Application of the configuration-interaction method and the random phase approximation to the Ab Initio calculation of electronic excitation energies of H2o

By performing calculations on H₂ O similar to the calculations of Dunning and McKoy on C₂H₄ and HCHO [1a], it is shown that the singlet excited states of the water molecule cannot be adequately represented by an ab initio calculation (no semiempirical elements) using a valence-like basis set (either minimum basis or double zeta) of Slater-type orbitals. The triplets which are the lowest states of their symmetry appear to be described more accurately than the other states, and the lowest four triplet excitation energies are calculated to be 8.1, 8.4, 9.5, and 10.3 eV. The implications for the applicability of simple molecular orbital theory of the type commonly applied are discussed.     

Electronic structure and the hydrogen-shift isomerization of hydrogen nitryl HNO2

Summary Electronic structure of hydrogen nitryl HNO₂, a yet not identified entity, and the path of its possible isomerization totrans-HONO have been investigated byab initio SCF and MRD-CI computations using the 6-31G** basis set. HNO₂ isC _2v -symmetric and its ground state (¹ A ₁) is less stable thantrans-HONO by 66 kJ/mol (with the SCF vibrational zero-point energy correction). The lowest two excited singlet states (¹ A ₂ and¹ B ₁) are nearly degenerate, their vertical excitation energies being predicted to be 4.8 eV. The isomerization path is traced by the CASSCF procedure and the activation barrier height is evaluated by the CI treatment. HNO₂ in its ground state isomerizes totrans-HONO by maintaining the planar (C _s-symmetric) structure. The activation energy is calculated to be 171 kJ/mol, which is clearly lower than the calculated H-N bond energy (253 kJ/mol). The transition state seems to be more adequately described as an interacting system of proton and the nitrite anion rather than as a pair of two fragment radicals.     

Semiempirical AM1 electrostatic potentials and AM1 electrostatic potential derived charges: A comparison with ab initio values

Electrostatic potentials calculated from AM1 wave functions have been compared with ab initio STO-3G values and qualitative agreement has been found. Atomic charges derived from AM1 electrostatic potentials for both experimental and AM1 optimized geometries are of comparable quality with STO-3G potential derived charges. These results suggest that the AM1 electrostatic potential may be useful both in its own right and also for deriving atomic charges for use in molecular dynamics studies.     

A charge-conserving approximation method for ab initio calculations

A new method is presented for approximate ab initio calculations in quantum chemistry. It is called CCAM (charge conserving approximation method). The calculation method does not include the use of empirical parameters. We use Slater type orbitals as basis set, replacing STO's by STO-2G functions to evaluate three- and four-center integrals and making the STO-2G two-orbital charge distributions have the same total charge as STO. The results are presented for test calculations on five molecules. In view of these results, CCAM is better than ab initio calculations over STO-6G in the results on total energies, kinetic energies and occupied orbital energies. In atomic populations, dipole moments and unoccupied orbital energies, CCAM is also satisfactory. We estimate that CCAM would be as fast as ab initio calculations over STO-2G in evaluating molecular integrals.     

Geometry optimization and electronic structures of molecules H_3AXAH_3

The geometries of molecules H_3AXAH_3(X=O,S,Se and A=C,Si)have been optimizedusing STO-3G ab initio calculations and gradient method and the results are in good agreement withreported experimental values.From the STO-3G optimized geometries,we have also calculated theelectronic structures of these molecules using 4-31G and 6-31G basis sets to obtain the MO energies.atomic net charges and dipole moments.The ionization potentials calculated by 6-31G basis set are ingood agreement with experimental values.     

Study of hydrogen bonding interactions relevant to biomolecular structure and function

Ab initio molecular orbital calculations were used to study hydrogen bonding interactions and interatomic distances of a number of hydrogen bonded complexes that are germane to biomolecular structure and function. The calculations were carried out at the STO-3G, 3-21G, 6-31G*, and MP2/6-31G* levels (geometries were fully optimized at each level). For anionic species, 6-31 + G* and MP2/6-31 + G* were also used. In some cases, more sophisticated calculations were also carried out. Whenever possible, the corresponding enthalpy, entropy, and free energy of complexation were calculated. The agreement with the limited quantity of experimental data is good. For comparison, we also carried out semiempirical molecular orbital calculations. In general, AM1 and PM3 give lower interaction enthalpies than the best ab initio results. With regard to structural results, AM1 tends to favor bifurcated structures for O? H-O and N? HO types of hydrogen bonds, but not for hydrogen bonds involving O-H? S and S-H? O, where the usual hydrogen bond patterns are observed. Overall, AM1 geometries are in general in poor agreement with ab initio structural results. On the other hand, PM3 gives geometries similar to the ab initio ones. Hence, from the structural point of view PM3 does show some improvement over AM1. Finally, insights into the formation of cyclic or open formate–water hydrogen bonded complexes are presented. © 1992 by John Wiley & Sons, Inc.     

A theoretical study of the rotational isomers of nitrosomethanol by semiempirical (AM1) and ab initio methods

The conformational potential energy surface as a function of the two internal torsion angles in C-nitrosomethanol has been obtained using the semiempirical AM1 method. Optimized geometries are reported for the local minima on this surface and also for the corresponding points on the HF/6-31G, 6-31G*, and 6-31G** surfaces. All methods predict cis and trans minima which occur in degenerate pairs, each pair being connected by a transition state of C_s symmetry. The AM1 structures are found to compare well with the corresponding ab initio structures. Ab initio HF/6-31G and HF/6-31G* harmonic vibrational frequencies are reported for the cis and trans forms of nitrosomethanol. When scaled appropriately the calculated frequencies are found to compare well with experimental frequencies. The ab initio calculations predict the energy barrier for cis → trans isomerization to be between 5.8 and 6.5 kcal/mol with the trans → cis isomerization barrier lying between 2.3 and 6.5 kcal/mol. The corresponding AM1 energy barriers are around 1 kcal/mol lower in energy. The ab initio calculations predict the barrier to conversion between the two cis rotamers to be very small with the AM1 value being around 1 kcal/mol. Both AM1 and ab initio calculations predict interconversion between trans rotamers to require between 1.2 and 1.4 kcal/mol.     

A theoretical study on the structure and properties of phenothiazine derivatives and their radical cations

Semiempirical CNDO, AM1, PM3 and ab initio HF/STO-3G, HF/3-21G(d), and HF/6-31(d) methods were employed in the geometry optimization of the phenothiazine and the corresponding radical cation. The results obtained from the PM3 performances were as good as those from the ab initio calculations in the structure optimization of both phenothiazine and phenothiazine radical cation. The PM3 method was used to optimize the structures of a series of N-substituted phenothiazine derivatives and their radical cations. The PM3-optimized results were then analyzed with the ab initio calculation at the 6-311G(d,p) level, which yielded the total energy, frontier molecular orbitals, dipole moments, and charge and spin density distributions of the phenothiazine derivatives and their radical cations.     

Optimized Structures and Relative Stabilities of the Carboranes from Ab Initio Calculations

Ab initio calculations at the STO-3G level were performed on almost all of the possible isomers for the entire series of closo-carboranes, C₂B_n-2H_n, 5 ? n ? 12. Geometry optimizations using the gradient method were also included in all calculations. We report here the relative energies obtained for the various isomers as well as the optimized structures. These calculations confirm our previous predictions of relative stabilities obtained from topological charge stabilization. Comparisons of our structures with those from experimental data provide us with a measure of reliability for bond distances obtained using ab initio SCF MO calculations at the STO-3G level. Results from the geometry optimization substantiated the experimentally known fluxional behavior of the 8 and 11 atom polyhedra.     

Electronic spectrum and photodissociation of ClONO in comparison to BrONO

A theoretical study of the low-lying singlet and triplet states of ClONO is presented. Calculations of excitation energies and oscillator strengths are reported using multireference configuration interaction, MRD-CI, methods with the cc-pVDZ + sp basis set. The calculations predict the dominant transition, 4(1)A' <-- 1(1)A', at 5.70 eV. The transition 2(1)A' <-- 1(1)A', at 4.44 eV, with much lower intensity nicely matches the experimental absorption maximum observed around 290 nm (4.27 eV). The potential energy curves for both states are found to be highly repulsive along the Cl-O coordinate implying that direct and fast dissociation to the Cl + NO2 products will occur. Photodissociation along the N-O coordinate is less likely because of barriers on the order of 0.3 eV for low-lying excited states. A comparison between the calculated electronic energies related to the two dominant excited states of ClONO and BrONO indicates that the transitions lie about 0.6 eV higher if bromine is replaced by chlorine. The stratospheric chemistry implications of ClONO and BrONO are discussed.     

Comparison of AM1 and ab initio calculation of the carbon-carbon bond rotation in ethylene glycol diacetate

The C-C glycol bond rotational energy in ethylene diacetate as a polyester model was compared using the semiempirical method AM1 and an ab initio method with an STO-3G basis set. The results were qualitatively much different depending on the method used. Ab initio calculations showed the expected minima at 180 and near 60 (69.6) degrees dihedral angle with maxima at 0 and 120 degrees. The AM1 rotational curve indicated an apparent minimum at a 90 degree dihedral angle, a shallow, apparent maximum at 180 degrees and an apparent maximum at 0 degrees which could not be confirmed as minima or maxima via frequency calculations. Ethylene diacetate analog compounds with one or two ester oxygens replacing methylene group(s) gave curves with AM1 having the general shape for ethylene diacetate by the ab initio method, indicating a parameterization problem for the otherwise very useful AM1 to correctly handle a compound with only two carbons between the two electronegative oxygen atoms thus rendering this method currently unsuitable for examination of rotational energy barriers of such polyester model compounds.     

Molecular ion LiHe: ab initio study

High level ab initio calculations are performed on the molecular ion LiHe⁺. Potential energy curves for the low-lying singlet and triplet electronic states are calculated using the multi-reference configuration interaction and single-reference coupled cluster methods with large basis sets. The corresponding dipole moments and transition dipole moments functions are also determined. The basic spectroscopic properties and excitation energies of the electronic states are derived from rovibrational bound state calculations.     

Theoretical studies on ammonia oxide and its unimolecular reactions

The unimolecular reactions of ammonia oxide H₃NO, isomerization and dehydrogenation, are investigated by ab initio MO calculations with the 4-31G basis set. The geometries and energies of the reactant, transition states and products have been determined on the singlet potential energy surface. The reaction ergodography along the intrinsic reaction coordinate (IRC) for the two reactions have been performed. The vibrational frequency correlation diagram of the two reactions are analyzed along the IRC.     

Electron donor-acceptor complexes: Evaluation of MNDO as a computational tool to probe intermolecular interactions

Donor-acceptor pairs form EDA complexes that exist as conformational isomers exhibiting different ground-state and photochemical properties. We have sought a rapid, general, and accurate quantum mechanical computational method to generate potential energy surfaces that are representative of the donor-acceptor intermolecular interactions at the self-consistent field (SCF) level. The semiempirical molecular orbital (MO) method MNDO has been compared to ab initio methods to assess its behavior with respect to energy, dipole moment and ionization potential shifts. MNDO correctly distinguishes between repulsive and bound EDA complex states at the SCF level and produces potential curves that are smooth and free of spurious minima or cusps. MNDO curves are systematically more repulsive than those for ab initio STO-3G calculations; calculated interaction energies exhibit a mean absolute deviation of 2.90 kcal/mol. MNDO appears to provide a reliable qualitative estimate of the nondispersion portion of the interaction energy. Limitations and errors arising from minimal basis sets, single determinants, and neglect of dispersion are discussed.     

DFT/TDDFT Studies on the Electronic Structures and Spectral Properties of Carbazole‐based Blue Light‐emitting Dendrimers

The purpose of this paper is to provide an in‐depth investigation of the electronic and optical properties of two series of carbazole‐based blue light‐emitting dendrimers, including 1 – 6 six oligomers. These materials show great potential for application in organic light‐emitting diodes as efficient blue‐light and red‐light emitting materials due to the tuning of the optical and electronic properties by the use of different electron donors (D) and electron acceptors (A). The geometric and electronic structures of these compounds in the ground state are calculated using density functional theory (DFT) and the ab initio HF, whereas the lowest singlet excited states were optimized by ab initio single excitation configuration interaction (CIS). All DFT calculations are performed using the B3LYP functional on 6‐31G* basis set. The outcomes show that the highest occupied molecular orbitals (HOMOs), lowest occupied molecular orbitals (LUMOs), energies gaps, ionization potentials, electron affinities and reorganization energies of each molecular are affected by different D and A moieties and different substitute positions.     

Assignment of electronic transitions by geometry optimization

Adiabatic excitation energies, excited state geometries, excited state charges, bond orders and dipole moments have been obtained for HCN, CO₂,H₂CO, HFCO, F₂CO, ethylene, trans-butadiene, furan, pyrrole and uracil using the SINDO1 semi-empirical method with configuration interaction. Our results generally agree with those ofab initio calculations and experiment satisfactorily. Geometry optimization is found to mix configurations differing in their allowedness in vertical excitation from the ground state, which in turn helps in the assignment of spectral transitions. TheV excited singlet state of trans-butadiene and various excited states of furan, pyrrole and uracil have been found to be appreciably non-planar. The single and double CC bonds are found to exchange positions due to the lowest triplet and singlet transitions of furan and pyrrole. The first triplet and first singlet transitions of uracil have been found to be of π-π* and π-σ* types respectively in agreement with recent experimental findings. On leave of absence from the Department of Physics, Banaras Hindu University, Varanasi-221005, India     

Spiro[4.4]nonatetraene and its Positive and Negative Radical Ions: Molectronic Structure Investigations

The electronic structure of spiro[4.4]nonatetraene 1 as well as that of its radical anion and cation were studied by different spectroscopies. The electron‐energy‐loss spectrum in the gas phase revealed the lowest triplet state at 2.98 eV and a group of three overlapping triplet states in the 4.5 – 5.0 eV range, as well as a number of valence and Rydberg singlet excited states. Electron‐impact excitation functions of pure vibrational and triplet states identified various states of the negative ion, in particular the ground state with an attachment energy of 0.8 eV, an excited state corresponding to a temporary electron attachment to the 2b₁ MO at an attachment energy of 2.7 eV, and a core excited state at 4.0 eV. Electronic‐absorption spectroscopy in cryogenic matrices revealed several states of the positive ion, in particular a richly structured first band at 1.27 eV, and the first electronic transition of the radical anion. Vibrations of the ground state of the cation were probed by IR spectroscopy in a cryogenic matrix. The results are discussed on the basis of density‐functional and CASSCF/CASPT2 quantum‐chemical calculations. In their various forms, the calculations successfully rationalized the triplet and the singlet (valence and Rydberg) excitation energies of the neutral molecule, the excitation energies of the radical cation, its IR spectrum, the vibrations excited in the first electronic absorption band, and the energies of the ground and the first excited states of the anion. The difference of the anion excitation energies in the gas and condensed phases was rationalized by a calculation of the Jahn‐Teller distortion of the anion ground state. Contrary to expectations based on a single‐configuration model for the electronic states of 1 , it is found that the gap between the first two excited states is different in the singlet and the triplet manifold. This finding can be traced to the different importance of configuration interaction in the two multiplicity manifolds.     

Simulations of internal rotation potential energies for substituted ethanes

Two approaches to the simulation of internal rotation potential energies in substituted ethanes are formulated for general applications. Called the vicinal Fourier coefficient and vicinal pair energy methods, they differ only in form. The latter procedure has the advantage of yielding energy terms that represent pairwise interactions between vicinal substitutents. As numerical examples, the potential energies of ethane and five of its simple methyl and chloro derivatives are employed to simulate the corresponding energies of two higher derivatives of the series. The initial energy data were calculated by the molecular mechanics method (MM2) with geometry optimizations and the ab initio MO procedure (STO-3G) with standard geometries. Results indicate that simulated energies are reasonably accurate for the flexible-rotor model (MM2) and extremely accurate for the rigid-rotor model (STO-3G). Deviations appear to be systematic and may be rationalized on the basis of molecular structure.     

Forward Electron Scattering in Benzene; Forbidden Transitions and Excitation Functions

A systematic study of vibronic excitation in benzene via forward electron scattering was carried out using a novel type of a trochoidal electron spectrometer. Energy-loss spectra in the energy range 1.0–9.5 eV, with residual energies 0.03–20 eV as well as excitation functions for individual vibrational levels of some of the triplet and singlet states are presented and discussed. Following observations were made. (1) A new s-type Rydberg series with quantum defect δ = 0.86. (2) Additional information on the complex 6–6.5 eV band. (3) A new core excited shape resonance at 6.5 eV. (4) A narrow Feshbach resonance at 5.87 eV, The new spectrometer is suggested as a tool for routine study of forbidden transitions and negative ion states in organic molecules.