Insight into Anticorrosion Mechanism of Ampicillin on Mild Steel in Acidic Environment: A Combined Experimental and Theoretical Approach

Table 3

Proton affinity (PA) and gas basicity (GB) values were carried out by using the B3LYP/6-31+G(d, p) method.

Compound	AMP
Protonation site	N9	N12	N18	O19	O20	O24	S14
PA (kcal/mol)	203.3	213.1	227.4	225.3	205.0	198.6	203.3
GB (kcal/mol)	211.5	221.1	235.2	233.2	212.4	206.1	210.6
ΔPA	−24.1	−14.3	0.0	−2.1	−22.4	−28.8	−24.1
ΔGB	−23.7	−14.1	0.0	−2.0	−22.8	−29.1	−24.6

ΔPA = PA (less stable) − PA (most stable). ΔGB = GB (less stable) – GB (most stable).