Myself, I only have a casual interest in chemistry, but when you are also the son of a Ph.D inorganic chemist who spent his career on developing chemicals to clean things, you pick some things up.

Chloramine actually refers to a (very) large number of chemical compounds, the principal defining factor being a nitrogen-chloride bond somewhere in the structure. The simplest such compound is monochloramine (NH 2 Cl), an ammonia molecule with a hydrogen replaced by chlorine; it is generally seen in dilute aqueous solution, but in its pure form it is a gas at room temperature (25*C). So, distilling water by boiling at high temperature and feeding the vapor into an open-ended still will reduce monochloramine concentrations if present. However, monochloramine is used as a disinfectant because it will stay in water longer than "free chlorine" (chlorine from ionic compounds), and is less harmful to larger organisms (including plants). So, just putting a bucket of chloraminated water out in the sun will have a lesser effect than with chlorinated water (the free chlorine will exit the water at a greater rate than evaporation, while chloramines are much more content to sit in solution).

However, when you hear about "chloramines" outside of municipal chlorination, you are generally hearing about organic chloramines; these are compounds with an NHCl group or two, bound to one or more organic (H-O-C) molecules. These are by-products of disinfection by chlorine and/or quaternary ammonium compounds (quat sanitizers; based on ammonia, they'll pick up chlorine atoms easily); the chlorine (or chloramine) latches on to chunks of important enzymes or other key organic compounds in microbes, and the microbe dies due to the destruction, leaving the organic chloramine floating around (along with many more as the bacteria decomposes in the water). Organic chloramines are often aromatic; A pool that has the distinctive "chlorine smell" is actually one with a high concentration of organic chloramines that are evaporating. These are not incredibly stellar things to be drinking or to be feeding your plants, and don't disinfect well, although if your municipality has chlorinated water of any kind you will have SOME organic chloramines in it (except in the totally implausible case of there being nothing in the water to kill). Again, boiling water to distill it will reduce organic chloramine concentration, more so than with monochloramine alone.

Putting water out in the sun reduces overall chloramines indirectly, by consumption of monochloramine into organic chloramines; the sunlight and heat will cause the more aromatic of the organic chloramines to boil off. But, open standing water attracts more microbes, which the chlorine or chloramine kills and dissolves, then those organic chloramines boil off, more microbes come in, and the cycle repeats itself until there's no more chloramine in the water. But, by the time the chloramine runs out the water's pretty full of microbes, especially hardier ones that didn't get poisoned by the decreasing level of chloramine. So, don't drink water you've left sitting out, and be careful about eating things fresh from the garden when you've watered them with dechloraminated water. The microbes that were tough enough to survive the dechloramination process are not going to be things you want in your body.