Melting point suppression in new lanthanoid(III) ionic liquids by trapping of kinetic polymorphs: an in situ synchrotron powder diffraction study

The complexes (N(4444))(3)[Ln(dcnm)(6)] (Ln = La-Nd, Sm; N(4444) = tetrabutylammonium) display a decrease in the melting point upon fast cooling from a melt, which is shown by in situ synchrotron based X-ray powder diffraction to be due to the formation of a second, less thermodynamically stable, polymorph.     

Distortion-product otoacoustic emission suppression growth in normal and noise-exposed rabbits

This study investigated noise-induced changes in suppression growth (SG) of distortion product otoacoustic emissions (DPOAEs). Detailed measurements of SG were obtained in rabbits as a function of f2 frequencies at four primary-tone levels. SG measures were produced by using suppressor tones (STs) presented at two fixed distances from f2. The magnitude of suppression was calculated for each ST level and depicted as contour plots showing the amount of suppression as a function of the f2 frequency. At each f2, SG indices included slope, suppression threshold, and an estimate of the tip-to-tail value. All suppression measures were obtained before and after producing a cochlear dysfunction using a monaural exposure to a 2-h, 110-dB SPL octave-band noise centered at 2 kHz. The noise exposure produced varying amounts of cochlear damage as revealed by changes in DP-grams and auditory brainstem responses. However, average measures of SG slopes, suppression thresholds, and tip-to-tail values failed to mirror the mean DP-gram loss patterns. When suppression-based parameters were correlated with the amount of DPOAE loss, small but significant correlations were observed for some measures. Overall, the findings suggest that measures derived from DPOAE SG are limited in their ability to detect noise-induced cochlear damage.     

Sound speed correction in ultrasound imaging

A constant sound speed of 1.54 mm/micros is generally used by ultrasound imaging systems for delay and timing. However, the body's sound speed in-homogeneity can lead to defocusing and increased clutter. To provide an improvement using standard transducers, the sound speed used in delay and timing was computed using different sound speeds. We observed improvement in lateral resolution and clutter in phantom, OB, abdominal, and breast imaging. In OB and abdominal imaging using a 4 MHz curved array, 1.48 mm/micros provided higher image quality in many situations. In breast with an 8 MHz linear array, 1.44 mm/micros provided better images in some cases. To provide an automated way to determine and adjust the sound speed used by the imaging system, an algorithm was developed that determines the sound speed that produces the best overall lateral image quality by analyzing the spatial frequency content in a single B-mode frame of channel data using images reconstructed using various trial sound speeds. The metric produced correlates well with the observed best lateral image quality.     

Determination of the rate constant for the radical-radical reaction NCO(X2Pi) + CH3(X2A2 ') at 293 K and an estimate of possible product channels

The rate constant for the reaction of the cyanato radical, NCO(X2Pi), with the methyl radical, CH3(X2A2' '), has been measured to be (2.1 +/- 1.3(-0.80)) x 10(-10) cm3 molecule(-1) s(-1), where the uncertainty includes both random and systematic errors at the 68% confidence level. The measurements were conducted over a pressure range of 2.8-4.3 Torr of CH4 and at a temperature of 293 +/- 2 K. The radicals were generated by the 248-nm photolysis of ClNCO in a large excess of CH4. The subsequent rapid reaction, Cl + CH4, generated the CH3 radical. The rate constant for the Cl + CH4 reaction was measured to be (9.2 +/- 0.2) x 10(-14) cm3 molecule(-1) s(-1), where the uncertainty is the scatter of one standard deviation in the data. The progress of the reaction was followed by time-resolved infrared absorption spectroscopy on single rovibrational transitions from the ground vibrational level. Multiple species were detected in these experiments, including NCO, CH3, HCl, C2H6, HCN, HNC, NH, and HNCO. Temporal concentration profiles of the observed species were simulated using a kinetic model, and rate constants were determined by minimizing the sum of the squares of the residuals between experimental observations and model calculations. Both HCN and HNC seem to be minor products (<0.3% each) of the NCO + CH3 reaction. The peak concentrations of NH and HNCO were small, accounting for <1% of the initial NCO concentration; however, their temporal profiles could not be fit by the model kinetics. The observed C2H6 temporal profile always peaked at significantly higher concentrations than the model predictions, and several reaction models were constructed to help explain these observations. The most likely product channel seems to be the recombination channels, producing CH3NCO and CH3OCN.     

Excited-state structural dynamics of cytosine from resonance Raman spectroscopy

Cytosine, a nucleobase found in both DNA and RNA, is known to form photoproducts upon UV irradiation, damaging the nucleic acids and leading to cancer and other diseases. To determine the molecular mechanism by which these photoproducts occur, we have measured the resonance Raman spectra of cytosine at wavelengths throughout its 267 nm absorption band. Self-consistent analysis of the resulting resonance Raman excitation profiles and absorption spectrum using a time-dependent wave packet formalism yields both the excited-state structural changes and electronic parameters. From this analysis, we have been able to determine that, at most, 31% of the reorganization energy upon excitation is directed along photochemically relevant modes.     

Role of donor and secondary interactions in the structures and thermal properties of alkaline-earth and rare-earth metal pyrazolates

The addition of neutral coligands to reduce the aggregation and improve the volatility of potential heavy alkaline-earth metal chemical vapor deposition (CVD) precursors has typically resulted in liberation of the coligand upon heating. A new series of dinuclear alkaline-earth and rare-earth metal pyrazolates, bis[bis(3,5-di-tert-butylpyrazolato)(tetrahydrofuran)calcium] (1), bis[bis(3,5-di-tert-butylpyrazolato)(tetrahydrofuran)strontium] (2), and bis[bis(3,5-di-tert-butylpyrazolato)bis(tetrahydrofuran)barium] (3), have been obtained from our previous donor-free oligonuclear complexes [{M(3,5-tBu2pz)2}n] (5, M = Ca, n = 3; 6, M = Sr, n = 4; 7, M = Ba, n = 6) by treatment with tetrahydrofuran (THF). Compounds 1-3, as well as the europium analogue bis[bis(3,5-di-tert-butylpyrazolato)(tetrahydrofuran)europium(II)] (4), can also be prepared by direct reaction of the metals and pyrazole in THF and anhydrous liquid ammonia. Recrystallization from hexane led to single crystals of 2-4, while the powder diffraction pattern of 1 revealed it to be isostructural with the previously published bis[bis(3,5-di-tert-butylpyrazolato)(tetrahydrofuran)ytterbium(II)] (8), providing important insight into differences and similarities between the two groups of metals. Detailed structural analysis of the compounds reveals secondary interactions including pi-bonding and agostic interactions, which are considered essential in stabilizing the metal complexes. The direct comparison of structural features and thermal properties (as evaluated by thermogravimetric analysis and sublimation studies) of the donor-free oligonuclear and the donor-containing dinuclear species offers a better understanding of the role of donors and secondary interactions.     

The Effect of Tryptophan on UV-induced DNA Photodamage

Trp–DNA adducts resulting from UV irradiation of pyrimidine bases and nucleotides in the presence of tryptophan (Trp) have been the subject of previous research. However, the relative yield of the adducts compared with the UV screening effect of Trp has not been previously considered. To determine whether Trp–DNA adduct formation or absorption “screening” by Trp is the predominant process when DNA solutions are irradiated with UV light in the presence of Trp, we irradiated Trp-containing DNA oligonucleotide solutions with UVC light and incubated aliquots of those solutions with molecular beacons (MBs) to detect the damage. We observed a rapid decay of fluorescence of the MBs for pure DNA solutions, thereby indicating damage. However, in the presence of Trp, the fluorescence decay is prolonged, with time constants that increase exponentially with Trp concentration. The results are discussed in terms of a beneficial in vivo cellular protection rather than harmful adduct formation and suggest a net sacrificial absorption of UV light by Trp which actually protects the DNA from UV damage.     

Metal–π Interactions Dominate in the Solid‐State Structures of Molecular Heterobimetallic Alkali‐Metal–Europium(II) Aryloxo Complexes

Treatment of Eu metal, 2,6‐diphenylphenol (HOdpp), and MOdpp (M=Na, K) at elevated temperature in the presence of mercury afforded heterobimetallic complexes which were structurally characterized after crystallization from toluene. The structures of [MEu(Odpp)₃]?nPhMe (M=Na, n=1, 1 ; K, n=2.5, 2 ) consist solely of bridging aryloxide ligands and feature extensive π‐Ph–metal interactions. Rather than a heterobimetallic species, treatment of Eu metal and HOdpp with LiOdpp under similar conditions afforded a number of products, including a mixed‐valent europium complex, [Eu₂(Odpp)₃][Eu(Odpp)₄]?4 PhMe ( 3 ). The structural framework of the [Eu₂(Odpp)₃]⁺ cation of 3 is similar to that of the molecular heterobimetallics 1 and 2 , including the presence of π‐Ph–Eu interactions. The reluctance of the reaction to provide a Eu/Li heterobimetallic complex was exemplified by the simultaneous crystallization of [Eu₂(Odpp)₄]?PhMe ( 6 ) and the homoleptic cubane [Li₄(Odpp)₄]?2 C₆H₁₄ ( 5 ) from toluene/hexane.     

Mono‐, Di‐, Tri‐ and Tetranuclear Rare Earth Complexes Obtained Using a Moderately Bulky Aryloxide Ligand

Redox transmetallation ligand exchange reactions involving a rare earth metal, 2,4,6‐trimethylphenol (HOmes), and a diarylmercurial afford rare earth aryloxo complexes, which are structurally characterized. Both the lanthanoid contraction and the identity of the reaction solvent are found to influence the outcome of the reactions. Using THF in the reaction affords a dinuclear species [Ln₂(Omes)₆(thf)₄]?2THF (Ln=La 1 , Nd 2 ) for the lighter rare earth metals, while a mononuclear species [Ln(Omes)₃(thf)₃] (Ln=Sm 3 , Tb 5 , Er 6 , Yb 7 , Y 8 ) is obtained for the heavier rare earth elements. Surprisingly, there is no change in metal coordination number between the two structural motifs. A divalent trinuclear linear complex [Eu₃(Omes)₆(thf)₆] 4 is obtained for Eu, and features solely bridging aryloxide ligands. Using DME as the reaction solvent affords [La(Omes)₃(dme)₂] 9 from the reaction mixture, and [Ln₂(Omes)₆(dme)₂]?PhMe (La 10 , Nd 11 ) and [Y(Omes)₃(dme)₂] 14 following crystallization of the crude product from toluene. The dinuclear species [Eu₂(Omes)₄(dme)₄] 12 contains two unidentate and two chelating DME ligands, and contrasts the linear structure of 4 . Treatment of HOmes and HgPh₂ with Yb metal in DME affords the mixed valent Yb^II/III complex [Yb₂(Omes)₅(dme)₂] 13 , which is stabilized by an intramolecular π‐Ph–Yb interaction, and is a rare example of a mixed valent rare earth aryloxide. Treatment of Er metal with HOmes at elevated temperature (solvent free) affords the homoleptic [Er₄(Omes)₁₂] 15 , which consists of a tetranuclear array of Er atoms arranged in a ‘herringbone’ fashion; the structure is stabilized by intramolecular π‐Ph–Er interactions. Reaction of La metal with HOmes under similar conditions yields toluene insoluble “La(Omes)₃”, which affords 1 following extraction with THF.