4 research outputs found
Gas Phase Acidity Measurement of Local Acidic Groups in Multifunctional Species: Controlling the Binding Sites in Hydroxycinnamic Acids
The
applicability of the extended kinetic method (EKM) to determine
the gas phase acidities (GA) of different deprotonable groups within
the same molecule was tested by measuring the acidities of cinnamic,
coumaric, and caffeic acids. These molecules differ not only in the
number of acidic groups but in their nature, intramolecular distances,
and calculated GAs. In order to determine independently the GA of
groups within the same molecule using the EKM, it is necessary to
selectively prepare pure forms of the hydrogen-bound heterodimer.
In this work, the selectivity was achieved by the use of solvents
of different vapor pressure (water and acetonitrile), as well as by
variation of the drying temperature in the ESI source, which affected
the production of heterodimers with different solvation energies and
gas-phase dissociation energies. A particularly surprising finding
is that the calculated solvation enthalpies of water and the aprotic
acetonitrile are essentially identical, and that the different gas-phase
products generated are apparently the result of their different vapor
pressures, which affects the drying mechanism. This approach for the
selective preparation of heterodimers, which is based on the energetics,
appears to be quite general and should prove useful for other studies
that require the selective production of heterodimers in ESI sources.
The experimental results were supported by density functional theory
(DFT) calculations of both gas-phase and solvated species. The experimental
thermochemical parameters (deprotonation Δ<i>G</i>, Δ<i>H</i>, and Δ<i>S</i>) are in
good agreement with the calculated values for the monofunctional cinnamic
acid, as well as the multifunctional coumaric and caffeic acids. The
measured GA for cinnamic acid is 334.5 ± 2.0 kcal/mol. The measured
acidities for the COOH and OH groups of coumaric and caffeic acids
are 332.7 ± 2.0, 318.7 ± 2.1, 332.2 ± 2.0, and 317.3
± 2.2 kcal/mol, respectively
Intramolecular C–C Coupling Reactions of Alkynyl, Vinylidene, and Alkenylphosphane Ligands in Rhodium(III) Complexes
Electrophilic
attack with methyl trifluoromethanesulfonate
or tetrafluoroboric acid, to new alkynyl rhodium complexes containing
alkenylphosphanes, leads to butenynyl coupling products or to the
unprecedented rhodaphosphacycle complexes [Rh(η<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>){κ<sup>4</sup>-(<i>P</i>,<i>C</i>,<i>C</i>,<i>C</i>)-<i><sup>i</sup></i>Pr<sub>2</sub>PCH<sub>2</sub>C(CH<sub>2</sub>)C(CH<sub>2</sub>R)CC(R)}][BF<sub>4</sub>] (R = Ph (<b>11a</b>), <i>p</i>-tol (<b>11b</b>)). These complexes <b>11a</b>,<b>b</b> can
be explained as a result of the coupling of three organic fragments
in the molecule, the alkynyl, the vinylidene, generated <i>in
situ</i> by reaction with HBF<sub>4</sub> (<b>A</b>), and
the C–C double bond from the alkenylphosphane. DFT computational
studies on the formation of complex <b>11a</b> suggest the [2
+ 2] intramolecular cycloaddition between the double bond of the allylphosphane
and the Cα–Cβ of the vinylidene <b>A</b> as
the most plausible pathway for this reaction
Acidities of <i>closo</i>-1-COOH-1,7‑C<sub>2</sub>B<sub>10</sub>H<sub>11</sub> and Amino Acids Based on Icosahedral Carbaboranes
Carborane
clusters are not found in Nature and are exclusively
man-made. In this work we study, both experimentally and computationally,
the gas-phase acidity (measured GA = 1325 kJ·mol<sup>–1</sup>, computed GA = 1321 kJ·mol<sup>–1</sup>) and liquid-phase
acidity (measured p<i>K</i><sub>a</sub> = 2.00, computed
p<i>K</i><sub>a</sub> = 1.88) of the carborane acid <i>closo</i>-1-COOH-1,7-C<sub>2</sub>B<sub>10</sub>H<sub>11</sub>. The experimental gas-phase acidity was determined with electrospray
tandem mass spectrometry (ESI/MS), by using the extended Cooks kinetic
method (EKM). Given the similar spatial requirements of the title
icosahedral cage and benzene and the known importance of aminoacids
as a whole, such a study is extended, within an acid–base context,
to corresponding <i>ortho</i>, <i>meta</i>, and <i>para</i> amino acids derived from icosahedral carborane cages,
1-COOH-<i>n</i>-NH<sub>2</sub>-1, <i>n</i>-R with
{R = C<sub>2</sub>B<sub>10</sub>H<sub>10</sub>, <i>n</i> = 2, 7, 12}, and from benzene {R = C<sub>6</sub>H<sub>4</sub>, <i>n</i> = 2, 3, 4}. A remarkable difference is found between the
proportion of neutral versus zwitterion structures in water for glycine
and the carborane derived amino acids
Phyllanthus emblica
The experimentally observed stereomutation of spiro-1,2-oxaphosphetanes
is shown by DFT calculations to proceed through successive M<sub>B2</sub> or M<sub>B4</sub> and M<sub>B3</sub> mechanisms involving two, four,
and three Berry pseudorotations at phosphorus, respectively. Oxaphosphetane
decomposition takes place in a single step via a polar transition
state. The calculated activation parameters for this reaction are
in good agreement with those determined experimentally