Probing ultrafast vibrational dynamics of intramolecular hydrogen bonds with broadband infrared pump-probe spectroscopy

Probing ultrafast vibrational dynamics of intramolecular hydrogen bonds with broadband infrared pump-probe spectroscopy

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Accepted Manuscript Probing ultrafast vibrational dynamics of intramolecular hydrogen bonds with broadband infrared pump-probe spectroscopy Madhumitha Balasubramanian, Anthony Reynolds, Tyler Blair, Munira Khalil PII: DOI: Reference:

S0301-0104(18)30985-6 https://doi.org/10.1016/j.chemphys.2018.11.018 CHEMPH 10250

To appear in:

Chemical Physics

Received Date: Revised Date: Accepted Date:

7 September 2018 25 November 2018 25 November 2018

Please cite this article as: M. Balasubramanian, A. Reynolds, T. Blair, M. Khalil, Probing ultrafast vibrational dynamics of intramolecular hydrogen bonds with broadband infrared pump-probe spectroscopy, Chemical Physics (2018), doi: https://doi.org/10.1016/j.chemphys.2018.11.018

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Probing ultrafast vibrational dynamics of intramolecular hydrogen bonds with broadband infrared pump-probe spectroscopy Madhumitha Balasubramanian, Anthony Reynolds, Tyler Blair, and Munira Khalil*

*

Corresponding author. Email address: [email protected] (Munira Khalil)

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Department of Chemistry, University of Washington, Box 351700, Seattle, WA 98195, USA.

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ABSTRACT In this study we use ultrafast infrared (IR) pump-probe spectroscopy to study the vibrational dynamics of two intramolecular hydrogen bonded complexes, 10hydroxybenzo[h]quinoline (HBQ) and 2-(2′-hydroxyphenyl)benzothiazole (HBT) dissolved in carbon tetrachloride. We pump the νOH mode and probe across 1800 cm-1 to 3300 cm-1 using a broadband IR probe pulse. Both systems exhibit large anharmonicities (>250 cm-1), which are characteristic of medium strong hydrogen bonded systems. Additionally, we observe the coherent beating of low-frequency modes of 248 cm-1 and 118 cm-1 across the νOH stretch of HBQ and HBT, respectively. Anharmonic frequency calculations at the DFT level identify these low-frequency structural modes as in-plane bending modes, which modulate the intramolecular hydrogen bonding distance. These findings provide evidence that the anharmonic nature of the fundamental νOH stretch is primarily dictated by the coupling to low-frequency structural modes and these motions play an important role in the hydrogen bonding dynamics.

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1. Introduction Understanding the microscopic interactions governing the vibrational dynamics of inter- and intramolecular hydrogen bonds is important for elucidating the structure-function relationships of molecular complexes in biology, chemistry, and material science. Of particular interest is learning how the transfer of the proton from a donor to an acceptor species is modulated by the intramolecular vibrational motions of the molecules and the surrounding solvent on the ground and electronic excited states. Intramolecular proton transfer in the electronic excited state has been widely studied for developing dye molecules and molecular switches. Additionally, excited state intramolecular proton transfer (ESIPT) is a fundamental chemical process crucial in proton coupled electron transfer processes in photochemistry and photobiology [1, 2]. Vibrational spectroscopy of the OH stretching (νOH) mode is an excellent reporter of the structural dynamics of inter- and intramolecular hydrogen bonds in the gas and condensed phases [3-5]. In this study we use femtosecond (fs) broadband IR (BBIR) spectroscopy to identify low-frequency vibrations anharmonically coupled to the νOH modes of two intramolecular hydrogen bonded complexes,

10-Hydroxybenzo[h]quinoline

(HBQ)

and

2-(2′-

Hydroxyphenyl)benzothiazole (HBT), dissolved in CCl4. Identifying and measuring the coupling of low frequency vibrations to the high frequency νOH mode is crucial for accurately describing the lineshape of the broad νOH mode and for understanding

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the structural dynamics of the hydrogen bond and microscopic details of proton transfer in the ground and electronic excited states. The photophysics of HBQ has been studied experimentally using steady-state absorption, femtosecond fluorescence up-conversion, and transient absorption, and theoretically to understand the microscopic details of the excited state intramolecular proton transfer process resulting in the enol-keto tautomerization [6-16]. Timeresolved measurements on HBQ in solution show that the ESIPT process occurs within 13 fs and the relaxation of the keto state includes coherent vibrational motion from several skeletal modes between 100 – 500 cm-1 [8, 12-14]. The experimental results are supported by simulations of the ESIPT process in HBQ dissolved in a nonpolar solvent [11]. Excited state intramolecular proton transfer in HBT has also been studied both experimentally and theoretically [8, 17-27]. In contrast to HBQ, the proton transfer process takes 62 fs in HBT and does not show an isotope dependence [8]. Transient IR spectroscopy and accompanying calculations suggest the coupling of electron motion along with the proton transfer following UV excitation [19]. Similar to HBQ, the relaxation on the photoexcited keto state of HBT involves the motion of several low-frequency skeletal modes which modulate the O...N distance [8, 23, 25]. As described above, the photophysics of HBQ and HBT have been the subject of several studies. However, the vibrational dynamics of the νOH mode in the

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electronic ground state have not been studied for HBQ and HBT. We note that deuterated version of the HBT (DBT) in toluene has been studied using femtosecond IR pump-probe spectroscopy and the results show an oscillatory feature in the relaxation of the OD stretching vibration corresponding to a low-frequency vibrational mode at 118 cm-1. This wave packet motion has been attributed to a lowfrequency mode that modulates the length of the OD bond [28]. The ground state studies on DBT results follow in the footsteps of various computational and excited state studies mentioned previously and point to the importance of anharmonically coupled low- and high-frequency motions in intramolecular hydrogen bonded complexes. This study uses femtosecond IR pump-probe spectroscopy to probe the ground state vibrational spectroscopy of νOH mode in HBQ and HBT. We use our broadband IR (BBIR) source to probe the entire fundamental νOH stretch and the transition from the first excited state to the second excited state (νOH1→2). The pumpprobe results elucidate the low-frequency motions coupled to the fundamental νOH vibration.

The experimental results are compared with anharmonic ab initio

calculations to understand how the anharmonic vibrational couplings dictate the vibrational dynamics of the νOH stretch in intramolecular hydrogen bonded complexes. 2. Methods

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HBQ and HBT were obtained commercially from TCI chemicals and Sigma Aldrich, respectively, and used without further purification. Both molecules were dissolved in anhydrous carbon tetrachloride (CCl4), purchased from Sigma Aldrich, at a concentration of ~300 mM. The vibrational spectra of the νOH mode in the HBQ and HBT samples were obtained using a JASCO FT/IR-4100 spectrometer with 2 cm-1 resolution. The solution samples were placed in a commercial sample cell which utilizes CaF2 windows separated by a 250 μm thick Teflon spacer. The use of CCl4 as the solvent minimizes spectral interference from the solvent across the vibrational spectrum of the νOH. The pump-probe experiments utilized femtosecond mid-infrared laser pulses produced using a commercial Ti-Sapphire oscillator and regenerative amplifier, which generated 40 fs pulses centered at 800 nm at a repetition rate of 1 kHz with pulse energies of 4 mJ/pulse. The fundamental 800 nm pulses were split and used as pump, probe, or gating pulses (for pulse characterization). The IR pump pulses were generated in a home-built, β-BBO based, double-pass optical parametric amplifier (OPA) followed by difference frequency generation (DFG) in a 500 μm thick AgGaS2 crystal. For HBQ, the IR pump pulse was centered at 2611 cm-1 with a bandwidth of 530 cm-1 (full width at 10%) as shown by the blue spectrum in Fig. 1(a). For the HBT sample, the pump pulse was centered at 2888 cm-1 with a bandwidth of 550 cm-1 (full width at 10%). The pump pulses were temporally

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characterized using the cross-correlation frequency resolved optical gating (XFROG) technique [29]. Briefly, the IR pump pulse was mixed with a 800 nm pulse in a 100 μm thick Type I Lithium Niobate (LiNbO3) crystal and the resultant sum frequency generated (SFG) signal was spectrally dispersed with a spectrometer (Thorlabs CCS200) as a function of the time delay between the input pulses. The XFROG measurement revealed that the IR pump pulses were ~60 fs in duration. The 800 nm reference field used in the XFROG measurement was separately characterized by spectral phase interferometry for direct electric field reconstruction (SPIDER) and used as an input in the X-FROG pulse retrieval algorithm. The BBIR probe pulse was generated by using the previously described process of filamentation of the fundamental (800 nm, 3.5 mJ) and the second harmonic (400 nm) pulses in a pressure controlled gas cell [30]. The BBIR beam is isolated from the 800 nm and 400 nm pulses by transmission through a 250-μm thick silicon (Si) wafer at Brewster’s angle before being collimated. The BBIR probe pulse was temporally compressed using the 4f deformable mirror (DM) pulse shaper that has been outlined previously [31]. To help with the temporal compression 2 mm antireflection (AR) coated Germanium was added in the BBIR path along with 1 mm CaF2 to compensate for the sample cell and other transmissive optical elements in the probe beam path.

A genetic algorithm was used to optimize the pulse

compression through a feedback loop that maximizes the detected SFG intensity

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generated from mixing the 800 nm gating pulse and BBIR pulse in a LiNbO3 crystal. For the HBQ and HBT pump-probe experiments, the pressure in the gas cell was set at 1000 Torr which tuned the probe spectrum to center it at 2600 cm-1 with a spectral width greater than 1500 cm-1 (full width at 10% max) as seen in Figure 1 (black traces, right axis). The Fourier transform of the BBIR spectrum is plotted in Fig. S1 of the supplementary information and shows minor oscillations in the wings (less than 0.2% beyond 200 fs of the primary pulse) resulting from the CO2 absorption at ~ 2350 cm-1. The electric field reconstruction of the BBIR was performed using XFROG and resulted in pulse width of ~30fs. Any residual chirp in the BBIR pulse is corrected using the instrument response function measured using the pump-probe data for the solvent, CCl4 (see Fig. S2). The pump and probe beams with the same polarization were focused noncollinearly into the sample using an off-axis parabolic mirror (f = 101.5 mm). The focused pump beam had a spot size of ~150 μm and pulse energy of 400 nJ/pulse. The probe pulse was focused to a spot size of <100 μm and pulse energy of ~50nJ/pulse. The time delay,t, between the pump and probe pulses is adjusted with a computer-controlled translation stage (Newport, XMS100). The pump pulse is blocked after the sample and the pump-probe signal is dispersed at the focal plane of a 0.19 m spectrometer (Triax 190, Horiba Jobin Yvon, 75 grooves/mm grating) using a 2 × 64 pixel mercury cadmium telluride (MCT) array detector (IR0144,

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Infrared Systems Development). The signal was collected using band pass filters (either 2.5–5.1 μm or 4.5–8.5 μm) to eliminate the higher diffraction orders from the grating in the spectrometer as a result of the broad bandwidth of the BBIR pulse. A total of 6 grating positions (centered at 2700 nm, 3350 nm, 4000 nm, 4650 nm, 5300 nm and 5950 nm) were used to piece together the spectrally dispersed BBIR probe pulse. The number of grating positions is dictated by the groove-density on the grating, desired spectral resolution in ω3 and the number of available pixels in the MCT detector. The averaged data consists of ~80000 shots for both HBQ and HBT. The instrument response calculated from a solvent only sample is 180 fs across the BBIR probe spectrum. A chopper was placed in the pump arm allowing the collection of the weak pump-probe signal at 500 Hz and discriminating it from the much stronger probe pulse. Density functional theory (DFT) calculations of HBQ and HBT were conducted in the Gaussian 16 software package using the unrestricted Becke, 3parameter, Lee-Yang-Parr (u-B3LYP) functional and 6-311++G (d,p) basis set [32]. A polarizable continuum model was used to account for the solvation of these compounds in CCl4. Following geometry optimization vibrational frequencies were calculated using Gaussian’s standard anharmonic frequency calculation settings. An integration grid of 150 radial points and 770 angular points was utilized to ensure

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accuracy of the optimized equilibrium structures and subsequent anharmonic calculations [33]. 3. Results and Discussion The FTIR spectra of HBQ and HBT dissolved in CCl4 are plotted in Figure 1. The νOH peak is centered at 2771 cm-1 and 2930 cm-1 for HBQ and HBT, respectively. It is well known that the high frequency νOH mode of hydrogen bonded complexes is red-shifted with respect to the νOH stretching vibration in non-hydrogen bonded systems and is significantly broadened [3]. The blue shift of the νOH mode for HBT compared to HBQ is representative of the strength of the intramolecular hydrogen bonding, which is greater in the latter complex. X-ray crystallography studies of HBQ have measured two independent molecules in the crystal structure with the

Figure 1: Solvent (CCl4) subtracted FTIR spectra of the HBQ (a) and HBT (b) compounds in red (left axis, red). The molecular structures of the compounds (HBQ and HBT) are shown above the respective FTIR spectra. The normalized pump and probe spectra are shown in blue-dotted and black-dashed respectively (right axis, black). The dashed black line is a trace of the broadband mid-IR spectral content of the probe pulse. The dotted blue line is a trace of the DFG mid-IR spectral content of the pump pulse.

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O...N distance across the intramolecular hydrogen bond ranging from 2.562-2.573 Å classifying it as a medium strong hydrogen-bonded complex [34]. While the crystal structure of HBT has not been published, our computed structures of HBT and HBQ give O...N distances of 2.628 and 2.600 Å, respectively. This agrees with the intramolecular H-bond being stronger for HBQ. The lineshapes of the νOH mode for HBQ and HBT are complex and span a large spectral region from 2300 to 3280 cm-1. The νOH mode overlaps with the aromatic νCH modes that can be observed as the sharp feature at 3061 cm-1 and 3069 cm-1 for HBQ and HBT, respectively. In the HBQ νOH spectrum, spectroscopic features with 60 cm-1 spacing are seen across 2600-2900 cm-1. The HBT νOH spectrum has similar features with variable spacing of 80-100 cm-1 across 2770 - 2990 cm-1. The complex lineshapes of the νOH mode in HBQ and HBT are similar to those seen in other interand intramolecular hydrogen bonded species such as the cyclic dimers of acetic and benzoic acid and intramolecular complexes like phthalic acid monomethylester (PMME-d) [28, 35-41]. Hydrogen bonding systems are characterized by the νOH mode having a large bandwidth and the substructure seen in HBQ and HBT is attributed commonly to Frank-Condon-like progressions due to anharmonic coupling to low frequency structural modes [42, 43]. The broad nature of the νOH mode in H-bonding species, poses experimental challenges for coherently pumping and probing the sample of interest. The recent incorporation of BBIR sources in

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pump-probe and 2D IR experiments has provided greater insight into H-bonding dynamics [44, 45].

In this work, the BBIR source allows for probing the

fundamental (0→1) and excited state (1→2) transitions of the intramolecular νOH mode in HBQ and HBT.

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Figure 2: The normalized spectrally dispersed pump-probe spectra (ΔA) of 300 mM HBQ (a) and 300 mM HBT (b) in CCl4. The contours are plotted at 10% intervals from -1 to +1.

The IR spectrally dispersed pump-probe spectra of HBQ and HBT dissolved in CCl4 solvent are shown in Figure 2 where the signal frequency (ω3) is plotted as a function of the pump-probe time delay (t2). For both samples, the signal decays very quickly. The negative bleach signal is centered at the peak of the FTIR transition. The positive excited state absorption signal is red-shifted from the bleach and corresponds to the (1→2) transition. The lack of pump-probe data at ~2300 cm-1

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Commented [A1]: Revised figure with color bar.

results from the strong absorption of the pump-probe spectra due to atmospheric CO2 absorption (see BBIR spectrum in Fig. 1). In dispersed IR pump-probe experiments, the frequency difference between the bleach and excited state absorption signals is a measure of the vibrational anharmonicity when the linewidths of the individual vibrational transitions are smaller than the anharmonicity. This is not the case for hydrogen bonded systems; because of this fact and the overlapping CO2 absorption, we estimate that the anharmonicity of the νOH mode in HBQ and HBT is greater than 250 cm-1. Similar phenol–OH(OD)-pyridine systems found that the anharmonicities of the νOH vibration were between 180-250 cm-1 [46]. The decay of the pump-probe signal in Fig. 2 reports on the vibrational dynamics of the νOH mode in HBQ and HBT. We fit the signal frequencies with a biexponential decay function (see Fig. S3). The IR pump-probe data from both samples exhibit a very fast decay, which is fit with a timescale less than 100 fs. These fast dynamics are within the instrument response function of the experiment. The fits to the data reveal a longer decay, which is attributed the νOH vibrational dynamics. The fits to the bleach frequencies (between 2700 cm-1 to 2900 cm-1) for HBQ, yielded decay times ranging from 2.3±0.5 ps to 1.0±0.3 ps. The decrease in vibrational dynamics across the 200 cm-1 range for HBQ could be due to ultrafast structural heterogeneity or other dynamical processes [47]. The fits to the signal frequencies corresponding to the 1→2 transition (between 2000 cm-1 to 2200 cm-1) resulted in decay times of

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0.8±0.2 ps. Based on our fitting, we note that the vibrational dynamics involving the 1→2 transition are faster than those associated with the fundamental transition for HBQ.

The differing time-scales could result from coupling to different

vibrational modes in the molecule as discussed below. Similarly, the HBT data exhibits slightly different timescales for the bleach frequencies and excited state absorption frequencies. The timescales for excited-state absorption frequencies between 1800 cm-1 to 2000 cm-1 are ~1.4 ps. The timescale for the decay of the bleach frequencies (between 2950 cm-1 to 3100 cm-1) have time scales in the range from 1.5±0.4 ps to 1.1±0.1 ps. Unlike the case of HBQ, there is no variation in the timescales across the probe frequency window. Previous IR pump-probe study of the νOD mode in DBT dissolved in toluene had measured slower vibrational relaxation dynamics with decay time scales ranging from 3-6 ps [28]. The faster decay of the νOH mode in HBT compared to the previous study in DBT could be due to H/D isotope effects or access to different vibrational relaxation pathways. The longer vibrational decay time scales of the νOH mode in HBQ and HBT are reminiscent of the lifetime of the OH stretch which is typically ~1 ps for hydrogen bonding liquids and in inter- and intramolecular complexes. We note that for our medium strong H-bonded systems, the ~1 ps decay time could be associated with thermal equilibration dynamics of a vibrationally hot molecule and decay of the lowfrequency oscillations.

We also see a non-zero offset in our pump-probe

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experiments on longer timescales (> 6 ps), which we attribute to equilibration dynamics, perhaps related to the relaxation of a “hot” ground state [48]. A complete description of the vibrational dynamics of the OH stretch would require measuring the anharmonic coupling to the other vibrational modes in the system, to develop a quantitative understanding of the relaxation pathways possible in HBQ and HBT. The strong coupling of the νOH mode with νCO and ring modes has been noted in similar systems [49-51]. This is not the subject of this work. Instead we focus on the low-frequency oscillations observed in the pump-probe spectra.

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The low-frequency oscillations are clearly seen in the residual of the biexponential

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fits described above. In Figure 3, an example is shown for a particular frequency in

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the bleach signal for the HBQ and HBT samples. The residual traces are Fourier

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transformed and the power spectra are plotted in the insets of Figures 3(a) and (b).

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The power spectrum of the oscillatory signals in the HBQ sample reveal frequencies

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at 245 and 220 cm-1. The pump-probe signal for HBT at 2900 cm-1 shows strong

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oscillations (Figure 3(b)). The power spectrum of the residual trace reveals an

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intense feature at 118 cm-1 and weaker overlapping features at 170 and 220 cm-1. The 245 cm-1 mode seen in our ground state IR pump-probe work in HBQ has been observed in the ESIPT process where the proton transfer resulted in the coherent

Figure 3: Oscillatory signals in the spectrally dispersed IR pump-probe spectra of HBQ (a) and HBT (b). The pump-probe signals (black) are plotted along with the bi-exponential fit (grey). The residual signal calculated by subtracting the data from the best fit is plotted in red. The amplitude of the Fourier transform of the residual signal is plotted in the inset. The grey lines and their relative intensities are the results from the ab initio anharmonic frequency calculations (see text and Table 1). The HBQ and HBT pump-probe signals are measured at 2771 cm-1 and 2900 cm-1 respectively.

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excitation of several vibrational modes including one at 242 cm-1 [12, 14]. The 118 cm-1 mode seen in our ground state IR pump-probe work in HBQ has also been observed in the IR pump-probe measurements on DBQ and in the ESIPT studies, where a vibrational mode is coherently excited on the excited state keto surface at ~100 cm-1 [12, 28, 52]. The differences in frequencies of the vibrational modes in the ground and excited state reflect the difference in molecular structure and the effects of tautomerization following ESIPT. To eliminate the possibility of the lowfrequency modes arising from the CCl4, the Raman modes of the solvent should be compared with the frequencies noted above. Previous studies have measured the Raman modes of CCl4 at 218 cm-1, 315 cm-1, and 464 cm-1 [53]. These solvent modes are a possibility for the presence of the shoulder at 220 cm-1 in HBQ data (inset of Fig. 3(a)). The presence of low-frequency oscillations on νOH modes of other medium strong intramolecular H-bonded systems have been observed previously and have been discussed as the manifestation of the anharmonic coupling between the high and low-frequency vibrational modes in the time-domain [28,3637].

Our analysis of the low-frequency motions (described below) and the

accompanying DFT based anharmonic frequency calculations demonstrate how the low-frequency modes modulate the high frequency νOH stretch. The analysis described in Fig. 3 for a single detection frequency is repeated across the signal frequencies spanning the fundamental νOH transition. The resulting

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correlation spectra plotted as ω2 vs. ω3 are shown in Figure 4. These spectra provide

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a map of the coupling of the low-frequency modes oscillating during the pump-probe

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delay (t2) with the detected signal frequency (ω3) revealing how the high-frequency

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νOH stretch is modulated by low-frequency vibrational modes [36]. The correlation

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spectrum for HBQ (Figure 4(a)) shows the presence of the 245 cm-1 mode across the

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fundamental region of the high frequency νOH stretch. For this mode, the strongest

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coupling is seen in the wings of the IR pump pulse. Previous femtosecond transient

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absorption studies probing vibrational coherence have seen reduced amplitude of the

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oscillations in the center of the linear spectrum and observed phase flips in their

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Figure 4: (a) Correlation spectrum of HBQ displaying the coupling of the high-frequency νOH mode with various low frequency modes. The top panel plots the FTIR spectrum of HBQ (red solid line) and the intensity of the IR pump spectrum (blue dotted line). The inset shows the structure of HBQ with the calculated dipole vectors associated with the low-frequency mode at 241 cm-1. (b) Correlation spectrum of HBT displaying the coupling of the high-frequency νOH mode with various low-frequency modes. The top panel plots the FTIR spectrum of HBT (red solid line) and the intensity of the IR pump spectrum (blue dotted line). The inset shows the structure of HBT with the calculated dipole vectors associated with the low frequency mode at 108 cm-1.

pump-probe signals [54-56]. Our current signal-to-noise and the residual chirp in our BBIR pulse do not allow for the measurement of the phase of the oscillatory signal and we cannot comment on the phase- flips at the edges of the linear

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absorption spectra. We note that the previous IR pump-probe studies of DBQ have observed the phase-flip for the oscillatory pump-probe signal [28]. Figure 4(b) shows the correlation spectrum for HBT. We see that the νOH stretch is most strongly coupled to the vibrational mode at 118 cm-1 and less strongly coupled to modes at 180 cm-1 and 220 cm-1. The amplitude of the 118 cm-1 oscillation is stronger at the center of the pump pulse spectrum. We do not pump the FTIR spectrum at the blue edge due to the limited bandwidth in our pump pulse. We note that the mode at 220 cm-1 could arise from the solvent response (mode at 218 cm-1) or from the solute. Our ab initio frequency calculations find a mode at 240 cm-1 (see Table 1), which could correspond to the 220 cm-1 vibration seen in Figure 4(b). The DFT calculations are useful in visualizing the low-frequency modes and describing how they modulate the high-frequency νOH stretch. The anharmonic Table 1: Description of the relevant low-frequency modes calculated for HBQ and HBT with the anharmonic correction (). (ip = in-plane, oop = out-of-plane) HBQ

HBT

j (cm-1)/ βijj (cm-1) Intensity (a.u.)

j (cm-1) Intensity (a.u.)

/ βijj (cm-1)

'Butterfly motion', Symmetric oop bend

86.89/1.59

-18.94

64.58/0.98

102.18

Symmetric oop bend

208.06/0.03

5.13

202.59/0.62

-17.80

(-Py) or β(-C3H3NS), Symmetric oop 220.02/3.81 bend

21.69

239.62/0.04

-45.48

'Cogwheel motion', Symmetric ip bend 241.81/3.38 (see Fig. 4)

59.46

108.47/0.97

-67.95

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calculations provided cubic coupling terms (βijj) of low-frequency structural modes (j) to the high frequency νOH mode (i). Table 1 highlights the low frequency modes that are most strongly coupled to the νOH mode and contribute to the correlation spectra shown in Figure 4. The anharmonic frequency calculations for HBQ reveal that a mode at 242 cm-1 is coupled to the νOH mode. We note that the frequency of this mode matched closely with the peak seen at 248 cm-1 in Figs. 3(a) and 4(a). The calculations describe this mode to have a ‘cogwheel’ motion or a symmetric in-plane bending motion and the dipole vectors for the motions of the individual atoms in HBQ are plotted in Fig. 4(a). We note that this low-frequency mode modulates the O...N distance, which shows how this motion would affect H-bonding and proton transfer in the HBQ molecule. The angle between the transition dipole moment of the calculated 242 cm-1 mode and the calculated νOH mode is 22.13°. Among the modes listed in Table 1, the 242 cm-1 mode has the highest cubic coupling followed by the mode at 220 cm-1 (symmetric out-of-plane bend).

The experimental

correlation spectrum for HBQ in Fig. 4(a) also shows a mode at 220 cm-1, which is less strongly coupled than the mode at 248 cm-1. The calculations also yield a νOH fundamental frequency of 2716 cm-1 which closely matches the fundamental frequency of the FTIR at 2771 cm-1. Complete results with all the relevant modes (0 – 400 cm-1) from the anharmonic calculations are shown in the Supplementary

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Information (Table S1) along with illustrations of the vibrational motions (see Figs. S4 and S5). The anharmonic frequency calculations for HBT finds a strongly coupled lowfrequency mode at 102 cm-1 involving the cogwheel motion and modulating the O...N distance in HBT. We assign the experimentally determined low-frequency mode at 118 cm-1 to this vibrational motion (see inset of Fig. 4(b)). The angle between the transition dipole moment of the 102 cm-1 mode and the calculated νOH mode is 34.55°. From Table 1, the symmetric out-of-plane bend has the highest cubic coupling strength and is calculated at 65 cm-1. We are unable to resolve this mode from our experimental data. In comparison to HBQ, the calculated frequencies of the structural modes in HBT do not quantitatively agree with the experimental data. The greater structural flexibility of the molecule could account for the apparent discrepancy. The experimentally determined correlation of the νOH mode with the lowfrequency modes in HBQ and HBT complex strongly suggest that anharmonic coupling through low-frequency structural modes gives rise to the broad FTIR lineshapes seen in Figure 1. The clear difference in the strength of the oscillatory signals between HBT and HBQ in the fundamental region may be attributed to the difference in the skeletal moieties of the two molecules. HBQ has a fairly planar structure due to the three rings that force the molecule to be planar while HBT has a

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single bond connecting the rings (between the proton donor and acceptor) that allows for more structural flexibility.. The additional flexibility in bringing the H donor and acceptor atoms in HBT can be the reason behind the observed increased intensity of the similar cogwheel motion between the two molecules.. The differences in the observed vibrational dynamics of the νOH mode and the anharmonic couplings with low-frequency skeletal modes could also result from the presence of the sulfur atom in HBT. The shared electron density across the intramolecular hydrogen bond will be affected by the presence of the sulfur atom in HBT and this could affect the vibrational dynamics of the νOH stretch in the ground state. It has recently been shown in Ref. 19 that the excited state proton transfer process can be described as a photoinduced proton coupled electron transfer process, which suggests that modulation of the electron density across the intramolecular hydrogen bond could affect the ground state nuclear dynamics of the νOH mode. 4. Summary In summary, this study reveals how the low-frequency vibrations modulating the distance between the donor and acceptor atoms in intramolecular hydrogen bonded systems play an important role in the vibrational dynamics of the high-frequency νOH mode. Femtosecond BBIR experiments measuring the vibrational dynamics of the νOH mode in prototypical intramolecular hydrogen bonded complexes, HBQ and HBT revealed that low-frequency structural modulations at 245 cm-1 for HBQ and

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112 cm-1 for HBT are anharmonically coupled to the OH mode. Anharmonic frequency calculations agreed with the experimental results. Future polarizationselective IR pump-probe experiments could reveal additional low-frequency modes coupled to the high-frequency stretches. Previous femtosecond studies on HBQ and HBT have implicated the same low-frequency modes in the ultrafast proton transfer process. Our studies reveal that these modes are important descriptors of the Hbonding dynamics on the electronic ground state. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Acknowledgements This work is supported by the National Science Foundation (NSF) (Grant no. CHE 1565759). M.K. Thanks the Camille and Henry Dreyfus Foundation for fellowship support.

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