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The hydrogen atom channel in the photodissociation of HNCO
CHEMICALPHYSICSLETTERS
Volume 206, number 1,2,3,4
30 April 1993
The hydrogen atom channel in the photodissociation of HNCO Whikun Yi and Richard Bersohn Departmentof Chemistry,Columbia University,New York,NY 10027, USA Received 4 December 1992;in final form 15 February 1993
In the photodissociation of HNCO at 193.3and 212.6 nm the main channel is NH(a ‘A) t CO but the channel HtNCO has also been observed with a yield of 0.050f 0.006 at 193.3MI and 0.13+ 0.0 1 at 212.6 nm. Tbe average kinetic energy of the H atoms at 193.3and 212.6 nm is 10.7+ 1.5 and 11.4? 0.6 kcal/mol respectively. Results are compared with similar data recently repotted for HN,.
1.
Introduction
Isocyanic acid (HNCO ) is a sixteen valence eleo tron molecule with ground state C, geometry. In the ground state the NC0 group is linear or nearly so but the HNC angle is 125” [ 11. The UV (200-282 nm) and VUV ( 120-200 nm) spectrum have been measured by Dixon and Kirby [ 21 and Okabc [ 31 rcspectively. The UV absorption at lower energies exhibits vibrational and even rotational structure which disappears at higher energies. In the UV region the two possible spin-allowed-photodissociation channels are: HNCO+hv+NH(a’A)+CO, AE= 119 kcal/mol ,
0)
HNCO+hv-+H+NCO,
2. Experimental
AE= 115 kcal/mol .
(lb)
Classicalphotochemical studies showed that the major dissociation channel for HNCO involves the breaking of the C-N bond. However, real evidence for the breaking of the N-H bond has been elusive [4,5]. Nevertheless HNCO photodissociation is a good source of NC0 which is produced by a secondary reaction, NH+HNCO+NH,+NCO
rescence (LIF) . In the earliest studies by Drozdoski et al. the NH was shown to be primarily in the u=O sublevelof the a ‘Astate [ 61. Recently, an upper limit for the branching ratio, NCO/NH of dissociation fragments was reported to be 0.10 at 193 nm [ 71. Detailed measurements have been made on the disposal of energy in translation, vibration and rotation of NH(a’A) and CO(X ‘2) fragments of channel (la) [6,8-111. The present study was carried out to see if, in analogy with the isoelectronic HNNN, channel ( lb) exists and, if so, to measure its quantum yield and the average kinetic energy of the H atom. These experiments are similar to and interesting to compare with those reported recently for HNNN [ 121.
_
(2)
The channel ( 1a) has been extensively studied under collision-free conditions by laser-induced fluo-
WV light for excitingthe H atom fluorescencewas generated by a four-wave mixing technique [ 13,141. A Lambda Physik 201 MSC excimer laser simultaneously pumped two dye lasers, one of variable frequency near 845 nm and the other of fixed frequency at 425.0 nm. The 425.0 nm light was frequency doubled in a BBO crystal cut at an angle of 75” so that 425 nm light entering normal to one surface would have the maximum probability of being doubled. The energy of two 212.6 nm photons is resonant with a ‘p [ 1/210 state in Kr. Therefore the 212.6 and 845 nm light were focused together into a Kr cell by using
0009-2614/93/t 06.00 8 1993 Elsevier Science Publishers B.V. All rights reserved.
365
Volume 206, number 1,2,3,4
CHEMICALPHYSICSLETTERS
an achromatic lens, thus generating 121.6 nm (L,) light. The experiment exploited the coincidence that the 212.6 nm light used to generate the WV probing light exciting the H atom from the 1s to the 2p state could also be used as a pump light to dissociate the HNCO. The average time between the pump and probe pulses is therefore rather less than the 20 ns width of the fundamental pulse. Under these short time conditions pressures of 50-80 mTorr were sufficient to guarantee collision-free conditions including absence of secondary reactions. When 193.3 nm light was used to photodissociate, the intensity of the 2 12.6nm light and HNCO pressure was reduced and no signal was seen when the ArF laser was blocked. The LIF signal of the H atoms was detected by an EMR solar blind photomultiplier (PMT) whose output pulse was fed to a boxcar averager where it was normalized by a pulse from another solar blind PMT which monitored the VUV exciting light. The output of the boxcar averager was sent to a computer to calculate the average translational energy of the H atoms.
3. Results Atomic hydrogen LIF was in fact seen when dissociation was at 193.3 or 212.6 nm. No matter what the kinetic energy of the H atom the NC0 must be in its ground X 211electronic state. A 193.3 nm photon (147.9 kcal/mol) does not have enough energy to excite the A(%) state of NCO, which is 2.85 eV above the ground state. Fig. 1 is the LIF spectrum of nascent H atoms generated from the photodissociation of HNCO at 212.6 nm. The average kinetic energy of the H atoms is determined from their Doppler broadened spectra as usual. The absorption frequency of H atoms moving toward the probe laser at velocity u, is shifted by v,,u,/c from its center frequency vo. Thus the second moment of the symmetric LIF excitation curve taken with respect to the center is ( v,,/c)~( vz ). If the velocity distribution is isotropic, then the average kinetic energy of the H The value so atom, (ET)= jm(v’) =tm(vt). measured is 11.4f0.6 kcal/mol. The total translational energy release including the recoil of the NC0 radical is (43/42) ( 11.4) = 11.7 kcal/mol. At 193.3 366
3OAprill993
121.6 nm
wrvslrn6th
(nm)
Fig. 1. Laser-induced fluorescence spectrum of H from the photodissociation of HNCO at 2 12.6nm.
nm the averagetotal kinetic energy is 11.Ot 1.5 kcall mol. The dissociation energy calculated from the heats of formation of HNCO and NC0 ( - 24.9 f2.8 and 36.9 kcal/mol, respectively) is 114.8+ 2.8 kcal/mol. The available energy is the proton energy plus the initial internal energy of the HNCO moleculeless the bond dissociation energy, that is, EAvL= 134.5+ 1.O- 114.8= 20.2 kcal/mol. The average fraction of the available energy released as translation, cfT> = 0.58&0.10. At 193.3 nm cf,} is only 0.32kO.08. These values are distinctly smaller than the values for HN3 which were 0.83 and 0.89 at 193 and 248 nm respectively [ 121. The absolute quantum yield, &(HNCO) for the process HNCO+H+NCO was obtained by comparison with the quantum yield of H2S which was assumed to be 1 [ 151, The ratio of the LIF signal of H atoms from HNCO to that from HIS was measured. The following equation was then used to calculate &(NCO): AH(HNCO) AH(&S)
~HNCOPHNCO& (NCO) =
~H~~H&IH(&S)
(3) '
and Pi are the molar extinction coefficient, area under the LIF excitation curve and pres-
where Ei, Ai
CHEMICAL PHYSICS LETTERS
Volume 206, number 1,2,3,4
sure of compound i. The molar extinction coefficients for the two wavelengths, 193.3 and 2 12.6 nm are taken from the literature, CHN~=60and80and eHls=832 and 1164M-‘cm-‘respectively [16,17]. The final results are quantum yields h(HNC0) = 0.050f0.006 and 0.13&0.01 at 193.3 and 212.6 nm respectively, showingthat channel ( la) exists but is minor. An upper limit of 0.10 was previously reported [ 71 for 193.3 nm excitation consistent with our measurement of 0.0502 0.006. The 121.6 nm light produced by this four-wave mixing difference technique is so intense that molecules which absorb strongly at this wavelength can exhibit a two-photon effect in which one Lyman alpha photon is absorbed and dissociates the molecule and the second Lyman alpha photon is absorbed by the product H atom. The molar extinction coeff% cient of HNCO at 121.6 nm is approximately 400 times larger than that at 2 12.6 nm [ 1,161. To check that this was not a problem, dissociation was carried out by a 193.3 nm laser attenuated so that its output, about 0.5 mJ was about the same as the 2 12.6 nm laser and the 121.6 nm light, delayed by 100 ns was generated by frequency tripling of 364.8 nm light generated by a YAG laser. An H signalwas seen only when the system was exposed to the light of both lasers. This is a reasonableresult because the probe light was about IO4 times weaker than the dissociating light.
4. Discussion It is interesting to compare the partitioning of the
30 April 1993
availableenergy between rotational, translationaland vibrational modes in both HNCO and HNNN at different photon energies. Allavailable data are summarized in table 1. According to the simple impulsive model the fraction of energy released as rotation of NH is given by
(4a)
=pNC(rN
sin2%INN/~NH)
,
(4b)
for HNCO and HNNN dissociations respectively. In these equations c(NCis the reduced mass of atoms N and C, INHis the moment of inertia of NH( a ‘A), I is the distance between the N bound to H and the center of mass of the leaving CO or NZgroup and yi/k is the
ijk angle at the instant of dissociation.
The NH dissociated from HNNN has less rotational energy than the NH from HNCO. From eqs. (4) we can deduce that the angle y~pn:is smallerthan the angle yHNNassuming that both angles are larger than 90”. This result is also consistent with the observations of Gericke et al. on HNNN and ours on HNCO. The average fraction of energy released as translation in HNCO was 0.32 and 0.58 at 193.3and 2 12.6 nm respectively whereas for HNNN the fraction was 0.83 and 0.89 at two different wavelengths. If the I-INNgroup of HNNN is more nearly linear in the transition state, repulsion of the H atom as it leaves generates less rotation of the NS fragment and therefore more translationalenergy as compared with HNCO.
Table I Energyreleaseon photodissociationof HNCO and HNNN HNCO+H+NCO
HNNN+H+NNN
&)(H-NCO)=0.32?0.08 G>(H-NCO)=0.56&0.10 HNCO+NH(a’A)
(193.3 nm) (212.6 nm)
tC0
#&#INCO)=O.12 (23Onm) [9] #&&-INCO) =0.20 (193 nm) [7] (f,),(HNCO)=O.l4(193nm) [8] cfr>(HN-CO)=0.93 (230nm) [ll]
&)(H-NNN)=0.83 Cfr>(H-NNN)=0.89
(193nm) [121 (248 nm) [12]
HNNN+NH(a ‘A) tN, cf,)&-INNN)=O.34 (283nm) [17] cf,),(HNNN)=O.O4 (248 nm) [18] cfv)Nn(HNNN)=0.06 (248 nm) [18] (jr) (HN-NN) ~0.45 (248 nm) [ 181 Cf,)(HN-NN)=0.60 (283 nm) 1171
367
Volume 206,number1,2,3,4
CHEMICAL PHYSICS LETTERS
A similar argument can be used to compare hco withy. Wecompare ~~)co(HNCO)=0.20with &),(HN,)=0.34andalso Cf,)(HN-CO)=0.93 with (fT) (HN-NN) = 0.45 and 0.60. Both comparisons are consistent with the assumption that ba is a larger angle than yNNN,i.e. more nearly linear. Dixon and Kirby deduced from rotational structure that in the upper state the NC0 group is stronglybent and the angleyNcomay be as small as 120” [ 21. Their observation was made at 37500 cm-’ but in the present experiments at 47000 cm- ’predissociation may be so rapid that the equilibrium angle is irrelevant. HMCO has now been shown to be one of a set of molecules of the form HABC which on irradiation dissociate in two different ways, mostly as HA t BC but with a nonnegligible fraction forming H t ABC. The set includes HNNN, HOOH and HOCH3which have quantum yields for H atom formation at 193 nm of 0.15, 0.12, and 0.86 [12,19,20]. In summary, the H atom channel for the phots dissociation of HNCO has been established at least at 193.3 and 212.6 nm. At the latter wavelength the quantum yield is 0.13 &0.01 and the average translational energy release is 11.7 kcal/mol or 58 -t 10% of the available energy. These data are qualitatively in agreement with previous work which showed that in the excited state both the HNC and the NC0 groups are strongly bent. Very recently Neumark and co-workers [ 2 1 ] have reported evidence that the heat of formation of NC0 is 30.5+ 1 and not 36.9 kcal/mol as found by Spigkmin and Chandler [ II]. Unfortunately, our data do not shed any light on this discrepancy. A decrease in heat of formation of a fragment just means an increase in available energy. As the average kinetic energy is much less than the available energy, we can say nothing about the true value of the latter. A detailed investigation of the internal state distribution of the NC0 partner might resolve the problem.
368
30 April 1993
Acknowledgement This research was supported by the US National Science Foundation.
References [ 1 ] K. Yamada, J. Mol. Spectry. 79 ( 1980) 323. [Z] R.N.DixonandG.H. Kirby, Trans.FaradaySoc. 64 (1968) 911. [3] H. Okabe, J. Chem. Phys. 53 (1970) 3507. [4] W.D. Woo1eyandR.A. Back,Can. J. Chem. 46 (1968) 295. [S] J.N. Bradley, J.R. Gilbert and P. Svejda, Trans. Faraday Soc.64 (1968) 911. [6] W.S. Drozdoski, A.P. Baronavski and J.R. McDonald, Chem. Phys. Letters 64 (1979) 421. [7] T.A. Spiglanin, R.A. Perry and D.W. Chandler, J. Phys. Chem. 90 (1986) 6184. [8] G.T. Fujimoto, M.E. Umsteadand M.C. Lin, Chem. Phys. 65 (1982) 197. [9] T.A. Spiglanin and D.W. Chandler, J. Chem. Phys. 87 (1987) 1577. [lo] T.A. Spiglanin, R.A. Perry and D.W. Chandler, J. Chem. Phys. 87 (1987) 1568. [ 11) T.A. Spiglanin and D.W. Chandler, Chem. Phys. Letters 141 (1987) 428. [ 121 K.H. Gericke, M. Lock and F.J. Comes, Chem. Phys. Letters 186 (1991) 427. [ 131 G.C. Bjorklund, IEEE J. Quantum Electron. 11 (1975) 187. [ 141 T. Tsuchizawa, K. Yamanouchi and S. Tsuchiya, J. Cbem. Phys. 92 (1990) 1560. [ 151 R.E. Continetti, B.A. Balko and Y.T. Lee, Chem. Phys. Letters 182 ( 199 1) 420. [ 161 J.W. Rabalais, J.R. McDonald and S.P. McGlynn, J. Chem. Phys. 51 (1969) 5103. [ 171 J. Chu, P. Marcus and P.J. Dagdigian, J. Chem. Phys. 93 ( 1990) 257. [ 181 F. Rohrer and F. Stuhl, J. Chem. Phys. 88 (1988) 4788. [ 191 U. Gerlach-Meyer, E. Linnebach, K. Kleinermanns and J. Wolfrum, Chem. Phys. Letters 133 (1987) 113. [201 S. Satyapal, J. Park, B. Katz and R. Bersohn, J. Chem. Phys. 91 (1989) 6873. [21] D.R. Cyr, R.E. Continetti, R.B. Metz, D.L.0sbomandD.M. Neumark, J. Chem. Phys. 97 (1992) 4937.
Volume 206, number 1,2,3,4
30 April 1993
The hydrogen atom channel in the photodissociation of HNCO Whikun Yi and Richard Bersohn Departmentof Chemistry,Columbia University,New York,NY 10027, USA Received 4 December 1992;in final form 15 February 1993
In the photodissociation of HNCO at 193.3and 212.6 nm the main channel is NH(a ‘A) t CO but the channel HtNCO has also been observed with a yield of 0.050f 0.006 at 193.3MI and 0.13+ 0.0 1 at 212.6 nm. Tbe average kinetic energy of the H atoms at 193.3and 212.6 nm is 10.7+ 1.5 and 11.4? 0.6 kcal/mol respectively. Results are compared with similar data recently repotted for HN,.
1.
Introduction
Isocyanic acid (HNCO ) is a sixteen valence eleo tron molecule with ground state C, geometry. In the ground state the NC0 group is linear or nearly so but the HNC angle is 125” [ 11. The UV (200-282 nm) and VUV ( 120-200 nm) spectrum have been measured by Dixon and Kirby [ 21 and Okabc [ 31 rcspectively. The UV absorption at lower energies exhibits vibrational and even rotational structure which disappears at higher energies. In the UV region the two possible spin-allowed-photodissociation channels are: HNCO+hv+NH(a’A)+CO, AE= 119 kcal/mol ,
0)
HNCO+hv-+H+NCO,
2. Experimental
AE= 115 kcal/mol .
(lb)
Classicalphotochemical studies showed that the major dissociation channel for HNCO involves the breaking of the C-N bond. However, real evidence for the breaking of the N-H bond has been elusive [4,5]. Nevertheless HNCO photodissociation is a good source of NC0 which is produced by a secondary reaction, NH+HNCO+NH,+NCO
rescence (LIF) . In the earliest studies by Drozdoski et al. the NH was shown to be primarily in the u=O sublevelof the a ‘Astate [ 61. Recently, an upper limit for the branching ratio, NCO/NH of dissociation fragments was reported to be 0.10 at 193 nm [ 71. Detailed measurements have been made on the disposal of energy in translation, vibration and rotation of NH(a’A) and CO(X ‘2) fragments of channel (la) [6,8-111. The present study was carried out to see if, in analogy with the isoelectronic HNNN, channel ( lb) exists and, if so, to measure its quantum yield and the average kinetic energy of the H atom. These experiments are similar to and interesting to compare with those reported recently for HNNN [ 121.
_
(2)
The channel ( 1a) has been extensively studied under collision-free conditions by laser-induced fluo-
WV light for excitingthe H atom fluorescencewas generated by a four-wave mixing technique [ 13,141. A Lambda Physik 201 MSC excimer laser simultaneously pumped two dye lasers, one of variable frequency near 845 nm and the other of fixed frequency at 425.0 nm. The 425.0 nm light was frequency doubled in a BBO crystal cut at an angle of 75” so that 425 nm light entering normal to one surface would have the maximum probability of being doubled. The energy of two 212.6 nm photons is resonant with a ‘p [ 1/210 state in Kr. Therefore the 212.6 and 845 nm light were focused together into a Kr cell by using
0009-2614/93/t 06.00 8 1993 Elsevier Science Publishers B.V. All rights reserved.
365
Volume 206, number 1,2,3,4
CHEMICALPHYSICSLETTERS
an achromatic lens, thus generating 121.6 nm (L,) light. The experiment exploited the coincidence that the 212.6 nm light used to generate the WV probing light exciting the H atom from the 1s to the 2p state could also be used as a pump light to dissociate the HNCO. The average time between the pump and probe pulses is therefore rather less than the 20 ns width of the fundamental pulse. Under these short time conditions pressures of 50-80 mTorr were sufficient to guarantee collision-free conditions including absence of secondary reactions. When 193.3 nm light was used to photodissociate, the intensity of the 2 12.6nm light and HNCO pressure was reduced and no signal was seen when the ArF laser was blocked. The LIF signal of the H atoms was detected by an EMR solar blind photomultiplier (PMT) whose output pulse was fed to a boxcar averager where it was normalized by a pulse from another solar blind PMT which monitored the VUV exciting light. The output of the boxcar averager was sent to a computer to calculate the average translational energy of the H atoms.
3. Results Atomic hydrogen LIF was in fact seen when dissociation was at 193.3 or 212.6 nm. No matter what the kinetic energy of the H atom the NC0 must be in its ground X 211electronic state. A 193.3 nm photon (147.9 kcal/mol) does not have enough energy to excite the A(%) state of NCO, which is 2.85 eV above the ground state. Fig. 1 is the LIF spectrum of nascent H atoms generated from the photodissociation of HNCO at 212.6 nm. The average kinetic energy of the H atoms is determined from their Doppler broadened spectra as usual. The absorption frequency of H atoms moving toward the probe laser at velocity u, is shifted by v,,u,/c from its center frequency vo. Thus the second moment of the symmetric LIF excitation curve taken with respect to the center is ( v,,/c)~( vz ). If the velocity distribution is isotropic, then the average kinetic energy of the H The value so atom, (ET)= jm(v’) =tm(vt). measured is 11.4f0.6 kcal/mol. The total translational energy release including the recoil of the NC0 radical is (43/42) ( 11.4) = 11.7 kcal/mol. At 193.3 366
3OAprill993
121.6 nm
wrvslrn6th
(nm)
Fig. 1. Laser-induced fluorescence spectrum of H from the photodissociation of HNCO at 2 12.6nm.
nm the averagetotal kinetic energy is 11.Ot 1.5 kcall mol. The dissociation energy calculated from the heats of formation of HNCO and NC0 ( - 24.9 f2.8 and 36.9 kcal/mol, respectively) is 114.8+ 2.8 kcal/mol. The available energy is the proton energy plus the initial internal energy of the HNCO moleculeless the bond dissociation energy, that is, EAvL= 134.5+ 1.O- 114.8= 20.2 kcal/mol. The average fraction of the available energy released as translation, cfT> = 0.58&0.10. At 193.3 nm cf,} is only 0.32kO.08. These values are distinctly smaller than the values for HN3 which were 0.83 and 0.89 at 193 and 248 nm respectively [ 121. The absolute quantum yield, &(HNCO) for the process HNCO+H+NCO was obtained by comparison with the quantum yield of H2S which was assumed to be 1 [ 151, The ratio of the LIF signal of H atoms from HNCO to that from HIS was measured. The following equation was then used to calculate &(NCO): AH(HNCO) AH(&S)
~HNCOPHNCO& (NCO) =
~H~~H&IH(&S)
(3) '
and Pi are the molar extinction coefficient, area under the LIF excitation curve and pres-
where Ei, Ai
CHEMICAL PHYSICS LETTERS
Volume 206, number 1,2,3,4
sure of compound i. The molar extinction coefficients for the two wavelengths, 193.3 and 2 12.6 nm are taken from the literature, CHN~=60and80and eHls=832 and 1164M-‘cm-‘respectively [16,17]. The final results are quantum yields h(HNC0) = 0.050f0.006 and 0.13&0.01 at 193.3 and 212.6 nm respectively, showingthat channel ( la) exists but is minor. An upper limit of 0.10 was previously reported [ 71 for 193.3 nm excitation consistent with our measurement of 0.0502 0.006. The 121.6 nm light produced by this four-wave mixing difference technique is so intense that molecules which absorb strongly at this wavelength can exhibit a two-photon effect in which one Lyman alpha photon is absorbed and dissociates the molecule and the second Lyman alpha photon is absorbed by the product H atom. The molar extinction coeff% cient of HNCO at 121.6 nm is approximately 400 times larger than that at 2 12.6 nm [ 1,161. To check that this was not a problem, dissociation was carried out by a 193.3 nm laser attenuated so that its output, about 0.5 mJ was about the same as the 2 12.6 nm laser and the 121.6 nm light, delayed by 100 ns was generated by frequency tripling of 364.8 nm light generated by a YAG laser. An H signalwas seen only when the system was exposed to the light of both lasers. This is a reasonableresult because the probe light was about IO4 times weaker than the dissociating light.
4. Discussion It is interesting to compare the partitioning of the
30 April 1993
availableenergy between rotational, translationaland vibrational modes in both HNCO and HNNN at different photon energies. Allavailable data are summarized in table 1. According to the simple impulsive model the fraction of energy released as rotation of NH is given by
(4a)
=pNC(rN
sin2%INN/~NH)
,
(4b)
for HNCO and HNNN dissociations respectively. In these equations c(NCis the reduced mass of atoms N and C, INHis the moment of inertia of NH( a ‘A), I is the distance between the N bound to H and the center of mass of the leaving CO or NZgroup and yi/k is the
ijk angle at the instant of dissociation.
The NH dissociated from HNNN has less rotational energy than the NH from HNCO. From eqs. (4) we can deduce that the angle y~pn:is smallerthan the angle yHNNassuming that both angles are larger than 90”. This result is also consistent with the observations of Gericke et al. on HNNN and ours on HNCO. The average fraction of energy released as translation in HNCO was 0.32 and 0.58 at 193.3and 2 12.6 nm respectively whereas for HNNN the fraction was 0.83 and 0.89 at two different wavelengths. If the I-INNgroup of HNNN is more nearly linear in the transition state, repulsion of the H atom as it leaves generates less rotation of the NS fragment and therefore more translationalenergy as compared with HNCO.
Table I Energyreleaseon photodissociationof HNCO and HNNN HNCO+H+NCO
HNNN+H+NNN
&)(H-NCO)=0.32?0.08 G>(H-NCO)=0.56&0.10 HNCO+NH(a’A)
(193.3 nm) (212.6 nm)
tC0
#&#INCO)=O.12 (23Onm) [9] #&&-INCO) =0.20 (193 nm) [7] (f,),(HNCO)=O.l4(193nm) [8] cfr>(HN-CO)=0.93 (230nm) [ll]
&)(H-NNN)=0.83 Cfr>(H-NNN)=0.89
(193nm) [121 (248 nm) [12]
HNNN+NH(a ‘A) tN, cf,)&-INNN)=O.34 (283nm) [17] cf,),(HNNN)=O.O4 (248 nm) [18] cfv)Nn(HNNN)=0.06 (248 nm) [18] (jr) (HN-NN) ~0.45 (248 nm) [ 181 Cf,)(HN-NN)=0.60 (283 nm) 1171
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Volume 206,number1,2,3,4
CHEMICAL PHYSICS LETTERS
A similar argument can be used to compare hco withy. Wecompare ~~)co(HNCO)=0.20with &),(HN,)=0.34andalso Cf,)(HN-CO)=0.93 with (fT) (HN-NN) = 0.45 and 0.60. Both comparisons are consistent with the assumption that ba is a larger angle than yNNN,i.e. more nearly linear. Dixon and Kirby deduced from rotational structure that in the upper state the NC0 group is stronglybent and the angleyNcomay be as small as 120” [ 21. Their observation was made at 37500 cm-’ but in the present experiments at 47000 cm- ’predissociation may be so rapid that the equilibrium angle is irrelevant. HMCO has now been shown to be one of a set of molecules of the form HABC which on irradiation dissociate in two different ways, mostly as HA t BC but with a nonnegligible fraction forming H t ABC. The set includes HNNN, HOOH and HOCH3which have quantum yields for H atom formation at 193 nm of 0.15, 0.12, and 0.86 [12,19,20]. In summary, the H atom channel for the phots dissociation of HNCO has been established at least at 193.3 and 212.6 nm. At the latter wavelength the quantum yield is 0.13 &0.01 and the average translational energy release is 11.7 kcal/mol or 58 -t 10% of the available energy. These data are qualitatively in agreement with previous work which showed that in the excited state both the HNC and the NC0 groups are strongly bent. Very recently Neumark and co-workers [ 2 1 ] have reported evidence that the heat of formation of NC0 is 30.5+ 1 and not 36.9 kcal/mol as found by Spigkmin and Chandler [ II]. Unfortunately, our data do not shed any light on this discrepancy. A decrease in heat of formation of a fragment just means an increase in available energy. As the average kinetic energy is much less than the available energy, we can say nothing about the true value of the latter. A detailed investigation of the internal state distribution of the NC0 partner might resolve the problem.
368
30 April 1993
Acknowledgement This research was supported by the US National Science Foundation.
References [ 1 ] K. Yamada, J. Mol. Spectry. 79 ( 1980) 323. [Z] R.N.DixonandG.H. Kirby, Trans.FaradaySoc. 64 (1968) 911. [3] H. Okabe, J. Chem. Phys. 53 (1970) 3507. [4] W.D. Woo1eyandR.A. Back,Can. J. Chem. 46 (1968) 295. [S] J.N. Bradley, J.R. Gilbert and P. Svejda, Trans. Faraday Soc.64 (1968) 911. [6] W.S. Drozdoski, A.P. Baronavski and J.R. McDonald, Chem. Phys. Letters 64 (1979) 421. [7] T.A. Spiglanin, R.A. Perry and D.W. Chandler, J. Phys. Chem. 90 (1986) 6184. [8] G.T. Fujimoto, M.E. Umsteadand M.C. Lin, Chem. Phys. 65 (1982) 197. [9] T.A. Spiglanin and D.W. Chandler, J. Chem. Phys. 87 (1987) 1577. [lo] T.A. Spiglanin, R.A. Perry and D.W. Chandler, J. Chem. Phys. 87 (1987) 1568. [ 11) T.A. Spiglanin and D.W. Chandler, Chem. Phys. Letters 141 (1987) 428. [ 121 K.H. Gericke, M. Lock and F.J. Comes, Chem. Phys. Letters 186 (1991) 427. [ 131 G.C. Bjorklund, IEEE J. Quantum Electron. 11 (1975) 187. [ 141 T. Tsuchizawa, K. Yamanouchi and S. Tsuchiya, J. Cbem. Phys. 92 (1990) 1560. [ 151 R.E. Continetti, B.A. Balko and Y.T. Lee, Chem. Phys. Letters 182 ( 199 1) 420. [ 161 J.W. Rabalais, J.R. McDonald and S.P. McGlynn, J. Chem. Phys. 51 (1969) 5103. [ 171 J. Chu, P. Marcus and P.J. Dagdigian, J. Chem. Phys. 93 ( 1990) 257. [ 181 F. Rohrer and F. Stuhl, J. Chem. Phys. 88 (1988) 4788. [ 191 U. Gerlach-Meyer, E. Linnebach, K. Kleinermanns and J. Wolfrum, Chem. Phys. Letters 133 (1987) 113. [201 S. Satyapal, J. Park, B. Katz and R. Bersohn, J. Chem. Phys. 91 (1989) 6873. [21] D.R. Cyr, R.E. Continetti, R.B. Metz, D.L.0sbomandD.M. Neumark, J. Chem. Phys. 97 (1992) 4937.