Electron capture detector response and dissociative electron attachment cross sections in chloroalkanes and chloroalkenes

Electron capture detectors (ECDs) are widely used in gas chromatography to detect electronegative compounds. In this work, we examine the connections between the ECD response and the cross sections for dissociative electron attachment (DEA) determined from low energy electron beam studies in the chloroalkane family, stressing in particular the role of temporary anion state energies. We show that attachment rate coefficients computed from these cross sections are well correlated with ECD response, and that the latter decreases exponentially with increasing energies of the lowest anion states. ECD measurements are also carried out in monochloroalkanes substituted with unsaturated ethenyl and phenyl moieties, and the response is shown to depend strongly on the mixing between the unsaturated pi* and the C-Cl sigma* temporary anions as exhibited by the vertical attachment energies (VAEs) of these states. The results show good correlations between the chloroalkene and phenyl chloride ECD responses and the VAEs for the mixed states.     

Orbital energies and negative electron affinities from density functional theory: Insight from the integer discontinuity

Orbital energies in Kohn-Sham density functional theory (DFT) are investigated, paying attention to the role of the integer discontinuity in the exact exchange-correlation potential. A series of closed-shell molecules are considered, comprising some that vertically bind an excess electron and others that do not. High-level ab initio electron densities are used to calculate accurate orbital energy differences, Deltavarepsilon, between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO), using the same potential for both. They are combined with accurate vertical ionization potentials, I(0), and electron affinities, A(0), to determine accurate "average" orbital energies. These are the orbital energies associated with an exchange-correlation potential that averages over a constant jump in the accurate potential, of magnitude Delta(XC)=(I(0)-A(0))-Deltavarepsilon, as given by the discontinuity analysis. Local functional HOMO energies are shown to be almost an order of magnitude closer to these average values than to -I(0), with typical discrepancies of just 0.02 a.u. For systems that do not bind an excess electron, this level of agreement is only achieved when A(0) is set equal to the negative experimental affinity from electron transmission spectroscopy (ETS); it degrades notably when the zero ground state affinity is instead used. Analogous observations are made for the local functional LUMO energies, although the need to use the ETS affinities is less pronounced for systems where the ETS values are very negative. The application of an asymptotic correction recovers the preference, leading to positive LUMO energies (but bound orbitals) for these systems, consistent with the behavior of the average energies. The asymptotically corrected LUMO energies typically agree with the average values to within 0.02 a.u., comparable to that observed with the HOMOs. The study provides numerical support for the view that local functionals exhibit a near-average behavior based on a constant jump of magnitude Delta(XC). It illustrates why a recently proposed DFT expression involving local functional frontier orbital energies and ionization potential yields reasonable estimates of negative ETS affinities and is consistent with earlier work on the failure of DFT for charge-transfer excited states. The near-average behavior of the exchange-correlation potential is explicitly illustrated for selected systems. The nature of hybrid functional orbital energies is also mentioned, and the results of the study are discussed in terms of the variation in electronic energy as a function of electron number. The nature of DFT orbital energies is of great importance in chemistry; this study contributes to the understanding of these quantities.     

Reaction energetics on long‐range corrected density functional theory: Diels–Alder reactions

The possibility of quantitative reaction analysis on the orbital energies of long‐range corrected density functional theory (LC‐DFT) is presented. First, we calculated the Diels–Alder reaction enthalpies that have been poorly given by conventional functionals including B3LYP functional. As a result, it is found that the long‐range correction drastically improves the reaction enthalpies. The barrier height energies were also computed for these reactions. Consequently, we found that dispersion correlation correction is also crucial to give accurate barrier height energies. It is, therefore, concluded that both long‐range exchange interactions and dispersion correlations are essentially required in conventional functionals to investigate Diels–Alder reactions quantitatively. After confirming that LC‐DFT accurately reproduces the orbital energies of the reactant and product molecules of the Diels–Alder reactions, the global hardness responses, the halves of highest occupied molecular orbital (HOMO)‐lowest unoccupied molecular orbital (LUMO) energy gaps, along the intrinsic reaction coordinates of two Diels–Alder reactions were computed. We noticed that LC‐DFT results satisfy the maximum hardness rule for overall reaction paths while conventional functionals violate this rule on the reaction pathways. Furthermore, our results also show that the HOMO‐LUMO gap variations are close to the reaction enthalpies for these Diels–Alder reactions. Based on these results, we foresee quantitative reaction analysis on the orbital energies. © 2012 Wiley Periodicals, Inc.     

Theoretical and experimental investigation on the electronic properties of the shuttlecock shaped and the double-decker structured metal phthalocyanines, MPc and M(Pc)2 (M = Sn and Pb)

The molecular geometries, electronic structures, and excitation energies of tin and lead phthalocyanine compounds, SnPc, PbPc, Sn(Pc)(2), and Pb(Pc)(2), were investigated using the B3LYP method within a framework of density functional theory (DFT). The geometries of SnPc, PbPc, Sn(Pc)(2), and Pb(Pc)(2) were optimized under C(4v), C(4v), D(4d), and D(4d) molecular symmetries, respectively. The excitation energies of these molecules were computed by the time-dependent DFT (TD-DFT) method. The calculated results for the excited states of three compounds other than the unknown Pb(Pc)(2) corresponded well with the experimental results of electronic absorption spectroscopy. The non-planar C(4v) molecular structure of SnPc and PbPc influences especially on the orbital energy of the HOMO-1 through mixing of the s-type atomic orbital of the central metal atom to the π system of the Pc ring in an anti-bonding way; however, the HOMO and the LUMO have little effect of the deviation from the planar structure because they have no contribution from the atomic orbital of the central metal. This orbital mixing pushes up the orbital energy of the HOMO-1, and reduces the energy of the metal-to-ligand charge transfer band of SnPc and PbPc. The calculated results also reproduced well the excitation profile of Sn(Pc)(2), which was quite different from that of SnPc. The strong interactions between the π-type orbitals of two Pc moieties altered the electronic structure resulting in the characteristic excitation profile of Sn(Pc)(2). In addition, this caused a reduction of about 0.8 eV in the ionization potential as compared to usual MPcs including SnPc, which was consistent with the experimental results.     

Total dissociative electron attachment cross sections of selected amino acids

Total dissociative electron attachment cross sections are presented for the amino acids, glycine, alanine, proline, phenylalanine, and tryptophan, at energies below the first ionization energy. Cross section magnitudes were determined by observation of positive ion production and normalization to ionization cross sections calculated using the binary-encounter-Bethe method. The prominent 1.2 eV feature in the cross sections of the amino acids and the closely related HCOOH molecule is widely attributed to the attachment into the -COOH pi* orbital. The authors discuss evidence that direct attachment to the lowest sigma* orbital may instead be responsible. A close correlation between the energies of the core-excited anion states of glycine, alanine, and proline and the ionization energies of the neutral molecules is found. A prominent feature in the total dissociative electron attachment cross section of these compounds is absent in previous studies using mass analysis, suggesting that the missing fragment is energetic H-.     

Temporary anion states and dissociative electron attachment in diphenyl disulfide

The temporary anion states of gas-phase diphenyl disulfide are characterized by means of electron transmission (ET) and dissociative electron attachment (DEA) spectroscopies. The measured energies of vertical electron attachment are compared to the virtual orbital energies of the neutral state molecule supplied by MP2 and B3LYP calculations with the 6-31G basis set. The calculated energies, scaled with empirical equations, reproduce satisfactorily the attachment energies measured in the ET spectrum. The first anion state of diphenyl disulfide is stable, thus escaping detection in ETS. The vertical and adiabatic electron affinities, evaluated with B3LYP/6-31+G calculations as the energy difference between the neutral and anion states, are predicted to be 0.37 and 1.38 eV, respectively. The anion current displayed in the DEA spectrum has a sharp and intense peak at zero energy, essentially due to the C6H5S- negative fragment. In agreement, according to the calculations, the localization properties of the first anion state are strongly S-S antibonding, and the energetic requirement for its dissociation along the S-S bond is fulfilled even at zero energy.     

Electron attachment to dye-sensitized solar cell components: cyanoacetic acid

The energies of electron attachment associated with temporary occupation of the lower-lying virtual orbitals of cyanoacetic acid (CAA), proposed as a possible component of dye-sensitized solar cells, and its derivative methyl cyanoacetate (MCA) are measured in the gas phase with electron transmission spectroscopy (ETS). The corresponding orbital energies of the neutral molecule, supplied by B3LYP/6-31G(d) calculations and scaled using an empirically calibrated linear equation, are compared with the experimental vertical attachment energies (VAEs). The vertical and adiabatic electron affinities are also evaluated at the B3LYP/6-31+G(d) level as the anion/neutral total energy difference. Dissociative electron attachment spectroscopy (DEAS) is used to measure the total anion current as a function of the incident electron energy in the 0-4 eV energy range, and the negative fragments generated through the dissociative decay channels of the molecular anion are detected with a mass filter. In both compounds only two intense fragment anion currents are observed, that due to loss of a hydrogen atom from the molecular anion ([M - H](-)) and that due to formation of CN(-). In CAA the former signal displays a very sharp feature at 0.68 eV, assigned to a vibrational Feshbach resonance arising from coupling between a dipole bound anion state and a temporary σ* anion state.     

Theoretical calculation about the valence and Rydberg excited states of hydrogen cyanide

The singlet and triplet excited states of hydrogen cyanide have been computed by using the complete active space self-consistent field and completed active space second order perturbation methods with the atomic natural orbital (ANO-L) basis set. Through calculations of vertical excitation energies, we have probed the transitions from ground state to valence excited states, and further extensions to the Rydberg states are achieved by adding 1s1p1d Rydberg orbitals into the ANO-L basis set. Four singlet and nine triplet excited states have been optimized. The computed adiabatic energies and the vertical transition energies agree well with the available experimental data and the inconsistencies with the available theoretical reports are discussed in detail.     

Excited state properties, fluorescence energies, and lifetime of a poly(fluorene-pyridine) copolymer, based on TD-DFT investigation

The structural and electronic properties of the fluorene-pyridine copolymer (FPy)(n), (n = 1-4) were investigated theoretically by means of quantum mechanical calculations based on density functional theory (DFT) and time-dependent DFT (TD-DFT) using the B3LYP functional. Geometry optimizations of these oligomers were performed for the ground state and the lowest excited state. It was found that (FPy)(n) is nonplanar in its ground state, whereas a more pronounced trend toward planarity is observed in the S(1) state. Absorption and fluorescence energies have been extrapolated to infinite chain length making use of their good linearity with respect to 1/n. An extrapolated value of 2.64 eV is obtained for vertical excitation energy. The S(1)<--S(0) electronic excitation is characterized as a highest occupied molecular orbital to lowest unoccupied molecular orbital transition and is dominating in terms of oscillator strength. Fluorescence energies and radiative lifetime were calculated as well. The obtained results indicate that the fluorescence energy and radiative lifetime of (FPy)(n) are 2.16 eV and 0.38 ns, respectively. The decrease of fluorescence energy and radiative lifetime with the increase in the chain length is discussed.     

QM/MM study of the absorption spectra of DsRed.M1 chromophores

We report geometries and vertical excitation energies for the red and green chromophores of the DsRed.M1 protein in the gas phase and in the solvated protein environment. Geometries are optimized using density functional theory (DFT, B3LYP functional) for the isolated chromophores and combined quantum mechanical/molecular mechanical (QM/MM) methods for the protein (B3LYP/MM). Vertical excitation energies are computed using DFT/MRCI, OM2/MRCI, and TDDFT as QM methods. In the case of the red chromophore, there is a general blue shift in the excitation energies when going from the isolated chromophore to the protein, which is caused both by structural changes and by electrostatic interactions with the environment. For the lowest ππ* transition, these two factors contribute to a similar extent to the overall DFT/MRCI shift of 0.4 eV. An enlargement of the QM region to include active‐site residues does not change the DFT/MRCI excitation energies much. The DFT/MRCI results are closest to experiment for both chromophores. OM2/MRCI and TDDFT overestimate the first vertical excitation energy by 0.3–0.5 and 0.2–0.4 eV, respectively, relative to the experimental or DFT/MRCI values. The experimental gap of 0.35 eV between the lowest ππ* excitation energies of the red (cis‐acylimine) and green (trans‐peptide) forms is well reproduced by DFT/MRCI and TDDFT (0.32 and 0.37 eV, respectively). A histogram spectrum for an equal mixture of the two forms, generated by OM2/MRCI calculations on 450 snapshots along molecular dynamics trajectories, matches the experimental spectrum quite well, with a gap of 0.23 eV and an overall blue shift of about 0.3 eV. DFT/MRCI appears as an attractive choice for calculating excitation energies in fluorescent proteins, without the shortcomings of TDDFT and computationally more affordable than CASSCF‐based approaches. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

Intermolecular potentials based on symmetry-adapted perturbation theory with dispersion energies from time-dependent density-functional calculations

Recently, three of us have proposed a method [Phys. Rev. Lett. 91, 33201 (2003)] for an accurate calculation of the dispersion energy utilizing frequency-dependent density susceptibilities of monomers obtained from time-dependent density-functional theory (DFT). In the present paper, we report numerical calculations for the helium, neon, water, and carbon dioxide dimers and show that for a wide range of intermonomer separations, including the van der Waals and short-range repulsion regions, the method provides dispersion energies with accuracies comparable to those that can be achieved using the current most sophisticated wave-function methods. If the dispersion energy is combined with (i) the electrostatic and first-order exchange interaction energies as defined in symmetry-adapted perturbation theory (SAPT) but computed using monomer Kohn-Sham (KS) determinants, and (ii) the induction energy computed using the coupled KS static response theory, (iii) the exchange-induction and exchange-dispersion energies computed using KS orbitals and orbital energies, the resulting method, denoted by SAPT(DFT), produces very accurate total interaction potentials. For the helium dimer, the only system with nearly exact benchmark values, SAPT(DFT) reproduces the interaction energy to within about 2% at the minimum and to a similar accuracy for all other distances ranging from the strongly repulsive to the asymptotic region. For the remaining systems investigated by us, the quality of the SAPT(DFT) interaction energies is so high that these energies may actually be more accurate than the best available results obtained with wave-function techniques. At the same time, SAPT(DFT) is much more computationally efficient than any method previously used for calculating the dispersion and other interaction energy components at this level of accuracy.     

Theoretical Studies on the Structure and Spectrum of Imidazole-Chloranil Charge Transfer Complex

UV-Vis absorption spectra of the molecular complex formed by imidazole (Im) and chloranil (CA) were measured in chloroform. The stoichiometry of the imidazole-chloranil (Im-CA) complex was determined as 1:1 by applying Benesi-Hildebrand's equation and Job's continuous variation method. Density function theory (DFT) and MP2 calculations were performed to study the structures and the binding energies of the Im-CA complex. The calculations located four conformations (denoted as S1-S4) for the Im-CA complex, two edge(Im)-to-face(CA) linked and two edge(Im)-to-edge(CA) linked. It was found that the edge-to-face conformers are more stable than the edge-to-edge ones. The bonding characteristics of these conformers were investigated with natural population analysis (NPA), topological analysis of electron density, and natural bond orbital (NBO) analysis. It was revealed that the edge-to-face conformers are charge-transfer (CT) complexes whereas the edge-to-edge conformers are the hydrogen bond complexes. For the most stable conformation of the Im-CA complex (S1), the charge transfer interaction of the imidazole n(N15) lone pair orbital with the chloranil ∏*(C1=O7) orbital plays a crucial role in the Im-CA binding, and the binding is further strengthened by the O7… H20 hydrogen bond. The electronic excitation energies of the complex (S1) were calculated with time-dependent DFT (TDDFT), and the observed UV-Visible spectrum of the complex was analyzed based on the computed results.     

Charge stripping and the site of cationization of substituted aromatic compounds

The site of protonation, methylation and ethylation of anilines, phenols and thiophenols can be determined by tandem mass spectrometry (MS/MS). The extent to which the cation attachment products formed from these compounds undergo charge stripping is related to the site of cation attachment. The formation of doubly charged ions for phenols and anilines is favored when cation attachment occurs on the ring. For thiophenols alkylation occurs almost exclusively on the substituent, and yet an abundant stripping peak is recorded. This behavior is opposite to that seen for anilines and phenols and is accounted for in terms of orbital energies in the three systems. The site of cation attachment, as deduced by charge stripping, agrees with conclusions drawn from the fragmentation behavior of these ions upon collision-induced dissociation.     

Enhanced stability by three-dimensional aromaticity of endohedrally doped clusters X(10)M(0/-) with X = Ge, Sn, Pb and M = Cu, Ag, Au

The group 14 clusters encapsulated by coinage metals in neutral and anionic states X(10)M(0/-) (X = Ge, Sn, Pb and M = Cu, Ag, Au) are investigated using quantum chemical calculations with the DFT/B3LYP functional and coupled-cluster CCSD(T) theory. Addition of transition metals into the empty cages forms high symmetry endohedral structures, except for Ge(10)Ag(0/-). In agreement with experiments available for X(10)Cu, the D(4d) global minima of the anions are calculated to be magic clusters with large frontier orbital gaps, high vertical and adiabatic detachment energies, and large embedding energies and binding energies as compared to those of the empty cages X(10)(2-). The enhanced stability of these magic clusters can be rationalized by the three-dimensional aromaticity.     

稠环芳烃菲电子激发态的含时密度泛函理论研究

用含时密度泛函理论TD-DFT及组态相关CIS、含时TD-HF方法对菲的UV光谱进行了理论研究,在几何构型优化的基础上,计算了其垂直电子激发能．计算表明,基函数的选择对激发能的计算影响较大,而不同的密度泛函交换一相关势对其影响较小,这和轨道能级的系统误差相互抵消有关．对菲的电子跃迁能的计算,与实验结果比较相一致,理论与实验的误差和不同的实验之间的误差在同一个数量级之内,显示TD-DFT方法比CIS,TD-HF方法更适合电子激发能的计算．     

Electronic structure and stability of binary and ternary aluminum‐bismuth‐nitrogen nanoclusters

The electronic structure and stability in binary and ternary aluminum‐bismuth‐nitrogen nanoclusters up to six atoms are studied using density functional theory (DFT). The lowest energy geometries were obtained by sampling the geometrical space with a Monte Carlo method and geometry optimizations, at DFT level, with M06L functional. The clusters stability is analyzed using formation and fragmentation energies. Our results show that a high concentration of nitrogen presents a tendency to form nitrogen clusters. highest occupied molecular orbital‐lowest unoccupied molecular orbital gaps show the well‐known oscillation as the number of atoms is increased. Bonding between Al, Bi, and N has mainly a π character. Bismuth and aluminum atoms tend to promote high multiplicity states in small clusters. These new binary and ternary materials provide a potential new field in optoelectronics and high energetic material compounds. © 2014 Wiley Periodicals, Inc.     

Computations of 36 tautomer/isomer equilibria of different lactams

Gas-phase energies of 36 tautomer/isomer pairs of 18 six-membered N-heterocyclic compounds were computed quantum chemically. Among the considered B3LYP, BH&HLYP, BH&HLYP(G), and PW6B95 DFT functionals, the latter two provide accurate tautomer/isomer pair energies with root-mean-square deviations (rmsd) relative to experiments of 0.2 and 0.3 kcal/mol, respectively. Since only few (namely five) experimental data are available, 15 tautomer/isomer pair energies were computed with the very precise QCISD(T)(quadruple-ζ) method serving as reference. Relative to this reference the PW6B95 DFT functional is slightly superior to the BH&HLYP(G) functional, yielding an rmsd of 0.7 and 0.8 kcal/mol, respectively. In contrast to BH&HLYP(G), the PW6B95 DFT functional yields also accurate tautomer/isomer pair energies if zwitterionic structures are involved. The tautomer/isomer pair states possess different amounts of aromaticity. This is characterized by nucleus-independent chemical shift (NICS) values. The tautomer/isomer pair reference energies, from which the energies computed with PW6B95 are subtracted, correlate linearly with the corresponding differences in the NICS values. This correlation is used to construct a correction term for the pair energies computed with PW6B95, yielding tautomer/isomer pair energies with rmsd of 0.3 kcal/mol with respect to the more CPU time demanding QCISD(T)(quadruple-ζ) method.     

Quantum versus classical electron transfer energy as reaction coordinate for the aqueous Ru2+/Ru3+ redox reaction

Applying density functional theory (DFT)-based molecular dynamics simulation methods we investigate the effect of explicit treatment of electronic structure on the solvation free energy of aqueous Ru²⁺ and Ru³⁺.Our approach is based on the Marcus theory of redox half reactions, focussing on the vertical energy gap for reduction or oxidation of a single aqua ion. We compare the fluctuations of the quantum and classical energy gap along the same equilibrium ab initio molecular dynamics trajectory for each oxidation state. The classical gap is evaluated using a standard point charge model for the charge distribution of the solvent molecules (water). The quantum gap is computed from the full DFT electronic ground state energies of reduced and oxidized species, thereby accounting for the delocalization of the electron in the donor orbital and reorganization of the electron cloud after electron transfer (ET). The fluctuations of the quantum ET energy are well approximated by gaussian statistics giving rise to parabolic free energy profiles. The curvature is found to be independent of the oxidation state in agreement with the linear response assumption underlying Marcus theory. By contrast, the diabatic free energy curves evaluated using the classical gap as order parameter, while also quadratic, are asymmetric reflecting the difference in oxidation state. The response of these two order parameters is further analysed by a comparison of the spectral density of the fluctuations and the corresponding reorganization free energies.     

Valence electronic structure of benzo-2,1,3-chalcogenadiazoles studied by photoelectron spectroscopy and density functional theory

The He I photoelectron spectra of benzo-2,1,3-thia-, selena-, and telluradiazole were measured, and the observed ionization bands were assigned by comparison with the results of DFT calculations. Whereas the B3LYP exchange-correlation functional provided orbital energies that permitted a preliminary assignment by application of Koopman's theorem, a more-accurate interpretation was established by calculation of the vertical ionization energies with the PW91 functional and analysis of the correlation of energy levels along the homologous series. This strategy clarified earlier disagreements in the assignment of the spectrum of benzo-2,1,3-thiadiazole.     

Mechanistic insights into triterpene synthesis from quantum mechanical calculations. Detection of systematic errors in B3LYP cyclization energies

Most quantum mechanical studies of triterpene synthesis have been done on small models. We calculated mPW1PW91/6-311+G(2d,p)//B3LYP/6-31G* energies for many C30H51O+ intermediates to establish the first comprehensive energy profiles for the cationic cyclization of oxidosqualene to lanosterol, lupeol, and hopen-3beta-ol. Differences among these 3 profiles were attributed to ring strain, steric effects, and proton affinity. Modest activation energy barriers and the ample exothermicity of most annulations indicated that the cationic intermediates rarely need enzymatic stabilization. The course of reaction is guided by hyperconjugation of the carbocationic 2p orbital with parallel C-C and C-H bonds. Hyperconjugation for cations with a horizontal 2p orbital (in the plane of the ABCD ring system) leads to annulation and ring expansion. If the 2p orbital becomes vertical, hyperconjugation fosters 1,2-methyl and hydride shifts. Transition states leading to rings D and E were bridged cyclopropane/carbonium ions, which allow ring expansion/annulation to bypass formation of undesirable anti-Markovnikov cations. Similar bridged species are also involved in many cation rearrangements. Our calculations revealed systematic errors in DFT cyclization energies. A spectacular example was the B3LYP/6-311+G(2d,p)//B3LYP/6-31G* prediction of endothermicity for the strongly exothermic cyclization of squalene to hopene. DFT cyclization energies for the 6-311+G(2d,p) basis set ranged from reasonable accuracy (mPW1PW91, TPSSh with 25% HF exchange) to underestimation (B3LYP, HCTH, TPSS, O3LYP) or overestimation (MP2, MPW1K, PBE1PBE). Despite minor inaccuracies, B3LYP/6-31G* geometries usually gave credible mPW1PW91 single-point energies. Nevertheless, DFT energies should be used cautiously until broadly reliable methods are established.