Kalathingal T. Giju

An ab initio and matrix isolation infrared study of the 1:1 C2H2–CHCl3 adduct

Abstract The details of weak C–H⋯π interactions that control several inter and intramolecular structures have been studied experimentally and theoretically for the 1:1 C2H2–CHCl3 adduct. The adduct was generated by depositing acetylene and chloroform in an argon matrix and a 1:1 complex of these species was identified using infrared spectroscopy. Formation of the adduct was evidenced by shifts in the vibrational frequencies compared to C2H2 and CHCl3 species. The molecular structure, vibrational frequencies and stabilization energies of the complex were predicted at the MP2/6-311+G(d,p) and B3LYP/6-311+G(d,p) levels. Both the computational and experimental data indicate that the C2H2–CHCl3 complex has a weak hydrogen bond involving a C–H⋯π interaction, where the C2H2 acts as a proton acceptor and the CHCl3 as the proton donor. In addition, there also appears to be a secondary interaction between one of the chlorine atoms of CHCl3 and a hydrogen in C2H2. The combination of the C–H⋯π interaction and the secondary Cl⋯H interaction determines the structure and the energetics of the C2H2–CHCl3 complex. In addition to the vibrational assignments for the C2H2–CHCl3 complex we have also observed and assigned features owing to the proton accepting C2H2 submolecule in the acetylene dimer.

X-ray crystallographic and theoretical studies of substituted phenyl 3,5-di[N-methyl] carbamoyl-1,4-dihydropyridines: Targets for Ca2+ antagonist

A series of 4(x-substituted phenyl)-1,4-dihydropyridines (x=2—CF3 (1), 2-CH3 (2), 2-OCH3 (3) and 2,4-Cl (4)) with a new substituent, the N-methylcarbamoyl (CONHCH3) group at C3 and C5 are crystallographically characterized and a comparison has been made with important conformational parameters obtained theoretically. The dihydropyridine rings are in shallow boat conformation. The phenyl substituent orientation is synperiplanar. Both the carbonyl groups are oriented anticlinal in 1, 2 and 3; but in 4, one is synclinal and the other synperiplanar with the adjacent double bond. The presence of solvent molecules in 1 (CH3OH), 2 (CH3OH), and 3 (H2O) has significantly changed the hydrogen bonding pattern. Theoretical studies at the semiempirical AM1 MO level reproduces the general features of the structures. The near planarity of the DHP ring and the orientation of the phenyl substituent make 1 and 2 encouraging targets for pharmacological, study. Crystallographic Data:1: a = 8.793(2), b = 29.962(5), c = 8.215(2) Å, β = 115.28(2)°, Monoclinic, P21/c;2: a = 8.799(2), b = 15.789(3), c = 14.074(2) Å, β = 100.25(2)°, Monoclinic, P21/n; 3: a = 8.347(1), b = 8.986(1), c = 13.749(2) Å, α = 97.50(1), β = 94.78(1), γ = 101.38(1)° Triclinic, P1¯4: a = 12.928(3), b = 14.506(3), c = 9.740(2) Å, Orthorhombic, Pca21.

An ab initio MO study of perfluoro-effect in thermal cyclization of buta-1,3-diene to cyclobutene

Abstract Ab initio theoretical studies at SCF and MP2 levels with various basis sets on thermal cyclization of perfluorobuta-1,3-diene ( 3 ) to perfluorocyclobutene ( 4 ) confirm the experimentally observed reversal of relative stabilities on perfluorination of buta-1,3-diene ( 1 ) and cyclobutene ( 2 ). The transition structures follow Hammonds postulate. The effect of fluorine substitution on sp 2 and sp 3 hybridized carbon centres are analysed using the relative energies of mono-, di-, tri- and tetra- fluoroderivatives of butadiene and cyclobutene ( 5–18 ) at HF/6-31G ∗ level. The reversal of relative stabilities of perfluorination is attributed to the increasing destabilization on polyfluorination at sp 2 -C.