BioPHYSICAL chemistry

In general, for absorption, an electron in an orbital with the angular

momentum l must make a transition to an orbital with an angular

momentum of l+1. Likewise, an electron that makes a transition from

a higher-energy orbital with angular momentum lmust end up in an

orbital with the angular momentum l−1 due to the emission of a photon.

It follows that some transitions are allowed whereas others are forbidden.

The restrictions on the transitions that are allowed are summarized as a

selection rule. For electrons, transitions are allowed when the change in

angular momentum lis 1 and correspondingly the change in mlis 0 or 1.

Research direction: hydrogen economy

Hydrogen is the simplest of atoms with one electron in orbit around a

single proton. It is the lightest and third most abundant element on the

Earth’s surface. Why has there been such a significant focus on this ele-

ment by scientists and politicians? The reason is relatively simple. Global

energy consumption is increasing dramatically, driven by rising standards

of living and growing populations worldwide. This leads to the challenge

of potentially doubling the global production of energy within 50 years

coupled with increasing demands for clean energy sources that do not

add more carbon dioxide and other pollutants to the environment, to

minimize effects such as global warming. Today, the major source of fuel

for transportation, though, is petroleum, with the accompanying release

of unwanted exhaust fumes. By allowing hydrogen and oxygen to

combine, hydrogen-based fuel systems are touted as generating enough

energy to power a car without the production of exhaust fumes, only

water (Hoffert et al. 2002; US Department of Energy 2003). Whereas hybrid

cars will improve the fuel efficiency of a car, hydrogen-based cars use

fuel cells that will eliminate the need

for petroleum. In the USA, funds have

been allocated to both academic institutes

and industrial companies to produce

hydrogen-based cars by the year 2015,

replacing both gasoline-based and hybrid

cars as the means of transportation

(Figure 12.10).

The attractiveness of hydrogen as an

energy-storage material arises from the

fact that hydrogen can be oxidized to

water with the release of energy and then

recovered from water with the input

of energy while generating no environ-

mentally detrimental side products. To be

economically competitive with the present

254 PART 2 QUANTUM MECHANICS AND SPECTROSCOPY

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0

2000 2005 2010 2015 2020 2025 2030 2035 2040 2045 2050

Fraction of total vehicle miles or new vehicle sales (%)Ye a r

Conventional

Hybrid

Fuel Cell

Figure 12.10A graph showing relative projected
production of cars with different fuel systems.