The CBS-QB3 method was used to calculate the gas-phase free energy difference between 20 phenols and their respective anions, and the CPCM continuum solvation method was applied to calculate the free energy differences of solvation for the phenols and their anions. The CPCM solvation calculations were performed on both gas-phase and solvent-phase optimized structures. Absolute pKa calculations with solvated phase optimized structures for the CPCM calculations yielded standard deviations and root-mean-square errors of less than 0.4 pKa unit. This study is the most accurate absolute determination of the pKa values of phenols, and is among the most accurate of any such calculations for any group of compounds. The ability to make accurate predictions of pKa values using a coherent, well-defined approach, without external approximations or fitting to experimental data, is of general importance to the chemical community. The solvated phase optimized structures of the anions are absolutely critical to obtain thi...

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Absolute pKa determinations for substituted phenols

The elimination of specific environmental and industrial contaminants, which are hazardous at only part per million to part per billion concentrations, poses a significant technological challenge. Adsorptive materials designed for such processes must be engendered with an exceptionally high enthalpy of adsorption for the analyte of interest. Rather than relying on a single strong interaction, the use of multiple chemical interactions is an emerging strategy for achieving this requisite physical parameter. Herein, we describe an efficient, catalytic synthesis of diamondoid porous organic polymers densely functionalized with carboxylic acids. Physical parameters such as pore size distribution, application of these materials to low-pressure ammonia adsorption, and comparison with analogous materials featuring functional groups of varying acidity are presented. In particular, BPP-5, which features a multiply interpenetrated structure dominated by <6 A pores, is shown to exhibit an uptake of 17.7 mmol/g at 1 b...

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Ammonia capture in porous organic polymers densely functionalized with Brønsted acid groups.

Gas-phase infrared multiple photon dissociation (IRMPD) spectra have been recorded for the conjugate bases of a series of amino acids (Asp, Cys, Glu, Phe, Ser, Trp, Tyr). The spectra are dominated by strong symmetric and antisymmetric carboxylate stretching modes around 1300 and 1600 cm−1, respectively. Comparison of the experimental spectra with spectra calculated at the DFT level suggests a carboxylate structure for all species investigated, which is in contrast with what has recently been suggested in this journal for deprotonated cysteine [J. Am. Chem. Soc. 2007, 129, 5403−5407]. In addition, the IR spectrum of the conjugate base of tyrosine is also unambiguously that of a carboxylate ion and not that of a phenoxide ion. In sharp contrast with the conjugate bases of other amino acids investigated here, the aspartate and glutamate anions show very broad, hardly resolved spectral features. We present qualitative experimental evidence indicating that this can be attributed to the formation of a proton br...

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Gas-phase IR spectroscopy of deprotonated amino acids.

The effect of substituents on the electrochemistry of metal-free phthalocyanines was examined for 17 phthalocyanine compounds. This work also provides new information about the electron-donating or -withdrawing nature of various substituents, namely, alkoxy, alkylthio, alkyl, alkynyl, phenyloxy, and phenylthio groups attached to the phthalocyanine system, from the viewpoint of electrochemistry. Most of the effects of peripheral and nonperipheral substitution and changes in the ring (π-conjugated system) size on the electrochemistry of metal-free phthalocyanines can be reasonably explained by considering the energy levels of frontier molecular orbitals of the corresponding compounds, which were obtained by calculations using the semiempirical PM3 method.

Electron-donating or -withdrawing nature of substituents revealed by the electrochemistry of metal-free phthalocyanines.

Mechanistic studies form the basis for a better understanding of chemical processes, helping researchers develop more sustainable reactions by increasing the yields of the desired products, reducing waste production, and lowering the consumption of resources and energy overall. Conventional methods for the investigation of reaction mechanisms in solution include kinetic studies, isotope labeling, trapping of reactive intermediates, and advanced spectroscopic techniques. Within the past decade, electrospray ionization mass spectrometry (ESI-MS) has provided an additional tool for mechanistic studies because researchers can directly probe liquid samples by mass spectrometry under gentle conditions.Specifically, ESI-MS allows researchers to identify the molecular entities present in solution over the course of a chemical transformation. ESI-MS is particularly useful for investigations of organic reactions or metal catalysis that involve ionic intermediates. Accordingly, researchers are increasingly using ESI...

Applications of electrospray ionization mass spectrometry in mechanistic studies and catalysis research.

Gas-phase proton-transfer equilibria: A I H + A2= A I + A2H, involving some 50 benzoic acids and phenols and various other standard acids with known acidities were measured with a pulsed electron beam high ion source pressure mass spectrometer. The equilibrium constants K were used to obtain the free energy changes and establish the relative and absolute gas-phase acidities of the benzoic acids and phenols. The gas-phase acidities of the substituted benzoic acids and phenols are compared with the corresponding aqueous acidities. The reasons for the very different attenuation of the substituent effects in water and Me2SO are examined. Ortho effects in the gas phase and specific hydrogen bonding effects are also discussed. The electron affinities of some phenolic radicals are determined from known bond dissociation energies and the gas-phase acidities. Linear free energy relationships have had a profound impact on the development of physical organic chemistry. Thus the classification of substituent effects on chemical reactions done in connection with Hammett type plots led to many of the now widely used concepts of bonding and electronic effects.] The ability of phenols and benzoic acids to ionize as Brqnsted acids in the experimentally easily accessible pH range made them the natural choice as references for free energy relationships.2 The vast majority of reaction types, including the reference reactions that have been successfully correlated by linear free energy relationship plots, are processes occurring in solution. Since ionic solvation is an integral and dominant part of the energetics of these reactions, interpretations of free energy correlations purely in terms of electronic arguments involving the reactants but not the solvent has been a perennial area of concern. One pertinent example is the finding that in most cases the differences in free energy of ionization of substituted phenols and benzoic acids are due to a change of ent r ~ p y . ' ~ Yet, i f solvation effects are not taken into account, electronic effects of the substituents should show up in the enthalpy and not in the entropy change. T o separate solvent effects from the intrinsic molecular effects of the substituents one needs to know the free energy and enthalpy change for the same process occurring in the dilute gas phase. Fortunately such data have recently become available from studies of gas-phase ion molecule reac t iok8 Information on the intrinsic acidities and basicities is obtained from studies of the gas-phase proton-transfer reactions A I H + A2= A I + A2H (1) In earlier work9*Io nly the sign of the free energy change could be obtained from the obsefved occurrence or nonoccurrence of the reaction in the forward direction. Many important results on gas-phase acidity orders were obtained with this technique.' ' I 3 More recently, the study of ion equilibriaI4 was extended to proton-transfer reactions in ICR mass spectrome t e r ~ , ' ~ high-pressure pulsed ion sources,16 and flowing afterglow apparatus.I7 The resulting determinations of K , AGO, and A H o have provided already many significant data. It is interesting to note that recentlyt* results from a b initio MO methods using relatively simple STO-3G basis sets have predicted energy changes (AE2) for the positive ion protontransfer reactions (eq 2) which are in very good agreement with gas-phase ion equlibria data ( A H * ) . Thus an important independent contribution for these reactions can be expected from theoretical calculations. On the other hand the negative ion systems (eq 1) are electron crowded systems in which electron repulsion and electron correlation are much more important. These systems require much more accurate calculations, larger basis sets, and electron correlation corrections. Therefore, unfortunately, numerous and good theoretical calculations of AE2 cannot be expected in the near A brief reportI9 on the gas-phase acidities of the monosubstituted benzoic acids was presented earlier from this laboratory. The present publication presents more extensive documentation of these early results and combines them with newly obtained results for the substituted phenols20 in a more comprehensive treatment of substituent effects on the acidity of benzoic acids and phenols. Experimental Section The acidity data obtained in this work were measured using a pulsed electron beam high-pressure mass spectrometer, which has been described previously.2' Briefly, a typical experiment is carried out as follows. A concentrated methanol solution containing a known molar ratio of the two acids of interest is prepared by dissolving accurately weighed amounts of the acids into methanol. This solution is injected with a syringe through a septum into a 5-L bulb containing I a tm of the buffer gas methane. NF3 (50 cm3) at I a tm are also injected into the bulb. The bulb and inlet system to the ion source are maintained at 150-200 "C to ensure complete vaporization of the rather involatile benzoic acids and phenols. Typically the bulb contains 1 atm of methane, 3 Torr each of the acids of interest, and I O Torr of NF3. After allowing sufficient time for mixing, this gaseous mixture is passed in slow flow through the ion source. The flow out of the bulb is controlled by means of an all metal valve. The gas mixture flows through I O mm i.d. tubing in and out of the ion source and then passes via a flow control capillary into a pump. The pressure in the flow system and ion source is maintained at some constant value in the 2-5 Torr range. Viscous flow results under these conditions. Therefore no fractionation of the minor components (Le., the acids) should occur and the partial pressure ratios of the acids in the ion source should be identical with those in the storage bulb. A confirmation of the expected lack of fractionation could be obtained by observing that the measured equilibrium constant K I changed by only 5% after 85% of the gas had flowed out of the 5-L storage bulb. The ratio of the neutral acids in the bulb was varied by a factor from 2 to 5 in separate experiments where new methanol solutions were prepared and injected. The equilibrium constants were found to be independent of the concentration ratios used. Ionization of the sample is accomplished by a short (-10 p s ) pulse of 2000-eV electrons, which creates some I O 7 ions and electrons. The N F3 undergoes dissociative electron attachment to produce F-. The fluoride ion rapidly abstracts protons from the benzoic acids and Journal of the American Chemical Society / 99:7 / March 30, I977 D ow nl oa de d by T A R B IA T M O D A R R E S U N IV o n A ug us t 1 9, 2 00 9 Pu bl is he d on M ar ch 1 , 1 97 7 on h ttp :// pu bs .a cs .o rg | do i: 10 .1 02 1/ ja 00 44 9a 03 2

T. B. McMahon

Papers

Intrinsic acidities of substituted phenols and benzoic acids determined by gas-phase proton-transfer equilibriums