The -OH proton in ^1H-NMR produces a broad signal primarily due to rapid proton exchange with other acidic protons in the solution, such as those from water or other alcohol molecules. This fa

Here are the answers to your questions:

1. The -OH proton in $^1$H-NMR produces a broad signal primarily due to rapid proton exchange with other acidic protons in the solution, such as those from water or other alcohol molecules. This fast exchange averages out the spin states of adjacent nuclei, leading to decoupling and a broadened, often featureless, peak instead of a sharp multiplet.
2. EPR spectroscopy is often performed at low temperatures to increase the population difference between electron spin energy levels, which enhances signal intensity, and to reduce spin relaxation rates and molecular motion, leading to sharper signals and better resolution of anisotropic effects. Low temperatures also help to stabilize transient free radicals.