Molecular Constants of MnF6 −2 Anion

Abstract

The normal coordinate analysis of XY₆ type of anions and molecules employing different force fields is well established.^1,2 Recently, the vibrational spectra and complete frequency assignment of MnF₆ ^?2 has been reported by Flint.³ Using the available vibrational data the force constants have been evaluated employing three potential models viz. general valence force field (GVFF), Urey-Bradley force field (UBFF) and orbital valence force field (OVFF). The mean amplitudes of vibration and Bastiansen-Morino shrinkages have also been computed at two temperatures.     

Vibrational Parameters of Some Mixed Tetracoordinate Halogenomercurate(II) Complexes

Abstract

The normal coordinate analysis has been performed for the mercury(II) halogeno complexes HgX₃Y²? (X≠Y=C1, Br, I) using latest Raman spectral data for the first time. The general quadratic valence force field has been used in this study. The results in turn are utilized to investigate the useful vibrational parameters such as compliance constants and mean amplitudes of vibration. The trends among the vibrational parameters are discussed and the bond properties of the mercury(II) and related complexes are examined.     

Scaled DFT Force Constants and Vibrational Spectrum of Cyclopropylamine

Abstract

The molecular structure and harmonic vibrational frequencies of cyclopropylamine have been calculated using the B3LYP density functional method with the 6‐31G(2d,2p) basis set. The scaled DFT force field gives very good reproduction of the experimental vibrational frequencies. Several of the vibrational fundamental modes assigned previously are reassigned on the basis of the B3LYP/6‐31G(2d,2p) method and the scaled force field calculations. The optimized scaling factors were used to scale the B3LYP/6‐31G(2d,2p) force field of cyclopropane and cyclopropylamine (–ND₂) molecules.     

A refined potential energy surface and the rovibrational states for the electronic ground state of ozone

A potential energy surface for the electronic ground state of ozone has been optimized by using a variational procedure with the exact vibrational Hamiltonian in bond length-bond angle coordinates. In the optimization, the ab initio force field of Borowski, P., Andersson, K., Malmquist, P.-A., and Roos, B. O., 1992, J. chem. Phys., 97, 5568 is taken as the starting point, and the recent observed vibrational band origins up to 4900 cm^-1 reported by Floud, J.-M., Barbe, A., Camy-Peyret, C., and Plateaux, J. J., 1996, J. molec. Spectrosc., 177, 34 are involved. The root mean square error of this fit for the 39 observed vibrational energy levels is 0.83 cm^-1. In order to test the refined potential, the rovibrational energy levels up to J = 15 are calculated and compared with the observed values.     

Force field calculations of acetonitrile using CNDO/force method

The redundancy-free internal valence force field (RFIVFF) of acetonitrile is reported using CNDO/force method. The initial force field is set up by taking the interaction and bending force constants from CNDO force field and transferring stretching force constants from the force fields of chemically related molecules. The final force field is obtained by refining the initial force field using vibrational harmonic frequencies of CH₃CN,¹³CH₃CN, CH₃ ¹³CN, CH₃C¹⁵N, CD₃CN and CD₃ ¹³CN. The final force field thus obtained is found to be excellent on the basis of frequency fit and potential energy distribution.     

Vibrational Spectroscopic and Theoretical Studies of Urea Derivatives with Biochemical Interest: N,N′-Dimethylurea,N,N,N′,N′-Tetramethylurea,and N,N′-Dimethylpropyleneurea

Abstract

Mid-infrared, far-infrared, and Raman vibrational spectroscopic studies were combined with density functional theory (DFT) calculations and normal coordinate force field analyses for N,N′-dimethylurea (DMU), N,N,N′,N′-tetramethylurea (TMU), and N,N′-dimethylpropyleneurea (DMPU: IUPAC name 1,3-dimethyltetrahydropyrimidin-2(1H)-one). The equilibrium molecular geometry of DMU (all three conformers), TMU, and DMPU and the frequencies, intensities, and depolarization ratios of their fundamental infrared (IR) and Raman vibrational transitions were obtained by DFT calculations. The vibrational spectra were fully analyzed by normal coordinate methods as well. A starting force field for DMPU was obtained by adapting corresponding force constants for DMU and TMU, resulting after refinements in the stretching force constants C=O (7.69, 7.30, 7.68 N·cm^?1), C–N (5.16, 5.55, 5.05 N·cm^?1), and C-Me (5.93, 4.00, 4.22 N·cm^?1) for DMU, TMU, and DMPU, respectively. The dominating conformer of liquid DMU was identified as trans-trans, strong intermolecular hydrogen bonding was verified in solid DMU, and weak dipole–dipole association was found in liquid TMU and in DMPU. Special attention was paid to analyzing the methyl group frequencies, which revealed deviations from local C_3v symmetry. A linear correlation was found between the CH stretching force constants and the inverse of the CH bond lengths (1/r ²). The averaged NH stretching frequencies of gaseous, dissolved, and solid urea and of DMU, with variations for hydrogen bonding of different strength, are linearly correlated to the NH stretching force constants. Characteristic skeletal vibrations were assigned for a broad variety of urea derivatives and also for pyrimidine derivatives, which all contain the N₂C=O entity. The very strong IR bands of C=O stretching (1,676 ± 40 cm^?1) and asymmetric CN₂ stretching (1,478 ± 60 cm^?1), and the very intense Raman feature of symmetric CN₂ stretching or ring breathing (757 ± 80 cm^?1), can be recognized as fingerprint bands also for the pyrimidine derivatives cytosine, thymine, and uracil, which all are nucleobases in DNA and RNA nucleotides.     

Vibrational Spectra and Normal Coordinate Analysis for an Imidazole Bridged Copper,Zinc Binuclear Complex

Abstract

The normal coordinate analysis of a model compound for Cu, Zn-SOD: [(dtma)CuImZn(dtma)]CIO₄ · 2.5H₂O (where dtma = 4-diethylenetriamineacctate; Im = imidazolate) has been carried out by using a Urey-Bradley force field. According to the molecular structure, 240 internal coordinates were established and 174 theoretical vibrational frequencies were calculated. Due to introducing an appropriate set of internal coordinates and force constants in the course of calculation, the calculated frequencies agree well with the observed values, with the average difference 4.40cm^?1 and the maximum deviation 24.2cm^?1 between them, although the structure of the title compound is complicated. Some structural and spectral properties are here discussed.     

Determination of force fields for two conformers of nitromethane bycndo/Force method

cndo/Force method is used to evaluate the redundancy free internal valence force fields for two conformers of nitromethane. The initial force field is set up by taking the interaction and bending force constants from this method and transferring the stretching force constants from the force fields of chemically related molecules. The final force field is obtained by refining the initial force field using vibrational frequencies of isotopic speciesviz CH₃NO₂, CD₃NO₂, CH₃ ¹⁵NO₂ and CH₃N¹⁸O₂. The final force field thus obtained is reasonable on the basis of frequency fit and potential energy distribution. The barrier to internal rotation is found to be 0.048 kcal mol⁻¹.     

Vibrational Spectroscopic Studies of Molecules with Biochemical Interest: The Cysteine Zwitterion

Abstract

The L-cysteine zwitterions in the orthorhombic crystal structure and in aqueous solution, including the deuterated isotopologues HSCD₂CH(NH₃ ⁺)COO^?, DSCH₂CH(ND₃ ⁺)COO^?, and DSCD₂CH(ND₃ ⁺)COO^?, have been studied by mid-infrared, far-infrared, and Raman spectroscopy. Density functional theory (DFT) calculations were performed for an equilibrium molecular geometry of the cysteine zwitterion to obtain vibrational frequencies of fundamental modes, infrared (IR) and Raman intensities, and the depolarization ratio of the Raman bands and combined with normal coordinate force field analyses. The force field obtained for dissolved (in H₂O and D₂O) cysteine, based on the 4 × 36 experimental fundamental modes of the four isotopologues, was successfully transferred to the two conformers in the solid state. The experimentally observed multiple bands (generally doublets) of L-cysteine and its deuterated isotopologues in the solid state were interpreted based on the coexistence of two conformers in the unit cell. The calculated frequencies were used for full assignments of the fundamental IR and Raman vibrational transitions, including an attempt to interpret all low-frequency vibrations (below 400 cm^?1) of the zwitterion also in the solid state. In particular, the hydrogen bonding effects on conformation, bond lengths, and force constants were studied, including those of the distorted NH₃ ⁺ amino group. The –S-H and -S-D stretching vibrations were found to be local modes, not sensitive to deuterium substitution of the -CH₂ and -NH₃ ⁺ groups in the molecule or to the H(D)-S-C-C torsional angle. The two major -S-H or -S-D stretching bands observed in the solid state correspond to different S-H/D bond lengths and resulted in the force constants K _SH = 3.618 N·cm^?1 and 3.657 N·cm^?1 for the SH^… S and SH^… O hydrogen-bonded interactions. A remarkable result was that the S(H)^… O interaction was weaker than the S(H)^… S interaction in the solid state and even weaker in aqueous solution, K _SH = 3.715 N·cm^?1, possibly due to intramolecular interactions between the thiol and amino groups. A general correlation between the S-H/D bond length and vibrational frequency was developed, allowing the bond length to be estimated for sulfhydryl groups in, for example, proteins. The C-S stretching modes were fitted with different C-S stretching force constants, K _CS = 3.213 and 2.713 N·cm^?1, consistent with the different CS bond lengths for the two solid-state conformers.     

Theoretical study of the phonon–phonon scattering mechanism and the thermal conductive coefficients for energetic material

Abstract

The elastic constants and force constants of an energetic material named as 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) are calculated by first-principles method under the GGA+vdW functional. Based on them, a physical picture to describe the phonon–phonon scattering mechanism is obtained, and a method to evaluate the thermal conductive tensor is developed. The resulting thermal conductivities are in agreement with the available experiments. We find that 20 vibrational modes play important roles in determining the heat exchange process of TATB. The vibrational direction, symmetry, displacement field and the detailed information of the key vibrational modes are obtained.     

Neutron inelastic scattering spectrum and valence force field for neopentane

The neutron inelastic scattering spectrum of neopentane has been measured between 10 and 2000 cm^?1. A partial reassignment of the normal modes of vibration is presented. A vibrational analysis based on the Snyder and Schachtschneider valence force field has been carried out using the frequencies of neopentane and neopentane-d₁₂. The calculated neutron spectrum, using our new force field, is found to be in good agreement with the observed neutron data.     

The computed force constants and vibrational spectra of cubane

The geometry, complete harmonic force field, and dipole moment derivatives of cubane, C₈H₈, have been calculated at the Hartree-Fock level using a 4–21 Gaussian basis set. The infrared and Raman spectra of cubane and four deuterated derivatives were calculated and compared with previously observed spectra. A set of five scale factors for the calculated force constants was then derived by least-squares fitting of the fundamental vibrational frequencies calculated from the scaled force field to the frequencies obtained by direct experimental measurement. The resulting scaled quantum-mechanical (SQM) force field, containing 73 unique elements, is believed to give an accurate representation of the harmonic vibrational potential of cubane. In most cases, the spectral assignments previously made from purely empirical considerations were confirmed, but a few corrections are proposed. The only major alteration is for an A_2u mode revised to appear at 1030 cm^?1 in the undeuterated molecule. Coriolis constants and approximate infrared intensities are also calculated.     

Charge transfer interaction and vibrational spectral investigation of 2-amino-5-nitropyridinium sulfamate

Abstract

A hetero aromatic nonlinear optical crystal, 2-amino-5-nitropyridinium sulfamate was synthesized by slow evaporation method. An optimized molecular structure and theoretical vibrational spectra were obtained and analyzed by use of density functional theory. The observed vibrational wavenumbers in the experimental spectra were compared with the calculated results. Scaled quantum mechanical force field methodology with normal coordinate analysis was used for the assignment of experimental infrared and Raman spectra. The second harmonic generation and photoluminescence properties of the crystal were studied. The charge transfer interaction between the donor acceptor moieties were obtained by means of natural bond orbital analysis.     

Force field calculation for inplane vibrations of ethylene usingcndo/Force method

cndo/Force method is used to evaluate redundancy-free internal valence force field (rfivff) for inplane vibrations of ethylene. The bending force constants, the stretch-band and bend-bend interaction force constants are predicted reasonably well in magnitude and sign by this method; whereas stretching force constants and stretch-stretch interactions are overestimated. Initial force field is set up by transferring stretching force constants from structurally-related molecules and including the rest of the force constants fromcndo force field. The field so constructed is subjected to refinement by the least square method. A total of 64 vibrational frequencies of C₂H₄, C₂D₄, C₂H₂D₂ and their¹³C isotopic modifications are used to determine force field containing 15 parameters. The final force field is found to be reasonable on the basis of frequency fits, potential energy distribution and band assignments.     

Vibration-rotation spectra and the harmonic force field of HOCl

Vibration-rotation spectra of HOCl have been measured at a resolution of 0.05 cm^?1 to determine vibration rotation constants, and 35–37 Cl isotope shifts in the vibration frequencies. The spectrum of DOCl has also been recorded, and a preliminary analysis for the band origins has been made. The vibrational frequency data and centrifugal distortion constants have been used to determine the harmonic force field in a least-squares refinement; the force field obtained also gives a good fit to data on the vibrational contributions to the inertial defect. The equilibrium rotational constants of HOCl have been obtained, and an equilibrium structure has been estimated.     

Sextic force field of nitrous oxide

The sextic force field in the curvilinear internal coordinates has been studied for the nitrous oxide molecule from the spectroscopic data of ¹⁴N₂¹⁶O, ¹⁴N¹⁵N¹⁶O, and ¹⁵N¹⁴N¹⁶O. The bands below 6600 cm⁻¹ have been used. The force constants in the internal coordinates are converted to those in dimensionless normal coordinates by two successive transformations. The vibration Hamiltonian matrix for each symmetry species of a given isotopic species has been constructed from the harmonic oscillator basis functions, and it is then diagonalized numerically to give the vibrational energy levels and the wavefunctions. The latter have been used for the evaluation of ratational constants. The least-squares refinement has been very successful in the present study, and it is shown that the general quartic force field supplemented by the quintic and sextic stretching diagonal force constants estimated from the Morse function, provided that the terms up to sextic are kept in the dimensionless normal coordinate space, well reproduces the spectroscopic constants such as the vibrational levels, rotational constants, l-type doubling constants, and centrifugal distortion constants. The spectroscopic constants of the isotopic molecules which are excluded from the refinement process are also in good agreement with the computed ones. The bond dissociation energies of the NN and NO bonds estimated from the present results have been critically examined.     

The microwave spectrum,harmonic force field,and structure of difluorodichloromethane

Very large numbers of rotational transitions have been accurately measured for ¹²CF₂³⁵Cl₂, ¹²CF₂³⁵Cl³⁷Cl, and ¹³CF₂³⁵Cl₂, and have been analyzed for rotational constants and quartic centrifugal distortion constants. The distortion constants have been combined with vibrational wavenumbers (both from the literature and from the present work), and with ab initio force constants also evaluated in the present work, to give an approximate harmonic force field. The rotational constants and force field have been used to evaluate ground state effective, substitution, and ground state average structures for the molecule.     

Vibrational fluorescence of CN− centers in KCl by electronic excitation of neighbouring Tl+ centers

Abstract

Infrared emission from higher vibrational states of CN^? in KCl has been achieved by optically exciting nearby Tl⁺ ions with an excimer laser. The electronic vibrational energy transfer, already known from F_H(CN^?) in alkali halides, leads to population of vibrational states up to at least the tenth level.     

Formate anion: The physical force field

Unequivocal assignments for all of the vibrational modes of the formate ion have been made from isotopic shifts measured in the Fourier transform infrared and Raman spectra of polycrystalline samples of the sodium salts of HCO₂^?, H¹³CO₂^?, DCO₂^?, and D¹³CO₂^?. Literature frequencies for DCO¹⁸O^? and DC¹⁸O₂^? have been added to the above results for the unique determination of the 10 force constants in the general harmonic force field. Product rule relations among the harmonic frequencies are shown to interfere seriously with the determination of the physical force field by the standard normal coordinate computation. The least-squares refinement is found to be particularly sensitive to identified types of anharmonic error in the frequencies. Refinements characterized as Φ^aH and Φ^aD improve reliability by elimination of the deuterium isotopic shift for νCH. The physical force field is interpreted in terms of the geometric and electronic structure.