Methane activation by laser-ablated V, Nb, and Ta atoms: Formation of CH3-MH, CH2=MH2, CHMH3-, and (CH3)2MH2

Methane activation by group 5 transition-metal atoms in excess argon and the matrix infrared spectra of reaction products have been investigated. Vanadium forms only the monohydrido methyl complex (CH3-VH) in reaction with CH4 and upon irradiation. On the other hand, the heavier metals form methyl hydride and methylidene dihydride complexes (CH3-MH and CH2=MH2) along with the methylidyne trihydride anion complexes (CHMH3-). The neutral products, particularly the methylidene complex, increase markedly on irradiation whereas the anionic product depletes upon UV irradiation or addition of a trace of CCl4 or CBr4 to trap electrons. Other absorptions that emerge on irradiation and annealing increase markedly at higher precursor concentration and are attributed to a higher-order product ((CH3)2MH2)). Spectroscopic evidence suggests that the agostic Nb and Ta methylidene dihydride complexes have two identical metal-hydrogen bonds.     

Infrared spectra of CH(3)-MoH, CH(2)=MoH(2), and CH(triple bond)MoH(3) formed by activation of CH(4) by molybdenum atoms

Reaction of laser-ablated Mo atoms with CH(4) in excess argon forms the CH(3)-MoH, CH(2)=MoH(2), and CH(triple bond)MoH(3) molecules, which are identified from infrared spectra by isotopic substitution and density functional theory frequency calculations. These simple methyl, methylidene, and methylidyne molybdenum hydride molecules are reversibly interconverted by alpha-H transfers upon visible and ultraviolet irradiations. The methylidene dihydride CH(2)=MoH(2) exhibits CH(2) and MoH(2) distortion and agostic interaction to a lesser degree than CH(2)=ZrH(2). Molybdenum methylidyne trihydride CH(triple bond)MoH(3) is a stable C(3v) symmetry molecule.     

Infrared spectra and density functional calculations of CH2 = MHX and CH[triple bond]MH2X complexes prepared in reactions of methyl halides with Mo and W atoms

The simple methylidene and methylidyne complexes (CH2=MHX and CH[triple bond]MH2X; X = F, Cl, Br, and I) are prepared in reactions of laser-ablated Mo and W atoms with the methyl halides and investigated by matrix infrared spectroscopy and density functional theory calculations. These complex structures are photoreversible: visible irradiation converts the methylidene complex to the methylidyne complex, and UV irradiation reverses this effect via alpha-hydrogen migration. While the higher oxidation state complexes are readily formed regardless of halogen size, the Mo methylidyne complex is relatively less favored with increasing halogen size, and the W complex shows the opposite tendency. The group 6 metal methylidenes are predicted to have the most agostically distorted structures among the early transition-metal methylidenes. The computed carbon-metal bond shortens with increasing halogen size for both the methylidene and methylidyne complexes. Harmonic and anharmonic frequencies computed by DFT converge on the experimental values and thus provide support for the identification of these new Mo and W complexes.     

CH3--MoF], [CH2=MoHF], and [CH[triple bond]MoH2F] formed by reaction of laser-ablated molybdenum atoms with methyl fluoride: persistent photoreversible interconversion through alpha-hydrogen migration and agostic interaction

Simple molybdenum methyl, carbene, and carbyne complexes, [CH3--MoF], [CH2=MoHF], and [CH[triple chemical bond]MoH(2)F], were formed by the reaction of laser-ablated molybdenum atoms with methyl fluoride and isolated in an argon matrix. These molecules provide a persistent photoreversible system through alpha-hydrogen migration between the carbon and metal atoms: The methyl and carbene complexes are produced by applying UV irradiation (240-380 nm) while the carbyne complex is depleted, and the process reverses on irradiation with visible light (lambda>420 nm). An absorption at 589.3 cm(-1) is attributed to the Mo--F stretching mode of [CH3--MoF], which is in fact the most stable of the plausible products. Density functional theory calculations show that one of the alpha-hydrogen atoms of the carbene complex is considerably bent toward the metal atom (angle-spherical HCMo=84.5 degrees ), which provides evidence of a strong agostic interaction in the triplet ground state. The calculated C[triple chemical bond]Mo bond length in the carbyne is in the range of triple-bond values in methylidyne complexes.     

Infrared spectra of CH3-CrH, CH3-WH, CH2=WH2, and CH[triple bond]WH3 formed by activation of CH4 with Cr and W atoms

Laser-ablated W atoms react with CH4 in excess argon to form the CH3-WH, CH2=WH2, and CH[triple bond]WH3 molecules with increasing yield in this order of product stability. These molecules are identified from matrix infrared spectra by isotopic substitution. Tungsten methylidene and methylidyne hydride molecules are reversibly interconverted by alpha-H transfers upon visible and ultraviolet irradiations. Matrix infrared spectra and DFT/B3LYP calculations show that CH[triple bond]WH3 is a stable molecule with C3v symmetry, but other levels of theory were required to describe agostic distortion for CH2=WH2. Analogous reactions with Cr gave only CH3-CrH, which is calculated to be by far the most stable product.     

Infrared spectra of CH3-MF and several fragments prepared by methyl fluoride reactions with laser-ablated Cu, Ag, and Au atoms

Reactions of laser-ablated, excited group 11 metal atoms with CH(3)F isotopomers have been carried out, leading to the generation of CH(3)-MF and CH(2)F-M complexes for Cu, Ag, and Au in addition to smaller complexes for gold. The products in the infrared spectra identified on the basis of their frequencies, isotopic shifts, and correlation with DFT calculated frequencies reveal that M-F insertion by the coinage metals and H atom release readily occur. The relatively low dissociation energies of CH(3)-AuF to give several smaller Au complexes are consistent with the observation of these fragments. The C-Au bonds of CF-AuH and CH(2)-AuF exhibit considerable π character, and the methylidene CH(2)-AuF contains a true double bond. In contrast, the bond orders of CH(2)-Au and CH(2)-AuH are lower, indicating that F bonded to Au contracts the gold 5d orbitals for better overlap with the carbon 2p orbital for π bonding.     

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.     

Infrared spectra of CH3-MH, CH3-M, and CH3-MH- prepared via methane activation by laser-ablated Au, Ag, and Cu atoms

Methane activation by laser-ablated, excited Group 11 metal atoms has been carried out, leading to generation of CH(3)-MH, CH(3)-M, and CH(3)-MH(-), which are identified in the product infrared spectra on the basis of isotopic shifts and correlation with DFT calculated frequencies. The products reveal that C-H insertion by excited Au, Ag, and Cu readily occurs, and subsequent hydride-detachment or electron addition also follows. Each type of product has similar photochemical properties regardless of the metal. DFT computed energies reveal facile hydride dissociation and high electron affinities for the insertion complexes. The methyl metal species have the shortest C-M bonds, consistent with their highest calculated effective bond order, and the CH(3)-MH complexes have higher electron affinities than the metal atoms.     

Tetrahydrometalate species VH(2)(H(2)), NbH(4), and TaH(4): matrix infrared spectra and quantum chemical calculations

Laser ablated V, Nb, and Ta atoms react with molecular hydrogen in excess neon at 4 K to give vanadium, niobium, and tantalum dihydrides that further react with H(2) to form VH(2)(H(2)), NbH(4), and TaH(4). The reaction products are identified by deuterium and deuterium hydride isotopic substitution. DFT and CCSD theoretical calculations are used to predict energies, geometries, and vibrational frequencies for these novel metal hydrides complex and molecules. The vanadium dihydride hydrogen complex, VH(2)(H(2)), is identified, while the niobium and tantalum tetrahydrides, NbH(4) and TaH(4,) with D(2d) symmetry structures are confirmed. Reactions of group 5 metal atoms with H(2) condensing in solid hydrogen gave VH(2)(H(2)) and the higher tetrahydride-hydrogen complexes NbH(4)(H(2))(4) and TaH(4)(H(2))(4).     

Formation and characterization of thorium methylidene CH2=ThHX complexes

Laser-ablated thorium atoms react with methyl fluoride to give the CH2=ThHF molecule as the major product observed and trapped in solid argon. Infrared spectroscopy, isotopic substitution, and density functional theoretical frequency calculations confirm the identification of this methylidene complex. The four strongest computed absorptions (Th-H stretch, Th=C stretch, CH2 wag, and Th-F stretch) are the four vibrational modes observed. The CH2=ThHCl and CH2=ThHBr species formed from methyl chloride and methyl bromide exhibit the first three of these modes in the infrared spectra. The computed structures (B3LYP and CCSD) show considerable agostic interaction, similar to that observed for the Group 4 CH2=MHX (M = Ti, Zr, Hf) methylidene complexes, and the agostic angle and C=Th bond length decrease slightly in the CH2=ThHX series (X = F, Cl, Br).     

Infrared spectra of methylidynes formed in reactions of re atoms with methane, methyl halides, methylene halides, and ethane: methylidyne C-H stretching absorptions, bond lengths, and s character

Rhenium carbyne complexes (HC identical with ReH 3, HC identical with ReH 2X, HC identical with ReHX 2, [X = F, Cl, and Br] and CH 3C identical with ReH 3) are produced by reactions of laser-ablated Re atoms with methane, methyl halides, methylene halides, and ethane via oxidative C-H(X) insertion and alpha-hydrogen migration in favor of the carbon-metal triple bond. The stabilities of the carbyne complexes relative to other possible products are predicted by DFT calculations. The diagnostic methylidyne C-H stretching absorptions of HC identical with ReH 3 and its mono- and dihalo derivatives are observed on the blue sides of the precursor C-H stretching bands, and the frequency decreases and the bond length increases in the order of H, F, Cl, and Br, following the decreasing s character in hybridization for the C-H bond. The dihalo methylidynes have higher C-H stretching frequencies and s characters than the monohalo species. The rhenium methylidynes have C s structures, and as a result the HC identical with ReH 3 and CH 3C identical with ReH 3 complexes have two equivalent shorter and one longer Re-H bonds, as compared to the tungsten methylidyne HC identical with WH 3 with three equivalent W-H bonds.     

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.     

Formation of CH3TiX, CH2=TiHX, and (CH3)2TiX2 by reaction of methyl chloride and bromide with laser-ablated titanium atoms: photoreversible alpha-hydrogen migration

The simple methylidene (CH2=TiHX) and Grignard-type (CH3TiX) complexes are produced by reaction of methyl chloride and bromide with laser-ablated Ti atoms and isolated in a solid Ar matrix, and they form a persistent photoreversible system via alpha-hydrogen migration between the carbon and titanium atoms. The Grignard-type product is transformed to the methylidene complex upon UV (240 nm < lambda < 380 nm) irradiation and vice versa with visible (lambda > 530 nm) irradiation. More stable dimethyl dihalide complexes [(CH3)2TiX2] are also identified, whose relative concentration increases upon annealing and at high methyl halide concentration. The reaction products are identified with three different groups of absorptions on the basis of the behaviors upon broadband photolysis and annealing, and the vibrational characteristics are in a good agreement with DFT computation results.     

Millimeter wave spectrum of bromomethyl radical, CH2Br

The rotational spectra of the two isotopic species of the bromomethyl radical, CH2 79Br and CH2 81Br, have been observed in their ground electronic state 2B1 in the 180-470 GHz frequency region, corresponding to a-type transitions from N=8-7 to N=21-20. The radical was produced by hydrogen abstraction of methylbromide (CH3Br) either by chlorine or by fluorine atoms in a free space cell. Hyperfine structure due to the bromine nucleus has been resolved in the observed spectra, and the rotational constants as well as the fine and hyperfine interaction constants were accurately determined for both isotopomers. The inertial defect was determined to be 0.028 96(20) and 0.028 95(20) amu A(2), for CH2 79Br and CH2 81Br, respectively, suggesting a planar structure. By fixing the [angle]HCH bond angle at 124.5 degrees , an effective molecular structure can be derived as r0(CBr)=1.848 A and r0(CH)=1.084 A. A comparison of the molecular structure of various halogen-substituted methyl radicals with respect to the planarity of these radicals is discussed.     

CH3SH与HOCl分子间氢键和卤键的结构与性质

在DFT-B3LYP及MP2/6-311＋＋G**水平上分别求得CH3SH…HOCl氢键复合物和CH3SH…ClOH卤键复合物势能面上的稳定构型. 频率分析表明, 与单体HOCl相比, 在两种复合物中, Cl(9)—O(7)和H(8)—O(7)键伸缩振动频率发生显著的红移. 经MP2/6-311＋＋G**水平计算的含基组重叠误差(BSSE)校正的气相中相互作用能分别为－19.23和－6.85 kJ&#8226;mol－1. 自然键轨道理论(NBO)分析表明, 在CH3SH…ClOH卤键复合物中, 引起Cl(9)—O(7)键变长的因素包括2种电荷转移: (i)孤对电子LP[S(1)]1→σ*[Cl(9)—O(7)]; (ii)孤对电子LP[S(1)]2→σ*[Cl(9)—O(7)], 其中孤对电子LP[S(1)]2→σ*[Cl(9)—O(7)]转移占主要作用, 总的结果是使σ*[Cl(10)—O(11)]的自然布居数增加, 同时O(7)和Cl(9)原子s成分均增加的杂化重优具有与电荷转移作用相同的“拉长效应”; 在CH3SH…HOCl氢键复合物中也存在类似的电荷转移, 但是O(7)原子的再杂化效应不同于前者. 自然键共振理论(NRT)进行键序分析表明, 在氢键复合物和卤键复合物中, H(8)—O(7)和Cl(9)—O(7)键的键序都减小. 通过分子中原子理论(AIM)分析了复合物中氢键和卤键的电子密度拓扑性质.     

Ab initio study of the F + CH3NHNH2 reaction mechanism

The F + CH(3)NHNH(2) reaction mechanism is studied based on ab initio quantum chemistry methods as follows: the minimum energy paths (MEPs) are computed at the UMP2/6-311++G(d,p) level; the geometries, harmonic vibrational frequencies, and energies of all stationary points are predicted at the same level of theory; further, the energies of stationary points and the points along the MEPs are refined by UCCSD(T)/6-311++g(3df,2p). The ab initio study shows that, when the F atom approaches CH(3)NHNH(2), the heavy atoms, namely N and C atoms, are the favorable combining points. For the two N atoms, two prereaction complexes with C(s) symmetry are generated and there exists seven possible subsequent reaction routes, of which routes 1, 2, 5, and 7 are the main channels. Routes 1, 2, and 5 are associated with HF elimination, with H from the amino group or imido group, and route 7 involves the N-N bond break. Routes 3 and 6 with relation to HF elimination with H from methyl, and route 4 involved the C-N bond break, are all energetically disfavored. For the C atom, the attack of F results in the break of the C-N bond and the products are CH(3)F + NHNH(2). This route is very competitive.     

Preparation of CH3TiF and (CH3)2TiF2 from the reaction of CH(3)F with laser-ablated Ti atoms

Laser-ablated Ti atoms react with CH(3)F upon condensation with excess argon to form primarily CH(3)TiF and (CH(3))(2)TiF(2). Irradiation in the UV region promotes alpha-hydrogen rearrangement of CH(3)TiF to CH(2)=TiHF and increases the yield of (CH(3))(2)TiF(2). Annealing to allow diffusion and reaction of more CH(3)F markedly increases the yield of (CH(3))(2)TiF(2). This shows that the CH(3)TiF + CH(3)F reaction is spontaneous and that triplet state CH(3)TiF is an extremely reactive molecule. B3LYP calculations are extremely effective in predicting vibrational frequencies and isotopic shifts for CH(3)TiF and (CH(3))(2)TiF(2) and thus in confirming their identification from matrix infrared spectroscopy.     

(CH3)2S与HOCl分子间的卤键和氢键相互作用

在DFT-B3LYP/6-311++G**水平上分别求得(CH3)2S…ClOH卤键复合物和(CH3)2S…HOCl氢键复合物势能面上的稳定构型. 频率分析表明, 与单体HOCl相比, 在两种复合物中, 10Cl—11O和12H—11O键伸缩振动频率发生显著的红移. 经MP2/6-311++G**水平计算的含基组重叠误差(BSSE)校正的气相中相互作用能分别为-11.69和-24.16 kJ·mol-1. 自然键轨道理论(NBO)分析表明, 在(CH3)2S…ClOH卤键复合物中, 引起10Cl—11O键变长的因素包括两种电荷转移: (i) 孤对电子LP(1S)1→σ*(10Cl—11O); (ii) 孤对电子LP(1S)2→σ*(10Cl—11O), 其中孤对电子LP(1S)2→σ*(10Cl—11O)转移占主要作用, 总的结果是使σ*(10Cl—11O)的自然布居数增加0.14035e, 同时11O原子的再杂化使其与10Cl成键时s成分增加, 即具有与电荷转移作用同样的“拉长效应”; 在(CH3)2S…HOCl氢键复合物中也存在类似的电荷转移, 但是11O原子的再杂化不同于前者. 自然键共振理论(NRT)进行键序分析表明, 在卤键复合物和氢键复合物中, 10Cl—11O和12H—11O键的键序都减小. 通过分子中原子理论(AIM)分析了复合物中卤键和氢键的电子密度拓扑性质.     

Theoretical study of the structures, stability and vibrational spectra of the nitrous acid complexes with CH4

The structures, stability and vibrational spectra of the binary complexes CH4...HONO-trans and CH4...HONO-cis have been investigated using ab initio calculations at the SCF and MP2 levels with 6-311++G(d,p) basis set and B3LYP calculations with 6-31G(d,p) and 6-31+G(d,p) basis sets. Full geometry optimization was made for the complexes studied. It was established that the complex CH4...HONO-trans is more stable by 0.41 kcal mol(-1) than the complex CH4...HONO-cis. The accuracy of the ab initio calculations have been estimated by comparison between the predicted values of the vibrational characteristics (vibrational frequencies and infrared intensities) and the available experimental data. It was established, that the methods, used in this study are well adapted to the problem under examination. The predicted values with the B3LYP calculations are very near to the results, obtained with 6-311++G(d,p)/MP2. The changes in the vibrational characteristics of methane and trans-, cis-nitrous acid upon formation of the hydrogen bond show that the complexes CH4...HONO-trans and CH4...HONO-cis have geometry in which the OH group interacts with a methane molecule forming a single hydrogen bond. This fact is confirmed by relatively strong perturbation of the OH stretching vibration to lower frequencies and an increase of the infrared intensity of this vibration up to three times upon hydrogen bonding.     

Infrared and DFT investigations of the XC[triple bond]ReX3 and HC[triple bond]ReX3 complexes: Jahn-Teller distortion and the methylidyne C-X(H) stretching absorptions

The XC[triple bond]ReX3 complexes (X = F, Cl) are produced by CX(4) reaction with laser-ablated Re atoms, following oxidative C-X insertion and alpha-halogen migration in favor of the carbon-metal triple bond and are identified through the observation of characteristic absorptions in the argon matrix infrared spectra and comparison with vibrational frequencies calculated by density functional theory. The methylidyne C-F and C-Cl stretching absorptions are observed near 1584 and 1328 cm-1, and the C-H stretching modes for HC[triple bond]ReX3 at 3104 and 3097 cm(-1), respectively, which are substantially higher than the precursor stretching modes and in agreement with the general trend that higher s-orbital character in carbon hybridization leads to a higher stretching frequency. The Jahn-Teller effect in the doublet-state XC[triple bond]ReX3 and HC[triple bond]ReX3 complexes gives rise to distorted structures with Cs symmetry and two equivalent longer Re-X bonds and one slightly shorter Re-X bond.