Improving on equilibrium isotropic nuclear shielding constants

Isotropic nuclear shielding constants at the equilibrium molecular structure σ_eq and zero‐point vibrational corrections (ZPVCs) to σ_eq are evaluated using the B3LYP/aug‐cc‐pVTZ level of theory, as well as the KT2/aug‐cc‐pVTZ level of theory. Various scaling factors and systematic corrections are obtained by linear regression to experimental shielding constants. Comparisons of the scaled and systematically corrected equilibrium and vibrationally averaged shielding constants reveal that, at the 99% confidence level, the ZPVCs via second‐order perturbation theory do not improve the agreement of B3LYP/aug‐cc‐pVTZ and KT2/aug‐cc‐pVTZ calculated shielding constants with experiment. This holds true when the same analysis is applied to CCSD(T)/aug‐cc‐pCV[TQ]Z calculated σ_eq of Teale et al. [Journal of Chemical Physics 2013, 138, 024111]. In addition, at the 99% confidence level, B3LYP/aug‐cc‐pVTZ and KT2/aug‐cc‐pVTZ scaled and systematically corrected shielding constants are found to be statistically no different from CCSD(T)/aug‐cc‐pCV[TQ]Z calculated σ_eq. The use of scaling factors and systematic corrections could thus provide a cheaper but yet reasonably accurate alternative for the study of nuclear shielding constants of larger systems.     

Effects of introduction of α‐carboxylate,N‐methyl,and N‐formyl groups on intramolecular cyclization of o‐quinone amines: Density functional theory‐based study

o‐Quinone amines, which are relevant to various biological processes, can undergo spontaneous intramolecular cyclization (ring closure reaction by amino‐terminated hydrocarbon side chain) that deactivates them toward another possible reactions, that is, thiol binding. Density functional theory‐based calculation is employed for obtaining the potential energy curves along the C? N bond formation in the intramolecular cyclization of various o‐quinone amines, viz., dopaminequinone, dopaquinone, N‐methyl‐dopaminequinone, N‐formyl‐dopaminequinone, and the corresponding methylene‐inserted analogues. The activation barrier is decreased by introduction of α‐carboxylate and N‐methyl group whereas increased by introduction of N‐formyl group. A negative correlation between the activation barriers and the level of highest occupied molecular orbital is pointed out. Furthermore, the methylene‐inserted analogues show decreased activation barriers. This is explained by reduction of steric repulsion in the transition state.     

Synthesis and spectroscopic studies of highly fluorescent,solvatochromic diastereomers with differentially stacked bithiophene-substituted quinoxaline rings

Diastereomeric C-shaped molecules containing closely stacked bithiophene-substituted quinoxaline rings were synthesized and characterized by NMR, UV–vis absorption, and fluorescence spectroscopy. The unique geometry of each diastereomer resulted in different degrees of π-overlap between the bithiophene-substituted quinoxaline ring chromophores, modulating their spectroscopic properties. The donor-acceptor nature of this chromophore gave rise to its positive solvatochromism. ¹H NMR and UV–vis absorption spectroscopy confirmed the existence of π-π interactions in the ground state between the quinoxaline rings in both molecules but between the bithiophene rings only in the syn isomer. They exhibited significant emission maxima bathochromic shifts, a strong, positive solvatochromism, increased band broadening, and larger Stokes shifts when compared to a compound with an unstacked chromophore. Additionally, the syn isomer consistently showed λ_max,em value red-shifts and larger band broadening and Stokes shifts compared to the anti isomer due to the greater π-overlap in the syn isomer.     

Two small molecular propellers and their rotational potential energy surfaces

Molecular propellers based upon the twisting of a disulfide bond are analyzed here as the locomotion source for fullerene nanoparticles. The HC(CCHSSHCC)₃CH and related HC(CCHSSNC)₃CH bicyclic compounds are optimized and linked to pyracyclene functioning as a model fullerene surface. It is shown that steric hinderance from the hydrogen atoms on both the bottom of the propeller blade and the linker to the fullerene surface can have significant effects on the rotational potential energy surface. Replacing the bottom CH groups on the molecular propeller with nitrogen atoms not only reduces these barriers significantly, but this action creates a strongly dipolar molecule in HC(CCHSSNC)₃CH. Such a system would be responsive to and controllable with an external, rotating, magnetic or electric field. Endohedral fullerenes have known applications for targeted delivery, especially in nanomedicine. Providing further control with molecular propellers could enhance the feasibility and use of these technologies.     

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 Å and an N–CO bond length of 1.387 Å. The calculated transition state for inversion of the amide ring retains very little amide resonance (N–CO, 1.440 Å). Some of this loss is compensated through tightening of the N–Si bond (2.422 Å), 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.     

Nematic order by disorder in spin-2 Bose-Einstein condensates

We show that quantum and thermal fluctuations in spin-2 Bose-Einstein condensates lift the accidental degeneracy of the mean-field phase diagram. Fluctuations select the uniaxial (square biaxial) nematic state for scattering lengths a4>a2 (a4相似文献   

The use of physical and virtual infrastructures for the validation of algal cryopreservation methods in international culture collections

Two cryopreservation methods, colligative cryoprotection coupled with controlled cooling and vitrification-based, encapsulation-dehydration were validated by five members of the EU research infrastructure consortium, COBRA, and two independent external validators. The test strain Chlorella vulgaris SAG 211-11b was successfully cryopreserved using two-step cooling employing passive (Mr Frosty) and Controlled Rate Freezers (CRF) attaining the desired recovery target within 15% of the median viability level (94%). Significant differences (p < 0.05) between cooling regimes were observed where Mr Frosty was more variable (Inter-Quartile Range being 21.5%, versus 13.0% for CRF samples). Viability assessment using fluorescein diacetate gave significantly (P < 0.0001) higher survival than growth in agar with median values being 96% and 89%, respectively. On employing encapsulation-dehydration, greater variability between some validators was observed, with six labs observing recovery in 100% of the beads (84-95% of cells surviving) and one lab observing survival in 80% of the treated beads. Bead disruption followed by algal growth in agar was considered the most reliable and accurate method of assessing cell survival for encapsulation-dehydration.     

A minimalist approach to understanding the efficiency of mononuclear Zn(II) complexes as catalysts of cleavage of an RNA analog

Mononuclear complexes between Zn(2+) and the following four macrocycles were prepared: 1,4,7,10-tetraazacyclododecane (1), 1-oxa-4,7,10-triazacyclododecane (2), 1,5,9-triazacyclododecane (3) and 1-hydroxyethyl-1,4,7-triazacyclononane (4). The pH rate profiles of values of the observed second-order rate constant log (k(Zn))(app) for Zn(X)(OH(2))-catalyzed cleavage (X = 1, 2, 3 and 4) of 2-hydroxypropyl-4-nitrophenyl phosphate (HpPNP) show downward breaks centered at the pK(a) for ionization of the respective zinc bound water. At low pH, where the rate acceleration for the catalyzed reaction is largest, the stabilizing interaction between the catalyst and the bound transition state is 5.7, 7.4, 7.4 and 5.9 kcal mol(-1) for the reactions catalyzed by Zn(1)(OH(2)), Zn(2)(OH(2)), Zn(3)(OH(2)) and Zn(4)(OH(2)), respectively. The interactions between the metal cation and the macrocycle cause either a modest increase or reduction in transition state stabilization compared with 6.6 kcal mol(-1) stabilization for catalysis by Zn(OH(2))(6). The best Zn(II)-macrocycle catalysts are those for which the interactions between the metal ion and macrocycle are the weakest. Inhibition studies show that each of the four catalysts form complexes with phosphate and oxalate dianions with a much higher affinity than diethyl phosphate monoanion, consistent with stronger interaction of the catalysts with the transition state dianion compared with the substrate monoanion HpPNP. The pH-dependence of methyl phosphate inhibition of Zn(2) catalyzed cleavage of HpPNP shows that only the Zn(2)(OH(2)) species binds the inhibitor. This result is consistent with a mechanism that has Zn(2)(OH(2)) as the active catalytic species.     

Design and synthesis of an array of selective androgen receptor modulators

We describe the design, using shape comparison and fast docking computer algorithms, and rapid parallel synthesis of a 1300 member array based on GSK7721, a 4-aminobenzonitrile androgen receptor (AR) antagonist identified by focused screening of the GSK compound collection. The array yielded 352 submicromolar and 17 subnanomolar AR agonists as measured by a cell-based reporter gene functional assay. The rapid synthesis of a large number of active compounds provided valuable information in the optimization of AR modulators, which may be useful in treating androgen deficiency in aging males.     

Ruthenium(II)-catalyzed cyclization of azabenzonorbornadienes with alkynes

The ruthenium-catalyzed cyclization of azabenzonorbornadienes with alkynes leads to an unanticipated dihydrobenzoindole framework. Depending on the structure of the alkyne and the Ru catalyst, either a dihydrobenzoindole and/or a [2+2] cycloaddition product could be formed. Cp*Ru(COD)Cl was found to be an active catalyst for the cyclization of an azabenzonorbornadiene with a propargylic alcohol to produce the dihydrobenz[g]indole as a single regio and stereoisomer in good yield. For other alkynes, selective formation of the dihydrobenz[g]indole is possible by using a cationic Ru catalyst, [Cp*Ru(CH3CN)3]PF6.