UV photolysis of α-cyclohexanedione in the gas phase

Theoretical study on the photooxygenation and photorearrangement reactions of 3-hydroxyflavone

The mechanisms of three photodegradation reactions of 3-hydroxyflavone – its photosensitized oxygenation, photooxygenation with ³O₂ and photorearrangement into an indanedione derivative – have been investigated by computing the free energy profiles.

Photodissociation dynamics of enolic 1,2-cyclohexanedione at 266, 248, and 193 nm: mechanism and nascent state product distribution of OH

The photodissociation dynamics of 1,2-cyclohexanedione (CHD), which exists in enolic form in gas phase, is studied using pulsed laser photolysis (LP)-laser induced fluorescence (LIF) "pump-and-probe" technique at room temperature. The nascent state distribution of the OH radical, formed after initial photoexcitation of the molecule to it is (π, π*) and Rydberg states, is determined. The initial (π, π*) and Rydberg states are prepared by excitation with the fourth harmonic output of Nd:YAG (266 nm)/KrF (248 nm) and ArF (193 nm) lasers, respectively. The ro-vibrational distribution of the nascent OH photofragment is measured in collision-free conditions using LIF. The OH fragments are formed in the vibrationally cold state at all the above wavelengths of excitation but differ in rotational state distributions. At 266 nm photolysis, the rotational population of OH shows a curvature in Boltzmann plot, which is fairly described by two types of Boltzmann-like distributions characterized by rotational temperatures of 3100 ± 100 and 900 ± 80 K. However, at 248 nm photolysis, the rotational distribution is described by a single rotational temperature of 950 ± 80 K. The spin-orbit and Λ-doublets ratios of OH fragments formed in the dissociation process are also measured. The average translational energy in the center-of-mass coordinate, partitioned into the photofragment pairs of the OH formation channels, is determined to be 12.5 ± 3.0, 12.7 ± 3.0, and 12.0 ± 3.0 kcal/mol at 266, 248, and 193 nm excitation, respectively. The energy partitioning into various degrees of freedom of products is interpreted with the help of different models, namely, statistical, impulsive, and hybrid models. To understand the nature of the dissociative potential energy surface involved in the OH formation channel, detailed ab initio calculations are performed using configuration interaction-singles (CIS) method. It is proposed that at 266 nm photolysis, the OH fragment is formed from two different excited state structures, one with a strong H bonding, similar to that in the ground state, and another without effective H bonding, whereas, at 248 nm photodissociation, it seems that the OH formation occurs mainly from the excited state, which lacks effective H-bonding. At 193 nm excitation, the initially prepared population in the Rydberg state crosses over to a nearby σ* repulsive state along the C-O bond, from where the dissociation takes place. The exit barrier for the OH dissociation channel is estimated to be 14 kcal/mol. The existence of dynamical constraint due to strong hydrogen bond in the ground state is effectively present in the dissociation process at 266 and somewhat deficient at 248 nm photolysis.

Chemical Composition and Biological Activities of Soldiers of the Brazilian Termite Species, Nasutitermes macrocephalus (Isoptera: Natutitermitinae)

The defensive secretion of the frontal gland from termite soldiers is a mixture of monoterpenes, sesquiterpenes and diterpenes, the latter being the most representative. Analyses of the dichloromethane extract from soldiers of the Brazilian termite, Nasutitermes macrocephalus (Silvestri, 1903) (Isoptera, Nasutitermitinae), described for the first time, allowed to identify the presence of two monoterpenes (α-pinene and limonene) and two sesquiterpenes (β-trans- caryophyllene and α-selinene) by GC-EIMS, and the isolation of one rippertane and six trinervitane diterpenes by RP-HPLC. The chemical structures of the purified compounds were elucidated by interpretation of their spectroscopic data (1D and 2D NMR, EIMS, HRESIMS, and specific optical rotation) and the complete unequivocal assignment of the 3α-hydroxy-trinervita-1(15),8(19)-dien-2-one (6) was included in this paper, to complement the lack of information in the literature. Antibacterial, antifungal and cytotoxicity against cancer cell lines activities were evaluated. In particular, the compounds 2α,3β-dihydroxy- trinervita-1(15),8(19)-diene (2) and 3α-hydroxy-15-rippertene (7) exhibited the better activities against the clinically isolated Gram-positive bacterium methicillin-resistant Staphylococcus aureus BMB 9393, both with a MIC value of 31.2 μg mL−1. This is the first description of a rippertane diterpene (7) as an antibacterial agent.