Structure and vibrational spectra of mononitroalkanes

Structures and force fields for several mononitroalkane molecules were determined by ab initio quantum-chemical methods. The data obtained were used for calculation of the frequencies and modes of normal vibrations. Potentialities of different methods (RHF, MP2, and B3LYP) and basis sets for estimation of the structures and spectra were studied.     

Molecular structure,conformational mobility,vibrational spectra,and thermochemistry of peroxyacetic acid: an ab initio and density functional study

The structure of the peroxyacetic acid (PAA) molecule and its conformational mobility under rotation about the peroxide bond was studied by ab initio and density functional methods. The free rotation is hindered by the trans-barrier of height 22.3 kJ mol^–1. The equilibrium molecular structure of AcOOH (C _s symmetry) is a result of intramolecular hydrogen bond. The high energy of hydrogen bonding (46 kJ mol^–1 according to natural bonding orbital analysis) hampers formation of intermolecular associates of AcOOH in the gas and liquid phases. The standard enthalpies of formation for AcOOH (–353.2 kJ mol^–1) and products of radical decomposition of the peroxide — AcO^· (–190.2 kJ mol^–1) and AcOO^· (–153.4 kJ mol^–1) — were determined by the G2 and G2(MP2) composite methods. The O—H and O—O bonds in the PAA molecule (bond energies are 417.8 and 202.3 kJ mol^–1, respectively) are much stronger than in alkyl hydroperoxide molecules. This provides an explanation for substantial contribution of non-radical channels of the decomposition of peroxyacetic acid. The electron density distribution and gas-phase acidity of PAA were determined. The transition states of the ethylene and cyclohexene epoxidation reactions were located (E _a = 71.7 and 50.9 kJ mol^–1 respectively).     

Vibrational spectra of 1,1,3,3-tetramethyl-2-nitroguanidine and their interpretation with the use of scaling of quantum-chemical force field

The IR (4000–50 cm⁻¹) and Raman (3500–170 cm⁻¹) spectra of solid 1,1,3,3-tetramethyl-2-nitroguanidine (TMNG) were obtained. The spectra were interpreted using the scaling of the TMNG quantum-chemical force field in the B3LYP/6-311G(d,p) approximation. Transferable scale factors necessary for the interpretation of spectra of more complex related compounds were determined. The scaled harmonic force field is supposed to be used in the analysis of the available gas-phase electron diffraction data for TMNG. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 495–498, March, 2008.     

Structure and spectra of 1,3-dioxanes. II. Microwave spectrum, structural parameters, and ab initio calculations of 2-methyl-1,3-dioxane

The microwave spectra of five isotopomers with the ¹³C and ¹⁸O natural abundance isotopes of the 2-methyl-1,3-dioxane molecule (22–50 GHz) were studied. Rotational transitions of a and c types with 4 ≤ J ≤ 12 were identified. The rotational constants and the substitution r _s and effective r _o structural parameters of the molecule were determined. Ab initio calculations on 2-methyl-1,3-dioxane were performed with molecular structure optimization. The results of quantum-chemical calculations at different levels are compared with experimental data.     

Structure and vibrational spectra of dicyclopentadienylzinc. A DFT study

The quantum-chemical DFT calculations of the Cp₂Zn structure confirm the conclusion made earlier from the vibrational spectra that the sandwich structure (⁵-C₅H₅)₂Zn (A) is not energetically favorable and more favorable are the close in energy -structure (⁵-C₅H₅)(¹-C₅H₅)Zn (B) and -structure (¹-C₅H₅)₂Zn (C). The vibrational spectra of structures B and C with the DFT-derived force fields were calculated. A comparison of the calculated spectra of the isolated Cp₂Zn molecules with the experimental data gives no way of deciding between the B and C structures. It is most likely that the molecule is nonrigid and experiences a strong influence from the nearest environment in solution or in the crystalline state.     

AM1 studies of photoelectron spectra

Deep-lying π- and σ-orbitals of 10-alkylphenothiazines were studied by photoelectron spectroscopy and quantum-chemical AM1 calculations. It was demonstrated that in 10-ethylphenothiazine the lone electron pair of the S atom interacts with the π-system of the aromatic fragments. The π-MOs, whose energies are a function of the dihedral angle between the planes of the benzene rings of phenothiazines and are independent of the degree of pyramidality of the N atom, were found. The differences in the energy of these MOs were used for estimating equilibrium dihedral angles of tricyclic molecules in the gaseous phase. These values differ only slightly from those observed in the solid phase. The replacement of the hydrogen atom at position 10 by the methyl group leads to a decrease in the dihedral angle, leaving the orientation of the substituent unchanged. The orbital energies of phenothiazines, which were calculated by the AM1 method, adequately reflect the order of changes in the ionization potential. However, contributions of the two highest occupied π-MOs to the total charges on the N and S atoms are inconsistent with the experimental data. For Part 10, see Ref. 1. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1542–1548, August, 1998.     

Calculating molecular vibrational spectra beyond the harmonic approximation

We present a new approach for calculating anharmonic corrections to vibrational frequency calculations. The vibrational wavefunction is modelled using translated Hermite functions thus allowing anharmonic effects to be incorporated directly into the wavefunction whilst still retaining the simplicity of the Hermite basis. We combine this new method with an optimised finite-difference grid for computing the necessary third and fourth nuclear derivatives of the energy. We compare our combined approach to existing anharmonic models—vibrational self-consistent field theory (VSCF), vibrational perturbation theory (VPT), and vibrational configuration interaction theory (VCI)—and find that it is more cost effective than these alternatives. This makes our method well-suited to computing anharmonic corrections for frequencies in medium-sized molecules. Contribution of the Mark S. Gordon 65th Birthday Festschrift Issue.     

Simulation of IR and Raman spectra of p-hydroxyanisole and p-nitroanisole based on scaled DFT force fields and their vibrational assignments

This work deals with the vibrational spectroscopy of p-hydroxyanisole (PHA) and p-nitroanisole (PNA) by means of quantum chemical calculations. The mid and far FT-IR and FT-Raman spectra were recorded in the condensed state. The fundamental vibrational frequencies and intensity of vibrational bands were evaluated using density functional theory (DFT) with the standard B3LYP/6-31G* method and basis set combination and were scaled using various scale factors which yield a good agreement between observed and calculated frequencies. The vibrational spectra were interpreted with the aid of normal coordinate analysis based on scaled density functional force field. The results of the calculations were applied to simulate infrared and Raman spectra of the title compounds, which showed excellent agreement with the observed spectra.     

The use of the hybrid density functional theory (DFT) approach for calculation of vibrational spectra: nitromethane

The molecular geometry of nitromethane was optimized and its force field and vibrational spectrum were calculated by the BECKE3LYP method. The accuracy of optimization of the geometry of MeNO₂ obtained by this method using the 6–311G(d,p) and 6–311++G(d,p) basis sets is not poorer than that obtained at the second-order Møller-Plesset level of perturbation theory (MP2). The vibrational frequencies of nitromethane and its d₁, d₂, and d₃ isotopomers obtained by the BECKE3LYP method are in much better agreement with the experimental data than those calculated at the MP2 level using the same basis set. The average absolute error of calculations performed without the use of any scaling factors is ～2% for frequencies; the maximum deviation is ～4%.     

Cryosynthesis,structure, and IR spectra of aluminum chloride—nitromethane molecular complexes

The products of codeposition of aluminum chloride and nitromethane were studied by low-temperature IR spectroscopy (80—200 K). Density functional (B3LYP/6-31G* and B3LYP/6-31+G**) quantum-chemical calculations of the geometry and vibrational frequencies of aluminum chloride, nitromethane, and AlCl₃·MeNO₂ and 2AlCl₃·MeNO₂ complexes were carried out. Comparison of the experimental and calculated IR spectra suggests that the preferred geometry of the 2AlCl₃·MeNO₂ complex is such that one AlCl₃ molecule is coordinated to an O atom of the nitro group, while the other AlCl₃ molecule forms an Al...Cl bridge.     

Scaling of quantum-mechanical molecular force fields

Comparative analysis of various methods of empirical scaling of quantum-mechanical harmonic molecular force fields has been performed. The efficiency of using each particular scaling technique was shown to depend on the theoretical level of the quantum-mechanical calculation. The Pulay method of scaling (congruent transformation of the force constant matrix) is applicable in the case where the relative accuracies of determination of diagonal and off-diagonal quantum-mechanical force constants are approximately equal. This requirement is satisfied for a quantum-mechanical force field determined close to the HartreeFock limit. This makes it possible to carry out its correction with maximum retention of the peculiarities inherent in the molecule under investigation.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 840–807, April, 1996.     

Theoretical studies of force fields and IR spectra of isocytosine

The optimized geometries, complete harmonic force fields, and infrared intensities of isocytosine tautomers, amino‐hydroxy and amino‐oxoN(1)H, were calculated at the ab initio Hartree–Fock level using the 6‐31G* basis set. The theoretical force fields were scaled by empirical scale factors, which were determined by fitting to the IR spectrum of the amino‐oxo form and were then transferred to the amino‐hydroxy form. The average deviations between experimental and computed frequencies are 7.6 cm⁻¹ for amino‐oxo and 9.5 cm⁻¹ for amino‐hydroxy, respectively. The assignments of the fundamental frequencies and the transferability of the force constant scale factors are also presented. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 53–60, 1999     

Vibrational spectra and force constants ofS-methyl-N,N-dimethyldithiocarbamate

The IR and Raman spectra ofS-methyl-N,N-dimethyldithiocarbamate were studied in different phase states. The frequencies and vibration modes of normal vibrations were analyzed, and the erroneous assignments made in some previous works were corrected. The force constants of the molecule were estimated using the model of the generalized valenceforce field.Translated fromIzyestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1108–1110, June, 1995.This work was carried out with partial financial support from the International Science Foundation.     

Structure and spectra of 1,3,2-dioxaphospholenes. 1. Microwave spectra of 2-chloro-4,5-dimethyl-1,3,2-dioxaphospholene and its isotopomers in the ground and excited vibrational states

The microwave spectrum of 2-chloro-4,5-dimethyl-1,3,2-dioxaphospholene (1) was studied in the frequency range from 7 to 53 GHz. Rotational transitions of the parent molecule in the ground and eleven excited vibrational states and those of its mono-substituted ³⁷Cl, ¹³C_Me, and ¹³C_Cycle isotopomers in the ground vibrational state were identified. Rotational constants and partial r _s-structure were obtained. The quartic centrifugal distortion constants, dipole moment components _a = 3.8D and _c = 0.24D (the total dipole moment is 3.81D), and the ³⁵Cl quadrupole coupling constants were determined for the parent molecule. The fine structure of the microwave transitions in the parent molecule was analyzed under the assumption of noninteracting methyl groups. The height of the barrier to internal rotation (V ₃₀ = V ₀₃ = 665 cm^–1) and the frequency of torsional vibrations ( = 167 cm^–1) were found. The frequencies of two lowest vibrational modes corresponding to deformation vibrations of the five-membered ring were estimated (100 cm^–1) from the relative intensities of rotational transitions for different vibrational states.     

Molecular structure of ErCl<Subscript>3</Subscript> and YbCl<Subscript>3</Subscript> according to the data of the simultaneous electron diffraction and mass spectrometric experiment

The saturated vapors of ErCl₃ and YbCl₃ were studied in a simultaneous electron diffraction and mass spectrometric experiment at 1165 K and 1170 K, respectively. In the vapors of these compounds, we found up to 3 mol.% dimers along with the monomers. The parameters of the r _g effective configuration of the monomer molecules were determined. For ErCl₃ and YbCl₃, the internuclear distances r _g(Ln-Cl) were 2.436(5) Å and 2.416(5) Å, and the bond angles ∠_g(Cl-Ln-Cl) were 117.0(10)° and 117.2(10)°, respectively. The equilibrium configurations and vibration frequencies of the monomer and dimer molecules were calculated by the HF, B3LYP, and MP2 methods using the combination of the ECP_D energy-consistent quasirelativistic core potential, including 4f electrons [Kr4d ¹⁰4f ⁿ ], and the contracted [5s4p3d] valence basis set for Er and Yb atoms and the MIDIX [4s3p1d] basis set for Cl atoms. The parameters of the effective r _g configuration of the monomer molecules corresponding to the temperature of the experiment were calculated. The difference between the calculated equilibrium r _e(Ln-Cl) and temperature-averaged r _g(Ln-Cl) distances was found to be 0.001–0.002 Å and did not exceed the error of the r _g(Ln-Cl) parameter determined in the electron diffraction experiment. The experimental parameters of the r _g structure were shown to be consistent with the idea about the planar equilibrium geometrical configuration of ErCl₃ and YbCl₃ molecules.     

Ab initio study of the structure, force fields, and vibrational spectra of alkali metal tellurites

The equilibrium geometrical parameters, force fields, vibration frequencies, and intensities in the IR molecular spectra of M₂TeO₃ (M-Li, Na, K) are found by the Hartree — Fock ab initio method in the bases complemented with polarization and diffusion functions using relativistic effective core potentials. The relative energies of alternative molecular configurations and the energies of the dissociation M₂TeO₃→ M2O + TeO2 are refined using Möller — Plessett second-order perturbation theory. It is found that the chemical bond in these molecules can be approximated by the scheme (M⁺)₂[TeO₃]^2- The equilibrium nuclear configuration has C_s symmetry and corresponds to the bis-bidentate coordination of M₊ cations by the pyramidal anion [TeO₃]^2-. The mono-bidentate structures of C_s symmetry correspond to the saddle points on the potential energy surface of the molecules. The calculated IR molecular spectrum of K₂TeO₃ agrees with the experimental spectrum obtained by the matrix isolation method by J. Ogden et al. [J. Chem. Soc. Dalton Trans., 1957 (1997)]. Based on the results of the previous ab initio studies of alkali metal sulfites and selenites, the barrier of the intramolecular rearrangement (bb)→(mb)→(bb)’ was found to lower in the series Li₂XO₃→Na₂XO₂→K₂XO₃ (X = S, Se, Te) and M₂TeO₃→M₂SeO₃→M₂SO₃. The degree of deformation of the XO fragment in the M₂XO₃ molecules decreases when the M atom is replaced by its heavier analog (Li→Na→K).     

A force field study of diacetylene molecule by kinetic constant method

The potential constants of diacetylene molecule has been evaluated using kinetic constants. The other molecular constants such as the generalised vibrational mean amplitudes, shrinkage constants, Coriolis coupling constants and centrifugal distortion constants are also calculated using the vibrational frequencies and the results discussed.     

Quantum-chemical study of the electronic structure and reactivity of 2-methyl-4H-thieno[3,2-b]pyrrole-5-carboxylic acid methyl ester

We have used the MNDO approximation to carry out a quantum-chemical study showing that the selectivity of acylation of 2-methyl-4H-thieno[3,2-b]pyrrole-5-carboxylic acid methyl ester under Friedel—Crafts reaction conditions in the presence of AlCl₃ depends more on the electron density distribution in the complexes than on the structure parameters. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 291–298, February, 2006.