CH Stretching Region: Computational Modeling of Vibrational Optical Activity

Abstract

Most organic compounds provide vibrational spectra within the CH stretching region, yet the signal is difficult to interpret because of multiple difficulties in experiment and modeling. To better understand various factors involved, the ability of several harmonic and anharmonic computational approaches to describe these vibrations was explored for α-pinene, fenchone, and camphor as test compounds. Raman, Raman optical activity (ROA), infrared absorption (IR), and vibrational circular dichroism (VCD) spectra were measured and compared to quantum chemical computations. Surprisingly, the harmonic vibrational approach reasonably well reproduced the measured spectral patterns, including the vibrational optical activity (VOA). The CH stretching, however, appeared to be more sensitive to the basis set and solvent variations than lower-frequency vibrations. For a higher accuracy in frequencies and spectral shapes, anharmonic corrections were necessary. Accurate harmonic and anharmonic force fields were obtained with the mPW2PLYP double-hybrid functional. A limited vibrational configuration interaction (LVCI) where the CH stretching motion was decoupled from other vibrations provided the best simulated spectra. A balanced harmonic oscillator basis set had to be used, containing also states indirectly interacting with fundamental vibrations. A simpler second-order perturbational approach (PT2) appeared less useful. The modeling provided unprecedented agreement with experimental vibrational frequencies; spectral shapes were reproduced less faithfully. The possibility of ab initio interpretation of the CH spectral region for relatively large molecules further broadens the application span of vibrational spectroscopy.

Cocaine Hydrochloride Structure in Solution Revealed by Three Chiroptical Methods

“…Most of the frequencies obtained at the B3PW91 level are closer to experiment than those obtained at the mPW2PLYP level, in spite of the latter methodb eing more sophisticated [19] and computationally more expensive. Furthermore, the differences between computed and experimental frequencies are rather minor (typically within 0-20 cm À1 )a nd can be explained by intrinsic error of the computationalm ethods, both treating the correlation energy rather incompletely,a nd by anharmonic interactions [20] not included in the modeling. However,t he calculated frequencies do not differ much,e xcept for the ND stretching mode.…”

Vibrational spectroscopy of Methyl benzoate

“…hydrogen-bonded complexes with surrounding water. Among others, the discrete variable representation (DVR), [21][22][23] diffusion quantum Monte Carlo (DQMC), [24][25][26] vibrational configuration interaction (CI), [27][28][29] and vibrational self-consistent field (VSCF) [30][31][32][33] methods proved their applicability to study anharmonic effects in systems with varying sizes. The main problem of anharmonic spectroscopic calculations is that different vibrational modes are not mutually separable like in the harmonic approximation.…”

Relative intensity correction of Raman optical activity spectra facilitates extending the spectral region

“…Raman optical activity (ROA) is a relatively novel chiroptical variant of vibrational spectroscopy that is rich in structural information but experimentally rather difficult. The advances both in experimental techniques [multichannel charge‐coupled device (CCD) detectors, holographic edge and notch filters, simultaneous acquisition of signals at both circular polarizations and fiber optics] and theoretical calculations [density functional theory (DFT) procedures, advances in electron correlation description and anharmonic corrections] have made ROA applicable to a wide range of chemistry, biochemistry and biophysics‐related problems. On the basis of the increased quality of experimental ROA data, it becomes interesting to pay closer attention to numerical intensities.…”

“…in 1975 using an instrument recording one spectral point at a time. There are also recent ROA data on C–H and C–D vibrations . With current multichannel spectrometers, measurements in the extended spectral range can be realized by either compressing the spectrum along the CCD detector or by combining data from several shorter spectral subregions, each of them measured separately with a region‐optimized arrangement, particularly grating .…”

“…Such well‐defined data having higher numerical accuracy could provide suitable material for testing quality of methods used for simulation of vibrational spectra. In relation to our previous study, we report comparison between corrected and uncorrected experimental spectra and theoretical simulations. This is easier for the Raman spectra, as their calculations are considerably less complicated.…”