Structure Determinations Organic Compounds

The standard r% length (see Part I, Chapter 1, p. 15) for a carbon-carbon single bond in a gas molecule has been taken to be the 1.534 0.001 A found in ethane, CaHe. Results close to this value have been obtained for the n-paraffins x = 4—7, but the bond in propane, QHg, seems to 

A procedure has been developed in which electron-diflraction intensities and spectroscopic rotational constants for several isotopic species are used as joint observables in least-squares refinements. The refined parameters are reduced to zero-point average values which are designated by different symbols depending on how the parameters were derived (see Part I, 

Chapter 1, p. 18) r° if the parameters were derived from electron-diffraction intensities alone rz if they were derived from rotational constants alone r v if they were derived utilizing a combination of both types of data. The method includes a way of converting rg to r ° distances, but it does not provide a procedure for converting r to r , the isotope-substitution distance from microwave spectroscopy. The first structures to be refined by the above workers using these methods were those of butadiene, acrolein, and glyoxal. The principal results for these planar molecules, in which the double bonds are situated trans to each other, are listed in Table 2. This work establishes with certainty for the first time the differences between the C-C bond lengths in these three molecules. 

Konaka, and T. lijima. Bull. Chem. Soc. Japan, 1969, 42, 2148. 

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Structure Determinations Organic Compounds Table 1 Comparison of bond-length hypotheses for C-C bonds ... 

Before the advent of NMR spectroscopy infrared (IR) spectroscopy was the mstrumen tal method most often applied to determine the structure of organic compounds Although NMR spectroscopy m general tells us more about the structure of an unknown com pound IR still retains an important place m the chemist s inventory of spectroscopic methods because of its usefulness m identifying the presence of certain functional groups within a molecule... 

A type of spectroscopy used in chemical analysis and the determination of the structure of organic compounds and polymers. 

The structural chemistry of dienes and polyenes is extremely diverse and intricate since about 12% of all determined structures of organic compounds contain two or more double... 

How does structure determine organic reactivity, 35, 67 Hydrated electrons, reactions of, with organic compounds, 7,115 Hydration, reversible, of carbonyl compounds, 4, 1 Hydride shifts and transfers, 24, 57... 

The 5 major spectroscopic methods (MS, UV, IR, H NMR and NMR) have become established as the principal tools for the determination of the structures of organic compounds, because between them they detect a wide variety of structural elements. 

DETERMINiNG THE STRUCTURE OF ORGANiC COMPOUNDS FROM SPECTRA... 

Chapter 8 Determining the Structure of Organic Compounds from Spectra... 

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. 

M. Beroza, Determination of the Chemical Structure of Organic Compounds at the Microgram Level by Gas Chromatography, Accts. Chem. Res. 3, 33 (1970). 

It is important to note that the molecular structure of organic compounds has a determined effect on the oxidation rate constants. For example, if a compound is "saturated" with four chlorine atoms per carbon atom such as tetrachloroalkane, its reactivity rate with hydroxyl radicals is expected to be significantly lower than that of an "unsaturated" compound with only two or three chlorine atoms such as di- and trichloroalkanes. 

Mass spectrometers provide a wealth of information concerning the structure of organic compounds, their elemental composition and compound types in complex mixtures. A detailed interpretation of the mass spectrum frequently allows the positions of the functional groups to be determined. Moreover, mass spectrometry is used to investigate reaction mechanisms, kinetics, and is also used in tracer work. 

Q Combine the chemical shifts, integrals, and spin-spin splitting patterns in NMR spectra with information from infrared and mass spectra to determine the structures of organic compounds. Problems 13-47 and 48... 

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