Tetrafluorobenzyne thermochemistry: experiment and theory

Gas-phase thermodynamic properties of 1,2,3,4-tetrafluorobenzyne (1 H-(2)) were determined by Fourier transform mass spectrometry and ab initio and density functional theory methods. 1,2,3,4-Tetrafluorobenzyne radical anion was generated by abstraction of a proton and a hydrogen atom upon reaction of 1,2,3,4-tetrafluorobenzene (1) with O(-.). The resulting structure was confirmed by converting it to a species which could be independently prepared. Bracketing results provided the proton affinity of 1,2,3,4-tetrafluorobenzyne radical anion and the electron affinities of 1,2,3,4-tetrafluorobenzyne and 1,2,3,4-tetrafluorophenyl radical. These measured values were combined in a thermodynamic cycle to provide the heat of hydrogenation of 1 H(2) (DeltaH degree (hyd) = 367 +/- 18 kJ mol(-1)) and the first and second CH bond dissociation energies of 1 (481 +/- 11 and 321 +/- 13 kJ mol(-1)). The same approach failed for the meta and para isomers, but their energetics were examined using B3LYP and CCSD(T) computations.     

Theoretical investigations into the intermediacy of chlorinated vinylcobalamins in the reductive dehalogenation of chlorinated ethylenes

The reductive dehalogenation of perchloroethylene and trichloroethylene by vitamin B(12) produces approximately 95% (Z)-dichloroethylene (DCE) and small amounts of (E)-DCE and 1,1-DCE, which are further reduced to ethylene and ethane. Chloroacetylene and acetylene have been detected as intermediates, but not dichloroacetylene. Organocobalamins (RCbls) have been proposed to be intermediates in this process. Density functional theory based approaches were employed to investigate the properties of chlorinated vinylcobalamins and chlorinated vinyl radicals. They reveal that all vinyl radicals studied have reduction potentials more positive (E degrees >or= -0.49) than that of the Co(II)/Co(I) couple of B(12) (E degrees = -0.61 V), indicating that any (chlorinated) vinyl radicals formed in the reductive dehalogenation process should be reduced to the corresponding anions by cob(I)alamin in competition with their combination with Co(II) to yield the corresponding vinylcobalamins. The computed Co-C homolytic bond dissociation enthalpies (BDEs) of the latter complexes range from 33.4 to 45.8 kcal/mol. The substituent effects on the BDEs are affected by the stabilities of the vinyl radicals as well as steric interactions between (Z)-chloro substituents and the corrin ring. The calculated E degrees values of the cobalamin models were within approximately 200 mV of one another since electron attachment is to a corrin ring pi-orbital, whose energy is relatively unaffected by chloride substitution of the vinyl ligand, and all were >500 mV more negative than that of the Co(II)/Co(I) couple of B(12). Reduction of the base-off forms of vinyl- and chlorovinylcobalamin models also involves the corrin pi* orbital, but reduction of the base-off dichlorovinyl- and trichlorovinylcobalamin models occurs with electron attachment to the sigma(Co)(-)(C*) orbital, yielding calculated E degrees values more positive than that of the calculated Co(II)/Co(I) couple of B(12). Thus, cob(I)alamin is expected to reduce these base-off vinyl-Cbls. Heterolytic cleavage of the Co-C bonds is much more favorable than homolysis (>21 kcal/mol) and is significantly more exergonic when coupled to chloride elimination.     

Chemoselective ligation applied to the synthesis of a biantennary N-linked glycoform of CD52

We report here a strategy for the synthesis of N-linked glycopeptide analogues that replace the glycosidic linkages extending from the core pentasaccharide with thioethers amenable to construction by chemoselective ligation. The key building block, a pentasaccharide-Asn analogue containing two thiol residues, was incorporated into CD52 by 9-fluorenylmethoxycarbonyl (Fmoc)-based solid-phase peptide synthesis. An undecasaccharide mimetic was then readily generated by alkylation of this glycopeptide with an N-bromoacetamido trisaccharide. The rapid assembly of a complex type N-linked glycopeptide mimetic was accomplished using this technique.     

Photoionization and photodissociation dynamics of the B 1sigmau + and C 1Piu states of H2 and D2

The photoionization and photodissociation dynamics of H(2) and D(2) in selected rovibrational levels of the B (1)Sigma(u) (+) and C (1)Pi(u) states have been investigated by velocity map ion imaging. The selected rotational levels of the B (1)Sigma(u) (+) and C (1)Pi(u) states are prepared by three-photon excitation from the ground state. The absorption of fourth photon results in photoionization to produce H(2)(+) X (2)Sigma(g)(+) or photodissociation to produce a ground-state H(1s) atom and an excited H atom with n >or= 2. The H(2) (+) ion can be photodissociated by absorption of a fifth photon. The resulting H(+) or D(+) ion images provide information on the vibrational state dependence of the photodissociation angular distribution of the molecular ion. The excited H(n >or= 2) atoms produced by the neutral dissociation process can also be ionized by the absorption of a fifth photon. The resulting ion images provide insight into the excited state branching ratios and angular distributions of the neutral photodissociation process. While the experimental ion images contain information on both the ionic and neutral processes, these can be separated based on constraints imposed on the fragment translational energies. The angular distribution of the rings in the ion images indicates that the neutral dissociation of molecular hydrogen and its isotopes is quite complex, and involves coupling to both doubly excited electronic states and the dissociation continua of singly excited Rydberg states.     

Computational strategies for evaluating barrier heights for gas-phase reactions of lithium enolates

Gas-phase activation energies were calculated for three lithium enolate reactions by using several different ab initio and density functional theory (DFT) methods to determine which levels of theory generate acceptable results. The reactions included an aldol-type addition of an enolate to an aldehyde, a proton transfer from an alcohol to a lithium enolate, and an S(N)2 reaction of an enolate with chloromethane. For each reaction, the calculations were performed for both the monomeric and dimeric forms of the lithium enolate. It was found that transition state geometry optimization with B3LYP followed by single point MP2 calculations generally provided acceptable results compared to higher level ab initio methods.     

PHOTOTRANSFORMATION KINETICS OF UNDEGRADED OAT AND PEA PHYTOCHROME INITIATED BY LASER FLASH EXCITATION OF THE RED-ABSORBING FORM

Abstract— Phototransformation at 2°C of the red-absorbing form of phytochrome (Pr) to the far-redabsorbing form (Pfr) was studied with both undegraded oat ( Avena sativa L., cv. Garry) and undergraded pea Pisum sativum L., cv. Alaska) phytochrome. Phototransformation was initiated by a 15-ns laser pulse with maximum emission near 600 nm and output power of 30 mJ. The first resolvable transformation intermediate exhibited relative to Pr a maximum absorbance increase near 700 nm and was fully present at the earliest time measured, which was 60 ns after the flash. This intermediate absorbance decayed by two reactions for oat phytochrome (half-lives of 11 and 140 μs assuming parallel reactions) and by three for pea phytochrome (half-lives of 14, 280 and 1600 μs assuming parallel reactions). The kinetics of the slowest reaction for pea phytochrome, however, might be somewhat distorted by an instrument artifact. The appearance of the far-red-absorbing phytochrome, as monitored by absorbance increase at 720 nm, occurred by at least two reactions for both oat (half-lives of 47 and 250 ms assuming parallel reactions) and pea (half-lives of 170 and 770 ms assuming parallel reactions) phytochrome. The possibility of slower reactions was not tested. Assays for possible proteolysis of the phytochrome samples studied here indicated that the presence of degraded phytochrome could not account for the observed multiphasic kinetics except possibly for one phase of the triphasic intermediate decay seen with pea phytochrome.     

A tensiometric study of diamond (111) and (110) faces

Tensiometric contact angle measurements of diamond surfaces in both the air:water and octane:water systems reveal moderate hydrophobicity, as might be expected of a surface with a minority of oxygen-containing groups. High-temperature reaction with oxygen or hydrogen leads to large changes in the contact angles. Spectroscopic methods show a “one-third” coverage by oxygen. The response to pH and adsorption of ferric ion suggests that the oxygen is present as an hydroxyl group. Steric resistance to higher oxide coverage, because of the small lattice constant of the crystal, may be an explanation for the natural hydrophobicity of the diamond surface.     

Isomer-specific spectroscopy and conformational isomerization energetics of o-, m-, and p-ethynylstyrenes

The infrared and ultraviolet spectroscopy of o-, m-, and p-ethynylstyrene isomers (oES, mES, and pES) were studied by a combination of methods, including resonance-enhanced two-photon ionization (R2PI), UV-UV hole-burning spectroscopy (UVHB), resonant ion-dip infrared spectroscopy (RIDIRS), and rotationally resolved fluorescence excitation spectroscopy. In addition, the newly developed method of stimulated emission pumping-population transfer spectroscopy (SEP-PTS) was used to determine the energy threshold to conformational isomerization in m-ethynylstyrene. The S(1) <-- S(0) origin transitions of oES and pES occur at 32 369 and 33 407 cm(-1), respectively. In mES, the cis and trans conformations are calculated to be close in energy. In the R2PI spectrum of mES, the two most prominent peaks (32672 and 32926 cm(-1)) were confirmed by UVHB spectroscopy to be S(1) <-- S(0) origins of these two conformers. The red-shifted conformer was identified as the cis structure by least-squares fitting of the rotationally resolved fluorescence excitation spectrum of the origin band. There are also two possible conformations in oES, but transitions due to only one were observed experimentally, as confirmed by UVHB spectroscopy. Density functional theory calculations (B3LYP/6-31+G) predict that the cis-ortho conformer, in which the substituents point toward each other, is about 8 kJ/mol higher in energy than the trans-ortho isomer, and should only be about 5% of the room temperature population of oES. Ground-state infrared spectra in the C-H stretch region (3000-3300 cm(-1)) of each isomer were obtained with RIDIRS. In all three structural isomers, the acetylenic C-H stretch fundamental was split by Fermi resonance. Infrared spectra were also recorded in the excited electronic state, using a UV-IR-UV version of RIDIR spectroscopy. In all three isomers the acetylenic C-H stretch fundamental was unshifted from the ground state, but no Fermi resonance was seen. The first observed and last unobserved transitions in the SEP-PT spectrum were used to place lower and upper bounds on the barrier to cis --> trans isomerization in m-ethynylstyrene of 990-1070 cm(-1). Arguments are given for the lack of a kinetic shift in the measurement. The analogous trans --> cis barrier is in the same range (989-1065 cm(-1)), indicating that the relative energies of the zero-point levels of the two isomers are (E(ZPL)(cis) - E(ZPL)(trans))= -75 to +81 cm(-1). Both the barrier heights and relative energies of the minima are close to those determined by DFT (Becke3LYP/6-31+G) calculations.     

2,2′‐[2,3‐Di­hydro‐2‐(prop‐2‐enyl)‐1H‐isoindole‐1,3‐diyl­idene]bis(propane­di­nitrile)–tetra­thia­fulvalene (1/1), TCPI–TTF

The title complex, C₁₇H₉N₅·C₆H₄S₄, contains π‐deficient bis(di­nitrile) and TTF mol­ecules stacked alternately in columns along the a‐axis direction; the interplanar angle between the TTF molecule and the isoindolinyl C₄N[C(CN)₂]₂ moiety is 1.21 (4)°. The N‐allyl moiety in the TCPI mol­ecule is oriented at an angle of 87.10 (10)° with respect to the five‐membered C₄N ring, and the four C[triple‐bond]N bond lengths range from 1.134 (3) to 1.142 (3) Å, with C—C[triple‐bond]N angles in the range 174.3 (3)–176.9 (2)°. In the TTF system, the S—C bond lengths are 1.726 (3)–1.740 (3) and 1.751 (2)–1.763 (2) Å for the external S—C(H) and internal S—C(S) bonds, respectively.