Volume 234, Issue 7-9 - Special Issue dedicated to Friedrich Temps on the occasion of his 65th birthday; Guest Editors: Gernot Friedrichs, Nils Hansen

The rate coefficient for the unimolecular decomposition of CH 3 OCH 2 , k 1 , has been measured in time-resolved experiments by monitoring the HCHO product. CH 3 OCH 2 was rapidly and cleanly generated by 248 nm excimer photolysis of oxalyl chloride, (ClCO) 2 , in an excess of CH 3 OCH 3 , and an excimer pumped dye laser tuned to 353.16 nm was used to probe HCHO via laser induced fluorescence. k 1 ( T , p ) was measured over the ranges: 573–673 K and 0.1–4.3 × 10 18 molecule cm −3 with a helium bath gas. In addition, some experiments were carried out with nitrogen as the bath gas. Ab initio calculations on CH 3 OCH 2 decomposition were carried out and a transition-state for decomposition to CH 3 and H 2 CO was identified. This information was used in a master equation rate calculation, using the MESMER code, where the zero-point-energy corrected barrier to reaction, Δ E 0,1 , and the energy transfer parameters, ⟨Δ E down ⟩ × T n , were the adjusted parameters to best fit the experimental data, with helium as the buffer gas. The data were combined with earlier measurements by Loucks and Laidler ( Can J. Chem. 1967 , 45, 2767), with dimethyl ether as the third body, reinterpreted using current literature for the rate coefficient for recombination of CH 3 OCH 2 . This analysis returned Δ E 0,1 = (112.3 ± 0.6) kJ mol −1 , and leads to k1∞(T)=2.9×1012$k_{1}^{\infty}(T)=2.9\times{10^{12}}$ ( T /300) 2.5 exp(−106.8 kJ mol −1 /R T ). Using this model, limited experiments with nitrogen as the bath gas allowed N 2 energy transfer parameters to be identified and then further MESMER simulations were carried out, where N 2 was the buffer gas, to generate k 1 ( T , p ) over a wide range of conditions: 300–1000 K and N 2 = 10 12 –10 25 molecule cm −3 . The resulting k 1 ( T , p ) has been parameterized using a Troe-expression, so that they can be readily be incorporated into combustion models. In addition, k 1 ( T , p ) has been parametrized using PLOG for the buffer gases, He, CH 3 OCH 3 and N 2 .

Methyl-Crotonate (MC, ( E )-methylbut-2-enoate, CH 3 CHCHC(O)OCH 3 ) is a potential component of surrogate fuels that aim to emulate the combustion of fatty acid methyl ester (FAME) biodiesels with significant unsaturated FAME content. MC has three allylic hydrogens that can be readily abstracted under autoignition and combustion conditions to form a resonantly-stabilized CH 2 CHCHC(O)OCH 3 radical. In this study we have utilized photoionization mass spectrometry to investigate the O 2 addition kinetics and thermal unimolecular decomposition of CH 2 CHCHC(O)OCH 3 radical. First we determined an upper limit for the bimolecular rate coefficient of CH 2 CHCHC(O)OCH 3 + O 2 reaction at 600 K ( k ≤ 7.5 × 10 −17 cm 3 molecule −1 s −1 ). Such a small rate coefficient suggest this reaction is unlikely to be important under combustion conditions and subsequent efforts were directed towards measuring thermal unimolecular decomposition kinetics of CH 2 CHCHC(O)OCH 3 radical. These measurements were performed between 750 and 869 K temperatures at low pressures (<9 Torr) using both helium and nitrogen bath gases. The potential energy surface of the unimolecular decomposition reaction was probed at density functional (MN15/cc-pVTZ) level of theory and the electronic energies of the stationary points obtained were then refined using the DLPNO-CCSD(T) method with the cc-pVTZ and cc-pVQZ basis sets. Master equation simulations were subsequently carried out using MESMER code along the kinetically important reaction pathway. The master equation model was first optimized by fitting the zero-point energy corrected reaction barriers and the collisional energy transfer parameters ΔEdown,ref$\Delta{E_{{\text{down}},\;{\text{ref}}}}$ and n to the measured rate coefficients data and then utilize the constrained model to extrapolate the decomposition kinetics to higher pressures and temperatures. Both the experimental results and the MESMER simulations show that the current experiments for the thermal unimolecular decomposition of CH 2 CHCHC(O)OCH 3 radical are in the fall-off region. The experiments did not provide definite evidence about the primary decomposition products.

Diethyl ether (DEE) has been proposed as a biofuel additive for compression-ignition engines, as an ignition improver for homogeneous charge compression ignition (HCCI) engines, and as a suitable component for dual-fuel mixtures in reactivity-controlled compression ignition (RCCI) engines. The combustion in these engines is significantly controlled by low-temperature (LT) chemistry. Fundamental studies of DEE LT oxidation chemistry and of its influence in fuel-mixture oxidation are thus highly important, especially at elevated pressures. Elevated pressure speciation data were measured for the LT oxidation of DEE, of its similarly-structured linear five-heavy-atom hydrocarbon fuel ( n -pentane), and of a mixture of the two fuels in a jet-stirred reactor (JSR) in the temperature range of 400–1100 K and at various pressures up to 10 bar. The pressure influence on the LT oxidation chemistry of DEE was investigated by a comparison of the measured profiles of oxidation products. The results for DEE and n -pentane were then inspected with regard to fuel structure influences on the LT oxidation behavior. The new speciation data were used to test recent kinetic models for these fuels [Tran et al., Proc. Combust. Inst. 37 (2019) 511 and Bugler et al., Proc. Combust. Inst. 36 (2017) 441]. The models predict the major features of the LT chemistry of these fuels well and could thus subsequently assist in the data interpretation. Finally, the LT oxidation behavior of an equimolar mixture of the two fuels was explored. The interaction between the two fuels and the effects of the pressure on the fuel mixture oxidation were examined. In addition to reactions within the combined model for the two fuels, about 80 cross-reactions between primary reactive species generated from these two fuel molecules were added to explore their potential influences.

The irreversible dimerization of polycyclic aromatic hydrocarbons (PAHs) – typically pyrene (C 16 H 10 ) dimerization – is widely used in combustion chemistry models to describe the soot particle inception step. This paper concerns itself with the detection and identification of dimers of flame-synthesized PAH radicals and closed-shell molecules and an experimental assessment of the role of these PAH dimers for the nucleation of soot. To this end, flame-generated species were extracted from an inverse co-flow flame of ethylene at atmospheric pressure and immediately diluted with excess nitrogen before the mixture was analyzed using flame-sampling tandem mass spectrometry with collision-induced fragmentation. Signal at m/z  = 404.157 (C 32 H 20 ) and m/z  = 452.157 (C 36 H 20 ) were detected and identified as dimers of closed-shell C 16 H 10 and C 18 H 10 monomers, respectively. A complex between a C 13 H 9 radical and a C 24 H 12 closed-shell PAH was observed at m/z  = 465.164 (C 37 H 21 ). However, a rigorous analysis of the flame-sampled mass spectra as a function of the dilution ratio, defined as the ratio of the flow rates of the diluent nitrogen to the sampled gases, indicates that the observed dimers are not flame-born, but are produced in the sampling line. In agreement with theoretical considerations, this paper provides experimental evidence that pyrene dimers cannot be a key intermediate in particle inception at elevated flame temperatures.

The influence of water molecules on the kinetics of urea condensation reactions was studied with high-level quantum chemical methods and statistical rate theory. The study focuses on the production of biuret, triuret, and cyanuric acid from urea because of their relevance as unwanted byproducts in the urea-based selective catalytic reduction (urea-SCR) exhaust after treatment of Diesel engines. In order to characterize the potential energy surfaces and molecular reaction pathways, calculations with explicitly-correlated coupled-cluster methods were performed. It turned out that the reactions proceed via pre-reactive complexes and the inclusion of one or two water molecules into the condensation mechanisms leads to a decrease of the energy barriers. This effect is particularly pronounced in the production of biuret. Due to the pre-reactive equilibria, the rates of the overall reactions can increase or decrease by incorporating water into the mechanism, depending on the temperature and water concentration. Under the conditions of urea-SCR, the studied reactions are too slow to contribute to the observed byproduct formation.

Ammonia is a potential fuel for the storage of thermal energy. Experimental data were collected for homogeneous ammonia combustion: ignition delay times measured in shock tubes (247 data points in 28 datasets from four publications) and species concentration measurements from flow reactors (194/22/4). The measurements cover wide ranges of temperature T , pressure p , equivalence ratio φ and dilution. The experimental data were encoded in ReSpecTh Kinetics Data Format version 2.2 XML files. The standard deviations of the experimental datasets used were determined based on the experimental errors reported in the publications and also on error estimations obtained using program MinimalSplineFit. Simulations were carried out with eight recently published mechanisms at the conditions of these experiments using the Optima++ framework code, and the FlameMaster and OpenSmoke++ solver packages. The performance of the mechanisms was compared using a sum-of-square error function to quantify the agreement between the simulations and the experimental data. Ignition delay times were well reproduced by five mechanisms, the best ones were Glarborg-2018 and Shrestha-2018. None of the mechanisms were able to reproduce well the profiles of NO, N 2 O and NH 3 concentrations measured in flow reactors.

Simplified representations of branching fractions for thermal unimolecular two-channel reactions are discussed. The dissociation of formaldehyde serves as an illustrative example. Quantum-corrected classical trajectory calculations on an ab initio potential energy surface are combined with master equation calculations for collisional energy transfer. The treatment accounts for roaming atom dynamics. The dependence of the channel branching fractions on the bath gas pressure and temperature, on the collision efficiencies, and on the difference of channel threshold energies, are explored. It is discussed to what extent the derived simplified representations of channel branching fractions can be generalized.

Cation and anion formation from molecular SF 6 and SF 6 clusters is reported in the F 1s-excitation regime (680–720 eV) using tunable synchrotron radiation. Stable products are identified by mass spectrometry, while the tunablility of the soft X-rays source is used to determine mechanistic details on the formation of the detected charged products. Characteristic differences are observed between product formation from the isolated molecule as well as from clusters for both, cations and anions. Specifically, dication formation, such as SF 2 2+ , is only observed for molecular precursors, which are formed with high efficiency at the F 1s → t 2g -transition. SF 6 − is efficiently formed from clusters upon F 1s → t 1u -transition. Mechanistic details on the formation of stable products are discussed.

High-resolution rotational spectroscopy was used to investigate the conformational landscape of methyl-3,3,3-trifluoropyruvate, a small, partially-fluorinated molecule, which is of interest because of its chemical properties and reactivity in contrast to the unfluorinated species. Methyl 3,3,3-trifluoropyruvate is also subject to two possible large amplitude motions of the methyl and trifluoromethyl group. However, only the methyl rotor gives rise to the tunneling splitting specific to individual conformers. In the rotational spectrum measured in the frequency region from 6 to 27 GHz, the identified conformers, s - cis and s - trans , were fitted to experimental accuracy, resulting in the accurate determination of the vibrational ground state rotational constants A0=2185.0582736 MHz${A}_{0}=2185.05827\left(36\right)\text{\hspace{0.17em}MHz}$, B0=1023.300 3117 MHz${B}_{0}=1023.30031\left(17\right)\text{\hspace{0.17em}MHz}$, and C0=803.52028795 MHz${C}_{0}=803.520287\left(95\right)\text{\hspace{0.17em}MHz}$ for the s - cis conformer, and A0=2706.9024(49) MHz${A}_{0}=2706.9024\left(49\right)\text{\hspace{0.17em}MHz}$, B0=864.889 539(81) MHz${B}_{0}=864.889539\left(81\right)\text{\hspace{0.17em}MHz}$, and C0=746.532 896(71) MHz${C}_{0}=746.532896\left(71\right)\text{\hspace{0.17em}MHz}$ for the s - trans conformer. Additionally the barrier heights of the methyl rotor V3CH3=363.11694 cm−1${V}_{3}\left({\text{CH}}_{3}\right)=363.116\left(94\right){\text{\hspace{0.17em}cm}}^{-1}$ and V3(CH3)=389.290(80) cm−1${V}_{3}\left({\text{CH}}_{3}\right)=389.290\left(80\right){\text{\hspace{0.17em}cm}}^{-1}$ were obtained for the s - cis and s - trans conformer, respectively.

Shock-tube experiments have been performed to investigate the thermal decomposition of octamethylcyclotetrasiloxane (D4, Si 4 O 4 C 8 H 24 ) and hexamethylcyclotrisiloxane (D3, Si 3 O 3 C 6 H 18 ) behind reflected shock waves by gas chromatography/mass spectrometry (GC/MS) and high-repetition-rate time-of-flight mass spectrometry (HRR-TOF-MS) in a temperature range of 1160–1600 K and a pressure range of 1.3–2.6 bar. The main observed stable products were methane (CH 4 ), ethylene (C 2 H 4 ), ethane (C 2 H 6 ), acetylene (C 2 H 2 ) and in the case of D4 pyrolysis, also D3 was measured as a product in high concentration. A kinetics sub-mechanism accounting for the D4 and D3 gas-phase chemistry was devised, which consists of 19 reactions and 15 Si-containing species. The D4/D3 submechanism was combined with the AramcoMech 2.0 (Li et al., Proc. Combust. Inst. 2017, 36, 403–411) to describe hydrocarbon chemistry. The unimolecular rate coefficients for D4 and D3 decomposition are represented by the Arrhenius expressions k total/D4 ( T ) = 2.87 × 10 13 exp(−273.2 kJ mol −1 / RT ) s −1 and k total/D3 ( T ) = 9.19 × 10 14 exp(−332.0 kJ mol −1 / RT ) s −1 , respectively.

Self-assembled monolayers (SAMs) of azobenzene (AB) functionalized alkyl thiols on gold diluted with simple alkyl thiols provide a straightforward way to photochromic surfaces with high and tunable photoswitching efficiency. Trans-cis isomerization of the AB molecule changes the physical properties of the surface, including the nonlinear optical (NLO) response. Vibrational sum-frequency generation (VSFG) spectroscopy as a nonlinear type of laser spectroscopy offers surface- and orientation-sensitive insight into the molecular structure of mixed SAMs. In this study, VSFG as well as ultraviolet-visible (UV/Vis) spectroscopy has been employed to investigate the morphology, molecular structure, and NLO response of mixed SAMs with systematically varied surface composition. Methylazobenzene (MeAB) has been used as the molecular switch with the methyl substituent serving as orientational VSFG marker. Both short-chain and long-chain alkyl thiol co-ligands have been used to gain insight into the interplay between SAM structure and sterical constraints that are known to limit the free switching volume. Underlining the dominating role of sterical effects for controlling photochromic properties, a strong inhibition of the photoswitching efficiency and NLO response has been observed for the SAMs with an alkyl thiol co-ligand long enough to spatially extend into the layer of the MeAB chromophore. Overall, with <12% signal change, the relative NLO switching contrasts remained low in all cases. VSFG spectral trends clearly revealed that the presumably higher photoswitching efficiency upon dilution with the co-ligand is counteracted by a loss of structural order of the chromophore.

The extent of contact and solvent shared ion pairs of phosphate groups with Na + , Ca 2+ and Mg 2+ ions in aqueous environment and their relevance for the stability of polyanionic DNA and RNA structures is highly debated. Employing the asymmetric phosphate stretching vibration of dimethyl phosphate (DMP), a model system of the sugar-phosphate backbone of DNA and RNA, we present linear infrared, femtosecond infrared pump-probe and absorptive 2D-IR spectra that report on contact ion pair formation via the presence of blue shifted spectral signatures. Compared to the linear infrared spectra, the nonlinear spectra reveal contact ion pairs with increased sensitivity because the spectra accentuate differences in peak frequency, transition dipole moment strength, and excited state lifetime. The experimental results are corroborated by long time scale MD simulations, benchmarked by density functional simulations on phosphate-ion-water clusters. The microscopic interpretation reveals subtle structural differences of ion pairs formed by the phosphate group and the ions Na + , Ca 2+ and Mg 2+ . Intricate properties of the solvation shell around the phosphate group and the ion are essential to explain the experimental observations. The present work addresses a challenging to probe topic with the help of a model system and establishes new experimental data of contact ion pair formation, thereby underlining the potential of nonlinear 2D-IR spectroscopy as an analytical probe of phosphate-ion interactions in complex biological systems.

Organic substitutes for ruthenium and iridium complexes are increasingly finding applications in chemical syntheses involving photoredox catalysis. However, the performance of these organic compounds as electron-transfer photocatalysts depends on their accessible photochemical pathways and excited state lifetimes. Here, the UV-induced dynamics of N-phenyl phenoxazine, chosen as a prototypical N-aryl phenoxazine organic photoredox catalyst, are explored in three solvents, N,N-dimethyl formamide, dichloromethane and toluene, using ultrafast transient absorption spectroscopy. Quantum chemistry calculations reveal the locally excited or charge-transfer electronic character of the excited states, and are used to assign the transient electronic and vibrational bands observed. In toluene-d 8 , complete ground-state recovery is (31 ± 3) % by internal conversion (IC) from the photo-excited state (or from S 1 after IC but before complete vibrational relaxation), (13 ± 2) % via direct decay from vibrationally relaxed S 1 (most likely radiative decay, with an estimated radiative lifetime of 13 ns) and (56 ± 3) % via the T 1 state (with intersystem crossing (ISC) rate coefficient k ISC  = (3.3 ± 0.2) × 10 8  s −1 ). In dichloromethane, we find evidence for excited state N-phenyl phenoxazine reaction with the solvent. Excited state lifetimes, ISC rates, and ground-state recovery show only modest variation with changes to the solvent environment because of the locally excited character of the S 1 and T 1 states.

Dibenzofuran (DBF) has attracted much attention from scientists recently since it is applied as a photoluminescence material and pharmaceutically active compound. Since the polychlorinated derivatives are highly toxic and manifest photostability in the environment. Femtosecond transient absorption spectroscopy associated with quantum chemical calculations are employed to investigate the ultrafast excited state dynamics of dibenzofuran from the S 2 state in 1,4-dioxane and ethanol, respectively. Following excitation at a wavelength of 266 nm, the S 2 state is firstly populated in the Franck–Condon region and preserves the planar molecular structure of the ground state. The observed increase of the transient absorption spectra of the excited state within the first several picoseconds indicates a geometry relaxation occurring on the S 2 potential energy surface. The subsequent kinetic traces of excited state absorption show that the S 2 state in the adiabatic region decays to the S 1 state through a fast internal conversion, followed by intersystem crossing to the T 1 state with a decay time of tens and hundreds of picoseconds in ethanol and 1,4-dioxane, respectively. Finally, the deactivation processes from the S 1 or T state are slow and take place on a time scale of about 20 ns.

A general framework for the simulation of ultrafast pump-probe time resolved experiments based on Born-Oppenheimer molecular dynamics (BOMD) is presented. Interaction of the molecular species with a laser is treated by a simple maximum entropy distribution of the excited state occupancies. The latter decay of the electronic excitation into the vibrations is based on an on-the-fly estimation of the rate of the internal conversion, while the energy is distributed in a thermostat-like fashion. The approach was tested by reproducing the results of previous femtosecond studies on ethylene, naphthalene and new results for phenanthrene.

Excited-state intramolecular proton transfers (ESIPT) are one of the fastest reactions in chemistry (<100 fs) which – among other features like high photostability – makes them an important reaction class for molecular switches. ESIPTs can be coupled with double bond rotation/isomerization, so that molecules can act as “molecular cranes”, facilitating long-range proton transfer. A versatile model system is 7-hydroxy-4-methylquinoline-8-carbaldehyde (HMQCA): it features two proton-accepting sites, two stable ground-state isomers and should allow for easy derivatization. There is also experimental and theoretical reference data available, however, only for static properties, e.g. ground-state IR spectra or potential energy surface scans. In this contribution we show the results of full-dimensional surface-hopping molecular dynamics (MD) of HMQCA after photo-excitation, employing semiempirical quantum mechanics coupled to floating-occupation configuration interaction. The results support the potential of HMQCA as prototype system for directed proton transport by ESIPT.

The thermally induced stereochemical interconversion between the trans and cis isomers of [Co(acac) 2 (N 3 )(py)] in liquid solution is investigated with time-resolved Fourier-transform infrared spectroscopy. The complex is synthesized stereo-selectively in its trans -form. Upon dissolution of the trans -form, the kinetic build-up of the cis -form is evidenced by the spectro-temporal evolution of the FTIR-spectrum. The individual isomer-specific component spectra are in good agreement with calculated spectra obtained from density functional theory. The rate constants of the forward and backward reactions responsible for the trans - cis isomerization equilibrium are derived from the kinetic traces in combination with existing thermochemical data from the literature. Moreover, the temperature-dependence of the rate constants are in line with Arrhenius activation energies of (122 ± 8) kJ/mol and (109 ± 8) kJ/mol for the forward and backward reactions, respectively. DFT-calculations suggest that the stereochemical rearrangement is caused by a pyridine rebound mechanism involving penta-coordinated square-pyramidal [Co(acac) 2 (N 3 )]-intermediates.