4. Conclusions
4.2. Ideas and plans for further work
The experimental work on this thesis was finished in the first quarter of year 2006.
Since then the author continues to work in the same laboratory carrying out research related to SIFT-MS but not any longer as a part of the topic of the doctoral thesis. The legacy SIFT instrument has worked hard well over 25 years and is now being dismantled. In September 2006 a new SIFT-MS instrument was commissioned in our laboratory, this is a new generation of instrumentation commercially manufactured (Profile 3, Instrument Science Limited, Crewe, England) as a transportable unit weighing 120 kg with external dimensions of 80 x 60 x 60 cm based on a flow tube 5 cm long. We have started several research projects using this new instrument including research of processes within the microwave plasma ion source and improvement of detection limits for quantitative real time breath analysis. From January 2007 we are starting new projects concerning the research of ion chemistry of Titan atmosphere and a project in collaboration with an explosives manufacturer Synthesia for detection of explosives and products of their combustion. We have initiated pilot projects in breath analysis of healthy volunteers, that already resulted in two papers already published, see the author’s C.V., and we are staring collaboration with microbiology and experimental medicine institutes. Other than this we intend to follow up the research of diffusion processes in the smaller flow tube.
81
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A1. Dryahina K., Polášek M., Španěl P. A selected ion flow tube, SIFT, study of the ion chemistry of H3O+, NO+ and O2+• ions with several nitroalkanes in the presence of water vapour.
International Journal of Mass Spectrometry 239 (2004) 57-65.
International Journal of Mass Spectrometry 239 (2004) 57–65
A selected ion flow tube, SIFT, study of the ion chemistry of H
3O
+, NO
+and O
2+•ions with several nitroalkanes in the presence of water vapour
Kseniya Dryahina, Miroslav Pol´aˇsek, Patrik ˇSpanˇel∗
J. Heyrovsk´y Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejˇskova 3, 18223 Prague 8, Czech Republic
Received 17 August 2004; accepted 22 September 2004 Available online 28 October 2004
Abstract
We have carried out a selected ion flow tube (SIFT) study of the reactions of H3O+, NO+and O2+•with the following six nitroalkanes:
nitromethane, nitroethane, 1-nitropropane, 2-nitropropane, 1-nitrobutane and 2-methyl-2-nitropropane. The primary purpose of this work was to extend the kinetics database to allow these compounds, M, to be analysed in air by selected ion flow tube mass spectrometry, SIFT–MS.
Some nitroalkanes are used as industrial solvents and some are component of agricultural agents that are known health hazards. The initial step in all the H3O+reactions is exothermic proton transfer to produce MH+ions, which are seen to be the only products for the two smallest nitroalkanes, but for the isomers of nitropropane and nitrobutane, fragmentation of the nascent MH+ions occurs. NO+reacts with the four smallest compounds via association resulting in NO+M product ions, whilst for the isomers of nitrobutane the C4H9+hydrocarbon ion is produced. The reaction of O2+•with nitromethane proceeds via charge transfer giving M+•as the major product, whilst the O2+• reactions with all the remaining nitroalkanes in this study lead to a single hydrocarbon ion product CnH2n+1+. The secondary chemistry of the ion products with H2O and with M, which is relevant to SIFT–MS applications, is fully described, with the interesting finding that water cluster ions of the kind MH+(H2O)3containing three water molecules are formed at 300 K. The mechanisms of the reactions are described with the aid of ab initio calculations of the ion energetics that were not previously available for some of the ions involved in the chemistry.
© 2004 Elsevier B.V. All rights reserved.
Keywords: SIFT; Nitropropane; Nitromethane; SIFT–MS
1. Introduction
Nitroalkanes are commonly used as solvents in indus-trial construction and maintenance, printing, highway main-tenance and food packaging. They have been widely used as chemical intermediates, industrial solvents, and as compo-nents of inks, paints, varnishes and other coatings[1]. Poten-tial occupational exposure to nitro compounds occurs during their manufacture and use. Nitromethane, along with other nitroalkanes, has been detected in cigarette smoke[2]. Ni-tromethane and 2-nitropropane are considered to be carcino-genic to humans and animals[3,4]. High concentrations of nitroalkanes (above 600 ppm) are acutely toxic and have
pro-∗Corresponding author. Tel.: +420 2 6605 2112; fax: +420 2 858 2307.
E-mail address: spanel@seznam.cz (P. ˇSpanˇel).
duced industrial fatalities[5]. Occupational exposure limits set by the US Department of Labour are 100 ppm for ni-tromethane and nitroethane and 25 ppm for both isomers of nitropropane[6]. Selected ion flow tube (SIFT)–MS[7]has the potential for accurate on-line real time measurement and monitoring of concentrations of these nitroalkanes in air.
SIFT–MS may also be feasible analytical method for bi-ological monitoring via breath analysis of the exposure of workers to the most widely used nitroalkane, 2-nitropropane.
Human uptake of nitroalkanes occurs mainly through the lungs. In experimental studies on animals (rats)[5]it has been shown that 2-nitropropane is not only rapidly absorbed via the lungs but also via the peritoneal cavity and the gastrointestinal tract; however, there is no satisfactory information on absorp-tion via the skin. 2-Nitropropane is not retained in the body for more than a few hours, since it is rapidly metabolised,
1387-3806/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.ijms.2004.09.007
58 K. Dryahina et al. / International Journal of Mass Spectrometry 239 (2004) 57–65
mainly to acetone, nitrite salts and possibly some isopropyl alcohol[8]. These metabolites and 2-nitropropane are then rapidly lost from the body by further metabolic transforma-tion, exhalation and excretion[5].
The data also may be of some relevance to atmospheric ion chemistry. In situ measurements of the ion composition of the atmosphere indicate that CH3NO2is also present as a component of complex hydrated H3O+based clusters[9].
In order to establish the library of kinetic data and to un-derstand the trends of reactivity of the common SIFT–MS precursor ions H3O+, NO+, O2+•, a series of studies of the reaction of these ions with different classes of molecules has been carried out; see the recent studies of the reactions of diols and the references there in[10]. The objective of the present study is to extend this library of data by providing information on the trends of reactivity of several nitroalka-nes with the SIFT–MS precursor ions. As the information on proton affinities (PAs) is not available for all the nitroalka-nes included, the study was complemented by their ab initio calculations. Details of the reaction mechanisms involving fragmentation were elucidated by information from parallel study using collisionally activated dissociation (CAD) in a sector mass spectrometer.
Protonated parent molecules that are produced in the pri-mary reactions of H3O+with small molecules can bind one or more water molecules under the typical conditions of the SIFT experiments, viz. helium carrier gas, temperature 300 K, pressure 100 Pa. Notable exception are protonated hy-drocarbons that do not associate efficiently with H2O under these SIFT conditions[11]. The maximum number of bound water molecules is characteristic of the different classes of compounds. Thus ketones, ethers and esters typically bind only a single water molecule [12], but aldehydes [13], al-cohols [14] and carboxylic acids [15] can bind two H2O molecules. The biggest number of added water molecules observed to date in SIFT experiments at 300 K is three for H3O+ [16] and CH3CNH+ [17]. As it will be shown later, the protonated nitroalkanes also are seen to add three water molecules.
2. Methods
2.1. Selected ion flow tube (SIFT)
The well-known SIFT technique for the determination of the rate coefficients and ion product distributions for the re-actions of H3O+, NO+ and O2+• ions with organic com-pounds has been described in numerous previous publications [18,19], so it is sufficient here to summarise it as follows.
Precursor ions (H3O+, NO+and O2+•) are generated in a mi-crowave discharge ion source, mass selected by a quadrupole mass filter and then injected as into helium carrier gas, flow-ing at a velocity of 170 m/s in the Prague SIFT. The reactant vapours of interest (in these experiments, nitromethane, ni-troethane, 1-nitropropane, 2-nitropropane, 1-nitrobutane and
2-methyl-2-nitropropane) are then introduced into the ion swarm/carrier gas where they react with the chosen precur-sor ion species. Measurement of the absolute flow rates of their neat vapours into the carrier gas of the SIFT instrument, required for determination of absolute rate coefficients[18], cannot be achieved to an accuracy better than 20% due to the surface activity of these compounds. In these circumstances, the established procedure is to measure relative rate coeffi-cients of the H3O+, NO+and O2+•ions with the compound.
Thus, the reactant nitroalkane is introduced as its saturated vapour in helium by introducing a small droplet of liquid ni-troalkane in a sealed plastic bag, which is then inflated with helium. This helium/nitro compound vapour mixture is then introduced into the helium carrier gas/precursor ion swarm of the SIFT via a flow meter in the usual way. The H3O+, NO+ and O2+•ions are injected simultaneously into the carrier gas by switching the injection mass filter to the total ion mode [20].
The relative concentrations of the primary and product ions of the reactions are determined from the count rates recorded by a downstream quadrupole mass spectrometer with a pulse counting single channel electron multiplier de-tector. This can be operated in the full scan mode (FSM) over a predetermined m/z range to obtain a spectrum of the reactant and product ions or in the multi-ion mode (MIM) in which the spectrometer is switched and dwells on selected reactant/product ions as their count rates are determined[20].
The FSM is primarily used to identify the product ions and the MIM is used to accurately determine the rate coefficients and the product ion distributions[20,21]. Primary product branching ratios are determined by extrapolation of the mea-sured ion count rate ratios towards zero reactant flow rate [18].
The relative rate coefficients for the reactions of the three precursor ions with the given molecule are then determined from the slopes of the plots of logarithm of ion count rates against the vapour mixture flow rate. It can be justifiably assumed that the proton transfer reactions of H3O+ with the nitroalkane compounds are exothermic and proceed at their collisional rates, because the proton affinities of the ni-troalkanes exceed the proton affinity of water molecules[22]
by >20 kJ/mol[23]. Collisional rate coefficients, kc, for the H3O+reactions have been calculated from the polarizabilities and the dipole moments of the reactant nitroalkanes using the parameterised trajectory formulation of Su and Chesnavich [24]. Values of the dipole moments and of the polorizabilities are from[25]with the single exception of 2-nitropropane, the polarizability of which is unknown, but we estimate it to be the same as the polarizability of its isomer 1-nitropropane.
The rate coefficients, k, for the NO+and O2+•reactions are then experimentally derived from the relative decay rates of the three ion species (see the previous successful applications of this procedure, for example, in[12,13]).
In SIFT–MS analyses of moist air samples, it is impor-tant to know if any of the product ions, R+, of the analytical reactions undergo association with water molecules[16]. To
K. Dryahina et al. / International Journal of Mass Spectrometry 239 (2004) 57–65 59
Fig. 1. Experimental SIFT data tracing the ion chemistry that occurs when H3O+ions are injected into the helium carrier gas into which a small steady flow of nitromethane and variable flows of water vapour are introduced. The count rates, c/s, for the precursor and product ions indicated are plotted on a semilogarithmic scale as a function of the number density of water molecules [H2O] in the carrier gas.
investigate this, controlled amounts of an air/water vapour mixture were introduced into the carrier gas whilst monitor-ing the R+ ions using the MIM mode (seeFig. 1). The rate coefficients for association reactions that occur can be esti-mated from the dependencies of the [R+·H2O]/[R+] count rate ratio on the H2O molecule number density in the carrier gas, which is calculated from the distribution of the H3O+ ions and its hydrates H3O+·(H2O)1,2,3. This procedure has been discussed previously in a paper on the influence of humidity on SIFT–MS analyses[16].
2.2. Collisionally activated dissociation (CAD), mass spectrometry
In order to elucidate mechanisms of the fragmentation observed in the O2+•reactions, additional information was used from collisionally activated dissociation mass spectra recorded on ZAB2-SEQ hybrid tandem mass spectrome-ter equipped with chemical ionisation (CI) ion source. The ions of interest were generated by self-chemical ionisation (self-CI) using the standard ion source conditions (0.5 mA, 100 eV, 150◦C) at the pressure of 6 × 10−3Pa measured at the ion source diffusion pump intake. The ions were ex-tracted from the ionisation chamber, accelerated to 8 keV, and products of their collisional activation in the first and second field-free regions were analysed using the B/E = const. linked scan and mass-analysed ion kinetic energy (MIKE) scan, respectively. Helium was used as a collision gas.
2.3. Quantum chemical calculations
Standard ab initio calculations of proton affinities of nitroalkanes were preformed using the Gaussian 98 suite of programs [26]. Geometries were optimised with den-sity functional calculations using Becke’s hybrid functional
(B3LYP) [27] and the 6-31+G(d,p) basis set. Stationary points were characterized by harmonic frequency lations as local minima (all frequencies real). The calcu-lated frequencies were scaled by the factor of 0.963 [28]
and used to obtain zero-point energy (ZPVE) corrections and enthalpies. For selected species, the geometries were also optimised at the MP2/6-31+G(d,p) level of theory. Im-proved energies were obtained by single-point calculations at two levels of theory. The first set of energies resulted from B3LYP/6-311+G(3df,2p) calculations. The second set of energies was calculated using the G2(MP2) method[29]
consisting of MP2/6-311+G(3df,2p), MP2/6-311G(d,p), and QCISD(T)/6-311G(d,p) calculations that were combined to provide effective QCISD(T)/6-311+G(3df,2p) energies cor-rected for ZPVE and the number of valence electrons. Ac-cording to definition, the proton affinity of molecule M is the negative enthalpy change of the hypothetical protonation re-action H++ M→MH+at 298 K. Therefore, the geometries of the lowest energy conformers of neutral nitroalkanes and their O-protonated ionic counterparts were found and used for accurate single-point energy calculation at two levels of theory (vide supra).
3. Results
3.1. Proton affinities of nitroalkanes
To the best of our knowledge, proton affinities are only available for nitromethane and nitroethane[22]. Since there is also very few theoretical values available[30,31], as a part of the present study we have calculated ab initio the pro-ton affinities of nitroethane, 1-nitropropane, 2-nitropropane, 1-nitrobutane and 2-methyl-2-nitropropane. The results are summarized inTable 1. For the three compounds where previ-ous theoretical or experimental values are available the agree-ment is within the estimated uncertainty of the theoretical method,±7 kJ/mol[29].
3.2. Rate coefficients for the H3O+, NO+and O2+• reactions
The calculated collisional rate coefficient, kc, for the re-actions of H3O+ with the nitroalkanes and the derived rate coefficients, k, for the corresponding NO+and O2+•reactions are listed inTable 2. As can be seen, the k values for the NO+ reactions increase with the number of carbon atoms in the nitroalkane molecules. The k values for the O2+•reactions are close to their respective kcvalues (within the 20% exper-imental error). The only significant exception is the reaction of O2+•with nitromethane, the k value being somewhat lower at 0.75 kc.
Previous measurements are available for the reaction of H3O+with nitromethane (k reported as 4.1±1 cm3s−1[32]
and 4.11±1 cm3s−1[33]), agreeing well with our calculated kc.
60 K. Dryahina et al. / International Journal of Mass Spectrometry 239 (2004) 57–65 Table 1
Calculated proton affinities (298 K) of nitroalkanes in kJ/mol
Molecule B3LYP/6-31+G(d,p) B3LYP/6-311+G(3df,2p) G2(MP2)a Previousc
Nitromethane 740 745 747b 754.6
Nitroethane 761 767 766 765.7
1-Nitropropane 769 774 772
2-Nitropropane 774 780 778 777.1
1-Nitrobutane 773 779 776
2-Methyl-2-nitropropane 788 793 790
a Uncertainty estimated as mean average deviation is±7 kJ/mol[29].
b Value taken from[31].
c The values of proton affinities for nitromethane and nitroethane are from[22,33]and that for 2-nitropropane is from[30].
Table 2
Rate coefficients for the reactions of H3O+, NO+and O2+•with the nitroalkanes indicated
Molecule M (Da) αa(10−24cm3) µ(D) H3O+[kc] (10−9cm3s−1) NO+kb[kc] (10−9cm3s−1) O2+•kb[kc] (10−9cm3s−1)
Nitromethane 61 7.37 3.46 [4.6] 0.4 [3.9] 2.9 [3.8]
Nitroethane 75 9.63 3.65 [4.8] 2.4 [4.0] 4.6 [3.9]
1-Nitropropane 89 8.5 3.66 [4.6] 2.5 [3.9] 3.6 [3.8]
2-Nitropropane 89 8.5±1 3.73 [4.7] 2.6 [3.9] 3.3 [3.8]
1-Nitrobutane 103 10.4 3.59 [4.6] 3.4 [3.9] 4.4 [3.8]
2-Methyl-2-nitropropane 103 10.3 3.71 [4.7] 4.2 [3.9] 4.2 [3.8]
Also given are their molecular weights, M, in Daltons, Da, their polarisabilities,α, in units of 10−24cm3and their permanent dipole moments,µin Debye, D.
The collisional rate coefficients, kc, calculated using the parameterised trajectory formulation of Su and Chesnavich[24]are given in square brackets.
a The knownαandµvalues[25]are shown in regular type. The estimatedαvalue is shown in italic.
b The rate coefficients, k, for the NO+and O2+•reactions are experimentally derived by the procedure described in Section2. The absolute and relative uncertainties in these calculated rate coefficients are±30% and±15%, respectively.
The products of the reactions and their percentage (in parenthesis) are given in Table 3together with the known ionisation energies[34]of the nitroalkanes. The trends in re-activity will be discussed briefly in the following subsections.
3.3. The H3O+reactions
These reactions proceed initially via the formation of a nascent protonated parent molecule, MH+, and this is the only observed product ion for the nitromethane and nitroethane re-actions. Partial fragmentation of the nascent MH+ions occurs for both isomers of nitropropane leading to the formation of the C3H7O+ion; similarly, both isomers of nitrobutane lead to C4H9+ions (seeTable 3).
There is an obvious difference in the products of the re-action of 1-nitropropane and 2-nitropropane. The rere-action of
Table 3
Primary product ions and their percentage (in parenthesis) for the reactions of H3O+, NO+and O2+•with the nitroalkanes indicated
Molecule IE (eV)a Productions
H3O+ NO+ O2+•
Nitromethane CH3NO2 11.08 CH3NO2·H+(100) CH3NO2·NO+(100) CH3NO2+(90); NO+(10) Nitroethane C2H5NO2 10.90 C2H5NO2·H+(100) C2H5NO2·NO+(100) C2H5+(100)
1-Nitropropane C3H7NO2 10.78 C3H7NO2·H+(95); C3H7O+(5) C3H7NO2·NO+(100) C3H7+(100) 2-Nitropropane (CH3)2CHNO2 10.74 C3H7NO2·H+(40); C3H7O+(60) C3H7NO2·NO+(100) C3H7+(100) 1-Nitrobutane n-C4H9NO2 10.71 C4H9NO2·H+(90); C4H9+(10) C4H9NO2·NO+(85); C4H9+(15) C4H9+(100) 2-Methyl-2-nitropropane (CH3)3CNO2 C4H9NO2·H+(5); C4H9+(95) C4H9+(100) C4H9+(100)
a The values of the ionisation energy are from[47].
1-nitrpropane proceeds thus:
H3O+ +C3H7NO2→C3H7NO2·H+ + H2O; 95% (1a) H3O+ +C3H7NO2
→C2H5CHOH+ + (HNO +H2O); 5% (1b) Here the major product is the protonated parent molecule.
Both reactions (1a) and (1b) are exothermic. The reac-tion exothermicities of reacreac-tions (1a) and (1b) calculated at the G2(MP2) level of theory are −rH298= 84 and 46 kJ/mol respectively. The latter exothermicity is in rela-tively good agreement with value of 60 kJ/mol calculated for the C2H5CHOH+product structure using data from[35](see Scheme 1). Note that the identity of the neutral products can-not be directly determined using SIFT, thus we always list the
K. Dryahina et al. / International Journal of Mass Spectrometry 239 (2004) 57–65 61
Scheme 1. The nitro-to-nitrite group isomerization by an alkyl cation mi-gration for (a) protonated 1-nitropropane C3H7NO2·H+; (b) protonated 2-nitropropane (CH3)2CHNO2·H+.
most exothermic neutral products and indicate the possibility of different products by parentheses.
In contrast to this, in the reaction of 2-nitropropane the closed shell C3H7O+fragment is the major product.
H3O+ + (CH3)2CHNO2
→(CH3)2CHNO2·H+ +H2O; 40% (2a)
H3O+ + (CH3)2CHNO2
→(CH3)2COH+ +(HNO + H2O); 60% (2b) The exothermicities of (reaction (2a) and (2b) were calcu-lated at the G2 level of theory to be 90 and 93 kJ/mol, respec-tively. It should be noted that structure of oxygen-protonated propanal, C2H5CHOH+, was considered as the product in reaction (2b) whereas protonated acetone, (CH3)2COH+, is considered as the product in reaction (2b).
From the mechanistic viewpoint, reactions (1b) and (2b) require some rearrangement of the reactants which allows C O bonds to form. The nitro-to-nitrite group isomeriza-tion (Scheme 1) by an alkyl caisomeriza-tion migraisomeriza-tion may serve as a plausible explanation. However, it cannot be excluded that such rearrangement takes place in a reaction complex with H2O, since methyl cation transfer has already been described in CH3NO2+•[36], CH3CO+[37]and CH3NN+[38] com-plexes with various atoms and small molecules. Especially in the case of larger systems, after the proton transfer the H2O leaving the reaction complex could be trapped for a time suf-ficiently long to be involved in further reaction, a step-wise mechanism, in which the proton transfer (1a) and (2a) takes place as the first step. After the H2O leaves the reaction com-plex, the vibrationally excited C3H7NO2·H+rearranges and dissociates unimolecularly to the products (Scheme 1) as it will be discussed in the following section.
It has been found that a methyl group migrating along the NO2H moiety in protonated nitromethane must proceed over large energy barriers, and so virtually no isomerization was observed during collisional activation[31]. However,
migra-tion of larger alkyl camigra-tions should be easier due to their higher stability, as was actually observed in protonated nitroethane [39]. Stability of the migrating alkyl cations can be espe-cially important when comparing the reactivity of 1- and 2-nitropropane since the higher stability of a migrating 2-propyl cation can result in higher stabilization of the corresponding transition state. So in the case of 1-nitropropane, the frag-mentation shown inScheme 1a requires migration of a less stable 1-propyl cation over a correspondingly higher energy barrier making the dissociation of C3H7NO2·H+to C3H7O+ and HNO much slower. Therefore, protonated 1-nitropropane substantially prevails among the products, whereas in the case of 2-nitropropane, isomerization via the stable 2-propyl cation migration becomes faster, which increases the fraction of C3H7O+and HNO products.
The final step of the proposed reaction mechanism (Scheme 1) involves hydrogen migration from the propyl to the NO group in protonated propyl nitrite. Similar to methyl nitrite[31], protonation of an ether oxygen atom in propyl nitrites leads to the most stable tautomer. Our cal-culations showed, that C3H7O(H)NO+ ions are in fact a complexes of propanols with NO+, as it is indicated by the O N bond lengths which are 1.98 and 1.95 ˚A in 1-propyl nitrite and 2-propyl nitrite, respectively. The migration of the most labile ␣-hydrogen to NO+ is considered to take place producing appropriately C3H7O+ ion and a neutral HNO. This, which from the formal point of view is hy-dride anion abstraction, was found to be the only reaction channel in the 1- and 2-propanol reactions with NO+ [14].
Since the reaction of propanol with NO+is fast (it proceeds at the collisional rate [14]), the nitro-to-nitrite group rear-rangement is most probably the rate-determining step (in the fragmentation shown inScheme 1) that controls the product distribution.
From the G2(MP2) enthalpies of reactions (1a), (1b), (2a) and (2b) it follows that the dissociation of C3H7NO2H+ to CH3CH2CHOH+ and HNO is 38 kJ/mol endothermic and dissociation of (CH3)2CHNO2·H+to (CH3)2COH+and HNO is 3 kJ/mol exothermic. Although the calculation of the isomerization barriers along the reaction path is beyond the scope of this work, from the nature of the SIFT experiment it can be concluded that none of these barriers can exceed the protonation exothermicity reduced by the internal and kinetic energy carried away by the H2O. There are similar differences between the branching ratios of 1-nitrobutane and 2-methyl-2-nitropropane reactions. For 1-nitrobutane the major product is the protonated parent molecule and C4H9+is only a minor product:
H3O+ +C4H9NO2→C4H9NO2·H+ + H2O; 90% (3a) H3O+ +C4H9NO2→C4H9+ + (HONO +H2O); 10%
(3b) For the 2-methyl-2-nitropropane reaction the protonated parent molecule is the minor product and C4H9+is the major