Critical interpretation of CH– and OH– stretching regions for infrared spectra of methanol clusters (CH3OH)n (n = 2–5) using self-consistent-charge density functional tight-binding molecular dynamics simulations

Abstract

Vibrational infrared (IR) spectra of gas-phase O-H⋅⋅⋅O methanol clusters up to pentamer are simulated using self-consistent-charge density functional tight-binding method using two distinct methodologies: standard normal mode analysis and Fourier transform of the dipole time-correlation function. The twofold simulations aim at the direct critical assignment of the C-H stretching region of the recently recorded experimental spectra [H.-L. Han, C. Camacho, H. A. Witek, and Y.-P. Lee, J. Chem. Phys. 134, 144309 (2011)]. Both approaches confirm the previous assignment (ibid.) of the C-H stretching bands based on the B3LYP/ANO1 harmonic frequencies, showing that ν3, ν9, and ν2 C-H stretching modes of the proton-accepting (PA) and proton-donating (PD) methanol monomers experience only small splittings upon the cluster formation. This finding is in sharp discord with the assignment based on anharmonic B3LYP/VPT2/ANO1 vibrational frequencies (ibid.), suggesting that some procedural faults, likely related to the breakdown of the perturbational vibrational treatment, led the anharmonic calculations astray. The IR spectra based on the Fourier transform of the dipole time-correlation function include new, previously unaccounted for physical factors such as non-zero temperature of the system and large amplitude motions of the clusters. The elevation of temperature results in a considerable non-homogeneous broadening of the observed IR signals, while the presence of large-amplitude motions (methyl group rotations and PA-PD flipping), somewhat surprisingly, does not introduce any new features in the spectrum.

A systematic benchmarking of computational vibrational spectroscopy with DFTB3: Normal mode analysis and fast Fourier transform dipole autocorrelation function

“…Without a sufficient conformational sampling, the FT‐DAC method predicted three different spectral signatures in the ∼3600 cm –1 region. To overcome this issue, the FT‐DAC spectra based on multiple independent trajectories were obtained . The convergence of the FT‐DAC spectra with different levels of conformational sampling was monitored.…”

“…This includes by decomposing the spectral peaks as a sum of effective vibrational modes extracted from the FT‐VAC or by describing the localized vibrational modes as a linear combination of nonredundant internal coordinates . Nishimura et al used the FT‐TAC of symmetrized internal vibrational modes in the prediction of vibrational spectra of methanol dimer . Bowman et al and Kaledin et al developed driven molecular dynamics approach to perform full normal mode analysis without calculating the Hessian matrix for larger biomolecular systems.…”

“…As the molecules in our database are relatively rigid, we decided to assign the FT‐DAC peaks to the closest NMA vibrational modes. If the mean absolute percentage error (MAPE) between NMA and FT‐DAC vibrational mode is more than 3% (estimated MAPE) or the assignment is ambiguous (e.g., peak overlaps), we rechecked the FT‐DAC peak attributes with the FT‐TAC of the localized vibrational modes, following the work by Nishimura et al Out of 307 vibrational modes, 19 FT‐DAC vibrational modes were deviated from the NMA vibrational modes by more than 3% (MAPE). They were listed in the Supporting Information Table S1.…”

Comparative studies of IR spectra of deprotonated serine with classical and thermostated ring polymer molecular dynamics simulations

“…Several methods have been proposed to characterize the FT-DAC spectra peaks (see detailed discussions in Ref. 26 ). Among them, Nishimura et al implemented the Fourier transform of the localized vibrational mode autocorrelation (FT- ν N AC) function to characterize the vibrational bands in methanol dimer.…”

A combination of near infrared and mid-infrared spectroscopy to improve the determination efficiency of active components in Radix Astragali

“…According to the literature, the characteristic absorption of O-H appears at 5550-4550 cm À1 . 37,38 This broad peak is mainly caused by the combined frequency of stretching vibration and bending vibration. It also contains the bending vibration of C-H and is often used for quantitative analysis.…”