Competition between intersystem crossing and internal conversion in isolated large molecules

The excess energy and deuteration dependence of the radiationless decay rate in “isolated” aromatic hydrocarbons (anthracene, 9,10-dimethylanthracene, phenanthrene and fluorene) suggest that S₁→S₀ internal conversion dominates over S₁→T intersystem crossing for molecules with very large excess vibrational energies.     

Deuterium Isotope Effect on Excess Enthalpies of Methanol or Ethanol and Their Deuterium Derivatives at 298.15 K

Excess enthalpies of six binary mixtures of CH₃ OD+CH₃ OH, CH₃ OD+CD₃ OD, CD₃ OD+CH₃ OH, C₂ D₅ OD+C₂ H₅ OH, C₂ D₅ OD+C₂ H₅ OD, C₂ H₅ OD+C₂ H₅ OH have been determined over the whole range of mole fractions at 298.15 K in order to know the isotopic effect on hydrogen-bonding accurately, although there are many reports on the differences in the strength of hydrogen-bonding between OH and OD. All excess enthalpies measured are very small and endothermic. The mixtures of CH₃ OD+ CH₃ OH, and C₂ D₅ OD+C₂ H₅ OH showed the largest excess enthalpies among each methanol and ethanol mixtures. The difference of intermolecular interaction between OH and OD in methanol and ethanol was almost same value of (1.82±0.04) J mol^-1 Excess enthalpies of 1,4-dimethylbenzene+1,3-dimethylbenzene and 1,4-dimethylbenzene+1,2-methylbenzene were measured by three different principle calorimeters at 298.15 K in order to know the precision of calorimetry for a small enthalpy change. This revised version was published online in July 2006 with corrections to the Cover Date.     

Spectroscopic Properties of Morin in Various CH3OH–H2O and CH3CN–H2O Mixed Solvents

The specific fluorescence properties of morin (3,2′,4′,5,7‐pentahydroxyflavone) were studied in various CH₃OH–H₂O and CH₃CN–H₂O mixed solvents. Although the dihedral angle is large in the S₀ state, morin has an almost planar molecular structure in the S₁ state owing to the very low rotational energy barrier around the interring bond between B and the A, C ring. The excited state intramolecular proton transfer (ESIPT) at the S₁ state cannot occur immediately after excitation, S₁ → S₀ fluorescence can be observed. Two conformers, Morin A and B have been known. At the CH₃OH–H₂O, Morin B will be the principal species but at the CH₃CN–H₂O, Morin A is the principal species. At the CH₃OH–H₂O, owing to the large Franck–Condon (FC) factor for S₂ → S₁ internal convernal (IC) and flexible molecular structure, only S₁ → S₀ fluorescence was exhibited. At the CH₃CN–H₂O, as the FC factor for S₂ → S₁ IC is small and molecular structure is rigid, S₂ → S₀ and S₁ → S₀ dual fluorescence was observed. This abnormal fluorescence property was further supported by the small pK₁ value, effective delocalization of the lone pair electrons of C(2′)–OH to the A, C ring, and a theoretical calculation.     

Interaction energy and the shift in OH stretch frequency on hydrogen bonding for the H2O → H2O,CH3OH → H2O,and H2O → CH3OH dimers

The ability to use calculated OH frequencies to assign experimentally observed peaks in hydrogen bonded systems hinges on the accuracy of the calculation. Here we test the ability of several commonly employed model chemistries—HF, MP2, and several density functionals paired with the 6‐31+G(d) and 6‐311++G(d,p) basis sets—to calculate the interaction energy (D_e) and shift in OH stretch fundamental frequency on dimerization (δ(ν)) for the H₂O → H₂O, CH₃OH → H₂O, and H₂O → CH₃OH dimers (where for X → Y, X is the hydrogen bond donor and Y the acceptor). We quantify the error in D_e and δ(ν) by comparison to experiment and high level calculation and, using a simple model, evaluate how error in D_e propagates to δ(ν). We find that B3LYP and MPWB1K perform best of the density functional methods studied, that their accuracy in calculating δ(ν) is ≈ 30–50 cm^?1 and that correcting for error in D_e does little to heighten agreement between the calculated and experimental δ(ν). Accuracy of calculated δ(ν) is also shown to vary as a function of hydrogen bond donor: while the PBE and TPSS functionals perform best in the calculation of δ(ν) for the CH₃OH → H₂O dimer their performance is relatively poor in describing H₂O → H₂O and H₂O → CH₃OH. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

Study on the method of chemical trapping for formyl intermediates

The method of chemical trapping for formyl intermediates has been studied, with syngas conversion to ethanol over rhodium-based catalysts as the diagnostic reaction concerned, and CH₃I as the trapping reagent. Two species of acetaldehyde, i.e., CH₃CHO and CH₃CDO, were produced in the trapping reaction following CO + 2D₂ reaction. It was shown that the formation of CH₃CHO in the trapping reaction resulted from dehydrogenation of CH₃ from CH₃I to give H, which induced the formation of CH₃CHO in the presence of CO and CH₃ So there may be two pathways for the formation of CH₃CDO in the trapping reaction: one, methylation of DCO adspecies; the other, deuteration of CH₃ CO formed by CO insertion into CH₃ The catalyst surface was purged with Ar following CO + 2D₂ reaction before the trapping reaction was performed. By means of this modified method of chemical trapping for formyl intermediates, CH₃CDO was found to be mainly derived from the methylation of DCO adspecies. Accordingly, it could be concluded that formyl is a C₁ intermediate in the syngas conversion to ethanol over rhodium-based catalysts.     

The decomposition of ethanol vapour by infrared radiation from a pulsed HF-laser

The decomposition of ethanol vapour induced by infrared radiation from a pulsed HF-laser has been studied as a function of pressure. At high pressures, above 10 torr, the main primary processes appear to be:C₂H₅OH → H₂ + CH₃CHO,C₂H₅OH → C₂H₄ + H₂O,C₂H₅OH → CH₃ + CH₂OHin a ratio of 3:2:1 which is independent of pressure. At low pressures the process yielding C₂H₄ and H₂O becomes dominant. The results suggest that the high pressure behaviour involves a “thermal” decomposition with collisional processes dominating, whereas at low pressures the decomposition is due to multiple photon absorption which at the lowest pressures approaches a collision-free unimolecular decomposition.     

Ab initio calculations on the barrier height for the hydrogen addition to ethylene and formaldehyde. The importance of spin projection

Heats of reaction and barrier heights have been computed for H + CH₂CH₂ → C₂H₅, H + CH₂O → CH₃O, and H + CH₂O → CH₂OH using unrestricted Hartree-Fock and Møller–Plesset perturbation theory up to fourth order (with and without spin annihilation), using single-reference configuration interaction, and using multiconfiguration self-consistent field methods with 3-21G, 6-31G(d), 6-31G(d,p), and 6-311G(d,p) basis sets. The barrier height in all three reactions appears to be relatively insensitive to the basis sets, but the heats of reaction are affected by p-type polarization functions on hydrogen. Computation of the harmonic vibrational frequencies and infrared intensities with two sets of polarization functions on heavy atoms [6-31G(2d)] improves the agreement with experiment. The experimental barrier height for H + C₂H₄ (2.04 ± 0.08 kcal/mol) is overestimated by 7?9 kcal/mol at the MP2, MP3, and MP4 levels. MCSCF and CISD calculations lower the barrier height by approximately 4 kcal/mol relative to the MP4 calculations but are still almost 4 kcal/mol too high compared to experiment. Annihilation of the largest spin contaminant lowers the MP4SDTQ computed barrier height by 8?9 kcal/mol. For the hydrogen addition to formaldehyde, the same trends are observed. The overestimation of the barrier height with Møller-Plesset perdicted barrier heights for H + C₂H₄ → C₂H₅, H + CH₂O → CH₃O, and H + CH₂O → CH₂OH at the MP4SDTQ /6-31G(d) after spin annihilation are respectively 1.8, 4.6, and 10.5 kcal/mol.     

Far-infrared studies of ethanols and fluoroethanols trapped in low-temperature matrices

Ethanol, its deuterated derivatives (C₂H₅OD, C₂D₅OD) and fluoroethanols (CFH₂CH₂OH, CF₃CH₂OH) have been isolated in low temperature matrices and investigated in the far-infrared region. From the concentration dependency of the observed bands and from studies of the pure alcohols in the gaseous, liquid, and solid phase it was found that the hydrogen bond stretching frequencies ν_σ associated with the hydrogen bonded system OH?O appear in the 100 – 160 cm^?1 range. At higher M/A ratios the OD(OD) torsion modes τ_OH are dominating and were identified in the 200 – 300 cm^?1 region. The influence of various matrix materials like argon, krypton, xenon, nitrogen and methane on the low frequency spectra of ethanol have also been studied. It was found, that nitrogen and methane matrices produce significant changes in the far infrared spectra.     

The √t law in solid-phase reactions. Analysis of the hypothesis on rate constant distribution

The reaction C₂H₅ + O₂ → C₂H₅O₂ in glassy methanol-d₄ and the H-atom abstraction by CH₃, C₂H₅, and n-C₄H₉ radicals in C₂H₅OH + C₂D₅OH and CD₃CH₂OH + C₂D₅OH glassy mixtures have been studied by electron spin resonance. The analysis of the dependence of the reaction rates on the concentration of O₂ (oxidation) and C₂H₅OH, CD₃CH₂OH (H-atom abstraction) has shown that the √t law is not conditioned by the existence of regions characterized by different rate constants.     

Selective Population of the n, π* and π, π* Triplet States of 1-Indanones

The two components of the dual phosphorescence of 1-indanone ( 1 ) and six related ketones ( 2–7 ) possess different excitation spectra exhibiting the vibrational progression characteristic of the S₀ → S₁ (n, π*) transition (shorter-lived emission) and two bands of the S₀ → S_{2 and 3} (π,π*) 0–0 transitions, respectively. The most favorable intersystem crossing routes are S₁ (n, π*) → T (n, π*) and S_2,3 (π*) → T (π, π*). Internal conversion to S₁ competes more effectively with S (π, π*) → T (π, π*) intersystem crossing only from higher vibrational levels of the S₂ and S₃ states.     

Angular dependence of 1H- and 2H-NMR spectra of water and methanol absorbed in cellulose triacetate film

¹H- and ²H-NMR spectra of water (H₂O and D₂O) and methanol (CH₃OD and CD₃OH) absorbed in cellulose triacetate films have been observed as a function of the angle θ between the film surface and the magnetic field. ¹H-NMR signals of H₂O and CH₃OD are doublets and triplets due to dipole interactions, respectively. ²H-NMR signals of D₂O, CD₃OH, and CH₃OD are doublets due to quadrupole splittings. The magnitudes of these splittings change depending on θ. The analysis of the angle-dependent patterns indicates that the motionally averaged axes of the dipole and the quadrupole moments orient in the direction perpendicular to the film surface. The alignment of water and/or methanol molecules originates from the film morphology, which is anisotropic in the perpendicular direction. From the angle dependence of the chemical shift, the volume diamagnetic susceptibility of the film is estimated to be 0.44 ppm.     

Chromatographic performance of deuterated solvents in reversed phase micro high performance liquid chromatography

The chromatographic performance of the deuterated solvents, CD₃OD and D₂O, has been investigated in reversed-phase micro high performance liquid chromatography. The chromatographic performance of CD₃OD is only slightly superior to that of CH₃OH. However, the performance of D₂ is significantly superior to that of H₂O, separation of aromatics being improved by about 30%. D₂ is a particularly powerful solvent for the separation iof deuterated and non-deuterated compounds.     

Temperature dependence of the nonradiative decay of indoles in solution

The nonradiative decay rate k_nr. of some indole derivatives has been measured as a function of temperature in three solvents : n-heptane, methanol and water. A temperature-independent component (attributed to S₁ → T₁ intersystem crossing) and a temperature-dependent component (attributed to S₁ → S₀ internal conversion) are present in all case. In aqueous solutions, our results indicate the existence of a second temperature-dependent process which can be identified with photoionization.     

Ionization constants for deuterium oxide in deuterium oxide-organic mixtures

A potentiometric method using a glass electrode has been applied to determination of ionization constants for deuterium oxide (D₂O) in binary mixtures of D₂O with dioxane, tetrahydrofuran, acetone, dimethylsulfoxide, CH₃CH₂OD, and CH₃OD at 25°C. The results are compared with values of ionization constants for H₂O obtained previously in the corresponding H₂O-organic mixtures, and the isotope effect is shown to be small. Further calculations with the D₂O results show that the first five solvents mentioned above are neither appreciably acidic nor basic in D₂O solution, but that CH₃OD shows slightly acidic behavior (pK_a=16.0±0.3).     

Volumetric properties of mixtures of water and methanol H/D-isotopomers between 5 and 45°C

Densities of H/D-isotopomers mixtures of water (H₂O, D₂O) and methanol (CH₃OH, CD₃OH, CH₃OD, and CD₃OD) over the full range of compositions were measured at 5, 15, 25, 35, and 45°C. Results have been used to calculate molar volumes, excess molar volumes, apparent molar volumes, and isotope effects of the mixtures. The volumetric properties are discussed in terms of the structural changes in water-methanol solutions under the influence of isotope substitution.     

Identification of an OH,CH combination band in the near infrared spectrum of ethanol

The identification of an OH_stretch/CH_stretch combination band in the near infrared (n.i.r.) spectrum of ethanol is based on comparison of the calculated positions of overtone and combination bands with the n.i.r. spectra of C₂H₅OH, C₂H₅OD, CH₃OH and CH₃OD.     

An energy-resolved study of the fragmentation of ionized 1-Penten-3-ol

A detailed energy-resolved study of the fragmentation of CH₂?CHCH(OH)CD₂CD₃ (1-d₅) has been carried out using metastable ion studies and charge exchange techniques, combined with collision-induced dissociation studies to establish the structures of fragment ions. At low internal energies (metastable ions) the molecular ion of 1-d₅ rearranges to the 3-pentanone structure and fragments by loss of C₂H₅ or C₂D₅ leading to the acyl structure, [CH₃CH₂C?O]⁺ or [CD₃CD₂C?O]⁺, for the fragment ion. However, with increasing internal energy of the molecular ion this rearrangement process decreases rapidly in importance and loss of C₂D₅ by direct cleavage, leading to [CH₂?CHCH?OH]⁺, becomes the dominant fragmentation reaction. As a result the [C₃H₅O]⁺ ion seen in the electron impact mass spectrum of 1-penten-3-ol has predominantly the protonated acrolein structure.     

Physical and chemical properties of hydroxyflavones. V. Deuteroxyflavones from acetoxyflavones by imidazole-catalyzed deacetylation

A recently described procedure for imidazole-catalyzed deacetylation of acetoxyflavones has been modified by effecting hydrolysis in a mixture of deuterium oxide and deuterioethanol (C₂H₅OD), or in deuterium oxide and deuteriomethanol (CH₃OD). Application of the new procedure to monoacetoxyflavones has resulted in the monodeuteroxyflavone. 5-Deuteroxyflavone has been obtained also by hydrolysis of a rubidium salt in deuterium oxide. Imidazole-catalyzed hydrolysis of four diacetoxyflavones in a mixture of deuteriomethanol and deuterium oxide is described. Infrared spectral data are presented for the deuteration products, with emphasis on OD stretching and vibrational modes. Comparison of spectra of appropriate monohydroxyflavones and monodeuteroxyflavones permits assignment of δ (OH) bands in four hydroxyflavone spectra.     

Analysis of deuterium effects on the rate of multiphonon relaxation in hydrated cesium chloromanganate crystals (CsMnCl3-2H2O, -2D2O)

The assumption that OH (OD) stretch vibrations in CsMnCl₃-2H₂O and -2D₂O govern the rate of the ⁴T₁ → ⁶A₁ multiphonon transitions in Mn²⁺ is shown to account quantitatively for the observed 16-fold increase in the ⁴T₁ lifetime upon replacing H₂O by D₂O. The argument is generalized to include other coordination compounds.     

Neutralization-reionization mass spectrometry (NRMS). Structural information from vertical neutralization and reionization efficiencies

The neutralization-reionization mass spectra of alkane radical ions indicate significant differences between the structures and geometries of alkane molecules and their molecular ions, confirming recent ab initio predictions. Ionic isomers that are indistinguishable by collisionally-activated dissociation because of easy interconversion can be characterized by neutralization-reionization if the corresponding neutrals show different reactivities, as is demonstrated for the [C₂H₅]⁺/C₂H₅˙ system and for [C₂H₄O₂]⁺˙ isomers. For identification of mixtures of more than one neutral species, the relative efficiency for reionizing each neutral must be determined; e.g. the O₂ reionization efficiency of ˙CH₂OH radicals is ～4 times greater than that of CH₃O˙. This information and reference reionization spectra of CH₃O˙ and ˙CH₂OH show that metastable or collisionally activated methyl acetate cations lose CH₃O˙, not ˙CH₂OH as previously reported; the newly-formed CH₃O˙ undergoes partial (～20%) isomerization to ˙CH₂OH in the ～10^?6s before reionization. Similar results are obtained for [B(OCH₃)₃]⁺˙.