The Position of Absorption Bands

The frequency of a stretching vibration—the amount of energy required to stretch a bond—depends on the strength of the bond and the masses of the bonded atoms. The stronger the bond, the greater the energy required to stretch it. The frequency of the stretching vibration is also inversely related to the mass of the atoms joined by the bond thus, heavier atoms vibrate at lower frequencies. 

Lighter atoms show absorption bands at larger wavenumbers. C—H 

The approximate wavenumber of an absorption band can be calculated from the following equation derived from Hooke s law, which describes the motion of a vibrating spring  

Robert Hooke s drawing of a blue fly appeared in Micrographia, the first book on microscopy, published by Hooke in 1665. 

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Many other measures of solvent polarity have been developed. One of the most useful is based on shifts in the absorption spectrum of a reference dye. The positions of absorption bands are, in general, sensitive to solvent polarity because the electronic distribution, and therefore the polarity, of the excited state is different from that of the ground state. The shift in the absorption maximum reflects the effect of solvent on the energy gap between the ground-state and excited-state molecules. An empirical solvent polarity measure called y(30) is based on this concept. Some values of this measure for common solvents are given in Table 4.12 along with the dielectric constants for the solvents. It can be seen that there is a rather different order of polarity given by these two quantities. 

Additional bands may appear because of ring bending vibrations. The position of absorption bands for more highly substituted naphthalenes and other polynuclear aromatics are summarized by Colthup et al. (1990) and by Conley (1972). 

This weak transition is due to the promotion of an electron from the non-bonding molecular orbital n to an anti-bonding tt orbital. This transition is usually observed in molecules that contain a heteroatom as part of an unsaturated system. The most common of these bands corresponds to the carbonyl band at around 270 to 295 nm, which can be easily observed. The molar absorption coefficient for this band is weak. The nature of the solvent influences the position of absorption bands because the polarity of the bond is modified during absorption. For example, ethanal Amax = 293 nm (e = 12 in ethanol as solvent). 

The valence and coordination symmetry of a transition metal ion in a crystal structure govern the relative energies and energy separations of its 3d orbitals and, hence, influence the positions of absorption bands in a crystal field spectrum. The intensities of the absorption bands depend on the valences and spin states of each cation, the centrosymmetric properties of the coordination sites, the covalency of cation-anion bonds, and next-nearest-neighbour interactions with adjacent cations. These factors may produce characteristic spectra for most transition metal ions, particularly when the cation occurs alone in a simple oxide structure. Conversely, it is sometimes possible to identify the valence of a transition metal ion and the symmetry of its coordination site from the absorption spectrum of a mineral. 

In the spectra of a-substituted naphthalenes, the bands for the isolated hydrogen and the two adjacent hydrogen atoms of /3-naphthalenes are replaced by a band for three adjacent hydrogen atoms. This band is near 810-785 cm 1. Additional bands may appear because of ring bending vibrations (see Table 2.3). The position of absorption bands for more highly substituted naphthalenes and other polynuclear aromatics are summarized by Colthup et al. (1990) and by Conley (1972). 

All three methods give similar values of interfacial potentials typical results for some of micelles and vesicles are listed in Table 3. Also listed are estimates of interfacial dielectric constants (e), determined by comparing the position of absorption bands of solvatochromic indicators in the surfactant assemblies with that of reference 1,4-dioxane water mixtures with known e values. More generally, luminescence probe analysis [49], thermal leasing [50] and absorption spectroscopy [47, 51] are techniques that have all been utilized to measure local polarities in micelles and vesicles. It is important to note that these methods presume knowledge of the loca-... 

Infrared spectra are measured in units of frequency or wavelength. The wavelength is measured in micrometers, fim, or microns, n (i fi = 1 x 10 cm). The positions of absorption bands are measured in frequency units by wavenumbers, v, which are expressed in reciprocal centimeters, cm corresponding to the number of cycles of the wave in each centimeter. 

The NIR spectra of solid compounds 6-10 were measured [101]. The positions of absorption bands of solid [Np02(Pic)(H20)2] and complex particle [Np02(Pic)] in solution were found similar. It may be concluded that the coordination spheres of Np atom are close in both cases, that is, in solution the picolinate ion coordinates to Np with the formation of chelate ring. Alternatively, significant shifts of absorption band to lower energies were observed in spectra of neptunium nicotinate and isonicotinate as the result of the formation of cation-cation structures in solid 9 and 10. 

The intensities and the positions of absorption bands may also be influenced by solvent molecules. Electrostatic dipole interactions, and specific interactions that lead to formation of complexes based on hydrogen bonds and of charge-transfer type complexes, result in changes in the spectra of chemical compounds. 

In discussing the positions of absorption bands in relation to the splittings of the d orbitals, it is convenient and common practice to use the same unit, the reciprocal centimeter or wave number, abbreviated cm-1, for both the unit of frequency in the spectra and the unit of energy for the orbitals. With this convention, we see that the spectrum of Fig. 20-14 tells us that A0 in [Ti(H20)6]3+ is 20,000 cm"1. Since there are 83.7 cm"1 per kJ, this means that the splitting energy is 240 kJ mol"1, which is comparable with the usual... 

Bond order affects bond strength, so bond order affects the position of absorption bands. A C C bond is stronger than a C = C bond, so a C C bond stretches at a higher frequency ( 2100cm ) than does a C=C bond ( 1650cm ). C—C... 

Catalan (1995) has developed a set of polarity parameters known as the solvent bipolarity-polarizability (SPP) scale. Like the tt scales, the SPP parameters are based on the abihty of the solvent to shift the positions of absorption bands in a test molecule used as a probe. The effect, known as solvatochromism, utilizes 2-(N,Af-dimethylamino)-7-nitrofluorene (DMANF) by measuring the shift in the absorption spectrum as a series of solvents is used. The value of SPP for each solvent is calculated from the relationship... 

The position of absorption band is compared with the ionization potential of the hydrocarbon A... 

On the whole these data testify that in many cases the change of the medium affects comparatively little the physical characteristics, relative reactivity and relative stability of the stable carbocations. Hence, the sensitivity of the PMR and NMR- C spectra of arenium ions to the medium effects is small the positions of absorption bands of carbocations including the mesitylenium ion for solutions in acids and for the gas phase are surprisingly close and the influence of the solvent on the kinetic data on the degenerate rearrangements of arenium ions by 1,2-shifts of CHj groups is not appreciable (see Sect. IV.2.B). 

Many other measures of solvent polarity have been developed." One of the most useful is based on shifts in the absorption spectrum of a reference dye. The positions of absorption bands in general are sensitive to solvent polarity because the electronic distribution, and therefore the polarity, of the excited state is different... 

A number of workers have studied the influence of electron-attracting and -withdrawing substituents on the position of absorption bands (see reference [ ] in the Appendix). An interesting use of solvent effects to identify the carbonyl frequencies of pyridines also has been reported (see reference in the Appendix). 

Chemical information can also be derived from the position of absorption signals measured on an energy basis and the fine structure of such absorption signals. The position of absorption bands is referred to as the chemical shift and is measured relative to a standard substance. The effect is... 

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