Basic Information for IR
Method of structure determination based upon the amount of infrared light absorbed by a compound. The frequencies indicate stretching and bending between atoms. Absorption of infrared photon produces the excitation of the molecule to a higher vibrational quantum state and this excitation results in absorption of infrared light.
Intensity of IR Absorption
For stretching vibration, a photon is absorbed and the molecule is excited to a higher vibrational quantum state only if this vibration results in a change in bond dipole. The bond dipole is based upon bond length and the difference in charge of the bonded atoms. The bond length changes as the bond vibrates. The difference in charge is determined by the electronegativity of the bonded atoms. Atoms that are not identical have a difference in electronegativity and will have a charge difference. No photon is absorbed if the atoms have equal electronegativity.
Bonds with Similar IR Frequencies
The energy of the photon needed to excite the molecule to a higher quantum level is based upon the masses of the bonded atoms and the bond strength. Similar functional groups have similar atoms and thus will absorb photons of similar energies.
In symmetric stretch, the atoms move toward or away from the central atom. In asymmetric stretch, one atom moves toward the central atom and one atom moves away from the central atom. Symmetric in-plane bend resembles scissoring. Asymmetric in-plane bend resembles rocking. Symmetric out-of-plane bend resembles twisting. Asymmetric out-of-plane bend resembles wagging.
U = C + 1 - 2H + 2N. U = unsaturation number. C = number of carbons. H = number of hydrogens + halogens. N = number of nitrogens + phosphorus. The interpretation of unsaturation number follows. U = 0; no double bonds, triple bond, or rings. U = 1; 1 double bond or ring. U = 2; 2 double bonds, 2 rings, 1 triple bond, or 1 double bond + 1 ring. U = 3; 3 double bonds, 3 rings, 1 double bond + 2 rings, 2 double bonds + 1 ring, 1 triple bond + 1 double bond, or 1 triple bond + 1 ring. U = 4; benzene. U = 5 (4 + 1); benzene + 1 double bond or ring. U = 6 (4 + 2); benzene + 2 double bonds or 2 rings or 1 triple bond or 1 double bond + 1 ring.
Zones and Trends for IR Interpretation
The five zones for IR interpretation are described. Distinct trends appear in an IR spectrum for alcohol, alkyne, alkane, aryl, vinyl, aldehyde, carboxylic acid, alkyne, nitrile, carbonyl functional groups, alkene, and amine
IR Absorption Frequencies
The IR absorption frequencies are given for hydrocarbons, alcohols and phenols, ethers, aldehydes and ketones, carboxylic acids, esters, acid halides, anhydrides, amides, amines, alkyl halides.
IR Problem Solving
Calculate the unsaturation number and give interpretation. Using the formula and interpretation of the unsaturation number, predict possible functional groups present. Look for frequencies for the functional groups in the table. Once all of the frequencies have been identified, draw the structure. Examples are illustrated.
Mass Spectroscopy Background
Compound is injected into high vacuum. Molecules are ionized. Only positive ions are recorded in the mass spectrometer. The ions are sorted according to mass. The abundance of ions of each mass is recorded.
Mass Spectroscopy Definitions and Trends
Molecular ion is the original molecule loses one electron. Base peak is the tallest peak in the spectrum. Mass to charge ratio is the mass divided by charge; usually corresponds to the weight of the fragment. Cl: M + 2 is one-third the size of M. Br: M + 2 is equal in size to M. N: odd molecular ions.
Mass Spectroscopy Cleavage Reactions
The cleavage reactions of alcohols, amines, ethers, aldehydes, and ketones are illustrated. McLafferty rearrangements are explained.
Mass Spectroscopy Problem Solving
Draw out the structure. Cleavage can occur between most single bonds. Draw out all possibilities. Review cleavage reactions in prior section for additional structures. Add up the atomic weights of the atoms to determine the mass / charge ratio. Look for these fragments in the list.