As a chemist, I can provide you with a general explanation of how the molecular structure of water changes with temperature and pressure, and how quantum mechanics can help explain these changes. However, please note that a detailed quantum mechanical analysis would require advanced computational methods and is beyond the scope of this response.Water H2O is a polar molecule with a bent structure, consisting of two hydrogen atoms bonded to a central oxygen atom. The bond angle between the hydrogen-oxygen-hydrogen atoms is approximately 104.5 degrees, and the bond length between the oxygen and hydrogen atoms is about 0.96 angstroms .As temperature and pressure change, the molecular structure of water can be affected in several ways:1. Temperature: As the temperature increases, the kinetic energy of the water molecules also increases. This leads to increased vibrations and rotations within the molecule, which can cause fluctuations in bond lengths and angles. At higher temperatures, the average bond length may increase slightly due to the increased thermal motion of the atoms. However, these changes are generally small and reversible when the temperature returns to its original value.Quantum mechanics can help explain these changes through the concept of vibrational and rotational energy levels. As the temperature increases, the water molecules gain energy and can be excited to higher vibrational and rotational energy levels. This increased energy can cause the bond lengths and angles to fluctuate.2. Pressure: When pressure is increased, the water molecules are forced closer together. This can lead to changes in the hydrogen bonding network between water molecules, which in turn can affect the bond lengths and angles within individual water molecules. Generally, an increase in pressure will cause a slight decrease in bond lengths and a slight change in bond angles.Quantum mechanics can help explain these changes through the concept of electron density and molecular orbitals. As pressure increases, the electron density around the oxygen atom in water can be affected, leading to changes in the molecular orbitals that describe the bonding between the oxygen and hydrogen atoms. This can result in changes to the bond lengths and angles.In summary, the molecular structure of water can change with varying temperature and pressure, and quantum mechanics can help explain these changes through concepts such as vibrational and rotational energy levels, electron density, and molecular orbitals. However, a detailed quantum mechanical analysis would require advanced computational methods and is beyond the scope of this response.