Relationship between hydrogen bond network dynamics of water and its terahertz spectrum

Water is the source of all life. The understanding of the terahertz absorption spectrum of water is the prerequisite for the application of terahertz technology to biomedicine. The choice of terahertz frequency is essential for achieving the biological effects of terahertz with high efficiency and low energy consumption. The complex hydrogen bond network of water possesses a broad terahertz absorption peak. Therefore, it is necessary to study the relation between the dynamics of the hydrogen bond network of water and its terahertz absorption spectrum. However, the research in this field is still lacking. Using molecular dynamics simulation methods, the terahertz absorption spectra of different water models at room temperature and pressure are studied in this work. Furthermore, taking the temperature as a variable, the dependence of the terahertz absorption spectrum of water on the strength of the hydrogen bond network is explored. It is found that rising temperature makes the terahertz absorption spectrum of the hydrogen bond network red-shift, indicating that the center frequency of the spectrum is strongly correlated with the strength of the hydrogen bond. Further studies show that there is a linear relationship between the hydrogen bond lifetime of water and the center frequency of vibration absorption peak of the hydrogen bond network. The underlying mechanism can be disclosed by imitating the hydrogen bonds in the hydrogen bond network as springs then using the spring oscillator model. These findings are conducive to understanding in depth the complex hydrogen bond network dynamics in water and promoting the study of terahertz biological effects.