Comprehensive Quantum Chemical Characterization of the Astrochemically Relevant HC_nH Chain Family: An Attempt to Aid Astronomical Observations

Abstract

In this paper, the results of a comprehensive theoretical investigation of neutral HC n H 0 and charged HC n H ± chains are reported using state-of-the-art compound model chemistries (W1BD, G4, CBS-QB3, CBS-APNO, CBS-4M) and single point methods (including "gold standard" CCSD(T)). The properties envisaged include electronic and chemical bonding structure data, enthalpies of formation, adiabatic and vertical ionization (IP), and electron attachment (EA) energies, gas phase acidities, C−H bond strengths, and chain "deprotecting" (HC n H → H + C n + H) energies. The best theoretical estimates based on composite models reported here for the various properties investigated agree with the available experimental data and are better than all previous theoretical estimates. To exemplify, for the enthalpies of formation, the G4 composite model achieves a RSMD (root square mean deviation) of 0.28 kcal mol −1 and a MUD (maximum unsigned deviation) of 0.45 kcal mol −1 . Most importantly from the astrochemical perspective, it is predicted that the stable HC n H − anions with n ≤ 8 are nonlinear. Having sufficiently large dipole moments, the anion HC n H − cis conformers are potential candidates for detection in space via rovibrational spectroscopy.

Critical analysis of radical scavenging properties of atorvastatin in methanol recently estimated via density functional theory

“…Tables 1, S2, and S3) we performed unrestricted (UB3LYP) calculations. Therefore, it is worth mentioning that, similar to other similar cases [19, 20, 21, 22], we can safely rule out spin contamination artifacts (cf. Table 1).…”

Why Ortho- and Para-Hydroxy Metabolites Can Scavenge Free Radicals That the Parent Atorvastatin Cannot? Important Pharmacologic Insight from Quantum Chemistry

“…BDE for H-atom abstraction at positions 1-OH, 2-OH, and 3-OH in ATV, o-ATV and p-ATV is basically the same. The differences between the values calculated by us for ATV, o-ATV, and p-ATV amounting to at most 0.5 kcal/mol are certainly irrelevant; recall that we showed recently [20] that even for much smaller molecules in vacuo DFT/B3LYP calculations with the largest Pople basis set 6-311++G(3df,3pd) are far away from “chemical” accuracy (∼ 1 kcal/mol). In fact, p-ATV’s numerical value of PA=58.2 kcal/mol somewhat differs from ATV’s (and o-ATV’s) PA=61.5 kcal/mol, but if heterolytic O-H bond cleavage were to occur in p-ATV, it would rather occur at position 1-OH, which has a substantially smaller value PA=23.8 kcal/mol.…”

Why Ortho- and Para-Hydroxy Metabolites Can Scavenge Free Radicals That the Parent Atorvastatin Cannot? Important Pharmacologic Insight from Quantum Chemistry

“…From experience with much smaller molecules and much simpler chemical structures (e.g., ref. [ 20 ]) we had to learn that achieving this accuracy for bond dissociation enthalpies and proton affinity (BDE and PA, quantities entering the discussion that follows) is often illusory even for extremely computationally demanding state-of-the-art compound model chemistries (CBS-QB3, CBS-APNO, G4, W1BD); see, e.g., Figure 10 of ref. [ 20 ].…”

“…[ 20 ]) we had to learn that achieving this accuracy for bond dissociation enthalpies and proton affinity (BDE and PA, quantities entering the discussion that follows) is often illusory even for extremely computationally demanding state-of-the-art compound model chemistries (CBS-QB3, CBS-APNO, G4, W1BD); see, e.g., Figure 10 of ref. [ 20 ]. DFT-calculations done by us and by others [ 21 ] revealed that, e.g., errors in ionization potential can amount up to 0.7 eV (16 kcal/mol) even when employing the functional B3LYP and the largest Pople basis set 6-311++G(3df,3pd).…”