Hydrogen bonding in solid alcohols

JournaZof Molecular Structure

ElsevierPublishing Comptiny, Amsterdam - Printed in the Netherlands

HYDROGEN

309

BONDING IN SOLID ALCOHOLS*

R. J. JAKOBSEN, J. W. BRASCHANDY. MlKAWA Coltimbus Laboratories, Battelle Memorial Institute, Columbus, Ohio 43201 (U.S.A.)

INTRODUCTION The extreme broadness of the OH stretching vibration is one of the most characteristic features of JR spectra of hydrogen-bonded molecules. Bandwidths of several hundred wave numbers occur1*2 and the broadness is seemingly independent of the phase (gas, liquid, or solid) in which the hydrogen bonding occurs. In fact, most of the studies of this phenomenon attempt to explain why it is common to all physical states and types of hydrogen bonds. Pimentel and. McClellan’ and Sheppard3 reviewed these studies, and pointed out that each explanation has its limitations. In a study of the effects of high pressure on hydrogen-bonded systems, we obtained polarized IR spectra of single crystals of several alcohols at various pressures. The unexpected results in the OH stretching region led us to study spectra of partially-deuterated samples of these alcohols. From these data we concluded: 1) The OH stretching vibration of hydrogen-bonded solid alcohols is not inherently broad. 2) The breadth usually observed is a result (direct or indirect) of nearestneighbor or first-order coupling of the vibrations of OH groups in the hydrogenbonded chain of the crystal. 3) Perhaps most important, coupling is not the primary cause of broadening in liquid alcohols, and therefore, the cause of broadening is not the same in liquid and solid. Explanations for the breadth must take differences between physical states into account.

The alcohols in this study were commercial products dried over magnesium * Preset&xi in pati at the Euchem meeting on Hydrogen B.onding, Elmau, Germany, April, 1967, and in part at the XIII Colloquium Spectroscopicurn Intemationale, Ottawa, Canada, June, 1967. J. Mol. Structurk, 1 (1967-68) 309-321

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Y. MIKAWA

sulfate and ‘used without further purification. The deuterated alcohols were prepared by exchange with D,O, extraction with an organic solvent, and drying over magnesium sulfate. All the alcohols used are liquids at room temperature. Single crystals of each alcohol were grown in a diamond-window highpressure cell by techniques previously described4_ The ease with which polymorphism can be detected in the pressure cell has been demonstrated5* 6. Polarized

spectra were obtained on these single crystals using an AgCl pile polarizer. Some of the deuterated alcohols were studied as single crystals in the pressure

However, because of strong diamond absorption in the OD stretching region, -most of the deuterated alcohols were studied as polycrystalline films in a conventional low-temperature Dewar cell. All spectra were run on a Perkin-Elmer 521 spectrophotometer. cell.

RESULTS

A. Alcohols with an even number of carbon atoms Polarized JR spectra in the OH stretching region of single crystals of ndecanol are shown in Fig. 1 at low and at high pressures. At low pressure, the

Fig. 1. Polarized spectra of n-decanol single crystal. A and B are curves obtained with polarizer settings 90” apart. Upper curves at low pressure, lower &ves at high @zssure. Orientation of polarizer with respect to crystallographic axis i.snot known. J. Mol. Structure, _l (1967-68)

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ALCOHOLS

v(OH) is completely separated by polarization into a sharp band at~3410 cm-l and a broad band near 3320 cm-‘. This was not expected, since it was previously reported3 that for hydrogen-bonded systems, all of v(OH) has the same polarization properties. This obviously is not true for n-decanol nor did we find -it true for any of the pure alcohols we studied. At high pressures, both v(OH) components of n-decanol shift to lower frequencies, but the broad 3320 cm-’ component shows the greater shift- Since these are single-crystal spectra, we are not concerned with mixtures of polymorphic forms. In addition, visual observation through a microscope is maintained during all pressure changes to detect any polymorphic transitions. Frequency shifts and change in half-bandwidth of v(OH) with increasiug pressure are shown in Fig. 2 for n-decanol single crystals. The greater frequency

-

3 OH3410

‘I;3,

P

1

‘\, ‘1 I II

vm-’

0

I

I

-

I 0

‘

I

I IO0 cm-’

Fig. 2. Frequencyshifts and half-band widths of v(OH) as a function of pressure. n-Decanol single crystal.

shift of the 3320 cm-’ component is clearly seen. There is a definite, small increase in half-bandwidth of the 3410 cm-i component with increasing pressure and a possible slight increase in the broad 3320 cm-’ component, but half-bandwidths of the latter component are difficult to measure due to overlap with the CH stretching vibrations. However it is clear that any increase in half-bandwidth is small considering the ML200 cm- 1 frequency shift (we have observed similar pressure-induced v(OH) shifts in several compounds with no change in bandwidth). Fig. 2 also emphasizes the difference in bandwidth between the 3410 and.the 3320 cm-’ components of v(OH). Thus the spectra of n-decanol show certain features: two different v(OH) frequencies with widely-different bandwidths, difIerent polarizations, and different frequency shifts with pressure. In order to help ex$& this behavior we prepared samples of. partially-deuterated. .n-decanol with varkus OI!) : OH ratios,. tt, hak been Well established’. that intermolecular -interactions due to. coupling between

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R. J. JAKOBSEN, J. W. BRASCH, Y. MIKAWA

1

b

/

J

d

/ .a8

I

F

I Fig. 3. Effects of deuterationon v(OH) of solid decanol. OH : OD is, respectively:(a) 100 :0, (b)85:15,(~)45:55,(d)25:75.

identical molecular systems can best be detected by isotopic dilution and mixed crystal studies. Fig. 3 shows such pertinent spectral data for various degrees of deuteration of n-decanol. These spectra are of low-temperature solids. In the upper spectrum of the pure OH compound, v(OH) is clearly split into one broad component and one sharp component, with the sharp component at higher frequency. This is identical to the spectrum observed in the high-pressure cell. With increasing dilution by n-decanol-OD, the v(OH) sharp-broad doublet collapses and at high dilution (75 % OD) nearly becomes a sharp singlet. At the same time, v(OD) changes from a narrow singlet at low concentration of OD to a sharp-broad doublet at high concentration, analogous to the shape of Y(OH) in the pure OH compound. Thus, when there is a sufficiently dilute mixture of one isotopic substituent, the respective stretching band is a sharp singlet. At high isotopic concentrations, however, splitting is apparent with ultimate resolution into a sharp-broad doublet. These deuteration studies leave no doubt that the appearance of two bands is due to coupling or crystal splitting. Falk and Whalley’ observed similar behavior in their study of crystalline methanol and its deuterated derivatives. J. Mol. Structure, I (1967-68)309-321

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Here we must stress a less obvious, but very important, point. The ultimate singlet OH band is quite narrow. For n;decanol we have measured a 30-4-O cm-’ half-bandwidth for v(OH) even when the deuteration is not sufficient to compIetely decouple the OH vibration. A 25-30 cm-l half-bandwidth was measured for the decoupled v(OD) vibration. These are extremely narrow bandwidths considering that these are stru~~gl’ hydrogen-bonded systems. We thus found the same behavior for n-decanol that Bertie and Whalley’ have reported for ice: that is, the OH stretching vibration, even when strongly hydrogen-bonded, is not inherently broad.

Fig. 4. Polarized spectra of single-crystal partially-deuterated n-decanol. (a) and (b) are curves obtained with polarizer settings 90” apart. Orientation of polarizer with respect to crystallographic axis is not known.

Fig. 4 shows polarized spectra of a single crystal of deuterated n-decanol. The degree of deuteration is similar to that of the lower spectrum of Fig. 3, i.e., about 75 oAOD. The decoupled OH stretching vibration in Fig. 4 does not show the polarization changes previously seen in Fig. 1 for the pure OH compound. A sharp-broad v(OH) doublet with different polarization properties was observed in each of the three alcohols we have studied with an even number of carbon atoms. These were n-decanol, ethanol, and n-hexanol. We have detected only one polymorph of each of these alcohols regardless of whether the solid was produced by lowering the temperature or increasing the pressure. 3.

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B. Alcohols with an odd number of carbon atoms In the case of the alcohols with an odd number of carbon atoms, complications arise from polymorphism. We have isolated single crystals of three different polymorphs of n-nonanol and n-undecanol. X-ray diffraction investigations’Oy I1 have shown that there are three crystal forms (o(, 8, y) for the normal alcohols with 16-18 carbon atoms. Tasumi et al.” have shown that alcohols C12-C37 give IR spectra which can be related to either the p or y polymorph, and give spectral characteristics which can be used for distinguishing the two forms. The three polymorphs we detected are labeled B, G, and X. The B and G forms give spectra quite similar to those of the /? and y forms, respectively, reported by Tasumi et al.“, except for the vibrations involving the OH group. Therefore we label the forms B and G because we are not certain that they correspond to those reported by Tasumi et al12. The B and G solids were obtained both at low temperature and at high pressure. The X polymorph was obtained only at high pressures. Spectra of the three polymorphic solids of undecanol are shown in Fig. 5. We are not concerned in this paper with particular differences in the polymorphs or in their lower-frequency spectra, and we include Fig. 5 only to show that the spectra clearly differentiate the three forms. Fig. 6 shows polarized spectra of v(OH) in the three polymorphic solids. In each case, two components are obvious although the clear separation seen in decanol was not obtained. Frequencies of the two components differ for the three solids as seen in Table 1, and different band shapes are probable in the different solids although lack of separation makes this inconclusive. This lack of clean separation could result from either improper orientation of the crystal for polarization data, or the unknown effects of different crystal structures on hydrogen bond TABLE

1

FREQUENCIES

(cm-l)

OF v(OH)

OF SINGLECRYSTALSOFODD-CARBON

Polarizer setting

X form, undecanol

G form, undecanol

B form, nonanol

O0 90”

3170 3330,317o

3270,3 170 Sh 3270 Sh, 3 170

3280 3280, w 3220

ALCOHOLS

structure. In the case of the X-polymorph, it is clear that the absorption is a sharpbroad doublet, but for the other two polymorphs, this is not certain. For the B-polymorph, the 3220 cm- ’ band is broader than the 3280 cm-’ component, but we cannot measure half-bandwidths accurately. Spectra of partially-deuterated samples of undecanol and nonanol, rep.resenting two of the three solids, are shown in Fig. 7. The results are completely ‘analogous.to those previously discussed for decanol. When decoupled by isotopic J. Mol. Sfructure, 1

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c

I

I

I

Fig.

I

I 600

1000

1400

5. Spectra of three polymorphic

cm-’

solids of undecanol. (a) X form, (b) G form, (c) B form. n

A

b

cl

i

t

t91k:>. )-jI

3400

I

I

3000

*

&

I

3400

I

I

3000

-

A

I

3400

I

I

3000

cm-’

Fig. 6. Polarized spectra of single crystals of three polymorphic solids of odd-number-carbon alcohols. (a) X form, undecanol; (b) G form, undecanol; (c) B form, nonanol. 0” and 90” refer to polarizer settings. Orientation of polarizer with respect to crystallographic a&s is not known. J. Mol. Srrucrure, 1 (1967-68) 309-321

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R_ J_ JAKOBSEN,

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BRASCH,

Y.

MIKAWA

dilution, the doublet v(OH) (or v(OD)) collapses td a singlet whose frequen?y is between those of the coupled doublet. In the upper spectrum of undec&2ol, G polymorph (70 % OD) in Fig. 7, the asymmetry on the high-frequency sic@ of the main peak at 3150 cm-’ indicates that the deuteration has not proceeded far enough to completely decouple v(OH).

I

b

a

I

3200

I

1

2400

*

cm4

I 3200

I

I

2400

I

Fig. 7. Effects of deuteration on v(OH) of different polymorphic solids of odd-number-carbon alcohols.

(a) Undecanol,

G form.

(b) Normal,

B form.

Decoupling to a singlet is not as obvious in the spectra of nonanol in Fig. 7 because of low-frequency shoulders seen on v(OH) (v(OD)) in both spectra. These absorptions (- 3200 and 2370 cm-’ in the upper spectrum, 3170 and - 2300 cm-’ in the lower spectrum) are due to the presence of small amounts of the G polymorph in the polycrystalline samples. The agreement of these frequencies with those of the G polymorph is, by itself, good evidence for polymorphic impurities in the nonanol sample. However, the single-crystal spectra offer the best evidence of this (Fig. 6) since the low-frequency shoulder (3170 cm-‘) has never been observed in spectra of single crystals of the B form. Polarized spectra of a single crystal of the X solid of partially-deuterated undecanol are shown in Fig. 8. Isotopic dilution was sufficient in this sample (- 70 %) to collapse the doublet structure of v(OH), but the breadth of v(OH) indicates there is still some coupling. Nevertheless, this nearly-decoupled v(OH) shows no polarization effects even though changes in the remainder of the spectrum pro% that polarizer and crystal were- prc&rly oriented for polar&ation measurementsJ. Mol.-Structure,

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2800

IN SOLID

*

I

A

317

ALCOHOLS

1400

I

I

!

1 1000

I 600

cm-’

Fig. 8. Polarized spectra of single-crystal, X form of partially-deuterated undecanol. 0” and 90” refer to polarizer settings. Orientation of polarizer with respect to crystallographic axis is not known. C. Liquid alcohols One other experiment is summarized in Fig. 9, which shows half-bandwidths of OH and OD stretching vibrations in liquid decanol. Similar data were obtained for other liquid alcohols. A slight decrease in bandwidth is observed as isotopic

OH/OD

hi2

100

/

0

a5

/

15

I

I

I

I

75

/

25

I

1

.45

/

55

I

I

40

/so

I

1

:,

I 2OOcm-’

15 /

I

I

I

I

I

I

I 85

OD

I

I

I

:,

IdO cnii~

Fig_ 9. Half-bandwidths of v(OH) .and v(OD) in partially-deuterated liquid decanol. J. iU&. Srructure, 1 (196+-68) 309-321

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dilution increases, but the resultant band is still very broad and alsp structureless. This indicates that coupling does affect the bandwidth of liquids; but only to a small extent.

DISCUSSION

In the preceding section we have shown that all the solid afcohols studied have two OH stretching vibrations. These bands may vary in frequency or intensity from one polymorphic form to another, but all polymorphs give two vibrations. These two vibrations collapse to a singlet when decoupled by isotopic dilution. In addition, the coupled OH doublet generally consists of one sharp and one broad component which give different frequency shifts with increasing pressure. The two components of the coupled doublet have different polarization properties, but the decoupled singlet shows no polarization effects. These characteristics are consistent with only one explanation, i.e., a hydrogen-bonded polymeric chain in which OH stretching vibrations are coupled through

nearest-neighbor

or first-order

coupling.

Consider

a planar

hydrogen

ROH, with the hydrocarbon portion (R) of the alcohol treated as a point mass as illustrated in Fig. 10. There are two vibrations due to the phase bond

chain,

V OH IN-PHASE

V

OH

OUT-OF-PHASE

Fig. 10. OH stretching vibrations in hydrogen-bond chain of a solid alcohol. (0) hydrocarbon portion (R) of alcohol molecule; (a) oxygen atoms; (0) hydroxyl hydrogen atoms.

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relationship of adjacent OH movements along the hydrogen bond chain. For the in-phase vibration, both protons are moving in the same direction, i.e., in the direction of the hydrogen bond. For the out-of-phase vibration, the protons move in opposite directions, one toward the hydrogen bond and one away from the hydrogen bond. We would expect the out-of-phase vibration to be at higher frequencies than the in-phase vibration. It is readily apparent from Fig. 10 that the in-phase vibration would favor a tautomerization, whereas the opposite movement of the protons in the out-ofphase vibration would not favor such a tautomerization. It is not unreasonable that such a tendency toward tautomerization would lead to a broadened absorption band structure for the in-phase vibration, while the lack of tautomeric contribution in the out-of-phase vibration would lead to a more normal, narrow band. The different pressure-induced shifts we observed also fit this explanation_ Compression results in shorter hydrogen bond distances which would lead to low-lrequency shifts for both components of v(OH). The in-phase vibration would be greatly facilitated by the consequently stronger hydrogen bond (shorter 0 - - - 0 distance). However, a stronger hydrogen bond would impede the out-of-phase OH vibration by opposing proton movement away from the hydrogen bond. Thus; a lowfrequency shift with pressure is expected for the out-of-phase vibration, but the magnitude of the shift would be less than that for the in-phase vibration. The resultant transition moments of the two components of v(OH) would be very nearly at right angles to each other, accounting for the experimentallyobserved polarization data for the coupled vibrations. The decoupled vibrations would also have transition moments at right angles due to orientation o.f the OH groups, but the decoupled vibrations wouid be completely equivalent and would show no changes with polarization. To reiterate these conclusions: coupling through first-order or nearestneighbor interaction results in splitting of the OH stretching vibration. Tautomeric tendencies contribute to only one component and lead to a broad absorption band and a greater frequency shift with increasing pressure for that component; the resultant transition moments of the two components would be near right angles to each other and the respective absorption would be strongly polarized: Nearest-neighbor coupling is the underlying reason for the breadth of v(OH) in solid alcohols. We must emphasize here that the coupling leads to an increased v(OH) bandwidth in two ways: (1) The coupling leads to splitting of the OH stretching vibration and this alone causes increased bandwidth; (2) the coupling favors a tendency towards tautomerism which also contributes to increased bandwidth for some components. We have found no way to explain the observed pressure, polarization, and mixed crystal data that does not involve the tautomerization argument as presented above and illustrated in Fig. lo_ Tautomerism was also involved in explaining spectral differences in polymorphic solids of formic acid6. Kishida and J. Mol. Structure,

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Nakamoto13 invoked a similar tautomeric effect in order to fit normal coordinate

calculations to observed frequencies for the cyclic dimer of formic acid. Bellamy and Rogasch r4 have shown that a tautb’meric mechanism is feasible for dimeric * systems such as Zthiapyrodone. Obviously, the extreme limit of these tautomeric structures is essentially equivalent to the mechanism, favored by Cannon” of proton transfer across the barrier in a double minimum potential function. However, we believe that broad-

ening is caused primarily by coupling, which does not require that any degree of tautomerism occur.* A tendency towards tautomerism then contributes substantially to further broadening of the band, but coupling is a necessary condition for the tautomerism to take place. The primary significance of these conclusions is their implication for future research. There is now increasing evidence that first-order coupling and a tendency towards tautomerism are general phenomena in highly-ordered hydrogen-bonded systems. This paper demonstrates their occurrence in solid alcohols, which are infinite chains of hydrogen-bonded polymers. Other work has indicated their occurrence in solid formic acid6 and solid phenol16 which also involve hydrogenbonded polymeric chains. Similar phenomena are strongly indicated in the cyclic dimer structure of formic acid13 occurring in the vapor state and in solution. Preliminary work” indicates the same is true for higher molecular-weight carboxylic acids which are dimeric in all physical states. Far-m work in our laboratory” and in otherslg* 2o does not give experimental support to theories of broadening in v(OH) through interaction with hydrogen-bond vibrations such as v(OH * . * 0). Likewise, no evidence has been found in the alcohol study to suggest band broadening from combinations of v(OH) with other fundamentals intensified by Fermi resonance. Fermi resonance has been established in formic21 and acetic acids22, but its effect on bandwidth is not clear. While the implications are clear for these highly-ordered hydrogen-bonded systems, major advances should also be possible in disordered systems such as liquid alcohols and phenols. The isotopic dilution experiments showed that first-order coupling does not cause band broadening in liquid alcohols, although such interaction contributes slightly to the observed breadth of v(OH). Obviously, then, different mechanisms operate in liquid and in solid alcohols. A major problem in explaining v(OH) bandwidths has been the generally believed necessity of devising a common theory for both liquids and solids. It is now clear that this is not the proper goal. Explanations for band broadening in liquids irivolving kT energy, thermal collisions and/or relaxation effects can no longer be discounted simply because they cannot be applied to solids. * We do not imply that the extreme limit of tautomerism, i.e. proton transfer, is ever reached. We are onIy concerned with the fact that for the in-phase vibration the movement of the protons is towards the tautomeric structure.

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SUMMARY

Polarized IR spectra of single crystals of solid alcohols have shown that the OH stretching vibration, v(OH), is always split into two components. Spectra of pattially-deuterated solid alcohols have demonstrated that the appearance of two’ bands is due to crystal splitting. This splitting can be explained by nearest-neighbor or first-order coupling of the vibrations of adjacent OH groups along the hydrogen bond chain. The decoupled OH stretching vibration of these alcohols gives a narrow singlet absorption with a half bandwidth as low as 30 cm-‘. Thus, v(OH) of solid alcohols is not inherently broad, but gains breadth as a result (direct or indirect) of the coupling between OH groups. A study of partially-deuterated liquid alcohols indicates that coupling plays only a small part in the v(OH) bandwidth in liquids. Therefore, different mechanisms control the OH bandwidth in liquid and solid alcohols_

REFERENCES

1 H. E. HALLAM in M. DAVIES (Ed.), Infrared Spectroscopy and Molecular Srructare, Elsevier, Amsterdam, 1963, pp. 410411. 2 G. C. PIMENTEL AND A. L. MCCLELLAN, The Hydrogen Bond, W. H. Freeman and Co., San Francisco, 1960, pp. 102-l 11 and 246-267. 3 N. SHEPPARD, in D. HAD~I (Ed.), Hydrogen Bonding, Pergamon Press, New York, 1959, pp. 85-106. 4 J. W. BRASCH, Spectrochim. Acta, 21 (1965) 1183. 5 J. W. BRAXH, J. Chem. Phys., 43 (1965) 3473. 6 Y. MIKAWA, R. J. JAKOESENAND J. W. BRASCH, 3. Chem. Phys., 45 (1966) 4750. 7 H. 5. HRO~TOWSKIAND G. C. PIMENTEL,J. Chem. Phys., 19 (1951) 661. 8 M. FALK AND E. WHALLEY, J. Chem. Phys., 34 (1961) 1554. 9 J. E. BERTIEAND E. WHALLEY, J. Chem. PJzys., 40 (1964) 1646. 10 S. ABRAHAMSSON,G. LAR~~ON AND E. VON SYLOW, Acra Cryst., 13 (1960) 770. 11 T. SETO, Mem. Coil. Sci. Kyoto Univ., Ser. A, 30 (1962) 89. 12 T. TASUMI, T. SHIMANOUCHI,A. WATANABE AND R. GOTO, Spectrochim. Acra, 20 (1964) 629. 13 S. KISHIDA AND K. NAKAMOTO, J. Chem. Phys., 41 (1964) 1558. 14 L. J. BELLAMY AND P. E. ROGAXH, Proc. Roy. Sot. (London), Ser. A, 257 (1960) 98. 15 C. G. CANNON, Spectrochim. Acta, 10 (1958) 341. 16 J. W. BRASCH, Y. MIKAWA AND R. J. JAKOBSEN,Proceedings of the XIII Colloquium Spectroscopicum Inrernationale, in press. 17 Y. MIUAWA, 3. W. BRASZH AND R. J. JAKOBSEN, Specrrochim. Acta, submitted for publication. 18 J. W. BRASCH,R. J. JAKOBSENAND Y.MIKAWA, paper presented at the 9th European Congress on Molecular Spectroscopy, Madrid, Spain, 1967. 19 G. STATZ AND E. LIPPERT, Ber. Bunsenges. Physik. Chem., in press. 20 C. PERCHARDAND A. NOVAK, J. Mol. Spectry., in press. 21 Y. MIKAWA, J. W. BRASCH AND R. J. JAKOBSEN,J. Mol. Specrry., in press. 22 M. HAURIE AND A. NOVAK, Specrrochbn. A&a, 21 (1965) 1217. J. Mol. Structure,

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