The synthesis and spectral characterization of the (𝐸)-3-benzylideneindolin-2-one is reported. The compound crystallized in the monoclinic system with space group P 2₁/𝑛 with cell coordinates 𝑎 = 3.9849 (2) Å, 𝑏 = 22.2236 (9) Å, 𝑐 = 12.2501 (5) Å, 𝛽 = 95.0535 (12)° , 𝑉 =1080.64 (8) Å 3 , and 𝑍 = 4. In the crystal, molecules are packed in chains formed via weak intermolecular C13–H13A· · · O1 and N1-H1N1. . .O1 hydrogen bonding. The relative stabilities of the two tautomeric isomers of 3 are calculated by DFT/B3LYP method using 6-311G(d,p) basis set in gas phase and in solution. The quantum chemical calculations, NMR studies and the XRD analyses showed that the keto form T0 is the only form that could exist in gas, solution and solid phases respectively. The calculated geometric parameters of the dimer molecule showed better agreement with the XRD data than those obtained for single isolated molecule. This shed light on the effect of intermolecular interactions on the calculated geometric parameters. MEP study showed that, the O-atom and the NH proton are the most reactive H-acceptor and H-donor sites, respectively. The N-H...O H-bonding interactions increased the negative charge at the O-atoms and the positive charge of the NH protons compared to the monomer unit.

Crystals of 7,11-bis(2,4-dichlorophenyl)-2,4-dimethyl-2,4-diazaspiro[5.5]undecane -1,3,5,9-tetraone were grown in polar solvents and subjected to single crystal X-ray diffraction. The molecular crystal is Triclinic, P-1, 𝑎 = 8.3734 (19) Å, 𝑏 = 12.382 (3) Å, 𝑐 = 12.871 (3) Å, 𝛼 = 66.639 (7)° , 𝛽 = 85.148 (7)°, 𝛾 = 70.690 (6)° , 𝑉 = 1154.5 (5)ų, 𝑍 = 2, D_calc = 1.519 g cm⁻³. The optimized molecular structure of the studied compound using B3LYP/6-311G(d,p) method showed good agreement with the X-ray structure. The electronic and spectroscopic properties of the title compound were predicted. The NBO calculations were used to calculate the natural atomic charges at the different atomic sites as well as the intramolecular charge transfer (ICT) interactions among the most significant natural orbitals. The high LP(N)→ BD^*(2)C-O ICT interaction energies indicate strong electron delocalization from the lone pair of the N-atoms of the pyrimidinetrione ring to the adjacent carbonyl groups. In contrast, the small LP(O)→ BD^*(1)C-H stabilization energies (E⁽²⁾) indicated weak C-H—O interactions. Experimentally, the studied compound showed the most intense electronic transition band at 232 nm which is calculated using TD-DFT method as a shoulder at 231.3 nm (f=0.0832) and it belongs to H-3/H-1→L+1 and H-2→L+2 excitations. The GIAO calculated $_{}^{1}$H and $_{}^{13}$C NMR chemical shifts showed good correlations with the experimental data.