Showing papers in "Journal of Physical Chemistry A in 2002"

Theoretical Studies of the Ground and Excited Electronic States in Cyclometalated Phenylpyridine Ir(III) Complexes Using Density Functional Theory

Abstract: The ground state and low-lying excited electronic states in the Ir(III) complex Ir(ppy)3, and in the related complexes Ir(ppy)2(acac) and Ir(ppy)2(bza), are studied using density functional theory techniques [where ppy = 2-phenylpyridine, acac = acetoylacetonate, and bza = benzyolacetonate]. Ir complexes of ppy have been the subject of numerous photophysical absorption and luminescence experiments and have been examined as potential donors in organic light emitting diodes (OLEDs). The electronic properties of the neutral molecules, in addition to the positive and negative ions, are studied using the B3LYP functional. Optimized geometries are compared to experimentally observed structures. Excited triplet and singlet states are examined using time-dependent density functional theory (TDDFT). The calculated energies of the lowest triplet state (2.4−2.6 eV) and lowest singlet state (2.6−2.7 eV) in the three complexes are in good agreement with experimental absorption spectra and luminescence studies. All of ...

Polarizable Continuum Model (PCM) Calculations of Solvent Effects on Optical Rotations of Chiral Molecules

Abstract: A new theory of solvent effects on the optical rotations of chiral molecules is presented. The frequency-dependent electric dipole−magnetic dipole polarizability, βαβ(ν), is calculated using density functional theory (DFT). Solvent effects are included using the polarizable continuum model (PCM). DFT/PCM calculations of sodium D line specific rotations, [α]D, have been carried out for seven conformationally rigid chiral organic molecules (fenchone, camphor, α-pinene, β-pinene, camphorquinone, verbenone, and methyloxirane) for a diverse set of seven solvents (cyclohexane, carbon tetrachloride, benzene, chloroform, acetone, methanol, and acetonitrile). The predicted variation in [α]D for the solvents cyclohexane, acetone, methanol, and acetonitrile are in excellent agreement with experiment for all seven molecules. For the solvents carbon tetrachloride, benzene, and chloroform, agreement is much poorer. Since only electrostatic solute−solvent interactions are included in the PCM, our results lead to the con...

A quantum mechanical investigation of the relation between impact sensitivity and the charge distribution in energetic molecules

Abstract: Quantum mechanically determined electrostatic potentials for isosurfaces of electron density of a variety of CHNO explosive molecules are analyzed to identify features that are indicative of sensit...

On the Enthalpy of Formation of Hydroxyl Radical and Gas-Phase Bond Dissociation Energies of Water and Hydroxyl

Abstract: In a recent letter (J. Phys. Chem. A, 2001, 105,1), we argued that, although all major thermochemical tables recommend a value of (OH) based on a spectroscopic approach, the correct value is 0.5 kcal/mol lower as determined from an ion cycle. In this paper, we expand upon and augment both the experimental and theoretical arguments presented in the letter. In particular, three separate experiments (mass-selected photoionization measurements, pulsed-field-ionization photoelectron spectroscopy measurements, and photoelectron-photoion coincidence measurements) utilizing the positive ion cycle to derive the O−H bond energy are shown to converge to a consensus value of the appearance energy AE0(OH+/H2O) = 146117 ± 24 cm-1 (18.1162 ± 0.0030 eV). With the most accurate currently available zero kinetic energy photoionization value for the ionization energy IE(OH) = 104989 ± 2 cm-1, corroborated by a number of photoelectron measurements, this leads to D0(H−OH) = 41128 ± 24 cm-1 = 117.59 ± 0.07 kcal/mol. This corres...

Photoreactions on LaTiO2N under Visible Light Irradiation

Abstract: Photochemical reactions on LaTiO2N, a perovskite-type oxynitride, were examined. Under visible-light irradiation (420 nm < λ < 600 nm), LaTiO2N reduced H+ into H2 and oxidized H2O into O2 in the presence of a sacrificial electron donor (methanol) or acceptor (Ag+) by the band gap transition (2.1 eV). Oxidation of water proceeded with little degradation of the oxynitride, whereas partial substitution of Ca2+ for La3+ of LaTiO2N and modification by IrO2 colloid markedly suppressed degradation of the oxynitride and increased O2 evolution efficiency.

State-Selected and State-to-State Ion−Molecule Reaction Dynamics

Abstract: A unique triple-quadrupole double-octopole (TQDO) photoionization mass spectrometer has been developed for total cross section measurements of state-selected ion−molecule reactions. By employing this TQDO apparatus, we have recently examined the absolute total cross sections for a series of state-selected ion−molecule reactions involving Ar+(2P3/2,1/2), O+(4S, 2D, 2P), and organosulfur ions (CH3SH+, CH3CH2SH+, and CH3SCH3+) in their ground states. The cross section measurements, together with product ion kinetic energy analyses, have provided convincing evidence that the Ar+(2P3/2,1/2) + CO2 (CO, N2, O2) reactions proceed via a charge-transfer predissociation mechanism. The comparison of absolute cross sections for product ions formed in the dissociative charge transfer of Ar+(2P3/2,1/2) + CO2 (CO, N2, O2) and those produced in photoionization of CO2 (CO, N2, O2) suggests that product ions formed by dissociative charge transfer are also produced by photoionization via a similar set of excited predissociat...

Blue-shifting hydrogen bonds.

Abstract: In this paper we put forward the idea that the various improper, blue-shifting hydrogen bond systems discussed in the literature are all of essentially the same nature and occur because of three ne ...

Computing Redox Potentials in Solution: Density Functional Theory as A Tool for Rational Design of Redox Agents

Abstract: High-level density functional theory in combination with a continuum solvation model was employed to compute standard redox potentials in solution phase for three different classes of electrochemically active molecules: small organic molecules, metallocenes, and M(bpy)3x (M = Fe, Ru, Os; x = +3, +2, +1, 0, −1). Excellent agreement with experimentally determined redox potentials is found with an average deviation of approximately 150 mV when four different solvents commonly in use for electrochemical measurements were included. To obtain quantitative agreement between theory and experiment, the use of a large basis set is crucial especially when the redox couple includes anionic species. Whereas the addition of diffuse functions improved the results notably, vibrational zero-point-energy corrections and addition of entropy effects are less important. The computational protocol for computing redox potentials in solution, which has been benchmarked, is a powerful and novel tool that will allow a molecular-l...

Quantitative characterization of the local electrophilicity of organic molecules. Understanding the regioselectivity on Diels-Alder reactions

Abstract: Regional electrophilicity at the active sites of the reagents involved in polar Diels−Alder processes may be described on a quantitative basis using an extension of the global electrophilicity inde...

Hydrogen Bond Dynamics in Water and Ultrafast Infrared Spectroscopy

Abstract: Molecular dynamics simulations are used to examine two key aspects of recent ultrafast infrared experiments on liquid water dynamics. It is found that the relation between the OH stretch frequency and the length of the hydrogen bond in which the OH is involved, currently assumed to be one-to-one, is instead characterized by considerable dispersion and that the time scale currently interpreted in terms of a stochastic modulation by the surrounding solvent of a highly frictionally damped hydrogen bond system is shown to be governed by hydrogen bond-breaking and -making dynamics, whereas the motion of an intact hydrogen-bonded complex is underdamped in character.

Red- versus Blue-Shifting Hydrogen Bonds: Are There Fundamental Distinctions?

Abstract: The OH covalent bond of conventional hydrogen bonds of the OH···O type undergoes a lengthening as a result of the complex formation, and the OH stretching frequency shifts to the red; the same is t...

Theoretical Calculation of pKa Using the Cluster−Continuum Model

Abstract: The pKa's of 17 species from −10 to 50 were calculated using the ab initio MP2/6-311+G(2df,2p) level of theory and inclusion of solvent effects by the cluster−continuum model, a hybrid approach that combines gas-phase clustering by explicit solvent molecules and solvation of the cluster by the dielectric continuum. In addition, the pure continuum methods SM5.42R and PCM were also used for comparison purposes. Species such as alcohols, carboxylic acids, phenol, acetaldehyde and its hydrate, thiols, hydrochloric acid, amines, and ethane were included. Our results show that the cluster−continuum model yields much better agreement with experiment than do the above-mentioned pure continuum methods, with a rms error of 2.2 pKa units as opposed to 7 pKa units for the SM5.42R and PCM methods. The good performance of the cluster−continuum model can be attributed to the introduction of strong and specific solute−solvent interactions with the molecules in the first solvation shell of ions. This feature decreases the...

Reactions of Gaseous Mercury with Atomic and Molecular Halogens: Kinetics, Product Studies, and Atmospheric Implications

Abstract: Rapid depletions of atmospheric mercury (Hg0) and ozone coinciding with high halogen concentrations have recently been observed in several high-Arctic and sub-Arctic regions. The lack of kinetic data on the halogen-initiated reactions of elemental mercury precluded the drawing of any conclusions on the chemistry of these mercury depletion events. We carried out extensive kinetic and product studies on the reactions of gaseous Hg0 with molecular and atomic halogens (X/X2 where X = Cl, Br) at atmospheric pressure (750 ± 1 Torr) and room temperature (298 ± 1 K) in air and N2. Kinetics of the reactions with X/X2 was studied using relative and absolute techniques by cold vapor atomic absorption spectroscopy (CVAAS) and gas chromatography with mass spectroscopic detection (GC-MS). The measured rate constants for reactions of Hg0 with Cl2, Cl, Br2, and Br were (2.6 ± 0.2) × 10-18, (1.0 ± 0.2) × 10-11, <(0.9 ± 0.2) × 10-16, and (3.2 ± 0.3) × 10-12 cm3 molecule-1 s-1, respectively. The reaction products were analy...

HOMO−LUMO Gap as an Index of Molecular Size and Structure for Polycyclic Aromatic Hydrocarbons (PAHs) and Asphaltenes: A Theoretical Study. I

Abstract: Theoretical calculations are presented for the effect on the HOMO−LUMO gap due to the successive addition of aromatic rings and their different distributions, isomers, for polycyclic aromatic hydrocarbons (PAHs). The study is based on ZINDO/S calculations. PAHs with 1−14 fused aromatic rings (FAR) are considered. The results of these calculations are addressed to a currently existing controversy regarding the number of FAR in asphaltene structures. Asphaltenes are considered as polycyclic aromatic compounds similar to PAHs but containing heteroatoms and alkyl side chains. The theoretical results are compared with fluorescence emission (FE) experimental data. It is found that the asphaltene experimental FE range does not necessarily correspond to different chromophores with different number of FAR but may be different isomers with the same number of FAR. Also, the effect of the presence of alkyl chains and heteroatoms in the asphaltene structures on the HOMO−LUMO gap is almost negligible. We conclude that ...

Practical Approaches To Construct RS-HDMR Component Functions

Abstract: A general set of quantitative model assessment and analysis tools, termed high-dimensional model representations (HDMR), has been introduced recently for improving the efficiency of deducing high-dimensional input−output system behavior. HDMR is a particular family of representations where each term in the representation reflects the independent and cooperative contributions of the inputs upon the output. When data are randomly sampled, a RS (random sampling)-HDMR can be constructed. To reduce the sampling effort, different analytical basis functions, such as orthonormal polynomials, cubic B splines, and polynomials may be employed to approximate the RS-HDMR component functions. Only one set of random input−output samples is necessary to determine all the RS-HDMR component functions, and a few hundred samples may give a satisfactory approximation, regardless of the dimension of the input variable space. It is shown in an example that judicious use of orthonormal polynomials can provide a sampling saving o...

A new approach to vibrational analysis of large molecules by density functional theory: wavenumber-linear scaling method

Abstract: Normal vibration calculations were performed on 164 basic organic and inorganic compounds by the density functional method of B3LYP/6-311+G** using the wavenumber-linear scaling (WLS) method, in which essentially only one parameter is involved for the whole wavenumber region The average deviation of the WLS-scaled calculated wavenumbers from the observed wavenumbers for a total of 1223 normal modes was 34% The present results and the previous results on 205 compounds have shown the high performance of the WLS method in vibrational analysis of a variety of compounds The effect of anharmonicity on the observed wavenumbers was examined on the basis of the calculated wavenumbers for 224 diatomic molecules and ions in comparison with their experimental harmonic wavenumbers The results indicate that the larger positive deviations of the unscaled calculated wavenumbers from the observed values at higher wavenumbers are attributed almost exclusively to the neglect of anharmonicity in the calculated wavenumbe

Solvation Dynamics of Coumarin-153 in a Room-Temperature Ionic Liquid

Abstract: Room-temperature ionic liquids are rapidly emerging as a new class of medium that is ideally suited for carrying out chemical reactions and various other applications. In this paper, steady-state and time-resolved fluorescence behavior of coumarin-153 (C153) have been reported in a room-temperature ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate or [BMIM][BF4]. The polarity of the solvent, as estimated from the fluorescence maximum of C153, is found to be 48.9 on the ET(30) polarity scale. The fluorescence decay behavior of C153 in [BMIM][BF4] is found to be dependent on the monitoring wavelength. Although the time dependence of fluorescence in the blue region of the spectrum is represented by a multicomponent decay, that in the red region of the spectrum consists of clear growth followed by decay. The time-dependent evolution of the fluorescence spectrum of C153, constructed from the decay profiles at several wavelengths, indicates biphasic solvation dynamics similar to that observed in the c...

Ultrafast Dynamics of Porphyrins in the Condensed Phase: II. Zinc Tetraphenylporphyrin†

Abstract: Femtosecond spectroscopic studies of zinc tetraphenylporphyrin (ZnTPP) in benzene and dichloromethane are reported, combining both fluorescence up-conversion and transient absorption measurements. The purpose is to investigate the initial electronic and vibrational relaxation of the S_1 and S_2 excited states, in a system in which interference from solvent rearrangement is insignificant as evidenced by the small Stokes shift in the fluorescence. Excitation of the low-lying singlet excited state (S_1) results in nanosecond relaxation, while excitation to S_2, the Soret band, leads to multiple electronic and vibrational relaxation time scales of S_2 and S_1 populations, from hundreds of femtoseconds to tens of picoseconds. The systematic and detailed studies reported here reveal that the Soret fluorescence band decays with a lifetime in benzene of 1.45 ps for excitation at 397 nm, while emission monitored at the same wavelength, but for two-photon 550 nm excitation, decays biexponentially with 200 fs and 1.0 ps time constants. In addition, the Soret fluorescence decay lifetime for 397 nm excitation is distinctly longer than the rise time of S_1 fluorescence for the same excitation, which varies with wavelength. These observations are consistent with the model presented here in which the Soret band structure consists of absorption from S_0 to two manifolds of states with distinct electronic and vibrational couplings to S_1 and higher electronic states. To compare with literature, we also measured the S_2 lifetime in dichloromethane and found it to be 1.9 ps, a lengthening from its value in benzene. However, the transient fluorescence intensity is greatly reduced. These observations in dichloromethane provide evidence of an ultrafast (<100 fs) channel for electron transfer from ZnTPP to dichloromethane for a subset of excited molecules in favorably oriented contact with the solvent, that is, a bifurcation of population. Finally, solvent-induced vibrational relaxation of the S_1 population following internal conversion from S_2 occurs over a range of time scales (picoseconds to tens of picoseconds) depending on the wavelength (fluorescence or transient absorption), and the observed rate indeed changes with solvent.

Accurate Prediction of Acidity Constants in Aqueous Solution via Density Functional Theory and Self-Consistent Reaction Field Methods

Abstract: We have developed a protocol for computing the acidity constant (pKa) of organic compounds via ab initio quantum chemistry and continuum solvation methods. Density functional (DFT) calculations employing large basis sets are used to determine the gas-phase deprotonation energies. Solvation effects are treated via a self-consistent reaction field (SCRF) formalism involving accurate numerical solution of the Poisson−Boltzmann equation. Dielectric radii are parametrized for each functional group of interest to optimize solvation free energy calculations for neutral and charged species. While the intrinsic accuracy of these approaches is quite impressive (errors on the order of a few kcal/mol), it is not quite good enough to achieve the target accuracy that we have set for pKa prediction of 0.5 pKa units. Consequently, two further empirical parameters, scaling and additive factors, are determined for every functional group of interest by linear fitting directly to pKa data for a training set. With this additi...

Spectroscopic, Electrochemical, and Photochemical Studies of Self-Assembled via Axial Coordination Zinc Porphyrin−Fulleropyrrolidine Dyads†

Abstract: Spectroscopic, redox, and photochemical behavior of self-assembled donor−acceptor dyads formed by axial coordination of zinc tetraphenylporphyrin, (TPP)Zn, and fulleropyrrolidine bearing either pyridine or imidazole coordinating ligands were investigated. The UV−vis, 1H NMR, and ESI-mass spectral studies, as well as computational studies, revealed supramolecular 1:1 dyad formation between the electron donor [(TPP)Zn] and the electron acceptor, fulleropyrrolidine entities. The determined formation constant K values followed the order o-pyridyl ≪ m-pyridyl ≃ p-pyridyl ≪ N-phenyl imidazole entities of the fulleropyrrolidine. The evaluated thermodynamic parameters revealed stable complexation with complex dissociation enthalpies ranging between 26 and 32 kJ mol-1. The 1H NMR studies revealed axial coordination of the pyridine or imidazole ligands to the central zinc of (TPP)Zn, while the ESI-Mass spectral studies performed in CH2Cl2 matrix revealed the expected molecular ion peak of the self-assembled dyads. ...

Conformational Change of Poly(N-isopropylacrylamide) during the Coil−Globule Transition Investigated by Attenuated Total Reflection/Infrared Spectroscopy and Density Functional Theory Calculation†

Abstract: A conformational change in the coil−globule transition of poly(N-isopropylacrylamide) (PNiPA) was investigated by Fourier transform infrared (FT-IR) spectroscopy with attenuated total reflection (ATR) method and density functional theory (DFT) calculations. ATR/IR spectra of PNiPA in an aqueous solution change dramatically in the vicinity of the coil−globule transition temperature (θ temperature). Below the θ temperature, unimodal peaks are observed at 1624 cm-1 in the amide I region and at 1562 cm-1 in the amide II region, respectively. Above the θ temperature, a new peak appears abruptly near 1653 cm-1 in the amide I region and the amide II band shifts gradually to a lower frequency by 6 cm-1. In the amide III region, the relative intensity of a band at 1173 cm-1 is weaker than that of a band at 1155 cm-1 at lower temperatures, but it becomes larger during the coil−globule transition of PNiPA. DFT calculation for dimer models of PNiPA suggests that the amide I band at 1624 cm-1 is assigned mainly to a s...

Blueing, bleaching, and blinking of single CdSe/ZnS quantum dots

Abstract: Room temperature time-resolved luminescence measurements on single CdSe/ZnS quantum dots (QDs) are presented.Luminescence spectra were recorded over time periods of up to 30 min with a time resolution down to 6 ms.A clear 30 ± 40 nm blue shift in the emission wavelength is observed for QDs in ambient air, before the luminescence stops after about 2 ± 3 min due to photobleaching. The blue shift is absent in a nitrogen atmosphere and photobleaching occurs after much longer times, 10 ± 15 min.These observations are explained by photoinduced oxidation.During illumination in the presence of oxygen the surface of CdSe is oxidized, which effectively results in shrinkage of the CdSe core diameter by almost 1 nm and, consequently, in a blue shift.This also influences the blinking behavior.The faster fading of the luminescence in air suggests that photoinduced oxidation results in the formation of nonradiative recombination centers at the CdSe/ CdSeOx interface.In a nitrogen atmosphere photoinduced oxidation is prevented by the absence of oxygen.A surprising observation is a higher initial light output for CdSe/ZnS QDs in air.We explain this higher light output by a fast reduction of the lifetime of the long-lived defect states of CdSe/ZnS QDs by oxygen.

One- and Two-Photon Spectroscopy of Donor−Acceptor−Donor Distyrylbenzene Derivatives: Effect of Cyano Substitution and Distortion from Planarity

Abstract: The one- and two-photon spectroscopic properties of four symmetrically substituted donor−acceptor−donor distyrylbenzenes with either di-n-butyl- or diphenylamino donor groups and cyano acceptor groups are reported. It has been found that the position of the substitution of the electron-withdrawing cyano groups on the central phenylene ring as compared to the vinylene bond strongly affects the observed properties. In particular, the molecules with cyano substitution on the α-carbon of the vinylene linkage are characterized by weak fluorescence, short fluorescence lifetimes, and two-photon cross sections (δ) that are comparable to analogous molecules with no acceptor groups. In contrast, the molecules with acceptor substitution on the central phenylene ring are strongly fluorescent and have δ values roughly twice those of the vinyl-substituted molecules. These results are discussed in terms of the larger deviation of the conjugated backbone from planarity and the smaller distance between the donors and acce...

Reactive Uptake of Ozone by Aerosol-Associated Unsaturated Fatty Acids: Kinetics, Mechanism, and Products

Abstract: The heterogeneous reaction between ozone and oleic and linoleic acids, prevalent components of both marine and urban organic aerosol, were studied in a flow reactor using electron impact and chemical ionization mass spectrometry. Liquids and frozen liquids were used as proxies for atmospheric aerosol. The reactive uptake coefficients, γ, were determined to be (8.3 ± 0.2) × 10-4 and (1.2 ± 0.2) × 10-3 for liquid oleic and linoleic acid respectively and (5.2 ± 0.1) × 10-5 and (1.4 ± 0.1) × 10-4 for frozen oleic and linoleic acid, respectively. Although, the reacto-diffusive length is estimated to be rather small in the liquid experiments, <10 nm, a clear indication of the participation of subsurface layers in the uptake is observed. This is in contrast to uptake by the frozen acids where the reaction is limited to the surface. Aldehydes were identified as the major volatile reaction products: 1-nonanal was detected following reaction with oleic acid, 2-nonenal, 4-nonenal, and 1-hexanal were detected follow...

DFT Calculations of the Electron Affinities of Nucleic Acid Bases: Dealing with Negative Electron Affinities

Abstract: To better understand the cause of the diversity in reported values of the electron affinities (EAs) for DNA bases, we performed a series of DFT (B3LYP functional) calculations at different basis set sizes. Through investigation of (1) trends in the values of EAs, (2) the excess electron spin distribution of the anion radical dependence on basis set size, (3) effect of the excess electron on ZPEs, we are able to identify the features of a basis set that allows for dipole-bound and continuum states to compete with molecular states for the electron. Smaller basis sets that confine the excess electron to the molecule allow for reasonable estimates of relative valence electron affinities excluding dipole-bound states and suggest the order of adiabatic valence electron affinities to be U ≈ T > C ≈ I (hypoxanthine) > A > G with G nearly 1 eV less electron affinic than U. Combining the best estimates from theory and experiment we place the adiabatic valence electron affinities of the pyrimidines as zero to +0.2 e...

Ultrafast Dynamics of Porphyrins in the Condensed Phase: I. Free Base Tetraphenylporphyrin †

Abstract: With femtosecond resolution, using fluorescence up-conversion and transient absorption, we have carried out measurements on free base tetraphenylporphyrin (H_2TPP) in benzene solution, pumping with ∼1300 cm^(-1) of excess vibrational energy in each of the Soret, Q_y, and Q_x bands, and also pumping the lowest vibrational band of Q_y. From these studies, made for different excitations and at different detection wavelengths, we provide a model for describing the elementary intramolecular processes in the Soret, Q_y, and Q_x electronic manifolds, with the following order of time scales and couplings: electronic (femtosecond), vibrational (femtosecond−picosecond), and singlet−triplet (nanosecond). These dynamical electronic and vibrational relaxation pathways in a molecule with small dipole in nonpolar solvents can be studied without interference from solvent reorganization, as indicated by the small Stokes shift of fluorescence. Vibrationally excited Soret → {Q_y,Q_x} and Q_y → Q_x electronic relaxation occurs in less than 100 fs, within our resolution, as evidenced by the immediate rise of Q_x fluorescence after Soret (397 nm) and Q_y (514 and 550 nm) excitation. There are generally three distinguishable ultrafast relaxation time scales within the Q_x state, which are assigned to intra- and intermolecular vibrational relaxation processes leading to thermal equilibrium in Q_x, the lowest excited singlet state. The measured time scales are as follows: 100−200 fs for intramolecular vibrational energy redistribution, 1.4 ps for vibrational redistribution caused by elastic collision with solvent molecules, and 10−20 ps for thermal equilibration by energy exchange with the solvent. Decay of the equilibrated Q_x population occurs on the nanosecond time scale by intersystem crossing to the triplet state.

Innershell absorption spectroscopy of amino acids

Abstract: We present comprehensive measurements of the C (carbon) K edge near-edge X-ray absorption (NEXAFS) spectra of all 20 amino acids commonly occurring in nature. Qualitative trends among the spectra of amino acids with similar chemical character are identified and spectral features are compared with extensive ab initio calculations. The contributions of individual units and substitutional groups have been determined to explore their fingerprinting character using the building block concept. Several such units are found. Two that give particularly clear features in the C Is NEXAFS spectra are the carboxyl group (which can be clearly identified by a pronounced structure due to the C 1s-->pi*c-o transition with maximum at 288.65(5) eV) and modified phenol rings in aromatic amino acids (which give sharp C 1s-->pi*c=o structures). The latter transitions are located around 285 cV, and their shape is specific for each aromatic amino acid. Other building blocks, such as the CNHn group and the CH, CC, CO, CN pair bonds, are also identified, although their characteristic features are less pronounced in the C K edge spectra than the carboxylic and aromatic structures. This study provides the basis for rigorous assignment of the NEXAFS spectra of the amino acids, and will be helpful in developing X-ray absorption spectroscopy for quantitative analysis of proteins.

Droplet Evaporation and Discharge Dynamics in Electrospray Ionization

Abstract: We present measurements of the distributions of droplet size and charge along with, for selected droplets, the variation of droplet size and charge with time for electrosprays of methanol, acetonitrile, and water, as well as for methanol at different polarities and electrolyte concentrations. These measurements are performed using a new technique for measuring droplet size and charge that uses phase Doppler interferometry for obtaining droplet size and inferring droplet charge from comparison of measured and calculated droplet mobility in a constant electric field. For selected droplets, multiple measurements of the size and charge are performed by repeated reversal of the drift field. This “ping-pong” experiment tracks droplet size and charge for loss of up to 99.9% of the initial droplet volume. We observe that droplet instability, referred to as a discharge event, mainly occurs near or above the Rayleigh limit of charge, resulting in a charge loss of 15−20% for methanol and acetonitrile and 20−40% in t...

Excited State Intramolecular Proton Transfer and Metal Ion Complexation of 2-(2‘-Hydroxyphenyl)benzazoles in Aqueous Solution

Abstract: The excited-state intramolecular proton transfer (ESIPT) of a series of water-soluble 2-(2‘-hydroxyphenyl)benzazole derivatives has been studied under physiological conditions using absorbance and steady-state emission spectroscopy. At neutral pH in the presence of 0.1 M ionic background, the fluorescence properties of these derivatives differ substantially compared to previously reported data in nonaqueous solvents. The ESIPT process is disrupted, presumably due to intermolecular hydrogen bonding with surrounding water molecules combined with increased stabilization of the trans-rotamer, which cannot undergo the ESIPT process. The emission spectrum of the benzimidazole derivative depends significantly on the solvent polarity, as revealed by titrations with Zn(II) in methanol, ethanol, and under physiological conditions. Inhibition of ESIPT via metal coordination shows a significant wavelength shift together with a substantial ratio increase by a factor of 13.7. Titration of the benzoxazole derivative wit...

A Photon Echo Peak Shift Study of Liquid Water

Abstract: The first photon echo peak shift study of liquid water is presented. Spectral diffusion within the OH stretching absorption band of HDO in D2O takes place on many time scales with a slow component on the order of 5−15 ps. This indicates that fluctuations of local structure of the hydrogen bond network in liquid water are relatively long-lived. Vibrational relaxation of the excited state populates a state which is spectroscopically different from the initial ground state. This leads to an strong enhancement of the peak shift and allows spectral diffusion to be measured for delay times beyond the limit determined by the population relaxation time T1. The observed signals are discussed with the help of model calculations.