), and excited-state Monte Carlo liquid simulations, adding new results from symmetry-adapted-cluster configuration interaction (SAC-CI) and TD-DFT calculations. Invariably, gas-phase molecular dimers are identified as stable local minima on the S(1) surface with energies less than those for dissociated molecular products. Lower-energy biradical PCET minima are also identified that could lead to ground-state recombination and hence molecular dissociation, dissociation into radicals or ions, or hydration reactions leading to ring cleavage. For pyridine.water, the calculated barriers to PCET are low, suggesting that this mechanism is responsible for fluorescence quenching of pyridine.water at low energies rather than accepted higher-energy Dewar-benzene based "channel three" process. Owing to (n,π*) excitation localization, much higher reaction barriers are predicted for the diazines, facilitating fluorescence in aqueous solution and predicting that the as yet unobserved fluorescence from pyridazine.water and pyrimidine.water should be observable. Liquid simulations based on the assumption that the solvent equilibrates on the fluorescence timescale quantitatively reproduce the observed spectral properties, with the degree of (n,π*) delocalization providing a critical controlling factor.     

Speciation analysis of chromium in drinking water samples by ion-pair reversed-phase HPLC–ICP-MS: validation of the analytical method and evaluation of the uncertainty budget

The approach presented in this article refers to the modification of a method for the detection and quantitative determination of chromium species in water by high-performance liquid chromatography inductively coupled plasma mass spectrometry. The main aim of this work was to establish a detailed validation of the analytical procedure and an estimation of the budget of measurement uncertainty which was helpful in recognizing the critical points of the presented method. As a result of the method validation experiment, the obtained limit of quantification, repeatability and intermediate precision were satisfied for the quantification Cr(III) and Cr(VI) in water matrices. The trueness of the method was verified via an estimation of the recovery of the spiked real samples. The recovery rate of both determined analytes was found to be between 93 and 115 %. Considering that the validation of the method and the evaluation of measurement uncertainty are crucial for quantitative analysis, the above-mentioned assessment of the uncertainty budget was performed in two different ways: a modelling approach and a single-laboratory validation approach. The measurement uncertainties of the results were found to be 4.4 and 7.8 % for Cr(III), 4.2 and 7.9 % for Cr(VI) using the classical concept and method validation data, respectively. This paper is the first publication to presenting all the steps needed to evaluate the measurement uncertainty for the speciation analysis of chromium species. In summary, the obtained results demonstrate that the method can be applied effectively for its intended use.