Rotational spectroscopy overview

Recent developments in rotational spectroscopy [19] have provided an accurate study of a number of hydrogen bonded dimers in the gas phase. Experimental information is thus available nowadays on the molecular characteristics of such systems. Due to major advances in the field of computational chemistry, a number of theoretical calculations were performed (for an overview on the subject see references [1,20,21]). 

An overview of the theoretical background and computational requirements needed for the accurate evaluation of the spectroscopic parameters of relevance to rotational spectroscopy is given. The accuracy obtainable from state-of-the-art quantum chemical calculations is mainly discussed by means of signihcant examples, which also allows us to stress the importance of the interplay of theory and experiment in the field of rotational spectroscopy. 

Dipole Moment The dipole moment, which has intrinsic importance as a molecular property, has a twofold importance as it provides information on line intensities as well as on the type of transitions observable. Rotational spectroscopy by means of the Stark effect can accurately determine it, and quantum chemistry can support the corresponding investigation. The basic computational requirements for the accurate evaluations of this property are, as usual, the employment of correlated methods in conjunction with large basis sets, and we refer the reader to the brief overview given in the section concerning the quantum chemical calculations as well as to the references there cited [68-70]. 

The fundamental requirement for the existence of molecular dissymmetry is that the molecule cannot possess any improper axes of rofation, the minimal interpretation of which implies additional interaction with light whose electric vectors are circularly polarized. This property manifests itself in an apparent rotation of the plane of linearly polarized light (polarimetry and optical rotatory dispersion) [1-5], or in a preferential absorption of either left- or right-circularly polarized light (circular dichroism) that can be observed in spectroscopy associated with either transitions among electronic [3-7] or vibrational states [6-8]. Optical activity has also been studied in the excited state of chiral compounds [9,10]. An overview of the instrumentation associated with these various chiroptical techniques is available [11]. 

An overview of the apphcation of NMR to the field of liquid crystals over the past 50 years has been presented and the important aspects of proton and deuterium NMR have been delineated. The use of liquid crystals as solvents in NMR spectroscopy has been reviewed." Reviews on NMR studies of orientational order with 20 references, rotational diffusion of liquid crystals in the nematic phase and NMR spectroscopy in liquid crystals and membranes are available. A review on the application of spectroscopic methods to liquid crystalline phases has appeared during the period under report and it includes a discussion of the NMR methods. NMR and other methods used to determine the order parameters of nematics have been reviewed. ... 

The purpose of this chapter is to provide an overview of a rather wide array of experimental techniques that can tell us about the electronic structure of molecules. Some of these techniques, such as photoelectron (PE) spectroscopy, which is based on Einstein s photoelectric effect, are generally applied to gas-phase molecules. They can give high-resolution spectra, providing information about molecular vibrations and even, in a few cases, rotations. At the other end of the scale, UV/vis spectroscopy, particularly as applied to transition-metal complexes in solution, involves broad bands, and although it is an important and widely-used method, the information it gives is limited. Emission spectroscopy of transition-metal compounds has also become important. 

Vibrational, Rotational and Raman Spectroscopies Historical Overview... 

This report presents an overview of the literature on the use of EPR spectroscopy to study ionic liquids as solvents. After a short in overview on the history and the outstanding properties of ionic liquids, the report focusses on EPR investigations of rotational correlation times obtained with various spin probes, on biradicals, on electron selfexchange reactions as well as on synthetic and mechanistic aspects of ionic liquids. 

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