Spectra and structure of silicon compounds. XVII Raman and infrared spectra and vibrational assignment for hexamethyldisilane and ab initio calculations for disilane and hexamethyldisilane

The IR spectra of gaseous and solid hexamethyldisilane between 4000 and 25 cm⁻¹ and the far-IR spectrum of the liquids from 450 to 25 cm⁻¹ have been recorded. The Raman spectra have been recorded from 3500 to 10 cm⁻¹ for all three physical phases. Assisted by ab initio calculations, the vibrational spectrum of hexamethyldisilane has been assigned under D_3d symmetry and the results of a normal coordinate analysis are discussed. No spectral features indicative of free internal rotation have been observed. Gradient ab initio calculations have been carried out for the disilane and hexamethyldisilane molecules using different types of basis sets. The structural parameters, rotational constants, unscaled and scaled frequencies and harmonic force constants have been reported for both disilane and hexamethyldisilane.     

Photoemission from matrix-isolated nickel atoms

UV photoelectron spectra from Ni atoms isolated in a xenon matrix at 9 K have been measured for the first time using Hell (40 8 eV). Two peaks have been found at 2.8 and 4.6 eV below E_F, which are interpreted as being due to different d⁸s¹ final states of the Ni ion. including a relaxation shift of 2.0 eV. The existence of atomic Ni species in the matrix has been verified by in situ optical absorption spectra.     

FT-Raman and infrared spectra,r0 structural parameters,ab initio calculations and vibrational assignment for nitryl chloride

The FT-Raman spectra (2000-30 cm⁻¹) of liquid and solid nitryl chloride, ClNO₂, along with the infrared spectra (2000-80 cm⁻¹) of the gas and solid have been recorded. All six fundamentals are confidently identified and the potential energy distributions determined from the force fields obtained from ab initio calculations. Several different basis sets have been utilized to determine the harmonic frequencies and force constants which are compared to the previously reported valence force constants. Structural parameters have been calculated with these basis sets including electron correlation with MP2, MP3 and MP4 perturbation. The calculated equilibrium structural parameters are compared to the experimental r₀ structural parameters. The spectra of the solid indicate that there are at least two molecules per primitive cell. All of these results are compared to the corresponding quantities for some similar molecules.     

Microwave spectroscopy and nuclear magnetic resonance study of the molecular geometry of 2-fluorophenylisocyanate

Analysis of the microwave spectra of the vibrational ground state of 2-fluorophenyl-isocyanate in the gaseous state shows the existence of a single planar cis conformer. Rotational spectra of the excited torsional states out of the plane and spectra of in-plane vibrations have also been studied. MINDO III type theoretical calculations corroborate the experimental results. Similarly, the configuration of 2-fluorophenylisocyanate in the liquid phase has been studied by NMR using the chemical shift reagent, Eu(fod)₃. Induced paramagnetic shifts have been measured on the ¹³C spectra, for different quantities of the reagent. The results clearly show a higher paramagnetic shift for the carbon bonded to the fluorine atom than for the other nuclei of the aromatic cycle (with the exception of the quaternary carbon), indicating the probable presence of the cis isomer.     

Vibrational spectra and structure of gaseous and solid dimethylboric anhydride

The Raman spectra of gaseous, liquid and solid dimethylboric anhydride (CH₃)₂BOB(CH₃)₂ have been recorded from 10–3500 cm^?1. The IR spectra from 4000–30 cm^?1 have also been recorded. The spectra of the gaseous phase have been interpreted in terms of C₂ symmetry implying a bent B-O-B skeleton with the B(CH₃)₂ groups twisted and consistent with a rather larger barrier to internal rotation about the B-O bonds. The spectra of the crystalline state, however, suggest that the molecular symmetry is altered upon solidification. Isotopic substitution of the oxygen atom by ¹⁸O confirmed that the B-O-B skeleton is linear in the solid state, and the spectra have been interpreted in terms of D_2h molecular symmetry.     

Umsetzung von metall- und metalloidverbindungen mit mehrfunktionellen molekülen : XIII. Herstellung von thioamidoboranen durch SiN-spaltung

N-Trimethylsilylthioamides with the trimethylsilyl group in the N-position react with haloboranes and haloorganylboranes with elimination of the halosilane to give monomeric and dimeric thioamidoboranes, respectively. The monomeric compounds mostly exist as mixtures of cis-trans isomers. Borotrope equilibria have also been found. ¹H, ¹¹B and ¹⁹F NMR spectra, mass spectra and IR spectra are reported.     

Infrared and electronic absorption spectra of some trisubstituted benzenes

The electronic absorption spectra of 2,3-, 2,4-, 2,5-, 2,6- and 3,4-difluorobenzonitriles, 3,4-difluoroaniline and 3,4-difluoroanisole in the ultraviolet region in vapour phase have been recorded on medium quartz and Hilger large quartz spectrographs and on a Hitachi model 150-20 UV-VIS ratio recording spectrophotometer. All the molecules investigated have exhibited two π* ← π band systems corresponding to ¹B_2u ← ¹A_1g (λ2600 Å) and ¹B_1u ← ¹A_1g (λ2100 Å) systems of benzene. The infrared absorption spectra of all the molecules studied have also been recorded and analysed. These infrared data have been taken to help analyse the u.v. spectra of the molecules studied.     

13C-NMR study of cresyl benzoates

The proton-decoupled ¹³C FT—NMR spectra of para-, meta- and ortho-cresyl benzoates have been recorded and the observed signals have been assigned following the substituent additivity rule. The important features of diagnostic value of these spectra are discussed.     

Absolute and relative emission cross sections for electron impact on CH3F,CH2F2 and CHF3

Emission spectra following electron impact on CH₃F, CH₂F₂ and CHF₃ at various energies have been investigated in the spectral region from 200 nm to 700 nm. Emission thresholds and excitation functions for atomic and molecular fragments have been determined. Absolute emission cross sections were obtained for two band systems, ^{A 2} Δ → X ² Π, C ² Σ⁺ → X ² Π, observed in the emission spectrum of CH₃F and for the H-Balmer radiation, H _α ? H _γ, in the spectra of all compounds. The continuous emission between 200 nm and 400 nm in the spectra of CH₂F₂ and CHF₃ has been examined systematically. It was found that CF₂(Ã) is the main precursor in both spectra.     

Vibrational spectra and torsional analyses of cis-2,3-dimethyloxirane and trans-2,3-dimethyloxirane

The Raman spectra of cis-2,3-dimethyloxirane and trans-2,3-dimethyloxirane in the vapor, liquid, and polycrystalline solid phases are reported for the region between 25 and 3100 cm^?1. The IR spectra of these two compounds between 80 and 4000 cm^?1 in the vapor and polycrystalline solid phases are also reported. In the IR and Raman spectra of gaseous trans-2,3-dimethyloxirane a total of eight torsional transitions have been observed. In the Raman spectrum of the cis compound in the vapor phase, four torsional transitions have been observed. From these experimental data, periodic barriers to the methyl torsional motions have been calculated to be 905 ± 7 cm^?1 (2.5 kcal mol^?1) for the trans molecule and 617 ±5 cm^?1 (1.76 kcal mol^?1) for the cis molecule. Additionally, complete vibrational assignments based on band contours, depolarization values, and group frequencies are proposed for both molecules and gas-phase thermodynamic functions have been calculated. These results are compared to the corresponding quantities for some similar molecules.     

Decay rates and γ-rays spectra of HeHμ systems

Radiative and nonradiative decay of charge non-symmetrical molecular ions ^3,4He ^1,2Hμ have been investigated. Decay rates in both channels and γ-ray spectra for the states of the total angular momentum L = 0, 1, 2 have been calculated using the hyperspherical “surface” functions method. Isotopic dependence of the decay rates is discussed. A comparison of the calculated γ-ray spectra with the recent experiments indicates the ^3,4Hedμ decay from the L = 1 state.     

Synthesis,spectral and computational studies of some N-acyl-t(3)-isopropyl-r(2),c(6)-bis-2′-furylpiperidin-4-one oximes

Four N-acyl-t(3)-isopropyl-r(2),c(6)-bis-2′-furylpiperidin-4-one oximes 1–4 were synthesized and the high resolution ¹H and ¹³C NMR spectra have been recorded at various temperatures and analyzed. The spectra reveal the presence of two rotameric forms (syn and anti) in solution. ¹H-¹H COSY and ¹H-¹³C COSY spectra have been recorded to assist the assignment of the signals for the syn and anti isomers of 1–4. Coupling constants predict an equilibrium mixture of boat form B₁ and alternate chair form CA for 1–4. The effect of varying the substituents at nitrogen and at C(4) on the ¹H and ¹³C chemical shifts has been analyzed in detail. The molecular structures of the N-acyl derivatives 1–4 were also determined using AM1 Hamiltonian and the results have been compared to the results derived from spectral studies. Mass spectra have also been recorded for 1–4.     

Microwave rotational spectra of alkaline earth monohalides: CaI and BaI

Rotational spectra in the ²Σ⁺ ground state of CaI and BaI have been measured in the 100 GHz range. Precise Dunham coefficients Y_ik and potential constants a_i can be derived from these spectra. Effects of unresolved hyperfine structure have to be considered in the interpretation of the observed γ-splitting.     

13C Pulse fourier transform NMR of menthol stereoisomers and related compounds

¹³C NMR spectra of menthol stereoisomers have been determined. The correlations of chemical shifts of these ring carbons with those of stereoisomeric 2-isopropylcyclohexanols are examined. Observed chemical shifts of 1-Me carbons are compared with those predicted from the chemical shifts of stereoisomeric 1-methyl-4-t-butylcyclohexanes. ¹³C NMR spectra of menthyl acetates, and cis and trans p -menthanes have also been examined.     

Raman spectra of gases: XIV. Hexachlorodisiloxane

The Raman spectra (50–1200 cm^?1) of gaseous, liquid, and solid (Cl₃Si)₂O have been recorded. The infrared spectra of the gas and solid have been recorded from 55–2000 cm^?1 . The spectra of the gas have been interpreted in detail on the basis of C_2v symmetry with the A₁ skeletal Si-O-Si bend assigned at 63 cm^?1. The spectra gave evidence that there are structural changes upon condensation of the gas and the Si-O-Si angle approaches linearity in the solid state. The opening of this angle is probably due to crystal packing factors.     

Electronic absorption spectra of 5- and 6- fluoroindoles

The electronic absorption spectra of 5-and 6-fluoroindoles, corresponding to the λ2850 Å system of indole, have been recorded in the vapour phase and analysed, assuming C_s symmetry for the molecules. The observed band system in both the molecules which lies in the region λ3100–2680 Å has been identified as a π* ← π transition corresponding to a ¹A′ ← ¹A′ transition. The i.r. spectra of these molecules have also been recorded and analysed and the results are used in the analyses of the electronic spectra. These show that there is not much change in the shape and very little change in size of either of these molecules in the excited electronic states.     

Vibrational circular dichroism,absolute configuration and predominant conformations of volatile anesthetics: 1,2,2,2-tetrafluoroethyl methyl ether

Vibrational absorption and circular dichroism spectra of (−)-1,2,2,2-tetrafluoroethyl methyl ether have been measured in CCl₄ solution in the 2000–900 cm⁻¹ region. These spectra are compared with the ab initio predictions of absorption and VCD spectra obtained with density functional theory using B3LYP/6-31G* and B3PW91/6-311G(2d) basis sets for different conformers of (R)-1,2,2,2-tetrafluoroethyl methyl ether. The results suggest that the trans-conformer of 1,2,2,2-tetrafluoroethyl methyl ether is predominant in the solution phase and that the (−)-enantiomer has the R-configuration.     

Nature of light-absorbing pigments from Brazilian lichens identified by Raman spectroscopy

Light-absorbing pigments from different chemical classes have been identified from the lichens Usnea sp. and Crocodia aurata using Raman spectroscopy supported by quantum mechanical DFT calculations. Raman spectra were obtained directly from the lichen tissues as well as from isolated extracts. Usnic acid, a chemomarker of Usnea spp has been reported together with a minor constituent, namely stictic acid, which has been unambiguously identified by ¹H and ¹³C NMR spectral analysis. The structures of calycin and pulvinic dilactone isolated from Crocodia aurata have been confirmed by single crystal X-ray diffraction. The ubiquitous carotenoids have been characterized by FT-Raman and dispersive Raman microimaging in tissues of Usnea sp. and C. aurata, respectively. The Raman map has revealed the presence of a mixture of carotenoids heterogeneously distributed in the upper layer of C. aurata. In this work we have demonstrated that Raman spectroscopy can be used to monitor aromatic and conjugated polyenic pigments in different layers of lichen tissues.     

Circular dichroism spectra of trans-chalcone epoxides

The electronic absorption and CD spectra of (−)-trans-chalcone epoxide and its derivatives with methyl and alkoxy substituents at the ortho-positions of the aromatic rings have been measured. The spectra have been assigned with help of the energies, oscillatory strengths, and rotatory strengths of the singlet transitions obtained from DFT calculations. The features of the CD spectra, indicative of the absolute configuration, are the carbonyl n–π¹ band and two further strong bands assigned to the overlapping signals of π–π¹ and n_epoxy–π¹ excitations.     

The infrared,Raman and NMR spectra,conformational properties and values for the ideal gas thermodynamic functions of cis- and trans-1,2-cyclobutanedicarbonitrile

The infrared spectra above 400 cm⁻¹ and the Raman spectra above 30 cm⁻¹ have been recorded for the cis- and trans- 1,2-cyclobutanedicarbonitriles as melts. ¹H and ¹³C NMR data are also reported. The observed spectral data, and notably the Raman depolarization data, are consistent with C₂ symmetry for the trans isomer and C₁ symmetry for the cis isomer. The probable conformations of each compound have been proposed, and evidence is put forward that the cis compound exists with a relatively small degree of ring puckering. Complete vibrational assignments have been proposed for both compounds apart from the cis ring puckering frequency which was assumed to be too low to observe. Values for the thermodynamic functions of the two compounds as ideal gases have been calculated and the possible errors in these values have been described.