Geometry molecules, comparative

Conformational energy differences for a small selection of acyclic and cyclic molecules obtained from 6-31G, EDF1/6-31G, B3LYP/ 6-3IG and MP2/6-31G models are provided in Tables 14-2 to 14-5, respectively. Results from exact geometries are compared with those obtained using structures from MMFF, AMI and 6-3IG calculations. 

The properties of a molecule that are most frequently calculated are geometry, energy (compared to that of other isomers), and spectra. Why is it more of a challenge to calculate simple properties like melting point and density Hint is there a difference between a molecule X and the substance X ... 

Table 7.9 UV spectra (as transition energies in eV) of acetone, acetaldehyde, and formaldehyde, calculated by time-dependent DFT, using Gaussian 98 [78]. The results of using MP2/6-311+G [110] and (calculations by the author) AMI geometries are compared both sets of calculations are single-point B3P86/6-311++G. For each molecule only 6 transitions, all singlets, are shown. The number of positive and negative deviations from experiment and the mean absolute errors are given...

As one can see, the SLG-MINDO/3 in general improves the description of molecular geometry as compared to the SCF-MINDO/3 method. Obvious deterioration takes place only for the ammonia molecule (particularly for the valence angle). However, in other cases, transition to the SLG wave function improves the values of the valence angles. This is seen in the example of the hydrogen peroxide molecule. 

As discussed in Chapter 1, the full three-dimensional structure of a compound can be optimized with almost any of the quantum computational techniques. Since most quantum chemical computations are still performed on a single molecule in the gas phase, these computed structures can be most readily compared to gas-phase experimental structures. In the following chapters, we will present a number of case studies where computed and experimental geometries are compared. To get a sense of the quality of computed geometries, a few selected cases are discussed next. 

The geometry of CH4 could be square planar, with the four H atoms at the comers of a square and the C atom at the center of the square. Sketch this geometry and compare its stability with that of a tetrahedral CH4 molecule. 

Franck-Condon geometry ) as compared to the minimum of that surface is 32 kcal/mol. One of Ohmine s primary goals was to determine how the excess energy flowed into the solvent thereby resulting in the relaxation of the ethylene molecule to its minimum energy configuration on the triplet surface. 

Relative and absolute descriptors also differ from each other at a more fundamental level. If we compare two molecules (either their nuclear geometries or their electron densities), the result will normally depend on how they are oriented relative to each other. In contrast to absolute descriptors, relative descriptors are not invariant if one of the molecules compared is rigidly translated or rotated. Therefore, the proper use of relative descriptors must be accompanied by some sort of optimization in the superposition between two structures. Yet, a maximum superposition (e.g., by minimizing the rms deviation of paired atoms) may not produce a relative orientation that is most relevant in a given comparison of molecular shapes. This is a problem with no unique solution and is still under much research. [See discussions in Ref. 27.]... 

Part of the difficulty with the question is an assumption about the form of the answer. We are conditioned to seek an answer in a simple language involving orbitals, their overlap, their bonding and repulsion. We look for an answer in terms of individual electrons, or at least individual one-electron orbitals. Unfortunately, these are approximations. In reality the behaviour of all of the electrons within a molecule is indivisible. We shall, at several points in this book, arrive at the conclusion that an orbital model may well not be capable of providing an answer to a question. One has to carry out detailed and accurate calculations for a variety of geometries and compare the results. Whilst such calculations are available for lighter... 

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