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The involvement of ring hydrogen atoms in the reaction C6H5NH2+. → C5H6+. + HNC
Volume 147, number 4
CHEMICAL PHYSICS LETTERS
10 June 1988
THE INVOLVEMENT OF RING HYDROGEN ATOMS IN THE REACTION &HsNH;’ -X5H6”tHNC Jeffrey A. ZIMMERMAN and Rebecca M. O’MALLEY I Department ofChemistry, UniversityofSouth Florida, Tampa, FL 33620, USA Received 8 March 1988
We report electron impact and 266 nm MPI data obtained using an aniline/aniline-2.3,4,5,6-d, mixture which indicate that both the rapid and the metastable decomposition reactions &H,NH$ -CsHg. t HNC in the aniline system have two components. The major component involves the loss of a hydrogen atom originally attached to the nitrogen atom in the aniline molecule whereas the minor component involves the loss of a hydrogen atom originally attached to the ring. The relative rate constants for the two metastable decomposition reactions in aniline-2,3,4,5,6-d, have been measured.
1. Introduction The metastable decomposition reaction of the aniline radical cation
has been well studied by single photon [ 1 ] and multiphoton ionization (MPI) [ 2-41 and also by electron impact [5-71. In the single photon PIPECO study, Baer and Carney [ 1] concluded that the ionic and neutral products in the reaction were the cyclopentadienyl cation and hydrogen cyanide molecule respectively, which lie at a calculated thermodynamic threshold energy of 10.43 eV above the ground state of the neutral aniline molecule. Baer and Camey’s RRKM/QET calculations on the rate of this reaction as a function of threshold energy were found to give a best tit to their experimental data for a threshold energy of 11.2 eV and a loose transition state. This implies a reverse activation energy of about 0.8 eV. They pointed out that there appeared to be an inconsistency in this as a loose transition state is usually associated with a reaction having no reverse activation energy. Later, using an ion-trapping technique, Lifshitz et al. [6] measured a millisecond appearance energy limit of 11.26 eV for the reaction, in close agreement with Baer and Carney’s calculated transition state energy, and they proposed ’ TO whom correspondence should be addressed.
that the neutral product in the reaction was not HCN, but the isomer HNC. The reaction then has a calculated thermodynamic threshold energy of 11.07 eV. Burgers et al. [ 71 confirmed experimentally that the identity of the neutral produced in the reaction is HNC and not HCN by comparing the collision-induced dissociation spectrum of the neutral metastable product with that of source generated HCNhC’ ions. More recently the cyclopentadienyl structure of the ionic product has also been confirmed using collision-induced dissociation of the metastable C,H,f ’ product ion [8]. In this paper we present electron impact and 266 nm MPI data obtained using an aniline/aniline2,3,4,5,6-d, mixture which indicate clearly that this reaction has two components. The major component involves the loss of a hydrogen atom originally attached to the nitrogen atom in the aniline molecule, whereas the minor component involves the loss of a hydrogen atom originally attached to the ring.
2. Experimental The data were obtained using a laser/time-of-flight mass spectrometer apparatus which has been modified so that spectra can be obtained when ionization is initiated either by the electron beam or the laser. Details of the equipment have been described elsewhere [ 9 I. Ion draw-out in the mass spectrometer is
0 009-2614/88/$ 03.50 0 Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )
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achieved with a system of three grids. The first of these, the ion draw-out grid, initiates ion acceleration and in laser experiments is pulsed to a voltage of -200 V at a known and variable time after the laser light crosses the source. The delay time (time lag) between the laser light crossing the source and the application of the draw-out pulse can be set at 0.1 ps intervals in the range O-5.0 ps. In this way the allowed reaction time for metastable decomposition reactions can be varied. After draw-out, ion acceleration is completed in two stages with the second grid held at a constant potential of - 1500 V and the third held at - 2700 V. The accelerated ions enter a 1 m field-free flight tube and are separated by mass according to their flight times. The signal due to each pulse of ions produced’by the laser is captured by a waveform recorder and downloaded to a signal averager. Data from between 200 and 500 laser pulses were averaged to obtain the spectra reproduced here, which were digitised using a 10 ns time interval. For the metastable product ion intensity studies the data were digitised using a 2 ns time interval to obtain maximum sensitivity and reproducibility. The MPI experiments at 266 nm were carried out using the fourth harmonic of the Nd : YAG laser described previously [ 9 1, In these experiments the laser beam was used either unfocused and directed through a 0.26 cm diameter orifice or focused with a 250 mm focal length plane-convex lens. The aniline used was research grade. Aniline-d, (98%) and anilineJ5N (99%) were obtained from MSD Isotopes, St. Louis, MO. All were used without further purification. The integrity of the labelling in the aniline-d, was checked by NMR and found to show no significant ring-attached hydrogen or nitrogen-attached deuterium. The approximately 1 : 1 mixture used throughout these experiments was subjected to several freeze-pump -thaw cycles before introduction into the mass spectrometer via a needle valve.
3. Results and discussion
10 June 1988
a1 70 eV
m/z
20
30
401
50 ,
60 70 80 1 90 I
I
100
b) 20 eV
I
m/r
20
I
I
I
30
40
50
I
I
1
I
L-
I
60 70 80 90 100
Fig. 1. Electron impact spectra of an approximately 1 : 1 aniline/aniline-d, mixture. (a) 70 eV, (b) 20 eV with insert of expanded mass range m/z= 65-72.
(CsH5NH: ‘) and m/z=98 (C6D5NH2+‘) with the most intense fragment ions being observed at m/z=66 (C,H,$‘) and m/z=71 (C5D5H+‘). The observation of these fragment ions indicate the occurrence of the expected fragmentation reaction in the aniline system, C6HSNH:’ (m/z=93) -+CSH6+’(m/z=66)+HNC,
3.1. Electron impact Fig. 1 shows for comparison purposes the 70 and 20 eV electron impact spectra of an approximately 1: 1 aniline/aniline-d, mixture. The 70 eV spectrum (fig. la) shows large molecular ions at m/z=93 316
and the analogous reaction in the aniline-d, system, C6D5NH:’ (m/z=98) +C5D5H+ (m/z=71)
tHNC.
However, close inspection of the spectrum shows that
CHEMICAL PHYSICS LETTERS
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in this approximately 1: I aniline/aniline-d, mixture (which actually contains slightly more of the aniline-&, as evidenced by the relative intensities of the molecular ion signals) the intensity of the fragment ion at m/z=71 arising from the aniline-d, is smaller than would be expected by comparison with the intensity of the m/z=66 ion. Furthermore, the intensity of the m/z=70 ion is rather larger than would be expected from comparison with the m/z=65 intensity. This observation is demonstrated more dramatically in fig. lb which is a spectrum of the same mixture recorded at a nominal electron energy of 20 eV. Under these ionizing conditions the internal energy of the molecular ion is much lower and the non-deuterated molecular ion fragments almost exclusively to form C>Hs+’ (m/z= 66). The corresponding fragmentation for the deuterated molecular ion is observed as two peaks occurring at m/z=71 and m/z= 70 in the approximate ratio of 1.O: 0.4, indicating the occurrence of two reactions in the aniline-d, system, C6DSNH: ’ (m/z=98) +CsDsH+’
(m/z=71)
tHNC
and C6D5NH:’ (m/z=98) --+C5D4H:’ (m/z=70)
tDNC.
These data show clearly that under electron impact conditions the fragmentation reaction of the molecular ion of aniline to produce CSH6+. and HNC has two components. The major component of the reaction involves the loss of a hydrogen originally attached to the nitrogen atom, while the minor, but nevertheless significant component involves the loss of an originally ring-attached hydrogen. Our electron impact experimental conditions do not allow for clearly defined observation of metastable decomposition reactions and it is quite possible that even at the low ionizing energy conditions of 20 eV the majority of the fragmentation observed is due to rapid decomposition and not metastable decomposition of the molecular ion. To investigate the metastable decomposition process data were obtained using MPI at 266 nm under experimental conditions such that we could be sure that we were observing product ions formed on the microsecond time scale.
3,2. Metastable C,(H,D),’ nm
10 June 1988
formation: MPI at 266
It is known from our previous studies on the MPI and fragmentation of aniline at 266 nm [4] that it is possible, using a low intensity laser beam, to weight the distribution of the molecular ion energy such that a large fraction of the molecular ions are formed by the absorption of three photons by the neutral molecule. Under these conditions this large fraction of the molecular ions have energies of approximately 13.6 eV, placing them exactly in the energy range for this decomposition to take place on the microsecond time scale. The reaction can then be followed in real time by altering the time delay between ion production by the laser pulse and ion draw-out. Fig. 2 shows 266 nm MPI spectra of the same aniline/aniline-d, mixture recorded using the laser beam focused (fig. 2a) and unfocused (fig. 2b). The time-lag value for both of these spectra was 0.7 ps which means that a major part of the metastable decomposition reaction will be complete [4]. Fig. 2a was obtained using a 0.18 mJ, focused laser beam and shows some similarities with the 70 eV electron impact spectrum. The molecular ions are observed plus the expected major fragments at m/z=66 and 7 1. More intense lower mass fragments are observed under these focused laser conditions than under electron impact. The majority of the fragment ions detected under these focused laser conditions are expected to be formed in rapid decomposition reactions initiated as the result of the absorption of at least four 266 nm photons. In contrast, in fig. 2b, the unfocused low intensity beam gives rise mainly to molecular ions and fragment ions formed by the metastable decomposition reaction initiated by the absorption of three photons by the neutral molecule. As for the electron impact case, the spectrum shows clearly that the metastable fragmentation reaction in the deuterated aniline again has two components. In this case the measured minor component is somewhat smaller, the m/z= 7 1: m/z= 70 ratio being approximately 1.O: 0.3. The expanded inset in fig. 2b shows that in addition to the unexpectedly large peak at m/z= 70 the group of peaks associated with the decomposition of the deuterated aniline does not have quite the same pattern of that due to the non-deuterated aniline. 317
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ated aniline, identified as mainly C5H: from “Nlabelled observations, is probably distributed between CSD: (m/z= 70), C,D,H+ (m/z=69) and C5D3H2+ (m/z= 68) in the deuterated system. The recorded m/z=70 intensity therefore probably contains some intensity due to C4ND3H: and C5D: ions. However, consideration of the probable intensities of these two ions indicates they are likely to be minor components of the overall intensity.
a) 266 nm focused
3.3. Me&stable m/z= 70 intensity as a function of reaction time
m/z
20
30
40
50
60 70 80 90 100
bl 266 nm unfocused
m/z
To check that the ion at m/z= 70 is indeed a product of the metastable decomposition reaction its intensity as a function of reaction time on the microsecond time scale was monitored. The laser beam was used focused for this experiment to allow comparison of the peak height of the suspected metastable product ion with peak heights of rapidly formed fragment ions with similar mass. This was necessary to separate out change in peak height as a function of reaction time due to time-lag focusing and ion loss effects [ lo] from change due to the occurrence of a microsecond time scale reaction. Fig. 3 shows a plot of peak height for m/z= 65 (from aniline in the mixture) and m/z= 69 and 70 (from aniline-d, in the mixture) as a function of time lag in the range O-2.5 ys. Under these focused conditions
1
20
30
40
50
60 70 80 SO 100
Fig. 2. 266 nm MPI spectra of an aniline/aniline-d, mixture at a time lag of 0.7 bs. (a) Focused beam, 0.18 mJ/pulse; (b) unfocused beam, 0.5 mJ/pulse, with insert of expanded mass range
m/2=65-72.
-
60’
E E nl
Z
.c 40' x
Most noticeably the m/z=67 peak in the non-deuterated system does not translate to an equivalent mjz=72 peak in the deuterated system. Comparison of aniline and aniline-15N 266 nm MPI spectra has shown that m/z=67 is almost 100% C,NH$. . In the deuterated system this ion is probably distributed between the ions &ND: ’ (m/z=72), C,ND,H+’ (m/z=71) and C4ND3H:’ (m/z=70). This explains the low m/z=72 intensity. Similarly the very small intensity m/z= 65 in the non-deuter318
a,
.---c-
20.'
m/r=69
0.5
10
1.5
20
2.5
time-lag(crs) Fig. 3. Plot of absolute peak height of m/z=65from aniline and m/z= 69 and m/z= 70 from aniline-d, as a function of time-lag delay. Focused 266 nm MPI at 1.0 mJ/pulse (ion loss effects important).
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CHEMICAL PHYSICS LETTERS
m/z=65 is the rapidly formed CSH: fragment ion and m/z=69 is the deuterated analogue of this, i.e. CsD4H+. (m/z=69 cannot be a &(D,H)g’ metastable product ion as this would require three hydrogen atoms.) All three peaks, m/z= 6569 and 70 increase to a maximum value and then decrease as the reaction time is allowed to increase. The time-lag position of the maxima in the curves increase with increasing mass as would be expected from the combined ion focusing and ion loss effects [ lo]. However, the m/z=70 intensity, which at low reaction times is less than that of m/z=65, increases more rapidly as the reaction time increases, overtakes the m/z=65 intensity and remains the most intense ion at the longest reaction times recorded. This indicates clearly that even under these more highly focused conditions the m/z=70 intensity has a major component which is the product of a microsecond time scale reaction. 3.4 Measurement of the relative rate constants for the two aniline-d5 reactions It was impossible to measure the absolute rate constants for the two reactions identified in the aniline-ds system. To make an absolute rate constant measurement using this experimental method requires the measurement of a reference non-metastable reaction product ion to normalize and correct for ion focusing and ion loss effects as we have described previously [ 41. In measuring the absolute rate constant for the reaction C6H5NH: ’ + Cs HZ ’ +HNC in aniline, m/z=65 was taken as the reference ion [ 41. The analogous ion in aniline-d, is m/z= 70, which is a product ion, and therefore cannot be used as a reference ion. The relative rate constants of the two reactions were therefore measured in the following way. MPI spectra of the same aniline/aniline-d, mixture were recorded as a function of reaction time using an unfocused, low intensity (0.25 mJ) laser beam at 266 nm. We know that under these unfocused conditions ion loss is negligible at reaction times of less than 1.5 ps [ 111. Jon focusing effects due to time lag remain but are expected to be very similar for the ions under consideration as they are close in mass (m/z= 66, 70, 7 1). Fig. 4 shows a plot of the intensities of the three metastable product ions as a func-
m/z=66
.
1.0
2.0
time-lag (ps) Fig. 4. Plot of peak heights of m/z=66 from aniline and m/z= 70 and m/z= 7 1 in aniline-d, as a function of time-lag delay. Unfocused, 266 nm MPI at 0.25 ml/pulse (ion loss effects negligible).
tion of reaction time over the range O-2.4 ps. (These data have been adjusted so that they reflect the relative peak heights that would be observed in an exactly 1 : 1 aniline/aniline-d, mixture.) The peak heights bf all three of the metastable ions rise and reach a plateau as the reaction time is increased and the reactions approach completion. Assuming that the reactions are first order and can be represented by the equation A+B+C, where A is the molecular ion and B the metastable product ion, the appropriate integrated rate equation is B=B,(
1-epkr),
where B, is the intensity of the metastable product ion at infinite reaction time. The integrated rate equation can be used in the form -In ( 1 -B/B, ) = kt to obtain k by plotting -ln( 1 -B/B,) versus t. In this case no effort was made to compensate for timelag focusing and B, values for each reaction were obtained by averaging peak heights for the product at the plateau. Fig. 5 shows plots of -In ( 1-B/B,) versus t for each of the reaction products. From the slopes of the . < straight lmes obtained k,, z kb6 and k,o/k,, x 0.75. Taking the absolute value for kb6of ( 1.74 + 0.4) x lo6 319
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10 June 1988
5. Conclusions Both the metastable and the rapid fragmentation reaction C6H,NH: . -+C5H6+. + HNC in aniline have been shown to have two components. The major component involves the loss from the molecular ion of a hydrogen atom originally attached to the nitrogen atom of the neutral molecule while the minor component involves the loss of a hydrogen atom attached to the ring. In aniline-2,3,4,5,6-d, the rate constant for the loss of DNC has been found to have a value equal to approximately 0.75 that for the loss of HNC. time-lag@s) Fig. 5. Plotsof -ln( 1-B/B,) versus reaction time for m/z=66, m/z=‘lOand m/z=71. (Seetext.)
s- ’ reported previously [ 41 this gives k,, x 1.7X lb6 s-l and kTO%1.3 x lo6 s- ‘. The smaller rate constant for the production of C5D4HZ+’ as compared to CSDSH+’ in aniline-& is not surprising as this reaction must involve the breaking of at least one C-D bond.
4. Mechanistic implications The ratio of the metastable HNC : DNC loss recorded for MPI at 266 nm is approximately 1 : 0.3 and the corresponding rapid decomposition (electron impact) is approximately 1 : 0.4. Complete randomisation of the H and D atoms in the molecular ion before decomposition would lead to a value of HNC : DNC loss of 1 : 2.5. The experimental data indicate that this is’clearly not the case. A mechanism which involved randomisation of the H atoms attached to the N atom and the two ortho H atoms would lead to a value of HNC : DNC loss of 1 : I. Again, this does not fit with the experimental data. It seems most likely that the recorded data indicate the existence of two reactions, possibly with different threshold energies, which contribute to the overall products.
320
Acknowledgement The author would like to thank Dr. Tomas Baer for helpful discussions and corroborating experimental data taken on the aniline-2,3,4,5,6-& system.
References [ 1 ] T. Baer and T.E. Camey, J. Chem. Phys. 76 (1982) 1304. [ 2,] R. Proch, D.M. Rider and R.N. Zare, Chem. Phys. Letters 81 (1981) 430. [ 3 ] H. Kuhlewind, H.J. Neusser and E.W. Schlag, J. Chem. Phys. 82 (1985) 5452. [4] J.A. Zimmerman, R.M. O’Malley and J.E. Weinzierl, Intern. J. Mass Spectrom. Ion Processes 76 (1987) 257. [ 5) P.N. Rylander, S. Meyerson, E.L. Eliel and J.D. McCollum, J. Am. Chem. Sot. 85 (1963) 2723. [ 61 C. Lifshitz, P. Gotchiguian and R. Roll&, Chem. Phys. Letters 95 (1983) 106. [7] P.C. Burgers, J.L. Holmes, A.A. Mommers and J.K. Terlouw, Chem. Phys. Letters 102 (1983) 1. [ 81 R. Flammang, P. Meyrant, A. Maquestiau, E.E. Kingston and J.H. Beynon, Org. Mass Spectrom. 20 (1985) 253. [ 91 W.B. Martin and R.M. O’Malley, Intern. J. Mass Spectrom. Ion Processes 59 (1984) 277. [ lo] W.B. Martin and R.M. O’Malley, Intern. J. Mass Spectrom. Ion Processes 77 (I 987) 203. [ 111 J.A. Zimmerman and R.M. O’Malley, unpublished work
CHEMICAL PHYSICS LETTERS
10 June 1988
THE INVOLVEMENT OF RING HYDROGEN ATOMS IN THE REACTION &HsNH;’ -X5H6”tHNC Jeffrey A. ZIMMERMAN and Rebecca M. O’MALLEY I Department ofChemistry, UniversityofSouth Florida, Tampa, FL 33620, USA Received 8 March 1988
We report electron impact and 266 nm MPI data obtained using an aniline/aniline-2.3,4,5,6-d, mixture which indicate that both the rapid and the metastable decomposition reactions &H,NH$ -CsHg. t HNC in the aniline system have two components. The major component involves the loss of a hydrogen atom originally attached to the nitrogen atom in the aniline molecule whereas the minor component involves the loss of a hydrogen atom originally attached to the ring. The relative rate constants for the two metastable decomposition reactions in aniline-2,3,4,5,6-d, have been measured.
1. Introduction The metastable decomposition reaction of the aniline radical cation
has been well studied by single photon [ 1 ] and multiphoton ionization (MPI) [ 2-41 and also by electron impact [5-71. In the single photon PIPECO study, Baer and Carney [ 1] concluded that the ionic and neutral products in the reaction were the cyclopentadienyl cation and hydrogen cyanide molecule respectively, which lie at a calculated thermodynamic threshold energy of 10.43 eV above the ground state of the neutral aniline molecule. Baer and Camey’s RRKM/QET calculations on the rate of this reaction as a function of threshold energy were found to give a best tit to their experimental data for a threshold energy of 11.2 eV and a loose transition state. This implies a reverse activation energy of about 0.8 eV. They pointed out that there appeared to be an inconsistency in this as a loose transition state is usually associated with a reaction having no reverse activation energy. Later, using an ion-trapping technique, Lifshitz et al. [6] measured a millisecond appearance energy limit of 11.26 eV for the reaction, in close agreement with Baer and Carney’s calculated transition state energy, and they proposed ’ TO whom correspondence should be addressed.
that the neutral product in the reaction was not HCN, but the isomer HNC. The reaction then has a calculated thermodynamic threshold energy of 11.07 eV. Burgers et al. [ 71 confirmed experimentally that the identity of the neutral produced in the reaction is HNC and not HCN by comparing the collision-induced dissociation spectrum of the neutral metastable product with that of source generated HCNhC’ ions. More recently the cyclopentadienyl structure of the ionic product has also been confirmed using collision-induced dissociation of the metastable C,H,f ’ product ion [8]. In this paper we present electron impact and 266 nm MPI data obtained using an aniline/aniline2,3,4,5,6-d, mixture which indicate clearly that this reaction has two components. The major component involves the loss of a hydrogen atom originally attached to the nitrogen atom in the aniline molecule, whereas the minor component involves the loss of a hydrogen atom originally attached to the ring.
2. Experimental The data were obtained using a laser/time-of-flight mass spectrometer apparatus which has been modified so that spectra can be obtained when ionization is initiated either by the electron beam or the laser. Details of the equipment have been described elsewhere [ 9 I. Ion draw-out in the mass spectrometer is
0 009-2614/88/$ 03.50 0 Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )
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achieved with a system of three grids. The first of these, the ion draw-out grid, initiates ion acceleration and in laser experiments is pulsed to a voltage of -200 V at a known and variable time after the laser light crosses the source. The delay time (time lag) between the laser light crossing the source and the application of the draw-out pulse can be set at 0.1 ps intervals in the range O-5.0 ps. In this way the allowed reaction time for metastable decomposition reactions can be varied. After draw-out, ion acceleration is completed in two stages with the second grid held at a constant potential of - 1500 V and the third held at - 2700 V. The accelerated ions enter a 1 m field-free flight tube and are separated by mass according to their flight times. The signal due to each pulse of ions produced’by the laser is captured by a waveform recorder and downloaded to a signal averager. Data from between 200 and 500 laser pulses were averaged to obtain the spectra reproduced here, which were digitised using a 10 ns time interval. For the metastable product ion intensity studies the data were digitised using a 2 ns time interval to obtain maximum sensitivity and reproducibility. The MPI experiments at 266 nm were carried out using the fourth harmonic of the Nd : YAG laser described previously [ 9 1, In these experiments the laser beam was used either unfocused and directed through a 0.26 cm diameter orifice or focused with a 250 mm focal length plane-convex lens. The aniline used was research grade. Aniline-d, (98%) and anilineJ5N (99%) were obtained from MSD Isotopes, St. Louis, MO. All were used without further purification. The integrity of the labelling in the aniline-d, was checked by NMR and found to show no significant ring-attached hydrogen or nitrogen-attached deuterium. The approximately 1 : 1 mixture used throughout these experiments was subjected to several freeze-pump -thaw cycles before introduction into the mass spectrometer via a needle valve.
3. Results and discussion
10 June 1988
a1 70 eV
m/z
20
30
401
50 ,
60 70 80 1 90 I
I
100
b) 20 eV
I
m/r
20
I
I
I
30
40
50
I
I
1
I
L-
I
60 70 80 90 100
Fig. 1. Electron impact spectra of an approximately 1 : 1 aniline/aniline-d, mixture. (a) 70 eV, (b) 20 eV with insert of expanded mass range m/z= 65-72.
(CsH5NH: ‘) and m/z=98 (C6D5NH2+‘) with the most intense fragment ions being observed at m/z=66 (C,H,$‘) and m/z=71 (C5D5H+‘). The observation of these fragment ions indicate the occurrence of the expected fragmentation reaction in the aniline system, C6HSNH:’ (m/z=93) -+CSH6+’(m/z=66)+HNC,
3.1. Electron impact Fig. 1 shows for comparison purposes the 70 and 20 eV electron impact spectra of an approximately 1: 1 aniline/aniline-d, mixture. The 70 eV spectrum (fig. la) shows large molecular ions at m/z=93 316
and the analogous reaction in the aniline-d, system, C6D5NH:’ (m/z=98) +C5D5H+ (m/z=71)
tHNC.
However, close inspection of the spectrum shows that
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in this approximately 1: I aniline/aniline-d, mixture (which actually contains slightly more of the aniline-&, as evidenced by the relative intensities of the molecular ion signals) the intensity of the fragment ion at m/z=71 arising from the aniline-d, is smaller than would be expected by comparison with the intensity of the m/z=66 ion. Furthermore, the intensity of the m/z=70 ion is rather larger than would be expected from comparison with the m/z=65 intensity. This observation is demonstrated more dramatically in fig. lb which is a spectrum of the same mixture recorded at a nominal electron energy of 20 eV. Under these ionizing conditions the internal energy of the molecular ion is much lower and the non-deuterated molecular ion fragments almost exclusively to form C>Hs+’ (m/z= 66). The corresponding fragmentation for the deuterated molecular ion is observed as two peaks occurring at m/z=71 and m/z= 70 in the approximate ratio of 1.O: 0.4, indicating the occurrence of two reactions in the aniline-d, system, C6DSNH: ’ (m/z=98) +CsDsH+’
(m/z=71)
tHNC
and C6D5NH:’ (m/z=98) --+C5D4H:’ (m/z=70)
tDNC.
These data show clearly that under electron impact conditions the fragmentation reaction of the molecular ion of aniline to produce CSH6+. and HNC has two components. The major component of the reaction involves the loss of a hydrogen originally attached to the nitrogen atom, while the minor, but nevertheless significant component involves the loss of an originally ring-attached hydrogen. Our electron impact experimental conditions do not allow for clearly defined observation of metastable decomposition reactions and it is quite possible that even at the low ionizing energy conditions of 20 eV the majority of the fragmentation observed is due to rapid decomposition and not metastable decomposition of the molecular ion. To investigate the metastable decomposition process data were obtained using MPI at 266 nm under experimental conditions such that we could be sure that we were observing product ions formed on the microsecond time scale.
3,2. Metastable C,(H,D),’ nm
10 June 1988
formation: MPI at 266
It is known from our previous studies on the MPI and fragmentation of aniline at 266 nm [4] that it is possible, using a low intensity laser beam, to weight the distribution of the molecular ion energy such that a large fraction of the molecular ions are formed by the absorption of three photons by the neutral molecule. Under these conditions this large fraction of the molecular ions have energies of approximately 13.6 eV, placing them exactly in the energy range for this decomposition to take place on the microsecond time scale. The reaction can then be followed in real time by altering the time delay between ion production by the laser pulse and ion draw-out. Fig. 2 shows 266 nm MPI spectra of the same aniline/aniline-d, mixture recorded using the laser beam focused (fig. 2a) and unfocused (fig. 2b). The time-lag value for both of these spectra was 0.7 ps which means that a major part of the metastable decomposition reaction will be complete [4]. Fig. 2a was obtained using a 0.18 mJ, focused laser beam and shows some similarities with the 70 eV electron impact spectrum. The molecular ions are observed plus the expected major fragments at m/z=66 and 7 1. More intense lower mass fragments are observed under these focused laser conditions than under electron impact. The majority of the fragment ions detected under these focused laser conditions are expected to be formed in rapid decomposition reactions initiated as the result of the absorption of at least four 266 nm photons. In contrast, in fig. 2b, the unfocused low intensity beam gives rise mainly to molecular ions and fragment ions formed by the metastable decomposition reaction initiated by the absorption of three photons by the neutral molecule. As for the electron impact case, the spectrum shows clearly that the metastable fragmentation reaction in the deuterated aniline again has two components. In this case the measured minor component is somewhat smaller, the m/z= 7 1: m/z= 70 ratio being approximately 1.O: 0.3. The expanded inset in fig. 2b shows that in addition to the unexpectedly large peak at m/z= 70 the group of peaks associated with the decomposition of the deuterated aniline does not have quite the same pattern of that due to the non-deuterated aniline. 317
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ated aniline, identified as mainly C5H: from “Nlabelled observations, is probably distributed between CSD: (m/z= 70), C,D,H+ (m/z=69) and C5D3H2+ (m/z= 68) in the deuterated system. The recorded m/z=70 intensity therefore probably contains some intensity due to C4ND3H: and C5D: ions. However, consideration of the probable intensities of these two ions indicates they are likely to be minor components of the overall intensity.
a) 266 nm focused
3.3. Me&stable m/z= 70 intensity as a function of reaction time
m/z
20
30
40
50
60 70 80 90 100
bl 266 nm unfocused
m/z
To check that the ion at m/z= 70 is indeed a product of the metastable decomposition reaction its intensity as a function of reaction time on the microsecond time scale was monitored. The laser beam was used focused for this experiment to allow comparison of the peak height of the suspected metastable product ion with peak heights of rapidly formed fragment ions with similar mass. This was necessary to separate out change in peak height as a function of reaction time due to time-lag focusing and ion loss effects [ lo] from change due to the occurrence of a microsecond time scale reaction. Fig. 3 shows a plot of peak height for m/z= 65 (from aniline in the mixture) and m/z= 69 and 70 (from aniline-d, in the mixture) as a function of time lag in the range O-2.5 ys. Under these focused conditions
1
20
30
40
50
60 70 80 SO 100
Fig. 2. 266 nm MPI spectra of an aniline/aniline-d, mixture at a time lag of 0.7 bs. (a) Focused beam, 0.18 mJ/pulse; (b) unfocused beam, 0.5 mJ/pulse, with insert of expanded mass range
m/2=65-72.
-
60’
E E nl
Z
.c 40' x
Most noticeably the m/z=67 peak in the non-deuterated system does not translate to an equivalent mjz=72 peak in the deuterated system. Comparison of aniline and aniline-15N 266 nm MPI spectra has shown that m/z=67 is almost 100% C,NH$. . In the deuterated system this ion is probably distributed between the ions &ND: ’ (m/z=72), C,ND,H+’ (m/z=71) and C4ND3H:’ (m/z=70). This explains the low m/z=72 intensity. Similarly the very small intensity m/z= 65 in the non-deuter318
a,
.---c-
20.'
m/r=69
0.5
10
1.5
20
2.5
time-lag(crs) Fig. 3. Plot of absolute peak height of m/z=65from aniline and m/z= 69 and m/z= 70 from aniline-d, as a function of time-lag delay. Focused 266 nm MPI at 1.0 mJ/pulse (ion loss effects important).
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m/z=65 is the rapidly formed CSH: fragment ion and m/z=69 is the deuterated analogue of this, i.e. CsD4H+. (m/z=69 cannot be a &(D,H)g’ metastable product ion as this would require three hydrogen atoms.) All three peaks, m/z= 6569 and 70 increase to a maximum value and then decrease as the reaction time is allowed to increase. The time-lag position of the maxima in the curves increase with increasing mass as would be expected from the combined ion focusing and ion loss effects [ lo]. However, the m/z=70 intensity, which at low reaction times is less than that of m/z=65, increases more rapidly as the reaction time increases, overtakes the m/z=65 intensity and remains the most intense ion at the longest reaction times recorded. This indicates clearly that even under these more highly focused conditions the m/z=70 intensity has a major component which is the product of a microsecond time scale reaction. 3.4 Measurement of the relative rate constants for the two aniline-d5 reactions It was impossible to measure the absolute rate constants for the two reactions identified in the aniline-ds system. To make an absolute rate constant measurement using this experimental method requires the measurement of a reference non-metastable reaction product ion to normalize and correct for ion focusing and ion loss effects as we have described previously [ 41. In measuring the absolute rate constant for the reaction C6H5NH: ’ + Cs HZ ’ +HNC in aniline, m/z=65 was taken as the reference ion [ 41. The analogous ion in aniline-d, is m/z= 70, which is a product ion, and therefore cannot be used as a reference ion. The relative rate constants of the two reactions were therefore measured in the following way. MPI spectra of the same aniline/aniline-d, mixture were recorded as a function of reaction time using an unfocused, low intensity (0.25 mJ) laser beam at 266 nm. We know that under these unfocused conditions ion loss is negligible at reaction times of less than 1.5 ps [ 111. Jon focusing effects due to time lag remain but are expected to be very similar for the ions under consideration as they are close in mass (m/z= 66, 70, 7 1). Fig. 4 shows a plot of the intensities of the three metastable product ions as a func-
m/z=66
.
1.0
2.0
time-lag (ps) Fig. 4. Plot of peak heights of m/z=66 from aniline and m/z= 70 and m/z= 7 1 in aniline-d, as a function of time-lag delay. Unfocused, 266 nm MPI at 0.25 ml/pulse (ion loss effects negligible).
tion of reaction time over the range O-2.4 ps. (These data have been adjusted so that they reflect the relative peak heights that would be observed in an exactly 1 : 1 aniline/aniline-d, mixture.) The peak heights bf all three of the metastable ions rise and reach a plateau as the reaction time is increased and the reactions approach completion. Assuming that the reactions are first order and can be represented by the equation A+B+C, where A is the molecular ion and B the metastable product ion, the appropriate integrated rate equation is B=B,(
1-epkr),
where B, is the intensity of the metastable product ion at infinite reaction time. The integrated rate equation can be used in the form -In ( 1 -B/B, ) = kt to obtain k by plotting -ln( 1 -B/B,) versus t. In this case no effort was made to compensate for timelag focusing and B, values for each reaction were obtained by averaging peak heights for the product at the plateau. Fig. 5 shows plots of -In ( 1-B/B,) versus t for each of the reaction products. From the slopes of the . < straight lmes obtained k,, z kb6 and k,o/k,, x 0.75. Taking the absolute value for kb6of ( 1.74 + 0.4) x lo6 319
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5. Conclusions Both the metastable and the rapid fragmentation reaction C6H,NH: . -+C5H6+. + HNC in aniline have been shown to have two components. The major component involves the loss from the molecular ion of a hydrogen atom originally attached to the nitrogen atom of the neutral molecule while the minor component involves the loss of a hydrogen atom attached to the ring. In aniline-2,3,4,5,6-d, the rate constant for the loss of DNC has been found to have a value equal to approximately 0.75 that for the loss of HNC. time-lag@s) Fig. 5. Plotsof -ln( 1-B/B,) versus reaction time for m/z=66, m/z=‘lOand m/z=71. (Seetext.)
s- ’ reported previously [ 41 this gives k,, x 1.7X lb6 s-l and kTO%1.3 x lo6 s- ‘. The smaller rate constant for the production of C5D4HZ+’ as compared to CSDSH+’ in aniline-& is not surprising as this reaction must involve the breaking of at least one C-D bond.
4. Mechanistic implications The ratio of the metastable HNC : DNC loss recorded for MPI at 266 nm is approximately 1 : 0.3 and the corresponding rapid decomposition (electron impact) is approximately 1 : 0.4. Complete randomisation of the H and D atoms in the molecular ion before decomposition would lead to a value of HNC : DNC loss of 1 : 2.5. The experimental data indicate that this is’clearly not the case. A mechanism which involved randomisation of the H atoms attached to the N atom and the two ortho H atoms would lead to a value of HNC : DNC loss of 1 : I. Again, this does not fit with the experimental data. It seems most likely that the recorded data indicate the existence of two reactions, possibly with different threshold energies, which contribute to the overall products.
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Acknowledgement The author would like to thank Dr. Tomas Baer for helpful discussions and corroborating experimental data taken on the aniline-2,3,4,5,6-& system.
References [ 1 ] T. Baer and T.E. Camey, J. Chem. Phys. 76 (1982) 1304. [ 2,] R. Proch, D.M. Rider and R.N. Zare, Chem. Phys. Letters 81 (1981) 430. [ 3 ] H. Kuhlewind, H.J. Neusser and E.W. Schlag, J. Chem. Phys. 82 (1985) 5452. [4] J.A. Zimmerman, R.M. O’Malley and J.E. Weinzierl, Intern. J. Mass Spectrom. Ion Processes 76 (1987) 257. [ 5) P.N. Rylander, S. Meyerson, E.L. Eliel and J.D. McCollum, J. Am. Chem. Sot. 85 (1963) 2723. [ 61 C. Lifshitz, P. Gotchiguian and R. Roll&, Chem. Phys. Letters 95 (1983) 106. [7] P.C. Burgers, J.L. Holmes, A.A. Mommers and J.K. Terlouw, Chem. Phys. Letters 102 (1983) 1. [ 81 R. Flammang, P. Meyrant, A. Maquestiau, E.E. Kingston and J.H. Beynon, Org. Mass Spectrom. 20 (1985) 253. [ 91 W.B. Martin and R.M. O’Malley, Intern. J. Mass Spectrom. Ion Processes 59 (1984) 277. [ lo] W.B. Martin and R.M. O’Malley, Intern. J. Mass Spectrom. Ion Processes 77 (I 987) 203. [ 111 J.A. Zimmerman and R.M. O’Malley, unpublished work