While chemistry is an experimental science, theory (or calculation) is now playing a role of increasing importance with the use of the computer and the development of new calculation methods.
The key to theoretical chemistry is molecular quantum mechanics, which deals with the transference or transformation of energy on a molecular scale. Soon after the formulation of quantum mechanics in the 1920s, it was recognized that, in principle, the application of quantum mechanical principles could lead to accurate predictions of many chemical phenomena. This approach of studying chemistry is called ab initio¢wLatin for 'from the beginning', an approach independent of any experiment other than for the determination of fundamental constants such as the mass and charge of an electron.
Yet although the quantum mechanical principles for understanding the electronic structure of matter had been recognized, the mathematics involved in the application of these principles was intractable at best in the 50 years that followed. But with the steady development of new theoretical and computational methods, as well as the availability of bigger and faster computers with reasonable price tags in the last 20 years or so, calculations have sometimes become more accurate than experiments, or at least accurate enough to be useful to experimentalists. Calculations are also less costly, less time-consuming, and easier to control. And computational results often serve as a guide to experimental chemists attempting to synthesize or discover new molecules.
An indication that calculations in chemistry have been receiving increasing attention in the science community was provided by the award of the 1998 Nobel prize in chemistry to Profs. J.A. Pople and W. Kohn for their contributions to quantum chemistry.
In a project conducted by Profs. Li Wai Kee and So Suk Ping of the University's Department of Chemistry, ab initio calculations were employed to study the structures and energetics of novel chemical species, in particular, chemical systems known as 'transient reaction intermediates'. As their name suggests, the lifetime of these species is exceedingly short, usually in the order of 0.000,000,000,000,1 second. Hence, experimental investigations of them are extremely difficult and expensive, and high-level calculations offer the least costly, and very likely, the most reliable way to study them. Indeed, as testified by the project, in the study of transient intermediates, experiment and calculation frequently complement each other.
The CUHK researchers, in collaboration with their counterparts from Iowa State University in the US and the University of Science and Technology of China, studied many novel chemical systems using the earmarked grant of HK$502,000 from the Research Grants Council.
In the first part of the project the researchers studied unstable and short-lived novel chemical species. In one representative study, they attempted to predict the structure of the cation GeH7+ (a species consisting of one germanium atom and seven hydrogen atoms with an overall charge of +1), which has never been observed experimentally, using the analogous species CH7+ (one carbon atom with seven hydrogen atoms with a charge of +1) and SiH7+ (one silicon atom with seven hydrogen atoms with a charge of +1) which are known to exist. The cations, i.e., ions with a positive charge, are considered analogous because germanium, carbon, and silicon are elements of the same family. GeH7+ is expected to be detected in the foreseeable future since its analogous species have already been detected.
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| Figure 1 |
It is recognized that the eight atoms in GeH7+ may be arranged in numerous ways and that GeH7+ will adopt a structure with the least electronic energy simply because the most comfortable configuration or shape for any object is the one requiring the least energy. The researchers' calculations indicated that this species has the structure shown in Figure 1, similar to that of SiH7+ but different from that of CH7+. Furthermore they predicted that it only takes about 5 kcal mol-1 of energy, slightly more than room temperature, to break up GeH7+ into GeH5+ and H2. This means that GeH7+ is not a very stable species but one which should exist long enough for experimental detection and characterization, as it has been found that SiH7+, with a corresponding dissociation energy of about 4.6 kcal mol-1, can be observed spectroscopically.
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| Figure 2 |
In the second part of the project, Profs. Li and So targeted novel chemical compounds which were pollution causing such as organosulfur compounds. Experiments have shown that the organosulfur compound CH3SSCH3 breaks up into CH3, whose structure is well known to chemists, and the sulfur-containing fragment, CH3S2, whose structure is unknown. CH3S2 has many possible structures, some of which are shown in Figure 2. Combining experimental data from Iowa State University and their own computational results, the CUHK researchers deduced that the CH3S2 fragment has the structure shown in Figure 2 (a). This knowledge is useful for understanding the dissociation channels of the air pollutant CH3SSCH3.
In a related experiment by the Iowa State researchers, light was shone on CH3SSCH3 to simulate what happens in the atmosphere when the sun shines on pollutants in the air. This caused a reaction known as 'dissociative photoionization' wherein CH3SSCH3 broke up into the fragments CH3 and CH3S2+. (Note that the only difference between CH3S2+ and CH3S2, mentioned earlier, is that the CH3S2+ has one fewer electron.) Once again, while experimentalists knew that CH3S2+ was formed, they did not know its structure. Valance theory, which explains relations between atoms in compounds, predicts many structures for CH3S2+, some of which are shown in Figure 3. By combining the quantitative data in the dissociative photoionization experiment with calculations, the researchers identified the structure of CH3S2+ to be the one shown in Figure 3 (c).
Another organosulfur compound related to CH3SSCH3 is HSCH2CH2SH. The researchers, using calculations, discovered that HSCH2CH2SH requires 77 kcal mol-1 to split up into two CH2SH units, and 72 kcal mol-1 to fragment into HS and CH2CH2SH. The latter dissociation is more likely to occur since it requires less electronic energy. Such 'bond energies' are important properties of these compounds.
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Figure 3
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Prof. Li Wai Kee, professor of chemistry, obtained his BS from the University of Illinois in 1964 and his Ph.D. from the University of Michigan in 1968. He joined the Department of Chemistry of The Chinese University in the same year. |
Ab initio calculations provide both qualitative and quantitative results for novel chemical species. Many of these results are important in their own right and, more significantly, they are often useful in helping researchers to interpret and analyse the data they obtain from experiments done in their laboratories. Knowledge about the structure and energetics of air polluting compounds also has important implications for environmental chemists in their attempt to reduce air pollution.
Profs. Li and So were motivated to do the project because doing chemistry with computers is economical and it provides accurate results as well as reliable predictions. As the subjects of their studies are transient species, their computational results should prove to be helpful to experimentalists.
A total of 17 publications in international journals have resulted from this project.
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Prof. So Suk Ping, professor in the Department of Chemistry, obtained his B.Sc. and B.Sc.Sp. Hon. from the University of Hong Kong in 1963 and 1964 respectively, and his Ph.D. from McMaster University in 1969. He joined the Department of Chemistry of The Chinese University in the same year. |