Blue- and Red-Shifting CH···O Hydrogen Bonded Complexes between Haloforms and Ethers: Correlation of Donor νC−H Spectral Shifts with C−O−C Angular Strain of the Acceptors

Blue- and Red-Shifting C-H⋯O Hydrogen Bonds of Cyclic Ethers with Haloforms: Effect of Ring-Size and Consistency with Bent's Rule

A DFT-based computational study is carried out to delve into the interplay between hyperconjugation and rehybridization effects underlying the formation of blue- or red-shifting H-bonds (HBs) in 1 : 1 complexes of cyclic ethers (HB acceptor) of varying ring-size with haloforms, CHF3 and CHCl3 (HB donor). The calculations reveal that with decreasing angular strain (increasing ring-size) of the cyclic ethers, the extent of blue-shift increases for 1 : 1 complexes with CHF3, while a reverse sequence is observed with CHCl3, eventually leading to a red-shifting HB in the oxepane : CHCl3 complex. It is noted that the trend in the shift of C-H stretching fundamental is not mirrored by the C-H bond length or interaction energies for both the systems studied, that is, the low sensitivity of the changes on the strain on the O-atom of HB acceptor (cyclic ethers) is to be emphasized.

Factors contributing to halogen bond strength and stretch or contraction of internal covalent bond

The halogen bond formed by a series of Lewis acids TF₃X (T = C, Si, Ge, Sn, Pb; X = Cl, Br, I) with NH₃ is studied by quantum chemical calculations. The interaction energy is closely mimicked by the depth of the σ-hole on the X atom as well as the full electrostatic energy. There is a first trend by which the hole is deepened if the T atom to which X is attached becomes more electron-withdrawing: C > Si > Ge > Sn > Pb. On the other hand, larger more polarizable T atoms are better able to transmit the electron-withdrawing power of the F substituents. The combination of these two opposing factors leaves PbF₃X forming the strongest XBs, followed by CF₃X, with SiF₃X engaging in the weakest bonds. The charge transfer from the NH₃ lone pair into the σ*(TX) antibonding orbital tends to elongate the covalent TX bond, and this force is largest for the heavier X and T atoms. On the other hand, the contraction of this bond deepens the σ-hole at the X atom, which would enhance both the electrostatic component and the full interaction energy. This bond-shortening effect is greatest for the lighter X atoms. The combination of these two opposing forces leaves the T-X bond contracting for X = Cl and Br, but lengthening for I.

The Adsorption Characteristics of Uranium(VI) from Aqueous Solution on Leonardite and Leonardite-Derived Humic Acid: A Comparative Study

The humic substance is a low-cost and effective adsorbent with abundant functional groups in remediating uranium (U) (VI)-contaminated water. In this research study, leonardite together with leonardite-derived humic acid (L-HA) was used to eliminate U(VI) from water under diverse temperatures (298, 308, and 318 K). L-HA showed a higher adsorption volume for U(VI) than leonardite. U adsorption was varied with pH and increased with temperature. The adsorption kinetics of L-HA had a higher determination coefficient (R²) for pseudo-second-order (R² > 0.993) and Elovich (R² > 0.987) models, indicating possible chemisorption-assisted adsorption. This was further supported with the activation energies (15.9 and 13.2 kJ/mol for leonardite and L-HA, respectively). Moreover, U(VI) equilibrium adsorption on leonardite was better depicted with the Freundlich model (R² > 0.970), suggesting heterogeneous U(VI) adsorption onto the leonardite surface. However, U(VI) adsorption onto L-HA followed the Langmuir equation (R² > 0.971), which implied the dominant role of monolayer adsorption in controlling the adsorption process. Thermodynamic parameters, including standard entropy change (ΔS⁰ > 0), Gibbs free energy (ΔG⁰ < 0), and standard enthalpy change (ΔH⁰ > 0), suggested a spontaneous and endothermal adsorption process. In addition, ionic species negatively affected U(VI) adsorption by leonardite and L-HA.

A theoretical investigation on the complexes of B3O3H3 with acetylene and its substituted derivatives

The complexes of B₃O₃H₃ with acetylene and its substituted derivatives C₂HX (X=H, F, Cl, Br, CH₃, NH₂) are explored by theoretical calculations using MP2 and M06-2X methods with aug-cc-pVDZ as the basis set. The different molecular electrostatic potential features of B₃O₃H₃ with benzene (C₆H₆) and borazine (B₃N₃H₆) result in different stable structures of the complexes with C₂HX. The geometric analysis indicates that the global minimum is the parallel stacked (PS) structure with the linear C-C triple bond lies on the top of B₃O₃H₃, and this is much different from that of the B₃N₃H₆...C₂HX complex, in which the C-H bond directs towards the B₃N₃H₆ ring (T-shaped). The H-bonded structure where the C-H interacts with the O atom of B₃O₃H₃ is found to be a local minimum. Besides, there is an additional X…O halogen-bonded structures for the B₃O₃H₃…C₂HX (X=Cl, Br) complexes. As for the PS geometry, when one H atom in C₂H₂ is replaced by different X groups, the binding energies of the complexes increases in the order of F<H<Cl<Br<CH₃<NH₂, which is mainly determined by the strength of the π-hole bond. The nature of interaction in these complexes is further studied by the atoms-in-molecules (AIM) and noncovalent interaction (NCI) analysis. The natural bond orbital (NBO) analysis indicates that the main orbital interaction arises from LP_X→BD*_B‑O for B₃O₃H₃…C₂HX (X=Cl, Br, NH₂), and BD_C‑C→BD*_B‑O for B₃O₃H₃…C₂HX (X=H, CH₃) complexes. Complexes of B₃O₃H₃ with acetylene and its substituted derivatives.

Kinetics and Thermodynamics of Uranium (VI) Adsorption onto Humic Acid Derived from Leonardite

Humic acid (HA) is well known as an inexpensive and effective adsorbent for heavy metal ions. However, the thermodynamics of uranium (U) adsorption onto HA is not fully understood. This study aimed to understand the kinetics and isotherms of U(VI) adsorption onto HA under different temperatures from acidic water. A leonardite-derived HA was characterized for its ash content, elemental compositions, and acidic functional groups, and used for the removal of U (VI) from acidic aqueous solutions via batch experiments at initial concentrations of 0-100 mg·L^-1 at 298, 308 and 318 K. ICP-MS was used to determine the U(VI) concentrations in solutions before and after reacting with the HA. The rate and capacity of HA adsorbing U(VI) increased with the temperature. Adsorption kinetic data was best fitted to the pseudo second-order model. This, together with FTIR spectra, indicated a chemisorption of U(VI) by HA. Equilibrium adsorption data was best fitted to the Langmuir and Temkin models. Thermodynamic parameters such as equilibrium constant (K₀), standard Gibbs free energy (ΔG⁰), standard enthalpy change (ΔH⁰), and standard entropy change (ΔS⁰), indicated that U(VI) adsorption onto HA was endothermic and spontaneous. The co-existence of cations (Cu²⁺, Co²⁺, Cd²⁺ and Pb²⁺) and anions (HPO₄^2- and SO₄^2-) reduced U(VI) adsorption. The high propensity and capacity of leonardite-derived HA adsorbing U(VI) suggests that it has the potential for cost-effective removal of U(VI) from acidic contaminated waters.

Blue- and Red-Shifting Hydrogen Bonding: A Gas Phase FTIR and Ab Initio Study of RR'CO···DCCl3 and RR'S···DCCl3 Complexes

Blue-shifting H-bonded (C-D···O) complexes between CDCl₃ and CH₃HCO, (CH₃)₂CO, and C₂H₅(CH₃)CO, and red-shifting H-bonded (C-D···S) complexes between CDCl₃ with (CH₃)₂S and (C₂H₅)₂S have been identified by Fourier transform infrared spectroscopy in the gas phase at room temperature. With increasing partial pressure of the components, a new band appears in the C-D stretching region of the vibrational spectra. The intensity of this band decreases with an increase in temperature at constant pressure, which provides the basis for identification of the H-bonded bands in the spectrum. The C-D stretching frequency of CDCl₃ is blue-shifted by +7.1, +4, and +3.2 cm^-1 upon complexation with CH₃HCO, (CH₃)₂CO, and C₂H₅(CH₃)CO, respectively, and red-shifted by -14 and -19.2 cm^-1 upon complexation with (CH₃)₂S and (C₂H₅)₂S, respectively. By using quantum chemical calculations at the MP2/6-311++G** level, we predict the geometry, electronic structural parameters, binding energy, and spectral shift of H-bonded complexes between CDCl₃ and two series of compounds named RCOR' (H₂CO, CH₃HCO, (CH₃)₂CO, and C₂H₅(CH₃)CO) and RSR' (H₂S, CH₃HS, (CH₃)₂S, and (C₂H₅)₂S) series. The calculated and observed spectral shifts follow the same trends. With an increase in basicity of the H-bond acceptor, the C-D bond length increases, force constant decreases, and the frequency shifts to the red from the blue. The potential energy scans of the above complexes are done, which show that electrostatic attraction between electropositive D and electron-rich O/S causes bond elongation and red shift, and the electronic and nuclear repulsions lead to bond contraction and blue shifts. The dominance of the two opposing forces at the equilibrium geometry of the complex determines the nature of the shift, which changes both in magnitude and in direction with the basicity of the hydrogen-bond acceptor.

Removing uranium (VI) from aqueous solution with insoluble humic acid derived from leonardite

The occurrence of uranium (U) and depleted uranium (DU)-contaminated wastes from anthropogenic activities is an important environmental problem. Insoluble humic acid derived from leonardite (L-HA) was investigated as a potential adsorbent for immobilizing U in the environment. The effect of initial pH, contact time, U concentration, and temperature on U(VI) adsorption onto L-HA was assessed. The U(VI) adsorption was pH-dependent and achieved equilibrium in 2 h. It could be well described with pseudo-second-order model, indicating that U(VI) adsorption onto L-HA involved chemisorption. The U(VI) adsorption mass increased with increasing temperature with maximum adsorption capacities of 91, 112 and 120 mg g^-1 at 298, 308 and 318 K, respectively. The adsorption reaction was spontaneous and endothermic. We explored the processes of U(VI) desorption from the L-HA-U complex through batch desorption experiments in 1 mM NaNO₃ and in artificial seawater. The desorption process could be well described by pseudo-first-order model and reached equilibrium in 3 h. L-HA possessed a high propensity to adsorb U(VI). Once adsorbed, the release of U(VI) from L-HA-U complex was minimal in both 1 mM NaNO₃and artificial seawater (0.06% and 0.40%, respectively). Being abundant, inexpensive, and safe, L-HA has good potential for use as a U adsorbent from aqueous solution or immobilizing U in soils.

Antagonistic Interplay Between an Intermolecular CH···O and an Intramolecular OH···O Hydrogen Bond in a 1:1 Complex Between 1,2-Cyclohexanedione and Chloroform: A Combined Matrix Isolation Infrared and Quantum Chemistry Study

Matrix isolation infrared spectra of a weak C-H···O hydrogen-bonded complex between the keto-enol form of 1,2-cyclohexanedione (HCHD) and chloroform have been measured. The spectra reveal that the intramolecular O-H···O H-bond of HCHD is weakened as a result of complex formation, manifesting in prominent blue shift (∼23 cm^-1) of the ν_O-H band and red shifts (∼7 cm^-1) of ν_C═O bands of the acceptor (HCHD). The ν_C-H band of donor CHCl₃ undergoes a large red shift of ∼33 cm^-1. Very similar spectral effects are also observed for formation of the complex in CCl₄ solution at room temperature. Our analysis reveals that out of several possible iso-energetic conformational forms of the complex, the one involving antagonistic interplay between the two hydrogen bonds (intermolecular C-H···O and intramolecular O-H···O) is preferred. The combined experimental and calculated data presented here suggest that in condensed media, conformational preferences are guided by directional hyperconjugative charge transfer interactions at the C-H···O hydrogen bonding site of the complex.

Evidence of blue-shifting N–H⋯N hydrogen bonding despite elongation of the N–H bond

N–H⋯N hydrogen bonding between pyrrole and N₂ results blue shift of ν_N–H accompanied by an increase in the N–H bond length.

A theoretical investigation on the conformation and the interaction of CHF₂OCF₂CHF₂ (desflurane II) with one water molecule

The conformation and the interaction of CHF₂OCF₂CHF₂ (desflurane II) with one water molecule is investigated theoretically using the ab initio MP2/aug-cc-pvdz and DFT-based M062X/6-311++G(d,p) methods. The calculations include the optimized geometries, the harmonic frequencies of relevant vibrational modes along with a natural bond orbital (NBO) analysis including the NBO charges, the hybridization of the C atom and the intra- and intermolecular hyperconjugation energies. In the two most stable conformers, the CH bond of the F2HCO- group occupies the gauche position. The hyperconjugation energies are about the same for both conformers and the conformational preference depends on the interaction between the non-bonded F and H atoms. The deprotonation enthalpies of the CH bonds are about the same for both conformers, the proton affinity of the less stable conformer being 3 kcal mol−1 higher. Both conformers of desflurane II interact with water forming cyclic complexes characterized by CH…O and OH…F hydrogen bonds. The binding energies are moderate, ranging from −2.4 to −3.2 kcal mol−1 at the MP2 level. The origin of the blue shifts of the ν(CH) vibrations is analyzed. In three of the complexes, the water molecule acts as an electron donor. Interestingly, in these cases a charge transfer is also directed to the non bonded OH group of the water molecule. This effect seems to be a property of polyfluorinated ethers.

Strong hyperconjugative interactions in isolated and water complexes of desflurane: a theoretical investigation

Ab initio MP2/aug-cc-pvDZ and density functional B3LYP calculations with the 6-311++G(d,p) basis set are performed to investigate the conformation of desflurane (CHF2OCHFCF3), its acidity/basicity and its interaction with one water molecule. The calculations include the optimized geometries, the harmonic frequencies of relevant vibrational modes, the binding energies with water, and a detailed natural bond orbital (NBO) analysis Iincluding the NBO charges, the hybridization of the C atoms and the intra- and intermolecular hyperconjugations. The relative energies of the two most stable conformers are discussed as a function of the total hyperconjugative energies resulting from the interaction of lone pairs of the O and F atoms to the different antibonding orbitals of desflurane. The proton affinity is the same for both conformers but the acidity of the CH bond is larger for the less stable conformer. The binding energies of the complexes of two desflurane conformers with one water molecule range from -2.75 to -3.23 kcal mol(-1). Depending on the structure of the complexes, the CH bonds involved in the interaction are contracted or elongated. The σ*(CH) occupation predominates over the hybridization effect in determining the CH bond length. There is an unexpected charge transfer to the external OH bond of the water molecule. This effect is in good agreement with theoretical data on the complexes between fluorinated dimethyl ethers and water and seems to depend on the number of F atoms implanted on the ether molecule.

Weak H-bonds. Comparisons of CH···O to NH···O in proteins and PH···N to direct P···N interactions

Whereas CH···O H-bonds are usually weaker than interpeptide NH···O H-bonds, this is not necessarily the case within proteins. The nominally weaker CH···O are surprisingly strong, comparable to, and in some cases stronger than, the NH···O H-bonds in the context of the forces that hold together the adjacent strands in protein β-sheets. The peptide NH is greatly weakened as proton donor in certain conformations of the protein backbone, particularly extended structures, and forms correspondingly weaker H-bonds. The PH group is a weak proton donor, but will form PH···N H-bonds. However, there is a stronger interaction in which P can engage, in which the P atom, not the H, directly approaches the N electron donor to establish a direct P···N interaction. This approach is stabilized by the same sort of electron transfer from the N lone pair to the P-H σ* antibond that characterizes the PH···N H-bond.