2635
J. Phys. Chem. 1995, 99, 2635-2645
Competing Bond Fission and Molecular Elimination Channels in the Photodissociation of CHJNH~ at 222 nm G. C. G. Waschewsky, D. C. Kitchen, P. W. Browning, and L. J. Butler* The James Franck Institute and Department of Chemistry, The University of Chicago, Chicago, Illinois 60637 Received: September I , 1994@ This paper presents the first experimental investigation under collisionless conditions of the competing photodissociation channels of methylamine excited in the first ultraviolet absorption band. Measurement of the nascent photofragments' velocity distributions and preliminary measurements of some photofragments' angular distributions evidence four significant dissociation channels at 222 nm: N-H, C-N, and C-H bond fission and H2 elimination. The data, taken on photofragments from both methylamine and methylamine-d2, elucidate the mechanism for each competing reaction. Measurement of the emission spectrum of methylamine excited at 222 nm gives complementary information, evidencing a progression in the amino wag (or inversion) and combination bands with one quantum in the methyl (umbrella) deformation or with two quanta in the amino torsion vibration. The emission spectrum reflects the forces in the Franck-Condon region which move the molecule toward a ciscoid geometry. The photofragment kinetic energy distributions measured for CH3ND2 show that hydrogen elimination occurs via a four-center transition state to produce HD and partitions considerable energy to relative product translation. The reaction coordinates for N-H and C-N fission are analyzed in comparison to that for ammonia dissociation from the state and with reference to ab initio calculations of cuts along the excited state potential energy surface of methylamine which show these reactions traverse a small barrier in the excited state from a Rydbergkalence avoided crossing and then encounter a conical intersection in the exit channel. The measured kinetic energy distribution of the C-N bond fission photofragments indicates that the NH2 (NDz) product is formed in the 2A1 state; the C-N fission reactive trajectories thus remain on the upper adiabat as they traverse the conical intersection. The mechanism for C-H bond fission is less clear; most of the kinetic energy distribution indicates the reaction evolves on a potential energy surface with no barrier to the reverse reaction, consistent with dissociation along the excited state surface or upon internal conversion to the ground state, but some of the distribution reflects more substantial partitioning to relative translation, indicating that some molecules may dissociate via a repulsive triplet surface. In general, the photofragment angular distributions were anisotropic, but the measured p -0.4 f 0.4 for C-N bond fission indicates dissociation is not instantaneous on the time scale of molecular rotation. We end with analyzing why in methylamine three other primary dissociation channels effectively compete with N-H fission while in CH30H and CH3SH primarily 0 - H and S-H fission, respectively, dominate.
A
A
I. Introduction The photofragmentation pathways of CH3NH2, methylamine, excited in its first ultraviolet absorption band peaking at 215 nm are very different from those in isoelectronic CH30H or its analog CH3SH. Whereas those molecules evidence primarily O(S)-H bond fission, with a minor C-S fission channel in CH3SH, upon excitation in the first ultraviolet absorption band, in methylamine several primary bond fission and molecular elimination channels compete. Although the primary photofragmentation pathways have been extensively studied, the results were not always in agreement. The most recent and definitive study of photodissociation of methylamine from the first absorption band is by Michael and Noyes in 1963.' Using broad-band radiation from a Hanovia S-100 (194-244 nm), they detected four major dissociation pathways: CH3NH,
-
+ CH,NH -H + CH,NH,
-
H
AH' = 102.6 kcal mol-'
(1)
AH0 = 94.6 kcal mol-'
(2)
+ NH,
AH' = 79.5 kcal mol-'
(3)
+ CH3N
AH' = 35.0 kcal mol-'
(4)
CH3 H,
By measuring the amount of molecular hydrogen formed after photolysis, with and without radical scavengers, they determined @
Abstract published in Advance ACS Abstracts, February 1, 1995.
that reaction 1 accounted for at least 75% of the dissociation and (2) for 10% of (1). Since the scavengers were unable to suppress H2 formation completely, Michael and Noyes estimate the yield of reaction 4 to be 0.1 at most. Similarly, reaction 3 was assumed to account for the presence of methane and ethane in the postphotolysis mixture, from secondary reactions of methyl radicals. The yield for (3) was estimated to be very small-less than 0.05-but not zero. Thus, upon excitation to the fist excited state, the C-H and N-H bonds break much more readily than the weaker C-N bond, the reverse of what occurs upon thermal decomposition in the ground electronic state. To begin to understand this branching, we next review what is known about the lowest excited electronic potential energy surface of methylamine. Michael and Noyes review the theoretical and experimental literature on methylamine photochemistry prior to 1963. Although the experimental studies on methylamine since then have used primarily vacuum-ultraviolet photolysis light, in the second and third absorption there have been more recent electronic structure calculations on the excited states of methylamine relevant to photodissociation in the first UV absorption band?s8 Briefly, two singlet electronic states have been observed by U V absorption spectro~copy.~-~' The first absorption band (190-240 nm) corresponds to excitation of an electron to the nN 3s Rydberg ~ t a t e . ~The . ~ ~absorption spectrum shows structure assigned to two long vibrational progressions of the amino wagging and methyl rocking vibrations." Similarly, the second band corresponds to excitation to the nN 3p Rydberg
-
0022-3654/95/2099-2635$09.00/0 0 1995 American Chemical Society
-
Waschewsky et al.
2636 J. Phys. Chem., Vol. 99, No. 9, 1995
-
state. This band also shows vibrational structure, the source of which has not yet been assigned. Like ammonia, the nN 3s state is predissociated by an nN u* valence state leading to N-H and C-N bond fission. Kassab and co-workers used ab initio calculations to generate potential energy curves along the N-H and C-N bond fission channels in the first excited state. Their potential energy curves show small barriers in the excited state along both the C-N and N-H fission reaction coordinates. The barriers can be viewed as resulting from an avoided Rydberg-valence configuration curve crossing, providing an adiabatic pathway for N-H and C-N fission in an excited state that has Rydberg character in the Franck-Condon region. They suggest that although thermally the C-N bond is weaker than the N-H bond, in the excited state the barrier to C-N fission is higher than the barrier to N-H fission, explaining why the N-H bond might break more readily than the C-N bond upon excitation to the first excited state. In contrast, thermal and infrared multiphoton dissociation (IRMPD) from the ground state results in fission of the C-N bond13 and H2 eliminati~n,’~ not fission of the N-H bond. The IRMPD experiments of Xiang and Guillory on the C-N fission channel measure a statistical translational energy distribution of the NH2 radicals, implying a negligible barrier to the reverse reaction on the ground state potential energy surface. Whereas the electronic structure calculations above present the excited state in an adiabatic picture, spectroscopistsinterpret the lifetime broadened structure in the absorption spectrum in an equivalent approximately diabatic picture, describing the excited state (in the Franck-Condon region) as a strongly predissociated Rydberg state. Due to the rapid predissociation lifetimes, it could be too short-lived to exhibit a significant fluorescence quantum yield. Indeed, upon excitation at 206.2 or 253.7 nm, Freeman et al. did not detect any fluorescence from methylamine, in contrast to the fluorescence seen in trimethylamine at both 206.2 and 253.7 nm.2 A glance at the absorption spectrum confirms that methylamine barely absorbs at 253.7 nm. Absorption spectrum and force field calculations by Tsuboi and co-workerslI show that the frst excited state of methylamine is planar about the nitrogen, as in ammonia. Tsuboi and coworkers assign the structure in the absorption spectrum to excited state amino wagging and methyl rocking vibrations. Like ammonia, which is also planar in the excited state,15 methylamine has a conical intersection in the exit channel of the N-H and C-N bond fission channels. The ab initio calculations by Kassab and co-workers done at ciscoid geometry also suggest this. (The planar and ciscoid geometries of methylamine are depicted in Figure 1. Note that both the “planar” and ciscoid geometries are planar about the C-N bond. Dunn and Morokuma observe a very small barrier to rotation between the two geometries.) At planarkiscoid geometry, excited state products correlate with the ground state methylamine and ground state products correlate with excited state methylamine, resulting in conical intersections in the C-N and N-H bond fission channels. In studies of ammonia photodissociation in the first absorption band, the N-H fission reaction coordinate also has a small barrier to N-H bond fission near the Franck-Condon region and a conical intersection further along the reaction coordinate.’6-20 No similar studies have been done for methylamine yet. We first considered a molecular beam study of methylamine because the experimental results of Michael and Noyes showed that, upon excitation to the first excited electronic state, the strongest bond broke more readily than the weakest bond in the molecule, while the theoretical results of Kassab and coworkers implied that dissociation occurred directly from the
-
H
Figure 1. Geometries of methylamine corresponding to (top) ground
state equilibrium, (middle) ciscoid excited state equilibrium suggested by Kassab et al. (ref 7), and (bottom) planar excited state equilibrium used by Tsuboi et al. (ref 11) and Dunn and Morokuma (ref 8). Dunn and Morokuma find the ciscoid and planar structures have close to the same energy. excited state, where the barrier to N-H fission was lower than that to C-N fission. Thus, methylamine presented the opportunity to examine competing reaction channels where one might be able to predict the branching between dissociation channels by considering the barriers on the adiabatic excited state potential energy surface. Unlike the competition between the Woodward-Hoffmann forbidden C-X (X = Br, C1) bond fission channels in excited state bromoacetyl chloride and bromopropionyl chloride,21,22in which the expected branching is reversed by nonadiabatic recrossing of the barrier to C-Br fission, in methylamine the Rydberg-valence avoided crossing resulting in the barriers to N-H and C-N fission should be relatively strongly avoided, so the branching should be more amenable to predictions assuming adiabatic dissociation. In this paper we present the results of crossed laser-molecular beam and emission spectroscopy experiments on the photodissociation of methylamine excited to the nN 3s singlet state. Under collisionless conditions, we identified primary dissociation products from all four dissociation channels inferred by Michael and Noyes (eqs 1-4). At this wavelength, the contribution from C-H bond fission is more prominent than found in their broad-band excitation experiments. Our data on isotopically substituted methylamine show that H:! elimination occurs via a four-center process, as in methanol dissociation:
-
CH3NH,
-
CH2=NH
+ H,
(5)
The measured photofragment kinetic energy distributions give evidence that C-N and N-H bond fission occurs directly from the excited state, not through internal conversion to a vibrationally hot ground state, and indicates that C-N bond fission produces electronically excited NH2 fragments. In complement to the beam experiments, the emission spectrum we measured probes the forces in the Franck-Condon region felt by photoexcited methylamine. Strong emission to the ground state amino wagging and torsion vibrations, as well as methyl deformations, shows that the molecule is reacting to an excited state where the minimum-energy geometry is planar/ ci~coid.~~*J
Photodissociation of CH3NH2
J. Phys. Chem., Vol. 99, No. 9, 1995 2637
11. Experimental Method primary photofragments. Methylamine: The bimodal signal A. Molecular Beam Experiments. To measure the photoobserved at mle+ = 30, C W + , after 900 000 shots evidences fragment velocities and angular distributions from the photoboth primary C-H fission and primary N-H fission. The much dissociation of methylamine, CH3NH2, and methylamine-dz, stronger signal at d e + = 29, CH3N+, after 250 000 shots, and CH3ND2, we use a crossed laser-molecular beam a p ~ a r a t u s . ~ ~ . ~d ~e + = 27, HCN+, after 250 OOO shots, evidences H2 elimination Upon photodissociation with a pulsed excimer laser, neutral in addition to N-H and C-H fission. The signal observed at dissociation products scatter from the crossing point of the laser de' = 15, CH3+ and NH', after 700000 shots, was good and the molecular beam with velocities determined by the vector despite high background and results from C-N fission products sum of the molecular beam velocity and the recoil velocity as well as daughter ions from N-H and C-H fission products. Unfortunately, the high background counts at mle+ = 1, H+, imparted in the dissociation. Those scattered into the 1.5" acceptance angle of the differentially pumped detector travel made the signal for hydrogen, the other N-H and C-H fission 44.1 cm to an electron bombardment ionizer and are ionized product, barely discernible after 600 000 shots. High background also obscured the extremely weak signal at mle+ = 16, by 200 eV electrons. After mass selection with a quadrupole mass filter, the ions are counted with a Daly detector and CHq or NH2, after 600000 shots. Methylamine-dz: For multichannel scalar with respect to their time-of-flight (TOF) deuterated methylamine, we saw analogous signal at d e + = from the interaction region after the dissociating laser pulse. 32, CH2ND2+, after 500 000 shots, evidencing C-H fission, at mle+ = 31, CHjND+ and CHND2+, after 300 000 shots, After subtracting the calibrated ion flight time, forward convoluevidencing N-D fission as well as C-H fission, at mle+ = 30, tion fitting of the TOF spectrum determines the distribution of CH2ND+, after 200 000 shots and mle+ = 27, HCN+, after energies imparted to relative product translation in the dissocia250 000 shots, evidencing HD elimination and C-H and N-D tion. The angular distribution of the scattered photofragments fission. Signal observed at mle' = 18, ND2+, after 500 000 is obtained with a linearly-polarized photolysis beam by shots results almost exclusively from C-N fission, while the measuring the variation in signal intensity with the direction of much weaker signal at mle+ = 15, CH3+, after 500 000 shots, the electric vector of the laser in the molecular beaddetector evidences N-D fission as well as momentum matching the scattering plane. ND2+ signal from C-N fission. Due to high background, no The molecular beam was formed by expanding a 20% mixture signal was seen after 350000 shots at mle+ = 3, HD+. Nor of methylamine or methylamine-d2 in He at a stagnation pressure was any signal discernible after 500 000 shots at mle+ = 17, of 300 Torr through a 0.076 mm diameter nozzle heated to 60 CH3D'. "C. To reduce the effect of hydrogen exchange between B. Emission SpectroscopyMeasurement. We measure the methylamine-dz and H20 adsorbed to vessel and delivery tubing walls, methylamine-dz was introduced into the system and emission spectrum of methylamine using apparatus already described in detail e l ~ e w h e r e , thus ~ ~ . ~only ~ briefly described allowed to equilibrate for at least 15 min before evacuating the here. To generate the 222 nm light, we double the output of a system again. The peak beam velocity was 1.35 x lo5 c d s Lambda Physik FL3002 dye laser pumped with the 308 nm with a full width at half-maximum of 14.8%. To measure the velocity of the parent molecular beam in situ, the molecular output of a Lambda Physik EMG103MSC excimer laser (XeCl). beam source was rotated to point into the detector and a chopper Coumarin 440 or 450 is used with an intracavity etalon to wheel raised into the beam. To measure the velocities of the produce 444 nm light, which is then doubled in a @-barium neutral photofragments, the molecular beam source is rotated borate (BBO) crystal. The resulting 222 nm light is directed to a different angle in the plane containing the beam and detector into a stainless steel flow cell where it excites room temperature axis and perpendicular to the laser beam propagation direction. gaseous methylamine. We flow 600 mTorr of methylamine Laser polarization angles and molecular beam source angles are (Matheson, 95%) and flush the side arms with 6 Torr of helium. given here with respect to the detector axis, one defined as The emission is collected at 90" to the propagation of the positive with clockwise rotation and the other as positive with excitation beam and detected with an EG&G 1455B-700-HQ counterclockwise rotation. optical multichannel analyzer (OMA). The detector collects a Time-of-flight and angular distribution measurements were 50 nm wide spectrum for each exposure with a resolution of made on methylamine and methylamine-& photofragments at 75 cm-' near 222 nm. For each exposure of 150 s (1500 laser 222 nm. For most measurements the source angle was shots), we gate the image intensifier of the OMA with a 500 ns maintained at 10" with respect to the detector axis. The source gate synched to the laser. We then sum 32 exposures to obtain angle was increased to 15" for spectra taken at mle+ = 18, a spectrum averaged over 48 000 shots. ND2+, and mle' = 15, CH3+ and NH', and further to 20" for 111. Results and Analysis spectra taken at mle+ = 3, HD', and mle+ = 1, H+. The unpolarized laser power from a Questek 2640 excimer was When we excite methylamine at 222 nm, several dissociation typically 30 mllpulse, with the light focused to a 4.0 mm2 spot channels are energetically a~ailable:~ size at the crossing region of the laser and molecular beam. H, CH2=NH AHo = 35.0 kcal mol-' CH,NH, Quadrupole resolution was adjusted to 1.0 amu fwhm for mle+ (6) = 29 (CH3N+), 30 ( C W ' or CH2ND+), 31 (CH3ND+), and 2H2 HCN AHo = 37.8 kcal mol-' (7) 32 (CHzND2+) and to 0.8 amu for mle+ = 15 (CH3+ and NH+) and mle+ = 18 (NDz+). CH, NH AHO = 73.6 kcal mol-' (8) For the anisotropy measurements, we disperse the unpolarized laser light into two linearly polarized components with a singleCH3 NH, AHo = 79.5 kcal mol-' (9) crystal quartz Pellin-Broca and use the horizontal component, rotating the polarization into the desired direction with a halfH CH,NH, AHO = 94.6 kcal mol-' (10) wave retarder. The polarization-dependent signal, integrated in many repeated short scans and alternating between each laser H CH,NH AHO = 102.6 kcal mol-' (1 1) polarization direction, required no additional normalization to laser power or detector efficiency. Polarized spectra typically 2H2 HNC AHo = 120.8 kcal mol-' (12) were taken at 6 mllpulse. For both methylamine and methylamine-&, signal was We see evidence for primary C-N, C-H, and N-H fission, observed at several parent and daughter ions of the neutral as well as H2 elimination (eqs 9, 10, 11, and 6). In addition to
-
-
+ +
+ +
+ + +
2638 J. Phys. Chem., Vol. 99, No. 9, 1995
Waschewsky et al.
1.2 CH3NH2, 222 nm 1
m / e + = 30, CNH4+
-B 0.8 E
. 0.6 c
-
5 0.4
z 0.2 0 1.2
o
-B 0.0 E
4
=-
0
CH3ND2, 222 nm
1
10
15
20
25
30
6 (kcellmol)
m/e+= 31 CH3ND+ and CHND2+
Figure 3. Center-of-mass product translational energy distributions, P(ET)’s,for the C-H (filled circles) and N-H(D) (open circles) bond fission channels in methylamine at 222 nm. The P(ET)’sare derived
0.6
from forward convolution fitting the CN&+ signal in the upper frame of Figure 2 . The shaded area represents the translational energy range which is undetermined by our data (see text).
0.4
h
z 0.2 0
100
5
200
300 400 500 600 tlme of arrlval (p)
700
Figure 2. Laboratory time-of-flight spectrum of the photofragments detected at CN&+ from methylamine (upper) and CH3ND+ and CHND2’ from methylamine-d2 (lower)photodissociated at 222 nm with an unpolarized laser. The source angle was 10’. The signal results from a combination of primary C-H and N-H(D) bond fission and is fit with P(ET)’sshown in Figure 3. Identification of the two channels is based upon the shifting to earlier arrival times of the higher energy component in the deuterated sample, an observation consistent with recoil from the heavier D atom fragment.
identifying the primary dissociation channels occumng in methylamine, we used isotopically substituted methylamine, CH3ND2, to determine that the mechanism for H2 elimination is a four-center process analogous to the H2 elimination mechanism in CH,NH2
-H2 +
CH2=NH
(13)
The photofragment recoil kinetic energy distributions observed for the C-N and N-H bond fission channels indicate that these dissociation pathways evolve on the excited state surface, not through internal conversion to the ground state. The kinetic energy distributions peak well away from zero, providing evidence that the bond fission reactions proceed on a potential energy surface with barriers to the reverse reaction. Although the exit barrier along the ground state potential energy surface is nonzero for some of these reactions, it is not large enough to explain the observed kinetic energy distributions. The measured kinetic energy distribution for H2 elimination, in contrast, could result from dissociation upon internal conversion to a vibrationally hot ground state, as the ground state exit channel barrier is estimated at 26 kcal/mol.13 The kinetic energy distribution for C-H bond fission is peaked near zero, consistent with dissociation from the ground state or from an excited state with a negligible barrier to the reverse reaction. A. Molecular Beam Photofragmentation Data: Identification of Product Channels. 1. C-H and N - H Bond Fission Channels. To identify contributions from N-H and C-H bond fission in methylamine photodissociated at 222 nm, we measured the arrival times of the photofragments from both methylamine, CH3NH2, and methylamine-d2, CH3ND2. The TOF signal at d e + = 30, CH3NH+/CH2NH2+, from the photodissociation of CH3NH2 shown in the top frame of Figure 2, must result from the loss of H from either the carbon or the nitrogen. It is best fit by a combination of two translational energy distributions
(P(&)’s), one peaking near zero kinetic energy the other at 15 kcaymol. To separately determine these two distributions and to identify which distribution results from N-H fission and which results from C-H fission, we photodissociated deuterated methylamine, CH3ND2. Assuming the same total kinetic energy release for N-H(D) fission in the two isotopes, momentum conservation requires that the CH3ND photofragment gets a larger share of the total kinetic energy as it is momentum matched to mass 2 (D atom) rather than mass 1 (H atom). Indeed, the TOF signal at m/e+ = 3 1, CH3NDf/CHND2+, from the photodissociation of CH3ND2 shown in the bottom frame of Figure 2 shows the CH3ND photofragment from N-D fission has shifted to considerable earlier arrival times than the CH3NH photofragment from N-H fission. We still see the CH2ND2 fragment from C-H bond fission at a daughter ion, CHND2+; its arrival times are barely shifted from that in the upper spectrum of Figure 2. Thus, the slower CN€& photofragments in the upper frame are from C-H fission and the faster photofragments are from N-H fission. The P(&)’s for C-H bond fission and N-H(D) bond fission determined from fitting both these sets of data are shown in Figure 3, with the faster distribution corresponding to N-H fission. We cannot detect CH2NH2(D2) photofragments from C-H fission partitioning less than 4 kcal/mol to translation; thus, we shade over the low-energy portion of that P(ET) as it is undetermined by our data. Note that the recoil kinetic energy distribution for N-H fission is peaked at kinetic energies far from zero, consistent with dissociation along a potential energy surface with a barrier to the reverse reaction. We will return to this observation later when we discuss the dynamics of N-H bond fission along an excited state potential energy surface. 2 . The H2 Elimination Channel. To detect signal from H2 elimination from CH3NH2 and HD elimination from CH3ND2, we looked for signal at the parent ion of the CH2NH and the CH2ND photofragments, respectively, in Figure 4, top and bottom. These spectra also show contributions from daughter ions of the N-H and C-H bond fission photofragments, but we have separately determined those time of arrival distributions in Figure 2. The CH2NH signal in the top frame can adequately be fit with contributions from only the C-H and N-H bond fission channels, making the addition of the H2 elimination contribution somewhat arbitrary. The CH2ND signal in the lower frame, however, requires the contribution from the HD elimination channel. We fit the signal at mle’ = 30, CH2ND+, in the bottom frame of Figure 4 by adding an HD elimination channel with the P(&) shown in Figure 5 to the N-D and C-H fission channels uncovered in the CNH4+ signal. We added the additional constraint that the same P(&) must fit the H2
J. Phys. Chem., Vol. 99, No. 9, 1995 2639
Photodissociation of CH3NH2
100
7
.
.
I
.
.
.
-
I
- . ... I
80:
0 0
I
0 0
8
p
0
:o
20 -
' . 40
0 0
0 0 0 0
0
O o * . . l * * . ' . ' " ~ ' ' ' O -
...
100 80
I . . . I . . . I . . ~ ~ . ~ . . . I . . ' I . . . I . . .
C H ~ N H 222 ~ , n'm H2 ellminatlon
o O o
-
:
-
0 0 0 0 0 L
1
0
0
. . ' ' . . ~ ' . " . . . ' . . " ' ' . ' . . . v . ~ ~ 2
4
8 10 ET ( k c a l / m o i ) 6
12
14
18
Figure 5. Center-of-mass product translational energy distribution, P(ET),for the four-center Hz elimination channel in methylamine at 222 nm. The P(&) is derived from forward convolution fitting the portion of the CNH3+ and CHzND+ signal in Figure 4 not accounted for by primary N-H(D) and C-H bond fission.
elimination signal in the top frame of Figure 4. Although we cannot give a reliable estimate of the fraction of molecules which undergo four-center H2 elimination, the strong contribution this signal makes to the spectra in Figure 4 indicates it is a significant process. Here again, the translational energy distribution is peaked away from zero, but for four-center H2 elimination, the barrier to reverse reaction on the ground state is far from negligible and would partition energy to both H2 vibration and translation. Xiang and Guillory estimate the exit barrier for this elimination to be 26 kcdm01.l~ Our measured translational energy distribution, which peaks at 4 kcavmol, with a maximum well under 26 kcavmol, could thus result from either dissociation dynamics on the ground state potential energy surface after internal conversion or from dynamics on the excited state potential energy surface, which also has a barrier to the reverse reaction.
'
.
CH3NH2, 222 nm C-N fission
0 0 0
.
-
~
Waschewsky et al.
2640 J. Phys. Chem., Vol. 99, No. 9, 1995
-z
'
g
m / e + = 32, CH2NDp+
1.1 0.7
0.3
L
I -0.1 .n
I:
1.2
:. . .
1:
I .
- - . . -- . . - . I
I
W3NH \ O
0
I
- '.
. I .
-...
CH3NH2,222 nm
:
m/e+ = 1 5
1
-dc 0.8
e 5
0.6
0.4
A I
z 0.2 0
100
200
300 400 500 tlme of arrival ( p )
600
700
Figure 9. Laboratory time-of-flight spectrum of the photofragments detected at CH3+ and NH+ from methylamine (upper) and CH3' from methylamine-d2 (lower)photodissociated at 222 nm with an unpolarized laser. The source angle was 15". Most of the signal results from primary C-N, N-H, and C-H bond fission products and their daughter ions; however, the fast (-150 ps) edge of the upper frame and the signal around 340 ps in the lower frame cannot be assigned.
the discussion to analyze the dynamics through the conical intersection along the C-N bond fission reaction coordinate. 4 . Accounting for the Remaining Signal. Using the kinetic energy distributions determined from the data above, we tried to fit all the remaining spectra with linear combinations of the four dissociation channels identified above (the weighting of the photofragment signals at parent and daughter ions is unknown, so was arbitrarily adjusted to fit the data; often the fit is not unique). While the rather poor signal at mle+ = 32, CHzND2' (Figure 8), is adequately fit by the lower energy P(&) in Figure 3, consistent with its assignment to the C-H bond fission channel, the mle+ = 15, CH3' and NH+,spectra (Figure 9) showed signal at very early arrival times near 150 pus which could not be fit by any of the four dissociation channels determined above. For the signal from CH3NH2 photodissociation in the top frame of Figure 9, the large slow portion of the signal observed at mle+ = 15, CH3+ and NH+, is fit by a combination of the P(ET)'sfor N-H fission (for which the CH3NH product can give two daughter ions at mle+ = 15) and
C-H bond fission. The small fast portion of the signal in Figure 9 is partially accounted for by the P(&) for C-N bond fission shown in Figure 7, but no photofragment from the four dissociation channels identified thus far has arrival times corresponding to the signal near 150 ps. The only additional energetically allowed dissociation channel that could result in the remaining mle+ = 15 signal near 150 ps arrival time is methane elimination. The signal to noise at mle+ = 15 is poor at best, but could be fit by a translational energy distribution peaking near 12 kcal/mol and extending to almost 40 kcal/mol. There was extremely poor signal to noise at both mle' = 16, C&, from methylamine, or m/e+ = 17, CH3D, from methylamine-&, so we were unable to confirm this assignment by detecting methane product at its parent ion. It may also be that the fast CH3+ signal results from secondary photodissociation of primary products from the other fission channels. Note also that for the above analysis we have assumed that the energy partitioned to product translation in C-N bond fission in CH3NH2 is the same as that determined from the ND2 product from C-N fission in CH~NDZ, an assumption which may not hold. In fitting the signal in the lower frame of Figure 9 from CH3N D 2 , we are left with one more problem. While most of the slow part of the signal at CH3+ is fit by the P(&) for N-D fission, a small contribution, indicated by a question mark in the figure, from an additional channel is necessary to fit all the signal. A good fit to this signal can be achieved by assuming contamination of the sample by CH3NDH and using the P(ET) for N-H fission in Figure 3. Despite attempts to reduce hydrogen exchange between the methylamine-dp and water adsorbed to system walls, a small amount of this contamination may be inevitable. B. Angular Distribution. In this section we analyze the angular distributions of the CNI& fragments from N-H and C-H fission in methylamine and the N D p fragments from C-N fission in methylamine-&. To obtain the angular distributions for N-H and C-H fission from the CN& fragment signal, we integrated only the part of the signal corresponding to N-H or C-H fission, as determined from the unpolarized TOF spectrum shown in the upper frame of Figure 2 . We redid the measurements of the CN& signal once in an attempt to improve signal to noise. Although the second measurement was not an improvement, we do see the same general trend for N-H fission. The two measurements are contradictory for C-H fission, however, so these angular distribution measurements should be regarded as preliminary. Figure 10 shows the integrated CN& fragment signal for N-H fission vs @lab, the angle between the laser electric vector and the detector axis. The best fit to the photofragment angular distribution is obtained by varying the anisotropy parameter, /3, in the classical electric dipole expressionz9
Because &M is the angle between the recoil direction of the photofragment in the center-of-mass frame and the electric vector of the light in the laboratory frame, fitting the data involves converting between the center-of-mass and laboratory frame using the measured molecular beam velocity and the P(&) derived from the unpolarized data. We can predict the expected values for the anisotropy parameter, /3, for N-H, C-H, and C-N fission, assuming dissociation occurs before the molecule has a chance to rotate or to distort from the ground state geometry. The n 3s transition is believed to be A' in ~ y m m e t r y and , ~ ~the ~~~~~ transition dipole moment is expected to be in the plane of symmetry and nearly perpendicular to the C-N bond. (Our
-
J. Phys. Chem., Vol. 99, No. 9, 1995 2641
: CH,ND,,
200
0
30
60 90 120 150 Lab. Polarization Angle
180
Gaussian 9230 calculations put the transition dipole moment between 3" and 14" from perpendicular, depending on basis set used.) The anisotropy parameter is related to the angle a between the dissociating bond and the transition dipole moment by the expression31
,B = 2P,(cos a)
: 0
Figure 10. Laboratory angular distributionsof the C W + signal which results from primary N-H bond fission in methylamine photodissociated at 222 nm with linearly polarized light. 0 is the angle of the laser electric vector with respect to the detector axis (measured in the opposite sense of rotation as the source angle). The data points represent the experimental TOF signal, integrated between 249 and 294 ps, measured at four different laser polarization angles. Line fits show the predicted change in detected scattered signal intensity with laser polarization angle obtained, after transformation from the centerof-mass to laboratory frame, with three trial anisotropy parameters: /3 = -0.2, /3 = -0.3, andp = -0.4. The error bars show f l u , calculated by taking into account that we subtracted 16 channels of background from the total integrated signal plus background, where the background per channel was determined for the signal at each angle by averaging over 31 background channels. The average backgroundchannel ranged from 1419 to 1464 counts for the four measurements.
(15)
where P2 is the second Legendre polynomial. To find a,we used the ground state geometry determined from microwave spectroscopy by Takagi and K ~ h m a as , ~well ~ as geometry optimization calculations of our own. Assuming that each of the two N-H bonds has an equal chance to break, we find a p = 2P2(cos 62" to 68") = -0.35 to -0.55 for N-H fission. Our earliest data set showed a p e -0.5 for N-H fission, while for the second set, with better signal to noise, we measure the /3 x -0.3 f 0.2, shown in Figure 10. Both measurements roughly fit the prediction. Thus, the observed angular distribution could be rationalized on the basis of prompt dissociation from a geometry close to the equilibrium geometry in the ground state. However, the fact that vibrational structure is resolvable in the UV absorption spectrum implies that the lifetime of the excited molecule must be more than a few periods of the amino wag, and the vibrational analysis of that spectrum has shown that forces on the excited state potential energy surface induce both the amino wag and torsion about the C-N bond. If the amino wags and torsion are indeed the first motions of the excited molecule, then this could influence the angle between the transition dipole moment and the bond recoil axis. In fact, if the first motion of the molecule is toward the planarkiscoid geometry, then we would expect an a closer to perpendicular Cp = -1.0) for the N-H bond fission. The measured p e -0.3, then, implies a less than prompt dissociation from an almost planarkiscoid geometry. Figure 11 shows the integrated ND2 fragment signal for C-N fission in methylamine-dz vs @lab. For C-N fission we would predict p = 2P2(cos 76" to 87") = -0.83 to -0.99 if we use the same transition dipole moment and geometry for methylamine-& as for methylamine. The measured p x -0.4 f 0.4
222 nm
30
60 90 120 150 Lab. Polarlzatlon Angle
180
Figure 11. Laboratory angular distributions of the ND2+ signal which results from primary C-N bond fission in methylamine-dz photodissociated at 222 nm with linearly polarized light. 0 is the angle of the laser electric vector with respect to the detector axis. The data points represent the experimental TOF signal, integrated from 147 to 336 pus, measured at three different laser polarization angles. Line fits show the predicted change in detected scattered signal intensity with laser polarization angle obtained, after transformation from the center-ofmass to laboratory frame, with three trial anisotropy parameters: /3 = -0.2, /3 = -0.4, and 3/ = -0.6. The error bars show f l u , calculated by taking into account that we subtracted 64 channels of background from the total integrated signal plus background, where the background per channel was determined for the signal at each angle by averaging over 101 background channels. The average backgroundkhannel ranged from 1935 to 1955 counts for the four measurements. is consistent with a small amount of molecular rotation before dissociation. In the same way, we can also predict the p's for C-H fission. If we assume each of the C-H bonds also has equal probability of breaking, then p = (0.33)2P2(cos 10" to 21") (0.67)2Pzs (COS63" to 68") = (0.33)(1.6 to 1.9) (0.67)(-0.38 to -0.57) = 0.27 to 0.25 for C-H fission. Two sets of data were collected for C-H fission, as for N-H fission. The results were unfortunately contradictory, one giving a p 0.3 and the other a p -0.3. Thus, the poor signal to noise in these preliminary measurements renders us unable to distinguish whether or not the in-plane C-H bond breaks preferentially over the other two. C. Emission Spectrum. Figure 12 shows the emission collected upon excitation at 222 nm. Although the spectrum is congested, the peaks in the early part of the emission spectrum are easily assigned to emission from the excited CH3NHz molecule to several vibrational levels in the ground electronic state. Most prominent are a progression in vg, the amino wag (or inversion), and a combination band of the amino wag with one quantum in V6, the methyl (umbrella) deformation. Although emission to states with odd quanta in the amino torsion vibration, ~ 1 5is, forbidden by symmetry, two quanta appear in combination with the amino wag along a progression. We see one quantum each in V I , the N-H stretch, vg,the C-H stretch, v7, the methyl rock, and V8, the C-N stretch.33 Prominent in the spectrum is emission to vibrational modes such as the amino wag, amine torsion, and methyl (umbrella) deformation, which indicate motion upon the excited state surface which brings the molecule to a planarkiscoid geometry. Tsuboi and co-workers,ll in an analysis of the UV absorption spectrum determined that the excited state is planar about the nitrogen, with the plane of symmetry perpendicular to the plane formed by the carbon and the amine group as shown in the bottom frame of Figure 1. They assign the structure in the absorption spectrum to amino wagging and methyl rocking vibrational modes in an excited state bound in these coordinates. Tsuboi et aZ.'s force field calculation shows a potential energy surface strongly canted toward the planar geometry upon vertical excitation. In a slight amendment to Tsuboi et d ' s work, Kassab and co-workers' propose a ciscoid excited state equi-
+
+
Waschewsky et al.
2642 J. Phys. Chem., Vol. 99, No. 9, 1995
16000 14000
Methylamine
222 nm
12000
.-Lh
10000
u)
al
-
8000
c C
6000
v6
= CH,
v9
=
def. (umbrella)
-.
4000
2000
v1
NH, inversion
= NH, torsion
0
Figure 12. Emission spectrum of dmociating methylamine excited at 222 nm. Peaks are assigned according to vibrationalenergies and assignments from ref 33. OMA pixel to wavenumber calibration is determined by matching mercury lamp emission lines. librium geometry, shown in the middle frame of Figure 1, in which the plane of symmetry includes the amine hydrogens and one of the hydrogens bonded to carbon. It is not surprising to see emission to vibrational modes that mirror this change to an equilibrium geometry planar about the C-N bond. Whether the torsion about the C-N bond is caused by a potential sloping toward a ciscoid geometry, a loosening of the torsional potential, or a coupling to other motions which bring the molecule to planar geometry is not possible to determine.
65,000
v I cm-‘
IV. Discussion In the following discussion, we first analyze the evidence that the competing bond fission channels result from dissociation along the excited potential energy surface, not upon intemal conversion to the ground electronic state. Then we focus on the C-N and N-H bond fission channels, for which ab initio calculations of the excited state reaction coordinates are available. The data and analysis elucidate the nature of the excited state bond fission reaction coordinate for these two bond fission channels in comparison with that for N-H bond fission in ammonia photodissociation. The next section discusses the probable mechanism for C-H bond fission consistent with our data; we use an analogy with the excited state C-H bond fission reaction coordinate in methanol to understand our results. Finally, we speculate on the mechanism of the H2 elimination channel. The discussion ends with summarizing our understanding of the competition between the bond fission channels in CH3NH2 in comparison with CH30H and CH3SH based on the barriers on the adiabatic excited state potential energy surface and the conical intersection in the N-H fission exit channel. A. Evidence Supporting an Excited State Dissociation Mechanism over Internal Conversion to the Ground Electronic State. The measured photofragment velocities show that all channels except C-H fission have a barrier to the reverse reaction; the kinetic energy distributions for C-N bond fission, N-H bond fission, and H2 elimination peak well away from zero, indicating that the potential energy surface is repulsive after the transition state. Although this is expected for concerted
0 0
Figure 13. Ground and first excited state potential energy surfaces for ammonia showing the region of intersection along the N-H bond fission coordinate as it varies with the out-of-plane angle 0. Reproduced with permission from ref 19. Copyright 1989 American Institute of Physics. reactions (the estimated ground state exit channel barrier for H2 elimination is 26 kcal13),usually one can assume that simple bond fission reactions in the ground electronic state do not have a barrier to the reverse reaction. In this case, dissociation via intemal conversion to the ground state would be inconsistent with the measured C-N and N-H kinetic energy distributions, but for this system we must be pore careful. As with N-H bond fission in ammonia (see Figure 13), there are conical intersections along the ground state C-N and N-H fission reaction paths. C-N bond fission on the ground state potential energy surface of methylamine correlates adiabatically to excited state NH2 product with no barrier to the reverse reaction, but at bent geometries it correlates to ground state NH2 product with an exit barrier that depends on how planar the molecule is about the nitrogen during the bond fission. Thus, on the basis of the contour of the ground state potential energy surface alone, we cannot exclude the possibility that the photofragments’ kinetic energy partitioning results from a small exit barrier along the
Photodissociation of CH3NH2 ground state reaction coordinate. However, Xiang and Guillory's infrared multiphoton dissociation experiments show a statistical kinetic energy distribution for C-N bond fission in the ground electronic state. Thus, the influence of the exit barrier to C-N bond fission in altering the partitioning to relative kinetic energy is negligible; dissociation in the ground electronic state still results in a kinetic energy distribution peaked near zero. Because the P(ET)measured here for C-N bond fission peaks well away from zero, dissociation clearly is not proceeding along the ground state potential energy surface after intemal conversion. Similarly, the recoil kinetic energies for N-H fission indicate the molecule is experiencing repulsive forces on an excited state potential energy surface during dissociation. In the next section, we review what is known about the C-N and N-H excited state bond fission reaction coordinates and analyze our product energy partitioning with regard to an excited state dissociation mechanism. B. C-N and N-H Bond Fission on the Excited State Potential Energy Surface. The dynamics for dissociation of the C-N and N-H bonds in methylamine excited at 222 nm involve three important regions of the excited state potential energy surface: the Franck-Condon region, dominated by Rydberg character and characterized by an equilibrium planar1 ciscoid geometry different from that in the ground electronic state; the transition state region formed by the avoided crossing between the Rydberg electronic configuration and electronic configurations repulsive in the C-N and N-H bonds, respectively; and the conical intersection in the exit channel, which if traversed adiabatically correlates to excited state products and if traversed diabatically can give ground state products. We detail below what insight the measurements reported here give into the dynamics in each of these regions of the excited state potential energy surface for C-N and N-H bond fission. 1. Geometry Changes in the Franck-Condon Region. The emission spectra evidence the early dynamics of methylamine excited at 222 nm in the Rydberg region of the excited state potential energy surface. Motion in the amino wag and torsion about the C-N bond move the molecular geometry toward the equilibrium geometry in the excited state, a planarlciscoid structure depicted in Figure 1. This dynamics is consistent with Tsuboi et al.'s analysis of the structure in the absorption spectrum, which they assign to excited state amino wagging and methyl rocking vibrations. Vertical excitation puts the molecule into the excited state with five quanta of amino wag. In the Franck-Condon region, the excited state is predominantly Rydberg in character and thus bonding in the C-N and N-H stretch, as is ammonia in the N-H stretch (see Figure 13). 2. Barriers along the C-N and N-H Fission Reaction Coordinatesffom a RydberglValence Avoided Crossing. In ab initio calculations7of the excited state potential energy surface, from the ciscoid geometry, cuts along the C-N stretch and N-H stretch show a shallow well in the Franck-Condon region followed by a small barrier formed by an avoided crossing of the Rydberg configurationwith valence electronic configurations of the C-N and N-H bonds, respectively. After the transition state, the valence character of the electronic configuration gives a contour that is repulsive in the C-N bond along the C-N bond fission reaction coordinate and repulsive in the N-H bond along the N-H fission reaction coordinate. These repulsive forces after the transition state result in the fast kinetic energy distributions of the C-N and N-H fission reaction products measured in these experiments. Although electronic structure calculations by Kassab et al.' and, more recently, Dunn and Morokuma8 give the barrier to excited state C-N bond fission as just higher than that energetically allowed at 222 nm, the kinetic energy distributions
J. Phys. Chem., Vol. 99, No. 9, 1995 2643 measured in these experiments strongly indicate both N-H and C-N fission result from an excited state dissociation mechanism. Thus, we assume that these notoriously difficult excited state calculations overestimate the energy at the C-N transition state relative to the ground state of methylamine. The general shape of the excited state potential energy surface calculated by those workers is, however, in complete agreement with the experimental results presented here, and the relative barrier heights Dunn and Morokuma calculate for the C-N (5.8 eV) and N-H (5.5 eV) fission reaction coordinates in the excited state correctly predict that N-H bond fission should dominate C-N bond fission upon excitation in the first absorption band as found in Michael and Noyes experiments.' (C-N bond fission would dominate N-H fission upon intemal conversion to the ground state.) We should note that the quantum yield for C-N bond fission in our experiments at 222 nm is probably considerably larger than the 0.05 reported in the broad-band photodissociation experiments of Michael and Noyes; it is possible that excited state C-N bond fission is energetically allowed at 222 nm but not at much lower energies in the first absorption band. 3. Traversing the Conical Intersection in the Exit Channel. Along the repulsive region of the C-N and N-H fission reaction coordinates near planar/ciscoid geometries after the transition state, the dissociative trajectories must traverse a conical intersection much like that for N-H bond fission in ammonia shown in Figure 13. The figure shows that excited state NH& correlates with ground state NH2 only at planar geometries; any deviation from planarity removes the symmetry restriction, and electronically excited NH3 correlates adiabatically with excited state products. Photolysis at wavelengths below the threshold energy of NH2(A) results in ground state products, NHz(ji'); molecules dissociating on the upper adiabatic cone must undergo a nonadiabatic transition to the lower adiabat to dissociate. When the photolysis energy exceeds the threshold energy for production of A state NH2, a significant fraction of the dissociative trajectories traverse the conical intersection adiabatically, staying on the upper cone and forming state N H 2 p~oduct.'~The resulting NH& fragments are rotationally excited as they result from trajectories which traverse areas of the potential energy surface with considerable out-of-plane bending. The present measurements on the dissociation of CH3N H 2 probe the dissociation of the N-H and C-N bonds through analogous conical intersections. If the molecule traverses the conical intersection adiabatically along the C-N fission reaction coordinate, it results in N H 2 product in the A electronic state. From the bond dissociation energy and the relative energy of NHz(ji') and NH2(&,18,19,28 we can predict the maximum kinetic energy available to the ground and excited state products of C-N bond fission, Eavdl = hv - Do(C-N) - Eelec(NHz product). For ground state products Ea"& = 128.7 - 79.5 kcaYmol = 49.2 kcaumol, while for excited state products Eavail = 128.7 - 79.5 - 31.6 kcaY mol = 17.6 kcdmol. Our measured translational energy distribution for C-N bond fission peaks near 4 kcdmol and shows no dissociation events giving recoil energies greater than 17 kcdmol, suggesting that C-N bond fission produces only excited state products. Indeed, the measured kinetic energy distribution stops just short of the energetic limit for formation of A state N H 2 product.34 Even if, as in ammonia, half the available energy were partitioned to intemal energy of the NH2 product, we would expect much higher kinetic energies in the C-N fission photofragments if NHz(ji') were formed.35 The data thus suggest that all the C-N dissociative trajectories cross the region of the conical intersection adiabatically; this is not too surprising in view of the fact that the molecule is consider-
a
2644 J. Phys. Chem., Vol. 99, No. 9, 1995
ably excited in the NH2 wag as it dissociates though the conical intersection, so it accesses regions of the conical intersection where the upper and lower adiabats are widely split, and the relative velocity through the conical intersection is, because of the reduced mass, slower than that for N-H bond fission in ammonia. As in the ammonia photodissociation, the kinetic energy distribution peaks at lower energies than the maximum allowed; the wagging motion generated in the Franck-Condon region likely produces rotationally excited NH2 fragments. Along the N-H bond fission reaction coordinate, the relative velocity through the crossing is faster so one might expect some diabatic formation of ground state CH3NH product; indeed, the translational energy distribution extends to the limit of the available energy for producing CH3NH in the ground electronic state. For molecules that cross the conical intersection adiabatically, the upper adiabat correlates to excited state CH3NH products which may not be energetically allowed at 222 nm. In ciscoid geometry, Kassab et al.’s calculations gives the asymptotic energy for producing electronically excited CH3NH products at higher energy than the barrier to C-N fission. Thus, unlike C-N bond fission which can proceed easily at nonciscoid geometries to excited state NH2 fragments, because the excited state CH3NH* is energetically (or maybe dynamically) inaccessible, trajectories which cross the N-H bond fission conical intersection adiabatically are forced to turn back so that only ground state products of N-H bond fission result. This pathway through the conical intersection may also provide a way for dissociativetrajectories to access the bound region of the ground state potential energy surface to result in H2 elimination as well as N-H fission. C. The C-H Bond Fission Channel. The kinetic energy distribution for this channel peaks athear zero and extends to almost 12 kcaymol, partitioning on average 4.1 kcallmol of the available 34.2 kcallmol into relative translational energy. The distribution peak near zero kinetic energy indicates that no significant repulsive forces act on the C-H stretch after the transition state; the potential energy surface on which C-H bond fission occurs does not have a barrier to the reverse reaction for a significant fraction of the dissociation events. Both dissociation after internal conversion to the ground electronic state or dissociation directly on the excited state surface are consistent with the low kinetic energy part of this distribution. Dunn and Morokuma’s recent calculationss show that the reaction coordinate for C-H bond fission in the excited state is similar to that for C-H fission in excited CH30H,27 correlating to excited state CHzNHz product with a high barrier and a very flat potential after the barrier. Whether this excited state channel is energetically allowed or not in methylamine has been the subject of two recent calculations.8~36Although Dunn and Morokuma’s results give an energy of 6.1 eV required to dissociate via this excited state pathway, it is within 0.3 eV of the transition state for excited state C-N bond fission, which the experiments show is energetically allowed. Thus, it is possible the C-H excited bond fission pathway in methylamine is energetically allowed, and the flat potential along this pathway is consistent with the measured photofragment kinetic energy distribution. A more accurate determination of the C-H bond fission product anisotropy would help distinguish whether the reaction pathway is on the excited state or follows internal conversion to the ground electronic state, as the latter should have an isotropic angular distribution, but we do not yet have a reliable measurement of this anisotropy. The significant branching to C-H bond fission does indicate that the rate of C-H fission is comparable to the rates of C-N and N-H fission, suggesting that it too must occur on the excited state surface unless it results from trajectories trapped in a region of
Waschewsky et al. the potential energy surface from which it cannot access the other transition states. We should note that the C-H bond fission P(&) does show some dissociation events that partition as much as 12 kcallmol to relative translation; it may be that these result from accessing a repulsive triplet state from the excited state, as suggested by Dunn and Morokuma’s analysis. D. The Hydrogen Elimination Channel. The measured translational energy distribution for H2 elimination is peaked away from zero, indicating a barrier to the reverse reaction. The reverse barrier along the ground state reaction coordinate is estimated to be 26 kcdmol by Xiang and G~illory,’~ so it is possible that H2 elimination occurs on the ground state surface after internal conversion. It is also possible that H2 elimination evolves on the excited state potential energy surface. Our data on methylamine-& show that H2 elimination occurs via a fourcenter transition state; it is a 1,2-elimination similar to that in m e t h a n 0 1 . ~ ~Buenker ~~ et al. have calculated the excited state reaction coordinate for 1,2-H2 elimination in methanol;27their results show that both the ground state and excited state pathways have a substantial barrier to the reverse reaction, but the barrier to the forward reaction is much lower in the excited state than in the ground state (although higher if both are referenced to the ground electronic state). If similar barriers exist for H2 elimination in methylamine, then an excited state dissociation might have a low enough barrier to allow it to compete with N-H and C-H fission on the excited state surface, and the repulsive region of the potential after the excited state barrier would partition considerable kinetic energy into product translation as observed in our data. As in the C-H bond fission channel, branching to the hydrogen elimination channel suggests that its rate must be comparable to the rates of C-N and N-H bond fission, which occur on the excited state. Hydrogen elimination from the excited state would be consistent with this branching, or else the HZelimination must result from a trajectory which travels quickly to the ground state, such as through a conical intersection. It may be that some trajectories starting toward excited state N-H fission are turned back and upon crossing through the conical intersection access a part of the ground state potential energy surface leading to H2 elimination. Our emission spectrum, as well as UV absorption spectra, suggests that there is initially a large amount of vibrational excitation in the photoexcited molecules. The emission spect” shows emission to amino wagging, methyl umbrella, and amine torsion vibrations; all of these indicate the molecule is moving through the geometries which would be necessary for the formation of the four-center 1,Zelimination transition state. E. Competition between the Bond Fission Channels in CHaH2 in Comparison with CH30H and CHBH. In the analogous molecules, CH30H and CH3SH, the 0-H and S-H bonds break almost exclusively, with very little branching to C - 0 or C-S fission, while in methylamine not only C-N bond fission but also H2 elimination and C-H bond fission compete effectively with N-H bond fission. Comparison of the excited state potential energy surfaces along the N-H and C-N fission reaction coordinates with the analogous ones in CH30H and CH3SH shows why. In the first excited state, potential energy surfaces for m e t h a n 0 1 ~ and ~ ~ ~methyl j ~ ~ mercaptan25are purely repulsive in the 0-H and S-H bond fission coordinates, while the C - 0 and C-S channels have small barriers. The force of the repulsive surface plus the smaller mass of the H atom favors 0 - H or S-H fission over 0 - C or S-C fission. Indeed 0-H fission is the dominant photodissociation pathway in methanol, with very little C-0 f i ~ s i o n .Similarly, ~ ~ ~ ~ ~photoexcitation of methyl mercaptan in its first UV absorption band results exclusively in S-H fission, with a small amount of C-S fission
J. Phys. Chem., Vol. 99, No. 9, 1995 2645
Photodissociation of CH3NH2 at the higher energies in the first absorption band.25 In contrast, in methylamine, instead of a purely repulsive surface there is a small barrier to N-H fission in the excited state. Thus, even in the Franck-Condon region, the N-H dissociation is slowed, allowing passage over the small barrier to C-N fission to compete. In addition, once a trajectory crosses the small Rydberghalence barrier to N-H fission, it must still traverse the conical intersection in the exit channel to result in N-H bond fission products. If the trajectory stays on the upper adiabat through the conical intersection and excited state N-H bond fission products are not energetically accessible for that trajectory, the trajectory will turn back toward the conical intersection. Diabatic passage through the conical intersection on the way back results in an efficient internal conversion to the ground electronic state, from which Hz elimination can compete efficiently. In CH30H and CH3SH, trajectories do not have to pass through a conical intersection after traversing the small barriers near the Franck-Condon region as the CH30 and CH3S asymptotic products are doubly degenerate. Because the singlet excited state is bonding in the FranckCondon region in the C-H, N-H, and C-N fission reaction coordinates but the potential well in the excited state is quite shallow, this system provides the opportunity to test the limits of statistical transition state theories. By accessing the excited state via infraredultraviolet double-resonance excitation, tuning the infrared photon to either the C-H or N-H fundamental in the ground state, we can influence the early dynamics in the Franck-Condon region of the excited state surface. If vibrational energy is not effectively randomized in the excited state, the double-resonance excitation could effectively promote one of the bond fission channels over the other.
Acknowledgment. This work was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, U.S. Department of Energy, under Grant DE-FG02-92ER14305. L.J.B. gratefully acknowledges the support of an Alfred P. Sloan Research Fellowship. We thank K. M. Dunn and K. Morokuma for providing us with the results of their work in ref 8 prior to publication and R. N. Dixon and M. N. R. Ashfold for providing us with an original of their previously published figure reproduced in Figure 13 here. References and Notes (1) Michael, J. V.; Noyes, W. A. J . Am. Chem. Soc. 1963, 85, 1228. (2) Freeman. C. G.: McEwan. M. J.: Claridee. R. F. C.; Phillius. L. F. Chem. Phys. Lett. 1971, 8, 77. (3) Haak, H. K.; Stuhl, F. J . Phys. Chem. 1984, 88, 3627. (4) Kenner, R. D.; Rohrer, F.; Stuhl, F. Chem. Phys. Lett. 1985, 116, 374. (5) Gardner, E. P.; McNesby, J. R. J . Phys. Chem. 1982, 86, 2646. (6) Magenheimer, J. J.; Vamerin, R. E.; Timmons, R. B. J . Phys. Chem. 1969, 73, 3904. (7) Kassab, E. Gleghom, J. T.; Evleth, E. M. J. Am. Chem. Soc. 1983, 105, 1746. (8) Dum, K. M.; Morokuma, K. Manuscript in preparation. (9) Robin, M. B. Higher Excited States of Polyatomic Molecules I; Academic Press: New York, 1974; p 216. (10) Calvert, J. G.; Pitts, Jr., J. N. Photochemistry; Wiley: New York, 1966; p 45 1. (11) Tsuboi, M.; Hirakawa, A. Y.; Kawashima, H. J . Mol. Spectrosc. 1969, 29, 216. L
(12) Hubin-Franskin, M.-J.; Delwiche, J.; Tollet, F.; Furlan, M.; Collin, J. E. J. Phys. B: At. Mol. Opt. Phys. 1988, 21, 189. (13) Xiang, T.-X.; Guillory, W. A. J . Chem. Phys. 1986, 85, 2019. (14) (a) Emeleus, H. J.; Jolley, L. J. J . Chem. Soc. 1935, 929. (b) Strauss, 0. P.; Lown, J. W.; Gunning, H. E. In Comprehensive Chemical Kinerics;Bamford, C. H., Tipper, C. F. H., Eds.; Elsevier: New York, 1972. (15) Herzberg, G. Molecular Spectra and Molecular Structure: I l l . Electronic Spectra and Electronic Structure of Polyaromic Molecules; Van Nostrand: Princeton, 1966; p 463. (16) Vaida, V.; Hess, W.; Roebber, J. L. J. Phys. Chem. 1984,88,3397. (17) McCarthy, M. I.; Rosmus, P.; Wemer, H.-J.; Botschwina, P.; Vaida, V. J. Chem. Phys. 1987, 86, 6693. (18) Biesner, J.; Schnieder, L.; Schmeer, J.; Ahlers, G.; Xia, X.; Welge, K. H.; Ashfold, M. N. R.; Dixon, R. N. J . Chem. Phys. 1988, 88, 3607. (19) Biesner, J.; Schnieder, L.; Ahlers, G.; Xie, X.; Welge, K. H.; Ashfold, M. N. R.; Dixon, R. N. J . Chem. Phys. 1989, 91, 2901. (20) Dixon, R. N. Chem. Phys. Lett. 1988, 4, 377. (21) Person, M. D.; Kash, P. W.; Butler, L. J. J . Chem. Phys. 1992, 97, 355. (22) Kash, P. W.; Waschewsky, G. C. G.; Butler, L. J.; Francl, M. M. J . Chem. Phys. 1993, 99,4479. (23) For details, see: Person, M. D. Ph.D. Thesis, Department of Chemistry, University of Chicago, 1991. (24) The universal detector was introduced by: Lee, Y. T.; McDonald, J. D.; LeBreton, P. R.; Herschbach, D. R. Rev. Sci. Instrum. 1969,40, 1402. (25) Jensen, E.; Keller, J. S.; Waschewsky, G. C. G.; Stevens, J. E.; Graham, R. L.; Freed, K. F.; Butler, L. J. J . Chem. Phys. 1993, 98, 2882. (26) Browning, P. W.; Jensen, E.; Waschewsky, G. C. G.; Tate, M. R.; Butler, L. J.; Hessler, J. P. J . Chem. Phys., in press. (27) Buenker, R. J.; Olbrich, G.; Schuchmann, H.-P.; Schurmann, B. L.; von Sonntag, C. J . Am. Chem. Soc. 1984, 106, 4362. (28) Johns, J. W. C.; Ramsay, D. A,; Ross, S. C. Can. J. Phys. 1976, 54, 1804. (29) Zare, R. N. Mol. Photochem. 1972, 4, 1. (30) Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. GAUSSIAN 92, Revision C; Gaussian, Inc.: Pittsburgh, PA, 1992. (31) The expression p = 2Pz(cos a)is given in: Busch, G. E.; Wilson, K. R. J. Chem. Phys. 1972, 56, 3638. Yang, S. C.; Bersohn, R. J . Chem. Phys. 1974, 61, 4400. (32) Takagi, K.; Kohma, T. J . Phys. SOC.Jpn. 1971, 30, 1145. (33) Gray, A. P.; Lord, R. C. J . Chem. Phys. 1957, 26, 690. (34) For C-N bond fission in methylamine, the soft radical impulsive model predicts a partitioning of 83.5% of the available energy to translation. Thus, we would expect ground state products with an average kinetic energy of 41.3 kcal/mol and excited state products with 14.8 kcal/mol. The soft radical impulsive model is discussed in: Holdy. K. E.; Klotz, L. C.; Wilson, K. R. J. Chem. Phys. 1970, 52, 4588. (35) Assuming, as in ammonia, the geometry change in the excited state partitions a great deal of intemal energy to the NH2 photofragments, we would still not expect such low kinetic energies for the C-N bond fission fragments if ground state NH2 were formed. Biesner er al. find that 53% of the available energy was partitioned to intemal energy of the NH2 formed from ammonia at the highest excitation where only ground state NH2 is formed. If the partitioning to intemal energy of the NH2 products from C-N fission in methylamine is the same as in N-H fission in ammonia, we wGuld expect average kinetic energies of 23 kcaVmol if ground state NH2(X) products were formed. The average kinetic energy we measure is 6.9 kcdmol. This is much closer to the 8.3 kcal/mol average kinetic energy expected if the NH2 products are formed in the excited state, and the dissociation still partitions 53% of the 17.6 kcaYmol available energy (the available energy after subtracting the NH2 electronic energy) to intemal vibration and rotation. (36) In a private communication of their preliminary calculations, R. J. Buenker, G. Hirsch, and H.-P. Liebermann find the 2A’ state of CHzNH2 lies 72.3 kcal/mol above the ground state of the radical, almost 25 kcaY mol higher than Dunn and Morokuma’s result. (37) Marston, C. C.; Weide, K.; Schinke, R.; Suter, H. U. J . Chem. Phys. 1993, 98, 4718. (38) Satyapal, S.; Park, J.; Bersohn, R.; Katz, B. J . Chem. Phys. 1989, 91, 6873.
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