Low pressure diamond syntheses had seemed thermodynamically impossible. Now diamond syntheses under low pressures is very easy. However, the development of the theoretical aspects was rather slow. A chemical pump model, proposed previously by one of the authors, J.-T. Wang, together with J.-O. Carlsson, gives explanations about the thermodynamics and mechanisms of chemical vapor deposition diamond syntheses under low pressures. Based on the chemical pump model carbon atoms are pumped by excited chemical particles (such as super-equilibrium hydrogen atoms) from the stable graphite phase to the metastable diamond phase. Several sets of chemical pump reactions (thermodynamically coupled reactions) for diamond growth are described in this paper. In spite of the importance of thermodynamic coupled reactions in biochemistry, low pressure diamond syntheses are important developments in ordinary chemistry. Influences of the chemical pump effect on the gas phase are also discussed in this paper. Theoretical calculated results and predictions agree with the experimental observations.