The change of the proton magnetic shielding in red- and blue-shifted linear hydrogen-bonded complexes

The change in the proton magnetic shielding constant of FH and FArH on the formation of the vibrationally red-shifted FH... Rg and blue-shifted FArH... Rg (Rg = Ne, Ar). complexes was determined by GIAO ab initio computations at various levels of theory. The blue-shifted FArH... N₂ and red-shifted FArH... P₂ complexes were also studied. The characteristic downfield shift of the isotropic proton magnetic resonance in red-shifted hydrogen-bonded complexes is smaller in the blue-shifted complexes. In FArH... Ne and FArH... N₂ the proton NMR actually shifts to higher fields on complexation. These results are rationalized by considering the changes in the magnetic and electric contributions to the proton shielding in FH and FArH.     

Two isomers of 2,4‐di­benzyl‐8‐azabicyclo­[3.2.1]­octan‐3‐ol

The crystal structures of the title compounds, 2α,4α‐di­benzyl‐3α‐tropanol (2α,4α‐di­benzyl‐8‐methyl‐8‐aza­bi­cyclo­[3.2.1]­octan‐3α‐ol), C₂₂H₂₇NO, (I), and 2α,4α‐di­benzyl‐3β‐tropanol (2α,4α‐di­benzyl‐8‐methyl‐8‐aza­bi­cyclo­[3.2.1]­octan‐3β‐ol), C₂₂H₂₇NO, (II), show that both compounds have a piperidine ring in a chair conformation and a pyrrolidine ring in an envelope conformation. Isomer (I) is asymmetric, the benzyl groups having different orientations, whereas isomer (II) is mirror symmetric, and the N and O atoms, the C atom attached to the hydroxy group, and the methyl C atom attached to the N atom lie on the mirror plane. In the crystal structures of both (I) and (II), the mol­ecules are linked together by intermolecular O—H⋯N hydrogen bonds to form chains that run parallel to the a direction in (I) and parallel to b in (II).     

Multidimensional infrared spectroscopy of water. I. Vibrational dynamics in two-dimensional IR line shapes

In this and the following paper, we describe the ultrafast structural fluctuations and rearrangements of the hydrogen bonding network of water using two-dimensional (2D) infrared spectroscopy. 2D IR spectra covering all the relevant time scales of molecular dynamics of the hydrogen bonding network of water were studied for the OH stretching absorption of HOD in D2O. Time-dependent evolution of the 2D IR line shape serves as a spectroscopic observable that tracks how different hydrogen bonding environments interconvert while changes in spectral intensity result from vibrational relaxation and molecular reorientation of the OH dipole. For waiting times up to the vibrational lifetime of 700 fs, changes in the 2D line shape reflect the spectral evolution of OH oscillators induced by hydrogen bond dynamics. These dynamics, characterized through a set of 2D line shape analysis metrics, show a rapid 60 fs decay, an underdamped oscillation on a 130 fs time scale induced by hydrogen bond stretching, and a long time decay constant of 1.4 ps. 2D surfaces for waiting times larger than 700 fs are dominated by the effects of vibrational relaxation and the thermalization of this excess energy by the solvent bath. Our modeling based on fluctuations with Gaussian statistics is able to reproduce the changes in dispersed pump-probe and 2D IR spectra induced by these relaxation processes, but misses the asymmetry resulting from frequency-dependent spectral diffusion. The dynamical origin of this asymmetry is discussed in the companion paper.     

Density functional theory investigation of the polarizability and second hyperpolarizability of polydiacetylene and polybutatriene chains: treatment of exact exchange and role of correlation

The static polarizability and second hyperpolarizability of increasingly large polydiacetylene and polybutatriene (PBT) chains have been evaluated using the optimized effective potential for exact exchange (OEP-EXX) method developed by Yang and Wu [Phys. Rev. Lett. 89, 143002 (2002)], where the unknown part of the effective potential is expressed as a linear combination of Gaussian functions. Various conventional atomic orbital basis sets were employed for the exchange potential (X basis) as well as for the Kohn-Sham orbitals [molecular orbital (MO) basis]. Our results were compared to coupled-perturbed Hartree-Fock (CPHF) calculations and to ab initio correlated values obtained at various levels of approximation. It turns out that (a) small conventional basis sets are, in general, unsatisfactory for the X basis; (b) the performance of a given X basis depends on the MO basis and is generally improved when using a larger MO basis; (c) these effects are exaggerated for the second hyperpolarizability compared to the polarizability; (d) except for the second hyperpolarizability of PBT chains, using 6-311++G** for the X basis gives reasonable agreement with the CPHF results for all MO basis sets; (e) our results suggest that in the limit of a complete X basis the OEP-EXX values may approach the CPHF data; and (f) in general, the quality of a given conventional X basis degrades with the length of the oligomer, which correlates with the fact that the number of X basis functions becomes a smaller fraction of the number required to reproduce exactly the finite-basis-set Hartree-Fock energies. Linear and especially nonlinear electric field responses constitute a very stringent test for assessing the quality of functionals and potentials; appropriately tailored basis sets are needed to describe the latter. Finally, this study further highlights the importance of electron correlation effects on linear and nonlinear responses, for which correlated functionals with OEP are required.     

Aryl Nitro Reduction with Iron Powder or Stannous Chloride under Ultrasonic Irradiation

The selective reduction of aryl nitro compounds in the presence of sensitive functionalities, including halide, carbonyl, nitrile, and ester substituents, under ultrasonic irradiation at 35 kHz is reported in yields of 39–98%. Iron powder proved superior to stannous chloride with high tolerance of sensitive functional groups and high yields of the desired aryl amines in relatively short reaction times. Simple experimental procedure and purification also make the iron reduction of aryl nitro compounds advantageous over other methods of reduction.     

A Stereoselective Synthesis of (±) Exobrevicomin; Synthetic Applications of Gamma-Alkoxy Allylboronates

A synthesis of the naturally occurring bicyclic acetal, “exobrevicomin” is described. The requisite threo stereochemistry of keto diol precursors is obtained selectively by condensation of a gamma-alkoxy allylboronate with an appropriate aldehyde.     

The multifarious world of transition metal hydrides

Transition metal (TM) hydrides display a remarkable range of bonding types, encompassing classical M-H moieties, dihydrogen complexes containing the eta 2-H2 ligand, and trihydrides which display quantum mechanical site exchange. Furthermore, C-H, Si-H and B-H moieties can bind to TM centres in an eta 2-manner, to give sigma-bond complexes with a spectrum of M...H contributions. In addition to these primary bonding modes, TM complexes also indulge in a wide spectrum of hydrogen-bonding interactions, including both M...H-X and the unique type M-H...H-X. This review begins with a historical perspective of the development of TM hydride chemistry, and proceeds to focus on three significant developments of the past two decades: the discovery of sigma-bond and dihydrogen complexes, the involvement of TM hydrides in hydrogen bonding, and the role played by quantum mechanical phenomena in the chemistry and dynamics of TM hydrides. The account concludes with an overview of the inter-relationship between these apparently disparate novel aspects of TM hydride chemistry.     

Second Dissociation Constant of Bis-[(2-hydroxyethyl)amino]acetic Acid (BICINE) and pH of Its Buffer Solutions from 5 to 55 °C

The values of the second dissociation constant, K ₂, and related thermodynamic quantities of the ampholyte bis[(2-hydroxyethyl)amino]acetic acid (BICINE) have been determined at temperatures from 5 to 55 ^∘C. The pH values of six equimolal buffer solutions, and four buffer solutions having ionic strengths (I = 0.16 mol⋅kg⁻¹) similar to those in blood plasma, have been evaluated at 12 temperatures from 5 to 55 ^∘C using the Bates–Guggenheim convention. The liquid junction potentials (E _j ) between the buffer solutions of BICINE and saturated KCl solution of the calomel electrode at 25 and 37 ^∘C have been estimated by measurement with a flowing junction cell. These values of E _j have been used to ascertain the operational pH values at 25 and 37 ^∘C. The pK ₂ values at 25 and 37 ^∘C are 8.333 and 8.156, respectively. The thermodynamic quantities associated with the second acid dissociation have been calculated from the values of pK ₂ as a function of temperature. The zwitterionic buffer BICINE was shown to be useful as a pH standard in the region close to that of blood serum.     

Selected-area growth of carbon nanotubes by the combination of focused ion beam and chemical vapor deposition techniques.

In this article, we report a technique for growing carbon nanotubes in a more controllable fashion, which enables us to synthesize nanotubes directly in various forms of designed patterns. This nanofabrication process is based on a combination of focused ion beam (FIB) and chemical vapor deposition (CVD) techniques. In this process, arrays of conductive patterns were first deposited on silicon substrates by directing a gaseous compound (C(9)H(16)Pt) via the capillary needle-sized nozzles within a FIB system. The substrates were then coated with catalyst and further modified by the FIB to localize the position of the catalyst. Finally, the growth of carbon nanotubes on the designed substrates was carried out by CVD of hydrocarbon gases. This fabrication technique has the advantage of positioning carbon nanotubes in selected locations. This may open up opportunities for the direct synthesis of carbon nanotubes onto almost any substrate material, thus allowing fabrication of carbon nanotube-based devices.     

Sugar radicals formed by photoexcitation of guanine cation radical in oligonucleotides

This work presents evidence that photoexcitation of guanine cation radical (G+*) in dGpdG and DNA-oligonucleotides TGT, TGGT, TGGGT, TTGTT, TTGGTT, TTGGTTGGTT, AGA, and AGGGA in frozen glassy aqueous solutions at low temperatures leads to hole transfer to the sugar phosphate backbone and results in high yields of deoxyribose radicals. In this series of oligonucleotides, we find that G+* on photoexcitation at 143 K leads to the formation of predominantly C5'* and C1'* with small amounts of C3'*. Photoconversion yields of G+* to sugar radicals in oligonucleotides decreased as the overall chain length increased. However, for high molecular weight dsDNA (salmon testes) in frozen aqueous solutions, substantial conversion of G+* to C1'* (only) sugar radical is still found (ca. 50%). Within the cohort of sugar radicals formed, we find a relative increase in the formation of C1'* with length of the oligonucleotide, along with decreases in C3'* and C5'*. For dsDNA in frozen solutions, only the formation of C1'* is found via photoexcitation of G+*, without a significant temperature dependence (77-180 K). Long wavelength visible light (>540 nm) is observed to be about as effective as light under 540 nm for photoconversion of G+* to sugar radicals for short oligonucleotides but gradually loses effectiveness with chain length. This wavelength dependence is attributed to base-to-base hole transfer for wavelengths >540 nm. Base-to-sugar hole transfer is suggested to dominate under 540 nm. These results may have implications for a number of investigations of hole transfer through DNA in which DNA holes are subjected to continuous visible illumination.