The observed decrease in the energy difference (E) of the isomeric structures (HNQ-isomer-azo and HNQ- isomer) indicates that the N2/H-N1 hydrogen bond participates noticeably in stabilizing the keto- tautomer.
However under the standard conditions the title compounds are pure keto- tautomers which is consistent with the experimental results.
They indicate that the absence of the N2/H-N1 H-bond decreases the stability of the keto- tautomer significantly which confirms that this five-membered H-bond interaction (despite it is being weak) is a critical factor.