Spin-Orbit coupling in aromatic hydrocarbons: A semiempirical evaluation of the radiative lifetimes of naphtalene and anthracene

Singlet-triplet spin-orbit matrix elements, which govern the lowest ³B_1u ← ¹A_g transition in typical aromatic molecules like naphtalene and anthracene, are calculated with INDO molecular orbitals and the conventional spin-orbit one-electron hamiltonian. The correct order of magnitude of the triplet radiative lifetimes is obtained for the two molecules, when INDO MO coefficients are referred to a symmetrically orthogonalized basis. The possibility of using the semiempirical Hamiltonian is explored using ab initio wave-functions for a few test cases. Reasonably accurate doublet-doublet and singlet-triplet matrix elements have been computed.     

Empirical force-field and ab initio calculations on delocalized open chain cations

Force-field calculations are reported for large delocalized cations. The results for the geometries, heats of formation, and π-electron densities agree well with MP2(full)/6–31G^* ab initio calculations. Both methods give similar results for the distortion of the carbon skeletons of unsubstituted cations by hyperconjugating methyl groups. Because of the SCF treatment of π-interactions, the MMP2 force-field technique enables calculations of resonance energies in delocalized cations. The additional resonance stabilization produced by extending conjugation is directly related to the π-charge on the carbon at which a vinyl group is substituted. The good agreement of MMP2 results for nonbonded resonance effects in large delocalized cationic π-systems with ab initio data suggests that MMP2 can be used to study the influence of these interactions in cationic π-systems too large to be calculated by correlated ab initio methods. © 1997 by John Wiley & Sons, Inc.     

Inclusion of hydrogen p orbitals in the semiempirical calculation of NMR parameters. I. Influence on the FPT INDO spin–spin coupling constants

Hydrogen 2p orbitals have been introduced into the basis set to calculate the Fermi contact term of spin–spin coupling constants using the FPT INDO method. Different coupling constants show different sensitivity to these hydrogen polarization functions. Some improvements are found for molecules containing N or F. Calculations of proton-proton geminal coupling constants give more negative results than those of FPT INDO , yielding a better agreement with experimental values. The π-transmission mechanism is notably exaggerated.     

Theoretical studies on the diazacyclopentadienylidenes: An analysis of aromatic versus nonaromatic systems and singlet versus triplet reactivity

Ab initio molecular orbital calculations have been carried out on the various electronic states of 2,3- ( 6 ), 2,4- ( 7 ), 2,5- ( 8 ), and 3,4-diazacyclopentadienylidene ( 9 ) at the fully geometry optimized 6–31G* level, with single point calculations being carried out at the MP2/6–31G* level. The calculated geometries are interpreted in terms of the degree of occupancy and the nature of the π and σ-nonbonded MO's. At the 6–31G* level the five π-electron, π,σ-triplet states were calculated to be considerably lower in energy. At the MP2/6–31G* level, however, with 7 the six π-electron singlet state is calculated to lie only slightly above the five π-electron triplet (0.4 kcal mole⁻¹), whereas with 8 and 9 the aromatic six π-electron singlet states are calculated to be lower in energy (9.0 and 8.1 kcal mole⁻¹). With 3 and 9 the aromatic six π-electron σ-triplet states lie only 3.6 and 5.5 kcal mole⁻¹ above the lowest energy states. It is concluded that, in general, the energy gained by having an electron in a lower energy σ-type MO instead of a higher energy π MO effectively offsets the energy gained by having an aromatic π system. The results are discussed in terms of the observed chemistry of 6–9 and their substituted systems.     

The thioketone–enethiol tautomerism of aliphatic thiocarbonyls: An ab initio study

Approximate minimum energy reaction paths have been calculated for two thioketone–enethiol tautomeric systems using an ab initio SCF–MO method. The calculations indicate nearly equal stabil ties of the isolated tautomers in both systems and an energy barrier of ca. 85 kcal/mol for their interconversion. This barrier is expected to be significantly lower in solution as a result of solvent–solute interactions.     

Deorthogonalization of atomic originals in the CNDO approach

The procedure for deorthogonalization (D) of atomic orbitals in the semiempirical CNDO approach is reviewed. For comparative studies, CNDO/2, CNDO/2D, and STO -3G calculations of molecular dipole moments and Mulliken populations are carried out on 35 prototype molecules containing H, C, N, O, and F atoms. The calculated values are assessed on the basis of how well they agree with experimental trends, chemical bonding theories, and ab initio molecular orbital (MO) values. Results of analyses indicate that the CNDO/2D values for dipole moments are in reasonable agreement with experimental values, and those for net atomic charges and electron populations bear greater resemblance to the ab initio (STO -3G and 6-31G**) values than the original CNDO/2 values. These findings, together with those of previous investigators, demonstrate unequivocally the advantages of incorporating deorthogonalization into routine CNDO/2 or INDO calculations as a means to obtain reasonable estimates of charge distributions.     

A simplex optimized INDO calculation of 13C chemical shifts in hydrocarbons

The variable-size simplex optimization method is used to reparametrize the I + A and β parameters of an INDO approximation to the perturbed Hartree–Fock calculation of ¹³C chemical shifts in hydrocarbons. The absolute shifts for 39 nuclei in a set of molecules containing up to four carbons are reproduced within a standard error of 9.9 ppm for an unconstrained optimization and to a standard error of 10.0 ppm for an optimization constrained to yield gross atomic charges in agreement with double-zeta ab initio calculations.     

The value of the π-bond order–bond length relationship in geometry prediction and chemical bonding interpretation

The π-bond order–bond length relationship is reintroduced to the literature and extended to heteronuclear bonds by presenting graphs derived solely by theoretical methods. π-bond order and overlap population results for carbon–carbon, carbon–nitrogen, and carbon–oxygen bonds obtained from ab initio STO -3G calculations using theoretically-optimized geometries are reported for a series of pteridines and for a wide range of small organic molecules. The order–length correlation graphs are used in predicting the “intrinsic” single bond lengths for sp² – sp² and sp – sp hybridized C? C, C? N, and C? O bonds, and in evaluating the relative importance of hybridization, π-electron delocalization and bond polarization effects in causing bond shortening in conjugated and hyperconjugated molecules. The calculated value of the π-bond order for a given bond in a molecule is shown to be relatively insensitive to moderate geometry changes: Hence, a use for the correlation graphs in geometry prediction is suggested. Some results for the extended 4-21G basis set are also presented.     

Localized molecular orbitals and chemical binding in five-membered heterocycles. II

The localized molecular orbitals (LMOs) of thiophene, furan, and pyrrole are derived from ab initio 4-31G wavefunctions using Boys' criteria for localization. From the transferability point of view, these LMOs are classified as (i) completely different and nontransferable LMOs (these are the lone-pair orbitals on O and N on one hand and those on S on the other hand), (ii) chemically similar lone pairs and inner shells on O and N (of furan and pyrrole, respectively), and (iii) chemically equivalent C? C, C?C, and C? H LMOs in the three heterocycles. The sp³ hybridization of the L core of sulfur, its appreciable polarization, and considerable involvement in bonding in the C? S bond region have been discussed. The present investigation indicates the limitation of the application of semiempirical MO methods to molecules that contain second-row atoms due to both the appreciable core—valence and π-σ interactions involving such atoms. Qualitative investigation of aromaticity and reactivity of the studied heterocycles agrees satisfactorily with experimental observations and shows that conclusions drawn based solely on static factor considerations (charge distribution in the noninteracting molecules) might very well be misleading and such factors determine the ease rather than the final orientation of the substituent.     

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.     

A comparative study of the structure and bonding of HOO,HOS, HSO,and HSS radicals by CNDO/2 and INDO methods

Equilibrium geometries, force constants, barriers to linearity, charge distributions, dipole moments, and electron spin density of HOO, HOS, HSO, and HSS radicals are calculated by CNDO/2 and INDO methods using respectively the original and some recently introduced scheme of parametrization. Three sets of calculations, namely, CNDO/2(sp), CNDO/2(spd), and INDO, are performed, and the results are compared with the ab initio and experimental values, wherever available. A good agreement is obtained for geometry in the case of CNDO/2 (sp) and INDO calculations. The performance of CNDO/2 (spd) calculations in this regard is quite unreliable. The stretching force constants are considerably overestimated by all the methods, while the bending force constants are in reasonable agreement with the ab initio values. With respect to dipole moments, the CNDO/2 values are in better agreement with the ab initio results than the INDO values. In all the cases, the dipole moment vector directions are in complete disagreement with the ab initio predictions.     

An optimum strategy for solution chemistry using semiempirical molecular orbital method: Importance of description of charge distribution

For the purpose to execute direct dynamics calculation in solution chemistry, we propose an optimum strategy for solution chemistry using semiempirical molecular orbital (MO) method with neglect of diatomic differential overlap (NDDO) approximation with specific solution reaction parameters (SSRP), i.e., the NDDO‐SSRP method. In this strategy, the empirical parameters of the semi‐empirical MO method were optimized individually for target molecule or ion by reference to the ab initio MO calculation data for many configurations on the potential energy surface near the reaction path. For demonstration, the NDDO‐SSRP method was applied to two molecules and two ions (OH^?, H₂O, NH₃, NH₄⁺) at their equilibrium states in aqueous solution, respectively. Accordingly, it was verified that both the potential energy surface and the charge distribution of these solutes in aqueous solution are dramatically improved to reproduce themselves accurately at ab initio MO calculation level. In conclusion, it is expected that the NDDO‐SSRP method should become quite useful for dynamic and statistical applications to chemical reaction systems in solution. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

Photophysics,Excited‐state Double‐Proton Transfer and Hydrogen‐bonding Properties of 5‐Deazaalloxazines

The photophysical properties of 5‐deazaalloxazine and 1,3‐dimethyl‐5‐deazaalloxazine were studied in different solvents. These compounds have higher values of fluorescence quantum yields and longer fluorescence lifetimes, compared to those obtained for their alloxazine analogs. Electronic structure and S₀–S_i transitions were investigated using the ab initio methods [MP2, CIS(D), EOM‐CCSD] with the correlation‐consistent basis sets. Also the time‐dependent density functional theory (TD‐DFT) has been employed. The lowest singlet excited states of 5‐deazaalloxazine and 1,3‐dimethyl‐5‐deazaalloxazine are predicted to have the π, π* character, whereas similar alloxazines have two close‐lying π, π* and n, π* transitions. Experimental steady‐state and time‐resolved spectral studies indicate formation of an isoalloxazinic excited state via excited‐state double‐proton transfer (ESDPT) catalyzed by an acetic acid molecule that forms a hydrogen bond complex with the 5‐deazaalloxazine molecule. Solvatochromism of both 5‐deazaalloxazine and its 1,3‐dimethyl substituted derivative was analyzed using the Kamlet–Taft scale and four‐parameter Catalán solvent scale. The most significant result of our studies is that the both scales show a strong influence of solvent acidity (hydrogen bond donating ability) on the emission properties of these compounds, indicating the importance of intermolecular solute–solvent hydrogen‐bonding interactions in their excited state.     

Ab initio MO Calculations of benzene + TCNE and naphthalene + TCNE complexes with STO-3G π-split basis set

Ab initio SCF MO calculations have been carried out on benzene + TCNE (tetracyanoethylene) and naphthalene + TCNE complexes with the STO -3G, STO -3G π-split (STO -3G for π orbitals and a split basis for π orbitals), and 4–31G basis sets. The interaction energy, gross charges, dipole moment, and the electron density in the middle plane of the complexes have also been evaluated. The STO -3G π-split basis set is appropriate for the calculation of large π–π stacking complexes from two points of view, production of reliable results and ease of computations. The approximation scheme based on the semiorthogonalized orgitals is revealed to be very efficient to save CPU time and storage in such calculations. The stable conformation and the charge-transfer interaction of the two complexes are discussed on the basis of the calculated quantities.     

A CNDO/INDO molecular orbital formalism for the elements H to Br. applications

The CNDO/INDO molecular orbital formalism introduced in the preceding paper has been applied to a large number of atom combinations up to bromine under the inclusion of the first transition metal series. The results are compared with experimental data (geometries, ionization potentials, dipole moments) or with the results of sophisticatedab initio calculations (one electron energies, net charges, atomic populations). The semiempirical model reproduces for a wide range of molecules the experimental andab initio data with remarkable success.     

Interbenzylic 19F,19F spin-spin coupling constants in bis(fluoromethyl) arenes

Five- to eleven-bond F,F spin coupling is observed in bis(fluoromethyl)arenes. According to INDO MO calculations, these couplings are transmitted mainly via the aromatic π-system. The low preference of 1,4-(CH₂F)₂-benzene for the conformation in which the C? F bonds are perpendicular to the aromatic ring is shown by the strong increase of⁷J(FF) upon double ortho-substitution.     

A quantum-mechanical study of the interaction of glyoxal with guanine

A quantum molecular study by the SCFab initio method of the interaction of glyoxal with guanine provides for the formation of a stable covalent adduct in which the glyoxal fragment forms a complementary cyclic ring attached to the imino N₁ and amino N₂ atoms of guanine with the concomitant migration of the N-bonded H atoms to the oxygens of glyoxal. The reaction should proceed in two steps. The most plausible mechanism involves as the first step the interaction of a carbonyl group of glyoxal with the amino group of guanine followed by a similar interaction at the imino group of guanine, rather than the reverse order of interactions. The respective energy barriers are 49.7 and 63.9 kcal/mole. The intermediate product is also more stable when the adduct occurs first at N₂:30.7 kcal/mole versus 17.9 kcal/mole for the adduct at N₁.     

Localized molecular orbital studies of certain compounds with [A_3X_3] six-membered rings

The localized molecular orbitals (LMOs) of certain quasi-aromatic organic and inor-ganic molecules with six-membered rings have been calculated by virtue of the ab initio methodusing STO-3G and 4-31G basis sets as well as the CNDO/2 method. It is shown that there existsextensively-delocalized p-pπ bonding in these quasi-aromatic systems. The localized pictures ofthe π-type LMOs for the heterocyclic and homocyclic systems from the σ-π localization schemeare discussed. The Generator Orbital approach is utilized to account for the bonding patterns.     

Reactivity of molecular oxygen on the surface of ionic crystals

A possibility of multiplicity change for ground‐state molecular oxygen adsorbed on the surface of regular and doped broad‐gap ionic crystals was considered in the framework of cluster approximation by using SCF MO LCAO quantum chemical methods [semiempirical INDO approximation and ab initio calculation with the 6‐311G** basis set taking into account the correlation effects on the level of second‐order Meller–Plesset perturbation theory (MP2)]. The formation energetics of cyclic products of addition reactions of dioxygen in different multiplet states to furan and cis‐butadiene in the gas phase and on the surface of ionic crystals was considered. (These reactions are typical for the O₂ singlet state in the gas phase.) It is shown that the presence of sites with high effective charge on the crystal surface can result in a situation not requiring, as in the gas phase, multiplicity change in the transition of a system from an initial to the final state, which can significantly affect the kinetic parameters of the reactions. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002