Imaging in Chemical Dynamics - American Chemical Society

Chapter 17

Photoisomerization and Photodissociation Dynamics of the NCN, CNN, and HNCN Free Radicals Ryan T. Bise, Alexandra A. Hoops, Hyeon Choi, and Daniel M . Neumark Downloaded by COLUMBIA UNIV on September 20, 2012 | http://pubs.acs.org Publication Date: October 18, 2000 | doi: 10.1021/bk-2001-0770.ch017

1

Department of Chemistry, University of California, Berkeley, CA 94720 and Chemical Sciences Division, Lawrence Berkeley National Laboratories, Berkeley, CA 94720

The photodissociation spectroscopy and dynamics of the N C N , C N N and H N C N radicals have been investigated by fast beam photofragment translational spectroscopy. For all three radicals, N loss was determined to be the dominant dissociation channel. However, minorCNchannels were observed for the N C N and C N N radicals. Dissociation energies have been measured directly for each radical, providing substantially improved heats of reaction. The translational energy distributions for N C N and H N C N show well-resolved structure corresponding to vibrational excitation of the N photofragment. For C N N , the vibrational structure of the N photoproduct could not be resolved due to extensive rotational excitation of this fragment. The photofragment branching ratios and translational energy distributions suggest that theNCNand H N C N radical dissociate through cyclic intermediate states of approximate C symmetry while the C N N radicals dissociate via bent intermediate states. 2

2

2

2v

I. Introduction The N C N , C N N , and H N C N radicals have all been proposed as important intermediates in both combustion and interstellar chemistry.(l-4) The C N N and H N C N radicals have been suggested as possible intermediates in the formation of NO, providing low-energy pathways for the cleavage of molecular nitrogen to produce Ν atoms, which are subsequently oxidized to produce nitric oxide.(2) In an effort to further characterize the potential energy surfaces of these radicals, we have Corresponding author.

296

© 2001 American Chemical Society

In Imaging in Chemical Dynamics; Suits, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

297

Downloaded by COLUMBIA UNIV on September 20, 2012 | http://pubs.acs.org Publication Date: October 18, 2000 | doi: 10.1021/bk-2001-0770.ch017

investigated their photodissociaton dynamics. These radicals possess multiple lowlying excited states and fragmentation pathways, see Table I, providing potentially rich systems for photodissociation measurements. While the formation of molecular nitrogen from C N N seemingly involves simple bond cleavage, substantial bond rearrangement is required for the H N C N and N C N radicals. Using the technique of fast beam photofragment translational spectroscopy we have obtained structured photodissociation cross-sections, product branching ratios, and detailed internal energy distributions, yielding accurate dissociation energies and insight into isomerization pathways and fragmentation mechanisms of these radicals.

II. Experimental The fast beam photofragment translational spectrometer, Figure 1, has been described in detail elsewhere;(5-7) only a brief description will follow. In this experiment, we generate a clean source of neutral radicals by mass-selectively photodetaching a beam of stable negative ions. The neutrals are then photodissociated by a second laser. Vibrationally and rotationally cold negative ions are prepared using a pulsed discharge source,(8) then accelerated to 8 keV, and separated temporally by a Bakker time-of-flight (TOF) mass spectrometer.(9,10) The ion of interest is selectively photodetached by a pulsed laser. To produce vibrationally cold neutral radicals, the detachment energy, based upon the recent photoelectron studies of Clifford et al.{ 11,12) and Taylor et α/.(13), is set < 100 meV above threshold. In the case of N C N , a detachment energy of 2.8 eV was selected to produce N C N in the Χ Σ ~ state exclusively, while a detachment energy of 4.03 eV was used to populate both the Χ Σ ~ and states.(14) Undetached ions are deflected out of the beam path. 3

3

ion source

1

mass

; detachment* dissociation

TOF A

z.

/

TPS detector

Figure 1. Fast radical beam translational spectrometer. The dotted line separates the radical production section on the left from the photodissociation experiment on the right. In the dissociation region, the neutrals are intersected by an excimer-pumped dye laser with frequency doubling capabilities. When resonant with a predissociative transition, a fraction of the neutral molecules dissociate yielding photofragments In Imaging in Chemical Dynamics; Suits, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.


298

detected directly by either the retractable T O F or TPS (time and position sensing) microchannel plate detector assemblies in Figure 3. A n aluminum strip is positioned at the center of each detector to prohibit any undissociated radicals from impacting the detector, so that any observed signal is from recoiling photofragments. The spectroscopy of the dissociative electronic state is examined by monitoring the total yield of photofragments at the T O F detector as the dissociation laser is scanned. Once the dissociation spectroscopy has been examined, a second type of experiment, which probes the dissociation dynamics, can be performed. In this detection scheme both photofragments from a single parent radical are detected in coincidence using a time-and-position sensitive (TPS) detector of the type developed by de Bruijn and Los,(15) located at either 1 or 2m from the interaction region. Our implementation of this detection scheme has been described in detail elsewhere.(5,6) The TPS detector records the positions and difference in time of arrival of the two photofragments from a single dissociaton. This information is then used to determine the photofragment masses, their relative translational energy E and the scattering angle θ between the relative velocity vector and the electric vector of the polarized dissociation laser. The photofragment mass resolution is m/Am « 10 while the translational energy resolution for these experiments is Δ E I E = 2.0. Tl

T

T

III. Results A. NCN

32000

34000

36000

38000

40000

42000

44000

Photon Energy (cm* )

Figure 2. Photofragment Yield Spectrum of the Β Σ~ X%band. 3

The

vibrational comb denotes the symmetric stretch 1JJ progression,


299 1. Photofragment Yield Spectrum The photofragment yield (PFY) spectra for the Β Χ l

and d A

l

u

3

(la)

N ( X Z ; ) + C( P)

2.54 ±0.03

(lb)

N ^ X ' Z p + CCD)

3.80 ±0.03

(Ic)

C N ( X Z * ) + N( S)

4.56 ±0.03

(Ha)

N ( X ' Z ; ) + C( P)

1.22 ±0.03

(lib)

N . i X ' Z p + Ci'D)

2.47 ±0.03

(He) did)

C N ( X X ) + N( S) N ^ X ' E p + CCS)

3.24 ± 0.03

(lie) (Hf)

C N ( A n ) + NCS) C N ( X E ) + N( D)

4.39 ±0.03 5.62 ±0.03

(Ilia)

Ν (Χ'Σ;)

1

J

2

i

4

3

2

2

+

4

3.90 ±0.03

2

2

+

2

HNCN(X A") 2

+Œ(x n) 2

2

(Ob)

2.78 ±0.03 2.87 ±0.05

(Hie)

HNC(X'Z ) + N( S)

3.45 ±0.05

(Hid)

N ( X ' X p +CH(a Z")

3.50 ±0.03

(Hie)

H( S) + NCN(X Z;)

3.72 ±0.04

(Illf)

+

4

4

2

J

3

3

2

+

Ν Η ( Χ Σ ) + ΟΝ(Χ Σ )

4.86 ±0.06

All values are based upon this work and NIST-JANAF thermochemical tables.(20) 2. Photofragment mass distributions At the photon energies explored in this study three possible dissociation channels are accessible, see Table I. A l l three channels are accessible following excitation of the 1Q, η > 4 transitions of the Β Σ ^ 4.9 eV, the photofragment mass ratio of low translational energy fragments was found to be 14:26, indicating the onset of a new Ν + C N dissociation channel. The photofragment mass distributions were found to be 12:28 for the c ^ 0 K

2

2


307 •

•

•

•

•

•

ί

1|

W

3 6 0 2 7 e

•

•

•

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•

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ι

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•

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0|=η

Y

1

Ε|*Μοη

ι :

=

•

Λ

I

•

I

1

ι

1

ι

Φ "

»|Α

V I|

3.9093 e y


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ι

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4

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3

5

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V

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0.0 0.2 0.4 0.6 0.8

1.0

3

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E ^ = 4.157 eV

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0.2 0.4 0.6 0.8

\ i

1.0

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1.2

1.4

Translational Energy (eV) Figure 9. P(E ) distributions for the CH + N dissociation channel of the HNCN radical. E denotes the transition energy and the combs correspond the vibrational energy levels of the N photofragment. T

2

photon

2

IV. Discussion The P(E ) distributions for the N C N , C N N , and H N C N radicals reveal that the mechanism of photofragmentation is complicated and involves considerable geometry changes along the dissociation pathway. The dynamics of the N C N radical are particularly surprising. The excited states for the N C N radical are all linear, yet the dominant reaction pathways are the N loss channels indicating that strongly bent or cyclic transition states are involved in the dissociation pathway. The C( P) products are the dominant dissociation channel (> 90%) for the c ! ^ state, even though spin allowed C(*D) products are energetically accessible by more than 0.9 eV, while only C( D) products are observed from the c r A state. T

2

3

1

!

u

A l l of the N C N P(E ) distributions reveal well resolved structure corresponding to N vibrational excitation. The extended vibrational excitation in the N photoproduct is consistent with a bent or cyclic intermediate state with the N - N bond T

2

2


308

forming and C - N bonds breaking at relatively large N - N distances. The rotational distributions peak between 20-40 quanta. The limited rotational excitation suggests that the dissociation at the transition state does not produce significant torque on the N photofragment, consistent with a cyclic, C -type dissociation pathway. Recent calculations by Martin et α/.(28) on the linear, bent and cyclic structures of N C N and C N N provide support for a C dissociation mechanism. Their calculations within the C point group have located a local minimum energy structure with A symmetry and a transition state with B i symmetry at respective energies of 3.01 and 4.76 eV above the linear ground state of N C N , see Figure 10. The Χ Σ ~ 2

2v

2 v

2 v

3

3

2


3

6 5-

3

CN(X^*) + NfS)

B,

43H

Ά

3

-—\.

2

N + C( P) 2

Ά,

H

N C N

Dissociation Coordinate (C ) 2v

Figure 10. Energy level and correlation diagram of the relevant electronic states of the NCN radical along a C dissociation coordinate. The energetic positions of the a A , t r E g , Α Π , B E , c U , and a A states are based upon previous 2v

!

3

3

g

Μ

l

U

l

u

u

7

3

3

experimental spectroscopic studies discussed in the text. The A;, A , and B states are from ab inito calculations by Martin and coworkers (Ref. 27) and the product state energies are from NIST-JANAF thermochemical tables (Ref. 20). 2

3

3

2

3

state adiabatically correlates to the B i state while the B E state correlates to the A state, providing a low-energy pathway through a cyclic intermdiate to products N + C( P). The Α Π state, although above the dissociation asymptote, cannot dissociate since it does not correlate with the lower energy A state and does not have enough energy to access cyclic intermediates of either B i or A i symmetry. The formation of C( P) products from the c Π state clearly indicates that the dissociation mechanism involves intersystem crossing (ISC) to a triplet surface. The U

2

2

3

3

Μ

3

2

3

3

3

1

ιι


309

1

l

c Π 4.9

U

eV, comprising 25 ± 10% of the total dissociation signal. The rotational distribution peaks at J = 10, less than half that observed for the N loss channels. The limited rotational excitation and positive photofragment anisotropy (β=0.9) are consistent with collinear dissociation. The ability of the N +C( P) channel to dominate when linear dissociation channels are available proves that the coupling of the linear excited states to intermediate cyclic states is highly efficient. Unlike the N C N radical, the C N N radical can access either N or C N products 2

3

2

2

3

3

through simple bond cleavage. Since both the Α Π and Β Σ ~ states are linear, one might expect the dissociation to proceed along a linear pathway producing only low rotational quantum levels for the diatomic fragment. However, rotational excitation of the N fragment is so large that it obscures the underlying vibrational structure. The extensive rotational excitation (J > 40) suggests that the C N N radical dissociates via a bent transition state, which produces torque onto the molecular fragment. Analogous to the N C N radical, the H N C N radical displays resolved vibrational structure of the N photofragment. Formation of CH( n) + N products from H N C N requires significant structural rearrangement involving an H atom shift to the central C and bending of the N C N backbone to allow N - N bond formation. The N(*S) channels are not observed, indicating that isomerization is much faster than intersystem crossing. It is particularly surprising that we are able to resolve vibrational structure since two diatomic photofragments are produced and hence, a large number of product rotational states are accessible. The limited rotational excitation implies a cyclic transition state, just like N C N . Several theorists have investigated the role of cyclic H C N in the reaction of CH( FI) + N - » N( S) + HCN,(29-36) and propose that a bound c - H C N 2

2

2

2

2

2

4

2


2

310

intermediate allows the above spin-forbidden reaction to occur.(30) Walch calculates the minimum energy structure of c-HCN to be 0.55 eV below the CH + N asymptote and estimates the exit barrier to be 0.79 eV above this asymptote.(35) We observe photofragment signal from the origin of the B A ' « - X A * band (T = 28993.766 cm* ),(25) indicating that the exit barrier must be < 0.8 eV, in good agreement with the results of Walch. We cannot directly determine the barrier to isomerization of HNCN to c-HCN ; however, this barrier must also be located < 0.8 eV above the dissociation limit. The HNCN radical is energetically more stable than H C N and c-HCN by 1.74 ± 0.05 eV(27,37) and 2.23 ± 0.09 kcal/mol(35) respectively, suggesting that this radical may play an important role in the reaction of N + CH( n), particularly at higher temperatures where passage over activation barriers becomes more facile. 2

2

2

2

0

1

2

2

2

2


2

V. Conclusions The photodissociation spectroscopy and dynamics of the CNN, NCN, and HNCN free radicals have been investigated by fast beam photofragment spectroscopy. For all of these systems, we have observed dissociation from multiple excited states. In addition to refining the thermodynamics of these radicals, the photofragment internal energy distributions indicate that these radicals isomerize to cyclic or bent intermediate states prior to dissociation. We hope that future theoretical studies of the isomerization pathways of these radicals and direct spectroscopic investigation of the diazirinyl radical (c-HCNN) and diazirinylcarbene (c-CNN) will allow a more thorough investigation of the global potential energy surface.

Acknowledgements This research is supported by the Director, Office of Energy and Research, Office of Basic Energy Sciences, Chemical Sciences Division, of the U.S. Department of Energy under Contact No. DE-AC03-76F00098.

References 1. 2. 3. 4. 5.

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