Structure determination, experimental spectroscopic methods

Electron diffraction is the other of the two important sources of gas-phase structural data. As discussed by Hedberg (this volume), the intensity of electrons scattered by molecules is modulated by the interatomic distances, both bonded and nonbonded. Since interatomic distances enter explicitly into electron diffraction determinations, the method is in some ways more direct than spectroscopy. Moments of inertia are functions of Cartesian coordinates of individual atoms rather than distances between atoms. On the other hand, electron diffraction is much more susceptible to experimental error than spectroscopic techniques.15 Problems with structural determinations by spectroscopic methods often stem almost entirely from model error, whereas in electron diffraction both experimental and model error are important. Experimental and model error in electron diffraction are discussed elsewhere in this volume by Hedberg, and we shall confine ourselves here to definitions of the various structural parameters that arise in electron diffraction studies and the relationships among them and spectroscopic quantities. 

There is considerable need for exploration of interaction effects If SCSs are to be used for signal assignments or structure determinations, it is essential to know about alterations of SCSs by interactions with other substituent(s) to avoid misinterpretations. Additionally, interaction effects provide valuable information about the o-electron distribution and its dependence on structure, since it is well known that 13C chemical shifts are highly sensitive to changes in the geometry and/or electronic state of the molecule. This research area is not easily accessible experimentally by other spectroscopic methods, at least for larger molecules, which are also beyond the reach of most theoretical calculations. 

Such ambiguity and also the low structural resolution of the method require that the spectroscopic properties of protein fluorophores and their reactions in electronic excited states be thoroughly studied and characterized in simple model systems. Furthermore, the reliability of the results should increase with the inclusion of this additional information into the analysis and with the comparison of the complementary data. Recently, there has been a tendency not only to study certain fluorescence parameters and to establish their correlation with protein dynamics but also to analyze them jointly, to treat the spectroscopic data multiparametrically, and to construct self-consistent models of the dynamic process which take into account these data as a whole. Fluorescence spectroscopy gives a researcher ample opportunities to combine different parameters determined experimentally and to study their interrelationships (Figure 2.1). This opportunity should be exploited to the fullest. 

Infrared (IR) spectroscopy was the first modern spectroscopic method which became available to chemists for use in the identification of the structure of organic compounds. Not only is IR spectroscopy useful in determining which functional groups are present in a molecule, but also with more careful analysis of the spectrum, additional structural details can be obtained. For example, it is possible to determine whether an alkene is cis or trans. With the advent of nuclear magnetic resonance (NMR) spectroscopy, IR spectroscopy became used to a lesser extent in structural identification. This is because NMR spectra typically are more easily interpreted than are IR spectra. However, there was a renewed interest in IR spectroscopy in the late 1970s for the identification of highly unstable molecules. Concurrent with this renewed interest were advances in computational chemistry which allowed, for the first time, the actual computation of IR spectra of a molecular system with reasonable accuracy. This chapter describes how the confluence of a new experimental technique with that of improved computational methods led to a major advance in the structural identification of highly unstable molecules and reactive intermediates. 

The case of coherent tunneling is invariably studied experimentally by spectroscopic methods. For example, the neutron scattering structure factor determining the spectral line shape is equal to... 

The experimental methods presented in Part A permit us to determine the number of each type of carbon present in a molecule and the spatial relationships between protons, which are critical to the elucidation of the two- and, perhaps, three-dimensional structure of that molecule. As we have seen, this new information, combined with that obtained from chemical shifts, coupling constants, and other spectroscopic methods, might be sufficient to determine... 

Experimental study of the double layer is not limited to thermodynamics. A variety of spectroscopic methods have been applied to determine the structure and composition of the double layer. Two of these, namely, second-harmonic generation and vibrational sum frequency spectroscopy, have already been described in section 8.11. Other important techniques are based on the absorption of electromagnetic radiation when it is transmitted through or reflected at the interface. Finally, the scattering of X-rays and neutrons at interfaces has proven to be a valuable tool for obtaining atomic level information about the interface. In the following section some of these methods are outlined in more detail. 

In the columns identifying the experimental method, MW stands for any method studying the pure rotational spectrum of a molecule except for rotational Raman spectroscopy marked by the rot. Raman entry. FUR stands for Fourier transform infhired spectroscopy, IR laser for any infiured laser system (diode laser, difference frequency laser or other). LIF indicates laser induced fluorescence usually in the visible or ultraviolet region of the spectrum, joint marks a few selected cases where spectroscopic and diffraction data were used to determine the molecular structure. A method enclosed in parentheses means that the structure has been derived from data that were collected by this method in earlier publications. The type of structure determined is shown by the symbols identifying the various methods discussed in section II. V/ refers to determinations using the Kraitchman/Chutjian expressions or least squares methods fitting only isotopic differences of principal or planar moments (with or without first... 

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