共查询到20条相似文献,搜索用时 15 毫秒
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Dr. Kirsten Schwing Dr. Holger Fricke Dr. Kristina Bartl Dr. Jolanta Polkowska Prof. Dr. Thomas Schrader Prof. Dr. Markus Gerhards 《Chemphyschem》2012,13(6):1576-1582
The functionality of bioactive molecules sensitively depends on their structure. For the investigation of intrinsic structural properties, molecular beam experiments combined with laser spectroscopy have proven to be a suitable tool. Herein we present an analysis of the two isolated tripeptide model systems Ac‐Phe‐Tyr(Me)‐NHMe and Boc‐Phe‐Tyr(Me)‐NHMe. For this purpose, mass‐selective combined IR/UV spectroscopy is applied to both substances in a molecular beam experiment. The comparison of the experimental data with DFT calculations, including different functionals as well as dispersion corrections, allows an assignment of both tripeptide models to β‐turns formed independently from the protection groups and supported by the interaction of the two aromatic chromophores. 相似文献
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Gas‐Phase Infrared Spectroscopy of Substituted Cyanobutadiynes: Roles of the Bromine Atom and Methyl Group as Substituents 下载免费PDF全文
Dr. M. Merced Montero‐Campillo Prof. Dr. Otilia Mó Prof. Dr. Manuel Yáñez Dr. Abdessamad Benidar Dr. Cédric Rouxel Dr. Nicolas Kerisit Dr. Yann Trolez Dr. Jean‐Claude Guillemin 《Chemphyschem》2016,17(7):1018-1024
The IR spectra of 5‐bromo‐2,4‐pentadiynenitrile (Br?C≡C?C≡C?CN) and 2,4‐hexadiynenitrile (CH3?C≡C?C≡C?CN), a compound of interstellar interest, have been recorded within the 4000–500 cm?1 spectral region and calculated by means of high‐level ab initio and density functional calculations. Although the calculated structures of both compounds are rather similar, there are very subtle differences, mainly in the strength of the C≡C bond not directly bound to the substituent. These subtle bonding differences are reflected in small, but not negligible, differences in the electron density at the corresponding bond critical points, and, more importantly, are reflected in the IR spectra. Indeed, the IR spectrum for the bromine derivative presents two well‐differentiated strong bands around 2250 cm?1, whereas for the methyl derivative both absorptions coalesce in a single band. These bands correspond in both cases to the coupling between C≡C and C≡N stretching displacements. A third, very weak, band also associated with C≡C and C≡N coupled stretches is observed for the bromine derivative, but not for the methyl one, owing to its extremely low intensity. 相似文献
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Rafael C. González Cano Helena Herrera Prof. José L. Segura Prof. Juan T. López Navarrete Dr. M. Carmen Ruiz Delgado Prof. Juan Casado 《Chemphyschem》2012,13(17):3893-3900
Herein we investigate the conformational and electronic properties of the 2,2′;3′,2“‐terthiophene ( B3T ) unit as the building block of thiophene dendrimeric materials. By means of DFT ground electronic state dihedral energy profiles, we get insight in the flexibility of B3 T as the prominent feature promoting the 3D arrangement. The presence of diverse conformers is explored by Raman and 1H NMR spectroscopies. A theoretical estimation of the Raman and 1H NMR spectra over the most energetically accessible conformers is found to be crucial for the appropriate assignment of the major conformer population derived from the experimental spectra. We show that energy barriers for the interconversion of conformers also play a role. Finally, the impact on the optical spectra (absorption and emission) of the α–α and α–β connections is studied and addressed by scanning the properties of the relevant low‐lying excited states. These studies highlight the relevance of the architecture of the basic molecular unit to understand charge and exciton behavior in organic semiconductors, particularly for those useful in photovoltaics. 相似文献
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β‐turns belong to the most important secondary structure elements in proteins. On the basis of density functional calculations, vibrational Raman optical activity signatures of different types of β‐turns are established and compared as well as related to other signatures proposed in the literature earlier. Our findings indicate that there are much more characteristic ROA signals of β‐turns than have been hitherto suggested. These suggested signatures are, however, found to be valid for the most important type of β‐turns. Moreover, we compare the influence of different amino acid side chains on these signatures and investigate the discrimination of β‐turns from other secondary structure elements, namely α‐ and 310‐helices. 相似文献
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Vibrational Circular Dichroism and Theoretical Study of the Conformational Equilibrium in (−)‐S‐Nicotine 下载免费PDF全文
Pilar Gema Rodríguez Ortega Prof. Manuel Montejo Prof. Juan Jesus López González 《Chemphyschem》2015,16(2):342-352
We report an extensive study of the molecular and electronic structure of (?)‐S‐nicotine, to deduce the phenomenon that controls its conformational equilibrium and to solve its solution‐state conformer population. Density functional theory, ab initio, and molecular mechanics calculations were used together with vibrational circular dichroism (VCD) and Fourier transform infrared spectroscopies. Calculations and experiments in solution show that the structure and the conformational energy profile of (?)‐S‐nicotine are not strongly dependent on the medium, thus suggesting that the conformational equilibrium is dominated by hyperconjugative interactions rather than repulsive electronic effects. The analysis of the first recorded VCD spectra of (?)‐S‐nicotine confirmed the presence of two main conformers at room temperature. Our results provide further evidence of the hypersensitivity of vibrational optical activity spectroscopies to the three‐dimensional structure of chiral samples and prove their suitability for the elucidation of solution‐state conformer distribution. 相似文献
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Heteronuclear NMR Spectroscopy as a Surface‐Selective Technique: A Unique Look at the Hydroxyl Groups of γ‐Alumina. 下载免费PDF全文
Dr. Mostafa Taoufik Dr. Kai C. Szeto Dr. Nicolas Merle Dr. Iker Del Rosal Dr. Laurent Maron Dr. Julien Trébosc Dr. Grégory Tricot Dr. Régis M. Gauvin Dr. Laurent Delevoye 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(14):4038-4046
The surface hydroxyl groups of γ‐alumina dehydroxylated at 500 °C were studied by a combination of one‐ and two‐dimensional homo‐ and heteronuclear 1H and 27Al NMR spectroscopy at high magnetic field. In particular, by harnessing 1H–27Al dipolar interactions, a high selectivity was achieved in unveiling the topology of the alumina surface. The terminal versus bridging character of the hydroxyl groups observed in the 1H magic‐angle spinning (MAS) NMR spectrum was demonstrated thanks to 1H–27Al RESPDOR (resonance‐echo saturation‐pulse double‐resonance). In a further step the hydroxyl groups were assigned to their aluminium neighbours thanks to a {1H}‐27Al dipolar heteronuclear multiple quantum correlation (D‐HMQC), which was used to establish a first coordination map. Then, in combination with 1H–1H double quantum (DQ) MAS, these elements helped to reveal intimate structural features of the surface hydroxyls. Finally, the nature of a peculiar reactive hydroxyl group was demonstrated following this methodology in the case of CO2 reactivity with alumina. 相似文献
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Anna I. Arkhypchuk Dr. Andreas Orthaber Viorica Alina Mihali Dr. Andreas Ehlers Prof. Koop Lammertsma Dr. Sascha Ott 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(41):13692-13704
The reaction of a {W(CO)5}‐stabilized phosphinophosphonate 1 , (CO)5WPH(Ph)? P(O)(OEt)2, with ethynyl‐ ( 2 a – f ) and diethynylketones ( 7 – 11 , 18 , and 19 ) in the presence of lithium diisopropylamide (LDA) is examined. Lithiated 1 undergoes nucleophilic attack in the Michael position of the acetylenic ketones, as long as this position is not sterically encumbered by bulky (iPr)3Si substituents. Reaction of all other monoacetylenic ketones with lithiated 1 results in the formation of 2,5‐dihydro‐1,2‐oxaphospholes 3 and 4 . When diacetylenic ketones are employed in the reaction, two very different product types can be isolated. If at least one (Me)3Si or (Et)3Si acetylene terminus is present, as in 7 , 8 , and 19 , an anionic oxaphosphole intermediate can react further with a second equivalent of ketone to give cumulene‐decorated oxaphospholes 14 , 15 , 24 , and 25 . Diacetylenic ketones 10 and 11 , with two aromatic acetylene substituents, react with lithitated 1 to form exclusively ethenyl‐bridged bisphospholes 16 and 17 . Mechanisms that rationalize the formation of all heterocycles are presented and are supported by DFT calculations. Computational studies suggest that thermodynamic, as well as kinetic, considerations dictate the observed reactivity. The calculated reaction pathways reveal a number of almost isoenergetic intermediates that follow after ring opening of the initially formed oxadiphosphetane. Bisphosphole formation through a carbene intermediate G is greatly favored in the presence of phenyl substituents, whereas the formation of cumulene‐decorated oxaphospholes is more exothermic for the trimethylsilyl‐containing substrates. The pathway to the latter compounds contains a 1,3‐shift of the group that stems from the acetylene terminus of the ketone substrates. For silyl substituents, the 1,3‐shift proceeds along a smooth potential energy surface through a transition state that is characterized by a pentacoordinated silicon center. In contrast, a high‐lying transition state TS(E′–F′)R=Ph of 37 kcal mol?1 is found when the substituent is a phenyl group, thus explaining the experimental observation that aryl‐terminated diethynylketones 10 and 11 exclusively form bisphospholes 16 and 17 . 相似文献
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Halil Aktas J. Chris Slootweg Dr. Andreas W. Ehlers Dr. Martin Lutz Dr. Anthony L. Spek Prof. Dr. Koop Lammertsma Prof. Dr. 《Angewandte Chemie (International ed. in English)》2009,48(17):3108-3111
Do the twist : The reaction of in situ generated phosphinidenes with phosphaalkynes is a facile route to the new metal‐coordinated η3‐diphosphavinylcarbene 1 , which shows facile ligand‐exchange reactions and undergoes an unprecedented rearrangement that involves phosphinidene complex 2 and η3‐phosphaalkenylphosphinidene complex 3 , the 1,3 isomer of 1 .
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Dr. Josep M. Anglada Dr. Ramon Crehuet Prof. Joseph S. Francisco 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(50):18092-18100
High‐level ab initio and Born–Oppenheimer molecular dynamic calculations have been carried out on a series of hydroperoxyalkyl (α‐QOOH) radicals with the aim of investigating the stability and unimolecular decomposition mechanism into QO+OH of these species. Dissociation was shown to take place through rotation of the C?O(OH) bond rather than through elongation of the CO?OH bond. Through the C?O(OH) rotation, the unpaired electron of the radical overlaps with the electron density on the O?OH bond, and from this overlap the C=O π bond forms and the O?OH bond breaks spontaneously. The CH2OOH, CH(CH3)OOH, CH(OH)OOH, and α‐hydroperoxycycloheptadienyl radical were found to decompose spontaneously, but the CH(CHO)OOH has a decomposition energy barrier of 5.95 kcal mol?1 owing to its steric and electronic features. The systems studied in this work provide the first insights into how structural and electronic effects govern the stabilizing influence on elusive α‐QOOH radicals. 相似文献
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David Zanders Goran Ba
i Dominique Leckie Oluwadamilola Odegbesan Jeremy Rawson Jason D. Masuda Anjana Devi Sen T. Barry 《Angewandte Chemie (International ed. in English)》2020,59(33):14138-14142
Attempted preparation of a chelated CoII β‐silylamide resulted in the unprecedented disproportionation to Co0 and a spirocyclic cobalt(IV) bis(β‐silyldiamide): [Co[(NtBu)2SiMe2]2] ( 1 ). Compound 1 exhibited a room‐temperature magnetic moment of 1.8 B.M. and a solid‐state axial EPR spectrum diagnostic of a rare S= configuration for tetrahedral CoIV. Ab initio semicanonical coupled‐cluster calculations (DLPNO‐CCSD(T)) revealed the doublet state was clearly preferred (?27 kcal mol?1) over higher spin configurations only for the bulky tert‐butyl‐substituted analogue. Unlike other CoIV complexes, 1 had remarkable thermal stability, and was demonstrated to form a stable self‐limiting monolayer in preliminary atomic layer deposition (ALD) surface saturation experiments. The ease of synthesis and high stability make 1 an attractive starting point to investigate otherwise inaccessible CoIV intermediates and for synthesizing new materials. 相似文献
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Dr. Srivari Chandrasekhar Kakita Veera Mohana Rao Mallikanti Seenaiah Police Naresh Ambure Sharada Devi Dr. Bharatam Jagadeesh 《化学:亚洲杂志》2014,9(2):457-461
Hybrid peptidic oligomers comprising natural and unnatural amino acid residues that can exhibit biomolecular folding and hydrogen‐bonding mimicry have attracted considerable interest in recent years. While a variety of hybrid peptidic helices have been reported in the literature, other secondary structural patterns such as γ‐turns and ribbons have not been well explored so far. The present work reports the design of novel periodic γ‐turns in the oligomers of 1:1 natural‐α/unnatural trans‐β‐norborenene (TNAA) amino acid residues. Through DFT, NMR, and MD studies, it is convincingly shown that, in the mixed conformational pool, the heterogeneous backbone of the hybrid peptides preferentially adopt periodic 8‐membered (pseudo γ‐turn)/7‐membered (inverse γ‐turn) hydrogen bonds in both polar and non‐polar solvent media. It is observed that the stereochemistry and local conformational preference of the β‐amino acid building blocks have a profound influence on accessing the specific secondary fold. These findings may be of significant relevance for the development of molecular scaffolds that facilitate desired positioning of functional side‐chains. 相似文献
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Barbara Chiavarino Dr. Maria Elisa Crestoni Prof. Simonetta Fornarini Prof. Francesco Lanucara Joel Lemaire Dr. Philippe Maitre Dr. 《Chemphyschem》2009,10(3):520-522
An anti trihydroxycarbenium ion is revealed to be the gas‐phase structure of protonated carbonic acid by IR multiple‐photon dissociation spectroscopy (see picture for calculated structure and comparison of experimental and computed spectra). Deprotonation yields anti‐H2CO3 with a nominal gas‐phase basicity of 724 kJ mol?1.
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Dr. Klara J. Jonasson Dr. Alexey V. Polukeev Dr. Rocío Marcos Prof. Mårten S. G. Ahlquist Prof. Ola F. Wendt 《Angewandte Chemie (International ed. in English)》2015,54(32):9372-9375
Despite significant progress in recent years, the cleavage of unstrained C(sp3)? C(sp3) bonds remains challenging. A C? C coupling and cleavage reaction in a PC(sp3)P iridium pincer complex is mechanistically studied; the reaction proceeds via the formation of a carbene intermediate and can be described as a competition between α‐hydrogen and α‐alkyl elimination; the latter process was observed experimentally and is an unusual way of C(sp3)? C(sp3) bond scission, which has previously not been studied in detail. Mechanistic details that are based upon kinetic studies, activation parameters, and DFT calculations are also discussed. A full characterization of a C? C agostic intermediate is presented. 相似文献
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Gas‐Phase Infrared Spectra of Three Compounds of Astrochemical Interest: Vinyl,Allenyl, and Propargyl Isocyanides 下载免费PDF全文
Dr. Abdessamad Benidar Prof. Dr. Didier Bégué Dr. Falk Richter Prof. Dr. Claude Pouchan Prof. Dr. Mohammed Lahcini Dr. Jean‐Claude Guillemin 《Chemphyschem》2015,16(4):848-854
Isocyanides, isomers of the cyanides detected in the interstellar medium, are also possible components of this medium. The infrared spectra (5000–500 cm?1) of gaseous vinyl isocyanide, allenyl isocyanide, and propargyl isocyanide have been recorded at 0.1 cm?1 resolution. When prepared on a gram scale to produce a partial pressure of 10 mbar after evaporation in the cell, these three isocyanides, which have previously been reported to be kinetically unstable, do not display any sign of decomposition when recording the spectra. Geometry optimizations and harmonic and anharmonic vibrational frequencies were calculated using the LCCSD(T) method with the cc‐pVTZ basis set. Anharmonic frequencies of fundamental, overtone, and combination transitions were calculated using a variational approach implemented in the P Anhar.v2.0 code, to assign the experimental data for each compound. These results improve our knowledge of these under‐investigated compounds and pave the way for other physicochemical studies on functionalized isocyanides. 相似文献
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From a Si3‐Cyclopropene to a Si3S‐Bicyclo[1.1.0]butane to a Si3S‐Cyclopropene to a Si3S2‐Bicyclo[1.1.0]butane: Back‐and‐Forth,and In‐Between 下载免费PDF全文
Dr. Vladimir Ya. Lee Dr. Olga A. Gapurenko Shogo Miyazaki Prof. Dr. Akira Sekiguchi Prof. Dr. Ruslan M. Minyaev Prof. Dr. Vladimir I. Minkin Prof. Dr. Heinz Gornitzka 《Angewandte Chemie (International ed. in English)》2015,54(47):14118-14122
Compact and highly reactive bicyclo[1.1.0]butanes constitute one of the most fascinating classes of organic compounds. Furthermore, interplay of bicyclo[1.1.0]butanes with their valence isomers, such as buta‐1,3‐dienes and cyclobutenes, is among the fundamental pericyclic transformations in organic chemistry. Herein we report the back‐and‐forth interconversion between the cyclotrisilenes and thiatrisilabicyclo[1.1.0]butanes, allowing for the synthesis of novel representatives of such classes of highly reactive organometallics. The peculiar structural and bonding features of the newly synthesized compounds, as well as the mechanism of their isomerization, were verified both experimentally and computationally. 相似文献
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Claudia Loerbroks Dr. Roberto Rinaldi Prof. Dr. Walter Thiel 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(48):16282-16294
The molecular understanding of the chemistry of 1,4‐β‐glucans is essential for designing new approaches to the conversion of cellulose into platform chemicals and biofuels. In this endeavor, much attention has been paid to the role of hydrogen bonding occurring in the cellulose structure. So far, however, there has been little discussion about the implications of the electronic nature of the 1,4‐β‐glycosidic bond and its chemical environment for the activation of 1,4‐β‐glucans toward acid‐catalyzed hydrolysis. This report sheds light on these central issues and addresses their influence on the acid hydrolysis of cellobiose and, by analogy, cellulose. The electronic structure of cellobiose was explored by DFT at the BB1 K/6‐31++G(d,p) level. Natural bond orbital (NBO) analysis was performed to grasp the key bonding concepts. Conformations, protonation sites, and hydrolysis mechanisms were examined. The results for cellobiose indicate that cellulose is protected against hydrolysis not only by its supramolecular structure, as currently accepted, but also by its electronic structure, in which the anomeric effect plays a key role. 相似文献