Analytical techniques

what is wavelength?

the distance between adjacent peaks of the electric field

what is wave frequency?

the number of waves (cycles) passing any point each second

give the equation for frequency

frequency (Hz) = velocity of light (2.998 ×10¹⁰ cm/s) / wavelength (cm)

what is wavenumber?

used to measure the IR absorbance of a molecule

directly proportional to energy

larger range (easier to see the shape of absorptions in the spectra)

what are the 3 principles of IR spectroscopy?

1. quantum theory - molecules and atoms can only exist on certain energy levels

2. molecules can only absorb energy as a quantum that corresponds to the energy difference between two different energy levels

3. the energy of IR radiation enables molecules to transfer between their vibrational and rotational states

when will IR light be absorbed?

when the frequency of IR light = the frequency of bond stretching

bonds stretch when they absorb energy - amount of stretching is influenced by bond strength and mass of the atoms

what kind of bonds do not absorb in the IR?

symmetrical non-polar bonds

as stretching along the bond axis does not change the dipole moment

what kind of bonds absorb higher wavenumbers in IR spectra?

stronger bonds

bonds with lighter atoms

(as they vibrate at a higher frequency)

what is an IR spectra a plot of?

the amount of transmitted light versus its wavenumber

(has a fingerprint and functional group region)

what is the fingerprint region of an IR spectra?

area of the spectra between 1500-500 cm^-1

contains a complicated series of absorptions, mainly due to bending vibrations within the molecule

different for every compound - helps differentiate between isomers

would an ester or a ketone have more activity in the fingerprint region of its IR spectra?

an ester

due to resonance/more bending in molecule

how does wavenumber change with increasing percent s-character in a molecule?

also increases

list bonding, anti-bonding and lone pair electrons from lowest to highest in energy

bonding, lone pair, anti-bonding

what are absorptions in UV-visible spectroscopy caused by?

electron transition to anti-bonding molecular orbital (from bonding MO)

how does UV-visible spectroscopy work?

light is absorbed which excites electrons to transition between orbitals (from ground state to excited state)

measures the absorption or transmission of light in the ultraviolet (190-400nm) and visible (400-700nm) regions

what is transmittance?

the proportion of light passing through the sample (%)

ratio of transmitted (I) to incident intensity (I₀)

what is the Beer-Lambert law?

states that the absorbance of light by a solution is directly proportional to its concentration and the (that the light travels through)

(fails at high concentrations - samples must be <0.1M)

what factors influence UV-Vis absorbance?

1. measurement wavelength

2. solvent

3. pH of the solution if is analyte is an acid or a base

4. temperature

what is mass spectrometry?

a technique used for measuring the molecular weight and determining the molecular formula of an organic compound

what types of mass spec exist for gas, liquid and solid phases?

gas - electron impact (EI) or chemical ionisation (CI)

liquid - electrospray (ES)

(solid - atmospheric pressure chemical ionisation or matrix assisted laser desorption ionisation)

how does an EI mass spectrometer work?

a molecule is highly vaporised and ionised by bombardment with a beam of high-energy electrons

(electron beam ionises the molecule by causing it to eject an electron)

what is the species formed when an electron beam in EI mass spec ionises the molecule, and what does it represent?

a radical cation, symbolised as M⁺⁺

represents the molecular weight of M

(however since M is unstable, it decomposes to form fragments of radicals and cations that have a lower molecular weight than M⁺⁺)

what is mass spec a plot of?

the amount of each cation (its relative abundance) versus its mass to charge ratio (m/z, where m is mass and z is charge)

since z is almost always +1, m/z actually measures the mass of the individual ions

what is the tallest peak in mass spectrum called?

the base peak

(also the M peak usually)

how do you read a mass spectrum?

identify base peak (tallest)

molecular ion peak (M⁺ - furthest right, representing total molecular mass)

interpret fragmentation patterns (structural fragments from where bonds have broken)

what does the presence of an odd-numbered molecular ion (eg, 53)?

indicates that a compound has an odd number of nitrogen atoms (nitrogen rule)

(N usually forms 3 bonds but has an even atomic mass (14) - to satisfy its valence, it requires an extra H atom, which shifts the molecular weight by one)

what are the two common isotopes of Cl and what ratio do they occur in naturally?

³⁵Cl and ³⁷Cl

3:1 ratio

how are low and high resolution mass spectrometers different?

low - report m/z values to the nearest whole number (mass of a given molecular ion can correspond to many different molecular formulas)

high - measure m/z ratios to four decimal places (making it possible to determine the single molecular formula that gives rise to a molecular ion)

what is the source of NMR?

radio waves

(which have long wavelengths, and low energy and frequency)

what is nuclear magnetic resonance (NMR)?

used to characterise organic molecules by identifying carbon-hydrogen frameworks within molecules

how does NMR work?

involves the excitation of nuclei by electromagnetic radiation in the radio-frequency range of the electromagnetic spectrum

for a nucleus to absorb energy in this way, it must have the quantum mechanical property of spin

describe the 4 principles of NMR

1. a spinning proton creates a magnetic field

2. in a magnetic field there are two energy states for a proton: a lower energy state with the nucleus aligned in the same direction as the applied (external) magnetic field, and a higher energy state in which the nucleus is aligned against this

3. the energy difference between the two nuclear spin states corresponds to the low frequency radiowave region of the electromagnetic spectrum

4. when an external energy source (hv) that matches the energy difference between the two energy states is applied, energy is absorbed, causing the nucleus to ‘spin flip’ from one orientation to another

what 2 variables characterise NMR instruments?

1. the strength of the applied magnetic field (measured in tesla T)

2. the frequency (v) of radiation used for resonance (measured in hertz Hz)

what are the basic principles of NMR, EI and IR spec?

NMR - nuclei with spin are placed in a strong magnetic field and irridated with radiofrequencies

EI - molecules are bombarded with high-energy electrons, causing them to ionise and fragment

IR - IR radiation causes covalent bonds to vibrate

what solvent is typically used to dissolve the sample for NMR spec?

CDCl₃

what gives the position of an NMR signal?

the chemical shift of the x-axis

measured in ppm

give the equation for chemical shift (in ppm)

observed chemical shift (in Hz) downfield from TMS / v of the NMR spectrometer (in MHz)

what 4 different features of a ¹H NMR spectrum provide information about a compound’s structure?

1. number of signals

2. position of signals

3. intensity of signals

4. spin-spin splitting of signals

what does the number of signals indicate on a ¹H NMR spectrum indicate?

the number of different protons in a molecule

(ie the number of different proton environments)

what does the position of signals (chemical shift) on a ¹H NMR spectrum indicate?

indicates the electron environment surrounding a specific nucleus

reveals how shielded or deshielded a nucleus is by nearby electrons

if electrons shield the nucleus - absorption shifts upfield (to right)

decreased electron density deshields a nucleus, absorption shifts downfield (to left)

what does the intensity of NMR signals indicate?

the area under an NMR signal is proportional to the number of absorbing protons (gives a ratio, not the absolute number of electrons)

what does the spin-spin splitting of signals on a NMR spectrum indicate?

occurs when the magnetic field of a nucleus is influenced by the spins of neighbouring, non-equivalent nuclei

what is the n+1 rule for splitting patterns on NMR spectra?

n is the number of chemically equivalent protons that neighbour a given proton

(ie if no neighbouring protons = singlet, 3 neighbouring protons = quartet)