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A theoretical and experimental study of the HPCl radical: the Ã2A′→X̃2A″ visible emission spectrum
8 December 2000
Chemical Physics Letters 331 (2000) 483±488
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A theoretical and experimental study of the HPCl radical: ~ 2A0 ! X ~ 2A00 visible emission spectrum the A M.J. Bramwell, D.M. Rogers, J.J.W. McDouall, J.C. Whitehead * Department of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, UK Received 22 September 2000; in ®nal form 10 October 2000
Abstract ~ 2 A0 ! X ~ 2 A00 visible emission spectrum of HPCl. The spectrum shows a series We report the ®rst observation of the A of blue-degraded bands with two prominent progressions in the ground state bending vibration. Ab initio calculations are used to help to assign the observed spectrum. The experimentally derived constants are T0 21 305 cmÿ1 , x002 868 cmÿ1 , x02 622 cmÿ1 and x03 529 cmÿ1 compared with calculated values of T0 23 518 cmÿ1 , x002 893 cmÿ1 , x02 670 cmÿ1 and x03 524 cmÿ1 . The bending progression results from a substantial change in the bond angle between the ground and electronically excited states from 96° to 118°. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction It has long been recognised that there are similarities in geometrical and electronic structures between molecules of dierent chemical composition that have the same number of valence electrons [1]. In this paper, we report the ®rst observation of the visible emission spectrum for the radical HPCl which belongs to the HAB family of triatomic radicals having 13 valence electrons, which we interpret with the aid of ab initio cal~ 2 A0 ! X ~ 2 A00 emission spectra for culations. The A the HSO radical have been studied in the range 520±960 nm in a low-pressure chemiluminescent O/O3 /H2 S ¯ow system by Schurath et al. [2] and the corresponding system for the HNF radical has been reported by Lindsay et al. [3] in the visible region of the spectrum (400±650 nm) in an
*
Corresponding author. Fax: 44-161-275-4598. E-mail address: [email protected] (J.C. Whitehead).
F2 /N2 H4 ¯ow. There have been no reports of the emission spectra for other 13 valence electron HAB species but ab initio calculations have been performed for the species HPF [4] and HNCl [5] to aid experimentalists in identifying the spectra for these species. For HPF, the calculations suggest ~ 2 A0 state that because of avoided crossings, the A has only a small number of bound vibrational levels which might make its production, and hence observation dicult. ~ 2 A0 states of the corresponding 13 elecThe A tron HA2 species lie at much lower energies and ~ 2 A0 ! X ~ 2 A00 emission is found in the near the A infrared at 950±2100 nm for HS2 [6] and at 1200± 2300 nm for HO2 [7]. For the HAB systems, the emission spectrum is dominated by progressions in the bending mode and the calculations show that there is a signi®cant change in the bond angle upon electronic excitation from 101° to 123° for HNF [8], 103° to 133° for HNCl [5] and 96° to 111° for HPF [4]. In contrast, there is little or no change in the bending angle for the HA2 species, HS2 and
0009-2614/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 1 2 1 9 - 7
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M.J. Bramwell et al. / Chemical Physics Letters 331 (2000) 483±488
HO2 , where the emission consists almost exclusively of progressions in the HA±A stretching vibration [6,7]. Lindsay et al. [3] have attempted to rationalise the dierences between the HAB and HA2 species in the ground and electronic states in terms of the respective molecular orbitals and the eect of dierences in the electronegativity between the A and B atoms where a signi®cant change in electronegativity correlates with a change in bond angle upon excitation. 2. Experimental The details of the experimental arrangement involving a low-pressure ¯ow system have been given previously [9]. Atomic hydrogen was generated by a 2450 MHz microwave discharge of a ¯ow of molecular hydrogen. Phosphorous trichloride (Fisons, >98.5% purity) was degassed thoroughly by repeated freeze±pump±thaw cycles and the vapour was introduced coaxially into the hydrogen ¯ow giving a total cell pressure of 2 mbar. The resulting luminescence in the spectral region 200±800 nm was recorded by a 0.5 m scanning monochromator (SPEX 1870C) at a resolution of 0.32 nm with a cooled photomultiplier tube (EMI 9659QB). The spectrometer was calibrated with a mercury resonance lamp.
out consisted of the full valence shell (13 electrons in nine orbitals). With this choice of space, the A00 ground state and the A0 excited state occur as the ®rst and second roots of the CASSCF secular problem, respectively. 4. Results and Discussion ~ 2 A0 ~ 2 A00 and A The optimised geometries of the X states of HPCl are shown in Fig. 1. Table 1 lists the calculated harmonic vibrational frequencies. At the CASSCF/6-311G(2df,2pd) level, Te is calculated to be 23641 cmÿ1 and T0 23518 cmÿ1 , which is approximately 10% larger than the experimental value (see below). This agreement could be improved by including further electron correlation in the calculations. However, our purpose is to assist in assigning the spectrum and the calculations we report are adequate for this. The emission appears as a green glow and the corresponding spectrum is displayed in Fig. 2 and the positions of the band maxima are given in Table 2. The spectrum consists of a single band system comprising a series of blue-degraded bands
3. Computational Complete active space self-consistent ®eld (CASSCF) calculations were carried out using the 6-311G(d,p) and 6-311G(2df,2pd) bases [10±17]. All molecular one- and two-electron integrals and integral derivatives were evaluated using the GA U S S I A N 98 suite of programs [18] to which we have interfaced our codes. All CASSCF energy and gradient calculations were run using appropriate options within our valence bond programs [19]. Geometries were fully optimised using analytical gradients and the optimisation procedures within GA U S S I A N 98 and characterised by calculating harmonic vibrational frequencies. The frequencies were obtained by ®nite dierences of the energy gradients. The active space used through-
and degree) calculated at the CASSCF/6Fig. 1. Geometries (A 311G(2df,2pd) and 6-311G(d,p) (in parenthesis) levels for the ~ 2 A0 states of HPCl. ~ 2 A00 and A X
M.J. Bramwell et al. / Chemical Physics Letters 331 (2000) 483±488
485
Table 1 ~ 2 A states of HPCl calculated at the CASSCF level ~ 2 A and A Harmonic vibrational frequencies (cmÿ1 ) of the X Frequency ~ 2 A00 X
Mode
6-311G(d,p)
6-311G(2df,2pd)
x1 x2 x3
P±Cl stretch Bend H±P stretch
486 893 2295
505 893 2281
~ 2 A0 A x1 x2 x3
P±Cl stretch Bend H±P stretch
501 670 2270
524 672 2236
Fig. 2. Emission spectrum in the spectral region 400±600 nm, resulting from the interaction of the products of a microwave discharge in hydrogen with PCl3 vapour at a total pressure of 2 mbar. The monochromator bandwidth was 0.32 nm.
in the spectral region 400±600 nm. Under conditions of lower PCl3 ¯ow, a dierent glow is obtained which is purple in colour corresponding to emission in the spectral region 400±500 nm. This originates from a completely dierent band system which can also be obtained by the interaction of the products of a microwave discharge of nitrogen, helium or argon with PCl3 vapour indicating that, for this purple glow, the emitter contains only phosphorous and chlorine. Analysis of the resulting spectrum is in progress and shows that the emission does not correspond to any known band
systems of P2 , P3 or Cl2 and is probably an electronically excited state of PCl or PCl2 [20]. Accordingly, we conclude that the emitter responsible for the green glow displayed in Fig. 2 uniquely depends on the presence of hydrogen as well as phosphorous and chlorine. The calculated ground state vibrational frequencies for HPCl (Table 1) are close to the corresponding values calculated and measured for the molecule PH2 Cl [21], as might be expected given the similarities between the species. However, the ®rst electronic absorption bands for PH3 and PCl3 lie at wavelengths of
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M.J. Bramwell et al. / Chemical Physics Letters 331 (2000) 483±488
Table 2 Assignments and wavenumbers of the bands observed in the ~ 2 A0 ! X ~ 2 A00 emission spectrum of HPCl A Assignment
Wavenumber cmÿ1
0 3 1 ! 0 0 0 0 2 1 ! 0 0 0 0 1 1 ! 0 0 0 0 0 1 ! 0 0 0 0 1 0 ! 0 1 0 0 0 0 ! 0 0 0 0 0 1 ! 0 1 0 0 0 0 ! 0 1 0 0 0 1 ! 0 2 0 0 0 0 ! 0 2 0 0 0 1 ! 0 3 0 0 0 0 ! 0 3 0 0 0 1 ! 0 4 0 0 0 0 ! 0 4 0 0 0 1 ! 0 5 0 0 0 0 ! 0 5 0
23 708 23 100 22 477 21 844 21 561 21 313 20 956 20 437 20 084 19 566 19 205 18 695 18 347 17 832 17 486 16 984
190 and 220 nm, respectively [22,23], suggesting that the corresponding absorption for PH2 Cl will lie at similar wavelengths. Thus, we can reject PH2 Cl as being responsible for the visible emission that we observe. There are distinct similarities between the present spectrum and the emission spectra of HNF ~ 2 A0 ! X ~ 2 A00 reported by Lindsay et al. [3] A produced by the interaction of hydrazine and molecular ¯uorine. The HNF spectrum shows a series of blue-degraded bands in the spectral region 400±600 nm. A correspondence can also be drawn with the emission spectrum of HSiCl produced by ¯ash photolysis of SiH3 Cl studied by Herzberg and Verma [24] which consists of a series of violet-shaded bands extending from 410 to 480 nm. By analogy, we conclude that the emission which we observe originates from the triatomic radical HPCl which is the second row isovalent analogue of HNF and has one more electron than HSiCl. Following the assignments of HNF and the isoelectronic system HS2 [6], we attribute the ~ 2 A0 ! X ~ 2 A00 system. emission to the A The spectrum shows two extended vibrational progressions (denoted (a) and (b) in Fig. 2) each of six bands with spacings of 870 cmÿ1 which we identify with the ground state bending vibration (0 m002 0). These two progressions are displaced by 530 cmÿ1 . A frequency of this magnitude
could relate either to the bending vibration or the PCl stretching vibration in the excited state. Based on the theoretical calculations for which results are given in Table 1, we identify this frequency with the excited state P±Cl stretch, x03 . There is a third progression (denoted (c) in Fig. 2) to the blue of the two main progressions with four bands that share a common origin with progression (a) at 21 844 cmÿ1 and have a spacing of 620 cmÿ1 which the calculations identify as the excited state bending vibration, x02 . We assign the progression (a) to 0 0 1 ! 0 m002 0 and progression (b) to 0 0 0 ! 0 m002 0. The progression (c) is assigned to 0 m02 1 ! 0 0 0. Using linear regression, we obtain the following vibrational constants for these states; T0 21 305 6 cmÿ1 , x002 868 2 cmÿ1 , x02 622 4 cmÿ1 and x03 529 8 cmÿ1 , where the quoted errors represent one standard deviation. These can be compared with the corresponding calculated values of T0 23 518 cmÿ1 , x002 893 cmÿ1 , x02 670 cmÿ1 and x03 524 cmÿ1 . The relative ordering of the bending frequencies, x2 , in the two electronic states is in agreement with the experimental results for HNF [3] and HSiCl [24] and the theoretical calculations for HPF [4] and HNCl [5]. The remaining major band at 21 561 cmÿ1 (marked with an asterix) has an oset from the origin of 258 cmÿ1 . The theoretical predictions of Table 1 identify this oset as the dierence between the upper and lower state vibrational frequencies of the H±P±Cl bending mode (calculated Dx2 221 cmÿ1 ). Thus we assign this band to be the 0 1 0 ! 0 1 0 transition. Our observation of a dominant progression in the bending mode indicates that there is a substantial change in the bond angle upon excitation ~ 2 A0 state. This is to the electronically excited A supported by the calculated change in bond angle of 22°, close to the values of 22°, 25° and 30° calculated for HNF [8], HPF [4] and HNCl [5]. The smaller progression in the PCl stretching mode observed in the experimental spectrum suggests a change in the PCl bond length between the two states although the calculations imply that this We do not obtain any change is small (0.036 A). information about the H±P bonding in the two
M.J. Bramwell et al. / Chemical Physics Letters 331 (2000) 483±488
states as there are no bands in the spectrum attributable to the H±P stretching mode. The calculated geometries (Fig. 1) indicate that electronic excitation induces the following changes DRP±Cl ÿ0:036 A; in geometry: DRH±P ÿ0:014 A; DhHPCl 22:05°. As noted above, the valence angle is the most aected. The electronic con®gu~ 2 A0 states of HPCl dier ~ 2 A00 and A rations of the X by a single excitation which may be denoted as ~ 2 A0 : ~ 2 A00 ! . . . a00 2 a0 1 A . . . a0 2 a00 1 X The change in geometry is easily rationalised by inspection of the form of these molecular orbitals, shown in Fig. 3. The a00 molecular orbital shows no contribution on H and is clearly antibonding with respect to the P±Cl bond. The a0 molecular orbital is antibonding with respect to the H±P and P±Cl bonds, but shows a signi®cant bonding interaction between H and Cl. When this orbital is doubly occupied, the bonding interaction between H and Cl is enhanced and so gives a smaller bond angle, but longer H±P and P±Cl bond distances in the
487
~ 2 A00 state. Lindsay et al. [3] note that, for HNF, X ~ 2 A0 and X ~ 2 A00 states arise from the Renner± the A Teller distortion of a linear 2 P state and that the singly occupied orbital a0 in the excited state strongly favours a bent con®guration whilst the single occupied a00 orbital in the ground state does not signi®cantly favour bending. It is interesting to speculate on the mechanism by which HPCl is formed in an electronically excited state. The reactions of hydrogen atoms with PCl3 have been studied using infrared chemiluminescence detection of the HCl product [25,26]. The reaction is thought to proceed via two channels. Firstly, the formation of HCl by H-atom abstraction H PCl3 ! HCl PCl2 H PCl2 ! HCl PCl H PCl ! HCl P The presence of vibrationally excited HCl that is not thermodynamically allowed by the above
Fig. 3. Plots of the highest lying (a) a00 and (b) a0 orbitals of HPCl obtained at the CASSCF/6-311G(2df,2pd) level.
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M.J. Bramwell et al. / Chemical Physics Letters 331 (2000) 483±488
reactions [25] and the observation of bimodal HCl rotational distributions [26] leads to the suggestion of an exchange mechanism for the alternative production of HCl H PCl3 HPCl3 ! HPCl2 Cl H HPCl3 ! HCl HPCl2 Within the uncertainties of the thermochemistry, the reaction of H atoms with HPCl2 can produce electronically excited HPCl over the wavelength range that we observe H HPCl2 ! HPCl HCl Additional HPCl might also come from reaction of electronically excited H atoms produced by the discharge with PCl3 H PCl3 ! HPCl 2 Cl or Cl2
5. Conclusions Chemiluminescence from the reactions of the products of a hydrogen discharge with PCl3 vapours has been attributed to emission from the ~ 2 A0 ! X ~ 2 A00 system of previously unreported A HPCl. The spectrum shows a series of blue-degraded bands with two prominent progressions in the ground state bending vibration. Ab initio calculations are used to help assign the observed spectrum. Harmonic vibrational frequencies calculated at the CASSCF/6-311G(2df,2pd) level are in semi-quantitative agreement with experiment and suggest that the two observed vibrational progressions arise from the valence angle bend and the P±Cl stretching modes. The prominent bending state progression results from a substantial change in the bond angle upon excitation to the elec~ 2 A0 states which is calculated to tronically excited A be 22°. Acknowledgements M.J.B. and D.M.R. thank EPSRC for the provision of Studentships.
References [1] A.D. Walsh, J. Chem. Soc. (1953) 2260. [2] U. Schurath, M. Weber, K.H. Becker, J. Chem. Phys. 67 (1977) 110. [3] D.M. Lindsay, J.L. Gole, J.R. Lombardi, Chem. Phys. 37 (1979) 333. [4] B.L. Sch urmann, D.B. Knowles, G. Hirsch, R.J. Buenker, Chem. Phys. Lett. 145 (1988) 529. [5] B.L. Sch urmann, R.J. Buenker, Can. J. Chem. 63 (1985) 3264. [6] K.J. Holstein, E.H. Fink, J. Wildt, F. Zabel, Chem. Phys. Lett. 113 (1985) 1. [7] K.H. Becker, E.H. Fink, P. Langen, U. Schurath, J. Chem. Phys. 60 (1974) 4623. [8] M. Peric, S. Peyerimho, R.J. Buenker, Can. J. Chem. 61 (1983) 2500. [9] H.S. Braynis, D. Raybone, J.C. Whitehead, J. Chem. Soc. Faraday Trans. 283 (1987) 627. [10] R. Krishnan, J.S. Binkley, J.A. Pople, J. Chem. Phys. 72 (1980) 650. [11] A.D. McLean, G.S. Chandler, J. Chem. Phys. 72 (1980) 5639. [12] A.J.H. Wachters, J. Chem. Phys. 52 (1970) 1033. [13] P.J. Hay, J. Chem. Phys. 66 (1977) 4377. [14] K. Raghavavchari, G.W. Trucks, J. Chem. Phys. 91 (1989) 1062. [15] R.C. Binning Jr., L.A. Curtiss, J. Comput. Chem. 11 (1990) 1206. [16] L.A. Curtiss, M.P. McGrath, J.-P. Blandeau, N.E. Davis, R.C. Binning Jr., L. Radom, J. Chem. Phys. 103 (1995) 6104. [17] M.P. McGrath, L. Radom, J. Chem. Phys. 94 (1991) 511. [18] M.J. Frisch, et al., in: Gaussian Inc., Pittsburgh, PA, 1998. [19] N.O.J. Malcolm, J.J.W. McDouall, J. Comput. Chem. 15 (1994) 1357. [20] M.J. Bramwell, Ph.D. Thesis, University of Manchester, 1993. [21] H. Beckers, H. B urger, R. Kuna, M. Paplewski, J. Chem. Phys. 101 (1994) 5585. [22] M. Halmann, J. Chem. Soc. (1963) 2853. [23] M.J. McAdams, B.R. Russell, Chem. Phys. Lett. 18 (1973) 402. [24] G. Herzberg, R.D. Verma, Can. J. Phys. 42 (1964) 395. [25] M.A. Wickramaaratchi, D.A. Setser, B. Hildebrandt, B. Korbitzer, H. Heydtmann, Chem. Phys. 84 (1984) 105. [26] B. Hildebrandt, H. Vanni, H. Heydtmann, Chem. Phys. 84 (1984) 125.
Chemical Physics Letters 331 (2000) 483±488
www.elsevier.nl/locate/cplett
A theoretical and experimental study of the HPCl radical: ~ 2A0 ! X ~ 2A00 visible emission spectrum the A M.J. Bramwell, D.M. Rogers, J.J.W. McDouall, J.C. Whitehead * Department of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, UK Received 22 September 2000; in ®nal form 10 October 2000
Abstract ~ 2 A0 ! X ~ 2 A00 visible emission spectrum of HPCl. The spectrum shows a series We report the ®rst observation of the A of blue-degraded bands with two prominent progressions in the ground state bending vibration. Ab initio calculations are used to help to assign the observed spectrum. The experimentally derived constants are T0 21 305 cmÿ1 , x002 868 cmÿ1 , x02 622 cmÿ1 and x03 529 cmÿ1 compared with calculated values of T0 23 518 cmÿ1 , x002 893 cmÿ1 , x02 670 cmÿ1 and x03 524 cmÿ1 . The bending progression results from a substantial change in the bond angle between the ground and electronically excited states from 96° to 118°. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction It has long been recognised that there are similarities in geometrical and electronic structures between molecules of dierent chemical composition that have the same number of valence electrons [1]. In this paper, we report the ®rst observation of the visible emission spectrum for the radical HPCl which belongs to the HAB family of triatomic radicals having 13 valence electrons, which we interpret with the aid of ab initio cal~ 2 A0 ! X ~ 2 A00 emission spectra for culations. The A the HSO radical have been studied in the range 520±960 nm in a low-pressure chemiluminescent O/O3 /H2 S ¯ow system by Schurath et al. [2] and the corresponding system for the HNF radical has been reported by Lindsay et al. [3] in the visible region of the spectrum (400±650 nm) in an
*
Corresponding author. Fax: 44-161-275-4598. E-mail address: [email protected] (J.C. Whitehead).
F2 /N2 H4 ¯ow. There have been no reports of the emission spectra for other 13 valence electron HAB species but ab initio calculations have been performed for the species HPF [4] and HNCl [5] to aid experimentalists in identifying the spectra for these species. For HPF, the calculations suggest ~ 2 A0 state that because of avoided crossings, the A has only a small number of bound vibrational levels which might make its production, and hence observation dicult. ~ 2 A0 states of the corresponding 13 elecThe A tron HA2 species lie at much lower energies and ~ 2 A0 ! X ~ 2 A00 emission is found in the near the A infrared at 950±2100 nm for HS2 [6] and at 1200± 2300 nm for HO2 [7]. For the HAB systems, the emission spectrum is dominated by progressions in the bending mode and the calculations show that there is a signi®cant change in the bond angle upon electronic excitation from 101° to 123° for HNF [8], 103° to 133° for HNCl [5] and 96° to 111° for HPF [4]. In contrast, there is little or no change in the bending angle for the HA2 species, HS2 and
0009-2614/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 1 2 1 9 - 7
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M.J. Bramwell et al. / Chemical Physics Letters 331 (2000) 483±488
HO2 , where the emission consists almost exclusively of progressions in the HA±A stretching vibration [6,7]. Lindsay et al. [3] have attempted to rationalise the dierences between the HAB and HA2 species in the ground and electronic states in terms of the respective molecular orbitals and the eect of dierences in the electronegativity between the A and B atoms where a signi®cant change in electronegativity correlates with a change in bond angle upon excitation. 2. Experimental The details of the experimental arrangement involving a low-pressure ¯ow system have been given previously [9]. Atomic hydrogen was generated by a 2450 MHz microwave discharge of a ¯ow of molecular hydrogen. Phosphorous trichloride (Fisons, >98.5% purity) was degassed thoroughly by repeated freeze±pump±thaw cycles and the vapour was introduced coaxially into the hydrogen ¯ow giving a total cell pressure of 2 mbar. The resulting luminescence in the spectral region 200±800 nm was recorded by a 0.5 m scanning monochromator (SPEX 1870C) at a resolution of 0.32 nm with a cooled photomultiplier tube (EMI 9659QB). The spectrometer was calibrated with a mercury resonance lamp.
out consisted of the full valence shell (13 electrons in nine orbitals). With this choice of space, the A00 ground state and the A0 excited state occur as the ®rst and second roots of the CASSCF secular problem, respectively. 4. Results and Discussion ~ 2 A0 ~ 2 A00 and A The optimised geometries of the X states of HPCl are shown in Fig. 1. Table 1 lists the calculated harmonic vibrational frequencies. At the CASSCF/6-311G(2df,2pd) level, Te is calculated to be 23641 cmÿ1 and T0 23518 cmÿ1 , which is approximately 10% larger than the experimental value (see below). This agreement could be improved by including further electron correlation in the calculations. However, our purpose is to assist in assigning the spectrum and the calculations we report are adequate for this. The emission appears as a green glow and the corresponding spectrum is displayed in Fig. 2 and the positions of the band maxima are given in Table 2. The spectrum consists of a single band system comprising a series of blue-degraded bands
3. Computational Complete active space self-consistent ®eld (CASSCF) calculations were carried out using the 6-311G(d,p) and 6-311G(2df,2pd) bases [10±17]. All molecular one- and two-electron integrals and integral derivatives were evaluated using the GA U S S I A N 98 suite of programs [18] to which we have interfaced our codes. All CASSCF energy and gradient calculations were run using appropriate options within our valence bond programs [19]. Geometries were fully optimised using analytical gradients and the optimisation procedures within GA U S S I A N 98 and characterised by calculating harmonic vibrational frequencies. The frequencies were obtained by ®nite dierences of the energy gradients. The active space used through-
and degree) calculated at the CASSCF/6Fig. 1. Geometries (A 311G(2df,2pd) and 6-311G(d,p) (in parenthesis) levels for the ~ 2 A0 states of HPCl. ~ 2 A00 and A X
M.J. Bramwell et al. / Chemical Physics Letters 331 (2000) 483±488
485
Table 1 ~ 2 A states of HPCl calculated at the CASSCF level ~ 2 A and A Harmonic vibrational frequencies (cmÿ1 ) of the X Frequency ~ 2 A00 X
Mode
6-311G(d,p)
6-311G(2df,2pd)
x1 x2 x3
P±Cl stretch Bend H±P stretch
486 893 2295
505 893 2281
~ 2 A0 A x1 x2 x3
P±Cl stretch Bend H±P stretch
501 670 2270
524 672 2236
Fig. 2. Emission spectrum in the spectral region 400±600 nm, resulting from the interaction of the products of a microwave discharge in hydrogen with PCl3 vapour at a total pressure of 2 mbar. The monochromator bandwidth was 0.32 nm.
in the spectral region 400±600 nm. Under conditions of lower PCl3 ¯ow, a dierent glow is obtained which is purple in colour corresponding to emission in the spectral region 400±500 nm. This originates from a completely dierent band system which can also be obtained by the interaction of the products of a microwave discharge of nitrogen, helium or argon with PCl3 vapour indicating that, for this purple glow, the emitter contains only phosphorous and chlorine. Analysis of the resulting spectrum is in progress and shows that the emission does not correspond to any known band
systems of P2 , P3 or Cl2 and is probably an electronically excited state of PCl or PCl2 [20]. Accordingly, we conclude that the emitter responsible for the green glow displayed in Fig. 2 uniquely depends on the presence of hydrogen as well as phosphorous and chlorine. The calculated ground state vibrational frequencies for HPCl (Table 1) are close to the corresponding values calculated and measured for the molecule PH2 Cl [21], as might be expected given the similarities between the species. However, the ®rst electronic absorption bands for PH3 and PCl3 lie at wavelengths of
486
M.J. Bramwell et al. / Chemical Physics Letters 331 (2000) 483±488
Table 2 Assignments and wavenumbers of the bands observed in the ~ 2 A0 ! X ~ 2 A00 emission spectrum of HPCl A Assignment
Wavenumber cmÿ1
0 3 1 ! 0 0 0 0 2 1 ! 0 0 0 0 1 1 ! 0 0 0 0 0 1 ! 0 0 0 0 1 0 ! 0 1 0 0 0 0 ! 0 0 0 0 0 1 ! 0 1 0 0 0 0 ! 0 1 0 0 0 1 ! 0 2 0 0 0 0 ! 0 2 0 0 0 1 ! 0 3 0 0 0 0 ! 0 3 0 0 0 1 ! 0 4 0 0 0 0 ! 0 4 0 0 0 1 ! 0 5 0 0 0 0 ! 0 5 0
23 708 23 100 22 477 21 844 21 561 21 313 20 956 20 437 20 084 19 566 19 205 18 695 18 347 17 832 17 486 16 984
190 and 220 nm, respectively [22,23], suggesting that the corresponding absorption for PH2 Cl will lie at similar wavelengths. Thus, we can reject PH2 Cl as being responsible for the visible emission that we observe. There are distinct similarities between the present spectrum and the emission spectra of HNF ~ 2 A0 ! X ~ 2 A00 reported by Lindsay et al. [3] A produced by the interaction of hydrazine and molecular ¯uorine. The HNF spectrum shows a series of blue-degraded bands in the spectral region 400±600 nm. A correspondence can also be drawn with the emission spectrum of HSiCl produced by ¯ash photolysis of SiH3 Cl studied by Herzberg and Verma [24] which consists of a series of violet-shaded bands extending from 410 to 480 nm. By analogy, we conclude that the emission which we observe originates from the triatomic radical HPCl which is the second row isovalent analogue of HNF and has one more electron than HSiCl. Following the assignments of HNF and the isoelectronic system HS2 [6], we attribute the ~ 2 A0 ! X ~ 2 A00 system. emission to the A The spectrum shows two extended vibrational progressions (denoted (a) and (b) in Fig. 2) each of six bands with spacings of 870 cmÿ1 which we identify with the ground state bending vibration (0 m002 0). These two progressions are displaced by 530 cmÿ1 . A frequency of this magnitude
could relate either to the bending vibration or the PCl stretching vibration in the excited state. Based on the theoretical calculations for which results are given in Table 1, we identify this frequency with the excited state P±Cl stretch, x03 . There is a third progression (denoted (c) in Fig. 2) to the blue of the two main progressions with four bands that share a common origin with progression (a) at 21 844 cmÿ1 and have a spacing of 620 cmÿ1 which the calculations identify as the excited state bending vibration, x02 . We assign the progression (a) to 0 0 1 ! 0 m002 0 and progression (b) to 0 0 0 ! 0 m002 0. The progression (c) is assigned to 0 m02 1 ! 0 0 0. Using linear regression, we obtain the following vibrational constants for these states; T0 21 305 6 cmÿ1 , x002 868 2 cmÿ1 , x02 622 4 cmÿ1 and x03 529 8 cmÿ1 , where the quoted errors represent one standard deviation. These can be compared with the corresponding calculated values of T0 23 518 cmÿ1 , x002 893 cmÿ1 , x02 670 cmÿ1 and x03 524 cmÿ1 . The relative ordering of the bending frequencies, x2 , in the two electronic states is in agreement with the experimental results for HNF [3] and HSiCl [24] and the theoretical calculations for HPF [4] and HNCl [5]. The remaining major band at 21 561 cmÿ1 (marked with an asterix) has an oset from the origin of 258 cmÿ1 . The theoretical predictions of Table 1 identify this oset as the dierence between the upper and lower state vibrational frequencies of the H±P±Cl bending mode (calculated Dx2 221 cmÿ1 ). Thus we assign this band to be the 0 1 0 ! 0 1 0 transition. Our observation of a dominant progression in the bending mode indicates that there is a substantial change in the bond angle upon excitation ~ 2 A0 state. This is to the electronically excited A supported by the calculated change in bond angle of 22°, close to the values of 22°, 25° and 30° calculated for HNF [8], HPF [4] and HNCl [5]. The smaller progression in the PCl stretching mode observed in the experimental spectrum suggests a change in the PCl bond length between the two states although the calculations imply that this We do not obtain any change is small (0.036 A). information about the H±P bonding in the two
M.J. Bramwell et al. / Chemical Physics Letters 331 (2000) 483±488
states as there are no bands in the spectrum attributable to the H±P stretching mode. The calculated geometries (Fig. 1) indicate that electronic excitation induces the following changes DRP±Cl ÿ0:036 A; in geometry: DRH±P ÿ0:014 A; DhHPCl 22:05°. As noted above, the valence angle is the most aected. The electronic con®gu~ 2 A0 states of HPCl dier ~ 2 A00 and A rations of the X by a single excitation which may be denoted as ~ 2 A0 : ~ 2 A00 ! . . . a00 2 a0 1 A . . . a0 2 a00 1 X The change in geometry is easily rationalised by inspection of the form of these molecular orbitals, shown in Fig. 3. The a00 molecular orbital shows no contribution on H and is clearly antibonding with respect to the P±Cl bond. The a0 molecular orbital is antibonding with respect to the H±P and P±Cl bonds, but shows a signi®cant bonding interaction between H and Cl. When this orbital is doubly occupied, the bonding interaction between H and Cl is enhanced and so gives a smaller bond angle, but longer H±P and P±Cl bond distances in the
487
~ 2 A00 state. Lindsay et al. [3] note that, for HNF, X ~ 2 A0 and X ~ 2 A00 states arise from the Renner± the A Teller distortion of a linear 2 P state and that the singly occupied orbital a0 in the excited state strongly favours a bent con®guration whilst the single occupied a00 orbital in the ground state does not signi®cantly favour bending. It is interesting to speculate on the mechanism by which HPCl is formed in an electronically excited state. The reactions of hydrogen atoms with PCl3 have been studied using infrared chemiluminescence detection of the HCl product [25,26]. The reaction is thought to proceed via two channels. Firstly, the formation of HCl by H-atom abstraction H PCl3 ! HCl PCl2 H PCl2 ! HCl PCl H PCl ! HCl P The presence of vibrationally excited HCl that is not thermodynamically allowed by the above
Fig. 3. Plots of the highest lying (a) a00 and (b) a0 orbitals of HPCl obtained at the CASSCF/6-311G(2df,2pd) level.
488
M.J. Bramwell et al. / Chemical Physics Letters 331 (2000) 483±488
reactions [25] and the observation of bimodal HCl rotational distributions [26] leads to the suggestion of an exchange mechanism for the alternative production of HCl H PCl3 HPCl3 ! HPCl2 Cl H HPCl3 ! HCl HPCl2 Within the uncertainties of the thermochemistry, the reaction of H atoms with HPCl2 can produce electronically excited HPCl over the wavelength range that we observe H HPCl2 ! HPCl HCl Additional HPCl might also come from reaction of electronically excited H atoms produced by the discharge with PCl3 H PCl3 ! HPCl 2 Cl or Cl2
5. Conclusions Chemiluminescence from the reactions of the products of a hydrogen discharge with PCl3 vapours has been attributed to emission from the ~ 2 A0 ! X ~ 2 A00 system of previously unreported A HPCl. The spectrum shows a series of blue-degraded bands with two prominent progressions in the ground state bending vibration. Ab initio calculations are used to help assign the observed spectrum. Harmonic vibrational frequencies calculated at the CASSCF/6-311G(2df,2pd) level are in semi-quantitative agreement with experiment and suggest that the two observed vibrational progressions arise from the valence angle bend and the P±Cl stretching modes. The prominent bending state progression results from a substantial change in the bond angle upon excitation to the elec~ 2 A0 states which is calculated to tronically excited A be 22°. Acknowledgements M.J.B. and D.M.R. thank EPSRC for the provision of Studentships.
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