Comparing experimental and ab initio

In order to illustrate how the optical response of clusters can serve as a fingerprint of their structural properties, let us first compare experimental and ab initio findings for the Na9" cluster (Figure 1). Two isomers close in energy have been found (AE ss 0.03 e V) the first one, with C2, symmetry, arises from the tetrahedral structure of Nag by one-atom capping the second one, with C3, symmetry, represents a deformed section of the fee lattice. However, their optical properties differ... 

The original (and most widely-used) versions of MNDO, AMI, and PM3 do not use d orbitals. Hence they might be expected to show reduced accuracy for elements in the second-row (computational chemists lingo) and beyond like, P, S, Cl, Br, and I, and cannot be used for transition metals. Actually, because of appropriate parameterization AMI and PM3 are able to treat monovalent Cl, Br and I as standard elements (C, H, O, N, F), and they handle divalent S reasonably well. To make them able to work better with elements in the second row and beyond, and/or to handle transition metals (note that in Zn, Cd, and Hg the d electrons are not normally involved in bonding), d orbitals have been incorporated into some SE methods. MNDO/d [53] uses d orbitals for some post-first row nonmetals and has been parameterized for several transition metals. Some versions of SPARTAN [54] have PM3 (tm), PM3 with d orbitals for many transition metals. PM3 (tm) geometries have been compared with experimental and ab initio ones ... 

PES representation, students are asked to predict the reaction path. Locations, geometries, and energies of the two minima and of the transition state are also extracted from the PES graph (Figure 4). The AMI and PM3 results are compared with experimental and ab initio data. The last assignment is devoted to the molecular electrostatic potential (MEP) and HOMO of five organic molecules, which are calculated using AMI and displayed as 2D contours plots in HYPERCHEM. For each molecule, the more favorable sites for electrophilic attack are deduced from the position of the MEP minima and from the HOMO localization. Differences are discussed and results are con elated with gas phase proton affinity. 

Experimental and ab initio characterizations of the 2l( IT) potential minima are given in Table IV. It is difficult to compare the calculations without resolved spin-orbit components to the experimental data since changing from to... 

Figure 3. X-ray absorption at the Ti K-edge of TS-1 samples, experimental data (full lines) and ab initio calculation (dotted lines) b) to f) the calculation are compared with the [l.5]TS-l experimental data left hand side (dTi.o = l -80 A, dsi-o = l-63 A, Ti-O-Si angle of c.a. 163° and 3 Si second neighbors) and right htind side, mixture of sites 30% with angle of 154° and 70% with angle of lb s .

The PM3 and ab initio calculations have been employed to compare mercuronium and bromonium ions 55 and 52111. Experimental comparison of the mechanism of the oxymercuration and bromination has also been made (see the section on bromination)142. 

Table I. Calculated principal values of 13C chemical shift tensors compared to experimental values and ab initio results (the reference U.F. denotes a private communication from Ulrich Fleischer, Ruhr University Bochum) ...

Nucleophilic vinylic substitutions of 4//-pyran-4-onc and 2,6-dimethyl-4//-pyran-4-one with a hydroxide ion in aqueous solution were calculated by the density functional theory (B3LYP) and ab initio (MP2) methods using the 6-31+G(d) and 6-31G (d) basis sets. The aqueous solution was modelled by a supermolecular approach, where 11 water molecules were involved in the reaction system. The calculations confirmed a different addition-elimination mechanism of the reaction compared with that in the gas phase or non-polar solution. Addition of OH- at the C(2) vinylic carbon of the pyranone ring with an activation barrier of 10-11 kcalmol-1 (B3LYP) has been identified as the rate-determining step, in good quantitative and qualitative agreement with experimental kinetics. Solvent effects increase the activation barrier of the addition step and, conversely, decrease the barrier of the elimination step.138... 

The photoelectron spectra of 2-oxazolidinone and a series of related compounds (49 X = CH2, S or NH, Y = O, S or Se) have been determined and the experimental ionization potentials compared with energy levels calculated by CNDO/2 and ab initio methods <80JST(69)151>. 

Solid state 2H NMR has been used to obtain detailed structural information for the amide and carboxylic acid hydrogen sites in a single crystal of the model peptide N-acetyl-D,L-valine [134]. Both the amide and carboxylic acid hydrogens are involved in intermolecular hydrogen bonds. The results were compared with experimental data obtained for acetylanilide [135] and ab initio calculations for glycylglycine [136]. 

Semi-empirical NDDO methods have been used for the study of various heterocycles <1995JMT51>. A belief that highly accurate values are not essential for valid conclusions to be drawn and the time advantage of semi-empirical over ab initio methods were factors governing the choice of these procedures. In general, there is reasonable agreement between structures and energies computed and experimental or ab initio values. The semi-empirical structural data have been discussed and compared with previously determined structures (Table 1). 

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