New Hydroxylated Cyclic and Acyclic Silylenes Via DFT

Abstract

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 (1H, 1’H, 2H, 2’H, 3H, 3’H, 3”H, 3”K-H, 4H, 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 (ΔEs-t = −19.03 eV) and band gap (ΔEH-L = −2.01 eV) and the highest nucleophilicity (N = 4.02 eV), chemical potential (μ = −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 (ρ(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 (E2 = 7.85 kcal/mol) for 4′ which is in consistent with the lowest bond length of IHB (1.84 A). Furthermore, the infrared (IR) spectroscopy indicates the lowest vibrational frequency of O-H bond νOH(νOH= 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.