J. Phys. Chem. 1987, 91, 2051-2057
2051
Rearrangement Processes In Exclted Hydrogen-Bonded Ammonia-Water Complexes. An ab Inltlo Study Gottfried Kohler* Institut fur theoretische Chemie und Strahlenchemie, University of Vienna, A-1090 Vienna, Austria
and Rudolf Janoscbek Institut fur theoretische Chemie, University of Graz, A-8010 Graz, Austria (Received: August 8, 1986; In Final Form: November 25, 1986)
Vertical transition energies in hydrogen-bonded ammonia-water complexes and rearrangement processes in their excited states were studied by ab initio calculations. The aim of the work was to model spectral shifts caused by hydrogen bonding and excited-state intermolecular interactions. If the excitation is located primarily on the proton acceptor then the complex is dissociative in the excited state; in the alternative case the complex becomes more tightly bound. Complexes between excited planar ammonia as proton donor and a water molecule as acceptor represent the global minimum on the excited-state surface. An alternate relaxed excited-state structure conforms to a loosely bound complex between planar ammonia and a water molecule, the nitrogen and oxygen atoms as nearest neighbors (binding energy -13.0 kJ/mol). Hydrogen atom transfer along the hydrogen bond from water to ammonia leads to a structure mimicked as a complex between an ammonium and a hydroxyl radical.
Introduction The interest in the fate of molecular Rydberg excited states has greatly increased in recent years.'-' Experimental results4v5 and quantum chemical studies" demonstrate the Rydberg character of the lowest excited states of ammonia and water molecules. As the excited diffuse orbitals correlate directly with antibonding valence orbitals along the bond dissociation coordinate, these molecules undergo photofragmentation.@ When the bond length increases the excited orbital changes from almost Rydberg to valence in character. This was also demonstrated for the bond rupture processes in the methylated compounds, methylamine and methanol, recently.lO~ll Contrary to the photofragmentation process that dominates the photochemistry of primary and secondary amines, tertiary amines show strong luminescence when excited in the gas phase as well as in sol~tion.'~-'~The lowest excited states are, however, likewise of predominant Rydberg character, demonstrated by a large pressure effect on the gas-phase absorption spectrum and confirmed by a b initio c a l c ~ l a t i o n s . ' ~ J ~ Gas-phase absorption spectra of these molecules become diffuse and shift slightly to higher energies as the pressure of a perturber gas increa~es.4'~ For inert perturbers this effect can be rationalized by the topology of the potential energy surface describing the (1) Mulliken, R. S. Acc. Chem. Res. 1976, 9, 7. (2) Sandorfy, C. Top. Curr. Chem. 1979.86, 91. (3) Malrieu, J. P. Theor. Chim. Acta (Berlin) 1981, 59, 251.
(4) Robin, M. B. Higher Excited States of Polyatomic Molecules; Academic: New York, 1974; pp 208-276. (5) Ashfold, M. N. R.; Macpherson, M. T.; Simons, J. P. Top. Curr. Chem. 1979,86, 1. (6) Runau, R.; Peyerimhoff,S. D.; Buenker, R. J. J. Mol. Spectrosc. 1977, 68, 253. (7) Rianda, R.; Frueholz, R. P.; Goddard 111, W. A. Chem. Phys. 1977, 19, 131. (8) Muller, J.; Canuto, S. Chem. Phys. Lett. 1980, 70, 236. (9) Buenker, R. J.; Peyerimhoff, S.D. Chem. Phys. Lett. 1974,29,253. (10) Kassab, E.; Gleghorn, J. T.; Evleth, E. M. J. Am. Chem. SOC.1983, 105, 1746. (1 1) Buenker, R. J.; Olbrich, G.; Schuchman, H.-P.; Schiirmann, G. L.; von Sonntag, C. J. Am. Chem. Soc. 1984, 106,4362. (12) Halpern, A. M. Chem. Phys. Lett. 1970, 6, 296. (13) Muto, Y . ;Nakato, Y.; Tsubomura, H. Chem. Phys. Lett. 1971, 9, 597. (14) Cureton, C. G.; Hara, K.; OConnor, D. V.; Phillips, D. Chem. Phys. 1981, 63, 31. (15) Tannenbaum, E.; Coffin, E. M.; Harrison, A. J. J. Chem. Phys. 1953, 21, 311. (16) Avouris, P.; Rossi, A. R. J . Phys. Chem. 1981, 85, 2340.
interaction of the molecule in its Rydberg excited state and a ground-state perturber. While the ground state should be attractive because of van der Waals forces, penetration into the extended Rydberg space leads to exchange repulsion in the excited The approach of a perturber kids, therefore, to a shift to higher energies, thus causing statistical asymmetric broadening of the transition and loss of structure at high pressures. In cases, however, where strong local interactions, e.g. hydrogen bonding, between solute and solvent molecules occur, these contribute largely to the interaction energy and determine predominantly the topology of the potential energy surface. Ground-state hydrogen bonding between ammonia and water molecules has been subject to several quantum chemical studies.'*-*O Since ammonia is the better proton acceptor (Le. the stronger base) than water, the configuration OH-.N (GI, Figure 1) shows a higher binding energy than structures like N H - 0 in which the water molecule is the acceptor (e.g. G3). Cyclic and bifurcated configurations correspond rather to a saddle point than to a local minimum.2o In hydrogen-bonding solvents like alcohols and water the absorption spectra of ammonia and amines are generally blue shifted when compared to inert solvents or also ethers, where no hydrogen bonding occurs.21 This blue shift should be associated with the dominance of OH-N hydrogenbonded complexes in such solutions. In Rydberg excited states one electron shows a diffuse, spatially extended distribution so that the cationic core determines predominantly intermolecular interactions. Experimental evidence for these specific interactions and for the rearrangement of the solvation shell in the excited states can be obtained from spectral shifts, Stokes' loss, and quenching processes of the fluorescence of tertiary amines. The emission spectrum of triethylamine lies, for instance, at 4.49 eV (corresponding to 277 nm) in the gas phase, whereas it is at 4.40 eV (282 nm) in n-hexane ~olution.'~-'~ This spectral shift increases, however, in slightly polar solvents like diethyl ether (at 3.98 eV equivalent to 312 nm) and tetrahydrofuran (3.68 eV equivalent to 348 nm).22 In protic solvents (17) Kassab, E.; Gleghorn, J. T.; Evleth, E. M. Chem. Phys. Lett. 1980, 70, 151. (18) Kollman. P. A.: Allen. L. C. J . Am. Chem. SOC.1971. 93. 4991. (19j Diercksen, G. H.F.; K;aemer, W. P.; von Nissen, W. Theor.'Chim. Acta 1972, 28, 67. (20) Kerns, R. C.; Allen, L. C. J . Am. Chem. Soc. 1978, 100, 6587. (21) Stevenson, D. P.; Coppinger, G. M.; Forbes, J. W. J. Chem. Phys. 1961.83.4350. (22) Halpern, A. J . Phys. Chem. 1981, 85, 1682.
0022-3654/87/2091-2051$01.50/0 0 1987 American Chemical Society
2052
Kohler and Janoschek
The Journal of Physical Chemistry, Vol. 91, No. 8. 1987
t'
TABLE I: Vertical Transition Energies for the Lowest Excited States of Ammo& and Water in the Ground-State Equilibrium Geometry and Planar Ammonia in the Excited-State ('A2'') Geometry, N-H Distance = 1,96 nu" excitation energy, eV this transition work exp 6 ammonia (C3J 'A, gs 0.00 7.37 6.39 'A, (3al 4al(3s)) 6.92 'Al 8.84 7.91 2e(3pXJ) 'E(3a1 8.64 'E 9.47 8.14 8.86 11.57 10.15 2A, (3a1 a) ammonia ( D S h )
-
'Al gs lA;';(laF (la; 'A; 'E" (la;
'Et,
2e'(3pX,))
'A,' (la; 'Al'
2a,"(3pZ))
1'
emission is also shifted to lower energies and also strongly q u e n ~ h e d . ' ~ *These ~ ~ * ~shifts ~ were associated with exciplex formation between the excited amines and solvent molecule^.^^-^^ Such structures should correlate with minima on the excited-state surface. To our knowledge only limited quantum chemical studies on intermolecular interactions in Rydberg-like excited states are available. The excited water dimer was studied, applying a perturbation technique.25 More importantly, however, some and in mixed results on hydrogen transfer in ammonia ammonia-water complexes27were reported recently. In the present paper ab initio studies on excited complexes of ammonia and water molecules are reported with the aim to rationalize the effect of complexation on the spectra and to model specific interactions in the excited state. It is expected to obtain model structures for local intermolecular interactions in Rydberg excited states. Especially for tertiary amines the knowledge of such excited structures should help to explain the photophysics of this class of fluorescing saturated molecules in the condensed phase. Excited-state bond dissociation processes were, therefore, not considered. Computational Methods Starting from a b initio ground-state calculations the formalism of improved virtual orbitals (IVO) was applied to obtain excited-state energies and wave f ~ n c t i o n s . ~ ~This - ~ ~ concept is (23) Kbhler, G. Chem. Phys. Lett. 1986, 226, 260. (24) Kbhler, G. J. Photochem. 1986, 35, 189. (25) Van Hemert, M.;van der Avoird, A. J. Chem. Phys. 1979, 71,5310. (26) Cao, H.2.;Evleth, E. M.; Kassab, E. J. Chem. Phys. 1984,81, 1512. (27) Evleth, E. M.; Cao, H. Z . ; Kassab, E. Phorophysics and Photochemistry above 6 eV, Lahmani, F., Ed.; Elsevier: Amsterdam, 1985; p 479. (28) Hunt, W. J.; Gcddard, W. A. Chem. Phys. Lett. 1969, 3, 414. (29) Huzinaga, S.; Arnau, C. J. Chem. Phys. 1971, 54, 1948. (30) Janoschek, R. Exrifed States in Organic Chemistry and Biochemistry, Pullman, B., Goldblum, N., Ed.; Reidel: Dordrecht, 1977; p 419.
0.00 6.1 1 5.46 7.52 7.37 8.33 7.62
0.00
2Bl. (1b,
Figure 1. Geometries of the ammonia-water complexes referred to as GI (water as the proton donor), G2, and G3(ammonia as the proton donor) in the text.
3a1')
2Al (3al
--
-) m)
8.52 7.87 10.34 10.01 10.88 10.03 1 1.23 11.13 11.63 10.85 13.73 15.59
7.4 7.2 9.1 9.7 10.0 10.2 12.6 14.7
"The ground-state total energies are -56.182801 au for NH3 (C'"), -56.167460 au for NH, (D3,+), and -76.004099 au for H 2 0 . gs denotes ground state.
equivalent to a limited CI where only those singly excited configurations are incorporated, which result from the propagation of one electron from one specified orbital, doubly occupied in the ground state.,' In this treatment the concept of molecular orbitals is maintained also in the excited states, and facilitates therefore the interpretation of their properties. Since excitation is treated in Koopmans' approximation, errors are in a well-known order of magnitude. These stem from reorganization and correlation effects due to excitation, which mutually compensate widely, and this justifies, therefore, their neglect.30 The diffuse function augmented basis set 6-31+G* was used in order to achieve proper wave functions for ground and excited states.32 The exponents for diffuse s and p sets on N and 0,0.021 and 0.028, respectively, were found by optimizing the total energy in the excited states of NH3 and HzO. The experimental geometries of ammonia and water were taken as the equilibrium geometries (NH,: LHNH = 106.7', H-N distance = 1.913 au;33H 2 0 : LHOH = 103.9O, 0-H distance = 1.812 a ~ ~ These ~ ) .geometries were applied when not otherwise stated. For planar ammonia the N H bond length was taken at the minimum of the total energy in the excited state which was at 1.96 au. Three main geometrical conformations are investigated in this paper (GI, G,, and G3 in Figure 1). In the hydrogen-bonded structures G I and G,, both considered with linear hydrogen bonds, the molecular fragments are characterized by their proton donor and acceptor properties. Whereas in G I ammonia acts as the proton acceptor, in G3 ammonia is the donor. G 2 is a structure (31) (32) (33) (34) 1139.
Bjbrna, N. J . Phys. B 1973, 6 , 1412. Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. Benedict, W. S.; Plyler, E. K. Can. J . Phys. 1957, 35, 1235. Benedict, W. S.; Gailar, N.; Plyler, E. K. J. Chem. Phys. 1956, 24,
The Journal of Physical Chemistry, Vol. 91, No. 8, 1987 2053
Hydrogen-Bonded Ammonia-Water Complexes
TABLE Ik Transition Energies (AE)and Binding Energies (BE) in the Lowest Excited States of OH.. -N Ammonia-Water Complexes (GI, Figure 1) for a Distance' of 5.86 au between Nitrogen and Oxygen Atomsb
E
Io.u.1 -
complex transition
r
--
'A' gs 'A' (sa' 'A' lA" (2a" 'All 'A' (8a' 'A'' (sa' 3A' 3AYf 'A' (8a''A' 'A'' (2a" 3A0 'A' (7a'
- 1a.u.l -0.U
-132.16-
-132.18-
- -o.La
1
I
I
I
I
I
A
!!j 6 7 8 9 10 -mla.u.l Figure 2. Energy curves for the variation of the intermolecular separation (distance N to 0)for the ammonia-water complex, geometry G1,in the ground ('A') and two excited states 'A' and 'A". The energies of the two uppermost orbitals 8a' and 2a" are shown by broken lines.
essential in the excited state emerging out of GIby a rotation of the water molecule.
Results and Discussion Vertical Spectra of Ammonia and Water. Calculated vertical transition energies for isolated ammonia and water molecules in their equilibrium ground-state geometry are compiled in Table I in order to compare them with data for the hetero dimers. Detailed discussions of the vertical spectra of both molecules were given p r e v i o ~ s l y . " ~The ~ * ~transition ~~ energies are generally too high by approximately 1 eV, when compared to experimental data and to theoretical studies using extended CI. This is already well-known and due to the IVO method applied, but the ordering of the transitions and also the relative energetic positions of the excited states are reproduced correctly.35 The vertical transition energies for planar ammonia in the excited-state equilibrium geometry, NH = 1.96 au, are also presented in Table I. In the planar excited state the bond length increases compared to 1.86 au in the planar ground state.6 Hydrogen-Bonded Complexes of the Type OH-N. The energy as a function of the NO distance in the ground and lowest excited states (I,' and lA" in C,) for OH-N hydrogen-bonded ammonia water complexes (structure GIin Figure 1) is plotted in Figure 2. The binding energy (BE) at the equilibrium distance of 5.9 au is BE = -28.9 kJ/mol (Table 11). In this structure the hydrogen bond is linear and considerably more stable by 14.7 kJ/mol compared with the NH-0 structure (seeTable 111). The present ground-state properties agree satisfactorily with those in the literature (BE = -26.3 kJ/mol at an equilibrium distance of 5.979 au19). The highest occupied orbital (HOMO) 8a' (C,) in the hydrogen-bonded complex can be described as the bonding combination of essentially the ammonia lone pair orbital (3al in C3Jand the 3al and lb2 orbitals of water (C2").As the molecules approach in forming the complex its energy decreases and reaches a minimum near the equilibrium separation (see Figure 2). The ionization potential of the complex increases, therefore, in comparison with isolated ammonia. The next doubly occupied orbital, 2a", is essentially the HOMO, l b l (Cb), in the water molecule. Its energy increases as the NO separation decreases (Figure 2) and (35) Wadt, W.
R.;Goddard 111, W. A. Chem. Phys.
1976, 18, 1
8.56
'A, 'A1 gs gs
gs
gs
BE, kJ/mol -28.9 33.1 34.4
-25.1
9.50 9.53
'E
'B, 'B, gs
9.30
'E
gs
34.8
gs
gs
102.7 58.8
'A2
-28.1
7.99
-16.3 34.7
9.32 lla') loa')
sa')
--
-
sa')
3a")
lA" (2a" SA!/ 'A' (2a" 'A' 2A' (8a' 2A" (2a"
V
8.02 7.58
loa')
-
- -0.50 -132.20
sa')
AE, eV 0.0
separated molecules NH3 H20 gs gs
1 la')
3a")
m)
m)
9.75 10.35 10.22 10.99 10.36
'A, 'A, gs gs gs gS
10.63
gs
'B,
10.57
gS
'B1
10.86
gs
'AI
10.21
gs
'A,
10.20
.'A2 'A,
'Al
-8.8
-18.0 -3.4 -86.7 -83.0 -103.5 -90.5
12.36
12.80
OPotential minimum in the ground state, see Figure 2. bBE is referred to the separated molecules (the respective states of NH3 and H 2 0 are given in columns 3 and 4, gs denotes the ground state). TABLE III: Transition Energies ( P E ) and Binding Energies (BE) in the Lowest Excited States of NH.. -0 Ammonia-Water Complexes (C, Figure 1) for a Distance of 6.14 au between Nitrogen and Oxygen Atoms'
complex transition
'
'A' gs 'A' (8a' 'A' 'A" @a' 3Af) 'A' (8a' 3A' 'A' (7a' 'A' 'A' (8a' 'A' *A' (8a' 2A'' (7a'
-
9a')
--
3a")
AE,eV
0.0
7.22 6.82 8.39 8.23
loa')
9a') 1 la')
m) m)
8.66 8.62 8.73 8.09 9.03 8.42 10.86
separated molecules NH, H20
BE, kJ/mol -15.5
gs
gs
'A, 'A1
gs
-30.6 -25.6
'E )E 'E
gs gs gs gs
'E
gs
gs gS 'AI 'Al
'Bl
'BI gS gS
-59.1 -54.9 2.9 -18.0 4.6 5.9 2.5
-51.8
14.14
OBE is referred to the separated molecules (the respective states of NH3 and H20 are given in columns 3 and 4, gs denotes the ground state). at small separations both the 8a' and 2a" orbitals are nearly degenerate. This decrease of the orbital energy should arise from the increase of charge on the water moiety, as a net transfer of 0.06 charge units was obtained. Propagation of an electron from either of these two orbitals into the lowest Rydberg orbital 9a' gives two excited states of symmetry 'A' and lA". The totally symmetric state is repulsive along the hydrogen bond coordinate, whereas the other one has bonding character. The influence of the formation of a hydrogen bond on the vertical spectra at the equilibrium separation is shown in Table 11. These data point out that the lowest transitions, located primarily on ammonia, shifts uniformly to higher energies, whereby singlet-triplet splitting remains nearly unaffected. The degenerate transitions in ammonia split slightly into the two components 'A' and lAf'. On the other side, transitions located on the water moiety remain nearly unchanged. Only the excitation into the Rydberg 3p, orbital is more affected by the adjacent ammonia molecule.
2054
Kohler and Janoschek
The Journal of Physical Chemistry, Vol. 91, No. 8, 1987 I
EI
hu.I
\r \
--131.87 131~~61 -131.884
A
-131.894
-10.0
log 9
0.0
-5.0
5.0
10.0
N I I
NO = 5.67 b
H
O
I
,
0.0 5.0 a.u.
-5.0
I 1
n 3-
-
1
1
I
NO = 5.67 b 2ov-9O’
N
H O
i
1 I
1
I
,
,
5I
31
, -15.0
I
I
-5.0
-10.0
0.0
,
I
I
I
5.0
10.0
15.0
20.0
-30
0.u.
-20
-10
0
10
20
f-
Figure 3. Plots of the orbital densities along the OH-N axis for the lowest excited orbitals of the separated molecules NH3 and H 2 0 (4al), and the complex (9a’) for the excitations 8a’ 9a’ and 2a” 9a‘ in the given geometry at an intermolecular separation of 5.67 au.
Figure 4. Energy curves for the variation of the angle y (ammonia inversion) for the ground and lowest excited states at a N-0 distance of 5.9 au. y is defined in the text. The energies of the uppermost orbitals are shown by broken lines.
These results are consistent with the effect of hydrogen bonding on the experimentally obtained absorption spectra of amines. It was found generally that the long wavelength transition shifts to higher energies as the amines are complexed with water or alc o h o l ~ . ’ ~This effect is reproduced correctly by these model calculations. In Figure 3 the electron density of the 4al Rydberg orbital of pyramidal ammonia along the z axis and that of water along the OH bond are plotted for the lowest transitions. These plots demonstrate the spatial extension of the electron density. For ammonia the typical charge distribution expected for the Rydberg type 3s function with two nodal surfaces of nearly spherical symmetry is obtained. In the excited water molecule the charge density is not nearly spherical symmetric demonstrating the antibonding intravalence character of the ‘B1state of water. Density plots for the excited orbitals of the complex are also 9a’, in Figure shown for both transitions, 8a’ 9a’ and 2a” 3. The excited orbitals of the complex remain essentially diffuse. 9a’ The Rydberg character of the excited orbital for the 2a” transition increases relative to that of isolated water, and this can be associated with the decrease in the ionization potential after complexation. The excited orbitals of such complexes can be described as diffuse orbitals, extending over the whole complex, represented primarily by a combination of Rydberg orbitals of the subsystems. Ammonia Inversion in Complexes of the Type OH-N. It is well accepted that ammonia adopts a planar configuration in its Therefore, the influence of aggregation excited state (Table on the ammonia inversion was investigated and the results are plotted in Figure 4. The angle of pyramidality y is thereby defined as the angle between an NH bond and the plane of the hydrogen atoms (Figure 1). For isolated ammonia the calculations yielded a ground-state inversion barrier of 18.9 kJ/mol compared with the experimental barrier height of 24.3 kJ/m01.~~For the ground-state mixed dimer in geometry G ,the energy curve shows an asymmetric double minimum for this inversion, in good agreement with studies,
reported previously, using a more extended basis set.I9 At the position of the energy barrier and at the upper minimum the complex is unstable against dissociation. The energy of the second minimum is 7.5 kJ/mol higher than that for the separated subsystems. The shape of excited-state energy curves depends on their descent from locally excited states of the subsystems. In the case that the excited states are primarily located on the amine moiety, i.e. they originate from a propagation of one electron out of the 8a‘ orbital, the energy curves of the excited binary complex as a function of the angle of pyramidality show minima-near the planar configuration (Figure 4). For the lowest excited state this minimum is, however, well above the energy of the separated subsystems (i.e. the excited ammonia in the relaxed planar configuration and ground state water) by 50.3 kJ/mol. The minimum in the energy curve for the excited state is positioned at a slightly negative value of y, contrary to the maximum of the barrier in the ground state. This small asymmetry and the shift of the barrier is the result of the perturbation by the attached water molecule. The instability of the binary complex in the excited state against dissociation into the subsystems can be rationalized by the charge distributions differing from that in the ground state. The ground-state hydrogen bond gains most of its stability from electrostatic interactions. That is, a positively charged hydrogen atom of water points toward the nitrogen lone pair of ammonia. In the excited state, one electron is propagated from the localized ammonia lone pair into a diffuse Rydberg-like orbital. The main electrostatic interaction in the excited state can be described essentially by the interaction of the positively charged ammonia core and a water hydrogen atom. This interaction is clearly repulsive. Contrary to the ‘A’, the energy curve for the IA”, correlating with a local excited state of water, follows essentially the ground-state energy curve. The activation energy is higher in the excited state and the upper minimum appears only as a shoulder. In the ground-state equilibrium geometry both excited states, the lowest ‘A’ and the ‘A’’, are very close in energy, but they are well separated in the planar structure and the ‘A’’ is at an even higher loa’) state. Since for the ‘A” energy than the next ‘A’ (8a’
-
-
-
-
-
I).436
(36) Rauk, A.; Allen, L. C.; Clementi, E. J . Chem. Pkys. 1970, 52,4337
-
Hydrogen-Bonded Ammonia-Water Complexes
The Journal of Physical Chemistry, Vol. 91, No. 8, 1987 2055
-131.93
-131.92 -132.1Li
Ia.u.1 -0.30
EIMO) Ia.u.1
-132.10
- -
-0.10 A
I
-0.50
W
-132.20 I
I
1.5
2.0
I
I
I
J
2.5 3.0 3.5 OH-bond length ia.u.1
Figure 6. Energy curves for the shift of the hydrogen atom along the ON axis in geometry G, at a fixed N-O distance of 5.9 au for the ground and two lowest excited states. The energies of the two highest doubly occupied orbitals are shown by broken lines.
ammonia. Since in this mutual orientation, excitations, centered on the water moiety, are shifted considerably to higher energies, excited states are situated above the lowest four states located on ammonia. The transition energy for excitation of the lowest state of this type is 8.38 eV in the complex compared to 7.37 eV in free water. On the other side, the lowest transition is found at 5.85 eV, at a lower energy than in isolated planar ammonia (6.1 1 eV, Table I). This transition energy should correspond to the energy where emission from such complexes could be found. Such small shifts of the fluorescence spectrum were found recently for 1:l complexes of triethylamine with alcohols and ether molecules, and the formation of such complexes was followed kinetically.23 The flat potential energy curves indicate, however, that the geometry of such complexes is very flexible and the stoichiometry not well defined. Therefore, the kinetics of the formation and the stability of such complexes are significantly determined by steric effects. Excited-State Proton Transfer along the Hydrogen Bond. Ground-state proton transfer along the hydrogen bond in various hetero dimers is generally characterized by large positive reaction energie~.~’I t is, however, well-known that proton-transfer equilibria are often at the alternate position in the excited state when compared to the ground state.38 Furthermore, the deuterium isotope effect, observed in the quenching of saturated amine fluorescence by alcohols, indicates significant participation of modes involving hydrogen motions in the quenching mechanism.24 From this point of view, we studied proton migration in excited complexes of structure G, (Figure l), although such a reaction appears at first rather unlikely. That is because of the positively charged core of excited ammonia. The dissociation process [NH3--H20]* NH4’ + OH’ was discussed re~ently.~’Hydrogen migration at almost unchanged N-0 distance was, however, not studied in detail. The results obtained for the model of proton transfer in the mixed complex are presented in Figure 6. Herein the energy
-
~~~
(37) Beyer, A.; Karpfen, A.; Schuster, P. Top. Curr. Chem. 1984, 120, 1.
~
~
(38) Shizuka, H. Acc. Chem. Res. 1985, 18, 141, and references cited herein.
2056
Kohler and Janoschek
The Journal of Physical Chemistry, Vol. 91, No. 8, 1987
curves for the ground and lowest excited states are drawn over the proton displacement along the line connecting nitrogen and oxygen atoms for the fixed equilibrium N O separation of 5.67 au. The ground state displays the behavior already described above. The barrier height for the transfer of the proton from water to ammonia is near to 250 kJ/mol. Additionally, the energies of the two uppermost occupied molecular orbitals, Sa’ and 2a”, are shown in Figure 6. When the proton is moved from oxygen to nitrogen, both orbitals become located on the OH- subsystem. The energies for the lowest excited states, ‘A’ and IA”, show minima near the ground-state equilibrium geometry (Figure 6). For both states the energy increases as the proton migrates toward the ammonia moiety, the ascent being considerably steeper for the lowest excited state, ‘A‘, and, therefore, the curves cress near the minimum position. Since the ‘A’ state is centered on ammonia, proton motion needs the rupture of the OH bond of water. The binding energy as well as the electrostatic repulsion of the proton by the positively charged nitrogen hinders proton migration and causes the strong increase of the energy of the system. The second excited state correlates with the lowest excited state of isolated water molecule. As this Rydberg state is dissociative, hydrogen shift along the hydrogen bond is only hindered by repulsion by the attached ammonia molecule, rationalizing the flat energy curve for the second excited state. Since the excited states split at the crossing point when the only symmetry element of the complex is destroyed, the excited-state surfaces should show a conical intersection. Motions, like rotational vibration of the water molecule about the N O axis, should facilitate, therefore, the conversion between the two states. In this fixed geometry proton migration is a thermally activated process. The activation energy is 25.1 kJ/mol related to the local minimum of the second excited state, but 62.9 kJ/mol related to the lowest vertically excited state. Beyond the barrier, the energy descends steeply and the energy surface shows a deep minimum, when the hydrogen atom is near the ammonia moiety. Here the two excited states are nearly degenerate, as they arise from the propagation of the electron from the degenerate orbitals located on the OH- group into the excited 9a’ Rydberg orbital. The orbital density plots show that the Rydberg excited orbital is primarily centered on NH4’. The electron density distribution is widely identical with that in Figure 9a’ transition. 3 for the excited orbital obtained in the Sa’ The minimum of the excited-state surface lies well below the energy of dissociation into the two molecular subunits. This structure might be also stable against dissociation into NH4’ and OH’ radicals as variation of the ON separation, when the hydrogen is situated in the vicinity of ammonia, gave a very flat minimum at a N-O distance of 5.57 au; the binding energy is when negative only small. The most stable geometry is obtained as the four hydrogens adopt a tetrahedral geometry around the nitrogen atom (LHNH = 109.5O) and the bond to the attached hydrogen is slightly elongated (1.96 au compared to 1.93 au for the others). At the minimum, the OH bond is collinear with the O N connection line and its length is 1.SO au. For this geometry the total energy is -131.9341 au. The geometries of the NH4’ and OH’ subunits in this complex are very near to those of the ammonium and hydroxyl radicals, as obtained r e ~ e n t l y . ~ ~ , ~ ~ Summation over the atomic gross charges in both excited states gave -0.35 charge units for NH4 and -0.9 charge units for (NHJ. This is in accordance with the interpretation of the excitation as the propagation of one electron from the OH- ion into a Rydberg orbital, primarily centered on the ammonia moiety. Hence, the complex has a structure which can be described as a complex between a hydroxyl and an ammonium radical. Consequently, the excited-state reaction can be interpreted as hydrogen atom transfer along the hydrogen bond. This is in good agreement with the earlier study in which the dissociation of the complex into the
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(39) Lathan, W. A.; Hehre, W. J.; Curtiss, L. A,; Pople, J. A. J . A m . Chem. SOC.1971, 93, 6317. (40) Haviliak, S.; King, H. F. J . Am. Chem. SOC.1983,105, 4.
E Ia.u.1 1 -131.91-
- 131.92- 131.93-131.94-131.95-
- 131.96-
- 132.1 6-
- 132.17 -
I
- 132.1 0 -, - 132.19-
- 132.20I
I
1
I
I
I
I
V A
q
-
Figure 7. Dependence of the total energy of the ground and lowest excited states of the ammonia-water complex in geometry G 3 as a function of the ON distance for pyramidal and planar ammonia.
two radical subunits was shown.27 The “radical complex” has a considerably lower transition energy, Le. 4.33 eV, than the other structures. This stems from the large stabilization in the excited state, but, more importantly, from the high activation for proton transfer in the ground state. Ammonia as Proton Donor in Hydrogen-Bonded Complexes of the Type NH-0. Hydrogen-bonded complexes in which ammonia is the proton donor can be used as model systems for hydrogen bonding in primary and secondary amines. For this purpose, the vertical spectra in such structures are described below. The dependence of the total energies of ground and lowest excited states of these complexes as a function of the N O separation is given in Figure 7. The ground state shows a minimum at a N - 0 distance of 6.14 au and a binding energy of -15.5 kJ/mol, in good agreement with results of recent studies.I9 Vertical transition energies for this equilibrium configuration are presented in Table 111. The transition energies between states centered on the ammonia moiety are shifted to lower energies, whereas those on water are now found at higher energy values. This effect separates the excitations of the water and ammonia moieties more strongly than they are separated in the isolated molecules. The magnitude of the shift of the energetic position is greatly influenced by the symmetry of the state which is considered. This is also shown by the stabilization energies for the various excited states, given in Table 111. For excited complexes with planar ammonia the equilibrium separation between the nitrogen and oxygen atoms was found at a N - 0 distance of 5.5 au. The binding energy is -16.2 kJ/mol. In this geometry the global minimum on the excited-state surface of ammonia-water complexes is adopted, when bond dissociation processes are not considered. Hydrogen atom transfer in the excited state exhibits a large positive reaction energy. The upper shoulder, corresponding to the proton in the vicinity of water, is 140 kJ/mol higher than the lower minimum in the excited state, compared to 520 kJ/mol in the ground state. N H - 0 complexes in their lowest excited states contrasts to OH.-N complexes in respect to their stability. This stems from the fact that ammonia excited states are found at lower energies than those of water. Thus, excited hydrogen-bonded complexes
J. Phys. Chem. 1987, 91, 2057-2062 are stable/unstable when the excitation is located primarily on the proton donor/acceptor, respectively. Conclusion
This study was undertaken in order to model the effects of ground-state hydrogen bonding on the electronic spectra and complex formation in excited states. The excited states of ammonia and water, the subunits of the studied complexes, are of predominant Rydberg character with excited orbitals of large spatial e x t e n ~ i o n . Bond ~ , ~ ~dissociation processes were not considered in this paper. The excited states of the complex can be correlated with those of the individual subsystems. If the excitation is located primarily on the proton acceptor then the complex becomes dissociative in the excited state. Alternatively, when the excitation is located on the donor, the stability of the ground state is retained. In the first case the transition energy is shifted considerably to higher energies whereas in the second case a shift to lower energies results. The most stable ground-state configuration, in which ammonia is the acceptor, is, therefore, unstable against dissociation into the subunits in the excited state. The structure in which ammonia is the donor is stable and the hydrogen-bonded complex of planar
2057
ammonia as donor and water as acceptor conforms to the total minimum on the excited-state surface. Two higher lying minima were observed which are most important for exciplex formation of tertiary amines. The first is obtained by a rotational relaxation of the water molecule in OH-N (GI) in order to direct a doubly occupied water lone pair to the singly occupied ammonia lone pair orbital. This gives a rather loosely bound complex with not well-defined stoichiometry. A second relaxation mechanism involves hydrogen atom transfer, along the hydrogen bond. The resulting configuration can be described as a complex between an ammonium and a hydroxyl radical. Excited-state interactions can be understood in terms of electrostatic interactions of the positively charged core of that moiety, on which the excitation is primarily located, with its polar counterpart. Acknowledgment. G. K. appreciates the continued help and encouragement by N. Getoff. The supply of sufficient computer time by the computer centers of the Universities of Graz and Vienna is gratefully acknowledged. Registry No. Ammonia, 7664-41-7; water, 7732-18-5.
Exclted-State Behavior of Phenylethynyldisilanes: An Intramolecular Charge-Transfer Emission Haruo Shizuka,* Katsuhiko Okazaki, Hideaki Tanaka, Masayuki Tanaka, Department of Chemistry, Gunma University, Kiryu, Gunma 376, Japan
Mitsuo Ishikawa, Department of Synthetic Chemistry, Kyoto University, Kyoto 606, Japan
Minoru Sumitani, and Keitaro Yoshihara Institute for Molecular Science, Okazaki 444, Japan (Received: September 4, 1986)
Photochemical and photophysical properties of phenylethynyldisilanes in MP (methylcyclohexane/isopentane,3: 1 v/v) have been studied by means of picosecond and nanosecond spectroscopyalong with steady-state experiments. Intramolecular charge transfer in the excited singlet state of phenylethynyldisilanesoccurs very rapidly (
