Adiabatic ionization of water clusters: Nonempirical dynamic model

To determine the probability and mechanism of water cluster ionization initiated by the absorption of energy equal to the adiabatic ionization potential, the evolution of the vibrationally excited ring-like water tetramer was studied. The simulations were carried out in terms of the classical dynamics approach in the Born-Oppenheimer approximation. The adiabatic potential of the system and the forces acting on the nuclei were calculated in the second order of the Möller-Plesset perturbation theory with the use of the extended double-zeta 6-31++G** basis set. The initial states of the cluster system differed in the energy distribution over the intra-and intermolecular vibrational degrees of freedom. The initial conditions that promote the formation of an H₃O ... H₂O ... OH sequence of fragments, when the vertical electron detachment requires the energy equal to the adiabatic ionization potential of the system, are found.     

Theoretical study of ionization and one‐electron oxidation potentials of N‐heterocyclic compounds

A number of density functionals was utilized to predict gas‐phase adiabatic ionization potentials (IPs) for nitrogen‐rich heterocyclic compounds. Various solvation models were applied to the calculation of difference in free energies of solvation of oxidized and reduced forms of heterocyclic compounds in acetonitrile (AN) for correct reproduction of their standard oxidation potentials. We developed generally applicable protocols that could successfully predict the gas‐phase adiabatic ionization potentials of nitrogen‐rich heterocyclic compounds and their standard oxidation potentials in AN. This approach is supported by a MPW1K/6‐31+G(d) level of theory which uses SMD(UA0) approximation for estimation of solvation energy of neutral molecules and PCM(UA0) model for ionized ones. The mean absolute derivation (MAD) and root mean square error (RMSE) of the current theoretical models for IP are equal to 0.22 V and 0.26, respectively, and for oxidation potentials MAD = 0.13 V and RMSE = 0.17. © 2013 Wiley Periodicals, Inc.     

氟代乙烯阳离子的理论研究

用B3LYP和MP2方法及6-31G(d, p)、6-31＋G(d, p)、6-311G(d, p)和6-311＋G(d, p) 基组，对六种氟代乙烯阳离子做了理论研究，优化了它们的基电子态的结构，计算了对应分子的垂直电离势（VIP）和绝热电离势(AIP).结果表明，与具有非平面结构的乙烯阳离子不同，六种氟代乙烯阳离子都只具有平面结构；与分子结构相比，离子结构的C－C键增长， C－F键缩短， CCF键角变小. 自然布居分析计算表明，这些离子的正电荷主要分布在与F原子相连的C原子和各H原子上. B3LYP/6-311＋G(d, p) 级别上计算的各分子的VIP和AIP值和实验值符合得很好. 使用含弥散基函数的基集可以明显提高这类分子的电离势的计算精度.     

Two-color resonantly enhanced multiphoton ionization and zero-kinetic-energy photoelectron spectroscopy of jet-cooled indan

We report studies of supersonically cooled indan using two-color resonantly enhanced multiphoton ionization and two-color zero-kinetic-energy photoelectron spectroscopy. With the aid of ab initio and density-functional calculations, vibrational modes of the first electronically excited state of the neutral species and those of the cation have been assigned, and the adiabatic ionization energy has been determined to be 68458 +/- 5 cm(-1). Similar to the ground state and the first electronically excited state of the neutral molecule, the ground state of the cation is also proven to be nonplanar, with an estimated barrier of 213 cm(-1) and a puckering angle of 15.0 degrees. These conclusions will be discussed in comparison with a previous study of an indan derivative 1,3-benzodioxole.     

Localized orbital corrections for the calculation of ionization potentials and electron affinities in density functional theory

This paper describes the extension of a previously reported empirical localized orbital correction model to the correction of ionization potential energies (IP) and electron affinities (EA) for atoms and molecules of first and second row elements. The B3LYP localized orbital correction version of the model (B3LYP-LOC) uses 22 heuristically determined parameters that improve B3LYP DFT IP and EA energy calculations on the G2 data set of 134 molecules from a mean absolute deviation (MAD) from experiment of 0.137 to 0.039 eV. The method significantly reduces the number of outliers and overall MAD to error levels below that achieved with G2 wave function based theory; furthermore, the new model has zero additional computational cost beyond standard DFT calculations. Although the model is heuristic and is based on a multiple linear regression to experimental errors, each of the parameters is justified on physical grounds, and each provides insight into the fundamental limitations of DFT, most importantly the failure of current DFT methods to accurately account for nondynamical electron correlation.     

Observation of rotamers of m-aminobenzoic acid: zero kinetic energy photoelectron and hole-burning resonantly enhanced multiphoton ionization spectroscopy

We report studies of supersonically cooled m-aminobenzoic acid using two-color resonantly enhanced multiphoton ionization (REMPI) and two-color zero kinetic energy (ZEKE) photoelectron spectroscopy. Two conformers have been identified and characterized using the hole-burning method in the REMPI experiment. With the aid of ab initio and density functional calculations, vibrational modes of the first electronically excited state (S(1)) of the neutral species and those of the ground state cation (D(0)) have been assigned, and the adiabatic ionization potentials have been determined for both conformers. The REMPI spectra are dominated by in-plane motions of the substituents and ring deformation modes. A propensity of Deltav=0, where Deltav is the change in vibrational quantum number from the S(1) to the D(0) state, is observed in the ZEKE spectra. The origin of this behavior is discussed in the context of electron back donation from the two substituents in the excited state and in the cationic state. Comparisons of these results with those of p-aminobenzoic acid will be analyzed.     

Core ionization potentials from self-interaction corrected Kohn-Sham orbital energies

We propose a simple self-interaction correction to Kohn-Sham orbital energies in order to apply ground state Kohn-Sham density functional theory to accurate predictions of core electron binding energies and chemical shifts. The proposition is explored through a series of calculations of organic compounds of different sizes and types. Comparison is made versus experiment and the "DeltaKohn-Sham" method employing separate state optimizations of the ground and core hole states, with the use of the B3LYP functional and different basis sets. A parameter alpha is introduced for a best fitting of computed and experimental ionization potentials. It is found that internal parametrizations in terms of basis set expansions can be well controlled. With a unique alpha=0.72 and basis set larger than 6-31G, the core ionization energies (IPs) of the self-interaction corrected Kohn-Sham calculations fit quite well to the experimental values. Hence, self-interaction corrected Kohn-Sham calculations seem to provide a promising tool for core IPs that combines accuracy and efficiency.     

Structures and heats of formation of the neutral and ionic PNO, NOP, and NPO systems from electronic structure calculations

High level ab initio electronic structure calculations using the coupled cluster CCSD(T) method with augmented correlation-consistent basis sets extrapolated to the complete basis set limit have been performed on the PNO, NOP, and NPO isomers and their corresponding anions and cations. Geometries for all species were optimized up through the aug-cc-pV(Q+d)Z level and vibrational frequencies were calculated with the aug-cc-pV(T+d)Z basis set. The most stable of the three isomers is NPO and it is predicted to have a heat of formation of 23.3 kcal/mol. PNO is predicted to be only 1.7 kcal/mol higher in energy. The calculated adiabatic ionization potential of NPO is 12.07 eV and the calculated adiabatic electron affinity is 2.34 eV. The calculated adiabatic ionization potential of PNO is 10.27 eV and the calculated adiabatic electron affinity is only 0.24 eV. NOP is predicted to be much higher in energy by 29.9 kcal/mol. The calculated rotational constants for PNO and NPO should allow for these species to be spectroscopically distinguished. The adiabatic bond dissociation energies for the P[Single Bond]N, P[Single Bond]O, and N[Single Bond]O bonds in NPO and PNO are the same within approximately 10 kcal/mol and fall in the range of 72-83 kcal/mol.     

氯代苯阳离子的密度泛函理论研究

采用B3LYP方法及6-311G(d,p)和6-311+G(d,p)基组,对12种氯代苯阳离子进行了理论研究,优化其电子基态的结构,计算了对应分子的垂直电离势(VIP)和绝热电离势(AIP).依据Jahn-Teller理论,确定了1,3,5-C6H3Cl3+和C6Cl6+离子分别具有C2v(2B1)和D2h(2B2g)结构(对应分子分别为D3h和D6h结构).其余10个离子的构型的对称点群与对应分子相同,但构型参数值有明显差别.用B3LYP方法计算的各氯代苯分子的垂直电离势和绝热电离势与实验值符合得很好.     

TD-CI simulation of the strong-field ionization of polyenes

Ionization of ethylene, butadiene, hexatriene, and octatetraene by short, intense laser pulses was simulated using the time-dependent single-excitation configuration-interaction (TD-CIS) method and Klamroth's heuristic model for ionization (J. Chem. Phys.2009, 131, 114304). The calculations used the 6-31G(d,p) basis set augmented with up to three sets of diffuse sp functions on each heavy atom as well as the 6-311++G(2df,2pd) basis set. The simulations employed a seven-cycle cosine pulse (ω = 0.06 au, 760 nm) with intensities up to 3.5 × 10(14) W cm(-2) (E(max) = 0.10 au) directed along the vector connecting the end carbons of the linear polyenes. TD-CIS simulations for ionization were carried out as a function of the escape distance parameter, the field strength, the number of states, and the basis set size. With a distance parameter of 1 bohr, calculations with Klamroth's heuristic model reproduce the expected trend that the ionization rate increases as the molecular length increases. While the ionization rates are too high at low intensities, the ratios of ionization rates for ethylene, butadiene, hexatriene, and octatetraene are in good agreement with the ratios obtained from the ADK model. As compared to earlier work on the optical response of polyenes to intense laser pulses, ionization using Klamroth's model is less sensitive to the number of diffuse functions in the basis set, and only a fraction of the total possible CIS states are needed to model the strong field ionizations.     

Accurate ionization potentials for UO and UO2: a rigorous test of relativistic quantum chemistry calculations

Accurate ionization potential (IP) measurements provide essential thermodynamic information and benchmark data that can be used to evaluate the validity of electronic structure models. Calculations of the first IP of UO2 using relativistic methods consistently predict values that are approximately 0.7 eV higher than the accepted experimental value. The present measurements validate the theoretical calculations and show that the previous determinations corresponded to the ionization of thermally excited molecules. Similarly, new measurements of the IP for UO show that the currently accepted value is too low by 0.4 eV.     

Excited state proton transfer in guanine in the gas phase and in water solution: a theoretical study

Theoretical investigations were performed to study the phenomena of ground and electronic excited state proton transfer in the isolated and monohydrated forms of guanine. Ground and transition state geometries were optimized at both the B3LYP/6-311++G(d,p) and HF/6-311G(d,p) levels. The geometries of tautomers including those of transition states corresponding to the proton transfer from the keto to the enol form of guanine were also optimized in the lowest singlet pipi* excited state using the configuration interaction singles (CIS) method and the 6-311G(d,p) basis set. The time-dependent density function theory method augmented with the B3LYP functional (TD-B3LYP) and the 6-311++G(d,p) basis set was used to compute vertical transition energies using the B3LYP/6-311++G(d,p) geometries. The TD-B3LYP/6-311++G(d,p) calculations were also performed using the CIS/6-311G(d,p) geometries to predict the adiabatic transition energies of different tautomers and the excited state proton transfer barrier heights of guanine tautomerization. The effect of the bulk aqueous environment was considered using the polarizable continuum model (PCM). The harmonic vibrational frequency calculations were performed to ascertain the nature of potential energy surfaces. The excited state geometries including that of transition states were found to be largely nonplanar. The nonplanar fragment was mostly localized in the six-membered ring. Geometries of the hydrated transition states in the ground and lowest singlet pipi* excited states were found to be zwitterionic in which the water molecule is in the form of hydronium cation (H3O(+)) and guanine is in the anionic form, except for the N9H form in the excited state where water molecule is in the hydroxyl anionic form (OH(-)) and the guanine is in the cationic form. It was found that proton transfer is characterized by a high barrier height both in the gas phase and in the bulk water solution. The explicit inclusion of a water molecule in the proton transfer reaction path reduces the barrier height drastically. The excited state barrier height was generally found to be increased as compared to that in the ground state. On the basis of the current theoretical calculation it appears that the singlet electronic excitation of guanine may not facilitate the excited state proton transfer corresponding to the tautomerization of the keto to the enol form.     

氟代吡啶阳离子的理论研究

Cations of fluorinated pyridines（pentafluoropyridine,2,6-difluoropyridine,and 2-fluoropyridine）have been studied by using density functional B3LYP method in conjunction with 6-31G（d,p）,6-311G（d,p）,6-31+G（d,p）,and 6-311+G（d,p）basis sets. B3LYP geometry optimization and frequency analysis calculations indicate that the pentafluoropyridine cation,2,6-difluoropyridine cation,and 2-fluoropyridine cation have C2v,C2v,and Cs structures in the 2A2,2A2,and 2A" ground states,respectively. The calculated geometries of the cations and the parent molecules were compared. The natural population analysis calculations at the B3LYP level with different basis sets were performed on the three cations and the three parent molecules. The isotropic hyperfine coupling constants in the three cations（radicals）were calculated. The vertical and adiabatic ionization potential（VIP and AIP）values of the pentafluoropyridine,2,6-difluoropyridine,and 2-fluoropyridine molecules were calculated by using the B3LYP method,and the calculated VIP values are in excellent agreement with experiment.     

Theoretical study of the spectroscopically relevant parameters for the detection of HNP(q) and HPN(q) (q = 0, +1, -1) in the gas phase

High level ab initio electronic structure calculations at different levels of theory have been performed on HNP and HPN neutrals, anions, and cations. This includes standard coupled cluster CCSD(T) level with augmented correlation-consistent basis sets, internally contacted multi-reference configuration interaction, and the newly developed CCSD(T)-F12 methods in connection with the explicitly correlated basis sets. Core-valence correction and scalar relativistic effects were examined. We present optimized equilibrium geometries, harmonic vibrational frequencies, rotational constants, adiabatic ionization energies, electron affinities, vertical detachment energies, and relative energies. In addition, the three-dimensional potential energy surfaces of HNP(-1,0,+1) and of HPN(-1,0,+1) were generated at the (R)CCSD(T)-F12b∕cc-pVTZ-F12 level. The anharmonic terms and fundamentals were derived using second order perturbation theory. For HNP, our best estimate for the adiabatic ionization energy is 7.31 eV, for the adiabatic electron affinity is 0.47 eV. The higher energy isomer, HPN, is 23.23 kcal∕mol above HNP. HPN possesses a rather large adiabatic electron affinity of 1.62 eV. The intramolecular isomerization pathways were computed. Our calculations show that HNP(-) to HPN(-) reaction is subject to electron detachment.     

Importance of selecting proper basis set in quantum mechanical studies of potential energy surfaces of carbohydrates

An extensive quantum mechanical study of a water dimer suggests that the introduction of a diffuse function into the basis set, which significantly reduces the basis set superposition error (BSSE) in the hydrogen bonding energy calculation, is the key to better calculations of the potential energy surfaces of carbohydrates. This article examines the potential energy surfaces of selected d -aldo- and d -ketohexoses (a total of 82 conformers) by quantum mechanics (QM) and molecular mechanics (MM) methods. In contrast to the results with a smaller basis set (B3LYP/6-31G** 5d), we found at the higher level calculation (B3LYP/6-311++G(2d,2p)//B3LYP/6-31G** 5d) that, in most cases, the furanose forms are less stable than the pyranose forms. These discrepancies are mainly due to the fact that intramolecular hydrogen bonding energies are overestimated in the lower level calculations. The higher level QM calculations of the potential energy surfaces of d -aldo- and d -ketohexoses now are more comparable to the MM3 results. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 1593–1603, 1999     

Electronic structure and spectroscopy of nucleic acid bases: ionization energies, ionization-induced structural changes, and photoelectron spectra

We report high-level ab initio calculations and single-photon ionization mass spectrometry study of ionization of adenine (A), thymine (T), cytosine (C), and guanine (G). For thymine and adenine, only the lowest-energy tautomers were considered, whereas for cytosine and guanine we characterized the five lowest-energy tautomeric forms. The first adiabatic and several vertical ionization energies were computed using the equation-of-motion coupled-cluster method for ionization potentials with single and double substitutions. Equilibrium structures of the cationic ground states were characterized by DFT with the ωB97X-D functional. The ionization-induced geometry changes of the bases are consistent with the shapes of the corresponding molecular orbitals. For the lowest-energy tautomers, the magnitude of the structural relaxation decreases in the following series, G > C > A > T, the respective relaxation energies being 0.41, 0.32, 0.25, and 0.20 eV. The computed adiabatic ionization energies (8.13, 8.89, 8.51-8.67, and 7.75-7.87 eV for A, T, C, and G, respectively) agree well with the onsets of the photoionization efficiency (PIE) curves (8.20 ± 0.05, 8.95 ± 0.05, 8.60 ± 0.05, and 7.75 ± 0.05 eV). Vibrational progressions for the S(0)-D(0) vibronic bands computed within double-harmonic approximation with Duschinsky rotations are compared with previously reported experimental photoelectron spectra and differentiated PIE curves.     

Photolysis of allene and propyne in the 7-30 eV region probed by the visible fluorescence of their fragments

The photolysis of allene and propyne, two isomers of C(3)H(4), has been investigated in the excitation energy range of 7-30 eV using vacuum ultraviolet synchrotron radiation. The visible fluorescence excitation spectra of the excited neutral photofragments of both isomers were recorded within the same experimental conditions. Below the first ionization potential (IP), this fluorescence was too weak to be dispersed and possibly originated from C(2)H or CH(2) radicals. Above IP, three excited photofragments have been characterized by their dispersed emission spectra: the CH radical (A (2)Delta-X (2)Pi), the C(2) radical (d (3)Pi(g)-a (3)Pi(u), "Swan's bands"), and the H atom (4-2 and 3-2 Balmer lines). A detailed analysis of the integrated emission intensities allowed us to determine several apparition thresholds for these fragments, all of them being interpreted as rapid and barrierless dissociation processes on the excited potential energy surfaces. In the low energy range explored in this work, both isomers exhibit different intensity distributions in their fragment emission as a function of the photolysis energy, indicating that mutual allene<-->propyne isomerization is not fully completed before dissociation occurs. The effect of isomerization on the dissociation into excited fragments is present in the whole excitation energy range albeit less important in the 7-16 eV region; it gradually increases with increasing excitation energy. Above 19 eV, the fragment distribution is very similar for the two isomers.     

Resonantly enhanced multiphoton ionization and zero kinetic energy photoelectron spectroscopy of 2-indanol conformers

We report studies of a supersonically cooled 2-indanol using two-color resonantly enhanced multiphoton ionization (REMPI) and two-color zero kinetic energy (ZEKE) photoelectron spectroscopy. In the REMPI experiment, we have identified three conformers of 2-indanol and assigned the vibrational structures of the first electronically excited state for the two major conformers. Conformer Ia contains an intramolecular hydrogen bond between the -OH group and the phenyl ring, while conformer IIb has the -OH group in the equatorial position. We have further investigated the vibrational spectroscopy of the cation for the two major conformers using the ZEKE spectroscopy. The two conformers display dramatically different vibrational distributions. The ZEKE spectrum of conformer Ia shows an extensive progression in the puckering mode of the five member ring, indicating a significant geometry change upon ionization. The ZEKE spectra of conformer IIb are dominated by single vibronic transitions, and the intensity of the ZEKE signal is much stronger than that of conformer Ia. These results indicate an invariance of the molecular frame during ionization for conformer IIb. We have performed ab initio and density functional theory calculations to obtain potential energy surfaces along the dihedral angle involving the -OH group for all three electronic states. In addition, we have also calculated the vibrational distribution of the ZEKE spectrum for the puckering mode of the five member ring. Not only the vibrational frequencies but also the intensity distributions for both conformers have been reproduced satisfactorily. The adiabatic ionization energies have been determined to be 68 593+/-5 cm(-1) for conformer Ia and 68 981+/-5 cm(-1) for conformer IIb.