Bent molecular configuration

The observation of a bent Cr-H-Cr bond in the tetraethylammonium salt without an accompanying substantial deformation of the linear architecture of the nonhydrogen atoms in the [Cr2(CO)io(M2-H)] monoanion reflects the inherent flexibility of the bond. The deformability of the[M2(CO)io(M2-H)] monoanion species to adopt an appreciably bent, staggered carbonyl structure was first reported by Bau and co-workers (23) from neutron diffraction studies of two crystalline modifications of the electronically equivalent, neutral W2(CO)9(NO)(m2-H) molecule. Subsequent x-ray diffraction studies (15) of the analogous [W2(CO)io(m2-H)] monoanion found that the nonhydrogen backbone can have either an appreciably bent structure for the bis(triphenylphosphine)-iminium salt or a linear structure for the tetraethylammonium salt, with the W-W separation 0.11 A less in the bent form. Crystal packing forces probably were responsible (15) for the different molecular configurations of the monoanion in the two lattices. In solution, however, all known salts of the [W2(CO)io(m2-H)] monoanion exhibit the same three-band carbonyl ir absorption spectrum char-... 

Figure 9 Variation of orbital energies in HAH molecule on going from 90° bent molecule to linear molecule. The classification of states, built from s and p atomic orbitals, is discussed in the main text. The steep rise in the curve joining ai and favours the bent molecular form for H2O, whereas with four valence electrons, as in BeH2 or HgH2, the linear configuration is favoured. This argument is based on an intimate relation, which Walsh assumed, between the sum of orbital energies and total energy. Density theory in its simplest form supplies such a relation, namely equation (84). The figure is a schematic version of that of Walsh,46 who noted that the line 180° must be either a maximum or a minimum...

The molecular configuration is assumed to be linear, since experimental evidence indicates that other transition metal dichlorides are linear (4, 5, 6, 7, 8, ) even though some fluorides are bent (1 0). The bond distance is assumed to be the same as that of ZrCl (g). The electronic levels and quantum weights are estimated to be the same as those of TiCl2(g) (2 ) ... 

Inasmuch as TRH is believed to exert its hormonal function after binding to the plasma membrane of the pituitary cell, which secretes thyrotropin, the molecular configuration of the hormone is of interest because such configuration will influence the binding properties. Molecular biologists believe that the molecule is bent to form a hairpin turn, which is then stabilized by two specific hydrogen bonds [41]. 

Schematic and simplified representations of the CO2 molecular orbitals are shown in Fig. 1.3. Figure 1.4 illustrates qualitatively the variation of the MO energies with the change of OCO bond angle (diagram of Walsh [19]) and the correlation of symmetry representations of the molecular orbitals of CO2 among the point groups associated with the linear (Dooh) and bent (C2v) configuration.

First it was argued to be due to a chiral molecular configuration characteristic of the particular type of bent-shape molecules, such as twisted or propeller shape (conformational chirality). The concept of conformational chirality was supported by simulations by Earl et al. [61], and was demonstrated by the observation that doping calamitic cholesteric liquid crystal by achiral bent-core molecules can lead to a decrease of the helical pitch, indicating an enhanced rotatory power of the mixture [62]. Unfortunately there is no proof that the decrease of the pitch is not due to a decrease of the twist elastic constant caused by the addition of bent-core units. Although the conformational chirality is usually not questioned in the solid B4 phase [20], its role has been questioned by Walba et al. [20] by arguing that these chiral conformations have very short lifetime, therefore they average out in fluid smectic, such as SmCP or SmCo phases. 

Structure. The straiued configuration of ethylene oxide has been a subject for bonding and molecular orbital studies. Valence bond and early molecular orbital studies have been reviewed (28). Intermediate neglect of differential overlap (INDO) and localized molecular orbital (LMO) calculations have also been performed (29—31). The LMO bond density maps show that the bond density is strongly polarized toward the oxygen atom (30). Maximum bond density hes outside of the CCO triangle, as suggested by the bent bonds of valence—bond theory (32). The H-nmr spectmm of ethylene oxide is consistent with these calculations (33). 

Triatomic species can be linear, like CO2, or bent, like O3. The principles of orbital overlap do not depend on the identity of the atoms involved, so all second-row triatomic species with 16 valence electrons have the same bonding scheme as CO2 and are linear. For example, dinitrogen oxide (N2 O) has 16 valence electrons, so it has an orbital configuration identical to that of CO2. Each molecule is linear with an inner atom whose steric number is 2. As in CO2, the bonding framework of N2 O can be represented with sp hybrid orbitals. Both molecules have two perpendicular sets of three tt molecular orbitals. The resonance structures of N2 O, described... 

The estimation of f from Stokes law when the bead is similar in size to a solvent molecule represents a dubious application of a classical equation derived for a continuous medium to a molecular phenomenon. The value used for f above could be considerably in error. Hence the real test of whether or not it is justifiable to neglect the second term in Eq. (19) is to be sought in experiment. It should be remarked also that the Kirkwood-Riseman theory, including their theory of viscosity to be discussed below, has been developed on the assumption that the hydrodynamics of the molecule, like its thermodynamic interactions, are equivalent to those of a cloud distribution of independent beads. A better approximation to the actual molecule would consist of a cylinder of roughly uniform cross section bent irregularly into a random, tortuous configuration. The accuracy with which the cloud model represents the behavior of the real polymer chain can be decided at present only from analysis of experimental data. 

Oxygen vacancy centre C02 Another electron centre, C02" , molecular ion (C2v symmetry), is a triatomic AB2 type molecule having 17 electrons and a bent configuration with a bond angle of 134° O2- is removed from the electron centre of C033 . 

Consult the molecular orbital diagram in Fig. 6.16 and predict whether BHj will be linear or bent. What would you predict for the excited state configuration, 2[Pg.672]

Out of each pair of states correlated in this way only one corresponds to an energy minimum. For example, simple molecular orbital theory predicts that CH2 will be linear or nearly linear in its lowest triplet state (Lennard-Jones and Pople, 1951 Walsh, 1953a) the stable state is then 3ZB and the bent configuration then represents the turning-points of vibrational motion in the bending mode of the electronic state. 

Anselmi et al. (02JCS(P2)1525), very recently published the conformational analysis and dynamics of cA/ rafts-4-methylcyclohexyl tetrahydro-pyranyl ethers (cf. Scheme 12) and compared their structures with the floral odors of the compounds. The cis isomer 40(cis), endowed with a main white flower note, has an bent, oval molecular shape. The trans derivatives 40(trans) and 41, exhibit different odors, possess an extended structure of cylindrical molecular shape. Brenna et al. (02CJC714) reexamined the configuration/conformation of rose oxide analogues. However, the conformational analysis provided poorer results than published previously (78JPC303) without even citing the previous paper. 

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