2877
J. Phys. Chem. 1991, 95, 2877-2881 was assessed in the form of the projection xl of u, onto the collision trajectory, y. These factors are incorporated into a collisional coupling term, Cif,for transfer from level i to level f.
ci, = CWYrI(Xay)*A”’ Y
Here w., is the geometrical weighting factor for collision trajectory y (shown in Figure 10). The collisional coupling term takes the place of the sum over surface atom displacements in the vibrational matrix element and fits into the propensity rules as another multiplicative factor. In applying this model to pDFB, we assume the geometrical shape drawn for benzene. Figure 10 shows the representative collision trajectories chosen for pDFB by direct comparison to those chosen for benzene by computer simulation. Simple symmetry arguments were used to translate the “benzene trajectories” into “pDFB trajectories”. Consider the collision trajectories BH and BF (degenerate trajectories in the benzene case), both out-of-plane trajectories impinging on the carbon-carbon bonds as shown in Figure 10. These clearly have different weighting factors and, in fact, w(BF) = 2w(BH) because there are twice as many “targets” for BF than for BH. The sum of w(BF) and w(BH) should equal the value of
w(B) for benzene, Le., w(B) = 0.15. Thus, w(BH) and w(BF) are 0.05 and 0.10, respectively. Collision type B in benzene affects the doubly degenerate mode 10 and xFo is 0.34. Mode 10 is not degenerate in pDFB but splits into u9 (loa) and ~ 1 (lob). 7 They will be affected by ccllision trajectories BF and BH, respectively. Normal vibrations 9 and 17 are shown in Figure 6. A collision trajectory impacting pDFB on the carbon-carbon bond (where both carbons are attached to a hydrogen) may induce atomic displacements along normal mode 9 while not inducing displacements along normal mode 17. The converse is true for a trajectory impacting the pDFB on a carbon-carbon bond where one carbon is attached to a fluorine, Bp Thus, we assign the following projections for pDFB. = 0.34 xp = 0.00
xf%= 0.00
xrf
= 0.34
The trajectory projections for the other normal modes shown in Figure 10 were determined in a comparative manner and are collected in Table IX. The resulting collisional coupling terms for Av = 1 changes in all 30 normal modes are given in Table VI. Registry No. pDFB, 540-36-3; Ar, 7440-37-1; He, 7440-59-7.
Intramolecular Hydrogen Bond in the Ground and Excited Electronic States of 2-Hydroxyquinoline. A Study Using High-Resolution Laser Spectroscopy A. Held, D. F. Plusquellic, J. L. Tomer, and D. W. Pratt* Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received: October 9, 1990)
2-Hydroxyquinoline (2HQ) is known to exist in two tautomeric forms, the lactim (“enol”) and lactam (“keto”), which exhibit different electronic spectra. Reported herein are spectra of the isolated molecule at full rotational resolution which show (1) that the higher frequency electronic origin is that of the lactim form, (2) that in the ground electronic state (So) the more stable rotamer of the lactim has an 0-H bond that is cis with respect to the naphthalene frame, (3) that this rotamer exhibits an out-of-plane intramolecular hydrogen bond involving the OH group and the ring nitrogen, and (4) that on electronic excitation of this rotamer to the SIstate there is only a small shift of the hydroxy hydrogen doward the nitrogen atom. Additionally, it is found (5) that the SI S, transition is mainly long-axis polarized, demonstrating that the SIstate is principally m* in character. The interpretations of these results and their implications for the acid-base properties of 2HQ in both electronic states are discussed.
-
Introduction In the introduction to their classic text, The Hydrogen Bond, Pimentel and McClellan state that “it is hardly an exaggeration to say that in the chemistry of living systems the hydrogen bond is as important as the carbon-carbon bond.”’ Later, describing the application of different spectroscopic techniques to hydrogen-bonded substances, they note that “it is to be expected that the UV-visible spectrum of a molecule may be altered by the formation of a hydrogen bond if the chromophoric portion of the molecule is perturbed.”* And, still later, they point out that changes in such spectra “furnish the only experimental evidence available concerning the hydrogen-bonding properties of electronically excited state^."^ These remarks are as true today as they were 30 years ago. Thus, in honoring the memory of George Pimentel with this special issue, it seems appropriate to focus on one molecule in which there is an intramolecular hydrogen bond, (1) Pimentel, G. C.; McClellan, A. L. The Hydrogen B o d , W . H. Freeman: San Francisco, 1960; p 9. (2) Pimcntel, G. C.; McClcllan, A. L. The Hydrogen Bond; W . H . Freeman: San Francisco, 1960; p 157. (3) Pimentel, 0.C.; McClellan, A. L. The Hydrogen Bond; W. H . Freeman: San Francisco, 1960; p 164.
0022-3654/91/2095-2877$02.50/0
2-hydroxyquinoline, and to show how the properties of this bond in both the ground and an excited electronic state are revealed by high-resolution optical spectroscopy in the gas phase. 2-Hydroxyquinoline (2HQ), a weak base in its electronic ground state, is known to exist in two tautomeric forms, the lactim (2quinolinol, I) and the lactam (2-quinolinone, 11). The two forms
..
@Qr=m U
I
I1
are related by H-atom transfer from or to the oxygen atom to or from the ring nitrogen. Additionally, the lactim exhibits rotational isomerism, there being two possible orientations of the 0-Hbond with respect to the naphthalene frame, cis-(Ia) and tray-(Ib). H
Ia
m
Of the two tautomers, the lactim (I) is slightly (- 1 I~J/mol)~ more 0 1991 American Chemical Society
Held et al.
2878 The Journal of Physical Chemistry, Vol. 95, No. 7, 1991
stable in the gas phase, and the lactam (11) is significantly more stable in solution, especially in polar solvents (-20 kJ/mol in ~ a t e r ) owing ,~ to the stabilization of a zwitterionic resonance structure. Of the two rotamers of the lactim, Ia, with the potential to form an intramolecular hydrogen bond involving the ring nitrogen, should be more stable than Ib in the electronic ground state of the isolated molecule. Electronic excitation of 2HQ could produce significant changes in the properties of these isomers. Both mr* and n7r* excitations are possible. mr* excitation of the lactim could, as in 2hydroxynaphthalene (2HN),6 markedly increase the acidity of the hydroxy group, whereas the heterocyclic nitrogen in 2HQ is expected to become a stronger base in the mr* state. Intramolecular proton transfer might result. The m*state of the lactam should lie at lower energy than the corresponding state of the lactim. And n?r* excitations could have very different consequences, in both tautomeric and rotameric forms. The putative intramolecularly bonded hydrogen atom, located between the two functional groups in Ia, is in a unique position to sense these effects, and to move in response to subtle changes in the neighboring electronic distributions caused by the absorption of light. Nimlos et al.’ have examined the absorption and emission spectra of 2HQ in the collision-free environment of a supersonic jet. Two electronic origins were observed, at 29 112 and 31 349 cm-I, and assigned to the lactam and lactim forms, respectively. The authors’ assignments were based primarily on similarities between the vibrational structure built upon the higher energy origin, in both absorption and emission, and the corresponding vibronic features in the spectrum of 2HN. In this report, we describe high-resolution studies of the higher frequency band in a molecular beam which demonstrate, unambiguously, that it is the electronic origin of the cis-lactim. Our strategy is the same as that employed in a recent study of 1/2HN.8 We base our assignment on a comparison of the rotational constants of the zero-point vibrational levels of 2HQ and 2DQ, the hydroxy-deuterated 2-hydroxyquinoline. These constants are obtained from analyses of fully resolved spectra of this band in both molecules. The absolute center-of-mass positions of the substituted atom in both electronic states are determined from this comparison. We also determine the polarization properties of this band from the rotational analysis. The data provide new information about the hydrogen-bonding properties of 2HQ in both its ground and electronically excited states. Experimental Section Both the low-resolution supersonic jet and high-resolution molecular beam spectrometers have been described el~ewhere.~.~ In the high-resolution experiments, the sample was heated to 300 “C in a quartz nozzle, expanded in 500 Torr of argon through a 240-pm pinhole, and excited by the intracavity doubled CW ring dye las& either 10 or 100 cm downstream of the nozzle. A typical laser power was 0.5 mW. 2HQ was obtained from Aldrich and used without further purification. 2DQ was prepared by exchange of 2HQ with D20. The isotopic purity of the 2DQ sample was 98+%. Results The rotationally resolved fluorescence excitation spectrum of the 0: band in the SI So transition of 2HQ, located at 31 339.846 f 0.002 cm-I, is shown in Figure 1. Also examined was the isotopically substituted form of 2HQ (2DQ) at 3 1 339.802
-
A
B
I
939538300
959543000
II
1 I ll
C
939540830
-
939542080
FREQUENCY (MFz)
Figure 1. Rotationally resolved fluorescence excitation spectrum of the 0;band in the SI So transition of the cis enol form of 2-hydroxyquinoline. Panel A shows the overall experimental band contour, which spans approximately 7 cm-’. Panel B is an intermediate expansion of A. The strong sets of lines are the K subband Q-branch transitions with K” = 3 (right) through K”= 8 (left). The bottom panel shows an expanded portion of the Q branch (K”= 6) at full experimental resolution, and the corresponding computer simulation. f 0.002 cm-’. Deuterium substitution of the hydroxy hydrogen produces a red shift of 0.044 cm-’. Parts A, B, and C of Figure 1 show the rotationally resolved spectrum of 2HQ in three successive horizontal scale expansions. Panel 1A is the overall experimental spectrum consisting of over 2000 resolved lines spanning approximately 7 cm-I. Panel 1 B is an intermediate expansion of 1A showing resolved K-family substructure in the Q-branch (N = 0, AK = 0). The rotationally resolved spectra of 2HQ and 2DQ were analyzed using a rigid rotor model following previously described fitting strategies.1° The Hamiltonian for a rigid, near prolate top is
with (4) Beak, P. Acc. Chem. Res. 1977, 10, 186. (5) Cook,M. J.; Katritzky, A. R.; Linda, P.; Tack, R. D., J. Chem. Soc., Perkin Trans. 2 1973, 1080. (6) Ireland, J. F.; Wyatt, P. A. H.Ado. Phys. Org. Chem. 1976, 12, 131. (7) Nimlos, M. R.;Kelley, D. F.; Bernstein, E. R. J. Phys. Chem. 1987, 91, 6610. (8) Johnson, J. R.; Jordan, K. D.; Plusquellic, D. F.; Pratt, D. W.J. Chem. Phys. 1990, 93, 2258. ( 9 ) Majewski, W. A.; Plusquellic,D. F.; Pratt, D. W. J. Chem. Phys. 1989, 90, 1362.
A,=A-(B+C)/2 Aj = ( B
+ C)/2
a = ( B - C)/2 (10) Champagne, B. B.; Pfanstiel, J. F.; Plusquellic, D. F.; Pratt, D. W.; van Herpen, W. M.; Meerts, W. L. J. Phys. Chem. 1990, 94, 6.
The Journal of Physical Chemistry, Vol. 95, No. 7, 1991 2879
Electronic States of 2-Hydroxyquinoline TABLE I: Inertial Parameters of 2-Hydroxyquinoline (ZHQ) and 2-Hydroxyquinoline-OD ( Z D Q ) in Their Ground and First Excited Singlet States
band energy and character
SI
SO
observed
2HQ 31339.846 cm-I,” 0 = t 3 9 f 3O A” B” C“
2835.gb 865.1’ 663.6’ K -0.814 AI” -0.44c
2DQ 31339.1302-~,”0 = f 3 9 f 3O
C”
2195.9‘ 851.3’ 652.8’
K
-0.815
A”
B”
TABLE III: Comparison of Observed and Calculated Rotational Constants and Hydroxy Hydrogen Atom Coordinates of the Tautomeric Forms of So 2-Hydroxyquinoline
AI” -0.2Ic
AA
AC AK A(AI)
-91.3’ -0.6’ -5.4’ -0.012 -0.16c
AA AB AC AK A(A0
-91.1’ 0.36 -4.9’ -0.013 -O.lOc
AB
‘t0.002 cm-I. ’In megahertz; f0.1 MHz. cIn amu A2; f0.13 amu A2.
A’’,
MHz
B”, MHz C”, MHz 1x1, A
Ivl, A IZL A
2835.8 865.7 663.6 3.15 1.65 0.34
calculated’ enol (Ia) keto (11) 2893.9 871.1 669.6 3.30 1.65 0.00
2887.0 870.6 668.9 1.20 2.01 0.00
a From ab initio cahlations using GAUSSIAN 86 (3-21G basis). Maximum forces in the distance and angle coordinates in the final optimization were less than 0.0004 hartree/bohr and 0.0004 hartree/radian, respectively. In these calculations, the molecule was constrained to be planar, but all other coordinates were optimized.
substitution in a nonplanar asymmetric top molecule the coordinates of the substituted atom are given by
TABLE II: Experimentally Determined Coordinates (in the COM Coordinate System) of the Hydroxy Hydrogen Atom in the So and SI Electronic States of 2-Hydroxyquinoline’ 1x1, A 3.15 f 0.02 3.06 f 0.02 -0.09
state SO
SI
A(SI-So)
Ivl, A
lzl, A
1.65 f 0.03 1.71 f 0.03 0.06
0.34 f 0.15 0.39 f 0.13 0.05
O x , y , and z are parallel to the a, b, and c inertial axes, respectively. See Figure 2.
in representation IIIr. Fits of the spectra were made using the derivative approximation and a standard least-squares analysis. The standard deviations of the fits are under 9.0 MHz, significantly less than the experimental line widths. Panel 1C shows a portion of 1B at full experimental resolution along with the corresponding calculated simulation. All lines in the spectrum of 2HQ exhibit widths (fwhm) of 18 f 1.0 MHz and are lifetime limited. The rotational temperature determined from the fit is 9f1K. The experimental spectrum of 2HQ is best fit as a hybrid band, exhibiting both a-type (parallel) and b-type (perpendicular) character. The 0;band of 2HQ was found to be 60 f 5% a type and 40 i 5% b type. The angle between the optical transition moment and the a inertial axis is related to the relative intensitities, Z(a) and Z(b) by
e = tan-2 (I(b)/Z(o))= f 3 9
f 3O
Table I summarizes the experimentally derived band origins, band characters and inertial parameters of both vibronic states of 2HQ and 2DQ. The ground- and excited-state rotational constants have been determined to an accuracy of fO.l MHz ( A 4 etc., are the differences in the A values, etc. for the two states, A ’ - A”, etc.). These values are very similar to those for the corresponding levels of 2HN, cis and tram8 The inertial defects of the ground and excited states are both small and negative suggesting a rigid, planar structure for both states. The small positive change in the inertial defect between the deuteriumsubstituted and parent molecules indicates the hydroxy hydrogen’s contribution to the zero-point vibrations of the ground state is out-of-plane. Further, comparison of AZ(H) and AZ(D) in the So and SIstates indicate the hydrogen’s out-of-plane contribution is the same for the ground and first excited states. The changes in moments of inertia resulting from a single isotopic substitution of an atom can be used to determine the coordinates of that atom measured from the center-of-mass of the parent molecule. The equations relating these moments of inertia are those developed by Kraitchman.” For one isotopic ~~~~~~
( 1 1) Gordy, W.;
Cook, R. L. Microwave Molecular Specrra, 3rd
Wiley-Interscience: New York, 1984.
ed.;
where APx = (l/2)(-AZx
+ AIy + AZ,)
APy = (l/2)(-AZy
+ AZ, + AZx) + AZ, + AIy)
AP, = (1/2)(-AZ2
Here, the AZi (i = x , y, z ) are the differences in moments about the ith inertial axis between the parent and substituted molecule, and p is the reduced mass. Specifically, p = ( M A m ) / ( M Am), where Am is the additional mass of the isotope and M is the total mass of the parent molecule. We have applied these equations to 2HQ using the data in Table I. The results are listed in Table 11. (The errors in the coordinates are inversely related to the distances of the substituted atom from the respective inertial axes.) It will be noted that whereas bl and IzI are the same, within experimental error, in the two electronic states, 1x1 decreases by 0.09 f 0.04 A on excitation of the SIstate. We also note that the hydroxy hydrogen is out-of-plane (121 # 0) in both states. Clearly, the inertial defect is insensitive to this displacement.
+
Discussion Our first task is to use these data to determine which of the three isomers, Ia, Ib, or 11, is the species responsible for the observed band in the SI So electronic spectrum of 2HQ. Toward this end, we have optimized the geometries of the electronic ground states of Ia and I1 by performing ab initio calculations using a 3-21G basis set. We then used these geometries to calculate the Sorotational constants of both isomers. The results are listed in Table 111. Comparing these results to experiment, we find reasonable agreement in both cases. Thus, it is not possible to distinguish I from I1 on the basis of the rotational constants alone. Comparison of the observed and calculated hydroxy hydrogen atom coordinates does distinguish the possibilities. This also is shown in Table 111. The experimental values of 1x1, bl, and 121 are in excellent agreement with those calculated for the enol structure Ia, and in poor agreement with those calculated for the keto structure 11. Thus, we conclude that the species responsible for the high-frequency band in the S, So spectrum of 2HQ is the lactim (enol) tautomer, I.
-
-
2880 The Journal of Physical Chemistry, Vol. 95, No. 7, 1991
Held et al. n
n
v
W
IVa
’ \
Figure 2. Experimentally determined center-of-mass H-atom coordinates in the enol form of cis-2-hydroxyquinoline, superimposed on the So geometry of 2-hydroxyquinoline calculated by ab initio methods (3-21G basis).
We also can determine which of the two lactim rotamers (Ia
or Ib) is responsible for the high-frequency band using these data. Plotted in Figure 2 are the experimentally determined hydroxy hydrogen atom coordinates superimposed on the ab initio geometry of the enol form of 2HQ in its So state. Clearly, the 0-H bond is cis (Ia) with respect to the naphthalene frame, not trans (Ib). In independent experiments, we have searched carefully for the bands of the trans enol a t both high and low resolution. If two conformers existed in equilibrium, we would expect to find a duplication of the cis enol vibronic spectrum, shifted in energy by the S, Soenergy difference of the two forms. This behavior has been observed in both 1H N and 2HNe8 Varying the sample temperature up to 120 ‘C, and the backing pressure down to 400 Torr, we could find no evidence of a second isomer of the lactim form of 2HQ. Thus,if it exists, the trans enol must lie at an energy in excess of 1000 cm-I above the cis enol in the ground state. Several factors may contribute to the difference in the ground state energies of the cis and trans enol forms of 2HQ. One is that the trans rotamer may be destabilized by the repulsion of the lone pairs on oxygen and nitrogen. Unfortunately, our experiments provide no direct information about the importance of this effect. A second factor is that the cis rotamer may be stabilized by intramolecular hydrogen bond formation involving the ring nitrogen and the hydroxy hydrogen. Surprisingly, our experiments are sensitive to this effect. The hydroxy hydrogen atoms in other aromatic molecules such as phenolI2 and cis- and tram-2HN8J3 have been shown to lie in the molecular plane. But our experiments on 2HQ show that the hydrogen atom in the ground state is out of plane, by k0.34A with respect to the naphthalene frame. Now, it is well-known by chemistsI4 that the nitrogen atom in quinoline bears additional negative charge relative to the corresponding carbon atom in naphthalene. This is because there are, in quinoline, additional ionic resonance structures of the type IIIa and IIIb in which the nitrogen atom has two electrons in its A orbital.
-
IIIa
IIIb
This contributes additional A electron density above and below the naphthalene plane to which the electropositive hydrogen will be attracted, forcing it above or below the aromatic ring (IV). (12) Larsen, N. W. J . Mol. Srrucr. 1979, 51, 175. (13) Fitting the previously published data on cis- and rrans-2HN to Kraitchman’s equations for a nonplanar asymmetric top gives values of 121 = fO.13 10.13 and iO.12 10.14 A, respectively. (14) Set,for example: Salem, L. Molecular Orbiral Theory of Conjugated Systems; W. A. Benjamin: New York, 1966, p 43.
Nb
Thus, we believe that the observed “perpendicular” displacement of the hydroxy hydrogen offers convincing proof of the existence of an intramolecular hydrogen bond in the cis enol form of 2HQ, despite a relatively unfavorable four-member pseudoring arrangement of the affected atoms. To further examine these issues, we have performed a complete geometry optimization of the cis enol rotamer at the 6-31G* 1e~el.l~ The calculation gives a value for the COH bond angle in Ia of 107.9’. This value is significantly less than that for the corresponding angle in cis-ZHN, 113.5’ (6-31G*). The relevant bond distances in So 2HQ are rNQ= 2.25, rNH= 2.26, and rOH= 0.95 A, placing the hydrogen atom within hydrogen-bonding range of the ring nitrogen. These data suggest, then, that an in-plane intramolecular hydrogen bond involving the hydroxy hydrogen and the ring nitrogen exists in the cis enol rotamer of 2HQ. However, the out-of-plane displacement of the hydroxy hydrogen is not reproduced by this calculation. [The calculation does suggest that weak hydrogen bonds involving ring hydrogens exist, since the angles LHC2C3= 117.6’ (adjacent to oxygen) and LC,C8H = 118.1“ (adjacent to nitrogen) are less than those of other CCH bond angles in the ring (120 f lo)]. Further theoretical studies of these effects are in progress. Our final task is to explain why, while the experiments reveal that there is a shift of the hydrogen atom toward the ring nitrogen upon electronic excitation, this shift is very small (-0.1 A in the x direction). Some important clues to the origin of this behavior are provided by considering the nature of the SI So transition in 2HQ. We note, first, that the transition is clearly m*. Two pieces of evidence support this view. The observed band is redshifted with respect to the m* transition of quinolineI6 by 845 cm-I, a value that is comparable to the red shift of the corresponding band of cis-2HN with respect to the electronic origin of naphthalene, 11 15 cm-1.8 Additionally, we have found that the in-plane orientation of the optical transition moment in cis enol 2HQ is 0 = f39’. This value, with the positive sign, is intermediate between the corresponding values in naphthalene (0’) and cis-2HN (+66°),8 the expected result if all transitions are principally AT* in character. (Resonance structures like 111 should decrease the value of 0 relative to that in cis-SHN, since there is less ?T orbital charge density at C2.) We do not know whether the n** transition of 2HQ lies at still lower energy, as it appears to in quinoline itself,16or whether the formation of an intramolecular hydrogen bond in the cis enol form of 2HQ shifts the nr* transition to higher energy, as has been proposed for intermolecular hydrogen bond f0rmation.l’ Given the TA* character of the S I So transition, it is at first sight surprising that electronic excitation does not result in a larger change in the position of the hydrogen atom in the cis enol form of 2HQ. We believe, however, that there is a simple explanation for this observation. Recent a b initio calculations8 have shown that whereas substitution in the 1-position of naphthalene has a relatively small effect on the symmetry properties of the relevant MO’s, substitution in the 2-position produces a major change in the position of their nodal planes. In 2HN, the nodal planes of 44and &*, the major contributors to the S,-So transition, bisect the CI-C, bond.8 A similar effect should occur in 2HQ, with one
-
-
(15) GAUSSIAN 90, Revision F. Frisch, M. J.; Head-Gordon, M.; Trucks, G. W.; Foresman, J. B.; Schlegel, H. B.; Raghavachari, K.; Robb, M.; Binkley, J. S.; Gonzulez, C.; Defrees, D. J.; Fox, D. J.; Whiteside. R. A.; Seeger, R.; Melius, C. F.; Baker, J., Martin, R. L.; Kahn, L.R.; Stewart, J. J. P.; Topiol, S.;Pople, J. A. Gaussian, Inc.: Pittsburgh, PA, 1990. (16) Hiraya, A.; Achiba, Y.; Kimura, K.; Lim, E. C. J. Chem. Phys. 1984, 81, 3345 and references therein. (17) Felker, P. M.; Zewail, A. H. Chem. Phys. Lerr. 1983, 94, 454 and references therein.
288 1
J . Phys. Chem. 1991,95,2881-2888 nodal plane extending into the region between the ring nitrogen and the cis-OH group (V). Now, Nagaoka and ceworkersI* have H
,. \
# .
, ,. .
, *
.
a
V shown that an impressive collection of experimental results on the photochemical reactions of organic molecules can be explained on the basis of the nodal patterns of the wave function in the excited state. Applying this principle to the possibility of proton transfer in the enol form of 2HQ, we immediately see that this process is inhibited by the existence of the partial node between the cis-OH group and the ring nitrogen in the SI state. Thus, m* excitation, though it might increase the acidity of the OH group and the basicity of the ring nitrogen, does not produce a significant displacement of the hydrogen because of the existence of this node. A stronger N-H hydrogen bond cannot be formed, and the 0 - H bond cannot be weakened, at least at low excitation energies in the SI manifold.' Consistent with this speculation is the fact that only a very small (red) shift of the electronic origin (18) Nagaoka, S.;Nagashima, U. J. Phys. Chem. 1990, 94, 1425 and references therein.
in cis-2HQ (enol) is observed on deuteration of the OH group. (c-2HN exhibits a much larger red shift, 4 . 0 cm-1).8 This picture thus satisfactorily accounts for the relatively small motion of the hydrogen atom in the SIstate. Summarizing, we have used the technique of high-resolution molecular beam laser spectroscopy to determine the absolute position of the intramolecularly hydrogen bonded hydroxy hydrogen atom in the So and SIstates of the lactim form of 2hydroxyquinoline. We have found that, despite its favorable position, and the AT* character of the SI So transition, there is only a small shift of this atom toward the nitrogen atom on electronic excitation. We also have found that the hydrogen atom lies above (or below) the naphthalene plane, in both electronic states. These results have been interpreted as being dictated by subtle but important changes in the neighboring electronic distributions caused by the absorption of light. Thus, "the chromophoric portion of the molecule is perturbedw2by the formation of a hydrogen bond, and these perturbations, in both electronic states, are revealed by the UV spectrum of the molecule, albeit at high resolution.
-
Acknowledgment. We thank K. D. Jordan for helpful discussions. This work has been supported by NSF. The calculations were performed on the Cray Y/MP at the Pittsburgh Supercomputing Center. Registry No. Dl,7782-39-0; 2-quinolinone, 59-31-4; 2-quinolinol, 70254-42- 1.
Spectroscopic Evldence for Near-Resonant Intermolecular Energy Transfer in the Vibrational Predlssociation of C2H2-HX and C2H2-DX (X = CI, Br, and I)Complexes D. C. Dayton, P. A. Block, and R. E. Miller* Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599 (Received: October 1 1 , 1990)
Infrared laser molecular beam spectroscopy has been used to study a number of C2H2-HX and C2H2-DX complexes. In all cases the asymmetric acetylenic stretch is excited by using an F-center laser while the optothermal method is used to detect the resulting decrease in molecular beam intensity caused by vibrational predissociation of the complex. The spectra are fit to a rigid rotor asymmetric top Hamiltonian to determine the associated molecular constants. Of particular interest is the fact that, for a number of these systems, the vibrational predissociation lifetimes are rather short, despite the remoteness of the intramolecular vibration from the intermolecular bond. The isotopic dependence of these lifetimes suggest that localized intermolecular resonances are responsible for this seemingly anomalous behavior.
Introduction The vibrational predissociation of weakly bound complexes has been studied using a wide variety of experimentalI4 and theoreticals9 techniques, and yet, there still remain many aspects of this unimolecular dissociation process which are not clearly understood. In fact, despite the low energies characteristic of these processes, the associated dynamics is surprisingly rich. This richness can be understood if one considers that the dissociation of these complexes depends directly upon the subtleties of the intramolecular/intermolecular vibrational couplings,' as well as on the translational, rotational, and vibrational energy levels available to the photodissociation fragments.6 Much of our intuitive understanding of vibrational predissociation in these complexes is based upon the use of Fermi's Golden Rule, namely 7-1
= C(*,JVI*")
where 7-l is the vibrational predissociation rate and *Author to whom all correspondence should be addressed.
*,,,and 9"
are the final continuum-state and quasi-bound-state wave functions, respectively.6 The coupling term (V")connects the initially excited intramolecular vibration to the motions of the van der Waals bond. In the early discussions of vibrational predissociation the main emphasis was centered upon the overlap between the quasi-bound-state and continuum-state wave functions, the argument being that if the continuum wave function was highly oscillatory (large translational energy) then the overlap with the (1) Levy, D. H. Adu. Chem. Phys. 1981, 47, 323. ( 2 ) Miller, R. E. Science 1988, 240, 447; Acc. Chem. Res. 1990, 23, 10. ( 3 ) Nesbitt, D. J. Chem. Reu. 1988, 88. 843. ( 4 ) Waterland, R. L.; Skene, J. M.; Lester, M.I. J . Chem. Phys. 1988, 89, 7211. ( 5 ) Cline, J. I.; Sivakumar, N.; Evard, D. D.; Breler, C.R.; Reid, B. P.; Halberstadt, N.; Hair, S.R.; Janda, K.C. J . Chem. Phys. 1989, 90,2605. ( 6 ) Ewing, G. E. Faraday Discuss. Chem. Soc. 1982, 73, 325. Ewing, G. E. J . Chem. Phys. 1980, 72,2096. Ewing, G. E. J. Phys. Chem. 1987, 91, 4662. ( 7 ) LeRoy, R. J.; Corey, G. C.; Hutson, J. M. Faraday Discuss. Chem. Soc. 1982, 73, 339. (8) Hutson, J. M. In Dynamics of Polyatomic Van der Waals Molecules; Halberstadt, N., Janda, K. C., Eds.; Plenum: New York, 1990. ( 9 ) Beswick, J. A.; Jortner, J. J . Chem. Phys. 1981, 74, 6125.
0022-3654/91/2095-2881$02.50/0 0 1991 American Chemical Society
