Polyatomic molecules water molecule

Polyatomic molecules vibrate in a very complicated way, but, expressed in temis of their normal coordinates, atoms or groups of atoms vibrate sinusoidally in phase, with the same frequency. Each mode of motion functions as an independent hamionic oscillator and, provided certain selection rules are satisfied, contributes a band to the vibrational spectr um. There will be at least as many bands as there are degrees of freedom, but the frequencies of the normal coordinates will dominate the vibrational spectrum for simple molecules. An example is water, which has a pair of infrared absorption maxima centered at about 3780 cm and a single peak at about 1580 cm (nist webbook). 

Although we have been able to see on inspection which vibrational fundamentals of water and acetylene are infrared active, in general this is not the case. It is also not the case for vibrational overtone and combination tone transitions. To be able to obtain selection mles for all infrared vibrational transitions in any polyatomic molecule we must resort to symmetry arguments. 

Potassium is a metal, and the polyatomic anion, C104 is a nonmetal therefore, the compound is an ionic solid at room temperature. When the compound is dissolved in water, the ionic bond between the cation, K+, and the polyatomic anion, Cl()4, is broken due to the polarity of the water molecule, resulting in the two aqueous ions, K+ and C104 . 

When exposed to electromagnetic radiation of the appropriate energy, typically in the infrared, a molecule can interact with the radiation and absorb it, exciting the molecule into the next higher vibrational energy level. For the ideal harmonic oscillator, the selection rules are Av = +1 that is, the vibrational energy can only change by one quantum at a time. However, for anharmonic oscillators, weaker overtone transitions due to Av = +2, + 3, etc. may also be observed because of their nonideal behavior. For polyatomic molecules with more than one fundamental vibration, e.g., as seen in Fig. 3.1a for the water molecule, both overtones and... 

The terms mode-selective and bond-selective dissociation refer to the control of the dissociation products in VMP. The terms are usually used as synonyms although, strictly speaking, the former should refer to selective preexcitation of a vibrational mode and the latter to the resulting selective bond cleavage. Control of the dissociation products in VMP has been extensively reviewed [28-31] and our discussion will focus on molecules studied (or continued to be smdied) after the latest comprehensive review was published [31], An exception will be a short overview on the VMP of water isotopologues since it was the extensive theoretical and experimental investigations of these molecules, in particular H2O and HOD, that opened a new era of detailed smdies of state-to-state photodissociation out of specific rovibrationally excited states of polyatomic molecules. 

The bond energy and bond-dissociation energy are the same for the bond in a diatomic molecule but are different for a bond in a polyatomic molecule. For example, the bond-dissociation energy for the O—H bond in the water molecule (splitting H20 into H + OH) is 119.9 kcal/mole and that for the O—H bond in the OH radical is 101.2 kcal/mole. Their average, 110.6 kca.l/mole, is the O—H bond energy. 

For polyatomic molecules, it is important to distinguish between a polar molecule and a polar bond. Although each bond in a polyatomic molecule may be polar, the molecule as a whole will be nonpolar if the dipoles of the individual bonds cancel one another. For example, the two 8+C—O8- dipoles in carbon dioxide, a linear molecule, point in opposite directions, so they cancel each other (30). As a result, C02 is a nonpolar molecule even though its bonds are polar. The electrostatic potential diagram (31) illustrates this conclusion. In contrast, the two 8-0—H8+ dipoles in H20 lie at 104.5° to each other and do not cancel, so H20 is a polar molecule (32). This polarity is part of the reason why water is such a good solvent for ionic compounds. 

Polyatomic molecules have more complex microwave spectra, but the basic principle is the same any molecule with a dipole moment can absorb microwave radiation. This means, for example, that the only important absorber of microwaves in the air is water (as scientists discovered while developing radar systems during World War II). In fact, microwave spectroscopy became a major field of research after that war, because military requirements had dramatically improved the available technology for microwave generation and detection. A more prosaic use of microwave absorption of water is the microwave oven it works by exciting water rotations, and the tumbling then heats all other components of food. 

The vibrational motion of polyatomic molecules encompasses all nuclei in the molecule and, as long as the displacement from the equilibrium configuration is sufficiently small, it can be broken down into the so-called normal-mode vibrations (see Appendix E). In special cases these vibrations take a particular simple form. Consider, e.g., a partially deuterated water molecule HOD. In this molecule, the H OD and HO-D stretching motions are largely independent and the normal modes are, essentially, equivalent to the local bond-stretching modes. To that end, consider the following reaction that has been studied experimentally [6,7] as well as theoretically [8] ... 

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