Quantum Chemistry (Preface)
The Road to the Quantum Chemistry
In the late seventeeth century, Isaac Newton discovered classical mechanics, the laws of motion of macroscopic objects. In the early twentieth century, physics found that classical mechanics does not correctly describe the behavior of very small particles such as the electron and nuclei of atoms and molecules; the behavior of such particles is described by a set of laws called quantum mechanics.
Quantum chemistry applies quantum mechanics to problem in chemistry. The influence of quantum chemistry is felt in a all branches of chemistry. Physical chemists use quantum mechanics to calculate (with the aid of stastical mechanics) thermodynamic properties (for example, entropy, heat capacity) of gases; to interpret molecular spectra, thereby allowing experimental determination of molecular properties (for example, bond lenghts and bond angles, dipole moments, barriers to internal rotation, energy differences between conformational isomers); to calculate molecular properties theoritically; to calculate properties of transition states in chemical reaction, thereby allowing estimation estimation of rates constant; to understand intermolecular forces; and to deal with bonding in solids.
Organic chemists use quantum mechanics to estimate the relative stabilities of molecules, to calculate properties of reaction intermediate, to investigate the mechanisms of chemical reaction, to predict aromaticity of compounds, and to analyze NMR spectra. Analythical chemists use spectroscopic methods extensivey. The frequencies and intensities of lines in a spectrum can be properly understood and interpreted only through use of quantum mechanics. Inorganic chemists use ligand field theory, an appriximate quantum-mechanical method, to predict and explain the properties of transition-metal complex ions. Although the large size of biologically important molecules makes quantum-mechanical calculations of them extremely difficult, biochemists are beginning to benefit from quantum-mechanical studies of conformations of biological molecules, enzyme-substrate binding, and solvation of biological mollecules.
(Taking from “Levine, I.N. 1991. Quantum Chemistry Fourth Edition. New Jersey : Prentice-Hall, Inc”, page 1)