Ab initio calculations of vibronic activity in phosphorescence microwave double resonance spectra of p-dichlorobenzene

The phosphorescence spectrum of p-dichlorobenzene has been calculated using multiconfiguration self-consistent-field wave functions and the quadratic response technique. Attention has been paid to the intensity distribution of the singlet–triplet (³B_1u¹A_g) transition through a number of vibronic subbands. The second order spin–orbit coupling (SOC) contribution to the spin splitting of the ³B_1u (^3*) state is found to be almost negligible, and the calculations therefore provide a good estimate for the zero-field splitting (ZFS) parameters based only on the electron spin–spin coupling expectation values. Nuclear quadrupole resonance constants for the different Cl isotopes are also calculated to accomplish the ZFS assignment. The electric dipole activity of the spin sublevels in the triplet–singlet transitions to the ground-state vibrational levels is estimated by calculations of derivatives using distorted geometries which are shifted from the equilibrium position along different vibrational modes. A vibrational analysis of the phosphorescence spectrum, based on the SOC-induced mixing of the singlet and triplet states calculated along different vibrational modes, provides reasonable agreement with experimental data.Acknowledgment O. R.-P. would like to thank the European MOLPROP network for support. The authors thank Alexander Baev for fruitful discussions. This work was supported by the Swedish Royal Academy of Science (KVA).     

Quantum chemical calculation of force constants and vibrational spectra

As a result of the development of direct derivative methods and improved computational facilities, ab initio quantum chemical calculations have become an increasingly important source of information for the determination of molecular force constants. Within the Hartree-Fock (H-F) SCF model and using moderate size basis sets such calculations are now economically feasible for molecules of up to 2o–3o atoms. At this level of theory, harmonic diagonal force constants are overestimated by 1o–3o%, corresponding to 5–15% in the frequencies. However, the largely systematic errors can be accounted for by simple empirical corrections. The resulting SQM (Scaled Quantum Mechanical) force fields are probably the most reliable ones available at present for larger molecules. Calculated infrared intensities are semi-quantitatively correct. Beyond the H-F model, large scale calculations including electron correlation give great improvements in the force constants, but there are still residual errors of a few percent.     

Selective excitation effects in microwave optical double resonance and phosphorescence spectroscopy of molecular crystals

The photoexcitation routes used to produce molecular crystal, triplet states are shown to have important optical and microwave spectral consequences. 2-benzoylpyridine crystals at 4.2 K have T₁ → S₀ phosphorescence spectra showing line width dependence on whether initial production of the T₁ state is through direct T₁ → S₀ absorption, or through S₁ ← S₀ absorption followed by S₁ → T₁ intersystem crossing. Striking differences are seen in the optically detected zero-field resonance spectra.     

Quantum chemical calculation of vibrational spectra of large molecules--Raman and IR spectra for Buckminsterfullerene

In this work we demonstrate how different modern quantum chemical methods can be efficiently combined and applied for the calculation of the vibrational modes and spectra of large molecules. We are aiming at harmonic force fields, and infrared as well as Raman intensities within the double harmonic approximation, because consideration of higher order terms is only feasible for small molecules. In particular, density functional methods have evolved to a powerful quantum chemical tool for the determination of the electronic structure of molecules in the last decade. Underlying theoretical concepts for the calculation of intensities are reviewed, emphasizing necessary approximations and formal aspects of the introduced quantities, which are often not explicated in detail in elementary treatments of this topic. It is shown how complex quantum chemistry program packages can be interfaced to new programs in order to calculate IR and Raman spectra. The advantages of numerical differentiation of analytical gradients, dipole moments, and static, as well as dynamic polarizabilities, are pointed out. We carefully investigate the influence of the basis set size on polarizabilities and their spatial derivatives. This leads us to the construction of a hybrid basis set, which is equally well suited for the calculation of vibrational frequencies and Raman intensities. The efficiency is demonstrated for the highly symmetric C(60), for which we present the first all-electron density functional calculation of its Raman spectrum.     

Spectroscopic applications of three-level microwave double resonance

The versatility of a computer controlled double resonance microwave spectrometer has been exploited in three widely differing ways; for quantitative chemical analysis at the percentage level of specific constituents of a gas mixture; for the spectroscopy of forbidden transitions and wideband spectral searches for allowed transitions in torsional states of acetaldehyde; and in the development of techniques for the Fourier transform spectroscopy of both probed and pumped transitions in a three-level system.     

Quantum chemical calculation of gas-phase borenium ions

Conclusions A CNDO/2 calculation of C₃H₇NB⁺ borenium ions predicts the greater thermodynamic stability of cyclic structures in comparison with the corresponding acyclic structures.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2619–2620, November, 1987.     

Quantum chemical calculation of infrared spectra of acidic groups in chabazite in the presence of water

The changes in the spectra of the acidic group in chabazite are studied by quantum chemical calculations. The zeolite is modeled by two clusters consisting of eight tetrahedral atoms arranged in a ring and seven tetrahedral atoms coordinated around the zeolite OH group. The potential energy and dipole surfaces were constructed from the zeolite OH stretch, in-plane and out-of-plane bending coordinates, and the intermolecular stretch coordinate that corresponds to a movement of the water molecule as a whole. Both the anharmonicities of the potential energy and dipole were taken into account by calculation of the frequencies and intensities. The matrix elements of the vibrational Hamiltonian were calculated within the discrete variable representation basis set. We have assigned the experimentally observed frequencies at approximately 2900, approximately 2400, and approximately 1700 cm(-1) to the strongly perturbed zeolite OH vibrations caused by the hydrogen bonding with the water molecule. The ABC triplet is a Fermi resonance of the zeolite OH stretch mode with the overtone of the in-plane bending (the A band) and the overtone of the out-of-plane bending (the C band). In the B band the stretch is also coupled with the second overtone of the out-of-plane bending. The frequencies at approximately 3700 and approximately 3550 cm(-1) we have assigned to the OH stretch frequencies of a slightly perturbed water molecule.     

Quantum chemical definition and calculation of oxidation number

A new quantum chemical definition of oxidation number is proposed, in the present paper, as a direct generalization of the corresponding classical definition. According to the proposed general definition, the oxidation number can be calculated by use of molecular orbital data and a population analysis method or by use of other quantum chemical methods. For the practical calculation, we present a corresponding concrete calculation procedure within the framework of the maximum overlap population principle, which is very simple and very easy to use. The calculated numerical results are, on the whole, in good agreement with chemists' intuitive concepts of chemical bonding.     

Quantum chemical calculation of hydration for alkali metal salts

Calculations using the GAUSSIAN 03 program package were performed to determine the hydrated structures of lithium, potassium, LiCl, NaCl, and sulfonic cation exchanger in the form of alkali metal ions. For ions with positive hydration, the distance between the radii of the first and second spheres of hydration water is larger than in pure water; for ions with negative hydration, this distance is smaller. Salts with different types of hydration for cation and anion have hydrated structures similar to those of ions with negative hydration.     

Quantum chemical calculation for the inhibitory effect of compounds

The effects of the molecular structure on the corrosion inhibition efficiency are investigated by nine methods of calculations. The selected thio compounds were previously identified as corrosion inhibitors for mild steel in the 1.0 M HCl solution. The electronic properties such as highest occupied molecular orbital (EHOMO) energy, lowest unoccupied molecular orbital (ELUMO) energy, dipole moment (μ), and Fukui indices are calculated and discussed. Results show that the corrosion inhibition efficiency increase with the increase in both EHOMO and μ values, respectively, and decrease in ELUMO. QSAR approach is utilized in this study; a good relationship is found between the experimental corrosion inhibition efficiency (IE_exp, %) and the theoretical corrosion inhibition efficiency (IE_theor, %). The calculated inhibition efficiency is found closer to the experimental inhibition efficiency with a coefficient of correlation (R ²) of 0.875.     

Time-resolved four-level double resonance in the microwave spectrum of formaldehyde

The technique of transient double resonance has been applied to the (2₁₁ ← 2₁₂)_s-(3₁₂ ← 3₁₃)_p system in the microwave spectrum of formaldehyde. An effective broadening parameter γ*/2πp=22.96±0.3 MHz Torr^?1 was measured and theory is presented to relate this to other measured multipole relaxation rates.     

Nuclear magnetic resonance spectra under chemical polarization conditions

Russian Chemical Bulletin -     

Nature of the chemical bond of hydrogen and oxygen atoms with PT(100) surface: Quantum chemical calculation and disappearance potential spectra

Electronic structures of clean, hydrogen covered, and oxygen covered Pt(100)-(1×1) surface have been calculated. Both absorbates form surface subzones localized below the metal conduction band but overlapping partially with. Furthermore, the local density of states (LDOS) reveals a peak of the resonant state on the absorbed atom which is narrower for hydrogen than for oxygen. The comparison of LDOS on absorbed and Pt atoms shows that subzone surface states are responsible for the covalent component of the chemical bond between absorbed and platinum atoms, while resonant states make the ionic contribution. The obtained LDOS were used to calculate disappearance potential spectra. Theoretical spectra are well consistent with experimental ones.     

Transient nutation and molecular echoes in microwave double resonance experiments

Transient phenomena occurring in time resolved microwave-microwave double resonance experiments when the pump power is switched on and off have been investigated both theoretically and experimentally. Special attention has been paid to the case when both fields are saturating and exactly resonant. A calculation using the evolution operator formalism has been carried out to explain the experimental results. Transient nutations similar to those observed by the Stark modulation method have been observed and theoretically explained. A new phenomenon, the driven induction decay, has also been observed when the pump power is switched on. Moreover, double resonance photon echoes have also been exhibited. The application of these methods to relaxation studies is discussed.     

Vibronic absorption, fluorescence, and phosphorescence spectra of psoralen: a quantum chemical investigation

Excited state potential energy hypersurfaces of 7H-furo[3,2-g][1]benzopyran-7-one (psoralen) have been explored employing (time-dependent) Kohn-Sham density functional theory. At selected points, we have determined electronic excitation energies and electric dipole (transition) moments utilizing a combined density functional/multireference configuration interaction method. Spin-orbit coupling has been taken into account employing an efficient, non-empirical spin-orbit mean-field Hamiltonian. Franck-Condon factors have been computed for vibrational modes with large displacements in the respective Dushinsky transformations. The simulated band spectra closely resemble experimental band shapes and thus validate the theoretically determined nuclear structures at the S(0), S(1), and T(1) minima. In the S(1) (pi(HOMO)-->pi*(LUMO)) state, the lactone bond of the pyrone ring is significantly elongated. From excited vibrational levels of the S(1) state a conical intersection between a (pi-->sigma*) excited state and the electronic ground state may be energetically accessible. Fast non-radiative decay via this relaxation pathway could explain the low fluorescence quantum yield of psoralen. The T(1) (pi(HOMO-1)-->pi*(LUMO)) exhibits a diradicaloid electronic structure with a broken C(5)-C(6) double bond in the pyrone ring. A variational multireference spin-orbit configuration interaction procedure yields a phosphorescence lifetime of 3 s, in excellent agreement with experimental estimates.     

On the calculation of phosphorescence oscillator strength

It is pointed out that one of the two commonly used expressions for calculating the oscillator strength of phosphorescence transitions is incomplete. By using the adequate transition operator, a correction term is added to one of the two expressions which then become analytically equivalent up to second order in the fine structure constant.     

Monte Carlo calculation of resonance self-shielding factors for epithermal neutron spectra

In this work, the resonance self-shielding factor is calculated by means of the Monte Carlo technique for different materials (Au, Co, Mn), geometries (circular foils and wires) and incidence of neutrons (isotropic field and collimated beam). The results are compared with the values obtained by other authors.     

取代基咪唑啉与铁原子化学吸附作用能的量子化学计算

用量子化学方法计算了６个咪唑啉型化合物及其与铁原子的化学吸附作用能,探讨了这种作用能与缓蚀性能的关系。得到咪唑啉环上氮与铁的配位键长、双原子作用能以及重叠集居数。研究发现具有ｐ－π共轭体系的咪唑啉以及在环上引入供电子基团或取代芳烃,能增强氮与铁原子的化学吸附作用力。计算结果可为设计性能较好的新型咪唑啉缓蚀剂提供有用的信息。     

Quantum chemical predictions of IR spectra and thermodynamic properties of tetrazole derivates

We have predicted the optimized geometries, infrared spectrum, and thermodynamic properties for six tetrazole derivates at density functional theory and second-order Møller–Plesset perturbation theory (MP2) level with the 6-31G** basis set. Their heats of formation were calculated accurately using the designed isodesmic and isogyric reactions. The computed total energies, heat of formation, and enthalpy of combustion consistently indicate the stability: 5-(2,4,6-trinitrophenyl)-2H-tetrazole > 5-(2,4,6-trinitrophenyl)-1H-tetrazole > 5-(2,4,6-trinitrophenyl)-5H-tetrazole The similar result for the isomers of 5-(2,4,6-trinitrobenzyl)-tetrazole: 5-(2,4,6-trinitrophenyl)-2H-tetrazole > 5-(2,4,6-trinitrophenyl)-1H-tetrazole > 5-(2,4,6-trinitrophenyl)-5H-tetrazole.     

Quantum chemical studies of FT-IR and Raman spectra of methyl 2,5-dichlorobenzoate

In this paper, experimental and theoretical studies on the molecular structure and vibrational spectra of methyl 2,5-dichlorobenzoate (MDCB) are presented. Fourier transform infrared and Raman spectra of the title molecule in the solid phase were recorded and analyzed. The geometrical parameters were calculated using DFT (B3LYP) with 6-311G(d,p) and 6-311++G(d,p) basis sets, and compared with the experimental data. The vibrational frequencies, infrared intensities and Raman scattering activities were also reported. The detailed assignments were given based on the total energy distribution of the vibrational modes, calculated with scaled quantum mechanics method. The observed and calculated frequencies are found to be in good agreement.