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Kinetics and spectroscopy of the attachment of t-butylamine to Hg(6 3P0). Dimer emission from two electronic systems
Volume 49, number 1
CHEMICAL PHYSICS LETTERS
KINETICS AND SPECTROSCOPY OF THE AITACHMENT
1 July 1977
OF GBUTYLAMINE TO Hg(6 3Po)_
DIMER EMISSION FROM TWO ELECTRONIC SYSTEMS
Anthony
B. CALLEAR, David R. KENDALL
Physìcal Chemistry Department Gmbridge. UK
and Lucjan KRAUSE z
and Churchill College. Ukiversìty of Cambridge,
Received 13 April 1977
Excitation of mixtures of the vapours of Hg and t-butylamine, A, with 2537 A Hg resonance radiation sensitises luminescence in the near ultraviolet and visiblc regions. The spectroscopy and kinetics of the emission have been examined, using
both cw and pulsed excitarion. The following scheme interprets the observations: Hgt3Po) + A + HgA*, HgA++ HgH + R, HgA+ .-. hu + Hg + A, HgA+ f M + HgA* f M, HgA* -+ hu + Hg + A, HgA* + A * HgAzf. HgAz -+ hu + Hg + 2A. Removal of Hq(3Po) occurs solely in a bimolecular reaction, rate coefficient = 3.9 (-c 0.4) X 1 OS” cm3 molecule-’ s-1. Thc vibrationally and electronically excited complex HgA+(F fi< 1 ns) decays principally by u radiationless reaction, probably forming HgH. The collisionally relaxed complex HgA* (T= 2.75 (~~0.2) PS) decays predominantly
by emission via a broad band, hmax LJ 3700 A. With high pressures of amine, a second molecule attaches to the complex to form the dimer, HgA$, which exhibits an emission spectrum with a double maximum, apparently because it populates two electronic states; the dimer has
a much shorter lïfetime than the monomer. HgA*.
1. Introduction
2. Experimentai
The mechanism of the Hg-photosensitised luminescence of NH3 is now understood in some detail; of special interest is the specrroscopy and kinetics of the clustering attachrnent of NH, to Hg(3Po) (hereafter Hg,) [l-9]_ To widen the understandïng of these molecular processes, we have chosen to investigate in detail the Hg-photosensitised reactions of t-butylamine. Some aspects of its behavïour turn out to be analogous to the Hg-NH, system. However, remarkable differences have been discovered and an outline of the results is described here. The occurrence of luminescente resulting from reactions of Hg0 wïth t-butylamine has already been reported as part of a general survey of the behaviour of amines [lO,ll].
Tbe experimental method for the kinetic measurements has been described [S] . The flash source is a transverse discharge lamp with 5 electrode pairs, repetitively pulsed with an input energy of 0.1 J. The reaction vessel was constructed with a filter jacket which contained either BrI or Cl2 according to the spectra1 region of observation. The fluorescente was detected photoelectrically after dispersion with a Hilger and Watts medium quartz spectrograph and was averaged over = 103 events. Spectral profiles were recorded with cw excitation from an Hg resonance lamp. The spectral sensitivity of the spectrograph-photomultiplier combination was calibrated with a tungsten lamp, the fdament temperature of which was measured wïth an optical pyrometer. The kïnetic experïments were conducted at 294 (22) K. The spectra1 profdes were recorded 299 (k2) K.
2 On leave from the University of Windsor, Canada.
29
Volume 3.
49. numbcr
CHERIICAL
1
PHYSICS
LETTERS
1 July 1977
Results
3.I_ Í%e hhetics The kinetics have been analysed by monitoring the decay of the sensitised emission following termination of the exciting flash. As in the Hg-NH, system, the lurninescence is delayed and there is negligible emission arising directly from reactions of Hg(3P,). With a trace of amine in a large excess of Na, we have monitored both the Hg0 decay rate (from the delayed 2537 A emission) and also the decay rate of the Hg-amine luminescente, and the time profiles are indistinguishable. In fig. 1, measured relaxation rates with 0.010 torr o! amine are plotted against pressure of NI. The addition of Ni- has no signifìcan; effect on the decay rates which are very much faster than for the same partial pressure of NH3_ Similarly we have found that addition of large pressures of Ar is without effect on the decay profiles, however, added Ar does greatly intensify the luminescente. The kinetic behaviour is rather different from the Hg”-NH, system in which the relaxation of Hg, obeys predominantly termolecular kinetics with total pressures above 10 torr. These differences reflect the behaviour of the complex formed between Hg0 and the amine in a bimolecular collision Hg0 i- A + HgA+. lf A = NH,, at zero pressure the predominant mode of decay of HgAf is to redissociate to Hg0 + A, so Gat collisional stabilisation of HgAf increases the removal rate of Hgo_ However, if A = t-butylamine, the pre-
N2 Pressurf
/ torr
Fig. 1. Relaxntion times of the sensitised lumïnescence with 0.010 torr of t-butylamine and various prestures of added Nz.
30
%,I
\
50
1 100
Pressure of t-Buíylamine/tm Fim =. 2. Varintion of the reciprocnl relaxzztion times of thc sensitised emission with pressure of f-butylamine; a decrease of hfetime with increasing dimer concentration.
dominant
of HgA+ at zero pressure must be a process whïch does not regenerate Hgo,
reaction
radiationless probably
HgA+ + HgH -i-R . Inert gas collisions remove energy from HgA+ to produce the stabilised HgA*. The collisional stabilisation does not affect the removal rate of Hgo, but it is responsible for the collisionally induced enhancement of the luminescente intensity. With amine pressures between 0.1 and a few torr, the decay rate of the luminescente is practically constant, corresponding to a mean lifetime of 2.75 (20.2) p for the stabilised monomer, HgA*. With increasing pressure of amine, however, the rate of decay ïncreases, as indicated in fig. 2, the lifetime falling to ~1 ps at a pressure of 100 torr. 3.2. í%e spectroscopy The spectra1 dïstributions, which were fìrst examïned on film, are shown in fig. 3. Even though in 1 torr of amine most of the Hg(3P1) is quenched, the intensity of the luminescente is weak because only a smal1 fraction of the HgA+ is collisionaily stabilised. With a trace of amine and a large excess of N2, an intense emission due to the stabilïsed monomer is observed. As the amine pressure is increased above 20 torr, a red-shift and broadening are apparent, which _ is similar to the behaviour of the Hg*-NH3 system though the broadening is more pronounced in the present case. The profiles were recorded photoelectrically and were reduced to relative intensities (quanta per wavenumber) by division by the quantal sensitivity of the
Volume
49, number
1
CHERIICAL
Fig. 3. Spectra1 distributlons SPec*a
a-J
are 21%
150.
and intensities
~00~50~
20.
10,5,
PHYSICS
LETTERS
1 July 1977
(10 min exposure) with various pressures of t-butkiamine. Thc pressurw (in torr) I’or 2, 1 and 0.36. In k, N2 at 100 torr 1~s been added to 0.36 torr of.m~me. Thc wwz-
lengt11 scale is A X 10m2.
spectrometer. After normalisation, the spectra then exhibit an isosbestic point of common intersection, fig. 4, whïch is characteristic of a superposition of just two basic functions. These are the profïles “a” and “d” of fig. 4, due respectively to stabilised HgA* monomer and HgAZ dimer. Additions of large pressures of N2 to the amine at intermedïate pressures of about 50 torr does not affect the intensity distribution of the observed profiles, which demonstrates that the reaction
HgA* + A * HgA;
,
is in thermodynamic equilibrium. With an amine pressure of ~60 torr, the two components are emitted with equal intensity. With low amine pressures and without addition of foreign gas, a weak emission can be detected down to a2700 a, the carrier of which appears to be the unsjabilised HgA+. A similar high frequency component arïses in the Hg*-NH3 system [9] _ 31
V&me
CHEMICAL
49, number I
Fig. 4. Normdised
profries
of the spectral
distribution
PHYSICS
LETTERS
showing
intersection. Spectrum “a” correspondsto the stabilisedmonomer, HgA*. Spectra “b” and “c” were recorded with amine pressures respectively of 50 and 214 torr. Spectrum “d” corresponds to the HgAz dimer. the isosbestic
Fig. 5. Schematic
potential
diagram
of the HgAz dimer.
4. Discussion The results of fig. 1 correspond to a bimolecular rate coefficient of 3.9 (20.4) X 101l* cm3 molecule-1 s-l for removal of Hgo by t-butylamine. From the initial studies of these reactions [ 10,l l] , a bimolecular rate coefficient Qf 5.5 X lo-l2 cm3 molecuIe-l s-l anc! a termolecular rate coefficient of 3 X 10d30 cmG molecule-2 s-l (A as third body) were derived; these rate coefficients correspond to a relaxation time of 500 us for 0.010 torr of t-butylamine with addition of small pressures of foreign gas. The discrepancy of -60 fold with the results of fig. 1 is outside the usual limits of experimental error, and we find no evidence for removal of HgO in a termolecular reaction. At high pressures the quarrtum yield iS approximately the same as the Hg-NH, system. With amine pressures up to 300 torr and with total pressures 250 torr, the spectra of the Hg photosensitised luminescence analyse as a superposition of only two basic components, due to HgA* and HgA$. It may be supposed that the formation of larger clusters is hindered by +&ehydrocarbon groups, though the Hg*NH3 system behaves similarly at elevated temperature 171
l
It is interesting to discover that the dimer spectrum exhibits a double maximum. For the most part, the bonding is between the N atom lone pair of electrons 32
and the 6s orbital of IIg(3P); these complexes are amongst the simplest of known coordination compounds. In the linear H3NHgNH3 structure, the MJ = -1 states of Hg(‘P1) become the components of an E state. However, in the t-butylamine complexes the degeneracy is removed. It appears that bonding must be particularly strong in one of these states, as indicated schematically in fig- 5. At ambient temperature, the radiative decay of the dimer is fairly evenly distributed between two electronic systems, as determined by the thermodynamic populations and the two spontaneous emission rates. It is notable that HgAz does have a shorter lifetime than HgA* , as seen from the results of fig. 2. We plan to test this hypothesis of emission via two electronic states by examining any effect of temperature on the spectral profile of the dlrner.
Acknowledgement We thank Dr. A.J. Stone for valuable discussions. We are grateful to the Science Research Council for the award of a Studentship to D.R.K. and for an equipment grant; also to NATO for the award of a Senior Fellowship to L-K.
Volume
49, number
1
CHEMICAL
References [l] [Z]
[3] ]4] [S]
C.G. Freeman, M,J. McEwan, R.F.C. Clz_;dge and L.F. Phlllips, Chem. Phys. Letters 9 (1971) 578. CG. Freeman, M.J. McEwan, R.F.C. Claridge and L.F. Phlllips, Trans. Faraday Sec. 67 (1971) 2004. A.B. Callear anti J.C. McGurk, Chem. Phys. Letters 7 (1970) 491. A.B. CaBear, J.H. Connor and J. Koskikallio, J. Chem. Sec. Faraday 1170 (i974) 1542. A.B. Harker and C.S. Burton, J. Chem. Phys 63 (1975) 885.
PHYSICS
LETTERS
1 July 1977
[6] K. Luther, H.R. Wendt and H.E. Huruiker, Chem. Phys. Letters 33 (1975) 146. [ 71 A.B. Callear and J.C. Connor, J. Chem. Sec. Faraday 11 70 (1974) 1667. [S] T. Hikida, T. Ichimura and Y. hlori, Chem. Phys. Letters 27 (1974) 548. [9] A.B. CaIlear and CG. Freeman, Chem. Phys. Letters 45 (1977) 204. [ 101 R.H. Newmar., CG. Freeman, M.J. McEwan, R.F.C. Claridge and L.F. Phlllips, Trans. Faraday Sec. 67 (197 1) 1360. [ 1 l] CG. Freeman, M.J. McEwan, R.F.C. Clarïdge and L.F. Phillips, Trans. Faraday Sec. 67 (1971) 3247.
33
CHEMICAL PHYSICS LETTERS
KINETICS AND SPECTROSCOPY OF THE AITACHMENT
1 July 1977
OF GBUTYLAMINE TO Hg(6 3Po)_
DIMER EMISSION FROM TWO ELECTRONIC SYSTEMS
Anthony
B. CALLEAR, David R. KENDALL
Physìcal Chemistry Department Gmbridge. UK
and Lucjan KRAUSE z
and Churchill College. Ukiversìty of Cambridge,
Received 13 April 1977
Excitation of mixtures of the vapours of Hg and t-butylamine, A, with 2537 A Hg resonance radiation sensitises luminescence in the near ultraviolet and visiblc regions. The spectroscopy and kinetics of the emission have been examined, using
both cw and pulsed excitarion. The following scheme interprets the observations: Hgt3Po) + A + HgA*, HgA++ HgH + R, HgA+ .-. hu + Hg + A, HgA+ f M + HgA* f M, HgA* -+ hu + Hg + A, HgA* + A * HgAzf. HgAz -+ hu + Hg + 2A. Removal of Hq(3Po) occurs solely in a bimolecular reaction, rate coefficient = 3.9 (-c 0.4) X 1 OS” cm3 molecule-’ s-1. Thc vibrationally and electronically excited complex HgA+(F fi< 1 ns) decays principally by u radiationless reaction, probably forming HgH. The collisionally relaxed complex HgA* (T= 2.75 (~~0.2) PS) decays predominantly
by emission via a broad band, hmax LJ 3700 A. With high pressures of amine, a second molecule attaches to the complex to form the dimer, HgA$, which exhibits an emission spectrum with a double maximum, apparently because it populates two electronic states; the dimer has
a much shorter lïfetime than the monomer. HgA*.
1. Introduction
2. Experimentai
The mechanism of the Hg-photosensitised luminescence of NH3 is now understood in some detail; of special interest is the specrroscopy and kinetics of the clustering attachrnent of NH, to Hg(3Po) (hereafter Hg,) [l-9]_ To widen the understandïng of these molecular processes, we have chosen to investigate in detail the Hg-photosensitised reactions of t-butylamine. Some aspects of its behavïour turn out to be analogous to the Hg-NH, system. However, remarkable differences have been discovered and an outline of the results is described here. The occurrence of luminescente resulting from reactions of Hg0 wïth t-butylamine has already been reported as part of a general survey of the behaviour of amines [lO,ll].
Tbe experimental method for the kinetic measurements has been described [S] . The flash source is a transverse discharge lamp with 5 electrode pairs, repetitively pulsed with an input energy of 0.1 J. The reaction vessel was constructed with a filter jacket which contained either BrI or Cl2 according to the spectra1 region of observation. The fluorescente was detected photoelectrically after dispersion with a Hilger and Watts medium quartz spectrograph and was averaged over = 103 events. Spectral profiles were recorded with cw excitation from an Hg resonance lamp. The spectral sensitivity of the spectrograph-photomultiplier combination was calibrated with a tungsten lamp, the fdament temperature of which was measured wïth an optical pyrometer. The kïnetic experïments were conducted at 294 (22) K. The spectra1 profdes were recorded 299 (k2) K.
2 On leave from the University of Windsor, Canada.
29
Volume 3.
49. numbcr
CHERIICAL
1
PHYSICS
LETTERS
1 July 1977
Results
3.I_ Í%e hhetics The kinetics have been analysed by monitoring the decay of the sensitised emission following termination of the exciting flash. As in the Hg-NH, system, the lurninescence is delayed and there is negligible emission arising directly from reactions of Hg(3P,). With a trace of amine in a large excess of Na, we have monitored both the Hg0 decay rate (from the delayed 2537 A emission) and also the decay rate of the Hg-amine luminescente, and the time profiles are indistinguishable. In fig. 1, measured relaxation rates with 0.010 torr o! amine are plotted against pressure of NI. The addition of Ni- has no signifìcan; effect on the decay rates which are very much faster than for the same partial pressure of NH3_ Similarly we have found that addition of large pressures of Ar is without effect on the decay profiles, however, added Ar does greatly intensify the luminescente. The kinetic behaviour is rather different from the Hg”-NH, system in which the relaxation of Hg, obeys predominantly termolecular kinetics with total pressures above 10 torr. These differences reflect the behaviour of the complex formed between Hg0 and the amine in a bimolecular collision Hg0 i- A + HgA+. lf A = NH,, at zero pressure the predominant mode of decay of HgAf is to redissociate to Hg0 + A, so Gat collisional stabilisation of HgAf increases the removal rate of Hgo_ However, if A = t-butylamine, the pre-
N2 Pressurf
/ torr
Fig. 1. Relaxntion times of the sensitised lumïnescence with 0.010 torr of t-butylamine and various prestures of added Nz.
30
%,I
\
50
1 100
Pressure of t-Buíylamine/tm Fim =. 2. Varintion of the reciprocnl relaxzztion times of thc sensitised emission with pressure of f-butylamine; a decrease of hfetime with increasing dimer concentration.
dominant
of HgA+ at zero pressure must be a process whïch does not regenerate Hgo,
reaction
radiationless probably
HgA+ + HgH -i-R . Inert gas collisions remove energy from HgA+ to produce the stabilised HgA*. The collisional stabilisation does not affect the removal rate of Hgo, but it is responsible for the collisionally induced enhancement of the luminescente intensity. With amine pressures between 0.1 and a few torr, the decay rate of the luminescente is practically constant, corresponding to a mean lifetime of 2.75 (20.2) p for the stabilised monomer, HgA*. With increasing pressure of amine, however, the rate of decay ïncreases, as indicated in fig. 2, the lifetime falling to ~1 ps at a pressure of 100 torr. 3.2. í%e spectroscopy The spectra1 dïstributions, which were fìrst examïned on film, are shown in fig. 3. Even though in 1 torr of amine most of the Hg(3P1) is quenched, the intensity of the luminescente is weak because only a smal1 fraction of the HgA+ is collisionaily stabilised. With a trace of amine and a large excess of N2, an intense emission due to the stabilïsed monomer is observed. As the amine pressure is increased above 20 torr, a red-shift and broadening are apparent, which _ is similar to the behaviour of the Hg*-NH3 system though the broadening is more pronounced in the present case. The profiles were recorded photoelectrically and were reduced to relative intensities (quanta per wavenumber) by division by the quantal sensitivity of the
Volume
49, number
1
CHERIICAL
Fig. 3. Spectra1 distributlons SPec*a
a-J
are 21%
150.
and intensities
~00~50~
20.
10,5,
PHYSICS
LETTERS
1 July 1977
(10 min exposure) with various pressures of t-butkiamine. Thc pressurw (in torr) I’or 2, 1 and 0.36. In k, N2 at 100 torr 1~s been added to 0.36 torr of.m~me. Thc wwz-
lengt11 scale is A X 10m2.
spectrometer. After normalisation, the spectra then exhibit an isosbestic point of common intersection, fig. 4, whïch is characteristic of a superposition of just two basic functions. These are the profïles “a” and “d” of fig. 4, due respectively to stabilised HgA* monomer and HgAZ dimer. Additions of large pressures of N2 to the amine at intermedïate pressures of about 50 torr does not affect the intensity distribution of the observed profiles, which demonstrates that the reaction
HgA* + A * HgA;
,
is in thermodynamic equilibrium. With an amine pressure of ~60 torr, the two components are emitted with equal intensity. With low amine pressures and without addition of foreign gas, a weak emission can be detected down to a2700 a, the carrier of which appears to be the unsjabilised HgA+. A similar high frequency component arïses in the Hg*-NH3 system [9] _ 31
V&me
CHEMICAL
49, number I
Fig. 4. Normdised
profries
of the spectral
distribution
PHYSICS
LETTERS
showing
intersection. Spectrum “a” correspondsto the stabilisedmonomer, HgA*. Spectra “b” and “c” were recorded with amine pressures respectively of 50 and 214 torr. Spectrum “d” corresponds to the HgAz dimer. the isosbestic
Fig. 5. Schematic
potential
diagram
of the HgAz dimer.
4. Discussion The results of fig. 1 correspond to a bimolecular rate coefficient of 3.9 (20.4) X 101l* cm3 molecule-1 s-l for removal of Hgo by t-butylamine. From the initial studies of these reactions [ 10,l l] , a bimolecular rate coefficient Qf 5.5 X lo-l2 cm3 molecuIe-l s-l anc! a termolecular rate coefficient of 3 X 10d30 cmG molecule-2 s-l (A as third body) were derived; these rate coefficients correspond to a relaxation time of 500 us for 0.010 torr of t-butylamine with addition of small pressures of foreign gas. The discrepancy of -60 fold with the results of fig. 1 is outside the usual limits of experimental error, and we find no evidence for removal of HgO in a termolecular reaction. At high pressures the quarrtum yield iS approximately the same as the Hg-NH, system. With amine pressures up to 300 torr and with total pressures 250 torr, the spectra of the Hg photosensitised luminescence analyse as a superposition of only two basic components, due to HgA* and HgA$. It may be supposed that the formation of larger clusters is hindered by +&ehydrocarbon groups, though the Hg*NH3 system behaves similarly at elevated temperature 171
l
It is interesting to discover that the dimer spectrum exhibits a double maximum. For the most part, the bonding is between the N atom lone pair of electrons 32
and the 6s orbital of IIg(3P); these complexes are amongst the simplest of known coordination compounds. In the linear H3NHgNH3 structure, the MJ = -1 states of Hg(‘P1) become the components of an E state. However, in the t-butylamine complexes the degeneracy is removed. It appears that bonding must be particularly strong in one of these states, as indicated schematically in fig- 5. At ambient temperature, the radiative decay of the dimer is fairly evenly distributed between two electronic systems, as determined by the thermodynamic populations and the two spontaneous emission rates. It is notable that HgAz does have a shorter lifetime than HgA* , as seen from the results of fig. 2. We plan to test this hypothesis of emission via two electronic states by examining any effect of temperature on the spectral profile of the dlrner.
Acknowledgement We thank Dr. A.J. Stone for valuable discussions. We are grateful to the Science Research Council for the award of a Studentship to D.R.K. and for an equipment grant; also to NATO for the award of a Senior Fellowship to L-K.
Volume
49, number
1
CHEMICAL
References [l] [Z]
[3] ]4] [S]
C.G. Freeman, M,J. McEwan, R.F.C. Clz_;dge and L.F. Phlllips, Chem. Phys. Letters 9 (1971) 578. CG. Freeman, M.J. McEwan, R.F.C. Claridge and L.F. Phlllips, Trans. Faraday Sec. 67 (1971) 2004. A.B. Callear anti J.C. McGurk, Chem. Phys. Letters 7 (1970) 491. A.B. CaBear, J.H. Connor and J. Koskikallio, J. Chem. Sec. Faraday 1170 (i974) 1542. A.B. Harker and C.S. Burton, J. Chem. Phys 63 (1975) 885.
PHYSICS
LETTERS
1 July 1977
[6] K. Luther, H.R. Wendt and H.E. Huruiker, Chem. Phys. Letters 33 (1975) 146. [ 71 A.B. Callear and J.C. Connor, J. Chem. Sec. Faraday 11 70 (1974) 1667. [S] T. Hikida, T. Ichimura and Y. hlori, Chem. Phys. Letters 27 (1974) 548. [9] A.B. CaIlear and CG. Freeman, Chem. Phys. Letters 45 (1977) 204. [ 101 R.H. Newmar., CG. Freeman, M.J. McEwan, R.F.C. Claridge and L.F. Phlllips, Trans. Faraday Sec. 67 (197 1) 1360. [ 1 l] CG. Freeman, M.J. McEwan, R.F.C. Clarïdge and L.F. Phillips, Trans. Faraday Sec. 67 (1971) 3247.
33