Hammond

A challenging task in material science as well as in pharmaceutical research is to custom tailor a compound s properties. George S. Hammond stated that the most fundamental and lasting objective of synthesis is not production of new compounds, but production of properties (Norris Award Lecture, 1968). The molecular structure of an organic or inorganic compound determines its properties. Nevertheless, methods for the direct prediction of a compound s properties based on its molecular structure are usually not available (Figure 8-1). Therefore, the establishment of Quantitative Structure-Property Relationships (QSPRs) and Quantitative Structure-Activity Relationships (QSARs) uses an indirect approach in order to tackle this problem. In the first step, numerical descriptors encoding information about the molecular structure are calculated for a set of compounds. Secondly, statistical and artificial neural network models are used to predict the property or activity of interest based on these descriptors or a suitable subset. 

This relation between reaction endothermicity or exothermicity is known as the Hammond... 

B. L. Hammond, W. A. Lester, Jr., P. J. Reynolds, Monte Carlo Methods in Ah Initio Quantum ChemistryScientific, Singapore (1994). 

Many programs allow the user to input a weighting factor (i.e., to give a structure that is 70% of the way from reactants to products). This allows the application of the Hammond postulate that the transition structure will look more like the reactants for an exothermic reaction and more like the products for an endothermic reaction. 

Quantum Monte Carlo (QMC) methods are computations that use a statistical integration to calculate integrals which could not be evaluated analytically. These calculations can be extremely accurate, but often at the expense of enormous CPU times. There are a number of methods for obtaining excited-state energies from QMC calculations. These methods will only be mentioned here and are explained more fully in the text by Hammond, Lester, and Reynolds. 

Table 1. Reaciivicy of reducing agents towards functional groups (adapted from J. B. Hendrickson, O. J. Cram, and G. S. Hammond, Organic Chemistry, 3rd ed., McGraw-Hill 1970, with modi(icaiiotis. ...

Inferring the structure of a transition state on the basis of the reactants and prod nets of the elementary step m which it is involved is a time honored practice m organic chemistry Speaking specifically of transition states George S Hammond suggested that... 

Hammond made his proposal in 1955 while at Iowa State University He later did pio neering work in organic pho tochemistry at Caltech... 

FIGURE 13 5 Diagram of a nuclear magnetic resonance spectrometer (Reprinted with permis Sion from S H Pine J B Hendrickson D J Cram and G S Hammond Organic Chemistry 4th ed McGraw Hill New York 1980 p 136)... 

Hammond s postulate (Section 4 8) Pnnciple used to deduce the approximate structure of a transition state If two states such as a transition state and an unstable intermediate de rived from it are similar in energy they are believed to be similar in structure... 

A combination of the two dioxaborinanes, (2) and (3), is marketed as a fuel microbiocide by Hammonds Euel Additives, Inc., Houston, Texas, under the trademark BIOBORJE. Annual U.S. production and consumption is estimated at 140—230 metric tons (28). 

---