Topic 11: Measurement & Data Processing

• identifies the bonds in a molecule - since different bonds absorb different frequencies

• the gaseous molecules are shot with e-s to form cation fragments • the peak in the mass spectrum with the greatest mass comprises of ions that passed through unscathed • but other peaks appear as well (the cation fragments) • the ‘fragmentation patterns’ are then used to analyze the composition of the molecule • REMEMBER! generally the fragment that gives the most stable ion is formed!

• tells us how unsaturated the molecule is - the number tells us how many H2 molecules can be added

• IR is absorbed by polar bonds and will cause stretching/bending • the change in energy produces a change in the molecule’s dipole moment • the intensity of the absorption depends on the bond polarity (more polar = more stretch/bend observed)

• IR is absorbed by polar bonds and will cause stretching/bending • the change in energy produces a change in the molecule’s dipole moment • the intensity of the absorption depends on the bond polarity (more polar = more stretch/bend observed)

• polar bonds have separate areas of partial positive and negative charges • this allows the electric field component to excite vibrational energy

• symmetric stretch/bend takes less energy than asymmetric • multiple bonds need higher frequencies e. g. ≡ 𝐶 − 𝐻 has higher wavenumber (≈3300) than = 𝐶 − 𝐻 (≈3100); and = 𝐶 − 𝐻 has higher wavenumber than −𝐶 − 𝐻 (≈2900) • in terms of hybridization, sp hybridization has 50% s-orbital character, sp2 has 33% s-orbital character and sp3 has 25% s-orbital character

• uses radio waves to see how much energy is required to stimulate H+ in an element • using radio waves to see how much energy it is required to stimulate the protons (H+ ) in an element. Hydrogen atoms in different position in the molecule will require different amount of energy

• a proton has a spin that creates a magnetic field, just like a magnet bar • proton in its low energy state spins parallel to an external magnetic field (its N gets attracted to the southern magnetic pole, which is geographically the North Pole, and its S gets attracted to the northern magnetic pole, which is geographically the South Pole) • in NMR, energy is applied in the form of radio waves to flip the direction of magnetic field of hydrogen atoms • when these flipped protons flip back again to its low energy state, each one gives out energy that is recorded in the NMR spectrometer

tells you the number of possible “chemical environments” a hydrogen can be in

i. e. integration trace - tells you how many protons are sending the signal - essentially reveals the ratio of H in the molecule

• the energy required to flip the proton in relation to energy required to flip TMS (tetramethylsilane, Si(CH3)4) • value depends on the environment

AKA spin-spin splitting/coupling • the signals/peaks are actually split, not straight • this is because a proton’s magnetic field is influenced by neighboring protons • neighboring protons may spin in the same direction (thus intensifying the magnetic field) or in the opposite direction (thus decreasing the magnetic field) • this is because each proton can spin either up or down with equal probability • number of splitting = number of neighbor protons + 1

• it sends X-ray radiation on a sample at an angle θ • we can deduce the bond length, bond angle, and the structure from the reflected radiation (either constructive or destructive)

• it has one peak; all hydrogens are in identical chemical environment • it is inert and has low boiling point (easily separated from other compounds) • silicon absorbs different wavelengths from carbon-hydrogen bonds (so it doesn’t interfere)

chemical shift = (shift in relation to TMS / frequency of spectrometer) * 10^6