Molecular electronic effects

Fan, F.R.F. et al., Structure-dependent charge transport and storage in self-assembled monolayers of compounds of interest in molecular electronics Effects of tip material, headgroup, and surface concentration, J. Am. Chem. Soc. 126, 4035-4042, 2004. Liu, B. et al., Electron transfer at self-assembled monolayers measured by scanning electrochemical microscopy, J. Am. Chem. Soc. 126, 1485-1492, 2004. 

Sandstrom et al. (65) evaluated the Kj value for 4,5-dimethyl-A-4-thiazoline-2-thione (46) in water (Scheme 19) K-j= 10. A-4-Thiazoline-2-thiones are less basic in the first excited state (61) than in the ground state, so application of Forster s cycle suggests that the thione form is even more favored in the first excited state. Huckel molecular orbital (HMO) calculations suggest that electronic effects due to substitution in... 

The BDE theory does not explain all observed experimental results. Addition reactions are not adequately handled at all, mosdy owing to steric and electronic effects in the transition state. Thus it is important to consider both the reactivities of the radical and the intended coreactant or environment in any attempt to predict the course of a radical reaction (18). AppHcation of frontier molecular orbital theory may be more appropriate to explain certain reactions (19). 

Electronics Production of circuit boards (producing contacts in boreholes), modified electrolytic condensers, modified field effect transistors, molecular electronics (unidirectional conductors), photostructural lacquers based on ICPs (electron beam lithography), novel photoluminescent diodes (LED), data storage (e.g. spatially resolved eleclrochromics)... 

Molecular mechanics calculations don t explicitly treat the electrons in a molecular system. Instead, they perform computations based upon the interactions among the nuclei. Electronic effects are implicitly included in force fields through parametrization. 

Neglect of electrons means that molecular mechanics methods cannot treat chemical problems where electronic effects predominate. For example, they cannot describe processes which involve bond formation or bond breaking. Molecular properties which depend on subtle electronic details are also not reproducible by molecular mechanics methods. 

Hiickel models of molecular electronic structure enjoyed many years of popularity, particularly the r-electron variants. Authors sought to extract the last possible amount of information from these models, perhaps because nothing more refined was technically feasible at the time. Thus, for example, the inductive effect was studied. The inductive effect is a key concept in organic chemistry a group R should show a - -1 or a —I effect (according to the nature of the group R) when it is substituted into a benzene ring. 

During the photoelectron emission event there are electronic relaxation effects occurring, which are usually divided into intra- and inter-molecular relaxation effects. These effects can be rationalized in a classical picture as follows. An elec-... 

These major trends in act can be qualitatively predicted using semi-empirical molecular orbital calculations. However, the methods fail to adequately predict some electronic effects, remote substituent effects and the influence of hydrogen bonding. Higher level ah initio or DFT calculations provide a better indication of trends in these circumstances. 

Macropolycyclic ligands, 2,942 classification, 2,917 metal complexes binding sites, 2, 922 cavity size, 2,924 chirality, 2, 924 conformation, 2,923 dimensionality, 2, 924 electronic effects, 2, 922 shaping groups, 2,923 structural effects, 2,922 molecular cation complexes, 2,947 molecular neutral complexes, 2,952 multidentate, 2,915-953 nomenclature, 2,920 Macro tetrolide actins metal complexes, 2,973 Macrotricycles anionic complexes, 2,951 cylindrical... 

Prehminary AMI calculations carried out with the MOPAC program on 18 and related molecules suggest that there are atomic orbital contributions from the heteroatom (e.g., S in 18) to the frontier molecular orbitals. It is conceivable, therefore, that there is negative hyper conjugation involving specific orbitals of S and the P centers in 18. This electronic effect may explain the unusual stabiUty towards oxidation of 18 and other heteroatom functionaUzed primary bisphosphines as described above [51]. 

The work described in this paper is an illustration of the potential to be derived from the availability of supercomputers for research in chemistry. The domain of application is the area of new materials which are expected to play a critical role in the future development of molecular electronic and optical devices for information storage and communication. Theoretical simulations of the type presented here lead to detailed understanding of the electronic structure and properties of these systems, information which at times is hard to extract from experimental data or from more approximate theoretical methods. It is clear that the methods of quantum chemistry have reached a point where they constitute tools of semi-quantitative accuracy and have predictive value. Further developments for quantitative accuracy are needed. They involve the application of methods describing electron correlation effects to large molecular systems. The need for supercomputer power to achieve this goal is even more acute. 

This approach did not seem to be as satisfactory for those sulfamates having heteroatom substituents (hetero-sulfamates). Spillane suggested that the various electronic effects of the hetero-atoms probably introduce an additional variable that is apparently absent, or constant, for the carbosulfamates. Because molecular connectivity correlates structure with molecular volume and electronic effects, Spillane included molecular connectivity, (computed for the entire molecule, RNHSOO to the four variables, x, y, z, and V, and applied the statistical technique of linear-discrimination analysis to 33 heterosulfamates (10 sweet, 23 not sweet). A correlation of >80% was obtained for the x, z, x subset 5 of the 33... 

Following a description of femtosecond lasers, the remainder of this chapter concentrates on the nuclear dynamics of molecules exposed to ultrafast laser radiation rather than electronic effects, in order to try to understand how molecules fragment and collide on a femtosecond time scale. Of special interest in molecular physics are the critical, intermediate stages of the overall time evolution, where the rapidly changing forces within ephemeral molecular configurations govern the flow of energy and matter. 

Adsorption potential shifts are higher at the air/solution than at the Hg/solution interface. This aspect has been discussed in terms of nonlocal electronic effects in the metal surface and different molecular orientation atthetwo interfacee. " "... 

The link between UpophiUcity and point charges is given by intermolecular electrostatic interactions (Sections 12.1.1.2, 12.1.3 and 12.1.4 address this topic) and ionization constants. The mathematical relationships between Upophilicity descriptors and pKjS are discussed in detail in Chapter 3 by Alex Avdeef. Here, we recall how pKj values are related to the molecular electron flow by taking the difference between the pfCj of aromatic and aUphatic amines as an example. The pfCa of a basic compound depends on the equilibrium shown in Fig. 12.2(A). A chemical effect produces the stabilization or destabiUzation of one of the two forms, the free energy difference (AG) decreases or increases and, consequently. 

Mac Rae EG (1956) Theory of solvent effects of molecular electronic spectra frequency shifts. J Chem Phys 61 562-572... 

A common feature of the various methods that we have developed for the calculation of electronic effects in organic molecules is that they start from fundamental atomic data such as atomic ionization potentials and electron affinities, or atomic polarizability parameters. These atomic data are combined according to specific physical models, to calculate molecular descriptors which take account of the network of bonds. In other words, the constitution of a molecule (the topology) determines the way the procedures (algorithms) walk through the molecule. Again, as previously mentioned, the calculations are performed on the entire molecule. 

The scope of the SH reaction encompasses sp-sp2 (alkenyl, aryl (29),143 heteroaryl) and sp-sp couplings (the modified Cadiot-Chodkiewicz reaction).142 Iodides are most frequently used as electrophilic coupling partners, though the use of bromides, triflates (30),144 or even some reactive chlorides (31)145 is also possible. Due to the low steric bulk of the acetylenic unit, as well as its exceptional ability in the transduction of electronic effects, the SH reaction is well suited for construction of new (e.g., star-like) molecular architectures through polysubstitution (32).146... 

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