Gas-phase Ionization Potentials Research Articles

Ionization Patterns and Chemical Reactivity of Cytosine-Guanine Watson-Crick Pairs.

Gas-phase and aqueous vertical ionization potentials, vIPgas and vIPaq respectively and measurements of the molecular electrostatic and local ionization maps calculated at the DFT/B3LYP-D3/ 6-311+G** level of theory and the C-PCM reaction field model for single- and double-stranded CpG and 5MeCpG pairs show that the vIPaq for single- and double-stranded pairs of C-G and 5MeC-G are practically the same, in the range of 5.79 to 5.81 eV. The aqueous adiabatic ionization potentials for single-stranded CpG and 5MeCpG are 5.52 eV and 5.51 eV respectively and they reflect the nuclear reorganization that takes place after the abstraction of the electron. The aqueous adiabatic ionization energy values that correspond to the CpG+. radical cation and the hydrated electron, e-,, being at infinite distance, adIPaq+Vo, are 3.92 eV and 3.91 eV respectively with (Vo=-1.6 eV) Analysis of data suggest that the HOMO-LUMO energy gap in the hard/soft-acid/base (HSAB) concept cannot be used a priori to determine the effect of cytosine methylation on the guanine enhanced oxidative damage in DNA.

Ab Initio Composite Approaches for Heavy Element Energetics: Ionization Potentials for the Actinide Series of Elements.

The first, second, and third gas-phase ionization potentials have been determined for the actinide series of elements using an ab initio composite scalar and fully relativistic approach, employing the coupled cluster with single, double, and perturbative triple excitations (CCSD(T)) and Dirac Hartree-Fock (DHF) methods, extrapolated to the complete basis set (CBS) limit. The impact of electron correlation and basis set choice within this framework are examined. Additionally, the first three ionization potentials were obtained using an ab initio heavy element correlation-consistent Composite Approach (here referred to as α-ccCA). This is the first utilization of a ccCA for actinide species.

Temperature-dependent oxidation of BSCAPE molecule in methanol medium

Temperature-dependent solvation free energy and oxidation by free energy of ionization of 2-Phenylethyl (2E)-3-(1-benzenesulfonyl-4,5-dihydroxyphenyl) acrylate (BSCAPE) in methanol medium are the concerns of the present work. This molecule is a relevant phenolic acid enclosing multiple bioactivities. The explicit, implicit and discrete-continuum models of solvation were used. The methanol molecules were coordinated to this acid to form cluster complexes. The dual method M06-2X/6–31++G(d,p)//B3LYP/6-31G(d) was employed along with basis set superposition error correction. The results show that, the free energy of coordination and solvation are distant. Both quantities increase with temperature. From discrete-continuum treatment, there is non-spontaneity of solvation process, while coordination yielded spontaneity and non-spontaneity at cold and hot room temperatures, respectively. The ionization potential in gas phase, decreases with temperature. All the solvation models yielded lower ionization potential than that of gas phase. Thus, it follows that, the increase of temperature and methanol medium favours the oxidation of BSCAPE. Consequently, this favours its metabolism processes.

Direct observation of photoionization dynamics in solution phase induced by femtosecond two-photon excitation

In solution phase, the solute can be photo-ionized in the lower excitation energy than its ionization potential in gas phase. Therefore, the specific interaction is expected to be exist between the surrounding media and higher excited (Sn) state of the solute. In order to elucidate such polarization effect of solvent on the photoionization process, femtosecond double-pulse excitation was applied to direct detection of low-energy photoionization dynamics of a phenylenediamine derivative in solution phase. From the results of the transient absorption change, in polar solvent, it is clearly indicated that photoionization does not proceed directly from the Sn state, but through specific intermediate state. Moreover,

Effects of Hydration on the Photoionization Threshold Energy of DNA Components and on the Activation Barriers for Guanine Methylation by Dimethyl Sulfate

The evaluation of nucleotide photoionization threshold energies provides useful information for understanding mechanisms of carcinogenesis. A comparison of aqueous photoionization energies with gas-phase ionization potentials indicates that hydration lowers these energies by 3.31 to 3.83 eV for the nucleobases, 2.92 to 3.11 eV for the nucleosides and alters the ionization events for the nucleotides. Nucleotides in gas-phase are ionized at the phosphate moiety but hydration favors base ionization, which is consistent with experimental data indicating that at 77 K in aqueous perchlorate glasses, the primary photoionization pathway involves base ionization followed by deprotonation. The gas-phase ionization potentials were calculated using the B3LYP/6-31+G∗ density functional method. Solvation energy calculations were performed employing the SM8 continuum solvation model. The photoionization energies in aqueous solution are estimated to be 4.78, 5.08, and 4.96, and 5.26 eV for 5’dGMP-, 5’dCMP-, 5’dAMP-, and 5’dTMP-, respectively. For the test molecules indole and tryptophan the aqueous photoionization energies (4.26 and 4.18 eV, respectively) are in agreement with the experimental values (4.35 and 4.45 eV). The SN2 transition states of the methylation reaction of guanine with dimethyl sulfate, the vapors of which may cause severe inflammation and necrosis of the eyes, mouth, and respiratory tract of humans and tumors in the nasal passages, lungs, and thorax of animals, were examined in the gas- and aqueous- phases at N7, N3 and O6 sites employing the same theoretical approach. Results show that the transition states in water are influenced by steric interactions, and the activation barriers are lower by 4.0 to 11.0 kcal/mol than in the gas-phase.

Predictably tuning the frontier molecular orbital energy levels of panchromatic low band gap BODIPY-based conjugated polymers

Here we report the synthesis, electrochemical properties, and optical properties of five novel π-conjugated alternating copolymers based on the BODIPY core. These polymers were synthesized via the Sonogashira polymerization reaction and contain BODIPY units alternating with comonomers such as 9,9-bis(2-ethylhexyl)-9H-fluorene (FL), 9-(2-ethylhexyl)-9H-carbazole (CBz), 2,2′-bithiophene (BT), 4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b']dithiophene (CPDT) and 4-(2-ethylhexyl)-4H-dithieno[3,2-b:2',3'-d]pyrrole (DTP). These comonomers were rationally chosen based on their gas phase ionization potential (IP) values estimated by density functional theory (DFT) calculations. Cyclic voltammetry on drop-cast thin films, as well as solutions of these polymers, revealed that the highest occupied molecular orbitals (HOMOs) of the resulting polymers correlated well with the ionization potentials (donor strength) of the comonomers. On the contrary, the lowest unoccupied molecular orbital (LUMO) energy levels of all copolymers were fairly invariant, independent of the comonomer used. This suggests that the BODIPY moiety provides the primary influence on the LUMO levels of the polymer. In addition to the experimentally determined HOMO/LUMO energy levels bearing good correlation with theoretical estimates, all polymers were found to possess broad absorption spectra covering the entire visible range, thus making them truly panchromatic. These polymers provide us with a toolset to tune the frontier molecular orbital energy levels, while retaining the low band gap and broad absorption of these polymers.

A DFT study on nucleophilicity and site selectivity of nitrogen nucleophiles

Nucleophilicty of 32 nitrogen nucleophiles belonging to the families of amine, amide, carbamate, amidine, and pyridine have been calculated at the DFT/B3LYP/6-31G (d, p) level of theory using the gas-phase ionization potential based nucleophilicity index N. The calculated gas phase nucleophilicities of the pyridine series correlate well with Mayr’s nucleophilicity values calculated on the basis of experimental quantities and the inverse of electrophilicity index. Local nucleophilicity trend and site selectivity have been analyzed using local nucleophilicity index N k and the philicity index ω k - . In most cases both the two local indices reproduced the experimentally observed trend.

Theoretical prediction of ionization/oxidation potentials in conjugated polymers

Various density functional theory (DFT) functionals and semiempirical techniques were used to predict the ionization potentials of selected conjugated polymers. Ionization potentials at infinite chain lengths were estimated using Meier fit on oligomer data. Calculated gas-phase ionization potentials with BHandH functional showed good correlation with the experimental data. The results from the semiempirical techniques do not compare as favorably as the ones obtained from DFT methods. The data fitting allowed us to estimate the size of “effective ionization length”, which spanned over 20–30 double bonds in the conjugated backbone of the polymer in question.

A further exploration of a nucleophilicity index based on the gas-phase ionization potentials

An empirical nucleophilicity index based on the gas-phase ionization potentials has been recently shown to be useful categorizing and settling the nucleophilicity power of a series of captodative ethylenes reacting in cycloaddition reactions (L.R. Domingo, E. Chamorro, P. Pérez, Journal of Organic Chemistry 73 (2008) 4615–4624). In the present work, the applicability of such model is tested within a broader series of substituted alkenes, substituted aromatic compounds and simple nucleophilic molecules. This index obtained within a Koopman’s theorem framework has been evaluated here in both gas and solution phases for several well-known nucleophiles. These results are found to be linearly correlated. Finally, the feasibility of the predictive character of this index has been discussed in comparison to the available experimental nucleophilicities of some amines in water. These results further support and validate the usefulness of such approximation in the modeling of the global nucleophilicity.

Matrix‐assisted laser desorption/ionization mechanism study with dihydroxybenzoic acid isomers as matrices

Desorption and ionization efficiencies of matrix-assisted laser desorption/ionization (MALDI) for various biomolecules with different dihydroxybenzoic acid isomers were studied. No clear relationships were observed between MALDI biomolecule signals vs. gas-phase basicity, proton affinity and ionization potential. This indicates the the gas-phase protonation mechanism is not adequate to explain the observed results.

Computational Electrochemistry: The Aqueous Ru3+|Ru2+ Reduction Potential

We present results of density functional calculations for the standard reduction potential of the Ru3+|Ru2+ couple in aqueous solution. The metal cations are modeled as [Ru(H2O)n]q+ surrounded by continuum solvent (q = 2, 3; n = 6, 18). The continuum model includes bulk electrostatic polarization as well as atomic surface tensions accounting for the deviation of the second or third hydration shell from the bulk. After consideration of 37 density functionals with 5 different basis sets, it has been found that hybrid and hybrid meta functionals provide the most accurate predictions for the [Ru(H2O)n]q+ geometries and for the corresponding reduction potential in comparison with available experimental data. The gas-phase ionization potentials of [Ru(H2O)n]2+ calculated by density functional theory are also compared to results of ab initio computations using second-order Møller−Plesset perturbation theory. The difference in solvation free energies of Ru3+ and Ru2+ varies from −10.56 to −10.99 eV for n = 6 and from −6.83 to −7.45 eV for n = 18 depending on the density functional and basis set quality. The aqueous standard reduction potential is overestimated when only the first solvation shell is treated explicitly and is underestimated when the first and second solvation shells are treated explicitly.

Computational estimates of the gas-phase basicities, proton affinities and ionization potentials of the six isomers of dihydroxybenzoic acid

The gas-phase basicities (GBs), gas-phase proton affinities (PAs) and ionization potentials (IPs) of all six isomers of dihydroxybenzoic acid have been calculated using density functional theory at the B3LYP/6-311++G(2df,p)//B3LYP/6-31+G** level. A detailed conformational analysis of each isomer was performed, and the calculated thermodynamic properties were Boltzmann averaged over all conformations. Respectively, the GBs and the gas-phase PAs vary from 803.8 and 832.5 kJ mol−1 for the least basic species (3,5-DHB) to 830.1 and 861.4 kJ mol−1 for the most basic isomer (2,4-DHB). The reported GBs and gas-phase PAs of 2,3-DHB and 2,4-DHB, are in excellent agreement with previous experimental measurements. Agreement for the 2,5-DHB and 3,4-DHB isomers are not as good, but still close to or within the experimental error estimates. The calculated values for the GB and gas-phase PA of 2,6-DHB and especially 3,5-DHB are significantly outside the experimental error brackets. Repeating these calculations on the lowest energy conformation of each isomer at the MP2/6-311++G(2df,p)//MP2/6-31+G** level yielded significantly worse results. Our results indicate that protonation in all isomers takes place on the carboxylic sites. The vertical IPs vary from 8.14 eV for 2,5-DHB to 8.56 eV for 2,4-DHB.

Non-planarity and solvent effects on structural and polarizability properties of cytosine tautomers

We report equilibrium geometries, relative stabilities, electronic and vibrational polarizabilities of cytosine tautomers and of reference compounds phenol, aniline and pyrimidine, in the gas phase and in solution, calculated by conventional correlated ab initio (CCSD, CCSD(T) and MP2) and density functional (B97-1) methods, using a variety of basis sets. Gas phase ionization potentials, electron affinities and torsional potentials for the internal rotation of the –OH and –NH2 groups, were obtained at MP2 and B97-1 levels of theory. The results show that the structures of the tautomers are non-planar in the gas phase and become planar as the polarity of the solvent increases, in contrast with aniline where the amino group remains non-planar also in the polar CH3CN solvent, what has a correspondence in the NH2 inversion barrier and NH2 wagging vibration lower in cytosine than in aniline. To constrain NH2 to be planar does not produce important changes in the relative energies, polarizabilities, ionization potential and electron affinities of the tautomers in vacuo. Polarizabilities are consistently influenced by solvent effects. The –OH and –NH2 groups show much higher π interaction with the heterocyclic ring than with the phenyl ring, what determines barriers for internal rotation of the groups much higher in cytosine than in phenol and aniline. Hardness and polarizability profiles for the –OH and –NH2 rotation do not follow, however, the maximum hardness and the minimum polarizability principles, respectively.

A theoretical study on bond dissociation energies and oxidation potentials of monolignols

Density functional theory methods were utilized to study the redox reactivities of a number of monolignols. A large number of bond dissociation energies, gas-phase ionization potentials, and aqueous oxidation potentials were obtained, which compared favorably with the available experimental data. The structure–activity relationships for these quantities were investigated. The implications of these data to the biosynthesis of lignins were also discussed.

Conformational Study of Glycine Amide Using Density Functional Theory

Seven stable stationary points, corresponding to three pairs of mirror-image conformers and one Cs symmetry conformer, have been located on the potential energy surface (PES) of neutral glycine amide at the B3LYP/6-311++G** level of theory. Accurate geometric structures, relative stabilities, and harmonic vibrational frequencies have been investigated. More importantly, the intramolecular H-bond formed from the amide to the amine plays a key role in stabilizing the global minimum, as observed in alanine amide and has been discussed qualitatively from the viewpoint of the structures, charge distributions, and vibrational analyses. As an important supplement in property for glycine amide, other property parameters, such as gas-phase basicity (GB), proton affinity (PA), and ionization potential (IP), have been predicted. The Boltzmann equilibrium distributions for the seven conformers have also been discussed qualitatively through the calculations of Gibbs free energy at various temperatures. At room temperature, the equilibrium compositions are mainly composed of conformers I and II exclusively, i.e., about 75.02% and 23.28%, respectively. As a tentative study, the conformational behaviors in aqueous solution have been explored using the Onsager model within the self-consistent reaction field (SCRF) method at the same level employed in the gas phase. Computational results indicate that the global minimum should be conformer I regardless of whether in the gas phase or in aqueous solution, which is different from the previous theoretical reports. Moreover, the consistent results in relative energy using higher-level computations, including the MP2, MP3, MP4SDQ, and CCSD(T) methods employing the Dunning's correlation consistent basis set aug-cc-PVDZ, indicate that the B3LYP/6-311++G** level of theory may be applied to the analogous systems.

Method for Predicting Photocatalytic Oxidation Rates of Organic Compounds

In designing a photocatalytic oxidation (PCO) system for a given air pollution source, destruction rates for volatile organic compounds (VOCs) are required. The objective of this research was to develop a systematic method of predicting PCO rate constants by correlating rate constants with physical-chemical characteristics of compounds. Accordingly, reaction rate constants were determined for destruction of volatile organics over a titanium dioxide (TiO2 ) catalyst in a continuous mixed-batch reactor. It was found that PCO rate constants for alkanes and alkenes vary linearly with gas-phase ionization potential (IP) and with gas-phase hydroxyl radical reaction rate constant. The correlations allow rates of destruction of compounds not tested in this research to be predicted based on physical-chemical characteristics.

Excited State Dynamics of Methyl Viologen. Ultrafast Photoreduction in Methanol and Fluorescence in Acetonitrile

The photophysical and photochemical deactivation pathways of electronically excited methyl viologen (1,1‘-dimethyl-4,4‘-bipyridinium, MV2+) were studied in several polar solvents at room temperature using a variety of ultrafast time-resolved and steady-state spectroscopic techniques. The results highlight the very strong electron accepting character of the lowest singlet excited state of MV2+. Transient absorption was measured between 270 and 740 nm as a function of delay time after excitation of the strong π−π* transition of MV2+ by a 150 fs, 265 nm pump pulse. In methanol, the radical cation of methyl viologen (MV•+) appeared within our time resolution, indicating that forward electron transfer from a nearby donor quenches electronically excited MV2+ in < 180 fs. Identical dynamics within experimental uncertainty were observed for the chloride salt of MV2+ and for the salt prepared with tetrafluoroborate counterions. This latter “superhalide” ion has a condensed-phase detachment threshold that is too high to permit oxidation by the excited state of MV2+. Thus, electron transfer does not take place within an associated MV2+-counterion complex in methanol but results instead from oxidation of a solvent molecule. Photoreduction of MV2+ in methanol is a novel example of ultrafast electron-transfer quenching of a photoexcited acceptor in an electron-donor solvent. This is the first demonstration that a hydrogen-bonding solvent can serve as the electron donor in an ultrafast intermolecular ET reaction. Decay of the initial MV•+ population and simultaneous recovery of ground-state MV2+ with a characteristic time constant of 430 ± 40 fs were observed immediately after the pump pulse and assigned to back electron transfer in the geminate radical pair. Despite the high rate of back electron transfer, a significant fraction of the initial radical pairs avoid recombination, and a finite yield (∼12%) of MV•+ ions is observed at delay times > 2 ps. There was no evidence of photoreduction when the solvent was acetonitrile or water. Both of these solvents have high gas-phase ionization potentials that prevent oxidation by excited MV2+. The transient absorption signals indicate, however, that very different excited-state decay channels exist in these two solvents. In aqueous solution, an unknown nonradiative decay process causes decay of excited MV2+ with a time constant of 3.1 ps in H2O and 5.3 ps in D2O. In acetonitrile, on the other hand, the transient absorption decays hundreds of times slower and fluorescence is observed. This is the first report of an efficient radiative decay pathway for MV2+ in fluid solution. The excited-state absorption spectrum (S1→SN spectrum) of MV2+ was measured in acetonitrile and the fluorescence was characterized by time-correlated single-photon counting and steady-state measurements. The fluorescence quantum yield is 0.03 ± 0.01 and the lifetime in acetonitrile at room temperature is 1.00 ± 0.04 ns. The fluorescence is efficiently quenched by electron transfer from added quenchers with gas-phase ionization potentials lower than about 10.8 eV. Using the measured emission spectrum, the excited-state reduction potential is determined to be E° (MV2+*/MV•+) = 3.65 V, confirming the highly oxidizing character of this photoexcited dication.

Positive Hole Transfer Upon Irradiation of Frozen Alkylbenzene Solutions in Freonic Matrices at 77 K

The process of positive hole transfer between alkylbenzene molecules of various structures occurring upon irradiation in electron-scavenging (freonic) matrices at 77 K was studied by ESR. The factors controlling the efficiency and direction of this process were analyzed. In the case of the ethylbenzene–toluene pair, it was shown that the direction of hole transfer depends on the conformation of the ethylbenzene radical cation. If the conformation effects are absent, the hole transfer occurs in accordance with the gas-phase ionization potentials (IP) and appears to be efficient, provided that the difference in the IP values is greater than 0.3 eV.

Monodisperse poly(triacetylene) oligomers extending from monomer to hexadecamer: joint experimental and theoretical investigation of physical properties

A series of monodisperse Et3-Si-end-capped poly(triacetylene) (PTA) oligomers ranging from monomer to hexadecamer was prepared by a fast and efficient statistical deprotection-oxidative Hay oligomerization protocol. The PTA oligomers exhibit an increasingly deep-yellow color with lengthening of the pi-conjugated backbone, feature excellent solubility in aprotic solvents, and exhibit melting points up to > 22 degrees C for the hexadecameric rod. This new dramatically extended oligo(enediyne) series now enables to investigate the evolution of the physico-chemical effects in PTAs beyond the linear 1/n versus property regime into the higher oligomer region where saturation becomes apparent. We report the results of joint experimental and theoretical studies, including analysis of the 13C NMR spectra, evaluation of the linear (UV/ Vis) and nonlinear [third-harmonic generation (THG) and degenerate four-wave mixing (DFWM)] optical properties, and characterization of the redox properties with cyclic and steady-state voltammetry. Up to the hexadecameric rod, an increasingly facile one-electron reduction step is observed, showing at the stage of the dodecamer, a leveling off tendency from the linear correlation between the inverse number of monomer units and the first reduction potential. The effective conjugation length (ECL) determined by means of UV/Vis spectroscopy revealed a pi-electron-delocalization length of about n = 10 monomeric units, which corroborates well with the oligomeric length for which in the 13C NMR spectrum C(sp2) and C(sp) resonances start to overlap. Third-harmonic generation (THG) and degenerate four-wave mixing (DFWM) measurements revealed for the second-order hyperpolarizability gamma a power law increase gammma-alpha-n(a) for oligomers up to the octamer with exponential factors a= 2.46+/-0.10 and a=2.64+/-0.20, respectively, followed by a smooth saturation around n = 10 repeating units. The power law coefficient a calculated with the help of the valence effective Hamiltonian (VEH) method combined to a sum-over-states (SOS) formalism corroborates well with the values found by both THG and DFWM experiments. Up to the PTA heptamer, INDO (intermediate neglect of differential overlap)-calculated gas-phase ionization potentials and electron affinities obey a linear relationship as a function of the inverse number of monomer units displaying a strong electron-hole symmetry. The onset of saturation for the electron affinity is calculated to occur around the octamer, in accordance with experimentally obtained results from electrochemical measurements.

Analysis of Charge-Transfer Complex Absorption Spectra and the Applicability of the Mulliken two-State Model

Complete absorption spectra, free from interference due to an unbound acceptor and donor, of several charge-transfer complexes have been examined. These spectra serve as a significant test to the applicability of the Mulliken two-state model for this series of complexes. the appearance of multiple new absorption bands and the trend in the observed oscillator strength of the CT transitions and the localized excitation within the complex spectrum require significant expansions of Mulliken's simple two-state model. Multiple new CT absorption bands are frequently encountered and their appearance has been explained in terms of multiple ion-pair states participating in CT interactions. the relative positions of these bands are predicted based on the gas-phase ionization potentials of the donors and the electron affinity of the acceptor in excellent agreement with the observed spectra achieved. the integrated absorption intensities of the localized bands within each CT complex spectrum have been measured and used as an indicator of the extent on intensity borrowing by the CT transition. These measurements show that for the better donors used in this study, i.e. hexamethylbenzene and pentamethylbenzene, intensity borrowing is not significant. However, for the poorer donors used, an increasing contribution of the LE within the CT absorption bands is observed. a multi-state model, which includes the localized excited state of the acceptors well as additional ion-pair states, is the minimum required to describe these complexes.