Fixed-dipole moment effects

An efficient a-sialylation that took advantage of the highly reactive sialyl donors having C5 cyclic imides, especially phthalimide (NPhth), by virtue of the fixed-dipole moment effects, was also reported [42], For example, the use of the sialyl donor 4 and acceptor 6 at -78°C supplied the sialoside 7 as a only with 92% yield on 50 mg scale. The scale-up in batch process, however, significantly decreased the yield and stereoselectivity. This decrease in sialylation efficiency might be a result of... 

Nuclear spin weightings are given by gv. For H2CO states that are symmetric with respect to a rotation by -tt about the symmetry axis, gv = 0.25 for the antisymmetric states gv = 0.75. Here we will be considering the 10 K spectrum for H2CO, where only the ground vibrational state is populated. For this vibrational state, the symmetric and antisymmetric rotation-vibration states are those for which K is even and odd, respectively (91). The effective dipole moment operator p,/.ff is obtained from p,/. via the transformation of Eq. (34). To transform pA, we first reexpress it in terms of the body-fixed dipole moment operators pa, described above, by... 

The interaction of a fixed dipole moment with a polarizable medium is given by eq. (14.61). This, however, is not an SCRF model, as the dipole moment and stabilization are not calculated in a self-consistent way. When the back-polarization of the medium is taken into account, the dipole moment changes, depending on how polarizable the molecule is. Taking only the first-order effect into account, the stabilization is given by eq. (14.62). 

An obvious aim of simulations and theoretical studies is to understand the mechanisms by which the structures observed in experiment (or simulations) evolve. A first explanation of how chains can attract each other in ER fluids was provided by Halsey and Toor [257,297,298]. These authors remark that once chains have grown sufficiently large to span the electrodes, they behave effectively as infinite chains, due to the image charges. If these chains were perfectly ahgned touching spheres with fixed dipole moments in the field direction (no thermal effects) two such (infinite) chains would repel each other if in register and attract each other when offset by one radius in the field direction. However, the electric field created by the perfect chain... 

The theoretical approach based on the HNC integral equation is described in the context of ionic specificity. Two levels of description of the water medium are considered. Within the Primitive Model (continuous solvent), ionic specificity is introduced via effective, solvent-averaged, dispersion forces. The agreement with experimental data in bulk or at air-water interfaces is only partial and illustrates the limits of that approach. Within the Born-Oppenheimer model, the molecular HNC equation is solved with an explicit description of the solvent molecules (SPC water). Ionic and solvent profiles in bulk and at interfaces are enriched by short-range osdUated structures. The ionic polaris-ability is introduced via the self-consistent mean-field theory, the polarisable ions carrying an effective, fixed dipole moment. The study of the air-water interface reveals the limits of the conventional HNC approach and the needs for improved integral equations. 

The result of all of the vibrational modes contributions to la (3 J-/3Ra) is a vector p-trans that is termed the vibrational "transition dipole" moment. This is a vector with components along, in principle, all three of the internal axes of the molecule. For each particular vibrational transition (i.e., each particular X and Xf) its orientation in space depends only on the orientation of the molecule it is thus said to be locked to the molecule s coordinate frame. As such, its orientation relative to the lab-fixed coordinates (which is needed to effect a derivation of rotational selection rules as was done earlier in this Chapter) can be described much as was done above for the vibrationally averaged dipole moment that arises in purely rotational transitions. There are, however, important differences in detail. In particular. 

H2 quadrupole moment, <72(re) at the fixed equilibrium position, and thus the long-range coefficient of the quadrupole-induced dipole component, Eq. 4.3, is about 5% too small relative to the proper vibrational average, <12 = (v = 0 < 2(r) f = 0) [216, 217, 209], A 5% difference of the dipole moment amounts to a 10% difference of the associated spectral intensities. Furthermore, the effects of electron correlation on this long-range coefficient can be estimated. Correlation increases the He polarizability by 5% but decreases the H2 quadrupole moment by 8% [275], a net change of-3% of the leading induction term B R). 

In molecular quantum mechanics, we often find ourselves manipulating expressions so that one of a pair of interacting operators is expressed in laboratory-fixed coordinates while the other is expressed in molecule-fixed. A typical example is the Stark effect, where the molecular electric dipole moment is naturally described in the molecular framework, but the direction of an applied electric field is specified in space-fixed coordinates. We have seen already that if the molecule-fixed axes are obtained by rotation of the space-fixed axes through the Euler angles (, 6, /) = >, the spherical tensor operator in the laboratory-fixed system Tkp(A) can be expressed in terms of the molecule-fixed components by the standard transformation... 

An applied electric field (E) interacts with the electric dipole moment (p,e) of a polar diatomic molecule, which lies along the direction of the intemuclear axis. The applied field defines the space-fixed p = 0 direction, or Z direction, whilst the molecule-fixed q = 0 direction corresponds to the intemuclear axis. Transformation from one axis system to the other is accomplished by means of a first-rank rotation matrix, so that the interaction may be represented by the effective Hamiltonian as follows ... 

Optical excitations quite often generate considerable changes in fixed partial charges, usually described in terms of the difference solute dipole Amo ( 0 refers here to the solute). Chromophores with high magnitudes of the ratio Amo/Rl, where Rq is the effective solute radius, are often used as optical probes of the local solvent structure and solvation power. High polarizability changes are also quite common for optical chromophores, as is illustrated in Table 2. Naturally, the theory of ET reactions and optical transitions needs extension for the case when the dipole moment and polarizability both vary with electronic transition ... 

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