CO Bond Length Research Articles

The O1/H3-preferred 1:1 H-bonding and the electron-cloud migration induced by H-bonding or non-specific interactions: A systematic study on the interactions between dimethyl phthalate and 1-, 2- or 3-alkanol (C2-C6)

H-bonding and non-specific interactions between dimethyl phthalate (DMP) and different human body substances were considered to be mainly responsible for various health dysfunctions. However, the manner of H-bonding and the adaptability of the structure of DMP that may underpin the H-bonding/non-specific interactions have been difficult to assess and remain elusive. In this study, we set out to gain an insight into the subjects above through a systematic investigation on the interactions between DMP and 1-, 2- or 3-alkanol (C2 - C6). We calculated the H-bond distances, the CO and CO bond lengths and other selected structural essentials at B3LYP 6-311G(d) level. We recorded the infrared spectra Band I and Band II of CO of DMP when dissolving DMP into 11 different alkanol/n-Hexane binary solvent systems. We created and fit the equations to delineate the dependence of the absorption intensity ratio of Band II to Band I (AII/AI) on the volume-ratio of the alkanol component (x). Based on the equations, we obtained the percent of the major alkanol associates (−p), the binding constant of the 1:1 and 1:2 DMP-alkanol complexes (K1 and K2) and the ratio of the molar absorptivity of H-bonded CO to the molar absorptivity of non-bonded CO (εhb/εns). The −p, K1, K2 and εhb/εns values confirmed the calculated results. It can be concluded that H-bonds mainly form the 1:1 complexes at O1 and H3, leading to species O5-H1⋯O1 = C1 for 3-alkanols and species O5-H1⋯O1 = C1 and Ph-H3⋯O5 for 2-alkanols. The differences of the affinity of H-bonding were mainly attributed to the steric hindrance and to a lesser extent to the hydrophobic interactions and the multi-H-bond cooperativity. The electron-cloud of the DMP molecule was supposed to migrate mainly in the E1 or E4 mode to adapt to the H-bonding or non-specific interactions, respectively.

An accurate full-dimensional permutationally invariant potential energy surface for the interaction between H2O and CO.

The interaction between H2O and CO has been the subject of numerous experimental and theoretical investigations for a long time due to their important roles in various environments, such as the atmosphere, combustion of hydrocarbons, and the interstellar medium. In this work, the first full-dimensional accurate potential energy surface (PES) was developed for the CO + H2O system based on ca. 102 000 points calculated at the level of an explicitly correlated coupled-cluster method with single, double, and perturbative triple excitations with the augmented correlation-consistent polarized triple zeta basis set (CCSD(T)-F12a/AVTZ) using the permutation invariant polynomial-neural network (PIP-NN) method. The geometries, energies, and frequencies of the two complex wells, CO-H2O and OC-H2O, and one transition state connecting them, as well as some interconversions between different conformers, are accurately reproduced by the PES, thanks to the small fitting error of only 1.08 meV. With full-dimensional degrees of freedom considered in the PES, we found that there exist strong dependences of the CO and OH bond lengths on the OC-H2O and CO-H2O interaction energies, which is not possible in reduced dimensional PESs. Finally, classical dynamics was carried out to study the energy transfer between H2O and CO with different initial vibrational energies in H2O and different vibrational states in CO.

Vibrational spectroscopy of a crystallographically unsettled uranyl carbonate: Structural impact and model

Room-temperature vibrational and photoluminescence (PL) spectra of a natural, rare hydrated calcium copper uranyl carbonate mineral, voglite (Ca2Cu(UO2)(CO3)4·6H2O) are recorded and discussed in details. Vibrational spectroscopy gives information about the structure of voglite, which is still missing due to its unknown crystallographic features. By comparison with other uranyl carbonates and sulfates, a strong Raman line occurring at 834 cm−1 is assigned to the ν1(UO2)2+ symmetric stretching vibration rather than to the ν2(CO3)2− out-of-plane bending vibration. The ν3(UO2)2+ antisymmetric stretching vibration is tentatively identified at 897 cm−1 from infrared (IR) spectroscopy. Several well resolved bands found at 1074,1092, 1381, 1566 cm−1 in the Raman and 1046, 1114, 1145, 1376, 1426, 1510, 1561 cm−1 in the IR are ascribed to symmetric and antisymmetric stretching motions of the carbonate units. The presence of all these intense vibrational bands points to different CO bond lengths. The infrared water band is well structured, suggesting a few different OH moieties in the crystal. Original micro-PL spectra show a manifold of vibronic features whose energy spacing is close to the frequency of the symmetric OUO stretching vibration and confirms the uranium origin of the most intense Raman band. The study suggests that voglite structure has no inversion centers, a low symmetry, and contains molecular units similar to those of the parent phases, andersonite or liebigite, like uranyl tricarbonate clusters (UTC). The existence of these UTCs in voglite is confirmed by density functional theory calculations. A new assignment of all vibrational modes is proposed.

Water-soluble Cobalt(II) & Cobalt(III) complexes supported by new triazine Schiff base ligands: Synthesis, structure and biological evaluation

A new class of triazine ligands (E)-2-(2-(6–methyl-5-oxo-2,5-dihydro-1,2,4-triazin-3-yl)hydrazono)propanoic acid hydrate (HL1.H2O) and (Z)-2-(((E)-4-amino-6-methyl-5-oxo-4,5-dihydro-1,2,4-triazin-3(2H)ylidene)hydrazono)propanoic acid (H2L2) has been synthesized by the condensation reaction of pyruvic acid with diaminoguanidine and triaminoguanidine respectively. The corresponding Schiff base cobalt complexes [Co(L1)2].2H2O (1) and [Co(HL2)(L2)].H2O (2) have also been synthesized and characterized by analytical, thermal, spectroscopic and diffraction studies. Strong field ligand results low spin Co(III) centre in 2, which was evidenced by the shorter bond length of Co(III) complex. In H2L2 there is a choice of coordination modes based on distinct sets of donor atoms, both of which are seen in complex 2, involving either an –NH2 group on position 4 of the triazine ring, or via a ring nitrogen of the triazine itself. The deprotonation of one version of L2 allows the formation of the ligand field stabilized low spin Co(III) in 2. In complex 1, each ligand binds to the metal via pyruvate oxygen, azomethine nitrogen and triazine nitrogen forming two five-membered stable chelate rings. In complex 2, the coordination sphere assembled by two types of coordinating atoms from the same ligand with different conformation. Their binding ability and mode of binding with CT-DNA and BSA was studied by UV- absorption, fluorescence and CD spectroscopy. Density Functional Theory (DFT) studies provide further insights into the mode of binding, structure and mechanism. The HOMO and LUMO energy gap values indicate that both the complexes are prone to interact with CT-DNA and BSA. We have also performed molecular docking calculations to understand the mode of binding and the corresponding results confirm our experimental findings.

Balancing Steric and Electronic Effects in Carbonyl–Phosphine Molybdacarboranes

Analysis of the literature structures [(CO)(PPh3)2MC2B9H11] and [(CO)2(PPh3)MC2B9H11] suggests that in [L3MC2B9H11] metallacarboranes the trans influence of CO is greater than that of PPh3. Extending this study to the [L4MC2B9H11] system the new molybdacarboranes [3,3,3‐(CO)3‐3‐PPh3‐3,1,2‐closo‐MoC2B9H11] (2), [1,2‐Me2‐3,3,3‐(CO)3‐3‐PPh3‐3,1,2‐closo‐MoC2B9H9] (3) and trans‐[3,3‐(CO)2‐3,3‐(PPh3)2‐3,1,2‐closo‐MoC2B9H11] (4) were prepared and fully characterised. Consideration of the exopolyhedral ligand orientations (ELO) in 2 confirms that, in terms of trans influence, CO > PPh3 in [L4MC2B9H11] also. The ELO is effectively reversed in 3 through intramolecular steric crowding between the cage CH3 groups and the PPh3 ligand. The dicarbonylbis(triphenylphosphine) compound 4 has effective Cs symmetry with one CO ligand trans and the other CO ligand cis to the cage C–C connectivity. Unexpectedly the Mo–CO bond lengths are equal. DFT calculations on 4 reproduce this unusual result, but suggest that in the less‐crowded PH3 analogue, the Mo–CO bond length trans to cage C would be about 0.2 Å shorter than that trans to cage B. To test this prediction, the analogous PEt3 complex was prepared as cis and trans structural isomers 5 and 6. The cis isomer 5 is quantitatively converted into the trans isomer 6 when heated to reflux in THF. In 6 the Mo–CO bond more trans to cage C is about 0.2 Å shorter than that which is more trans to cage B, in line with the DFT prediction.

High-resolution Fourier transform emission spectroscopy of the A∼2Πi-X∼2Πi band of the OCS+ ion.

High resolution Fourier transform emission spectroscopy of the A∼2Πi-X∼2Πi band of the OCS+ ion was performed in the UV region to observe the ν1 (CO stretch) progression bands (υ1 = 0 → 2-5) for both the Ω=3/2 and 1/2 spin components. Accurate molecular constants including the rotational constants, B0 = 0.194 765(13) and 0.187 106(13) cm-1, and the spin-orbit interaction constants, A0 = -381.0(56) and -126.5(56) cm-1, were determined for the X∼2Π and A∼2Π states, respectively, by the simultaneous analysis of the observed progression bands. The CO bond length (rCO = 1.2810 Å) for the A∼2Π state, derived from the rotational constant B0 and Franck-Condon factors, is longer by 0.1756 Å than that (1.1054 Å) for the X∼2Π state, while the CS bond length for the A∼2Π state is shorter by 0.0905 Å than that for the X∼2Π state. Pure rotational transition frequencies in the ground X∼2Π state are predicted, as well as transition frequencies of the ν1 fundamental band, with the present molecular constants.

Selenium carboxylic acids betaine; 3,3′,3″-selenotris(propanoic acid) betaine, Se(CH2CH2COOH)2(CH2CH2COO)

Attempts to prepare [Se(CH2CH2COOH)3]+Cl− from Se(CH2CH2COOH)2 and H2CCHCOOH in concentrated hydrochloric acid, for the corresponding sulfonium salt, led exclusively to the Se-betaine, Se(CH2CH2COOH)2(CH2CH2COO). The Se-betaine crystallises in the space group P2l/c with the cell dimensions at 223 K, a = 5.5717(1), b = 24.6358(4), c = 8.4361(1) Å, β = 104.762(1)°, V = 1119.74(3) Å3, Z = 4, Dcalc = 1.763 Mgm− 3, μ = 3.364 Mm−1. The structure refined to RI = 0.0223 for 2801 reflections with Fo > 4σ(Fo). In the crystalline state the molecule is intermolecularly linked to neighbouring molecules by a number of hydrogen bonds; a very strong carboxylic-carboxylate bond with an O⋯O distance of 2.4435(16) Å, a medium strong carboxylic-carboxylate bond with an O⋯O distance of 2.6431(16) Å and several weak O⋯H(CH2) with O⋯C distances between 3.2 and 3.3 Å. In the carboxylic group involved in the very strong hydrogen bond the O⋯H bond is antiperiplanar to the CO bond while the OH bond is periplanar to the CO bond in the second carboxylic group. Based upon the CO bond lengths and the elongation of the OH bond involved in the strong hydrogen bond one may describe the compound as strongly linked units of Se(CH2CH2COOH)(CH2CH2COO)2 rather than Se(CH2CH2COOH)2(CH2CH2COO). The selenium atom forms two strong intramolecular 1,5-Se⋯O contacts, with a carboxylate oxygen atom, 2.9385(12) Å, and with a carboxylic oxygen atom, 2.8979(11) Å. To allow for these contacts the two organic fragments have been forced into the periplanar conformation. The molecule is only slightly asymmetric with regard to the CSeC bond angles but is very asymmetric with regard to the torsion angles.

Ab initio computational study of 1-methyl-4-silatranone and attempts at its conventional synthesis

1-Methyl-4-silatranone could exhibit the structural aspects of a typical silatrane including a short N–Si bond distance reflecting a dative bond. But given the significant amide resonance in a [3.3.3] bridgehead bicyclic lactam, the lone pair could be shared with the carbonyl group leading to a very long N–Si bond, essentially a “non-silatrane.” Ab initio calculations (MP2/6-311 + G*) predict that ground state conformations of this molecule are best regarded as lactams rather than silatranes, the most stable having a calculated N–Si bond length of 2.902 A and an N–CO bond length of 1.387 A. The calculated transition state for inversion of the amide ring retains very little amide resonance (N–CO, 1.440 A). Some of this loss is compensated through tightening of the N–Si bond (2.422 A), leading to a net energy of activation of ca 8 kcal/mol. Attempts to synthesize 1-methyl-4-silatranone using conventional pathways successful for 1-methylsilatrane [condensations employing N,N-bis(2-hydroxyethyl)glycolamide in place of tris(2-hydroxyethyl)amine] were unsuccessful. This is due to the net loss in resonance energy of the amide reactant relative to that in the [3.3.3] system, the essential absence of the N–Si dative bond, and the rigidity introduced by the planar amide linkage in the starting material. A more likely pathway to successful synthesis should be formation of the amide linkage in the final step.

Computational study on fused five membered heterocyclic compounds containing tertiary oxygen

The structure, stability and electronic properties of two fused five membered heterocyclic compounds containing tertiary oxygen have been investigated with quantum chemistry computation. The results show that 3H-4λ3-furo[1,2-a]furan (M1) and 2a1λ4-oxacyclopenta[cd]pentalene (M2) both own three CO bonds and especially in M2 the three CO bonds and angles are approximately identical. The two compounds are comparatively stable and M2 with three pentacycles is more stable than compound M1 according to the CO bond lengths, bond orders, Egap, electronegativity (χ), hardness (η), electrophilicity (ω), and aromaticity indexes. The aromaticity indexes such as NICS(0), NICS(1) and NICS(1)zz show that they both have strong aromaticity. The natural bonding orbital (NBO) analysis shows that oxygen atom takes hybrid orbitals composed of s and p orbital components to form σ(CO) bonds in compounds M1 and M2. The second order perturbation analysis shows that the occurrence of intramolecular charge transfer within the molecule exists in compound M1.

Dispersed Fluorescence Spectroscopy of Jet-Cooled Isobutoxy and 2-Methyl-1-butoxy Radicals.

We report dispersed fluorescence (DF) spectra of the isobutoxy and 2-methyl-1-butoxy radicals produced by photolysis of corresponding nitrites in supersonic jet expansion. Different vibrational structures have been observed in the DF spectra when different vibronic bands in the laser-induced fluorescence (LIF) spectra of each radical were pumped, which suggests that those vibronic bands be assigned to different conformers. Spectra simulated using calculated vibrational frequencies and Franck-Condon factors well reproduce the experimentally observed ones and support the assignment of the vibronic bands in the LIF spectra to the two lowest-energy conformers of each radical. DF spectra obtained by pumping the B̃ ← X̃ origin bands of the LIF spectra are dominated by CO stretch progressions because of the large difference in CO bond length between the ground (X̃) and the second excited (B̃) electronic states. Furthermore, with non-CO stretch bands pumped, the DF spectra are dominated by progressions of combination bands of the CO stretch and the pumped modes as a result of Duschinsky mixing. Ã-X̃ separation of both conformers of the isobutoxy radical has also been determined in the experiment.

How π back-donation quantitatively controls the CO stretching response in classical and non-classical metal carbonyl complexes.

The CO stretching response upon coordination to a metal M to form [(L) n M(CO)] m complexes (L is an auxiliary ligand) is investigated in relation to the σ donation and π back-donation components of the M-CO bond and to the electrostatic effect exerted by the ligand-metal fragment. Our analysis encompasses over 30 carbonyls, in which the relative importance of donation, back-donation and electrostatics are varied either through the ligand in a series of [(L)Au(CO)]0/+ gold(i) complexes, or through the metal in a series of anionic, neutral and cationic homoleptic carbonyls. Charge-displacement analysis is used to obtain well-defined, consistent measures of σ donation and π back-donation charges, as well as to quantify the σ and π components of CO polarization. It is found that all complexes feature a comparable charge flow of σ symmetry (both in the M-CO bonding region and in the CO fragment itself), which is therefore largely uncorrelated to CO response. By contrast, π back-donation is exceptionally variable and is found to correlate tightly with the change in CO bond distance, with the shift in CO stretching frequency, and with the extent and direction (C → O or C ← O) of the CO π polarization. As a result, we conclusively show that π back-donation can be an important bond component also in non-classical carbonyls and we provide the framework in which the spectroscopic data on coordinated CO can be used to extract quantitative information on the π donor properties of metal-ligand moieties.

CO adsorption on a silica bilayer supported on Ru(0001)

Silica bilayers are built up of two layers of corner sharing SiO4–tetrahedra and constitute an inert ultra-thin membrane supported on the Ru(0001) surface. We have investigated the adsorption of CO on that system using DFT with inclusion of dispersion corrections. The molecules adsorb at the interface between the SiO2 film and Ru(0001) surface. The estimated barrier for diffusion of CO through the silica bilayer is around 0.5eV. The CO bond length, the C–O stretching frequency and the silica–ruthenium distance depend strongly on the CO coverage. The band observed at 2051cm−1 in previous experiments can be assigned to a CO coverage of around 0.5 ML on Ru(0001), with the silica bilayer floating above the CO molecules.

Cobalt(II) complexes containing N′-substituted N,N′,N-bis((1H-pyrazol-1-yl)methyl)amine ligands: The formation of four-coordinate or five-coordinate complexes as a function of the N′-substitution group in N,N′,N-bis((1H-pyrazol-1-yl)methyl)amine

N,N-bis((1H-pyrazol-1-yl)methyl)(cyclohexyl)methanamine (LA), N,N-bis((1H-pyrazol-1-yl)methyl)-2,6-diethylbenzenamine (LB), N,N-bis((1H-pyrazol-1-yl)methyl)-4-bromobenzenamine (LC), N,N-bis((1H-pyrazol-1-yl)methyl)cyclohexanamine (LD), N,N-bis((1H-pyrazol-1-yl)methyl)(furan-2-yl)methanamine (LE), N,N-bis((1H-pyrazol-1-yl)methyl)-2-methoxyethanamine (LF), and N,N-bis((1H-pyrazol-1-yl)methyl)-3-(methylthio)propan-1-amine (LG) with [CoCl2·6H2O] in ethanol yield a novel series of Co(II) chloride complexes; i.e. [LnCoCl2] (Ln=LA–LG), respectively. The molecular structures of [LnCoCl2] (Ln=LA–LC) and [LnCoCl2] (Ln=LD–LG) exist as monomeric four-coordinated and five-coordinated complexes, respectively. [LnCoCl2] (Ln=LA–LC) showed a distorted tetrahedral geometry involving non-coordination of the nitrogen atom of the N′-substitution amine moiety and the Co(II) centre, resulting in the formation of an eight-membered chelate ring. The geometry at each Co(II) centre in [LnCoCl2] (Ln=LD–LG) were best described as a distorted trigonal bipyramid involving coordination of the nitrogen atom of the N′-substitution amine group and the Co(II) centre. Specifically, a distorted trigonal bipyramid was achieved in [LDCoCl2] through a coordinative interaction of the nitrogen atom of the cyclohexylamine unit and the Co(II) atom with an σ plane of symmetry based on the bond length of Co–Ncyclcohexylamine [2.466(4)Å]. Moreover, [LDCoCl2] showed the highest activity for methyl methacrylate (MMA) polymerization in the presence of modified methylaluminoxane (MMAO) at 60°C and yielded poly(methylmethacrylate) (PMMA) with a high molecular weight (Mw) and narrower polydispersity index (PDI) compared to the other Co(II) complexes. However, all Co(II) complexes produced syndiotactic (PMMA), characterised using 1H NMR spectroscopy, with ca. 0.70.

Ab initio theoretical reinvestigation of the ground and excited state properties of silylated coumarins: Good candidates for solid state dye lasers and dye-sensitized solar cells

We present ab initio theoretical calculations of various properties of the ground and excited states of basic coumarin (1) and its derivatives: 4-methylcoumarin (2), 7-aminocoumarin (3), 7-amino-4-methylcoumarin or coumarin 120 (4), 4-trifluoromethylcoumarin (5), 7-amino-4-trifluoromethylcoumarin or coumarin 151 (6), silylated coumarin 120 (7) and silylated coumarin 151 (8). We calculate the following: (i) ground and excited state dipole moments (ii) energies and locations of HOMOs and LUMOs (iii) SCF total energies of ground state (iv) excitation energies with oscillator strengths for first six excited states (v) CO and CN bond lengths in ground and excited states (vi) ground state thermodynamic and electronic properties. The ground and excited state properties of coumarins 1–8 are obtained within the framework of density functional theory using B3LYP and long-range-corrected (LRC) ωB97X-D functionals with 6–31G(d,p) basis set. A detailed comparative analysis of different photo physical and electronic properties of silylated and unsilylated coumarins is made. On the basis of theoretical results we find many interesting features of silylation process and we can conclude that silylation will result in better long-term photo and thermodynamic stability compared to its unsilylated counterpart due to increase in the values of thermodynamic parameters like SCF total energy, G0 and H0, etc. Therefore, silylated molecules may become good candidates for solid state dye lasers and dye sensitized solar cells. In contrast, we find that both the functional B3LYP and LRC-ωB97X-D predict nearly the same results for electronic, thermodynamic and photo physical properties of studied coumarins 1–8 in their ground states but B3LYP hybrid functional severely overestimates excited state dipole moments, underestimates vertical excitation energies, oscillator strengths, CO and CN bond lengths of studied coumarins. On the basis of our theoretical results we conclude that LRC-ωB97X-D functional must be used for prediction of excited state properties of a molecule.

Facile synthesis of CoX (X = S, P) as an efficient electrocatalyst for hydrogen evolution reaction

HER catalytic activity of CoX (X = S, P) nanocatalysts prepared through a facile and controllable synthesis by the chemical conversion of thin Co(OH)2 nanoplates was studied. The better HER performance of CoP could be derived from its intrinsically positive charged nature of the metal center Co, the long bond length of Co–P and the abundant catalytic active sites toward HER.

A new copper(II) Schiff base complex containing asymmetrical tetradentate N2O2 Schiff base ligand: Synthesis, characterization, crystal structure and DFT study

A new copper (II) Schiff base complex, CuL1, was prepared from the reaction of asymmetrical Schiff base ligand of L1 and Cu(OAC)2 (L1=salicylidene imino-ethylimino-pentan-2-one). The Schiff base ligand, L1, and its copper (II) complex, CuL1, have been characterized by elemental analysis (CHN) and FT-IR and UV–vis spectroscopy. In addition, 1H NMR was employed for characterization of the ligand. Thermogrametric analysis of the CuL1 reveals its thermal stability and its decomposition pattern shows that it is finally decomposed to the copper oxide (CuO). The crystal structure of CuL1 was determined by the single crystal X-ray analysis. The CuL1 complex crystallizes in the monoclinic system, with space group P21/n and distorted square planar coordination around the metal ion. The Schiff base ligand of L1 acts as a chelating ligand and coordinates via two nitrogen and two oxygen atoms to the copper (II) ion with C1 symmetry. The structure of the CuL1 complex was also studied theoretically at different levels of DFT and basis sets. According to calculated results the CO bond length of the salicylate fragment is slightly higher than that in the acetylacetonate fragment of ligand, which could be interpreted by resonance increasing between phenyl and chelated rings in ligand in relative to the acetylacetonate fragment.

CO interaction with Cu(I)-MCM-22 zeolite: density functional theory investigation

MCM-22 zeolite has been widely used in many applications for catalysis and adsorption. Especially, this material exchanged with Cu+ cation (Cu(I)-MCM-22) is an active catalyst in green chemical reaction, such as decomposition of NO and N2O. The local geometry of Cu+ in vicinity of Al (III) replacement in six different Si (IV) sites and CO interaction with the most stable Cu+ in each Al site were explored using periodic density functional theory (DFT) method. The CO stretching frequencies were computed applying the ω/r scaling method in which frequencies were determined at high quantum level (couple cluster) and CO bond length calculated at DFT level. The results showed that Cu+ cation located in the channel wall position and intersection position coordinated with 3 or 2 framework oxygen atoms, respectively, before CO adsorption and Cu+ cation coordinated with 2 framework oxygen atoms after CO adsorption. The interaction energies between CO and Cu+ cation were in range - 148 to -195 kJ.mol-1 and CO frequencies exhibit two peaks at 2151 and 2159 cm-1 in good agreement with experimental data. This investigation brought us to understand the Cu+ location in MCM-22 and CO adsorption in Cu(I)-MCM-22 zeolite.

Solvent induced conformational fluctuation of alanine dipeptide studied by using vibrational probes

The solvation effect on the three dimensional structure and the vibrational feature of alanine dipeptide (ALAD) was evaluated by applying the implicit solvents from polarizable continuum solvent model (PCM) through ab initio calculations, by using molecular dynamic (MD) simulations with explicit solvents, and by combining these two approaches. The implicit solvent induced potential energy fluctuations of ALAD in CHCl3, DMSO and H2O are revealed by means of ab initio calculations, and a global view of conformational and solvation environmental dependence of amide I frequencies is achieved. The results from MD simulations with explicit solvents show that ALAD trends to form PPII, αL, αR, and C5 in water, PPII and C5 in DMSO, and C5 in CHCl3, ordered by population, and the demonstration of the solvated structure, the solute–solvent interaction and hydrogen bonding is therefore enhanced. Representative ALAD–solvent clusters were sampled from MD trajectories and undergone ab initio calculations. The explicit solvents reveal the hydrogen bonding between ALAD and solvents, and the correlation between amide I frequencies and the CO bond length is built. The implicit solvents applied to the ALAD–solvent clusters further compensate the solvation effect from the bulk, and thus enlarge the degree of structural distortion and the amide I frequency red shift. The combination of explicit solvent in the first hydration shell and implicit solvent in the bulk is helpful for our understanding about the conformational fluctuation of solvated polypeptides through vibrational probes.

DFT study of the rearrangement of 5-oxymethyl-1,3-oxathiolane-2-imine to thiiran-2-ylmethyl carbamate

Rearrangement of 5-oxymethyl-1,3-oxathiolane-2-imine under alkali conditions was studied by B3LYP/6-311+G(d) method. Calculations showed that only the pathway leading to formation of thiirane is possible. Oxymethyl-1,3-oxathiolane-2-imine intermediate can be formed by deacylation of the corresponding acyloxymethyl derivative or after cyclization of unstable 2-oxyranylmethyl N,N-dimethylthiocarbamate. The deacylation pathway includes anionic species whereas the rearrangement via thiocarbamate goes through zwitterionic and neutral species. For the first way, the rearrangement of oxymethyl-1,3-oxathiolane-2-imine to (2-imino-1,3-dioxolan-4-yl)methanethiol through bicyclic transition state has the highest energy barrier. For the second way, the rearrangement of 2-oxyranylmethyl thicarbamate to oxymethyl-1,3-oxathiolane-2-imine has the highest energy barrier. The anionic and neutral bicyclic structures are differing in CO and CS bond lengths and in ellipticity of the carbon–nitrogen double bond.

Chelated Assisted Metal-Mediated N–H Bond Activation of β-Lactams: Preparation of Irida-, Rhoda-, Osma-, and Ruthenatrinems

2-Azetidinones substituted with pyridine (2a), quinoline (2b), isoquinoline (2c), imidazole (2d), and benzimidazole (2e) at the 4-position of the four-membered ring have been prepared in order to synthesize tribactams containing a transition metal and its associated ligands, LnM, at the 2-position of the tricyclic skeleton. The developed procedure is compatible with a wide range of transition-metal starting complexes. Thus, the iridium and rhodium dimers [M(η5-C5Me5)Cl2]2 react with 2a–e, in the presence of sodium acetate, to afford irida- and rhodatrinems (1a–j) containing the half-sandwich d6 metal fragments M(η5-C5Me5)Cl (M = Ir, Rh). The reactions of [M(μ-OMe)(η4-COD)]2 (M = Ir, Rh) with 2a lead to irida- and rhodatrinems (1k,l) with the d8 moieties M(η4-COD). The coordination sphere and oxidation state of the metal center in these compounds can be modified, without affecting the 2-azetidinone backbone, by means of substitution and oxidative addition reactions. As a proof of concept, metallatrinems with the M(CO)2 (M = Ir (1m), Rh (1n)) and Ir(Me)I(CO)2 (1o) units are also reported. Osmatrinems 1p,q containing the d4 metal fragment OsH3(PiPr3)2 have been obtained starting from the d2 hexahydride OsH6(PiPr3)2, by reaction with 2a,b, whereas the treatment of the tetrahydroborate complexes MH(η2-H2BH2)(CO)(PiPr3)2 (M = Os, Ru) with 2a yields osma- and ruthenatrinems (1r,s) containing six-coordinate bis(phosphine) d6 metal fragments. The IR stretching frequency of the lactamic carbonyl, the bent angle between the five- and four-membered rings of the tricycle, and the N–CO bond length in the lactamic ring are clearly infuenced by the LnM fragment.