▁methods ▁to ▁be ▁ 1 2 5 ▁or ▁ 2 1 0 ▁k J / m ol . ▁ ▁The ▁direct ▁t aut omer isation ▁is ▁ener get ically ▁not ▁fav oured . ▁There ▁are ▁other ▁possible ▁mechan isms ▁for ▁this ▁t aut omer isation . ▁ ▁D imer isation ▁▁ 2 - Py rid one ▁and ▁ 2 - h ydro x yp y rid ine ▁can ▁form ▁dim ers ▁with ▁two ▁hydro gen ▁b onds . ▁ ▁A ggreg ation ▁in ▁the ▁solid ▁state ▁In ▁the ▁solid ▁state ▁the ▁dim er ic ▁form ▁is ▁not ▁present ; ▁the ▁ 2 - py rid ones ▁form ▁a ▁hel ical ▁structure ▁over ▁hydro gen ▁b onds . ▁ ▁Some ▁subst it uted ▁ 2 - py rid ones ▁form ▁the ▁dim er ▁in ▁solid ▁state , ▁for ▁example ▁the ▁ 5 - m eth yl - 3 - car bon it rile - 2 - py rid one . ▁The ▁determ ination ▁of ▁all ▁these ▁structures ▁was ▁done ▁by ▁X - ray ▁cry stal log raph y . ▁In ▁the ▁solid ▁state ▁the ▁hydro gen ▁is ▁located ▁closer ▁to ▁the ▁o xygen ▁so ▁it ▁could ▁be ▁considered ▁to ▁be ▁right ▁to ▁call ▁the ▁colour less ▁cry st als ▁in ▁the ▁flask ▁ 2 - py rid one . ▁ ▁A ggreg ation ▁in ▁solution ▁In ▁solution ▁the ▁dim er ic ▁form ▁is ▁present ; ▁the ▁ratio ▁of ▁dim er isation ▁is ▁strongly ▁dependent ▁on ▁the ▁polar ity ▁of ▁the ▁sol vent . ▁Pol