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Chemistry > Spectroscopic Techniques and Applications > Flashcards

Spectroscopic Techniques and Applications Flashcards

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1
Q

spectroscopy

A

study of interaction of electromagnetic radiation with matter

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2
Q

principle of spectroscopy

A

the principle is based on the measurement of spectrum of a sample containing atoms or molecules

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3
Q

spectrum

A

graph of intensity of absorbed or emitted radiation by sample verses frequency or wavelength

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4
Q

instrument to measure the spectrum of a compound

A

spectrometer or spectrophotometer

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5
Q

absorption spectroscopy

A

an analytical technique which concerns with the measurement of absorption of em radiation

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6
Q

emission spectroscopy

A

an analytical technique in which emission of a particle or radiation is dispersed according to some property of emission and the amount of dispersion is measured

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7
Q

absorption spectrum

A

a spectrum which consists of dark lines produced by the absorption of incident radiation

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8
Q

emission spectrum

A

spectrum which consists of bright lines produced by the emission of absorbed radiation

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9
Q

atomic spectrum

A

atoms interact with em radiation to give atomic spectra

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10
Q

why is atomic spectra referred to as line spectrum

A

since atoms have limited number of energy levels, atomic spectrum consist of lines

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11
Q

molecular spectra

A

molecules interact with em radiation to give molecular spectra

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12
Q

why does the resulting molecular spectrum consist of many lines which are close together, making a band

A

since molecules possess a number of energy levels, large number of such energy transitions are possible

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13
Q

em radiation

A

radiation having electronic and magnetic components along with them, like visible light, X rays,, microwaves, radio waves etc

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14
Q

photons

A

em radiation consists of discrete package of energy

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15
Q

em spectrum

A 
arrangement of different types of em radiations in the order of increasing frequencies or decreasing wavelength
Radiowaves
Microwaves
Infrared
Visible light
UV
X rays
Gamma
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16
Q

types of energy levels

A

rotational, vibrational, electronic

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17
Q

electronic energy levels

A

molecule possess electronic energy levels which is associated with transition of an electron from the ground state level to the excited state by the absorption of photons of of suitable energy
- in visible or UV region

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18
Q

vibrational energy levels

A

the molecule possess vibrational energy when the center of gravity does not change during the to and fro motion of the nucleus of the molecule
- near IR region

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19
Q

rotational energy levels

A

molecule rotates about an axis perpendicular to the inter nuclear axis passing through the center of gravity of the molecule
- in microwave region

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20
Q

Born-Oppenheimer approximation

A

total energy of the molecule is given by the sum of electronic energy, vibration energy, rotational energy, translation energy

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21
Q

Beer Lambert’s Law / Absorption Law

A

The law gives the linear relationship between absorbance of em radiation and conc of an absorber by the equation
A=(sigma)cl

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22
Q

How is UV-VIS region formed

A

results from interaction of em radiation in UV-VIS region with atoms and molecules

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23
Q

3 types of species showing electronic transitions and give UV-VIS spectrum

A

organic compounds
inorganic compounds
coordination compounds

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24
Q

Why does electronic spectrum in UV-VIS region appear broad

A

since the electronic energy level has a number of vibrational energy levels in it and each of the vibrational energy level has a number of rotational energy levels in it, along with electronic transition vibrational and rotational transitions also occur simultaneously

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25
Q

What are HOMO and LUMO

A

HOMO: the one with lower energy out of occupied orbitals
LUMO: the lowest among unoccupied molecular orbitals

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26
Q

why should there be less energy gap between the bonding and anti bonding orbitals

A

the max absorption is moving to longer wavelengths as the amount of delocalisation increases. so, there should be less energy gap coz amount of delocalization increases

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27
Q

absorbance

A

measure of amount of light absorbed

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28
Q

chromophore

A

isolated covalently bonded group that shows a characteristic absorption in the UV or visible region irrespective of the fact whether colour is produced or not

29
Q

auxochrome

A

atom or group of atoms which does not give raise to an absorption band on its own, but it is a colour enhancing group

30
Q

bathochromic shift ( red shift)

A

a shift of lambda max to longer wavelengths due to the presence of auxochrome or by the change of solvent

31
Q

hyposochromic shift ( blue shift)

A

a shift of lambda max to shorter wavelengths due to loss in configuration in acid medium

32
Q

hyperchromic and hypochromic shifts

A

hyperchromic shifts:
normal increase in intensity of absorption band due to the presence of auxochrome
hypochromic shifts:
a normal decrease in intensity of absorption band due to the presence of some groups like alkyl groups

33
Q

applications of electronic spectrum

A

1) characterization of aromatic compounds
2) determination of impurities
3) process of purification can be controlled
4) det of unknown conc
5) det of molecular weight
6) study of kinetics of chem rn
7) engg researchm industrial analysis, clinical labs etc

34
Q

vibrational spectroscopy

A

concerned with the study of mainly the vibrational motions of molecules. It arises due to the interaction of molecular dipole moment of matter with em radiation

35
Q

essential requirement for molecule to give IR spectrum

A

dipole moment of the molecule must change during vibration

36
Q

IR active molecules

A

IR spectrum is given by diatomic molecules with permanent dipole moment like HCl and molecules with a fluctuating dipole moment like CO2

37
Q

Why is IR spectrum also known as vibrational rotational spectrum

A

IR spectroscopy involves transitions in various vibrational energy levels of the molecule. since rotational energies are smaller than vibrational energies, both vibration and rotation occur simultaneously

38
Q

natural frequency of vibration

A

the movement of atoms and chemical bonds are like a spring and ball. This characteristic vibration is called natural frequency

39
Q

criteria for compound to absorb IR radiation

A

1) correct wavelength of radn

2) change in dipole moment

40
Q

number of vibrational modes in CO2

A

4

41
Q

number of vibrational modes in water vapour

A

3

42
Q

2 kinds of fundamental vibrations

A

1) stretching

2) bending

43
Q

modes of vibration in CO2

A

1) symmetrical C=O stretch with a wavenumber of 1332cm-1
2) asymmetrical C=O stretch with a wavenumber of 2351cm-1
3) C=O in plane bending with a wavenumber of 671cm-1
4) C=O out of plane bending with a wavenumber of 671cm-1

44
Q

why is symmetrical mode of CO2 considered as IR inactive

A

does not change the dipole moment of CO2

45
Q

modes of vibration H2O molecule

A

1) symmetrical stretch with a wavenumber of 3652cm-1
2) asymmetrical stretch with a wavenumber of 3756cm-1
3) Bending with a wavenumber of 1596cm-1

46
Q

application of IR spectrum

A

det of:

1) force constant of diff bonds
2) shape and symmetry of molecules
3) bondlength
4) identification of unknown compounds
5) impurity detection and qualitative analysis
6) study of chem reaction
7) structural elucidation of org compounds
8) distinguish between intra and inter molecular H bonding
9) identification of functional grps in org molecules
10) tautomeric eqm can be studied

47
Q

NMR spectroscopy

A

it is related to the magnetic properties of nucleus of atoms and molecules

48
Q

larmour frequency

A

((gamma)Bo)/(2pi)

49
Q

gyromagnetic ratio

A

nuclear mag moment/angular momentum

50
Q

NMR spectrum

A

plot of intensity of NMR signals verses mf with reference to TMS ( tetra methyl silane a reference material)

51
Q

NMR active and NMR inactive nuclei

A

active:
1) odd Z odd A
2) odd A even A
3) even Z odd A
inactive:
1) even Z even A

52
Q

chemical shift

A

difference between resonance frequency of the observed proton and TMS hydrogen

53
Q

why is TMS used as the reference

A

it has a conveniently located well defined absorption

54
Q

unit of chemical shift

A

delta

55
Q

define delta

A

proportional difference from the reference standard. it is expressed in ppm

56
Q

what causes shift in NMR signals

A

shielding and deshielding of proton

57
Q

factors affecting chemical shift

A

1) presence of electronegative grps
2) magnetic anisotropy of pi systems
3) hydrogen bonding

58
Q

magnetic anisotropy

A

non uniform magnetic field

59
Q

spin-spin splitting / coupling

A

fine splitting of the peaks in NMR spectrum due to the interaction between protons on adjacent atoms

60
Q

general rules of spin-spin interactions

A

1) chemically equivalent protons do not show
2) only non equivalent protons couple
3) protons on adjacent Cs normally couple
4) protons separated by 4 or more bonds will not couple

61
Q

why is there no spin spin interaction between the -OH proton and the other protons

A

in solution H+ ion ionizes easily and leaves the molecule

62
Q

applications of MRI

A

1) radiology to visualize internal structures
2) uses powerful mf, radio frequency pulses and a computer to produce detailed pics of body structures
3) images can then be examined on a computer system, transmitted electronically, printed or copied to a CD

63
Q

principle of MRI

A

1) new medical version of NMR
2) clinical MRI uses mag properties of H present in body cells
3) protons interact with large external mf and radio waves to produce highly detailed images of the human body

64
Q

instrumentation of MRI

A

1) patient is placed in large external mf (0.5 - 1.5T)
2) H nuclei align with the mf
3) mf applied on nuclei produce nuclear energy levels with spacing
4) radio frequency pulse is applied perpendicular to mf which leads to transitions between nuclear energy levels
5) after the removal of RF, the absorbed radn is reemitted
6) the signal produced : free induction decay response signal
7) measured and processed to obtain 3D grey scale MR imgs

65
Q

Larmour precession

A

the external mf applied on the nuclei produce nuclear energy levels with spacing

66
Q

Applications of MRI

A

1) Anatomical imaging
2) measuring physiological functions
3) flow measurements and angiography
4) tissue perfusion studies
5) structural diagnosis
6) conformational analysis
7) keto-enol tautomerism
8) H bonding
9) reaction velocities

67
Q

structural diagnosis by NMR specrtum

A

1) the number of main NMR signals= no. of grps of equivalent protons
2) the peaks should have consistent chemical shifts
3) intensity of peaks should be proportional to the number of protons of various kinds

68
Q

conformational analysis

A

measurement of relative population of various conformations of a molecule

Chemistry (10 decks)

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