Electronic Structure. Dipole Moment

Prior to systematic studies of N2F4, the structure and the electron distribution of X2Y4 molecules (including N2F4) were discussed in terms of bond order and electronegativity [1] and by a Huckel MO formalism which considered a- and/or jt-electron delocalization [2]. 

The bond indices obtained from CNDO calculations were given as functions of the dihedral angle [8], and were correlated with the corresponding bond energies and bond lengths [9]. 

The dipole moment i 0.26D of gauche-N2F4 (fx = 0 for trans-N2F4) was obtained from measurements of the Stark effect in the microwave spectrum of gaseous N2F4 [10]. Theoretical values are i = 0.23 D (CNDO) [3] and i 0.4 D (SCF-IO) [7,11]. The sign of i is unknown. 

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The electronic structure of fluorenes and the development of their linear and nonlinear optical structure-property relationships have been the subject of intense investigation [20-22,25,30,31]. Important parameters that determine optical properties of the molecules are the magnitude and alignment of the electronic transition dipole moments [30,31]. These parameters can be obtained from ESA and absorption anisotropy spectra [32,33] using the same pump-probe laser techniques described above (see Fig. 9). A comprehensive theoretical analysis of a two beam (piunp and probe) laser experiment was performed [34], where a general case of induced saturated absorption anisotropy was considered. From this work, measurement of the absorption anisotropy of molecules in an isotropic ensemble facilitates the determination of the angle between the So Si (pump) and Si S (probe) transitions. The excited state absorption anisotropy, rabs> is expressed as [13] ... 

Electronic structure ir-electron distribution dipole moments... 

Electronic structure n-electron distribution dipole moment... 

It has been shown recently by Kapturkiewicz and co-workers [14] that the analysis of the CT absorption CT <— So and the radiative and radiationless charge recombination processes CT So (Figure 4) in selected D-A n-n interacting systems sterically hindered to coplanarity (such as 9-anthryl and 9-acridyl derivatives of aromatic amines [14a,b], carbazol-9-yl derivatives of aromatic nitriles [14c] and ketones [14d] and D-A derivatives of indoles [14e] or phenoxazines and phe-nothiazines [14f]) in terms of the theory of photoinduced ET processes in absorption [52, 53] and emission [53-55] and Mulliken and Murrell models of molecular CT complexes [56, 57] leads to the determination of the quantities relevant for the rate of the radiative ET processes (exemplified by the CT absorption and emission) and to the estimation of the electronic structure and molecular conformation of the states involved in the photoinduced ET. A similar approach can be applied to describe the properties of the fluorescent singlet CT states and phosphorescent triplet CT states [58]. It should be pointed out that the relatively large values of the electronic transition dipole moments of the CT fluorescence indicate a non-... 

The comparison of the Mfiu values with those of Mabs allows one to obtain information about the changes in the electronic structure and molecular conformation between the Franck-Condon excited state initially reached upon excitation and the solvent-equilibrated fluorescent state [14]. Electronic transition dipole moments are mainly determined by the direct interactions between the lowest CT state and the ground state (So), and by the contributions from the locally excited configurations [14, 54, 56, 57]. For example, for the fluorescent CT state one can obtain... 

The structure of nitrate esters was further characterized by techniques of Raman spectroscopy, Infrared spectroscopy, Electron Diffraction, dipole moment, and X-ray. The results supported the finding of no hyperoxide structure in nitrate esters. Symmetrical structure was demonstrated for nitrate esters. 

Adsorbates can physisorb onto a surface into a shallow potential well, typically 0.25 eV or less [25]. In physisorption, or physical adsorption, the electronic structure of the system is barely perturbed by the interaction, and the physisorbed species are held onto a surface by weak van der Waals forces. This attractive force is due to charge fiuctuations in the surface and adsorbed molecules, such as mutually induced dipole moments. Because of the weak nature of this interaction, the equilibrium distance at which physisorbed molecules reside above a surface is relatively large, of the order of 3 A or so. Physisorbed species can be induced to remain adsorbed for a long period of time if the sample temperature is held sufficiently low. Thus, most studies of physisorption are carried out with the sample cooled by liquid nitrogen or helium. 

In order to describe the second-order nonlinear response from the interface of two centrosynnnetric media, the material system may be divided into tlnee regions the interface and the two bulk media. The interface is defined to be the transitional zone where the material properties—such as the electronic structure or molecular orientation of adsorbates—or the electromagnetic fields differ appreciably from the two bulk media. For most systems, this region occurs over a length scale of only a few Angstroms. With respect to the optical radiation, we can thus treat the nonlinearity of the interface as localized to a sheet of polarization. Fonnally, we can describe this sheet by a nonlinear dipole moment per unit area, -P ", which is related to a second-order bulk polarization by hy P - lx, y,r) = y. Flere z is the surface nonnal direction, and the... 

The interaction of the electron spin s magnetic dipole moment with the magnetic dipole moments of nearby nuclear spins provides another contribution to the state energies and the number of energy levels, between which transitions may occur. This gives rise to the hyperfme structure in the EPR spectrum. The so-called hyperfme interaction (HFI) is described by the Hamiltonian... 

Also produced in electronic structure sunulations are the electronic waveftmctions and energies F ] of each of the electronic states. The separation m energies can be used to make predictions on the spectroscopy of the system. The waveftmctions can be used to evaluate the properties of the system that depend on the spatial distribution of the electrons. For example, the z component of the dipole moment [10] of a molecule can be computed by integrating... 

Phosphine is a colourless gas at room temperature, boiling point 183K. with an unpleasant odour it is extremely poisonous. Like ammonia, phosphine has an essentially tetrahedral structure with one position occupied by a lone pair of electrons. Phosphorus, however, is a larger atom than nitrogen and the lone pair of electrons on the phosphorus are much less concentrated in space. Thus phosphine has a very much smaller dipole moment than ammonia. Hence phosphine is not associated (like ammonia) in the liquid state (see data in Table 9.2) and it is only sparingly soluble in water. 

The first empirical and qualitative approach to the electronic structure of thiazole appeared in 1931 in a paper entitled Aspects of the chemistry of the thiazole group (115). In this historical review. Hunter showed the technical importance of the group, especially of the benzothiazole derivatives, and correlated the observed reactivity with the mobility of the electronic system. In 1943, Jensen et al. (116) explained the low value observed for the dipole moment of thiazole (1.64D in benzene) by the small contribution of the polar-limiting structures and thus by an essentially dienic character of the v system of thiazole. The first theoretical calculation of the electronic structure of thiazole. benzothiazole, and their methyl derivatives was performed by Pullman and Metzger using the Huckel method (5, 6, 8). 

Diels-Alder reactions, 4, 842 flash vapour phase pyrolysis, 4, 846 reactions with 6-dimethylaminofuKenov, 4, 844 reactions with JV,n-diphenylnitrone, 4, 841 reactions with mesitonitrile oxide, 4, 841 structure, 4, 715, 725 synthesis, 4, 725, 767-769, 930 theoretical methods, 4, 3 tricarbonyl iron complexes, 4, 847 dipole moments, 4, 716 n-directing effect, 4, 44 2,5-disubstituted synthesis, 4, 116-117 from l,3-dithiolylium-4-olates, 6, 826 electrocyclization, 4, 748-750 electron bombardment, 4, 739 electronic deformation, 4, 722-723 electronic structure, 4, 715 electrophilic substitution, 4, 43, 44, 717-719, 751 directing effects, 4, 752-753 fluorescence spectra, 4, 735-736 fluorinated derivatives, 4, 679 H NMR, 4, 731 Friedel-Crafts acylation, 4, 777 with fused six-membered heterocyclic rings, 4, 973-1036 fused small rings structure, 4, 720-721 gas phase UV spectrum, 4, 734 H NMR, 4, 7, 728-731, 939 solvent effects, 4, 730 substituent constants, 4, 731 halo... 

The physical data index summarizes the quantitative data given for specific compounds in the text, tables and figures in Volumes 1-7. It does not give any actual data but includes references both to the appropriate text page and to the original literature. The structural and spectroscopic methods covered include UV, IR, Raman, microwave, MS, PES, NMR, ORD, CD, X-ray, neutron and electron diffraction, together with such quantities as dipole moment, pX a, rate constant and activation energy, and equilibrium constant. 

Azulene does have an appreciable dipole moment (0.8 The essentially single-bond nature of the shared bond indicates, however, that the conjugation is principally around the periphery of the molecule. Several MO calculations have been applied to azulene. At the MNDO and STO-3G levels, structures with considerable bond alternation are found as the minimum-energy structures. Calculations which include electron correlation effects give a delocalized n system as the minimum-energy structure. ... 

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