2214
J. Phys. Chem. 1990, 94, 2274-2279
Ultravlolet Resonance Raman Spectroscopy of Formamide: Evidence for n-r* Interferences and Intermolecular Vibronic Coupling Peter Hildebrandt,',s Masamichi Tsuboi,*%* and Thomas G.Spire*,+ Department of Chemistry, Princeton University, Princeton, New Jersey 08544- 1009, and Faculty of Sciences and Engineering, Iwaki- Meisei University, Iwaki, Fukushima 970, Japan (Received: May 26, 1989; In Final Form: September 12, 1989)
Raman spectra of formamide in H,O and D 2 0 have been examined at 15 excitation wavelengths between 192 and 520 nm. The excitation profiles are structured, even though the first allowed electronic transition (m*) is at 183 nm. This structure is interpreted as resulting from interference between the s-s* state and the quasi-forbidden n-s* state at -220 nm. This interpretation is supported by model calculations that qualitatively reproduce the structure in the profiles, including a deep trough for the 609-cm-' N-C-0 bending vibration. This trough is attributed to a large change in the N-C-0 angle in the n-r* excited state. The ultraviolet resonance Raman spectra reveal two new bands, at 1742 and 797 cm-I, whose molar intensities are concentration-dependent. These bands are attributed to coupled C=O stretching and N C - O bending vibrations in formamide aggregates. They show excitation profile peaks at 220 nm, attributable to vibronic coupling of the n-s* and s-r* states in the aggregate.
Introduction This work was motivated by the discovery' that the 609-cm-I Raman band of formamide, assignable to its N-C-0 skeletal bending vibration, disappears when the excitation wavelength is changed from 514.5 to 257.3 nm. It was suggested that this deenhancement results from an interference in the Raman scattering tensor between contributions from the n-r* electronic state, located at 220 nm,* and the s-s* state at 183 nm.233 Information about the properties of formamide's n-a* state is useful in connection with the elucidation of the physical properties of biologically important molecules, including peptides, uracil, and thymine derivatives. It has recently become possible to extend Raman spectroscopy deep into the ultraviolet region due to adWe have therefore examined vances in laser formamide Raman spectra with deep UV excitation and have found that the N-C-O bending band does reappear with excitation at 200 and 192 nm. Construction of UV excitation profiles for all of the formamide Raman bands has revealed clearly detectable structure, which can likewise be attributed to interference between n-r* and s-s* scattering contributions. In this study we describe these results and provide model calculations that reproduce the excitation profile structure qualitatively. In addition, we have discovered two new formamide Raman bands, at 1742 and 797 cm-l, which appear only in UV excitation. These bands are shown via a dilution study to be associated with molecular aggregates, the vibrational mode frequencies of which are displaced from the monomer values by intermolecular coupling. Uniquely, these bands show excitation profile peaks at 220 nm, the location of the n-s* transition. Their enhancement is suggested to result from vibronic coupling of the n T * and s-r* states of the aggregates. Experimental Section Formamide was purchased from Sigma (ACS grade) and was used without further purification. Aqueous solutions were made up with 1 M sodium sulfate, whose 980-cm-' Raman band was used as an internal intensity standard. UV Raman spectra were obtained from a free jet recirculating sample arrangement. The Raman scattered light was collected in a 135O backscattering geometry and focused onto the entrance slit (300 Fm) of a 126-111 single monochromator (Spex 1269) which was equipped with a solar blind photomultiplier tube (Hamamatsu UH 166). Further details of the apparatus are described e l ~ e w h e r e .UV ~ Raman *Authors to whom correspondence should be addressed. 'Princeton University. !Iwaki-Meisei University. Present address: Max-Planck-Institut fur Strahlenchemie, stiftstrasse 34-36. D-4330 Mulheim/Ruhr I , West Germany.
0022-3654/90/2094-2274$02.50/0
TABLE I: Observed Raman Frequencies and Assignments for Formamide in 1 M Aqueous Solutions (H20 and D20) in H 2 0 in D20
freq, cm-1 1742 1692 1603 1392 1324 1100
1050 197 609
assignment freq, cm-' aggregate vib 1728 C=O stretch 1667 NH, scissor CH in-plane bend 1399 C-N stretch 1352 NH, rock 1050 CH out-of-plane def aggregate vib OCN bend
918 193 540
assignment aggregate vib C=O stretch CH in-plane bend C-N stretch CH out-of-plane def ND, rock aggregate vib OCN bend
spectra were accumulated for 1 s per point in -0.07-0.04-A increments corresponding to about 1 cm-I per point in the range between 273 and 192 nm. About five repetitive scans were added to improve the signal-to-noise ratio. Twelve excitation wavelengths were generated by multiplequantum (n)anti-Stokes H2 shifting of the output of a Quanta-Ray DCR 1A Nd:YAG laser, using the second, third, and fourth harmonics: 532 nm (253, 229, 209, and 192 nm for n = 5, 6, 7, and 8), 355 nm (273,246,223, and 204 nm from n = 2, 3,4, and 5 ) , and 266 nm (240, 218, and 200 nm from n = 1, 2, and 3). In addition, C W excitation was provided at 520 and 407 nm by a Spectra Physics 171 Krf laser and at 325 and 442 nm by a Liconics He-Cd laser. CW-excited Raman spectra were measured with conventional equipment consisting of a Spex 1401 double monochromator, a cooled RCA 31024A photomultiplier tube, and an Ortec 93 15 photocounting system. These spectra were accumulated in 1-cm-l increments and a total dwell time of 3 s per cm-I. The spectral slit width was 7 cm-I, and the laser power at the sample was between 10 and 100 mW.
Results Figures 1 and 2 show Raman spectra of formamide (1 M) in H 2 0 and D,O excited at 218 and 520 nm, respectively. Band ( 1 ) Hirakawa, A. Y.; Tsuboi, M. In Vibrational Spectra and Structure; Durig, J. R., Ed.; Elsevier: Amsterdam, 1983; Vol. 12, p 145. ( 2 ) Basch, H. 9.; Robin, M. 9.; Keubler, N. A. J . Chem. Phys. 1968,49, 5007. (3) Nagakura, S. Mol. Phys. 1960, 3, 105. (4) Asher. S. A.; Yohnson, C. R.; Mutfaugh, J. Reo. Sci. Instrum. 1983, 5 4 , 1657. ( 5 ) Fcdor, S. P. A,; Rava, R. P.; Copeland. R. A.; Spiro, T. G. J . Raman Spectrosc. 1986, 17, 411 (6) Mayne, L. C.; Ziegler, L. D.; Hudson, B. J . Phys. Chem. 1985. 89. 3395 ~~
~
(7) Chinsky, L.; Laigle. A.; Peticolas, W. L.; Turpin, P. Y. J . Chem. Phys. 1980, 72. 3134.
0 1990 American Chemical Society
Raman Spectra of Formamide in H 2 0 and DzO
FA, 218nm
The Journal of Physical Chemistry, Vol. 94, No. 6, 1990
FA, 520nm
*
N
" p
N
s
I "
1
H2O
m
c
P
m
I
1275
I
I
700
I
I
I
io00
1300
1600
Acm-'
Figure 2. Raman spectra of formamide (1 M) excited at 520 nm in H20
and DzO.
20 760
lobo
1300
-
1600
A c m-I
Figure 1. Raman spectra of formamide (1 M) excited at 218 nm in H20, D20,and ethanol (EtOH, peaks marked E). 1.0 3
assignments are given in Table I, based on a normal-coordinate and on the basis analysis using a b initio MO force of an examination of the contours of corresponding infrared bands of formamide vapor.I0 Two bands are seen with 218-nm, but not with 520-nm, excitation, at 1742 and 797 cm-I in H 2 0 and at 1728 and 793 cm-I in DzOor at 1742 and 790 cm-' in ethanol (Figure 1). The intensities of these bands were observed to decrease, relative to other Raman bands of formamide, when the solution was diluted; thus, they do not arise from impurities in the sample. In Figure 3 the molar intensity of the 1742-cm-I band is plotted against formamide concentration in H20. There is a clear concentration dependence, which implicates formamide aggregates as the source of the 1742-cm-I band. Raman spectra of liquid formamide (Figure 4) show the 1742-cm-I band to completely replace the 1668-cm-l band seen in the 442-nm-excited spectrum, and the 797-cm-l band is also seen. We infer that the 1742- and 797-cm-I bands arise from hydrogen-bonded aggregates of the formamide molecules. We searched the infrared spectra of formamide in D20for the corresponding 1728-cm-' band but were unable to find it. Solutions having concentrations of 0.125, 0.25, and 0.5 M were examined in 100-, 50-, and 25-pm CaF2 cells. Difference spectra among these samples failed to show any trace of a band at 1728 (8) Sugawara, Y . ;Hamada, Y . ;Hirakawa, A. Y . ;Tsuboi, M.; Kato, S.; Morokuma, K. Chem. Phys. 1980, 50, 105. (9) Wojcik, M.; Hirakawa, A. Y . ;Tsuboi, M.; Kato, S.; Morokuma, K. Chem. Phys. Leu. 1983, 100, 523. (10) Sugawara, Y . ;Hamada, Y . ;Tsuboi, M. Bull. Chem. SOC.Jpn. 1983, 56, 1045.
-/
Figure 3. Concentration dependence of the 1742-cm-' Raman band of formamide. J,, denotes the intensity of the 1742-cm-l band normalized to the molar intensity of the 980-cm-I band of sulfate. C, is the overall
concentration of formamide. cm-'. We infer that this mode, as well as the 1742-cm-I mode in H20, is infrared-inactive. Figures 5 and 6 show Raman spectra of aqueous formamide excited at various wavelengths. We note that the 609-cm-I NCO skeletal bending vibration is seen with excitation at 407 nm (also 520 nm, Figure 1) and again at 200 and 192 nm, but not at intermediate wavelengths, as noted previously for 254-nm excitation.] Absolute Raman cross sections were determined with reference to the intensity of the 980-cm-' band of sulfate, using the sulfate cross sections given by Dudik et al." and Fodor et a1.I2 Figure 7 shows the excitation profiles constructed in this way. They all show a sharp rise toward the shortest wavelength, but there is structure superimposed on this general trend. Thus, the 1692-, 1603-, 1392-, 1324-, and 1100-cm-' band profiles all show (1 1) Dudik, J. M.; Johnson, C. R.; Asher, S. A. J . Chem. Phys. 1985,82, 1732. (12) Fodor, S . P. A.; Copeland, R. A,; Spiro, T. G. J . Am. Chem. SOC. 1989, 1 1 1 , 5509.
2276 The Journal of Physical Chemistry, Vol. 94, No. 6, 1990
Hildebrandt et al.
FA, liqu. 4 4 2 nm
325nm
7ci
IO00
1300
1
I '
1600
ncm-l
Figure 4. Raman spectra of pure liquid formamide excited at 442 and 218 nm
407 nm
I
I1
v
g
192 nm
700
I
1
I
1000
1300
1600
ncm-'
Figure 6. Raman spectra of aqueous solution of formamide excited at different wavelengths. C,, was 1.0 M at 229 nm but 3.0 M at 252, 325, and 407 nm. Csulfatc was 1.0 M in each case. 200 nm
N
E
'i 1
209 nm
218 rm
700
1
IO00
1300
attribute all of the structure in the formamide excitation profiles to the interplay between the n-x* and x-x* transitions and their consequences for the Raman scattering tensor.
Discussion n-r* Interference Effects. Interference effects in excitation profiles arise from the fact that the Raman intensity is proportional to the square of the scattering tensor, which has contributions from all of the electronic excited states. These contributions can have positive or negative signs and can reinforce or suppress one another. Interference effects are especially notable in the vicinity of a weak electronic transition which lies not far below a strong one. In that case the scattering contributions from the local state and the higher lying one may be comparable in magnitude and interfere maximally. This kind of interference can lead to an "antiresonance", an intensity minimum near the position of the weak electronic state.I3 This situation is conveniently analyzed by restricting attention to the A term scattering contributions from two electronic states in a simple one-dimensional approach:
1600
A cm-'
Figure 5. Raman spectra of aqueous solution of formamide excited a t different wavelengths. CFAwas 0.095 a t 192 and 200 nm but 1 .O M at 209 and 2 I8 nm. CsUlfalc was 1 .O M in each case.
a shallow trough in the 200-260-~m-~region, while the 6 0 9 - ~ m - ~ band displays a deep trough, no intensity being detectable below 325 but above 209 nm. On the other hand, the 1742- and 797-cm-I bands show excitation profile peaks at 220 nm. This is the wavelength of a weak absorption band in neat formamide, which is assigned to the n-r* transition.z This is the only electronic transition below the allowed T-T* transition at 183 We nm7.233
where 1 and 2 are the two interfering electronic states, with pure electronic transition moments P, vibrational levels k in the mode of interest, and damping factors I?. Auk is the detuning interval between the laser frequency and the frequency of the level k in the state 1 or 2 . In the simplest approximation, the Franck-Condon products ( I l k ) and (k10) can be evaluated on the assumption that the (13) Stein, P.; Miskowski, V.; Woodruff, W. H.; Griffin, J. P.; Werner, K . G.; Gaber, B. P.; Spiro, T. G. J . Chem. Phys. 1967, 64, 2159.
Raman Spectra of Formamide in
H20and D20
The Journal of Physical Chemistry, Vol. 94, No. 6,1990 2211
Excitation wavelength, nm
250
200
'
xlO-28L
I
1
I
300 1
I
1
1
400
1
1
1
500 1
'
\
-
HCONH, in
004-
YO
-
:x16'8
----
7
7lo00
-
-=l-oo
----
f
c 0
t - 0
0 300 Jexc 5' Figure 9. Theoretical excitation profiles calculated for different Az values, calculated in the same way as for Figure 8.
1
1
1
1
50000
1
1
1
1
1
1
1
1
1
1
1
1
1
.
2oooo
40000 x)ooo Excitation wavenumber, cm"
Figure 7. Excitation profiles of the eight Raman bands of HCONH2. Absolute Raman cross sections are plotted against the excitation wavelength. i', ; ',
hZ=0.2
r=v
Figure 10. Two examples of theoretical excitation profiles given by eq I . (A) On the assumption that rl = IOU,Azl = -0.05,r2= 8u, Az2 = -1.5, €,/e2 = 200, and (ueI0- us,,) - ( y e p - ug0) = 1 9 . 4 ~( u = 609 cm-'). (B)r, = 5u, A Z ~ = 0.5, r2 = 1.5v, A Z ~= I , 4 c 2 = 200, - ug0) (u+ - ugo) = 1 0 . 9 ~( u = 1100 cm-I).
and Q is the normal coordinate. The harmonic vibrational eigenfunction is
8
,
' ,
i
I
'
1,
('
';
'!/
Figure 8. Two examples of theoretical excitation profiles. Calculated (( I l k ) ( k l O ) ) / ( A v k ir) are plotted against Auk - kv, where Auk = vet - vgo - vclc = (u, + ku) - v - ucxc = uo0 - vcxc+ ku, which is
values of
xk
+
given by the unit of the vibrational fBr0equency, u.
vibrational mode is harmonic and has the same frequency in the ground (g) and the excited (e) states, but the potential minimum is shifted by
AZ = Z,
- zg
where z is a dimensionless vibrational coordinate z = y1I2Q; y =
4r2v/h
where
(3) We now consider the scattering amplitude, a,in the vicinity of resonance with the lower lying electronic state, 2. In Figures 8 and 9 the real and imaginary part of the amplitude are plotted separately, for illustrative purposes, using different values of the shift parameter Az and assuming for convenience that r = v . It can be seen that the interference is destructive if the potential shift is in the same direction for both excited states (same sign for the Franck-Condon products), since the energy denominator changes sign at the local resonance. In Figure 10 we plot excitation profiles calculated with eq 1 for two transitions of energies 51 600 and 39 700 cm-l (slightly lower than the band centers of the 183- and 220-nm formamide absorption bands) and a h12/k22ratio of 200 (the formamide
2278 The Journal of Physical Chemistry, Vol. 94, No. 6, 1990 absorption band intensity ratio), with r = v ; 1 refers to the a-a* state and 2 to the n-a* state. The right panel (B) was obtained with origin shifts of Azl = -0.5 and Az, = -1, with rl = 1Ov and r2= 8 ~ The . shape of the profile bears a qualitative resemblance to that of the 1 100-1692-cm-' formamide bands (Figure 7). (The same results are obtained if the shift values are positive.) The deep trough calculated for the left panel (A) was obtained by increasing l z , to -1.5 and decreasing Azl to -0.05, with I'l = 5v and r2= 1.51). We consider this last calculation to be illustrative of the actual situation for the formamide 609-1"' band, whose excitation profile must also have a deep trough near the n-a* transition (Figure 7). A large change in the OCN angle is expected in the n-a* state. For the isoelectronic molecule HCOF the vibrational structure of the n-a* absorption band in the 270-200-nm region is dominated by the OCF angle bending mode (451 cm-I), and the OCF angle has been estimated to change from 121' in the ground state to 106-1 13' in the n-a* excited state.I4 We infer from the model calculation that a similarly large OCN angle change would produce the observed trough in the excitation profile provided that the angle decreased to a smaller extent in the a-a* excited state. Support for this inference comes from a previous analysis of HCSNH2, in which the SCN angle was estimated to decrease by 4' in the a-a* state.15 We note that a similar situation may pertain to HCONHCH3, for which the 620-cm-' O C N bending Raman band decreases in intensity, relative to the 960-cm-' N-CH3 stretch when the laser excitation is changed from 488 to 266 nm.I6 Likewise, the relative intensity of the 668-cm-' OCN bending mode of HCON(CH3), decreases between 514.5 and 257.3 111n.l' On the other, there is no observable influence of the N-methylacetamide n-a* transition on any of the excitation profiles including that of the 628-cm-' C=O bending Perhaps substitution of the methyl group on the carbonyl C atom suppresses the OCN angle change in the n-a* state. Intermolecular Origin of the 1742- and 797-cm-' Bands. The dilution experiments establish that the 1742- and 797-cm-I bands, which are uniquely enhanced in the deep UV, arise from formamide aggregates. The frequencies do not correspond to overtone or combination frequencies of the assigned formamide fundamental vibrations. The 1742-cm-' frequency is in a range where assignment to C 4 stretching is the only reasonable one. This band shifts 14 cm-I in D,O, vs 25 cm-I for the 1692-cm-l band, which is also assigned to C=O stretching. The assignment of the 797-cm-' band is more problematic. It might have OCN bending character, but it is much higher than the 609-cm-l OCN bending mode and shifts much less in D 2 0 (4 vs 69 cm-l). In the ensuing paragraphs we consider the character of the 1742-cm-' band. If formamide formed a dimer, two C=O stretching modes would be expected with frequencies separated by the intermolecular interaction, most likely caused by transition dipole coupling between the two C=O groups.19 I f the dimer were centrosymmetric, then one of the modes (in-phase C=O stretching) would be Raman-active, while the other (out-of-phase) would be infrared-active. Both bands under consideration, however, appear in the Raman spectrum, and the 1742-cm-' band is infrared-inactive. Because each NH2 group of formamide can form two hydrogen bonds, a hydrogen-bonded network is possible and indeed is observed in the crystal structure,20which is illustrated in Figure 1I . The Bravais cell of this structure has four formamide molecules arranged in C2, symmetry (factor group). There should be four carbonyl stretching vibrations, two Raman-active (A, and (14) Giddings, L. E., Jr.; Innes, K. K. J . Mol. Spec[rosc. 1961, 6, 528. ( 1 5) Tsuboi, M.; Hirakawa, A. Y.; Hoshino, T.; Ishiguro, T.: Kimura, K.; Katsumata, S. J . Mol. Spectrosc. 1976, 63, SO.
(16) Harada. T.; Takeuchi, H. Personal communication; to be published. (17) Kudo, H . Master Thesis, University of Tokyo, 1975. (1s) Dudik. S. P. A.; Johnson, C. R.; Asher, S. A. J . Chem. Phys. 1985, 89, 3805. (19) Tsuboi, M.; Nishimura, Y. In Raman Spectroscopy-Linear and Nonlinear: Lacombe, .I.. Huong, P. V., Eds.: Wiley: New York, 1982; p 683. (201 Lmdell. .1.. Post. B. Acta Crysfallogr. 1954, 7, 559.
Hildebrandt et al.
\
Figure 11. Arrangement of the molecules in the crystal structure of f~rmamide.'~
B,) and two infrared-active (A, and given by2'
+b+c = VO + b - c
v(A,) = u(BJ
VO
BJ. Their frequencies are
v(A,) = v(B,) =
YO VO
-b-c
-b
+c
where
v = [K(&
+ po)]1/2/2ac,
b=
VO(kd/K),
c = v,(k,/K)
K is the C=O stretching force constant, while kd and ki are interaction force constants for the stretching of two carbonyl bonds within a ring dimer and between adjacent dimer units, respectively. F~ and lcare the reciprocal masses of the oxygen and carbon atoms. If we examine liquid formamide (Figure 4), which is likely to retain some of the structural units of the crystalline material, we find the two Raman bands at 1668 and 1472 cm-' and a single infrared band at 1681 If transition dipole coupling is the mechanism for splitting the C = O frequencies, then the interaction constants are expected to have negative value^.'^ This would place the B, frequency above the A, frequency and the A, frequency above the B, frequency, these pairs differing only in the sign of c, the interdimer interaction term. Consequently, we assign the 1668- and 1742-cm-' Raman bands to the A, and B, modes, respectively. On the same argument, the B, frequency should be higher than the A, frequency, while the A,, frequency should be higher than the B, frequency, these pairs differing only in the sign of 6, the intradimer coupling term. Therefore, we assign the 1668-cm-] infrared band, which lies between the two Raman bands, to the B, mode. With this choice of frequencies, the above equations can be solved to give vo = 17 1 1.5 cm-', b = -6.5 cm-I, and c = -37 cm-l. The predicted frequency for the A, mode is 1755 cm-I. There is no infrared band at this position, but the transition dipole moment may be small since the individual C = O dipoles should largely cancel in the A, eigenvector. This analysis indicates that the hydrogen bonds between the dimeric units are stronger than the intradimer hydrogen bonds (Figure 11). The situation is somewhat different in concentrated aqueous solution. In D 2 0 the formamide C=O stretching band is 1668 cm-' in the infrared spectrum, coincident with the lower frequency Raman band (1667 cm-I). An effective coupling unit of two C=O oscillators is therefore suggested, although a centrosymmetric dimer is still ruled out by the selection rules. A 2-fold linear aggregate (probably helical) is suggested, as shown in Figure 12, with water molecules completing the H-bond network. The (21) Shimanouchi, T.; Tsubai, M.; Miyazawa, T. J . Chem. Phys. 1961,35, 1597. (22) Evans, J. C. J . Chem. Phys. 1954, 22, 1228.
J. Phys. Chem. 1990, 94, 2279-2283
2279
enhancement). In the formamide aggregates the electronic transitions should also split into A and B symmetry components. These can be mixed vibronically via a B symmetry vibration. We propose that the 1742-cm-l aggregate C=O stretching mode mixes the A component of the n-x* state with the B component of the x-x* state, or vice versa, accounting for the resonance with the n-x* transition. Since the 797-cm-I band shows a similar resonance, it is also assigned to a B symmetry aggregate mode, although the detailed composition is uncertain. The 200-nm peak in the 1742-cm-I profile may likewise be due to n-x*/x-x* vibronic mixing, this time resonant with the r - ~ *state.
it
H-f-H
Figure 12. An aggregate of formamide molecules, which is postulated to occur in concentrated aqueous solution.
Bravais cell would have two formamide molecules, which would form A-type (symmetric) and B-type (antisymmetric) combinations of C=O stretches. These are assigned to the lower and higher frequency Raman bands, respectively. Thus, the 1742-cm-l Raman band in aqueous solution is assigned to the B-type vibration of the proposed linear aggregate. Now we turn to the question of the unique excitation profile peak at 220 nm observed for the 1742- and 797-cm-' Raman bands. Since the 1742-cm-' band is assigned to a B symmetry mode, in both liquid formamide and aqueous solution, its enhancement is expected to arise from vibronic coupling (B term
Conclusions The formamide Raman excitation profiles reveal an upward trend toward the first x-x* transition at 183 nm, but a superimposed structure in the region of the n-x* transition at 220 nm. Most of the bands show a shallow trough at wavelengthsjust above 220 nm, which can be modeled as resulting from destructive interference in the Raman tensor and between the locally resonant weak n-x* transition and the much stronger higher lying x-x* transition. A deep trough seen for the 609-cm-' OCN bending mode is consistent with a larger interference effect arising from a large decrease in the OCN angle in the n-a* excited state, consistent with evidence from related molecules. Deep UV excitation reveals new Raman bands, at 1742 and 797 cm-I for liquid formamide and in aqueous solutions. These are demonstrated to arise from aggregates and are assigned to B symmetry modes of the aggregate. The excitation profile maxima seen uniquely for these bands at 220 nm are consistent with vibronic mixing of the A component of the n-x* state with the B component of the x-x* state, or vice versa. Acknowledgment. This work was supported by NIH Grant GM 25158 (to T.G.S.). P.H. was the recipient of an Otto-Hahn stipend provided by the Max-Planck-Gesellschaft, FRG. Registry No. Formamide, 75-1 2-7.
Nonadiabatic Unimolecular Reactions. 4. Isolated State Decay in the Fragmentation of the Formaldehyde Cation J. C. Lorquet* and T. Takeuch? Dgpartement de Chimie, Universitg de Li?ge, Sart- Tilman. B-4000 Licge 1 Belgium (Received: June 6, 1989; In Final Form: September 25, 1989) ~
The dissociation (by hydrogen loss) of the first excited state (A2BI)of the formaldehyde cation as a function of its vibrational excitation is analyzed. The reaction has been_experimentally observed as a metastable dissociation. Its mechanism involves an electronic predissociation by the ground X2B2state in the tunneling regime. A statistical treatment recently developed for nonadiabatic interactions accounts for the low value of the rate constant as well as for a surprisingly large isotope effect that persists over an energy interval of ca. 0.7 eV. The method requires a partitioning sf the set of degrees of freedom. The CO bond stretch (which assumes very different equilibrium distances in electronic states A and B) introduces a Franck-Condon factor in the expression of the rate constant. The remaining degrees of freedom are treated collectively and give rise to a RRKM-like expression in which the nonadiabatic transition probability is introduced as a transmission coefficient. The hydrogen loss of H2CO+ appears to be the first well-established case of isolated state decay by noncommunicating electronic states, i.e., a process where the rate-limiting step is internal conversion and not dissociation.
I. Introduction Much insight into the dynamics of unimolecular reactions has been gained by the study of the dissociation processes of ionized molecules in a mass spectrometer.'" In particular, the development of photoion-photoelectron coincidence spectroscopy (PIPECO) has been at the origin of a real breakthrough in our 'Permanent address: Department of Chemistry, Nara Women's University, Nara 630, Japan.
0022-3654/90/2094-2279$02.50/0
understanding of that field. Another important source of information results from the fact that slow dissociation processes (Le., (1) Lifshitz, C. Adu. Mass Spectrom. 1978, 7 A , 3; J . Phys. Chem. 1983, 87, 2304; Int. Rev. Phys. Chem. 1987, 6, 35; Adu. Mass Spectrom. 1989, 1 1 , 3.3
113.
(2) Bowers, M . T., Ed. Gas Phase Ion Chemistry; Academic Press: New York, 1979, 1984; Vols. 1-3. (3) Derrick, P. J.; Donchi, K. F. In Chemical Kinetics; Bamford, C. H., Tipper, C. F. H., Eds.; Elsevier: Amsterdam, 1983; Vol. 24.
0 1990 American Chemical Society
