MCAT Organic Chemistry - Spectroscopy

500

Spectroscopy

measures the energy differences between the possible states of a molecular system by determining the frequencies of electromagnetic radiation absorbed by the molecules

magnetic resonance imaging (MRI)

measure ¹H–NMR spectra of water molecules in different environments in the body; multiple cross-sectional scans of the patient’s body are taken, and the various chemical shifts of absorbing protons are translated into specific shades of grey which produces a picture that shows the relative density of specific types of protons

dark area = waters

light area = fattier tissue

Infrared (IR) spectroscopy

measures molecular vibrations, which helps estimate types of bonds; infrared light is passed through a sample, and the absorbance is measured; for an absorption to be recorded, the vibration must result in a change in the bond dipole moment

range: 2500 to 25,000 nm = 4000 to 400 cm⁻¹

infrared light range

700 nm to 1 mm

wavenumber

analog of frequency

= 1/λ

units: cm⁻¹

molecular vibration

bond stretching, bending, or combinations of different vibrational modes; include twisting and folding; Symmetric stretches do not show up in IR spectra because they involve no net change in dipole movement

fingerprint region

complex vibration patterns, caused by the motion of the molecule as a whole, in the 1500 to 400 cm⁻¹ range; the specific absorbance pattern is characteristic of each individual molecule

hydroxyl group (O−H) IR

broad (wide) peak

3300 cm⁻¹ for alcohols

3000 cm⁻¹ for carboxylic acids (carbonyl pulls electron density → shifts absorption lower)

carbonyl group (C=O) IR

sharp (deep) peak

around 1700 cm⁻¹

amine group (N−H) IR

same region as O−H bonds, but have a sharp peak

around 3300 cm⁻¹

transmittance

the amount of light that passes through the sample and reaches the detector

IR spectra

plotted as percent transmittance vs. wavenumber; maximum absorptions appear as the bottoms of valleys on the spectrum

ultraviolet-visible (UV-vis) spectroscopy

passing ultraviolet light through a sample that is usually dissolved in an inert, nonabsorbing solvent, and recording the absorbance, caused by electronic transitions between orbitals; wavelength of maximum absorbance, which tells us the extent of conjugation within conjugated systems

UV spectra

plots absorbance vs. wavelength

HOMO–LUMO gap

energy gap between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO); lower energy gap = longer wavelengths = more conjugation = more excitable

Conjugation

molecules with unhybridized p-orbitals, can be excited by ultraviolet light; shifting the absorption spectrum, resulting in higher maximum wavelengths; larger conjugated molecules may even absorb light in the visible range → coloured compounds

Nuclear magnetic resonance (NMR)

certain atomic nuclei have magnetic moments that tend to align either with or against the direction of an applied field; irradiated with radiofrequency pulses that match the energy gap between the two states, exciting some nuclei depending on an atom’s magnetic environment

α-state

Nuclei with magnetic moments that are aligned with the magnetic field; lower energy

β-state

Nuclei with magnetic moments that are aligned against the magnetic field; higher energy

chemical shift (δ)

standardized method of plotting the NMR spectrum using an arbitrary variable with units of parts per million (ppm) of spectrometer frequency

NMR spectra

plot of frequency vs. absorption of energy

downfield

towards a larger chemical shift; increases to the left; signals become relatively deshielded

tetramethylsilane (TMS)

calibration standard/reference peak to mark 0 ppm

chemically equivalent

protons that have the same magnetic environment; lead to the same peak

integration

the area under the peaks; the ratio of different peaks corresponds exactly to the ratio of protons that produced each peak

deshielding

electron density away from the proton, leaving it more vulnerable to the magnetic field

spin–spin coupling (splitting)

two protons in such close proximity to each other that are not magnetically identical; the magnetic environment of one group of protons can be affected by another

n + 1 rule

if a proton has n protons that are three bonds away, it will be split into n + 1 peaks

coupling constant (J)

magnitude of this splitting, measured in hertz

multiplet

peaks that have more than four splits

methyl group (-CH₃) ¹H-NMR

0.9 ppm

methylene group (-CH₁) ¹H-NMR

1.25 ppm

methine gorup (-CH) ¹H-NMR

1.5 ppm

aldehyde ((C=O)-H) ¹H-NMR

9 - 10 ppm

carboxylic acid ((C=O)-OH) ¹H-NMR

10.5 - 12 ppm

aromatic hydrogen (Ar-H) ¹H-NMR

6.0 - 8.5 ppm

sp³ hybridized carbons ¹H-NMR

0.0 to 3.0 ppm

sp² hybridised carbons ¹H-NMR

4.6 - 6.0 ppm

sp hybridised carbon ¹H-NMR

2.0 - 3.0 ppm