Second-row Hydrides Research Articles

Theoretical prediction of closed-shell paramagnetism for scandium and yttrium hydride.

Following chemical intuition, one would expect that all closed-shell molecules are diamagnetic. However, it is known that this is not the case for some second-row hydrides with low-lying unoccupied orbitals due to an unquenching of the total angular momentum in the presence of an external magnetic field. In this article, the transition-metal hydrides ScH and YH are investigated, assuming a similar unquenching effect involving low-lying unoccupied and orbitals formed from the metal orbitals rather than the orbitals. We are comparing results obtained with various quantum-chemical methods (HF, CCSD, CCSD(T), CCSDT) and basis sets. The obtained positive values for the magnetizabilities clearly indicate paramagnetic behavior. Vibrational effects on the magnetizability tensor are also considered, but these effects are small and do not change the overall conclusion that both ScH and YH are further examples for closed-shell paramagnetism.

Structure and Bonding of Second-row Hydrides

The structure and bonding of the hydrides of boron, carbon, nitrogen and oxygen are described using the concepts of the valence state and hybrid atomic orbitals. The tetrahedral arrangement of orbitals in CH4, NH3 and H2O is explained. A rationalization is proposed for the unique bridged structure of diborane B2H6.

A Systematic CCSD(T) Study of Long-Range and Noncovalent Interactions between Benzene and a Series of First- and Second-Row Hydrides and Rare Gas Atoms

Binding energies, potential energy curves, and equilibrium intermonomer distances describing the interaction between benzene and a series of first- and second-row hydrides and rare gas atoms are calculated using coupled-cluster theory with single, double, and perturbative triple excitations (CCSD(T)) in conjunction with a large augmented quadruple-zeta basis set (aug-cc-pVQZ). These benchmark results are accurate to within one eighth of 1 kcal/mol and, as such, provide a reliable foundation for the development and testing of more approximate methods for calculating long-range and noncovalent interactions.

Theoretical Study on the Substitution Reactions of Silylenoid H2SiLiF with CH4, NH3, H2O, HF, SiH4, PH3, H2S, and HCl

DFT calculations at the B3LYP/6-31G(d,p) level have been performed to explore the substitution reactions of silylenoid H(2)SiLiF with XH(n) hydrides, where XH(n) = CH(4), NH(3), H(2)O, HF, SiH(4), PH(3), H(2)S, and HCl. We have identified a previously unreported reaction pathway on each reaction surface, H(2)SiLiF + XH(n) --> H(3)SiF + LiXH(n-1), which involved the initial formation of an association complex via a five-membered cyclic transition state to form an intermediate followed by the substituted product H(3)SiF with LiXH(n-1) dissociating. These theoretical calculations suggest that (i) there is a very clear trend toward lower activation barriers and more exothermic interactions on going from left to right along a given row in the periodic table, and (ii) for the second-row hydrides, the substitution reactions are more exothermic than for the first-row hydrides and the reaction barriers are lower. The solvent effects were considered by means of the polarized continuum model (PCM) using THF as a solvent. The presence of THF solvent disfavors slightly the substitution reaction. Compared to the previously reported insertions and H(2)-elimination reactions of H(2)SiLiF and XH(n), the substitution reactions should be most favorable.

Ab Initio Calculations of pKaValues of Transition-Metal Hydrides in Acetonitrile

A well-defined ab initio protocol was developed which could be used to calculate the pKa values of a variety of transition-metal hydrides in acetonitrile with a precision of 1.5 pKa units. The gas-phase energies were calculated using a two-layer ONIOM method where the core layer was handled by the CCSD(T) theory. In addition, the C-PCM solvation model previously developed for small organic molecules was confirmed to be reliable for predicting the solvation energies of bulky organometallic complexes. Using the protocol, we studied the structure−property relationships for the acidities of the Fe-, Co-, and Ni-group metal hydrides. It was found that the pKa values of these hydrides varied considerably with respect to the metal centers. The third-row metal hydrides always appeared to be the least acidic, as compared to the first- and second-row metal hydrides. On the other hand, the relative acidity between the first- and second-row metal hydrides was strongly dependent on the chemical formulas of the metal complexes. Finally, linear correlations were observed between the pKa values of metal hydrides and the NBO (natural bond orbital) charges carried by the chelating phosphorus atoms.

CALCULATIONS WITH CORRELATED MOLECULAR WAVE FUNCTIONS: HF, MP2 AND DFT CALCULATIONS ON SECOND-ROW DIATOMIC HYDRIDES

The performance of the previously proposed double zeta valence quality plus polarization functions (DZP) and augmented DZP (ADZP) basis sets is tested at the Hartree–Fock, second-order Møller–Plesset, and density functional levels of theory for the electronic ground state of the second-row diatomic hydrides. Total energy, second-order correlation energy, dissociation energy, bond length, vibrational frequency, and dipole moment are calculated and compared with results obtained with popular basis sets reported in the literature. It is shown that the DZP and ADZP basis sets in general provides better accuracy for a similar number of basis functions.

Theoretical study on H2 elimination pathways of silylenoid H2SiLiF with CH4, NH3, H2O, HF, SiH4, PH3, H2S, and HCl

Ab initio molecular orbital calculations have been performed to explore the reaction potential energy surfaces of silylenoid H2SiLiF with XH n hydrides, where XH n = CH4, NH3, H2O, HF, SiH4, PH3, H2S, and HCl. We have identified a previously unreported reaction pathway on each reaction surface, H2SiLiF + H-XH n − 1 → H n XSiLiF + H2, which involves H2 elimination following the initial formation of an association complex via a four-membered ring transition state to form the substituted three-membered ring silylenoid H n XSiLiF and a H2 molecule. This theoretical calculations suggest that (i) for H2 eliminations there is a very clear trend toward lower activation barriers and more exothermic interactions on going from left to right along a given row in periodic table, and (ii) for the second-row hydrides, the H2 elimination reactions are less exothermic than for the first-row hydrides and the reaction barriers are lower for X–S and Cl. Compared to the insertions of H2SiLiF into XH n , the H2 elimination pathways should be unfavorable with higher barrier and lower exothermic.

The interpretation of molecular magnetic hyperfine interactions

Investigations of the hyperfine structure in the excited electronic states of several free radical species have revealed shortcomings in the currently accepted values used for the theoretical interpretation of such interactions. We introduce updated reference atomic values from a combination of experimental observations and ab initio calculations. The latter are at Hartree-Fock and multireference configuration interaction levels of theory and several atomic test cases are discussed. Furthermore, ground and excited electronic state hyperfine coupling constants are calculated using both levels of theory for a range of first- and second-row diatomic hydride and nonhydride radicals. These results, together with a selection of other experimental measurements are then compared with experimental data where available, and the implications of the revised interpretation are discussed.

The local form of the source function as a fingerprint of strong and weak intra- and intermolecular interactions

This work introduces a local form for the source function, from each atom, for the electron-density value at a given point. The source function enables one to equate the value of the electron density at any point within a molecule to a sum of atomic contributions and thus to view properties of the density at representative points, such as the bond critical points, from a new perspective. The local form of the function introduces further detail. When plotted along a bond path and with reference to the bond critical point (b.c.p.), the source function shows which regions of the atoms involved in the bonding are accumulating or removing electronic charge at the b.c.p. The local form of the source function therefore represents an interesting fingerprint of a given bonding interaction. The local source may be expressed as a sum of two contributions, related to the kinetic energy density and electronic potential energy density, respectively. This approach gives further physical insight into why an atomic region is accumulating or removing charge at the b.c.p. The local form of the source function is applied to the study of the second-row diatomic hydride series and of a number of prototypical hydrogen-bonded systems. Differences in the local source contributions to the density at bond critical points due to chemical bonding (deformation density) and crystallization (interaction density) are also explored and found to be more informative and experimentally detectable than are the corresponding changes for the bond-critical-point properties of weak intermolecular interactions. This result might be of potential interest when judging the data quality of a charge-density experimental determination. Although the present paper deals with electron densities derived from theoretical computations only, both the source function and its local form should also be easily obtainable from a charge-density quality X-ray diffraction experiment.

Binding of the metal ions Cr +, Mn 2+, and Fe 3+ to the second-row hydrides BeH 2, BH 3, NH 3, OH 2, FH, and their associated anions BeH −, BH 2−, NH 2−, OH −, F −: a computational study

We report results from a computational study of the binding in complexes formed from one of the transition metal ions Cr +, Mn 2+, or Fe 3+, each of which has five valence electrons outside an Argon core, and one of the second-row hydrides (BeH 2, BH 3, NH 3, OH 2, FH) or their associated anions (BeH −, BH 2 −, NH 2 −, OH −, F −). The high-spin (sextuplet) form of each of these monoliganded complexes is found to be lowest in energy. The metal cation consistently behaves as a Lewis acid in both the high- and low-spin configurations of these complexes in that there is a net transfer of electron density from the ligand to the metal ion. The underlying mechanisms that result in this transfer of charge from different types of hydride and anion ligands to Cr +, Mn 2+, and Fe 3+ are discussed.

Dramatic response effects in the photoionization of the second-row hydrides: A time-dependent density functional investigation

Cross section and asymmetry parameter profiles for the ionizations of the valence and core shells of the second-row hydrides have been calculated by the time-dependent density functional theory (TD-DFT) and Kohn–Sham (KS) schemes. The importance of the screening effects has been pointed out by the comparison of the KS and TD-DFT results and has been analyzed in detail for the HCl molecule. The results have been discussed with respect to the nature of the ionized orbitals along the series in order to assess the observed trends. This behavior of the Cooper minima in the outer valence molecular orbitals has been considered in detail both in the cross section and asymmetry parameter profiles. The TD-DFT results, compared with the experimental data, show quantitative agreement with the experiment for the outer-valence and core ionizations. A deterioration of the quality of the TD-DFT results is still present in the inner-valence region.

Binding of the two-valence-electron metal ions Sc(+), Ti(2+), and V(3+) to the second-row hydrides BeH(2), BH(3), NH(3), OH(2), and FH: a computational study.

We report results from a computational study of the binding in complexes formed from one of the transition-metal ions Sc(+), Ti(2+), or V(3+), each of which has two valence electrons outside an argon core, and one of the second-row hydrides FH, OH(2), NH(3), BH(3), or BeH(2). The complexes that involve the electron-rich ligands FH, OH(2), and NH(3) have strong ion-dipole components to their binding. There are large stabilization energies for sigma-interactions that transfer charge from occupied lone-pair natural bond orbitals on the F, O, or N atom of the (idealized) Lewis structure into empty non-Lewis orbitals on the metal ions; these interactions effectively increase electron density in the bonding region between the metal ion and liganded atom, and the metal ions in these complexes act in the capacity of Lewis acids. The complexes formed from the electron-poor hydrides BH(3) and BeH(2) consistently incorporate bridging hydrogen atoms to support binding, and there are large stabilization energies for interactions that transfer charge from the Be-H or B-H bonds into the region between the metal ion and liganded atom. The metal ions in Sc(+)-BeH(2), Ti(2+)-BeH(2), Ti(2+)-BH(3), and V(3+)-BH(3) act in the capacity of Lewis acids, whereas the scandium ion in Sc(+)-BH(3) acts as a Lewis base.

C−H···X Hydrogen Bonds of Acetylene, Ethylene, and Ethane with First- and Second-Row Hydrides

The structures and binding energies of a series of C−H···X hydrogen-bonded complexes involving acetylene, ethylene, and ethane as proton donors and the first- and second-row hydrides CH4, NH3, OH2, FH, PH3, SH2, and ClH as proton acceptors have been determined. Geometries were optimized with both the MP2 and the B3-LYP methods in conjuction with the 6-311+G(3df,2p) basis set. In general, we note good agreement between MP2 and B3-LYP hydrogen-bonded structures. However, for some very weakly bound complexes larger differences exist, particularly in the r(H···X) distance, and in these instances the MP2 results are determined (from comparative CCSD(T) calculations) to be more reliable. The CCSD(T)/6-311+G(3df,2p) binding energies (De), which include corrections for basis set superposition errors, are very similar for the MP2 and B3-LYP geometries, reflecting the relative insensitivity of De to geometry for weakly bound complexes. The C−H···X hydrogen-bond strength (D0) shows a considerable dependence on the a...

Effect of correlating core orbitals in calculations of nuclear spin–spin couplings

Electron correlation effects on nuclear spin–spin coupling tensors arising from core-valence interaction were investigated with ab initio calculations based on multiconfigurational self-consistent field (MCSCF) reference states, an adequate basis set, and large restricted active spaces. Calculations of first and second-row hydrides were performed to examine the effect of including the core and semicore orbitals in the active molecular orbital space. The effects of using a multireference wave function instead of a single-reference one, as well as different numbers of electrons in the virtual orbitals, were considered. In second-row hydrides, the inclusion of semicore orbitals is found to be necessary for accurate calculations of spin–spin couplings involving the heavy nucleus. A smaller but nonnegligible effect is observed also for the hydrogen–hydrogen coupling. Comparison is made with nuclear shielding and electric field gradient, for which the effect is considerably smaller. Recommendations for wave functions for calculations of spin–spin couplings are given.

Ab Initio Gas-Phase Acidities of NaH, MgH2, and AlH3

We present ab initio estimates of the gas-phase acidities ΔHacid of the second-row metal hydrides NaH, MgH2, and AlH3. Our estimates are based on frozen-core CCSD(T) calculations using augmented triple- and quadruple-ζ correlation-consistent basis sets that include core polarization functions; these single-point calculations are extrapolated to the complete basis set limit and then adjusted to incorporate core−valence correlation effects, vibrational zero-point energy contributions, and thermal corrections. The final ΔHacid values we obtain at T = 298 K are 345.26 kcal/mol for NaH, 363.83 kcal/mol for MgH2, and 372.93 kcal/mol for AlH3; the estimated uncertainty in these values is ±0.8 kcal/mol. Our results suggest that the current NIST−JANAF recommended bond dissociation energy for NaH is too high by roughly 2 kcal/mol.

Density functional calculations of photoionization with an exchange-correlation potential with the correct asymptotic behaviour

The effect of the choice of the exchange correlation potential on the cross section and asymmetry parameter profiles is analysed. The VWN potential is compared with respect to the LB94, which displays the correct Coulomb tail. The effect of the two possible choices of the electron configuration (ground state, GS, or transition state, TS) is also considered. The comparison with respect to the experimental data shows that in the outer-valence region the VWN TS and the LB94 GS are the best choices for first-row hydrides and N2, while for core ionization LB94 GS performs much better. The discrepancies in the second-row hydrides are not recovered by the LB94 potential, and are ascribed to the screening effect which can be taken into account by a time-dependent extension of the theory. Because of accuracy and computational economy, LB94 GS appears to be a decidedly superior choice in comparison with the VWN TS potential.

Theoretical Study of the Germylene Insertion Reaction into the A-H Bond of First-Row and Second-Row AHn Hydrides

The in ser tion of dimethylgermylene into the A -H bond of A H n hy drides is cal cu lated us ing the CCSD(T) method in comparsion with the den sity func tional the ory (B3LYP) with the 6-311G* ba sis set. Th e B3LYP val ues re pro duce the CCSD(T) re sults very well. The pres ent the o ret i cal cal cu la tions sug g est that (a) for germylene in ser tions there is a very clear trend to ward lower ac ti va tion bar ri ers and more exo ther mic in ter ac tions on go ing from left to right along a given row, and (b) for the sec ond-row hy drides, the in ser tion re ac tions are more exo ther mic than for the first-row hy drides, and the re ac tion bar ri ers are lower.

Dihydrogen Bonding in Main Group Elements: A Case Study of Complexes of LiH, BH3, and AlH3with Third-Row Hydrides

A systematic investigation of the occurrence of dihydrogen bonds in the main group elements is taken up. The complexes of LiH, BH3, and AlH3 with the third-row hydrides, viz. HCl, H2S, and PH3, as well as their dimers are studied at the ab initio MP2 level of theory and compared with the corresponding second-row hydrides. The dihydrogen bonds in the third-row dimers are weaker than those in the second-row ones, with the H···H distance being greater than 2.4 Å in some complexes and dimers. The existence of a bond between such neighboring hydrogens is established on the basis of topographical analysis of electron density. The energy of dihydrogen bonds in the dimers of third-row complexes is similar to the conventional weak H-bonds. The decomposition analysis of interaction energy of dimers shows predominance of an electrostatic component followed by polarization and charge transfer. The present study also suggests reinvestigation of the structure of phosphine−borane (BH3PH3) using the IR and NMR techniques.

Theoretical Study on the Silylidyne Insertion into NH3, H2O, HF, PH3, H2S, and HCl

The reaction paths of the silylidyne insertion into NH3, H2O, HF, PH3, H2O, HF, and HCl have been studied by means of ab initio molecular orbital calculations incorporating electron correlation with Moller−Plesset perturbation theory up to the second and fourth order and using polarization basis sets augmented by diffuse functions. All these reactions involve the initial formation of an intermediate complex followed by a hydrogen-migration process via a transition state. Analyses for the reactivity of the six substrates in the SiH insertion reactions indicate that the reaction with the hydride of the right-hand group has a less stable complex and is more exothermic than with the hydride of the left-hand group and that the reaction with the second-row hydride has a lower overall barrier and is less exothermic than with the first-row hydride. Comparison with the CH and SiH2 insertion reactions has been made, and it is noted that the CH, SiH2, and SiH insertion reactions have similar reaction processes. The energetic results (for the barriers and reaction enthalpies) indicate that the SiH radical is less reactive than the CH radical in the insertion reactions.

An ab initio study of vibrational corrections to the electrical properties of the second-row hydrides

An ab initio study of vibrational corrections to the electrical properties of the second-row hydrides