Postulation of Molecular Structures

Neutral Loss Only a limited number of neutral fragments of low mass which are eliminated in decompositions of molecular ions. Examples are H, H2, CH3 and OH. Therefore, the presence of a major ion below the molecular ion at an improbable interval (eg, loss of 4 to 14, 21 to 25 amu) will indicate that the latter is not the molecular ion Postulation of Molecular Structures The. postulation of the structure of an unknown molecule is based on several major kinds of general structural information available in the mass spectmm. McLafferty (Ref 63) suggests the following systematic approach ... 

Today s understanding of information pathways has arisen from the convergence of genetics, physics, and chemistry in modern biochemistry. This was epitomized by the discovery of the double-helical structure of DNA, postulated by James Watson and Francis Crick in 1953 (see Fig. 8-15). Genetic theory contributed the concept of coding by genes. Physics permitted the determination of molecular structure by x-ray diffraction analysis. Chemistry revealed the composition of DNA. The profound impact of the Watson-Crick hypothesis arose from its ability to account for a wide range of observations derived from studies in these diverse disciplines. 

NM R experiments (ID and 2D, homonuclear and heteronuclear) are the preeminent techniques for the determination of molecular structures. However, careful application and analysis of mass spectral data can provide sufficient information to postulate tentative structures. In this respect, the application of tandem MS experiments, sometimes in conjunction with selective derivatization of the unknown compound, can be very informative about the stmcture. The high-resolution mass spectral data are critical to the support of NMR-deduced stmctures by providing molecular formulae for unknowns. 

Careful application of the Hammond postulate allows us to ascertain the structure of the transition state under certain conditions if we know something about the structure and energy of the next prior or later consecutive species, which could bean unstable intermediate, reactant, or product. Proposed originally by George Hammond,41 the postulate declares, "if two states, as for example, a transition state and an unstable intermediate, occur consecutively during a reaction process and have nearly the same energy content, their interconversion will involve only a small reorganization of molecular structure."... 

The information from such experiments, of which there are now many examples(2,3), is important to our understanding of both dynamics and structure. First, since the dynamics of the initially excited state are reflected in spectral line widths, factors which control the rate of vibrational energy flow within molecules can be studied by recording spectra as a function of molecular structure. Fast randomization of this energy is a postulate of statistical unimolecular rate theories. On the... 

It is always tempting for the chemist to interpret observations on SARs in mechanistic terms and to postulate means by which the biological activity could be brought about through some feature of molecular structure or chemical activity which seems significant in the... 

A number of molecular structural parameters for trivinylborane, B(CH=CH2)3, are shown in Table 2. The most acceptable molecular model is a planar dynamic one, in which steric strain is reduced by distortion of the vinyl groups. Since the molecule is probably planar, some C B 7r-electron delocalization was postulated this was corroborated by the value for the C=C bond length. 

Scientists dedicated to probing the nature of molecular forces undoubtedly shared in the excitement surrounding the introduction of matrix and wave mechanics in 1926. Postulated to contain all information available about a given physical system, the Schrodinger equation was in essence a complete description, and thus it seemed obvious the equation should serve as the basis for theoretical treatments of molecular structure, valency, and reactivity. Yet the new theory offered only the elegant, aloof formalism of the stationary state ... 

Chapters 7 and 8 discuss spin and identical particles, respectively, and each chapter introduces an additional postulate. The treatment in Chapter 7 is limited to spin one-half particles, since these are the particles of interest to chemists. Chapter 8 provides the link between quantum mechanics and statistical mechanics. To emphasize that link, the ffee-electron gas and Bose-Einstein condensation are discussed. Chapter 9 presents two approximation procedures, the variation method and perturbation theory, while Chapter 10 treats molecular structure and nuclear motion. 

---