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1.
By deuteration of the OH group, it has been shown that two bands, at 1665 and 1630 cm−1 (CCl4), in the IR spectra of 5-hydroxychromones are associated with the H-bonded CO stretching vibration. Nuclear deuteration of 5-hydroxy-2-methylchromone (1) under acidic conditions gave a tri- and a hexadeutero derivative; the latter, isotopically substituted at C-3, shows a single CO band at 1649 cm−1. Hydrolysis of 3-acetyl-5-hydroxy-2-methylchromone with sodium carbonate in deuterium oxide furnished 5-hydroxy-2-trideuteromethylchromone-3-d which also exhibits a single CO absorption. Partial incorporation of O18 into the CO group of 1 results in a single νC=O18 at 1593 cm−1. It is suggested that the doublet CO absorption of 5-hydroxychromones arises from a Fermi resonance involving a low-energy vibrational mode of the vinyl proton on the nuclear C-3 position. 相似文献
2.
Quantitative IR solution data in carbon tetrachloride and chloroform are recorded for the CO and OH regions of 31 chromones. In the 1580–1700 cm−1 region, 5-hydroxychromones show three main maxima, the two of highest frequency, at 1663 ± 3 cm−1 and 1630 ± 5 cm−1 in CCl4 (1661 ± 2 cm−1 and 1627 ± 5 cm−1 in CHCl3), being sufficiently intense as to possess high CO character. Typically, 5-alkoxychromones exhibit two intense maxima in this region, 1663 ± 3 cm−1 and 1613 ± 7 cm−1 in CCl4 (1657 ± 2 cm−1 and 1608 ± 12 cm−1 in CHCl3). Diagnostically useful changes in contour and principal peak positions can be seen for substituted and annellated 5-hydroxychromones. In the 2500–3650 cm−1 region, the stretching frequencies of OH groups at the most commonly encountered positions (C-5, C-7, and 2-CH2OH) in natural chromones, are identified. 相似文献
3.
4.
Pamela L. Broaddus-Lawrence Kathleen Treole Robert B. McCabe Rose L. Allen Louise Toppin 《Journal of voice》2000,14(1):58-71
The purpose of the present study was to determine the effects of vocal hygiene education on the vocal hygiene behaviors and perceptual vocal characteristics of untrained singers. Eleven adult untrained singers served as subjects. They attended four 1-hour class sessions on vocal hygiene, including anatomy and physiology of the phonatory mechanism, vocally abusive behaviors, voice disorders commonly seen in singers, and measures to prevent voice disorders. Pre- and postinstruction surveys were used to record subjects' vocal abuses and their perceptions of their speaking and singing voice. They also rated their perceived value of vocal hygiene education. Results revealed minimal changes in vocal hygiene behaviors and perceptual voice characteristics. The subjects did report a high degree of benefit and learning, however. 相似文献
5.
Binding states and sticking coefficients of CO and H2 on clean and oxide covered (110) planes of Pt are examined using flash desorption mass spectrometry to characterize binding states and Auger electron spectroscopy (AES) to characterize oxide densities. It is found that on the oxide both adsorbates have new binding states with significantly higher binding energies than on the clean surface. For H2 the binding states associated with the clean surface are also shifted to higher energies as the oxide coverage increases. The oxide state for H2 desorbs with first order kinetics, and isotope exchange experiments are used to examine exchange between isotopes and between states. The initial sticking coefficients for CO are 1.0 and 0.85 on clean and oxidized surfaces, and the initial sticking coefficient for H2 increases from 0.15 on the clean surface to 0.28 on the oxidized surface. Enhanced bonding on the oxide is interpreted in terms of models involving microfacets, electronic structure alteration, and compound formation. 相似文献
6.
John M. Adams Keith Martin Richard W. McCabe 《Journal of inclusion phenomena and macrocyclic chemistry》1987,5(6):663-674
Suitably modified smectite clays can be very selective catalysts for a wide range of organic reactions. While it has long been known that such materials can act as Bronsted and Lewis acids, it has been shown recently that they are also effective Diels-Alder catalysts. A selection of illustrative reactions is given which emphasises their wide range of use, their selectivity, and the ease of work-up after reaction. In each case, mechanistic information is presented, e.g., on the site of reaction (whether interlayer or surface), rate determining steps, etc. The regiochemical consequences of the restricted reaction space are stressed.Based on material presented at the Fourth International Symposium on Inclusion Phenomena and the Third International Symposium on Cyclodextrins, Lancaster, U.K., 20–25 July 1986. 相似文献
7.
Padraic S. Nagle Amila Kahved?i? Thomas McCabe Isabel Rozas 《Structural chemistry》2012,23(2):315-323
We present the crystal structures of the chloride salts of the mono-guanidinium 1 (–CH2CH2– linker) and the bis-isouronium 2 (–O– linker) that have been resolved by us indicating that these compounds are diprotonated in the solid state as informed
by the counterions positions. To determine the pK
a values of these compounds as well as those of their analogues 3 (mono-2-aminoimidazolinium with a –CH2CH2– linker) and 4 (mono-guanidinium with a –O– linker), the corresponding UV–Vis titrations were carried out. Thus, in aqueous solution compounds
1, 3 and 4 were present as mono-cationic species while derivative 2 was a bis-cation. 相似文献
8.
The synthesis and photophysical evaluation of three diaryl thiourea-based anion receptors (4–6) for comparison with their urea counterparts (1–3) is outlined. These anion receptors posses an acetamide functionality on one of the aryl groups and an electron-withdrawing CF3 group on the other. By varying the position of the acetamide group, in the o-, m- and p-positions of 4–6, respectively, the anion binding ability was both tuneable and found to be, in some cases, significantly different from that seen for the urea analogues 1–3. The binding affinities of the receptors 4–6, as well as the binding stoichiometries, were evaluated using UV–vis absorption spectroscopy in MeCN. However, these receptors were not sufficiently emissive to quantify the anion recognition using fluorescence. The results confirmed strong binding of these receptors to anions such as fluoride, acetate, phosphate, pyrophosphate and chloride. Nevertheless, the overall results obtained did not conform to the anticipated trends seen for 1–3, which is most likely due to the enhanced binding affinity of the thiourea analogues 4–6. The binding interactions were also investigated by using 1H NMR which confirmed that these receptors interacted with the anions in a stepwise manner, where the primary anion binding interaction occurred at the thiourea side, which led to an activation of the acetamide moiety towards the second anion binding interaction, an example of an allosteric activation mode. 相似文献
9.
S -nitrosothiols have many biological activities and may act as nitric oxide (NO) carriers and donors, prolonging NO half-life in vivo. In spite of their great potential as therapeutic agents, most S -nitrosothiols are too unstable to isolate. We have shown that the S -nitroso adduct of N -acetylcysteine (SNAC) can be synthesized directly in aqueous and polyethylene glycol (PEG) 400 matrix by using a reactive gaseous (NO/O2 ) mixture. Spectral monitoring of the S–N bond cleavage showed that SNAC, synthesized by this method, is relatively stable in nonbuf-fered aqueous solution at 25°C in the dark and that its stability is greatly increased in PEG matrix, resulting in a 28-fold decrease in its initial rate of thermal decomposition. Irradiation with UV light (λ= 333 nm) accelerated the rate of decomposition of SNAC to NO in both matrices, indicating that SNAC may find use for the photogeneration of NO. The quantum yield for SNAC decomposition decreased from 0.65 ± 0.15 in aqueous solution to 0.047 ± 0.005 in PEG 400 matrix. This increased stability in PEG matrix was assigned to a cage effect promoted by the PEG microenvironment that increases the rate of geminated radical pair recombination in the homolytic S–N bond cleavage process. This effect allowed for the storage of SNAC in PEG at −20°C in the dark for more than 10 weeks with negligible decomposition. Such stabilization may represent a viable option for the synthesis, storage and handling of S -nitrosothiol solutions for biomedical applications. 相似文献
10.
Emma E. McCabe 《Journal of solid state chemistry》2010,183(3):624-143
The new phases Ba2LaMNb4O15: M=Mn, Fe were prepared by solid state reaction at 1100 °C. They have the tetragonal tungsten bronze structure, space group P4/mbm, at room temperature. The two octahedral sites show partial order of M and Nb with preferential occupancy of the smaller B(1) sites by M. Both phases have high permittivities 90±15 over the range 10-320 K. Ba2LaFeNb4O15 is highly insulating with bulk conductivity ?10−8 ohm−1 cm−1 at 25 °C and tan δ?0.001 over the range 100-320 K and at 105 Hz. Solid solutions between these new phases and the compositionally and structurally related relaxor ferroelectric Ba2LaTi2Nb3O15 show gradual loss of ferroelectric behaviour attributed to replacement of polarisable Ti4+ by a mixture of (Mn, Fe)3+ and Nb5+. 相似文献