Spectroscopy, UV-VISIBLE spectroscopy, Infrared Spectroscopy, Nuclear Magnetic Resonance spectroscopy, Mass Spectroscopy.

Spectroscopy, Elementary Organic Spectroscopy, Electromagnetic Spectrum, UV-visible spectroscopy, IR spectroscopy, NMR Spectroscopy, Mass Spectroscopy. Instrumention, application, Chemical shift, coupling constant, Absorption and intensity shifts

1. Spectroscopy

2. Electromagnetic Radiations Electromagnetic radiation consists of electromagnetic waves, which are synchronized oscillations of electric and magnetic fields. In a vacuum, electromagnetic waves travel at the speed of light. They are depending on the frequency of oscillation, different wavelengths of electromagnetic spectrum are produced. In homogeneous, isotropic media, the oscillations of the two fields are on average perpendicular to each other and perpendicular to the direction of energy and wave propagation, forming a transverse wave. The position of an electromagnetic wave within the electromagnetic spectrum can be characterized by either its frequency of oscillation or its wavelength. Electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy.[1] Types of EMR include radio waves, microwaves, infrared, (visible) light, ultraviolet, X-rays, and gamma rays, all of which are part of the electromagnetic spectrum.

3. Units Wavelength: The distance between the two adjacent crests or troughs in a particular wave. Wavenumber: I t is the reciprocal of wavelength and calculated as per centimeter. Frequency: The number of waves which can pass through a point in one second. Energy: E=hv; here, h= Planck’s constant; v= frequency of radiation in cycles per second.

4. Electromagnetic Spectrum: It is the full range of electromagnetic radiation, organized by frequency or wavelength. The spectrum is divided into separate bands, with different names for the electromagnetic waves within each band.

5. Ultra-violet and Visible Spectroscopy Ultraviolet (UV) spectroscopy related with absorption spectroscopy or reflectance spectroscopy in part of the ultraviolet and the full, adjacent visible regions of the electromagnetic spectrum. The only requirement is that the sample absorb in the UV-Vis region, i.e. be a chromophore. Absorption spectroscopy is complementary to fluorescence spectroscopy. Parameters of interest, besides the wavelength of measurement, are absorbance (A) or transmittance (%T) or reflectance (%R), and its change with time . A UV-vis spectrophotometer is an analytical instrument that measures the amount of ultraviolet (UV) and visible light that is absorbed by a sample. UV-vis spectrophotometers work by passing a beam of light through the sample and measuring the amount of light that is absorbed at each wavelength. The amount of light absorbed is proportional to the concentration of the absorbing compound in the sample. Most molecules and ions absorb energy in the ultraviolet or visible range, i.e., they are chromophores. The absorbed photon excites an electron in the chromophore to higher energy molecular orbitals, giving rise to an excited state.[7] For organic chromophores, four possible types of transitions are assumed: π–π*, n–π*, σ–σ*, and n–σ* . Transition metal complexes are often colored (i.e., absorb visible light) owing to the presence of multiple electronic states associated with incompletely filled d orbitals.

6. Lambert’s Law: When a beam of monochromatic radiation passes through a homogenous absorbing medium, the rate of decrease of intensity of radiation with thickness of absorbing medium is proportional to the intensity of the incident radiation. -dI/dx= kI where I= intensity of radiationafter passing; dI/dx= rate of decrease of intensity of radiation with thickness of the absorbing medium; k proportionality constant or absorption coefficient. Beer’s Law: When a beam of monochromatic radiation is passed through a solution of an absorbing substance, the rate of decrease of intensity of radiation with thickness of the absorbing solution is proportional to the intensity of incident radiation as well as the concentration of the solution. -dI/dx= k’Ic where k’= molar absorption coefficient; c= concentration of the solution in moles per litre; -dI/dx= dI/dx= rate of decrease of intensity of radiation with thickness of the absorbing medium.

7. Measurement of Absorption Intensity: The change in spin quantum number of an electron causes forbidden in singlet to triplet transition. The orbitals with variations in symmetry allowed forbidden symmetry between them. Also, thew intensity of absorbing radiations are directly related with transition. Instrfumentation: A spectrophotometer works on the measurement of transitted light radtions through given samples. It generally consists of light source, monochromator, detector, amplifier and the recording devices. Light Source: Tungsten filament lamp (Red light, 375 mμ) and hydrogen-deuterium discharge lamp (Below 375 mμ ) . Simple spectrophotometer: 220-800 mμ. The region below 200 mμ is called vacuum UV region. They regulated by two beams of light with equal intensities. The monochromatic light beams passed through the sample and refernece solvent placed in silica cells/quartz cells . these are the example of double beam spectrophotometer. The comparision of absorbed intensities with whole wavelength helps in measurements.

8. Shifts in UV-visible spectroscopy: [1] Bathochromic effect or Red shift: the axuchrome or the change of sovent causes shift of absorption maximum towards longar wavelength. [2] Hypsochromic shift or Blue shift: The change in solvent or removal of conjugation causes shift towards shorter wavelength of absorption maximum. [3] Hyperchromic shift : The auxochrome causes increased intensity of absrption maximum. [4] Hypochromic shift : The functiona group introduction causes decreased the intensity of absorption maximum due to aletration in molecular structure. Types of absorption bands : K* bands, R * bands, B-band, E-bands. Solvent effect : The solvents used in UV-visible spectroscopy may causes the change in the wavelengths towards shorter to longar absorption maximum or vice-versa. Temperature : The temperarture directly responsible for vibrational and rotational energies. The increase in the tenperature increases the vibrational and rotational energies.

9. Conjugated Dienes: Conjugated dienes are two double bonds separated by a single bond. e.g. 1,3-butadiene, 1,4-pentadiene. Inthis the energy level of higher aoccupied molecular orbital (HOMO) is raised and that of the lowest unoccupied molecular orbital (LUMO) is decreased. Parent value [butadiene/a cyclic conjugated diene]= 217mμ Acyclic Triene= 245 m μ Homoannular conjugated diene= 253 mμ Heteroannular conjugated diene= 215mμ Alkyl substituent/ ring residue= 5m μ Exocyclic double bond= 5mμ Double bond extending conjugation: 30mμ -OR= +6mμ -SR= +30mμ -Cl*, -Br *= +5 mμ -NR 2 = +60 m μ

10. Applications of UV-Visible spectroscopy It helps in the detection of functional groups. It identifies the extent of conjugation im polyenes. This spectroscopy easily differentiate between the conjugated and non-conjugated compounds. The UV-visible spectroscopy helps in the identfication of an unknown compound. It helps in the examination of Polynuclear hydrocarbons. It easily elucidate the structure of vitamins A or K. This technique easily detect the preference over two Tuatomeric forms of a molecule. For Identification of a compound in different solvents. To the determination of configurations of Geometrical isomers. This distinguishes between Equitorial and Axial Conformations. It can be used in the determination of strength of hydrogen bonding. To study hindered rotation and conformational analysis.

11. Infra-Red pectroscopy Infrared spectroscopy (IR spectroscopy or vibrational spectroscopy) is the measurement of the interaction of infrared radiation with matter by absorption, emission, or reflection . It is used to study and identify chemical substances or functional groups in solid, liquid, or gaseous forms. It can be used to characterize new materials or identify and verify known and unknown samples. The method or technique of infrared spectroscopy is conducted with an instrument called an infrared spectrometer (or spectrophotometer) which produces an infrared spectrum. An IR spectrum can be visualized in a graph of infrared light absorbance (or transmittance) on the vertical axis vs. frequency, wavenumber or wavelength on the horizontal axis. Typical units of wavenumber= cm −1 . The IR region situated in between 2.5 μ to 15 μ . Near IR region= 0.8 μ to 2.5 μ . Far IR region= 15 μ to 200 μ . Principle: Vibarational-rotaional spectra accompanied by a change in dipole moment . Theory: The IR spectroscopy causes change in the vibrational energy due to; (a) Mass of the atom of a molecule; (b) Bond strength; (c) Arrangements of atoms within the molecule.; (d) The enantiomers have similar IR spectra. [1] Stretching vibrations: The distance between two atoms increases or decreases but the atoms remains in the same bond axis. Its is of two types: (i) Symmetric stretching: Movement of the atoms towards the same direction. (ii) Asymmetric stretching: One atom move towards central atom while other departs from it.

12. Vibrational Frequency: vibrational frequency or wave number depends upon: (i) Bond strength and (ii) reduced mass. The vibrational frequency of a band increases when the bond strength increases and also when the reduced mass of the system decreases. Hooke's Law states: [i] he vibrational frequency is proportional to the strength of the spring; the stronger the spring, the higher the frequency. [ii] The vibrational frequency is inversely proportional to the masses at the ends of the spring; the lighter the weights, the higher the frequency. Hooke's Law in IR spectroscopy means: Stronger bonds absorb at higher frequencies. Weaker bonds absorb at lower frequencies. Bonds between lighter atoms absorb at higher frequencies. Bonds between heavier atoms absorb at lower frequencies.

13. [2] Bending Vibrations: The positions of the atoms change with respect to the original bond axis. (i) Scissoring: Two atoms approach each other. (ii) Rocking: Movement of the atoms in the same direction. (iii) Wagging: Two atoms move ‘up and down’ the plane with respect to the central atom. (iv) Twisting: One atom move up while other moves down the plane with respect to the central atoms.

14. Factors influencing vibrational frequencies: Coupled vibrations and farmi resonance: The coupled vibrations occurs at higher frequencies of asymmetric vibrations. The resonance occurs due to overtone colision of fundamental mode in different vibrations is called as Farmi Resonance. Electronic effects: This effect are caused by Inductive effect, Mesomeric effect and Field effects . The change in the substitutents of nearby group causes changes in the absorption frequencies of that group. Hydrgen bonding: H-bonding with strong bonding force causes decreased frequency shifts through greater absorption shift towards lowered wave number. Bond angles: Higher the bond order larger is the band frequency. A C-C triple bond is stronger than a C=C bond , so a C-C triple bond has higher stretching frequency than does a C=C bond .

15. Instrumentation

16. IR Source= Nernst glower, A rod of silicon carbide. Monochromator= NaCl or certain alkali metal halides . Absorbance (at particular frequency of a given sample): A= log (I /I) . Also transmittance, T= I/I or A= log (1/T) . Sampling techniques: [1] Solids: Solids are examined after mixing or grounded with KBr . They can be determined a mull or a paste. Mulling reagents : Nujol, Hexachlorobutadiene, Chloro-fluro carbon oil . Nujol is the mixture of liquid paraffinic hydrocarbons with high molecular weights. [2] Liquids: Liquids are inserted between two NaCl plates before IR spectra. Water containing liquids are examined through calcium fluride . [3] Gases: Gases placed inside the NaCl cell befre IR introduction. The gas cell is 10 cm long . [4] Solutions: Solvents usually incorporated for solution preparations before examine in IR instrument are Chloroform, Carbon tetrachloride and Carbon disulphide . Finger print region: The 500 - 1500 cm- 1 range is called the fingerprint region. The region are rich in many absorption bands . This region of the spectrum is almost unique for any given compound. This can be caused by bending vibrations and stretching vibrations of C-C, C-O and C-N bonds . Types: (i) 1500-1350 cm -1 ; (ii) 1350-1000 cm -1 ; (iii) Below 1000 cm -1 .

17. Important feature in IR Spectroscopy: The ring strain in cycloalkanones and carbonyl stretching frequency , that showed increase in C=O stretching frequency with decrease in the ring size . Resonance splitting: The atoms having two bonds oscillating it can cause change in the frequencies due to coupling and relative phasing . Group Frequencies: The light atoms impart high vibrations and heavy atoms impart low frequency . Presence of both in a molecule can cause resonance and group frequencies shifted drametically. Field Effects: The polar group in association with carbonyl group causes inductive effect and field effect on the stretching frequency of the carbonyl group. Adsorption and Force Constant: Adsorption causes restricts in the rotaional movemnets of the adsorbate. This effects vibrational frequency and to get estimate about the strength of bond with the adsorbent.

18. Applications of IR Spectroscopy It helps in identification of an Organic compound by comparing its spectrum with already known spectrum. IR spectroscopy help in structure determination of an unknown compound. The important functional groups are absorbs with chracteristic wave number. It helps in the qualitative analysis of functional groups. It also predicts the types of hydrogen bonding present in a molecule through comparision of their bond strength. The quantitative analysis can be estimated through measuring intensities of absorption bands and determining the optical density of the compounds. IR spectra helps to understand the chemical reaction through comparing absorption bands at each reaction steps. This helps in the determination of keto-enol tautomerism. The IR spectra helps to establish the structure of complex molecules. The relative stability of various conformations of cyclic compounds can be determined by this technique. The cis and trans isomers are easily disctinguish and helps in geometrical isomerism studies. The Gauche and staggered conformations can be detected through this technique. The impurity in the sample/compound can be detected through comparision with the authentic sample or reference.

19. Nuclear Magnetic Resonance Spectroscopy It includes the re-orientation of atomic nuclei (Proton) with non-zero nuclear spins in an external applied magnetic field. These occurs after absorption of electromagnetic radiation in the radio frequency region from around 4-900 MHz. This mediated by isotopic atomic nuclei and applied external magnetic field. The active nucleus give readings about chemical nature, its functional groups and exixtance of nearby nuclei of a same molecule. Principle: The alignment (polarization) of the magnetic nuclear spins in an applied, constant magnetic field. The perturbation of this alignment of the nuclear spins by a weak oscillating magnetic field, usually referred to as a radio-frequency (RF) pulse. Detection and analysis of the electromagnetic waves emitted by the nuclei of the sample as a result of this perturbation. Spin +1/2 Spin -1/2 Magnet N S N S S N Spinning particle have Magnetic field (+1/2). Change in spinning direction changes direction in the magnetic field (-1/2) Magnetic field of aligned and non-aligned proton

20. If we consider a spinning top, its behave as a slower waltz like motion around the vertical axis and called as precessional motion . The spinning top presessing around the verical axis of earth’s gravitaional field. Gyroscopic motion is the precessing arises from the interaction with earth’s gravity that is vertically downwards. ω= γ H ; ω= angular precessional velocity, H = applied field in gauss, Gyromagnetic ratio ( γ) = 2πμ/ hI . Here, μ= magnetic moment of the spinning bar magnet, I= spin quantum number of the spinning magnet, h= Plank’s constant. Hence γ H = 2πν . ν= frequency. Therefore, A ngular precessional velocity, ω= 2πν . Precessional frequency: The number of revolutions per second made by the magnetic moment vector of the nucleus around the external field H . Precessional frequency of the spinning nucleus: It is a transition from one spin state to another induce by frequency of electromagnetic radiations in megacycles per second.

21. Relaxation Processes: {1} Spin-spin relaxation-Mutual exchange of spins by two processing nuclei in close proximity to each other. {2} Spin-lattice relaxation (Longitudinal relaxation)- The total energy of the nucleus tranfered in the form of additional translation, vibrational and roational energy to the molecular lattice. Quadrupole Relaxation: The prominent relaxation that takes place for nuclei having I > ½ . The anisotropic interaction between non-spherical, electrically quadrupole nuclei and the electric field gradient at the nucleus by electric charge distribution causes an asymmetric positive charge disribution and facilitate broad signals. － ＋ ＋ ＋ ＋ ＋ ＋ ＋ ＋ ＋ － － － － － － － － － － － － －

22. Instrumentation

23. Applications of NMR Spectroscopy It helps in knwoing the structural isomers. The type of hydrogen bonding can be easily detected. The aromaticity can be detected by NMR spectrum. It also distinguishes the Cis-Trans and Conformers. NMR spectra also identifys the electronegative atom or group. The double bond can be detected due to its resonance in the applied field. It helps in the quantitative analysis of the compounds.

24. Mass Spectroscopy The technique is very accurate to calculate the molecular masses of the given compound and its elemental composition. This technique is used to measure the mass-to-charge ratio of ions . The results are presented as a mass spectrum , a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is used in many different fields and is applied to pure samples as well as complex mixtures. In a typical MS procedure, a sample, which may be solid, liquid, or gaseous , is ionized, for example by bombarding it with a beam of electrons. This may cause some of the sample's molecules to break up into positively charged fragments or simply become positively charged without fragmenting. These ions (fragments) are then separated according to their mass-to-charge ratio, for example by accelerating them and subjecting them to an electric or magnetic field: ions of the same mass-to-charge ratio will undergo the same amount of deflection. The ions are detected by a mechanism capable of detecting charged particles, such as an electron multiplier . Results are displayed as spectra of the signal intensity of detected ions as a function of the mass-to-charge ratio. The atoms or molecules in the sample can be identified by correlating known masses (e.g. an entire molecule) to the identified masses or through a characteristic fragmentation pattern.

25. Ion Source: The ionisation of given sample are very crucial in Mass spectroscopy. The amount of energy needed to convert sample into the ions are called as ionisation potential . The bombardment of electrons helps in the production of ion. This are achieved through electrically heated tungsten filament with introducing pressure of 10 -6 . The Mass Analyzer: The produced ions are accelerated into the mass analyser, where they are sorted on the basis of their mass-charge ratio (m/e or m/z) . These are accelerated throughout the analyser that have portions created through slits or plates , where high velocity of ions are achieved and they are separated on the basis of mass-charge ratio (m/e or m/z). Ion Detection System: The separated ions are then measured and sent to a data system where the m/z ratios are stored together along with their relative abundance. A mass spectrum is simply the m/z ratios of the ions present in a sample plotted against their intensities. Each peak in a mass spectrum shows a component of unique m/z in the sample, and heights of the peaks connote the relative abundance of the various components in the sample Time-of-flight: The time-of-flight (TOF) analyzer uses an electric field to accelerate the ions through the same potential, and then measures the time they take to reach the detector. If the particles all have the same charge, their kinetic energies will be identical, and their velocities will depend only on their masses.

26. Instrumnetation

27. The Nitrogen Rule: According to this rule a molecule of even numbered molecular mass must contain no nitrogen atom or an even number of nitrogen atoms. An odd numbered molecular mass requires an odd number of nitrogen atoms. Mass Spectrum: It is recodring of charged fragmnets detected through bombardment of electrons.It also denoted the relative abundances [Total ion current] of the molecular ion. Base peak are the highest peak in the spectrum. McLafferty Rearrangement: This reaction observed in mass spectrometry during the fragmentation or dissociation of organic molecules . It is sometimes found that a molecule containing a keto-group undergoes β-cleavage, with the gain of the γ-hydrogen atom.

28. Applications of Mass Spectroscopy It helps in the detection of molecular masses/ weight. This technique determines the molecular fomulas on the basis of relative abundances of m-z ratio. Mass spectrometry has both qualitative and quantitative uses. Identifying unknown compounds, determining the isotopic composition of elements in a molecule, and determining the structure of a compound by observing its fragmentation. Increased sensitivity over most other analytical techniques because the analyzer, as a mass-charge filter, reduces background interference, Excellent specificity from characteristic fragmentation patterns to identify unknowns or confirm the presence of suspected compounds, Information about molecular weight, Information about the isotopic abundance of elements, Temporally resolved chemical data.

29. References https://en.wikipedia.org/wiki/Ultraviolet%E2%80%93visible_spectroscopy. Mendes, E.; Duarte, N. Mid-Infrared Spectroscopy as a Valuable Tool to Tackle Food Analysis: A Literature Review on Coffee, Dairies, Honey, Olive Oil and Wine. Foods 2021, 10, 477. Siesler, Heinz W. "Basic principles of near-infrared spectroscopy." Handbook of near-infrared analysis. CRC press, 2007. 25-38. https://en.wikipedia.org/wiki/Nuclear_magnetic_resonance_spectroscopy. https://en.wikipedia.org/wiki/J-coupling. https://en.wikipedia.org/wiki/Mass_spectrometry. https://en.wikipedia.org/wiki/McLafferty_rearrangement. Sharma YR. Elementary Organic Spectroscopy. Published by S. CHAND & Company Pvt. Ltd. Revised edition. ISBN: 978-81-219-2884-7.