Spectral response of crystalline acetanilide and $N$-methylacetamide: Vibrational self-trapping in hydrogen-bonded crystals

Femtosecond pump-probe and Fourier transform infrared spectroscopy is applied to compare the spectral response of the amide I band and the NH-stretching band of acetanilide (ACN) and $N$-methylacetamide (NMA), as well as their deuterated derivatives. Both molecules form hydrogen-bonded molecular crystals that are regarded to be model systems for polypeptides and proteins. The amide I bands of both ACN and NMA show a temperature-dependent sideband, while the NH bands are accompanied by a sequence of equidistantly spaced satellite peaks. These spectral anomalies are interpreted as a signature of vibrational self-trapping. Two different types of states can be identified in both crystals in the pump-probe signal: a delocalized free-exciton state and a set of localized self-trapped states. The phonons that mediate self-trapping in ACN and deuterated ACN are identified by their temperature dependence, confirming our previous results. The study shows that the substructure of the NH band in NMA (amide $A$ and amide $B$ bands) originates, at least partly, from vibrational self-trapping and not, as often assumed, from a Fermi resonance.

Figure 1

(a) Absorption spectrum of the CO band of crystalline ACN, showing a temperature-dependent doublet. The peak at $1650{\text{cm}}^{-1}$ corresponds to a self-trapped state and the one at $1666{\text{cm}}^{-1}$ to a free exciton. (b) Absorption spectrum of the NH band, consisting of a main peak at $3295{\text{cm}}^{-1}$ (free exciton) and a sequence of satellite peaks (self-trapped states). The proposed schemes of potential energy surfaces are depicted on the right-hand side. $E_{BD}$ represents the binding energy of the self-trapped state.

Figure 2

IR spectrum of the NH band of ACN, ACN-${\mathrm{D}}_8$, NMA, and NMA-${\mathrm{D}}_6$ at high temperatures (a)–(d) and at low temperatures (e)–(h). All four crystals show similar band shapes, consisting of a main peak and a sequence of three to four satellite peaks. The CH modes, the amide I overtone (A) and the $B$ modes are not part of the NH mode. The thick lines are for parallel polarization of the IR light with respect to the hydrogen-bonded chain and the thin lines for perpendicular polarization. For NMA-${\mathrm{D}}_6$ the perpendicular spectra are magnified by a factor of 10, for ACN by a factor of 4. The self-trapped states are marked by black bars and the free-exciton peak by FE.

Figure 3

IR spectrum of the amide I band of crystalline (a) ACN and (b) NMA at three different temperatures. The vertical line marks the position of the temperature-dependent sideband. The IR light was polarized parallel to the hydrogen-bonded chain.

Figure 4

The transient response of ACN after impulsive excitation for a probe frequency $\left(3200{\text{cm}}^{-1}\right)$ which coincides with one of the self-trapped states. The temperature is varied from 77 to $293\mathrm{K}$.

Figure 5

(a) and (b) Absorption spectra of crystalline ACN, showing the NH band for 293 and $77\mathrm{K}$. (d) and (e) The 2D plot of the absolute value of the Fourier transformation of the transient pump-probe signal after impulsive excitation. ${\omega }_{pr}$ is the frequency of the probe pulse and $\omega$ results from the Fourier transformation of the pump-probe signal with respect to delay time $t_{pu,pr}$ between pump and probe pulses. (c) and (f) Vertical cuts through the 2D spectrum at frequencies corresponding to the satellite peaks.

Figure 6

Temperature dependence of the low-frequency Raman modes $(\circ )$ in ACN, taken from Johnston et al. (Ref. 34), and of the beating frequencies observed in the pump-probe experiment $(•)$. The temperature dependence of the beating frequencies perfectly matches that of the Raman modes.

Figure 7

The transient signal of ACN and NMA after selective excitation of the free-exciton peak. In the case of ACN the signal recovers on a $400\text{fs}$ time scale at $293\mathrm{K}$ and on a $600\text{fs}$ time scale at $77\mathrm{K}$. In NMA the signal relaxes on a $1\text{ps}$ time scale.

Figure 8

(a) The transient signal of ACN-${\mathrm{D}}_8$ after impulsive excitation for a probe frequency which coincides with the free exciton, showing pronounced oscillations. (b) The Fourier-transform spectra show a sharp band at $47{\text{cm}}^{-1}$ and a broadband at $80{\text{cm}}^{-1}$.

Figure 9

Sections of the NH band of (a) ACN, (b) NMA-${\mathrm{D}}_6$, and (c) NMA. Pump-probe response after selective excitation of the high frequency band (d)–(f) and of the second satellite peaks (g)–(i) for $400\text{fs}$ $(•)$ and $4\text{ps}$ $(\circ )$ delay times between the pump and the probe pulse. The arrows indicate the position of the narrow-band pump pulse. All three molecules show qualitatively similar responses.