Group 4 transition metal CH2=MF2, CHF=MF2, and HC/MF3 complexes formed by C-F activation and alpha-fluorine transfer

Group 4 transition metal methylidene difluoride complexes (CH2=MF2) are formed by the reaction of methylene fluoride with laser-ablated metal atoms and are isolated in an argon matrix. Isotopic substitution of the CH2F2 precursor and theoretical computations (B3LYP and CCSD) confirm product identifications and assignments. Our calculations indicate that the CH2=MF2 complexes have near C2v symmetry and are considerably more stable than other possible products (CH2(mu-F)MF and CHF=MHF). The primary reaction exothermicity provides more than enough energy to activate the initial bridge-bonded CH2(mu-F)MF products on the triplet potential energy surface to complete an alpha-F transfer to form the very stable CH2=MF2 products. Analogous experiments with CHF3 produce CHF=TiF2, which is not distorted at the C-H bond, whereas the heavier group 4 metals form lower-energy triplet HC/MF3 complexes, which contain weak degenerate C(p)-M(d) pi-bonding interactions. Comparisons are made with the CH2=MHF methylidene species, which showed considerable agostic distortions.     

Reagents for electrophilic amination: a quantum Monte Carlo study

Electrophilic amination is an appealing synthetic strategy to construct carbon-nitrogen bonds. The authors explore the use of the quantum Monte Carlo method and a proposed variant of the electron pair localization function--the electron pair localization function density--as a measure of the nucleophilicity of nitrogen lone pairs as a possible screening procedure for electrophilic reagents.     

Decoupling of exchange and persistence times in atomistic models of glass formers

With molecular dynamics simulations of a fluid mixture of classical particles interacting with pairwise additive Weeks-Chandler-Andersen potentials, we consider the time series of particle displacements and thereby determine the distributions for local persistence times and local exchange times. These basic characterizations of glassy dynamics are studied over a range of supercooled conditions and were shown to have behaviors, most notably decoupling, similar to those found in kinetically constrained lattice models of structural glasses. Implications are noted.     

Matrix infrared spectroscopic studies of the MH-C2H3 and MH2-C2H2 intermediates in the reactions of ethylene with laser-ablated group 5 metal atoms

Reactions of ethylene with laser-ablated group 5 metal atoms in excess argon have been carried out during codeposition at 8 K, and the matrix infrared spectra of intermediate products have been investigated. Oxidative C-H insertion of the transition metal into a C-H bond occurs and beta-hydrogen transfer follows to form the dihydrido complexes (MH2-C2H2). In the Ta spectra, the dihydrido complex is the primary product, whereas the Nb and V spectra reveal absorptions from both the insertion (MH-C2H3) and dihydrido complexes. The insertion and dihydrido complexes identified here are in fact the reaction intermediates in the hydrogen elimination of ethylene proposed in previous reaction dynamics studies. Calculations also show that the higher oxidation-state complex becomes more stable relative to the insertion product going down the group 5 family.     

Sodium hydride clusters in solid hydrogen and neon: infrared spectra and theoretical calculations

Laser-ablated sodium atom reactions with H2 have been investigated in solid molecular hydrogens and neon. The NaH molecule and (NaH)2,3,4 clusters were identified by IR spectra with isotopic substitution (HD and D2) and comparison to frequencies calculated by density functional theory and the MP2 method. The use of para-hydrogen enriched samples provides evidence for a (H2)nNaH subcomplex surrounded by the solid hydrogen matrix cage. The ionic rhombic (NaH)2 dimer is characterized by strong absorptions at 761.7, 759.1, and 757.0 cm(-1), respectively, in solid neon, para-hydrogen, and normal hydrogen matrices. The cyclic sodium hydride trimer and tetramer clusters are also observed. Although the spontaneous reaction of two Li and H2 to form (LiH)2 occurs on annealing in solid H2, the formation of (NaH)2 requires near uv photoexcitation.     

Infrared spectra of the CH3-MX and CH2-MHX complexes formed by reactions of laser-ablated group 3 metal atoms with methyl halides

Reactions of laser-ablated group 3 metal atoms with methyl halides have been carried out in excess of Ar during condensation and the matrix infrared spectra studied. The metals are as effective as other early transition metals in providing insertion products (CH3-MX) and higher oxidation state methylidene complexes (CH2-MHX) (X = F, Cl, Br) following alpha-hydrogen migration. Unlike the cases of the group 4-6 metals, the calculated methylidene complex structures show little evidence for agostic distortion, consistent with the previously studied group 3 metal methylidene hydrides, and the C-M bond lengths of the insertion and methylidene complexes are comparable to each other. However, the C-Sc bond lengths are 0.013, 0.025, and 0.029 A shorter for the CH2-ScHX complexes, respectively, and the spin densities are consistent with weak C(2p)-Sc(3d) pi bonding. The present results reconfirm that the number of valence electrons on the metal is important for agostic interaction in simple methylidene complexes.     

Formation and characterization of the photochemically interconvertible side-on and end-on bonded dioxygen-iron dioxide complexes in solid argon

Two interconvertible iron dioxide-dioxygen complexes were prepared and characterized by matrix isolation infrared absorption spectroscopy as well as theoretical calculations. Iron atoms react with O2 to form the inserted FeO2 molecule in solid argon only upon UV-visible light irradiation. Annealing allows the dioxygen molecules to diffuse and to react with FeO2 and form the side-on and end-on bonded dioxygen-iron dioxide complexes, (eta2-O2)FeO2 and (eta1-O2)FeO2. The side-on bonded structure is a peroxide complex having a singlet ground state with a nonplanar C2v symmetry. The end-on bonded isomer is characterized to be a superoxide complex with a planar 3A' ' ground state. These two isomers are photoreversible, that is, near-infrared light (lambda > 850 nm) induces the conversion of the side-on bonded (eta2-O2)FeO2 complex to the end-on bonded (eta1-O2)FeO2 isomer and vice versa with red light irradiation (lambda > 600 nm).     

Cyanides and Isocyanides of Zinc,Cadmium and Mercury: Matrix Infrared Spectra and Electronic Structure Calculations for the Linear MNC,NCMCN, CNMNC,NCMMCN, and CNMMNC Molecules

Cadmium atoms from laser ablation react with cyanogen, NC=CN, in excess argon during co-deposition at 4 K, and even more on UV irradiation of the cold samples. Final annealing to 35 K increases bands at 2187.3 and 2089.2 cm⁻¹ at the expense of weaker bands at 2194.6 and 2092.2 cm⁻¹ through addition of another cadmium atom. Reaction products were identified by comparison with B3LYP and CCSD(T) computed frequencies and energies, through frequency differences between Zn and Cd products, and by cyanogen isotopic substitution. The CN radical, ZnNC, and CdNC were observed on sample deposition. Hg arc ultraviolet (UV) irradiation activates the insertion of Cd and Zn to form the NCCdCN, CNCdNC, NCZnCN and CNZnNC molecules. Next annealing increased the dimetal products NCCdCdCN, CNCdCdNC, NCZnZnCN, and CNZnZnNC at the expense of their single metal analogs. Laser ablated mercury amalgam also produced NCHgCN, NCHg−HgCN, CNHgNC and CNHg−HgNC. The Group12 metals form both cyanide and isocyanide products, and the argon matrix also traps the higher energy but much more intensely absorbing isocyanides. In the isocyanide case bond polarity results in very intense infrared absorptions. Group 12 metals produce shorter M−M bonds in the dimetal cyanides NCM−MCN and isocyanides CNM−MNC than in the M−M itself, and their computed M−M bond lengths compare favorably with those measured for dimetal complexes stabilized by large ring containing molecular ligands.     

Cyanides,Isocyanides, and Hydrides of Zn,Cd and Hg from Metal Atom and HCN Reactions: Matrix Infrared Spectra and Electronic Structure Calculations

Zinc and cadmium atoms from laser ablation of the metals and mercury atoms ablated from a dental amalgam target react with HCN in excess argon during deposition at 5 K to form the MCN and MNC molecules and CN radicals. UV irradiation decreases the higher energy ZnNC isomer in favor of the lower energy ZnCN product. Cadmium and mercury atoms produce analogous MCN primary molecules. Laser ablation of metals also produces plume radiation which initiates H-atom detachment from HCN. The freed H atom can add to CN radical to produce the HNC isomer. The argon matrix also traps the higher energy but more intensely absorbing isocyanide molecules. Further reactions with H atoms generate HMCN and HMNC hydrides, which can be observed by virtue of their C−N stretches and intense M−H stretches. Computational modeling of IR spectra and relative energies guides the identification of reaction products by providing generally reliable frequency differences within the Zn, Cd and Hg family of products, and estimating isotopic shifts using to ¹³C and ¹⁵N isotopic substitution for comparison with experimental data.     

Combined triple and double bonds to uranium: the N≡U=N-H uranimine nitride molecule prepared in solid argon

Reactions of laser-ablated U atoms with N(2) and H(2) mixtures upon codeposition in excess argon at 5 K gave strong NUN and weak UN infrared absorptions and new bands at 3349.7, 966.9, 752.4, and 433.0 cm(-1) for the unusual new U(V) molecule N≡U=N-H, uranimine nitride, containing both triple and double bonds. This identification is based on D and (15)N isotopic substitution and comparison with frequencies computed by density functional theory for the (2)Δ ground state NUNH. Calculated bond lengths are compared to those of the (1)Σ(g)(+) ground state of U(VI) uranium dinitride N≡U≡N, the (2)Φ ground state of the isoelectronic nitride oxide N≡U=O, and the (3)A ground state of the U(IV) uranimine dihydride HN=UH(2) molecule, which have all been prepared in solid argon matrices. Mulliken bond orders based on the CASSCF orbitals for N≡U=N-H are 2.91, 2.19, and 1.05, respectively. Here, the terminal nitride is effectively a triple bond, just as found for N≡U≡N. The solid argon matrix is a convenient medium to isolate reactive terminal uranium nitrides for examination of their spectroscopic properties.