Unit 3 - Spectroscopy & Spectrometry

Spectroscopy

Interaction of light with different molecules

Microwaves vibrates

OH bonds

UV Waves breaks

Pi bonds

X-Rays Breaks

Sigma bonds

Wavelength is

Inversely proportional to energy

Frequency is

Proportional to energy

NMR

Uses radio waves to find what’s bound to what

IR

Uses infrared to find what functional groups present as every bond type absorbs a specific photon

UV-VIS

Uses visible + UV to find conjugated pi systems

Matter on a macro scale exhibits

Continuous behaviour

Matter on a molecular scale exhibits

Wave-like, quantum behaviour

% Transmittance

How much light get through

IR Absorption Specrum

% transmittance vs wavenumber

Absorption Bands

Peaks in a IR plot

Wavenumber ranges

400 - 4000 1/cm

Wavenumber is

Proportional to energy

Diagnostic Region

Above 1500 1/cm

Fingerprint Region

Below 1500 1/cm, hard to analyze

Characteristics in IR

• Wavenumber

• Intensity

• Shape

Stronger bonds have

Higher stretching frequency, and wavenumber

More bonds, and similar orbitals have

More overlap and thus higher wavenumber

Terminal Alkenyl & Alkynyl Bonds

>3000 1/cm

Bonds to H

4000 - 2700 1/cm

Triple Bonds

2300 - 2100 1/cm

Double Bonds

1800 - 1600 1/cm

Single Bonds

<1600

Resonance & Delocalized pi electrons have

Weaker pi bonds and thus decrease wavenumber

Larger mass difference have

Lower stretching frequency (very light bonds = large wavenumber)

More polar moelcules have

Higher wavenumber because more interaction between stronger electric field from oscillating dipoles that photon from IR can bump into

Complete Symmetry Creates

No signal because no electrical fields from lack of EN

C-H Bonds

Add up and give strong signals

Narrow SIgnal

No intermolecular forces

Broad SIgnal

Intermolecular forces

OH Bonds show up

Very broad because H bonding weakens and vary in a molecule, leading to varying wavenumber

Free OH Bonds create

Shorter intensity

COOH

Weaker from more efficient H bonding and variety of a broad OH, and sharp carbonyl

Amines

Less broad peaks than alcohol because H bonding is less efficient

Tertiary Amine has

No peaks

Secondary Amines have

Single Peaks

Primary Amines have

Doublets

NH₂ H bond vibrations

• Symmetrically, higher energy

• Asymmetrical, lower energy (2 peaks)

Mass Spectrometry

Determines molar mass and formula of a compound

Mass Spectrometry Process

1. Vaporize in a vacuum, ionized with electrons, fragment

2. Electrical field spreads ions by mass

3. Detectors reads them

Radical Cation

Molecular with an unpaired electron

Molecular Ion

Radical cations stays intact

Mass to Charge Ratio

M/z (usually +1)

Relative Abundance

Tallest peak is 100% and everything else is relative

The most right cluster in mass spectrometry is

The molecule with varying lines the isotopes of the molecule

Carbon Isotopes

12C and 13C

Chlorine Isotopes

35Cl and 37Cl

Bromine Isotopes

79Br and 81Br

If molecules has odd molar mass

Odd number of nitrogens

Degree of Unsaturation

A pi bond and ring counts as degree of unsaturated, and number of hydrogens decrease by 2

Hydrogen Deficiency Index (HDI)

1:1 ratio where 1HDI = 1 degree of saturation

Halogen HDI

Treated as 1 HDI

NItrogen HDI

Increase number of hydrogens by 1

Oxygen HDI

Doesn’t affect it

HDI Formula

½(2C + 2 + N - H - X)

Nuclear Magnetic Resonance (NMR)

Manipulate nucleus with electromagnetism

B spin sate

Opposes B_^o, higher energy state

a spin state

Allign with B_o, lower energy state

Energy Gap

Difference between alpha and beta spin state (really small)

Stronger Magnetic Field

Greater energy gap

More electrons

Smaller energy gap because shielded electrons create a stronger opposing F_B, causing smaller B_o

Less electrons

Bigger energy gap because deshielded electrons have weak opposing F_B, more delta⁺

Resonance

Spinning in sequence

Common NMR Solution

Deteurium chloroform

Characteristics of NMR

• Number of signals

• Signal location

• Signal intensity (integration)

• Signal shape

Higher Chemical Shift

Higher energy, delta positive, deshielded, larger gaps, closer to pi electrons

Lower Chemical Shift

Lower energy, delta negative, smaller gap

Number of SIgnals

Each peak represents a unique H environment and what it’s bounded to

Counting Number of Signals

Determine the number of hydrogens that have unique atoms and number of hydrogens bound to it

Protons nearby EN atom

Creates a larger shift because it’s more positive and deshielded

Diamagnetic Anisotropy

Electron in a pi system creates a F_B which goes through atoms orthogonal, increasing F_B

Pi Systems

Higher shift (more energy needed to flip), and closer to the pi system causes more deshielding

Signal Intensity/Integration

Area under the peak shows relative number of p

Integration Calculations

Divide by lowest integration number, round and multiple to nearest whole number for empirical formula

Symmetrical Molecules

Don’t give absolute hydrogen count

Multiplicity/Coupling

Number of peaks in a signal indicates how many hydrogens are in neighboring carbons

Doublet

1 H atom on neighboring carbon from hydrogens spinning up or down, creating two different electrical fields

Singlet

No H atoms on neighbouring C

Triplet

2 H atoms on neighbouring C because coupling of H atoms has 3 outcomes with higher chance of spinning different directions 

Quartet

3 H atoms on neighbouring C with coupling creating 4 outcomes and mix of spin up or down is more likely

Multiplicity Rules

1. Hydrogens bounded to the same carbon don’t couple because equivalent magnetic field

2. To couple, H must be 3 bond distance apart

3. Coupling doesn’t happen with O-H and N-H because H bonding is too quick to catch

1500–1680 cm^-1

Double bonded carbon

1680–1800 cm^-1

Ketones, Carbonyl, Amides

2900-3200 cm^-1

sp³ C-H

3200-3600

Alcohol or Amine

1-1.5ppm HNMR

Alkyl

1.5-3.0ppm HNMR

Allylic

3.0-4.5ppm HNMR

Carbon bound to hetero atom

4.5-6.5ppm HNMR

Vinylic

6.5-8.5ppm HNMR

Aromatic

8.5-10ppm HNMR

Aldehyde or COOH

0-50ppm C NMR

Alkyl sp³

50-100ppm C NMR

sp³ bound to O or F, triple bond to carbon or nitrogen

100-160ppm C NMR

sp² carbon, double bond

160-190ppm C NMR

Carbonyl and hetero atom

190-220ppm C NMR

Ketones and aldehydes

Integration Value for Mono Substitued Aromatic

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