Quantum mechanics provides us information about both the nuclear position and the distribution of an atom. This information is deducted based on the study of arrangements and interactions of electron and nuclei in a molecular system. Quantum mechanics differs from molecular mechanics in a way that it doesn’t require the use of parameters help in drug designing. That is it studies the structure of a drug without any reference to the chemical bonds.
The mathematics of the wave motion of electrons is exploited to have a better understanding of the atomic and molecular structure of the drugs. In one approach, nuclei are arranged in the space and the corresponding electrons are spread all over the system in a continuous electron density and computed by solving the Schrodinger equation.
Quantum Mechanics can be explained by two methods
- Ab initio method
Ab initio is computational method of quantum physics used in drug designing. It is limited to tens of atoms and is best performed using super computers. It can be applied to organics, organic metallic, molecular fragments etc. Therefore it can be used to study ground, transition and exited states of atoms.
- Semi-Emperical method
The semi emperical method of quantum mechanics is limited to hundreds of atoms. It can be applied to organics, organo metallic and small oligomers such as peptides, nucleotides and saccharides. It is therefore used in the study of ground transition and exited states of atoms.
Quantum mechanics differs from classical physics or Newtonian physics as it explains energy, momentum, angular momentum aspects at a atomic or subatomic level. As it is observed that the classical physics cannot be applied at the subatomic level. Thereby, quantum physics came into to existence explaining the particle and wave nature of objects at microscopic scales.
The basis of quantum mechanics and it’s use in drug design can be explained by the double slit experiment. Conducted by Thomas Young in the year 1801, this experiment explains the wave – particle duality of light and sub atomic particles. It demonstrates that light and matter can display characteristics of both classically defines wave and particles. It therefore displays the the nature of quantum mechanical phenomena.
In the basic version of double slit experiment, a beam of light is passed through a plate which has two parallel double slits and the light passing through the slits is observed on a screen behind the plate. The same experiment ahs been conducted using electrons and passing them through the double slit plate. The results showed interference patterns on the screen. Both constructive and destructive interference were observed. Therefore it was inferred that it is the wav and the particle nature of the light that had produced interference patterns on the screen. The wave nature of the matter or light causes it to pass through both the slits producing the wave patterns on the screen . In another such experiment, Only one photon was passed and their was no such strict path followed by the photons. It was seen to pass through either of the slit , when the experiment were conducted repeatedly.
Therefore this explains the wave- particle duality of light and matter. Low intensity experiments show that even an individual electro when passing through a prism or a slit will interfere with each other. The lower the mass of the particle the higher the wavelength it will produce. Therefore , based on the above studies we understand that Newtonian mechanics is not the fundamental descriptor of motion.
An electron being really small in size emits a much larger wavelength. An electron can be demonstrated as a circular standing wave surrounding the nucleus. Any circular standing wave must have an integer number of waves to exist. The higher the , the more the energy. This can be explained by the Bohr model, where when an electron is hit by a photon it is prompted to a higher energy level ( electron excitation) thereby increasing the number of wavelengths.
Quantum Physics was explained mathematically by Erwin Schrondinger as a system of 3D wave patterns. Schrondinger equation computes the wave function that pertain to one moment of time to the probability amplitudes that pertain to another.