Ab initio calculations on the hydrogen bonding interactions among pseudoephedrinium cation isomers and methacrylic acid

The hydrogen bonding interactions between both 1R-2R and 1S-2S isomers of pseudoephedrinium cation (Drug) and methacrylicacid (MAA) were calculated using Gaussian 98 software. Eight possible forms for the H-bond interactions were computed at HF and B3LYP levels of theory with 6-31G(d) and 6-31+G(d,p) basis sets and for all of the molecules the B3LYP/6-31G* provides the highest stabilization energies. The hydrogen bonding energies, E $_{(H-bond)}$ , were obtained from the equation E $_{(H-bond)}$ =E $_{(complex)}$ – [E $_{(Drug}}$ + E $_{(MAA)}$ ] and among four computational methods, B3LYP/6-31G(d) provides the highest hydrogen bonding energy for all of the complexes. The nuclear quadrupole coupling constants (χ) for $^{17}$ O, tetrahedral $^{14}$ N nuclei were calculated about 10.0, 0.5–1.0 MHz and for $^{2}$ H atoms varied from nearly 150.0 kHz to almost 350.0 kHz.