Reactions of the hydroperoxide anion with dimethyl methylphosphonate in an ion trap mass spectrometer: evidence for a gas phase alpha-effect

Benchmark ab initio characterization of the complex potential energy surfaces of the HOO- + CH3Y [Y = F, Cl, Br, I] reactions

The α-effect is a well-known phenomenon in organic chemistry, and is related to the enhanced reactivity of nucleophiles involving one or more lone-pair electrons adjacent to the nucleophilic center. The gas-phase bimolecular nucleophilic substitution (SN2) reactions of α-nucleophile HOO- with methyl halides have been thoroughly investigated experimentally and theoretically; however, these investigations have mainly focused on identifying and characterizing the α-effect of HOO-. Here, we perform the first comprehensive high-level ab initio mapping for the HOO- + CH3Y [Y = F, Cl, Br and I] reactions utilizing the modern explicitly-correlated CCSD(T)-F12b method with the aug-cc-pVnZ [n = 2-4] basis sets. The present ab initio characterization considers five distinct product channels of SN2: (CH3OOH + Y-), proton abstraction (CH2Y- + H2O2), peroxide ion substitution (CH3OO- + HY), SN2-induced elimination (CH2O + HY + HO-) and SN2-induced rearrangement (CH2(OH)O- + HY). Moreover, besides the traditional back-side attack Walden inversion, the pathways of front-side attack, double inversion and halogen-bond complex formation have also been explored for SN2. With regard to the Walden inversion of HOO- + CH3Cl, the previously unaddressed discrepancies concerning the geometry of the corresponding transition state are clarified. For the HOO- + CH3F reaction, the recently identified SN2-induced elimination is found to be more exothermic than the SN2 channel, submerged by ∼36 kcal mol-1. The accuracy of our high-level ab initio calculations performed in the present study is validated by the fact that our new benchmark 0 K reaction enthalpies show excellent agreement with the experimental data in nearly all cases.

Origin of the α-Effect in SN 2 Reactions

The α-effect is a term used to explain the dramatically enhanced reactivity of α-nucleophiles (R-Y-X:- ) compared to their parent normal nucleophile (R-X:- ) by deviating from the classical Brønsted-type reactivity-basicity relationship. The exact origin of this effect is, however, still heavily under debate. In this work, we have quantum chemically analyzed the α-effect of a set of anionic nucleophiles, including O-, N- and S-based normal and α-nucleophiles, participating in an SN 2 reaction with ethyl chloride using relativistic density functional theory at ZORA-OLYP/QZ4P. Our activation strain and Kohn-Sham molecular orbital analyses identified two criteria an α-nucleophile needs to fulfill in order to show α-effect: (i) a small HOMO lobe on the nucleophilic center, pointing towards the substrate, to reduce the repulsive occupied-occupied orbital overlap and hence (steric) Pauli repulsion with the substrate; and (ii) a sufficiently high energy HOMO to overcome the loss of favorable HOMO-LUMO orbital overlap with the substrate, as a consequence of the first criterion, by reducing the HOMO-LUMO orbital energy gap. If one of these two criteria is not fulfilled, one can expect no α-effect or inverse α-effect.

Adsorptive Removal of Nerve Agent Gases by Carbon Nanotubes: A Density Functional Theory Study

Density functional theory (DFT) studies were performed to evaluate the adsorption behavior and electronic response of (4,4) carbon nanotubes (CNTs) to the organophosphorus nerve agents 3,3-dimethylbutan-2-yl methyl phosphono fluoridate (Soman), pinacolyl methyl phosphonate (SOS), diethyl fluorophosphates (SAS-F) and diethyl chlorophosphate (SAS-Cl). The calculations were performed using the triple numerical plus polarization (TNP) as the basis set with an orbital cutoff of 4.5 Å. The electronic exchange and correlation effects were analyzed by generalized gradient approximation (GGA) with the BLYP parameterization. The studied systems were fully optimized and adsorption energy (E ad), interaction distances, geometric and electronic structures were investigated. According to the obtained relatively high E ad, it was shown that Soman, SOS, SAS-Cl and SAS-F more likely to be absorbed on the CNTs surfaces, introducing an interesting candidate for chemisorption of the nerve agent gas molecules. As a result, the order of increasing of the E ad values of the studied systems were |E ad SAS-F/CNT| > |E ad SAS-Cl/CNT| > |E ad SOS/CNT| > |E ad Soman/CNT| systems. The calculated partial density of states (PDOS) of the adsorption systems confirmed the strong electrons interaction between the nerve agent molecules and the CNTs surfaces. The obtained results indicated the potential application of CNTs in the design and fabrication of protective low-cost gas filters against toxic odorless nerve agent gases.

Hydrazinolysis of aryl cinnamates and related esters: the α-effect arises from stabilization of five-membered cyclic transition state

A kinetic study is reported for nucleophilic substitution reactions of Y-substituted-phenyl cinnamates (1a–1h) with a series of primary amines including hydrazine in H2O containing 20 mol % DMSO at 25.0 °C. The Brønsted-type plot for the reaction of 2,4-dinitrophenyl cinnamate (1a) is linear with βnuc = 0.57 except hydrazine, which exhibits positive deviation from the linear correlation (i.e., the α-effect). The Brønsted-type plots for the reactions of 1a–1h with hydrazine and glycylglycine (glygly) are also linear with βlg = –0.71 and –0.87, respectively, when 1a is excluded from the linear correlation. Thus, the reactions have been concluded to proceed through a concerted mechanism on the basis of the linear Brønsted-type plots and magnitudes of the βnuc and βlg values. The α-effect shown by hydrazine is dependent on electronic nature of the substituent Y in the leaving group, e.g., it increases as the substituent Y becomes a weaker electron-withdrawing group (or as basicity of the leaving aryloxide increases), indicating that the α-effect is not due to destabilization of the ground state but mainly due to stabilization of the transition state. A five-membered cyclic TS structure, which could increase nucleofugality of the leaving aryloxide through H-bonding interaction, has been proposed to account for the leaving-group dependent α-effect found in this study. The theories suggested previously to rationalize the α-effect found for the related systems are also discussed.

Medium effect on the α-effect for nucleophilic substitution reactions of p-nitrophenyl acetate with benzohydroxamates and m-chlorophenoxide in DMSO–H2O mixtures as contrasts with MeCN–H2O mixtures: comparing two very different polar aprotic solvent components

A kinetic study is reported on nucleophilic substitution reactions of p-nitrophenyl acetate (1a) with three α-effect nucleophiles, benzohydroxamate (BHA–), p-methylbenzohydroxamate (MBHA–), and p-methyl-N-methylbenzohydroxamate (M2BHA–), and a reference nucleophile, m-chlorophenoxide (m-ClPhO–), in DMSO–H2O mixtures of varying compositions at 25.0 ± 0.1 °C. Second-order rate constants for the reactions with BHA– and MBHA– decrease upon addition of DMSO to the reaction medium up to 60 mol % DMSO and then increase thereafter only a little. In contrast, M2BHA– and m-ClPhO– become much more reactive as the DMSO content in the medium increases. Such contrasting medium effects on reactivity are consistent with the report that hydroxamic acids behave as OH acids in H2O but as NH acids in dipolar aprotic solvents (e.g., DMSO and MeCN). It has been concluded that BHA– and MBHA– form an equilibrium of a reactive form I with less reactive species II in DMSO–H2O mixtures and the position of the equilibrium is dependent on solvent compositions. BHA– and MBHA– exhibit the α-effect in H2O but not in in 90 mol % DMSO. In contrast, the α-effect yielded by M2BHA– increases steeply up to 70 mol % DMSO and then levels off thereafter.

Nucleophilic Substitution (SN 2): Dependence on Nucleophile, Leaving Group, Central Atom, Substituents, and Solvent

The reaction potential energy surface (PES), and thus the mechanism of bimolecular nucleophilic substitution (SN 2), depends profoundly on the nature of the nucleophile and leaving group, but also on the central, electrophilic atom, its substituents, as well as on the medium in which the reaction takes place. Here, we provide an overview of recent studies and demonstrate how changes in any one of the aforementioned factors affect the SN 2 mechanism. One of the most striking effects is the transition from a double-well to a single-well PES when the central atom is changed from a second-period (e. g. carbon) to a higher-period element (e.g, silicon, germanium). Variations in nucleophilicity, leaving group ability, and bulky substituents around a second-row element central atom can then be exploited to change the single-well PES back into a double-well. Reversely, these variations can also be used to produce a single-well PES for second-period elements, for example, a stable pentavalent carbon species.

The α-effect in the SNAr reaction of 1-(4-nitrophenoxy)-2,4-dinitrobenzene with anionic nucleophiles: effects of solvation and polarizability on the α-effect

A kinetic study on SNAr reactions of 1-(4-nitrophenoxy)-2,4-dinitrobenzene (1a) with various anionic nucleophiles in 80 mol% water – 20 mol% DMSO at 25.0 °C is reported. The Brønsted-type plot for the reaction of 1a with a series of substituted phenoxides and HOO− results in an excellent linear correlation with βnuc = 1.17. However, OH− exhibits dramatic negative deviation from the Brønsted-type plot, while N3−, C6H5S−, and butane-2,3-dione monoximate (Ox−) deviate positively from linearity. HOO− is 680-fold more reactive than OH− but does not exhibit the α-effect. In contrast, Ox− is 166-fold more reactive than isobasic 4-Cl−C6H4O− and exhibits the α-effect. Differential solvation effects have been suggested to be responsible for the α-effect in this study, i.e., Ox− exhibits the α-effect, since it is 5.7 kcal/mol less strongly solvated than 4-Cl−C6H4O− in the reaction medium, while HOO− does not show the α-effect due to a strong requirement for partial desolvation before nucleophilic attack. The highly enhanced reactivity of polarizable N3− and C6H5S− and extremely decreased reactivity of nonpolarizable OH− are in accord with the hard–soft acid and base principle.

The α-effect and competing mechanisms: the gas-phase reactions of microsolvated anions with methyl formate

The enhanced reactivity of α-nucleophiles, which contain an electron lone pair adjacent to the reactive site, has been demonstrated in solution and in the gas phase and, recently, for the gas-phase S(N)2 reactions of the microsolvated HOO(-)(H2O) ion with methyl chloride. In the present work, we continue to explore the significance of microsolvation on the α-effect as we compare the gas-phase reactivity of the microsolvated α-nucleophile HOO(-)(H2O) with that of microsolvated normal alkoxy nucleophiles, RO(-)(H2O), in reactions with methyl formate, where three competing reactions are possible. The results reveal enhanced reactivity of HOO(-)(H2O) towards methyl formate, and clearly demonstrate the presence of an overall α-effect for the reactions of the microsolvated α-nucleophile. The association of the nucleophiles with a single water molecule significantly lowers the degree of proton abstraction and increases the S(N)2 and B(AC)2 reactivity compared with the unsolvated analogs. HOO(-)(H2O) reacts with methyl formate exclusively via the B(AC)2 channel. While microsolvation lowers the overall reaction efficiency, it enhances the B(AC)2 reaction efficiency for all anions compared with the unsolvated analogs. This may be explained by participation of the solvent water molecule in the B(AC)2 reaction in a way that continuously stabilizes the negative charge throughout the reaction.

Investigating the α-effect in gas-phase S(N)2 reactions of microsolvated anions

The α-effect-enhanced reactivity of nucleophiles with a lone-pair adjacent to the attacking center-was recently demonstrated for gas-phase S(N)2 reactions of HOO(-), supporting an intrinsic component of the α-effect. In the present work we explore the gas-phase reactivity of microsolvated nucleophiles in order to investigate in detail how the α-effect is influenced by solvent. We compare the gas-phase reactivity of the microsolvated α-nucleophile HOO(-)(H2O) to that of microsolvated normal alkoxy nucleophiles, RO(-)(H2O), in reaction with CH3Cl using a flowing afterglow-selected ion flow tube instrument. The results reveal enhanced reactivity of HOO(-)(H2O) and clearly demonstrate the presence of an α-effect for the microsolvated α-nucleophile. The association of the nucleophile with a single water molecule results in a larger Brønsted βnuc value than is the case for the unsolvated nucleophiles. Accordingly, the reactions of the microsolvated nucleophiles proceed through later transition states in which bond formation has progressed further. Calculations show a significant difference in solvent interaction for HOO(-) relative to the normal nucleophiles at the transition states, indicating that differential solvation may well contribute to the α-effect. The reactions of the microsolvated anions with CH3Cl can lead to formation of either the bare Cl(-) anion or the Cl(-)(H2O) cluster. The product distributions show preferential formation of the Cl(-) anion even though the formation of Cl(-)(H2O) would be favored thermodynamically. Although the structure of the HOO(-)(H2O) cluster resembles HO(-)(HOOH), we demonstrate that HOO(-) is the active nucleophile when the cluster reacts.

Dissection of activation parameters in the bell-shaped α-effect following solvent modulation (DMSO-H2O media)

This paper comprises results of our investigation of the α-effect phenomenon for the reaction of O-p-nitrophenyl thionobenzoate (PNPTB) with butane-2,3-dione monoximate (Ox(-), α-nucleophile) and p-chlorophenoxide (p-ClPhO(-), normal-nucleophile) in DMSO-H2O mixtures of varying compositions at 15.0 °C, 25.0 °C, and 35.0 °C. The reactivity of Ox(-) and p-ClPhO(-) increases significantly as the DMSO content in the medium increases, although the effects of medium on reactivity are not the same for the reactions with Ox(-) and p-ClPhO(-). Ox(-) exhibits the α-effect in all solvent compositions and temperatures. The α-effect increases up to 50 mol % DMSO and then decreases thereafter, resulting in a bell-shaped α-effect profile. Dissection of the activation parameters (i.e., ΔH(‡) and TΔS(‡)) has revealed that the bell-shaped α-effect behavior is due to entropy of activation differences rather than enthalpy terms, although the enthalpy term controls almost entirely the solvent dependence of the reaction rate. Differences in the transition-state (TS) structures for the reactions with Ox(-) (a six-membered cyclic TS) and p-ClPhO(-) (an acyclic TS) are consistent with the entropy-dependent α-effect behavior.

The α-effect exhibited in gas-phase S(N)2@N and S(N)2@C reactions

In order to explore the existence of α-effect in gas-phase S(N)2@N reactions, and to compare its similarity and difference with its counterpart in S(N)2@C reactions, we have carried out a theoretical study on the reactivity of six α-oxy-Nus (FO(-), ClO(-), BrO(-), HOO(-), HSO(-), H2NO(-)) in the S(N)2 reactions toward NR2Cl (R = H, Me) and RCl (R = Me, i-Pr) using the G2(+)M theory. An enhanced reactivity induced by the α-atom is found in all examined systems. The magnitude of the α-effect in the reactions of NR2Cl (R = H, Me) is generally smaller than that in the corresponding S(N)2 reaction, but their variation trend with the identity of α-atom is very similar. The origin of the α-effect of the S(N)2@N reactions is discussed in terms of activation strain analysis and thermodynamic analysis, indicating that the α-effect in the S(N)2@N reactions largely arises from transition state stabilization, and the "hyper-reactivity" of these α-Nus is also accompanied by an enhanced thermodynamic stability of products from the n(N) → σ*(O-Y) negative hyperconjugation. Meanwhile, it is found that the reactivity of oxy-Nus in the S(N)2 reactions toward NMe2Cl is lower than toward i-PrCl, which is different from previous experiments, that is, the S(N)2 reactions of NH2Cl is more facile than MeCl.

Theoretical investigation of experimentally determined α-εffects — The role of electronics

Recent experimental reports involving both α-nucleophiles and normal nucleophiles have reported both the presence and absence of an α-effect. In ester systems, such as dimethylmethylphosphonate (DMMP), a small α-effect is reported, but the reference point is a stationary point of the potential energy surface that must rearrange to acquire the near attack conformation (NAC) necessary for the Sn2 pathway to proceed. The second type of study involves use of highly fluorinated alkoxides as normal nucleophiles and reports no α-effect. This paper employs linear free energy plots in an investigation of electronic effects in methyl formate SN2 reactions, using high-level computations of transition states for determination of energy barriers.

Rapid monitoring of sulfur mustard degradation in solution by headspace solid-phase microextraction sampling and gas chromatography mass spectrometry

A method using headspace solid-phase microextraction (HS-SPME) followed by gas chromatography/mass spectrometry (GC/MS) analysis has been developed to gain insight into the degradation of the chemical warfare agent sulfur mustard in solution. Specifically, the described approach simplifies the sample preparation for GC/MS analysis to provide a rapid determination of changes in sulfur mustard abundance. These results were found to be consistent with those obtained using liquid-liquid extraction (LLE) GC/MS. The utility of the described approach was further demonstrated by the investigation of the degradation process in a complex matrix with surfactant added to assist solvation of sulfur mustard. A more rapid reduction in sulfur mustard abundance was observed using the HS-SPME approach with surfactant present and was similar to results from LLE experiments. Significantly, this study demonstrates that HS-SPME can simplify the sample preparation for GC/MS analysis to monitor changes in sulfur mustard abundance in solution more rapidly, and with less solvent and reagent usage than LLE.

Pitfalls in assessing the α-effect: reactions of substituted phenyl methanesulfonates with HOO(-), OH(-), and substituted phenoxides in H(2)O

Toward resolving the current controversy regarding the validity of the α-effect, we have examined the reactions of Y-substituted phenyl methanesulfonates 1a-1l with HOO(-), OH(-), and Z-substituted phenoxides in the gas phase versus solution (H(2)O). Criteria examined in this work are the following: (1) Brønsted-type and Hammett plots for reactions with HOO(-)and OH(-), (2) comparison of β(lg) values reported previously for the reactions of Y-substituted phenyl benzenesulfonates 2a-2k with HOO(-) (β(lg) = -0.73) and OH(-) (β(lg) = -0.55), and for those of 1a-1l with HOO(-) (β(lg) = -0.69) and OH(-) (β(lg) = -1.35), and (3) Brønsted-type plot showing extreme deviation of OH(-) for reactions of 2,4-dintrophenyl methanesulfonate 1a with aryloxides, HOO(-), and OH(-), signifying extreme solvation vs different mechanisms. The results reveal significant pitfalls in assessing the validity of current interpretations of the α-effect. The extreme negative deviation by OH(-) must be due, in part, to the difference in their reaction mechanisms. Thus, the apparent dependence of the α-effect on leaving-group basicity found in this study has no significant meaning due to the difference in operating mechanisms. The current results argue in favor of a further criterion, i.e., a consistency in mechanism for the α-nucleophiles and normal nucleophiles.

Enhanced reactivity of RC[triple bond]CZ- (R = H and Cl; Z = O, S, and Se) and the influence of leaving group on the alpha-effect in the E2 reactions

The enhanced reactivity exhibited by six pseudo-alpha-bases, RC[triple bond]CZ(-) (R = H and Cl; Z = O, S, and Se) in gas-phase E2 reactions with ethyl chloride was examined at the G2(+) level. It is found that anomalous reactivity is observed despite the fact that these chalcogen bases do not possess adjacent lone-pair electrons. The influence of the halide leaving groups on the alpha-effect and the origin of the alpha-effect in the E2 reactions of ethyl halides are investigated and discussed.

Reactions of alpha-nucleophiles with alkyl chlorides: competition between S(N)2 and E2 mechanisms and the gas-phase alpha-effect

Reaction rate constants and deuterium kinetic isotope effects for the reactions of BrO(-) with RCl (R = methyl, ethyl, isopropyl, and tert-butyl) were measured using a tandem flowing afterglow-selected ion flow tube instrument. These results provide qualitative insight into the competition between two classical organic mechanisms, nucleophilic substitution (S(N)2) and base-induced elimination (E2). As the extent of substitution in the neutral reactants increases, the kinetic isotope effects become increasingly more normal, consistent with the gradual onset of the E2 channel. These results are in excellent agreement with previously reported trends for the analogous reactions of ClO(-) with RCl. [Villano et al. J. Am. Chem. Soc. 2006, 128, 728.] However, the reactions of BrO(-) and ClO(-) with methyl chloride, ethyl chloride, and isopropyl chloride were found to occur by an additional reaction pathway, which has not previously been reported. This reaction likely proceeds initially through a traditional S(N)2 transition state, followed by an elimination step in the S(N)2 product ion-dipole complex. Furthermore, the controversial alpha-nucleophilic character of these two anions and of the HO(2)(-) anion is examined. No enhanced reactivity is displayed. These results suggest that the alpha-effect is not due to an intrinsic property of the anion but instead due to a solvent effect.