Calculation of the size dependence of the ionization energy and autoionization energy of Hg-clusters

To study the transition from van der Waals to metallic bonding we calculate the size dependence of the ionization energy and 5d→6p autoionization energy of Hg _n -clusters using a parametrized LCAO model. Our results are in good qualitative agreement with experiment. Comparison with experimental results suggests that electron correlations play an important role for the transition from localized (van der Waals-like) to delocalized (covalent or metallic) electronic states occuring in Hg _n atn?13–19.     

Dissociation energy and ionization potential of the molecule CF

The gaseous equilibrium S + CF₂ = SF + CF was studied over the temperature range 1851 to 2232 K by mass spectrometry, and the derived enthalpy change was used to evaluate the heat of formation of CF ΔH₂₉₈ = 58.0 ± 2.4 kcal/mol (2.52 ± 0.10 eV), and the dissociation energy D₀⁰ (CF) = 130.8 ± 2.4 kcal/mol (5.67 ± 0.10 eV). The new thermochemical data indicate a slightly higher stability for CF than earlier determinations. Direct measurement by electron impact yielded a value of 9.17 ± 0.10 eV for the vertical ionization potential of CF, in agreement with an indirect result obtained from the photodissociative ionization of C₂F₄.     

Internal energy distribution in electrospray ionization

Internal energies and energy distributions were studied using the 'survival yield' method developed previously. In addition to conventional benzylpyridinium salts, protonated esters (fragmenting by rearrangement) and protonated leucine enkephalin were also used, extending the validity of the technique. Fragmentation processes were studied in the cone voltage region and modeled by the RRKM-based MassKinetics program. The results show that the shapes of the energy distributions are similar to thermal distributions. The mean internal energies are very similar for all compound classes studied, and show a linear increase with collision energy in the 10-50 eV region.     

Negative shake-up energy in core ionization

In recent years unusual multipeak structures have been detected in the core electron ionization spectra of certain highly polar aromatic molecules. In this work the ionization spectrum of para-nitroaniline, NO₂C₆H₄NH₂, in the Nls energy region is calculated by ab initio methods taking configuration interaction between the single hole-hole configurations and manifold of two-hole-one-particle excited configurations into account. The splitting of the Nls(NO₂) line observed in the ESCA spectrum of para-nitroaniline is reproduced by the calculation. The calculation shows that in the presence of the core hole the lowest π → π* excitation energy is negative. This negative shake-up energy is responsible for the multi-peak structure of the Nls(NO₂) line. It is shown that the computational results can be simply rationalized by considering the charge distribution of the highest occupied and lowest unoccupied valence orbitals. The close analogy between the multi-peak phenomena in the K-shell ESCA spectra of nitroaniline (and related highly polar aromatic compounds) and molecules chemisorbed on metal surfaces is emphasized.     

MRD Cl calculation of the first and second ionization potential of OH

Multireference Cl calculations are reported for OH(²Π). OH⁺ (³Σ⁻), and OH²⁺ (⁴Σ⁻) using several basis sets. Ionization potentials derived from the potential curves are compared to previous calculations and to experimental values. We show that the errors obtained from the calculations can be traced to the difficulty in obtaining an accurate treatment of the ionization energy of the oxygen atom. No experimental artifact needs to be invoked to explain the discrepancy with the experimental results.     

On the use of UHF orbitals in ionization energy calculations

The UHF Hamiltonian and simple Löwdin-like annihilators are formulated in the second quantization formalism. The so formulated Hamiltonian was employed in many-body Rayleigh-Schrödinger perturbation theory to evaluate the corrections to the UHF orbital energies.     

电喷雾离子源中样品离子化能量转移理论的初探

电喷雾离子源（electrospray ionization,ESI）不仅可以用于小分子的检测,也能够用于蛋白质、多肽等大分子的研究。本文通过对离子化过程的系统分析,提出了基于能量最低原理的离子化过程能量转移理论。样品分子在由液相转移到气相形成离子化气体的过程中受到静电力、分子间的范德华力等多种力的作用。样品的离子化是多种力共同作用的结果,在不同的离子化阶段,不同形式的力的作用也不尽相同。电荷在样品表面蒸发和多电荷离子的形成之间存在竞争。对不同结构的分子,分子形态、构象改变导致的两相间转移Gibbs自由能变化不同,可能导致离子蒸发、大分子形成多电荷离子、产生链弹射等行为。离子化能量转移理论不仅能够对已有的3种理论加以简化统一,也可以说明溶剂、电解质离子等在离子化过程中的作用,为优化不同结构与形态样品的质谱检测、了解离子化的真实过程提供了一种可能的依据。     

Dissociation of toluene cation: a new potential energy surface

The potential energy surface (PES) for the formation of tropylium and benzylium ions from toluene cation (1) has been explored theoretically. Quantum chemical calculations at the B3LYP/6-311++G and G3//B3LYP levels were performed. A pathway to form o-isotoluene (5-methylene-1,3-cyclohexadiene) cation (5) from 1 was found. The isomerization occurs by two consecutive 1,2-H shifts from CH(3) to the ortho position of the aromatic ring via a distonic benzenium cation (2), which is also an intermediate in the well-known isomerization of 1 to cycloheptatriene cation (4). Since the barrier for the formation of 2 is the highest in the two isomerization pathways, 1, 4, and 5 are interconvertible energetically prior to dissociation. The benzylium ion can be produced via 5 as well as from 1 and the tropylium ion via 4. Rice-Ramsperger-Kassel-Marcus model calculations were carried out based on the obtained PES. The result agrees with previous experimental observations. From a theoretical analysis of kinetics of the isomerizations and dissociations, we suggest that 5 plays an important role in the formation of C(7)H(7)(+) from 1.     

Analytical second derivatives of the energy in MNDO methods

Analytical second derivatives of the energy are derived and efficiently implemented for semiempirical MNDO-type methods including AM1, PM3, and MNDO/d. A new algorithm for the simultaneous solution of several CPHF equations is proposed in which the amount of memory required is independent of the number of iterations. The analytical approach is faster than the numerical approach typically by a factor of 5 and exhibits a reliable convergence over a wide range of molecules. The asymptotic memory and secondary storage requirements of the reported procedure can be as low as O(N²) without significant degradation of the performance. © 1996 by John Wiley & Sons, Inc.     

Thermal energy distribution observed in electrospray ionization

A new method was developed which fits a thermal internal energy distribution to ions formed by electrospray ionization. The molecular ion survival yield was measured and determined by RRKM calculations as a function of temperature. The ('characteristic') temperature was determined when the calculated and measured molecular ion survival yields were equal. The 'characteristic' temperatures were very similar (average RSD errors were 8%) for a set of analogous compounds (benzylpyridine salts), and the resulting thermal internal energy distributions were close to those determined by De Pauw's method. The validity of the method was also checked performing blackbody infrared radiation and on-resonance excitation experiments on a Fourier transform ion cyclotron resonance instrument with benzylpyridine salts and leucine enkephalin. The results strongly suggest that internal energy distributions in electrospray ionization are very close to thermal distributions. It was found that the characteristic temperature increases linearly with the cone voltage. It is suggested that the characteristic temperature can be used as a quantitative measure to control and standardize conditions in electrospray ionization. Copyright 1999 John Wiley & Sons, Ltd.     

Structure and charge-state dependence of the gas-phase ionization energy of proteins

Photoionization of protein ions: The ionization energy of polyprotonated protein cations in the gas phase measured using VUV synchrotron radiation appears to be correlated with the charge state z of the protein and its tertiary structure. A simple electrostatic model accounts for the results and also shows predictive capabilities to derive a mean radius R(m) of the protein ion from the ionization energy, and vice versa.     

The energy of the homogeneous electron gas to second order

The energy of an infinite, homogeneous electron gas is examined by second order perturbation theory using a Hartee-Fock rather than a noninteracting particle unperturbed state. The second order energy still diverges for small promotions k , albert than as ln|ln k| rather than as In k.     

The multiphoton ionization spectrum of toluene in a supersonic free jet: internal rotation of the methyl group

The S₁← S_o electronic transitions of toluene involving also some internal rotational levels were observed for the first time in the multiphoton ionization spectrum in a supersonic jet. A large population in several low-lying internal rotational levels and a strong coupling between electronic motion and the internal rotation are suggested.     

Wave-mechanical theory of field ionization and field-ion energy distributions

Although field-ion energy distributions have been measured for many years, no theoretical treatment has ever been published that is strictly compatible with the axioms of quantum mechanics. This article shows how a fully wave-mechanical theory of field ionization and field-ion energy distributions can be constructed, based on the idea that surface field ionization is the fermion analogy for Dirac's treatment of photon emission. The theory is formal and suggests, as expected, that existing quasiclassical treatments are essentially satisfactory in normal circumstances.The discussion comprises of three main steps. The first is to clarify the various definitions of energy and potential used in field-ion theory, and to codify the corresponding energy diagrams. The second is to derive and expression for the rate-constant for transfer from one vibrational state to another. The third is to sum over initial states to derive a formal expression for the ion energy distribution. The article concludes by discussing the possible development of this theory.     

An efficient approach for self-consistent-field energy and energy second derivatives in the atomic-orbital basis

Based on self-consistent-field (SCF) perturbation theory, we recast the SCF and the coupled-perturbed SCF (CPSCF) equations for time-independent molecular properties into the atomic-orbital basis. The density matrix and the perturbed density matrix are obtained iteratively by solving linear equations. Only matrix multiplications and additions are required, and this approach can exploit sparse matrix multiplications and thereby offer the possibility of evaluating second-order properties in computational effort that scales linearly with system size. Convergence properties are similar to conventional molecular-orbital-based CPSCF procedures, in terms of the number of derivative Fock matrices that must be constructed. We also carefully address the issue of the numerical accuracy of the calculated second derivatives of the energy, in order to specify the minimum precision necessary in the CPSCF procedure. It is found that much looser tolerances for the perturbed density matrices are adequate when using an expression for the second derivatives that is correct through second order in the CPSCF error.