Theoretical calculations of intramolecular hydrogen bond of the 2-Amino-2, 4, 6-cycloheptatrien-1-one in the gas phase and solution: Substituent effects and their positions

The density functional theory (DFT) method with 6-311++G** basis set has been used to calculate the intramolecular hydrogen bond, molecular structure, vibrational frequencies, nuclear quadrupole resonance (NQR) parameters, (HNMR)-H-1, and resonance parameters of 2-Amino-2, 4, 6-cycloheptatrien-1-one (2-amino tropone) and its 18 derivatives in 5 positions. The natural bonding orbital (NBO) and quantum theory of atoms in molecules (QTAIM) analyses have been studied. The strongest and weakest hydrogen bonds exist for NO2 substituent in R-3 position and OH in R-7 position, respectively. In general, the substituted systems in position 3 indicate the stronger hydrogen bond in comparison with the parent molecule (R = H), while, it is comparatively weaker for position 5. The energy of the N-H center dot center dot center dot O interaction is found to be medium in strength (E-HB* = -25.106 kJ mol(-1) to -38.560 kJ mol(-1)). The low rho, positive del(2)rho values and H-C > 0 show that the nature of O center dot center dot center dot H bonding is electrostatic. Also, our theoretical results show that the hydrogen bond strength in solution phase and the first singlet excited state is weaker in comparison with the gas phase ground state.