Spectroscopy

IHD Formula

Index of Hydrogen Deficiency, could be due to rings or bonding

IHD = 1/2(2c+2-h-x+n)

Degree of IHD based on the bond

Double bond =1

Triple Bond=2

Ring=1

Aromatic/Benzene=4

What does spectroscopy examine?

how matter interacts with various EMR. The energy of light

Radio Waves

Absorbed by nuclei to reverse spin state, causing nuclear transition in external magnetic field

Types of EMR

Radio, Microwaves, Infrared, Visible, UV, X-ray, Gamma rays

Microwaves

cause changes in molecular rotational energy, gives info on bond length

Infrared

causes bonds to vibrate, stretch/bend, giving info on functional groups

Visible and Ultraviolet

Give electronic transitions, give info on energy levels

X-rays

High energy causes electrons to be removed from inner energy levels

Crystallography uses wavelengths having inter-atomic distances, to make diffraction patterns for the molecular structure (bond-angle/distance)

Example of X-ray usage?

Showed the DNA structure

Gamma rays

High energy

What is mass spectrometry used for?

Determines relative molecular masses, individual abundances or different atomic isotopes

What is Ionization?

Highly energetic collisions between incident species, and electron gun causes molecules to break up into smaller fragments

What does a mass spectrometer do?

VIADD

Vaporization, make gas and atomize

Ionization, using e bombardment to knock off electrons

Acceleration, accelerates using e field

Deflection using magnetic B field

Detection, detects the angle of the hit based on deflection

How to fragmentation patterns provide info on compound’s structure?

Largest fragment is the molecular peak- peak meaning greatest molar mass

The detected product has a positive charge

Smaller peaks from daughter ions, the most stable ions being broken off

What do we do with the info on peak in mass spectrum?

The peak is the molecule with the highest mass, and it’s the parent ion, use it as a baseline for what the other ions have lost

The molecular mass of the ion lost tells u what functional group was lost, and if not, the molecular mass of the ion present tells u what’s present

What is the parent ion, or peak?

The peak is the part of the mass spectrum that has the highest mass- its the full paent ion

What does the deflection in a mass spectrometer depend on?

Depends on mass/charge

Lighter atoms with a higher positive charge deflected more

Heavier atoms with a lower charge deflected less

What does infrared radiation do?

causes molecular vibrations, like certain bonds bending and stretching, gives info on functional groups

What do bonds have to do with spring theory?

Covalent bonds are springs, which vibrate at natural frequencies based on bond strength and atomic mass

Fundamental vibration frequency?

lighter atoms vibrate at higher frequencies than heavier ones, multiple bonds are tighter springs that vibrate at higher frequencies, more energy to stretch than single bonds

Why do molecules absorb different frequencies?

because the energy needed for the molecule to vibrate depends on bond ethalpy

What is the wavenumber and vibrational transition?

Frequency cited as the reciprocal of wavelength is the wavenumber (cm-1)

vibrational transitions correspond to definite energy levels, like stretching, bending and twisting

What type of stretch do diatomic molecules have?

one molecular vibration of stretching

What causes increasing vibration frequency?

Larger bond enthalpy, and smaller mass, like HCl which has the highest frequency

How does the vibration of polyatomic species compare to diatomic species?

polyatomic species have several modes of vibration unlike diatomic, which only has one vibration of stretching

they can be symmetric stretch, asymmetric stretch, or symmetric bend

What happens to different molecules with different bonds?

different vibration types

What is needed for IR to be absorbed?

to absorb infrared, there must be a change in the molecular dipole movement

Must form a dipole from vibrations

Only polar covalent bonds interact, non-polar can’t, because the symmetric stretch is not dipole

Summing up what infrared radiation needs to be asborbed

polar to be absorbed, non-polar passes thru

What does an IR spectra plot?

transmittance (%) vs. wavenumber, and the baseline is 100% transmittance, only dipping down when absorbed by a bond that is polar due to the molecule dipole movement caused by vibrations

How to tell which bond is present based on spectra?

The location will have a sharp peak, and if the absorption is really broad, suggests OH

Other suggestions based on the spectra?

C triple bond C is 2100

C=O is 1800

C-H 3000

O-H 3400

C=C 1600

C-O 1300

If broad also OH

Units of wavenumber, or c

What types of vibration can a molecule have to make it active?

If diatomic, only vibration is stretching IR active

For polyatomics, all forms have active IR, like symmetric stretch and bend, and asymmetric stretch

What happens on a molecular level when IR radiation is absorbed?

IR is only absorbed when there is a change to the molecular dipole movement, or only on polar molecules

So the absorption of IR causes a change to the molecular dipole movement, creating dipoles via bending or stretching via vibrations