Dissociation of Energy-Selected 1, 1-Dimethylhydrazine Ions

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Dissociation of Energy-Selected 1,1-Dimethylhydrazine Ions Zsolt Gengeliczki,† Sampada N. Borkar,‡ and Ba´lint Szta´ray*,‡ Department of Chemistry, Stanford UniVersity, Stanford, California 94305, and Department of Chemistry, UniVersity of the Pacific, Stockton, California 95211 ReceiVed: February 26, 2010; ReVised Manuscript ReceiVed: March 29, 2010

The unimolecular dissociation of 1,1-dimethylhydrazine ions was studied by threshold photoelectron photoion coincidence spectroscopy (TPEPICO). Time-of-flight distributions and breakdown curves were recorded in the photon energy range of 9.5-10.4 eV. The 0 K appearance energies of the fragment ions were extracted by modeling the experimental data with rigid activated complex (RAC-) RRKM theory. It was found that the data could be well-reproduced with a single TS for each dissociation channel if two different H-loss channels were assumed, one corresponding to a C-H and the other to a N-H bond dissociation. Once the appearance energies were established, heats of formation of the fragment ions could be derived. The heat of formation of the neutral molecule was computed by applying composite ab initio methods (G3, CBS-APNO, W1U) on a series of isodesmic reactions between methyl hydrazines and methyl amines. Introduction Hydrazine and its methyl derivatives have industrial use of paramount importance. They are blowing agents in preparing polymer foams, precursors to catalysts and pharmaceuticals, and are used in large quantities as rocket fuels.1,2 1,1-dimethylhydrazine (1,1-DMH) is a toxic, volatile, hygroscopic liquid that readily absorbs oxygen and carbon dioxide and mixes with water, ethanol, and kerosene. When combined with dinitrogen tetraoxide (N2O4), methyl hydrazines are used as an efficient rocket fuel.3,4 In order to better understand and accurately model the reactions of these fuels, the knowledge of precise gas-phase thermochemical data (heats of formation of the neutral and ionic species) is necessary. Threshold photoelectron photoion coincidence spectroscopy (TPEPICO) is a useful tool to determine ionic bond dissociation energies and, indirectly, the gas-phase heats of formation of neutral and ionic species. It has been shown in numerous studies that in the case of a small kinetic shift, RRKM theory can be used to extract 0 K appearance energies from experimental TPEPICO data.5,6 This technique has been applied to various inorganic7–10 and organic11–13 compounds. An important element of these studies is that only one transition state (TS) is applied over a range of several eVs of ion internal energies in the RRKM calculations. In their recent study, Boulanger et al. showed that the dissociation of 1,1-DMH ions can be modeled with reasonable accuracy only by using variational transition-state theorem (VTST),14 that is, assigning multiple TSs as a function of the ion internal energy. The VTST approach is generally superior to RRKM, but in many cases, it is not necessary and just introduces more fitting parameters. In that work, TPEPICO experiments were carried out on 1,1-DMH, and two mass losses were observed upon photoionization, a loss of 15 amu and of 1 amu. The former was assigned to a methyl loss, and the latter was assigned to hydrogen loss from one of the methyl groups by breaking the C-H bond. The possibility of the hydrogen * To whom correspondence should be addressed. E-mail: bsztaray@ pacific.edu. † Stanford University. ‡ University of the Pacific.

loss from one of the amino groups was ruled out on the basis of its presumably higher activation barrier and the lack of deuterium loss from deuterated 1,1-DMH ions in the TCID experiments. Since it is somewhat surprising for the RRKM theory to perform so poorly in reproducing the unimolecular dissociation rates of 1,1-DMH ions and because the knowledge of precise gas-phase thermochemical data of this system is of considerable importance, a new set of TPEPICO experiments were carried out on 1,1-DMH on an instrument with a limited wavelength range but with (a) a better control of the ion internal energies with the new cooled inlet system and (b) a direct way to measure the unimolecular dissociation rates. The experimental data are modeled assuming a competing N-H dissociation channel to demonstrate that the RRKM theory with one TS for each dissociation channel can be used to properly model the experiments. To establish the heats of formation of the various fragment ions, the gas-phase heat of formation of the neutral precursor is also needed. Donovan et al. reported the gas-phase heat of formation of 1,1-DMH to be 83.3 ( 3.6 kJ/mol in 1960.15 Since then, several computational studies have been published, and the reported heats of formation vary from 77 to 94 kJ/mol.16–19 Boulanger et al. based their thermochemical data on their G3 value of 94 kJ/mol.16 Isodesmic reactions proved to be powerful tools in determining neutral heats of formation.20–22 In a combined TPEPICO and quantum chemical study, Bodi et al. established the heats of formation of alkyl amines with an uncertainty of (2.0 kJ/mol11,12 by building an isodesmic reaction network. The reaction enthalpies were calculated with the G3, CBS-APNO, and W1U composite methods, and well-known heats of formation of certain species in the network were used as anchors of the thermochemical network. In the present study, isodesmic reactions between the neutral gas-phase methyl hydrazines and methyl amines are constructed to establish a precise and accurate value of the gas-phase heat of formation of 1,1-DMH. Appearance energies and thermochemical data of the fragment ions have been reported by other authors.23–26 These studies include photoionization and electron ionization studies. The

10.1021/jp1017604  2010 American Chemical Society Published on Web 05/05/2010

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reported values, however, differ by over 1 eV because of the neglect of kinetic and thermal shifts. The adiabatic ionization energy of 1,1-DMH was measured to be 7.29 ( 0.04 eV by Meot-Ner et al. in high-pressure mass spectrometry experiments.27 We used this value in the data analysis, in agreement with the NIST compilation by Lias et al,28 because photoionization techniques tend to overestimate the adiabatic ionization energy of amines due to the large change in geometry upon ionization.16,24,28–30 Experimental and Theoretical Approach TPEPICO Experiments. 1,1-DMH was purchased from Sigma-Aldrich and used without further purification. The sample was introduced into the ionization chamber through a hypodermic needle. The TPEPICO apparatus has been described in detail elsewhere,5,13,31 and only a brief overview is given here. In the ionization region, the sample is ionized by the incident light from a hydrogen discharge lamp dispersed by a 1 m normal incidence vacuum UV monochromator. The photon energy resolution is 8 meV at a photon energy of 10 eV. The energies are calibrated against the hydrogen Lyman-R ´ The electrons are extracted from resonance line at 1215.67 Å. the center of a 12 mm wide ionization region by a 20 Vcm-1 field. This is followed by a second acceleration region with a terminal electron energy of 74 eV and a 13 cm long field-free drift region. The applied voltages are designed to velocity focus threshold electrons onto a 1.4 mm aperture at the end of the electron drift region, where a Channeltron detects them. Hot electrons are focused into concentric rings with diameters determined by their initial velocity perpendicular to the extraction axis. A fraction of these hot electrons is collected by a second Channeltron located next to the central Channeltron. The effect of the hot electrons, that is, energetic electrons with a zero perpendicular velocity component, can now be eliminated by subtracting a fraction of the pure hot electron signal from the central signal. The energy resolution of the threshold electron detection is at least 5 meV, better than the resolution of the monochromator. The reflectron time-of-flight (ReTOF) system consists of single acceleration and deceleration fields, in which the ions are accelerated to 104 eV in the first 5 cm long acceleration region and travel 40 cm in the first drift region. The ions are then reflected and travel through another 35 cm second drift region before being collected by a tandem multichannel plate ion detector. Ions that dissociate in the first drift region do not penetrate as deeply into the reflectron as parent ions and are thus separated from the parent ions. The drift peak appears as a sharp, symmetric peak just after the corresponding metastable daughter ion peak. In the linear time-of-flight (LinTOF) setup, ions are accelerated to 104 eV in the first 5 cm long acceleration region and to 260 eV in a short second acceleration region, after which they travel approximately 26 cm in the first drift region. The ions are then decelerated and travel through a 7.5 cm second drift region before being collected by a tandem multichannel plate ion detector. The deceleration serves to separate ions which have dissociated in the first drift region from ions which do not dissociate. The drift peak appears as a broad peak at a higher TOF than the parent ion. The TPEPICO study of 1,1-DMH was carried out using both the LinTOF and the ReTOF experimental setups. The main advantage of the ReTOF mass spectrometer is its mass resolution, which is important in the study of H-losses. The bond

Gengeliczki et al. dissociation energies derived from the experiments are identical, as discussed later. In a third set of experiments, to exercise better control on the internal energy distribution of the photoions and, due to sharper peaks in the TOF spectra, to allow a partial deconvolution of the [M-H]+ peak from the precursor ion signal, a chilled effusive sample inlet system was also utilized. In this setup, the sample vapors travel approximately 10 cm in a temperature-controlled copper block before they enter a temperature-controlled ionization chamber. These experiments were carried out at a -40 °C sample temperature with the linear TOF setup. TPEPICO Data Analysis. The experimental data are modeled in terms of the statistical theory of unimolecular decay developed by Rosenstock et al.32 as the quasi-equilibrium theory (QET) and by Rice, Ramsperger, Kassel, and Marcus, known as the RRKM theory.33–35 The RRKM rate constant at an internal energy (E) is given by

σNq(E - E0) k(E) ) pF(E)

(1)

where F(E) is the density of states of the molecule, Nq(E - E0) is the number of states of the TS at excess energy E - E0 above the barrier E0, σ is the symmetry number, and p is Planck’s constant. Because of the width of the ions’ energy distribution, the rate constants associated with dissociation at a given photon energy also have a significant distribution that has to be taken into account.5,8,36 The ion internal energy distribution is derived from the thermal energy distribution of the neutral molecule and ionization and photon energies, with the approximation that the threshold photoionization cross section is constant over the thermal energy range. This approximation was demonstrated to be valid in our recent temperature-controlled TPEPICO experiments.13 Densities and sums of vibrational states are calculated by the direct count method,36,37 which assumes harmonic oscillators. In some cases, the large-amplitude internal rotations of the methyl group have to be treated separately. Whether the mode is a vibration, a hindered rotation, or a free rotation depends on the barrier, which differs for the neutral molecule, ion, TS, and final products. Therefore, one of the difficult aspects in modeling reaction dynamics and determining thermal energy distributions is the role of internal rotations. For methyl amines, Bodi et al. found12 that for the correct modeling of the internal energy distributions, the internal rotations of the methyl group needed to be treated separately as hindered rotors in monomethylamine, but in the larger trimethyl amine, they had to be treated as vibrations. In the present study, we found that when every mode in the molecule was treated as a vibration, the modeled internal energy was in agreement with the experiments. Also, even more importantly, it was carefully checked that the extracted appearance energies only changed well within the experimental accuracy when certain modes were treated as rotors or vibrators. For the models involving internal rotation, the barriers to internal methyl rotations were obtained at the MP2/6-311G** level by fixing the optimized bond lengths, scanning the appropriate dihedral angle, and carrying out a constrained optimization at each value of the dihedral angle. Quantum Chemical Calculations. DFT and ab initio quantum chemical calculations were carried out to obtain the

Dissociation of Energy-Selected 1,1-Dimethylhydrazine Ions

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initial parameters of the data analysis (see above) and to estimate the heat of formation of 1,1-DMH. Equilibrium geometries of the neutral molecule and the dissociating ion were computed at the MP2/6-311G(d,p) level and were justified by the absence of imaginary vibrational frequencies. These geometries were used to calculate the thermal energy distribution of the neutral molecule and the dissociating ion in the data analysis. In order to estimate the TS vibrational frequencies for each dissociating channel, relaxed potential energy curves were calculated at the B3LYP, MP2, CCSD, and CCSD(T) levels applying the 6-311G(d,p) and cc-pVTZ basis sets. At the coupled cluster levels of calculations, the geometries were taken from the MP2 calculations, and only single-point energy calculations were carried out at the coupled cluster levels. Heats of formation of the fragment ions can be derived from the heat of formation of the neutral molecule and the appearance energies extracted from the experiments. Several different values of the gas-phase heat of formation of 1,1-DMH were reported in the literature. In the present work, reaction enthalpies of isodesmic reactions between methyl hydrazines and methyl amines were calculated. This approach has proved to provide accurate heats of formation of organic molecules.11,12,20–22 The following set of isodesmic reactions was used

N2H4-n(CH3)n + NH3-m(CH3)m f N2H4-n+1(CH3)n-1 + NH3-m-1(CH3)m+1 (2) where n ) 0-4, m ) 0-3, and l ) 1-3. Reaction enthalpies for these 32 reactions were computed with the G3, CBS-APNO, and W1U composite methods. The W1U method was applied only to molecules containing less than five heavy atoms. The heats of formation of N2H4,38,39 NH3,19,39,40 and NH3-m(CH3)m11,12 species were taken from the literature and are listed in Table 2. Initial 298 K values for the heats of formation of the methyl hydrazines were taken from the literature (see Table 2) and were converted to 0 K values by using W1U and CBSAPNO H298K - H0K values. The 0 K heats of formation of the methyl hydrazines were then adjusted so that the smallest difference between the calculated and the derived reaction enthalpies was obtained according to the following error function

εi )

[∆rH(calc) - ∆rH(from exp. ∆fH)]2 σ

∑ δj

(3)

j

∆rH(calc) denotes the best ab initio reaction heat (W1U, where all species contain no more than four heavy atoms, and CBSAPNO otherwise). The σ parameter was chosen to reflect the expected accuracy of the calculations and was chosen to be 2 for W1U and 3 for CBS-APNO. The numerator is the sum of the reported uncertainties of the supplementary species in the reaction. Results and Discussion Experimental Data. Time-of-flight spectra of energy-selected 1,1-DMH ions were collected in the photon energy range of 9.50-10.30 eV with both the linear TOF (LinTOF) and the reflectron (ReTOF) setup. To have sufficient mass resolution to integrate the H-loss peak areas, the use of the ReTOF setup was unavoidable. However, the loss of daughter ions in a slow dissociation is always a concern in a low-extraction voltage

Figure 1. Sample time-of-flight distributions obtained at room temperature (a) in the LinTOF and (b) in the reflectron TOF experiments. The solid line represents the calculated time-of-flight distributions, while the dots are the experimental points.

ReTOF setup, and that is why appearance energies were extracted from both experiments. The extracted appearance energies are in an excellent agreement within the experimental uncertainties. Typical TOF spectra and the breakdown curves (i.e., relative ion abundances) are shown in Figures 1-4. The experimental data are plotted as dots, while the solid lines show the modeled breakdown curves and TOF distributions. TOFs in the linear setup are shorter, and the mass resolution is significantly worse. However, the presence of the molecular ion (m/z ) 60; 18.1 µs in LinTOF, 83.4 µs in ReTOF), the hydrogen

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Figure 2. Breakdown curves modeled with only two dissociation channels. Satisfactory match between the experimental and calculated breakdown curves could not be obtained over the entire photon energy range. In the upper, a looser transition state was assumed, while in the lower, tighter transition states were assumed.

Figure 3. Breakdown curves modeled with three dissociation channels. In the linear TOF experiments (upper figure), mass resolution was not enough to deconvolute the signals of the [M-H]+ and M+ ions. In the ReTOF experiments (lower figure), the signal of the [M-H]+ is deconvoluted. In the inset, the shift of the center of mass of the hydrogen loss peak is shown.

loss (m/z ) 59; 17.9 µs, 82.7 µs), and the methyl loss (m/z ) 45; 15.6 µs, 72.2 µs) are evident with both setups. In the ReTOF spectra, the additional peak (drift peak) at around 74 µs is due to the daughter ions formed between the ion acceleration region and the reflectron. In the LinTOF spectra, the drift peak at around 18.6 µs corresponds to ions losing the methyl group while flying in the first drift region, before deceleration into the second drift region. The 15 amu loss and the 1 amu loss can be modeled as parallel steps; therefore, the observed reactions are as follows

The relative ion abundances extracted from the TOF distributions are plotted as breakdown curves in Figure 3. No daughter ions were observed below the photon energy of 9.55 eV, and the parent ion signal disappeared above 9.95 eV. The low-temperature TPEPICO data on the title compound offer two advantages over the room-temperature measurements; (a) at lower temperature, the low-energy part of the breakdown curves is significantly different, and this provides a redundant set of experimental data to check the validity of the RRKM model; and (b) at lower temperature, the TOF peaks are somewhat sharper, which could allow a deconvolution of the H-loss peak from the fragment ion signal. Figure 4 shows the low-temperature experimental breakdown curves along with the results of the RRKM modeling. Below 9.95 eV, the ratio of the H-loss peak to the precursor ion signal was too low to allow an accurate deconvolution; therefore, only the sum of the M+ and the [M-H]+ ion intensities are plotted. Compared to the room-temperature LinTOF curves at low energies, the data points are shifted to higher photon energies (smaller thermal shift), while at the highest photon energies, the data are almost identical. TPEPICO Data Analysis. In the present study, three possible dissociation channels were assumed

(CH3)2NNH2 + hV f (CH3)NNH+ 2 + CH3 + e

(4) f C2H7N+ 2 + H + e

(5)

As shown in Figure 1, the daughter ion TOF distributions that are due to the methyl loss are asymmetric at low photon energies and gradually become symmetric with increasing photon energy. This indicates that the low-energy ions dissociate while traveling in the first acceleration region of the ion optics. The TOF distributions due to the hydrogen loss are presented in Figure 1. The mass resolution is not enough to distinguish between the dissociation in the acceleration and drift regions. Instead, we observe a shift in the peak center. The shift of the center of mass is shown in Figure 3 as an inset. At low photon energies, the center of mass of the peak is at 82.75 µs, and at high energy, it is at 82.66 µs.

+ (CH3)2NNH+ 2 f (CH2)(CH3)NNH2 + H

(6)

f (CH3)NNH+ 2 + CH3

(7)

f (CH3)2NNH+ + H

(8)


Figure 4. Breakdown curves determined from the low-temperature LinTOF experiments. Below 9.95 eV, the signals of the [M-H]+ and M+ ions could not be deconvoluted with reasonable accuracy, and therefore, the total [M-H]+ and M+ ion intensities are plotted against the [M-Me]+ ion intensities. Open circles stand for the molecular ion, (CH3)2NNH2+, plus the hydrogen-loss fragment ions, C2H7N2+, while open triangles stand for the methyl-loss fragment ion, CH3NNH2+. Above 9.95 eV, the filled circles stand for the molecular ion, filled downward triangles for CH3NNH2+, and filled upward triangles for C2H7N2+. At higher photon energies, the broken lines show the individual contribution of the two H-loss channels. The dotted curves show the same breakdown curves modeled at room temperature to illustrate the effect of the low sample temperature.

The data analysis was based on the RRKM modeling of the dissociation rates assuming only one TS for each dissociation channel over the available ion internal energy range as outlined above. MP2 calculations predict a saddle point on the potential energy surface (see Figures S1a-c in the Supporting Information), and TSs for data analysis were calculated at the MP2/6311G(d,p) level. Vibrational frequencies were calculated for the obtained geometries, and one imaginary frequency was found for each TS, namely, the critical oscillator. The accuracy of the TS vibrational frequencies was not a real concern because they are fitted to the experimental data during the data analysis. Symmetry parameters of the unimolecular dissociation pathways are not always trivial to determine. One can argue that there are two equivalent methyl groups, six equivalent H atoms in the methyl groups, and two equivalent H atoms in the -NH2 group. Therefore, the symmetry numbers should be 2, 6, 2 for the reactions 6-8, respectively. On the other hand, the symmetry number should be determined in the TS,41 in which H atoms are not equivalent, and the symmetry number should be 2 for reaction 6.12 The data analysis was carried out in both ways, and it was found that symmetry number did not have an effect on the extracted appearance energies; it only affected the assumed TS vibrational frequencies that were fitted to the experiments. Therefore, the assumed symmetry numbers do not affect the thermochemical data, and they could not be determined from these experiments. When the amine-hydrogen-loss channel (eq 8) was excluded from the modeling, the experimental breakdown curves could not be reproduced using RRKM; this is in good agreement with the study of Boulanger et al.14 In Figure 2a, the breakdown curves could be properly reproduced up to the photon energy of 10.0 eV. In this case, the bond dissociation energies obtained by extrapolating to 0 K with the RRKM theory are 2.31 and 2.36 eV for the hydrogen loss and methyl loss, respectively.

J. Phys. Chem. A, Vol. 114, No. 20, 2010 6107 By using a model with tighter TSs, the breakdown curves could be properly modeled above 10.0 eV (Figure 1b). In this case, however, the calculated breakdown curves very poorly reproduce the experimental curves in the low-energy region. The derived 0 K bond dissociation energies with this model are 2.25 and 2.29 eV. When the hydrogen loss from the amino group (eq 8) was included in the model, the experimental breakdown curves and the TOF distributions could be reproduced using RRKM over the entire experimental ion internal energy range. In Figure 2, one can see that simulation underestimates the relative abundance of the [M-H]+ ions in the 9.6-9.7 eV photon energy range. The breakdown curve data are extracted from the TOF distributions, and at very low energies, the [M-H]+ ion signal can be deconvoluted from the large molecular ion peak only with significant error. However, parallel to the breakdown curves, the TOF distributions are also modeled in the data analysis, and the simulated TOF distributions are in very good agreement with the experimental data (for a sample curve at the lowest photon energy, see Figure 4b). Therefore, we can conclude that the relative abundance of the [M-H]+ is properly reproduced in the TOF simulations even if they are less perfectly reflected in the breakdown curves. The extracted bond dissociation energies are listed in Table 1. Because of the better mass resolution, the ReTOF data are considered to be more precise; therefore, only these numbers were used in deriving the thermochemical data, as discussed later. However, we mention that the bond dissociation energies extracted from the LinTOF and ReTOF experiments agree within experimental error. The errors in these values were estimated by the following method. The dissociation threshold in question was scanned by steps of 20 cm-1 until the overlap integral between the calculated and the experimental breakdown curves suddenly decreased. During the scan, the other barriers were fixed, but the TS frequencies corresponding to the scanned barrier were optimized to compensate for the change in the E0. By this method, the following 0 K bond dissociation thresholds were obtained: 2.299 ( 0.015 eV (2.311 eV in LinTOF) for the carbon-hydrogen bond dissociation, 2.356 ( 0.010 eV (2.354 eV in LinTOF) for the nitrogen-carbon bond breaking, and 2.62 ( 0.03 eV (2.60 eV in LinTOF) for the nitrogen-hydrogen bond breaking. The latter dissociation threshold has the highest uncertainty. This is expected because the HNN(CH3)2+ ion has a relatively low contribution to the ion signal, as shown in Figure 2. The low-temperature TPEPICO data were also modeled using the same kinetic model. Since at low energies the H-loss signal could not be deconvoluted from the precursor ion signal, these data are not used to alter the suggested values of the bond energies; rather they are used to further validate the single transition-state RRKM model. In the models, only the data points above 9.95 eV were used in the fitting, where the area of the H-loss peaks was independently determined. The bond energies that best reproduce the experimental breakdown curves were 2.289 ( 0.015 eV for the carbon-hydrogen bond dissociation, 2.349 ( 0.020 eV for the nitrogen-carbon bond breaking, and 2.65 ( 0.04 eV for the nitrogen-hydrogen bond breaking. The accuracy of the H-loss bond energies is affected by the fact that the most important part of the breakdown curve of the first M-H loss is at low energies where, even at a lower temperature, the LinTOF measurement was insufficient for a complete deconvolution of the fragment versus precursor signal. However, all of these numbers are within the error of the ReTOF results, and furthermore, the sum of the M+ and the [M-H]+ ion intensities are reproduced very

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TABLE 1: Calculated and Experimental Dissociation Energies (eV) reaction

CCSD cc-pVTZ

CCSD(T) cc-pVTZ

G3

CBS APNO

W1U

H2NN(CH3)CH2-H H2NN(CH3)-CH3 H-HNN(CH3)2

2.27 2.33 2.60

2.18 2.33 2.51

2.21 2.38 2.59

2.28 2.48 2.65

2.23 2.43 2.61

exp. 2.299 ( 0.015 2.356 ( 0.010 2.62( 0.03

TABLE 2: Ancillary and Derived Heats of Formation (kJ/mol) Neutral Species 0K H(g)a H2(g)a C(graphite)a CH3 NH3b N2H4c CH3NH2d (CH3)2NHd (CH3)3Nd

218.00

298 K

H0K - H298K

lit.g,m

216.03 8.47 1.05 10.5

149.9 -38.9 ( 0.35 109.34 ( 0.50 -7 ( 1.5 3.1 ( 1.5 1.4 ( 2.0

146.7 ( 0.3 -45.9 ( 0.35 95.18 ( 0.50 -21.8 ( 1.5 -18.8 ( 1.5 -27.2 ( 2.0

Methyl Hydrazines (CH3)HNNH2e (CH3)2NNH2e (CH3)HNNH(CH3)e (CH3)2NNH(CH3)e (CH3)2NN(CH3)2e

112.2 ( 2.2 107.7 ( 2.2 120.2 ( 2.4 112.7 ( 2.3 116.7 ( 2.3

90.7 ( 2.2 79.7 ( 2.2 92.7 ( 2.4 78.6 ( 2.3 76.4 ( 2.3

13.65 16.65 17.18 20.01 23.35

94.6 ( 0.6,h 81,i 107,j 102.8k 83.3 ( 3.6,l 77,i 94,j 86.3,d 90.0m 92.0h

Ionic Species (CH3)2NNH2+f (CH3)NNH2+f (CH2)(CH3)NNH2+f (CH3)2NNH+f

811.1 ( 4.5 887.9 ( 2.4 815.0 ( 2.7 845.9 ( 3.7

784.8 ( 4.5 869.6 ( 2.4 790.4 ( 2.7 821.6 ( 3.7

18.31 12.62 15.80 16.15

831 (0 K), 804 (298 K)n 906 (0 K), 893 (298 K)n 822 (0 K), 801 (298 K)n

69.5l

a See ref 39. b See refs 19, 39, and 40. c See refs 38 and 39. d See refs 11, 12, 38, and 39. e From the isodesmic reaction network between methyl hydrazines and methyl amines. f Present TPEPICO appearance energies combined with the heat of formation of the neutral 1,1-dimethylhydrazine. g Refers to 298 K, except otherwise noted. h See ref 19. i See ref 43. j See ref 16. k See ref 17. l See ref 18. m See ref 15. n See ref 14.

accurately even though these points are not included in the fitting. Figure 4 also shows the effect of the lower temperature in the modeling; the dotted lines correspond to the same RRKM model calculated for a room-temperature sample. There is a very significant deviation from the low-temperature model at low photon energies, while the branching ratios are largely unaffected at high photon energies. In order to derive thermochemical data for the fragment ions, two things are needed; (1) the derived bond dissociation energies have to correspond to the energy difference between the molecular ion and the completely dissociated ions with known heats of formation, and (2) the heat of formation of the neutral molecule has to be known within a reasonable uncertainty. This latter will be discussed later. The DFT potential energy surfaces have no reverse barrier (see Supporting Information). However, the ab initio methods (MP2, CCSD, CCSD(T)) predict reverse barriers for the H-loss reactions. As electron correlation is taken into account at higher levels, the reverse barrier decreases from 0.40 (MP2) to 0.15 eV (CCSD(T)) for the C-H bond breaking and from 0.92 (MP2) to 0.53 eV (CCSD(T)) for the N-H bond breaking. It is possible that higher-order coupled cluster calculations would further decrease this value, but extrapolation cannot be based on these calculations. In Table 1, experimental dissociation thresholds are compared to the calculated energy differences between the molecular ion and the completely dissociated fragment ions and neutral ligands. The values obtained by the composite methods (G3, CBS-APNO, and W1U) that contain extrapolation to infinite order of electron correlation and complete basis set are also listed. From this comparison, one can see that the predicted

reverse barriers are most likely artifacts of the calculations. One explanation for this can be that the TS has multireference nature due to the radical-radical interaction, where multireference methods (CASPT2, CASSCF) would have to be used to obtain exact potential energy surfaces.42 It has therefore been assumed that there is no reverse barrier on the potential energy surface, or it is lower than the uncertainty of the dissociation barrier and cannot be determined from these experiments. This is especially true for the hydrogen loss from the amino group, in which case all but the CCSD(T) values are within the experimental uncertainty. Gas-Phase Heat of Formation of the Neutral 1,1-DMH. The heats of formation of the neutral methyl hydrazines are listed in Table 2. The 0 K heats of formation were estimated by using 32 isodesmic reactions between methyl amines and methyl hydrazines as outlined above. Then, the 0 K heats of formation were converted into 298 K heats of formation by using W1U and CBS-APNO H298K - H0K values. From the isodesmic reaction network, the 0 K gas-phase heat of formation of 1,1-DMH was determined to be 107.7 ( 2.2 kJ/mol. It was found that when the optimized heats of formation were changed by (2.0%, the error as defined in eq 3 doubled. Therefore, a relative uncertainty of (2.0% is assumed for the 0 K heats of formation of 1,1-DMH and the other methyl hydrazine derivatives. From the reaction network, the heats of formation of the other methyl hydrazine derivatives were also determined. These values are listed in Table 2 for completeness. The above-derived 0 K heats of formation can be converted into 298 K values


∆fH298K ) ∆fH0K -

∑ (H298K - H0K)elements +

(H298K - H0K)molecule

(9)

where the H298K - H0K values of the elements were taken from the NIST-JANAF Thermochemical Tables39 (8.468 kJ/mol, H2; 1.051 kJ/mol, Cgraphite; 8.670 kJ/mol, N2), and the H298K - H0K of 1,1-DMH was calculated from the vibrational frequencies at the highest available level of theory (W1U for species containing less than five heavy atoms and CBS-APNO otherwise). With these values, the 298 K heat of formation of 1,1-DMH is calculated to be 79.7 ( 2.2 kJ/mol, which is within the experimental error of the value of 83.3 ( 3.6 kJ/mol reported by Donovan et al.15 In the earlier TPEPICO work, a value of 94 kJ/mol (298 K) was determined from G3 calculations,14,27 and this value was used in the derivation of the heats of formation of the 1,1-DMH molecular ion and its fragments ions. However, our highly redundant method of determining the heat of formation of 1,1-DMH is more accurate because the network of isodesmic reactions involves more than one well-established anchor of the energy scale. Thermochemical Data. As discussed above, the RRKMbased dissociation onsets of the observed unimolecular reactions can be used to derive the heats of formation of the various fragment ions. The 0 K gas-phase heat of formation of neutral 1,1-DMH is 107.7 ( 2.2 kJ/mol. This can be combined with the adiabatic ionization energy of 7.29 ( 0.04 eV reported by Meot-Ner et al.27 Therefore, the 0 K heat of formation of the 1,1-DMH molecular ion is 811.1 ( 4.5 kJ/mol. This, combined with the RRKM-based 0 K dissociation onsets of the observed fragment ions of 1,1-DMH provides the following heats of formation: ∆fH0K((CH2)(CH3)NNH2+) ) 815.0 ( 2.7 kJ/mol, ∆fH0K((CH3)NNH2+) ) 887.9 ( 2.4 kJ/mol, and ∆fH0K((CH3)2NNH+) ) 845.9 ( 3.7 kJ/mol. The uncertainties of these values largely propagated from the estimated error of the heat of formation of the neutral 1,1DMH. As in the TPEPICO experiment, the fragment ions’ appearance energies are determined and not the ionic bond energies. The relative large uncertainty in the ionization energy does not directly propagate into the derived heats of formation; it only has an indirect effect on the accuracy of the RRKM model. The above-derived 0 K heats of formation can be converted into 298 K values using eq 9; these numbers are listed in Table 2. Conclusions Threshold photoelectron photoion coincidence spectroscopy experiments were carried out on 1,1-dimethylhydrazine; methylloss and hydrogen-loss channels were observed in the TOF distributions. It was shown that the unimolecular dissociation of 1,1-dimethylhydrazine ions can be adequately modeled with RRKM theory when three dissociation channels are included in the kinetic model. In the model, it was assumed that the hydrogen atom can dissociate from either a methyl group or the amino group. Using these three parallel pathways (one methyl- and two hydrogen-loss dissociations), the experimental data was reproduced over the whole experimental energy range at two sample temperatures. With only one H-loss pathway, either the low-energy or the high-energy part of the calculated breakdown curves significantly deviated from the experimental data. The 0 and 298 K heats of formation of five neutral methyl hydrazine derivatives were computed by building an isodesmic reaction network in which 32 reactions between methyl hydra-

J. Phys. Chem. A, Vol. 114, No. 20, 2010 6109 zines and methyl amines were included. These results suggest that the originally reported experimental 298 K gas-phase heat of formation of 1,1-DMH is accurate within its claimed error, even though this value contradicted earlier quantum chemical calculations. The 0 K gas-phase heat of formation of the neutral 1,1-DMH was combined with the adiabatic ionization energy and the appearance energies of the fragment ions to obtain the 0 and 298 K heats of formation of the fragment ions. These values are 10-25 kJ/mol lower than the previously reported numbers. Acknowledgment. The measurements were carried out on the TPEPICO apparatus built in the laboratory of Professor Tomas Baer at the University of North Carolina at Chapel Hill, supported by the grants from the U.S. National Science Foundation, the U.S. Department of Energy, and the Hungarian Science Fund (OTKA #71644). This experiment was donated to the University of the Pacific in 2009, for which we are eternally grateful. B.Sz. gratefully acknowledges the support of the ACS Petroleum Research Fund. Supporting Information Available: Figure S1. Potential energy surfaces calculated at different levels of theory. Figure S2. Breakdown curves modeled with treating methyl vibrations as internal rotors. Table S1. Best parameter set used for modeling the breakdown curves and the time-of-flight distributions. Table S2-S4. Details of the G3, CBS-APNO, and W1U calculations on the isodesmic reaction network of the neutral methyl hydrazines. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Chen, X. W.; Zhang, T.; Xia, L. G.; Li, T.; Zheng, M. Y.; Wu, Z. L.; Wang, X. D.; Wei, Z. B.; Xin, Q.; Li, C. Catalytic Decomposition of Hydrazine over Supported Molybdenum Nitride Catalysts in a Monopropellant Thruster. Catal. Lett. 2002, 79 (1-4), 21–25. (2) Schirmann, J. P.; Bourdauducq, P. Hydrazine. Ullmann’s Encyclopedia of Industrial Chemistry; Wiley: 2001. (3) Catoire, L.; Chaumeix, N.; Pichon, S.; Paillard, C. Visualizations of Gas-Phase NTO/MMH Reactivity. J. Propul. Power 2006, 22 (1), 120– 126. (4) Sutton, G. P.; Ross, D. M.; Biblarz, O. Rocket Propulsion Elements, 7 ed.; John Wiley & Sons: New York, 2001. (5) Baer, T.; Szta´ray, B.; Kercher, J. P.; Lago, A. F.; Bodi, A.; Shull, C.; Palathinkal, D. Threshold Photoelectron Photoion Coincidence Studies of Parallel and Sequential Dissociation Reactions. Phys. Chem. Chem. Phys. 2005, 7, 1507–1513. (6) Ervin, K. M. Experimental Techniques in Gas-Phase Ion Thermochemistry. Chem. ReV. 2001, 101 (2), 391–444. (7) Szta´ray, B.; Baer, T. Dissociation Dynamics and Thermochemistry of Energy-Selected CpCo(CO)2+ Ions. J. Am. Chem. Soc. 2000, 122, 9219– 9226. (8) Szta´ray, B.; Baer, T. Consecutive and Parallel Dissociation of Energy-Selected Co(CO)3NO+ Ions. J. Phys. Chem. A 2002, 106, 8046– 8053. ´ .; Pongor, Cs. I.; Bodi, A.; Szta´ray, B.; Baer, T. (9) Re´ve´sz, A Manganese-Chalcocarbonyl Bond Strengths from Threshold Photoelectron Photoion Coincidence Spectroscopy. Organometallics 2006, 25, 6061–6067. (10) Gengeliczki, Z.; Szta´ray, B.; Baer, T.; Iceman, C.; Armentrout, P. B. Heats of Formation of Co(CO)2NOPR3, R ) CH3 and C2H5, and Its Ionic Fragments. J. Am. Chem. Soc. 2005, 127 (26), 9393–9402. (11) Bodi, A.; Kercher, J. P.; Bond, C.; Meteesatien, P.; Szta´ray, B.; Baer, T. Photoion Photoelectron Coincidence Spectroscopy of Primary Amines RCH2NH2 (R ) H, CH3, C2H5, C3H7, i-C3H7): Alkylamine and alkyl Radical Heats of Formation by Isodesmic Reaction Networks. J. Phys. Chem. A 2006, 110 (50), 13425–13433. (12) Bodi, A.; Szta´ray, B.; Baer, T. Dissociative Photoionization of Mono-, Di- And Trimethylamine Studied by a Combined Threshold Photoelectron Photoion Coincidence Spectroscopy and Computational Approach. Phys. Chem. Chem. Phys. 2006, 8, 613–623. (13) Kercher, J. P.; Stevens, W.; Gengeliczki, Zs.; Baer, T. Modeling Ionic Unimolecular Dissociations from a Temperature Controlled TPEPICO Study on 1-C4H9I Ions. Int. J. Mass Spectrom. 2007, 267, 159–166.

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JP1017604

Dissociation of Energy-Selected 1, 1-Dimethylhydrazine Ions

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