Ab initio calculations of the ultraviolet resonance Raman spectra of uracil

An equation been derived to calculate, ab initio, the frequencies and intensities of a resonant Raman spectrum from the transform theory of resonance Raman scattering. This equation has been used to calculate the intensities of the ultraviolet resonance Raman spectra from the first π-π* excited state of uracil and 1,3-dideuterouracil. The protocol for this calculation is as follows: (1) The force constant matrix elements in Cartesian coordinate space, the vibrational frequencies, and the minimum energy ground and excited state geometries of the molecule are calculated ab initio using the molecular orbital program Gaussian 92, (2) the force constants in Cartesian coordinates are transformed into force constants in the space of a set of 3N – 6 nonredundant symmetrized internal coordinates, (3) the G matrix is constructed from the energy minimized ground state Cartesian coordinates and the GFL = LΛ eigenvalue equation is solved in internal coordinate space, (4) the elements of the L and L^?1 matrices are calculated, (5) the changes in all of the internal coordinates in going from the ground to the excited state are calculated, and (6) these results are used in combination with the transform theory of resonance Raman scattering to calculate the relative intensities of each of the 3N – 6 vibrations as a function of the exciting laser frequency. There are no adjustable parameters in this calculation, which reproduces the experimental frequencies and intensities with remarkable fidelity. This indicates that the Dushinsky rotation of the modes in the excited state of these molecules is not important and that the simplest form of the transform theory is adequate. © 1995 John Wiley & Sons, Inc.     

Normal Coordinate Analysis in Cartesian Space II. Planar Symmetric XYn Molecules

Force constants of planar symmetric XY_n type molecules in the Cartesian space have been derived in terms of the vibrational frequencies. The force constants and normal coordinates of some planar XY₃ molecules with available isotoptic data are worked out. Further study of infrared absorption spectra of square planar XY₄ complex ions will be needed for the present normal coordinate analysis.     

Polyatomic,anharmonic, vibrational–rotational analysis. Application to accurate ab initio results for formaldehyde

A computer program SURVIB is described for calculating vibrational anharmonicity constants for polyatomic molecules. The program requires as input a grid of calculated energies in the vicinity of a stationary point. This grid is fit, in a least squares sense, to a polynomial function of the internal coordinates. This analytic representation of the energy surface is employed in a normal mode analysis, and the energy is reexpanded as a polynominal function of the normal mode coordinates (expressed as vectors in the mass-weighted atomic Cartesian coordinate space). The resulting coefficients are used in a second-order perturbation theory analysis to obtain the vibrational anharmonicity constants. Also reported is an application of this program to formaldehyde employing ab initio, RHF , MP 2, MP 3, and RHF -CI calculations. The spectroscopic constants obtained for H₂CO are in good agreement with experimentally derived values recently reported by Reisner.     

Scaled Quantum Mechanical Force Fields for the Isoelectronic Molecules CF3X (X = SiH3, PH2, SH,Cl )

The vibrational properties of the CF₃X (X = SiH₃, PH₂, SH, Cl) series of molecules were studied by means of density functional theory (DFT) techniques. After obtaining the optimized geometrical parameters and conformations, the frequencies corresponding to the normal modes of vibration and the associated force constants were calculated. The original force fields in Cartesian coordinates were transformed to local symmetry coordinates and subsequently scaled to reproduce the experimental frequencies. Some trends observed in the experimental data could be explained on the basis of the calculated atomic charges.     

Calculation of molecular vibrations: Selective scaling factors for semiempirical force constants

The complete force constant matrices of a set of 50 aliphatic and aromatic hydrocarbons are calculated at the density functional theory B3LYP/6–31+G(d, p) and semiempirical PM3 levels of theory. After transformation from Cartesian to nonredundant internal coordinates, the errors in the semiempirical force constants are systematically analyzed. The force constants of the C(SINGLE BOND)C stretching coordinates can be easily corrected by a second-order fit. Thus, only two parameters are needed to reduce the mean error from 21.2 to 1.23%. The errors of other internal coordinates, particulary those including torsional modes, exhibit a larger diversity. The performance of the correction scheme in predicting vibrational spectra is shown for several examples including buckminsterfullerene (C₆₀). © 1997 John Wiley & Sons, Inc. J Comput Chem 18 : 2050–2059, 1997     

Normal Coordinate Analysis in Cartesian Space III. The Symmetric Nonplanar XYn Type Molecules

The normal vibrations of tetrahedral hydride and octahedral hexafluoride have been studied in the Cartesian coordinates. Equations relating the force constants with the vibrational frequencies are derived by using symmetry coordinates. Numerical values of the force constants and normal coordinates of these molecules are tabulated.     

Quantum-chemical study on nitrosonium complexes of Bi- and polycyclic aromatic compounds

The affinities of bi- and polycyclic aromatic compounds for nitrosonium ion (A_NO⁺ A_{NO^ + } ) were calculated at the RI-MP2/L1 and DFT/PBE/3z levels of theory. Both methods gave generally similar geometric parameters of nitrosonium complexes. The formation of pincer complexes in which the NO⁺ ion is linked to two aromatic rings is more energetically favorable than the formation of analogous complexes through external binding of NO⁺ to one aromatic ring. Linear correlations were found between the calculated A_NO⁺ A_{NO^ + } values of aromatic compounds and experimental equilibrium constants for the reactions of these compounds with nitrosonium ion in solution. Structural peculiarities of the examined nitrosonium complexes are discussed.     

Application of the Seth-Paul-Van Duyse equation—I : A simple relation defining new X+(R) substituent constants on the basis of electrophilic σ+ constants

The Seth-Paul-Van Duyse equation (SPVDE) correlating the CO stretching frequencies of a great number of R₁COR₂ molecules with X (R) substituent constants has been reinvestigated. A simple empirical relation defining new X⁺(R) constants on the basis of electrophilic σ constants has been derived: X⁺ (R) = 0·238 σ⁺ + 1·077. The three earlier reported relationships between the X(R) constants and Hammett σ values have been replaced by this single relationships and a set of new X⁺(R) constants has been calculated. The new X⁺(R) constants applied to 287 R₁COR₂ compounds containing the para- or meta-substituted benzene rings fit very well the SPVDE. It has been possible to extend the SPVDE to various aromatic systems.     

Green's Function Determination of Force Constants-H2O as Example

Treating isotopically substituted molecule as a perturbed system, Green's function for the perturbation are constructed and related to the force field of vibration. By spectral representation, Green's function is diagonalized in the normal coordinates. Then transforming back to the Cartesian coordinates, the Cartesian force constants are generated without solving the secular equation directly. The relations between the internal force constants and the Cartesian force constants ate given and complete internal force field can be obtained. The results for H₂O are discussed.     

Calculation of the frequencies and intensities in the infrared and raman spectra of cyclopropenone

By using the values of the vibrational frequencies of normal and deuterated cyclopropenone (II-d₀ , II-d₂ ) and ¹⁶ O-and ¹⁸ O-substituted dimethylcyclopropenone (III) as -well as the infrared and Raman intensities of II a consistent set of force constants has been derived for the cyclopropenone skeleton. The derived values show that the zwitterionic form makes a substantial contribution to the electronic ground state of the molecule. The combined frequency and intensity calculation - simulation of the infrared and the Raman spectrum - is shown to be a good method for making a proper assignment of calculated and observed vibrations and deriving realistic sets of force constants.     

Kinetics of the reactions of acenaphthene and acenaphthylene and structurally-related aromatic compounds with OH and NO3 radicals,N2O5 and O3 at 296 ± 2 K

The kinetics of the atmospherically important gas-phase reactions of acenaphthene and acenaphthylene with OH and NO₃ radicals, O₃ and N₂O₅ have been investigated at 296 ± 2 K. In addition, rate constants have been determined for the reactions of OH and NO₃ radicals with tetralin and styrene, and for the reactions of NO₃ radicals and/or N₂O₅ with naphthalene, 1- and 2-methylnaphthalene, 2,3-dimethylnaphthalene, toluene, toluene-α,α,α-d₃ and toluene-d₈. The rate constants obtained (in cm³ molecule^?1 s^?1 units) at 296 ± 2 K were: for the reactions of O₃; acenaphthene, <5 × 10^?19 and acenaphthylene, ca. 5.5 × 10^?16; for the OH radical reactions (determined using a relative rate method); acenaphthene, (1.03 ± 0.13) × 10^?10; acenaphthylene, (1.10 ± 0.11) × 10^?10; tetralin, (3.43 ± 0.06) × 10^?11 and styrene, (5.87 ± 0.15) × 10^?11; for the reactions of NO₃ (also determined using a relative rate method); acenaphthene, (4.6 ± 2.6) × 10^?13; acenaphthylene, (5.4 ± 0.8) × 10^?12; tetralin, (8.6 ± 1.3) × 10^?15; styrene, (1.51 ± 0.20) × 10^?13; toluene, (7.8 ± 1.5) × 10^?17; toluene-α,α,α-d₃, (3.8 ± 0.9) × 10^?17 and toluene-d₈, (3.4 ± 1.9) × 10^?17. The aromatic compounds which were observed to react with N₂O₅ and the rate constants derived were (in cm³ molecule^?1 s^?1 units): acenaphthene, 5.5 × 10^?17; naphthalene, 1.1 × 10^?17; 1-methylnaphthalene, 2.3 × 10^?17; 2-methylnaphthalene, 3.6 × 10^?17 and 2,3-dimethylnaphthalene, 5.3 × 10^?17. These data for naphthylene and the alkylnaphthalenes are in good agreement with our previous absolute and relative N₂O₅ reaction rate constants, and show that the NO₃ radical reactions with aromatic compounds proceed by overall H-atom abstraction from substituent-XH bonds (where X = C or O), or by NO₃ radical addition to unsaturated substituent groups while the N₂O₅ reactions only occur for aromatic compounds containing two or more fused six-membered aromatic rings.     

Theoretical study of dynamical properties on reaction path in molecular internal coordinates

The dynamical properties on reaction path (IRC) in internal coordinates have been obtained, which in. clude ω_K (frequencies orthogonal to IRC), L_K (vibrational modes), B_(KF) (coupling constants between the IRC and vibra tions orthogonal to it), B_(KL) (coupling constants between every two vibrations orthogonal to IRC). A set of theory of teac. tion path in molecular internal coordinates has been also constructed. The dynamical properties, including ω_K, B_(KF) B_(KL) of the reaction H~1O~2H~3 H~4→H~1O~2 H~3H~4 have been calculated, which explicitly explain the interaction, chang ing trend and contribution of each chemical bond (including bond angle) in the reaction.     

Equilibrium structure and spectroscopic constants of maleic anhydride

The quadratic, cubic, and semi-diagonal quartic force fields of maleic anhydride have been calculated at the MP2 level of theory employing the cc-pVTZ basis set. The spectroscopic constants derived from the force field are in excellent agreement with the corresponding experimental values. The semi-experimental equilibrium structure has been derived from experimental ground state rotational constants and rovibrational corrections calculated from the cubic force field. This semi-experimental equilibrium structure is in excellent agreement with the ab initio structures computed at the CCSD(T) level of theory and it is closer to the ab initio structure than the purely experimental (or empirical) structures r ₀, r _m⁽¹⁾, and r _m⁽²⁾ obtained by microwave spectroscopy as well as the equilibrium structure derived from gas-phase electron diffraction data.     

The Reactivity of the Nitrate Radical (•NO3) in Aqueous and Organic Solutions

Rate constants for the nitrate (^•NO₃) radical reaction with alcohols, alkanes, alkenes, and several aromatic compounds were measured in aqueous and tert‐butanol solution for comparison to aqueous and acetonitrile values from the literature. The measured trends provide insight into the reactions of the ^•NO₃ radical in various media. The reaction with alcohols primarily consists of hydrogen‐atom abstraction from the alpha‐hydroxy position and is faster in solvents of lower polarity where the diffusivity of the radical is greater. Alkenes react faster than alkanes, and their rate constants are also faster in nonpolar solution. The situation is reversed for the nitrate radical reaction with the aromatic compounds, where the rate constants in tert‐butanol are slower. This is attributed to the need to solvate the NO₃⁻ anion and corresponding tropylium cation produced by the ^•NO₃ radical electron transfer reaction. A linear correlation was found between measured rate constants in water and acetonitrile, which can be used to estimate aqueous nitrate radical rate constants for compounds having low water solubility.     

Photoinitiation. IV. The effect of lewis acid on the vinyl monomer polymerization photoinitiated by aromatic hydrocarbons

Polymerization of acrylonitrile photoinitiated by naphthalene, anthracene, phenanthrene, and pyrene is accelerated by an admixture of zinc (II) chloride, acetate, or nitrate. The effect of zinc (II) salts on the rate of pyrene-photoinitiated polymerization of acrylonitrile leads to an increase in this rate in the order Zn/OCOCH_3/2 < ZnCl₂ < Zn/NO_3/2. The maximum polymerization rate is achieved at the molar ratio [ZnCl₂]/([ZnCl₂] + [pyrene]) approximately 0.7. In contrast to the photoinitiated polymerization of acrylonitrile, the methyl methacrylate admixture of zinc (II) chloride exerts a smaller effect on the polymerization rate. In the pyrene-photoinitiated polymerization of styrene an admixture of zinc (II) chloride retards the polymerization rate. Fluorescence of aromatic hydrocarbon in the system acrylonitrile–aromatic hydrocarbon is efficiently quenched by zinc (II) chloride. Stern–Volmer constants determined for pyrene (80 dm³ mole^?1), phenanthrene (66 dm³ mole^?1), and naphthalene (49 dm³ mole^?1) are higher by about 2–3 orders of the Stern–Volmer constants for fluorescence quenching of aromatic hydrocarbons by acrylonitrile in the absence of ZnCl₂. The fluorescence of anthracene in acrylonitrile is not quenched by ZnCl₂. The acceleration effect of Zn (II) salts on the polymerization of acrylonitrile photoinitiated by aromatic hydrocarbons depends on two factors: an increase in the ratio of the rate constant of the growth and termination reactions, k_p/k_t, and an increase in the quenching constant of fluorescence of aromatic hydrocarbon, k_q, by the complex {acrylonitrile…ZnCl₂}. ZnCl₂ thus influences both the growth and initiation reactions of the polymerization process.     

Fast and Tunable Phosphorescence from Organic Ionic Crystals

Crystalline diphosphonium iodides [MeR₂P-spacer-R₂Me]I with phenylene ( 1 , 2 ), naphthalene ( 3 , 4 ), biphenyl ( 5 ) and anthracene ( 6 ) as aromatic spacers, are photoemissive under ambient conditions. The emission colors (λ_em values from 550 to 880 nm) and intensities (Φ_em reaching 0.75) are defined by the composition and substitution geometry of the central conjugated chromophore motif, and the anion-π interactions. Time-resolved and variable-temperature luminescence studies suggest phosphorescence for all the titled compounds, which demonstrate observed lifetimes of 0.46–92.23 μs at 297 K. Radiative rate constants k_r as high as 2.8×10⁵ s⁻¹ deduced for salts 1 – 3 were assigned to strong spin-orbit coupling enhanced by an external heavy atom effect arising from the anion-π charge-transfer character of the triplet excited state. These rates of anomalously fast metal-free phosphorescence are comparable to those of transition metal complexes and organic luminophores that utilize triplet excitons via a thermally activated delayed fluorescence mechanism, making such ionic luminophores a new paradigm for the design of photofunctional and responsive molecular materials.     

Ab initio study of low-lying electronic states of the PF2 radical

The equilibrium geometries, excitation energies, force constants, and vibrational frequencies of the low-lying electronic states X²B₁, ²A₁, ²B₂, and ²A₂ of the PF₂ radical have been calculated at the MRSDCI level with a double zeta plus polarization basis set. Our calculated geometry, force constants, and vibrational frequencies for the X²B₁ state are in good agreement with experimental data. The electronic transition moments, oscillator strengths for the ²A₁ → X²B₁ and ²A₂ → X²B₁ transitions, and radiative lifetimes for the ²A₁ and ²A₂ states are calculated based on the MRSDCI wave functions. © 1994 by John Wiley & Sons, Inc.     

An ab initio potential energy surface and spectroscopic constants for the XΣg state of NO2

A three-dimensional potential energy function has been calculated for the X¹Σ⁺_g state of NO⁺₂ from ab initio MRD-CI data. With this PE function, converged vibrational calculations have also been performed for ten vibrational states, with the aid of a computer program developed in the present work for this purpose. The calculated harmonic frequencies, vibrational term values and rotational constants are in good agreement with experimental data.     

Die Kristallstruktur des l, 6-8, 13-Bis-oxido-[14]-annulens

Crystals of 1,6-8, 13-bis-oxido-[14]-annulene are monoclinic, a = 9,29, b = 9,74, c = 11,46 Å, β = 90,0°, space group P 2₁/c, with 4 molecules in the unit cell. The structure has been solved by direct methods and refined by full-matrix least-squares analysis of three-dimensional intensity data. The cis-configuration of the two oxygen bridges is confirmed, and the observed molecular parameters are in full accord with the aromatic properties of the molecule.     

Adsorption of aromatic compounds from solutions in <Emphasis Type="Italic">n</Emphasis>-heptane by modified nanoporous silica

The adsorption of anisole, anthracene, benzoic acid, benzene, naphthalene, phenanthrene, phenol, and carbon tetrachloride from dilute solutions in n-heptane on the modified nanoporous silica sorbents, Silasorb C18 and Phenyl-Silasorb, at 298 K is investigated by developing liquid chromatography. The Henry law constants and Helmholtz energy of adsorption for these aromatic compounds are calculated from their retention values. The adsorption isotherms of the aromatic compounds are calculated from their chromatographic peaks with allowances made for longitudinal smearing. A correlation is made between the Henry law constants calculated from the retention values of the aromatic compounds and from their adsorption isotherms. The influences of the surface modification of the silica adsorbents and the molecular structure of the aromatic compounds on their elution from the chromatographic column with n-heptane as the eluent are discussed. Contributions of functional groups in the molecules of the aromatic compounds to the Helmholtz energy of adsorption on different surfaces are estimated.