Molecular structure spectroscopy

Mack, J., Wilmot, G., Symp. Molecular Structure Spectroscopy, Ohio... 

Lang DR, Davies JA, Lopes LGF, et al. A controlled NO-releasing compound synthesis, molecular structure, spectroscopy, electrochemistry, and chemical reactivity of R,R,5 )5 -fra s-[RuCl(NO)(cyclam)]5+ (1,4,8,11-tetraazacyclotetradecane). Inorg Chem. 2000 39 2294. 

Metallocorroles Molecular Structure, Spectroscopy and Electronic States Christoph Erben... 

The theoretical explanation of the existence of molecular structure is based on the concept of the adiabatic potential surface (APS) - a total energy function of fixed nuclear coordinates for molecular systems. As the APS links together molecular structure, spectroscopy, chemical thermodynamics and kinetics on a common basis, the methods predicting the shape of the APS belong to the most productive ideas of chemical physics ... 

This spectrum is called a Raman spectrum and corresponds to the vibrational or rotational changes in the molecule. The selection rules for Raman activity are different from those for i.r. activity and the two types of spectroscopy are complementary in the study of molecular structure. Modern Raman spectrometers use lasers for excitation. In the resonance Raman effect excitation at a frequency corresponding to electronic absorption causes great enhancement of the Raman spectrum. 

The vibrational states of a molecule are observed experimentally via infrared and Raman spectroscopy. These techniques can help to determine molecular structure and environment. In order to gain such useful information, it is necessary to determine what vibrational motion corresponds to each peak in the spectrum. This assignment can be quite difficult due to the large number of closely spaced peaks possible even in fairly simple molecules. In order to aid in this assignment, many workers use computer simulations to calculate the vibrational frequencies of molecules. This chapter presents a brief description of the various computational techniques available. 

Nuclear magnetic resonance (NMR) spectroscopy is a valuable technique for obtaining chemical information. This is because the spectra are very sensitive to changes in the molecular structure. This same sensitivity makes NMR a difficult case for molecular modeling. 

Section 13 20 IR spectroscopy probes molecular structure by examining transitions between vibrational energy levels using electromagnetic radiation m the 625-4000 cm range The presence or absence of a peak at a charac tenstic frequency tells us whether a certain functional group is present Table 13 4 lists IR absorption frequencies for common structural units... 

It is not the purpose of this book to discuss in detail the contributions of NMR spectroscopy to the determination of molecular structure. This is a specialized field in itself and a great deal has been written on the subject. In this section we shall consider only the application of NMR to the elucidation of stereoregularity in polymers. Numerous other applications of this powerful technique have also been made in polymer chemistry, including the study of positional and geometrical isomerism (Sec. 1.6), copolymers (Sec. 7.7), and helix-coil transitions (Sec. 1.11). We shall also make no attempt to compare the NMR spectra of various different polymers instead, we shall examine only the NMR spectra of different poly (methyl methacrylate) preparations to illustrate the capabilities of the method, using the first system that was investigated by this technique as the example. 

King, G. W. (1964) Spectroscopy and Molecular Structure, Flolt, Rinehart and Winston, New York. Kuhn, FI. G. (1969) Atomic Spectra, Longman, London. 

In the chapter on vibrational spectroscopy (Chapter 6) 1 have expanded the discussions of inversion, ring-puckering and torsional vibrations, including some model potential functions. These types of vibration are very important in the determination of molecular structure. 

With this as background, we will now discuss spectroscopic techniques individually. NMR, IR, and UV-VIS spectroscopy provide complementary information, and all are useful. Among them, NMR provides the information that is most directly related to molecular- structure and is the one we ll examine first. 

Figure 17.7 Molecular structures of (a) CIF3 and (b) BrFa as determined by microwave spectroscopy. An X-ray study of crystalline CIF3 gave slightly longer distances (171.6 and 162.1 pm) and a slightly smaller angle (87.0 "). (c) Structure of laCl showing planar molecules of approximate Da symmetry.

Figure 17.16 Molecular structure and dimensions of gaseous molecules of chlorine oxides as determined by microwave spectroscopy (CI2O and CIO2) or electron diffraction (CI2O7).

Contributions in this section are important because they provide structural information (geometries, dipole moments, and rotational constants) of individual tautomers in the gas phase. The molecular structure and tautomer equilibrium of 1,2,3-triazole (20) has been determined by MW spectroscopy [88ACSA(A)500].This case is paradigmatic since it illustrates one of the limitations of this technique the sensitivity depends on the dipole moment and compounds without a permanent dipole are invisible for MW. In the case of 1,2,3-triazole, the dipole moments are 4.38 and 0.218 D for 20b and 20a, respectively. Hence the signals for 20a are very weak. Nevertheless, the relative abundance of the tautomers, estimated from intensity measurements, is 20b/20a 1 1000 at room temperature. The structural refinement of 20a was carried out based upon the electron diffraction data (Section V,D,4). 

The determination of molecular structure by microwave spectroscopy (Section VII,A), the measurement of acid-base equilibria by ICR (Section VII,F), and theoretical calculations, especially accurate for the gas phase, are the main novelties of this section. 

Experimental (Microwave Spectroscopy)" and Calculated Molecular Structures OF THE Parent 1, 2-Dithiete (241)"... 

The infoiination obtained from an application of IR spectroscopy to a surface investigation includes the molecular structure, orientation, chemical reaction, conformation, crystallinity, and so on. 

Interactive to learn to utilize 13C NMR spectroscopy to deduce molecular structures. 

The value of infrared spectra for identifying substances, for verifying purity, and for quantitative analysis rivals their usefulness in learning molecular structure. The infrared spectrum is as important as the melting point for characterizing a pure substance. Thus infrared spectroscopy has become an important addition to the many techniques used by the chemist. 

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