Σ-complex Structures Research Articles

Paddle Ball Dynamics during Conversion of a Rh–Methyl Hydride Complex to a Rh–Methane σ-Complex through Reductive Coupling

We used quasiclassical direct dynamics simulations to examine reaction pathways for protonation-induced reductive coupling of a pincer (PONOP)Rh–methyl hydride complex. These dynamics simulations revealed that the majority of trajectories (>90%) emerging from a vibrationally averaged velocity distribution of the Rh–methyl hydride reductive coupling transition state led to the Rh–methane σ-complex. Only a few trajectories (<10%) dynamically deviated and did not sample this σ-complex structure and resulted in direct methane reductive elimination/dissociation. This indicates that the Rh–methane σ-complex is directly dynamically connected to the Rh–methyl hydride reductive coupling transition state and is not the result of methane rebound due to a solvent cage. Unexpectedly, trajectories that resulted in a Rh–methane σ-complex occurred with paddle ball motion where methane began to leave the Rh coordination sphere but then returned to the σ-complex, and so the distinction between reductive coupling and elimin...

Predicting Regioselectivity in Nucleophilic Aromatic Substitution

We have investigated practical and computationally efficient methods for the quantitative prediction of regioisomer distribution in kinetically controlled nucleophilic aromatic substitution reactions. One of the methods is based on calculating the relative stabilities of the isomeric σ-complex intermediates using DFT. We show that predictions from this method can be used quantitatively both for anionic nucleophiles with F(-) as leaving group, as well as for neutral nucleophiles with HF as leaving group. The σ-complex approach failed when the leaving group was Cl/HCl or Br/HBr, both for anionic and neutral nucleophiles, because of difficulties in finding relevant σ-complex structures. An approach where we assumed a concerted substitution step and used such transition state structures gave quantitatively useful results. Our results are consistent with other theoretical works, where a stable σ-complex has been identified in some cases, whereas others have been indicated to proceed via a concerted substitution step.

Solvation-Induced σ-Complex Structure Formation in the Gas Phase: A Revisit to the Infrared Spectroscopy of [C6H6–(CH3OH)2]+

Structures of the [C(6)H(6)-(CH(3)OH)(2)](+) cluster cation are investigated with infrared (IR) spectroscopy. While the noncovalent type structure has been confirmed for the n = 1 cluster of [C(6)H(6)-(CH(3)OH)(n)](+), only contradictory interpretations have been given for the spectra of n = 2, in which significant changes have been observed with the Ar tagging. In the present study, we revisit IR spectroscopy of the n = 2 cluster from the viewpoint of the σ-complex structure, which includes a covalent bond formation between the benzene and methanol moieties. The observed spectral range is extended to the lower-frequency region, and the spectrum is measured with and without Ar and N(2) tagging. A strongly hydrogen-bonded OH stretch band, which is characteristic to the σ-complex structure, is newly found with the tagging. The remarkable spectral changes with the tagging are interpreted by the competition between the σ-complex and noncovalent complex structures in the [C(6)H(6)-(CH(3)OH)(2)](+) system. This result shows that the microsolvation only with one methanol molecule can induce the σ-complex structure formation.

Infrared and Electronic Spectroscopy of Benzene−Ammonia Cluster Radical Cations [C6H6(NH3)1,2]+: Observation of Isolated and Microsolvated σ-Complexes

We report infrared (IR) and electronic spectra of benzene-ammonia cluster radical cations [C(6)H(6)(NH(3))(n)](+) (n = 1 and 2) in the gas phase to explore cluster structures and chemical reactivity of the simplest aromatic radical cation with base (nucleophile) molecules. The electronic spectra in the visible region indicate that these cluster cations no longer have the benzene cation chromophore as a result of an intracluster reaction. Analyses of the IR spectra, on the basis quantum chemical calculations and the vibration-internal rotation analysis, reveal that both [C(6)H(6)(NH(3))(1,2)](+) form σ-complex structures, in which the ammonia moiety is covalently bonded to the benzene moiety due to the intracluster nucleophilic addition. For [C(6)H(6)(NH(3))(2)](+), it is also shown that the second ammonia molecule solvates the σ-complex core via a N-H···N hydrogen bond. Such σ-complex structures are generally supposed to be a key intermediate of aromatic substitution reactions. The observed mass spectra and energetics calculations, however, show that [C(6)H(6)(NH(3))(n)](+) systems are inert for aromatic substitutions. The present experimental observations indicate the inherent stability of these σ-complex structures, even though they do not show the aromatic substitution reactivity.

An ab initio study on structures and isomerization of magnesium chlorosilylenoid H 2SiClMgCl

The structures and isomerization of magnesium chlorosilylenoid H 2SiClMgCl were investigated by ab initio molecular orbital theory for the first time. Four equilibrium structures and three isomeric transition states were located and fully optimized at the G3MP2B3 level. For more accurate structural analysis, the B3LYP/cc-pVTZ and QCISD/6-311+G* calculations were performed, respectively. Based on the B3LYP/6-31G(d) optimized geometries, 29Si chemical shifts and harmonic frequencies of various structures were obtained. Isomerization paths for isomers were confirmed by the intrinsic reaction coordinate (IRC) calculations. Tetrahedral, three-membered ring and p-complex structures are suggested to be experimentally detectable ones. σ-Complex structure has the highest energy and will not exist. The solvent effects were considered by means of the polarizable continuum model (PCM) using THF as a solvent.

Density functional theory studies on structural isomers and bonding of catecholborane adducts of Group 5 metallocene (Nb, Ta) hydride complexes

Density functional theory calculations at the Becke3LYP level have been performed to study structural isomers of borane adducts of Cp 2M(H) (M=Nb, Ta). Results of calculations show that O- and N-substituted borane adducts can have various structural isomers including boryl, hydridoborate and σ-complex structures. H-, alkyl- and Cl-substituted borane adducts are found to adopt hydridoborate structures. The structural feature is closely related to how the electron-deficient boron center is electronically saturated.

Theoretical studies on the structures and isomerization of methylenelithoflurosilylenoid H 2CSiLiF

The main characters of the potential energy surface of the methylenelithoflurosilylenoid (H 2CSiLiF) are studied by ab initio calculations at the G2(MP2) level. Four equilibrium structures, a p-complex, a three-membered ring, a σ-complex and a silene, and three isomerization transition states are located, and isomerization reaction paths are investigated and confirmed by the intrinsic reaction coordinate (IRC) calculations. The calculations have shown that nonplanar p-complex structure has the lowest energy and should be experimentally detectable among four equilibrium structures of the H 2CSiLiF. The silene and σ-complex structures have high energies and are easy to isomerize to the three-membered ring structure with lower energy, but in fact, they will not exist. Also, the structural characteristics and bonding properties of various structures are analyzed in this Letter.

The transition state of electrophilic aromatic substitution in the gas phase

Electrophilic aromatic substitution without solvent molecules is a one-step reaction. This is the result of an MP2/6-31+G** analysis of the benzene–HF–BF3 supermolecule. There is a qualitative agreement of this with an estimation given by Tomasi etal. in 1997 (Theoret. Chim. Acta, 1977, 45, 127). We report the structures of the BF3–HF–benzene complex and the degenerate Cs transition structures of the H exchange. However, considering the zero-point vibrational energies in order to get Δ‡H°(0), a broad transition state region including the C2v structure becomes evident. It demonstrates the large out-of-plane mobility of the reaction cycle. The cationic subunits of the zwitterionic transition structures resembles the well-known σ-complex structure. It is shown that the prolongation of the C–H bonds at the protonated C atom of benzene may lead to a small HCH angle and a transcyclic H···H bonding in the transition structure.