▁exact ▁position ing ▁is ▁difficult ), ▁and ▁IR - spect ro sc opy , ▁which ▁shows ▁that ▁the ▁C = O ▁longitud inal ▁frequency ▁is ▁present ▁whilst ▁the ▁O - H ▁frequencies ▁are ▁absent . ▁ ▁T aut omer ism ▁in ▁solution ▁ ▁The ▁determ ination ▁of ▁which ▁of ▁the ▁two ▁t aut omer ic ▁forms ▁is ▁present ▁in ▁solution ▁has ▁been ▁the ▁subject ▁of ▁many ▁publications . ▁The ▁energy ▁difference ▁appears ▁to ▁be ▁very ▁small ▁and ▁is ▁dependent ▁on ▁the ▁polar ity ▁of ▁the ▁sol vent . ▁Non - pol ar ▁sol vent s ▁favour ▁the ▁formation ▁of ▁ 2 - h ydro x yp y rid ine ▁whereas ▁polar ▁sol vent s ▁such ▁as ▁alco h ols ▁and ▁water ▁favour ▁the ▁formation ▁of ▁ 2 - py rid one . ▁ ▁The ▁energy ▁difference ▁for ▁the ▁two ▁t autom ers ▁in ▁the ▁gas ▁phase ▁was ▁measured ▁by ▁IR - spect ro sc opy ▁to ▁be ▁ 2 . 4 3 ▁to ▁ 3 . 3 ▁k J / m ol ▁for ▁the ▁solid ▁state ▁and ▁ 8 . 9 5 ▁k J / m ol ▁and ▁ 8 . 8 3 ▁k J / m ol ▁ ▁for ▁the ▁liquid ▁state . ▁ ▁T aut omer isation ▁mechanism ▁A ▁The ▁single ▁mole cular ▁t aut omer isation ▁has ▁a ▁forb idden ▁ 1 - 3 ▁sup raf a cial ▁transition ▁state ▁and ▁therefore ▁has ▁a ▁high ▁energy ▁bar rier ▁for ▁this ▁t aut omer isation , ▁which ▁was ▁calculated ▁with ▁theoretical