Isomeric Yield Research Articles

Surface-Isomerization Dynamics of trans-Stilbene Molecules Adsorbed on Si(100)-2×1

Photoinduced trans-cis isomerization studies of stilbene molecules in the gas phase have led to a precise understanding of the corresponding molecular dynamics. Yet, when such molecules are adsorbed on surfaces, these reactions are expected to be strongly modified as compared to what is know in the gas phase. In this work, a low temperature (5 K) scanning tunneling microscope (STM) is used to image the trans-stilbene molecules deposited on a Si(100)-2 x 1 surface at 12 K. trans-Stilbene undergoes conformational changes during the adsorption process such that four different stilbene conformers are observed: trans-stilbene (TS), cis-stilbene (CS), and two new conformers I(1) and I(2). Furthermore, electronic excitation of individual stilbene molecules, by means of tunnel electrons, is shown to activate specific reversible molecular surface isomerization (TS <--> I(1) and CS <--> I(2)) combined with diffusion across the surface. Calculated STM topographies, using the tight binding method, indicate that the CS and TS molecules are physisorbed. The molecular conformations of the surface isomers I(1) and I(2) are suggested to be analogous to transient states conformations of the stilbene molecule when stabilized by the silicon surface. The measurements of the molecular surface isomerization and diffusion reaction yields are used to build a qualitative potential energy surface of the various stilbene reactions. The molecular surface-isomerization dynamics is shown to be influenced by the type of dopant (n or p). This is related to surface charging, which reveals modifications in the stilbene ionization potential.

Isomeric yield ratios in nucleiIr190andEu150,152

Isomeric yield ratios $^{190}\mathrm{Ir}{}^{m2,m1+g}$, $^{150}\mathrm{Eu}{}^{m,g}$, $^{152}\mathrm{Eu}{}^{m1,m2}$ have been measured in $(\ensuremath{\gamma},n)$-reactions for the end point energies of bremmsstrahlung photons about 12, 12.5, and 16 MeV for $^{190}\mathrm{Ir}{}^{m2,m1+g}$ and 12 MeV for $^{150}\mathrm{Eu}{}^{m,g}$, $^{152}\mathrm{Eu}{}^{m1,m2}$. Experimental values of the isomeric yield ratios have been compared with the theoretical values, which were calculated by using codes TALYS and MCEM. The low influence of nonstatistical effects has been observed for all these energies.

Hopping Technology in Relation to α-Acids Isomerization Yield, Final Utilization, and Stability of Beer Bitterness

A detailed study of α-acids isomerization kinetics was undertaken by performing pilot-scale brewing experiments with T90 hop pellets, nonisomerized hop extract, and nonisomerized hop extract plus hop residue. In addition, a brew was prepared by adding preisomerized hop extract at the onset of wort boiling. Compared with the use of nonisomerized hop extract, significantly higher isomerization yields were obtained when vegetative material was included in the hopping. However, when vegetative material was present final α-acids utilization was compromised by large losses of iso-α-acids postboiling. Evidence of reverse isomerization of iso-α-acids during wort boiling was not found when preisomerized hop extract was applied. The results obtained using quantitative HPLC profiling confirmed that both α-acids and iso-α-acids were not stable upon beer aging. In particular, trans-iso-α-acids underwent rapid degradation in finished beer. Therefore, beers prepared with preisomerized hop extract containing relatively less-sensitive trans-isomers, and thus more cis-isomers, show improved bitterness stability upon aging compared with conventionally hopped beers.

Spallation and fission products in the (p+ 179Hf) and (p+ natHf) reactions

Production of Hf and Lu high-spin isomers has been experimentally studied in spallation reactions induced by intermediate energy protons. Targets of enriched 179Hf (91%) and natHf were bombarded with protons of energy in the range from 90 to 650 MeV provided by the internal beam of the Dubna Phasotron synchrocyclotron. The activation yields of the reaction products were measured by using the γ-ray spectroscopy and radiochemistry methods. The production cross-sections obtained for the 179m2Hf, 178m2Hf and 177mLu isomers are similar to the previously measured values from the spallation of Ta, Re and W targets. Therefore, the reactions involving emission of only a few nucleons, like (p,p′), (p,p′n) and (p,2pn), can transfer high enough angular momentum to the final residual nuclei with reasonable large cross-sections. A significant gain in the isomeric yields was obtained when enriched 179Hf targets were used. The mass distribution of the residual nuclei was measured over a wide range of masses and the fission-to-spallation ratio could be deduced as a function of the projectile energy. Features of the reaction mechanism are briefly discussed.

Why Does trans-Azobenzene Have a Smaller Isomerization Yield for ππ* Excitation Than for nπ* Excitation?

A realistic dynamics simulation study is reported for the photoisomerization of trans-azobenzene. The isomerization follows both n pi* (the HOMO --> LUMO) and pi pi* (the HOMO-1 --> LUMO) excitations. The simulation finds that for the pi pi* excitation, the relaxation of the S(pi pi*) state is immediately followed by double excitation, (pi)(2)(pi*)(2). The decay from the S((pi)(2)(pi*)(2)) state to the S(0) state can occur at partially twisted structure, which favors the formation of the trans isomer. Multiple decay channels are found at about twisted structure for both n pi* and pi pi* excitations. Decay at about twisted geometry leads to the formation of either cis or trans isomer. Opening of the decay channel at partially twisted structure accounts for the smaller isomerization yield for the pi pi* excitation.

Isomerization of Lactose-Derived Oligosaccharides: A Case Study Using Sodium Aluminate

Galactooligosaccharides (GOS) obtained during the enzymatic hydrolysis of lactose contain large amounts of glucose, galactose, and unreacted lactose, which do not have prebiotic properties and increase the calorific value of the product. In this work, the isomerization of the GOS mixture by the action of sodium aluminate has been studied. During the reaction, lactose, glucose, and galactose were isomerized to lactulose, fructose, and tagatose, respectively, and in addition allolactose, 6-galactobiose, and 6'-galactosyl-lactose were also converted to the corresponding keto-sugars. The effect of time, temperature, and aluminate/initial lactose ratio has been studied. After 9 h at 40 degrees C and molar ratio aluminate/lactose 3:1, the isomerization yield was >60%, and the amount of final carbohydrates was close to 90% of the initial product. This process considerably decreases the amount of lactose, glucose, and galactose.

Influence of Laser Pulse Parameters on Dynamical Processes during Azobenzene Photoisomerization

When a molecule is subjected to a short intense laser pulse, the ensuing dynamical processes depend qualitatively on the pulse parameters, including duration, frequency, and fluence. Here we report studies of cis to trans photoisomerization of azobenzene following femtosecond-scale laser pulses which are relatively short (10 fs) or long (100 fs) and which have a central frequency matched to either the first excited state (S1, or HOMO to LUMO in a molecular orbital picture) or the second (S2, or HOMO-1 to LUMO). The results presented here demonstrate that photoisomerization involves a rather intricate sequence of connected steps, with the nuclear and electronic degrees of freedom inextricably coupled. One important feature is the de-excitation required for the molecule to achieve its new ground-state after isomerization. If the primary excitation is to S1, then we find that only a single HOMO/LUMO avoided crossing is required and that this crossing occurs halfway along a rotational pathway involving the central CNNC dihedral angle. If the primary excitation is to S2, then the same HOMO/LUMO avoided crossing is observed, but it must be preceded by another avoided crossing that permits transfer of holes from the HOMO-1 to the HOMO, so that the HOMO is then able to accept electrons from the LUMO. We find that this earlier crossing can occur in either of two geometries, one near the cis configuration and the other near the trans. The fact that S2 (pi pi*) isomerization requires two steps may be related to the fact that isomerization yields are smaller for this (UV) excitation than for the S1 (n pi*, visible-light) excitation.

Isomeric yield ratios for the formation of 44m,gSc in the 45Sc(γ,n), natTi(γ,xnp), natFe(γ,xn5p) and natCu(γ,xn8p) reactions with 2.5 GeV bremsstrahlung

We measured the isomeric yield ratios for the 44m,gSc isomeric pairs produced from four different photonuclear reactions 45Sc(γ,n) 44m,gSc, natTi(γ,xn1p) 44m,gSc, natFe(γ,xn5p) 44m,gSc, and natCu(γ,xn8p) 44m,gSc by using the activation method. The high purity natural Sc, Ti, Fe, and Cu metallic foils in disc shape were irradiated with uncollimated 2.5 GeV bremsstrahlung beams of the Pohang Accelerator Laboratory. The induced activities in the irradiated foils were measured by the high-resolution γ-ray spectrometry with a calibrated high-purity Germanium (HPGe) detector. In order to improve the accuracy of the experimental results the necessary corrections were made in the gamma activity measurements and data analysis. The obtained isomeric yield ratios for the 45Sc(γ,n) 44m,gSc, natTi(γ,xn1p) 44m,gSc, natFe(γ,xn5p) 44m,gSc and natCu(γ,xn8p) 44m,gSc reactions are 0.25 ± 0.03, 0.43 ± 0.05, 1.38 ± 0.14, and 1.89 ± 0.21, respectively. The present result for the natCu(γ,xn8p) 44m,gSc reaction is in good agreement with the existing data. Our results for the 45Sc(γ,n) 44m,gSc, natTi(γ,xn1p) 44m,gSc, and natFe(γ,xn5p) 44m,gSc reactions are the first measurements at 2.5 GeV bremsstrahlung. The obtained results are compared with the corresponding values found in the literature. The relation between the isomeric yield ratios and the complexity of the photonuclear reactions is discussed.

Ultrafast Pump−Probe Study of the Primary Photoreaction Process in pharaonis Halorhodopsin: Halide Ion Dependence and Isomerization Dynamics

Halorhodopsin is a retinal protein that acts as a light-driven chloride pump in the Haloarchaeal cell membrane. A chloride ion is bound near the retinal chromophore, and light-induced all- trans --> 13- cis isomerization triggers the unidirectional chloride ion pump. We investigated the primary ultrafast dynamics of Natronomonas pharaonis halorhodopsin that contains Cl (-), Br (-), or I (-) ( pHR-Cl (-), pHR-Br (-), or pHR-I (-)) using ultrafast pump-probe spectroscopy with approximately 30 fs time resolution. All of the temporal behaviors of the S n <-- S 1 absorption, ground-state bleaching, K intermediate (13- cis form) absorption, and stimulated emission were observed. In agreement with previous reports, the primary process exhibited three dynamics. The first dynamics corresponds to the population branching process from the Franck-Condon (FC) region to the reactive (S 1 (r)) and nonreactive (S 1 (nr)) S 1 states. With the improved time resolution, it was revealed that the time constant of this branching process (tau 1) is as short as 50 fs. The second dynamics was the isomerization process of the S 1 (r) state to generate the ground-state 13- cis form, and the time constant (tau 2) exhibited significant halide ion dependence (1.4, 1.6, and 2.2 ps for pHR-Cl (-), pHR-Br (-), and pHR-I (-), respectively). The relative quantum yield of the isomerization, which was evaluated from the pump-probe signal after 20 ps, also showed halide ion dependence (1.00, 1.14, and 1.35 for pHR-Cl (-), pHR-Br (-), and pHR-I (-), respectively). It was revealed that the halide ion that accelerates isomerization dynamics provides the lower isomerization yield. This finding suggests that there is an activation barrier along the isomerization coordinate on the S 1 potential energy surface, meaning that the three-state model, which is now accepted for bacteriorhodopsin, is more relevant than the two-state model for the isomerization process of halorhodopsin. We concluded that, with the three-state model, the isomerization rate is controlled by the height of the activation barrier on the S 1 potential energy surface while the overall isomerization yield is determined by the branching ratios at the FC region and the conical intersection. The third dynamics attributable to the internal conversion of the S 1 (nr) state also showed notable halide ion dependence (tau 3 = 4.5, 4.6, and 6.3 ps for pHR-Cl (-), pHR-Br (-), and pHR-I (-)). This suggests that some geometrical change may be involved in the relaxation process of the S 1 (nr) state.

Excited state isomerization in a new ruthenium chelating sulfoxide complex

Electrochemical and photochemical studies of a photochromic complex with a chelating sulfoxide are reported. Electrochemical data do not support evidence for isomerization following electrochemical oxidation. However, irradiation of solutions containing [Ru(bpy) 2(PhNSO)](PF 6) 2 (bpy is 2,2′-bipyridine, PhNSO is N-benzylidine-2-(ethylsulfinyl)ethanamine) demonstrate excited state S → O isomerization. The S-bonded absorption maximum is 382 nm, while the O-bonded isomer features absorption maxima at 360 and 486 nm. The quantum yield of isomerization is 0.033 and the S-bonded isomer is emissive at low temperature (77 K). These data and an electronic structural model explaining this reactivity are introduced.

Isomeric yield ratios of 238U and 232Th photofission products

The isomeric ratios of fragments of photofission of 238U and 232Th have been determined experimentally at the bremsstrahlung end-point of 50 and 3500 MeV. Results of measurements enabled us to estimate the mean angular momenta of primary fragments and to compare them with the published data.

Isomeric yield ratios for the reaction 142Nd(γ, n)141m,g Nd

Isomeric yield ratios for the reaction 142Nd(γ, n)141m,g Nd

Asymmetric diene synthesis of bicyclo[2.2.2]octenes in the presence of Lewis acids

Asymmetric diene synthesis of substituted bicyclo[2.2.2]octenes from (−)-menthyl acrylate and cyclohexa-1,3-diene in the presence of achiral Lewis acid catalysts (TiCl4, AlCl3, BBr3, AlCl3 · OEt2, BBr3 · OEt2) was studied. The influence of various factors on the isomeric composition, yield, and enantiomeric purity of the compounds prepared was examined.

Single-particle spin effect on fission fragment angular momentum

Independent isomeric yield ratios (IYR) of 128Sb, 130Sb, 132Sb, 131Te, 133Te, 132I, 134I, 136I, 135Xe, and 138Cs have been determined in the fast neutron-induced fission of 243Am using the radiochemical and γ-ray spectrometric technique. From the IYR, fragment angular momenta (J rms) have been deduced using the spin-dependent statistical model analysis. From the J rms-values and experimental kinetic energy data deformation parameters (β) have been deduced using the pre-scission bending mode oscillation model and the statistical model. The J rms- and β-values of fission fragments from the present and earlier work in the odd-Z fissioning systems ( 238Np * , 242Am * and 244Am * ) are compared with the literature data in the even-Z fissioning systems ( 230, 233Th * , 233, 234, 236, 239U * , 239, 240, 241, 242Pu * , 244Cm(SF), 245, 246Cm * , 250Cf * and 252Cf(SF)) to examine the role of single-particle (proton) spin effect. It was observed that i) in all the fissioning systems J rms- and β-values of the fragments with spherical 82n shell and even-Z products are lower than the fragments away from the spherical neutron shell and odd-Z products, which indicate the effect of nuclear structure. ii) For both even-Z and odd-Z fission products J rms-values increase with Z F 2/A F due to increase in Coulomb torque. iii) The J rms- and β-values of even-Z fission products are comparable in all the fissioning systems. However, for odd-Z fission products they are slightly higher in the odd-Z fissioning systems compared to their adjacent even-Z fissioning systems. This is possible due to the contribution of the extra single-particle (proton) spin of the odd-Z fissioning systems to their odd-Z fragments. iv) The yield-weighted fragment angular momentum and elemental yields profile shows an anti-correlation in even-Z fissioning systems but not in the odd-Z fissioning systems.

Mechanisms of Molecular Response in the Optimal Control of Photoisomerization

We report on adaptive feedback control of photoinduced barrierless isomerization of 1,1'-diethyl-2,2'-cyanine in solution. We compare the effect of different fitness parameters and show that optimal control of the absolute yield of isomerization (photoisomer concentration versus excitation photons) can be achieved, while the relative isomerization yield (photoisomer concentration versus number of relaxed excited-state molecules) is unaffected by adaptive feedback control. The temporal structure of the optimized excitation pulses allows one to draw clear mechanistic conclusions showing the critical importance of coherent nuclear motion for the control of isomerization.

Quantum optimal control of HCN isomerization

Quantum optimal control of HCN isomerization is studied including all vibrational degrees of freedom and with rotation treated in the sudden approximation. The nuclear dynamics toolkit (NDT) technique is used for solving the control equations. The NDT is shown to be successful in reducing the computational expense in the large-scale optimal control calculations by shifting the resource burden from the processor to the storage. The C–N stretching mode was found to be important in the isomerization dynamics. Modest isomerization yields were found when keeping the field intensity below levels, where severe electronic disturbances could occur. Significant molecular orientational dependence upon the yield is found, with some orientations producing high yields while others giving little product when exposed to a single optimal field.

Azobenzene as Conformational Switch in Model Peptides

The photoinduced isomerization of azobenzene between the extended (trans) and compact (cis) conformations is reversibly triggered by light of two differing wavelengths. The resulting changes in molecular geometry have been extensively utilized to photoswitch transformations in chemical species reversibly for applications in optoelectronic devises as well as to photocontrol conformational states in (bio)polymers. The high isomerization yield, remarkable photostability and ultrafast kinetics (few ps) of azobenzene are well suited for the design of small, defined model systems that allow detailed folding studies to be carried out both experimentally and theoretically on the same molecules. In our and other laboratories such systems were recently obtained with cyclic peptides of defined conformational preferences as well as with alpha-helical and beta-hairpin peptides. These should, by comparison of simulation and experiment, permit an assessment and improvement of the theoretical description on the one hand and a detailed interpretation of the ultrafast conformational dynamics on the other. The phototriggered changes in conformational states lead to concurrent changes in biophysical properties that can be exploited in the photocontrol of biochemical and biological events, as exemplarily discussed with redox-active cyclic bis-cysteinyl peptides and receptor ligands.

IR laser manipulation of cis↔trans isomerization of 2-naphthol and its hydrogen-bonded clusters

The cis<-->trans isomerization reaction has been carried out for 2-naphthol and its hydrogen (H) bonded clusters by infrared (IR) laser in the electronic excited state (S1) in supersonic jets. A specific isomer in the jet was pumped to the X-H stretching vibration in the S1 state, where X refers to C, O, or N atom, by using a stepwise UV-IR excitation, and the dispersed emission spectra of the excited species or generated fragments were observed. It was found that the isomerization occurs only in the H-bonded clusters but a bare molecule does not exhibit the isomerization in the examined energy region of Ev< or =3610 cm(-1), indicating a reduction of the isomerization barrier height upon the H bonding. The relative yield of the isomerization was observed as a function of internal energy. The isomerization yield was found to be very high at the low IR frequency excitation, and was rapidly reduced with the IR frequency due to the competition of the dissociation of the H bond within the isomer. Density-functional theory (DFT) and time-dependent DFT calculations were performed for estimating the barrier height of the isomerization for bare 2-naphthol and its cluster for electronic ground and excited states. The calculation showed that the isomerization barrier height is highly dependent on the electronic states. However, the reduction of the height upon the hydrogen bonding was not suggested at the level of our calculation.

Methoxy-Substituted Stilbenes, Styrenes, and 1-Arylpropenes: Photophysical Properties and Photoadditions of Alcohols

The photochemistry of trans-stilbene and four methoxy-substituted stilbene derivatives has been investigated in a variety of solvents. The fluorescence of all five trans isomers was quenched by 2,2,2-trifluoroethanol (TFE). Upon irradiation of the five substrates in TFE, the products derived from photoaddition of the solvent were detected. Nuclear magnetic resonance spectroscopy of the products formed by irradiation in TFE-OD indicated that the proton and nucleophile are attached to two adjacent atoms of the original alkene double bond. Irradiation of the corresponding methoxy-substituted styrenes and trans-1-arylpropenes in TFE produced the analogous solvent adducts. The photoaddition of TFE proceeded with the general order of reactivity: styrenes > trans-1-arylpropenes > trans-stilbenes. Transient carbocation intermediates were observed following laser flash photolysis of the stilbenes in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP). The results are consistent with a mechanism that involves photoprotonation of the substrates by TFE or HFIP, followed by nucleophilic trapping of short-lived carbocation intermediates. Compared to the other stilbene derivatives, trans-3,5-dimethoxystilbene displayed a large quantum yield of fluorescence and a low quantum yield of trans-cis isomerization in polar organic solvents. The unique photophysical properties of trans-3,5-dimethoxystilbene are attributed to formation of a highly polarized charge-transfer excited state (mu(e) = 13.2 D).

Isomerization and dissociation dynamics of HCN in a picosecond infrared laser field: A full-dimensional classical study

We report a full-dimensional study of the classical dynamics of HCN-->HNC isomerization and of HCN rovibrational dissociation driven by a strong but nonionizing picosecond infrared laser field. The dynamics of the isolated molecule and of the molecule in liquid Ar have both been studied. Our theoretical and numerical results show that when all degrees of freedom are accounted for the field induced molecular dynamics can be totally different from what was found in previous studies, where the HCN molecule is restricted to a plane containing the external field. It is shown that as HCN is driven by an infrared laser field, the rotation of the H atom around the C-N bond provides an important and highly efficient energy absorption mechanism. In the presence of a monochromatic picosecond infrared laser field with an intensity of 10(13) W/cm(2), this energy absorption mechanism generates considerable HCN-->HNC isomerization yield or high rovibrational dissociation yield without molecular preorientation or prealignment. Our study of the field induced isomerization and dissociation dynamics of the same system in liquid Ar shows that the picosecond isomerization dynamics is insignificantly affected by the surrounding atomic liquid whereas the dissociation yield may be greatly suppressed in a high density liquid. The implications of this study for full-dimensional quantum dynamics of multiphoton rovibrational excitation and dissociation of triatomics are briefly discussed.