Molecular compounds fundamental unit

The electronic spectra of the five-membered ring compounds have been intensively studied by the experimental and theoretical works. These molecules are fundamental units in many important biological systems. Furthermore, their excitation spectra are benchmark examples for theoretical studies of molecular excited states [51,55-58]. For furan and thiophene, various types of excitation spectra were measured the vacuum ultraviolet (VUV) spectrum, electron energy-loss (EEL) spectrum and magnetic circular dichroism (MCD) spectrum. The SAC-Cl method offered consistent interpretations of these electronic spectra [51-53]. 

Therefore, 58.44 grams of sodium chloride will have the same number of fundamental units as 32.00 grams of O2, and 18.02 grams of H2 O whose formulas describe molecules. So, in the case of substances which exist as molecules, we can call the sum of atomic masses the molecular mass. But how shall we name the mass of sodium chloride equivalent to those masses. We know that the designation, molecule, for NaCl is not appropriate. However, if we describe the mass of the fundamental unit representing the composition of any compound by using the expression formula mass, such difficulties are avoided. Table 4-1 lists the formulas and formula masses of several representative compounds. 

After establishment of the fundamental laws of chemistry, units like gram-atom or gram-molecule, were used to specify amounts of chemical elements or compounds. These units are directly related to atomic weights and molecular weights. These units refer to relative masses. The advent of mass spectrometry showed that the atomic weights arise from mixtures of isotopes. Intermittently two scales, a chemical scale and a physical scale were in use. In 1960, by an agreement between the International Union of Pure and Applied Physics (lUPAP) and the International Union of Pure and Applied Chemistry (lUPAC), this duality was eliminated. 

While we have dealt with the atomic weights of elements here and in Chapter 1, we have not dealt with the weights of molecules and formula units, which are the fundamental particles of compounds. First, by way of review, it is important to distinguish between molecular (or covalent) compounds and ionic compounds. Molecular compounds are those that consist of distinct individual particles called molecules. Molecules interact with one another in a sample of a molecular compound, but only in a rather mild way. Ionic compounds are those that consist of indistinct particles, which are most appropriately referred to as formula units, because a formula represents the simplest ratio between the cations and anions that constitute the compound (and not a distinct particle). Ionic compounds, on the atomic scale, consist of a three-dimensional array of cations and anions, all of which strongly interact with each other. Thus there is no such particle as a molecule in these cases. Figure 7.1 depicts the difference between these two kinds of compounds. 

FIGURE 7.1 Left, an example of a three-dimensional array of ions (no molecule). Right, an example of a molecule, the fundamental unit of a molecular compound. (From Kenkel, J., Kelter, P., and Hage, D., Chemistry An Industry-Based Introduction with CD-ROM, CRC Press, Boca Raton, FL, 2001. With Permission.)... 

An atom is the fundamental unit of an element. A molecule is the fundamental unit of a molecular compound. A molecule is composed of atoms in chemical combination. 

When a compound is irradiated with monochromatic radiation, most of the radiation is transmitted unchanged, but a small portion is scattered. If the scattered radiation is passed into a spectrometer, we detect a strong Rayleigh line at the unmodified frequency of radiation used to excite the sample. In addition, the scattered radiation also contains frequencies arrayed above and below the frequency of the Rayleigh line. The differences between the Rayleigh line and these weaker Raman line frequencies correspond to the vibrational frequencies present in the molecules of the sample. For example, we may obtain a Raman line at 1640 cm-1 on either side of the Rayleigh line, and the sample thus possesses a vibrational mode of this frequency. The frequencies of molecular vibrations are typically 1012—1014 Hz. A more convenient unit, which is proportional to frequency, is wavenumber (cm-1), since fundamental vibrational modes lie between 4000 and 50 cm-1. 

It should also be recalled that a full electrochemical, as well as spectroscopic and photophysical, characterization of complex systems such as rotaxanes and catenanes requires the comparison with the behavior of the separated molecular components (ring and thread for rotaxanes and constituting rings in the case of catenanes), or suitable model compounds. As it will appear clearly from the examples reported in the following, this comparison is of fundamental importance to evidence how and to which extent the molecular and supramolecular architecture influences the electronic properties of the component units. An appropriate experimental and theoretical approach comprises the use of several techniques that, as far as electrochemistry is concerned, include cyclic voltammetry, steady-state voltammetry, chronoampero-metry, coulometry, impedance spectroscopy, and spectra- and photoelectrochemistry. 

Currently, metal-polypyridine units are the molecular building blocks of choice whenever a compound with special electro- and/or photo-activity is to be designed. The research emphasis has somewhat shifted from fundamental studies of electron transfer reactivity and excited state properties of individual complexes to the design of new functional molecules and supermolecules with predetermined properties. 

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