Temperature-dependent reaction kinetics of NH (a1Δ)

1. Introduction The reactionsof NH(a’A) have attracted experimental [l-19] and tbcoreticaI[ZO-241 interestover the past decade, primarily as a rest& of their utility in exploring the mechanistic details of their gas- or Iiquid-phase reactio& with hydrogen and simple hydtrocarbons;experimentaland theoretical comparisons with NH(X32-) reactions [25-333 are also of interest. NH(a’A) reaction rates have been measured for some gas-phase [1,2,4-7,9,12,19] and liquid-phase [8,1Ct,ll,13,15,16-181 reactions, but uncertainties in the mechanisms. have hampered comparison with the isockctronic species, CH&At) and G(‘D). Ab initio SCF [20] and CI (21-241 caIcuIations have been .us&I to gain ~tiechtkistic insight by .energy pathways for searching for minimum NH(X38-) and NH(a’A) reactions with hydra. gen, methane, and ethylenc_‘GcncraIIy,NH(a*A) is predicted to insert into hydrogen and cycloadd to ethylene with no energy barrier [21]_- However, gas-phase experimentaI evidence [9,12]-_suggests -that hydrogen abstraction from aIkanes is a cornpetitive channd in spite of substam@ activation’ barrierswhich are predicted [22,24] for the H, and CH, -cuom& .. _ -~ -’

’ NRL/NRC

Postdoctoral Rescarcfi,&~&~-

: .‘. ..

:

In liquid-phase reactions of olefms .wiih NH(a’A), addition to doubIe bonds, insertion into. CH bonds, and electronic quenching have been inferred from experimental observations [~0,15]. Insertion aIso appears to play a role in the IicIuidphase isobutane reaction, and electronic quenching has been postulated in aim&r liquid-phase rcactions[S]_-Competition among the insertion, abstractiod, and ckctronic quenching channels in gas-phase NH(a’A) reactions is undetermined relative .to analogous CH,(‘&,) and O(‘D) reactions [3&37], i: and may possibly be revealed in temper-at&-de-pcndence studies, which have not been previously undertaken.We report activation energiesand second-order rate constants. for gas-phase NH(a*A) with simple hydrocarbons. Results’are compared with available theoretical predictions and the., mechanistic conclusions of. other eXperimental ..~ studies_ --.. ._ . . :NH((X3&) and me&able [38,39] Y NH(a’A) were produced by 266 run photolysis of hydrazine ~_~.. 2, Experimental -.

176

J-W.

Coxamf

/ Tcmpcmnm -&pen&~

_and hydrazoic acid respectiveIy_The hydrazoic acid. prepared according to a standard technique [40k aas stored in 10 E #ass buIbs_ No pusification of HN, was attempted. since mass spectral anaIysis revealed onIy a negible amount of NH, as an impurityThe precursor and reactant gases were diluted with helium to obtain proportions appropriate for optimaI kinetic scans. and to minim& NH(atA) iosses from cohisions with HNs_ Helium is known to be a poor NH(&) quencher [39]_ Typical gas ciiiutions were 1 z 100 HN,: He and 1: lo-20 x-eactant I He_ Calibrated Rowmeters and TyIan elcctronic mass flowmeters were used to adjust partiaI pressures while slowly flowing the component gases_ The totltl reactant ceII pressure was brought to 5 Torr with helium for most experiments, and was measured with an MKS capacitance manometer_ The experimental cell is a stainless-steel cross enclosed in a forced convection oven. The photolysis Iaser (Quantei Ndr YAG. fourth harmonic. -20 ml at 266 mu) and probe laser (lambda Physik XeCI excimer-pumped FL2002 dye laser, frequency-doubkd sulforhodamine 640) beams

counterpropagate coIIinearIy tbrougb the cell_ T%e probe laser excites either the NH(c’H * a’A) O-O Q(2) transition at 325-78 nm or the NH(A311 6

X 32-) 1-O R,(O) transition at 304.85mn. thereby interrogating NH(ah) or NH(X%-) concentratiOIlS_ scanning of the ffuoresccn ce excitation laser at varying pump to probe laser delays demonstrated

the NH to be vibrationally and rotationally thcxmakcd to the ambient c&I temperature at relative delay times appropriate to the experimental measurements_ Scattered Iaser Iight was separated from the probe Iaser induced fluorescence sigmd by combinations of solution. dielectric and glass fibers. The photolysis laser puke served to trigger the boxcar integrator (PAR model 160) The boxcar integrator detection gate and the probe laser firing are slaved together_ The delay between the photolysis laser and the detection gate is scanned using the intemaI boxcar scan_ The L.lF signai thus probes the radical concentration as a function of time following the photolysis puke_ The HN,

rmaicnz kinaia

of NH(a ‘A]

precursor concerktion was aIways maintained at Ieast ten times below that of the reactant g& to ensure pseudotirst-order reaction conditions. Careful decay mcasurenients were made in the

absence of reactant gas to ensure that neither radical-radical

reactions nor radical diffusion con-

tributed significantly to the measured decay &mes_ A h&oratory minicomputer controlled the data acquisition and storage, and carried out analysis of the pseudo-first-order signal decays Measurements of radical disappearance rates as a function of reactant gas pressure provided the second-order rate constants_ These experiments were carried out as a function of temperature in the range 255 c T < 600 K to determine the dependence of the reaction rates on temperature_ The temperature dependence of the bimolecular rates were fit to a simple Arrhenius model. k, = A exp( - EJkT).

This reaction rate has been measured by numerous authors by several techniques including chemihunin escence, flash photolysis and laser pump and probe experiments similar to those described here_ We have remeasured the rate at room temperature to cahbrate our technique against prior studies The rate we m easurc (131 f 024) x lo-‘Ocn? s-I_ is compared to the several earlier measurements in table l_ The present value is in reasonable agreement with earlier studies by Paur and Bair [2] and by McDonald et al. [.5], and with the much more recent measurements of Rohrer and StuhI [39]. AII these measurements are slower than and outside the reported uncertainties of the rate reported by Piper et al. [7& Because this is a very fast reaction at room temperature, we have chosen not to study its temperatrrre dependence_ 3.2. NH(a ‘A) reaction with H,

This reaction should be the prototypical reaction for study with the NH(a’A) radical. It is the most tractable for detailed theoretical study, and has been carefully considered in ab in&o SCF Ci

__._J_W_-Cox 2r& /. Tmpu&re+pad&~

cai&tions by Fueno _and &-workers in recentpapers 121,241.The& is also a large btiy of both theory and eq&iment on the isoeI&~&i~ ieacargution of O(‘D) 4 Hz + OH + H_ - MqdeF ments based both on experimental and theoretical evidence make a strong case for the

reaction

conclude that this reaction has a zero activation energy to the formation of NH, which lies at

136 k&/mole

lower energy_ Fueno and co-

[22,24] have also considered the abstraction process. They conclude that the reaction pro-

workers

ceeds

maintaining

a planar C, structure along the

reaction coordinate The formation of the transition stafe implied in fig- 1 is calculated to require an activation energy of 83 kcal/mole_

Both the insertion and abstraction reaction channeIs would be expected to lead either directly or indirectly to formation of NH= + H in the gas phase as the energized NH, molecule formed by Tzbie 1 GsphYe ColIisicm

rates Jmd Arrhenius

.p$& ..T

4E=136

1-_.

a’ NH

i

-.

-E 83

l:;.

: ! ; :

..

_~

_ -_

y

Ref. this work -’ 121

iSI

I’

NH3

Niioh)f CH4 . .c-\ : T :T : 4Ei227.6 ‘.. f .--

nE=IIz

_I

;

: : :

EO=I56 Nn;?*=H3

.:

C”3NH2

Fi&1. !%hematic representations fortheNH(a%)

insertion and

abstractionrcacfionswith hydrogen(top) and methane (bottom). The produclsand acrivation enetgy barriers depict the

calculations in refs

[21,24]_ Exothcrmicities are cakulated

10 pnmcdwithout anxti%ation energy_

the insertion reaction lies 28.2 kcaI/mole above the NH2(X’B,)+ H product channel. Within experimental error, the temperature-dependent

A (10-“cm-’

s-1)

13.1 k4.8 93 93 50-9

E (kcal/moIe)

s-1)

-

1s 1-o t2 0.46* 0.04

-

-

Hz

0.34 +: 0.02

6.8 + 0.6 -

1_54+0.10 -

CH.

Cl91 this work -’

0.38 f OXU

8.3 fO_6

1.87 +O.OS

I-2 *02

CA

is] thisxwxk=’

cis-2-burene methyl &etyIene ~Auunanain

&is a-ark 1’ 151 this work 0) &i&O&.-’

41 88

-II

ti& fmm &is work are scdstical only and arc ~20.

0.83 f0.10 -0_26*0_18 -

16 _t2 6.0&U

&03 *O-S

3.8 +rO_8 .25 c4 14

from

cq3cliwnLal thermodynamic dara [43.44] and are ConsisePt with the ab initio results_The addition rexdons are cdculaccd

El this work a)

CZH,

i . -.i_ -:I

.’

L;

k(30025 K) (Io-“cn?

-~~~.~77.’

2 *l-l

panme ters for reactions of NH(rh)

Pm= Hw

NH(a’&+.H;!.-______L_

of

o(‘D) with H, proceeding by both insertion and abstraction mechauisms [4X42]_ The energetics of the NH(a’A) reaction are shown in fig. 1, r;ueno et al. consider the insertion reaction to be the typical example of an orbitalsymmetry-allowed process (21]_ It is calculated to proceed along a minimum energy path to directly insert the N atom between the two hydrogen atoms maintaining a C, or Cz, symmetryFueno et aL

B

ya&ion kid&s of NH(a ‘A)

.:r-.: ..

_I..,

_ 0th~

25

is

Q_Q *o-15

17

24

0.0 *02

err=s.

shere awilahle~sre

f2u-

~.

~.

data measured in this work can be fit by a simple Arrhenius form with an activation energy of l-54 + O-f0 kcd/moIe and pre-exponential A factor of (6_sio_6)x1o-‘rcm3 s-r_ it is somewhat disaancerting that the measured room-temperature rate constant and activation energy are not in accord with the experimentrd results for O(‘D) j35] and CH,(‘A,) [34] and the calculation of Fueno et al We see no reason to expect, a priori, that there shouId be a positive activation energy associated with the insertion channel- Based on our measurements we are not, however_ able to distinguish between an entrance channtf barrier for the insertion reaction and a Iower barrier to abstraction than that calculated by Fueno- Considering the data and the fit to the Arrhenius plot, we conclude that a competing, zero activation energy reaction cannot account for mom than z: 15% of the total reaction at room temperature- Because of this, and by anaI~~y to the reactions of O(‘D) and CH,(‘A,) we prefer the insertion mechanism for this reaction-

We studied the temperature dependence of the NH(a%) reaction with methane and with propane as typicaI of huger linear aIkanes_ The reaction with CH, is very similar to that with H,, i-e_. we observe a room-temperature rate of (3-S If 0-4)x I(-)-‘Z&f s-1 and an activation energy of 1.87 + O-05 kcaI/mole The Arrhenius plot for the temperature dependence of the second-order rate is shoun in fig 2_ The NH(a%) reaction with methane has aIso been studied theoreticahy by Fuemo et aI_ [24]_ The transition state for the abstraction appears much Iike that of the H, case- Fucno concludes that formation of the transition state is associated with an activation energy of 13-6 kcaI/mole FVesumably the insertion channeI is associated with a zero activation enerSS as was computed to be the case with the Hz reaction The ene~~etics of the two reaction paths for CH, are depicted in fig 1 and compared with those of Hz_ The formation of the energized methyhunine complex formed by the insertion channei Iies well above the CH, f NH, product channel (-27-6 kcaI/moIe)_ In the gas

4c3f+e

5

4-

20

24

28

32

36

4.0

phase, although one might expect a longer RRKM lifetime for the complex. one would expect that it wouId be difficult to stabilize the insertion product against dissociation_ Because of the increased probability of stabilizing the CH, energized adduct over that with H,. we extended the temperature-dependence: studies to -17“C_ The experimental data are consistent with a simple Arrhenius mechanism These observations again argue against a significant (> 25%) zero activation energy insertion channei for this reaction at room temperature_ The reaction of NH(a’A) with propane was studied as being typical of the gas-phase reaction

-._. lf_k_ Car et aL /

T&peratu&-went -.

of larger alkanes. At room temperature the rate is .>~. about
reaction f&tics

4

NH& ‘A)

,. _

: .. _:._ _. _.. _- __? -:.

ii9

with _tbose calcuIated_.The higt?-pressure~me+urements of. Fueno- and co-workers [9,12]‘&e inter:preted as m&ii&g a substantial contribution from the insertion channeI_ We are .precluded in these studies-. from. operating at .’ l&h &-bmues by quenching bf the NH(c’lT) fluores&&_

.-

3.4_-hW(a ‘A) reaction with oie&s The temperature dependences of the reactions of NH(a’A) with ethylene, cis-2-butene and methyl acetylene were measured_ The room-temperature rates, activation energges and A factors are shown in table 1 and the Arrhenius plot for cis-2-butene is shown in fig 2. The reaction with each of the olefins is very fast, taking place at near the gas kinetic collision frequency_ While there is some scatter in the data, the temperature coefficients (300-600 K) for each of the reactions is consistent with a zero activation energy process. The NH(a’A) reaction with small olefins has been studied in the condensed phase [10,15,18] by final product analyses_ These authors have concluded that addition to double bonds and insertion into C-H bonds are active reaction channels. Jacox and Milligan [45] have observed stabie adducts consistent with cycloaddition across the C-C double bond in matrix isolation studies. Fueno et al [Zl] in *heir extensive ab initio, SCF CI study have carefully considered the reaction of NH(a’A) and NH(X32-) with ethylene_ The cycloaddition of NH(a’A) across the double bond, where the ethylene carbon atoms and NH retain a C, symmetry along the reaction coordinate, is a completely orbital-symmetry-allowed process to form the cyclic ethylenimine-This reaction is concluded to proceed without an activation energy barrier and is = 103 ksal,/moIe exothetic. Whether the. adduct can be collision&ly stabilized in the gas phase or whether it fragments is unclear from our studies. The measured ma&on rates are independent of total pressure in the l-20 Torr range_ The reaction rates with acetylenes and olefins are significantly faster than reactions with similar aIkanes_ This observation combined with the zerotemperature coefficient for the reactions leads us to favor the mechanism . invohing .-addition of

: j

NH(a’A) across the carbon-carbon multiple bond_ This addition could be followed by coIIisionaI stabilization of the cyclic addition product, or hytiOcn migration and formation of a xu-iety of stable products or radicals:

Tabk 2 NH(h) - NH(X3Z) quenchingratesfor coUision with rcacthr and non-reactive gases

w 3q AK Ht

AH = - 107 kcal/mde; -

H&=NH

-f- CH,,

AH = = 40 f 20 kcaI/moIe; -

NH2--CH=CH,, AH = - 114 + 5 kcaI/moIe;

-

HN=CH-CH,. AH = - 113 + 20 kcaI/moIe;

-

NH2 f C-H,, AH = - 24 i_ 5 kcaI/moIe.

3_5_ Fornuuiorz and raction of tVH(XJZ-) Because ehzctronic quenching. NH(a ‘A) + NH(X32-), has been concluded to be a relatively efficient process in scveraI of the NH(a’A) reaction-product studies d&usscd in sections 33 and 3-4 we carried out a few studies of both cohisioninduced intersystem crossing and relative rcactivities of the X(32-) state Drozdoski et al_ [6] studied the reaction of NH(a’A) + 02(%-) by monitoring the rise time of NH(X32-) by Iaser-induced fIuoresccnce_ They conduded that the rate is 155 X lo-” ems s-r_ We have ofthisprocess repeated these measurements using HN, as a precursor and arrive at essentiaIIy the same vaIue for the rate Rohrcr and StuhI [39] have recently estimated the collision-induced intersystcm cross ing rate for NH(ah) by Ar and He to bc in the range of (l-10) x lo-t6 ems s-t by monitoring the quenching of the NH(a’A)+ NH(X32-) phosphorescence Using laser-induced fhtorescen ce to monitor NH(3Z-) we cannot detect production of NH( 32-) from quenching of NH(a’A) by either of these rare gases The Limit of sensitivity which we expect for our measurements is = 5 X lo-l6 cm3

)

Ref.

kJcn2 s-1)

this work [6]

155X IO-” (1-10)x10-*~ <1-IO)xlo-‘~

1391 t391

a.

thisawk

HL

this wxk

<5x10-‘4

C3Hs

this

<4x10-‘3

-=4x10-”

a-cxk

based upon the signals observed for the NH(X 32-) production from Xe coIIisions. In similar experiments we attempted to detect NH(X32-) from the quenching of NH(a’A) by collision with Hz, CH, and C,Hs. Again. we could detect no laser-induced fIuorcscence signal from ground state NH_ In table 2 we have estimated the upper limits to the rates based upon these experiments_ There are relatively few measurements of reactive rates or NH(X3Z-) avaiIabIe_ The reaction rates with hydrocarbons arc considerably lower than might be expected based upon comparison with reactions of isoelectronic species q3P) and CHI(3A)_ In preliminary measurements of NH(X32-) with both aIkanes and olefins we estimate the rates to be considerably Icss than lo-l5 ems s-’ at room temperature. Preparations are currently underway to study these rates in a high-temperature fast-flow reactor in the 600-1200 K range and in a lower-temperature reactor by tong path length infrared absorption. The rate of the NH(X32-) reaction with NO is reasonably fast and has been measured by Gordon et aI. [29] and by Hansen et aI_ [31]_ We have repeated this mcasummcnt for comparison with the earher works The rEsults arc in reasonable agreement as shown in table 3.

s- ’

l-able 3 Reaction rate of NH(X ‘Z-

) with NO

Rcf_

k
P9l

4_7_+0_8 38 4-820.2

I311

Ibis w0s-k

s-’

>

4. ConcIusion

~[2] R.J. F&r and RJ. Bair. Intein. J_Chah_ Kinetics 8 (7976)

:.

We have measured the rates of -the ga&hase reactions of NH(arA) with H2 and several saturated. and unsaturated hydrocarbons as a function of temperature over the range 250-600 IL. Ethylene. cis-2-butene and methyl acetylene ~&act at near the gas kinetic collision rate at all temperatures studied. Our resuits for. all ~three‘of these unsaturated hydrocarbons are consistent with a zero activation energy. These reactions are concluded to most likely involve cycle-addition of NH(ah) across the carbon-carbon multiple bond_ The H,, methane and propane reactions show marked temperature dependence in the range of this study_ The activation energies measured for the reaction of NH(a’A) with Hs and CH, are 0.8-2 kcal/mole_ These observations contrast strongly with the results for O(‘D) [36,37] and CH2( ‘A,) [34.35] and the calculations of Fueno et al_ We are not able to distinguish between an entrance channel barrier to insertion or a smaller than calculated barrier to abstraction. Measurements

in different

experimental

temperature

and

pressure regimes may aid in the elucidation of the mechanism_ Higher temperature experiments could probe the onset of a higher activation energy channel that would he identified as abstraction_ Studies of the H, reaction at much higher pressures with more efficient buffer gases could lead to the isolation of the NH, product of the insertion channelIn additios measurements of the rotational distributions of the product NH, could, as in the case of O( ‘D). provide mechanistic information_ Finally, we have studied the electronic quenching of NH(atA) by monitoring the appearance of NH (X 32-) by laser-induced fhrorescen ce_ Under our experimental conditions, l-200 mTorr hydrocarbon, we observe no production of ground state NH_ This is not attributable to rapid reaction of NH( ‘Z-) as we have determined an upper limit for its reaction at room temperature with both alkanes and alkenes of -C lo-” cn? s-‘.

References [I] FU. Tourand EJ. Bair. J. Phomchan. 1 (1973)

255.

yi

139. [3] J.R McDonald, ;G. Miller and AP. Etaron~vski Chcriie Phys. utters 51 (1977) 57_ 141AP_ Baronavski. RG_-Miller and J-R-McDonald Chem Phyx 30 (1978) 119. [Sl JR McDonaW RG_ Milk and AI’_ Flaronavski Che!ne Phys. 30 (1978) 133; (61 W_S_ Drozdoski. AP_ Baronavski and JR McDonald, Chem Phya Letters 64 (1979) 421. [7] LG_ Piper. RH_ Krech and RL Taylor. J_ Chem Phya (1980) 791_ [S] S. Tsunasbima, J_ Hamada. M_ Houa and S Sara. EulL C&m_ Sot Japan 53 (19SO) 2443_ [9] 0. Kajimoto and T. Fucno. Chem. Phys_ Letters 8G (1961) 434. [lo] T. Kiramura. S. Tsunashima and S. Sate. Bull. Chcm Sot. Japan 54 (1981) 55_ ill] S-Thima. T_ Kiramura and S. Sara. BuB. Ghan Sot Japan 54 (1981) 2869. [12] 0. Kondo. J. hiiyata. 0. Kajimoro and T_ Fucno. Chem Phys_ Letters 88 (1982) 424. 1131J. Hamada. S. Tsunashima and S. Sato. Bull. Chcm Sot. Japan 55 (1982) 1739. [14] J_ ICawxi. S. Tsunashima and S. Sam BnlL Cbenr Sot Japan 55 (1982) 3312 [IS] J. Hamada. S. Tsunasbima and S. Sate. Bull. Cbexn Sot Japan 56 (1983) 662 [16] S. Kodama. Bull. Chcm Sot. Japan 56 (1983) 2348. [17] S. Kodama. Bull. Chem Sot. Japan 56 (1983) 2355. [la] S. Kodama. BuB. Chem &x Japan 56 (19S3) 2363. [X9] F_ Stubi. private conununication 1201W-L Haina and LG. Csiamadia, Thcoret. Chim Acta 31 (1973) 283s [21] T_ Fueno. V_ BonaCic-Kouteckjrand J. Kouteck$. J. Am Chem_ Sot 105 (1983) 5547. [22] T_ Fueno. O_ Kajimoco and V. Bona&f-Koutecky. Ann. Rev_ Okazaki NatL Res_ In% Okazaki. Japan (1983) p_ 22 1231T. Fueno. K_ Yamaguchi and 0. Kondo. Arm Rev. Okaaaki. NatL Rea Inst. Okazaki. Japan (1983) p_ 22_ [24] T_ Fueno. 0. Kajimoto and V. Bonatic-Koutecky, J. Am Chem Sot_ 106 (1984) 4061. 1251D-W_ Cornell. RS Berry and W_ Lwowski. J_ Am Chem Sot_ 88 (1966) 544 [26] GM Meabum and S. Gordon. J_ Ph- Chcm_ 72 (1968) 1592 [27] K_A Manrd and EJ_ Bair. J. Chcm. Phys- 49 (1968) 324s. [zp] M.P. Nadier. V-K Wang and X-G_ Kaskan. J. Phys. Cban 74 (1970) 917. [ZS] S_ Gordon. W. Mulac and P. Nangia. J. Phya. Cbern 75 (1971) 2087. [30] J_N_ MulvihiB and LF_ Phillips Chem Phya Letters 33 (1975) 608. C Zerzsrh and F_ SruhL Chem [31] I_ Hansen. K H&@aus. Phys_ Leuers 42 (1976) 370. ‘321 C. Zcusch and I. Hansen. &r-B unsenga. Physili Chem 82 (1978) 830.

1391 F- Rohna- and F- Smhl, C&m_ Phys Lcttas Iii 0984) 234_ [40) B. Krakow. RC Loid ind G-0. +ly. j- MoL Spccuye 27
kinclics (w&y.

New York.

WiX Enus. V-B_ Parker. Rli Schumm. I_ Hakw. SM Baikjr. KL Churny and RI_ Nutc&J_ Phys Cban Rcf_ Data If (1982) 2 ~~S]A~J~andDEhiilligasJ~~cbansoC85 (iW3) 27s

[?4] DD- Wn