# Physical Chemistry-I: Quantum Chemistry for Himachal Pradesh PSC 2020

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Quantum Chemistry - Basis Of Quantum Mechanics, Role Of Quantum Mechanics (Physical Chemistry)

## Contents

What is Quantum Mechanics?

Role of Quantum Mechanics in Chemistry

Failure of Classical Mechanics

Basis of Quantum Mechanics

Summary

## Topics to be Learnt

Understand the basis of quantum mechanics

Understand the role of quantum mechanics in chemistry

Learn about the origin of quantum mechanics

Identify the phenomena responsible for failure of classical mechanics

Appreciate the importance of quantum mechanics

## What is Quantum Mechanics?

Quantum Mechanics is a theoretical science of microscopic matter (electrons and nuclei in atoms and molecules). It gives the laws of motion of microscopic objects (the way classical mechanics gives for macroscopic objects)

Quantum Mechanics is the greatest intellectual developments of 20th century.

Quantum Mechanics is the science which takes both particle and wave character (dual behavior) of matter into consideration.

Quantum Mechanics is made up of two words QUANTUM + MECHANICS.

The term â€śMechanicsâ€ť refers to the science of motion of the body. Mechanics word comes because we are dealing with motion of the particle.

The other word is â€śQuantumâ€ť which is Latin word for amount and in modern conventions is used to represent smallest possible discrete unit of any physical property\

## With this Discussion, the Following Questions Might Arise in Your Mind

In this paper, each of the above aspect of Quantum Mechanics will be discussed.

Role of Quantum Mechanics in Chemistry

Quantum Mechanics has found application in all the fields of science viz., chemistry, biology, physics and other integrated fields.

Quantum chemistry applies fundamentals of quantum mechanics to problems in chemistry. Quantum chemistry explains theoretically structure (S), bonding (B) and reactivity (R) in chemical entities

The influence of quantum chemistry is evident in all branches of chemistry. Some important areas of chemistry where quantum mechanics is used are as follows:

Physical chemistry applies the results of quantum mechanics to calculate thermodynamic properties, molecular properties etc. for bulk matter via statistical mechanics.

Organic chemistry utilizes quantum mechanics to estimate the relative stabilities of molecules, to calculate properties of reaction intermediates, to investigate the mechanisms of chemical reactions, and to analyze NMR spectra.

Analytical chemistry uses spectroscopic methods extensively which stem from fundamentals of quantum chemistry. All the spectra are understood and interpreted via thorough understanding of the basics of quantum mechanics.

Inorganic chemistry utilizes ligand field theory which is an approximate quantummechanical method used to predict and explain the properties of transition-metal complex ions. Entire atomic structure comes from Quantum Mechanics â€“ orbtials, shape of orbitals, bonding etc.

Biochemistry where quantum mechanical studies on biological molecules, enzyme substrate binding etc., have been carried out successfully.

In all, one can say that Quantum Mechanics explains SBR of Chemistry taking into account the role and behavior of electrons. This field revolves around studying and interpreting the behavior of electrons. All the chemical problems are solved quantum mechanically with rigorous examination of electrons and their behavior in the respective environment. Indeed, quantum mechanics has had great success in providing a general theoretical framework for chemistry.

## Failure of Classical Mechanics

Till the end of 19th century, Classical or Newtonian Mechanics was considered to be the only right and undisputed theoretical science.

But soon some new experimental phenomena were observed which could not be explained by Classical Mechanics.

This includes:

Black Body Radiation

Photoelectric effect

Compton effect

Atomic Spectrum

Heat Capacity of Solids

Black Body Radiation:

A Black Body is an idealized physical body that absorbs and emits radiations of all frequencies.

Classical mechanics gives an equation which describes the intensity of blackbody radiation as a function of wavelength, Î» (or frequency, Î˝) for a fixed temperature - the result is known as the Rayleigh-Jeans law.

Although Rayleigh-Jeans law works for higher wavelengths but fails to correctly reproduce experimental trends at shorter wavelengths; this divergence for low wavelengths is called the Ultraviolet Catastrophe.

On 14th December 1900, Max Planck at the meeting of German Physical Society proposed an equation similar to the Rayleigh-Jeans equation to explain black- body radiation with an adjustable parameter h.

Planck found that for h = 6.626 x 10-34 J s, the experimental data could be reproduced. Planck was able to derive this formula by assuming that the energies of the oscillations of electrons which gave rise to the radiation were proportional to integral multiples of the frequency, i.e

where Î˝ is the frequency of radiation and h is Planckâ€™s constant. This limitation of energy to discrete values is called the quantization of energy. This was the first use of notion of quantization of energy.

Quantization refers to a situation where physical quantities, like position, energy and momentum can have only certain discrete values.

Radiation consists of discrete packets of energy called Quanta. It is the basic unit (â€śquantumâ€ť) of electromagnetic radiation.

Why this phenomenon was misinterpreted by classical mechanics? This is because classical mechanics considered an electromagnetic radiation solely wave phenomena. However, Planck held that radiant energy (or radiation) is made up particle like components known as â€śquantumâ€ť along with wave picture of radiation.

-Mayank