Orientational and low-frequency (0–450 cm−1) dynamics of methyl methacrylate: OHD-RIKES measurements and DFT calculations

Abstract This paper presents results of a study in which optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES) and density functional theory (DFT) were used to glean information about the low-frequency (0–450 cm−1) intramolecular and intermolecular dynamics and picosecond reorientational dynamics of methyl methacrylate (MMA) as a function of temperature. Based on DFT calculations, the two intramolecular bands at 220 and 380 cm−1 in this spectral region are assigned, respectively, to the (O)CH3 torsional and C C out-of-plane modes of the cis-conformer of MMA. In the 2–40 ps time range, the OHD-RIKES response of MMA can be well fit by a biexponential decay function with a fast component (relaxation time τ1) associated with intermolecular dynamics, and a slow component (relaxation time τ2) associated with collective reorientation. The temperature dependence of τ2 can be understood in terms of a modified Debye-Stoke-Einstein (DSE) equation with the rotation of MMA molecules being governed by slip boundary conditions, which is consistent with MMA being a non‑hydrogen-bonded liquid. τ1 and τ2 are found to be linearly correlated with each other, which is attributed to τ1 being dominated by structural relaxation and therefore to motional narrowing. The low-frequency broad band in the 0–200 cm−1 region of the Kerr spectrum of MMA was analyzed in terms of a multicomponent line-shape model, with the sum of the first three components being primarily associated with the intermolecular modes of MMA. With increasing temperature, the intermolecular band shifts to lower frequency and narrows with increasing temperature. The shift to lower frequency is attributed to the softening of the effective intermolecular potential due to decreasing density with increasing temperature and the narrowing of the intermolecular band due to motional narrowing. Finally, it is shown that the Kerr spectrum of MMA provides support for the interpretation of the main band of the low-frequency reduced-Raman spectrum of poly(methyl methacrylate) (PMMA) being due to the librational motion of the monomer units in the polymer. This explanation is physically reasonably based on fits of the multicomponent line-shape model to the spectra of PMMA and MMA. It is proposed that this librational motion is associated with the torsional motion of the methoxycarbonyl pendant group on the polymer chain.