Molar absorptivity hydrogen bonding

AT1 cm ). and therefore, these dyes are excitable not only with red (635 or 670-nm) but also with blue (380, 405, and 470 nm) diode lasers or LEDs (Fig. 1). Carbonyl containing substituents such as 1,3-indanedione, cyanoacetic ester, barbituric, and thiobarbituric acid form intramolecular H-bond with the polymethine hydrogens of the squaraine bridge. As a result, the molar absorptivities and quantum yields of these dyes are substantially decreased. 

In an excellent paper, Zhao et al. [29] assembled a carefully reviewed literature set of human absorption data on 241 drugs. They showed that a linear regression model built with 5 Abraham descriptors could fit percent human absorption data reasonably well (r2 = 0.83, RMSE = 14%). The descriptors are excess molar refraction (E), polarizability (S), hydrogen bond acidity (A), hydrogen bond basicity (B), and McGowan volume (V), all related to lipophilicity, hydrophilicity, and size. In a follow-on paper, data on rat absorption for 151 drugs was collected from the literature and modeled using the Abraham descriptors [30]. A model with only descriptors A and B had r2 = 0.66, RMSE = 15%. 

Symmetry-forbidden transitions. A transition can be forbidden for symmetry reasons. Detailed considerations of symmetry using group theory, and its consequences on transition probabilities, are beyond the scope of this book. It is important to note that a symmetry-forbidden transition can nevertheless be observed because the molecular vibrations cause some departure from perfect symmetry (vibronic coupling). The molar absorption coefficients of these transitions are very small and the corresponding absorption bands exhibit well-defined vibronic bands. This is the case with most n —> n transitions in solvents that cannot form hydrogen bonds (e 100-1000 L mol-1 cm-1). 

Zhao and coworkers [53] also constructed a linear model using the Abraham descriptors. The MLR model possesses good correlation and predictability for external data sets. In this equation, E is an excess molar refraction (cm3/mol/ 10.0) and S the dipolarity/polarizability, A and B are the hydrogen bond acidity and basicity, respectively, and V is the McGowan characteristic volume (cm3/ mol/100). The large coefficients of A and B indicate too polar molecules having poor absorption. 

One of the models that has had considerable success for predicting solvation processes of dipoles in non-hydrogen-bonded solvents is the dielectric continuum model [5,14]. In this model, the amount of solvation will depend on the dipole density— that is, the molar concentration and strength of dipoles. While the position of the absorption maximum is not directly related to the energy of solvation that a molecule experiences, one would expect the two to be very strongly correlated. However, for the three different... 

Novartis uses the In Silico Profiling web tool. Available properties include the octanol water partition coefficient log P, molar refractivity, flexibility index, hydrogen bonding characteristics and molecular polar surface area. Various drug properties, such as intestinal absorption, BBB permeability or Plasma Protein Binding (PPB) are calculated based on in-house models. 

At a constant cell path length, Beer s law shows that the absorbance of radiation through a medium is proportional to the concentration of the solute. Beer s law is strictly valid only for monochromatic radiation. Stray light (i.e., scattered radiation), which reaches the detector without having passed through the desired beam path, molecular interactions such as hydrogen bonding, which varies with the sample concentration, and other instrumental factors such as slit width, all affect molar absorptivity and result in some deviations from Beer s law. For an accurate analysis of the concentration of an unknown sample, it is usually necessary to first create a calibration curve from standard... 

The absorption and emission maxima of SA in different states of protonation are compared in Table 3. The data show that Stokes shifts of about 6000 cm are usual for compounds in which Intramolecular proton transfer does not occur. Concentrated solutions of the acids where dimerization can occur show very large shifts of about 11,000 cm l, regardless of whether Intramolecular hydrogen bonds are formed or not. (The association of SA has not been studied in alcoholic solvents in benzene there Is considerable association even at 80°C (apparent M/M = 1.4 in 0.01 molar solution)(33). 

Figure 7 plots the absorbance of the OH stretching mode of phenol in the 1 % phenol in toluene solution as a function of path length. The absorption coefficient was estimated to be 2.44x10- . This value was applied to estimate the molar concentration of phenol in the pore of cast-crystalhzed 6e-sPS and sPPMS. The molar concentration of phenol at equilibrium state is listed in Table 3. The concentration of phenol in the sPS/m-xylene cast film was the largest of the fom films. Since the concentration was estimated from the 3540 cm- hydrogen bonding-free band, these solvent molecules were absorbed in the pores of crystal. If the crystallinity is known, the number of phenol molecules in the pores of the crystals can be calculated. So the next section will deal the quantification of phenol molecules in the pores. 

Temperatnre- and concentration-dependent variations in self-association of 1-octanol have also been studied by two-dimensional (2-D) Fourier transform near-infrared correlation spectroscopy." The population of the free OH groups increases with temperature, reportedly reaching 13% at 80°C. The molar absorptivities of the first and second overtones of the monomer were found to be similar in several non-hydrogen-bonding solvents such as carbon tetrachloride, heptane, and octane. The first overtone s absorptivity was about 1.7 1/mol-cm, which agrees with Goddu s data. The absorptivity of the second overtone is about one twentieth that of the first. 

---