Structure, vibrational frequencies, ionization energies, and photoelectron spectrum of the para-benzyne radical anion

Equilibrium structure, vibrational frequencies, and ionization energies of the para-benzyne radical anion are characterized by coupled-cluster and equation-of-motion methods. Vibronic interactions with the low-lying excited state result in a flat potential energy surface along the coupling mode and even in a lower-symmetry C_2v structures. Additional complications arise due to Hartree–Fock instabilities and near-instabilities. The magnitude of vibronic interactions was characterized by geometrical parameters, charge localization patterns and energy differences between the D_2h and C_2v structures. The observed trends suggest that the C_2v minimum predicted by several theoretical methods is an artifact of incomplete correlation treatment. The comparison between the calculated and experimental spectrum confirmed D_2h structure of the anion, as well as accuracy of the coupled-cluster and spin-flip structures, frequencies and normal modes of the anion and the diradical. Density functional calculations (B3LYP) yielded only a D_2h minimum, however, the quality of the structure and vibrational frequencies is poor, as follows from the comparison to high-level wave function calculations and the calculated spectrum. The analysis of charge localization patterns and the performance of different functionals revealed that B3LYP underestimates the magnitude of vibronic interactions due to self-interaction error. Contribution to the Mark S. Gordon 65th Birthday Festschrift Issue.     

Ab initio study on the structures, energies, and vibrational frequencies of acetone complexes with metal monocations and dications

The structures, interaction energies, and vibrational frequencies of acetone and its complexes with proton and various metal monocations/dications such as H⁺, Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Be²⁺, Mg²⁺, Ca²⁺, and Zn²⁺ have been investigated from the ab initio calculations. The linear or bent structure for the cation-acetone complexes has been found to be deeply influenced by the amount of charge transfer. The amount of red-shift of CO stretching frequencies of acetone is almost equivalent regardless of the kind of alkali metal monocations. This behavior can be attributed to the electrostatic nature of the interactions rather than orbital interactions. This phenomenon has been supported by the highest occupied molecular orbitals of the acetone complexes investigated, charge transfer based on natural bond orbital (NBO) atomic charges, and the inversely proportional behavior of the interaction energies to the interatomic distances r(M⁺O).     

Force field for computation of conformational energies,structures, and vibrational frequencies of aromatic polyesters

A CFF93¹ all-atom force field for aromatic polyesters based on ab initio calculations is reported. The force field parameters are derived by fitting to quantum mechanical data which include total energies, first and second derivatives of the total energies, and electrostatic potentials. The valence parameters and the ab initio electrostatic potential (ESP) derived charges are then scaled to correct the systematic errors originating from the truncation of the basis functions and the neglect of electron correlation in the HF/6-31G* calculations. Based on the force field, molecular mechanics calculations are performed for homologues of poly(p-hydroxybenzoic acid) (PHBA) and poly(ethylene terephthalate) (PET). The force field results are compared with available experimental data and the ab initio results. © 1994 by John Wiley & Sons, Inc.     

Molecular structure,vibrational frequencies and ionization potential of tin dihalides: An ab initio SCF and CI study

The geometrical parameters for SnX₂ (X = Cl, Br, I) and SnX₂⁺ have been optimized at the SCF and Cl levels using non-empirical pseudopotentials and a basis set of double-zeta plus polarization quality. For the neutral molecules the geometrical parameters are in agreement with the experimental values, the difference being of only 2%. Vibrational frequencies and ionization potentials are also in agreement with experiment.     

Solvent-dependent vibrational frequencies and reorganization energies of two merocyanine chromophores

Absorption and resonance Raman spectra have been measured over a wide range of solvents for two merocyanine dyes containing the indoline ("Fischer" base) electron donor group with different accepting groups. One appears to be near the cyanine limit (equal contributions of the neutral and zwitterionic resonance forms to both ground- and excited-state structures) based on electrooptic absorption data showing a very small dipole moment change upon electronic excitation. The resonance Raman spectra of both molecules show significant frequency shifts and intensity redistributions that evolve monotonically with increasing solvent polarity and are consistent with increasing zwitterionic character of the ground-state structure. The vibrational reorganization energies of both molecules, obtained by simulating the absorption band shapes, are smaller in polar solvents than in nonpolar or weakly polar ones, consistent with a more cyanine-like structure at higher solvent polarities. However, the vibrational reorganization energies of both molecules exceed 700 cm(-1) in all solvents, larger than in many true cyanine dyes, and the optical absorption maxima do not correlate well with either solvent polarity or vibrational reorganization energy. This indicates some limitations to the structural conclusions that can be reached from the two-state model for pi-conjugated donor-acceptor systems.     

Molecular orbital theory study on surface complex structures of phosphates to iron hydroxides: calculation of vibrational frequencies and adsorption energies

Quantum mechanical calculations were applied to resolve controversies about phosphate surface complexes on iron hydroxides. Six possible surface complexes were modeled: deprotonated, monoprotonated, and diprotonated versions of bridging bidentate and monodentate complexes. The calculated frequencies were compared to experimental IR frequency data (Persson et al. J. Colloid Interface Sci. 1996, 177, 263-275; Arai and Sparks J. Colloid Interface Sci. 2001, 241, 317-326.). This study suggests that the surface complexes change depending on pH. Four possible species are a diprotonated bidentate complex at pH 4-6, either a deprotonated bidentate or a monoprotonated monodentate complex at pH 7.5-7.9, and a deprotonated monodentate complex at pH 12.8. In addition, reaction energies were calculated for adsorption from aqueous solution to determine relative stability to form a monoprotonated monodentate complex and a deprotonated bidentate complex. According to these results, the monoprotonated monodentate complex should be favored. Vibrational frequencies of the monoprotonated monodentate and deprotonated bidentate complexes were analyzed with electronic effects on the Fe-OP and H-OP bonds.     

Scaling factors for vibrational frequencies and zero-point vibrational energies of some recently developed exchange-correlation functionals

The scaling factors for the vibrational frequencies and zero-point vibrational energies evaluated at various combinations of recently developed exchange-correlation functionals and various basis sets are reported. The exchange-correlation functionals considered are B972, B98, HCTH, OLYP, O3LYP, G96LYP, PBE0 and VSXC functionals; the basis sets employed are 3-21G, 6-31G*, 6-31G**, 6-31+G, 6-311G*, 6-311G**, 6-311G(df,p), 6-311+G(df,p), cc-pVDZ and aug-cc-pVDZ. The experimental harmonic frequencies of 122 small molecules and the zero-point vibrational energies of 39 small molecules are used to determine the scaling factors through the least-square fitting procedure. It was found that the scaling factors do not depend significantly on the basis sets considered. The vibrational frequency scaling factors evaluated by using the B98 and PBE0 functionals are recommended on the basis of smallest root mean square error. The zero-point vibrational energy scaling factors evaluated from the B972 functional with Pople's double-zeta basis set and the HCTH functional with Pople's triple-zeta basis set are recommended on the basis of smallest root mean square error.     

Theoretical study of energies,structures, and vibrational spectra of the carbonic acid–hydroperoxy radical complexes

In this work, we present the results for the first time of our study on hydrogen‐bonded H₂CO₃–HO₂ complexes (structures 1, 2) by means of ab initio molecular orbital theory. These complexes are important intermediates in the reaction of the hydroperoxy radical and cabonic acid. We calculated that these structures are a six‐membered ring. We found that the binding energy of two complexes are 5.8 and 9.3 kcal/mol using the CCSD(T) method. We also calculated the vibrational and rotational frequencies for these complexes. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Ab initio Hartree-Fock and density functional theory study on molecular structures, energies, and vibrational frequencies of conformations of 2-hydroxy-3-nitropyridine and 3-hydroxy-2-nitropyridine

The optimized molecular structures, vibrational frequencies and corresponding vibrational assignments of conformations of 2-hydroxy-3-nitropyridine and 3-hydroxy-2-nitropyridine molecules have been investigated using ab initio Hartree-Fock (HF) and density functional theory (B3LYP) methods with 6-311++G(d,p) basis set. The comparison of the experimental and calculated spectra of the molecules have shown that they exist in two conformations with the two OH bond angles (110 degrees and 250 degrees ) respective to the CO bond in the ground state and their energy curves having two minimums have been drawn.     

方酸构象和振动光谱的从头算研究

在6-31G^*^*水平上对方酸(3, 4-二羟基-3-环丁烯-1, 2-二酮)三种构象异构体进行了SCF计算。结果表明ZZ型异构体最稳定, ZE型次之。用等键反应能量分析方酸的稳定性, 并与苯作比较, 讨论方酸的芳香性。在6-31G水平上计算了方酸三种构象的振动频率。     

Mo studies of polymers. I. Use of MNDO to calculate geometries,vibrational frequencies and the electronic band structures of polymers; formalism and application to polyethylene

A general treatment of linear polymers is developed using MNDO and the tight binding approximation. Analytical expressions are derived for first derivatives of the energy. These are used to calculate geometries vibration frequencies and elastic moduli. Application of this treatment to polyethylene gave results in good agreement with experiment and also a reasonable account of its electronic band structure.     

Empirical equations for the bond energies and vibrational frequencies at chemisorptive bonds on surfaces

Empirical equations derived for bond energies and force constants of gaseous molecules are applied to chemisorptive bonds on surfaces. For two adsorbed atoms from the same family of the periodic table, A and B, the chemisorptive bond energies, E, to the same metal, M, can be approximated by E_A–M/E_B-M ≈ (E_A2/E_B2)^{$\text{1}\text{2}$}, where E_A2 and E_B2 are the bonds energies of diatomic molecules A₂ and B₂, respectively The corresponding vibrational frequencies, ν, can be approximated by ν²_A–M/ν²_B-M ≈ (m_B/m_A)(F_A2/F_B2)^{$\text{1}\text{2}$} · m_A and m_B are the masses of atoms A and B, respectively;F_A2 and F_B2 are the force constants of molecules A₂ and B₂, respectively. These relations are applied to the chemisorption of halogens on metals and showed good agreement with experiment.     

Structures,vibrational frequencies,topologies, and energies of hydrogen bonds in cysteine‐formaldehyde complexes

The hydrogen bonding interactions between cysteine (Cys) and formaldehyde (FA) were studied with density functional theory regarding their geometries, energies, vibrational frequencies, and topological features of the electron density. The quantum theory of atoms in molecules and natural bond orbital analyses were employed to elucidate the interaction characteristics in the Cys‐FA complexes. The intramolecular hydrogen bonds (H‐bonds) formed between the hydroxyl and the N atom of cysteine moiety in some Cys‐FA complexes were strengthened because of the cooperativity. Most of intermolecular H‐bonds involve the O atom of cysteine/FA moiety as proton acceptors, while the strongest H‐bond involves the O atom of FA moiety as proton acceptor, which indicates that FA would rather accept proton than providing one. The H‐bonds formed between the CH group of FA and the S atom of cysteine in some complexes are so weak that no hydrogen bonding interactions exist among them. In most of complexes, the orbital interaction of H‐bond is predominant during the formation of complex. The electron density (ρ_b) and its Laplace (?²ρ_b) at the bond critical point significantly correlate with the H‐bond parameter δR, while a linearly relationship between the second‐perturbation energy E(2) and ρ_b has been found as well. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Dissociation and multiple ionization energies for five polycyclic aromatic hydrocarbon molecules

We have performed density functional theory calculations for a range of neutral, singly, and multiply charged polycyclic aromatic hydrocarbons (PAHs), and their fragmentation products for H-, H(+)-, C(2)H(2)-, and C(2)H(2)(+)-emissions. The adiabatic and vertical ionization energies follow linear dependencies as functions of charge state for all five intact PAHs (naphthalene, biphenylene, anthracene, pyrene, and coronene). First estimates of the total ionization and fragmentation cross sections in ion-PAH collisions display markedly different size dependencies for pericondensed and catacondensed PAH species, reflecting differences in their first ionization energies. The dissociation energies show that the PAH(q+)-molecules are thermodynamically stable for q ≤?2 (naphthalene, biphenylene, and anthracene), q?≤?3 (pyrene), and q?≤?4 (coronene). PAHs in charge states above these limits may also survive experimental time scales due to the presence of reaction barriers as deduced from explorations of the potential energy surface regions for H(+)-emissions from all five PAHs and for C(2)H(2)(+)-emission from naphthalene--the smallest PAH.     

Coupled calculation of vibrational frequencies and intensities: Part IX. Intensities of methane,ethane, propane,and methionine calculated with the MNDO method

The absolute IR and Raman intensities of methane, ethane and propane are calculated using a combination of a normal coordinate analysis with the MNDO and CNDO/II methods. For the IR intensities the agreement between calculated and experimental observed intensities is better for MNDO than for CNDO/II. The results of both methods are similar for the Raman intensities. The change of the absolute intensities with the change of torsion angles can be used to treat conformational problems of the biomolecule methionine.     

Theoretical study of the vibrational frequencies of carbon disulfide

A benchmark comparison for different computational methods and basis sets has been presented. In this study, five computational methods (Hartree–Fock (HF), MP2, B3LYP, MPW1MP91, and PBE1PBE) along with 18 basis sets have been applied to optimize the geometry of carbon disulfide (CS₂), and further calculate the vibrational frequencies of the optimized geometries. The differences between the calculated frequencies and corresponding experimental data are used to evaluate the efficiency of each combination of computational method and basis set. The comparison of frequency difference indicates that B3LYP generally gives the best prediction of frequencies for CS₂, whereas the other two density functional theory (DFT) methods, i.e., MPW1PW91 and PBE1PBE, often give parallel results. Although MP2 predicts the frequencies with accuracy almost as good as those from DFT methods, in a particular case, HF calculation outperforms MP2 as well as MPW1PW91 and PBE1PBE for prediction of the frequency of asymmetrical stretching for CS₂. © 2013 Wiley Periodicals, Inc.     

An ab initio study on the geometries and vibrational frequencies of hydrazoic acid and methyl azide

The geometric parameters for hydrazoic acid and methyl azide were optimized at the HF/6-31G^** and MP2/6-31G^** levels and the vibrational frequencies of the compounds were calculated by use of these optimized geometries. The experimental frequencies are assigned on the basis of the calculated results. The effects of deutero-substitution and substitution of hydrogen in HN₃ by a methyl group are also discussed.     

An evaluation of SAM1 calculated vibrational frequencies

SAM1 and AM1 calculations were carried out on 41 molecules for the purpose of comparing the ability of these semiempirical methods to compute vibrational frequencies. For 442 vibrational modes, the results show that SAM1 reproduces all frequencies with an unsigned average error of 9.0% compared with 9.6% for AM1. The average signed error for SAM1 is less than 1%, while that for AM1 is about 5%. Except in the case of non-H atom stretching modes, the signed errors were near zero, indicating that no special scaling adjustment is needed. However, non-H atom stretching frequencies have highly systematic errors for both methods. The average signed error for SAM1 is + 14% which is quite close to the unsigned error at 16%. In such an obvious case, a systematic correction can be applied with some confidence. It can be said that the SAM1 non-H stretching frequencies are predicted to be too high by 14%, and should thus be scaled by a multiplicative factor of 0.86. A similar multiplicative correction of 0.15 can be applied in the case of AM1 to vibrations of this type.