Direct measurement of the fine-structure interval and g_J factors of singly ionized atomic carbon by laser magnetic resonance

We present the results of laser magnetic resonance measurements performed on the ground ^2P state of singly ionized atomic carbon (C_II). The 2^P_(3/2) ← ^2P_(1/2) fine-structure intervals of both ^(12)C^+ and ^(13)C^+ have been determined with a precision of approximately 1 ppm, and the g_J factors to approximately one part in 10^4. Specifically, we find that g_(J=(1/2)) = 0.66576(11) and g_(J=(3/2)) = 1.33412(11), while for ^(12)C^+ ΔE_0(^2P_(3/2) ← ^2P_(1/2))= 1900536.9(1.3) MHz, with ΔE_0(^2P_(3/2) ← ^2P_(1/2)) = 1900545.8(2.1) and ΔE(^2P(3/2) ← ^2P_(1/2), F = 2 ← 1) = 1900466.1(2.3) MHz in ^(13)C^+. The highly precise values of the ^(12)C_II and ^(13)C_II fine-structure intervals verify the already secure far-infrared astronomical identification of C^+ and should allow the interstellar (^(12)C / ^(13)C) ratio to be unambiguously determined in a number of environments

Irinotecan: Electron transfer mechanism in CNS disorders: Electron affinity, ROS, and SAR

A recent article deals with promising use of irinotecan in treatment of Angelman Syndrome, a neurological disorder. The present report provides mechanistic evidence for involvement of electron transfer based on preliminary data from computational studies on electron affinity. The drug and the topotecan analog are related to camptothecin, a well­known anticancer drug. The protonated forms are better at electron affinity. Lactone hydrolysis may provide carboxyl for intramolecular protonation. The active phenol metabolits may also play a role. Structure­activity relationship (SAR) in addressed. These results buttress the prior hypothesis dealing with irinotecan mechanism in central nervous system toxicit

Explicit solvation modeling in the accurate TD-DFT prediction of absorption spectra for natural nucleobases and fluorescent nucleobase analogues

Computational modeling of the absorption and emission properties of organic fluorophores is very useful for understanding their photophysics and improving fluorophore design, but it is challenging to do accurately. Methodologies based on density functional theory have been tested for accuracy in the quantitative simulation of experimental absorbance spectra of the longest wavelength π→π^* and n→π^* transitions for several natural nucleobases and nucleobase analogues to arrive at a tractable approach for relatively reliable prediction of excitation wavelengths in new analogues. Good performance was obtained for TD-B3LYP/aug-cc-pVDZ//B3LYP-D3(BJ)/cc-pVDZ calculations, with SMD implicit solvation plus explicit solvent at likely H-bonding sites. For spectra in aqueous solution, the explicit waters at sites with substantial difference in HOMO/LUMO shift the calculated absorption wavelengths by as much as 20 nm. This methodology yielded a root mean square error in absorbance λ_max values of 10 nm with a maximum absolute error of 16 nm for 16 transitions in aqueous solutions of 14 molecules (including the natural nucleobase monomers) in the range 219-442 nm; errors are even smaller for a more limited set of transitions in 1,4-dioxane. Qualitative trends in absorbance intensities are faithfully modeled, although the root mean square relative error in the absolute intensities is 76%. Although limited to non-charge-transfer transitions, the methodology is easily implemented and successfully describes the long-wavelength absorption spectra of many fluorescent nucleobase analogues such as tricyclic cytosines featuring substituents with a wide range of Hammett parameters

CO adsorption on transition metal clusters: trends from density functional theory

This work reports for the first time the trends for carbon monoxide (CO) chemisorption on transition metal clusters present in supported metal catalysts. In particular, the energetic, structural and infrared adsorption characteristics of linearly (atop) CO adsorbed on transition metal nano-clusters of less than 10 Å in size were explored. Spin-unrestricted density functional theory (DFT) calculations were employed to explore the trends of CO adsorption energy (AM–CO) and C–O vibrational frequency (νCO) for clusters composed of Ni, Cu, Ru, Rh, Pd, Ag, Ir, Pt and Au. The effects of the transition metal electronic structure onto the adsorption energy of CO and the vibrational stretching frequency of C–O, and how these chemical parameters can be correlated to the catalytic activity of transition supported metal catalysts that involve the adsorption, surface diffusion, and C–O bond dissociation elementary steps in heterogeneous catalytic surface reactions, are discussed. Our findings show that an increase of the electronic d-shell occupancy and the principal quantum number (n) in transition metals causes an increase in the vibrational stretching frequency of the C–O bond. This trend is inconsistent with the classical Blyholder model for the metal–carbonyl bond

Kinetic Stability of Si2C5H2 Isomer with a Planar Tetracoordinate Carbon Atom

Dissociation pathways of the global minimum geometry of Si2C5H2 with a planar tetracoordinate carbon (ptC) atom, 2,7-disilatricyclo[4.1.0.01,3]hept-2,4,6-trien-2,7-diyl (1), have been theoretically investigated using density functional theory and coupled-cluster (CC) methods. Dissociation of Si-C bond connected to the ptC atom leads to the formation of 4,7-disilabicyclo[4.1.0]hept-1(6),4(5)-dien-2-yn-7-ylidene (4) through a single transition state. Dissociation of C-C bond connected to the ptC atom leads to an intermediate with two identical transition states and leads back to 1 itself. Simultaneous breaking of both Si-C and C-C bonds leads to an acyclic transition state, which forms an acyclic product, cis-1,7-disilahept-1,2,3,5,6-pentaen-1,7-diylidene (19). Overall, two different products, four transition states, and an intermediate have been identified at the B3LYP/6-311++G(2d,2p) level of theory. Intrinsic reaction coordinate calculations have also been done at the latter level to confirm the isomerization pathways. CC calculations have been done at the CCSD(T)/cc-pVTZ level of theory for all minima. Importantly, all reaction profiles for 1 are found be endothermic in Si2C5H2. These results are in stark contrast compared to the structurally similar and isovalent lowest-energy isomer of C7H2 with a ptC atom as the overall reaction profiles there have been found to be exothermic. The activation energies for Si-C, C-C, and Si-C/C-C breaking are found to be 30.51, 64.05, and 61.85 kcal mol−1, respectively. Thus, it is emphasized here that 1 is a kinetically stable molecule. However, it remains elusive in the laboratory to date. Therefore, energetic and spectroscopic parameters have been documented here, which may be of relevance to molecular spectroscopists in identifying this key anti-van’t-Hoff-Le Bel molecule

A diffuse reflectance infrared Fourier-transform spectra and density functional theory study of CO adsorption on Rh/gamma-Al2O3

The vibrational frequencies and bond dissociation energies of carbon monoxide adsorbed to various rhodium clusters are computed, and the diffuse reflectance infrared Fourier-transform (DRIFT) spectrum of carbon monoxide adsorbed onto Rh/γ-Al2O3 is reported. The computations reveal that the spectral features that have previously been assigned to symmetric and antisymmetric vibrational modes of isolated dicarbonyl species on Rh can also be assigned to vibrational coupling of adjacent linearly bound carbonyl groups (atop M−CO). We explore, in small rhodium clusters, the effects of binding configuration, surface coverage, metal−support charge transfer, and cluster dimension on the vibrational frequency of the C−O bond and the adsorption energy of CO to rhodium. In particular, we find a linear correlation between the vibrational frequency of the carbon−oxygen bond with respect to the metal−carbonyl bond adsorption energy as well as the carbon-oxygen bond length for small rhodium clusters. A semiquantitative relationship is derived that may be used to calculate bond dissociation enthalpies of rhodium−carbonyl bonds with the use of crystallographic or IR data associated to the rhodium−carbonyl bond

Theoretical Studies of SiC4H2 Isomers Delineate Three Low-Lying Silylidenes Are Missing in the Laboratory

Eleven isomers of SiC4H2 lying within 50 kcal mol−1 have been theoretically investigated using density functional theory and high-level coupled-cluster methods. Among them, four isomers, 1-ethynyl-3-silacycloprop-1(2)-en-3-ylidene (1), diethynylsilylidene (2), 1-sila-1,2,3,4-pentatetraenylidene (4), and 1,3-butadiynylsilylidene (5), have already been identified in the laboratory. The current investigation reports three low-lying (3), 4-sila-2-methylenebicyclo[1.1.0]but-1(3)-en-4-ylidene (6), and 3-ethynyl-1-silapropadienylidene (7)] and three high-lying (>1 eV) silylidenes [2-sila-(didehydrovinylidene)cyclopropene (1 eV) silylidenes [2-sila-(didehydrovinylidene)cyclopropene (8), an isomer with a planar tetracoordinate carbon (ptC) atom (10), and 1-ethynyl-1-silapropadienylidene (11)], which remain elusive in the laboratory to date. Isomer 9 also contains a ptC atom, which turned out to be a transition state at all levels. Though all isomers are polar (μ ≠ 0), rotational spectrum is available only for 4. Using matrix isolation, three isomers (1, 2, and 5) have been trapped in the laboratory at 10 K. Considering the astrochemical relevance of silicon−carbide clusters in the interstellar medium, the current theoretical data demand new molecular spectroscopic studies on SiC4H2. Surprisingly, unlike the isovalent C5H2 isomers, where the bent carbenes are yet to be identified in the laboratory, the bent silylidenes (2 and 5) have been trapped in the case of SiC4H2. In both the cases, molecules with transannular C-C and/or Si-C bonds remain elusive, though they lie in the low-lying region. Using suitable precursors, whether these peculiar geometries (especially 3 and 6) would be identified or not in the laboratory needs to be addressed by molecular spectroscopists. The present investigation documents structural and spectroscopic information of SiC4H2 isomers, which may compliment future molecular spectroscopic observations including radioastronomical searches