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H-Bonded Dimers: Librational Substructure of the OH Band of Proton Donors

H-Bonded Dimers: Librational Substructure of the OH Band of Proton Donors

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that could contribute simultaneously: one factor is local inhomogeneity via

different bond angles and/or lengths considered above. The other mechanism is energetic inhomogeneity of the band via combination tones between

the OH stretch and low-lying bridge bond vibrations. In associated alcohols

like ethanol or in water (see below) there is evidence for the first mechanism from time-resolved spectroscopy (78,79,89–91) and from numerical

investigations (92–94). Intimations of the second process were obtained

from theoretical investigations (95–97). For hydrogen-bonded dimers in a

polymer film the temperature dependence of transient spectral hole burning

strongly suggested that variation of the local environments accounts for

the large broadening of the proton-donor OH band (98). To tackle this

important question in more detail in the liquid phase, the special alcohol 2,2dimethyl-3-ethyl-3-pentanol (DMEP) was investigated by conventional and

time-resolved IR spectroscopy. Quantum statistical simulations of molecular clusters show that even in the neat liquid DMEP only monomers and

open dimers are present (99). The simple explanation is steric hindrance

by the CHn groups; obviously for the same reason the H bond of the open

dimer of DMEP is weaker as compared to ethanol dimers. The absence of

larger oligomers and the more well-defined local structure of DMEP dimers

make the system well suited for detailed study.

Examples of transient spectra taken with excitation adjusted to

the frequency position of proton acceptors/monomers at T D 295 K are

shown in Fig. 20 (isotropic component) (100). At early delay times of

1 ps (Fig. 20a) a sample bleaching at the frequency of the proton

acceptors/monomers 3625 cm 1 and, most important, at the proton

donor position 3520 cm 1 is readily seen. Simultaneously an induced

absorption builds up below 3480 cm 1 , representing red-shifted ESA of

species in the v D 1 level. Soon thereafter at 2 ps (Fig. 20b) the spectral

components have increased in amplitude, while at a later delay time of

10 ps (Fig. 20c — note different ordinate scale) the induced bleaching of

the sample dominates the transient spectrum at a reduced amplitude level.

To interpret the fast dynamics related to the bleaching of the donor band that

proceeds with a time constant <1 ps below our temporal resolution, it is

important to know the reorientational motion of the donor-acceptor H bond.

The information is derived from the measured induced dichroism with

probing at the excitation frequency. Reorientational relaxation proceeds

with time constants of 4 and 8 ps for the proton acceptor and donor

molecule, respectively (101). The (weak) H bond between two DMEP

molecules significantly slows down the reorientation of the bonded OH

group by a factor of 2 relative to the nonbonded species. It is concluded

Copyright © 2001 by Taylor & Francis Group, LLC

Figure 20 Isotropic transient spectra taken at 295 K and excitation of the

OH-stretching vibration of proton acceptors/monomers are shown in this figure.

Data are measured at 1 ps (a), 2 ps (b), and 10 ps (c), while excitation is adjusted

to 3625 cm 1 , marked by an arrow. Migration of the vibrational quantum from

the directly excited proton acceptor to the proton donor can be clearly seen from

the spectra. Measured filled squares, solid line is the sum of the various spectral

components indicated by different line styles.

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that the initial fast bleaching of the donor absorption cannot be explained

by structural changes, i.e., bond breaking.

Further transient spectra taken with an excitation frequency of

3430 cm 1 (see Fig. 21) show no measurable absorption change at the

proton acceptor frequency 3625 cm 1 , i.e., some cooperative effect does

not account for the fast spectral dynamics shown in Fig. 20. The bleaching

of the proton donor absorption while exciting proton acceptor OH groups

is explained by vibrational energy transfer, strongly supported by the

observation of further red-shifted ESA in Fig. 20. The finding corresponds

to the notion of energy migration in ethanol oligomers after excitation of

internal OH groups (see above).

The analysis of the bleaching of the proton donor absorption in

Fig. 20b suggests two spectral subcomponents (see calculated curves). To

investigate the effect in detail, transient spectroscopy at four different

excitation frequencies within the proton donor OH-stretching band was

performed at 260 K (102). One example is shown in Fig. 21. The

isotropic signal component is plotted, i.e., population dynamics without

a reorientational contribution is measured; excitation is carried out at

3430 cm 1 for OH groups in the red wing of the dimer band. The

conventional absorption spectrum of the sample is also presented in Fig. 21c

(dash-dotted line, right-hand ordinate scale). It consists of a narrow line

at 3625 cm 1 , attributed to absorption of monomers and proton acceptor

groups, and a broader band at 3505 cm 1 of asymmetrical shape. The latter

represents the OH groups with proton-donor function in dimers, as shown

by comparison with other diluted alcohols (80).

It is interesting to see in the transient spectrum of Fig. 21a at early

times the double-peak of a bleaching structure. The band has already relaxed

to a single maximum with shoulder for tD D 0 (Fig. 21b), followed by

a shift of the transient band towards 3500 cm 1 (Fig. 21c). Again, the

bleaching of the sample for > 3350 cm 1 reflects the depletion of the

vibrational ground state v D 0 upon excitation. Correspondingly, induced

absorption of molecules in the first excited state is observed at frequencies of <3350 cm 1 . The time-resolved spectrum is readily attributed to

a discrete substructure of the transient band, as suggested by the multiple

maxima of the bleaching and ESA. The bleaching feature can be well

described by a time-dependent spectral hole at the excitation frequency

with approximately Lorentzian shape and width of 45 š 5 cm 1 as well

as additional components assigned to satellite holes with different time

evolutions to account for the complex, rapidly changing band shape. The

data suggest an approximately constant frequency spacing of 35 š 5 cm 1

Copyright © 2001 by Taylor & Francis Group, LLC

Figure 21 Conventional spectrum (c; dash-dotted line; right-hand ordinate scale)

and transient spectra of a 2 M DMEP and CCl4 mixture (isotropic signal) in the

OH-stretching region taken at 260 K for different delay times: 2 ps (a), 0 ps

(b), and 2 ps (c); excitation adjusted to 3430 cm 1 (marked by a vertical arrow);

experimental points, calculated curves.

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between the subcomponents. Satellite maxima are also seen in the induced

absorption and assigned to inverse spectral holes, but with a different

frequency distance of 45 š 5 cm 1 . The decomposition of the spectra is

indicated in Fig. 21 by calculated curves. Corresponding spectral holes

(inverse holes) are plotted with the same line style.

We have investigated the transient spectra of DMEP solutions in the

delay interval 2 to 10 ps for further excitation frequencies in the range

3430–3555 cm 1 . The data (not shown) are consistently fitted with a set

of Lorentzian-shaped spectral lines (transient holes and inverse holes) by

determining the minimum deviation numerically (65). Some results are

summarized in Table 3. The transition frequency of holes v D 0 ! v D

1 is denoted by 01 , while 12 is that of inverse holes v D 1 ! v D

2 . Spectral widths of 45 š 5 cm 1 and 55 š 5 cm 1 for the holes and

inverse holes, respectively, account for the experimental data. In order to

interpret the spectral holes and satellite holes in the sample bleaching, it

is important to recall that reorientational and similarly structural relaxation proceeds slowly with a time constant of 8 ps and cannot explain

the fast dynamics in Fig. 21. Consequently the spectral components are

interpreted in terms of an energetic substructure, i.e., combination tones

between the proton donor OH stretch and a low-frequency hydrogen bridge

bond vibration. The constant frequency spacing of the measured spectral

holes suggests that one mode is dominant and is considered in our simplified physical picture in the following. A Franck-Condon–like situation,

illustrated by Fig. 22, is assumed with significant anharmonic coupling

between the high-frequency OH-stretching vibration (quantum number v)

of the donor molecule and the low-frequency mode of the bridge (quantum

numbers n, n0 ; frequency nn0 ). Anharmonic frequency changes of nn0 ,

on the other hand, with quantum number n are considered to be small

Table 3 Frequency Positions of the Spectral Holes and Inverse Holes as

Determined by Fitting the Time-Resolved Spectra Taken at Various Delay Times,

Excitation Frequencies, and Temperatures of 260, 298, and 343 K



(cm 1 )

(cm 1 )

n D 2

n D 1

n D 0







n D




n D




n D




The spectral width of the components derived from fitting of the sample bleaching and

the excited-state absorption amounts to 45 and 55 cm 1 , respectively. All numbers with an

accuracy of š5 cm 1 .

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Figure 22 Simplified energy level scheme that accounts for the spectral observations with librational substructure (horizontal lines, n, n0 ) of the OH-stretching

levels (solid curves, v); the thick vertical arrow represents an example for the excitation process with n D 2; thin solid and dashed vertical arrows denote possible

probing transitions for different n for 01 and 12 transitions, respectively.

compared to the measured holewidths and are omitted. In addition to the

selection rules of the harmonic case, v D 1, n D n0 n D 0, transitions

n D š1, š2, . . . must be included (see Fig. 22). Tentatively we assign the

maximum of the conventional absorption band to a superposition of n D 0

transitions, suggesting the value OH D 3505 cm 1 . In the time-resolved

measurements, the pump pulse promotes a subensemble of molecules from

thermally populated 0, n levels to a modified set 1, n0 , the population

changes depending on the individual cross sections. In Fig. 22 the situation

for pumping at 3430 cm 1 is considered corresponding to n D 2 transitions (thick vertical arrow). Since the n’s are lowered in the excitation step,

excess population of the lower quantum numbers is generated compared to

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the Boltzmann distribution. The perturbed occupation of the low-frequency

mode gives rise to subsequent relaxation and transient bandshape changes,

i.e., spectral holes and, correspondingly, inverse holes for v D 1 ! v D 2

probing transitions. In this picture the frequency spacing of the components

is equal to the mode transition frequencies in the v D 1 state ( 01 transitions, bleaching) and v D 2 level ( 12 transitions, ESA), respectively. The

depicted probing transitions in Fig. 22 are to be taken as examples only.

The measured induced transmission change at 3430 cm 1 is equal to the

sum over all 01 -probing transitions with n D 2 for the case assumed

in the figure.

This interpretation is supported by the temporal evolution of

the spectral components at, for example, 3475 cm 1 n D 1 and

3435 cm 1 n D 2 as derived from a decomposition of the measured

transients. The decay times of the two components are determined to

4.5 š 0.5 ps and 3 š 0.5 ps, respectively. Two mechanisms obviously

contribute to the time constants: population decay of the OH-stretching

mode and population redistribution among the bridge bond vibration. Since

the excess population of the lower n levels is transferred to larger n, probing

with n D 2 yields a faster decay compared to n D 1, as indicated

by the data. For larger delay times the system does not return exactly to

its initial situation, since the deposited OH-excitation energy thermalizes.

From the temporal evolution of the ESA component at 3240 cm 1 derived

from fitting of the transient spectra, we determine a lifetime of the proton

donor OH stretch of 3 š 0.5 ps.

Tentatively we assign the low-frequency mode to the bending vibration of the hydrogen bridge bond, the frequency value nn0 ' 35 cm 1

referring to the OH level v D 1. In the low-frequency Raman spectrum of

neat DMEP a band around 27 cm 1 with width of approximately 50 cm 1

shows up that may be attributed to the OH Ð Ð Ð H-bending mode in the

v D 0 state (G. E. Walrafen, unpublished). The difference between the two

numbers may be related to the different OH levels, v D 0, and 1. For

comparison, in water a band located at 50 cm 1 was proposed to represent

the bridge-bending mode [103]. With regard to the notably weaker H bond

of the present dimers, our interpretation appears reasonable.

The physical situation is supported by a comparison of computed

bandshapes with the measured conventional dimer absorption, the temperature dependence of which is well described. Figure 23 illustrates the situation with three spectra of a 2 M DMEP solution in CCl4 taken at 260 K (a),

298 K (b), and 343 K (c). The measured data points can be fitted well with

the already determined spectral components listed in Table 3. Again, the

Copyright © 2001 by Taylor & Francis Group, LLC

Figure 23 Here we show conventional absorption spectra of the 2 M DMEP

and CCl4 mixture in the OH-stretching region at the three indicated temperatures

of 260 K (a), 298 K (b), and 343 K (c). The spectral components derived from

time-resolved spectroscopy and denoted by different line styles account well for

the measured data.

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different spectral components are depicted with different line styles. A shift

of the peak of the distribution is noted from the component at 3505 cm 1

(260 K) to the one at 3545 cm 1 (343 K). This is related to an overall

weakening of the H bond with increasing temperature, which results from

excitation of the low-frequency bridge bond vibrations to higher quantum

numbers in correspondence to the result of the fitting of the proton donor

OH band.

E. Investigations of Isotopic Water Mixtures

In the previous sections evidence for spectral holes in the absorption band

related to the OH-stretching mode of H-bonded alcohols was discussed

and interpreted in terms of different H-bonded structures (associated

ethanol) or combination tones between the OH-stretch and an H-bridge

bond vibration (DMEP). It is possible to use the same techniques for

experiments on water, the most abundant liquid in nature. Water has been

investigated by a variety of experimental techniques during recent decades

(1,2). Nevertheless, a clear physical picture of the structure and structural

relaxation of water was not established because time-resolved experiments

were lacking. Contradicting results were derived from dielectric relaxation,

nuclear magnetic resonance, and Raman spectroscopy. From Rayleigh

scattering, for example, time constants for structural relaxation on the 1 ps

time scale were reported as derived from fitting of the linewidth (104).

The potential of the OH-stretching vibration of water as a local probe

for hydrogen bonding was realized in early investigations (1). Its relation to the strength of hydrogen bonds was utilized for infrared (105,106)

and Raman (107) spectroscopic investigations in order to identify different

structural components from observed temperature or pressure changes. The

first time-resolved investigation with two-color infrared spectroscopy was

performed for the isotopic mixture HDO in D2 O with the moderate time

resolution of ³10 ps pulses. Evidence was reported for three major spectral

components in the range 3340–3520 cm 1 at room temperature (89). The

recent advance of IR pulse generation allows one to tackle the question of

structural relaxation in water in much more detail (108).

Experimental proof of the inhomogeneous broadening of the OHstretching band of water is straightforward, following the lines of the

ethanol studies discussed above (89). Because of its simpler OH spectrum

with only one stretching mode above 3000 cm 1 and in order to minimize

local heating effects, it is advantageous to investigate highly diluted HDO

in D2 O.

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The low concentration of 0.8 M ensures that the OH mode is utilized

as an ultrafast local probe for hydrogen bonds while quasi-resonant energy

transfer to neighboring OH groups is negligible.

To focus on the existence of spectral holes, we first show transient

spectra in Fig. 24 for excitation around the peak position of the OH band

at tD D 0 and three temperature values (91). The displayed anisotropic

component emphasizes the primary features of the molecular excitation that

maintain the linear polarization of the pump pulse. At T D 273 K (Fig. 24a)

the spectral bleaching is mainly governed by a rather broad component II

(dashed curve) while two further contributions I and III give minor contributions to the wings of the measured band around the pump frequency.

A spectral hole is obviously not observed. The broad induced absorption

around 3160 cm 1 is interpreted as excited-state absorption. Changing to

room temperature (Fig. 24b) the spectrum looks distinctly different with

a narrow component riding on a broad bleaching band. A spectral hole

(dotted curve) is inferred from the data in addition to the components I–III

already observed in Fig. 24a. At a higher temperature of 343 K (Fig. 24c)

a similar band shape is measured apart from a smaller amplitude. The main

contributions to the bleaching are assigned to component III (long-dashed)

and the spectral hole (dotted). The latter is well described by a Lorentzian

shape with width 45 š 10 cm 1 and located at the frequency position of the

pump pulse, as verified by additional measurements with varying excitation

in the blue wing of the OH absorption.

The spectral hole is attributed to a depletion of the vibrational ground

state and corresponding population of the first excited state of a molecular

subensemble. The lifetime of the hole is determined from the temporal

evolution of its peak amplitude derived from fitting of the respective transient spectra taken at different delay times, yielding h D 1 š 0.4 ps. The

time constant represents information on spectral changes initiated by structural relaxation with component III. The width of the spectral hole and

its lifetime differ from the results for weak ethanol solutions. Because the

spectral holes in water are only seen within component III located in the

blue wing of the OH band of HDO, the information on structural relaxation

refers to species with weaker hydrogen bonds. The weaker bonds seem to

allow for a broader variety of OH frequencies (bond lengths/angles) leading

to inhomogeneous broadening of component III.

In order to identify different spectral components within the OH

band of HDO more clearly, additional transient spectra with a variation

of Pu (109) are presented in Fig. 25, where the respective components are

predominantly excited. The dependence of the OH bleaching at 275 K and

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H-Bonded Dimers: Librational Substructure of the OH Band of Proton Donors

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