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A carbon-halogen bound fission succeeds gas-phase protonation of 1-fluoro- and 1-chloroadamantane. The dissociative proton attachment reactions have been established. 1-chloroadamantane is less basic in the gas phase than 1-fluoroadamantane. An ion-dipole complex is evident between the adamantyl cation and the associated hydrogen halide molecule.

A study of spectroscopic data on the gas-phase proton affinities (PAs) of 32 C-substituted pyrazoles and N-methyl pyrazoles as well as of 12 C-methyl- substituted pyrazoles reveal a simple additive pattern of substituent effects. Protonation energies calculated by ab initio 6-31G/6-31G methods are shown to correlate linearly with the experimental PAs. The analysis of substituent effects on PAs in terms of polarizability and field and resonance contributions suggests that, except for 3-aminopyrazole, the 3(5)-substituent does not play a major role in pyrazole's tautomeric behavior.

A theoretical study of the interaction between melamine and CO 2 was carried out using density functional theory (DFT) with the B3LYP-D3(BJ)/aug-cc-pVTZ level of theory. The presence of anions interacting with melamine transforms the weakly bonded tetrel complexes into adducts. Thus, melamine acts as an FLP (frustrated Lewis pair) with acid groups (NHs as hydrogen bond donors) and a base group (N of the triazine ring). The application of the relative energy gradient formalism (REG) along the reaction coordinate has demonstrated that the ability of the melamine-anion systems to capture CO 2 is linked to its capacity to polarize the CO 2 molecule. These results have been confirmed by placing the melamine:CO 2 complex in a uniform electric field with different strengths.

Multiply charged complexes bound by noncovalent interactions have been previously described in the literature, although they were mostly focused on organic and main group inorganic systems. In this work, we show that similar complexes can also be found for organometallic systems containing transition metals and deepen in the reasons behind the existence of these species. We have studied the structures, binding energies, and dissociation profiles in the gas phase of a series of charged hydrogen-bonded dimers of metallocene (Ru, Co, Rh, and Mn) derivatives isoelectronic with the ferrocene dimer. Our results indicate that the carboxylic acid-containing dimers are more strongly bonded and present larger barriers to dissociation than the amide ones and that the cationic complexes tend to be more stable than the anionic ones. Additionally, we describe for the first time the symmetric proton transfer that can occur while in the metastable phase. Finally, we use a density-based energy decomposition analysis to shine light on the nature of the interaction between the dimers.

The structure, stability, and bonding characteristics of dimers and trimers involving BX 3 and AlX 3 (X = H, F, Cl) in the gas phase, many of them explored for the first time, were investigated using different DFT (B3LYP, B3LYP/D3BJ, and M06-2X) and ab initio (MP2 and G4) methods together with different energy decomposition formalisms, namely, many-body interaction-energy and localized molecular orbital energy decomposition analysis. The electron density of the clusters investigated was analyzed with QTAIM, electron localization function, NCIPLOT, and adaptive natural density partitioning approaches. Our results for triel hydride dimers and Al 2 X 6 (X = F, Cl) clusters are in good agreement with previous studies in the literature, but in contrast with the general accepted idea that B 2 F 6 and B 2 Cl 6 do not exist, we have found that they are predicted to be weakly bound systems if dispersion interactions are conveniently accounted for in the theoretical schemes used. Dispersion interactions are also dominant in both homo- and heterotrimers involving boron halide monomers. Surprisingly, B 3 F 9 and B 3 Cl 9 C 3 v cyclic trimers, in spite of exhibiting rather strong B-X (X = F, Cl) interactions, were found to be unstable with respect to the isolated monomers due to the high energetic cost of the rehybridization of the B atom, which is larger than the two- and three-body stabilization contributions when the cyclic is formed. Another important feature is the enhanced stability of both homo- and heterotrimers in which Al is the central atom because Al is systematically pentacoordinated, whereas this is not the case when the central atom is B, which is only tri- or tetra-coordinated.

Mössbauer and X-ray photoelectron spectroscopies (XPS) are complemented with high-level quantum-chemical computations in the study of the geometric and electronic structure of the paramagnetic salt of the metallacarborane sandwich complex [Fe(1,2-C 2 B 9 H 11 ) 2 ]Cs = FeSanCs. Experimental 57 Fe isomer shifts and quadrupole splitting parameters are compared with the theoretical prediction, with good agreement. The appearance of two sets of Cs(3d) doublets in the XPS spectrum, separated by 2 eV, indicates that Cs has two different chemical environments due to ease of the Cs (+) cation moving around the sandwich complex with low-energy barriers, as confirmed by quantum-chemical computations. Several minimum-energy geometries of the FeSanCs structure with the corresponding energies and Mössbauer parameters are discussed, in particular the atomic charges and spin population and the surroundings of the Fe atom in the complex. The Mössbauer spectra were taken at different temperatures showing the presence of a low-spin Fe atom with S = 1/2 and thus confirming a paramagnetic Fe III species., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

N -Methyl-2-methoxymethylanilines 1 bearing various 5-substituted-pyrimidin-2-yl groups were prepared, and their rotational behaviors were explored in detail. It was revealed that the rotational barriers around two N-Ar bonds increase in proportion to the electron-withdrawing ability of substituents X at the 5-position.

The reaction of NH -indazoles with formaldehyde in aqueous hydrochloric acid has been experimentally studied by solution and solid-state nuclear magnetic resonance (NMR) and crystallography. The mechanism of the formation of N 1 -CH 2 OH derivatives was determined. For the first time, 2-substituted derivatives have been characterized by multinuclear NMR. Theoretically, calculations with gauge-invariant atomic orbitals (GIAOs) at the Becke three-parameter (exchange) Lee-Yang-Parr B3LYP/6-311++G(d,p) level have provided a sound basis for the experimental observations. The first X-ray structures of four (1 H -indazol-1-yl)methanol derivatives are reported.

Frustrated Lewis pairs (FLPs) based on nitrogen heterocycles (pyridine, pyrazole, and imidazole) with a silane or germane group in the α-position of a nitrogen atom have been considered as potential molecules to sequestrate carbon dioxide. Three stationary points have been characterized in the reaction profile: a pre-reactive complex, an adduct minimum, and the transition state connecting them. The effect of external (solvent) or internal (hydroxyl group) electric fields in the reaction profile has been considered. In both cases, it is possible to improve the kinetics and thermodynamics of the complexation of CO 2 by the FLP and favor the formation of adducts.

In the present paper, we report the quantitative evaluation of the electron density shift (EDS) maps within different complexes. Values associated with the total EDS maps exhibited good correlation with different quantities such as interaction energies, E int , intermolecular distances, bond critical points, and LMOEDA energy decomposition terms. Besides, EDS maps at different cutoffs were also evaluated and related with the interaction energies values. Finally, EDS maps and their corresponding values are found to correlate with E int within systems with cooperative effects. To our knowledge, this is the first time that the EDS has been quanitatively evaluated.

Nine 3,5-dihydro-4 H -pyrido[2,3- b ][1,4]diazepin-4-ones ( 17 - 25 ), some of which contain fluoro-substituents, have been regiospecifically prepared by reaction of 2,3-diaminopyridines with ethyl aroylacetates. In two cases, open intermediates have been isolated and these are related to the reaction pathway. The X-ray crystal structure of 1-methyl-4-phenyl-3,5-dihydro-4 H -pyrido[2,3- b ][1,4]diazepin-4-one ( 23 ) has been solved (formula, C 15 H 13 N 3 O; crystal system, monoclinic; space group, C 2/ c ). This is an asymmetric unit constituted by a single nonplanar molecule and its conformational enantiomer due to the presence of the seven-membered diazepin-2-one moiety, which introduces a certain degree of torsion in the adjacent pyridine ring. The 1 H, 13 C, 15 N, and 19 F NMR spectra were obtained and the chemical shifts, together with those of the previously published 1,3-dihydro-2 H -benzo[ b ][1,4]diazepin-2-ones ( 1 - 16 ), i.e., a total of 544 values, were successfully compared with the chemical shifts calculated at the gauge invariant atomic orbital (GIAO)/Becke, three-parameter, Lee-Yang-Parr (B3LYP)/6-311++G(d,p) level. The seven-membered ring inversion barrier in 5-benzyl-2-phenyl-3,5-dihydro-4 H -pyrido[2,3- b ][1,4]diazepin-4-one ( 25 ) was determined and, in conjunction with the data from the literature, compared with the B3LYP/6-311++G(d,p) computed values. This allowed the determination of several structural effects. The rotation about the exocyclic N1-CR bond was also calculated and its dynamic properties were discussed., Competing Interests: The authors declare no competing financial interest.

Pnicogen bonds, which are weak noncovalent interactions (NCIs), can be significantly modified by the presence of beryllium bonds, one of the strongest NCIs known. We demonstrate the importance of this influence by studying ternary complexes in which both NCIs are present, that is, the ternary complexes formed by a nitrogen base (NH 3 , NHCH 2 , and NCH), a phosphine (fluorophosphane, PH 2 F) and a beryllium derivative (BeH 2 , BeF 2 , BeCl 2 , BeCO 3 , and BeSO 4 ). Energies, structures, and nature of the chemical bonding in these complexes are studied by means of ab initio computational methods. The pnicogen bond between the nitrogen base and the phosphine and the beryllium bond between the fluorine atom of fluorophosphane and the beryllium derivative show large cooperativity effects both on energies and geometries, with dissociation energies up to 296 kJ mol -1 and cooperativity up to 104 kJ mol -1 in the most strongly bound complex, CH 2 HN:PH 2 F:BeSO 4 . In the complexes between the strongest nitrogen bases and the strongest beryllium donors, phosphorus-shared and phosphorus-transfer bonds are found.

A conceptual DFT study of the dissociation of anionic and neutral phosphonate dimers has been carried out. In addition, the dianion complexes have been studied in the presence of two solvents, water and tetrahydrofuran. The dissociation of the dianion complexes in the gas phase and in solution present a maximum along the reaction coordinate that is not present in the neutral-neutral and anion-neutral complexes. The principal chemical descriptors (chemical potential, reaction electronic flux, hardness, and global electrophilicity index) do not show changes in their trends along the dissociation profiles even when there is an energy maximum in the case of the anion-anion complexes.

High-level G4 ab initio calculations allowed us to show that C 4 H 4 (BeX) 4 (X = H, Cl) derivatives behave as rather efficient electron capturers due to their ability to trap the extra electron through the formation of a four-membered beryllium ring. This finding is in agreement with previous work showing the ability of highly electron-deficient atoms, such as beryllium, to lead to multicenter one-electron bonds. In our particular case, the formation of the four-center bond is characterized, in very good harmony, by different topological methods such as quantum theory of atoms in molecules (QTAIM), the electron localization function (ELF), and the noncovalent interactions (NCI) approach and is accompanied by large electron affinity values, around 300 kJ·mol -1 , in the gas phase. Preliminary results may anticipate that the ability of groups of beryllium atoms to trap electrons decays on going to bigger systems.

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out in search of equilibrium dimers on (XCP) 2 potential energy surfaces, for X = CN, Cl, F, and H. Five equilibrium dimers with D ∞ h , C ∞ v , C s , C 2 h , and C 2 symmetries exist on the (ClCP) 2 potential energy surface, four on the (FCP) 2 and (HCP) 2 surfaces, and three on the (NCCP) 2 surface. These dimers are stabilized by traditional halogen, pnicogen, and tetrel bonds, and one of them by a hydrogen bond. The binding energies of the dimers (XCP) 2 vary from 3.0 to 22.0 kJ·mol -1 , with the strongest and weakest bonds found for complexes on the (NCCP) 2 surface. The binding energies of the linear D ∞ h and C ∞ v dimers on each surface differ by no more than 1.0 kJ·mol -1 , except for (NCCP) 2 , which has D ∞ h and C ∞ v complexes with binding energies of 3.0 and 11.0 kJ·mol -1 , respectively. The highly symmetric complexes with D ∞ h and C ∞ v symmetry are found on all surfaces and are the most weakly bound complexes on each surface. The structures of these dimers, the nature and strengths of charge-transfer interactions, the molecular graphs, and the molecular electrostatic potentials are useful for determining the type of intermolecular bond that stabilizes the dimers. EOM-CCSD spin-spin coupling constants 1p J (P-P) for complexes with P···P pnicogen bonds and D ∞ h symmetry are the largest coupling constants, ranging from 119 to 170 Hz. These increase with decreasing distance and follow a second-order trendline. The nature of the spin-spin coupling constants of these complexes is consistent with the type of noncovalent bond that stabilizes the dimers.

MP2/aug'-cc-pVTZ calculations have been performed in search of complexes, molecules, and transition structures on the HN(CH)SX:CO 2 potential energy surfaces, for X = F, Cl, NC, CN, CCH, and H. Complexes stabilized by traditional N···C tetrel bonds and O···S chalcogen bonds exist on all surfaces and are bound relative to the isolated monomers. Molecules stabilized by an N-C covalent bond and an O···S chalcogen bond are found when X = F, Cl, and NC, but only the HN(CH)SF:CO 2 molecule is bound. The binding energies of these complexes correlate with the O-S distance but not with the N-C distance. Binding energies of complexes rotated by 90° about the N···C tetrel bond and by 90° about the O···S chalcogen bond provide estimates of these bond energies. Charge-transfer energies across tetrel and chalcogen bonds correlate with the N-C and O-S distances, respectively. As a function of the N-C distance, equation-of-motion coupled cluster singles and doubles spin-spin coupling constants 1t J (N-C) for complexes and transition structures and 1 J (N-C) for molecules describe the evolution of the N···C tetrel bonds in the complexes and transition structures to N-C covalent bonds in the molecules. The O···S chalcogen bond gains some covalency in the transition structures and again in the molecules but does not become a covalent bond.

Weak and strong noncovalent interactions such as tetrel bonds and alkaline-earth bonds, respectively, cooperate and get reinforced when acting together in ternary complexes of general formula RN··· SiH 3 F···MY, where MY is a Be or Mg derivative and RN is a N-containing Lewis base with different hybridization patterns. Cooperativity has been studied in the optimized MP2/aug'-cc-pVTZ ternary complexes by looking at changes on geometries, binding energies, 29 Si NMR chemical shifts, and topological features according to the atoms in molecules theoretical framework. Our study shows that cooperativity in terms of energy is in general significant: more than 40 kJ/mol, and up to 83.6 kJ/mol in the most favorable case. The weakest the isolated interaction, the strongest the reinforcement in the ternary complex; in this sense, the tetrel bond is shortened enormously, between 0.3 and 0.6 Å. This dramatic reinforcement of the tetrel bond is also nicely reflected in the positive variations of the 29 Si chemical shifts in all the ternary complexes. At the same time the ternary complexes are characterized by the presence of totally planar silyl group, due to the pentacoordination of the Si atom. Both the hybridization of the N base and the geometry imposed by the alkaline-earth ligands have a strong influence on the binding energies, as they modify the donor ability of N and the Lewis acid character of the alkaline-earth metal.

Ab initio MP2/aug'-cc-pVTZ (where MP2 is Møller-Plesset perturbation theory) calculations have been carried out on four series of complexes, H 2 XP:ClF:HCl, H 2 XP:ClF:HF, H 2 XP:ClCl:HF, and H 2 XP:ClCl:HCl, to answer the question raised in the title of this paper. When X is F or Cl, binary complexes containing a P(V) molecule hydrogen bonded to an acid are found on all potential surfaces except H 2 ClP:ClF:HF, where an ion-pair complex exists. Ion-pair complexes also result from the optimization of H 2 XP:ClF:HF for X = NC, CN, and H. Changing the central molecule from ClF to ClCl has a dramatic effect on the nature of the optimized complexes when the substituents are NC, CN, and H. On the potential surfaces H 2 XP:ClCl:FH for X = NC and CN, open ternary complexes stabilized by a pnicogen bond and a hydrogen bond are found. Optimization of H 3 P:ClCl:FH leads to an ion pair. For H 2 (NC)P:ClCl:HCl and H 2 (CN)P:ClCl:HCl, cyclic ternary complexes stabilized by pnicogen, halogen, and hydrogen bonds result from optimization. Optimization of H 3 P:ClCl:HCl leads to a reaction in which H 2 ClP and a second HCl molecule are formed, and the resulting cyclic ternary complex is stabilized by two hydrogen bonds and a pnicogen bond. Thus, the type of complex resulting from the optimization of the starting ternary complex H 2 XP:ClY:HZ depends on the nature of the central molecule ClF or ClCl, the terminal molecule HCl or HF, and the substituent X. It is not possible to simply turn around the terminal HZ molecule in complexes H 2 XP:ClF:ZH for Z = F and Cl to give H 2 XP:ClF:HZ, thereby replacing a halogen bond by a hydrogen bond. Complexes H 2 XP:ClCl:HZ for X = NC and CN are stable complexes, but the corresponding halogen-bonded complexes H 2 XP:ClCl:ZH are not.

A set of complexes L 2 HB···BeX 2 (L = CNH, CO, CS, N 2 , NH 3 , NCCH 3 , PH 3 , PF 3 , PMe 3 , OH 2 ; X = H, F) containing a boron-beryllium bond is described at the M06-2X/6-311+G(3df,2pd)//M062-2X/6-31+G(d) level of theory. In this quite unusual bond, boron acts as a Lewis base and beryllium as a Lewis acid, reaching binding energies up to -283.3 kJ/mol ((H 2 O) 2 HB···BeF 2 ). The stabilization of these complexes is possible thanks to the σ-donor role of the L ligands in the L 2 HB···BeX 2 structures and the powerful acceptor nature of beryllium. According to the topology of the density, these B-Be interactions present positive laplacian values and negative energy densities, covering different degrees of electron sharing. ELF calculations allowed measuring the population in the interboundary B-Be region, which varies between 0.20 and 2.05 electrons upon switching from the weakest ((CS) 2 HB···BeH 2 ) to the strongest complex ((H 2 O) 2 HB···BeF 2 ). These B-Be interactions can be considered as beryllium bonds in most cases.

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out to investigate the six unique cyclic quaternary complexes FH:FH:FH:FH, FH:FH:FH:FCl, FH:FH:FCl:FCl, FH:FCl:FH:FCl, FH:FCl:FCl:FCl, and FCl:FCl:FCl:FCl stabilized by F-H···F hydrogen bonds and F-Cl···F halogen bonds. The binding energies of these complexes decrease as the number of FH molecules decreases, and therefore as the number of hydrogen bonds decreases, indicating that hydrogen bonds are primarily responsible for stabilities. Nonadditivities of binding energies are synergistic for complexes with 4, 3, and 2 FH molecules, but antagonistic for those with 1 and 0 FH molecules. In addition to depending on changes in F-F, F-H, and F-Cl distances, complex binding energies are also influenced by two sets of angular parameters. These include the external F-F-F angles which must sum to 360° in these cyclic structures, and the internal H-F-F angles for hydrogen bonds and F-Cl-F angles for halogen bonds, which measure the deviation from linearity of these bonds. Transition structures present the barriers to converting an equilibrium structure to an equivalent equilibrium structure on the potential surfaces. These barriers increase as the number of FH molecules decreases. EOM-CCSD spin-spin coupling constants 2h J(F-F) across hydrogen bonds in complexes tend to increase with decreasing F-F distance. They increase dramatically in transition structures, but show no dependence on the F-F distance. The one-bond coupling constants 1h J(F-H) are relatively small and negative in complexes, increase dramatically, and are positive in transition structures. 1 J(F-H) values are greatest for the covalent F-H bond. Coupling constants 1x J(F-Cl) across halogen bonds are relatively small and positive in complexes, and increase dramatically in transition structures. The largest values of 1 J(F-Cl) are found for covalent bonds.

The ability of Be 3 as a hydrogen bond acceptor has been explored by studying the potential complexes between this molecule and a set of hydrogen bond donors (HF, HCl, HNC, HCN, H 2 O, and HCCH). The electronic structure calculations for these complexes were carried out at the MP2 and CCSD(T) computational levels together with an extensive NBO, ELF, AIM, and electrostatic potential characterization of the isolated Be 3 system. In all the complexes, the Be-Be σ bond acts as electron donor, with binding energies between 19 and 6 kJ mol -1 . A comparison with the analogous cyclopropane:HX complexes shows similar binding energies and contributions of the DFT-SAPT energetic terms. A blue-shift of the harmonic frequencies of Be 3 is observed upon complexation.

Forty-two peri-substituted naphthalene derivatives presenting chalcogen weak interactions were studied. They correspond to O···Y interactions, Y being O, S, and Se. While the O atom bears H or CH 3 substituents (OH and OCH 3 groups), the Y atom is substituted by H, F, and CN to explore the effect of these electron-donating and electron-withdrawing substituents on the chalcogen bond strength. The effect of F and CH 3 substituents on positions ortho/para (2,4,5,7 of the naphthalene ring) was also studied. Optimizations were performed at the MP2/aug-cc-pVDZ, and binding energies were performed at the MP2/aug-cc-pVDZ followed by an MP2/CBS estimation. The main properties studied were geometries, energies (E b , E iso , and E def ), the molecular electrostatic potential, electron density shifts, natural bond order E(2) energies, and the relationship between these properties.

Ab initio MP2/aug'-cc-pVTZ calculations were performed to identify equilibrium complexes and molecules and the transition structures that interconvert them, on the potential energy surfaces of a series of seven binary systems that have nitrogen heterocyclic carbenes (NHCs) as the electron-pair donors to CO 2 . Seven of the NHCs form complexes stabilized by C···C tetrel bonds, and six of these seven are also stabilized by a secondary interaction between an O of CO 2 and the adjacent N-H group of the carbene. Six of the seven NHCs also form stable molecules with C-C covalent bonds, and with one exception, these molecules have binding energies that are significantly greater than the binding energies of the complexes. Charge-transfer stabilizes all of the NHC:CO 2 complexes and occurs from the C lone pair of the carbene to the CO 2 molecule. The six complexes that have secondary stabilizing interactions are also stabilized by back-donation of charge from the O to the adjacent N-H group of the carbene. Transition structures present barriers to the interconversion of complexes and molecules. With one exception, the barrier for converting a molecule to a complex is much greater than the barrier for the reverse reaction. Atoms in Molecules bonding parameters, shifts of IR C-O stretching and O-C-O bending frequencies, changes in NMR 13 C chemical shieldings, and changes in C-C and C-O coupling constants as 1t J(C-C) and J(C-O) for complexes and transition structures become 1 J(C-C) and 2 J(C-O) for molecules, are all consistent with the changing nature of the C···C tetrel bond in the complex through the transition state to a covalent C-C bond in the molecule.

Ab initio MP2/aug'-cc-pVTZ calculations were performed to investigate tetrel-bonded complexes formed between CO 2 and the aromatic bases pyridine, the diazines, triazines, tetrazines, and pentazine. Of the 23 unique equilibrium azine:CO 2 complexes, 14 have planar structures in which a single nitrogen atom is an electron-pair donor to the carbon of the CO 2 molecule, and 9 have perpendicular structures in which two adjacent nitrogen atoms donate electrons to CO 2 , with bond formation occurring along an N-N bond. The binding energies of these complexes vary from 13 to 20 kJ mol -1 and decrease as the number of nitrogen atoms in the ring increases. For a given base, planar structures have larger binding energies than perpendicular structures. The binding energies of the planar complexes also tend to increase as the distance across the tetrel bond decreases. Charge transfer in the planar pyridine:CO 2 complex occurs from the N lone pair to a virtual nonbonding orbital of the CO 2 carbon atom. In the remaining planar complexes, charge transfer occurs from an N lone pair to the remote in-plane π*C-O orbital. In perpendicular complexes, charge transfer occurs from an N-N bond to the adjacent π*O-C-O orbital of CO 2 . Decreases in the bending frequency of the CO 2 molecule and in the 13 C chemical shielding of the C atom of CO 2 upon complex formation are larger in planar structures compared to perpendicular structures. EOM-CCSD spin-spin coupling constants 1t J(N-C) for complexes with planar structures are very small but still correlate with the N-C distance across the tetrel bond.

The proton affinity (PA) on the oxygen atom in silanol and siloxane derivatives is enhanced by the formation of tetrel bonds with small Lewis bases [B···R 3 SiOH, B···R 3 SiOSiR 3 , B···R 3 SiOSiR 3 ···B; B = H 2 O, CO, NH 3 , HCN, H 2 S; R = H, Me], as shown by MP2/jul-cc-pVTZ calculations. The complexed systems become more basic than ether and other carbon-related compounds, and even more basic than pyridine in some specific cases, reaching values up to 959.4 kJ/mol (H 3 N···H 3 SiOSiH 3 ···NH 3 complex). Changes on PAs are directly related to very large binding energies for the protonated species. Topological methods and the natural bond orbital scheme are used to rationalize the observed trends. The PA enhancement should be taken into account when dealing with silanols and siloxanes in different environments.

2-Propyl-1H-benzimidazole (2PrBzIm) is a small molecule, commercially available, which displays a curious behavior in the solid state. 2PrBzIm, although devoid of chirality by fast rotation about a single bond of the propyl group in solution, crystallizes as a conglomerate showing chiroptical properties. An exhaustive analysis of its crystal structure and a wide range of experiments monitored by vibrational circular dichroism spectroscopy eliminated all possibilities of an artifact. What remains is a new example of the unexplained phenomenon of persistent supramolecular chirality.

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out to identify stable complexes and molecules and the transition structures that interconvert them on the potential surfaces of ten singlet carbene bases acting as electron-pair donors to CO 2 . The carbene bases include cyclic C(NHCH) 2 or NHC, C(NH 2 ) 2 , an oxygen heterocyclic carbene C(OCH) 2 or OHC, C(OH) 2 , C(CH 3 ) 2 , cyclic C 3 H 2 , CCCH 2 , CCl 2 , CCH 2 , and CF 2 . Carbene:CO 2 complexes stabilized by C···C tetrel bonds have been found on all potential surfaces, whereas carbene-CO 2 molecules stabilized by C-C covalent bonds have been found on eight surfaces. Three of these molecules have open structures with C 2v symmetry, whereas the remaining have cyclic three membered C-O-C rings with C s symmetry. The transition structures which connect the complex and the molecule are bound on three of the potential surfaces. Whether the transition structure is bound or unbound relative to the carbene and CO 2 depends on the relationship among C-C distances at the three stationary points on the surface. Charge-transfer interactions stabilize carbene:CO 2 complexes. The primary charge transfer in complexes arises from electron donation from the carbene lone-pair to the CO 2 molecule. There is also back-donation of charge from CO 2 to the carbene in three complexes. Systematic changes in bonding properties occur as complexes go through transition structures and become molecules. EOM-CCSD inter- and intramolecular C-C and C-O spin-spin coupling constants have been computed and compared for complexes and molecules. A search of the CSD database found the (NH 2 ) 2 C-CO 2 structure and 17 NHC-CO 2 derivatives. Computed bond distances and angles have been compared with experimental data.

Ab initio MP2/aug'-cc-pVTZ calculations have been performed on the binary complexes XY:PH 3 for XY = ClCl, FCl, and FBr; and PH 3 :N-base for N-base = NCH, NH 3 , NCF, NCCN, and N 2 ; and the corresponding ternary complexes XY:PH 3 :N-base, to investigate P···N pnicogen bond formation through the lone-pair hole at P in the binary complexes and P···N pnicogen-bond formation assisted by P···Y halogen bond formation through the σ-hole at Y. Although the binary complexes PH 3 :N-base that form through the lone-pair hole have very small binding energies, they are not equilibrium structures on their potential surfaces. The presence of the P···Y halogen bond makes PH 3 a better electron-pair acceptor through its lone-pair hole, leading to stable ternary complexes XY:PH 3 :N-base. The halogen bonds in ClCl:PH 3 and ClCl:PH 3 :NCCN are traditional halogen bonds, but in the remaining binary and ternary complexes, they are chlorine- or bromine-shared halogen bonds. For a given nitrogen base, the P···N pnicogen bond in the ternary complex FCl:PH 3 :N-base appears to be stronger than that bond in FBr:PH 3 :N-base, which is stronger than the P···N bond in the corresponding ClCl:PH 3 :N-base complex. EOM-CCSD spin-spin coupling constants for the binary and ternary complexes with ClCl and FCl are also consistent with the changing nature of the halogen bonds in these complexes. At long P-Cl distances, the coupling constant 1x J(P-Cl) increases with decreasing distance but then decreases as the P-Cl distance continues to decrease, and the halogen bonds become chlorine-shared bonds. At the shorter distances, 1x J(P-Cl) approaches the value of 1 J(P-Cl) for the cation + (Cl-PH 3 ). The coupling constants 1p J(P-N) are small and, with one exception, are greater in ClCl:PH 3 :N-base complexes compared to that in FCl:PH 3 :N-base, despite the shorter P-N distances in the latter.

Two-armed imidazolium-based anion receptors have been prepared. The central 2,7-disubstituted naphthalene ring features two photoactive anthracene end-capped side arms with central 2-bromoimidazolium or hydrogen-bonding imidazolium receptors. Combined emission and (1)H and (31)P NMR studies carried out in the presence of a wide variety of anions reveal that only HP2O7(3-), H2PO4(-), SO4(2-), and F(-) anions promoted noticeable changes. The halogen receptor 6(2+)·2PF6(-) acts as a selective fluorescent molecular sensor for H2PO4(-) anions, since only this anion promotes the appearance of the anthracene excimer emission band, whereas it remains unchanged in the presence of the other tested anions. In addition this halogen receptor behaves as a chemodosimeter toward HP2O7(3-) anion, through its transformation into the corresponding bis-imidazolone after debromination by the action of the basic anion. The association constant values of the halogen-bonding complexes in a competitive solvent CD3CN/MeOD (8/2) mixture with H2PO4(-) and SO4(2-) anions are higher than those found for the hydrogen-bonding counterpart. In contrast, in the less competitive CH3CN solvent higher binding affinity for anions corresponds to the hydrogen-bonding receptor 7(2+)·2PF6(-). In addition, the receptor 6(2+)·2PF6(-) represents a useful alternative as an imaging agent in living cells in a wide range of emission wavelengths.

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out on B4H4 and B4(CH3)4 to investigate the base properties of these molecules with Td symmetry. Each face of the tetrahedral structure of B4H4 and B4(CH3)4 is stabilized by a two-electron, three-center B-B-B bond. The face uses these two electrons to act uniquely as an electron-pair donor for the formation of stable hydrogen-bonded and halogen-bonded complexes with C3v symmetry. The hydrogen-bonded complexes are B4H4:HY and B4(CH3)4:HY, with HY = HNC, HF, HCl, HCN, and HCCH; the halogen-bonded complexes are B4H4:ClY and B4(CH3)4:ClY, with ClY = ClF, ClCl, ClNC, ClCN, ClCCH, and ClH. The absolute values of the binding energies of the hydrogen-bonded complexes B4(CH3)4:HY and of the halogen-bonded complexes B4(CH3)4:ClY are significantly greater than the binding energies of the corresponding complexes with B4H4. The binding energies of each series correlate with the distance from the hydrogen-bonded H atom or halogen-bonded Cl atom to the centroid of the interacting face. Charge transfer stabilizes all complexes and occurs from the B2-B3-B4 orbital of the face to the antibonding H-X orbital of HY in hydrogen-bonded complexes and to the antibonding Cl-X orbital of ClY in halogen-bonded complexes, with X being the atom of Y that is directly bonded to either H or Cl. For fixed HY, EOM-CCSD spin-spin coupling constants J(X-B1) are greater than J(X-Bn) for complexes B4H4:HY, even though the X-B1 distances are longer. B1 and Bn are the atoms at the apex and in the interacting face, respectively. Similarly, for complexes B4H4:ClY, J(Cl-B1) is greater than J(Cl-Bn). In the halogen-bonded complexes, both coupling constants correlate with the corresponding distances.

A theoretical study of the cooperativity in linear chains of (H3SiCN)n and (H3SiNC)n complexes connected by tetrel bonds has been carried out by means of MP2 and CCSD(T) computational methods. In all cases, a favorable cooperativity is observed, especially in some of the largest linear chains of (H3SiNC)n, where the effect is so large that the SiH3 group is almost equidistant to the two surrounding CN groups and it becomes planar. In addition, the combination of tetrel bonds with other weak interactions (halogen, chalcogen, pnicogen, triel, beryllium, lithium, and hydrogen bond) has been explored using ternary complexes, (H3SiCN)2:XY and (H3SiNC)2:XY. In all cases, positive cooperativity is obtained, especially in the (H3SiNC)2:ClF and (H3SiNC)2:SHF ternary complexes, where, respectively, halogen and chalcogen shared complexes are formed.

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out to determine the structures, binding energies, and bonding properties of complexes involving the cation (H2C═PH2)(+) and a set of sp-hybridized nitrogen bases including NCCH3, NP, NCCl, NCH, NCF, NCCN, and N2. On each (H2C═PH2)(+):N-base surface, four types of unique equilibrium structures exist: a complex with a P···N pnicogen bond formed through the π system of (H2C═PH2)(+) (ZB-π); a complex with a P···N pnicogen bond formed through the σ system of (H2C═PH2)(+) (ZB-σ); a hydrogen-bonded complex with a P-H···N hydrogen bond (HB); and a tetrel-bonded complex with a C···N bond (TB). Binding energies of complexes stabilized by the same type of intermolecular interaction decrease in the order NCCH3 > NP > NCCl > NCH > NCF > NCCN > N2. For a given base, binding energies decrease in the order ZB-π > HB > ZB-σ > TB, except for a reversal of HB and ZB-σ with the weakest base N2. Binding energies of ZB-π, HB, and ZB-σ complexes increase exponentially as the corresponding P-N distance decreases, but the correlation is not as good between the binding energies of TB complexes and the intermolecular C-N distance. Charge-transfer energies stabilize all complexes and also exhibit an exponential dependence on the corresponding intermolecular distances. EOM-CCSD spin-spin coupling constants (1p)J(P-N) for ZB-π and ZB-σ complexes, and (2h)J(P-N) for HB complexes increase quadratically as the corresponding P-N distance decreases. Values of (1t)J(C-N) for TB are small and show little dependence on the C-N distance. (1)J(P-H) values for the hydrogen-bonded P-H bond in HB complexes correlate with the corresponding P-H distance, whereas values of (1)J(P-H) for the non-hydrogen-bonded P-H correlate with the P-N distance.

A series of new peroxisome proliferator activated receptors (PPARs) chiral ligands have been designed following the accepted three-module structure comprising a polar head, linker, and hydrophobic tail. The majority of the ligands incorporate the oxazolidinone moiety as a novel polar head, and the nature of the hydrophobic tail has also been varied. Docking studies using the crystal structure of an agonist bound to the ligand binding domain of the PPARα receptor have been performed as a tool for their design. Suitable synthetic procedures have been developed, and compounds with different stereochemistries have been prepared. Evaluation of basal and ligand-induced activity proved that several compounds showed agonist activity at the PPARα receptor, thus validating the oxazolidinone template for PPAR activity. In addition, two compounds, 2 and 4, showed dual PPARα/PPARγ agonism and interesting food intake reduction in rats.

Ab initio MP2/aug'-cc-pVTZ calculations were performed to investigate the pnicogen-bonded complexes F(4-n)H(n)P(+):N-base, for n = 0-3, each with a linear or nearly linear F-P···N alignment. The nitrogen bases include the sp(3) bases NH3, NClH2, NFH2, NCl2H, NCl3, NFCl2, NF2H, NF2Cl, and NF3 and the sp bases NCNH2, NCCH3, NP, NCOH, NCCl, NCH, NCF, NCCN, and N2. The binding energies vary between -20 and -180 kJ·mol(-1), while the P-N distances vary from 1.89 to 3.01 Å. In each series of complexes, binding energies decrease exponentially as the P-N distance increases, provided that complexes with sp(3) and sp hybridized bases are treated separately. Different patterns are observed for the change in the binding energies of complexes with a particular base as the number of F atoms in the acid changes. Thus, the particular acid-base pair is a factor in determining the binding energies of these complexes. Three different charge-transfer interactions stabilize these complexes. These arise from the nitrogen lone pair to the σ*P-F(ax), σ*P-F(eq), and σ*P-H(eq) orbitals. The dominant single charge-transfer energy in all complexes is N(lp) → σ*P-F(ax). However, since there are three N(lp) → σ*P-F(eq) charge-transfer interactions in complexes with F4P(+) and two in complexes with F3HP(+), the sum of the N(lp) → σ*P-F(eq) charge-transfer energies is greater than the N(lp) → σ*P-F(ax) charge-transfer energies in the former complexes, and similar to the N(lp) → σ*P-F(ax) energies in the latter. The total charge-transfer energies of all complexes decrease exponentially as the P-N distance increases. Coupling constants (1p)J(P-N) across the pnicogen bond vary with the P-N distance, but different patterns are observed for complexes with F4P(+) and complexes of the sp(3) bases with F3HP(+). These initially increase as the P-N distance decreases, reach a maximum, and then decrease with decreasing P-N distance as the P···N bond acquires increased covalent character. For the remaining complexes, (1p)J(P-N) increases with decreasing P-N distance. Complexation increases the P-F(ax) distance and (1)J(P-F(ax)) relative to the corresponding isolated ion. (1)J(P-F(ax)) correlates quadratically with the P-N distance.

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out on cationic pnicogen-bonded complexes F4P(+):N-base and F3HP(+):N-base, with linear F(ax)-P···N and H(ax)-P···N, respectively. The bases include the sp(3)-hybridized nitrogen bases NH3, NClH2, NFH2, NCl2H, NCl3, NFCl2, NF2H, NF2Cl, and NF3, and the sp bases NCNH2, NCCH3, NP, NCOH, NCCl, NCH, NCF, NCCN, and N2. The binding energies of these complexes span a wide range, from -15 to -180 kJ mol(-1), as do the P-N distances, which vary from 1.89 to 3.11 Å. There is a gap in the P-N distances between 2.25 and 2.53 Å in which no complexes are found. Thus, the equilibrium complexes may be classified as inner or outer complexes based on the value of the P-N distance. Inner complexes have P···N bonds with varying degrees of covalent character, whereas outer complexes are stabilized by intermolecular P···N bonds with little or no covalency. Charge-transfer stabilizes these pnicogen-bonded complexes. For complexes F4P(+):N-base, the dominant charge-transfer interaction is from the lone pair on N to the σ*P-F(ax) orbital. In addition, there are three other charge-transfer interactions from the lone pair on N to the σ*P-F(eq) orbitals, which taken together, are more stabilizing than the interaction involving σ*P-F(ax). In contrast, the dominant charge-transfer interaction for complexes F3HP(+):N-base is from the lone pair on N to the σ*P-F(eq) orbitals. Computed EOM-CCSD Fermi-contact terms are excellent approximations to the total spin-spin coupling constants (1p)J(P-N) and (1)J(P-H(ax)), but are poor approximations to (1)J(P-F(ax)). (1p)J(P-N) values increase with decreasing P-N distance, approach a maximum, and then decrease and change sign as the P-N distance further decreases and the pnicogen bond acquires increased covalency. (1)J(P-F(ax)) values for F4P(+):N-base complexes increase with decreasing distance. Although the P-H(ax) distance changes very little in complexes F3HP(+):N-base, patterns exist which suggest that changes in (1)J(P-H(ax)) reflect the hybridization of the nitrogen base and whether the complex is an inner or outer complex.

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out on the pnicogen-bonded homodimers (PH2X)2 and the binary complexes H2XP:PYH2, for X, Y = F, Cl, OH, NC, CCH, CH3, CN, and H. The binding energies of these complexes are influenced by the nature of the X,Y pair, the intermolecular distance, the relative orientation of the interacting molecules, and the charge-transfer energies from the lone pair of one P to the σ-hole of the other. Binary complexes with X,Y = F, Cl, OH, and NC, as well as the homodimers, have a trans arrangement of the P-A and P-A' bonds with respect to the P···P bond, with A and A', the atoms of X and Y, respectively, bonded to the P atoms. The trendlines for the homodimers in plots of the binding energy versus the P-P distance, and the binding energy versus the total charge-transfer energy, exhibit better correlations than the trendlines for the binary complexes. The trendlines for the homodimers mark the boundary of the region in which points for the binary complexes appear. Pnicogen-bond radii for P in PH2X molecules have been determined from the P-P distances in the homodimers. The sum of these radii provides an excellent approximation to the P-P distance in the corresponding binary complex. EOM-CCSD spin-spin coupling constants (1p)J(P-P) have also been computed for all complexes. Coupling constants for the dimers and binary complexes exhibit a similar linear increase as the P-P distance decreases.

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out on the pnicogen-bonded complexes H(n)F(5-n)P:N-base, for n = 0-5 and nitrogen bases NC(-), NCLi, NP, NCH, and NCF. The structures of these complexes have either C(4v) or C(2v) symmetry with one exception. P-N distances and interaction energies vary dramatically in these complexes, while F(ax)-P-F(eq) angles in complexes with PF5 vary from 91° at short P-N distances to 100° at long distances. The value of this angle approaches the F(ax)-P-F(eq) angle of 102° computed for the Berry pseudorotation transition structure which interconverts axial and equatorial F atoms of PF5. The computed distances and F(ax)-P-F(eq) angles in complexes F5P:N-base are consistent with experimental CSD data. For a fixed acid, interaction energies decrease in the order NC(-) > NCLi > NP > NCH > NCF. In contrast, for a fixed base, there is no single pattern for the variations in distances and interaction energies as a function of the acid. This suggests that there are multiple factors that influence these properties. The dominant factor appears to be the number of F atoms in equatorial positions, and then a linear F(ax)-P···N rather than H(ax)-P···N alignment. The acids may be grouped into pairs (PF5, PHF4) with four equatorial F atoms, then (PH4F, PH2F3) with F(ax)-P···N linear, and then (PH3F2 and PH5) with H(ax)-P···N linear. The electron-donating ability of the base is also a factor in determining the structures and interaction energies of these complexes. Charge transfer from the N lone pair to the σ* P-A(ax) orbital stabilizes H(n)F(5-n)P:N-base complexes, with A(ax) either F(ax) or H(ax). The total charge-transfer energies correlate with the interaction energies of these complexes. Spin-spin coupling constants (1p)J(P-N) for (PF5, PHF4) complexes with nitrogen bases are negative with the strongest bases NC(-) and NCLi but positive for the remaining bases. Complexes of (PH4F, PH2F3) with these same two strong bases and H4FP:NP have positive (1p)J(P-N) values but negative values for the remaining bases. (PH5, PH3F2) have negative values of (1p)J(P-N) only for complexes with NC(-). Values of (1)J(P-F(ax)) and (1)J(P-H(ax)) correlate with the P-F(ax) and P-H(ax) distances, respectively.

A theoretical study of the halogen-bonded complexes (A-X···C) formed between halogenated derivatives (A-X; A = F, Cl, Br, CN, CCH, CF3, CH3, H; and X = Cl, Br) and a nitrogen heterocyclic carbene, 1,3-dimethylimidazole-2-ylidene (MeIC) has been performed using MP2/aug'-cc-pVDZ level of theory. Two types of A-X:MeIC complexes, called here type-I and -II, were found and characterized. The first group is described by long C-X distances and small binding energies (8-54 kJ·mol(-1)). In general, these complexes show the traditional behavior of systems containing halogen-bonding interactions. The second type is characterized by short C-X distances and large binding energies (148-200 kJ·mol(-1)), and on the basis of the topological analysis of the electron density, they correspond to ion-pair halogen-bonded complexes. These complexes can be seen as the interaction between two charged fragments: A(-) and (+)[X-CIMe] with a high electrostatic contribution in the binding energy. The charge transfer between lone pair A(LP) to the σ* orbital of C-X bond is also identified as a significant stabilizing interaction in type-II complexes.

We have designed a series of triazolium-pyrene-based dyads to probe their potential as fluorescent chemosensors for anion recognition through combinations of hydrogen and halogen bonding. Cooperation between the two distinct noncovalent interactions leads to an unusual effect on receptor affinity, as a result of fundamental differences in the interactions of halogen and hydrogen bond donor groups with anions. Absorption, emission spectrophotometries and proton and phosphorus NMR spectroscopies indicate that the two interactions act in concert to achieve the selective binding of the hydrogen pyrophosphate anion, a conclusion supported by computational studies. Hence, as clearly demonstrated with respective halogen- and hydrogen-bonding triazolium receptors, the integration of a halogen atom into the anion receptor at the expense of one hydrogen-bonding receptor greatly influences the anion recognition affinity of the receptor. The association constant values of the halogen-bonding complexes are larger than the hydrogen-bonding counterpart. Thus, halogen bonding has been exploited for the selective fluorescent sensing of hydrogen pyrophosphate anion. Halogen bonding has been demonstrated to increase the strength of hydrogen pyrophosphate binding, as compared to the hydrogen-bonded analogue. Grimme's PBE-D functional, which adequately reproduces the pyrene stacking energies, has been successfully applied to model the affinity for anions, especially hydrogen pyrophosphate, of the new receptors.

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out on the halogen-bonded complexes H2XP:ClF and H2XP:Cl2, with X = F, Cl, OH, NC, CN, CCH, CH3, and H. H2XP:ClF complexes are stabilized by chlorine-shared halogen bonds with short P-Cl and significantly elongated Cl-F distances. H2XP:Cl2 complexes with X = OH and CH3 form only chlorine-shared halogen bonds, while those with X = H, NC, and CN form only traditional halogen bonds. On the H2FP:Cl2, H2(CCH)P:Cl2, and H2ClP:Cl2 potential surfaces small barriers separate two equilibrium structures, one with a traditional halogen bond and the other with a chlorine-shared bond. The binding energies of H2XP:ClF and H2XP:Cl2 complexes are influenced by the electron-donating ability of H2XP and the electron accepting ability of ClF and ClCl, the nature of the halogen bond, other secondary interactions, and charge-transfer interactions. Changes in electron populations on P, F, and Cl upon complex formation do not correlate with changes in the chemical shieldings of these atoms. EOM-CCSD spin-spin coupling constants for complexes with chlorine-shared halogen bonds do not exhibit the usual dependencies on distance. (2X)J(P-F) and (2X)J(P-Cl) for complexes with chlorine-shared halogen bonds do not correlate with P-F and P-Cl distances, respectively. (1X)J(P-Cl) values for H2XP:ClF correlate best with the Cl-F distance, and approach the values of (1)J(P-Cl) for the corresponding cations H2XPCl(+). Values of (1X)J(P-Cl) for complexes H2XP:ClCl with chlorine-shared halogen bonds correlate with the binding energies of these complexes. (1)J(F-Cl) and (1)J(Cl-Cl) for complexes with chlorine-shared halogen bonds correlate linearly with the distance between P and the proximal Cl atom. In contrast, (2X)J(P-Cl) and (1X)J(P-Cl) for complexes with traditional halogen bonds exhibit more normal distance dependencies.

A theoretical study of the minima and connecting transition states of the neutral complexes formed by alkaline-metal and alkaline-earth-metal derivatives of imidazolate and benzimidazolate anions has been carried out using B3LYP/6-31+G(d,p), B3LYP/6-311+G(3df,2p), and G3B3 methods. Two and three nondegenerated minima and two and four TS structures have been identified for imidazole and benzimidazole derivatives, respectively. The most stable minima of the alkaline-metal derivatives of both systems correspond to the metal interacting with the imidazole ring, whereas in the alkaline-earth-metal derivatives, the preferred minima depend on the substituent. A remarkable feature of some minima is the fact that some of the metal-aromatic interactions follow the classical π-cation pattern, even though the global structure corresponds to a neutral salt, constituting a class of noncovalent interaction of great interest in the chemistry of aromatic and heterocyclic complexes. A CSD search has confirmed that the two bonding modes, N-σ and π, are present in the solid phase. The π mode has been analyzed by comparison with other azoles.

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out to investigate the pnicogen-bonded complexes H2YP:X(-), for X,Y = Cl, NC, F, CCH, and CH3. Of the 36 possible complexes, only 21 are unique equilibrium structures. All substituents form (H2XPX)(-) complexes with symmetric X-P-X bonds. The P-A ion-molecule pnicogen bonds in these and some additional complexes have partial covalent character, while some P-A' covalent bonds have partial ion-molecule character. A and A' are the atoms of X and Y, respectively, which are directly bonded to P. Complexes with these types of bonds include the symmetric complexes (H2XPX)(-), H2(CH3)P:F(-), H2(CCH)P:F(-), H2FP:NC(-), H2FP:Cl(-), H2FP:CN(-), and H2(NC)P:Cl(-). Charge transfer from A to the P-A' σ* orbital stabilizes H2YP:X(-) complexes and leads to a reduction of the negative charge on X. For fixed X, the smallest negative charge occurs in the symmetric complex. Then, for a given X, the order of decreasing negative charge with respect to Y is CH3 > CCH > CN (bonded through C) > F > NC (bonded through N) > Cl, which is also the order of decreasing P-A distance. EOM-CCSD spin-spin coupling constants (1p)J(P-A) differentiate between shorter ion-molecule pnicogen bonds with partial covalent character and longer P···A ion-molecule pnicogen bonds. Similarly, coupling constants (1)J(P-A') differentiate between longer covalent P-A' bonds with partial ion-molecule character and shorter P-A' covalent bonds.

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out in search of equilibrium structures with P···Cl pnicogen bonds or halogen bonds on the potential energy surfaces H2FP:ClY for Y = F, NC, Cl, CN, CCH, CH3, and H. Three different types of halogen-bonded complexes with traditional, chlorine-shared, and ion-pair bonds have been identified. Two different pnicogen-bonded complexes have also been found on these surfaces. The most electronegative substituents F and NC form only halogen-bonded complexes, while the most electropositive substituents CH3 and H form only pnicogen-bonded complexes. The halogen-bonded complexes involving the less electronegative groups Cl and CN are more stable than the corresponding pnicogen-bonded complexes, while the pnicogen-bonded complexes with CCH are more stable than the corresponding halogen-bonded complex. Traditional halogen-bonded complexes are stabilized by charge transfer from the P lone pair to the Cl-A σ* orbital, where A is the atom of Y directly bonded to Cl. Charge transfer from the Cl lone pair to the P-F σ* orbital stabilizes pnicogen-bonded complexes. As a result, the H2FP unit becomes positively charged in halogen-bonded complexes and negatively charged in pnicogen-bonded complexes. Spin-spin coupling constants (1X)J(P-Cl) for complexes with traditional halogen bonds increase with decreasing P-Cl distance, reach a maximum value for complexes with chlorine-shared halogen bonds, and then decrease and change sign when the bond is an ion-pair bond. (1p)J(P-Cl) coupling constants across pnicogen bonds tend to increase with decreasing P-Cl distance.

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out to investigate the pnicogen bonded complexes formed between the acids O═PH3, S═PH3, HN═PH3, and H2C═PH3 and the bases NH3, NCH, N2, PH3, and PCH. All nitrogen and phosphorus bases form complexes in which the bases are lone pair electron donors. The binding energies of complexes involving the stronger bases NH3, NCH, and PH3 differentiate among the acids, but the binding energies of complexes with the weaker bases do not. These complexes are stabilized by charge transfer from the lone pair orbital of N or P to the σ*P═A orbital of X═PH3, where A is the atom of X directly bonded to P. PCH also forms complexes with the X═PH3 acids as a π electron donor to the σ*P═A orbital. The binding energies and the charge-transfer energies of the π complexes are greater than those of the complexes in which PCH is a lone pair donor. Whether the positive charge on P increases, decreases, or remains the same upon complex formation, the chemical shieldings of (31)P decrease in the complexes relative to the corresponding monomers. (1p)J(P-N) and (1p)J(P-P) values correlate best with the corresponding P-N and P-P distances as a function of the nature of the base. (1)J(P-A) values do not correlate with P-A distances. Rather, the absolute values of (1)J(P-O), (1)J(P-S), and (1)J(P-N) decrease upon complexation. Decreasing (1)J(P-A) values correlate linearly with increasing complex binding energies. In contrast, (1)J(P-C) values increase upon complexation and correlate linearly with increasing binding energies.

A theoretical study of the complexes formed by monosubstituted phosphines (XH2P) and the methyl radical (CH3) has been carried out by means of MP2 and CCSD(T) computational methods. Two minima configurations have been obtained for each XH2P:CH3 complex. The first one shows small P-C distances and, in general, large interaction energies. It is the most stable one except in the case of the H3P:CH3 complex. The second minimum where the P-C distance is large and resembles a typical weak pnicogen bond interaction shows interaction energies between -9.8 and -3.7 kJ mol(-1). A charge transfer from the unpaired electron of the methyl radical to the P-X σ* orbital is responsible for the interaction in the second minima complexes. The transition state (TS) structures that connect the two minima for each XH2P:CH3 complex have been localized and characterized.

Uncatalyzed racemization processes in atropisomeric diphenyl-like frameworks are classically described as the result of the rotation around the pivotal single bond linking two planar frameworks. Severe constraints leading to more or less distorted transition states account for the experimental barrier to atropenantiomerization. In 1988, one of us hypothesized that, in N-aryl-2(1H)-pyrimidin-(thi)ones, a ring-opening/ring-closure process was contributing to the observed racemization process accounting for the lower barriers in the sulfur analogues than in oxygen analogues. Now, a series of six novel 6-amino-5-cyano-1,4-disubstituted-2(1H)-pyrimidinones 5a-5f and two 6-amino-5-cyano-4-p-tolyl-1-substituted-2(1H)-pyrimidinethiones 6a and 6b were synthesized and characterized through spectroscopic and X-ray diffraction studies. Semipreparative HPLC chiral separation was achieved, and enantiomerization barriers were obtained by thermal racemization. The rotational barriers of 6-amino-5-cyano-1-o-tolyl-4-p-tolyl-2(1H)-pyrimidinone (5b) and 6-amino-5-cyano-1-(naphthalen-1-yl)-4-p-tolyl-2(1H)-pyrimidinone (5e) were found to be 120.4 and 125.1 kJ·mol(-1) (n-BuOH, 117 °C), respectively, and those of the corresponding thiones were 116.8 and 109.6 kJ·mol(-1) (EtOH, 78 °C), respectively. DFT calculations of the rotational barriers clearly ruled out the classical rotation around the pivotal bond with distorted transition states in the case of the sulfur derivatives. Instead, the ranking of the experimental barriers (sulfur versus oxygen, and o-tolyl versus 1-naphthyl in both series) was nicely reproduced by calculations when the rotation occurred via a ring-opened form in N-aryl-2(1H)-pyrimidinethiones.

This work targets the structural preferences of D-ribose and 2-deoxy-D-ribose in water solution and solid phase. A theoretical DFT (B3LYP and M06-2X) and MP2 study has been undertaken considering the five possible configurations (open-chain, α-furanose, β-furanose, α-pyranose, and β-pyranose) of these two carbohydrates with a comparison of the solvent treatment using only a continuum solvation model (PCM) and the PCM plus one explicit water molecule. In addition, experimental vibrational studies using both nonchiroptical (IR-Raman) and chiroptical (VCD) techniques have been carried out. The theoretical and experimental results show that α- and β-pyranose forms are the dominant configurations for both compounds. Moreover, it has been found that 2-deoxy-D-ribose presents a non-negligible percentage of open-chain forms in aqueous solution, while in solid phase this configuration is absent.

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out on complexes H2C═(X)P:PXH2, for X = F, Cl, OH, CN, NC, CCH, H, CH3, and BH2. Three sets of complexes have been found on the potential surfaces. Conformation A complexes have A-P···P-A approaching linearity, with A the atom of X directly bonded to P. Conformation B complexes have A-P···P linear, but the P···P═C orientation of H2C═PX may differ significantly from linearity. Conformation C complexes are unique, since the pnicogen bond involves π-electron donation and acceptance by H2C═PX. The order of binding energies of the three conformations of H2C═(X)P:PXH2 is C > A > B, with two exceptions. Although the binding energies of conformation C complexes tend to be greater than the corresponding conformation A complexes, intermolecular distances in conformation C tend to be longer than those in conformation A. Charge transfer stabilizes H2C═(X)P:PXH2 complexes. The preferred direction of charge transfer is from H2C═PX to PXH2. In conformations A and B, charge transfer occurs from a P lone pair on one molecule to an antibonding σ* orbital on the other. However, in conformation C, charge transfer occurs from the π orbital of H2C═PX to the σ*P-A orbital of PXH2, and from the lone pair on P of PXH2 through the π-hole to the π*P═C orbital of H2C═PX. Changes in charges on P upon complexation do not correlate with changes in (31)P chemical shieldings. Computed EOM-CCSD spin-spin coupling constants correlate with P-P distances. At each distance, the ordering of (1p)J(P-P) is A > B > C. Binding energies and spin-spin coupling constants of conformation A complexes of (PH2X)2, H2C═(X)P:PXH2, and (H2C═PX)2 with A-P···P-A approaching linearity have been compared. For complexes with the more electronegative substituents, binding energies are ordered (PH2X)2 > H2C═(X)P:PXH2 > (H2C═PX)2, while the order is reversed for complexes formed from the more electropositive substituents. A plot of ΔE(PH2X)2/ΔE(H2C═PX)2 versus ΔE[H2C═(X)P:PXH2]/ΔE(H2C═PX)2 indicates that there is a systematic relationship among the stabilities of these complexes. Complexes (PH2X)2 tend to have larger spin-spin coupling constants and shorter P-P distances than H2C═(X)P:PXH2, which in turn have larger coupling constants and shorter P-P distances than (H2C═PX)2, although there is some overlap. Complexes having similar P-P distances have similar values of (1p)J(P-P).

In this article, we report a comprehensive theoretical study of halogen, chalcogen, and pnicogen bonding interactions using a large set of pure and hybrid functionals and some ab initio methods. We have observed that the pure and some hybrid functionals largely overestimate the interaction energies when the donor atom is anionic (Cl(-) or Br(-)), especially in the halogen bonding complexes. To evaluate the reliability of the different DFT (BP86, BP86-D3, BLYP, BLYP-D3, B3LYP, B97-D, B97-D3, PBE0, HSE06, APFD, and M06-2X) and ab initio (MP2, RI-MP2, and HF) methods, we have compared the binding energies and equilibrium distances to those obtained using the CCSD(T)/aug-cc-pVTZ level of theory, as reference. The addition of the latest available correction for dispersion (D3) to pure functionals is not recommended for the calculation of halogen, chalcogen, and pnicogen complexes with anions, since it further contributes to the overestimation of the binding energies. In addition, in chalcogen bonding interactions, we have studied how the hybridization of the chalcogen atom influences the interaction energies.