Brief Summary
Alright, so this video is all about understanding molecular structure using different types of spectroscopy. It starts with why we need spectroscopy, then goes into UV-Vis, IR, NMR, explaining the theory, instrumentation, and how to interpret the data. Key takeaways include understanding electronic transitions, vibrational modes, chemical shifts, and spin-spin coupling.
- Spectroscopy helps determine molecular formula and structure.
- UV-Vis spectroscopy deals with electronic transitions.
- IR spectroscopy focuses on vibrational modes.
- NMR spectroscopy is based on the magnetic properties of atomic nuclei.
Introduction to Spectroscopy
So, the video kicks off by explaining why spectroscopy became a thing. Earlier methods to find out a compound's molecular formula were a pain because they needed loads of the compound and took ages. Spectroscopy, on the other hand, needs only a tiny amount, like milligrams, and gives you the answer super quick. This is especially useful when you're making new drugs or chemicals and need to confirm you've got the right stuff without wasting loads of material.
Basic Principles of Spectroscopy
Spectroscopy works by looking at how electromagnetic radiation interacts with matter. When light hits a substance, its electrons and nuclei do different things. Electrons can jump to higher energy levels (absorption), or fall back down and release energy (emission). Different types of spectroscopy study these phenomena. Absorption spectroscopy is when a particle absorbs energy and goes to a higher level. Emission spectroscopy is when a particle emits energy and goes to a lower level.
UV-Vis Spectroscopy: Electronic Transitions
UV-Vis spectroscopy is all about electronic transitions. When a molecule absorbs UV or visible light, its electrons jump from lower to higher energy levels. The amount of energy needed depends on the wavelength of the light. UV-Vis range is 200 to 800 nanometers. The type of transition depends on the energy absorbed. High energy means electronic transitions, medium energy means vibrational transitions, and low energy means rotational transitions.
Terms in UV-Vis Spectroscopy
Some key terms to remember are wavelength (distance between two crests), frequency (number of waves per second), and wavenumber (number of waves per unit length). Frequency is measured in Hertz (Hz), and wavenumber is measured in cm⁻¹.
Electronic Transitions in Detail
UV-Vis spectroscopy looks at transitions of valence electrons, which are the ones hanging out in the outermost shell. These electrons jump from the ground state to an excited state. UV-Vis uses light from 200 to 800 nm. If a molecule absorbs light in this range, it means electrons are jumping between energy levels. Electronic transitions happen within electronic levels, which contain vibrational levels, which in turn contain rotational levels.
Electronic Energy Levels
When you take a UV-Vis spectrum, you usually dissolve the compound in a solvent. The solvent blurs the fine details of the spectrum because it interacts with the compound. Electronic levels are studied first, then vibrational, then rotational. The amount of energy absorbed determines which transition occurs. High energy means electronic, medium means vibrational, and low means rotational.
Types of Electronic Transitions
UV-Vis spectroscopy looks at electronic transitions, which involve electrons jumping between molecular orbitals. These orbitals include sigma (σ), pi (π), and non-bonding (n) orbitals. When electrons jump, they go from occupied orbitals to unoccupied ones.
Examples of Electronic Transitions
Let's look at some examples. For a saturated compound, the possible transitions are σ to σ*. For an unsaturated compound, you can have σ to σ*, π to π*, n to σ*, and n to π*. To figure out the transitions, draw the energy levels and fill them with electrons. Then, see where the electrons can jump.
Energy Order of Electronic Transitions
The order of energy for these transitions is important. The general (but not always correct) order is σ to σ* > n to σ* ≈ π to π* > n to π*. A more accurate order is σ to σ* > n to σ* > π to π* > n to π*.
Selection Rules for Electronic Transitions
For a transition to happen, it needs to be allowed by selection rules. These rules depend on the symmetry of the orbitals involved. Allowed transitions have high intensity, while forbidden transitions have low intensity. Sigma to sigma star and pi to pi star are allowed, while n to sigma star and n to pi star are forbidden.
Types of Compounds and Their Transitions
Let's look at some specific types of compounds. Alkanes only have sigma bonds, so they show σ to σ* transitions. Alcohols, ethers, and halides have lone pairs, so they show n to σ* transitions. Alkenes and alkynes have pi bonds, so they show π to π* transitions.
Absorption Spectrum
An absorption spectrum shows how much light a compound absorbs at different wavelengths. To get a good spectrum, you need to consider the compound's structure, the possible transitions, and the order of energy levels.
Chromophores and Auxochromes
Chromophores are groups in a molecule that are responsible for UV-Vis absorption, like double or triple bonds. Auxochromes are groups that, when attached to a chromophore, change the absorption properties. Auxochromes usually increase the intensity of absorption.
Bathochromic Shift, Hypsochromic Shift, Hyperchromic Effect, and Hypochromic Effect
These are terms to describe how the absorption spectrum changes. A bathochromic shift (red shift) is a shift to longer wavelengths. A hypsochromic shift (blue shift) is a shift to shorter wavelengths. A hyperchromic effect is an increase in intensity. A hypochromic effect is a decrease in intensity.
Vibrational Spectroscopy: IR Spectroscopy
IR spectroscopy is all about molecular vibrations. When a molecule absorbs infrared light, its bonds vibrate in different ways. The IR region is from 4000 to 400 cm⁻¹. For a molecule to be IR active, its dipole moment must change during vibration.
IR Active Molecules
A molecule is IR active if its dipole moment changes during vibration. This means that symmetrical molecules like H₂ or O₂ are IR inactive because their dipole moment doesn't change. However, molecules like HCl are IR active because their dipole moment changes as they vibrate.
Fingerprint Region
The fingerprint region in an IR spectrum is from 1500 to 500 cm⁻¹. This region is unique to each molecule and can be used to identify it. Above 1500 cm⁻¹, you find group frequencies, which correspond to specific bonds like O-H, N-H, C=O, etc.
Types of Molecular Vibrations
Molecules can vibrate in different ways, including stretching and bending. Stretching can be symmetric or asymmetric. Bending can be scissoring, rocking, wagging, or twisting.
Modes of Vibration
To figure out how many vibrational modes a molecule has, use the formula 3N - 6 for non-linear molecules and 3N - 5 for linear molecules, where N is the number of atoms.
Example: CO₂ Vibrational Modes
CO₂ is a linear molecule, so it has 3N - 5 = 4 vibrational modes. These include symmetric stretching (IR inactive), asymmetric stretching (IR active), and two bending modes (IR active).
Frequency of Vibration
The frequency of vibration depends on the force constant (k) and the reduced mass (μ) of the bond. The formula is ν = (1/2πc)√(k/μ), where c is the speed of light.
Energy Levels and Selection Rules
The energy levels for vibrational transitions are quantized. The selection rule is Δv = ±1, meaning that a molecule can only jump to the next higher or lower vibrational level.
Zero Point Energy
Even at absolute zero, a molecule still has some vibrational energy, called the zero-point energy. This energy is equal to (1/2)hν.
Microwave Spectroscopy: Rotational Spectroscopy
Rotational spectroscopy looks at the rotational energy levels of molecules. For a molecule to be microwave active, it must have a permanent dipole moment.
Moment of Inertia
The moment of inertia (I) is a measure of a molecule's resistance to rotation. For a diatomic molecule, I = μr², where μ is the reduced mass and r is the bond length.
Rotational Energy Levels
The energy levels for rotational transitions are quantized. The energy is given by E = (h²/8π²I)J(J+1), where J is the rotational quantum number.
Selection Rules
The selection rule for rotational transitions is ΔJ = ±1. This means that a molecule can only jump to the next higher or lower rotational level.
Spacing Between Lines
The spacing between lines in a rotational spectrum is constant and equal to 2B, where B is the rotational constant.
NMR Spectroscopy: Nuclear Magnetic Resonance
NMR spectroscopy is based on the magnetic properties of atomic nuclei. It involves applying radio frequency radiation to a sample in a magnetic field.
Nuclear Spin
Atomic nuclei have a property called spin, which is quantized. The spin quantum number (I) can be 0, 1/2, 1, 3/2, etc. Nuclei with I = 0 are NMR inactive.
NMR Active Nuclei
NMR active nuclei include ¹H, ¹³C, ¹⁹F, and ³¹P. The most common NMR experiment is proton NMR (¹H NMR).
Nuclear Magnetic Moments
In the absence of an external magnetic field, nuclear spins are randomly oriented. When a magnetic field is applied, the spins align either with or against the field.
Energy Levels
The energy difference between the spin states is proportional to the strength of the magnetic field.
Instrumentation
An NMR spectrometer consists of a magnet, a radio frequency source, and a detector. The sample is placed in the magnetic field and irradiated with radio frequency radiation. When the frequency matches the energy difference between the spin states, absorption occurs.
Chemical Shift
Chemical shift is the difference in resonance frequency between a nucleus and a reference compound. It is caused by the electrons surrounding the nucleus, which shield it from the applied magnetic field.
Shielding and Deshielding
Shielding occurs when the electrons surrounding a nucleus reduce the effective magnetic field experienced by the nucleus. Deshielding occurs when the electrons surrounding a nucleus increase the effective magnetic field experienced by the nucleus.
TMS as a Reference
Tetramethylsilane (TMS) is used as a reference compound in NMR spectroscopy. It has a chemical shift of 0 ppm.
Factors Affecting Chemical Shift
The chemical shift of a nucleus depends on its electronic environment. Electronegative atoms deshield nearby nuclei, increasing their chemical shift.
Spin-Spin Coupling
Spin-spin coupling is the interaction between the spins of neighboring nuclei. It causes the signals in an NMR spectrum to split into multiplets.
Multiplicity
The multiplicity of a signal is determined by the number of neighboring nuclei. The n+1 rule states that a nucleus with n neighboring nuclei will be split into n+1 peaks.
Pascal's Triangle
Pascal's triangle can be used to predict the relative intensities of the peaks in a multiplet.
Predicting NMR Spectra
To predict the NMR spectrum of a compound, first determine the number of different types of protons. Then, determine the chemical shift and multiplicity of each signal.
MRI
Magnetic resonance imaging (MRI) is a medical imaging technique that uses NMR to create images of the inside of the body.
