P. Klaeboe

Conformational studies by vibrational spectroscopy: a review of various methods

Abstract The more important methods employed in conformational studies by infrared and Raman spectroscopy have been discussed. They include experimental methods to identify the observed vibrational bands with one or more conformers: crystallization under low temperature and high pressure conditions, variable temperature spectroscopy in the vapour and liquid states and matrix isolation spectroscopy with the hot nozzle method, including the effects of annealing and of photolysis. Differential scanning calorimetry is included, and certain molecules which have two or more solid phases each with a separate conformer are mentioned. An additional technique is spectral recordings of the compound as a solute in solvents of different polarity. Sometimes the formation of clathrates may favour one conformer. Experimental determinations of enthalpies and barriers using variable temperature studies and the analysis of torsional or puckering potentials are described. Computational methods including normal coordinate calculations, molecular mechanics and ab initio quantum chemical methods are included. The methods have been illustrated with compounds from the literature including substituted ethanes, cyclohexanes, cyclobutanes, neopentanes and related molecules.

The vibrational spectra, molecular structure and conformations of organic azides. I. A survey

Abstract A number of organic monoazides (RN 3 ) have been synthesized in which R is: (1) a saturated group, CH 3 , C 2 H 5 , nC 3 H 7 ; (2) an olefinic group, allyl, butadiene; (3) an acetylenic group, NCCH 2 , HCCCH 2 , CH 3 CCCH 2 . Two additional unsaturated diazides (CH 2 C(N 3 )C(N 3 )C(N 3 )CH 2 and N 3 CH 2 CCCH 2 N 3 ) were prepared. The compounds (most of them very explosive) were studied by IR and Raman spectroscopy in the liquid, in solution and in the solid state, and by matrix isolation technique in IR. The spectra were interpreted in terms of one or in some cases two or more conformers and assigned with the aid of normal coordinate analysis. UV photolysis experiments in nitrogen matrices at 12 K were carried out and the reactions monitored by FTIR. The intermediate products could in some cases be identified as imines. Six of the azides were investigated by gaseous electron diffraction and the molecular structures established. The azide group was situated gauche to the hydrocarbon skeleton in NCCH 2 N 3 , HCCCH 2 N 3 and CH 3 CCCH 2 N 3 . In the butadienes CH 2 C(N 3 )CHCH 2 and CH 2 C(N 3 )C(N 3 )CH 2 the azide group was syn to the adjacent CC bond, while in H 2 CCHCH 2 N 3 at least two conformers were detected. Model calculations on the smaller azides by ab initio quantum chemical methods were used to establish trends in the geometry and force fields of the azide group.

The vibrational spectra, molecular structure and conformation of organic azides: Part III. 2,3-Diazido-1,3-butadiene

Abstract A sample of 2-azido-1,3-butadiene was synthesized from 4-bromo-1,2-butadiene and tetramethylguanidinium azide. Although the sample is highly explosive, we succeeded in making a structure determination by gaseous electron diffraction. IR spectra of the vapour, of the matrix isolated species in argon at 15 K, and of an amorphous and crystalline solid at 90 K were recorded. A Raman spectrum of the liquid, including semiquantitative polarization data, was obtained at 240 K. The title compound was found to be planar with the CNN angle 117° oriented syn to the adjacent CC double bond, the NNN angle was ca. 177° oriented anti to the CN bond. The following bond distances (ra) were obtained: NN(N), 114.3; NN(C), 125.3; CN, 143.4; CC, 135.0; and CC, 146.7 pm. No additional conformers were observed in the vapour, liquid, amorphous or crystalline states.

Spectroscopic studies of ethylene-propylene copolymers in the near- and mid-infrared regions

Abstract Methods were derived to determine the monomer composition in block (heterophasic) ethylene-propylene copolymer samples by means of infrared spectroscopy in the mid- (MIR) and near-infrared (NIR) regions. Reference samples of known composition, ranging from 3.3 to 70.4 mol% of ethylene, having approximately the same crystallinity, were employed to standardize the method by which three commercial grades of copolymers were analysed. In the 6200-4500 and 4500-3600 cm−1 ranges, 20 and 23 absorption bands, respectively, were observed, but none of these were pure polyethylene bands. In spite of extended band separation and the use of band areas, the best accuracy in this region was obtained using peak heights. Approximately ten polypropylene bands and two polyethylene bands were separated in the MIR region (1200-600 cm−1), and band areas were successfully employed. The CO2 vapour band at 668 cm−1 interfered with the polyethylene doublet at 720 and 730 cm−1. Additional studies were carried out at low temperatures in the range 300-90 K to study the effect of temperature on the calibration graphs. When the PE bands at 730 and 720 cm−1 were ratioed against the PP band at 900 cm−1 as a function of temperature, a sharp break in the curve was observed at the position of the glass transition temperatures.

The virbrational spectra, molecular structure and conformations of organic azides: Part V. 3-Azidopropyne (propargylazide)

Abstract 3-Azidopropyne and 3-azidopropyne-1-[ 2 H] have been synthesized; the structure was determined by electron diffraction from the vapour and computed by ab initio Hartree—Fock SCF calculations. IR spectra of the vapour, of the cooled solid, of the matrix isolated species in argon and nitrogen at 13 K, and of the solid at 90 K were recorded. Raman spectra of the cooled liquid and of the amorphous and crystalline solid were also obtained. From the spectroscopic investigation it is evident that the title compound exists as only one conformer in all states of aggregation. The electron diffraction results and the theoretical calculations show unambiguously the conformation to the gauche around the CN bond with a dihedral angle of 37° (8) from syn and with the NNN angle, 169° (4), oriented anti to the CN bond. The following bond distances ( r a ) between the heavy atoms were obtained: r CC = 121.6(7) pm, r CC = 148.1(13) pm, r CN = 146.4(13) pm, r NN = 124.9(7) pm and r NN = 113.7(6) pm.

Infrared study of CO adsorbed on Co/γ-Al2O3 based Fischer–Tropsch catalysts; semi-empirical calculations as a tool for vibrational assignments

The adsorption of CO(g) on a Co/γ-Al2O3 catalyst has been studied by diffuse reflectance infrared spectroscopy. The effects of pressure, temperature and the addition of O2(g) and H2(g) have been investigated. Vibrational bands can be assigned to specific carbonyl species on the reduced Co catalyst, at low surface concentration, via a theoretical model. The basis for the model is an empirical relation between the number of valence electrons in transition metal carbonyl complexes and the corresponding carbonyl stretching frequency. This relation has been extended to metallic surfaces and combined with results from extended Huckel calculations.

Conformation and vibrational spectra of 1,5-hexadiyne (bipropargyl)

Abstract The infrared spectra of 1,5-hexadiyne (bipropargyl) as a vapour, liquid and crystalline solid and in several solvents have been recorded. Raman spectra were obtained for the liquid (including polarization measurements), the crystal and for several solutions. The data were interpreted in terms of two conformers, trans and gauche , in the vapour and liquid states and one, the trans , in the crystalline form. Interpretation of the spectra in terms of the conformational equilibria was not straightforward, and firm conclusions could not be drawn from the spectra alone. Vibrational assignments were made, supported by normal coordinate calculations.

The conformations of bromomethyl dimethyl silane and bromomethyl dimethyl silane-d1 as studied by vibrational spectroscopy and by ab initio calculations

Abstract Bromomethyl dimethyl silane, CH 2 Br(CH 3 ) 2 SiH (BDS), and its deuterated analogue, CH 2 Br(CH 3 ) 2 SiD (BDSD), were synthesised for the first time. Raman spectra of both liquids were recorded at various temperatures between 295 and 160 K and spectra of the amorphous and crystalline solids were obtained. Infrared spectra were recorded in the vapour phase and in the amorphous and crystalline states in the range 4000–60 cm −1 . Additional infrared spectra of the compounds isolated in argon and nitrogen matrices were recorded at 5 K before and after annealing to temperatures in the range 15–35 K. The compounds exist in anti and gauche conformers due to restricted rotation of the CH 2 Br group. Raman temperature studies gave an average Δ H conf 0 ( gauche–anti ) of 0.6±0.3 kJ mol −1 , anti being the low energy conformer in the liquid and in the matrices. Two different crystals were observed for these molecules after careful annealing of the amorphous solid in the range 110–150 K: one crystal (I) containing molecules in the gauche conformation and one crystal (II) with molecules in the anti conformation. For the parent molecule, both of these crystals were repeatedly investigated in the infrared and in the Raman spectra, while crystallisation was much more difficult to achieve for the deuterated compound. However, two different crystals (I, III) both containing the gauche conformer, was observed in the Raman spectra and one with anti (II) in the infrared spectrum of the deuterated compound. Ab initio calculations at the HF/6-311G* level gave vibrational frequencies and Raman and infrared intensities for both conformers. Complete assignments were made for the anti and gauche conformer spectra of both molecules.

High temperature infrared spectra of dimeric and monomeric AlBr3, AlI3 and GaCl3 in the vapour phase

Abstract The i.r. spectra of aluminium bromide, aluminium iodide and gallium chloride vapours were measured in the region 50–700 cm −1 with an evacuable Fourier transform spectrometer by transmission and emission techniques. Evacuable cells of nickel were employed having windows of type IIa diamond and sealed with o-rings of gold. The dimer spectra were interpreted in terms of D 2 h symmetry and seven of the eight i.r. active fundamentals were assigned. The monomer spectra were interpreted in terms of D 3 h symmetry and the three i.r. active fundamentals were assigned. Valence force fields were derived for the three dimers and three monomers based upon the present i.r. frequencies and on Raman data reported previously.

Condensed aromatics—XIV. Fluoranthene

Abstract The molecular vibrations of fluoranthene are examined. A complete set of independent symmetry coordinates is constructed. Simple force-field approximations developed for condensed aromatic hydrocarbons are adapted to both the in-plane and out-of-plane vibrations. The calculated frequencies were used as an aid in the assignment of experimental fundamentals. Experimental spectral data were recorded in both Raman and i.r., including a far-i.r. experiment down to about 100 cm −1 . A tentative, almost complete assignment for the experimental fundamentals is presented.