制藥工程外文資料翻譯--ch3+與一系列同環(huán)和異環(huán)分子的氣相反應(yīng)（英文）

1、Gas Phase Reactions of CH3+ with a Series of Homo- and Heterocyclic MoleculesL. Dalila Fondren, Nigel G. Adams,* and Leah StavishDepartment of Chemistry, UniVersity of Georgia, Athens, Georgia 30602ReceiVed: October 15,

2、2008; ReVised Manuscript ReceiVed: NoVember 10, 2008In gas phase ion chemistry, the growth of larger molecules is known to occur through association of ions and neutrals. Where the ion attaches to the neutral is importan

3、t because it can influence the possibility of additional associations, effectively enabling or terminating further molecular growth. This was investigated by using a Selected Ion Flow Tube (SIFT) at 300 K to study the re

4、actions of CH3+ with the following series of single- ring homocyclic and heterocyclic molecules: benzene (C6H6), cyclohexane (C6H12), pyridine (C5H5N), pyrimidine (C4H4N2), piperidine (C5H11N), 1,4-dioxane (C4H8O2), fura

5、n (C4H4O), pyrrole (C4H5N), and pyrrolidine (C4H9N). Most of the reactions, except for 1,4-dioxane, pyrrole, and pyrrolidine, proceed at the gas kinetic rate. In the ion product distributions, charge transfer, hydride io

6、n abstraction, proton transfer, fragmentation, and association were observed. In particular, proton transfer is seen to be small in all cases even though these channels are energetically favorable. Association is appreci

7、able when the molecules are aromatic (except for furan) and nonexistent when there are no π electrons in the ring. CH3+ ions are an important intermediate in molecular synthesis in interstellar clouds and in the Titan io

8、nosphere and ring molecules have also been detected in these media. The significance of the studied reactions to these media is discussed.IntroductionIn gas phase ion chemistry, the growth of larger molecules occurs main

9、ly through association of the ions and neutrals. Where the ion attaches to the neutral is important because it can affect the possibility of additional associations, effectively enabling or terminating further molecule g

10、rowth. Association reactions are important in the interstellar medium (radiative association1) and the ionosphere of Titan (collisional associa- tion2) where ion-neutral associations are thought to lead to the formation

11、of benzene and PAHs.3-5There is a lack of experimental data exploring association reactions of larger hydrocarbons and because of this, the possible importance of association in models of Titan’s atmosphere has been unde

12、r exploration. In this work, a series of reactions of the methyl cation, CH3+, with homo- and heterocyclic neutral molecules is investigated. The structures of the neutral molecules are shown in Figure 1. The association

13、 product channel is explored as a function of the presence of π electrons and the type and/or amount of heterocyclic substitution. CH3+ is an important ion in Titan’s ionosphere and the interstellar medium and reactions

14、involving this ion figure significantly in ensuring the thorough modeling of the chemistry occurring in these regions. Association of CH3+ to any of these molecules would be of interest as a mechanism for creating more m

15、assive molecules.Experimental SectionA selected ion flow tube (SIFT) was used to study this series of ion-neutral reactions between CH3+ and benzene (C6H6), cyclohexane (C6H12), pyridine (C5H5N), pyrimidine (C4H4N2), pip

16、eridine (C5H11N), 1,4-dioxane (C4H8O2), furan (C4H4O), pyrrole (C4H5N), and pyrrolidine (C4H9N). The SIFT method has been described extensively in the literature6-8 and this willnot be repeated here. Specific to this exp

17、eriment, the CH3+ was formed by injecting methane into a low-pressure ionization source. After mass selection in a quadrupole mass filter, the ions were injected into the flow tube at an ion energy of ～20 eV to minimize

18、fragmentation of the CH3+. Although a low ion injection energy was used it was not possible to completely eliminate further fragmentation of CH3+ and because of that the following ions were present in the flow tube at th

19、e average indicated percentage when compared to CH3+: C+ (3%), CH+(5%), CH2+ (7%), and CH4+ (6%). Pyridine, pyrrole, pyrrolidine, furan, 1,4-dioxane, benzene, and cyclohexane were obtained from Sigma-Aldrich with puritie

20、s of >99.9%, 98%, 99.5+%, 99+%, 99.5%, 99.5%, and 99%, respectively. Pyrimidine and piperidine were obtained from Alfa Aesar with manufactured purities of 99%. Benzene was obtained from Fisher Scientific with a purity

21、 of 99.5%. To eliminate dissolved gases, the liquids were further purified before use by several cycles of freeze- pump-thaw. The neat vapors proved difficult to work with due to condensation of the vapors in the neutral

22、 reactant system and on the flow tube walls, so a 1% mixture of the reactant neutral in helium was used. This dilution was accounted for when determining the rate coefficients. Ion product distributions and rate coeffici

23、ents were determined in the usual way.6,7,9 The ion product distributions are accurate to (5 in the percentage and the rate coefficients are accurate to (30% due to the sticky nature of these gases. All reactions were st

24、udied at 298 K. Mass discrimination in the detection quadrupole was corrected as before.8ResultsThe rate coefficients for all reactions studied are given in Table 1. While it can be seen that most of the reactions occur

25、within experimental error of the gas kinetic rate there is a tendency for the experimental rates to be less than gas kinetic. This feature has not been seen in reactions of other ions involving the same neutrals. The 1,4

26、-dioxane and pyrrolidine * Corresponding author. E-mail: adams@chem.uga.edu.J. Phys. Chem. A 2009, 113, 592–598 59210.1021/jp8091336 CCC: $40.75 ? 2009 American Chemical Society Published on Web 12/17/2008While the catio

27、n-π interaction is responsible for the increased association with aromatic molecules it is not an accurate way to describe the structure of the associated complex when dealing with CH3+ as the cation. Several theoretical

28、 calculations involving CH3+ with various aromatic molecules have been undertaken21-24 and show that CH3+ does not have a long-lived π interaction with benzene; instead the σ interaction is more energetically favorable.

29、A molecular dynamics study determined that the reactants were brought together by cation-π electron attraction; however, at a certain intermolecular separation, the CH3+, regardless of the initial approach, quickly forms

30、 a σ complex with a carbon in the ring.23 This fast-forming complex rapidly isomerizes to give the final stable σ complex where the hydrogen attached to the same carbon as the newly attached methyl cation shifts to the p

31、ara position. Miklis et al. explained the difference in reactivity of the CH3+ ion to ions that form π complexes (such as metal ions and the NH4+ ammonium ion) by pointing out that the unsaturated nature of the ion can p

32、romote different reactions.22 It is likely that the interaction of an unsaturated species is not as strongly dependent on electro- static forces as saturated species. Therefore, describing such interactions as cation-π i

33、s misleading. Hence, the term cation-π interaction will not be invoked and all associations of CH3+ with the aromatic neutrals studied in this paper are believed to result in a σ interaction. Fragmentation product channe

34、ls were also of great interest in this study. In an attempt to understand how each fragment was formed, it was necessary to study known fragmentation procedures in the literature. Rules governing unimolecular dissociatio

35、n, used most widely in interpreting electron impact (EI), are well-known;25 however, there is less information available on how the internal energy imparted during an ion-molecule reaction will be distributed and which b

36、onds will break. One area of the literature where this is considered is chemical ionization mass spectrometry (CI). Fundamentally, CI occurs through ion-molecule reactions and though the reason CI is used differs from th

37、e reason ion-molecule reactions are investigated in this study, the basic principles are the same for both. If the reactions in this study are considered in terms of chemical ionization then information about fragmentati

38、on mech- anisms becomes available.11In these studies there are several product channels that do not involve fragmentation but might be the starting points forfragmentation. These are hydride abstraction, charge transfer,

39、 proton transfer, and association. By investigating the fragments formed, it is possible to identify the starting point as one of the four listed above. It is known in CI that if there is internal energy left over after

40、a charge transfer reaction, then fragmentation reactions will resemble those observed in EI.11 Some differences between the spectrums of CI versus EI relating to the relative abundances of the fragment ions formed are li

41、kely to occur. This arises because, in EI, a wide range of internal energy is imparted to the molecule upon ionization while in CI the energy imparted is restricted by the difference in the recombination energy of the pr

42、imary ion and the ionization energy of the neutral. This allows for different fragmentation mechanisms to be favored in the two situations. Therefore, a major product ion in one process may only be a minor product ion in

43、 the other process. If the starting point for fragmentation is proton transfer then it is possible that the internal energy will be localized around the point of proton attachment; however, it is also possible for the in

44、ternal energy to randomize throughout the molecule and break the weakest bond. It is likely that the fragmentation product of the proton transfer will be the result of the elimination of an even-electron stable molecule

45、resulting in an even-electron product ion.11 The fragmentation that occurs if the starting point is hydride abstraction or association is not well-known. In the case of hydride abstraction while it seems logical that the

46、 atom at which the H- is lost will be the starting point for fragmentation, it is also possible for the internal energy to break the weakest bond just as with proton transfer.The EI spectrum of each neutral, obtained fro

47、m the NIST Web site, was compared to the fragments formed in each reaction. Fragmentation peaks seen in both spectra indicate that the fragment formed in our reactions could be initiated via charge transfer and if the pe

48、aks were not seen in both spectra then the fragment was not formed via charge transfer. In some cases, the fragments formed in our reactions, but not seen in the EI spectrum, only differ by an H-atom. It is possible that

49、 these fragments are created by unimolecular dissociation mech- anisms, but with a starting point that has one extra hydrogen such as proton transfer, or a starting point with one less hydrogen such as hydride abstractio

50、n. In discussing each neutral in detail, an attempt has been made to determine the starting point of each fragmentation channel and how it was formed.Figure 2. Decay of CH3+ and the rise of the product ions for the CH3+