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In Chapter 16, we discussed fats. Double bonds made all the difference. Fats with double bonds cannot change their shape around the double bond. Polyunsaturated fats have multiple double bonds. All naturally occurring fats, except those from ruminants, have cis double bonds. That means the fat molecule is bent at the double bond. However, chemical processing of polyunsaturated fats to produce monounsaturated fats generates trans double bonds. Fats with trans double bonds are called trans fats. The trans fat molecule is straight at a trans double bond rather than bent. This difference in shape, which is produced by the properties of the quantum mechanical covalent double bond, has a substantial influence on the biological activity of the molecules. Trans fats have been linked to a variety of deleterious effects on human health. The shapes of biomolecules, such as proteins, are central to biology. The shapes of molecules are controlled by the quantum mechanical interactions between atoms that give rise to different types of molecular orbitals and different types of bonds. Therefore, the processes of life are controlled by quantum mechanics.

CARBON DIOXIDE IS A GREENHOUSE GAS BECAUSE OF QUANTUM EFFECTS

We have seen that the greenhouse effect produced by carbon dioxide, which is driving global climate change, is fundamentally quantum mechanical in nature. Carbon dioxide is a perfect storm of quantum effects that produces a vicious greenhouse gas. Hot objects give off light that we call black body radiation. The colors given off by a hot object could not be explained with classical theory. In fact, the classical theory was so far off that it was called the Ultraviolet Catastrophe because the theory predicted that an infinite amount of energy would be given off in the ultraviolet portion of the spectrum by any hot object. Clearly a hot object does not and cannot give off an infinite amount of energy. So the failing of classical theory was monumental. In 1900 Planck made the first use of the idea of quantized energy levels for electrons in matter to explain black body radiation. He was able to derive a formula for the colors given off by a hot object that matched experiments virtually perfectly. The hotter an object is, the more high-energy photons it emits. However, Planck’s quantum theory showed that the amount of energy is not infinite and told us precisely how much light is emitted at each color. Stars are very hot, so they give off light in the visible and ultraviolet portions of the spectrum. An example of the black body spectrum for our Sun is shown in Figure 9.1. The Sun is a medium hot star. It appears slightly yellow. Very hot stars are blue and cooler stars than our Sun are red.

The Earth also emits black body radiation, but because it is very cool compared to a star, we cannot see the light the Earth emits with our eyes. The Earth’s black body spectrum is shown in Figure 17.1. The light emitted by the Earth is in the infrared, that is, the long wavelength (the low energy) part of the spectrum. Without the atmosphere, all of the black body radiation emitted by the Earth would go into space, and our world would be much colder, perhaps too cold for human life. However, the atmosphere absorbs some of the black body radiation, trapping the heat, and keeping the Earth warm. Most of the heat trapping is done by water, which has quantized rotational energy levels that can undergo transitions in the very far infrared (long wavelengths and low energy). We have not mentioned quantized rotations before. Here is where your quantum intuition comes into play. We have talked about quantized electronic energy levels and quantized vibrational energy levels. Classical objects can rotate, for example, a top. The classical energy associated with rotation is continuous. Spin the top a little bit faster, and the energy is increased a little bit. It should come as no surprise that molecules in the gas phase, such as water vapor in the air, can rotate, and because