Search

We report a joint negative ion photoelectron spectroscopy (NIPES) and quantum chemical computational study on glycine-chloride/bromide complexes (denoted Gly·X−, X = Cl/Br) in close comparison to the previously studied Gly·I− cluster ion. Combining experimental NIPE spectra and theoretical calculations, various Gly·X− complexes were found to adopt the same types of low-lying isomers, albeit with different relative energies. Despite more congested spectral profiles for Gly·Cl− and Gly·Br−, spectral assignments were accomplished with the guidance of the knowledge learned from Gly·I−, where a larger spin–orbit splitting of iodine afforded well-resolved, recognizable spectral peaks. Three canonical plus one zwitterionic isomer for Gly·Cl− and four canonical conformers for Gly·Br− were experimentally identified and characterized in contrast to the five canonical ones observed for Gly·I− under similar experimental conditions. Taken together, this study investigates both genericity and variations in binding patterns for the complexes composed of glycine and various halides, demonstrating that iodide-tagging is an effective spectroscopic means to unravel diverse ion-molecule binding motifs for cluster anions with congested spectral bands by substituting the respective ion with iodide. [ABSTRACT FROM AUTHOR]

Five CHB(11)X(6)Y(5)(-) carborane anions from the series X = Br, Cl, I and Y = H, Cl, CH(3) were generated by electrospray ionization, and their reactivity with a series of Brønsted acids and electron transfer reagents were examined in the gas phase. The undecachlorocarborane acid, H(CHB(11)Cl(11)), was found to be far more acidic than the former record holder, (1-C(4)F(9)SO(2))(2)NH (i.e., DeltaH degrees (acid) = 241 +/- 29 vs 291.1 +/- 2.2 kcal mol(-1)) and bridges the gas-phase acidity and basicity scales for the first time. Its conjugate base, CHB(11)Cl(11)(-), was found by photoelectron spectroscopy to have a remarkably large electron binding energy (6.35 +/- 0.02 eV) but the value for the (1-C(4)F(9)SO(2))(2)N(-) anion is even larger (6.5 +/- 0.1 eV). Consequently, it is the weak H-(CHB(11)Cl(11)) BDE (70.0 kcal mol(-1), G3(MP2)) compared to the strong BDE of (1-C(4)F(9)SO(2))(2)N-H (127.4 +/- 3.2 kcal mol(-1)) that accounts for the greater acidity of carborane acids.

Gas-phase acidity and hydrogen-deuterium (H/D) exchange experiments are carried out on the conjugate base of cysteine with four different deuterated alcohols. The results have shown that the side-chain thiol in cysteine is more acidic than the carboxyl group by 3.1 kcal [mao.sup.-1] and hence deprotonated cysteine is predicted to be a thiolate ion.

Density functional theory was used to examine the solvation number and aggregation state of several alkyllithium compounds in clusters with tetrahydrofuran molecules coordinated to each lithium atom. The number of coordinated solvent molecules in the monomer and dimer show significant temperature dependence, as does the aggregation state of the alkyllithiums.

A thermodynamic cycle is used to carry out several proton affinity, as well as electron-binding measurements, which are then used to effectively and accurately measure the cycloalkane and cycloalkene carbon-hydrogen (C-H) bond dissociation energies in the gas phase. The results show that the normally cited bond energies for such cyclic hydrocarbons are much smaller than their actual values, while a vast difference is observed between the actual and calculated values of their bond strengths.

The heat of hydrogenation of phenylcyclobutadiene is determined via a thermodynamic cycle by carrying out gas-phase measurements on 1-phenylcyclobuten-3-yl cation. The results are compared to G3, G3(MP2), and B3LYP computations and represent the first experimental measurements of the energy of a monocyclic cyclobutadiene.

A linear relation between delta-H(super degree) (sub acid)(HX), EA[X(sub .)], and BDE(HX) is shown for specific classes of compounds and only one of these quantities is needed to determine the others. The intercepts obtained from plots of delta-H(super degree) (sub acid)(HX) vs EA[X(sub .)] provide bond energies which correspond to BDE(HX) when EA[X(sub .)] = 0.

A negative ion photoelectron spectra resulting from the decarboxylation of nine zwitterionic pyridinium dicarboxylates (D(x,y)) is presented. The study shows that the loss of CO2 from the zwitterions and from oxygen-protonated OMP (orotidine 5'-monophosphate) is retarded by the presence of an additional anionic group. It is found that the formation of a zwitterion intermediate in the enzyme-catalyzed conversion of OMP to UMP (uridine 5'-monophosphate) has less energetic impact.

The alpha,2-, alpha,3- and alpha,4-dehydrophenoxide anions are regiospecifically generated by collisional activation of o-, m-, and p-nitrobenzoate. The alpha,2 and alpha,4 isomers are synthesized by reacting o-benzyne radical anion with carbon dioxide and electron ionization of p-diazophenol.

Mechanistic and energetic aspects for the conversion of camphor to 5-exo-hydroxycamphor by the compound I iron-oxo species of cytochrome P450 are discussed from B3LYP DFT calculations. The reaction occurs in a two-step manner along the lines that the oxygen rebound mechanism suggests.

Nine isomeric pyridinium dicarboxylate zwitterions are investigated by photoelectron spectroscopy and compared to their nonzwitterion counterparts. Observations revealed that in the nonzwitterion analogues, the Coulomb repulsion between the negative charge on the carboxylate group and the N lone pair is also significant.

The electron affinity of 3-tert-butyl-1-bicyclo [1.1.1] pentyl radical (14.8 plusminus 3.2 Kcal/mol) is measured by bracketing as well as the acidity of 1-tert-butylbicyclo [1.1.1] pentane (408.5 plusmns 0.9) is determined by the DePuy kinetic method. The bridgehead C-H bond dissociation energy (109.7 plusmns 3.3 Kcal/mol) is much greater than a typical tertiary C-H bond (~ 96 Kcal /mol) and more typical of an aromatic or alkene C-H bond.

A chemical model study of the ring scission of diastereomeric spiropentylcarbinyl radicals is described and two structures for the identity of the enzyme- catalyzed FAD adducts resulting from spiropentylacetic acid-CoA (SPA-CoA) are proposed. Two structures for the enzyme-catalyzed FAD adducts resulting from spiropentylacetic acid-CoA, a synthetic byproduct of fatty acid metabolism are proposed.

Research is presented concerning the use of electrospray ionization and sequential fragmentation in a Fourier transform mass spectrometer for the transfer of acenaphthyne dicarboxylate from solution into a gas phase.

The cyclopropenyl anions with 3-aryl substitutions are unstable in the gas phase during their transition to 6-substituted 3-indenyl anions. The acidity of 3-(4-methylphenyl)cyclopropene is determined by the DePuy method and the pH value agrees with that ab initio value.

Research is done on the molecular properties and electronic structures for three isomers of oxocyclohexadienylidene, which were examined using multiconfiguration SCF (MCSCF) wave functions. It is concluded that the MCSCF(8,8)/6-31+G* electron spin densities for both ortho and para isomers are very close to the experimental values of the phenoxyl radical.

The generation of benzocyclobutadiene radical anions in the gas phase is discussed.

Gas-phase dianion formation as a system for the synthesis of protected reactive intermediates, is discussed.

Research describes the synthesis of benzocyclobutenyl diazirine and the conjugate base of benzocyclobutadiene. Structural aspects and thermochemical data on the carbon-hydrogen bond energy and acidity of the conjugate species are presented.

The formation and characterization of zwitterionic ions and neutral counterparts in the gas phase is described.

The gem-dichloro ketone containing the complete tricyclic skeleton is reduced to prepare the tricyclo[4.2.0.0(sup)1,3]oct-4-ene following an intramolecular cyclization sequence procedure. The reactivity and thermal stability of this bridged spiro hydrocarbon is discussed.

Results discuss the Fourier transform mass spectrometer analyzed gas-phase acidities and reactivities of the three positions in benzocyclobutene. Thermochemical data show that the fused cyclobutane ring exhibits a slight acidifying effect at the alpha-position and the benzylic ion is the most stable of the three anions studied.

The deprotonation of thiirane, thiirane S-oxide, and thiirane S,S-dioxide produces the cyclic conjugate bases of thiirane and thiirane S,S-dioxide. In the case of thiirane S-oxide, a ring opening isomerization to vinyl sulfenate anion is formed.