CHarges from ELectrostatic Potentials

An improved method was developed by Chirlian and Francl and called CHELP (CHarges from ELectrostatic Potentials). Their method, which uses a Lagrangian multiplier method for fitting the atomic charges, is fast and noniterative and avoids the initial guess required in the standard least-squares methods. In this approach, the best least-squares fit is obtained by minimizing y ... 

Sigfridsson E, Eyde U (1998) Comparison of methods for deriving atoirric charges from electrostatic potential and moments. J Comput Chem 19 377-395... 

A critical evaluation of the various methods for the derivation of point charges from electrostatic potentials is in Breneman, C. M. Wiberg, K. B. Determining atom-centered monopoles from molecular electrostatic potentials. The need for high sampling density in formamide conformational analysis, J. Comp. Chem. 1990,11, 361-373. 

Electrostatic interactions can guide alkylation under certain conditions. Examine the electrostatic potential map of the potassium enolate of ethyl acetoacetate. Is carbon or oxygen more electron rich Are electrostatic interactions likely to favor addition of oxygen or carbon Examine atomic charges and electrostatic potential maps for diethylsulfate, ethyl chloride, ethyl bromide and ethyl iodide, pay attention to the backside of the electrophilic carbon. Order the systems from most to least electron poor. Which reaction is most likely to be guided by electrostatics Least likely Can the experimental results be fully explained on this basis ... 

Next, examine the dianion resulting from double deprotonation of ethyl acetoacetate. Draw all the resonance contributors needed to describe this ion. Next, examine the geometry, atomic charges and electrostatic potential map for dianion. Are you able to decide which resonance contributors are important and which are not ... 

Compare atomic charges and electrostatic potential maps between reactants and transition state. Is there charge transfer from one of the reactants to the other (count the migrating hydrogen as part of propene) If so, what is the direction of the transfer Why (See also Chapter 21, Problem 3.)... 

Chirlian, L. E., and M. M. Francl. 1987. Atomic Charges Derived from Electrostatic Potentials A Detailed Study. J. Comp. Chem. 8, 894. 

The modern view of HX addition is that H+ is transferred from HX to the alkene to give a carbocation. The major product is the one derived from the more stable carbocation. Compare the energies of 1-propyl and 2-propyl cations (protonated propene), 2-methyl-1-propyl and 2-methyl-2-propyl cations (protonated 2-methylpropene), and 2-methyl-2-butyl and 3-methyl-2-butyl cations (protonated 2-methyl-2-butene). Identify the more stable cation in each pair. Is the product derived from this cation the same product predicted by Markovnikov s rule Is the more stable carbocation also the one for which the positive charge is more delocalized Compare atomic charges and electrostatic potential maps for one or more pairs of carbocations. 

We start by considering the origin of the dipole moment, which represents the lowest order nonzero term in a Taylor series expansion of the electrostatic potential arising from a neutral body (i.e., a molecule). For an assembly of n discrete charges, the electrostatic potential at a coordinate r may be written... 

L E Chirlian, M M Francl (1987) Atomic charges derived from electrostatic potentials - a detailed study, J Comp Chem 8(6) 894-905... 

Masamura, M. (2000). Error of atomic charges derived from electrostatic potential. Struct. Chem., 11,41-5. [166]... 

Ramos, M. N., Gussoni, M., Castiglioni, C., and Zerbi, G., Ab initio counterpart of infrared atomic charges. Comparison with charges obtained from electrostatic potentials, Chem. Phys. Lett. 151,397 2(1988). 

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