Consecutive Dissociation Research Articles

Photo-Induced Tautomerism of Isocytosine in Aqueous Solution when Irradiated with UVC Light

It was found that the irradiation of aqueous solution of isocytosine with UVC light provokes an oxo-hydroxy phototautomerism of the compound with a rate constant of 5.29× 10-3 min-1. It was observed a backward reaction, after removing the UV light source, with a rate constant of 0.12×10-3 min-1. Two mechanisms of the phototautomerism were investigated at the B3LYP/aug-cc-pVDZ theoretical level in water surroundings (PCM). The first one showed a consecutive dissociation and association of a proton through conical intersections S0/S1 whose structures were located at the same theoretical level in the gas phase. It occurs along the 1πσ* excited-state reaction pathway. The more probable mechanism includes an excited-state H-transfer supported by a water molecule as a catalyst. It occurs along the 1ππ* excited-state reaction pathway which we found over the IRC ground-state energy curve. The water molecule drastically reduces the energy barrier in the ground state as well in the excited state.

Chemical weathering mechanism of Albite-rich rocks in Grottoes under an acidic environment: An atomistic view from ReaxFF simulation

This current ReaxFF simulation provides atomic-level monitoring of the reaction details as well as dynamics of Albite under acidic solution over a nanosecond time scale. 30% H-O bond dissociation in 10% H2SO4 solution and ∼ 0.9% Si-O/Al-O bonds breakage in Albite during 20 ps was evaluated according to the time evolution of number of H-O bonds, Si-O and Al-O bonds at a higher temperature of 323 K. The breakage of Si-O/Al-O bonds within the tetrahedron configuration makes Na atoms be less restricted within the network and generates open channel for Na migration, resulting in further diffusion to the Albite-water interface and complete release into the aqueous medium. A consecutive dissociation of H2SO4 can further accelerate the formation of SO42- products as the H dissociate from H2SO4 molecules. The migrated Na cations tend to combine with SO42- and form Na-SO4 ions pair, leading to a complicated zigzag motion in 3D space and a migration trend towards the aqueous medium along Z-axis direction and a sharp peak at 2.3 Å in Na-O pair RDF.

Interactive network of the dehydrogenation of alkanes, alkenes and alkynes – surface carbon hydrogenative coupling on Ru(111)

The reaction mechanisms of the dehydrogenation and retrosynthesis of alkanes, the consecutive dissociation of methane, ethane, ethene and ethyne, as well as propane, propene and propyne, on the fcc Ru(111) surface has been computed.

Application of triple quadrupole tandem mass spectrometry to the bioanalysis of collision-induced dissociation-resistant cyclic peptides – Ultra-sensitive quantification of the somatostatin-analog pasireotide utilizing UHPLC-MS/MS

Cyclic peptides are considered collision-induced dissociation (CID)-resistant due to immobile protons, and the necessity of at least two consecutive dissociation reactions to produce fragments with deviating m/z values. Therefore, the bioanalysis of cyclic peptides by tandem mass spectrometry (MS/MS) poses a major challenge, especially on triple quadrupole (TQ) instruments. One of these peptides is the somatostatin analog pasireotide, a cyclic hexapeptide administered to treat Cushing’s disease and acromegaly. To support oral formulation development, sub-therapeutic quantification of pasireotide is highly beneficial. Regardless of the considered CID-resistance, we investigated the CID-characteristics of pasireotide and subsequently developed an ultra-sensitive UHPLC-MS/MS assay with a lower limit of quantification (LLOQ) of 5 pg/mL (4.9 pM) when using 100 μL of plasma and validated it according to the guidelines of the FDA and EMA. The achieved sensitivity, which is the highest thus far reported, demonstrates that TQ-MS/MS is a feasible approach to sensitive quantification of cyclic peptides despite their CID-resistance. Pasireotide was fast and efficiently extracted by protein depletion via precipitation using acetonitrile. Correlation coefficients > 0.99 were achieved for all calibration curves with linear regression. Inter-run and intra-run accuracy ranged from 89.4 to 99.3 % with corresponding precision of ≤ 7.5 % in the calibrated range, and from 94.6 to 105.6 % with corresponding precision of ≤ 14.5 % at the LLOQ. Quantification of 10-fold diluted samples showed an accuracy of 90.8 % and corresponding precision of 4.0 %. The assay was applied to the quantification of pasireotide plasma concentrations after intravenous administration to beagle dogs.

Method of UV-Metric and pH-Metric Determination of Dissociation Constants of Ionizable Drugs: Valsartan

Valsartan is used for treating high blood pressure, congestive heart failure and to increase the chances of living longer after a heart attack and to reduce the mortality rate for people with left ventricular dysfunction following a heart attack. Regression analysis of the pH-spectra with REACTLAB and of the pH-titration curve with ESAB determined two close consecutive dissociation constants. MARVIN and ACD/Percepta predicted two protonation sites. In water a soluble anion L2− forms two sparingly soluble species LH−, LH2. Although adjusted pH less affected the spectral changes in the chromophore, $${\text{p}}K_{a1}^{\text{T}}$$ = 3.70 ± 0.12, $${\text{p}}K_{a2}^{\text{T}}$$ = 4.82 ± 0.08 at 25 °C and $${\text{p}}K_{a1}^{\text{T}}$$ = 3.44 ± 0.08, $${\text{p}}K_{a2}^{\text{T}}$$ = 4.67 ± 0.02 at 37 °C in an aqueous phosphate buffer were determined. By regression analysis of potentiometric pH-titration curves and $${\text{p}}K_{a1}^{\text{T}}$$ = 3.51 ± 0.01, $${\text{p}}K_{a2}^{\text{T}}$$ = 4.63 ± 0.01, at 25 °C and $${\text{p}}K_{a1}^{\text{T}}$$ = 3.44 ± 0.03, $${\text{p}}K_{a2}^{\text{T}}$$ = 4.51 ± 0.03 at 37 °C in an aqueous medium were estimated. Positive enthalpy values ΔH0(pKa1) = 10.33 kJ·mol−1, ΔH0(pKa2) = 17.70 kJ·mol−1 showed that the dissociation process was endothermic. The standard state Gibbs energy changes are ΔG0(pKa1) = 20.03 kJ·mol−1, ΔG0(pKa2) = 26.43 kJ·mol−1 at 25 °C and the ΔS0 at 25 °C and 37 °C are (ΔS0(pKa1) = − 32.56 J·K−1·mol−1, ΔS0(pKa2) = − 29.26 J·K−1·mol−1 at 25 °C and ΔS0(pKa1) = − 30.01 J·K−1·mol−1, ΔS0(pKa2) = − 25.92 J·K−1·mol−1 at 37 °C.

UV-vis action spectroscopy and structures of hydrogen-rich 2′-deoxycytidine dinucleotide cation radicals. A difficult case

Hydrogen-rich cation radicals of 2′-deoxycytidine dinucleotide, (dCC + 2H)+●, were generated by electron transfer dissociation of their doubly charged complexes with crown ethers and found to be stable in the gas phase. The (dCC + 2H)+● ions were calculated to be formed with ca. 280 kJ mol-1 internal energy that was rapidly dissipated below a 130 kJ mol-1 energy threshold for the lowest-energy dissociation by loss of water. UV-vis action spectroscopy was applied to monitor several photodissociation channels of (dCC + 2H)+● and to compose spectra that showed distinct absorption bands at 220, 270, 290, and 350 nm, and a broad featureless band at 500-700 nm. UV-vis action spectra were also obtained for the (dCC ‒ H2O + 2H)+● cation radical that showed bands at 225, 270, 330, 430, and 530 nm. Born-Oppenheimer molecular dynamics and density functional theory (DFT) calculations were employed to generate a complete set of tautomeric (dCC + crown + 2H)2+ precursor dications and identify the lowest-energy structures in the gas phase and aqueous solution that were the cytosine N-3-H tautomers. Calculations also indicated that cytosine cations and radicals in the lowest-energy (dCC + 2H)+● ions were N-3-H tautomers. Absorption spectra of several (dCC + 2H)+● tautomers were obtained by time-dependent DFT calculations that provided a limited match with the action spectrum. In contrast, a satisfactory match was obtained for the action and absorption spectra of (dCC ‒ H2O + 2H)+● fragment ions. (dCC + 2H)+● represents a difficult case because of a multitude of cation and radical tautomers to be analyzed and the propensity for consecutive photofragment dissociation upon photon absorption.

A Search for the Protonation Model with Thermodynamic Dissociation Constants and (Extra)-Thermodynamics of Nilotinib Hydrochloride (TASIGNA)

Nilotinib hydrochloride (AMN107, TASIGNA, Novartis) is used to treat adults with chronic myeloid leukemia (CML), a type of leukemia. It is a novel, orally active BCR-ABL tyrosine kinase inhibitor derived from aminopyrimidine that is 30 times more effective against CML cells than is Imatinib. The nonlinear regression of the A versus pH spectra with REACTLAB and SQUAD84 and of the pH-titration curve with ESAB determined the four close and consecutive dissociation constants in the 11 steps of the newly proposed procedure. Prediction of pKa performed by MARVIN, PALLAS and ACD/Percepta determined the protonation sites. The sparingly soluble nilotinib hydrochloride denoted as L forms four water-soluble LH+, $${\text{LH}}_{2}^{2 + }$$ , $${\text{LH}}_{3}^{3 + }$$ , and $${\text{LH}}_{4}^{4 + }$$ cations. Although the adjusted pH has less effect on the absorbance changes in the chromophore, four thermodynamic dissociation constants were reliably determined: $${\text{p}}K_{\text{a1}}^{\text{T}}$$ = 3.60 ± 0.04, $${\text{p}}K_{\text{a2}}^{\text{T}}$$ = 4.42 ± 0.07, $${\text{p}}K_{\text{a3}}^{\text{T}}$$ = 4.71 ± 0.04, and $${\text{p}}K_{\text{a4}}^{\text{T}}$$ = 4.84 ± 0.03 at 25 °C and $${\text{p}}K_{\text{a1}}^{\text{T}}$$ = 3.61 ± 0.11, $${\text{p}}K_{\text{a2}}^{\text{T}}$$ = 4.29 ± 0.18, $${\text{p}}K_{\text{a3}}^{\text{T}}$$ = 4.49 ± 0.02, and $${\text{p}}K_{\text{a4}}^{\text{T}}$$ = 5.05 ± 0.03 at 37 °C, and by regression analysis of potentiometric titration curves with ESAB, $${\text{p}}K_{\text{a1}}^{\text{T}}$$ = 3.74 ± 0.01, $${\text{p}}K_{\text{a2}}^{\text{T}}$$ = 4.05 ± 0.01, $${\text{p}}K_{\text{a3}}^{\text{T}}$$ = 4.25 ± 0.01, and $${\text{p}}K_{\text{a4}}^{\text{T}}$$ = 4.91 ± 0.20 at 25 °C and $${\text{p}}K_{\text{a1}}^{\text{T}}$$ = 3.63 ± 0.03, $${\text{p}}K_{\text{a2}}^{\text{T}}$$ = 3.96 ± 0.03, $${\text{p}}K_{\text{a3}}^{\text{T}}$$ = 4.18 ± 0.03, and $${\text{p}}K_{4 1}^{\text{T}}$$ = 4.81 ± 0.05 at 37 °C. Positive enthalpy values ΔH° at 25 °C showed that the dissociation process is endothermic and is accompanied by heat absorption. Inasmuch as the entropy values of the dissociation process ΔS° at 25 °C and 37 °C were negative, the dissociation process is reversible.

Dissociation of hydrazine on tetrahedral Ni4 cluster by density functional theory

Sequential dissociation of hydrazine has been studied on Ni4 cluster applying density functional theory (DFT) methods. Two routes of dissociation have been discussed. The first path (A) is the consecutive dissociation of four N–H bonds to form surface N2 via intermediate product diimide, N2H2. The second path (B) is the dissociation of N–N bond to form two NH2 species leading to formation of surface ammonia. The second elementary steps for both the routes show the highest activation energy barrier; for example in path A, N2H3 + H → N2H2 + 2H, EAct is 1.19 eV and in path B, 2NH2 → NH + H+NH2, EAct is 1.71 eV. These are the rate-determining steps. NBO analysis shows that adsorption of hydrazine and ammonia is due to strong delocalisation of the lone pairs to the higher energy states of the cluster. Adsorption and dissociation of hydrazine and ammonia are thermodynamically feasible at standard conditions. Formation of N2 is a slow exothermic process, whereas N–N bond dissociation to form two NH2 species is a faster and highly exothermic process. NH species binds by two Ni–N covalent bonds. N species binds the cluster by three Ni–N bonds and three strong lone pair delocalisation at three-fold site. Removal of these intermediates needs higher energy of activation. Thus, the formation and dehydrogenation of ammonia are slow and lengthy processes. New catalysts could be designed in such a way that N–N bond might not be dissociated, which is happening due to absence of lone pair of N1(LP) to Rydberg orbital of N2(RY*) delocalisation or vice versa.

Plasma Catalysis: Distinguishing between Thermal and Chemical Effects

The goal of this study is to develop a method to distinguish between plasma chemistry and thermal effects in a Dielectric Barrier Discharge nonequilibrium plasma containing a packed bed of porous particles. Decomposition of CaCO3 in Ar plasma is used as a model reaction and CaCO3 samples were prepared with different external surface area, via the particle size, as well as with different internal surface area, via pore morphology. Also, the effect of the CO2 in gas phase on the formation of products during plasma enhanced decomposition is measured. The internal surface area is not exposed to plasma and relates to thermal effect only, whereas both plasma and thermal effects occur at the external surface area. Decomposition rates were in our case found to be influenced by internal surface changes only and thermal decomposition is concluded to dominate. This is further supported by the slow response in the CO2 concentration at a timescale of typically 1 minute upon changes in discharge power. The thermal effect is estimated based on the kinetics of the CaCO3 decomposition, resulting in a temperature increase within 80 °C for plasma power from 0 to 6 W. In contrast, CO2 dissociation to CO and O2 is controlled by plasma chemistry as this reaction is thermodynamically impossible without plasma, in agreement with fast response within a few seconds of the CO concentration when changing plasma power. CO forms exclusively via consecutive dissociation of CO2 in the gas phase and not directly from CaCO3. In ongoing work, this methodology is used to distinguish between thermal effects and plasma–chemical effects in more reactive plasma, containing, e.g., H2.

Selective Oxidation of n‐Butane over Vanadium‐Phosphorus Oxide: Oxygen Activation and Dynamics

AbstractThe complex microkinetics of the selective oxidation of n‐butane to maleic anhydride (MAN) over vanadyl pyrophosphate‐based (VPP) catalysts has been widely investigated so far, however, they are still under debate. We present novel results of kinetic analyses and isotope labeling to shed light on the activation of gas‐phase oxygen and the dynamics of oxygen species on the catalyst surface and in the bulk. We suggest the existence of at least two catalytically relevant surface oxygen species responsible for selective and unselective activation of n‐butane, which are formed by adsorption and consecutive dissociation of O2 at the active site. The formation of selective [O] from unselective [O2] is likely promoted by co‐fed H2O, which increases the MAN selectivity. Regarding the VPP bulk serving as a reservoir for active [O] species it is concluded that the dynamic exchange is limited to the amorphous surface layer forming under reaction conditions, whereas the crystalline bulk is not involved into the reaction. Labelling studies are heavily influenced by vital 16O/18O scrambling of MAN isotopes with the reaction product H2O.

A Sequential Debonding Fracture Model for Hydrogen‐Bonded Hydrogels

ABSTRACTHydrogen bonds are known to play an important role in prescribing the mechanical performance of certain hydrogels such as polyether‐based polyurethanes. The quantitative contribution of hydrogen bonds to the toughness of polymer networks, however, has not been elucidated to date. Here, a new physical model is developed to predict the threshold fracture energies of hydrogels physically crosslinked via hydrogen bonds. The model is based on consecutive and sequential dissociation of hydrogen‐bonded crosslinks during crack propagation. It is proposed that the scission of hydrogen bonds during crack propagation allows polymer strands in the deformation zone to partially relax and release stored elastic energy. The summation of these partial chain relaxations leads to amplified threshold fracture energies which are 10–45 times larger than those predicted by the classical Lake–Thomas theory. Experiments were performed on a hydrophilic polyurethane hydrogel where urea additions were used to control the density of hydrogen bonds. The measured fracture energies were in good agreement with the calculated values. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 1287–1293

Multiwavelength UV-metric and pH-metric determination of the dissociation constants of the hypoxia-inducible factor prolyl hydroxylase inhibitor Roxadustat

The dissociation constant pKa of the Roxadustat with UV-VIS spectra, molar absorption coefficients and protonation equilibria are the key physicochemical parameters influencing many biopharmaceutical characteristics. Roxadustat is an orally bioavailable, hypoxia-inducible factor prolyl hydroxylase inhibitor with potential anti-anemic activity. It belongs to Active Pharmacenutical Ingredients, which have acidic/basic functionalities, their ionization state is controlled by solution pH and acid dissociation constants. Nonlinear regression of the pH-spectra with programs REACTLAB and SQUAD84 and of the pH-titration curve with ESAB determined four multiple consecutive dissociation constants with the protonation scheme. A sparingly soluble neutral molecule LH3 of Roxadustat was dissociated to the soluble anions LH2−, LH2− and L3− or protonated to the cation LH4+ in an aqueous medium. The graph of molar absorption coefficients of variously protonated species according to wavelength shows that the spectra of two anions LH2− and LH2− are nearly the same in colour. The Roxadustat spectrum exhibited five sharp isosbestic points, which were related to the LH2−/L3− equilibrium. Four consecutive thermodynamic dissociation constants were estimated using UV-metric data pKTa1 = 3.60(04), pKTa2 = 5.62(14), pKTa3 = 7.66(16), pKTa4 = 9.08(02) at 25 °C and pKTa1 = 3.60(04), pKTa2 = 5.73(10), pKTa3 = 7.52(10), pKTa4 = 8.99(02) at 37 °C and using pH-metric data pKTa1 = 4.33(09), pKTa2 = 6.57(11), pKTa3 = 8.88(05), pKTa4 = 9.03(04) at 25 °C and pKTa1 = 4.25(09), pKTa2 = 6.49(10), pKTa3 = 8.80(06), pKTa4 = 9.00(05) at 37 °C The positive values of the enthalpy ΔH0 showed that the dissociation process is endothermic and the positive values of the Gibbs free energy ΔG0 at 25 °C indicated that the dissociation process was not spontaneous, which also was confirmed by a negative value of the entropy ΔS0. Four macro-dissociation constants of Roxadustat and six protonation locations were predicted by MARVIN.

Multiwavelength UV-metric and pH-metric determination of the multiple dissociation constants of the lesinurad

Potentiometric and spectrophotometric pH-titrations of the lesinurad for three consecutive dissociation constants determination were compared. Lesinurad is a selective inhibitor of uric acid reabsorption as part of a combination of medicines to treat high levels of uric acid in blood, also called hyperuricemia. Nonlinear regression of the pH-spectra with REACTLAB and SQUAD84 and of the pH-titration curve with ESAB determined three multiple close dissociation constants. The protonation scheme of lesinurad was suggested. A sparingly soluble anion L− of lesinurad was protonated to the still soluble species LH, LH2+ and LH32+ in pure water. Three consecutive thermodynamic dissociation constants were estimated pKTa1 = 2.09, pKTa2 = 4.25, pKTa3 = 6.58 at 25 °C and pKTa1 = 1.96, pKTa2 = 4.16, pKTa3 = 6.32 at 37 °C by UV-metric spectra analysis. The graph of molar absorption coefficients shows that the spectrum of species LH2+ and LH vary in colour, while protonation of chromophore LH2+ to LH32+ has less influence on chromophores in the lesinurad molecule. Three multiple thermodynamic dissociation constants of 1 × 10−4 M lesinurad were determined by the pH-metric analysis pKTa1 = 2.39, pKTa2 = 3.47, pKTa3 = 6.17 at 25 °C and pKTa1 = 2.08, pKTa2 = 3.29, pKTa3 = 6.03 at 37 °C. The values of enthalpy ΔH0(pKa1) = 19.19 kJ mol−1, ΔH0(pKa2) = 13.29 kJ mol−1, ΔH0(pKa3) = 38.39 kJ mol−1, show the dissociation process is endothermic. The positive values of ΔG0(pKa1) = 11.93 kJ mol−1, ΔG0(pKa2) = 24.26 kJ mol−1, ΔG0(pKa3) = 37.56 kJ mol−1 at 25 °C indicate that the dissociation process of pKa2 is not spontaneous, which was confirmed by its value of entropy ΔS0(pKa1) = 24.37 J mol−1, ΔS0(pKa2) = −36.79 J mol−1, ΔS0(pKa3) = 2.79 J mol−1. Three macro-dissociation constants of lesinurad and protonation locations were predicted by MARVIN and ACD/Percepta.

Multiple dissociation constants of the Intepirdine hydrochloride using regression of multiwavelength spectrophotometric pH-titration data

Spectrophotometric and potentiometric pH-titrations of the Neurotransmitter Intepirdine hydrochloride INN·HCl for three dissociation constants determination were compared. A nonlinear regression of the pH-spectra (REACTLAB, SQUAD84) and of the pH-titration curve (ESAB) determined three multiple dissociation constants. A sparingly soluble neutral molecule LH of INN·HCl was capable of protonation to form the still soluble three cations LH2+, LH32+ and LH43+ in pure water. Although the change of pH somewhat less affected changes in the chromophore, three consecutive thermodynamic dissociation constants were estimated pKTa1=5.64, pKTa2=7.31, pKTa3=8.85 at 25°C and pKTa1=5.51, pKTa2=7.15, pKTa3=8.77 at 37°C. The graph of molar absorption coefficients of variously protonated species according to wavelength shows that the spectrum of species LH2+ and LH vary in colour, while protonation of chromophore LH2+ to LH32+ and LH43+ has less influence on chromophores in Intepirdine hydrochloride molecule. As the spectral response on the chromophore in the INN·HCl molecule is not large, it was necessary to test a reliability whether it is possible to estimate the dissociation constants even from such small spectrum changes. Three multiple thermodynamic dissociation constants of 3×10−4M INN·HCl were determined by the regression analysis of potentiometric titration curves pKTa1=5.14, pKTa2=8.38, pKTa3=9.33 at 25°C and pKTa1=5.17, pKTa2=8.31, pKTa3=9.07 at 37°C. The macro-dissociation constants of INN·HCl were estimated according to the chemical structure analyzed by two pKa predictors, the MARVIN and ACD/Percepta programs. The resulting protonation scheme of INN·HCl was suggested.

Sequence analysis of cyclic polyester copolymers using ion mobility tandem mass spectrometry

Tandem mass spectrometry (MS2) via collisionally activated dissociation (CAD), coupled with ion mobility (IM) separation, has been employed to determine the sequence of macrocyclic polyester copolymers with thermoresponsive properties. The polyesters were synthesized by polyesterification of succinic acid with two diols functionalized with different, bioinspired amide pendants. MS analysis confirmed the formation of cyclic and linear chains composed of two different polyester repeat units, N and P. MS2 experiments on the sodiated macrocyclic trimer P3 showed that fragmentation encompasses ring opening by 1,5-H rearrangement over an ester group and consecutive dissociation of the ring-opened species via the same mechanism. Investigation of the macrocyclic trimers N1P2 and N2P3 revealed that ring opening occurs randomly at any of the ester groups of the macrocycle. Armed with these data, the sequence of the macrocyclic tetramer N2P2 was elucidated based on the dimeric NN, NP (or PN), and PP fragments generated by CAD of the sodiated ion. The observed fragment distribution provided evidence that the tetramer had predominantly a block architecture, viz. cyclo-NNPP. Preference for the block arrangement of the monomers was also found for a linear oligomer with four diol units. The IM dimension enabled the examination of singly sodiated cyclic or linear chains that overlapped with higher charge states of larger oligomers. The described sequencing strategy should be generally applicable to polymers with ester groups in the backbone. The architectural information deduced by such sequencing would significantly facilitate structure/property correlations and the design of copolymers with the desired physical properties.

The Femtochemistry of a Ferracyclobutadiene.

The eminent role of metallacyclobutadienes as catalytic intermediates in organic synthesis and polymer chemistry is widely acknowledged. In contrast, their photochemistry is as yet entirely unexplored. Herein, the photo-induced primary processes of a ferracyclobutadiene tricarbonyl complex in solution are revealed by femtosecond mid-infrared spectroscopy. The time-resolved vibrational spectra expose an ultrafast substitution of a basal CO ligand by a solvent molecule in a consecutive dissociation-association mechanism. Following optical excitation, the system relaxes non-radiatively to the triplet ground state from which a CO is expelled. Since the triplet state is bound with respect to Fe-CO cleavage, the dissociation can only occur from vibrationally excited states. The excitation energy, vibrational relaxation, and intersystem crossing to the singlet ground state control the primary quantum yield for formation of the ferracyclic dicarbonyl-solvent product complex.

The Femtochemistry of a Ferracyclobutadiene

AbstractThe eminent role of metallacyclobutadienes as catalytic intermediates in organic synthesis and polymer chemistry is widely acknowledged. In contrast, their photochemistry is as yet entirely unexplored. Herein, the photo‐induced primary processes of a ferracyclobutadiene tricarbonyl complex in solution are revealed by femtosecond mid‐infrared spectroscopy. The time‐resolved vibrational spectra expose an ultrafast substitution of a basal CO ligand by a solvent molecule in a consecutive dissociation–association mechanism. Following optical excitation, the system relaxes non‐radiatively to the triplet ground state from which a CO is expelled. Since the triplet state is bound with respect to Fe−CO cleavage, the dissociation can only occur from vibrationally excited states. The excitation energy, vibrational relaxation, and intersystem crossing to the singlet ground state control the primary quantum yield for formation of the ferracyclic dicarbonyl–solvent product complex.

An experimental study on the formation behavior of single and binary hydrates of TBAB, TBAF and TBPB for cold storage air conditioning applications

Thermal storage can be applied to air conditioning systems to shift the demand on electricity grids to decrease the peak load. It is also a feasible backup for solar cooling systems to supply on-site loads during solar outages. Although chilled water has been widely used for thermal storage, phase change materials offer greater energy storage density than chilled water. Semi-clathrate hydrates, having large heat of fusion and phase transition temperatures in the range of 5–10°C, are proposed for thermal storage in air conditioning applications. This work presents an experimental study on the formation behavior of semi-clathrate hydrates based on tetra-n-butylammonium bromide (TBAB), tetra-n-butylammonium fluoride (TBAF) and tetra-n-butylphosphonium bromide (TBPB). The experiments vary the salt mass fraction from 10wt% to 40wt%. Single salt hydrates and binary salt mixtures are studied at various proportions. Furthermore, surfactant (sodium dodecyl sulfate, 0.05–0.5wt%) and nanoparticles (TiO2, 20–80nm) are employed to aid hydrate formation. The effect of the temperature difference driving force and memory effect on the hydrate formation are examined through consecutive formation and dissociation cycles. The formation temperature, maximum induction temperature and induction time are measured. It is observed that the supercooling and induction time differ for different salt hydrates. Both these parameters can be modified for air conditioning applications by suitable additives and proper operating conditions. Images of crystal morphology indicate that the columnar crystals of TBAB and TBAF are more compact than the hexagonal crystals of TBPB.

On the formation, stability, and dissociation of peptide radicals after femtosecond electron transfer from alkali metal atoms

In femtosecond encounters between peptide cations and alkali metal atoms, electron transfer occurs from the latter to the former. The peptide radicals are born in the same structure as their precursors and undergo subsequent dissociation on a microsecond to millisecond time scale. The fragmentation pattern is very similar to that observed from other electron-induced dissociation techniques: electron capture dissociation (ECD) where free electrons are captured and electron transfer dissociation (ETD) where the electron donor is an atomic or molecular anion. Dissociation channels involve either (i) loss of a neutral fragment (hydrogen or ammonia) and in some cases consecutive dissociation or (ii) breakage of the peptide backbone, i.e., NCα bond dissociation. We have dubbed the collisional electron transfer technique Electron Capture Induced Dissociation (ECID) to distinguish it from its relatives, ECD and ETD. Here we review ECID results with focus on the competition between dissociation channels and dependence on peptide size and conformation, crown-ether tagging and methylation of ammonium groups, microsolvation, phosphorylation, and the electronic state from which dissociation takes place. While ECID cannot compete with ECD or ETD as an analytical technique, it has provided a wealth of fundamental knowledge on the fragmentation of peptide radicals from work on simple model systems. Its combination with MALDI time-of-flight instruments may, however, show some potential use in the future as ECID does not rely on precursor ions being doubly charged (or higher charged) as neutral fragments can efficiently be converted to anions in secondary collisions.

A ReaxFF-based molecular dynamics study of the pyrolysis mechanism of polyimide

Reactive Force Field (ReaxFF) molecular dynamics simulation is first employed in Kapton-type polyimide pyrolysis process. A series of ReaxFF simulations are carried out to study the pyrolysis mechanisms of polyimide. The effects of temperature on the distribution of various products are investigated. Carbon dioxide and cyano radical are the dominant products, and a large number of acetylene molecules are first discovered at high temperature. Other small molecular products including water, carbon monoxide and hydrogen are detected in the ultimate products as well. Formation mechanisms of dominant products (CO2 and CN) are discussed in detail for the first time based on the simulation trajectories. The cleavage of C–N bond on the imide ring is the common initial step for forming CO2 and CN. The structure (–C(=O)–O–C–) is important and conducive for generating CO2, whereas the consecutive dissociation of C–N on the imide ring and the de-carbonization reaction from the benzene are integral for forming CN. The apparent activation energy and pre-exponential factor for pyrolysis of polyimide extracted from the ReaxFF simulations are well consistent with experimental results. The present work demonstrates that ReaxFF simulation is a promising method to elucidate detailed chemical reaction mechanisms in polymer pyrolysis.