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How does chlorine substitution on acetonitrile affect the internal SN2 isomerization of proton-bound pairs (ClCH2CN)(ROH)H+ (R=CH3, C2H5, C3H7)?
Abstract
The unimolecular reactions of proton-bound pairs of chloroacetonitrile and a variety of alcohols (methanol, ethanol, n- and i-propanol) were studied in order to probe the impact of chloro-substitution on both the internal SN2 rearrangement leading to dehydration and simple hydrogen-bond cleavage reactions. Chloro-substitution on acetonitrile does not significantly affect the height of the highest energy isomerization barrier which governs the dehydration reaction but does reduce the relative energy of the transition state that leads to the formation of the high energy intermediate ion (CH3CH⋯R⋯OH2)+. Kinetic modeling of the unimolecular reactions with RRKM theory showed that this latter reaction is a key step in governing the ratio of the two types of reactions observed in the MI mass spectra even though it is not the rate-limiting step for the overall isomerization mechanism leading to dehydration. Chloro-substitution also reduces the proton affinity of acetonitrile resulting in the lowest energy simple-bond cleavage reaction products to be ROH2++ClCH2CN in all cases.
The metastable ion decomposition (MID) spectra of C6HnSi+ (n = 6–8) generated by electron ionization of phenylsilane were obtained using mass-analyzed ion kinetic energy spectrometry (MIKES). H and H2 losses were the main channels in the MID of C6H8Si+ (1). The dominant product ion in the MIDs of C6H7Si+ (2) and C6H6Si+ (3) was C6H5Si+ (4) formed by H2 and H losses, respectively. Density functional theory calculations were performed to investigate isomerizations and dissociations of 1–3 at the B3LYP/6-311++G(d,p) level. RRKM model calculations were performed to understand their dissociation kinetics. These theoretical results predict that the consecutive dissociation 1 →3 (+ H2) →4 + H dominates at low energies, while 1 →2 (+ H) →4 + H2 at high energies. It has been found that several isomers of 1–3 including ion–molecule complexes, C6H6·SiHx+ (x = 0–2), play important roles in the dissociation of 1. Reaction mechanisms are proposed for the formation of several isomers of 1–4 from the phenylsilane molecular ion.
A common rearrangement reaction for gas-phase proton-bound molecular pairs corresponds to an internal SN2 reaction that results in the loss of a small neutral molecule. For pairs (RCN)(ROH)H+, the energies of the two transition states (TSa and TSb) and the intermediate complex (IC) in the isomerization reaction (relative to the proton-bound pair, in kJ mol1) can be estimated using the following relationships: E(TSa) = 87 9(n) 0.33(ΔPA), E(IC) = 83 9(n) 0.33(ΔPA), and E(TSb) = 107 9(n) 0.10(ΔPA), where 87, 83, and 107 kJ mol1 are the values for (CH3CN)(CH3OH)H+. Here, n is the number of stablizing alkyl groups on the central SN2 carbon and ΔPA is the difference between the proton affinity of the migrating moiety and that for the base system (in this case, CH3CN). For the analogous pairs (ROH)(R′OH)H+, only the first value in each expression is different (98, 94, and 121 kJ mol1, respectively, calculated for (CH3OH)2H+).Key words: proton-bound molecular pairs, isomerization, internal SN2 reaction, energetics, metastable ions.
There has been increasing interest in the gas-phase reactivity of alkyl nitrates because of their well-known applications as explosives and because of their role in atmospheric and in marine processes. This manuscript describes an experimental study by FT-ICR techniques of the gas-phase reactions of OH(-) and F(-) with methyl and ethyl nitrate. For methyl nitrate, the main reaction channel is found to be an elimination process promoted by abstraction of an α proton from the methyl group. Nucleophilic displacement of nitrate anion through an S(N)2 process at the carbon center is also found to be an important reaction channel with methyl nitrate. In ethyl nitrate, formation of NO(3)(-) is greatly enhanced and this is attributed to the ease of an E2-type elimination process promoted by proton abstraction at the β position of the ethyl group. Theoretical calculations at the MP2/6-311+G(3df,2p)//MP2/6-31+G(d) level of theory are consistent with the relative importance of the reaction channels and suggest that these reactions proceed through a double well potential. The calculations also predict that nucleophilic attack by OH(-) at the nitrogen center (Sn2@N) is energetically the preferred pathway but experiments with (18)OH(-) showed no evidence for this channel. Single-point calculations reveal a strong preference for approach to the carbon center and may explain the lack of reactivity at the nitrogen center. Calculations were also carried out for NH(2)(-) and SH(-) to establish the reactivity pattern to provide a better understanding of environmentally relevant nitrate esters.
Mass spectrometry and ab initio calculations have been employed to investigate the unimolecular decompositions of proton-bound dimers consisting of acetonitrile with n- and i-propanol. Common to both systems is a competition between dissociation of the hydrogen bond in the proton-bound dimer and isomerization to (CH3CNR)(H2O)+ [R = CH2CH2CH3 and CH(CH3)2]. The minimum-energy-reaction pathways for the isomerization in these two systems, as well as those for the dimers containing methanol and ethanol, are presented and compared. The dominant isomerization pathway for these ions is an internal SN2 reaction that proceeds via a stable intermediate CH3CN⋯ROH2+ ion [R = CH3, CH2CH3, CH2CH2CH3 and CH(CH3)2]. The mass spectra for the four butanol-containing dimers (n-, s-, i- and t-butanol) follow similar behavior.
This book provides a penetrating and comprehensive description of energy selected reactions from a theoretical as well as experimental view. Three major aspects of unimolecular reactions involving the preparation of the reactants in selected energy states, the rate of dissociation of the activated molecule, and the partitioning of the excess energy among the final products, are fully discussed with the aid of 175 illustrations and over 1,000 references, most from the recent literature. Examples of both neutral and ionic reactions are presented. Many of the difficult topics are discussed at several levels of sophistication to allow access by novices as well as experts. Among the topics covered for the first time in monograph form is a discussion of highly excited vibrational/rotational states and intramolecular vibrational energy redistribution. Problems associated with the application of RRKM theory are discussed with the aid of experimental examples. Detailed comparisons are also made between different statistical models of unimolecular decomposition. Both quantum and classical models not based on statistical assumptions are described. Finally, a chapter devoted to the theory of product energy distribution includes the application of phase space theory to the dissociation of small and large clusters. The work will be welcomed as a valuable resource by practicing researchers and graduate students in physical chemistry, and those involved in the study of chemical reaction dynamics.
Solutions of coupled differential equations for the decay rates of energy-selected ions which competitively dissociate and isomerize show that under appropriate circumstances two-component decay rates can be expected. Although the requirements for the observation of such non-exponential decay is rather restrictive, it has now been observed in a number of cases, including the dissociation of butene and pentene ions, as well as 1,4-oxathian, 1,4-dithian and dioxan. Photoelectron–photoion coincidence spectroscopy has been used to energy-select ions and to measure the rate of dissociation. Examples of such non-exponential decay rates are presented for the above-mentioned ions.
This study assesses a variety of computational methods for calculating the binding energies of proton-bound clusters containing a nitrile. The binding energies of (HCN)2H+, (HCN)(NH3)H+, (HCN)(H2O)H+ and (HCN)(HF)H+ were calculated at the HF, MP2 and B3-LYP levels of theory with a variety of basis sets, and the results compared to previous work employing G2 based methods. Reliable binding energies could be obtained using the MP2/6-31+G(d) and B3-LYP/6-31+G(d) levels of theory. The results were extended to a total of thirteen proton-bound dimers for which G2 values are available, and four multiply hydrated clusters (HCN)(H2O)2H+, (HCN)(H2O)3H+, (CH3CN)(H2O)2H+ and (CH3CN)(H2O)3H+.
The metastable proton-bound dimer of acetonitrile and methanol, (CH3CN)(CH3OH)H+, exhibits two unimolecular reactions on the microsecond time scale, a simple bond cleavage reaction to form CH3CNH+ and CH3OH, and the loss of water to form CH3CNCH3+. The latter process is preceded by the isomerization of the proton-bound dimer to a second isomer, (CH3CNCH3)(H2O)+. Collision-induced dissociation mass spectrometry was used to identify the two sets of reaction products, and the results of isotopic labeling experiments suggest that there is a rate-limiting isomerization step going from the proton-bound dimer to the second complex. The competition between the two channels was modeled with ab initio calculations and RRKM rate theory to obtain relative energies for the reaction surface. The transition structure for the rate-determining isomerization could not be located with ab initio calculations, and so its relative energy was estimated using RRKM theory. The 0 K binding energy of the (CH3CN)(CH3OH)H+ complex was calculated to be 121 kJ mol-1 at the G2 level of theory (relative to the dissociation products CH3CNH+ and CH3OH).
The proton-bound dimer of acetonitrile and ethanol, (CH3CN)(CH3CH2OH)H+, exhibits three unimolecular reactions on the microsecond time scale: two simple bond cleavage reactions to form CH3CNH+ + CH3CH2OH and CH3CH2OH2+ + CH3CN, and the loss of water to form CH3CNCH2CH3+. The latter process is preceded by the isomerization of the proton-bound dimer to a second isomer, (CH3CNCH2CH3)(H2O)+. The competition between the simple dissociation reactions and the isomerization reaction was modeled with ab initio calculations and RRKM theory to obtain relative energies for the reaction surface. The 0 K binding energy of the (CH3CN)(CH3CH2OH)H+ complex was calculated to be 152 kJ mol-1 at the G2(MP2,SVP) level of theory (relative to the dissociation products CH3CNH+ and CH3CH2OH). The isomerization barrier for the proton-bound dimer was estimated to be 22 kJ mol-1 lower than CH3CNH+ + CH3CH2OH. The greater polarizability of the ethyl group is believed to stabilize this transition structure over that found for the (CH3CN)(CH3OH)H+ ion (which was previously estimated to lie 6 kJ mol-1 below CH3CNH+ and CH3OH).
The NIST Chemistry WebBook (http://webbook.nist.gov) is an Internet site that provides access to chemical and physical property data both from NIST and other sources. The site was established in 1996 and has grown to encompass a wide variety of thermochemical, ion energetics, solubility, and spectroscopic data. The thermochemical data available include enthalpies of formation, enthalpies of phase transitions, and heat capacities. Thermochemical properties of many reactions that support enthalpy of formation values are provided. Automated tools are used to check data prior to its inclusion in the web site. Most of the collections in the site provide extensive coverage of the literature in their field and include relevant metadata such as the experiment type or important auxiliary data. These features make the site an excellent tool for data evaluation. A major goal of the project was to provide convenient access to all types of chemical data. Several challenges were encountered in the development of systems and conventions for concisely and accurately displaying chemical data on the Internet. The next phase of the evolution of the site will be the addition of tools to aid researchers in getting data from the site. Data from the site have found applications in industrial, research, and educational settings. Usage patterns for the site will be discussed.
A detailed mass spectrometric and thermochemical investigation was made of the C-2,H-2,N family of radicals and cations. The following thermochemical data were obtained: Delta(f)H degrees(298)((CH2CN)-C-+) = 291 +/- 2 kcal mol(-1), Delta(f)H degrees(298)((CH2NC)-C-+) = 282-293 kcal mol(-1), Delta(f)H degrees(298)(HCCHN+) less than or equal to 272 kcal mol(-1), Delta(f)H degrees(298)((CH2CN)-C-.) = 58 +/- 3 kcal mol(-1), Delta(f)H degrees(298)((CH2NC)-C-.)= 96 +/- 3 kcal mol(-1), and Delta(f)H degrees(298)(HCCHN.) approximate to 81 kcal mol(-1). Mass spectrometry showed that the precursor molecules CH3CN, CH3NC, and 1-H-1,2,3-triazole all produce the lowest energy cyclic ion upon dissociative ionization. The linear (CH2CN)-C-+ ion could only be made from ICH2CN, a situation similar to the isoelectronic C-3,H3+ isomers. The above thermochemistry was used to explore pi-delocalization in the linear cations and radicals. Only (CH2CN)-C-. exhibits delocalization of the radical center while delocalization of charge occurs in both linear cations.
Methyl cation transfer reactions between methanol and protonated methanol, protonated acetonitrile, and protonated acetaldehyde have been investigated experimentally by low-pressure FT-ICR mass spectrometry. The temperature dependencies of the rate constants for these reactions were determined in an Arrhenius-type analysis to obtain activation energies, enthalpies, and entropies of activation. The enthalpies of activation were determined to be -16.9 (0.6, -16.5 (0.6, and -18.4 (0.7 kJ mol -1 for the methanol/protonated methanol, methanol/protonated acetonitrile, and methanol/protonated acetaldehyde reactions, respectively. These values agree quite well with ab initio-calculated values. The entropies of activation were found to be quite similar for all three reactions within experimental uncertainty, which is expected due to the similar transition-state structures for all reactions. Ab initio potential energy surfaces calculated at the MP2/6-311G** level and basis set are reported for the three reactions. For the methanol/protonated acetonitrile and methanol/ protonated acetaldehyde reactions, isomerization of the initially produced proton-bound dimer to a methyl-bound complex is suggested prior to methyl cation transfer. The barrier for the first isomerization is predicted to be significantly lower than the barrier for methyl cation transfer such that it does not interfere with the experimental determination of the latter.
Given a positiver integer m and an ordered k-tuple c = (c1, ··· , ck) of not necessarily distinct positive integers, then any ordered k-tuple s = (s1, ··· , sk) of nonnegative integers such that m = ∑ki-1 sici is said to be a partition of m restricted to c. Let Pc(m) denote the number of distinct partitions of m restricted to c. The subroutine COUNT, when given a k-tuple c and an integer n, computes an array of the values of Pc(m) for m = 1 to n. Many combinatorial enumeration problems may be expressed in terms of the numbers Pc(m). We mention two below.
Scaling factors for obtaining fundamental vibrational frequencies, low-frequency vibrations, zero-point vibrational energies (ZPVE), and thermal contributions to enthalpy and entropy from harmonic frequencies determined at 19 levels of theory have been derived through a least-squares approach. Semiempirical methods (AM1 and PM3), conventional uncorrelated and correlated ab initio molecular orbital procedures [Hartree?Fock (HF), M?ller?Plesset (MP2), and quadratic configuration interaction including single and double substitutions (QCISD)], and several variants of density functional theory (DFT:? B-LYP, B-P86, B3-LYP, B3-P86, and B3-PW91) have been examined in conjunction with the 3-21G, 6-31G(d), 6-31+G(d), 6-31G(d,p), 6-311G(d,p), and 6-311G(df,p) basis sets. The scaling factors for the theoretical harmonic vibrational frequencies were determined by a comparison with the corresponding experimental fundamentals utilizing a total of 1066 individual vibrations. Scaling factors suitable for low-frequency vibrations were obtained from least-squares fits of inverse frequencies. ZPVE scaling factors were obtained from a comparison of the computed ZPVEs (derived from theoretically determined harmonic vibrational frequencies) with ZPVEs determined from experimental harmonic frequencies and anharmonicity corrections for a set of 39 molecules. Finally, scaling factors for theoretical frequencies that are applicable for the computation of thermal contributions to enthalpy and entropy have been derived. A complete set of recommended scale factors is presented. The most successful procedures overall are B3-PW91/6-31G(d), B3-LYP/6-31G(d), and HF/6-31G(d).