4308
J . Phys. Chem. 1991, 95, 4308-43 13
MPI Ion-Current and Photoelectron Spectra of Jet-Cooled p -Phenylenediamines H. Ozeki, K. Okuyama, M. Takahashi, and K. Kimura* Institute for Molecular Science and The Graduate University for Advanced Studies, Okaraki 444, Japan (Received: November 9, 1990) One- and two-color (1 + 1) resonant ionization experiments were carried out for p-phenylenediamine (PD), N,N-dimethyl-p-phenylenediamine(DMPD), N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) under conditions of isolated molecules. An MPI ion-current spectrum of PD shows many sharp bands consisting of several vibrational progressions with the SIorigin (0:) at 29 824 cm-l, while the photoelectron spectra due to the ground state (Do) of PD+ show a few progressions with the O+ peak at 54 640 cm-'. All these progressions have been interpreted in terms of the a vibrational modes in the SIand D states of PD. The vibrational frequencies of the NH2 out-of-plane mode are 489 cm-t (So), 598 cm-I (SI),and 1034 cm-P (Do). An MPI spectrum of DMPD shows a well-resolved vibrational structure in the low-energy region and many congestion bands in the higher region; while that of TMPD shows no vibrational structure. These facts suggest that the geometrical changes in the SIstate increase with the number of CHI,groups. For a mixture of PD with acetonitrile in jets, a preliminary result supporting a molecular complex has been obtained, indicating a red shift of 207 cm-l in the SI region of PD and a progression of 24 cm-I, which is probably due to an intermolecular vibrational mode of the PD-acetonitrile complex.
Introduction Ion-current measurements in multiphoton ionization (MPI) provide a high-resolution spectroscopy for studying resonant excited states in the gas phsae.' In a last decade, "resonantly enhanced multiphoton ionization" (REMPI) combined with photoelectron spectroscopy (PES) has been applied to various molecules for studying their ionic states produced through resonant excited states, as shown in review paper^.^.^ Many applications with this technique have been demonstrated in this laboratory.2 In the present work, we extended our REMPI experiments to pphenylenediamines to obtain new spectroscopic information about vibrational modes in the lowest excited singlet as well as in the cation ground states (Do). states (SI) The compounds studied are p-phenylenediamine (PD), N,Ndimethyl-p-phenylenediamine(DMPD), and N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD), which are well-known electron donors in charge-transfer (CT) complexes. The electron donating property becomes stronger with increasing number of methyl groups. Recently a study of resonance Raman spectroscopy of PD and its cation in solution has been reported by Ernstbrunner et al.,' providing spectroscopic information about vibrational modes and frequencies in their ground states. Most spectroscopic studies of these compounds have so far been carried out in solution^.^ However, no spectroscopic studies have been published for jet-cooled p-phenylenediamines under the conditions of isolated molecules. It is interesting to apply high-resolution MPI and LIF techniques to these jet-cooled molecules for studying their electronically excited states and ionic states in detail, in connection with future comprehensivestudies of gaseous CT complexes. Here, LIF means "fluorescence excitation". In REMPI experiments, the vibrational distribution in the cation largely depends on the resonant vibronic level. In other words, a Franck-Condon-favorable vibrational distribution of the cation is produced from the resonant state. Therefore, if we select a well-established vibronic level as a resonant excited state, then it is possible to interpret the photoelectron vibrational structure in terms of the cation vibrational modes. Several monosubstituted Johnson, P. M. Annu. Reu. Phys. Chem. 1981,32, 139. ( 2 ) (a) Kimura, K. Adu. Chem. Phys. 1985,60, 161. (b) Kimura, K. Int. Reu. Phys. Chem. 1987, 6, 195. (3) (a) Pratt, S.T.; Dehmer, P. M.; Dehmer, J. L. In Aduances in Multi-Photon Processes and Spectroscopy; Lin, S. H., Ed.; World Scientific: Singapore, 1988; Vol. 4, p 69. (b) Compton, R. N.; Miller, J. C. In Laser Applications in Physical Chemistry; Evans, D. K., Ed.;Marcel Dekker: New York, 1989; p 221. (4) Ernstbrunner, E. E.; Girling, R. B.; Grossman, W. E.; Mayer, L. E.; Willalms. K. P. J.; Hcster, R. E. J . Raman Spectrosc. 1981, 10, 161. (5) Nagakura, S.In Excited States; Lim, E . C., Ed.; Academic Press: New York, 1975; Vol. 2, p 321. (1)
0022-365419 112095-4308$02.50/0
benzenes such as toluene, phenol, aniline, and chlorobenzenehave been studied along this lines6 Under such circumstances, we considered it interesting to apply the REMPI technique for obtaining vibrational information of these electron-donor molecules in the SIstate as well as in the cation in the ground state. For PD, we have studied an MPI ion-current spectrum in detail, and by selecting three vibronic SI levels, we have successfully identified several photoelectron vibrational peaks due to the cation. The vibrational information thus obtained will be helpful for future studies of CT complexes obtainable under the conditions of isolated molecules. In the present paper, we report MPI results of jet-cooled pphenylenediamines for the first time, putting emphases on the following: (1) the MPI ion-current and photoelectron spectra of PD and its vibrational assignments in the SI and Do states, (2) the MPI ion-current spectra of DMPD and TMPD, and (3) some preliminary results that show spectroscopic evidence for complex formation between PD and acetonitrile. Experimental Section Apparatus and Technique. The apparatus and experimental techniques used in the present work are essentially the same as those previously The following one-color and two-color measurements were carried out under supersonic-jet conditions: (1) one-color MPI ion-current and LIF measurements, (2) onecolor time-of-flight (TOF) photoelectron measurements, and (3) two-color (1 + 1') ion-current and threshold photoelectron measurements. Samples. Each solid sample contained in a pulse nozzle was heated to have sufficient vapor pressure. PD, DMPD, and TMPD were vaporized at 85,45, and 30 OC,respectively. Helium was used as a carrier gas at 2280 Torr. In complex formation of PD with acetonitrile under jet-cooled conditions, we adjusted the acetonitrile pressure to find the optimum condition of complex formation. The samples of the p-phenylenediamines (Hayashi Kasei) were purified by vacuum sublimation several times. (6) (a) Meek, J. T.; Long, S. R.; Reilly, J. P. J . Phys. Chem. 1982, 86, 2309. (b) Anderson, S. L.; Rider, D. M.; a r e , R. N. Chem. Phys. Len.1982, 93, 1 1 . (c) Meek, J. T.; Sekreta, E.; Wilson, W.; Viswananthan. K. S.;Reilly, J. P. J . Chem. Phys. 1985, 82, 1741. (d) Anderson, S. L.; Goodman, L.; Krogh-Jespersen, K.; Ozkabak, A. G.; Zare, R. N.; Zheng, C-f.J . Chem. Phys. 1985,82, 5329. (7) (a) Achiba, Y . ;Sato, K.; Shobatake, K.; Kimura, K. J . Chem. Phys. 1983,78,5474. (b) Achiba, Y.; Hiraya, A.; Kimura, K. J. Chem. Phys. 1984, 80,6047. (c) Sato, K.; Achiba, Y.; Kimura. K. J . Chem. Phys. 1984,80,57. (d) Achiba, Y . ;Kimura, K. Chem. Phys. 1989, 129, 1 1 . (8) Achiba, Y . ;Kimura, K. In Aduances in Multi-Photon Processes and Spectroscopy; Lin, S. H.; Ed.; World Scientific: Singapore, 1988; Vol. 5 , p 317.
0 199 1 American Chemical Society
MPI Spectra of Jet-Cooled p-Phenylenediamines Acetonitrile (spectral grade) was used without further purification. We also tried to use other solvents such as acetone and benzene to produce molecular complexes with PD in jets, but we could not detect any complex formation. Laser Systems. The second harmonic of a Nd:YAG laser (Spectra Physics GCR-3) was used to pump a dye laser (Quanta Ray PDL-I) with either pure DCM dye, pure LDS-698 dye, or a mixture of DCM and LDS-698 dyes. The visible laser output was then frequency-doubled with a KD*P crystal in a Quanta Ray W E X system, generating UV light in the regions 305-335 nm (DCM), 333-347 nm (LDS 698), and 330-342 nm (DCM + LDS 698). The laser output was 8-11s pulses at 10 Hz. In the two-color experiments, two dye lasers (PDL-1 and PDL-3) with DCM and/or LDS-698 dyes were simultaneously pumped by the second harmonic of the Nd:YAG fundamental to generate 'excitation" and 'ionization" sources. The excitation source ( w , ) was the same as that used in the one-color experiments, while the ionization source ( w 2 ) was generated by mixing the dye laser output with the Nd:YAG fundamental (1.064 pm). The UV laser beam was lead to the vacuum chamber through a 350-mm focal length lens. The excitation and ionization lasers were led to the ionization region from the opposite direction under the focusing conditions that the beam size of the ionization laser was several times larger than that of the excitation beam to maintain resonant enhancement. MPI and LIF Measurements. In the one-color experiments, MPI ion-current and LIF measurements were simultaneously carried out. A lens collecting fluorescence and an ion detector were located on opposite sides, perpendicular to the plane of the laser beam and the nozzle direction. Ions were collected by an extracting electrostatic field, and then detected with a channel multiplier (Murata Ceratron). Ion currents were amplified by a current amplifier (Keithley 427) and boxcar integrators (Brookdeal9425 and 9415) and recorded on a chart recorder (National VP-6537A) as a function of scanning time. In the LIF measurements, the total fluorescence was collected by two lenses with focal lengths of 200 mm at an acceptance solid angle of 0.45 steradian, and detected by a photomultiplier (Hamamatsu Photonics R-928). The LIF signals thus obtained were processed by the data acquisition system as used in the MPI measurements. Furthermore, two-color (1 + 1') ion-current measurements were carried out at an extracting voltage of 25 V/cm, in connection with the two-color (1 + 1') threshold photoelectron measurements, which are mentioned later. TOF Photoelectron Measurements. Measurements of timeof-flight (TOF) photoelectron spectra were carried out with an electron drift tube with a flight length of 50 cm. The electron flight tube was mounted along the direction perpendicular to the molecular beam. Electrons traveling through the drift tube were detected by a multichannel plate (Galileo). To avoid the effect of Earth's magnetic field upon photoelectrons, the magnetic field was shielded by surrounding the TOF drift tube and the ionization region by Permalloy The electron signals thus obtained were detected by a fast transient recorder (Biomation 6500, 2 ns, 6 bits/channel) and then transferred into a microcomputer data acquisition system. Photoelectron signals were accumulated for a t least 30000 laser shots to obtain an adequate signal-to-noise ratio. The energy resolution in the TOF photoelectron measurements was estimated to be 4.8 meV from an observed bandwidth. 7\uo-Color Threshold Photoelectron Measurements. Threshold photoelectron spectra were measured in t w m l o r (1 + 1') resonant ionization experiments with a capillary-type analyzer recently developed in this laboratory. The detail of our threshold photoelectron analyzer is described el~ewhere.~ Briefly, the analyzer consisted of (1) a capillary plate through which threshold photoelectrons (with nearly zero kinetic energies) were collected, (2) a set of electric plates with metal meshes to apply an appropriate voltage across the ionization region to collect threshold photoelectrons through the capillary plate, and (3) an electron multiplier (9) Takahaski, M.; Okuyama, K.; Kimura, K. J . Mol. Srrucr., in press.
The Journal of Physical Chemistry, Vol. 95, No. 11, 1991 4309
I
29800
I
1
.
30200 30600 Wavenumber icm-1
1
31000
'
1
31400
.
Figure 1. One-color MPI and LIF spectra of jet-cooled pphenylenediamine (PD) in the 29 500-3 1 700-cm-1region, indicating various vibronic levels of the SIstate: (a) MPI ion-current spectrum; (b) LIF spectrum. Possible vibrational assignments are indicated on individual peaks.
TABLE I: Results for MPI Ion-Current Spectrum of PD ~
~~
energy assnmt shift' intens energy 0 100 31123 29824 0'' 5 31132 30 203 378 440 160 31149 30265 6al 29 31 228 30291 466 598 166 31260 30422 X' 5 31285 30 48 1 656 11 31299 30 624 800 78 31365 812 30637 1' 34 31380 30 643 819 20 31415 868 30 693 30705 6a2 880 161 31447 22 31455 30732 908 17 31502 949 30774 10 31516 30 801 976 9 31544 30 820 995 10 31561 30 844 1020 30861 6a1X1 1037 277 31568 68 31584 1193 31018 X2 33 31606 1240 31 064 31076 6a111 1251 104 31633 40 31669 31 082 1257 32 1285 31 110 a
assnmt shift# intens 7al 1298 65 1307 41 1324 76 6a3 1403 81 X11' 1435 93 1460 25 6a2X1 1474 100 1540 18 1555 26 1590 26 12 1622 35 6a1X2 1630 197 1677 64 6a211 1691 136 1720 58 6a17a1 1736 138 1744 42 1759 51 6a4 1781 20 1808 26 1844 105
In cm-l units.
(Murata Ceratron) to detect threshold photoelectrons. In collecting threshold photoelectrons through the capillaries, we applied an electric pulse field of 4 V/cm across the ionization region a t a delay time of 400 ns after each laser shot. No electric field was applied during the photoionization to avoid 'field ionization", which lowers the apparent ionization potential. The ionization laser (w2) was scanned a t a speed of 0.005 nm/s, while the excitation laser ( w J was kept at a constant wavelength to perform the S,excitation. The Ceratron output signal was amplified, and averaged in the same way as in the ion-current measurements. Measurements of threshold photoelectron spectra were carried out with an energy resolution of f 2 cm-', which was confirmed by recording a photoelectron rotational peak due to NO+ obtained by (1 + 1') resonant ionization through the A state (u' = 0, N' = 7) of NO. Results and Discussion p-Phenylenediamine. MPI and LIF Spectra. An MPI ioncurrent spectrum of jet-cooled PD observed in the region 29 500-31 700 cm-l (315-338 nm) is shown as spectrum a in Figure 1, indicating a well-resolved vibronic structure with the first distinct band appearing a t 29 824 cm-I, which is assigned to the SIorigin. No hot bands were detected below this band under our jet-cooled conditions. A total of 43 MPI bands were observed in the MPI spectrum (Figure 1). Their energies and
Ozeki et al.
4310 The Journal of Physical Chemistry, Vol. 95, No. 11, 1991 of Vibrational Frequeacies of PD
TABLE II: C o q " Symmtry)
vibrational freauency, cm-' gas phase' sohtionb
~
mode
Sn
v(NH2) v(CH) b(NH2)/ring 8a (ring mode) 7a (v(CN)/ring) 9a (B(CH)) 5 (y(CH)/out-of-plane) 4NHJ 1 (ring breathing) 6a (in-plane def) 10b (out-of-plane def) r(NH2)d
1270 1190 996 844 463 489
St
DnC
SO
DoC 1526 1646 1428 1186
816 453
1643 1600 1257 1179 1014 938 845 468
172P 1677* 1298
812 440 466e 598
838 466 415
b
loo0
2d00
f'
1034
'Present work. *Reference 4. 'Do refers to the ground-state cation. dThis mode is often denoted X in the text. #Tentative.
relative intensities are summarized in Table I. A LIF spectrum obtained for jet-cooled PD is shown as spectrum b in Figure 1. As seen from Figure 1, the MPI and LIF spectra resemble each other in vibrational frequency and intensity distribution. As far as we know, no MPI and LIF spectra have so far been published for PD. Vibrational Assignments in the SIState. The 16 MPI bands with relatively strong intensities in Figure 1 may be assigned to the fundamental, overtone, and combination bands of the vibrational modes "I", '6a", "7a", and "X". The following progressions have been identified: (1) [0,6a', 6a2, 6a3, 6a4]; (2) [0, XI, X2]; (3) [0, l', 12,];(4) [0,7a', 7a16a']; ( 5 ) [0, l', l'X']; (6) [0,6a'l', 6a21']; (7) [0, 6a'X1, 6a'X2]. Here, 0 denotes the SIorigin. Throughout the present paper, the Lord notationlois used for the vibrational modes characteristicto the skeletal benzene ring; mode 1 is 'ring breathing", mode 6a is "in-plane deformation", and mode 7a is the "v(CN)/ring mode". Mode X should be assigned to the NH2 out-of-plane mode I'(NH2) for the reasons mentioned later. These four vibrational modes (1,6a, 7a, and X) have been confirmed from the observed vibrational progressions (Figure 1). The fundamental frequencies of these modes are summarized in Table 11. In addition to the progressions mentioned above, there are several more peaks with relatively strong intensity in Figure 1. For these bands, we have made the following tentative assignments on the basis of the normal-mode calculations reported by Ernstbrunner et a1.4 (1) The band appearing at 30291 cm-' (0 466 cm-I) is due to "lob" (out-of-plane deformation), since its possible closest frequency in the ground state is 415 cm-I. (2) The two bands at 31 502 cm-' (0 + 1677 cm-I) and 31 544 cm-l (0 + 1720 cm-') are due to the vibrational modes 8a (ring mode) and 6(NHz) (amino group bending mode), respectively. The corresponding ground-state vibrational frequencies are 1600 and 1643 cm-' in solution! (3) The band at 31 260 cm-I (1435 cm-l) may be due to the overtone of a non-a, vibrational mode with two quanta, since there are no other a, ground-state vibrational modes in the region 1300-1 600 cm-'. From the long progressions of 6a and X in Figure 1, it is suggested that the changes in equilibrium geometry between the So and the SIstates are relatively large along the directions of the 6a and X normal coordinates. Concerning the molecular structure of PD, we can consider two rotational isomers that have the two NH, umbrellas at cis and trans positions. According to a gas-phase electron diffraction study," the molecule exists in only the trans form. Any clear evidences for the two isomers were not obtained from the present MPI and LIF experiments and the single-vibronic-level dispersed
+
~
(10) VarrranyL G.in Assignmetusfor VibmrfoMISpectra of 700 Benzene Drriuarives; Academic Pres% New York, 1969. (1 1) Colapietro, M.; Domenicano, A.; Portalone, G.; Schultz. G.: Hargittai, 1. J . Phys. Chem. 1987, 91, 1728.
ION INTERNAL ENERGY /cm-l
Figure 2. Excited-state photoelectron spectra of jet-cooled PD,obtained by (1 + 1) resonant ionization through three vibronic levels of the SI state: (a) 00 level, (b) 6al level, and (c) X1level. The spectra are shown as a function of the ion internal energy. Possible vibrational assignments of the cation are indicated on individual peaks.
fluorescence (SVL DF) spectra mentioned in the next section. Therefore we have concluded that the molecule exists only in the trans form under jet-cooled conditions. Ground-State 'Vibrational Modes Revealed from SVL DF Spectra. As shown in Table 11, the PD molecule has twelve normal vibrational modes with a, symmetry. Ernstbrunner et ala4have carried out normal-mode calculations for PD and its cation, indicating that the vibrational frequency of the amino group outof-plane mode "l'(NH2)" is 332 cm-' in the ground state (SO)of PD. For this vibrational mode, however, no experimental frequency is reported in the resonance Raman s t ~ d y . ~ As mentioned in the preceding sections, we have observed the new vibrational mode denoted by X in the SIand Do states of PD. In order to clarify the nature of the vibrational mode X, we considered it important to observe its ground-state vibrational frequency by analyzing SVL DF spectra of PD under the same jet-cooled conditions. Several SVL DF spectra of jet-cooled PD have previously been observed by O.K. at Tohoku University,'z as described briefly in the Appendix. On the basis of the SVL DF spectra observed by the X' excitation (see also Figure 6), we have clearly found that the vibrational frequency of the X mode in the So state is 489 cm-', which is considerably larger than the calculated frequency (332 cm-I) obtained from the normal-mode calculations of Emstbrunner et a1.4 However, all other vibrational frequencies deduced from the SLV DF spectra, except for those of mode X, are in good agreement with the calculated frequencies and the solution data of Ernstbrunner et aL4 Therefore, we have concluded that mode X should be attributed to the mode I'(NH2). Photoelectron Spectra and the Cation Vibrational Levels. The following two kinds of photoelectron experiments were carried out for PD. One is one-color (1 + 1) photoelectron measurements with a TOF analzyer, and the other is two-color (1 + 1') threshold photoelectron measurements. The latter measurements were performed mainly for the photoelectron energy calibration. In the (1 + 1) resonant ionizations, the photoelectron spectra are expected to show Franck-Condon-favorable vibrational structures. Three kinds of TOF photoelectron spectra which were obtained through the Oo, 6a1, and XI levels of the S,state are shown in Figure 2. As seen from Figure 2, the well-resolved ~
~~
(12) Okuyama, K.; Ito, M. Unpublished results.
The Journal of Physical Chemistry, Vol. 95, No. 1 1 , 1991 4311
MPI Spectra of Jet-Cooled p-Phenylenediamines TABLE III: Comparison of Experimental and Calculated Photoelectron Energies of P D and Vibrational Assignments
excitation 00"
6al
photoelectron band 1
2 3 4 1
2 3
XI
1
2 3 4
energy* exptl calcdd O.6& 0.621 0.52, 0.520 0.499 0.464 0.42, 0.419 0.67, 0.674 0.572 0.573 0.47, 0.472 0.772 0.638
0.539 0.436
0.769 0.641
0.540
0.439
Y
I
I
Ill
assnmt O+ 1"
6a+I 6a+'l+l 6a+11+2
'0
X+'
X+'l+I X+'1+*
'2hu = 59650 cm-I. b2hu = 60530 cm-'. e2hv = 60845 cm-'. dGas-phase vibrational frequencies given in Table I1 used for modes I+, 6a+ and X+. TABLE IV: Energy Levels of the PD Cation Obtained by Two-Color Ion-Current Measurements' levelb vib freqb assnmt 54 640 O+ 55 093 453 6a+ 55 673 1033 X+ 55 456 816 I+ 55910 1270 56 490 1850 1+x+
OCorrected for field ionization (see text).
M", ,Me
cm-l units.
vibrational peaks appear, suggesting that the ionization transitions take place from the optically prepared single vibronic levels of the SIstate. The photoelectron kinetic energies (K,)obtained for the individual bands in Figure 2 are summarized in Table 111, together with the ionization energies as well as their possible vibrational assignments. The ionization energy (E,) of producing the ith vibronic level of the cation is given by E, = 2hv - K,,where hv is the one-photon excitation energy corresponding to the resonant state and Ki is the photoelectron energy. The K,values in Table I11 were calibrated with the measurements of the (1 1') ioncurrent measurements, and then further calibrated with the field-free threshold photoelectron measurements. Table IV summarizes our final data on the energy levels of the PD cation. If there is no large displacement along vibrational coordinates between the resonant and ionic states, it is expected that a photoelectron vibrational band due to the Au = 0 propensity rule will be observed. As indicated in Figure 2, the Au = 0 photoelectron peaks corresponding to the 6a+' and X+'vibrational levels are clearly observed. Besides these modes, a vibrational progression of the ring-breathing mode is observed in Figure 2a. Spectrum a in Figure 2 indicates four photoelectron peaks. The first peak can be assigned to the origin O+ of the PD cation. The adiabatic ionization energy (I,) was found to be I, = 54640 f 8 cm-I = 6.774 0.001 eV
+
from the present two-color threshold photoelectron measurements. During the present photoionization experiments of PD, no electric field was applied to protect highly excited Rydberg states from "field ionization", which causes underestimation of the ionization energy. (Only at a delay time of 400 ns after each laser shot, we applied an electric pulsed field of 4 V/cm to collect threshold photoelectrons, as mentioned before.) So far there are no literatures available for comparison with the observed adiabatic ionization energy of PD. In photoelectron spectrum a in Figure 2, the second and fourth bands may be assigned to the fundamental and overtone of the respectively. The long ring-breathing mode, namely I+' and vibrational progression of the ring-breathing mode indicates that T electron removal makes the benzene skeletal larger. The third peak appearing at 1270 cm-l in spectrum a could not be assigned at present.
I
29300
29600 29400 Wavenumber / cm-1
29100
Figure 3. MPI ion-current spectrum of jet-cooled DMPD in the 29250-29650-cm-l region, obtained by (1 + 1) resonant ionization through the SIstate. The spectrum clearly shows a low-frequency vibrational progression of 23 cm-l.
Spectrum b in Figure 2 consists of the fundamental of the 6a+ mode and its combination bands with the 1' mode. The 6a+ frequency of the cation seems to be almost unchanged from that of the SI and Do states. From this fact, it is seen that the Au = 0 propensity rule seems to hold and the cation equilibrium structure is not distorted in the direction of this normal coordinate. In spectrum c, the first two bands may be assigned to O+ and X+', the third and fourth ones to the combination bands X+'1+' and X+l 1 +z, respectively. The resulting vibrational frequencies of PD+ (Do) are summarized also in Table 11, compared with the frequencies of the So and SIstates. It should be mentioned that the X+ frequency (1034 cm-l) of PD+ is remarkably larger compared with those of SI(489 cm-') and So (598 cm-I). Since this mode is the NH2 inversion motion, PD+ is considered to have a much larger barrier for inversion motion than the neutral PD molecule. In the aniline molecule, according to a REMPI study of Meek et al.," the frequency of the inversion motion (al symmetry) of the cation (Do) with two quanta is 1307 cm-', which is remarkably and the S, state (419 larger than those of the SIstate (758 ~ m - 9 . 'Therefore, ~ concerning the inversion motion, the situation for PD+ is quite similar to that for the aniline cation. Summarizing these results, we have obtained experimental evidence that there is a relatively large molecular deformation between the neutral molecule and the cation of PD. It should also be pointed out that the above-mentioned adiabatic ionization energy (6.774 eV) is much smaller than the vertical ionization energy (I, = 7.61 eV) reported in a He1 photoelectron study by Palmer et al.14 This large energy difference seems to be also consistent with the large molecular deformation. MPI Spectra of Jet-Cooled DMPD and TMPD. Figure 3 shows an MPI ion-current spectrum obtained with jet-cooled DMPD in the region 29 250-29 650 cm-', which is in the vicinity of the SIorigin, indicating the following features: (1) The first sharp peak appearing a t 29356 cm-' (3.640 eV) may be assigned to the SIorigin. (2) The first four peaks constitute a progression with a very low frequency of 23 cm-l, as indicated on the figure. (3) Many other sharp bands are observed with good reproducibility in the low-energy region. The present MPI measurements were extended up to 30200 cm-', showing a congestion of numerous bands, as shown by spectrum b in Figure 4, in which the MPI spectra of PD, DMPD, and TMPD are compared with one another on the same energy scale. The SIorigin of DMPD (29 356 cm-l, X = 340.64 nm) is 468 cm-l red shifted from that of PD. The vibrational progression of 23 cm-' in Figure 3 may be due to a "flexible" vibration such as the internal rotation of the CH, groups in DMPD. It is interesting to note that many sharp and well-resolved bands are observed in the low-energy region in Figure 3 and congestion takes place without spectral broadening with increasing excess (13) Christofferson,J.; Hollas, J. M.; Kirby, G. H. Mol. Phys. 1969, 16, 441. (14) Palmer, M. H.; Moycs, W.; Spiers, M.; Ridyard, J. N. A. J . Mol. Struct. 1978, 53, 235.
Ozeki et al.
4312 The Journal of Physical Chemistry, Vol. 95, No. 1 1 , 199'1
I cg
NU,
I
MPI
28800
29100
29600
(29821 Cm-')
I I,,
.
,
3odco
Wavenumber Icm-1
Figure 4. MPI and LIF spectra of jet-cooled PD, DMPD, and TMPD, compared on the same energy scale. Asterisks indicated in the DMPD spectra mark impurity peaks due to PD. energy (see also Figure 4b). The complicated spectral features of DMPD in the low-energy region are in striking contrast to the spectrum of PD, which shows no peaks in the same region. The many sharp bands observed in DMPD suggest that relatively large transition, structural change takes place upon making the Sl-So compared with the case of PD. An MPI spectrum of TMPD obtained in the region 28 80029800 cm-' is shown in Figure 4(c), indicating that no distinct SIorigin is identified. The intensity gradually increases to show a very broad band with no resolved structure. Concerning the SIorigin, it is probably located below 28 800 cm-' (A = 347 nm), since a significant MPI signal seems to start below the region shown in Figure 4(c). The SIorigin was not identified from the present MPI ion-current measurements. In addition to the MPI spectra, we tried to measure LIF spectra for both DMPD and TMPD. However, in contrast to our results for PD, we were not able to detect any LIF spectra for jet-cooled DMPD and TMPD molecules (see Figure 4). This fact suggests that in DMPD and TMPD the nonradiative relaxation processes are much faster than the radiative process, in contrast to the case of PD. From these results for DMPD and TMPD, we may conclude that there should be large displacements in the equilibrium structure between the So and SI states. Molecular Complex of pdhenylenediamine with Acetonitrile. In solution photochemistry, a weak CT complex formed between a strong electron-donating molecule and an electron-accepting molecule in the ground state is known to show a strong CT character at an electronically excited singlet or triplet CT state, through which "ionic photodissociation" takes place, producing the corresponding radical cation and anion.1s Our initial attemp was to obtain spectroscopic information about gas-phase CT complexes under isolated conditions from MPI ion-current experiments. In the present MPI experiments, we tried to detect molecular CT complexes of PD under jet-cooled conditions by mixing appropriate solvent molecules such as acetonitrile, acetone, and benzene, which are known as electron acceptors. These three solvents are aprotic, and the formation of hydrogen-bonded complexes is not expected. Figure 5 shows an MPI ion-current spectrum obtained with a mixture of PD and acetonitrile in the vicinity of the SIorigin of PD. The first significant peak appearing at 29618 cm-',which is 207 cm-'red-shifted from the SIorigin of PD, has been assigned to the SIorigin of the molecular complex formed between PD and acetonitrile. As seen from Figure 5 , a long vibrational progression with a spacing of 24 cm-' was observed, attributable to an Yintermolecular"vibrational mode. The whole spectral feature due to the complex formation is extremely weak in MPI intensity compared with that of free PD. The partial pressure range of acetonitrile for providing sharp MPI (15)
Kimura, K. Reo. Cham. Intermad.
1979, 2, 321.
Figure 5. MPI ion-current spectrum obtained for a mixture of PD and acetonitrile under jet-cooled conditions in the 29 550-29 850-cm-' region, indicating a low-frequency vibration progression due to complex formation. The spectral origin of the complex is red-shifted by 207 cm-l from that of the free PD. SVL-
DF OOExc.
(a)
YH2
6apXe I ,6a:
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xe 5?
I
I
I
28800
28400
(b)
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29600
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/I 29600
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(
(d)
30100
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29400
29800 30300 WAVENUMBER / cm-1
30800
Figure 6. SVL DF spectra observed for PD by excitation to four vibronic levels of the SIstate:' (a) Oo; (b) 6a'; (c) X'; (d) 6a'X'. The observed peaks correspond to the vibrational levels in the ground state.
peaks due to the molecular complex is considerably narrow; the optimum partial pressure was 0.8 Torr at a carrier gas pressure of 2280 Torr. For a mixture of PD with acetone or benzene, which are less polar solvents than acetonitrile, we were not successful in observing any sharp bands due to a molecular complex. From these experimental results, it may be suggested that molecular complexes of PD with acetone and benzene in the gas phase are probably unstable. The binding energy of the PDacetonitrile complex in the electronic ground state is probably small, comparable to that of van der Waals complexes, namely, less than 100 cm-I. On the contrary, in the electronically excited state studied here, the binding energy of the PD-acetonitrile complex should be several times larger than that in the ground state, as suggested from the observed red shift of 207 cm-l. The stabilization of this complex in its excited state may be due to an increase of CT character or hydrogen-bonding character. Excitation of a weak CT complex (D-A) in the ground state gives rise to a strong CT complex; in other words, the excited-state
J. Phys. Chem. 1991, 95, 4313-4318 complex is expressed by D+-A-, where D and A mean an electron donor and an electron acceptor, respectively. Therefore, it is interesting to observe vibrational frequencies for the PD-acetonitrile complex in the SIstate for studying its CT character. If the PD-acetonitrile complex has a strong CT character in the excited state, its vibrational frequencies should approach those of the PD cation rather than those of the neutral PD molecule. In the present work, however, we were not successful in observing any significant MPI signals for the PD-acetonitrile complex in the vicinity of other vibronic levels of PD in the SIstate. We need further MPI experiments for studying vibrational structures of molecular complexes of PD under the conditions of isolated molecules. Although our present spectroscopic results on the PD-acetonitrile complex are only preliminary, this is the first article reported on spectroscopic experiments of charge-transfer complex systems under the conditions of isolated molecules. Appendix. Analysis of SVL Dispersed Fluorescence Spectra Since SVL DF spectra had been measured for jet-cooled PD,'* in the present work we have analyzed the spectra to obtain gasphase vibrational frequencies of the electronic ground state (So). So far only solution data have been reported by Ernstbrunner et a1.4 In Figure 6, the SVL DF spectrum obtained at the Oo level is shown by spectrum a, indicating a well-resolved vibronic structure and a mirror image relationship with the MPI spectrum shown in Figure 1. Comparing with the solution data? we can easily assign several peaks of spectrum a to the a, modes of the benzene ring (1, 5, 6a, 7a, 9a, and lob). The ground-state vibrational frequencies are given also in Table 11. The peak indicated by Xy is located at 489 cm-' with respect to ,!O showing a relatively strong intensity. In solution, however, no such vibrational frequency is reported. This indicates that vibrational mode X seems to be sensitive to the circumstance around the PD molecule. The first overtone of X is not observed
4313
in the vicinity of the frequency-doubled region (-980 cm-l) in all the spectra, in spite of the relatively intense peak of its fundamental. A large vibrational anharmonicity is expected for mode X. Therefore, we have assigned mode X to the symmetric inversion motion of the NH2 groups; that is, the out-of-plane mode denoted as I'(NH2). The vibronic assignments of the SI manifold in the LIF spectrum in Figure 1 have been performed on the basis of the SVL DF spectra. The most important problem is concerned with the assignments of the 440 and 598 cm-' bands in the LIF spectrum. The corresponding SVL DF spectra are shown by spectra b and c in Figure 6, respectively. In spectrum b (440 cm-' excitation) in Figure 6, a strong peak assignable to XI is observed, while the peaks due to 6a1 and 6a2 are almost missing. Furthermore, in spectrum c (598 cm-' excitation) in Figure 6, the peak due to XI is missing, but a long progression of 6a is observed with an intensity distribution similar to that in spectrum a. The two MPI bands appearing at 440 and 598 cm-' should be attributed to the vibrational modes 6a and X, respectively, or vice versa. In the general case where the ground-state potential is similar to the excited-state one, the Av = 0 propensity rule should hold, so that one may straightforwardlyassign the 440 cm-' band to X' and the 598 cm-' band to 6a'. In the present case, however, it is considered that such a Av = 0 rule does not hold, since there should be a large molecular deformation along the 6a and X coordinates, as seen from spectrum a in Figure 6. Furthermore, spectrum d in Figure 6 shows a combination band at 440 + 598 cm-', which is definitely assigned to 6alX'. A peak due to 6aiX\, which is expected from the Av = 0 rule completely disappears in this spectrum. The intensity distributions of the 6a and X progressions in the spectrum should be superimposed on those of the individual components. From this fact we may conclude that the 440 cm-' band is assigned to 6a'; the 598 cm-' band, to XI. Our conclusion has been supported by simple Franck-Condon calculations of the 6a progression.
Atomic Rhodium and Niobium Insertion into the C-H Bond of Methane Margareta R. A. Blomberg,* Per E. M. Siegbahn, and Mats Svensson Institute of Theoretical Physics, University of Stockholm, Vanadisvagen 9, S-1 I346 Stockholm, Sweden (Received: August I , 1990)
Accurate calculations have been performed for the addition reaction between methane and the second-row transition-metal atoms rhodium and niobium. The most interesting result obtained is that for the rhodium insertion there is nearly no barrier on the low-spin potential energy surface, which contains the MHCH3complex. A remarkable similarity in reaction energetics and the recently studied rhodium complexes RhCl(PH3)zand RhCp(L) between the low-spin state of the rhodium atom (zD) (L = CO, PHJ is noted. In particular, CH,-Rh a-complexes are formed as precursors in all the reactions, which is of key im rtance for a low barrier. The electronic mechanism responsible for the low barrier is an efficient mixing between the 4dpand the 4d8s1-stateswhich are both low-lying states of the rhodium atom. The results for the niobium atom, on the contrary, show a rather high barrier for the insertion with results similar to those of the previously studied nickel atom insertion.
Introduction The activation of C-H bonds in saturated hydrocarbons has been a topic of much interest in recent years, both experimentally'-" and theoretically.H Even though H-H and C-H bonds
have similar bond strengths, the C-H bond in alkanes turns out to be very diffkult to break, whereas the H-H bond in H2 is readily activated by most transition metals. The intermolecular insertion
( 1 ) (a) Janowicz, A. H.; Bergman, R. G. J . Am. Chem. Soc. 1982,104, 352. (b) Janowicz, A. H.; Bergman, R. 0. J. Am. Chem. Soc. 1983,105, 3929. (2)Jones, W.D.; Feher, F. J. J . Am. Chem. Soc. 1982,104,4240. (3) (a) Hoyano, J. K.; Graham, W. A. G. J . Am. Chcm. Soc. 1982,104, 3723. (b) Hoyano. J. K.; McMaster, A. D.; Graham, W. A. G. J . Am. Chem. Soc. 1983, 105,7190. (4) Perspectives in the Selective Activorion of C-H ond C-C Bonds in Saturated Hydrocarbonr; Meunier, B., Chaudret, B., Eds.; Scientific Affairs Division-NATO Brussels, 1988.
Quantum. Chem. 1983,23,855.
(5) Blomberg, M.; Brandemark, U.; Petterson, L.; Siegbahn, P. Int. J.
(6) Blomberg, M. R. A.; Brandemark, U.; Siegbahn, P. E. M. J . Am. Chem. Soc. 1983,105, 5557. (7)(a) Low, J. J.; Goddard 111. W. A. J . Am. Chem. Soc. 1984,106,8321. (b) Low, J. J.; Goddard 111, W. A. Orgonomctollfcs 1986,5,609. (c) Low, J. J.; Goddard 111, W. A. J. Am. Chem. Soc. 1984,106,6928.(d) Low, J. J.; Goddard 111, W. A. J . Am. Chem. Soc. 1986, 108,6115. (8) Koga, N.; Morokuma, K. J. Phys. Chcm., in press. (9)Ziegler, T.; Tschinke. V.; Fan, L.; Becke, A. D. J . Am. Chcm. Soc. 1989,Ill, 9177.
0022-365419112095-4313$02.50/0 0 1991 American Chemical Society
