What Do You Mean By Theory of Molecular Spectra?
The essential resource that is available for investigating the molecular structure is the theory of molecular spectra. The discrete energy states of a certain molecule can be derived directly from the theory. Molecular energy levels and molecules in an excited state might seem more complicated to understand than atomic energy levels. This is because of the reason that molecules have the ability to rotate and vibrate. The energy which is associated with such motions of rotation and vibration lie in different ranges. You can study these different ranges separately to avoid complications and understand the subject matter easily.
Understanding Spectroscopy
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Spectroscopy or molecular spectroscopy is basically the area of science that deals with the emission, absorption, and scattering of electromagnetic radiation by molecules and atoms in phases such as solid, liquid, and gas. Spectroscopy also plays a crucial role in understanding the results of spectroscopic experiments and the development of the laws of quantum mechanics. The aim of spectroscopy is to make the discussion more concrete about the quantum mechanical description of the molecular properties and the laws of quantum mechanics.
Electromagnetic Spectrum
Before you delve into understanding molecular spectroscopy in detail, first, you will need to get a solid understanding of the subject of the electromagnetic spectrum. It is because spectroscopy is defined as the scientific area that deals with the emission, absorption, and scattering of electromagnetic radiation by molecules and atoms. Visible electromagnetic radiation is termed light, although here, you can use the terms electromagnetic radiation, radiation, and light, interchangeably.
One of the essential aspects of physical chemistry is to recognize the interaction of molecules with the surroundings. With molecular spectroscopy, you can clearly imagine how a polyatomic and diatomic molecule interacts with its surroundings. Further, you can study how any polyatomic or diatomic molecule interacts when studying the wavelength, frequency, energy, wave number, and molecular process. The absorption properties of any polyatomic or diatomic molecule in different regions can also be seen from the electromagnetic spectrum.
Information of molecules is clearly provided by the electromagnetic spectrum about if there are electronic transitions, rotational transition, or vibrational transition. A molecule or a group of molecules is usually read by seeing the absorption of microwave radiation that provides transitions between energy levels which are rotational. Additionally, when a molecule or group of molecules can absorb infrared radiation, it can provide the transition between the vibrational levels, which is followed by the transition between rotational levels of energy.
Electromagnetic Radiation
Electromagnetic radiation is simply a form of energy. Waves and particles both describe the behaviour of electromagnetic radiation. The dual models involving wave and particle behaviour give us an informative description of electromagnetic radiation. Although, the exact nature of it still remains unclear since the development of the laws of quantum mechanics at the beginning of the 20th century.
Properties of Electromagnetic Radiation
When the refraction of electromagnetic radiation occurs, it passes from a medium to another, and it can be explained in the best way possible by describing light as a wave.
Properties like absorption and emission can be better explained when treating light as a particle.
Vibrational Transitions Accompanied by Rotations
While studying spectroscopy, it is important to know how each of the peaks of rotational transitions and vibrational transitions correlates with the molecular processes. In the first place, a molecule’s rotation is possibly affected by the molecule’s vibrational transition. This happens because of a change in bond length which is responsible for rotational transitions that might occur.
Now that we know the basics of spectroscopy look at some of the basic solved examples that will help you ace your exams.
Solved Examples
Q1. Among the Options, Which One Can be Considered a Photographic Negative of an Emission Spectrum?
Option 1. Emission spectra
Option 2. Absorption spectra
Option 3. Atomic spectra
Option 4. Spectrum
Answer: Option 2. Absorption spectra
Q2. Choose the Term Which is Used for a Band of Colours.
Option 1. Spectra
Option 2. Spectral analysis
Option 3. Spectrum
Option 4. Dispersion
Answer: Option 3. Spectrum
Q3. Choose the One Which Is the Analysis of Electromagnetic Radiation.
Option 1. Spectroscopy
Option 2. Astronomy
Option 3. Kaleidoscopy
Option 4. Anatomy
Answer: Option 1. Spectroscopy
Q4. in an Atomic Emission Spectrum, Which One of These Lines is Present?
Option 1. Light
Option 2. Bright
Option 3. Brown
Option 4. Black
Answer: Option 2. Bright
Q5. Choose the One Which Has Lower Energy.
Option 1. Initial state energy level
Option 2. Excited-state energy level
Option 3. Ground state energy level
Option 4. None of these
Answer: Option 3. Ground state energy level
FAQs on Theory of Molecular Spectra
1. What is the theory of molecular spectra?
The theory of molecular spectra explains how molecules absorb or emit electromagnetic radiation to produce a spectrum. Unlike atoms which produce distinct line spectra, molecules produce band spectra. This is because molecules have more complex internal energy states, including electronic, vibrational, and rotational energy levels. When a molecule transitions between these energy levels, it results in a group of closely packed spectral lines that form a band, providing valuable information about the molecule's structure, bond lengths, and energy.
2. What is the fundamental origin of molecular spectra?
The origin of molecular spectra lies in the transitions between quantised energy levels within a molecule. According to quantum mechanics, a molecule can only exist in discrete energy states. When a molecule absorbs a photon of a specific energy, it jumps to a higher energy level (vibrational, rotational, or electronic). Conversely, when it drops from a higher to a lower energy level, it emits a photon. The collection of energies (or wavelengths) of these absorbed or emitted photons constitutes the molecule's spectrum, which acts as a unique fingerprint. This is conceptually similar to the principles explained in Bohr's Atomic Model but applied to the more complex energy structure of molecules.
3. How is a molecular spectrum different from an atomic spectrum?
The primary difference lies in their appearance and the information they provide. An atomic spectrum consists of sharp, well-defined lines because it only involves electronic transitions between energy levels of an atom. In contrast, a molecular spectrum consists of broad bands. Each band is actually a collection of many closely spaced lines. This complexity arises because, for each electronic transition in a molecule, there are numerous possible changes in its vibrational and rotational energy states, leading to a much richer and more complex spectrum. You can learn more about the basics of spectra from the page on Atomic Spectra.
4. What are the main types of molecular spectra?
Molecular spectra are primarily classified into three types based on the energy of the transitions involved:
Rotational Spectra: These arise from transitions between rotational energy levels of a molecule. They are observed in the microwave region and provide information about bond lengths and molecular geometry.
Vibrational-Rotational Spectra: These result from transitions between vibrational energy levels, with accompanying changes in rotational levels. They are typically found in the infrared region and are used to identify functional groups in a molecule.
Electronic Spectra: These are caused by transitions of electrons from one molecular orbital to another. These are the highest energy transitions and are observed in the visible and ultraviolet (UV-Vis) regions, often giving substances their colour.
5. Why do molecules exhibit band spectra instead of the line spectra seen in atoms?
Molecules exhibit band spectra because their total energy is a sum of electronic, vibrational, and rotational energies, all of which are quantised. When an electron jumps between electronic levels (a high-energy transition), the molecule can also simultaneously change its vibrational and rotational states. Since there are many possible vibrational and rotational levels associated with each electronic level, a single electronic transition is accompanied by a multitude of smaller energy transitions. These numerous transitions have very similar energies, resulting in a series of closely packed spectral lines that are unresolved by most spectrometers and thus appear as a continuous band. In contrast, atoms lack these vibrational and rotational degrees of freedom, leading to simple line spectra.
6. What is the practical importance of studying molecular spectra?
The study of molecular spectra, or spectroscopy, is a powerful analytical tool with wide-ranging applications. Its importance includes:
Chemical Analysis: Identifying unknown compounds by matching their unique spectral fingerprint.
Structural Determination: Determining molecular structure, bond lengths, and bond angles.
Astrophysics: Analysing the chemical composition of stars, planets, and interstellar gas clouds by examining the light they emit or absorb. You can explore this further in the topic of Astrophysics.
Environmental Monitoring: Detecting pollutants in the air and water by their characteristic absorption spectra.
7. How does the Born-Oppenheimer approximation help in understanding the theory of molecular spectra?
The Born-Oppenheimer approximation is a fundamental concept that greatly simplifies the analysis of molecular systems and their spectra. It assumes that the motion of the atomic nuclei and the motion of the electrons can be treated separately. This is a valid assumption because nuclei are much heavier and move far more slowly than electrons. By 'clamping' the nuclei in a fixed position, we can solve for the electronic energy levels first. Then, we can treat the electronic energy as a potential for the nuclear motion (vibration and rotation). This separation allows us to consider the total molecular energy as a simple sum: E_total ≈ E_electronic + E_vibrational + E_rotational, making the interpretation of complex molecular spectra much more manageable.
