Vibrational Modes and Phonon and Thermodynamic Properties of the Metaboric Acid Polymorphs α-, β-, and γ-(BOH)₃O₃ within a Density Functional Theory Framework

Abstract

A combined study of vibrational and thermodynamic properties of metaboric acid (BOH)O crystal polymorphs α, β, and γ were obtained through density functional theory (DFT) calculations in an attempt to resolve the conflicting assignments that currently exist in the literature for them. A complete correlation between the normal-mode assignment and vibrational signatures to distinguish particular features of each metaboric acid polymorph, in particular, those related to motions of the planar layers in α-(BOH)O, with a level of detail surpassing essays based on previous published experimental works has been achieved. Besides, no DFT-based research work was published early on the (BOH)O polymorph vibrational properties, and our DFT-simulated infrared and Raman spectra for all metaboric acid polymorphs agree very well with experiment. Comparison of the previously published experimental IR and Raman spectroscopic results with predictions from higher levels DFT calculations allows identification of the in-plane and out-of-plane B-O bending modes. For example, the strongest measured (DFT-calculated) Raman modes of α-(BOH)O at 591 and 797 cm (599 and 810 cm) are identified as vibrational signatures of breathing BO/A in-plane modes, while the shoulder in the lattice modes region at 135 (143) cm is the vibrational signature of the bending BO/B out-of-plane mode. Phonon-dispersion bands and their respective phonon densities of states were also evaluated for each system, as well as temperature-dependent curves for entropy, enthalpy, free energy, heat capacity, and Debye temperature. Phonon dispersion curves are singular for each (BOH)O species, and a consistent gap decrease between the lowest and highest frequency vibrational bands was observed. The DFT-based calculations also revealed that the noncovalent interactions prevalent in the α and β crystals lead to significant differences with respect to the thermodynamic properties in comparison with the γ phase.

Cocrystals of 2-Aminopyrimidine with Boric Acid—Crystal Engineering of a Novel Nonlinear Optically (NLO) Active Crystal

“…with 2-aminopyrimidine modes) at 1550-1370 cm −1 and 1250-1150 cm −1 (νBO and δBOH modes), 850-650 cm −1 (γBO 3 modes), and 550-510 cm −1 (δOBO and δBOH modes). The shape of these bands, especially in the IR spectrum of AMP(H 3 BO 3 ) 2 (see Figure 8), is very similar to that of crystalline boric acid [32] (see Figure S3, Supplementary Materials). δCH, νC-NH2 Abbreviations and Greek symbols used for vibrational modes: rg, ring; s, symmetric; ν, stretching; δ, deformation or in-plane bending; γ, out-of-plane bending; ρ, rocking; ω, wagging; τ, twisting; and X = N, O.…”

“…The presence of boric acid molecules in the crystal structure is clearly evident as strong to medium intensity bands (partly overlapping with 2-aminopyrimidine modes) at 1550-1370 cm −1 and 1250-1150 cm −1 (νBO and δBOH modes), 850-650 cm −1 (γBO3 modes), and 550-510 cm −1 (δOBO and δBOH modes). The shape of these bands, especially in the IR spectrum of AMP(H3BO3)2 (see Figure 8), is very similar to that of crystalline boric acid [32] (see Figure S3, Supplementary Materials). with 2-aminopyrimidine modes) at 1550-1370 cm −1 and 1250-1150 cm −1 (νBO and δBOH modes), 850-650 cm −1 (γBO 3 modes), and 550-510 cm −1 (δOBO and δBOH modes).…”

“…The number of expected normal modes of the studied crystals was determined by nuclear site group analysis [31]. The crystals of (2-AMP)3(H3BO3)2 and 2-AMP(H3BO3)2 belong to the space groups P3221 ( IR and Raman spectra (see Figures 7 and 8 and Tables 3 and 4) were assigned according to results from our quantum chemical calculations and recently published study on boric acid [32]. The assignment of the bands of stretching and out-of-plane bending vibrations of the N-H and O-H groups involved in hydrogen bonding is based on correlation curves [33,34] concerning the position of the vibrational bands and appropriate hydrogen bond lengths.…”

“…The assignment of the bands of stretching and out-of-plane bending vibrations of the N-H and O-H groups involved in hydrogen bonding is based on correlation curves [33,34] concerning the position of the vibrational bands and appropriate hydrogen bond lengths. IR and Raman spectra (see Figures 7 and 8 and Tables 3 and 4) were assigned according to results from our quantum chemical calculations and recently published study on boric acid [32]. The assignment of the bands of stretching and out-of-plane bending vibrations of the N-H and O-H groups involved in hydrogen bonding is based on correlation curves [33,34] concerning the position of the vibrational bands and appropriate hydrogen bond lengths.…”

“…The crystals of (2-AMP) 3 (H 3 BO 3 ) 2 and 2-AMP(H 3 BO 3 ) 2 belong to the space groups P3 2 21 (D 6 3 ) and C2/c (C 6 2h ) with 27 atoms (Z = 3) and 15 atoms (Z = 4) per asymmetric unit, respectively. In contrast to atoms N22, C21, C23, and H23 (see Figure 2), which occupy three-fold Wyckoff positions b(C 2 ), all remaining atoms of the asymmetric unit of (2-AMP) 3 Symmetry analysis of the optical vibrational modes (see Table 2) gave 14A 1 (Ra) + 15A 2 (IR) + 29E(IR,Ra) representations for the external modes, 59A 1 (Ra) + 61A 2 (IR) + 120E(IR,Ra) representations for the internal modes of (2-AMP) 3 Tables 3 and 4) were assigned according to results from our quantum chemical calculations and recently published study on boric acid [32]. The assignment of the bands of stretching and out-of-plane bending vibrations of the N-H and O-H groups involved in hydrogen bonding is based on correlation curves [33,34] concerning the position of the vibrational bands and appropriate hydrogen bond lengths.…”