Accurate structure, thermodynamic and spectroscopic parameters from CC and CC/DFT schemes: the challenge of the conformational equilibrium in glycine

Abstract

The structures, relative stabilities, and infrared spectra of the six low-energy conformers of glycine have been characterized using a state-of-the-art quantum-mechanical approach allowing the bond distances, conformational enthalpies and vibrational frequencies to be determined well within the chemical accuracy. Transition state structures governing interconversion among the different energy minima have also been characterized. In detail, the gas-phase thermodynamic properties (at 15 K and 410 K) of the glycine conformers considered have been obtained with a 1 kJ mol(-1) accuracy, and it has been shown that the employment of DFT geometries usually reduces such accuracy by at most 0.1 kJ mol(-1). Regarding molecular structures, the use of two different composite schemes allowed us to further confirm the suitability of a rather cost-effective approach and provide geometrical parameters with an overall accuracy better than 0.002 Å for distances and 1 degree for angles. Thanks to a hybrid CC/DFT approach, the infrared spectra of all conformers considered and of several deuterated isotopologues have been reproduced (when experimental data were available) or predicted with an accuracy of 10 cm(-1). Finally, the joint thermodynamic and spectroscopic investigation allowed us to shed some light on the possible observation of elusive conformers. On the whole, the high accuracy of the computational results allows us to draw a fully consistent interpretation of the available experimental data and to obtain a more complete characterization of the potential energy surface of glycine.

Aiming at an accurate prediction of vibrational and electronic spectra for medium‐to‐large molecules: An overview

“…As SNSD. [26,114,184,201] Up to now, focus has been given on improving the vibrational states' energies, hence the band positions. To get band-shapes comparable to experiment, it is necessary to treat correctly the transition intensities as well.…”

“…A good illustration is the case of glycine, for which the three lowest energy conformers (Ip/ttt, IIn/ccc, and IIIp/tct, see Figure 24) have been concomitantly identified through their IR vibrational features in the low-temperature matrix environment. [201,217,218] In this condition, the identification of each conformer is based on the analysis of the intense vibrational transitions that remarkably differ in frequency from one conformer to another, for instance the C@O stretch, which are highlighted in Figure 24. The simulation of the overall spectra, which can be directly compared with experiment, is then performed by applying weighted contributions of each conformer according to their abundance obtained from the theoretical Boltzmann population at experimental conditions, such as the temperature.…”

“…However, the full HRAO approach is able to provide relative free energies and theoretical Boltzmann population at 410 K for all conformers in excellent agreement with experimental estimates (i.e., Ip 5 77%, IIn 5 18%, IIIp 5 6%, see reference 201 for the details). The overall simulated spectrum obtained by weighting the contribution of each conformer according to its abundance, is also depicted in Figure 24 and Theoretical IR spectra of the most stable conformers of glycine computed at the CC/DFT level [201] : single contributions from Ip/ttt, IIn/ccc, and IIIp/tct, and their sum weighted with relative abundances at 410 K (Ip-IIn-IIIp) compared to the experimental IR spectrum recorded in low-temperature Ar matrix. [217] Red, blue, and green arrows mark the bands assigned to the Ip/ttt, IIn/ccc, and IIIp/tct conformers, respectively.…”

Correct Modeling of Cisplatin: a Paradigmatic Case

“…Even more accurate results, rivaling the most refined experimental techniques, can be obtained by employing composite schemes 12. We now have determined the equilibrium structure and harmonic vibrational frequencies of cisplatin with a “cheap” computational protocol,13 which is detailed in the Supporting Information (SI).…”

Correct Modeling of Cisplatin: a Paradigmatic Case

“…[12] We now have determined the equilibrium structure and harmonic vibrational frequencies of cisplatin with a"cheap" computational protocol, [13] which is detailed in the Supporting Information (SI). As to vibrational frequencies, best-estimated harmonic values have been combined with anharmonic effects at the DFT level (B3PW91-D3 [14] and B2PLYP-D3 [15] functionals), thus leading to the well-documented hybrid CCSD(T)/DFT approach (see for example Ref.…”