Approaches to Molecular Structure

R. F. Hout, Jr., W. J. Pietro, W. J. Hehre, A Pictorial Approach to Molecular Structure and Reactivity John Wiley Sons, New York (1984). 

Testa, B., Kier, L. B., Carrupt, P. A. A systems approach to molecular structure, intermolecular recognition, and emergence-dissolvence in medicinal research. Med. Res. Rev. 1997, 77, 303-326. 

Levine, R. D. (1988), Lie Algebraic Approach to Molecular Structure and Dynamics, in Mathematical Frontiers in Computational Chemical Physics, Springer-Verlag, New York. 

A final quasi-thermodynamic approach to molecular structure effects has been proposed by Good et al. (17-18) which relates molar volumes and the Girifalco-Good interaction parameter, as... 

Alternate double and single bonds are often used in drawing aromatic structures, although it is fully understood these form a closed loop (tc-system) of electrons. The reason is that these classical structures are used in the valence bond approach to molecular structure (as canonical forms), and they also permit the use of curly arrows to illustrate the course of reactions. 

THE MOLECULAR-OBBITAL AND EQUIVALENT-ORBITAL APPROACH TO MOLECULAR STRUCTURE... 

At the same time molecular orbital (MO) methods were seeing a rapid development, also because of increased computational ability. These, at least on the surface, appear to provide a simpler approach to molecular structure calculations. Nevertheless, Matsen and Browne[32] made a forceful case for the use of MCVB methods, indicating the difficulties... 

Proposed shortly after the VB theory, the MO theory became the most popular approach to molecular structure calculations, mainly because this theory is much more amenable than VB to computer implementation. As a consequence, there is a great number of results of MO calculations on many chemical systems. With the improvement of the numerical techniques and of auxiliary interpretative tools by many research groups, together with the wide availability of computer codes, MO theory was soon established as the computational (and for some also the conceptual) approach to the molecular structure problem. Due to its widespread use, MO theory is frequently pushed beyond its conceptual limits. In this section we will briefly outline some aspects of MO theory and highlight its physico-chemical interpretation. 

Nalewajski RF, Korchowiec J (1996) Charge Sensitivity Approach to Molecular Structure and Chemical Reactivity. World Scientific, Singapore prepared for publication... 

Bartlett RJ (1989) Coupled-cluster approach to molecular structure and spectra a step toward predictive quantum chemistry. J Phys Chem 93 1697-1708... 

A second problem occurs as a result of the possible stereochemical non-activity of the lone pair of electrons associated with the Group 15 element, and this is a problem common to the heavier elements of the neighbouring main groups in the Periodic Table. The VSEPR approach to molecular structure of the compounds of these heavier elements is tantamount to saying that d orbitals become part of any hybridization scheme and all the valence electrons are stereochemically active. 

R. J. Bartlett, /. Phys. Chem., 93, 1697 (1989). Coupled-Cluster Approach to Molecular Structure and Spectra A Step Toward Predictive Quantum Chemistry. 

Pictorial approaches to molecular structure and reactivity based on computer-generated models of electron density distributions (as well as the... 

The elucidation of the structure of cyclopeptidic indoles in a mixture could be determined by analyzing the B/E/CAD spectra of molecular ions. The FD spectra of this mixture indicated the presence of high-molecular-weight ions, including m/z 593 and 573. It was impossible to separate this mixture, even with liquid chromatography, as a result of the similar structures of these molecules and the mixture had to be studied as it was (about 1 pg of sample). Collisions on these ions led to side-chain cleavage, which led to an interesting approach to molecular structure. Table 23 lists the principal ions observed in B/E/CAD spectra from the M molecular ions produced in FD conditions. 

These experimental electron density distributions are in accord with the VB, MO, and DFT descriptions of chemical bonding, but are not easily applied to the determination of the relatively small differences caused by substituent effects that are of primary interest in interpreting reactivity. As a result, most efforts to describe electron density distribution rely on theoretical computations. The various computational approaches to molecular structure should all arrive at the same correct total electron distribution, although it might be partitioned among orbitals differently. The issue we discuss in this section is how to interpret information about electron density in a way that is chemically informative, which includes efforts to partition the total electron density among atoms. These efforts require a definition (model) of the atoms, since there is no inherent property of molecules that partitions the total electron density among individual atoms. 

Theoretical approaches to molecular structure design require accurate estimates of physical and transport properties. These are derived commonly iiom the principles of thermodynamics and transport phenomena, and using molecular simulations. Since the literature abounds with estimation methods, reference books and handbooks are particularly useful sources. One of the most widely used. Properties of Gases and Liquids (Poling et al., 2001), provides an excellent collection of estimation methods and data for chemical mixtures in the vapor and liquid phases. For polymers. Properties of Polymers (van Krevelen, 1990) provides a collection of group-contribution methods and data for a host of polymer properties. 

Bartlett, R. J., Tlie coupled-cluster approach to molecular structure and spectra A step towards predictive quantum chemistry. J. Phys. Chem. 93 xxx (1989). Summarizes some of the important advances made in the field since the publication of Bartlett s 1981 review (see above). 

The resonance approach to molecular structure is one way of explaining the bonding in benzene. An alternative, but equivalent,... 

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