Excitation and dissociation of 3-chloro-3-methyldiazirine and 1

3184

J. Phys. Chem. 1980, 84,3184-3187

Excitation and Dissociation of 3-Chloro-3-methyldiazirine and I-Pyrazoline by Low-Energy Electron Impact Michael T. H. Liu," Department of Chemistry, University of Prince Edward Island, Charlottetown, Prince Edward Island, Canada C 1A 4P3

Tohoru Tanaka, Takahiro Hlrotsu, Kiyoshl Fukul, I w a o Fujlta, and Keiji Kuwata" Department of Chemistry, Facuity of Science, Osaka University, Toyonaka, Osaka, 560 Japan (Received: December 16, 1979; In Final Form: June 25, 1980)

Emission spectra of the excited nitrogen molecule in the C3n,state, the excited CN radical in the B2Z+state, and the excited CH radical in the A2A state were observed by electron impact on 3-chloro-3-methyldiazirine and 1-pyrazoline. On the basis of the threshold values in the excitation functions for this excited fragment the lower limit of heat of formation of 1-pyrazolinewas evaluated. The spin multiplicitiesof the excited precursors or fragments of dissociation were deduced from the shape of excitation function curves near the threshold. The vibrational structure of the excited nitrogen molecules formed in the dissociation of 1-pyrazoline and 3-chloro-3-methyldiazirineshows more vibrational excitation of the nitrogen molecule formed from the former as compared to that from the latter.

Introduction The emission spectra of excited atoms, molecules, and ions from simple inorganic1 and organic molecules2 obtained by using electron-molecular crossed-beam techniques have been investigated extensively. In these studies the products could be detected with high sensitivity provided they were in the excited states and had allowed radiative transitions. Excitation by electron impact has a different selection rule3 from that of photon i m p a ~ t ; ~ excitation from ground-state singlet to states with higher multiplicity is feasible in electron impact. Spectroscopic studies on the emission from molecules under electron impact provides useful information regarding the highly excited states of these molecules and the fragmentation from these states. The relative yields of the fragment ions have been determined from the emission of the excited species, and fundamental knowledge of the various primary processes of electron impact can be obtained. In the fragmentation of 3-chloro-3-methyldiairine and 1-pyrazolineby low-energy electron impact -0.48-1.93 MJ mol-l (5-20 eV) the emissions from N2(C3TI,), CN(B2P), and CH(A2A)and the threshold energies were measured. The nature of the primary processes in the formation of these species from the parent molecules is discussed in the light of their relative cross sections, threshold energies, and thermochemical and kinetic data. The relative distribution in the vibrational levels of the excited nitrogen molecules (C311,) formed from 1-pyrazoline is discussed on the basis of the experimental data on the vibrational structure of the emission. Experimental Section 3-Chloro-3-methyldiazirine was prepared from acetamidine hydrochloride according to the procedure of Graham.5 Since the material is explosive, only millimole quantities were prepared and it was stored at -78 0C.6 Oxidation of pyrazolidine with mercury oxide in heptane' gave 1-pyrazoline (5.2 X lo3 Pa) which was stored at -78 "C. The NMR spectrum of 1-pyrazoline sample showed the absence of a detectable amount of byproductas The electron impact study was performed under constant electron current by using an apparatus designed for electron impact at low energy.gi10For the detection of light 0022-3654/80/2084-3184$01 .OO/O

emission from the fragments of the electron impact the photon-counting technique was employed. The width of the electron energy was estimated to be ca. f0.012 MJ mol-l (k0.12 eV). Calibration of the electron energy was done with molecular nitrogen, using the peak positionll of the excitation function for the C311, state of the nitrogen molecule. Although the electron energy was corrected only with the nitrogen molecule the electron-energy scale has been shown to be reliable in the energy region of the present study. This was observed by comparison of the experimental results with reference data for small molecules such as CO and NH2. The shift of electron energy due to the change in the work function of metal electrodes could be kept constant, if the effect of the adsorbed molecules from the gas phase is small. The shift due to the space charge is practically constant under constant electron current. For the diazirine compound the electron energy tended to shift gradually. This was due to the change in the work function of either the stainless-steel electrodes or the oxide cathode by adsorption of the diazirine molecule on the surface. After a long exposure to the diazirine molecule the electron energy became constant. Thus, calibration of electron energy could be established. This unsteady situation was absent in the case of pyrazoline. Product analysis by mass spectroscopy or gas chromatography was not done in the present study. Results and Discussion The emission spectrum of the diazirine compound in the region 300-450 nm is shown in Figure 1. In addition to the emission from the excited CN and CH radicals, relatively weak emission bands at (l,O), (O,O), and (0,l) for excited nitrogen Nz(C311,) were observed. With higher resolution, bands at (2,3) and (1,2) were also seen. The (0,O) emission band of the excited nitrogen at 337.1 nm rises steeply a t an electron energy of 1.21 MJ mol-l (12.5 eV). Following a maximum at 1.37 MJ mol-' (14.2 eV) it decreases gradually (Figure 2). The intensity of the CN(B2X+-+X2Z+)emission at 386 nm increases gently and changes its slope at 1.69 MJ mol-I (17.5 eV) (Figure 3, upper). A threshold energy of ca. 1.35

0 1980 American Chemlcal Society

The Journal of Physical Chemistry, Vol. 84, No. 24, 1980 3185

Electron Impact Study of Molecular Dissociation

DI a z I r ine

..

CN ( B-XI

.:

.' L

..

k>

a a a

t m a

U

Y

>

t u)

z W

4

400

350

300

I

WAVELENGTH /nm

z

Figure 1. Emission spectrum observed in the electron impact of 3-chloro-3-methyldiazirine at an electron energy of 2.2 MJ mol-' (23 eV). Pressure in the collision chamber was 4.7 X I O 4 Pa (3.5 X IO-' torr), and the bandpass; of the monochromator was 10 nm.

CH3, N, Cl,l, CI N

L

10

'r:

*

;.

.

.*

I

12

IO

N, (C311)

-+

14 16 ELECTRON

18 20 22 ENERGY /eV

Figure 3. Upper, excltation function curve for the exclted CN (B22+) radical famed from : M l o r ~ t h y l d i i z i r i n e .Lower, excitation function curve for the excited CH (A'A) radical formed from 3-chloro-3methyldiazirine.

.

. '

,, ...e*.

*.

,

I

20 ELECTRON

1

I

I

I

I

e'.

I

I

30

I

,

,

I

40

ENERGY /eV

FI ure 2. Excitation funlction curve for the excited nitrogen molecule (C 11,) formed from 3-c:hloro-3-methyldiazirine.

0

M J mol-l (14 eV) was obtained by extrapolation of the curve in the range of electron energy from 1.5 (16 eV) to 2 MJ mol-l (21 eV). The relatively strong emission of the CH(A2A+X211) transition at 431.5 nm is shown by the curve for the intensity, which seems to be partitioned into two slopes above and below 1.'7 MJ mol-l (18 eV) (Figure 3, lower). A threshold energy of ca. 1.3 MJ mol-l(13.5 eV) was obtained by extrapolation of the linear part of the curve with an electron energy ranging from 1.3(13 eV) to 1.7 MJ mol-l (18 eV). From mass spectral data,5 in which N2+was the most intense peak, emission from either N2+or N2excited states could be expected at an electron energy of several tens of an electronvolt per mole. This electron impact study was performed by mass spectroscopy. Only the relatively weak emission bands from N2(C3nU) were observed at an impact electron energy of 2.4 MJ mol-1 (25 eV). The weakness of these emission bands is probably due to the partitioning of the reaction energy into several vibrational sublevels of the C311, states. Emission from the N2+ion would be

observed at a higher electron energy than the threshold ionization energy of N2. The threshold value of 1.21 MJ mol-' (12.5 eV) for the N2 emission is clearly higher than the energy for the excitation of N2 from the singlet ground state X12;,+to the C311, state by 0.15 MJ mol-l. This indicates that pE15 0.15 MJ mol-l, where AEl is the dissociation energy, for the following hypothetical processes:

' 1 AN -

CH3\

CH3 C:'

CI

CI

N2(X12;,+) N:1(C311,)

/

+

N,(XZ' :)

A€,

(11

AE2 = 1.06 MJ mol-l l1 (2)

threshold energy I,hEl

+ AE2

(3)

These processes are given for the convenience of calculation of the threshold energy and do not mean that a two-step formation of N2(C3n,) actually occurs. In the kinetic measurements of the thermal decomposition of the diazirine compound, a value of 130 kJ mol-I for the activation energy E, has been obtained.12 This indicates that AEl I E, = 130 kJ mol-l, and the value agrees with the present value obtained by electron impact. The excitation cross-section curve shown in Figure 2 for the (0,O)band of excited nitrogen at 337.1 nm quickly rises to a maximum of 1.35 MJ mol-l (14 eV). At this point the emission intensity decreases and then increases again above an electron energy of ca. 1.9 MJ mol-l (20 eV). The curve could be reproduced by the superimposition of a sharp peak at 1.37 MJ molW1(14.2 eV) and a gentle rise which starts above the threshold. The sharp peak, just above the excitation threshold in the excitation crosssection curve, is known to be a characteristic feature of an

3186

The Journal of Physical Chemistry, Vol. 84, No. 24, 1980

Llu et al.

/I I I

1

1

1

1

1

1

1

1

1

20 ELECTRON

IO

1

1

1

,

1

30

1

1

1

,

40 /eV

ENERGY

FI ure 5. Excitationfunction curve for the excited nitrogen molecule (C formed from 1-pyrazoline.

!nu)

I-pyratoline CN(B-X)

300

I

I

I

350

400

450

..

WAVELENGTH / nm

..

Flgure 4. Emission spectrum observed in the electron impact of 1-pyrazoline at the electron energy of 2.25 MI mot' (23.3 eV). Pressure in the collision chamber was 3.3 X Pa (2.5 X torr), and bandpass of the monochromator was 20 nm.

electron exchange excitation from the ground state to a state with a different spin m~ltiplicity.l~-'~ Thus, the spin state of excited 3-chloro-3-methyldiazirine is shown to be a triplet. If the compound is in the triplet state, then the fragment CH3CC1 formed by the dissociation of the excited diazirine compound might be in the singlet state. The other fragment is an excited nitrogen molecule (C3&). The broad rise of flat extent in the cross-section curve above the threshold has been identified as a feature which results from excitation to a state with spin multiplicity similar to the ground state. Thus, according to the spin conservation rule, when the excited diazirine compound is in the singlet state the spin state of the fragment CH3CCl might be triplet.ls It follows that the threshold value for this excitation cannot be given due to the superimposition of the sharp peak near the threshold. The excitation giving the fragment CH3CC1in the triplet state is predominant in the energy region higher than 1.9 MJ mol;' (20 eV), whereas the process for producing singlet CH3CClis most important near the threshold energy. The excitation cross-section curves for CN and CH (Figure 3) show emission intensity gradually increasing up to 2.2 MJ mol-l (23 eV). In the emission spectrum of pyrazoline with an electron energy of 2.25 MJ moP(23.3 eV) (Figure 4),three emission bands (l,O), (O,O), and (0,l) of excited nitrogen N2(C31T,) were observed in addition to bands of the CH and CN radicals. The excitation cross-section curve shown in Figure 5 for the (0,O) band of excited nitrogen at 337.1 nm has a threshold value of 1.19 MJ mol-l (12.3 eV) with a maximum of 1.37 MJ mol-l (14.2 eV). On the basis of this curve, the spin state of excited pyrazoline was inferred to be a triplet. Hence, the fragment molecule might be in a singlet state. The energy of dissociation AE3 in the following processes was estimated to be AI%, I0.13 MJ mol-l with the observed

3

40

30

a a

- I-

>

CH ( A - X )

t v)

Z

w

c

z

-

.. C . '

0

i

LJ

...p.. ...

*

*.e

I

I

I

IO

I

I 20

I

I

I

ELECTRON

I

I 30

I

ENERGY

I

I

I

I 40

I

I

/eV

Figure 8. Upper, excitation function curve for the excited CN (B2z+) radical formed from 1-pyrazoline. Lower, excitation function curve for the excited CH (A2& radlcal formed from I-pyrazoline.

value of 1.19 MJ mol-' for the threshold energy. The low limit for the heat of formation of 1-pyrazoline in the following eq 4 and 5 was found to be 0.15 MJ molT1 H

A CH ,z

HZ

L

-

-

CH,,

+

N,(X'Z,)

AE,

(4)

N

N2(X1B,)

Nz(C311,)

.hEz = 1.06 MJ mol-l (5)

threshold energy 2 AE3 + AE2

(6)

if the experimental threshold energy of 1.19 MJ mol-l and the estimated value of 0.28 MJ mol-l for the heat of for-

~

Electron Impact Study of Molecular Dissociation

Figure 7. Vibrational structure of the emission from the excited nbogen molecules

(~~14).

mation of a lbiradical product C3H6were taken into account. Procedures a and b assume that the biradical AHfO(biradical)= iVIfO(propane) f 2E(C-H) 2AH?(hydrogen atom) = --24.8219 + 2 X 98.01’ - 2 X 52.095l’ = 67.0 kcal mol-l (a) AHfo(biradical) = AHfo(cyclopropane) + E(C-C) - SE = 12.74l’ + 83.2l’ - 27.620 = 68.3 kcal mol-l (b) product C3H6in the! dissociation of pyrazoline is a linear species -CH2CH2CHs..Consequently, the value for its heat of formation M,“(biradical) is estimated to be 280 kJ mol-l. E(C-H) is the bond energy of the C-H bond in hydrocarbons, E(C-C) is the bond energy for the C-C bond in hydrocarbons, and SE denotes the strain energy of the cyclopropane ring.20 The excited pyrazoline was deduced to be in the singlet state in the energy region higher than 1.9 MJ mol-l(20 eV) for the impact electron. Thus the spin state of the product C3H6might be triplet. The possible threshold value for this excitation could be slightly higher than that of the excitation producing the sharp peak. The excitation cross-section curves for CN and CH (Figure 6) display

The Journal of Physical Chemistry, Vol. 84, No. 24, 1980 3187

emission intensities which gradually increase up to 3.9 MJ mol-l (40 eV). Both curves show the same excitation feature regarding the spin state. Figure 7 shows the emission spectra of nitrogen molecules in the electron impact of 3-chloro-3-methyldiazirine, 1-pyrazoline, and nitrogen. In the case of 1-pyrazoline, when the intensity of the (0,O) band was compared with the intensity of the (1,O) band (where the weak (2,l) band is partly superimposed), the ratio of population of the u’ = 1level to that of the v’ = 0 level was estimated to be 0.86 by using the branching ratio of 0.520, calculated from the theoretical Franck-Condon factors for the transitions. The same procedure, applied to the electron impact of 3chloro-3-methyldiazirine,shows the population ratio to be 0.77. The intenuity ratio of the excited nitrogen molecule became constant at 0.46 in the region of electron energy higher than 1.46 MJ mol-l (15 eV). From this value the population ratio was estimated at 0.66. It follows that the population ratio of the u = 1 level to the u = 0 level decreased in the sequence of l-pyrazoline, 3-chloro-3methyldiazirine, and nitrogen. The upper limits for energy of dissociation for 1pyrazoline and 3-chloro-3-methyldiazirine were found to be similar. Hence, in the dissociation of 3-chloro-3methyldiazirine, the partitioning of the energy content into the vibration of the N-N bond is poor when compared to that of pyrazoline.

References and Notes (1) For example, 13%F. Holland and W. B. Maier, 11, J . Chem. Phys., 58, 5229 (197:!);H. Buberl and F. W. Froben, J. Phys. Chem., 75, 769 (1971). (2) For example, C). A. Vroom and F. J. de Heer, J. Chem. Phys., 50, 573 (1969);K. Fukui, I. Fujita, and K. Kuwata, Bull. Chem. SOC. ~ p n . 45, , 2278 (1972). (3) For example, J. P. Doering, J. Chem. Phys., 42, 395 (1965). (4) A. H. Laufer and H. Okabe, J. Phys. Chem., 78, 3504 (1972). (5) W. H. Graham, J. Am. Chem. Soc., 87, 4396 (1965). (6) M. T. H. Llu, Chem. Eng. News, 3,(Sept. 9, 1974). (7) R. J. Crawford, A. Mishra, and R. J. Dummel, J . Am. Chem. Soc., 88, 3959 (1966). (8) R. J. Crawford, R. J. Dummel, and A. Mlshra, J. Am. Chem. Soc.,

87, 3023 (196!5). (9) K. Fukui, I. Fujita, and K. Kuwata, Shitsuryo Bunsekl, 23, 105 (1975). (IO) K. Fukui, I. Fujlta, and K. Kuwata, J. Phys. Chem., 81, 1252 (1977). (11) T. 0. Finn, J. F. M. Aarts, and J. P. Doerlng, J. Chem. Phys., 58, 5632 (1972). (12) M. R. Bridge, H. M. Frey, and M. T. H. Liu, J. Chem. SOC. A , 91 (1969). (13) J. R. Oppenhelmer, Phys. Rev., 32, 361 (1928). (14) H. S.W. Massoy and C. B. 0. Mohr, Proc. R . SOC.London, Ser. A , 132, 805 (1931). (15) W. G. Penney, Phys. Rev., 39, 467 (1932). (16)R. D. Bates, A. Fundamlnsky, H. S. W. Massey, and J. W. Leech, Phil. Trans. R . Sac. London, Ser. A , 243, 93 (1950). (17) V. I. Ochkur, Sov. Phys. JETP, 18, 503 (1964). (18) Cf. K. J. Laiiler, “The Chemical Kinetics of Excited States”, Clare& Press, Oxford, 1955,p 21. (19) H. M. Rosenstock, K. Draxl, B. W. Seiner, and J. T. Herron, J. Phys. Chem. Ref. Data, 8, Suppi. No. 1 (1977). (20)J. N. Knowlton and F. D. Rossini, J. Res. Nafl. Bur. Stand., 43, 113 (1949).