J. Phys. Chem. 1994,98, 1791-1795
1791
The Structure of Acetaldehyde in Its First Excited Singlet State: Experimental and Theoretical Investigations J. M. Price,? J. A. Mack, G. v. Helden, X. Yang,* and A. M. Wodtke’498 Department of Chemistry, University of California, Santa Barbara, California 93106 Received: September 7 , 1993; In Final Form: December 6, 1993@
Medium-resolution laser induced fluorescence excitation spectra of jet-cooled acetaldehyde and fully deuterated acetaldehyde have been recorded using a pulsed molecular beam apparatus. Analysis of the rotationally resolved SI.= SOorigin yields quantitative information regarding the structure for the S1 excited state of the molecule. Ab initio, self-consistent-field calculations have been performed as a check on the uniqueness of the fit obtained to the experimental data. In accord with previous predictions, the electronic excitation is found to be an a* .=n C-type (I)transition. This results in a loss of planarity in the CCOH group, and an extension of the CO bond length relative to the ground-state form. The position of the methyl hydrogens also changes from an eclipsed to a staggered conformation with the CO bond. The excited-state rotational constants for thevibrationless level are found to be A’= 1.596 f 0.007, B’= 0.334 f 0.005, and C’= 0.305 f 0.006 for the CH3CHO isotope and A’ = 1.036 f 0.007, B’ = 0.286 f 0.006,and C’ = 0.253 f 0.006 for the CD3CDO isotope.
Introduction The desire to understand the role of molecular structure in chemistry is arguably the central motivation of modern chemical research, and it is for this reason that so much effort in molecular spectroscopy has gone toward resolving rotational transitions, from which molecular structures can be experimentally derived. Two classes of molecules that have received a great deal of attention are the carbonyl-containing aldehydes and ketones. Because of their photochemical activity, the first excited singlet and triplet states of these molecules are particularly important. Surprisingly, one of the simplest members of this family of molecules, acetaldehyde or CH3CH0, has resisted detailed spectroscopic analysis of its first excited electronic state. As for essentially all aldehydes and ketones, the UV absorption system for acetaldehyde is assumed to be an A* (r n C-type transition, similar to that found in formaldehyde. This would give rise to a large extension of the CO bond in the excited electronic state and a puckering of the CCOH portion of the molecule from a planar to a pyramidal structure. Low-resolution data suggest that the excited-state CCOH group is nonplanar,’ and ab initio studies have also indicated that the nature of the excitation is analogous to that of formaldehyde.2 But, as a point of fact, there is little or no direct evidence for the A* n picture for this molecule. Resolved emission from molecular beam fluorescence excitation spectra has not produced the strong progression in the CO stretching vibration in the excited state that would unambiguously indicate a profound lengthetling of this bond. Further, heavy spectral congestion above Evib = 600 cm-l in the excited electronicstate makes assignmentsof the structure that is observed difficult. A determination of the excited-state structure would certainly help settle the question of the nature of the electronic excitation in this molecule. Unfortunately, because of rotational congestion in the spectrum, unambiguous assignments of rotationally resolved spectra have only been made for beam conditions where a single rotational state was populated. Molecular constants derived from such data are not accurate enough for an unambiguous structural determination.
* Author to whom correspondence should be addressed. Present address: SRI International, Molecular Physics Division, 333 Ravenswood Ave., Menlo Park, CA 94025. t Present address: Department of Chemistry c/o Y. T. Lee, University of California, Berkeley, CA 94270. 8 National Science Foundation Presidential Young Investigatorand Alfred P. Sloan Research Fellow. Abstract published in Advance ACS Abstracts, January 15, 1994. 0022-3654I94 12098-1791$04.50/0
In addition to providing information about the nature of the transition to the SIsurface, an accurate geometry for the excited state would be very helpful in understanding the complicated stimulated emission pumping (SEP) spectrum of this molecule. Preliminary results from our laboratory have shown a very high level of complexity in the spectrum of this molecule at state densities where similarly sized molecules (e.g. propynal) show comparatively simple and easily assigned emission spectra.4 Such spectral complexity could well be due to the acceleratinginfluence of methyl torsion on intramolecular vibrational redistribution5 (IVR), as has been observed for a number of other systems includingp-fluorotoluene.6 However,spectralcomplexityby itself does not provide sufficient evidence for IVR. All electronic emission spectroscopies(includingSEP) are subject to the effects of Duschinsky rotation,’ which can also give rise to complex emission spectra. This effect arises from the fact that the definitions of the normal coordinates of the excited and ground states for polyatomic molecules are, in general, different. The clearest example of this phenomenon is a linear-to-bent electronic excitation in a triatomic, where a bending vibration in the ground state transforms to a rotation in the excited state. This phenomenon can introduce substantial complexity into the emission spectrum of a polyatomic molecule even when the vibrations of both electronic states are free from anharmonic coupling effects. The Franck-Condon intensity distributions in such systems where a large geometry change occurs in the excited state can be quite complex. In order to be able to distinguish the effects of Dushinsky rotation from the more interesting effects of anharmonic mixing, it is necessary to obtain accurate equilibrium structures and force constant matrices for both electronic states. Such molecular properties may be taken conveniently from ab initio calculations if it can be shown that these methods deliver physically realisticinformation. Compared to ground states, the results of ab initio calculations for electronically excited states are in general less certain. The purpose of this paper is to present, at once, the most complete set of experimental data yet obtained which can be used to derive structural information about acetaldehyde’s first excited electronic state and to test if ab initio calculationsare of the required quality to be used for the characterization of the Duschinsky effect in acetaldehyde. To this end, we have recorded rotationally resolved fluorescence excitation spectra for acetaldehydeand its fully deuterated analog seeded in a pulsed molecular beam. Under the conditions of the present study, a sufficient number of rotational transitions are 0 1994 American Chemical Society
Price et al.
1792 The Journal of Physical Chemistry, Vol. 98, No. 7,1994
observed to obtain improved rotational constants and information concerning the structure of the first excited state.
Acetaldehyde Excitation Spectrum 2.10 1.89
Experimental Section
1.68
Acetaldehyde (Aldrich 99.9%) or its deuterated derivative (Alderich,98.9%) was purified by several cycles of freeze-pump thaw vacuum distillation prior to being used in a 1% mixture with He carrier gas (Spectra Gasses 99.995). A pulsed and seeded molecular beam expansion was formed through the 0.75-mm nozzle of a General valve operating at 10 Hz. The chamber was pumped by a 6-in. diffusion pump. Backing pressure of 40 psig lead to a static chamber pressure of 5.5 X Torr during operation. Approximately 1 cm downstream from the nozzle the pulsed molecular beam was crmed with the output of an excimer pumped dye laser (Lambda Physik FL3002, spectral resolution 0.22*cm-I, pulse duration 20 ns, pulse energy 2 mJ) timed to interact with the rising edge of the gas pulse. Fluorescence from the excited molecules was detected at right angles to the crossing plane by a high-sensitivity photomultiplier tube (Hamamatsu R212UH). An f/2 imaging scheme employing a 3-in. focal length UV Quartz convex lens was used to improve the fluorescence collection efficiency. Scattered light from the laser was minimized by a combination of Brewster windows, light baffles, and a laser cutoff filter in front of the phototube. The signal from the photomultiplier tube was processed by a combinationof a home-built, X 10,fast preamplifier and a boxcar averager (Stanford Research, SR250) with a gate width of 100 ns for CHJCHO and -500 ns for CD3CDO. Output from the boxcar averager was stored on an IBM PC/AT compatible computer. Frequency calibration of the laser was accomplished with an Fe/Ne hollow cathode lamp, recording the lamp’s Optogalvanic spectrum. This allowed absolute wavelength determination with an uncertainty less than f l cm-I. Uncertainties in the relative frequenciesof peak maxima are estimated from the optogalvanic spectrum to be 10.015 cm-I.
-
44 I
w4
0.00 29763.0
I
n
29767.0
29771.0
29775.0
29779.0
29783.0
Frequency (cm-1)
F’lgure 1. Rotationally resolved molecular beam excitation spectra of acetaldehyde(CHICHO): (A) observed spectrum; (C) stick spectrum determined by optimization with respect to 42 assigned transitions; (B) convolution of stick spectrum with a Lorentzian function and intensity calculation assuming a C-type band. Acetaldehyde Excitation Spectrum
2.10 1.89
I
4
0.84 0.63 0.42 0.21
Results and Discussion The molecular structure of acetaldehyde presented here was derived in three steps. First, an accurate determination of the rotational constants for the different isotopes was made from rotationally resolved fluorescence excitation spectra. This determinationinvolveda fit of the observed transitions to a simulation calculated from a rigid-rotor asymmetric top Hamiltonian. For this multidimensional problem, the method of simplex optimizationg was used to find the best fit to the observed transitions. Next, the molecular structure was adjusted using a similar procedure to fit the optimized rotational constants obtained in the first step for both isotopes. Finally, as a test of the uniqueness of the fit obtained, ab initio SCF calculationswere performed for comparison. No attempt was made to “tailor” either the output of the ab initiocalculationorthe data analysis to obtainagreement. On the contrary, in this case we have made a blind comparison between experimentalmeasurementsand theoretical predictions. The rotationally resolved fluorescence excitation spectra of the origin bands for the two isotopes of acetaldehyde appear in Figures 1 and 2. In the CD3CDO spectrum, an overlappingband from a small amount of CH3CHO contamination (the 14; band; reported origin 29 804 cm-l 3) in the sample is present which complicatesthe appearance of the spectrum. (Features assigned to contaminants are marked with an asterisk.) Features from the CD3CDO spectrum which went into the structure determination were limited to those on the high-frequencyside of the band contour, where there was minimal contribution from the band of the lighter isotope. Rotational assignments of the observed transitions and calculated values for their transition frequencies appear in Tables 1 and 2. Our excited state and previously
0.00 89809.00
29811.00
29813.00
29815.00
29817.00
29819.00
Frequency (cm-1)
Figure 2. Rotationally resolved molecular beam excitation spectra of
acetaldehyde (CD3CDO): (A) observed spectrum; (C) stick spectrum determined by optimization with respect to 18 assigned transitions; (B) convolution of stick spectrum with a Lorentzian function and intensity calculationassuming a C-type band. (Peaksattributed to contamination by CH3CHO are marked with an asterisk.) published ground-state geometrical parameters for the molecule are given in Table 3. Rotati0~1Assignments and Determination of Constants. The ground-state structure and rotational constants of acetaldehyde and all of its deuterated isotopes have been well characterized by microwave and infrared spectroscopy.10 A near prolate asymmetric top, acetaldehyde has an asymmetry parameter K of -0.9545.11 In our determination of the excited-state constants, the ground-state constants were held fixed at values established in the literature (A = 1.887, B = 0.339, and C = 0.303).’0 Figure 1A shows the excitationspectrum obtained for the CH3CHO origin band. For these measurements, the expansion conditions were adjusted to obtain a “warmer” molecular beam environmentthan has been used in previous studies. The warmer temperature of the molecular beam allows for the measurement of numerous transitions from which accurate constants may be derived. Transitions have been assigned from ground state J levels as high as J = 6. The rotational temperature for molecules in the beam was found to be 3 K in the J quantum number and 13 K in the K, pseudoquantum number.12
Acetaldehyde in Its First Excited Singlet State
The Journal of Physical Chemistry, Vol. 98, No. 7, I994
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TABLE 1: Measured and Calculated Asymmetric Rotor Transitions for Acetaldehyde (CH3CHO) (SI So c-Type (I) Band, 0; Vibronic Transition) ground state a”
b” C”
Rotational Constants (cm-l) excited state 1.881 0.339 0.303
a’
b’ c’
29 164.14 29 164.22 29 164.84 29 166.11 29 161.49 29 168.20 29 168.88 29 169.55 29 169.58 29 769.62 29 169.89 29 110.49 29 110.19 29 111.11 29 111.39 29 112.36 29 112.21 29 112.33 29 113.03 29 113.10 29 114.31 29 115.06 29 115.90 29 115.94 29 116.50 29 116.61 29 111.08 29 111.29 29 111.64 29 118.24 29 118.24 29 118.86 29 779.48 29 719.49 29 119.98 29 119.98 29 180.60 29 180.60 29 181.13 standard deviation: 0.031
a
0.04 -0.04 0.01 -0.01 0.03 0.06 0.04 0.05 0.02 -0.02 0.04 0.02 0.03 0.03 0.06 -0.01 0.08 0.02 0.00 0.03 0.05 0.05 0.02 -0.02 0.04 -0.07 -0.01 -0.04 -0.04 -0.01 -0.01 -0.03 -0.04 -0.05 0.02 0.02 -0.02 -0.02 0.04
Band origin 29 171.10 cm-’.
Figure 1C shows a stick spectrum calculated using the rigidrotor asymmetric top Hamiltonian to find the frequenciesfor the spectroscopictransitions. This Hamiltonian has the form 1 1 H = ?(A + c)J2 + $A - c) h ( K )
where
h ( K ) = J,Z
TABLE 2: Measured and Calculated Asymmetric Rotor Transitions for Acetaldehyde (CDfcM)) (SI So c-type (I) Band, 0: Vibronic Transition) ground state
1.596 0.334 0.305
Measured vs Calculated Transitions (*Type Band)@ transition obsd (vac. cm-l) Calcd A (obsd - calcd) 29 164.18 29 164.18 29 164.85 29 166.16 29 161.52 29 168.26 29 168.92 29 169.60 29 169.60 29 169.60 29 169.93 29 110.5 1 29 710.82 29 111.14 29 111.45 29 112.35 29 112.35 29 112.35 29 113.03 29 113.13 29 114.42 29 775.11 29 115.92 29 115.92 29 116.54 29 116.54 29 111.01 29 111.25 29 111.60 29 118.23 29 118.23 29 118.83 29 779.44 29 119.44 29 180.00 29 780.00 29 180.58 29 180.58 29 181.17
-
+ ,162 -J,Z K
J, Ja, Jb, and Jc are the total angular momentum operator and the projection operators of the total angular momentum on the a, b, and c molecular axes.13J4 K is Ray’s asymmetry parameter. Expanding this Hamiltonian in the prolate top basis set and diagonalizing yields the eigenvalues for a given electronic state of the asymmetric top. After the allowed transitions for a given band type (in this case a c-type transition) are determined, the relative positions of the transitions may be found by simply taking the energy difference between theappropriateground- and excitedstate eigenvalues. The intensities of the spectroscopictransitions were found by transforming the symmetric top direction cosine matrices (which
1793
a
’’
b” C”
Rotational Constants (cm-l) excited state 1.16116 0.28613 0.25122
1.036 0.286 0.253
a’
b‘ c’
Measured vs Calculated Transitions (C-type Band)@ transition obsd (vac. cm-I) calcd A (obsd - calcd)
-.
312 404 000-110 413 50s 101 211 312 322 313 321 303 413 110’220 111 221 312 422 50s 61s 313 423 413 523 221 331 220 330 514 624 321 431 51s 625
----+
+
-+
-
+
+
@
29 810.01 29 810.30 29 810.51 29 810.89 29 811.04 29 811.23 29 8 12.08 20 812.20 29 812.20 29 813.16 29 813.35 29 813.35 29 813.63 29 813.86 29 813.86 29 814.10 29 8 14.41 29 814.68
29 810.09 29 810.21 29 810.55 29 810.84 29 811.02 29 811.23 29 812.04 29 812.18 29 812.21 29 813.19 29 813.33 29 813.38 29 813.68 29 813.88 29 813.81 29 814.18 29 814.42 29 814.63 standard deviation: 0.031
-0.02 0.03 0.03 0.05 0.02
0.00 0.03 0.02 -0.02 -0.03 0.02 4-04 -0.05 -0.02 -0.02
-0.08 0.05 0.05
Band origin 29 808.95 cm-’.
give rise to the familiar Hoenl-London factors for symmetric tops) to the same basis that diagonalized the asymmetric top Hamiltonian matrices for the ground and excited states. This transformation yields the asymmetric top direction cosine matrix for given ground- and excited-state K’S. The elements of this matrix, when squared and multiplied by the appropriate Boltzmann weight for a given temperature yielded the intensities of the transitions between the two states connected by each matrix element. The constants which go into these calculations include the rotational constants for the ground and excited states, the electronic band origin, and the rotational temperatures in J and K,. Since the ground-state constants are well-known, the frequencies of assigned transitions from the experimental data could be fit by varying the excited-state rotational constants. In this work we made use of the simplex9optimization scheme for multidimensions to solve for the excited-state rotational constants. Here, the calculation described above for finding the transition frequencies was iteratively performed, and the constants for the calculation were systematically varied using the simplex algorithm to minimize the x* of the fit between the positions for the calculated lines and the set of 40 assigned lines from the excitation spectrum. In any multidimensional search, local minima in the error function can be a problem. Our approach in this case was to limit the phase space of rotational constants to be searchedand to use numerous starting configurations.Values sampled by the simplex algorithm were restricted to f20% of the reported values of Nobel and Lee, and 50 randomly selected starting configurations within this interval were chosen. In this way the values of the rotational constants were derived which best fit the observed transitions. The standard deviation of the fit to the spectrum for CH3CHO was found to be 0.035 cm-1. Figure 1B shows a convolution of the calculated spectrum by a Lorentzian line width function to simulate the appearance of the experimentaldata. The convoluted calculationwas especially helpful in making rotational assignments as the spectrum is particularly congested. The quality of the fit obtained in this way using a pure c-type formalism for the transition shows that
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1794 The Journal of Physical Chemistry, Vol. 98, No. 7, 1994
Price et al.
TABLE 3 Geometrical Parameters for Acetaldehyde in Its Ground and First Excited States experimental (this work)“ theoretical (this work) Bond Lengths (A) C=O 1.343 f 0.01 1.363 1.499 C-C 1.500 k 0.01 1.106.f 1.080 C-H (aldehydic) 1.0871 1.085 C-H (methyl) C-C-O H-C (aldehydic) with CCO plane C-C-H (methyl)
Bond Angles (deg) 115.2 f 0.2 32 f I 110.6.f
114.0 38 110.6e
H-C-H (methyl) C-C-H (aldehydic)
108.q ll2f6
108.Y 119.7
accepted around-state valueb
*
1.213 0.010 1.504 f 0.010 1.106 0.010 1.091 0.0056 1.085 zk 0.005d 124.05 A 0.5
0.0 110.6 f 0 3 110.3 O S d 108.9 0.5 114.9 0.5
*
Dihedral Angles (deg) H(ald)CO-COC(aldehydic out-of-plane angle) 134k8 139 0 H(meth)CC-CCO(methyl torsional angle) 55.98 55.9 0 a The optimized structurethat best fit both isotopes gave A = 1.597 (1.596), B = 0.338 (0.334), and C = 0.300 (0.304) for CH3CHO and A = 1.039 (1.036), B = 0.288 (0.286), and C = 0.247 (0.253) for CDaCDO compared to the measured values determinedseparatelyfrom the spectra in parenthe-scs. Harmony, M. D.; Laurie, V.W.; Kuczkowski, R. L.; Schwendeman, R. H.; Ramsay, D. A.; Lovas, F. J.; Lafferty, W.J.; Maki, A. G.J. Phys. Chem. Ref. Data 1979, Vol. 8 No.3, 619. For eclipsed methyl hydrogen. For noneclipsed methyl hydrogen. Average of three slightly different values. fGround-state value used in fit. 8 Ab initio value from present work used in fit. the origin band, unlike many of the vibronic transitions involving excited vibrational states, is nearly a pure C-type transition. A similar analytical procedure was followed for the fully deuterated isotope, where a standard deviation of 0.036 cm-l to the fit was obtained for 20 assigned transitions. Uncertainties reported in these constants are the change in each constant required to degrade the standard deviation to the fit by a factor of 2. Experimental Determination of the Structure. The attempt to derive the excited-state structure of the molecule involved a “forward convolution”scheme similar to the one described above for the determination of the rotational constants. In this case, a comparison was made between the rotational constants calculated from a randomly selected structure to those derived above from the fit to the spectra of both isotopes. Since an insufficient number of isotopes were used to directly obtain the positions of all of the atoms in the molecule, a rigorous structural determination is not, strictly speaking, possible. Nevertheless, quantitative information regarding the excited-state structure can beobtained from the sixrotationalconstants of the twoisotopes if one employs some chemically reasonable assumptions. Certain assumptions would be required of any optimization scheme: to avoid sampling nonphysical regions of structural phase space in the searching algorithm, constraints have to be placed on the bond lengths and angles that the optimization routine is allowed to sample. The assumptions used here involve the notion that the change in geometry is localized for the most part on the CCOH portion of the molecule. The bond lengths and angles of the methyl group are not expected to be much affected since the ?r* n electronic excitation is mostly associated with the C = O chromophore. Consequently, we have fixed the relative positions of the methyl carbon and the three methyl hydrogen atoms at their ground-state values. A similar argument holds for not varying the aldehydic C-H bond length. In formaldehyde, where a complete structural determination of the excited state has been made, it was found that the aldehydic C-H bond length changed only by as little as 0.002 & * 5 well within the error of this work. So it seems reasonable to assume that the aldehydic C-H bond length in acetaldehyde also remains essentially unchanged in the excited state. With the aforementioned assumptions, only four chemically significant structural parameters remain, two bond lengths and a bond angle, which define the position of the heavy atoms, and an out-of-heavy-atom-plane bending angle for the aldehydic
hydrogen. We had originally hoped to derive information about the methyl torsional angle; however, the data are insensitive to this degree of freedom, and we have fixed this parameter at the results of our ab initio calculation (please see below). All theoretical and experimental information suggests that this degree of freedom has a slightly higher barrier to rotation (668 cm-l) than that of the ground state and that the equilibrium structure is staggered with respect to the C=O bond axis. We also derive a staggered configuration in our ab initio calculations. As stated above, the method for determining the structure is analogous to the derivation of the rotational constants. For a given structure, diagonalization of the moment of inertia tensor for. each isotope yielded the rotational constants. The simplex optimization scheme was used to vary the internal coordinates of the molecule to achieve the best agreement between the fit and the experimentally derived constants. Again, local minima in the error function (x2of the fit between the constants from the diagonalization of the moment of inertia tensor and those derived from the spectrum) can be a problem in the determination unless numerous starting configurations are used for the optimization and the bond lengths and angles are restricted. In this case, over 30 starting configurations were selected with the bond lengths restricted to f- 1&20% of their ground-state values and the bond angles limited to f20-40%. A weighting factor was used in the simplex fitting to the constants, giving more weight to the constants of the lighter isotope. This was a statistical factor, reflecting the difference in the number of assigned transitions between the two excitation spectra. Again, the reported error limits are the change in a parameter necessary to degrade the standard deviation of the fit by a factor of 2. Table 5 lists the derived structural parameters and compares these quantities to the theoretical results (please see below) and measured ground-state values. An important question which can now be addressed is the fundamental nature of the electronic transition. We find that there is strong evidence for a localized excitation and that the changes in the geometry are consistent with an expected T * += n transition. These data are most sensitive to the positions of the heavy atoms as they have the largest effect on the moment of inertia. These results show immediately that the aldehydic CCOH group is pyramidal since the CCO angle is much reduced from the 120’ expected for sp2 hybridization of the central carbon atom. The angle of the aldehydiccarbon-hydrogen bond also provides support for this notion, making an angle of 32O with respect to the plane of the three heavy atoms. Further support for the T* n nature
Acetaldehyde in Its First Excited Singlet State of the transition is found in the fact that the C = O bond is significantly lengthened over that of the ground electronic state ( 1 . 3 4 ~1.21 A). Our assumption that the transition is localized on one end of the molecule also seems to find support in that the change in the C-C bond length is less than +1.2% whereas the C=O bond length changes by +10.2%. This offers some validation for leaving the positions of the atoms in the methyl group at their ground-state values in the fitting procedure. Ab Initio Calculation of the Structure. In order to help shed light on the uniqueness of this structure and to test the capability of ab initio theory to calculate excited states of medium-sized polyatomics, ab initio calculations of the excited electronic state of both acetaldehyde and propynal were carried out at the SCF level (restricted open shell hartree-fock, 6-31G(d,p) basis with full geometry optimization using the GAUSSIAN 92 program). The calculations for the excited state of propynal using this basis set were compared to previous experimenW and gave results for the structure of the first excited state accurate to within 0.03 A in the bond lengths and 3 O in the bond angles. The results from the ab initio calculation for acetaldehyde are presented in Table 3 and compare quite favorably with those derived from the observed emission spectra, falling within the stated error bars of the experimental values. This agreement offers support for both the experimentally derived structure and the limited set of assumptions used regarding the localized nature of the excitation discussed above. The calculations show that acetaldehyde exhibits a pyramidal structure for the CCOH portion of the molecule in the SIexcited state and that the methyl hydrogens move to a staggered configuration relative to the C=O bond. In theground state, the molecule has a planar CCOH group and an eclipsed conformation for the methyl hydrogens. These results further support the work of Noble and Lee,’ whose analysis of the lowresolution fluorescence excitation spectrum first suggested that the CCOH group was nonplanar.
Conclusions This paper presents new measurements which support the notion that the acetaldehyde SI+ SOabsorption is an u* .+ n C-type (I) transition. It is the first experimental work which offers bond lengths and angles for the excited state of the molecule. The CCOH portion of the molecule is shown to be nonplanar, and the C = O bond is substantially lengthened from its ground-statevalue. Excited-state calculations at the SCF level (restricted open shell HartreeFock) were found to be in good agreement with experiment. This work also shows the high quality of HFSCF level calculations in determining structural properties of acetaldehyde in its first excited state. This information will be extremely useful in the analysis of forthcoming stimulated emission pumping experiments on acetaldehyde.
The Journal of Physical Chemistry, Vol. 98, No. 7. 1994 1795
Acknowledgment. A.M.W. wishes to thank the Alfred P. Sloan Research Foundation and the Camille and Henry Dreyfus Foundation. The workwas supported by the Petroleum Research Fund Grant No. 2403 1-AC6, the National Science Foundation Presidential Young Investigator Award CHE-8957978, and the National Science Foundation Atmospheric Chemistry Division Grant ATM-8922214. This work was also made possible by the Santa Barbara Laser Pool under NSF Grant No. CHE-8411302. Special thanks go to Professor M. T. Bowers for the use of an IBM RISC station. References and Notes (1) Baba, M.; Hanazaki, I.; Nagashima, U. J . Chem. Phys. 1985,82, 3938. ( 2 ) Baba, M.; Nagashima, U.; Hanazaki, I. J. Chem. Phys. 1985,83, 3514. (3) Noble, M.; Lee, E. K. C. J. Chem. Phys. 1984,81, 1632. (4) Rogaski, C. A.; Price, J. M.; Mack, J.; Yang, X.;Wodtke, A. M. J . Chem. Phys., submitted for publication. (5) Price, J. M.;Mack, J. A.; Wodtke, A. M. Siimulaied Emission Pumping Spectroscopy of Acetaldehyde from 0 io 4000 c m l ; Manuscript in preparation. (6) Longfellow, R. J.; Parmenter, C. S.J . Chem. Soc., Faraday Trans. 1988,84,1499. (7) Sharp, T. E.; Rosenstock, M. M. J . Chem. Phys. 1964,4I,11,3453. Duschinsky, F. Acta Physicochim URSS 1937, 7, 551. (8) Determined from the narrowmt isolated line of the acetaldehyde spectrum (2, 110). (9) A description of the simplex method of optimization and helpful exampleprograms may be found in: Press, W. H.; Flannery, B. P.; Teukolsky, S.A,; Vetterling, W. T. Numerical Recipes in Pascal; Cambridge University Press: Cambridge, U.K., 1986. (IO) Klib, R. W.; Lin, C. C.; Wilson, E. B., Jr. J. Chem. Phys. 1957,26(6), 1965. Iijima, T.; Kimura, M. Bull. Chem. Soc. Jpn. 1969,42, 2159. (1 1) Ray’s asymmetry parameter is defined as
-
(2B - A - C) (A-C) which becomes -1 for a prolate symmetric top (B = C) and +1 for an oblate symmetric top (B = A ) , varying between these two values for the asymmetric cases: Ray, B. S.Z . Physik 78, 74. (12) Asymmetric top energy levels are labeled by the J quantum number and the limiting prolate and oblate K quantum numbers for symmetric tops: JK,A, whereK. and Kcare the prolate and oblate limiting Kquantum numbers, respectively. E.g., 321 is a level of an asymmetric rotor which for a prolate top would correspond to J = 3,K = 2 and for an oblate top would correspond to J = 3, K = 1. K is not a good quantum number for an asymmetric rotor. (13) Kroto, H.W. Molecular Roiaiion Spectra; John Wiley and Sons, Ltd.: New York, 1975. Townes, C. H.; Schawlow, A. L. Microwave Spectroscopy; Dover Publications: New York, 1975. (14) King, G.W.; Hainer, R. M.; Cross, P. C. J . Chem. Phys. 1943,II, 27. Cross, P. C.; Hainer, R. M.; King, G. W. J. Chem. Phys. 1944,12,210. (15) Clouthier, D. J.; Ramsay, D. A. Annu. Rev. Phys. Chem. 1983, 34, 31. (16) Brand, J. C. D.; Callomon, J. H.; Watson, J. K. G. Discuss. Faraday Sot. 1963,35, 175. Lin, C. T.; Moule, D. C. J. Mol. Spectrosc. 1971,38, 136. Williams, D. R. J . Chem. Phys. 1971,55, 4578. K =
