The Al+ -H(2) cation complex: rotationally resolved infrared spectrum, potential energy surface, and rovibrational calculations

The infrared spectrum of the Al(+)-H(2) complex is recorded in the H-H stretch region (4075-4110 cm(-1)) by monitoring Al(+) photofragments. The H-H stretch band is centered at 4095.2 cm(-1), a shift of -66.0 cm(-1) from the Q(1)(0) transition of the free H(2) molecule. Altogether, 47 rovibrational transitions belonging to the parallel K(a)=0-0 and 1-1 subbands were identified and fitted using a Watson A-reduced Hamiltonian, yielding effective spectroscopic constants. The results suggest that Al(+)-H(2) has a T-shaped equilibrium configuration with the Al(+) ion attached to a slightly perturbed H(2) molecule, but that large-amplitude intermolecular vibrational motions significantly influence the rotational constants derived from an asymmetric rotor analysis. The vibrationally averaged intermolecular separation in the ground vibrational state is estimated as 3.03 A, decreasing by 0.03 A when the H(2) subunit is vibrationally excited. A three-dimensional potential energy surface for Al(+)-H(2) is calculated ab initio using the coupled cluster CCSD(T) method and employed for variational calculations of the rovibrational energy levels and wave functions. Effective dissociation energies for Al(+)-H(2)(para) and Al(+)-H(2)(ortho) are predicted, respectively, to be 469.4 and 506.4 cm(-1), in good agreement with previous measurements. The calculations reproduce the experimental H-H stretch frequency to within 3.75 cm(-1), and the calculated B and C rotational constants to within approximately 2%. Agreement between experiment and theory supports both the accuracy of the ab initio potential energy surface and the interpretation of the measured spectrum.     

Spectroscopy and potential energy surface of the H2-CO2 van der Waals complex: experimental and theoretical studies

A 4-D ab initio potential energy surface is calculated for the intermolecular interaction of hydrogen and carbon dioxide, using the CCSD(T) method with a large basis set. The surface has a global minimum with a well depth of 212 cm(-1) and an intermolecular distance of 2.98 A for a planar configuration with both the O-C-O and H-H axes perpendicular to the intermolecular axis. Bound state calculations are performed for the H(2)-CO(2) van der Waals complex with H(2) in both the para and ortho spin states, and the binding energy of paraH(2)-CO(2)(50.4 cm(-1)) is found to be significantly less than that of orthoH(2)-CO(2)(71.7 cm(-1)). The surface supports 7 bound intermolecular vibrational states for paraH(2)-CO(2) and 19 for orthoH(2)-CO(2), and the lower rotational levels with J< or = 4 follow an asymmetric rotor pattern. The calculated infrared spectrum of paraH(2)-CO(2) agrees well with experiment. For orthoH(2)-CO(2), the ground state rotational levels allowed by symmetry are found to have (K(a), K(c))=(even, odd) or (odd, even). This somewhat unexpected fact enables the previously observed experimental spectrum to be assigned for the first time, in good agreement with theory, and indicates that the orientation of hydrogen is perpendicular to the intermolecular axis in the ground state of the orthoH(2)-CO(2) complex.     

Rotationally resolved infrared spectrum of the Li+-D2 cation complex

The infrared spectrum of mass selected Li(+)-D(2) cations is recorded in the D-D stretch region (2860-2950 cm(-1)) in a tandem mass spectrometer by monitoring Li(+) photofragments. The D-D stretch vibration of Li(+)-D(2) is shifted by -79 cm(-1) from that of the free D(2) molecule indicating that the vibrational excitation of the D(2) subunit strengthens the effective Li(+)cdots, three dots, centeredD(2) intermolecular interaction. Around 100 rovibrational transitions, belonging to parallel K(a)=0-0, 1-1, and 2-2 subbands, are fitted to a Watson A-reduced Hamiltonian to yield effective molecular parameters. The infrared spectrum shows that the complex consists of a Li(+) ion attached to a slightly perturbed D(2) molecule with a T-shaped equilibrium configuration and a 2.035 A vibrationally averaged intermolecular separation. Comparisons are made between the spectroscopic data and data obtained from rovibrational calculations using a recent three dimensional Li(+)-D(2) potential energy surface [R. Martinazzo, G. Tantardini, E. Bodo, and F. Gianturco, J. Chem. Phys. 119, 11241 (2003)].     

Rotationally resolved infrared spectrum of the Na(+)-D2 complex: an experimental and theoretical study

The infrared spectrum of mass-selected Na(+)-D(2) complexes is recorded in the D-D stretch vibration region (2915-2972 cm(-1)) by detecting Na(+) photofragments resulting from photo-excitation of the complexes. Analysis of the rotationally resolved spectrum confirms a T-shaped equilibrium geometry for the complex and a vibrationally averaged intermolecular bond length of 2.461 A?. The D-D stretch band centre occurs at 2944.04 cm(-1), representing a -49.6 cm(-1) shift from the Q(1)(0) transition of the free D(2) molecule. Variational rovibrational energy level calculations are performed for Na(+)-D(2) utilising an ab initio potential energy surface developed previously for investigating the Na(+)-H(2) complex [B. L. J. Poad et al., J. Chem. Phys. 129, 184306 (2008)]. The theoretical approach predicts a dissociation energy for Na(+)-D(2) of 923 cm(-1) with respect to the Na(+)+ D(2) limit, reproduces the experimental rotational constants to within 1-2%, and gives a simulated spectrum closely matching the experimental infrared spectrum.     

Rotational Phenomena on Substituent Phenyl Rings of (η5-Cyclopentadienyl)(η4-Tetraphenylcyclobutadiene)Cobalt

We used extended Hückel calculations, variable-temperature, homonuclear long-range shift-correlated 2-D ¹H NMR and dynamic NOE measurements to investigate rotational phenomena of substituent phenyl rings on (η⁵-cyclopentadienyl)(η⁴tetraphenylcyclobutadiene)cobalt (1). Two closely related compounds, (η⁵-cyclopentadienyl)(η⁴-1,3-diphenylcyclobutadiene)cobalt (A) and (η⁵-cyclopentadienyl)(η⁴-1,2-diphenylcyclobutadiene)cobalt (A′), were prepared. Energy minima appeared at conformations of which the dihedral angles between phenyl and cyclobutadiene rings are about 30° for I and 0° for A according to extended Hückel calculations. In 1, ortho protons of phenyl rings belong to one set of multiplet in ¹H NMR; meta and para protons belong to the other. It was supported by a long-range coupling 2-D ¹H NMR and NOE experiments. A sharp line due to phenyl rings was observed in the low-temperature ¹H NMR spectrum of A, which indicates that the five protons are magnetically equivalent at that temperature.     

S0 and S1 state structure, methyl torsional barrier heights, and fast intersystem crossing dynamics of 5-methyl-2-hydroxypyrimidine

We report the analysis of the S1<--S0 rotational band contours of jet-cooled 5-methyl-2-hydroxypyrimidine (5M2HP), the enol form of deoxythymine. Unlike thymine, which exhibits a structureless spectrum, the vibronic spectrum of 5M2HP is well structured, allowing us to determine the rotational constants and the methyl group torsional barriers in the S0 and S1 states. The 0(0)(0), 6a(0)(1), 6b(0)(1), and 14(0)(1) band contours were measured at 900 MHz (0.03 cm(-1)) resolution using mass-specific two-color resonant two-photon ionization (2C-R2PI) spectroscopy. All four bands are polarized perpendicular to the pyrimidine plane (>90% c type), identifying the S1<--S0 excitation of 5M2HP as a 1nπ* transition. All contours exhibit two methyl rotor subbands that arise from the lowest 5-methyl torsional states 0A" and 1E". The S0 and S1 state torsional barriers were extracted from fits to the torsional subbands. The 3-fold barriers are V3" = 13 cm(-1) and V3' = 51 cm(-1); the 6-fold barrier contributions V6" and V6' are in the range of 2-3 cm(-1) and are positive in both states. The changes of A, B, and C rotational constants upon S1 <--S0 excitation were extracted from the contours and reflect an “anti-quinoidal” distortion. The 0(0)(0) contour can only be simulated if a 3 GHz Lorentzian line shape is included, which implies that the S1(1nπ*) lifetime is ~55 ps. For the 6a(0)(1) and 6b(0)(1) bands, the Lorentzian component increases to 5.5 GHz, reflecting a lifetime decrease to ~30 ps. The short lifetimes are consistent with the absence of fluorescence from the 1nπ* state. Combining these measurements with the previous observation of efficient intersystem crossing (ISC) from the S1 state to a long-lived T1 (3nπ*) state that lies ~2200 cm(-1) below [S. Lobsiger, S. et al. Phys. Chem. Chem. Phys. 2010, 12, 5032] implies that the broadening arises from fast intersystem crossing with k(ISC) ≈ 2 × 10(10) s(-1). In comparison to 5-methylpyrimidine, the ISC rate is enhanced by at least 10 000 by the additional hydroxy group in position 2.     

A Photoluminescent Bimetallic Zinc-pyridinedicarboxylate：Diverse Supramolecular Motifs from Distinct Hydrogen Bonds

A cyclic bimetallic metal-organic complex [Zn（C）（H-fmpdc）（H2O）]2·2H2O （fmpdc = 4-（furan-2-yl）-2,6-dimethylpyridine-3,5-dicarboxylate） was synthesized and characterized by single-crystal X-ray analysis. The compound crystallizes in orthorhombic, space group Pbca with a = 12.905（2）, b = 14.774（3）, c = 16.833（3）A, V= 3029.4（10）A^3 Z = 4, Dc = 1.644 g/cm^3, F（000） = 1616, R = 0.0347 and wR = 0.0956 （I 〉 2σ（I））. There exist diverse supramolecular motifs （1-D, 2-D and 3-D） from distinct hydrogen bonds in the crystal structure of the title compound. The furanyl group has obvious contribution to the red-shift in the photoluminescent spectrum of the H2fmpdc ligand. The title compound 1 shows strong photoluminescence with emission maximum at 2 = 402 nm （λex.max = 367 nm）.     

FTIR detection of protonation/deprotonation of key carboxyl side chains caused by redox change of the Cu(A)-heme a moiety and ligand dissociation from the heme a3-Cu(B) center of bovine heart cytochrome c oxidase

FTIR spectral changes of bovine cytochrome c oxidase (CcO) upon ligand dissociation from heme a(3)() and redox change of the Cu(A)-heme a moiety (Cu(A)Fe(a)()) were investigated. In a photosteady state under CW laser illumination at 590 nm to carbonmonoxy CcO (CcO-CO), the C-O stretching bands due to Fe(a3)()(2+)CO and Cu(B)(1+)CO were identified at 1963 and 2063 cm(-)(1), respectively, for the fully reduced (FR) state [(Cu(A)Fe(a)())(3+)Fe(a3)()(2+)Cu(B)(1+)] and at 1965 and 2061 cm(-)(1) for the mixed valence (MV) state [(Cu(A)Fe(a)())(5+)Fe(a3)()(2+)Cu(B)(1+)] in H(2)O as well as in D(2)O. For the MV state, however, another band due to Cu(B)(1+)CO was found at 2040 cm(-)(1), which was distinct from the alpha/beta conformers in the spectral behaviors, and therefore was assigned to the (Cu(A)Fe(a)())(4+)Fe(a3)()(3+)Cu(B)(1+)CO generated by back electron transfer. The FR-minus-oxidized difference spectrum in the carboxyl stretching region provided two negative bands at 1749 and 1737 cm(-)(1) in H(2)O, which were apparently merged into a single band with a band center at 1741 cm(-)(1) in D(2)O. Comparison of these spectra with those of bacterial enzymes suggests that the 1749 and 1737 cm(-)(1) bands are due to COOH groups of Glu242 and Asp51, respectively. A similar difference spectrum of the carboxyl stretching region was also obtained between (Cu(A)Fe(a)())(3+)Fe(a3)()(2+)Cu(B)(1+)CO and (Cu(A)Fe(a)())(5+)Fe(a3)()(2+)Cu(B)(1+)CO. The results indicate that an oxidation state of the (Cu(A)Fe(a)()) moiety determines the carboxyl stretching spectra. On the other hand, CO-dissociated minus CO-bound difference spectra in the FR state gave rise to a positive and a negative peaks at 1749 and 1741 cm(-)(1), respectively, in H(2)O, but mainly a negative peak at 1735 cm(-)(1) in D(2)O. It was confirmed that the absence of a positive peak is not caused by slow deuteration of protein. The corresponding difference spectrum in the MV state showed a significantly weaker positive peak at 1749 cm(-)(1) and an intense negative peak at 1741 cm(-)(1) (1737 cm(-)(1) in D(2)O). The spectral difference between the FR and MV states is explained satisfactorily by the spectral change induced by the electron back flow upon CO dissociation as described above. Thus, the changes of carboxyl stretching bands induced both by oxidation of (Cu(A)Fe(a)()) and dissociation of CO appear at similar frequencies ( approximately 1749 cm(-)(1)) but are ascribed to different carboxyl side chains.     

EPR Studies of Oxo-Molybdenum(V) Complexes with Sulfur Donor Ligands: Implications for the Molybdenum Center of Sulfite Oxidase

A series of six-coordinate compounds containing a chelating dithiolate coordinated to the [LMo(V)O](2+) unit (L = hydrotris(3,5-dimethyl-1-pyrazolyl)borate) have been characterized by EPR spectroscopy as models for the molybdenum centers of pterin-containing molybdenum enzymes. The structure of LMoO(bdt) (1) (bdt = 1,2-benzenedithiolate) has been determined by X-ray crystallography; the space group is P2(1)/n with a = 10.727(1) ?, b = 14.673(2) ?, c = 15.887(2) ?, beta = 100.317(4) degrees and Z = 4. Compound 1 exhibits distorted octahedral stereochemistry; the terminal oxo group and the sulfur atoms are mutually cis to one another. The Mo=O distance is 1.678(4) ?, and the average Mo-S distance is 2.373(2) ?. The EPR parameters for 1, determined from simulation of the frozen-solution spectrum, are g(1) = 2.004, g(2) = 1.972, g(3) = 1.934 and A(1)((95,97)Mo) = 50.0 x 10(-)(4), A(2) = 11.4 x 10(-)(4), A(3) = 49.7 x 10(-)(4) cm(-)(1). The EPR parameters for several LMo(V)O{S(CH(2))(x)()S} compounds (x = 2-4) with saturated chelate skeletons are similar to those of 1, indicating that it is the coordinated S atoms and not unsaturation of the chelate skeleton that gives rise to the large g values for 1. The presence of g components larger than the free-electron value is ascribed to low-energy charge transfer transitions from the filled sulfur pi orbitals to half-filled Mo d orbitals. The EPR spectrum of [LMo(V)O{S(2)P(OEt)(2)}](+) shows an unusually large isotropic (31)P hyperfine splitting of 66.1 x 10(-)(4) cm(-)(1) from the noncoordinated phosphorus atom. The frozen-solution EPR spectra of the low-pH and high-pH forms of sulfite oxidase have been reinvestigated in D(2)O and the anisotropic g and A((95,97)Mo) parameters determined by simulation of the spectrum arising from the naturally abundant Mo isotopes (75% I = 0, 25% I = (5)/(2)). The EPR parameters for the low-pH form are g(1) = 2.007, g(2) = 1.974, g(3) = 1.968 and A(1) = 56.7 x 10(-)(4), A(2) = 25.0 x 10(-)(4), A(3) = 16.7 x 10(-)(4) cm(-)(1). The EPR parameters for the high-pH form are g(1) = 1.990, g(2) = 1.966, g(3) = 1.954 and A(1) = 54.4 x 10(-)(4), A(2) = 21.0 x 10(-)(4), A(3) = 11.3 x 10(-)(4) cm(-)(1). These are the first determinations of the complete A((95,97)Mo) hyperfine components for an enzyme that possesses an [Mo(VI)O(2)](2+) core in its fully oxidized state.     

Phototransposition reactions of arylboronate esters in acetonitrile and 2,2,2-trifluoroethanol

The phototransposition (para, meta, ortho) reactions of the arylboronate esters 4-, 3-, and 2-(4',4',5',5'-tetramethyl-1',3',2'-dioxaborolanyl)toluenes (1, 2, and 3, respectively) in both acetonitrile and 2,2,2-trifluoroethanol (TFE) using 254 nm irradiation have been examined. The irradiations resulted in steady-state compositions of para (5%), meta (19%), and ortho (76%) isomers in acetonitrile starting from the ortho isomer and para (12%), meta (54%), and ortho (35%) isomers in TFE starting from the para isomer. Analysis of the (13)C NMR spectrum of the product mixture obtained from the photochemistry of the para isomer selectively deuterated at C3 and C5 (1d(2)()) revealed that the boron-substituted carbon is the active one in the phototransposition reactions in both acetonitrile and TFE. Similar results were observed for irradiations of 1 in cyclohexane. Fluorescence spectra, singlet-state lifetimes, and Stern-Volmer quenching of fluorescence with 2,3-dimethyl-1,3-butadiene indicated that the excited singlet states of these three isomers were spectroscopic minima and that the excited singlet state was the reactive one for 3 in acetonitrile.     

Self-assembled decanuclear Na(I)2Mn(II)4Mn(III)4 complexes: from discrete clusters to 1-D and 2-D structures, with the Mn(II)4Mn(III)4 unit displaying a large spin ground state and probable SMM behaviour

The synthesis, magnetic characterization and X-ray crystal structures are reported for five new manganese compounds, [Mn(III)(teaH(2))(sal)]·(1/2)H(2)O (1), [Na(I)(2)Mn(II)(4)Mn(III)(4)(teaH)(6)(sal)(4)(N(3))(2)(MeOH)(4)]·6MeOH (2), [Na(I)(2)Mn(II)(4)Mn(III)(4)(teaH)(6)(sal)(4)(N(3))(2)(MeOH)(2)](n)·7MeOH (3), [Na(I)(2)Mn(II)(4)Mn(III)(4)(teaH)(6)(sal)(4)(N(3))(2)(MeOH)(2)](n)·2MeOH·Et(2)O (4) and [K(I)(2)Mn(II)(4)Mn(III)(4)(teaH)(6)(sal)(4)(N(3))(2)(H(2)O)(2)](n)·5MeOH (5). Complex 1 is a mononuclear compound, formed via the reaction of Mn(NO(3))(2)·4H(2)O, triethanolamine (teaH(3)) and salicylic acid (salH(2)) in a basic methanolic solution. Compound 2 is a mixed-valent hetero-metallic cluster made up of a Mn(8)Na(2) decanuclear core and is formed via the reaction of sodium azide (NaN(3)) with 1. Compounds 3-5 are isolated as 1- or 2-D coordination polymers, each containing the decanuclear Mn(8)M(2) (M = Na(+) or K(+)) core building block as the repeating unit. Compound 3 is isolated when 1 is reacted with NaN(3) over a very short reaction time and forms a 1-D coordination polymer. Each unit displays inter-cluster bridges via the O-atoms of teaH(2-) ligands bonding to the sodium ions of an adjacent cluster. Increasing the reaction time appears to drive the formation of 4 which forms 2-D polymeric sheets and is a packing polymorph of 3. The addition of KMnO(4) and NaN(3) to 1 resulted in compound 5, which also forms a 1-D coordination polymer of the decanuclear core unit. The 1-D chains are now linked via inter-cluster potassium and salicylate bridges. Solid state DC susceptibility measurements were performed on compounds 1-5. The data for 1 are as expected for an S = 2 Mn(III) ion, with the isothermal M vs. H data being fitted by matrix diagonalization methods to give values of g and the axial (D) and rhombic (E) zero field splitting parameters of 2.02, -2.70 cm(-1) and 0.36 cm(-1) respectively. The data for 2-5, each with an identical Mn(II)(4)Mn(III)(4) metallic core, indicates large spin ground states, with likely values of S = 16 (±1) for each. Solid state AC susceptibility measurements confirm the large spin ground state values and is also suggestive of SMM behaviour for 2-5 as observed via the onset of frequency dependent out-of-phase peaks.     

Analysis of the torsional spectrum of monodeuterated methanol CH2DOH

An analysis of the torsional spectrum of monodeuterated methanol CH(2)DOH is presented. Twenty nine torsional subbands have been assigned in the 20-800 cm(-1) region. The newly assigned subbands and those already available in the literature were analyzed with a theoretical approach accounting for internal rotation of an asymmetrical CH(2)D methyl group. Seventy six subband centers were reproduced with an rms value of 0.09 cm(-1). Spectroscopic parameters corresponding to the generalized inertia tensor and to the hindering potential were determined as well as rotation-torsion distortion constants.     

Exo-coordination-based supramolecular silver(I) complexes of S2O macrocycles: effect of ligand isomerism on the structural diversity

An isomeric series of S2O macrocycles incorporating a xylyl group at the ortho (L1), meta (L2), and para (L3) positions were employed to examine the influence of the ring rigidity on silver(I) coordination modes in resulting supramolecular complexes (1-3); L1 and L3 afforded sandwich (1; Ag:L1 = 1:2) and infinite 1-D (3) complexes, respectively; otherwise, L2 gave the 1-D polymer (2a), 2:3 club sandwich (2b), and unique 2:4 bridged dinuclear complex (2c) complexes, in which their topologies vary with the solvent used.     

Polymeric Iodoplumbate Incorporating Copper Iodide Complex Cation Layer:Structure and Theoretical Study of a New Semi-conductive Hybrid:{[Cu(Ⅱ)(bipy)_2I][PbI_3](H_2O)_2}_n

A new iodoplumbate polymer incorporating copper iodide complex cation {[Cu(Ⅱ)(bipy)2I][PbI3](H2O)2}n 1(bipy = 2,2'-bipyridine) has been synthesized and structurally determined.It crystallizes in the triclinic system,space group P1 with a = 7.979(4),b = 14.538(11),c = 15.853(8),α = 110.77(2),β = 97.955(18),γ = 104.88(2)°,V = 1607.3(17)3,Z = 2,C20H16CuI4N4O2Pb,Mr = 1122.72,Dc = 2.320 g/cm3,F(000) = 1006,μ(MoKα) = 9.753,the final R = 0.0627 and wR = 0.1741 for 4846 observed reflections with Ⅰ 2σ(Ⅰ).Structural analysis indicates that 1 consists of 2-D {[Cu(Ⅱ)(bipy)2I]}nn+ cation layers(based on π-π interaction and hydrogen bonds) and [PbI3]nn-polyanions.The C-H···I hydrogen bonds between {[Cu(Ⅱ)(bipy)2I]}nn+ cation layers and [PbI3]nn-polyanions lead to the formation of an interesting 3-D network.Optical absorp-tion spectrum indicates that 1 is a semiconductor,which is further validated by DFT calculation.Its electronic structure is also discussed.     

Millimeter-wave spectroscopy of H(2)C=CD: Tunneling splitting and ortho-para mixing interaction

The H(2)C=CD isotopic species of vinyl radical produced in a supersonic jet expansion by ultraviolet laser photolysis was studied by millimeter-wave spectroscopy. Due to the tunneling motion of the α deuteron, the ground state is split into two components, 0(+) and 0(-). Tunneling-rotation transitions connecting the lower (0(+)) and upper (0(-)) components of the tunneling doublet were observed in the frequency region of 184-334 GHz, including three R- and two Q-branch transitions. Three and two pure rotational transitions in the K(a)=0 and 1 stacks, respectively, were also observed for each of the 0(+) and 0(-) states in the frequency region of 52-159 GHz. Least-squares analysis of the observed frequencies for the tunneling-rotation and pure rotational transitions with well resolved hyperfine structures yielded a set of precise molecular constants, among which the tunneling splitting in the ground state was determined to be ΔE(0)=1187.234(17)?MHz, which is 1/14 that for H(2)C=CH. The potential barrier height derived from the observed tunneling splitting by an analysis of the tunneling dynamics using a one-dimensional model is 1545?cm(-1), consistent with the value 1568?cm(-1) obtained for the normal vinyl. The observed spectrum was found to be perturbed by a hyperfine interaction connecting ortho and para levels. The constant for the interaction, which we call the ortho-para mixing Fermi contact interaction, has been determined to be δa(F) ((β))=68.06(53)?MHz. This is believed to be the first definite detection of such an interaction. By this interaction the ortho and para states of H(2)C=CD are mixed up to about 0.1%. The constant is more than 1000 times larger than spin-rotation interaction constants that cause ortho-para mixing in closed shell molecules and suggests extremely rapid conversion between the ortho and para nuclear spin isomers of H(2)C=CD.     

Differentiation of isomeric substituted diaryl ethers by electron ionization and chemical ionization mass spectrometry

A series of isomeric substituted diaryl ethers, i.e., 2- or 4-NO2, 5- FC6H3OC6H4 (4-R), where R=H, COCH3, COOCH3, NO2, CHO, OCH3 etc., which comprise ortho and para isomers with respect to the position of the nitro group are studied under GC-EI-MS and CI-MS conditions. The EI mass spectra of ortho and para isomers show distinct fragment ions, where the [MOH]+ and [MOHO]+ ions specifically appeared in all spectra of the ortho isomers (I), whereas the para isomers (II) contain [MO]+ and [MNO]+ ions. The [MOHCO]+ and [MOHNO]+ ions in I, and [MNO2]+ ion in II are the other specific fragment ions observed but feasibility of these fragment ions are found to depend on the nature of the substituent (R). The substitution (R) effect is also clearly reflected in the formation of fragment ions due to sigma-cleavage process with or without hydrogen migration. Similar differences in the formation of specific fragment ions are also observed in ortho and para isomers of substituted aryl naphthyl ethers. The methane/CI of isomeric compounds resulted in the same set of fragment ions, but prominent differences are observed in the relative abundance of [MHNO]+, which is relatively higher in para isomers compared with corresponding ortho isomer.     

Photochemistry of the three carboxypyridines in water: a pH dependent reaction

The photochemistry of ortho, meta and para-carboxypyridines (pK(a)(1)= 1.0-2.1 and pK(a)(2)= 4.7-5.3) in aqueous medium was studied by laser-flash photolysis and product studies. At pH < pK(a)(1), hydroxylated compounds are produced with low quantum yields. Within the pH range 4-7, ortho and meta isomers undergo dimerization together with decarboxylation with a quantum yield showing a very sharp maximum around pK(a)(2)([small phi](max)= 0.09 and 0.01, respectively) while the para isomer is photostable. End-of-pulse transients assigned to triplet states were detected by laser-flash photolysis at pH < pK(a)(1) and pH > 4. Additionally, the carboxypyridinyl radicals were detected as secondary intermediates at pH < pK(a)(1) and 4 < pH < 7 and the OH-adduct radicals at pH < pK(a)(1). This is in favour of an electron transfer reaction between triplet and starting compound producing a charge transfer species. The radical anion would escape as carboxypyridinyl radical while the radical cation may add water at pH < pK(a)(1) yielding the OH-adduct radical or may undergo decarboxylation at pH > 4. The high quantum yield of phototransformation of the ortho isomer at pH > 4 is due to an easy decarboxylation process. A reaction scheme is proposed accounting for the dependences of [small phi] on both the pH and the carboxypyridines concentration. This study points out the distinct pattern of reactivity of carboxypyridines depending on the ionisation state of starting compounds and isomeric substitution.     

The rotational spectrum and structure for the argon-cyclopentadienyl thallium van der Waals complex: experimental and computational studies of noncovalent bonding in an organometallic pi-complex

The rotational spectrum of a noble gas-organometallic complex was measured using a pulse molecular beam Fourier transform microwave spectrometer. Rotational transitions for the neutral argon-cyclopentadienyl thallium weakly bound complex were measured in the 4-9 GHz range. Analysis of the spectrum showed that the complex is a prolate symmetric-top rotor with C(5V) symmetry. The experimentally determined molecular parameters for Ar-C(5)H(5) (205)Tl are B=372.4479(3) MHz, D(J)=0.123(2) kHz, and D(JK)=0.45(2) kHz. For Ar-C(5)H(5) (203)Tl, B=373.3478(5) MHz, D(J)=0.113(3) kHz, and D(JK)=0.37(3) kHz. Using a pseudodiatomic model with Lennard-Jones potential yields an approximate binding energy of 339 cm(-1). The argon atom is located on the a-axis of the C(5)H(5)Tl monomer, directly opposite from the thallium metal atom. The measured separation distance between argon and the cyclopentadienyl ring is R=3.56 A. The overall size of the cluster is about 6 A, measuring from argon to thallium. Relatively small D(J) and D(JK) centrifugal distortion constants were observed for the complex, indicating that the structure of Ar-C(5)H(5)Tl is somewhat rigid. MP2 calculations were used to investigate the possible structures and binding energies of the argon-cyclopentadienyl thallium complex. Calculated, counterpoise corrected binding energies are evaluated at R=3.56 A for Ar-C(5)H(5)Tl range from 334 to 418 cm(-1). The experimental binding energy epsilon=339 cm(-1) for Ar-C(5)H(5)Tl falls within this range. The higher-level MP2/aug-cc-pVTZ-PP (thallium)/aug-cc-pVTZ(Ar, C, H) calculation with variable R yielded R(e)=3.46 A and binding energy of 535 cm(-1). Our estimated binding energy for argon-cyclopentadienyl thallium is very similar to the binding energy of argon-benzene. Calculations for the new van der Waals complexes, Ar(C(5)H(5)Tl)(2) and (C(5)H(5)Tl)(2), have been obtained, providing further information on the structures and bonding properties of previously observed cyclopentadienyl thallium polymer chains. The calculated intermolecular distance R(Tl-Cp)=3.05 A for the (CpTl)(2) chain subunit (Cp is cyclopentadienyl, C(5)H(5)) is slightly longer than the measured x-ray value R(M-Cp)(M=Tl)=2.75 A. The x-ray distance R(Tl-Tl)=5.5 A for the chain structure is almost identical to the calculated R(Tl-Tl)=5.51 A for the (C(5)H(5)Tl)(2) dimer.     

Accurate partition function for acetylene, 12C2H2, and related thermodynamical quantities

The internal partition function (Q(int)) of ethyne (acetylene), (12)C(2)H(2), is calculated by explicit summation of the contribution of all individual vibration-rotation energy levels up to 15,000 cm(-1). The corresponding energies are predicted from a global model and constants reproducing within 3σ all 18,415 published vibration-rotation lines in the literature involving vibrational states up to 8900 cm(-1), as produced by Amyay et al. [J. Mol. Spectrosc. 267, 80 (2011)]. Values of Q(int), with distinct calculations for para and ortho species are provided from 1 to 2000 K, in step of 1 K. The total internal partition function at 298.15 K is 104.224387(47) or 416.89755(19), with the nuclear degeneracy spin factors taken as 1/4:3/4 (astronomer convention) or 1:3 (atmospheric convention), respectively, for para:ortho species. The Helmholtz function, Gibbs enthalpy function, entropy, and specific heat at constant pressure are also calculated over the same temperature range. Accuracies as well as the missing contribution of the vinylidene isomer of acetylene in the calculations are discussed.     

Experimental observation of spin delocalisation onto the aryl-alkynyl ligand in the complexes [Mo(C≡CAr)(Ph2PCH2CH2PPh2)(η-C7H7)]+ (Ar = C6H5, C6H4-4-F; C7H7 = cycloheptatrienyl): an EPR and ENDOR investigation

The paramagnetic aryl-alkynyl complexes [Mo(C≡CAr)(dppe)(η-C(7)H(7))](+) (dppe = Ph(2)PCH(2)CH(2)PPh(2); Ar = C(6)H(5), [1](+); C(6)D(5), [2](+); C(6)H(4)-4-F, [3](+); C(6)H(4)-4-Me, [5](+)) and [Mo(C≡CBu(t))(dppe)(η-C(7)H(7))](+) [4](+), have been investigated in a combined EPR and ENDOR study. Direct experimental evidence for the delocalisation of unpaired spin density over the framework of an aryl-alkynyl ligand has been obtained. The X-band solution EPR spectrum of the 4-fluoro derivative, [3](+), exhibits resolved hyperfine coupling to the remote para position of the aryl group [a(iso)((19)F) = 4.5 MHz, (1.6 G)] in addition to couplings attributable to (95/97)Mo, (31)P and (1)H of the C(7)H(7) ring. A full analysis of the (1)H ENDOR spectra is restricted by the low g anisotropy of the system which prevents the use of orientation selection. However, inter-comparison of the (1)H cw-ENDOR frozen solution spectra of [1](+), [2](+), [4](+) and [5](+), combined with spectral simulation informed by calculated values derived from DFT investigations, has facilitated estimation of the experimental a(iso)((1)H) hyperfine couplings of [1](+) including the ortho, ±3.7 MHz (±1.3 G) and para, ±3.9 MHz (±1.4 G) positions of the C(6)H(5) substituent of the aryl-alkynyl ligand.