Electron In Methanol Research Articles

Shallow and deep trap states of solvated electrons in methanol and their formation, electronic excitation, and relaxation dynamics.

We present condensed-phase first-principles molecular dynamics simulations to elucidate the presence of different electron trapping sites in liquid methanol and their roles in the formation, electronic transitions, and relaxation of solvated electrons (emet−) in methanol. Excess electrons injected into liquid methanol are most likely trapped by methyl groups, but rapidly diffuse to more stable trapping sites with dangling OH bonds. After localization at the sites with one free OH bond (1OH trapping sites), reorientation of other methanol molecules increases the OH coordination number and the trap depth, and ultimately four OH bonds become coordinated with the excess electrons under thermal conditions. The simulation identified four distinct trapping states with different OH coordination numbers. The simulation results also revealed that electronic transitions of emet− are primarily due to charge transfer between electron trapping sites (cavities) formed by OH and methyl groups, and that these transitions differ from hydrogenic electronic transitions involving aqueous solvated electrons (eaq−). Such charge transfer also explains the alkyl-chain-length dependence of the photoabsorption peak wavelength and the excited-state lifetime of solvated electrons in primary alcohols.

Experimental and theoretical studies on isatin-Schiff bases and their copper(II)-complexes: Syntheses, spectroscopy, tautomerism, redox potentials, EPR, PXRD and DFT/TDDFT

Abstract The isatin-Schiff bases 3-(o/p-tolylimino)indolin-2-one (HL1 and HL2) react with the copper(II) acetate or nitrate to give the bis[3-(o/p-tolylimino)indole-2-olato-κ2N,O]copper(II) (1 and 2) via lactam (L)- to enol (E)-tautomerism in solution. IR spectra show solely the L-tautomer at solid state, while both tautomers in solution. 1H NMR results indicate a dynamic tautomerization equilibria between the L- and E-tautomers in solution with varying ratios of ca. 83/17, 60/40 and 54/46 (L/E) within 0.5, 5 and 24 h, respectively. Cyclic voltammograms demonstrate two quasi reversible one electron charge transfer processes for the Cu(II)/Cu(I) and Cu(I)/Cu(0) couples in acetonitrile, respectively. PXRD patterns at 5–50° (2θ) correspond well to the crystalline nature with various degrees of crystallinity for the ligands and complexes. X-band EPR spectrum shows an intense broad band at relatively high field region with isotropic nature. Magnetic moment values (μeff. = 1.56–1.63 μB) indicate paramagnetic nature of the copper(II)-complexes with one unpaired electron in methanol. Optimized structures by DFT show the L-form relatively stable than the E-form by 19.76 kcal/mol, while compounds HL2 and 2 are slightly stable than HL1 and 1 by 0.35 and 0.79 kcal/mol, respectively. Electronic spectra by TDDFT strongly support the experimental results in methanol.

A Simple ab Initio Model for the Solvated Electron in Methanol.

The solvation structure of a solvated electron in methanol is investigated with ab initio calculations of small anion methanol clusters in a polarized dielectric continuum. We find that the lowest-energy structure in best agreement with experiment, calculated with CCSD, MP2, and B3LYP methods with aug-cc-pvdz basis set, is a tetrahedral arrangement of four methanol molecules with OH bonds oriented toward the center. The optimum distance from the tetrahedron center to the hydroxyl protons is ∼1.8 Å, significantly smaller than previous estimates. We are able to reproduce experimental radius of gyration Rg (deduced from optical absorption), vertical detachment energy, and resonance Raman frequencies. The electron paramagnetic resonance g-factor shift is qualitatively reproduced using density functional theory.

Examination of the formation process of pre-solvated and solvated electron in n-alcohol using femtosecond pulse radiolysis

Abstract The formation process of pre-solvated and solvated electron in methanol (MeOH), ethanol (EtOH), n -butanol (BuOH), and n -octanol (OcOH) were investigated using a fs-pulse radiolysis technique by observing the pre-solvated electron at 1400 nm. The formation time constants of the pre-solvated electrons were determined to be 1.2, 2.2, 3.1, and 6.3 ps for MeOH, EtOH, BuOH, and OcOH, respectively. The formation time constants of the solvated electrons were determined to be 6.7, 13.6, 22.2, and 32.9 ps for MeOH, EtOH, BuOH, and OcOH, respectively. The formation dynamics and structure of the pre-solvated and solvated electrons in n -alcohols were discussed based on relation between the obtained time constant and dielectric relaxation time constant from the view point of kinetics. The observed formation time constants of the solvated electrons seemed to be strongly correlated with the second component of the dielectric relaxation time constants, which are related to single molecule motion. On the other hand, the observed formation time constants of the pre-solvated electrons seemed to be strongly correlated with the third component of the dielectric relaxation time constants, which are related to dynamics of hydrogen bonds.

Dynamics of electron solvation in methanol: Excited state relaxation and generation by charge-transfer-to-solvent.

The charge-transfer-to-solvent dynamics (CTTS) and excited state relaxation mechanism of the solvated electron in methanol are studied by time-resolved photoelectron spectroscopy on a liquid methanol microjet by means of two-pulse and three-pulse experiments. In the two-pulse experiment, CTTS excitation is followed by a probe photoejection pulse. The resulting time-evolving photoelectron spectrum reveals multiple time scales characteristic of relaxation and geminate recombination of the initially generated electron which are consistent with prior results from transient absorption. In the three-pulse experiment, the relaxation dynamics of the solvated electron following electronic excitation are measured. The internal conversion lifetime of the excited electron is found to be 130 ± 40 fs, in agreement with extrapolated results from clusters and the non-adiabatic relaxation mechanism.

Photoelectron spectroscopy of solvated electrons in alcohol and acetonitrile microjets

Photoelectron spectra of solvated electrons in methanol, ethanol, and acetonitrile microjets are reported. Solvated electrons are generated in and detached from microjets using two photons from single nanosecond laser pulses at wavelengths ranging 266 to 213 nm. We find vertical binding energies of 3.38 ± 0.11 eV in methanol and 3.38 ± 0.10 eV in ethanol. Two features are observed in acetonitrile at 2.61 ± 0.11 eV and 3.67 ± 0.15 eV, attributed to the solvated and dimer-bound binding geometries respectively. These results are compared to previous work on solvated cluster anions and alkali-doped solvent clusters.

Direct Measurement of Vertical Electron Binding Energies of Solvated Electrons in Methanol and Ethanol

Abstract Time-resolved photoelectron spectroscopy at ultralow kinetic energy was applied to liquid beams of NaI solutions in methanol and ethanol. Solvated electrons were formed from I− ions in these solutions by charge transfer to solvent reactions. The vertical electron binding energies of the solvated electrons were determined for the first time. Both of the binding energies were found to be 3.10 ± 0.1 eV. This is in reasonable agreement with the dielectric continuum model of solvated electrons using the previously measured vertical binding energy of a hydrated electron. This indicates that the cavity radii of solvated electrons in water, methanol, and ethanol are approximately the same (0.33–0.35 nm).

A new electron-methanol molecule pseudopotential and its application for the solvated electron in methanol

A new electron-methanol molecule pseudopotential is developed and tested in the present paper. The formal development of the potential is based on quantum mechanical calculations on the electron-methanol molecule model in the static exchange approximation. The computational model includes a steep confining potential that keeps the otherwise unbound excess electron in the vicinity of the methanol molecule. Using the Phillips-Kleinman theorem we introduce a smooth pseudowave function of the excess electron with the exact eigenenergy and correct asymptotic behavior. The nonlocal potential energy operator of the model Hamiltonian is then replaced to a local potential that reproduces the ground-state properties of the excess electron satisfactorily. The pseudopotential is then optimized in an analytically simple functional form to fit this approximate local potential in conjunction with the point charges and the geometry of a classical, all-site methanol-methanol interaction potential. Of the adjustable parameters, the parameters for the carbon and the methyl hydrogen atoms are optimized, while those for the oxygen and the hydroxyl hydrogen are taken from a previous electron-water molecule pseudopotential. A polarization term is added to the potential a posteriori. The polarization parameters are chosen to reproduce the experimental position of the optical absorption spectrum of an excess electron in mixed quantum-classical molecular dynamics simulations. The energetic, structural and spectroscopic properties of the solvated electron in a methanol bath are simulated at 300 K and compared with previous solvated electron simulations and available experimental data.

Resonance Raman Spectrum of the Solvated Electron in Methanol: Simulation within a Cluster Model

The microsolvation of the CH(3)OH(2) hypervalent radical in methanol clusters has been investigated by density functional theory. It is shown that the CH(3)OH(2) radical spontaneously decomposes within methanol clusters into protonated methanol and a localized solvated electron cloud. The geometric and electronic structures of these clusters as well as their vibrational frequencies have been characterized. Resonance Raman intensities, associated with the s --> p transition of the unpaired electron, have been estimated for CH(3)OH(2)M(n) (M = CH(3)OH, n = 1-3) clusters. It is shown that with increasing cluster size the simulated spectra converge toward the resonance Raman spectrum of the solvated electron in methanol measured recently by Tauber and Mathies (J. Am. Chem. Soc. 2004, 126, 3414). The results suggest that CH(3)OH(2)M(n) clusters are useful finite-size model systems for the computational investigation of the spectroscopic properties of the solvated electron in liquid methanol.

The precursors of the solvated electron in methanol studied by femtosecond pump-repump-probe spectroscopy

Using UV photoionization and delayed near-infrared reexcitation pulses, a novel time-, frequency-, and polarization-resolved pump-repump-probe spectroscopy is conducted in the probing range of 450-2400 nm with improved experimental accuracy. Both the generation process and relaxation dynamics following selective repumping of intermediate species of the solvated electron are investigated and analyzed self-consistently with the help of a kinetic model. New insight in the intermediates of the trapped electron is gained leading to a unique microscopic picture.

Electron Trapping in Polar-Solvated Zeolites

Of current interest in our laboratory is the nature of photoinduced processes in the cavities of zeolites completely submerged in polar solvents, or polar-solvated zeolites (PSZ). The present study addresses the nature of electron trapping in PSZ with emphasis on the zeolites NaX and NaY. Free electrons were generated by two-photon, pulsed-laser excitation of either pyrene or naphthalene included in zeolite cavities. Trapped electrons were monitored by diffuse transmittance, transient absorption spectroscopy at visible wavelengths. In anhydrous alcohols, electron trapping by Na(4)(4+) ion clusters was observed in both NaX and NaY. The resulting trapped electrons decayed over the course of tens of milliseconds. No evidence for alcohol-solvated electrons was found. More varied results were observed in solvents containing water. In NaX submerged in CH(3)OH containing 5% or higher water, species having microsecond lifetimes characteristic of solvated electrons were observed. By contrast, a 2 h exposure of NaY to 95/5 CH(3)OH/H(2)O had no effect on electron trapping relative to anhydrous CH(3)OH. The difference between NaX and NaY was explained by how fast water migrates into the sodalite cage. Prolonged exposure to water at room temperature or exposure to water at elevated temperatures was necessary to place water in the sodalite cages of NaY and deactivate Na(4)(4+) as an electron trap. Additional studies in NaY revealed that solvent clusters eventually become lower energy traps than Na(4)(4+) as the water content in methanol increases. In acetonitrile-water mixtures, electron trapping by Na(4)(4+) was eliminated and no equivalent species characteristic of solvated electrons in methanol-water mixtures was observed. This result was explained by the formation of low energy solvated electrons which cannot be observed in the visible region of the spectrum. Measurements of the rate of O(2) quenching in anhydrous solvents revealed rate constants for the quenching of ion cluster trapped electrons that were 2-4 times higher than that for pyrene triplets. In NaX, the rate constant in methanol was 10(4) times smaller than that in cyclohexane, showing greater inhibition of O(2) reactivity in the medium of PSZ. The results of this study point out the conditions under which Na(4)(4+) is active as an electron trap in PSZ and that water must be present in the sodalite cage to produce solvated electrons in the supercage.

Ionization of methanol: monitoring the trapping of electrons on the fs time scale

Femtosecond spectroscopy of the photogeneration of electrons in methanol is demonstrated from the visible to the mid-infrared utilizing polarization and fs-time resolution. After two photon excitation at 9.1 eV clear evidence is provided for a preferred trapping of the electrons with τ tr=100 fs within H-bonded methanol chains oriented preferentially parallel to the applied pump field. Trapping proceeds in contrast to the case of water along an excited p ′-state under same experimental conditions.

Non-linear response and hydrogen bond dynamics for electron solvation in methanol

Non-equilibrium and equilibrium adiabatic mixed quantum-classical molecular dynamics computer simulations of the solvation dynamics of an excess electron in methanol are reported. We develop the connection between the multiple time scales reflected in solvent response and individual physical phenomena, such as the radial collapse of the electron and structural relaxation of the hydrogen-bonding network of the solvent. The significant role of the latter aspect appears responsible for the breakdown of the linear response approximation for the relaxation of the adiabatic ground state energy of the excess electron.

The role of specific solvent modes in the non-radiative relaxation of an excess electron in methanol

Mixed quantum-classical molecular dynamics simulations have been used to examine the contributions of specific solvent degrees of freedom to the non-adiabatic coupling leading to relaxation of the first electronic excited state of an excess electron in methanol. Compared to water, we find that the presence of a reduced number of high frequency modes per solvent molecule and of a smaller number of molecules per unit volume combine to produce a three-fold decrease in the magnitude of the non-adiabatic coupling in methanol. The first solvation shell dominates the electronic coupling, most strongly through rotational and vibrational motions involving the hydrogen atom of the methanol molecule. Considering these results, the excited state lifetime in methanol is estimated to be an order of magnitude longer than in water, of the order of 7ps. Combined with the result of our previous work (Mosyak et al., J. Chem. Phys. 109 (1998) 6390), which reports relatively much slower solvation dynamics following ground state photoexcitation, compared to water, the current results lead to a firm conclusion regarding corresponding recent experimental data on excited state relaxation as well as classic experiments on solvated electron formation in alcohols. We conclude that the longest time dynamics can be identified with the excited state lifetime, rather than solvation dynamics of the neoformed ground state. It is predicted that for excess electrons in higher alcohols, the non-adiabatic transition should be increasingly induced primarily by molecules of the first solvation shell. This dominance of the first shell coupling should lead to a lifetime that continues to increase to a value at least four times its value in methanol, also consistent with the experiment.

Nonadiabatic molecular dynamics simulation of photoexcitation experiments for the solvated electron in methanol

Nonadiabatic quantum molecular dynamics simulations have been performed to simulate the pump-and-probe photoexcitation experiments of the ground state equilibrium solvated electron in methanol carried out by Barbara et al. [Chem. Phys. Lett. 232, 135 (1995)]. We have characterized both the time evolution of the quantum solute, the solvated electron, and the solvation response of the classical methanol bath. The quantum energy gap provides an excellent tool to gain insight into the underlying microscopic details of the solvation process. The solvent response is characterized for both processes by a fast Gaussian component and a biexponential decay. The present results suggest that the residence time of the solvated electron in the first excited state is substantially longer than inferred from the cited experiments. The experimentally observed fast exponential portion of the relaxation more likely corresponds to the adiabatic solvent response than to the lifetime of the excited state electron. By comparing to photoexcitation simulations in water, it is shown that the simulated excited state lifetime is about three times longer in methanol than in water, predicting a less substantial increase than a recent calculation based on nonadiabatic coupling elements alone. Hydrogen-bonding statistical analysis provides interesting additional details about the dynamics. We find that the hydrogen-bonding network is significantly different in the first solvent shell around the electron in ground and first excited states, the distribution around the latter, larger and more diffuse, ion resembling more that of the pure liquid. Transformation of the corresponding hydrogen bonding structures takes place on a 1 ps time scale.

Solvation dynamics of an excess electron in methanol and water

Molecular dynamics simulation has been used to explore the nature of solvation dynamics for an excess electron in methanol and in water. We perform the analysis within the linear response theory and show that nonlinear corrections are small in both cases. The response function characterizing solvent relaxation after electron photoexcitation and that following the subsequent nonradiative transition are modeled and found to behave very similarly in methanol, in contrast to water. For methanol, each is comprised of an extremely short Gaussian inertial component of small amplitude and a bi-exponential diffusive decay. A relatively fast ∼1 ps exponential accounts for approximately half of the solvent relaxation and is followed by a slower ∼7 ps relaxation of comparable magnitude, a solvation response that is rather similar to that reported previously for relatively large molecules in methanol. Spectral densities of energy gap fluctuations for the equilibrium ground and excited state trajectories show that translational motion dominates solvation. Relaxational processes in methanol have been compared with the results for water. In contrast to methanol, librational motions of solvent molecules significantly influence aqueous solvation dynamics, especially following excited state decay. This difference is reflected in the relaxational processes, which are an order of magnitude slower in methanol than in water.

Detailed Investigations of the Pump−Probe Spectroscopy of the Equilibrated Solvated Electron in Alcohols

Pump−probe spectroscopy of the equilibrated solvated electron in methanol and ethanol has been studied with ∼300 fs time resolution. At low pump power the observed dynamics are assigned to s → p excitation and subsequent relaxation of a localized solvated electron. In contrast, at high pump power, two-photon absorption apparently produces mobile “conduction band” electrons, which are subsequently trapped and relax at a remote site from the initial equilibrated electron. The two-photon excitation is also observed to induce an ultrafast proton-transfer reaction from the solvent.

A dynamical analysis of energy level fluctuations for an excess electron in methanol

The dynamics of energy level fluctuations for the equilibrium solvated electron in methanol are evaluated via adiabatic ground state quantum molecular dynamics simulations. The dynamics are analyzed in terms of apparent normal mode spectral densities and contributions from such mechanical elements as solvent librational and translational motions. It is found that fluctuations of the individual energy levels have substantial contributions from translational, librational and vibrational modes. However, the energy gap is completely dominated by the low frequency translational modes of the solvent. This is in contrast to the electron in water, where a significant component arises also from librational motion.

Equilibrium structure, fluctuations, and spectroscopy of a solvated electron in methanol

Structural and spectroscopic properties of the equilibrium excess electron solvated in methanol are evaluated via adiabatic quantum molecular dynamics simulations employing an electron-methanol pseudopotential based on that of Zhu and Cukier et al. [J. Chem. Phys. 98, 5679 (1993)]. The solvated electron is localized in a roughly spherical ground electronic state of 2.5 Å radius. The electron is surrounded by approximately six or seven methanol molecules in the first solvent shell with mostly O–H bond orientation. Due to the strong electron-methanol coupling, solvent fluctuations result in substantial fluctuations of energy, size, and shape of the electron. The influences of radial fluctuations and asymmetric distortions of solvent cavities are illustrated on both the solvation structure and the optical spectrum. The optical-absorption spectrum of the solvated electron is dominated by transitions to three clearly nondegenerate p-like excited states. The fluctuations influence the s-p energy gap and modulate the p-p energy splitting leading to the calculated broad, featureless optical band. Comparison between systems containing flexible and rigid methanol molecules is also reported. Unlike in water, characteristic fluctuations of approximately 20-ps time scale can occur in methanol. The structural and optical consequences of these long timescale fluctuations are examined.

Alternative Mechanisms for Solvation Dynamics of Laser-Induced Electrons in Methanol

A multiphoton ionization study of neat methanol with subpicosecond 2-eV laser pulses has been previously reported. A hybrid electron solvation mechanism combining both a stepwise transition between two electronsolvent configuration states and a continuous first-order blue shift of the electron absorption spectra was found to closely fit the experimental data. If substantial absorption from free electrons is assumed in this spectral region, we find that another model comprising thermalization prior to a stepwise branching localization without blue-shifting spectra fits equally well at all wavelengths. However, these two models display considerable differences between their respective kinetic parameters, especially the electron localization time. Furthermore, for the nonshifting model, this calculated localization time is considerably longer than that for electron hydration in neat water. We suggest independent studies such as ultrafast electron scavenging experiments before adopting a particular mechanism for electron solvation in methanol.