Computational studies into urea formation in the interstellar medium

Formation routes, involving closed shell, radical, and charged species for urea, have been studied using computational methods to probe their feasibility in the interstellar medium. All reactions involving closed shell species were found to have prohibitive barriers. The radical–radical reaction possesses a barrier of only 4 kJ mol−1, which could be surmountable. A charged species based route was also investigated. A barrier of only 8 kJ mol−1 was found in that case, when a partial water ice shell was included

Identifying electron transfer coordinates in Donor-Bridge-Acceptor systems using mode projection analysis

We report upon an analysis of the vibrational modes that couple and drive the state-to-state electronic transfer branching ratios in a model donor-bridge-acceptor system consisting of a phenothiazine-based donor linked to a naphthalene-monoimide acceptor via a platinum-acetylide bridging unit. Our analysis is based upon an iterative Lanczos search algorithm that finds superpositions of vibronic modes that optimize the electron/nuclear coupling using input from excited-state quantum chemical methods. Our results indicate that the electron transfer reaction coordinates between a triplet charge transfer state and lower lying chargeseparated and localised excitonic states are dominated by asymmetric and symmetric modes of the acetylene groups on either side of the central atom in this system. In particular, we find that while a nearly symmetric mode couples both the charge-separation and charge-recombination transitions more or less equally, the coupling along an asymmetric mode is far greater suggesting that IR excitation of the acetylene modes preferentially enhances charge recombination transition relative to charge-separation

Kinetic resolution of 2-Aryl-4-methylenepiperidines toward enantioenriched functionalizable piperidine fragments

The base n-BuLi with sparteine allows a kinetic resolution of N-Boc-2-aryl-4-methylenepiperidines. The 2,2-disubstituted products and recovered starting materials were isolated with high enantiomeric ratios. From VT-NMR spectroscopy and DFT studies, the rate of rotation of the N-Boc group is fast. Lithiation and trapping of the enantioenriched starting materials gave 2,2-disubstituted piperidines with retention of stereochemistry. Functionalization of the 4-methylene group led to a variety of 2,4-disubstituted piperidines without loss of enantiopurity that could be useful building blocks for drug discovery

Tribochemical nanolithography: selective mechanochemical removal of photocleavable nitrophenyl protecting groups with 23 nm resolution at speeds of up to 1 mm s−1

We describe the mechanochemical regulation of a reaction that would otherwise be considered to be photochemical, via a simple process that yields nm spatial resolution. An atomic force microscope (AFM) probe is used to remove photocleavable nitrophenyl protecting groups from alkylsilane films at loads too small for mechanical wear, thus enabling nanoscale differentiation of chemical reactivity. Feature sizes of 20–50 nm are achieved repeatably and controllably at writing rates up to 1 mm s−1. Line widths vary monotonically with the load up to 2000 nN. To demonstrate the capacity for sophisticated surface functionalisation provided by this strategy, we show that functionalization of nanolines with nitrilo triacetic acid enables site-specific immobilization of histidine-tagged green fluorescent protein. Density functional theory (DFT) calculations reveal that the key energetic barrier in the photo-deprotection reaction of the nitrophenyl protecting group is excitation of a π–π* transition (3.1 eV) via an intramolecular charge-transfer mechanism. Under modest loading, compression of the adsorbate layer causes a decrease in the N–N separation, with the effect that this energy barrier can be reduced to as little as 1.2 eV. Thus, deprotection becomes possible via either absorption of visible photons or phononic excitation transfer, facilitating fast nanolithography with a very small feature size

Turning intercalators into groove binders: synthesis, photophysics and DNA binding properties of tetracationic mononuclear ruthenium(ii)-based chromophore-quencher complexes.

The synthesis of two new tetracationic mononuclear RuII complexes containing the tetrapyridyl [3,2-a:2',3'-c:3'',2''-h:2''',3'''-j] phenazine ligand in which the uncoordinated site has been converted into a dicationic ethylene-bipyridyldiylium unit is reported. The structure of the complexes is fully assigned through detailed NMR studies and, in one case, through an X-ray crystallography study. Voltammetry, optical spectroscopy and computational studies confirm that the bipyridyldiylium moiety has a low-lying reduction that quenches the 3MLCT-based emission usually observed in such systems. The new complexes interact with DNA in a quite different manner to their dicationic analogues: they both bind to duplex DNA with micromolar affinity through groove binding. These observations are rationalized through a consideration of their structural and electronic properties

Ultrafast photoinduced charge transport in Pt(II) donor-acceptor assembly bearing naphthalimide electron acceptor and phenothiazine electron donor

Visible light-induced charge transfer dynamics were investigated in a novel transition metal triad acceptor–chromophore–donor, (NDI–phen)Pt(II)(–C[triple bond, length as m-dash]C–Ph–CH2–PTZ)2 (1), designed for photoinduced charge separation using a combination of time-resolved infrared (TRIR) and femtosecond electronic transient absorption (TA) spectroscopy. In 1, the electron acceptor is 1,4,5,8-naphthalene diimide (NDI), and the electron donor is phenothiazine (PTZ), and [(phen)Pt(–C[triple bond, length as m-dash]C–Ph–)], where phen is 1,10-phenanthroline, represents the chromophoric core. The first excited state observed in 1 is a 3MLCT/LL′CT, with {Pt(II)–acetylide}-to-phen character. Following that, charge transfer from the phen-anion onto the NDI subunit to form NDI−–phen–[Pt–(C[triple bond, length as m-dash]C)2]+–PTZ2 occurs with a time constant of 2.3 ps. This transition is characterised by appearance of the prominent NDI-anion features in both TRIR and TA spectra. The final step of the charge separation in 1 proceeds with a time constant of [similar]15 ps during which the hole migrates from the [Pt–(C[triple bond, length as m-dash]C)2] subunit to one of the PTZ groups. Charge recombination in 1 then occurs with two distinct time constants of 36 ns and 107 ns, corresponding to the back electron transfer to each of the two donor groups; a rather rare occurrence which manifests that the hole in the final charge-separated state is localised on one of the two donor PTZ groups. The assignment of the nature of the excited states and dynamics in 1 was assisted by TRIR investigations of the analogous previously reported ((COOEt)2bpy)Pt(C[triple bond, length as m-dash]C–Ph–CH2–PTZ)2 (2), (J. E. McGarrah and R. Eisenberg, Inorg. Chem., 2003, 42, 4355; J. E. McGarrah, J. T. Hupp and S. N. Smirnov, J. Phys. Chem. A, 2009, 113, 6430) as well as (bpy)Pt(C[triple bond, length as m-dash]C–Ph–C7H15)2, which represent the acceptor-free dyad, and the chromophoric core, respectively. Thus, the step-wise formation of the full charge-separated state on the picosecond time scale and charge recombination via tunnelling have been established; and the presence of two distinct charge recombination pathways has been observed

Comparative study of the effect of fuel deoxygenation and polar species removal on jet fuel surface deposition

The effect of near-complete deoxygenation and polar species removal on deposition propensity of a Jet A-1 fuel type, with marginal thermal oxidative stability was studied in a laboratory scale approach. The fuel deoxygenation was carried out via nitrogen purging and two types of bespoke zeolites were used separately in a packed bed reactor for partial polar separation. The treated fuel samples were assessed individually for deposition propensity, using “High Reynolds Thermal Stability(HiReTS)” test device. It was found that when the concentration of hydroperoxides in fuel is relatively high, polar removal is more effective way than the fuel deoxygenation in reducing carbona- ceous deposits. Furthermore, competitive adsorption of dissolved O 2 with polar species was studied for a model fuel doped with a few polar species, as well as for the Jet A-1 with marginal thermal stability, in the packed bed reactor with zeolite 3.7Å. The polar species added to the model fuel share the same functional groups as those in Jet A-1 with a strong im pact on fuel thermal degradation and surface deposition. These include hexanoic acids, heaxnol, hexanal, hexanone, phenyl amine (aniline), butylated hydroxytoluene(BHT), dibutyl disulfide and Fe naphthenate. A one-dimensional model for calculation of dissolved O 2 adsorption in the packed bed reactor was built using COMSOL Multiphysics. The modelling results were in good agreement with the induction period prior to the beginning of the O 2 adsorption, as well as the different stages of O 2 uptake during the competitive adsorption between dissolved O 2 and polar species in the Jet A-1 fuel. The calculation showed a discrep- ancy with the experimental results beyond the second phase of O 2 adsorption. More theories, assumptions and physical sub-models are required to build a more robust pre- dictive model. A new chemical reaction pathway based on the self-reaction of hydroperoxides was proposed as part of “Basic Autoxidation Scheme(BAS)” to justify the relatively high deposition propensity of the marginal fuel after near-complete deoxygenation. The vi- ability of this reaction pathway was supported by the quantum chemistry calculations

Density Functional Theory calculations on copper-mediated peroxide decomposition reactions. Implications for jet fuel autoxidation

The presence of metal impurities in jet fuel can lead to a reduction in the thermal stability of the fuel. Density functional theory (DFT) calculations are reported on the reactions of hydroperoxides with both bare Cu(I) ions and Cu(naphthenate). The reaction of Cu(naphthenate) and cumene hydroperoxide forms one product complex. The release of alkoxy radicals (RO●) from the product complex is energetically feasible. This provides a low-energy route to radical formation when compared to hydroperoxide fission. The reaction mechanisms reported here for the copper-catalyzed hydroperoxide decomposition can be used to improve current chemical kinetic models for fuel autoxidation

Synthesis of enantioenriched spirocyclic 2-arylpiperidines via kinetic resolution

Kinetic resolution of N-Boc-spirocyclic 2-arylpiperidines with spiro substitution at C-4 was achieved with high enantiomeric ratios using the chiral base n-BuLi/sparteine. Cyclopropanation or metallaphotoredox catalysis were used to access the piperidines, which could be further functionalised without loss of enantiopurity, highlighting their use as potential 3D fragments for drug discovery

On the formation of urea in the ISM

Potential routes to the formation of urea were investigated using electronic structure methods. The most likely pathways involve either the reaction of the formamide and amine radicals or involve protonated isocyanic acid as a starting point