Descriptors quantum mechanical

Quantum calculations can be done easily for small systems, but large systems require great computing power calculation times for N electron systems can scale as much as depending on the level of theory. An alternative way to treat large systems is to use QM solutions for simpler analogues or subspecies. Using quantum mechanics has worked well with gas-phase systems but is far more difficult for condensed phases. 

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Breindl et. al. published a model based on semi-empirical quantum mechanical descriptors and back-propagation neural networks [14]. The training data set consisted of 1085 compounds, and 36 descriptors were derived from AMI and PM3 calculations describing electronic and spatial effects. The best results with a standard deviation of 0.41 were obtained with the AMl-based descriptors and a net architecture 16-25-1, corresponding to 451 adjustable parameters and a ratio of 2.17 to the number of input data. For a test data set a standard deviation of 0.53 was reported, which is quite close to the training model. 

Quantum mechanical descriptors (e.g. HOMO-LUMO energy gap) 3D structure See Section 2.7.4... 

Table 1 Calculation of some molecular-based descriptors for BOA, DIMBOA and MBOA. Physicochemical descriptor like logP (partition coefficient between octanol and water) constitutional descriptors like the number of a specified atoms or bonds (number of carbons, hydrogens, oxygens, nitrogens, single and aromatic bonds, the total number of atoms and bonds) and molecular weight quantum-mechanical descriptors like HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital).

George R. Famini and Leland Y. Wilson, Linear Free Energy Relationships Using Quantum Mechanical Descriptors. 

CODESSA Semichem Inc. www. semichem. com/codessa. html Topological, geometric, constitutional, thermodynamic, electrostatic, quantum mechanical descriptors... 

Famini, G.R. and Wilson, L.Y., Linear free energy relationships using quantum mechanical descriptors, in Reviews in Computational Chemistry, Vol. 18, Lipkowitz, K.B. and Boyd, D.B., Eds., Wiley-VCH, New York, 2002, pp. 211-255. 

Linear Free Energy Relationships Using Quantum Mechanical Descriptors... 

This chapter provides a tutorial focused on the uses of quantum mechanical descriptors in linear free energy relationships (LFERs). Often, LFERs derived with empirically based (i.e., experimental) descriptors are superior in quality to those derived with quantum mechanical descriptors. However, theoretically based LFERs have some advantages including ease of calculation. The QM... 

Partitioning the electron density in real space Opposite to other quantum mechanical descriptors such as wavefunction or density-matrix, electron density is a well defined and observable quantity at both subatomic and macroscopic scales. 

Considering all the above mentioned fundamental nature of the physico-chemical process of protonation and its probable relationship with the quantum mechanical descriptors, we suggest an ansatz for the computation of the proton affinity in terms of these theoretical descriptors. The physico-chemical process and the energetic effect must entail the above stated four parameters. To derive an explicit relation to compute the proton affinity in terms of the above stated descriptors, we suggest... 

A multi linear regression (Nantasenamat et al. 2009) is performed using MinitablS (MinitablS) to compute the correlation coefficients C, Ci C2, C3 and C4 by plotting experimental PA along the abscissa and the values of the quantum mechanical descriptors along the ordinate. The computed correlation coefficients C, Cl, C2, C3 and C4, for aU the sets are tabulated in Table 14.1. 

Thereafter, invoking the suggested ansatz, Eq. 14.6, and putting the quantum mechanical descriptors and the correlation coefficients in the Eq. 14.6, we have computed the PA s of six sets of carbon compounds. The comparative study of theoretically evaluated and experimentally determined PA s of the Set lASet 6 is performed in the Tables 14.2-14.7 respectively. 

The first stage of the QSAR method consists of compiling molecular descriptors for a very large number of lead compounds. Descriptors such is molar mass, molecular dimensions and volume, and relative solubihty in water and nonpolar solvents are available from routine experimental procedures. Quantum mechanical descriptors determined by calculations of the type described in Chapter 10 include bond orders and HOMO and LUMO energies. 

The latter, as mentioned, did not require supercomputing resources, except perhaps occasionally to generate quantum mechanical descriptors. Mainly, however, the training had the concomitant benefit of exposing more medicinal chemists, including younger ones, to what could be achieved with the current state of the art of computational chemistry applied to molecular design. 

Recent day, quantum mechanics becomes very popular to explain the mechanistic features of bio-active molecules. There are several quantum chemical descriptors through which we can predict reaction mechanism and as well as stmcture activity relationship of munerous bioactive molecules. A number of excellent reviews have been pubhshed on the application of quantum chemical descriptors in SAR/SPR studies [24—26]. To determine the equilibrium geometry, the molecular force field and to compute the quantum mechanical descriptors of the dmg molecules, some suitable quantum mechanical method are invoked [27]. 

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