CN stretching and NO2 deformation vibrations and normal coordinate analysis of m-dinitrobenzene and m-dinitrobenzene-d4

Two bands appear for each CN stretching and nitro deformation vibration in the infrared and Raman spectra of m-dinitrobenzene and m-dinitrobenzene-d₄. The 907 cm^?1 bending mode in the vibrational spectra of m-dinitrobenzene undergo 30 cm^?1 upward shift upon d₄ substitution. A normal coordinate analysis pointed out that the 937 cm^?1 bending and 727 cm^?1 CN stretching vibrations as well as 18b CD in-plane deformation are mixed to a great extent. The other nitro bending mode undergo also an inverse isotopic effect (2 cm^?1 upward shift) due to coupling with the 18a CD in-plane deformation vibration.     

High resolution infrared analyses of fundamentals and overtones in isotopic ketenes

Rovibrational structure in the ν₅, ν₆, ν₇, ν₈ fundamentals and 2ν₅, 2ν₆ overtones in each of H₂¹²C¹²CO, H₂¹²C¹³CO, H₂¹³C¹²CO has been assigned and analysed. The effects of Fermi resonance between ν₇ and ν₂+ν₈ are corrected for, and a set of accurate unperturbed vibrational frequencies and ¹³C isotopic frequency shifts therefrom is obtained. The changing effects of the major Coriolis interactions involving the out-of-plane fundamentals, ν₅ and ν₆, are manifest in varying isotopic displacements of equivalent Q branch structures between the isotopic species studied, from which accurate isotopic vibrational shifts are obtained. Variable anharmonicity constants associated with the vibration frequencies of 2ν₅ and 2ν₆ give evidence of major Fermi resonance interactions between both overtones and the ν₄ fundamental, which they straddle.     

Crystal field effects on the ground and lowest excited triplet states of benzene isotopes in a borazine host crystal

The phosphorescence spectra of the C₆H₆C₆H₅D₁p-C₆H₄D₂symp-C₆H₃D₃, C₆D₆ and ¹³CC₅D in a borazine host crystal are analyzed at high resolution. The spectral lines are sharp (～2 cm^?1 wide) indicating that the impurity molecules occupy a unique site in the borazine lattice which is probably substitutional. The phonon sidebands are weak,giving clean, well-resolved spectra much like those of isotopic mixed crystals. In contrast, however, the crystal field effects on the ground state vibrational levels are much smaller than those found for isotopic mixed crystals. The gas-to-crystal shifts are very small, the vibrational degeneracies are not removed and orientational splittings are only observable for a few select vibrational levels. For most vibrational levels and for the derivation of selection rules one can asume the effective crystal site symmetry to be D_3d. The data provide the first conclusive evidence that the splitting observed in the benzene phosphorescence spectrum results from a distortion of the molecule when excited to the zeroth vibrational level of the T₁ state. Furthermore, the data suggest that the distortion is intrinsic in nature (i.e.,is not caused by the crystal field).     

Tapes of water hexamer clusters in the interlayer space of a 2D MOF: Structural, spectroscopic and computational insight of the confined water

The synthesis, crystal structure and characterization of [Er₂(C₄H₄O₄)₃(H₂O)₄]·6H₂O, with particular emphasis on the structural details and the vibrational behavior of the highly organized sub-net of water are reported. X-ray structural analysis reveals that the hybrid framework, which can be classified as belonging to the I⁰O² type, acts as a host of an infinite tape of six-membered water rings with distorted chair conformation. The water network extends between the hybrid layers helping the close packing and the stabilization of the structure through H-bond interactions, with further development of the supramolecular tridimensional structure. The self-assembly of the hydration water molecules is conditioned by the hybrid host, forcing them to adopt a less stable conformation when compared with hexagonal ice. The vibrational spectra of partially and completely dehydrated samples as well as of partially isotopic diluted ones have been obtained and analyzed. In order to guide the assignment of the vibrational spectra a theoretical calculation was performed at the B3LYP/6-31G^∗∗ level for a model consisting of three water clusters.     

A theoretical study of the structure and vibrations of 2,4,6-trinitrotolune

Theoretical calculations of the structure, internal rotations and vibrations of 2,4,6-trinitrotolune, TNT, in the gas phase were performed at the B3LYP/6-31G* and B3LYP/6-311+G** levels of theory. Two genuine energy minimum structures were found. In both structures the 4-nitro group is planar to the phenyl ring, while the 2,6-nitro groups are slightly out of plane with the phenyl ring due to steric interaction with the methyl group. The two structures are related by internal rotations of the methyl and 2, or 6-nitro group. The lowest energy route for interconversion between them is a concerted motion of the methyl group and 2 or 6 nitro group in a ‘cog wheel’ type of mechanism. The geometry of the low energy structure A is closest to that observed in the crystal structures of TNT, where all three nitro groups are out of plane with the phenyl ring. FTIR and Raman spectra of solid TNT and ¹³C, ¹⁵N enriched TNT are presented and assigned with the help of the B3LYP/6-311+G** calculations on A. The lower level B3LYP/6-31G* calculation fails to predict the correct vibrational coupling between the nitro and phenyl groups. The B3LYP/6-311+G** calculation gives a good prediction of the nitro vibrations and the isotopic shifts observed for TNT isotopomers.     

Vibrational coupling between the nitro groups in 1,4-dinitrobenzene and 1,3-dinitrobenzene molecules: ab initio and normal coordinate analyses treatments

Ab initio quantum chemical computations have been performed for 1,4-dinitrobenzene and 1,3-dinitrobenzene and their ¹⁵N isotope-labeled derivatives at the 3-21G, 6-31G and 6-31G^* basis set levels. The results have been compared with the experiment: the best coincidence has been obtained by the 6-31G method for 1,4-dinitrobenzene and by the 6-31G^* method for 1,3-dinitrobenzene. The isotopic frequency shifts of the nitro group bands, induced by ¹⁴N–¹⁵N labeling and the vibrational coupling between the nitro groups, calculated with GAMESS software, have been in accordance with the measured ones. A normal coordinate analysis has been carried out for the 1,4-dinitro- benzene molecule using a slightly modified force field of 1,3-dinitrobenzene, reasonable fit between the observed and calculated frequencies has been obtained. The results support the conclusion that the electronic contribution to the vibrational coupling between the two nitro groups in the studied dinitrobenzenes is negligible: thus, it may be suggested that the dynamical coupling will predominate.     

The vibrational spectrum and normal coordinate analysis of chlorodifluoromethane,CHClF2

The vibrational spectrum of chlorodifluoromethane has been reexamined. The i.r. and Raman spectra of the three isotopic species, ¹²CHClF₂, ¹³CHClF₂ and ¹²CDClF₂, have been obtained with better accuracy and higher resolution than in previous work. An SCF ab initio geometry optimization and quadratic force field calculation have been performed, and this force field has been further optimized to best fit the observed frequencies. The absorption spectrum of the ν₄, 2ν₆ region has been recorded at high resolution (0.002 cm⁻¹) and an analysis of the Fermi resonance perturbation has been carried out.     

Isotopic mixed crystal exciton spectra in the far infrared: Naphthalene

The far infrared vibrational exciton spectra of isotopic mixed crystals of naphthalene-h₈ and d₈ were studied. The two observed translational phonon modes were determined to fall into the amalgamated band limit while the lowest energy B_3u, A_u and B_1u vibrational exciton bands were found to be in the separated band limit. The lowest energy B_3u “butterfly” mode with its large (15 cm^?1) exciton splitting was found to agree well with CPA calculations of mixed crystal spectra. A peak at 185 cm^?1 was also assigned as a peak in the vibrational exciton density-of-states of the B_3u mode.     

Low frequency raman spectra of isotopic mixed benzophenone crystals

In this paper, we present an experimental study of vibrational lattice modes in isotopic mixed crystals of benzophenone-h₁₀ and -d₁₀. Our results are discussed using theoretical and experimental data concerning other molecular isotopic mixed crystals. The spectra show two regions; in the first (ν < 100 cm^?1) we did not observe an interaction between the various vibrational lattice bands; in the second region (ν > 100 cm^?1), the interaction appears clearly. The 111.5 cm^?1 and 73 cm^?1 torsional modes of benzophenone-h₁₀ have the same behaviour as the external modes throughout the whole concentration range (0–100% of benzophenone-d₁₀).     

D+D 2 Quasiclassical rate constant calculations on parallel computers

Summary The calculation of rate constant values of theH+H ₂ reaction for an extended range of excited vibrational states of the diatomic molecule and temperatures is relevant to the modeling ofH ^– sources. To investigate the effect of isotopic substitutions on the efficiency of vibrational deexcitation processes, we extended the calculations to theD+D ₂ system. These calculations were carried out using a program restructured to run on a shared memory vector and parallel computer. The dependence of the efficiency of vibrational deexcitation processes from both the initial vibrational state and temperature of reactants is reported. Restructuring strategies adopted for implementing the program on both shared and distributed memory computers as well as speedups achieved on both types of machines are also discussed.     

Infrared spectra and structure of carbanions: XVIII. A semiempirical study of carbanions of cyanoacetic acid derivatives

The IR spectra of cyanoacetic acid, ethyl cyanoacetate and cyanoacetamide as well as those of related mono- and, whenever possible, dianions have been studied in dimethyl-sulphoxide (DMSO) and DMSO-d₆.The observed nitrile and carbonyl absorption frequencies correlate linearly with the corresponding Wiberg bond indices given by CNDO/2 calculations with full geometry optimization. These calculations predict carbanionic structures throughout except in the case of the dinegative ion of cyanoacetamide, which could be considered as originating from the aminoacetylenic tautomer of NCCH₂CONH₂. Parallel MINDO/3 calculations, however, predict that the latter dianion is again a carbanion. This result is in reasonable agreement with normal coordinate calculations and the experimental isotopic shifts of vibrational frequencies of the dianion ¹⁵NCCH^?CONH^?.     

Study of the ν4-2ν8 Fermi resonance in heavy isotopic species of CH3CN by the Modified Winther Method

The Modified Winther Method is used to calculate the Fermi coupling constant W for the resonance between ν₄ and 2ν₈ vibrational levels in four heavy isotopic species (¹³C, ¹⁵N) of gaseous CH₃CN. The change of isotopic mass is used as a frequency scanning variable. Reliable values of W (5.41 cm⁻¹)and of the anharmonicity of 2ν⁰₈ (−12.5 cm⁻¹) are obtained.     

Structural relaxations and energy effects of intramolecular motions inN,N-dimethylnitramine: A theoretical study

The equilibrium geometry of theN,N-dimethylnitramine molecule and changes in the energy and structural parameters due to the internal rotation of the nitro group and the inversion of the N atom in the amino fragment were calculated by the restricted Hartree-Fock (RHF) method and at the second-order Møller-Plesset (MP2) level of perturbation theory with inclusion of electron correlation using the 6–31 G^* and 6–31 G^** basis sets. The one-dimensional potential functions of these motions calculated at the RHF/6–31 G^* level were approximated by a truncated Fourier and power series, respectively. The frequencies of torsional and inversion transitions were determined by solving direct vibrational problems for a non-rigid model,i.e., taking into account the molecular geometry relaxation. The equilibrium conformation of the molecular skeleton ofN,N-dimethylnitramine is nonplanar. Transition states of the internal rotation of the nitro group and inversion of the amine N atom are characterized by pronounced concerted changes in its bond angles and the length of the N?N bond. In the MP2/6–31 G^* approximation, the height of the barrier to internal rotation calculated taking into account the difference in the zero-point vibrational energies is equal to 9.7 kcal mol^?1. Inversion in the amino fragment is accompanied by a relatively small energy change at the barrier height of ?1.0 kcal mol^?1 calculated in the same approximation.     

A vibrational study of the infrared spectrum of dibromodifluoromethane,CBr2F2

The i.r. spectrum of natural dibromodifluoromethane has been recorded at medium resolution in liquid phase and at moderately high resolution in vapor phase. A number of regions have been analysed and the vibrational and vibrational—rotational features of the observed spectral structures interpreted. The frequency values pertaining to the fundamentals of the bromine isotopic species have been either directly measured or evaluated by “difference” bands and their isotopic splitting checked by a normal coordinates calculation. Absorptions due to the minor component ¹³CBr₂F₂ have also been observed and in part assigned.     

Spectroscopic Identification of Hydrogen Bond Vibrations and Quasi-Isostructural Polymorphism in N-Salicylideneaniline

The ortho-hydroxy aryl Schiff base 2-[(E)-(phenylimino)methyl]phenol and its deutero-derivative have been studied by the inelastic incoherent neutron scattering (IINS), infrared (IR) and Raman experimental methods, as well as by Density Functional Theory (DFT) and Density-Functional Perturbation Theory (DFPT) simulations. The assignments of vibrational modes within the 3500–50 cm⁻¹ spectral region made it possible to state that the strong hydrogen bond in the studied compound can be classified as the so-called quasi-aromatic bond. The isotopic substitution supplemented by the results of DFT calculations allowed us to identify vibrational bands associated with all five major hydrogen bond vibrations. Quasi-isostructural polymorphism of 2-[(E)-(phenylimino)methyl]phenol (SA) and 2-[(E)-(phenyl-D₅-imino)methyl]phenol (SA-C₆D₅) has been studied by powder X-ray diffraction in the 20–320 K temperature range.     

Reaction of nitrogen dioxide with iron tetraphenylporphyrinate nitro complex containing the trans-1-methylimidazole ligand

Electron and IR spectroscopy, as well as the data on the isotopic analog ¹⁵NO₂, were used for the studies of the reaction of nitrogen dioxide (NO₂) with thin layers of the six-coordinated Fe^III-porphyrin nitro complex with trans-1-methylimidazole ligand Fe(TPP)(1-MeIm)(NO₂) (TPP is the meso-tetraphenylporphyrinate dianion), which is a heme-modelling system. It was shown that, like in the case of the five-coordinated nitrite complex, additional portions of NO₂ promote the nitrite-to-nitrate transformation, accompanied by the evolution of NO. The reaction leads to the formation in the layer of five- and six-coordinated Fe-porphyrin nitrate complexes and a small amount of imidazole complexes Fe^III(TPP)(1-MeIm)₂ with the outersphere nitrate anion. The latter complex became predominate after treatment of this layer with additional portion of 1-methylimidazole.     

Vibrational spectroscopic and ab initio molecular orbital studies of the normal and 13C-labelled trifluoromethanesulfonate anion

Infrared and Raman spectra of both normal and ¹³C-labelled tetrabutylammonium trifluoromethanesulfonate (triflate) [Bu₄N·CF₃SO₃] and mercury triflate Hg(CF₃SO₃)₂ were recorded at room temperature. The observed isotopic frequency shifts and bandwidths in the vibrational spectra of the triflate anion were taken into account in the assignments of the vibrational modes of the triflate anion. These assignments were supported by ab initio Hartree—Fock self-consistent field (HF-SCF) calculations of vibrational frequencies and normal modes for the triflate anion.     

Diode laser spectroscopy of lithium chloride at elevated temperatures

Diode laser spectroscopy of lithium chloride in the gas phase (850°C) has been carried out in the region 621 to 693 cm⁻¹. Transitions of three isotopic combinations of lithium chloride (⁷Li³⁵Cl, ⁷Li³⁷Cl and ⁶Li³⁵Cl) have been measured with a nominal accuracy of ± 0.001 cm⁻¹. The results were combined with existing microwave data and fitted to the usual Dunham equation to produce accurate values of the pure vibrational parameters (Y₁₀, Y₂₀, etc.) and a number of high-order parameters.