Potential energy surface of the photolysis of isocyanic acid HNCO

The dissociation curves of the photolysis of the isocyanic acid HNCO→HN+CO corresponding to the ground state (S0), the first triplet excited state (TO and the first singlet excited state (S1) have been studied respectively at the UHF/6-311G** and CIS/6-311G** levels using ab initio method. The energy surface crossing points, S1/T1 T1/S0 and S1/S0, have been found and the characteristics of the energy minimum crossing point were given, based on which, the changes of the crossing points' geometries along the lower electronic energy surface and its end-result have been located according to the steepest descent principle. The computational result indicates thatthe photolysis of the isocyanic acid HNCO→HN + CO has three competitive reaction channels ((A)-(C)), and from the kinetic piont of view, channel (A) is the most advantageous.     

Infrared spectrum of 4-methoxypicolinic acid N-oxide: computation of asymmetric O-H stretching band

In this article we studied the strong intramolecularly hydrogen-bonded system 4-methoxypicolinic acid N-oxide. The potential energy surface V = V(rOH,rOO) and the corresponding dipole moment function were calculated using the DFT B3LYP/6-31+G(d,p) level of approximation. The time-independent vibrational Schr?dinger equation was solved using a rectangular grid basis set and shifted Gaussian basis set. The vibrational spectrum and metric parameters were also calculated. Effects of deuteration were considered. The calculated vibrational spectra were compared with the experimental spectra. The vibrational transition corresponding to asymmetric O-H stretching that occurs at about 1400 cm-1 compares well with the experimentally assigned O-H asymmetric stretching band centered at 1380 cm-1. The corresponding asymmetric O-D stretching band was predicted to be at 1154 cm-1, while the experimental O-D band was not assigned due to its very low intensity. Several overtones and hot transitions of significant intensities were located in the vicinity of the fundamental O-H stretching frequency, effectively broadening the infrared absorption attributed to the O-H stretching mode. This is in a good agreement with the observed broad protonic absorptions found in the infrared spectra of the title compound and its analogs. We have shown that the Gaussian basis set is the method of choice for a two-dimensional vibrational problem that requires several hundreds of vibrational basis functions and when high accuracy of the eigenvalues is required or when extending the calculations to more vibrational degrees of freedom. We have also demonstrated that for a large number of basis functions the Gramm-Schmidt orthogonalization procedure outperforms symmetric and canonical orthogonalization schemes.     

Infrared and Raman spectra and vibrational assignments for 4-(dimethylamino)benzaldehyde and its zinc complex

The vibrational spectra of 4-(dimethylamino)benzaldehyde (DABA) and its zinc complex are presented and vibrational assignments are proposed. Significant differences between the spectra of the parent molecule and the complex are observed. Examination of the form of the normal modes facilitates explanation of proposed frequency shifts of ring modes upon complexation. The spectra of the complex are interpreted by employing a quinonoid-type structure for the DABA ligand.     

Infrared spectrum of Hg(OH)2 in solid neon and argon

Mercury(II) hydroxide molecules have been prepared upon mercury arc lamp irradiation of Hg, H(2), and O(2) mixtures in solid neon and argon. The strongest three infrared absorptions are identified through isotopic substitution (D(2), HD, (18)O(2), (16)O(18)O) and comparison to frequencies from DFT calculations. The isolated Hg(OH)(2) molecule is stable and has a linear O-Hg-O linkage in a C(2) structure with an 86 degrees dihedral angle. However, in aqueous solution Hg(2+) and 2OH(-) may form an Hg(OH)(2) intermediate, which eliminates water and precipitates solid HgO: The solid Hg(OH)(2) compound is not known.     

Monothiodibenzoylmethane: Structural and vibrational assignments

The vibrational structure of the title compound (1,3-diphenyl-3-thioxopropane-1-one, TDBM) was studied by a variety of experimental and theoretical methods. The stable ground state configuration of TDBM was investigated by infrared (IR) absorption measurements in different media, by linear dichroism (LD) polarization spectroscopy of samples partially aligned in a stretched polymer matrix, and by Raman spectroscopy. The investigation of the metastable photoproduct of TDBM was based on the previously published spectrum of the product trapped in argon matrix [Y. Posokhov, A. Gorski, J. Spanget-Larsen, F. Duus, P.E. Hansen, J. Waluk, Chem. Phys. Lett. 350 (2001) 502]. The observed vibrational spectra were compared with theoretical transitions obtained with B3LYP/cc-pVTZ density functional theory (DFT). The results leave no doubt that the stable ground state configuration of TDBM corresponds to the intramolecularly hydrogen bonded-enol form (e-CCC), and that the photoproduct corresponds to an “open”, non-chelated enethiol form (t-TCC), thereby supporting the previous conclusions by Posokhov et al. No obvious indications of the contribution of other forms to the observed spectra could be found.     

Potential energy scans and vibrational assignments of cyclopropanecarboxylic acid and cyclopropanecarboxamide

The structural stability and internal rotations in cyclopropanecarboxylic acid and cyclopropanecarboxamide were investigated by the DFT-B3LYP and the ab initio MP2 calculations using 6-311G** and 6-311+G** basis sets. The computations were extended to the MP4//MP2/6-311G** and CCSD(T)//MP2/6-311G** single-point calculations. From the calculations the molecules were predicted to exist predominantly in the cis (C=O group eclipses the cyclopropane ring) with a cis-trans barrier of about 4-6kcal/mol. The OCOH torsional barrier in the acid was estimated to be about 12-13kcal/mol while the corresponding OCNH torsional barrier in the amide was calculated to be about 20kcal/mol. The equilibrium constant k for the cis<-->trans interconversion in cyclopropanecarboxylic acid was calculated to be 0.1729 at 298.15K that corresponds to an equilibrium mixture of about 85% cis and 15% trans. The vibrational frequencies were computed at the DFT-B3LYP level. Normal coordinate calculations were carried out and potential energy distributions were calculated for the low energy cis conformer of the molecules. Complete vibrational assignments were made on the basis of normal coordinate calculations and comparison with experimental data of the molecules.     

Infrared band intensities and bond polarities: Part 4. The bond-bending vibration of acetylene

Bond moment theory which takes rehybridization effects into account is used to analyze the band intensity of the π_u vibrational mode of acetylene. A value of μ_b(C XXX H) = 0.82 D is obtained which is in accord with that of ≈0.22 D, sense C XXX H, previously obtained for the CH bond in ethylene.     

Thioacetylacetone: structural and vibrational assignments.

Thioacetylacetone and its variously deuterated isotopomers have been investigated using electronic and vibrational spectroscopy combined with quantum chemical calculations. Thioacetylacetone is known for its photochromic properties, but the structures of the initial and final forms have been the subject of a long debate. Analysis of the IR spectra recorded in low-temperature argon and xenon matrices, room-temperature solutions, and in the gas phase has allowed us to establish the nature of the photochromic species and of its precursor. Similar to the case of another beta-thioxoketone, monothiodibenzoylmethane, the photo-product has been assigned to the nonchelated SH exo-rotamer of the (Z)-enethiol tautomeric form, whereas the dominant ground-state species corresponds to the chelated (Z)-enol tautomeric form. Detailed vibrational assignments have been proposed for both forms based on quantum chemical calculations and polarization experiments. In the case of the chelated (Z)-enol species prevailing in the ground state, a second-order perturbative anharmonic analysis at the B3LYP/cc-pVTZ level indicated strong anharmonic effects associated with the intramolecular hydrogen bond, leading to a shift of more than 600 cm-1 of the wavenumber of the OH-stretching vibration. A small fraction of the SH endo-rotameric chelated (Z)-enethiol form was also detected under unperturbed conditions. The (Z)-enethiol form can be converted into the (Z)-enol form by irradiation at 290 nm.     

Infrared spectrum of the novel electron-deficient BH(4) radical in solid neon

Laser-ablated boron reacts with hydrogen on condensation in excess neon to give BH4 radical, BH4- anion, and B2H6 as the major products. Identifications are based on 10B and D substitution, DFT frequency calculations, and comparison to previous spectra. Infrared spectra of BH4 support the C2v structure deduced from previous ESR spectra and theoretical calculations with two normal B-H bonds and two long B-H bonds for this novel electron-deficient radical. NBO analysis suggests that the two long B-H bonds and the H- -H bond are one-electron bonds.     

The vibrational spectrum of methyltrioxorhenium(VII)

The inelastic neutron scattering and high-resolution FT-Raman spectra of methyltrioxorhenium are reported. Literature assignments of the modes below 1600 cm(-1) are largely confirmed with the exceptions of the methyl torsion and the ReO3 bending modes. The methyl torsion is observed for the first time at 200 cm(-1), while the band at 230 cm(-1) assigned previously to the torsion is shown to be a component of the factor group splitting of the solid. In the FT-Raman spectrum, the bands that derive from the E modes in the isolated molecule are split by the Cs site symmetry of the crystal. This allows an unambiguous assignment of these modes to be made. On this basis, the assignment of the ReO3 bending modes is reversed with the asymmetric mode at 325 cm(-1) and the symmetric mode at 276 cm(-1).     

The vibrational spectrum of (trimethylenemethane)iron tricarbonyl

A vibrational assignment for the trimethylenemethane molecule, in [C(CH₂)₃]Fe(CO)₃, has been proposed for the first time. CC₃ stretches were found at 1348 (E) and 918 cm⁻¹ (A₁), with skeletal deformations at 802 cm⁻¹ (A₁, out-of-plane), and 471 cm⁻¹ (E, in plane). The Fe(CO)₃ vibrations were closely similar to those in analogous complexes.     

On the vibrational assignments and force constants of tetrasulphurtetraimide, (SNH)4

Infrared and Raman spectra of (S¹⁵NH)₄ and (S¹⁵ND)₄ have been recorded for the first time. The spectra are compared with those of (S¹⁴NH)₄ and (S¹⁴ND)₄ reported earlier. The observed isotopic shifts support the vibrational assignments and force constants published in 1977 by Steudel and Rose, while the alternative assignments published in 1978 by Turowski et al. and by Domingo and Orza are shown to be partly in error.     

Rearrangement of Langmuir-Blodgett stearic acid films and band assignments in deuterated stearic acid monolayers

Band assignments in the C-D stretching region of straight chain hydrocarbon species are derived from the spectra of stearic acid monolayers on gold. The fatty acid molecules reorient with respect to the metal surface as the films age. Correlating the changes in the IR spectra of both the undeuterated and deuterated acids allows one to identify the vibrational modes of the latter based on the accepted assignments of the former. The CD₂ asymmetric and symmetric stretches are observed at 2194 and 2086 cm^–1, respectively. Bands at 2212 and 2221 cm^–1 are attributed to asymmetric in-plane and out-of-plane CD₃ stretches. Assignments of several other features in this region are given while one band remains unassigned.     

Infrared and raman spectra,vibrational assignments,normal coordinate analysis,and ab initio calculations of methyltrifluoromethyldisulfide and bis(trifluoromethyl)disulfide

The infrared and Raman (3500-35 cm^–1) spectra of gaseous and solid methyltrifluoromethyldisulfide, CF₃SSCH₃, and bis(trifluoromethyl)disulfide, CF₃SSCF₃, have been recorded. Additionally, the Raman spectra of the neat liquids have been obtained and qualitative depolarization values have been measured. These vibrational data have been interpreted, for both molecules, on the basis that the C-S-S-C dihedral angle is approximately 90°. Vibrational assignments are given for both molecules and are supported by normal coordinate calculations utilizing ab initio Hartree-Fock gradient calculations with the 3-21G^* basis set to obtain the frequencies for the normal modes and potential energy distributions. The CH₃ and CF₃ torsional modes have been observed at 140 and 48 cm^–1, respectively, for CF₃SSCH₃, from which periodic barriers of 485 cm^–1 (1.39 kcal mol^–1) and 853 cm^–1 (2.44 kcal mol^–1), respectively, have been calculated. Complete equilibrium geometries have been determined for both molecules by ab initio calculations employing both 3–21G and 6–31G basis sets. The structural parameters for bis(trifluoromethyl)disulfide are compared to those suggested from electron diffraction studies. The results are compared to corresponding quantities obtained for some similar molecules.Taken in part from the thesis of M. M. Bergana which will be submitted to the Department of Chemistry in partial fulfillment of the Ph.D. degree.     

Infrared band intensities and bond polarities: Part 5 - XCl4 molecules (X = C,Si, Ge,Sn)

Gas phase band intensities of v₃ and v₄ of SiCl₄, GeCl₄ and SnCl₄ have been derived from dispersion curves measured using a Michelson interferometer in the range 1000 to 10 cm^?1. The results obtained are discussed in relation to the corresponding liquid phase band intensities. Bond polar properties are derived from the observed intensities using a modified Gribov method, the moments obtained being 0.9 D for SiCl, 1.5 D for GeCl and 2.7 D for SnCl.     

Infrared spectrum of poly(vinylidene fluoride)

Two crystalline forms (α and β) of poly(vinylidene fluoride) were studied by infrared spectroscopy. The spectral differences permitted the study of the transformation and the ratio of the two forms. The ordinary \documentclass{article}\pagestyle{empty}\begin{document}$ \vec G,\vec F $\end{document} matrix method was used to calculate the fundamental mode with a Urey-Bradley type potential field, and a preferred set of the force constants was obtained.     

Coupled calculation of vibrational frequencies and intensities

The combination of normal coordinate analysis with intensity calculations gives quantitative information about molecular force fields and the assignments of vibrational frequencies. Calculations of vibrational intensities by means of a standard CNDO/2 version give rise to satisfactory results for the IR intensities. However, the calculated Raman intensities often differ strongly from the experimental data. Inclusion of 2p-polarization functions on hydrogen in the usually used valence basis set is quite successful to obtain improved molecular polarizabilities as well as Raman intensities.     

The vibrational spectrum of (maleic anhydride)iron tetracarbonyl

On the basis of solid-phase IR and Raman spectra, with some solution data for the IR, a reasonably complete vibrational assignment has been made for the modes of maleic anhydride in (maleic anhydride)iron tetracarbonyl. Shifts in v(C=C) and δ(CH) are consistent with a strong interaction with the metal, but relatively little coupling between the modes. More restricted assignments were made for modes associated with the (maleic anhydride)iron and Fe(CO)₄ fragments.