New Hydroxylated Cyclic and Acyclic Silylenes Via DFT

We have scrutinized thirteen new derivatives of cyclic and acyclic silylenes and compared their structural and thermodynamic parameters, at M06-2X/6-311++G** level of theory. The cyclic three- and five-membered silylenes include (2-hydroxy)cyclopropasilylene-2-ene (1), (2,3-dihydroxy)cyclopropasilylene-2-ene (2), (2-hydroxy)cyclopentasilylene-2,4-diene (3), and (2,5-dihydroxy)cyclopentasilylene-2,4-diene (4). The acyclic isomers consist of hydroxypropa-2-silylene (1(')), (1,3-dihydroxy)propa-2-silylene (2(')), (2-hydroxy)penta-3-silylene (3(')), (2-hydroxy)penta-3-silylene-1,4-diene (3('')), (2,4-dihydroxy)penta-3-silylene (4(')), and (2,4-dihydroxy)penta-3-silylene-1,4-diene (4('')). In addition, keto forms of 3('') (3(K)(")) and 4('') (4(K1)(") and 4(K2)(")) along with protonated forms of silylenes (1(H), 1(H)('), 2(H), 2(H)('), 3(H), 3(H)('), 3(H)("), 3(K-H)("),4(H), 4(H)('), 4(H)("), 4(K1-H)("), and 4(K2-H)(")) are investigated for determining their proton affinities (PAs) and intramolecular hydrogen bondings (IHBs). The results show that 4(') shows the lowest singlet-triplet energy gap (Delta Es-t = -19.03 eV) and band gap (Delta EH-L = -2.01 eV) and the highest nucleophilicity (N = 4.02 eV), chemical potential (mu = -3.38 eV), and PA (382.85 kcal/mol) which correlates with its strongest IHB. Atoms in molecules (AIM) analysis represents the highest electron density (rho(r) = 0.033) at bond critical point (BCP) of IHB in 4('). The natural bond orbital (NBO) analysis shows the highest value of second-order perturbation stabilization energy (E-2 = 7.85 kcal/mol) for 4(') which is in consistent with the lowest bond length of IHB (1.84 angstrom). Furthermore, the infrared (IR) spectroscopy indicates the lowest vibrational frequency of O-H bond nu(OH)(nu(OH)= 3542.87 cm(-1)) which verifies the strong IHB of 4('). The overall order of the IHB strength is 4(') > 4(") > 4(H)(') > 4(H)(") > 2(') > 2(H)(').