Abstract
Near-infrared (12,500–4,000 cm−1; 800–2,500 nm) spectroscopy is the hallmark for one of the most rapidly advancing analytical techniques over the last few decades. Although it is mainly recognized as an analytical tool, near-infrared spectroscopy has also contributed significantly to physical chemistry, e.g., by delivering invaluable data on the anharmonic nature of molecular vibrations or peculiarities of intermolecular interactions. In all these contexts, a major barrier in the form of an intrinsic complexity of near-infrared spectra has been encountered. A large number of overlapping vibrational contributions influenced by anharmonic effects create complex patterns of spectral dependencies, in many cases hindering our comprehension of near-infrared spectra. Quantum mechanical calculations commonly serve as a major support to infrared and Raman studies; conversely, near-infrared spectroscopy has long been hindered in this regard due to practical limitations. Advances in anharmonic theories in hyphenation with ever-growing computer technology have enabled feasible theoretical near-infrared spectroscopy in recent times. Accordingly, a growing number of quantum mechanical investigations aimed at near-infrared region has been witnessed. The present review article summarizes these most recent accomplishments in the emerging field. Applications of generalized approaches, such as vibrational self-consistent field and vibrational second order perturbation theories as well as their derivatives, and dense grid-based studies of vibrational potential, are overviewed. Basic and applied studies are discussed, with special attention paid to the ones which aim at improving analytical spectroscopy. A remarkable potential arises from the growing applicability of anharmonic computations to solving the problems which arise in both basic and analytical near-infrared spectroscopy. This review highlights an increased value of quantum mechanical calculations to near-infrared spectroscopy in relation to other kinds of vibrational spectroscopy.
Highlights
Near-infrared spectroscopy has distinguished itself by a remarkable evolution from an undervalued section of vibrational spectroscopy to one of the presently most widespread modern analytical techniques with strong prospects for further expansion (Ciurczak and Drennen, 2002; Siesler et al, 2002; Cozzolino, 2014; Huck, 2014, 2016a; Ozaki et al, 2017)
The aim of the present review is to provide a comprehensive introduction to theoretical NIRS and to expound its distinctiveness and exceptional significance for the contemporary progress of applied spectroscopy (Graphical Abstract)
The 2D-COS technique allows the expanding of the spectral information onto two-dimensions, basically elucidating the correlations in the spectral variations and visualizing straightforwardly the complex dependencies which would be otherwise difficult to trace in linear NIR spectra
Summary
Near-infrared spectroscopy (commonly abbreviated as NIRS) has distinguished itself by a remarkable evolution from an undervalued section of vibrational spectroscopy to one of the presently most widespread modern analytical techniques with strong prospects for further expansion (Ciurczak and Drennen, 2002; Siesler et al, 2002; Cozzolino, 2014; Huck, 2014, 2016a; Ozaki et al, 2017). The primary parameters of the resulting bands, wavenumbers and intensities are ruled by anharmonic effects with intermode anharmonicity playing the most substantial role In this sense, NIRS distinctively sets itself apart from the other kinds of vibrational spectroscopy (mid-infrared, MIR; far-infrared, FIR; Mantsch and Naumann, 20101 and Raman) in which the major chemical information originates from fundamental vibrational transitions. NIRS distinctively sets itself apart from the other kinds of vibrational spectroscopy (mid-infrared, MIR; far-infrared, FIR; Mantsch and Naumann, 20101 and Raman) in which the major chemical information originates from fundamental vibrational transitions Unlike these latter ones, which put the harmonic approximation into good use, theoretical NIRS unequivocally requires computationally intensive anharmonic approaches. Throughout the following sections, the underlying phenomena and key correspondences are thoroughly explained, together with a brief history sketch reaching the most recent accomplishments and future prospects, outlining a complete overview of the highly promising and boundary-crossing development currently taking place in the field of NIRS
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