Side-on Isomer Research Articles

Vibrational Spectroscopy and Structural Analysis of V+(C2H6)n Clusters (n = 1-4).

The vibrational structure and binding motifs of vanadium cation-ethane clusters, V+(C2H6)n, for n = 1-4 are probed using infrared photodissociation spectroscopy in the C-H stretching region (2550-3100 cm-1). Comparison of spectra to scaled harmonic frequency spectra obtained using density functional theory suggests that ethane exhibits two primary binding motifs when interacting with the vanadium cation: an end-on η2 configuration and a side-on configuration. Determining the denticity of the side on isomer is complicated by the rotational motion of ethane, implying that structural analysis based solely on Born-Oppenheimer potential energy surface minimizations is insufficient and that a more sophisticated vibrationally adiabatic approach is necessary to interpret spectra. The lower-energy side-on configuration predominates in smaller clusters, but the end-on configuration becomes important for larger clusters as it helps to maintain a roughly square-planar geometry about the central vanadium. Proximate C-H bonds exhibit elongation and large red-shifts when compared to bare ethane, particularly in the case of the side-on isomer, demonstrating initial effects of C-H bond activation, which are underestimated by scaled harmonic frequency calculations. Tagging several of the clusters with argon and nitrogen results in nontrivial effects. The high binding energy of N2 can lead to the displacement of ethane from a side-on configuration into an end-on configuration. The presence of either one or two Ar or N2 can impact the overall symmetry of the cluster, which can alter the potential energy surface for ethane rotation in the side-on isomer and may affect the accessibility of low-lying electronic excited states of V+.

IR-Laser Ablation of Potassium Cyanide: A Surprisingly Simple Route to Polynitrogen and Polycarbon Species.

Pulsed laser irradiation of solid potassium cyanide (KCN) produces, besides free nitrogen and carbon atoms, the molecular species KN and KC which are potential candidates for interstellar species of potassium. Additionally, N3, N3 −, KN3, C3, C3 −, and KC3 are produced and isolated in solid noble gases as well as in solid N2. Molecular potassium nitrene (KN) reacts with dinitrogen in neon and argon matrices after photochemical excitation (λ=470 nm) forming molecular end‐on (C ∞v) and side‐on (C 2v) potassium azide isomers. The side‐on isomer (C 2v) is thermodynamically favored at the CCSD(T)/ma‐def2‐TZVP level of theory. It can be obtained from the end‐on isomer by UV‐irradiation (λ=273 nm).

Side-On Sulfur Monoxide Complexes of Tantalum, Niobium, and Vanadium Oxyfluorides.

Side-on sulfur monoxide complexes of tantalum, niobium, and vanadium oxyfluorides OMF2(η2-SO) were prepared via the reactions of metal atoms and SO2F2 upon UV-vis irradiation in a cryogenic matrix. The product structures were identified by the characteristic infrared absorptions and isotopic frequency ratios of terminal M-O, F-M-F, and M-(SO) stretches, which were further supported by density functional theory calculations at the B3LYP level. All of the three complexes were predicted to have doublet ground states with the S-O bond nearly perpendicular to the terminal metal-oxygen bond. Although end-on bonded isomers with either M-OS or M-SO geometry are stable as well, they are higher in energy than the side-on isomers, and their vibrational frequencies do not match the experimental values. For the OMF2(η2-SO) complexes, the S-O bond length approaches that of SO-, but it is longer than that of neutral SO due to the electron transfer from the metal d orbital to the in-plane π* orbital of SO. The metal-SO bonding in the side-on complex is mainly ionic, but covalent interactions play some role between the two parts. On the basis of the calculation results, the OMF2(η2-SO) complexes can be considered as (OMF2)+(SO)- in which the unpaired electron is mainly located in the out-of-plane π* orbital of SO. In addition to the complexes containing SO ligand, a series of other stable isomers were obtained by the B3LYP calculations, and they were proposed as intermediates involved in the formation of the OMF2(η2-SO) products via fluorine and oxygen transfer reactions between metal atoms and SO2F2 under UV-vis irradiation.

Photoelectron spectroscopy of CoC2H2(-) and density functional study of Co(n)C2H2 (n = 1-3) anion and neutral clusters.

The anionic and neutral ConC2H2 (n = 1-3) clusters were investigated using anion photoelectron spectroscopy and density functional calculations. The adiabatic detachment energies and vertical detachment energies of ConC2H2(-) (n = 1-3) were determined. Our results show that the most stable geometries of anionic ConC2H2(-) (n = 1-2) and neutral ConC2H2 (n = 1-3) are composed of ConC2H clusters adsorbing a hydrogen atom on the top or bridge sites of Con, whereas Co3C2H2(-) consists of a five-member ring of Co3C2 carbide adsorbing two hydrogen atoms on two bridge sites of Co3. The reaction mechanisms show that the inserted isomer HCoC2H can convert into the vinylidene complex Co═C═CH2 via a side-on isomer M-η(2)-(C2H2).

Can an ancillary ligand lead to a thermodynamically stable end-on 1 : 1 Cu–O2adduct supported by a β-diketiminate ligand?

The finding that dioxygen binds end-on to the Cu(B) site in the crystal structure of a precatalytic complex of peptidylglycine alpha-hydroxylating monooxygenase has spurred the search for biomimetic model complexes exhibiting the same dioxygen coordination. Recent work has not only indicated that sterically hindered beta-diketiminate ligands (L(1)) could support side-on 1 : 1 Cu-O(2) adducts, but also that an end-on L(1)Cu(THF)O(2) structure occurs as an unstable intermediate in the oxygenation mechanism of the Cu(I) complex. In this work, density functional theory and multireference methods are used to determine the potential of ancillary ligands, X, other than THF to yield thermodynamically stable end-on L(1)CuXO(2) species. A diverse set of ligands X, comprising phosphines, thiophene, cyclic ethers, acetonitrile, para-substituted pyridines, N-heterocyclic carbenes, and ligands bearing hydrogen bond donors, has been considered in order to identify ligand characteristics which energetically favor end-on L(1)CuXO(2) over: a) reversion to the Cu(I) complex and dioxygen, b) isomerization to side-on L(1)CuXO(2), and c) decay to L(1)CuO(2) and X. Ancillary ligands with judiciously chosen degrees and orientation of steric bulk and which bear potential hydrogen bond donors to an end-on bound dioxygen moiety most favor oxygenation of L(1)CuX to yield end-on L(1)CuXO(2). Conversion to the side-on isomer can be deterred through the use of a sufficiently bulky ligand X, such as one that is at least the size of a 5-membered ring. Loss of X to give L(1)CuO(2) can be made prohibitively endergonic by employing ligands X which are highly electron donating and which backbond strongly with and sigma-donate significantly to copper.

Characterization of a 1:1 Cu−O2Adduct Supported by an Anilido Imine Ligand

Copper(I) complexes of sterically hindered anilido imine ligands o-C6H4{N(C6H3(i)Pr2)}{C(R)=NC6H3(i)Pr2}- (L(1), R = H; L(2), R = CH3) have been prepared and characterized by spectroscopic and X-ray crystallographic methods. These complexes are highly reactive with O2, and in the case of L2 the product of low-temperature oxygenation was fully characterized by spectroscopic, X-ray crystallographic, and computational methods. The resonance Raman spectrum features an isotope-sensitive vibration at 974 cm(-1) (Delta(18O) = 66 cm(-1)), consistent with assignment as an O-O stretch. Despite the asymmetric coordination environment provided by the supporting anilido imine ligand, the X-ray crystal structure confirms rather symmetric side-on binding of the O2 moiety to the copper center, and the O-O bond length of 1.392(2) Angstroms indicates that this intermediate has significant Cu(III)-peroxo character. Theoretical calculations support this interpretation and predict that while a fully optimized end-on singlet geometry can be obtained, it is higher in energy than the side-on isomer by 3.5 kcal mol(-1) at the CASPT2/TZP level.

Quantum Chemical Evaluation of Protein Control over Heme Ligation: CO/O2 Discrimination in Myoglobin

Control of O2 versus CO binding in myoglobin (Mb) is tuned by a distal histidine residue through steric and H-bonding interactions. These interactions have been evaluated via Car-Parrinello DFT calculations, whose efficiency allows full quantum mechanical treatment of the 13 closest residues surrounding the heme. The small (8 degrees ) deviation of the Fe-C-O bond angle from linearity results from the steric influence of a distal valine residue and not the distal histidine. H-bond energies were evaluated by replacing the distal histidine with the non-H-bonding residue isoleucine. Binding energies for CO and O2 decreased by 0.8 and 4.1 kcal/mol for MbCO and MbO2, in good agreement with experimental H-bond estimates. Ligand discrimination is dominated by distal histidine H-bonding, which is also found to stabilize a metastable side-on isomer of MbO2 that may play a key role in MbO2 photodynamics.

Role of ligand bending in the photodissociation of O2 vs CO-heme: a time-dependent density functional study.

Time-dependent DFT calculations reveal a strong dependence of low-lying excited states on the <Fe-O-O angle of a model O2-heme adduct and ready access to a metastable side-on isomer. The predicted state crossings provide a pathway for internal conversion to the ground state and account for the low quantum yield for O2 dissociation, while the side-on isomer can account for the observation of an unphotolyzable fraction at low temperature. For CO-heme, the angle-sensitive states lie above the photoaccessed Q state and do not participate in photodissociation.

A quantum chemical survey of metalloporphyrin-nitrosyl linkage isomers: insights into the observation of multiple FeNO conformations in a recent crystallographic determination of nitrophorin 4.

Using density functional theory-based geometry optimizations, we have searched for eta(1)-NO, eta(1)-ON (isonitrosyl), and eta(2)-NO (side-on bound NO) linkage isomers of a number of metalloporphyrin-NO complexes, M(Por)(NO)(L), where Por = porphinato dianion, M = Mn(II), Fe(II), Fe(III), Ru(II), Ru(III), Co(II), and Rh(II), and L = no ligand, SMe, Ph, and imidazole. The eta(1)-NO isomer had the lowest energy in all cases, and the isonitrosyl isomer was also located as a higher energy potential energy minimum in a number of cases. The eta(2)-NO isomer was only located as a minimum for Mn(II) (L = no ligand), Fe(III) (L = no ligand), and Ru(III) (L = Ph, imidazole, pyrdine), suggesting that an [MNO](6) electron count is important for stabilization of the eta(2) mode of ligation. However, in the presence of axial ligands L, the side-on isomers of [FeNO](6) complexes were not stable and opened up to an unusual geometry where the FeN(O) and NO vectors were tilted in opposite directions relative to the heme normal. Exactly such a geometry, as well as a "normal" upright geometry, has been observed in a recent crystallographic determination of nitrophorin 4 (Nature Struct. Biol. 2000, 7, 551), a salivary protein from the blood-sucking insect Rhodnius prolixus. Together, the calculated and experimental result illustrate the extreme softness of the FeNO potential energy surface toward various forms of tilting and bending deformations.

Theoretical and experimental studies on the ground state potential energy surface of C 2H 4O dications

The C 2H 4O 2+ ground state potential energy surface was examined by ab initio molecular orbital theory corrected for the effects of zero-point energies and for electron correlations by means of Møller-Plesset perturbation theory terminated to second order (MP2/6-31G) using 4-31G optimized geometries. The global minimum corresponds to H 3C-C-OH 2+ ( 1, C 3 v ), which is 14.7 kcal mol −1 more stable than the dication of ring-opened ethylene oxide, i.e. H 2C-O-CH 2 2+ ( 2, D 2 d ). Other stable C 2H 4O 2+ isomers are the perpendicular forms of the anti- and syn-vinyl alcohol ( 3 and 4, 15.8 and 22.8 kcal mol −1 less stable than 1, an oxygen analogue of allene, 5, i.e. H 2C-C-OH 2 2+ (28.7 kcal mol −1 relative to 1), and an oxygen analogue of propyne, 7, i.e. HC-C-OH 3 2+, which is substantially less stable than 1 (99.4 kcal mol −1). The end-on complex between water and acetylene, i.e HCCH-OH 2 2+ ( 6), is also found to be a stable species on the 4-31G potential energy surface (38.0 kcal mol −1 relative to 1), in contrast to its side-on isomer. However, investigations at higher levels of theory indicate that 6 should not be regarded as a true minimum structure. The dications of acetaldehyde ( 8), planar vinyl alcohol ( 9) and ethylene oxide ( 10) are not found to exist in potential minima. The thermochemical and kinetic stabilities of 1, 2 and 3 are evaluated in detail. Although these dications are thermochemically highly unstable, dissociation is prevented by substantial barriers, thus making observation in the gas phase feasible. In fact, charge-stripping mass spectrometry provides evidence that 1, 2 and 3 can indeed be generated, and a comparison of the experimentally derived Q min values with the calculated vertical ionization energies shows good agreement for the ions 2 and 3. It seems, however, quite impossible that C 2H 4O 2+ could ever be generated as viable chemical intermediates in solution because the dications will strip an electron or an atom from an adjacent neutral (or negatively charged) species with avidity.