M. Rogojerov

Vibrational polarization spectroscopy of fluorene: alignment in stretched polymers and nematic liquid crystals ☆

New linear dichroism spectra in the mid-IR region of fluorene aligned in stretched polymers and nematic liquid crystals are compared with earlier experimental and theoretical investigations. Linear dichroism spectra of molecules aligned in such anisotropic solvents are simple to obtain, especially for small and medium sized molecules. Most molecules obtain a satisfactory alignment by these methods and the spectra are simple to interpret. The information obtained from these spectra about vibrational transition moment directions is often crucial for vibrational assignments in molecules like fluorene. Even high quality calculations are still unable to provide safe assignments for all fundamentals in fluorene as long as they are only compared with traditional spectra. Linear dichroism spectra provide a separation of the experimental information according to symmetry classes (particularly striking for molecules of C 2v and similar symmetries); this reduces the assignment puzzle drastically. In the present case the result is an almost complete and safe assignment of all symmetry allowed invibrational fundamentals of fluorene. A number of earlier assignments, which had been questioned or were doubtful, have been confirmed or corrected.q 1999 Elsevier Science B.V. All rights reserved.

Reaction of 2-acetyl-indane-1,3-dione with aniline - Schiff base or enamine?

The structure of the product of the condensation reaction between 2-acetylindan-1,3-dione and aniline has been investigated in the gas phase, solution and solid state using a combination of quantum-chemical calculations, NMR and IR spectroscopies, and X-ray crystallography. Energetics of both isomers and all tautomers of the product of the reaction are predicted at HF/321G//HF/3-21G and HF/6-31G**//HF/3-21G levels. Geometric optimizations at HF/6-31G** level are also performed for favorable tautomers. Quantum-chemical calculations indicate that the enamine isomer, 2-[1-( N-phenylamino)-1-ethylidene]indane-1,3-dione is favored. This is confirmed by 13 C NMR spectroscopy in non-polar and polar solvents, and in the solid state by X-ray crystallography. q 2003 Elsevier Science B.V. All rights reserved.

Study of overlapped bands by means of reduced IR linear dichroic spectra

Abstract Overlapping of bands and accidental degeneracy often occur in the vibrational spectra of large molecules. One frequently finds in the literature, particularly on vibrational analysis of aromatic compounds, one and the same frequency for vibrations belonging to different symmetry classes without any clear experimental evidence for such an assignment. Reduced linear dichroic IR spectra open in principle the opportunity for checking such situations.

Reduced IR-LD spectra of substances with two conformations oriented in nematic solutions

Abstract Infrared linear dichroic (IR-LD) spectra of three substance (2-naphthaldehyde, terephthalaldehyde and terephthalophenone) whose molecules give rise to two conformers were studied in the present article. It was aimed to understand how both conformers of each molecule orient in a nematic solution and could they be distinguished by means of IR-LD spectra. For this purpose the substances to be studied were dissolved and oriented in the nematic phase ZLI-1695 (Merck). The results thus obtained show that IR-LD spectra can help in distinguishing between the spectral bands of the conformers.

Reduced LD spectra of benzonitrile, p-, m- and o-nitrobenzonitrile in nematic solutions

Abstract Reduced LD spectra of benzonitrile, p-, m- and o-nitrobenzonitrile were recorded in the nematic liquid crystal ZLI-1695 (Merck) according to the “stepwise reduction” method of Thulstrup and Eggers[1]. It was shown that the in-plane vibrations of the nitrogroup and the out-of-plane benzene ring vibrations of the low symmetric o- and m-compound arise from mutually orthogonal vibrational transition moments that can serve for fixing a Cartesian coordinate frame to which all other transitions moment can be referred.

Aggregation of 2‐Aminobenzimidazole—A Combined Experimental and Theoretical Investigation

An investigation of 2-aminobenzimidazole was carried out by calculations at HF, MP2, and DFT levels of theory and also by UV and IR spectroscopy. The quantum chemical calculations predict a full shift of the equilibrium towards the amino form, but the absorption spectra in different solvents distinctly show a two-component equilibrium system. Examination of possible equilibria in solution shows that an equilibrium between two dimeric forms of the amino tautomer of 2-aminobenzimidazole explains the spectral observations.

Determination of the average orientation of low symmetry planar molecules in anisotropic solvent from two quantum chemically calculated vibrational transition moments

Abstract A new procedure combining a known experimental approach with quantum chemical data is proposed for determination of the average orientation of planar molecules (possessing only C s symmetry) oriented in nematic solvents. It is based on quantum chemical calculation of the angle between two vibrational transition moments corresponding to in-plane vibrations giving rise to strong IR absorption bands with different dichroic ratios, which allows one to find the principal orientation axes of the oriented molecule. All other vibrational transition moments are then transformed into this coordinate system in order to obtain an improved description providing consistency between experimental and theoretical determinations of transition moment directions. The correction procedure is illustrated by the example of p -nitrobenzaldehyde.

Solid-state tautomerism in 2-carboxyindan-1,3-dione.

The structure of 2-carboxyindan-1,3-dione was investigated using a combination of quantum-chemical calculations and solid-state NMR and IR spectroscopy. Due to poor solubility of the compound in different solvents, no single crystals could be obtained. Two dimeric structures formed from the tautomers of 2-carboxyindan-1,3-dione are likely to coexist in the solid state. The dimers interconvert via intramolecular proton transfer in one of the tautomeric forms constituting the dimers. The energy barrier of the intramolecular proton transfer reaction is calculated as 5.82 kcal mol(-1) at the MP2/6-31++G level of theory.

Vibrational analysis of terephthalaldehyde from its IR and Raman spectra in isotropic and anisotropic solutions

Abstract Vibrational analysis of terephthalaldehyde (TPA) has been carried out on the basis of its IR and Raman spectra measured in isotropic and anisotropic (nematic liquid crystal) solvents. Polarization spectra were recorded for the purposes of the band assignment. Quantum chemical DFT (B3LYP/6-311G∗∗) calculations were performed for theoretical prediction of the band positions, intensities and polarization properties of IR absorption bands.

IR linear dichroic spectra of the tetrahedral molecules tetrapropyn-1-yl silicon and -tin in anisotropic solvents

Dissolving spherical molecules of Td symmetry in nematic liquid crystal leads to distortions thus giving rise to lowering of the molecular symmetry down to D2d and C3v: The molecules deformed in this way undergo orientation that leads to appearance of linear dichroism. This phenomenon is shown for the examples of tetrapropyn-1-yl silicon and -tin, Si(Cx C–C H3)4 and Sn(CxC–CH3)4. Its theoretical explanation has been found on the basis of a small distortion of the polarizability and quadrupole moment of the considered molecules in the nematic solution. q 2003 Elsevier B.V. All rights reserved.