Robert R. Squires

A reinvestigation of singlet benzyne thermochemistry predicted by CASPT2, coupled-cluster and density functional calculations

Abstract Recent CASPT2 calculations of the heats of formation of the isomeric benzynes by R. Lindh and M. Schutz[Chem. Phys. Lett. 258 (1996) 409] are re-examined. The unrealistically low value reported for p -benzene (132.7 kcal/mol) is shown to be an artifact of the use of incorrect CASSCF and CASPT2 energies for p -benzene, as well as a flawed isodesmic reaction analysis. Use of correct energies and an appropriate isodesmic reaction leads to excellent agreement between the calculated and measured heats of formation for p -benzene. The performance of coupled-cluster methods and density functional theory in predicting benzyne thermochemistry and singlet-triplet splittings is also evaluated.

Gas-phase acidities of carboxylic acids and alcohols from collision-induced dissociation of dimer cluster ions

Abstract The gas-phase acidities (Δ G acid ) of fifteen carboxylic acids and six alcohols have been determined in a flowing afterglow—triple quadrupole instrument from a correlation of the relative yields of anions A − and B − produced by collisionally activated dissociation of the mixed dimer (AHB) − . The acidity of 2,3-butanedione has also been determined, using thermal energy proton transfer bracketing reactions.

A tandem selected ion flow tube—triple quadrupole instrument

Abstract The design, operation and calibration of a selected ion flow tube (SIFT)—triple quadrupole instrument are described. A detailed examination of the gas-phase reaction between ClCH+2 and CH3Cl has been carried out in order to demonstrate some of the unique experimental capabilities of the new instrument. The primary reactions at room temperature and 0.6 Torr total pressure are thermoneutral Cl-isotope exchange and termolecular association to yield [ClCH2ClCH3]+, with apparent bimolecular rate coefficients of 6.6 x 10−11 and 4.1 x 10−11 cm3 s−1, respectively. Double-labelling experiments with ClCD+2 as the reactant ion identify hydride transfer as the mechanism for the observed Cl-isotope exchange. Collision-induced dissociation (CID) of the addition product yields ClCH+2 with a threshold energy of 31.1 ± 3.0 kcal mol−1. The relative yields of the35ClCH+2 and37ClCH2+ product ions produced by CID of the mass-selected (35Cl,37Cl) isotopomeric adduct have been measured as a function of the CH3Cl concentration in the flow reactor. Analysis of these data with a simple kinetic model indicates that approximately one third of the adduct-forming collisions are accompanied by Cl-exchange via hydride transfer within the collision complex. When the [ClCH2ClCH3]+ ions are formed in the flow tube by a “switching” reaction between ClCH+2(SCO) and CH3Cl, Cl-exchange does not occur, as shown by the complete retention of the original Cl-isotope in the ClCH+2 fragment ion produced by CID.

A flowing afterglow-triple quadrupole study of the mechanisms and intermediates in the gas-phase reactions of CH3OH+2 with CH3OH

Abstract The detailed mechanism of formation of protonated dimethyl ether in ionized gaseous methanol has been investigated with a flowing afterglow-triple quadrupole instrument. The structures and unimolecular decomposition reactions of isomeric cluster ions related to those observed in ionized methanol have been examined as models for the proposed bimolecular reaction intermediates. Results from isotope labeling experiments provide evidence for backside nucleophilic attack in the reaction between CH 3 OH + 2 and CH 3 OH, producing (CH 3 ) 2 OH + and H 2 O. Collision-induced dissociation of the protonated methanol dimer and trimer results in both desolvation and formation of protonated dimethyl ether, the latter presumably via rearrangement of the collisionally activated proton-bound cluster ion to the backside displacement precursor. Both CH 3 OH) 2 H + and (CH 3 OH) 3 H + undergo inefficient but exothermic bimolecular displacement reactions with CH 3 OH, producing (CH 3 ) 2 OH + (CH 3 OH) and CH 3 ) 2 OH + (CH 3 OH) 2 , respectively.

Generation of α-acetolactone and the acetoxyl diradical •CH2COO• in the gas phase

Abstract The formation of neutral [C 2 ,H 2 ,O 2 ] has been investigated by tandem mass spectrometry in a sector instrument and by energy-resolved collision-induced dissociation in a flowing afterglow-triple quadrupole apparatus. The neutral species are generated by two different methods: (i) neutralization of the distonic anion radical • CH 2 COO − by collisional electron detachment and (ii) collision-induced loss of halides X − concomitant with formation of [C 2 ,H 2 ,O 2 ] from α-haloacetate ions XCH 2 COO − . The tandem mass spectrometry results suggest that neutralization of • CH 2 COO − and high-energy collisional activation of α-haloacetate ions lead to a mixture of α-acetolactone, c -(CH 2 C(O)O), and the acetoxyl diradical, • CH 2 COO • . Low-energy collisions with α-chloroacetate ions in the triple quadrupole analyzer produce α-acetolactone exclusively at the dissociation threshold. From the dissociation threshold measured for the appearance of Cl − from ClCH 2 COO − the heat of formation of acetolactone is determined to be ΔH f,298 = −47.3 ± 4.7 kcal/mol.

Gas-phase thermochemical properties of the bicarbonate and bisulfite ions

Abstract A combined experimental and theoretical study of the gas-phase thermochemical properties of bicarbonate ion HOCO−2, bisulfite ion HOSO−2, and their conjugate acids is presented. The formation and qualitative identification of the sulfonate ion HSO−3 is also reported. Threshold energies for collision- induced dissociation of OH− ion from HOCO−2 and HOSO−2 have been determined to be 50.3 ± 2.5 and 61.3 ± 2.5 kcal mol−1 respectively, from which the standard heats of formation ΔH°f,298 [HOCO−2, g] = −177.8 ± 2.5 kcal mol−1 and δH°f,298 [HOSO−2] = −165.6 ± 2.5 kcal mol−1 are derived by means of simple thermochemical cycles. The measured HO− binding energies of CO2 and SO2 are in good agreement with ab initio calculations and with the expected values based on empirical correlations between Bronsted and Lewis basicities of negative ions. Reactions between bicarbonate ion and a series of neutral acids in the flowing afterglow indicate an apparent proton affinity of 356 ± 2 kcal mol−1. Bicarbonate ion reacts with H2S to produce both HS− and HS−(H2O), and the deuterium-labelled ion, DOCO−2, reacts also by H/D exchange. Bisulfite ion is found to have an apparent proton affinity of 337 ± 3 kcal mol−1 from its behavior in proton transfer reactions with a series of reference acids. The reaction between HOSO−2 and HCl yields Cl−(SO2) as the major ionic product, along with a small amount of Cl−. DOSO−2 does not exhibit any H/D exchange with HCl or carboxylic acids. Sulfonate ion can be formed at room temperature in the flowing afterglow either by oxygen atom transfer from NO2 to HSO−2, or by hydride transfer from CH3O− to SO3. High level ab initio calculations predict gas-phase acidities (ΔHacid) for carbonic acid (HO)2CO and sulfurous acid (HO)2SO of 339 and 330 kcal mol−1 respectively, and enthalpy changes for their dehydration of −5 and −2kcal mol−1 respectively. The origin of the large differences between the calculated and apparent basicities of HOCO−2 and HOSO−2 is proposed to be due, in part, to a dissociative neutralization mechanism wherein dehydration of the nascent carbonic acid and sulfurous acid molecules accompanies proton transfer.

Determination of the H2ONO+2 and CH3O(H)NO+2 bond strenghts and the proton affinities of nitric acid and methyl nitrate

Abstract The binding energies of water and methanol to NO + 2 have been measured to be 14.8 ± 2.3 and 19.2 ± 2.3 kcal/mol, respectively, using energy-resolved collision-induced dissociation of H 2 ONO + 2 and CH 3 O(H)NO + 2 in a flowing afterglow triple quadrupole apparatus. These values are used with literature thermochemistry to derive proton affinities for nitric acid and methyl nitrate; PA(HONO 2 ) = 177.7 ± 2.3 kcal/mol and PA(CH 3 ONO 2 ) = 175.0 ± 2.5 kcal/mol. These results are in good agreement with recent calculations by Lee and Rice, but only the methyl nitrate result is in agreement with experimental results of Cacace and co-workers.

Gas‐phase reactions of the benzyne negative ions

The reactions of o-, m- and p-benzyne anions and the phenide ion with a series of neutral reagents are described. The m- and p-benzyne anions display similar behavior towards Bronsted acids, CS 2 , N 2 O, NO and O 2 , which is analogous to that of phenide ion but clearly different from that of o-benzyne anion. The strongly basic and nucleophilic character of m- and p-benzyne anions dominates their reactivity, and radical-type reactions are generally not observed. Novel bifunetional reactions between m- and p-benzyne anions and both CS 2 and NO are observed in which two sequential S-atom abstractions and two NO additions, respectively, take place.

DETERMINATION OF THE PROTON AFFINITY AND ABSOLUTE HEAT OF FORMATION OF CYCLOPROPENYLIDENE

Abstract The proton affinity and absolute heat of formation of cyclopropenylidene (c-C 3 H 2 ) have been derived from the translational energy threshold for endothermic proton transfer from c-C 3 H 3 + to ammonia in a flowing afterglow triple quadrupole instrument: c-C 3 H 3 + + NH 3 → c-C 3 H 2 + NH 4 + . The cyclopropenium cation C 3 H 3 + was prepared in a helium flow reactor at room temperature by the reaction of ionized ethylene with acetylene, and from dissociative electron ionization of bromocyclopropane. The kinetic energy dependence of the cross-sections for proton transfer from this ion to ammonia and other neutral amines was characterized in a triple quadrupole mass analyzer. The endothermicity for the reaction with ammonia was determined to be 23.3 ± 1.8 kcal mol −1 . Combining this with the known proton affinity (PA) of ammonia (204.0 ± 1.0 kcal mol −1 ) gives a value for PA(c-C 3 H 2 ) of 227.3 ± 2.1 kcal mol −1 . From the measured proton affinity and the known heats of formation of c-C 3 H 3 + and the proton, the 298 K heat of formation of cyclopropenylidene is determined to be 119.5 ± 2.2 kcal mol −1 . This value is slightly higher than a previous experimental estimate, but is in good agreement with the 298 K heat of formation predicted by high level molecular orbital calculations.

Collisional activation of intramolecular nucleophilic displacement reactions: the formation of acetolactone from dissociation of α-haloacetate negative ions

Abstract The energetics and products of low energy collision-induced dissociation (CID) of a number of α-substituted acetate ions and β-substituted alkoxide ions have been investigated. Analysis of the appearance energies of the fragment ions produced yields limiting values for the endothermicities of the dissociation processes. These limits provide information about the neutral product and mechanism of dissociation. Several of these ions appear to undergo cyclization accompanied by intramolecular nucleophilic displacement in the course of dissociation. For example, the appearance energies measured for the production of X − by CID of XCH 2 CO − 2 (X = Cl, Br, I) indicate that the neutral C 2 H 2 O 2 product is the cyclic species acetolactone ( 1 ). Similarly, production of X − by dissociation of XCH 2 CH 2 O − (X = F, Cl, CH 3 O, CH 2 CHO) is accompanied by formation of the cyclic C 2 H 4 O species ethylene oxide ( 5 ).