Theoretical investigations into the bonding and separation properties of non-rigid, partially rigid, and rigid ligands derived from Et-Tol-PTA with trivalent lanthanides and actinides.

The separation of trivalent actinide elements from lanthanide elements represents one of the most formidable challenges within the context of nuclear waste partitioning and transmutation (P&T) processes. Consequently, we embarked on a systematic investigation aimed at elucidating the bonding properties and thermodynamic behavior of a N-ethyl-N-tolyl-2-amide-1,10-phenanthroline (Et-Tol-PTA) ligand in conjunction with trivalent actinide and lanthanide elements. This investigation involved the utilization of various density functional theory (DFT) methods and a comparative analysis between small-core pseudopotential basis sets and all-electron basis sets. It was found that well-performing results were achieved using the PBE0 functional in both bond length and thermodynamic energy calculations, with minimal impact being exerted by the basis set on the results. Furthermore, an exploration was carried out into the bonding and thermodynamic properties of trivalent actinides and lanthanides with ligands derived from Et-Tol-PTA, encompassing non-rigid (La), partially rigid (Lb, Lc), and rigid (Ld) ligands. Thermodynamically, advantages in the separation of Am(III)/Eu(III) were exhibited by Lb and Lc ligands, while excellent performance in the separation of Am(III)/Cm(III) was demonstrated by the La ligand. Analyses conducted using quantum theory of atoms in molecules (QTAIM), reduced density gradient (RDG), and natural bond orbital (NBO) methodologies revealed the presence of partial covalent character in the bonds between oxygen (O) and metal (M), as well as between nitrogen (N) and metal (M), with a higher degree of covalent character being observed in O-Am and N-Am bonds compared to O-Cm/Eu and N-Cm/Eu interactions.

Theoretical investigation on the ligands constructed from phenanthroline and five-membered N-heterocyclic rings for bonding and separation properties of Am(III) and Eu(III).

The ligands, derived from the combination of phenanthroline and various five-membered N-heterocyclic rings, were subject to a comprehensive investigation for their potential in the extraction and separation of actinides and lanthanides. This study employed DFT methods to thoroughly explore the properties of both phenanthroline (Ph) and the diverse five-membered N-heterocyclic rings (R1-R8). Additionally, tridentate ligands RlPh (l = 1-8) and tetradentate ligands RlPhRr (l, r = 1-8) were analyzed in detail, encompassing their electrostatic potential (ESP), protonation energy, coordination bonding with the metals Am(III) and Eu(III), and the thermodynamics of extraction separation for Am(III) and Eu(III). The findings highlight that the electrostatic potential (ESP) and binding capabilities of the five-membered N-heterocyclic ring units serve as effective predictors for the properties of intricate tridentate and tetradentate ligands, as well as their coordination bonding affinity with metals. The ligands' binding energy is closely associated with their ESP, and notably, the binding energy of tridentate and tetradentate ligands correlates well with the binding energies of their constituent structural units. The computational results reveal that the R2 unit, along with its corresponding tridentate ligand R2Ph and tetradentate ligands R2PhRr, exhibits the highest ESP, superior binding energies, and the strongest coordination bonding affinity with the metals. The theoretical calculations further identify several promising extractants for the effective separation of Am(III) and Eu(III). The tridentate ligands R1Ph, R7Ph, and R4Ph, and the tetradentate ligands R4PhR4, R6PhR6, R2PhR2, R1PhR5 and R3PhR6 were identified as having excellent separation performance for Am(III) and Eu(III). This study would provide insights for the design of extractants for the separation of Am(III) and Eu(III) by use of five-membered N-heterocyclic rings as structural units.

Theoretical Study on the Coordination and Separation Capacity of Macrocyclic N-Donor Extractants for Am(III)/Eu(III).

Designing ligands that can effectively separate actinide An(III)/lanthanide Ln(III) in the solvent extraction process remains one of the key issues in the treatment of accumulated spent nuclear fuel. Nitrogen donor ligands are considered as promising extractants for the separation of An(III) and Ln(III) due to their environmental friendliness. Four new macrocyclic N-donor hexadentate extractants were designed and their coordination with Am(III) and Eu(III), as well as their extraction selectivity and separation performance for Am(III) and Eu(III), were investigated by scalar relativistic density functional theory. A variety of theoretical methods have been used to evaluate the properties of the four ligands and the coordination structures, bonding properties, and thermodynamic properties of the complexes formed by the four ligands with Am(III) and Eu(III). The results of various wavefunction analysis methods including NBO analysis, quantum theory of atoms in molecules (QTAIM) analysis, and so on show that Am(III) has a stronger coordination ability with the ligands than Eu(III) due to the Am 5f orbitals more involved in bonding with the ligands than the Eu 4f orbitals, and the bonding environment of the N-donor in the ligand has a significant effect on its coordination ability of the metal ions. Thermodynamic analysis of the solvent extraction process shows that all of the four N-containing macrocyclic ligands have good extraction selectivity and separation performance for Am(III) and Eu(III). This study provides theoretical support for designing potential nitrogen-containing macrocyclic extractants with excellent separation performance.

Long-bonding and bonding nature in noble gas insertion compounds MNgBY of transition metal-boron bond.

The nature of inert gas bonding has always been an important topic. The bonds of noble gases cover the entire range of chemical bonds, from the weakest van der Waals forces, to non-covalent interactions, and to covalent bonds. Two types of methods were used to investigate the properties of chemical bonds in the inert gas inserted compound MNgBY with the transition metal M = Cu/Ag/Au and substituents Y = O/S/NH, one based on orbital analysis and the other based on electron density analysis. The NBO/NRT analysis shows that in these compounds there exists long-bonding striding the noble gas between the transitional metal and boron, similar to the noble gas insertion compounds HNgX of hydrohalide, and so a three-center four-electron bond exists among the M-Ng-B part. The electron density analyses show that the M-Ng bond between the metal Cu/Ag/Au and noble gas and the Ng-B bond in the Cu/Ag compounds are partial covalent but the Ng-B bond in Au compounds is a typical covalent bond. The large relativistic effects of Au cause the bonds in Au compounds shorter and stronger than the bonds in Ag/Cu compounds. The properties of the M-Ng and Ng-B bonds are not affected by substituents Y, but the bond lengths are sensitive to substituents.

RgnBe3B3+: theoretical investigation of Be3B3+ and its rare gas capability.

A series of Be3B3+ and its rare gas (Rg) containing complexes RgnBe3B3+ (Rg = He-Rn, n = 1-6) have been predicted theoretically using the B3LYP, MP2, and CCSD(T) methods to explore structures, stability, charge distributions, and nature of bonding. Both Be3B3+ and RgBe3B3+ are the global minima on the potential energy surfaces. In the RgnBe3B3+ complexes, the dissociation energy drops with the increase in number of Rg. Natural bond orbital (NBO) and topological analysis of the electron density (AIM) show that the Rg-Be bonds for Kr-Rn have some covalent character. The Rg-Be bond is stabilized dominantly by the Rg → Be3B3+ σ-donation from the valence p orbital of Rg to the vacant valence LUMO orbital of Rgn-1Be3B3+. Besides, other two π-donations also play important roles in stabilizing the Rg-Be bonds.

OBCN isomerization and noble gas insertion compounds of identical valence electron number species: stability and bonding.

A series of new noble gas (Ng) insertion compounds of the general type XNgX, XNgY and XNgY+ has been theoretically studied using ab initio and DFT methods herein. We first studied the isomerization process of the OBCN compound, and then investigated the bonding properties and stability of the compounds formed by inserting Ng into the single bond of the three low energy isomers by high-level ab initio calculations. The OBNgCN compounds are thermochemically stable with respect to all dissociation channels except for the processes of releasing OBCN/OBNC and free Ng. Furthermore, the two dissociation processes OBNgCN → Ng + OBNC and OBNgNC → Ng + OBCN are kinetically prohibited by the relatively high free energy barrier ranging from 22.7 to 31.7 kcal mol-1 except for the OBKrCN and OBKrNC analogues. And the adaptive natural density partitioning (AdNDP) analysis indicated that chemical bonding in OBNgCN compounds is realized via a delocalized 3-center 2-electron (3c-2e) σ-bond in the B-Ng-C moiety and a totally delocalized 5-center 2-electron (5c-2e) σ-bond in the whole O-B-Ng-C-N. Natural bond orbital (NBO) theory, atoms-in-molecules (AIM) and energy decomposition analysis (EDA) based on the molecular wavefunction revealed that the B-Ng bond and Ng-C bond have some covalent character in OBNgCN. In addition, the calculation and detailed bonding analysis on a large number of neutral and monocationic compounds with identical valence electron numbers to OBNgCN demonstrate that the two bonds directly linked to the Ng atoms have covalent properties in neutral compounds, whereas Ng forms one typical covalent bond and one partial covalent and partial ionic bond with the neighboring atoms in the monocationic compounds.

Noble gas inserted compounds of borazine and its derivative B3N3R6: structures and bonding.

Quantum chemistry computations were performed at the MP2 and B3LYP levels of theory using the basis sets aug-cc-pVDZ and def2-TZVPPD to study the noble gas (Ng) compounds formed by insertion of a Ng atom (Kr, Xe, Rn) into the B-H/F and N-H/F bonds of inorganic benzene B3N3H6 and its fluorine derivative B3N3F6. The geometrical structures were optimized and vibrational analysis was carried out to demonstrate these structures being local minima on the potential energy surface. The thermodynamic properties of the formation process of Ng compounds were calculated. A series of theoretical methods based on the wavefunction analysis, including NBO, AIM and ELF methods and energy decomposition analysis, was used to investigate the bonding nature of the noble gas atoms and the properties of the Ng compounds. The N-Ng bond was found to be stronger than the B-Ng bond, but the B-Ng bond is of typical covalent character and σ-donation from the Ng atom to the ring B atom makes the predominant contribution towards stability of the B-Ng bond. NICS calculation shows that these Ng-containing compounds are of weak π-aromaticity.

Compounds with Rare Gas-Selenium/Tellurium Bonds: A Theoretical Investigation on FRgLF n and FRgLF n-1+ (Rg = Kr-Rn, L = Se and Te, n = 1, 3, and 5).

A new type of interesting insertion compounds FRgLF n (Rg = Kr-Rn, L = Se and Te, n = 1, 3 and 5) and ionic FRgLF n-1+ obtained through the insertion of a rare gas atom into the selenium fluorides and tellurium fluorides have been explored theoretically using MP2, CCSD(T), and PBE0 calculations. These predicted species were examined to present the optimized geometries, vibrational modes, molecular properties, thermodynamic and kinetic stabilities and bond nature. The optimized structures are without imaginary frequencies and metastable. In neutral FRgLF n, F-Rg bonds should be of ionic character with large dissociation energy ranging from 150-200 kcal mol-1 that could be best described by F-(RgLF n)+. Rg-L bonds have some covalent character with lower interaction energies within the range 25-40 kcal mol-1. In FRgL+ and FRgLF2+, the bonding nature of the F-Rg and Rg-L bonds are somewhat similar to that of the neutral compounds. In FRgLF4+, the F-Rg bond could be of partial covalent type but the Rg-L bond could be considered as an ionic bond.

Hydrogen bond strengthening between o-nitroaniline and formaldehyde in electronic excited states: A theoretical study.

To study the hydrogen bonds upon photoexcited, the time dependent density function method (TD DFT) was performed to investigate the excited state hydrogen bond properties of between o-nitroaniline (ONA) and formaldehyde (CH2O). The optimized structures of the complex and the monomers both in the ground state and the electronically excited states are calculated using DFT and TD DFT method respectively. Quantum chemical calculations of the electronic and vibrational absorption spectra are also carried out by TD DFT method at the different level. The complex ONA⋯CH2O forms the intramolecular hydrogen bond and intermolecular hydrogen bonds. Since the strength of hydrogen bonds can be measured by studying the vibrational absorption spectra of the characteristic groups on the hydrogen bonding acceptor and donor, it evidently confirms that the hydrogen bonds is strengthened in the S1/S2/T1 excited states upon photoexcitation. As a result, the hydrogen bonds cause that the CH stretch frequency of the proton donor CH2O has a blue shift, and the electron excitations leads to a frequency red shift of NO and NH stretch modes in the o-nitroaniline(ONA) and a small frequency blue shift of CH stretch mode in the formaldehyde(CH2O) in the S1 and S2 excited states. The excited states S1, S2 and T1 are locally excited states where only the ONA moiety is excited, but the CH2O moiety remains in its ground state.

Noble gas supported boron-pentagonal clusters B5Ngn3+: exploring the structures and bonding.

A novel type of trivalent BNg five-membered cational species B5Ngn3+(Ng = He~Rn, n = 1~5) has been found and investigated theoretically using the B3LYP and MP2 methods with the def2-QZVPPD and def2-TZVPPD basis sets. The geometry, harmonic vibrational frequencies, bond energies, charge distribution, bond nature, aromaticity, and energy decomposition analysis of these structures were reported. The calculated B-Ng bond energy is quite large (the averaged bond energy is in the range of 209.2~585.76 kJ mol-1) for heavy rare gases and increases with the Ng atomic number. The analyses of the molecular wavefunction show that in the BNg compounds of heavy Ng atoms Ar~Rn, the B-Ng bonds are of typical covalent character. Nuclear independent chemical shifts display that both B53+ and B5Ngn3+(n=1~5) have obvious aromaticity. Energy decomposition analysis shows that these BNg compounds are mainly stabilized by the σ-donation from the Ng valence p orbital to the B53+ LUMO. These findings offer valuable clues toward the design and synthesis of new stable Ng-containing compounds.

Monocyclic aromatic compounds BnRgn(n-2)+ of boron and rare gases.

The monocyclic compounds (BRg)3+(D3h), (BRg)42+(D4h), (BRg)53+(D5h) and (BRg)64+(D6h) formed between boron and rare gases Rg (He-Rn) are theoretically predicted to be stable structures and have π-aromaticity with a delocalized nc-2e π-system. For heavier rare gases Ar-Rn, the B-Rg bond energy is quite high and ranges from 15 to 96 kcal mol-1, increasing with the ring size and the atomic number of rare gases; the B-Rg bond length is close to the sum of covalent radii of B and Rg atoms; NBO and AIM analyses show that the B-Rg bonds for Ar-Rn have a typical covalent character. The B-Rg bond is stabilized mainly by σ-donation from the valence p orbital of Rg to the vacant valence orbital of the boron ring. We searched for a large number of isomers for the systems of Ar and found that the titled monocyclic compounds (BAr)3+(D3h), (BAr)42+(D4h) and (BAr)53+(D5h) should be global energy minima. For (BAr)64+ the global energy minimum is an octahedral caged structure, but the titled monocyclic compound is the secondary stable local energy minimum. The energy and thermodynamic stability of the ring BnRgn(n-2)+ cations indicate that these rare gas compounds may be viable species in experiments.

Substituent effects on the properties of the hemi-bonded complexes (XH2P···NH2Y)(+) (X, Y=H, F, Cl, Br, NH2, CH3, OH).

Ab initio calculations have been performed to study the structures, binding energies, and bonding properties of the hemi-bonded binary complexes (XH2P···NH2Y)(+) with the substituents X and Y being H, F, Cl, Br, NH2, CH3, and OH. The P···N interactions in these open-shelled systems have typical pnicogen bond characteristics but much stronger than the usual pnicogen bonds in closed-shell systems. This P···N bond can be strengthened by an electron-withdrawing substituent X or an electron-donating substituent Y, the bonding energy varies from 17 kcal mol(-1) of (CH3H2P···NH2F)(+) to 54 kcal mol(-1) of (FH2P···NH2CH3)(+). A nearly linear X-P···N arrangement is required by the pnicogen bond P···N and results in a strong hyperconjugation and charge transfer from the N lone pair to the X-P σ* antibond orbital for α spin, the P···N interaction is described as a single-electron σ bond of ß spin. The AIM and NBO analyses revealed that the P···N bonds in the majority of the hemi-bonded complexes are partly covalent in nature. Graphical Abstract The P···N interactions in the open-shelled systems (XH2P···NH2Y)(+) (X, Y=H, F, Cl, Br, NH2, CH3, OH) with bonding energy of 17~54 kcal mol(-1) have typical pnicogen bond characteristics but much stronger than the usual pnicogen bonds in closed-shell systems. This P···N bond can be strengthened by an electron-withdrawing substituent X or an electron-donating substituent Y.

Theoretical Predictions of C3v Symmetric Three-H-Bridged Noble Gas Compounds NgBeH3BeR, NgBeH3BR(+), and NgBH3BR(2).

A new series of stable noble gas-Lewis acid compounds NgBeH3BeR, NgBeH3BR(+), and NgBH3BR(2+) (R = F, H, CH3, Ng = He-Rn) with three 3c-2e H-bridged bonds have been predicted by use of the PBE0 and MP2 methods. The Ng-Be/B bonds are strong and have large binding energies 35-130, 9-38, and 4-13 kcal/mol for the doubly charged cations, singly charged cations, and neutral molecules, respectively. The binding energy and strength of the Ng-Be/B bonds increase largely from He to Rn but are insensitive to electronegativity of the substituent R. The Ng-B bonds in NgBH3BR(2+) should be typical covalent bonds and the Ng-Be bonds in NgBeH3BR(+) for heavy Ng atoms Kr, Xe, and Rn have some covalent character. The three bridging-H atoms have characteristic infrared vibrational modes with large IR intensity to be detected in spectroscopy experiments.

A comparative study of the chalcogen bond, halogen bond and hydrogen bond S⋯O/Cl/H formed between SHX and HOCl.

Ab initio quantum chemistry methods were used to analyze the noncovalent interactions between HOCl and SHX (X = F, CN, NC, Cl, Br, NO2, CCH, CH3, H). Three energetic minimal configurations were characterized for each case, where the S center acts as a Lewis acid interacting with O to form a chalcogen bond, as well as a Lewis base interacting with Cl or H of HOCl to form halogen bond and hydrogen bond, respectively. An electronegative substituent such as F, CN, NC and NO2 tends to form a stronger chalcogen bond, while an electropositive substituent such as CCH, CH3 and H is inclined to form a more stable H-bonded complex. The chalcogen-bonded, halogen-bonded and H-bonded complexes are stabilized by charge transfers from Lp(O) to σ*(SX), from Lp(S) to σ*(ClO), and from Lp(S) to σ*(HO), respectively. As a result, the SHX unit becomes positively charged in halogen-bonded and hydrogen-bonded complexes but negatively charged in chalcogen-bonded complexes. Theory of atoms in molecules, natural bond orbital analysis, molecular electrostatic potential and localized molecular orbital energy decomposition analysis were applied to investigate these noncovalent bonds.

Theoretical study on effects of hydrogen bonding on the ring stretching modes of pyridine.

Pyridine generally acts as the proton acceptors in the hydrogen bonding interaction by using its lone pair n(N) or pi-electrons. Some previous research indicated that for the N-type H-bond, the ring breathing mode v(1), the N-para-C stretching mode v(6a) and the meta-CC stretching mode v(8a) of pyridine showed a frequency blueshift but the triangle mode v(12) had no change in frequency. Both electrostatic interaction and charge transfer caused by intermolecular hyperconjugation n(N)-->sigma( *)(HX) have contributions to the frequency blue shifts, while charge transfer is predominant at equilibrium intermolecular distance. An intramolecular hyperconjugation between the lone pair n(N) and the two sigma( *)(meta-CC) orbitals in the pyridine ring provides a reasonable interpretation for the effect of charge transfer on the ring stretching modes upon formation of the N-type H-bonding.

Electronic properties of multifurcated bent hydrogen bonds CH3...Y and CH2...Y.

H-bonding angle angleYHX has an important effect on the electronic properties of the H-bond Y...HX, such as intra- and intermolecular hyperconjugations and rehybridization, and topological properties of electron density. We studied the multifurcated bent H-bonds of the proton donors H3CZ (Z = F, Cl, Br), H2CO and H2CF2 with the proton acceptors Cl(-) and Br(-) at the four high levels of theory: MP2/6-311++G(d,p), MP2/6-311++G(2df,2p), MP2/6-311++G(3df,3pd) and QCISD/6-311++G(d,p), and found that they are all blue-shifted. These complexes have large interaction energies, 7-12 kcal mol(-1), and large blue shifts, delta r(HC) = -0.0025 --0.006 A and delta v(HC) = 30-90 cm(-1). The natural bond orbital analysis shows that the blue shifts of these H-bonds Y...HnCZ are mainly caused by three factors: rehybridization; indirect intermolecular hyperconjugation n(Y) -->sigma*(CZ), in that the electron density from n(Y) of the proton acceptor is transferred not to sigma*(CH), but to sigma*(CZ) of the donor; intramolecular hyperconjugation n(Z) -->sigma*(CH), in that the electron density in sigma*(CH) comes back to n(Z) of the donor such that the occupancy in sigma*(CH) decreases. The topological properties of the electron density of the bifurcated H-bonds Y...H2CZ are similar to those of the usual linear H-bonds, there is a bond critical point between Y and each hydrogen, and a ring critical point inside the tetragon YHCH. However, the topological properties of electron density of the trifurcated H-bonds Y...H3CZ are essentially different from those of linear H-bonds, in that the intermolecular bond critical point, which represents a closed-shell interaction, is not between Y and hydrogen, but between Y and carbon.

Chemical origin of blue- and redshifted hydrogen bonds: intramolecular hyperconjugation and its coupling with intermolecular hyperconjugation.

Upon formation of a H bond Y...H-XZ, intramolecular hyperconjugation n(Z)-->sigma*(X-H) of the proton donor plays a key role in red- and blueshift characters of H bonds and must be introduced in the concepts of hyperconjugation and rehybridization. Intermolecular hyperconjugation transfers electron density from Y to sigma*(X-H) and causes elongation and stretch frequency redshift of the X-H bond; intramolecular hyperconjugation couples with intermolecular hyperconjugation and can adjust electron density in sigma*(X-H); rehybridization causes contraction and stretch frequency blueshift of the X-H bond on complexation. The three factors--intra- and intermolecular hyperconjugations and rehybridization--determine commonly red- or blueshift of the formed H bond. A proton donor that has strong intramolecular hyperconjugation often forms blueshifted H bonds.

Theoretical investigation of hydrogen bonds between CO and HNF2, H2NF, and HNO.

Ab initio quantum mechanics methods were applied to investigate the hydrogen bonds between CO and HNF2, H2NF, and HNO. We use the Hartree-Fock, MP2, and MP4(SDQ) theories with three basis sets 6-311++G(d,p), 6-311++G(2df,2p), and AUG-cc-pVDZ, and both the standard gradient and counterpoise-corrected gradient techniques to optimize the geometries in order to explore the effects of the theories, basis sets, and different optimization methods on this type of H bond. Eight complexes are obtained, including the two types of C...H-N and O...H-N hydrogen bonds: OC...HNF2(C(s)), OC...H2NF(C(s) and C1), and OC...HNO(C(s)), and CO...HNF2(C(s)), CO...H2NF(C(s) and C1), and CO...HNO(C(s)). The vibrational analysis shows that they have no imaginary frequencies and are minima in potential energy surfaces. The N-H bonds exhibit a small decrease with a concomitant blue shift of the N-H stretch frequency on complexation, except for OC...HNF2 and OC...H2NF(C1), which are red-shifting at high levels of theory and with large basis sets. The O...H-N hydrogen bonds are very weak, with 0 K dissociation energies of only 0.2-2.5 kJ/mol, but the C...H-N hydrogen bonds are stronger with dissociation energies of 2.7-7.0 kJ/mol at the MP2/AUG-cc-pVDZ level. It is notable that the IR intensity of the N-H stretch vibration decreases on complexation for the proton donor HNO but increases for HNF2 and H2NF. A calculation investigation of the dipole moment derivative leads to the conclusion that a negative permanent dipole moment derivative of the proton donor is not a necessary condition for the formation of the blue-shifting hydrogen bond. Natural bond orbital analysis shows that for the C...H-N hydrogen bonds a large electron density is transferred from CO to the donors, but for the O...H-N hydrogen bonds a small electron density transfer exists from the proton donor to the acceptor CO, which is unusual except for CO...H2NF(C(s)). From the fact that the bent hydrogen bonds in OC(CO)...H2NF(C(s)) are quite different from those in the others, we conclude that a greatly bent H-bond configuration shall inhibit both hyperconjugation and rehybridization.