Anharmonic DFT Study of Near-Infrared Spectra of Caffeine: Vibrational Analysis of the Second Overtones and Ternary Combinations

Abstract

Anharmonic quantum chemical calculations were employed to simulate and interpret a near-infrared (NIR) spectrum of caffeine. First and second overtones, as well as binary and ternary combination bands, were obtained, accurately reproducing the lineshape of the experimental spectrum in the region of 10,000–4000 cm−1 (1000–2500 nm). The calculations enabled performing a detailed analysis of NIR spectra of caffeine, including weak bands due to the second overtones and ternary combinations. A highly convoluted nature of NIR spectrum of caffeine was unveiled, with numerous overlapping bands found beneath the observed spectral lineshape. To properly reflect that intrinsic complexity, the band assignments were provided in the form of heat maps presenting the contributions to the NIR spectrum from various kinds of vibrational transitions. These contributions were also quantitatively assessed in terms of the integral intensities. It was found that the combination bands provide the decisively dominant contributions to the NIR spectrum of caffeine. The first overtones gain significant importance between 6500–5500 cm−1, while the second overtones are meaningful in the higher wavenumber regions, particularly in the 10,000–7000 cm−1 region. The obtained detailed band assignments enabled deep interpretation of the absorption regions of caffeine identified in the literature as meaningful for analytical applications of NIR spectroscopy focused on quantitative analysis of caffeine content in drugs and natural products.

Anharmonic Aspects in Vibrational Circular Dichroism Spectra from 900 to 9000 cm–1 for Methyloxirane and Methylthiirane

“…Also in the Δν = 3 region, the agreement with experiment, both in terms of energy and intensity, is remarkable. Though the simulation of NIR spectra in the three-quanta region within GVPT2 framework is gaining popularity in literature, 68 − 70 it should be noted here that these are among the first GVPT2 VCD spectra reported. At Δν = 3, combinations and overtones of normal modes combine together to give the local-mode picture 16 as first proposed by Lehmann 71 and embraced by others 5 ( vide infra ).…”

Development of ANN Models for Prediction of Physical and Chemical Characteristics of Oil-in-Aqueous Plant Extract Emulsions Using Near-Infrared Spectroscopy

“…There are additional variances in the amplitude of typical wavelength bands extending from 900 to 1050 nm, 1100 to 1300 nm, and 1400 to 1699 nm. The C-H and N-H 3rd overtones, the C-H 2nd overtone, and the O-H and N-H 2nd overtones, respectively, correspond to selected spectral areas [38,39]. The wavelength region from 1400 to 1699 nm, which is unique for the superposition of the O-H bonds, shows the most obvious variations in the spectral profiles of both sample types.…”

Contribution in the quality assessment of Myrrh

“…8430-8300 (the second overtone of CH stretching vibrations at 8300 cm −1 that corresponds to methyl (-CH3), methylene (-CH2), and olefin (-CH=CH-) bonds (Grassi et al, 2021)7367-7320 , 7250-7220 assigned to the C-H stretching vibration, (Bai et al, 2023), or CH in plane bending and ring in plane bending. 6009-5900, 5800-5815, 5690, 5560, 5530-5360, 5250-5040, 5000-4820 (C=O stretching) (Grabska et al, 2021) 4400-4370, 4310-4280 represent CH and CH2 s overtones3920-3800 A VIP plot showed the importance of the variable in descending order (Fig. 22) A VIP score of 0.9 was chosen as a threshold value for the variables that significantly contribute to accurate prediction.…”