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Experimental and theoretical study of the dissociation energies D0(H2NH) and D0(H2N+H) and other related quantities
CHEMICAL
10 March 1995
PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 234 (1995) 450-454
Experimental and theoretical study of the dissociation energies Do(HzN-H) and Do(HzN+-H) and other related quantities Fei Qi a, Liusi Sheng a, Yunwu Zhang a, Shuqin Yu b, Wai-Kee Li c National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, People's Republic of China b Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, People's Republic of China c Department of Chemistry, The Chinese University ofHong Kong, Shatin, N.T., Hong Kong
Received 2 November1994; in final form 5 January 1995
Abstract
Combining the techniques of molecular beam and vacuum ultraviolet synchrotron radiation photoionization mass spectroscopy, we have measured the ionization energy of NH 3 (10.16 + 0.02 eV) and the appearance potentials of H + (18.57 +0.05 eV) and NH~- (15.75 +0.02 eV). From these data, we have obtained dissociation energy values for Do(H2N-H) (4.97 + 0.05 eV), D0(H2N+-H) (5.59 + 0.02 eV) and Do(H2N-H +) (8.41 + 0.05 eV) and the ionization energy IE(NH 2) (10.78 + 0.05 eV). High-level ab initio calculations lead to results that are in good agreement with the experimental findings.
1. Introduction The precise determination of the N - H bond dissociation energy in ammonia, D0(HzN-H), is of fundamental importance in both physical and biological sciences. Yet despite numerous studies, the experimentally observed values for this quantity are only in fair agreement with each other (see the first entry in Table 2). Currently, the experimental procedures for the determination of D0(H2N-H) may be broadly classified into two categories. The first is Fourier transform ion cyclotron resonance mass spectroscopy (FT/ICRMS). Making use of the principles of chemical equilibrium, two reference acids are selected and the gaseous acidity of NH3, AHacid(NH3), is determined by interpolation. The dissociation energy can then be calculated, D o ( R - H ) = AHacio(Rn ) + EA(R) - I E ( H ) ,
(1)
where EA(R) is the electron affinity of radical R and IE(H) is the ionization energy of the hydrogen atom. Employing this method, Brauman and Blair obtained a D 0 ( H z N - H ) value of 4.77 eV [1]. Two years later, Bohme and co-workers obtained a slightly different value of 4.60 eV [2]. On the other hand, an alternative procedure is photoionization mass spectrometry (PIMS). The photoionization process involved is AP(R+) R - H + hv ~R++ H + e . (2) The dissociation energy D0(R-H) can be obtained upon measuring the appearance potential of R +, AP(R+), and ionization energy of R, IE(R), D o ( R - H ) = A P ( R +) - I E ( R ) .
The D0(H2N-H) values determined by PIMS are 4.53 [3] and 4.63 eV [4]. Based on the same basic
0009-2614/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0009-2614(95)00059-3
(3)
F. Qi et al. / Chemical Physics Letters 234 (1995) 450-454
principle, in 1987, Lee and co-workers [5] first employed synchrotron radiation to determine the C - H bond dissociation energies in C2H 2 and C z H 4. The photoionization process and the energy relationship concerned are Apm +) RH + hu , R + H++ e-, (4) D0(R-H ) =AP(H+)-IE(H). (5) Since IE(H) is well known (13.598 eV), we only need to measure AP(H ÷) in process (4) in order to calculate D0(R-H). Also, synchotron radiation has the advantages of being highly intense and having a wide wavelength range. It is particularly suitable for photoionization processes requiring photons with wavelengths smaller than 100 nm. In the present work, we have studied the photoionization of NH 3 with synchrotron radiation and measured AP(H +) accurately. Furthermore, we have also measured IE(NH 3) and AP(NH~-). From these measurements, the bond dissociation energies D o ( H 2 N - H ) , Do(HzN+-H) and Do(HzN-H +) and ionization en-
with a thickness of 0.5 mm was inserted into the beam path to eliminate second- and higher-order radiation in the wavelength range beyond 1050 A. A molecular beam was formed by supersonic expansion of sample gas from a pulsed nozzle of 0.5 mm diameter into the ion chamber through a 1 mm skimmer. The beam was ionized at about 70 mm downstream of the nozzle by the focused syn(a) ~ .z. _z o z ='~
..:.~ NH + ..,,'. 3 .•.~.:. • : .... "•... ".".nr-. ~0.1~°v ;.: • | ,'T.A:~....~"
I--
•la
z
w
,,-
T
1150
1170
1190
~aso (b)
i~.':..~,~... ~ "~.,~o z ~'~¢e --. to
NH +
a
,,,,11,
%•
z
W ->
~d~=,• 15.75oV
~=
'%%
2.1. Photoionization measurements
,
,
720
The experimental apparatus has been described previously elsewhere [6], and hence it is only briefly outlined here. Synchrotron radiation from the Hefei 800 MeV light source is monochromized with a 1 m Seya-Namioka monochromator equipped with three spherical gratings (2400 I/mm coated with gold, 1200 l/mm with iridium and 600 l/mm with aluminum) covering the wavelength range from 350 to 6000 ,~. Both pre- and post-focusing mirrors are gold-coated toroidal ones. The absolute wavelength scale was calibrated to better than 1 ,~ by recording Ne and Ar ionization potentials. The wavelength resolution of the monochromator was set at 1 A over the wavelength range from 350 to 1500 .~ used in these experiments. A lithium fluoride cutoff filter
12a0
,,
o_
2. Experimental and theoretical methods
1210
WAVELENGTH(A)
ergy IE(NH 2) can be calculated. To complement
these experimental studies, we have also carried out high-level ab initio computations to determine the energies of these dissociation and ionization processes.
451
J
,
,
,
i
,
,
,
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740
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,
,
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800
780
WAVELENGTH(A) (c) ~ z ~ o_ -~ ~ " tO
".::t•• , ~°~"'°"~•'%'~--~..*'" ~. ;"."" .~..~.% o='•
•
• "~-=g
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•
"%,. ~857.v "¢'~"%l .
.
.
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.
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.
.
.
700
WAVELENGTH(A) Fig. 1. Photoionization from N H 3.
and (c)H ÷
efficiencycurves of (a)
NH~-, (b) NH~-
452
F. Qi et al. / Chemical Physics Letters 234 (1995) 450-454
Table 1 Total energies " (E 0) and unscaled zero-point vibrational energies b (ZPVE) of the various species studied in this work
31G(d), This level was chosen since it is the level to which the Gaussian-2 procedure [7] is approximated. All calculations were carried out on an I B M
Species NH3 (C3v; ~A1) NH~ (C~; 2A~) NH 2 (C2v; 2B 1) NH~- (C2v; 3B 1) NH~ (C2v; 1Aj) NH (C~v; 3E) H (2S)
R S 6 0 0 0 / 3 4 0 workstation using the G A U S S I A N 92 [8] package of programs.
E 0 (hartree) -56.47114 -56.09985 -55.79203 -55.38416
-55.24419 - 55.13916 -0.49981
ZPVE (kJ tool 1) 92.7 88.0 51.4 46.1 48.8 20.1 -
3. R e s u l t s a n d d i s c u s s i o n
a Calculated at the QCISD(T)/6-311 + G(3df, 2p)//MP2/631G(d, p) level. b Calculated at the MP2/6-31G(d, p)level.
The photoionization efficiency curves for the formation of H +, N H ~ and NH~- are displayed in Fig. 1. The threshold for the formation of each ion is clearly discernible. From these curves, the A P s for
chrotron radiation. The sample gas N H 3 ( > 99.9% purity) was mixed with Ne carrier gas (pressure ratio, NH 3 : Ne = 1 : 5) in this work. The pressure of the ionization chamber was about 5 × 10 5 Pa when the b e a m was turned on. As our experiments were carried out under the condition of a supersonic beam, the temperature of the system was quite low, of the order of 101 K. Hence it is not necessary to make any corrections for thermal effects. Ions were detected with a quadrupole mass spectrometer with a channeltron electron multiplier. The apparatus was provided with a computerized control and data acquisition system. The m o n o c h r o m a t o r was u s u a l l y scanned with a wavelength increment of 0.5 A and the time of data acquisition for each point was 10 to 20 s d e p e n d i n g on ion abundance,
the three ions can be measured: A P ( H ÷) = 18.57 _ 0.05 eV, A P ( N H ~ - ) = 15.75 _ 0.02 eV and AP(NH~-) = 10.16 + 0.02 eV. On the computational side, the total energies ( E 0) calculated at the Q C I S D ( T ) / 6 - 3 1 1 + G(3df, 2 p ) l e v e l and the unscaled zero-point vibrational energies ( Z P V E ) calculated at the M P 2 ( f u l l ) / 6 - 3 1 G ( d ) level of the various species are s u m m a r i z e d in Table 1. The energies ( A E ) of the various dissociations and ionizations are listed in Table 2. In this Table, two A E values are reported: A E 1 is simply the difference of the E 0 of the various species concerned, while A E 2 includes the scaled Z P V E correction. The scaling factor e m p l o y e d here is 0.9646 [13]. With the available appearance potentials, the dissociation energies D 0 ( H 2 N - H ) , D 0 ( H 2 N + - H ) and
2.2. A b initio c a l c u l a t i o n s
Do(H 2 N - H +) can be easily calculated, D o ( H 2 N - H ) = A P ( H +) - I E ( H )
The ab initio level adopted in the present work was Q C I S D ( T ) / 6 - 3 1 1 + G(3df, 2 p ) / / M P 2 / 6 -
= 4.97_+ 0.05 eV,
(6)
Table 2 Calculated (AE) and experimental energies of the various dissociations and ionizations studied in this work Process
AE 1 (eg) a
AE 2 (eV) b
(i) NH 3 ~ NH 2 + H (ii) NH~-~ NH~-(3B~) + H (iii) NH~ --* NH~-(1A1) + H (iv) NH~-~ NH 2 + H ÷ (v) NH 2 ~ NH + H (vi) NH 3 ~ NH~ + e (vii) NH 2 ~ NH~(3BI) + e (viii) NH 2 ~ NH~(1A1) + e
4.88 5.87 9.68 8.38 4.16 10.10 11.10 14.91
4.47 5.45 9.29 8.01 3.85 10.02 11.05 14.88
Without zero-point vibrational energy correction. b Including scaled zero-point vibrational energy correction. c Ref. [2]. d Ref. [4]. e Ref. [1]. f Ref. [9]. g Ref. [3]. h This work. i Ref. [10]. J Ref. [11]. k Ref. [12].
Exp.(eV) 4.60 + 0.50 c, 4.63 ± 0.01 d, 4.77 + 0.09 e, 4.54 f, 4.53 g, 4.97 + 0.05 h 5.57+0.03 ~,5.59 + 0.02 h 8.41 + 0.05 h 3.95 ___0.02 ~ 10.07 + 0.01 i, 10.16 + 0.01 J, 10.069 + 0.010 k, 10.16 + 0.02 ~ 11.14:t:0.01 d, 10.78+0.05 h
F. Qi et al. / Chemical Physics Letters 234 (1995) 450-454
D o ( H 2 N + - H ) = AP(NH~-) - IE(NH3) = 5.59-4-0.02 eV, D o ( H 2 N - H + ) = AP(H +) - I E ( N H 3 ) = 8.41 _+ 0.05 eV.
(7)
(8)
The theoretical values for these quantities as listed in Table 2 are in fair agreement with the experimental findings. Also, it is noted that the Gaussian-1 and Gaussian-2 values for D 0 ( H 2 N - H ) are 4.59 [14] and 4.62 eV [7], respectively, As can be seen in Table 2, the D 0 ( H 2 N - H ) value measured in the present work is higher than the ones in the literature. In our opinion, our result is closer to the actual value than the others. This is because vacuum ultraviolet photons are highly energetic; a single photon is capable of ionizing the molecule, or even dissociatively ionizing the molecule. Additionally, the wavelengths can be scanned continuously, Hence the threshold values can be measured accurately. We also note here that Berkowitz and Ruscic [15] have mentioned that the measurement of the H + appearance potential will, due to kinetic shift, lead to a higher value for the dissociation energy. It is important to point out that, in accordance with the Franck-Condon principle, photoionization is a vertical process. The threshold value measured in a photoionization process is not necessarily equal to the adiabatic ionization energy, especially when the structures of the ground state and the ionized state differ markedly. Still, the measured threshold value may be taken as the upper bound of the adiabatic energy, It is noted that the two experimental methods outlined in Section 1 have obvious shortcomings. In the F T / I C R M S method, we need to select two reference acids with known gaseous acidity. Also, to obtain D0(R-H) from AHacid, we first need to determine EA(R). This is not easily accomplished for the case of NH 2 [4]. Finally, the equilibrium process is difficult to control. In the PIMS method briefly described above, D0(R-H) can be obtained once IE(NH 2) and AP(NH~) are measured. However, measuring IE(NH 2) accurately is also difficult to realize [4]. In view of these factors, the appearance potentials measured in the present work should lead to more reliable results than the ones already reported in the literature,
453
In detecting H +, if the process RH + h v ~ R - + n + (9) takes place, the AP(H +) measured would be lower than that for process (4); the difference between the two values is EA(NH2). In the present work, we failed to detect the existence of NH 2. Hence process (9) has not been considered here. Since synchrotron radiation has a wide wavelength distribution, there may exist a certain amount of higher-order harmonics upon monochromatization. Indeed, how to overcome the interference of higher-order harmonics is one of the technical problems that need to be addressed. In our experiments, we employed different filters for different wavelength regions. In measuring AP(H+), the photoionization light source chosen was the second-order harmonics of the synchrotron radiation after passing through a 1200 l / m m grating. Even though the light source generated in this manner may consist of some third-order harmonics, their intensity should be low and the threshold was clearly observed. In measuring AP(NH~-), we chose the first-order harmonics of the radiation after passing through the same grating and LiF filter. Such an arrangement effectively filtered out the higher-order harmonies. In measuring AP(NH~), we selected the first-order harmonics of the radiation after passing through a 2400 I / m m grating without any filter. We found that the threshold was still clearly observable. This indicates that the interference caused by higher-order harmonics produced by this grating in this wavelength region was negligible. In order to eliminate the effect of higher-order harmonics, Lee and co-workers [5] employed a rare gas filter. However, such a procedure is not practical in our case. Attention is now turned to the dissociation energies D0(NH~-H) and D0(NHz--H+). For the former, our result is in excellent agreement with that of Riiede et al. [10], measured with threshold photoelectron photoion coincidence spectroscopy. For the latter, no other experimental result is available for comparison. Additionally, our theoretical D0(HN-H) value is in fair agreement with that obtained by Gibson et al. using PIMS [4]. The Gaussian-1 and Gaussian-2 values for this quantity are 3.96 [14] and 4.00 eV [7], respectively. Our observed data do not lead to an experimentally determined value for this quantity.
454
F. Qi et al. / Chemical Physics Letters 234 (1995) 450-454
The IE(NH3) measured in the present work is also in excellent agreement with the currently accepted value [11] in the literature. The fact that our D 0 ( H e N - H +) and IE(NH 3) values agree with resuits obtained with different experimental techniques lends confidence to our result for D0(H2N-H). While our D0(NH~-H) of 5.59_+ 0.03 eV is in excellent agreement with that of Riiede and co-workers (5.57 ___0.03 eV), our measured IE(NH 3) (10.16 + 0.02 eV) and AP(NH~) (15.75 _+ 0.02 eV) are slightly different from theirs (10.07 _+ 0.01 and 15.60 _+ 0.02 eV, respectively). Indeed, the values of these two measurements in the literature are rather scattered: we can also find 10.16 + 0.01 [11] and 10.069_+ 0.010 eV [12] for IE(NH 3) as well as 15.76 _+ 0.02 eV for AP(NH~) [16]. It would be difficult to pinpoint the reason for these different experimental observations. In addition, IE(NH 2) can be calculated readily with our observed data, IE(NH2) = A P ( N H ~ ) - D 0 ( H e N - H )
that are, in general, in good agreement with experimental findings.
Acknowledgement WKL wishes to thank the support of a Hong Kong University and Polytechnic Grants Committee earmarked grant for research (Account No. 221600080). The other authors are grateful to the Chinese National Science Foundation for financial support.
References [1] J.I. Brauman and L.K. Blair, J. Am. Chem. Soc. 93 (1971) 3911. [2] D.K. Bohme, R.S. Hemsworth and H.W. Rundle, J. Chem. Phys. 50 (1973)77. [3] J.A.R. Samson, G.N. Haddad and L.D. Kilcoyne, J. Chem. Phys. 87 (1987) 6416. [4] S.T. Gibson, J.P. Greene and J. Berkowitz, J. Chem. Phys. 83 (1985) 4319. [5] H. Shiromaru, Y. Achiba, K. Kimura and Y.T. Lee, J. Phys.
= 1 0 . 7 8 + 0.05 eV. (10) This value is lower than that obtained by Gibson and co-workers [4] using PIMS. Finally, it is noted that the NH~- cation considered so far has the ground electronic state 3B I. In
Chem. 91 (1987) 17. [6] Y.-W. Zhang, Synchrotron Radiation News 1 (1988) 12; v.-w. Zhang, L.-S. Sheng, D.-Q. Wang and L. Tao, Chinese J. Chem. Phys. 5 (1992)321.
Table 2, we have also included the calculated values of Do[(1A1)H2N+-H] and IE[NH 2 ~ NH~(1A~) + e ]. These results await experimental confirmation.
[8] M.J. Frisch, G.W. Trucks, M. Head-Gordon, P.M.W. Gill, M.W. Wong, J.B. Foresman, B.G. Johnson, H.B. Schlegel,
4. Conclusion
[7] L.A. Curtiss, K. Raghavachari, G.W. Trucks and J.A. Pople, J. Chem. Phys. 94 (1991) 7221.
M.A. Robb E.S. Replogle, R. Gomperts, J.L. Andres, K. Raghavachari, J.S. Binkley, C. Gonzalez, R.L. Martin, D.J. Fox, D.J. DeFrees, J. Baker, J.J.P. Stewart and J.A. Pople, GAUSSIAN 92, Revision C (Gaussian, Pittsburgh, 1992).
Employing the technique of vacuum ultraviolet
[9] K.C. Smyth and J.l. Brauman, J. Chem. Phys. 56 (1972)
synchrotron radiation photoionization mass spectrometry, combined with ultrasonic molecular beam apparatus, we have measured the ionization energy o f N H 3 (10.16 + 0.02 eV) a n d t h e appearance potentials of H ÷ and NH~. Subtracting IE(H) f r o m
[10] R. Riiede, H. Troxler, Ch. Beglinger and M. Jungen, Chem. Phys. Letters 203 (1993) 477.
AP(H+), w e h a v e o b t a i n e d a v a l u e o f 4.97 _ 0.05 eV for D0(H 2N-H). Since IE(H) is precisely known, this method for the determination of D 0 ( H e N - H ) should be more direct and accurate. At the same time, we have also obtained D o ( H z N + - H ) = 5.59 + 0.02 eV, D 0 ( H e N - H ÷) = 8.41 ___ 0.05 eV and IE(NH z) = 10.78 ___ 0.05 e V . Finally, it is n o t e d high-level ab initio calculations give rise to results
4620.
[11] S.G. Lias, J.F. Liebman, J.L. Holmes, R.D. Levins and W.G. Mallard, J. Chem. Phys. Ref. Data, 17, Suppl. No. 1 (1988). [12] R. Locht, K. Hottmann, G. Hagenow, W. Denzer and H. Baumg~irtel, Chem. Phys. Letters 190 (1992) 124. [13] J.A. Pople, A.P. Scott, M.W. Wong and L. Radom, Israel. J. Chem. 33 (1993) 345. [14] J.A. Poplc, M. Head-Gordon, D.J. Fox, K. Raghavachari and L.A. Curtiss, J. Chem. Phys. 90 (1989)5622. [15] J. Berkowitzand B. Ruscic, in: Vacuumultravioletphotoionization and photodissociation of molecules and clusters, ed. C.Y. Ng (World Scientific, Singapore, 1991) pp. 1-41. [16] R. Locht, Ch. Servais, M. Ligot, Fr. Derwa and J. Momigny,
Chem. Phys. 123 (1988) 443.
10 March 1995
PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 234 (1995) 450-454
Experimental and theoretical study of the dissociation energies Do(HzN-H) and Do(HzN+-H) and other related quantities Fei Qi a, Liusi Sheng a, Yunwu Zhang a, Shuqin Yu b, Wai-Kee Li c National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, People's Republic of China b Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, People's Republic of China c Department of Chemistry, The Chinese University ofHong Kong, Shatin, N.T., Hong Kong
Received 2 November1994; in final form 5 January 1995
Abstract
Combining the techniques of molecular beam and vacuum ultraviolet synchrotron radiation photoionization mass spectroscopy, we have measured the ionization energy of NH 3 (10.16 + 0.02 eV) and the appearance potentials of H + (18.57 +0.05 eV) and NH~- (15.75 +0.02 eV). From these data, we have obtained dissociation energy values for Do(H2N-H) (4.97 + 0.05 eV), D0(H2N+-H) (5.59 + 0.02 eV) and Do(H2N-H +) (8.41 + 0.05 eV) and the ionization energy IE(NH 2) (10.78 + 0.05 eV). High-level ab initio calculations lead to results that are in good agreement with the experimental findings.
1. Introduction The precise determination of the N - H bond dissociation energy in ammonia, D0(HzN-H), is of fundamental importance in both physical and biological sciences. Yet despite numerous studies, the experimentally observed values for this quantity are only in fair agreement with each other (see the first entry in Table 2). Currently, the experimental procedures for the determination of D0(H2N-H) may be broadly classified into two categories. The first is Fourier transform ion cyclotron resonance mass spectroscopy (FT/ICRMS). Making use of the principles of chemical equilibrium, two reference acids are selected and the gaseous acidity of NH3, AHacid(NH3), is determined by interpolation. The dissociation energy can then be calculated, D o ( R - H ) = AHacio(Rn ) + EA(R) - I E ( H ) ,
(1)
where EA(R) is the electron affinity of radical R and IE(H) is the ionization energy of the hydrogen atom. Employing this method, Brauman and Blair obtained a D 0 ( H z N - H ) value of 4.77 eV [1]. Two years later, Bohme and co-workers obtained a slightly different value of 4.60 eV [2]. On the other hand, an alternative procedure is photoionization mass spectrometry (PIMS). The photoionization process involved is AP(R+) R - H + hv ~R++ H + e . (2) The dissociation energy D0(R-H) can be obtained upon measuring the appearance potential of R +, AP(R+), and ionization energy of R, IE(R), D o ( R - H ) = A P ( R +) - I E ( R ) .
The D0(H2N-H) values determined by PIMS are 4.53 [3] and 4.63 eV [4]. Based on the same basic
0009-2614/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0009-2614(95)00059-3
(3)
F. Qi et al. / Chemical Physics Letters 234 (1995) 450-454
principle, in 1987, Lee and co-workers [5] first employed synchrotron radiation to determine the C - H bond dissociation energies in C2H 2 and C z H 4. The photoionization process and the energy relationship concerned are Apm +) RH + hu , R + H++ e-, (4) D0(R-H ) =AP(H+)-IE(H). (5) Since IE(H) is well known (13.598 eV), we only need to measure AP(H ÷) in process (4) in order to calculate D0(R-H). Also, synchotron radiation has the advantages of being highly intense and having a wide wavelength range. It is particularly suitable for photoionization processes requiring photons with wavelengths smaller than 100 nm. In the present work, we have studied the photoionization of NH 3 with synchrotron radiation and measured AP(H +) accurately. Furthermore, we have also measured IE(NH 3) and AP(NH~-). From these measurements, the bond dissociation energies D o ( H 2 N - H ) , Do(HzN+-H) and Do(HzN-H +) and ionization en-
with a thickness of 0.5 mm was inserted into the beam path to eliminate second- and higher-order radiation in the wavelength range beyond 1050 A. A molecular beam was formed by supersonic expansion of sample gas from a pulsed nozzle of 0.5 mm diameter into the ion chamber through a 1 mm skimmer. The beam was ionized at about 70 mm downstream of the nozzle by the focused syn(a) ~ .z. _z o z ='~
..:.~ NH + ..,,'. 3 .•.~.:. • : .... "•... ".".nr-. ~0.1~°v ;.: • | ,'T.A:~....~"
I--
•la
z
w
,,-
T
1150
1170
1190
~aso (b)
i~.':..~,~... ~ "~.,~o z ~'~¢e --. to
NH +
a
,,,,11,
%•
z
W ->
~d~=,• 15.75oV
~=
'%%
2.1. Photoionization measurements
,
,
720
The experimental apparatus has been described previously elsewhere [6], and hence it is only briefly outlined here. Synchrotron radiation from the Hefei 800 MeV light source is monochromized with a 1 m Seya-Namioka monochromator equipped with three spherical gratings (2400 I/mm coated with gold, 1200 l/mm with iridium and 600 l/mm with aluminum) covering the wavelength range from 350 to 6000 ,~. Both pre- and post-focusing mirrors are gold-coated toroidal ones. The absolute wavelength scale was calibrated to better than 1 ,~ by recording Ne and Ar ionization potentials. The wavelength resolution of the monochromator was set at 1 A over the wavelength range from 350 to 1500 .~ used in these experiments. A lithium fluoride cutoff filter
12a0
,,
o_
2. Experimental and theoretical methods
1210
WAVELENGTH(A)
ergy IE(NH 2) can be calculated. To complement
these experimental studies, we have also carried out high-level ab initio computations to determine the energies of these dissociation and ionization processes.
451
J
,
,
,
i
,
,
,
L
760
740
,
,
,
i
,
,
,
i
820
800
780
WAVELENGTH(A) (c) ~ z ~ o_ -~ ~ " tO
".::t•• , ~°~"'°"~•'%'~--~..*'" ~. ;"."" .~..~.% o='•
•
• "~-=g
H÷
•
"%,. ~857.v "¢'~"%l .
.
.
500
.
J
.
.
.
54o
.
i
.
580
.
.
.
i
.
.
.
.
6ao
L
.
66o
.
.
.
700
WAVELENGTH(A) Fig. 1. Photoionization from N H 3.
and (c)H ÷
efficiencycurves of (a)
NH~-, (b) NH~-
452
F. Qi et al. / Chemical Physics Letters 234 (1995) 450-454
Table 1 Total energies " (E 0) and unscaled zero-point vibrational energies b (ZPVE) of the various species studied in this work
31G(d), This level was chosen since it is the level to which the Gaussian-2 procedure [7] is approximated. All calculations were carried out on an I B M
Species NH3 (C3v; ~A1) NH~ (C~; 2A~) NH 2 (C2v; 2B 1) NH~- (C2v; 3B 1) NH~ (C2v; 1Aj) NH (C~v; 3E) H (2S)
R S 6 0 0 0 / 3 4 0 workstation using the G A U S S I A N 92 [8] package of programs.
E 0 (hartree) -56.47114 -56.09985 -55.79203 -55.38416
-55.24419 - 55.13916 -0.49981
ZPVE (kJ tool 1) 92.7 88.0 51.4 46.1 48.8 20.1 -
3. R e s u l t s a n d d i s c u s s i o n
a Calculated at the QCISD(T)/6-311 + G(3df, 2p)//MP2/631G(d, p) level. b Calculated at the MP2/6-31G(d, p)level.
The photoionization efficiency curves for the formation of H +, N H ~ and NH~- are displayed in Fig. 1. The threshold for the formation of each ion is clearly discernible. From these curves, the A P s for
chrotron radiation. The sample gas N H 3 ( > 99.9% purity) was mixed with Ne carrier gas (pressure ratio, NH 3 : Ne = 1 : 5) in this work. The pressure of the ionization chamber was about 5 × 10 5 Pa when the b e a m was turned on. As our experiments were carried out under the condition of a supersonic beam, the temperature of the system was quite low, of the order of 101 K. Hence it is not necessary to make any corrections for thermal effects. Ions were detected with a quadrupole mass spectrometer with a channeltron electron multiplier. The apparatus was provided with a computerized control and data acquisition system. The m o n o c h r o m a t o r was u s u a l l y scanned with a wavelength increment of 0.5 A and the time of data acquisition for each point was 10 to 20 s d e p e n d i n g on ion abundance,
the three ions can be measured: A P ( H ÷) = 18.57 _ 0.05 eV, A P ( N H ~ - ) = 15.75 _ 0.02 eV and AP(NH~-) = 10.16 + 0.02 eV. On the computational side, the total energies ( E 0) calculated at the Q C I S D ( T ) / 6 - 3 1 1 + G(3df, 2 p ) l e v e l and the unscaled zero-point vibrational energies ( Z P V E ) calculated at the M P 2 ( f u l l ) / 6 - 3 1 G ( d ) level of the various species are s u m m a r i z e d in Table 1. The energies ( A E ) of the various dissociations and ionizations are listed in Table 2. In this Table, two A E values are reported: A E 1 is simply the difference of the E 0 of the various species concerned, while A E 2 includes the scaled Z P V E correction. The scaling factor e m p l o y e d here is 0.9646 [13]. With the available appearance potentials, the dissociation energies D 0 ( H 2 N - H ) , D 0 ( H 2 N + - H ) and
2.2. A b initio c a l c u l a t i o n s
Do(H 2 N - H +) can be easily calculated, D o ( H 2 N - H ) = A P ( H +) - I E ( H )
The ab initio level adopted in the present work was Q C I S D ( T ) / 6 - 3 1 1 + G(3df, 2 p ) / / M P 2 / 6 -
= 4.97_+ 0.05 eV,
(6)
Table 2 Calculated (AE) and experimental energies of the various dissociations and ionizations studied in this work Process
AE 1 (eg) a
AE 2 (eV) b
(i) NH 3 ~ NH 2 + H (ii) NH~-~ NH~-(3B~) + H (iii) NH~ --* NH~-(1A1) + H (iv) NH~-~ NH 2 + H ÷ (v) NH 2 ~ NH + H (vi) NH 3 ~ NH~ + e (vii) NH 2 ~ NH~(3BI) + e (viii) NH 2 ~ NH~(1A1) + e
4.88 5.87 9.68 8.38 4.16 10.10 11.10 14.91
4.47 5.45 9.29 8.01 3.85 10.02 11.05 14.88
Without zero-point vibrational energy correction. b Including scaled zero-point vibrational energy correction. c Ref. [2]. d Ref. [4]. e Ref. [1]. f Ref. [9]. g Ref. [3]. h This work. i Ref. [10]. J Ref. [11]. k Ref. [12].
Exp.(eV) 4.60 + 0.50 c, 4.63 ± 0.01 d, 4.77 + 0.09 e, 4.54 f, 4.53 g, 4.97 + 0.05 h 5.57+0.03 ~,5.59 + 0.02 h 8.41 + 0.05 h 3.95 ___0.02 ~ 10.07 + 0.01 i, 10.16 + 0.01 J, 10.069 + 0.010 k, 10.16 + 0.02 ~ 11.14:t:0.01 d, 10.78+0.05 h
F. Qi et al. / Chemical Physics Letters 234 (1995) 450-454
D o ( H 2 N + - H ) = AP(NH~-) - IE(NH3) = 5.59-4-0.02 eV, D o ( H 2 N - H + ) = AP(H +) - I E ( N H 3 ) = 8.41 _+ 0.05 eV.
(7)
(8)
The theoretical values for these quantities as listed in Table 2 are in fair agreement with the experimental findings. Also, it is noted that the Gaussian-1 and Gaussian-2 values for D 0 ( H 2 N - H ) are 4.59 [14] and 4.62 eV [7], respectively, As can be seen in Table 2, the D 0 ( H 2 N - H ) value measured in the present work is higher than the ones in the literature. In our opinion, our result is closer to the actual value than the others. This is because vacuum ultraviolet photons are highly energetic; a single photon is capable of ionizing the molecule, or even dissociatively ionizing the molecule. Additionally, the wavelengths can be scanned continuously, Hence the threshold values can be measured accurately. We also note here that Berkowitz and Ruscic [15] have mentioned that the measurement of the H + appearance potential will, due to kinetic shift, lead to a higher value for the dissociation energy. It is important to point out that, in accordance with the Franck-Condon principle, photoionization is a vertical process. The threshold value measured in a photoionization process is not necessarily equal to the adiabatic ionization energy, especially when the structures of the ground state and the ionized state differ markedly. Still, the measured threshold value may be taken as the upper bound of the adiabatic energy, It is noted that the two experimental methods outlined in Section 1 have obvious shortcomings. In the F T / I C R M S method, we need to select two reference acids with known gaseous acidity. Also, to obtain D0(R-H) from AHacid, we first need to determine EA(R). This is not easily accomplished for the case of NH 2 [4]. Finally, the equilibrium process is difficult to control. In the PIMS method briefly described above, D0(R-H) can be obtained once IE(NH 2) and AP(NH~) are measured. However, measuring IE(NH 2) accurately is also difficult to realize [4]. In view of these factors, the appearance potentials measured in the present work should lead to more reliable results than the ones already reported in the literature,
453
In detecting H +, if the process RH + h v ~ R - + n + (9) takes place, the AP(H +) measured would be lower than that for process (4); the difference between the two values is EA(NH2). In the present work, we failed to detect the existence of NH 2. Hence process (9) has not been considered here. Since synchrotron radiation has a wide wavelength distribution, there may exist a certain amount of higher-order harmonics upon monochromatization. Indeed, how to overcome the interference of higher-order harmonics is one of the technical problems that need to be addressed. In our experiments, we employed different filters for different wavelength regions. In measuring AP(H+), the photoionization light source chosen was the second-order harmonics of the synchrotron radiation after passing through a 1200 l / m m grating. Even though the light source generated in this manner may consist of some third-order harmonics, their intensity should be low and the threshold was clearly observed. In measuring AP(NH~-), we chose the first-order harmonics of the radiation after passing through the same grating and LiF filter. Such an arrangement effectively filtered out the higher-order harmonies. In measuring AP(NH~), we selected the first-order harmonics of the radiation after passing through a 2400 I / m m grating without any filter. We found that the threshold was still clearly observable. This indicates that the interference caused by higher-order harmonics produced by this grating in this wavelength region was negligible. In order to eliminate the effect of higher-order harmonics, Lee and co-workers [5] employed a rare gas filter. However, such a procedure is not practical in our case. Attention is now turned to the dissociation energies D0(NH~-H) and D0(NHz--H+). For the former, our result is in excellent agreement with that of Riiede et al. [10], measured with threshold photoelectron photoion coincidence spectroscopy. For the latter, no other experimental result is available for comparison. Additionally, our theoretical D0(HN-H) value is in fair agreement with that obtained by Gibson et al. using PIMS [4]. The Gaussian-1 and Gaussian-2 values for this quantity are 3.96 [14] and 4.00 eV [7], respectively. Our observed data do not lead to an experimentally determined value for this quantity.
454
F. Qi et al. / Chemical Physics Letters 234 (1995) 450-454
The IE(NH3) measured in the present work is also in excellent agreement with the currently accepted value [11] in the literature. The fact that our D 0 ( H e N - H +) and IE(NH 3) values agree with resuits obtained with different experimental techniques lends confidence to our result for D0(H2N-H). While our D0(NH~-H) of 5.59_+ 0.03 eV is in excellent agreement with that of Riiede and co-workers (5.57 ___0.03 eV), our measured IE(NH 3) (10.16 + 0.02 eV) and AP(NH~) (15.75 _+ 0.02 eV) are slightly different from theirs (10.07 _+ 0.01 and 15.60 _+ 0.02 eV, respectively). Indeed, the values of these two measurements in the literature are rather scattered: we can also find 10.16 + 0.01 [11] and 10.069_+ 0.010 eV [12] for IE(NH 3) as well as 15.76 _+ 0.02 eV for AP(NH~) [16]. It would be difficult to pinpoint the reason for these different experimental observations. In addition, IE(NH 2) can be calculated readily with our observed data, IE(NH2) = A P ( N H ~ ) - D 0 ( H e N - H )
that are, in general, in good agreement with experimental findings.
Acknowledgement WKL wishes to thank the support of a Hong Kong University and Polytechnic Grants Committee earmarked grant for research (Account No. 221600080). The other authors are grateful to the Chinese National Science Foundation for financial support.
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= 1 0 . 7 8 + 0.05 eV. (10) This value is lower than that obtained by Gibson and co-workers [4] using PIMS. Finally, it is noted that the NH~- cation considered so far has the ground electronic state 3B I. In
Chem. 91 (1987) 17. [6] Y.-W. Zhang, Synchrotron Radiation News 1 (1988) 12; v.-w. Zhang, L.-S. Sheng, D.-Q. Wang and L. Tao, Chinese J. Chem. Phys. 5 (1992)321.
Table 2, we have also included the calculated values of Do[(1A1)H2N+-H] and IE[NH 2 ~ NH~(1A~) + e ]. These results await experimental confirmation.
[8] M.J. Frisch, G.W. Trucks, M. Head-Gordon, P.M.W. Gill, M.W. Wong, J.B. Foresman, B.G. Johnson, H.B. Schlegel,
4. Conclusion
[7] L.A. Curtiss, K. Raghavachari, G.W. Trucks and J.A. Pople, J. Chem. Phys. 94 (1991) 7221.
M.A. Robb E.S. Replogle, R. Gomperts, J.L. Andres, K. Raghavachari, J.S. Binkley, C. Gonzalez, R.L. Martin, D.J. Fox, D.J. DeFrees, J. Baker, J.J.P. Stewart and J.A. Pople, GAUSSIAN 92, Revision C (Gaussian, Pittsburgh, 1992).
Employing the technique of vacuum ultraviolet
[9] K.C. Smyth and J.l. Brauman, J. Chem. Phys. 56 (1972)
synchrotron radiation photoionization mass spectrometry, combined with ultrasonic molecular beam apparatus, we have measured the ionization energy o f N H 3 (10.16 + 0.02 eV) a n d t h e appearance potentials of H ÷ and NH~. Subtracting IE(H) f r o m
[10] R. Riiede, H. Troxler, Ch. Beglinger and M. Jungen, Chem. Phys. Letters 203 (1993) 477.
AP(H+), w e h a v e o b t a i n e d a v a l u e o f 4.97 _ 0.05 eV for D0(H 2N-H). Since IE(H) is precisely known, this method for the determination of D 0 ( H e N - H ) should be more direct and accurate. At the same time, we have also obtained D o ( H z N + - H ) = 5.59 + 0.02 eV, D 0 ( H e N - H ÷) = 8.41 ___ 0.05 eV and IE(NH z) = 10.78 ___ 0.05 e V . Finally, it is n o t e d high-level ab initio calculations give rise to results
4620.
[11] S.G. Lias, J.F. Liebman, J.L. Holmes, R.D. Levins and W.G. Mallard, J. Chem. Phys. Ref. Data, 17, Suppl. No. 1 (1988). [12] R. Locht, K. Hottmann, G. Hagenow, W. Denzer and H. Baumg~irtel, Chem. Phys. Letters 190 (1992) 124. [13] J.A. Pople, A.P. Scott, M.W. Wong and L. Radom, Israel. J. Chem. 33 (1993) 345. [14] J.A. Poplc, M. Head-Gordon, D.J. Fox, K. Raghavachari and L.A. Curtiss, J. Chem. Phys. 90 (1989)5622. [15] J. Berkowitzand B. Ruscic, in: Vacuumultravioletphotoionization and photodissociation of molecules and clusters, ed. C.Y. Ng (World Scientific, Singapore, 1991) pp. 1-41. [16] R. Locht, Ch. Servais, M. Ligot, Fr. Derwa and J. Momigny,
Chem. Phys. 123 (1988) 443.