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Electron-impact dissociation of HNCO: Formation of NH(A3Π, c1Π) and NCO(A2Σ+, B2Π)
Chemical Physics X9 (1954) 51-57 -North-Holiand. Amsterdam
51
ELEaRON-IMPACT DISSOCIATION OF HNCO: FORMATION OF NH(A31T, c ‘II) AND NCO(A*Z +, B ‘II)
Rccehed 26 January 19X4
Emission spectra of the NH(A3fI-X3s-). NH(ctn-a%). NCO(A’, “+-X’fI) and NCOrB’II-X’IT) systems were obsened by electron impact on HNCO up to 40 eV. The NH(c’ ff) is shown to be formed directly. wuh the onset at 9 1 _cO 6 eV. The onsets of NCO(A’P’ ) and NCO(B’iT) formatron are found to be S 3~0 6 and 9 2kO 6 rV. respechvely. The observed internal energy distributtons for NH arc compared with those predicted stattstrcally. The precursor states forming NH(A). NH(c), NCO(A) and NCO(B) are discussed with the ard of a schematic correiatron diagram based on a semi-empirical calculation (CNDO/S).
1. Introduction
The electromc states and spectra of isocyanic acid (HNCO) have been investigated both theoretically and experimentally (l-61. In the VUV absorption of HNCO, Okabe [3] has reported that NH(c’fl) and NCO(A’Z+) are formed directly but that NH(A31i) is produced only by a secondary process. Drozdoski et al. [5] investigated the formation of NH and NC0 radicals from the photolysis of HNCO at 193 nm using laser-induced fluorescence. The rotational energy distribution of NH(a’A) thus determined can be represented by a Boltzmamr temperature of 1097 & 30 K, which is considerably colder than the statistically expected value. Recently, Fujimoto et al. [6] Investigated the formation of the CO radical from the photoiysis of HNCO at 193 nm by means of CO iaser resonance absorption. The vibrational energy distribution of the CO(X’Z+) state thus observed is in close agreement with the value predicted statistically on the- assumption that NH(a) + CO(X) are the photodissociation products_ This result has indicated that the photolysis of HNCO and subsequent energy-partitioning processes can be treated to a first approximation by using simple statistical models. However, they have concluded that even the adjustment of the
data of NH(a) reported by Drozdoski et al. [S] considering predusociation cannot account for the difference between the experimental distribution and the distribution predicted statistically. These studies of dissociation dynamics and the assignment of the parent electronic transition thus determined are restricted to relatively low-energy regions. In regions of higher energy the dissociation of molecules can easily be realized by electron impact. In electron-impact dissociation of HNCO, however, very little is known about the formation of NCO. Fukui et al. [7] reported that NH(A) is formed directly but that no NH(c) is formed. The question is raised why no NH(c) is formed by electron impact on HNCO because NH(c) is a direct product of HNCO photolysis, and because almost all of the optically allowed processes can occur in electron impact. Furthermore, both NH(A) and NH(c) are formed in direct dissociations by electron impact on HN, [7,8], which is isoelectronic with HNCO. Dissociation dynamics may prevent the formation of NH(c) by electron impact on HNCO in contrast to the direct formation of NH(c) by photolysis. In order to further characterize the dynamics of formation of NH(A, c) and NCO(A, B) we have measured the excitation function of the NH(A-X), NH(c-a), NCO(A-X) and NCO(B-X) emissions
0301-0104/84/$03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
I_ Tokue, Y. Ito / Electron -mpacr dmoctarion of HNCO
52
produced analyzed
by electron impact on HNCO and the vibrational and rotational distribu-
tions of the NH(A) and NH(c) states. The results of this study show that NH(c) is formed directly via an optically allowed reaction_
2. Experimental The apparatus and the experimental conditions have been described in a previous paper [9]. The pressure of the sample gas in the collision region during the emission measurement was controlled to (2-6) X 10m5 Torr by d variable leak valve. The HNCO sample was prepared by heating a mixture of KOCN and excess stearic acid at 95 o C under vacuum [6]. It was dried over PZOs and collected at liquid-nitrogen temperature. After being distilled trap-to-trap at = - 60°C several times to remove CO, and other impurities, it was stored in a gas reservoir at a pressure less than 10 Torr to avoid polymerization. A spectral band pass of 0.17 nm fwhm mas used for the measurement of the emission spectra. The excitation functions of the NH(A-X), NH(c-a), NCO(B-X) emissions were measured with a beam current controlled to lo-40 &A. A spectral band pass of 0.32 nm fwhm 1%~ used for the measurement of the excitation function. The energy scale for the impinging electron was calibrated using a maximum on the cross section for the 1-O band of the N,(C 311,-B 3JIIs) emission by Finn et al. [lo]. The N?(C-B) emission from mixed N, and the NH and NC0 emissions from HNCO were measured alternately.
3. Results 3. I. Emission specrm The emissions in the region 250-500 nm obtained by electron impact on HNCO belong to the hydrogen Balmer lines. the CN(B’S+-X ?C*), NH(A311-X ‘2-) and NH(c’lI-a%) bands. In addition. t\\o weak broad emissions observed are attributable to the NCO(A’Z*-X ‘II) and
NCO(B’II-X “II) bands [3,11,12]. The emission spectrum which covers the region 324-346 nm consists of two heavily overlapping bands corresponding to the O-O band of the NH(c-a) system and the O-O and l-l bands of the NH(A-X) system. The rotational structure can be resolved only partly due to the limited resolution. Therefore, the observed spectrum IS compared with band envelopes simulated on a microcomputer. The procedure of the band-envelope analysis has been reported in detail (131. The observed spectrum at an impact energy of 21 eV is compared in fig. 1 with synthetic envelopes_ This analysis of the intensities of these NH emission bands provides information about the rotational distribution and the relative vibrational population, P,._,/Pc,._,( PI/P,), of the NH radical in the A3n and c ‘II states independently. The analysis also provides information about the relative formation rate. P(c’lI)/P(A311). after the dissociative excitation of HNCO. The parameters used in the best-fit simulated spectra are shown in figs. 2 and 3. The results derived from this simulation can be summarized as follows: (1) Formation of NH(c) is considerably smaller than that of NH(A) particularly near the onset. (2) The vibrational population ratio, PI/PO, of NH(A) is 0.24-0.30, which is nearly independent of the Impact energy_ The PI/PO for NH(c) is zero becduse of the predissociation of the u’ 2 1 levels of the c state [14]. (3) In the U’ = 0 level of the A state the rotational distribution corresponds to a Boltzmann temperature of 6000 +- 500 K, which is independent of the impact energy. The rotational distribution of the A state can be reproduced with this effective Boltzmann temperature. but the observed distribution above N’ = 25 for the d =0 level decreases more steeply than this statistical distribution because of the predissociation [14]. The rotational temperature of the u’ = 1 level is assumed to be equal to that of the u’ = 0 level. (4) The rotational distribution of the LJ’= 0 level of the c state for the electron energies > 19 eV can be represented by an effective Boltzmann temperature. 2700 f 700 K. This is obscured at electron energies =S 16 eV due to large error bars.
_I_ Tokue. Y Iro / Electrqn -Impact dissoctatiqn _of HNCO
I
53
I
325
330
335 340 Wavelengthlnm
345
Fig I. The observed spectrum of NH (dots) at an impact energy of 21 eV with 0.17 nm fwhm resolution_ The trace IS the best-fit synthetic spectrum (see text)_
3.2. Excitation ftmctrons The excitation functions of the NH(A-X) and NH(c-a) emissions are shown in fig. 4. The ob-
served spectra and the excitation function of the NH(c-a) emission show that NH(c) is formed directly; the onset of NH(c) is found to be 9.1 f 0.6 eV. The onset of NH(A), 7.5 t 0.5 eV, is in good agreement with the v&e, 7.4 eV, reported by Fukui
et al. [7]. Furthermore,
the second
rise in the
plot for NH(A) appears near 22 eV. The excitation functions of the NCO(A-X) and NCO(B-X) emissions are shown in fig. 5. The onsets of
(a ) NH(c)/NH(A) 06
T/K Ic
i 6000
1-j ,___;I.:; 1I_,] 0
10
Electron
20
-
UOO-
2000-
30
energy/eV
Fig 2 (a) Relative formation rate, P(c)/P(A). and (b) relative vibrational population PI/P0 of NH(A) The horizontal solid and broken lines indicate the statistically predicted values and those predicted from the Franck-Condon factors. respectively
Ol
0
10
Electron
%ergy/eV
I
30
Fig. 3. The rotationaI temperatures for NH: 0 for NH(A) 0 for NH(c).
and
I. Tokue. Y. Ito / Electron-unpact drssocration of HNCO
54
_.
_.
(b) -z‘SC:__-_-_-_-__--
iI
“NC0
“-NC0
20 Electron energy/&
0
FIN. 4. NH em~sston mte_umes versus electron ener~zr (a) the P
branch of the NH(A-X) emission at 340 nm. and (b) the R-branch bAndheadof the O-Oband of the NH(c-a) emission dt 324 nm. NCO(A) and NCO(B) are found to be 8.3 f 0.6 and 9.2 t 0.6 eV. respectively. In electron Impact at low-energy regions the selection rule for dipole transition is not valid; particularly. a spin-forbidden transition c&n occur through an electron-exchange excitation. It is well known that the cross section of an electron-exchange excitation has a sharp maximum just above the onset and decreases rapidly with increasing energy of the impinging electron. while an opticallly allowed excitation has a broad maximum in
(b)
_ _ __-_-;--_-_--_ __r*-_-= _ -_ ______ _____-.-___-I-c. - _i _-- _ (0) -_ .*
-_--
--
i .--
Electron
20
CS
40
40
energy/eV
Fig 5. NC0 emissionintenwirs versus electron energy: (a) the OO’O-00’0 band of the NCO(A-X) emissionat 438 5 nm, md (b) the 00’0~0010 bmd of the NCO(B-X) em&on at 315 nm.
NH-CO
“NC0
Fig. 6. A schematIc correlatton diagram (point group C,) of the smglet and tnplet states of HNCO with NH + CO or H + NCO. The excited HNCO states at the Franck-Condon region are shown.
the cross section above the onset. Thus,it appears that NH(A) near the onset is produced via an optically forbidden excitation. whereas NH(c), NCO(A) and NCO(B) are produced via an optically allowed excitation.
4_ Discussion 4.1. Eiectrotlic states of HNCO
and products
The ground state of HNCO is known to have a trans bent structure (point group C,) [l&16]. The electronic energies and the conformation of the excited HNCO have been investigated both experimentally [1,2] and theoretically [17]. Dixon and Kirby [l] have observed the four progressions between 281 and 229 nm in the UV absorption spectrum_ The rotational analysis of the progression around 266 nm indicates that the excited state is probably a ‘A” state; the excitation energy of 4.65 eV from the ground state of HNCO is estimated from the band origin. In the UV absorption spectra of HNCO and related compounds, Rabalais et al. [2] have concluded that the assignment of the 3A’ + ‘AI transition provides a rational identification of the Dixon-Kirby bands. In addition, they have observed the ‘A” + ‘A’ transition at 210 nm and the shoulder of the ‘A” -‘A’ transition at 182.5 run.
1. Tokue, Y. Ito j Elecrron-cm&t
Nevertheless, -the available experimental and theoretical information about the electronic states of HNCO in the 610 eV region is insufficient_ The energy levels of the singlet and triplet HNCO states are therefore estimated from a semi-empiri: cal calculation (CNDO/S) on the assumption that the Franck-Condon principle is valid. The stritctural parameters (5) determined by Fusina and Mills [18] were used for the equilibrium geometry in the CNDO/S calculation. The calculated configuration for the ground stateof HNCO is (1-7)a’2 la”’ (8-9)a’2 2a”‘, which is the same as that of previous works [2,17]. The excitation energies (in eV) computed are (experimental values [1,2] are given m parentheses): S-2(5.9) for the ‘A” state: 6.1 for ‘A’; 6.8(6.8) for ‘A”; and 5.2(4_65) for 3A”. The energy levels of the NH + CO products or H + NC0 products are derived using the electronic energy [19] and the heat of formation [20,21]. Recently, Sullivan et al. [22] have determined a new value for AHt2,,( HNCO), -0.61 + 0.03 eV, combining their results for AH,O,,,(NCO) with Okabe’s H-NC0 dissociation threshold [3]. The NH(X ‘Z-) + CO(X I,+) ground state thus obtained lies 3.11 eV above the ‘A’ ground state of HNCO. A schematic correlation diagram of the singlet and triplet states of HNCO with the NH + CO products or the H + NC0 products thus obtained is shown in fig. 6, taking account of the non-crossmg rule. 4.2 Internai
energy
distribution
of NH(A,
c)
The main dissociation processes leading to the formation of NH(A) and NH(c) from HNCO near their thresholds are as follows: HNCO
+ CO(X’z+)
+ NH#H),
AH = 6.81 -f 0.11 eV; + CO(X’Z+)
0)
+ NH(c’lT),
AH = 8.53 + 0.11 eV_
(2)
The AH values represent the thrcshold energies beyond which dissociation is possible. Process (1) is a spin-forbidden reaction, while process (2) is a spin-allowed reaction. The molecular structure of the precursor state forming NH(A) in equilibrium is not known. Re-
dissoeiatioitof HNCO
55
gardless of the conformation of this precursor state, we car- estimate the intensity of the NH vibrational transition by use of the elementary form of the Franck-Condon principle_ The relative vibrational population of NH(A) is calculated by use of the Morse function for NH(A) [19] and the harmonic function for the N-H potential of the ground-state HNCO [23] disregarding the N-C-O motion. The vibrational population ratio, = 0.08, derived from the Franck-Condon factor is much lower than the observed value (see fig. 2). This suggests that the precursor forming NH(A) has a very different geometry in equilibrium from the ground state of HNCO and a dynamical effect enhances the vibrational energy distribution in the formation of NH(A). The rotational temperatures of the d = 0 level of the NH(A) and NH(c) states and the PI/P0 of NH(A) produced by electron impact on HNCO are compared with those predicted by a simple statistical model based on information theory [24]: PI/P,,
P,(N)
a: (2N+
PI/P, = c (1 -fr v
l)C(l -_& D
-f,)3’2,
-fL>“Z
(1 -f,K L’
where P,(N) is the rotational distribution of the u’ = 0 level of NH, fN is the fractional rotational energy (E,/E,,) of NH, and fi and fv are the fractional vibrational energies of NH and CO, respectively. The available energy for the formation of NH(A), E,, = 0.7 eV, is given by the difference between the observed onset of NH(A) and the calculated threshold for process (1). This model assumes complete randomization of the available energy among all degrees of freedom in the products. The predicted vibrational population ratio, PJP,, = 0.126, is about a half of the observed value (see fig. 2); also, the predicted rotational temperature of the v’ = 0 level of the NH(A) state, 3500 +400 K, is about one half of the observed temperature. This suggests that the internal energy of NH(A) is selectively enhanced in the dissociation of the precursor state of HNCO. On the other hand, the observed rotational temperature of the u’ = 0 level of the NH(c) state, 2700 f 700 K, appears to coincide with the statistically predicted value, 2300-3050 K, using Eay = 0.6 eV_
I. Tokue.
56
4.3.
Formation
of
NCO(A,
Y. 120 /
Electron -Impact dlssoctarion of HNCO
B)
The main dissociation processes leading to the formation of NCO(A) and NCO(B) near their thresholds are: HNCO -
H(ls) -I- NCO(A’Z+).
from HNCO AH = 7.77 eV;
(3) -+ H(ls) + NCO(B’II).
AH = 8.89 eV.
(4) These are spin-allowed reactions_ The onsets of NCO(A) and NCO(B) are = 0.4 eV higher than the AH of the corresponding reactions_ This suggests that the internal energy of NCO(A. B) is strongly enhanced_ However. we have not attempted a population analysis of the NCO(A-X) and NCO(B-X) bands because of their weak intensitiesDixon [11.12] has discussed the possible electronic states of NC0 that can be formed from the low-lying states of N + CO or 0 f CN. The correlation diagram of the singlet HNCO states with the H + NC0 products is incomplete since little is known about these NC0 states except for A’Z’ and B’II. Woolley and Back [25] have investigated the photolysis of HNCO at 206 nm and found that the NH + CO and H + NC0 products are equally important_ The formation of non-linear HNCO from H(ls) and NCO(X’II) leads to the ‘A’ and ‘A’ species by resolving these states into a species with C, point group. By comparison of the energy of the incident photon with the electronic energies of HNCO and products. the precursor dissocrating into H(lsj+NCO(X) is estimated to be the ‘A” state near 5.2 eV and/or the ‘A’ state near 6.1 eV. The observed onset of NCO(A) indicates that the precursor forming H(ls) + NCO(A) products Lies7.7-8.9 eV above the ground-state of HNCO. The formation of non-linear HNCO from H(ls) and NCO(A’S’) leads to the ‘A’ species by resolving these states into a species with C, point group. The ‘A’(lla’ -9a’) state near 8.6 eV appears to correlate with the H(ls) + NCO(A) products. since the next ‘A’ state is = 2.4 eV higher than the product state. Similarly, the precursor state forming H(ls)+ NCO(B) products is ex-
petted to tie in the region 8.6-9.8 eV. The formation of non-linear HNCO(C,) from H(lsj + NCO(B’II) leads to the ‘A’ and ‘A” species. The ‘A”(12a 6 2a”) state near 9.6 eV seems to be the precursor dissociating into H(ls) + NCO(B), since the next ‘A” state is = 1.5 eV higher than the product state_ It is difficult to identify the ‘A’ precursor, since several ‘A’ states have electronic energies higher than 10 eV_ 4.4. Formation of NH(A,
c)
The observed onset of NH(A) indicates that the precursor forming NH(A) + CO(X) products lies 7.0-8.0 eV above the ground state of HNCO. The formation of non-linear HNCO from NH(A311) and CO(X’C+) leads to the 3A’ and 3A” species by resolving these states into a species with C, point group. On the basis of the non-crossing rule the -‘A’(3a” +- 2a”) state near 5.4 eV and the 3A”(3a” - 9L11)state near 6.8 eV appear to correlate with NH(A) + CO(X)_ Dissociation into NH(A) + CO(X) can be expected from the 3A”(3a” +- 9a’) state. because the calculated energy for the ‘A’(3a” - 2a”) state is = 1.5 eV lower than that of the NH(A) f CO(X) product state. However. the calculated electronic energy of the 3A” precursor state (6.8 eVj forming NH(A) coincides with the value corresponding to the NH(A) + CO(X) product state and cannot account for the difference between the observed onset of NH(A) and the calculated threshold of reaction (1). Two possible origins of this difference are: First, the accuracy of the CNDO/S calculation is insufficient and the 3A” precursor state is higher than the calculated value: second. the ‘A” state (6.8 eV) gives rise to avoided curve crossing with the ‘A” state (7.5 eVj. which results in a potential barrier. The two possible origins are consistent with the fact that the internal energy distribution of NH(A) is considerably higher than the statistically expected value, and thereupon we have not been able to identify the origin of the above difference_ The observed onset of NH(c) indicates that the precursor dissociating into the NH(c) -t- CO(X) products lies 8.5-9.7 eV above the ground state of HNCO. Combination of NH(c) and CO(X) in group C, yields the ‘A’ and ‘A” species. On the
I_ Toh-ue, Y. Ito / Electron-tmpacr dirsocranonof HNCO
basis of the non-crossing rule we suppose that the ‘A“(3a” 6 9a’) state near 6.8 eV and the ‘A’(lla’ - 9a’) state near 8.6 eV correlate with the NH(c) -f-CO(X) product state. By comparison of these electronic energies with the observed onset of NH(c), the ‘A’ state near 8.6 eV is estimated to be the precursor dissociating into NH(c)+ CO(X) products. The preceding discussion indicates that the ‘A’ state near 8.6 eV correlates with both NH(c)+ CO(X) and H(ls) + NCO(A). The very small formation rate of NH(c) near its onset, despite that it is a spin-allowed transition, may be caused by a predominant competing channel. This competing channel appears to be the formation of NCO(A). However, the formation of NCO(B) can compete with the formation of NH(c), since the onset of NH(c) is 0.8 eV higher than that of NCO(A) and coincides with that of NCO(B).
5. Conclusion It is concluded that NH(A) near its onset is produced via a spin-forbidden excitation, but that NH(c), NCO(A) and NCO(B) are produced via a spin-allowed excitation. The present study shows that NCO(A) is formed from a ‘A’ state of HNCO near 8.6 eV and NCO(B) is formed from a ‘A” state or a ?4’ state which lies at an energy > 9.0 eV. SimiIarly, NH(A) is formed from a 3A” state of HNCO (2 7 eV) with or without avoided curve crossing and that NH(c) is formed from a ‘A’ state near 8.6 eV. The formation of NH(c) seems to compete with that of NCO(A) or NCO(B), which is predominant. An ab initio calculation is planned for a better understanding of the electronic structure and the equilibrium geometry of the excited states of HNCO and NCO. The interna energy distribution of NH(A) is considerably higher than the value expected statistically. Even if this simple statistical model cannot be applied to the dissociation of such a small molecule as HNCO, this model may be used as a convenient criterion for energy-partitioning processes resulting from electron-impact dissociation, namely, for prediction of whether the available energy is completely randomized among all
57
degrees of freedom in the products or a specific mode is enhanced_ Acknowledgement
The authors are grateful to Professor K. Kuchitsu of The University of Tokyo for his many valuable discussions. References [I] RN
Divan and G_H i&by, Trans. Faraday Sot 64 (1968) 2002. [2] J-W. Rabalais. J R. McDonald and S P. McGI~M. J. Chrm Phys 51 (1969) 5103. [3] H. Okabe, J Chem Phys. 53 (1970) 3507. 141 J.W. Rabalais. J.R. hicDonJd. V. Schcrr and S P. McGlynn, Chem Rev. 71 (1971) 73. [5] W.S Drozdoski, A.P Baronavski and JR McDonald, Chem Phys Letters 64 (1979) 421. [6] G T. Fujimoto. M E. Umstead and M C. Lin. Chcm. Phys 65 (1982) 197. (71 K. Fukm, I. FuJlta and K Kuwata. J. Phys. Chem. 81 (1977) 1252 PI I. Tokue and Y Ito. Chem Phys. 79 (1983) 383 [91 I. Tokue, M. Ikarashi, S. Takizawa and Y. Ito, Bull Chem Sot. Japan 56 (1983) 583 WI T G. Finn. J EM Aarts and J P Doenng, J. Chem. Phys. 56 (1972) 5632 r111 R.N. Dixon. Phi Tram Ro> Sot. London A252 (1960) 165. WI RN. Dixon. Can. J. Phys. 38 (1960) 10 I131 I. Tokue and M. Iwai. Chem. Phys 52 (1980) 47 (141 W H Smith. J. Brozozowski and P. Erman. J. Chem Phys. 64 (1976) 4628. WI A-D McLean. G H. Loew and D S. Berkowttz, J. Mol Spectry 72 (1978) 430. WI K Yamada. J. Mol. Specny. 79 (1980) 323. J Chem 55 (1977) 149s 1171 A RaukandPF.Alewood.Can WI L. Fusina and LM. Mdls. J. Mol. Spectry 86 (1981) 485. t191 K P. Huber and G. Iierzberg, Molecular spectra and moIecuIar structure. Vol. 4. Constants of diatomic molecules (Van Nostrand-Reinhold. New York, 1979). Stull, ed. JANAF thermochemical tables (Dow PO1 DR. Chenucal. Michigan. 1965). Addenda (1966. 1967) 1211 L.G Piper. J Chem. Phys 70 (1979) 3417. WI B J Sullivan. G P. Smith and D R. CrosIey. Chem. Phys. Letters 96 (1983) 307. I231 M. Carlotti. G. di Lonardo, G. Galloni and A_ Trombeth. J Mol Spectry 62 (1976) 192. [24] R.B. Bernstein and R.D. Levme. in: Advances m atomic and molecular physics. ed D R Bates (Academic Press, New York. 1975) p. 215. [25] W.D. 295.
Woolley and RA
Back, Can. J. Chem. 46 (1968)
51
ELEaRON-IMPACT DISSOCIATION OF HNCO: FORMATION OF NH(A31T, c ‘II) AND NCO(A*Z +, B ‘II)
Rccehed 26 January 19X4
Emission spectra of the NH(A3fI-X3s-). NH(ctn-a%). NCO(A’, “+-X’fI) and NCOrB’II-X’IT) systems were obsened by electron impact on HNCO up to 40 eV. The NH(c’ ff) is shown to be formed directly. wuh the onset at 9 1 _cO 6 eV. The onsets of NCO(A’P’ ) and NCO(B’iT) formatron are found to be S 3~0 6 and 9 2kO 6 rV. respechvely. The observed internal energy distributtons for NH arc compared with those predicted stattstrcally. The precursor states forming NH(A). NH(c), NCO(A) and NCO(B) are discussed with the ard of a schematic correiatron diagram based on a semi-empirical calculation (CNDO/S).
1. Introduction
The electromc states and spectra of isocyanic acid (HNCO) have been investigated both theoretically and experimentally (l-61. In the VUV absorption of HNCO, Okabe [3] has reported that NH(c’fl) and NCO(A’Z+) are formed directly but that NH(A31i) is produced only by a secondary process. Drozdoski et al. [5] investigated the formation of NH and NC0 radicals from the photolysis of HNCO at 193 nm using laser-induced fluorescence. The rotational energy distribution of NH(a’A) thus determined can be represented by a Boltzmamr temperature of 1097 & 30 K, which is considerably colder than the statistically expected value. Recently, Fujimoto et al. [6] Investigated the formation of the CO radical from the photoiysis of HNCO at 193 nm by means of CO iaser resonance absorption. The vibrational energy distribution of the CO(X’Z+) state thus observed is in close agreement with the value predicted statistically on the- assumption that NH(a) + CO(X) are the photodissociation products_ This result has indicated that the photolysis of HNCO and subsequent energy-partitioning processes can be treated to a first approximation by using simple statistical models. However, they have concluded that even the adjustment of the
data of NH(a) reported by Drozdoski et al. [S] considering predusociation cannot account for the difference between the experimental distribution and the distribution predicted statistically. These studies of dissociation dynamics and the assignment of the parent electronic transition thus determined are restricted to relatively low-energy regions. In regions of higher energy the dissociation of molecules can easily be realized by electron impact. In electron-impact dissociation of HNCO, however, very little is known about the formation of NCO. Fukui et al. [7] reported that NH(A) is formed directly but that no NH(c) is formed. The question is raised why no NH(c) is formed by electron impact on HNCO because NH(c) is a direct product of HNCO photolysis, and because almost all of the optically allowed processes can occur in electron impact. Furthermore, both NH(A) and NH(c) are formed in direct dissociations by electron impact on HN, [7,8], which is isoelectronic with HNCO. Dissociation dynamics may prevent the formation of NH(c) by electron impact on HNCO in contrast to the direct formation of NH(c) by photolysis. In order to further characterize the dynamics of formation of NH(A, c) and NCO(A, B) we have measured the excitation function of the NH(A-X), NH(c-a), NCO(A-X) and NCO(B-X) emissions
0301-0104/84/$03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
I_ Tokue, Y. Ito / Electron -mpacr dmoctarion of HNCO
52
produced analyzed
by electron impact on HNCO and the vibrational and rotational distribu-
tions of the NH(A) and NH(c) states. The results of this study show that NH(c) is formed directly via an optically allowed reaction_
2. Experimental The apparatus and the experimental conditions have been described in a previous paper [9]. The pressure of the sample gas in the collision region during the emission measurement was controlled to (2-6) X 10m5 Torr by d variable leak valve. The HNCO sample was prepared by heating a mixture of KOCN and excess stearic acid at 95 o C under vacuum [6]. It was dried over PZOs and collected at liquid-nitrogen temperature. After being distilled trap-to-trap at = - 60°C several times to remove CO, and other impurities, it was stored in a gas reservoir at a pressure less than 10 Torr to avoid polymerization. A spectral band pass of 0.17 nm fwhm mas used for the measurement of the emission spectra. The excitation functions of the NH(A-X), NH(c-a), NCO(B-X) emissions were measured with a beam current controlled to lo-40 &A. A spectral band pass of 0.32 nm fwhm 1%~ used for the measurement of the excitation function. The energy scale for the impinging electron was calibrated using a maximum on the cross section for the 1-O band of the N,(C 311,-B 3JIIs) emission by Finn et al. [lo]. The N?(C-B) emission from mixed N, and the NH and NC0 emissions from HNCO were measured alternately.
3. Results 3. I. Emission specrm The emissions in the region 250-500 nm obtained by electron impact on HNCO belong to the hydrogen Balmer lines. the CN(B’S+-X ?C*), NH(A311-X ‘2-) and NH(c’lI-a%) bands. In addition. t\\o weak broad emissions observed are attributable to the NCO(A’Z*-X ‘II) and
NCO(B’II-X “II) bands [3,11,12]. The emission spectrum which covers the region 324-346 nm consists of two heavily overlapping bands corresponding to the O-O band of the NH(c-a) system and the O-O and l-l bands of the NH(A-X) system. The rotational structure can be resolved only partly due to the limited resolution. Therefore, the observed spectrum IS compared with band envelopes simulated on a microcomputer. The procedure of the band-envelope analysis has been reported in detail (131. The observed spectrum at an impact energy of 21 eV is compared in fig. 1 with synthetic envelopes_ This analysis of the intensities of these NH emission bands provides information about the rotational distribution and the relative vibrational population, P,._,/Pc,._,( PI/P,), of the NH radical in the A3n and c ‘II states independently. The analysis also provides information about the relative formation rate. P(c’lI)/P(A311). after the dissociative excitation of HNCO. The parameters used in the best-fit simulated spectra are shown in figs. 2 and 3. The results derived from this simulation can be summarized as follows: (1) Formation of NH(c) is considerably smaller than that of NH(A) particularly near the onset. (2) The vibrational population ratio, PI/PO, of NH(A) is 0.24-0.30, which is nearly independent of the Impact energy_ The PI/PO for NH(c) is zero becduse of the predissociation of the u’ 2 1 levels of the c state [14]. (3) In the U’ = 0 level of the A state the rotational distribution corresponds to a Boltzmann temperature of 6000 +- 500 K, which is independent of the impact energy. The rotational distribution of the A state can be reproduced with this effective Boltzmann temperature. but the observed distribution above N’ = 25 for the d =0 level decreases more steeply than this statistical distribution because of the predissociation [14]. The rotational temperature of the u’ = 1 level is assumed to be equal to that of the u’ = 0 level. (4) The rotational distribution of the LJ’= 0 level of the c state for the electron energies > 19 eV can be represented by an effective Boltzmann temperature. 2700 f 700 K. This is obscured at electron energies =S 16 eV due to large error bars.
_I_ Tokue. Y Iro / Electrqn -Impact dissoctatiqn _of HNCO
I
53
I
325
330
335 340 Wavelengthlnm
345
Fig I. The observed spectrum of NH (dots) at an impact energy of 21 eV with 0.17 nm fwhm resolution_ The trace IS the best-fit synthetic spectrum (see text)_
3.2. Excitation ftmctrons The excitation functions of the NH(A-X) and NH(c-a) emissions are shown in fig. 4. The ob-
served spectra and the excitation function of the NH(c-a) emission show that NH(c) is formed directly; the onset of NH(c) is found to be 9.1 f 0.6 eV. The onset of NH(A), 7.5 t 0.5 eV, is in good agreement with the v&e, 7.4 eV, reported by Fukui
et al. [7]. Furthermore,
the second
rise in the
plot for NH(A) appears near 22 eV. The excitation functions of the NCO(A-X) and NCO(B-X) emissions are shown in fig. 5. The onsets of
(a ) NH(c)/NH(A) 06
T/K Ic
i 6000
1-j ,___;I.:; 1I_,] 0
10
Electron
20
-
UOO-
2000-
30
energy/eV
Fig 2 (a) Relative formation rate, P(c)/P(A). and (b) relative vibrational population PI/P0 of NH(A) The horizontal solid and broken lines indicate the statistically predicted values and those predicted from the Franck-Condon factors. respectively
Ol
0
10
Electron
%ergy/eV
I
30
Fig. 3. The rotationaI temperatures for NH: 0 for NH(A) 0 for NH(c).
and
I. Tokue. Y. Ito / Electron-unpact drssocration of HNCO
54
_.
_.
(b) -z‘SC:__-_-_-_-__--
iI
“NC0
“-NC0
20 Electron energy/&
0
FIN. 4. NH em~sston mte_umes versus electron ener~zr (a) the P
branch of the NH(A-X) emission at 340 nm. and (b) the R-branch bAndheadof the O-Oband of the NH(c-a) emission dt 324 nm. NCO(A) and NCO(B) are found to be 8.3 f 0.6 and 9.2 t 0.6 eV. respectively. In electron Impact at low-energy regions the selection rule for dipole transition is not valid; particularly. a spin-forbidden transition c&n occur through an electron-exchange excitation. It is well known that the cross section of an electron-exchange excitation has a sharp maximum just above the onset and decreases rapidly with increasing energy of the impinging electron. while an opticallly allowed excitation has a broad maximum in
(b)
_ _ __-_-;--_-_--_ __r*-_-= _ -_ ______ _____-.-___-I-c. - _i _-- _ (0) -_ .*
-_--
--
i .--
Electron
20
CS
40
40
energy/eV
Fig 5. NC0 emissionintenwirs versus electron energy: (a) the OO’O-00’0 band of the NCO(A-X) emissionat 438 5 nm, md (b) the 00’0~0010 bmd of the NCO(B-X) em&on at 315 nm.
NH-CO
“NC0
Fig. 6. A schematIc correlatton diagram (point group C,) of the smglet and tnplet states of HNCO with NH + CO or H + NCO. The excited HNCO states at the Franck-Condon region are shown.
the cross section above the onset. Thus,it appears that NH(A) near the onset is produced via an optically forbidden excitation. whereas NH(c), NCO(A) and NCO(B) are produced via an optically allowed excitation.
4_ Discussion 4.1. Eiectrotlic states of HNCO
and products
The ground state of HNCO is known to have a trans bent structure (point group C,) [l&16]. The electronic energies and the conformation of the excited HNCO have been investigated both experimentally [1,2] and theoretically [17]. Dixon and Kirby [l] have observed the four progressions between 281 and 229 nm in the UV absorption spectrum_ The rotational analysis of the progression around 266 nm indicates that the excited state is probably a ‘A” state; the excitation energy of 4.65 eV from the ground state of HNCO is estimated from the band origin. In the UV absorption spectra of HNCO and related compounds, Rabalais et al. [2] have concluded that the assignment of the 3A’ + ‘AI transition provides a rational identification of the Dixon-Kirby bands. In addition, they have observed the ‘A” + ‘A’ transition at 210 nm and the shoulder of the ‘A” -‘A’ transition at 182.5 run.
1. Tokue, Y. Ito j Elecrron-cm&t
Nevertheless, -the available experimental and theoretical information about the electronic states of HNCO in the 610 eV region is insufficient_ The energy levels of the singlet and triplet HNCO states are therefore estimated from a semi-empiri: cal calculation (CNDO/S) on the assumption that the Franck-Condon principle is valid. The stritctural parameters (5) determined by Fusina and Mills [18] were used for the equilibrium geometry in the CNDO/S calculation. The calculated configuration for the ground stateof HNCO is (1-7)a’2 la”’ (8-9)a’2 2a”‘, which is the same as that of previous works [2,17]. The excitation energies (in eV) computed are (experimental values [1,2] are given m parentheses): S-2(5.9) for the ‘A” state: 6.1 for ‘A’; 6.8(6.8) for ‘A”; and 5.2(4_65) for 3A”. The energy levels of the NH + CO products or H + NC0 products are derived using the electronic energy [19] and the heat of formation [20,21]. Recently, Sullivan et al. [22] have determined a new value for AHt2,,( HNCO), -0.61 + 0.03 eV, combining their results for AH,O,,,(NCO) with Okabe’s H-NC0 dissociation threshold [3]. The NH(X ‘Z-) + CO(X I,+) ground state thus obtained lies 3.11 eV above the ‘A’ ground state of HNCO. A schematic correlation diagram of the singlet and triplet states of HNCO with the NH + CO products or the H + NC0 products thus obtained is shown in fig. 6, taking account of the non-crossmg rule. 4.2 Internai
energy
distribution
of NH(A,
c)
The main dissociation processes leading to the formation of NH(A) and NH(c) from HNCO near their thresholds are as follows: HNCO
+ CO(X’z+)
+ NH#H),
AH = 6.81 -f 0.11 eV; + CO(X’Z+)
0)
+ NH(c’lT),
AH = 8.53 + 0.11 eV_
(2)
The AH values represent the thrcshold energies beyond which dissociation is possible. Process (1) is a spin-forbidden reaction, while process (2) is a spin-allowed reaction. The molecular structure of the precursor state forming NH(A) in equilibrium is not known. Re-
dissoeiatioitof HNCO
55
gardless of the conformation of this precursor state, we car- estimate the intensity of the NH vibrational transition by use of the elementary form of the Franck-Condon principle_ The relative vibrational population of NH(A) is calculated by use of the Morse function for NH(A) [19] and the harmonic function for the N-H potential of the ground-state HNCO [23] disregarding the N-C-O motion. The vibrational population ratio, = 0.08, derived from the Franck-Condon factor is much lower than the observed value (see fig. 2). This suggests that the precursor forming NH(A) has a very different geometry in equilibrium from the ground state of HNCO and a dynamical effect enhances the vibrational energy distribution in the formation of NH(A). The rotational temperatures of the d = 0 level of the NH(A) and NH(c) states and the PI/P0 of NH(A) produced by electron impact on HNCO are compared with those predicted by a simple statistical model based on information theory [24]: PI/P,,
P,(N)
a: (2N+
PI/P, = c (1 -fr v
l)C(l -_& D
-f,)3’2,
-fL>“Z
(1 -f,K L’
where P,(N) is the rotational distribution of the u’ = 0 level of NH, fN is the fractional rotational energy (E,/E,,) of NH, and fi and fv are the fractional vibrational energies of NH and CO, respectively. The available energy for the formation of NH(A), E,, = 0.7 eV, is given by the difference between the observed onset of NH(A) and the calculated threshold for process (1). This model assumes complete randomization of the available energy among all degrees of freedom in the products. The predicted vibrational population ratio, PJP,, = 0.126, is about a half of the observed value (see fig. 2); also, the predicted rotational temperature of the v’ = 0 level of the NH(A) state, 3500 +400 K, is about one half of the observed temperature. This suggests that the internal energy of NH(A) is selectively enhanced in the dissociation of the precursor state of HNCO. On the other hand, the observed rotational temperature of the u’ = 0 level of the NH(c) state, 2700 f 700 K, appears to coincide with the statistically predicted value, 2300-3050 K, using Eay = 0.6 eV_
I. Tokue.
56
4.3.
Formation
of
NCO(A,
Y. 120 /
Electron -Impact dlssoctarion of HNCO
B)
The main dissociation processes leading to the formation of NCO(A) and NCO(B) near their thresholds are: HNCO -
H(ls) -I- NCO(A’Z+).
from HNCO AH = 7.77 eV;
(3) -+ H(ls) + NCO(B’II).
AH = 8.89 eV.
(4) These are spin-allowed reactions_ The onsets of NCO(A) and NCO(B) are = 0.4 eV higher than the AH of the corresponding reactions_ This suggests that the internal energy of NCO(A. B) is strongly enhanced_ However. we have not attempted a population analysis of the NCO(A-X) and NCO(B-X) bands because of their weak intensitiesDixon [11.12] has discussed the possible electronic states of NC0 that can be formed from the low-lying states of N + CO or 0 f CN. The correlation diagram of the singlet HNCO states with the H + NC0 products is incomplete since little is known about these NC0 states except for A’Z’ and B’II. Woolley and Back [25] have investigated the photolysis of HNCO at 206 nm and found that the NH + CO and H + NC0 products are equally important_ The formation of non-linear HNCO from H(ls) and NCO(X’II) leads to the ‘A’ and ‘A’ species by resolving these states into a species with C, point group. By comparison of the energy of the incident photon with the electronic energies of HNCO and products. the precursor dissocrating into H(lsj+NCO(X) is estimated to be the ‘A” state near 5.2 eV and/or the ‘A’ state near 6.1 eV. The observed onset of NCO(A) indicates that the precursor forming H(ls) + NCO(A) products Lies7.7-8.9 eV above the ground-state of HNCO. The formation of non-linear HNCO from H(ls) and NCO(A’S’) leads to the ‘A’ species by resolving these states into a species with C, point group. The ‘A’(lla’ -9a’) state near 8.6 eV appears to correlate with the H(ls) + NCO(A) products. since the next ‘A’ state is = 2.4 eV higher than the product state. Similarly, the precursor state forming H(ls)+ NCO(B) products is ex-
petted to tie in the region 8.6-9.8 eV. The formation of non-linear HNCO(C,) from H(lsj + NCO(B’II) leads to the ‘A’ and ‘A” species. The ‘A”(12a 6 2a”) state near 9.6 eV seems to be the precursor dissociating into H(ls) + NCO(B), since the next ‘A” state is = 1.5 eV higher than the product state_ It is difficult to identify the ‘A’ precursor, since several ‘A’ states have electronic energies higher than 10 eV_ 4.4. Formation of NH(A,
c)
The observed onset of NH(A) indicates that the precursor forming NH(A) + CO(X) products lies 7.0-8.0 eV above the ground state of HNCO. The formation of non-linear HNCO from NH(A311) and CO(X’C+) leads to the 3A’ and 3A” species by resolving these states into a species with C, point group. On the basis of the non-crossing rule the -‘A’(3a” +- 2a”) state near 5.4 eV and the 3A”(3a” - 9L11)state near 6.8 eV appear to correlate with NH(A) + CO(X)_ Dissociation into NH(A) + CO(X) can be expected from the 3A”(3a” +- 9a’) state. because the calculated energy for the ‘A’(3a” - 2a”) state is = 1.5 eV lower than that of the NH(A) f CO(X) product state. However. the calculated electronic energy of the 3A” precursor state (6.8 eVj forming NH(A) coincides with the value corresponding to the NH(A) + CO(X) product state and cannot account for the difference between the observed onset of NH(A) and the calculated threshold of reaction (1). Two possible origins of this difference are: First, the accuracy of the CNDO/S calculation is insufficient and the 3A” precursor state is higher than the calculated value: second. the ‘A” state (6.8 eV) gives rise to avoided curve crossing with the ‘A” state (7.5 eVj. which results in a potential barrier. The two possible origins are consistent with the fact that the internal energy distribution of NH(A) is considerably higher than the statistically expected value, and thereupon we have not been able to identify the origin of the above difference_ The observed onset of NH(c) indicates that the precursor dissociating into the NH(c) -t- CO(X) products lies 8.5-9.7 eV above the ground state of HNCO. Combination of NH(c) and CO(X) in group C, yields the ‘A’ and ‘A” species. On the
I_ Toh-ue, Y. Ito / Electron-tmpacr dirsocranonof HNCO
basis of the non-crossing rule we suppose that the ‘A“(3a” 6 9a’) state near 6.8 eV and the ‘A’(lla’ - 9a’) state near 8.6 eV correlate with the NH(c) -f-CO(X) product state. By comparison of these electronic energies with the observed onset of NH(c), the ‘A’ state near 8.6 eV is estimated to be the precursor dissociating into NH(c)+ CO(X) products. The preceding discussion indicates that the ‘A’ state near 8.6 eV correlates with both NH(c)+ CO(X) and H(ls) + NCO(A). The very small formation rate of NH(c) near its onset, despite that it is a spin-allowed transition, may be caused by a predominant competing channel. This competing channel appears to be the formation of NCO(A). However, the formation of NCO(B) can compete with the formation of NH(c), since the onset of NH(c) is 0.8 eV higher than that of NCO(A) and coincides with that of NCO(B).
5. Conclusion It is concluded that NH(A) near its onset is produced via a spin-forbidden excitation, but that NH(c), NCO(A) and NCO(B) are produced via a spin-allowed excitation. The present study shows that NCO(A) is formed from a ‘A’ state of HNCO near 8.6 eV and NCO(B) is formed from a ‘A” state or a ?4’ state which lies at an energy > 9.0 eV. SimiIarly, NH(A) is formed from a 3A” state of HNCO (2 7 eV) with or without avoided curve crossing and that NH(c) is formed from a ‘A’ state near 8.6 eV. The formation of NH(c) seems to compete with that of NCO(A) or NCO(B), which is predominant. An ab initio calculation is planned for a better understanding of the electronic structure and the equilibrium geometry of the excited states of HNCO and NCO. The interna energy distribution of NH(A) is considerably higher than the value expected statistically. Even if this simple statistical model cannot be applied to the dissociation of such a small molecule as HNCO, this model may be used as a convenient criterion for energy-partitioning processes resulting from electron-impact dissociation, namely, for prediction of whether the available energy is completely randomized among all
57
degrees of freedom in the products or a specific mode is enhanced_ Acknowledgement
The authors are grateful to Professor K. Kuchitsu of The University of Tokyo for his many valuable discussions. References [I] RN
Divan and G_H i&by, Trans. Faraday Sot 64 (1968) 2002. [2] J-W. Rabalais. J R. McDonald and S P. McGI~M. J. Chrm Phys 51 (1969) 5103. [3] H. Okabe, J Chem Phys. 53 (1970) 3507. 141 J.W. Rabalais. J.R. hicDonJd. V. Schcrr and S P. McGlynn, Chem Rev. 71 (1971) 73. [5] W.S Drozdoski, A.P Baronavski and JR McDonald, Chem Phys Letters 64 (1979) 421. [6] G T. Fujimoto. M E. Umstead and M C. Lin. Chcm. Phys 65 (1982) 197. (71 K. Fukm, I. FuJlta and K Kuwata. J. Phys. Chem. 81 (1977) 1252 PI I. Tokue and Y Ito. Chem Phys. 79 (1983) 383 [91 I. Tokue, M. Ikarashi, S. Takizawa and Y. Ito, Bull Chem Sot. Japan 56 (1983) 583 WI T G. Finn. J EM Aarts and J P Doenng, J. Chem. Phys. 56 (1972) 5632 r111 R.N. Dixon. Phi Tram Ro> Sot. London A252 (1960) 165. WI RN. Dixon. Can. J. Phys. 38 (1960) 10 I131 I. Tokue and M. Iwai. Chem. Phys 52 (1980) 47 (141 W H Smith. J. Brozozowski and P. Erman. J. Chem Phys. 64 (1976) 4628. WI A-D McLean. G H. Loew and D S. Berkowttz, J. Mol Spectry 72 (1978) 430. WI K Yamada. J. Mol. Specny. 79 (1980) 323. J Chem 55 (1977) 149s 1171 A RaukandPF.Alewood.Can WI L. Fusina and LM. Mdls. J. Mol. Spectry 86 (1981) 485. t191 K P. Huber and G. Iierzberg, Molecular spectra and moIecuIar structure. Vol. 4. Constants of diatomic molecules (Van Nostrand-Reinhold. New York, 1979). Stull, ed. JANAF thermochemical tables (Dow PO1 DR. Chenucal. Michigan. 1965). Addenda (1966. 1967) 1211 L.G Piper. J Chem. Phys 70 (1979) 3417. WI B J Sullivan. G P. Smith and D R. CrosIey. Chem. Phys. Letters 96 (1983) 307. I231 M. Carlotti. G. di Lonardo, G. Galloni and A_ Trombeth. J Mol Spectry 62 (1976) 192. [24] R.B. Bernstein and R.D. Levme. in: Advances m atomic and molecular physics. ed D R Bates (Academic Press, New York. 1975) p. 215. [25] W.D. 295.
Woolley and RA
Back, Can. J. Chem. 46 (1968)