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Investigation of Structure and Dynamics in a Photochromic Molecular Crystal by NMR Crystallography
摘要： A photochromic anil, N-(3,5-di-t-butylsalicylidene)-4-amino-pyridine, has been studied by single-crystal X-ray diffraction, multinuclear magic-angle spinning NMR and first-principles density functional theory (DFT) calculations. Interpretation of the solid-state NMR data on the basis of calculated chemical shifts confirms the structure is primarily composed of molecules in the ground-state enol tautomer, while thermally-activated cis-keto and photoisomerised trans-keto states exist as low-level defects with populations that are too low to detect experimentally. Variable temperature 13C NMR data reveals evidence for solid-state dynamics which is found to be associated with fast rotational motion of t-butyl groups and 180° flips of the pyridine ring, contrasting the time-averaged structure obtained by X-ray diffraction. Comparison of calculated chemical shifts for the full crystal structure and an isolated molecule also reveals evidence for an intermolecular hydrogen-bond involving the pyridine ring and an adjacent imine carbon which facilitates the flipping motion. The DFT calculations also reveal that the molecular conformation in the crystal structure is very close to the energetic minimum for an isolated molecule, indicating that the ring dynamics arise as a result of considerable steric freedom of the pyridine ring and which also allows the molecule to adopt a favourable conformation for photochromism.
关键词： NMR crystallography,photochromic,anil,solid-state NMR,DFT calculations,molecular dynamics
更新于2025-09-09 09:28:46
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Raman Characterization of Phenyl-Derivatives: From Primary Amine to Diazonium Salts
摘要： The objective of the present work is to use Raman spectroscopy for characterizing, the fate of phenyl-derivatives, from phenyl-amines to aryl-diazonium derivatives (ADD). Four ADD were investigated: (i) benzene diazoniumtetrafluoroborate (DS), (ii) 4-decyl benzene diazoniumtetrafluoroborate (DS-C10H21), (iii) 4-carboxybenzene diazoniumtetrafluoroborate (DS-COOH) and (iv) 4-(aminoethyl) benzene diazoniumtetrafluoroborate (DS-(CH2)2NH2). Raman investigation of the above ADD confirmed the existence of an N≡N bond stretching in the range of 2285-2305 cm-1. Moreover, the strong band related to CH in plane-bending and C-N-stretching modes in the 1073-1080 cm-1 range, is a signature of phenyl derivatives stemming from ADD. Furthermore, we analyzed and discuss the H-N-(ring) symmetric stretching modes and the ring-N, as well as the benzene-ring vibrational modes, the C-H related vibrations and the functions in para-position carried by the aromatic ring. The effect of structural changes, the conformational rearrangements from amines to ADD and the influence of the substituent located in the para-position on Raman modes, were examined as well. Finally, Raman experiments supported by Density Functional Theory (DFT) modeling allowed us to determine the crystalline structure of DS-COOH.
关键词： Raman,Diazonium salts,Synthesis and characterization,DFT calculations,Phenyl-amines
更新于2025-09-04 15:30:14
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Bis[1]benzothieno[1,4]thiazines - Planarity, Enhanced Redox Activity and Luminescence by Thieno-Expansion of Phenothiazine
摘要： Twofold Buchwald-Hartwig aminations selectively furnish all three regioisomers of bis[1]benzothieno[1,4]thiazines and X-ray structure analyses and DFT calculations were corroborated for correlation of their electronic properties. All regioisomers outscore the parent compound phenothiazine with respect to a low lying oxidation potential and reversible redox activity. The anti-anti bis[1]benzothieno[3,2-b:2',3'-e][1,4]thiazines possess lowest oxidation potentials in this series and display pronounced green luminescence in solution (?F ≈ 20%) and in the solid state. Syn-anti regioisomers are only weakly luminescent in solution, but show aggregation induced emission enhancement and solid state luminescence. Most interestingly, found by X-ray structure analyses anti-anti derivatives reveal an amazingly coplanar structure of the pentacyclic anellated 1,4-thiazine system, emphasizing a structural similarity to heteroacenes. The calculated theoretical nucleus-independent chemical shifts additionally suggest that the 8?-electron core systems can be considered as first electronically unbiased anellated 1,4-thiazines with antiaromatic character.
关键词： Heterocycles,1,4-Thiazines,Fluorescence,Buchwald-Hartwig coupling,Cyclic voltammetry,Absorption,Antiaromaticity,Aggregation induced emission enhancement,Polymorphism,DFT calculations
更新于2025-09-04 15:30:14
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[ACS Symposium Series] Raman Spectroscopy in the Undergraduate Curriculum Volume 1305 || Quantitative Analysis of Xylene Mixtures Using a Handheld Raman Spectrometer
摘要： Xylene isomers (meta-xylene, ortho-xylene, and para-xylene) have been extensively analyzed by different instrumental methods. In this chapter, xylene isomers and their mixtures were analyzed qualitatively and quantitatively by Raman spectroscopy. First, we identified peaks that discriminated each isomer along with peaks that remain unchanged regardless of isomeric form. We used theoretical calculations to aid in the peak assignments. Then, we tested the efficacy of the technique in qualitatively and quantitatively apportioning isomeric mixtures of xylenes.
关键词： Xylene isomers,DFT calculations,Raman spectroscopy,quantitative analysis,handheld Raman spectrometer
更新于2025-09-04 15:30:14
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Towards simulation at picometer-scale resolution: Revisiting inversion domain boundaries in GaN
摘要： Motivated by recent high resolution results on the inversion domain boundaries (IDB) in gallium nitride, we re?ne by ab initio DFT calculations the well established atomic model IDB? derived by Northrup et al. This allows us to recover these experimental results obtained by coherent x-ray diffraction and showing small additional shifts of the polarity domains, in particular 8 pm shift along the hexagonal direction. The in?uence of boundary conditions and electrostatic ?elds (IDB-IDB and IDB-surface interactions) on the results and the existence of metastable solutions is carefully discussed to stress the accuracy of the method. These results demonstrate a cross-talk between advanced characterization tools and state-of-the-art ab initio calculations that opens perspectives for the structural analysis of defects in the picometer range.
关键词： picometer resolution,ab initio DFT calculations,coherent x-ray diffraction,gallium nitride,inversion domain boundaries
更新于2025-09-04 15:30:14
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Spectroscopic and electronic structure characterization of hydrogen bonding in 2-Bromohydroquinone
摘要： 2-Bromohydroquinone species form both intra- and inter-molecular H-bonds. The evidences for the H-bonds have been observed in the solid-phase Mid IR (4000?400 cm-1) and Near IR (8000?5000 cm-1) absorption spectra with two intensely broad bands near 3263 and 3222 cm-1 and a broad absorption in 7000?5200 cm-1 with a band maximum at 6335 cm-1. Dimer and trimer models with DFT’s B3LYP and BP86 functionals using 6-31G(d) and 6-311++G(d,p) basis sets have been proposed for the ?O?H···O bonding in 2-Bromohydroquinone. Out of the four predicted conformers, one trans and one cis conformers with nearly 1:1 Boltzmann populations, are separated by very small energy difference of 0.03 kcal/mol and this is confirmed by two chemical shifts at 9.0 and 9.3 ppm from an 1H NMR spectrum. On the basis of Quantum Theory of Atoms in Molecules (AIM) and reduced density gradient plots of electron densities and its isosurfaces by non-covalent interactions (NCI) method, the influence of the ?O?H···Br, weak van der Waals and ?O?H···O bond interactions on the stability of dimer and trimer structures have been analysed. From the natural bond orbital (NBO) analysis, it is explained that the formation of H-bonding may be attributed to charge transfer from the lone pair orbital n(O) of the base Oxygen into the vacant antibonding orbital σ*(OH) of the acid OH in the ?O?H···O bonding. The Near IR concentration-dependent spectra demonstrate that the sample of 2-Bromohydroquinone is composed of ?O?H···O bonded species and dissociate into non-bonded species as the mole fraction of the solvent (acetonitrile/1,4-dioxane) is increased. The band observed at 7095 cm-1 at 0.09 M is identified as the first overtone of the ?O?H stretching mode of the weak complex of 2-Bromohydroquinone and acetonitrile. The solvent effects on all the observed solution-phase NIR spectra are not ruled out.
关键词： H-bonding,AIM,NCI,IR spectra,2-Bromohydroquinone,DFT calculations
更新于2025-09-04 15:30:14
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Differential quenching of the angular momentum of the B and Q bands of a porphyrin as a result of extended ring π-conjugation
摘要： A novel porphyrin, whose p-system has been extended via the presence of two additional carbon–carbon triple bonds on opposite meso-positions and by fusion of a single naphthalene unit simultaneously bridging the third meso-position and the b-carbon of one of the pyrroles, has been synthesized in good yield. Absorption, magnetic circular dichroism, emission, and theoretical spectra are reported for the fused and unfused trans-naphthalene free base and zinc porphyrins. The fusing of one of the naphthalene moieties results in significant changes to the absorption spectrum and, very unusually, the bridged meso-b-pyrrole fusion results in quenching of the MCD Faraday pseudo-A term in the porphyrin’s B band (S2). This unique effect was interpreted as resulting from the origin of the electronic structure of the second excited state (the B state). The x and y polarizations are completely mixed by the electronic effects of the non-symmetric extended conjugation of the p ring. Analysis of the origin of the MCD signal indicates that the presence of this novel mixed polarization leads to negligible angular momentum in the important B state. To our knowledge, this is the first report in which the magnetic moment in a porphyrin’s intensely absorbing B band has been quenched while the angular momentum in the Q band, the first excited state, remains as normal. This implies that the photophysical properties of the B state are likely very different than those of the Q state, which has novel and significant implications for applications, especially in non-linear spectroscopy.
关键词： TD-DFT calculations,Extended porphyrin,Magnetic Circular Dichrosim (MCD),Q band,B band,angular momentum
更新于2025-09-04 15:30:14
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Electronic and magnetic properties of Al-doped WS <sub/>2</sub> monolayer under strain
摘要： Magnetic properties of Al-doped WS2 monolayer under strain are investigated by ab initio methods. Without strain, the Al-doped WS2 monolayer is a nonmagnetic nano material. We applied strain to Al-doped WS2 monolayer from –10% to 10%. Under tensile strain, ranging from 3% to 10%, the system always keeps nonmagnetic and changes from semiconductor to metal-like material. The magnetic moment gets a maximum value of 3.03 lB at –3% compressive strain. However, the magnetic moment of system decreases to zero gradually as the compressive strain increases. The coupling among the 3s states of Al, 5d states of W and 3p states of S is responsible for the strong strain effect on the magnetic properties. Our studies predict Al-doped WS2 monolayer under strain to be candidates for application in spintronics.
关键词： strain,DFT calculations,nonmagnetic metal,WS2 monolayer
更新于2025-09-04 15:30:14
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Aurophilicity and Photoluminescence of (6-Diphenylpnictogenoacenaphth-5-yl)gold Compounds
摘要： The organotin precursors 6-Br-Ace-5-SnBu3 (6, Ace = acenaphthyl) and 6-Ph2E-Ace-5-SnBu3 (7a: E = P; 7b: E = As; 7c: E = Sb) were prepared and used for the synthesis of organogold complexes, namely, the homodinuclear arylgold(I) species (6-Ph2E-Ace-5-Au)2 (8a: E = P; 8b: E = As; 8c: E = Sb), arylgold(III) dichloride 6-Ph2P-Ace-5-AuCl2 (9), diarylgold(III) chloride [trans-(6-Ph2P-Ace-5-)2Au]Cl ([10]Cl) as well as the heterodinuclear gold complexes 6-Ph2P(AuX)-Ace-5-Au(AsPh3) (11a: X = Cl; 11b: X = Br). Compounds 8a - 8c, 11a, and 11b show significant aurophilic interactions, which are related to their photoluminescence upon exposure to UV light. In the solid state, 11a and 11b show green-yellow emission, whereas 8a - 8c are non-emissive. In solution under an argon atmosphere, all five compounds 8a - 8c as well as 11a and 11b show green-yellow emission. The organogold complexes were characterized by multinuclear NMR spectroscopy, photoluminescence spectroscopy, single-crystal X-ray crystallography as well as (TD-)DFT calculations including real-space bond indicators derived from atoms-in-molecules (AIM), the electron localization indicator (ELI-D), and the non-covalent interaction (NCI) index.
关键词： DFT calculations,aurophilicity,photoluminescence,X-ray crystallography,NMR spectroscopy,organogold complexes
更新于2025-09-04 15:30:14
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De novo design of organic photocatalysts: bithiophene derivatives for the visible-light induced C-H functionalization of heteroarenes
摘要： Herein, we report the de novo synthesis and characterization of a series of substituted bithiophene derivatives as novel inexpensive organic photocatalysts. DFT calculations were used to predict a priori their absorption spectra and redox potentials, which were then confirmed with empirical data. The photocatalytic activity of this novel class of organic photoredox catalyst was demonstrated in two visible-light mediated strategies for the C-H functionalization of heteroarenes. The implementation of these strategies in a continuous-flow photo-microreactor afforded moderate to excellent yields within few minutes of reaction time. Due to their straightforward synthesis, low cost and good photocatalytic properties we believe that the proposed bithiophene derivatives could be employed as a new class of organic photoredox catalysts.
关键词： Organic photocatalysts,C-H functionalization,Photoredox catalysis,DFT calculations,Continuous flow catalysis
更新于2025-09-04 15:30:14