Perturbing the O-H O Hydrogen Bond in 1-oxo-3-hydroxy-2-propene.

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out to identify and characterize equilibrium structures and transition structures on the 1-oxo-3-hydroxy-2-propene: Lewis acid potential energy surfaces, with the acids LiH, LiF, BeH2, and BeF2. Two equilibrium structures, one with the acid interacting with the C=O group and the other with the interaction occurring at the O-H group, exist on all surfaces. These structures are separated by transition structures that present the barriers to the interconversion of the two equilibrium structures. The structures with the acid interacting at the C=O group have the greater binding energies. Since the barriers to convert the structures with interaction occurring at the O-H group are small, only the isomers with interaction occurring at the C=O group could be experimentally observed, even at low temperatures. Charge-transfer energies were computed for equilibrium structures, and EOM-CCSD spin-spin coupling constants 2hJ(O-O), 1hJ(H-O), and 1J(O-H) were computed for equilibrium and transition structures. These coupling constants exhibit a second-order dependence on the corresponding distances, with very high correlation coefficients.

Complexes H2 CO:PXH2 and HCO2 H : PXH2 for X=NC, F, Cl, CN, OH, CCH, CH3 , and H: Pnicogen Bonds and Hydrogen Bonds.

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out to investigate H2 CO : PXH2 pnicogen-bonded complexes and HCO2 H : PXH2 complexes that are stabilized by pnicogen bonds and hydrogen bonds, with X=NC, F, Cl, CN, OH, CCH, CH3 , and H. The binding energies of these complexes exhibit a second-order dependence on the O-P distance. DFT-SAPT binding energies correlate linearly with MP2 binding energies. The HCO2 H : PXH2 complexes are stabilized by both a pnicogen bond and a hydrogen bond, resulting in greater binding energies for the HCO2 H : PXH2 complexes compared to H2 CO : PXH2 . Neither the O-P distance across the pnicogen bond nor the O-P distance across the hydrogen bond correlates with the binding energies of these complexes. The nonlinearity of the hydrogen bonds suggests that they are relatively weak bonds, except for complexes in which the substituent X is either CH3 or H. The pnicogen bond is the more important stabilizing interaction in the HCO2 H : PXH2 complexes except when the substituent X is a more electropositive group. EOM-CCSD spin-spin coupling constants 1p J(O-P) across pnicogen bonds in H2 CO:PXH2 and HCO2 H : PXH2 complexes increase as the O-P distance decreases, and exhibit a second order dependence on that distance. There is no correlation between 2h J(O-P) and the O-P distance across the hydrogen bond in the HCO2 H : PXH2 complexes. 2h J(O-P) coupling constants for complexes with X=CH3 and H have much greater absolute values than anticipated from their O-P distances.

Hydrogen bonds and halogen bonds in complexes of carbones L→C←L as electron donors to HF and ClF, for L = CO, N2, HNC, PH3, and SH2.

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out to determine the structures and binding energies of the carbone complexes in which the carbone L→C←L acts as an electron pair donor to one and two HF or ClF molecules, for L = CO, N2, HNC, PH3, and SH2. The binding energies increase with respect to the ligand in the order CO < NN < CNH ≪ PH3 < SH2, and increase with respect to the acid in the order HF < 2 HF < ClF < 2 ClF. The complexes with the ligands CO, N2 and PH3 have C2v symmetry while those with CNH and SH2 have Cs symmetry, except for H2S→C←SH2:2HF which has C2 symmetry and a unique structure among all of the carbone complexes. F-H and Cl-F stretching frequencies in the complexes decrease as the F-H and Cl-F distances, respectively, increase. EOM-CCSD spin-spin coupling constants 2hJ(F-C) increase with decreasing F-C distance. Although the F-HC hydrogen bonds gain some proton-shared character in the most tightly bound complexes, the hydrogen bonds remain traditional hydrogen bonds. 1xJ(Cl-C) values indicate that the ClC halogen bonds have chlorine shared character even at the longest distances. 1xJ(Cl-C) then increases as the Cl-C distance decreases, and reaches a maximum for chlorine-shared halogen bonds. As the Cl-C distance further decreases, the halogen bond becomes a chlorine-transferred halogen bond.

Mutual Influence of Pnicogen Bonds and Beryllium Bonds: Energies and Structures in the Spotlight.

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 (NH3, NHCH2, and NCH), a phosphine (fluorophosphane, PH2F) and a beryllium derivative (BeH2, BeF2, BeCl2, BeCO3, and BeSO4). 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, CH2HN:PH2F:BeSO4. In the complexes between the strongest nitrogen bases and the strongest beryllium donors, phosphorus-shared and phosphorus-transfer bonds are found.

Calculated coupling constants 1 J(X-Y) and 1 K(X-Y), and fundamental relationships among the reduced coupling constants for molecules Hm X-YHn , with X, Y ═ 1 H, 7 Li, 9 Be, 11 B, 13 C, 15 N, 17 O, 19 F, 31 P, 33 S, and 35 Cl.

Equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) calculations have been performed to determine coupling constants 1 J(X-Y) for 65 molecules Hm X-YHn , with X,Y ═ 1 H, 7 Li, 9 Be, 11 B, 13 C, 15 N, 17 O, 19 F, 31 P, 33 S, and 35 Cl. The computed 1 J(X-Y) values are in good agreement with available experimental data. The reduced coupling constants 1 K(X-Y) have been derived from 1 J(X-Y) by removing the dependence on the magnetogyric ratios of X and Y. Patterns are found for the reduced coupling constants on a 1 K(X-Y) surface that are related to the positions of X and Y in the periodic table.

Unusual Complexes of P(CH)3 with FH, ClH, and ClF.

Ab initio MP2/aug'-cc-pVTZ calculations have been performed to determine the structures and binding energies of complexes formed by phosphatetrahedrane, P(CH)3, and HF, HCl, and ClF. Four types of complexes exist on the potential energy surfaces. Isomers A form at the P atom near the end of a P-C bond, B at a C-C bond, C at the centroid of the C-C-C ring along the C3 symmetry axis, and D at the P atom along the C3 symmetry axis. Complexes A and B are stabilized by hydrogen bonds when FH and ClH are the acids, and by halogen bonds when ClF is the acid. In isomers C, the dipole moments of the two monomers are favorably aligned but in D the alignment is unfavorable. For each of the monomers, the binding energies of the complexes decrease in the order A > B > C > D. The most stabilizing Symmetry Adapted Perturbation Theory (SAPT) binding energy component for the A and B isomers is the electrostatic interaction, while the dispersion interaction is the most stabilizing term for C and D. The barriers to converting one isomer to another are significantly higher for the A isomers compared to B. Equation of motion coupled cluster singles and doubles (EOM-CCSD) intermolecular coupling constants J(X-C) are small for both B and C isomers. J(X-P) values are larger and positive in the A isomers, negative in the B isomers, and have their largest positive values in the D isomers. Intramolecular coupling constants 1J(P-C) experience little change upon complex formation, except in the halogen-bonded complex FCl:P(CH3) A.

Can a Cl-H···F Hydrogen Bond Replace a Cl···F Halogen Bond? H2XP:ClY:ZH versus H2XP:ClY:HZ for Y, Z = F, Cl.

Ab initio MP2/aug'-cc-pVTZ (where MP2 is Møller-Plesset perturbation theory) calculations have been carried out on four series of complexes, H2XP:ClF:HCl, H2XP:ClF:HF, H2XP:ClCl:HF, and H2XP: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 H2ClP:ClF:HF, where an ion-pair complex exists. Ion-pair complexes also result from the optimization of H2XP: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 H2XP:ClCl:FH for X = NC and CN, open ternary complexes stabilized by a pnicogen bond and a hydrogen bond are found. Optimization of H3P:ClCl:FH leads to an ion pair. For H2(NC)P:ClCl:HCl and H2(CN)P:ClCl:HCl, cyclic ternary complexes stabilized by pnicogen, halogen, and hydrogen bonds result from optimization. Optimization of H3P:ClCl:HCl leads to a reaction in which H2ClP 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 H2XP: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 H2XP:ClF:ZH for Z = F and Cl to give H2XP:ClF:HZ, thereby replacing a halogen bond by a hydrogen bond. Complexes H2XP:ClCl:HZ for X = NC and CN are stable complexes, but the corresponding halogen-bonded complexes H2XP:ClCl:ZH are not.

Potential Energy Surfaces of HN(CH)SX:CO2 for X = F, Cl, NC, CN, CCH, and H: N···C Tetrel Bonds and O···S Chalcogen Bonds.

MP2/aug'-cc-pVTZ calculations have been performed in search of complexes, molecules, and transition structures on the HN(CH)SX:CO2 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:CO2 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 1tJ(N-C) for complexes and transition structures and 1J(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.

What Types of Noncovalent Bonds Stabilize Dimers (XCP)2, for X = CN, Cl, F, and H?

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, Cs, C2h, and C2 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 1pJ(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.

N C and S S Interactions in Complexes, Molecules, and Transition Structures HN(CH)SX:SCO, for X = F, Cl, NC, CCH, H, and CN.

Ab initio Møller-Plesset perturbation theory (MP2)/aug'-cc-pVTZ calculations have been carried out in search of complexes, molecules, and transition structures on HN(CH)SX:SCO potential energy surfaces for X = F, Cl, NC, CCH, H, and CN. Equilibrium complexes on these surfaces have C1 symmetry, but these have binding energies that are no more than 0.5 kJ·mol-1 greater than the corresponding Cs complexes which are vibrationally averaged equilibrium complexes. The binding energies of these span a narrow range and are independent of the N-C distance across the tetrel bond, but they exhibit a second-order dependence on the S-S distance across the chalcogen bond. Charge-transfer interactions stabilize all of these complexes. Only the potential energy surfaces HN(CH)SF:SCO and HN(CH)SCl:SCO have bound molecules that have short covalent N-C bonds and significantly shorter S…S chalcogen bonds compared to the complexes. Equation-of-motion coupled cluster singles and doubles (EOM-CCSD) spin-spin coupling constants 1tJ(N-C) for the HN(CH)SX:SCO complexes are small and exhibit no dependence on the N-C distance, while 1cJ(S-S) exhibit a second-order dependence on the S-S distance, increasing as the S-S distance decreases. Coupling constants 1tJ(N-C) and 1cJ(S-S) as a function of the N-C and S-S distances, respectively, in HN(CH)SF:SCO and HN(CH)SCl:SCO increase in the transition structures and then decrease in the molecules. These changes reflect the changing nature of the N…C and S…S bonds in these two systems.

Complexes of O=C=S with Nitrogen Bases: Chalcogen Bonds, Tetrel Bonds, and Other Secondary Interactions.

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out to investigate chalcogen-bond formation through the σ-hole at S and tetrel-bond formation through the π-hole at C in complexes of OCS with a series of nitrogen bases. The binding energies of chalcogen- and tetrel-bonded complexes with the sp-hybridized bases correlate exponentially with the N-S and N-C distances, respectively. The presence of secondary interactions between an N-H or C-H group of an sp2 -hybridized base and OCS in chalcogen-bonded complexes decreases the correlation between binding energies and the N-S distance. These secondary interactions are stronger in the tetrel-bonded complexes with the sp2 bases, particularly in the isomers of OCS:imidazole and OCS : N2 H2 , where they may be described as distorted N-H⋅⋅⋅O or N-H⋅⋅⋅S hydrogen bonds. Charge-transfer interactions are consistent with the nature of the primary and secondary interactions in these complexes. The in-plane OCS bending frequencies are blue-shift in the chalcogen-bonded complexes, and red-shifted in the tetrel-bonded complexes. EOM-CCSD spin-spin coupling constants 1c J(N4-S) across chalcogen bonds have absolute values less than 9.0 Hz, while the two-bond coupling constants 2c J(N4-C) do not exceed 4.0 Hz. These are greater in absolute value that the one-bond coupling constants 1t J(N4-C) across tetrel bonds that are less than 0.5 Hz at much shorter N-C distances.

Hydrogen and Halogen Bonding in Cyclic FH(4- n):FCl n Complexes, for n = 0-4.

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.

Complexes of CO2 with the Azoles: Tetrel Bonds, Hydrogen Bonds and Other Secondary Interactions.

Ab initio MP2/aug'-cc-pVTZ calculations have been performed to investigate the complexes of CO2 with the azoles pyrrole, pyrazole, imidazole, 1,2,3- and 1,2,4-triazole, tetrazole and pentazole. Three types of complexes have been found on the CO2:azole potential surfaces. These include ten complexes stabilized by tetrel bonds that have the azole molecule in the symmetry plane of the complex; seven tetrel-bonded complexes in which the CO2 molecule is perpendicular to the symmetry plane; and four hydrogen-bonded complexes. Eight of the planar complexes are stabilized by Nx···C tetrel bonds and by a secondary interaction involving an adjacent Ny-H bond and an O atom of CO2. The seven perpendicular CO2:azole complexes form between CO2 and two adjacent N atoms of the ring, both of which are electron-pair donors. In three of the four hydrogen-bonded complexes, the proton-donor Nz-H bond of the ring is bonded to two C-H bonds, thereby precluding the planar and perpendicular complexes. The fourth hydrogen-bonded complex forms with the strongest acid pentazole. Binding energies, charge-transfer energies and changes in CO2 stretching and bending frequencies upon complex formation provide consistent descriptions of these complexes. Coupling constants across tetrel bonds are negligibly small, but 2hJ(Ny-C) across Nz-H···C hydrogen bonds are larger and increase as the number of N atoms in the ring increases.

Carbenes as Electron-Pair Donors for P⋠⋠⋠C Pnicogen Bonds.

Ab initio MP2/aug'-cc-pVTZ calculations were performed on the P⋅⋅⋅C pnicogen-bonded complexes of the singlet carbene molecules C(NH2 )2 , C(OH)2 , and cyclic C(OCH)2 [OHC] with H2 XP molecules, with X=F, Cl, NC, OH, CH3 , CN, CCH, and H. The H2 XP:C(NH2 )2 and H2 XP:C(OH)2 complexes have Cs symmetry and two different structures: one in which the symmetry plane of the complex and the local symmetry plane of the carbene are non-coplanar, and the other in which they are coplanar. The non-coplanar H2 XP:C(NH2 )2 and H2 XP:C(OH)2 complexes arise only when X is one of the more electronegative substituents. Coplanar H2 XP:C(NH2 )2 complexes form when X is one of the more electropositive substituents, whereas coplanar H2 XP:C(OH)2 complexes exist for all X. H2 XP:C(NH2 )2 and H2 XP:C(OH)2 are stabilized by covalent P-C bonds or P⋅⋅⋅C pnicogen bonds, but co-planar H2 (CH3 )P:C(OH)2 and H3 P:C(OH)2 are stabilized by O-H⋅⋅⋅P hydrogen bonds. The H2 XP:OHC complexes have non-coplanar structures that are also stabilized by P-C covalent bonds or pnicogen bonds. The H2 (CH3 )P:OHC and H3 P:OHC complexes in which the symmetry plane of the complex and the local symmetry plane of the carbene are perpendicular are stabilized by P⋅⋅⋅π bonds with P acting as the electron-pair donor to the OHC π system. The H2 XP:C(NH2 )2 , H2 XP:C(OH)2 , and H2 XP:OHC complexes are described in terms of their binding energies, charge-transfer energies, electron density properties, and equation-of-motion coupled cluster singles and doubles spin-spin coupling constants.

Using one halogen bond to change the nature of a second bond in ternary complexes with PCl and FCl halogen bonds.

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out to determine the effect of the presence of one halogen bond on the nature of the other in ternary complexes H2XP:ClF:ClH and H2XP:ClF:ClF, for X = F, Cl, H, NC, and CN. The PCl bonds remain chlorine-shared halogen bonds in the ternary complexes H2XP:ClF:ClH, although the degree of chlorine sharing increases relative to the corresponding binary complexes. The FCl bonds in the ternary complexes remain traditional halogen bonds. The binding energies of the complexes H2XP:ClF:ClH increase relative to the corresponding binary complexes, and nonadditivities of binding energies are synergistic. In contrast, the presence of two halogen bonds in the ternary complexes H2XP:ClF:ClF has a dramatic effect on the nature of these bonds in the four most strongly bound complexes. In these, chlorine transfer occurs across the PCl halogen bond to produce complexes represented as (H2XP-Cl)+:-(F:ClF). In the ion-pair, the cation is also halogen bonded to the anion by a ClF- halogen bond, while the anion is stabilized by an -FCl halogen bond. The central ClF molecule no longer exists as a molecule. The binding energies of the ternary H2XP:ClF:ClF complexes are significantly greater than the binding energies of the H2XP:ClF:ClH complexes, and nonadditivities exhibit large synergistic effects. The Wiberg bond indexes for the complexes H2XP:ClF, H2XP:ClF:ClH, and H2XP:ClF:ClF, and the cations (H2XP-Cl)+ reflect the changes in the P-Cl and Cl-F bonds. Similarly, EOM-CCSD spin-spin coupling constants are also consistent with the changes in these same bonds. In particular, 1xJ(P-Cl) in H2XP:ClF complexes becomes 1J(P-Cl) in the ternary complexes with chlorine-transferred halogen bonds. A plot of these coupling constants shows a change in the curvature of the trendline as chlorine-shared halogen bonds in H2XP:ClF:ClH become chlorine-transferred halogen bonds in H2XP:ClF:ClF. 1xJ(F-Cl) coupling constants also reflect changes in the nature of FCl halogen bonds.

Carbon-Carbon Bonding between Nitrogen Heterocyclic Carbenes and CO2.

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 CO2. 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 CO2 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:CO2 complexes and occurs from the C lone pair of the carbene to the CO2 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 13C chemical shieldings, and changes in C-C and C-O coupling constants as 1tJ(C-C) and J(C-O) for complexes and transition structures become 1J(C-C) and 2J(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.

Azines as Electron-Pair Donors to CO2 for N···C Tetrel Bonds.

Ab initio MP2/aug'-cc-pVTZ calculations were performed to investigate tetrel-bonded complexes formed between CO2 and the aromatic bases pyridine, the diazines, triazines, tetrazines, and pentazine. Of the 23 unique equilibrium azine:CO2 complexes, 14 have planar structures in which a single nitrogen atom is an electron-pair donor to the carbon of the CO2 molecule, and 9 have perpendicular structures in which two adjacent nitrogen atoms donate electrons to CO2, 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:CO2 complex occurs from the N lone pair to a virtual nonbonding orbital of the CO2 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 CO2. Decreases in the bending frequency of the CO2 molecule and in the 13C chemical shielding of the C atom of CO2 upon complex formation are larger in planar structures compared to perpendicular structures. EOM-CCSD spin-spin coupling constants 1tJ(N-C) for complexes with planar structures are very small but still correlate with the N-C distance across the tetrel bond.

Lone-Pair Hole on P: P···N Pnicogen Bonds Assisted by Halogen Bonds.

Ab initio MP2/aug'-cc-pVTZ calculations have been performed on the binary complexes XY:PH3 for XY = ClCl, FCl, and FBr; and PH3:N-base for N-base = NCH, NH3, NCF, NCCN, and N2; and the corresponding ternary complexes XY:PH3: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 PH3: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 PH3 a better electron-pair acceptor through its lone-pair hole, leading to stable ternary complexes XY:PH3:N-base. The halogen bonds in ClCl:PH3 and ClCl:PH3: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:PH3:N-base appears to be stronger than that bond in FBr:PH3:N-base, which is stronger than the P···N bond in the corresponding ClCl:PH3: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 1xJ(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, 1xJ(P-Cl) approaches the value of 1J(P-Cl) for the cation +(Cl-PH3). The coupling constants 1pJ(P-N) are small and, with one exception, are greater in ClCl:PH3:N-base complexes compared to that in FCl:PH3:N-base, despite the shorter P-N distances in the latter.

Carbenes as Electron-Pair Donors To CO2 for C···C Tetrel Bonds and C-C Covalent Bonds.

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 CO2. The carbene bases include cyclic C(NHCH)2 or NHC, C(NH2)2, an oxygen heterocyclic carbene C(OCH)2 or OHC, C(OH)2, C(CH3)2, cyclic C3H2, CCCH2, CCl2, CCH2, and CF2. Carbene:CO2 complexes stabilized by C···C tetrel bonds have been found on all potential surfaces, whereas carbene-CO2 molecules stabilized by C-C covalent bonds have been found on eight surfaces. Three of these molecules have open structures with C2v symmetry, whereas the remaining have cyclic three membered C-O-C rings with Cs 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 CO2 depends on the relationship among C-C distances at the three stationary points on the surface. Charge-transfer interactions stabilize carbene:CO2 complexes. The primary charge transfer in complexes arises from electron donation from the carbene lone-pair to the CO2 molecule. There is also back-donation of charge from CO2 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 (NH2)2C-CO2 structure and 17 NHC-CO2 derivatives. Computed bond distances and angles have been compared with experimental data.

Halogen Bonding Involving CO and CS with Carbon as the Electron Donor.

MP2/aug'-cc-pVTZ calculations have been carried out to investigate the halogen-bonded complexes formed when CO and CS act as electron-pair donors through C to ClF, ClNC, ClCl, ClOH, ClCN, ClCCH, and ClNH2. CO forms only complexes stabilized by traditional halogen bonds, and all ClY molecules form traditional halogen-bonded complexes with SC, except ClF which forms only an ion-pair complex. Ion-pair complexes are also found on the SC:ClNC and SC:ClCl surfaces. SC:ClY complexes stabilized by traditional halogen bonds have greater binding energies than the corresponding OC:ClY complexes. The largest binding energies are found for the ion-pair SC-Cl⁺:-Y complexes. The transition structures which connect the complex and the ion pair on SC:ClNC and SC:ClCl potential surfaces provide the barriers for inter-converting these structures. Charge-transfer from the lone pair on C to the σ-hole on Cl is the primary charge-transfer interaction stabilizing OC:ClY and SC:ClY complexes with traditional halogen bonds. A secondary charge-transfer occurs from the lone pairs on Cl to the in-plane and out-of-plane π antibonding orbitals of ClY. This secondary interaction assumes increased importance in the SC:ClNH2 complex, and is a factor leading to its unusual structure. C-O and C-S stretching frequencies and 13C chemical shieldings increase upon complex formation with ClY molecules. These two spectroscopic properties clearly differentiate between SC:ClY complexes and SC-Cl⁺:-Y ion pairs. Spin-spin coupling constants 1xJ(C-Cl) for OC:ClY complexes increase with decreasing distance. As a function of the C-Cl distance, 1xJ(C-Cl) and ¹J(C-Cl) provide a fingerprint of the evolution of the halogen bond from a traditional halogen bond in the complexes, to a chlorine-shared halogen bond in the transition structures, to a covalent bond in the ion pairs.