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PubMed1992

Theoretical IR spectra of ionized naphthalene.

We report the results of a theoretical study of the effect of ionization on the IR spectrum of naphthalene, using ab initio molecular orbital theory. For that purpose we determined the structures, band frequencies, and intensities of neutral and positively ionized naphthalene. The calculated frequencies and intensities allowed an assignment of the most important bands appearing in the newly reported experimental spectrum of the positive ion. Agreement with the experimental spectrum is satisfactory enough to take into consideration the unexpected and important result that ionization significantly affects the intensities of most vibrations. A possible consequence on the interpretation of the IR interstellar emission, generally supposed to originate from polycyclic aromatic hydrocarbons (PAHs), is briefly presented.

Chemical Phenomena↗

PubMed2004

Electron impact ionization in helium nanodroplets: controlling fragmentation by active cooling of molecular ions.

Reported here is a study of the effects of liquid helium cooling on the fragmentation of ions formed by electron impact mass ionization. The molecules of interest are picked up by the helium nanodroplets as they pass through a low pressure oven. Electron impact ionization of a helium atom in the droplet is followed by resonant charge transfer to neighboring helium atoms. When the charge is transferred to the target molecule, the difference in the ionization potentials between helium and the molecule results in the formation of a vibrationally hot ion. In isolation, the hot parent ion would undergo subsequent fragmentation. On the other hand, if the cooling due to the helium is fast enough, the parent ion will be actively cooled before fragmentation occurs. The target molecule used in the present study is triphenylmethanol (TPM), an important species in synthetic chemistry, used to sterically protect hydroxyl groups. Threshold PhotoElectron PhotoIon COincidence (TPEPICO) experiments are also reported for gas-phase TPM to help quantify the ion energetics resulting from the cooling effects of the helium droplets.

Journal Article↗

PubMed2005

High ionization potential conjugated polymers.

We report the synthesis of a series of poly(p-phenylene ethynylene)s (PPEs) with high ionization potentials and associated high excited-state electron affinities. Their photophysical properties were investigated using steady-state and time-resolved fluorescence techniques. The ionization potentials of the polymer thin films were determined using ultraviolet photoelectron spectroscopy (UPS), and those with the highest ionization potentials displayed high sensitivity for the detection of electron-donating aromatic compounds. The effects of sterics, chemical structure, and electronic properties on the polymers' sensory responses were investigated by fluorescence quenching experiments in both solution and solid thin films. In addition, we report that in some cases the excited-state charge-transfer complexes (exciplexes) of the PPEs with analytes were observed. These latter effects provide promising opportunities for the formation of sensitive and selective chemical sensors.

Journal Article↗

PubMed2006

Polymerization of ionized acetylene clusters into covalent bonded ions: evidence for the formation of benzene radical cation.

Since the discovery of acetylene and benzene in protoplanetary nebulae under powerful ultraviolet ionizing radiation, efforts have been made to investigate the polymerization of ionized acetylene. Here we report the efficient formation of benzene ions within gas-phase ionized acetylene clusters (C2H2)n+ with n = 3-60. The results from experiments, which use mass-selected ion mobility techniques, indicate that the (C2H2)3+ ion has unusual stability similar to that of the benzene cation; its primary fragment ions are similar to those reported from the benzene cation, and it has a collision cross section of 47.4 A2 in helium at 300 K, similar to the value of 47.9 A2 reported for the benzene cation. In other words, (C2H2)3+ structurally looks like benzene, it has stability similar to that of benzene, it fragments such as benzene, therefore, it must be benzene!

Journal Article↗

PubMed2005

pH of solution greatly affects sorption of ionizable compounds into low-density polyethylene film.

Sorption of ionizable compounds of pyridines and aromatic carboxylic acids into low-density polyethylene (LDPE) film was investigated as a function of pH ranging from 4 to 7. The sorptions for pyridines were increased with increasing pH. Within the range examined, pH 7 was observed to promote the highest degree of sorption. When the pH increased by one unit from 6 to 7, the sorptions for 2-propylpyridine and 2,4,6-trimethylpyridine were both increased approximately 3.8 and 10.5 times, respectively. At pH 4, the sorption for the pyridines entirely disappeared. In contrast, the sorptions for aromatic carboxylic acids increased with decreasing pH. Within the range examined, pH 4 was observed to promote the highest degree of sorption. The magnitude of sorption for pyridines and carboxylic acids apparently depended on the affinity (delta(c)) of these compounds for LDPE film. Another factor affecting the sorption at various pHs was the pK(a) of these compounds; the sorption greatly decreased with the ionization degree of these compounds. To elucidate the phenomena, the following thermodynamic sorption equation was applied: S = S(0)gamma exp[V(v){(delta(w) - delta(v))(2) - delta(c)(2))/RT}. The plots of ln S for pyridines vs the term of the equation gave an insufficient relationship (r = 0.519). In contrast, taking into account the ratio (chi(i)()) of concentration of un-ionized compound to total concentration, the improvement for the equation was made: S' = S/chi(i)() = S(0)gamma exp[V(v){(delta(w) - delta(v))(2) - delta(c)(2))/RT}. The plots of ln S' vs the term of the equation gave a better relationship (r = 0.884). Furthermore, the equation was also applicable for the sorption behavior of carboxylic acids into LDPE (r = 0.769).

Adsorption↗

PubMed2005

Calculation of ionization potentials of small molecules: a comparative study of different methods.

With the help of various theoretical methods, ionization potentials (IPs) have been computed for a panel of small molecules containing atoms of group 14, 15, or 16 and representing different singly, doubly, or triply bonded systems with or without an interacting heteroatom lone pair. Comparison of experimental IP values to theoretical results indicates that (i) the standard outer valence green function (OVGF), density functional theory (DFT), and DeltaSCF methods lead to rather accurate values, (ii) the CASPT2 method systematically underestimates IPs, (iii) the method of deducing IPs from a shift of some standard DFT eigenvalue spectrum is a straightforward approach leading to rather accurate IPs, (iv) the eigenvalue spectrum obtained with the so-called statistical average of different orbital model potential (SAOP) exchange-correlation model potential is an efficient approach leading directly to quite accurate IPs, and (v) a good prediction of the IP spectrum can be obtained from the shifted excitation spectra of the system calculated by the time-dependent DFT (TD-DFT) method. It is also shown that the TD-DFT calculations of the ionized species bring a significant improvement over the calculations of the neutral molecules, indicating that a great part of the electronic relaxation is already taken into account (in a similar way for all ionizations). Finally, in the case of TD-DFT calculations of neutral molecules, the statistical average of different orbital model potential (SAOP) functional does not lead to significantly better results than the B3LYP functional.

Journal Article↗

PubMed2005

Collision-energy-resolved penning ionization electron spectroscopy of HCOOH, CH3COOH, and HCOOCH3 by collision with He*(2(3)S) metastable atoms.

Penning ionization of formic acid (HCOOH), acetic acid (CH3COOH), and methyl formate (HCOOCH3) upon collision with metastable He*(2(3)S) atoms was studied by collision-energy/electron-energy-resolved two-dimensional Penning ionization electron spectroscopy (2D-PIES). Anisotropy of interaction between the target molecule and He*(2(3)S) was investigated based on the collision energy dependence of partial ionization cross sections (CEDPICS) obtained from 2D-PIES as well as ab initio molecular orbital calculations for the access of a metastable atom to the target molecule. For the interaction potential calculations, a Li atom was used in place of He*(2(3)S) metastable atom because of its well-known similarity in interaction with targets. The results indicate that in the studied collision energy range the attractive potential localizes around the oxygen atoms and that the potential well at the carbonyl oxygen atom is at least twice as much as that at the hydroxyl oxygen. Moreover we can notice that attractive potential is highly anisotropic. Repulsive interactions can be found around carbon atoms and the methyl group.

Journal Article↗

PubMed2005

On the use of different dielectric constants for computing individual and pairwise terms in poisson-boltzmann studies of protein ionization equilibrium.

Poisson-Boltzmann (PB) models are a fast and common tool for studying electrostatic processes in proteins, particularly their ionization equilibrium (protonation and/or reduction), often yielding quite good results when compared with more detailed models. Yet, they are conceptually very simple and necessarily approximate, their empirical character being most evident when it comes to the choice of the dielectric constant assigned to the protein region. The present study analyzes several factors affecting the ability of PB-based methods to model protein ionization equilibrium. We give particular attention to a suggestion made by Warshel and co-workers (e.g., Sham et al. J. Phys. Chem. B 1997, 101, 4458) of using different protein dielectric constants for computing the individual (site) and the pairwise (site-site) terms of the ionization free energies. Our prediction of pK(a) values for several proteins indicates that no advantage is obtained by such a procedure, even for sites that are buried and/or display large pK(a) shifts relative to the solution values. In particular, the present methodology gives the best predictions using a dielectric constant around 20, for shifted/buried and nonshifted/exposed sites alike. The similarities and differences between the PB model and Warshel's PDLD/S model are discussed, as well as the reasons behind their apparently discrepant results. The present PB model is shown to predict also good reduction potentials in redox proteins.

Animals↗

PubMed2005

Interplay between Auger and ionization processes in nanocrystal quantum dots.

We study the interplay between Auger effects and ionization processes in the limit of strong electronic confinement in core/shell CdSe/ZnS semiconductor nanocrystal quantum dots. Spectrally resolved fluorescence decay measurements reveal a monotonic increase of the photoluminescence decay rate on excitation density. Our results suggest that Auger recombination accelerates ionization processes that lead to the occupation of dark, nonemissive nanocrystal states. A model is proposed in the quantized Auger regime describing these experimental observations and providing an estimate of the Auger assisted ionization rates.

Journal Article↗

PubMed2006

Ionization energy and electronic structure of polycytidine.

Ribonucleic acid (RNA) polycytidine (poly rC) homopolymer thin films were prepared on highly oriented pyrolytic graphite (HOPG) substrates. The films were grown from aqueous solution directly in a vacuum in multiple steps with use of an electrospray (ES) deposition system. Before poly rC deposition and after each deposition step the sample was characterized with X-ray and ultraviolet photoemission spectroscopy (XPS, UPS). Evaluation of the UP-spectra sequence allowed the determination of ionization energy and highest occupied molecular orbital (HOMO) electronic structure, as well as the charge injection barriers between HOPG and poly rC. Comparison with earlier results on polyadenosine (poly rA) indicates significant differences between ionization energies (poly rC: 8.1 eV; poly rA: 6.8 eV) and orbital alignment at the graphite interface. The larger ionization energy of poly rC results in a larger hole injection barrier and a smaller electron injection barrier relative to the HOPG Fermi level.

Electrons↗

PubMed2006

Transition-state characterization of the ammonia ionization process in aqueous solution via the free-energy gradient method.

For the ionization process of ammonia in aqueous solution, the transition-state (TS) structure was fully optimized for the first time on the free-energy surface (FES) by applying the free-energy gradient (FEG) method combined with a hybrid quantum mechanical and molecular mechanical molecular dynamics (QM/MM-MD) method. In aqueous solution, the ionization process was found to proceed by way of a clear TS (R(N1-H5) = 1.512 A), which does not exist in the gas phase. The free-energy (FE) of activation for ionization obtained was 14.7 kcal/mol, within the classical approximation, via the QM/MM-MD FEG method, and is found to be in good agreement with 9.57 kcal/mol estimated from the TS theory using the experimental value of the rate constant. Apart from the dynamic correction, it is indicated that the theoretical value would be improved to be 10.28 kcal/mol if the electronic-state calculation could be executed at the B3LYP/6-31G(d) level of theory.

Ammonia↗

PubMed2006

Collision-energy-resolved Penning ionization electron spectroscopy of thiazole and benzothiazole: study of ionic states and anisotropic interactions between a metastable He(23S) atom and hetero cyclic compounds.

Anisotropic interactions between a metastable He(2(3)S) atom and aromatic heterocyclic compounds (thiazole and benzothiazole) as well as their electronic structures were studied by means of collision-energy/electron-energy resolved two-dimensional Penning ionization electron spectroscopy combined with ab initio molecular orbital calculations. Different collision-energy dependence of partial ionization cross sections (CEDPICS) were clearly observed for different ionic states depending on anisotropic extents of molecular orbitals from which an electron is removed. It was found that thiazole and benzothiazole most strongly attract a He(2(3)S) atom around the region where the nitrogen lone pair orbital extends. For another heteroatom, sulfur, it is relatively weak, but a certain attractive interaction was found for the directions perpendicular to the molecular plane. Benzothiazole was shown to widely attract a He(2(3)S) atom in the out-of-plane directions, since the benzene moiety showed a deeper potential well than the five-membered ring. Assignments of the ionic states including shake-up states were also discussed from observed CEDPICS and ab initio molecular orbital calculations. In particular, for the satellite bands, a negative collision energy dependence of the band intensity was well supported by a configuration-interaction calculation that assigns the satellite bands to be the ionization from pi orbitals accompanying pi-pi or n-pi excitations.

Benzothiazoles↗

PubMed2006

Proton transfer mediated by the vibronic coupling in oxygen core ionized states of glyoxalmonoxime studied by infrared-X-ray pump-probe spectroscopy.

The theory of IR-X-ray pump-probe spectroscopy beyond the Born-Oppenheimer approximation is developed and applied to the study of the dynamics of intramolecular proton transfer in glyoxalmonoxime leading to the formation of the tautomer 2-nitrosoethenol. Due to the IR pump pulses the molecule gains sufficient energy to promote a proton to a weakly bound well. A femtosecond X-ray pulse snapshots the wave packet route and, hence, the dynamics of the proton transfer. The glyoxalmonoxime molecule contains two chemically nonequivalent oxygen atoms that possess distinct roles in the hydrogen bond, a hydrogen donor and an acceptor. Core ionizations of these form two intersecting core-ionized states, the vibronic coupling between which along the OH stretching mode partially delocalizes the core hole, resulting in a hopping of the core hole from one site to another. This, in turn, affects the dynamics of the proton transfer in the core-ionized state. The quantum dynamical simulations of X-ray photoelectron spectra of glyoxalmonoxime driven by strong IR pulses demonstrate the general applicability of the technique for studies of intramolecular proton transfer in systems with vibronic coupling.

Journal Article↗

PubMed2006

Vibrational structures of dimethyl sulfide and ethylene sulfide cations studied by vacuum-ultraviolet mass-analyzed threshold ionization (MATI) spectroscopy.

Adiabatic ionization energies of dimethyl sulfide (DMS) and ethylene sulfide (thiirane) are both accurately and precisely determined to be 8.6903 +/- 0.0009 and 9.0600 +/- 0.0009 eV, respectively, by vacuum-UV mass-analyzed threshold ionization (MATI) spectroscopy. Also reported are vibrational frequencies of DMS and thiirane monocations. Simulations using a Franck-Condon analysis based on ab initio molecular structures reproduce the experimental findings quite well. Detailed vibrational structures are discussed with the aid of ab initio calculations. Ionization-induced structural changes provide the information about the role of the sulfur nonbonding orbital in the geometrical layout of the title compounds.

Journal Article↗

PubMed2001

Characterization of encapsulating supramolecules by using CSI-MS with ionization-promoting reagents.

Guanidine nitrate, DMF, DMA, and DMSO are proven to be efficient ionization-promoting reagents in coldspray ionization mass spectrometry (CSI-MS) for the self-assembled cage Pt compound 1 in aqueous solution. These reagents made it possible to characterize the structures of encapsulated guest molecules in the cage. Detailed structure determinations of the cage compound encapsulating various kinds of guest molecules by using positive CSI-MS with ionization-promoting reagents are described.

Journal Article↗

PubMed1989

Iontophoretic transport of a homologous series of ionized and nonionized model compounds: influence of hydrophobicity and mechanistic interpretation.

An in vitro study was carried out to elucidate the mechanisms controlling iontophoretic transport. The investigation focused on three areas, including the nature of the permeant (state of ionization and hydrophobicity), skin structures (hair follicle distribution and stratum corneum), and various parameters influencing iontophoresis (current, permeant concentration, and competitive ion effects). The data indicate that iontophoretic-facilitated transport is essentially pore mediated and that the transport of ionized and nonionized molecules may be enhanced through the pore-type pathway. The data presented show that iontophoresis has a detrimental effect on the lipoidal transport pathway and that the transport of more hydrophobic nonionized molecules is decreased compared with passive diffusion. The iontophoretic enhancement values decreased linearly with increasing alkyl chain length of n-alkanols. The iontophoretic permeability coefficients of ionized n-alkanoic acids was shown to decrease with increasing permeant hydrophobicity.

Administration, Cutaneous↗

PubMed1989

The determination of microscopic ionization constants of a substituted piperazine using estimates from model compounds.

The microscopic ionization constants of 1-[3-(4-chlorophenyl) propyl] piperazine (I) were determined using piperazine (P) and 1,4-bis[3-(4-chlorophenyl)propyl] piperazine (II) as models for one of the ionizations. The macroscopic ionization constants for all three compounds were measured potentiometrically, and the micro constants of P and II, which are symmetrical molecules, were calculated from their macro constants. Only one micro constant of either P or II was suitable as a model for elucidating the micro constants of I.

Chemical Phenomena↗

PubMed1993

Effect of receiver fluid pH on in vitro skin flux of weakly ionizable drugs.

In in vitro skin permeation experiments, the pH of viable epidermis is readily conditioned by the receiver fluid. For weakly ionizable compounds, the flux determined experimentally thus depends on the receiver fluid pH. The purpose of the present work is to characterize this pH effect, since nonphysiological conditions have often been used in the receiver fluid to enhance the solubility of the subject compounds. A transport model was developed to analyze the above-mentioned pH effect of the receiver fluid on the steady state flux of weakly ionizable drugs. The results showed that the skin flux had a strong dependence on pH for those compounds with high intrinsic partition coefficients. Experimentally, this pH effect was observed with a model acid and a model base. The skin flux was found to have a profound dependence on the receiver fluid pH. This dependence also correlates with the octanol/water partition coefficient of the molecule. It was concluded that the use of a physiological receiver fluid would be crucial for a realistic estimation of transdermal potential. The results also suggested that, for weakly ionizable compounds with high partition coefficients, the viable epidermis could be a significant transport barrier for systemic absorption.

Chemical Phenomena↗