Analysis of the normal modes of molecules with D3h symmetry: Infrared spectra of monomeric s-triazine and cyanuric acid

Normal modes of s-triazine and cyanuric acid were calculated at the DFT(B3LYP)/6-311++G(d,p) level. These modes were analyzed in terms of potential energy distribution (PED), computed using a specially designed set of symmetry coordinates. The normal-mode analysis was described in detail and the PED matrix definition used in the calculations was provided. Particular attention has been devoted to the degenerate vibrations. The experimental infrared spectra of s-triazine and cyanuric acid isolated in low-temperature Ar matrices have been recorded and interpreted by comparison with the theoretically predicted normal modes. In the spectrum of matrix-isolated s-triazine, the IR bands originating from ¹³C and ¹⁵N isotopologues with one of the ring atoms substituted by a rare isotope were detected. These bands were identified thanks to the excellent agreement between the experimentally observed and theoretically predicted isotope shifts.     

Damped torsional oscillator model for polymer molecules

At present the widely used model for explaining viscoelastic and dielectric properties of polymer solutions is that of Rouse and Bueche. Here the polymer molecule is considered as an array of Gaussian subunits, each of which acts as an entropy spring. The motion of these segments is damped by the viscous drag of the surrounding solvent (RB model). An alternative model is presented, in which the segments are torsional oscillators consisting of two or three backbone links, and the damping is due to hindered internal rotation (DTO model). The mathematical treatment of these two models is essentially identical, but the physical interpretation of the constants used is very different. The DTO model has previously been applied by one of us to the interpretation of viscoelastic data. It is here applied to the interpretation of dielectric loss data. It is shown that dielectric measurements in dilute solution should very readily discriminate between the two approaches. Finally it is shown that the relaxation time computed from the DTO model is in closer agreement with published NMR data on poly(propylene oxide) 2025, than is the RB relaxation time. The postulates of the DTO model appear to be confirmed for this low molecular weight polymer. An even more sensitive distinction should be available by studies of the relaxation time as a function of polymer concentration.     

A theory of nuclear motion in polyatomic molecules based upon the morse oscillator: I. Application to the stretching overtone spectra of water and benzene molecules

A theory of molecular vibrations is presented which is based upon the Morse, as opposed to the harmonic, oscillator. As a first application of this model, the stretching overtone spectra of the benzene and water molecules have been calculated, these examples being chosen on account of the high degree of anharmonicity characterizing their potentials and the availability of suitable experimental data.     

Fundamental and overtone infrared spectra of CF3I

The i.r. spectra of CF₃I between 250 and 5000 cm⁻¹ have been measured with a resolution of 0.3–1 cm⁻¹ and with optical path lengths up to 4 bar·10 m. A few bands of a sample cooled down to 180 K and several bands of a sample enriched in ¹³CF₃I were also recorded. Values are assigned to most of the anharmonic x_ij and g_ii constants of ¹²CF₃I. Serious perturbations occur however by Fermi resonance between ν₁ and ν₂ + ν₃ and between ν₁ and 2ν₅, whose analysis is still very preliminary.     

Anharmonic overtone and combination states of glycine and two model peptides examined by vibrational self-consistent field theory

In this paper, the application of the vibrational self-consistent field (VSCF) and correction-corrected VSCF methods for calculating anharmonic parameters, including transition frequency, transition intensity and dipole, and vibrational anharmonicity of 3N-6 normal modes for formamide, glycine, N-methylacetamide and their deuterated derivatives are explored mainly at the level of density functional theory. The computed fundamental anharmonic frequencies are found to be in reasonable agreement with experimental results. Diagonal anharmonicities of the second overtone states were examined for multiple normal modes, whose magnitudes were found to correlate well with those of the first overtone states in the three small molecules. The results show that the VSCF-based approach can be utilized to predict anharmonic parameters of higher vibrational states that are essential to understanding multi-pulse infrared nonlinear experiments of peptides.     

Anharmonic analysis of the vibrational spectra of some cyanides and related molecules of astrophysical importance

A detailed analysis of the vibrational spectra of carbonyl cyanide, diethynyl ketone and acetyl cyanide has been conducted in harmonic and anharmonic approximations. RHF, MP2 and density functional theory (DFT) methods with 6-311++G(2df,2p) basis sets and B3LYP functionals have been employed. Spectroscopic constants such as anharmonicity constants, rotational and centrifugal distortion constants, rotation-vibration coupling constants and Coriolis coupling coefficients have been calculated for each molecule and compared with the experimental data, where available. A close agreement between the calculated and experimental values of the spectroscopic constants has been obtained. Complete assignments have been provided to the fundamental bands, overtones and combination tones of the molecules. Density functional theory based anharmonic frequencies compare well with the experimental frequencies within +/-18 cm(-1) on an average. RHF and MP2 methods, however, give much higher values for the frequencies that need scaling even in the anharmonic approximation.     

Deriving the 3D structure of organic molecules from their infrared spectra

The representation of the 3D structure of a molecule by a radial distribution function (RDF) code is described. The use of the RDF code for the simulation of an infrared spectrum by a counterpropagation (CPG) neural network is shown. Furthermore, a CPG network can also be operated in reverse mode: on input of an infrared spectrum an RDF code is obtained for which a 3D structure can be searched in a database. An empirical modelling process is used to refine this 3D structure to obtain a three-dimensional model of the molecular structure that corresponds to the infrared spectrum.     

A pure-dephasing model for overtone lineshapes

A model is developed for calculatinq the pure-dephasing (T₂) contribution to overtone lineshapes in isolated molecules. The linewidth is attributed to time-dependent fluctuations in the frequency of the overtone transition, resulting from the vibrational motions in the molecule, and no transfer of energy (intramolecular vibrational distribution) occurs in this picture. Numerical calculations for 2,3,5,6-paradeuterobenzene are presented.     

Homoleptic lanthanide triple-deckers of 5,15-diazaporphyrin with D2h symmetry

Two novel homoleptic diazaporphyrinato lanthanide sandwich com-plexes M(2)(DAP)(3) [M = Ce (1), Eu (2); DAP= 2,8,12,18-tetra-ethyl-3,7,13,17-tetramethyl-5,15-diazaporphyrinate] were synthesized by a one-pot reaction of the corresponding M(acac)(3).nH(2)O (acac = acetylacetonate) with metal-free D(2)(h) type diazaporphyrin, H(2)DAP. The spectroscopic and electrochemical properties are compared with those of complexes based on D(4)(h) symmetry porphyrin ligands.     

Calculation of ionization potentials of polycyclic hydrocarbons of symmetry D6h

Institute of Physical Organic Chemistry and Coal Chemistry, Academy of Sciences of the Ukrainian SSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 29, No. 6, pp. 131–134, November–December, 1988.     

A simple rule for prediction of intensities of vibrational overtone spectra

The intensities of vibrational overtone absorption transitions are described in terms of vibronic coupling of the ground molecular state to excited electronic configurations. Model calculations indicate an important role of nuclear geometry of excited electronic states relative to the ground state in determination of molecular overtone spectra. A simple rule for qualitative predictions of the overtone spectra for diatomic molecules or local bond modes of polyatomic molecules is proposed.     

Computational high-frequency overtone spectra of the water-ammonia complex

We have computed vibrational high-frequency overtone spectra of the water-ammonia complex, H(2)O-NH(3), and its isotopomers. The complex has been modeled as two independently vibrating monomer units. The internal coordinate Hamiltonians for each monomer unit have been constructed using exact gas phase kinetic energy operators. The potential energy and dipole moment surfaces have been calculated with the explicitly correlated coupled cluster method CCSD(T)-F12A and the valence triple-ζ VTZ-F12 basis around the equilibrium geometry of the complex. The vibrational eigenvalues have been calculated variationally and the eigenfunctions obtained have been used to compute the intensities of the absorption transitions. In H(2)O-NH(3), the water molecule acts as the proton donor and its symmetry is broken. The hydrogen-bonded OH bond oscillator undergoes a large redshift and intensity enhancement compared to the free hydrogen bond. Broken degeneracy of the asymmetric vibrations, quenched inversion splittings, and blueshift of the symmetric bending mode are the most visible changes in the ammonia unit.     

Vibrational overtone spectra of chloroanilines--evidence for intramolecular hydrogen bonding in o-chloroaniline

The near infrared vibrational overtone absorption spectra of liquid phase aniline and chloroanilines are reported. The analysis of the observed CH and NH local mode mechanical frequency values shows that intramolecular hydrogen bonding occurs between NH2 group and chlorine atom in o-chloroaniline. This observation supports the conclusion drawn from microwave spectroscopic studies reported earlier.     

Effects of torsion on O-H stretch overtone spectra and direct overtone photolysis of methyl hydroperoxide

We use laser photoacoustic spectroscopy to obtain overtone spectra at three through six quanta of O-H stretch excitation (3nu(OH)-6nu(OH)) for methyl hydroperoxide (MeOOH). Extending the spectral regions beyond our previous work reveals new features that can be attributed to transitions involving torsion about the O-O bond. Experimental spectral profiles (3nu(OH)-6nu(OH)) and cross sections (3nu(OH)-5nu(OH)) at room temperature show a good agreement with the simulated spectra that we obtain from ab initio calculations employing a vibration-torsion model at 298 K. A Birge-Sponer analysis yields experimental values for the O-H stretch frequency (omega=3773+/-15 cm(-1)) and anharmonicity (omegax=94+/-3 cm(-1)). We also detect OH radicals by laser-induced fluorescence and present photodissociation action spectra of MeOOH in the regions of 4nu(OH) and 5nu(OH). While the spectral profile at 5nu(OH) mimics the photoacoustic spectrum, the peak intensity for transitions to torsionally excited states is relatively more intense in the action spectrum at 4nu(OH), reflecting the fact that the 4nu(OH) excitation energy is below the literature dissociation energy (D0=42.6+/-1 kcal mol(-1)) so that features in the action spectrum come from thermally populated excited states. Finally, we use our calculations to assign contributions to individual peaks in the room-temperature spectra and relate our findings to a recent dynamics study in the literature.     

Computer-aided determination of relative stereochemistry and 3D models of complex organic molecules from 2D NMR spectra

A method for elucidation of the relative stereoconfiguration of natural product molecular structures and their 3D models based on NOE data and the application of a genetic algorithm is described. The method is applicable mainly for rigid polycyclic structures commonly encountered in natural products. It is demonstrated that the technique of simulated annealing cannot be easily used when dealing with low-weight fused ring molecules but the application of a genetic algorithm is proven successful. Examples of a typical genetic algorithm workflow and the optimization of the algorithmic parameters are discussed. The efficiency of the approach developed here is demonstrated on the complex natural products of both Taxol^® (C₄₇H₅₁NO₁₄) and brevetoxin B (C₅₀H₇₀O₁₄).     

Transition energies and oscillator strengths for electronic spectra of non planar conjugated molecules. I. Polyphenyls

Ultraviolet spectra of terphenyls and quaterphenyls are theoretically investigated by Molecules in Molecules (MIM) method and compared with experiment. Oscillator strengths are evaluated using the gradient operator and the mixed dipole-gradient operator. The results are compared with those obtained by the dipole-length operator. The calculation of oscillator strengths via the gradient operator is shown to be in general the most successful. All the components of the primitive integral, necessary for non planar -electron systems, are reported.

---

Zusammenfassung Die Ultraviolettspektren von Terphenylen und Quaterphenylen wurden mit Hilfe der Molekül im Molekül (MIM) Methode theoretisch untersucht und mit den experimentellen Resultaten verglichen. Mit einem Gradient-Operator und einem gemischten Dipol-Gradient-Operator wurden die Oszillatorenstärken berechnet und mit Rechnungen mit Dipollängen-Operatoren verglichen. Es wird gezeigt, daß die Berechnung von Oszillatorenstärken mit dem Gradient-Operator im allgemeinen am erfolgreichsten ist. Es werden alle Komponenten des primitiven Integrals, die für nicht planare -Elektronensysteme nötig, angegeben.

---

Résumé Les spectres dans l'ultraviolet des triphényles et des quadriphényles sont étudiés théoriquement par la méthode «Molécules dans les molécules» (MIM) et comparés à l'expérience. Les forces oscillatrices sont évaluées en utilisant l'opérateur gradient et l'opérateur mixte gradient dipolaire. Les résultats sont comparés avec ceux obtenus avec l'opérateur longueur dipolaire. Le calculades, forces oscillatrices à l'aide de l'opérateur gradient s'avère plus convenable. On reproduit toutes les composantes de «l'intégrale primitive», nécessaires pour les systemes d'électrons non plans.