J. Phys. Chem. 1987, 91, 5242-5241
5242
Mass-Selected Two-Color Multiphoton Ionization of the Hydrogen-Bonded Complex C6H,0H-N(CH,),: Generation of the Protonated Ion HN(CH,),+ Naohiko Mikami,* Itaru Suzuki, and Akihiro Okabe Department of Chemistry, Faculty of Science, Tohoku Unicersity, Sendai 980, Japan (Received: April 16, 1987)
Photoionization and photodissociation of the hydrogen-bonded 1: 1 complex of C6H50Hwith N(CHJ3, prepared in a supersonic expansion, have been studied by mass-selected photoionization spectroscopy using the two-color multiphoton ionization technique. The complex was initially excited to its S, state and subsequently ionized to its cation by changing the photon energy for the photoionization. A large reduction (ca. 2 eV) of the ionization potential of phenol due to formation of the complex was found from the photoionization yield spectrum of the complex cation generation. In the yield spectrum a remarkable dip region was observed where the complex cation yield is reduced by an efficient generation of the protonated amine fragment ion, HN(CH3)3+.It was found that the protonated ion is generated from the electronically excited state of the complex cation, which is located at about 3.03 eV from its ground state and about 0.9 eV lower than the dissociation limit for HN(CH3),+ and the electronically excited phenoxy1 radical C6H50*. The mechanism of the photodissociation involving the proton transfer is discussed on the basis of an energy scheme obtained.
Introduction Proton transfer is one of the most important reactions in a hydrogen-bonded system and has been extensively studied in both gas and condensed phases. In ionization mass spectroscopic studies of hydrogen-bonded clusters in the gas phase using vacuum-UV photoionization or electron impact ionization, the protonated ions of the constituent molecule and/or of the smaller size clusters are commonly observed.'-4 The protonated ions are known to be generated by unimolecular dissociation accompanying the proton transfer in the parent cluster ion. However, the mechanism of the generation of protonated ions has not been well u n d e r ~ t o o d . ~ . ~ Excitation by means of vacuum-UV light or electron impact brings the clusters directly to high-energy excited states of their ions, which are located far above the dissociation energy of the hydrogen bond of the ground-state ions. Since electronic and vibrational relaxations from these excited states contribute to promote many dissociations of the ions, it is quite difficult to find the most efficient route to produce a particular protonated ion. In order to determine the protonation mechanism, it is highly desired to study the proton transfer in low-lying states of hydrogen-bonded cluster ions. Recently, electronic spectra of hydrogen-bonded complexes of phenol with various proton acceptors have been studied under an isolated molecular condition generated by a supersonic free jet.' Very recently, photoionization spectra of these complexes have been studied,* showing that the dissociation energy of the hydrogen bond of the complex ion is quite large. These studies made it possible for us to investigate dissociation processes of hydrogen bond in the complex ion. In this paper, we report a mass-selected multiphoton ionization spectroscopic study of the photodissociation process of the cation of the complex of phenol with trimethylamine, under an isolated molecular condition. The 1:l complex, prepared in a supersonic free expansion. was excited by the first laser to its lowest singlet excited S, state and was subsequently photoionized by the second ( 1 ) Ng, C. Y. Aduances in Chemical Physics: Prigogin, 1.: Rice, S. A , , Eds.: Wiley: New York, 1983: Vol. 2. (2) Ceyer, S. T.: Tiedemann, P.W.: Mahan. B. H.: Lee, Y . T. J . Chem. 1979. 70. -Phv.c -,--- , 14. (3) Ng, C. Y.: Trevor, D. J.: Tiedemann, P. W.; Ceyer, S. T.: Kronebusch, P. L.: Mahan, B. H.; Lee, Y. T. J . Chem. Phys. 1977, 67. 4235. (4)Shinohara, H.; Nishi, N.: Washida, N. J . Chem. Phys. 1985, 83. 1939: Chem. Phys. Lett. 1984, 106, 302. ( 5 ) Sato, K.: Tomoda, S.: Kimura. K.: Iwata, S. Chem. Phys. L e f t . 1983, 95, 579. (6) Cao, H. 2.; Evleth, E. M.: Kassab, E. J. Chem. Phys. 1984, 81. 1512. ( 7 ) Abe, H . : Mikami, N.: Tto, M. J . Phys. Chem. 1982, 86, 1768. (8) Gonohe, N.: Abe, H.: Mikami, N.: Ito, M. J . Phys. Chem. 1985. 89. 3642. -
3
0022-3654/87/2091-5242$01.50/0
laser. An efficient generation of the protonated ion, Le., trimethylammonium, HN(CH3),+, from the complex ion was found in a particular ionization energy region. The dissociation mechanism of the hydrogen bond accompanied by the proton transfer of the complex ion was discussed. We also report the fluorescence excitation and dispersed fluorescence spectra of the neutral complex, which provides us with dissociation energies of the hydrogen bond in the ground So and the SI states. The dissociation energy of the complex was found to be useful in correlating the electronic states of the neutral and cationic complexes with the corresponding dissociation limits. The complex of C 6 H 5 0 H with N(CH,), is one of the most fundamental hydrogen-bonded systems of large molecules and has been thoroughly studied in the condensed phase.' Since the complex ion in the condensed phase is substantially stabilized by a number of solvent molecules, the mechanism of the dissociation process is drastically changed by the surrounding solvent molecules, depending upon their stabilization characteristics. Such a solvent effect complicates the dissociation route, even if the dissociation is known to occur in solution. The present work investigates the dissociation process including the proton transfer without solvent perturbation. The dissociation process of the complex ion in the isolated molecular condition is regarded as a half-reaction of the ion-molecule gas-phase reaction. In this respect the present study, even though it is qualitative, will be helpful for understanding the ion-molecule reaction with low-energy collision.
Experimental Section The hydrogen-bonded complex was prepared by a pulsed supersonic free expansion of the gaseous mixture of C,H,OH and N(CH,), seeded in helium. Helium gas was mixed with trimethylamine vapor by passing the gas through 28% water solution of amine; the resulting gaseous mixture stored in a reservoir was used as a carrier gas of phenol vapor for the free expansion. The estimated seeding ratio of amine and phenol in helium gas was 1 : 1 :250 with a total pressure of about 3 atm. The fluorescence excitation and dispersed fluorescence spectra of the jet-cooled complex were obtained by the apparatus reported elsewhere.'O The second harmonic of a nitrogen laser pumped dye laser (Molectron UV-24 + DL-14, Coumarin 540 A) was used for the excitation of the complex to its S, state. The fluorescence excitation spectrum was obtained by monitoring the total fluorescence, and the dispersed fluorescence spectrum was observed (9) Pimentel, G . C.: McClellan, A. L. The Hydrogen Bond; W . H. Freeman: San Francisco, 1960. ( I O ) Mikami, N.; Hiraya, A,: Fujiwara, I.: Ito. M. Chem. P h i s . Lett. 1980. 7 4 , 53 1 ,
C 1987 American Chemical Society
Multiphoton Ionization of C6H50H-N(CH3)3 la)
The Journal of Physical Chemistry, Vol. 91, No. 20, 1987 5243
(b)
Excimer
Laser
0
00
8"
N(CH3'3
Dhenol
I
35500
36000
h u , ENERGY/cm-'
-
Figure 2. Fluorescence excitation spectrum of the S , So transition of C,H50H-N(CH,), complex cooled in a supersonic free expansion. The 0,O transition appears at 35 532 and f 2 cm-I, which is 820 cm-l lower than the 0,O of free phenol (36352 cm-I). Figure 1. (a) Schematic diagram of the experimental setup for massselected two-color photoionization spectroscopy. (b) Schematic view of the photoionization region and the subchamber. Laser beams were led into the photoionization region along the direction perpendicular to the figure plane indicated by 0.
with a monochomator (Nalumi; 750 mm, F = 6.7). The photocurrent of a photomultiplier (Hamamatsu R928) was preamplified and averaged by a boxcar integrator (Brookdeal 9415). The experimental setup for the mass-selected two-color photoionization spectroscopy is schematically shown in Figure 1. The vacuum chamber consists of a main chamber for the free expansion and a subchamber for the detection system. The main chamber was evacuated by an oil diffusion pump (1 200 L/s, 6 in.) backed up with a mechanical booster pump system (3100 L/min). The chamber was also evacuated by a turbomolecular pump (370 L/s for He) that was placed a t the dead end of the jet stream. The subchamber, which was equipped with a quadrupole mass filter (Extranuclear 4-270-9) and a channel electron multiplier (Murata, Ceratron), was connected to the main chamber through an ion entrance hole of 8 mm in diameter and evacuated by a small turbomolecular pump (55 L/s for He). The averaged backing pressure of the main chamber and the subchamber with the pulsed and 5 X Torr, nozzle operation a t 10 Hz was about 2 X respectively. This backing pressure was sufficiently low to avoid the generation of secondary ionized species by collision. The output of a XeCl excimer laser (Lambda Physik EMG 102 MSC) was split by a beam splitter to pump two dye lasers. The second harmonic of the first dye laser (Lambda Physik FL-2002, Coumarin 540A, KDP) was used as a light source vl for the S, Soexcitation of the complex. The unfocused light beam with a beam diameter of about 5 mm crossed the jet 20 mm downstream from the nozzle. The fundamental output of the second dye laser (Molectron DL-14) was used as a photoionizing light source u2. Several dye solutions were used to cover a wide range (470-360 nm) of wavelengths for the photoionizing light. Dyes used were Coumarin 450, Coumarin 440, Bis-MSB, PBBO, BBQ, Butyl-PBD, and DMQ. The u2 light beam was led into the main chamber from the opposite direction from the v l beam and coaxially focused by a lens with a focal length of 250 mm into the region where the jet and the u1 laser crossed. Cations generated by the photoionization were repelled from the crossing region by a positively charged plate to the entrance hole of the subchamber which was placed in a direction perpendicular to both the jet stream and the laser beam. An electric field strength between the repeller and the grounded entrance hole was about 12 V/cm. In front of the entrance hole an ion deflector with three-stage electrodes was placed to improve the collection efficiency of ions. The accelerated ionic species was selected by the mass filter and detected by the electron multiplier. The ion current was amplified by a current amplifier and averaged by a digital boxcar integrator system (PAR 4420). The averaged current intensity was normalized with respect to the power spectrum of the v 2 which was simultaneously monitored by a photomultiplier. The normalization of the ion current with respect +-
to the u2 laser power was carried out by a point-by-point method at each wavelength of v2. Calibration of the mass filter was done by using the mass numbers of known fragment species of benzene, which are generated by the photoionization with an intense laser light. The resolution M / A M of the mass filter was found to be about 150 at M = 78 amu. In the photoionization experiment, the following points were carefully examined: (1) Collisional ionization or dissociation of originally photoionized species was examined. Under a much lower vacuum condition than that described above, species collisionally ionized or dissociated by collision were actually observed. These could, however, be discriminated from the originally photoionized species by examination of the difference in their time of flight. Since the creation of the ionic species by collision occurs after the photoionization, a longer flight time to the detector is required for species induced by collisions under the given accelerating condition. (2) The relative intensity of the ion current due to the fragment ion with respect to that due to the complex ion depends upon the repeller voltage used. This dependence results from the difference in the ion collection efficiency of the two ionic species with different Mle's. Because of this ambiguity in relative intensity, we did not obtain the branching ratio of the fragmentation but restricted ourselves to observation of the yield spectrum a t a constant repeller voltage. (3) In order to avoid photoionization due to multiphoton ionization by each of the two dye lasers, light intensities of both lasers were adequately reduced by optically neutral density filters.
Results and Discussion Dissociation Energies ofthe Complex in the Soand SIStates. We first present the fluorescence excitation and the dispersed fluorescence spectra of the neutral complex and discuss the dissociation energies of the complex in the Soand SI states. Figure 2 shows the fluorescence excitation spectrum of the jet-cooled complex in the region of the origin of the SI So transition. The most intense band at 35 532 cm-' is assigned as the 0,Otransition of the complex. A large red shift (820 cm-I) of the 0,O transition of the complex compared to that of free phenol (at 36 352 cm-I) indicates that two moieties are bound by hydrogen bonding in which phenol acts as a proton donor and trimethylamine as a proton acceptor. This is in accord with our previous conclusion derived from other hydrogen-bonded complexes of phenol with various proton acceptorsS8The red shift means that the dissociation energy of the SI complex is larger by 820 cm-' than that of the
-
SO. The band at 0+148 cm-' can be assigned as a vibronic band involving an intermolecular vibration of the hydrogen bond because of its low frequency which has never been observed in free phenol." The low-frequency vibration of 148 cm-' was assigned to the fundamental stretching vibration of the hydrogen bond in the SI state, the assignment being confirmed from the dispersed fluorescence spectrum obtained by exciting this band, as given ( 1 1 ) Bist, H. D.; Brand, J. C. D.; Williams, D.R. J . Mol. Specrrosc. 1967, 24, 413.
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The Journal of Physical Chemistry, Vol. 91, No. 20, 1987
Mikami et ai.
f 1
"
"
I
"
"
'
C6H50H IPiO1=68600
I
I I 1
60600 PHOTON
55000
500 RELATIVE ENERGY dV/cm? Figure 3. Dispersed fluorescence spectra of the SI So transition of the complex, obtained by excitation of (a) the 0,O and (b) the 0+148-cm-' bands shown in Figure 1. Assignment of the vibronic bands coupled with intramolecular vibrations of the phenol moiety are given in (a), and the bands due to the intermolecular vibration ( u = 0 4) are given in (b). 1000
II
I
'32 0
-
-
below. The bands a t 0+280 and 0+313 cm-' in the excitation spectrum were assigned to vibronic bands involving the overtone of the stretching vibration. The assignment was also confirmed from their dispersed fluorescence spectra. The doublet structure is presumably due to Fermi resonance with higher overtones of some other intermolecular vibrations. A weak band at 0+111 cm-l was assigned to be a vibronic band coupled with another intermolecular vibration. The frequency of the corresponding vibration in the So state was found from the dispersed fluorescence spectrum to be 99 cm-l. The vibronic structure other than the low-frequency bands is essentially the same as that of free phenol. Bands at 0+492 and 0+790 cm-' of the complex correspond to the vibronic bands 0+476(6a1) and 0+783(12l) cm-' of free phenol. The good correspondence in the vibronic structure indicates that the SI excitation is localized in the phenol moiety and proton transfer does not occur in the SI complex. Figure 3 shows the dispersed fluorescence spectra of the complex obtained by the excitation of the 0,O and 0+148-cm-l bands. In addition to the intramolecular vibrations of the phenol moiety in the So state (indicated in the figure), the progression of a lowfrequency vibration of about 130 cm-' is identified. In the spectrum of the excitation of the 0+148-cm-I band, higher members of the progesssion (up to the fourth quanta) were observed. The dispersed fluorescence spectra shows that the 148-cm-' fundamental in the SI corresponds to the 132-cm-I vibration in the So, which is reasonably referred to the fundamental vibration of the hydrogen-bond stretching mode.I2 It is seen from Figure 3 that the frequency difference between the adjacent bands of the progression decreases with increase of the quantum number, showing the anharmonicity of the vibration of 130 cm-l. The energy level C(u) of the vth quanta of this vibration is expressed13 by G(u) = 133.5(u + I/*) - l.O(c in reciprocal centimeters. This anharmonicity of the stretching vibration of the hydrogen bond means that the two moieties are bound by a dissociative potential energy surface with respect to the intermolecular distance. This stretching vibration is adequately expressed by the motion in which the distance between the two moieties changes along the direction parallel to the hydrogen bond. One can, therefore, regard it as a one-dimensional stretching vibration of two mass points, each mass of which is equivalent to that of phenol and of amine. Assuming the Morse function for the potential energy of the vibration, the dissociation energy
+
(12) Abe, H.; Mikami, N.; Ito, M.; Udagawa, Y. J . Phys. Chem. 1982,
86,2567. (13) Herzberg, G . Molecular Spectra and Molecular Structure; Van Nostrand Reinhold: h'ew York, 1950; Voi. I .
'
lL 65600 E N E R G Y h i U , * U 2 ) /cm"
I 70000
Figure 4. (a) Two-color photoionization yield spectrum for the complex cation C,H5OH-N(CH3),' where the Sl(Oo)state was excited by the first laser vI fixed at 3 5 5 3 2 cm-] and the ionizing laser v 2 was scanned. Tentatively divided regions I, 11, and 111 are indicated (see text). The enhanced yield spectrum near the threshold (X5) in the region I is shown. (b) The corresponding yield spectrum of free phenol, where the Sl(Oo) state of C,H,OH was excited.
De was readily obtained from the observed coefficients. The De of the complex was then obtained to be 4500 f 1000 cm-' for the So state. A large error in the estimation comes from a lack of the higher overtone levels as well as from the error involved in the spectral measurements of the low-frequency vibration in the dispersed fluorescence spectra. The dissociation energy of the SI complex was also found to be 5320 A 1000 cm-I, by taking a sum of De in So and the red shift (820 cm-I) observed in the excitation spectrum. Photoionization of the Complex and Dissociation Energy of the Complex Ion. Using the two-color two-photon ionization technique, we also obtained the multiphoton ionization (MPI) spectrum of the SI So transition of the complex, as a function of the photon energy, v l , of the first laser, where the second laser was fixed at a suitable wavelength for ionization. The (1 + 1) MPI spectrum was essentially the same as the fluorescence excitation spectrum and is not shown here. When u I was fixed at the 0,O band of the S , Sotransition of the complex and u2 was scanned for the ionization, the photoionization yield spectrum of the SI complex was obtained. Figure 4 shows the photoionization yield spectrum of the complex obtained by observing the ion current due to the complex ion ( M = 153 amu). The yield gradually increases toward higher energy with a vague onset at a total photon energy h(u, u 2 ) of about 56 400 cm-I. This onset represents an upper limit of the adiabatic ionization potential IP(0) of the complex, as discussed below. The indistinct threshold followed by the slow rise of the ion current is characteristic of hydrogen-bonded complexes of phenol with other proton acceptors.* The characteristic feature of the yield spectra of the hydrogen-bonded complexes has been explained as being due to a large change in the intermolecular distance at the time of ionization. The indistinct onset of the photoionization yield spectrum is attributed to extremely small Franck-Condon (FC) factors between the zero-point level of the SI complex and the low-lying vibrational levels of the complex ion. The transition to the zero-point level of the complex ion, Le., IP(O), may not be observed. It is therefore recognized that the observed threshold energy at 56 400 cm-l represents an upper limit of IP(0) of the complex. When the Sl(Oo)of free phenol was excited by Y , , on the other hand, a distinct threshold due to the two-color photoionization of phenol was observed at 68 600 f 10 cm-I, as shown in Figure 4, which was already reported in a previous paper.8 This threshold energy coincides well with the known IP(0) of phen01.I~ A great
-
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+
(14) Kimura, K.; Katsumata, S.; Achiba, Y.; Yamazaki, T.; Iwata. S. Handbook o/ Her Photoionization Spectra of Fundatnental Organic Molecules; Japan Scientific Societies Press/Halsted Press: Tokyo/Yew York. 1981.
Multiphoton Ionization of C,HSOH-N(CH3)3
The Journal of Physical Chemistry, Vol. 91, No. 20, 1987 5245
HN (CH3;1
z
El 0
50
100 150 MASS NUMBER / amu
Figure 5. Mass spectrum after photoionization of the complex C,HsOH-N(CH3),. Total photon energy, h(ul u Z ) , was fixed at 60700 cm-', where the S,(Oo) state was excited by hul (=35 532 cm-I).
+
difference (ca. 12 200 cm-I) of the IP between free phenol and the complex represents a large stabilization energy resulting from the complex formation in the ionic state. By adding the Deof the So complex (4500 cm-I) to the difference, the lower limit of the dissociation energy of the complex ion for the dissociation into C6HSOH+and N(CH,), is found to be about 16 700 cm-I. In going to the higher energy side of the threshold, the gradual and monotonous increase of the yield was seen for the photon energy up to about 60000 cm-I. We call this part region I, as indicated in Figure 4. In the higher energy side of region I, however, we found a remarkable decrease of the yield starting from about 60000 cm-' and a steep increase at about 62000 cm-I. At about 60 700 cm-', the yield exhibits a distinct minimum. We call this part region 11. On the higher energy side, the yield becomes large and approaches a limit at about 64 000 cm-I. This part is called region 111. The photoionization yield spectrum shows, therefore, a distinct dip of the yield in region I1 superimposed on a general trend of increasing yield throughout the three regions. The photoionization of the complex ion in region I results in low-lying vibronic states with a small degree of F C overlap from the Sl(Oo)state of the neutral complex, as stated above. The asymptotic approach to the limit value of the yield in region I11 means that the photoionization occurs in the FC region, that is, in the vertical ionization from the SI complex. Without unusual processes, therefore, we also expect a monotonous increase of the yield in region 11, determined by the FC overlap. In order to find the origin of the dip in region 11, we observed the mass spectrum of the photoionized species. Figure 5 shows the photoionized mass spectrum resulting from the selective ionization of the complex at the photoionization energy h(ul u2) of 60700 cm-I. The peak at 153.0 amu is due to the complex ion, and another intense peak at 60.0 amu can be assigned to the fragment of trimethylammonium ion, HN(CH3)3+. The result is in sharp contrast to the mass spectrum obtained by the photoionization in region I, where only the complex ion was observed and no fragmention was found. The mass spectrum in region I11 is essentially the same as that in region 11, except that the complex ion is dominant. Neither the nonprotonated ion N(CH3),+ (59.0 amu) nor the phenol ion C6HSOH+(94.0 amu) was observed in these photoionization regions. It is apparent from the above results that the ionization of the SI complex in region I1 produces the protonated ion HN(CHJ3+. Since the non-proton-transferred structure of the SI state is evident, the proton transfer must occur in the ionization process from the SI complex. We shall first be concerned with the question of what state is responsible for the dissociation leading to HN(CH3)3f. For this purpose, we observed the laser power dependence of the photoionization yield. Figure 6 shows the u2 power dependence of yield for HN(CH3)3+and that of the complex ion in region 11. The power dependences for both species are quite different from each other. The HN(CH3),+ yield shows an approximately quadratic power dependence, while the complex ion yield increases linearly in the low-power region although its inclination becomes smaller in the high-power region. The u 1 power dependences of
+
.5
u2 LASER POWER
1.0
0
.5
u2 LASER POWER
1.0
Figure 6. (a) Laser power dependence of the HN(CH,),+ ion current. Observed points expressed in relative scales are indicated by open circles, and a quadratic curve is shown by the solid line. (b) Laser power dependence of the parent complex ion current. Open circles show observed points, and the solid line indicates an appropriate curve connecting the
40 0
I E&
.5 0
C6H50H-N(CH3\
C6H50H , N(CH3$
-0.0
/--I SSOClATl ON COORDINATE Figure 7. Energy diagram of C6H50H-N(CH3)3complex and its ion.
both species, on the other hand, were found to be linear, though their dependences are not shown here. The observed power dependences show that the complex ion is generated by absorption of single photon of u2 from the SI complex, but absorption of one more photon of v 2 is required for the generation of HN(CH3)3+from the complex ion. The nonlinear power dependence of the complex ion in the high-power region where the H N ( C H & + yield dominates means that the complex ion yield is subjected to competition between the generation from the SI complex and the destruction into HN(CH3)3+. We have, therefore, concluded that HN(CH3)3+is generated by photodissociation from the excited state of the complex ion which is excited by the one-photon absorption of u2. In this respect the dip in region I1 of Figure 4 represents the absorption spectrum of the complex ion being superimposed on the photoionization spectrum of the complex. Energy Levels of the System. Figure 7 shows the energy level diagram of the complex, its ion, and their dissociation limits. The dissociation energies of the neutral complex in the Soand SI states were given in a previous section. IP(0) of free C6H50Hand N(CH3), molecules is known to be 8.4Si4 and 8.12'' eV, re(15) Robinson, J . W., Handbook of Spectroscopy; C R C Press: Boca Raton, FL, 1974; Vol. 1.
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The Journal of Physical Chemistry. Vol. 91, No. 20, 1987
spectively. Being taken into account of the large reduction in IP(0) of the complex (16 700 cm-I), the ground state of the complex ion was assumed to be located a t about 2.07 eV lower than IP(0) of free phenol. In this diagram is also shown the dissociation limit of 7.73 eV for C 6 H 5 0and HN(CH3),+, determined by the heat of formation of phenoxyl radical, C,H,O, and trimethylammonium, HN(CH,),+, with respect to the ground state of free C 6 H 5 0 H and N(CH,), molecules. The heat of formation was obtained from the known enthalpy changes of the gas-phase reactions of C 6 H , 0 H C,H,O H ( A H = f 3 . 7 4 eV),I6 H H+ e- (1P = + I 3.60 eV),I5 and H+ N(CH,), HN(CH3)+ (proton affinity = -9.61 eV)." Other fragmentations require much greater energy, and they are omitted in the figure. As seen from the figure, there are three channels possible for the low-energy dissociation of the complex ion: (i) C6HSOH+ generation due to a simple dissociation of the hydrogen bond, (ii) N(CH3),+ generation accompanied by electron transfer from the amine to phenol ion, and (iii) HN(CH3),+ production resulting from the proton transfer from the phenol ion. On the basis of the energy level diagram given in Figure 7, the characteristics of the photoionization yield spectrum and the mass spectrum may be explained as follows: the near-threshold photoionization of the SI complex generates the complex ion which has an energy far below any dissociation energy of the three channels described above. This is denoted by route I in Figure 7 and corresponds to the photoionization yield spectrum of region I in Figure 4, where no fragmentation was found. The photoionization in region TI, where the v 2 photon energy of more than 24 500 cm-' (3.03 eV) was used, leads to the generation of H N (CH3)3+. Even with this photoionization energy, the complex ion cannot gain enough energy for the dissociation of any channel as seen in the figure. However, the power-dependence experiment indicates that the generation of HN(CH3),+ is induced by absorption of one more photon for the complex ion. The excited state of the complex ion is at about 3.03 eV above its ground state, that is, a t about 84 500 cm-I from the So neutral complex. This energy is sufficiently large to promote the dissociation of all channels. This process is indicated by route I1 i n Figure 7. In the photoionization of region 111, the complex ions are dominant because of the vertical ionization from the SI state. In region I11 the transition to the excited state of the complex ion is also possible, but the photon energy of huz is too large for its vertical transition, as illustrated by route 111 in Figure 7. The Excited State of the Complex Ion. As described in a previous section, the excited state of the complex ion is responsible for the dissociation. Although the excited state of the complex ion is energetically possible to take all three dissociation channels. the mass spectrum indicates that only the dissociation process for HN(CH3)3+generation preferentially occurs from the excited state. Therefore, the excited state of the complex ion must strongly couple with the dissociation continuum leading to the fragments of C6H,0 and HN(CH,),+. This suggests that in the excited state the positive charge is mainly localized on the amine side in the complex. Two extreme cases are considered for the excited state of the complex ion: one is the complex of the electronically excited phenoxyl radical bound by trimethylammonium, Le., C6HSO*HN(CH3)3+and the other is the complex of the electronically excited phenol with trimethylamine ion, i.e., C6H50H*-N(CH3)3+. The former correlates with the dissociation limit for free C6H50* and HN(CH3)3C. Although the electronically excited state of the phenoxyl radical in the vapor phase has not been reported, the lowest excited state of phenoxyl radical in solution is known to be located at about 25 000 cm-' (3.10 eV) above its ground state, C6H,0.'* Assuming the same value for the vapor, the dissociation limit for C6HjO* and HN(CH3),+ is located at 10.83 eV as shown in Figure 7. This dissociation limit is 0.9 eV higher than the
+
-
+
+
-
-
(16) DeFrees, D. J.; Mclver. R . T., Jr.; Hehre. W. J. J . A m . Chern. SOC. 1980, 102, 3334. (17) Aue, D. H.; Bowers, M. T. Gas Phase Ion Chemistry; Bowers, M. T.. Ed.; Academic: New York, 1979; Vol. 2. (18) Tripathi, G . N.R.; Schuler, R. H . Chem. P h j s . Lett. 1982. 88. 2 5 3 : J . Chem Phvr 1984, 81, I13
Mikami et a1 estimated energy (84 500 cm-]) of the excited state of the complex ion. In the latter case, on the other hand, the expected limit of the dissociation into the S , state of free phenol (C6HSOH*at 4.50 eV) and N(CH3)3+is located at 12.62 eV. which is extremely higher than the excited state of the complex ion. It may be very difficult to correlate such a high-energy dissociation limit with the excited state of the complex ion. In addition to this difficulty, the formation of the hydrogen bond between N(CH3),+ and C 6 H 5 0 Hcan not gain a large stabilization energy, because the hydrogen bond to the nonbonding electron of nitrogen substantially increases the ionization potential of N(CH,),. The latter case is thus inadequate as a model for the expected structure of the excited state. It is concluded therefore that the excited state is characteristic of the complex ion of the electronically excited phenoxyl radical and trimethylammonium, C6HSO*-HN(CH3)3+, which implies that the proton of the phenol is transferred to the amine side. Proton- Transfer Mechanism. Before discussing the protontransfer mechanism, we shall briefly discuss the electronic structure of the ground-state complex ion. As shown in Figure 7, there are three low-lying dissociation limits around 8 f 0.5 eV which may be correlated with the ground-state complex ion. However, the ground-state complex ion can not be correlated with one of the dissociation limits that leads to free C 6 H 5 0 H and N(CH3),+, because the hydrogen bond to the nonbonding electron of the N atom increases the ionization potential of amine as pointed out in a previous section. It might rather be correlated with some other higher excited state. Since the remaining two limits are possible to correlate with the ground-state complex ion, two different structures can be expected with respect to the location of the proton, that is, C6H,0H+-N(CH3)3 and C,HSO-HIV(CH3),+. These structures are, in reality, isomeric so that the potential energy surface of the ground state is more complicated than that of the excited state. The potential energy depends on the isomeric coordinate, i.e., the proton-transfer coordinate, as well as on the two different dissociation coordinates. Since the proton affinity of N(CH3)3is the largest among that of the methyl derivatives of ammonia and is presumably larger than that of C 6 H 5 0 ,the equilibrium C6H50Hf-N(CH3)3 ~t C6HSO-HN(CH3)3+in the complex ion will be displaced to the latter. The proton-transferred structure, therefore, seems to be preferred for the ground-state complex ion. In cases of complex ions of hydrogen-bonded dimers of small molecules, for example, ions of water dimer or of ammonia dimer, the proton-transferred structure is known to be more stable than the non-proton-transferred structure.19 According to the expected structure of the ground-state ion, the proton transfer through the hydrogen bond is involved in the photoionization process from the SI complex in which the hydrogen is located on the phenol side. Although the photoionization from the S, complex prepares the ground-state ion in the FC states which are characterized by the non-proton-transferred structure, allowance must be made for inframolecular vibrational relaxation (IVR) with respect to the proton-transfer coordinate in the ground-state ion. Consequently the proton-transferred structure becomes dominant after the IVR from the FC state. As far as the complex ion having an insufficient promotion energy for its dissociation produced by the photoionization, the protonated ion can not be generated regardless of the proton location in the complex ion. Since the protonated ion is produced from the excited state in the present system, the overall protontransfer process which starts with the excitation of the phenol moiety and ends with HN(CH,),+ production is determined by the nature of the excited state. The excited state of the ion is coupled preferentially with the continuum 1s ing above the dissociation limit for C6H,0 and HN(CH3),+ because of the same protonation characteristics. The electronic and vibrational relaxations from the excited state provide an opportunity for the (19) Tomoda, S.; Kimura, K. Chem. Phys. Lett. 1984, I l l . 434; Chem. Phy.c.. Lett. 1985, 121. 159.
5247
J . Phys. Chem. 1987, 91, 5247-5251 predissociation of the ground-state complex ion, leading to the fragmentation into ChHsO an HN(CH3)3+. Most of the excess energy after the dissociation may be converted into vibrational energies of the phenoxyl radical in the ground state.
(4) Correlation between energy levels of the complex cation and the dissociation limit suggests the following photodissociation route: C,H jOH-N(CH,),
Summary and Conclusions (1) Characterization of the neutral complex C6HsOH-N(CH3)3 was done by fluorescence spectroscopy. Dissociation energy of the neutral complex was found to be 4500 A 1000 and 5320 A 1000 cm-' for the So and SI states, respectively. Those states are characteristic of the hydrogen-bonded heterodimer of C6HsOH with N(CH3),. (2) Two-color photoionization provides the yield spectrum for the complex cation generation via the S, state of the neutral complex. The threshold energy of the yield spectrum shows an upper limit of adiabatic ionization potential of the complex (ca. 56 400 cm-') which is extremely reduced by the complex formation from that of free phenol. The dissociation energy was found to be about 16700 cm-' (2.07 eV) in the ground-state complex cation. (3) The protonated fragment ion HN(CH3)3+is efficiently generated from the photoionized complex cation whose excited state is found to be responsible for the protonated fragmentation.
hui
C,HSOH*-N(CH3)3
hY2 ---*
dissocn
C6HsO-HN(CH3)3+ -!% C6HsO*-HN(CH3)3+ C6HSO + HN(CH3)3+ (5) The above process occurs at hu2 photon energy of about 25400 cm-I, which corresponds to the local excitation of the phenoxyl radical in the complex ion. (6) In conclusion, it is has been demonstrated that a combination of the mass-selected photoionization and the fluorescence spectroscopies is quite useful for investigation of the dissociation process of the complex ion.
Acknowledgment. We are grateful to Prof. Mitsuo Ito for valuable discussions and encouragement during this work. We also thank Y . Sugahara for assistance in the early stage of this work. Registry No. HN(CH3)3+,16962-53-1;C6H,0H, 108-95-2;N(CH3),, 75-50-3.
Adsorption of Carbon Monoxide on ZSM-5 Zeolites. Infrared Spectroscopic Study and Quantum-Chemical Calculations Leonid M. Kustov, Vladimir B. Kazansky, The N . D . Zelinsky Institute of Organic Chemistry. Academy of Sciences of USSR, Leninsky Prospekt 47, I 1 791 3 Moscow, USSR
Stanislav Beran, Ludmila Kubelkovi,* and Pave1 Jiru The J . Heyrovskj Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, M6chova 7 , 121 38 Prague 2, Czechoslovakia (Received: August 25, 1986)
Low-temperature adsorption of CO was studied on H-ZSM-5 zeolites modified by dehydroxylation, ionic exchange with A13+,and impregnation with A1203and on Na-ZSM-5 and CaH-ZSM-5 zeolites. It was found that interaction of CO with framework OH groups results in the formation of a hydrogen-bonded CO complex whose O H bond frequency is decreased by 310-320 cm-l compared with that of free hydroxyls. For the less acidic framework hydroxyls in large cavities of H70Na30:Y zeolite the observed shift is 275 cm-I. With ZSM-5 zeolites, at least six types of electron-accepting sites are observed originating from nonframework A1 species (bands of CO in the interaction complex: 2132, 2222,2202, 2195, and 2198 cm-I) and the A1203microcrystalline phase (CO band at 2153 cm-I). The CO bond orders calculated by the CNDO/2 method for the CO interaction complexes with models of surface sites increase in the following order: >0-CO < >OH-CO r +AI-CO Na-CO < alumina-CO z Ca-CO < >Si-CO < Al(cationic)-CO. A correlation between the calculated bond orders of CO and the observed vibrational frequencies of CO-forming interaction complexes is drawn.
Zeolites have found broad application as acid-type catalysts, whose proton-donor and electron-acceptor acidic sites of different nature are commonly regarded as the active centers. Proton-donor sites can be investigated directly by infrared spectroscopy (IRS) in the region of the OH bond stretching vibrations, where the band shifts resulting from the formation of hydrogen-bonded complexes of hydroxyls with adsorbed weak bases characterize their proton-donor ability (acid strength). However, I R spectroscopic identification and characterization of electron-accepting acid sites can be performed only indirectly using the adsorption of test molecules.' Carbon monoxide has often been employed to study aprotic sites of various adsorbents and catalysts.'-'' For zeolites, however, (1) Ward, J . W. Zeolite Chemistry and Catalysis; ACS Monograph 171; Rabo, J., Ed., American Chemical Society: Washington, DC, 1976; p 118.
0022-3654/87/2091-5247$01 .50/0
the field of its application has long been limited to cationic forms predominantly of Y zeolite^.^-'^ As for the Z S M family, only (2) Angell, C. L.; Schaffer, P. C. J . Phys. Chem. 1966, 70, 1413. (3) Fenelon, P. J.; Rubalcava, H. E. J . Chem. Phys. 1969, 51, 961. (4) Rabo, J. A.; Angell, C. L.; Schomaker, V. Proc. Int. Congr. Catal., 4th 1971, 2, 96. (5) Huang, Y . Y , J . A m . Chem. SOC.1973, 95, 6636. (6) Bregadze, T. A.; Seleznev, V. A.; Kadushin, A. A,; Krylov, 0. V. Izu. Akad. Nauk SSSR, Ser. Khim. 1973, 2701. (7) Egerton, T. A,; Stone, F. S . J . Chem. Soc., Faraday Trans. 1 1973, 69, 22. (8) Huang, Y . Y. J . Catal. 1974, j2, 482. (9) Kasai, P. H.; Bishop, Jr., R . J.; McLeod, Jr., D. J . Phys. Chem. 1978, 82. 279. (10) Dombrowskii, D.; Dyakonov, S. S.;Kiselev, A. V.; Lygin, V . I . Kinet. Katal. 1978, 19, 1067. ( 1 1 ) Ballivet-Tkatchenko, D.; Courdusier, G. Inorg. Chem. 1979, 18, 558. (12) Lokhov, Yu. A,; Davydov, A. A. Kinet. Katal. 1980, 21, 1515, 1523.
Q 1987 American Chemical Society
