4794
J. Phys. Chem. 1992,96, 4794-4801
be due to both the methines and the corollaries of the ethyl protons of OEP in the protoporphyrin IX system of Mb and Hb. A problem concerns the coupling due to HN1, which we previously assigned in Mb and Hb on the basis of deuterium exchange results. In the model compounds, we see a smaller coupling value ("1.6 MHz, DD') at g, than in the protein (e2.6 MHz). This could be due to differences in the changes in molecular geometry between the model compounds and the proteins upon cooling to e5 K. Acknowledgment. This work was supported by a grant from the Deutsche Forschungsgemeinschaft (Hu 24818- 1). Helpful discussions with Dr. R. Kappl as well as the support of H. Reinhard in simulating the I4N ENDOR spectra are gratefully acknowledged.
References and Notes (1) (a) Smith, T. D.; Pilbrow, J. R. Cmrd. Chem. Reu. 1981,39,295. (b) Jones, R. D.; Summerville, D. A.; Basolo, F. Chem. Rev. 1979, 79, 139. (2) Hoffman, B. M.; Petering, D. H. Proc. Natl. Acad. Sci. U.S.A. 1970, 67, 637. (3) Yonetani, T.; Yamamoto, H.; Iizuka, T. J. Biol. Chem. 1974, 149, 2168. (4) Gupta, R. K.; Mildvan, A. S.; Yonetani, T.; Srivastava, T. S. Biochem. Biophys. Res. Comm. 1975,67, 1002. (5) Hori,H.; Ikeda-Saito, M.;Yonetani, T. J. Biol. Chem.1982,257,3636. (6) Chien, J. C. W.; Dickinson, L. C. Proc. Natl. Acad. Sci. U.S.A. 1972, 69, 2783. (7) Dickinson, L. C.; Chien, J. C. W. Proc. Natl. Acad. Sci. U.S.A. 1980, 77, 1235. ( 8 ) HtJhn, M.; Hiittermann, J. J . Biol. Chem. 1982, 257, 10554.
(9) Walker, F. A. J. Am. Chem. SOC.1970, 92, 4235. (10) Walker, F. A. J . Magn. Reson. 1974, 15, 210. (1 1) Lubitz, W.; Winscom, C. J.; Dielgruber, H.; Miweler, R. Z . Naturforsch. 1987, 42a, 970. (12) (a) Baumgarten, M. Ph.D. Thesis, Freie Universitit Berlin, Germany, 1988. (b) Baumgarten, M.; Lubitz, W.; Winscom, C. J. Chem. Phys. Lett. 1987, 133, 102. (13) Hori, H.; Ikeda-Saito, M.; Froncisz, W.; Yonetani, T. J . Biol. Chem. 1983, 258, 12368. (14) Hiittermann, J.; Stabler, R. In Electron Magnetic Resonance of Disordered Systems; Yordanov, N. D., Ed.; World Scientific: Singapore, 1989; pp 127-148. (15) Inubushi, T.; Yonetani, T. Biochemistry 1983, 22, 1894. (16) Perutz, M. F.; Fermi, G.;Luisi, B.; Shaanan, B.; Liddington, R. C. In Cold Spring Harbor Symp. Quant. Biol. 1987, LII, 555. ( I 7) Hiittermann, J.; Kappl, R. In Metal Ions in Biological Systems; Sigel, H., Ed.; Marcel Dekker: New York, 1987; Vol. 22, pp 1-80. (18) Ohlmann, J. Diploma Thesis, Universitat des Saarlandes, 1991. (19) Hiittermann, J.; Ohlmann, J.; Schaefer, A.; Gatzweiler, W. Inr. J. Radial. Biol. 1991, 59, 1297. (20) Henderson, T. A. Ph.D. Thesis, University of Rochester, Rochester, NY, 1986. (21) Hurst, G. C.; Henderson, T. A.; Kreilick, R. W. J . Am. Chem. SOC. 1985, 107, 7294. (22) Ovchinnikov, I. V.; Konstantinov, V. N. J. Magn. Reson. 1978, 32, 179. (23) Iwasaki, M. J . Magn. Reson. 1974, 16,417. (24) Cullen, D. L.; Meyer, E. F. Acta Crystallogr. 1976, 32, 2259. (25) Little, R. G.; Ibers, J. A. J. Am. Chem. SOC.1974, 96, 4452. (26) Scheidt, W. R. J. Am. Chem. SOC.1974, 96, 90. (27) Dwyer, N. P.; Madura, P.; Scheidt, W. R. J . Am. Chem. Soc. 1974, 96, 4815. (28) Hartmann, H.; Parak, F.; Steigemann, W.; Petsko, G. A.; Ringe Ponzi, D.; Frauenfelder, H. Proc. Natl. Acad. Sci. U.S.A. 1982, 79, 4967.
Resonance Raman Spectra and Excltatlon Profiles of (Z)-1,3,5-Hexatriene Vapor Bjarne Amstrup, Department of Chemical Physics, The Technical University of Denmark, DTH-301, DK-2800 Lyngby, Denmark
Frans W. Langkilde, Department of General Chemistry, Royal Danish School of Pharmacy, 2 Universitetsparken, DK-2100 Copenhagen, Denmark
Krzysztof Bajdor: and Robert Wilbrandt* Department of Environmental Science and Technology, Riso National Laboratory, DK-4000 Roskilde, Denmark (Received: November 19, 1991; In Final Form: January 27, 1992)
-
UV resonance and preresonance Raman spectra of (Z)-1,3,5-hexatriene vapor in resonance with the 1 'Al 1 'B,transition, using 11 excitation wavelengths in the region 262-232 nm, are reported. The resulting excitation spectra for the observed fundamental vibrational modes, overtones, and combinations are presented. The spectra are interpreted on the basis of theoretical calculations using the time-dependent wave-packet propagation formalism. A best fit for the absorption spectrum and resonance Raman excitation profiles is calculated by parametrizing a model for the excited-state potential energy surface. Temperature effects and Duschinsky mixing are included in the model. The potential energy surface of the 1 'B1excited singlet state is found to have a local minimum at the planar Z geometry.
I. Introduction Polyenes are of general interest because of their importance in biological systems. Examples are retinal in the process of vision, carotenoids in photosynthesis, and vitamin A. Studies of the short, unsubstituted polyenes with two (butadiene), three (hexatriene), or four (octatetraene) C=C double bonds are of particular interest in view of their use as computationally tractable models for the larger biological systems. 'On leave from Institute of Physical Chemistry of the Polish Academy of Sciences, ul. Kasprzaka 44/52, 01-224 Warszawa, Poland. Present address: Industrial Chemistry Research Institute, ul. Rydygiera 8, 01-793 Warszawa, Poland.
0022-3654/92/2096-4794$03.00/0
One of the most discussed issues' is the electronic structure of the ground and excited singlet states of polyenes. In particular, the interpretation of the 1 '4 1 'B, optically allowed transition and the question of relative state ordering of the 1 'B, and 2 'Ag states (under C2,symmetry) and 1 'B1and 2 'A, states (under C2, symmetry) of butadiene and hexatriene has been discussed both theoretical1y2-l6and e~perimentally.l~-~~ There is now recent experimental e ~ i d e n c e that ~ ~ -the ~ ~state ordering in butadiene and hexatriene is the same as in the longer polyenes; i.e., the spectroscopic 2 lA, (2 'Al) state is lower in energy than the 1 IB, (1 IBI) state. Furthermore, energies of both valence and Rydberg states of hexatriene have been determined by electron energy loss spectroscopy.21s2s-26
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0 1992 American Chemical Society
Raman Spectra of (Z)-1,3,5-Hexatriene Vapor
The Journal of Physical Chemistry, Vol. 96, No. 12, I992 4795
The ground state of 1,3,5-hexatriene has been studied by various graphic grating). A gated intensified OMA (optical multichannel analyzer, OSMA IR4-700, Spectroscopy Instruments) with 700 techniques. The geometry of the E and Z (or trans and cis) active channels was used as detector, and data handling was isomers of 1,3,5-hexatriene has been determined by electron performed on a PDPll/23 computer. The spectral resolution d i f f r a c t i ~ n , ~and ~ * ~the * vibrational structure of (E)- and 1,3,5-hexatriene has been investigated by both t h e ~ r e t i c a l ' ~ ~ ~using ~ a slit width of 0.2 mm was ca. 25 cm-' at 230 nm. The frequency calibration was carried out using emission lines from and e ~ p e r i m e n t a l ~ ~ methods. - ~ ~ - " ~ The question of whether an Osmium hollow cathode lamp. (2)-hexatriene is strictly planar in the ground state is still a matter Each spectrum was recorded by averaging 16000-20 OOO pulses of d i s c ~ s s i o n . ~ * * ~ ~ (about 1 h of measurement), and two to three spectra were The photochemistry of hexatriene has been studied by a number combined to cover the spectral range MOO0 cm-I. To check the of groups.465s The dominant photochemical process upon direct possible conversion of Z-HT into E-HT and other irradiation irradiation of 1,3,5-hexatriene in the gas phase has been shown to be the formation of the other isomer and of 1,3-~yclohexadiene.~ products during the experiments, the samples were investigated after the Raman experiments by UV/vis absorption spectroscopy As discussed in several reviews,5660resonance Raman specand by GC analysis. No depletion of Z-HT could be detected troscopy is a powerful method to study potential energy surfaces by absorption spectroscopy, and GC showed the presence of 2.4% of molecules in ground and excited electronic states. Among the E-HT after irradiation of a sample cell with 300 000 laser pulses. unsubstituted polyenes, butadiene,22and hexatriNo other products of irradiation were detected by GC. From this ene6- have been studied using excitation wavelengths in resoit can be concluded that even weak bands in the resonance Raman nance or preresonance with the 1 'A, 1 'B, transitions. Of particular interest in the context of the present paper is the work spectra observed are due to (Z)-1,3,5-hexatriene. by Myers and co-workers on (E)-1,3,5-hexatriene in the gas Resonance Raman spectra were corrected for (i) the sensitivity phase6$and in Furthermore, time-resolved resonance variation across the diode array and (ii) the dependence on Raman studies of the lowest excited triplet state of 1,3,5-hexatriene wavelength of the detection system. These corrections were made and deuteriated and methylated derivatives have been published by the use of a calibrated deuterium lamp (Optronics UV-40) as intensity standard. Furthermore, the intensity of Raman bands by our g r o ~ p . ~ ~ - ~ ~ had to be corrected (iii) for the absorbance of ground-state In the present paper we report resonance Raman spectra of hexatriene in the sample cell. These corrections were calculated (Z)-1,3,5-hexatriene (or trans,cis,rrans-1,3,5-hexatriene) (Z-HT), excited in resonance with the 1 'Al 1 'B1transition in the gas from the measured absorption spectrum of Z-HT, the used wavelength of excitation, and the path length (measured by means phase. Excitation profiles are obtained by recording resonance of a microscope) from the focused laser beam to the exit window Raman spectra at 11 excitation wavelengths ranging from ca. 262 of the sample cell in direction of the monochromator entrance slit. to 232 nm. The absorption spectrum of 2-HT is remeasured, and absorption and resonance Raman spectra are modeled using the The molar absorption coeffcent of 2-HT in the gas phase was timedependent approach developed originally by Lee, Heller, and determined from the measured maximum decadic absorption c ~ - w o r k e r s . ~ The ~ ~ ~results - ~ ~ are compared with those obtained coefficient by determining the amount of Z-HT in the glass bulb by Myers and Pranata6$in a similar study of (E)-1,3,5-hexatriene. and the volume of the bulb. In this procedure Z-HT was transferred on a vacuum line from the glass bulb to another glass II. Experimental Methods tube, and a known volume of n-hexane added as solvent. The concentration of Z-HT was then determined by GC by comparison 1,3,5-Hexatriene was obtained from Aldrich as a mixture of with a solution of known concentration of Z-HT. It should be the isomers. The isomers were separated as described previously76 mentioned that, compared with the previously reported vapor-phase with some modifications. 2 - H T was isolated from the mixture resonance Raman spectra of E-HT!$ the concentration of Z-HT via a Diels-Alder cycloaddition of the E isomer to maleic anhyused in the present work was higher by about a factor of 20. This dride. Five grams of the isomer mixture was added to 10 g of higher vapor pressure was found necessary in order to obtain good well-ground maleic anhydride and 200 ppm hydroquinone. The resonance Raman spectra. It appears that the sensitivity of our stirred and N2-purged mixture was allowed to react for 2 h at room spectrometer in the UV region is considerably lower than that temperature and was then kept at -20 'C overnight. 2 - H T was of ref 65, presumably due mainly to a lower efficiency of our distilled off the reaction mixture on a vacuum line at room temholographic grating. Furthermore, attempts to determine the perature and collected in a receiver cooled by liquid N2. The absolute Raman cross section relative to that of a methane product was purified by several freeze-pumpthaw cycles. The standard failed because of low laser power and reduced detection purity of 2 - H T obtained in this way was 99.3% as determined sensitivity. by gas chromatography (GC). For the absorption and Raman measurements, Z-HT was transferred via a vacuum line and III. Theoretical Methods expanded into a 500-mL glass bulb equipped with a magnetic The theoretical treatment follows closely that given in ref 65, stirrer attached to a Suprasil fluorescence cuvette (5 X 5 X 40 which is based on the work of Heller and ~ o - w o r k e r s . ~This -~~*~~ mm). 2-HT pressures (- 1 mmHg) were adjusted to give decadic implies (i) the adiabatic and Condon approximation, (ii) the absorption coefficients of about 3 cm-'at the absorption maximum assumption of a single excited electronic state and the neglect of (241.8 nm), except for the sample used for the preresonance the nonresonant term, and (iii) the assumption of harmonic Raman spectrum for which the decadic absorption coefficient was ground- and excited-state potentials. The excited-state potential 2 cm-' at 262 nm. energy surface was studied by modeling the absorption spectrum Ground-state absorption measurements were performed on a and Raman excitation profiles using Heller's time-dependent Philips PU 8800 UV/vis spectrophotometer with a spectral resformalism. Briefly, the resonance Raman cross section at exciolution of 0.1 nm using an evacuated bulb as reference. tation energy EL and scattered photon energy Es is given by Resonance Raman excitation wavelengths in the range 232-262 nm were obtained by Raman shifting the output of a frequency-doubled dye laser, pumped by the second harmonic of a Nd:YAG laser (5-Hz repetition rate, 15-11s pulse duration). The dyes used were Rhodamine 6G, Rhcdamine 610, and Rhodamine exp[i(EL + t i ) t / h - l ~ / h ] 1 ~ 8 ( E+ L ti - Es - tf) (1) 640. Pulse energies ranged from 10 to 30 pJ/pulse. The laser while the corresponding absorption cross section is beam was focused to approximately 0.1 mm on the sample cell, resulting in an intensity of approximately 8-25 MW cm-2. Q*(EL) = Scattered Raman light was collected by reflecting optics (Cassegrain telescope) and imaged through a polarization scrambler -2*a E L ( ~ z ~ P i ~(ili(?)) ~ d t exp[i(EL + c,)t/h - I'ltl/h] 3h on the entrance slit of a home-built Czerny-Turner grating monochromator (600-mm focal length, 2400 grooves/" hole (2)
(a-
-
-
4796 The Journal of Physical Chemistry, Vol. 96, No. 12, 1992
Amstrup et al.
Here, M is the electronic transition length, Piis the Boltzmann population of state l i ) , a is the fine structure constant, ti and tf are the energies of vibrational states li) and V),I' is the electronic homogeneous line width, and l i ( t ) ) = exp[-iHt/h]li) where H is the excited-state vibrational Hamiltonian. As our experimetnal setup was calibrated in terms of intensity, we have substituted the term Es3ELin eq 1 by Es4 in the calculations. Nonzero temperature was treated as described by Myers and Pranata6s and Heller and m w ~ r k e r s . ~Briefly, ~ ? ~ ~the thermal population of low-frequency vibrational modes was taken into account by their Boltzmann factors in eqs 1 and 2. As li(t)) is simple to calculate for harmonic states only if li) is the ground state, higher oscillator eigenstates were expressed as linear combination of complex Gaussians through the formalism
ipj(q - qj) + ipjqj/2
+ 2rinj/Nl
(3)
where qj = (2n)'J2 cos (Zrj/N)
(4a)
pj = -(2n)'i2 sin ( 2 ~ j / N )
(4b) Ten basis states (N = 10) were used, giving a good fit to oscillator eigenstates up to n = 3. Based on the linearity of the Schrbdinger equation, l i ( t ) ) and the appropriate overlaps for thermally populated vibrational states were calculated by propagating each Gaussian component separately and then summing the overlaps. Thermal population is of importance only for vibrational modes being Franck-Condon active due to a change either in molecular equilibrium geometry or in vibrational frequency. In the present calculations a Boltzmann average was taken over 18 initial states, through n = 1 in vI2 (a, CCC deformation at 389 cm-I) and n = 2 in v I 3(a, CCC deformation at 175 cm-I) and vi9 (a, C-C torsion at 143 cm-I). Time integrals were carried out using Simpson's rule. The integration was stopped after 800 steps at 0.2 ps.
IV. Results Absorption Spectrum. The experimental absorption spectrum of (Z)-1,3,5-hexatriene vapor in the spectral region 38 000-46000 cm-I, measured at a pressure of 1 mmHg, is shown in Figure 1 together with the excitation wavelengths used in the recording of raonance Raman spectra. The general shape of the spectrum and position of maxima are in good agreement with the data reported in refs 17 and 79. We determined a maximum molar absorption coefficient of t = 56 650 M-I cm-I. Our values for the molar absorption coefficient are within 10% of those reported by Orchard and ThrushS2at 228.8 and 253.7 nm. However, they are about 4 times higher than those reported by Gavin et al." A similar discrepancy has been found by Myers and co-workers6s for (E)-1,3,5-hexatriene recently, and we agree with their conclusion that ref 17 must be in error. It should be noticed that our room temperature spectrum is rather similar to the free jet absorption spectrum of Z-HT reported by Leopold et The main differences are a slightly increased bandwidth at room temperature and a small change in relative intensities of the 0-0 and 0-1 bands (at 39 709 and 41 356 cm-I), the 0-0 band being relatively weaker at r c " temperature than in the jet. Integration of the absorption spectrum down to a wavelen th of 225 nm yielded an electronic transition length of 1.30 . The cutoff wavelength of 225 nm may seem somewhat high, and the used value of the transition length thus may be somewhat low. However, at lower wavelengths Rydberg states may begin to contributes0 to the absorption spectrum, making the determination of the transition length more uncertain. Raman Spectra. Figure 2 shows an overview of resonance Raman spectra of Z-HT vapor in the region 0-4000cm-l, recorded using 11 excitation wavelengths ranging from 262 to 232 nm. The relative scaling is chosen such that all spectra are normalized relative to the strong band at 1630 cm-I. Figure 3 shows greater details for 252.05-nm excitation in the region 0-2000 cm-I. An
1
38000
4#x
42000
44ooo
46000
WAVE NuMBE R / c ~ - ~ Figure 1. Experimental absorption spectrum of (Z)-hexatriene(pressure 1 mmHg, temperature 298 K) with indication of wavelengths used for
excitation of resonance Raman spectra. The maximum molar absorption coefficient is 56 650 M-' em-'.
artifact, due to rotational Raman scattering in the hydrogen Raman shifter, masked the region around 590 cm-I. Additional experiments (excitation wavelength 252.5 nm, not shown here) without the use of the Raman shifter (by frequency doubling of the dye laser using a BBO crystal) revealed a weak vibrational band at 573 cm-I, which is masked in the spectra of Figures 2 and 3. Vibrational Assignments. The equilibrium geometry and force field of Z-HT have been calculated using different theoretical approaches such as a b i n i t i ~ and ~ ~ ~emiempiricall~+~~~~~*~~ ,~~ methods. These calculations, together with the classical observations by Lippincott et a1.4I and the recent studies by McDiarmid and S a b l j i and ~ ~ ~Langkilde et a1.,44provide a reliable assignment of the Raman and I R spectra of Z-HT. We shall in the following adopt the numbering and assignments of the most recent a b initio calculation by Tasumi and co-worker~.'~Table I lists wavenumbers and assignments of observed resonance Raman bands together with values from the Small differences in wavenumbers are due to a lower spectral resolution in the present work using pulsed UV excitation as compared with visible excitation in combination with commercial Raman spectrometers in refs 39 and 44. All combination and overtone bands having substantial intensity are listed in Table I. There is little question about the assignment of bands at 177, 389,882,1084,1248,1320,1397,1575, and 1630 cm-I as totally symmetric fundamentals. Of the remaining bands, most are assigned as overtones or combinations involving these modes or non-totally symmetric modes, and one weak band (1030 cm-I) of probably at symmetry is observed. But also some of the bands assigned as totally symmetric fundamentals may receive some additional intensity from combination bands. For example, the difference v5 - vIz (1630-389 cm-I) contributes to v9 (1248 cm-') and, correspondingly, v9 v l 2 contribute to us. Even though the strongly Franck-condon active v I 2CCC bending mode (389 cm-') is populated only to a small extent a t room temperature, it does contribute significantly as difference band to the intensity of some observed Raman bands, e.g., the bands at 473-490,855,930,1248, 2096-21 18,2793-2820, and 2872-2890 cm-I. All bands above 1700 cm-I are assignable as overtones or combination bands involving one or more of the three most intense bands vs (1630 cm-'), v8 (1320 cm-I), and v9 (1248 cm-l). Yet, most of the observed Raman bands are assignable in more than one way. For a number of observed bands an assignment to overtones and combination bands of totally symmetric modes is unlikely or
+
The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 4797
Raman Spectra of (2)-1,3,5-Hexatriene Vapor I
I
1
?
232.78 nm
234.60 nm
uh
237.48
JL
-
2JI.01 nm
JL
241.81 nm
e
.
3
9 nm
%248*11
JL
250.24 nm
&
nm
L-.JL253*"
4000
0
Figure 2. Overview of preresonance and resonance Raman spectra of (Z)-hexatriene vapor with indicated excitation wavelengths. An artifact (from rotational Raman scattering in the Raman shifter) is observed around 590 cm-I. All spectra are normalized relative to the 1630-cm-I band. All spectra are corrected for self-absorption, sensitivity variation of the diodes in the diode array, and wavelength dependence of the detection system.
2000
WAVENUMB E R / C ~
0
Figure 3. Details in the region 0-2000 cm-I for the resonance Raman spectrum excited at 252.05 nm.
not possible. Such bands are those at 288, 473-490, 573, 658, 1030, and 1160-1 177 cm-I. These bands can all, with the exception of the 1030-cm-' band, be assigned to overtones and
combination bands of non-totally symmetric modes. In this way, the 288-cm-l band is tentatively assigned to 249 ( C - C torsion), a contribution to the 473-490-cm-' band to 298 + vI9 ( C - C and C-C torsions), a contribution to the band at 573 cm-I to 4v19 (C-C torsion), a band at 658 cm-' to 2v18(C=C torsion), and a contribution to the band at 1160-1 177 cm-I to 2~~~(CH wag). Hence, considerable activity of non-totally symmetric, in particular torsional, modes is observed. Excited-State Potential Energy Surface. To model the excited-state potential energy surface, an initial set of displacement parameters Ai was taken from the work of Myers and Pranata65 on (E)-hexatriene. Ground- and excited-statefrequencies of totally symmetric modes were assumed identical. The transition length was fmed at 1.30 A, as determined from the integrated absorption spectrum. The zero-zero energy was taken as 39 709 cm-' (from the measured absorption spectrum), and the homogeneous line width was set to 185 cm-I, as this value gave the best agreement with the observed absorption spectrum. The displacements for us, vg, and u l 2 were then fitted to reproduce the absorption spectrum, As, As, and A9 determining the relative height of the second peak around 41 350 cm-l. Then the ratio ASIA9 was adjusted to best fit the two absorption bands between 42 500 and 43 000 cm-I. After this crude fit of the absorption spectrum, parameters were further refined to match the observed excitation
4198 The Journal of Physical Chemistry, Vol. 96, No. 12, I992
Amstrup et al.
TABLE I: Resonrace Raman Band8 Obsened below 3400 cm-’and Assipmeats this work intd SYm lit.“ lit! 170 (155) 394
W
573
W
(155)
W
(332) (394)
(331) (392)
884
883
658 778 855 882 930 950 1030 1068-1 085
W
m
assigntC
166
177 288 389 473-490
392
W
W W W W
W W W
1160-1 177
W
1248
S
1320 1396-1 41 7
m W
1455 1575 1630
sh
1708 1785-1 81 2
W W
1966 202 1
W
2096-21 18
W
2205
W
2275 2406
W W
2489-2505
W
2563
W
265 1
W
2715
W
2793-2820
W
2812-2890
m
2950
W
3026
W
3256
S
1032 1082 (394) (585) 1246
1084 (392) (590) 1247
1315 1397
1318 1397
1578 1623
1580 1626
W
S
m
“Reference 44. bReferencc 39. CNumberingof modes as in ref 39. v = stretch, 6 = in-plane bend, 6, = in-plane rock, 6, = in-plane scissoring, T = torsion, yw = out-of-plane wag. dIntensity denoted qualitatively from the spectrum excited at 252.05 nm.
profiles. Two alternative levels of approximation were used. First, the best fit (fit 1 in Table 11) to both the absorption spectrum and the measured excitation profiles was determined (by manual
trial and error) by varying the displacement parameters for inplane vibrational modes only, assuming zero temperature and no Duschinsky rotation between normal modes. Second, a fit (fit
The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 4799
Raman Spectra of (Z)-1,3,5-Hexatriene Vapor
TABLE II: Ground- and Excited-State Potential Parameters for IZ)-1.3.5-Hexatriene' and (Ebl.3.5-Hexatriene (E)-1,3,5-hexatriene (ref 65) (Z)-1,3,5-hexatriene (this work) mode
us,cm-I
we, cm-I
1630 1575 1397 1319 1248 1084 884 389 175
1630 1575 1397 1319 1248 1084 884 389 175
330 143 580
290 70 420
lAlb fit 1 1.27 0.40 0.21 0.42 0.79 0.06 0.21 0.59 0.35 nid nid nid
IAr fit 2 1.30 0.40 0.21 0.38 0.76 0.06 0.21 0.65 0.35
0 0 0
wg, cm-I
we, cm-I
I4
1635 1581 1403 1290
1635 1581 1403 1290
1.32 0.38 0.085 0.485
1192 934 444 354 686 246 93
1192 934 444 354 450 110 98
0.82 0.23 0.23 0.55 0
0 0
9709 cm-I, homogeneous line width r = 1 5 cm-l, transition length M = 1.30 A, displacement between ground- and excited-state potenGl minima in ground-state dimensionless normal coordinates. bDisplacements in a , (in-plane) modes only, T = 0 K, no Duschinsky rotation. cIncluding non-totally symmetric modes, T = 298 K (n = 0, 1 for uI2, n = 0, 1, 2 for ~ 1 3 n, = 0, 1, 2 for ~19). Duschinsky rotation parameter &J between v5 and vg = -8' and between u I 8 and v I 9 = 15'. The conversion between ground-state (qgl, qs2) and excited-state (qel, qe2) diqensionless normal coordinates is given by
The sign of A is relevant only in the presence of Duschinsky rotation. us and us were assumed to have displacements of the same sign. (They both have considerable CC double-bond character, and the sign is in agreement with ref 13. It should be noted, however, that identical results are obtained upon a simultaneous change of the sign of Ag and 6.)dNot included in calculation.
3,
I
2.5 I
I
2.5
.-$
2
c
-c 1.5 % -" 1 c
2
.5
3$000
40000 42000 44000 WAVENUMBER (cm - )
46000
Figure 4. Experimental (dashed) and calculated (solid) absorption spectrum of (2)-hexatriene using in-plane modes (al symmetry) only, T = 0 K, and no Duschinsky rotation. Parameters as listed in Table 11, fit 1.
2 in Table 11) including frequency shifts in non-totally symmetric modes, finite temperature (298 K), and Duchinsky mixing was attempted. Three combinations of Duschinsky mixing among in-plane modes were considered: us with v8, v5 with v9, and v8 with v9. Of these, the mixing between v5 and v8 clearly yielded the best results. Furthermore, the mixing between the modes vI8 and ~ 1 9 was included in order to obtain Raman intensity in the combination band observed at 473 cm-I. Duschinsky rotation parameters of -8' between vs and v8 and of 15" between v18 and vi9 were used. While the influence of Duschinsky rotation on excitation profiles was limited, we in particular found an influence on the peak at 241.6 nm of absorption spectrum. Of the remaining fitting parameters, the values for Aula, A6, Alo, and A13 are subject to considerable uncertainty. The displacements 4,Alo, and A13were determined from combination bands or weak shoulders only, and the frequency shift for v I 8 could not be derived from its first as this band was partly masked by an artifact overtone at 658 cm-', (see above). The results of the fitted absorption spectra are seen in Figures 4 (fit 1) and 5 (fit 2) and of the resonance Raman excitation profiles in Figures 6 (fit 1) and 7 (fit 2). In order to visualize the vibronic contributions to the absorption spectra, calculations with I' = 2 cm-I are shown in Figures 4 and 5 as well. In Figures 6 and 7 all excitation profiles are normalized relative to the calculated one for the 1630-cm-' band. The profile of this mode
Figure 5. Experimental (dashed) and calculated (solid) absorption spectrum of (2)-hexatriene using parameters as listed in Table 11, fit 2, including non-totally symmetric modes, finite temperature, and Duschinsky rotation.
could not be determined experimentally due to the lack of an internal standard.
V. Discussion Of the two fits proposed, fit 2, including temperature effects, frequency shifts in non-totally symmetric modes, and Duschinsky mixing, shows considerably better agreement with the absorption spectrum and clearly improved excitation profiles. In general, the results for (Z)-hexatriene are qualitatively similar to those for (Qhexatriene. In particular, the excited-state surface is bound with respect to all torsions, implying a local minimum on the excited-state potential energy surface a t or near the planar geometry. If the frequency in v I 8is changed from 290 cm-l to 5 cm-l (a very weakly bound potential), the calculated relative intensities of the combination band at 473 cm-' and the overtone at 658 cm-' increase by more than a factor of 10, far beyond the experimental limits of error. A notable difference is the increased activity in the skeletal bending vibration v12 in 2-HT as compared to E-HT, probably being due to the somewhat strained angle C2C3C3'in the ground state of Z-HT, which can relax in the excited state because of the weakening of the central CC bond. For both T(C=C) (vI8) and 7(C-C) (~19)a decrease in frequency was assumed in the excited state. The last assumption may seem surprising in view of an expected increase in the bond order of
4800 The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 IW
40
140
30
55
38000
43000
WAVENUMBER/C~ Figure 6. Experimental (open squares) and calculated (solid lines) resonance Raman intensities as function of exciting wavelength (excitation spectra) for selected vibrational modes of (Z)-hexatriene. The excitation profile of the group of bands at 1575-1708 cm-’ is the actual calculated Raman cross section in units of A2/molecule. For the other bands the intensity is given as fraction of the excitation profile for the 15751708-cm-l bands in units of percent. The calculation is based on in-plane (a, symmetry) modes only, T = 0 K, and no Duschinsky mixing, using the parameters of fit 1 , listed in Table 11. 0.mo
Q
m
7
WAVENUMBER/cm-l Figure 7. Experimental (open squares) and calculated (solid lines) resonance Raman intensities as function of exciting wavelength (excitation spectra) for selected vibrational modes of (a-hexatriene. The excitation profile of the group of bands at 1575-1708 cm-’ is the actual calculated Raman cross section in units of .&*/molecule. For the other bands the intensity is given as fraction of the excitation profile for the 15751708-cm-l bands in units of percent. The calculation is based on parameters of fit 2, listed in Table 11, including non-totally symmetric modes, finite temperature, and Duschinsky mixing.
the C+3 bond. However, vI9 has some double-bond character,39 and there is some mixing with V I 8 in the excited state. These considerations, together with the fact that the semiempirical calculations of Hemley et al.I3 show a small decrease in the frequency of vI9 upon excitation, led us to choose a decrease in frequency. Yet, an increase in the freuqency of v19 affects ex-
Amstrup et al. citation profiles only little. In general, our derived displacement parameters Ai are in qualitative agreement with those reported by Hemley et al.13 In particular, the most active modes are v5, vg, and vI2, followed by and us with somewhat lower intensities. The homogeneous line width r = 185 cm-’, required to reproduce the absorption spectrum in this work, is considerably larger than that (r = 130 cm-I) derived by Hemley et al.I3 in the fit to the free jet absorption s p t r u m . This could be possibly due to the fact that we miss contributions of some low-frequency modes and that our calculation is restricted to harmonic potentials. A similar discrepancy was found by Myers and co-workers in their studies of E-HT65and recently Z-HT.82 Finally, it is worth noticing that the finding of minima on the excited-state potential energy surface of hexatriene at planar geometries is not limited to the excited singlet state but that planar minima also have been found for the lowest excited triplet state from results of time-resolved resonance Raman experiment^.^^^^^
Acknowledgment. We thank Professors J. P. Dah1 and 0. S. Mortensen for helpful discussions and Dr. K. B. Hansen for continual support with the equipment; Prof. Anne B. Myers for communicating yet unpublished results on 2-HT to us and for helpful comments; and Prof. Richard Mathies and his group for hospitality during the stay of B.A. in Berkeley. This work was supported partly by the Danish Natural Science Research Council and the Danish Academy of Sciences.
References and Notes (1) Hudson, B. S.; Kohler, B. E.; Schulten, K. In Excited Stutes; Lim, E. C., Ed.; Academic Press: New York, 1982; Vol. 6, p 1 . (2) Schulten, K.; Karplus, M. Chem. Phys. Lett. 1972, 14, 305. (3) Schulten, K.; Ohmine, I.; Karplus, M. J. Chem. Phys. 1976,64,4422. (4) Tavan, P.; Schulten, K. J . Chem. Phys. 1979, 70, 5407. (5) Nascimento, M. A. C.; Goddard, W. A. Chem. Phys. 1979, 36, 147. (6) Lasaga, A. C.; Aerni, R. J.; Karplus, M. J . Chem. Phys. 1980, 73, 5230. (7) Ohmine, I.; Morokuma, K. J. Chem. Phys. 1980, 73, 1907. (8) Dinur, U.; Hemley, R. J.; Karplus, M. J . Phys. Chem. 1983,87, 924. (9) Hemley, R. J.; Dawson, J. I.; Vaida, V. J. Chem. Phys. 1983,78,2915. (10) Said, M.; Maynau, D.; Malrieu, J.-P. J . Am. Chem. Soc. 1984, 106, 580. (1 1 ) Aoyagi, M.; Osamura, Y.; Iwata, S. J . Chem. Phys. 1985,83, 1140. (12) Goldbeck, R. A.; Switkes, E. J . Phys. Chem. 1985, 89, 2585. (13) Hemley, R. J.; Lasaga, A. C.; Vaida, V.; Karplus, M. J . Phys. Chem. 1988. 92. 945. ( 1 4 ) Zerbetto, F.; Zgierski, M. Z.; Negri, F.; Orlandi, G. J. Chem. Phys. 1988,89, 3681. (15) Cave, R. J.; Davidson, E. R. J . Phvs. Chem. 1988. 92. 614. (16) Cave, R. J.; Davidson, E. R. C h e k Phys. Lett. ’1988, 148, 190. (17) Gavin, R. M.; Risemberg, S.; Rice, S. A. J . Chem. Phys. 1973, 58, 3160. (18) Gavin, R. M.; Rice, S. A. J . Chem. Phys. 1974, 60, 3231. (19) Granville, M. F.; Kohler, B. E.; Snow, J. B. J. Chem. Phys. 1981,75, 3765. (20) Leopold, D. G.; Pendley, R. D.; Rcebber, J. L.; Hemley, R. J.; Vaida, V. J . Chem. Phys. 1984,81,4218. (21) McDiarmid, R.; Sabljic, A.; Doering, J. P. J. Am. Chem. Soc. 1985, 107, 826. (22) Chadwick, R. R.; Gerrity, D. P.; Hudson, B. S . Chem. Phys. Lett. 1985, 115, 24. ( 2 3 ) Buma, W. J.; Kohler, B. E.; Song, K. J . Chem. Phys. 1990,92,4622. (24) Buma, W. J.; Kohler, B. E.; Song, K. J . Chem. Phys. 1991,94, 6367. (25) Flicker, W. M.; Mosher, 0. A.; Kuppermann, A. Chem. Phys. Lett. 1977. 45, 492. (26) Doering, J. P.; Sabljic, A.; McDiarmid, R. J . Phys. Chem. 1984,88, 835. (27) Haugen, W.; Traetteberg, M. Acta Chem. Scond. 1966, 20, 1726. (28) Trztteberg, M. Acta Chem. Scund. 1968, 22, 2294. (29) Warshel, A.; Karplus, M. J . Am. Chem. Soc. 1972, 94, 5612. (30) Warshel, A.; Karplus, M. Chem. Phys. Lett. 1972, 17, 7 . (31) Petelenz, P.; Petelenz, B. J. Chem. Phys. 1975, 60, 3482. (32) Mains, G. J.; George, P.; Trachtman, M.; Brett, A. M.; Bock, C. W. J . Mol. Struct. 1977, 36, 317. (33) Warshel, A.; Dauber, P. J . Chem. Phys. 1977, 66, 5417. (34) Bock, C. W.; George, P.; Trachtman, M. J . Mol. Struct. (THEUCHEM) 1984, 109, 1 . (35) Bock, C . W.; Panchenko, Y.N.; Krasnoshchiokov, S. V.; Pupyshev, V. I . J . Mol. Strucr. (THEOCHEM) 1986, 148, 131. (36) Hemley, R. J.; Brooks, B. R.; Karplus, M. J. Chem. Phys. 1986,85, 6550. (37) Fogarasi, G.; Szalay, P. G.; Liescheski, P. P.; Boggs, J. E.; Pulay, P. J . Mol. Struct. (THEOCHEM) 1987, 151, 341. (38) Szalay, P. G.;Karpfen, A.; Lischka, H. J. Chem. Phys. 1987, 87, 3530.
4801
J . Phys. Chem. 1992,96,4801-4804 (39) Yoshida, H.; Furukawa, Y.; Tasumi, M. J . Mol. Struct. 1989, 194, 279. (40) Langkilde, F. W.; Wilbrandt, R.; Brouwer, A. M. J . Phys. Chem. 1990, 94, 4809. (41) Lippincott, E. R.; Kenney, T. E. J . Am. Chem. Soc. 1962,843641. (42) Furukawa, Y.;Takeuchi, H.; Harada, I.; Tasumi, M. J. Mol. Struct. 1983, 100, 341. (43) Panchenko, Y.N.; Csaszar, P.; T W k , F. Acra Chim. Hung.1983, 113, 149. (44) Langkilde, F. W.; Wilbrandt, R.; Nielsen, 0. F.; Christensen, D. H.; Nicolaisen, F. M. Spectrochim. Acra 1987, 43A, 1209. (45) McDiarmid, R.; Sabljic, A. J . Phys. Chem. 1987, 91, 276. (46) Srinivasan, R. J . Am. Chem. Soc. 1961,83, 2806. (47) Srinivasan, R. J. Am. Chem. Soc. 1962, 84, 3982. (48) Srinivasan, R. J . Chem. Phys. 1963, 38, 1039. (49) Datta, P.; Goldfarb, T. D.; Boikess. R. S. J . Am. Chem. SOC.1971, 93, 5189. (50) Vroegop, P. J.; Lugtenburg, J.; Havinga, E. Tetrahedron 1973, 29, 1393. (51) Orchard, S.W.; Thrush, B. A. Proc. R . SOC.London, A 1974,337, 257. (52) Orchard, S. W.; Thrush, B. A. Proc. R . SOC.London, A 1974,337, 243. (53) Jacobs, H. J. C.; Havinga, E. In Advances in Photochemistry; Pitts, Jr., J. N., Hammond, G . S.,Gollnick, K., Eds.; John Wiley: New York, 1979; Vol. 11, p 305. (54) Lewis, F. D.; Teng, P. A.; Weitz, E. J. Phys. Chem. 1983,87, 1666. ( 5 5 ) Lewis, F. D.; Weitz, E. Ace. Chem. Res. 1985, 18, 188. (56) Siebrand, W.; Zgierski, M. Z. In Excited Slates; Lim, E. C., Ed.; Academic Press: New York, 1979; Vol. 4, p 1. (57) Champion, P. M.; Albrecht, A. C. Annu. Reu. Phys. Chem. 1982,33, 353. (58) Champion, P. M. In Biological Applications of Raman Spectroscopy; Spiro, T. G., Ed.; Wiley & Sons: New York, 1988; Vol. 3, p 249. (59) Hudson, B.; Kelly, P. B.; Ziegler, L. D.; Desiderio, R. A.; Gerrity, D. P.; Hess, W.; Bates, R. In Advances in Loser Spectroscopy; Garetz, B. A.,
Lombardi, J. R., Eds.; Wiley: New York, 1986; Vol. 3, p 1. (60) Myers, A. B.; Mathies, R.A. In Biological Applications of Raman Spectroscopy; Spiro, T. G., Ed.; Wiley & Sons: New York, 1987; Vol. 2, p 1. (61) Ziegler, L. D.; Hudson, B. S. J . Chem. Phys. 1983, 79, 1197. (62) Sension, R. J.; Mayne, L.; Hudson, B. J . Am. Chem. SOC.1987, 109, 5036. 163) Sension. R. J.: Hudson. B. S. J . Chem. Phvs. 1989. 90. 1377. (64j Myers, A. B.; Mathies,'R. A.; Tannor, D. j.; Heller; E.'J. J . Chem. Phys. 1982, 77, 3857. (65) Myers, A. B.; Pranata, K. S . J . Phys. Chem. 1989, 93, 5079. (66) Ci, X.;Pereira, M. A.; Myers, A. B. J. Chem. Phys. 1990, 92,4708. (67) Langkilde, F. W.; Jensen, N.-H.; Wilbrandt, R. J. Phys. Chem. 1987, 91. 1040. (68) Langkilde, F. W.; Jensen, N.-H.; Wilbrandt, R.; Brouwer, A. M.; Jacobs, H. J. C. J . Phys. Chem. 1987, 91, 1029. (69) Negri, F.; Orlandi, G.; Brouwer, A. M.; Langkilde, F. W.; Wilbrandt, R. J. Chem. Phvs. 1989. 90. 5944. (70) Wilbraidt, R.; Langklde, F. W.; Brouwer, A. M.; Negri, F.; Orlandi, G. J. Mol. Struct. 1990, 217, 151. (71) Langkilde, F. W.; Wilbrandt, R.; Moller, S.; Brouwer, A. M.; Negri, F.; Orlandi, G. J. Phys. Chem. 1991, 95, 6884. (72) Nenri. - . F.:. Orlandi. G.: Brouwer. A. M.:. Lannkilde. . F. W.: Moller. S.;Wilbrandt, R. J. Phys.'Chem. 1991,'95, 6895. (73) Lee, S.-Y.; Heller, E. J. J. Chem. Phys. 1979, 71, 4777. (74) Heller, E. J.; Sundberg, R. L.; Tannor, D. J . Phys. Chem. 1982,86, 1822. (75) Tannor, D. J.; Heller, E. J. J . Chem. Phys. 1982, 77, 202. (76) Hwa, J. C. H.; de Benneville, P. L.; Sims, H. J. J. Am. Chem. SOC. 1960, 82, 2537. (77) Heller, E. J. J. Chem. Phys. 1975, 62, 1544. (78) Davis, M. J.; Heller, E. J. J . Chem. Phys. 1979, 71, 3383. (79) Minnaard, N. G.; Havinga, E. R e d . Trav. Chim. 1973, 92, 1179. (80) Sabljic, A.; McDiarmid, R. J . Chem. Phys. 1986, 84, 2062. (81) Zerbetto, F.; Zgierski, M. Z. Chem. Phys. Lett. 1988, 143, 153. (82) Ci, X.;Myers, A. B. J. Chem. Phys., submitted for publication.
HCN Dlmers: Photoelectron Spectrum of Iminoacetonitrile Richard A. Evans,lPSylvie M. Lacombe,lb Maryse J. Simon,lb Genevieve Wister-Guillouzo,lb and Curt Wentrup*.'" Department of Chemistry, The University of Queensland, Brisbane, Queensland, Australia 4072, and Laboratoire de Physico- Chimie Moldculaire, UA CNRS 474, Universitd de Pau, F-64000 Pau, France (Received: November 19, 1991; In Final Form: February 18, 1992)
The photoelectron spectrum of iminoacetonitrile (HN=CHCN), obtained by gentle pyrolysis of the sodium salt of 1cyanoformamide tosylhydrazone at 200 OC, is reported. The first (vertical) ionization potentials are 11.60, 12.34, 13.09, 13.84, and 14.51 eV. These are compared with those of N-cyanomethanimine (CH2=N-CN).
Introduction Iminoacetonitrile (HN=CH-CN) (1) has captivated the imagination of organic chemists for over a century.2 However, it is only recently that an efficient synthesis of this molecule has become a ~ a i l a b l e . ~It is of considerable interest as a potential interstellar and prebiotic molecule. Both HCN and HNC are widely distributed in interstellar c10uds.~ HCN, (CN),, and cyanoacetylene have been detected in the atmosphere of Titan,s and HCN and/or the CN radical on Halley and other comets and on the asteroid Chiron.6 Iminoacetonitrile is widely theorized to be the prebiotic HCN dimer which, via further reaction with HCN to give diaminomaleonitrile, adenine, and HCN polymer, can function as a building block for amino acids, purines, and pyrimidine^.^ We have recently described a high yielding synthesis3 of H N = C H C N (1) (as a mixture of 2 and E isomers ((Z)-l and (E)-1)by thermal decomposition of the sodium salt of 1-cyanoformamide tosylhydrazone (2) at 200 OC and reported the IR, NMR, and mass ~ p e c t r a . There ~ ~ ~ is another formal HCN dimer, N-cyanomethanimine (H2C=N-CN) (3), which has also been characterized by mass infrared,j and millimeter wave ~pectroscopy.~J~ The photoelectron (PE) spectrum of 3 has
been obtained from two different precursors (4 and 5) by Pfister-Guillouzo" and by Bock and Wentrupl* and their co-workers.
. N1
2
H,
c-k N
N3
800K
HzC=N
-2N,-
4
1E
1z
\ CN
880K
-N2
.CzH4 3
0 \
I
N
N=N 5
Herein we report the PE spectrum of 1 and compare it with that of 3. This data is of importance for (i) a full description of the molecules and (ii) investigations of gas-phase structures and thermochemistry of C2H2Nz'+ Experimental Section
The sodium salt of 1-cyanoformamide tosylhydrazone (2) was prepared as previously de~cribed.~
0022-3654/92/2096-480 1$03.OO/O 0 1992 American Chemical Society
