ultraviolete (UV)

INTRODUCTION

When we speak of a molecule as being raised to a higher electronic level we mean that An electron has been chansed from one orbital to another orbital of higher energy. This electron can be of any kinds we have encountered –a σ electron, a π electron or  an n electron. In Ultraviolet region we are confined only to the excitation of the comperatevely loosely held n and π electrons.

     Various types of transitions

Out of the above mentioned transitions only n → π^* and π → π^* are of use to the analytical chemist working on the ultraviolet spectrophotometer.

                                                                  n → π^*

                                                                 π → π

When light – either visible or ultraviolet – is absorbed by valence (outer) electrons. These Electrons are promoted from their normal (ground) states to higher energy (excited) states .The energies of the orbitals involved in electronic transitions have fixed values. Because energy is quantised, it seems safe to assume that absorption peaks in a UV/visible spectrum will be sharp peaks. However, this is rarely, if ever, observed. Instead the spectrum has broad peaks .This is because there are also vibrational and rotational energy levels available to absorbing materials.

Ultraviolet-visible spectroscopy or ultraviolet-visible spectrophotometery (UV-Vis or UV/Vis) involves the spectroscopy of photons in the UV-visible region.

 There is an interaction between UV visible light and sample which is in solution form. As a result of this interaction some photons(photons of UV-Vis EMR) are absorbed and this absorption of UV-Vis  is measured by an instrument named UV visible spectrophotometer.

Uv visible is low energy EMR hence generally no ionization is take place but electronic transition of lone pair and π electron take  place (200-800 nm)

QUANTUM MECHANICS

Quantum mechanics (QM) is a set of scientific principles describing the known behavior of energy and matter that predominate at the atomic and subatomic scales. QM gets its name from the notion of a quantum, and that quantum value is the Planck constant. The wave–particle duality of energy and matter at the atomic scale provides a unified view of the behavior of particles such as photons and electrons. While the notion of the photon as a quantum of light energy is commonly understood as a particle of light that has an energy value governed by the Planck constant, what is quantized for an electron is the angular momentum it can have as it is bound in an atomic orbital. Electrons have certain properties of particles and certain properties of waves. Electrons have mass and charge like particles. Because they are so small and are moving so fast, electrons have no defined position. Their location is best described by wave mechanics (i.e. a three dimensional wave) and a wave equation called the Schrödinger equation. Solutions of the Schrödinger equation are called wave functions and are represented by the Greek letter psi. Each wave function describes a different orbital. There are many solutions to the Schrödinger equation for a given atom.

Atomic orbitals:- the region in space where an electron is likely to be found called an orbital.there are different kinds of orbitals , which have different sizes and different shapes, and which are disposed about the nucleus in specific ways. The particular kind of orbital that an electron occupies depends upon the energy of the electron.

CONCEPT OF MOLECULAR ORBITALS

Theory of molecular orbitals comes under  the preview of quantum mechanism. Erwin schrodinger formulated a wave equation which has a series of solutions called wave functions, each corresponding to a different energy level for the electron.

In molecules, as an isolated atoms,electrons occupy orbitals and in accordance with much the same “rules”. These molecular orbtials are considered to be centered about many nuclei, perhaps covering the entire molecule; the distribution of nuclei and electrons is simply the one that results in the most stable molecule.

Construcyive interference of two wave function leads to high probability density between the nuclei , leading to bonding orbital whilein antibonding MOs wave function interfare destructively leading to low electron density that is greater repultions

Thus electrons in a bonding MOs tend to hold the atoms together and electrons in antibonding tend to force atoms apart.

Linear Combination Of Atomic Orbitals [LCAO]

As we know that the number of formed molecular orbitals is equal to the number of component atomic orbitals. AOs may overlap either axially or laterally. This can be understood diagrammatically as shown below

Axial overlaping

                                     P­_AO                                                                       P_AO

                                                 _{φA                                                                        φB}

Axial Overlaping

 

               Addition mode                                                              Subtraction mode

               More stable                                                                    less stable

               Less energy                                                                    more energy         

          _{φA + φB = ψ1                                                                                            φA – φB = ψ2}

               Bonding MO                                                                 Anti bonding MO

            

                  

Max. electron density                                                                         zero electron density  

Lateral overlapping

                                                         P­_AO                         P_AO

                                                                           _{φA                        φB}

lateral Overlaping

 

                Addition mode                                                            Subtraction mode

                 More stable                                                                 less stable

                 Less energy                                                                 more energy          

           _{φA + φB = ψ1                                                                                     φA – φB = ψ2}

Bonding MO                                                                   Anti bonding MO

                                                      

 

                                              

                                                                                                                        zero electron density

ENERGY LEVELS OF BONDING AND ANTI-BONGING  MOLECULAR ORBITALS

Molecular orbirtal Diagram

The two P atomic orbitals combine to form two molecular orbitals, one bonding  and one antibonding .

ELECTRONIC CONFIGURATION OF SOME MOLECULES

Ethylene . configuration of π electrons in the ground state and the excited state

For the π electrons of ethylene, there are two molecular orbitals since there are two linear combinations of the two component p orbitals. The broken line in the figure indicates the non-bonding energy levels. Below it lies the bonding orbitals, π and above it lies the antibonding orbitals π^*. Normally a molecule exist in the state of lowest energy ,the ground state. But absorption of  light of the right frequency in the ultraviolet region rises a molecule to an exicted state, a state of higher energy, in the ground state of ethylene both π electrons are in the π orbital, this configuration is specified as π², where the superscript tells the no. of electrons in the orbital. In the excited state one electron is in the π orbital and the other still of opposite spin- is in the π^*orbital, this configuration, ππ^* is naturally the less stable since only one electron helps to hold the atoms together, while other tend to force them apart.

1,3- butadiene.configuration of π electrons in the ground state and the first excited state

For 1,3-butadiene,with 4 component p orbitals, there are 4 MO for π electrons.the ground state has the configuration ψ₁²  ψ₂², i.e. there are two electrons in each of the bonding orbitals ψ₁ &  ψ_2.the higher of these ψ_2, resembles two isolated π orbitals,although it is of somewhat lower energy.orbital ψ₁ encopasses all four carbons, this delocalization provides the net conjugated system.absorption of light of the right frequency rises one electron to ψ₃

                                ψ₁²  ψ₂²            hv                  ψ₁²  ψ₂ ψ₃

                                ground state                            lowest excited state

HOMO  refers to Highest Occupied Molecular Orbital

LUMO  refers to Lowest Unoccupied Molecular Orbital

The two orbitals best matched in energy will be the highest occupied molecular orbitals or HOMO.on one reactant,and the lowest energy unoccupied MO,or LUMO on the other.

               

POSSIBLE HOMO-LUMO COMBINATIONS

HOMO	LUMO	RESULT
An occupied n orbital	An empty n orbital	Bond formation only
An occupied n orbital	A σ* orbital	Bond formation and bond rupture
An occupied n orbital	A π* orbital	Bond formation and bond rupture
A  π orbital	An empty orbital	Bond formation and bond rupture
A  π orbital	A  σ *orbital	Bond formation and bond rupture
A  π orbital	A  π*  orbital	Bond formation and bond rupture
A  σ orbital	An empty  orbital	Bond formation and bond rupture
A σ orbital	A  σ * orbital	Bond formation and bond rupture
A σ  orbital	A  π*  orbital	Bond formation and bond rupture

Transition probability

It is not essential that exposure of a compound to ultraviolet or visible light must always gives to an  electronic transition. On the other habd, the provavility of a particular electronic transition has found to depenЄd upon the value of molar extinction coefficient and certain other factors. According ransitions can be divided into two categories.

(i)                 Allowed transitions

(ii)               Forbidden transitions

(i)Allowed transitions – these are transitions having molar coefficient 10⁴   or more. These are generally designated as π → π transitions.for  example in 1,3-butadiene which exhibits absorption at 217nm has  Є_max value 21000 represents an allowed transition.these transition are mainly favoured due to symmetry relationship.for eg.

1,3- butadiene absorbs at 217nm and has molar absorptivity of 21000

 (ii)Forbidden transitions­ – these are transitions for which Є_max is generally less than 10⁴ . for example transition of saturated aldehyde  showing weak absorption near 290nm and having Є_max 100 has been a forbidden transition.for eg.

Carbonyl group absorbs at 300nm and a molar absorptivity of 10-100

TRANSITIONS  IN  ULTRAVIOLET  SPECTROSCCOPY

Electronic transition in uv-visible spectroscopy which are important are 

n → π^*  & π → π^*transitions.

(a) n → π^* transitions. - In this transition, an electon of unshared electron pair

  on a hetero atom is excited to π^* antibonding orbital. This transition involves least amount of energy than all the transitions and therefore, this transition  gives rise to an absorption band at longer wabelengths. In saturated aliphatic ketones, e.g., the  n → π^* transitions around 280 nm is the lowest energy transitions. This n → π^* transitions is “forbidden” by symmetry considerations, thus the intensity  of the band due to this transition is low, although the wavelength is long(lower energy).

(b) π → π^*  transitions -  this transition is available in compounds with unsaturated centres, e.g., simple aalkenes, aromatics,carbonyl compounds, etc. this transition requires lesser energy then transition.in a simple alkene, although several transitions are available , the lowest energy transition is the π → π transition and a absorption band around 170nm-190nm in unconjugated alkenes is due to this transition in the case of , e.g., saturated ketones, the most intense band around 150nm is due to π → π transition.

DESIGNATION OF BANDS

K- Band

One may designate the uv absorption bands by using electronic transitions or the letter designation. The band due to π → π transitions in a compound with conjugated  π system is usually intense (Є_max.>10000) and is frequently referred to as the k-band (german- konjugierte). The examples of the compounds in which k-band appears are butadiene, mesityl oxide. Benzene itself displays three absorption bands at 184,204 and 256nm and of these the band at 204nm is often designated as k-band, and this used in other benzenes as well.

Eg. Conjugated die ne , triene , polyene, enones and aromatic rings

R- Band

 The n → π^* transition (R-band german radikalartig) in compounds with single chromatographic groups i.e., carbonyl or nitro are forbidden with Є value less than 100.

In conjugated systems the energy separation between the ground and excited states is reduced and the system then absorbs at longer wavelengths and with a greatly increased intensity (k-band is intense and at longer wavelength). Moreover, due to the lessening of the energy gap, the n → π^* transition due  to the presence of the heteroatom and lone pair i.e. the r-band also undergoes a red shift with little change in intensity.

Eg. Acetone, acroline, methyl vinyl ketone, acet aldehyde, acetophenone, croton aldehyde

B-Band

These bands are observed in aromatic compounds and hetero aromatic compounds. Here B refers to Benzenoid bands

Eg. Benzene, tolune, acetophenone, benzoic acid, napthelene, styrene

E- Bands

Such band originate due to electronic transition in the benzenoid system of the ethylinic part enclosed in cylic conjugation. Here E refers to Ethylinic. These are further classified as E1 and E2

Eg. Benzene, nepthelene, anthracene, quinolene

DIFFERENT EFFECTS

1.      Effect of solvent

The transitions of polar bonds, like c=o but not ethylene, are affected by solvent polarity.as solvent polarity is increased, π → π bands undergo red shifts. This is so since excited state is more polar than the ground state and hence stabilization is greater relative to the ground state with two n electrons receives greater stabilization than than the excited state with only n electron. These opposite trends are clear by examining the data of mesityl oxide. There is more on shift of bands with solvents.

2.      Effect of conjugation

Absorption in near UV that is above 200 nm is invariably associated with the presence of unsaturated groups or atoms with unshared pairs of electrons the saturated hydrocarbon which do not have these structural elements absorve below 200nm reason, not of much significance for structural study of organic compounds. Thus interstically a complex steroid molecule cholest-4- ene- 3 one is easily recognized to have an  α-ß   unsaturated keton moiety, similar to that in mesityl oxide by their spectral resemblance.

π → π* transition(K-band)                                                                     (for cholest-4en-3 one)                                                                                                                 n → π* transition(R-band)                                                                                                             (for mesityl oxide)

                              Λ (in nm)

3.      Effect of PH

In alkaline PH π → π transition is more favoured and in acidic PH n → π^* transition is  more favoured.for eg.

 (1)     P-nitrophenol                                           p-nitrophenol (+M effect increased)

                                                                       Alkaline PH

                                                                        π → π transition more favoured

                                                                        less energy is required

                                                                        absorption would take place at longer                  

                                                                        wavelength

(2)      p-amino phenol                                   P-amino phenol

                                                                      Acidic PH

                                                                       More energy required

                                                                      Blue shift

                                                                   Absorption at shorter wavelength