Molecular Spectroscopy

What is molecular Spectroscopy?? INTRODUCTION What is molecular Spectroscopy?? Applications: (in petroleum chemistry) Components of Spectroscopy Light: electromagnetic radiations Molecular behavior classification of spectroscopy Techniques involved Analysis 2

What is spectroscopy?? Spectroscopy is the study of interaction of light with matter More specifically, it is the study of properties of matter through its interaction with different frequency component of the light (electromagnetic radiation) With light we are not looking the molecule directly……instead we observe the INTERACTION of light with different types of molecules_______ Each type of spectroscopy (different light frequency)------gives a picture (our result), known as SPECTRUM--------- which gives lots of informations!!!

Introduction to spectroscopy The Information we get are: molecular structure (molecular symmetry, bond distances, bond angles), properties of molecules (electronic distribution, bond strength, intra- and inter-molecular spectra and concentration of sample Molecular Structure (molecular geometry, bond-distance, bond angles) Properties (electronic configuration, bond strength, etc) Concentration of mixture Identification concentration Sample of mixture Incident light 4

Applications of spectroscopy Spectroscopic techniques are some of the most widely used analytical methods in the world today. Properties of organic, inorganic molecules Petroleum products are hydrocarbons, i.e., they contain at least hydrogen and carbon—often other elements such as nitrogen, oxygen, halogens, phosphorus and sulfur. Most of the bonding involved in organic compounds is covalent. 5

Applications of spectroscopy The physical and chemical properties of these compounds are highly structure-related. Attachment of small groups of atoms known as functional groups to a hydrocarbon can yield a new substance with properties--at least in part--characteristic of that group. What type of hydrocarbon is present in this mixture?? Molecular Structure (bond distance, bond angles, total number and types of bonds present in the molecule), functional group attached

The immediate questions we want to address are: What is LIGHT?? What does light do to sample?? What is molecular sample? How do we produce a spectrum for a sample? What exactly a spectrum can give informations?

Light (electromagnetic radiations) Light is Electromagnetic radiation, which travel in the form of waves in a defined direction.  Energy of EMR is expressed in the form of WAVELENGTH (λ-lambda) The distance between two waves is known as wavelength Crest 9 Trough

Electromagnetic Radiation Visible light is just one form of electromagnetic radiation. Light travels in space as a wave. In vacuum, speed of light is constant and given the symbol “c”, c = 3.00 x 108 m/s. Light waves have amplitude, frequency, and wavelength. Wavelength () : distance between consecutive crests. Frequency () : number of waves that pass a given point in one second (SI Unit is s-1 or Hz). c = and  = c/

Which of the above waves has are distances – S.I. unit is the meter Which of the above waves has the higher frequency?

Light (electromagnetic radiations) TYPES: The gamma rays that are emitted by spent nuclear fuel. The x-rays that a doctor uses to visualize your bones, the ultraviolet light that causes a painful sunburn when you forget to apply sun block. The microwaves that you use to heat up your frozen food stuff, and the radio-frequency waves that bring music to listen to FM or AM radio. The different properties of the various types of electromagnetic radiation are due to differences in their wavelengths, and the corresponding differences in their energies: shorter wavelengths correspond to higher energy.  12

The Electromagnetic Spectrum

What does light to do to sample?? Basically, we characterize how a sample modifies light entering in it. E.g. Can Absorb light (absorption spectroscopy) Can emit light (emission spectroscopy)

Absorption spectroscopy

Absorption spectroscopy

Absorption spectroscopy

Atomic Emission Spectroscopy The substances in a solution are suctioned into an excited phase where they undergo vaporization, and are broken down into small fragmented atoms by discharge, flame or plasma. By exposing these atoms to such temperatures they are able to “jump” to high energy levels and in return, emit light. The versatility of atomic absorption an analytical technique (Instrumental technique) has led to the development of commercial instruments. Three main types of emission Atomic Emission (with thermal excitation), AES Atomic Absorption, (with optical photon unit) AAS Atomic Florescence, AFS

Principle of Emission Spectroscopy As with fluorescence, the atomic emission is a result of electrons dropping from an excited state to lower states. Following atomization, a small percentage of the atoms absorb sufficient energy from the flame so as to be promoted to an excited state. As with molecules in fluorescence, these atoms quickly return to a lower state, and light corresponding to the energy that is lost in the process is generated. It is this light that our eye perceives.

AES experiment set-up

What is molecular sample? The electrons and nuclei of atoms and molecules may exist in only certain specific energy levels. They can absorb energy and get excited to the higher energy levels example: diatomic molecule

How do we produce a spectrum for a sample?---the basic idea In a spectroscopy experiment, electromagnetic radiation of a specified range of wavelengths is allowed to pass through a sample containing a compound of interest. The sample molecules absorb energy from some of the wavelengths, and as a result jump from a low energy ‘ground state’ to some higher energy ‘excited state’.  Other wavelengths are not absorbed by the sample molecule, so they pass on through.  A detector on the other side of the sample records which wavelengths were absorbed, and to what extent they were absorbed.

Schematic diagram of spectrometer Io= Intensity of Incident (falling) light I= intensity of light after absorption

How do we produce a spectrum for a sample?---the basic idea Here is the key to molecular spectroscopy:  a given molecule will specifically absorb only those wavelengths which have energies that correspond to the energy difference of the transition that is occurring.  Thus, if the transition involves the molecule jumping from ground state A to excited state B, with an energy difference of ΔE, the molecule will specifically absorb radiation with  wavelength that corresponds to ΔE, while allowing other wavelengths to pass through unabsorbed. 

How does spectrum look like?

How does spectrum look like?

Molecular energies An isolated molecule possesses: translational energy by virtue of the motion of the molecule as a whole  (ii) rotational energy due to bodily rotation of the molecule about an axis passing through the center of gravity of molecule. (iii) vibrational energy due to periodic displacement of its atoms from their equilibrium positions. (iv) electronic energy since the electrons associated with each atom and bond are in constant motion. the total energy of a molecule can be expressed as the sum of the constituent energies, that is Etotal = Etrans + Erot + Evib + Eelec 29

molecule. A few examples are shown in Table I. The electrons and nuclei of atoms and molecules may exist in only certain specific energy levels, that is, they area quantized. They can absorb only photons having certain energies or wavelengths. The energies of light absorbed by a molecule can be related to motions (energy modes) of the molecule. A few examples are shown in Table I. 30

Absorbed electromagnetic radiations Energy Molecular motions Absorbed electromagnetic radiations Energy Rotation Microwave, infrared Low Vibrations Infrared Moderate Electronic transitions Visible, UV radiations High 31

Radiations and Transitions Change of Spin

Characteristic Electronic Transitions L mol-1 cm-1 Structure Determination of Organic Compounds 33

UV Absorption of Conjugated Alkenes e units = L mole-1 cm-1 Increasing conjugation gives: longer wavelength absorption more intense absorption

b-Carotene 11 double bonds lmax = 460 nm (e = 139,000)

Application of Spectroscopy in Petroleum Hydrocarbons / Oil Detection with UV/Vis Spectrometry Detection of the presence of hydrocarbons and oils in water is an important application in some industrial waste waters (petrochemical applications, but also food industry, e.g. in edible oil production) as well as in environmental monitoring (river, lake, seawater). Hydrocarbons and oil are groups of substances and should thus be considered sum parameters. However, they do not represent the total amount of organic materials present in the water, but a more specific fraction.

Application of Spectroscopy in Petroleum Hydrocarbons Hydrocarbons are substances originating from or produced out of crude oil and natural gas. This group comprises a huge amount of different molecules, ranging from aromatic substances (e.g. benzene) to linear alkanes (e.g. hexane). The signal in the UV-absorp­tion spectrum for these substances varies with their chemical structure, but it is known that depending on the application about 30 - 80 percent of the substances that contribute to the parameter Total Hydrocarbons produce a useful optical signal, either by conjugated double-bounds, or by chromophore functional groups.

Application of Spectroscopy in Petroleum The signal in the UV-absorption spectrum for hydrocarbons varies with their chemical structure. Depending on the application about 30 - 80 % of the substances produce a useful optical signal.

Application of Spectroscopy in Petroleum Aromatic compounds Common aromatic compounds present in crude oils in­clude benzene , alkylbenzenes, alkylnaphthalenes, biphenyl, and aromatic steranes, all of them well detect­able in the low ppb range in practical applications except highly turbid or strongly colored waters. The polycyclic aromatic hydrocarbons, which are rated high priority pollutants because of their toxicity and their ubiquity.

aromatic content -> responsible for the main absorption signal Application of Spectroscopy in Petroleum The possibility of measurement of hydrocar­bons / oil in water using UV/Vis spectrometry strongly depends on the composition of the oil, and more specifically on the following points: aromatic content -> responsible for the main absorption signal size of the molecules -> determines miscibility

IMPORTANCE OF MOLECULAR SPECTROSCOPY Molecular spectroscopy helps us to give electronic structure of the molecule and thus explain the various possible reactions that can take place. Spectroscopic techniques are some of the most widely used analytical methods in the world today. understanding of the molecular processes, molecular interactions, chemical system

Summary Study of the detailed molecular structure through interaction of light with sample is known as spectroscopy Light is EMR, consisting of various regions, depending upon different energies, each region is capable of interaction Interaction causes molecular changes by absorption/emission of light This produces a spectrum, which gives identification and analysis