Panpan Zhou

An “off–on–off” fluorescent probe for the sequential detection of Zn2+ and hydrogen sulfide in aqueous solution

A novel selective fluorescent chemosensor (L1) based on 8-aminoquinoline was synthesized and characterized. The sensor exhibited remarkable selectivity for Zn2+ in the presence of other cations in aqueous solution. Density functional theory calculations on L1 and the L1–Zn complex are consistent with the experimental results. Once combined with Zn2+, the complex L1–Zn displayed high specificity for H2S. As HS− is equivalent to H2S in physiological solution, HS− was selected to represent H2S in this work. Among various anions, only HS− induced revival of the fluorescence of L1. Signal transduction occurs via reversible formation–separation of complex L1–Zn and ZnS. L1 and the sequential complex L1–Zn have ideal chemical and spectroscopic properties that satisfy the criteria for further Zn2+ and H2S sensing in biological and environmental applications.

Competition and cooperativity of σ-hole and π-hole intermolecular interactions between carbon monoxide and bromopentafluorobenzene

Theoretical investigations of the interactions between carbon monoxide (CO) and bromopentafluorobenzene (C6F5Br) have been carried out at both M06-2X/6-311++G(2d,2p) and M06-2X/aug-cc-pVTZ levels of theory. Because both C and O atom-ends of CO show negative electrostatic potential, they can favorably interact with the positive electrostatic potential generated by the σ-hole of Br and the π-hole of the aromatic ring of C6F5Br, yielding four different dimers and trimers. Their structures, spectroscopy and nature were systematically studied. The competition between the interactions involving the σ-hole of the Br atom and the π-hole of C6F5Br with the C and O atom-ends of CO was illustrated for the dimers and trimers, and meanwhile, the cooperativity between the two components of the trimers was elaborated. In addition, the experimental FT-IR and fluorescence spectra were measured for C6F5Br and the mixed C6F5Br and CO system without and with the solvent of hexane to gain information on the formation of molecular complexes between C6F5Br and CO. The results presented in this work are beneficial for the understanding of the competition and cooperativity of σ- and π-hole intermolecular interactions.

Solid-state identity of 2-hydroxynicotinic acid and its polymorphism

2-Hydroxynicotinic acid (2-HNA), a derivative of nicotinic acid, was found to exist in four polymorphs. In the solid state, 2-HNA is actually present as its tautomer, 2-oxo-1,2-dihydro-3-pyridinecarboxylic acid (2-ODHPCA). The polymorphism stems from distinctive packing arrangements of the molecules, and the formation of the distinct polymorphs can be affected by using various acidic additives. The thermal behaviors of the four polymorphs indicate that form I is the most stable at elevated temperature, form II converts into form I during heating, and forms III and IV transform into form II when heated. Theoretical studies showed that 2-ODHPCA is more energetically favored than 2-HNA. Condensed Fukui functions and the dual descriptor were used jointly to examine the local preference of hydrogen bonding in the crystal. Lattice energy calculations were conducted to further evaluate energetic properties of the system.

HBT-based chemosensors for the detection of fluoride through deprotonation process: experimental and DFT studies

When searching to develop fluoride chemosensors based on O–H⋯F, we discovered that an HBT-based fluorophore containing a hydroxyl group was easily synthesized and displayed excellent fluorescence properties. 4-(benzothiazol-2-yl)-phenol (L1H) was found to facilitate the monitoring of fluoride and showed ratiometric fluorescence changes. It is worth noting that an aldehyde group in conjugation with the HBT-based fluorophore core at an adjacent position to the hydroxyl group (i.e. 5-(benzothiazol-2-yl)-2-hydroxybenzaldehyde, L2H) would elevate the sensitivity towards fluoride immensely. Spectroscopic studies indicated that L1H and L2H interacted with a fluoride anion, which involved a two-step reaction: hydrogen bond formation and deprotonation. Deprotonation of the chemosensors by a fluoride anion enhanced the electron-donating ability of the phenolic O− to the HBT core acceptor and facilitated an intramolecular charge transfer process, resulting in a red shift in both UV-vis absorption and fluorescence spectra. The mechanism of L2H binding with fluoride was confirmed by 1H NMR titration experiments and DFT computational calculations.

Cooperative halogen bonds in V-shaped H3N·X1X2·X3Y (X1, X2, X3 = Cl and Br; Y = F, Cl and Br) complexes

A series of V-shaped molecular complexes formed by NH3, X1X2 and X3Y (X1, X2, X3 = Cl, Br; Y = F, Cl, Br) molecules via two halogen bonds (i.e., N⋯X1 and X2⋯X3 interactions) have been investigated at the MP2/aug-cc-pVTZ level of theory to obtain their optimized geometries, stretching modes and interaction energies. Molecular electrostatic potential was used to illustrate how X1 and X2 act as the halogen bond donor and acceptor in N⋯X1 and X2⋯X3 interactions, respectively. The evaluation of the binding distances, interaction energies and the electron density at the bond critical points of the halogen bonds reveals the existence of cooperativity between the two halogen bonds. Subsequently, the concepts of pair interaction and pairwise non-additive contributions to the total interaction energy, and the cooperativity factor were further employed to assess the cooperativity. The formation mechanisms of these complexes were analyzed based on the contour maps of the Laplacian (∇2ρ) of electron density. Energy decomposition analysis suggests that electrostatic force is the main net contribution to the stability of these complexes. The work would provide valuable insights into the design of related halogen-bonded complexes.

Polymorphism and solid-to-solid phase transitions of a simple organic molecule, 3-chloroisonicotinic acid

Three polymorphs (I, II, and III) have been discovered for 3-chloroisonicotinic acid. The torsion angle between the aromatic ring and the carboxylic acid in form I differs from that of forms II and III, which are similar. All three polymorphs form hydrogen-bonded chains based on the acid–pyridine heterosynthon. Despite the conformational similarity between forms II and III, the hydrogen-bonded chains in form II alternate in direction while those in form III all point in the same direction. Study of the phase behaviors of the three forms by differential scanning calorimetry, hot-stage microscopy, and thermogravimetric analysis revealed two solid-to-solid phase transitions from the metastable forms II and III to the most stable form I. Sublimation of 3-chloroisonicotinic acid also led to form I. A higher-temperature polymorph seemed to be possible but remained elusive. Lattice energy and hydrogen bonding strength calculations provided further insight into the stability of the polymorphs. A search of conformational space for the molecule suggested possibly additional polymorphs of this simple compound. The system may be valuable for further solid-state structure–property relationship studies.

Mechanism of selective C–H cyanation of 2-phenylpyridine with benzyl nitrile catalyzed by CuBr: a DFT investigation

The mechanism of selective C–H cyanation of 2-phenylpyridine catalyzed by CuBr was investigated using the DFT method at the B3LYP/6-31+G(d,p) level, and the integral equation formalism polarized continuum model (IEFPCM) was applied to simulate the solvent effect. The computational results suggested that 2-phenylacetonitrile can convert into benzoyl cyanide under O2 conditions via two paths (a and b), and also, 2-phenylacetonitrile can first react with the O2− anion to yield 2-hydroxy-2-phenylacetonitrile, and then 2-hydroxy-2-phenylacetonitrile goes through oxidative dehydrogenation to yield benzoyl cyanide via four different paths (c, d, e and f). The other part reaction of the conversion of 2-phenylpyridine to 2-(pyridin-2-yl)-benzonitrile catalyzed by CuBr can go through three paths (g, h and i) which involve the coordination of CN− and the N atom of 2-phenylpyridine with Cu cations, and then the processes of addition and oxydehydrogenation reactions lead to the final product 2-(pyridin-2-yl)benzonitrile. In addition, another path (j) without the participation of CuBr could also occur. The results could provide valuable insights into these types of interactions and related ones.

Linear σ-hole⋯CO⋯σ-hole intermolecular interactions between carbon monoxide and dihalogen molecules XY (X, Y = Cl, Br)

Carbon monoxide can interact with two dihalogen molecules XY (X, Y=Cl, Br) in the form of X(Y)⋯COX(Y)⋯CO⋯X(Y)X(Y) trimeric complex, and their nature and characteristics were investigated at MP2/aug-cc-pVDZ level without and with counterpoise method, together with single point calculations at CCSD(T)/aug-cc-pVDZ level. The optimized geometries, stretching modes and interaction energies of a series of X(Y)⋯COX(Y)⋯CO⋯X(Y)X(Y) trimeric complexes were obtained and discussed. The cooperativity in these complexes was evaluated. EDA analyses reveal that the electrostatic interaction is the dominant net driving force in each trimer, but the contributions of other interactions like exchange, dispersion and polarization interactions are also important. QTAIM and NCI analyses confirm the existence of attractive halogen-bonding interactions. Additionally, EDDMF analysis was employed for the component dimers of these trimers, which indicates that the formation of halogen-bonding interactions is closely related to the charge shift and the rearrangement of electronic density in the formation of these complexes. The results would provide valuable insight into for these linear halogen bonds.

Preferred formation of the carboxylic acid–pyridine heterosynthon in 2-anilinonicotinic acids

The carboxylic acid–carboxylic acid homosynthon and carboxylic acid–pyridine heterosynthon are two competing supramolecular synthons in 2-anilinonicotinic acids that possess both carboxylic acid and pyridine functionalities. Previously we demonstrated that carboxylic acid–pyridine heterosynthons can be selectively formed in crystals by chemically introducing bulky functional groups to the aniline ring of the molecules. In this study we show that with the same philosophy, but a different strategy, i.e., adding substituent groups to the nitrogen bridging the two aromatic rings, we can also achieve the preferential formation of the carboxylic acid–pyridine heterosynthon over the carboxylic acid–carboxylic acid homosynthon. This is a new case of how molecular conformation can affect intermolecular interactions and consequent crystal packing.

Quantitative relationships between bond lengths, stretching vibrational frequencies, bond force constants, and bond orders in the hydrogen-bonded complexes involving hydrogen halides

To uncover the correlation between the bond length change and the corresponding stretching frequency shift of the proton donor D–H upon hydrogen bond formation, a series of hydrogen-bonded complexes involving HF and HCl which exhibit the characteristics of red-shifted hydrogen bond were investigated at the MP2/aug-cc-pVTZ, M062X/aug-cc-pVTZ, and B3LYP/aug-cc-pVTZ(GD3) levels of theory with CP optimizations. A statistical analysis of these complexes leads to the quantitative illustrations of the relations between bond length and stretching vibrational frequency, between bond length and bond force constant, between stretching vibrational frequency and bond force constant, between bond length and bond order for hydrohalides in a mathematical way, which would provide valuable insights into the explanation of the geometrical and spectroscopic behaviors during hydrogen bond formation.