On the infrared spectrum of malonaldehyde, a tunneling hydrogen-bonded molecule

JOURNAL OF MOLECULAR SPECTROSCOPY

96, 146- 155 (1982)

On the Infrared Spectrum of Malonaldehyde, A Tunneling Hydrogen-Bonded Molecule C. J. SELISRAR’

AND R. E. HOPIMANN

Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221 Measurements of the medium-resolution infrared spectrum of malonaldehyde and a doubly deuterated derivative are presented. Experimental evidence for a tunneling planar asymmetric hydrogen-bonded structure is presented and interpretations are made within a molecular symmetry group description of the molecule. INTRODUCTION

Malonaldehyde (OHC-CH,-CHO) has become an important molecule for a detailed study of the intramolecular hydrogen bond. Under low-pressure conditions, malonaldehyde vapor apparently consists (l-4) of only the intramolecularly hydrogen-bonded enol tautomer. This behavior proves to be a decisive experimental advantage and this, coupled with the chemical simplicity of the molecule, provide a unique case for a detailed spectroscopic examination of intramolecular hydrogenbonding between identical atoms (oxygen). Questions concerning the vibronic ground-state symmetry and bond distances of the molecule have elicited recent theoretical (5-14) and experimental (Z-4,15-21) investigations. Experimental work on the isolated molecule (dilute vapor phase) has been only very recently reported (Z-4, 16, 21, 22). A central question which has emerged concerns the structural symmetry of the electronic ground state: Is the planar molecule symmetric (C,, point group) or asymmetric (C, point group)? The vibronic ground state of the molecule has been shown (I, 4) to be planar. The presence of tunneling between equivalent planar asymmetric structures of the molecule apparently occurs through a relatively low potential energy barrier, and this is depicted in Fig. 1. In this paper we report measurements of the medium-resolution infrared spectrum of malonaldehyde and a deuterium isotope derivative. Experimental evidence for a tunneling malonaldehyde structure is presented and interpreted using a molecular symmetry group description (23). EXPERIMENTAL

DETAILS

Ambient temperature (23°C) spectra of the vapors of isotopic malonaldehydes were recorded on a Beckman IR- 12 spectrophotometer fitted with a Wilks’ 20-m variable-path absorption cell having KBr windows. Medium-resolution (-0.2 cm-‘) ’ To whom correspondence

should be addressed.

0022-2852/82/l 10146-10$02.00/O Copyright0 1982 by Academic F’res, Inc. All rights of reproductionin any form reserved.

146

MALONALDEHYDE

INFRARED

147

SPECTRUM

/\

FIG. 1.The tunneling motion of planar intramolecularly

hydrogen-bonded

malonaldehyde

spectra were taken on purified (2, 22) samples of malonaldehyde whose pressures were varied from - 50-400 mTorr. Under these conditions, the only identified vapor species was the enol tautomer. Wavenumbers reported for band measurements were vacuum-corrected and standardized relative to IUPAC calibration lines (24). Vapor-phase hydrogen/deuterium exchange reactions were carried out by adding varying small partial pressures of dry DC1 (g) (99% D, Stohler Isotope Chemicals Comp.) to malonaldehyde vapor after seasoning the cell with DC1 (g). Under these conditions, deuterium exchange was found to be immediate and both the hydrogenbond hydrogen and the /3carbon hydrogen atoms were exchanged (1-4). This doubly exchanged deuterium isotope will be henceforth denoted by dZ. Additional experimental details may be found in our previous papers on malonaldehyde (2, 3, 22). RESULTS AND DISCUSSION 1.

Theoretical Description and Selection Rules

The need to employ a molecular symmetry group (23) theoretical description, i.e., one which can include a large amplitude structural nonrigidity (tunneling), is dictated by the observation by Wilson and co-workers (I, 4) and by us (2, 3) that tunneling leaves distinct impressions on the recorded spectra of this interesting molecule. The symmetry group (23) of the rovibronic Hamiltonian of hydrogen-bonded tunneling malonaldehyde is of order 4. Adopting the nuclear labels shown in Fig. 1, the elements of this group, G4, are: the permutation of equivalent nuclei (l2) (34) (56); the inversion of all coordinates through the molecular center-of-mass, E *; the permutation-inversion, (l2) (34) (56)*; and, the identity element, E. The group G4 is isomorphous with the point group CZ, and by convention (23) we choose the labeling of the irreducible representations of Gq to be those of isomorphous C,,. Adopting CZulabels does not mean that the equilibrium structure of malonaldehyde has a twofold axis of symmetry. It does, however, ensure that, as the molecule traverses the tunneling coordinate (vide infra), the most symmetric structure (“C,,“) attained would be point group labeled identically to that using G,. The inclusion of tunneling between two equivalent asymmetric molecular structures (Fig. 1) can be thought of as imparting a large amplitude nonrigidity to the molecule and we assume that this lies in one of the 3N - 6 normal coordinates (23, 2.5, 26). Following Bunker (23), we then denote this by writing the zero-order rotation-vibration Hamiltonian as H:, where the zero-order

= H%Q,, . . . 5 Q~N-7;PI + Htb(x, 0, 4, P), vibrational

Hamiltonian,

Ht is of dimension

3N - 7 and

148

SELISKAR

AND

HOFFMANN

parametrically dependent on the tunneling coordinate, p. The rotation-tunneling Hamiltonian, HO’ is four dimensional: three Eulerian angles and p. Thus, a rough model for tunneling malonaldehyde would be one in which there were 3N - 7 = 20 regular normal modes of vibration and one, the tunneling mode, which could couple to the remaining 3N - 7 in varying degrees. In addition, since the moments of inertia vary considerably as one traverses the tunneling coordinate, p, rotation and tunneling cannot, strictly speaking, be separated. The infrared selection rules could be thought of in terms of the instantaneous asymmetric (C,) and symmetric (C2J structures attained along the tunneling coordinate. In this case in-plane vibrations would appear as a or b type or a, b type in rotational profile; out-of-plane vibrations would be c type in profile. Decomposition of the 3N-6 modes of vibration would lead to either 15a’ 0 6~” (C,) or 8a, 0 2a2 0 4b, 0 7b2 (Cl”) in the usual manner. The use of the molecular symmetry group, G4, description largely avoids the dilemma posed by considering these limiting structures of malonaldehyde. Although the formal decomposition of the vibrational modes in G4 is 8ai 0 2a2 0 4b, 0 6b2 0 lb* (tunneling) and this is superficially identical to the Czv result, the meaning is quite different (23). The symmetry labeling of the vibrational levels is shown in Fig. 2, where the various G4 symmetry species of the 3N - 7 vibrations are shown together with those of the tunneling mode. The rovibrational selection

01

fundomentol

b,

fundamental

bl

fundamental

a2

fundamental

FIG. 2. Diagram of infrared selection rules. The dashed vertical lines denote transitions expected to be weak on the basis of consideration of the intensities for a limiting symmetric, Czu, structure. Additional details may be found in Ref. (3).

MALONALDEHYDE

I800

1600

1400

INFRARED

1200 WAVENUMBER

149

SPECTRUM

1000

800

600

400

, cm-1

FIG. 3. The medium-resolution survey spectrum of malonaldehyde (normal isotope) from 1800 to 400 cm-’ at a vapor pressure of 270 mTorr and pathlength of 0.75 meters.

rules may be obtained from Fig. 2 by appending the usual asymmetric top rotational levels and their labeling in G4. Rovibrational levels are then interconnected by I’* = a2 in Gq as shown by Bunker (23). Finally, it is to be noted that the energy levels in the tunneling mode are independent of the degree of excitation in the remaining 3N - 7 vibrational modes only in the zero-order description and, in practice, one might expect these levels to be somewhat sensitive to the particular vibrational excitation to which they are coupled. 2. General Features of the Infrared Spectrum A portion of the survey spectrum of the normal isotope of malonaldehyde is shown in Fig 3. The spectrum is remarkable in two respects. First, the presence of sharp-featured infrared bands is in marked contrast to that found for acetylacetone (27) whose hydrogen-bonded enol structure is very similar to that of malonaldehyde. Evidently near-free rotation of the methyl groups in acetylacetone blends out any sharply defined rovibrational features. Second, there appears to be a bewildering variety of bands of different rotational contour. Indeed, few of the observed bands appear to have rotational contours expected for a rigid asymmetric top. A closer examination of the bands in Fig. 3 does reveal that there are two general types of band profiles. One type indicative of a- and/or b-selection rules is composed of relatively weak P-branch intensity, one or more Q-branch spikes of intensity and strong R-branch intensity. Examples of this include the bands at 1596 and 1360 cm-‘. Wide variation in the finer details of these a, b-type bands may be found, however, and this is obvious from a comparison of the bands at 1660, 1360, 1246, and 5 12 cm-’ in Fig. 3. The fact that bands of hybrid a, b intensity are found (see the band at 1246 cm-‘) leads one to conclude with certainty that the molecule is not only planar (1, 4) but asymmetric on a vibrational time scale. It is notable that no clean b-type band has been found in the spectrum of any of the isotopes of malonaldehyde to date. A sharp Q branch and weak P and R branches are expected for a c-type band of

150

SELISKAR

AND

HOFF’MANN

a rigid planar asymmetric top and this type of band constitutes the second class of bands to be found in the spectrum (Fig. 3). Examination of the shape of the prominent band at 768 cm-’ reveals that it differs little from that expected of a rigid rotating asymmetric planar molecule. The measured rotational profiles of a dominantly u-type and a c-type band are shown in more detail in Fig. 4. The. 1596-cm-’ band (Curve 1) of the normal isotope represents absorption terminating on the II = 1 level of the C=C stretching/C-H bending fundamental, The band shows a strong R-branch intensity/sharp (unsplit) Q-branch spike/weak P-branch intensity rotational profile. This overall rotational profile is computable using the Pierce algorithm (28) and rigid rotor a-type selection rules and the ground-state constants of Wilson and co-workers (I, 4 ). Further analysis of this computation shows that the stronger R-branch intensity of the band arises from a summing of bunches of strong qrR transitions (AK, = 0, AK+, = + 1; AJ = + 1) in the region of 6- 10 cm-’ above the band origin. This pileup of intensity does not occur in the related qpP transitions since they are spread out over a relatively larger wavenumber interval. That the rigid rotor selection rules allow computation of this u-type band profile should not be taken as evidence for the complete lack of a b-type intensity component in this and similar bands since the latter intensity would contribute significant P, R intensity in only the wings of the rotational envelope where comparison with computation is error prone. Rather, these results indicate that the 1596-cm-’ band, and the bands of similar contour, represent absorptions to t, = 1 vibrational levels not strongly coupled to the tunneling motion. Thus, the 0+-O- tunneling interval of their ground and upper-vibrational states are effectively equal. A measured c-type contour is shown as Curve 2 in Fig. 4. The large sharp Qbranch component of this transition (to the II = 1 level of the C,-H out-of-plane bend) dwarfs the associated P- and R-branch intensities. The general features of this band are also computable using c-type rigid rotor selection rules (28). Thus, there is nothing in this rotational profile which differs qualitatively from that expected for absorption to a b, vibrational level of a rigid CzVmolecule. Careful examination

rr

1650

1600

1550 WAVENUMBER

FIG. 4. The rotational

800 ( cm-

contourS of the I596- and 768-cm-’

750

700

)

infrared bands at medium

resolution.

MALONALDEHYDE

u

INFRARED

SPECTRUM

151

540

WAVENUMBER

(cm-‘)

FIG. 5. The rotation contours of the 5 I2-cm-’ ring deformation vibration for the normal isotope (Curve I) and for d2 (Curve 2).

of the Q-branch spike at 767.8 cm-’ reveals that there is a satellite Q branch to lower wavenumber (-764.5 cm-‘) which is a third to a quarter as intense as the large spike. Although our resolution is marginal, the d2 isotope also appears to have a very similar satellite Q branch in the analogous c-type band at 579.1 cm-‘. Given the identification of low-lying vibration-tunneling levels in both normal and d2 isotopes (Z-3) at 250-300 cm-’ and the corresponding Boltzmann factor of -0.3. these satellite Q-branches might well be c-type Av = 0 hot bands in either an a’ or a2 vibration attached to the Co-H out-of-plane bending vibration. In contrast to the well-defined infrared fundamental absorptions discussed above, those found, for example, at 1660 and 1269 cm-’ in the normal isotope represent transitions which, although largely b-type in contour, possess hybrid character. These two particular bands suffer little shift on deuterium exchange and may be attributed to the a, C=O stretch and a, C-C/C0 stretch fundamentals, respectively. Given the apparent confusion of rotational structure in these bands at the prevailing resolution not much more can be said about them. The matter of the position of the OH stretching vibration is complicated by the apparent lack of any definitive absorption band which can be attributed to it. Infrared spectra taken in the region 4000 to 1800 cm-’ during a deuterium isotope exchange titration of the OH hydrogen and C,-H hydrogen atoms of the normal isotopic vapor show that there is a broad featureless region of strong absorption between 2850 and 3100 cm-’ which shifts to a similar absorption between 2350 to 2600 cm--’ upon introduction of deuterium into the hydrogen bond. Lacking a more definitive spectral trait, we tentatively assign this intensity to the OH/OD stretching fundamental of the hydrogen-bonded molecule. We note that similar behavior has been found in other hydrogen-bonded molecules (27. 29, 30). 3. Evidence of Tunneling in the Spectrum Tunneling leaves distinct impressions on the spectrum of malonaldehyde. The most obvious example of this may be found in the behavior of the 5 12-cm-’ band

152

SELISKAR AND HOFFMANN

on deuterium isotope substitution (Fig. 5). This band, in all probability, corresponds to a transition to the u = 1 levels of a bz ring deformation vibration. Concentrating, momentarily, on only the most prominent features in Curve 1, the overall rotational profile is found to be overwhelmingly a type in contour with a split Q branch at 5 12.8 and 506.7 cm-‘. The (unresolved) P- and R-branch maxima are separated by -20 cm-’ and this is also consistent with that calculated (20.0 cm-‘) using the SethPaul algorithm (31) and the rotational constants of Wilson and co-workers (I, 4). Introduction of deuterium into the molecule by simple exchange to produce the d2 isotope (Curve 2) results in two remarkable changes. First, the entirety of the band shifts by only - 10 cm-’ and this argues strongly for the skeletal-atom vibration assignment. Second, the reduction of the splitting of the Q branch from -6 cm-’ to effectively zero is strong evidence that these two features be attributed to tunneling and not to some accidental resonance interaction, Reference to Fig. 2 shows the 6cm-’ splitting in the 5 12-cm-’ band may be interpreted in two ways. The first interpretation would associate the 5 12.8/506.7 cm-’ intensity with transitions originating in the 0+/O- tunneling levels of the ground state; the second would associate them with O-/O+, respectively. In the former case the 0+/O- level separation in the v = 1 level of the 5 12-cm-’ vibration would be 6 cm-’ less than that in the ground state; in the latter the opposite would obtain. Deuterium exchange is known (4) to reduce the ground-state 0+-O- tunneling level separation from 26 + 10 to 2.88 cm-‘. There is also experimental evidence (4) that the ground state of d2 has the tunneling motion nearly frozen out. Relying on this information, the most plausible explanation for the 5 12-cm-’ band is that the 0+-O- splitting in the upper, o = 1, state of the b2 ring vibration is 6 cm-’ less than that in the ground state in the normal isotope. Deuterium exchange would then be thought of reducing both the upperand lower-vibrational states’ 0+-O- splitting to effectively zero. It would thus be concluded that the b2 ring fundamental makes it more difficult for malonaldehyde to tunnel between equivalent planar structures.

410

390 WAVENUMBER

370 (cm-‘)

FIG. 6. The rotational contour of the 390-cm-’ band of the normal isotope recorded with 280 mTorr malonaldehyde and 8.25-m pathlength.

MALONALDEHYDE

INFRARED

153

SPECTRUM

The bands found at 1454 (Fig. 3) and 390 cm-’ (Fig. 6) may be variants of the 5 12-cm-’ band discussed above. In each case, the rotational profile appears to be dominantly a type. Our spectrum of d2 shows a prominent a-type band at 1368 cm-‘, which can be associated with the 1454-cm-’ band in the normal isotope. The 1454-cm-’ band has Q-branch components at 1453.9 and 1438.2 cm-‘. In the case of the 390-cm-’ band (Fig. 6), the spikes of intensity at 39 1.O and 383.7 cm-’ appear to be members of a split Q branch. Our present experimental capabilities do not allow us to trace this latter band in the d2 isotope. It is, however, to be noted that the 390-cm-’ band corresponds nicely with the microwave intensity estimates (4) of the low-lying vibration-tunneling levels in malonaldehyde. The bands at 1596 and 1000 cm-’ are characteristic a-type bands in the normal isotope (Figs. 3, 4). On deuterium exchange these bands shift to - 1540 and -975 cm-‘, respectively, and their rotational envelopes become much less well-defined a, h-type hybrid bands. While this dramatic change in contour is consistent with the microwave evidence for the tunneling motion being frozen out on deuterium exchange (4) there appears to be no ready explanation why these a-type bands should behave so differently from that at 5 12 cm-’ already discussed. Nonetheless, there can be little doubt that these effects are to be attributed to changes in the tunneling motion of the molecule on deuteration. Reference to Fig. 3 shows that there is a sharp Q-branch spike of intensity at 965.8 cm-‘, which is undoubtedly the central rotational feature of a strong c-type band belonging to an out-of-plane fundamental. A careful analysis of the deuterium exchange reactions to produce d2 strongly suggests that the weaker spike of intensity at 927.1 cm- ’ (Fig. 3) is a related Q branch.2 The d2 isotope shows additional strong c-type bands at 672.2 and 770.5 cm-‘, neither of which possess a similar satellite spike of intensity. Assignment of the strong band at 874.8 cm-’ in the normal isotope as c type in contour leads us to associate it with that at 672.2 cm-’ in dz. Thus, we associate the c-type band at 965.8 cm-’ with that at 770.5 cm-’ in dz. Given the intensity of these c-type bands it seems only reasonable to assign them to b, fundamental vibrations. It is not, however, clear what the 965.8/927.1 cm-‘, 39 cm-‘, interval is to be assigned to. Microwave intensity estimates (4) do not apparently indicate that there is a vibrational level at this interval above the ground state. Given the sensitivity of this 39-cm-’ interval to deuterium exchange and the relative intensities of the 965% and 927.1-cm-’ spikes, it seems plausible that it could correspond to a difference between the ground vibrational and excited vibrational 1‘/ 1~ intervals in the tunneling mode. Although this interpretation is speculative, there must be some such explanation for the many spikes of intensity (Fig. 3) which occur in the spectrum of the normal isotope. Clearly more isotopic derivatives and higher-resolution spectra are badly needed. A tabulation of the measured strong bands for normal and dideuterated malonaldehyde is presented in Table I. Tentative assignments of the fundamental vibrations were made by recourse to the published work on acetylacetone (27) and the isotope shift behavior of the analogous bands in malonaldehyde. Traces of the spectra recorded for the dideuterated isotope may be obtained from the authors on request. ’ The infrared spectrum of the C, monodeuterium for us by Dr. R. W. Duerst. also has these Q-branch

derivative of the malonaldehyde. features at 836 and 801 cm-‘.

kindly

provided

154

SELISKAR AND HOFFMANN TABLE I Prominent Infrared Bands for Malonaldehyde Band

Position1 cm-1

contour Intensity2

Type

Assignment

3100(2310)

m,br

CD-H

stretch

3056(3045)

m,br

Cm-H

stretch

2850-3100(2350-2600)

m,br

b2

O-H

stretch

s,br

C,-H

1660(1659)

AB

s

al

1596(1540)

ACAB)

s

b2 C=C/C-H stretch/bend

1454(1368)

A

In

b2

1360

A

m

1269(1270)

AB

s

1030

A

m

lOOO(975)

ACAB)

s

b2

C-H

bl

fund.

2861(2856)

stretch C-O

stretch

fund.

+“,h”-”

966(770)

C

s

927

C

w

875(672)

C

s

bl

fund.

bend

768(579)

C

s

bl

CB-H

bend

512(497)

A

m

b2

ring

def.

390

A

m

1.

Band position for dideuterated

2.

s

=

strong,

to

nearest wavenumber; malonaldehyde.

m = medium,

w = weak,

values br

in

parentheses

are

= broad

CONCLUSION

There are two principal conclusions to be drawn from our studies on the infrared spectrum of monaldehyde vapor. First, the presence of rotation-vibration features in the infrared fundamental bands attributable to tunneling is consistent with microwave results (I, 4) and our observations are interpretable within a molecular symmetry group description (23). Second, the structure of the normal isotope of malonaldehyde, on an infrared time scale, is planar and asymmetric with the two equivalent asymmetric structures interconnected by the tunneling motion. ACKNOWLEDGMENTS The authors thank Professors E. B. Wilson and R. L. Redington for helpful discussions and for communicating their results prior to publication. Special thanks are given to Dr. R. W. Duerst for many helpful discussions and for sending us some of his results prior to publication. This research was funded, in part, by Research Corporation in a grant to C.J.S. RECEIVED:

June 15, 1982 REFERENCES

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SPECTRUM

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