Time-resolved IR spectroscopy of N-methylthioacetamide: trans-->cis isomerization upon n-pi and pi-pi excitation and cis-->trans photoreaction

Time-resolved infrared spectroscopy was used to study the photoisomerization of N-Methylthioacetamide (NMTAA) in D2O in both the cis-->trans and the trans-->cis direction upon selective excitation of the n-pi (S1) and pi-pi (S2) electronic transitions. While isomerization and the return to the ground state takes place on two distinct time scales (cis isomerization is 30-40%, independent of the electronic state excited, while the cis-->trans isomerization proceeds with a 60-70% quantum efficiency. These results support a mechanism by which isomerization takes place via one common intermediate state independent of electronic excitation energy and initial conformation.     

The infrared-driven cis-trans isomerization of HONO. II: Vibrational relaxation and slow isomerization channel

In a recent paper [R. Schanz et al., J. Chem. Phys. 122, 044509 (2005)], we investigated the IR-driven cis-trans isomerization of HONO in a Kr matrix with the help of femtosecond IR spectroscopy. We found that isomerization occurs on a 20 ps time scale, however, with a cis-->trans quantum yield of only 10% that is significantly below the value reported in the literature (close to 100%). At the same time, we concluded that vibrational energy has not completely dissipated out of the molecule at the maximum delay time we reached in this study (500 ps). In order to verify whether additional, slower reaction channels exist, we extend the study here to delay times up to 100 ns. At a temperature of 32 K, we indeed find an additional isomerization channel on a 2 ns timescale, which increases the total cis-->trans quantum yield to approximately 30%. The trans-->cis quantum yield is approximately 7%. There is still a discrepancy between the quantum yields we observe and the literature values, however, we provide experimental evidence that this discrepancy is due to the different temperatures of our study. Vibrational cooling occurs on a 20 ns time scale, and cascades in a highly nonstatistical manner through one single normal mode (most likely the ONO bending mode nu(5)). Intermolecular energy dissipation into the rare gas matrix is more efficient than intramolecular vibrational energy redistribution and the matrix environment can certainly not be considered a weak perturbation.     

cis/trans photoisomerization of secondary thiopeptide bonds

The reversible cis/trans photoisomerization of secondary thiopeptide bonds has been systematically studied with UV-visible absorption, capillary electrophoresis, 1H NMR spectroscopy, and circular dichroism methods. It was found that the concentration of the cis conformers could be increased from less than 1 % in the thermal equilibrated solution to up to 20 % in the photostationary state. The rotational barriers of the thiopeptide bond and the pH dependence of the isomerization rates were also studied. The quantum yields of the trans-->cis and the cis-->trans processes were determined from photokinetic analysis.     

Direct calculation of interconversion barriers in dynamic chromatography and electrophoresis: Isomerization of captopril

Dynamic capillary electrophoresis (DCE) and direct calculation of the rate constants of isomerization has been applied to determine the cis-trans isomerization barriers of the angiotensin-converting enzyme inhibitor captopril. The separation of the rotational cis-trans isomeric drug has been performed in an aqueous 50 mM borate buffer at pH 9.3. Interconversion profiles featuring plateau formation, peak-broadening, and peak coalescence were observed. To determine the rate constants of the forward and backward reaction (k(cis-->trans) and k(trans-->cis)) of the isomerization process in dynamic capillary electrophoresis, a novel straightforward calculation method using the experimental parameters plateau height, h(plateau), peak width at half height w(h), the total migration times of the cis-trans isomers t(R) and the electroosmotic break-through time t(0) as well as the peak ratio of the cis-trans isomers is presented for the first time. From temperature dependent measurements the rate constants k(cis-->trans) and k(trans-->cis) and the kinetic activation parameters DeltaG( not equal), DeltaH( not equal), and DeltaS( not equal) of the cis-trans isomerization of captopril were obtained. From the activation parameters the isomerization barriers of captopril at 37 degrees C under basic conditions were calculated to be DeltaG( not equal) (cis-->trans) = 90.3 kJ.mol(-1)and DeltaG( not equal) (trans-->cis) = 90.0 kJ.mol(-1*).     

Determination of the cis-trans isomerization barrier of enalaprilat by dynamic capillary electrophoresis and computer simulation

Dynamic capillary electrophoresis (DCE) and computer simulation of the elution profiles with the stochastic model has been applied to determine the isomerization barriers of the angiotensin converting enzyme inhibitor enalaprilat. The separation of the rotational cis-trans isomeric drug has been performed in an aqueous 20 mM borate buffer at pH 9.3. Interconversion profiles featuring plateau formation and peak broadening were observed. To evaluate the rate constants k(cis-->trans) and k(trans-->cis) of the cis-trans isomerization from the experimental electropherograms obtained by dynamic capillary electrophoresis, elution profiles were analyzed by a simulation with iterative convergence to the experimental data using the ChromWin software which requires the total migration times of the individual isomers t(R), the electroosmotic break-through time t(0), the plateau height h(plateau), the peak widths at half height of the individual isomers w(h), as well as the peak ratio of the isomers as experimental data input. From temperature-dependent measurements between 0 degrees and 15 degrees C the thermodynamic parameters Delta G, Delta H and Delta S, the rate constants k(cis-->trans) and k(trans-->cis) and the kinetic activation parameters Delta G*, Delta H*, and Delta S* of the cis-trans isomerization of enalaprilat were obtained. From the activation parameters the isomerization barriers at 37 degrees C were calculated to be Delta G* (trans-->cis) = 87.2 kJ.mol(-1) and Delta G*(cis-->trans) = 91.9 kJ.mol(-1).     

Simulation of the photodynamics of azobenzene on its first excited state: comparison of full multiple spawning and surface hopping treatments

We have studied the cis-->trans and trans-->cis photoisomerization of azobenzene after n-->pi* excitation using the full multiple spawning (FMS) method for nonadiabatic wave-packet dynamics with potential-energy surfaces and couplings determined "on the fly" from a reparametrized multiconfigurational semiempirical method. We compare the FMS results with a previous direct dynamics treatment using the same potential-energy surfaces and couplings, but with the nonadiabatic dynamics modeled using a semiclassical surface hopping (SH) method. We concentrate on the dynamical effects that determine the photoisomerization quantum yields, namely, the rate of radiationless electronic relaxation and the character of motion along the reaction coordinate. The quantal and semiclassical results are in good general agreement, confirming our previous analysis of the photodynamics. The SH method slightly overestimates the rate of excited state decay, leading in this case to lower quantum yields.     

The photoisomerization mechanism of azobenzene: a semiclassical simulation of nonadiabatic dynamics

We have simulated the photoisomerization dynamics of azobenzene, taking into account internal conversion and geometrical relaxation processes, by means of a semiclassical surface hopping approach. Both n-->pi* and pi-->pi* excitations and both cis-->trans and trans-->cis conversions have been considered. We show that in all cases the torsion around the N==N double bond is the preferred mechanism. The quantum yields measured are correctly reproduced and the observed differences are explained as a result of the competition between the inertia of the torsional motion and the premature deactivation of the excited state. Recent time-resolved spectroscopic experiments are interpreted in the light of the simulated dynamics.     

First-principle computational study on the full conformational space of L-threonine diamide, the energetic stability of cis and trans isomers

First-principle computations were carried out on the conformational space of trans and cis peptide bond isomers of HCO-Thr-NH2. Using the concept of multidimensional conformational analysis (MDCA), geometry optimizations were performed at the B3LYP/6-31G(d) level of theory, and single-point energies as well as thermodynamic functions were calculated at the G3MP2B3 level of theory for the corresponding optimized structures. Two backbone Ramachandran-type potential energy surfaces (PESs) were computed, one each for the cis and trans isomers, keeping the side chain at the fully extended orientation (chi1=chi2=anti). Similarly, two side chain PESs for the cis and trans isomers were generated for the (phi=psi=anti) orientation corresponding to approximately the betaL backbone conformation. Besides correlating the relative Gibbs free energy of the various stable conformations with the number of stabilizing hydrogen bonds, the process of trans-->cis isomerization is discussed in terms of intrinsic stabilities as measured by the computed thermodynamic functions.     

Matrix isolation infrared and ab initio studies on conformers of 3-fluoropropene

The infrared spectra of the cis and gauche conformers of 3-fluoropropene, CH₂CHCH₂F, were studied in Ne, Ar, Kr and Xe matrices. An infrared-induced cis to gauche rotamerization was found in Ar, Kr and Xe matrices. A thermal interconversion process was also found. Its direction was dependent upon the host, being the same as that of the IR process in Kr but reverse in Ar and Xe. In Ar and Xe matrices considerable site-splitting occurs in the IR spectra and a detailed analysis of the processes in different sites is given. An energy difference of 2.5±0.3 kJ mol⁻¹ between the cis and gauche species was obtained on assuming that the gas phase equilibrium between the conformers is trapped upon deposition. A slow dark process from cis to gauche conformer was observed in Kr matrices at temperatures above 15 K, possibly due to tunnelling. Ab initio calculations were carried out on 3-fluoropropene. The torsional potential energy curve and spectra of the conformers were calculated at the MP2(full)16-31G** level and were compared with the experimental results.     

A fast photoswitch for minimally perturbed peptides: investigation of the trans-->cis photoisomerization of N-methylthioacetamide

Thio amino acids can be integrated into the backbone of peptides without significantly perturbing their structure. In this contribution we use ultrafast infrared and visible spectroscopy as well as state-of-the-art ab initio computations to investigate the photoisomerization of the trans form of N-methylthioacetamide (NMTAA) as a model conformational photoswitch. Following the S2 excitation of trans-NMTAA in water, the return of the molecule into the trans ground state and the formation of the cis isomer is observed on a dual time scale, with a fast component of 8-9 ps and a slow time constant of approximately 250 ps. On both time scales the probability of isomerization to the cis form is found to be 30-40%, independently of excitation wavelength. Ab initio CASPT2//CASSCF photochemical reaction path calculations indicate that, in vacuo, the trans-->cis isomerization event takes place on the S1 and/or T1 triplet potential energy surfaces and is controlled by very small energy barriers, in agreement with the experimentally observed picosecond time scale. Furthermore, the calculations identify one S2/S1 and four nearly isoenergetic S1/S0 conical intersection decay channels. In line with the observed isomerization probability, only one of the S1/S0 conical intersections yields the cis conformation upon S1-->S0 decay. A substantially equivalent excited-state relaxation results from four T1/S0 intersystem crossing points.     

Photoelectron energy distribution measurements from benzene in rare gas matrices

Photoelectronic energy distributions (PED's) are presented for benzene molecules embedded in solid Ar, Kr and Xe rare gas matrices obtained with monochromatized synchrotron radiation for selective excitation in the range hν = 8 eV to 15 eV. For photon energies in the transparent region of the matrices direct emission from the occupied benzene initial states in the band gap of the matrix is observed. Energy transfer from the matrix exciton states to the benzene guest molecules takes place when the host exciton states are excited. The PED's show that energy of unrelaxed excitons is transferred and that transfer to initial states just at the ionization threshold is favoured.     

HXeCCH in Ar and Kr matrices

HXeCCH molecule is prepared in Ar and Kr matrices and characterized by IR absorption spectroscopy. The experiments show that HXeCCH can be made in another host than the polarizable Xe environment. The H-Xe stretching absorption of HXeCCH in Ar and Kr is blueshifted from the value measured in solid Xe. The maximum blueshifts are +44.9 and +32.3 cm(-1) in Ar and Kr, respectively, indicating stabilization of the H-Xe bond. HXeCCH has a doublet H-Xe stretching absorption measured in Xe, Kr, and Ar matrices with a splitting of 5.7, 13, and 14 cm(-1), respectively. Ab initio calculations for the 1:1 HXeCCHcdots, three dots, centeredNg complexes (Ng = Ar, Kr, or Xe) are used to analyze the interaction of the hosts with the embedded molecule. These calculations support the matrix-site model where the band splitting observed experimentally is caused by specific interactions of the HXeCCH molecule with noble-gas atoms in certain local morphologies. However, the 1:1 complexation is unable to explain the observed blueshifts of the H-Xe stretching band in Ar and Kr matrices compared to a Xe matrix. More sophisticated computational approach is needed to account in detail the effects of solid environment.     

Probing highly efficient photoisomerization of a bridged azobenzene by a combination of CASPT2//CASSCF calculation with semiclassical dynamics simulation

Mechanism of phototriggered isomerization of azobenzene and its derivatives is of broad interest. In this paper, the S(0) and S(1) potential energy surfaces of the ethylene-bridged azobenzene (1) that was recently reported to have highly efficient photoisomerization were determined by ab initio electronic structure calculations at different levels and further investigated by a semiclassical dynamics simulation. Unlike azobenzene, the cis isomer of 1 was found to be more stable than the trans isomer, consistent with the experimental observation. The thermal isomerization between cis and trans isomers proceeds via an inversion mechanism with a high barrier. Interestingly, only one minimum-energy conical intersection was determined between the S(0) and S(1) states (CI) for both cis → trans and trans → cis photoisomerization processes and confirmed to act as the S(1) → S(0) decay funnel. The S(1) state lifetime is ～30 fs for the trans isomer, while that for the cis isomer is much longer, due to a redistribution of the initial excitation energies. The S(1) relaxation dynamics investigated here provides a good account for the higher efficiency observed experimentally for the trans → cis photoisomerization than the reverse process. Once the system decays to the S(0) state via CI, formation of the trans product occurs as the downhill motion on the S(0) surface, while formation of the cis isomer needs to overcome small barriers on the pathways of the azo-moiety isomerization and rotation of the phenyl ring. These features support the larger experimental quantum yield for the cis → trans photoisomerization than the trans → cis process.     

Dynamic heterogeneities in glassy polymers

The photoinduced isomerization of molecules incorporated in a glassy polymer matrix exhibits a wide spectrum of quantum yields. The source of the spectrum is matrix heterogeneities. The kinetics of the photoisomerization of 1-naphthyl-p-azomethoxybenzene in poly(ethyl methacrylate) and poly(n-butyl methacrylate) films is first used to study the rearrangement of the environments of photochromic molecules. The nonequilibrium distribution of cis molecules over the spectrum is obtained via conversion of trans molecules with the highest quantum yield into the cis form with the use of 405-nm light. The kinetics of attainment of the photostationary ratio for concentrations of cis and trans isomers under the action of light with a wavelength of 546 nm is studied through variation in the pause between the conversion of molecules into the cis form and the beginning of the studied process. It is shown that reversible changes in the structure of polymer matrices occur at a high rate at temperatures much lower than the glasstransition temperature.     

Photoswitchable luminescence of rhenium(i) tricarbonyl diimines

The synthesis, characterization, and X-ray crystal structures of [Re(diimine)(CO)(3)(dpe)](PF(6)) (dpe = 1,2-di(4-pyridyl)ethylene) compounds are reported. The cis-dpe complexes exhibit yellow luminescence after UV excitation, whereas the trans-dpe counterparts are nonluminescent. The luminescence quantum yields of the cis-dpe complexes are strongly dependent on the identity of the diimine ligand. Irradiation (350 nm) of the trans-dpe complexes induces trans --> cis dpe-ligand isomerization with quantum yields on the order of 0.2, and this process leads to an on-switching of yellow luminescence. After long 350-nm irradiation times, a steady state composed of roughly 70% cis- and 30% trans-dpe complexes is reached. The reverse cis --> trans photoisomerization reaction is induced by irradiating the cis-dpe complexes at 250 nm, switching off the yellow luminescence. For 250-nm excitation, photodecomposition of the [Re(diimine)(CO)(3)(dpe)](+) complexes competes efficiently with photoisomerization.     

Ti2: accurate determination of the dissociation energy from matrix resonance Raman spectra and chemical interaction with noble gases

UV-visible and resonance Raman spectra of Ti(2) isolated in Ar, Kr, and Xe matrices at temperatures of 10 K were measured by using the 514 nm line of an Ar ion laser. The data show that the Ti(2) molecule interacts strongly with Xe, leading to a significant weakening of the Ti[bond]Ti bond strength. The f(Ti[bond]Ti) force constant decreases in the series Ar>Kr>Xe, from 232.8 Nm(-1) in Ar and 225.5 Nm(-1) in Kr to 199.7 Nm(-1) in Xe. Additional experiments in an Ar matrix containing 2 % of Xe indicate the formation of a molecule of the formula Ti(2)Xe. Our spectra for Ti(2) in an Ar matrix give evidence for several previously not observed members of the Stokes progression. The sum of experimental data allows for an improved estimation of the dissociation energy on the basis of a LeRoy-Bernstein-Lam analysis. A dissociation energy of 1.18 eV was derived from this analysis. The UV-visible data give evidence of the vibrational levels of an excited state of Ti(2).     

Mechanistic study of trans⇆cis isomerization of the substituted azobenzene moiety bound on a liquid‐crystalline polymer

A mechanistic study of the trans?cis isomerization of the azobenzene moiety in a side‐chain liquid‐crystal polymer system was carried out with six liquid‐crystalline polymethacrylates in which different electron‐withdrawing substituents were attached to the para‐positions of the azobenzene chromophores. Compared to the non‐nitro‐substituted azo polymers, the nitro‐substituted azo polymers exhibited two quite different behaviors: an extraordinarily high reaction rate of the thermal cis–trans isomerization and an unexpected composition of cis–trans isomers obtained from the photochemical trans–cis isomerization process. A potential energy profile for the isomerization process was established on basis of the structures of the proposed transition states and was employed to elucidate the reaction mechanism. The results confirmed that the nitro‐substituted azo polymer system proceeded via a rotation mechanism in either direction of the trans?cis isomerization reaction, whereas the non‐nitro‐substituted species were more likely to follow an inversion mechanism. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2296–2307, 2001     

Diatomic monocations and dications containing germanium and a rare gas atom.

Diatomic dications and monocations GeY2+ and GeY+, where Y = Ne, Ar, Kr or Xe, have been detected on the microsecond time scale. They are characterized via appearance energies, charge stripping of GeY+ ions, and mass-analysed ion kinetic energy spectroscopy. The formation of GeAr+ is found to involve an excited state of Ar in a Hornbeck-Molnar process.     

Photodissociation of formyl fluoride in rare gas matrixes

Photodissociation of formyl fluoride (HCOF) is studied in Ar, Kr, and Xe matrixes at 248 and 193 nm excitation by following spectral changes in the infrared absorption spectra. In all matrixes, the main photodissociation products are CO/HF species, including CO-HF and OC-HF complexes and thermally unstable CO/HF species (a distorted CO/HF complex or a reaction intermediate), which indicate negligible cage exit of atoms produced via the C-F and C-H bond cleavage channels. However, the observation of traces of H, F, CO, CO(2), F(2)CO, FCO, and HRg(2)(+) (Rg = Kr or Xe) in Kr and Xe matrixes would imply some importance of other reaction channels too. The analysis of the decay curves of the precursor shows that dissociation efficiency of HCOF increases as Ar < Kr < Xe, the difference being the factor of 10 between Ar and Xe. Moreover, HCOF dissociates 20-50 times faster at 193 nm compared to 248 nm. Interestingly, whereas the CO/HF species are stable with respect to photolysis in Ar, they photobleach in Kr and Xe matrixes at 248 and 193 nm, even though the first excited states of CO and HF are not energetically accessible with 193 and 248 nm photons. In krypton matrix, the photodissociation of CO/HF species at 248 nm is observed to be a single photon process. Quantum chemical calculations of electronic excitation energies of CO-HF and OC-HF complexes show that the electronic states of HF and CO mostly retain their diatomic nature in the pair. This clearly demonstrates that photodissociation of CO/HF complexes is promoted by the surrounding rare gas lattice.