UV-Vis Spectroscopy- Introduction

***Introduction***

UV radiation and Electronic Excitations

The difference in energy between molecular bonding, non-bonding and anti-bonding orbitals ranges from 125-650 kJ/mole. This energy corresponds to EM radiation in the ultraviolet (UV) region, 100-350 nm, and visible (VIS) regions 350-700 nm of the spectrum.

Process

Using IR we observed vibrational transitions with energies of 8-40 kJ/mol at wavelengths of 2500-15,000 nm. For purposes of our discussion, we will refer to UV and VIS spectroscopy as UV.
In UV spectroscopy, the sample is irradiated with the broad spectrum of the UV radiation. If a particular electronic transition matches the energy of a certain bandof UV, it will be absorbed. The remaining UV light passes through the sample and is observed. From this residual radiation a spectrum is obtained with “gaps” at these discrete energies – this is called an absorption spectrum

Observed electronic transitions

The lowest energy transition (and most often obs. by UV) is typically that of an electron in the Highest Occupied Molecular Orbital (HOMO) to the Lowest Unoccupied Molecular Orbital (LUMO). For any bond (pair of electrons) in a molecule, the molecular orbitals are a mixture of the two contributing atomic orbitals; for every bonding orbital “created”from this mixing (s, p), there is a corresponding anti-bonding orbital of symmetrically higher energy (s*, p*). The lowest energy occupied orbitals are typically the s; likewise, the corresponding anti-bonding s*orbital is of the highest energy. p-orbitals are of somewhat higher energy, and their complementary anti-bonding orbital somewhat lower in energy than s*. Unshared pairs lie at the energy of the original atomic orbital, most often this energy is higher than por s(since no bond is formed, there is no benefit in energy)

Although the UV spectrum extends below 100 nm (high energy), oxygen in the atmosphere is not transparent below 200 nm .Special equipment to study vacuumor far UVis required .Routine organic UV spectra are typically collected from 200-700 nm. This limits the transitions that can be observed:

Selection Rules

1. Not all transitions that are possible are observed
2. For an electron to transition, certain quantum mechanical constraints apply –these are called “selection rules”
3. For example, an electron cannot change its spin quantum number during a transition –these are “forbidden”
4. Other examples include:

• the number of electrons that can be excited at one time
• symmetry properties of the molecule
• symmetry of the electronic state

To further complicate matters, “forbidden”transitions are sometimes observed (albeit at low intensity) due to other factors

Band Structure

Unlike IR (or later NMR), where there may be upwards of 5 or more resolvable peaks from which to elucidate structural information, UV tends to give wide, overlapping bands. It would seem that since the electronic energy levels of a pure sample of molecules would be quantized, fine, discrete bands would be observed –for atomic spectra, this is the case. In molecules, when a bulk sample of molecules is observed, not all bonds (read –pairs of electrons) are in the same vibrational or rotational energy states. This effect will impact the wavelength at which a transition is observed –very similar to the effect of H-bonding on the O-H vibrational energy levels in neat samples.When these energy levels are superimposed, the effect can be readily explained – any transition has the possibility of being observed

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Ultra Violet & Visible (UV/Vis.) Spectroscopy & its Types

Spectroscopy Process

1.  In UV spectroscopy, the sample is irradiated with the broad spectrum of the UV radiation 
2. If a particular electronic transition matches the energy of a certain band of UV, it will be absorbed.
3. The remaining UV light passes through the sample and is observed From this residual radiation a spectrum is obtained with “gaps” at these discrete energies – this is called an absorption spectrum

Types of Ultra Violet-Visible Spectroscopy

• Acoustic resonance spectroscopy: It is based on sound waves primarily in the audible and ultrasonic regions.

• Auger spectroscopy is a method used to study surfaces of materials on a micro scale. It is often used in connection with electron microscopy.

• Coherent anti-Stokes Raman spectroscopy (CARS) is a recent technique that has high sensitivity and powerful applications for in vivo spectroscopy and imaging.

• Correlation spectroscopy encompasses several types of two-dimensional NMR spectroscopy.

• Deep-level transient spectroscopy measures concentration and analyses parameters of electrically active defects in semiconducting materials

• Dual polarisation interferometry measures the real and imaginary components of the complex refractive index

• Electron phenomenological spectroscopy measures physico-chemical properties and characteristics of electronic structure of multi-component and complex molecular systems.

• Fourier transform spectroscopy is an efficient method for processing spectra data obtained using interferometers. Fourier transform infrared spectroscopy (FTIR) is a common implementation of infrared spectroscopy. NMR also employs Fourier transforms.

• Inelastic electron tunnelling spectroscopy (IETS) uses the changes in current due to inelastic electron-vibration interaction at specific energies that can also measure optically forbidden transitions.

• Laser-Induced Breakdown Spectroscopy (LIBS), also called Laser-induced plasma spectrometry (LIPS)

• Mass spectroscopy is an historical term used to refer to mass spectrometry.

• Mössbauer spectroscopy probes the properties of specific isotopic nuclei in different atomic environments by analyzing the resonant absorption of gamma-rays

• Neutron spin echo spectroscopy measures internal dynamics in proteins and other soft matter systems

• Photoacoustic spectroscopy measures the sound waves produced upon the absorption of radiation.

• Pump-probe spectroscopy can use ultra fast laser pulses to measure reaction intermediates in the femto-second timescale.

• Raman optical activity spectroscopy exploits Raman scattering and optical activity effects to reveal detailed information on chiral centres in molecules.

• Thermal infrared spectroscopy measures thermal radiation emitted from materials and surfaces and is used to determine the type of bonds present in a sample as well as their lattice environment. The techniques are widely used by organic chemists, mineralogists, and planetary scientists.

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