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Abstract: The hydrogen bonding interactions between both 1R-2R and 1S-2S isomers of pseudoephedrinium cation (Drug) and methacrylic acid (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
Author keywords: Ab initio computations, pseudoephedrinium cation, hydrogen bonding interactions, NQR