Everything from the 2 lessons of this topic on a few pages: the key idea of each lesson, the diagram to remember, the points to learn and the traps examiners set. Read it, cover it, test yourself.
Covalent bonds are always vibrating (stretching and bending). A bond absorbs infrared radiation whose frequency matches its natural vibration frequency, provided the vibration changes the dipole of the molecule. Positions are quoted as wavenumber in cm−1, and each bond absorbs in a characteristic range listed in the Data Booklet, for example O–H in alcohols 3200–3600 cm−1 (broad), O–H in carboxylic acids 2500–3000 cm−1 (very broad) and C=O 1670–1740 cm−1. A spectrum plots transmittance against wavenumber, so every absorption is a dip. The fingerprint region below 1500 cm−1 is unique to each compound and is matched against a database. The same absorption by C=O and O–H bonds in CO2 and H2O is what makes them greenhouse gases.
Spectra are drawn with wavenumber decreasing from left to right, and absorptions point down. Do not call every peak near 3000 an O–H: C–H at 2850–2950 is in almost every compound. The acid O–H is very broad and lower (2500–3000) than the alcohol O–H (3200–3600). An ester has C=O and C–O but no O–H. You cannot identify a compound from the fingerprint region by eye: it needs a database match.
In the spectrometer a molecule loses an electron to form the molecular ion, M+. The M+ peak, at the highest m/z apart from its isotope peaks, gives the Mr. Some molecular ions break into a fragment ion and a neutral radical; only the ion is detected, so peaks such as 15 (CH3+), 29 (C2H5+ or CHO+) and 43 (C3H7+ or CH3CO+) reveal pieces of the structure. About 1.1% of carbon is 13C, so a small M+1 peak appears and the number of carbon atoms is n = 100 × (M+1 height) / (1.1 × M height). An M+2 peak one third the height of M shows one Cl atom (35Cl : 37Cl = 3 : 1); M and M+2 of equal height show one Br atom (79Br : 81Br = 1 : 1).
The M+ peak is the last significant peak before its own small isotope peaks, not the tallest (the tallest is the base peak). In the M+1 formula use peak heights, not m/z values, and put the M+1 height on top. Only ions are detected, never the radical lost. A 3 : 1 pair two units apart means chlorine; 1 : 1 means bromine, so do not swap them.