On-Line NMR Spectroscopic Reaction Kinetic Study of Urea–Formaldehyde Resin Synthesis

Quantitative on-line NMR spectroscopy is used to study the kinetics of the reaction of aqueous formaldehyde and urea. The investigation focuses on the formation of low molecular mass compounds during the methylolation step. The experiments were carried out at overall formaldehyde to urea molar ratios between 1:1 and 4:1, pH values between 6 and 8, and temperatures between 313 and 353 K. The experimental data were used to develop a kinetic model based on the true species concentrations. The model describes the experimental data well and can be used to predict the composition of the reacting mixture of aqueous formaldehyde and urea during the methylolation step as a function of time

Multistate-Mediated Rearrangements and FeCl₂ Elimination in Dinuclear FePd Complexes

Mass spectrometric, spectroscopic, and computational characterization of a novel bifunctional iron–palladium complex proves a change of coordination upon solvation. Collisional excitation reveals FeCl₂ and HCl elimination in a solvent-modulated competition. Hereby, <i>syn</i> and <i>anti</i> isomers, identified by theoretical calculations, favor and disfavor FeCl₂ elimination, respectively. The FeCl₂ elimination likely proceeds by chlorido and Cp ligand exchange among the metallic centers in a concerted, ballet-like manner. A multitude of stationary points were identified along the computed multistep reaction coordinates of the three conceivable spin states. The quintet state shows a static Jahn–Teller type relaxation by a tilt away of the Cp ligand at the iron center. The direct singlet–quintet spin crossover is an unprecedented assumption, leaving behind the triplet state as a spectator without involvement. The FeCl₂ elimination would decrease catalytic activity. It is kinetically hindered within a range of applicable temperatures in conceivable technical applications

NMR Spectroscopic Study of the Aldoxane Formation in Aqueous Acetaldehyde Solutions

Crotonaldehyde is an interesting intermediate in the chemical industry. It is usually produced from aqueous acetaldehyde in a two step process in which the first step is carried out under basic and the second step under acidic conditions. It is commonly assumed that acetaldehyde is converted in the first step to acetaldol and that acetaldol is subsequently dehydrated in the second step to crotonaldehyde. We demonstrate by ¹H and ¹³C NMR spectroscopic studies that acetaldol is hardly present in the reacting solutions at lower temperatures and that the key intermediate is aldoxane (2,6-dimethyl-1,3-dioxane-4-ol). For the first time, data on the chemical equilibrium of the aldoxane formation in aqueous acetaldehyde solutions is provided. Furthermore, preliminary information on the kinetics of that reaction is presented

Monoalkylcarbonate Formation in Methyldiethanolamine–H₂O–CO₂

In this work, the monoalkylcarbonate ((<i>N</i>-hydroxyethyl)­(<i>N</i>-methyl)­(2-aminoethyl) hydrogen carbonate) formation in the system methyldiethanolamine (MDEA)–water (H₂O)–carbon dioxide (CO₂) is investigated by nuclear magnetic resonance (NMR) spectroscopy. Aqueous solutions containing 0.4 g/g of MDEA were loaded with CO₂ in valved NMR tubes, and the composition of the liquid phase in equilibrium was determined <i>in situ</i> at 298 K at pressures up to 11 bar. By two-dimensional NMR, the presence of monoalkylcarbonate was verified, which has been widely overlooked in the literature so far. The experimental data of this work and reevaluated NMR data obtained in previous work of our group were used to calculate chemical equilibrium constants of the proposed monoalkylcarbonate formation. A model taken from the literature that describes the solubility of CO₂ in aqueous solution of MDEA and the corresponding species distribution is extended so that it can account for the monoalkylcarbonate in the liquid phase as well. The extended model is validated using NMR data in the temperature range 273–333 K. The study shows that more than 10 mol % of the absorbed CO₂ is bound as monoalkylcarbonate under conditions relevant for technical applications

Online ¹H NMR Spectroscopic Study of the Reaction Kinetics in Mixtures of Acetaldehyde and Water Using a New Microreactor Probe Head

Mixtures of acetaldehyde and water are reactive multicomponent systems because poly­(oxymethylmethylene) glycols are formed. A study on the kinetics of the formation of these oligomers was carried out using a new microreactor NMR probe head that combines online flow ¹H NMR spectroscopy with microreaction technology. The study covers temperatures between 278 and 298 K and pH values between 3.5 and 10.3. From the peak areas in the ¹H NMR spectra, quantitative results for the conversion of acetaldehyde were obtained. On the basis of the new data, a reaction kinetic model was developed and numbers for the kinetic constants of poly­(oxymethylmethylene) glycol formation were determined together with a correlation that describes their dependence on the temperature and pH value

Exploring the Gas-Phase Activation and Reactivity of a Ruthenium Transfer Hydrogenation Catalyst by Experiment and Theory in Concert

This study elucidates structures, activation barriers, and the gas-phase reactivity of cationic ruthenium transfer hydrogenation catalysts of the structural type [(η⁶-cym)­RuX­(pympyr)]⁺. In these complexes, the central ruthenium­(+II) ion is coordinated to an η⁶-bound <i>p</i>-cymene (η⁶-cym), a bidentate 2-R-4-(2-pyridinyl)­pyrimidine ligand (pympyr) with R = NH₂ or N­(CH₃)₂, and an anion X = I^–, Br^–, Cl^–, or CF₃SO₃^–. We present infrared multiple-photon dissociation (IR-MPD) spectra of precursors (before HCl loss) and of activated complexes (after HCl loss), which elucidates C–H activation as the key step in the activation mechanism. A resonant two-color IR-MPD scheme serves to record several otherwise “dark” bands and enhances the validity of spectral assignments. We show that collision-induced dissociation (CID)-derived activation energies of the [(η⁶-cym)­RuX­(pympyr)]⁺ (R = N­(CH₃)₂) complexes depend crucially on the anion X. The obtained activation energies for the HX loss correlate well with quantum chemical activation barriers and are in line with the HSAB concept. We further elucidate the reaction of the activated complexes with D₂ under single-collision conditions. Quantum mechanical simulations substantiate that the resulting species represent analogues for hydrido intermediates formed after abstraction of H⁺ and H^– from isopropanol, as postulated for the catalytic cycle of transfer hydrogenation by us before

Cyclopentadienide Ligand Cp^C– Possessing Intrinsic Helical Chirality and Its Ferrocene Analogues

The novel chiral cyclopentadiene-type ligand Cp^CH is accessible from dibenzosuberenone in a five-step sequence with overall yields of 64%. NMR spectroscopy as well as DFT calculations prove that the racemization of this compound is slow at room temperature. By deprotonation of Cp^CH and subsequent reaction with appropriate iron­(II) precursors, the novel ferrocene derivatives (Cp^C)₂Fe and (Cp^C)­Fe­(⁴Cp) are accessible in good yields. The latter could structurally be characterized by means of single-crystal X-ray crystallography. Mössbauer spectroscopy proves the ferrocene nature of (Cp^C)₂Fe and (Cp^C)­Fe­(⁴Cp), and electrochemical investigations carried out with (Cp^C)­Fe­(⁴Cp) show that the compound is, as expected, more easily oxidized than ferrocene

Cyclopentadienide Ligand Cp^C– Possessing Intrinsic Helical Chirality and Its Ferrocene Analogues

The novel chiral cyclopentadiene-type ligand Cp^CH is accessible from dibenzosuberenone in a five-step sequence with overall yields of 64%. NMR spectroscopy as well as DFT calculations prove that the racemization of this compound is slow at room temperature. By deprotonation of Cp^CH and subsequent reaction with appropriate iron­(II) precursors, the novel ferrocene derivatives (Cp^C)₂Fe and (Cp^C)­Fe­(⁴Cp) are accessible in good yields. The latter could structurally be characterized by means of single-crystal X-ray crystallography. Mössbauer spectroscopy proves the ferrocene nature of (Cp^C)₂Fe and (Cp^C)­Fe­(⁴Cp), and electrochemical investigations carried out with (Cp^C)­Fe­(⁴Cp) show that the compound is, as expected, more easily oxidized than ferrocene

Cyclopentadienide Ligand Cp^C– Possessing Intrinsic Helical Chirality and Its Ferrocene Analogues

The novel chiral cyclopentadiene-type ligand Cp^CH is accessible from dibenzosuberenone in a five-step sequence with overall yields of 64%. NMR spectroscopy as well as DFT calculations prove that the racemization of this compound is slow at room temperature. By deprotonation of Cp^CH and subsequent reaction with appropriate iron­(II) precursors, the novel ferrocene derivatives (Cp^C)₂Fe and (Cp^C)­Fe­(⁴Cp) are accessible in good yields. The latter could structurally be characterized by means of single-crystal X-ray crystallography. Mössbauer spectroscopy proves the ferrocene nature of (Cp^C)₂Fe and (Cp^C)­Fe­(⁴Cp), and electrochemical investigations carried out with (Cp^C)­Fe­(⁴Cp) show that the compound is, as expected, more easily oxidized than ferrocene

Cyclopentadienide Ligand Cp^C– Possessing Intrinsic Helical Chirality and Its Ferrocene Analogues

The novel chiral cyclopentadiene-type ligand Cp^CH is accessible from dibenzosuberenone in a five-step sequence with overall yields of 64%. NMR spectroscopy as well as DFT calculations prove that the racemization of this compound is slow at room temperature. By deprotonation of Cp^CH and subsequent reaction with appropriate iron­(II) precursors, the novel ferrocene derivatives (Cp^C)₂Fe and (Cp^C)­Fe­(⁴Cp) are accessible in good yields. The latter could structurally be characterized by means of single-crystal X-ray crystallography. Mössbauer spectroscopy proves the ferrocene nature of (Cp^C)₂Fe and (Cp^C)­Fe­(⁴Cp), and electrochemical investigations carried out with (Cp^C)­Fe­(⁴Cp) show that the compound is, as expected, more easily oxidized than ferrocene